Novel aligned hemp fibre reinforcement for structural biocomposites: Porosity, water absorption, mechanical performances and viscoelastic behaviour
Introduction
The potential of natural fibres as reinforcement in composite materials is well recognized due to their attractive mechanical properties which enhance the possibility of producing eco-friendly materials. Natural fibres such as hemp and flax are already used in the automotive industry to reduce weight, cost and environmental impact. Hemp is an upcoming European industrial crop [1], with good mechanical fibre properties which can be cultivated with a low consumption of fertilizers and almost no pesticides [2]. Concerning the matrix in the composites, the industrial trend for natural fibre composites is giving more importance to a thermoplastic matrix, rather than a thermosetting matrix [1], [3].
Polylactic acid (PLA) is the most important biothermoplastic for applications requiring biodegradability. It shows also quite good properties appropriate for applications that do not require long-term durability or high mechanical performance at higher temperatures. The mechanical properties of the PLA can be improved by using reinforcements like natural fibres in order to increase its potential use in many industrial applications [4], [5]. Therefore, because of the attractive properties of hemp fibre, it was used as reinforcement for PLA composite in the presented study.
The main application of natural fibres is today mainly in non-structural composites as they are mostly available as randomly oriented nonwovens [1], [3], [6]. The fibre orientation (i.e. alignment of the fibres) must be controlled to ensure that the fibre mechanical properties are efficiently utilized in order to attract industrial interest as an alternative to the traditionally applied synthetic fibres (e.g. glass fibres). It is evident that PLA/hemp fibre composites can compete with glass fibre composite regarding stiffness, whereas for tensile and impact strength, the properties are still not on a satisfactory level [7], [8], [9], [10], [11], [12]. Previous studies have demonstrated that the full reinforcement potential of natural fibres can be exploited in bio-composites if an aligned fibre orientation is used [12], [13]. Natural fibres are naturally discontinuous; therefore natural fibre reinforcements reported so far are based on twisted spun staple yarns, which are produced by spinning methods, mainly ring spinning. These spun yarns tend to be highly twisted, which leads to fibre misalignment due to their helical paths around the yarn axis. This misalignment contributes negatively to the mechanical properties of the resultant composites. Another negative impact of yarn twist is that it tightens the yarn structure, rendering resin impregnation difficult [14]. Therefore, in the textile industry a broad range of techniques for the alignment of natural fibres have been developed and optimised to produce yarns with controlled fibre orientations by reducing or replacing twist in yarns. Goutianos and Peijs [14] tried to produce flax yarns with the minimal level of twist for manufacturing aligned composites. Shah et al. [15] used a sizing agent to substitute the use of twist in roving and yarn. Zhan and Miao [13] studied the effect of wrapped spun yarn with low twist for reinforcement purpose.
Our previous study [12] investigated the mechanical properties of composites manufactured from PLA/hemp co-wrapped hybrid yarn prepregs. Here we used continuous PLA filaments, which were used to wrap a low twist hemp yarn. The composites made from the hybrid yarn with higher wrapping density showed improvements of mechanical properties due to lower porosity. However, the porosities of composites were between 6 and 9 vol.%, and the porosity fraction was still high even if the fibre volume fraction was low, which could be due to several factors. Porosity is difficult to avoid in natural fibre composites and influences on the composite properties, yet how to control the porosity has so far only received limited attention. Thus, research on how to decrease the amount of porosity is warranted.
In the current paper, we discuss the development new hybrid yarns with low twist for high performance natural fibre-reinforced composites suitable for use in structural or semi-structural applications and with lower amount of porosity. The overall was to study the mechanical properties of these novel aligned hemp fibre yarn composites and investigate the effect of a range of relevant parameters such as prepreg type such as nonwovens and hybrid yarn prepregs with different off-axis angles (0°, 45° and 90°) and fibre treatment.
Section snippets
Materials
Two types of staple fibres were used in this study: hemp and PLA fibres. The PLA staple fibre, provided by Trevira GmbH (Hattersheim, Germany), had a fineness of 1.7 dtex and a mean fibre length of 38 mm. Based on the manufacturer’s information; the PLA fibres were made from PLA Polymer 6202D from NatureWorks®, Cargill Dow LLC (Minnetonka, USA). This thermoplastic has a density of 1.24 g/cm3, a melt temperature of 160–170 °C, and a glass transition temperature of 60–65 °C. The hemp in the form of
Single fibre tensile test
In order to evaluate the effect of moulding conditions, the mechanical properties of heat and alkali treated single hemp fibres were determined, see Table 1. The alkali treatment improves fibre tensile strength and modulus which helps to improve the properties of the composites [18], [19]. This treatment possibly orients fibrils along the direction of tensile forces by removing hemicelluloses from fibre, resulting in better load sharing between the fibrils [21]. Heat treatments greatly reduce
Conclusions
Unidirectional hemp reinforced PLA composites were produced from PLA/hemp yarn using compression moulding. We investigated the influences of different orientation especially off-axial direction of hemp fibre as well as alkali treatment; focusing on the determining void%, water absorption, mechanical and thermo-mechanical properties. The properties of PLA composites were significantly improved compared to the neat PLA matrix. The alkali treated hemp/PLA yarn gave maximum improvement in
Acknowledgment
The authors thank Tommy Martinsson, Swedish School of Textiles, University of Borås (Sweden) for his assistance in preparing hybrid yarns.
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