Highly filled composites from anao-silica and benzoxazing-Epoxy copolymer

Abstract

Epoxy act as diluent for benzoxazine resin. A synergistic behavior with the maximum glass transition temperature value was found at the benzoxazine-epoxy composition of 80:20 mass ratio. However, the nano-SiO2 showed better interfacial adhesion with benzoxzaine than copolymer matrix. Therefore, highly filled polybenzoxazine nanocomposites filled with nano-SiO2 particles were investigated for their mechanical and thermal properties as a function of filler loading. The nanocomposites were prepared by high shear mixing followed by compression molding. A very low A-stage viscosity of benzoxazine monomer gives it excellent processability having maximum nano-SiO2 loading as high as 30wt% (18.8vol%) with negligible void content. Moreover, thermal analysis of the curing process of the compound of the PBA-a/nano-SiO2 composites was found to be autocatalytic in nature with average activation energy of 79–92 kJ mol−1. Microscopic analysis (SEM) performed on the PBA-a/nano-SiO2 composite fracture surface indicated a nearly homogeneous distribution of the nano-scaled silica in the polybenzoxazine matrix. In addition, the enhancement in storage modulus of the nano-SiO2 filled polybenzoxazine composites was found to be significantly higher than that of the recently reported nano-SiO2 filled epoxy composites. The dependence of the nanocomposites’ modulus on the nano-SiO2 particles content is well fitted by the generalized Kerner equation. Furthermore, the relatively high micro-hardness of the PBA-a/nano-SiO2 composites up to about 600 MPa was achieved. Finally, the substantial enhancement in the glass transition temperature (Tg) of the PBA-a/nano-SiO2 composites was also observed with the △Tg up to 16oC at the nano-SiO2 loading of 30wt%. The effects of the nano-filler on their thermal degradation kinetics were investigated using non-isothermal thermogravimetric analysis (TGA). The DTG curves revealed three stages of decomposition process in the neat PBA-a while the first peak at low temperature was absent in its nanocomposites. As a consequence, the maximum degradation temperature of the nanocomposites shifted significantly to higher temperature as a function of the nano-SiO2 contents. Moreover, the degradation rate for every degradation stage was found to decrease with increasing amount of the nano-SiO2. From the kinetics analysis, dependence of activation energy (Ea) of the nanocomposites on conversion () indicates a complex reaction with the participation of at least two different mechanisms. From Coat-Redfern and integral master plot methods, the average Ea and pre-exponential factor (A) of the nanocomposites showed systematically higher value than that of the PBA-a, likely from the shielding effect of the nanoparticles. For example, at the main degradation stage, the neat PBA-a has Ea and lnA of 116 kJ/mol and 13.6 while the 10wt% nano-SiO2 filled PBA-a has Ea and lnA of 157 kJ/mol and 19.1. The main degradation mechanism of the PBA-a was determined to be a random nucleation type with one nucleus on the individual particle (F1 model) while that of the PBA-a nanocomposite was best described by diffusion- controlled reaction (D3 model).