On the mechanical properties, deformation and fracture of a natural fibre/recycled polymer composite
Introduction
During the past decade, increasing environmental awareness new global agreements, and international governmental policy and regulations have been the driving force behind renewed interest in natural fibre reinforced thermoplastics. The attractiveness of a plant-based fibre as an alternative reinforcement material comes from its high specific strength and stiffness, natural availability, and environmental ‘friendliness’ (flax fibre is bio-degradable and carbon dioxide neutral) [1], [2]. This class of composite is considered, therefore, as a candidate material for selection in engineering design and application; for example, the manufacture of car panels of various kinds in the automotive industry.
Recently published research into the use of natural fibre–polymer composites has focused mainly on material having a polypropylene matrix [3], [4], [5], [6], [7], [8], [9]. We are not aware at this point in time of any related work where the matrix is polyethylene. In this paper, we discuss the mechanical, deformation and fracture behaviour of a flax fibre/recycled high density polyethylene (HDPE) laminate, where the fibre is randomly oriented in a non-woven mat within layers of the composite construction.
The principal constituent of flax fibres is cellulose (60%). Other main constituents include hemicellulose (15%), pectin (2–3%), lignin (2%), and waxes (1%) [10], [11]. Flax is a bast fibre which is found in the stalks of dicotyledenous plants, notably in the stem of the plant Linium usitatissimum. Naturally occurring flax consists of bundles of fibre. A schematic of the structure of flax from its stem to the microscopically fine elementary fibre was presented by Van Den Oever et al. [3] and is shown in Fig. 1. The individual fibre is composed of technical fibres bonded together in part by a weak pectin and lignin interphase. On a finer scale, these technical fibres are composed of 40 or more elementary fibres (sometimes called ultimate fibres, fibre cells or simply fibres), some 15 μm in diameter and length between 20 and 50 mm. These elementary fibres are bound together by a stronger pectin interphase [3]. Flax absorbs water and has a polar surface meaning that adhesion with a non-polar thermoplastic will be weak. Coupling agents were, however, not used in this study.
Elementary fibres have a tensile modulus (or Young's modulus), of up to 80 GPa and a tensile strength of about 1.5 GPa. These figures compare favourably with 72 and 2.5 GPa, respectively, for glass fibre (Table 1), especially when the low density of flax is taken into account. On the other hand, flax fibre is much less competitive when its strength and stiffness are compared to that of an aramid fibre (such as Kevlar®). In practise, however, most of the flax fibres used in composites are so-called technical fibres or fibre bundles. These technical fibres have much lower mechanical properties than elementary fibres with typical strength values of around 600–700 MPa and a Young's modulus of 50–60 GPa.
Furthermore, there is the added attractiveness of the natural fibre composite having a matrix of a recycled polymer. Natural fibres tend to degrade near the processing temperature of most thermoplastics and thermal degradation during processing not only limits the number of polymers that can serve as a matrix system (mainly polyolefins) but gives concern with respect to re-processing. Hence, problems related to thermal degradation during re-processing may significantly lower the mechanical performance and therefore also eco-performance of natural fibre composites. This is the case with re-cyclable polymers like polyolefins. On the other hand, the upgrading of recycled plastics—which are close to the end of their lifetime—with natural fibres is clearly an environmentally sound option because of the clear advantages with respect to end-of-time disposal by incineration or thermal re-cycling.
Section snippets
Material fabrication, experimental and specimen design
Fabrication of the flax fibre mat/recycled high density HDPE composite is by film stacking and compression moulding. Initially, the fibre is dried at 75 °C for 24 h. Then, thin sheets of HDPE (about 0.5 mm thick and diameter of some 180 mm) are manufactured from recycled bottles in the form of granules weighing 17 g, spread over an aluminium plate and compressed in a hot-press. A pressure of 6 MPa is applied for 5 min and the temperature is 166 °C. The moulding is then air cooled. These sheets
Tensile measurements and stress–strain behaviour
Comparison between the averaged tensile stress–strain curves of the flax/HDPE (10% fibre by volume) composite compared to the recycled HDPE alone is informative (Fig. 4). The composite shows a lower strain to failure, about 5% compared to 20% or more for recycled HDPE. Whilst there is a modest increase only in yield stress and tensile strength, the tensile modulus increases from 1.2 to 8 GPa.
Next, compare the stress–strain curves of the family of flax fibre composite (Fig. 5). As fibre content
Conclusions and final remarks
In comparison to flax fibre/PP, the flax fibre/recycled HDPE composites is attaining similar mechanical properties and reaching useful values. Whilst there has been no direct comparison with a similar composite made of virgin HDPE, there is no reason to believe that the use of recycled HDPE as a matrix has had deleterious effects upon the tensile strength, stiffness, and toughness. A number of deformation and fracture mechanisms have been identified as the origin of the order of magnitude
Acknowledgements
We would like to thank Mr Alan Heaver and Mr John Street for their invaluable input in the mechanical testing and scanning electron microscopy phases of this investigation. We would like to thank Reprise Limited for the supply of HDPE granules made from recycled bottles.
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