Investigation on Thermal and Ablation Characteristics of EPDM-Filler Composites

Abstract

The research work is related to the understanding of the effects of fillers loadings on thermal and ablation performances of ethylene–propylene–diene terpolymers (EPDM). EPDM filled with particulate and fibrous fillers are widely used as internal thermal insulator in space vehicles. The most widely used fibrous filler is asbestos. But due to the health hazards posed by asbestos and requirements of space technology, efforts have been initiated for its replacement. In current research, the effects of silica, kevlar and cork on thermal and ablation performances of EPDM based thermal insulators have been investigated. Various batches of thermal insulations were prepared by compounding EPDM with various percentages of silica, kevlar and cork in the presences of other necessary ingredients such as curing agents, accelerators and process aids etc. It was observed that cork and kevlar in the absences of silica did not import enough reinforcement to EPDM to fulfill the requirements of thermal insulations for space vehicles to withstand the stresses during handlings, operations and launching etc. Therefore; thermal and ablation performance of the EPDM batches containing silica only was investigated. Experimental evidences showed that silica had not only enhanced mechanical properties but also improved thermal and ablation performance of EPDM based thermal insulations. However, silica also increased density and thermal conductivity of the thermal insulations, which were the limitations of EPDM loaded with high concentration of silica as far as thermal insulation for space application was concerned. vii In the next phase of research the concentration of silica was limited to 10 Phr (parts per hundred parts of rubber) and kevlar was added at various concentrations in order to remove the drawbacks of high density and high thermal conductivity of silica filled EPDM. Kevlar loadings significantly enhanced thermal and ablation performance of silica-EPDM based thermal insulation by forming a tough char layer on the surface of the insulator. However, the same problem of high density and high thermal conductivity occurred. Kevlar also significantly decreased the elongation of the EPDM based thermal insulator. Cork, a hydrocarbon material with low density and low thermal conductivity was added in various Phr to EPDM loaded with 10 Phr of each silica and kevlar. Cork loadings not only enhanced thermal and ablation performance of the kevlar-silica filled EPDM but significant reduction in thermal conductivity and density was also achieved. Physico- mechanical, thermal and ablation performances of indigenous model insulation based on EPDM filled with hybrid fillers, cork, kevlar and silica were compared with EPDM filled with silica and asbestos. It was observed that model insulation not only exhibited better mechanical, thermal and ablation performances than asbestos based thermal insulation but also significant reduction in thermal conductivity and density was achieved. From the experimental data onset of decomposition temperature (Tonset) and temperature at which maximum degradation occurs (Tmax) of the model insulation for space vehicles were determined. The Flynn-Wall-Ozawa model was successfully used for the determination of activation energy required for thermal decomposition of the insulation. The experimental data obtained at various heating rates were fitted to existing models for the determination of kinetic mechanism of the thermal decomposition of the model insulation. The results showed that model insulation was decomposed according to random nucleation which followed the general mechanism proposed in random nucleation, Avrami Erofe’ev equation. Based on the concept of thermal decomposition by random nucleation, a general decomposition scheme consisting of various reactions was proposed and a kinetic model for thermal decomposition was developed. The developed kinetic model verified that the thermal decomposition of the model insulation was according to random nucleation as the experimental data best fitted to the model equation.