JOURNAL OF MATERIALS SCIENCE LETTERS 19, 2 0 0 0, 2151 – 2153 The effect of a chemical treatment on the structure and morphology of coir fibers V. CALADO, D. W. BARRETO Escola de Quimica, Universidade Federal o Rio de Janeiro, Centro de Tecnologia, Bl. E, Cidade Universit ´ aria, Rio de Janeiro, Brazil J. R. M. D’ALMEIDA Materials Science and Metallurgy Department, Pontif´ıcia Universidade Cat´ olica do Rio de Janeiro, Rua Marqu ˆ es de S ˜ ao Vicente, 225 - 22453-900 - Rio de Janeiro-RJ, Brazil E-mail: dalmeida@rdc.puc-rio.br Natural fiber reinforced resin matrix composites have to overcome many challenges in order to be commonly used as engineering materials. A common drawback of these composites is the large variability of their me- chanical properties, in many instances associated with low mean values for their properties [1, 2]. These char- acteristics are due to the intrinsic variability of the prop- erties of the fibers, the susceptibility of natural fibers against moisture and poor wetting of the fibers by the resin matrix. Considerable work has been done to in order to improve the strength of fiber/matrix interface [3–6], but much work has yet to be done in order to guarantee reliability to natural fiber composites. Coconut is among one of the most abundant and worldwide spread plantations in tropical countries, with an estimated annual world production of 42 million tons of leaves [7]. Fibers can be obtained from the leaflets or from the fruits. Fruit fibers are now commercially used, blended with natural rubber latex in the produc- tion of seat cushion parts in automobiles [8]. Common to other natural fibers, the use of coir fibers as reinforce- ment in resin matrix composites is limited by the poor interfacial adhesion between the fibers and thermoset- ting polymers [3]. The chemical treatment of such fibers changes their polarity (and therefore increases the fiber- resin adhesion) and may then be a way to overcome this limitation. In this paper the effect of a chemical treatment on the structure and surface morphology of coir fibers was investigated. The first step of the chemical treatment was the removal of the lignin from the surface of the fibers, roughening their surface. The presence of a rougher fiber surface can increase the adhesion by promoting a mechanical interlock between the fibers and the resin matrix [9]. The raw coir fibers were soaked in a solu- tion of 2% sodium sulfite (Na 2 SO 3 ), and sonicated for 2 h on an ultrasonic apparatus. The vibration produced by the sonication is believed to enhance the penetration of the reactants into the fibers, improving the efficacy of the treatment. The fibers were then washed to neu- tral pH and dried at room temperature, 23 ± 2 ◦ C, for 2–3 days. The removal of lignin by sulfite treatment is a stan- dard procedure in the paper industry. Sodium sulfite reacts with the aromatic rings of the lignin, forming water-soluble lignosulfonates [10] which are then eas- ily removed from the fiber core, Fig. 1. The second step of the treatment was done to reduce the number of free hydroxyl groups of cellulose. This will result in the reduction of the polarity of cellulose molecules and in the improvement of its compatibility with the common thermosetting matrices used in com- posites [4] (Fig. 2). The fibers were soaked in a solution of 100 parts of acetic anhydride per 1 part of sulfuric acid and sonicated for 3 h. The reaction was kept at room temperature during 12 h to allow a better diffu- sion of the reactants into the fiber core, sonicated again for 2 h and, next, kept at room temperature for 3 h. The fibers were then washed to neutral pH and dried at room temperature (23 ± 2 ◦ C). Infrared spectrometry (IR) and scanning electron mi- croscopy (SEM) were used to analyze the effect of the treatment on the surface structure and morphology of the fibers. The IR spectra of the treated and untreated Figure 1 Scheme of the sulfonation reaction between lignin and sodium sulfite. Figure 2 Scheme of the acetylation reaction of cellulose. 0261–8028 C  2000 Kluwer Academic Publishers 2151