Thermal stability and ablation properties of silicone rubber composites

Effects of incorporaiton of clay and carbon fiber (CF) into a high temperature vulcanized (HTV) silicone rubber, i.e., poly(dimethylsiloxane) (PDMS) containing vinyl groups, on its thermal stability and ablation properties were explored through thermogravimetric analyses (TGA) and oxy-acetylene torch tests. Natural clay, sodium montmorillonite (MMT), was modified with a silane compound bearing tetra sulfide (TS) groups to prepare MMTS4: the TS groups may react with the vinyl groups of HTV and enhance the interfacial interaction between the clay and HTV, MMTS4 layers were better dispersed than MMT layers in the respective composites with exfoliated/intercalated coexisting morphology. According to TGA results and to the insulation index, the HTV/MMTS4 composite was more thermally stable than HTV/MMT. However, addition of CF to the composites lowered their thermal stability, because of the high thermal conductivity of CF. The time elasped for the composite specimen, loaded with a constant weight, to break off after the oxy-acetylene flame bursts onto the surface of the specimen was employed as an index for an integrated assessment of the ablation properties, simultaneously taking into consideration the mechanical strength of the char and the rate of decomposition. The elapsed time incresed in the order of : HTV < HTV/CF approximate to HTV/ MMTS4 < HTV/CF/MMTS4 approximate to HTV/MMT < HTV/CF/MMT. This order was different from the incresing order of the thermal stability determined by TGA results and the insulation index. The decreased degree of crosslinking of the composite with MMT may be unfavourable for the formation of a mechanically string char and could lead to early rupture of the HTV/MMTGS(4) specimen. (c) 2008 Wiley Periodicals, Inc.