Silicone Elastomers: Types, Properties and Uses Demystified

Silicone elastomers have become indispensable across industries, valued for their extreme temperature tolerance, flexibility, and chemical inertness. However, confusion persists around defining silicone elastomers and distinguishing their types and applications. This article will demystify these versatile high-performance polymers by examining the composition, categories, and uses of silicone elastomers compared to other silicone rubbers.

What Are Silicone Elastomers?

Silicone elastomers are polymeric materials composed of silicone polymers and reinforcing fillers, crosslinked to form elastic networks. Their backbone contains alternating silicon and oxygen atoms, with organic side groups attached to silicon. The unique chemistry of silicone enables its rubber-like flexibility and elasticity.

Silicone elastomers can be classified into three main categories based on their base polymer viscosity and curing processes:

High Consistency Rubbers (HCR)

HCRs are solid, high viscosity silicones requiring conventional rubber processing like extrusion, calendering, or compression molding. They cure via free radical reactions, producing robust elastomers with high tensile strength. Common applications of HCR include seals, gaskets, tubing, and catheters.

Liquid Silicone Rubbers (LSR)

LSRs are low viscosity, easily pourable silicones capable of injection molding and 3D printing. Their addition cure system enables rapid processing. LSRs provide excellent tear resistance and dielectric properties. They find wide use in electrical insulation, medical devices, and electronics.

Room Temperature Vulcanized (RTV) Rubbers

RTVs are moderate viscosity silicones, applied by coating, pouring or molding. Their condensation cure enables forming elastomers at room temperature. RTVs can adhere to many substrates, making them ideal sealants and encapsulants. Common RTV applications include molds, casts, and adhesives.

Distinguishing Silicone Elastomers from Silicone Rubber

Silicone elastomers differ from general silicone rubbers in their molecular structure and processing methods:

  • Silicone elastomers are typically lower molecular weight polymers with lower viscosities. This allows liquid state processing.
  • Silicone rubbers have higher molecular weights and viscosities, requiring mechanical methods like extrusion and compression molding.
  • Silicone elastomers undergo addition or condensation curing, while silicone rubbers primarily utilize peroxide or platinum curing systems.
  • The main application difference is silicone elastomers’ dominance in liquid injection molding and moldmaking.

So in summary, silicone elastomers represent lower viscosity, addition or condensation cured silicones, while silicone rubber refers to higher viscosity peroxide or platinum cured silicones.

Relationships Between Silicone Elastomers and LSR

Liquid silicone rubber (LSR) is a type of silicone elastomer with its own distinct properties and advantages:

  • LSRs have very low viscosities, allowing easy flow into molds. Other elastomers have higher viscosities.
  • LSRs utilize rapid addition cure chemistries. Other elastomers use slower condensation curing.
  • LSRs can be readily injection molded and 3D printed. Other elastomers require coating or press molding.
  • LSRs exhibit very high tear strength and flexibility. Properties vary more with other elastomers.

So in summary, LSR represents a specialized sub-category of silicone elastomers optimized for injection molding applications requiring precision, flexibility, and fast process cycles.

Key Applications of Silicone Elastomer Types

The different silicone elastomer varieties are each suited to specific applications:

HCR Applications

  • Seals, gaskets, o-rings demanding compression set resistance
  • Wire insulation requiring high dielectric strength
  • Catheters and tubing needing flexibility combined with strength
  • Vibration dampening mounts withstanding high continuous temperatures

LSR Applications

  • Molded electronics requiring precision and dimensional stability
  • Medical devices like seals, stoppers, valves needing flexibility and an autoclave-compatible material
  • Components needing transparency, like light pipes and diffusers
  • 3D printed wearables and cushions requiring soft durometer and tear resistance

RTV Applications

  • Mold making to capture precise detail and dimensional accuracy
  • Adhesive bonding of silicone to itself and many substrate materials
  • Conformal coatings and encapsulants providing protective barriers
  • Thermal insulation demanding minimal shrinkage after curing

Choosing the Optimal Silicone Elastomer

With numerous silicone elastomer grades available, consider factors like:

  • Viscosity needs for dispensing or flow during processing
  • Cure chemistry compatibility with required processing methods
  • Desired mechanical properties like tensile strength, elongation, and hardness
  • Whether transparency or opaque coloring is required
  • Thermal stability requirements across operating temperatures
  • Flammability rating and regulatory compliance needs

Discussing application requirements with an experienced silicone manufacturer ensures identifying the optimal silicone elastomer for your product needs.

In summary, taking time to understand the compositions, properties, and applications of silicone elastomer types assists in selecting the ideal material for your challenging elastomer needs.