The effect of fiber treatment on the mechanical properties of unidirectional sisal-reinforced epoxy composites

doi:10.1016/S0266-3538(01)00046-X

Composites Science and Technology

Volume 61, Issue 10, August 2001, Pages 1437-1447

https://doi.org/10.1016/S0266-3538(01)00046-X Get rights and content

Abstract

The present paper investigates the effect of fiber treatment on the mechanical properties of unidirectional sisal-reinforced epoxy composites. Treatments including alkalization, acetylation, cyanoethylation, the use of silane coupling agent, and heating were carried out to modify the fiber surface and its internal structure. As indicated by infrared spectroscopy, X-ray diffraction and tensile tests, variations in composition, structure, dimensions, morphology and mechanical properties of the sisal fibers can be induced by means of different modification methods. When the treated fibers were incorporated into an epoxy matrix, mechanical characterization of the laminates revealed the importance of two types of interface: one between fiber bundles and the matrix and the other between the ultimate cells. In general, fiber treatments can significantly improve adhesion at the former interface and also lead to ingress of the matrix resin into the fibers, obstructing pull-out of the cells. As a result, the dependence of laminate mechanical properties on treatment methods becomes complicated. On the basis of a detailed analysis, the relationship between optimized fiber treatment and performance improvement of sisal composites was proposed.

Introduction

The use of natural vegetable fibers as reinforcements in polymer composites to replace synthetic fibers like glass is presently receiving increasing attention because of the advantages, including cost effectiveness, low density, high specific strength, as well as their availability as renewable resources [1], [2], [3]. Owing to the poor wettability and adsorbability towards polymers resulting from the hydrophilicity of plant fibers, however, the adhesion between the fibers and polymer matrices is generally insufficient. To improve the interfacial bonding, either surface modification of the fibers [4] or plasticization of the fibers [5] can be carried out.

It is worth noting that chemical composition and cell structure of natural fibers are quite complicated. Each fiber is essentially a composite in which rigid cellulose microfibrils are embedded in a soft lignin and hemicellulose matrix (Fig. 1). In addition, the microfibrils are helically wound along the fiber axis to form ultimate hollow cells. Uncoiling of these spirally oriented fibrils consumes large amounts of energy and is one of the predominant failure modes. As a result, pretreatment of the fibers would result in chemical and structural changes not only on the fiber surface but also in the distinct cells, which in turn also influences the properties of the fibers and composites.

Possessing high content of cellulose and high tensile strength in comparison with other natural fibers [6], sisal fiber in chopped, continuous and woven forms has been shown to be suitable for application in polymer composites [7]. For purposes of having a thorough understanding of the mechanical role of sisal fibers in composites and the effects of fiber treatment, unidirectional sisal/epoxy laminates (instead of short-fiber composites which are more difficult to analyze owing to the complexity of their microstructure) with chemically and physically modified fibers are produced and evaluated in this paper. In particular, it is expected that the relationship between mechanical behavior and interfacial adhesion can be known.

Section snippets

Materials

All of the materials employed in this work were obtained from commercial sources and used as received. Untreated sisal fibers (UTSF) with diameters ranging from 100 to 200 μm were provided by Dongfanghong State Farm in Guangdong. The epoxy resin was a product of the Guangzhou East Chemistry Company (E51, molecular weight=392). A mixture of tetraethylenepentamine (TEPA) and acrylonitrile (1.2/1 by weight ratio) was used as the curing agent.

Alkali-treated sisal-fiber (ATSF)

Sisal fiber was immersed in a solution of 2% NaOH for 4

Treatments induced variation in sisal fiber

Sisal fiber, principally consisting of 65.8% cellulose, 9.9% lignin and 12.0% hemicellulose, has been shown to be very sensitive to caustic soda [10]. In agreement with other lignocellulose fibers, the significant weight loss of sisal fiber after alkali treatment (Table 1) can be ascribed to the partial dissolution of hemicellulose [11]. This is further convinced by the FTIR curves in Fig. 2, where the carbonyl band at 1730 cm⁻¹ corresponding to hemicellulose disappears when the fiber is

Conclusions

1. Sisal fibers can be effectively modified by chemical and physical treatments. Chemical methods usually bring about an active surface by introducing some reactive groups, and provide the fibers with higher extensibility through partial removal of lignin and hemicellulose. In contrast, thermal treatment of the fibers can result in higher fiber stiffness due to the increased crystallinity of hard cellulose.

2. Adhesion at the interface between sisal bundles and matrix and that between ultimate

Acknowledgements

The financial support by the National Natural Science Foundation of China (Grant: 59725307), the Key University Teachers Training Program of the Ministry of Education of China, the Team Project of the Natural Science Foundation of Guangdong, and the Talent Training Program Foundation of the Higher Education Department of Guangdong Province are gratefully acknowledged.

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The effect of fiber treatment on the mechanical properties of unidirectional sisal-reinforced epoxy composites

Abstract

Introduction

Section snippets

Materials

Alkali-treated sisal-fiber (ATSF)

Treatments induced variation in sisal fiber

Conclusions

Acknowledgements

Polymer

Eur. Polym. J.

Carbon

Prog. Polym. Sci.

Compos. Sci. Technol.

Comp. Sci. Technol.

Properties and modification methods for vegetable fibers for natural fiber composites

J. Appl. Polym. Sci.

Natural vegetable fiber/plasticized natural vegetable fiber-a candidate for low cost and fully biodegradable composite

Adv. Compos. Lett.