OK
The material selection platform
Plastics & Elastomers
The material selection platform
Plastics & Elastomers

Silicone Rubber: Complete Guide on Highly Durable Elastomer

Silicone rubber is a high-performance elastomer characterized by an unusual combination of properties. These properties range from high temperature performance to durability, excellent electrical insulation properties as well as its transparency.

But, what makes Silicone rubber a high performance material in several sectors? What are its key properties and applications? Get detailed technical information on Silicone rubbers here and hence, filter out your possibilities to evaluate a wide array of material properties to find the suitable material meeting demanding performance specifications.

Overview

What Makes Silicone Rubber Versatile?

What Makes Silicone Rubber Versatile?

Silicone rubber is a durable & highly-resistant elastomer (rubber-like material) composed of silicone (polymer) containing silicon together with other molecule like carbon, hydrogen and oxygen. Its structure always comprises siloxane backbone (silicon-oxygen chain) and an organic moiety bound to the silicon.

Hence, the properties of silicone rubber can vary greatly depending on the:
  • Organic groups (methyl, vinyl, phenyl, trifluoropropyl or other groups)
  • Chemical structure

As compared to organic rubber, silicone rubber has Si-O bond in its structure, and hence, it has better:
  • Heat resistance
  • Chemical stability
  • Electrical insulation
  • Abrasion resistance
  • Weatherability as well as Ozone resistance

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

Silicone Rubber’s special features, are hence, originated from its unique molecular structure that they can carry both inorganic and organic properties.

Silicone Rubber Molecular Structure
With these unique characteristics, silicone rubber is widely used in industries such as aerospace, automotive, construction, medical, E&E, food processing etc. Overall silicone rubbers are used in various applications as elastomers, adhesives & sealants, potting, and encapsulating compounds as well as in coatings, lubricants etc.

Commercial grade Silicone rubbers were first introduced by Dow Corning (now 100% Dow subsidiary) in 1943. Today, silicone rubbers are manufactured by several companies.

Different Types & Method Used to Synthesize Silicone Rubbers

Different Types & Method Used to Synthesize Silicone Rubbers

The organic groups in silicone rubbers may be methyl, vinyl, phenyl or other groups. According to ASTM D1418 standard, which covers a system of general classification or nomenclature for rubber and rubber lattices, silicone rubbers are classified as:

  • Methyl Group – Also known as dimethylsilicone elastomer/rubber or simply 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 referred as PMQ and it has excellent low temperature performance

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

  • Methyl, Phenyl and Vinyl Groups – It is referred as PVMQ as well and known for its excellent low temperature performance.

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

Apart from its molecular structure, another factor for classifying silicone rubber are viscosity and method employed for their processing. Silicone rubber is available in three main forms:

  • Solid Silicone Rubber or High Temperature Vulcanized, HTV - Solid silicone rubber contains polymers with a high molecular weight and relatively long polymer chains. They are available in uncured form and required traditional rubber processing techniques.

  • Liquid Silicone Rubber, LSR - Liquid silicone rubber contains polymers of lower molecular weight and hence shorter chains. It has better flow properties. It is processed on specially designed injection molding and extrusion equipment.

  • 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 where their hardness range of very soft to medium. They are available for potting, encapsulations, sealants etc.

Liquid Silicone Rubber maintains mechanical properties over a wide range of temperatures (from -50°C to 250°C). This heat-cured elastomer provides excellent optical clarity, durability and design freedom. This innovative transparent material serves diverse applications such as high-power LED lighting, electronics, automotive lighting and many others.

Method of Synthesis


Overall, silicone rubbers synthesis majorly involves three steps i.e. preparation of chlorosilanes followed by hydrolysis and then polymerization yielding silicone elastomers.

Today, silicones are obtained commercially from chlorosilanes prepared following the direct process of Rochow. The reaction giving chlorosilanes takes place in a fluidised bed of silicon metal powder in which flows a stream of methylchloride, usually 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
Mixture of Different Silanes
(Source: Dow Corning)

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

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

This heterogeneous and exothermic reaction gives formally a disilanol “Me2Si(OH)2” [2] which readily condenses, with HCl acting as a catalyst, to give 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 too short a chain for most applications. They must be condensed (linears) or polymerized (cyclics) and crosslinking to obtain elastomers.


Liquid Silicone Rubber


Liquid Silicone Rubber is a low viscosity and high purity thermoset elastomer maintaining mechanical properties over a wide range of temperatures (from -50°C to 250°C). This heat-cured elastomer is an excellent solution to consider if you're looking for highly optical clarity combined with long-term durability in harsh environment (high temperature, UV etc...).

Benefits of Liquid Silicone Rubber

Liquid Silicone Rubber is exclusively processed by injection molding and heat-cured during the molding process. The main advantage of LSR is the possible integration of multiple parts into one, allowing drastic cost reduction. This material highly boosts productivity by decreasing cycle time, reducing material waste and using smaller machines.

» Learn about Liquid Silicone Rubber in Detail

High-Temperature-Vulcanizing (HTV)


Heat-curable, high-temperature-vulcanizing (HTV) elastomers are higher viscosity rubbers that are mixed and processed like other elastomers. They are cured at elevated temperature, either by means of organic peroxides or with platinum catalyst.

Key Properties of Silicone Rubbers

Key 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


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

Additives and Fillers for Silicone Rubber

Additives and Fillers for Silicone Rubber

Silicone rubber generally contain various additives, crosslinkers and fillers to obtain high performance material for 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 mechanically stable cured product

  • Fillers - Pyrogenic silica with very high BET surface areas and Quartz are most commonly used reinforcing and non-reinforcing fillers respectively. Fillers are used to produce modified silicone rubber with high tear strength or increase conductivity behavior (Carbon black).

  • Stabilizers – They are mainly added to silicone rubber to enhance its the heat resistance.

  • Flame Retardants - Additives such as platinum compounds, carbon black, aluminium trihydrate, zinc or ceric compounds are used to increase fire resistance of silicone rubbers

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

Silicone Rubber Vs. Thermoplastics Vs. TPEs

Silicone Rubber Vs. Thermoplastics Vs. TPEs

Comparison With Silicone Rubber Benefits
Latex
  • Lot-to-lot consistenct 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. )

Integrating LSR with Thermoplastic Injection Molding

Integrating LSR with Thermoplastic Injection Molding

Injection molding of liquid silicone rubber (LSR) has caught increasing attention of late. Processing this material 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 and 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 which functions similar to that for thermoplastic injection molding but the barrel and screw assembly is designed specifically for LSR with a shorter L/D ratio than for thermoplastic.

  • The two-component LSR material is pumped to the injection unit using a meter/mix device with a static mixer mounted at the feed throat to aid in mixing and/or additives dispersion.

  • The LSR is injected into a mold, which 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 platens to ensure 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 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, and is used for applications such as integrating gaskets into connectors or other similar type applications.

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
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.


Find Suitable Silicone Rubber Grade


View a wide range of silicone rubber grades available today, analyze technical data of each product, get technical assistance or request samples.

Key Applications

Suppliers

Brands

Leave a comment


Want to comment?

No Account yet?

Register
Rate this Content
1 Comments on "Silicone Rubber: Complete Guide on Highly Durable Elastomer"
Jan van K Jul 25, 2021
Good information about all types of silicone rubber

Back to Top