What Is Silicone Rubber?
Composition
Silicone rubber is a synthetic elastomer composed of three main components: (A) a high molecular weight polyorganosiloxane polymer, (B) reinforcing silica fillers, and (C) a curing agent or crosslinking agent. The polyorganosiloxane polymer typically has a viscosity of at least 100 mPa.s at 25°C and contains alkenyl groups for crosslinking. Common fillers include fumed silica, precipitated silica, and surface-treated silica. Curing agents can be organic peroxides or organohydrogen polysiloxanes used with platinum catalysts for hydrosilylation curing.
Formulation and Additives
In addition to the main components, silicone rubber formulations may contain various additives to impart desired properties, such as silicone oils for processability, thixotropic agents, adhesion promoters, condensation catalysts, and other fillers like calcium carbonate, iron oxide, or clays. Specific additives like alkoxysilanes, disilanol compounds, and encapsulated platinum catalysts can improve properties like compression set resistance, plasticity reversion, and cure speed.
Properties of Silicone Rubber
- Excellent thermal stability, weatherability, electrical insulation, and chemical resistance over a wide temperature range (-90°C to 300°C).
- Low surface energy, low toxicity, high optical transparency, and good flame resistance.
- High flexibility, compression set resistance, and mechanical strength which can be tailored by varying fillers like zinc oxide, acicular minerals, carbon nanotubes, etc.
- Tunable thermal conductivity by adding fillers like aluminum.
- Adhesion promotion and selective adhesion using special adhesion promoters.
Uses & Benefits of Silicone Rubber
Automotive Industry
- Turbocharged engines: Fluorosilicone rubber provides flexibility and high-temperature resistance for turbocharger hoses and gaskets.
- Sealing and insulation: Silicone rubber’s heat resistance, weatherability, and electrical insulation properties make it ideal for automotive seals, gaskets, and wire insulation.
Electrical and Electronics
- Insulation and protection: It offers excellent electrical insulation, UV resistance, and seismic resistance for electrical insulation, cable jacketing, and protective coatings.
- Lightweight and durable: Its components are lighter and more durable than traditional materials, with low fire propagation and erosion resistance.
Consumer Products
- Processability and clarity: Liquid silicone rubber (LSR) enables easy processing and transparency for consumer products like kitchen utensils and baby products.
- Mechanical properties: LSR provides excellent mechanical properties, making it suitable for seals, gaskets, and other consumer applications.
Synthesis of Silicone Rubber
Silicone rubber is typically synthesized through a condensation-curing process involving the following key components:
- Base Polymer
- Linear diorganopolysiloxane with alkenyl groups (e.g., vinyl) at the terminals or along the polymer chain
- Molecular weight and vinyl content tailored for desired properties
- Crosslinking Agent
- Organohydrogenpolysiloxane containing Si-H groups
- Reacts with vinyl groups on the base polymer during curing
- Reinforcing Filler
- Fumed or precipitated silica with high surface area (≥50 m²/g)
- Enhances mechanical properties and heat resistance
- Additional Fillers
- Titanium dioxide for UV resistance and opacity
- Carbon black for conductivity
- Metal oxides (e.g., cerium oxide) for improved heat resistance
- Curing Catalyst
- Platinum-based compounds
- Facilitates the hydrosilylation reaction between Si-H and vinyl groups
The synthesis typically involves mixing the base polymer, fillers, and other additives, followed by the addition of the crosslinking agent and curing catalyst. The curing reaction can be accelerated by heat or moisture, depending on the specific formulation.
Latest innovations
Novel Silicone Rubber Compositions
- Emulsion Compositions: Emulsion compositions containing diorganopolysiloxane, reinforcing silica filler, smectite clay, emulsifier, and cross-linking agent have been developed for producing silicone rubber without dehydration. The smectite clay improves processability and properties.
- Fabric/Cloth Printing Compositions: Silicone rubber compositions with improved printing properties and workability for forming patterns on fabric or cloth have been formulated, containing specific silicone polymers, silica, silicone oil, thixotropic agent, and hardening catalyst.
- Calcined Filler Compositions: Silicone rubber compositions incorporating calcined mineral fillers like fumed silica, precipitated silica, calcium carbonate, iron oxide, or clay have been developed, offering improved thermal conductivity and environmental stability.
Advanced Processing Techniques
- Emulsion Curing: Methods for manufacturing silicone elastomers from aqueous silicone emulsions have been introduced, involving emulsifying vinyl-containing organopolysiloxane, organohydrogenpolysiloxane, and platinum catalyst, followed by heating to form a cross-linked latex.
- Kneading and Heat Treatment: A process for preparing silicone rubber compositions involves kneading components at different temperatures, followed by heat treatment to improve silica dispersibility and compression set resistance without separate heat treatment.
Performance and Application Enhancements
- Thermal Conductivity: Silicone rubber compositions with high thermal conductivity have been developed by incorporating zinc oxide fillers prepared by firing zinc hydroxide or zinc carbonate at specific temperatures.
- Adhesion and Selectivity: Adhesive and selective adhesive silicone rubbers containing special adhesion promoters have been formulated for improved bonding capabilities.
- Electrically Conductive/Semiconductive: Electrically conductive and semiconductive silicone rubbers have been developed by incorporating carbon black fillers, enabling applications in electronics and related fields.
Sustainable and Specialty Innovations
- Environmentally Friendly: Low VOC and solvent-free silicone rubber compositions have been developed to address environmental concerns.
- Silicone Sponges: Addition-cured silicone rubber sponges have been produced without using organic blowing agents, offering potential applications in insulation and cushioning.
- Reactive Hot-Melts: Silicone reactive hot-melt compositions have been introduced, enabling new applications in adhesives and sealants.
These innovations in silicone rubber compositions, processing techniques, performance enhancements, and sustainable/specialty products demonstrate the continuous advancements in this versatile material to meet evolving application requirements across various industries.
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