Natural fi bers, especially lignocellulosic fi bers extracted from plants, are gaining attention as polymer-matrix composite (PMC) reinforcements due to their comparative advantages over synthetic fi bers. Natural fi bers are relatively low cost, renewable, and biodegradable. Their production systems are associated with low equipment wear and are energy effi cient. In addition, the incorporation of lignocellulosic fi bers into PMCs may signifi cantly improve some mechanical properties. This article presents an overview of the advantages and drawbacks of applying natural fi bers, some of them relatively unknown, as reinforcements of PMCs. The mechanical behavior of composites incorporated with selected fi bers is discussed in terms of the effect of surface micromorphology and the fi ber/matrix interaction.
By means of dimensional selection of natural lignocellulosic fibers, based on precise diameter measurements, it was recently possible to obtain fibers with relatively higher tensile strength. The present article overviews works on the statistical evaluation, through the Weibull analysis, of the ultimate tensile stress of eight lignocellulosic fibers: sisal, ramie, curaua, jute, bamboo, coir, piassava and buriti. It is shown that, for all of these fibers, the tensile strength holds an inverse relationship with the fiber diameter. Statistically this relationship conforms to a hyperbolic type of analytical equation, which discloses the possibility of unusually high strength fibers to be selected in association with very small diameters. A structural analysis using scanning electron microscopy offered an explanation to the strengthening mechanisms responsible for the superior performance of these dimensionally selected fibers.
The fibers extracted from the piassava palm tree, scientifically known as Attalea funifera, are among the stiffest lignocellulosic fibers being considered for polymer composite reinforcement. Characterization of piassava composites have been carried out for different polymeric matrices and mechanical tests. In this work the tensile properties of DGEBA/TETA epoxy matrix composites reinforced with up to 30% in volume of continuous and aligned piassava fibers were evaluated. Tensile specimens post-cured at 60 o C for 4 hours were room temperature tested and the corresponding fracture analyzed by scanning electrons microscopy. The results showed a decrease in both the tensile strength and the elastic modulus of the composites up to 30% with an increase at 40% of piassava fibers to values above those of the pure epoxy. The fracture analysis revealed a weak fiber/matrix interface, which could account for the comparative low performance of these composite in tensile tests up to 30% of volume fraction. The relatively large amount of stronger piassava fibers accounts for the better performance of the composite with 40% in volume fraction.
The piassava fiber is one of the most rigid natural lignocellulosic fibers, which has since the last decade been investigated as possible reinforcement for polymeric composites. In the present work, the tensile properties of polyester composites reinforced with piassava fibers with thinner diameters were investigated. Composites with volume fractions up to 30% of continuous and aligned piassava fibers were tensile tested at room temperature to evaluate the ultimate strength, elastic modulus and total strain. For each volume fraction, individually separated fibers with the smaller diameters were tested. The results indicated that the tensile properties tend to improve with increasing volume fraction of the thinner piassava fibers. The role played by the fiber/matrix interaction was analyzed by scanning electron microscopy.
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