Rice husk derived silica aerogel in unsaturated polyester composites

Abstract

Unsaturated polyester resins (UPR) are the most widely used thermosets for composite applications. Besides fiber materials, solid fillers are also used extensively with UPR as reinforcement to enhance thermal and mechanical properties. Recently, the use of high porosity solid such as silica aerogels (SA) in polymer resins is gaining interest. SA is a unique class of nano-porous material with extremely low bulk density and high specific surface area. The addition of SA in polymer resins had resulted in enhanced composite materials having excellent thermal insulation, heat resistance, flame retardancy and lightweight. However, there are two major problems which hindered the production of SA polymer composites on an industrial scale; Firstly, the high cost of conventional SA which depends on expensive chemicals as precursor and secondly, the problem of adsorption of polymer into the SA nanopores which results to the loss of product properties. As solutions, two approaches were implemented in this study; first was to reduce the production cost via rice husk ash (RHA) as potential silica source for SA synthesis. Second, to prevent resin insertion into SA nano-pores, a novel coating method of the SA particles with polyvinyl alcohol was proposed to provide an impermeable layer to the structure of the SA. Through these approaches, this study aimed to investigate on how the physical, thermal and mechanical behaviours of the UPR composites are affected by changes in certain characteristics of the SA as the filler such as the porosity, particle sizes, surface coating and hybridization. As benchmarking, the amount of filler in UPR was fixed at 30% of volume fraction. The composites were characterized by various methods such as thermogravimetric analysis, differential scanning calorimetry, scanning electron microscopy, uniaxial tensile and compressive tests, dynamic mechanical analysis, hot-disk thermal analyzer and ASTM D635-14 standard for horizontal burning rate. Evaluation of the SA produced from RHA revealed comparable properties to the conventional SA with density of 0.07 g/cm3, surface area up to 600–700 m2/g and thermal conductivity as low as 0.04 W/mK. The coating of the SA particles of diameters around 2.5 ± 0.5 mm using a fluidized bed coating technique had resulted in closed–pores core–shell aerogel (CSA) particles with measured shell thickness of between 10-50 µm. For UPR composites filled with silica, the composite containing SA as porous filler was at least 23% and 55% lower in density and thermal conductivity than the composite filled with non-porous filler (precipitated silica) respectively. For the same volume fraction of SA, the improvement in composite’s thermal insulation and thermal stability were found to be more for larger SA particles. However, increased in particle size also results in decreased of mechanical properties. For the same particle size, the composite with CSA particles showed a 50% higher of compressive strain and 10 to 12% lower for burning rate as compared to the composite with uncoated SA particles. The CSA particles show reinforcing effects on most of the properties studied, except tensile due to weak filler-matrix bonding. Finally, the combination of SA with alumina trihydrate (ATH) in UPR revealed a synergistic effect during thermal degradation as evidenced by higher thermal stability of the SA/ATH composite when compared to the composite containing only ATH.