Why silicone is so popular

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29.09.2018

Rubbers are not all the same. There are many different types of rubber with different characteristics suitable for a variety of uses (see Overview of different elastomer materials and Elastomer mechanical properties and how they are measured). Starting from a basic rubber, manufacturers and producers are able to design unlimited rubber compounds optimized to suit a specific application.  

While the majority of elastomers like EPDM, FKM or NBR are unknown to the end user, silicone rubber is a material just about everyone knows.

Although silicone is inferior to other elastomers when it comes to mechanical properties, there are applications in virtually every industry. Silicone products are found in the food, medical and pharmaceutical sectors. Baby feeding bottle teats and pacifiers are made of silicone, as are diving goggles. In the technical industry, silicone crops up frequently as a material for seals or membranes in dynamic applications. In the automotive sector, it is used for hoses, sheathing and cable insulation.   

The reason for that is explained in this blog.

Benefits and properties of silicone rubber

Silicone has a distinct advantage over other types of rubber because its mechanical properties remain relatively stable over a wide temperature range whereas the same properties in many other materials deteriorate significantly in cold or heat. For example, an EPDM may appear to be superior to a silicone compound according to the mechanical properties listed in the technical information, but these are given for room temperature and for high or low temperatures the reverse situation occurs. Silicone is heat resistant in air from -80 °C to 250 °C. This is the property that makes it popular for seals as silicone’s very low compression set, typical for silicone, takes effect.

Due to its excellent ozone, UV and weather resistance, silicone compounds are also often used in outdoor applications.

In addition, silicone is exceptionally fire resistant and a good electrical insulator and conductor.  

Silicone is chemically resistant against, for example, plant and animal fats, hot water and alcohol. Problems do occur with acids, alkalis, fuels, ketones, and steam. Chemical resistance is something that needs consideration depending on the particular application.

Resistance to fuels and mineral oil can be significantly improved by the addition of fluorine; the silicone is then referred to as fluorosilicone (in fact, the methyl groups are replaced by fluoroalkyl groups). Fluorosilicone consequently is a favorite for static seals in the automotive sector, where a wide temperature range has to be catered for.

The gas permeability of silicone is also very high.

Special properties required by the medical sector

In addition to its industrial applications, for decades silicone has been the preferred elastomer in the medical sector. Components made of silicone are used in short-term implants (less than 30 days in medical devices of class IIa) and long-term implants (30 days or more in medical devices of class IIb). They perform critical functions in devices such as cardiac catheters, cardiac pacemakers, ventilators, neurostimulators and defibrillators. 

The silicone rubber used for long-term implants is offered by only a few producers worldwide (e.g. NuSil Technology). The blends are manufactured under the strict requirements of the US Food and Drug Administration (FDA) and special care has to be taken to ensure the high hygiene requirements are fulfilled and that the components are manufactured in a clean room.

An important advantage of silicone is that it is bio-compatible and as such it is generally safe for humans. The biocompatibility of a silicone compound is often evidenced by its USP class VI rating (USP stands for United States Pharmacopeia) or by tests in accordance with the stricter (DIN EN) ISO 10993 directive. The (DIN EN) ISO 10993 is used mainly for testing medical devices that are implanted long-term or permanently in the human body. The lower classification according to USP of class VI is adequate for applications implanted for a shorter time.  

In addition, because it can be used in a wide temperature range from about -80 °C to about 250 °C, silicone can be steam sterilized (heating in an autoclave). Products made from silicone can be freed of any living micro-organisms, their spores, and viruses, etc. 

The good electrical insulation qualities of silicone are also particularly important in the medical field.

Thanks to its high stability inside the human body, silicone provides very good protection for critical components and due to the above characteristics is preferred for functional parts.

Different types of silicone rubber

Basically, the difference between silicone rubber and other organic elastomers is that its main chains, which have an inorganic structure, do not consist of carbon compounds, but are formed from a combination of silicon and oxygen atoms, with pyrogenic silica primarily utilized as the filler, to create the good properties.

Silicone rubbers are classified into three groups according to the state of its aggregates and their vulcanization temperature:

Silicone rubbers with a solid raw material are called HTV (High-Temperature Vulcanizing) or HCR (High Consistency Rubber). They are usually vulcanized at high temperatures between 140 °C and 200 °C. Crosslinking is performed by peroxides or addition reactions and platinum compounds are used as the catalyst.  

Liquid silicone or LSR (Liquid Silicone Rubber) is a (viscous) liquid and consists of two components mixed together just before processing. Crosslinking is performed by addition reactions at temperatures similar to the HTV varieties and the crosslinking generally is much faster.

Both types of silicone can be colored.

Finished elastomeric parts made from HTV silicone and LSR silicone differ very little in their properties.

The third group is the so-called RTV silicones (Room Temperature Vulcanizing). In these, the crosslinking can take place at room temperature. They are popular as sealing compounds or for the production of prototypes. They are available both as single and two-component systems.

Outline of the properties of silicone (VMQ) and liquid silicone (LSR)

Finally a further summary of the properties of silicone (VMQ) and liquid silicone (LSR) is intended.

Please be aware that this is only a general guide and the presentation should not be used to make a definitive choice. Altering the formulas of silicones can affect their properties either positively or negatively and as such they may differ from the description. 

The rating ranges from ☆☆☆☆☆ (very poor) to ★★★★★ (very good).

 Mechanical characteristics:  
 Hardness range: 10 Shore A to 90 Shore A
 Tensile strength: ★★☆☆☆
 Elongation at break: ★★☆☆☆
 Tear strength: ★★☆☆☆
 Compression set at high temperatures:               ★★★★★
 Compression set at low temperatures: ★★★★★
 Rebound resilience: ★★★☆☆
 Abrasion resistance: ★☆☆☆☆
 Thermal properties:  
 Low-temperature flexibility: ★★★★★
 Thermal resistance: ★★★★★
 (Chemical) resistance:  
 Gasoline: ★☆☆☆☆
 Mineral oil (100° C): ★★★★☆
 Acids: ★★☆☆☆
 Alkalis: ☆☆☆☆☆
 Water (100° C): ★☆☆☆☆
 Harsh weathering and ozone: ★★★★★
 UV/light: ★★★★★

If you require more information on properties and chemical resistance, or if you have any questions about a particular application, please feel free to contact us at any time.  

If you have any questions or comments on this blog, or in future would like us to focus on a specific aspect of elastomers, feel free to contact us by email at info@hepako.de

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