Flexural properties of hemp fibre reinforced polylactide and unsaturated polyester composites

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Abstract

In this work, flexural strength and flexural modulus of chemically treated random short and aligned long hemp fibre reinforced polylactide and unsaturated polyester composites were investigated over a range of fibre content (0–50 wt%). Flexural strength of the composites was found to decrease with increased fibre content; however, flexural modulus increased with increased fibre content. The reason for this decrease in flexural strength was found to be due to fibre defects (i.e. kinks) which could induce stress concentration points in the composites during flexural test, accordingly flexural strength decreased. Alkali and silane fibre treatments were found to improve flexural strength and flexural modulus which could be due to enhanced fibre/matrix adhesion.

Introduction

Natural fibres such as hemp, sisal, jute, kenaf, flax and coir are cheap, abundant and renewable. Due to low densities, these strong and stiff fibres have the potential to produce polymer composites with similar specific properties to those of synthetic fibres [1], [2]. Natural fibres consist mainly of cellulose, hemicellulose and lignin in different proportions. One of the most important distinguishing features of these fibres is the presence of cell walls. The basic fibrous building element of the cell wall may be regarded the microfibril. The cell walls differ in their composition (i.e. crystalline and amorphous cellulose content) and orientation (i.e. spiral angle). The spiral angle of fibrils and the content of cellulose are generally determine the mechanical properties of natural fibres. However, cell walls of natural fibres contain defects, known as kink bands and micro-compressive defects [3]. Hughes et al. [4] have shown that micro-compressive defects can be present in both ‘green’ and ‘processed’ hemp fibres. In another report, Davies and Bruce [5] observed that tensile properties of the flax and nettle fibres decreased as the number of defects increased.

Fibre/matrix adhesion plays an important role in the mechanical performance of composites. However, natural fibres are covered with non-cellulosic components (e.g. pectin and wax) which cause poor interfacial bonding/adhesion with polymer matrices. Therefore, for better adhesion with matrices, fibres are chemically treated to remove the non-cellulosic components [6], [7], [8]. Chemical treatment also brings about an active surface by introducing some reactive groups [8], [9].

Generally, polymer matrices used for fibre reinforced composites are divided into two groups namely thermoset and thermoplastic. In this work, polylactide (PLA), a bioderived thermoplastic polyester and a synthetic unsaturated thermoset polyester (UPE) were used as matrices. Recent developments in the manufacturing of lactic acid (i.e. monomer of PLA) economically from agricultural products (e.g. corn and potato) have placed PLA at the forefront of the emerging biodegradable plastic [10]. On the other hand, UPE resins are the most frequently used thermoset matrices owing to their low cost and adaptability to be transformed into large composite structures. Due to the commercial potential for natural fibre reinforced polymer composites in automotive applications and building construction as well as demands for environmentally friendly materials [11], the development of PLA and UPE based composites for many applications is an interesting area of research as appeared in many research papers. For instance, Serizawa et al. [12] used kenaf fibres to fabricate PLA composites by extrusion and injection moulding. They showed that flexural modulus increased from 4.5 to 7.6 GPa as the fibre content increased from 0 to 20 wt%; however, flexural strength decreased from 132 to 93 MPa. In another report, Shibata et al. [13] treated abaca fibres with acetic anhydride, butyric anhydride, alkali and cyanoethylation to reinforce PLA matrix by melt mixing and injection moulding. They found that flexural strength decreased with increased fibre content (0–20 wt%) but flexural modulus increased. They also observed that flexural strength and flexural modulus of treated fibre composites did not increase significantly compared with those of untreated fibre composites.

Sèbe et al. [14] fabricated non-woven hemp fibre mat reinforced UPE composites by resin transfer moulding. They found that flexural strength and flexural modulus of the composites increased with increased fibre content (0–36 wt%). At 36 wt% fibre content, flexural strength and flexural modulus increased by 220% and 100%, respectively, compared to UPE only samples (flexural strength and flexural modulus were approximately 30 MPa and 3 GPa, respectively). In another study, Hill and Abdul Khalil [15] used non-woven mats of random coir and oil palm fibres (0–55 wt%) to reinforce UPE by compression moulding. Flexural strength of all the composites was found to be lower than UPE only samples (52 MPa) at all fibre contents but flexural modulus increased. Composites reinforced with 45 wt% coir fibres had the highest flexural modulus (4.97 GPa).

The works reviewed in this section indicate that flexural properties of natural fibre reinforced PLA and UPE composites can greatly be influenced by the variety of fibres, fibre treatments and composite processing. However, there is no comprehensive explanation regarding the influence of fibre defects on the failure mechanism of natural fibre reinforced composites in flexural test. In this work, influence of fibre defects on the flexural properties is analysed. In addition, the effect of fibre treatments and fibre contents on the flexural properties is discussed.

Section snippets

Materials

NatureWorks® PLA 4042D (from NatureWorks LLC, USA) was used as thermoplastic matrix. A standard unsaturated polyester resin (Crystic P489 from Nuplex, New Zealand) was used as thermoset matrix. The industrial hemp fibres were supplied by Hemcore Ltd., UK. [3-(2-aminoethyl amino)propyl]trimethoxy silane coupling agent was purchased from Aldrich. All other chemicals used were of analytical grade obtained from local commercial sources.

Fibre treatment

Prior to treatment, untreated fibres (FB) were washed with hot

Flexural properties of PLA/hemp fibre composites

The average flexural strength and flexural modulus of the PLA/hemp fibre (untreated and treated short fibre) composites as a function of fibre content are presented in Fig. 1a and b, respectively. As can be seen in Fig. 1a, the flexural strength decreased with increased fibre content. This behaviour is different than the relationship between strength and fibre content as can be expected according to the ‘rule of mixtures’ models [18]. In addition, the above finding is not consistent with the

Conclusions

PLA could be reinforced with a maximum of 30 wt% fibres using conventional injection moulding, but could not be processed at higher fibre contents due to poor melt flow of the compounded materials. It was also observed that flexural modulus of both the short and long fibre reinforced PLA composites increased with increased fibre content, however, flexural strength decreased with increased fibre content. This reduction of flexural strength could be due to fibre defects (kinks) which act as stress

Acknowledgement

The financial support from Biopolymer Network Ltd., New Zealand for this work is greatly acknowledged.

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