Introduction to Silicone Rubber
Silicone rubber is an inorganic synthetic polymer composed of a backbone of alternating silicon and oxygen atoms (siloxane), with organic side groups (e.g., methyl, vinyl, or phenyl) attached to the silicon. It is produced via polymerization of silane precursors, followed by cross-linking with peroxides, platinum catalysts (for addition curing), or condensation methods. Developed in the 1940s, Silicone rubber comes in various forms: high-temperature vulcanizing (HTV), room-temperature vulcanizing (RTV), liquid silicone rubber (LSR), and fluorosilicone (FVMQ) for enhanced chemical resistance. The properties can be tailored by varying the side groups and cross-link density. It is compounded with fillers like silica for reinforcement, pigments, and additives for specific enhancements. Silicone is non-toxic, odorless, and available in medical-grade formulations compliant with FDA, USP Class VI, or ISO 10993 standards.
Characteristics of Silicone Rubber
Silicone rubber stands out for its wide service temperature range and inertness. Here are its key characteristics:
- Mechanical Properties: Silicone has moderate tensile strength (5-12 MPa), high elongation at break (200-800%), and excellent tear resistance in some grades. It offers superior flexibility and compression set resistance, even after prolonged deformation. Resilience is high, with good vibration damping, but it has lower abrasion resistance compared to organic rubbers like NBR.
- Chemical Resistance: Excellent resistance to water, oxygen, ozone, UV radiation, and weathering due to its inorganic backbone. It withstands dilute acids, bases, alcohols, and many polar solvents but has poor resistance to hydrocarbons, oils, strong acids (e.g., concentrated sulfuric acid), and silicone fluids, which can cause swelling. Fluorosilicone variants improve fuel and oil resistance.
- Thermal Properties: Exceptional temperature range from -60°C to +250°C (or up to +300°C for short periods in heat-resistant grades). It maintains elasticity at low temperatures without embrittlement and resists thermal degradation, with low thermal conductivity for insulation purposes.
- Durability and Aging: Outstanding long-term stability, with minimal aging or hardening over time. It has high dielectric strength (electrical insulation), low flammability (self-extinguishing in some grades), and biocompatibility, making it safe for skin contact or implantation. Gas permeability is high, which can be advantageous or a drawback depending on the application.
- Processing and Cost: Versatile processing via injection molding (LSR), extrusion, compression molding, or casting (RTV). It has good mold release and can be overmolded onto plastics or metals. Silicone is more expensive than general-purpose rubbers (e.g., 5-10 times costlier than SBR) but offers value through durability and minimal maintenance.
- Other Properties: Transparent or easily colorable, non-stick surface, low toxicity, and sterilizable by autoclave, gamma radiation, or ethylene oxide. It can be vulcanized peroxide-free for clean applications. Specific gravity is low (1.1-1.5), contributing to lightweight parts. However, it has poor adhesion without primers and can accumulate static charge.
In formulations, Silicone is often reinforced with fumed silica and may be blended with other materials for hybrid properties, though it's rarely mixed with organic rubbers due to incompatibility.
Applications of Silicone Rubber
Silicone's versatility and safety profile make it suitable for high-reliability applications:
- Medical and Healthcare: Used in implants (e.g., catheters, pacemaker leads), seals for medical devices, tubing, and prosthetics due to its biocompatibility and sterilizability. RTV silicones are common in wound dressings and dental molds.
- Electronics and Electrical: In keypads, seals for connectors, insulators, and potting compounds for circuit protection, leveraging its dielectric properties and thermal stability.
- Automotive and Aerospace: Employed in gaskets, O-rings, spark plug boots, and vibration mounts for engines, resisting heat and weathering.
- Consumer Products: In kitchenware (e.g., baking molds, spatulas), baby products (pacifiers), and seals for appliances due to its food-grade safety and non-stick nature.
- Industrial Seals and Gaskets: For pumps, valves, and HVAC systems in non-oil environments, providing reliable sealing over wide temperatures.
- Other Uses: Adhesives and sealants (RTV), foam for insulation, and coatings for textiles or wires. LSR is popular in optics and 3D printing molds.
Silicone rubber production is eco-friendly with low VOC emissions, and recyclable variants are emerging. It's a premium material with global consumption in millions of tons, driven by healthcare and electronics growth.
