Abstract
The production of hot-pressed composite materials based on unsaturated polyester resin reinforced with rice husk is described. A correlation equation relating the tensile and compressive mechanical properties as a function of both rice husk concentration and the pressure applied to the mold was obtained.
1.INTRODUCTION
Polymer composites have been studied for many years and constitute a very active area for researchers in materials science. The reinforcing agents mostly used in engineering composite materials include fibreglass, boron, carbon, and silicon carbide fibres [3, 4] among others. An alternative for producing low cost composite materials is by using natural fibres which grow abundantly in Nature and can be commercially exploited because of their low cost and ease of handling [5, 6] . In fact, one of the most rapidly growing areas of the materials manufacturing is precisely that of natural reinforcedreinforced polymers. [1, 2, 11] A number of natural fibres have been employed as reinforcements, and one can mention cotton, [11] pineapple-leaf fibre, [7] sisal fibres, [8] wood fibres, [9] palm fibre, [12] bagasse fibre, [10] henequen fibres, etc. On the other hand , one of the main natural by-products in Mexico and many other rice-producing countries is rice husk, which has a very interesting silica-rich composition, that makes this agricultural by-product a very suitable candidate for being used in a composite material. Accordingly, in the present work the preparation, mechanical properties as well as their modelling, of polyester resin-based, rice husk-reinforced composites, are presented.
EXPERIMENTAL 2.1 Raw Materials
Rice husk (type A-70) was obtain from the rice industry San Jose in Jojutla, Morelos, Mexico. The polyester resin and benzoyl peroxide catalyst were purchased from Baker Co.
Composites preparation
The samples were prepared as follows: 2% of Benzoyl peroxide catalyst by weight was added to the polyester resin. Then, this resin is mixed homogeneously in a industrial blender at 100 rpm. with rice husk in the adequate concentration. The final mix is poured into a circular mold of a hydraulic hot press. Once the mixture was introduced there, pressure was applied to the mold, keeping it for 10 min. to allow the crosslinking reaction to complete.
Characterisation techniques
Tensile and compressive samples were obtained by of the composites as a function of the rice husk concentration and the pressure applied to the mold. Table 2 shows both the corresponding correlation equations and experimental factors of statistical significance. In general, the statistical errors are not significant, however, rice husk concentration and pressure applied to the mold (factor X and factor Y, respectively) are indeed significant, statistically 2ULJLQ ) ) 6LJQLILFDQW" 6LJQLILFDQW" U@ITDG@Ã )DFWRU ; <(6 <(6 )DFWRU < <(6 12 ,QW;< <(6 <(6 8PHQS@TTDW @Ã )DFWRU ; <(6 <(6 )DFWRU < <(6 <(6 ,QW;< <(6 <(6
RESULTS AND DISCUSSION
1: Tensile (s t ) and Compressive (s c ) properties in the rice husk-polymer resin composites
Numerical Analysis of the Mechanical Properties of Rice Husk-Poyester Composites
speaking . The factor XY represents the change of both rice husk concentration (X) and mold pressure (Y) at the same time. F represents the empirical statistical error, with a confidence of 95% -F(0.05)-and 99% -F(0.01). In each case, 5 experiments were averaged.
From the statistical analysis, the empirical equations (1) and (2) were obtained, as follows:
Eqn. 1 describes the tensile properties of the composite, whereas Eqn. 2 corresponds to the compressive properties. There, s t is the tensile strength (kg/cm . On the other hand, Fig.1 shows graphically the tensile behaviour of the rice husk-polymer resin composites. It is clear that, in general, the higher the rice husk concentration the poorer the mechanical properties. This is due to the poor fibre-matrix interface.
As it can be observed in Fig. 1 , the best tensile properties can be obtained at two points, that is, high pressures and low rice husk concentration and low pressures and low concentration. Due to the production cost, low pressures in the mold are preferred. On the other hand, Fig. 2 shows that the best compressive properties can be obtained by applying low pressures and low rice husk concentration. Therefore these conditions are recommended for future studies because they provide the best tensile and compressive properties in the rice husk-polymer resin composites.
It is important to mention that the properties of the present composites were better as compared to 
CONCLUSIONS
The development of this material opens an exciting area of research in materials science in which agricultural byproducts, which have been traditionally neglected by materials scientist, can provide interesting opportunities for producing novel materials. In fact, to demonstrate the feasibility of designing tailored materials by simple compounding was one of the main goals of the present work.

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