Effects of perfluorooctyltriethoxysilane coupling agent on the properties of silica filled PTFE composites

Fused amorphous SiO2 filled polytetrafluoroethylene (PTFE) microwave substrate composites were manufactured. The composition of all the samples was 57 wt% - SiO2 and 43 wt% PTFE. The effects of compound silane coupling agents on the properties of the SiO2 filled PTFE composites were investigated, including density, water absorption, dielectric properties and temperature coefficient of dielectric constant. Compound coupling agents x wt% KH550+ (1.5-x) wt% F8261 (x: mass ratio to SiO2 ceramic, x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) were used to pre-treat SiO2 fillers and the performance of composites were improved with appropriate amounts of coupling agents. The SEM results show that SiO2 particles modified with compound coupling agents are well dispersed in PTFE polymer and exhibit a strong connection with PTFE. The dielectric constant of all the composites shows excellent frequency stability within the frequency from 1 to 18 GHz. The SiO2 ceramic filler modified with 1.1 wt% F8261 and 0.4 wt% KH550 simultaneously has the highest contact angle and the lowest surface energy, besides, the composite achieves a most compact structure with a maximum density of 2.059 g/cm(3). At the same time, this composite obtains optimal properties, including good dielectric properties (epsilon(r) similar to 2.89, tan delta similar to 0.0007), acceptable water absorption of 0.2%, temperature coefficient of dielectric constant (tau(epsilon)) of 32.32 ppm/degrees C. Moreover, the ratio of experimental and theoretical dielectric constant can achieve 98.86%, which is far greater than that in previous reports.