Processing of unidirectional hemp fiber reinforced composites with micro-braiding technique

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Abstract

Natural fiber reinforced biodegradable composites are expected as alternatives of fossil fuel oil based composites. In case of using poly(lactic acid) as a matrix of natural fiber composites, It is difficult to impregnate reinforcement fiber yarns because of high viscosity of molten PLA. In this study, continuous hemp fiber reinforced thermoplastic composites were obtained using intermediate materials fabricated through a micro-braiding technique. Unidirectional composite plates were fabricated by hot press molding under various molding conditions, including molding temperature, pressure and time. At molding temperature of 190 °C, good impregnation was achieved, which result in good mechanical properties. At the molding temperature more than 210 °C hemp fiber and matrix thermally decomposed. Considering the tensile and shear properties, 190 °C–1 MPa–4 min is expected as an optimum molding condition.

Introduction

Plastic materials have been widely used because of their superior properties such as light weight and good processability. On the other hand, 21% of waste plastics are burnt up and 33% of them are landfilled. In addition, the impact of green house gases is a serious global-scale problem. Under such serious conditions, composites which are derived from renewable resources, such as natural fibers, attract much attention. Natural fiber composite materials have been developed in the past decade in an attempt to find alternatives to carbon and/or glass fiber reinforced fossil fuel-based polymeric materials.

Short natural fiber is easily impregnated with thermoplastics and is used for continuous molding, such as injection molding. Thus, many investigations have been conducted on randomly oriented short natural fiber reinforced thermoplastic composites by injection molding. Previous investigations on short natural fiber composites are summarized in Table 1. Arbelaiz et al. [1] used flax, Li et al. [2] and Sain et al. [3] used hemp fibers, Rana et al. [4] and Karmaker and Youngquist [5] used jute fibers and Tou [6] and Rowell et al. [7] used kenaf fibers. They used polypropylene (PP) as a matrix and tensile strength of the composites is about 50 MPa in average.

PP used in the previous investigations is not biodegradable plastics. In case of usage of biodegradable plastics for the natural fiber composites, the composites degrade after landfill. Therefore, it may lead to one of the solutions for the issue of disposal ground depletion. Poly(lactic acid) (PLA) is suitable for a matrix of the natural fiber composite materials, because it is naturally-derived biodegradable plastics. Short natural fiber/PLA composites also have been investigated. Oksman et al. [8] and Bodros et al. [9] used flax fibers and Bax and Mussing [10] used cordenka fibers. They used compression molding and obtained the composites which have tensile strength of about 50 MPa in average. As for injection molding, Pan et al. [11] and Nishino et al. [12] used kenaf fibers, Sawpan et al. [13] used hemp fibers and Bax and Mussig [14] and Ganster et al. [15] used cordenka fibers. Tensile strength of the composite was also about 50 MPa in average.

Strength and modulus of short fiber reinforced composites are lower. Therefore, unidirectionally-aligned long fibers are expected as reinforcements. Table 2 shows investigations on long natural fiber composites. The investigations for long natural fiber composites include hemp/epoxy [16], henequen/high density polyethylene [17], sisal/polyester [18] and straw/polyester [19]. Tensile strength of the composites is about 80 MPa in average. From the results, they are not so improved comparing to short natural fiber composites. As for investigations on long natural fiber/PLA composites, Graupner et al. [20] used hemp fiber and Plackett et al. [21] used jute fiber. Tensile strength of the composites is also 80 MPa in average and not so improved. As examples of the composites with improved strength, Madsen molded hemp/polyethylene terephthalate by filament winding [22] and molded flax/PP by film stacking method under vacuum [23] and Ochi [24] fabricated kenaf/PLA composites by compression molding of prepregs. Tensile strength of the composites was about 220 MPa in average.

As shown above, investigations of long fiber reinforced PLA composites were very limited because of high viscosity of molten thermoplastic PLA. Unlike thermoset polymers, thermoplastic polymers have relatively higher viscosity. Therefore, it is difficult to impregnate thermoplastic resin into long fiber yarns.

In order to improve impregnation of thermoplastic resin into fiber yarns, Sakaguchi et al. used a micro-braiding method to supply flexible material design on continuous fiber reinforced thermoplastic composites [25]. With this method, high matrix impregnation and good fiber dispersion can be expected since the reinforcing fibers and the matrix fibers can be combined as one braided yarn. The method also allows many design choices of thermoplastic matrices. Furthermore, braided yarn was supplied as a continuous yarn, which enables a following weaving process. The method also enables impregnation and shape forming at the same time. In other words, heating process is just once, which prevents thermal degradation of fiber and/or matrix and reduces the energy consumed during molding.

It is well known that the mechanical properties of a continuous fiber reinforced thermoplastic composite are affected by processing conditions such as pressure, temperature, holding time. In this study, hemp fiber/PLA composites were molded at several conditions. Those composites were characterized on tensile and shear tests. From the results, the effect of molding condition on the mechanical properties and the optimum molding condition are investigated.

Section snippets

Molding method of natural fiber reinforced composite

The reinforcement used in this study was hemp spun yarn (tex: 590). The matrix material was biodegradable PLA fiber yarn (tex: 55, Toray). An intermediate material, called as Micro-Braided Yarn (MBY), was fabricated using a micro-braiding technique. A continuous hemp fiber yarn was used as the straightly inserted axial yarn, and matrix fiber yarns were braided around the reinforcing hemp fiber yarn, as shown in Fig. 1. The hemp fiber weight fraction in MBY was about 53%.

Fabrication of

Moldability

In order to evaluate impregnation, cross-sectional observation was conducted. The results of cross-sectional observation are shown in Fig. 5. Fig. 6 shows the enlargement of un-impregnated and impregnated area. Black region in the figure corresponds with un-impregnation region. In case of molding temperature 170 °C, many un-impregnation regions were observed. Otherwise, favorable impregnation was obtained in case of molding temperature more than 190 °C. This is due to higher viscosity of molten

Conclusion

In the present study, hemp fiber reinforced PLA composites were molded using a micro-braiding technique. Optimum molding conditions was investigated through cross-sectional observation, tensile and in-plane shear testing. We obtained the following conclusion:

  • 1.

    At molding temperature of 190 °C, good impregnation was achieved. Hemp fibers decomposed at molding temperature of 230 °C.

  • 2.

    Tensile strength increased with increasing molding temperature from 170 °C to 190 °C, whereas strength decreased at the

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