Effects of particle size distribution of silica on properties of PTFE/SiO2 composites

The properties of Polytetrafluoroethylene (PTFE) filled with two kinds of binary mixtures of silica particles with different size distribution (i.e., 2 μm + 15 μm and 5 μm + 15 μm) were investigated at a fixed total filler content of 62 wt.%. The effects of particle size distribution of silica on the properties of PTFE/SiO2 composites were investigated, including density, moisture absorption, dielectric properties (e r , tan δ), coefficient of thermal expansion (CTE) and temperature coefficient of dielectric constant (τ e ). Scanning electron microscopy results show that small size silica of appropriate content filled the holes between large size silica, contributing to compact microstructure. However, remarkable agglomeration of SiO2 fillers was observed with too much small size silica and resulted in lower density. As the filler was comprised of 31 wt.% 2 μm and 31 wt.% 15 μm SiO2, the composite showed a compact structure with a high density of 2.157 g cm−3. At the same time, optimal properties were obtained for PTFE/SiO2 composite, including excellent dielectric properties (e r ~ 2.99, tan δ ~ 0.002), coefficient of thermal expansion of 18.68 ppm C−1, acceptable water absorption of 0.05% and low temperature coefficient of dielectric constant of 11.43 ppm C−1.