This paper provides a comprehensive overview on different surface treatments applied to natural fibres for advanced composites applications. In practice, the major drawbacks of using natural fibres are their high degree of moisture absorption and poor dimensional stability. The primary objective of surface treatments on natural fibres is to maximize the bonding strength so as the stress transferability in the composites. The overall mechanical properties of natural fibre reinforced polymer composites are highly dependent on the morphology, aspect ratio, hydrophilic tendency and dimensional stability of the fibres used. The effects of different chemical treatments on cellulosic fibres that are used as reinforcements for thermoset and thermoplastics are studied. The chemical sources for the treatments include alkali, silane, acetylation, benzoylation, acrylation and acrylonitrile grafting, maleated coupling agents, permanganate, peroxide, isocyanate, stearic acid, sodium chlorite, triazine, fatty acid derivate (oleoyl chloride) and fungal. The significance of chemically-treated natural fibres is seen through the improvement of mechanical strength and dimensional stability of resultant composites as compared with a pristine sample.

Chemical treatments on plant-based natural fibre reinforced polymer composites: An overview

Biodegradable composites based on lignocellulosic fibers—An overview

Cellulosic/synthetic fibre reinforced polymer hybrid composites: A review

There has been much effort to provide eco-friendly and biodegradable materials for the next generation of composite products owing to global environmental concerns and increased awareness of renewable green resources. An increase in the use of natural materials in composites has led to a reduction in greenhouse gas emissions and carbon footprint of composites. In addition to the benefits obtained from green materials, there are some challenges in working with them, such as poor compatibility between the reinforcing natural fiber and matrix and the relatively high moisture absorption of natural fibers. Green composites can be a suitable alternative for petroleum-based materials. However, before this can be accomplished, there are a number of issues that need to be addressed, including poor interfacial adhesion between the matrix and natural fibers, moisture absorption, poor fire resistance, low impact strength, and low durability. Several researchers have studied the properties of natural fiber composites. These investigations have resulted in the development of several procedures for modifying natural fibers and resins. To address the increasing demand to use eco-friendly materials in different applications, an up-do-date review on natural fiber and resin types and sources, modification and processing techniques, physical and mechanical behaviors, applications, life-cycle assessment, and other properties of green composites is required to provide a better understanding of the behavior of green composites. This paper presents such a review based on 322 studies published since 1978.

A review of natural fiber composites: properties, modification and processing techniques, characterization, applications

The development of plant fibre composite is on the rise for a wide range of applications. Probably a single most important aspect with respect to the formulation of plant fibre composites with superior mechanical performance is the optimization of the interfacial bonding between the reinforcing plant fibre and polymer matrix. While the interface plays a pivotal role in determining the mechanical properties, e.g. transferring the stress and distributing the bond, it is among the least understood components of the composite. This paper presents an overview of the compatibility between the heterogeneous constituents of plant fibre composite, various modification approaches aiming at overcoming the incompatibility and refining the interfacial adhesion of the composite, interfacial bonding mechanisms, and the assessment of interface structure and bonding. It has been found that 1) the physical and chemical incompatibility between the fibre and matrix, leading to poor dispersion, weak interfacial adhesion and ultimately inferior composite quality, could be overcome through strategical modifications; 2) inter-diffusion, electrostatic adhesion, chemical reactions and mechanical interlocking are in general responsible for the interfacial bonding and adhesion of plant fibre composites; and 3) a thorough knowledge of structure-property relationship of the composite could be established by conducting a set of direct and indirect interfacial assessments. This paper finishes with some critical suggestions and future perspectives, underscoring the roles of composite material researchers and engineers for the further in-depth studies and up-scale commercialization of plant fibre composite.

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Interface and bonding mechanisms of plant fibre composites: An overview

Hybrid composites prepared by the incorporation of two or more different types of fibres into a single polymer matrix deserve much attention. This method of hybridisation of composites offers a profitable procedure for the fabrication of products while the resulting materials are noted for their high specific strength, modulus and thermal stability. The influence of the relative composition of short sisal/glass fibres, their length and distribution on the tensile properties of short sisal/glass intimately mixed polyethylene composites (SGRP) was examined. Different compositions of sisal and glass such as 70/30, 50/50 and 30/70 have been prepared with varying fibre lengths in the range of 1–10 mm. Emphasis has also been given to the variation of fibre–matrix adhesion with several fibre chemical modifications. Chemical surface modifications such as alkali, acetic anhydride, stearic acid, permanganate, maleic anhydride, silane and peroxides given to the fibres and matrix were found to be successful in improving the interfacial adhesion and compatibility between the fibre and matrix. The nature and extent of chemical modifications were analysed by infrared spectroscopy while improvement in fibre–matrix adhesion was checked by studying the fractography of composite samples using a scanning electron microscope. Assessment of water retention values has been found to be a successful tool to characterize the surface of the stearic acid modified fibres. It was found that the extent of improvement in tensile properties of SGRP varied with respect to the nature of chemical modifications between fibre and matrix. Improved mechanical anchoring and physical and chemical bonding between fibre and polyethylene matrix are supposed to be the reasons for superior tensile strength and Young's modulus in treated composites. Several secondary reasons such as high degree of fibre dispersion and reduced hydrophilicity in chemically modified fibres also are believed to play a role. Among the various chemical modifications, the best tensile strength and modulus was exhibited by the SGRP with benzoyl peroxide treated fibres. This is attributed to the peroxide-initiated grafting of polyethylene on to the fibres. Copyright © 2004 Society of Chemical Industry

Effect of fibre length and chemical modifications on the tensile properties of intimately mixed short sisal/glass hybrid fibre reinforced low density polyethylene composites

The properties of diglycidyl ether of bisphenol-A epoxy resin toughened with poly(ether sulfone ether ketone) (PESEK) and poly(ether sulfone) (PES) polymers were investigated. PESEK was synthesised by the nucleophilic substitution reaction of 4,4’-difluorobenzophenone with dihydroxydiphenylsulfone using sulfolane as solvent and potassium carbonate as catalyst at 230 °C. The T
 g–composition behaviour of the homogeneous epoxy resin/PESEK blend was modelled using Fox, Gordon–Taylor and Kelley–Bueche equations. A single relaxation near the glass transition of epoxy resin was observed in all the blend systems. From dynamic mechanical analysis, the crosslink density of the blends was found to decrease with increase in the thermoplastic concentration. The storage modulus of the epoxy/PESEK blends was lower than that of neat resin, whilst it is higher for epoxy/PES blends up to glass transition temperature, thereafter it decreases. Scanning electron microscopic studies of the blends revealed a homogeneous morphology. The homogeneity of the blends was attributed to the similarity in chemical structure of the modifier and the cured epoxy network and due to the H-bonding interactions between the blend components. The fracture toughness of epoxy resin increased on blending with PESEK and PES. The increase in fracture toughness was due to the increase in ductility of the matrix. The thermal stability of the blends was comparable to that of neat epoxy resin.

Diglycidyl ether of bisphenol-A epoxy resin–polyether sulfone/polyether sulfone ether ketone blends: phase morphology, fracture toughness and thermo-mechanical properties

Composites of isotactic polystyrene with different loading of titanium dioxide were prepared by melt mixing in a Brabender Plasticorder at a rotor speed of 60 rpm at 180 °C. Mechanical properties of the composites were measured by various techniques and most of the properties were found to be increasing with filler loading. Tapping mode atomic force microscopy was used to study the surface characteristics of the composites. Various AFM analytical parameters like power spectral density, roughness analysis and section analysis were measured. The mechanical properties of the composites were correlated with analytical data obtained from the AFM studies. The focused ion beam technique was also used to characterize the dispersion of the filler in the polymer matrix.

Selvin P. Thomas

Papers

Effect of fibre length and chemical modifications on the tensile properties of intimately mixed short sisal/glass hybrid fibre reinforced low density polyethylene composites

Diglycidyl ether of bisphenol-A epoxy resin–polyether sulfone/polyether sulfone ether ketone blends: phase morphology, fracture toughness and thermo-mechanical properties

Mechanical, atomic force microscopy and focussed ion beam studies of isotactic polystyrene/titanium dioxide composites