Effect of Hybridization and Chemical Modification on the Water-Absorption Behavior of Banana Fiber–Reinforced Polyester Composites Laly A. Pothan, 1 Sabu Thomas 2 1 Department of Chemistry, Bishop Moore College, Mavelikara 690110, Kerala, India 2 School of Chemical Sciences, Mahatma Gandhi University, Priyadarshini Hills, P.O. 686560, Kottayam, Kerala, India Received 8 August 2001; accepted 26 September 2003 ABSTRACT: The water sorption characteristics of banana fiber–reinforced polyester composites were studied by im- mersion in distilled water at 28, 50, 70, and 90°C. The effect of hybridization with glass fiber and the chemical modifica- tion of the fiber on the water absorption properties of the prepared composites were also evaluated. In the case of hybrid composites, water uptake decreased with increase of glass fiber content. In the case of chemically modified fiber composites, water uptake was found to be dependent on the chemical treatment done on the fiber surface. Weight change profiles of the composites at higher temperature indicated that the diffusion is close to Fickian. The water absorption showed a multistage mechanism in all cases at lower tem- peratures. Chemical modification was found to affect the water uptake of the composite. Among the treated compos- ites the lowest water uptake was observed for composites treated with silane A1100. Finally, parameters like diffusion, sorption, and permeability coefficients were determined. It was observed that equilibrium water uptake is dependent on the nature of the composite and temperature. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 91: 3856 –3865, 2004 Key words: banana fiber; diffusion; chemical modification; composites; glass fibers INTRODUCTION The plant cell wall is a highly versatile composite material that is the basis of products from wood and paper to paint thickeners and explosives. The mesh- work of insoluble cellulose fibrils embedded in a ma- trix of gelatinous sugar polymer is responsible for the high tensile strength of the material. These lignocellu- losic materials have slowly been introduced as rein- forcement materials in polymeric matrices. These composites have appreciable mechanical properties, which make them good candidates in low-load– bear- ing applications. However, for many applications, knowledge of the water absorption properties is important. Since the 1970s, tremendous efforts have been made to under- stand the mechanism of moisture absorption in poly- mers and composites and to improve their moisture resistance. Besides moisture, most applications also expose the material to a wide range of temperatures. As a result, the temperature effect of moisture absorp- tion is also an important field of investigation. Natural fibers are potentially hydrophilic because of the pres- ence of –OH groups. This hydrophilicity in turn affects the long-term mechanical properties of the composite, when natural fibers are used as reinforcement in var- ious polymeric matrices. Moisture diffuses into poly- mers to different degrees depending on a number of molecular and microstructural aspects. 1 The main fac- tors that affect the diffusion process are as follows: 1. Polarity of the molecular structure, that is, the presence of chemical groups capable of forming hydrogen bonds with water. 2. Degree of crosslinking. 3. Presence of residual monomers or other water- attacking groups. 4. Crystallinity. The water permeability of an overall composite, however, is decided mainly by the nature of the fi- bers. 2 Many matrix resins also absorb moisture revers- ibly by Fickian diffusion. Complexities involved in resin chemical structure and microstructure attributed to crosslinking density, polarity, and impurities cause non-Fickian processes to occur. 3 The absorbed water causes plasticization of the matrix 4 and also cracking through swelling. 5 In addition, the diffused moisture may reduce the bond strength by breaking the bonds in the matrix. Incorporation of vegetable reinforce- ment in polymeric matrices aggravates the complexity of the situation. The polymeric matrix and the rein- forcement differ significantly in their independent re- sponses to moisture. Expressions relating the compos- Correspondence to: S. Thomas (sabut@.vsnl.com). Journal of Applied Polymer Science, Vol. 91, 3856 –3865 (2004) © 2004 Wiley Periodicals, Inc.