DEVELOPMENT OF POLYMER - BASED SILICEOUS PARTICULATE COMPOSITES FOR SOLAR ENCAPSULANT APPLICATIONS

Abstract

Development of polymeric composites by the addition of reinforcing fillers has invoked much interest in recent years. The present study aims to develop High Density polyethylene (HDPE) matrix composites reinforced with silica powder particles extracted from rice husk ash (RHA). Silica powder was extracted from RHA by burning, thoroughly washed and dried rice husk into ash in an enclosed drum, the ash was thereafter conditioned in a muffle furnace at a temperature of 650 oC; the conditioned ash was leached with various concentrations (0.5, 1.0, 1.5 and 2.0M) of sodium hydroxide (NaOH) in a water shaker bath at 100 oC for 1 hour, the solution was filtered with ashless filter paper and the filterate was precipitated with concentrated hydrochloric acid (HCl ) to obtain the desired silica gel with pH 7 and incubated for 48 hours to promote silica gel formation. The silica gel so produced was separated from soluble salt solution by vacuum filteration. The silica gel was dried in an air blast oven at 150 oC for 48 hours and ground into powder. The presence of silica in the powder was confirmed with Energy Dispersive X-ray Spectrometer (EDX). The chemical composition of the silica increases with increase in the concentration of NaOH. The EDX pattern of silica powder produced at 2.0 M NaOH treatment shows the composition of silica to be 96.28 wt.%. Horiba dynamic light scattering particle size analyzer also revealed the average particle size of the silica gel to be 500 nm. The composites were developed by blending silica powder, titania powder, HDPE and Maleic Anhydride Polypropylene (MAPP) in predetermined proportions. The mixtures were melt compounded with a laboratory Rapra single screw extruder at temperature of 200 oC to 230 oC. The extrudates were ground with a grinding machine and were poured into tensile and rectangular moulds from where tensile and rectangular samples were produced respectively using Carver laboratory press at 230 oC for 10 minutes under applied pressure of 0.2 kPa. The materials were then water cooled at 20 oC min-1. Ethylene Vinyl Acetate ( EVA) containing 30 wt.% Vinyl Acetate, 5 wt. % MAPP and 65 wt. % HDPE was also prepared in the same way for comparative investigation. Samples were prepared for tensile, impact, hardness, flexural and water absorption tests which were carried out in accordance with ASTM D638, ISO 179, ISO 868, ASTM D790 and ISO 175, respectively. Thermogravimetric analysis and Scanning Electron Microscopic analysis of neat HDPE, EVA and the developed composites were also carried out. Optimum mechanical properties were achieved at 4 wt. % particulate silica reinforced HDPE. The results of the water absorption test showed that the amount of water absorbed by the composites increases as the silica particles content increases. Composite sample with the least water absorption was from sample that contains 2 wt.% SiO2 with an average value of 0.16 %. Above 4 wt. % silica content, particles agglomerate and the HDPE chains is raised accordingly, thereby, decreasing the efficiency of the interlayer and interfacial stress transfer. The results of the thermal-degradation analysis of the neat HDPE, EVA and HDPE/SiO2/TiO2 composites shows that HDPE/SiO2/TiO2 composites with 10 wt.% SiO2 has the maximum thermal degradation temperature of 382.38 oC. It was observed from Scanning Electron Microcope (SEM) images of the composites that there is good dispersion of the silica particles at lower weight fractions of 2-4 wt. %.