Annual Plant: Sources of Fibres, Nanocellulose and Cellulosic Derivatives: Processing, Properties and Applications 9819924723, 9789819924721

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Composites Science and Technology

Ramzi Khiari Mohammed Jawaid Mohamed Naceur Belgacem   Editors

Annual Plant: Sources of Fibres, Nanocellulose and Cellulosic Derivatives Processing, Properties and Applications

Composites Science and Technology Series Editor Mohammad Jawaid, Laboratory of Biocomposite Technology, Universiti Putra Malaysia, INTROP, Serdang, Malaysia

This book series publishes cutting edge research monographs comprehensively covering topics in the field of composite science and technology. The books in this series are edited or authored by top researchers and professional across the globe. The series aims at publishing state-of-the-art research and development in areas including, but not limited to: . . . . . . . . . . . .

Conventional Composites from natural and synthetic fibers Advanced Composites from natural and synthetic fibers Chemistry and biology of Composites and Biocomposites Fatigue damage modelling of Composites and Biocomposites Failure Analysis of Composites and Biocomposites Structural Health Monitoring of Composites and Biocomposites Durability of Composites and Biocomposites Biodegradability of Composites and Biocomposites Thermal properties of Composites and Biocomposites Flammability of Composites and Biocomposites Tribology of Composites and Biocomposites Applications of Composites and Biocomposites

Review Process The proposal for each volume is reviewed by the main editor and/or the advisory board. The chapters in each volume are individually reviewed single blind by expert reviewers (at least two reviews per chapter) and the main editor. Ethics Statement for this series can be found in the Springer standard guidelines here - https://www.springer.com/us/authors-editors/journal-author/journal-aut hor-helpdesk/before-you-start/before-you-start/1330#c14214

Ramzi Khiari · Mohammed Jawaid · Mohamed Naceur Belgacem Editors

Annual Plant: Sources of Fibres, Nanocellulose and Cellulosic Derivatives Processing, Properties and Applications

Editors Ramzi Khiari Department of Textile Higher Institute of Technological Studies of Ksar Hellal, University of Monastir Monastir, Ksar Hellal, Tunisia Grenoble INP University Grenoble Alpes, CNRS Grenoble, France

Mohammed Jawaid Laboratory of Biocomposite Technology Institute of Tropical Forestry and Forest Products (INTROP) Universiti Putra Malaysia, UPM Serdang, Selangor, Malaysia

Mohamed Naceur Belgacem Grenoble INP University Grenoble Alpes, CNRS Grenoble, France

ISSN 2662-1819 ISSN 2662-1827 (electronic) Composites Science and Technology ISBN 978-981-99-2472-1 ISBN 978-981-99-2473-8 (eBook) https://doi.org/10.1007/978-981-99-2473-8 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Preface

Natural fibre consumption is constantly increasing around the world, even in countries where wood resources are very limited. Due to that, Academicians and Industries show interest in the exploitation of agricultural residues or marine biomass which are likely to represent a new source of cellulosic fibres for various applications. This strategy has already been adopted for several agricultural wastes available in different countries, such as Portugal, India, Malaysia, Iran, Sudan and/or Tunisia. The search for new substitute materials, which respect the environment (in any case regarding the rational recovery of biomass waste), represents one of the interests of our research and constitutes one of its main objectives for this book. This book is devoted to presenting the advantage to use annual plants as sources for the production of fibres and cellulose derivatives with a focus on their processing, properties and applications. The special features in this book comprise illustrations and tables that summarize up-to-date information on research carried out on the production of fibres and cellulose derivatives using several methods and/or characterizations tools. This work collects also information and knowledge of a new way to the preparation of cellulosic derivatives and description of the concepts and architecture of fibre from different sources. As the title indicates, the book will emphasize new challenges for the synthesis characterization and properties of valorization of annual plants at multiscale applications such as fibre, nanocellulose and as a macromolecule. Our book covers the void for the need of one stop reference book for researchers. Leading researchers from industry, academia, government and private research institutions across the globe contributed to this book. Academics, researchers, scientists, engineers and students in the field of lignocellulosic biomass will benefit from this book which is highly application-oriented. Moreover, it will provide a cutting-edge research from around the globe in this field. Current status, trends, future directions, opportunities, etc. will be discussed in detail, making it friendly for beginners and young researchers.

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We are thankful to all the authors who contributed book chapters to this edited book and made our imaginary thoughts into reality. Lastly, we are thankful to the Springer Nature team for their continuous support at every stage to make it possible to publish this book on time. Monastir, Ksar Hellal/Tunisia Serdang, Malaysia Grenoble, France

Ramzi Khiari Mohammed Jawaid Mohamed Naceur Belgacem

Contents

Characterization and Comparative Evaluation of Structural, Chemical, Thermal, Mechanical, and Morphological Properties of Plant Fibers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sakib Hossain Khan, Md Zillur Rahman, Mohammad Rejaul Haque, and Md Enamul Hoque Chemical Characterization of Natural Species and Study of Their Application for Papermaking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sara Saad, Manel Elakremi, Faten Mannai, Ramzi Khiari, Anis Tlili, and Younes Moussaoui Recent Developments in Pretreatment Strategies on Annual Plant Residues for Bioethanol Production: Technological Progress and Challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Imen Ben Atitallah and Tahar Mechichi Nanocellulose from Date Palm: Production, Properties and Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Karima Ben Hamou, Fouad Erchiqui, Youssef Habibi, and Hamid Kaddami

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Preparation and Characterization of Cellulose Nanofibril from annual Plant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 Soumia Boukind, El-Houssaine Ablouh, Zineb Kassab, Fatima-Zahra Semlali Aouragh Hassani, Rachid Bouhfid, Abou El Kacem Qaiss, Mounir El Achaby, and Houssine Sehaqui Characterization of Nanocellulose Obtained from Cactus . . . . . . . . . . . . . 145 Anass Ait Benhamou, Zineb Kassab, Fatima-Zahra Semlali Aouragh Hassani, El-Houssaine Ablouh, Rachid Bouhfid, Abou El Kacem Qaiss, Amine Moubarik, Houssine Sehaqui, and Mounir El Achaby

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Corn Crop Residues as Source to Obtain Cellulose Nanocrystals . . . . . . . 169 Marcus Felippe de Jesus Barros, Samir Leite Mathias, Robson Valentim Pereira, and Aparecido Junior de Menezes Recent Nanocelullose Applications for Sustainable Agriculture—A Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185 Manuel Peña-Ortiz, Esther Rincón, Luis Serrano, and Araceli García Alternative Adhesives for Composites Made of Annual Plants . . . . . . . . . 215 Eugenia Mariana Tudor Annual Plant Reinforced Biocomposite Fiberboards—Investigation on Mechanical Properties . . . . . . . . . . . . . . . . 241 Sazedur Rahman, Muhammad Ifaz Shahriar Chowdhury, and Md Enamul Hoque Nanocomposites with Cellulose Nanocrystals Extracted from Annual Plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301 El-Houssaine Ablouh, Adil Bahloul, Zineb Kassab, Aziz Faissal, Rachid Bouhfid, Abou El Kacem Qaiss, Houssine Sehaqui, Mounir El Achaby, and Fatima-Zahra Semlali Aouragh Hassani Advances in the Production of Cellulose Nanomaterials and Their Use in Engineering (Bio)Plastics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333 Gabriel Banvillet, Mathieu Bugaut, Estelle Doineau, Aurélie Taguet, Nicolas Le Moigne, and Orlando J. Rojas An Overview on the Pharmaceutical Applications of Nanocellulose . . . . . 395 Rabab Kamel and Nermeen A. Elkasabgy Preparation and Characterization of Cellulosic Derivatives from Annual Plant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413 Mohamed Hamid Salim, Zineb Kassab, Fatima-Zahra Semlali Aouragh Hassani, El-houssaine Ablouh, Rachid Bouhfid, Abou El Kacem Qaiss, Houssine Sehaqui, and Mounir El Achaby Structure and Properties of Cellulose and Its Derivatives . . . . . . . . . . . . . . 443 Zineb Kassab, Adil Bahloul, Fatima-Zahra Semlali Aouragh Hassani, El-Houssaine Ablouh, Rachid Bouhfid, Abou El Kacem Qaiss, Houssine Sehaqui, and Mounir El Achaby Activated Carbon from Agricultural Waste for the Removal of Pollutants from Aqueous Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 465 Rimene Dhahri, Mongi Ben Mosbah, Ramzi Khiari, Anis Tlili, and Younes Moussaoui

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Valorization of Annual Plants in Removing Synthetic Dyes . . . . . . . . . . . . 485 Laila Laasri Schinus Molle: Currently Status and Opportunity . . . . . . . . . . . . . . . . . . . . 535 Abir Razzak, Ramzi Khiari, Younes Moussaoui, and Naceur Belgacem

About the Editors

Dr. Ramzi Khiari is a senior lecturer at the Higher Institute of Technological Studies in Ksar-Hellal (Monastir, Tunisia) in the department of Textile Engineering. He was graduated in 2005 at the National Engineering School of Monastir in the specialty of Textile Chemistry, before getting a Master degree (2007) from the same institution. Then, he performed a sandwich Ph.D. thesis (2010) between ENIM from University of Monastir in Tunisia and Grenoble INP in France. Finally, in 2017, he has got the diploma of “Habilitation Universitaire” from University of Monastir and 2020 he has got the diploma “Habilitation à diriger des Recherches” in Grenoble INP. His research interests focus on the valorization of biomass at multiscale levels namely: fibers, nanocellulose, lignin, hemicelluloses and their used as potential raw material in several industrial applications (Textile, papermaking, polymeric materials, composites and nanocomposites). A particular focus is given for vegetal biomass from annual plants, and particularly agricultural residues and industrial wastes. During his career, he coordinated several research projects, mainly projects with industry, and supervised various post-graduation students. He has been member of scientific committees, organizing committees and participated in national/international evaluation boards. e-mail: [email protected]

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Dr. Mohammed Jawaid is currently working as Senior Fellow (Professor) at Biocomposite Technology Laboratory, Institute of Tropical Forestry and Forest Products (INTROP), Universiti Putra Malaysia (UPM), Serdang, Selangor, Malaysia, and also has been Visiting Professor at the Department of Chemical Engineering, College of Engineering, King Saud University, Riyadh, Saudi Arabia since June 2013. Recently he joined as Distinguish Visiting Professor at Malaysian Japan International Institute of Technology (MJIIT), Kuala Lumpur. He is Director of Start-up Company-FiberStrong Pvt ltd and Research advisor of Kenaf Venture Global (KVG), Malaysia. He has more than 20 years of experience in teaching, research, and industries. His area of research interests includes hybrid composites, lignocellulosic reinforced/filled polymer composites, advance materials: graphene/nanoclay/fire retardant, modification and treatment of lignocellulosic fibers and solid wood, biopolymers and biopolymers for packaging applications, nanocomposites and nanocellulose fibers, and polymer blends. So far, he has published 55 books, 75 book chapters, more than 350 peer-reviewed international journal papers, and several published review papers under top 25 hot articles in science direct during 2013–2018. He also obtained 6 Patents and 5 Copyrights. H-index and citation in Scopus are 75 and 25233 and in Google scholar, H-index and citation are 88 and 34218. He is founding Series Editor of Composite Science and Technology, Sustainable Materials and technology, and Smart Nanotechnology Book Series from Springer-Nature. He worked as guest editor of special issues of SN Applied Science, Current Organic Synthesis and Current Analytical Chemistry, International Journal of Polymer Science, IOP Conference Proceeding. He also in Editorial Board Member of Journal of Polymers and The Environment, Journal of Plastics Technology, Applied Science and Engineering Progress Journal, Journal of Asian Science, Technology and Innovation and the Recent Innovations in Chemical Engineering. He is also life member of Besides that, he is also reviewer of several high-impact international peer-reviewed journals of Elsevier, Springer, Wiley, Saga, ACS, RSC, Frontiers, etc. Presently, he is supervising 8 Ph.D. students and 30 Ph.D. and

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13 Master’s students graduated under his supervision in 2014–2022. He has several research grants at university, national, and international levels on polymer composites of around 3 million Malaysian ringgits (USD 700,000). He also delivered plenary and invited talks in international conferences related to composites in India, Turkey, Malaysia, Thailand, the United Kingdom, France, Saudi Arabia, Egypt, and China. Besides that, he is also a member of technical committees of several national and international conferences on composites and material science. Dr. Mohammed Jawaid received Excellent Academic Award in Category of International Grant-Universiti Putra Malaysia-2018 and also Excellent Academic Staff Award in industry High Impact Network (ICAN 2019) Award. Beside that Gold Medal-Community and Industry Network (JINM Showcase) at Universiti Putra Malaysia. He also Received Publons Peer Review Awards 2017, and 2018 (Materials Science), Certified Sentinel of science Award Receipient-2016 (Materials Science) and 2019 (Materials Science and Cross field). He is also Winner of Newton-Ungku Omar Coordination Fund: UK-Malaysia Research and Innovation Bridges Competition 2015. Recently he recognized with Fellow and Charted Scientist Award from Institute of Materials, Minerals and Mining (IOM), UK. He is also life member of Asian Polymer Association, and Malaysian Society for Engineering and Technology. He has professional membership of American Chemical Society (ACS), and Society for polymers Engineers (SPE), USA. e-mail: [email protected] Prof. Mohamed Naceur Belgacem received his Engineer diploma in wood science at the Saint Petersburg’s Forest Academy in 1986 (Russia). He then got a Ph.D. in Material Engineering and Science from the Polytechnic Institute of Grenoble, France in 1991. He was then awarded a fellowship from the National Research Council of Canada to carry out a post-doctoral training for two years in Polytechnic School of Montreal (Canada) as a researcher in the field of surface and interface phenomena in cellulose-based composite materials. On his return, he worked in R&D department in industry for 5 years, before moving to University of Beira Interior in Portugal where he spent three years (from 1997

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to 2000) as an associate professor. Dr. Belgacem was nominated full professor in 2000 at the Polytechnic Institute of Grenoble, France. He has published over 250 scientific papers, including 2 books and ~25 book chapters with around 10000 citations (H-Index > 75). Professor Belgacem is one of the three Editors-in-chief of an Elsevier Journal: « Industrial Crops and Products » (ISSN: 0926-6690 http://ees.elsevier.com/indcro/). This task was performed from 2007 to 2020. e-mail: naceur. [email protected]

Characterization and Comparative Evaluation of Structural, Chemical, Thermal, Mechanical, and Morphological Properties of Plant Fibers Sakib Hossain Khan, Md Zillur Rahman, Mohammad Rejaul Haque, and Md Enamul Hoque Abstract As a result of their widespread availability and wide range of technical applications, environmentally friendly natural fiber-based composites are attracting much attention. Unlike synthetic fibers, natural fibers have a wide range of advantages that make them a viable alternative. A rise in concern for the environment, coupled with the possible depletion of global petroleum supplies, has raised the interest in using more renewable resources instead of non-renewable ones to create new products. As a result, the design of new products by combining manmade and natural fibers is promising. On the other hand, natural fibers have become increasingly popular because of their eco-friendly nature. It is essential to understand their behavior to get the most benefits from these fibers. This book chapter aims to provide a comparative evaluation of plant fibers’ mechanical and physical properties. Other characterization investigations such as Fourier transform infrared spectrum, X-ray, and thermogravimetric analyses are also discussed. Keywords Plant fiber · Tensile strength · Modulus · Fiber treatment · SEM · FTIR · XRD · TAG

S. H. Khan · M. Z. Rahman · M. R. Haque Department of Mechanical Engineering, Ahsanullah University of Science and Technology (AUST), Dhaka 1208, Bangladesh e-mail: [email protected] M. Z. Rahman e-mail: [email protected] M. E. Hoque (B) Department of Biomedical Engineering, Military Institute of Science and Technology (MIST), Dhaka 1216, Bangladesh e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 R. Khiari et al. (eds.), Annual Plant: Sources of Fibres, Nanocellulose and Cellulosic Derivatives, Composites Science and Technology, https://doi.org/10.1007/978-981-99-2473-8_1

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1 Introduction Due to environmental concerns, recycling and environmental safety are becoming more vital for introducing new materials and products (Karimah et al. 2021). Designing products and materials to be recyclable or, more broadly, eco-friendly is becoming a trend in the industry (Vinod et al. 2020). Since glass fibers harm the environment, it makes sense to utilize lignocellulose fibers as an ecologically friendly option for reinforcing engineered polymeric materials. Natural fibers, including flax, hemp, kenaf, jute, and sisal, offer many advantages over E-glass fibers in structural and non-structural applications, economy, and the environment. Natural fibers have been increasingly applied in producing composite structures to replace traditional reinforcements due to their unique mechanical and physical properties and benign nature to the environment (Wang et al. 2022; Rahman et al. 2017a, 2016), as illustrated in Fig. 1. Natural fiber composites have begun to take the place of synthetic fiber composites due to a growing demand for environmentally friendly alternatives (Bismarck and Lampke 2005). Traditional manufacturing facilities for natural fibers are widespread in many developing nations, ensuring significant numbers of countryside employment (Chand and Fahim 2021). The roots, stems, leaves, fruits, and other components of plants may all be used to make natural fibers (Smole et al. 2013). Natural fibers offer many benefits over synthetic fibers due to their volume, accessibility, and low prices (Thyavihalli Girijappa et al. 2019). As the natural fibers are recyclable, biodegradable, and lightweight, they are used in constructing biodegradable green composites (Saha et al. 2021; Lamberti et al. 2020; Kozlowski et al. 2004; Cattani and Baruque-Ramos 2015; Rahman 2022). Due to technological advancements, manmade materials used in boat hulls, bathtubs, and archery bows may be replaced by natural fibers. Composite materials, such as plastics reinforced with glass or carbon fibers, are liable for significant climate change. However, these materials can be utilized in high-performance applications such as aerospace and motorsports because of their high strength and capability to endure elevated temperatures. There are several benefits to using glass fiber, including low cost, high strength, etc. However, they have one major drawback; they cannot

Fig. 1 Comparison between natural fiber, glass fiber, and carbon fiber (Faruk et al. 2014). (Reused with permission, license number: 5372610913229)

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Fig. 2 Classification of natural fibers (Milosevic et al. 2020)

decompose in the environment. In contrast, natural fibers are abundant, sustainable, affordable, reusable, and harmless. They are less dense and have a smaller ecological footprint than man-made fibers. They are made to resist the harsh conditions of the natural environment. Depending on where they come from, natural fibers may be categorized into three groups: plant, animal, and mineral (Fig. 2). Several types of plant fibers are available, including bast (e.g., jute and flax), leaf (e.g., abaca and pineapple), seed (e.g., cotton and kapok), and fruit (e.g., coir and oil palm). Reduced density, high strengthto-weight ratio, and low weight of natural fibers make them excellent candidates for lightweight composites and reinforcements (Rahman et al. 2017b, 2018). The mechanical characteristics of fibers are influenced by their microstructure and chemical composition, with the fiber’s diameter and length being the most important variables that influence its strength. Fibers with more cellulose and crystallinity have more substantial strength, and the opposite is true for lignin. In addition, fiber structural traits vary depending on the species affecting the fibers’ density and mechanical properties (Karimah et al. 2021). The natural fibers exhibit hydrophilicity due to the hemicellulose, and the fiber surfaces can be affected by various contaminants. However, several chemical treatments may help overcome these limitations, which are discussed in this book chapter.

2 Chemical Characterization Lignocellulosic fibers are the world’s most abundant renewable bioresource. Lignocellulosic materials are composed of cellulose, lignin, hemicellulose, extracts, ash,

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and minerals (Lennartsson et al. 2011; Thakur and Thakur 2015). The composition may vary for the same fiber depending on the growth conditions and locations (Frollini et al. 2000). A fundamental constituent of plant cell walls is the fibrous, stiff, water-insoluble polymer cellulose. It is also an alternative to fossilfuel-based polymers since cellulose is a biodegradable, biocompatible, and renewable natural polymer. The cellulose polymer comprises monomers linked together by glycosidic oxygen bridges and is generated through condensation. Every monomer block in cellulose is corkscrewed 180 degrees about its surroundings in a linear homopolymer comprised of β-1,4-linked glucopyranose units (Rowell and Rowell 1996). When natural fibers reinforce hydrophobic matrices, many hydroxyl groups in cellulose provide them hydrophilicity, resulting in a poor interfacial region and moisture absorption barrier (Ali et al. 2018). The hydrogen bonds that bind hemicellulose to cellulose fibrils are most likely responsible for its tight fit. In contrast to cellulose, hemicellulosic polymers are highly branched, completely amorphous, and have relatively low molecular weights. Hemicellulose is hygroscopic and water-soluble due to its unfolded construction, which contains several hydroxyl and acetyl units (Kozlowski et al. 2004). Phenylpropane units comprise the bulk of the long-chain heterogeneous polymer called lignin, which is water-insoluble and most often connected by ether bonds. After cellulose, lignin is the most common organic polymer on the planet. Secondary cell walls of plants include lignin, which contributes to the stiffness and hydrophobicity of cells that sustain loading and transport water (Saha et al. 2009; Nair et al. 2017). Natural fibers’ hydrophilicity may impact their interfacial characteristics with the polymer matrix and limit their reinforcing agent capability. The natural fiber interface can be enhanced by applying chemical treatments (Fig. 3) (Sanal et al. 2019). Chemicals may activate hydroxyl groups or efficiently create mechanical interlocks with the matrix. Chemical coupling agents are often multifunctional compounds. The initial step is to react with celluloses’ hydroxyl groups, and the following step is to react with the functional groups of the matrix. Polymer matrix adhesion may be improved by chemically altering natural fibers (Gunti et al. 2016). Materials can be coupled in various ways, including by removing incapacitated boundary layers, generating a resilient layer, constructing a highly crosslinked interface between polymer and substrate, enhancing wetting and establishing covalent bonds between polymer and substrate, and changing the substrate surface acidity (Bledzki and Gassan 1999). Environmental factors such as humidity, sunshine, or bacteria may degrade composites, which is a significant concern. In addition, there may be detrimental impacts on the effective stress transfer, beginning with the matrix toward the fiber due to the low resistance of fiber to water absorption (Punyamurthy et al. 2012). Because of this, it is crucial to thoroughly investigate the water absorption properties of natural fiber composites to ascertain both the long-term outcomes of water absorption and the durability of composites exposed to water for extended periods. The hydrophilic character of natural fibers reduces their potential as reinforcing agents; chemical modifications are suggested to enhance the fiber-polymer matrix interfacial adhesion. It is possible to lower the moisture absorption of fibers by

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Fig. 3 Different types of treatments on natural fibers (Nirmal et al. 2014; Rahman et al. 2016). (Reused with permission, license number: 5372620858780)

various chemical treatments, such as alkali treatment, acetylation, methylation, and cyanoethylation (Bledzki and Gassan 1999; Weyenberg et al. 2003). Different modifications can also be performed, including removing weak boundary layers, altering the acidity-alkalinity of fibers, constructing a flexible and robust layer on the surface, and utilizing a chemical agent to modify the chemistry of natural fibers during chemical treatments. When reinforcing polymers, alkaline treatment is a standard chemical treatment for natural fibers. Hydrogen bonding in the network configuration is disrupted by alkaline treatment, increasing surface roughness. This process depolymerizes cellulose and reveals the crystallites by removing some of the lignin, wax, and oils that coat the outside of the cell enclosure (Mohanty et al. 2001). The ionization of the hydroxyl group to the alkoxide is aided by the inclusion of sodium hydroxide solution in natural fiber (Agrawal et al. 2000). Its effectiveness is determined by the nature and amount of the alkaline solution, duration of treatment, and temperature (Bassyouni and Hasan 2015). Excess delignification of the fiber may occur if the alkali concentration exceeds the recommended level, damaging the fiber (Hamidon et al. 2019). The formation of a rough surface and the improved aspect ratio of the composites lead to improved fiber-matrix interfacial adhesion.

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The degree of the change increases as the NaOH concentration rises. Mercerization removes binding components (i.e., lignin and hemicellulosic chemicals), which alter the chemical structure of plant fibers, the degree of polymerization, and the molecular orientation of cellulose crystallites. As a result, chemical modification directly impacts the fine cellulose structure of plant fibers. Because of this, mercerization and other alkali treatments significantly influence the mechanical properties of fibers, particularly fiber strength and stiffness (Jähn et al. 2002). In addition to reducing the density and rigidity of the interfibrillar zone, removing hemicellulose enhances the ability of fibrils to reorganize themselves in response to tensile deformation (Bledzki and Gassan 1999). Pores of fiber become substantially transparent after alkalization because waxy residues and impurities are eliminated from the fiber exterior (Rokbi et al. 2011). Alkali treatment can break the fiber bundles into fibers, increasing the contact areas exposed for interaction with the matrix (Bledzki and Gassan 1999). Bonding agents are removed from alkali-treated fibers, resulting in a better fitting of cellulose chains, which results in a high proportion of crystallinity index (Varma et al. 1984). Treatment with NaOH solution leads to a decrease in spiral angle and a rise in molecular orientation. The elimination of non-cellulosic materials causes significant randomness in the crystallite orientation. Theoretically, when molecule orientation increases, fiber elastic moduli should rise (Sreenivasan et al. 1996). The breakdown of lignin is also expected to coarsen the fiber surface via the mercerization process. Mercerized fibers are more prone to capillary action, which raises surface tension when they contact the matrix and eventually provides a stronger adhesion. The treatment of sisal fibers with sodium hydroxide lowers porosity and enhances mechanical strength and water resistance (Bisanda 2000). The tensile characteristics of natural fibers were dramatically enhanced after alkali treatment as it delivers an improved interlocking between fiber and matrix. Brígida et al. (2010) studied the effects of NaOCl, NaOCl/NaOH, and H2 O2 treatments on the coconut fiber structure, composition, and characteristics (Brígida et al. 2010). The fibers treated with NaOCl/NaOH exhibit less hemicellulose, exposing more cellulose, while the thermal stability decreases. Although H2 O2 treatment is the most effective in removing wax and fatty acid deposits, it does not affect the chemicals on the surface. Alkali treatment of the jute fiber reacts with the hemicellulose, and thus the mesh structure is destroyed, and the fibers are divided into smaller filaments. The width of the fibers is reduced when the fibers are broken. Breaking down composite fiber bundles into tiny pieces enhances the polymer wetting contact area, resulting in a more effective wetting process. As a result, the fiber-matrix interaction becomes stronger (Ray and Sarkar 2001). A strong alkali treatment causes the abaca fiber to twist and the cell wall to swell. Alkali concentrations of 10 and 15% fragment the abaca fiber bundle into constituent fibers, removing hemicelluloses, pectin, and lignin. (Cai et al. 2015) The combined NaCl and alkali-treated bagasse fibers improve mechanical characteristics and thermal properties compared to raw sugarcane bagasse fibers (Mohit 2019). It is possible to enhance the mechanical properties and environmental resistance of fiber/polymer composites by treating fibers with silane appropriately (Xie et al.

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2010). The kenaf and pineapple leaf fibers treated with NaOH-silane eliminate noncellulosic materials, diminish their hydrophilicity, and exhibit improved tensile properties compared to untreated fibers (Asim et al. 2016). After treatment with saline and NaOH, the sugar palm fibers and thermoplastic polyurethane (TPU) matrix have better interfacial bonding (Atiqah et al. 2018). Flax-epoxy interface modification using silane coupling agents successfully enhances interfacial strength and increases the composites’ flexural strength (Weyenberg et al. 2003). The crystallinity of sisal fiber treated with a silane coupling agent decreases. The tensile and flexural strengths of the alkali and saline-treated fibers are marginally higher than those of the untreated sisal fiber. The coupling agent can easily migrate through alkali-treated sisal fibers, and its reaction with cellulose may lead to an even greater interfacial connection (Zhou et al. 2014). Surface treatment of henequen fiber with silane improves interfacial stress transfer performance; however, it does not increase fiber wetting (ValadezGonzalez et al. 1999). Saline-treated oil palm fiber exhibits the lowest incidence of crystallization and the highest degree of stability. Untreated composites have low thermal stability; hence chemical treatments should be performed to enhance them (Agrawal et al. 2000). The acetylation treatment can alter the hydrophilic nature of natural fibers into hydrophobicity by replacing hydroxyl groups with a hydrophobic acetyl group (CH3 CO) (Ferreira et al. 2019). Cellulosic fibers are typically plasticized using acetic anhydride and acetic acid (esterification method) (Ouarhim et al. 2019). Acetylation treatment on sisal fiber exhibits improved fiber-matrix adhesion, and a rougher and more uneven surface morphology, leading to enhanced tensile strength (Mokaloba and Batane 2014). Acetylation treatment on jute fiber results in a substantial rise in acetyl groups. Because of this, the treated jute is considerably hydrophobic and does not absorb water. Acetylated jute’s thermal stability is lower than that of raw jute (Teli and Valia 2013).

3 Structural Aspects 3.1 Chemical Composition Natural fibers comprise cellulose, hemicellulose, lignin, pectin, and waxes (Table 1) and have a multiscale structure (Fig. 4) (Bledzki and Gassan 1999). A fiber’s chemical content is influenced by its age, location, development rate, and the kind of plant species. The amount of cellulose in a plant fiber determines its stiffness and strength (Baley 2002). Microtubular plant fibers have cell walls around a central conduit called a lumen, which helps plant fibers absorb water. The fibers consist of different layers of microstructures (Brett et al. 1990). The fiber cell wall is not uniform. The primary cell wall (P) is the initial wall formed at the time of cell development, followed by the secondary cell wall (S) containing microfibrils, which have three levels (S1, S2, and S3). Semicrystalline cellulose microfibrils encased in a

8

S. H. Khan et al.

hemicellulose/lignin matrix of variable proportion constitute the cell walls (Bismarck et al. 2001). The primary cell wall regulates the pace and path of cell development and cell-to-cell contacts. The secondary cell wall provides mechanical strength (Ray and Sain 2017). The lignin can be found in the exterior cell enclosure of plant fibers, whereas cellulose microfibrils and hemicellulose can be seen inside lignin cellulose (Zhang et al. 2015). Natural fibers are composite materials whose cellulose chains are arranged spirally, indicating that cellulose chains are directed along the tangential direction (Yu et al. 2005). Plant fibers have a complex chemical composition and structure. Rigid cellulose microfibril-reinforced amorphous lignin and hemicellulose matrices are the primary building blocks of the fiber. Plant fibers contain a few water-soluble chemicals; however, cellulose, hemicellulose, and lignin are the primary constituents (Bledzki and Gassan 1999). The wax content in raw fibers impacts the fiber-matrix interfacial adhesion during composite manufacturing, while the lignin content substantially influences the fiber structure, characteristics, and morphology (Mohanty et al. 2000). Cellulose consists of both crystalline and amorphous structures. Microstructural factors such as cellulose and cellulose crystallinity directly affect the mechanical performance of natural plant fibers. The rigidity of crystalline cellulose is much superior to that of any other component. Plant fibers with increased cellulose content and cellulose crystallinity are suitable for structural applications (Petroudy 2017). Wood and plant fibers have distinct chemical compositions. Cellulose crystallinity in wood fibers is lower than in plant fibers, with an average of 55–70% and 90–95%, respectively (Madsen and Gamstedt 2013). The stiffness and strength of fibers are closely linked to their cellulose content (Thygesen et al. 2007). The crystallinity index of plant fibers rises as the delignification period of the fiber increases (Wang et al. 2018). The cellulose content and degree of crystallinity of plant fibers may be altered by physical and chemical treatments (Ahmad et al. 2019). One of the most prevalent chemical treatments (i. e., alkaline treatment) of natural fibers enables the elimination of lignin and hemicellulose partly, improving the crystallinity and roughness of the fiber. (Faruk et al. 2012). The alkaline peroxide treatment enhances plant fibers’ cellulose crystallinity and surface area (Seo et al. 2019). One of the essential crystalline structure factors is the degree of cellulose crystallinity. Increasing the proportion of crystalline to amorphous increases the stiffness and reduces the flexibility of cellulose fibers (Gümüskaya et al. 2003). The tensile strength of a fiber is affected by its crystallinity. Fewer and smaller fiber molecules are packed into tight and narrow pores when crystallinity is high, resulting in strong molecular bonds (Wang et al. 2016). With poor crystallinity, fibers will be more susceptible to chemical reactions, making them less resistant to stress, which results in lower strength (Reddy and Yang 2009). Crystallinity also affects the ability of fiber to absorb water since amorphous areas may take up more water. The crystallinity changes when well-organized native cellulose microstructures are disrupted, and more amorphous cellulose structures grow. The presence of amorphous cellulose dramatically enhances flexibility and elongation. Additionally, amorphous cellulose contains more accessible OH− groups that might serve as active sites for the material synthesis process to introduce

Hemi-cellulose (%)

82.70–90

30–47.7

Cotton

Coir

0.15–0.25

3

10

20–25

63–64

56–63

Banana

19.5

4–28

8–13

Abaca

70–82

Palf

10–14

66–78

59–77.6

Sisal

68.6–76.2

Ramie

Henequen

13.1–16.7

45–57

Kenaf

4–18

13.6–22.4

53–91

61–71.5

Hemp

11–20.6

Jute

Cellulose (%)

64–72

Fibers

Flax

Table 1 Chemical composition of various plant fibers Pectin (%)

–

0.4–1.2

1

–

–

–

0–10

0–1.9

0–0.6

0–0.2

1–17

1.8–2.3

Lignin (%)

40–45

7–9

5

5–12.7

8–13.1

8–20

0.6–0.7

18–21.50

12–13

1–21

2–2.5

Wax (%)

–

0.6

3

–

–

0–2

0–2

0–0.3

0–0.8

0–0.5

0–1

1.5–1.7

Moisture content (wt.%)

–

7.85–8.5

0.7

10–12

11.8

–

10–22

7.5–8

6.2–12

12.5–13.7

0–10

7.9–10

References

Esmeraldo, et al. 2010)

Liu and Fang 2018)

Saragih et al. 2018)

Pujari et al. 2013)

Daud 2014)

Cazaurang-Martinez et al. 1991)

Ekundayo and Adejuyigbe 2019)

Pandey and Ramie fibre: part I. 2007)

Millogo et al. 2015)

Alam et al. 2014)

Liu et al. 2017)

Yan et al. 2014)

Characterization and Comparative Evaluation of Structural, Chemical, … 9

10

S. H. Khan et al.

Fig. 4 MFA in the microstructure of plant cell walls and cell wall sub-layers (Melelli et al. 2020; Ramakrishnan et al. 2015). (Reused with permission, license number 5372621255980)

novel functions or enhance the mechanical characteristics of the material (Rumi et al. 2021). In terms of mechanical properties, water absorption, fire resistance, and biodegradability, the influence of the chemical composition of natural fiber is substantial. The cellulose percentage of abaca fiber is 63%. Abaca fiber has a higher cellulose percentage than hemp and banana fiber. Outer layers and fibers benefit significantly from the strength and rigidity of cellulose. The presence of hemicellulose may diminish the strength of the fiber. The hydrolysis of hemicellulose is more straightforward than that of cellulose. Abaca fiber’s hemicellulose concentration is roughly 20–25%. Fiber flexibility is also affected by the amount of lignin. Fibers with low cellulose content are more likely to have high lignin content. The high lignin content causes the fibers to be brittle, thus providing lower fracture strain (Mohanty et al. 2005). There is a link between chemical content and mechanical characteristics of sugar palm fiber, with cellulose, lignin, and hemicelluloses significantly improving the tensile strength, modulus, and elongation at break, respectively (Ishak et al. 2013).

3.2 Microfibrillar Angle The microfibrillar angle (MFA) refers to the angle microfibrils create about the fiber axis (Nunes et al. 2017) (see Fig. 4). The microfibrillar angles of various plant fibers

Characterization and Comparative Evaluation of Structural, Chemical, …

11

are shown in Fig. 5. The cellulose fibers consist of microfibrils where the cellulose molecules are laterally fixed by hydrogen bonding between the hydroxyl groups (Siró and Plackett 2010). Amorphous structures link crystalline cellulose to form microfibrils (Nunes et al. 2017). The microfibrils are organized in a helix shape in overlapping tubular layers (Chand and Fahim 2021). The microfibrillar angle plays a critical role in affecting the intrinsic fiber strength (Bhattacharyya et al. 2015; Mohamed et al. 2018; Kumar Ramamoorthy, et al. 2019). Cellulose fibers may be strong and ductile depending on the microfibrillar angle (Donaldson 2008). Because microfibrils’ orientation mainly determines the fiber’s tensile strength, a low MFA signifies higher mechanical characteristics (Meylan 1972). Reduced MFA indicates that the microfibrils of the cellulose fiber are well aligned relative to the fiber axis, suggesting lower fiber elongation (Reddy and Yang 2009). It is common to change the microfibrillar angle to accompany a change in the diameter of the fiber, which affects the impact strength. The initial modulus decreases with diameter for the same applied stress, resulting in excellent elongation. This higher elongation is due to the increased microfibrillar angle coupled with a higher fiber diameter (Satyanarayana et al. 1986). When fibers with greater cellulosic content and lesser microfibrillar angle are drawn under tension, they show increased ultimate tensile strength and higher modulus at the beginning of intracellular fracture. Fibers with lower cellulose content and higher microfibrillar angle have lower ultimate tensile strength and initial modulus with intercellular fracture. Fibers with a lesser cellulose percentage and an increased microfibrillar angle have excellent ductility (Bismarck and Lampke 2005). Because the fibrils may twist when stretched, fibers

Fig. 5 Microfibrillar angle of various plant fibers (Petroudy 2017; Satyanarayana et al. 1986; Chakravarty and Hearle 1967; Devi et al. 2011; Mohanty and Fatima xxxx; Radzi et al. 2022)

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S. H. Khan et al.

with an increased MFA show more resilience than those with a lower MFA (Kouko et al. 2019). The MFA is inversely correlated with Young’s modulus in various flax fibers (Bourmaud et al. 2013).

3.3 Density Because of their hollow and cellular structure, plant fibers are excellent acoustic and thermal insulators (Sorieul et al. 2016). The hollow structure significantly decreases fiber bulk density and weight (Chand and Fahim 2021). The density of various natural fibers is illustrated in Fig. 6. Natural fibers possess reduced densities in their natural condition; however, they are generally increased through processing. Nonetheless, these fibers are far less dense than inorganic fibers (i.e., glass fibers), even at their highest density. As a result of their low density, they are suitable for applications where weight is a major issue (Clemons and Caulfield 2005). Natural fibers are added to polymers to enhance their mechanical properties (stiffness and strength) without increasing their density excessively (Pickering et al. 2016). Renewable and CO2 -neutral resources, such as plants, are used in the reinforcing fibers of natural fiber composite materials (NFCs). Natural fibers have a lower density than synthetic fibers, allowing more design freedom in composite structures (Kim 2012). It is essential to comprehend the fundamental physical–mechanical characteristics of natural fibers to achieve their best possible use. Flax fiber bundles with larger diameters have larger lumens. When the lumen dimension increases, the porosity becomes more visible, and the fiber density decreases (Stamboulis et al. 2001). After 6% NaOH treatment, both hemp and kenaf fibers demonstrate a minor increase in density, suggesting cell wall compactness (Aziz and Ansell 2004).

3.4 Aspect Ratio The fiber length and aspect ratio (length to diameter ratio) are vital for the reinforcing action. Geometric characterizations may influence mechanical properties (i.e., aspect ratio), making it an essential parameter to understand. Low stress is transmitted to the reinforcement if the aspect ratio of the fiber is lower than the critical aspect ratio. To avoid dispersion issues due to intertwining among fibers, they should not be excessively long or too short to hinder stress transfer and weaken the overall structure (Mohanty et al. 2005). Cellulosic fibers are more flexible and less likely to break during handling than other fibers. Even after processing, the distribution of fiber lengths will stay consistent. The smaller size of the fiber ends is subjected to reduced stress because of the high aspect ratio. Low stress will be transmitted to the structure if the aspect ratio of the fiber is lower than the critical aspect ratio. Natural plant fiber possessing a shorter aspect ratio is not subjected to stress that exceeds its strength, so unbroken fibers are removed from the matrix when the composite breaks (Mohanty

Characterization and Comparative Evaluation of Structural, Chemical, …

13

Fig. 6 Density of various plant fibers (Ramakrishnan et al. 2015; Sanjay et al. 2015; Alkbir et al. 2016)

et al. 2005). The poor strength of the coir fiber is primarily due to its considerable number of cells with low aspect ratios, while sisal and hemp have cells with high aspect ratios and big lumen sizes. In addition to cellulose content and microfibrillar angle, the cell aspect ratio may also be used to develop structure–property relations in natural fibers (Stevens 2002). Tensile strength decreases with increasing fiber loading in natural fiber composites after a certain fiber weight fraction (Rahman 2021, 2017). The fiber size does not significantly influence the strength of composites. Generally, fibers with a uniform circular cross-section and a specific aspect ratio exhibit high performance (Mohanty et al. 2005). Stresses transferred from the polymer matrix are greatly decreased in the irregularly shaped fibers with a low aspect ratio. Hemp fibers do not lose as much length as flax fibers after processing; however, hemp fibers tend to fibrillate more quickly and drop in diameter (Peltola et al. 2011). During treatment, the fiber aspect ratio of hemp fiber is enhanced, allowing for more stress transfer from matrix to fiber. In the case of flax, the aspect ratio decreases, lowering the reinforcing effectiveness of fiber. The aspect ratio of composites significantly affects its mechanical properties. Because it gives greater room for interaction with the matrix, a higher aspect ratio leads to superior tensile, flexural, and impact properties. Short-length fibers tend to operate as fillers instead of reinforcing components in composites due to their lower aspect ratios (Yılmaz 2014). Examining their component phases and interfacial regions is necessary to understand how fiber-reinforced composites behave mechanically. Fiber aspect ratio, fiber modulus and strength, fiber-matrix adhesion, and matrix toughness are essential when manufacturing a composite. Composites with inferior properties may result from varying only a few of these parameters (Ramaswamy et al. 2003).

14

S. H. Khan et al.

Fig. 7 Mechanical properties of various plant fibers (Ramakrishnan et al. 2015; Sanjay et al. 2015; Alkbir et al. 2016; Danso et al. 2015; Abiola et al. 2014; Chokshi 2020)

4 Mechanical Properties Plant fibers have much poorer mechanical characteristics than the most extensively used, reinforcing glass fibers. Nevertheless, the particular characteristics, such as specific strength and stiffness, are equivalent to those of glass fibers (Wambua et al. 2003). Generally, people consider products made from renewable resources to be ecologically beneficial. Eco-balances comparing glass and plant fiber mats from seed generation and raw material extraction to final fabric found that the energy consumption for mats made from plant fiber is significantly reduced (Diener and Siehler 1999). The essential characteristics that influence the comprehensive behavior of fibers are the structure, microfibrillar angle, cell size and defects, and chemical content of plant fibers (Mukherjee and Satyanarayana 1986). Cellulose content and microfibrillar angle are both factors that control the average fiber tensile strength. Increases in cellulose content lead to greater tensile strength and Young’s modulus in plant fibers (Flemming et al. 1999). Cellulose microfibrils are the reinforcement of natural fibers surrounded by hemicelluloses and lignin. The microfibrils inside the hemicellulose and plant cell walls’ lignin may be either void shapes or nearly

Characterization and Comparative Evaluation of Structural, Chemical, …

15

square cross-sections (Park et al. 2006). After applying a load, the microfibril axis is brought into line with the fiber axis. For cellulose microfibrils to break, a matrix loses contact with the supporting fibrils, and the hydrogen bonding weakens in the cellulose microfibrils. The less cellulose in the material, the lower the tensile strength (Komuraiah et al. 2014). The crystalline structure of cellulose emerges from significant hydrogen bonding amidst its chains. Cellulose is capable of coping with excessive stress. Because of their amorphous form and heterogeneity, hemicelluloses may barely respond to moderate stresses (Bledzki and Gassan 1999). The stiffness of fibers is determined by the alignment of cellulose microfibrils with the fiber axis. Plant fibers show excellent ductility when microfibrils are spirally aligned to the fiber axis. If the microfibrils are arranged parallel to the fiber axis, the fibers show a rigid behavior and possess high tensile strength. Natural fibers have lower tensile strengths and Young’s modulus than E-glass fibers (Esfandiari 2007). Compared to synthetic fibers, natural fibers’ specific strength and modulus are equivalent to, or surprisingly better than, those of E-glass fibers (Rohit and Dixit 2016).

4.1 Mechanical Properties of Untreated Plant Fibers Abaca is a natural fiber with outstanding mechanical properties due to its cell wall structure. The cellulose, hemicellulose, and lignin of abaca fiber affect its mechanical properties. Moreover, the fiber stiffness and Young’s modulus are correlated with the cellulose content. In addition, the elongation is inversely related to cellulose content. Abaca fiber is stiffer due to its high cellulose content (Saragih et al. 2018). Henequen fibers have a slight elongation at break. They are strong and may be used to strengthen composites. These fibers exhibit excellent hardness and resilience (Esmeraldo et al. 2010; Fiore et al. 2015). Due to its high cellulose content and low microfibril angle, PALF fiber has outstanding physical properties. PALF fibers are equivalent to jute fibers in bending and twisting stiffness (Wang et al. 2019). The abaca fiber is often regarded as the toughest plant fiber available. Its tensile strength is triple that of cotton and doubles that of sisal (Biagiotti et al. 2004). Oil palm fibers are mostly composed of cellulose and a significant amount of lignin. The fibers have a hollow cross-section and are very permeable. They have a variety of diameters. The mechanical characteristics of the fiber are affected by all of these parameters. The tensile strength of oil palm fibers is inferior to other natural fibers (Fan et al. 2012). Cotton has lower toughness and initial modulus than hemp fibers; however better fracture strain, resilience, and ductility than hemp. Due to the high lignin content, coir is tough, strong, and long-lasting. Coir is a very tough natural fiber compared to other fibers. Sisal fiber’s tensile characteristics are not consistent throughout its length. Elongation at break is larger in fibers derived from the root or bottom of the leaf, which have lower tensile strength and modulus. The fibers exhibit greater stiffness and strength in the middle of the fiber, while the fibers extracted from the end exhibit intermediate characteristics (Guo and Wu 2008). The ramie fibers typically demonstrate excellent properties; however, when soaked in water, the tensile

16

S. H. Khan et al.

strength, flexural strength, and impact strength of ramie/epoxy composites decrease significantly (Kokot et al. 2002). Kenaf fibers are hard to deal with because they are rough and easily breakable. The strength at which it disintegrates is comparable to poor-quality jute and is only marginally reduced when soaked in water (Li et al. 2021). Jute fibers have lower tensile strength than flax or hemp fiber (Bismarck and Lampke 2005).

4.2 Mechanical Properties of Treated Plant Fibers The chemical treatment of bagasse fibers improves tensile characteristics by eliminating the wax coating from the fiber exterior. The waxy material is detrimental to fiber-matrix adhesion and surface wetting. The NaOH treatment enhances the stiffness of the fibers, improving the tensile characteristics (Prasad et al. 2020). During the tensile test, the sticky substance prevents the cellulose chains from reorganizing themselves compactly along the path of tension. Moderate alkali treatment removes sticky hemicellulose and lignin from bamboo fibers, increasing cellulose crystallinity, tensile strength, and modulus (Zhang et al. 2018). The bamboo fibers retain tensile strength at increased NaOH concentrations; however, their stiffness decreases significantly (Ray and Sarkar 2001). The growth in the lumen size of flax fiber increases the porosity of the fiber and decreases its density. Significant defects cause fracture rise as fiber length increases (Stamboulis et al. 2001). The coir composite is fragile without immersion and breaks with lower stresses than composites submerged in water (Samal and Ray 1997). When coir fibers are alkali treated, the surface roughness of the fibers rises, enhancing the mechanical interlocking between the fibers and the matrix of composites (Abiola et al. 2014). For the coir fiber treated with 2% NaOH solution, the composite increases tensile and flexural strength by 26 and 15%, respectively. In the case of 5%, NaOH treated fiber composite, the flexural strength increases by 17% more than untreated fiber composites. The tensile strength of bleached coir composite is somewhat lower than that of untreated fiber composites. Regarding flexural strength, the bleached fiber polyester composites outperform untreated fiber by 20% (Rout et al. 2001). The tensile strength is affected by the amount of lignocellulosic fiber and the kind of coupling agent used. Other mechanical properties, such as stiffness, elongation at break, and toughness, are determined mainly by the amount of lignocellulosic fiber. Silane-treated composites demonstrate the most remarkable mechanical performance because of substantial contact between the matrix and lignocellulosic fibers at the interface (Colom et al. 2003). The combined alkali and dilute epoxy treatment on flax fibers substantially impact the composite’s tensile mechanical characteristics. The combined treatment also enhances flexural strength and stiffness (Weyenberg et al. 2003). Surface alteration of sisal fiber with macromolecular coupling agents increases tensile strength while reducing impact strength, implying that surface treatment improves the compatibility of sisal fiber and polylactide and ensures efficient stress transfer between the fibers and matrix (Li et al. 2011). Compared to untreated

Characterization and Comparative Evaluation of Structural, Chemical, …

17

sisal fiber composites, mercerized sisal fiber composites demonstrate a noticeable increase in flexural properties. This modification changes the fiber/matrix interfaces due to increased mechanical interlocks caused by alterations in surface morphology and improved wettability caused by the partial or entire exclusion of amorphous fiber components (i.e., lignin and surface contaminants) (Ganan et al. 2005). The treated kenaf fibers increase the tensile strength and modulus of the orientated long fibers composites by 11% and 3.5%, respectively, compared to untreated kenaf composites. The hydrophilic kenaf fibers treated with 6% NaOH solution improve the interaction with the hydrophobic polymer. Even after fiber chemical treatment, the randomly oriented short fiber composites have lower tensile strength than untreated kenaf composites. The random distribution of fibers in a polymer matrix results in the formation of stress concentrators that lead to the early failure of composites (Fiore et al. 2015). Composites made from chemically treated jute fibers outperform raw jute composites with tensile strength, elongation at break, void content, and interfacial adhesion. Chemically treated jute fibers may have greater matrix-fiber interfacial adhesion, improving the tensile characteristics of composites (Wang et al. 2019). Biagiotti et al. (2004) concluded that almost all of the critical characteristics of composites might be enhanced by adequately treating the fibers. The ductility of natural fiber is influenced by its elongation at break. The ductility of natural fiber is measured by its elongation at break, called fracture strain. Plant fibers’ specific modulus and fracture strain are superior to those of synthetic fibers (i.e., E-glass) in composites. When natural plant fibers are added to a polymer matrix, the fracture strain is reduced; however, tensile modulus increases. The fracture strain tradeoff often shows tensile strength improvements (Mohanty et al. 2005). The fracture strain of composites decreases as fiber content increases. Moreover, fiber-matrix interfaces are less prone to deformation due to increased stiffness.

5 FTIR Spectrum Analysis FTIR (Fourier-transform infrared spectroscopy) is used to determine plant fibers’ chemical compositions and chemical bonds (Fan et al. 2012). Many studies have been conducted on analyzing cellulose hydrogen bonds using various approaches, with FTIR emerging as among the most effective (Guo and Wu 2008; Kokot et al. 2002; Li et al. 2021; Li 2022; Yi et al. 2021, 2022a, 2022b; Guo 2022; Gao et al. 2021). The chemical contents of untreated and treated natural fibers can be identified using FTIR. FTIR also ensures hemicellulose removal from plant fiber after alkaline treatment (Mwaikambo and Ansell 2002) and effective coupling between the fiber and the matrix (Mohanty et al. 2001). The FTIR spectra of untreated and treated jute fibers are shown in Fig. 8, while Table 2 presents the stretching/bonding band of various plant fibers. The major constituent in natural fibers is cellulose, a cementing substance within the cell enclosure. The cellulose molecules are in microfibrils with a strong crystalline structure due to significant hydrogen bonding between cellulose chains. A

18

S. H. Khan et al.

Fig. 8 FTIR spectra of untreated and treated jute fibers (Thomas et al. 2015). (Reused with permission, license number: 5372871151641)

broad band around 3400 cm−1 is a hydrogen-bound O–H stretching vibration. The OH band of the treated flax fiber is somewhat sharper than its untreated OH band. The ethylene plasma treatment converts certain hydroxyl groups (O–H) in flax fiber to ether groups. Hydrogen bonds between molecules weaken due to the changes in hydroxyl groups (Lee et al. 2003). The hydroxyl stretching band stays almost unaltered for flax fibers throughout the transmission of FTIR spectra of acylation reactions, suggesting that a substantial proportion of flax fiber OH groups are not involved in the process (Zini et al. 2003). The presence of acetyl/propionyl groups in the fiber components for treated flax and hemp fibers can be determined by FTIR analysis. Hydrophobicity of the fiber increases by acetylation and propionylation treatments. Thus, a decrease in hydrophilicity after esterification is observed; however, fiber crystallinity slightly reduces due to the rise in esterification reaction in amorphous content (Tserki et al. 2005). NaOH treatment eliminates amorphous chemicals and increases the crystallinity index of fiber bundles (Troedec et al. 2008). Alkali and acetyl treatments of hemp fibers eliminate the hemicellulose and lignin from the fiber exterior; however, silane shows no response on the fiber surface. The processes of alkalization and acetylation reduce the hemicellulose and lignin of the fiber. Cellulose microfibrils on the fiber exterior are also exposed during these processes. As a result, silane treatments can coat fiber surfaces and fill the voids between individual microfibrils (Kabir et al. 2013). The proportion of cellulose in banana fibers rises after the steam explosion and subsequent bleaching due to a rise in crystallinity in fibers. The amount of lignin and hemicellulose reduces from raw to bleached fibers. The lignin peak decreases from raw to bleached fibers as most lignin components are destroyed during the steam explosion and subsequent bleaching (Cherian et al. 2008). The FTIR spectrum of banana fibers exhibits an aromatic character, indicating lignocellulosic containing cellulose, hemicelluloses, and lignin (Subramanya et al. 2017). The O–H stretching occurs around 3500 cm−1 , and its intensity decreases due

1736.5 1384.1

1737.4

1374.2

1608.3

C=O

C–H bending

C=C

1654

1384.1 1653.9

2924.2

3447.2

2903.8

C–H vibration

2920.5

3448

3349.9

O–H

Sisal (Mwaikambo and Ansell 2002)

Hemp (Mwaikambo and Ansell 2002)

Stretching/bonding Pineapple (cm−1 ) leaf fiber (PALF) (Samal and Ray 1997)

Table 2 FTIR of various plant fibers

1653.8

1384.1

1737.2

2918.8

3447.9

Jute (Mwaikambo and Ansell 2002))

1596.1

1383.6

1741.1

2918.1

3419.7

–

1418

1740

2882

3342

–

1371.43

1728.28

3026.41

3435.34

–

1372

1750

2980

3300

1593

1732

2924

3400

Kapok Bamboo Coir Cotton Abaca (Mwaikambo (Afrin et al. (Chukwunyelu (Chung (Punyamurthy and Ansell et al. 2016) et al. 2004) et al. 2014) 2012) 2002)

Characterization and Comparative Evaluation of Structural, Chemical, … 19

20

S. H. Khan et al.

to fibers with fewer OH groups on their surfaces, resulting in a more hydrophobic nature. The CH stretching vibration is responsible for the peak at 2900 cm−1 . In the vicinity of 1638 cm−1 , the carbonyl group (C=O) stretching is responsible for an absorption band. Treatment with alkaline removes the hemicelluloses in the date palm fibers, as shown by the reduced absorption in the carbonyl region of the FTIR spectra of treated and untreated samples. The soda treatment on the date palm fibers has minimal effect on their chemical structure (Alawar et al. 2009). The alkali treatment removes natural and manufactured impurities from the fiber surface, producing a rough surface. Despite FTIR evidence of chemical bonding, silane-treated ramie fibers and matrix show a lower interfacial bonding strength than alkali-treated ramie fibers. In particular, the absorption peak ascribed to the Si-cellulose bond is 855 cm−1 and 865 cm−1 . FTIR spectra of the untreated and treated pineapple leaf fibers exhibit a single strong band at about 1244 cm−1 which corresponds to C–O stretching of lignin, whereas a strong and sharp band around 1730 cm−1 corresponds to the ester and carbonyl functional (C=O) groups in lignin and hemicellulose, which reduce as the NaOH concentration rises. Untreated jute has an absorption band at 1032 cm−1 that may contain cyclic alcohol. The siloxane interaction with the hydroxyl groups in the jute cellulose may raise this band to 1051 and 1055 cm−1 , depending on the alkali or siloxane treatment. This finding provides a confirmation of oligomeric siloxanes’ affinity for jute fibers (Seki 2009). The chemical composition of cellulose I and cellulose II nanofibers from cotton fibers, partly altered by NaOH treatment, demonstrates no visible change following acidic hydrolysis. There is no difference between cellulose I and II in hydrolysis rates (Li et al. 2014). Hemicellulose, lignin, and unwanted components can be eliminated from the kenaf fiber surface by NaOH treatment. The H-bonding degree decreases with increasing NaOH concentration (El-Shekeil et al. 2012).

6 X-ray Diffraction Analysis The crystallinity of cellulose in plant fibers is an essential feature. Plant fibers with a high cellulose crystallinity are preferred for composite applications since crystallinity is related to fiber strength and stiffness. The intrinsic cellulose crystallinity of plant fibers should not be damaged during any treatment. When utilizing a Cu Ka X-ray source, cellulose is identified by XRD peaks at 2 h = 15.5, 16.5, and 22.8, which correspond to (1 0 1), (0 0 2), and (0 0 4) reflections, respectively. The (0 0 2) reflection is the primary crystalline peak of cellulose I. Cellulosic materials’ crystalline index (CI) may be calculated using an X-ray diffractogram and the peak height approach provided by the following empirical Equation (Nam et al. 2016). %CI = [1 − (Iam /I002 )] × 100

(1)

Characterization and Comparative Evaluation of Structural, Chemical, …

21

where I 200 is the maximum intensity of the (0 0 2) lattice plane at a 2θ angle between 22 and 23° and I am is the intensity of diffraction of the amorphous material at an angle of 2θ (around 18°). XRD patterns can be used to determine a material’s crystallinity. The diffraction intensity is in the range of 5–60° of 2θ with a scanning speed of 0.02°/s for the untreated, treated, and polylactic acid-coated sisal fiber. The crystallinity indices of various plant fibers after alkali treatment are presented in Table 3. Due to the PLA coating, alkali-treated sisal fibers with coating have a higher crystallinity index than pure sisal or treated sisal fibers. The crystallinity index of coated sisal fibers is higher than the other two because it eliminates the amorphous phase of sisal fiber. Tensile strength improves with an increase in the crystallinity index (Sahu and Gupta 2019). After the alkali treatment of jute fiber, the degree of crystallinity rises. The improvement in the strength of the peaks is used to measure the level of crystallinity development by NaOH treatment. This may be due to eliminated hemicelluloses, which typically split the cellulose chains, resulting in the development of new hydrogen bonds between specific cellulose chains (Sinha and Rout 2009). Hemp fibers have a greater crystallinity index than sisal fibers, although hemp fibers contain less lignin, pectin, and wax. The crystallinity index of hemp fibers increases after surface treatment, whereas it decreases for the sisal fibers after treatment. Due to the elimination of pectin and other impurities following treatment, the crystallinity of hemp fiber increases. The reduction in crystallinity index and, thus, the mechanical properties of sisal fiber are connected to the permanent transformation from cellulose I to cellulose II (Hajiha et al. 2014). Non-crystalline cellulose and amorphous fiber components, including lignin, pectin, and hemicellulose, are responsible for the low peak intensity in untreated pineapple leaf fibers. Cellulose I peak is more prominent in caustic solution-treated materials. Few NaOH hydrates impact the cellulose crystal structure at low concentrations. The treatment temperature significantly influences the alkaline treatment’s elimination of amorphous chemicals. Following treatment, the enhanced crystallinity of the fiber is caused by the cellulose chains being more tightly packed (Jaramillo-Quiceno et al. 2018). The crystallinity index of flax fibers is influenced by the temperature, time, and NaOH solvent content. The crystallinity index decreases with the rise in NaOH concentration while the treatment temperature and duration remain the same. Amorphous cellulose in the flax increases when it is mercerized with NaOH, reducing crystallinity. Increasing the treatment temperature increases the crystallinity index while maintaining the same treatment duration and NaOH solvent content. The crystallinity index decreases as treatment time increases while maintaining the same temperature and concentration. The crystallinity index of flax fibers is raised from 72 to 77 after alkaline treatment. Eliminating cementing elements leads to a larger stacking of cellulose chains, increasing the crystallinity index of alkaline-treated flax fiber (Cao et al. 2012). The molecular structure of bamboo fibers treated with 50% alkali is not uniform, reducing crystallinity. Randomness in molecular structure may result from cellulose chain breakdown when alkali concentrations are high. The mechanical characteristics of adequately oriented fibers are superior to those with a low or medium degree of orientation. Mechanical performances deteriorate for a higher percentage of alkali-treated bamboo fibers because

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Table 3 Crystallinity index and alkali treatment Natural fibers

Crystallinity index Crystallinity index Crystallinity index References (untreated) (alkali treatment (alkali treatment for 5 min) for 5 to 20 min)

Flax

72.2

60.9

Cao et al. 2012)

Banana fibers

52

–

Subramanya et al. 2017)

Coir

35.02

43.09

32.05

Bakri et al. 2018)

Cotton

90.3,90

–

–

Nam et al. 2016; Santana et al. 2019)

Hemp

87.87

87.61

81.34

Mwaikambo and Ansell 2002)

Sisal

70.90

68.39

77.52

Mwaikambo and Ansell 2002)

Jute

71.39

77.79

78.32

Mwaikambo and Ansell 2002)

Kapok

45.75

48.73

53.74

Mwaikambo and Ansell 2002)

Pineapple leaf 58.6,50,64 Ramie Fiber

68.20

Abaca fiber

52

Sena Neto et al. 2013) 58

84.79

Li and Yu 2014)

47

Cai et al. 2016)

this causes the crystallinity to decline and increases molecular structure randomness (Das and Chakraborty 2008).

7 Microscopic Analysis Scanning electron microscopy (SEM) is used to examine and analyze a material’s microstructural and morphological properties (Hearle and Simmens 1973; Rahman 2013; Fakirov et al. 2014). Natural fibers have been studied extensively for their morphological characteristics, including chemical treatments (Bismarck et al. 2001; Mwaikambo and Ansell 2002; Sreekala et al. 1997), structural studies (Barkakaty 1976), and thermal analysis (Wypych et al. 2007). The morphology of plant fibers is often compared to one another. Sisal fiber microstructure consists of many fiber cells with primary, secondary, and tertiary cell walls and lumens (cavity). The middle lamellae, composed of lignin and hemicellulose, connect each fiber-cell, as illustrated in Fig. 9a, b. The number of fiber cells, cell wall thickness, and fiber diameter distinguish different fiber varieties as individual fiber has unique mechanical characteristics (Fidelis et al. 2013).

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Fig. 9 Sisal microstructure showing a fiber-cells with a lumen and middle lamellae and b detail of middle lamellae and cell walls (Fidelis et al. 2013) (Reused with permission, license number: 5372880443214), and c banana microstructure (Subagyo and Chafidz 2018; Alavudeen et al. 2015) (Reused with permission, license number: 5372890740806)

Banana fiber has a cellular structure covered in scales with an intact interior (Fig. 9c). The parallel lines on the fiber surface along the fiber length are because of the bundle construction of the fiber, which comprises many fibrils in each bundle. The cross-section of the fiber confirms the multi-cellular structure along with the lumen or cavity in the cross-section (Subagyo and Chafidz 2018). Cell wall thickness is roughly 1.25 μm for banana fibers, with diameters between 18 and 30 mm. The lengths of the fibers range from 2.7 to 5.5 mm (Ansell and Mwaikambo 2009). Flax fiber along the length determines the diameter of the fiber. A cross-section does not determine the fiber size; however, its diameter profile can be obtained by evaluating it throughout its entire length. Fiber size fluctuates greatly depending on where the observation area is located (Karine, et al. 2007). The surface topography alters because of removing non-crystalline elements of the fibers, possibly hemicellulose, binder lignin, and waxy substances (Fig. 10) (Amiri et al. 2015). Although similar in shape, natural fibers vary in the inner lumen diameter, the number of lumens, the amount and structure of fiber cells, and the thickness of the secondary cell walls. The coir fiber cells are thin and consist of voids with thick cellulose walls (Fig. 11) (Fidelis et al. 2013). The coir fibers have 13–14 mm diameters and a cell-wall thickness span of 8 mm from the thin wall (Ansell and Mwaikambo 2009). Munawar et al. (2007) studied the cross-sections of several nonwood plant fiber bundles, including abaca, coir, pineapple leaf, sisal, kenaf, and ramie fiber, using the SEM. Each fiber is different in terms of bundle shape and size, lumen number and size and actual cross-sectional area. Coir fiber demonstrates a circular cross-section, and the rest of the fibers demonstrates an irregular cross-sectional shape. Ramie fiber exhibits a lower bundle diameter than the other fibers, but sisal fiber has a higher bundle diameter (Fig. 16). The surfaces of the sisal fibers alter substantially, indicating that some parenchyma cells are lost (Fig. 17). The transition from a rough to smooth surface suggests that surface contaminants are removed, and elongated fibers cells are separated because of the extraction of cementing elements, such as lignin and hemicellulose, after the

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Fig. 10 Flax fiber microstructure: a untreated and b alkali-treated (Amiri et al. 2015) (Open access)

Fig. 11 Cross-sectional structure of coir fiber and jute fiber (Fidelis et al. 2013) (Reused with permission, license number: 5372880443214)

NaOH treatment. The effective contact surface area has increased due to the increased spacing of the elongated fibers cells (Fig. 17b) (Martins et al. 2004). The surface of untreated jute (Fig. 18a) contains numerous contaminants and other materials such as wax, pectin, and lignin. As displayed in Fig. 18b, many bumpy patches appeared on the alkali-treated jute surface. This demonstrates that contaminants and other compounds on the jute surface are eliminated following alkali treatment, resulting in a cleaner and rougher jute surface than the untreated sample

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Table 4 Cross-sectional morphologies of the plant fibers (Munawar et al. 2007) Fiber

Cross-section

Fibers are thin-walled (~2 mm thick) with a diameter of 22 mm and a length of approximately 20 mm (Ansell and Mwaikambo 2009)

Figure 12

Jute fiber bundles are irregular with small lumens. Fibers are 5–20 mm in Figure 13 diameter, and the size of the bundles alters greatly (Ansell and Mwaikambo 2009) Kenaf is a multicellular typed fiber. The kenaf lumen has slightly rounded Figure 14 corners, similar to the lumens of the jute fiber. Kenaf fiber bundles have thick cell walls and approximately 20–25 individual fibers (Anuar et al. 2019) Hemp fiber bundles are 30–90 cm in length and 5–17 mm in diameter (Ansell and Mwaikambo 2009)

Figure 15

Sisal fiber bundles are irregular in shape, and lumen size alters significantly (Ansell and Mwaikambo 2009)

Figure 16

Fig. 12 Kapok fibers (Ansell and Mwaikambo 2009) (Reused with permission, license number: 5372900479111)

Fig. 13 Jute fiber bundles (Ansell and Mwaikambo 2009) (Reused with permission, license number: 5372900479111)

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Fig. 14 Kenaf fiber bundles (Ansell and Mwaikambo 2009) (Reused with permission, license number: 5372900479111)

Fig. 15 Hemp fiber bundles (Ansell and Mwaikambo 2009) (Open access)

(Fig. 18a). Because lignin and hemicelluloses are removed, the alkali-treated jute has a rougher surface morphology, resulting in improved fiber crystallinity (Nam et al. 2015). There is no degradation of flax fiber’s non-fibrous stem tissue with the strong bonding of the fibers (Fig. 19a). The cellulose, pectic, hemicellular tissues, and lignin are all present in the fiber bundles. Figure 19b shows that the tissues can partly stick to the fiber bundle’s surface at a lower percentage of NaOH. Alkali treatment (over 5%) eliminates any remaining parenchyma cells and non-fibrous stem tissue from refined flax fiber bundles. Refinement begins inside the fiber bundles, as shown by the presence of clean fiber bundles (Fig. 19c). The cell wall layers of the elementary

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Fig. 16 Sisal fiber bundles (Ansell and Mwaikambo 2009) (Reused with permission, license number: 5372900479111)

Fig. 17 SEM images of a untreated sisal fibers, b 5% NaOH treated, and c 10% NaOH treated (Martins et al. 2004). (Reused with permission, license number: 5372910181694)

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Fig. 18 SEM images of jute fiber surface: a Untreated and b alkali-treated (Jabbar et al. 2017) (Reused with permission, license number: 5372920375465)

Fig. 19 Alkali treatment of flax fiber: a untreated, b 5% NaOH, c 10% NaOH, d 15% NaOH, and e 20% NaOH (Jähn et al. 2002) (Reused with permission, license number: 5372940406865)

fibers shrink and partly break, indicating that the flax fibers contain a more significant amount of amorphous cellulose (Jähn et al. 2002). The reduction in fiber diameter confirms the results of the alkaline surface treatment, which cleans the fiber’s surface. Stripping of lignin, hemicellulose, and other chemicals forms an outer coating on cellulose, eventually reducing the fiber diameter. This removal of impurities improves interfacial adhesion by increasing surface roughness (Shibata et al. 2005). A greater amount of fiber-to-matrix adhesion is reported after the alkali treatment of the fibers. Because of the wax on the surfaces of the sisal fiber, the untreated fibers are smooth. The surface is rougher following treatment with NaOH. The NaOH treatments on banana fibers result in excellent surface modifications compared to sisal and pineapple fibers (Buitrago Uribe et al. 2015). Coating with the silane coupling agents forms a smooth silane coating on the fiber surface (Zhou et al. 2014). The superior adhesion of treated sisal and oil palm fibers can be observed, and the alkali-treated fibers show higher tensile characteristics than untreated fibers. Bonding agents and fiber treatment are necessary to improve fiber-matrix adhesion (Jacob et al. 2004). Both primary and tertiary wall delamination can happen in the case of sisal fiber. Due to this delamination between fiber cells, the stress–strain curve shows a slight non-linear region (Silva et al. 2008). The fracture behavior of coconut fiber reinforced composites depends on varied fiber orientations and static loading circumstances (Badri et al. 2017). After the tensile

Characterization and Comparative Evaluation of Structural, Chemical, …

29

test, untreated and treated flax fibers lack significant fibril splitting, resulting in the fiber’s brittle behavior (Liotier et al. 2019). Instead of increasing tensile strength, lignin in the cell wall matrix stiffens the plant. Compared to other bast and leaf fibers, the shear modulus of sisal fiber is greater due to the higher concentration of lignin in the fiber bundles (Yu et al. 2005). Flax fiber’s quality decreases as lignification increases because the fibers become stiffer and coarser (Sharma 1986). Due to insufficient binding material, fibers can be pulled out of the bundle’s outer layer (mostly lignin and pectin). However, fiber pull-out may not be seen in the center of the alkalized fiber bundle fracture failures, suggesting adequate binding materials in the structure. Shattered fiber bundles may also show insignificant fibril splitting and fiber bending. Because the jute fiber bundle fails under different stresses, fiber possesses cell wall imperfections that lead to increased stress levels and eventually collapse (Mwaikambo 2009). A significantly high fiber loading leads to fiber-to-fiber interaction, generating cracks that spread over the interface between fibers, eventually imparting lower strength (Zhiming et al. 2017). A soda treatment of the date palm tree removes contaminants considerably from the fiber surface and increases fiber fibrillation. The pits on the fiber surface rise with increasing the soda concentration. As soda concentration increases, the solution attacks the primary structural components of the fiber, resulting in more pits on the fiber surface. The tensile strength declines due to this additional weakening of fiber strength (Alawar et al. 2009). Using alkali to remove lignin and hemicellulose from abaca fibers increases roughness (Punyamurthy et al. 2012). Kenaf fibers are cleansed of contaminants and have an enhanced surface roughness after treatment with NaOH (Fig. 20). El-Shekeil et al. (2012) reported that fiber-matrix adhesion seems to be excellent for untreated fiber-matrix. For the case of treated kenaf, gaps between fibers and the matrix and fiber pull-outs during tensile failure indicate poor fiber-matrix adhesion. Alkali treatment degrades the fiber-matrix adhesion. The silane coupling agent promotes uninterrupted interaction between the lignocellulosic fibers and high-density polyethylene to a great extent than the untreated fibers. There are no matrix residuals on the surface of the untreated fibers. This clearly illustrates the untreated lignocellulosic fibers’ poor adhesion to the matrix (Colom et al. 2003). The uninterrupted fiber surfaces after alkali treatments are shown in Fig. 21a. The fiber surfaces are stripped of the lignin and hemicellulose coatings by NaOH treatments. Furthermore, the acetylated fiber surfaces (Fig. 21b) illustrate a splitting of cellulose microfibrils and a smoother surface than the untreated fibers because of the loss of the coatings from the cellulose surface (Kabir et al. 2013). Figure 21c shows the development of coatings on fiber surfaces in silane-treated fiber. The coatings also covered the gaps between the cellulose microfibrils, making the fiber surfaces smoother (Faruk et al. 2014). The interaction with acetyl removes the wax and cuticle from the fiber surface, resulting in a finer texture. Reduced moisture retention, elimination of non-crystalline components, formation of a fine fiber surface, and improved load transfer efficiency are benefits of acetylation treatment of flax and hemp fibers (Fig. 21d–e) (Bledzki et al. 2008).

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Fig. 20 Kenaf fiber surface: a untreated and b alkali-treated (Aziz and Ansell 2004) (Reused with permission, license number: 5372920622415)

Fig. 21 SEM micrographs of a alkalized, b acetylated, and c salinized fiber surface (Kabir et al. 2013). (Reused with permission, license number: 5372920890531), and SEM micrographs of treated acetylated fiber: d flax and e hemp (Tserki et al. 2005) (Reused with permission, license number: 5372921194895)

When composites are fractured under tension, unmercerized sisal fibers are pulled out of the matrix. Interfacial failure between fibers and matrix occurs initially, accompanied by fiber breakage because the fiber-matrix interface has a lower interfacial adhesion strength. Fiber pull-out lengths in mercerized fiber composites are substantially less than those in untreated fiber composites (see Fig. 22a), even though most fibers are still attached to resin throughout the testing. Mercerization of natural fibers improves resin adhesion (Fig. 22c) (Kim and Netravali 2010).

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Fig. 22 SEM images of the fracture surface of sisal fiber composite: a unmercerized, b slackmercerized, and c mercerized (Kim and Netravali 2010). (Reused with permission, license number: 5372930126771)

8 Thermogravimetric Analysis Thermogravimetric analysis (TGA) monitors the weight change while a sample is continuously heated to assess its thermal stability and proportion of volatile components. The weight is measured as a function of rising temperature. The measurement is generally performed in air or an inert environment like helium or argon (Rajisha et al. 2011). Figure 23 shows the TGA curves of untreated and treated hemp fiber bundles. The temperature of cellulose breakdown rises slightly to 410 °C after treatment with NaOH. This is because amorphous cellulose is easily destroyed by alkali compared to crystalline cellulose. As a result, following treatment, more crystalline and heat-resistant cellulose molecules remain (Troedec et al. 2008). The addition of inorganic material impacts the stability of the cellulose/hydrous niobium oxide hybrid, increasing plasticity and reducing its mechanical characteristics. In addition, excessive moisture content causes poor resin wetting and weak interfacial bonding between the fibers and matrix (Aziz and Ansell 2004). Cotton fiber heated below 100°C loses 8% of its original weight because moisture is drawn out of the fiber. Decomposition of a-cellulose, the primary component of cotton fibers, occurs at 345 °C, resulting in significant weight loss (Shahedifar and Rezadoust 2013). Thermal depolymerization of hemicellulose and cleavage of cellulose glycosidic bonds occur between 220 and 300 °C in treated and untreated banana fibers. Lignin contributes to the broad peak from 200 to 500 °C, while cellulose breakdown occurs between 275 and 400 °C (Deepa et al. 2011). Thermal characterization of raw and soda-treated date palm fibers shows that soda-treated fibers withstand thermal deterioration better than raw fibers. Soda treatment is known to eliminate natural and synthetic contaminants, causing surface roughness and fiber fibrillation. Soda treatment also increases the thermal stability of natural fibers (Alawar et al. 2009). The incorporation of pineapple fiber improves the thermal stability of composites, and its decomposition temperatures are also higher than jute fiber (Sapuan et al. 2011; Dash et al. 2002). The TGA of various plant fibers is presented in Table 5.

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Fig. 23 Comparison of the TGA of untreated and treated hemp fiber bundles (Troedec et al. 2008) (Reused with permission, license number: 5372930543515)

9 Conclusions Natural fiber composites have been more prevalent in their applications in recent years. There are several areas where natural fiber-based composites are used in place of conventional materials, such as automotive, marine, construction, and aerospace. Light-weight products and strict environmental restrictions have prompted the development of materials based on plant fibers. However, some drawbacks of plant fibers, such as poor interfacial adhesion, excessive water absorption, and thermal instability, need to be addressed. Reactive surface treatments and chemical modifications of natural fibers and the creation of polymer interlayers on fiber surfaces can enhance

• Hemicellulose degradation at 297 °C • Cellulose degradation at 365 °C

Polyurethane (PU)

Epoxy, polylactic acid, and ABS • Thermal decomposition of the hemicellulose and pectin at 220 °C and 320 °C, respectively • Degradation of cellulose between 320 °C to 500 °C

Epoxy

Sisal fiber

Banana fiber

Jute fiber Lignin is the first component to disintegrate during heat degradation, starting between 155 and 169 °C, followed by hemicellulose, which begins to degrade between 230 and 307 °C. As a result of the cellulose breakdown, considerable weight loss occurs at the temperature range of 323 to392 °C after the hemicellulose decomposition

Thermal degradation

Matrix

Fiber

Table 5 TGA of various fibers

Jute fiber has a lower breakdown temperature than epoxy resin. The degradation temperature may be altered to a higher temperature due to the integration of this lingo-cellulose jute fiber into the polymer matrix

The addition of banana fibers in the polylactic acid matrix results in a lower degradation temperature when compared to polylactic acid, indicating banana fibers’ poor thermal stability, which may have aided the deformation of the crystalline structure of polylactic acid at elevated temperature Compared to pure resin, the inclusion of natural fiber in the thermoplastic matrix increases thermal stability

Chemical treatment enhances the thermal stability of the sisal fiber composite

Thermal properties

(continued)

Raghavendra et al. 2014; Asha, et al. 2017)

Jandas et al. 2011)

Głowi´nska et al. 2017; Chowdhury et al. 2022)

Reference

Characterization and Comparative Evaluation of Structural, Chemical, … 33

The primary lignin degradation occurs at the broad temperature range from 200 °C Lignin loses ~50% of its weight in the temperature range of 400–650 °C

Ramie fiber

Polyurethane

• Because of the moisture in the fibers, the initial transition occurs from 40 °C to 130 °C • The second transition (the decomposition temperature range of 195 to 360 °C) is related to the degradation of cellulosic substances such as hemicellulose and cellulose • The third stage of the decomposition (360–470 °C) was due to the degradation of non-cellulosic materials in the fibers

Pineapple leaf fiber (PALF) Polycarbonate (PC)

Bamboo fiber

Thermal degradation

Matrix

Epoxy, polyester, and vinyl ester • Decomposition of cellulosic components at 199–399 °C: • Decomposition of lignin at 364–499 °C

Fiber

Table 5 (continued) Thermal properties

Reference Chen et al. 2018; Asrafuzzaman„ et al. 2021)

When compared to fractionated lignin, isolated lignin has higher thermal stability. The fractionation of lignin results in the breakdown of lignin macromolecules into smaller molecules, decreasing thermal stability. Because lignin is more thermally stable than polyurethane resin, it may be employed as a synthesis agent for the resin

Handika et al. 2021)

(continued)

The onset temperature for thermal Threepopnatkul et al. 2009; Asim et al. degradation is lower in PALF 2015) composites. Because of the poor thermal stability of PALF, composites have low thermal stability compared to PC resin. The poor thermal stability of high PALF content leads to the onset temperature at a lower temperature than the other composites

Untreated individual bamboo fibers begin to decompose at a lower temperature than treated bamboo fibers with low NaOH concentrations. This is due to the presence of hemicellulose in untreated fibers since the primary component xylose in hemicellulose decomposes at roughly 180–280 °C

34 S. H. Khan et al.

Flax fibers

Abaca fibers

Matrix

PLA

Fiber

Table 5 (continued) Thermal degradation

Abaca fibers remain intact from 100 °C to roughly 250 °C. The thermal breakdown of hemicellulose and the glycoside link of the cellulose molecule occurs after the first weight loss stage. The most considerable mass loss occurred at a temperature of 367 °C, which corresponds to the thermal breakdown of cellulose

Flax fibers degrade in three phases, with the first stage occurring up to 150 °C and releasing absorbed moisture, followed by cellulosic and hemicellulosic components degrading at a faster rate over 250 °C. The third and final stage begins around 375 °C, which is the onset temperature for non-cellulosic materials, with the material being completely converted to ashes at roughly 600 °C, with just a few residues

Thermal properties

The thermal stability of abaca fibers is up to 250 °C, which allows them to use in various thermoplastic polymers with lower processing temperatures

Flax fibers increase the thermal durability of PLA by lowering the temperature at which thermal breakdown occurs. The inclusion of flax fibers may also affect the glass transition temperature, raising it to 30 wt.% fiber content, bringing the allowable range for application to over 100 °C

Reference

Souza and d’Almeida 2014)

Santulli 2022; Fong, et al. 2015)

Characterization and Comparative Evaluation of Structural, Chemical, … 35

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natural fibers’ mechanical, thermal, physical, and interfacial properties. Their characterizations using an array of analyses, such as XRD, FTIR, SEM, and TGA, can also give insight into the natural fibers’ crystallinity, chemical compositions, morphologies, and thermal stability.

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Chemical Characterization of Natural Species and Study of Their Application for Papermaking Sara Saad, Manel Elakremi, Faten Mannai, Ramzi Khiari, Anis Tlili, and Younes Moussaoui

Abstract The fast growth of pulp and paper companies around the world causes a huge demand for lignocellulosic raw materials. During the last three decades, the pulp and paper manufacturer, especially in countries with poor forest resources, has been spurt to use non-wood fiber as a raw material for cellulose extraction. Nonwood sources including annual and perennial plants have been promising renewable sources to manufacture pulp and paper with acceptable behaviors. The chapter paper intends to give a general background to the employment of attractive lignocellulosic biomass for the manufacture of high-quality paper and pulp. A broad description of the chemical composition of mentioned lignocellulosic biomass is made with conventional wood and other non-wood fiber sources. To assess the suitability of the above plant as a feedstock for paper making, pulp processing, and delignification have been evaluated and presented. Annual and perennial pulps have high-quality deliberated fibers. Handmade papers’ physical and mechanical properties have been studied, discussed, and compared with papers from wood and other non-wood resources. The annual and perennial plants could have fiber sources available to papermakers in arid S. Saad · F. Mannai Laboratory for the Application of Materials to the Environment, Water and Energy (LR21ES15), Faculty of Sciences of Gafsa, University of Gafsa, Gafs, Tunisia S. Saad · M. Elakremi · F. Mannai · Y. Moussaoui (B) Faculty of Sciences of Gafsa, University of Gafsa, Gafs, Tunisia e-mail: [email protected] M. Elakremi · Y. Moussaoui Organic Chemistry Laboratory (LR17ES08), Faculty of Sciences of Sfax, University of Sfax, Sfax, Tunisia R. Khiari Higher Institute of Technological Studies (ISET) of Ksar-Hellal, University of Monastir, Monastir, Ksar Hellal, Tunisia e-mail: [email protected] University of Grenoble Alpes, CNRS, Grenoble INP, LGP238000 Grenoble, France A. Tlili Univ Lyon, Université Lyon 1, CNRS, CPE-Lyon, INSA, 43 Bd du 11 Novembre 1918, 69622 Villeurbanne, France © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 R. Khiari et al. (eds.), Annual Plant: Sources of Fibres, Nanocellulose and Cellulosic Derivatives, Composites Science and Technology, https://doi.org/10.1007/978-981-99-2473-8_2

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regions in the future. At the end of their life cycle, these products are recycled either for reutilization or as a resource for energy.

1 Introduction Natural cellulosic fiber resources are pertinent materials to satisfy the demands of human beings; their utilization was related to the evolution of new technologies. Formerly, forest resources were severely exploited, especially to cover the demand for firewood, construction, and furniture, has contributed to the depletion of this natural material. Tracking policy advice on sustainable forest management, the world’s pulp and papermaking sector have tried to make supplies of wood sustainable. Since the 1960’s, alternative solution to this issue has been used. These goods are recycled as a raw material for recovered papers and boards at the end of their lifecycle, either for resale or as an energy solution. Indeed, the recovery of an alternative resource of cellulosic fibers derived from renewable biomass offers a practical and effective way to address the challenge, particularly for countries with limited forest supplies. Furthermore, paper requirement is directly linked to the type of paper and the region in which it is produced. The paper industry is attempting to use approximately 60% of fiber from wood as a substitute for non-wood ones, agricultural residues and annual plants (Abd El-Sayed et al. 2020) such as sugarcane biogas, kenaf, rice, papyrus, sorghum, bamboo, wheat, reeds, jute, alfa … (Belgacem and Gandini 2005). Many studies were conducted over a long period of time. They were devoted to researching the exploitation of alternative sources of fiber from agricultural waste and/or annual plants. In particular, these are annual plants such as: the roots of Astragalus armatus (Moussaoui et al. 2011), Sorghum stalks (Jiménez et al. 1993), Cynara cardunculus L. (Antunes et al. 2000; Gominho et al. 2001), as well as agricultural residues including rapeseed straw (Mousavi et al. 2013), vine stems (Mansouri et al. 2012), banana tree residues (Rosal et al. 2012), Sugarcane bagasse (Agnihotri et al. 2010), date palm rachis (Khiari et al. 2010) and olive tree residues (Díaz et al. 2005), …. The aim of this chapter is to supply a general background on the use of attractive natural species biomass for the production of high-quality pulp and lightweight paper.

2 Chemical Composition of Some Lignocellulosic Raw Materials The chemical composition of lignocellulosic feedstock for papermaking must be determined, indeed, the chemical composition of the raw material affects its papermaking suitability and needs to be determined before pulping, etc.… (Gominho et al. 2016; Sharma et al. 2020). Bundles of cellulosic fibers within the cell walls of the feedstock are the main components in making pulp and paper (Cassab 1998). Species

Chemical Characterization of Natural Species and Study of Their … Ash

49

0.5-10%

Hemicellulose 10-35%

Cellulose 30-55%

Lignin 15-30%

Extractives 5-20%

Fig. 1 Chemical composition of biomass

differ in cell wall amount and composition. The characteristics of the resulting pulp are determined by the type of the pulp raw material and manufacturing process used. Although the chemical composition of lignocellulosic feedstocks, such as wood or non-woody biomass, differs from species to species, a fibrous feedstock contains 38– 50% cellulose, 15–25% lignin, 5–10% extractives, and 5–10% inorganics, with the rest consisting of a mixture of non-glucose polymeric carbohydrates (hemicellulose) (Bajpai 2016; Tarasov et al. 2018; Chen et al. 2020) (Fig. 1).

2.1 Cellulose The most important constituent of fibrous raw materials is cellulose (C6 H10 O5 )n . The purity of cellulose determines the strength and longevity of the paper. The degree of polymerization of cellulose units is represented by n. The amount of cellulose unit’s repetition in pulp samples varies according to the chemical and mechanical treatment applied during the pulping and bleaching process. The –OH group in the cellulose molecule exhibits a variety of different behaviors. The cellulose chain’s – OH groups participate in hydrogen bonding. The ultra-structure of cellulose is formed as a result of these hydrogen bondings. The –OH groups of cellulose can hydrogen bond with the –OH groups of water, making cellulose hydroscopic in nature. These water molecules can adhere to the cellulose surface (Heinze 2015). The purity of the cellulose determines the strength and longevity of the fiber and paper. Because of the presence of high alpha cellulose, paper made from pure cellulose is more durable. Cellulose is divided into β-, γ- or α-cellulose based on its degree of polymerization

50

S. Saad et al.

(DP) and purity. The cellulose content of softwoods and hardwoods ranges from 65 to 75%, while the α-cellulose content varies from 45 to 55%.

2.2 Hemicellulose Hemicelluloses are heteropolysaccharides that are not fibrous. In dilute sodium hydroxide solution, hemicelluloses having a degree of polymerization about 15 ± 03 are soluble. They hydrolyze to produce arabinose, galactose, glucose and mannose. Depending on the species, the amount of hemicellulose in the raw material ranges from 10 to 25%. Hemicellulose content in agro residues is higher than in soft and hardwoods. It is preferable in order to maintain the hemicelluloses during the pulping process to increase pulp yield and strengthen fiber bonds. Hemicelluloses, on the other hand, are undesirable in dissolving grade pulps. Before pulping, acid-hydrolysis is used to reduce the amount of hemicelluloses (Gao et al. 2014). Hemicellulose has a much lower molecular weight than cellulose, which is a long-chainpolymer. It has low crystallinity and largely dissolves in the chemical pulp. Also, the presence of hemicelluloses causes the swelling of fiber (Hosseinaei et al. 2012).

2.3 Lignin Lignin is a polyphenolic substance. It is eliminated selectively during cooking, liberating the cellulose fibers. The pulping and bleaching techniques used in paper production are primarily designed to remove lignin (Fernández-rodríguez et al. 2017). The amount of lignin in pulp and paper determines its brightness (Baucher et al. 2003). The remaining lignin in the pulp has an impact on the paper’s characteristics and contributes to its rigidity (Esteves et al. 2020). The chemical dose, time, and temperature during delignification are all determined by the lignin content. As lignin content increases in a feedstock, the demand for chemical loading increases, and the cooking time for the pulping process increases. For non-woody plants, due to the lower lignin content, there is less chemical demand for pulping and bleaching (Matsakas et al. 2019).

2.4 Extractives In quantitative terms, the extractives represent fats, waxes, gums, resins, and phytosterols in biomass. The nature of extractives differs depending on the raw material. Woody raw materials contain resin acids, essential oils and sterols, whereas nonwoody ones involve coloring matter and tannins. The solubility in alcoholbenzene stipulate extractives existence, which, affects pulp properties and causes

Chemical Characterization of Natural Species and Study of Their …

51

defects in paper manufacturing processes related to pitch issues (Anupam et al. 2016).

2.5 Ash Ash reflects the existence of mineral compounds. The amount of ash content is depended on the nature of the feedstock. The main cause of high inorganic content comes down to silica found in the lumen and leaves of the plant (Mukome al. 2013).

2.6 Alkali, Cold and Hot Water Solubility The solubility in water is a method used to evaluate sugars, tannins, gums, coloring agents, and starches contents in wood and pulp. It influences the yield of the pulp. For this reason, it is recommended that the water-soluble content in raw material and pulp should be low as possible. While the increased solubility of hot water suggests that the chemicals used in pulp making will penetrate more easily. A higher solubility in 1% alkali causes a lower pulp yield from the kraft and soda pulping and suggests that the pulp is more subject to deterioration during storage (Gulsoy and Tufek 2013).

2.7 Chemical Composition of Some Lignocellulosic Natural Species Extractives were derived through the use of different liquids according to common standards. Both hot and cold water, 1% NaOH and ethanol-toluene solubility were monitored by TAPPI standard methods: T207 cm-08, T212 om-07and T204 cm-07. The ash content was determined according to the standard method T211 om-07. The Klason lignin, holocellulose and a-cellulose, were respectively quantified using T222 om-06, Wise et al. process (1946) and T203 cm-99. Table 1 summarizes previously published data on cellulosic materials like wood, non-wood, and annual plants. These species are known for their important levels of extractives, lignin, and cellulose. It could be seen as a viable supply of cellulose for the synthesis of lignocellulosic fibers for papermaking applications, due to the acceptable amount of holocellulose. The extractives in hot and cold water is very high in non-wood such as Opuntia ficus-indica (24–36.3%), Pituranthos chloranthus (25–26.7%), Retama raetam (32– 31.5%) Saccharum spontaneum (17.8–18.4%) and annual plants like Astragalus armatus (26.2–33%) and Amaranth (23.5–28%) compared with that found in hardwood such as in Vine stems (8.2–13.9%), Brutia pine (2.2–2.8%) and Trema orientalis (2.4–4.9%). The solubility in 1% NaOH was very high in some sources of annual

52

S. Saad et al.

Table 1 Chemical composition of some lignocellulosic biomass CW

HW

1% AB NaOH

Ash

lign

Hol

Cell

Vine stems (Mansouri et al. 2012)

8.2

13.9

37.8

11.3

3.9

28.1

64.4

35

Retama monosperma (Moreno-Jiménez et al. 2008)

–

3.8

16.9

5.03

–

21.5

71.7

43

Wood

Brutia pine (Akgül et al. 2007)

2.2

2.8

16.1

1.94

0.4

26.1

75.5

47

Trema orientalis (Jahan et al. 2010)

2.4

4.9

21.4

0.89

1.1

24.1

–

45.0

Pine sawdust (Tarrés et al. 2017)

–

–

–

2.04

–

30.60 67.36 55.06

Sundari (Mun et al. 2011)

–

–

26.80

10.30 2.60 21.80 76.00 36.10

Bine (Mun et al. 2011)

–

–

18.80

9.40

1.20 28.30 82.90 35.80

Keora (Mun et al. 2011)

–

–

20.30

5.10

1.60 28.50 83.00 38.40

Gewa (Mun et al. 2011)

–

–

26.70

5.60

3.20 19.50 77.80 38.30

Kakra (Mun et al. 2011)

–

–

25.00

6.50

1.00 27.60 76.32 35.30

Pashur (Mun et al. 2011)

–

–

45.70

13.90 1.60 17.20 81.00 30.10

Opuntia ficus-indica (Mannai et al. 2016)

24.0

36.3

29.6

9.8

5.5

4.8

64.5

53.6

Date palm rachis (Khiari et al. 2010)

5

8.1

20.8

6.3

5

27.2

74.8

45

Pituranthos chloranthus (Ferhi et al. 2014a)

25

26.7

49

9.5

5

17.6

62

46.5

Stipagrostis pungens (Ferhi et al. 2014b)

19.3

20.5

42.9

4.8

4.6

12

71

44

Posidonia oceanica (Khiari et al. 2010)

7.3

12.2

16.5

10.4

12

29.8

61.8

40

Retama raetam (Ferhi et al. 2014a)

32

31.5

47

10

3.5

20.5

58.7

36

Nitraria retusa (Ferhi et al. 2014a)

23

25.5

40

3

6.2

26.3

52

41

Ailantus Altissima (Ferreira et al. 2013)

–

–

–

1.8

0.9

21.2

–

49.7

–

Non-wood

Jute Stick (Suman et al. 2022)

–

–

Saccrarum officinarum (Ferdous et al. 2020)

8.4

12.13 39.6

Saccharum spontaneum (Ferdous et al. 2020)

17.82 18.49 38.09

–

1.0

24.1

63.7

38.4

2.2

0.67 20.4

62.2

39.3

2.01

1.07 19.8

66.2

43.3

(continued)

Chemical Characterization of Natural Species and Study of Their …

53

Table 1 (continued) CW

HW

1% AB NaOH

Ash

lign

Hol

Cell

59.5

35.1

Zea Mays (Ferdous et al. 2020)

14.62 18.13 38.92

0.87

4.48 19.7

Durian Rinds (Marsol et al. 2015)

–

–

5.50 10.90 47.10 31.60

Citronella grass (Sharma et al. 2022)

15.68 21.50 28.20

6.31

8.20 25.10 63.50 38.10

13

3

–

–

Annual plants Astragalus armatus (Moussaoui 26.2 et al. 2011) Oat straw (Jiménezet al. 1990)

33

32.7

16.7

54

35

13.2

15

41.8

4.4

7

11

–

37

Amaranth (Fiserova et al. 2006) 23.5

28

46.8

2.51

12

13.2

58.4

32

Wheat straw (Tozluoglu et al. 2015)

–

12.27 43.58

4.01

6.49 17.28 76.2

39.72

Ailantus Altissima (Ferreira et al. 2013)

–

–

–

1.8

0.9

21.2

49.7

Rice straw (Mousavi et al. 2013)

–

–

–

0.56

9.2

21.90 60.70 41.20

Rapeesed straw (Mousavi et al. – 2013)

–

–

1.63

3.46 16.00 78.90 41.66

ribbon retted jute (Jahan et al. 2016)

4.70

30.60

1.70

2.00 23.90 71.90 38.50

4.30

–

CW, cold water solubility (%); HW, hot water solubility (%); AB, solubility in etnol toluene (%); 1% NaOH, 1% sodium hydroxide solubility (%); Hol, holocelllose (%); lign, Klason lignin (%); Cell, Cellulose (%).

plants such as Oat straw (41.8%), Amaranth (46.8%), ribbon retted jute(30.6%) and in non-wood plants like Retama raetam (47%), Nitraria retusa (40%) and Saccrarum officinarum (39.6%) compare to some wood such as Pinus pinaster (7.98%), Brutia pine (16.1%) and Bine (18.8%). The Higher alkali solubility indicates fungal decomposition or deterioration in raw material as a result of ambient physical or chemical conditions, indicating that the feedstock cannot be stored for an extended period of time (Sharma et al. 2022). The proportions of klason lignin were typical for annual plants, wood and non-wood which are less than 30%, whereas in comparison to wood such as Kakra (27.6%) and Pine sawdust (30.6%), non-wood such as Opuntia ficus-indica (4.8%) and Durian Rinds (10.9%) may be simply cooked with a reduced chemical charge and at a moderate temperature due to its lower lignin content (Sharma et al. 2022). For the ash content was less than 15%, in this chapter we should note that when it comes to pulp refining and chemical recovery, a higher ash concentration causes problems. It also causes interference with the use of hydrogen peroxide and oxygen throughout the bleaching process as trace elements, and it accumulates in the pulp, negatively affecting its physical strength attributes (Ferdous et al. 2020). Finally, the amount of holocellulose and cellulose were typical for wood, non-wood

54

S. Saad et al.

and annual plants which are higher than 50% for holocellulose, such as for Retama monosperma (71.7%) Date palm rachis (74.8%) and Wheat straw (76.2%); and for cellulose were higher than 35% such as for Pinus pinaster (55.9%), Pituranthos chloranthus (46.5%) and Zea mays (44.08%). These high levels of cellulose allows these plants to be valorized such a source of lignocellulosic fibers for paper and/or cellulose derivatives applications. Solubility in NaOH and water, extractives, ash and lignin have been classed as qualities with lowest values are beneficial for paper manufacture, whereas holocellulose and α-cellulose have greater values that are advantageous for papermaking (Anupam et al. 2015).

3 Pulping Processes and Fiber Properties 3.1 Pulping Processes Pulping consists in isolating cellulosic fibers from wood or other woody plants or other sources of fibers, while preserving as much as possible their mechanical, optical and morphological properties and while trying to eliminate the lignin. There are three main processes: mechanical process, semi-chemical process and chemical process.

3.1.1

Mechanical Process

The separation of cellulosic fibers by mechanical methods such as grinding and defibration with grinding wheel or disc. This treatment is very aggressive, degrades the fibers, which results in the poor quality pulp (Sayda and Taylor 2003). This degradation can be limited by hot defibration. Mechanical separation leads to have an excellent yield of about 90 to 95%. This is why mechanical pulps are also called high yield pulps. These pulps keep in their structure of almost all the original wood. They have a high lignin content, which explains their use in short-lived paper types (Bajpai 2018).

3.1.2

Semi-chemical Process

There are intermediate processes called semi-chemical (mixed) for which the delignification operation is conducted in a very gentle way and the pulping is completed by mechanical treatment (Rudi et al. 2016). These processes are rarely used nowadays, they are mainly found in fluting because the fibers are individualized and have kept a little lignin content which makes them rigid and gives them both a capacity to be thermoformed and a quality of resistance to compression. This process allows obtaining

Chemical Characterization of Natural Species and Study of Their …

55

a high yield, about 70 to 80%, and produces stiff fibers and it was used to manufacture corrugated paperboard, cardboard roll cores, and containers (Forouzanfar et al. 2016).

3.1.3

Chemical Process

The delignification of the fibers allows eliminating an important part of the lignin. It allows individualizing the wood and all kinds of fibers. There are 3 types of chemical process that can take into account the nature of the fibers and the specific chemicals (Corcelli et al. 2018; Bajpai 2018).

Soda Pulping Soda pulping process consists of two alkaline phases, a solid phase, and a liquid phase. According to historical evidence, soda pulping was the first industrially adopted pulping method. It entailed the use of sodium carbonate and sodium hydroxide in the cooking process (Marin et al. 2017). Longer cooking time, Low pulp yield, chemical charges, high temperature, and relatively lower paper strength properties are all disadvantages of the soda process (Marin et al. 2017). Because soda-based pulps provide opacity, absorbency, high bulk, and printability, they are most appropriate for paper grades where pulp strength is not an issue (Sharma et al. 2020).

Soda-Anthraquinone (AQ) Pulping The use of anthraquinone (AQ) during this pulping process solves the problem of the low yield of soda pulps. In fact, this process speeds up lignin removal and shields the aldehyde groups of carbohydrates from the alkaline peeling reaction. Superior bleach ability, higher pulp production, and sometimes better paper strength qualities have been recorded with soda-AQ pulp (Omer et al. 2019).

Kraft Pulping In German, the word "kraft" means strength. North America is the most common location for this sort of pulping (Gopal et al. 2019). Despite the low yield, kraft pulping recovery technology has been proved. Linerboard, xerographic, and food boards are just a few of the goods made from kraft pulp. Sodium hydroxide and sodium sulfide are the two chemicals used in kraft pulping (Deniz et al. 2004). This process has been around for over a century and continues to dominate the paper industry. This process is still used to produce nearly 70% of the world’s pulp (Yoon and Van Heiningen 2008; Qing et al. 2016; Álvarez-Díaz et al. 2018; Gordobil et al.

56

S. Saad et al.

2019). This process is the extremely worldwide, representing 84% of the chemical pulp production and 63% of the total pulp production (Fearon et al. 2020).

3.2 Fiber Properties The morphological parameters of the produced fibers are used to evaluate the quality of the obtained pulps (Mannai et al. 2016). Table 2 lists the characteristics of the fibers extracted from different lignocellulosic materials and different pulping process. The viscosity of the bleached pulp of date palm rachis was equal to 15.7 mPas and a degree of polymerization (DP) around 1203 (Khiari et al. 2010) and for wheat straw pulp the viscosity was 536mPas and the DP was equal to 1381 (Espinosa et al. 2016). These values of DP are ideal for papermaking application and correspond to values commonly found in fibres derived from annual plants. A decrease in the DP generates poor mechanical strength of the paper (Małachowska et al. 2021). From Table 2, it appears that the width of Vine stems was comparable to that of fibers obtained from other plants, despite their significantly shorter length about 0.6 mm. Whereas, for Opuntia ficus-indica, the length and width were 764 and 38 μm, respectively. And for Zea Mays, the length and width were 870 and 17.7 μm, respectively (Ferdous et al. 2020). These properties influence the aspect ratio, which its increase reinforces the entanglement of the fibers in the sheet of paper and consequently a pronounced resistance (Mansouri et al. 2012). Furthermore, the Pinus radiata pulp has low fine element content (7.4%), and a good drainability of pulp, which was about (13°SR), can be explained in part by the low fine element content. This value was lower than other plants like date palm rachis (Khiari et al. 2010), bamboo (Khristova et al. 2006) and Opuntia ficus-indica (Mannai et al. 2016). These properties recommending that the cited material can produce paper sheets (Mannai et al. 2016). Besides, the remarkable discrepancies in pulp yields probably related to the pulping process applied. For example, Amode and Jeetah (2021) observed that the Kraft pulping gave the lowest yield for obtaining pulp (17.80%).

4 Papermaking Process and Properties of Paper from Natural Species 4.1 Papermaking Process The papermaking process is divided into different stages starting with the stock preparation, then the creation of a wet end, the elimination of liquid water by pressing, after that the evaporation of water by evaporation to strengthen the product, finally the finish and coating of the dry product (Holik 2006). The paper was manufactured using 2 operations wet-end and dry-end. The conversion of the processed pulp into a

25.5 8.5

–

–

600

17

0.76

38

16.3

Kappa number

Viscosity (mPas)

DP

Schopper Riegler degree (zSR)

Fibre length (mm)

Fibre width (μm)

Fine elements (% in length)

Viscosity (mPas)

19.84

10.7

Kappa number

Soda–AQ 41.5

Soda–AQ

Saccrarum officinarum (Ferdous et al. 2020)

9.2

24.6

0.59

16

Pulping yield (%) 64.2

Pulping process

45

41.4

Pulping yield (%)

Juste Stick (Jahan and Akhtaruzzaman 2018)

Soda–AQ

Soda–AQ

Pulping process

800

Vine stems (Mansouri et al. 2012)

Opuntia ficus-indica (Mannai et al. 2016)

Table 2 Main properties of pulps obtained from natural species

25.39

44.47

Soda–AQ

Saccharum spontaneum (Ferdous et al. 2020)

12.2

20.9

0.77

17

1399

18.1

8.7

50

Kraft

Eucalyptus (Mishra et al. 2010)

9.95

32.82

Soda–AQ

Zea Mays (Ferdous et al. 2020)

30.8

22.3

0.89

14

1203

15.7

54

44.8

Soda–AQ

Date palm rachis (Khiari et al. 2010)

100

51.11

Chemi-mechanical

Durian Rinds (Marsol et al. 2015)

7.4

31.6

1.9

13

1423

20.5

10.6

55

Soda–AQ

Pinus radiata (Mishra et al. 2010)

536

38.6

70

(continued)

Soda–AQ

Wheat straw (Espinosa et al. 2016)

2.7

16.1

0.939

32

–

16.2

15.4

54.8

Soda–AQ

Sarkanda (Sharma et al. 2020)

Chemical Characterization of Natural Species and Study of Their … 57

1.03 19.45 30

Fibre length (mm)

Fibre width (μm)

Fine elements (% in length)

43.43

15.23

1.67

58.6

17.7

0.87 128.59

Durian Rinds (Marsol et al. 2015)

Wheat straw (Espinosa et al. 2016)

26.9

20.1

51

Zea Mays (Ferdous et al. 2020)

1381

Saccharum spontaneum (Ferdous et al. 2020)

Schopper Riegler 31 degree (zSR)

Saccrarum officinarum (Ferdous et al. 2020)

DP

Juste Stick (Jahan and Akhtaruzzaman 2018)

Table 2 (continued)

58 S. Saad et al.

Chemical Characterization of Natural Species and Study of Their …

59

paper product is carried out with a paper production machine. At the beginning, the water percentage exceeds 99%. Then, the water will be drawn by gravity into vacuum chambers and vacuum rollers. The continuous sheet was then passed through more rollers, which compress the fibers and remove any remaining water. The machines used to make paper are highly sophisticated in terms of technology. Four dryer machines are commonly used for thin sheets and double wire formers, on the other hand thick or multi-layer sheets are manufactured by cylindrical board machines. The largest size of the machines used can reach up to 10 m in width and length of 120 m (Bajpai 2016). Despite the differences in design, all paper machines are made up of the same components: dryer section, Head box, press section, reel and wire section (Fig. 2). A sheet of paper is made as follows: The fibers lay next to each other and on top of each other on the wire. Simultaneously, the water is sucked from the bottom and remains 80% water after filtering. The strip of paper passes between rollers (hard rubber, granite, or steel) using a highly absorbent felt. Then it passes through the drying section which is made up of steam-heated drying cylinders. In this section, additional equipments can be used to impart special properties to the paper. For example, a durable solution of starch or synthetic-based material is applied with a size press to the pre-dried paper web (Bajpai 2016).

Papermaking Process

Headbox

Pope Reel

Fig. 2 Papermaking process

Press Part

Wire

Post Dryers

Size Press

Pre Dryers

60

S. Saad et al.

4.2 Properties of Paper from Natural Species Table 3 listed some physic-mechanical proprieties of paper from different plants. Basis weight or weight per unit area is a fundamental property of paper. The basis weight changes with moisture content. The basis weight of Opuntia ficus-indica paper (65.2 g m2 ) was equivalent to the one of Alfa stems paper (Marrakchi et al. 2011) and Napier (Reddy et al. 2014). The calliper or thickness of papers from Alfa stems paper was higher compared to other types of paper and this is due to the fiber width. The density or bulk of paper is calculated from the basis weight and calliper. The formula for density or bulk is thickness (mm) x basis weight (g/m2 ) × 1000. The bulk (2.36 cm3 /g) obtained for Alfa stems was the highest and this is due to the reduced thickness. The strength of paper is the main interesting behaviour of paper. However, paper from Alfa and Opuntia ficus-indica (Mannai et al. 2018) stems have the lower elongation. The high tear index observed for papers obtained from Opuntia ficusindica (Mannai et al. 2018) (12 mN m2 /g) and Alfa stems (Marrakchi et al. 2011) (9.2 mN m2 /g) provide high-quality paper. In addition, the burst index obtained for paper from Opuntia ficus-indica (Mannai et al. 2018) (5.8 kPa m2/g) was slightly higher to that of other plants such as Bamboo (Marrakchi et al. 2011) (2.02 kPa m2 /g) and Eucalyptus globus (Ferreira et al. 2013) (1.71 kPa m2 /g). Obtained paper with high brust index from Opuntia ficus-indica is due to the use of pulps with long fibers (Mannai et al. 2018).

5 Conclusion Paper production is increasing globally as the world’s population grows and their consumption of goods increases. The market for some grades, such as graphic papers, is declining, but new opportunities will emerge, particularly in specialty papers and packaging. Pulp and paper manufacturing is the process of converting fibers, typically from wood but also from other plants, into pulp and then into a variety of products. Pulp and paper are made from a variety of lignocellulosic biomasses. This includes hardwood and softwood trees, agricultural wastes (wheat straw, bagasse, rice straw). The raw material used for pulp manufacturing has a considerable impact on the chemical composition, pulping, and overall papermaking process. Various raw materials and pulping technologies for pulp and paper manufacturing are discussed in this chapter. Physi-chemical components impact on pulping and papermaking was also described. In a brief, this chapter summarizes the pulp and papermaking process evaluation concepts. Non-wood plant availability is a reality in many places, but more study is needed to replace wood fibers with these source materials. The major axes to work on are pulp quality, cost pulp reduction, and process optimization. Furthermore, as the Earth warms, plant and wood species will change: introduced species will increase, and

65.2

1.02

220

2.64

65.2

135

2.07

0.55

535

1.83

5.8

Basis weight (g/m2 )

Thickness (μm)

Bulk (cm3 /g)

Elongation (%)

Specific energy (mJ/g)

Young modulus (109 × Pa)

Burst index (kPa m2 /g)

1.62

2.21

144

Astragalus armatus (Ferhi et al. 2014a)

Physical and Opuntia mechanical ficus-indica proprieties (Mannai et al. 2018)

1.44

3.86

207

0.87

1.86

126

67.4

Pituranthos chloranthus (Ferhi et al. 2014b)

–

2.51

221

1.09

2.21

141

63.9

4

1.8

–

0.51

2.36

153

65

1.86

–

–

–

–

128.59

58.87

Date palm Alfa stems Durian rachis (Marrakchi Rinds (Khiari et al. 2011) (Marsol et al. 2010) et al. 2015)

Table 3 Paper properties obtained from some sources of lignocellulosic fibers

2.02

–

–

1.5

127.8

67

4.98

–

–

2.14

138.4

64.62

2.02

–

–

–

–

–

Citronella Napier Bamboo grass (Reddy (Marrakchi (Sharma et al. 2014) et al. 2011) et al. 2022)

2.58

107

–

0.621

–

–

Tall wheatgrass (Małachowska et al. 2021)

(continued)

1.71

–

–

1.65

–

63.8

Eucalyptus globus (Ferreira et al. 2013)

Chemical Characterization of Natural Species and Study of Their … 61

3.57

417

3.37

12.0

Air 229.2 permeability (cm3 /s Pa m2 )

1.57

12.65

7.7

Tears index (mN m2 /g)

Breaking length (km)

Dry zero-span breaking length (km)

Wet zero-span breaking length (km)

7.64

10.3

Astragalus armatus (Ferhi et al. 2014a)

Physical and Opuntia mechanical ficus-indica proprieties (Mannai et al. 2018)

Table 3 (continued)

7.65

10.5

3.85

202

3.38

Pituranthos chloranthus (Ferhi et al. 2014b)

10.8

13.4

–

450

–

8.8

10.7

3.75

859

9.2

Date palm Alfa stems rachis (Marrakchi (Khiari et al. 2011) et al. 2010)

–

–

–

–

Durian Rinds (Marsol et al. 2015)

–

3.57

8.23

–

3.75

–

6.25

3.44

Citronella Napier Bamboo Tall grass (Reddy (Marrakchi wheatgrass (Sharma et al. 2014) et al. 2011) (Małachowska et al. 2022) et al. 2021)

–

5.6

Eucalyptus globus (Ferreira et al. 2013)

62 S. Saad et al.

Chemical Characterization of Natural Species and Study of Their …

63

wood quality will decline owing to insect assaults or storms. By employing the fibers derived from these resources, the pulp and paper sector may play a significant role in environmental protection. To lessen the impact of agriculture, agro-residues must also be considered. As a result, natural fibers of many kinds, not just wood fibers, are of interest. Their use will become increasingly significant, and the pulp and paper sector must prepare to deal with non-wood fibers.

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Recent Developments in Pretreatment Strategies on Annual Plant Residues for Bioethanol Production: Technological Progress and Challenges Imen Ben Atitallah and Tahar Mechichi

Abstract Plant residues stand for a potential source of low-cost and renewable cellulose fibers for bioethanol production. Due to recalcitrance nature of these materials, pretreatment corresponds to an intrinsic step for the reduction of cellulose crystallinity and lignin removal. Effective bioconversion with least inhibitory compounds production is critical for an efficient overall ethanol yield. This chapter reviews the multiple types of pretreatments applied for lignocellulosic biomass. It describes the methods of microbial cells immobilization, as well as the effects of biological detoxification on pretreated hydrolysate and subsequent fermentation process. Furthermore, the development of genetic and evolutionary approaches for the enhancement of key microbial traits, including robustness against various inhibitors compounds and stressful environmental conditions, are discussed, highlighting the increasing trend towards consolidated bioprocessing technology. Keywords Plant residues · Pretreatment · Bioethanol · Cell immobilization · Consolidated bioprocessing · Genetic engineering

1 Introduction The depletion of fossil fuel resources along with growing environmental pollution have prompted the hunt for alternate energy sources. During the past few decades, bioethanol has been considered as the most frequently used alternative fuel (Ben Atitallah et al. 2020). It can be invested in terms of gasoline additive or a standalone fuel for internal combustion engines (Ntaikou et al. 2018) The use of renewable materials is expected to act as a powerful agent in reducing CO2 emissions and I. Ben Atitallah (B) · T. Mechichi Laboratory of Biochemistry and Enzyme Engineering of Lipases, National School of Engineers of Sfax, University of Sfax, BP 1173, 3038 Sfax, Tunisia e-mail: [email protected] I. Ben Atitallah Higher Institute of Biotechnology of Beja University of Jendouba, Beja, Tunisia © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 R. Khiari et al. (eds.), Annual Plant: Sources of Fibres, Nanocellulose and Cellulosic Derivatives, Composites Science and Technology, https://doi.org/10.1007/978-981-99-2473-8_3

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solving the global warming problem (Ben Atitallah et al. 2019). Recently, more investigation on the valorization of plant residues into bioethanol has whetted worldwide interest, particularly owing to the positive effects from both environmental and economic perspectives (Devi et al. 2022). Lignocellulosic biomass refers to an abundantly renewable resource on the globe, mainly consisted of cellulose, hemicellulose and lignin, determining the high resistance of plant material (Arora et al. 2019; Liu et al. 2019). It has been estimated that about 5300 million tons of dry biomass per year of lignocellulosic biomass is generated annually worldwide (Jiang et al. 2017). Indeed, its valorization into bioethanol production can possibly ease the stresses to the natural resources as well as shift reliance on fossil fuels to sustainable energy sources (Sarsaiya et al. 2019). The utilization of lignocellulosic materials requires pretreatment for disintegrating the close inter-component connection between major constituents of the plant cell wall (Soltanian et al. 2020). The aims of pretreatment are to fractionate the main components of the feedstock and to reveal the cellulose for enzymatic saccharification (Bhatia et al. 2020). Therefore, various physical, chemical, physicochemical and biological pretreatments are performed to ensure the fractionation of the complex structure of lignocellulosic biomass (Antonopoulou et al. 2015). Nevertheless, most of pretreatments often display side reactions entailing toxic by-products including (i) furanic aldehydes (i.e. furfural, 5-hydroxy methyl furfural (5-HMF)), (ii) phenolic derivatives, and (iii) weak acids (i.e., acetic acid, formic acid, levulinic acid), that are inhibitory to microorganisms and enzymes (Jönsson and Martín 2016). Indeed, the pretreatment step choice is of paramount importance as it affects the performance of the entire conversion of lignocellulosic materials into biofuels (Kumar et al. 2020). To shun negative effects of various inhibitors during the fermentation process, biomass hydrolysate requires detoxification step (Moreno et al. 2015). Fermentation processing and simultaneous detoxification proves to be a more practical and cost-effective strategy for converting lignocellulosic biomass. Researchers are also focusing on other strategies, including cells immobilization, evolutionary and genetic engineering of microbes to increase their resistance against various inhibitory compounds (Karagoz et al. 2019; Bhatia et al. 2020; Adegboye et al. 2021). This chapter exhibits a critical review on the recent updates progress in lignocellulosic biomass pretreatment technologies, cells immobilization, biological detoxification as well as strategies for increase both tolerance to inhibitors and ethanol yields.

2 Lignocellulosic Biomass Structure and Composition Lignocellulosic biomass, called also cellulosic biomass, represents the most sustainable feedstock for biorefineries to respond to the ever-growing energy demand. Lignocellulosic materials involve numerous different types of residues of plant origin, such as wheat bran, barely straw, date palm fibers, cereal stalks, sugarcane bagasse, softwood which involve cellulose, lignin and hemicellulose in different ratios (Ben

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Fig. 1 Different components of lignocellulosic biomass and generated inhibitory by-products

Atitallah et al. 2022a, b; Liu et al. 2019). Nevertheless, its composition ranges from one plant species to another and in response to cultivation conditions (Bhatia et al. 2020). The consumption of lignocellulosic biomass at large scale does not compete with food and feed supply making it a sustainable feedstock for second generation bioethanol supply (Zabed et al. 2016; Soltanian et al. 2020). Lignocellulose stands for an inevitable part of the plant cell wall mainly consisting of three major interwoven polymeric components: cellulose (40–50%), Hemicellulose (25–35%), lignin (15– 20%) (Fig. 1) (Putro et al. 2016; Sharma et al. 2019). Cellulose and hemicellulose, as two main polysaccharides in lignocellulosic biomass, are powerfully entangled with lignin to form a complex lignocellulosic matrix. Lignocellulosic biomass also contains trace amounts of extractive fractions such as proteins, tannins and pectin (Baruah et al. 2018; Arora et al. 2019). Cellulose is a linear homo-polymer consisting of repeated units of D-glucopyranose related by β → 1, 4-glycosidic bonds (Saini et al. 2015; Shabih et al. 2018). The cellulose molecule represents the structural basis of the plant cells and the most abundant natural biopolymers found on the planet (Sharma et al. 2019). In woody biomass, the degree of polymerization is around 4000–6000 Dglucopyranose. The long linear chain of cellulose polymers is packed and arranged in parallel to form cellulose microfibrils. Cellulose possesses a high structural stability and is insoluble in the most of standard solvents. It is highly resistant to enzymatic attack due to the strong hydrogen bonding among the hydroxyl groups which produces a high crystalline structure (Li et al. 2018; Abraham et al. 2020). Previous research works revealed that the crystalline structure of cellulose displays two forms called Iα and Iβ (O’Sullivan 1997). The triclinic unit of cellulose Iα has just one chain, whereas the monoclinic unit of cellulose I β has two chains for generating more intramolecular hydrogen bonds, making it more stable and resistant to degradation. Cellulases are capable of hydrolyzing the more accessible amorphous cellulose, but not the less accessible crystalline portion (Bhatia et al. 2020). Hemicellulose is composed of highly branched heteropolymer of various monomers involving hexoses (β-D-glucose, α-D-galactose, β-D-mannose) and

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pentoses (mainly β-D-xylose and α-L-arabinose) (Shabih et al. 2018). These monomers are attached together by β-1,4-glycosidic and β-1,3-glycosidic bonds. The hemicellulose chains may also contain other compounds namely α-D-galacturonic, α-D-glucuronic acid and α-D-4-O-methylgalacturonic acid. Contrarily to cellulose, hemicellulose is largely hydrophilic and amenable to hydrolysis owing to its amorphous nature (Abraham et al. 2020). Its composition ranges in softwood which includes Galactoglucomannan and arabinoglucuronoxylan as well as hardwood involving glucuronoxylan (Saha 2003). Hemicellulose plays the role of a physical barrier, preventing cellulases from accessing cellulose. The addition of enzymes (hemicellulose) and the elimination of hemicellulose with pretreatment procedures accelerates cellulose hydrolysis. Lignin corresponds to a highly complex aromatic polymer including multiple hydroxycinnamyl alcohol monomers, called lignin precursors or monolignols (Liu et al. 2019). There are three types of monolignols, namely p-coumaryl, sinapyl, and conifineryl alcohols that are composed of the precursor p-hydrophenyl (H), syringyl (S) and guaiacyl (G), respectively. The monolignols are linked together through various ether (α-O-4, β-O-4, and 4-O-5) and carbon–carbon (C–C) bonds (β-β, β-5, and 5–5) (Zabed et al. 2019). This phenylpropanoid unit of lignin refers to the major hindrance of lignocellulosic biomass breakdown since it supplies structural support, impermeability, and protection against enzymatic activity (De Bhowmick et al. 2018). Lignin degradation mainly results in the production of phenolic inhibitors, which largely impacts the techno-economic operation of bioethanol supply from lignocellulosic biomass (Nguyen et al. 2017; Jönsson and Martín 2016). To take advantage from lignocellulosic feedstocks to the highest possible extent, effective fractionation is required before any kind of bioconversion. Depolymerization of cellulose and hemicellulose, as well as the breaking of the lignin seal, can be noticed when an appropriate pretreatment procedure is used. This facilitates the next liberation and consumption of fermentable sugars (hexoses and pentoses) that can be converted from microbes to biofuels, resulting in increased yields and productivities (Antonopoulou et al. 2015). Therefore, pretreatment process is of crucial importance in terms of the biomass fractionation and the enzymatic hydrolysis step. Nevertheless, most of pretreatment methods produce a big variety of inhibitory compounds, i.e. furanic aldehydes (furfural, 5-hydroxy methyl furfural (5-HMF)), weak acids and phenolic compounds.

3 Pretreatment of Lignocellulose Biomass Pretreatment represents an intrinsic step in the biochemical route of biomass conversion in view of the complex structure and persistent nature of the cell wall components in the biomass (Mussatto and Dragone 2016; Kumar and Sharma 2017). Thus, pretreatment is extremely helpful in surmounting the natural recalcitrance of lignocellulosic biomass by the disarray of lignin and breaking down of lignocellulosic biomass into its components (Mussatto and Dragone 2016; Kumar and Sharma 2017).

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Fig. 2 Pretreatment methods for biomass hydrolysis

Main objectives of a feasible and efficient pretreatment process are: (a) reducing the cellulose crystallinity, (b) expanding the surface area of biomass, (c) removing the lignin seal of holocellulose, (d) limiting the generation of inhibitory products, as well as (e) improving the formation of fermentable sugars during the enzymatic hydrolysis step (Hassan et al. 2018; Kumari and Singh 2018). Moreover, the feasibility of pretreatment is a crucial issue to achieve a higher yield of fuel. The choice of pretreatment can affect to a great extent the overall cost and configuration of the lignocellulosic feedstock based biorefinery. To date, several pretreatment methods (i.e. chemical, physical, biological and combined) have been elaborated and evaluated to disassociate their structure and remove lignin (Fig. 2) (Carrere et al. 2016). Table 1 depicts recent bioethanol production studies from lignocellulosic substrates after various pretreatment methods.

3.1 Physical Pretreatments Primary treatment methods that are physically based include mechanical grinding, irradiation (X-ray, electron-beam, gamma-ray), and thermolysis (J˛edrzejczyk et al. 2019). Basically, this type of pretreatment affects biomass by rising surface area and pore volume, reducing cellulose polymerization and crystallinity, hydrolysis of hemicelluloses, and partial depolymerization of lignin (Rajendran et al. 2017). Physical pretreatments are inefficient, harmful to the environment, and unsuitable for commercial use (Seidl and Goulart 2016).

H2 SO4 (2%) H2 SO4 Na2 CO3 (11%) NaOH (15%) NaOH (0.5%)

NaOH/H2 O2 NaOH (0.5%)/H2 O2 (0.5%)

Wheat straw Rice straw

Wheat straw Sugarcane bagasse Sugarcane bagasse

Wheat straw Date palm fibers

Sorghum bagasse Wheat straw Sweet sorghum stalks Rice straw

Rice straw Triticale straw Wheat straw

Acid

Alkaline

Alkaline-peroxide

Organosolv

Ionic liquid 1-H-3-Methylmorpholinium chloride ([HMMorph][Cl]) 1-Ethyl-3-methylimidazolium acetate Triethyl ammonium hydrogen sulfate [TEA][HSO4 ]

Butanol (25%) Ethanol (60%) Ethanol (50%) Choline chloride-based solvent

Chemical agent

Feedstock

Type of pretreatment

120 °C; 5 h 120 °C; 2 h 130 °C; 3 h

200 C°,60 min 190 °C; 60 min 100 °C; 30 min 60–121 °C 30 min-12 h

50 °C; 3–15 h 80 °C; 24 h then 80 °C; 24 h

75 °C; 10–85 min 175 °C; 1.5 h 60 °C; 20 min

180 °C; 10 min 100 °C; 2 h

Pretreatment condition

30 °C; 24 h 37 °C; 96 h 30 °C;48 h

35 °C; 72 h 50 °C, 72 h – 37 °C; 72 h

37 °C; 96 h 30 °C; 48 h

30 °C; 96 h 37 °C; 10 h 30 °C; 96 h

30 °C; 72 h 30 °C; 72 h

Fermentation condition

Table 1 Bioethanol production from lignocellulosic biomasses pretreated with different of pretreatment methods

64% 10.64 g/dm3 43.1 g/L

61.9 g/l – 65.7% 36.7 g/l

31.1 g/l 6.04 g/l

65 g/l 8.8 g/l 17.26 g/l

0.44 g/l 40.6 g/l

Bioethanol production

Mohammadi et al. (2019) Smuga-Kogut et al. (2019) Ziaei-Rad et al. (2021)

Teramura et al. (2018) Smit and Huijgen (2017) Ostovareh et al. (2015) Kumar et al. (2016a)

Yuan et al. (2018b) Ben Atitallah et al. (2022b)

Yuan et al. (2018a) Carvalho et al. (2016) Hilares et al. (2017)

Nosratpour et al. (2018) Zhu et al. (2015)

References

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3.2 Chemical Pretreatments Chemical pretreatment of lignocellulosic materials can be carried out using alkali and acid. Relying upon the type of chemical used, the pretreatment can display various effects on the structural components of the lignocellulosic material. Acid can be used either at the dilute or concentrate stages, providing certain advantages and drawbacks over each other. Dilute acid is favored over concentrated for industrial applications since it is inexpensive, hydrolyzes effectively hemicelluloses into monomeric sugars, resulting in increased enzyme accessibility and improved cellulose conversion (Sievers et al. 2017; Tizazu and Moholkar 2018). This pretreatment has been commonly applied, as. So far, a variety of organic and inorganic acids have been utilized, under multiple temperatures, reaction times, concentrations and pressures. The use of dilute sulfuric acid (H2 SO4 ) to convert hemicellulose to simple sugars with high yields has proven successful (Kuglarz et al. 2018; Yu et al. 2019). Uses of other inorganic acids like hydrochloric (HCl), phosphoric (H3 PO4 ), nitric (HNO3) acid have also been employed but their use is limited due to less effective lignin removal (Kumar et al. 2020). Furthermore, some organic acids, including fumaric and maleic acids are invested for biomass pretreatment (Zabed et al. 2017). Notably, acid pretreatment can be performed via short-term pretreatment at high temperature (160–200 °C), or long-term pretreatment requiring low temperature (60–120 °C) (Esteghlalian et al. 1997). A neutralization of the pH after the pretreatment is necessary for the fermentation process. The basic drawback of acid pretreatment is the generation of 5-hydroxymethylfurfural (5-HMF) from glucose degradation, as well as furfural, acetic acid and formic acid from xylose degradation. These compounds may be further converted to levulinic acid and entail the repolymerization of furans and pseudolignins from phenolic derivatives arising from degradation of lignin (Jönsson et al. 2013; Monlau et al. 2014). The presence and amount of these compounds rely on the raw material, as well as on the severity of the used pretreatment (Antonopoulou et al. 2015). Thus, to determine the optimal pretreatment scheme for lignocellulosic biomass, a wide range of conditions selection needs to be investigated, taking into account the fractionation of the lignocellulosic content as well as the potential of generated inhibitors. Alkaline pretreatment, through the use of various alkalis including NaOH, Ca(OH)2 , and NH3 proved to be efficient for partial lignin removal (Carrere et al. 2016). Basically, during alkaline pretreatment, apart from lignin breakdown, an improved of the solubilization of hemicellulose to its oligomers is equally conducted, while cellulose is affected to a smaller degree. In comparison with dilute acid pretreatment, alkaline pretreatment is carried out at lower pressure and temperature to avoid releasing furanic compounds into the pretreatment slurry (Liu et al. 2019). Nevertheless, phenolics may be formed referring to lignin decomposition at high pH levels. Therefore, the reliability and effectiveness of alkaline pretreatment refers to the characteristics of the lignocellulosic biomass, the alkali utilized and the reaction conditions implemented to the process. Owing to its strong reactivity to lignin, NaOH stands for one of the powerful bases that has been extensively examined for the

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pretreatment of lignocellulosic biomass. Its importance with respect to various feedstocks such as corn stover, rice straw, sugarcane bagasse, and cotton stalk is obvious (Wang et al. 2016; Zhao et al. 2018). In contrast, the use of H2 O2 as an oxidative agent, which produces free radicals (HO and HOO) and molecular oxygen upon decomposition, is typically carried out under mild operating conditions, resulting in high cellulose accessibility and enzymatic hydrolysis efficiency, entailing in turn to increased glucose yields (Ben Atitallah et al. 2022a, b). Recently, research has demonstrated that alkaline hydrogen peroxide treatment (AHP) can be quite effective in terms of big variety of lignocellulosic biomass types (i.e. corn stover, rapeseed straw, date palm fibers, willow sawdust and rice hulls), greatly improving delignification and enhancing enzymatic hydrolysis of hollocellulose (Ben Atitallah et al. 2022a, b). Moreover, compared to previous chemical pretreatment procedures, this kind of pretreatment can be effective at very moderate temperatures, necessitating only little energy consumption, while the generation of inhibitory compounds is significantly reduced. Notably, the lignin removal efficiency accomplished through combining sodium hydroxide (NaOH) and hydrogen peroxide (H2 O2 ) is powerfully influenced by the pH of the treatment, which fosters reactive oxygen radical formation, disrupting the complex matrix of lignocellulose and exposing cellulose, thereby enhancing its dissolution capability. Alkaline hydrogen peroxide treatment (AHP) is performed under mild conditions such as atmospheric pressure and low temperature (below 50 °C); which results in the formation of a relatively low concentration of inhibitors (Dutra et al. 2018; Ho et al. 2019).

3.3 Physico-chemical Pretreatments Physico-chemical pretreatment takes use of its activity by combining chemical and physical factors. Pretreatment with liquid hot water (LHW), steam explosion (SE) (Rodríguez et al. 2017), ammonia fiber explosion (AFEX) (Bonner et al. 2016), CO2 explosion, soaking aqueous ammonia (SAA), ammonia recycling percolation (ARP), wet oxidation, and other technologies have been introduced to date under this type of treatments. Physicochemical methods work on biomass through rising the accessible surface area, decreasing cellulose crystallinity, and removing hemicelluloses and lignin from lignocellulose. Steam explosion (SE) technology has been the subject of several studies often related to bioethanol production with a big variety of feedstocks, namely olive tree pruning (Cara et al. 2008), wheat straw (Ballesteros et al. 2006), and poplar (Negro et al. 2003). During physicochemical pretreatment, the use of chemicals or extreme culture conditions are commonly required. Relying on the applied conditions, inhibitors may be generated in different amounts. In this regard, biological pretreatment proves to be a more environmentally friendly alternative. It rests on the special abilities of a small group of filamentous fungi, known as white-rot fungi (WRF), which are able to degrade lignin, through utilizing a set of extracellular lignolytic enzymes, without generating inhibitors.

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3.4 Biological Pretreatments Notably, biological pretreatment methods display several merits over physicochemical ones. This refers basically to the fact that they are performed under mild conditions, with low energy demands and without the release of toxic compounds. The synthesis of lignolytic enzymes like laccase (Lac), manganese peroxidase (MnP), and lignin peroxidase (LiP) has a major impact on the efficiency of delignification. Phanerochaete chrysosporium, Trametes versicolor, Trichoderma reesei, Trichoderma viride, and Aspergillus niger stands for the selective fungi, which have been invested to mitigate the recalcitrance of lignocellulosic biomass (Sharma et al. 2019). Among them, the fungi T. versicolor and T. reesei are regarded as the most outstanding for cellulase production and pretreatment of lignocellulosic biomass (Bischof et al. 2016; Shirkavand et al. 2017). Numerous bacteria have been invested in the biological pretreatment process including Bacillus sp., Clostridium sp., Streptomyces sp., Thermomonospora sp., and Cellulomonas sp. (Sharma et al. 2019). The major disadvantage of biological pretreatment is the low hydrolysis rate of most lignocellulosic materials in comparison with that obtained by other technologies. A duration of two to five weeks may be needed for sufficient delignification (Zabed et al. 2019).

3.5 Combined Pretreatments The combination of various pretreatment methods corresponds to a promising alternative pretreatment strategy, with pertinent results in lignocellulosic fractionation (Sindhu et al. 2016). Indeed, the combination of fungal pretreatment with liquid hot water (Wang et al. 2012), acid pretreatment (Ma et al. 2010), or steam explosion (Sawada et al. 1995) proved to yield enhanced enzymatic saccharification of lignocellulosic biomass.

4 Biological Detoxification of Lignocellulosic Hydrolysate Biological detoxification included the use of microorganisms and/or their enzymes to detoxify and remove the inhibitory compounds (Bhatia et al. 2020). RomeroGarcía et al. (2022) showed that biological treatment by Saccharomyces cerevisiae was the best detoxification method for olive stone liquors. Additionally, detoxified hydrolysate inoculated with Candida boidinii for xylitol generation (38 g/l xylitol, equivalent to 63% yield). Singh et al. (2019) engineered strains of Acinetobacter baylyi ADP1 to produce a consortium capable to simultaneously metabolize two inhibitors (4-hydroxybenzoate and benzoate). After the detoxification step using ADP1-derived mutants, Kluyveromyces marxianus converted the glucose remaining after detoxification and generated 36.6 g/L ethanol. Panda and Maiti (2019) studied

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the effect of feeding strategy on simultaneous inhibitors detoxification and generation of cellulase (CMCase and FPase) enzyme by Trichoderma reesei using acid pretreated solid rice straw. Results showed that CMCase and PFase activity production increased 10-and 14-folds, respectively along with xylose assimilation from the hydrolysate and simultaneous detoxification (Panda and Maiti 2019). The newly isolated bacterium Bordetella sp. BTIITR allowed the degradation of 100%, 94% and 82% of furfural, 5-hydroxymethylfurfural (5-HMF) and acetic acid, respectively, present in the lignocellulosic liquor in 16 h of incubation time (Singh et al. 2017). He et al. (2016) demonstrated that the conversion rate of furfuraldehydes, acetic acid, and phenolic compounds were considerably improved using the fungus Amorphotheca resinae ZN1, which yielded the decrease in terms of biodetoxification time (from 96 to 36 h). Biodetoxification by A. resinae ZN1 seems a promising approach for inhibitory compounds removal from pretreated lignocellulose feedstock invested for ethanol production (He et al. 2016). Zhang et al. (2017) asserted that the stability as well as the activity of cellulolytic enzymes increased significantly in Escherichia sp. HHQ-1-Trichoderma reesei consortia degrading system using in-situ detoxification with regard to a single strain system. The use of ferulic acid degrading E. coli HHQ-1 with T. reesei enhanced the generation of reducing sugars and ethanol effectively; proving that in-situ detoxification using microbial consortia is a worthwhile strategy (Zhang et al. 2017). The application of laccases has been widely investigated as powerful and environmentally friendly technology in terms of eliminating and/or adjusting the lignin polymer, as well as in decreasing the phenolic content of pretreated lignocellulosic hydrolysates, thereby boosting subsequent saccharification and conversion yields (Suman et al. 2018). Giacobbe et al. (2019) investigated the likelihood of two laccases from Pleurotus ostreatus rPOXA1b to detoxify a delignify milled brewer’s spent grain and found that it up to 94% of phenols was reached. Liu et al. (2021) identified a new laccase LAC-Yang1 from a white-rot fungus strain P. ostreatus, which was able to degrade and detoxify chlorophenols (2,6-dichlorophenol and 2,3,6-trichlorophenol). Various chemical mediators such as 2,20-azino-bis (3 ethylbenzothiazoline-6-sulfonic acid) (ABTS), violuric acid (VLA), N-hydroxyacetanilide, or N-hydroxyphthalimide (HPI) have been used to enhance the oxidation of recalcitrance compounds (Fillat et al. 2017).

5 Cell Immobilization Microorganisms are typically utilized as free cells for ethanol production. Nevertheless, during batch fermentation process, the specific growth rate of free cells might be influenced by several factors linked to either or product substrate concentration. During last few decades, ethanol production by immobilized microbial cells has undergone excessive investigation in order to surmount the substrate and product inhibition on the cells as well as to improve the productivity and yield of ethanol (Kourkoutas et al. 2004). According to Groboillot et al. (1994), the main benefits of

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yeast immobilization include increased ethanol yield and cellular stability, as well as lower process costs referring to the efficiency of and reutilization. Nikoli´c et al. (2010) investigated the influence of immobilization of S. cerevisiae var. ellipsoideus on bioethanol generation from corn meal hydrolyzates. They found that the immobilization with Ca-alginate appears as a more favorable method as higher ethanol tolerance and productivity as well as lower substrate inhibition were observed. Cells immobilization technologies offers several advantages over conventional free cells systems (Kourkoutas et al. 2004; Rakin et al. 2009; Zabed et al. 2017): (a) increased stability of biocatalyst, (b) reduced inhibition of high substrate concentration, (c) enhanced ethanol yield and volumetric productivity, (d) improved substrate uptake, (e) decreased product inhibition, (f) reduced risk of contamination due to high cell densities, (g) decreased energy demand, and (h) cells recycling. S. cerevisiae stands for the most frequently used yeast used microorganism in cellulosic ethanol production. In fact, cells of this yeast can be immobilized through two major immobilization techniques namely self-immobilization and immobilization with support materials. Various cell-supporting materials have been proposed for immobilization including Ca-alginate, agarose, γ-alumina, k-carrageenan gel, cellulose, polyacrylamide, chitosan, and sorghum bagasse (Nikoli´c et al. 2010). Mishra et al. (2016) studied the immobilization of S. cerevisiae, to maximize ethanol production in repeated batch fermentation, through entrapment in Ca-alginate followed by optimization of alginate concentration, hardening time, temperature bead size, and substrate concentration. Novel and low-cost immobilization materials including the use of filamentous fungi, are considered as a key solution for ethanol production investing immobilized systems (Karagoz et al. 2019). Self-immobilization of cells is defined as a non-sexual, homotypic, reversible and multivalent technology of cells aggregation, where strain cells aggregate by interactions to each other forming biofilms or flocs. Compared to traditional immobilization methods, this type of cells immobilization is more appropriate for the generation of bulk products, such as bioethanol, as it presents many benefits: it is technically easier and cost effective, eliminates contamination from support materials, offers better cell growth, and allows the control of the required concentration in the reactors (Zabed et al. 2017; Karagoz et al. 2019).

6 Consolidated Bioprocessing (CBP) Technology CBP (Fig. 3) corresponds to an innovative strategy allowing sustainability for cellulosic ethanol generation derived from plants residues. This strategy involves combining the three main steps (enzyme production, hydrolysis of pretreated biomass in fermentable sugars, fermentation) into a single bioprocessing system. Recently, CBP was demonstrated as a potential timesaving and cost-effective approach for 2nd generation ethanol production, referring to the use of one single reactor as well as the

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Fig. 3 A simplified schematic of consolidated bioprocessing for cellulosic ethanol production from lignocellulosic residues

avoided cost of exogenous enzymes necessary for the hydrolysis process in simultaneous saccharification and fermentation (SHF), separate hydrolysis and fermentation (SSF) and simultaneous saccharification and co-fermentation (SSCF). The features recommended for CBP microorganisms are: efficient production of enzymes, high hydrolysis rate, potential ethanol production (yield and productivity), co-fermentation of C6 and C5 sugars, ethanol tolerance, resistance to inhibitory compounds, and environmental stress conditions such as low pH, high temperature, and high osmotic pressure (Olguin-Maciel et al. 2020). In order to accomplish these objectives, there are different possibilities: engineer lignocellulose degrading microorganisms (i.e., Clostridium phytofermentans, Clostridium thermocellum, and Caldicellulosiruptor bescii), which produce hemicellulase and cellulase but with low ethanol efficiency and engineer ethanologenic yeast (S. cerevisiae) (Kumar et al. 2016b). Nevertheless, so far, most of genetically engineered strains found having the capacity to perform CBP are far below efficient alcohol production expectations.

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7 Evolutionary Engineering Evolutionary engineering/adaptive evolution, represents a prominent experimental technique imitating nature’s own engineering principle through variation and selection. The methodology of evolutionary selection seems to offer multiple benefits as an efficient approach to select microorganisms with biotechnologically significant traits such as improved enzyme properties, novel catabolic activities, and resistance to stress conditions (Zhang et al. 2019; Kocaefe-Öz¸sen et al. 2022). The adaptive evolution approach was applied by Wang et al. (2018) for the improvement the inhibitor resistance of Corynebacterium glutamicum S9114. The modified strain displayed an increased conversion rate for various inhibitory compounds namely 5-HMF, furfural, syringaldehyde, vanillin, 4-hydroxybenzaldehyde and acetic acid. Recently, yeast strains with increased resistance to triticale straw (Smith et al. 2014) as well as corn stover hydrolysate (Almario et al. 2013) are investigated as illustrative examples of evolutionary engineering.

8 Genetic and Metabolic Engineering To overcome the challenges coupled with commercializing of bioethanol production, namely less ethanol yield, low tolerance to high temperatures and inhibitory by-products, recombinant microorganisms with high resistance to many inhibitory factors has been investigated using genetic engineering technology (Jönsson and Martín 2016). The latter is a powerful biotechnological tool based on the use of recombinant DNA method in order to up-regulate the stress tolerance genes so as to surmount the inhibitory compounds effect. Among ethanologenic microorganisms, S. cerevisiae corresponds to the world’s most invested ethanol producer yeast due to its robustness and its tolerance to several stressful conditions (Mohd -Azhar et al. 2017). However, generally, wild strains are unable to ferment pentoses (C5 sugars) released from the degradation of hemicellulose. Yeasts from genera Candida, Pichia, Pachysolen, and Schizosaccharomyces proved to be able of fermenting pentoses towards ethanol (Antonopoulou 2020). Strategies for engineering ethanol-producers with the co-fermentation of pentoses and hexoses are quite significant in terms of the development of successful procedures for converting plant residues to bioethanol. Furthermore, molecular approaches have been elaborated to reduce the carbohydrate metabolizing pathway to minimal reactions through the use of gene knockout mutations in genetically engineered E. coli for both pentose and hexose fermentations to produce ethanol with high yield (Trinh et al. 2008; Clomburg and Gonzalez 2010). Hasunuma et al. (2014) reported that the amplification of transaldolase and alcohol dehydrogenase in S. cerevisiae, cultivated in a lignocellulosic hydrolysate, entailed in an increase of the ethanol yield owing to enhanced performance in the presence of furfural. For the improvement of ethanol production from cellulosic hydrolysate, Lee et al. (2017) engineered in industrial

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S. cerevisiae JHS200 through Cas9 (CRISPR associated protein 9)-based genome editing. Consequently, ethanol concentration achieved 55.5 g/L. Moreover, Wang et al. (2013) engineered Escherichia coli for enhanced furfural resistance in order to improve resistance to sugarcane bagasse hydrolysate. Engineering of acetic and formic acid resistance of S. cerevisiae, through the enhancement of transaldolase and formate dehydrogenase activities, resulted in enhanced tolerance to rice straw hydrolysate (Sanda et al. 2011). In order to mitigate the furfural toxicity for biochemical production in E. coli, Song et al. (2017) employed simultaneous expression of pncB and nadE genes for the enhancement supply of NAD(P)H through the nicotine amide salvage pathway. According to Argyros et al. (2011), genetically modified strains of T. saccharolyticum and C. thermocellum produced 38 g/L cellulosic ethanol in 146 h. In another study, cellulolytic enzymes (BGL1 from Aspergilus aculeatus and EGII from T. reesei) emerged on the surface of a recombinant K. marxianus strain, allowing efficient ethanol production (20.4 g/L) from β-glucan at high temperatures (45 °C) (Yanase et al. 2010). Suo et al. (2019) engineered Clostridium tyrobutyricum by the co-expression of sdr and groESL from Clostridium beijerinckii NCIMB 8052 to simultaneously enhance tolerance of furfural tolerance and phenolic compounds displayed in dilute-acid lignocellulosic hydrolysates.

9 Conclusion Plant residues-based biorefinery corresponds to a renewable and environmentally beneficial strategy yielding an outstanding economic merit. Significant progress has recently been accomplished in terms of establishing efficient pretreatment procedures, engineering microorganisms, and immobilizing microbial cells, in order to sustain lignocellulosic biomass-derived ethanol and make it commercially viable and profitable. The selection and engineering of biocatalysts with increased tolerance have undergone much progress with respect to inhibitory compounds presented in the hydrolysate. Furthermore, consolidated bioprocessing (CBP) technique has been largely improved as a promising and valuable strategy since it simplifies the overall operation process for cellulosic ethanol production. However, supplementary research efforts are highly needed in order to further refine the efficiency of bioenergy production in a more cost-effective way.

References Abraham A, Mathew AK, Park H, Choi O, Sindhu R, Parameswaran B, Pandey A, Park JH, Sang BI (2020) Pretreatment strategies for enhanced biogas production from lignocellulosic biomass. Bioresour Technol 301:122725. https://doi.org/10.1016/j.biortech.2019.122725 Almario MP, Reyes LH, Kao KC (2013) Evolutionary engineering of Saccharomyces cerevisiae for enhanced tolerance to hydrolysates of lignocellulosic biomass. Biotechnol Bioeng 110:2616– 2623. https://doi.org/10.1002/bit.24938

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Nanocellulose from Date Palm: Production, Properties and Applications Karima Ben Hamou, Fouad Erchiqui, Youssef Habibi, and Hamid Kaddami

Abstract With the growing demand for more eco-friendly, sustainable, and renewable materials, natural fibers arouse a tremendous interest both from a scientific standpoint as well as industrial prospect. With a high annual production generating huge amounts of by-products of pruning, date palm (Phoenix dactylifera L.) stands as one of the most available sources of natural fibers particularly in arid and semiarid regions. Moreover, nanocellulose is emerging as efficient low-cost nanomaterials with attractive chemical and physical attributes allowing their applications as precursors for the design of biomaterials for sophisticated applications. In conjuncture with increasing interest in developing sustainable economies through the substitution of fossil resources by renewable feedstock, interest in nanocellulose does not seem to wane and they will continue to attract considerable attention within the scientific community and industries. The quest for new sources of cellulose fibers particularly non-woody feedstock to extract novel nanocellulose substrates to fulfill the high demand is therefore primary necessity. This chapter collates the recent developments achieved on the production of nanocellulose substrates from the date palm tree and their potential applications. Keywords Palm date · Nanocellulose fiber · Application · Cellulose · Outlook

K. Ben Hamou Faculty of Sciences, Moncton University, Shippagan Campus, Canada F. Erchiqui Laboratory of Biomaterials, Université du Québec en Abitibi-Témiscamingue, Quebec, Canada Y. Habibi University Mohamed IV Polytechnic, Benguerir, Morocco H. Kaddami (B) Faculty of Sciences and Technologies, Laboratory IMED-Lab, Cadi Ayyad University, Marrakech, Morocco e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 R. Khiari et al. (eds.), Annual Plant: Sources of Fibres, Nanocellulose and Cellulosic Derivatives, Composites Science and Technology, https://doi.org/10.1007/978-981-99-2473-8_4

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1 Introduction Date palm tree (Phoenix dactylifera L.) is one of the Middle East and North Africa’s most important non-woody renewable feedstock used primarily for its fruit, fibre and as a construction material. The application of date palm fiber for industrial applications as a reinforcement in polymeric and inorganic matrices is relatively new. To understand the advantages and limitations of using date palm fibers as reinforcement in composites, it is vital to understand the chemical, physical, mechanical, and thermal properties of date palm fibers. In this context, cellulosic fibers are gaining popularity as a reinforcement material for composite materials, owing to their low cost, low density, biodegradability, availability, specific interesting module, and ability to be recycled (Bendahou et al. 2010; Sbiai et al. 2012; Ghori et al. 2018). Recently, it has been emphasized that by using appropriate chemical and mechanical treatments, it is possible to produce fibrous materials with one or two dimensions in the nanometer range from any naturally occurring source of cellulose, which opens the door to new applications (Vardanyan et al. 2014; Dufresne 2018; Desmaisons et al. 2018). Cellulose nanocrystals (CNCs) and nanofibrillated cellulose (NFCs) are the two most extracted and characterised cellulose nanofibers. They differ in terms of morphological properties and crystallinity, but NFCs have a higher aspect ratio in general. Several methods for extracting cellulose from agricultural biomass resources and converting it into nanocellulose have been reported (Bendahou et al. 2009a, 2010; Benhamou et al. 2014; Ouled Ltaief et al. 2021). The date palm is a cellulose-rich tree; fibers can be extracted from four parts of the tree: the mesh, spadix stems, midribs, and leaflets and it has been reported that nanocellulose derived from various parts of the date palm tree differ in structure, morphology, degree of polymerization, and surface properties (Isogai 2013). This chapter emphasizes the significance agricultural wastes from date palm tree in the production of nanocellulose to reduce environmental impact. It also shows some opportunities of valorization of these wastes for a sustainable development. On the other hand, it provides an overview of the literature describing the morphological and cell-wall ultrastructure of date palm tree fibers (Phoenix L. dactylifera) as well as their chemical compositions. The production of nanocellulose from date palm tree will be the subject of the following parts. In addition, the physicochemical properties of isolated nanocellulose will be discussed. Finally, the potential of using these nanocellulose in a variety of applications will be investigated. The information gathered will then be used to draw conclusions about the studies that have been conducted and potential areas that could be further investigated.

2 Date Palm Tree (Phoenix dactylifera L.) Date palm (Phoenix dactylifera L.), known since ancient times, is a palmae (or Arecaceae) family tree native to the tropics and subtropics. It has been known since ancient times. Its origin would be in western India or in the Persian Gulf region but it can be found throughout North Africa, the Sahara, from the Atlantic to the

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Red Sea, as well as the Middle East and east to the Indus. It makes up the oases’ distinctive vegetation. Its preferred area is between the 15th and 30th degrees of north latitude. Further north it can be cultivated but does not bear fruit or gives poor fruit. It was introduced to all five continents, particularly America from the sixteenth century and Australia in the nineteenth century. It is the object of cash crops in North Africa, the Middle East and the United States (California, Arizona) (Hadrami et al. 1998). Since ancient civilizations, the date palm has represented the fruit tree of the desert in which it plays a major socioeconomic and environmental role and support for indigenous populations (Goaman et al. 1993). The mean yield of date production is important as it varies between 18 and 50 kg of dates per tree. As for its ecological role, this specie constitutes the basic structure of oasis agronomy. In fact, the date palm creates a microclimate that is essential for the good development of the underlying crops. In addition, its cultivation is one of the main agricultural activities of these environments, thus allowing the stabilization and subsistence of indigenous populations whose livelihood depends on the products and by-products generated by palm tree. A wide range of by-products is generated by the date palm, whose exploitation is limited to domestic and semi-industrial practices: handicrafts, construction, energy production and livestock feeding (Chehma 2001).

2.1 Anatomy and Morphology There are over 2,600 species of palm trees. It might be expected that it is a tree that has a trunk while it is a Monocotyledon that does not contain wood or trunk but has a stipe. In addition, it is a dioecious plant therefore, containing male and female palms. The date palm has a large erect trunk with a stout, brown-colored central portion. It has one terminal bud, and the leaf sheaths are overlapping. The trunk has a width of about 25 to 30 cm and a length of about 10 m (Hodel 2009) (Fig. 1). Both the asexual and sexual methods are used to propagate the date palm (Khierallah and Bader 2007; Hadrami and Hadrami 2009; Rajmohan 2011). The asexual method involves the shoot being grown from seed, while the sexual method involves an offshoot that appears on the trunk near the tree’s base. Some date palms have leaf bases that deteriorate over time, leaving old specimens with a smooth trunk, whereas others retain their prominence throughout the tree’s life. (Dowson 1923). The crown of the date palm is composed of approximately fifty new pinnate, rigid lanceolate leaves that can grow up to 4.5 m long, though individuals with a dense terminal crown can have up to 120 fronds. The rachis of the leaves is moderately rigid (Paszke 2019) with numerous leaflets with sharp points up to 40 cm long on one side. Young and mature leaves grow upward and sideways in general, while dead, slowly abscising leaves turn downward, However, the appearance of the fronds varies greatly between species (Dowson 1923; Hadrami and Hadrami 2009). Several erect spadices are frequently found between the leaves, which will eventually turn into pendulous stalks containing dates (Dowson 1923; Paszke 2019). The quantity and quality of dates are closely related to the number of green leaves in the crown’s

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Fig. 1 Schematic representation of parts of a date palm. (Taken from https://ian.umces.edu/)

center, as well as the removal of old ones that are less or no longer active. A morphological investigation was carried out by Bendahou et al. (2009b) to investigate the morphology and ultrastructure of the cell walls that form the leaflets and rachis of the date palm (phoenix L. dactylifera). Figure 2 shows scanning electron micrographs (SEM) of transversal cross-sections of the palm leaflet and rachis, respectively. In both the leaflets and the rachis, these micrographs clearly show three major components: parenchyma cells, fibers (died cells, mature cells, tracheids, or sclerenchyma fibers), and conducting vessels. However, the distribution of these three elements varies between the leaflets and the rachis. A regular repartition of slots is observed for the leaflets (Fig. 2a). Many bundles of fibres are dispersed and separated by chlorenchyma cells between two successive slots. The detailed structure of the slots, as well as the fiber bundles in the leaflets, is shown in SEM observations in Fig. 2b and c. The semi-symmetrical structure of slots is represented as a large bundle of vessels wrapped in a bundle of fibres (or surrounded by fibers). The vessel’s width can reach approximately 25 μm, while the diameter of the fibre ranges from 5 to 10 μm. The parenchyma cells have many faces and can grow to be 30 μm in diameter. SEM morphological analysis of a transverse section of the date palm rachis reveals that the rachis is composed of the same elements observed in the structure of the leaflets. but the microstructure is different. As discussed for the leaflets, the vessels and fibers form a structure similar to a slot (Fig. 2d–f). This slot is made up of a vessel bundle wrapped in a fiber bundle. Parenchyma cells separate these bi-component structures. It should be noted that the rachis structure is denser than the leaflets

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Fig. 2 Scanning electron micrographs of the leaflets (a–c), and the rachis (d–f) of Phoenix Dactylifera-L. (Bendahou et al. 2009b)

(fewer parenchyma cells). From a composition standpoint, it is worth noting that the leaflets in the P. dactylifera palm account for 46.6 wt% of the dry weight, while the rachis accounts for 53.4 wt%. The constituents and chemical composition of the leaflets and rachis of P. dactylifera palm are depicted in Fig. 3.

Fig. 3 Chemical composition of leaflets and rachis of Phoenix dactylifera palm, (a) as % of dry matter (Bendahou et al. 2007)

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The leaflets contain a significant amount of lignin (27 wt%) in addition to minerals (6.5 wt%). The protein content is low (2 wt%), and the main constituents are polysaccharides (59.5 wt%), including cellulose (33.5 wt%). On the other hand, the rachis contains less lignin (14 wt%) and more protein (6 wt%). The polysaccharides are the main components, with cellulose accounting for the majority (44 wt%) of the total rachis. As previously demonstrated by Bendahou et al. (2007), the non-cellulosic polysaccharides from both leaflets and rachis belong to the xylans family and have 4O-methyl-glucuronic acid side chains. The 4-O methylglucuronic acid molar ratios in all xylan fractions extracted from both leaflets and rachis are typical of higher plants.

2.2 Production Status Many studies have been conducted on fibers derived from date palm trees. The majority of attempts have been made to use these fibers as reinforcing composites (Al-sulaiman 2003; Kaddami et al. 2006; Sbiai et al. 2010; Agoudjil et al. 2011), as environmentally friendly flocculants or filters in water treatment (Riahi et al. 2009; Khiari et al. 2010), and as composts (Saadaoui et al. 2013; El Fels et al. 2014). However, due to the COVID-19 crisis, which included a lockdown, the closure of dry borders and ports, and traffic restrictions (Hafez and Attia 2020; Hashem et al. 2020) many people became interested in these by-products. Alternative feedstuffs and locally available feed ingredients could be used to improve a possible solution. Date by-products are the most well-known by-products of arid and desert areas, and they are occasionally used in livestock and poultry diets. Recent research has revealed that these by-products are rich in polysaccharides and lignin (Bendahou et al. 2007; Sbiai et al. 2010, 2011; Khiari et al. 2010; Benhamou et al. 2015; Belgacem et al. 2020), making them as promising resources for industrial purposes. It would be better to just provide the production capacity in tons, or hectares that are cultivated before jumping the influence of climate changes on such production. You can’t talk about what are the expectations and influence of climate change without providing a snapshot of the actual production. Date palm (Phoenix dactylifera L.), as part of the Arecaceae family, is grown on a large scale throughout the world (Fig. 4) and is heavily influenced by climate change and soil conditions. Although continuous climate change has a significant impact on date palm efficiency, identifying appropriate generation regions for date palm under changing climatic conditions remains the main production areas. Farooq et al. (2021) used CLIMEX model to forecast date palm distribution area expansion/contraction under current and future climatic conditions. A1B and A2 climate change scenarios were used, and production suitability was predicted for three time periods [i.e., 2030 (early century), 2050 (mid-century), and 2100 (late century)]. The model estimated a significant suitable area (71.21%) for date palm cultivation under current climatic conditions. Climate change appeared to have no effect on production areas until the

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Fig. 4 The current worldwide distribution of P. dactylifer (Taken from https://doi.org/10.1371/jou rnal.pone.0048021.g001) (Shabani et al. 2012)

early twentieth century. Range contraction (8 and 10% decline under A1B and A2 scenarios, respectively) was predicted for the mid-century in suitable areas. Nonetheless, severe range contraction was predicted in late-century production areas (27.98 and 33% decline under A1B and A2 scenarios, respectively). In the late twentieth century, most of the climatically suitable areas that existed in the early twentieth century became unsuitable. Furthermore, the model predicted a shift to the north in date palm production areas. Due to a stronger warming trend, the range contraction was greater in the A2 climate change scenario than in the A1B one. According to Farooq and co-authors’ findings (Farooq et al. 2021), Numerous areas become suitable for date palm cultivation. Date palm cultivation could thus be expanded in these areas to increase output. Future orchards should be planned in the most suitable areas to avoid the negative effects of climate change on date palm production in the country.

2.3 Traditional Uses of Date Palm The date palm tree serves multiple functions, including fruit, fiber, shelter, and fuel. For generations, dates have been used for commercial purposes. Each branch of the tree has the potential to generate economic returns by increasing the income of poor rural people. Its trunk provides timber; its leaves provide roofing materials; its leaf midribs provide material for crates and furniture; its leaflets are made into baskets; its leaf bases are used for fuel; its fruit stalks provide rope and fuel; its fibers are processed into cordage and packing material, and its seeds can be ground and fed to livestock. The fruits can be processed into syrup, alcohol, and vinegar. Because the extraction method is harmful to the tree, only trees that produce little fruit are

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used for sap. When palm trees are cut down, the tender terminal bud is harvested and eaten as a salad. Dates keep well in storage, allowing small farmers to maximize crop yields. Encouragement of date processing into a variety of products can lead to new demand and market opportunities for producers and actors along the supply chain. Fresh and dried fruits are staples in traditional diets, snacks, and ingredients in confectionery products. In terms of human nutrition, dates, like raisins and figs, are high in carbohydrates. Dates contain a wide range of essential nutrients and are a good source of potassium. Ripe dates contain about 80% sugar and the rest is protein, fiber, and trace elements like boron, cobalt, copper, fluorine, magnesium, manganese, selenium, and zinc. When compared to other types of fruit, such as figs and dried plums, dates have the highest antioxidant content. Dates have a glycemic index ranging from 30.5 to 49.7, making them appealing for lowering blood glucose and insulin levels (Brost and Committee). Aside from nutritious and savory fruits, the date palm yields a variety of by-products that Asians primarily use for domestic purposes. There are four types of fibers in this perennial tree: leaf fibers in the peduncle, bast fibres in the stem, wood fibers in the trunk, and surface fibers around the trunk (Kriker et al. 2005). In practice, the production of these by-product fibers may equal or even exceed that of the crop. Because the palm produces an average of 12–15 new leaves per year, the same number can be pruned and removed as part of its maintenance (Agoudjil et al. 2011). On the other hand, there are over 100 million date palm trees in the world (Al-Kaabi et al. 2005), which are found in hot spots across North Africa, the Sahara from the Atlantic to the Red Sea, the Middle East, and as far east as the Indus. They are grown in the Canary Islands, the northern Mediterranean, the southern United States, and Australia, among other places. In Morocco, where there are approximately 4.45 million palm trees, using date palm by-products as a raw material source for industrial uses is a promising idea (Saadaoui et al. 2013).

3 Date Palm Tree Lignocellulose Fiber 3.1 Composition Lignocellulosic materials, which include agricultural wastes, forestry residues, grasses, and plant crops, are the most promising feedstock for modern industrial societies as a natural and renewable resource. A substantial amount of such materials are produced as waste by-products from various agricultural industries (Pérez et al. 2002). Unfortunately, the majority of lignocellulosic biomass is discarded or burned as a source of energy. Because of their renewable nature and composition (primarily composed of three major units: cellulose, hemicellulose, and lignin), lignocellulosic biomass has recently piqued the interest of scientific and industrial communities, and numerous research efforts are underway to develop added-value products such as fuels, drop-in chemicals, and materials from lignocellulosic biomass (Sbiai et al. 2010, 2012; Ofori-Boateng and Lee 2013; Asgher et al. 2013; Seantier et al. 2016).

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As a result, vast amounts of lignocellulosic biomass are nowadays converted into a variety of high-value products through many processing technologies combining chemical, physical, microbial and enzymatic pathways (Millati et al. 2011; Iqbal et al. 2013; Irshad et al. 2013; Ofori-Boateng and Lee 2013; Asgher et al. 2013). Among the most attractive and straightforward applications, lignocellulosic materials have a significant potential for composites production. The term lignocellulosic material refers to a porous, hydrated, three-dimensional bio-composite made up of an interconnected network of 40–60% cellulose, 20–40% hemicelluloses, and 10–25% lignin (Sun et al. 2016; Dahadha et al. 2017). The middle lamella, which is made of lignin and provides adhesion between the layers, is the cell wall’s outermost layer. The thin primary wall after the middle lamella is characterized by the largely random orientation of cellulose microfibrils. Hemicelluloses are tightly associated with cellulose fibrils in the primary cell wall and form a loadbearing network that provides the mechanical function to the plant (Fig. 5). The remaining cell wall domain is the secondary cell wall, which has three layers: S1, S2, and S3, as previously stated (Fig. 5). The S1 and S3 layers are mostly made up of microfibrils glued together by lignin and hemicellulose “matrices”. The thick S2 layer, which is the richest in cellulose, is arguably the most important in determining the cell’s and thus the plant’s properties at a macroscopic level. The lumen is the void space in the cell’s interior that reflects the space available for water conduction.

3.2 Chemical and Physical Structures The use of all the three components effectively would have a significant impact on economic viability. Except for cotton bolls, cellulose fibers are embedded in a matrix of other structural biopolymers, primarily hemicellulose and lignin.

3.2.1

Cellulose

Cellulose is the most abundant polysaccharide in the biosphere because it is the primary building material for most plants. It is a linear homopolymer made up of glucan chains with repeating (1–4)-D-glucopyranose units linked together by Oglycosidic bonds (Klemm et al. 1998). Cellulose is a relatively stable polymer due to its strong intra and intermolecular hydrogen bonding. It has no melting point and cannot be dissolved in ordinary solvents. The cellulose polymer chain’s structure allows it to form a variety of highly ordered crystalline domains (Klemm et al. 2005). The crystalline cellulose polymorphs are known as cellulose I, II, III, and IV (Klemm et al. 1998). The naturally occurring crystalline structure of cellulose, cellulose I, has two polymorphs (triclinic I and monoclinic I structures), and the proportions of these two differ depending on the source of cellulose (Atalla and VanderHart 1984). The chemical formula for cellulose is (C6 H10 O5 )n , and the structure of one polymer chain is shown in Fig. 5.

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Fig. 5 A diagrammatic representation of the lignocellulose structure

3.2.2

Hemicelluloses

Hemicelluloses are polysaccharide molecules that act as a matrix for cellulose microfibrils engaged in molecular interactions such as hydrogen bonds and van der Waals forces. Aside from structural properties, hemicellulose may also be used for cell signaling and reserve substances. Xyloglucans are major components of higher plant dicotyledon hemicelluloses, comprise 20% of the dry weight primary cell-wall material (Kamide 2005), whereas, they constitute only about 2% of the dry matter in monocotyledons. Indeed, the major hemicellulose components in monocotyledons are xylans and -(1-3), -(1-4)-glucans, which constitute about 15–20% of the dry weight cell-wall mass. Xyloglucans, like xylans, are linked to cellulose microfibrils by hydrogen bonds.

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Lignin

Lignin is the most complex fraction, representing approximately 10–25% of biomass by weight. It is a long-chain, heterogeneous polymer composed primarily of phenylpropane units linked together via ether bonds. It is a long-chain, heterogeneous polymer composed mostly of phenyl-propane units linked by ether bonds. Lignin acts as a glue by filling the gap between and around the cellulose and hemicellulose complexions with polymers. Because it is found in all plant biomass, it is considered a by-product or residue in the bio-ethanol manufacturing process.

3.2.4

Extraction Processes

The four major families of plant fibre extraction processes are the physical process, mechanical process, chemical process, and biological process. The correct process is determined by the plant’s type and age, as well as the extracting organ. Multiple processes must be interconnected in some cases. In the literature, several techniques for extracting fibres from various parts of the date palm have been reported. Each technique has its own unique strengths and weaknesses. Extraction techniques may use mechanical means, while others rely on chemical or biological treatments to separate cellulose fibres from other constituents such as lignin and hemicellulose. It is worth noting that the extraction technique has a significant impact on the final characteristics and performances of the isolated fibres. The extraction yields, purity grades, and morphological properties are the defining parameters for a successful extraction protocol. For a long time, the effect of pretreatment of lignocellulosic fibers has been recognised by McMillan (McMillan 1994). The goal of the pretreatment is to remove some of the lignin and hemicellulose, soften the cellulose fibres, and increase the porosity of the material. Pretreatment must meet the four criteria (Mosier et al. 2005): (i) improve sugar release or the ability to form sugars later by enzymatic hydrolysis; (ii) avoid carbohydrate degradation or loss; (iii) avoid by-product formation inhibiting subsequent hydrolysis and fermentation processes; and (iv) be cost-effective. Physical, physicochemical, chemical, and biological processes, as well as combinations of these, have been applied to lignocellulosic fibres derived from palm trees.

4 Date Palm Tree Nanocelluloses and Its Modification The complex of enzymes involved in the bio-synthesis of cellulose chains brings together approximately 36 individual cellulose molecules during the biosynthesis to form larger units known as elementary fibrils or microfibrils, which are then packed into larger units known as microfibrillated, or nanofibrils of, cellulose (Habibi et al. 2010). Elementary fibrils have a diameter of about 5 nm, whereas microfibrillated cellulose has a diameter ranging from 20 to 50 nm. Each microfibril is like a flexible

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hair strand, with cellulose crystals connected by disordered amorphous domains along the microfibril axis (Azizi Samir et al. 2005). The ordered regions are cellulose chain packages held together by a strong and complex network of hydrogen bonds (Habibi et al. 2010). Two types of nanocellulose are commonly considered as a result of this ultrastructural organisation. The extraction pathways for (i) NanoCrystalline Cellulose (NCC) and (ii) Nanofibrillated Cellulose (NFC) are distinct: NFCs are generally liberated upon mechanical shearing, whereas NCC results from acid-catalyzed hydrolysis of the amorphous parts.

4.1 Preparation and Pre-treatments To disintegrate wood fibres, nanocellulose production methods combine chemical, mechanical, and enzymatic treatments (Araki et al. 1998; Nakagaito and Yano 2005). Based on the treatment, fibrils with diameters of 5–30 nm and lengths in micrometres or rod-like crystals, with diameters of 5–20 nm and lengths ranging from tens to hundreds of nanometers to micrometres, can be obtained. The dimensions of nanocellulose (fibril/crystal) vary greatly depending on the type of the treatment and source of the cellulose fibers, but they are always nanosized in at least one dimension (Dufresne 2013). Microfibrillated cellulose (MFC) was the first name given to the fibrillar material derived from wood (Herrick et al. 1983), which was later referred to as nanofibrillated cellulose (NFC) or cellulose nanofibrils (CNF) (Klemm et al. 2011; Isogai 2013). Cellulose nanowhiskers or simply whiskers have been used for the nanosized crystals. The terms nanocrystalline cellulose (NCC) and cellulose nanocrystals (CNC) are now more commonly adopted (Fleming et al. 2001; De Souza Lima and Borsali 2004; Azizi Samir et al. 2005). Recently, a few studies have been carried out to obtain pure cellulose fibres from specific parts of the date palm and then use them as starting materials to extract micro/nano-cellulose substrates. Microcrystalline cellulose, for example, was extracted from date seeds through alkali treatment, bleaching with sodium hypochlorite, and the purified fibers were further subjected to acid hydrolysis using hydrochloric acid (2.5 N) at 105 °C for 45 min. Type I cellulose was obtained with a crystallinity index of 62%, which was increased to 72% after acid hydrolysis (Abu-Thabit et al. 2020). A similar study isolated microcrystalline cellulose from a date palm fruit bunch stalk by first bleaching it, then alkaline treatment, and finally acid hydrolysis. It was found that the resulting cellulose fibers exhibited diameters from 21 to 96 μm and lengths exceeding 200 μm. Furthermore, the cellulose obtained had a crystallinity index of 79.4%. Using dilute acid, alkali treatment, and bleaching with acetic acid, hydrogen peroxide, and sulfuric acid, Galiwango et al. have isolated cellulose from waste date rachis, leaflets, and fibers. With an average crystallinity index of 52.27%, alpha-cellulose was produced at a yield of more than 70% (Galiwango et al. 2019). Bendahou et al. (2010) isolated microfibrillated cellulose and cellulose nanocrystals from date rachis and leaflets. Cellulose fibres were isolated from these parts through a series of treatments, including hemicellulose and lignin removal with 2wt% of sodium hydroxide (NaOH) and bleaching

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with sodium chlorite (NaClO2 ). Furthermore, microfibrillated cellulose was obtained using a high-pressure microfluidizer, and cellulose nanocrystals were obtained using sulfuric acid treatment. It has also been reported that rod-shaped nanoparticles (CNC) with average diameters of 6.1 nm and lengths of 260 nm were obtained. Boufi et al. (2013) reported the isolation of nanofibrillated cellulose from alfa and rachis of date palm tree, first by treating the biomass with alkali treatment at a higher temperature and then bleaching it to purify cellulose, followed by oxidation with 2,2,6,6tetramethylpiperidin-1-oxyl) (TEMPO) in a second step. The purified fibers were then homogenized under high pressure to produce cellulose nanofibers (CNF) with widths of 20–50 nm and lengths of 200–1000 nm. The purified fibers were further hydrolyzed with a high sulfuric acid content (65 wt%) to produce cellulose nanocrystals with average widths of 15–25 nm and lengths of 150–250 nm. Similarly, date rachis was used to extract nanocellulose fibers via sequential oxidation with TEMPO in order to obtain shape-controlled fibers, and cellulose fibers with widths ranging from 20 to 30 nm were obtained depending on the oxidation time and specific reaction conditions (Benhamou et al. 2014).

4.2 Properties and Applications Nanocellulose has unique intrinsic properties that make it appealing for a variety of applications. They have a high specific area, flexibility, and crystallinity, as well as a high concentration of hydroxyl groups, as previously stated. All these properties influence their interactions between them and with surrounding media whether in liquid suspension, as composite filler or as a neat film.

4.2.1

Viscoelastic Properties

The viscoelastic response of nanocellulose suspensions is affected by particle shape, dimension, crystalline structure, and surface properties, as well as solvents and additives in the suspensions (Shojaeiarani et al. 2019). Thus, the rheology of nanocellulose suspensions is critical to understanding the structure and properties of nanocelluloses to use them at their full potential. Despite diversified literature on the rheological properties of various types of nanocellulose substrates, there are only a few studies on the rheological properties of nanocellulose isolated from date palm tree (Phoenix dactylifera L.) wastes. Indeed, Benhamou et al. (2014) has investigated the rheological properties of TEMPO-oxidized cellulose nanofibers extracted from the rachis of the palm tree and the authors related the influence of the oxidation extend to the variation of the rheological properties. However, the study was limited to aqueous nanofibers suspensions.

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Thermal Super-Insulating Properties

Energy production has become a global challenge and considerable efforts are deployed to not only develop alternative and renewable energy sources but also at developing materials, particularly for construction for better energy saving and minimizing energy losses. Many development efforts are focused on producing insulating materials from agricultural and industrial wastes including fibers derived from palm tree. In this regard, Oushabi et al. (2015) investigated the possibility of incorporating waste data palm fibers as a component of insulation materials used in the refrigeration and air conditioning industries, and their findings show that the thermal conductivity measured in atmospheric pressure showed that the material retains good properties when compared to other natural and synthetic insulating materials, with a thermal conductivity value of 0.041 W/m K, implying that the date palm fibers are useful. Seantier et al. (2016) published a recent study on the production of bioaerogel materials based on various combinations of bleached cellulose fibers (BCF) and TEMPO-oxidized nanocellulose in the form of cellulose nanofibers (CNFs) or cellulose nanocrystals (CNCs) isolated from the date palm tree. The resulting aerogels were tested for thermal insulation, and the related properties have shown that the thermal conductivity value as low as 23 mW m−1 K−1 was obtained for BCF/CNFs systems. The observed structure can be described as stacked nanofiber films encircling BCF. The interactions between the two types of fibers have been shown to promote the formation of meso and nanopores when BCF and nanofillers are combined. The nanofiller films have been shown to be effective at confining air in bio-aerogels via the Knudsen effect and significantly lowering the thermal conductivity of binary bio-aerogels. On the other hand, Bendahou et al. (2015) investigated the effect of cellulose nanofibers isolated from date palm on thermal conductivity, and mechanical properties of nanosized-based hybrid materials. As shown in Fig. 6, this new type of biocomposite demonstrated good thermal and mechanical properties, allowing it to be used as thermal insulation materials. Another study found that when compared to other natural materials, date palm wood is a good candidate for the development of efficient and safe insulating materials (Benmansour et al. 2014).

4.2.3

Electrical Properties

Numerous research studies have shown that adding nanoparticles to a polymer matrix improves the dielectric properties (electric and magnetic energy storage and dissipation Ben Amor et al. 2009; Kadimi et al. 2014, 2019; Boufi et al. 2014; BenHamou et al. 2020). Several studies have been published on the thermal and dielectric properties of nanocellulose-based nanocomposites (Ladhar et al. 2013, 2014). Ladhar et al. (2015) investigated the electrical properties of series of natural rubber (NR) matrices loaded with varying amounts of nanofibrillated cellulose extracted from the rachis of date palm tree. Their findings revealed that NFCs have a minor impact on the electrical conductivity of NR-NFC nanocomposites. Accordingly, the presence of water molecules

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Fig. 6 The effect of various solutions’ nanozeolith (NZs) concentrations on (A): thermal properties, (of the bleached cellulose (BCF) and nanocellulose (NFC and NCC) aerogels (Bendahou et al. 2015)

linked to cellulose nanocrystals has a significant influence on the α dipolar relaxation and interfacial polarisation processes. The activation energy for relaxation increased with filler concentration due to the reinforcing effect. The evolution of activation energy for interfacial relaxation reveals a critical point between 2.5 and 7.5%. Hammami et al. (2020), recently investigated the morphology, dielectric, and electrical behavior of a series of PVAc matrix charged with different cellulose nanocrystals content extracted from date palm rachis nanocomposites. It was concluded that the use of these nanocomposites as separators for ionic batteries could be of great potential. The addition of cellulose nanocrystals isolated from date palm rachis improved the dielectric and mechanical properties of polycaprolactone diol-based polyurethanes (Ouled Ltaief et al. 2021).

4.2.4

Applications in Composites

Because of their recyclability, biodegradability, compatibility, and nontoxic behavior, sustainable and eco-friendly materials derived from natural sources are attracting a great deal of attention. Nanocellulose-based green composites have recently received a lot of attention from researchers due to their lightweight, low density, and excellent physical properties. According to Dufresne (2017) the mechanical properties of nanocelluloses are a significant asset; CNC’s tensile strength exceeds 10 GPa (10 times that of steel), and despite its low density, its tensile modulus averages 130 GPa. The aspect ratio, which is defined as the length to width ratio, is a critical parameter for nanocellulose materials. It oversees anisotropic phase formation and reinforcing properties. A method for predicting these properties could reduce the

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number of sources tested. Table 1 summarises the physical properties reported for cellulose nanocrystals obtained from various sources of films. The aspect ratio of date palm tree’s CNC is clearly higher than that of the cotton, wood and sugar cane bagasse cellulose nanocrystals. Julien Bras et al. investigated the effect of the CNC aspect ratio on the mechanical properties of nanopapers made from dispersions of these CNC in water by casting method. The results showed that the young modulus of the nanopaper, in general, increases with the aspect ratio of the CNC and the nanopaper of date palm tree CNC shows high stiffness which is partly due to its high aspect ratio. However, numerous studies have investigated and proven that the aspect ratio of date palm nanocellulose is intermediate in comparison to other natural nanoparticle types, which can account for its use in a wide range of nanocomposite applications. (Bendahou et al. 2010; Benhamou et al. 2015, 2020). Because natural rubber has poor mechanical properties, nanocellulose materials isolated from various parts of date palm trees were investigated to modify and improve their properties. The thermomechanical behavior of nanocomposite films made of natural rubber and cellulosic nanoparticles (CNC and NFC) extracted from the rachis of the date palm tree was investigated. The higher aspect ratio and entanglement potential of nanofibers improved the mechanical properties of the rubber matrix more than nanocrystals. It was also demonstrated that the presence of residual lignin, extractive substances, and fatty acids on the surface of NFC promotes higher levels of Table 1 Physical properties of cellulose nanocrystals from various sources (adopted from Bras et al. (2011) Natural fibers/plants

Dimensions of extracted nanocellulose

References

Young’s modulus (GPa) of CNC based nanopapersa

42

(Bendahou et al. 2009b; Benhamou et al. 2015)

7.70 ± 1.15

5.2

46.5

(Siqueira et al. 2010b)

2.41 ± 0.31

200

10

20

(Beck-Candanedo et al. 2005)

0.40 ± 0.06

96.7

7.5

12.9

(Dufresne et al. 1997)

1.50 ± 0.24

Length (nm)

Diameter (nm)

L/D

Palm tree rachis

260

6.1

Luffa cylindrica

242

Hardwood Sugar cane bagasse Tunicin

1000

15

66.7

(Favier et al. 1995) 15

Cotton

170

15

11.3

(De Souza Lima and Borsali 2004)

2.13 ± 0.32

Wheat straw

225

5

45

(Dufresne et al. 1997)

6

Capim dourado

300 ± 93

4.5 ± 0.86

67

(Siqueira et al. 2010a)

10.9 ± 1.6

a

The Young moduli presented are those of nanopapers made of the corresponding NCC

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adhesion with the polymeric matrix, thereby improving the mechanical properties of the composites (Bendahou et al. 2009b, 2010). Similar results were found by Fiorote et al. (2019) and Ladhar et al. (2017). The incorporation of cellulose nanofibers and nanocrystals isolated from the rachis of the date palm tree into acrylic-based latex improved the mechanical properties of the resulting nanocomposites based on cellulose nanofibers compared to those based on cellulose nanocrystals (CNC), which was attributed to the higher aspect ratio of cellulose nanofibers (Boufi et al. 2013). Benhamou et al. investigated the use of TEMPO-oxidized cellulose nanofibers and cellulose nanocrystals isolated from date palm rachis at loadings ranging from 2.5 to 10 wt% to reinforce polycaprolactone diol-based polyurethanes (Benhamou et al. 2015). The TEMPO-NFC outperformed nanocrystals in the mechanical and thermal properties, as well as the dispersibility in the polyurethane matrix.

5 Conclusions Date palm fibers exhibit mechanical, physical, and thermal properties comparable to those of most used natural fibers. Many parts of the date palm tree, such as the midribs, spadix stems, leaflets, and mesh, could be used to extract fibers. With the emergence of new bio industries, there is ample opportunity for a holistic utilization date palm, aiming at an efficient and effective date palm waste management, in date palm growing countries. Date palm fibers have a high potential for replacing many other fibers in many applications, including the production of sound insulating panels, thermal insulation, and reinforcement to many materials, due to their exceptional properties. Moreover, several studies have shown that fiber derived from date palm tree can be used in the production of various composites based on both thermoplastics and thermoset polymers. The current chapter discussed the recovery of nanocellulose from date palm by-products and the importance of determining the physicochemical properties of nanocellulose isolated from date palm lignocellulosic materials. The scientific findings presented in this chapter are expected to highlight the importance of physicochemical characterization of nanocellulose as well as the potential of agro-industrial wastes generated from palm tree as cellulose reservoirs.

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Preparation and Characterization of Cellulose Nanofibril from annual Plant Soumia Boukind, El-Houssaine Ablouh, Zineb Kassab, Fatima-Zahra Semlali Aouragh Hassani, Rachid Bouhfid, Abou El Kacem Qaiss, Mounir El Achaby, and Houssine Sehaqui

Abstract Cellulose nanofibrils (CNF) exhibit desirable chemical and physical characteristics, which expand their application range. The eco-friendly nature of cellulose nanofibrils makes them a suitable alternative to meet the current environmental requirements of sustainable development. Therefore, researchers have recently focused their attention on cellulose nanofibrils (CNF) and their composites since the introduction of CNF enables advanced properties and novel uses. In the field of materials, these efforts are reflected in the desire to offer new bio-based materials in the short term capable of replacing toxic or non-biodegradable materials while providing at least equivalent properties. Over the last decades, more research has shown considerable potential for cellulose nanofibrils isolation from various annual plant sources. These have been utilized as fillers to enhance the performance of nanocomposites. This chapter will resume previous research on the production methods adopted for CNF production from annual plants and the suitable characterization approaches used. The present chapter will focus on covering the production processes and the evaluation of the published data on the different mechanical, structural, morphological, and thermal characteristics of the resulting materials assessed through morphological (SEM, TEM, AFM), thermal (TGA/DTG, DSC), structural (XRD, fibrillation degree) and rheological analysis. This chapter is structured in three parts. The first part provides generalities about CNF from the structure to the applications. The second part will cover the chemical and mechanical disintegration approaches usually adopted. The last section focuses on the characterization S. Boukind · E.-H. Ablouh (B) · Z. Kassab · F.-Z. S. A. Hassani · M. El Achaby · H. Sehaqui (B) Materials Science, Energy and Nano-engineering (MSN) Department, Mohammed VI Polytechnic University (UM6P), Lot 660 – Hay Moulay Rachid, 43150 Benguerir, Morocco e-mail: [email protected] H. Sehaqui e-mail: [email protected] E.-H. Ablouh · R. Bouhfid · A. E. K. Qaiss Composites and Nanocomposites Center (CNC), Moroccan Foundation for Advanced Science, Innovation and Research (MAScIR), Rabat Design Center, Rue Mohamed El Jazouli, Madinat El Irfane, 10100 Rabat, Morocco

© The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 R. Khiari et al. (eds.), Annual Plant: Sources of Fibres, Nanocellulose and Cellulosic Derivatives, Composites Science and Technology, https://doi.org/10.1007/978-981-99-2473-8_5

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methods used in literature. It seeks to establish the link between the structure and the properties of CNF, which allows a better understanding of the behavior and provides essential indications for future up-scaling production. Keywords Annual plants · Cellulosic fibers · Cellulose nanofibrils (CNF) · Nanocomposites · Extraction of cellulose nanofibrils · Characterization of cellulose nanofibrils

1 Introduction Cellulose is considered the most abundant biopolymer on earth, with an annual natural production estimated to be 1011 –1012 tons (Nechyporchuk et al. 2016a; Rol et al. 2019a). This renewable and biodegradable compound is naturally produced by photosynthesis and constitutes 40–50% of the planet’s total biomass reserves (El Achaby et al. 2019; Yi et al. 2020). Cellulose is primarily stored in various natural sources and living species such as plants, animals, and bacteria (Eichhorn et al. 2010; Tingaut et al. 2012; Dufresne 2013; Zhang et al. 2016; Yu et al. 2021). However, wood pulp (about 40–50% of cellulose) and cotton (>90% of cellulose) (Illy et al. 2015; Pennells et al. 2020; Ablouh et al. 2021) remain the primary commercial sources (Nechyporchuk et al. 2016a; Rajinipriya et al. 2018) for the industrial production of cellulose, primarily due to their abundance worldwide. Yet, more efforts are ongoing to identify new sources of cellulose biomass as a feedstock to replace costly cotton linters and wood pulp, which are now restricted according to conservative environmental requirements. Annual plants (e.g., sisal, hemp, wheat straw) are progressively replacing wood as an alternative source of cellulosic products, considering the environmental impact, the economic purpose, and the greater yield of cellulose (El Achaby et al. 2018; Kassab et al. 2020a). They are also preferred for easy cultivation, transportation, and low chemical and energy consumption during the pulping process. These natural resources have major chemical constituents such as cellulose, lignin, hemicellulose, and other extractives (Varshney and Naithani 2011). For the last decade, cellulose nanofibrils (CNF) have attracted much interest due to their unique features that can benefit various application fields. Despite their numerous properties, CNF industrialization remained limited until recently, thanks to effective methods that successfully reduced energy consumption during mechanical treatment. CNF are conventionally produced through different mechanical treatments using disintegrating equipment, e.g., a microfluidizer, homogenizer, or grinder. The production of these cellulosic nanofibrils is notoriously energy-intensive due to the large high amount of surface hydroxyl groups that generate extensive hydrogenbonding interactions amongst the nanofibrils. Therefore, several research studies proposed new pathways to make CNF production more straightforward and less expensive. Accordingly, two approaches have been suggested for this aim, including novel mechanical methods and chemical pre-treatments. Most of these investigations

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were primarily focused on the second alternative since it affords additional properties (Rezayati Charani et al. 2013; Wang et al. 2020). Numerous chemical pre-treatment procedures can be adopted, including TEMPO oxidation, carboxymethylation, phosphorylation, and enzymatic processes. These pre-treatments result in functionalized CNF with various additional characteristics and advanced functionalities. Cellulose nanofibrils are commonly used in multiple applications, such as membranes for drug delivery, barrier films, water treatment, reinforcing material, and optical media (Kalia et al. 2009; Belbekhouche et al. 2011; Sehaqui et al. 2016, 2018). Recently, more book chapters and review articles on the production and characterization of CNF have been published (Lavoine et al. 2012; Isogai 2013; Osong et al. 2016), some of which dedicate considerable importance to the market issues and industrialization processes (Chauve and Bras 2014; Lindström and Aulin 2014). Thus, this chapter aims to introduce the different preparation methods of CNF, some related issues, and features of these nanofibrils.

2 Cellulose Nanofibrils (CNF): Definition, Properties, and Applications Annual plants are gaining increased attention as a source of cellulose due to their high yield of cellulose, looser fibril structure, and lower lignin content. As a result, the delignification and purification processes of such fibers are simplified and less hazardous, while fibrillation consumes less energy (Varshney and Naithani 2011). Cellulose nanofibrils (CNF) are promising natural nanoscaled materials that can be extracted from the cell walls of lignocellulosic feedstock through a mechanical process (Fig. 1). These nanofibrils represent a category of cellulose nanomaterial featuring high aspect ratios, nanoscale diameters, and high surface area. Typically, CNF exhibit a diameter range of 5–50 nm and are 1–3 µm in length (Kontturi et al. 2018). Turbak et al. (1983a) were the first to introduce the production of CNF as a novel cellulosic material by repeatedly passing a softwood pulp aqueous dispersion through a high-pressure homogenizer, leading to the formation of a viscous aqueous suspension of cellulose fibrils with a diameter of 25–100 nm. Due to the high shearing forces generated during this treatment, highly entangled networks of nanofibrils with amorphous and crystalline domains are formed. The obtained CNF displayed a high aspect ratio and shear-thinning and thixotropic behavior. Cellulose nanofibrils are usually produced with a concentration of less than 2% from wood or non-woody pulps (Sehaqui et al. 2010a). Nevertheless, several studies were conducted to minimize the energy consumption of the mechanical treatment to make industrial CNF production economically attractive. However, good management of CNF nanostructure is required to provide the necessary characteristics. This level of structural control necessitates a profound knowledge of how manufacturing affects the material. Using novel source materials and techniques would result in MFC with various properties (Iwamoto et al. 2008). The CNF aqueous suspensions

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Fig. 1 From cellulosic fibers to cellulose nanofibrils (CNF)

exhibit multiple characteristics depending on the manufacturing procedure. These suspensions may be transformed into other CNF products such as aerogels, hydrogels, films, or powders (Sehaqui et al. 2010a, 2011). As mentioned above, cellulose nanofibrils have high stiffness, a high lengthto-width ratio, and the potential to create networks via strong secondary bonding, such as hydrogen bonds. This provides considerable flexibility for the design of novel materials. Therefore, various applications are possible, ranging from dense nanopaper structures to extremely porous foams and reinforced polymer composites (Sehaqui et al. 2010b, 2011). There are numerous potential applications, including biomedical and pharmaceutical applications (e.g., tissue engineering scaffolds, drug carriers), acoustic and thermal isolation, food packaging, network reinforcement, and environmental applications (Benhamou et al. 2014; Blanco et al. 2018).

3 Production Methods of Cellulose Nanofibrils (CNF) 3.1 Chemical Pre-treatments Due to its structure, cellulose may be treated in various methods. The hydroxyl groups in its chain provide several options according to the modification adopted. Chemical changes may introduce charges into the cellulose surface, which is beneficial for nanofibrillation since it reduces energy consumption and clogging and offers new features to the CNF. Charged groups, for example, induce repulsive forces that decrease the cohesiveness of hydrogen bonds. The repulsive forces are generated by osmotic pressure, caused by the variation in ionic concentrations within and outside the fibers. Consequently, water diffuses into the fibers, lowering the osmotic pressure and causing fiber swelling. The interfibrillar cohesiveness is weakened, whereas cell wall breakdown is enhanced. Therefore, the cellulose nanofibrils can be readily separated and endowed with additional functions (Rol et al. 2019a). The chemical pretreatments of cellulose pulps facilitate mechanical defibrillation and minimize the

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Fig. 2 Examples of the chemical pre-treatments for the improvement of the mechanical disintegration of cellulose fibers

number of passes through the disintegration equipment. They include various chemical treatments, most importantly enzymatic hydrolysis (Henriksson et al. 2007) and oxidation of the cellulose surface (Saito et al. 2007). Depending on the production conditions, the cellulose fibers may be disintegrated to form flexible cellulose nanofibrils ranging from 5 nm to tens of nanometers (Nechyporchuk et al. 2016a). The literature described a wide range of CNF with various surface properties due to covalently linked chemical groups (Nechyporchuk et al. 2016a; Thomas et al. 2018). The following section summarizes the most used chemical pre-treatments described in the literature (Fig. 2) (Lavoine et al. 2012).

3.1.1

TEMPO Oxidation

As it is well established, the insertion of negatively charged groups into cellulosic fibers improves the nanofibrils delamination owing to the electrostatic repulsion generated between the charged cellulose nanofibrils (Montanari et al. 2005; Bäckström et al. 2012). TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical)-mediated oxidation is one of the earliest approaches adopted to introduce negatively charged groups into cellulose via oxidation of the cellulose fibers using 2,2,6,6-tetramethyl-1piperidinyloxy (TEMPO) and the most used pre-treatment (Saito et al. 2006, 2007), due to the uniform diameter ( 334.59 ºC. This improvement can be related to the elimination of thermally instable components (lignin and hemicellulose) (Tarchoun et al. 2019b; Ait Benhamou et al. 2021d). In the same trend, the as-extracted CNCs showed lower thermal profile than that of CMFs with a T onset at 273.54 °C and a T max at 340.87 °C. This decrease is related to the presence of sulfate groups that promotes an early degradation of cellulose (Camarero Espinosa et al. 2013; Frone et al. 2017). The determined thermal parameters of CNCs from CWS (273.54 °C and a T max at 340.87 °C) was found to be higher than those measured for CNCs from cactus leaves (T onset at 220 °C and a T max at 318 °C). This probably related to the applied process of CNCs preparation that exceeds 3 h compared to that used for CWS-CNC of 10 min, which probably fix a significant portion of negatively charged sulfated groups in the surface of the CNC. Also, the thermal parameters of CNFs from cactus cladodes skin (T onset at 251 °C and a T max at 312 °C) were also lower than those

Fig. 8 TGA (a) and DTG (b) curves of untreated fibers, extracted cellulose, and Sulfated-CNC from cactus seeds compared to MCC. Reproduced from (Ait Benhamou et al. 2022b) with permission

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Table 2 The characteristic parameters of raw and nanocellulose from different part of cactus plant Source and type

CrI (%)

Yield of CNC (%)

Zeta (-mV)

D (nm)

L (nm)

Tonset (°C)

Tmax (°C)

Refs.

CNCs Cactus waste seeds

86

25

−30.0

13 ± 3

419 ± 48

273

340

(Ait Benhamou et al. 2022b)

CNFs cactus skin

50

–

–

10–50

–

251

312

(Ramezani Kakroodi et al. 2015)

CNCs cactus leaves

74

32

−45.2

10–14

200–255

220

318

(Nagarajan et al. 2020)

reported for CNCs from CWS (Kassab et al. 2019b). These findings indicates that the thermal parameter depends on several factors including the preparation process, origin of cellulose source as well as the reagent concentration.

4.4 Morphological Properties To assess the morphological structure of cellulosic materials at various length scales, numerous microscopy techniques can be used to determine the roughness, fibers dimensions and the state of dispersion of particles. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM) can all be used to qualitatively observe and analyse particle dispersion at different lengths (Boussetta et al. 2021a, b). SEM, AFM and TEM have been previously used in few works to study the morphological behaviour of nanocellulose from some parts of cactus plant. For instance, Ait Benhamou et al., used both SEM and AFM to highlight the morphological structure of extracted micro and nanocellulose from CWS as illustrated in Fig. 9a and b (Ait Benhamou et al. 2022b). Authors showed that alkali and bleaching treatments had an effect on the structure of raw fibers leading to a total defibrillation with individual microfibers exhibiting a smooth surface due to the elimination of non-cellulosic compounds (Kassab et al. 2020b; Ait Benhamou et al. 2022a). Moreover, the measured average diameter was found to be 11 μm ± 1.4, which is quite comparable to other extracted CMF from different parts of cactus including trunks (Mannai et al. 2016) and cladodes (Ait Benhamou et al. 2021a) as well as other lignocellulosic fibers (Bahloul et al. 2020; Rasheed et al. 2020). Moreover, the sulfuric acid hydrolysis of CMF from CWS fibers was also discussed in their work. The authors stated that a majority of amorphous parts in CMFs were solubilized, and the crystalline ones were not affected, resulting in the

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Fig. 9 SEM observations of CMF from CWS (a), AFM observation of CNCs from CWS (b). Reproduced from (Ait Benhamou et al. 2022b) with permission. Low (c) and high magnification (d) TEM observations of prepared MFC from the skin of prickly pear cactus. Reproduced from (Habibi et al. 2009) with permission

preparation of a nanocrystals with a needle like shape as seen in AFM image Fig. 9b. The isolated CNCs exhibited an average diameter (D) of 13 ± 3 nm, and a length (L) of 419 ± 48 nm giving rise to an aspect ratio (L/D) of 32. It is worth noting that an aspect ratio larger than 13 induces the formation of an anisotropic phase inside the polymer matrix, resulting in nanocomposite materials with improved mechanical properties(Trache et al. 2017; Kassab et al. 2020a; Bahloul et al. 2021). In addition, the measured D and L of CNCs from CWS were also comparable to those reported

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for CNCs from Saharan aloe vera cactus leaves (Nagarajan et al. 2020) and Nopal Cactus (Vieyra et al. 2015). TEM observations were also used by Habibi et al., to confirm the successful production of CNFs from the prickly pear skin of cactus Fig. 9c and d (Habibi et al. 2009). The authors demonstrated that the mechanical disintegration of pure cellulose from OFI skins after successful passes through Manton Gaulin laboratory homogeniser leads to the production of individualized microfibrils or associated into bundles containing the crystalline and amorphous parts, thus a web like structure is formed as shown below. Its diameter is no more than 2 to 5 nm and it has a length of few microns, suggesting that the obtained CNFs were effectively generated after several pass of CMFs through the homogenizer. Similar outcomes have been published by Malainin et al. (2005). Authors also stated that CNFs were effectively prepared from OFI cladodes using the same mechanical procedure, resulting in a global defibrillation of the fibers with the presence of some bundles. The individual fibrils are almost 5 nm in width and a few micrometers in length. TEM observations were also used by Ramezani et al., to confirm the production of CNFs from the beavertail cactus skin using Masuko commercial grinder (Ramezani Kakroodi et al. 2015). They stated that the measured diameter was in the range of 10–50 nm with only a few micrometers in the length.

4.5 Colloidal Stability The colloidal stability of the prepared nanocellulose is a determining factor for their uses in dispersed systems with improved suspension stability to be used in nanocomposite materials films, foams, and aerogels (El Achaby et al. 2018a). The nanomaterial stability can be determined from conductometric titration and zeta potential (ζ) values that easily indicates their colloidal stability of the suspensions (Kalita et al. 2015; Bahloul et al. 2020). As known, the aggregation of nanocellulose will occur if the ζ value is in the range of -15 to 15 mV. Whereas if the value of ζ is higher than 30 mV and less than -30 mV then the suspension of nanocellulose is considered highly stable, due to the nanometric particles’ electrostatic repulsion (Cheng et al. 2020; Kassab et al. 2020c; Nagarajan et al. 2020). Ait benhamou et al., claimed a ζ value of the prepared CNCs from CWS of about -30 mV and a charge content of about 287.8 mmol·kg−1 indicating a highly stable nanocellulose suspension (Ait Benhamou et al. 2022b). In the same context, Nagarajan et al., prepared a CNCs from saharan aloe vera cactus leaves with a zeta potential value of -45,2 mV showing improved colloidal stability (Nagarajan et al. 2020). Contrastingly, none of the prepared CNFs from cactus plant in previous works have a charge content on their surfaces (Malainine et al. 2003, 2005; Habibi et al. 2009; Ramezani Kakroodi et al. 2015). This is a result of non-chemical modification of the CMFs prior to the mechanical disintegration. Worth mentioning that the functionalization of cellulose imparts their surface with beneficial functions that may

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reduce the consumed energy required for the preparation of nanocellulose resulting in improved colloidal stability during their processing.

4.6 Rheological Behaviour One of the most exciting properties of nanocellulose is their rheological properties, the assessment of their behavior is crucial for their combinations with other materials and can give important information for its possible use in several fields including, wood adhesives, cosmetics and food packaging that have been implemented in recent years (El Achaby et al. 2018b; Kayes et al. 2021). Habibi et al., mentioned that the prepared CNFs from prickly pear skins did not flocculate nor settle down (see above Fig. 3), where the rheological measurements indicated that the CNF suspension in water leads to the formation of a weak gel presenting a typical shear-thinning behavior (Habibi et al. 2009). Malainin et al. stated the same comments for CNFs from OFI cladodes showing a stable (nonflocculating) suspension (Malainine et al. 2005). It is crucial to remember that a CNF suspension below 1% may lead to a flocculated suspension that lose some of their rheological properties as previously reported for CNF from sugar beet pulp (Dinand et al. 1999). Unlike CNFs, CNCs exhibited a totally different rheological behavior. Ait benhamou et al., revealed that the suspension of CNC with a small concentration of 1% exhibited a viscous fluids-like behavior at lower frequencies while it transforms into gel-like system at higher frequencies (Ait Benhamou et al. 2022b). While it behaves as a viscoelastic solid (stiff gel) for concentrations up to 5% since the value of G’ is relatively independent of frequency and significantly higher than the value of G” over the entire investigated frequency range.

5 Potential Applications of Nanocellulose Recently, researchers from throughout the world paid close attention to nanocellulose because of their widespread applications in various fields, including water treatment, nanocomposites, as well as various other environmental applications. Moreover, it is extremely important to note that the application of nanocellulose for various applications can be involved in two separated pathways: the first one involves the use of chemically modified or unmodified as-prepared nanocellulose, and the other one consist of their use as a reinforcement in polymer nanocomposites (Johnsy George 2015). In the development of nanocomposite, nanocellulose have been tested as reinforcement in different matrices due to their surface charge density, high aspect ratio, mechanical strength, and thermal stability (Camarero Espinosa et al. 2013; Bilal et al. 2017; El-shafei et al. 2018). The study of El Achaby et al., showed an improved

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reinforcing capability of S-CNC in three biobased matrices including. The obtained results showed that the addition of S-CNCs significantly enhanced the mechanical properties of the prepared nanocomposite films (El Achaby et al. 2018d). Also, in the case of other semi-synthetic matrices, CNCs indicated remarkable improvement in terms of thermal and tensile properties, as well as optical transparency of nanocomposite-based poly-vinyl alcohol (El Achaby et al. 2018c). Whereas according to Blilid et al., phosphorylated nanocellulose from S-CNC increases the thermal and antibacterial properties of chitosan-based nanocomposite by creating a protective layer on the surface that partially avoid the total degradation of the composite while inhibiting the growth of different microorganisms on composite surfaces (Blilid et al. 2020). Regarding the environmental problems, heavy metals released into the water by different industries become the most hazardous water pollutants (Liu et al. 2015). Cellulose has been considered as a suitable material for water purification to substitute conventional adsorbent regarding its availability with low cost and its high sorption power (El Achaby et al. 2019; Lehtonen et al. 2020). Recently, the selectivity and affinity of cellulose towards heavy metals have been improved by chemical oxidation of S-CNC, owing to the presence of dialdehyde groups on their surface. For instance, Song et al. prepared dialdehyde S-CNC as crosslinking and reinforcing building block in keratin sponge for Pb2+ and Cd2+ removal (Song et al. 2019). The study showed significant improvement of metal sorption velocity, reaching 767 and 517 mg/g, removal of Pb2+ and Cd2+ , respectively. This adsorbent showed excellent recyclability. Also, Lehtonen et al. demonstrated in their work the remarkable selectivity of Fe3+ and Cu2+ using S-CNC as starting material to get phosphorylated nanocellulose as biosorbents (Liu et al. 2015). The study indicated the highest adsorption capacity reaching 99% removal of Fe3+ and Cu2+ from industrial effluent. Other potential applications examples are also reported recently in biomedicine (Trache 2018), food emulsion stabilization (Baek et al. 2018), biochemical separation (Suflet et al. 2006), piezoelectric effect (Csoka et al. 2012), wood adhesives (Khanjanzadeh et al. 2019) and protein adsorption (Oshima et al. 2011). In addition to that, it is important to conclude that nanocellulose play essential roles in developing many added values products used as ecofriendly materials. All of the aforementioned uses provided a superb indication of the need to value cactus plants as a reliable, affordable source for the production of cellulose in general and nanocellulose in particular.

6 Conclusions Nowadays, the production of cellulose nanomaterials from different sources is attracting a huge interest in different fields owing to their several fascinating physicochemical properties. Among several resources, cactus plant is considered as one of such interesting resources known by its abundancy in different zones on the globe. Historically, cactus plant showed several benefits either in biomedical application or food utilisation due to the presence of several compounds with different amounts

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in each part of the plant. This chapter’s main objective focuses on the characterization of nanocellulose from cactus. During this work, we have extensively detailed the origin, history, and social impact of cactus plant. We discussed the chemical composition of each part in the plant and showed the several protocols developed to extract nanocellulose. Finally, several characterization techniques were shown to better understand the successful obtention of nanocellulose from cactus and to easily understand its physicochemical properties. As the cactus plant is available with huge content, we expect that the production of nanocellulose from cactus may shows a new pathway other than its classic uses and could also play a critical role in the creation of products with high added value and attractive features in several fields. Acknowledgements The authors acknowledged the financial assistance of the Materials Science and Nanoengineering Department at Mohammed VI Polytechnic University.

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Corn Crop Residues as Source to Obtain Cellulose Nanocrystals Marcus Felippe de Jesus Barros , Samir Leite Mathias , Robson Valentim Pereira, and Aparecido Junior de Menezes

Abstract The corn cereal besides being a source of food for humans and used in the production of animal feed, is together with its agricultural residues a profitable and rich source of cellulose obtaining cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs). Its properties such as biodegradability, biocompatibility, biobased, high mechanical strength, specific surface area and thermal stability, transparency, viscosity modifier, make it possible to develop materials for various areas such as environmental, electronics, biomedicine, pharmaceutical, materials engineering and others; generating added value to the agricultural residues of the corn crop that was previously incinerated for energy generation and now it is of great scientific interest with applications in nanotechnology and nanoscience. In this chapter will be depicted the obtaining of CNCs from corn crop residues.

1 The Corn Plant Corn is one of the most consumed crops in the world. It is used in human food and is the main source for the production of animal feed, being fundamental for the production of animal protein (Saath and Fachinello 2018). The corn plant belongs to the Poaceae family (Graminae) and the C4-type plant group, first stable organic compound of photosynthesis is a 4-carbon molecule, it has a wide climate adaptation, its maximum productivity is expressed in high temperatures and incident solar radiation, in addition to the adequate amount of water during its production cycle. The corn plant is composed of a large system of fasciculated roots (in bundles), M. F. de Jesus Barros · S. L. Mathias · A. J. de Menezes (B) Grupo de Polímeros Provenientes de Fontes Renováveis (GP2FR), Universidade Federal de São Carlos–UFSCar, Campus Sorocaba, Rodovia João Leme Dos Santos, SP-264—Sorocaba-SP, CEP: 18052-780 São Paulo, Brasil e-mail: [email protected] R. V. Pereira Grupo de Eletroquímica e Polímeros Naturais (GEPN), Universidade Federal do Rio de Janeiro–UFRJ, Campus Macaé, Macaé-Rio de Janeiro, Brasil © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 R. Khiari et al. (eds.), Annual Plant: Sources of Fibres, Nanocellulose and Cellulosic Derivatives, Composites Science and Technology, https://doi.org/10.1007/978-981-99-2473-8_7

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stem, leaf and reproductive structure (Vega and Sadras 2003; Dupuy et al. 2015). The growing up of the corn plant its divided by stages, the vegetative ones (V) and the reproductive ones (R), the plant starts with the vegetative emergence stage (VE) and pass through V1 ~ V21 (each number representing the number of leaves with a collar around the stem) until the Tasseling stage (VT) where occurs the formation of the tassel, the beginning of the reproductive stage that is divided by 6 stages R1 ~ R6 (each number representing a maturity degree of the corn) (Abendroth et al. 2011). The harvesting of corn crops generates agricultural residues (straw and cob) that are generated in large quantities can be used in several areas, such as medicinal (Blackburn et al. 2006; Kwon et al. 2007; Volpicella et al. 2017), obtention CNCs enabling the application as nanofillers in composites (Kanwal et al. 2013; Trigui et al. 2013; Ghosh Dastidar and Netravali 2013; Zhang et al. 2016), bioenergy (Oslaj et al. 2010; Chandra et al. 2012; Berchem et al. 2017; Lizasoain et al. 2017), among other applications (Fig. 1) (Naef et al. 2006; Fescemyer et al. 2013). According to Food and Agriculture Organization of the United Nations (FAO), the five largest corn producers in the world and their respective production values are: 1st United States of America with 360,251,560 tons, 2nd China with 260,670,000 tons, 3rd Brazil with 103,963,620 tons, 4th Argentina with 58,395,811 tons and 5th Ukraine with 30,290,340 tons (FAOSTAT 2020; Coêlho 2021). And the projection for global corn production in 2022 is still uncertain because it didn’t yet consider the impacts of the conflict between Russia and Ukraine (both countries account for 20% of the world’s corn production and as war affects production, consequently, the price of the commodity is under pressure). The projection of the corn production for Brazil (the third largest producer in the world) is promising duo to the increase in the planting area and the high export demand (FAOSTAT 2020; Coêlho 2021; CONAB 2021; International Food Policy Research Institute 2022; Jacintho 2022). The state

Fig. 1 Properties and applications of nanocellulose

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of São Paulo (SP) which occupies the sixth position of the largest corn producers in Brazil will also have a promising 2021/22 crop with the production of 4,596 thousand tons of corn. The increase in production is also due to the increase in the planting area and the favorable corn market (Camargo et al. 2021; Coêlho 2021; CONAB 2022a, b). By the beginning of February 2022 approximately 7% of the planting area was harvested and productivity is within schedule (CONAB 2021). Figure 2 illustrates the map of Brazil with corn production from their respective federative units (CONAB 2022a). Considered a short-cycle, crop corn harvesting generates residues (straw and cob) that are a rich source of lignocellulosic material to produce CNCs. According to the Brazilian Agricultural Corporation (EMBRAPA) (Pessoa and Meirelles 2022), to estimate residue production in corn crop the ratio 1:1 is adopted, that is, for each ton of corn grain one tone of residue will be produced. According to Professor Tony J. Vyn of the Department of Agronomy at Purdue University, the percent of residues after corn grain removal depend on hybrid, management and environment. The total average residue weights production for the North American Corn Belt is approximately 45% (Weil and Vyn 2022).

Fig. 2 Map of Brazilian corn production (data obtained from Conab on March 22nd 2022. The data are available under Open Database License (ODbL) © contributors of the OpenStreetMap. www. opendatacommons.org)

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In short, due to the high production of corn, there is high generation of crop residues, making feasible the production of CNCs, generating added value to agricultural residue that are almost always incinerated for energy generation or are returned to the soil surface of the crop itself as a source of organic matter for the next plantation.

2 Composition of Vegetable Fibers It is one of the complex systems developed by nature that, both in its industrial applications and in its biological functions, are materials with high biodegradability used as a reinforcing agent. Its main components are cellulose, hemicellulose and lignin, but there are also alkaloids, simple carbohydrates, gums, fats, greases, pectins, polyphenols, resins, saponins and terpenes, among other components (Silva et al. 2009). The proportion of the constituents depends on how the fiber was obtained, its growth time and its origin (Moreira 2010). There is the consideration that plant fibers, due to their structure, are naturally occurring composites, as the cellulose fibrils (promote strength and stability) are held together by a matrix of hemicellulose and lignin-acts as a natural barrier to microbial degradation and serves for mechanical protection, as shown in Fig. 3 (John and Thomas 2008; Silva et al. 2009).

2.1 Cellulose Among the natural polymers, we can highlight the wide use of cellulose, a versatile and abundant biopolymer in nature. As it is the most abundant biopolymer, cellulose is the focus of numerous researches in the development of new functional materials and new micro and nanoscopic structures (de Morais et al. 2010; Eichhorn 2011; Klemm et al. 2011; Kiziltas et al. 2016). New structures can be obtained at the nanometer scale by isolating the fibrils and cellulose crystals and using them as reinforcement to improve the properties of composite materials (Bledzki 1999; Auad et al. 2008; Flauzino Neto et al. 2016). In plant cells, cellulose chains are polymerized from glucose chains and arranged into highly organized fibrillar structures called microfibrillated cellulose (MFC), which is composed of cellulose chains arranged parallel to each other and, together with lignin and hemicellulose, represents the main structural component of plants. The microfibrillary nature of cellulose was first confirmed in 1948 by FreyWyssling through electron microscopy observations and since then, several models to describe the arrangement of chains have been proposed (Frey-Wyssling 1954; Manley 1964). In summary, the model accepted and proven by experimental evidence is based on the formation of crystallites composed of cellulose chains in an extended form. The size and shape of crystallites vary according to the source from which the cellulosic material was extracted. Thus, the cellulose microfibrils are composed of

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Fig. 3 Representation of cellulose fiber and its components, cellulose microfibrils, hemicellulose (a glucuronoxylans, b arabinoglucuronoxylan and c arabinoxylan) and lignin. Adapted from (ACS 2012; Brethauer et al. 2020)

a region with highly ordered chains (crystalline phase) surrounded by a region with disordered chains (amorphous phase). The degree of crystallinity of the microfibrils can vary from 65 to 95% depending on the origin of the material from which it was extracted (Dufresne 2017). The Fig. 4 represents the basic chemical structure of cellulose composed of β1,4-glucopyranose rings and intramolecular intermolecular hydrogen bonds that are responsible for keeping the fibers together. In this structure, the adjacent monomers are arranged in such a way that the glycosidic bonds of oxygen point in opposite directions and the repeating unit of the cellulose polymer chain is composed of two β-glucopyranose rings rotated 180° in relation to each other forming the unit known as cellobiose. The cellobiose units are covalently linked forming a long chain of a linear homopolymer. The degree of polymerization of cellulose depends on the source of raw material, ranging from 10,000 to 15,000 units of glucopyranose (de Souza Lima and Borsali 2004; Klemm et al. 2005).

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Fig. 4 Schematic representation of intra/intermolecular hydrogen bonds of cellulose

2.2 Hemicellulose The hemicellulose biopolymer, also known as polyose, is a macromolecule with branched and amorphous structure exposing more hydroxyl functional groups, consequently, absorb more water and are more reactive. It’s a mixture of low molecular mass polysaccharides when compared with cellulose is formed by two or more units of sugars (β-D-glucose, β-D-xylose, β-D-mannose, α-D-galactose and α-Larabinose) and acids (glucuronic acid, galacturonic acid and methyl glucuronic acid) with a variety of structural arrangement (D’almeida 1988; Fornari Junior 2017; Wastowski 2018; Botaro et al. 2019). The most common hemicellulose found in grasses and cereals are: glucuronoxylans (A), arabinoglucuronoxylan (B) and arabinoxylan (C) (Fig. 3) (Ebringerová et al. 2005; Wastowski 2018). It functions as to keep also the celluloses chains together, defining structural properties and plant development (Fornari Junior 2017; Botaro et al. 2019). Corn cob is widely used to extract hemicelluloses. Some applications of hemicellulose are: thickeners, adhesives, emulsifiers, stabilizers, additive in the paper industry, ethanol production, furfural production (base molecule for various chemical products) and other applications (Wastowski 2018; Botaro et al. 2019).

2.3 Lignin The lignin biopolymer (Fig. 3) is a complex macromolecule consisting of an aliphatic and aromatic system composed of phenylpropane unit, in other words it is not considered a carbohydrate. The structural complexity of lignin is due to its extremely amorphous and branched structure, and have several functional groups: methoxyls, hydroxyls, carbonyls, carboxylics, ethers, esters and unsaturations (the presence of functional groups makes lignin very reactive). The precursors of lignin are: trans p-coumaryl alcohol, trans coniferyl alcohol and trans sinapyl alcohol. Its function is to fill the empty spaces between cellulose and hemicellulose by packing in a single fibrous set, increasing the stiffness of the cell wall, transport water and nutrients, reduce cell wall permeability to water and protect the plant against microorganisms (by predominating the phenolic structure, lignin acts as fungicide). Some applications

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of lignin are: dispersants, complexing agents, adhesives, phenolic resins, additives, binders, emulsifiers, adsorbents, surfactants and other applications (D’almeida 1988; Fornari Junior 2017; Wastowski 2018; Botaro et al. 2019).

3 Cellulose Nanocrystals (CNCs) They are high crystallinity α-cellulose particles with at least a dimension equal to or below 100 nm. These are the ordered domains of cellulosic fibers, isolated primarily through acid hydrolysis, and are so named because of their physical characteristics of stiffness, thickness and length (de Souza Lima and Borsali 2004; Shi et al. 2011). The first to report the obtaining of colloidal suspensions, which CNCs form, through acid hydrolysis was the Scandinavian Rånby (Rånby et al. 1949; Rånby 1951), before him, the researchers Nickerson and Habrle (Nickerson and Habrle 1947) observed that the hydrolysis of cellulose fibers with boiling hydrochloric or sulfuric acid attacked the disordered intercrystalline chains after a certain reaction time, but it was Mukherjee (Mukherjee et al. 1952; Mukherjee and Woods 1953) who, using Transmission Electron Microscopy (TEM), observed that after drying the suspension obtained in the hydrolysis there was the appearance of aggregates with a needle-shaped structure where they had the same crystalline structure of cellulose fibers. Currently, nanocrystals are obtained by several experimental methods, such as enzymatic hydrolysis, ultrasound assisted hydrolysis and by ionic liquids (Oksman et al. 2006; Filson et al. 2009), but the most used continues to be the hydrolysis in strong acids (Fig. 5), as they are supported by the fact that the crystalline regions, extreme molecular organization, are very little soluble in acids under the conditions in which they are used, on the other hand, the amorphous phase, due to its natural disorganization, favors the accessibility of acids, so the isolation of the nanocrystals is favored by the hydrolysis kinetics of higher speed in the amorphous phase in relation to the crystalline phase (Azizi Samir et al. 2005).

Fig. 5 Cellulose structure highlighting the crystalline and amorphous regions

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4 Using Corn Residues to Obtain Cellulose Nanocrystals The corn residue (R3) received a treatment before the CNCs extraction, this treatment consisting in a bath of acid chlorite to remove part of the lignin and a bath of sodium hydroxide to remove the hemicellulose, exposing the cellulose and favoring the acid hydrolysis that is used to remove the non-crystalline sequence of the cellulose, leaving the CNCs only (Schuerch 1968; TAPPI 203 cm-99 1999; TAPPI T222 om-02 2006). The cellulose content of the corn residue was determined through the chemical composition measure, reaching a mean percentage of 37.1 ± 2.3% and this isolation is corroborated by the Fourier Transformed Infrared Spectroscopy (FTIR) present in Fig. 6. The removal of lignin can be seen through the intensity’s decrease of the 1370 cm−1 peaks that are assimilated to the twists of the phenolic OH and the 1730 cm−1 that are referent to the C = O stretching, both in the lignin, the glycosidic bonds are maintained even after all the reactions (Smyth et al. 2017), showing that the cellulose remains intact, as can be corroborated by the Fig. 7 that represents the Scanning Electron Microscope (SEM) micrographs of the raw, holocellulose and alpha samples. In the acid hydrolysis of the Fig. 7(C)’s samples occurred the removal of the non-crystalline sequence of the cellulose. The resolution of a SEM cannot reach the nanoscale, so the morphological analysis can be carried out by TEM or Atomic Force Microscope (AFM), the nanocrystals generated by the corn crops is showed in the Fig. 8.

Fig. 6 FTIR spectra from the corn residue (R3)

Corn Crop Residues as Source to Obtain Cellulose Nanocrystals Fig. 7 SEM micrographies of the raw (a), holocellulose (b) and alphacellulose (c) samples

Fig. 8 AFM images of the nanocrystals from corn crop residue

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Studies showed that the average length of the CNCs from corn stalk can variate from 40 to 940 nm, the average diameter from 6 to 25 nm, leading us to an average aspect ratio of 1.6 to 160 depending on the methodology, time of hydrolysis and acid utilized (Souza and Quadri 2014; Huang et al. 2017; Smyth et al. 2017; Mathias et al. 2019). The Fig. 8 shows us an estimate average length of 66.7 nm and aspect ratio of 24.7. The aspect ratio (L/D) of the CNCs shows a notable influence on the quality of reinforcement, a large aspect ratio confers a greater capacity of mechanical reinforcement and short extended nanocrystals ( 150 are usually reported, with very good properties for high carboxylate contents (Isogai et al. 2011). This work paved the way to new pretreatments, involving the grafting of various functional groups with different properties. Among them, cationization with a cationic nitrogen using quaternary ammonium salts (Olszewska et al. 2011), carboxymethylation (Wågberg et al. 2008), phosphorylation with phosphate groups (Ghanadpour et al. 2015), sulfoethylation (Naderi et al. 2017; Zhang et al. 2011) or esterification (Beaumont et al. 2021). The functional groups on the surface of CNF greatly influence their properties and compatibility with polymer matrices, as will be discussed in a following section. The last family of pretreatments considers all the chemical modifications of cellulose involving a ring-opening, i.e. a breakage of a covalent bond in the cyclic anhydroglucose unit by periodate (IO4 − ). This reaction was introduced in 1928 (Guthrie

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and Wolfrom 1962; Malaprade 1928), and its mechanism is therefore well-known. Sodium metaperiodate (NaIO4 ) cleaves the C2 -C3 bond in the AGU by oxidizing their vicinal hydroxyl groups into aldehyde groups, forming the so-called product 2,3-dialdehyde cellulose. This causes an important decrease of cellulose DP and crystallinity, which can be minimized using isopropanol as a radical scavenger (Painter 1988). Although the modified cellulose can be used as such with improved properties (Plappert et al. 2018), the two aldehydes can also be used as reactive sites for further modification, among which: (i) reduction with NaBH4 to produce dialcohol cellulose (Larsson et al. 2014), (ii) conversion of aldehyde groups into carboxylic acid groups with ClO2 (Liimatainen et al. 2012), (iii) creation of imine bonds by nucleophilic addition using an amine (Sirviö et al. 2014), and (iv) sulfonation using sodium metabisulfite (Liimatainen et al. 2013). Periodate oxidation is therefore a very efficient way to produce modified CNF with tuned chemical reactivities, and possible antimicrobial, barrier or proton-exchange properties. However, it must be noted that the chemicals used in the protocols are often toxic, thus limiting their industrialization possibility. The wide variety of available cellulose pretreatments for CNF production is a great asset to tune their properties for the desired application, especially in (nano)composites. But CNF morphological properties also play a major role in the mechanical properties of the final material in which they are used. The different mechanical fibrillation processes have a different effect on the resulting CNF morphology, as will be described in the next section.

2.1.2

Fiber Deconstruction by Fibrillation

The discovery of cellulose nanofibrils in 1983 was closely connected to the highpressure homogenization, initially tailored for the production of emulsions in the food industry. Herrick et al. (1983) and Turbak et al. (1983) understood that the action of mechanical forces on cellulose fibers led to their destructuration into elementary components, which paved the way to a better understanding of the fiber structure and the associated intermolecular bonds. Since then, many efforts have been directed to optimize fiber deconstruction, with the goal of reducing the energy consumption and obtaining truly individual nanoscale elements. These processes originate from industries such as food, pharmacy and cosmetics (homogenization, ultra-fine grinding, etc.), paper (refining), plastics and composites (extrusion, ultrasonication, etc.) or materials (cryo-crushing, ball milling, etc.). High-pressure homogenization processes have been continuously improved, and can be divided into two categories: (i) homogenizers with a valve, and (ii) microfluidizers with a chamber with a given geometry (100–400 μm wide, Y or Z shape). In these devices, the fibrillation is caused both by shear and normal forces, as well as compression and decompression. The mass of treated fibers (dry equivalent) can range from a few grams to a few hundred grams, the solid content during fibrillation is usually low (from 0.5 to 3 wt%), and the pressure varies between 500 and

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Fig. 3 Transmission electron microscopy and atomic force microscopy images of CNF after various mechanical fibrillation processes: a ultra-fine grinder (Qing et al. 2013), b microfluidizer (Qing et al. 2013), c twin-screw extrusion (Baati et al. 2017), d steam explosion (Cherian et al. 2010), e high speed blender (Boufi and Chaker 2016), f cryo-crushing (Wang et al. 2007a). Copyright Elsevier, American Chemical Society, Springer Nature

2000 bar. These processes are very popular in laboratories because they are very efficient in producing well-individualized elements with widths of several nanometers depending on the pretreatment (Fig. 3b) (Olszewska et al. 2011; Liimatainen et al. 2013). However, their specific energy consumptions, i.e. the specific electrical energy required for an efficient CNF production, remain relatively high. In the absence of cellulose pretreatments, a value of 20–30 MWh.t−1 is reported (Spence et al. 2011); this value decreases to around 5–10 MWh.t−1 with the use of carboxymethylation (Naderi et al. 2016) or enzymatic hydrolysis (Rol et al. 2018b). To improve the process flow and overcome the clogging issues often met with homogenizers, ultra-fine grinding was proposed in 1998 as an alternative for CNF production, by Taniguchi and Okamura (1998). Here, the fibrillation is caused by the rotation of a disk (rotor) against another (stator) at high speed (1000–2000 rpm), creating shear and compression forces. These forces are monitored by controlling the gap between the two disks. The viscosity of the fiber suspension plays a major role in the process, because it is expelled by centrifugal forces. However, processing of suspensions at high solid content (usually from 2 to 5 wt%) is possible, which contrasts with homogenization (Fig. 3a). Without pretreatments, energy consumptions of 1.5–3.5 MWh.t−1 were reported (Spence et al. 2011), as well as values of 5.25, 5.75 and 6.75 MWh.t−1 for softwood fibers, wheat straw and recycled newspaper, respectively (Josset et al. 2014). This process is also often used to quantify the

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efficiency of a pretreatment, and has been used with enzymatic hydrolysis (Nechyporchuk et al. 2015), TEMPO-oxidation (Qing et al. 2013), or co-grinding with fillers or tannins (Missio et al. 2020). Ultra-fine grinders and homogenizers are considered as “conventional” processes, as they were first used for CNF production and are suitable for studies at bench scales. However, their low flow (production) rates, poor processing and relatively high energy consumption limit them as suitable candidates for industrial use in CNF production. Among the other processes available for large scale CNF production, refining is one of the most promising ones. This process, well known in the papermaking industry, creates morphological and structural changes to the cellulosic fibers to improve the mechanical and barrier properties of papers. This is done by applying energy to the fibers (compression and shear forces) with the rotation of a rotor disk against a stator disk, both with specially designed geometries (succession of bars and grooves). Therefore, the destructuration mechanism is comparable to the ultrafine grinding mentioned above, with the difference that refining is tailored for larger quantities of fiber, creates various flows and forces in the process due to the bar geometry, and works at a higher fibrillation gap, of around 100 μm (Bordin 2008; Roux 2008). The main effects of refining on pulp properties are (i) the creation of new surfaces by external and internal fibrillation, (ii) the creation of new particles, and (iii) structural and surface modifications (Roux 2008; Gharehkhani et al. 2015). When used as a main fibrillation process for CNF production, it usually results in poorly fibrillated CNF bundles, with widths above 200 nm and a few nanoscale elements (Afra et al. 2013; Karande et al. 2011; Tonoli et al. 2012). Combined with an efficient pretreatment, such as periodate-chlorite oxidation, suspensions with abundant nanoscale elements can be obtained (width < 100 nm) and with a reduced energy consumption of 461 kWh.t−1 (Kumar et al. 2020). The efficiency and stability of the process are still two of the major issues to be addressed. Finally, other processes used for CNF production, often classified as “nonconventional”, are well described in the literature (Wang et al. 2021). Twin-screw extrusion, for example, has been used for this purpose by Suzuki et al. (2013) (fibrillation in-situ for compounding with thermoplastic polymers), and since 2015 by Ho et al. (2015) (fibrillation of cellulose fibers alone). These processes have a harsh effect on the cellulose structure, with a decrease of crystallinity and degree of polymerization, but reduce the energy consumption, and increase the flow rates and allow high solid contents (Rol et al. 2018b). They can notably be coupled with pretreatments such as enzymatic hydrolysis (Rol et al. 2020, 2017), TEMPO-oxidation (Baati et al. 2017; Trigui et al. 2020), cationization (Rol et al. 2019a) or phosphorylation (Rol et al. 2019b). Other mechanical fibrillation processes have been proposed for CNF production, such as steam explosion (Cherian et al. 2010; Deepa et al. 2011; Liu et al. 2017), ultrasonication (Santucci et al. 2016; Wang and Cheng 2009), high-speed blenders (Boufi and Chaker 2016; Uetani and Yano 2011), cryo-crushing (Wang et al. 2007a; Dufresne et al. 1997), ball milling (Zeng et al. 2020; Zhang et al. 2015) or aqueous counter collision (Kondo et al. 2014; Tsalagkas et al. 2018) (Fig. 3c–f). The number of available mechanical treatments for CNF production ensues from the

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constant aim to optimize the process, and several of them are promising as far as industrial adoption.

2.1.3

Towards Sustainable and Energy-Efficient Processes

For the industrial production of CNM, the main cost driver remains the cost of the raw material, whereas the second driver is the energy consumption of the processes (Assis et al. 2018). For an industrial feasibility, pretreatments also need to avoid the use of toxic chemicals. Both aspects (toxicity and energy consumption) are increasingly considered, and efforts are made toward sustainable processes. Several green pretreatments, such as enzymatic hydrolysis with enzyme cocktails, have gained interest over the past decade. There is a trend towards using several enzymes with complementary functions, with good results with e.g. the synergistic action of cellulase and xylanase (Long et al. 2017). A recent study by Cebreiros et al. also underline the potential of non-catalytic enzymes, such as swollenin (Cebreiros et al. 2021). This enzyme also has a strong synergism with xylanase, and enables the disruption of xylan’s loosely ordered structure (Gourlay et al. 2013), with a xylan hydrolysis of 84.6% in optimal conditions (Cebreiros et al. 2021). In turn, the efficient removal of hemicelluloses allows a higher cellulose accessibility as measured by Simon’s stain and WRV, which improves the penetration of catalytic enzymes in the fiber structure. Deep eutectic solvents and ionic liquids are also promising routes that rely on fiber swelling, with the use of recyclable solvent. Novel routes for cellulose functionalization are also often proposed, such as the recent reversible esterification reported by Beaumont et al. (2021) (Fig. 4a). The esterification reaction takes place in a water/acetone solvent, using N-succinylimidazole as an acylating agent, and results in CNF bearing carboxyl moieties that enhance their individualization. Interestingly, this reaction is reversible, with a mild alkaline treatment cleaving the ester and yielding the original hydroxyl group. Therefore, the authors claim that when the alkaline treatment is done after CNF individualization, it enables to recover the original network of hydrogen bonds between individual CNF, with extremely high mechanical properties. This method thus seems promising for obtaining either functionalized or unmodified CNF in an energy-efficient manner. Finally, optimizing the (bio)chemical pretreatments is necessary, but not sufficient alone. The key part of the process remains the fibrillation using various devices with a wide variety of geometries. Due to the large number of available processes, and the fact that CNF production has sparsely been studied from a process engineering perspective, very little information on the rheology and forces in those processes is available in the literature. In addition, it is commonly admitted that combining several processes has a beneficial effect on CNF morphology and energy consumption, with only few evidences in that direction (Qing et al. 2013; Rol et al. 2018b; Nakagaito and Yano 2004). In a recent study, Banvillet et al. (2023) proposed a systematic study of four processes alone or in combinations: refining, ultra-fine grinding, homogenization and twin-screw extrusion (TSE) (Fig. 4b). The quantification of CNF properties and energy consumption enabled a quantitative comparison. Although 8–14 nm wide

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Fig. 4 Schematic representations of a regioselective and reversible esterification as a novel CNF pretreatment (Beaumont et al. 2021), Copyright American Chemical Society, and b combinations of four mechanical fibrillation processes to reduce the energy consumption during CNF individualization (Banvillet et al. 2023), Copyright Springer Nature

CNF were obtained for all processes, the presence of residual micrometric fragments for refining and TSE proved the lower fibrillation efficiency, counterbalanced by a lower energy consumption. In addition, the authors found that process combinations are relevant only for some cases, e.g. refining followed by ultra-fine grinding that led to very homogeneous CNF with high transparency. In the case of TSE, a previous mechanical fibrillation was only detrimental, the process being more efficient when directly using enzymatically treated fibers. This was explained by the fixed gap inside this process, which is more sensitive to the modification of fiber morphology, aggregate size and suspension rheology. Overall, more work is needed in this field to understand, therefore optimize the physical phenomena at work during CNF production.

2.2 Cellulose Nanocrystals The discovery of CNC dates back to 1947, when Nickerson and Habrle (1947) observed that cellulose fibers subjected to sulfuric or hydrochloric acid hydrolysis led to a preferential degradation of disordered regions of cellulose. This work was carried on by Rånby (1951), who reported morphological analysis of CNC using electron microscopy and showed rod-like shapes and nanometric dimensions. Modern CNC production processes are still mainly based on this chemical reaction, i.e. the acid hydrolysis of cellulose by mineral or organic acids, followed by purification and individualization by sonication. Many alternative production routes also exist, which will be briefly described in this section and are summarized in Fig. 5. Several reviews on the subject are available, with detailed information on these routes (Vanderfleet and Cranston 2021).

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Fig. 5 Classification of cellulose fibers hydrolysis routes for CNC production, adapted with permission from Vanderfleet and Cranston (2021). The drawings indicate the usual state of dispersion and surface charge for each process family. AFM and TEM images reproduced with permission from Chen et al. (2015); Camarero Espinosa et al. (2013); Yu et al. (2013); Spinella et al. (2016); Braun and Dorgan (2009); Du et al. (2016); Chen et al. (2016); Sirviö et al. (2016); Tan et al. (2015); Miao et al. (2016); Zhou et al. (2018); Pereira and Arantes (2020). Copyright Springer Nature, Royal Society of Chemistry, American Chemical Society, Elsevier

2.2.1

Hydrolysis and Functionalization

The mechanism behind CNC production is the hydrolytic cleavage of the glycosidic bonds of cellulose chains by hydronium ions (H3 O+ ). The accessibility of cellulose to these ions is higher in the disordered regions, which are hydrolyzed first, leaving intact the crystalline parts. CNC therefore comprise the crystalline regions of cellulose elementary fibrils that remain after the hydrolysis, while the disordered regions are dissolved and removed by purification. This mass loss inevitably leads to lower process yields than for CNF, which are composed of these pristine elementary fibrils and their aggregates. The three main properties that are assessed to describe the efficiency of a CNC production process are (i) the crystallinity index, (ii) the nanoscale dimensions and (iii) the colloidal stability of CNC as aqueous suspensions (Vanderfleet and Cranston 2021). The use of sulfuric acid hydrolysis is still, by far, the preferred strategy for CNC production (Dufresne 2017a). Its high efficiency is due to the fact that sulfuric acid is a strong acid, meaning that the dissociation of hydronium ions is complete, favorizing a fast reduction of the degree of polymerization until the levelling off degree of polymerization LODP (Nickerson and Habrle 1947). Sulfuric acid also causes an

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esterification reaction on the hydroxyl group bore by the C6 of the AGU, resulting in the presence of half-ester sulfate groups on the surface of the obtained CNC, with an usual charge content of 0.1–0.3 mmol/g (Lin and Dufresne 2014). These groups drastically improve CNC colloidal stability due to electrostatic interactions. Based on a large number of studies, the optimal reaction conditions using sulfuric acid include an acid concentration of 64 wt%, 30 min hydrolysis time at a temperature of 40–45 °C (Vanderfleet and Cranston 2021). The acid to cellulose ratio is also of great importance, and a value around 0.1 g of pulp per mL of acid usually gives the best compromise between a harsh enough treatment, and the preservation of crystalline regions (Beck-Candanedo et al. 2005). The hydrolysis is then followed by a quenching step with water, a centrifugation step to remove the acid, followed by a dialysis step to remove any trace of residual acid. The CNC are then individualized using an ultrasonication device, yielding a well-dispersed CNC suspension. All these steps are used in other production routes described below, and a variation of one parameter can have a major effect on the yield or CNC properties (Chen et al. 2015). CNC obtained by this conventional method display lengths of 100–400 nm and widths of 5–20 nm depending on the cellulose source (Vanderfleet and Cranston 2021). A recent study by Babi et al. (2022) using fluorescence microscopy confirms a good correlation between the length of crystalline regions in the native cellulose and that of the resulting CNC. Overall, there is a trend towards tuning the reaction conditions to optimize the yield and CNC properties, which will be described in the following section. Other mineral acids can be used for the cellulose hydrolysis, such as hydrochloric acid (HCl), which is also a strong acid. By this means, higher yields can be reached compared to sulfuric acid, as high as 93% when using a hydrothermal treatment enabling the penetration of the acid (Yu et al. 2013). The yield can reach 97% when using vapor HCl (Kontturi et al. 2016), which will be described in the following section. However, CNC do not bear any surface charge with these methods, and therefore exhibit a poor colloidal stability. Other mineral acids combine the grafting of functional groups and high yields (76–80%), such as phosphoric acid (Camarero Espinosa et al. 2013). The presence of phosphate groups also leads to an enhanced thermal stability. However phosphoric acid is a weak acid, resulting in relatively unstable CNC, and a low phosphate content of 0.008–0.05 mmol/g (Vanderfleet et al. 2018). In order to reduce processes environmental impacts, and provide a wider variety of functional groups, the use of organic acids and acid blends has also been proposed. Oxalic acid and maleic acid can be used (Chen et al. 2016), with the advantage of grafting novel chemical functions compared to sulfuric and phosphoric acids, resulting in a good colloidal stability (Fig. 5). However low yields are reported with these methods (maximum 25%), mainly because these weak acids do not enable a total hydrolysis of the disordered regions of cellulose. The use of formic acid has also been reported, with a higher yield of 75% (Du et al. 2016). However, none of these routes results in a good balance between CNC properties (e.g. colloidal stability) and a high yield. To address this issue, acid blends seem to be a promising strategy: in this case, a strong acid is used to provide a sufficient proton concentration, while

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an organic acid is used for grafting functional groups by esterification. HCl can be coupled with citric, malonic or malic acids (Spinella et al. 2016), resulting in CNC with lengths of 220 nm and widths of 12 nm. The use of HCl and acetic acid has also been reported (Braun and Dorgan 2009), yielding slightly larger aggregates. Two mineral acids can also be coupled, such as sulfuric and phosphoric acids, resulting in presence of both sulfate and phosphate half-esters groups (Vanderfleet et al. 2019). This strategy of acid blends is considered to be a promising and growing research area, as it can lead to a high crystallinity, nanoscale dimensions, and a high colloidal stability with a relatively high yield (Vanderfleet and Cranston 2021). Several pretreatments for CNF production can also be used to produce CNC, when followed by a (thermo)mechanical treatment. For example, TEMPO-oxidation can be used for CNC production, by simply submitting TEMPO-oxidized fibers to sonication for 10–120 min (Zhou et al. 2018). The resulting CNC, with average lengths around 200 nm and widths of 3.5 nm, exhibit a drastically higher charge content (1.7 mmol/g) than sulfated CNC, resulting in a very high colloidal stability. In addition, they have a larger aspect ratio compared to their sulfated equivalent, and the process yield is also higher (69–94%). Another acid-free method that derives from a CNF pretreatment is the periodate oxidation (Yang et al. 2015a), that can be followed by reduction with NaBH4 to yield dialcohol cellulose (Leguy et al. 2018). The oxidation occurs preferentially in disordered regions of the cellulose elementary fibrils, that can be solubilized after heating the suspension at 80 °C for 6 h (Yang et al. 2015a) or using a sonication treatment (Leguy et al. 2018). The resulting CNC are described as sterically stabilized by hemiacetal linkages, with a core–shell structure with the presence of oligomers at their surface (Leguy et al. 2018). In addition, ionic liquids have been applied for CNC production (Man et al. 2011), with the sub-family of deep eutectic solvents (DES) being the most promising, as they are more environmentally friendly. The DES choline chloride/oxalic acid dihydrate, followed by a mechanical disintegration, results in CNC with lengths of 310 − 410 nm and widths of 9 − 17 (Sirviö et al. 2016). An optimization of the reaction parameters during a DES treatment has recently been proposed, using a design of experiments approach (Douard et al. 2021). The resulting yield and crystallinity remained slightly lower than the sulfuric acid reference (44% and 81%, respectively). Lastly, CNC can be produced without the use of acid, oxidizing agent or solvent, by solely using enzymatic hydrolysis. This method, very efficient to weaken cellulose fibers for CNF production, however is less efficient when targeting CNC. This is due to a lower accessibility to cellulose chains in the disordered regions compared to acids, and an overall less harsh hydrolysis action. CNC produced using enzymes are therefore generally rough and irregular (Domingues et al. 2016), with lengths of 100–600 nm and widths of 10–30 nm (Domingues et al. 2016; Tong et al. 2020). It should however be noted that this reaction can be done at high solids content, on agricultural residues, and avoiding a sonication treatment (Pereira and Arantes 2020), which are key considerations when targeting a large scale production.

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Towards a Greener Chemistry and Higher Yields

When considering CNC production at an industrial scale, the yield needs to be high enough for the process to be cost-effective. Even a medium yield of 50% means that half of the raw material is lost in the process as dissolved sugars in the acid, therefore hard to recover. This also causes difficulties to recycle the acid, although some efforts are being made to re-use it and valorize the sugars for biofuel production, notably in the process of CelluForce Inc. (Vanderfleet and Cranston 2021). But several strategies propose to increase the yield by tuning the process conditions, using a high reaction time and/or temperature. Yu et al. (2013) obtained CNC with lengths of approx. 250 nm and widths of 16 nm when using a liquid HCl hydrolysis at 110 °C for 3 h, while maintaining a crystallinity of 88.6%. An even higher yield of 97% was obtained by Kontturi et al. (2016), using HCl vapor at room temperature. The authors explain this result by the enhanced and rapid penetration of gaseous acid into the fibers, and with the hypothesis that in the absence of water a crystallization occurs simultaneously with cellulose hydrolysis (Fig. 6a). For now, these high yields are obtained at the expense of the CNC properties, resulting in unstable and aggregated CNC. The large number of process steps remains a major challenge for CNC production at an industrial scale, as well as the high energy consumption of the processes involved. A life cycle assessment determined that sonication requires a substantially higher energy than homogenization for the individualization of CNC (Li et al. 2013). Following this work, some studies have used homogenization as a main mechanical process, resulting in good CNC properties when coupled with, for example, cellulose esterification with oxalic acid dihydrate (Henschen et al. 2019). To go even further, there is a trend towards using one-step processes, which hydrolyze, functionalize and individualize cellulose into CNM at the same time. This is made possible by bringing mechanical energy (friction) to the raw material in the presence of reactants, in a process called mechanochemistry. This approach was used by Lu et al. (2015), who used acetic acid and 4-dimethylaminopyridine to esterify cellulose in a ball-milling device, reaching a degree of substitution of

Fig. 6 Schematic representations of a CNC production using gaseous HCl and the potential crystallization during the treatment (Kontturi et al. 2016), Copyright John Wiley and Sons, and b mechanochemistry process for the simultaneous esterification and hydrolysis of cellulose (Lu et al. 2015), Copyright Royal Society of Chemistry

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approx. 0.22 (Fig. 6b). More recently, Kang et al. proposed a cellulose esterification with hexanoyl chloride following a similar strategy (Kang et al. 2017). Although these processes resulted in CNC with nanometric dimensions, little information is reported on the colloidal stability their functional groups bring. In addition, such processes still require an additional ultrasonication step (Lu et al. 2015), or the use of an organic solvent (Kang et al. 2017). Despite this, many efforts are being deployed to combine existing CNC production processes with mechanochemistry (e.g. deep eutectic solvents). Overall, the processes that use dry conditions, high temperatures and mechanical energy are considered promising.

2.3 Two Families with Different Properties 2.3.1

Structure and Morphology

Although their chemical compositions are close, CNC and CNF have very different properties. CNC’s rigid structure and high crystallinity confer them high mechanical properties, while CNF’s high aspect ratio facilitates more entanglement in the final material. The following section summarizes the main properties that influence the characteristics of the final material, underlining the effects of CNM structures, morphologies and chemical properties. An overview of the key properties is given in Table 1. The distinction between CNF and CNC based on the sole differences of production process is not always sufficient. The aspect ratio (length/width) is often the most relevant property: CNC have aspect ratios of 5–30 (typical length of 50–350 nm), CNF have aspect ratios of 10–100, and above (typical length above 1 μm) (Foster et al. 2018). The CNC production processes consist mainly in the hydrolysis of disordered domains of cellulose fibrils, and the final material contains more crystalline domains than the original cellulose fibers, in proportion. CNC have thus crystallinity indices of 69–89% (Hamad and Hu 2010), while bleached kraft cellulose fibers have a crystallinity index around 55% (Ahvenainen et al. 2016). The crystallinity of CNF Table 1 Mechanical and structural properties of the two nanocellulose families. EL : longitudinal and ET : transverse moduli Material

Aspect ratio

Crystallinity Degree of EL (GPa) index (%) polymerization

ET (GPa)

Specific surface area (m2 .g−1 )

CNF

10–100

48–67

200–300

6.9

65–195

CNC

5–30

69–89

100–200

References (Foster (Ahvenainen (Rol et al. et al. 2018) et al. 2016; 2018b; Hamad Hamad and and Hu 2010) Hu 2010)

81–84 145–150

18–50

439–540

(Cheng et al. 2009; Iwamoto et al. 2009)

(Lahiji et al. 2010; Parvej et al. 2020)

(Brinkmann et al. 2016; Spence et al. 2010)

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is usually 48–67% (Ahvenainen et al. 2016), and depends both on the pretreatment and fibrillation process. For example, enzymatic hydrolysis is known to increase the CNF crystallinity by hydrolyzing the disordered regions of cellulose (Nechyporchuk et al. 2015), while periodate oxidation drastically reduces the crystallinity, due to important disruptions in the intermolecular hydrogen bonds network (Kim et al. 2000). The degree of polymerization is also affected by the production processes, and is usually lower for CNC than for CNF, again due to the cellulose chain cleavage by hydrolysis. This property must be precisely assessed: there is a clear correlation between a low degree of polymerization and low mechanical properties, notably tensile strength (Henriksson et al. 2008). The structural properties greatly influence the mechanical properties of CNM, that can be measured by atomic force microscopy (AFM). Nanoscale three points bending tests lead to elastic moduli in the range of 81–84 GPa for CNF (Cheng et al. 2009), and 145–150 GPa for CNC (Iwamoto et al. 2009), which is drastically higher than the modulus of a single plant fiber such as jute, cotton or flax, usually between 5–27 GPa (Dufresne 2017b). In comparison, the elastic moduli of Kevlar and carbon fibers are 60 GPa and 240–425 GPa, respectively. This places CNM as good candidates for mechanical reinforcement applications in the materials industry. The transverse modulus can also be measured with AFM force-distance curves, when compared to 3D finite element calculations of tip indentation (Lahiji et al. 2010). This method gives values between 18–50 GPa for CNC, while a value of 6.9 GPa has been reported for CNF (Parvej et al. 2020). A decrease of the size is also geometrically associated with an increase of the specific surface area. The latter is therefore higher for CNM than for individual wood fibers, for which values of 1–2 m2 .g−1 are usually measured (Fan et al. 1980). Values between 65–195 m2 .g−1 for CNF and 439–540 m2 .g−1 for CNC have been reported, though these values can greatly vary depending on the measurement technique (Dufresne 2017c). Specific surface area also plays a major role in the materials’ properties, because it defines the fraction of CNM in contact with each other (hydrogen bonds) or with the polymer matrix (adsorption or covalent bonds). As discussed in the previous sections, CNM suspensions are usually multi-scale, with residual fiber fragments or CNM bundles depending on the process. This heterogeneity negatively affects the specific surface area, therefore the reinforcement potential of CNM: it is one of the key challenges that need to be solved when processing CNM-reinforced nanocomposites, along with their dispersion inside the matrix.

2.3.2

Chemical Reactivity

The diversity of CNM production routes leads to an incredible variety of possible functional groups on the cellulose surface. Consequently, the chemical reactivity of CNM strongly depends on the production process, and is the second main characteristic affecting the materials’ properties, after CNM morphology and structure.

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Cellulose itself is strongly hydrophilic, due to the presence of three polar hydroxyl groups on the carbons C2 , C3 and C6 of the AGU. In addition, the presence of hemicelluloses, which also possess polar groups such as hydroxyl groups and carboxylic acids, tends to increase the hydrophilicity of cellulosic materials (Solala et al. 2020). Residual lignin tends to do the opposite and decreases their hydrophilicity, due to its relatively hydrophobic chemical nature (phenyl propane units with one or several methoxy groups) (Solala et al. 2020). The chemical composition of the raw material, and the presence of residues after purification processes, play therefore a major role in the chemical reactivity of CNM. Several CNM production processes leave the chemical structure of cellulose intact: enzymatic hydrolysis, fiber swelling (CNF) or HCl hydrolysis (CNC), for example. But most production routes involve the substitution of one or several hydroxyl groups by, among others, carboxylic, carboxymethyl, phosphate, sulfate, or aldehyde groups (Figs. 2 and 5). In the objective of elaborating CNM based polymer nanocomposites, the question is: “how can compatibility between CNM and organic polymer matrices be improved”? Indeed, when used as composites reinforcement, the main issue is the compatibility with hydrophobic polymers, CNM being hydrophilic due to their numerous hydroxyl groups. These issues are tackled in the following section on CNM functionalization in engineering (bio)plastics, as well as the effect of each (nano)composite production process on their mechanical properties.

3 Cellulose Nanomaterials in Engineering (Bio)Plastics 3.1 Modification of CNM for Thermoplastic Composites As described in the previous section, the surface polar groups and high specific surface area of cellulose nanomaterials (CNM) (Table 1) (Brinkmann et al. 2016; Spence et al. 2010; Gardner et al. 2008; Dufresne 2013; Moon et al. 2011) promotes a natural tendency for CNM to agglomerate when dispersed in thermoplastics (TPs), which are typically hydrophobic. This is of significance since CNM dispersion at the nanoscale is a key factor in the development of nanocomposites (Trache et al. 2020; Abitbol et al. 2016). Since CNM produced by chemical or enzymatic pretreatments allows for the insertion of reactive functional groups (Figs. 2 and 5), the challenges of compatibility and direct dispersion in TP matrices can be addressed. In this regard, the main CNM functionalization strategies include those applied prior (chemical, and physical–chemical) or during (coupling reactions) processing of the TPs (Fig. 7). Physical, enzymatic and chemical means to modify CNM after their exctraction (Rana et al. 2021; Ng et al. 2017) can enhance dispersion in TPs, but chemical and physical–chemical surface modifications are more typically used. They endow a range of functional groups and reactivity with the given TPs (Wang et al. 2020; Missoum et al. 2013), enhancing the compatibility of the CNM with polymer matrices, or limiting agglomeration by steric hindrance (Chanda and Bajwa 2021).

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Fig. 7 Main chemical and physical–chemical functionalization approaches to improve the compatibility/reactivity of CNM and their dispersion in thermoplastic matrices

Examples of surface modification include silanization and silylation, esterification, polymer grafting, adsorption of surfactants and wrapping with (co)polymers (Fig. 7). Herein, we focus on chemical and physical–chemical functionalization but physical (Trache et al. 2020; Chanda and Bajwa 2021) and enzymatic processes (Trache et al. 2020; Afrin and Karim 2017) are also relevant.

3.1.1

Chemical Functionalization

Silanization and silylation enable grafting organosilanes on the surface of CNM using alkoxysilane and silyl molecules, respectively. Organosilanes, which are commercially available, are versatile coupling agents that improve the interfacial compatibility between natural fibers and polymer matrices (Xie et al. 2010; Castellano et al. 2004). CNM modification can be carried out with water/alcohol media, e.g., avoiding hazardous organic solvents (Wang et al. 2016). The alkoxy moiety of organosilanes must first be hydrolyzed into silanols that react with CNM’s hydroxyl groups under appropriate conditions (temperature and time) (Robles et al. 2015; Kim et al. 2019). A variety of organosilanes can achieve covalent bonding according to the matrix: for polar matrices, aminosilanes improve the reactivity with the polymer chain end-groups (Xu et al. 2018; Yin et al. 2018). In the case of non-polar matrices, silanes

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bearing vinyl groups generate free radical grafting with the polymer (Mokhena and Luyt 2014). CNM dispersion in linear low density polyethylene (LLDPE) can be facilitated by grafting silane-bearing isocyanates (Anžlovar et al. 2020). Esterification involves the reaction of CNM hydroxyl groups with acids (anhydride and acid chloride) in the presence of catalysts, leading to ester groups formation on the CNM surface (Rana et al. 2021; Missoum et al. 2013). Surface esterification makes CNM hydrophobic, preventing aggregation in nonpolar matrices. A wide variety of acids has been reported for this purposed; for example, CNC has been modified with dodecanoyl chloride (Robles et al. 2015), benzoic acid (Shojaeiarani et al. 2018) or valeric acid using 4-dimethylamino pyridine as a catalyst (Shojaeiarani et al. 2019), improving CNC dispersion in polylactic acid (PLA). CNF modified with aliphatic, cyclic and branched chlorides have also been reported for making composites with high density polyethylene (HDPE) (Yano et al. 2018). Depending on the concentration of reagents and catalyst, the degree of substitution can vary (from 0.2 to 3) (Herrera et al. 2015). Recently, green esterification has been reported using acid solvents prior to CNM drying, avoiding the use of organic, hazardous counterparts (Shojaeiarani et al. 2018; Espino-Pérez et al. 2014). Esterification via acetylation involves the formation of hydrophobic acetyl groups on the surface of CNM, a process widely used to produce commercial cellulose derivatives (Missoum et al. 2013). This chemical modification is usually carried out in several steps using organic solvents, such as acetic acid and acetic anhydride with catalysts (Igarashi et al. 2018; Jonoobi et al. 2012). A simple, one-step reaction mechanism with simultaneous hydrolysis and acetylation of alkali-treated microcrystalline cellulose has been reported (Xu et al. 2020), leading to acetylated cellulose nanocrystals (ACNC) with tailorable size, shape, thermal stability and surface energy (Xu et al. 2020). The ACNC can be incorporated in polymers either by solvent casting (Tingaut et al. 2010), melt-mixing (Jonoobi et al. 2012) or using continuous processes by solvent-assisted centrifugation (Xu et al. 2016). ACNC have been incorporated into polyesters, such as polybutylene succinate (PBS) (Hu et al. 2015) or poly(3-hydroxybutyrate-co-4hydroxybutyrate) (PHB) (Gan et al. 2017). Overall, CNC dispersion in polyester matrices is substantially improved with the increased degree of acetyl substitution, as shown by Gan et al. using Raman mapping spectra (Gan et al. 2017). Polymer grafting uses an agent that is anchored on the surface (Roy et al. 2009; Wohlhauser et al. 2018). For CNM, two approaches can be highlighted: grafting to and grafting from (Roy et al. 2009). Grafting to involves covalent grafting of long polymer chains bearing reactive groups that react with CNM’s hydroxyl groups. Grafting from involves the growth of polymer chains from an initiating site on the CNM surface, for example, by atom transfer radical polymerization (ATRP) (Morandi et al. 2009; Zoppe et al. 2011), ionic or ring-opening polymerization. Grafting from is most widely used because it can be carried out during composite processing (“in-situ polymerization”). In addition, a well-controlled polymer graft length and high polymer grafting densities are possible. However, grafting from is limited by the type of polymer chains that can be grown. The monomers which have been used so far include (i) ε-caprolactone (Habibi et al. 2008; Goffin et al. 2011), Llactide (Goffin et al. 2011; Lizundia et al. 2016), D-lactide (Wu et al. 2016; Muiruri

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et al. 2017) by Ring Opening Polymerization, (ii) acrylic monomer (Wohlhauser et al. 2018) by surface-initiated free radical polymerization, (iii) styrene monomer (Missoum et al. 2013; Chanda and Bajwa 2021; Yi et al. 2008) by ATRP. Grafting to offers the advantage that a wide variety of polymer types can be grafted, with good control on molecular weight, given that it is pre-defined (before grafting onto CNM). However, low grafting densities are usually produced due to the steric hindrance of polymer chains (Chanda and Bajwa 2021; Clarke et al. 2019). The grafted CNM can be dispersed in polymer matrices by solvent and/or melting with polymer pellets (Goffin et al. 2011; Morelli et al. 2016a), leading to the given nanocomposites, such as those that use poly(ε-caprolactone)-grafted CNC (Habibi et al. 2008) or CNF (Zoppe et al. 2009) and PLA-grafted CNC (Lin et al. 2009). Some efforts report on silanization and oligomer grafting by first synthesizing polysiloxanes (via hydrolysis and condensation of alkoxysilanes), followed by grafting onto the surface of CNC for further dispersion in poly(butylene adipate-co-terephthalate) (PBAT) (Dhali et al. 2022).

3.1.2

Physical–Chemical Functionalization

Surfactants can be used to improve the compatibility of nanoparticles with polymeric matrices (Tardy et al. 2017). The anionic groups on CNM surfaces, originating from their chemical modification during production (sulfate, carboxylate, phosphate, etc.), allow ionization according to the pH value (Kalashnikova et al. 2012). Therefore, anionic (Bondeson and Oksman 2007a) or cationic surfactants can be adsorbed on CNM by electrostatic interactions. For example, Nagalakshaiah et al. adsorbed fatty ammonium salts on CNC to improve the affinity with polypropylene (PP) (Nagalakshmaiah et al. 2016). Fujisawa et al. proposed the adsorption of amine terminated-polyethylene glycol (NH2-PEG) on the surface of TEMPO-oxidized CNF via ion-exchange treatments followed by dispersion in poly(L-lactic acid) (PLLA) (Fujisawa et al. 2013). Overall, this strategy has been extensively explored for CNC and, to a lesser extent for CNF (CNC production often result in high surface charges, while common production processes lead to unmodified CNF). Non-ionic surfactants, including block copolymers, are also good candidates given that the hydrophilic blocks interact with CNM, e.g., by hydrogen bonding, and the hydrophobic block with the polymer matrix (HDPE (Sakakibara et al. 2016) or LLDPE (Nagalakshmaiah et al. 2016), for example). Other surfactants involve non-ionic interactions, such as lipophilic sorbitan monostearate, which was reported to improve the stability of CNC in organic solvents, before dispersion in polystyrene (PS) (Kim et al. 2009). Polymer coating or wrapping uses a principle similar to that of non-ionic surfactants, namely, adsorption of polymer chains on CNM surfaces. This enables a greater surface coverage compared to that achieved with surfactants. However, the CNM dispersion and adsorption in a dissolved coating polymer is often difficult to achieve. As polymer coating is obtained by a solvent route, water soluble polymers are often used in aqueous suspensions of CNM: poly(ethylene oxide), polyoxyethylene (Azouz et al. 2012; Lin and Dufresne 2013) or poly(vinyl alcohol) (PVOH or PVA)

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(Bondeson and Oksman 2007a). Some reports indicate polyoxyethylene-adsorbed CNC to enhance the thermal stability and compatibility with the polymer matrix (Alloin et al. 2011). The improved thermal stability of polyoxyethylene-adsorbed CNC was ascribed to the protective role of the adsorbed polyoxyethylene (PEO) chain. To improve the affinity between the CNC and the modifier, chemical and physical grafting with poly(ethylene glycol) followed by adsorption of PEO have been used. Finally, chain entanglement between PEO and PEG, forming a multi-layer surface, enhances the compatibility of CNC with non-polar PS matrices (Alloin et al. 2011).

3.1.3

Coupling Agents

Coupling agents directly functionalize CNM to compatibilize it with thermoplastics during processing. One of the most common coupling agents include maleic anhydride-functionalized polymers, such as those based on polypropylene (Peng et al. 2016; Wang et al. 2018), polyethylene or poly(lactic acid). Such polymers are added during melt processing whereby the maleic moiety interacts with the surface of the CNM, and the polymer chain entangle with the polymer matrix. Glycidyl methacrylate (GMA) has been used as coupling agent (Yang et al. 2015b), for example, by grafting GMA onto PLA in a pre-melting step and mixing the PLA-g-GMA in the molten state with CNC, in a second step to lead to a masterbatch.

3.2 CNM as Reinforcing Agents in Thermoplastic Composites CNM dispersion plays a key role in the development of the final properties of the given (nano)composites (Fig. 8). Two main routes to reinforce thermoplastics are discussed, namely, solvent and melt processing. The effect of CNM properties and the state of dispersion on the mechanical behavior and performance of thermoplastic composites is considered next.

3.2.1

Solvent Processing

CNM reinforced nanocomposites obtained by solvent processing mostly involve CNM suspensions in a solvent where the polymer matrix is dissolved. The removal of the solvent by evaporation (solvent casting or spin coating) leads to polymer/CNM films that are prepared at ambient conditions (at much lower temperatures than those used in melt mixing). One of the main advantages of solvent processing, is that sample preparation can be carried out over long time, allowing nanoparticles to disperse and self-organize into networks (Kargarzadeh et al. 2017) and to reach a percolation

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Fig. 8 Main processing routes to develop CNM-reinforced thermoplastic composites

threshold (Nascimento et al. 2018). Unfortunately, solvent processing is generally limited to the bench scales. Solvent processing first requires dispersing the nanoparticles and dissolving the polymer matrix in suitable solvents. The CNM suspensions properties depend on the affinity between the nanoparticles and the solvent (Bruel et al. 2019, 2018), the CNM volume fraction, CNM dimensions and surface charge, and the ionic strength of the solvent (Derakhshandeh et al. 2013; Shafeiei-Sabet et al. 2013; Goff et al. 2014; Bercea and Navard 2000). These parameters directly affect electro-repulsion and Brownian motion of CNM and thus their organization within the suspension. Mechanical mixing, ultrasonication, or microfluidization (Kargarzadeh et al. 2017) are usually applied before solvent evaporation. Mixing time in the preparation of the suspension (polymer, CNM, solvent) must be controlled in order to ensure homogeneous distribution and optimal interaction between the components (Ng et al. 2017). Bruel et al. (2018, 2019) presented 3D-plots with the Hansen parameters of several solvents, solvent mixtures and polymers using wood-based sulfuric acidhydrolyzed CNC as reference, i.e., to identify the best solvent to disperse CNC with the dissolved polymer. The best solvents for CNC dispersion, considering the polar interactions, were dimethyl sulfoxide (DMSO), formamide, water and ethanolamine. The non-polar interactions are predicted for solvents such as chloroform and dicholoromethane. For polymers, PVOH has a high affinity with the hydrophilic surface of CNC while PEG, PLA and poly(methyl methacrylate) (PMMA) present balanced affinity as far as the polar and non-polar interactions. Finally, PE and PP are assigned to hydrophobic surfaces on CNC.

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The formation of liquid crystal chiral nematic ordered phases in aqueous CNC suspensions (Revol et al. 1992) leads to microstructuration, affecting the flowbehavior. Isotropic-liquid crystal-gel transition of CNC suspensions has been studied as far as the complex rheological behavior (Bercea and Navard 2000; Menezes et al. 2009). Compared to CNC suspensions at the same concentration, aqueous suspensions of CNF show higher zero-shear modulus G0 and viscosity η0 (Li et al. 2015; Nechyporchuk et al. 2014). This is explained by the higher aspect ratio of CNF, as their length and flexibility allow the creation of entangled networks. On the contrary, CNC, which behave as rigid rods, align in the flow field in the aqueous medium or, under specific conditions, form Bouligand structures (Natarajan and Gilman 2018). Li et al. (2015) and Shafiei-Sabet et al. (2013) reported on the rheology of both CNC and CNF suspensions and found that it is strongly dependent on the conditions of acid hydrolysis used to obtain the CNM. Azouz et al. (2012) measured the complex viscosity as a function of shear rate at different PEO / CNC ratios, and found a critical concentration of 6 wt% CNC in the aqueous solution containing PEO, i.e. concentration at which all the available PEO chains were adsorbed on the CNC surface. SAXS experiments were conducted to investigate the dispersion of PEO-PPO-PEO triblock copolymer of CNC-containing aqueous suspensions. The adsorption of the triblock copolymer on the surface of the CNC did not change the overall dimension of the nanoparticles. However, it modified the mutual interactions between the nanoparticles and improved their dispersion in water (Nagalakshmaiah et al. 2016). Finally, the readers are directed to the review of Hubbe et al. (2007), which discusses the rheological behavior of CNC and CNF aqueous suspensions. Besides the rheology, suspension stability is most relevant to the preparation of composites. As expected, the turbidity of CNC suspensions depends on the pH and salt concentration, which also considers the given surface functional groups and their pKa in the case of carboxylate or sulfate groups, for example. It is therefore possible to control the surface charge and the dispersion of CNC in aqueous suspension by desulfation or post-sulfation treatments, and hence to control the properties of the suspension (Kalashnikova et al. 2012). Finally, the structures formed by CNM after water evaporation have been extensively studied and the reader is referred to Menezes et al. (2009). Several works report on the preparation of aqueous CNM suspensions with hydrosoluble polymers such as PVOH (Silvério et al. 2013; Kassab et al. 2019), PEO (Bossard et al. 2010) or carboxymethyl cellulose (CMC) (Boluk et al. 2012). To improve the dissolution of the polymer in water and improve the dispersion of CNM, mechanical, magnetic stirring or sonication are usually performed. But attention should be given to the possible effects of high shear, which can degrade the macromolecules and lead to molecular chain scissions (Bossard et al. 2010). Following solvent mixing, the drying step is crucial in dispersion of CNM in water-soluble polymers. Pereda et al. freeze-dried an aqueous suspension of PEO-CNC to prepare PEO-modified CNC films for subsequent extrusion with polyethylene (Pereda et al. 2014). CNC were modified by adsorbing PEO macromolecules of different molecular weights on the surface. The authors showed that PEO molecular weight changed

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the macroscopic structure of the freeze-dried CNC and freeze-drying was effective in avoiding extensive self-aggregation of CNC. Although CNM are readily dispersed in water, only a limited number of thermoplastics are hydrosoluble. Hence, organic solvents are needed for hydrophobic matrices. For this purpose, solvent exchange can be a valuable approach. For example, CNM can be dispersed in water, followed by solvent exchange with acetone, and then with the polymer solvent (Ng et al. 2017). For some solvents, such as DMF, chloroform or formic acid, it is possible to prepare CNM suspensions directly using the polymer solvent. The most commonly used solvents are dimethyl formamide (DMF) (PHB (Seoane et al. 2017) and PLA (Bagheriasl et al. 2018)), chloroform (PLA (Wu et al. 2018; Qian et al. 2018) and PHB (Zhang et al. 2019)), toluene (PLA (Paula et al. 2016)), pyridine (PC (Mariano et al. 2015)), and formic acid (polyamide, PA (Aitha and Vasanthan 2020; Osorio et al. 2021)). The initial concentration of CNM affects the final dispersion state within the polymer film: above a critical concentration, CNM tend to form agglomerates upon drying/evaporation of the solvent. This is due to the strong intermolecular hydrogen bonding between the CNM particles. This critical concentration depends on CNM characteristics and the solvent/polymer affinity. For example, in a CNC/PHB film prepared by solvent casting in chloroform, a serious agglomeration occurred, due to strong intermolecular interactions observed above 5 wt% CNC in the final composite (Zhang et al. 2019). Figure 9a shows a well-dispersed CNC, even at 5wt% in PHB using chloroform. In the case of CNF/PHB composites, agglomeration was observed at > 1 wt% of CNF (Fig. 9b). Regarding the dispersion of CNC in polyamide matrices using formic acid, a limiting CNC agglomeration concentration of < 1 wt% was found for composites prepared by spin coating (Osorio et al. 2021). In other cases, agglomeration occurred at > 5 wt% CNC in composites prepared by solvent casting (Aitha and Vasanthan 2020). Similarly applies to PLA matrices (Qian et al. 2018). Taken all together, the CNM source and the preparation protocols are noted to greatly influence the dispersion behavior. Obviously, the dispersion of CNM in polymers that are water insoluble is more challenging compared to water-soluble polymers (Viet et al. 2007; Oksman et al. 2016). Casting-evaporation is reported to be an efficient process for CNM dispersion, due to Brownian motion that allows molecular rearrangement and tends to facilitate the interactions between CNM and polymeric chains (Ng et al. 2017). Some strategies to improve the dispersion of CNM in non-water-soluble polymers include the use of surfactants or chemical functionalization, as described previously. Siqueira et al. (2010) found that functionalizing CNC with n-octadecyl isocyanate allowed dispersion in organic solvents and further processing as nanocomposite films (casting/evaporation) using a broad range of polymeric matrices. In another example, PLLA surface-grafted CNC were dispersed in PLLA by solvent casting in chloroform and toluene (Paula et al. 2016) and sulfated CNC (P-CNC) and lauric arginate modified CNC (F-CNC) were mixed with PLA by solvent casting (Chi and Catchmark 2017). The authors in the latter study showed the benefit of UV/Vis/NIR spectrophotometry (transmittance (%) versus the wavelength data) to observe that CNC (1 to 15 wt% concentration) showed a transmittance that was much higher for F-CNC

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1 wt%

5 wt %

(A)

With CNC

(B)

With CNF

Fig. 9 SEM images of impact cross-section of samples containing different amounts of CNC and CNF dispersed in PHB by solvent mixing in chloroform. Reproduced with permission from Zhang et al. (2019)

compared to that of P-CNC. This was the result of a better dispersion of the CNC that is more hydrophobic (Chi and Catchmark 2017). Unfortunately, most of the articles related to the incorporation of functionalized CNC in polymers by solvent processing do not characterize the dispersion state of the suspensions or films after solvent removal. The incorporation of CNC in polymer matrices affects the crystallization during solvent evaporation, for instance, larger crystallite sizes were observed for CNC in PLA (Qian et al. 2018). When good dispersion is achieved in PA6, CNC act as nucleation sites for polymer spherulites (Osorio et al. 2021), thus impacting their growth and structure (formation of the γ-crystalline phases (Aitha and Vasanthan 2020), and other properties). In conclusion, solvent processing is efficient to disperse CNM in polymer matrices. Suspensions containing CNM and water-soluble polymers are well reported and indicate a good dispersion, especially when CNM are modified by polymer adsorption. However, since polymers are most often not water soluble, organic solvents are needed for processing. To obtain a good dispersion after solvent/water removal, several parameters should be considered: evaporation process, the nature of the solvent, CNM concentration, matrix/CNM affinity, type and characteristics of CNM and whether they are functionalized or not.

3.2.2

Melt Processing

Melt processing has many advantages: it is reproducible, cost-effective and enables fast manufacturing that does not necessarily require the use of solvents in dealing with thermoplastics. In particular, continuous melt processing, as a route for the production of CNM-based nanocomposites, has attracted great interest over the last decade, especially due to the infrastructure that is readily available for scale-up (Oksman et al. 2016). Some processes use co-rotating twin-screw extrusion to incorporate CNM in thermoplastics, where the shear forces induced by the co-rotating screws

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are effective in breaking-up CNM aggregates in the polymer (Wang et al. 2020; Oksman et al. 2016). Dispersion of CNM by twin-screw extrusion The preparation of thermoplastic nanocomposites by twin-screw extrusion and the final dispersion state of nanoparticles within the polymer matrix are determined by several factors, i.e. feed rate and feeding protocol, screw speed and profile, specific mechanical energy developed along the process, barrel temperature and rheological behavior of the polymer (Wang et al. 2020; Vergnes 2019). While the effects of extrusion conditions have been extensively studied, for example for clay-based nanocomposites (Vergnes 2019), there is limited literature discussing the effect of processing conditions in CNM-based nanocomposites. Leszczy´nska et al. varied the screw speed (50 to 150 rpm) in dispersions of microfibrillated cellulose (MFC) in polyamide (PA) (Leszczy´nska et al. 2015) and found a better dispersion at the higher screw speeds. Moreover, the dispersion of CNM by melt extrusion at various loading rates, ranging from 1 to 30 wt%, has been studied for polyolefins (i.e. polyethylene (PE) (Boran et al. 2016), polypropylene (PP) (Kim et al. 2019; Nagalakshmaiah et al. 2016; Iyer et al. 2015; Yang and Gardner 2011; Khoshkava and Kamal 2014), polystyrene (PS) (Lin and Dufresne 2013), polyesters (i.e. polylactic acid (PLA) (Robles et al. 2015; Lizundia et al. 2016; Bondeson and Oksman 2007a; Arias et al. 2015; Bitinis et al. 2013; Dhar et al. 2016; Johari et al. 2016; Kamal and Khoshkava 2015; Khoshkava and Kamal 2013; Oksman et al. 2006; Raquez et al. 2012; Spinella et al. 2015; Yang et al. 2016), polybutylene succinate (PBS) (Hu et al. 2015), polycaprolactone (PCL) (Goffin et al. 2011; Kaldéus et al. 2019), polybutylene adipate terephthalate (PBAT) (Morelli et al. 2016a), polyamides (i.e. PA6 Clemons 2015; Rahimi and Otaigbe 2016), PA11 (Panaitescu et al. 2013; Peng et al. 2017), PA12 (Nicharat et al. 2015)) or natural polymers such as thermoplastic starch (TPS) (Fourati et al. 2020; Gray et al. 2018) and proteins. Although a satisfactory state of dispersion can be achieved at the microscopic scale, the shear forces induced by the extrusion process are not sufficient to deagglomerate and disperse CNM at the nanoscale in the molten polymers (Fig. 10). As discussed earlier, the surface energy and polarity of the polymer matrix influence the dispersibility of CNM. The required dispersion energy is reduced for more polar matrices such as PLA or TPS, due to a lower interfacial tension and a higher work of adhesion between the CNM and the molten polymer (Khoshkava and Kamal 2013). If the work of adhesion between the fillers and the polymer melt is high, the stress transfer efficiency during mixing is improved. The deagglomeration and dispersion of CNM during mixing thus requires for the work of adhesion (polymer/CNM) to be comparable to or higher than the cohesive strength of CNM agglomerates. Otherwise, only a redistribution of the CNM agglomerates in the matrix would occur. It was found that freeze-dried and spray-dried CNC do not lead to the same dispersion state upon incorporation in PP by melt-mixing, which highlights the structuring effect that occur upon drying (Khoshkava and Kamal 2014).

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Fig. 10 SEM images of cryo-fractured cross-section of films obtained by microcompounder, of a PP and b PP reinforced with 3 wt% of CNC. White dots are seen, assigned to CNC aggregates and red circles point holes that are attributed to pulled-out CNC aggregates. Reproduced with permission from Nagalakshmaiah et al. (2016), Copyright American Chemical Society

Overall, the direct dispersion of CNM at the nanoscale in thermoplastic matrices by twin-screw extrusion remains a challenge, and often requires CNM physical– chemical modification or the addition of processing aids, as discussed in previous sections. As far as processing, several options can be considered: pre-mixing, either dry, melt or solvent mixing prior extrusion; mixing CNM and polymer at low temperature, so-called chilled extrusion, to achieve high shear and compression forces; and direct incorporation of CNM suspensions during extrusion, the so-called wet compounding by using appropriate solvents (Wang et al. 2020; Oksman et al. 2016). The following sections discuss chemical modifications and the latter processes in the context of melt processing. CNM chemical modification and coupling agents Esterification and acetylation of CNM are used in several studies to achieve better dispersion states in thermoplastic matrices after a melt-mixing step (Wang et al. 2020; Shojaeiarani et al. 2018, 2019; Yano et al. 2018; Sapkota et al. 2014), as it is likely to modify surface energy and polarity of CNM and improve their physicochemical interactions with less polar matrices. This however requires a medium to high degree of substitution (DS), usually > 0.4, so that CNM is sufficiently modified, especially for apolar matrices such as polyolefins. Hu et al. used acetylated CNC (ACNC) as reinforcing and nucleating agents in PBS foams. The authors showed that the addition of ACNC stabilized the cell morphology. However, the addition of excess CNC (>5wt%) led to self-aggregation, deteriorating the foam properties (Hu et al. 2015). Jonoobi et al. showed no difference in the dispersion of unmodified and acetylated CNF in PLA composites prepared by twin screw extrusion (Jonoobi et al. 2010). The chain length of the grafted molecules should be long enough to induce steric barriers to limit cohesive interactions between CNM and to favor dispersion during melt extrusion. Finally, cellulose nanofibrils were modified with aliphatic ester groups with linear, cyclic and branched structures (Yano et al. 2018) and it was

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Fig. 11 X-ray CT images of injection molded HDPE and unmodified and modified CNF/HDPE composites showing the effect of CNF modification on its dispersion state in HDPE. Reproduced with permission from Yano et al. (2018)

shown that after melt-mixing the grafting of long linear chains (such as stearoyl, C18) resulted in the best dispersion of CNF in HDPE (Fig. 11) (Yano et al. 2018). Organosilanes have the advantage of their functional end groups that can be varied for suitable compatibility/reactivity with thermoplastic matrices. A commercial silane, the 3-aminopropyl triethoxysilane, was used to functionalize CNF followed by mixing in a PLA matrix using a mini-lab twin-screw extruder where an amino-silane was shown to enhance CNF dispersion (Robles et al. 2015). “Grafting from” process during in-situ polymerization is an effective way to improve dispersion, but this approach requires prohibitive protocols relative to current standards. In this area, a polar matrix (PLA) was combined with PLLAgrafted CNC prepared by the “grafting from”, to produce PLA/CNC nanocomposites (Lizundia et al. 2016). Two types of functionalization were compared, L-lactide ratios of 5:95 and 20:80, with the better dispersion achieved with the latter condition. 1 wt% grafted-CNC showed a relatively good dispersion state with a typical sea-island morphology, whereas at a concentration of 3wt% of grafted-CNC, aggregation occurred. In related efforts, significant improvement in the dispersion of PCLgrafted CNC was shown when incorporated by melt-mixing in PCL (Fig. 12) (Goffin et al. 2011). Meanwhile, nanocomposites based on a non-polar matrix (PS) and PEGgrafted CNC were demonstrated for the interfacial interactions, limited degradation

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Fig. 12 AFM images (phase) of a PCL + 8wt% of raw cellulose nanocrystals and b PCL + 8wt% of PCL-grafted- cellulose nanocrystals. Reproduced with permission from Goffin et al. (2011), Copyright Elsevier

of the nanoparticles during the high-temperature processing and good dispersion and compatibility (Lin and Dufresne 2013). Surfactants and polymer treatment involve additives such as low-molecular weight surfactants which facilitate the dispersion of CNM during the melt-mixing step (Nagalakshmaiah et al. 2016; Iwamoto et al. 2014a). Cationic surfactants (quaternary ammonium salt) can be added to PP/CNC during extrusion to improve the dispersion of CNC (Nagalakshmaiah et al. 2016), as was also the case of an acid phosphate ester of ethoxylated nonylphenol (Beycostat A B09) added in PLA/CNC (Bondeson and Oksman 2007a). Polymer coating or polymer wrapping of nanocelluloses show good compatibility at the cellulose/polymer interface and also between the modified polymer and the matrix, for example, in CNF/HDPE composites with fibrils modified with a PLMA-b-PHEMA diblock copolymer (Sakakibara et al. 2016). Other work has shown that the encapsulation of CNC reduces the number of agglomerates observable by electron microscopy in both apolar (PE) (Azouz et al. 2012) and polar (PLA) (Arias et al. 2015) matrices. The molecular weight of PEO coated onto CNC was shown to have an impact on the final dispersion of CNC in PLA, with best blends being those of low molecular weight PEO (L-PEO) and high PEO/CNC ratios (Arias et al. 2015). It was also observed that L-PEO functionalized CNC has a synergistic effect on the crystallization of PLA due to the plasticization of PLA with L-PEO and the enhanced nucleation induced CNC. Commercial coupling agents can be directly incorporated during melt extrusion, such as maleic anhydride grafted PP (MAPP) coupled with CNM and mixed with polyolefins. In such efforts, twin-screw extrusion was used to prepare nanocomposites of PP/MAPP and CNFs treated with polyoxyethylene nonylphenyl ether (PNE). It was shown that the dispersion of CNF and interfacial adhesion with PP were improved by treatment of CNFs with PNE and the use of MAPP (Iwamoto et al.

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2014a). In the case of more polar matrices, such as PLA, maleic anhydride grafted PLA (PLA-MA) was used to enhance the compatibility when extruding the polymer with the feeding of the CNC suspension (wet compounding) (Oksman et al. 2006). However, TEM analysis showed no improvement in the dispersion of CNC with PLAMA alone, and partial dispersion was obtained when the additives were PLA-MA and PEG (Wang et al. 2018). Pre-mixing by milling Pre-mixing by milling is used to facilitate the dispersion of CNM in polymer matrices. This pretreatment is usually carried out in dry conditions, where the CNM and the polymer (in powder or pellet forms) are ground to small sizes by mechanical collision and/or pulverization. To date, three types of milling have been described in the literature, namely, jet-milling (Rohner et al. 2018), cryo-milling and ball-milling (Venkatraman et al. 2019). For example, a CNF/PA11 system was prepared in a jetmill operating at a pressure of 2 bar and 5 g/min speed followed by melt extrusion (Rohner et al. 2018), showing a good dispersion of CNF in PA11. Pre-mixing by kneading Several studies report on kneaders to pre-mix the CNM and the polymer prior to extrusion, either in the molten state or below the melting temperature. Pre-mixing below the polymer melting temperature allows higher shear and compression forces (Wang et al. 2020). A study on a CNF/MAPP/PP system compared the influence of the residence time when pre-mixing powdered matrix and CNF with a batchtype kneader at 120 °C and 60 rpm, on the resulting microstructure and dispersion state of CNM within the PP matrix. The residence times ranged from 10 to 60 min. Then, the pre-mixed material was melt compounded with a twin-screw extrusion at 170 °C and 30 rpm and cut into pellets for injection molding (Iwamoto et al. 2014a). It was demonstrated that the pre-mixing residence time had a positive impact on the disintegration of CNF aggregates, with a maximum efficiency at 30 min pre-mixing. To enhance the CNM dispersion with this process, kneading has been combined with CNM surface modification and coupling agents. This has been shown in industrially relevant, up-scalable processes: wood fibers were direct-kneaded in the so-called “Kyoto process” (Igarashi et al. 2018) and alkenyl succinic anhydride (ASA) was used to treat the fibers (DS ranging from 0.22 to 0.57), then mixed with CaCO3 and HDPE powder in isopropanol. This mixture was then filtered, mixed and dried (30 wt% CNF) to further compounding with HDPE pellets by twin-screw extrusion at 140 °C to reach 10 wt% CNF content. With this procedure, a good fibrillation of ASA-treated fibers (DS 0.43) in HDPE was achieved, although several micron size materials were still present. The main advantage of this process is that it allows simultaneously the fibrillation of the dried fibers and uniform dispersion in polyolefins. Pre-mixing by solvent Another pre-mixing method uses polymer/CNM mixtures with a solvent, which is then dried and melt compounded (Shojaeiarani et al. 2019; Arias et al. 2015; Bitinis

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et al. 2013; Nicharat et al. 2015; Jonoobi et al. 2010; Zhang et al. 2014). For example, Jonoobi et al. prepared PLA/CNF mixtures, by combination with acetone-based CNF suspensions and PLA dissolved in acetone-chloroform, which were then cast to obtain PLA/CNF masterbatch films. The PLA/CNF composite films were then crushed and compounded with neat PLA using a twin-screw extruder. A relatively good dispersion at low CNF content (1 and 3 wt.%) was possible but small aggregates were visible at 5 wt.% (Jonoobi et al. 2010). Similarly, PLA, PBAT and PLA/PBAT blends containing CNC were processed by solvent casting in DMF followed by melt mixing (Mohammadi et al. 2021). The melt mixing step caused a significant reagglomeration of the CNC in neat polymer matrices. Meanwhile, a PLA/CNC mixture in chloroform was casted for solvent evaporation. The resulting PLA/CNC films were cut and dry mixed with natural rubber (NR) before being melt compounded by twin-screw extrusion. The thermal degradation of CNC was limited with this premixing method and the dispersion of CNC was improved, which was evidenced by rheological measurements that showed an increased storage modulus and complex viscosity (Bitinis et al. 2013). PLA/CNC nanocomposites were prepared by premixing CNC with PEO in water and introducing PEO/CNC blends into PLA by melt-blending. Good dispersion of PEO/CNC in PLA at the nanoscale level, as well as a good particle/matrix interfacial adhesion were obtained (Arias et al. 2015). However, given the use of PEO-coated CNC, it is difficult to conclude whether the improved CNC dispersion was due to the pre-mixing process, or to the modifying polymer (PEO) or both. Chilled extrusion The chilled extrusion process, also known as solid-state shear pulverization (SSSP) takes various forms in the literature, but it is basically a process whereby the materials are subjected to cooling rather than heating (Wang et al. 2020; Iyer et al. 2015; Blumer et al. 2019). Consequently, this technique requires special equipment, including specially designed extrusion screws that exert high shear and compression forces as well as cooling systems. The polymer is usually cooled below its melting temperature, if semi-crystalline, or below its glass transition temperature, if amorphous. This allows much higher shear and compression forces compared to those typical of melt extrusion, resulting in pulverization, fragmentation and intimate mixing of CNM and polymer powder. As previously discussed, the control of specific mechanical energy (SME) is one of the key factors affecting the dispersion of nanoparticles in polymer matrices. Chilled extrusion involves high SME and hence leads to a good CNM dispersion, even when mixed with polyolefins (Iyer et al. 2015; Iwamoto et al. 2014a) and with no need for chemical modification or the application of solvents. Hence, this technique has many advantages: single-step, solventless, continuous and industrially scalable. The preparation of CNC/polyolefins systems (PP and LDPE) by SSSP has been demonstrated (Iyer et al. 2015). Therein, CNC and polyolefins were pulverized at a screw speed of 200 rpm into an extrusion barrel maintained at −7 °C. As a result, a fine powder with 5–10 wt% of CNC was obtained with good dispersion at the nanoscale (SEM observations, Fig. 13). The potential of SSSP to break CNC agglomerates into

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Fig. 13 FE-SEM images of a 90/10 wt% of LDPE/CNC (melt mixing, MM), b 90/10 wt% of PP/CNC (MM) c 90/10 wt% of LDPE/CNC (solid-state shear pulverization, SSSP), d 90/10 wt% of PP/CNC (SSSP). Reproduced with permission from Iyer et al. (2015), Copyright Elsevier

individual nanocrystals within polyolefin matrices and without thermal degradation (which otherwise start at 150 °C due to the presence of sulfate end groups) are possible (Iyer et al. 2015; Wang et al. 2007b; Roman and Winter 2004). Wet compounding As described in a recent review (Clemons and Sabo 2021), wet compounding involves either pre-dispersion of CNM in water or the solvent, and then incorporation to the polymer during extrusion (“wet feeding”) or combining sequential fibrillation of cellulose fibers during processing (“wet extrusion fibrillation”). The former is mostly used to disperse CNC while the latter is used to produce and disperse CNF in polymers. These techniques do not require drying of the CNM suspension, which is the main cause of agglomeration. There are two different types of devices used for wet compounding: twin-screw extruder or batch mixer (Yasim-Anuar et al. 2020). Wet compounding using a twin-screw extruder was inspired by processing routes that aimed to defibrillate, compound and dry natural fibers or associated materials (MCC or CNF), in a single pass through the extruder (Suzuki et al. 2013, 2014; Soulestin et al. 2007). Several advantages can be listed in this process, such as cost reduction, lower melting point of the matrix due to a plasticizing effect of water/solvents, and reduction or avoidance of thermal degradation. Due to the presence of water/solvents, which evaporate during extrusion, it is not possible to carry out wet compounding with standard equipment. To allow the purge of vapors, a heated barrel with ventilation/exhaust openings is required; hence, excess of water/solvents may lengthen the processing time, for complete evaporation. Another drawback is the limited concentration of the CNM suspension that can be processed by wet feeding (thus, limiting CNM loading in the polymer), which requires the venting of large volumes of water (Clemons and Sabo 2021). The dispersion of wood-derived CNC into PA6 was compared by using dry compounding, wet compounding and solvent mixing. It was shown that the break-up of CNC agglomerates and their dispersion was better achieved by solvent mixing than wet compounding, which in turn yielded better results than dry compounding (Clemons 2015; Clemons and Sabo 2021). The nanofibrillation of CNF and its simultaneous incorporation in PP by wet compounding has been attempted (Suzuki et al. 2013, 2014) by MAPP addition as compatibilizer. It was possible to achieve good

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interfacial adhesion between polar hydroxyl groups of CNF and non-polar PP. SEM micrographs showed that the fibers were fibrillated (to nanometer/submicron widths) and were well dispersed in the PP matrix. Wet compounding using batch mixers allows processing of highly concentrated CNC suspensions, up to 15 wt% (Sapkota et al. 2017). The dispersion of CNF into PE / MAPE using liquid-assisted internal melt mixer and twin-screw extrusion were studied (Yasim-Anuar et al. 2020) and it was found that twin-screw extrusion led to better dispersion (SEM images mapped with EDS, Fig. 14). Some batch-mixer devices, such as High Speed Thermokinetic Mixer, are known to apply high frictional forces and attain shear rates of 104 s−1 and short mixing times (Gopakumar and Page 2005). Moreover, to improve the dispersion and the interfacial adhesion between CNM and polyolefins, coupling agents and additives are often added in the wet-compounding processes (MAPP, MAPE, quaternary ammonium salt, etc.). Well-dispersed CNF in polycaprolactone (PCL) has been demonstrated by wet feeding of a pre-mix of water-dispersed PMMA-latex, CNF and PCL powder. The favorable electrostatic interactions between CNF and the latex facilitated mixing of the components and prevented CNF agglomeration (Lo Re et al. 2018). Finally, water-borne copolymer compatibilizer containing hydrophilic quaternized ammonium groups have been used to strengthen ionic interactions with negatively charged

Fig. 14 SEM images and EDS cartographies of oxygen element which represents the CNF for a PE/PE-g-MA, increasing amounts of CNF into PE/PE-g-MA prepared by c, e and g liquid-assisted internal melt-blending (IMB), i, k and m liquid-assisted twin screw extrusion (TWS). Reproduced with permission from Yasim-Anuar et al. (2020), Copyright Elsevier

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CNF, and to develop core–shell CNF. The modified CNF was then incorporated in PCL by wet feeding, under conditions of improved dispersion and interfacial adhesion (Kaldéus et al. 2019).

3.2.3

Mechanical Performance

The mechanical properties of CNM nanocomposites and the influence of the polymer (matrix) type, the processing method and the modification of CNM are discussed in this section. These factors influence the CNM dispersion state, and therefore affect the mechanical performance of the obtained composites. In order to compare the data collected in the literature, herein we use the relative tensile mechanical properties, considering the reported values for neat polymers and their CNM-reinforced nanocomposites. From our database, Ashby diagrams were built to map out the evolution of the relative Young’s modulus (E relative = E composite / E matrix ) as a function of the relative yield strength (σ relative = σ composite / σ matrix ) of the CNM-reinforced nanocomposites. Figure 15a, b and c show the effect on the tensile mechanical properties of CNM composites according to the processing method (see also Fig. 8), shaping processes (injection and compression molding, solvent casting and cast extrusion) and matrix type, namely, polyolefins (PP, PE), polyamides (PA6, PA11, PA12), polyesters (PLA, PHA, PCL, PBAT) and polyvinyls (PVOH, PVAc). The effect of CNC and CNF modification is also considered. The amounts of CNC and CNF were in the range of 0.5 to 30 wt% and 0.1 to 30 wt%, respectively. It should be pointed out that some data points were not used, for instance, in given areas of the Ashby diagrams, either due to the very high CNM volume fraction (>20 wt%), or the possibility that the processing steps degraded the matrix, leading to poor relative tensile mechanical properties (20 mPa.s–1 . Fig. 8 The pre-treatments rheology and characterization procedures. Reproduced from (Pirich et al. 2019) with permission

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4.5 Surface Charge Density The cellulose dispersion stability is greatly influenced by the surface charges of cellulose materials (Tian et al. 2016; Bahloul et al. 2021). The higher the surface charge on the cellulose fibers or rods, the better the dispersive qualities. Surface charge analysis of cellulose materials has been conventionally performed via titration of the cellulose suspension. Several titrimetric techniques have been used to determine the surface charge and the content of functional groups on the surface of cellulose materials. It is critical to know if the charge groups act as strong acids or weak acids before using different titration techniques when analyzing the surface charge density of cellulose materials via titration. Cellulose aqueous solutions that act as strong acids are titrated directly by NaOH, producing a titration curve with an abrupt transition at the point of equivalence. For cellulose materials with weak acid groups, on the other hand, a specified amount of strong acid is introduced to the suspension to create a measurably reduced conductivity at the beginning of the titration. For all these approaches, when protons are absorbed by OH groups, the suspension’s conductivity falls until the equivalence point is reached. At the point of equivalence, sodium counterions have completely taken the place of all cellulose proton counterions. The conductivity of the suspension increases as more sodium hydroxide is additionally added because of the cellulose suspension contains free OH groups. As protons are more conductive than hydroxyl groups, the slope of the acidic zone is steeper than that of the basic region. As shown in Fig. 9, the molar surface charge of cellulose materials is graphically evaluated by plotting the conductivity of the cellulose suspension as a function of the volume of NaOH added. The surface charge content can be determined using the following equation: Charge content (mmol/kg) = C N a O H Wx VN a O H x106 ; where C NaOH corresponds to concentration NaOH (M), V NaOH corresponds to the volume used to titrate, W is the weight of the cellulose sample. According to the literature, conductometric titration could also contribute to the investigation of the chemical structure of surface functionalized cellulose. Benhamou et al. presented conductometric titration results of phosphorylated CMF which showed higher charge content of about 3133 mmol kg−1 in comparison to phosphorylated CNCs exhibited 254 mmol kg−1 (Ait Benhamou et al. 2021b).

4.6 Zeta Potential Measurement The zeta potential is a method that analyses the attraction or repulsion electrostatic intensity of cellulose materials in colloidal suspensions, and it measures the effective electrical charge on the surface of cellulosic material (Pirich et al. 2019; Wang et al. 2019; Sai Prasanna and Mitra 2020; Araya-Chavarría et al. 2022). A cellulosic suspension is introduced into a zeta potential measurement instrument, which provides a reading in mV, which could be a negative or positive number. The higher

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Fig. 9 Conductometric titration curves of a phosphorylated CNC and b phosphorylated CMF. Reproduced from (Ait Benhamou et al. 2021b) with permission

the zeta potential, the more stable the suspension will be because the charged particles tend to resist one other, overcoming the natural tendency for the cellulosic particles to agglomerate (Perumal et al. 2022). The electrostatic repulsion that occurs in the existence of charged chemical groups on the surface of micro or nanocellulose materials is what gives the cellulose materials suspension its colloidal stability. The colloidal stability gives essential aspects to the characteristics of cellulose materials in emulsions, foams, and nanocellulose aggregation, which need to be carefully controlled for optimal outcomes as a filler in nanocomposite materials. For instance, the existence of a negative charge with surface groups like carboxyl and sulfate groups results in the negative electrostatic layer on the CNCs and the spreading medium. If the zeta potential of cellulosic materials is between 15 and 15 mV, there is a chance that cellulosic material will agglomerate. For instance, Tian et al. studied the zeta potential of CNFs by varying the conditions of extraction like temperature, reaction

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time, and acid concentration (Tian et al. 2016). The highest zeta potential (−44.5 mV) obtained was for CNF treated with 55 wt% H2 SO4 at 45 °C (Tian et al. 2016). Generally, temperature, contaminants, and the pH value of the cellulose suspensions affect the zeta potential value.

5 Conclusions This chapter is an extensive review of the preparation and characterisation methods of cellulose and its derivatives from the annual plants. It emphasizes the use of annual plant fibers in the field of cellulose isolation. The aforementioned sources are not only widely accessible, profitable, and varied in characteristics, but they can also offer value-added nanomaterials that will generate a new economy and serve as the best raw material for nanocellulose production. The use of cellulose and its derivatives in daily life has proven beneficial in a variety of industries, including product packaging, paper and paperboard, food processing, health and sanitation products, paints, cosmetics, and so on. For many years, various cellulose manufacturing techniques have been fully improved, and implementation strategies that may focus on a larger scale are currently being developed. Furthermore, with growing ecological concerns and a shift toward sustainable materials, scientists should think creatively to develop new green methods of cellulose extraction and advanced characterization techniques taking into account more sustainable sources such as agriculture and food wastes. Acknowledgment The financial assistance of the Materials Science and nanoengineering (MSN) Department of the Mohammed VI Polytechnic University (UM6P), toward this research, is hereby acknowledged. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Structure and Properties of Cellulose and Its Derivatives Zineb Kassab, Adil Bahloul, Fatima-Zahra Semlali Aouragh Hassani, El-Houssaine Ablouh, Rachid Bouhfid, Abou El Kacem Qaiss, Houssine Sehaqui, and Mounir El Achaby

Abstract The widespread recognition of environmental, social, and financial imperatives, the hunt for environmentally friendly, sustainable technology, the escalating waste issue, environmental law requirements, and the depletion of fossil fuel supplies underlie the focus of scientific research on the development of eco-friendly materials. Lignocellulosic fiber is among the most studied natural materials due to its impressive economic and environmental characteristics. However, its use as an alternative to petroleum compounds requires in-depth knowledge of its structure, properties, and interactions with other materials of a different nature. However, even if these fibers have little interest in being used directly in composites, the constituent elements can still be exploited. Cellulose is the element that contributes the most to the structural properties of annual plants, is a good example. The first part of this chapter will describe the annual plant fibers and their structure and molecular organization. At the same time, the second part will deal with the cellulose’s physical-chemical characteristics and those of its derivatives. Keywords Annual plants · Cellulosic fibers · Cellulose derivative · Structure of cellulose · Properties of cellulose

Z. Kassab (B) · F.-Z. S. A. Hassani (B) · E.-H. Ablouh · H. Sehaqui · M. El Achaby Materials Science, Energy and Nano-engineering (MSN) Department, Mohammed VI Polytechnic University (UM6P), 43150 Benguerir, Morocco e-mail: [email protected] F.-Z. S. A. Hassani e-mail: [email protected] A. Bahloul Laboratoire d’Ingénierie et Matériaux, Faculté des Sciences Ben M’sik, Université Hassan II, B.P.7955 Casablanca, Morocco R. Bouhfid · A. E. K. Qaiss Composites and Nanocomposites Center (CNC), Moroccan Foundation for Advanced Science, Innovation and Research (MAScIR), Rabat Design Center, Rue Mohamed El Jazouli, Madinat El Irfane, 10100 Rabat, Morocco © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 R. Khiari et al. (eds.), Annual Plant: Sources of Fibres, Nanocellulose and Cellulosic Derivatives, Composites Science and Technology, https://doi.org/10.1007/978-981-99-2473-8_15

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1 Introduction The biomass’ main constituent is cellulose. From a modern perspective, cellulose is the most naturally occurring biopolymer on Earth since all living plants produce it. Generally, it can be defined as a stiff, fibrous, water-insoluble material crucial for preserving the shape of plant cell walls (Habibi et al. 2010; Kassab et al. 2020a). It may also be found in other species, like bacteria, fungi, and animal tunicates, albeit the quantity of cellulose in such species is much lower than that of plants (Kargarzadeh et al. 2017; El Achaby et al. 2018b). Annually in the biosphere, many living organisms generate cellulose at a rate that adds up to about 7.5 × 1010 tons (Habibi 2014; Kassab et al. 2020c), of which around 1.8 × 10 9 tons is industrially extracted from wood (El Achaby et al. 2018b). Thus, cellulose is a sustainable material that is worldwide accessible. Furthermore, it is a biodegradable component returning to the ecological system through simple decaying that is safe for all living beings. Due to the high modulus of their crystalline part, cellulose fibers serve as the main load-bearing component in trees and plants (El Achaby et al. 2018a). In combination with its low density, renewability and biodegradability, cellulose has a potential reinforcing capability compared to that of inorganic and synthetic fibers (Siqueira et al. 2010). The structure and characteristics of cellulose have been researched and emphasized in the literature by Anselme Payen in 1838 (El Achaby et al. 2018c). Cellulose is a linear polysaccharide containing of β-1,4-D-glucosidic repeating units (Ferreira et al. 2018). Each repeating unit contains two anhydroglucose units. In each anhydroglucose unit, there is one primary hydroxyl (OH) group at the carbon 6 (C6) position and two OH groups at the C2 and C3 positions. Cellulose has a hydrophilic character, although the strong inter- and intra-molecular hydrogen bonds limit the solubility of cellulose in water (Boujemaoui 2016). The majority of native celluloses are cellulose Iα and Iβ combinations. The monoclinic Iβ type is the allomorph found in the cellulose characteristic of annual plants, and the triclinic Iα allomorph predominates in algal-bacterial celluloses (Siqueira et al. 2010). The amounts of two allomorphs affect some cellulose fibers’ physical characteristics. The annealing process can change Iα into the Iβ form (Saxena and Brown 2005). A powerful and intricate network of intramolecular and intermolecular hydrogen bonds stabilizes the many crystal structures of cellulose, which come in various configurations. Cellulose has a wide range of molecule orientations and hydrogen-bonding networks. Depending on the source, extraction technique, or treatment, this results in the emergence of cellulose polymorphs or allomorphs. Cellulose has been identified in six interconvertible polymorphs: I, II, IIII , IIIII , IVI , and IVII (Siqueira et al. 2010). The biosynthesis of cellulose by plants and wood is still not fully understood. However, it has been suggested that cellulose chains are synthesized by protein complexes (also called rosettes) present in the cell wall. These rosettes contain six

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“lobes,” and each can synthesize six cellulose chains. The chains of poly-β-1,4-Dglucosyl congregate to form a fibril, a lengthy bundle of molecules that resembles a thread and is laterally supported by hydrogen bonds between molecules. (Siqueira et al. 2010). Once the chains are formed, they co-crystallize to form elementary nanofibrils with 3−5 nm wide. These nanofibrils contains both disordered amorphous and ordered crystalline parts. In wood, these CNF are hierarchically structured to form cellulose microfibrils (10-60 nm); which further assemble to build up cellulose cell/fibers (20−40 μm) and then forms the cellular structure of a tree or plant (Boujemaoui 2016; Ballesteros et al. 2018). Cellulose is of enormous economic significance since it is utilized as a feedstock in many sectors and produces a wide range of goods and materials, from packaging to the biomedical sector. (Eyley and Thielemans 2014; Habibi 2014; Trache et al. 2017). Due to the potential to create cellulose nanostructures with unique features not seen in bulk materials, cellulose has garnered more interest in recent decades. Considering how vital cellulose and its derivatives are to the economy, the present research attention is mainly on cellulose manufacture’s molecular structure and process in annual plants because it is still not fully understood. More focus is also given to the properties of cellulose derivatives. This chapter deals with cellulose structure and describes the most important features of cellulose’s chemistry and chemical technology and its derivatives and their properties.

2 Cellulose and Its Derivatives Sources Lignocellulosic biomass contains various naturally occurring organic components, most of which are plants or products derived from plants. (Phanthong et al. 2018). Lignin, hemicellulose, and cellulose, make up most of the cell wall composition of lignocellulosic biomass. Cellulose chain-based microfibrils are closely packed and are kept stable by hydrogen bonding. These fibrils are coated with lignin and joined by hemicelluloses, amorphous polymers of various sugars, and pectin (Menon and Rao 2012). However, due to the various species, kinds, and origins of lignocellulosic biomass, these three components’ composition and content vary (Langan et al. 2014). The primary component of lignocellulosic biomass, mainly concentrated at approximately 35–50 % in the plant cell wall, is cellulose (Phanthong et al. 2018). Wood, seeds, bast, cane, leaves, straw, fruit, tunicate, algae, fungus, bacteria, and minerals are just a few of the many sources from which cellulose can be produced (El Achaby et al. 2018d). The source is critical because it affects the size and properties of the extracted cellulose (Kargarzadeh et al. 2017). The composition of each varies, though; in addition to cellulose, lignin, and hemicelluloses, specific sources also contain waxes, pectin, and other elements that are water-soluble (Belouadah et al. 2015).

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3 Structure of Cellulose and Its Derivatives The physico-chemical characteristics of cellulose can be fully understood only through understanding the chemical structure of cellulose and its shape in the solid form. Understanding the impact of various substituents on cellulose’s chemical and physical characteristics and its derivatives necessitates a thorough studying the structural features of original cellulose. Three structural features must be differentiated when examining macromolecules of any kind (Fig. 1).

3.1 Molecular Structure Cellulose structure is classified as a single macromolecule, which evaluated to assess chemical composition, molecular mass, the presence of reactive sites, and potential intramolecular interactions. Payen is credited for discovering cellulose’s fundamental composition as early as in 1838 (El Achaby et al. 2018c), with a composition of carbon (44 to 45%), hydrogen 6.5% and oxygen; resulting in the following empirical formula C6 H10 O5 . However, it was still unknown what cellulose’s exact macromolecular structure was. Cellulose has D-anhydroglucopyranose units (AGU) on one end and a nonreducing C4-OH group on the other, while the terminating group is C1−OH, a reducing end with an aldehyde structure (Fig. 2). Its essential features are due to its molecular structure: Due to the donor group’s (−OH) strong reactivity, chirality, hydrophilicity, degradability, and chemical variability are all present (Boujemaoui 2016). The limitations of -linkage cause AGU units to spin 180 ° concerning one another in the solid-state. Three hydroxyls (OH) groups are present in every AGU unit. The molecule has an aldehyde group with reducing activity at its C−1 OH terminus. Through an intramolecular hemiacetal form, aldehyde groups create a pyranose ring. The OH (C4) on the opposite end of the chain, on the other hand, is an alcohol-borne OH ingredient and hence is referred to as the non-reducing end. The AGU unit occurs in the pyranose ring form. Fig. 1 Cellulose structural properties

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Fig. 2 The cellobiose unit in the molecular structure of cellulose

The cellulose polymer chain length varies depending on its origin. The degree of polymerisation (DP) of naturally occurring vascular plant cellulose, for example, is more significant than 10,000. The microcrystalline cellulose (MCC) employed in this work is a pure, highly crystalline cellulose obtained by acid hydrolysis with DP values ranging from 300−600. Isolated cellulose is always polydisperse in its most prevalent form. It combines molecules with the same fundamental makeup but different chain lengths, much like all polymers. As a result, cellulose’s molecular mass and DP may only be used as estimates (Heinze et al. 2018). Cellulose glycosidic connections are vulnerable to hydrolysis (Hakeem et al. 2015a). The hydroxyl groups play a crucial role in cellulose solubility. Cellulose is insoluble in water and most organic solvents (Hakeem et al. 2015a). The OH groups in the cellulose molecule can generate hydrogen bonds via two different processes. One way is by interactions between OH groups in the same molecule in the right places (intramolecular). These are found between the C2−OH and C6−OH groups and between C3-OH and endocyclic oxygen (Fig. 3a, X and Y). The other process occurs when C3-OH and C6-OH groups on adjacent cellulose chains contact (intermolecular) (Fig. 3, Z). The hydroxyl group at the C−3 and the oxygen of the pyranose ring were the first intramolecular hydrogen bonds to be described by Liang and Marchessault in the 1960s, and a second ’pair’ of intramolecular hydrogen bonds between the C−6 and C−2 of nearby AGUs were allegedly discovered by Blackwell et al. in the 1990s (Niemelä 2010; Heinze et al. 2018).

3.2 Supramolecular Structure Cellulose chains are prone to aggregation and the formation of highly organized structures and structural entities (Kassab et al. 2020b; El Bourakadi et al. 2021; Semlali Aouragh Hassani et al. 2022; Ait Benhamou et al. 2022). The cellulose molecule’s highly regular structure, rigidity of the molecular chain, and substantial hydrogen bonding capability encourage molecular alignment and aggregation (Salim et al. 2021; Ait Benhamou et al. 2021b; Ait Benhamou et al. 2021a). Despite this, the hydrogen bond’s precise structure remains unknown. Regardless of this understanding, the precise structure of this hydrogen bond network remains a topic of

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Fig. 3 Cellulose structures showing a the intramolecular hydrogen bonding between C2−OH and C6−OH (X), and C3−OH with endocyclic oxygen (Y); and b the intermolecular hydrogen bonding between C3−OH and C6−OH (Z) (supramolecular structure)

O

6

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HO

y O

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1

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H

OH

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O H

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H O

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debate. The history of cellulose’s supramolecular structure dates back to 1913 when Nishikawa and Ono used well-defined X-ray diffraction patterns to identify the structure of fibrous cellulose (Nitta 1962; Grisoni-Colli et al. 1967). According to this discovery, individual cellulose molecules prefer to arrange themselves in a highly organized fashion, resulting in a ‘paracrystalline’ form. The parallel cellulose chains are organized into crystallites and crystallite strands, which serve as the model’s basis, which are the fundamental components of the fibers (Hakeem et al. 2015b). The key contributions to the structure of cellulose are intermolecular hydrogen bonds between C6−OH and C3−OH of neighboring chains, which is recognized as the predominant component responsible for uniform packing (Grisoni-Colli et al. 1967; Kassab et al. 2020a; Salim et al. 2022). The great spatial regularity and availability of the hydroxyl groups, in turn, regulate the constancy of the interchain interactions. Because the order of molecules in cellulose fiber is not consistent across the structure, it is reasonable to presume that there are areas inside it with varying degrees of order. Today’s experimental data supports a two-phase paradigm in which the supramolecular structure is divided into two parts: amorphous and highly crystalline, with the middle ordered regions excluded (El Achaby et al. 2018d; Kassab et al. 2020a). This fringed fibril model is employed as a core idea in the current work to define the hierarchy of cellulose derivatives created via self-assembly and their crystallinity. Different X-ray methods, including as wideangle X-ray scattering, can be used to determine the crystallinity of cellulose and its derivatives (Nitta 1962; Grisoni-Colli et al. 1967). Additionally, NMR techniques have revealed to be an effective tool for determining the crystallinity of cellulose

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Table 1 Dimensions of cellulose polymorphs’ unit cells. (γa = lattice angle) Polymorph

a-axis (Å)

b-axis (Å)

c-axis (Å)

γ(deg)a

Cellulose I

7.85

8.17

10.34

96.4

Cellulose II

9.08

7.92

10.34

117.3

Cellulose III

9.9

7.74

10.3

122

Cellulose IV

7.9

8.11

10.3

90

(Heinze et al. 2018). Cellulose exists in several polymorphs (classes I, II, III, IV) that differ in their unit cell dimensions (Table 1) (Hakeem et al. 2015b). The current structures of cellulose polymorphs, particularly those of cellulose I and cellulose II, was developed in the 1970s using a mix of X-ray diffraction, model building, and conformational investigations (Siqueira et al. 2010). The polymorph structure of native cellulose is cellulose I, which may be found in two crystalline forms: Iα (in algae and bacteria) and Iβ (in higher plants) (French 2014; Mahmud et al. 2019). Intermolecular hydrogen bonding stabilizes the sheetlike structure of cellulose I. These bonds run parallel to the pyranose rings, and staggered stacking of the sheets along the cellulose chain axis by distances comparable to halfglucose rings is frequent in both crystalline forms. In native cellulose, the manner of staggering differs between the two allomorphs: continuous staggering occurs in cellulose I, whereas alternating staggering occurs in cellulose Iβ (Jiang and Hsieh 2013; French 2014; Jiang and Hsieh 2015; Huang et al. 2019). The distinct crystal structure polymorph of cellulose II is adopted by cellulose I when it is subjected to a highly alkaline solution or regenerates from a suitable solvent, in the case of the current study, ionic liquid (IL). When cellulose I fibrils are submerged in ammonia or amine solutions, the tiny molecules penetrate the fibrillar structure and cause it to expand, forming a complex structure. After the guest molecules have been washed and evaporated, the cellulose fibrils shorten and transform into cellulose III, a crystalline form (Nitta 1962; GrisoniColli et al. 1967). In an aprotic or nonpolar environment and because of its ability to assemble intricate structures with visitors molecules like ethylenediamine (EDA) and then release these molecules in the presence of a polar solvent, cellulose may be used as a molecular filter. Regenerated cellulose (i.e. cellulose II) fibers are stretched in hot baths to produce cellulose IV; this polymorph’s lattice structure is similar to that of cellulose I (Hakeem et al. 2015b; Salim et al. 2021; Ait Benhamou et al. 2021a).

3.3 Morphological Structure The structural entities created by cellulose molecules are covered at this level. The structures may get increasingly intricate as they grow in size. The presence of different

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cell wall layers and structures in cellulosic fibers is explored on a morphological level. It’s also looked at if there are any voids or interfibrillar interstices. A well-organized structure of fibrillar components makes up the morphological structure of cellulose. Fibril is the smallest unit, with a size ranging from 3−20 nm depending on the cellulose source. The fibrillar entities’ hierarchy in natural cellulose is organized in layers with different fibrillar textures. However, the organization into discrete layers does not occur in regenerated cellulose fibers. The constituent fibrils that make up these synthetic fibers are distributed reasonably randomly throughout the framework. Morphology of the cellulosic materilas can be studied by electron microscopy techniques such as Scanning electron microscopy, transmission electron microscopy, and atomic force microscopy, and can be used to identify the topology, morphology, and size of micro and nanocellulose. The most common approach for the morphological study of cellulosic fibers is scanning electron microscopy (SEM). The SEM picture generated by this study has a three-dimensional structure and may be used to examine cellulose materials (Silva et al. 2018; Beroual et al. 2021; Nehra and Chauhan 2022). SEM scans bulkier photos with a broader field of view and obtains a good image of the sample’s 3D structure using a shadow-relief effect of both secondary and backscattered electron contrast. For example, SEM pictures of raw alfa fiber bundles (Fig. 4a–b) reveal that they are made up of individual microfibers connected by lignin and other non-cellulosic components (Kassab et al. 2019). Chemical and mechanical treatments alter the morphology of fibers, changing their size and smoothness, as well as their size and smoothness. After many chemical treatments, the raw fiber bundle size was reduced and the surface smoothness enhanced (Fig. 4c–f). This is because non-cellulosic components, including hemicellulose, lignin, and waxes are removed. On the other hand, transmission electron microscopy (TEM) is a strong and unique nanoscale imaging tool that develops the sample’s structure with even higher resolution (Tian et al. 2016; Bahrami et al. 2018; Kusmono et al. 2020). Compared to SEM, it can control the sample’s size distribution and surface morphology; nevertheless, the analysis takes longer. Another distinction is that SEM allows for a broader sample area to be scanned at a higher magnification and resolution. As a result, both SEM and TEM may be used to evaluate nanocellulose size and shape and their degree of dispersion and aggregation. Another morphological analysis approach is atomic force microscopy (AFM), which employs a sharp tip that acts as a probe to engage with the external sample surface and map physical characteristics with nanometric accuracy while also quantifying the interaction between distinct structures (El Achaby et al. 2018d; Ait Benhamou et al. 2021b; Perumal et al. 2022). Differences in sample structure, modulus, or probe contact determine the imaging contrast of CNC or CNF. AFM has a high spatial resolution (0.1–100 nm) and requires little sample preparation, making it ideal for studying cell wall structure. AFM study of CNC from hemp stalks generated at various hydrolysis periods (Fig. 5a and b) (15 and 30 min, respectively) (Kassab et al. 2020a). Numerous researches have employed AFM analysis to reveal

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Fig. 4 SEM images of a, b raw fibers, c alkali-treated fibers, d cellulose microfibers, e oxidized cellulose fibers, and f cellulose nanofibers. Reproduced from (Kassab et al. 2019) with permission

that many factors impact the shape and size of nanocellulose, including temperature, time, extraction process pH value, pressure level, chamber size, and cellulose supply.

4 Properties of Cellulose Derivatives Due to its favorable inherent cellulose features, such as its strong biocompatibility, biodegradability, non-toxicity and eco-friendly, cellulose is a sustainable resource that has attracted growing interest in the last two decades (Arrakhiz et al. 2012; Chaouf et al. 2019; Bendahhou et al. 2020; Essaghraoui et al. 2021; Ablouh et al. 2022; Kassem et al. 2022; Risite et al. 2022; Salim et al. 2022). The essential properties of cellulose derivatives issue from annual plants are briefly discussed in this section, including chemical, mechanical, thermal and rheological properties.

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Fig. 5 AFM morphological analysis of a CNC15, b CNC30, and CNF. Reproduced from (Kassab et al. 2020a) with permission

4.1 Chemical Properties Cellulose derivatives are characterized by specific and extraordinary properties that allow applications other than native cellulose. First of all, it is obtained in a highly pure state, devoid of hemicelluloses, lignin, and pigments (Besbes et al. 2011). Understanding the molecular makeup of the cellulose molecule is necessary for understanding the chemical characteristics of cellulose and its derivatives. Three hydroxyl groups and an anhydroglucose unit are present in each monomer unit of cellulose molecule chains. Two types of nanocellulose derivatives are frequently characterized: (i) cellulose nanofibers (CNF) found by a chemical pretreatment and mechanical disintegration process, and (ii) cellulose nanocrystals (CNC) obtained by acidic treatment. To obtain cellulose nanofibers, many chemical surface functionalization reactions may be utilized to regulate the surface chemistry of natural cellulose without appreciably changing its bulk structure. For instance, oxidation, phosphorylation and esterification are common chemical functionalization of cellulose. Cellulose reactivity is facilitatedby the three hydroxyl groups in each monomer. The hydroxyl group at position C6 is 10 times more reactive than the C2 (Fig. 6), which is double that of

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Fig. 6 Chemical modification of cellulose by TEMPO oxidation reaction

position C3 (Lunardi et al. 2021). Given that the hydroxyl group in C6 is bonded to a carbon that is only attached to one alkyl group, in opposition to the carbon atoms carrying the OH groups in the second and third positions, which are bonded to two alkyl groups, this can be caused by the steric hindrance of each group (Jaffar et al. 2022). In the literature, TEMPO-mediated oxidation is a widely used chemical treatment for the preparation of cellulose nanofibers, which is based on the regioselective oxidation of C6 hydroxyl of the cellulosic chain, making the surface negatively charged (with COO- groups) and stimulating the formation of very stable suspension (Besbes et al. 2011). The TEMPO-oxidation treatement of cellulose fibers expressively decreases the number of passes necessary to bring about the nanofibrillation, in which the carboxylate (COO- ) content is an essential parameter that affects the yield of fibrillation and the transparency degree of the ensuing gel. Kassab et al. reported recently that cellulose microfibers extracted from alfa fibers were prepared using a TEMPO-mediated oxidation reaction (Kassab et al. 2019), by causing electrostatic repulsions and disrupting hydrogen bonds between the microfibers, such oxidation can individualize the fibers. Fig. 7 Digital images of aqueous suspensions of cellulose microfibers extracted from Juncus plant (CMF), TEMPO-oxidized CMF (T-CMF) and cellulose nanofibers (CNF). Reproduced from (Kassab et al. 2020d) with permission

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Fig. 8 Possible electrostatic repulsions between individual sulfated cellulose nanocrystals

The cellulose nanofibers suspension’s colloidal stability can be determined by zeta potential, due to the electrostatic repulsion that occurs because of charged chemical groups on nanofibers surface. For instance, Chen et al. showed that the zeta potential value of 0.1 wt% TEMPO-oxidized nanofibers suspensions was −67.0 ± 2.2 mV (Chen et al. 2018). Despite their different carboxylate content rate, the surface density of dissociated carboxylate groups was closely correlated with the ξ-potential values that the suspensions of TEMPO-oxidized CNF showed. Similar outcomes have been published by Kassab et al., when they demonstrate that The TEMPO-oxidized cellulose nanofibers suspension was stable compared to cellulose microfibers and TEMPO-oxidized cellulose microfibers (Kassab et al. 2020d) (Fig. 7). The literature has noted that cellulose can offer the opportunity to isolate cellulose nanocrystals with favorable chemical and physical properties. Sizable specific surface area, high crystallinity, biocompatibility and nontoxicity are characteristics of CNC (Jaffar et al. 2022), which are mainly extracted by acid hydrolysis of native cellulose fibers. This procedure is a fairly well-known chemical method for dissolving the amorphous cellulose components while maintaining the crystalline components. El Achaby et al. made a series of efforts to prepare cellulose nanocrystals from cellulose microfibers extracted from alfa fibers (El Achaby et al. 2018d). For that, cellulose microfibers were treated by acid hydrolysis and then dialysis against distilled water. The presence of sulfate groups was confirmed using conductometric titration measurements, which give information about cellulose nanocrystals’ surface charge densities (Fig. 8).

4.2 Thermal Stability The thermal stability of the cellulose derivatives (CNF and CNC) is quantified by using a thermogravimetric analyzer. The thermogravimetric analysis is carried out to

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Fig. 9 a TGA and b DTG curves of raw Juncus plant, cellulose microfibers and TEMPO-oxidized cellulose nanofibers. Reproduced from (Kassab et al. 2020d) with permission

predict the weight loss with temperature changes under the nitrogen or air environment. Numerous aspects of cellulose are inextricably linked to its molecular structure. Recently, significant efforts have been made to explore new chemical and physical treatments to obtain cellulose derivatives with good thermal stability. According to some reports, the breakdown of cellulose structure starts with the production of glucose units and progresses through the dehydration, oxidation, depolymerization, and disruption of the charred residue into gaseous components (Kassab et al. 2019; Ablouh et al. 2021). Kassab et al. studied the thermal stability of cellulose microfibers extracted from Juncus plant and TEMPO-oxidized cellulose nanofibers (Kassab et al. 2020d). The authors also stated that the TEMPO-oxidation of cellulose nanofibers influences the rate of their thermal deterioration. It was also reported that the Tonset was observed at 211.1 and 204.6 °C for TEMPO-oxidized CMF and CNF, which are lower than cellulose microfibers. It indicates that the Tmax1 and Tmax2 were observed at 235.1 and 286.7 °C for CNF, which are due to COONa decarbonisation, and the degradation of the main cellulose structure (Fig. 9) (Kassab et al. 2020d). Additionally, CNF’s smaller dimensions speed up their disintegration at lower thermal energies. Similar outcomes have been mentioned by Kassab et al. when they studied the thermal stability of cellulose nanofibers obtained from Alfa fibers, which indicate two degradation stages (Kassab et al. 2019). Decarboxylation of carboxylate groups added through TEMPO oxidation caused the initial degradation at about 249 °C, the second degradation (290 °C) was related to the degradation of cellulose. Thermal decomposition temperature for cellulose nanocrystals is usually around 200−300 °C. However, thermal stability can be influenced by physical or chemical modifications such as desulfation of CNC prepared using sulphuric acid hydrolysis treatement. A good example is described by Roman et al., which investigate the effect of introducing the sulfate groups on the CNC surface by controlling the sulfuric acid hydrolysis conditions (Roman 2004). It was evident that the cellulose nanocrystals’ thermal stability significantly decreased as the amount of sulfate groups on their surface increased. The presence of sulfate groups on the CNC surface stimulated

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the degradation processes, as revealed by the substantially lower thermal activation energy for degradation in CNC with high sulfate group concentrations. Kassab et al. have studied the thermal stability of the cellulose nanocrystals extracted from Afla fibers (Kassab et al. 2020a). This study showed how the acidic treatment could strongly influence cellulose degradation.

4.3 Rheological Properties In order to make cellulose gel suspensions and combine them with other polymers to research their potential applications in a variety of areas, such as food packaging, and coating, it is essential to understand their rheological characteristics. Generally, the rheological properties of cellulose gels were studied by determining the shear rate sweep, oscillatory strain sweep and frequency sweep using a parallel plate rotational rheometer (Ure et al. 2011; Shafiei-Sabe et al. 2012; Hamad and Hatzikiriakos 2013; Xu et al. 2020; Ait Benhamou et al. 2022). The linear viscoelastic zones are found by doing oscillatory strain sweeps. To explore the linear viscoelastic characteristics of these materials, oscillatory shear measurements are often taken at a strain after the oscillatory frequency measurements (Nechyporchuk et al. 2016). Numerous works have investigated the rheological properties of CNC and CNF extracted from annual plants to discuss the microstructural changes and the associated outcomes during preparation, handling, and applications (Pech-Cohuo et al. 2018; Pakutsah and Aht-Ong 2020; Kassem et al. 2021). Pakutsah et al. reported that Defibrillation cycles affect the CNF suspension’s stable viscosity as a function of shear rate. They suggested that prolonged defibrillation cycles caused increased viscosity throughout the shear rate range. This may be explained by the fact that when the number of defibrillation cycles increased, hydroxy groups of the CNF formed hydrogen bonds more readily (Fig. 10) (Pakutsah and Aht-Ong 2020).

Fig. 10 Variable defibrillation cycles-prepared cellulose nanofiber suspension: Viscosity as a function of shear rate (5, 10, 15, 20 and 30 cycles) a full shear rate range and b expanded narrow shear rate range. Reproduced from (Pakutsah and Aht-Ong 2020) with permission

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Fig. 11 The proposed cellulose nanofibers networks and TEM image visualization. Reproduced from (Pakutsah and Aht-Ong 2020) with permission

In the same study, a higher viscosity event at a low shear rate range was provided for cellulose nanofibers suspension compared to cellulose nanocrystals extracted from water hyacinth, probably due to robust entangled networks and physical crosslinking fueled by the nanofibers’ hydrogen bonds (Fig. 11). The concentration and temperature of aqueous suspensions affected the rheological properties of CNF and CNC. El Achaby et al, investigated how temperature and concentration (from 20−80 °C) affected the rheological characteristics of the cellulose nanocrystals that were produced from Miscanthus fibers by determination of steady-shear viscosity (El Achaby et al. 2018a). They reported that the viscosity increases as the cellulose nanocrystal’s concentration increases, which may be explained by the increase in the interconnection degree and collision of the CNC gel. At low concentrations of up to 3 wt%, cellulose nanocrystal suspensions are isotropic, but, at greater concentrations, their phases separate to form liquid crystalline and isotropic biphasic suspensions. Thus, a three-region behavior may be seen in the viscosity profile. Additionally, the CNC suspensions exhibit single shear thinning behavior throughout the range of shear rates because they function as rheological gels at even greater concentrations. Similarly, other factors such as ionic strength, temperature and pH of suspension also affect the flow and rheological properties of CNC and CNF suspensions.

4.4 Optical Properties Optical transparency or light transmittance is fascinating characteristic of nanosized cellulose. CNF and CNC permit the transmission of light because the visible light wavelength is higher than the size of nanofibers. Generally, the optical transparency

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of cellulose is improved by different chemical methods such as TEMPO-oxidation, esterification, methylation, acetylation…etc. As the functional group (carboxylate, methyl, …etc.) occupies more space due to its volume, it shoves the nearby nanocellulose chains and rises the volume as well as deteriorates the hydrogen bonding, which leads to relaxation of fibrils. Therefore, the refractive index of nanocellulose is enhanced, which allows more excellent transmittance of light (Fig. 12) (Ifuku et al. 2007). For cellulose nanofibers films, the transparency can be partially attributed to the difference in the size of the nanofibers, especially the diameter and width. In addition, the film transparency depends on the absorption of light, which can be affected by surface roughness. In comparison, cellulose nanocrystals possess better transmittance than cellulose nanofibers due to their high crystallinity and short length. For instance, Jikim et al. observed low light transparency of the cellulose nanofibers film compared to sodium carboxymethyl cellulose film (Kim et al. 2021). This lowest transparency can be affected by the dimensions of the cellulose fibrils, the degree of homogenization, and the surface roughness of the film. Fig. 12 The appearance of the various types of CNF and CNF film. a sodium carboxymethyl cellulose, b CNF obtained by carboxymethylation, c CNF obtained by enzyme hydrolysis, and d CNF obtained by TEMPO-mediated oxidation. Reproduced from (Kim et al. 2021) with permission

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5 Conclusions Cellulose and its derivatives extracted from the annual plant are the prevalent natural biopolymer, has dramatically impacted the world and society in wide-reaching areas. Its excellent structural and physicochemical qualities, including renewability, biocompatibility, low density, optical transparency, adjustable surface chemistry, biodegradability, and nontoxicity appropriate for various applications, have garnered it thus much attention. Several studies about cellulose and nanocellulose derivatives are found, for example, TEMPO-oxidized cellulose nanofibers and cellulose nanocrystals. It showed that the cellulose derivatives issue from the annual plant. Overall, this chapter deals with cellulose structure and describes the most important features of cellulose’s chemistry and chemical technology and its derivatives and their properties. It shows the structure, chemical, morphological, rheological, thermal and optical properties of cellulose and its derivatives issue from annual plants like Alfa, hemp, water hyacinth as potential raw material for cellulose derivatives, which constitute a strong alternative to wood-based nanocellulose, as both cellulose nanofibers and cellulose nanocrystals was easily attainable by modifying the extraction procedures.

References Ablouh E-H, Brouillette F, Taourirte M, Sehaqui H, El Achaby M, Belfkira A (2021) A highly efficient chemical approach to producing green phosphorylated cellulosic macromolecules. RSC Adv 11:24206–24216. https://doi.org/10.1039/D1RA02713A Ablouh EH, Kassab Z, Semlali Aouragh Hassani FZ, El Achaby M, Sehaqui H (2022) Phosphorylated cellulose paper as highly efficient adsorbent for cadmium heavy metal ion removal in aqueous solutions. RSC Adv 12:1084–1094. https://doi.org/10.1039/d1ra08060a Ait Benhamou A, Boussetta A, Kassab Z, Nadifiyine M, Hamid Salim M, Grimi N, EL Achaby M, Moubarik A (2021a) Investigating the characteristics of cactus seeds by-product and their use as a new filler in phenol formaldehyde wood adhesive. Int J Adhes Adhes 110: 102940. https:// doi.org/10.1016/j.ijadhadh.2021.102940 Ait Benhamou A, Kassab Z, Boussetta A, Salim MH, Ablouh E-H, Nadifiyine M, Qaiss AEK, Moubarik A, El Achaby M (2022) Beneficiation of cactus fruit waste seeds for the production of cellulose nanostructures: extraction and properties. Int J Biol Macromol 203: 302−311. https://doi.org/10.1016/j.ijbiomac.2022.01.163 Ait Benhamou A, Kassab Z, Nadifiyine M, Salim MH, Sehaqui H, Moubarik A, El Achaby M (2021) Extraction, characterization and chemical functionalization of phosphorylated cellulose derivatives from giant reed plant. Cellulose 28:4625–4642. https://doi.org/10.1007/s10570-02103842-6 Arrakhiz FZ, Elachaby M, Bouhfid R, Vaudreuil S, Essassi M, Qaiss A (2012) Mechanical and thermal properties of polypropylene reinforced with Alfa fiber under different chemical treatment. Mater Des 35:318–322. https://doi.org/10.1016/j.matdes.2011.09.023 Bahrami B, Behzad T, Zamani A, Heidarian P, Nasri-Nasrabadi B (2018) Optimal design of ozone bleaching parameters to approach cellulose nanofibers extraction from sugarcane bagasse fibers. J Polym Environ 26:4085–4094. https://doi.org/10.1007/s10924-018-1277-5

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Activated Carbon from Agricultural Waste for the Removal of Pollutants from Aqueous Solution Rimene Dhahri, Mongi Ben Mosbah, Ramzi Khiari, Anis Tlili, and Younes Moussaoui

Abstract With increasingly strict environmental protection legislation, industrial development leads to a high need for low-cost and high-performance adsorbents. Due to their availability and eco-friendly character as well as their high porosity, activated carbons are exploited in diversified applications such as adsorption of pollutants in the liquid or gas phase, as well as in catalysis, and energy or gas storage. Activated carbon designates carbon materials with a high porosity and surface area, which has numerous uses in environmental and industrial applications for the removal, recovery, isolation, and processing of several gaseous and liquid phase compounds. Hence, biomass lignocellulosic material derived from agricultural by-products has shown its potential as a valuable raw material for the production of activated carbon, mainly for its low-price availability. Many investigations on lignocellulosic materials as an efficient precursor for activated carbon preparation have been published by different researchers. This chapter illustrates an outline of the different methods of preparation of activated carbon and their available forms and their surface functions and their application in wastewater treatment. R. Dhahri · M. Ben Mosbah Laboratory for the Application of Materials to the Environment, Water and Energy (LR21ES15), Faculty of Sciences of Gafsa, University of Gafsa, Gafsa, Tunisia R. Dhahri · M. Ben Mosbah · Y. Moussaoui (B) Faculty of Sciences of Gafsa, University of Gafsa, Gafsa, Tunisia e-mail: [email protected] R. Khiari Higher Institute of Technological Studies (ISET) of Ksar-Hellal, University of Monastir, Monastir, Ksar Hellal, Tunisia e-mail: [email protected] University of Grenoble Alpes, F-38000 Grenoble, France A. Tlili Institute of Chemistry and Biochemistry (ICBMS–UMR CNRS 5246), Univ Lyon, Université Lyon 1, CNRS, CPE-Lyon, 69622 Villeurbanne, France Y. Moussaoui Organic Chemistry Laboratory (LR17ES08), Faculty of Sciences of Sfax, University of Sfax, Sfax, Tunisia © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 R. Khiari et al. (eds.), Annual Plant: Sources of Fibres, Nanocellulose and Cellulosic Derivatives, Composites Science and Technology, https://doi.org/10.1007/978-981-99-2473-8_16

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Keywords Activated carbon · Lignocellulosic biomass · Adsorption · Pollutants · Wastewater

1 Introduction Activated carbon (AC) is obtained from different types of carbon sources, such as residues or components of biomasses or animals, through physical or chemical activation. It is considered an ancient known adsorbent material (Gayathiri et al. 2022, Mariana et al. 2021). Currently, the increasing requirement for adsorbent materials is driving further research into the preparation of activated carbon from natural lignocellulosic residues (Kaushik et al. 2017). These wastes are widely available, low cost, and represent a potential source of eco-friendly renewable materials (Elhleli et al. 2018). The development of activated carbons from biomasses is considered one of the solutions envisaged for wastewater treatment and many other applications (Danish and Ahmed 2018). Activated carbons are well known and widely used in the industrial world (Machrouhi et al. 2018). The porous carbonaceous materials are obtained by pyrolysis of biomasses (Li et al. 2021), followed by controlled oxidation called activation which gives them a remarkably developed internal porosity. The study of pyrolysis conditions and activation process allows optimizing the porosity (Dhahri et al. 2021, Ahmed et al. 2019). Due to their high porosity, activated carbons are very good adsorbents that are generally used in aqueous or gaseous media for the retention of mineral or organic compounds (Dhahri et al. 2022a, Khadhri et al. 2019). In this context, there is a great attentiveness in focusing on the synthesis of activated carbons from biomass, in particular from agriculture, which is cheap and ecological. Over the years, several preparations of activated carbon were done from residues of the agricultural and agro-industrial sectors, such as rice husk (Khu and Thi 2014), animal manure (Idrees et al. 2018), Sugarcane Bagasse (Kaushik 2017), date palm petiole (Khadhri et al. 2019), Opuntia ficus indica (Elhleli et al. 2020), Posidonia oceanic (Saad et al 2014), palm kernel shell (Rashidi and Yusup 2017), lignin (Taleb et al. 2020), Coconut Shells (Huang et al. 2015). This chapter gives an overview of activated carbon as related to its preparation, characteristics, and application for wastewater treatment.

2 Composition of Lignocellulosic Biomass Cellulose, hemicelluloses and lignin are the predominant polymers encountered in the biomass (Fig. 1) (Ammar et al. 2018), and also contains small quantities of mineral substances (García 2018). Bio-waste can be processed by thermal and chemical treatment to produce a variety of high-value materials, including biofuels, bio-oils, biogas, and activated

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Fig. 1 Principal constitutions of lignocellulosic biomass: cellulose, hemicellulose and lignin

carbons. The composition of such raw materials, as determined by biochemical analysis, is presented in Table 1. Biomass involve mainly lignin, hemicelluloses and cellulose (Iwo et al. 2016), which is generated at different quantities. It is very required to realize the distinctive of certain material used for the intended application. As an example, Abbas and Ahmed (2016) proved that leaves are not appropriate to produce activated carbon, because of their reduced carbon content and high ash percentage. Some research has besides reported that under chemical activation, the activating agent decomposes the lignin more effectively than cellulose contained in the biomass (García 2018, Sharma et al. 2017). There is 10-40% approximately of lignin content in lignocellulosic biomass. Lignocellulosic biomass such as prickly pear seed (Dhahri et al. 2022b) and Saccrarum officinarum contain 34.2 and 39.3% of cellulose (Ferdous et al. 2020), respectively, and some other biomass such as Opuntia ficus-indica and Eucalyptus globulus can contain up to 53% of cellulose (Jimenez et al. 2008, Mannai et al. 2019). However, there are other materials, such as Cattle manure (Table 1), which contain only about 4.6 % of cellulose (Westerholm et al. 2012). Interestingly, the table also highlights that various parts of the same lignocellulosic material can have different proportions of cellulose such as Tomato leaves and stems, which contains 10.9 and 27% of cellulose, respectively (Tiryaki et al. 2014). On the other hand, hemicellulose is considered to be the second most abundant polymer after cellulose, this biomass fraction has a heterogeneous, ramified nature and an average molecular weight of