Mechanical and electrical properties of surface treated rice straw reinforced polypropylene composites

Abstract

Rice straw (RS) Polypropylene (PP) composites were prepared in different ratios (RS : PP wt%) of 10:90, 20:80, 30:70, 40:60 and 50:50 by extrusion moulding and hot press compression technique. The mechanical properties such as tensile strength (TS), bending strength (BS), impact strength (IS) and elongation at break (Eb%) have been examined. Alternating current (AC) conductivity, dielectric constant and dielectric loss tangent have been measured. Differential thermal analysis (DTA) and Thermogravimetric analysis (TGA) have been performed. Surface morphology of these composites have also been investigated by scanning electron microscopy (SEM). It is found that the composite made with 30% RS content produced good mechanical performance which is selected as optimised filler-matrix system for afterwards investigation. For further improvement, RS have been treated in the different ratio of urea solution (0.1, 0.3, 0.5, 1.0 and 2.0 wt%) to fabricate composites and observed that 1.0 wt% urea treated RS contained composites have exhibited highest mechanical properties. Thereafter, 1.0 wt% urea treated RS have been irradiated through gamma radiation with different doses for composite preparation. Rice Straw PP composites irradiate with 100 krad showed superior mechanical properties (TS = 36 MPa and Eb% = 63%) than those of prepared composites. The dielectric constant for RSPP, urea treated RSPP composite (URSPP) and gamma urea treated RSPP composites (GURSPP) measured over a frequency range of 1KHz -100KHz and in the temperature range of (299-373K) showed dependence on frequency, temperature. It is observed that dielectric constant decreases with frequency and increases with temperature. Dielectric constant increases for urea and gamma treated RSPP because of free radical formation and cross linking between fiber and PP molecules during gamma radiation. AC conductivity increases with frequency and temperature. Loss factor was also measured for RSPP, URSPP, GURSPP. RS fiber surface morphology showed smooth surface, where urea and gamma treated RS fiber showed rough surface. Surface roughness becomes higher for gamma radiation. Untreated RS fiber partly adheres to the polypropylene matrix, demonstrating a week fiber matrix adhesion. After Urea RS treated fiber was completely surrounded with the PP matrix proving a better fiber matrix adhesion. Fractured surface micrographs reveals more better adhesion of fiber and PP in GURSPP composites than URSPP and RSPP composites. In DTA thermographs show that, melting temperature (Tm) and degradation temperature (Td) are increased for treated composites. Analysis of TGA traces, GURSPP composites shows more thermal stability than RSPP.