Nonthermal Plasma Surface Modification of Materials 9819945054, 9789819945054

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Masaaki Okubo

Nonthermal Plasma Surface Modification of Materials

Nonthermal Plasma Surface Modification of Materials

Masaaki Okubo

Nonthermal Plasma Surface Modification of Materials

Masaaki Okubo Department of Mechanical Engineering Graduate School of Engineering Osaka Metropolitan University Sakai, Japan

ISBN 978-981-99-4505-4 ISBN 978-981-99-4506-1 (eBook) https://doi.org/10.1007/978-981-99-4506-1 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore Paper in this product is recyclable.

Preface

This book describes various types of nonthermal plasma surface modification technologies for materials at varying atmospheric-pressure and low-temperature conditions, and it is among the first to address these topics. Furthermore, this book aims to bridge the gap between fundamental and technical aspects with respect to industrial applications in material and plasma engineering. The main objective of this book is to provide readers with an easy-to-understand resource that outlines the foundations and application potential of nonthermal plasma surface modifications. In recent years, the surface modification of materials has gained considerable attention in the field of advanced manufacturing. The technology for the surface modification of materials can be classified into two groups: (1) reduced-pressure plasma treatment and (2) atmospheric-pressure plasma treatment. The latter has many advantages, such as compatibility with industry apparatus, because a reducedpressure environment can be avoided. Compared to other treatments, nonthermal plasma treatment offers the possibility of a strong and effective modification. Typically, surface treatment using fluoroplastics, such as perfluoroalkoxy alkane (PFA), polytetrafluoroethylene (PTFE; Teflon), and polychlorotrifluoroethylene (PCTFE), is considered an original, unique, and successful feature. Fluoroplastics have excellent characteristics such as chemical resistance, electrical insulation, heat resistance, flame resistance, and high gas barrier properties, and therefore, they are widely used. However, owing to their high hydrophobicity, they cannot be easily bonded to other materials without losing these characteristics. My group has solved this problem using our innovative atmospheric-pressure plasma hybrid processing technology and achieved high adhesiveness for the first time. Our technology is expected to be widely applied in the future. Nonthermal plasma surface modification is an interdisciplinary field that is physicochemical in nature and has applications in mechanical, electrical, and chemical engineering. Given that practitioners in these fields hail from diverse backgrounds, I strove to devise a technology that is sufficiently self-contained to be accessible to engineers, scientists, and students from many fields.

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The contents of the book are summarized as follows: Chapter 1 defines plasma and outlines the fundamentals of nonthermal plasma technologies. A method to generate atmospheric-pressure nonthermal plasma is described. Chapter 2 addresses the fundamentals of surface treatment technologies and characterization. The principle of plasma graft polymerization, evaluation method of surface treatment effects, and apparatus used for surface modification characterization are described. Chapter 3 describes the hydrophilic treatment of polymer surfaces and its applications. The principle and methods of plasma treatment and plasma graft polymerization treatment are described. Furthermore, the application of PTFE/plastic to millimeter-wave devices, development of organic light emitting diode (OLED) elements, and improved adhesion of fluoropolymer film to butyl rubber for biomedical applications are expounded. Chapter 4 describes the hydrophilic treatment technology for textiles, filters, and glass and its applications. The surface treatment of fibers, apparel, and deodorization technology using these treated materials are described. Chapter 5 covers the hydrophobic treatment of polymer surfaces. Diamond-like carbon (DLC) plasma surface treatment hydrophobic technology, catalyst surface treatment, and trends in other surface treatments with plasma are explained. Chapter 6 describes the hydrophobic treatment of plastic, glass, and metal surfaces and its applications. The principles of plasma cleaning and surface activation are explained. Several examples of the implementation of hydrophobicity by plasma cleaning and activation are described, and plasma corrosion-resistant chemical hybrid treatments are explained. Chapter 7 describes plasma and electron-beam technologies used for surface treatment applications. The principles of electron-beam technology for surface treatment and plasma jet sterilization of surfaces are described, and respective examples are provided. Chapter 8 describes measurement technology for functional groups generated by plasma treatment. The characterization and analysis of functional groups can be used to evaluate the effect of surface modification. Suitable technologies for the analysis of functional groups and chemical species formed by plasma treatment are described. Concluding remarks summarize the contents of this book and outline future prospects. The topics treated in this book are presented as self-contained descriptions derived from literature, and Appendix presents supporting information on Sakai city where this book was written, along with the principles of measurement apparatus for surface treatment. I hope technologists in industries, academic university professors, and graduate students in engineering alike find this book useful. The unique elements of this book can be expressed as follows: 1. Covers: Principle, apparatus, methods, and industry application examples of nonthermal plasma surface modification technologies. 2. Explains: Principle and methods of nonthermal plasma surface modification technologies, in particular, knowledge on the generation of atmospheric-pressure plasma is provided.

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3. Demonstrates: Successful industry application technologies for atmosphericpressure plasma surface treatment. 4. Introduces: Principle and methods of nonthermal plasma surface modification technologies for beginner engineers and graduate students. 5. Readers: I strongly recommend this book for all those who work or intend to work on the adhesion of poorly adhesive materials. Our research group has a history of more than 20 years of performing research projects on the surface modification for many companies toward the development of new machines. These projects have provided us with a wide range of exciting experiences. We aim to share our fascination with this technology through this book to enable scientists and engineers to engage in it successfully. I believe that our research work should be documented and it is vital that these technologies are conveyed to the future generations. Most of these projects were conducted at Osaka Prefecture University (currently Osaka Metropolitan University) in Sakai city, Osaka Prefecture, Japan. Sakai city has prospered in ocean trade for centuries. In the sixteenth century, it was called “Oriental Venice” or “Saccai” to Europeans, and it has been a prosperous international trade port and home to many industries. At present, Sakai city is an important industrial city in Japan with a large factory zone in the coastal area. Sakai has a long tradition of metal manufacturing. I am delighted to publish this book on the plasma surface treatment technology from Osaka Metropolitan University, which is located in this traditional Japanese city. This book covers recent developments in nonthermal technologies and their fundamental aspects. I have also described selected applications of surface modification technologies. While some of these technologies have reached the commercial stage, others are still in early development. This book provides technical details and test results rooted in fundamentals of plasma engineering. The author first wrote each chapter separately based on materials that have been published previously as scientific papers, reviews, and book chapters, and then, the contents were knit together to maintain comprehensive unity. I am grateful to many individuals who assisted during the preparation of this book. I thank Ms. Ayako Yoden for her meticulous typing of the handwritten manuscripts. It has also been a pleasure to work with the editor of this book Mr. Smith Ahram Chae and Mr. Rajesh Manohar, Project Coordinator—Total Service. Collaborations with many colleagues over a number of years have been very enriching and enjoyable. I have discussed the future prospects of various plasma treatment systems with Dr. Tomoyuki Kuroki and Dr. Haruhiko Yamasaki of Osaka Metropolitan University, Dr. Toshiaki Yamamoto and Dr. Hidekatsu Fujishima of Osaka Prefecture University, Dr. Keiichiro Yoshida of Osaka Institute of Technology, Dr. Takuya Kuwahara of Nippon Institute of Technology, and Dr. Hashira Yamamoto of Nihon Yamamura Glass Co., Ltd. Research studies performed with many students for over 20 years at Osaka Prefecture University and Osaka Metropolitan University have been enriching and enjoyable, and most of these are credited in this book through citations of their

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published work. I have truly enjoyed studying and performing experiments on plasma treatments. I hope that this book proves beneficial not only to material engineers and students but also to professionals in other fields such as electrical, mechanical, chemical, and environmental engineering who wish to gain essential knowledge on the emerging plasma surface modification technologies. Sakai, Japan

Masaaki Okubo

Contents

1 Fundamentals of Nonthermal Plasma Technologies . . . . . . . . . . . . . . . . 1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Generation of Atmospheric-Pressure Nonthermal Plasmas . . . . . . . 1.3 What Are Plasmas? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Types of Plasmas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 Pulse Corona Plasmas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6 Dielectric Barrier Discharge-Induced Plasmas . . . . . . . . . . . . . . . . . 1.7 High-Frequency Plasmas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.7.1 Surface Discharge Plasma . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.7.2 Radio-Frequency Plasma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.7.3 Microwave-Generated Plasma . . . . . . . . . . . . . . . . . . . . . . . . 1.8 Plasma Jet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.9 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 1 1 1 2 5 6 9 9 10 12 12 15 16

2 Fundamentals of Surface Treatment Technologies and Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Mechanism of Surface Modification . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Plasma Graft Polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Apparatus for Surface Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5 Surface Characterization Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.1 Contact Angle Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.2 Evaluating Adhesive Properties by Peeling Test . . . . . . . . . 2.5.3 Result of Surface Analysis by ESCA (XPS) . . . . . . . . . . . . . 2.5.4 Result of FTIR Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.5 Result of SEM Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

17 17 17 18 19 22 22 24 26 27 29 31 34

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3 Hydrophilic Treatment for Polymer Surfaces and Its Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Plasma Treatment and Plasma Graft Polymerization Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Plasma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2 Examples of Plasma Treatment Electrodes . . . . . . . . . . . . . . 3.2.3 Principle and Example of Hydrophilic Plasma Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.4 Plasma Surface Treatment and Plasma Graft Polymerization Surface Treatment Mechanism . . . . . . . . . . 3.2.5 Structure of the Three Electrodes with Different Potentials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.6 Principle of Atmospheric-Pressure Plasma Graft Polymerization and Adhesion Improvement Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Atmospheric-Pressure Plasma Graft Polymerization Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 Atmospheric-Pressure Plasma Graft Polymerization Apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2 Surface Treatment Evaluation for PTFE Metal Plating . . . . 3.3.3 XPS Analysis Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Applicability of PTFE/Plastics in Millimeter-Wave Devices . . . . . 3.4.1 Plastic Properties: Dielectric Constant, Dielectric Loss Tangent, and Hydrophobicity . . . . . . . . . . . . . . . . . . . . 3.4.2 Small High-Performance Millimeter-Wave Band Antennas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.3 Applicability to High-Frequency Coaxial Cables . . . . . . . . 3.4.4 Method of Copper Plating on PTFE and Results . . . . . . . . . 3.4.5 Surface Treatment of Dielectric Cable . . . . . . . . . . . . . . . . . 3.4.6 Method of Nickel Plating on PTFE and Results . . . . . . . . . . 3.4.7 Microfabrication of Nickel Plating on PTFE . . . . . . . . . . . . 3.4.8 Applicability to Radome . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.9 Plasma Hybrid Surface Treatment of Fiber-Reinforced Composite Materials . . . . . . . . . . . . . . . . . 3.5 Development of OLEDs on PCTFE . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.1 Flexible OLED Element . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.2 Peeling Strength for PCTFE . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.3 XPS Analysis Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.4 SEM Observation Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5.5 Prototype Fabrication Procedure for OLED Device on PCTFE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6 Improved Adhesion of Fluoroplastic Film to Butyl Rubber . . . . . . 3.6.1 Application Example: Prefilled Syringe . . . . . . . . . . . . . . . . 3.6.2 Butyl Rubber and PTFE Film Composite Material . . . . . . .

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3.6.3 Peeling Test of Fluoroplastic Film–Butyl Rubber Composite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.6.4 Peeling Strength of the Composite Material . . . . . . . . . . . . . 3.6.5 Molecular-Level Adhesion Mechanism Between Rubber and PTFE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Hydrophilic Treatment Technology for Textiles, Filters, and Glass and Its Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Surface Treatment of Textiles and Apparels . . . . . . . . . . . . . . . . . . . 4.2.1 Principle of Functional Surface Treatment . . . . . . . . . . . . . . 4.2.2 Experimental Apparatus and Methods . . . . . . . . . . . . . . . . . . 4.2.3 Experimental Results and Discussion . . . . . . . . . . . . . . . . . . 4.3 Deodorization Technology Using Low-Temperature Nonthermal Plasma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1 Plasma Deodorization Technology . . . . . . . . . . . . . . . . . . . . . 4.3.2 Methods for Producing and Measuring Performance of Functional Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.3 Experimental Results and Explanation . . . . . . . . . . . . . . . . . 4.4 Increased Glass Surface Hydrophilicity by Nonthermal Plasma Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.1 Definition of Contact Angle . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.2 Glass Surface Treatment Using Atmospheric-Pressure NTP Irradiation . . . . . . . . . . . 4.4.3 Glass Surface Hydrophilicity Dynamically Controlled by Nonthermal Plasma Actuator . . . . . . . . . . . . . 4.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Hydrophobic Treatment for Polymer Surfaces . . . . . . . . . . . . . . . . . . . . 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Preparing a Hydrophobic Material Surface by Fluorocarbon Plasma Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Radio-Frequency Plasma Reactors with Chemical Vapor Deposition Apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4 Nonthermal Plasma Technology for Surface Modification . . . . . . . 5.5 Reaction Between Plasma and Polymer . . . . . . . . . . . . . . . . . . . . . . . 5.6 Surface Hydrophobicity by Laser Microfabrication . . . . . . . . . . . . . 5.7 Diamond-Like Carbon-Based Plasma Surface Treatment . . . . . . . . 5.8 Plasma-Treated Catalyst Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.9 Trends in Other Plasma-Based Surface Treatments . . . . . . . . . . . . . 5.10 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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6 Hydrophobic Treatments for Plastic, Glass, and Metal Surfaces and Their Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Definition of Plasma and Its Characteristics . . . . . . . . . . . . . . . . . . . 6.2.1 Definition of Plasma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.2 Plasma Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.3 Thermal Equilibrium and Nonequilibrium Plasma . . . . . . . 6.2.4 Method to Evaluate Ionization Degree in Plasma . . . . . . . . 6.3 Principles of Plasma Cleaning and Surface Activation Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.1 Overview of Plasma Cleaning . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.2 Example of Electrode Systems for Reduced-Pressure Plasma Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.3.3 Cleaning Using Atmospheric-Pressure Plasmas . . . . . . . . . . 6.3.4 Effects of Atmospheric-Pressure Plasma Cleaning . . . . . . . 6.3.5 Remote Plasma Cleaning and Cleaning by Ozone . . . . . . . . 6.4 Example of Plasma Cleaning and Enhanced Activation of Hydrophilicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.4.1 Hybrid Plasma-Hydrophobic–Chemical Process . . . . . . . . . 6.4.2 Experimental Apparatus and Method . . . . . . . . . . . . . . . . . . 6.4.3 Test Results Obtained for Cleaning and Hydrophilicity and Discussion . . . . . . . . . . . . . . . . . . . . 6.4.4 Hydrophobic Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5 Hybrid Plasma–Anti-corrosion Process Treatment (Aluminum Plate Surface Treatment) . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.1 Ordinary and Hybrid Plasma Treatments . . . . . . . . . . . . . . . 6.5.2 Plasma Apparatus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.3 Treatment After Plasma Irradiation and Increased Anti-corrosion Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.4 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.5 Corrosion Test Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Plasma and Electron-Beam Technologies Used for Surface Treatment Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Plasma Hybrid Hydrophilic Treatment Process . . . . . . . . . . . . . . . . 7.2.1 Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.2 Examples of the Adhesion of Glass and PTFE . . . . . . . . . . . 7.3 Anti-fog Using Electron-Beam Irradiation Treatment Process . . . . 7.3.1 Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.2 Anti-fog Treatment Application Example . . . . . . . . . . . . . . . 7.4 Plasma Treatment for Medical Applications . . . . . . . . . . . . . . . . . . . 7.4.1 Surface Treatment for an Endoscope . . . . . . . . . . . . . . . . . . . 7.4.2 Plasma Jet Sterilization of Surfaces . . . . . . . . . . . . . . . . . . . .

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Contents

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7.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184 8 Measurement Technology for Functional Groups Generated by Plasma Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2 Analysis of Functional Groups Generated by Plasma Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.1 Functional Group Analysis of Plasma Graft-Polymerized Acrylic Acid Film on PTFE by FTIR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.2 Functional Group Analysis of Plasma Graft-Polymerized Acrylic Acid Film on PTFE by XPS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3 Analysis of Chemical Species Formed by Plasma Treatment . . . . . 8.3.1 Analysis of Byproducts During Treatment of Ammonia Gas and Acetaldehyde Gas with Plasma by FTIR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.2 Analysis of Byproducts During Treatment of CF4 Gas with ICP by FTIR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.3 Analysis of Byproducts During Treatment of Xylene Gas with Plasma by FTIR . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.4 Analysis of Byproducts During Plasma Treatment of TEOS by FTIR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

187 187 188

188

190 191

191 193 194 195 197 197

Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199 Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201 Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209

About the Author

Dr. Masaaki Okubo received his B.Eng., M.Eng., and Ph.D. degrees in mechanical engineering from the Tokyo Institute of Technology, Tokyo, Japan, in 1985, 1987, and 1990, respectively. He is Full Professor at the Department of Mechanical Engineering, Osaka Metropolitan University, Sakai, Japan. In April 2022, Osaka Prefecture University and Osaka City University were merged to form Osaka Metropolitan University. His previous positions include Associate Professor at Osaka Prefecture University, Assistant Professor at Tokyo Institute of Technology, and Assistant Professor at Tohoku University. He also served as Invited Professor at Tohoku University in 2015. His current research interests include environmental applications of nonthermal plasmas, particularly, nanoparticle control, electrostatic precipitators, aftertreatment of clean diesel engines and combustors, and surface treatment of materials and their biomedical applications. His works span multidisciplinary areas including electrical, chemical, and mechanical engineering. Dr. Okubo has published more than 230 peer-reviewed and invited papers in scientific journals and has authored 33 books. Dr. Okubo is Fellow of the Institute of Electrical and Electronics Engineers (IEEE) and the Japan Society of Mechanical Engineers (JSME). He was Chairman of the Environmental Engineering Division of the JSME in 2007. He served as Chairman of the Electrostatic Process Committee of the IEEE Industry Application Society during 2016–2018. He is Associate Editor of the IEEE Transactions on Industry Applications, Kansai Branch Chair of the Institute of Electrostatics Japan, and Editorial Board Member of the Journal of Electrostatics. He received the Environmental Engineering Achievement Award of the Environmental Engineering Division of the Japan Society of Mechanical Engineers in 2013.

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Chapter 1

Fundamentals of Nonthermal Plasma Technologies

1.1 Introduction In this chapter, the fundamentals of nonthermal plasma technologies are outlined. The definition of plasma, as well as various types of plasmas and plasma generation methods, is described. Further, a method to generate atmospheric-pressure nonthermal plasma is detailed.

1.2 Generation of Atmospheric-Pressure Nonthermal Plasmas Plasma is an ionic state of gas, and it can be generated using various methods. For example, when easily ionized metals such as Cs and K are introduced into a hightemperature gas stream of > 1000 °C, plasma is generated at atmospheric pressure. The plasma can be generated at reduced pressure because ionization occurs easily. Plasma treatment is performed to irradiate plasma to materials. In industrial applications, plasma is generated using a metal electrode that is energized by electric power as the insulation of gas is broken by the gradient of the electric field. Various types of plasma generation methods, especially those of atmospheric-pressure nonthermal (cold) plasma, are described in the following parts. The plasma is explained in detail in the next section.

1.3 What Are Plasmas? Figure 1.1 defines a plasma and details its generation process. The plasma is the fourth state of matter after solid, liquid, and gas. Upon applying energy, such as electrical energy, phase changes to solid, liquid, and gas occur. At atmospheric pressure, when © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 M. Okubo, Nonthermal Plasma Surface Modification of Materials, https://doi.org/10.1007/978-981-99-4506-1_1

1

2

1 Fundamentals of Nonthermal Plasma Technologies Energy

Energy

Gas Solid

Energy

Liquid Dissociation of a molecule Dissociation of an atom C* B+

A−

B+ B+

Plasma

e−

A− B+

e−

C*

Heavy particle A- Negative ion B+ Positive ion C* Atom (Radicals) e− Electron

Ionized gas Fig. 1.1 Definition and generation process of plasmas. Plasma is induced when electrical or heat energy is input to materials

electrical energy is introduced, the plasma is induced. Through the application of plasma to a gas state, molecular and atomic dissociation occurs, and the plasma is generated as a partially ionized gas. The plasma is composed of negative ions (A− ), positive ions (B+ ), atoms, radicals (C*), and electrons. Powerful activity may exist in the plasma, and by the application of plasma to materials, their surfaces are activated to usually show hydrophilic properties.

1.4 Types of Plasmas The most typical plasmas are classified in Table 1.1. Familiar plasmas are known as flames, aurora, lightning, fluorescent lights, etc. Four types of plasma are explained here. A high-temperature plasma is in a high-temperature state of ≥ 10,000 °C, and the electron temperature is equal to that of heavy particle temperature; examples are atmospheric-pressure arc discharge and nuclear fusion plasmas. A low-pressure plasma is generated by discharging under a pressure of several Pascals to several hundred Pascals; examples are plasma–chemical vapor deposition (CVD) and surface treatment. A nonequilibrium plasma is a state where the electron temperature is much higher than that of heavy particle temperature. The atmospheric-pressure lowtemperature or cold plasma is generated by accelerating only electrons with a small mass and high mobility under atmospheric pressure with a strong electric field, and it

1.4 Types of Plasmas

3

can be used for various purposes such as energy and environmental cleaning, surface treatment, medical treatment, and biological application. Various types of plasma generation are introduced. Figure 1.2 shows a lighting ball or plasma globe, which is used as a room interior feature for lightning or as a toy. A mixed rare gas such as neon at a low concentration is input into the glass sphere shell, and a high-voltage electrode exists at the center of the glass sphere. A high voltage is applied to the electrode, and a plasma filament is formed between the central electrode and the spherical glass shell, and a bright beam of light appears to be constantly extending. Figure 1.3 shows a pulse corona-induced plasma reactor of approximately 2 m in length. This type of plasma reactor consists of a coil-type outer ground electrode and a centered wire electrode. Five pairs are observed in the figure, and the exhaust Table 1.1 Classification and types of plasmas Familiar plasmas

Flames, aurora, lightning, fluorescent lights, etc.

Plasma types

Characteristics

High-temperature plasma

A plasma in a high-temperature state of ≥ 10,000 °C or higher, and electron temperature = heavy particle temperature. Examples: atmospheric-pressure arc discharge and nuclear fusion plasmas

Low-pressure plasma

Plasma generated by discharging under a pressure of several Pa to several hundred Pa. Examples: plasma–chemical vapor deposition (CVD) and surface treatment

Nonequilibrium plasma

Electron temperature >> heavy particle temperature

Atmospheric-pressure low-temperature or cold plasma

Plasma generated by accelerating only electrons with small mass and high mobility under atmospheric pressure with a strong electric field. It can be used for various purposes. Examples: energy and environmental cleaning, surface treatment, medical treatment, and biological application

Fig. 1.2 Lighting ball or plasma globe

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1 Fundamentals of Nonthermal Plasma Technologies

Fig. 1.3 Pulse corona-induced plasma reactor (length = 2 m) [2]

gas passes perpendicular to the coil electrodes. This type of reactor was used in an experiment on the decomposition of dioxins emitted from a garbage incinerator [1, 2]. The structure of the coil-type outer ground electrode and the centered wire electrode reactor is schematically shown in Fig. 1.4. Figure 1.5 shows another example of an atmospheric-pressure plasma photograph called the surface discharge, which is used for research on energy and environmental applications. The inner surface of a cylindrical reactor is covered by an electrode, and the inside of the wall of the cylinder is shown. A surface discharge-induced plasma is generated inside the cylinder near the surface electrode. This type of surface discharge-induced plasma apparatus is used for ozone generation aimed at NOx and SOx reduction using the plasma–chemical hybrid process [2]. In the following section, another type of surface discharge is explained.

1.5 Pulse Corona Plasmas

5

Fig. 1.4 Schematic diagram of a coil-type plasma reactor [2] Fig. 1.5 Surface discharge inside the cylinder reactor

1.5 Pulse Corona Plasmas The methods that are used to generate atmospheric-pressure low-temperature plasmas are explained. An atmospheric-pressure low-temperature plasma is generated by applying a high voltage to a pair of electrodes. The pulse corona and surface discharge methods are emerging as the most useful. The pulse corona method involves electrical discharge using a pulse power supply (~ 150 kV), and the dielectric barrier is not always required. Pulse high voltage is generated either by a mechanical rotary spark gap-type pulse power supply or by a

6

1 Fundamentals of Nonthermal Plasma Technologies

Fig. 1.6 Schematic diagram of a coaxial-type plasma reactor

semiconductor pulse power supply. Semiconductors such as insulated gate bipolar transistor (IGBT) and static induction (SI) thyristor or gap switches are used. Figure 1.6 shows a coaxial-type plasma reactor in the laboratory. The reactor is energized by a high-voltage pulse. Figure 1.7 shows voltage, current, and instantaneous power waveforms when using an IGBT-type pulse power supply. The diameter of the plasma reactor is 20 mm, and the distance between electrodes is 10 mm. The gas used is air, and the discharge average electric power is 11.9 W. The present method induces a very fast increase in voltage.

1.6 Dielectric Barrier Discharge-Induced Plasmas The ferroelectric packed-bed method or barrier-type packed-bed plasma reactor is known to intensify the plasma. Ferroelectric pellets such as barium titanate (BaTiO3 ) with a relatively high dielectric constant of ~ 10,000 are used. The power supply may be combined with a dielectric barrier as in the silent discharge method. Figure 1.8a, b

1.6 Dielectric Barrier Discharge-Induced Plasmas

7

Fig. 1.7 Voltage, current, and instantaneous power waveforms for the IGBT-type pulse power supply

shows a barrier-type packed-bed plasma reactor and a discharge plasma state in the packed region. In this case, the inner diameter is 20 mm, and the length of the plasma region is 260 mm. In the reactor, many pellets are packed in the glass tube. The surface of the pellets is treated, and it becomes hydrophilic. Therefore, the reactor can be used for the surface treatment of pellets and powder. Many requests are submitted for the surface treatment of powders. It is difficult to treat the surface of powders using the plasma jet torch because they easily fly and are released from the substrate. Therefore, the reactor is very convenient for this purpose. The plasma in the packed region is shown in the photograph of Fig. 1.8b, which was taken with a high-sensitive digital camera that is used for pictures of stars in space. Other types of plasma reactors such as wire plate and surface discharge reactors are explained later. Figure 1.9 shows voltage, current, and instantaneous power waveforms for the barrier-type packed-bed plasma reactor. In this graph, the horizontal axis expresses the elapsed time, the left vertical axis the voltage (kV) and current (mA × 10), and the right vertical axis expresses instantaneous power (W). Electrons are accelerated by the strong electric field among pellets inside the plasma reactor. Radicals with high chemical activities are generated by the collisions of electrons. Various chemical reactions are enhanced based on rich radical states. Energy injection to heavy particles (molecules, atoms, ions, and radicals) becomes lower, and larger energy is input to electrons. A nonequilibrium plasma is that in which the electron temperature

8

1 Fundamentals of Nonthermal Plasma Technologies

Fig. 1.8 Barrier-type packed-bed plasma reactor (a) and discharge plasma state in the packed region (b)

exceeds that of the heavy particle temperature, which is almost equal to the gaseous temperature.

Fig. 1.9 Voltage, current, and instantaneous power waveforms for the barrier-type packed-bed plasma reactor

1.7 High-Frequency Plasmas

9

Fig. 1.10 Silent discharge ozone generator plasma reactor: the table provides specifications of the reactor, and the diagram shows the structure of the reactor. The ozonizer consists of 30 plasma reactors

The specifications of a silent discharge ozone generator plasma reactor are shown in the table of Fig. 1.10, along with the structure of the reactor [3]. The ozone generator is used for surface cleaning.

1.7 High-Frequency Plasmas 1.7.1 Surface Discharge Plasma Sine wave high-voltage power supply (~ 150 kV) is typically used for dielectric barrier discharges. Low-frequency AC (60 Hz) is generally used for this purpose. Medium-frequency AC (~ 20 kHz) is frequently used for the ozonizer and plasma jet; however, as the temperature rises, the power consumption increases. High-frequency AC (> 1 MHz) is also used for generating high-frequency plasmas. Surface discharge is a typical example of medium-frequency AC (~ 20 kHz) applications. Several kHz to several ten kHz alternate high voltage is applied between striped discharge electrodes covered by ceramic dielectric material and lined with

10

1 Fundamentals of Nonthermal Plasma Technologies

Principle of surface discharge Discharge electrode Ceramic coat

Surface discharge Induction electrode

Ceramic High voltage

(a)

(b)

Fig. 1.11 Surface discharge technology. a Surface discharge element. b Principle of surface discharge

plate metal induction electrodes inside the ceramic. As a result, surface discharge is induced. The principle of surface discharge is shown in Fig. 1.11. The frequency, peak-to-peak voltage, and maximum power are 10 kHz, 10 kVp-p , and 25 W, respectively. When the material, such as a polymer film, is put on the surface of the reactor, the surface of the material is in contact with the surface of the plasma reactor and it is treated by the plasma induced near the edge of the discharge electrode. Figure 1.11a shows a rod that is used for surface discharge plasma generation. A pair of highvoltage electrodes exists on the surface of the rod and inside the rod as shown in Fig. 1.11b. Surface discharge-induced blue-colored plasma is generated on the surface of the rod in the air environment. The blue color is caused by the ionization of nitrogen included in the air. This type of plasma has been used for experiments on the conversion of carbon dioxides to fuel and nitrogen oxides (NOx ) reduction [4] as well as surface treatments.

1.7.2 Radio-Frequency Plasma When the discharge frequency increases in the range of MHz, both ions and electrons change the direction of the motion before reaching the other electrode, thereby trapping them in the interelectrode space. These trapped ions and electrons will form plasma to excite gas molecules, which is called a high-frequency or radiofrequency (RF) discharge. The electrodes generated by the RF plasma do not contact

1.7 High-Frequency Plasmas

11

contaminated gases, and hence, only clean plasma is obtained. The RF plasma can be generated by either inductively coupled plasma or capacitively coupled plasma. The inductively coupled plasma shown in Fig. 1.12a uses the coil outside the dielectric barrier reactor tube in which a high-frequency current is applied to induce the magnetic flux. The plasma can be generated in the azimuthal direction within the tube reactor. For this purpose, the most widely used frequency is 13.56 MHz; however, 4 or 2 MHz RF power supply is occasionally employed. The inductively coupled plasma is widely used because the reactor is simple to build. The nonthermal plasma generated at a low pressure has a wide range of applications such as CVD, polymerization, etching, surface modification, and environmental protection. When the plasma is generated at atmospheric pressure, a thermal plasma can be obtained, which is used for ultrafine particle generation and creating a plasma torch for plasma spraying. The generated capacitively coupled plasma uses high frequency and voltage. The electrode can be placed outside or inside the reactor. Figure 1.12b shows the electrode placed outside the reactor. When the electrode is placed inside the reactor, it can be wrapped by the dielectric material. The frequency used is a commercially available 13.56 MHz. The capacitively coupled plasma has less power consumption as compared to that of the inductively coupled plasma.

Fig. 1.12 Radio-frequency plasma reactors [2]. a Inductively coupled plasma reactor. b Capacitively coupled plasma reactor

12

1 Fundamentals of Nonthermal Plasma Technologies

1.7.3 Microwave-Generated Plasma The microwaves generated by a magnetron oscillator are introduced to the chamber using a microwave waveguide. The chamber is usually composed of an insulating material such as quartz or alumina tube. The plasma is induced inside the chamber by the radiation of the microwave. The typical frequency of the microwave is 2.45 GHz. When a transverse magnetic field is applied to the electric field, the electrons rotate around the magnetic field lines. When the frequency of the rotation (cyclotron frequency) is adjusted to that of the applied electric field, the rotation of the electron is accelerated because of resonance. This phenomenon is called “electron cyclotron resonance.” The initiation voltage of the discharge decreases at this state. The resonance frequency is generally 109 –1012 Hz (Fig. 1.13).

1.8 Plasma Jet Figure 1.14 shows an AC low-temperature plasma jet electrode called the corona electrode. The system comprises a pair of wire-type electrodes inside the torch. The electrode is made with a high temperature-resistant material such as tungsten and Inconel. The electrodes are supported by the insulator. High-voltage resistances are used for supplying current to electrodes. Gases such as air, Ar, and He flow from the top of the torch, and the plasma shower is induced between the electrodes. The surface of the sample is treated using this shower. The distance between the surface of the material and the torch d is variable, and it is optimally selected to increase the hydrophilicity of the surface. Figure 1.15 shows a treatment with three-electrode-type plasma jet apparatus. The torch consists of a gas flow channel nozzle cover, electrodes, metal plate, and ground. A plasma jet is generated with Ar gas. The Ar gas flow passes between the positive and negative electrodes. When a high voltage is applied to the electrodes, the discharge plasma is induced. A plasma flow is eluted from the outlet of the plasma torch and impinges, and the surface of the film, which is on the metal plate, is treated. The film is located among the three electrodes: red, blue, and green. A high-voltage pulse is applied (design values are 20 kHz and 24 kV) to the electrodes. The film to be treated is sandwiched among three different potential electrodes of red, blue, and green, and a strong plasma is applied to form a hydrophilic film and achieve high adhesion. The electrode spacing is ± 5 mm, and the distance to the conductor plate is 9 mm. The string treatment is possible with the configuration using acrylic acid vapor (graft polymerization). Hydrophilic treatment is possible to realize a higher adhesion to other materials. The optimum values for treatment are a distance of 5 mm between the positive and negative electrodes and a distance of 9 mm between the surfaces of the torch to the film. Figure 1.16 shows an electric circuit for the corona plasma generation apparatus (Plasma stream-PSC1002, Pearl Kogyo Co., Ltd., Osaka, Japan). The apparatus

1.8 Plasma Jet

13

Fig. 1.13 Microwave-induced plasma reactor for substrate surface modification [2]

comprises a DC power supply power factor improvement circuit, output voltage setup circuit, transformer drive circuit, and discharge electrodes controlling circuits with the central processing unit. First, 100 V AC is converted to 200 V DC and controlled using the output voltage setup circuit at 18–180 V DC. This DC voltage is switched using an IGBT pulse and increased using a pulse transformer. As shown in Fig. 1.16, the high-voltage output of 20 kHz and 24 kVp-p is generated between the needle electrodes that induce the plasma. Notably, 24 kVp-p is a design value, and the voltage is reduced to approximately 4 kVp-p . The induced plasma is called the gliding arc jet plasma, and it is used for surface treatment.

14

1 Fundamentals of Nonthermal Plasma Technologies

Air, oxygen gas High voltage

High-voltage resistance Electrode Direction of shower

d Sample

Fig. 1.14 Typical plasma jet electrode Fig. 1.15 Plasma jet apparatus among the three potential electrodes [5]. a Structure. b Plasma jet

1.9 Conclusions

15

Fig. 1.16 Electric circuit for the plasma jet. Output voltage = 24 kV, pulse frequency = 20 kHz, pulse modulation frequency = 60 Hz, pulse duty ratio = 1–100%, and input power = maximum 1000 W [5]

1.9 Conclusions This chapter provides a detailed exploration of plasmas, their generation, and their various applications. The contents of this chapter provide the basic knowledge for understanding the subsequent chapters. The contents are summarized as follows. 1. Plasma: This is an ionic state of gas and represents the fourth state of matter following solid, liquid, and gas. It is comprised of negative ions, positive ions, atoms, radicals, and electrons. It can be created under various conditions, and its interaction with materials can activate their surfaces, often making them hydrophilic. 2. Generation: Plasma can be generated using easily ionized metals or through electrically energized metal electrodes. The breakdown of gas insulation due to the electric field gradient allows for plasma generation. 3. Types of plasmas: Plasmas can exist in various forms: (1) High-temperature plasma: High-temperature state where electron and heavy particle temperatures are equal. (2) Low-pressure plasma: Generated under low pressure, used for applications like chemical vapor deposition. (3) Nonequilibrium plasma: Electron temperature is much higher than that of heavy particle temperature. (4) Atmospheric-pressure low temperature or cold plasma: Useful for energy, environmental cleaning, and medical applications.

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1 Fundamentals of Nonthermal Plasma Technologies

4. Generation methods: (1) Pulse corona plasmas: Achieved through electrical discharge using pulse power supply. (2) Dielectric barrier discharge-induced plasmas: Use ferroelectric pellets like barium titanate to enhance plasma generation. (3) High-frequency plasmas: Various methods are explained, including surface discharge plasma, radio-frequency plasmas, and microwave-generated plasma. (4) Plasma jet: Different designs of plasma jet electrodes and their applications are described. Throughout the chapter, various figures and diagrams are used to illustrate the concepts and apparatuses discussed. Overall, the chapter delves deep into the world of plasma technology, showcasing its importance in modern industrial and medical applications.

References 1. K. Yoshida, T. Yamamoto, T. Kuroki, M. Okubo, Pilot-scale experiment for simultaneous dioxin and NOx removal from garbage incinerator emissions using the pulse corona induced plasma chemical process. Plasma Chem. Plasma Process. 29(5), 373–386 (2009) 2. T. Yamamoto, M. Okubo, Advanced physicochemical treatment technologies, in Handbook of Environmental Engineering, vol. 5, ed. by L.K. Wang, Y.-T. Hung, N.K. Shammas (The Humana Press Inc., Springer, 2007), Chapter 4, Nonthermal Plasma Technology, pp. 135−294. 3. H. Fujishima, K. Takekoshi, T. Kuroki, A. Tanaka, K. Otsuka, M. Okubo, Towards ideal NOx control technology for bio-oils and a gas multi-fuel boiler system using a plasma-chemical hybrid process. Appl. Energy 111, 394–400 (2013) 4. M. Okubo, T. Kuwahara, New Technologies for Emission Control in Marine Diesel Engines, Butterworth-Heinemann, imprint of Elsevier, Paperback ISBN: 9780128123072, eBook ISBN: 9780128123089, 1–296 (2019) 5. M. Okubo, T. Onji, T. Kuroki, H. Nakano, E. Yao, M. Tahara, Molecular-level reinforced adhesion between rubber and PTFE film treated by atmospheric plasma polymerization. Plasma Chem. Plasma Process. 36, 1431–1448 (2016)

Chapter 2

Fundamentals of Surface Treatment Technologies and Characterization

2.1 Introduction In this chapter, the fundamentals of surface treatment technologies, including the principle underlying plasma graft polymerization, methods for evaluating the effects of surface treatment, and the apparatus required for surface modifications, are addressed. Further, these surface treatments are compared with a conventional method.

2.2 Mechanism of Surface Modification The mechanism of plasma surface modification is elucidated, along with the difference between corona discharge and plasma surface treatments. Corona discharge is a state of electrical discharge in a gas. A partial discharge is induced when a high voltage is applied to the sharp-edged metallic electrodes, such as metal needles, and the ionized state is considered a type of plasma. Surface treatment using this corona discharge is called corona discharge treatment, which is performed by applying either a steady (DC) or varying (pulse or AC) high voltage. Meanwhile, the word plasma is used for a wide range of ionization states. Plasma can also be prepared by methods other than the corona discharge, such as combustion and microwave heating, and the corresponding surface treatment is called plasma treatment. A surface can be modified and made hydrophilic by plasma treatment. Specifically, the plasma supplies electrons with high energies of 1–10 eV, which affect some chemical bonds on the material surface. For a metallic material, this treatment affects the oxidative surface layer and/or the oil layer. On polymer surfaces, the main chain or side chains are cleaved to generate radicals. Simultaneously, atmospheric gases, such as oxygen and nitrogen, as well as moisture, are decomposed and induce the formation of radicals. One reason that plasma treatment makes a surface hydrophilic is that electrons, radicals, and ions decompose some organic material on the surface, © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 M. Okubo, Nonthermal Plasma Surface Modification of Materials, https://doi.org/10.1007/978-981-99-4506-1_2

17

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2 Fundamentals of Surface Treatment Technologies and Characterization

such as oily residue and fats, which is often present as a fouling layer on surfaces. In addition, radicals reconnect on the surface and inside the gas to generate hydrophilic functional groups like hydroxyl (−OH), carbonyl (> C=O), and carboxyl (−COOH). Because the targeted material surface becomes hydrophilic, it is easier to adhere to other materials. However, the effects of such plasma surface treatments degrade with time, and therefore, treatment with permanent effects is required. The next section presents a technology for preparing permanent thin-film layers using plasma graft polymerization.

2.3 Plasma Graft Polymerization Plasma treatment is known to increase the hydrophilicity and/or adhesivity of the surface of materials like plastics, textiles, glass, and metals. However, as mentioned above, the effect of the plasma treatment on ordinary plastics disappears within a short time, typically only several days. Furthermore, only a negligible effect is observed on fluoropolymer plastics (e.g., Teflon® ). To overcome this shortcoming, plasma graft polymerization has been introduced as a permanent surface treatment method. A reduced-pressure plasma graft polymerization was not successful, whereas the atmospheric-pressure plasma graft polymerization resulted in treated film surfaces with excellent properties. The target film is a fluoropolymer plastic, because its adhesion is extremely difficult compared to other plastics, such as polycarbonate (PC), polyethylene terephthalate (PET), and ethylene–vinyl acetate (EVA). Fluoropolymer plastics (Teflon® ), such as polytetrafluoroethylene (PTFE), perfluoroalkoxy fluoroplastics (PFA), and polychlorotrifluoroethylene (PCTFE), exhibit excellent thermal and chemical stabilities and are electrically insulative and lower loss at high frequencies. In addition, they provide gas barrier properties, they cannot be burnt, and their surfaces have low friction coefficients. Various types of applications have made use of these excellent properties. However, adhering other substances to fluoropolymer plastics, laminating fluoropolymer films, and plating these plastics with metal are all very difficult. Improving the adhesion properties of Teflon, while maintaining these excellent characteristics, would extend its applications even further. The applications of these treatments are presented in further detail in Chap. 3. Table 2.1 shows the gas barrier properties of various plastic films including nylon, polyvinyl chloride (PVC), PET, ethylene tetrafluoroethylene (ETFE), polypropylene (PP), PFA, and PCTFE. Specifically, it lists the penetration efficiencies of water vapor (g/m2 /day at 40 °C, relative humidity (RH) = 35%, thickness = 100 µm), nitrogen gas (mL mm/m2 /day/atm), and oxygen gas (mL mm/m2 /24 h/atm). PCTFE exhibits very lower values. Figure 2.1 shows an example of a conventional flexible display. For superior moisture-proofing and gas barrier properties, the organic light emitting diodes (OLEDs) for such displays are fabricated on PCTFE. The lifetimes of these displays and other devices like electronic paper can be extended by improving the adhesiveness of this polymer. OLEDs on PCTFE are explained in Chap. 3.

2.4 Apparatus for Surface Treatment

19

Table 2.1 Gas barrier properties of various plastic films Type of a Water vapor transmittance g/m2 /day 40 °C, film RH = 35%, thickness = 100 µm

N2 transmittance mL mm/m2 /day/ atm

O2 transmittance mL mm/m2 /day/ atm

Nylon

40–100

0.2

0.7

PVC

7–20

1

7

PET

5–10

0.8

2

ETFE

4–8

30

60

PP

2–5

10

90

PFA

1–2

100

700

Neofron

0.1–0.2

0.5

2

Fig. 2.1 Example of a conventional flexible display (photo by Shekhar Sahu, licensed under CC BY-SA 2.0, https://flic.kr/p/85rm4z)

2.4 Apparatus for Surface Treatment PFA and PCTFE have excellent gas barrier properties for nitrogen, oxygen, and water vapor (i.e., moisture resistance), which enables extending the luminescence lifetime of the OLED devices that they encapsulate. In addition, improving the adhesion properties of these films will allow their application to electrical paper and flexible displays. As mentioned above, plasma graft polymerization under atmospheric pressure very effectively improves adhesion properties; many experiments have been carried out by the author’s team in laboratory settings with an apparatus, which is shown in Fig. 2.2 [1]. This apparatus consists of a plasma power supply, a cylinder containing 99.99% Ar gas connected to a flow meter, a draft chamber, plasma torches, a treatment chamber containing the film to be processed, and a container filled with acrylic acid (CH2 =CHCOOH) on a plate heater controlled by a thermo-controller. The power supply and plasma torches are in Plasma stream–PSC1002 system manufactured by Pearl Kogyo. Co., Ltd. The film is fixed in the frame and exposed to the

20

2 Fundamentals of Surface Treatment Technologies and Characterization

Flow meter Fan

Draft chamber 99.99% argon

Treatment chamber

Plasma torches Frame Plasma jet

PEARL

Film

Variac

Exhaust

Plasmastream - PSC1002 -

Plasma power supply

Thermo controller Acrylic acid

Plate heater

CH2=CHCOOH

Fig. 2.2 Schematic of the laboratory apparatus for atmospheric-pressure plasma polymerization treatment [1]

plasma jet in an environment of acrylic acid vapor. After plasma treatment in the vapor, the film can also be immersed in liquid acrylic acid if required. The acrylic acid is either pure (100%) or a 50% aqueous solution. All the treatments are carried out inside the chamber, which is maintained at 0.0% oxygen because the presence of oxygen may adversely affect the plasma graft polymerization. Figure 2.3 shows a photograph of a low-temperature plasma jet being applied to a piece of white paper. In this case, the jet is produced outside the chamber, and the air is used as the working gas. After irradiation with air plasma for approximately 10 s as shown in Fig. 2.3a, no burning occurs on the surface of the paper as shown in Fig. 2.3b. During the plasma application, the gas and surface temperatures can be measured using temperature-sensitive stickers, which have revealed a maximum temperature of 80 °C. This can be considered sufficiently low for plasma-treating soft surfaces. The mechanism underlying the plasma graft polymerization of fluorocarbon polymers can be explained as follows. Treating fluoropolymers like PFA, PTFE, and PCTFE is difficult because of the firm C–F bonds, whereas graft polymerization on PC, PI (polyimide), EVA, acrylonitrile–butadiene–styrene (ABS), and PET surfaces is comparatively easy. The principal chemical reactions for plasma graft polymerization can be expressed as follows. Application of atmospheric-pressure nonthermal plasma: R − F → R· + F·

(2.1)

R· + CH2 = CHCOOH → R − CH2 − C · HCOOH

(2.2)

2.4 Apparatus for Surface Treatment

21

Fig. 2.3 Low-temperature air plasma jet. a Being applied to a white paper, and b after irradiation for approximately 10 s

Graft polymerization (i.e., generation of surface hydrophilic film or layer): R· + n(CH2 = CHCOOH) → R − (CH2 − CHCOOH)n −

(2.3)

Films of several fluorocarbon polymers have been treated, including PTFE (Valqua, Ltd., Japan, thickness = 0.5 mm), PFA (Valqua, Ltd., Japan, thickness = 0.1 mm), and PCTFE (Daikin Industries, Ltd., Japan, Neofron, thickness = 0.2 mm). Typical treatment conditions can be summarized as follows: the gas mixture for corona discharge plasma treatment = Ar, nitrogen, and air; maximum flow rate = 100 L/min (variable); treatment time = 75 s (variable); discharge voltage frequency = 20 kHz, with a peak voltage of 12 kVp (designed value); pulse modulation frequency = 60 Hz; and duty ratio = 99%, which is considered equivalent to 100%. The plasma is first irradiated to samples. After the plasma irradiation is performed, the plasma-treated film is immersed in aqueous solutions of acrylic acid (100% or 50%).

22

2 Fundamentals of Surface Treatment Technologies and Characterization

The polymerization time (immersion time) is typically 30 min. The polymerization temperature is 41–47°C; otherwise, the film is plasma-treated under an acrylic acid vapor condition with no immersion.

2.5 Surface Characterization Methods Various laboratory techniques are used to evaluate treated fluorocarbon polymer materials. Their surface wettability is most often determined by contact angle measurements, whereas their adhesive properties are investigated by T-type peeling tests. X-ray photoelectron spectroscopy (XPS) is frequently used to analyze the atomic contents of the surface, and Fourier transform infrared (FTIR) spectroscopy probes the chemical structure. Finally, the surface morphology can be observed using scanning electron microscopy (SEM). The measurement principles of XPS, FTIR, and SEM are presented in Appendix.

2.5.1 Contact Angle Measurement Figure 2.4 shows how hydrophilicity is evaluated by measuring the contact angle. Typically, a small droplet of pure water is dropped onto the surface of the substrate. The hydrophilicity of the substrate surface is evaluated by measuring the contact angle θ shown in the figure. A large θ indicates strong hydrophobicity and weak wettability. Conversely, a small θ implies strong hydrophilicity, i.e., wettability. The mechanism underlying this method is explained in further detail in Chap. 3. Table 2.2 shows the contact angles in the cases of various treatments. The angles made by 5-µL droplets of pure water 1 s after the droplet impacts were measured on the surface using a contact angle meter (CA-150, Kyowa Interface Science Co., Ltd., Japan). The contact angles of untreated PTFE, PFA, and PCTFE are presented

Droplet θ

θ

θ

Substrate Small

Contact angle

Large

Large

Hydrophilicity

Small

Fig. 2.4 Method for evaluating hydrophilicity by contact angle measurements

2.5 Surface Characterization Methods

23

(a)

(b)

Fig. 2.5 Photographs of water droplets a after 1 s on the untreated PTFE surface (θ = 104.4°) and b after 3 min on a PTFE surface treated with Ar plasma (120 s) + acrylic acid vapor treatment (θ = 17.3°)

along with the corresponding surfaces with Ar plasma treatment, Ar plasma treatment combined with immersion in 100% pure acrylic acid, Ar plasma treatment combined with immersion in 50% acrylic acid, nitrogen plasma treatment combined with immersion in 100% pure acrylic acid, and Ar plasma treatment in the presence of acrylic acid vapor (concentration ~ 2000 ppm). For the untreated samples, the contact angles are in the range of 90.9–104.4°. After Ar plasma treatment, the angles decreased to 81.4–95.4°, implying increased wettability. Therefore, under the optimized plasma conditions, adhesion increased slightly. After the plasma application combined with acrylic acid exposure under both liquid and vapor conditions, the angles decreased greatly, reaching a minimum of 42.0°. For all three fluorocarbon polymers, the vapor treatment attained the lowest contact angle, suggesting it is the most effective method for producing a hydrophilic surface. Figure 2.5a, b shows photographs of water droplets on the untreated PTFE and on the surface treated with acrylic acid vapor, respectively. These photographs are recorded 1 s and 3 min after the water droplets impacted the surface, respectively. The contact angle on the untreated surface is θ = 104.4°, suggesting hydrophobic nature. Evidently, the combination of Ar plasma for 120 s and ~ 2000 ppm acrylic acid vapor drastically increases the wettability, as the contact angle decreases to θ = 17.3°, and this wettability is obtained after 3 min. Table 2.2 Contact angles (°) measured 1 s after the water droplet contacts the surface (unit °) Untreated Ar Ar plasma + Ar plasma + N2 plasma + Ar plasma plasma 100% acrylic 50% acrylic 100% acrylic + acrylic acid immersion acid immersion acid immersion acid vapor PTFE

104.4

95.4

62.2

72.5

–

49.4

PFA

94.3

84.3

49.7

70.0

77.4

43.7

PCTFE

90.9

81.4

42.0

–

–

42.0

24

2 Fundamentals of Surface Treatment Technologies and Characterization

2.5.2 Evaluating Adhesive Properties by Peeling Test The setup for the T-type peeling test, which is used to evaluate adhesion, is schematically illustrated in Fig. 2.6. Typically, a sample is bonded to a stainless steel (SUS) plate coated with a 250-µm-thick layer of epoxy adhesive (Konishi E-set, Konishi Co., Ltd., Japan), which is applied using a baker-type applicator with dimensions of 25 mm × 50 mm. The samples are sandwiched between a pair of iron plates of dimensions 25 cm × 18 cm. After curing at room temperature for a day under a 5-kgf (= 49 N) load, the plate is peeled off at a speed of 100 mm/min, while the peeling strength is measured. Figure 2.7 schematically shows a cross-section of the treated composite sample for adhesion testing. The fluoropolymer film is coated with a plasma graft polymerization layer, upon which an epoxy adhesion layer is deposited. A metal plate is adhered to this layer to prepare a composite sample. We confirmed in the peeling tests that the interface between the film and the graft polymerization layer, which is shown with a red line, is peeled off during the test. The strength of the adhesion is confirmed by the peeling test. Fig. 2.6 Evaluation of adhesion using a T-type peeling machine

Peeled interface

Metal plate Epoxy adhesive layer Plasma graft polymerization layer Fluoropolymer film

Fig. 2.7 Schematic of a cross-section of the composite fluoropolymer sample for adhesion testing

2.5 Surface Characterization Methods

25

Fig. 2.8 Example of bonding strength measurement for graft-polymerized PTFE [2]

Figure 2.8 shows an example of the measured relationship between the peeling strength (N/mm) and stroke in the peeling machine (mm) for the treatment of a PTFE sample [2]. It is noted that the measured peeling strength is equivalent to the bonding strength. The conditions for the plasma graft polymerization treatment, in this case, are as follows. The temperature of the acrylic acid bath is 55 °C, the flow rate of Ar is 40 L/min, and the transverse plasma torch speed is 4 mm/s. In this example, double-sided adhesive tape (KPS-25, 3M Japan Limited) is used instead of epoxy adhesive to adhere the treated film to the metal plate, and this tape is found to break in all six samples. This graph shows the average peeling strength for samples 1–6. Clearly, the peeling strength increases with increasing stroke length and becomes almost constant after the stroke reaches 15 mm. The averaged bonding strength for a stroke of 15–50 mm is 4.92 N/mm (bonding strength per unit sample width) in this case. The result is a sample prepared with an A4-size apparatus, which has the same principle as the system in Fig. 2.2 and explained in Chap. 3. For a sample with a width of 25 mm, a bonding strength of 123 N is attained. The adhesion test results are shown in Table 2.3. This table shows the peeling strength (N) per 25-mm sample width for treated PTFE, PFA, and PCTFE. The untreated samples clearly exhibit lower peeling strengths of 0.5–1.36 N/25 mm. The samples treated with Ar or air corona discharge and immersion in 50–100% acrylic acid demonstrated slightly higher peeling strengths of 13.2–18.6 N/25 mm. Meanwhile, the samples treated under Ar–acrylic acid vapor condition exhibit very high peeling strengths of 35.5–128 N/25 mm. The highest strength of 128 N/25 mm is attained with PTFE in the A4-size treatment apparatus, as described in Chap. 3. Figure 2.9 shows the peeling strength or load (N) as a function of the processing time for graft polymerization. Evidently, the optimum processing time for the polymerization of the acrylic acid monomer is approximately 4 min.

26

2 Fundamentals of Surface Treatment Technologies and Characterization

Table 2.3 Bonding property evaluation (peeling or bonding strength (N) per 25-mm sample width, the highest record achieved in March 2022 at Osaka Metropolitan University) Fluoropolymer film

Untreated

Ar corona treatment followed by immersion in 100% liquid acrylic acid

PTFE

1.36

16.4

–

PFA

0.56

18.6

13.2

PCTFE

0.5

13.3

–

a

Ar corona treatment followed by immersion in 50% liquid acrylic acid

Air corona treatment followed by immersion in 100% liquid acrylic acid

Ar corona treatment in acrylic acid vapor

–

128a

1.86

114

–

35.5a

Results are samples treated with the A4-size apparatus

Ar =100 L/min + acrylic acid vapor

Fig. 2.9 Variation in peeling strength or load with graft polymerization treatment time

2.5.3 Result of Surface Analysis by ESCA (XPS) To analyze the surface composition of polymer surfaces, elemental analysis by electron spectroscopy for chemical analysis (ESCA) or XPS is used. In general, the binding energy of electrons is the orbital energy specific to each element and its oxidation states; thus, the binding energy can reveal the type of element as well as its oxidation state. Therefore, by irradiating the sample surface with X-rays and measuring the energy of the generated photoelectrons, the constituent elements of the sample can be analyzed, which is the principle underlying ESCA or XPS. Figure 2.10 depicts an example of an XPS spectrum, showing the C1s peaks of a PFA film treated by Ar plasma plus immersion in 100% acrylic acid. In this graph, the horizontal axis shows the binding energy (eV), and the vertical axis is intensity (counts per second, cps). The recorded spectrum is split, revealing peaks attributed to –COO–, > C=O, –C–C–, –C–H–, and –C–O–, as shown in different colors. Some active bond radicals

2.5 Surface Characterization Methods

27

Fig. 2.10 XPS results of a treated PFA film

Table 2.4 XPS elemental analysis results of untreated and treated PFA films (unit atomic %) Atom Untreated Ar plasma + immersion in Ar plasma + immersion in Ar plasma + 100% acrylic acid 50% acrylic acid acrylic acid vapor C

29.1

62.0

60.3

65.9

O

0.1

38.0

35.2

33.4

F

70.8

0.0

4.5

0.7

of –COO–, > C=O, and –C–O– are known to be hydrophilic, while other peaks are hydrophobic, viz., –C–C– and –C–H–. The surface becomes hydrophilic with these active chemical functional groups such as –COO–, > C=O, and –C–O–. Table 2.4 shows the C, O, and F contents (atomic %), as determined by XPS elemental analysis of the untreated PFA film and the PFA films treated with Ar plasma combined with immersion in 50% or 100% liquid acrylic acid or in an acrylic acid vapor environment. Evidently, more F atoms are present in the untreated sample than in the treated ones, with only a small amount of O atoms, suggesting that surface treatments evidently decreased the number of F atoms. Figure 2.11a, b shows the C1s XPS results for untreated and treated PTFE films, respectively. Only the –CF2 – peak is observed for the untreated film, whereas after the plasma graft polymerization treatment, the –CF2 – disappears, and hydrophilic –COO–, –C–O–, and > C=O bonds appear on the surface, which suggests that treated surface is hydrophilic.

2.5.4 Result of FTIR Analysis Figure 2.12 shows a measured result of FTIR analysis for a PFA film. Transmittance in the vertical axis is measured for various rays with the wavenumber (1/λ, cm−1 ) in the horizontal axis for untreated PFA (PFA-blank), PFA treated with Ar plasma

28

2 Fundamentals of Surface Treatment Technologies and Characterization

(a)

(b) Fig. 2.11 XPS analysis (C1s peaks) results for a untreated and b plasma graft polymerizationtreated PTFE films [1]

2.5 Surface Characterization Methods

29

Fig. 2.12 FTIR spectra of untreated and treated PFA films [1]

followed by the acryl 100% liquid immersion (PFA-acryl-100%), and Ar plasma followed by the acryl 50% solution immersion (PFA-acryl-50%). Peaks are observed only for –CF2 – and –CF3 at approximately 1200 cm−1 for untreated PFA, whereas an additional –COOH peak appears at approximately 1800 cm−1 for PFA-acryl-50 and 100%, suggesting hydrophilic properties and thus improved adhesion properties.

2.5.5 Result of SEM Analysis Figure 2.13 shows SEM images of treated PFA surfaces. Figure 2.13a, b shows a part and another part of micrographs, respectively, of a surface treated by Ar corona discharge plus immersion in 100% acrylic acid. The scale bars are shown in each micrograph. The thickness of the layer is approximately 1 µm in this case. In these two figures, graft copolymer, which is firmly connected to the surface of the treated film at the molecular level, can be observed, along with the homopolymer, which can easily be removed from the surface by washing. Figure 2.13c shows the untreated surface of a PFA film. Comparing this image with those shown in Fig. 2.13a, b suggests that the treatment produces some uneven features. The four images shown in Fig. 2.14 are SEM images of untreated and treated PTFE films at magnifications of 2000× and 5000×. Some unevenness is observed on the surfaces of the untreated sample shown in Fig. 2.14a, b because of the cutting process during the manufacturing of the PTFE film. Figure 2.14c, d presents PTFE films treated with Ar corona discharge plus acrylic acid vapor treatment for 120 s. After treatment, the unevenness disappeared completely, and a very flat surface is obtained. However, a delaminated defect is observed in these photographs, which was caused by either the tweezers used to handle films or the friction with metal. This defect suggests that the generated layer of polyacrylic acid is not very robust, and care must be taken during the handling and transport of these films. Interestingly, the adhesive property hardly decreased after a year under atmospheric exposure.

30

2 Fundamentals of Surface Treatment Technologies and Characterization

Graft copolymer generation

(a)

(c)

(b) Fig. 2.13 SEM images of PFA films (magnification = 2000×). a, b Ar corona + immersion in 100% acrylic acid and c untreated surface [1]

Figure 2.15 presents SEM images of the surface of an untreated PTTE film (Fig. 2.15a), a PTFE film treated with plasma graft polymerization by an A4-size apparatus (Fig. 2.15b), and Na–NH3 -treated PTFE (Fig. 2.15c). In the Na–NH3 treatment, the sample film is immersed in liquid ammonia along with solid Na. This method is very effective for increasing adhesion properties, but it has a higher environmental load in terms of liquid solution waste than the plasma graft polymerization treatment. Furthermore, the Na–NH3 treatment turns the film surface brown, whereas the plasma treatment does not change the surface color. Almost equivalent adhesion improvement is obtained from the Na–NH3 treatment using a transparent adhesive layer on the film by the plasma graft polymerization. Finally, Fig. 2.16 shows medium-magnification SEM images of surfaces untreated and treated by the gas-phase nanopolymerization using the A4-size apparatus. Localized nanopolymer branches are evident on the treated surface. The detailed procedure for the treatment with this apparatus is explained in Chap. 3.

2.6 Conclusion

31

(a)

(b)

(c)

(d)

Fig. 2.14 SEM images of the a, b untreated PTFE films, and c, d PTFE films treated with Ar corona discharge + acrylic acid vapor treatment (120 s). Magnifications: a, c: 2000×; b, d 5000×

2.6 Conclusion Surface treatment technologies, specifically plasma treatments and plasma graft polymerization, play a crucial role in material science, offering a means to enhance and alter the properties of various substrates. While plasma treatments offer immediate hydrophilic properties to surfaces, their temporal nature necessitates more permanent solutions like plasma graft polymerization. This chapter provides foundational knowledge for understanding the mechanisms, applications, and advancements in these technologies, setting the stage for deeper dives into their applications in subsequent chapters. The contents are summarized as follows. 1. Surface modification mechanisms: Surface treatment technology fundamentally revolves around modifying surfaces to achieve desired properties. This chapter discusses two primary mechanisms: plasma surface modification and plasma graft polymerization. Plasma surface modification is a process in which materials are treated using corona discharge or other plasma sources, such as combustion and microwave heating. The process makes surfaces hydrophilic

32

2 Fundamentals of Surface Treatment Technologies and Characterization

(a)

(c)

(b) Fig. 2.15 SEM images of the surface of the PTFE films [3]. a Untreated, b treated by graft polymerization, and c treated by Na–NH3 (ordinary technology, brown, higher environmental load) Fig. 2.16 Medium-magnification SEM images of a untreated film and b film treated with gas-phase nanopolymerization (each scale interval = 0.2 µm) [4]

(a)

2.6 Conclusion

33

Fig. 2.16 (continued)

(b)

by decomposing organic residues like fats and oils and generating hydrophilic functional groups. However, the effects of plasma treatments are temporal and diminish over time, necessitating more permanent solutions. 2. Plasma graft polymerization: To address the ephemeral nature of plasma treatments, plasma graft polymerization offers a more enduring surface treatment solution. This is particularly vital when treating materials like plastics, which can lose their hydrophilicity shortly after treatment. Special attention is given to fluoropolymer plastics like Teflon, known for their inherent resistance to adhesion, which presents significant challenges. Plasma graft polymerization under atmospheric pressure has shown promise in enhancing adhesion properties without sacrificing the plastic’s inherent benefits. 3. Practical applications and treatments: Fluoropolymer plastics, such as Teflon variants, are of particular interest due to their unique properties, including thermal stability, electrical insulation, and low friction coefficients. Improving their adhesion properties can extend their applicability in areas like OLED displays, which benefits from enhanced moisture proofing and gas barrier properties. Practical experimentation with graft polymerization is detailed, discussing the specific apparatus and conditions utilized to achieve desired surface properties on various substrates. The latter half of this chapter delves into various methodologies used for surface characterization of treated fluorocarbon polymer materials. Here is a brief encapsulation: 4. Contact angle measurements: This method measures hydrophilicity via contact angles, with a larger angle indicating hydrophobicity and vice versa. Results from various treatments show that the combination of Ar plasma treatment and exposure to acrylic acid vapor produced the most hydrophilic surface, reducing contact angles significantly.

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2 Fundamentals of Surface Treatment Technologies and Characterization

5. Peeling tests: The T-type peeling test is introduced to gage adhesion. The study indicates that untreated samples possess considerably lower peeling strengths compared to those treated with Ar-acrylic acid vapor conditions. The latter exhibits considerably elevated peeling strengths, especially when processed using the A4-size treatment apparatus. 6. XPS analysis: XPS or ESCA helps in understanding the elemental composition of polymer surfaces. After treatments, there is a notable reduction in the number of F atoms on the polymer surface. Treated surfaces exhibit hydrophilic functional groups like –COO–, –C–O–, and >C=O, transforming their characteristics. 7. FTIR analysis: The analysis highlights the emergence of hydrophilic properties in treated PFA films. Peaks indicative of hydrophilic groups are observed in treated samples, suggesting augmented adhesion properties. 8. SEM analysis: Scanning electron microscopy reveals topographical alterations on treated polymer surfaces. Untreated PTFE films display some roughness due to manufacturing processes. Post Ar corona and acrylic acid vapor treatment, surfaces become notably flatter. It is noteworthy that the newly formed polyacrylic acid layer is not particularly sturdy and needs careful handling. An alternate method using Na-NH3 shows effective adhesion improvement but has environmental drawbacks and altered surface color. In contrast, plasma graft polymerization preserves the original color while achieving similar adhesive improvement. Lastly, gas-phase nanopolymerization yields unique nanopolymer branches on treated surfaces. In essence, various treatments can modify fluorocarbon polymer surfaces in distinct ways, with some methods providing a combination of improved wettability and adhesive properties. The chapter underscores the versatility and promise of these techniques in enhancing the surface characteristics of fluorocarbon polymers.

References 1. M. Okubo, M. Tahara, N. Saeki, T. Yamamoto, Surface modification of fluorocarbon polymer films for improved adhesion using atmospheric-pressure nonthermal plasma graftpolymerization. Thin Solid Films 516(19), 6592–6597 (2008) 2. T. Kuroki, M. Nakamura, K. Hori, M. Okubo, Effect of monomer concentration on adhesive strength of PTFE films treated with atmospheric-pressure nonthermal plasma graft polymerization. J. Electrostat. 108, 103526 (2020) 3. M. Okubo, M. Tahara, Y. Aburatani, T. Kuroki, T. Hibino, Preparation of PTFE film with adhesive surface treated by atmospheric-pressure nonthermal plasma graft polymerization. IEEE Trans. Ind. Appl. 46(5), 1715–1721 (2010) 4. M. Narita, M. Nakamura, T. Kuroki, M. Okubo, Adhesive polytetrafluoroethylene films fabricated via atmospheric nonthermal plasma graft polymerization. J. Adhes. Soc. Jpn. 57(7), 280–290 (2021)

Chapter 3

Hydrophilic Treatment for Polymer Surfaces and Its Applications

3.1 Introduction To make the surface of a material hydrophilic or to improve its adhesiveness, corona discharge treatment is often applied, as described in Chap. 2. Specifically, the material to be treated is passed between a pair of electrodes, at least one of which is sharp, and a high voltage is applied to the electrodes. In addition, the target surface is often passed through a noble gas, such as Ar or He, that is easily ionized, between these electrodes, thereby applying a low-temperature plasma jet (i.e., the ionized gas generated under atmospheric pressure) to the target object. This method is also called plasma treatment. Corona means a crown, and corona discharge is a general term used to describe the continuous discharge caused by a nonuniform electric field generated around a sharp electrode (e.g., a needle electrode). In this chapter, fluorine-based polymers as representative hydrophobic plastics that contain strong C–F bonds and are most difficult to adhere to other substances are discussed. Typical examples of these plastic materials are polytetrafluoroethylene (PTFE, –(CF2 –CF2 )n –), perfluoroalkoxy fluoroplastics (PFA, –(CF2 –CF2 )n –[CF2 – CF(OCF2 CF2 CF3 )]m –), and PCTFE (polychlorotrifluoroethylene, –(CF2 –CFCl)n –). Fluoropolymers are characterized by excellent chemical and thermal stabilities, as well as low coefficients of friction owing to their self-lubricating properties. However, this chemical stability also makes them difficult to adhere to other materials or to laminate them as a sheet or film with other materials using an adhesive. To overcome these limitations, various methods have been proposed for modifying the surface of fluoroplastics such that they can be adhered to other materials using adhesives. Among these reported methods, treatment by Na–NH3 solution is widely used. In this method, an immersion of the sample in Na and liquid NH3 pool is carried out for the surface treatment. With the treatment, it is known to chemically modify the surface almost completely to enable adhesion with other materials. However, this modification turns the surface of the fluoroplastic dark brown, which is undesirable in terms of appearance. In addition, detachment occurs owing to long-term exposure to ultraviolet light after bonding. Furthermore, metallic Na possibly remains on the © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 M. Okubo, Nonthermal Plasma Surface Modification of Materials, https://doi.org/10.1007/978-981-99-4506-1_3

35

36

3 Hydrophilic Treatment for Polymer Surfaces and Its Applications

film, which may be an obstacle, especially for medical rubber products that require strict quality control. Further, the use of fluorine films, which exhibit light resistance, chemical stability, and gas barrier properties, in flexible displays, electronic papers, sealing films, etc., has remained limited. In this chapter, the basics of plasma and its formation are explained using an example of an electrode system for plasma processing. Next, the principle underlying atmospheric-pressure plasma surface treatments is elucidated, and a specific method (an example of plasma combined treatment or plasma hybrid treatment) for the adhesive bonding of plastics is detailed. The details of the conventional plasma treatment and an atmospheric-pressure plasma graft polymerization treatment, both of which are based on the conventional blowing-type corona discharge treatment, are also presented. Atmospheric-pressure plasma graft polymerization (also called atmospheric-pressure plasma composite surface treatment), as developed by the author’s team [1, 2], is an innovative surface treatment method that effectively improves the adhesion of PTFE and surface plating while imposing an extremely low environmental load. This chapter only introduces the atmosphericpressure plasma combined surface treatment technology for improving fluoroplastic adhesion, as other plastics (such as polyethylene (PE), polypropylene (PP), polycarbonate (PC), and polyethylene terephthalate (PET)) are already relatively easy to bond. This treatment method improves the adhesiveness of fluoroplastic for application in high-frequency cables, automobile hose parts, and medical equipment. The author is currently researching such materials, and the synthesis and adhesion performance test results of adhesive fluoroplastics are presented. The possibility of applying PTFE and other types of fluoroplastics to millimeter-wave devices is discussed as an application example. First, the atmospheric-pressure plasma combined surface treatment, which enables plating on PTFE, is outlined. Next, the procedure for evaluating the effects of the surface treatment and plating on PTFE is presented. Finally, the plasma composite surface treatment of fiber-reinforced composite materials, which are characterized by small dielectric constants similar to that of PTFE and are used in radar domes, also known as “radomes,” is illustrated along with strength improvements of such composite materials. Next, the lamination of thin films on PTFE, metals, and PCTFE and their application to organic light emitting diode (OLED) devices [3] is described. A method by which monomers are polymerized in the vapor phase, while plasma is used to treat the fluoroplastic surface, is also proposed. This method was previously shown to remarkably improve the adhesion between butyl rubber and PTFE [4]. The working principle of the atmospheric-pressure plasma combined surface treatment equipment for bonding a fluoroplastic (Teflon) and its application in medical equipment and electronic equipment are then presented. The successful adhesion of fluoroplastic enabled by this surface treatment indicates the feasibility of improving the adhesion of other plastics simply by using this method. Here, the surface treatmentinduced adhesion between PTFE and metals and that between PTFE and butyl rubber [4] is mainly described.

3.2 Plasma Treatment and Plasma Graft Polymerization Treatment

37

3.2 Plasma Treatment and Plasma Graft Polymerization Treatment 3.2.1 Plasma As already mentioned in Chap. 1, when energy, such as electric power or heat, is applied to a substance, its phase can change from solid to liquid and from liquid to gas. When more energy (here, mainly high-voltage electrical energy) is given to a gas, its molecules dissociate because of the breakage of covalent bonds by highspeed electrons, and ionization of the atoms occurs owing to the attachment and ejection of electrons, leading to the formation of plasma. Plasma consists of negative ions, positive ions, atoms, active species (radicals), and electrons, which are much lighter than heavy particles. By applying a pulse high voltage and cooling using high-speed flowing gas, a state can be created in which many chemically active high-speed electrons exist while maintaining the gas at room temperature; this is called nonequilibrium plasma.

3.2.2 Examples of Plasma Treatment Electrodes Figure 3.1 shows the electrode systems of two types of typical plasma treatment equipment used for treating film surfaces. In a barrier discharge treatment system (Fig. 3.1a), at least one dielectric plate forms a barrier in the space between the electrodes. In this case, a constant voltage is applied to form plasma, which is subsequently applied to treat the surface of the target film transported by the roll. The film passes near one of the electrodes or near the center between the electrodes, and basically, both sides of the film are processed. This longstanding method has been applied to treat the surfaces of various plastics. In addition, it can be used to process relatively large surface areas such as those of films and sheets; however, this method is difficult to apply to three-dimensional and large objects. Figure 3.1b illustrates the formation of a plasma jet by a varying voltage, such as a pulse, across the sharp electrodes, between which a noble gas passes. In this method, which is known as the gliding arc discharge method, the plastic film transported by the roll passes through the jet area, and only the surface facing the jet is treated. The use of this method (gliding arc discharge method) for surface treatment is relatively new, but some products are known. Because this method involves a plasma jet, it can be used to process three-dimensional and/or large objects. However, to process a large surface area, the torch must be moved according to the size or shape of the surface to be processed, and multiple torches are often required. The appropriate choice between these plasma systems depends on the situation. In ordinary corona treatment equipment, the distance between the electrodes is set at 1–3 mm, and only the samples that can pass through this distance can be treated. This type of equipment is suitable for treating a film, for example, but not a thick board,

38

3 Hydrophilic Treatment for Polymer Surfaces and Its Applications Voltage Electrode Gas

Plasma

Dielectric plate

Roll

Film

Plate electrode Voltage

Roll

(a) Gas Plasma torch High Voltage

High Voltage Needle electrode Film

Plasma flow

Copper plate Roll

Roll

(b) Fig. 3.1 Examples of atmospheric plasma electrodes for plastic film surface treatment. a Barriertype electrodes. b Plasma jet-type electrodes

and it cannot be used at all for moldings with complicated shapes. To overcome these drawbacks, a blow-out-type corona discharge treatment device is used for such complex shapes [1]. This device uses a low-temperature plasma generated by cooling the arc discharge with a high-speed stream of gas and allows single-sided processing of thick materials as well.

3.2.3 Principle and Example of Hydrophilic Plasma Treatment The plasma treatment systems shown in Fig. 3.1 result in the remarkable hydrophilization of the surfaces of plastics, polymers, glass, and metals to improve their adhesive properties. As mentioned in Sect. 3.2.1, this surface modification and adhesion improvement occur because high-energy electrons (approximately 1–10

3.2 Plasma Treatment and Plasma Graft Polymerization Treatment

39

eV), generated by the atmospheric-pressure plasma discharge, dissociate the main chains and side chains of the bonds on the material surface, which usually bears an oxidized layer or an oil film on the surface, in the case of metals. This dissociation forms radicals on the surface, while the gas molecules are also dissociated into radicals. Moreover, organic substances like oils and fats that foul surfaces make these surfaces hydrophobic, but the electrons, radicals, and ions generated by the plasma decompose these organic substances, making the surface more hydrophilic. Further, recombination reactions between the radicals in the gas and on the surface form hydrophilic functional groups, such as hydroxyl groups (–OH), carbonyl groups (>C=O), carboxyl groups (–COOH), and aldehyde groups (>C=O), on the material surface. Both of these mechanisms (decomposing fouling layers and forming hydrophilic functional groups) contribute to the hydrophilicity of the surface and increase the surface free energy, thereby facilitating adhesion and joining with other materials. Sufficient wetting between two different phases is important for forming a strong bond between the two phases. The sufficient wetting is known from the value of the contact angle of a water droplet dropped on the surface. Meanwhile, the adhesion energy is a measure of the adhesion strength and represents the work per unit area required to detach the adherent system. The general relationship between the surface adhesion energy and the contact angle of a water droplet dropped on the surface is given in Young’s equation, as explained in Chap. 4. It indicates that a smaller contact angle results in a larger adhesion energy. Next, an example of plasma processing is described wherein a temporal change in the contact angle of a soda glass surface is observed after being treated using the barrier electrode-type atmospheric-pressure air plasma device in Fig. 3.1a. In this case, the initial contact angle of approximately 45° is reduced to approximately 3° after the surface is plasma-treated for 1 min; such a small angle indicates superhydrophilicity. Subsequently, the glass is left in the laboratory for five days, and the contact angle is measured every day. The hydrophilic effect of the plasma is found to decay with time, and the hydrophilicity is gradually lost. This is a commonly observed shortcoming of plasma treatment, because in most cases, materials treated by plasma alone exhibit gradually degrading adhesiveness if left in the ambient atmosphere, and the effect of the treatment vanishes within a few days to a week owing to the penetration and oxidation of the segments containing hydrophilic functional groups into the plastic. Therefore, a treated film must adhere to the required surface immediately after the plasma treatment. Furthermore, plastics with low chemical reactivities, such as fluorocarbon plastics as represented by PTFE, are rarely hydrophilized sufficiently by plasma treatment alone, suggesting the need for other surface treatment strategies for such polymers. To mitigate these problems, atmospheric-pressure plasma graft polymerization treatment has been introduced, which is a permanent treatment technology developed by the author’s group that improves the hydrophilicity of the surface of the fluoroplastics. Figure 3.2 shows photographs of water droplets on an aluminum surface before and after it is treated with plasma under atmospheric pressure using a jet-type device

40

3 Hydrophilic Treatment for Polymer Surfaces and Its Applications

such as that shown in Fig. 3.1b. As evident, spherical water droplets are formed due to the oils and fats present on the untreated surface. These contaminants are decomposed by the air plasma treatment, thus removing this source of hydrophobicity. Combined with the formation of new hydrophilic functional groups by the plasma reaction, as described above, this mechanism resulted in the hydrophilization of the aluminum surface. Fig. 3.2 Water droplets on an aluminum surface a before and b after atmospheric-pressure plasma treatment [5]

3.2 Plasma Treatment and Plasma Graft Polymerization Treatment

41

3.2.4 Plasma Surface Treatment and Plasma Graft Polymerization Surface Treatment Mechanism Notably, the surface treatment only using Ar gas, air, or other gases has been termed the “plasma solo surface treatment,” and that performed in a monomer gas atmosphere, such as acrylic acid, is called a “plasma hybrid surface treatment.” In general, subjecting the surfaces of plastics, glass, and metals to plasma-only surface treatments in the air often makes them strongly hydrophilic and improves their adhesiveness. However, this effect tends to be short-lived, and the plasma solo treatment surface only negligibly hydrophilizes plastics with low chemical reactivities, such as PTFE (a fluoroplastic), requiring a more permanent treatment that is effective for both ordinary and fluorocarbon plastics. As an example of such a plasma hybrid treatment, there has been a technology that monomers with unsaturated bonds like double bonds (e.g., acrylic acid CH2 =CHCOOH) are polymerized one after another on the surface of a material that has been activated by plasma under reduced pressure. This is called plasma graft polymerization and forms a permanent patch-like functional film. However, it is difficult to activate the surface enough with reduced-pressure plasma below atmospheric pressure. Focusing on this technique, the author’s group successfully developed an atmospheric-pressure plasma method to treat the surface of plastics more efficiently. In the plasma hybrid surface treatment method (also termed “plasma combined surface treatment”), treatments by chemicals (such as monomers and paints) and plasma are performed in parallel. The following section explains the principle, treatment method, and examples of the atmospheric-pressure plasma graft polymerization treatment, which enables strong bonding between a metal film and the surface of PTFE.

3.2.5 Structure of the Three Electrodes with Different Potentials Atmospheric-pressure plasma graft polymerization uses a plasma torch, as shown in Fig. 3.3, and a strong nonthermal plasma (NTP) flow is applied to the sample (plastic film) sandwiched between three electrodes with different potentials (sharp positive and negative electrodes and a grounded metal plate) in a monomer gas atmosphere (acrylic acid in this case). This unique patented method creates an ultrahigh-strength adhesive layer on the surface. A periodic voltage, such as a pulse high voltage, is applied across the sharp electrodes, between which a noble gas also passes, and the plastic film transported roll-to-roll passes through the ejection destination of the plasma jet, and only the side facing the jet is treated. Despite being new, this method has already been widely employed to treat the surfaces of different materials. The plasma jet allows the processing of three-dimensional and large objects, although the torch must be moved according to the size or shape of the surface to be processed,

42

3 Hydrophilic Treatment for Polymer Surfaces and Its Applications

Argon gas

Plasma torch Nozzle cover

Monomer gas atmosphere (Acrylic acid)

Electrodes Plasma flow

Grounded

Film (treated sample)

Metal plate

Fig. 3.3 Plasma torch for NTP flow hybrid treatment implemented using three electrodes, each with a different potential [2]

or multiple torches are required to process a large surface area. The cross-sectional dimensions of the nozzle of the apparatus shown in Fig. 3.3 are 38 mm × 38 mm, the electrodes are 6 mm apart, and the dimensions of the rectangular plasma outlet are 38 mm × 6 mm. In addition, a nozzle cover is attached to adjust the gas flow beyond the nozzle. The lower metal plate sandwiching the two electrodes and the film to be processed is grounded. Overall, this method enables effective plasma processing of films.

3.2.6 Principle of Atmospheric-Pressure Plasma Graft Polymerization and Adhesion Improvement Mechanism The principle underlying the atmospheric-pressure plasma graft polymerization of a hydrophilic monomer (acrylic acid, as an example of a double-bonded monomer), as briefly described in Chap. 2, is explained in further detail by the following reactions. Radical generation on the surface of fluoroplastics by plasma-induced electrons under atmospheric-pressure NTP followed by the graft polymerization: R − F → R· + F·

(3.1)

R· + CH2 = CHCOOH → R − CH2 − C · HCOOH

(3.2)

R· + n(CH2 = CHCOOH) → R − (CH2 − CHCOOH)n −

(3.3)

3.3 Atmospheric-Pressure Plasma Graft Polymerization Treatment

43

where R represents the main chain of a fluoroplastic composed of C, H, O, and F atoms, and R·, F·, and R–CH2 –C·HCOOH are radicals with unpaired electrons. Specifically, the electrons generated by the atmospheric-pressure plasma (energy ~ 5 eV, electron number density ~ 1017 m−3 ) collide with the monomers, as well as the material surface, and induce cleavage of the covalent bonds, leading to the formation of radicals. Although the C–C bonds in the main chain of PTFE might dissociate, the C–F bonds in reaction (3.1) are cleaved more frequently. F· is released into the atmosphere. Finally, reaction (3.3) affords the graft polymerization layer of a hydrophilic –(CH2 –CHCOOH)n – on the surface of the fluoroplastic R–F, which improves the adhesive properties of its surface, reaching a layer thickness of approximately 100 nm. X-ray photoelectron spectroscopy (XPS) analysis has been used to confirm the presence of the carboxyl functional group (–COO–). In many cases, adhesion mainly originates from hydrogen bonds, but the strength of a hydrogen bond depends on the functional group that contains it. In the oxygen carboxyl group (–COOH), the oxygen atom has strong negative polarity, and the terminal hydrogen has a positive polarity; thus, this functional group may induce a stronger hydrogen bond. In reduced-pressure plasma graft polymerization, which is the conventional technique, the energy of the generated electrons is relatively higher than that under atmospheric-pressure plasma, but the number density of the electrons is lower. However, to achieve a reduced-pressure environment, a vacuum chamber and vacuum pump are required for the plasma generation. However, the author’s group developed a method in which the atmospheric-pressure plasma contains a high concentration of the generated radicals, which has obtained a high adhesion efficiency. Because this treatment can be performed under atmospheric pressure, it can be easily incorporated into manufacturing lines with high productivity. Another advantage of this method is that it does not use harmful gases or solutions of heavy metals, as in the case of the Na–NH3 method, and, thus, has a negligible impact on the environment. The next section provides further details as well as some examples of atmospheric-pressure plasma graft polymerization.

3.3 Atmospheric-Pressure Plasma Graft Polymerization Treatment 3.3.1 Atmospheric-Pressure Plasma Graft Polymerization Apparatus Figure 3.4a shows a schematic of the atmospheric-pressure plasma graft polymerization treatment apparatus developed by the author’s group, and Fig. 3.4b shows an actual photograph of the equipment in the author’s laboratory. This setup, which can be used to treat an A4-size (210 mm × 297 mm) fluoroplastic film, can treat a variety of polymers, metals, and glasses with thicknesses ranging from 200 μm to 20 mm.

44

3 Hydrophilic Treatment for Polymer Surfaces and Its Applications

The apparatus is placed inside the draft chamber, as shown by dotted lines in the schematic. A single stainless steel evaporation vessel or a double vessel filled with liquid acrylic acid (monomer; FUJIFILM Wako Pure Chemical Industries, purity of 98 mass%) is placed on top of a heating plate equipped with a temperature regulator and a transformer to generate vapor inside the acrylic chamber. The monomer liquid is maintained at a constant temperature (~ 60 °C) and vaporized inside the acrylic treatment chamber, the concentration of which is approximately 1000–2000 ppm. Industrial Ar with a purity of 99.99% is used as the plasma-forming gas, and the Ar flow in the plasma torch is regulated at a constant flow rate of Q. The effects of varying Q are described in the next section. As shown in Fig. 3.3, a pair of wire electrodes with sharp tips is located inside the plasma torch. When a pulse-modulated high AC voltage (20 kHz and 24 kV) is applied, a gliding arc discharge is generated. In this case, the atmospheric-pressure low-temperature plasma jet formed by the gas discharge, with a temperature < 80 °C, acts on the film sample surface on top of the conveyor to initiate the surface treatment. Graft polymerization is performed in a mixed vapor environment of Ar and acrylic acid, with the plasma radicalizing the flowing Ar while simultaneously cleaving the C–F bonds on the surface. In this method, the acrylic acid vapor is ionized, and a violet-green plasma jet is generated. The plasma torch is moved in the transverse direction as the sample is transported by the conveyor in the longitudinal direction to treat the entire surface of the A4size (210 × 297 mm) film. To place the plasma torch under a positive-pressure Ar environment, Ar gas is also flowed into the acrylic chamber. In addition, to prevent air from entering the chamber through the entrance and exit of the film conveyor, gas injection-type curtains are installed at the entrance and exit. The total gas flow rate q in the chamber is 20 L/min. Furthermore, the chamber is thoroughly purged with Ar gas before the operation to remove any air from the acrylic chamber, and a gas concentration meter is used to confirm 0.0% concentration of oxygen inside the chamber.

3.3.1.1

Effects of Gas Flow Rate on the Treatment

Figure 3.5 shows photographs of the plasma jets in Ar flowing at various rates (30, 40, and 50 L/min) gas captured during the film treatment. The gap between the torch and the film surface is 10 mm. As the flow rate increases, the plasma jet evidently becomes stronger, and the contact between the plasma and film surface is minimum at Q = 30 L/min and maximum at Q = 50 L/min. However, the strongest adhesion is obtained at Q = 40 L/min, as explained in more detail later. A stable, uniform monomer environment is formed by optimizing the flow rate, which results in strong, stable adhesion. Moreover, the color of the fluoroplastic surface does not change in the present plasma treatment, in contrast with the brown appearance occurring with ordinary Na–NH3 treatment.

3.3 Atmospheric-Pressure Plasma Graft Polymerization Treatment

45 Exhaust

Fan

Draft chamber Flow meter For purge and gas curtain

Acrylic acid container

Valve

Treatment chamber NTP jet nozzle

Sample Brush and Ar curtain Conveyer

For NTP generation

Mass flow controller

Industrial Ar gas (purity = 99.99%)

Transverse direction Longitudinal Heater direction Stainless container

NTP jet

Thermocontroller

20-kHz pulse modulated AC high voltage power supply

(a)

(b) Fig. 3.4 Atmospheric-pressure plasma graft polymerization apparatus (A4-size film treatment). a Schematic and b photograph of the surface treatment of a fluoropolymer film, showing the moving plasma torch and plasma [3]

46

3 Hydrophilic Treatment for Polymer Surfaces and Its Applications

Fig. 3.5 Photographs of plasma jets with different Ar gas flow rates Q of a 30, b 40, and c 50 L/ min [2]

3.3.1.2

Two Types of Vapor Generation Methods

Figure 3.6 shows schematics of the strategies for introducing vapor into the prototype of the plasma graft polymerization surface treatment equipment. Graft polymerization is achieved by introducing a monomer, such as acrylic acid, into an Ar gas plasma jet formed between high-voltage electrodes (24 kV). In the conventional method (Fig. 3.6a), the acrylic acid vapor is introduced to the side of the Ar plasma jet from outside the chamber. Figure 3.6b shows an improved system that has been tested in recent years and is similar to the apparatus shown in Fig. 3.4. By installing an acrylic acid vapor generator inside the duct, the surface of the fluoroplastic film can be treated with a highly concentrated (up to 5000 ppm) and uniform acrylic acid vapor atmosphere. This modification results in a strong, stable adhesion performance, as described below. For example, the evaporation–diffusion method shown in Fig. 3.6b recently attained a maximum adhesion strength of 2.5 N/mm during a T-type peeling test using, whereas the apparatus shown in Fig. 3.6a provided a maximum strength of 1.4 N/mm.

3.3 Atmospheric-Pressure Plasma Graft Polymerization Treatment Fig. 3.6 Two configurations for introducing vapor into a plasma graft polymerization surface treatment apparatus for fluoroplastics: a vapor side injection and b vapor diffusion methods

47

Ar gas Acrylic acid vapor

Acrylic acid vapor

High voltage

High voltage

Electrodes Plasma Fluoropolymer film

(a) Ar gas

High voltage

High voltage

Acrylic acid vapor generator

Acrylic acid Electrodesvapor generator Plasma Fluoropolymer film

(b) 3.3.2 Surface Treatment Evaluation for PTFE Metal Plating 3.3.2.1

Contact Angle Measurement

The surface hydrophilicity of the treated fluoroplastic films can be evaluated through contact angle measurements, as introduced in Sect. 2.5.1. In this case, a 5-μL drop of purified water is dropped on the surface, and the contact angle is measured (Kyowa Interface Science, CA-VE) after 1 s. A small contact angle indicates a more hydrophilic surface. For example, the contact angles of the untreated PFA, PTFE, and PCTFE films are found to be 94°, 104°, and 91°, respectively, indicating their hydrophobicity. After treating these film surfaces using a laboratory prototype plasma hybrid surface treatment apparatus, the contact angles of PFA and PTFE decreased significantly to 44° and 39°, respectively. Interestingly, the contact angle has been observed to decrease rapidly with treatment time; for PTFE, it decreased

48

3 Hydrophilic Treatment for Polymer Surfaces and Its Applications

to 17° in 2 min and became saturated at only 10°, indicating that the surface became superhydrophilic.

3.3.2.2

T-Type Peeling Test

Figure 3.7 shows the experimental setup used for the T-type peeling test of the fluoroplastic films. In this example, the surface of a highly pure PTFE sample is subjected to the previously described surface treatment. After adhering a 25-mmwide stainless steel plate (length, L = 50 mm) to the treated sample using a two-part epoxy adhesive (Konishi E-set), the adhesive strength is evaluated by performing a T-type (90°) peeling test using a peeling test machine composed of a digital force gauge and an electrical measurement stand (ZTA – 100 N + MX2 − 500 N, Imada Co., Ltd., Japan). A nonadhesive area of the treated sample is held with a clamp, and the stainless steel plate is fastened by supporting metals and allowed to slide from left to right, while the peel angle is maintained at 90°. The peeling strength or adhesive strength is measured while the test piece is peeled by pulling the surface layer upward at a speed of 100 mm/min. Figure 3.8 shows a schematic of the crosssection of a laminated sample with a layered structure of PTFE film/plasma graft polymerization layer/epoxy adhesive layer/stainless steel plate. Based on the test results, it can be confirmed that a sufficiently strong adhesion occurs between the plasma graft polymerization layer and the epoxy adhesive layer as well as between the epoxy adhesive layer and the stainless steel metal plate layer. Therefore, peeling mainly occurs at the interface of the PTFE/graft polymerization layer, as shown with the red line. The bar graph in Fig. 3.9 shows an example of T-type peeling test measurements for PTFE films with a polyacrylic acid layer. The vertical axis shows the maximum peeling strength (value at the maximum load point) per 1 mm of the sample width. Test

Peeling direction Clamp Epoxy adhesive

Stainless plate

Supporting metal

Supporting metal Sliding possible

Fig. 3.7 a Schematic and b photograph of the T-type peeling test and apparatus wherein a PTFE film is adhered to a stainless steel plate and peeled at a rate of 100 mm/min [6]

3.3 Atmospheric-Pressure Plasma Graft Polymerization Treatment

49

Stainless steel plate Peeling interface

Epoxy adhesive layer Plasma graft polymerization layer Fluorocarbon plastic film

Fig. 3.8 Cross-section of a PTFE film laminated onto a stainless steel plate

Maximum peeling strength N/mm

pieces 1–3, which are prepared by the vapor diffusion method, exhibit a maximum peeling strength of 2 N/mm or more, whereas test pieces 4–6, prepared using vapor side injection, are peeled off at a strength of less than 1 N/mm. Thus, the maximum peeling strength of test pieces 1–3 is higher than that of test pieces 4–6 because of the stronger adhesion between the fluoroplastic film and the polyacrylic acid plastic layer. As described above, plasma treatment alone does not significantly improve the adhesiveness of fluoroplastics, unlike other polymer materials, and in the case of PTFE, the maximum adhesiveness is 0.27 N/mm under optimal plasma treatment conditions. Thus, both types of graft polymerization provided higher adhesion. In the untreated case, the peeling strength against PTFE is 0.012 N/mm, and adhesion itself is difficult. For another T-type peeling test, NTP graft polymerization is performed on PTFE (thickness = 200 μm) under the following experimental conditions: Ar main flow rates Q = 30, 40, or 50 L/min, acrylic acid temperature = 60 °C, and lateral head feed rate = 4 mm/s. Then, the peeling (adhesion) strength of the polymerized layer is measured. Six T-type peeling tests are performed at each flow rate, and the maximum values of the maximum and average bond strengths are observed at Q = 40 L/min. Figure 3.10 presents the displacement–peeling strength (load) curve at Q = 40 L/min.

Test No.

Fig. 3.9 Maximum peeling strength obtained between a stainless metal plate and PTFE film (values per 1 mm of sample width; test samples 1–3 are prepared by the vapor diffusion method, and test samples 4–6 are prepared with the vapor side injection method) [7]

50

3 Hydrophilic Treatment for Polymer Surfaces and Its Applications

Fig. 3.10 Displacement–peeling strength (load) curve of the PTFE–stainless steel-laminated sample prepared with an Ar flow fate of Q = 40 L/min. The vertical axis represents the peeling strength for a sample width of 1 mm. The acrylic acid temperature is 60 °C, the lateral speed is 4 mm/s, the maximum peeling strength is 5.12 N/mm, and the average peeling strength is 4.78 N/ mm [8]

The graph represents the average and standard deviations of the six measurements per condition. In the figure, the displacement on the horizontal axis exceeds the length of the adhesive area (30 mm) because of the strong adhesive force and film elongation. Further, the units (N/mm) on the vertical axis imply the peeling strength per 1 mm width. Evidently, the maximum peeling (adhesion) strength, in this case, is 5.12 N/ mm, and the average adhesion strength is 4.78 N/mm. This outstanding adhesive strength has not been achieved without using a conventional sodium-based solution surface treatment agent. The largest maximum and average adhesive strengths at Q = 40 L/min can be elucidated as follows. First, when the plasma flow rate is low, the adhesive strength is low owing to the insufficient activation of the plasma jet, monomer, and PTFE surface. When the flow rate is high, the impinging jet stream dilutes the monomer concentration near the film surface. Alternatively, because the plasma energy with respect to a unit gas flow rate decreases when the flow rate is too high, the adhesive strength decreases. Thus, the largest values are observed at Q = 40 L/min. Nonetheless, these results reveal that the adhesion of PTFE is remarkably improved by atmospheric-pressure plasma polymerization in a highly concentrated acrylic acid atmosphere in the vessel that contains the discharge plasma device and fluoroplastic substrate.

3.3 Atmospheric-Pressure Plasma Graft Polymerization Treatment

3.3.2.3

51

Effect of Acrylic Acid Evaporation Temperature on Adhesive Strength of Treated PTFE Films

To determine the optimal treatment conditions, the author’s team first studied the effect of the temperature at which acrylic acid is heated and evaporated on the adhesive strength [8]. Thus, PTFE has been treated at acrylic acid evaporation temperatures of 40, 45, 50, 55, and 60 °C. Experiments are not conducted above 60 °C because the flash point temperature of acrylic acid is 68 °C. At this temperature, the self-polymerization of acrylic acid becomes significant inside the evaporation vessel, which is dangerous. The speed at which the plasma torch moved in the transverse direction is maintained at 4 mm/s, which is the optimum speed for balancing strength and productivity, as discussed previously. The peeling mode in the test is interfacial peeling or cohesive failure between PTFE and the graft-polymerized layer. Figure 3.11a–e depicts the relationship between adhesive or peeling strength and the stroke of the peeling with the increased evaporation temperature for the acrylic acid. In each figure, data corresponding to six treated composites are averaged, and the standard deviations (± σ ) are shown, assuming that the data follow a normal distribution. In all cases, the samples are peeled off at the interface between the double-sided adhesive tape and PTFE. In these experiments, the films are elongated from their original length of 50 mm, and the films are released at different strokes in different cases. In Fig. 3.11a, the average peeling strength is 4.12 N/mm at strokes of 15–40 mm, and release occurs with a stroke of 45 mm. In Fig. 3.11b, the strength increases to 4.66 N/mm at strokes of 15–60 mm, and release occurs at a stroke of 75 mm. In Fig. 3.11c, the strength is greater than 4.87 N/mm at strokes of 15–60 mm, with the highest value of 5.4 N/mm. In this case, release occurs when the stroke is 75 mm. In Fig. 3.11d, the strength is 4.88 N/mm at strokes of 15–60 mm, with a maximum value of 5.1 N/mm. Similarly, in Fig. 3.11e, the strength is 4.90 N/ mm at strokes of 15–60 mm, with a maximum value of 5.1 N/mm, equal to that in Fig. 3.11d). Figure 3.12 presents the average peeling strength as a function of the acrylic acid evaporation temperature with the plasma torch moving at a constant speed of 4 mm/s. Evidently, increasing the evaporation temperature increased the average peeling strength. Under the given conditions, the maximum average peeling strength of 4.90 N/mm is observed at 60 °C, which might be attributed to the increase in the amount of acrylic acid evaporated on the PTFE surface due to the increase in monomer concentration during plasma treatment. In other words, the amount of monomer evaporated increases with increasing temperature. At 60 °C and 4 mm/s, the average concentration of acrylic acid vapors inside the chamber can be estimated by measuring the decrease in the mass of liquid monomer inside the evaporation vessels during treatment (~ 1500 ppm). Notably, the average adhesive strength at 55 °C is 4.88 N/mm, which is similar to that at 60 °C.

52 Fig. 3.11 Peeling strength and the stroke at which release occurs for various evaporation temperatures of acrylic acid (a 40 °C, b 45 °C, c 50 °C, d 55 °C, and e 60 °C) with the transverse speed of the plasma torch maintained at 4 mm/s. In all cases, the samples peel off at the interface between the double-sided adhesive tape and PTFE [8]

3 Hydrophilic Treatment for Polymer Surfaces and Its Applications

3.3 Atmospheric-Pressure Plasma Graft Polymerization Treatment 6 Average peeling strength (N/mm)

Fig. 3.12 Relationship between the average peeling strength and acrylic acid evaporation temperature when the plasma torch moves at a constant speed of 4 mm/s [8]

53

5

4 3 2 1 0 35

40

45

50

55

Acrylic acid temperature (

3.3.2.4

60

65

)

Effect of the Plasma Torch Speed on the Adhesive Strength of Treated PTFE Films

The plasma processing rate, as determined by the speed at which the plasma torch moves in the transverse direction, also affects the adhesive strength of the treated films. In addition, by shortening the processing time, the productivity of the treatment increases, and more samples can be treated in a given time period. PTFE films are thus treated by changing the transverse speed of the plasma torch to 1, 2, 4, 8, 12, 16, and 20 mm/s, while maintaining the acrylic acid evaporation temperature at the optimized value of 55 °C. Figure 3.13a–f depicts the relationship between the peeling strength and stroke as well as the stroke at which release occurs with increasing transverse torch speeds. The results corresponding to 4 mm/s and 55 °C are also depicted in Fig. 3.11d. Again here, the curves show the average standard deviations for six treated samples, as in Fig. 3.10. In all cases, the samples peel off at the interface between the doublesided adhesive tape and PTFE. At a transverse speed of 1 mm/s (Fig. 3.13a), the average adhesive strength is 4.93 N/mm at strokes of 15–60 mm, which is the highest among all the tested cases. In addition, the smallest standard deviation is observed. In Fig. 3.13b, c, the strengths are 4.49 and 4.44 N/mm at strokes of 15–60 mm, respectively, which are smaller than that in Fig. 3.13a. In Fig. 3.13d, the average strength is 4.04 N/mm at strokes of 15–50 mm, and the standard deviation is greater than that in Fig. 3.13a. In Fig. 3.13e, the average standard deviation is 2.23 N/mm at strokes of 15–30 mm, and when the stroke exceeds 30 mm, the strength drops sharply owing to film release. In Fig. 3.13f, the average strength is only 1.30 N/mm at strokes of 15–30 mm, and it decreased sharply at a short stroke of 35 mm. Figure 3.14 shows the relationship between the average peeling strength and the transverse speed of the torch at an acrylic acid evaporation temperature of 55 °C. Evidently, the peeling strength tends to decrease with the increasing torch speed,

Peeling strength (N/mm)

6 5.5 5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0

Peeling strength (N/mm)

6 5.5 5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0

(a)

Average

Standard deviation

(b)

Average

6 5.5 5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0

Standard deviation

(c)

Average

Standard deviation

Peeling strength (N/mm)

6

(d)

5.5 5 4.5 4

3.5 3 2.5

Average

2

Standard deviation

1.5

1 0.5 0

Peeling strength (N/mm)

Fig. 3.13 Peeling strength and stroke at which release occurs for different transverse torch speeds with the acrylic acid evaporation temperature maintained at the optimized value of 55 °C. In all cases, the samples peel off at the interface between the double-sided adhesive tape and PTFE. a 1 mm/s, b 2 mm/s, c 8 mm/s, d 12 mm/s, e 16 mm/s, and f 20 mm/s [8]

Peeling strength (N/mm)

3 Hydrophilic Treatment for Polymer Surfaces and Its Applications

6 5.5 5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0

Peeling strength (N/mm)

54

6 5.5 5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0

(e)

Average

Standard deviation

(f)

Average Standard deviation

0

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75

Stroke (mm)

3.3 Atmospheric-Pressure Plasma Graft Polymerization Treatment

N/mm

6

Average peeling strength

Fig. 3.14 Relationship between the average peeling strength and the transverse torch speed at an acrylic acid evaporation temperature of 55 °C [8]

55

5 4 3 2 1

0 0

5

10

15

Speed in transverse direction

20

25

mm/s

which reduces the plasma irradiation density per unit area of the sample, thus making graft polymerization difficult. The maximum average peeling strength in these experimental conditions is 4.93 N/mm (at 1 mm/s). In this case, the average concentration of acrylic acid vapors inside the chamber is ~ 1500 ppm. By manipulating the monomer evaporation temperature, the acrylic acid concentration can be controlled between 1000 and 2500 ppm.

3.3.2.5

Highest Performance Values of T-type Peeling Strength

Table 3.1 shows the largest T-type (90°) peeling strengths obtained thus far with this treatment method. The maximum peeling strength is determined from the maximum load point of the displacement–peeling strength curve, and large maximum strength of 128.0 N is recorded for PTFE. In general, PFA and PCTFE, both of which contain impurities or bonds other than C–F bonds, adhere more easily than PTFE. In Table 3.1, the maximum peeling strengths of treated PFA and PCTFE, i.e., 50.0 and 36.0 N, are lower than those of treated PTFE. However, if the plasma graft polymerization is optimized for these materials, values that surpass that of PTFE are expected, especially for PFA. Table 3.1 Various fluoroplastic T-type peeling test results (peeling strength, value per 25 mm width, the highest value obtained in January 2020) Peeling test results (N) Untreated (plasma solo treatment) After plasma graft polymerization PTFE

< 0.3(6.7)

PFA

< 0.6

50.0

PCTFE

< 0.6

36.0

128.0

56

3 Hydrophilic Treatment for Polymer Surfaces and Its Applications

Unlike other polymer materials such as PE, PP, PC, and PET, fluoroplastics do not show much improvement in adhesiveness with the solo plasma treatment, reaching only 6.7 N for PTFE, as shown in Table 3.1, even if the conditions of the plasma are optimized. By contrast, the plasma graft-polymerized PFA exhibited a strength of 50.0 N, which is more than 80 times higher than that of the untreated sample (0.6 N or less). Although the results cannot be compared because the evaluation methods are different, a strength of 9 N (converted to a width of 25 mm) was previously reported for a PFA film treated with sodium–tetrahydrofuran (Na/THF) solution [9]. Remarkably, the strength attained by the plasma polymerization method is more than five times higher. In the case of PFA, depending on the conditions, the adhesive strength is found to be comparable to that obtained after the sodium or naphthalene solution treatment (trade names: Tetraetch, Fluorobonder, etc.). Furthermore, the author’s team also conducted tests with various monomers, and acrylic acid exhibited the best performance. Thus, it can be concluded that the adhesive strength provided by this plasma hybrid method is sufficient for application in fluoroplastic endoscopes, chemical liquid injection tubes, and electronic substrates.

3.3.3 XPS Analysis Results The processed films are analyzed by XPS (using a Kratos ESCA-3300 instrument) to evaluate the bonding state and constituent elements of the sample surface. For the XPS analysis, the sample surface is irradiated with X-rays, and the energy of the generated photoelectrons is measured, as outlined in Sect. A.2. Figure 3.15a, b shows the XPS spectra (C1s ) of the untreated and plasma-grafted PTFE films, respectively. In Fig. 3.15a, the peak centered at 292 eV corresponds to –CF2 –(difluoromethylene group), which is the origin of the hydrophobicity. In Fig. 3.15b, the peaks centered at 289, 287.5, 286.5, and 285 eV can be ascribed to –COO–, –C=O, the alcohol group (–C–O–), and the methylene group (–CH2 –), respectively. In general, functional groups, such as –COO–, > C=O, and –C–O–, are incorporated into the polymer surface, and these groups make the surface hydrophilic and improve adhesion.

3.4 Applicability of PTFE/Plastics in Millimeter-Wave Devices A wide range of applications of fluoropolymers is currently being developed. However, layering and metal plating are difficult with these materials because they do not adhere easily to themselves or other materials. If layering and plating were possible, many applications in various fields could be realized. Examples of such applications include automobile-related electrical parts, such as car navigation systems and millimeter-wave radars, telecommunication devices, e.g., wireless

3.4 Applicability of PTFE/Plastics in Millimeter-Wave Devices

Fig. 3.15 XPS profile of the treated PTFE film surface (C1s peaks) [1]

57

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3 Hydrophilic Treatment for Polymer Surfaces and Its Applications

and optical communication devices, and the millimeter-wave coaxial cables used in various fields, including industrial device wiring and connection. The relative dielectric constant (εr ) and dielectric loss tangent (tan δ) of PTFE are both low; hence, antennas and cables that use PTFE as the internal dielectric material present extremely low transmission losses. Unfortunately, as PTFE is a low-adhesion material, the above-mentioned issues remain during metal coating processes, such as copper film adhesion and metal plating on the PTFE surface. To overcome these hurdles, the adhesiveness of PTFE/plastic can be improved using a plasma hybrid process to enable metal plating technology and its application to millimeter-wave devices, as described here.

3.4.1 Plastic Properties: Dielectric Constant, Dielectric Loss Tangent, and Hydrophobicity Figure 3.16 shows the εr and tan δ values of various plastics [10]. Among them, fluoroplastics, such as PTFE, PFA, and PCTFE, present the smallest dielectric constants and dielectric loss tangents, and liquid crystal polymers (LCPs) also exhibit similarly small values. In other words, these plastics exhibit low transmission losses when used as electronic substrates. Thus, the use of PTFE in millimeter-wave antennas is very promising and a particularly desirable goal. With the approaching era of nextgeneration 5G mobile telecommunication systems and automatic driving operations using millimeter-wave devices, a paradigm shift is also occurring in substrate materials for wireless communication devices. Specifically, conventional glass epoxy substrates and polyimide substrates are rapidly giving way to low-transmissionloss materials, such as LCPs and fluoroplastics. In addition, the latter materials are hydrophobic and hence biologically compatible with the human body, which also allows for their application in implantable electronic devices.

3.4.2 Small High-Performance Millimeter-Wave Band Antennas Examples of antennas developed for automobile radar systems using PTFE and LCPs are introduced in [11]. Fujitsu Ten Ltd. (currently, Denso Ten Ltd.) has been continuously developing radars for automobiles using the millimeter-wave band at 30– 300 GHz. Important electrical parts include the flat tri-plate antennas installed in automobiles and simple microstrip antennas. The flat antenna is indispensable for simultaneously realizing automobile mountability and scanning functions. Moreover, microstrip antennas, with their simple structure, are particularly effective for cost reduction. However, as microstrip antennas contain a plastic substrate, the transmission losses are high, which reduces their efficiency.

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59

Fig. 3.16 Relationship between the dielectric tangent (tan δ) and the relative dielectric constant (ε) for various plastics (made based on Fig. 3 in Ref. [10]. Some data are added)

Figure 3.17 schematically shows an example of a microstrip antenna structure, also called a printed antenna, which contains a plastic dielectric substrate interposed between a conductive feeder line and a grounded plate (copper foil). For high antenna performance, thin dielectric material with a low relative dielectric constant is required. This microstrip antenna is constructed with dimensions matching the resonance frequency, and it is a narrow-band, wide-beam antenna. The electrical properties, mechanical strength, environmental durability, etc., of the plastic substrates of microstrip antennas have been studied in detail. When an antenna is used in a millimeter-wave band, its electrical properties, principally its Fig. 3.17 Microstrip antenna structure

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3 Hydrophilic Treatment for Polymer Surfaces and Its Applications

dielectric constant and dielectric loss tangent, are particularly important. Because the frequency bandwidth becomes narrower with the increasing dielectric constant, a dielectric constant of approximately three or smaller is usually appropriate. Uezato et al. [11] describe LCPs and PTFE as target materials, and their results show that the losses are lower with PTFE, suggesting the possibility of realizing high-efficiency antennas using PTFE as the substrate material. However, PTFE is a poorly adhesive material, and unresolved issues remain when a copper coating is attached to its surface. To resolve these issues, a thin adhesive layer of hydrophilic polyacrylic acid (thickness ~ 100 nm) is deposited on PTFE by the atmospheric-pressure plasma hybrid treatment, as described in the next section. Consequently, microstrip antennas could potentially be constructed by adhering a metal film to the treated polymer surface or by performing electroless plating on this surface, followed by electrolytic plating. However, once this type of antenna is built, the electrical properties are expected to deteriorate slightly as the polyacrylic acid layer interposed between the metal and dielectric material is very thin.

3.4.3 Applicability to High-Frequency Coaxial Cables PTFE can be similarly applied to waveguides that supply microwaves or in coaxial cable assemblies with a high frequency corresponding to millimeter waves. Because LCPs and PTFE display both low εr and tan δ, coaxial cables that use either material as the internal dielectric exhibit extremely low transmission losses. Coaxial cables are used in a broad range of fields, including wireless and optical telecommunication systems, wiring and connection of industrial devices, and automobile-related parts, such as car navigation systems and millimeter-wave radars. In recent years, the applicable frequency bandwidth of coaxial cable assemblies has increased from several tens of GHz to several hundreds of GHz, and the bandwidth continues to widen. In response to these developments, high-frequency coaxial cable assemblies for millimeter waves using porous PTFE as the dielectric material are already being commercialized [12]. The structure of an ordinary coaxial electrical cable is shown in Fig. 3.18. The cable is composed of a centered conductor, such as copper wire; a dielectric material, such as porous PTFE; an outer surface conductor, such as a copper foil roll, copper coating, or copper braided wire; and a sheath, such as fluorinated ethylene propylene (FEP). When poorly adhesive PTFE is used as the dielectric material, the dielectric material and copper metal do not adhere well to each other. Therefore, a surface treatment is needed. The processes described below normally yield excellent characteristics in that regard. Porous PTFE, the plastic material with the smallest εr and tan δ values, is typically used. Furthermore, a uniform dielectric material is laid in the longitudinal direction on both the external surface and in voids using the paste pressout method. In addition, the outer conductor is formed of a spiral of silver-plated tape, wherein the tape width and roll pitch are strategically customized to ensure

3.4 Applicability of PTFE/Plastics in Millimeter-Wave Devices

61

Fig. 3.18 General coaxial cable structure

smooth signal flow. For low attenuation, silver-plated copper with low resistivity is often used. However, problems introduced by the poor adhesivity of PTFE remain in realizing high-performance coaxial cables. Thus, before plating copper on the PTFE surface, a thin adhesive layer of hydrophilic polyacrylic acid (thickness ~ 100 nm) can be grafted onto the surface using the atmospheric-pressure plasma hybrid treatment. Adhering a metal film to the surface or performing electroless plating, followed by electrolytic plating, enables the fabrication of a high-performance coaxial cable. Once this is accomplished, negligible deterioration of the electrical properties is expected, similar to the case of the microstrip antenna, because the polyacrylic acid layer inserted between the metal film and dielectric material is thin.

3.4.4 Method of Copper Plating on PTFE and Results Through trial and error, the author’s team successfully used a sensitizing–activating method to perform electroless copper plating on the surface of PTFE films treated by atmospheric-pressure plasma graft polymerization [13]. The specific procedure is described below and as shown in Fig. 3.19: (1) Sensitizing process: The plasma graft-polymerized PTFE film is immersed for 5 min in a mixed aqueous solution containing 20–40 g/L SnCl2 and 20–40 mL/ L HCL, followed by washing with purified water. (2) Activation process: The film is immersed for 5 min in a mixed aqueous solution containing 0.25–0.50 g/L PdCl2 and 2.5–5.0 mL/L HCL, followed by washing with purified water. (3) Alkaline cleaning process: To remove the tin from the film and metalize it with palladium, the film is cleaned with an alkaline solution. Specifically, the film is immersed in 10% NaOH at room temperature for 10 min and washed again with purified water. If the treated film is immersed in NaOH for a longer period of time, the polyacrylic acid polymerized layer begins to peel off, and the process fails. (4) Electroless plating process: After finishing the palladium metallization, the sample is immersed for 3 min in a copper plating solution (concentration of CuSO4 = 3.5 g/L, Rochelle salt KNaC4 H4 O6 ·4H2 O = 34 g/L, Na2 CO3 = 3.0 g/ L, NaOH = 7.0 g/L, and 37% formalin HCHO = 13 mL/L) under stirring. Then, it is washed with purified water and dried. To increase the thickness of the metal

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3 Hydrophilic Treatment for Polymer Surfaces and Its Applications Sensitizing

SnCl2 HCl Temperature Time

Activating Palladium chloride

PdCl2 HCl

20 40 20 40

g/L mL/L

Room temp. 5 min 0.25 0.5

2.5-5

Short-time alkari washing

10% NaOH Temperature Room temp. Time

Electroless plating

solution

g/L mL/L

Temperature Room temp. Time 5 min Alkari washing

solution

CuSO4 Rochelle salt Na2CO3 NaOH HCHO(37%) Water bath Immersion time

0.25 3.5 34 3 7 13

Copper plating solution

min g/L g/L g/L g/L mL/L

Copper ion reduction by catalyst Thickness increase by electrical plating

Room temp. 3 min

Fig. 3.19 Procedure for electroless copper plating on the treated PTFE with the sensitization– activation method

plating, conventional electrical plating can be performed after the electroless plating procedure. Figure 3.20a, b shows SEM (Nikon E-SEM2700) images of copper plating on untreated and treated PTFE film surfaces, respectively [13]. Many holes with diameters of 100–200 μm appear on the surface of the untreated sample as a result of the formation of bubbles caused by the hydrophobicity of the surface. In this case, uniform metal plating is impossible. Figure 3.20b shows the very smooth metalplated surface obtained for the treated film, with none of the holes observed on the untreated film. Electroless copper plating can be performed on four types of treated and untreated PTFE samples. Figure 3.21 shows the results when a stainless steel plate adheres to the surface of one of these samples during the previously described T-type peeling test. The horizontal axis shows the Ar gas flow rate used during the plasma treatment. The peeling strength of the untreated sample is less than 0.5 N in the figure, whereas the maximum strength for the treated sample is 23 N. This high strength is approximately 50 times larger than that of the untreated sample. The thermal durability of the copper plating at 300 °C and 1 min testing time is also confirmed.

3.4 Applicability of PTFE/Plastics in Millimeter-Wave Devices Fig. 3.20 SEM images comparing copper plating on PTFE surface: a copper on untreated PTFE surface and b electroless plating of copper after plasma graft polymerization treatment [13]

Fig. 3.21 Effect of Ar gas flow rate during plasma graft polymerization treatment on the peeling strength of electroless copper plating on treated and untreated PTFE [13]

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3.4.5 Surface Treatment of Dielectric Cable In this subsection, an electric cable, which is composed of centered dielectric material of PFA and an outer surface silver metal electrode, is explained. Such cable is fabricated by the electric cable treatment machine shown in Fig. 3.22. This system consists of the A4-size treatment machine explained previously and a pair of cable winding rollers with an electric motor. Surface treatment of cable dielectric materials is carried out with the apparatus shown in Fig. 3.23. The surface of the dielectric cable passes through the plasma region inside the plasma jet. As a result, the surface of the dielectric material becomes hydrophilic, and a metal coating can be easily attached to the outer surface. Process conditions are as follows. Ar flow rate = 40 L/min, acrylic acid evaporation temperature is 55 °C, and cable feed rate is 2 mm/s. Plasma torch-to-cable and cable-to-substrate distances are changed variously, 5–5 mm, 5– 10 mm, and 10–10 mm. The 5–5 mm gives the best performance. We investigated the optimum cable feed rate. It is 2 mm/s = 120 mm/min. It takes 4.2 h to process 30 m cable. The polymer layer thickness is approximately 300 nm for the highest adhesive strength. At least 10 plasma torches are required to increase the practical process feed rate to 1.5–3 m/min for industry applications. Figure 3.24 shows the fabricated prototype sample of high-frequency cable. The material is PFA, and the diameter is 2 mm. After the plasma hybrid treatment, the cable is immersed in a Dotite® conductive silver paste (D-500, Fujikura Kasei Co., Ltd.). The very uniform silver metal layer is created, and a peeling strength larger than 1 N/mm is confirmed.

Fig. 3.22 Electric cable treatment machine

3.4 Applicability of PTFE/Plastics in Millimeter-Wave Devices

65

Fig. 3.23 Schematics of the electric cable treatment machine Fig. 3.24 Outer view of a fabricated prototype sample of high-frequency dielectric cable

3.4.6 Method of Nickel Plating on PTFE and Results Again through trial and error, the author’s team used a catalyzer–accelerator method to perform electroless nickel plating on a PTFE film treated by atmospheric-pressure plasma graft polymerization [14]. The procedure consists of the following processes, as shown in Fig. 3.25: (1) Activation process: The film is immersed in Condilyzer FR Conku (concentration = 50 mL/L), a chemical that improves catalyst adhesion, for 5 min at 40 °C, followed by washing with purified water. (2) Catalytic process: A catalyzation treatment is performed with a mixed colloidal solution of Sn2+ and Pd2+ . The temperature of the solution is 35 °C with a sintering time of 6 min. Then, the film is washed with purified water. (3) Accelerator solution process: After immersing the film in an accelerator solution (an acidic solution with a concentration of 200 mL/L) at room temperature for 5 min, it is washed with purified water. (4) Electroless plating process: The film is immersed in an acidic nickel plating solution (Topnicolon TOM-S, concentration = 200 mL/L) at 80 °C. It is then

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3 Hydrophilic Treatment for Polymer Surfaces and Its Applications

Activation Condilyzer FR Conch (To improve catalyst adhesion), 50 mL/L, 40 , 5 min

colloid solution

↓Water-washing Catalytic process: Sn2+ and Pd2+ mixed colloid, 35 , 6 min

Acid solution washing

↓Water-washing Accelerator solution process: acidic solution, 200 mL/L, room temperature, 5 min ↓Water-washing Electroless plating process: acidic nickel-plating solution (TopNicolon TOM-S) 200 mL/L, 80

Nickel plating solution bath

Nickel ion reduction by catalyst Thickness increase by electroplating

↓Water-washing Finishing, increased thickness by electroplating Fig. 3.25 Procedure for electroless nickel plating on the treated PTFE with the catalyzer–accelerator method

washed with purified water and dried. To increase the thickness of the metal plating, electroplating can be performed after electroless metal plating. Figure 3.26 shows photographs of PTFE and PTFE treated by plasma graft polymerization after the nickel electroless plating procedure described above. Although no nickel plating occurred on the surface of untreated PTFE, the treated film is uniformly plated. In addition, the peeling strength of the treated and plated film reached over 1 N per 1 mm width. To test the flexibility of metal-plated PTFE films, repeated bending tests can be performed using a bending tester, as shown in Fig. 3.27. The sample is held by a rod for bending and two sample-holding rods. Figure 3.28 shows an SEM image of a metal-plated PTFE sample treated with plasma graft polymerization at an Ar flow rate of 40 L/min after the bending test. In this case, 100,000 bending cycles at a bending angle of approximately 170° are performed on the metal-plated PTFE film sample. The adhesiveness of the metal plating shown in this figure is fairly good, with no peeling observed.

3.4 Applicability of PTFE/Plastics in Millimeter-Wave Devices

67

Fig. 3.26 Electroless plating of nickel on a untreated and b plasma graft polymerization-treated PTFE films with width of 10 cm

3.4.7 Microfabrication of Nickel Plating on PTFE Patterns can be generated by photolithography on nickel-plated PTFE according to the following processes: (1) The sample is attached to a Si wafer with a diameter of 4 in.

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3 Hydrophilic Treatment for Polymer Surfaces and Its Applications

Rod for bending

Film sample

Rod for sample support

Rod for bending

Film sample

(a)

Rods for sample support

(b)

Fig. 3.27 Repeated bending tester machine: a overview and b detailed side view [14] Fig. 3.28 SEM image after repeated bending test of nickel-plated PTFE. About 100,000 cycles of bending are performed at a bending angle of approximately 170° [14]

(2) A positive-type resist (OFPR-800 50cP, Tokyo Ohka Kogyo Co., Ltd., Japan) is spin-coated on the surface (first pass: 500 rpm, 5 s; second pass: 4000 rpm, 20 s). The thickness of the film on the Si substrate is typically 1.2–1.5 μm. (3) The sample is prebaked in an oven at 90 °C for 15 min. (4) Mask exposure is performed at 18 mW/cm2 for 20 s using an extra-high-pressure mercury lamp. (5) The resist and nickel on the unexposed parts are dissolved using a developer liquid (tetramethyl ammonium hydroxide, NMD-3, 2.38%, Tokyo Ohka Kogyo Co., Ltd.) for 45 s. (6) The wafer is washed with flowing purified water for 2 min. The resist is completely removed.

3.4 Applicability of PTFE/Plastics in Millimeter-Wave Devices

69

(7) After blowing with nitrogen gas, the wafer is dried in an oven at 90 °C for 10 min. Figure 3.29a shows an example treated by this procedure. The numbers 20 and 50 shown in the photograph in the figure indicate that each line is drawn with a thickness of 20 μm at intervals of 50 μm. As shown in Fig. 3.29b, the PTFE on which the photolithography pattern is created is flexible and is thus appropriate for potential applications such as flexible print boards or GHz-bandwidth high-frequency antennas. Fig. 3.29 Photolithography on nickel-plated PTFE treated by atmospheric-pressure plasma polymerization. a Photolithographic pattern on nickel-plated PTFE, b photograph confirming its flexibility [14]

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3.4.8 Applicability to Radome Fiber-reinforced composite materials for aerospace applications have also been strengthened through an atmospheric-pressure plasma graft polymerization treatment. In particular, this section focuses on a device called a “radome,” which is a combination of “radar” and “dome.” Radomes are used to protect radar antenna systems from the outdoor environment, mainly from wind, rain, snow, sun, etc., in airports or on aircraft themselves. Figure 3.30 shows an example of the outer appearance of a radome at an airport. The use of a radome mitigates the effects of environmental conditions on the antenna system. Specifically, it provides wind speed, rain, and sunlight resistance. Therefore, deterioration of the antenna system over time can be minimized without the need for adding reinforcements. Therefore, the antenna systems can be used safely over longer terms. The materials used for radomes include glass fiber, PTFE, and fiber-reinforced composite materials, which allow radio waves to pass through easily. Moreover, the electrical properties can be customized for the frequency band used. In addition, the land radomes placed at airports are actually buildings and have access doors, lightning rods, aircraft obstacle lights, etc. Assuming that the radome is the final applied product, this section introduces several attempts to strengthen fiber-reinforced composite materials. Some trial production results using such plasma hybrid surface treatment are described. The treatment is performed to either make the fiber fabric hydrophilic or to modify the surface for improved adhesiveness. This research will broaden the applicability of

Fig. 3.30 Example of a radome at Osaka International Airport

3.4 Applicability of PTFE/Plastics in Millimeter-Wave Devices

71

organic-fiber-reinforced composite materials with low dielectric constants (~ 3) and high radio-wave permeabilities in aerospace structure materials.

3.4.9 Plasma Hybrid Surface Treatment of Fiber-Reinforced Composite Materials 3.4.9.1

Adhesion of Fiber-Reinforced Composite Materials

The surface modification and/or adhesion improvement can be assessed on fiber-reinforced composite materials with different dielectric constants, aiming for applications in radome structures. In these studies, plain weave fabrics composed of the following four types of organic fibers are used as samples: polypropylene fiber Innegra (Integrity Corporation), polyethylene fiber Dyneema (Toyobo Co., LTD.), polyacrylate fiber Vectran (Kuraray Co., LTD.), and poly(pphenylenebenzo)bisoxazole fiber Zylon (Toyobo Co., LTD.). In Sect. 3.4.9, plasma hybrid surface treatment of fiber-reinforced composite materials is explained with figures in [15, 16]. The device and apparatus are similar to those shown in Fig. 3.4. An A4-size (approximately 300 mm × 200 mm) sample is fixed to the conveyor belt of an atmospheric-pressure plasma graft polymerization apparatus. While the plasma torch moves horizontally at a speed of 4 mm/s, the surface is sprayed with a pulsemodulated AC Ar plasma jet (frequency 20 kHz; voltage 24 kV; pulse modulation frequency 60 Hz; pulse duty ratio 99%; gas flow rate 30 L/min). At that time, Ar (flow rate of 3 L/min) is bubbling in an acrylic acid monomer solution heated to 45 °C, and the resulting vapor is sprayed from the side of the jet. As the surface of the test material is radicalized, the carbon bonds are cleaved, and graft polymerization occurs. After the torch makes a roundtrip, the test material is moved 10 mm vertically by the conveyor belt, and this process is repeated to treat the entire A4-size surface. First, to evaluate the adhesiveness, a peeling test is performed on each of the materials, Dyneema, Vectran, and Zylon, which are either untreated or treated by plasma graft polymerization. A 50-mm-long part of the test sample, extracted at a width of 25 mm and length of 100 mm, is adhered to an aluminum plate coated with an epoxy adhesive (Konishi E-set) at a thickness of 250 μm, and the adhesive is cured by allowing it to rest at room temperature (~ 25 °C) for a day under a load of 5 N. Subsequently, the aluminum plate is peeled at a speed of 100 mm/min, and the peeling strength is measured at that time. Subsequently, the vacuum-assisted resin transfer molding (VaRTM) method can be used to trial-produce flat fiber-reinforced composite material test pieces using treated or untreated polymer samples by the following method. For the thickness to be approximately 2 mm, the test materials are layered with 6 fibers (Innegra), 10 fibers (Dyneema), 8 fibers (Vectran), and 12 fibers (Zylon), and after immersion in

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a matrix resin by vacuum aspiration, the materials are heated, and the resin is cured. Specifically, epoxy resin (XNR6815/XNH6815, Nagase ChemteX Corporation) is used as the matrix resin. The resin curing temperature and time are 80 °C and 2 h, respectively. Three test pieces (length × width for a peeling test, compression test, and three-point bending test: 250 mm × 25 mm, 80 mm × 12.7 mm, 90 mm × 12.7 mm, respectively) are cut from the fiber-reinforced composite material samples. A peeling test (ASTM D3039), a compression test (SCAMA SRM 1R-94), and a three-point bending test (ASTM D790) are performed at room temperature using a material universal test machine (AUTOGRAPH DCS-10T, Shimadzu Corporation) at speeds of 2, 1, and 1 mm/min, respectively.

3.4.9.2

Peeling Strength Results

Peeling strength per 25 mm (N)

The results of the peeling tests for the test materials are shown in Fig. 3.31. Evidently, the peeling strength (σ T ) increases after NTP graft polymerization of all the test materials. However, as shown in Fig. 3.32, σ T tends to decrease in the peeling test for the fiber-reinforced composite material specimens, which may be because the fiber surface may be slightly damaged during the plasma treatment, thereby reducing σ T . Meanwhile, the Dyneema sample shows lower heat resistance than the other tested materials and is thus easily influenced by the temperature during plasma irradiation (~ 150 °C); therefore, its σ T is much lower than those of the other tested materials. Considering the elastic modulus E, the relationship between the adhesion improvement by the plasma treatment and the fiber damage is different. In particular, the Dyneema sample with the lower heat resistance presents a large degree of fiber damage; therefore, E decreases. In contrast, the Zylon material, which has higher heat resistance, presents negligible fiber damage, and E is improved. In contrast, the plasma graft polymerization treatment increased the compression strength, σ c , of all four types of fiber-reinforced composite materials, as shown in 3000 2500

Untreated Treated

2000 1500 1000 500 0 Dyneema (PE)

Vectran (PAR)

Zylon (PBO)

Fig. 3.31 Peeling test results for three types of fiber-reinforced composite material specimens

3.5 Development of OLEDs on PCTFE

73

1000

60

Untreated Treated

50

800

40

600

30

400

20

200

10

0

Elastic modulus E (GPa)

Tensile strength σ T (MPa)

1200

0 Innegra (PP)

Dyneema (PE)

Vectran (PAR)

Zylon (PBO)

40

120 100

Untreated Treated

35 30

80

25 20

60

15

40

10 20

5

0

Elastic modulus E (GPa)

Compressive strength σ c (MPa)

Fig. 3.32 Tensile test results for four types of fiber-reinforced composite material specimens

0 Innegra (PP)

Dyneema (PE)

Vectran (PAR)

Zylon (PBO)

Fig. 3.33 Compression test results for four types of fiber-reinforced composite material specimens

Fig. 3.33, possibly because of the improved adhesion after the plasma graft polymerization. In the case of E, similar results to those of the tensile test are obtained. In the bending test results shown in Fig. 3.34, an increase in the bending strength σ b is observed. This increase is believed to be due to the improvement in the compression strength caused by the plasma graft polymerization treatment.

3.5 Development of OLEDs on PCTFE 3.5.1 Flexible OLED Element An organic light emitting material refers to an organic material that can emit light in response to being stimulated by some type of energy (such as voltage). With electroluminescence (EL), phosphors are excited by electrical energy, and when they are deactivated from the excited state, that energy is emitted as light, which can

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Fig. 3.34 Bending test results for four types of fiber-reinforced composite material specimens

be used for lighting and displays. In Japan, it is often called an organic EL; however, globally, such devices are generally called OLEDs as a type of light emitting diode. OLED devices are extremely vulnerable to moisture and oxygen in the air, and hightemperature processing is still sometimes involved in the fabrication of devices. Therefore, glass, which has gas barrier properties and heat resistance, has been used as a substrate material and sealing member. In recent years, OLED devices have become more flexible, but barrier films to replace glass are required to have very high barrier properties against water vapor, heat, and other sources of damage. The author’s team focuses on PCTFE (–(CF2 –CFCl)n –), a transparent fluoroplastic used for moisture-proof packaging of chemicals, which has better gas barrier properties than other materials. The author’s team attempted to use it as an OLED sealing film. If OLEDs can be formed on a flexible film such as PCTFE with high gas barrier properties (water vapor transmission rate of ~ 0.1–0.2 g/m2 /day, oxygen transmission rate of ~ 0.5 mL mm/m2 /day/atm), high performance will be achieved, thus enabling the realization of a long-life flexible OLED device. However, as PCTFE is a type of fluoroplastic, it has poor surface water repellency and adhesiveness, making it difficult to form an indium–tin oxide (ITO) electrode or EL layer on the surface. Therefore, when the aforementioned plasma graft-polymerized film is formed on the PCTFE surface, it is successfully layered and assembled into OLEDs. This section describes the basic surface properties of the treated PCTFE (adhesion strength, surface analysis results), then describes the actual prototyping process, and finally describes the process of assembling an OLED device on PCTFE. In Sect. 3.5, the development of OLEDs on PCTFE is explained with figures in Ref. [3].

3.5 Development of OLEDs on PCTFE

75

Fig. 3.35 Experimental results of peeling strength obtained for a stainless steel plate and PCTFE film (vertical axis is the value per 25 mm of sample width)

3.5.2 Peeling Strength for PCTFE Prior to making the OLED prototypes, the adhesive strength of the plasma hybridtreated PCTFE is investigated. Figure 3.35 shows the results of the 180° peeling test of the treated PCTFE. The PCTFE sample is very thin, and thus, it is difficult to perform the 90° peeling test. The vertical axis of the figure shows the peeling strength per 25 mm width of the sample, and the horizontal axis shows the peeling displacement; in total, six samples are tested. In the figure, open circles, the solid line, and error bars represent measured values, average value, and ± σ (σ: standard deviation), respectively. As shown in the figure, although the data are widely scattered, an average strength exceeding 25 N and a maximum strength of 37.5 N are attained for a 25mm-wide sample.

3.5.3 XPS Analysis Results XPS analysis is conducted to demonstrate that a layer of polyacrylic acid is graftpolymerized on the surface of the PCTFE films. Information on the chemical bonding of untreated and treated films is obtained through measurements performed using a spectrometer (PHI Quantera II, ULVAC-PHI, Ltd.) employing a mono Al Kα (1486.7 eV) X-ray source. Spectral measurements are performed at 15 kV, 50 W, and a pass energy of 112 eV, with an energy step of 0.1 eV. The C1s XPS spectrum of the untreated PCTFE film is obtained but not shown in figure. The peaks for –CF2 – (difluoromethylene group), –COOH (carboxyl group), – C=O (carbonyl group), and –C–C– (carbon) are detected at 292, 289.2, 287.2, and 285

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Fig. 3.36 XPS results of the treated PCTFE film surface (C1s peak)

eV, respectively, by splitting of the total C1s spectrum. The latter three peaks are due to trace contamination adhering to the PCTFE film. Although the peak of 290.7 eV is not identified, it is supposed that this peak indicates a functional group containing carbon, fluorine, and chlorine. The difluoromethylene group and functional group containing carbon, fluorine, and chlorine, which make the film hydrophobic, are the ingredients of PCTFE film. The peaks for the carboxyl group, carbonyl group, and carbon are due to a trace of contamination adhering to the PCTFE film. Figure 3.36 shows the C1s XPS spectrum of the treated PCTFE film. The peaks for –COOH, –C=O, –C–O– (alcohol), and –C–C– appear at approximately 288.8, 287.8, 286, and 284.4 eV, respectively, obtained by splitting the total C1s peak. Surface adhesion is generally improved when hydrophilic functional groups, such as carboxyl, carbonyl groups, and alcohol, are attached to the polymer surface. Furthermore, because –CF2 – is no longer detected on the surface, it is most likely covered by the plasma graft polymerization of acrylic acid.

3.5.4 SEM Observation Results The morphologies of the graft-polymerized surfaces are analyzed by SEM (S-4800, Hitachi High-Technologies Corporation, with magnifications of 1000–50,000×, an acceleration voltage of 3 kV, and a working distance of 8.6 mm). Figure 3.37a shows the surface of the untreated PCTFE film. Although some white spots, which represent contaminants, are observed, the surface is significantly flatter than that of the PTFE film. Figure 3.37b shows the surface of the PCTFE film treated by plasma graft

3.5 Development of OLEDs on PCTFE

77

polymerization. Fine unevenness, which could improve the adhesiveness of the film, is observed on the grafted polyacrylic acid layer.

Fig. 3.37 SEM images of the a untreated and b treated PCTFE film surfaces (10,000×, each interval on scale bar = 0.5 μm)

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3.5.5 Prototype Fabrication Procedure for OLED Device on PCTFE Figure 3.38 shows the procedure for fabricating OLEDs using the treated PCTFE film [3]. The procedure is as follows: (1) sputtering ITO, which is transparent and conductive, onto the treated PCTFE film; (2) preparing a sample (length = 24 mm and width = 20 mm) by cutting the treated PCTFE and fixing it on a glass plate of the same size because of its extremely thin thickness; (3) masking a portion of the ITO layer using a 3-mm-wide masking tape and then etching away the exposed ITO layer by submerging the sample in HCl + FeCl3 solution at 60 °C for 75 s, and then rinsing it with water, removing the masking tape, and completely drying the sample; (4) spin coating the sample with a 100 μL poly(3,4-ethylenedioxythiophene)-poly(4styrene sulfonate) (PEDOT-PSS) solution (500 rpm × 10 s and 1500 rpm × 60 s), and drying it in an oven at 120 °C for 30 min; (5) spin coating the sample with 100 μL of a green fluorescent dye solution (YMD-1, Yamada Chemical Company, Ltd., 1500 rpm × 3 s and 3000 rpm × 30 s), and drying it in the oven at 120 °C for 30 min; (6) masking a portion of the sample using masking tape; and (7) depositing cesium fluoride (CsF) on the exposed surface at a deposition rate of 0.01 nm/s (deposition thickness: 1.0 nm) using vacuum deposition equipment, then depositing Al on the CsF at a deposition rate of 0.2–2.35 nm/s (deposition thickness: 160 nm), and finally removing the masking tape from it. Generally, both sides of the OLED assembly should be covered with barrier materials to ensure a long operating life. However, these prototypes have been tested as a first step to confirm that OLEDs assembled on PCTFE films would emit light. Eventually, the OLEDs should also be covered completely with the PCTFE films. Figure 3.39 shows the luminescence testing apparatus. A light source evaluation system (GE-1100A, Otsuka Electronics Company, Ltd.) with a luminance camera (MC-948) and a high-sensitivity multi-channel photodetector (MCPD-9800) is used to measure luminance and analyze luminescence. The fabricated OLED is connected to a DC power supply and secured on a fixed plate 1 m from the luminance camera. The luminance is measured as the applied voltage increases gradually. The applied voltage, current, and luminance data are then transferred to a computer. Three OLEDs are prepared under the same conditions, each of which has three luminescence points. When a DC voltage is applied, the luminescence is confirmed by visual observation, and green luminescence can be observed in Fig. 3.40. Figure 3.41 shows the luminescence spectra of the samples, which exhibit a green peak at 512 nm. Some points on the LEDs similarly emitted light, while others did not. Figure 3.42 shows the luminance of the luminescence point as a function of the applied voltage. The operating voltage and luminance of OLEDs depend on the material and thickness of the luminescent layer and PEDOT-PSS layer. The luminance increases with increasing applied voltage, reaches a maximum value (349 cd/m2 ) at approximately 14.5 V, and decreases thereafter. The peak luminance varies depending on the sample and the luminescence points, and a maximum luminance of 420 cd/m2 has been achieved.

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Fig. 3.38 Procedure for fabricating OLEDs using the treated PCTFE film

The unstable performance and characteristics of the OLEDs may be caused by the roughness of the surface of the PCTFE films because the PCTFE films used in this case are very thin. However, the stable performance of OLEDs is attained when flat glass plates are used as substrates. Therefore, a flexible OLED device with a long operating life is expected to be realized by reducing the surface roughness of PCTFE films, precisely fabricating OLEDs, and covering the fabricated OLEDs with PCTFE films.

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Fig. 3.39 Luminescence measurement setup for the OLED on the treated PCTFE

3.6 Improved Adhesion of Fluoroplastic Film to Butyl Rubber 3.6.1 Application Example: Prefilled Syringe Next, in Sect. 3.6, the improvement of the adhesiveness of fluoroplastic film to butyl rubber and the trial production of a fluoroplastic film–butyl rubber composite is explained with figures and a table based on [4]. Figure 3.43 shows a prefilled syringe, which is a rubber medical product that is filled with liquid medicine to prevent mixing up medicines. In this case, the prototype composite can be used as a gasket, as shown in the figure, and this prefilled syringe has a gasket on the piston of the plunger. Because the surface of this gasket is in contact with the chemical solution for a long time, the elution of the rubber into the chemical solution must be prevented. Owing to the high chemical stability of PTFE, it would be suitable for protecting the gasket surface, and the plasma graft polymerization treatment can be applied to ensure the adhesion between the two materials. In addition, the coefficient of dynamic friction of the PTFE surface is extremely low (~ 0.04), which is also an advantage because the piston can operate smoothly thanks to the surface treatment on one side. For these purposes, the author’s team has investigated the improvement of adhesiveness to rubber by subjecting the surface of PTFE film to the plasma hybrid treatment.

3.6 Improved Adhesion of Fluoroplastic Film to Butyl Rubber Fig. 3.40 Photographs of the OLED a in the daylight and b in the dark emitting green light (2-B point)

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Fig. 3.41 Luminescence spectra of the OLED device on the treated PCTFE (2-B point) Fig. 3.42 Luminescence of the OLED device on the treated PCTFE as a function of applied voltage (1-A, 2-B, 3-A, and 3-B show luminescence points of the device)

Fig. 3.43 Application example: schematic of a prefilled medical syringe with a rubber gasket protected by a PTFE film

3.6.2 Butyl Rubber and PTFE Film Composite Material The PTFE film is treated by the plasma hybrid treatment with acrylic acid monomer using the A4-size treatment apparatus, as shown in Fig. 3.44. The C1s XPS spectrum of the PTFE surface is shown in Fig. 3.45, and the elemental analysis results of the

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Fig. 3.44 Photograph of the NTP jet nozzle in motion in the experimental apparatus for the atmospheric-pressure NTP jet graft polymerization surface treatment of A4-size fluorocarbon polymer films

PTFE films are shown in Table 3.2. SEM images of untreated and treated PTFE film surfaces at various magnifications are shown in Fig. 3.46. Figure 3.47a–c shows the FTIR spectra for (a) untreated PTFE, (b) butyl rubber, and (c) PTFE after the rubber is peeled off. In Fig. 3.47a for the untreated sample, specific peaks are observed at wavenumbers of approximately 1150–1200 cm−1 . In Fig. 3.47b for the rubber, specific peaks appear at approximately 680, 1020, 1450, 2990, and 3675 cm−1 . The peaks at 680, 1020, and 3675 cm−1 could be attributed to talc (Mg3 Si4 O12 (OH)2 ), which is a raw material of rubber. The peaks at 1450 and 2990 cm−1 could be assigned to –CH2 –. In Fig. 3.47c for the peeled PTFE, the weak peak at 1700 cm−1 could be caused by –COOH (carboxyl group), and similar peaks to those in Fig. 3.47b are observed. This result suggests that the rubber layer is not cleanly removed owing to the strong adhesion. Apparently, adherend failure occurs in the rubber layer itself. These results also confirm that the number of chemical bonds with F atoms on the surface greatly decreases after the surface treatment, which enables the formation of a flat adhesive hydrophilic layer. The treated PTFE film and unbridged (non-cross-linked) raw butyl rubber sheet (thickness = 2 mm) bonded well under the hot-press conditions of 150 °C for 40 min or 180 °C for 10 min. A photograph of the composite material cured at 180 °C for 10 min is shown in Fig. 3.48a, which displays significantly strong and uniform adhesion. It is confirmed that untreated PTFE films do not adhere to rubber at all. It is easy to peel off the film from the rubber by hand. Moreover, when the treatment is insufficient, several bubbles appear on the surface of the PTFE film. When the treatment is complete, however, the PTFE film adheres tightly to the rubber. Figure 3.48b shows a photograph of a composite material consisting of a PTFE film first treated by the Na–NH3 solution before adhering to the butyl rubber sheet. In this treatment, after the PTFE film is submerged for 5 s in the Na–NH3 solution (Na concentration = 1 mass%), it is washed with ethanol and pure water, yielding a PTFE film with a chemically activated surface. Although the adhesion is significantly improved, the surface morphology is altered considerably, and it is chemically damaged by the

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Fig. 3.45 XPS spectra of the C1s peak from the surface of the a untreated and b treated PTFE films

Table 3.2 Elemental analysis results of the untreated and plasma graft polymerization-treated PTFE films Atom composition %

Untreated

Plasma graft polymerization (acrylic acid)

C

33.51

72.95

O

–

23.20

F

66.49

2.45

N

–

0.95

Others

–

0.45

etching. Furthermore, the PTFE film surface appears brown, and hazardous sodium may be left on the surface. In this regard, the plasma jet treatment has several advantages over the Na–NH3 treatment, which yields a maximum peeling strength that is approximately the same or only slightly higher than that of the plasma-treated PTFE film.

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Fig. 3.46 SEM images of a, b untreated PTFE film and c, d PTFE film treated by plasma graft polymerization magnified at 5000× and 20,000× : a untreated surface (5000×, each interval on scale bar = 1 μm). b Untreated surface (×20,000, each interval on scale bar = 0.2 μm). c Treated surface (5000×, interval = 1 μm). d Treated surface (20,000×, each interval on scale bar = 0.2 μm)

Vulcanization (cross-linking) plasma adhesion between the fluoroplastic film and the rubber involves applying heat and pressure for a predetermined period while the films are in contact with each other to cross-link the rubber and bond it to the fluoroplastic film. Even when sulfur is not used as a cross-linking agent in this treatment for crude rubber, cross-linking is sometimes called “vulcanization” by custom. The time and temperature for cross-linking adhesion are set according to the need for cross-linking the uncross-linked rubber compound. In this prototype, the rubber material is cross-linked on the surface-modified fluoroplastic film described above under various conditions, namely a heating temperature of 130–180 °C and a treatment time of 5–90 min. Uncross-linked rubber sheets (halogenated butyl rubber, thickness = 2 mm) are prepared. Unvulcanized butyl rubber is placed on one side of a stainless steel mold (length 100 mm × width 100 mm × groove depth 1.95 mm), and a PTFE film treated with plasma graft polymerization with acrylic acid monomer is placed on the mold so that the treated surface is in contact with the rubber. This assembly is sandwiched between the other molds and placed between the presses of the hot press. The nipping margin necessary for fixing to the chuck of the peeling tester is secured by sandwiching an untreated PTFE film between the rubber and the treated PTFE film. No adhesive other than the cross-linking agent contained in the rubber is used for adhesion.

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Fig. 3.47 FTIR spectra for a untreated PTFE, b butyl rubber, and c PTFE after the rubber is peeled off

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87

Fig. 3.48 Photograph of composite materials cured at 180 °C for 10 min: PTFE treated by the a NTP graft polymerization and b Na–NH3 processes

3.6.3 Peeling Test of Fluoroplastic Film–Butyl Rubber Composite After preparing a composite of fluoroplastic film and butyl rubber cross-linked at a temperature of 150 °C, a treatment time of 40 min, and a treatment pressure of 1.6 MPa, a peeling test is performed with reference to JIS K 6854. The composite obtained by this treatment is cut to a width of 25 mm and a length of 50 mm, and the tensile strength is measured. Figure 3.49 shows the procedure and photographs of the PTFE–rubber composite material sample fitted to the peeling test apparatus. The peeling strength is very high at approximately 3.9 N per 1 mm sample width. Adherend failure of the rubber occurs if the peeling is enforced and the strength exceeds those of the film and/or rubber sheet themselves.

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Chuck

Rubber Chuck

(a)

(b)

(c)

Fig. 3.49 Peeling test of the rubber–PTFE composite: a test method, b photographs of the peeling test wherein peeling is enforced, and c failure of the adherend

3.6.4 Peeling Strength of the Composite Material Figure 3.50 shows the results of three peeling strength tests performed on the rubber– PTFE composite samples under the same experimental conditions. In these figures, the horizontal axis denotes the stroke of the peeling test, and the vertical axis corresponds to the peeling strength per 1 mm width of the 25-mm-wide PTFE film. In the experimental results shown in Fig. 3.50a, c, the peeling is stopped when the stroke reaches approximately 80 mm in order to release the tension due to elastic elongation of the rubber, which may otherwise damage the peeling strength test apparatus, whereas the peeling is not stopped in Fig. 3.50b. In Fig. 3.50a, c, peeling is restarted for strokes longer than approximately 80 mm. In these results, the peeling strength decreases when the peeling stops, and it increases again when it restarts. The relaxations of the peeling strength are due to the elastic properties of the rubber. Compared to the untreated sample (peeling strength = 0.02 N/mm), these three treated samples exhibited much higher adhesive strength. The peeling strength increases with increasing stroke and reaches maxima of 3.88, 3.69, and 3.53 N/mm at strokes of 77.6, 83.1, and 71.9 mm, respectively. On average, these values are approximately 185 times larger than that achieved with the untreated film (= 0.02 N/ mm).

3.6.5 Molecular-Level Adhesion Mechanism Between Rubber and PTFE Figure 3.51 shows the possible reaction mechanism responsible for the reinforced adhesion between rubber and treated PTFE. Butyl rubber is a compound copolymer of isobutylene (C(CH3 )2 =CH2 ) and a small amount of isoprene (CH2 =C(CH3 )– CH=CH2 ), which is a component of natural rubber. For the rubber curing process, an

3.6 Improved Adhesion of Fluoroplastic Film to Butyl Rubber Fig. 3.50 Relationship between peeling load and stroke in three similar peeling strength tests for the PTFE–rubber composite samples. a Sample 1 (maximum strength = 3.88 N/mm). b Sample 2 (maximum strength = 3.69 N/mm). c Sample 3 (maximum strength = 3.53 N/mm)

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additional cross-linking material is usually added to the raw rubber materials. In the present process, 2–di–n–butylamino–4, 6–dimercapto–1, 3, 5–triazine (C11 H20 N4 S2 , triazine dithiol) is used as a cross-linking material. The acrylic acid-modified PTFE (R–[CH2 –CHCOOH]l ) reacts with isobutylene, isoprene, and triazine dithiol under high-temperature (~ 180 °C) and high-pressure (~ 157 N/cm2 ) conditions during the rubber curing process. This results in a composite material of butyl rubber and PTFE, the interface of which is firmly attached by molecular-level adhesion through the cross-linking material. Hydrogen bonding could be considered an alternative connection mechanism between the treated PTFE and rubber. Figure 3.52 schematically shows how the rubber is chemically bonded to the treated PTFE with triazine dithiol. Strong adhesion between rubber and treated PTFE by triazine dithiol occurs to produce the composite material. In the process used here, the PTFE is fully treated by plasma graft polymerization, which is very effective for improving its surface and results in extremely strong reinforced adhesion. Although it may be the most likely explanation, further research is needed to obtain clear evidence of the underlying adhesion mechanism.

Fig. 3.51 Proposed reaction for the reinforced adhesion of rubber to treated PTFE with triazine dithiol as a linking material

3.7 Conclusions

91

Fig. 3.52 Molecular-level adhesion in the PTFE–rubber composite material

3.7 Conclusions This chapter has explained the basics of plasma treatment for improving the hydrophilicity of polymer surfaces through examples of electrode systems and equipment for atmospheric-pressure plasma treatment. It has also given an overview of the optimization of this plasma surface treatment and its current trends, along with the chemical liquid and vapor treatments proposed by the author’s team. The principles underlying the plasma hybrid surface treatment, which is employed to modify the surface of fluorine-based plastics by firmly bonding a functional thin film to the surface of the material to be processed, are elucidated, and some examples are presented. Furthermore, the mechanism underlying the plasma combined processinduced improvement of the adhesion of PTFE/plastic to metal is described. In addition, the effect of the surface treatment and plating technology on PTFE, its application, and the possibility of the development of millimeter-wave devices are discussed. The adhesion of fiber-reinforced composite material, i.e., fluorine plastic films, is demonstrated to enable the fabrication of an OLED prototype, and another composite material with butyl rubber for application to prefilled medical syringes is explained as well. In recent years, a company has announced a film in which a functional group is introduced into the fluoroplastic during the manufacturing process to change the structure itself and realize an adhesive fluoroplastic with an adhesion strength lower than 5 N/mm. This treatment technology is effective for PTFE as well as various other plastics. However, the surface of inorganic materials, such as metals and glass, can be permanently hydrophilized by the plasma hybrid treatment. In such a case, it is effective to introduce a silane coupling agent and perform a pretreatment to bridge the organic material. In the future, the author’s team will continue to optimize the treatment conditions and improve the equipment by using wider electrodes, while suppressing discoloration. The remarkably large peeling strength of 5 N/mm or more, achieved for the fluoroplastic, is certified by a researcher at a major company that manufactures

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medical equipment using Teflon, and it is also confirmed that the color of the fluoroplastic itself did not change. Furthermore, the peeling strength of 3.5 N or more per 1 mm width of the sample, achieved for the adhesion of PTFE–butyl rubber, can be realized through cross-linking, thus avoiding the use of an adhesive, although the processes of pressurization and heating are still essential. This is an unprecedented strength, and the author would like to elucidate the mechanism in the future work. Moreover, the feasibility of medical applications, such as endoscopes, syringes, dialysis machines, the adhesion of biocompatible materials, and the adhesion of glass, rubber, and Teflon, will be assessed, as well as other applications, such as solar cell panels, liquid crystal display panels, and organic EL elements. The author’s team also intends to continue to develop electronic devices, such as millimeter-wave devices, coaxial cables, and antennas, as well as electronic component materials, such as reinforcing fibers. Moreover, they will also strive to increase the number of successful applications of these materials in wide-ranging fields, as well as promote the application of plasma hybrid processing and manufacturing of the processing systems. In addition, collaborative research will be attempted to identify various other applications of such surface-treated fluoroplastics. Finally, the author’s team also plans to expand into roll-to-roll manufacturing and pursue applications in the aerospace field.

References 1. M. Okubo, M. Tahara, N. Saeki, T. Yamamoto, Surface modification of fluorocarbon polymer films for improved adhesion using atmospheric-pressure nonthermal plasma graftpolymerization. Thin Solid Films 516(19), 6592–6597 (2008) 2. K. Hori, S. Fujimoto, Y. Togashi, T. Kuroki, M. Okubo, Improvement in molecular-level adhesive strength of PTFE film treated by atmospheric plasma combined processing. IEEE Trans. Ind. Appl. 55(1), 825–832 (2019) 3. T. Kuroki, K. Nakayama, D. Nakamura, T. Onji, M. Okubo, Nonthermal plasma hybrid process for preparation of organic electro-luminescence fluoropolymer film devices. IEEE Trans. Ind. Appl. 51(3), 2497–2503 (2015) 4. M. Okubo, T. Onji, T. Kuroki, H. Nakano, E. Yao, M. Tahara, Molecular-level reinforced adhesion between rubber and PTFE film treated by atmospheric plasma polymerization. Plasma Chem. Plasma Process. 36, 1431–1448 (2016) 5. T. Yamamoto, A. Yoshizaki, T. Kuroki, M. Okubo, Aluminum surface treatment using three different plasma-assisted dry chemical processes. IEEE Trans. Ind. Appl. 40(5), 1220–1225 (2004) 6. M. Narita, Development of Fluororesin-Rubber Composite Material by Improving Adhesiveness Using Nonthermal Plasma Graft Polymerization Treatment, Master’s Thesis in Fiscal Year 2020, Department of Mechanical Engineering, Osaka Prefecture University (2020), pp. 1–162 (in Japanese) 7. M. Okubo, M. Tahara, Y. Aburatani, T. Kuroki, T. Hibino, Preparation of PTFE film with adhesive surface treated by atmospheric-pressure nonthermal plasma graft polymerization. IEEE Trans. Ind. Appl. 46(5), 1715–1721 (2010) 8. M. Narita, M. Nakamura, T. Kuroki, M. Okubo, Adhesive polytetrafluoroethylene films fabricated via atmospheric nonthermal plasma graft polymerization. J. Adhesion Soc. Jpn. 57(7), 280–290 (2021) 9. M. Kogoma, Applications of atmospheric-pressure glow plasma surface treatment of organic materials. J. Plasma Fusion Res. 79(10), 1009–1015 (2003). (in Japanese)

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10. T. Yasuda, Purasuchic zairyo no kakudoutokusei no shikenho to hyouka kekka , (English translated title: Test methods and evaluation results for dynamic characteristic of plastic materials ). Plastics Kogyo Chosa Kai Corp. 52(5), 79–84 (2001). (in Japanese) 11. Y. Kamisato, H. Yoshitake, M. Ikuno, M. Fujimoto, T. Yamawaki, Compact and highperformance millimeter-wave antenna. Fujitsu Ten Tech. Rep. 28(2), 19–25 (2010). (in Japanese) 12. Totoku Electric Co., Ltd., Flexible millimeter wave coaxial cable assemblies. Furukawa Electr. Rev. 120, 123–124 (2007). (in Japanese) 13. M. Okubo, M. Tahara, T. Kuroki, T. Hibino, N. Saeki, Plating technology for fluorocarbon polymer films using atmospheric-pressure nonthermal plasma graft polymerization. J. Photopolym. Sci. Technol. 21(2), 219–224 (2008) 14. T. Kuroki, M. Tahara, T. Kuwahara, M. Okubo, Microfabrication and metal plating technologies on polytetrafluoroethylene film surface treated by atmospheric pressure nonthermal plasma graft polymerization process. IEEE Trans. Ind. Appl. 50(1), 45–50 (2014) 15. T. Aoi, T. Kuroki, M. Tahara, M. Okubo, Improvement of strength characteristics of aerospace fiber reinforced composite materials using atmospheric pressure plasma-graft polymerization treatment. Trans. Inst. Electr. Eng. Jpn. 131A(5), 412–413 (2011). (in Japanese) 16. T. Aoi, T. Kuroki, M. Okubo, Strengthening of fiber-reinforced composite materials for aerospace aircraft by atmospheric pressure plasma graft polymerization treatment, in Atmospheric Pressure Plasma Generation Control and Applied Technology, revised version, supervised by M. Kogoma, Chapter 4, Section 6 (Science & Technology Co., Ltd., 2012), pp. 169–182 (in Japanese)

Chapter 4

Hydrophilic Treatment Technology for Textiles, Filters, and Glass and Its Applications

4.1 Introduction Research on the surface modification of materials using atmospheric-pressure nonthermal low-temperature plasma has been widely conducted for many industrial purposes such as adhesion, paintability, functionality improvements, and surface cleaning. Conventional plasma technologies use radio-frequency (RF) and microwave (MW)-frequency power sources, which limit the types of gases (mainly noble gases) that can be used and often require pressure reduction and gas cooling, in addition to consuming large amounts of power; therefore, these methods often result in low throughput. Conversely, the surface modification method using corona discharge-type atmospheric-pressure nonthermal plasma (NTP) technology (APNTPT) uses a relatively low frequency, has a simple apparatus, does not require depressurization, and consumes limited power. This method can overcome several drawbacks of conventional low-pressure plasma processing schemes. APNTPT systems often use NTP that is induced by a DC corona, pulse corona, or silent discharge (dielectric-barrier discharge). The AC dielectric-barrier discharge for ozone generation was invented by Siemens approximately 160 years ago. Conversely, APNTPT, which uses a higher-performance rapid pulse corona, was first reported by Masuda et al. [1]. Since then, many studies have been conducted on combustor exhaust gas purification, catalytic activation, diesel engine exhaust gas purification, direct decomposition of volatile organic compounds (VOCs) and odorous substances, and indoor air purification using NTP [2]. Furthermore, APNTPT has been used by researchers for modifying clothing, polymers, and metal surfaces. In this chapter, the permanent hydrophilization of textiles, apparels, filters, glass [3], and glass surfaces by APNTPT actuators is described. The dynamic control of hydrophilicity is also explained. First, the surface treatment of textiles and apparel is explained. Sweat generated during a workout lowers the temperature of the human body, which increases owing to the heat of vaporization. This causes discomfort and a feeling of being cold after exercising. To eliminate such discomfort, moisture-breathing apparel should © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 M. Okubo, Nonthermal Plasma Surface Modification of Materials, https://doi.org/10.1007/978-981-99-4506-1_4

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be developed to absorb sweat from the skin surface quickly and release it to the external environment. However, when one side of a hydrophobic fabric such as polyester is irradiated with low-temperature plasma and then chemically reacted with a hydrophilic chemical (polymer monomer) to form a strong bond (plasma graft polymerization), the fabric becomes water-repellent, breathes, and removes odors like activated charcoal does. Moreover, the fabric still exhibits these functionalities even after being washed dozens of times, and the intelligence and functionality can be further enhanced by repeated plasma irradiation. This fabric can be used to produce a functional intelligent sportswear that prevents body odor, is significantly breathable, and dissipates perspiration. The method of manufacturing this highly functional apparel was devised by the authors, and a wide range of applications other than apparels, such as interiors, nursing-care clothing, and air filters, are being investigated. The objectives of this research are to develop and improve a plasma surface treatment device for achieving a prototype for a comfortable, unprecedentedly thin, intelligent apparel that absorbs body odors and rapidly dissipates perspiration and to explain the results of this research and development. When one side of a hydrophobic fiber or fabric that repels water, such as polyethylene terephthalate (PET), is irradiated with a low-temperature plasma, followed by plasma graft polymerization treatment, a single-sided treated fabric is obtained. The fabric absorbs moisture and eliminates odors, like activated charcoal. This performance is not lost even after washing more than tens of times, and the functionality is further enhanced by repeated plasma irradiation. Using these fibers and fabrics, innovative deodorizing air cleaners and functional apparel can be manufactured. In this chapter, the new results are explained. Next, the surface treatment of functional filters is explained. In recent years, to further improve the living environment, especially in Japan, several small indoor air cleaners have been marketed. Most of them use an electrostatic precipitator (ESP) that uses a DC corona discharge of several kilovolts or a dust collection filter, and some of them have a particle collection efficiency of over 95%. Despite their frequent use, many of them exhibit a limited effect on the highly concentrated gaseous odors such as cigarette smoke (which consists of several hundred types of components such as ammonia and acetaldehyde). To overcome this challenge, the methods using a highvoltage pulse discharge plasma or plasma-activated deodorizing filter have attracted attention. In this chapter, the basic principles of plasma deodorization and plasmaactivated filters are described, and an overview of previous research is presented. The performance test results related to the simultaneous removal of fine particles are explained. Furthermore, in recent years, anti-fog or anti-cloud treatment of material surfaces has attracted attention in relation to the surface treatment of materials. Anticloud proofing includes the following processes: (1) achieving an oil-repellent and stain-proof surface; (2) achieving a super-water-repellent surface; (3) obtaining a hydrophilic surface; (4) water droplet absorption using a water-absorbent material; and (5) techniques such as maintaining the surface temperature above the dew point (using a heater, etc.). Anti-fogging technology is expected to become increasingly important in relation to the hydrophilization of glass and metal surfaces, and

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plasma treatment can be an effective technique. In relation to this, the permanent hydrophilization of glass surfaces by the atmospheric-pressure plasma polymerization treatment technology and the dynamic control of the hydrophilicity of the glass surface by the APNTPT actuator are explained.

4.2 Surface Treatment of Textiles and Apparels 4.2.1 Principle of Functional Surface Treatment In Sect. 4.2, the surface treatment of textiles and apparels are described with figures and a table based on Refs. [4, 5]. The functional gradient is explained for textiles and apparels using three models of the dispersion of sweat from the human body to the external environment through clothing, as shown in Fig. 4.1. Figure 4.1a shows the cross-section of a fully hydrophobic cloth represented by a synthetic fiber such as polyester. The cross-section exhibits a small sweat-penetrating region. Figure 4.1b shows the cross-section of a fully hydrophilic cloth represented by a natural fiber such as cotton. A straight and uniform water-penetrating region appears in the crosssection. Figure 4.1c shows the cross-section of a functionally graded cloth that is hydrophilic on the outer surface and hydrophobic on the inner surface, i.e., near the skin. The cross-section shows a nonuniform water-penetrating region that expands to the outer surface. When clothing made of this type of functionally graded cloth is worn, the penetrating region expands significantly on the outer surface, and the sweat evaporates because of the larger evaporation area. Apparels made from such a fabric are considerably effective in terms of perspiration absorption, which is achieved by effectively enlarging the permeation area of perspiration on the surface that interacts with the outside air and allowing perspiration to be easily dissipated. Several methods for preparing functionally graded fabrics are described here. Chemical coating is a conventional technique. The desired functionality can be achieved by coating one side of the cloth with hydrophilic or hydrophobic chemicals. However, this cloth has poor durability and easily loses its functionality after washing because the chemicals are simply coated. Furthermore, air permeability decreases significantly after this treatment. Another technology involves weaving using different threads. In this method, the inner and outer sides of the apparel are woven with hydrophobic and hydrophilic threads such as polyester and cotton, respectively. This type of functional textile or cloth has been developed and patented since the 1990s by a few Japanese companies. Although this type of cloth or textile exhibits sufficient durability and performance, its thickness cannot be easily reduced because of its complex structure. Furthermore, the manufacturing cost of this material is relatively high.

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Sweat penetrating region

Hydrophilic region

Hydrophobic region

Skin

Skin

Skin Sweatdroplet

(a)

(b)

(c)

Fig. 4.1 Schematic diagrams of sweat dispersal through functional fibers. a Cross-section of a fully hydrophobic cloth. b Cross-section of a fully hydrophilic cloth; c Cross-section of a functionally graded cloth

In this chapter, the manufacturing of such functional clothes or apparels using a low-temperature nonequilibrium plasma polymerization process is described. Lowtemperature nonequilibrium plasma refers to the plasma whose gas temperature is approximately equal to room temperature while its electron temperature (average kinetic energy) is significantly high (usually 10,000 K or higher), which damages the surface of cloth fibers owing to combustion reactions. Therefore, active groups (radicals) on the surface can be activated, and a polymerization chemical reaction can be achieved using a monomer. Figure 4.2 shows the chemical reactions in the graft polymerization procedure used by the authors, which involves a reaction between a polyester cloth and an acrylic acid monomer. After the inert gas plasma is irradiated on the surface of the polyester cloth, many active radicals are induced on the surface. Then, a monomer mist is immediately injected onto the polyester surface. Graft polymerization progresses on the surface. Because the monomer and polyester are chemically and firmly bonded, the functional layer on the surface is significantly durable. The use of the APNTPT in this process is desirable because the surface is thermally damaged if the gas temperature is high. Furthermore, this process is simple and does not require a reduced-pressure system involving an expensive and powerful vacuum pump. It is reported that a one-side-treated cloth can adsorb odorous gases in a manner like that of activated carbon because the surface morphology becomes porous. Further, the cloth thickness and manufacturing cost can be reduced because a thinner cloth can be treated with APNTPT after weaving.

4.2 Surface Treatment of Textiles and Apparels

99

Fig. 4.2 Graft polymerization reaction between polyester and acrylic acid monomer

Electron-beam (EB) polymerization technologies have been developed for separating nuclear fuel material from seawater and removing toxic gas. In APNTPT, the capital cost of the apparatus is considerably lower than that of the EB process, and the textile surface can be treated on one side because relatively low-energy electrons (~10 eV) are used, which do not pass through the textile. Conversely, in the EB process, the electron energy is too high (~1 meV) to treat only one side of the textile because the electrons can easily reach the other surface of the textile.

4.2.2 Experimental Apparatus and Methods 4.2.2.1

Improvement of the Atmospheric-Pressure Nonthermal Plasma Graft Polymerization Apparatus

Figure 4.3 shows a photograph of the side view and schematic of the surface treatment system using atmospheric-pressure NTP graft polymerization. The apparatus shown in Fig. 4.3a was first manufactured by Pearl Kogyo Co., Ltd., [6] for treating a sheet of PET cloth. In this study, this apparatus is improved to treat not only the cloth but also the sewn apparel itself, as shown in Fig. 4.3b. A polyester film is wound onto the rollers to prevent a chemical reaction on the untreated side. The sewn and finished apparels are fixed on the film. An RF (frequency of 13.56 MHz) glow discharge NTP jet consisting of a mixture of Ar and He gases is irradiated almost uniformly on the surface of the apparel or cloth. The plasma jet is induced by a nozzle comprising two parallel dielectric-barrier (insulating alumina layers) electrodes, as shown in Fig. 4.4. The distance from the plasma jet to the surface is varied between 0 and 15 cm. The RF power supply is accompanied by a matching box and tuner controller to minimize power reflection. The maximum width of the cloth for the apparatus is 50 cm, and its length is 3–10 m. The cloth movement velocity can be adjusted from 1 to 100 mm/ min.

100

4 Hydrophilic Treatment Technology for Textiles, Filters, and Glass …

Fig. 4.3 Photographs and schematics of the plasma graft polymerization apparatus. a Overview (left: plasma chamber; right: high-voltage power supply). b Schematic of the apparatus

Fig. 4.4 Shape of inductively coupled plasma electrode

4.2 Surface Treatment of Textiles and Apparels

101

After the plasma is applied, the surface is modified, and many active radicals are induced on the surface. Because these radicals have significantly short lifetimes and are easily oxidized, the monomer must be injected immediately after plasma exposure in an oxygen-poor environment. A hydrophilic monomer (acrylic acid) mist is continuously injected on the surface using pressurized nitrogen gas. The distance between the electrode and the monomer injector is 30 cm. The monomer reacts with the radicals and is firmly bonded to the cloth in accordance with the chemical reactions shown in Fig. 4.2. A highly durable hydrophilic layer is produced on the surface by plasma polymerization.

4.2.2.2

Treated Material

One of the treated materials is a piece of ordinary hydrophobic polyester (PET) cloth. Its properties are as follows: size = 40 cm × 23 cm; mass per unit area = 335 g/m2 ; thickness = 0.9 mm; elongation percentage = 20.3% (length direction), 18.3% (transverse direction); air permeability = 8.0 mL/cm2 /s. In this paper, this treated material is referred to as “cloth.” The other materials are two types of commercial sportswear, where one is made of 100% polyester (brand name: Whole Earth, Descente, Ltd.), which is referred to as “apparel A,” and the other is made of 65% polyester and 35% cotton (brand name: Sanrolly, Flex Japan Co.), which is referred to as “apparel B.” The thicknesses of apparels A and B are 0.17 and 0.14 mm, respectively. The conditions for plasma graft polymerization were optimized by trial and error. The parameters of the method are as follows: the resulting cloth movement velocity is 10 mm/min; the distance between the cloth and electrode is 0 mm (in contact); the ratio of He: Ar mixture is 10:2 L/min; the discharge power of the plasma is 0.77 kW for treating the tested cloth and 1.5 kW for treating apparels A and B; the flow rate of the injected acrylic acid is 100 mL/h; and moreover, the polymerization temperature is in the range 24–26 °C. The polymerization and plasma treatment are performed at atmospheric pressure and temperature. Upon the completion of the plasma polymerization, the unreacted polymers are removed from the surface by washing with warm water (approximately 40 °C) in an ultrasonicator (Sanyo Electric Co., Ltd., Japan). The washed cloth and apparels are dried completely in the laboratory and, consequently, become hydrophilic and hydrophilic on the outer and inner sides, respectively. To obtain a material that is hydrophilic on both sides, this process is performed first on one side and then on the other.

4.2.2.3

Moisture Adsorption Measurements

According to the Japanese Industrial Standard (JIS) for the water droplet falling test, the moisture adsorption or water penetration is measured for both sides of the untreated cloth, one-side-treated cloth, and apparels A and B. A pure water droplet of 10 μL is dropped on the surface from a height of 25 mm using a mechanical pipette (Eppendorf Reference, Eppendorf Corporation, operation volume = 2– 20

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4 Hydrophilic Treatment Technology for Textiles, Filters, and Glass …

μL). After 20 s, the photographs of the surfaces are captured. The water penetrations of the outer (treated or untreated) and inner (untreated) sides are compared based on the size of the area where the water droplet spreads.

4.2.2.4

Odor Removal Measurements

To examine the odor removal of the treated clothes, an adsorption test is performed using one of the typical offensive body odor components, i.e., ammonia. The outline of the procedure is as follows: 1030 ppm ammonia gas diluted with nitrogen in a cylinder is mixed with dry air prepared using an air compressor and passed through an air filter and dryer. The relative humidity of the dry air is sustained at 4% at room temperature. A pair of mass flow controllers is used to control the flow rate and concentration and prepare 50 ppm ammonia flowing at 1.0 L/min. The gas flows through the test section, where a test cloth (dimensions: 40 cm in width and 23 cm in length) is wrapped around a perforated polystyrene tube (outer diameter = 13 mm, length = 32 cm, diameter of the hole = 1 mm, and pitch of each hole = 1 cm). The gas flows through the holes in the tube and passes through the tested cloth. The concentration of ammonia before and after the test is measured using an ammonia concentration analyzer (XD-303, New Cosmos Electric Co., Ltd.) or gas detection tubes (GV-100, Gastec Co.) to examine the extent of odor removal.

4.2.2.5

Qualitative Evaluation for Treated Apparels

Qualitative evaluations (sensory test in fitting) were performed on the untreated and treated apparels A and B. The method used is as follows: either a twenty-threeyear-old or forty-two-year-old healthy Japanese man was asked to wear the apparel and ride a stationary bike (AF5900 Exercise Bike, Alinco, Inc.) at a medium pace and strength. After exercising, he was questioned about the comfort while sweating and the softness of the surface of the treated and untreated apparels. The tests were performed in a room at a constant temperature and humidity of 27 °C and 50%, respectively.

4.2.2.6

Quantitative Evaluation for Treated Apparels

To quantitatively evaluate the performance of the untreated and treated apparels, the following experiments were conducted. A healthy twenty-three-year-old Japanese man (height = 171 cm, weight = 60 kg) was chosen as the subject, and he was asked to wear either treated or untreated apparel. He wore no additional innerwear on the upper part of his body, and his skin was in direct contact with the apparel. A temperature and humidity sensor (TR-72S, T & D Corporation; temperature range for measurement = 0–50 °C, humidity = 10–95%) controlled by a notebook PC was used to record the temperature and humidity of the air between the apparel and skin

4.2 Surface Treatment of Textiles and Apparels Recording starts

103

40 min

10 min

10 min

7 min

23 min

Rest

Rest

Exercise

Rest

Fig. 4.5 Time periods of rest and exercise during the test

at intervals of 1 s. The sensor was attached to the back of the test subject on the inner side of the apparel and 15 cm below the neck. The exercise procedure is illustrated in Fig. 4.5. In each trial, the subject first drinks a specific amount of water (200 mL). After the subject rests for 10 min, the temperature and humidity are recorded. After further resting for 10 min, the subject rides a stationary bicycle at a medium pace and strength (strength = 6, velocity = 25 km/h, rate of calorie consumption = 10 kcal/min, and typical heart rate = 120 times/min) for 7 min. Following this, the subject rests for 23 min, and the measurements are completed. From the measured data, moisture adsorption is identified for the untreated and treated apparels A and B. The tests are conducted in a room at a constant temperature and humidity of 27 °C and 50%, respectively. In each trial, the experimental conditions are controlled and maintained constant as much as possible.

4.2.2.7

Vapor Dissipation Test for the Apparels

To quantitatively investigate the performance of functional apparel A, which is prototyped in this study, in terms of vapor dissipation against the outside air, the test setup is shown in Fig. 4.6. Pure water (180 mL) is placed in a cylindrical container (inner diameter: 61 mm, length: 130 mm), and a constant temperature and vapor pressure are maintained by a heater (TR-K, AS ONE) equipped with a constant temperature controller. To investigate the vapor dissipation of apparel A, a part of the apparel is attached to the top of an open container with the hydrophilic surface facing outward, and the change in humidity and temperature due to steam passing through the apparel is measured at a length L 2 from the surface. A temperature and humidity sensor (TR-72S, T & D Corporation; measurement temperature range: 0–50 °C, relative humidity: 10–95%) is set over the wear surface, and the data are transferred to a notebook computer for data processing. The experiment is conducted by changing the distance L 1 between the water surface and the apparel, the distance L 2 between the apparel and the sensor, and the pure water temperature T w to clearly differentiate

104

4 Hydrophilic Treatment Technology for Textiles, Filters, and Glass … Temperature &RHsensor Steam L

2 Data transfer

Wear Steam L Thermocouple

1 Pure water

Temperature controller

Stand Heater

Cylindrical container

Notebook NotePC PC

Fig. 4.6 Apparatus for measuring the moisture diffusion characteristics

between the untreated and treated apparels. The test is conducted in a room maintained at a temperature of 19 °C and humidity of 35% using an indoor air conditioner. Humidity and temperature may have changed locally during the experiment.

4.2.3 Experimental Results and Discussion 4.2.3.1

Moisture Adsorption

The external views of the one-side-treated apparels A and B are shown in Fig. 4.7. Figure 4.8 shows photographs of the droplets that are dropped on the untreated and one-side-treated cloths and the one-side-treated apparels A and B in the water droplet penetration test. All the photographs are captured 20 s after the droplets have fallen. On the surface, a flatter and larger wet region, rather than a droplet, indicates a more hydrophilic surface. Figure 4.8a, b shows that the treated cloth and apparel A, which is made of 100% polyester, exhibit significant one-sided hydrophilicity. Conversely, for apparel B, which is made of mixed fibers, the hydrophilicity increases on both surfaces after treatment, as shown in Fig. 4.8c. During polymerization, the monomer mist instantly reaches from one surface to the other because of the hydrophilicity or hygroscopicity of the cloth itself, and both sides are affected by the plasma. Consequently, the hydrophilicity increases on both sides of the sample (Fig. 4.8c).

4.2 Surface Treatment of Textiles and Apparels

105

Fig. 4.7 Photographs of the apparels: a Apparel A; b Apparel B

(a) Cloth

(b) Apparel A

(c) Apparel B

Outer

Inner

Untreated

Outer surface is treated

Untreated

Outer surface is treated 1 cm

Untreated

Outer surface is treated

Fig. 4.8 Results of the water drop test (appearance of the water drop 20 s after it is dropped)

4.2.3.2

Odor Control

Figure 4.9a shows the relationship between the ammonia concentrations after the gas is passed through the cloth and the elapsed time. The concentration is measured downstream from the cloth. The initial concentration of ammonia that is measured upstream from the cloth is set at 50 ppm. Clearly, this graph shows that the concentration for the cloth that is treated on both sides by APNTPT graft polymerization does not reach saturation even after 200 min; thus, the highest adsorption is obtained. The cloth treated with plasma graft polymerization on one side exhibits the second-highest ammonia adsorption, and the adsorption is saturated after 90 min. The untreated cloth and the one-sided plasma-treated cloth reach saturation after only 20 min. These results indicate that the APNTPT graft polymerization process imparts good odor control to the cloth. Scanning electron microscopy photographs of the cloth before and after the process showed that a porous structure is formed on the surface, like that

106

4 Hydrophilic Treatment Technology for Textiles, Filters, and Glass …

of activated carbon. This porous and chemically activated structure of acrylic acid on the surface can adsorb odors both physically and chemically. The total amounts of adsorbed ammonia are approximately 0.6, 2, and 6 mL for the untreated, one-sidetreated, and both-side-treated cloths, respectively. This type of high-performance adsorption exhibits high durability, which does not reduce even after the material is washed several dozen times using a washing machine. Further, the odor adsorption is regenerated after washing. Similarly, the measurement results for felt and Spabond™ are shown in Fig. 4.9b, c, respectively. This trend is like that shown in Fig. 4.9a. Thus, the treated fabric exhibited good odor adsorption because the formation of the graft polymerization layer produces a porous fiber surface like that of activated carbon, thereby improving the chemical adsorption. Moreover, such high adsorption is consistently exhibited even after multiple washings.

4.2.3.3

Qualitative Evaluation for Treated Apparels

Photographs of the treated apparels A and B are shown in Fig. 4.7. The results of the qualitative evaluation of the treated and untreated apparels based on the two subjects are summarized as follows. When apparel A is worn, some roughness is felt on the skin before the treatment because the material of 100% polyester is originally slightly rough. The roughness increases marginally after the APNTPT graft polymerization process. However, the wearing comfort increases significantly after apparel A is worn for some time before the exercise. The sweat dispersion increases significantly after the treatment. A similar sensory test was performed for functional wear that was prepared using the same surface treatment system and was worn by two female subjects. This result qualitatively agrees with these results. For apparel B, the comfort does not increase during exercise before or after treatment. Although the odor control may increase, no experiment is performed to confirm this.

4.2.3.4

Quantitative Evaluation for Treated Apparels

Figure 4.10a shows the results of the qualitative evaluation of apparel A, which has superior one-sided hydrophilicity. In this graph, the elapsed time is represented on the horizontal axis, whereas the relative humidity and temperature are represented on the left and right vertical axes, respectively. The relative humidity and temperature are measured by placing the sensor on the back of the subject at a position 15 cm below the neck and inside the apparel. This figure clearly shows that for the untreated apparel exhibiting both hydrophobic sides, the relative humidity is higher before exercising and increases significantly after exercising for 17 min. After approximately 30 min of exercising, the relative humidity decreased. Conversely, for the one-side-treated apparel A, the relative humidity began to decrease during exercise (after approximately 15 min), although the temperature remained constant. This may be caused by sudden ventilation or moisture dispersion to the environment owing to moisture

107

50 NH3 concentration before cloth = 50 ppm

40 30 Sample size: 40 cm 22 cm

20

Untreated Untreated One-side plasma One side solo plasma only One-side One side PG PGP Both-side PG Both sides PGP

10 0 0

50

100

150

200

Elapsed time (min)

NH3 concentration after felt (ppm)

(a) 50 NH3 concentration before felt = 50 ppm

40 30 Sample size: 40 cm 22 cm

20

Untreated Untreated One-side plasma One side solo plasma only One-side One side PG PGP Both-side Both sidesPG PGP

10 0 0

50

100

150

200

Elapsed time (min)

(b) NH3 concentration after spabond (ppm)

Fig. 4.9 Relationship between ammonia concentration and elapsed time downstream of a cloth, b felt, and c Spabond™

NH3 concentration after cloth (ppm)

4.2 Surface Treatment of Textiles and Apparels

50 NH3 concentration before spabond = 50 ppm

40 30 Sample size: 40 cm 22 cm

20

Untreated Untreated One-side plasma One side solo plasma only One-side One side PG PGP Both sidesPG PGP Both-side

10 0 0

50

100

150

Elapsed time (min)

(c)

200

108

4 Hydrophilic Treatment Technology for Textiles, Filters, and Glass …

penetration from the inner to the outer side. After the exercise, the relative humidity decreased rapidly, followed by a decrease in temperature. These results indicate the superior performance of the treated apparel. Figure 4.10b shows the result for the treated apparel B. No significant difference in moisture adsorption is observed in the one-side-treated and untreated apparels owing to the simultaneous increase in the hydrophilicity of both sides of the treated apparel B, as shown in Fig. 4.8c. These results indicate the effective dispersal of human sweat to the outside environment by wearing functional sportswear exhibiting hydrophilicity and hydrophobicity on the outer and inner sides, respectively.

100 90 80 70 60 50 40 30 20 10 0 0

Relative humidity

Temperature

Exercise

5

10

15

20

25

30

35

39 38 37 36 35 34 33 32 31 30 29 40

Temperature (°C)

Relative humidity (%)

: Untreated wear : Treated wear

Elapsed time (min)

(a)

100 90 80 70 60 50 40 30 20 10 0

Relative humidity

Temperature Exercise

0

5

10

15

20

25

30

Elapsed time (min)

(b)

35

39 38 37 36 35 34 33 32 31 30 29 40

Temperature (°C)

: Untreated wear : Treated wear

Relative humidity (%)

Fig. 4.10 Performance test results for changes in relative humidity and temperature between the apparel and skin when wearing prototype apparels a A (100% polyester) and b B (65% polyester, 35% cotton)

4.2 Surface Treatment of Textiles and Apparels

4.2.3.5

109

Results of Vapor Dissipation Tests for the Apparels

Figure 4.11a shows an example of the results of the vapor diffusion test against outside air for the 100% polyester apparel A that is prototyped in this study (distance L 1 = 85 mm between the water surface and the apparel, L 2 = 10 mm, pure water temperature T w = 36 °C). Figure 4.11a shows that the temperature and amount of vapor emitted from the treated apparel are lower than those of the untreated apparel. In the treated apparel, as the moisture dissipated, the hydrophilic surface on the outside expands, and the area exposed to the outside air increases. Therefore, the vapor is considered to have diffused satisfactorily, and the humidity and temperature at the sensor position decrease.

100 90 80 70 60 50 40 30 20 10 0 0

30 28

Relative humidity

26 24 22

Temperature

Temperature (°C)

Relative humidity (%)

: Untreated wear : Treated wear

20 2

4

6

8

10

12

14

Elapsed time (min)

(a) : Untreated wear : Treated wear

100 90 80 70 60 50 40 30 20 10 0 0

30 Relative humidity

28 26 24

Temperature

22 20 2

4

6

8

10

Elapsed time (min)

(b)

12

14

Temperature (°C)

Relative humidity (%)

Fig. 4.11 Result of the diffusion test: a L 1 = 85 mm, L 2 = 10 mm, T w = 36 °C; b L 1 = 195 mm, L 2 = 290 mm, T w = 100 °C

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4 Hydrophilic Treatment Technology for Textiles, Filters, and Glass …

Figure 4.11b shows the result for higher-temperature steam (L 1 = 195 mm, L 2 = 290 mm, T w = 100 °C). Although the steam environment of T w = 100 °C is rare, the experiment is conducted to investigate the dissipation of considerable steam. The figure shows that the treated apparel allows steam to pass through satisfactorily, while the untreated apparel exhibits poor transmission of steam during the first 2 min. After 2 min, a trend like that shown in Fig. 4.11a is observed. Moreover, the moisture permeable layer clearly expands on the surface, as shown in Fig. 4.1c, which occurs in the treated apparel A when compared to the other treated apparels.

4.3 Deodorization Technology Using Low-Temperature Nonthermal Plasma 4.3.1 Plasma Deodorization Technology 4.3.1.1

Principle of Plasma Deodorization

As explained in Chap. 1, plasma refers to the state of gas that is ionized by a strong electric discharge. Low-temperature nonequilibrium plasma (also called “NTP”), which is generated by applying a high voltage between electrodes, is particularly effective in improving the environment. However, the electron temperature is high (usually at or above 10,000 K), and the gas temperature is low (approximating room temperature); consequently, the plasma can be formed using low electrical energy wherein charged particles such as electrons, ions, and neutral radical particles typified by oxygen radicals coexist in the plasma. When this plasma is brought into contact with odorous-molecule-containing and harmful gases, it decomposes into water and carbon dioxide. Plasma deodorization utilizes this effect.

4.3.1.2

Types of Plasma Odor Control Systems

Plasma deodorization is predominantly performed using DC corona discharge in small indoor air cleaners, which are currently available in the Japanese market. However, in DC corona discharge, the area where the plasma is formed is limited to the vicinity of the discharge line; moreover, the energy efficiency and odor removal effect are low. The pulse and AC discharges obtained by rapidly switching the DC are effective for generating plasma along the entire flow path and are practically used in large-scale equipment, such as deodorizing equipment in garbage disposal facilities of local governments. Although several small devices have been commercialized, they are expensive to manufacture and harmful gases such as ozone and nitrogen oxides are generated. When considering the basic research on this type of technology, many studies have been conducted, such as a deodorizing device using a catalyst and the study conducted by our team.

4.3 Deodorization Technology Using Low-Temperature Nonthermal Plasma

4.3.1.3

111

Types of Plasma Odor Control Technologies

Plasma deodorization methods are classified into the following two types. The first is the indirect (remote) plasma method. The gas is passed into the plasma reactor, where rapid electrons are activated to form radicals, which are mixed with the gas containing odorous components to decompose and remove the odorous components. Moreover, as the radicals disappear within microseconds, measures such as rapid mixing to shorten the time required for mixing or clustering of radicals to prolong their life are necessary. As an example of the latter, indoor air cleaners that emit so-called radical negative-ion clusters into the environment are commercially available. Conversely, an ozone-injection-based deodorization system, which has long been used, is an indirect plasma method. Figure 4.12 shows an example of an ozone deodorization plant using plasma generation. Because ozone has a high deodorizing effect under high-humidity conditions, a gas scrubbing tower is installed, and ozone is blown into it to remove water-soluble odorous components simultaneously. Moreover, because high ozone concentrations are harmful, ozone is removed by heating or adsorption using activated carbon before the treated gas is discharged into the atmosphere.

4.3.1.4

Elements of the Plasma Odor Control System

Direct plasma-type indoor air cleaners, which are currently in practical use, can be understood by disassembling several devices, and they are often composed of a combination of the following five elements: (a) (b) (c) (d) (e)

Electrostatic precipitators (single stage or two stages); Plasma reactor; Particulate filters (such as nonwoven filters); Adsorbents, catalysts (activated carbon, manganese dioxide, etc.); Electrostatic filter.

Figure 4.13 shows a typical example of these elements. Elements (a), (c), and (e) are mainly effective for removing fine particles, such as dust, and elements (b) and (d) are effective for removing bad odors. The electrostatic filter (e) retains a positive or negative charge and has a high collection efficiency for fine particles. When a catalyst such as TiO2 photocatalyst is supported on elements (c) and (d), an offensive odor removal function is generated, or the performance is improved by the light activation of the NTP. Figure 4.14 shows an air cleaner [7] that combines an AC plasma reactor, a DC ESP, and a filter reported by the authors and is a combination of elements (a), (b), and (c). The plasma reactor decomposes or atomizes odors, and the ESP collects 99% or more of the particles. Removal efficiency of 95% or more is obtained for acetaldehyde, ammonia, and bad odor components of cigarette smoke. Moreover, Suda et al. [8] developed and commercialized a low-pressure loss air-cleaning system in which elements (b), (c), and (d) are arranged in a series with improving performance.

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4 Hydrophilic Treatment Technology for Textiles, Filters, and Glass …

(a) Ozone deodorization tower

Treated gas

Ozone destructor

Odorous gas Deodorization fan pH

Activated carbon adsorption tower

Circulation pump Ozone Compressor generator unit

Tap water

Feeding pump pH adjustment tank

Caustic soda storage tank

(b) Fig. 4.12 Ozone deodorizing plant using indirect plasma ozonizer method: a overview; b schematic representation

However, when developing high-performance clothing using plasma graft polymerization treatment, the authors identified that the plasma graft polymerization treatment of the particulate filter (element (c)) significantly improves deodorization. A functional filter exhibiting the enhanced effects of elements (c), (d), and (e) can be manufactured. Furthermore, the adsorption function could be improved by irradiating the filter with NTP by barrier discharge. The results for these functional filters are explained below.

4.3 Deodorization Technology Using Low-Temperature Nonthermal Plasma

113

Fig. 4.13 Elements of the indoor air-cleaning system: a electrostatic precipitator, b plasma reactor, c particulate filter (supporting catalyst), d adsorbent and catalyst, and e electrostatic filter Fig. 4.14 Air cleaner combining a plasma reactor and an electrostatic precipitator [7]

AC high voltage

DC high voltage

Flow

Flow

Non thermal plasma reactors

Two stage ESP

Filter

4.3.2 Methods for Producing and Measuring Performance of Functional Filters 4.3.2.1

Treated Filter Material

The three types of filter cloths (polyester fiber manufactured by Sintokogio, Ltd.), as listed in Table 4.1, are subjected to plasma treatment. These cloths are used not only as general-purpose fibers but also as bag filter cloths in the dry-type filtration dust collection technology. Tests are performed after cutting the cloths to dimensions of 40 × 23 cm.

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4 Hydrophilic Treatment Technology for Textiles, Filters, and Glass …

Table 4.1 Characteristics of tested textiles Type of textile

Mass per area g/m2

Thickness mm

Tensile strength kgf/5 cm width Longitudinal direction

Transverse direction

Air permeability mL/cm2 /s

Cloth

335

0.9

224

172

8.0

Felt

597

1.7

94

252

10.9

Spabond

260

0.62

90

60

11.0

4.3.2.2

Plasma Graft Polymerization

The plasma graft polymerization is performed as follows. First, as shown in Fig. 4.15, the fiber surface is irradiated with low-temperature nonequilibrium plasma to generate active radicals on the surface. Then, the pressure in the chamber is reduced, and an acrylic acid monomer (CH2 =CHCOOH) is sprayed to perform graft polymerization. Because plasma exhibiting a gas temperature approximating room temperature is used, the surface of the cloth fiber is not damaged by combustion reactions. Moreover, unlike in EB graft polymerization, monomers can be safely formed on one side of the fiber using a relatively simple apparatus to initiate polymerization. The reactions are shown in Fig. 4.2. The treatment is performed using the equipment shown in Fig. 4.3. This device is manufactured by Pearl Industrial Co., Ltd., and while winding a 50-cm-wide and 3–10-m-long cloth at 1–100 mm/min, a rare gas is generated on the cloth surface by high-frequency glow discharge. Atmospheric low-temperature nonequilibrium plasma (mixed gas of Ar + He) is uniformly irradiated by a parallel-plate dielectricbarrier nozzle electrode (Fig. 4.4). Subsequently, the acrylic acid mist is continuously Fig. 4.15 Plasma graft polymerization of fiber and monomer

Textile R R Plasma irradiation

R R R

Hydrophilic monomer

R R R

Graft polymerization

R R R R

Activated radicals

Plasma graft polymerized layer

4.3 Deodorization Technology Using Low-Temperature Nonthermal Plasma

115

sprayed to graft polymerize firmly with the fiber. The conditions for the plasma graft polymerization treatment are optimized by trial and error, and the following conditions are set: cloth feed velocity: 10 mm/min, the distance between the cloth and the electrode: 0 mm, noble gas mixture ratio: He = 10 L/min, Ar = 2 L/min, input power: 0.77 kW, the volume of sprayed acrylic acid: 100 mL, reaction temperature: 24–26 °C, and reaction process pressure: atmospheric pressure. After the treatment, the unreacted polymer adhering to the surface is washed and dried. The onesided processing is completed as described. For double-sided processing, the same processing technique is performed on the other side.

4.3.2.3

Nonthermal Plasma Treatment Method

In the NTP treatment, a copper mesh electrode is wrapped around the outer side of a heat-resistant glass tube (inner diameter: 30 mm, outer diameter: 33 mm), as shown in Fig. 4.16, in which a discharge wire (made of 1.5-mm-diameter stainless steel) is used. A filter is wrapped around the internal discharge wire and sealed, and a pulse high-voltage power supply (SI thyristor system, NGK Insulators, Ltd.) is used to generate a short 210-Hz ~ 600-ns-wide pulse and peak applied voltage of 30 kV. The plasma is irradiated with NTP for 20 min. Fig. 4.16 Nonthermal plasma reactor for filtering [9]

Discharge wire

Pulse high voltage power supply

Filter

Pyrex tube Silicon bushing

Copper mesh electrode

116

4.3.2.4

4 Hydrophilic Treatment Technology for Textiles, Filters, and Glass …

Method for Measuring Deodorization

To examine the deodorization of the filters treated with both plasma types, an adsorption characteristic test for ammonia, which is a typical component of body odor and other offensive odors, was conducted. Furthermore, when examining the washing durability, an ultrasonic electric washing machine was used to wash and dehydrate the fabric for approximately 30 min, which was then dried in the open air in winter for an entire day.

4.3.2.5

Method for Measuring Particle Removal

Dust collection measurements were conducted for the untreated and single- and double-side-treated felt filters. In the experiment, one of the filters was set in a filter holder (inner diameter = 55 mm), and monodisperse polystyrene latex particles (diameter = 0.3 or 0.6 μm) generated by a nebulizing aerosol generator (Model 307, TSI) were passed through it at 1 L/min. The number of particles before and after filtering was measured several times using a laser particle counter (Model TF-500, Kanomax Japan, Inc.), and the dust collection efficiency was calculated from the average value.

4.3.3 Experimental Results and Explanation 4.3.3.1

Electron Micrographs of Filter Surfaces

Figure 4.17 shows photographs of the surfaces of untreated and treated filters that are captured using a scanning electron microscope (SEM, S-4700, Hitachi). Platinum– palladium powder is sputtered on the surface in advance to obtain a conductive sample during imaging. Figures 4.17a–c show a single fiber at 5000 × magnification, while Figs. 4.17d–f show its surface at 20,000 × magnification. One scale of the Vernier in Fig. 4.17c, d is 1 and 0.1 μm, respectively. These photographs show that the fiber surface is initially roughened by the plasma graft polymerization treatment and further roughened by the NTP treatment. Such a change in surface shape is believed to have improved odor absorption. The deodorization measurements are shown in Fig. 4.9. The results show that the treated fabric has good odor adsorption because the surface becomes porous like activated carbon, as shown in the photograph in Fig. 4.17. By washing, the adsorbed ammonia is desorbed, and the adsorption is recovered. Furthermore, polyester fiber cloths retain such high adsorption even after dozens of washings. Figure 4.18 shows the results of an experiment investigating the effect of NTP treatment. The effective adsorption time of the plasma-irradiated filter (black circles) is considerably longer (300 min). The effective time is approximately quadruple that of plasma irradiation after the single-sided treatment (black triangles). Next, when

4.3 Deodorization Technology Using Low-Temperature Nonthermal Plasma

117

Fig. 4.17 Scanning electron microscopy photographs of the surface of the felt filter [9]: a untreated sample; b plasma graft polymerization treatment; c plasma graft polymerization followed by nonthermal plasma (NTP) irradiation (after washing); d untreated sample; e plasma graft polymerization treatment; and f plasma graft polymerization followed by NTP irradiation (after washing). Magnification is 5000 × for a–c (1 interval = 1 μm) and 20,000 × for d–f (1 interval = 0.1 μm)

examining the durability of the NTP treatment, the same figure shows the results of the adsorption experiment after the treatment is allowed to stand for one month. The figure shows that although the performance of the NTP treatment alone deteriorated sharply after one month (white circles), the performance of the fabric treated with both plasma graft polymerization and NTP on one side negligibly changes (white triangle points).

4.3.3.2

Dust Collection Measurements

Table 4.2 lists the results of the dust collection experiments using monodisperse fine particles (two types: 0.3 and 0.6 μm). From the table, one can observe that the dust collection efficiency increases in the following order: double-sided plasma graft polymerization treatment > no treatment > NTP treatment. Because collecting particles by applying a weak charge to an electrostatic precipitator improves the dust collection efficiency, this issue must be considered.

118

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Fig. 4.18 Time-dependent adsorption characteristics of NTP-treated filter [9]

Table 4.2 Dust collection efficiencies of untreated and treated bag filters (felt) for 0.3- and 0.6-μm polystyrene latex particles [9]

Type of filters

Number of collection efficiency % 0.3 μm

0.6 μm

Untreated

74.8

78.4

PG treatment on both sides

75.7

85.1

NTP treatment

67.2

70.8

4.4 Increased Glass Surface Hydrophilicity by Nonthermal Plasma Treatment 4.4.1 Definition of Contact Angle Next, the NTP irradiation of a glass surface and increased hydrophilicity are explained in Sects. 4.4.1 and 4.4.2 with figures based on Ref. [3]. Figure 4.19 shows a typical liquid droplet placed on a glass substrate. The force balance for the liquid droplet and glass substrate can be expressed by Young’s equation as follows: γL cosθ = γS − γSL ,

(4.1)

where γS is the surface tension of the glass, γ L is the surface tension of the liquid, γ SL is the surface tension between the liquid droplet and the glass, and θ is the contact angle. The contact angle (θ ) of a liquid droplet on the glass is closely related to the work of adhesion (W SL ) required to remove the droplet from the glass plate (Young–Dupre equation). WSL = γL (1 + cosθ ).

(4.2)

4.4 Increased Glass Surface Hydrophilicity by Nonthermal Plasma Treatment

119

Fig. 4.19 Contact angle θ between the glass surface and liquid droplet

Therefore, the contact angle provides a method for calculating the liquid-glass adhesion. Equations (4.1) and (4.2) show that when the surface tension (or surface energy) between the liquid droplet and glass (γSL ) increases, the contact angle (θ ) increases, which decreases the adhesion energy (W SL ) or hydrophobic characteristics. However, when the surface tension of the glass (γ S ) increases, the contact angle decreases, which increases W SL or the hydrophilic characteristics. The application of plasma increases surface energy, adhesion energy, wettability, and hydrophilicity.

4.4.2 Glass Surface Treatment Using Atmospheric-Pressure NTP Irradiation 4.4.2.1

Experimental Apparatus and Methods

Among the various types of APNTPTs, the silent or dielectric-barrier corona discharge was employed because the plasma can be easily and uniformly applied to the glass surface. A schematic of the experimental setup is shown in Fig. 4.20. The dielectric-barrier corona plasma reactor consists of two parallel round aluminum disks. The 73-mm-diameter upper disk electrode is hollowed with 30 2-mm-diameter pinholes. The lower disk electrode of the same diameter is covered with a dielectric material (1-mm-thick acrylic plate), where a 38-mm-wide, 26-mm-long, and 1-mm-thick sodium silicate glass sample is placed. This configuration is called the “corona reactor” because the bottom electrode that is covered with a dielectric barrier is a typical configuration for dielectric-barrier discharge, and the sharp-edged and pinholed upper electrode is typical in a corona discharge. The distance from the electrode to the glass surface is maintained at 3 mm. As a background gas, dry air at 4% relative humidity and 20 °C is primarily used by the compressor through the dryer. Pure nitrogen (99.99% purity) and He (99.995% purity) in the cylinders are also tested. The desired flow rate is obtained using a mass flow controller. The gas is supplied from the top shaft, passed through the 30 holes, and uniformly impinged on the glass sample. Then, the gas is dispersed outward and discharged into the holes of the bottom shaft. The electrodes are energized using a 60 Hz AC power supply (maximum of 20 kV and 30 mA) to generate microdischarges between electrodes.

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4 Hydrophilic Treatment Technology for Textiles, Filters, and Glass …

Fig. 4.20 Schematic of the experimental setup

The applied voltage and current waveforms are measured using an oscilloscope (TDS380P, Tektronix) through a voltage divider (P6015A, Tektronix) and a current probe. The effects of the background gas, flow rate Q, plasma treatment time, and applied peak-to-peak applied voltage V p-p on the contact angle are investigated to understand the fundamental characteristics of the glass surface using solo plasma and plasma–chemical combined processes. The contact angle of the glass samples is measured using a contact angle meter (CA-DT, Kyowa Interface Science Co., Ltd.). All the experiments are conducted at Osaka Prefecture University (currently, Osaka Metropolitan University, Nakamozu Campus) from January to February in Sakai, Japan. The experimental conditions are as follows: average rainfall for a month = 44 mm, no snow, average temperature = 5.0 °C, daylight hours for a month = 117.2 h, average atmospheric pressure = 1016 hPa, average relative humidity = 60% at 15 °C, and dew point = 7.3 °C.

4.4.2.2

Experimental Results and Discussions

Initially, the effects of background gases, such as air, nitrogen, and He, are investigated. The flow rate Q, plasma treatment time, and applied voltage V p-p are set to 1.0 L/min, 1 min, and 17 kV, respectively. All the results are averaged based on five experimental measurements. This is referred to as the basic treatment condition. The contact angle of the glass sample is initially 45° and is reduced to 3.3 and 4° for dry air and nitrogen, respectively, by plasma treatment, showing superior hydrophilic properties. However, when He gas is used, only 2.5 kV is applied, and the contact angle becomes 52°, resulting in no hydrophilicity. Because dry air shows the best results, it is used in subsequent experiments.

4.4 Increased Glass Surface Hydrophilicity by Nonthermal Plasma Treatment

121

Under basic conditions, except for voltage, the effect of the applied voltage is examined. Figure 4.21 shows the voltage-dependent contact angle of the glass surface. The applied voltage is varied from 0 to 15 kV. The contact angle is initially 45° and then decreases 4° at 13 kV. No apparent changes are observed at voltages greater than 13 kV. Next, the effect of the plasma treatment time is examined. The plasma treatment time is varied from 0 to 60 s. The results are shown in Fig. 4.22. The contact angle decreased from 45° to 10, 5, and < 4° after plasma exposure times of 10, 30, and 60 s, respectively. The contact angle of the glass sample does not change beyond 60 s of plasma treatment time. The experimental conditions are the same, while the flow rate is varied up to 5 L/ min. Because the residence time of the gas in the reactor decreases with increasing flow rate, maintaining an effective plasma is difficult for a high flow rate. Figure 4.23 Fig. 4.21 Voltage-dependent contact angle of the glass surface (background gas: dry air, flow rate Q = 1.0 L/min, plasma treatment time = 1 min)

Fig. 4.22 Effect of plasma treatment time on contact angle (background gas: dry air, flow rate Q = 1.0 L/min, V p-p = 15 kV)

122

4 Hydrophilic Treatment Technology for Textiles, Filters, and Glass …

shows the effect of the flow rate on the contact angle. As shown in Fig. 4.23, the flow rate did not affect the contact angle within the range of this experiment. The durability of the plasma-treated glass samples is investigated, as shown in Fig. 4.24. Three plasma-treated glass samples (V p-p = 13, 15, and 17 kV) are exposed to the outside environment for five days without windshield wiper operation. At the end of each day, a sample is taken, and the contact angle is measured. The contact angle clearly increases from 3.3° to in the range of 15–20° for all three samples after just one day. In fact, the contact angle exceeded 10° after 7 h. After five days, the hydrophilicity is completely lost in all the samples. To maintain a low contact angle, plasma-induced graft polymerization is considered but is not performed presently because of the low visibility of the glass. Fig. 4.23 Effect of flow rate on contact angle (background gas: dry air, plasma treatment time = 1 min, V p-p = 15 kV)

Fig. 4.24 Effect of exposure time on contact angle (background gas: dry air, flow rate Q = 1.0 L/min, plasma treatment time = 1 min)

4.4 Increased Glass Surface Hydrophilicity by Nonthermal Plasma Treatment

4.4.2.3

123

Role of Nonthermal Plasma in Improving Hydrophilicity

The mechanism by which plasma improves hydrophilicity is considered. The water in the silicate mixed during glass manufacturing exists as OH, which is predominantly bound to hydrogen atoms on the glass surface. When the surface is irradiated with plasma, the short-range (0.255 nm) O–OH bonds are cleaved by electron collisions, and more O and OH groups are on the surface. The electron energy of NTP is usually in the range of several electron volts, which is sufficient to break the hydrogen bond energy (0.2–0.3 eV). Several free OH groups are formed on the glass surface owing to the plasma irradiation, and the hydrophilicity is considered to have increased.

4.4.3 Glass Surface Hydrophilicity Dynamically Controlled by Nonthermal Plasma Actuator When droplets adhere to the surface of a vehicle windshield or rearview mirror during rainy weather, the light transmission or reflectance (and, thus, the external view) must be improved without using a mechanical wiper that interferes with the view. To achieve this effect, high voltage is applied from a high-voltage power supply to a transparent electrode installed on the surface or inside the windshield or rearview mirror to generate surface discharge plasma, and the shape or movement is measured for the water droplets on the surface. Alternatively, by controlling the hydrophilicity of the surface, the light transmission or reflectance can be improved to ensure external visibility. The principle of the system called the “NTP actuator” is explained [10].

4.4.3.1

Principle of Nonthermal Plasma Actuator

Figure 4.25 shows an explanatory diagram of this method. A plate or net-like back electrode is attached to the back of the glass, and the front electrode is a thin parallel straight line. The glass material is not limited if it is insulating and may be a film such as a polymer. The preferred electrode material is indium tin oxide, i.e., tin oxide-added indium oxide or a transparent conductor such as conductive plastic. Figure 4.26 shows a cross-section of Fig. 4.25a. By turning on switch 1, plasma is generated near the high-voltage electrode, and simultaneously, a gas flow called the “ion wind” is generated to the right. The plasma causes a hydrophilic glass surface, and as shown in Fig. 4.25b, the water droplets form a plate-like region, suppressing light scattering. Further, when the glass is a mirror, light reflectivity is maintained. This hydrophilicity is maintained for several hours after the plasma application is stopped, and if the hydrophilicity is lost, the plasma can be applied again. In fact, 2-mm-thick silicate glass is prepared with electrodes, as shown in Fig. 4.26, and a pulse high voltage exhibiting a peak of approximately 12 kV, 305 Hz, and a rise time of approximately 60 μs from a positive-pulse high-voltage power source

124

4 Hydrophilic Treatment Technology for Textiles, Filters, and Glass …

Fig. 4.25 Principle of glass surface hydrophilicity-based dynamic control using nonthermal plasma actuator

Alternate or pulse high-voltage power supply

High voltage electrode

Dielectric material (glass, etc.)

Plasma and ion wind

Grounded electrode

Switch 1

Fig. 4.26 Schematic showing the principle of the nonthermal plasma actuator (cross-sectional view)

is used to conduct an experiment. After preparing the prototype, the surface of the electrode is lightly polished with an abrasive to remove surface oils and fats and then washed with ethanol before starting the test. A high voltage is applied between the electrodes to generate a uniform surface discharge plasma near the electrodes. Before and after plasma generation, a 2-mm-diameter water droplet is gently dropped onto the glass surface and the contact angle is measured using a contact angle meter (CA-DT, Kyowa Interface Science Co., Ltd.). The contact angle, which is initially 54°, decreased to ≤ 7° near the electrode, and sufficient hydrophilicity is obtained.

4.5 Conclusions

125

Alternate or pulse high voltage power supply

High voltage electrode Plasma and ion wind

V2

V1 Dielectric material (glass, etc.)

Grounded electrode

Switch 2

Direct current high-voltage power supply

Fig. 4.27 Schematic showing the principle of the sliding nonthermal plasma actuator (crosssectional view)

However, after standing for one day, the contact angle returned to its original value, and the plasma must be reapplied. Thus, vehicle windshields and rearview mirror systems can be developed with innovative wipers that have no mechanically moving parts. Furthermore, such wipers can be used for cleaning owing to the improved hydrophilicity; for example, to wash away stains adhering to the surface of a solar battery panel plate by hydrophilizing the surface.

4.4.3.2

Sliding Discharge-Induced Plasma

Conversely, active research is being conducted to control the boundary layer of the flow on the wing surface of an airplane by dynamically displacing the surface discharge-induced plasma. As an example, Fig. 4.27 illustrates a schematic of an attempt to extend the region of plasma and ion wind by adding another electrode and a DC power source to the surface discharge electrode that is shown in Fig. 4.26. An AC or pulse power supply (amplitude V 1 = approximately 20 kV) is turned on to generate a surface discharge, and switch 2 is turned on to raise the DC voltage (V 2 = approximately 5 kV). A slide in the direction of the electrode is observed. This is called the “sliding discharge.” In Fig. 4.26, the length of the discharge region is approximately L = 10 mm; however, in Fig. 4.27, the plasma region is longer than approximately L = 40 mm. This technique can also be used to control the droplets and hydrophilicity of glass surfaces.

4.5 Conclusions The principle of single-sided plasma graft polymerization and the manufacturing method for high-performance fibers, apparels, and filters exhibiting odor removal and high-efficiency dust collection functionalities are explained. The experimental results

126

4 Hydrophilic Treatment Technology for Textiles, Filters, and Glass …

demonstrate the high absorption characteristics of ammonia gas and the improvement in the fine-particle collection characteristics. Furthermore, when one side of the fiber is hydrophilic, the effect can withstand dozens of washings and does not deteriorate even after approximately a year. The results of this study can be summarized as follows. The plasma graft polymerization equipment for processing functional apparel is improved and can be used to process cloth and apparel surfaces directly into functional gradients. The functionalities are measured for the processed fabric and two apparel types. The results confirm that the plasma graft polymerization treatment imparted good one-sided hydrophilicity and poor odor adsorption to the fabric. The processed apparel is sensorily evaluated, and the results confirm that the functional gradient apparel, which exhibits a skin-touching hydrophobic surface and hydrophilic outer surface, can maintain comfort during sports. A vapor dissipation test is conducted for the processed apparel, and the results confirm that the treated apparel diffused vapor well and that the moisture permeation layer expands on the surface. Next, the basic principle of plasma deodorization, conventional research, application equipment, plasma treatment method of the dust collection filter conducted by the authors, and performance test results are explained. A high-performance filter capable of simultaneous deodorization and particle collection is achieved using plasma treatment. For the NTP treatment alone, although the durability is inferior to that obtained using only the plasma graft polymerization treatment, the adsorption efficiency is considerably superior immediately after treatment. The plasma graft polymerization treatment also improves the efficiency of fine-particle removal. The high-performance filter described above should be applied in indoor air cleaners and water filters (adsorption of cations) in the future. Furthermore, the surface treatment modification technology of glass using APNTPT (hydrophilic approach) and the method of dynamic control of the hydrophilicity of the glass surface using the APNTPT actuator are explained. The potential for a plasma wiper is also shown.

References 1. J.S. Chang, P.A. Lawless, T. Yamamoto, Corona discharge processes. IEEE Trans. Plasma Sci. 19(6), 1152–1166 (1991) 2. M. Okubo, T. Kuwahara, New Technologies for Emission Control in Marine Diesel Engines, Butterworth-Heinemann, imprint of Elsevier, Paperback ISBN: 9780128123072, eBook ISBN: 9780128123089, 1–296 (2019) 3. T. Yamamoto, M. Okubo, N. Imai, Y. Mori, Improvement on hydrophilic and hydrophobic properties of glass surface treated by nonthermal plasma induced by silent corona discharge. Plasma Chem. Plasma Proc. 24(1), 1–12 (2004) 4. M. Okubo, N. Saeki, T. Taguchi, T. Yamamoto, Development of surface treatment apparatus for manufacturing functional wear using low-temperature plasma. Trans. Jap. Soc. Mech. Eng. 72A(714), 263–268 (2006). (in Japanese) 5. M. Okubo, N. Saeki, T. Yamamoto, Development of functional sportswear for controlling moisture and odor prepared by atmospheric pressure nonthermal plasma graft polymerization induced by RF glow discharge. J. Electrostat. 66(7–8), 381–387 (2008)

References

127

6. T. Yamamoto, M. Okubo, Advanced physicochemical treatment technologies, in Handbook of Environmental Engineering, ed. by L.K. Wang, Y.-T. Hung, N.K. Shammas vol. 5, Chapter 4 (The Humana Press Inc., Nonthermal Plasma Technology, Springer 2007), pp. 135–294 7. M. Okubo, T. Yamamoto, T. Kuroki, H. Fukumoto, Electric air cleaner composed of nonthermal plasma reactor and electrostatic precipitator. IEEE Trans. Ind. Appl. 37(5), 1505–1511 (2001) 8. H. Suda, T. Ueno, T. Yamauchi, Y. Sainomoto, Plasma discharge deodorizing system. Matsushita Electric. Works Techn. Rep. 76, 59–63 (2001). (in Japanese) 9. M. Okubo, T. Kuroki, Measurement and analysis techniques for functional groups and chemical species formed by plasma treatment, [Mizu] to kinousei porima ni kansuru zairyo settukei, saishin ouyo (English translation book title: [Water] and Material Design for Functional Polymers, Latest Applications) Technical Information Institute. Section 3, 53–60 (2021). (in Japanese) 10. M. Okubo, T. Kuroki, Method to increase the surface transparency and its application to windshield glass and rearview mirror devices, Japanese Patent No. 4793769 (2011.8.5)

Chapter 5

Hydrophobic Treatment for Polymer Surfaces

5.1 Introduction Although degreasing treatments using alkaline or acidic solutions are widely used to improve the adhesion between material surfaces and hydrophobic (water-repellent) or corrosion-resistant films, a dry surface treatment method is desirable because of liquid-waste disposal-related problems. One such dry method is “corona treatment,” in which the material to be treated passes between a pair of electrodes to which a high voltage is applied to modify the material surface. Another dry method is “plasma treatment,” in which surfaces are treated by passing easily ionizable noble gases such as argon or helium between the electrodes to generate a plasma (ionized gas) jet that can act on the target. In general, corona or plasma treatment can activate and often significantly hydrophilize the surfaces of polymers, plastics, glass, and metals. Radicals formed on the surface owing to the action of high-energy electrons (approximately 1–10 eV) generated by atmospheric-pressure plasma discharge are hydrophilic, and the free energy on the surface increases, causing adhesion and bonding with other materials. When plasma-treated surfaces are subjected to hydrophobic plasma treatment or coatings, the adhesion and durability of the hydrophobic and anti-corrosive agents are improved, and the hydrophobicity (water repellency) is maintained. However, if the plasma-treated hydrophilized material is left as is in ambient air, the radicals disappear owing to air oxidation, and the effect of adhesion to other materials is reduced and completely disappears within a few days to a week unless treatment with hydrophobic or corrosion-resistant agents is performed immediately after plasma treatment. In contrast, a technique has been developed in which an organic gas is ionized by a low-pressure nonequilibrium plasma to form a high-hardness diamond-like carbon (DLC) thin film on the surface of the material to be treated. The surface can be hydrophobized or hydrophilized by selecting a suitable organic gas. Although this permanent treatment increases the surface hardness, the container (or chamber) wherein the sample is placed must be maintained below atmospheric pressure.

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 M. Okubo, Nonthermal Plasma Surface Modification of Materials, https://doi.org/10.1007/978-981-99-4506-1_5

129

130

5 Hydrophobic Treatment for Polymer Surfaces

Fig. 5.1 Application examples of the plasma hybrid surface treatment technologies described in this chapter

This chapter describes the plasma hybrid surface treatments, as shown in Fig. 5.1. First, (1) plasma-fluorocarbon gas hybrid treatment is explained. In this method, a fluorocarbon-based gas is introduced to a low-pressure plasma, and hydrophobic fluorine is then introduced to the material surface by the so-called fluorocarbon plasma. The possibility of developing this method as an atmospheric-pressure plasma treatment is explained. Anti-fog is realized in this treatment. Next, (2) plasma-laser processing by laser microfabrication is introduced. Plasma is generated using a laser, and the hydrophobicity becomes apparent by the formation of surface irregularities. The targets are polymers, etc. Next, the principles and implementation examples of treating the inner surfaces of polyethylene terephthalate (PET) bottles with (3) plasma DLC treatment processing are explained. Additionally, the DLC treatment of medical stents and other surface treatments are described.

5.2 Preparing a Hydrophobic Material Surface by Fluorocarbon Plasma Treatment To prepare a hydrophobic material surface using plasma, a fluorocarbon-based gas such as methane tetrafluoride (CF4 ) is introduced to a reduced-pressure plasma, and hydrophobic fluorine is introduced to the material surface by the fluorocarbon plasma. This method has been reported [1, 2]. A typical system comprises a gas inlet, reaction vessel, vacuum pump, matching network, and power supply. Various reactors have been used for plasma processing. Internal electrodes are required for DC and lowfrequency glow discharges. A typical reactor is a bell jar equipped with circular or square electrodes, as shown in Fig. 5.2 [1]. At high frequencies, electrodes are often outside the reactor vessel. Multiple plasma processing parameters determine the rate

5.3 Radio-Frequency Plasma Reactors with Chemical Vapor Deposition …

131

Fig. 5.2 Schematic of bell jar-type plasma reactor [1]. A: Vacuum vessel; B: radio-frequency (RF, in MHz) electrodes; C: grounded electrode

Insulator

Gas inlet

To pump

at which the plasma deposits the polymer and the physicochemical properties of the deposited film and modified surface.

5.3 Radio-Frequency Plasma Reactors with Chemical Vapor Deposition Apparatus Radio-frequency (RF; an industrial frequency = 13.56 MHz) plasma is usually induced by the discharge generated by a high-frequency power supply. In this type of plasma reactor, the reactor is usually pumped down, and the pressure is reduced to stabilize the plasma. RF plasma reactors are usually classified as capacitively coupled plasma (CCP) reactors (Fig. 5.3) in which a capacitor is used as the plasma electrode [3] and inductively coupled plasma (ICP) reactors (Fig. 5.4) in which a coil is used as the plasma inductor [4]. Other reactors include the inner- and outer-electrode-type plasma reactors, which position the electrode inside or outside the vacuum chamber, respectively. These systems are driven by high-frequency power supplies and are equipped with a matching box to match the power supply to the reactor. These plasma reactors are usually depressurized using a vacuum pump, which requires the plasma to remain stable. In inner-electrode-type plasmas, the glow discharge between two parallel plates is often applied in many industries, as already described in Chap. 3, because it can produce high-quality large thin films at relatively low temperatures (in the range 150–300 °C). In this type of chemical vapor deposition (CVD) apparatus, thin-filmtransistor-based liquid crystal displays and amorphous-silicon-based solar batteries are manufactured, as shown in Fig. 5.3. The substrate is typically attached to a grounded electrode. To achieve a uniform film thickness, the material gas is injected

132

5 Hydrophobic Treatment for Polymer Surfaces

Fig. 5.3 Capacitively coupled RF plasma semiconductor processing reactor

Fig. 5.4 Inductively coupled RF plasma processing reactor

from the pinholes in the high-frequency voltage electrode and impinges on the substrate. Plastic substrates can be treated using this method. In the ICP reactor CVD apparatus, as shown in Fig. 5.4, the substrate is processed by the plasma inside the coil. The plasma CVD (P-CVD) system consists of a gas supply system (gas cylinders, pressure reducer valve, flow stop valves, purge lines, and mass flow control valves), plasma generation system (plasma reactor, vacuum chamber, and power supply), vacuum pumping system, and emission gas processing system. Figures 5.3 and 5.4 show the schematics of simple laboratory-scale P-CVD systems. Moreover,

5.4 Nonthermal Plasma Technology for Surface Modification

133

the RF plasma and corona discharge are used for etching and cleaning substrates, respectively.

5.4 Nonthermal Plasma Technology for Surface Modification A laboratory-scale atmospheric-pressure plasma reactor was constructed using a nanosecond pulse corona to demonstrate potential applications ranging from modifying the surface energy to removing surface organic films [4, 5]. For surface modification studies, bare aluminum, polyurethane, and photoresist-coated silicon substrates were selected to evaluate the surface energies. The critical surface energies of all the plasma-treated materials increased significantly. The effects of the gas composition and plasma treatment time were also investigated. The photoresist, ethylene glycol, and microsurfactant were used as the tested organic films. On the silicon substrate, the photoresist coating was etched at 9 nm/min, indicating that atmospheric-pressure plasma technology can be used to remove organic films. The apparatus and treatment procedure are as follows. A laboratory-scale atmospheric-pressure pulse corona plasma reactor is designed and constructed for surface modification and organic film removal. The plasma reactor comprises a wire in the cylindrical section and a bottom section designed to hold the sample. A centered discharge wire is attached to a pulse high-voltage source, and the stainless steel cylinder is grounded, as shown in Fig. 5.5. The wire-to-cylinder distance is 11.5 mm, and the effective tube length is 127 mm. The discharge wire is 0.2 mm in diameter and 100 mm long. A grounded stainless steel disk is placed downstream of the plasma reactor to mount the substrate to be treated or cleaned. The distance between the electrode wire and substrate can be adjusted by moving the disk stem. The gas flow rate is calibrated using a soap bubble flow meter and maintained at a total flow rate of 500 mL/min using 10% argon and 90% dry air or oxygen by volume. This corresponds to a flow velocity of 1.87 cm/s and a residence time of 3.1 s in the plasma reactor. The pulse repetition rate is maintained at 400 Hz, and the peak voltage is approximately 30 kV. The plasma reactor can be operated in either direct or remote plasma modes. In the remote plasma mode, the high-voltage discharge electrode is raised approximately 20 mm from the bottom of the grounded cylinder such that the electric field terminates within the reactor zone, resulting in no electron or ion bombardment on the substrate. Conversely, in the direct plasma mode, the wire electrode is lowered to the same level as or below the bottom of the grounded cylinder. The electric field from the dischargewire tip is terminated on the substrate surface so that electrons, ions, and radicals can bombard the substrate surface. The pulse corona reactor employs a positive DC power supply that is altered to produce a short pulse with a very rapid rise time (approximately 20 ns) through a rotating spark gap. The advantage of this arrangement is that a sharp-rise pulse corona

134 Fig. 5.5 Pulse nonthermal plasma reactor for surface treatment [4, 5]

5 Hydrophobic Treatment for Polymer Surfaces

Pulsed-corona power supply

produces a streamer corona, which generates radical species and free electrons, while producing a limited number of ions. The experimental results are listed in Table 5.1, which shows the gravimetric data obtained for the aluminum and polycarbonate samples before and after the plasma treatment. The first, second, and third columns show the weights before and after the coating and after the remote plasma exposure, respectively. The weight changes between each step are shown in parentheses. Polycarbonate materials can easily absorb water, thereby gaining weight. According to the gravimetric data listed in Table 5.1, organic film removal using the atmospheric-pressure plasma treatment appears to be marginally feasible. Direct plasma treatment appears to be more effective than remote plasma treatment for removing organic films, suggesting that the electron-energy and/or high-energy radicals are more important than lower-energy radicals for removing organic films. Three major conclusions are drawn from this work. First, on silicon substrates, photoresists can be etched using dry plasma. Second, organic film removal by atmospheric-pressure plasma treatment appears to be feasible based on gravimetric

5.5 Reaction Between Plasma and Polymer

135

Table 5.1 Gravimetric data obtained for aluminum and polycarbonate samples before and after plasma treatment [4, 5] Before coating After coating with micro-surfactant Plasma-treated Aluminum 1.19638 g

1.20593 g (+9.55 mg)

1.20509 g (−0.84 mg)

Remote plasma 1.21387 g

1.22300 g (+9.13 mg)

1.22288 g (−0.12 mg)

1.20400 g

–

1.20400 g

Direct plasma

0.64887 g

0.64794 g (−0.93 mg)

0.64586 g (−2.08 mg)

Control

0.64148 g

0.64151 g (+0.03 mg)

0.64155 g (+0.04 mg)

Direct plasma Control Polycarbonate

data. This seems clear even though the organic film removal rate could not be chemically analyzed using electron spectroscopy because of the difficulty in depositing a thin organic film on the substrate. Third, the direct plasma method appears to be more effective than the remote one for removing organic films.

5.5 Reaction Between Plasma and Polymer The reaction between the plasma and polymer can be classified as follows [1]: (1) Surface reactions: Functional groups and crosslinks are generated on the surface by the reactions between the vapor phase and surface species and between the surface species. These reactions occur, for example, during the plasma treatments with argon, ammonia, carbon monoxide, carbon dioxide, fluorine, hydrogen, nitrogen, nitrogen dioxide, oxygen, and water. (2) Plasma polymerization: The formation of a thin film on the polymer surface through the polymerization of organic monomers such as CH4 , C2 H6 , C2 F4 , and C3 F6 in plasma is called “plasma polymerization,” which involves reactions between gaseous, gaseous and surface, and surface species. (3) Cleaning and etching: The material is removed from the polymer surface by chemical reactions and physical etching of the surface to form a volatile product. Oxygen-containing plasmas are used to remove organic contaminants such as oligomers, antioxidants, anti-blocks, or mold-release agents from the polymer surface. Etching differs from cleaning because it removes the material from the surface. Oxygen- and fluorocarbon-containing plasmas are often used to etch polymers. A characteristic of fluorocarbon-containing plasmas is that surface reactions, etching, and plasma polymerization can occur simultaneously. The predominant reaction depends on the gas supply, operating parameters, and chemistry of the polymer substrate and electrodes.

136

5 Hydrophobic Treatment for Polymer Surfaces

Fig. 5.6 Principle of polymer surface treatment with fluorocarbon-containing plasma

Figure 5.6 shows the principle of the polymer surface treatment with the CF4 plasma. Cm Fn radicals play important roles as polymerization accelerators, silicon dioxide etchants, and recombination species during anisotropic etching. Halogen atoms, especially fluorine and chlorine, are the major etchants (i.e., nonpolymerizable etching species) for various materials. The ions and electrons can affect plasma–substrate interactions, and the impacts on these surfaces change the lattice surface bonds. Fluorine can be introduced to the surface by plasma polymerization. Owing to the strong bonding force between the carbon and fluorine atoms, the use of CF4 for treating the surface with lower electron-energy atmospheric-pressure plasma is relatively difficult. Efforts are ongoing to develop practical applications. Figure 5.7 shows an example of generating a hydrophobic polyimide surface using low-pressure CHF3 plasma [2]. The figure shows atomic force microscopy images of nanopatterning on a polyimide film surface and a photograph of the corresponding water droplet contact angle. As shown in Fig. 5.7a, the contact angle is 65° before the plasma treatment. However, in Fig. 5.7b, the surface became hydrophilic with a contact angle of ≤ 5° after the argon plasma etching. Next, after the CHF3 plasma treatment, as shown in Fig. 5.7c, the surface became superhydrophobic with a contact angle of 156°. The hydrophobic effect of the plasma surface treatment can be controlled by changing the irradiated plasma gas. A prototype microboard was developed for cell growth and protein manipulation by applying this process [2]. The process for manufacturing thin hydrophilic metal wires on hydrophobic polyimide film surfaces is important, and its outline is shown in Fig. 5.8. Procedures (a)–(e) are as follows: (a) hydrophilic nanoprotrusions are formed by plasma etching on the polyimide surface, (b) which is hydrophobized with CHF3 plasma. (c) The hydrophobic surface is laser ablated to generate grooves. (d) The polyimide is immersed in a dispersed silver particle solution to deposit the silver particles in the hydrophilic grooves. (e) The polyimide film is placed in a furnace, and the water in the solution is evaporated to prepare a hydrophilic metal-based fine-line pattern on the surface and obtain the substrate.

5.5 Reaction Between Plasma and Polymer

137

Fig. 5.7 Atomic force microscopy (AFM) images of nanopatterning using the plasma process and water contact angle [2]. (Root mean square (Rms) means surface roughness value.) a Before the plasma process. b After the plasma etching process. c After the CHF3 plasma treatment

Fig. 5.8 Schematic representation of fabrication processing [2]. a Nanosized bumps are formed by plasma etching on the polyimide surface. b CHF3 plasma treatment is performed to make the hydrophobic surface. c The laser ablation is performed on the hydrophobic surface to make a groove. d The polyimide is immersed in the silver diffused solution. e The polyimide is placed in a furnace to evaporate the water in the solution

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Micro unevenness

Film

(a) Before process

(b) After process

Fig. 5.9 Schematic of laser processing to form microprotrusions

5.6 Surface Hydrophobicity by Laser Microfabrication A superhydrophobic surface can be prepared by forming a fine structure on the material surface by laser processing, as illustrated in the schematic in Fig. 5.9. A laser is used to introduce a high-pressure plasma on the material surface and generate irregularities [6]. In a previous study, nano-/micromechanical microstructures were developed by irradiating a metal surface with femtosecond laser pulses. This process can also be applied to plastic surfaces. The multifunctional surface exhibits significantly enhanced broadband absorption, superhydrophobicity, and self-cleaning. The superhydrophobicity repelled dust from the formed surface structure, while conserving 30% of the droplet kinetic energy. This self-cleaning effect is confirmed by falling water droplets removing considerable dust from the surface. Multifunctional surfaces are useful for collecting light and repelling water and dust. Furthermore, this surface reportedly exhibits other highly desirable properties such as corrosion resistance, anti-icing properties, antibiotic adhesion, and self-hygiene.

5.7 Diamond-Like Carbon-Based Plasma Surface Treatment DLC can functionalize, hydrophilize, or hydrophobize polymer and metal surfaces. Figure 5.10 shows an example of a plasma device used for coating DLC on the inner surface of a PET bottle exhibiting a low oxygen permeability (0.001 mL/day per container) compared to that of a glass bottle [7]. DLC is a representative surface treatment that uses vacuum plasmas and is challenging to handle at atmospheric pressure because the raw material gas may explode. According to the results of a previous study, the film forms as follows: (1) An untreated PET bottle is stored in a usual PET bottle-shaped metallic chamber. (2) The internal pressure is reduced to approximately 0.1 Torr (= 0.13 kPa).

5.8 Plasma-Treated Catalyst Surfaces

139

Material gas

Vacuum exhaust Dielectric material Electrode

High-frequency power supply PET bottle

Gas tube

Fig. 5.10 Schematic of polyethylene terephthalate bottle, in which inner surface is coated with diamond-like carbon using low-pressure plasma (drawn based on Ref. [7])

(3) Hydrocarbon gas (acetylene, etc.) is supplied from a gas introduction pipe. (4) When high-frequency (13.56-MHz) power is applied to the chamber, the hydrocarbon gas becomes plasma. The ions and radicals collide with the PET bottle’s inner surface. (5) Consequently, a uniform high-gas-barrier DLC film is formed on the PET bottle’s inner surface. This PET bottle has been commercialized and marketed as an alternative wine-storage container to glass bottles that are widely used. Additionally, Teflon™-based medical parts (endoscopes, dialysis machine tubes, etc.) have conventionally been joined by caulking or crimping. Several adhesive joining technologies have been developed. Moreover, commercial DLC surfacetreated coronary stents and balloon catheters exhibiting both high biocompatibility and safety have been developed [8], and various functional groups can be introduced by irradiating DLC-coated substrates with ammonia and oxygen plasmas.

5.8 Plasma-Treated Catalyst Surfaces Plasma-based surface modification treatments have also been used to prepare catalysts. To modify catalyst surfaces, argon, oxygen, carbon dioxide, or a mixed gas is passed through a quartz tube, as shown in Fig. 5.11. The system uses a microwave (~GHz, 1-kW) power supply and waveguide, and the plasma flows at low pressure in ranges of 10–20 Torr (1830–1670 Pa) and 2–4 standard liters per minute (SLM) [9]. Ni(NO3 )2 -containing TiO2 pellets (approximately 6 mm in diameter) enclosed in a container downstream are treated with an argon plasma stream to generate a NiO/ TiO2 catalyst by decomposing the Ni(NO3 )2 solution. Because the argon plasmatreated catalyst surfaces can be used for the plasma reduction of carbon dioxide to

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Quartz glass tube

Ar, O2, CO2 Oil

Wave guide for plasma generation

Plasma flow Catalyst pellet

TiO2 + Ni(NO3)2 → NiO / TiO2 Fig. 5.11 Schematic of equipment used for treating catalyst surfaces with low-pressure microwave plasma (drawn based on Ref. [9])

carbon monoxide, the processing energy efficiency is approximately double that of only plasma processing.

5.9 Trends in Other Plasma-Based Surface Treatments Although atmospheric-pressure plasma processing is suitable for mass production, choosing an appropriate technique for applying high voltage between electrodes (electrostatic engineering) is important. Recently, electrospinning has attracted considerable research attention for application to electrostatic engineering wherein a high voltage is applied to a polymer solution in the spinning nozzle to form severalnanometer-diameter nanofibers and impart hydrophilicity or hydrophobicity to material surfaces. Furthermore, electrospraying, which is widely applied for atomization and nanotechnology, has also been reexamined [10]. The plasma treatment research trends, including research in areas other than hydrophobic surface treatments, are as follows. Surface treatments are also widely used for bonding automobile parts. Recent environmental standards have required vehicle body weights to be reduced to improve automobile fuel efficiencies. To satisfy these requirements, several automobile manufacturers are currently developing various methods, including automobile part resinification. However, this has stringent quality standards centered on the application of engineering plastics, which exhibit superior heat and chemical resistance compared to those of other plastics [11]. Plastics and metals, which are the main automobile part components, are joined using adhesives, and plasma surface treatments are important pretreatment methods.

References

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Sports shoes are one of the high value-added products that are manually produced by adhering certain polymers and rubber materials using adhesives; however, some materials cannot be joined using adhesives or ultraviolet treatment. Although pristine ethylene–vinyl acetate is a typical poor adhesive, plasma-treated ethylene–vinyl acetate can achieve high-strength adhesion. Moreover, plasma irradiation enables the pasteurization of living organisms and proteins [12] and effectively treats cancers such as melanomas in the human body [13].

5.10 Conclusions In this chapter, methods for introducing a fluorocarbon-based gas to a cyclone plasma and hydrophobic fluorine to a material surface are explained as plasma hybrid surface treatment examples. The water repellency/hydrophobicity can be controlled by switching the gas type. Next, hydrophobic surfaces using laser microfabrication are introduced. Plasma is generated using a laser, and the hydrophobicity becomes apparent by the formation of surface irregularities. Furthermore, as application examples, the principles and implementation of DLC on the inner surface of a PET bottle, DLC treatment of medical stents, and catalyst surface treatment are explained. In the future, plasma hybrid surface treatment will be applied to not only polymers but also various materials such as glass, metals, and ceramics. With increasing applications, plasma hybrid surface treatments will become more widespread.

References 1. C.-M. Chan, T.-M. Ko, H. Hiraoka, Polymer surface modification by plasmas and photons. Surf. Sci. Rep. 24(1–2), 1–54 (1996) 2. M. Kim, J. Noh, Fabrication of a hydrophilic line on a hydrophobic surface by laser ablation processing. Micromachines 9, 5 (2018). https://doi.org/10.3390/mi9050208 3. T. Hirao, T. Yoshida, S. Hayakawa, Hakumaku gijyutsu no shin choryu (English translated title: New Trend in thin film manufacturing technologies). Kogyo Chosakai Publishing Co. LTD. (1997). (in Japanese) 4. T. Yamamoto, M. Okubo, Advanced physicochemical treatment technologies, in Handbook of Environmental Engineering, ed. by L.K. Wang, Y.-T. Hung, N.K. Shammas, vol. 5, Chapter 4 (The Humana Press Inc., Springer, 2007), Nonthermal Plasma Technology, pp. 135–294 5. T. Yamamoto, J.R. Newsome, D.S. Ensor, Modification of surface energy, dry etching, and organic film removal using atmospheric-pressure pulsed corona plasma. IEEE Trans. Ind. Appl. 31(3), 494–499 (1995) 6. A.Y. Vorobyev, C. Guo, Multifunctional surfaces produced by femtosecond laser pulses. J. Appl. Phys. 117, 033103 (2015) 7. T. Kage, Garasu bin namino DLC coating PET bottle (English translated title: DLC-coated PET bottles comparable to glass bottles). Soc. Packag. Sci. Technol. Jap 19(6), 493–502 (2009). (in Japanese) 8. T. Nakatani, H. Takeuchi, A. Wada, S. Yamashita, Investigation of anti-corrosive performance of a Si-doped DLC-coated magnesium alloy stent deposited by RF-plasma CVD. J. Photopolym. Sci. Technol. 32(3), 511–517 (2019)

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9. G. Chena, V. Georgieva, T. Godfroid, R. Snyders, M.-P. Delplancke-Ogletree, Plasma assisted catalytic decomposition of CO2 . Appl. Catal. B 190, 115–124 (2016) 10. A. Jaworek, A.T. Sobczyk, Electrospraying route to nanotechnology: An overview. J. Electrostat. 66(3–4), 197–219 (2008) 11. T. Aoki, Adhesion technology of engineering plastic contributing to lightweighting of automobile. Nippon Gomu Kyokaishi 95(17), 262–267 (2017). (in Japanese) 12. T. Sato, K. Okazaki, T. Nakajima, S. Fujimura, T. Nakatani, Development of compact plasma sterilization device for contact lenses. Plasma Med. 11(1), 57–67 (2021) 13. G. Fridman, A. Shereshevsky, M.M. Jost, A.D. Brooks, A. Fridman, A. Gutsol, V. Vasilets, G. Friedman, Floating electrode dielectric barrier discharge plasma in air promoting apoptotic behavior in melanoma skin cancer cell lines. Plasma Chem. Plasma Process. 27, 163–176 (2007)

Chapter 6

Hydrophobic Treatments for Plastic, Glass, and Metal Surfaces and Their Applications

6.1 Introduction The irradiation of solid material surfaces with plasma is one of the most effective surface treatment techniques used for surface washing and cleaning, which has been used in various industries. Examples include surface cleaning in semiconductor processes, microchip manufacturing, cleaning optical components, packaging, printing, medical applications, and the adhesive bonding processes used for structures such as automobiles. However, the basic principles of cleaning have not necessarily been clarified in all these examples, and the principles of cleaning and surface modification may differ depending on the type of plasma process and target material (metal, plastics, ceramic, etc.) used. This complicates the problem. In general, there is a common opinion in industries that it is difficult to adopt technology for products if its basic principles are not fully understood. Therefore, they are often applied to evaluate the effectiveness of a certain effect. However, if the principles can be clarified, the range of applications of the technology will expand, and significant research efforts can be undertaken toward its future use. In particular, in the context of surface treatment, it is important to identify the radicals, bonds, and functional groups generated on the treated surface. Washing or cleaning using plasma activates the surface, and a water-repellent/ hydrophobic treatment can be sustained using a hybrid treatment process, which is performed immediately after the cleaning step. In particular, research on the surface modification of materials using atmospheric-pressure nonthermal low-temperature plasma has been widely conducted in recent years for many industrial purposes such as improving the paintability and adhesion of materials and surface cleaning. This chapter first describes the definition, classification, and characteristics of plasmas and subsequently explains a method for estimating the specific degree of ionization in plasma. The features and applications of plasma as a cleaning method have been described. Plasma cleaning is roughly divided into low- and atmosphericpressure plasma treatments. However, there is no difference between the two; the method is to apply the active gas species generated by the plasma onto the surface © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 M. Okubo, Nonthermal Plasma Surface Modification of Materials, https://doi.org/10.1007/978-981-99-4506-1_6

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of the material. First, the definition and characteristics of the plasma common to low- and atmospheric-pressure plasmas used for cleaning are described. Next, the principles of low- and atmospheric-pressure plasma cleaning are explained along with practical examples. Modification of the surfaces of glass and aluminum using APNPT (hydrophilic and hydrophobic approaches) [1, 2] is explained last.

6.2 Definition of Plasma and Its Characteristics 6.2.1 Definition of Plasma All substances change from solid to liquid and from liquid to gas when energy or heat is supplied. This is referred to as a phase change and occurs at a constant temperature. When energy is added to a gas, electrons emerge from the neutral gas particles and become ions. The state in which many ions and electrons are intermingled is known as “plasma,” as already explained in Chap. 1. The change from gas to plasma is based on an ionization reaction. The energy required for the reaction is in the range of 1–50 eV, which is generally much higher than the latent heat energy required for a phase change (0.01 eV). Therefore, the change from gas to plasma is not strictly classified as a phase change. Plasma is often called the fourth state, whereas solid, liquid, and gas are the other states of a substance. The characteristics of plasma will be explained in the next section.

6.2.2 Plasma Parameters Physical quantities such as the density and temperature of plasma are recognized as key parameters used to define its characteristics. Several types of densities and temperatures exist in plasma: electron number density ne , ion number density ni , gas molecule number density ng , electron temperature T e , ion temperature T i , and gas temperature T g . According to the definition of plasma, ne is usually equal to ni . However, T e is often higher than T i and T g , whereas T i is nearly equal to T g . Therefore, this state is called “nonequilibrium plasma.” In general, when T g is much lower than the combustion temperature (T c ~ 1000 °C), the state is called “nonthermal plasma.” Because the temperature of plasma is defined when the energy of each particle follows the Maxwell distribution, the energy distribution function of the particles in plasma is also important for characterizing plasma. Furthermore, the Debye length, the angular frequency of the plasma, the thickness of the ion sheath, and the space potential of the plasma are important parameters used to describe the state of plasma.

6.2 Definition of Plasma and Its Characteristics

145

6.2.3 Thermal Equilibrium and Nonequilibrium Plasma Plasma in a fully thermal equilibrium state satisfies the following five conditions: (a) The energy distribution of the particles follows the Maxwell–Boltzmann distribution. The average value of the energy distribution is defined as the plasma temperature. (b) The temperature of each particle is equivalent to the plasma temperature. (c) The excitation state is close to the Boltzmann distribution. (d) The particle number density becomes the reaction balance composition. (e) Electromagnetic waves are considered black-body radiation. With the exception of condition (e), it is relatively easy to realize conditions (a)–(d) in an arc discharge at atmospheric pressure. The state where conditions (a)– (d) are satisfied is called the local thermodynamic equilibrium (LTE). In general, physical properties such as the electrical conductivity and specific heat of plasma cannot be defined when the plasma is in a thermally nonequilibrium state. When the pressure is low, the electric field is strong, and three-body recombination is active. If an interface exists between the plasma and the solid, conditions (a)–(d) are often not satisfied, and the plasma is in a nonequilibrium state. In particular, it is called a two-temperature nonequilibrium plasma when the electron temperature is different from the temperature of the heavy particles. Plasma can be roughly divided into three types: high-temperature, low-pressure, and atmospheric-pressure plasma. A high-temperature plasma is a plasma in a hightemperature state of > 10,000 °C, and the heavy particle temperature is almost equal to the electron temperature. Examples include atmospheric-pressure arc discharge and the nuclear fusion plasma used for welding metals. A low-pressure plasma is generated by discharge at pressures of tens to hundreds Pa. Examples include plasma chemical vapor deposition (CVD), sputtering plasma, and process plasma. An atmospheric-pressure low-temperature plasma is generated by accelerating only electrons with a smaller mass and high mobility in the presence of a strong electric field upon applying a high voltage between electrodes at atmospheric pressure. The energy is typically in the range of 1–10 eV, which is much higher than the heavy particle temperature near room temperature. It can be used for various purposes such as environmental plasma, surface treatment, wastewater treatment, and medical applications. High-temperature plasma is also known as thermal plasma, and low-temperature plasma is called nonthermal plasma. Figure 6.1 shows that the electron temperature T e of low-pressure plasma can be higher than the gas temperature T g [3]. When electrons are accelerated in the presence of an electric field, they collide less frequently with heavy particles such as gas molecules and ions and are more likely to be accelerated. As a result, T e becomes higher. However, a low-pressure plasma has a relatively low electron density, and low-pressure conditions hinder the execution of the process. These advantages make it suitable for processing.

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Fig. 6.1 Relationship among electron temperature T e of a discharge plasma, gas temperature T g , and pressure (1 Torr = 133 Pa, identical plasma current conditions) [3]

6.2.4 Method to Evaluate Ionization Degree in Plasma A simple calculation method for the electron number density, which is important in atmospheric-pressure plasma experiments, is shown below [4]. If the number density of neutral particles in the gas before ionization is n0 (particles/m3 ), the number equal to Avogadro’s number = 6.022413 × 1023 in a volume of 22.4 L = 0.0224 m3 at 0 °C = 273.15 K at atmospheric pressure and the absolute temperature T (K), n0 can be calculated as follows: n0 =

6.022413 × 1023 273.15 . (particles/m3 ) 0.0224 T

(6.1)

On the other hand, the electron number density ne in plasma can be expressed using the following equation based on Saha’s equation: ( eε ) nenn (2π m e kT )3/2 2gi i = Ki = exp − ni h3 g0 kT

(6.2)

where nn is the number density of neutral particles after ionization, ni is the number density of ions, me is the electron mass (= 9.11 × 10–31 kg), k is the Boltzmann constant (= 1.38 × 10–23 J/K), T is the absolute temperature of the plasma (units: K), h is the Planck constant (= 6.63 × 10–34 m2 kg/s), gi is the ground-state statistical weight of ions, g0 is the ground-state statistical weight of neutral species, e is the electron charge (= 1.60217657 × 10–19 C), and εi is the ionization potential of the gas species (units: eV). From the definition of plasmas, ne = ni , and by solving the quadratic equation for ne obtained by substituting nn = n0 – ne in Eq. (6.2), ne can be obtained as expressed in the following equation:

6.3 Principles of Plasma Cleaning and Surface Activation Methods

ne =

/ −K i + K i 1 + 4 nK0i 2

147

(6.3)

For example, in the case of argon at atmospheric pressure and a temperature of 10,000 K, gi = 6, g0 = 1, εi = 15.76 eV, ne = 1.54 × 1022 particles/m3 , and the degree of ionization is α = ne /n0 × 100 = 2.1%. At 15,000 K, almost half of the gas molecules are ionized. By measuring the plasma temperature using spectroscopy, ne or the degree of ionization can be estimated using the above-mentioned technique. It should be noted that these values are obtained under various assumptions and are not exact values, but it is effective and important to conduct plasma treatment under conditions that maximize the degree of ionization. The principles of plasma cleaning and surface activation methods are described in the next section.

6.3 Principles of Plasma Cleaning and Surface Activation Methods 6.3.1 Overview of Plasma Cleaning A solid surface usually consists of multiple layers of contaminants, which have complex chemical and physical structures. The purpose of surface treatments is to remove this stratified surface contamination using reactive particles (ions, radicals, electrons, etc.) formed by plasmas. This layer of fouling is sometimes more than 1 µm thick and can be classified as either naturally occurring or anthropogenic. Examples of the former include those caused by organic matter, hydrocarbons, particles, and atmospheric moisture. Examples of the latter include those caused by lubricating oil, film, and excess grease after wet cleaning [5]. Surface cleaning is used as a pretreatment step for surface coating. Wet cleaning/ drying with an acid or alkaline solution is frequently performed prior to coating or painting; however, performing this step with plasmas is effective. On the other hand, plasma treatments may also be performed as a post-treatment step to complete the wet cleaning process. The wet cleaning process is generally used when the contamination layer is thick (~10 µm); however, challenges such as the need for a waste liquid treatment may be encountered. Plasma treatments are an environmentally friendly, dry, and nearly wasteless process. The rate of cleaning the surface by plasma etching is usually ≤ 0.5 mm/h, which is relatively slow and mainly suitable as a super-finishing technique. Based on the mechanism, there are two types of cleaning technologies: physical and chemical cleaning (surface modification) [6]. Although the two overlap and cannot be clearly distinguished, physical cleaning mainly refers to cleaning solids via mechanical action, which includes laser, sandblasting, ion etching, and compressed air blowing. This refers to the process of scraping and removing surface contamination through polishing. On the other hand, chemical cleaning refers to the process of

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removing impurities and dirt from the surface mainly via chemical action. This is a decomposition or dissolution method. Plasma cleaning, which is the subject of this section, has both physical and chemical cleaning properties. In the initial stages of plasma cleaning, “scraping” surface contamination and impurities takes place because of the collision of the high-energy particles (fast ions, neutral particles, radicals, or fast electrons) formed by the plasma with the surface. Targeted cleaning is recognized as the primary plasma cleaning action. Recently, the main feature of plasma treatment has been chemical cleaning, which uses the high reactivity of the plasma to induce a chemical reaction on the surface and decompose dirt components such as oil, moisture, and organic matter.

6.3.2 Example of Electrode Systems for Reduced-Pressure Plasma Treatment The technology of plasma cleaning, especially that of utilizing reduced- or lowpressure plasmas, is now almost established. Going back in history, Strong (1935) [7] reported that it could be used to clean the lens of an astronomical telescope before applying an aluminum coating to improve its adhesion properties. Through various improvements, it has become an indispensable technology for modern semiconductor chip manufacturing processes. Plasma surface treatment methods include heating (baking) and wet cleaning, and plasma cleaning methods include sputtering and etching. Etching is a process that removes the surface film with ions and radicals heated to tens of thousands of degrees using reactive plasmas [5, 8]. Etching is performed using fluorine gas (F2 ) and carbon tetrafluoride (CF4 ). An example of a cleaning device using a lowpressure plasma device has been reported [5]. In addition, it is possible to perform atmospheric-pressure surface treatment through the development of devices using easily ionized gases such as He and Ar. These noble gases have a particularly high metastable excited state when compared to other gases such as air. Therefore, they have a low ionization voltage and are easily ionized [9]. Figure 6.2a, b shows schematics of the typical systems used for generating lowpressure plasmas, called a capacitively coupled plasma (CCP) system. A DC voltage is applied between the electrodes, as shown in Fig. 6.2a, or plasma is formed using an RF (~MHz) power supply, as shown in Fig. 6.2b. The target, known as the substrate, is placed between the electrodes, and the surface is then cleaned. On the other hand, as shown in Fig. 6.3, the surface of the target can be cleaned by winding a coil several times around the outside of a container such as a quartz tube and forming a plasma using an RF power source. This device is called an inductively coupled plasma (ICP) device. Furthermore, a highly efficient plasma can be formed using microwave (MW, up to GHz) instead of radio waves, as shown in Fig. 6.4. The microwave plasma power

6.3 Principles of Plasma Cleaning and Surface Activation Methods

149

Direct-current plasma power supply Electrode

Plasma Gas Vacuum pump Substrate Exhaust

(a) Direct current plasma drive Radio-frequency plasma power supply Electrodes

Plasma Gas Vacuum pump Substrate Exhaust

(b) Radio-frequency plasma power drive Fig. 6.2 Substrate cleaning using a capacitively coupled plasma apparatus: a direct current and b RF plasma power drives

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Coil electrode

Radio-frequency plasma power supply

Plasma Gas Vacuum pump Substrate Exhaust Fig. 6.3 Substrate cleaning using an inductivity coupled plasma

supply transmits energy through a waveguide to the reactor, forming a plasma in the chamber and cleaning the surface of the target. In these systems, the target object is placed directly into the plasma formation space, and the surface of the object is cleaned. However, owing to the instability of plasma generation at the time of plasma formation, the formed radicals and ions are used “remotely” during the material surface treatment process when the plasma formation region and target object are separated. This is termed as remote plasma processing. Several devices that use an ICP, MW, and ozone processing have been reported, which will be described later. In an ICP plasma [10] cleaning system as well as a CCP system, a matching circuit is installed between the power supply and the coil to apply the maximum energy to the coil with a high frequency to form a plasma. As an example, it consists of two variable capacitors, C 1 and C 2 , as shown in Fig. 6.5. C 1 is semifixed, and C 2 can be adjusted for matching. The output impedance of the power supply is typically designed to be 50 Ω. The induction coil has a high impedance of several hundred ohms or more when no plasma is generated, but it drops sharply when the plasma is formed. Considering the problem of matching the load of Z = 16 + i30 (i is an imaginary unit) to the power supply of Z c = 50 Ω at f = 13.56 MHz, C 1 = 1.68 nF and C 2 = 510 pF are determined as the matched values using a Smith chart. A normal variable capacitor is used for C 1 , and a vacuum variable capacitor is usually used for C 2 , and they are electrically changed by servo-mechanisms for automatic

6.3 Principles of Plasma Cleaning and Surface Activation Methods Fig. 6.4 Substrate cleaning using a microwave plasma

151

Microwave plasma power supply

Wave guide tube

Plasma Gas Vacuum pump Substrate Exhaust

Fig. 6.5 Matching circuit with an ICP apparatus [10]

Matching circuit

C1 C2

Coil

matching. In other words, plasma irradiation is normally possible by simply turning the switch on.

6.3.3 Cleaning Using Atmospheric-Pressure Plasmas “Corona discharge treatment” and “plasma treatment” using electric discharges are effective methods to create a hydrophilic surface on the target material to improve its adhesion or to develop a water-repellent film to improve its water repellency. Strictly speaking, it is difficult to compare a “corona discharge treatment” and a “plasma treatment” because the plasma represents an ionized gas and the corona discharge is a phenomenon or means. Although plasmas can be formed by methods other

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than discharges (heating, laser, combustion, etc.), discharges are the most powerful and easiest means. Atmospheric-pressure plasma cleaning, in which the surface of the material is cleaned by irradiating it with atmospheric-pressure plasmas, can be easily conducted in the ambient atmosphere. Therefore, it is currently being studied to incorporate it into this process. Atmospheric-pressure plasmas can also be formed using air, oxygen, or nitrogen without the need to use noble gases. The electron temperature is lower than that of the low-pressure plasma, but the electron number density may be higher. Methods for generating atmospheric-pressure low-temperature plasmas include a pulse corona, silent discharge (dielectric barrier), a dielectric filling layer (packed bed), and creeping or surface discharge methods. The methods often used for surface treatment and cleaning are introduced below. Figure 3.1 in Chap. 3 shows the electrode system of two typical film surface treatments using plasmas and cleaning equipment. Figure 3.1a shows a barrier discharge treatment system in which a dielectric cover is used as a barrier for at least one pair of electrodes. A plasma is formed, through which the roll-fed film passes, and the surface is treated and cleaned. The film passes near one electrode or near the center of the electrodes, and both sides of the film are then processed. Figure 3.1b shows that a plasma jet is formed by applying an unsteady voltage such as a pulse between the sharp electrodes through which the noble gas passes, and the film transported by the roll passes through the ejection destination of the jet. Consequently, the surface is treated. In this case, only the surface impinged by the jet is treated. In the next section, the effects of the plasma cleaning are described.

6.3.4 Effects of Atmospheric-Pressure Plasma Cleaning By applying plasma to the surfaces of plastics, polymers, glass, metals, and paper in the treatment system shown in Fig. 6.6, the surfaces of the materials are cleaned and, in many cases, markedly hydrophilized, and their adhesive properties are improved [11]. The principles of cleaning, physical cleaning, and chemical cleaning, as described earlier, are conceivable. Specifically, the mechanism can be described as follows. The first effect is that the surface is scraped and cleaned via plasma etching, and at the same time, the unevenness increases, as shown in Fig. 6.6a. However, in many cases of atmospheric-pressure plasmas, the improvement in adhesion based on this effect is small, and the following two types of effects are significant. The second effect is that of the action of high-energy electrons (~1–10 eV) generated by the atmospheric-pressure plasma discharge on the bonding of the material surface (in the case of metals, the oxide layer on the surface) or oil film, which results in dissociation into radicals, as shown in Fig. 6.6b. On the other hand, molecules of atmospheric gases such as oxygen, nitrogen, and moisture are also dissociated, forming functional groups such as hydroxyl, oxygen, and nitrogen radicals, which adhere to the surface. Furthermore, if hydrocarbons, such as acrylic acid, are mixed into the plasma, recombination reactions between the radicals in the gas and on the

6.3 Principles of Plasma Cleaning and Surface Activation Methods

153

Plasma irradiation

Roughening

Material surface

Material surface

(a)

Plasma irradiation Hydrophilication

Material surface

Material surface

(b)

Plasma irradiation

Decomposition of organic substances

Organic matter Material surface

Material surface

(c) Fig. 6.6 Principles of substrate cleaning with an atmospheric-pressure plasma. a Roughening, b surface qualification by functional groups, and c removal of organic matter [11]

surface generate carbonyl (> C = O) and carboxyl (−COOH) groups on the target material surface. A large number of hydrophilic functional groups are modified and attached. This process is known as plasma combined processing (hybrid processing). For these reasons, the surface becomes hydrophilic, the free energy of the surface increases, and adhesion and joining with other materials becomes easy. The third effect is that the electrons, radicals, and ions formed by the plasma decompose organic substances such as oily residue and fats, which is often present as a fouling layer on surfaces, producing carbon dioxide and carbon monoxide, as

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shown in Fig. 6.6c. They are then converted to carbon gas and water vapor, which are subsequently removed. Consequently, the surface is cleaned and made hydrophilic. As described above, cleaning using plasmas integrates the etching effect, i.e., the effect of roughening the surface, modification by surface functional groups, and decomposition of oil, dirt, and water. At this time, it is difficult to analyze the radicals and bonds generated on the surface, making it difficult to analyze the detailed mechanism of the plasma cleaning process. It should be noted that coating the material surface after plasma cleaning is effective in improving the adhesion of a waterrepellent film. An example of plasma treatment (cleaning and hydrophilization) is described in the next section.

6.3.5 Remote Plasma Cleaning and Cleaning by Ozone A plasma in which the plasma source or electrode is distant from the cleaning target is called a remote plasma. Therefore, cleaning in which ozone (O3 ) formed by the plasma acts on the surface can be considered a type of remote plasma cleaning. This section describes the surface treatment and cleaning process using ozone. First, the principles of ozone generation are explained. Ozone has a strong oxidation power next to fluorine (F). Various applications have been developed utilizing the chemical activity of ozone. Surface treatment and cleaning are some of these methods. Discharge plasmas, electrolysis, and ultraviolet (UV) rays have been typically used as methods for generating ozone. The method using discharge plasmas has the highest generation energy efficiency (generation power efficiency), and the generation energy efficiency is approximately 100 g/kWh when ozone is generated using dry oxygen as the raw material. Among these discharge methods, silent discharge (dielectricbarrier discharge) is typically used. When air is used as the atmospheric gas instead of oxygen, the energy efficiency drops to ~ 50 g/kWh. This can be attributed to the large amount of energy consumed for the vibrational excitation of nitrogen molecules. A process that combines multiple processes such as UV light and ozone is called an advanced oxidation process (AOP). UV/O3 cleaning is a representative method used for the surface treatment of materials in an AOP. UV radiation is an electromagnetic wave with a wavelength in the range of 100–380 nm. However, it has a strong oxidation power. The UV/O3 cleaning method decomposes organic dirt into gaseous substances (CO2 , H2 O, etc.) by combining the effect of breaking the chemical bonds in organic compounds using UV radiation and the strong oxidation effect of ozone. UV as plasma light and O3 as active gas species are also formed during surface cleaning using atmospheric-pressure plasmas, as described in the previous section, but UV lamps are usually used as a strong UV radiation source. In particular, the formation of O3 by UV light with a wavelength of < 240 nm is remarkable. The combination of UV radiation and O3 has been studied for a long time, and it has been confirmed that contaminants such as cutting oil and acidic solder flux can be completely removed within 1 min under appropriate conditions.

6.4 Example of Plasma Cleaning and Enhanced Activation of Hydrophilicity

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As an example of ozone cleaning, a process for removing unnecessary photoresists adhering to a substrate surface during the plate-making process has been reported. The organic matter on the surface is decomposed into CO2 and H2 O by the ozone formed by an ozone generator or UV irradiation, and oxygen radicals (•O) are generated via the thermal decomposition of ozone or UV irradiation. In addition, ozone cleaning is used for the dry cleaning of parts such as semiconductor wafers, liquid crystal display (LCD) glass substrates, and electronic substrates. As described above, ozone cleaning is a powerful method, and it is expected that various effects and treatment technologies will be developed through this new AOP.

6.4 Example of Plasma Cleaning and Enhanced Activation of Hydrophilicity 6.4.1 Hybrid Plasma-Hydrophobic–Chemical Process In Sect. 6.4, the surface modification of glass using APNPT is described and results are explained based on the description and figures of Ref. [1]. A fundamental study has been conducted to investigate the improvements in the hydrophobic and hydrophilic properties of the glass surface for the possible elimination of the need for windshield wipers in automobiles.

6.4.2 Experimental Apparatus and Method Among the various types of atmospheric-pressure nonthermal plasma technologies reported to date, the silent corona method has been employed because the plasma can be easily and uniformly applied to the glass surface. A schematic of the experimental setup is shown in Fig. 4.20 of Chap. 4. The silent corona plasma reactor consists of two parallel round aluminum disks. The top disk electrode with a diameter of 73 mm is hollowed with 30 pinholes having a diameter of 2 mm. The bottom disk electrode of the same diameter is covered with a dielectric material (an acrylic plate with a thickness of 1 mm), where the glass sample (sodium silicate glass, width = 38 mm, length = 26 mm, and thickness = 1 mm) is placed. This configuration is called a silent corona reactor because the bottom electrode covered with the dielectric barrier is a typical configuration for silent discharge, and the upper electrode with sharp edges and pinholes is typical for corona discharge. The distance from the electrode to the glass surface is maintained at 3 mm. As a background gas, dry air with relative humidity (RH) of 4% and a temperature of 20 °C is primarily used in the compressor through a dryer. Pure nitrogen (N2 , 99.99% purity) and helium (He, 99.995% purity) are also tested. The desired flow rate is obtained using a mass flow controller. Gas is supplied from the top shaft, passes through the 30 holes, and uniformly impinges the

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glass sample. The gas is then dispersed outward and discharged into the holes of the bottom shaft. The electrodes are energized using a 60 Hz AC power supply (20 kV and 30 mA max.) to generate microdischarges between the electrodes. The effects of the background gas, flow rate Q, plasma treatment time, and peakto-peak applied voltage V p-p on the contact angle were investigated to understand the fundamental characteristics of the glass surface using the plasma alone and a plasma and chemical hybrid process. The contact angle of the glass samples is measured using a contact angle meter (CA-DT, Kyowa Interface Science Co., Ltd.). For the durability test, the windshield wiper for the automobile is applied to the glass sample continuously with a frequency of 33 rpm for 1.5 h. The glass sample is exposed to ambient conditions (January through February in Sakai city, Japan: average rainfall per month = 44 mm, no snow, average temperature = 5.0 °C, daylight hours per month = 117.2 h, average pressure = 1016 hPa, average relative humidity = 60% at 15 °C, and dew point = 7.3 °C) for the rest of the day. During the operation of the windshield wiper, water is applied to simulate a rainy day. This procedure is repeated for several days. The applied voltage and current waveforms are measured using an oscilloscope (Tektronix TDS380P) equipped with a voltage divider (Tektronix P6015A) and a current probe.

6.4.3 Test Results Obtained for Cleaning and Hydrophilicity and Discussion 6.4.3.1

Effects of the Plasma Parameters

Initially, the effects of background gases such as air, nitrogen, and helium were investigated. The flow rate Q, plasma treatment time, and applied voltage V p-p were set at 1.0 L/min, 1 min, and 17 kV, respectively. All results were obtained based on the average of five measurements throughout the experiments. This is referred to as the basic treatment condition. The contact angle of the glass sample was initially 45° and reduced to 3.3° for dry air and 4° for nitrogen upon execution of the plasma treatment process, showing excellent hydrophilic properties. However, when helium gas was used, only 2.5 kV was applied, and the contact angle became 52°, resulting in no hydrophilic properties. Because dry air showed the best results, it was used subsequently.

6.4.3.2

Test Results

Subsequently, the effects of the plasma treatment time and the change in the gas flow rate to 5 L/min were investigated. As a result, the contact angle increased from approximately 3.3° to > 10° after 7 h. After a day, the contact angle increased to 15–20°. After 5 days, the hydrophilicity is almost lost in all samples, and the contact

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Table 6.1 Procedure for preparing the four types of samples Sample 1

TAS coating → wiping off

Sample 2

Plasma → TAS coating → wiping off

Sample 3

TAS coating → plasma → wiping off

Sample 4

TAS coating → wiping off → plasma

Dotted line

Critical value for the replacement of windshield wiper

angle was the same as that of the untreated sample. In other words, the cleaning and enhanced hydrophilicity due to the plasma treatment alone does not last for a long time.

6.4.4 Hydrophobic Approach 6.4.4.1

Sample Preparation

A commercially available hydrophobic chemical solution used for wiperless automobile windshields provided by CCI Corporation, Japan, trialkoxysilane (TAS), has been used as a hydrophobic coating solution. Four types of glass samples were prepared, as listed in Table 6.1. For control sample 1, a glass surface is coated with hydrophobic TAS and wiped off with a cloth, and the contact angle is measured to obtain the baseline data. For sample 2, the plasma is initially applied, followed by coating with TAS. The sample is then wiped to remove the excess TAS coating. The plasma conditions are the same as those used in other experiments (a flow rate of 1.0 L/min and a plasma treatment time of 1 min) with the exception of the applied voltage of 20 kV. Because this experiment is carried out with a higher applied voltage, the effects of the plasma may increase owing to the increase in the number of high-speed electrons with higher voltage. For sample 3, the process is the reverse of that used for sample 2, i.e., coating with TAS is first carried out, followed by the plasma treatment. The glass surface is then wiped off with a cloth to remove the excess liquid. For sample 4, coating with TAS is first performed, and the excess coating is removed. The samples are then exposed to the plasma.

6.4.4.2

Durability Test

Durability tests were conducted using the procedures described earlier. All samples were placed outdoors and continuously rubbed with an automobile windshield wiper at a frequency of 33 rpm for 1.5 h while water was continuously applied to the sample. The sample was then left exposed for the rest of the day in January or February. This procedure was repeated for several days. The results are shown in Fig. 6.7. Sample 1

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Fig. 6.7 Durability test for the hybrid treatment using the plasma and TAS coating (dry air, Q = 1.0 L/min, plasma treatment time = 1 min, V p-p = 20 kV)

shows the baseline data and indicates a contact angle of 108° after the application of TAS, and it drops to 90° after 70 min of windshield wiper operation. A contact angle of 90° is set as the critical hydrophobic property. However, the durability of sample 2 increased to 330 min to reach the critical hydrophobic property, which is 4.7 times more durable than sample 1 (TAS coating alone). The durability test for sample 3 reached 400 min, during which the durability increased 5.7-fold. Sample 4 shows effective durability, which completely fades away. The glass sample is completely exposed to the plasma, resulting in enhanced hydrophilicity. It is clear from these results that the appropriate combination of the hydrophobic TAS coating and plasma can result in a fivefold increase in the durability of the glass coating. In addition, the visibility of the plasma and chemically treated glass is excellent.

6.4.4.3

Role of the Nonthermal Plasma in the Hydrophobic Liquid

The role of plasma in hydrophobic coatings (TAS) is currently being considered. The molecular structure of sodium silicate glass proposed by Schloze is shown in Fig. 6.8 [1]. In the glass manufacturing process, water in molten silicates exists as OH, which either combines with a hydrogen atom or remains unbonded. When the plasma is applied, O–OH bonds with a shorter distance (0.255 nm) are broken upon electron impact, and atomic O and OH radials appear on the glass surface, as shown in Fig. 6.9. The electron energy of nonthermal plasmas is in the range of 4–5 eV and is sufficient to break the hydrogen bond energy (0.2–0.3 eV). A similar phenomenon has been observed for different materials and identified as an oxygen peak (O1s ) using electron spectroscopy for chemical analysis (ESCA). The molecular structure of the glass surface when the TAS coating is applied after the plasma treatment is shown in Fig. 6.9. An H2 SO4 solution is used as the catalyst for the TAS coating. Chemically stable Si–O is generated on the glass surface, which is the main cause of the increased hydrophilicity. Simultaneously, a solution of MeOH (methanol or ethanol) is removed.

6.4 Example of Plasma Cleaning and Enhanced Activation of Hydrophilicity

159

Fig. 6.8 Chemical structure of sodium silicate glass

Fig. 6.9 Surface chemical reaction of the TAS coating combined with a plasma treatment

After the plasma exposure to the TAS-coated sample (sample 3), the molecular structure on the glass surface is almost the same as that observed for sample 2. The mechanism for the increased durability can be explained by the stable Si–O structure on glass (the same as in sample 2). However, when TAS is applied and the surface is wiped off, the surface structure is completely exposed to plasma and becomes hydrophilic, as in the case of sample 4.

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6.5 Hybrid Plasma–Anti-corrosion Process Treatment (Aluminum Plate Surface Treatment) 6.5.1 Ordinary and Hybrid Plasma Treatments An aluminum coil, as received from the manufacturer, uses grease or organic compounds on the aluminum surface to enhance detachment. The surface of an aluminum plate with hydrophilic and corrosion-resistant properties is often required for various mechanical parts or systems. The conventional process for treating aluminum surfaces employs a series of wet processes, as shown in Fig. 6.10: grease or organic compound removal from the uncoiled material, followed by drying at 200 °C; a chromate process to increase the corrosion resistance, followed by drying at 200 °C; and a final chemical treatment to achieve hydrophilic properties on the aluminum surface prior to recovery. However, the use of chromium (Cr+6 ) in the chromate process, which is widely used in aluminum treatment processes, creates environmental problems. Therefore, a simple and economical dry cleaning process used to replace the conventional aluminum wet-surface treatment, i.e., a nonthermal plasma, followed by chemical treatment is explored. This process eliminates several stages of surface preparation, as shown in Fig. 6.10. A cross-sectional view of the final aluminum surface is shown in Fig. 6.11. The use of nonthermal plasmas for plastic materials, steels, and various surface modifications has been studied, but very few studies have been conducted on aluminum surface treatment. In Sect. 6.5, results are explained based on the description and figures of Ref. [2]. Among several nonthermal plasma processes, two types of plasma jets (60 Hz AC plasma and RF plasma) and the pulse corona plasma

Fig. 6.10 Conventional wet process and dry plasma process for aluminum surface treatment

6.5 Hybrid Plasma–Anti-corrosion Process Treatment (Aluminum Plate …

161

Fig. 6.11 Cross-section of the aluminum surface

have been investigated to remove grease or organic compounds. Simultaneously, the aluminum surface reaches an excited state in order to activate the chemical bonds. The corrosion-resistant epoxy resin coating is directly applied to the aluminum surface without Cr+6 . The adhesion characteristics and surface corrosion of the aluminum surface were evaluated using an accelerated corrosion test.

6.5.2 Plasma Apparatus Figure 6.12a shows a cross-sectional view of the plasma jet using a 60 Hz AC power supply (10 kV, 0.06 A), in which air at atmospheric pressure is used as the feeding gas. The dimensions of the plasma jet port are 5 mm × 50 mm, and the distance between the plasma jet and the aluminum surface is maintained at 10 mm. Figure 6.12b illustrates the RF plasma jet (Pearl Kogyo Co., Ltd., Osaka, Japan), in which the plasma is generated between the jet nozzle and the aluminum surface. The flow rate of atmospheric-pressure helium is set at 1.0 L/min, and the RF power, which is rated at 3 kVA and 13.56 MHz, is set at 250 W and used to irradiate the sample. No uniform plasma is obtained when air is used. The RF power supply consists of a matching box and tuner controller to minimize power reflection. The plasma jet on the aluminum surface is maintained at 5 mm. An insulated-gate bipolar transistor (IGBT) pulse power supply (Masuda Research, Inc., Tokyo, Japan), in combination with magnetic pulse compression, is used as a third plasma device. A schematic of the experimental setup is shown in Fig. 6.12c. This pulse power supply can generate a 200 ns pulse width at 30 kV. The reactor consists of 10 knife edges that are contained in a 104mm-diameter aluminum disk and a 120-mm-high acrylic housing. The aluminum sample is placed either on the 3-mm-thick barrier surface or the 10 knife edges. The distance between the jet nozzle and the aluminum surface is set at 5 mm. Either air or helium is used as the feeding gas at a flow rate of 1.0 L/min. A power of 4.5 W with a pulse repetition rate of 300 Hz is used.

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Fig. 6.12 Various nonthermal plasma surface treatment device electrodes for aluminum plates: a AC plasma jet (air, 60 Hz, 10 kV, 60 mA), d = 10 mm, b RF plasma jet (helium, 13.56 MHz, 250 W), d = 5 mm, and c IGBT element pulse corona plasma (air, pulse width of 200 ns, 30 kV, 300 Hz, 4.5 W), d = 5 mm

6.5 Hybrid Plasma–Anti-corrosion Process Treatment (Aluminum Plate …

163

6.5.3 Treatment After Plasma Irradiation and Increased Anti-corrosion Effect After plasma exposure, the corrosion-resistant chemical (epoxy resin) without a chromate film is coated using a specific roller, which provides a coating that is approximately 2.5 µm thick. The corrosion resistance and adhesion force have been evaluated using the pressure cooker test, in which samples were placed in a chamber heated at 130 °C in an environment at 100% RH and 2.755 kg/cm2 . As a more severe and faster test, the copper-accelerated acetic acid salt spray (CASS) test, which is specified in JIS H8520, was performed. This test procedure is recommended for evaluating the corrosion resistance of plating. The test chamber is maintained in an extremely strong acidic environment for 8 h; that is, NaCl and CuCl2 are sprayed into the chamber to maintain a pH of 3.0, and the temperature is set at 63 ± 2 °C. All aluminum samples are cut to dimensions of 80 mm × 30 mm. The aluminum surface is evaluated using a scanning electron microscope (SEM). The corrosion area is scanned and analyzed using a computer.

6.5.4 Results and Discussion 6.5.4.1

60 Hz Plasma

When the aluminum plate is contaminated with grease or organic compounds, the contact angle is in the range of 90°–98°. When the contaminated aluminum is heated at 500 °C for 10 min in an oven, the grease is completely removed from the surface, and the contact angle changes to 65°–70°. The plasma exposure time is varied among 1, 3, and 5 min, and the contact angle is measured using a contact angle meter. Figure 6.13 shows the contact angle as a function of the elapsed time up to 60 min. Figure 6.14 shows the contact angle measurements over a long period of up to 5 days. The contact angle ranges from 7° to 12° depending on the plasma exposure time and the type of plasma used. The aluminum surface initially becomes hydrophilic upon plasma treatment, and the contact angle increases to 30° after 60 min. After 5 days, the contact angle further increased to 70°–80° irrespective of the plasma exposure time, indicating that although the hydrophilic properties are lost with time, the aluminum surface is completely cleaned during the plasma treatment process. The hydrophilic properties are attributed to the formation of OH groups on the aluminum surface. As time elapses, the decrease in the contact angle may be ascribed to the decrease in the excited state of the aluminum surface due to oxidation. The surface temperature of the aluminum sample reaches 97.8 °C when the plasma nozzle and sample distance are maintained at 10 mm and the plasma exposure time is set at 3 min. The surface temperature does not exceed 100 °C when the plasma exposure time is beyond 3 min. It is also confirmed that no organic compounds are removed when the samples are exposed to 100 °C in an oven, indicating that

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Fig. 6.13 Contact angle versus the elapsed time up to 60 min for the AC plasma

Fig. 6.14 Contact angle measurements conducted up to 5 days for the AC plasma

the effects of temperature on the surface cleaning process can be ignored in this experiment.

6.5.4.2

RF Plasma

Figure 6.15 shows the contact angle versus the elapsed time as a function of the plasma exposure time (10, 30, 45, and 60 s) using an RF plasma jet. Figure 6.16 also shows the long-term contact angle measurements conducted up to 5 days. It is clear from Figs. 6.15 and 6.16 that the contact angle initially reaches 6°–8° and gradually increases over time. Less than 30 s of plasma exposure does not affect the hydrophilic properties. The effectiveness of the contact angle is affected, even in the long-term exposure test. However, organics appear to be removed from the aluminum surface even after 10 s of plasma exposure. The contact angle is < 60° after 5 days. This number is slightly smaller than that of the contamination-free aluminum surface (65°–70°), which may be attributed to the spattering on the aluminum surface by the high-power plasma. The surface temperature of the aluminum sample reaches 137 °C when the plasma nozzle and sample distance are maintained at 5 mm and the plasma exposure time is

6.5 Hybrid Plasma–Anti-corrosion Process Treatment (Aluminum Plate …

165

Fig. 6.15 Contact angle versus the elapsed time as a function of the plasma exposure time for the RF plasma

Fig. 6.16 Long-term contact angle measurements conducted up to 5 days for the RF plasma

set at 3 min, as shown in Fig. 6.17. The aluminum samples are maintained at 140 °C in an oven, and no organic compounds are removed. Again, the effects of temperature on surface cleaning are ignored in this experiment. The decrease in the contact angle is also affected by the surface roughness of the aluminum sample. The hydrophilic property of the aluminum surface decreases with plasma exposure times of < 30 s. However, a plasma exposure time of 10 s appears to be sufficient for the removal of organic compounds, which may be attributed to the high RF plasma power.

6.5.4.3

Pulse Corona

Figure 6.18 shows the contact angle measurements versus the elapsed time as a function of the plasma exposure time (0.5, 1, and 3 min). Figure 6.19 also shows the longterm contact angle measurements conducted up to 5 days. Figure 6.18 shows that the contact angle is initially < 10° with plasma exposure times of > 1 min. This effectiveness is sustained for > 60 min. The contact angle observed after 5 days is lower than that of the clean aluminum surface because electrons directly impinge the aluminum surface in this arrangement. The low contact angle surface may be attributed to the increased surface roughness or etching effects on the aluminum surface. It is clear

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Fig. 6.17 Aluminum surface temperature observed for various RF plasma exposure times

that organic compounds are effectively removed from the aluminum surface using all three plasma devices with appropriate plasma exposure times because the plasma power is different for each plasma device. Fig. 6.18 Contact angle measurements versus the elapsed time as a function of the plasma exposure time for the pulse corona

Fig. 6.19 Long-term contact angle measurements conducted up to 5 days for the pulse corona

6.5 Hybrid Plasma–Anti-corrosion Process Treatment (Aluminum Plate … 0.5 0.45

Corrosion area (%)

Fig. 6.20 Corrosion areas after aluminum plasma treatments (1)– (7)

167

0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0

(1) (2) (3) (4) (5) (6) (7) 6.5.5 Corrosion Test Results The following seven cleaning methods have been used for organic removal: (1) 500 °C for 10 min in an oven, (2) alkali cleaning, (3) AC plasma cleaning for 3 min, (4) RF plasma cleaning for 28 s, (5) alkali and AC plasma cleaning for 3 min, (6) pulse corona cleaning for 3 min, and (7) conventional chromate cleaning. The epoxytype film thickness using the combined chromate process and the corrosion resistance film used for the conventional process is approximately 2.5 µm. A corrosion-resistant epoxy film is applied after the plasma cleaning step, whose thickness is ~ 2.4 µm. Three samples (30 mm × 80 mm) are prepared for each test, and a total of 21 samples are prepared. Initially, a pressure cooker test is performed and showed no distinct corrosion in any of the samples tested. Therefore, the CASS test, which uses more severe test conditions, is performed for 8 h. The surface condition is then analyzed using an SEM, and a corroded area of 600 mm2 of the sample is evaluated using a computer. The results are shown in Fig. 6.20. It is clear that the RF plasma with a 28 s exposure time has the lowest corrosion area. This implies that the corrosion resistance using RF plasma cleaning is significantly higher than that of the conventional chromate method, and simultaneously, the adhesion between the aluminum surface and the corrosion-resistant coating is significantly improved. The pulse corona process is nearly equal to that of the conventional process. Alkali cleaning followed by the AC plasma is next, followed by the AC plasma alone and alkali cleaning alone. Figure 6.21 shows an image of the aluminum plate surface after the CASS test with a sample width of 30 mm. It is obvious from the figure that the RF plasma treatment (28 s) is better than alkali cleaning because no corrosion holes are observed. Because 28 s of RF plasma cleaning followed by the epoxy treatment shows the best performance in the corrosion resistance test, the exposure time is varied to obtain the optimum operating conditions. The plasma exposure is varied among 10, 28, and 45 s. The results are shown in Fig. 6.22, in which the data for the conventional chromate wet process are also included. In the figure, (1), (2), (3), and

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(a)

(b)

Fig. 6.21 Image of the aluminum plate surface after a CASS test with a sample width of 30 mm. a Alkali cleaning, (2). b RF plasma cleaning for 28 s, (4)

(4) denote RF plasma cleaning with an exposure time of 10 s, RF plasma cleaning with an exposure time of 28 s, RF plasma cleaning with an exposure time of 35 s, and the conventional chromate treatment, respectively. It is apparent from the figure that the corrosion area decreases with increasing plasma exposure time. The RF plasma with more than 30 s of exposure time followed by coating with the epoxytype corrosion-resistant film shows better performance than the conventional wet process. Therefore, the conventional wet chemical process for aluminum treatment can be replaced with the newly developed plasma-assisted chemical process, which simplifies the entire process and makes it more economical. The RF plasma requires a significantly higher power consumption than the other two plasma devices, but this is insignificant when compared to the process improvement. However, a pulse corona discharge with prolonged plasma exposure may show a superior corrosion-resistant surface. It is well known that the use of polarized resins such as epoxy and phenol has strong adhesion properties for glass or metal surfaces. This is primarily because of the hydrogen bonds formed between the metals and epoxy resins. Therefore, resins bearing −COOH, −OH, and CO functional groups exhibit strong adhesion properties. Thereby, the enhanced presence of OH functional groups on an aluminum Fig. 6.22 CASS corrosion test conducted for 8 h (total corroded area and corrosion ratio of 600 m2 on the RF plasma exposure time)

6.6 Conclusions

169

Fig. 6.23 Structure of plasma-treated metal coated with polymer

surface due to the plasma treatment can increase the hydrophilic and adhesion properties and, thereby, the corrosion resistance. Figure 6.23 illustrates the structure of the plasma-treated metal coated with plastics. Etching or spattering by plasma may also increase the adhesion properties. However, the hydrogen bonds are dominant in the enhanced adhesion mechanism over splattering or van der Waals forces.

6.6 Conclusions The definition and characteristics of the plasma, the principles of cleaning treatments, examples of electrode systems and equipment, and an overview of atmospheric-pressure plasma cleaning have been presented. Examples of cleaning using atmospheric-pressure plasmas have been described for glass and aluminum substrates. A new technology for modifying glass surfaces using atmosphericpressure nonthermal plasma has been introduced. The glass surface shows excellent hydrophilic properties with a contact angle of < 4° upon irradiation with the nonthermal plasma. However, its durability is less than a few days. By irradiating and cleaning the surface of the material in advance with the plasma and then applying a hydrophobic agent, the glass surface can be made water-repellent. This is because the hydrophobic agent and glass are bonded more firmly. The temporal durability of the water-repellent effect can be improved by more than fivefold, indicating that it is at the level of practical use. Alternatively, the corrosion resistance of metal can be increased by applying an anti-corrosion agent to the metal after plasma treatment. This is because the anti-corrosion agent has a water-repellent action, and the metal is more strongly bonded. In the future, plasma cleaning technology will be developed as a pretreatment for the adhesion of dissimilar materials such as glass–plastic, metal–plastic, and glass–metal. The number of successful examples will be increased over a wide range, and the diffusion and practical application of the treatment system will be carried out.

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There are still several unexplored points with regard to the mechanisms for improving adhesion via plasma. It will be important to clarify their basis and organize them systematically in the future, and it will be important to continue to make efforts to clarify their principles. Although research on surface modification is often regarded as supplementary work, it should be positioned as important work for the creation of an industrial base. It is believed that collaboration between industry and academia will be extremely important in the coming years to elucidate the mechanisms of adhesion.

References 1. T. Yamamoto, M. Okubo, N. Imai, Y. Mori, Improvement on hydrophilic and hydrophobic properties of glass surface treated by nonthermal plasma induced by silent corona discharge. Plasma Chem. Plasma Proc. 24(1), 1–12 (2004) 2. T. Yamamoto, A. Yoshizaki, T. Kuroki, M. Okubo, Aluminum surface treatment using three different plasma-assisted dry chemical processes. IEEE Trans. Ind. Appl. 40(5), 1220–1225 (2004) 3. CFD-ACE-GUI™ Modules Manual Volume II V2003, ESI group (2004) 4. T. Yamamoto, M. Okubo, Y - T. Hung, R. Zhang et al. (2004), Advanced air and noise pollution control, in Handbook of Environmental Engineering, ed. by L.K. Wang, et al., Humana Press, Inc. , Totowa, NJ, USA. 2, Chapter 8, pp. 273–334 5. A. Belkind, S. Gershman, Plasma cleaning of surfaces. Vacuum Technology & Coating (2008) 46 (total 12 pages) 6. F. Mamiya, Senjyo Gijyutsu Gairon (English translated title: Introduction to cleaning technology). J. Vacuum Soc. Jap. 43(6), 631–641 (2000). (in Japanese) 7. J. Strong, On the cleaning of surfaces. Rev. Sci. Inst. 6, 97 (1935). https://doi.org/10.1063/1. 1751951 8. Hitachi High-Tech Corporation, Eching souchi toha? (English translated title: What is an etching device?); https://www.hitachi-hightech.com/jp/products/device/semiconductor/etch. html (reference date 2022.10.22) (in Japanese) 9. M. Kogoma, Generation and application of atmospheric glow discharge. J. Plasma Fusion Res. 79(10), 1000–1001 (2003). (in Japanese) 10. K. Chayama, ICP ni tsuite (English translated title: on the ICP (inductively coupled plasma)). http://kccn.konan-u.ac.jp/chemistry/ia/contents_05/05.html (reference date, 2022.10.22) (in Japanese) 11. SAKIGAKE Semiconductor Co., Ltd., Hyomen kaishitsu senjyo no genri (English translated title: Principles of surface modification and cleaning). https://sakigakes.co.jp/principle-plasma. html (Accessed 2018.8.1) (in Japanese)

Chapter 7

Plasma and Electron-Beam Technologies Used for Surface Treatment Applications

7.1 Introduction In this chapter, the plasma and electron-beam technologies used for surface treatment applications are described. Various topics are explained, except for those described in the preceding chapters. Figure 7.1 shows the content of the treatment processes described in this chapter. Firstly, (1) plasma hybrid hydrophilic treatment process has been explained, in which polytetrafluoroethylene (PTFE) and glass are used as targets. The detailed procedure for the plasma hybrid surface treatment process is described in the preceding chapters. Secondly, (2) electron-beam irradiation treatment process used to introduce anti-fog properties to material surfaces such as plastic and glass has been described. The electron beam is a state of plasma with high electron energy and can be used as an effective treatment process. Finally, the principles and examples of (3) plasma treatment for medical applications have been explained. Targets are endoscope and sterilization technology.

7.2 Plasma Hybrid Hydrophilic Treatment Process 7.2.1 Principles Plasma treatment is known as a dry surface treatment method, in which an ionized noble gas such as argon or helium generates a plasma jet that acts on an object to perform the surface treatment process. In general, the application of a corona or plasma to the surfaces of polymers, plastics, glasses, and metals activates the surfaces of these materials, which are often significantly hydrophilized. The radicals and functional groups formed on the surface due to the action of high-energy electrons (approximately 1–10 eV) generated by atmospheric-pressure plasma discharges have hydrophilic properties, and the free energy on the surface increases, facilitating adhesion and bonding with other materials. When the plasma-treated surface is subjected © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 M. Okubo, Nonthermal Plasma Surface Modification of Materials, https://doi.org/10.1007/978-981-99-4506-1_7

171

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Fig. 7.1 Plasma irradiation/electron-beam irradiation technologies described in this chapter

to hydrophobic chemical agents’ treatments or coatings, the adhesion and durability of the hydrophobic and anti-corrosive agents are improved, and the water repellency is maintained. However, if the material is left in air as it is after the plasma treatment process, the radicals totally disappear owing to oxidation, and the effect completely vanishes within a few days to a week. That is, the effects of adhesion with other materials and the hydrophobic plasma treatment or “plasma hybrid treatment” with a hydrophobic agent or corrosion-resistant agent are reduced unless it is performed immediately after the plasma treatment process.

7.2.2 Examples of the Adhesion of Glass and PTFE In the plasma hybrid treatment process for fluoropolymer and glass adhesion using the A4-size treatment apparatus, as shown in Fig. 7.2, the adhesion of the PFA, PTFE, and PCTFE can be significantly increased, as described in Chap. 3. On the other hand, the glass surface can be superhydrophilic by plasma irradiation using dielectric barrier discharge, as shown in Fig. 7.3. Figure 7.3 shows two types of surface treatments for glass. When plasma is irradiated to the glass surface, the surface becomes hydrophilic or hydrophobic to maintain the performance of the hydrophobic surface chemicals such as trialkoxysilane. These results and the mechanisms are explained in Chap. 6. Figure 7.4 shows a schematic of the adhesion procedure of a surface-treated PTFE film to NTP-treated sodium silicate glass. One of the applications of such a composite material is in the surface film of smartphones, which has oil-proofing and anti-fog properties. In this case, although the PTFE film adheres, a transparent PFA film is also possible. The procedure is as follows: (1) A PTFE sheet (0.2 mm thick, A4 size) is processed with the A4-size machine, and a sample with dimensions of 26 mm × 100 mm is cut from the sheet. (2) A transparent epoxy adhesive (Konishi, Epoclear) is applied to a glass plate (26 mm × 76 mm) with a thickness of 250 µm using a baker-type applicator.

7.2 Plasma Hybrid Hydrophilic Treatment Process

173

Fig. 7.2 A4-size plasma graft polymerization equipment

Superhydrophobic +Hydrophobic chemical (Trialkoxysilane)

Water droplet

Large contact angle(lifetime: longer)

Plasma irradiation OR

Small contact angle (lifetime: few days)

Glass

Superhydrophilic Fig. 7.3 Modifications of glass surfaces via plasmas. By irradiating the glass with the nonthermal plasma, the surface of the glass is modified to make the surface hydrophilic (small contact angle but a lifetime of a few days) or superhydrophobic (large contact angle and a lifetime longer) with the plasma hybrid application of the hydrophobic chemicals

5 kgf Epoxy adhesive Steel plate (18 25 cm)

Glass plate

PTFE

Glass plate PTFE

Fig. 7.4 Adhesion of PTFE and surface-treated glass

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(3) The PTFE sample is glued and left at room temperature for a day with a load of 5 kgf (= 49 N). Following these procedures, a fluorocarbon film–glass composite is successfully manufactured. Figure 7.5 shows the results of a 180° peeling test obtained for the composite. It should be noted that a T-type peeling test is impossible because the glass plate is rigid and easily breakable. Therefore, 180° peeling tests are conducted. In this graph, the horizontal axis represents the stroke, and the vertical axis represents the load per 26 mm sample width. Five samples are used in this test, and the width of the composite sample is 26 mm. The error bars express the standard deviation (±σ ) for the five samples tested. It is known from this graph that an average strength of 21 N per 26 mm and a maximum strength of 25 N per 26 mm (= 0.96 N/mm) are obtained. These values are sufficient for industrial applications. For applications in wide films, it is necessary to improve the treatment apparatus. Figure 7.6 shows the concept of a high-speed plasma hybrid processing apparatus with wide electrodes. Figure 7.6a shows a schematic of the conventional A4-size apparatus used for these films. Figure 7.6b shows the newly designed apparatus with wider electrodes. A plasma torch electrode that is approximately 10 times wider than the conventional electrode is prepared, and a tenfold increase in processing speed is demonstrated. Instead of physically wider electrodes, a magnetic field-assisted electrode system can be used in the apparatus.

Error bars: Standard deviation ±σ

Number of samples = 5 Sample width = 26 mm

Maximum load = 0.96 N/mm Fig. 7.5. 180° peeling test results (PTFE glass). Adhesion is performed using the epoxy adhesive with the applicator

7.2 Plasma Hybrid Hydrophilic Treatment Process

Spot electrode

175

Wide electrode

Film

Plasma

Plasma y direction

x direction

x direction

(a)

(b)

Fig. 7.6 High-speed plasma hybrid processing equipment with wider electrodes (concept): a present apparatus and b apparatus with wider electrodes

Figure 7.7 shows the magnetic field-assisted gliding arc discharge plasma system used for surface modification [1]. This is a novel design for a gliding arc plasma generator assisted by a magnetic field used for surface modification. The introduction of a magnetic field enhances the motion and reach of the gliding arc discharge. Consequently, a 19-cm-long gliding arc discharge plasma is successfully generated, which is effective for surface modification. The gliding arc discharge is a popular technique for generating a nonequilibrium plasma at atmospheric pressure and has been widely utilized in chemical and environmental industries. Previously, the author’s team has performed extensive work on the surface modification of fluorocarbon polymer films using the corona discharge plasma technique at atmospheric pressure. This corona discharge is considered to be a type of the gliding arc discharge on a pair of sharp needle electrodes. To improve the efficiency of the surface treatment process, the author’s team has designed a magnetic field-assisted gliding arc discharge plasma apparatus for surface modification. Schematics of the gliding arc discharge plasma generator are shown in Fig. 7.7a and b. Two electrodes, stainless steel water-cooled tubes with a diameter of 1.6 mm and a width of 19 cm, are placed between two permanent magnets. The shortest gap between electrodes is 0.5 cm. Argon is used as the processing gas. Power is delivered by a pulse DC supply with a maximum voltage V max of 10 kV and a frequency f of 40 kHz. The magnetic flux density B is approximately 0.25 T. In general, the gliding plasma is generated initially at the shortest gap between the two divergent electrodes, and thereafter, the length of the arc discharge increases owing to the flow of the gas. Both ends of the arc discharge move along the electrodes until the arc voltage exceeds the limit provided by the electrodes [1]. Images of the different stages of the gliding arc plasma are recorded using a digital camera in an oblique direction, as shown in Fig. 7.8. The exposure time of the camera is 2 ms. According to the voltage–current characteristics of the gliding arc discharge, the generation time of discharges (a), (b), (c), (d), (e), and (f) are estimated to be 1.0, 1.6, 2.0, 2.7, 3.2, and 5.0 ms, respectively. The generated gliding arc discharge consists of ionized gas and electron flows. According to Fleming’s left-hand rule, the gliding

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7 Plasma and Electron-Beam Technologies Used for Surface Treatment …

Fig. 7.7 Gliding arc discharge configuration: a side and b top views [1]

arc is pulled upward and stretched by the Lorentz force. In our case, the extended gliding arc plasma between the two electrodes is approximately 19 cm long, as shown in Fig. 7.8f, which is longer than that generated conventionally without a magnetic field. Furthermore, a magnetic field-assisted gliding arc discharge plasma has been applied to the surface modification of PTFE in an acrylic acid vapor atmosphere. The results of the graft polymerization process on PTFE showed that the adhesion of PTFE increased by approximately 20-fold. Further studies on the optimization of the operational parameters such as the gas flow rate, electrode shape, magnetic flux density, and power source supply on the discharge characteristics will further increase adhesion. The results show that this novel gliding arc discharge process is an effective method for surface modification over a large area.

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Fig. 7.8 Photographs of the magnetic field-assisted gliding arc discharge plasma at different stages in the discharge cycle: a 1.0, b 1.6, c 2.0, d 2.7, e 3.2, and f 5.0 ms. Ar flow rate is 20 NL/min [1]

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7.3 Anti-fog Using Electron-Beam Irradiation Treatment Process In recent years, anti-fog treatments of materials have attracted a lot of attention, which include (1) (2) (3) (4) (5)

hydrophobic surface coatings; surface oil repellents, stain prevention, and hydrophobic treatments; surface hydrophilization; the absorption of attached water droplets using a water-absorbent material; maintaining the surface temperature above the dew point; and so on. Technologies for realizing (1)–(5) include.

(1) plasma hybrid glass hydrophobic coatings; (2) fluorine-based gas plasma treatment, laser irradiation, and electron-beam irradiation treatment processes; (3) plasma hydrophilization treatment processes and plasma actuators with surface electrodes; (4) plasma graft polymerization/electron-beam graft polymerization; (5) heating using a surface heater. Anti-fog technology, which controls the surface hydrophilicity/water repellency of glass, plastic, and metal, will become increasingly important in the future, and plasma/laser/electron-beam irradiation is promising methods to achieve anti-fog. Plasma anti-fog technologies have been described in Chap. 5 in relation to hybrid glass surface treatment and CHF2 and hydrophobic laser treatment processes. These hydrophobization technologies are well suited to anti-fog technology. In this section, the hydrophilization of macromolecules using electron-beam irradiation is explained.

7.3.1 Principles An electron beam is a bundle of accelerated electrons in a high vacuum with a high voltage that irradiates the target substrate. The irradiation of the target material causes various reactions on the surface of the substance. Originally, electron-beam irradiation technology began with research on radiation deterioration and has been used in various industries such as surface treatment and environmental cleaning. The effects of electron-beam irradiation on plastics can be roughly divided into two types: (1) “radical generation,” in which radicals react between molecular chains to form a network structure via cleavage reactions, and (2) “graft polymerization,” in which radicals polymerize with another compound (monomer) bearing a functional group. Two parameters should be noted when radicalizing (hydrophilizing) the surface of a material using electron-beam irradiation. These are the absorbed dose and acceleration voltage. The absorbed dose represents the amount of energy absorbed by the

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179

material from the electron beam, generally reported in gray (Gy), where 1 Gy = 1 J/ kg. As the absorbed dose increases, the amount of energy absorbed by the material from the electron beam and the number of radicals increase. On the other hand, the acceleration voltage is the voltage that accelerates the electrons. As the voltage increases, the effect of the electron beam on the material tends to increase, and the acceleration voltage is typically reported in units of kilovolt (kV) or kilo electron-volt (keV), etc. The electron-beam irradiator accelerates the electrons emitted by heating the filament in a vacuum chamber at a high voltage of 100 kV to 10 MV in high vacuum, which then pass through a thin metal foil with a thickness of several tens of micrometers. It is a facility that can be removed into the atmosphere by allowing electrons to flow and is composed of complex technologies such as high-voltage, beam-engineering, and high-vacuum technologies. The irradiated part must shield the X-rays generated by the collision of electrons with the metal and is generally covered with lead or concrete. Electron-beam graft polymerization [2] is a technique in which a monomer is polymerized using electron-beam irradiation starting from a radical generated in the polymer chain. Using this technology, it is possible to impart a completely different functionality to the polymer used as the base material. The electron-beam graft polymerization process is shown in Fig. 7.9. The polymer substrate is irradiated with an electron beam to obtain a predetermined absorbed dose (generally several tens to several hundreds of kGy) to generate radicals, which react with a monomer bearing a functional group such as a vinyl group. Thus, the graft chains grew. The same polymerization treatment process has been used for plasma graft polymerization. However, because the energy of the electron beam is larger, electron-beam graft polymerization must be a double-sided treatment because the beam passes through the target substrate. In plasma/electron-beam graft polymerization, the monomer is chemically bonded to the base material, unlike ordinary surface treatment; therefore, almost no temporal deterioration (time dependency) in the hydrophilicity and anti-fogging properties is observed. It is also possible to easily perform graft polymerization inside the film or fiber. Electron-beam gun

Electron beam

Substrate

Monomer molecules

Graft polymerization chain

Fig. 7.9 Electron-beam graft polymerization process using electron-beam irradiation [2]

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7 Plasma and Electron-Beam Technologies Used for Surface Treatment …

7.3.2 Anti-fog Treatment Application Example As an example, the effect of electron-beam irradiation on the fogging of transparent polycarbonate plastic [3] is explained. The electron-beam irradiation processing device used is an electro-curtain processor (Type CB175/15/180L, Iwasaki Electric Co., Ltd., Japan), as shown in Fig. 7.10. The main feature of this device is that it can radiate electron beams in a nitrogen gas stream at atmospheric pressure. In addition, the irradiation process radiates the electron beam in a pulse manner on the conveyor, which significantly suppresses the increase in the surface temperature. The results show that the polycarbonate sample is cleared of cloudiness in a short time of 4.5 s after irradiation with 0.432 MGy. In other words, the electron-beam irradiation treatment reduced the clear time and obtained an anti-fogging effect, as in the case of inorganic materials. The wettability of the material toward water is likely to have a significant anti-fog effect. Therefore, a change in the surface free energy is obtained from the water contact angle. As a result, the surface energy before electron-beam irradiation is 90 N/m. On the other hand, this increases to 130 N/m after electron-beam irradiation with 0.432 MGy [3]. This result confirmed that the surface of the polycarbonate plastic is activated by the electron-beam irradiation treatment, improving wettability. It is presumed that this results in the anti-fog effect. Other examples of anti-fog treatment using electron-beam irradiation have been reported for dental mirrors, endoscope lenses, medical protective glasses, medical glasses, and ITO thin films. Fig. 7.10 Outline of electron-beam irradiation equipment (created based on Fig. 1 in Ref. [3])

Vacuum chamber Electron-beam gun (cathode) Foil window (anode) Treatment region (Electron beam)

N gas

Sample

7.4 Plasma Treatment for Medical Applications Fig. 7.11 Example of a medical endoscope (By melvil, CC BY-SA 4.0, https://commons.wikimedia. org/wiki/File:PENTAX_Col onoscope001.jpg)

181

Adhesion to forceps, metals, and needles

Marks, ticks

7.4 Plasma Treatment for Medical Applications 7.4.1 Surface Treatment for an Endoscope Figure 7.11 shows an example of a medical endoscope. Plasma surface treatment can be carried out to draw marks and ticks on the surface of the insertion tube because it is usually made using fluoroplastic and it is difficult to draw so that the ticks does not disappear due to friction without the surface treatment. Adhesion of the tube to the forceps, metals, and needles is also difficult without the surface treatment. Currently, PTFE surface treatment is used in medical equipment and instruments. Fluoroplastic tubes are often used for dialysis machines, body fluid transports. Surface treatments are often required to join fluoroplastic tubes.

7.4.2 Plasma Jet Sterilization of Surfaces Plasma sterilization is described in this subsection with figures based on Ref. [4]. Figure 7.12 shows a schematic of the experimental apparatus used for the sterilization process. The sterilization system was originally developed to achieve nonthermal plasma surface treatment by introducing a corona discharge-induced air plasma jet and pulse modulation as explained in Chap. 1. A plasma torch is placed over the test sample. It is equipped with a fan and discharge electrodes. The fan causes air to flow from the outside into the torch, generating a corona discharge-induced nonthermal plasma jet at the discharge electrodes. The gas temperature of the plasma jet is maintained below 50 °C, and the discharge electrodes comprise two wires. A highvoltage power supply that can generate a pulse-modulated high voltage is connected to the discharge electrodes.

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7 Plasma and Electron-Beam Technologies Used for Surface Treatment …

Fig. 7.12 Schematic of the experimental apparatus used for sterilization

The plasma jet blows onto the test sample on which a bacterial or yeast strain (E. coli or S. cerevisiae) is set, resulting in sterilization. The distance between the discharge electrodes and test pieces is 20 mm, the air flow rate is 40–50 L/min, and the flow velocity at the outlet of the torch is 7.0 m/s. The experiment is performed under ambient conditions at 25 °C with a relative humidity of 70% in a draft chamber. Figure 7.13 shows the results of the sterilization tests against E. coli. Images of the test samples and the number of bacteria colonies are shown in the figure. Although the concentration of active species at the center is probably higher than on the sides, it seems to be illuminated under the conditions at 3 and 4 s. A possible reason is that the initial concentration of E. coli near the center is higher; therefore, it requires a longer time for sterilization. There are 27 colonies on the nontreated test sample and three colonies on the test sample treated with plasma for 1 s. These results indicate an 88% reduction in the number of colonies. Furthermore, no colonies are observed on the test samples after treatment for 5 s.

7.4 Plasma Treatment for Medical Applications

Number of colonies

30 25

Tested papers 0s

1s

183

2s

3s

4s

5s

20 15 10 5 0 0

1

2

3

4

5

Duration of plasma application (s)

Fig. 7.13 Results of sterilization tests against E. coli

Figure 7.14 shows the results of the sterilization tests against S. cerevisiae. The test sample appears to be illuminated when the plasma jet irradiates for 5 s. This is attributed to the untreated backside colonies being slightly visible. The number of colonies on the untreated test samples is too large to count. However, only a single colony remained on the test sample after treatment for 1 s. These results indicate a significant reduction in the number of colonies. Moreover, no colonies are observed on the test sample treated for more than 3 s. Several tests are performed against E. coli and S. cerevisiae. Similar sterilization effects are observed for all the samples. When compared with other sterilization methods, complete sterilization is achieved in a shorter time of 5 s. It has been suggested that some active species such as ozone, superoxide anions (·O2 − ), hydroxyl radicals, and ultraviolet radiation contribute to the sterilization effect of nonthermal plasmas in tubes for medical purposes. Our results indicate that a similar effect occurred in the current study. In view of the results of the gas component measurements, it is suggested that the bacteria and yeast may be killed by the strong oxidation ability of ozone, even at the low concentrations found in these tests. NO2 is a strong oxidizing gas that reacts with water to produce nitric acid. Nitric acid can also be easily transformed into NO2 upon reaction with atmospheric oxygen. Although the sterilization mechanism of NO2 has not yet been reported, it is possible that the NO2 induced by the air plasma jet reacts with water or moisture from bacteria or yeast, resulting in the death of microorganisms because of its strong acidity and oxidizing ability. Furthermore, hydroxyl radicals have a strong oxidation ability. Therefore, they can also contribute to the sterilization process. More research on the effects on the cell membranes of nucleic acids is needed to fully understand the sterilization mechanism. All sterilization processes are performed at a duty cycle of 50%. The input power can be controlled by selecting the duty cycle. A moderate value of 50% is selected in the present study. The sterilization effects at different duty cycles should be studied as part of future work.

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Number of colonies

Extremely large

Tested papers 0s 1s

25

2s

3s

4s

5s

20 15 10 5 0 0

1

2

3

4

5

Duration of plasma application (s)

Fig. 7.14 Results of sterilization tests against S. cerevisiae

7.5 Conclusions Plasma and electron-beam technologies used for surface treatment applications have been described in this chapter. The hybrid plasma–hydrophilic treatment process has been explained, in which the targets are PTFE and glass. The detailed procedure for the hybrid plasma surface treatment process has been explained in the preceding chapter. The electron-beam irradiation treatment process used to introduce anti-fog properties to the material surfaces has been described based on the surface treatment method. An electron beam is a state of plasma with high electron energy, and effective treatment can be performed with its use. Finally, the principles and an example of its implementation in medical instruments have been explained. In the future, it will be applied to various materials such as polymers, glasses, metals, and ceramics, and it is expected that the spread and scope of the application of plasma/electronbeam irradiation treatments will be further expanded by increasing the number of successful cases.

References 1. Z. Feng, N. Saeki, T. Kuroki, M. Tahara, M. Okubo, Magnetic-field-assisted gliding arc discharge plasma for surface modification. IEEE Trans. Plasma Sci. 39(11), 2846–2847 (2011) 2. Y. Okumura, Denshi sen gurafuto jyugou ni yoru koubunshi no shinsui ka gijyutsu (English translated title: Polymer hydrophilization technology by electron-beam graft polymerization), Cho shinsui/shinyusei hyoumen no gijyutsu (English translated book title: Ultra-hydrophilic/ hydrophobic surface technology), section 15 (Science & Technology Co., Ltd. 2018), pp. 155– 160. (in Japanese)

References

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3. K. Oguri, T. Takahashi, M. Kadowaki, A. Tonegawa, Y. Nishi, Tomei polycarbonate jyushi no kumori ni oyobosu denshisen syousya no eikyo ni tsuite (English translated title: The effect of electron beam irradiation on the fogging of transparent polycarbonate plastic). J. Jap. Inst. Metals Mater. 68(8), 537–539 (2004). (in Japanese) 4. T. Kuwahara, T. Kuroki, K. Yoshida, N. Saeki, M. Okubo, Development of sterilization device using air nonthermal plasma jet induced by atmospheric pressure corona discharge. Thin Solid Films 523, 2–5 (2012)

Chapter 8

Measurement Technology for Functional Groups Generated by Plasma Treatment

8.1 Introduction It is fundamental to measure, analyze, and observe the functional groups generated by atmospheric-pressure plasma treatment, which is an effective technique to make the surface of a material hydrophilic or permanently hydrophobic. In this chapter, techniques used to measure, analyze, and observe the functional groups and chemical species generated by plasma treatment are described. Figure 8.1 shows an outline of a typical atmospheric-pressure plasma surface treatment device. An unsteady voltage, such as a pulse voltage, is applied between the sharp electrodes through which noble gas or the like passes, resulting in the formation of a plasma jet, and a substrate such as a polymer film, e.g., conveyed by a roller, passes between the jet and a surface. The jet collides with only one surface in this case. Although the use of this corona discharge electrode method for surface treatment is relatively new, some commercial products have been developed. Owing to the use of a plasma jet in this method, processing three-dimensional and large objects is possible. However, to process a larger area, the movement of the torch according to the size or shape of the processing surfaces and the arrangement of multiple torches are necessary. In atmospheric-pressure plasma surface treatment, these processing treatment systems have been used appropriately, depending on the situation. Plasma treatment improves the adhesiveness of the substrate surfaces of plastics, polymers, glass, and metals in the treatment system, as illustrated in Fig. 8.1, often making the surfaces of these materials remarkably hydrophilic. The main and side chains of the bonds on a material’s surface (in the case of metal, an oxide layer or oil film on the surface) dissociate into radicals by the action of high-energy electrons (approximately 1–10 eV) generated by the atmospheric-pressure plasma discharge. On the other hand, molecules of atmospheric gases, such as air and moisture, also dissociate and become radicals. The electrons, radicals, and ions formed by the plasma decompose organic substances such as fats and oils present as “dirt” on the surface of the material, rendering the surface hydrophilic. Moreover, hydrophilic © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 M. Okubo, Nonthermal Plasma Surface Modification of Materials, https://doi.org/10.1007/978-981-99-4506-1_8

187

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8 Measurement Technology for Functional Groups Generated by Plasma …

Fig. 8.1 Substrate surface treatment device using atmospheric-pressure low-temperature plasma (plasma torch)

Plasma gas 1

Atmospheric gas 2

Plasma jet Substrate

functional groups, such as hydroxyl groups (–OH), carbonyl groups (> C=O), and carboxyl groups (–COOH), are generated on the surface of the material by the radical formation in the gas phase or reactions between radicals. Hence, the surface becomes hydrophilic and the surface free energy increases, thereby facilitating bonding and association with other materials. To better understand the hydrophilization reaction described above, it is vital to obtain information on the byproducts generated on the surface of the substrate during plasma treatment using plasma gas 1 and atmospheric gas 2 in Fig. 8.1, as well as the functional groups generated. The plasma device is not limited to the one application illustrated in Fig. 8.1. I intend to present the results of Fourier transform infrared spectroscopy (FTIR) analysis of functional groups and byproducts and those of a representative X-ray photoelectron spectroscopy (XPS) analysis obtained from our experiments where plasma gas 1, atmosphere gas 2, or the substrate are varied.

8.2 Analysis of Functional Groups Generated by Plasma Treatment 8.2.1 Functional Group Analysis of Plasma Graft-Polymerized Acrylic Acid Film on PTFE by FTIR Using argon as plasma gas 1, acrylic acid (CH2 =CHCOOH) vapor as atmosphere gas 2, and polytetrafluoroethylene (PTFE; thickness: 100 µm, dimensions: 21 cm × 30 cm; Nippon Valker Industries, Ltd.) film as the substrate, an acrylic acid plasma graft-polymerized film on the substrate surface is produced. The acrylic acid used is purchased from FUJIFILM Wako Pure Chemical Corporation, with a purity of 98%. The film is analyzed using FTIR [1] to determine the functional groups, especially carboxyl groups (–COOH), which contribute to improved hydrophilicity and adhesiveness. The FTIR spectra are obtained using AVATAR 360 FTIR combined with single-reflection attenuated total reflection (Thermo Nicolet Co.) and the OMNIC

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189

control software. The treated sample (measurement sample) is a fluoroplastic PTFE film (thickness: 100 µm, dimensions: 21 cm × 30 cm, manufactured by Nippon Valker Industries, Ltd.). The FTIR spectra for the untreated PTFE and PTFE treated by atmosphericpressure plasma graft polymerization with acrylic acid vapor are shown in Fig. 8.2 (a: full spectra) and (b: magnified spectra). The spectrum labeled “Untreated” corresponds to the untreated sample, and the “Treated 1” spectrum to the sample treated by moving the plasma torch back and forth once in the lateral direction. This is a normal plasma processing condition. The spectrum labeled “Treated 2” is for the sample treated by moving the plasma torch for ten round trips to increase the thickness of the graft-polymerized layer and to increase and clarify the peak intensity. In Treated 1 and Treated 2 samples, a peak corresponding to a carboxyl group is detected at approximately 1700 cm−1 , which is absent in the spectrum of the untreated sample. This result shows that the treatment causes the formation of Fig. 8.2 FTIR spectra of PTFE treated by atmospheric-pressure plasma graft polymerization [1]. a Full spectra. b Magnified spectra

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8 Measurement Technology for Functional Groups Generated by Plasma …

–COOH (carboxyl groups) on the surface of the film, thereby improving hydrophilicity and adhesiveness. In addition, –CH2 – (methylene group) is detected in the treated samples at approximately 2850 and 2925 cm−1 , which do not appear in the untreated sample. Furthermore, peaks corresponding to –CF2 – (difluoromethylene group) and –CF3 (trifluoromethylene group) are detected in all the samples in the range of 1100–1200 cm−1 . This does not corroborate with XPS surface analysis, i.e., no peaks corresponding to –CF2 – and –CF3 are detected in the treated samples. FTIR provides a more in-depth analysis of the chemical structure compared to XPS. Considering that the graft-polymerized layer is thin (200–300 nm or less), the internal chemical structure can also be detected.

8.2.2 Functional Group Analysis of Plasma Graft-Polymerized Acrylic Acid Film on PTFE by XPS XPS or ESCA (ESCA5700, ULVAC-PHI, Ltd) analysis of the surface of the untreated and treated PTFE (thickness: 200 mm, dimensions: 21 cm × 30 cm; Nafron tape; Nichias Co., Ltd.) films is performed [2]. The acrylic acid used is purchased from FUJIFILM Wako Pure Chemical Corporation, with a purity of 98%. Figures 8.3a, b show the C1s XPS spectra of the untreated and treated PTFE films, respectively. Two acrylic acid evaporation vessels are used at a monomer temperature of 55 °C. In Fig. 8.3a, only the peak corresponding to –CF2 – (difluoromethylene group), which is the origin of the hydrophobicity of the film, is detected at 292 eV. On the other hand, as shown in Fig. 8.3b, the peak height of –CF2 – of the treated film is much smaller than that of the untreated film. Peaks corresponding to O– C=O (carboxyl group), > C=O (carbonyl group), –C–OH (alcohol), and –C–C– (carbon) are observed at approximately 288.8, 287.8, 286, and 284.4 eV, respectively. It is difficult to compare the XPS spectra obtained in the experiment with a single evaporation vessel shown in Ref. [3] to the result in Fig. 8.3 because the XPS devices used for these analyses are different. However, the same trends are observed in functional group analysis. Hydrophilic functional groups such as carboxyl, carbonyl, and alcohol groups reduce the contact angle of the treated surface. In general, when a hydrophilic functional group increases onto a polymer surface, the wettability of the surface improves, which enhances the reaction of the adhesive with the surface. As a result, the adhesiveness of the surface improves.

8.3 Analysis of Chemical Species Formed by Plasma Treatment

191

Fig. 8.3 C1s XPS spectra of the a untreated and b treated PTFE films [2]

8.3 Analysis of Chemical Species Formed by Plasma Treatment 8.3.1 Analysis of Byproducts During Treatment of Ammonia Gas and Acetaldehyde Gas with Plasma by FTIR Ammonia plasma is used to modify the surface of a material with amine groups. Acetaldehyde is a typical unsaturated hydrocarbon, and its plasma can modify the surface of materials with hydrocarbon functional groups. In Sect. 8.3.1, plasma gas 1 is used as ammonia-diluted gas or acetaldehyde-diluted gas, and the results of the measurement and analysis of the chemical species formed in the gas by plasma treatment are explained with a table and figures based on Ref. [4]. In the test, ammonia and acetaldehyde gas treatments are carried out using a barrier-type multilayer plasma reactor (BMPR), and the byproducts are identified and their concentrations measured. FTIR spectroscopy (FTS3000; Bio-Rad Laboratories, Inc.) with a mercury cadmium

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8 Measurement Technology for Functional Groups Generated by Plasma …

telluride detector, 100 mL-gas cell, and an optical path length of 2.4 m (manufactured by Infrared Analysis, Inc.) is used. The Bio-Rad Merlin and Bio-Rad Win-IR software packages are used for control and analysis, respectively. For the ammonia gas analysis experiment, a mixture of N2 adjusted to 950 ppm ammonia and dry air (relative humidity of 4%) is generated from a compressor through a filter and dryer. The diluted ammonia gas treated with BMPR is used for the analysis. Next, the simulated gas used in the acetaldehyde analysis experiment consisted of a mixture of 1020 ppm CH3 CHO adjusted with N2 and dry air (relative humidity of 4%) generated from the compressor through a filter and dryer. The diluted acetaldehyde gas treated with BMPR is analyzed under the following measurement conditions: resolution = 0.5 cm−1 , integration number = 128 times, and the concentration of ammonia or acetaldehyde is adjusted during the experiment by the mass-flow controller of each gas line. When the voltage at the plasma reactor is changed, the measurement is performed after waiting for approximately 2 min until steady state is obtained. Figure 8.4 shows the FTIR spectrum of the byproducts of ammonia removal at a flow rate of 10 L/min and an applied voltage of 8 kV. Peaks of O3 , HNO3 , HCOOH, CO, N2 O, CO2 , and particulate NH4 NO3 are detected, confirming that each component is generated by the plasma. It is also noted that O3 is generated at the highest concentration. Table 8.1 shows the variation in ammonia and HNO3 concentrations at each flow rate when the applied voltage is 8 kV. HNO3 is generated when all ammonia is removed at low flow rates (5.0 and 1.0 L/min), while at 20 L/min, ammonia is not completely removed and HNO3 is not generated. These results show that the HNO3 generated by the plasma immediately reacted with ammonia to generate NH4 NO3 . Figure 8.5 shows the FTIR spectrum of the byproducts of acetaldehyde removal at a flow rate of 10 L/min and an applied voltage of 8 kV. Peaks corresponding to Fig. 8.4 FTIR spectrum of the byproducts of ammonia plasma processing

Table 8.1 Change in the ammonia and HNO3 concentrations at each flow rate at an applied voltage of 8 kV

Flow rate L/min

NH3 ppm

HNO3 ppm

5.0

0

46.7

10

0

16.8

20

14

0

8.3 Analysis of Chemical Species Formed by Plasma Treatment

193

Fig. 8.5 FTIR spectra of byproducts of acetaldehyde plasma processing

Fig. 8.6 Concentration of byproducts as a function of applied voltage at a flow rate of 10 L/min

O3 , HNO3 , HCOOH, CO, N2 O, and CO2 are confirmed. The CO2 concentration cannot be determined because it is beyond the limit of detection. In addition, NO and NO2 cannot be confirmed because they overlap the wavenumber region of water. Figure 8.6 shows the generated concentrations of O3 , HNO3 , HCOOH, CO, and N2 O with respect to the applied voltage at a flow rate of 10 L/min. The results indicate that more O3 is generated than other byproducts. In addition to O3 , HNO3 and CO are generated at concentrations up to nearly 100 ppm. A maximum of 40 ppm HCOOH and N2 O is generated. Since harmful O3 can be removed using a catalyst such as MnO2 , it is optimal to operate at approximately 6 kV (9.9 W) for air cleaning, where almost no byproducts other than O3 are generated.

8.3.2 Analysis of Byproducts During Treatment of CF4 Gas with ICP by FTIR Carbon tetrafluoride (CF4 ) is used for etching and hydrophobic treatment of the surface of semiconductor substrates and is one of the most stable perfluorocarbons (PFCs) with high global warming potential. A CF4 (global warming potential of

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6500) treatment is conducted by directly introducing 100% CF4 gas (plasma gas 1) into an inductively coupled plasma (ICP) reactor. The effects of the flow rate, power, pressure, and O2 addition amount on the CF4 processing efficiency are investigated [5]. The equipment, control software, and analysis software used for FTIR are described in Sect. 8.3.1. In the experiment, N2 is used as the background gas, 100% CF4 gas as the measurement sample, and O2 , Ar, He, and N2 as the additive gases. The flow rate is adjusted using a mass-flow controller. These mixed gases are introduced into an ICP reactor, treated, and used as measurement samples. All the flow rates in the experiments are expressed in the standard state (20 °C, 0.1 MPa). The flow rate in the reactor is larger than in the standard state, approximately 2500 times at 20 °C and 40 Pa, and the residence time in the reactor is 0.01 s. The measurement conditions and methods are as follows: resolution = 0.5 cm−1 , integration number = 128, and the CF4 concentration during the experiment is adjusted by the mass-flow controller of each gas line. The sample is diluted 32.3 times with N2 . Figure 8.7 shows the FTIR spectra of each component of the exhaust gas after plasma treatment. The flow rates of CF4 and O2 are 0.2 and 0.4 L/min, respectively, and the pressure is 80 Pa. The power during plasma processing is set to 2.0 kW, and the CF4 processing efficiency is 96%. Peaks corresponding to CF4 , CO2 , CO, COF2 , HF, SiF4 , and H2 O are observed. HF is generated because a small amount of moisture in the air is mixed during sampling, and SiF4 is generated by etching a small amount of SiO2 in the alumina tube. It is considered that CO2 , CO, and COF2 are produced by the following reactions: CF4 + O2 → CO2 + 2F2

(8.1)

CF4 + O → CO + 2F2

(8.2)

CF4 + O → COF2 + F2

(8.3)

Although both COF2 and CO are harmful substances, they can be removed by passing over an adsorbent downstream of the dry vacuum pump. Although F2 and COF2 are corrosive gases, no corrosion was observed in the equipment within the condition of this experiment.

8.3.3 Analysis of Byproducts During Treatment of Xylene Gas with Plasma by FTIR It is possible to form a uniform pinhole-free thin film regardless of its shape using para-xylene, which polymerizes into a film when it is in contact with the surface of an object at room temperature. Using xylene C8 H10 as plasma gas 1, the treatment

8.3 Analysis of Chemical Species Formed by Plasma Treatment

195

Fig. 8.7 FTIR spectra of byproducts during CF4 plasma processing [5]

rate and byproducts are determined when xylene adsorbed on the adsorbent is treated with a nonthermal plasma flow [6]. The equipment, control software, and analysis software used for FTIR are described in Sect. 8.3.1. N2 is used as the background gas. There are three isomers of xylene: o-xylene (1,2-dimethylbenzene), m-xylene (1,3-dimethylbenzene), and p-xylene (1,4-dimethylbenzene). Liquid p-xylene (purity 99%) and liquid xylene mixture (purity 80%, o–:m–:p– = 22.4:38.7:20.0) are used to generate xylene gas. After adsorbing these gases on the hydrophobic adsorbents (HiSivTM 1000 and HiSivTM 3000, Union Showa Co., Ltd.) located in the pipe, the plasma flow containing radicals is passed to perform the plasma treatment. The plasma-treated gas is then subjected to FTIR under the following conditions: resolution = 0.5 cm−1 , integration number = 128, and no gas dilution. Nonthermal plasma is generated using an AC 60 Hz neon transformer as a power source. Figure 8.8 shows the FTIR spectrum obtained during the 90 min treatment. The H2 O, CO2 , CO, O3 , and N2 O peaks are detected. The wavenumber range of each peak is as follows: H2 O: 1300–2000 cm−1 , 3400–4000 cm−1 ; CO2 : 667 cm−1 , 2363 cm−1 ; CO: 2169 cm−1 ; O3 : 1053 cm−1 ; and N2 O: approximately 1300 cm−1 and 2580 cm−1 . Further, the N2 O concentration is determined to be 200 ppm.

8.3.4 Analysis of Byproducts During Plasma Treatment of TEOS by FTIR Tetraethyl orthosilicate (TEOS, Si(OC2 H5 )4 ) is used as a monomer in coating technology using the plasma chemical vapor deposition (CVD) method. In this test, TEOS is used as plasma gas 1 and processed with a high-frequency plasma device with 4 MHz frequency [7].

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Fig. 8.8 FTIR spectra of the byproducts of the xylene plasma treatment [6]

Fig. 8.9 FTIR spectra of the byproducts of TEOS processing [7]

The equipment, control software, and analysis software used for FTIR are described in Sect. 8.3.1. The background gas is the sampling gas downstream of the vacuum pump without TEOS. TEOS characteristics are obtained from the database of the National Institute Standards and Technology (NIST) webpage (http://webbook. nist.gov/chemistry/form-ser.html). The measurement sample is the exhaust gas when TEOS + oxygen gas is treated with the high-frequency plasma device, and the plasma power is varied (0, 100, 250, 400, and 600 W). The measurement conditions are as follows: resolution = 0.5 cm−1 , integration number = 128, and no gas dilution.

References

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Figure 8.8 shows the FTIR spectrum of the byproducts generated during TEOS treatment. As the plasma power increases from 0 to 600 W, the concentration of TEOS decreases, and acetylene is produced. In addition, the CO (e.g., at approximately 2169 cm−1 ) concentration increases with an increase in input power; at 600 W, approximately 2100 ppm is generated. The peak of the TEOS spectrum (e.g., at approximately 1157 cm−1 ) decreases as the input power increases. In addition, the processing efficiency increases to 99% at 600 W. The concentration of acetylene (e.g., at approximately 3322 cm−1 ) also increases with increasing input power up to 600 W, reaching approximately 2170 ppm. Additionally, SiH4 is not detected. Analysis of the spectra under each condition shows possible peaks that appear to correspond to ethylene in the vicinity of 2945–3200 cm−1 and 949 cm−1 . Since alcohol peak determination is difficult, it is not performed. In addition to SiH4 and SiF4 , Si2 H6 is a gaseous compound of Si; however, its waveform data is not obtained.

8.4 Conclusions The measurement, analysis, and observation results of the functional groups formed by plasma treatment, which is effective in making the surface of the material hydrophilic or hydrophobic, have been described. Owing to its important role in surface treatment, it is necessary to continue accumulating data in the future. In particular, in recent years, the author’s team has focused on atmospheric-pressure plasma hybrid surface treatment to improve the adhesiveness of fluoroplastics or Teflon for application in millimeter-wave electronic materials, medical instruments, and biocompatible materials, as well as related tests. It is also important to explain these results based on basic principles. In the future, the author’s team plans to measure, analyze, and observe the functional groups of various gases and materials.

References 1. M. Okubo, M. Tahara, Y. Aburatani, T. Kuroki, T. Hibino, Preparation of PTFE film with adhesive surface treated by atmospheric-pressure nonthermal plasma graft polymerization. IEEE Trans. Ind. Appl. 46(5), 1715–1721 (2010) 2. T. Kuroki, M. Nakamura, K. Hori, and M. Okubo, Effect of monomer concentration on adhesive strength of PTFE films treated with atmospheric-pressure nonthermal plasma graft polymerization. J. Electrostat. 108, 103526 (2020), 7p 3. K. Hori, S. Fujimoto, Y. Togashi, T. Kuroki, M. Okubo, Improvement in molecular- level adhesive strength of PTFE film treated by atmospheric plasma combined processing. IEEE Trans. Ind. Appl. 55(1), 825–832 (2019) 4. T. Kuroki, M. Okubo, T. Yamamoto, Indoor air cleaning technology using non-equilibrium plasma (odor removal in cigarette smoke). Trans. Jpn. Soc. Mechan. Eng. 67B(658), 1481–1486 (2001). (in Japanese)

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5. T. Kuroki, N. Saeki, M. Okubo, T. Yamamoto, Decomposition of CF4 exhaust gas from semiconductor manufacturing equipments by low pressure inductively coupled plasma (optimization of operating conditions and byproduct analysis). Trans. Jpn. Soc. Mechan. Eng. 70B(692), 1058–1063 (2004) 6. T. Kuroki, K. Hirai, R. Kawabata, M. Okubo, T. Yamamoto, Decomposition of xylene on adsorbent using nonthermal plasma with gas circulation. IEEE Trans. Ind. Appl. 46(2), 672–679 (2010) 7. M. Okubo, T. Kuroki, Measurement and analysis techniques for functional groups and chemical species formed by plasma treatment, [Mizu] to kinousei porima ni kansuru zairyo settukei, saishin ouyo (English translation book title: [Water] and material design for functional polymers, latest applications) Technical Information Institute. Section 3, 53–60 (2021). (in Japanese)

Concluding Remarks

The industrial applications of plasma technology are progressing in various fields, such as material surface modification treatment, environmental treatment, and bioapplication. Surface modification technology is used to control the hydrophilicity/ hydrophobicity of surfaces and interfaces of plastics, inorganic materials, metals, and composite materials for improving adhesiveness, cleaning, plating, functional electronic parts, liquid crystals, and producing semiconductors. Recently, a technology has been developed that uses plasma to modify the surface, achieving the bonding required for creating electrical circuit boards for high-speed communication. Additionally, another technology that enhances the adhesive strength among resins, metals, and composite materials to reduce the weight of automobiles has emerged. Under these circumstances, this book focuses on trends in surface modification technology for materials using plasma and is intended for front-line researchers/ engineers who are actively researching or applying this technology. It provides useful information for detailed technical methods in relevant technical fields. In each chapter, various topics regarding nonthermal plasma surface treatment technology for hydrophilic, hydrophobic, cleaning, adhesion improvement, anti-fog treatment, and sterilization technologies have been described. The creation of high-strength bonding interfaces is a common theme in this field, especially for the lamination of dissimilar materials. In this regard, joining and bonding metals and plastics is a key technology, and many papers in this special issue discuss this technology. For example, in the field of high-speed communication, the technique for laminating a thin metal film onto a flexible resin with high strength is important. This problem has already been solved by applying the technology for bonding thin metal films to rigid substrates, such as high-frequency strip antennas, to products. For the formation of thin metal films on flexible high-frequency cables and flexible substrates, it is necessary to achieve ultra-strong bonding with a peeling strength of the thin film of 10 N/mm or more.

© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 M. Okubo, Nonthermal Plasma Surface Modification of Materials, https://doi.org/10.1007/978-981-99-4506-1

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Concluding Remarks

The first advantage of plasma processing is its ability to easily reach ultrahigh temperatures, exceeding 10,000 °C, with high-temperature plasma. The second advantage is that the chemical reaction can be promoted regardless of whether the temperature is high or low while simultaneously changing the morphology of the surface. By realizing chemical and physical modification of the surface by highspeed electrons, positive and negative ions, and radicals generated by the plasma, super-strong adhesion can be achieved, which is otherwise difficult. Plasma can be generated relatively easily and utilized by high-voltage discharge with a relatively simple apparatus composed of electrodes and a power supply. It is expected that the material surface modification technology using plasma will continue to develop, with emerging applications to a wide range of fields, thereby becoming an indispensable manufacturing technology.

Appendix

A.1 Historical Image of Sakai City Humans settled in the region around present-day Sakai City, Osaka Prefecture, Japan, approximately 10,000 years ago. In the fourth and fifth centuries, the Imperial Court was established, and more than 100 emperor tombs were constructed in Sakai City. “Sakai” translates to “boundary” or “border” in English; the area was named as such because it is located at the boundary of three small prefectures that were established at the time. From the twelfth to fourteenth centuries, Sakai was developed as the main shipping base of western Japan. From the end of the fifteenth century to the end of the sixteenth century, when domestic wars were frequent in Japan, Sakai significantly developed as an international trade port. In the fifteenth and sixteenth centuries, it served as an import hub, i.e., an international trading city. The introduction of guns from Western countries is a typical example. Information was disseminated from the city to all of Japan. Although Japan was not open to foreign countries in this era, Sakai prospered as a unique independent city known as “Saccai” and formed an international trade base with countries such as China and European countries. A depiction of Sakai during its historical and prosperous times in the sixteenth and seventeenth centuries was drawn and published in a book written by the Dutch missionary and scholar Arnoldus Montanus. The image is shown in Fig. A.1. Several ocean-going international ships were observed near the Sakai Port. After the nineteenth century, rapid modernization occurred, including the development of modern industries, the expansion of the population and city area, and an increase in traffic, similar to other big cities in Japan. Although many ruins and prehistoric sites were lost in the bombings during World War II, Sakai continued to develop as an industrial city in Japan. By the early 1900s, the city accounted for more than 80% of Japan’s production of bicycle parts, and became internationally known as the town of bicycles. The bicycle industry continued to function after World War II, and the traditional techniques of Sakai’s craftsmen have been handed down to modern high-tech bicycle manufacturers such as Shimano Inc. © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 M. Okubo, Nonthermal Plasma Surface Modification of Materials, https://doi.org/10.1007/978-981-99-4506-1

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Fig. A.1 Historical illustration at Sakai Port

Additionally, there is an ancient emperor’s tomb near the university, which has been under the control of the Imperial Household Agency since A.D. 300. An example of this is presented in Fig. A.2. It has been recognized as the oldest human nation in Japan. The route that connects the Nara and Sakai Ports is the oldest in Japan and is renowned for its prosperity in ancient times.

A.2 Principle Explanation of XPS Measurement Figure A.3 illustrates the principle of XPS, or X-ray photoelectron spectroscopy, also known as electron spectroscopy for chemical analysis (ESCA), and Fig. A.4 shows a photograph of the XPS instrument. When a solid sample surface is irradiated with soft X-rays (AlKα or MgKα radiation) of specific energy in a vacuum, electrons are emitted from the sample surface owing to the photoelectric effect. These electrons are called photoelectrons. Because these photoelectrons have an energy value specific to the element, the composition can be determined by measuring their energy value. Photoelectrons are also emitted from deep regions, but they are not detected because they lose their kinetic energy owing to inelastic scattering before they reach the

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Fig. A.2 Kofun, tomb of Emperor Hanzei, the 18th legendary emperor of Japan (A.D. 336–410) (https://en.wikipedia.org/wiki/Emperor_Hanzei)

surface of the sample. Only photoelectrons emitted from a depth of several nanometers that escape from the sample surface without inelastic scattering are detected and used for component analysis The horizontal axis of the spectrum corresponds to the energy of the electrons. The binding energy is obtained by subtracting the kinetic energy of the photoelectrons from that of the irradiating soft X-rays. The ratio of elements can be determined based on the type of element and the signal intensity based on the binding energy of the detected electrons because the inner-shell electrons of various atoms have specific binding energies. The detectable elements range from Li to U. The detection limit varies depending on the element but is approximately 0.1%. The first step in the analysis is to determine the elements present on the surface. Therefore, we first performed wide-area scan measurements in the range of 0–1100 eV. In addition, it is desirable to perform broad spectrum measurements on samples for which the elements to be measured are known to obtain information such as surface contamination. After identifying the peaks based on the broad spectrum, a narrow scan measurement (usually near the most sensitive peak) of each detected element is performed. The surface composition can be obtained from the spectral intensity and sensitivity coefficient, and the chemical state of the element can be obtained from the chemical shift. The features of XPS (or ESCA) analysis include: (1) elemental analysis of several nanometers from the surface (qualitative and quantitative analysis); (2) analysis of elements other than H and He; (3) capable of nondestructive analysis; (4) can be measured regardless of the electrical conductivity of the sample; (5) the chemical state of an element can be examined using chemical shift (peak shift).

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Electron analyzer Lens system

X-ray source

Electron multiplier

Sample XPS spectrum Fig. A.3 Schematic of the XPS principle

Fig. A.4 Photograph of an XPS (ESCA) instrument (Model: ULVAC-Phi, ESCA5700) [1]

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Fig. A.5 Schematic of the SEM principle

Electron beam

Secondary electron for SEM images

X-ray for elemental analysis Reflected electron

Cathode light Sample

A.3 Principle Explanation of SEM Measurement A scanning electron microscopy (SEM) uses electromagnetic lenses to focus an electron beam to a very small diameter that cannot be achieved with light, irradiates the sample with this spot beam as a probe, and creates an image based on the signals obtained from the sample. The operating principle of SEM is shown in Fig. A.5. Figure A.6 shows the photograph of an SEM instrument. It is equipped with an electron gun to generate electrons, a focusing lens (condenser lens), an objective lens for obtaining a fine electron-beam flux, and a magnetic field deflector (scanning coil) for scanning the electron beam. The surface of the sample is scanned using an electron beam with a diameter of several nanometers. The sample is attached to the sample holder, and the electron beam is irradiated at the sample position to be observed. Depending on the shape of the sample, various signals, such as secondary electrons, backscattered electrons, X-rays, and light, are emitted from the region irradiated with the electron beam. SEM detects the secondary electrons, which account for the largest signal among these, and displays the differences in the number of secondary electrons as a difference in brightness on display.

A.4 Principle Explanation of FTIR Measurement Fourier transform infrared spectroscopy (FTIR) is a technique that qualitatively evaluates the composition of a sample based on the measurement results obtained by irradiating the sample with infrared light. It is possible to measure samples of various shapes, such as solids (films, powders, etc.), liquids, and gases. Figure A.7 illustrates the principle of FTIR. An interferometer is built into the FTIR. The infrared light emitted from the infrared light source into the interferometer is split by a beam splitter into two parts, one on the fixed mirror side and the other

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Fig. A.6 Photograph of an SEM instrument (Model: HITACHI, S-4800) [1]

on the moving mirror side. The fixed mirror does not move; only the moving mirror moves, and the light reflected by both mirrors is combined. This combined light is an interference wave (interferogram), with a phase difference according to the moving distance of the movable mirror. The sample is irradiated with this interference wave and detected by a detector. The structure of an FTIR instrument mainly consists of an infrared light source, an interferometer, a laser, and a detector. Because the window material used inside the device may fog under conditions of high humidity, it must be a sealed structure. The FTIR measurement results are presented in several chapters of this book. Typically, by Fourier transforming the interferogram, continuous data in the range of 4000–400 cm−1 on the horizontal axis and absorbance on the vertical axis can be obtained. This is called an IR spectrum. For example, functional groups such as methyl (–CH3 ), carbonyl (>C=O), and carboxyl (–COOH) groups absorb only light of a specific wavenumber, resulting in an absorbance peak in the IR spectrum. IR spectra are stored in databases known as libraries. Such an operation is called a library search. There are various types of libraries, such as those for polymers, food additives, and organic solvents; therefore, it is necessary to use them appropriately. The items that can be qualitatively characterized using FTIR spectroscopy include organic compounds, such as resins, rubber, paper, plastics, fibers, fabrics, and adhesives, and inorganic compounds, such as sulfur compounds, phosphorus compounds, silicon compounds, and carbonates. It should be noted that FTIR cannot analyze metal elements. There are two primary FTIR measurement methods: the transmission method and the attenuated total reflection (ATR) method. The transmission method is the most basic measurement method, and the absorption spectrum can be obtained from the

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Fig. A.7 Schematic of the principle of FTIR

Infrared light source Optical path difference Interference light

Moving mirror

Beam splitter

Sample Detector

Fixed mirror Michelson interferometer

ratio of the intensity of the incident and transmitted light. For films, the sample is placed between two cells for measurement. In the case of a liquid, it is similarly sandwiched between the cells and measured. In the case of a powder, a tablet is produced using a tableting machine, and the tablet is placed in a dedicated cell and measured. In addition, gases can be qualitatively and quantitatively determined using a gas cell. In the ATR method, the sample is brought into close contact with a crystal that has a higher refractive index than the sample and irradiated with infrared light. The absorption spectrum of the surface layer of the sample is obtained by measuring the total reflected light. It serves as a versatile measurement method because it can measure samples, such as films, liquids, and powders, without pretreatment.

Reference

1. M. Narita, Development of Fluororesin-Rubber Composite Material by Improving Adhesiveness Using Nonthermal Plasma Graft Polymerization Treatment, Master’s Thesis in Fiscal Year 2020, Department of Mechanical Engineering, Osaka Prefecture University (2020), pp. 1–162 (in Japanese)

© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 M. Okubo, Nonthermal Plasma Surface Modification of Materials, https://doi.org/10.1007/978-981-99-4506-1

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