Interface Engineering of Natural Fibre Composites for Maximum Performance

Nikolaos E Zafeiropoulos

• Publisher: Woodhead Publishing
• Year: 2011
• Print ISBN: 9781845697426
• eBook ISBN: 9780857092281
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

One of the major reasons for composite failure is a breakdown of the bond between the reinforcement fibres and the matrix. When this happens, the composite loses strength and fails. By engineering the interface between the natural fibres and the matrix, the properties of the composite can be manipulated to give maximum performance. Interface engineering of natural fibre composites for maximum performance looks at natural (sustainable) fibre composites and the growing trend towards their use as reinforcements in composites.Part one focuses on processing and surface treatments to engineer the interface in natural fibre composites and looks in detail at modifying cellulose fibre surfaces in the manufacture of natural fibre composites, interface tuning through matrix modification and preparation of cellulose nanocomposites. It also looks at the characterisation of fibre surface treatments by infrared and raman spectroscopy and the effects of processing and surface treatment on the interfacial adhesion and mechanical properties of natural fibre composites. Testing interfacial properties in natural fibre composites is the topic of part two which discusses the electrochemical characterisation of the interfacial properties of natural fibres, assesses the mechanical and thermochemical properties and moisture uptake behaviour of natural fibres and studies the fatigue and delamination of natural fibre composites before finishing with a look at Raman spectroscopy and x-ray scattering for assessing the interface in natural fibre compositesWith its distinguished editor and international team of contributors Interface engineering of natural fibre composites for maximum performance is an invaluable resource to composite manufacturers and developers, materials scientists and engineers and anyone involved in designing and formulating composites or in industries that use natural fibre composites. - Examines characterisation of fibre surface treatments by infrared and raman spectroscopy and the effects of processing and surface treatment - Reviews testing interfacial properties in natural fibre composites including the electrochemical characterisation of the interfacial properties of natural fibres - Assesses the mechanical and thermochemical properties and moisture uptake behaviour of natural fibres and studies the fatigue and delamination of natural fibre composites

Part I

Processing and surface treatments to compose the interface in natural fibre composites

1

Modifying cellulose fiber surfaces in the manufacture of natural fiber composites

A. Gandini, University of Aveiro, Portugal

M.N. Belgacem, Polytechnic institute of Grenoble, France

Abstract:

Several original methods for modifying the surfaces of cellulose fibers are reviewed including: (i) plasma discharge activation solvent-free grafting; (ii) reversible hydrophobic–hydrophilic tailoring of cellulose surface; (iii) ‘grafting from’ and ‘grafting onto’ fiber–matrix continuous chemical bonding composites through click-chemistry or living polymerization; (iv) layer-by-layer polyelectrolyte systems combined with the precipitation of metallic or metal oxides nanoparticles, in order to prepare hybrid materials with highly hydrophobic and lipophobic surfaces; and (v) facile gas–solid modifications bearing a green connotation. The variously grafted materials require a thorough characterization and special emphasis is given here to studies in which such techniques as contact angle measurements, cross polarization magic angle spinning (CP-MAS) ¹³C-nuclear magnetic resonance (NMR), secondary ion mass spectrometry (SIMS) and x-ray photoelectron spectroscopy (XPS), are used to assess the modifications.

Key words

cellulose fibers

chemical grafting

surface modification

1.1 Introduction

The exploitation of cellulose is a rapidly growing sector, not only for the production of high-volume commodities, such as textiles and paper, but also for novel high-added-value materials, such as functionalized fibers and reinforcing elements in natural fiber-based composite materials (Belgacem and Gandini, 2008a). Such a marked increase is the result of lignocellulosic fibers having several potential advantages, such as low density, a bio-renewable character, ubiquitous availability at low cost and, in a variety of forms, modest abrasivity and easy recyclability in the energy recovery stream. In fact, compared with glass-fiber based composites, cellulose-based composites do not leave any solid residue after their combustion at the end of their life cycle. Unfortunately, natural fibers also have some drawbacks because they possess a strong hydrophilic character, which gives rise to two major limitations when used as reinforcing elements in composite materials. These are: (i) their strong sensitivity to water and even moisture, which results in composites subject to loss of mechanical properties upon moisture adsorption, and (ii) their poor compatibility with the hydrophobic macromolecular matrices generally used in this context, which causes weak interfacial adhesion.

In order to overcome these major drawbacks associated with the possibility of poor performance in the corresponding composites, lignocellulosic fibers are generally submitted to various surface modifications. It follows, therefore, that the main aims of such specific surface modifications are, on the one hand, to provide an efficient hydrophobic barrier and, on the other, to minimize the interfacial energy between the fibers and the non-polar polymer matrix, thus generating optimum adhesion. The chemical processes applied for this purpose are the same as those used to prepare cellulose derivatives, but their impact is limited here to the macromolecular layers constituting the fiber surface, since, in this way, the mechanical properties of the reinforcing elements are preserved, whereas the surface properties are modified to ensure an optimum compatibility with the matrix.

Various modification strategies have been reported in the literature, since the improvement of the interfacial strength and of the hydrophobic character can be attained by different approaches, namely physical treatments such as corona, plasma, laser, vacuum ultraviolet and γ-radiation, and/or, chemical grafting by direct condensation, including (i) fiber–matrix surface compatibilization using hydrophobic moieties, (ii) co-polymerization with the matrix using bifunctional molecules capable of reacting with the OH groups of the cellulose surface and leaving their second function available for further exploitation, and (iii) grafting long chains appended to the fiber surface (by calling upon ‘ grafting from’ and/or ‘ grafting onto’ procedures) (Belgacem and Gandini, 2005, Belgacem and Gandini, 2008b, Belgacem and Gandini, 2009). The two latter strategies ideally generate a continuity of covalent bonds between the surface cellulose chains and the matrix, thus providing the best mechanical performance for a given system, because no irreversible fiber slippage is possible for these situations.

Various cellulose substrates have been employed in these studies, notably lignocellulosic fibers from numerous wood and annual plants (bleached, unbleached, mechanically treated, etc.), solution-regenerated continuous fibers having a regular diameter, cellophane films, microcrystalline powders (Avicel, Technocel, etc.), Whatman filter paper made from high-purity cellulose and laboratory-made paper sheets from different substrates.

The main characterization techniques usually applied to assess the occurrence and the extent of the modification include Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS), elemental analysis, contact angle measurements, inverse gas chromatography (IGC) and scanning electron microscopy (SEM). Recently, new emerging techniques, such as angle take-off photoelectron spectroscopy, cross-polarization magic-angle-spinning nuclear magnetic resonance (CP-MAS NMR), confocal fluorescence microscopy, secondary ion mass spectrometry (SIMS) and atomic force microscopy (AFM) have started to be used in this field.

Although we have published extensively on the state-of-the-art associated with this realm in recent monographs (Belgacem and Gandini, 2005, Belgacem and Gandini, 2008b, Belgacem and Gandini, 2009), the coverage of the two most recent contributions only went up to 2006, and hence the present update deals mostly with the advances, which are numerous, since then. In writing this chapter, we focused our attention on investigations in which the modification of the surface of the fibers for the purpose of preparing optimized composite materials was accompanied by convincing evidence of the actual occurrence and extent of that transformation. Studies dealing empirically with the incorporation of cellulose fibers into polymer matrices, and with the mechanical properties of the ensuing composites, without any proof of surface modification, were therefore left out, considering, moreover, that the present book contains chapters devoted to the mechanical properties of these composites. In addition, interesting reports are included on the surface modification of cellulose fibers aimed at generating specific functionalities, such as omniphobic properties, electrical conductivity, antimicrobial properties and absorption capacity.

1.2 Physical treatments

1.2.1 Plasma grafting

An interesting and environmentally sound surface treatment concerns the use of atmospheric air pressure plasma (AAPP), which was recently applied to various lignocellulosic fibers (abaca, flax, hemp and sisal) using treatments limited to a few minutes (Baltazar-Y-Jimenez & Bismarck, 2007). The wettability of the treated surface was determined using the capillary rise technique, whereas the changes in the surface chemistry were characterized by means of zeta-potential measurements. Some bulk properties, such as the fibers’ swelling behavior, were also assessed. The surface energy of the lignocellulosic fibers was found to remain practically constant, even for prolonged treatment times, with the exception of the abaca fibers, for which this parameter decreased with increasing AAPP treatment time. The ζ-potential measurements were performed as a function of pH and the ensuing results were found difficult to rationalize or to correlate with the nature of the treatments because, except for the abaca fibers, this parameter increased and became positive, a fact that is hard to associate with the oxidation mechanism induced by ...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributor contact details
• Part I: Processing and surface treatments to compose the interface in natural fibre composites
• Part II: Testing interfacial properties in natural fibre composites
• Index