Advanced Composite Materials: Properties and Applications 9783110574432, 9783110574401

Citation preview

Ehsan Bafekrpour (Ed.) Advanced Composite Materials: Properties and Applications

Ehsan Bafekrpour (Ed.)

Advanced Composite Materials: Properties and Applications

Managing Editor: Irmina Grzegorek Language Editor: Adam Tod Leverton

ISBN: 978-3-11-057440-1 e-ISBN: 978-3-11-057443-2

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License. For details go to http://creativecommons.org/licenses/by-nc-nd/3.0/. Library of Congress Cataloging-in-Publication Data A CIP catalog record for this book has been applied for at the Library of Congress. © 2017 Dr. Ehsan Bafekrpour (ed.) and chapters contributors Published by De Gruyter Open Ltd, Warsaw/Berlin Part of Walter de Gruyter GmbH, Berlin/Boston The book is published with open access at www.degruyter.com. Managing Editor: Irmina Grzegorek Language Editor: Adam Tod Leverton www.degruyteropen.com Cover illustration: © Shama Parveen, Sohel Rana, Raul Fangueiro

Contents Preface

XV

Carla Vilela, Ricardo J. B. Pinto, Ana R. P. Figueiredo, Carlos Pascoal Neto, Armando J. D. Silvestre, Carmen S. R. Freire 1 Development and applications of cellulose nanofibres based polymer nanocomposites 1 1.1 Introduction 1 1.1.1 Nanocelluloses: production, properties and applications 3 1.1.1.1 Nanofibrillated Cellulose (NFC) 4 1.1.1.2 Bacterial Cellulose (BC) 7 1.1.2 Nanocomposites Context 11 1.2 Nanocellulose based Composites 12 1.2.1 With natural polymers 12 1.2.1.1 Alginate based nanocomposites 13 1.2.1.2 Carrageenan based nanocomposites 15 1.2.1.3 Chitin/chitosan based nanocomposites 16 1.2.1.4 Cellulose derivatives based nanocomposites 17 1.2.1.5 Hemicelluloses based nanocomposites 19 1.2.1.6 Starch based nanocomposites 20 1.2.1.7 Pullulan based nanocomposites 22 1.2.1.8 Pectin based nanocomposites 23 1.2.1.9 Multipolysaccharide based nanocomposites 24 1.2.1.10 Natural rubber based nanocomposites 24 1.2.1.11 Polypeptides and proteins based nanocomposites 25 1.2.2 With synthetic polymers 29 1.2.2.1 Water soluble polymers 29 1.2.2.2 Thermoplastic (and thermosetting) polymers 32 1.3 Conclusions and Future Perspectives 43 References 44 Sebastian Heimbs, Tim Bergmann 2 Effect of prestress on the impact response of composite laminates 2.1 Introduction 66 2.2 Literature survey 67 2.3 Low velocity impact on preloaded composite plates 71

66

Materials and manufacturing 72 Test equipment and low velocity impact test conditions 73 Compressive preloading conditions 74 Test results and damage inspection 75 High velocity impact on preloaded composite plates with hard projectiles 79 2.4.1 Materials and manufacturing 79 2.4.2 Test equipment and high velocity impact test conditions 80 2.4.3 Test results and damage inspection 82 2.5 High velocity impact on preloaded composite plates with soft projectiles (bird strike) 86 2.5.1 Materials and manufacturing 87 2.5.2 Test equipment and high velocity impact test conditions 87 2.5.3 Test results and damage inspection 88 2.6 Modelling and simulation of impact on preloaded composite plates 89 2.6.1 Composite material modelling 90 2.6.2 Delamination modelling 92 2.6.3 Preload modelling 94 2.6.4 Low velocity impact simulation on preloaded composite plates 94 2.6.5 High velocity impact simulation on preloaded composite plates 99 2.7 Conclusion 106 References 107 2.3.1 2.3.2 2.3.3 2.3.4 2.4

Sang Yoon Park, Won Jong Choi 3 Production Control Effect on Composite Material Quality and Stability for Aerospace Usage 112 3.1 Introduction 112 3.1.1 Material selection criteria for new generation aircraft 112 3.1.2 Structural Requirements for Certification 117 3.2 Material Qualification Procedures 121 3.2.1 BBA (Building Block Approach) 121 3.3 Material Qualification Procedures 122 3.3.1 M&P Contribution 122 3.4 Material Property Development 123 3.4.1 Material Screening and Selection 124 3.4.2 Material and Process Specification Development 124 3.4.3 Allowables Development 125 3.5 Material and Process Control 130

3.6 An Example Study: Material Acceptance and Equivalency 3.6.1 Material Acceptance 133 3.6.2 Material Equivalency 134 3.6.3 Test Result and Analysis 137 3.7 QCs for Composite Part Manufacturing 140 3.7.1 Manufacturing and QCs Procedure 140 3.7.2 In-process QCs 141 3.8 Part Manufacturer Qualification 143 3.8.1 Property Equivalency Verification 144 3.8.2 Production Level Inspection 144 3.8.3 Engineering Compliance 146 3.9 Process Specification Guidelines 147 3.9.1 Work instructions 147 3.9.2 Material Requirements 148 3.9.3 Facility Requirements 151 3.9.4 Curing tools 154 3.9.5 Lamination 158 3.9.6 Process Equipment Requirements 167 3.10 Monitoring Procedures 181 3.11 Nonconforming Part 182 3.12 Types of Defects 182 3.13 Conclusions 187 References 188 Appendix 194

132

Guijun Xian, Hui Li 4 Hygrothermal Aging of an Ultraviolet Cured Glass-fiber Reinforced Acrylate Composite 195 4.1 Introduction 195 4.2 Raw materials and testing methods 197 4.3 Thermal aging of UV-GFRP without sustained bending 199 4.3.1 Water diffusion and uptake in UV-GFRP coupons 199 4.3.2 Variation of the glass transition temperatures 203 4.3.3 Variation of the tensile properties 204 4.3.3.1 Tensile properties of UV-GFRP immersed in water 204 4.3.3.2 Tensile properties of UV-GFRP immersed in alkaline solution 205 4.4 Aging of UV-GFRP under combined immersion and sustained bending at room temperature 205 4.4.1 Water uptake and diffusion 205

4.4.2 Variation of the glass transition temperatures 4.4.3 Variation of the tensile properties 209 4.5 Conclusions 210 References 211

208

Hui Mei 5 5.1 5.1.1

Thermal and Mechanical Properties of the Ceramic Matrix Composites 214 Modeling and Testing 215 Real-time Monitoring of Thermal Cycling Damage on the Basis of AE Technique 215 5.1.1.1 Monotonic Tensile Behavior 216 5.1.1.2 Strain Response during Thermal Cycles 217 5.1.1.3 Acoustic Emission Characterization of Damage Initiation and Evolution during Thermal Cycles 221 5.1.2 A Strain Evolution Model on the Basis of Physical and Chemical Damage Mechanisms 226 5.1.2.1 Strain Results and Analysis 227 5.1.2.2 Model Formulation 230 5.1.3 Measurement and Calculation of Thermal Residual Stress in CMCs 240 5.1.3.1 Monotonic Tensile Behaviors 241 5.1.3.2 Reloading-unloading Behaviors 243 5.1.3.3 Experimental Measurement of TRS 246 5.1.3.4 Analytical Calculation of TRS considering Matrix Cracking and Interfacial Sliding 250 5.1.3.5 Theoretical Prediction of TRS with Ideal Interfacial Bond and Noncracked Matrix 251 5.1.3.6 Comparison of TRS with a SiC/SiC System 252 5.2 Thermal Properties 253 5.2.1 Thermal Stresses Caused by Load and Displacement Constraints 253 5.2.1.1 Thermal Cycling Response Curves 254 5.2.1.2 Residual Properties 257 5.2.1.3 Microstructures 259 5.2.2 Temperature Gradient and Thermal Stresses 260 5.2.2.1 Definitions of Terms 261 5.2.2.2 Strain Response of C/SiC Composites 262 5.2.2.3 Residual Properties 264 5.2.2.4 Microstructures 265

5.2.3 Fiber Architecture and Thermal Stresses 267 5.2.3.1 Monotonic Tensile Behaviors 267 5.2.3.2 Strain Response of the Composites 269 5.2.3.3 Microstructures 271 5.3 Mechanical Properties 276 5.3.1 Strengthening and Toughening Behaviors after Heat Treatment 276 5.3.1.1 Heat Treatment 277 5.3.1.2 Mechanical Properties 278 5.3.1.3 Microstructures 280 5.3.2 Further Investigation on the Thickness of the PyC Interface 283 5.3.2.1 Post-heat Treatment Tensile Strengths 284 5.3.2.2 Modulus evolutions with HTT 288 5.3.2.3 Post-heat treatment toughness 290 5.3.2.4 AE behavior 291 5.3.2.5 Heat treatment strengthening and toughening mechanisms 293 5.3.3 Mechanical Hysteresis of C/SiC Composites with Different Fiber Preforms 296 5.3.3.1 Fiber Architectures 297 5.3.3.2 Thermal Crack Characterization 299 5.3.3.3 Mechanical Hysteresis Behavior Analysis 300 5.3.3.4 Axial TRS Comparison 305 5.3.3.5 UTS Prediction 307 5.3.4 High Temperature Tensile Properties and Oxidation Behaviors of C/SiC Bolts in A Simulated Re-entry Environment 308 5.3.4.1 C/SiC Bolts Specimen and Re-entry Environment Simulation 308 5.3.4.2 Tensile Properties and Oxidation Behaviors 309 References 316 Hassan M. El-Dessouky 6 Spread Tow Technology for Ultra Lightweight CFRP Composites: Potential and Possibilities 323 6.1 Introduction 323 6.2 Background 324 6.3 Fibre Tow Spreading by Oxeon 325 6.4 Air-Flow Tow Spreading Technology by Harmoni 329 6.5 Experimental Case Studies 332 6.5.1 Spread Tow-Thermoset Composites 332 6.5.2 Spread Tow-Thermoplastic Composites 334 6.5.2.1 Woven Spread Tows 334

6.5.2.2 UD and QI Spread Tows 6.6 Conclusions 347 References 348

342

Long-Cheng Tang, Li Zhao and Li-Zhi Guan 7 Graphene/Polymer Composite Materials: Processing, Properties and Applications 349 7.1 Introduction 349 7.2 Synthesis of graphene 352 7.2.1 Bottom-up approaches 352 7.2.2 Top-down approaches 353 7.2.2.1 Directly exfoliated graphene 353 7.2.2.2 Chemically oxidized/exfoliated graphene 354 7.3 Processing of graphene/polymer composites 357 7.3.1 Nature problem of integration of graphene into polymer 357 7.3.2 Surface modification of graphene 358 7.3.2.1 Non-covalent modification 358 7.3.2.2 Covalent modification 360 7.3.3 Mechanical methods for dispersing graphene into polymer 363 7.3.4 Fabrication of graphene/polymer composites 365 7.3.4.1 Solution mixing 365 7.3.4.2 In situ polymerization 366 7.3.4.3 Melt blending 366 7.3.4.4 Other methods 368 7.4 Properties of graphene/polymer composites 368 7.4.1 Mechanical properties 368 7.4.2 Electrical conductivity 373 7.4.3 Thermal conductivity 376 7.4.4 Thermal stability, glass transition temperature and dimensional stability 379 7.4.5 Other properties 380 7.5 Applications of graphene/polymer composite materials 382 7.5.1 Structural reinforcement materials 382 7.5.2 Functional materials 384 7.5.2.1 Sensors 384 7.5.2.2 Flexible conductor 388 7.5.3 Biomedical applications 390 7.6 Summary and outlook 391 References 393

Fabrizio Sarasini, Debora Puglia, José M. Kenny, Carlo Santulli 8 8.1

Injection moulding of plant fibre composites 420 Introduction to plant fibres as structural reinforcements in polymer matrix composites 421 8.2 Short fibre reinforced thermoplastic composites 425 8.2.1 Processing of natural fibre reinforced thermoplastic composites 426 8.2.1.1 Injection moulding of short fibre reinforced thermoplastic composites 428 8.3 Examples of production of injection moulded plant fibre composites 431 8.4 Conclusions 435 References 436 Ning Xie, Wenzhu Shao, Liang Zhen 9 Percolation in disordered conductor/insulator composites 440 9.1 Introduction 441 9.2 Preparation of disordered insulator/conductor composites 442 9.2.1 Ceramic/Metal composites 442 9.2.2 Carbon/Ceramic composites 443 9.2.3 Carbon/polymer composites 443 9.2.4 Carbon/Cement composites 444 9.3 Critical behavior of disordered insulator/conductor composites 445 9.3.1 Percolation phenomenon 445 9.3.2 Percolation threshold 448 9.3.3 Backbone 450 9.3.4 Critical exponents 453 9.3.5 Fractal characterization 457 9.3.6 Tortuosity 461 9.4 Summary and outlook 463 References 464 Hannah Böhrk 10 Heat Flux Reduction by Transpiration-Cooling of CMCs for Space Applications 468 10.1 A Space Application: Hypersonic Flight 471 10.2 Hot and Cooled Structure Thermal Response 476 10.3 Material Choice 481 10.4 Transpiration Cooling 486

10.5 Perspective and Concluding Remarks References 493

491

Pibo Ma, Gaoming Jiang, and Zhe Gao 11 The three dimensional textile structures for composites 497 11.1 Introduction 497 11.2 Three-dimensional woven structural composite 498 11.2.1 The manufacturing process of 3D woven structural textiles 498 11.2.1.1 Orthogonal woven structure 498 11.2.1.2 Woven interlocking structure 501 11.2.1.3 Woven honeycomb structure 502 11.2.1.4 Woven integrated hollow structure 503 11.2.2 The structure and properties of 3D woven structural composite 504 11.3 Three-dimensional braiding structural composites 506 11.3.1 The manufacturing process of 3D braiding structural textiles 506 11.3.1.1 2-step process 507 11.3.1.2 4-step process 508 11.3.2 The structure and properties of 3D braid structural composite 510 11.4 Three-dimensional knitted structural composites 512 11.4.1 The manufacturing process of 3D knitted structural textiles 512 11.4.1.1 Weft-knitting 513 11.4.1.2 Warp-knitting 516 11.4.2 The structure and properties of 3D knitted structural composite 520 References 523 Josmin P. Jose, Sabu Thomas 12 Cross-linked Polyethylene Nanocomposites for Dielectric Applications 12.1 Introduction 527 12.2 Candidate mechanisms leading to enhanced breakdown characteristics 529 12.3 Experimental evidences 530 12.4 It’s all interfaces 533 12.5 Theoretical models 536 12.6 Effect of polymer morphology on dielectric breakdown characteristics 539 12.7 Conclusions 542 References 543

527

Shama Parveen, Sohel Rana, Raul Fangueiro 13 Advanced Carbon Nanotube Reinforced Multi-scale Composites 545 13.1 Introduction 545 13.2 Fabrication Techniques of Multi-scale Composites 548 13.3 Types of Nanomaterials 549 13.4 Methods of CNT Incorporation Within Multi-scale Composites 550 13.4.1 Dispersion 550 13.4.1.1 Ultrasonication 551 13.4.1.2 Combination of Ultrasonication with Other Techniques 552 13.4.2 Growth of CNTs on Fibre Surface 554 13.4.3 Electrophoretic Deposition 555 13.4.4 Spraying Method 557 13.4.5 Transfer Printing 557 13.4.6 Chemical Grafting Process 558 13.5 Fabrication of Multi-scale Composites 559 13.6 Properties of Multi-scale Composites 560 13.6.1 Mechanical properties 560 13.6.2 Electrical and Thermal Conductivity 564 13.6.3 Electromagnetic Shielding 568 13.6.4 Self-sensing Properties 568 13.6.5 Coefficient of Thermal Expansion (CTE) 569 13.7 Applications of Multi-scale Composites 571 13.8 Conclusions 572 References 573

Preface Composite materials are a major growth area within advanced materials and the range of applications for such products continues to grow and increase in diversity with every new development. Composite products are highly in demand and reached sales of $21.2 billion globally in 2014. The top three market segments in 2014 were transportation, construction, pipes, and tanks. Other segments include energy, automotive, and aerospace. This has, therefore, appeared to me as a favourable opportunity to gather together a comprehensive edited collection to explain the recent advances in composite materials. This state-of-the-art book has been written by high-profile authors who have extensive experience and knowledge in the field of composite materials. The chapters in this collection would be useful for a wide range of audience: undergraduate and post-graduate students students, industrial professionals, materials scientists and researchers, and composite manufacturers. This book provides the reader with a wide range of information in the interdisciplinary subject area of composite materials. The book consists of thirteen chapters. It deals with two types of nanocomposites: graphene and carbon nanotube reinforced nanocomposites, their manufacturing, properties and applications. It also presents fibre reinforced composites and a comprehensive review of bio-composites. Furthurmore, it has a focus on thermal, mechanical and electrical properties of advanced composite materials. Dr. Ehsan Bafekrpour [email protected]

Carla Vilela, Ricardo J. B. Pinto, Ana R. P. Figueiredo, Carlos Pascoal Neto, Armando J. D. Silvestre, Carmen S. R. Freire*

1 Development and applications of cellulose nanofibres based polymer nanocomposites Abstract: In the last decade there has been an increasing interest in the search for biobased sources of novel functional composite materials for application in several domains, such as in packaging, automotive and aeronautic industries, structural materials and electronic devices. Cellulose fibres, due to their abundance and mechanical properties are among the most important resources for the development of biobased composite materials. More recently, cellulose nanofibres like nanofibrillated cellulose (NFC) and bacterial cellulose (BC) gained particular attention in this context because of their unique features and properties. NFC is obtained by disintegration of common plant fibres, using high-pressure homogenizers, and has a high aspect ratio and specific surface area combined with remarkable strength and flexibility, low thermal expansion, high optical transparency and specific barrier properties. BC is produced by several bacteria, as a tridimensional network of nano- and micro-fibrils and has higher purity, water holding capacity, crystallinity, tensile strength and Young’s modulus than conventional plant fibres. In this chapter, a comprehensive overview on the production, processing, properties and applications of cellulose nanofibres (NFC and BC) based polymer nanocomposites will be compiled and discussed. A vast collection of cellulose nanocomposites, such as those with other natural polymers and synthetic thermoplastic and thermosetting matrices, will be addressed aiming to establish the genuine potentialities of cellulose nanofibres in this field.

1.1 Introduction Cellulose is an ubiquitous natural polymer with some 1012 tons being produced each year. It represented and still represents a valuable resource for mankind; first, as a source of energy, shelter, clothing and other daily needs, then as a source of writing substrates (Gandini, 2011), and nowadays, as an inexhaustible source of new functional and sustainable materials for numerous applications (Klemm, 2005). Most cellulose is obtained from plants, where it represents the main structural element of cell walls; but it is also produced by a family of sea animals called

*Corresponding author: Carmen S. R. Freire, CICECO – Aveiro Institute of Materials, Department of Chemistry, Campus de Santiago, University of Aveiro, 3810-193 Aveiro, Portugal, E-mail: [email protected] Carla Vilela, Ricardo J. B. Pinto, Ana R. P. Figueiredo, Carlos Pascoal Neto, Armando J. D. Silvestre, CICECO – Aveiro Institute of Materials, Department of Chemistry, Campus de Santiago, University of Aveiro, 3810-193 Aveiro, Portugal

2 

 Development and applications of cellulose nanofibres based polymer nanocomposites

tunicates, several species of algae and some aerobic non-pathogenic bacteria, as well as through enzymatic and chemical methods (Klemm, 2005). Regardless of its origin, cellulose is a linear homopolysaccharide of β-D-glucopyranose units linked by β-(1→4) glycosidic bonds, varying essentially in purity, degree of polymerization and crystallinity index (Chawla, 2009). The molecular structure of this biopolymer is responsible for properties like hydrophilicity, due to the high density of hydroxyl groups; biodegradability, chain stiffness and broad chemical-modifying capability, through hydroxyl groups (Azizi Samir, 2005; Klemm, 2005; Verlhac, 1990). Hydroxyl groups are also the basis of the abundant intra- and inter-molecular hydrogen bonds within and between individual chains that promote their aggregation into cellulose fibres (Eichhorn, 2009; Gandini, 2008; Huber, 2011; Kalia, 2011; Pérez, 2005). In fact, the strong network of hydrogen bonds results in a highly crystalline structure and is further responsible for cellulose strength, and insolubility in most organic solvents (Eichhorn, 2009; Klemm, 2005). The discovery of nanoscale forms of cellulose (Klemm, 2011), viz. cellulose nanocrystals (CNC) (also known as cellulose whiskers), nanofibrillated cellulose (NFC) (previously referred to as microfibrillated cellulose, MFC) and bacterial cellulose (BC), increased the interest in the production and utilization of cellulosic materials, as emphasized by the almost countless number of scientific publications on the topic, e.g., (Abdul Khalil, 2012; Cowie, 2014; Dufresne, 2012, 2013; Durán, 2012; Eichhorn, 2009; Freire, 2013; Gama, 2013; Habibi, 2014; Isogai, 2013; Kalia, 2014; Khan, 2014; Klemm, 2009, 2011; Plackett, 2014; Shatkin, 2014; Shi, 2013; Siró, 2010; Thakur, 2015). Furthermore, it opened new perspectives for the development of sustainable nanomaterials as a result of the superior and original properties that these nanocellulose forms can convey to the ensuing materials. Thus, it is expected that the potential nanocellulose markets will widen, as discussed by Shatkin et  al. (Shatkin, 2014) and Cowie et  al. (Cowie, 2014), with i) paper and paper packaging, textiles, cement and automobile parts as the applications having the largest potential volume, ii) sensors, construction, aerospace materials, cosmetics, biomaterials, pharmaceuticals, and paint additives as low volume applications, and iii) electric and photonic structures as novel and emerging applications (Shatkin, 2014). The high volume and low volume applications are likely to exist within the next 5 to 10 years, while the novel uses have the potential to become significant but still have technological hurdles to be overcome before coming to market (Cowie, 2014; Shatkin, 2014). The intense R&D in the field of nanocelluloses is also leading several organizations around the world to invest in pilot-scale demonstration plants for the pursuit of nanocelluloses large-scale commercial production. For example, CelluForce (Canada) and Melodea (Israel) are commercializing nanocrystalline cellulose, whereas Borregaard Chemcell (Norway) and Stora Enso (Finland) are commercializing microfibrillated cellulose (MFC). In addition, Bio Vision (Canada) manufactures Nanocel™ (carboxymethylated nanocrystalline cellulose), while Jenpolymers (Germany) sells bacterial cellulose.

Introduction 

 3

In recent years, particular attention has also been devoted to the use of nanocelluloses, especially the nanofibrillar forms (NFC and BC), as reinforcing elements in nanocomposites with several polymeric matrices for applications in a panoply of domains. So, the purpose of the present chapter is to give a clear overview of the production, properties and applications of the nanofibrillar forms of cellulose, and to comprehensively cover some of the more stimulating results reported in recent years concerning the development and applications of cellulose nanofibres based polymer nanocomposites.

1.1.1 Nanocelluloses: production, properties and applications Nanocelluloses, i.e. cellulosic materials that possess at least one dimension in the nanometric scale (Klemm, 2011), are divided into three types depending on the dimensions, functions and preparation methods (Table 1.1): i) cellulose nanocrystals (CNC) prepared by acid hydrolysis of cellulose from different sources (Eichhorn, 2011), ii) nanofibrillated cellulose (NFC) produced via the disintegration of plant cellulose using intense mechanical treatments (combined with chemical and enzymatic treatments) (Abdul Khalil, 2014), as illustrated in Figure 1.1; and also iii) bacterial cellulose (BC) produced by aerobic bacteria (Klemm, 2005). Nevertheless, only the nanofibrillar forms of cellulose (NFC and BC) will be discussed in the present chapter. Details regarding cellulose nanocrystals have been discussed in the relevant literature (Eichhorn, 2011; Habibi, 2010; Moon, 2011). Table 1.1: Types of nanocelluloses (Abdul Khalil, 2014; Klemm, 2011). Type of nanocellulose

Typical sources

Cellulose nanocrystals (CNC)

Wood, cotton, hemp, flax, wheat Acid hydrolysis of cellulose from straw, Avicel, tunicin, cellulose many sources from algae and bacteria Diameter: 5-70 nm Length: 100–300 nm (from plant celluloses); 100 nm to several micrometers (from celluloses of tunicates, algae and bacteria) Wood, sugar beet, potato tuber, Delamination of wood pulp by hemp, flax mechanical pressure before and/ or after chemical or enzymatic treatment Diameter: 5–60 nm Length: several micrometers Low-molecular-weight sugars Bacterial synthesis and alcohols Diameter: 20–100 nm; different types of morphologies

Nanofibrillated cellulose (NFC)

Bacterial cellulose (BC)

Production and average size

4 

 Development and applications of cellulose nanofibres based polymer nanocomposites

Figure 1.1: From macroscopic cellulosic fibres into nanoscale fibrils. Reproduced with permission from (Pääkkö, 2007).

1.1.1.1 Nanofibrillated Cellulose (NFC) Nanofibrillated cellulose, referred as microfibrillated cellulose (MFC) or cellulose nanofibrils (CNF) (Plackett, 2014), consists of long, flexible and entangled nanosized cellulose fibrils with diameters in the range of 5–60 nm and several micrometers length (Klemm, 2011). It exhibits amorphous and crystalline domains, high aspect ratio and specific surface area (Lavoine, 2012). NFC can be obtained from a wide variety of cellulosic sources including wood or agricultural/forest crops or residues (Siró, 2010). So far, NFC has already been prepared from soft and hardwoods (Figure 1.2) (Abe, 2007, 2009; Besbes, 2011; Chen, 2011b; Henriksson, 2007; Okita, 2009; Qian, 2011; Rodionova, 2013; Saito, 2007; Spence, 2010; Stelte, 2009; Uetani, 2011), non-woody plants such as wheat straw (Alemdar, 2008; Chen, 2011c; Hrabalova, 2011; Kaushik, 2011), potato tubers (Dufresne, 2000), sugar beet pulp (Dinand, 1999; Leitner, 2007), banana rachis (Cherian, 2008; Deepa, 2011; Elanthikkal, 2010; Paul, 2008; Zuluaga, 2007, 2009), bamboo (Abe, 2010; Chen, 2011a, 2011c), Opuntia ficus-indica (Habibi, 2009; Malainine, 2005), Luffa cylindrical (Siqueira, 2010), seaweed (Razaq, 2009; Thiripura Sundari, 2012), among several others (Alila, 2013).

Introduction 

 5

Figure 1.2: Cellulose nanofibres obtained via nanofibrillation of never-dried Cryptomeria japonica pulp balloons. Reproduced with permission from (Uetani, 2011).

1.1.1.1.1 NFC production The production of MFC/NFC from wood fibres was first reported by Turbak et  al. (Turbak, 1983) and Herrick et  al. (Herrick, 1983) in the 1980s, by submitting dilute slurries of cellulose fibres from softwood pulp to high shear forces through the use of a Gaulin laboratory homogenizer. The ensuing individualized cellulose micro and/or nanofibrils were obtained in the form of aqueous suspensions bearing the appearance of highly viscous shear-thinning transparent gels, as depicted in Figure 1.3.

Figure 1.3: Picture of an enzymatically pre-treated 2 wt% microfibrillated cellulose aqueous suspension from eucalyptus. Reproduced with permission from (Lavoine, 2012).

Generally speaking, the production principle includes the delamination of cellulosic fibres under an intense high mechanical shearing action to release the NFC. Numerous mechanically induced de-structuring strategies have been reported to prepare NFC, with high-pressure homogenization, microgrinding and microfluidization (Figure 1.4) as the most commonly used mechanical treatments, and high-speed blending, cryocrushing, high-intensity ultrasonication, and steam explosion as the ones not yet amenable to scale-up (Abdul Khalil, 2014; Dufresne, 2012; Habibi, 2014; Kalia, 2014; Siró, 2010).

6 

 Development and applications of cellulose nanofibres based polymer nanocomposites

The high pressure homogenization process is an efficient method that consists in pumping dilute slurries of cellulose fibres at high pressure into a spring-loaded valve assembly (Figure 1.4a) that opens and closes in a rapid succession. The subjection of the fibres to this large pressure drop with shearing and impact forces promotes a high degree of fibrillation resulting in the production of NFC (Abdul Khalil, 2014; Siró, 2010). On the other hand, the microgrinding process, first developed by Masuko® (Tokyo, Japan), consists in the breakdown of the cell wall structure due to the shearing forces generated by two grinding stones (one static and another rotating) (Abdul Khalil, 2014; Lavoine, 2012), as depicted in Figure 1.4b. Finally, the microfluidization process (Figure 1.4c) comprises an intensifier pump to generate high pressure and an interaction chamber to defibrillate the fibres via shear and impact forces against colliding streams and the micro-channel walls (Abdul Khalil, 2014; Lavoine, 2012).

Figure 1.4: Operation schemes of a) high pressure homogenizer (reproduced with permission from (Dufresne, 2012)), b) microgrinder from Masuko Sangyo Co (reproduced with permission from (Kalia, 2014)), and c) microfluidizer® from Microfluidics Inc. (“http://www.microfluidicscorp.com/,” 2014)).

These fibrillation processes are often applied in association with chemical or enzymatic pretreatments (Habibi, 2014; Isogai, 2013; Kalia, 2014; Missoum, 2013; Siró, 2010), in order to increase the reactivity of the fibres by enhancing the accessibility of hydroxyl groups, increasing the inner surface, and breaking cellulose hydrogen bonds (Abdul Khalil, 2014; Szczęsna-Antczak, 2012); and thus, contributing to the improvement of the nanofibrillation level of the resulting NFCs and also to the reduction of the energy consumption associated with the large number of cycles in the mechanical shearing

Introduction 

 7

processes (Dufresne, 2012). It is worth mentioning that the use of pre-treatments and, more importantly, fibrillation techniques, affects the morphological characteristics of NFC. In addition, post-treatments of NFC have also been explored as a route to deal with cellulose hydrophilic nature via surface modification (e.g. acetylation, silylation, application of coupling agents and grafting (Siró, 2010)). In this context, NFC is now a commercially available product from various companies and organizations. In early 2011, the world’s first pilot plant for the production of NFC was launched in Stockholm by Innventia with a daily capacity of 100 kg. The aim of this initiative was to broaden the research to nanocellulose applications that require larger volumes of NFC. Ever since, other suppliers of NFC, such as UPM Fibril Cellulose (Finland) and Engineered Fibres Technology (USA), came into sight.

1.1.1.1.2 NFC properties and general applications As evoked above, NFC can be obtained from a wide variety of cellulosic sources and produced by applying different pre-treatments and fibrillation processes; thus, its properties are greatly influenced not only by the processing method but also by the cellulose fibers source. Nevertheless, NFC presents widths in the nanometric and lengths in the micrometric ranges, a high aspect ratio and specific surface area together with outstanding strength and flexibility, low thermal expansion, high optical transparency, and specific barrier properties (Dufresne, 2012, 2013; Klemm, 2011). In terms of applications, NFC has the potential to be used in a broad range of areas as a result of its outstanding intrinsic properties and the ability to form strong transparent films and porous dense aerogels (Lavoine, 2012). This nanoscale form of cellulose finds application in paper and paperboard formulations, e.g. as a rheology modifier (Dimic-Misic, 2013), strength additive (Eriksen, 2008), and component of food packaging (Khan, 2014). Additionally, NFC or its derivatives and/or composites are suitable for biomedical applications including e.g. drug delivery (Plackett, 2014), as hydrogels for the replacement of human nucleus pulpous (Borges, 2011), and prosthetic heart valves and vascular grafts (Cherian, 2011a). Other applications of modified NFC or composites include adsorbents for CO2 capture (Gebald, 2011), dissolved organic compounds (Maatar, 2013) and oil (Korhonen, 2011), battery separator (Klemm, 2011), among others.

1.1.1.2 Bacterial Cellulose (BC) Besides plants, cellulose is also biosynthesized by some bacteria, mainly those of the genera Gluconacetobacter, Sarcina and Agrobacterium (Klemm, 2001, 2005). Among the bacteria capable of producing cellulose, only those of the Gluconacetobacer genus (formerly named Acetobacter) are so far known to produce cellulose at commercial levels (Shi, 2014b). Gluconacetobacter xylinum was the first cellulose-producer

8 

 Development and applications of cellulose nanofibres based polymer nanocomposites

bacterium described in literature and until now has been used as a model organism for the study of cellulose biosynthesis and properties. It is a non-pathogenic rodshaped, obligate aerobic, Gram-negative bacterium capable of producing cellulose from several carbon and nitrogen sources (Chawla, 2009; Klemm, 2001). Such bacteria are ubiquitous in Nature, being naturally present wherever the fermentation of sugars takes place, for example, on damaged fruits, and also in unpasteurized juice, beer, and wine (Klemm, 2005). In fact, the production of cellulose by bacteria is an interesting method to achieve high purity cellulose; therefore, BC is preferred over its plant counterparts when high purity is aimed due to the minimum clean-up procedures required. In addition, the properties of the resulting cellulose (BC) can be tuned by selection of the bacterial strain, culture media composition and conditions, and through the inclusion of additives (Klemm, 2005).

1.1.1.2.1 BC production The biosynthesis of BC is a regulated multi-step process that involves the synthesis of the cellulose precursor, uridine diphosphoglucose (UDPGlc), followed by its polymerization into the β-1-4-glucan chains. These are afterwards released outwards through pores in the cell surface and assemble into ribbons of crystalline cellulose, whose interwoven nature produces the BC fibrous network (Bielecki, 2005). The reason for BC biosynthesis has been related to its ability to act like a shelter to the bacteria, protecting them against UV radiation, predators and contamination by heavy metals. BC also allows the diffusion of nutrients and maintains the aerobic conditions in the surrounding area necessary to cellular maintenance and growth (Iguchi, 2000; Klemm, 2005). BC has many applications which have stimulated its production at a commercial scale. However, one of the main problems that hampers this process is its low yield and high production costs, especially for low added value applications. In this context, efforts have been made towards the identification of cheap carbon and nutrient sources, namely agriculture and industrial wastes, such as wheat straw acid hydrolysate (Hong, 2011), sugar-cane molasses and corn steep liquor (El-Saied, 2008), grape skins aqueous extract and sulfite pulping liquor (Carreira, 2011), dry olive mill residue (Gomes, 2013) and pineapple agro-industrial residues (Algar, 2014), as alternative to the expensive conventional culture medium, in order to allow economically viable BC production. Another important point to be taken into account in BC production is the cultivation method employed, since this strongly affects the structure and the physical and mechanical properties of the final product. Therefore, the selection must be made according to BCs intended commercial applications (Sani, 2009). BC can be produced in static and agitated conditions. Static cultivation is the most common method, generating a highly hydrated membrane on the air-culture medium interface

Introduction 

 9

(Iguchi, 2000; Watanabe, 1998). However, its low productivity and long cultivation time together with the high working space and workload required, make the potential industrial scale production more expensive (Lin, 2013a; Shi, 2014b). In order to overcome such limitations, research has been focused on the development of efficient static culture reactors (Figure 1.5) among which the Horizontal Lift Reactor (HoLiR) (Kralisch, 2010), rotating disk bioreactor (Serafica, 2002), and the aerosol bioreactor (Hornung, 2007) are the most common. The alternative approach to BC production is through agitated conditions, which generates small pellets, fibres, irregular masses or spherical particles instead of membranes (Cheng, 2009b; Czaja, 2004; Hu, 2010). This method is considered the most suitable method for the commercial production of BC since, in comparison with static culture, it requires less space and a smaller work force, and higher production rates can be achieved (Czaja, 2004; Lin, 2013a).

Figure 1.5: Bacterial cellulose production under static and agitated conditions. Reproduced with permission from (Shi, 2014b).

Attempts to produce BC under agitated conditions have also faced many problems, including the i) spontaneous appearance of mutations in the bacterial strains, which causes a decline in cellulose production; ii) accumulation of BC in fibrous form during cultivation, which increases the viscosity of the broth desabling proper oxygen supply; and iii) an easy attachment of BC to the shaft of reactors, making it hard to recover it and to clean up the reactors (Czaja, 2004; Krystynowicz, 2002).

10 

 Development and applications of cellulose nanofibres based polymer nanocomposites

In order to overcome such drawbacks, more effective agitated culture reactors have been designed and tested for BC production (Figure 1.5), among which the spherical type bubble column bioreactor (Choi, 2009; Song, 2009), air-lift reactor (Chao, 2001a, 2001b, 1997) and modified air-lift reactor are the most well-known (Cheng, 2002). These reactors have improved the productivity of BC, for instance, after 72 hours of cultivation on the modified air-lift reactor three times more BC was produced than that attained using the stirred-tank reactor (Cheng, 2002); and this simplified the product extraction and the reactor clean-up process.

1.1.1.2.2 BC properties and general applications BC shows an ultrafine 3D network of nanofibres (Figure 1.6) with an average diameter 100 times thinner than that of common plant fibres. As a result, the BC membrane is a highly porous structure with significant permeability towards liquids and gases, and high water uptake (water content >90%) (Klemm, 2001).

Figure 1.6: Photograph of a wet BC membrane produced under static conditions and its scanning electron microscopy (SEM) image. Reproduced with permission from (Freire, 2013; Wei, 2011).

The nanometric dimensions of BC fibres result in a high surface area and aspect ratio that contribute to strong interactions with surrounding components, resulting, for example, in the retention of high amounts of water, strong interactions with other polymers and biomaterials, and fixation of different types of nanoparticles (Klemm, 2006), that is a fundamental aspect on the development of novel composite materials, as will be discussed later. BC nanofibres also show low density (Kalia, 2011), being proper for the development of lightweight materials, and high degree of polymerization (Pecoraro, 2008). BC has remarkable mechanical properties, with Young’s Modulus reaching up to 15 GPa (Chawla, 2009; Klemm, 2001; Yamanaka, 1989) and tensile strength around 200–300 MPa (Chawla, 2009; Klemm, 2005), and high thermal stability (with a maximum decomposition temperature ranging between 340–370°C) (Shi, 2012), to which contributes its high crystallinity index (60–80%) (Chawla, 2009;

Introduction 

 11

Czaja, 2006; Siró, 2010). Furthermore, the in situ biosynthesis of bacterial cellulose allows the development of uniform and smooth BC products with defined shapes, by introduction of appropriate molds into the culture media (Klemm, 2001; White, 1989). Finally, BC is biocompatible, non-toxic to animal cells and promotes cell attachment and proliferation (Jeong, 2010; Lina, 2011). As a result of its unique properties, BC has also been employed in several applications. The biomedical applications of BC are the most reported in the literature, with the potential to be used as wound dressing for the treatment of burns (Czaja, 2006), dermal and oral drug delivery systems (Silvestre, 2014) (e.g. lidocaine hydrochloride (Trovatti, 2011, 2012c), ibuprofen (Trovatti, 2012c) and caffeine (Silva, 2013)), temporary skin substitutes (Czaja, 2006), and also as a scaffold for tissue engineering (e.g. artificial blood vessels (Klemm, 2001; Wan, 2011), artificial cornea (Wang, 2010), heart valve prosthesis (Millon, 2006) and artificial bone (Zimmermann, 2011)). BC also finds applications in the food industry, where it can be employed as support for enzymes and cells immobilization (e.g. glucoamylase and wine yeast) (Nguyen, 2009; Ton, 2011; Wu, 2008) as well as potential thickening, stabilizing, gelling and suspending agent (Shi, 2014b). Finally, the remarkable mechanical properties and reinforcing potential, renewability, biodegradability and unique nanostructured porous network of BC make it a perfect candidate for polymer nanocomposites development, as will be later discussed.

1.1.2 Nanocomposites Context Nanocomposites are materials made from two or more structurally complementary materials, normally designated as the matrix and the reinforcing element, which remain individualized and in which one of the elements has at least one nanometric dimension, i.e., between 1 and 100 nm (Azeredo, 2009; Dufresne, 2008; Siró, 2010). The matrix is the continuous phase of the material in which the reinforcement element is embedded. The main purpose of the development of such composites is the possibility of obtaining materials with properties (e.g. mechanical, thermal, barrier, optical, etc) that cannot be obtained from the individual constituents (Dufresne, 2008). The development of BC and/or NFC based nanocomposites is a challenging field that has witnessed a sustained increase in momentum in the last few years. NFC and BC have a tremendous potential to be used in nanocomposite materials since they may allow the production of nanomaterials with: i) increased tensile strength, ii) decreased density, iii) improved barrier properties for sound and oxygen (or other gases), iv) optically transparent and/or colour specific layer coatings, v) biodegradability, and vi) renewability, that can replace current composites in many applications (Shatkin, 2014), or allow the development of entirely new materials with totally diverse and innovative functional properties.

12 

 Development and applications of cellulose nanofibres based polymer nanocomposites

Different strategies have been applied to obtain nanocellulose based nanocomposites, namely using natural and synthetic polymer matrices, as will be discussed latter. In the former case, polysaccharide matrices are of great interest since they are most often naturally compatible with cellulose (nano)fibers due to their structural similarity. This is the reason why simple “green procedures” such as casting of water-based suspensions or melting-mixing can be applied for the production of composites with other polysaccharides or their derivatives and nanocellulose fibres (Freire, 2013). The natural polymers which have been used to prepare nanocomposites with nanocellulose forms include, among others, polysaccharides such as alginate, carrageenan, cellulose derivatives, chitin/chitosan, pullulan, starch, pectin and hemicelluloses and also proteins, such as gelatin and soy. In addition, nanocellulose fibres have also been extensively used to reinforce synthetic polymeric matrices, however in this case the previous chemical modification of the nanocellulose fibres is often required to guarantee a good compatibility with the matrix, particularly with thermoplastic and thermosetting polymers. Poly(vinyl alcohol), poly(lactic acid), polyurethanes, acrylic resins, conductive polymers, are just some examples of synthetic polymers already combined with nanocellulose fibres. In this sense, the coming sections will include a critical appraisal of most relevant developments amongst the extensive literature published on the design of innovative NFC and BC nanocomposite materials with both natural polymers and synthetic thermoplastic and thermosetting matrices, with improved and functional properties.

1.2 Nanocellulose based Composites 1.2.1 With natural polymers Currently, the use of natural polymers in the development of new nanocellulose-based composites has garnered great attention due to the increasing availability and diversity of these biopolymers. Indeed (as for pristine nanocelluloses), the main advantages arising from their use are their potential biodegradability and biocompatibility, low toxicity, renewability and ecological nature. Therefore, it is not surprising that an extended range of studies using distinct polysaccharides as alginate (Cheng, 2009a; Chiaoprakobkij, 2011; Kirdponpattara, 2013; Lemahieu, 2011; Lin, 2012, 2014; Phisalaphong, 2008; Ramana, 2006; Sirviö, 2014; Zhang, 2011a; Zhou, 2007), carrageenan (Martínez, 2013; Savadekar, 2012), chitin/chitosan (Butchosa, 2013; Cai, 2009a; Ciechanska, 2004; Dubey, 2005; Fernandes, 2011, 2009, 2010; Hassan, 2010; Hosokawa, 1990, 1991; Jia, 2014; Kim, 2010; Kingkaew, 2014; Lai, 2014; Lin, 2013b; Liu, 2012a, 2011; Lu, 2013; Nge, 2010; Nordqvist, 2007; Pavaloiu, 2014b; Ul-Islam, 2011; Velásquez-Cock, 2014; Wang, 2012b), cellulose derivatives, namely carboxymethyl cellulose (Cheng, 2011, 2009a; Dimic-Misic, 2013; Haigler, 1982; Hirai, 1998; Ma, 2014; Olszewska, 2013; Pavaloiu,



Nanocellulose based Composites 

 13

2014a; Seifert, 2004), hydroxypropyl cellulose (Zimmermann, 2004), hydroxypropyl methylcellulose (Huang, 2011), cellulose acetate (Pircher, 2014) and cellulose acetate butyrate (Gindl, 2004); hemicelluloses such as xylan (Dammström, 2005, 2009; Hansen, 2012; Iwamoto, 2008; Linder, 2003; Mikkonen, 2012; Peng, 2011; Stepan, 2014; Weimer, 2000) and glucomannan (Lozhechnikova, 2014; Mikkonen, 2011; Tokoh, 1998); starch (da Silva, 2013; Ghosh Dastidar, 2013; Grande, 2008, 2009; Hietala, 2014; LópezRubio, 2007; Martins, 2009, 2012, 2013; Mondragón, 2008; Nainggolan, 2013; Orts, 2005; Plackett, 2010; Spence, 2011; Svagan, 2007, 2009; Wan, 2009; Woehl, 2010; Yang, 2014), pullulan (Trovatti, 2012a, 2012b), pectin (Agoda-Tandjawa, 2012; Chanliaud, 1999; Gu, 2012, 2013; McKenna, 2010; Ninan, 2013, 2014; Touzel, 2003); mixtures of distinct polysaccharides (Liu, 2013, 2014; Ninan, 2013; Tomé, 2013) as well as other natural polymers such as natural rubber (Abraham, 2012a, 2012b, 2013); polypeptides and proteins, such as polylysine (Gao, 2011, 2014; Zhu, 2010), soy proteins (Arboleda, 2013), collagen (Albu, 2014; Luo, 2008; Saska, 2012; Wiegand, 2006; Zhijiang, 2011a) and gelatin (Chang, 2012; Chen, 2013, 2014; Fadel, 2012; Jing, 2013; Kramer, 2006; Lin, 2009; Nakayama, 2004; Wang, 2011, 2012c) have been published in order to obtain bioinspired nanocomposites. As expected, the studies with NFC are fairly more recent in relation to those with BC due to the most recent controlled and large-scale production of the former material. However, both NFC and BC have been applied as reinforcing elements in order to improve the mechanical properties of the ensuing bionanocomposites, as well as (particularly in the case of using BC) other functional properties. There are some differences in the preparative methodologies of the nanocomposites based on the two nanocellulose forms. Typically, when BC is used as a membrane the other biopolymer can be either incorporated directly on the culture medium or later by dipping the membranes on a biopolymer solution. On the other hand, when BC is used in a disintegrated form, or for NFC the nanocomposite materials are typically obtained by mixing the dispersed nanofibers into the matrix.

1.2.1.1 Alginate based nanocomposites Alginate is a linear heteropolysaccharide with a chain composed of block copolymers of (1→4)-linked β-D-mannuronic and α-L-guluronic residues. Typically produced by brown seaweeds, this polyssacharide find numerous applications in biomedical science and engineering, and in food industry (Lee, 2012b). However, the mechanical performance of the obtained hydrogels and matrices are mechanically very weak, limiting their practical applications in the previously mentioned fields. The combination of this biopolymer with BC (Cheng, 2009a; Chiaoprakobkij, 2011; Kirdponpattara, 2013; Lin, 2014; Phisalaphong, 2008; Ramana, 2006; Zhang, 2011a; Zhou, 2007) or more recently with NFC (Lemahieu, 2011; Lin, 2012; Sirviö, 2014) appeared as an interesting and green alternative to overcome this gap, as well as to extend its areas of application.

14 

 Development and applications of cellulose nanofibres based polymer nanocomposites

The first studies on nanocellulose/alginate based nanocomposites were mainly focused on the effect of the addition of sodium alginate on BC production medium (Cheng, 2009a; Zhou, 2007). The results showed that the addition of alginate hindered the formation of large clumps of BC, enhanced cellulose production yield and water retention ability of the final nanocomposite membrane, but decreased BC crystallinity and crystal size; however, the impact on the mechanical properties was not explored in detail. However, recent studies demonstrated the enhancement of the mechanical properties imparted by the addition of BC to alginate, as reported by Chiaoprakobkij et  al. (Chiaoprakobkij, 2011) and Lin et  al. (Lin, 2014). The nanocomposites were prepared by mixing alginate with a disintegrated BC slurry or carboxymethylated BC followed by cross-linking of alginate with Ca2+ and finally processed by freeze-drying. The obtained materials presented organized three-dimensional network structures with improved mechanical strength and stability. These studies also showed meaningful enhancements of the swelling ratios of the nanocomposite materials (up to 212% for 75% carboxymethylated BC) (Lin, 2014). Following distinct approaches, using LiOH/urea/thiourea (Zhang, 2011a) or NaOH/ urea (Phisalaphong, 2008) aqueous systems to dissolve BC prior to mixing with alginate solutions, it was possible to prepare mechanically reinforced BC/alginate fibres by wetspinning, smooth membranes by casting and nanoporous membranes via supercritical drying. These materials have potentialities especially for wound dressing, tissue engineering regeneration, and as separation membranes for chemical processes. Other morphologies can be envisaged for BC/alginate nanocomposites. For example, Kirdponpattara and Phisalaphong (Kirdponpattara, 2013) described an interesting methodology that allowed the preparation of sponge-like materials also via cross-linking of alginate chains by calcium salts. These sponges were found to be very effective for yeast immobilization and it was demonstrated that they can be reused as a yeast cell carrier for ethanol fermentation (Figure 1.7). Amorphous alginate based sponge-like materials were also obtained by combination with NFC (Lin, 2012) following similar procedures to those described above. Here, oxidized NFC fibres with increased amount of carboxyl groups on their surface played a fundamental role in the structural and mechanical stability and strength of the resulting materials. The solvent casting method was applied to prepare nanocomposite cross-linked films based on NFC and alginate (Sirviö, 2014); additionally, by modulating NFC fibre sizes and contents it was possible to prepare films that showed excellent grease barrier properties and reduced water vapour permeability with potential for application in high strength packaging materials. In the same vein, Lemahieu et  al. (Lemahieu, 2011) reported a ground-breaking approach to prepare cross-linked NFC/alginate nanocomposite capsules on a scale of a few millimetres diameter. Furthermore, a patented concept (DOPE process) (Tiquet, 2008) was used to disperse the ensuing capsules on a thermoplastic material (BIOPLAST GF 106/02) by extrusion. However, the obtained composites displayed poor mechanical properties which were explained by the poor dispersion level and low integrity of the capsules.



Nanocellulose based Composites 

 15

Figure 1.7: SEM images of fresh BC/alginate carrier (0 h): (A) cross section and (B) surface; BC/ alginate carrier after 72 h of batch fermentation: (C) cross section and (D) a closer look of yeast cells in a hollow space. Reproduced with permission from (Kirdponpattara, 2013).

1.2.1.2 Carrageenan based nanocomposites Carrageenan, a water-soluble polysaccharide with a linear chain of partially sulphated galactans, is extracted from red seaweed and extensively used in foods, cosmetics, and pharmaceuticals. To the best of our knowledge, the preparation of nanocomposites based on carrageenan and nanocellulose forms is limited to the studies with NFC as reinforcing element (Martínez, 2013; Savadekar, 2012). These studies have demonstrated that the incorporation of a small amount of NFC (up to 1 wt%) followed by the addition of potassium salt to control the aggregation of carrageenan (Martínez, 2013), or simply by addition of glycerol as plasticizer (Savadekar, 2012), can improve significantly the mechanical properties of the nanocomposites. Furthermore, the water vapour and oxygen transmission rate tests carried out with these carrageenan based nanocomposite materials reveal their potential as biodegradable films for foodpackaging applications with improved barrier properties in relation to the control kappa-carrageenan film.

16 

 Development and applications of cellulose nanofibres based polymer nanocomposites

1.2.1.3 Chitin/chitosan based nanocomposites Chitin, the second most abundant polysaccharide in nature, is a linear high molecular weight polysaccharide consisting of β-(1→4)-linked 2-deoxy-2-acetamidoD-glucopyranose monomers. It is the main component of the exoskeleton of crustaceans, therefore widely available as a by-product of sea food processing industries (Rinaudo, 2006). Although plentiful, its use is still limited due to the poor solubility in most solvents and difficulty to process it into useful materials. This fact is also reflected in the low number of published papers of composites of chitin and nanocelluloses. To the best of our knowledge, Butchosa et al. (Butchosa, 2013) reported the only study regarding the preparation of BC/chitin nanocrystals composites. The obtained nanocomposites presented potentialities as substitutes for unfriendly antimicrobial compounds such as heavy metal nanoparticles and synthetic polymers. Chitosan (CH) is produced commercially by deacetylation of chitin and is readily soluble in diluted acidic aqueous solutions and therefore easily processed (Rinaudo, 2006). CH has been widely used for the development of nanocellulosebased Materials, as flexible films (Kingkaew, 2014; Ul-Islam, 2011; Velásquez-Cock, 2014) or as porous network structures (Cai, 2009a; Kim, 2010; Lai, 2014; Liu, 2011; Wang, 2012b). The first studies with BC have focused on the effect of the introduction of CH on the culture medium during its biosynthesis (Cai, 2009a; Ciechanska, 2004; Jia, 2014; Kim, 2010; Ul-Islam, 2011). Although the thermal stability of the materials has been improved, BC crystallinity decreased with the increase in CH content. Additionally, the BC/CH nanocomposites presented better biocompatibility than pure BC (Kim, 2010). A controversial aspect was the mechanical properties of the BC/CH nanocomposites since those studies (Cai, 2009a; Ciechanska, 2004; Jia, 2014; Kim, 2010; Ul-Islam, 2011) reported dissimilar results, probably owing to different preparation methodologies and also to distinct CH molecular weight distribution and content. BC/CH nanocomposites were also prepared by immersion of wet BC membranes in CH solutions followed by freeze-drying (Dubey, 2005; Lin, 2013b). The incorporation of CH led to a more compact 3D nanofibrillar network with smaller pore size, that promoted a decrease in the water swelling ratio, tensile strength and elongation at break; however, it increased Young’s modulus. Furthermore antibacterial properties were observed as a result of the presence of CH. It is noteworthy that with exception of the study reported by Dubey et al. (Dubey, 2005), which addressed the development of BC/CH membranes for pervaporative separation of EtOH/H2O azeotropic mixtures, all the reported studies on the application of BC/CH nanocomposites (Butchosa, 2013; Cai, 2009a; Ciechanska, 2004; Fernandes, 2009; Jia, 2014; Kim, 2010; Kingkaew, 2014; Lai, 2014; Lin, 2013b; Liu, 2011; Lu, 2013; Nge, 2010; Pavaloiu, 2014b; Ul-Islam, 2011; Velásquez-Cock, 2014; Wang, 2012b) have been focused on biomedical applications, namely as dressing material for treating different kinds of wounds, burns and ulcers and for



Nanocellulose based Composites 

 17

tissue engineering (tissue scaffolds and regeneration aids), taking advantage of the well known biological properties of CH (Rinaudo, 2006) and of the improvements in mechanical properties imparted to the final materials by BC. More recent studies demonstrated that the oxidation of BC to introduce carboxyl groups at the nanofibres surface (Lai, 2014; Nge, 2010), the immobilization of heparin on BC (Wang, 2012b), the use of distinct aqueous acids to dissolve CH (Velásquez-Cock, 2014), the use of hydroxypropyl chitosan (Lu, 2013) or water soluble CH derivatives (Fernandes, 2009) instead of CH, the modification of CH molecular weight (Fernandes, 2009; Kingkaew, 2014), and mixing with synthetic polymers as poly(vinyl alcohol) (Pavaloiu, 2014b) promote significant enhancements of specific features of the nanocomposites such as mechanical and antimicrobial properties. The first studies with NFC/CH nanocomposites were reported by Hosokawa and co-workers at early 90’s (Hosokawa, 1990, 1991). Biodegradable composite films with enhanced tensile strength were obtained by cast drying and using glycerol as a plasticizer to improve the flexibility and the softness of the films. The mechanical enhancements induced in CH based nanocomposites by the presence of NFC were also confirmed in other studies (Fernandes, 2010, 2011; Hassan, 2010; Nordqvist, 2007) that likewise emphasized the improvements in the thermal and barrier properties, as well as the films transparency; however, in these cases no plasticizers were applied. Finally, the effect of parameters such as CH molecular weight (Fernandes, 2010, 2011), the use of a water-soluble chitosan derivative (a quaternary ammonium derivative) (Fernandes, 2010, 2011), and NFC origin (Hassan, 2010) on the final properties of the nanocomposites have also been assessed. Following a completely different line of study, Liu and Berglund (Liu, 2012a) demonstrated the possibility of producing nanopaper structures combining a CH solution with montmorillonite and NFC hydrocolloids using a papermaking approach. Using CH as a flocculation agent, a nacre-like multi-layered structure was obtained where the addition of small amounts of NFC dramatically improved the strength and modulus of the CH-modified clay nanopaper composites. The fact that the filtration time during processing was dramatically reduced opened excellent perspectives when looking at a possible future scale-up of this process.

1.2.1.4 Cellulose derivatives based nanocomposites Cellulose derivatives are typically biocompatible polymers that can be used as thickeners, binding agents, film formers, suspension aids, surfactants, stabilizers, especially as additives in food, pharmaceutical, and cosmetic industries (Chang, 2011a). The interest in using cellulose derivatives in association with nanocellulose fibres was driven by the early work of Haigler et al. (Haigler, 1982) which reported the improvement of BC production in static culture by incubation of G. xylinum in a medium containing 0.1% of carboxymethylcellulose (CMC) for 24 h. Following a related procedure, Hirai et al. (Hirai, 1998) contributed to a better understanding

18 

 Development and applications of cellulose nanofibres based polymer nanocomposites

of these materials namely on the formation/assembly and the crystallization mechanism of the BC nanofibrils. The presence of CMC was more effective on producing nanofibrils with smaller sizes rather than on reducing their aggregation. This reduction demonstrated to be correlated with the decrease in mass fraction of cellulose Iα. After these early studies, several others explored the use of this negatively charged water-soluble cellulose derivative to prepare nanocomposite materials with BC (Cheng, 2011, 2009a; Ma, 2014; Pavaloiu, 2014a; Seifert, 2004) or with NFC (Dimic-Misic, 2013; Olszewska, 2013). The introduction of small amount of CMC in the BC culture medium promotes better water retention, metal-ion chelating capacity, thermal and mechanical stability of the ensuing nanocomposites, although it induces a decrease in the crystallinity of BC (Cheng, 2011, 2009a; Seifert, 2004). Similar results were obtained using distinct bacteria, such as Enterobacter sp, in the cultures (Ma, 2014). Nanocomposite hydrogels of these biopolymers can be also obtained by immersion of BC membranes into CMC solutions followed by drying at room temperature or cross-linking by addition of epichlorohydrin (Pavaloiu, 2014a). The CMC content was shown to have an influence on the water swelling and release properties of the hydrogels demonstrating the potential of those materials for controlled drug delivery systems. For NFC/CMC nanocomposites, Olszewska et al. (Olszewska, 2013) proposed an approach which involved the irreversible adsorption of a CMC derivative onto NFC fibrils surface in order to obtain strong hydrogen bonding between the two phases. For this purpose native cellulose fibrils surface were functionalized by adsorption of polyethylene glycol via non-ionic interactions. The adsorption of CMC was afterwards studied using a quartz crystal microbalance with dissipation. In another vein, NFC was also used as partial substitute in CMC based paper-coating colour formulations acting as a natural co-binder (Dimic-Misic, 2013). This influenced the rheological behaviour, viscosity, and elasticity of the coating which resulted in low friction between the water surrounded nanocellulose fibres and pigments, helping phase separation or increased mobility of the coating colour compared to regular CMC coatings. CMC was undoubtedly the most used cellulose derivative in this context, however other derivatives as hydroxypropylcellulose (Zimmermann, 2004), hydroxypropylmethylcellulose (Huang, 2011), cellulose acetate (Pircher, 2014) and cellulose acetate butyrate (Gindl, 2004) were also combined with NFC and BC, demonstrating the possibility of using cellulose derivatives with different chemical functionalities. Some examples include BC/hydroxypropylmethylcellulose nanocomposites that helped to overcome the rehydration limitations of dried BC while extending the storage time and possible applications in biofilm systems (Huang, 2011) and BC/cellulose acetate nanocomposite aerogels that significantly enhanced mechanical resistance towards compressive stress preserving the openporous BC morphology (Pircher, 2014).



Nanocellulose based Composites 

 19

1.2.1.5 Hemicelluloses based nanocomposites Hemicelluloses are the most abundant plant polysaccharides after cellulose. Pulp and paper processing, as well as cultivation of cereal crops continuously produce an enormous amount of by-products with high hemicellulose contents (Mikkonen, 2012; Peng, 2011; Stepan, 2014). Therefore, it is not surprising that different hemicelluloses, as xylans (Dammström, 2005, 2009; Hansen, 2012; Iwamoto, 2008; Linder, 2003; Mikkonen, 2012; Peng, 2011; Stepan, 2014; Weimer, 2000) and glucomannans (Lozhechnikova, 2014; Mikkonen, 2011; Tokoh, 1998) appear as highly promising renewable components for novel nanocomposites with BC and NFC. Several works developed with xylans from such distinct origins as rye (Mikkonen, 2012; Stepan, 2014), bamboo (Peng, 2011), birch wood (Hansen, 2012; Linder, 2003), aspen (Dammström, 2005, 2009), tobacco stalks (Weimer, 2000) and sitka spruce (Iwamoto, 2008) proved the high potential of hemicellulose as a matrix in nanocomposites with nanocelluloses. Dammström et al. (Dammström, 2005, 2009) using dynamic mechanical analysis (DMA) and dynamic Fourier transform infrared spectroscopy (FTIR) concluded that the interaction of plant cellulose and xylan in delignified aspen wood was stronger than in corresponding model compounds with BC. The softening behavior of the BC/ xylan nanocomposites was probably due to differences in the spatial organization of the components. Free-standing NFC/xylan films, with or without the addition of plasticizers such as glycerol, sorbitol or methoxypolyethylene glycol, have been prepared by casting at 23°C and 50% relative humidity for 10 days. The introduction of NFC resulted in the enhancement of their thermal stability and tensile strength (Hansen, 2012; Peng, 2011). In a different study, Mikkonen et al. (Mikkonen, 2012) exploited the use of selective enzymes to decrease the degree of substitution (DS) and polymerization (DP) of rye arabinoxylans (rAXs) in order to mimic the diversity of these naturally occurring xylans. Interestingly, the subsequent addition of NFC significantly increased the tensile strength of films with high molar mass rAXs, but not that of films from low molar mass rAXs. Acetylated rAXs were also used to prepare water and humidity resistant nanocomposite films through the incorporation of NFC (ranging from 1 to 10 wt%) (Stepan, 2014). This strategy was demonstrated to cause a decrease in water permeability; furthermore, the ultimate strength and the Young’s modulus were also improved, opening interesting perspectives for the creation of new and more resistant arabinoxylan packaging materials. Within mannans, O-acetyl galactoglucomannan (GGM) is a partially acetylated water-soluble hemicellulose, with the acetyl groups located randomly at C2 and C3 positions of the mannose units in the main chain (Sjöström, 1993). Tokoh et al. (Tokoh, 1998) reported the first use of this biopolymer in the preparation of nanocomposites with BC through its inclusion in a conventional Hestrin-Schramm culture medium

20 

 Development and applications of cellulose nanofibres based polymer nanocomposites

with G. xylinum. The addition of GGM prevents the tight aggregation of the cellulose nanofibrils and caused a morphological modification from a ribbon to a loose bundle structure. In addition, it was also observed a considerable change on the cellulose Iα/Iβ ratio (Iα content was reduced from 0.73 to 0.43). GGM matrices can also be reinforced with NFC in order to produce stronger and stiffer films (Mikkonen, 2011). For example, NFC/GGM nanocomposites, prepared using glycerol as plasticizer, showed lower softening and damping properties. Nevertheless, the hydrophilic nature of GGM and NFC restricts the use of the ensuing nanocomposites in applications that might involve direct contact with water or high moisture content. To overcome this limitation, GGM can be previously modified using either fatty acids or poly(dimethylsiloxane), producing amphiphilic derivatives that do not hinder the compatibility with NFC, but strongly reduce the water sensitivity of the material (Lozhechnikova, 2014).

1.2.1.6 Starch based nanocomposites Native starch consists of amylose (a linear homopolymer) and amylopectin (an extensively branched homopolymer) strictly composed of glucose units joined by glycosidic bonds. It can be converted into a thermoplastic material, known as thermoplastic starch (TPS), in the presence of water or other plasticizers (generally polyols, such as glycerol) (Martins, 2009). However, as other biopolymers, TPS has some drawbacks, such as poor water resistance and relatively poor mechanical properties. Over again its compounding with BC (da Silva, 2013; Grande, 2008, 2009; Martins, 2009; Nainggolan, 2013; Orts, 2005; Wan, 2009; Woehl, 2010; Yang, 2014) or NFC (Ghosh Dastidar, 2013; Hietala, 2014; López-Rubio, 2007; Martins, 2012, 2013; Mondragón, 2008; Orts, 2005; Plackett, 2010; Spence, 2011; Svagan, 2009, 2007) emerged as a promising strategy for improvement of the mechanical properties due to the good adhesion between the two polysaccharides. Potato (Grande, 2008, 2009; Martins, 2009; Orts, 2005; Yang, 2014), corn (Grande, 2008, 2009; Martins, 2009), wheat (Wan, 2009), or cassava (da Silva, 2013; Woehl, 2010) starches are some possible sources currently available and already studied concerning BC/starch nanocomposites. Furthermore, the use of Mater-Bi® (Nainggolan, 2013), a commercially available starch based polymeric material, was another possibility tested that reflects current commercial interest in this type of biomaterials. The first reports on the preparation of nanocomposite materials with BC and starch were developed by Grande et  al. (2008, 2009) using partially gelatinized starch. The starch granules (2% w/v) were added into the culture medium in order to allow the BC nanofibrils to grow in the presence of a starch phase. The plasticized BC/ TPS nanocomposites sheets were obtained by hot-pressing. It should be noted that significant differences on the mechanical properties of the BC/TPS nanocomposites were observed depending on the starch origin; with potato starch based material



Nanocellulose based Composites 

 21

displaying a similar behaviour compared to that observed with pure BC but superior regarding corn starch based nanocomposite samples (Figure 1.8).

Figure 1.8: Scheme of the BC/TPS production process; (a) Starch granules in suspension in the culture medium; (b) After autoclaving, the starch is partially gelatinized, amylose leaches and granules swell; (c) BC nanofibrils grow in the presence of the partially gelatinized starch; (d) After hot pressing, the nanocomposite shows interpenetrating networks of amylose and cellulose. Reproduced with permission from (Grande, 2009).

Woehl et  al. (Woehl, 2010) demonstrated that Trichoderma reesei endoglucanases can be used for BC hydrolysis decreasing its degree of polymerization without varying its crystallinity index or promoting a significant mass loss such as in acid hydrolysis or mechanical fibrillation. This eliminated less organized regions within fibres allowing a much better dispersion of BC, as well as reduction of defects in the surface of the fibres that could act as crack propagators. Treated BC was further used as reinforcement agent in glycerol-plasticized cassava starch bionanocomposites that were evaluated in terms of in vivo biocompatibility proving their potential in the biomedical field, such as wound dressing and temporary artificial skin. A single step approach that allowed the reinforcement of a TPS matrix during its gelatinization process involved the addition of disintegrated BC as the reinforcing agent and glycerol/water as the plasticizer using a melting mixer. The ensuing composites were prepared using variable BC loads (1-5% (Martins, 2009) and 7.8-22 wt% (Wan, 2009)) and displayed higher tensile strength and modulus for increasing BC fibre contents relatively to the unreinforced starch. Casting films of aqueous suspensions of disintegrated BC (along with NFC) and TPS were employed by Orts et al. (Orts, 2005). All nanocomposites presented a reinforcement effect; although those derived from BC did not improve the Young’s modulus to the same extent as those derived from NFC. Mondragón et al. (Mondragón,

22 

 Development and applications of cellulose nanofibres based polymer nanocomposites

2008) used husks of corncobs nanofibrils, sorbitol, and glyceryl monostearate (GMS) as surfactants to prepare TPS nanocomposites films by casting. The mechanical properties were improved due to the high interactions between the starch matrix and the nanofibres. However, the formation of amylose–GMS complexes, which increase the V-type crystalline structure, may also contribute to this enhancement. López-Rubio et al. (López-Rubio, 2007) verified that when NFC was added to maize amylopectin it was possible to obtain plasticizer-free amylopectin nanocomposites by casting with enough ductility to be easily handled. A process for the reinforcement of potato starch films with NFC that takes place at ambient temperature and pressure with water as the medium was reported by Svagan et  al. (Svagan, 2007, 2009). NFC was well dispersed in the starch matrix, yielding materials with high tensile strength, modulus, work of fracture, and reduced water vapour sorption (nanocomposites with 70 wt% NFC). Other interesting approaches followed for the preparation of mechanically reinforced starch films included the use of a twin-screw extrusion process with a single step that fibrillate cellulose fibres into nanosize range and at same time prepare the nanocomposites with TPS (Hietala, 2014) and a complete polyol free procedure (Plackett, 2010). An interesting environmentally friendly alternative has been developed for engineering waxy maize starch-based nanocomposite films reinforced with NFC applying a non-toxic and water-soluble cross-linker (1,2,3,4-butane tetracarboxylic acid) (Ghosh Dastidar, 2013). The cross-linking increased the stability of the films in water while NFC improved the flexibility without compromising other tensile properties including Young’s modulus and revealing potential commercial applications, such as packaging and coatings. Finally, as starch is a common paper additive, several authors have focused their efforts in the coating possibilities of starch/NFC blends for papermaking applications. Some examples include their use as reliable coating alternative to the traditional polyelectrolytes to fix distinct inorganic fillers (e.g. Ag (Martins, 2012), ZnO (Martins, 2013), CaCO3, and kaolin clay (Spence, 2011)) where in addition to the mechanical improvement other functional properties are attained.

1.2.1.7 Pullulan based nanocomposites Pullulan is a linear homopolysaccharide of glucose described as a α-(1–6) linked polymer of maltotriose units. This water soluble biopolymer, produced aerobically by certain strains of the polymorphic fungus Aureobasidium pullulans, was fairly explored relatively to the preparation of pullulan based-composites with nanocellulose forms. Trovatti et  al. used BC (Trovatti, 2012a) or NFC (Trovatti, 2012b) to prepare pullulan based homogeneous and translucent nanocomposite films, demonstrating an improvement of the thermal stability (up to 40 and 20°C on the maximum degradation temperature, respectively) of the films. Furthermore, when glycerol was used as a plasticizer, it was possible to substantially increase the flexibility and the



Nanocellulose based Composites 

 23

homogeneity of the films (increments of up to 8000% in the Young’s modulus and tensile strength for both NFC and BC nanocomposites). These studies emphasized the possible applications of these nanocomposites in several fields such as the food and pharmaceutical industries, namely as coatings or as packaging materials.

1.2.1.8 Pectin based nanocomposites Pectin is the gel-forming component in primary cell walls and intercellular regions of higher plants. It is a structural heteropolysaccharide containing partially methylesterified homo-galacturonan, rhamnogalacturonan I and II (Gu, 2012), which has been used to prepare several BC (Chanliaud, 1999; Gu, 2012, 2013; McKenna, 2010; Touzel, 2003) and NFC (Agoda-Tandjawa, 2012; Ninan, 2013, 2014) nanocomposites. The pioneering studies on the assembly of nanocelluloses and pectins were mainly focused on the way in which they interact. Some examples comprise the works with BC where the authors tried to mimic the plant cell wall assembly (Chanliaud, 1999) and the lignin polymerization process (Touzel, 2003). In both cases the formation of nanocomposites was achieved by adding pectin to the culture medium of BC at the inoculation time. In the first report the obtained nanocomposites were further chelated with Ca2+; the materials show that the pectin networks became more aggregated resulting in increased uniaxial tension and extensibility and decreased stiffness. In the second case a glass diffusion cell was used to allow the diffusion of both hydrogen peroxide and coniferyl alcohol into a BC/pectin membrane through dialysis membranes. The in vitro polymerization of coniferyl alcohol occurred within the composite membrane, and it was observed that pectin worked as a compatibilizer that induced a better dispersion of the synthetic lignin in the cellulose network and enhanced the proportion of alkyl-aryl-ether linkages in the polymer (Touzel, 2003). Following this line of research, McKenna et al. (McKenna, 2010) investigated the effects of distinct metal ions (e.g. aluminium, copper and ruthenium) on the saturated hydraulic conductivity of BC/pectin composites. BC composites in their natural hydrated state mimic the hydration state of primary plant cell walls and provide a useful model system for plant cell walls. The conductivity was reduced to 30-55% of the initial flow depending on the metal ion used as a result of changes in BC/pectin composites porosity. Pectin composition has also influence on the rheological and microstructural properties of NFC/pectin nanocomposites as evaluated by Agoda-Tandjawa et  al. (Agoda-Tandjawa, 2012). In fact, low methoxyl pectin (LM - degree of methyl esterification less than 50%) alone does not significantly modify the viscoelastic and microstructural properties of the nanocomposites. However, this limitation can be overcome by the addition of calcium and/or sodium ions which have synergistic effect that induced pectin gelation and contributed to the formation of stronger composite gels. Nevertheless as highlighted by Ninan et al. (Ninan, 2013, 2014), the use of NFC/

24 

 Development and applications of cellulose nanofibres based polymer nanocomposites

LM pectin systems without the presence of any cations was also plausible, opening interesting opportunities for composite scaffolds and wound dressing materials. This application was validated by the in vivo studies conducted on Sprague Dawley rats that showed the promotion of skin regeneration within ten days and significant collagen deposition (Ninan, 2014).

1.2.1.9 Multipolysaccharide based nanocomposites Nowadays, the combination of several biopolymers in order to obtain new nanocellulose-based biocomposites is an interesting and innovative approach to eventually surpass the individual limitations of each biopolymer, ending up with multicomponent nanocomposites with improved combinations of properties. In this vein, Tomé et  al. (Tomé, 2013) reported the preparation of thin films composed of TPS, CH and cellulose nanofibres (BC and NFC) by solvent casting of water based suspensions of the three components. It was demonstrated that TPS played a significant role on the thermal stability of the films while the mechanical properties were mainly governed by the nanocellulose content (NFC or BC) and the transparency and biological activity were imparted by CH. In a different study, Liu et  al. (Liu, 2013, 2014) described an original methodology to prepare cross-linked CH complexes with benzalkonium chloride by ionic gelation using tripolyphosphate for the nanoencapsulation of distinct biocides (methylisothiazolinone and benzalkonium chloride) and their adsorption on microfibrillated cellulose. These biocomposites were then used to enhance the antibacterial and mechanical properties of agar and sodium alginate films, respectively. Other interesting studies regarding the preparation of highly porous NFC/CMC/ pectin three-dimensional scaffolds by a freeze-drying process (Ninan, 2013, 2014) were described above. Merging their polyelectrolyte nature with pectin’s ability to enable immobilization of bioactive components or cells and the mechanical performance of NFC, the authors developed interesting nanocomposite scaffolds for tissue engineering applications. Gu and Catchmark (Gu, 2012, 2013) have studied BC/pectin/xyloglucan ternary systems with various ratios of xyloglucan and pectin. Xyloglucan and pectin caused morphological changes in cellulose assembly and improved the binding between the BC fibrils, however had a poor impact on the mechanical performance of the composites.

1.2.1.10 Natural rubber based nanocomposites Natural rubber is a biopolymer consisting of isoprene units (C5H8)n linked together in a 1,4-cis-configuration. This natural biopolymer has a high economic and strategic importance since it cannot be replaced by synthetic alternatives owing to its unique properties (e.g. resilience, elasticity, abrasion and impact resistance, efficient heat



Nanocellulose based Composites 

 25

dispersion and malleability at cold temperatures). Rubber trees (Hevea brasiliensis) are currently the only commercial source of natural latex (colloidal suspension of rubber particles) (van Beilen, 2007). To the best of our knowledge the only reports on the preparation of nanocomposites of natural rubber and nanocellulose were published by Abraham et  al. (Abraham, 2012a, 2012b, 2013) who used NFC, obtained from banana fibres by steam explosion, as reinforcing material. The authors studied the morphological, crystallographic and mechanical properties of the resulting nanocomposites obtained by casting (Abraham, 2012a). The NFC filler was evenly distributed on the nanocomposites and the increment of its content caused an increase of the Young’s modulus and of the tensile strength of materials, but decreased the characteristic rubber flexibility and crystallinity. Furthermore, the biodegradability (Abraham, 2012b) and the transport properties of the films using organic solvents (Abraham, 2013) were also evaluated. The nanocomposites have demonstrated to be fully biodegradable and it was observed that the presence of nanocellulose favoured the degradation of the inner part of the composite. The diffusion coefficient decreased as a function of the concentration of nanofibres contrary to the solvent resistance that increased due to the restriction caused by the hydrophilic hydroxyl groups against hydrophobic organic solvents.

1.2.1.11 Polypeptides and proteins based nanocomposites ε-Polylysine (EPL) is a natural peptide of L-lysine units produced by bacterial fermentation and it is usually applied as a natural and safe preservative in food products due to its antibacterial properties against gram-positive and gram-negative bacteria. The few studies on the preparation of nanocomposites of this biopolymer and BC reported their preparation by immersing BC tubes on an EPL solution (Zhu, 2010) or immersion BC membranes followed by cross-linking via a green methodology using procyanidin (Gao, 2011, 2014). In general, the antibacterial activity of the nanocomposites was enhanced with the increase of EPL concentration allowing the preparation of materials with improved antibacterial properties e.g. for applications in the food industry (Zhu, 2010, Gao, 2014) (Figure 1.9). The BC/EPL nanocomposites can act also as nanotemplates to induce cell adhesion and regeneration, making them promising biomaterials in bone tissue engineering (Gao, 2011). A variety of proteins have also been studied for the preparation of nanocellulose based composites, including soy protein (Arboleda, 2013), collagen (Albu, 2014; Luo, 2008; Saska, 2012; Wiegand, 2006; Zhijiang, 2011a) and gelatin (Chang, 2012; Chen, 2013, 2014; Fadel, 2012; Jing, 2013; Kramer, 2006; Lin, 2009; Nakayama, 2004; Wang, 2011, 2012c). Soy proteins (SPs), are a by-product of the soy oil industry currently exploited in applications such as animal feed. However, due to their abundance, low price, nonanimal origin and relatively long storage time and stability, these proteins possess high

26 

 Development and applications of cellulose nanofibres based polymer nanocomposites

potential for the development of composite materials. Despite being biodegradable and biocompatible, the practical use of SPs for the development of solid materials is still challenging because they tend to be brittle and possess limited mechanical strength. In order to overcome those limitations, the preparation of NFC/SPs nanocomposites has been reported (Arboleda, 2013). Specifically, SPs were investigated for the development of aerogels by freeze-drying of an SPs aqueous solution in which NFC was dispersed. The developed porous nanocomposites showed improved mechanical properties and water (and other solvents) absorption while maintaining their physical integrity. Another interesting aspect was that high soy protein content decreased the water sorption rate but increased the maximum loading capacity.

Figure 1.9: Photos and SEM images of (a) BC and (b) BC/EPL nanocomposite (EPL concentration of 0.5%) with the insets showing the respective TEM images; (c) antibacterial activity (halo test) of the nanocomposite against Escherichia coli (left column) and Staphylococcus aureus (right column). Adapted with permission from (Gao, 2014).



Nanocellulose based Composites 

 27

Collagen is the main structural protein of various connective tissues in animals, working as a physical scaffold that supports cell proliferation, but also enhances cell adhesion, and function (Saska, 2012; Zhijiang, 2011a). The studies concerning nanocellulose/collagen composites focused only on BC for the improvement of their mechanical and biological properties. Initially, BC/collagen composites were prepared by two methods namely the addition of type I collagen to the BC culture medium (Luo, 2008; Wiegand, 2006) or cyclic immersion of the wet BC membrane in a collagen solution followed by a freeze-drying process (Zhijiang, 2011a). SEM images showed that the morphology of these nanocomposites was more rugged and porous than BC. Furthermore, collagen polymeric chains were not only on the BC membrane surface but also could penetrate inside the tri-dimensional network. These nanocomposites were able to reduce the amount of selected proteases and interleukins (in vitro) and showed to possess a distinctive antioxidant capacity (Wiegand, 2006). Following the second approach, Albu et  al. (Albu, 2014) also prepared a series of nanocomposites using collagen in three different forms namely gel, solution and hydrolyzed collagen with bacterial cellulose. All the composites presented enhanced elastic modulus (an increase of 60 times for the BC based composites prepared with the collagen solution) even though they had less elongation at break. The outstanding values for the case of the nanocomposites obtained from the collagen solution were explained by the easiest penetration of collagen in the BC network. Nevertheless, the two methodologies mentioned above to prepare BC/collagen nanocomposites can present some disadvantages such as poor homogeneity, a noncontrollable collagen content and the denaturation of collagen during the purification process. To overcome these limitations Saska et  al. (Saska, 2012) developed a ground-breaking study that enabled a covalently bonded and homogeneous incorporation of collagen into the BC membrane, which were previously esterified with Fmoc (9-flourophenylmethoxycarbonyl)-glycine followed by cross-linking of type I collagen from rat tail tendons employing 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC). The elastic modulus and tensile strength of the resulting material slightly decreased, however cell viability/proliferation was not affected indicating that this composite allowed the in vitro development of the osteoblastic phenotype. A similar protein used was gelatin, a polypeptide derived from partial hydrolysis of native collagens (Chen, 2014). Gelatin films have a typical brittle behaviour which limits their application. Once more, substantial improvements in the properties of the materials can be achieved by combination with nanocellulose forms. For example, in the pioneering work of Nakayama et al. (Nakayama, 2004) BC/gelatin double-network hydrogels were successfully prepared, presenting enhanced mechanical strength and low mechanical frictional coefficients. This strategy presented good perspectives for new soft and wet materials as substitutes for artificial cartilage and other tissues. The mechanical strength and the

28 

 Development and applications of cellulose nanofibres based polymer nanocomposites

rehydration properties of these nanocomposites can be enhanced by cross-linking BC fibrils and gelatin using different cross-linkers as methacrylate (Kramer, 2006), procyanidins (Wang, 2011, 2012c), glutaraldehyde (Chen, 2014), transglutaminase and genipin (Chang, 2012). The rehydration ratio and rate can also be improved combining enzymatically modified gelatin (EMG) with BC (Lin, 2009) or HPMCmodified BC (Chen, 2013) by immersion or adsorption methods (Figure 1.10). These materials have potential use as rehydratable membranes with high storage ability and desirable rehydration properties to be applied as antibacterial food cloth, masks or wound dressings.

Figure 1.10: Images of the appearance of BC, BC/gelatin and BC/EMG before freeze-drying, after freeze-drying, and after rehydration. Adapted with permission from (Lin, 2009).

Via a laser patterning process, Jing et  al. (Jing, 2013) demonstrated that it was possible to introduce regular vertical parallel-channel pores (200–300 µm in diameter) into a BC membrane. Then, this porous membrane was modified with gelatin and hydroxyapatite that enwrapped the surfaces of nanofibres in the pore wall retaining the BC 3D network structure. The chemical and crystal structures of the matrix were not affected and the cytocompatibility tests showed that these scaffolds support cell attachment and proliferation besides keeping their viability. To the best of our knowledge, the only study about NFC/gelatin composites was reported by Fadel et  al. (Fadel, 2012) where the direct mixing of NFC and gelatin (using glutaraldehyde as a cross-linker) allowed the preparation of films with improved wet and dry tensile strength properties without significantly affecting their transparency. Furthermore, the addition of NFC reduced the permeability of water vapour across the cross-linked gelatin film and decreased its water absorption.



Nanocellulose based Composites 

 29

1.2.2 With synthetic polymers 1.2.2.1 Water soluble polymers Poly(vinyl alcohol), (PVA), has received a lot of attention in recent years for the development of cellulose nanofibres based composites because of its water solubility, good mechanical properties, biocompatibility and high compatibility with cellulose (Baker, 2012). NFC/PVA nanocomposites are essentially prepared by dispersion of nanofibres obtained from several biomass sources, normally at low contents (1-10%), into PVA aqueous solutions typically followed by solvent casting (Cheng, 2009c; Clemons, 2013; Frone, 2011; Kamphunthong, 2012; Lani, 2014; Li, 2013; Souza, 2010; Spoljaric, 2013a; Srithep, 2012; Virtanen, 2014; Wang, 2007; Zhou, 2012). One of the first studies with this synthetic polymer and NFC focused on the isolation of cellulose nanofibres (50-100 nm diameter and in the scale micrometre length) from a soybean source and their incorporation into PVA (as well as polypropylene and polyethylene as discussed later on in this chapter) (Wang, 2007). The evaluation of the mechanical properties showed an increase in tensile strength (from 65 MPa for pure PVA to 103 MPa for PVA reinforced with 5 wt% nanofibres) and in the Young’s modulus (from 2.3 GPa for pure PVA to 6.2 GPa for PVA reinforced with 5 wt% nanofibres). In another study, micro and nanoscales cellulose fibrils, generated from different biomass sources (regenerated cellulose fibres, pure cellulose fibres and microcrystalline cellulose) by ultrasonic treatment, were used to reinforce PVA (Cheng, 2009c). The mechanical performance of the ensuing PVA based composites was expressively improved by most of the small fibrils; however, the morphological characterization revealed that the dispersion of the cellulose nanofibrils was uniform on the surfaces but not totally in the matrix cross-section. Frone et  al. also used cellulose nanofibres obtained from microcrystalline cellulose by ultrasonication as reinforcement (at low contents 1-5 wt%) dispersed in PVA (Frone, 2011). In addition to the expected improvements in terms of mechanical properties, slightly higher onset degradation temperatures were also observed for the nanocomposites in comparison with neat PVA. More recently, Li et  al. (Li, 2013) also investigated the isolation of nanocellulose from bleached hard Kraft pulp and their reinforcing ability for PVA obtaining similar results in terms of mechanical properties improvements. In a quite different vein, nanofibrillated cellulose-PVA composites films were prepared by casting and physically foamed (using CO2 and/or water as foaming agents in a batch process) and their properties (including dynamic mechanical properties, crystallization behaviour and carbon dioxide solubility) were evaluated (Srithep, 2012). It was observed that the addition of NFC increased the tensile strength of the nanocomposites, however higher NFC contents led to lower thermal stability of the PVA based nanocomposites. As predicted, due to the water solubility of PVA and high hydrophilicity of NFC, these nanocomposites were sensitive to moisture

30 

 Development and applications of cellulose nanofibres based polymer nanocomposites

content and DMA analysis showed that at room temperature the E’ increased with the reducing moisture level. Regarding the CO2 solubility in the nanocomposites, it depends on their moisture content and decreased with the addition of NFC. Finally, the nanocomposites exhibited better and more anisotropic cell morphologies than neat PVA foamed films. The combination of PVA and nanofibrillated cellulose with additional components has also been investigated as a way to improve the properties of the nanocomposites or to introduce other functionalities. For instance, Clemons et al. studied the distribution of poly(acrylic acid) (PAA) in model laminates composed of NFC and PVA by FTIR chemical imaging (Clemons, 2013). PAA can improve the performance of the NFC reinforced PVA laminates by cross-linking with PVA matrix and the adhesion between PVA and NFC through ester linkages between them. However, it was observed that PAA migrates preferentially out of the PVA matrix and concentrates at the NFC layer leading to a weak interface region and consequently to a restricted stress transfer. In a different study, Dai et al. (Dai, 2014) prepared composite nanofibres of PVA and waterborne polyurethane reinforced with TEMPO-oxidized cellulose nanofibrils by electrospinning. SEM images revealed a highly homogeneous dispersion of the cellulose nanofibrils into the PVA/waterborne polyurethane matrix. These reinforced nanocomposites showed improved mechanical properties (44% increment in tensile strength for only 5 wt% of cellulose nanofibrils) and thermal stability, when compared to the pure PVA/waterborne polyurethane nanofibres and could be considered for application in tissue scaffolding and wound dressing materials. More recently, nanocomposites of PVA, NFC and montmorillonite clay (MTT) were prepared by solvent casting (Spoljaric, 2013a). Thermal cross-linking of the PVA matrix with PAA was also investigated. SEM results revealed an effective NFC and MTT dispersion throughout the nanocomposites matrix. The barrier properties against water and oxygen, and the thermal stability of the nanocomposites were improved by the addition of MTT and by the cross-linking, while the incorporation of NFC had essentially a pronounced impact on the mechanical properties. Lani et al. (Lani, 2014) produced nanocellulose fibres from empty fruit bunch and studied their reinforcing effect on PVA/starch blends. PVA/starch films reinforced with 5% (v/v) of nanocellulose fibres showed the best combination of properties (tensile strength of ~5.7 MPa, elongation at break of ~482% and water absorption of ~20%). The chemical modification of NFC has also been considered as a simple strategy to improve the mechanical properties of NFC/PVA based composites. For example, Virtanen et al. (Virtanen, 2014) used chemically modified (allylated and subsequently epoxidised with hydrogen peroxide) NFC at low contents (0.5-3%) as reinforcement in PVA matrices. The nanocomposites showed improved mechanical performance (474% and 224% improvements in Young’s modulus and tensile strength, respectively, for 1% addition of modified NFC) and good optical properties (83% visible light transmittance for 1% addition of modified NFC).



Nanocellulose based Composites 

 31

BC/PVA nanocomposites had also been extensively investigated; however, in this case distinct methodologies can be adopted, namely: i) the addition of PVA to the culture medium during BC biosynthesis (Gea, 2010a; Seifert, 2004), ii) the diffusion of a PVA solution into a BC membrane followed by drying (or other processing methodology) (Gea, 2010a; Qiu, 2012; Wang, 2010) and iii) the dispersion of disintegrated BC into a PVA solution followed by casting (Kibédi-Szabó, 2011). As an illustrative example, Seifert et al. (Seifert, 2004) reported that the addition of PVA (as well as other water soluble polymers as carboxymethylcellulose and methylcellulose, as already referred in the previous sections) into Hestrim-Schramm culture medium originates in general nanocomposites with improved water retention capability and copper ion sorption ability. Gea et  al. (Gea, 2010a) described BC/ PVA nanocomposites prepared by impregnation of a BC membrane with a PVA solution as well as by the in situ growth methodology. The in situ process proved to be more advantageous since it resulted in nanocomposites with superior mechanical and optical properties. The authors explained their different results based on a more homogeneous nanostructure obtained in the in situ method; however, the lower estimated content of PVA incorporated in BC network in this case (1.3% vs. 3.7%) had certainly the most relevant influence on the general performance of the nanocomposites. Anisotropic BC/PVA nanocomposites were obtained by cross-linking of a PVA matrix using a thermal processing methodology under an applied strain and with the addition of small amounts of BC nanofibres (e.g. 0.3 wt%) (Millon, 2008). The stressstrain properties of these nanocomposites closely match those of porcine aorta in both circumferential and axial directions and therefore present enormous potentialities as replacements for cardiovascular and other connective tissues. Subsequently, BC/PVA nanocomposite hydrogels mimicking aortic heart valve leaflet behaviour were also prepared by a similar methodology (optimal conditions: 15% PVA, 0.5 BC cycle 5, 75% initial tensile strain) (Mohammadi, 2011). In a different study, Wang et  al. (Wang, 2010) investigated the potential use of BC/PVA hydrogel nanocomposites, prepared by freezing-thaw method, as an artificial cornea replacement. The nanocomposites showed desirable properties as high water content (around 70%), high visible transmittance (up to 90%), improved mechanical strength (3.9 MPa for 12 wt% of BC) and appropriate thermal stability (Tonset of 252°C for 12 wt% of BC). In two different studies, layered BC/PVA nanocomposite hydrogels were also prepared by means of a physical method of freezing and thawing (Tan, 2011, 2012). The tensile strength and Young’s modulus of the hydrogels reached 1.74 and 7.82 MPa, respectively, when composed of 15 wt% of PVA and two layers of BC. In a different vein, Qiu and Netravali (Qiu, 2012) enhanced the mechanical properties and thermal stability of BC/PVA nanocomposites by chemical crosslinking of PVA matrix with glutaraldehyde, after BC membrane impregnation with PVA solution and drying. These nanocomposite materials represent good candidates for substituting conventional non-biodegradable plastics in several applications.

32 

 Development and applications of cellulose nanofibres based polymer nanocomposites

In a different study, the biodegradation of BC/PVA composites, prepared by solvent casting, was enhanced by the addition of chitosan (Kibédi-Szabó, 2011). This performance was attributed to the probable emulsifying effect of chitosan, originating more homogeneous composites, or to the presence of chitinases in the activated sludge used. However, no clear support for these assumptions was indicated in the report. Poly(ethylene glycol) (PEG), is another water soluble synthetic polymer with singular properties and renowned applications in several fields, and that has been also combined with both NFC (Wang, 2014c; Willgert, 2014) and BC (Cai, 2009b; Liu, 2012b) to produce innovative nanocomposites. For example, Wang et al. (Wang, 2014c) developed novel nanofibrous ultrafiltration membranes based on a nanocomposite barrier layer made of a cross-linked PEG matrix and cellulose nanofibres (~5 nm in diameter) prepared by UV radiation of functional EG monomers in the nanofibrous cellulose layer scaffold. The resulting membrane was hydrophilic and led to enhanced anti-fouling ability compared to similar commercial counterparts. In another study, Willgert et al. (Willgert, 2014) described the preparation of NFC reinforced composite polyelectrolytes for lithium ion battery applications composed of an ionic conductive PEG matrix. NFC was previously modified with acryloyl chloride and propionyl chloride aiming to create covalent bonds between the two phases upon the polymerization of acryloyl moieties, and consequent crosslinking between NFC and acryloyl modified PEG domains. Ionic conductivity of 5x10-5 S.cm-1 and an elastic modulus of 400 MPa were obtained for composite polyelectrolyte after swelling at 25°C. BC/PEG biocompatible porous nanocomposites were prepared by immersing a wet BC membrane in 1% PEG aqueous solution followed by a freeze-drying stage (Cai, 2009b). The thermal stability of the nanocomposites was improved from 263°C to 293°C, most probably due to the strong interactions established between BC nanofibrils and PEG chains. Though, as anticipated, the tensile properties of the nanocomposites tended to decrease when compared to BC since PEG can act as a plasticizer of the BC network structure. These porous scaffolds can be used for wound dressing or tissue engineering. BC/PEG composites have also been prepared by adding PEG, with distinct molecular weights, into G. xilinum culture medium (Liu, 2012b).

1.2.2.2 Thermoplastic (and thermosetting) polymers The strengthening of synthetic thermoplastic (and thermosetting) polymers with cellulose nanofibrils has also been widely investigated because of their widespread usage, high performance and simplicity of processing. However, in dissimilarity to the excellent compatibility between cellulose nanofibres and natural polymers (or water soluble polymers), one of the main difficulties tackled by researchers in this field is the poor adhesion between intrinsically polar cellulose nanofibres and the non-polar nature of most thermoplastic matrices. Several methodologies have been reported



Nanocellulose based Composites 

 33

in literature targeting to circumvent this constraint. Moreover, the use of harmful solvents in several processing steps (as for example in nanofibers modification towards compatibilization and often on the dissolution of these polymers for casting) is another relevant limitation. Here some illustrative examples for nanocellulose based composites with important synthetic thermoplastic (and thermosetting) polymers as poly(lactic acid) (PLA), poly(caprolactone) (PCL), poly(hydroxyalkanoates) (PHAs), acrylic polymers, epoxy resins, among others are appraised. PLA is a biodegradable aliphatic polyester produced from D- and/or L-lactic acid, commonly obtained from the fermentation of starch enriched products like sugar beet, wheat and corn (Auras, 2010). During the last decade, a colossal number of studies dealing with the reinforcement of PLA with cellulose nanofibres has been reported in literature (Abdulkhani, 2014; Almasi, 2014; Baheti, 2013, 2014; Eyholzer, 2012; Fox, 2013; Frone, 2013; Fujisawa, 2013; Iwatake, 2008; Jonoobi, 2012, 2010; Kim, 2009; Kose, 2013; Kowalczyk, 2011; Larsson, 2012; Lee, 2009, 2011, 2012c, 2012d; Li, 2010b, 2010a; Quero, 2010, 2012; Tingaut, 2010; Tomé, 2011b; Wang, 2014b). One of the first studies described the reinforcement of PLA using microfibrillated cellulose by premixing of cellulose with PLA using acetone and water followed by the evaporation of these solvents and then kneading of the mixture in a twin rotary roller mixer (Iwatake, 2008). The addition of 10 wt% of microfibrillated cellulose promoted an increase of 40% and 25% of the Young’s modulus and tensile strength, respectively. In a quite similar vein, NFC/PLA nanocomposites were prepared by twin screw extrusion, specifically by premixing a master batch with high content of NFC in PLA using a solvent mixture (chloroform/acetone 1:9) and diluting to final concentrations (1-5%) during the extrusion process (Jonoobi, 2010). Kim et al. (Kim, 2009) reported the preparation of transparent nanocomposites by adding a BC pellicle to a chloroform PLA solution followed by drying during several days. The tensile strength and Young´s modulus of the nanocomposites increased by 203% and 146%, respectively, when compared with the pure PLA matrix. Similarly, Tingaut et  al. (Tingaut, 2010) reported the preparation and characterization of bionanocomposites based on PLA and acetylated microfibrillated cellulose by solvent casting from chloroform suspensions. These nanocomposites showed improved filler dispersion, higher thermal stability and reduced water sensitivity when compared to those with unmodified cellulose nanofibres, obviously due to the decrease of the nanofibres surface energy and inherent compatibility with PLA. However, the use of harmful organic solvents in the studies mentioned above could limit to a great extent the potential application of the ensuing materials in areas as biomedicine and food packaging and preservation. More recently, Abdulkhani et al. (Abdulkhani, 2014) also prepared and characterized nanocomposites composed of PLA and acetylated NFC by solvent casting. In order to overcome this type of limitation, transparent nanocomposites prepared by the simple mechanical compounding of PLA and acetylated BC nanofibrils were developed (Tomé, 2011b). These nanocomposites showed upgraded mechanical

34 

 Development and applications of cellulose nanofibres based polymer nanocomposites

properties (increments of about 40% and 25% of the Young´s modulus and tensile strength, respectively, for 6 wt% of BC), thermal stability and water resistance. The same research group also investigated the heterogeneous acetylation of BC nanofibrils using ionic liquids as solvent and catalytic systems, aiming to reduce the environmental impact of this step on the preparation of nanocomposites with BC and PLA (Tomé, 2011a). Jonoobi et al. (Jonoobi, 2012) also studied the effect of acetylation of NFC on the properties PLA nanocomposites prepared by twin screw extrusion. However, the results were less expressive because no significant improvements were observed for nanocomposites with acetylated nanofibres compared to non-acetylated counterparts. NFC/PLA latex nanocomposites with high NFC content and nanopaper network structure were prepared by a papermaking method involving components mixing by using a wet mixing approach and films formation by filtration and hot pressing (Larsson, 2012). The use of PLA latex allowed a wet mixing of PLA with NFC and the aggregation of the hydrophilic nanocellulose fibres, that normally occur in organic solvents, is avoided. Moreover, the use of a water based process is also preferable from an environmental and end-use point of view. In a more fundamental vein, Zhang et al. (Zhang, 2011b) investigated the confined crystallization performance of acetylated BC/PLA nanocomposites obtained by compression molding. The outcomes showed that acetylated BC fibres favoured the crystallization of PLA at higher temperatures. In a similar line, Kose and Kondo (Kose, 2013) studied the size effect of cellulose nanofibres for enhancing the crystallization of PLA in nanocomposites by monitoring the crystallization rates of PLA using differential scanning calorimetry (DSC) and polarized optical microscopy. It was found that the smallest width of cellulose nanofibres does not promote an enhancement of PLA crystallization, probably because of self-assembly of those smaller cellulose nanofibres in the PLA matrix. The surface functionalization of cellulose nanofibres as a road to create PLA nanocomposites with upgraded properties has also been broadly explored in recent years. Apart from the previous examples of cellulose nanofibres acetylation, for instance, the functionalization with various organic acids rendered hydrophobic BC nanofibres (Lee, 2011, 2012c) and resulted in superior interfacial adhesion between the PLA matrix and BC (Lee, 2009). In this last study, an original methodology to compound BC and PLA based on thermally induced phase separation yielding a dry pre-extrusion nanocomposite was likewise established. In an additional study, the result of cross-linking the layered BC structure with glyoxal and the grafting with maleic anhydride on the physical properties of PLA based nanocomposites was described (Quero, 2012). Li et  al. (Li, 2010a) also described the BC surface grafting with maleic anhydride as a strategy to improve the mechanical properties of BC/PLA nanocomposites. A totally carbohydrate derived poly(lactide) copolymer was successfully used as compatibilizer to produce BC/PLA nanocomposites with better mechanical properties



Nanocellulose based Composites 

 35

(Lee, 2012d). The addition of only 5 wt% of this copolymer to a nanocomposite loaded with 5 wt% of BC caused relative improvements of the Young’s modulus (4%) and tensile strength (17%) when compared to the neat PLA or BC/PLA nanocomposites. More recently, Fujisawa et  al. (Fujisawa, 2013) reported the surface grafting of crystalline and ultrafine TEMPO oxidized cellulose nanofibrils with amino-PEG chains via ionic interactions to improve the dispersion into organic solvents and compatibilization with PLA (Figure 1.11). The tensile strength, Young’s modulus and work of fracture of grafted nanocellulose fibrils/PLA composites obtained by casting were remarkably improved at low cellulose addition levels ( 32% dimensional discrepancies > 14% process failures > 4% miscellaneous. Table 3.39 summarizes the effect of defects in the structural composite parts.

Figure 3.44: Structural nonconforming part records for Boeing 737 composite stabilizer (Aniversario et al., 1983). Table 3.39: Potential effect on structural performance for solid laminate. Discrepancy

Potential effect on structural performance

Delamination

Catastrophic failure due to loss of interlaminar shear strength. Typical acceptance criteria require the detection of delamination with a linear dimension larger than 6.4 mm. Degrades matrix-dominated properties. 1% porosity reduces strength by 5% and fatigue life by 50%. Strength degradation depends on stacking order and location. For [0/45/90/±45]2S laminate, strength is reduced 9% due to gap(s) in 0 ply and 17% due to gap(s) in 90ply. For 0 ply waviness in [0/45/90/±45]2S laminate, static strength reduction is 10% for slight waviness and 25% for extreme waviness. Fatigue life is reduced at least by a factor of 10. Static strength reduction of up to 50%. Strength reduction is small for notch sizes that are expected in service.

Voids Ply gap

Ply waviness

Scratched surfaces (i.e. sharp, narrow cuts or marks)



Types of Defects 

 185

Acceptance criteria for solid laminate for representative defects are summarized in Table 3.40. Table 3.40: Acceptance criteria for solid laminate. Discrepancy

Definition

Voids (see Figure 3.45(b))

Relatively large localized air, Maximum dimension not to exceed 13 mm water vapor or gas that have in any direction. been trapped and cured into a laminate (Refer to Figure 3.46)

Inclusions (see Figure 3.45(c))

Visible foreign material such as particles, chips and films (Refer to Figure 3.47)

Surface depressions (see Figure 3.45(d))

A localized indentation or low For faying surface area, all width for depth spot in a surface shall be less than 0.13 mm and depression length shall be not greater than 152 mm. Depression depth shall be less than 25% of total laminate thickness. No visible fiber damage is allowed.

Scratched surfaces (i.e. sharp, narrow cuts or marks)(see Figure 3.45(e)) Ply wrinkles or folds (see Figure 3.45(f))

Acceptable Limits

Single discrepancies: Maximum area not to exceed 161 mm2. Multiple discrepancies: Maximum area not to exceed 22 mm in any direction and the distance between inclusions shall be greater than 152 mm.

Scratches are allowed in the surface resin provided no fiber damage exists (i.e. no penetration into fiber) A ridge or fold-over of ply material

No visible ply wrinkles are allowed.

Ply gap (see Figure 3.45(g))

Ply splicing shall be minimized and any gap is not allowed. Butt splicing with a maximum gap of 0.0 to 1.5 mm.

Resin starved area (see Figure 3.45(h))

Area of a composite where the Any surface resin starvation where no resin has a non-continuous visible fibers are exposed (i.e. limited to coverage of the fiber. the surface ply). Resin starved area shall be less than 5% of total laminate area.

Delamination (see Figure 3.45(i))

Separation of laminate plies from each other (Refer to Figure 3.47)

Single discrepancies: Maximum dimension not to exceed 13 mm in any direction. Multiple discrepancies: For discrepancies exceeding 6.3 mm in length, the distance between delamination and voids shall be greater than 63 mm.

186 

 Production Control Effect on Composite Material Quality and Stability for Aerospace Usage

Figure 3.45: Typical defect types for solid laminate.

Figure 3.46: Void area formed in unidirectional carbon laminate.

Figure 3.47: Inclusion inserted in unidirectional carbon laminate.

Acceptance criteria for honeycomb core sandwich panel for representative defects are summarized in Table 3.41. Table 3.41: Acceptance criteria for honeycomb core sandwich panels. Discrepancy

Definition

Cell tear-out (see Figure 3.48(a)) Core splice (see Figure 3.48(b)) Node bond separation Nodes that come apart after being (see Figure 3.48(c)) bonded together by node adhesive. Frayed or burred areas Broken or loose fibers occurring at (see Figure 3.48(f)) machined edges or holes Edge waviness Core depression Core depression is a localized indentation or gouge in the core.

Acceptable Limits 161 mm2 max area or 13 mm max width from edge.

Number of nodes: