Silicone Rubber

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. The chemical formula is mSiO₂·nH₂O.

Melting Point 

Unlike most other rubbers, silicone rubber can withstand temperature extremes, from above 200°C and as low as minus 60°C. When it comes to melting points, silicone doesn’t melt due to temperature alone, regardless of how hot it gets.

If, for example, silicone rubber is heated to 150°C, even for an extended period of time, it should not change. When it is heated to 200°C, it will become harder and less stretchy over time, while if it’s heated above 300°C, it will quickly become harder and less stretchy. However, even when exposed to these high temperatures, it won’t melt.

It’s important to note, however, that the auto-ignition temperature of silicone is around 450°C, so heating it above this temperature is not recommended.

Properties of Silicone Rubbers

1. Heat Resistance

Heat resistance of silicone rubber is the one of its most excellent properties and provides the basis for its creation. Silicone rubber is far better than organic rubbers in terms of heat resistance. At 150℃, almost no alterations of properties take place that it may be used semi permanently. Furthermore, silicone rubber withstands use for over 10,000 consecutive hours even at 200℃ and, if used for a shorter term, it may also be used at 300℃ as well. Boasting this excellent heat resistance, silicone rubbers are widely used to manufacture rubber components and parts used in high-temperature places.

2. Cold Resistance

Cold resistance of silicone rubber is the finest among organic rubbers. It provides a critical reason behind the creation of silicone rubbers. Natural and ordinary rubbers demonstrate significant changes in formation depending on temperatures. They become soft at high temperatures and hard at low temperatures so that they may not be able to used any more. While other organic rubbers may only be used up to -20℃ or -30℃, silicone rubber maintains its elasticity between -55℃ and -70℃. Some of the products even withstand temperatures as extremely low as under -100℃.

3. Weatherability

Silicone rubber has superb ozone resistance. Due to corona-discharged ozone, other organic rubbers become soften at a higher speed, but silicone rubber is rarely affected. Furthermore, even long-term exposures to UV rays, winds, or rain silicone rubber's physical properties will not be changed substantially.

 

4. Electric Properties

Silicone rubber is being used for insulation materials at high temperature with its superior insulation properties. It is particularly known for wide range in temperature and volume resistance between 1014Ω cm and 1016Ω cm. Silicone rubber experiences lowest change in performance in wet condition and is the best fit for being used as insulation materials. By adding special conductive fillers, conductive silicone may also be manufactured. In particular, silicone rubber is strongly resistant against corona discharge compares to others, while being widely used for insulation purposes in high voltage environments. 

5. Electric Conductivity

Conductive silicone rubber is a compound comprising conductive materials such as carbon black, silver and copper. Depending on the type of silicone rubber, they range in resistance level from a few Ω cm to 103Ω cm. One of the properties is that its electric properties are not much affected by variance in temperatures. No rubber materials are not found yet to match the electric properties of silicone rubber over 200℃. Conductive silicone rubber is also being used for keyboard interfaces, antistatic parts, and shield materials for high voltage cables. 

6. Radiation Resistance

Compares to other organic rubbers, ordinary (dimethyl) silicone rubber has no special performance in terms of anti radiation. However, methyl phenyl silicone rubber adopting phenyl group in polymer molecules does have radiation resistance to be used for cables at nuclear power plants and connectors. 

 

7. Steam Resistance

Silicone rubber absorbs only 1% of moisture even after experiencing long exposure to water without being affected in mechanical strength or electric properties. Generally, silicone rubber does not deteriorate even after having contact with steam under atmospheric pressure. In high pressure steam over 150°…, siloxane polymer is cut off and rubber properties decline. Such a property may be improved by the composition of silicone rubber, selection of curing agent, and the post curing. Other modified products are also available with improved steam and boiling water resistance. 

8. Flame Retardancy

Silicone rubber does not easily burn when in contact with a flame, but would burn out consistently once ignited. However, by adding a small amount of flame retardant, it may become flame retardant and self-extinguisher. Flame retardant silicone rubbers presently in use would scarcely produce toxic gas during combustion since they do not contain organic halogen compounds discovered in organic polymers. 

9. Oil Resistance

Silicone rubber is inferior to ordinary organic rubber in oil resistance at room temperature. However, for automobiles or aircrafts that require high temperature resistance, it demonstrates higher performance. Even when in contact with automobile oil, silicone rubber does not inflate significantly by reason of swelling. It swells in non polar organic compounds such as benzene, toluene, and gasoline. But its materials do not disintegrate or dissolve unlike ordinary organic rubbers. If solvent is removed, it would be restored to the original conditions. 

10. Non Toxic

Silicone rubber is physiologically inert, and is thus used for baby nipple and stoppers in medical application. Silicone rubber is also very ideal elastomer for making swimming caps and goggles.

 

 

11. Thermal Conductivity

Silicone rubber has an excellent thermal conductivity property as it is filled with special heat conductive materials to give an excellent heat transfer. Its main function is to transfer the heat from the heat source to the heat sink and normally applied between them. It provides cushioning effect on components and very adhesive as it’s very soft. It also has a property of self-adhesion so no need to treat with any other adhesive material. 

12. Electromagnetic Absorption

Recently the technologies of electronic equipments are advancing at a very fast-growing. Due to this reason EMC has become one of the hot issue in the electronic industrial. Electromagnetic absorption material is manufactured by filling the high performance metal powder with silicone rubber. It absorbs electromagnetic wave and changes the electromagnetic wave into the heat then vanish it.

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Types of Silicone Rubber

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…)

Among the families of silicone rubber variations, there are three main forms of silicone rubber, depending on the processing method: liquid, room temperature vulcanized, and high temperature vulcanized.

Liquid silicone rubber (LSR) contains polymers of lower molecular weight and hence shorter chains. It can be extruded or injection molded with specialized machinery. This type of silicone rubber is more weather resistant and transparent.

RTV (room temperature vulcanized) 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.  It’s generally used for potting, sealant, and encapsulations. Silicone rubber molds are also made from RTV silicone rubber.

Solid Silicone Rubber or High Temperature Vulcanized (HTV) has long-chained polymers with the heaviest molecular weights. They are available in uncured form and required traditional rubber processing techniques.

Curing Method of Silicone Material 

In its uncured state, silicone rubber is a highly adhesive gel or liquid. In order to convert to a solid, it must be cured, vulcanized, or catalyzed. This is normally carried out in a two-stage process at the point of manufacture into the desired shape, and then in a prolonged post-cure process. It can also be injection molded.

Silicone rubber may be cured by a platinum-catalyzed cure system, a condensation cure system, a peroxide cure system, or an oxime cure system. For the platinum-catalyzed cure system, the curing process can be accelerated by adding heat or pressure.

Platinum-based cure system

In a platinum-based silicone cure system, also called an addition system (because the key reaction-building polymer is an addition reaction), a hydride- and a vinyl-functional siloxane polymer react in the presence of a platinum complex catalyst, creating an ethyl bridge between the two.[1] The reaction has no byproducts. Such silicone rubbers cure quickly, though the rate of or even ability to cure is easily inhibited in the presence of elemental tin, sulfur, and many amine compounds.

Condensation cure system

Condensation curing systems can be one-part or two-part systems.[3] In one-part or RTV (room-temperature vulcanizing) system, a cross-linker exposed to ambient humidity (i.e., water) experiences a hydrolysis step and is left with a hydroxyl or silanol group. The silanol condenses further with another hydrolyzable group on the polymer or cross-linker and continues until the system is fully cured. Such a system will cure on its own at room temperature and (unlike the platinum-based addition cure system) is not easily inhibited by contact with other chemicals, though the process may be affected by contact with some plastics or metals and may not take place at all if placed in contact with already-cured silicone compounds. The crosslinkers used in condensation cure systems are typically alkoxy, acetoxy, ester, enoxy or oxime silanes such as methyl trimethoxy silane for alkoxy-curing systems and methyl triacetoxysilane for acetoxy-curing systems. In many cases an additional condensation catalyst is added to fully cure the RTV system and achieve a tack-free surface. Organotitanate catalysts such as tetraalkoxy titanates or chelated titanates are used in alkoxy-cured systems. Tin catalysts such as dibutyl tin dilaurate (DBTDL) can be used in oxime and acetoxy-cured systems. Acetoxy tin condensation is one of the oldest cure chemistries used for curing silicone rubber, and is the one used in household bathroom caulk. Depending on the type of detached molecule, it is possible to classify silicone systems as acidic, neutral or alkaline.

Two-part condensation systems package the cross-linker and condensation catalyst together in one part while the polymer and any fillers or pigments are in the second part. Mixing of the two parts causes the curing to take place.

Once fully cured, condensation systems are effective as sealants and caulks in plumbing and building construction and as molds for casting polyurethane, epoxy and polyester resins, waxes, gypsum, and low-melting-temperature metals such as lead. They are typically very flexible and have a high tear strength. They do not require the use of a release agent since silicones have non-stick properties.

Peroxide cure system

Peroxide curing is widely used for curing silicone rubber. The curing process leaves behind byproducts, which can be an issue in food contact and medical applications. However, these products are usually treated in a postcure oven which greatly reduces the peroxide breakdown product content. One of the two main organic peroxides used, dicumyl peroxide (compare cumene hydroperoxide), has principal breakdown products of acetophenone and phenyl-2-propanol. The other is dichlorobenzoyl peroxide, whose principal breakdown products are dichlorobenzoic acid and dichlorobenzene.

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Advantage

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

1. Silicone rubber has higher heat resistance and chemical stability that help it in providing better electrical insulation.
2. It has high elasticity and compressibility as well as excellent resistance to cold temperatures.
3. 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.
4. Silicone rubber has outstanding oil resistance at high temperatures. From among other common types of synthetic rubbers, nitrile rubber and chloroprene rubber have somewhat higher oil resistance at temperatures lower than 100°C but at higher temperatures, silicone rubber is superior to all other rubber types.
5. Silicone rubber has exceptional weather resistance. Ozone created by corona discharge does not deteriorate silicone rubber. Thus, silicone rubber can be exposed to wind, rain and UV rays for longer periods without any change in its physical properties.
6. Thermal conductivity of silicone rubber is higher than most of the other rubbers. Silicone rubber with high thermal conductivity are used to make products like thermal interface sheets and heating rollers.
7. Silicone rubber is flame retardant and has the property of low smoke emission.
8. Excellent mechanical properties (high tear strength, high elongation)
9. Wide hardness range, Compression set resistance.

Rubber vs Silicone Rubber

Silicone and rubber are both elastomers, however, although they share some characteristics, they do have several differing properties.

The atomic structures of rubber and silicone rubber differ. These differences can be seen in how each material reacts to various environments.

If you heat natural rubber, it will withstand temperatures of up to around 80°C before it starts to melt and degrade. Silicone, on the other hand, can withstand temperatures of up to 230°C. As a result, silicone also offers a better level of flame resistance than rubber, making it the best choice for high temperature applications.

Silicone also has a better chemical resistance, weather resistance, and UV resistance than rubber. When used outdoors, you will notice that rubber begins to break down extremely quickly, unlike silicone rubber which can withstand the elements far more effectively.

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Applications and Uses

Silicone Rubber’s special features originated from its unique molecular structure that they can carry both inorganic and organic properties. With these unique characteristics, silicone rubber is widely used in industries such as aerospace, automotive, construction, medical, E&E, food processing etc.

Electronics/Electrical: Silicone rubber is used for making electrical insulating tapes, adhesives, sealants, circuit boards coating, varnishes, resins, lubricants, optical fiber coatings and many other semiconductor-grade silicon and silicon-source chemicals. Keyboards, keypads, hard coating housings for computers, telephones, facsimile machines, and home entertainment equipment are all made by silicone rubber.

Aerospace: Silicone rubber has excellent ability to resist extreme temperatures, which makes it critical in the aviation industry. Silicone adhesives are regularly used to seal the most vital parts of planes like fuel tanks, doors, windows, vent ducts, etc.

Automotive: Silicone rubber which is heat, oil and fuel-resistant is used for molding into a variety of durable parts used in automobile. There are sealants and adhesives, materials for noise, specialty lubricants, materials for vibration, harshness, automotive polishes made by silicone rubber.

Construction: In construction and building, adhesives, sealants and coatings made of silicone rubber make construction materials work better and longer. Silicone adhesives join diverse materials like glass, granite, concrete, steel, plastics etc. Silicone rubber is used for making construction adhesive /sealants, architectural coatings, silicone roof coatings, water repellents, concrete pavement joint sealants etc.

Food contact: Silicone rubber is highly inert and does not react with most chemicals and isn’t available to participate in biological processes allowing it to be used in many medical applications including medical implants. It is biocompatible, hypoallergenic, which makes it suitable for baby care products, and food contact in general. 

Medical products: Since silicone rubber is proven to be resistant to bacteria, it's commonly used in healthcare products. It's also essential for infant care products, manufacturing of prosthetics, and other medical equipment with high safety standards. Silicone rubber is popular for making medical-grade tubing, defoamers, adhesives and fluids.

Leather and Textiles: Another application of silicone rubber is in making waterproofing treatments and fiber chemicals.

Paints and Coatings: Silicone rubber is used in making paints, enamels, finishes and abrasion resistant coatings for plastics.

Personal and Household Care: Silicone rubber emulsions, fluids, surfactants, and powder treatments are important ingredients in skin lotions, hair care products, anti-perspirants, cosmetics, shaving creams, fabric treatments, starches, laundry products and more.

Office printing market: Silicone rubber for rollers has the advantages of low hardness, good flexibility, low compression set, long service life and good wear resistance,  anti-static,  and moisture resistance.

Gaskets and Seals: Silicone gaskets and seals are commonly used to prevent leaks between mechanical parts. In general, rubber is an excellent material because of its resistance to several liquids, its flexibility, and compressibility. Silicone is an excellent sealing material because of its resistance to chemicals and ability to withstand a range of temperatures.  

Explore some of the application in detail in the table below:

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Comparison of Similar Materials

The following table lists the material performance comparison of Silicone Rubber, Thermoplastics and TPEs: