Silicone Rubber: Types, Structure & Properties

What is silicone rubber?

Silicone rubber is a durable & highly-resistant elastomer (rubber-like material). It is composed of silicone (polymer) containing silicon together with other molecules. These molecules include carbon, hydrogen, and oxygen. Its structure always comprises of:

Siloxane backbone (silicon-oxygen chain)
Organic moiety bound to the silicon

Hence, the special features of silicone rubber originate from its unique molecular structure. This structure carries both inorganic and organic groups.

The first commercial silicone rubber was first introduced by Dow Corning in 1943 (now a 100% Dow subsidiary). These rubbers are now manufactured by several companies. View the complete list of silicone rubbers here »

How are silicone rubbers classified based on the organic groups?

The organic groups in silicone rubbers may be methyl, vinyl, phenyl, or other groups. The ASTM D1418 standard covers a system of general nomenclature for rubber and rubber lattices. This classification includes:

Methyl Group – Also known as dimethylsilicone elastomer/rubber or methyl silicone rubber. It is also referred by MQ.

Methyl and Phenyl Groups – Also known as methyl-phenylsilicone elastomer/rubber or phenylsilicone rubber. It is also referred by PMQ. It has an excellent low-temperature performance.

Methyl and Vinyl Groups – Also known as methylvinylsilicone elastomer/rubber. It is also referred by VMQ.

Methyl, Phenyl and Vinyl Groups – It is referred by PVMQ. It is also known for its excellent low-temperature performance.

Fluoro, Vinyl and Methyl Groups – Also known as fluorinated rubber or fluorosilicone rubber. It is also referred by FVMQ. They are highly resistant to chemical attacks (fuel, oil, solvent…).

How are silicone rubbers classified based on the molecular structure?

The molecular structure, viscosity, and processing methods are factors for classifying silicone rubbers. They are available in three main forms:

Solid Silicone Rubber or High-Temperature Vulcanized, HTV

Silicone solid rubbers contain polymers with a high molecular weight. They have relatively long polymer chains. They are available in an uncured form. They require traditional rubber processing techniques.

Heat-curable elastomers are higher-viscosity rubbers. They are mixed and processed like other elastomers. They are cured at elevated temperatures by means of organic peroxides or with a platinum catalyst.

Liquid Silicone Rubber, LSR

Liquid silicone rubbers contain polymers of lower molecular weight. They have shorter chains and better flow properties.

It is a low-viscosity and high-purity thermoset elastomer.
It maintains mechanical properties over a wide range of temperatures (from -50°C to 250°C).
This heat-cured elastomer provides:
- excellent optical clarity
- long-term durability in harsh environments (high temperature, UV, etc...)
- design freedom
It is a low-viscosity and high-purity thermoset elastomer.
This material highly boosts productivity by:
- decreasing cycle time
- reducing material waste
- using smaller machines
It is specially processed on injection molding and extrusion equipment.

The main advantage of LSR is the possible integration of multiple parts into one, allowing drastic cost reduction. This innovative transparent material serves diverse applications such as:

high-power LED lighting
electronics
automotive lighting, and many others.

Discover how this material can fit your application by going through this center dedicated to Liquid Silicone Rubber!

Room Temperature Vulcanized, RTV

RTV silicone rubber is a type of silicone rubber made from one-part (RTV-1) or two-component (RTV-2) systems. Their hardness ranges from very soft to medium. They are available for potting, encapsulations, sealants, etc.

What are the methods to synthesize silicone rubbers?

Overall, silicone rubbers synthesis majorly involves three steps.

Preparation of chlorosilanes
Followed by hydrolysis
Polymerization yielding silicone elastomers

Today, silicones are commercially obtained from chlorosilanes. They are prepared from the direct process of Rochow. The reaction giving chlorosilanes takes place in a fluidized bed. This bed is filled with silicon metal powder. A stream of methyl chloride flows through the bed. This usually takes place at temperatures of 250 to 350°C and at pressures of 1 to 5 bars. A copper-based catalyst is used.

A mixture of different silanes is obtained containing mainly the dimethyldichlorosilane, Me₂SiCl₂.

Mixture of Different Silanes
(Source: Dow Corning)

The dimethyldichlorosilane is separated via distillation. It is used as the monomer to produce Polydimethylsiloxanes. This is done by the hydrolysis of dimethydichlorosilane in the presence of excess water.

Linear & Cyclic Oligomer Synthesis Silicone Rubber
(Source: Dow Corning)

This heterogeneous and exothermic reaction gives formally a disilanol “Me₂Si(OH)₂” [2]. It readily condenses with HCl acting as a catalyst. This reaction gives a mixture of linear [3] or cyclic [4] oligomers by inter- or intramolecular condensation.

The linear and cyclic oligomers obtained by hydrolysis of the dimethyldichlorosilane have short chains for most applications. They must be condensed (linear) or polymerized (cyclic) and crosslinked to obtain elastomers.

What are the properties of silicone rubbers?

The strong Si-O chemical structure and high bond energy give Si Elastomers their unique performance properties. Several benefits of silicone rubbers include:

Wide service temperature range – excellent thermal and thermoxidative resistance (-Si-O-Si- binding energy is higher than C-C bonds)
Excellent resistance to attack by oxygen, ozone and sunlight
Readily resistant to electromagnetic and particle radiation (UV, alpha, beta and gamma rays)
Excellent non-stick and non-adhesive properties
Low toxicity
Flexible at low temperatures due to their low glass transition temperature (Tg).
Optical transparency
Good excellent insulation properties
Low chemical reactivity
High bio-compatibility
Excellent mechanical properties (high tear strength, high elongation)

Embrittlement Point	-60 to 70°C
Insulation resistance	1 - 100 TΩ.m
Thermal conductivity	0.2 W/mΩ.K
Volume resistivity	0.01 - 10 Ω.m
Tear Strength	9.8 kN/m
Water vapor permeability	15 - 51

Hence, the properties of silicone rubber can vary greatly depending on the organic groups and molecular structure.

Silicone rubbers can withstand temperatures (-50°C to 350°C) depending on the duration of exposure. Parts made of silicone rubber when exposed to wind, rain, and UV rays for long periods. This results in virtually no change in physical properties. Unlike most organic rubbers, silicone rubber is not affected by ozone as well.

As compared to organic rubber, silicone rubber has a Si-O bond in its structure, and hence, it has better:

Heat resistance
Chemical stability
Electrical insulation
Abrasion resistance
Weatherability
Ozone resistance

Comparison of Properties of Various Rubbers Using Natural Rubber as Reference
(Source: Shin-Etsu Silicone)

How inclusion of additives affect silicone rubbers?

Silicone rubbers generally contain additives to obtain high performance for a desired application.

Crosslinking - Silicone rubbers are cured/crosslinking either by:
- Peroxide crosslinkers (benzoyl peroxide, 2,4-dichlorobenzoyl peroxide, t-butyl perbenzoate, and dicumyl peroxide) or
- Platinum catalysts resulting in a mechanically stable cured product.

Fillers - Pyrogenic silica is a reinforcing filler with very high BET surface area. Quartz is a non-reinforcing filler. Used to produce parts with high tear strength or increase conductivity behavior (Carbon black).

Stabilizers - They are mainly added to silicone rubber to enhance its heat resistance.

Flame Retardants - Used to increase the fire resistance of silicone rubbers. Such additives include:
- Carbon black
- Aluminum trihydrate
- Zinc
- Platinum, and
- Ceric compounds

Pigments and colors - Unlike other rubbers, which are black, silicone rubbers offer high transparency options. Hence making it easily colorable with pigments as per application needs.

How to compare between silicone rubber vs. thermoplastics vs. TPEs?

Comparison With	Silicone Rubber Benefits
Latex	Lot-to-lot consistent due to controlled synthetic process, vs. organic lot-to-lot variations Superior bio-compatibility Higher clarity Better electrical insulation properties Stability over a broader temperature range
PVC	Inertness and absence of leachable additives Superior bio-compatibility Stability over a broader temperature ranges Superior sterilization properties
Polyurethane & Vinyl	Plasticizer- & toxin-free Superior bio-compatibility Broader temperature stability Lower compression set Better clarity Greater softness
Thermoplastic Elastomers (TPE)	Superior bio-compatibility Superior chemical resistance Lower durometer Lower compression set

(Source: Vesta, Inc. )

How to integrate LSR with thermoplastic injection molding?

Injection molding of liquid silicone rubber (LSR) has caught increasing attention of late. Processing requires special equipment and tooling as the LSR is vulcanized under heat.

In addition to an injection molding machine, a metering/mixing system is needed for the two LSR components. A specially designed mold is required to handle the material which is cured to about 160 to 200°C.

Most injection molding machines for LSR use a reciprocating screw injection unit. It functions like thermoplastic injection molding. But the barrel and screw assembly for LSR is specifically designed with a shorter L/D ratio than thermoplastics.

The two-component LSR material is pumped into the injection unit. This is done using a meter/mix device with a static mixer mounted at the feed throat. This process aids the mixing and/or additives dispersion.

The LSR is injected into a mold. It is typically heated by four to six electric mold heating zones for each mold half.

Adequate clamp force must be maintained throughout the molding cycle since, during the 10-100 seconds of curing, LSR will expand in volume by 1-2 percent, which is sufficient to cause flashing.

Pressure must also be maintained so that material is not permitted to migrate back through the sprue bushing into the nozzle.

Many LSR injection molding machines have water-cooled or insulated plates. This ensures that the high temperature of the mold is not transferred to other areas of the injection molding machine.

LSR Two-Shot Molding

Important advances are being made in the two-shot molding of LSR. This process typically involves injection molding a thermoplastic material such as PBT or nylon followed by co-molding or over-molding with an LSR. It is used for applications such as integrating gaskets into connectors or other similar types.

Two-Shot LSR Molding Process
LSR / LIM Injection (Thermoset)	Engineered Thermoplastic Injection
Cold material Cold runner system Sequentially valve gated Hot mold cavity	Hot material Hot runner system Cold mold

	LSR/LIM	Thermoplastic Material
Material Type	Thermoset	Thermoplastic
Typical Mold Temperatures	140-220°C	25-100°C
Typical Material Processing Temperatures	20-30°C	200-400°C
Typical Injection Pressures	7-35 bar	70-140 bar
Typical Cycle	30-60 sec	10-40 sec
Cure Time	25-55 sec	8-35 sec

However, the two-shot thermoplastic/LSR process presents certain challenges as the two materials process at significantly different temperatures. This results in a process in which one mold half is heated to cure the LSR, while the other half is cooled.

Two-Shot LSR Molding Process
(Source: Momentive )

A key factor for success in this process is thermal separation in the mold. Two separate injection units are required and a rotating table to index the molds or a robot to transfer the moldings from one mold half to the other in order to overmold the thermoplastic with LSR.

Are silicone rubbers recyclable?

Silicone rubber has limited recycling opportunities. However, pure silicone rubber is not easily recyclable. Uncontaminated silicone rubber can be sometimes ground and reused. Another option is to reuse silicone rubber products instead of recycling them. This in turn reduces waste and conserves resources.

Is silicone rubber toxic?

Many experts consider silicone rubbers as safe and non-toxic for applications that come in contact with food. This is because it is an inert material that does not react with most chemicals. This in turn makes it a popular choice for food-grade, medical, and other sensitive applications.

However, some additives used in the production of silicone rubber may have the potential to be harmful if not used properly. Additionally, some forms of silicone rubber may release volatile organic compounds (VOCs) during manufacturing or use. These compounds can be harmful if inhaled in high concentrations.

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Silicone Rubber: Complete Guide on Highly Durable Elastomer