Composites Part A: Applied Science and Manufacturing
Composite materials based on wood and nylon fibre
Introduction
In recent years, interest in the development of new composite materials derived from wood fibre and thermoplastic polymer matrices has grown markedly [1]. When combined with thermoplastic polymers, wood fibres produce composite materials that are lightweight, can be recycled, and offer high strength to weight ratios [2]. The combination of wood fibres and thermoplastic polymers presents a number of problems. In many instances incompatibility between the fibre and matrix results in an inferior interface that does not adequately transfer stress to the load bearing fibre. Therefore the mechanical properties of wood fibre reinforced polymers are dependent on the characteristics of the polymer–wood interface. Strong interfacial adhesion produces composites with more desirable mechanical properties [3], [4], [5].
Polypropylene (PP) is commonly used as a matrix in wood fibre based composites as it can be obtained inexpensively and is easily manufactured [6]. However, the combination of wood fibre and a PP matrix leads to poor mechanical properties, due to: (i) the incompatibility between wood fibre and PP resulting in weak interfacial adhesion and (ii) poor dispersion of the fibres in the PP matrix [7], [8], unless suitable coupling agents are added [5], thus making it more expensive.
Nylon is a more complex polymer than PP with polar groups attached to its polymer chain, which give Nylon a hydrophilic nature [6]. Due to the polar nature of both, it is expected that Nylon and wood may combine with strong adhesion resulting in a composite material with higher stiffness and strength. With the emphasis on recycling and environmentally friendly approaches to manufacturing, it is conceivable that Nylon may be obtained relatively inexpensively through the use of recycled Nylon, for example, fibres from stockings as described in this paper. For comparison, wood fibre/PP composites, manufactured by the same method, are also described.
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Starting materials
The eucalypt wood fibre was obtained from Weathertex in Raymond Terrace, NSW. The Nylon 6 fibre was obtained from stockings bought for this purpose (‘Solutions’ brand © Kmart Australia Ltd). The Nylon fibre was prepared by finely chopping the stockings with scissors. The PP was obtained from ICI Plastics in the form of pellets known commercially as ‘Propathane GWM 101’. The PP pellets were used in as supplied condition.
The wood fibre mass was originally converted from wood chips using a Mason
Fibre properties
Mechanical properties for the two polymers were obtained from the above tests, and are listed in Table 1. The elastic modulus of cellulose is approximately 150 GPa [4], however, for individual wood fibres the longitudinal modulus varies between 10 and 50 GPa, depending on the fibril angle and density [10]. It seems reasonable to assume a value of 30 GPa for the wood fibres employed here. No data for strength of individual wood fibres is available.
In total, 30 measurements of Nylon fibre diameter
Volume fraction of fibres
The relationship between volume fraction and weight fraction of wood fibre for Nylon boards can be accepted as linear, which indicates that the fractions, measured by the point counting technique, are representative of the wood fibre content. The gradient of the line should be indicative of the ratio of densities of the two phases. Since the densities of Nylon and cellulose are almost the same, the gradient of approximately 2:1 implies the apparent density of the wood phase to be a half that of
Conclusions
For the Nylon based composite materials, tensile strength and modulus of elasticity were found to increase significantly with the addition of a small amount of wood fibre. The increase indicated that efficient bonding occurred between the wood fibre and Nylon, increasing interfacial adhesion. No fibre pullout was observed during the SEM analysis of fracture surfaces.
The maximum strength and modulus of elasticity for the Nylon composites were achieved at a fibre weight fraction of 2.5%. After
Acknowledgements
The authors wish to acknowledge help from the following people: Dr Phil Evans, Forestry, ANU, who kindly supplied the wood fibre and provided access to his laboratory, Dr Roger Heady, EMU, ANU, for invaluable assistance with SEM, and Dr Ann Cowling, Statistical Unit, ANU, for providing the framework and assistance with statistical evaluation of the results.
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