Water Sorption Studies of Hybrid Biofiber-Reinforced Natural Rubber Biocomposites Maya Jacob, † K. T. Varughese, ‡ and Sabu Thomas* ,† School of Chemical Sciences, Mahatma Gandhi University, Priyadarshini Hills P.O. Kottayam, Kerala, India-686 560, Central Power Research Institute, Polymer Laboratory, Bangalore, India-560 080 Received April 19, 2005; Revised Manuscript Received May 31, 2005 Hybrid biofibers (sisal and oil palm) were incorporated into natural rubber matrix. The water absorption characteristics of the composites were evaluated with reference to fiber loading. The influence of temperature on water sorption of the composites is also analyzed. Moisture uptake was found to be dependent on the properties of the biofibers. The mechanism of diffusion in the gum sample was found to be Fickian in nature, while in the loaded composites, it was non-Fickian. Sisal and oil palm fibers were subjected to different treatments such as mercerization and silanation. The effect of chemical modification on moisture uptake was also analyzed. Chemical modification was seen to decrease the water uptake in the composites. The thermodynamic parameters of the sorption process were also evaluated. Activation energy was found to be maximum for the gum sample. 1. Introduction Lignocellulosic fiber-reinforced composite materials are used more and more in all kinds of applications, and they are turning out to be the apple of the eye of the scientific community. The major interest in plant fiber-reinforced composites is due to their favorable properties such as low specific density, high strength, and good acoustic insulating properties. Among the different natural fibers, sisal and oil palm fibers appear to be promising materials because of the high tensile strength of sisal fiber and the toughness of oil palm fiber. Therefore, any composite comprising these two fibers will exhibit the above desirable properties of the individual constituents. The properties of a hybrid composite mainly depend on the fiber content, length of individual fibers, orientation, extent of intermingling of fibers, fiber-to-matrix bonding, and arrangement of both the fibers. The strength of the hybrid composite is also dependent on the failure strain of individual fibers. Maximum hybrid results are obtained when the fibers are highly strain-compatible. One interesting area of study in the case of composites is the moisture uptake of fiber-reinforced composites. A major drawback of vegetable fiber-reinforced composites is their affinity toward moisture, leading to low degrees of adhesion between fiber and matrix. This limitation can be remedied in the form of chemical modifications. A large number of studies have been conducted to analyze the water uptake in natural fiber-reinforced composites. The water uptake behavior of sisal fiber-reinforced ben- zylated fir sawdust composites was investigated by Lu et al. 1 The experimental results indicated that water absorption behavior of the composites was mainly controlled by the reinforcing fiber and the fiber/matrix interfacial character- istics. In a similar study, Alvarez et al. 2 analyzed the water absorption behavior of composites made from a biodegrad- able matrix and alkaline-treated sisal fibers. The matrix in question was a commercial product called MaterBi-Y, which was based on a cellulose derivatives and starch system. The water absorption in natural fibers such as sisal, coir, luffa sponge, and cellulose (from pulp) reinforced polypro- pylene composites were investigated by Espert et al. 3 The authors carried out the experiments at three different tem- peratures: 23, 50, and 70 °C. The process of absorption of water was found to follow the kinetics and mechanisms described by Fick’s theory. In addition, the diffusivity coefficient was found to be dependent on the temperature as estimated by means of Arrhenius law. A decrease in tensile properties of the composites was demonstrated, showing a great loss in mechanical properties of the water-saturated samples compared to the dry samples. An attempt to study the moisture uptake characteristics of hybrid systems was performed by Mishra et al. 4 The composite systems chosen were sisal/glass and pineapple/ glass fiber-reinforced polyester composites. Composites were prepared by varying the concentration of glass fiber and by subjecting the biofibers to different chemical treatments. The authors observed that the water uptake of hybrid composites was less than that of unhybridized composites. In another interesting study, the water absorption behavior of short sisal fiber-reinforced polystyrene composites has been studied with special reference to fiber loading and * Corresponding author. E-mail: sabut@sancharnet.in. Tel: 91-481- 2730003, 91-481-2731036. Fax: 91-481-2561190. † Mahatma Gandhi University. ‡ Central Power Research Institute. 2969 Biomacromolecules 2005, 6, 2969-2979 10.1021/bm050278p CCC: $30.25 © 2005 American Chemical Society Published on Web 09/08/2005