Silicone rubber has a polysiloxane backbone instead of a carbon chain. Standard rubber analysis steps need adjustment, but the identification itself is easy.
Identification
- IR: very strong Si–O–Si bands between 1000 and 1100 cm⁻¹ and Si–CH₃ bands near 1260 and 800 cm⁻¹.
- Pyrolysis: cyclic siloxanes.
- Elemental: silicon in the ash that does not dissolve in acid.
- Variants: phenyl-methyl silicones (extra low-temperature range), fluorosilicone (FVMQ, with trifluoropropyl groups and extra fluid resistance) and vinyl-methyl silicone (the standard peroxide-curable type).
Fillers
Silicone gum is weak without reinforcement. Almost all silicone rubbers contain fumed silica as the reinforcing filler, often 20 to 50 phr, along with extending fillers such as ground quartz, and pigments. Because the polymer itself leaves silica-like ash when burned, the TGA and ash steps must separate silica filler from silicon released from the polymer. Many analysts treat silicone by extraction, TGA and then direct filler measurement after acid or solvent steps.
Cure type
- Peroxide-cured silicone (high consistency rubber): look for residue of peroxide decomposition products.
- Platinum-addition cured: very low residues, often with a small platinum content found by ICP.
- Condensation cured: tin or other catalyst in trace amounts.
Property profile
Compression set at high temperature, tear strength (which is relatively low), hardness, and low-temperature performance. Silicone stays flexible far below the limits of most other rubbers, with typical standard types working to about −55 °C and special types lower.
Why it matters for HPLT
For very cold sealing duty silicone and fluorosilicone are often candidates. They do not suit all oilfield fluids, but they have the lowest glass transition of the common rubbers, so reverse engineering a cold-service seal often ends with a silicone family answer.
