Inorganic and composite fibers: production, properties, and applications 9780081022283, 008102228X

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Inorganic and Composite Fibers

The Textile Institute Book Series Incorporated by Royal Charter in 1925, The Textile Institute was established as the professional body for the textile industry to provide support to businesses, practitioners and academics involved with textiles and to provide routes to professional qualifications through which Institute Members can demonstrate their professional competence. The Institute’s aim is to encourage learning, recognise achievement, reward excellence and disseminate information about the textiles, clothing and footwear industries and the associated science, design and technology; it has a global reach with individual and corporate members in over 80 countries. The Textile Institute Book Series supersedes the former ‘Woodhead Publishing Series in Textiles’, and represents a collaboration between The Textile Institute and Elsevier aimed at ensuring that Institute Members and the textile industry continue to have access to high calibre titles on textile science and technology. Books published in The Textile Institute Book Series are offered on the Elsevier web site at: www.elsevier.com/books-and-journals and are available to Textile Institute Members at a substantial discount. Textile Institute books still in print are also available directly from the Institute’s web site at: www.textileinstitute.org To place an order, or if you are interested in writing a book for this series, please contact Matthew Deans, Senior Publisher: [email protected]

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The Textile Institute Book Series

Inorganic and Composite Fibers Production, Properties, and Applications

Boris Mahltig Yordan Kyosev

An imprint of Elsevier

Woodhead Publishing is an imprint of Elsevier The Officers’ Mess Business Centre, Royston Road, Duxford, CB22 4QH, United Kingdom 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States The Boulevard, Langford Lane, Kidlington, OX5 1GB, United Kingdom © 2018 Elsevier Ltd. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein). Notices Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library ISBN: 978-0-08-102228-3 (print) ISBN: 978-0-08-102229-0 (online) For information on all Woodhead publications visit our website at https://www.elsevier.com/books-and-journals

Publisher: Matthew Deans Acquisition Editor: Brian Guerin Editorial Project Manager: Joshua Mearns Production Project Manager: Sojan P. Pazhayattil Cover Designer: Vistoria Pearson Typeset by SPi Global, India

Contributors Numbers in parenthesis indicate the pages on which the authors’ contributions begin.

M. Abdul Rauf Khan (105), Department of Physics, University of Azad Jammu & Kashmir, Muzaffarabad, Pakistan Pervaiz Ahmad (105), Department of Physics, University of Azad Jammu & Kashmir, Muzaffarabad, Pakistan; Department of Physics, Faculty of Science, University of Malaya, Kuala Lumpur, Malaysia Marcin Barburski (219), Technical University of Lodz, Lodz, Poland Robin B€ottjer (303), Faculty of Engineering Sciences and Mathematics, Bielefeld University of Applied Sciences, Bielefeld, Germany Holger Cebulla (131), TU Chemnitz, Chemnitz, Germany Andrea Ehrmann (303), Faculty of Engineering Sciences and Mathematics, Bielefeld University of Applied Sciences, Bielefeld, Germany Javad Foroughi (61), Intelligent Polymer Research Institute, University of Wollongong Australia, Wollongong, NSW, Australia Carsten Graßmann (303), Faculty of Textile and Clothing Technology, Hochschule Niederrhein—Niederrhein University of Applied Sciences, M€ onchengladbach, Germany Thomas Grethe (303), Faculty of Textile and Clothing Technology, Hochschule Niederrhein—Niederrhein University of Applied Sciences, M€ onchengladbach, Germany Christina Großerhode (303), Faculty of Engineering Sciences and Mathematics, Bielefeld University of Applied Sciences, Bielefeld, Germany Timo Grothe (303), Faculty of Engineering Sciences and Mathematics, Bielefeld University of Applied Sciences, Bielefeld, Germany Iren Juha´sz Junger (303), Faculty of Engineering Sciences and Mathematics, Bielefeld University of Applied Sciences, Bielefeld, Germany Mayeen Uddin Khandaker (105), Center for Biomedical Physics, School of Healthcare and Medical Sciences, Sunway University, Bandar Sunway, Selangor, Malaysia Katalin K€ uster (219), Hochschule Niederrhein University of Applied Sciences, M€onchengladbach, Germany Stepan V. Lomov (219), Department of Materials Engineering, KU, Leuven, Belgium

xi

xii Contributors

Boris Mahltig (1, 87, 165, 195, 277, 303), Faculty of Textile and Clothing Technology, Hochschule Niederrhein—Niederrhein University of Applied Sciences, M€onchengladbach, Germany Elizaveta Martynova (131), TU Chemnitz, Chemnitz, Germany Azadeh Mirabedini (61), Intelligent Polymer Research Institute, University of Wollongong Australia, Wollongong, NSW, Australia Uwe M€ohring (243), Textilforschungsinstitut Th€uringen-Vogtland e.V., Greiz, Germany Nawshad Muhammad (105), Interdisciplinary Research Centre in Biomedical Materials (IRCBM) COMSATS Institute of Information Technology, Lahore, Pakistan Andreas Neudeck (243), Textilforschungsinstitut Th€uringen-Vogtland e.V., Greiz, Germany Christopher Pastore (87, 165), Thomas Jefferson University, Kanbar College of Design, Engineering and Commerce, Philadelphia, PA, United States Jens Pusch (31, 53), Teijin Carbon Europe GmbH, Wuppertal, Germany Anne Schwarz-Pfeiffer (303), Faculty of Textile and Clothing Technology, Hochschule Niederrhein—Niederrhein University of Applied Sciences, M€ onchengladbach, Germany Jan Lukas Storck (303), Faculty of Engineering Sciences and Mathematics, Bielefeld University of Applied Sciences, Bielefeld, Germany Kristof Vanclooster (219), Department of Materials Engineering, KU; Siemens SISW, Leuven, Belgium Daria Wehlage (303), Faculty of Engineering Sciences and Mathematics, Bielefeld University of Applied Sciences, Bielefeld, Germany Bernd Wohlmann (31, 53), Teijin Carbon Europe GmbH, Wuppertal, Germany Yvonne Zimmermann (243), Textilforschungsinstitut Th€ uringen-Vogtland e.V., Greiz, Germany

Chapter 1

Introduction to Inorganic Fibers Boris Mahltig Faculty of Textile and Clothing Technology, Hochschule Niederrhein—Niederrhein University of € Applied Sciences, Monchengladbach, Germany

1.1 INORGANIC FIBERS This book is dedicated to inorganic fiber materials and fiber materials with significant inorganic content. The fibers containing an inorganic and as well an organic content are termed further as composite fibers. Various types of inorganic fibers are presented in the following chapters related to their specific fiber composition and material. However, before starting with a detailed view on each fiber, it should be explained what an inorganic fiber is. What could be a suitable definition for an inorganic fiber? These questions should be answered, especially in comparison to other fiber materials, which do not belong to the group of inorganic materials. The term “inorganic fiber” is related to the field of science known as “inorganic chemistry.” The field of inorganic chemistry summarizes the chemistry of all inorganic compounds and inorganic materials. For this, it can be stated that inorganic fibers are fibers built up by inorganic materials. In chemistry, usually materials or compounds are distinguished into inorganic and organic ones. Organic chemicals contain the chemical elements carbon and hydrogen bonded covalently together. Organic chemistry is also named the chemistry of carbon [1]. In addition to carbon and hydrogen, also other chemical elements can be part of an organic compound. These are most often elements like oxygen, nitrogen, chlorine, or sulfur. The simplest organic compound from the structural point of view is methane, CH4, and the simplest organic polymer is polyethylene. From the structural point of view, the simplest organic fiber is therefore a polyethylene fiber. Usually, compounds like carbon dioxide, CO2, carbonic acid, H2CO3, or carbon disulfide, CS2, are named inorganic compounds. Also, metal carbides and carbon in its elementary forms are considered to be inorganic compounds [2,3]. Inorganic and Composite Fibers. https://doi.org/10.1016/B978-0-08-102228-3.00001-3 © 2018 Elsevier Ltd. All rights reserved.

1

2

Inorganic and Composite Fibers

Typical organic compounds used for the production of traditional synthetic fibers are synthetic polymers like polyester, polypropylene, polyethylene, or polyamide. Natural fibers like cotton or wool are as well built up by organic compounds, which are cellulose or protein based. For this, both groups—synthetic and natural fibers—can be summarized under the head of organic fibers (Fig. 1.1). Inorganic fibers are those all types of fibers that are not built up by organic compounds. These inorganic materials building up fibers can be mainly sorted into three different subgroups, which are metals and alloys, metal or semimetal compounds, and carbon-based fibers. From a certain point of view, also fibers made from inorganic polymers could be named as inorganic fibers. In Fig. 1.1, in addition to the group of inorganic and organic fibers also an intermediate type, composite fibers, are mentioned. These composite fibers could be best described as fibers on the basis of organic materials containing as well high amounts of inorganic materials. The inorganic material is in these cases added to introduce new, advantageous, and functional properties to the fibers. Such composite fibers can be realized either by incorporation of inorganic particles in the organic fiber material or by coating the inorganic component onto the organic fiber. As a special example in this field, cellulosic fibers with embedded inorganic components exhibiting high X-ray absorption, as, for example, barium sulfate, can be mentioned. By this, fibers with X-ray protective properties are produced and X-ray protective clothes can be developed [4–6]. Such barium sulfate-containing fiber materials are also suitable to realize X-ray detectable fiber-based products for surgical dressings [7]. Composite fibers based on cellulose and inorganic components are described intensively in Chapter 12. Fibers with metal coatings and many of their applications are intensively presented in Chapter 11. These fiber materials coated with metals have electrical conductivity, EMI-shielding properties and, in some cases, even antimicrobial properties [8–14]. All these mentioned properties are related to the properties of the metal used for the coating of the fiber; so a transfer of metal properties to a fiber material is performed.

FIG. 1.1 Schematic overview of different fiber types. Especially, the subgroups of organic fibers are shown in detail, together with some typical examples.

Introduction to Inorganic Fibers Chapter

1

3

Another type of fiber, which could be understood as an intermediate between organic and inorganic fibers, are Preox fibers. Preox fibers are intensively described in Chapter 3. The Preox fibers are produced by partial pyrolysis of polyacrylnitrile fiber [15]. This pyrolysis could be described as the first step in the full conversion of polyacrylnitrile fiber into carbon fibers. Preox fibers are also known under the trade name PANOX-fibers supplied by SGLcarcon [16]. Fig. 1.2 and Table 1.1 give an overview of the different groups of inorganic fibers and related subgroups with a few examples. The different groups are further discussed in detail in the following sections and chapters. By this overview, it is clear that inorganic fibers are offered in a broad range of materials as fibers, monofilaments, staple fibers, short fibers, woven, and nonwovens. Compared to common organic fibers, the density of inorganic fibers is significantly higher. This statement is especially valid for some ceramic fibers and metal fibers, which are available as wires. Compared to common organic fibers, the prices of most inorganic fibers are also significantly higher. Table 1.2 gives a rough price overview comparing several ceramic fibers and organic fibers supplied by the same company [18]. The costs related to fiber length of the offered ceramic fibers are more than 10 times higher compared to the offered organic fibers. In Table 1.2, the price is given in € per fiber length (meter) and not by € per weight. Because of the higher density of ceramic fibers, the costs per weight are even higher for the ceramic fibers. A good summary of cost for other inorganic fibers such as different glass fibers, basalt, and carbon fibers can be found in the literature [17]. For different

FIG. 1.2 Schematic overview of different inorganic fiber types.

4

Inorganic and Composite Fibers

TABLE 1.1 Overview on Different Fiber Materials and Their Density

Type

Material

Specification

Density (g/cm3)

Reference

Glass

E-glass

Fiber

2.54

[17]

S-glass

Fiber

2.49

[17]

E-CR glass

Fiber

2.68

[17]

AR-glass

Fiber

2.68

[17]

Silica, SiO2

Monofilament

2.2

[18]

Alumina, Al2O3

Monofilament

3.9

[18]

Al2O3/SiO2 80/20

Monofilament

3.1

[18]

Al2O3/SiO2/B2O3 70/28/2

Monofilament

3.05

[18]

Al2O3/SiO2/B2O3 62/24/14

Monofilament

2.7

[18]

Silica/calcia/magnesia

Fabric

2.1

[18]

Zirconia stabilized with yttria; ZrO2/Y2O3

Fabric

5.9

[18]

Silicon carbide, SiC

Monofilament

3.2

[18]

Basalt

Fiber

2.6–2.8

[17]

Carbon

Fiber

1.76–2.15

[17, 18]

Copper

Wire

8.96

[18]

Silver

Wire

10.5

[18]

Iron

Wire

7.87

[18]

Boron

Wire

2.34–2.37

[18]

Polyester, PET

Fiber

1.3–1.4

[18]

Polyamide, PA66

Fiber

1.13

[18]

Aramid

Fiber

1.38

[18]

Polyphenylensulfide, PPS

Fiber

1.35

[18]

Polyetheretherketone, PEEK

Fiber

1.2–1.32

[18]

PTFE

Fiber

2.2

[18]

UHMW PE

Fiber

0.94

[18]

Regenerated cellulose

Fiber

1.44

[18]

Ceramic

Metal

Organic fibers

Introduction to Inorganic Fibers Chapter

1

5

TABLE 1.2 Overview on the Price of Some Ceramic Fiber Materials in Comparison to Common Organic Fiber Materials Type

Material

Price [€/m]

Ceramic

Alumina, Al2O3

7.3–14.8

Al2O3/SiO2 80/20

6.3

Al2O3/SiO2/B2O3 70/28/2

3.4

Al2O3/SiO2/B2O3 62/24/14

2.7

Silicon carbide, SiC

3.6–49.2

Polyester, PET

0.3–0.4

Polyamide, PA66

0.3–0.4

Aramid

0.3–2.1

Regenerated cellulose

0.3

Organic fibers

Given is the price for fiber length in meter [18].

glass and basalt fibers, costs in the range of 1–10 €/kg are mentioned. The price of carbon fibers is mentioned to be higher and is in the range of 15–30 €/kg [17] or 10–50 US$/kg [19]. Basalt fibers available as fiber strands or rovings are offered for 1–4 €/kg [20]. Fiber strands from E-glass are available for 3–14 €/kg, while carbon fiber strands at 54 €/kg are more expensive [21]. For different commercial ceramic oxide filaments high prices in the range of 270 up to 980 €/kg are reported. For nonoxide ceramic filaments even costs up to 9200 €/kg are mentioned [22]. Fabrics and nonwovens made from ceramic fibers are available for 1–6 US$/kg [19]. Because of these high costs, the application of inorganic fibers can be only justified if they exhibit outstanding and new properties that can be never reached by organic fibers. One aim of this book is therefore to present and emphasize the special properties of inorganic fibers and present also special applications, where these properties are demanded. One prominent property of most inorganic fibers is their high-temperature stability [15,23]. An overview of the maximum temperature of usage of several ceramic fiber materials compared to organic fibers is given in Table 1.3. The reported maximum temperature of usage is given by the supplier of the fibers. Of course, the mentioned temperatures are related to the type of use and the surrounding conditions. However, stated clearly the use of organic fibers is limited to temperatures not higher than 310°C. In comparison, inorganic ceramic fibers can be used at temperatures much higher than 1000°C. Inorganic fibers made from glass exhibit softening points in the range of 650–970°C, depending on their composition [15]. For this, the maximum temperature of usage of glass fiber materials is in between the values for organic

6

Inorganic and Composite Fibers

TABLE 1.3 Overview on Thermal Stability of Some Ceramic Fiber materials in Comparison to Organic Fiber Materials Maximum Temperature of Usage (°C)

Type

Fiber Material

Ceramic fibers

Alumina, Al2O3

1540/1600/ 1700

[18,24]

Al2O3/SiO2 80/20

1600

[18]

Al2O3/SiO2/B2O3 70/28/2

1350–1650

[18]

Al2O3/SiO2/B2O3 62/24/14

1200–1400

[18]

Zirconia stabilized with yttria; ZrO2/Y2O3

1930–2200

[18]

Polyamide, PA

110/115

[25]

Polyester, PET

150

[25]

Polyphenylensulfide, PPS

190/200/280

[15,25]

m-Aramide

200/220

[25]

Polyimide, PI

240/260

[25]

Polytetrafluoro ethylene PTFE, Teflon

250/260

[15,25]

Polyphenylenebenzobisoxazole PBO, Zyklon

310

[15]

Organic fibers

References

Given is the maximum temperature of usage.

fibers and ceramic fibers. However, even glass fibers are able to withstand two times or three times higher temperatures compared to organic fibers. In addition to the high thermal stability, inorganic fibers built up by metal oxide or semimetal oxide are as well stable against oxidative processes. Besides thermal stability, they exhibit therefore as well an absolute resistance against fire and flames [15]. For any kind of high-temperature application and especially for flame-resistant fabrics, inorganic fiber materials are an ideal choice [15,26]. Besides the resistance against heat, inorganic fibers have developed exhibiting excellent resistance against other influences as, for example, alkaline conditions [26,27]. The stability against any kind of organic solvent is as well an inherent property of inorganic fibers. Because of their stability against oxidation, many inorganic fibers are excellent substrates to carry photoactive materials. Photoactive materials like the titania-type anatase speed up oxidative processes under exposure to light [28]. They are photocatalysts and can be used

Introduction to Inorganic Fibers Chapter

1

7

for air purification or wastewater treatment [29–34]. Photocatalytic materials and fibers are intensively discussed in Chapter 13. However, if photoactive titania coatings are applied onto textiles made from organic fibers, the promoted oxidative process can also damage the organic textile substrate. A deposition of photocatalytic materials onto glass fiber fabrics is therefore advantageous. Inorganic fibers with a composition from titania and silica can even be produced with inherent strong photocatalytic properties [35]. A production process of those fibers is reported to start from polycarbosilane compounds [36]. One main field of applications of inorganic fibers is the construction of fiber-reinforced materials also named composite materials [15,37–42]. These materials are discussed as well in the following chapters. In fiber-reinforced materials, the inorganic fibers are surrounded by an organic polymer. These materials could be named composites built up by organic and inorganic components. An important influence on material properties and especially the material strength is here the interface between the inorganic fiber and the surrounding organic matrix. In fact, the interface has the task to keep both those different components together. For this, several methods have been developed to modify this interface with the aim for improvement of the interfacial adhesion between the fiber and the surrounding polymer matrix [43,44]. The surface composition and surface modification of inorganic fibers are therefore essential for their application in fiber-reinforced materials. For this, it is not only important to be aware about the composition of the fiber alone. Even more important is the knowledge about the fiber surface. Glass fibers are often modified with silane components, containing both inorganic and organic components in the same molecule. These silane compounds— also named silane coupling agents—are therefore ideal intermediate components to connect inorganic fibers with organic polymer matrices [45–47]. It has to be kept in mind that many properties of inorganic fibers are influenced by organic components deposited on the surface of the inorganic fibers. This is not only important for fiber-reinforced materials, but also for production processes like spinning, knitting, and weaving where an organic surface modification is often necessary to make the inorganic fibers processable. These issues are discussed especially in detail in Chapters 8 and 9.

1.2 METAL-BASED INORGANIC FIBERS This group summarizes all types of fibers built up by metals; they can be pure metals but also alloys. Chapter 10 is dedicated to metal fibers. Chapter 10 is especially related to fibers made from steel. The purpose of such metal fibers is mostly to transfer the special metal properties to a textile. Extraordinary metal properties are high conductivity for heat and electrical current. Electrically conductive fibers and textiles are often used in connecting electrical devices, sensors, or antennas to textile applications

8

Inorganic and Composite Fibers

[48]. Often, these applications are summarized under the buzz word “Smart Textiles” [49–51]. Related to the electric conductivity of metal fiber-containing materials is the property of shielding radio waves and microwaves. Products exhibiting these properties are often offered as shielding materials working against electrosmog [52–54]. Samples containing steel fibers offered for such purposes are presented in Fig. 1.3. Fig. 1.4 shows a microscopic image of a steel fiber recorded in high magnification. The shielding rate against microwaves given by the supplier (YShield GmbH, Ruhstorf, Germany) is 35 or 55 dB for radiation with a frequency of 1 GHz [55,56]. The application of steel fibers can be used as well for the production of antistatic textiles and carpets [57]. The steel grid shown in Figs. 1.3 and 1.4 is made from steel containing besides the metal iron as well the metals chromium and nickel. The composition of chemical elements on the surface of the metal fiber can be determined by the electron dispersive spectroscopy (EDS) method (Fig. 1.5) [58]. Beside the three metals iron, chromium, and nickel, also carbon can be detected on the surface of the steel fibers. Carbon can be detected, because it is a part of the steel composition but also because of the presence of oils or other surface-active components present on the surface of the steel fibers. These steel fibers are built up by several different metals; so they can be as well named as alloy fibers. Another alloy fiber with economic and technological relevance is the so-called Nitinol, built up by the metals titanium and nickel [59–62]. Nitinol materials are especially suitable for self-expanding stents and related to special properties such as shape memory and superelasticity [63]. Nitinol filaments can be used in knitting procedures to realize smart textiles with shape-memory properties [64]. An example for a Nitinol fiber is presented in Fig. 1.6. The related EDS spectrum is given in Fig. 1.7. The related elementary surface compositions of different Nitinol fibers are summarized in Fig. 1.8. The ratio of the metals nickel and titanium can vary for different types of Nitinol fibers. The differences can be caused by different surface treatments as, for example, mechanical polishing, or electropolishing [63]. The detection of carbon and oxygen on the surface of Nitinol fibers by EDS can be explained with different arguments such as the treatment of metal surfaces by oil for protective purposes. Also oxidative processes can occur on Nitinol surfaces. Materials and fibers containing the metals silver or copper are highly conductive and also have antimicrobial properties [65–68]. The antimicrobial properties of silver and copper metal are caused by the release of silver and copper ions from the metal surface into the surrounding medium [69–73]. Antimicrobial fibers and textiles are used for manifold applications, as, for example, wound bandages, treatment of atopic dermatitis, or sport textiles [74–76].

Introduction to Inorganic Fibers Chapter

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9

FIG. 1.3 SEM images of different samples containing steel fibers. First, sample of cotton fabric with added steel fibers—this product is distributed under the name Steel-Gray by YShield GmbH (Ruhstorf, Germany). Second, sample of a grid made from steel fibers—this product is distributed under the name HEG03 supplied by YShield GmbH (Ruhstorf, Germany). The shielding effect against radiation of 1 GHz is indicated directly in the images.

10

Inorganic and Composite Fibers

emishield

HL

D9,1 x1,0k

100 mm

FIG. 1.4 SEM image of a steel fiber in higher magnification. The fiber sample is taken from the product HEG03 supplied by YShield GmbH (Ruhstorf, Germany). 10000

C

Signal intensity (CPS)

8000

6000

Fe

Fe

4000

Cr 2000

Ni

Cr

Fe

Ni

0 0

1

2

3

4

5

6

7

8

Photon energy (keV) FIG. 1.5 EDS spectrum of a steel fiber. The chemical elements detected are directly indicated in the peaks in the spectrum. The fiber sample is taken from the product HEG03 supplied by YShield GmbH (Ruhstorf, Germany).

An example of a mesh produced from copper wires is shown in Fig. 1.9, together with a knitted fabric containing single polyamide fibers with silver coating. The SEM images are presented in low magnification to give an overview of the whole textile structure and in high magnification to support a

Introduction to Inorganic Fibers Chapter

1

11

2017.02.03 08:53 HL D6,3 x250

300 mm

2017.02.03 09:15 HL D5,9 x250

300 mm

2017.02.03 09:06 HL D6,3 x4,0k

20 mm

2017.02.03 09:27 HL D5,8 x4,0k

20 mm

FIG. 1.6 SEM images of two Nitinol fibers of different thicknesses (images left and right). The images are taken in different magnifications of 250 and 4000, to show as well the complete fiber and the surface topography in higher resolution.

detailed view of the specific surface topography of a single metal fiber. An ideal method to produce silver coatings on polyamide fibers is electrochemical deposition from aqueous solutions containing silver ions [77]. Such silver-coated polyamide fibers are especially introduced to realize antimicrobial textiles with high washing stability [68]. A special type of metal fibers are fibers made from boron. Compared to other metal fibers, the density of boron fibers is 2.3 g/cm3 significantly lower. It is even lower compared to the density of other commercial glass fibers (compare Table 1.1). For this, boron fibers can be used in the preparation of fiber-reinforced materials, with lightweight properties [24]. Boron fibers can be prepared by chemical vapor deposition CVD on a tungsten wire. During the CVD process, boron trichloride, BCl3, is reduced by hydrogen under the 1100—1250°C process temperature of the substrate [78]. Images from a cut boron fiber with a tungsten core of 5 μm diameter are shown in Figs. 1.10 and 1.11. The chosen measurement method

12

Inorganic and Composite Fibers

Signal intensity (CPS)

Ti

10000

O 5000

C

Ni Ni Ti Ni

0 0

2

4

6

8

Photon energy (keV) FIG. 1.7 EDS spectrum of Nitinol fiber (thicker fiber presented in SEM image—Fig. 1.6). The chemical elements related to the detected signals are indicated. Elements with content of less than 0.3 wt% are not indicated. This EDS spectrum is recorded under a magnification of 1000.

50

Nitinol; thin Nitino; thick

Surface content (wt-%)

40

30

20

10

0 Titanium

Nickel

Carbon

Oxygen

Detected chemical elements FIG. 1.8 Content of chemical elements on Nitinol fiber surfaces as detected by the EDS method. Elements with content of less than 0.3 wt.% are not indicated.

Introduction to Inorganic Fibers Chapter

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13

FIG. 1.9 SEM images taken from different fiber materials in different magnification. The images above show a mesh made of copper wires together with the surface topography of a single wire. The images below show knitted fabric containing silver-coated polyamide fibers.

FIG. 1.10 SEM image of a boron fiber with tungsten core.

14

Inorganic and Composite Fibers

FIG. 1.11 SEM-image of a boron fiber with tungsten core recorded in higher magnification together with the related EDS mapping showing the distribution of chemical elements.

allows the clear detection of the tungsten core due to the high density of this metal compared to the surrounding boron. The element distribution is also well depicted in the EDS mapping of the investigated boron fiber (Fig. 1.11). In the related EDS spectrum, the signal caused by the element boron is clearly distinguishable from the peak caused by carbon (Fig. 1.12).

Introduction to Inorganic Fibers Chapter

1

15

18000

Boron

Signal intensity (cps)

16000 14000 12000 10000

Carbon

8000

W

6000 4000 2000

W

O

W

0 0

1

2

3

4

Photon energy (keV)

FIG. 1.12 EDS spectrum of a boron fiber with tungsten core. The chemical elements detected are directly indicated at the peaks in the spectrum. This EDS spectrum is related to the SEM image and EDS mapping presented in Fig. 1.11.

1.3 INORGANIC FIBERS BASED ON METAL- OR SEMIMETAL COMPOUNDS The second group of inorganic fibers contains all fibers built up by metal- or semi-metal compounds. Fibers belonging to this group are often at first associated with the term “inorganic fiber”—probably due to the fact that this group contains glass fibers and ceramic fibers, which are well-known inorganic materials. Related to this second group of inorganic fibers are several chapters of this book, as Chapter 7 for glass fibers and Chapter 8 for ceramic fibers. Chapter 8 is mainly related to ceramic fibers made from oxidic materials, which are mainly oxides of aluminum and silicon. Beside oxidic-based ceramics also nonoxide ceramic materials are used for fiber production and application. A prominent example are here silicon carbide fibers (SiC fibers), which are presented in Chapter 5. Another type of non-oxide ceramic fibers is based on boron nitride (BN fibers), which are the topic of Chapter 6. The most prominent inorganic fiber is probably glass fiber made from E-glass. A fabric produced from E-glass in presented in Fig. 1.13. This E-glass fabric is offered by the company Culimeta (Textilglas-Technologie GmbH, Bersenbr€uck, Germany) and contains a weight per area of 200 g/m2 [79]. The related EDS spectrum of this E-glass fabric is presented in Fig. 1.14. The main components of the glass, silicon, aluminum, calcium, and oxygen, can be clearly detected. Further, the chemical element carbon is detected in significant amounts of 15 wt% on the surface of the glass fiber fabric (Table 1.4).

16

Inorganic and Composite Fibers

FIG. 1.13 SEM image of an E-glass fabric. The presented glass fiber fabric is supplied by the company Culimeta (Germany).

40000 Si

Signal intensity (cps)

30000

O

20000

Al

Ca

10000 C Ca 0 0

1

2 3 Photon energy (keV)

4

FIG. 1.14 EDS spectrum of an E-glass fabric, which is shown in Fig. 1.13 as a SEM image. The chemical elements detected are indicated by the related peaks in the spectrum.

TABLE 1.4 Content of Chemical Elements Determined on the Surface of Different Glass Fiber Fabrics Chemical Element; Content in wt% Si

Al

Ca

Mg

K

Fe

Na

C

O

E glass fabric HBO027

18.9  0.9

5.8  0.3

15.0  0.5

–

–

–

–

15.4  1.9

44.3  5.1

E glass fabric caramelized

22.7  1.0

7.5  0.4

18.1  0.6

0.6  0.1

–

–

–

3.5  0.6

48.6  5.8

Silica fabric preshrunk HSA018P

38.7  1.7

1.6  0.1

–

–

–

–

–

6.1  0.9

53.4  6.0

Silica fabric with vermiculite HSV0190

38.4  1.4

3.1  0.1

–

3.0  0.2

1.1  0.1

2.3  0.1

–

–

51.4  4.6

Silica fabric green preshrunk H55003

33.3  1.6

1.6  0.1

–

–

–

–

0.5  0.1

13.0  1.7

51.8  6.2

HBO019

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Inorganic and Composite Fibers

This detection of carbon can be explained by the presence of sizing agents and oils on the fiber surface. These agents are necessary to make the glass fiber processable in weaving production processes and also to introduce textile properties to the glass fiber fabric. A special product is caramelized glass fiber fabrics. These “caramelized” fabrics are thermally treated after a weaving process in a way that the sizing agent is partly decomposed. By this, a color change from white (glass fiber fabric) to brown (caramelized) is introduced, which is best described as caramel analogous to the famous sweets made from heat-treated sugar. Glass fiber fabrics can be also modified by inorganic materials. One appropriate method is here sol–gel technology allowing the application of silica nanosol coatings. This application of nanosol coatings is especially done to improve the abrasion fastness of glass fiber fabrics [80]. Silica fiber fabrics modified with vermiculites are also commercially available (Fig. 1.15). The vermiculite-containing silica fabric presented in Fig. 1.15 is supplied by the company Culimeta. The layered vermiculite structures can be clearly observed on the silica fibers. Vermiculites are layered mineral compounds built up mainly by silicon/aluminum oxide. Also, other metal oxides can be part of vermiculites in small amounts [81,82]. Vermiculite particles can be added to enhance the filter performance of fabrics from ceramic fibers. Also, an improvement in flame barrier properties is claimed [83,84]. Table 1.4 gives an overview of several commercially available glass and silica fiber fabrics together with their chemical surface composition. The chemical elements on the fabric surfaces are determined by the EDS method. For all fabric samples, the elements silicon, aluminum, and oxygen are detected. With the exception of the vermiculite-containing fabric, for all samples also carbon is determined on the fabric surface in significant amounts. The determination is done by EDS measurements and only elements with a content of 0.5 wt% or higher are recorded. The presented glass fiber fabrics are supplied by the company Culimeta (Germany). In addition to the both chapters related to fibers from metal oxides, two more chapters are dedicated especially to ceramic fibers, which are based on nonoxidic ceramics. These are Chapter 6 related to boron nitride, BN, and Chapter 5 related to silicon carbide, SiC [24,85,86]. One main difference between glass fibers and ceramic fibers is—beside their composition—their crystallinity. Ceramic fibers are of high crystallinity, while glass fibers are amorphous. Related to glass fibers are also basalt fibers made from natural stones. Basalt fibers are intensively described in Chapter 9. Analogous to glass fibers, basalt fibers are also mostly amorphous. However, they differ in composition due to their origin from natural stones, as basalt fibers contain a significant content of iron oxide [17,87–89]. This content of iron oxide gives the basalt fiber their typical dark coloration (Fig. 1.16).

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FIG. 1.15 Scanning electron microscopy (SEM) images of a silica fabric with vermiculite. The images are recorded in different magnifications. The presented fabric is supplied by the company Culimeta (Germany).

Beside their application in heat- and flame-resistant materials and their use in glass fiber-reinforced materials, glass fibers are used as optical fibers for light guiding [90]. This application is so important that the term glass fiber is often used as a synonym for optical fibers, even if the optical fibers are prepared from organic polymers.

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FIG. 1.16 Fabric made from basalt fibers (fibers supplied by Incotelogy GmbH, Basalt Fiber Technologies & Products, Pulheim, Germany).

Inorganic fibers are known for their high stability against many different environmental influences. For this, from the first view it seems to be nearly impossible that inorganic fiber could be biodegradable. However, biodegradable ceramic fibers prepared from silica sols are claimed in 2011 in an US patent [91]. Recently, in 2017 a further patent described bio-soluble inorganic fibers [92]. The term bio-soluble has probably the same meaning as the term biodegradable, but is perhaps more modern and more suitable to gain research funding. Although both patents claimed biodegradable fibers as own innovative invention, more than one decade earlier scientists published the realization of biodegradable inorganic silica fibers produced by using sol–gel technology with silica nanosols [93]. Such biodegradble inorganic silica fibers could be advantageous carrier systems for pharmaceuticals drugs as part of a controlled release arrangement [94, 95].

1.4 CARBON-BASED INORGANIC FIBERS The third group of inorganic fibers is based on carbon materials. Carbon in its elementary form belongs to the field of inorganic chemistry. For this, carbonbased fibers can be placed in the group of inorganic fibers [96]. However, it should be mentioned that some technicians and scientists from the textile field place carbon fibers also under the group of chemical fibers or synthetic fibers [97]. Other people from the textile area place carbon fibers into the group of high-tenancy inorganic fiber materials with high thermal stability [98].

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Carbon fibers are intensively discussed in Chapter 2. The production of commercially available carbon fibers is mostly done by the pyrolysis of organic fibers under controlled conditions. Most carbon fibers are produced by the pyrolysis of polyacrylonitrile, PAN, fibers [15,39]. However, also other methods for the production of carbon fibers are developed and reported [39]. One main reason for this development of alternative carbon fiber production is to reduce the cost of carbon fibers. One alternative method is the pyrolysis of lignin fibers or of fibers made from copolymers with the composition acrylonitrile–lignin [99]. The production of carbon fibers from PE-based precursor fibers is developed as a low-cost alternative. The polyethylene PE fibers are first produced by melt-spinning and then thermochemically treated with sulfuric acid to stabilize them for the following thermal treatment. This thermochemical treatment with sulfuric acid is done in 120°C hot concentrated sulfuric acid for 4 h. Following this, a thermal treatment is done to initiate carbonization to transform to carbon fiber. This heating regime uses temperatures from 250°C to 900°C [100]. Carbon fibers mainly used to build up fiber-reinforced materials, which are also named carbon matrix composites or carbon fiber composites. [15, 39,101,102]. The chemical element carbon occurs under ambient conditions mainly in two different types, which are graphite and diamond. These types are also named as the allotropes of carbon. These allotropes have been known since ancient times [103,104]. During the past decades, several new allotropic forms of carbon were discovered and produced. The first and probably the most prominent type in the series of discoveries are the fullerenes with the most prominent spherical species, C60 [104]. A few years later, carbon nanotubes, CNT, and graphene materials were discovered. These new carbon materials are in strong focus of actual research and attractive applications have been developed in different fields [105–110]. Due to the small size of C60 molecules, fullerenes are also named nanoallotropes of carbon. A very good description of various carbon nanoallotropes is given by Georgakilas et al. [111]. These authors classify carbon nanoallotropes with respect to the dimension of their shape. Zero-dimensional—0D allotropes—are fullerenes, carbon dots, or nanodiamonds. One-dimensional—1D allotropes—are carbon nanotubes, CNT, in their single wall or in multiwall modification. Two-dimensional—2D allotropes—are graphene, graphene oxide, or oligolayered graphene. [111]. In case of the 1D- and 2D allotropes, a prominent structural direction is already present, which could be compared with the structural direction of rigid-rod-like polymers as, for example, p-aramid or others [112–117]. For this, the direction for fiber and yarn production from carbon nanoallotropes is already given in their nanostructures. There are high expectations for outstanding properties realized by these fibers. It is supposed that low-weight fibers of high strength or fibers with electrical conductivity can be realized.

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In fact, this is a very innovative field of science and nearly every month new developments are published [118–122]. Chapter 4 introduces these materials in detail. These new materials are especially developed and dedicated to new fields of application. Yarns built up by CNTs are seen as lightweight cables for data transport and also as electromagnetic shielding materials [107]. As a further possible application, energy storage textiles made from graphene yarns are mentioned [123].

1.5 INORGANIC FIBERS BASED ON INORGANIC POLYMERS Various types of polymers are named inorganic polymers [124,125]. In comparison to traditional polymers which are named organic polymers, the backbone of the inorganic polymer is not built up by carbon atoms. Important types of inorganic polymers are polysilanes and polysiloxanes with silicon as the element building up the backbone of the polymer (see Fig. 1.17). Another type are polyphosphazenes with phosphorus and nitrogen as elements forming the polymer backbone (see Fig. 1.18) [124,125]. All three types—polysilanes, polysiloxanes, and polyphosphazenes—stand for high amounts of different polymers, because the side groups and side chains of these polymers can be varied in a very broad range. Most side groups are of an organic nature and the simplest side group is probably the methyl group –CH3 [124,125]. For this it can be stated, that in most cases the inorganic polymers contain an inorganic backbone but organic side groups. They are an intermediate placed between inorganic and organic chemistry.

FIG. 1.17 General chemical structures of polysilanes and polysiloxanes.

FIG. 1.18 General chemical structure of polyphosphazene.

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Polysilanes and polysiloxanes are used as precursor fibers for the production of SiC ceramic fibers [85,86]. They can be also used to realize water- and oilrepellent textiles. For this, the side group R (Fig. 1.17) is alkyl with groups OC2H5 to OC18H37 and some H atoms of this alkyl chain are substituted by fluorine atoms [126]. Polyphosphazenes exhibit different material properties depending on their attached groups [127]. As an example, microcrystalline thermoplastic fibers can be produced containing different glass temperatures and melting temperatures. Examples are given by Honarkar & Rahimi of fiber materials containing a glass temperature Tg of 8°C and a melting temperature Tm of +390°C. Another type of polyphoshazene fiber exhibits values, Tg of 66°C and a Tm of +242°C. Both examples show that the polyphosphazene fiber can cover a very broad temperature range (300–400°C) with similar material properties [127]. Polyphosphazenes are as well used as materials in electrospinning for the production of nanofibers. These nanofibers are especially dedicated to realize materials in biomedical applications [128–130].

1.6 SUMMARY AND CONCLUSION The term inorganic fibers can be used to describe many different types of fibers, of different compositions, properties and applications. These fibers are named inorganic fibers, because they are built up by inorganic matter which is part of inorganic chemistry. This definition is set in contrast to organic chemistry, which encompasses conventional synthetic, regenerated, and natural fibers. Inorganic chemistry encompasses materials with completely different properties, such as metals, ceramics, glass, carbon, etc. Fibers can be produced from all these very different inorganic materials by various production processes. Many inorganic fibers exhibit outstanding and special properties, as, for example, high strength, high thermal, and chemical stability or in case of metal fibers high conductivity and shielding properties against radio waves and microwaves. These outstanding properties cannot be reached by traditional organic fibers; so the production and application of inorganic fibers is justified even if their production procedures and their material costs are significant. The following chapters introduces different inorganic fibers and fibers with inorganic components. Production processes, material properties, and applications are summarized. Of course, the aim is to cover nearly all types of fibers and their applications. However, due to the broad range of different inorganic fiber materials and the limited size of a book, this aim cannot be reached completely. The decision regarding which fibers and aspects are presented is made by the editors and the authors to provide the best benefit for the reader. This book should be understood and used as a tool to gain fast and sustainable information in the area of inorganic fibers.

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ACKNOWLEDGMENTS For funding of the electronmicroscopic equipment, the author very gratefully acknowledges the FH-Basis program of the German federal state North-Rhine-Westphalia NRW. All product and company names mentioned in this chapter may be trademarks of their respective owners, even if without labeling.

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Chapter 2

Carbon Fibers Jens Pusch and Bernd Wohlmann Teijin Carbon Europe GmbH, Wuppertal, Germany

2.1 INTRODUCTION Carbon fibers can be defined as fibers with a carbon content of 90% or above. They are produced by thermal conversion of organic fibers with a lower carbon content such as polyacrylonitrile (PAN) containing several thousand filaments with a diameter between 5 and 10 μm. The main advantage of carbon fibers compared to other fibers are the high tensile strength, high stiffness, low density, and a high chemical resistance. All these advantages can be combined with an adequate (polymer resin) matrix material to give excellent mechanical properties of composite parts built from both. These composite components are lightweight with very high mechanical properties compared to parts made of metals like aluminum or other fiber-reinforced composites. This justifies the use of carbon fibers compared to other possible fibrous materials such as glass or organic fibers and metal. The main applications of carbon fiber-reinforced polymers include aerospace and defense, automotive, wind turbines, sport and leisure, and civil engineering. Especially the automotive sector is strongly growing with regard to lightweight constructions that reduce the energy consumption. This chapter gives a short overview of its development history, chemical structure, production process, finishing, processing issues, application areas, a market overview, and future trends.

2.2 HISTORY OF THE FIBER DEVELOPMENT AND USE The first commercially produced fibers with very high carbon content were made by thermal transformation of natural fibers such as cotton threads or bamboo slivers. These fibers were used in light bulbs as lamp filaments before tungsten became popular [1, 2]. First systematic work on high-performance carbon fibers made from PAN fabrics with a high tensile strength and a high modulus was done by the Union Carbide Corporation in the 1950s [3]. With Japanese Government support in the 1960s Japanese companies like Toho Rayon (later Teijin), Toray, Mitsubishi, and Nippon Carbon started strong development Inorganic and Composite Fibers. https://doi.org/10.1016/B978-0-08-102228-3.00002-5 © 2018 Elsevier Ltd. All rights reserved.

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activities and building up of production sites resulting in a worldwide lead in the production of PAN-based carbon fibers. From the late 1970s onwards further improved carbon fibers like T300 from Toray with a tensile strenght of 4000 MPa and a modulus of 400 GPa as well as intermediate modulus carbon fibers, such as IM 600 from Toho Rayon with a tensile strenght up to 6000 MPa started to get a wider use in the aerospace sector. An increase in the capacity and reduction in the cost price allowed the fiber to enter further markets in the 1980s and 1990s such as sport and leisure, civil engineering, wind turbines, and latest into the automotive market, where currently the biggest growth rates are accomplished [4, 5].

2.3 CHEMICAL STRUCTURE The majority of the currently commercially available carbon fiber is produced by oxidation and carbonization of PAN precursor as a continuous wound up tow. Each tow consists of several thousand continuous carbon filaments with a diameter of a few μm. To stick the filaments together, to protect the bundle during processing, and to enable a better linking to the resin matrix the bundle is coated with organic sizing. Fig. 2.1 shows a bundle of carbon fibers in a reflection electron microscope (REM) image. As the name suggests, carbon fibers contain mainly carbon above 90%. The second element is pnitrogen that can be found in commercially available carbon fibers in ranges between 3% and 7%. The remaining element in carbon fibers is oxygen with 700

3000–6000 MPa. The modulus of these fibers lies in the range of 250–500 GPa [18]. The values are less than the theoretically expectable values due to blemishes in the chemical structure and inhomogeneity in the stapling of the layers. Further on macroscopic defects reduce the obtained strength and modulus values of the fibers too. Both properties lead to the categorization of commercially available carbon fibers. The major properties of the fibers are presented in Table 2.1. In the application of carbon fibers these two properties play an important role, for example, in the production of lightweight and strong components in the aerospace sector. This covers the next important property of carbon fibers which is its low density as a result of its chemical structure with strong carbon backbone. The typical density of fibers currently lies in the area of 1.7–2.0 g/cm3. The low density combined with its strong mechanical properties in composite materials make it an ideal replacement for metals. They are especially attractive in aerospace sector where low weights are needed but lately also in the automotive sector where this property has become more and more attractive. Compared to other organic fibers carbon fibers also have a very low creep rate as the carbon backbone network is very strong. This makes them a stable base for fiber-reinforced composites with a wide range of thermal duration in which the creep rate will be low and the composite part thus stable. The macroscopic structure of the yarn with its layered graphite-like structure gives them also excellent vibration damping properties which are often needed in high-performance aerospace and industrial applications where strong oscillating powers are applied on the built parts. The chemical structure of carbon fibers and its macroscopic amorphous character results in the end also in a low fatigue compared to other fibers. The chemical properties of carbon fibers also add a great value to the broad applications they can be used for. In comparison with organic fibers they are chemically very inert and have a high resistance to acids, alkalis, and (organic) solvents, The reason again being its graphitic layer structure with the strong carbon backbone. Compared to metals they are noncorrosive supplying a long-term durability and reliability to the built parts.

38 Inorganic and Composite Fibers

The thermal properties of carbon fibers make them interesting for certain applications where a wider range of temperature exposure is needed. The low thermal expansion coefficients in the area of around 1  10 6/K axial and 10  10 6/K radial allows the use in more extreme thermal circumstances like again in aerospace. Also compared to metal low thermal conductivity is beneficial for certain applications. With regard to the electromagnetic and electrical properties carbon fibers have a low X-ray absorption, are nonmagnetic, and have a high electrical conductivity.

2.7 PROCESSING ISSUES Carbon fiber processing can be roughly divided into two groups: dry processing and wet processing/processing with resin. The classical way to process fibers dry is by weaving fabrics (Fig. 2.5). Weaving by definition is a method in which two distinct sets of yarns (warp and weft) are interlaced at right angles into a fabric. Depending on the pattern these two yarn sets are interlaced with each other and different types of fabrics with different patterns are created. Most important difference between the different fabric patterns is the mobility of the yarns with each other. This results in rather loose fabrics which can more easily be draped into complex shapes or ridged/stiff fabrics that keep its original structure. Carbon fibers consist of a bundle of several thousand filaments with a diameter of a few μm. Although the filaments are held together with a sizing the weaving of carbon fibers is not trivial. Filaments can easily break resulting in a poor fabric quality with a fuzzy appearance. Multiaxial fabrics or non-crimped fabrics (NCFs) are produced when layers of yarns are put together NOT by weaving (Fig. 2.6) [19]. Here different ankles are possible in which the layers are built up to each other. The layers of yarns

FIG. 2.5 Woven carbon fiber fabric.

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FIG. 2.6 Multiaxial carbon fiber fabric.

can be combined with fleeces, coatings, foams, or other materials. The drape ability of NCF again strongly depend on the buildup of the fabric and thus the way the fibers are aligned to each other. A lot of variations in buildup are easily possible. In unidirectional fabric/stitch-bonded fabrics also no weaving occurs to form a fabric. The yarns are laid down in one direction and the fabric is formed by stitch binding the yarns with each other. On comparing woven fabrics with NCF, the handling of woven fabrics is easier. But the strength and module of fiber-reinforced composites made from woven fabric is 10%–40% less than that made of NCF due to the curvature of the yarn. Also the wider variations of the nonwoven fabric buildup allows a much better buildup toward the structural and mechanical needs of a specific part that is built by mechanical exposure. Braiding means that a complex textile structure is formed when three or more yarns are interlaced with each other (Fig. 2.7). While weaving has with warp and weft two separate groups of yarns, in braiding all yarns are in the same group and cross each other in a zigzag pattern. Apart from classical braids which are long and narrow tow-like products todays braiding involves the seamless textile covering of complex 3D structures. After infusing the part with resin and sometimes removing the mold very strong and complex but lightweight three-dimensional (3D) parts can be produced. Preforms can be produced by hand draping or on specialized robots with thermo-activateable carbon fiber (CF) yarns or tapes. Thermoset prepreg tapes fall by sharp definition already under wet processing but are covered here for easier understanding. After production these preforms can be turned into parts by resin infusion. Advantage of an automatized way to create preforms is that fiber direction according to the load can be designed. The fiber reinforcement in the desired complex shapes from CF can be made to measure without major

40 Inorganic and Composite Fibers

FIG. 2.7 Braided carbon fiber fabric.

production of waste clippings from CF fabrics. The production is computer controlled which results in a higher degree of reproducibility compared to handmade preforms. Areas of higher mechanical stress for the part can be identified in the modeling of the part prior to its production. When these areas are localized they can easily get a higher reproducible fiber reinforcement by programming the robot. For the buildup of preforms by robot two different kinds of fiber layers exist: fibers laid parallel in a line and CF that are cut short and placed randomly. The last one is beneficial in modeling complex forms and infiltration of the preform. Parallel buildup preforms of course have a higher endurance on mechanical stress. Again modeling of the part and its mechanical stress decide which kind of preform/sort of fiber lay down is needed for the application. Robot-made preforms received lately a higher attention in the production of parts for the automotive and the aerospace sector. Another way of dry processing CF with only a small market volume is the production of CF fleece or specialty paper. For this application usually low or unsized CF is ripped into small filament pieces and pressed into flat structures. This method is quite suitable for recycling desized fibers. Also chopped and milled carbon fibers have a certain market in the plastic production area as depictured in Fig. 2.8. A very common way of wet processing/processing with resin of carbon fibers is the use of prepreg. “Prepreg” is an abbreviation for “pre-impregnated” meaning a fiber-reinforced fabric or layer of unidirectional carbon fibers is preimpregnated with a resin. Mostly the resin is from the thermoset group but thermoplastic resins are also available.

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FIG. 2.8 Milled (above left) and chopped carbon fibers.

FIG. 2.9 Thermoset carbon fiber prepreg.

Currently, thermoset prepregs are more commonly used for manufacturing prepreg composites (Fig. 2.9). Most of the resins used are primarily from the epoxy family but other resins are also available such as bismaleimide resins, phenolic resins, and benzoxazine-type resins. During the preparation of the prepreg the liquid resin impregnates into the fiber reinforcement and excess resin is removed. Then a partial curing of the epoxy resin takes place changing the resin from liquid to solid state known as the B-stage. In this stage the prepreg is usually tacky. In B-stage prepreg a resin is stopped from further cross-linking reaction and thus has a shelf life and is most often cured below room temperature. Otherwise the chance that the prepreg cross-links before being used increases. Thermoset prepregs usually come with a release film on both sides of the

42 Inorganic and Composite Fibers

fiber layer. It protects the prepreg during transit and preparations from sticking together against impurities. When using the prepreg it is cut into the desired shape, the release paper is removed, and the prepreg is then laid into the mold or tool. When the prepreg is brought to its final processing under suitable temperature and pressure it becomes slightly more liquid again before it is completely hardened. When it is completely cured the whole resin is crosslinked and forms an irreversible solid matrix. In thermoplastic prepregs the composite reinforcements (unidirectional tape or fabric) is pre-impregnated with thermoplastic resins (Fig. 2.10). A wide variety of polymers is available with PP, PET, PE, PPS, PEKK, and PEEK being the most common. The main difference in the handling between the thermoset and thermoplastic prepreg is that thermoplastic prepreg is stable at room temperature and thus generally does not have a limited shelf life. For processing the thermoplastic prepreg is heated and brought into shape. With heating the resin softens and the prepreg can be molded. Different from thermoset prepreg in which this step can take place only once and is irreversible, thermoplastic prepreg can be reheated and reshaped several times. The main advantage of prepregs is its handling as all compounds to build a part come in one material. Upside down the separate handling of fibers and resins is sometimes complicated and needs a lot of attention. Also the handling of a solid material is much easier compared to the use of liquid resin. Disadvantages include the storing of thermoset prepregs in the fridge, its short shelf life, and that the material is due to prior processing relatively expensive. Waste management during building of parts is thus very expensive with prepreg compared to the use of fibers and resins.

FIG. 2.10 Thermoplastic carbon fiber prepreg.

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In filament winding carbon fibers are wound under tension over a rotating mandrel. The process is used to make open cylinders or closed-end structures like pressure vessels or tanks. Also, pipes, golf clubs, ski sticks, bicycle components, and sailing boat masts are produced by this technique. In the process the mandrel rotates around the spindle while a delivery eye on a carriage for the fibers traverses horizontally in line with the axis of the rotating mandrel. Fibers are laid down in the desired pattern or angle. Before the fiber hits the mandrel it is impregnated with resin in a bath. When the desired structure is built up the resin is cured. This can be done by either placing the mandrel in an oven or placing radiant heaters at the mandrel. When the resin is fully cured the mandrel is removed and the hollow part is ready. For some products like gas bottles (Fig. 2.11) the mandrel is part of the finished item and is not removed. There the aluminum of the mandrel acts as gas diffusion protector. The process variables for winding which have a strong influence on the structure of the built part are fiber type, resin content, wind angle, tow or bandwidth, and thickness of the fiber bundle. The winding angle plays a major role in the stability of the part with low-angle patterns providing greater longitudinal/axial tensile strength. Pultrusion is a continuous process to manufacture with constant cross section. The meaning of the word comes from its two parts pull and extrusion. Different from extrusion in which material is pushed in the process in pultrusion the material is pulled. Main products that are produced are profiles, carcasses, and bars. Pultrusion of carbon fiber-reinforced material starts with the impregnation of the carbon-based material (fibers, fabrics, or braided strands) by either guiding it through a resin bath or by resin injection. It is then preformed and afterwards pulled through a heated stationary die to cure the matrix system. Resin materials are either classical thermoset matrices or thermoplastic resins. Resin infusion processes also play an important role in the processing of carbon fibers. Here a wide variety of processes exist that are tailor made for the desired outcome and the focused resin matrix. Processes are, for example, resin

FIG. 2.11 Gas bottle made by filament winding of carbon fibers.

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transfer molding (RTM), vacuum-assisted resin transfer molding (VARTM), reaction injection molding (RIM), and its further development structural reaction injection molding (SRIM) or Seemann composites resin infusion molding process (SCRIMP). In RTM the resin is pumped into a heated mold where the resin is hardened under the influence of temperature and pressure. To produce fiber-reinforced composites preforms of carbon fibers are initially placed into the mold. The VARTM is a variation of the RTM process, in which a vacuum is used to improve the flow and distribution of the resin into the mold. The RIM is another variation of the RTM process where the two reactive compounds of the resin are mixed prior to being injected into the mold, for example, polyurethane.

2.8 APPLICATION AREAS The aerospace and defense sectors are the most important field for carbon fiberreinforced products. Around 30% by volume (35.000 t) and 61% by price (nearly 11 billion US$) went to this sector in 2015 [7]. The strong difference between the price and the weight volume is due to much higher prices for composite materials in the aerospace industry with an average of around 310 US$ per kg compared to 90–100 US$ per kg in the other sectors. The difference is based on different manufacturing processes and quality requirements. This aerospace segment was also the start of the carbon fiber growth in the 1970s with only smaller nonstructural parts being made from carbon fiber-reinforced plastics. In the latest aircrafts of Airbus (A350) and Boeing (787 Dreamliner) around 50% by weight incl. structural parts like wing spars and fuselages are made from it [7]. Apart from wing spars and fuselage the trailing edge, along with the rear bulkhead, empennage, and unpressurized fuselage are made of composite material. Because this field is still new a lot of development is taking place to optimize the production processes. Another working point is the monitoring of structural aging of parts with high mechanical stress. Plans for the next-generation aircraft models see an even increased amount of carbon fiber-reinforced composites in the total weight of the aircraft with entire wings made of carbon fiber reinforcement to further the reduce fuel consumption. In helicopters also a major part of the structure is built from carbon fiberreinforced composites. Apart from civil aerospace spaceships like the SpaceX also make intense use of carbon fibers. In military application also the aerospace sector as well as various applications in vessels, rockets, and ballistics are strong. The share of carbon fibers in the automotive sector has increased strongly in the last few years. The share of the complete composite market was 25,000 t and 22% of the market by volume (or 2.2 billion US$, respectively, 12% by price) in 2015 [7]. Due to its high costs carbon fiber composites were used in the past only extensively in high-end automobile racing. Improved engineering methods

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with regard to the built parts allowed carbon fiber pieces strength in all directions by certain weaving methods and the need is in a certain direction to make it strong in a load-bearing direction. Its usage is in the safety cell chassis assembly but also few other parts are produced. With the increasing demand for fuel saving, automotive and electromobility carbon fibers have become the focus of high- and middle-class car builders. Lightweight composite parts significantly reduce the weight of cars and thus the fuel consumption. Also the cabin space can be increased by this method of building. BMW has started to introduce carbon fiber composites to not only mass serial production with its electrocars but also structural parts of their high-end models in the 5- and 7-series. The fibers are used in the passenger compartment as a hybrid construction method in making the car body. Other car producers are expected to follow with having several projects for this in the pipeline. Apart from the automotive the wind energy owns the biggest share in the civil engineering sector of the carbon fiber market. With 14,500 t and 13% of the market by volume (1.4 billion US$ or 8% by price) it alone was No. 3 in the carbon composite market in 2015 [7]. The need for renewable energy sources accelerates this development. The main application there is the production of the wind blades. The use of carbon fibers allows a lighter, longer, stiffer, and stronger and thus more efficient wind turbine. Today blade lengths are around 60 m but lengths up to 90 m are currently being planned. For this huge highly stressed structures composite materials that combine high strength and stiffness with lightweight are required which only carbon fiber containing composites can fulfill. Further industrial applications include large rollers and robot arms to high-precision engineering parts. Carbon fiber-reinforced composites are used in large-scale industrial machinery components like paper manufacturing rollers. They allow a huge weight reduction compared to metals in line with an increase in rigidity. Carbon fiber also increases resistance to wear and tear and make various industrial components stronger, more efficient, and more resilient. Also in engineering plastics carbon fibers are widely used. Many applications need composite components that are lighter, more compact, and more resistant to wear. Chopped carbon fibers can be used along with other resins such as nylon or polycarbonate to make composites that are ideally suited to meet these requirements. Carbon fibers added to resins reduce the weight of plastic parts and/or reduce their thickness. The electric conductivity of carbon fibers will also add an additional performance to plastic materials, preventing static loads and providing protection from electromagnetic interference. Carbon fibers are used for fabrication of carbon fiber microelectrodes. In this application typically a single carbon fiber with a diameter of 5–7 μm is sealed in a glass capillary. In marine application carbon composite is ideally suited to extreme marine environments. That is why it features heavily in masts, sailcloth, hulls, superstructures, drive shafts, and propellers.

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Carbon fiber is also well suited for use in medical facilities thanks to its X-ray permeability and nonmagnetic properties. It also has the perfect set of properties for sterilization of complex surgical instruments, making it a mainstay in modern operating theaters. Its high strength and lightweight also make it ideal for use in medical aids, such as artificial limbs and braces, wheelchairs, portable access ramps, and beds. The offshore oil and gas industry is increasingly using lightweight materials to replace steel. This is especially true in deeper and more remote waters, requiring new technologies to extract oil more efficiently from the existing fields. Furthermore, with higher working pressures, temperatures, and more aggressive crudes being extracted, the performance requirements of materials used under water are becoming increasingly demanding with, for example, longer length of the tubing where metal tubing reach the borders of resilience. With properties such as high tenacity, high strength, and high-tensile modulus, carbon fiber reinforces a wide range of deep water products. In studies and first applications it showed benefits in construction, it has also proved itself cost effective in a number of field applications such as strengthening concrete, masonry, steel, cast iron, and timber structures [16]. Its use in industry can be either for retrofitting to strengthen the existing structure or as an alternative reinforcing (or prestressing) material instead of steel from the outset of a project (Fig. 2.12). In addition new ways of constructions are possible as can be seen in the extension of the Stedelijk Museum in Amsterdam (Fig. 2.13), where in 2012 the largest composite fac¸ade was built [17]. Sport and leisure itself has a market share of 12% or 14,000 t by volume (or 1.3 billion US$ meaning 7% by sales) in the carbon composite market 2015 [7]. The material is finding use in a variety of pursuits, from motor racing to skiing. As carbon fiber increases in popularity and price falls, it is finding even more applications in the widest variety of sporting goods. We now see carbon fiber used in many kinds of racquets, skis, snowboards, hockey sticks, fishing poles, golf clubs, bicycles, surfboards, kites, shoes, and other sporting products. An overview of major applications of CF, its features and market volume is given in Table 2.2.

FIG. 2.12 Use of carbon fiber in the building sector to reinforce concrete structures.

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FIG. 2.13 Use of carbon fiber in the extension of the Stedelijk Museum in Amsterdam (right).

TABLE 2.2 Major Application Areas of Carbon Fibers, Its Important Features and Market Volume by Weight and Price [7]

Application Area

Application

Important Feature

Volume [Weight in tons]

Volume [price Mio US$]

Aerospace/ defense

All parts, e.g., structural, wings fuselage

Strength lightweight

35,200 (30%)

10.9 (61%)

Automotive

All parts, e.g., structural car body

Lightweight strength

25,500 (22%)

2.2 (12%)

Wind turbines

Blade

Lightweight strength

14,500 (13%)

1.4 (8%)

Marine

All parts, e.g., structural body or masts

Lightweight strength

1500 (1%)

0.1 (1%)

Civil engineering without wind turbines

Rollers, structural parts, plastic parts

Lightweight strength conductivity

5600 (5%)

0.4 (4%)

Others

Medical, concrete

Lightweight strength

20,200 (17%)

1.6 (9%)

Sports/leisure

Racquets, skis, hockey sticks, golf clubs

Lightweight strength

13,900 (12%)

1.3 (7%)

48 Inorganic and Composite Fibers

2.9 PRODUCERS AND MARKET AVAILABILITY The global carbon fiber demand and production was increasing continuously from the 1960s onwards. Since 2010 with a worldwide demand of 33 million tons of carbon fibers, this number has increased yearly by 10% and is expected to boom in the coming years. Driving force is the fuel saving of lightweight carbon fiber reinforced composites compared to metal in the aerospace and automotive sector as well as in the wind turbine market. Also the improved processing techniques of carbon fibers support this growth. Based on the industrial development of carbon fibers from the 1960s onwards the world’s largest carbon fiber producers are located in Japan: Toray, Teijin, and Mitsubishi Rayon. Together they hold more than half for the worldwide yearly production capacity of 130,000 t at different sites inside and outside Japan. SGL with its BMW joint venture activities had a big growth in the last few years. Apart from this an increase in Asian producers from Korea and China is seen in the last few years. The 10 leading carbon fiber producers make together nearly 90% of the worldwide capacity [7]. Carbon fiber production is an energy consuming process. Due to cheap energy costs in the United States with fracking and water power nearly 50% of the worldwide capacity of 130,000 t in 2016 is situated there followed by Japan with 25%, China with 13%, and Taiwan with nearly 9%. With regard to use, around 38% of the worldwide used carbon fiber-reinforced composites is in much demand in North America with a high focus on aerospace (Boeing) and defense. Europe has a share of 35% again with a lot for aerospace (Airbus) and also for automotive, wind energy, and civil industries. The Asia and the Pacific area have a stock of 23% with a lot in sport and in leisure and increasingly in aerospace and automotive sectors too.

2.10 FUTURE TRENDS The carbon fiber market has seen a constant annual growth of 10% per year since the worldwide economic crisis of 2008. This trend is expected to proceed in the coming years or even be accelerated. The production of carbon composites needs a lot of fossil resources for the production of precursors and the thermal conversion of the precursors. High efforts are underway to find renewable materials to produce the precursors and thermal conversion techniques that need less energy like microwave technology, for example. Regarding applications the aerospace and defense market, today the biggest consumer of carbon fibers, is expected grow further in that speed. Main driving force is reduction in the fuel consumption of the planes by weight reduction and exchange of metal party by carbon fiber-reinforced composite parts. The growing experience in building parts for planes with composite materials as well as improved processing techniques and materials allow the production of bigger, more complex, and

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challenging parts. While aircrafts till the 1990s had only up to 10% composite material built in the rate increased in modern aircrafts up to 50% and more with a tendency for further increase for future models. The latest aircrafts have the entire wings and fuselages built using carbon fiber-reinforced materials and also here high-mechanical power exposures are applied. Modern modeling methods allow a very good construction of these parts with additional reproducible reinforcements made to measure where it is needed. Apart from the fuel reduction the cost price also plays a major role in this area. A reduction in the production costs coupled with increased productivity as well-improved product properties are major elements of development projects in the aerospace sector today. The automotive market has seen in the previous years the biggest growth for the use of carbon composite materials. Driving force here is electromobility accomplished with the need to reduce fuel consumption. Again lighter carbon composite materials replace those made from metal with similar or even improved stability factors. Especially BMW with its electro mobility I-series has accelerated the usage of carbon fibers in the automotive sector. The passenger cabin and other major parts of the I-series cars are made from carbon fiber composites. The automotive sector is strongly prize driven. Due to its price the usage of carbon composite materials will be localized in the high-performance automotive sector. Moreover, the focus is on developing highly efficient production processes and reduction in raw materials that will allow the reduction in composite parts prices. Only then carbon fiber composites will find its way from luxury and sport cars into the middle-class segment. In the civil engineering sector the production of wind turbines and the building industry play a major role in the growth of the composite sector. The need for reducing fossil energy resources will let the wind energy sector further grow. In the construction sector the usage of carbon fibers and carbon fiber mats in concrete have the potential to grow strong in the future. Apart from the high price compared to steel reinforcement, carbon fibers offer big advantages regarding the durability and time and effort of assembling. Today it is already successfully used in the repair of bridges and other aged concrete constructions. Moreover, the much easier and flexible assembling compared to steel allows different tailor-made solutions for difficult tasks and saving of resources. With regard to the capacities of the newly built production lines the trend is to build them in the United States due to the cheap energy resources available there (fracking and water power). Thermoset matrices were for a long time the dominating systems for carbon composite. As on today the distribution between the thermoset and the thermoplastic is 3:1 by volume. With increasing interest in the automotive industry where short and flexible curing cycles are common as well as good formability and easy storage the use of thermoplastic resin is another big trend. Also the measure made production of preforms and parts with reduced waste production with novel techniques like the parts-via-preform technique (PvP) offers a lot of potential for the increase in carbon composite use in the future (Fig. 2.14).

50 Inorganic and Composite Fibers

FIG. 2.14 Use of carbon fiber in the PvP technology to build parts preform (A) and final part (B).

REFERENCES [1] J.W. Swan, British Patent 4933 (1880). [2] T.A. Edison, Incandescent lamps, US Patent 223, 398 (1880). [3] J.-B. Donet, T.K. Wang, S. Rebouillat, J. Peng, Carbon Fibers, Marcel Dekker, New York, 1998. [4] J.-K. Kim, Y.-W. Mai, Engineered Interfaces in Fiber Reinforced Composites, Elsevier, Amsterdam, 1998. [5] AKW–Industrievereinigung Verst€arkte Kunststoffe e.V. Publ, Handbuch Faserverbundkunststoffe/Composites, Springer Vieweg, Frankfurt, 2014. [6] L. Peebles, Carbon Fibers, CRS Press, Boca Raton, 1995.

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[7] T. Kraus, M. K€ uhnel, E. Witten, Composites Market Report 2016, 2016. Industrievereinigung Verst€arkte Kunststoffe. [8] J.L. Braun, K.M. Holtman, J.F. Kadla, Lignin-based carbon fibers: Oxidative thermostabilization of Kraft lignin, Carbon 43 (2005) 385–394. [9] F. S. Baker, N.C. Gallego, D.A. Baker, A.K. Naskar, U.S. Department of Energy, Vehicle Technologies Program (2009). [10] M. Heine (1988) Optimierung der Reaktionsbedingungen von thermoplastischen PolymerFasern zur Kohlenstofffaser-Herstellung am Beispiel Polyacrylnitril, PhD thesis, Falk € Otigheim. [11] E. Fitzer, L.M. Manocha, Carbon Reinforcements and Carbon/Carbon Composites, Springer, Berlin, 1998. [12] H. Huang, Fabrication and properties of carbon fibers, Materials 2 (2009) 2369–2403. [13] H.K. Geigl (1979) Studien zur Oberfl€achenchemie von Kohlenstofffasern und zur Entwicklung von Kohlenstoff-Holfasern, Universtit€at Karlsruhe, Fakult€at Chemie, PhD thesis. [14] L. Liubchev, M. Liubcheva, Z. Ovcharova, I. Mladenov, On some recent aspects of surface treatment of carbon fibres, J. Adhesion Sci. Technol. 6 (1992) 807–814. [15] S.-J. Park, Y.-H. Chang, Y.-C. Kim, K.-Y. Rhee, Anodization of carbon fibers on interfacial mechanical properties of epoxy matrix composites, J. Nanosci. Nanotechnol. 10 (2010) 117–121. [16] F. Schladitz, M. Curbach, Carbon concrete composite, in: K. Holschemacher (Ed.), 12. Tagung Betonbauteile–Neue Herausforderungen im Betonbau, Beuth Verlag, 2017, pp. 121–134. [17] Today, Tomorrow, Teijin, Vol. 11 (2012). [18] A. Foley, W. Frohs, T. Hauke, M. Heine, H. J€ager, S. Sitter, Carbon Fibers in Ullmanns’s Encyclopedia of industrial chemistry, Fibers, chapter 5, in: Synthetic Inorganic, Willey-VCH, Weinheim, 2008, pp. 291–312. [19] S.V. Lomov (Ed.), Non-crip fabric composites, In: M. Schneider (Ed.), Automated analysis of defects in non-crimp fabrics for composites, Woodhead Publishing, Oxford, 2011, pp. 103–114.

FURTHER READING [20] S.C. Bennet, D.J. Johnson, Proceeding of the Fifth London International Carbon and Graphite Conference London 18.-22.9.1978 London Society of Chemical Industry (1978) Vol 1. 377–386.

Chapter 3

Preox Fibers Jens Pusch and Bernd Wohlmann Teijin Carbon Europe GmbH, Wuppertal, Germany

3.1 INTRODUCTION Preox fibers or more exactly oxidized polyacrylonitrile fibers (OPFs) are an intermediate product of the carbon fiber process. Its importance for commercial applications and market volume is much lower compared to carbon fibers. Carbon fibers are produced by first oxidizing polyacrylonitrile (PAN) into preox fibers and then in a second step carbonizing them. For the production of preox fibers only the first step of the process, the oxidation of PAN, is performed. The produced fibers have high flame resistance without additional bromine/halide or organic phosphoric compounds and do not melt drip or support flame under fire exposure but chars. This makes them an ideal material for fire protection like welding blankets or other safety clothing, fleece insulation, and especially for carbon-carbon brake applications. A picture of the fiber is shown in Fig. 3.1, as a staple fiber it is depicted in Fig. 3.2. This chapter gives an overview about the history, the chemical structure, the production process, the properties, finishing/coating/functionalization, process issues, application areas, producers and market availabilities, and future trends of preox fibers.

FIG. 3.1 Oxidized polyacrylonitrile fiber (OPF). Inorganic and Composite Fibers. https://doi.org/10.1016/B978-0-08-102228-3.00003-7 © 2018 Elsevier Ltd. All rights reserved.

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54 Inorganic and Composite Fibers

FIG. 3.2 Oxidized polyacrylonitrile (OPF) stapled fiber.

3.2 HISTORY OF THE PREOX FIBER DEVELOPMENT AND ITS USE The development of preox fibers is strongly connected to that of carbon fibers as the production of preox fibers is part of the carbon fiber production process. Accelerated commercial production of the fiber was started in Japan in the 1960s [1, 2]. The most challenging application of the preox fibers is in the so-called carbon-carbon brakes. The first important civil and commercial applications of carbon-carbon brakes started in the mid-1970s with their use in the Concorde supersonic airliner. From this time onwards, the business for this type of high-performance brakes increased continuously and is today state of the art in the brake systems of aircraft, and has also found its way in high-end automotive sports applications after its first use in the 1979 Formula 1 [3].

3.3 CHEMICAL STRUCTURE 3.3.1 Preox Fibers Carbon fiber is produced by oxidation and carbonization of PAN precursor as a continuous wound-up tow. As the preox fiber is closely connected with this, the same first process step is used for its production. The oxidized PAN tows consist of several thousand continuous filaments with a diameter of a few micrometers. The filaments are held together in a bundle and protected during processing by an organic sizing coating. On an atomic scale, the fiber consists of aromatic heterocyclic polymeric structures with a carbon backbone and built in nitrogen. Also, oxygen is part of the structure in a lower content. The principal reaction structure is shown in Fig. 3.3. The exact structure of the material obtained after the oxidative cyclization of PAN has been a topic of multitude of investigations. Most likely, the chemical buildup of preox fibers is a mixture of several heterocyclic polymeric structures as the oxidative treatment can produce different

Preox Fibers Chapter

C

C

C

C

N

N

N

N

N

N

N

N

N

N

N

+O2

–H2O

N

N

N

N

N

55

C

C

N

N

3

FIG. 3.3 Chemical reaction of PAN toward OPF.

thermally stable structures. The polymeric chains can also undergo realignment, reorientation, and reaction along the fiber backbone forming a heterogeneous structure. The exact process is even today not exactly known [4, 5]. Its carbon content lies in the area of around 60% compared to carbon fibers that have a carbon content of 90% and above.

3.4 PRODUCTION PROCESS Preox fibers are, like carbon fibers, produced quasi-continuously (in lots up to several weeks) by thermal conversion of polymeric precursor fibers like PAN or petroleum pitch. In fact, it is a side product of the carbon fiber production and thus all bigger carbon fiber producers have this material in their product portfolio. Nowadays, only fibers spun from PAN play a commercially important role. For the PAN production only fossil resources are used. Intense research work is done to find replacements for PAN that are made from renewable resources instead. The spinning process of the PAN fiber and its stretching and tensioning is very important for the excellent material properties of the later produced carbon fibers, especially in modulus and in its strength. These material properties play a minor role in the application of preox fibers, therefor are the requirements for the PAN fiber of the preox fiber production less strict. The conversion of spun PAN fiber into preox fiber is realized in an oxidative stabilization of the PAN under a hot air atmosphere. At temperatures of up to 300°C, the conversion from PAN into preox fibers takes place exothermically. To smoothen the reaction with less extreme exothermic peaks and a reaction range in a wider temperature window copolymers are added. Pure PAN has a reaction enthalpy of 4000 J/g, a commercially used precursor with added copolymers of around 2500 J/g [6]. A too high copolymer content on the other side reduces fiber properties regarding tensile strength and modulus [7]. Usually, the PAN

56 Inorganic and Composite Fibers

Precursor

Oxidation (£ 300°C, Air)

Sizing

Preox fiber

FIG. 3.4 Production steps to manufacture OPF.

fiber undergoes a treatment of several increasing temperature steps until the maximal reaction temperature of up to 300°C is applied. The yarn tension plays an important role in the process and has some impact on the latter‘s molecular and structural material properties. Online quality control of the PAN precursor ensures process continuity and efficiency. After the conversion, the fiber can be coated with organic sizing to supply a better process ability in the following processing steps. The preox fiber is supplied depending on its further application with several thousand (usually around 300,000) filaments as tows or staple fibers. For the production of staple fibers, the produced tow is crimped and cut to specified lengths before packing. The production steps are shown in Fig. 3.4.

3.5 PROPERTIES Most important material properties of preox fibers are their high thermal stability, excellent chemical resistance, and very good electrical insulation properties. The tensile strength of around 200 MPa plays a minor role for its further applications. The high thermal stability of preox fibers can be seen in its limited oxygen index with values of 40%–50% while comparable other fibers like Aramid have values of 30% and less. Even when preox fibers are blended, the obtained fabrics can have an increased limited oxygen index than the pure flame sensible fiber. Above 400°C, the preox fiber slowly starts to decompose, and also at slightly higher temperatures as, for example, Aramid. The advantage of preox fibers compared to other fibers is that it does not undergo chemical decompositions with the exhaust of (toxic) smoke or as a result of the changing textile structure although with melting/dripping of the fibers. The fiber just slowly chars, which is self-extinguished and keeps its appearance, hand, and textile structure. The good electrical insulation is valued in an electrical resistivity of 8  108 Ω cm.

3.6 FINISHING/COATING/FUNCTIONALIZATION Preox fibers are normally coated to give them a better process ability in further processing like weaving. The exact type of finish coating again depends on the later application of the fiber and its chemical surroundings. Unlike carbon fibers, the surface of preox fibers in not fully smooth. Therefore, no initial surface finishing is necessary to make the fiber coatable.

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3.7 PROCESSING ISSUES Preox fibers are processed in various ways. Originally, it is produced as filament tows or wound-up yarns. Filament tows are usually further processed by cutting into stable fibers. Staple fibers again are used in nonwoven materials, but can also be turned into yarns or be dispersed in wet-laid nonwovens. Yarns are generally turned into fabrics by weaving. Preox fibers can be combined with other fibers to get fabrics with an increased thermal insulation, but on the other side they have good wearable properties for clothing.

3.8 APPLICATION AREAS Preox fibers are utilized due to its excellent thermal resistance in various applications. The most technically advanced application is their use in brake systems [3, 8–13]. Carbon-carbon brakes consist of a preox fiber reinforced with a carbon matrix [3]. They consist of a carbon rotor and a brake pad. As aircraft brakes, they are the primary reinforcement material. Processed in a conventional textile process these brake precursors are turned into a carbon-carbon composite with a high-temperature tolerance and, compared to metal, a significantly reduced weight. The greater energy absorption capacity of the carboncarbon material and its high thermal stability is the first advantage against metal in brake systems. This is extremely important in the landing process of planes where the plane needs to reduce its speed fast, thus a high kinetic energy transfer is accomplished through the brakes of the plane (Fig. 3.5) [9]. The several carbon-carbon plates that are built in such a brake system is shown in the figure. The specific heat of the carbon-carbon composite which is the amount of heat per unit mass required to raise the temperature by 1°C is two times as high as that of steel, making it better to absorb heat. This means carbon also heats up quicker and cools down quicker than steel because it has only one-fourth the

FIG. 3.5 Technical drawing of an aircraft brake (A) and a picture of such a brake (B). (Pictures property of UTC Aerospace Systems, with permission.)

58 Inorganic and Composite Fibers

density of steel and is a better heat conductor. Despite carbon material having high heat-absorbing properties, carbon-carbon cools down much quicker. Another advantage of carbon brakes compared to metal brakes is its longer life, which results in up to nearly twice as much possible landings of carboncarbon brakes compared to metal brakes. The third advantage is its lower weight (between 250 and 400 kg less than metal brakes) and thus a decreased fuel consumption and engine emissions like CO2 and a longer range of the aircraft with the same amount of fuel [9]. Also, other high-end brake systems in the motor sport take advantage of this system. The starting point for carbon-carbon aircraft brakes was as an application for high-end military aircraft brakes. One reason for the development was the lower weight and thus a longer possible range of the aircraft with the same amount of fuel. As second reason for the development was the higher energy absorption. Both reasons were stronger than the initially higher costs on the other side. The higher costs compared to classical metal brakes resulted in the fact that in the beginning of the development only military planes and bigger civil aircraft used carbon-carbon brakes where the cost amortization is easier achieved. Only newer production and refurbishment techniques reduce the costs for this kind of brakes and allowed the use in more applications as the competiveness increased. Nowadays, carbon-carbon brake systems are standard in all commercial bigger aircraft like the ones from Airbus and Boeing including the Boeing 747-400 and -400ER, 757-300, 767, 777, 787 Dreamliner, 747-8, and the McDonnel Douglas MD-11 and MD-90. Higher fuel prices and thus the focus of the aircraft industry on cost savings by reduced fuel consumption in the aircraft accelerated the trend. The wear behaviors of steel and carbon-carbon brakes are different. The wear of steel brakes is proportional to the kinetic energy absorbed by the brakes. The optimal durability of steel brakes is gained by having a large number of small, light brake applications with the brake to cool down in between. The mechanical load in cars favors this. The lifetime of carbon-carbon brakes generally depends on the number of brake incidents and not so much on the kinetic energy of the event. In aircraft, the mechanical load during landing is short, very strong but relatively seldom. This is the reason why carbon-carbon brakes are favored in the aerospace sector while steel brakes are more seen in the automotive sector. Carbon-carbon brake disks are produced by arranging the preox material in layers similar to felt and cut in shape and carbonized to be transferred into carbon fibers. Finally, they undergo two densification heat cycles at around 1000° C which last hundreds of hours. Hydrocarbon-rich gas is injected into the oven to fuse the carbon fiber layers together and form a solid material [9]. Preox fibers find further use in the field of fire- and heat-protective apparels. In combination with other fibers, clothing for fire fighters, military and police staff, steel workers, and racing drivers are produced with preox fibers. Their heat insulation properties in combination with low thermal conductivity make it the ideal material and its supply enhanced reliable and durable protection for the users of the fabric. Again its low specific weight is another wearing

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TABLE 3.1 Application Areas of Oxidized Polyacrylonitrile Fiber and it important Features Application Area

Application

Aerospace

l

l

Automotive

l l

Important Feature

Aircraft and (high-end sport racing) brakes Fire blocking insulation

l

Noise, heat, and vibration liners Fire blocking Insulation

l

l l

l

Industrial

l

Protective apparel

l

l l

Leisure

l

Fire blocking fabrics in furniture

l

Lightweight Heat resistance Excellent energy transfer Flame and heat resistance Insulation properties Flame and heat resistance Insulation properties Thermal conductivity Flame and heat resistance

advantage for the users. In the automotive sector, preox fibers are used as thermal and acoustic insulation materials, for example, in the engine compartment insulation, milled material can replace asbestos in friction lining. The fire protective properties and its low weight make it a valuable material to be used in home furnishing and for aircraft seats, respectively. Fire blocking preox fabrics can be part of those furniture buildups and prevent or delay the ignition or burning of other materials in those furniture like polyurethane foam or fabrics like nylon. The preox fabric provides efficient fire barrier and heat insulation. Further industrial applications of preox fibers are their use as welding blankets, packings, gaskets, bellows, and filters. A broad overview of its application areas is given in Table 3.1.

3.9 PRODUCERS AND MARKET AVAILABILITY Because the production of preox fibers is closely connected to that of carbon fibers, meaning it is a subprocess of the carbon fiber production, the producers are the same. Because of the industrial development of carbon fibers from the 1960s onwards, the world’s biggest carbon fiber producers are Japanese. Teijin Carbon, Toray (incl. its subsidiary Zoltek) and Mitsubishi Rayon as well as the German SGL are the worldwide biggest producers of preox fibers. Same as with carbon fibers due to the cheap energy resources in the United States the trend will be to produce more fibers in the United States.

60 Inorganic and Composite Fibers

3.10 FUTURE TRENDS Preox fibers have not undergone the same boom as the more expensive high-end carbon fibers. Constant growth is seen in the consumption of the fiber with increase in its application areas, but at the moment no trend is seen that would accelerate the use of the fibers. The use of carbon-carbon brake systems is limited by its relatively high price to high-end uses. Again the cheap energy prices in the United States will increase the production there close to its brother fiber carbon. For the PAN production, only fossil resources are used. Intense research work is done to find replacements for PAN that are made from renewable resources instead.

REFERENCES [1] J.-B. Donet, T.K. Wang, S. Rebouillat, J. Peng, Carbon Fibers, Marcel Dekker, New York, 1998. [2] L. Peebles, Carbon Fibers, CRS Press, Boca Raton, 1995. [3] J.G. Rao, K.H. Sinnur, R.K. Jain, Effect of weave texture of carbon fabric on mechanical, thermal and tribological properties of carbon/carbon aircraft brakes, Int. J. Compos. Mater. 5 (2015) 89–96. [4] P. Morgan, Carbon Fibres and Their Composites, Taylor & Francis, Boca Raton, 2005. [5] E. Fitzer, Polyaromaten mit Fasertextur—die modernen Hochleistungs-Carbonfasern, Acta Polymerica 41 (1991) 381–389. [6] M. Heine, Optimierung der Reaktionsbedingungen von thermoplastischen Polymer-Fasern zur € Kohlenstofffaser-Herstellung am Beispiel Polyacrylnitril, Falk Otigheim, PhD thesis, 1988. [7] J. Huang, D.G. Baird, J.E. McGrath, 245th ACS National Meeting and Exposition, 7.-11.4.2013, New Orleans Louisiana Virginia Polytechnic Institute and State University, New Orleans, 2013. [8] L. Rubin, Applications of carbon-carbon, in: J.D. Buckley, D.D. Edie (Eds.), Carbon-Carbon Materials and Composites, Ist ed., Noyes Publications, New Jersey, USA, 1993. [9] C. Byrne, Modern carbon composite brake materials, J. Comp. Mater. 38 (2004) 1837–1850. [10] S. Awasthi, J. Wood, Mechanical properties of extruded ceramic reinforced Al based composites, Adv. Ceramic. Mater. 35 (1988) 3449–3451. [11] V.I. Tefilov, Ceramic- and Carbon Matrix Composite, Chapman and Hall, London, 1995. [12] G.Q. Ming, H.B. Yun, H. Qrzhong, L.G. Hong, W.F. Qui, L. Ye, Trans. Nonferrus Met. Soc. China 12 (2002) 480–484. [13] M. Hao, R. Luo, Z. Hou, W. Yang, Q. Xiang, C. Yang, Effect of fiber-types on the braking performances of carbon/carbon composites, Wear 319 (2014) 145–149.

Chapter 4

Carbon Nanotube-Graphene Composites Fibers Azadeh Mirabedini and Javad Foroughi Intelligent Polymer Research Institute, University of Wollongong Australia, Wollongong, NSW, Australia

4.1 INTRODUCTION Developmental research in the field of multifunctional composite fibers has revealed numerous characteristics that promise great benefits to their possible use in a wide variety of devices and purposes ranging from biological applications such as implantable electrodes [1–6] and tissue scaffolds [7–12] all the way to accessory energy storage devices necessary to provide power to electronic devices and smart garments [13–18]. Due to their unique blend of properties, there has, therefore, been much interest and many attempts to produce a new class of composite fibers using organic conductors over the past decade to provide high-performance fibers which offer extra functions embedded in their structures compared with the traditional textile materials [19–31]. Such novel nanostructured fibers have been mostly obtained using nanocarbon materials especially carbon nanotubes (CNTs) and graphene, the most attractive oneand two-dimensional (1D and 2D) independent carbon crystals [32], which mainly composed of intrinsic covalent bonding of sp2-hybridized carbon atoms as well as their composites enables overall improvement of mechanical, thermal, and electrical properties of the host polymer. In addition, the synergistic effect of CNT and graphene layers in hybrid graphene/CNT materials causes substantial enhancement of the key properties of the final structure provides additional functions that were not found in any of the separate components [33]. The chemistry of fullerenes, geometric cage-like structures of carbon atoms that are composed of hexagonal and pentagonal faces, has been developed in the mid-1980s [34]. A few years later, their discovery led to the synthesis of CNTs. CNTs are now attracting a broad range of scientists and industries due to their fascinating physical and chemical properties such as lightweight, thermal stability, electrical conductivity and high strength up to 50 GPa, and stiffnesses of the order of 1 TPa. CNTs are considered as continuous cylinders formed by Inorganic and Composite Fibers. https://doi.org/10.1016/B978-0-08-102228-3.00004-9 © 2018 Elsevier Ltd. All rights reserved.

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wrapping graphene sheets with carbon atoms being covalently bonded to each other through sp2 hybridization [35]. They are further categorized into singlewalled CNT (SWNT) and multiwalled CNT (MWNT) depending on the number of concentrically rolled-up graphene sheets [36] with diameters of around 1 and 2–100 nm, respectively (Fig. 4.1A). An MWNT comprises a number of concentric SWNTs with an inter-wall spacing of about 0.34 nm [37]. As many previous studies suggest, the cylindrical structure of MWNTs with closed ends were first discovered by Iijima in 1991 [38] during high-resolution transmission electron microscopy (HRTEM) observation of soot generated from the electrical discharge between two carbon electrodes [39]. Iijima et al. (NEC Corporation, Japan) [40] and Bethune et al. (IBM, California) [41] then independently reported of the incidental discovery of SWNTs synthesis of single-walled nanotubes a few years later [42]. However, the first mention of the possibility of forming carbon filaments was actually reported in 1889 [42, 43], that is, more than a century ago in a patent in which was proposed of the use of such filaments in the light bulbs had just been presented by Edison the same year. However, this report and a few others published afterwards could hardly be considered as the first evidence for the growth of CNTs since the resolution of the available microscopy tools were barely able to image filaments smaller than few micrometers in diameter [42]. The deeper inside we go, it seems that observation of hollow graphitic carbon nanoscale structures similar to arrangements known for CNT is back to 1976 when Oberlin et al. published a paper in which a nanotube resembling an SWNT was imaged using transmission electron microscopy (TEM) [44]. It was not claimed to be so by the authors, though [42]. Graphene is a carbon-based material which is a layer of tightly packed 2D honeycomb crystal lattice of sp2-bonded carbon with a single-atom thickness [45] as shown in Fig. 4.1B. Graphene has shown extraordinary optical properties, thermal conductivity, and outstanding mechanical properties (Young’s

FIG. 4.1 (A) Carbon nanotubes (CNTs): single-walled CNTs (SWCNTs), (B) multiwalled CNTs (MWCNTs), and (C) honeycomb crystal lattice of sp2-bonded carbon with a single-atom thickness known as graphene. (Figure reprinted V. Choudhary, A. Gupta, Polymer/Carbon Nanotube Nanocomposites, in: S. Yellampalli (Ed.), Carbon Nanotub. Polym. Nanocompos, 2011, pp. 65–90, copyright @2011; InTech.)

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modulus up to 1 TPa). The theory behind the band structure of graphite was first introduced by theoretical physicist Wallace in 1947 [46, 47]. The concept was then further explored by Boehm et al., who described single-layer carbon foils in 1962 [48]. However, these models could not attract significant attention for nearly half a century. In the latter half of 2004, Novoselov et al. described a micromechanical cleavage method for developing individual graphene layers and further observed and investigated the unique properties of isolated 2D planes [49]. Oriented carbonaceous fibers and yarns such as CNT and graphene have shown to display improved properties compared with non-fibrillar forms of carbon-based materials. These features arise mainly from their low dimensions, well-oriented polymer chains, lightweight, and large surface area-to-volume ratio. This chapter mainly describes the history, structures, properties, processing routes and advanced fabrication techniques as well as the latest research for the development of pristine CNT and graphene fibers (GFs) with the focus on their fabrication methods. After a summary of the remarkable properties and of individual CNT and GFs, their assembly into novel hybrid graphene/CNT fibrillar structures composite fibers has been explored and discussed. The last section concludes with some perspectives on the challenges and opportunities of future work. We hope that this chapter provides a comprehensive overview of the field of pristine and composite fibers based on graphene and CNT and could attract more efforts to achieve further progress.

4.2 CNTS FIBERS CNT is an attractive 1D nanomaterial which shows extremely high strength [50–54], outstanding modulus [55, 56], low density, good chemical stability, and high thermal and electrical conductivities [54]. It is considered as one of the strongest materials in the world with strength between 50 and 100 GPa [24]. Experimental moduli, determined from isolated nanotubes, have been reported as 950 and 1800 GPa, for MWNTs and SWNTs, respectively [57]. In addition, their predicted thermal conductivity could be as high as 6000 Wm K 1. Moreover, SWNTs can show metallic or semiconducting properties depending on their chirality or conformation. The exceptional mechanical and physical properties showed by individual CNTs, demonstrated both by theoretical [50, 58, 59] and experimental [60–62] studies motivated several researchers in developing electrically conductive structures from them for varieties of applications [54, 63–66]. To fully utilize the excellent mechanical and physical properties of individual CNTs at a macroscopic level, it is desirable to fabricate various macroscopic CNT-based fibers. First, the geometric and structural aspects of anisotropy result in extraordinary mechanical properties when measured along the fiber axis. In addition, other physical properties, like electrical or thermal conductivity may be totally different when measured along or perpendicular to a fiber. Another unique property is the high surface area of

64 Inorganic and Composite Fibers

fibers, caused by their small diameter compared with their length, which provide the opportunity for them to be suitable for many practical applications and technologies [23, 67]. It is worth mentioning that this chapter considers a “fiber” as threadlike filaments with large L/Ds, possessing a diameter of tens of micrometers with the cross section comprising 105–106 individual nanotubes [68]. Thus, CNT fibers would combine the benefits of high-performance polymeric fibers with exceptional properties offered by a carbon-based material.

4.3 CURRENT PROCESSING TECHNOLOGIES OF CNT FIBERS AND YARNS Thus far, a number of methods have proposed to prepare fibers composed entirely of or a large fraction of nanotubes for a variety of applications. In view of CNTs as extremely strong and stiff polymer molecules, it is not surprising that the processing routes developed so far borrow concepts from polymer fiber-processing technologies [69]. Generally, the leading production techniques could be divided into two main categories: solution-spinning [70, 71] and solid-state spinning methodologies [66, 72–76]. In solution-based spinning, CNTs are spun from a lyotropic liquid crystalline (LC) suspension of nanotubes, through a process similar to that used for polymeric fibers [77]. Whilst, in solidspinning processes, CNT fibers could be mainly spun either from multiwall nanotubes grown on a substrate as semi-aligned mats [54] or an aerogel of SWCNT and double-walled CNT as they are formed in a chemical vapor deposition (CVD) reactor [78–80].

4.4 SOLUTION-SPINNING APPROACHES Solution-spinning or the so-called wet-spinning methods (Fig. 4.2) are known as one of the most successful methods to prepare pure and hybrid CNT-based nanostructured fibers and yarns [30] wherein a CNT dispersion is converted into a gel fiber by passing through a coagulation bath [24]. The processing of CNT wet-spun fibers does not require a subsequent cross-linking step of the precursor similar to the case of carbon fibers [82, 83]. Not surprisingly therefore, CNT

Take-up

As-spun fibre Spinning solution

Drawing zone Spinneret Coagulation bath

FIG. 4.2 Schematic representation showing the set up used for wet spinning of fibers [81]

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wet-spun fibers have already been applied in a wide variety of applications such as flexible electronics mainly as fiber-based sensors, transmission lines, microelectrodes, and solar cell [38, 71, 84–88]. Solution-spinning methods mainly include polymer-based coagulation spinning method, polymer-free spinning method, and LC spinning method [89].

4.5 POLYMER-BASED COAGULATION SPINNING METHOD The fabrication of hybrid CNT fibers via wet spinning dates back to less than two decades ago when Vigolo et al. published the very first report of the successful preparation of CNT fibers were successfully through spinning a SWNTs homogeneous dispersion [using sodium dodecyl sulfonate (SDS)] into a polyvinyl alcohol (PVA) coagulation bath [30]. They have described the preparation of homogenous dispersions of SWNTs in aqueous solutions of sodium dodecyl sulfate (SDS) which can encase individual nanotubes and stabilize them against van der Waals attractions. The CNT dispersion is then injected into the coflowing stream of a polymer solution that contains 5.0 wt% of PVA to form CNT ribbons. After the ribbons are washed and dried, most of the surfactants and polymers were removed. PVA was then utilized extensively as a polymeric coagulation bath for forming continuous CNT filaments [90–92]. Later on, replacing SDS surfactant with lithium dodecyl sulfate (LDS) this approach was modified by Baughman’s group to make CNT composite fibers with very high strength and thoughness [93, 94]. Besides, hot-drawing technique was utilized to improve the properties of CNT fibers. This posttreatment enhanced the crystallinity in PVA and created remarkable degree of alignment. The major issues with polymer-based spinning approach include a relatively high fraction of remaining polymer volume and short individual CNTs. Therefore, a compromise in electrical conductivity is typically observed particularly even for polymer-CNT fibers with high mechanical characteristics fabricated through this method. For instance, robust wet-spun CNT-PVA fibers with ultimate strength of 80 GPa and Young’s modulus of 1.8 GPa have indicated an electrical conductivity of only 2.5 S cm 1 due to the adverse effect of PVA as an insulating ingredient [93]. Removal of PVA through thermal annealing can enhance the electrical properties. However, the mechanical properties are sacrificed. Recently described wet-spinning approaches use CNT-polymer preformulated spinning dopes which eliminates the need for employing a polymer-based coagulation bath [95]. In this approach, CNT bundles are first dispersed properly in an appropriate solvent with the aid of either surfactants or biopolymers; the obtained dispersion is then added to a polymer solution to form the spinning solution. The wet spinning of CNTs produces continuous CNT fibers; however, homogeneous dispersion of CNTs in the solvent is necessary for proper spinning.

66 Inorganic and Composite Fibers

4.6 POLYMER-FREE SPINNING METHOD Not long ago, new approaches have been reported in which pure CNT fibers were spun without a matrix. Pure CNT fibers were successfully first spun from a CNT-fuming sulfuric acid solution [96]. However, the dispersion states of the nanotubes involve complicated phenomena, since the CNTs are produced in bundles or bundle aggregations. Therefore, the major challenge of all time was to make stable solutions in which the CNTs maintain properties inferior to the proposed properties of individual SWNTs. Considerable research efforts have already been devoted to develop processes for effective separation of individual nanotubes. The states are affected by at least two competitive interactions: (1) the interactions of van der Waals forces, among CNT threads and (2) the interactions between CNT threads and dispersion medium [97]. Among several method which have been explored by different research groups, stable pure CNT dispersions have been mainly prepared with the aid of solvents, superacids and surfactants. The three main approaches for preparation of CNT dispersions are described in detail further.

4.6.1 Dispersion of Nanotubes in Organic Solvents Previous studies from different research groups have reported of CNT dispersion in varieties of organic solvents. Liu et al. have demonstrated that individual SWNTs can be dispersed in N,N-dimethylformamide (DMF) [98]. In order to find an appropriate medium capable of solvating the nanotubes, Ausman et al. used five different solvents including N-methyl-2-pyrrolidone (NMP) [99]. It was also described that there are several critical parameters for choosing an appropriate solvent for dissolution of CNTs including high electron pair donation, low hydrogen bond donation as well as high solvatochromic parameter. Others have later on endeavored to produce stable dispersions of single nanotubes or small bundles by use of different organic solvents including but not limited to 1,2-dichlorobenzene (DCB), methanol, acetone, chloroform, etc. [100–102]. However, it was surprisingly found that the highest achievable concentration of CNT in most of the organic solvents was not higher than 0.1 mg mL 1. In 2009, a debundling approach was described by Bergin et al. utilizing two new solvents including cyclohexyl-pyrrolidone (CHP) and 1-benzyl-2-pyrrolidinone (NBenP) capable of dispersing CNT with high concentrations up to 3.5 mg mL 1 [103].

4.6.2 Dispersion of Nanotubes Using Surfactants Using surfactants was found to be a very effective approach to exfoliate and disperse SWNTs [97, 104–109] due to their ability to form micellar structures around individual CNTs. These CNT/micelle structures are kinetically stable because the surrounding surfactant molecules prevent CNTs from further

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aggregation [78]. The other advantage of using a surfactant is to disperse CNT in a reasonably high concentration of CNT (up to few mg/mL) at a relatively high level of debundling [110]. In a typical dispersion procedure, after the surfactant has been adsorbed on the nanotube surface, ultrasonication for minutes or hours (with ultrasonication tip or bath, respectively) may help a surfactant to debundle nanotubes by steric or electrostatic repulsions. A number of surfactants have been used to SWNTs in aqueous solutions including but not limited to sodium dodecyl benzenesulfonate (SDBS) [111, 112], sodium dodecyl sulphonate (SDS) [113, 114], LDS [115], tetradecyl trimethyl ammonium bromide (TTAB), and sodium cholate (SC). The chemical structures of these surfactants are presented in Fig. 4.3. The suitable concentration of surfactant is critical for the formation of a good dispersion. If it is lower than the desired, inadequate stabilization will be achieved. On the other hand, it has been reported that in case of adding very high concentrations of surfactant, the osmotic pressure of the excess micelles causes depletion induced aggregation [78]. In another study, it was demonstrated though that the effective concentration of surfactant is commonly higher than its critical micelle concentration (CMC) [110]. The dispersed amount of surfactant-assisted SWNT dispersions is plotted vs its concentration for six different surfactants as shown in Fig. 4.4.

Na

Na

Sodium dodecylbenzene sulfonate (SDBS)

Sodium dodecyl sulfonate (SDS)

Li

Lithium dodecyl sulfate (LDS)

Br

Tetradecyl trimethyl ammonium bromide (TTAB)

Na

Sodium cholate (SC)

FIG. 4.3 Chemical structures of some of the most effective surfactants to prepared stabilized CNT dispersions.

68 Inorganic and Composite Fibers

SC-NT

Fairy liquid-NT

60 40

Remaining NTs after centrifugation (%)

20 CMC=4–6.5 mg/mL

0

TTAB-NT

SDBS–NT

60 40 20 0

CMC=0.84 mg/mL

CMC=0.73 mg/mL

SDS-NT

60

LDS–NT

40 20 CMC=2.0–2.9 mg/mL

0 0

10

20

30 0

CMC=1.9–2.7 mg/mL 10

20

30

Surfactant conc (mg/mL) FIG. 4.4 Mass fraction of nanotubes remaining (%) after centrifugation as a function of surfactant concentration for the six surfactants. In all cases, the initial nanotube concentration (before CF) was 1 mg/mL. The literature value (or range) of the critical micelle concentration (CMC) of each surfactant is shown in each panel. (Figure reprinted from Z. Sun, V. Nicolosi, D. Rickard, S.D. Bergin, D. Aherne, J.N. Coleman, Quantitative evaluation of surfactant-stabilized single-walled carbon nanotubes: dispersion quality and its correlation with zeta potential, J. Phys. Chem. C 112 (2008) 10692–10699, Copyright© 2008; American Chemical Society.)

4.6.3 Dispersion of Nanotubes in SuperAcids Recently, researchers have demonstrated that highly conductive CNT fiber can be synthesized by a wet-spinning process through using an acidic dispersion. In this process, premade CNTs were dissolved or dispersed in a fluid, extruded out of a spinneret, and coagulated into a solid fiber by extracting the dispersant. An important variation of the original wet spinning method was the use of acid as solvent and standard coagulants like water, thereby simplifying the original method by avoiding surfactants in the CNT dispersion and eliminating polymer from the coagulation bath which both reduces electrical conductivity. Acid

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solvents have the unique ability to form liquid-crystalline dopes with a high concentration of SWNTs; in sulfuric acid, fibers are spun at 8 wt% SWNT concentration, over one order of magnitude higher than can be achieved by using surfactants [96]. Because of the spontaneous ordering in the liquid-crystalline phase, these fibers are highly aligned without the need for posttreatments. In another study, high-quality CNTs were dissolved in chlorosulfonic acid at a concentration of 2–6 wt% and filtered to remove particles, in order to form a spinnable liquid crystal dope. The spinning solution was extruded through a spinneret into a coagulant (water) to remove the acid [70]. The resulting high-performance multifunctional CNT fibers were prepared that combines the specific strength, stiffness, and thermal conductivity of carbon fibers with the specific electrical conductivity of metals. These fibers consist of bulk-grown CNTs and are produced by high-throughput wet spinning, the same process used to produce high-performance industrial fibers.

4.7 SOLID-STATE SPINNING METHOD Solid-state spinning (Fig. 4.5) is an alternative strategy, which allows avoidance of CNT dispersion in solvents and for various post-spinning processes to be applied with ease [116]. MWNT yarns could be prepared by drawing and twisting CNTs from a forest (Fig. 4.5A). The MWNT forest was synthesized by catalytic CVD using acetylene gas as the carbon source. CNTs, which are about 10 nm in diameter, are simultaneously drawn from the MWNT forest and twisted (Fig. 4.5B). The nanotube length in the forest is about 300 μm, resulting in yarn diameters between 1 and 60 μm, depending upon the forest width used for spinning. The twist is characterized by the helix angle (α), which depends directly upon the degree of twist and inversely on the yarn diameter (Fig. 4.6). The degree of twist is typically 20,000 turns m 1 [117]. Although significant progress has been made, the mechanical and physical properties of CNT fibers measured to date are still far below those for individual CNTs and even much lower than those for commercial carbon fibers [71]. In terms of mechanical properties, the various techniques have met with different degrees of success. Fibers produced by the LC route showed an encouraging stiffness of 120 GPa but only modest strengths on the order of 0.1 GPa. Fibers spun from CNT forest and subsequently twisted have now been made with strengths up to 1.9 GPa and stiffnesses up to 330 GPa. Until now, the highest strength reported for direct-spun CNT fiber was 2.2 GPa, and the highest stiffness reported was 160 GPa [83]. Despite the effort that has been made, however, fabrication of strong CNT fibers remains a great challenge [116]. A summary of CNT fiber production and properties using different methods is presented in Table 4.1.

70 Inorganic and Composite Fibers

CNT forest

Drawn CNT web

(A) Winding yarn guide Spindle pin

Winding Gear

Initial yarn guide

Spindle base

Bobbin

(B)

Yarn Spindle

Nanotube forest on substrate

FIG. 4.5 Schematic representation showing the spinning of CNT yarn from MCNT forest, (A) being CNTs drawing from forest and (B) the set up used for solid-sate CNT spinning [81].

4.8 FABRICATION OF CNT COMPOSITE FIBERS Many researchers have lately investigated improved processing techniques for the preparation of composite macroscopic fibrillar structures from CNT in which the CNT takes the role either as the guest or host component [8, 26, 65, 131, 143]. CNT particles could be added as guest nanofillers in another matrix and then become highly aligned to maintain high mechanical strength and electrical conductivity of individual CNTs [8, 143–147]. Composites of CNTs and other polymers can be generally synthesized by mixing CNTs and polymers in solutions or dispersing CNTs in polymer melts, once the used polymers are soluble or meltable. In an alternative method to produce composite materials, the related monomers may be first incorporated with CNTs, followed by in situ polymerizations. Various methods have been developed in recent years to efficiently disperse individual CNTs in a polymer matrix. Solution processing may be appropriate to prepare composite materials

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j=25°

2 µm

(A)

(B) FIG. 4.6 Scanning electron microscopy (SEM) micrographs of CNT yarn at (A) low and (B) higher magnification [81].

for thermoset and thermoplastic polymers, while melt blending is only applicable for thermoplastic polymers which can be heated to form melts. Exfoliation and uniform dispersion of CNTs with a polymer, with and without the help of a solvent is difficult. Therefore, there is a need for high external forces such as high-power ultra-sonication and mechanical stirring. On the other hand, the strong treatments could shorten and damage the CNTs structures [148]. Surfactants can be also used to assist with CNTs dispersion. The introduction of surfactants largely decreases the final properties, though [149]. To improve the structure uniformity and interaction between CNTs and polymers, monomers are often first incorporated with CNTs in either solution or melting processes, followed by in situ polymerizations. Using this method, higher content of CNTs can be achieved in the composites compared with the direct mixing with polymers. Moreover, it is particularly useful to insoluble and thermally instable thermoset and conductive polymers such as polypyrrole and polyaniline which can be grown onto CNTs by in situ electropolymerization [77].

4.9 GRAPHENE FIBERS The recent success in assembling graphene sheets into macroscopic fibers has inspired extensive interest in these materials because of the lower cost of GFs

Classification Spun from Aerogel

Fiber Preparation Technique

Sample Length

Strength (GPa)

Modulus (GPa)

Elongation (%)

Toughness (J g21)

Electrical Conductivity (S cm21)

Ref.

As-grown

2 mm

–

–

–

–

–

[118]

As-grown

Semicontinuous (3 cm)

–

–

–

–

–

[119]

As-grown

Semicontinuous (10–20 cm)

0.8

49–77