Silicone
Chemical Name: Vinyl-methyl polysiloxane
Common Names: Silicone, VMQ, PVMQ
ASTM D1418: VMQ
ASTM D2000 / SAE J200: FC, FE, GE
ISO/DIN 1629: VMQ
Silicone should be used as the material for a rubber molded part when temperature resistance is the most important factor in component performance. The added advantage of flexibility and weather resistance allow for additional deployment considerations. Typical use cases include medical devices and food applications. As a semi-organic elastomer, VMQ can be easily torn and is not an option for high wear applications.
Temperature range
-60°C ~ 200°C / -76°F ~ 392°F
Transient up to 230 °C / 445°F
Advantages
— High thermal resistance
— Good cold flexibility
— Good dielectric properties
— Very good resistance to oxygen and ozone
— The gas permeability at room temperature is higher than for other elastomers. This is particularly important for thin-walled diaphragms.
Average resistance in
Mineral oils (comparative to materials based on CR) and brake fluids on a glycol ether base. It can be used in water up to about +100 °C. Sufficient resistance in aqueous salt solutions, single and multiple value alcohols.
Not resistance in
Low molecular esters and ethers, aliphatic as well as aromatic hydro-carbons. Concentrated acids and alkalis, water and steam temperatures above approx. 100 °C have a destructive effect on the material.
Reference
The various silicone rubber types obtain their specific properties through the incorporation of vinyl, methyl, phenyl or fluoroalkyl groups. The silicone polymers that are used in practice are also differentiated on this basis:
— MQ or polydimethylsiloxane (PDMS): Two methyl groups are bound to the siloxane backbone.
— VMQ: In a polydimethylsiloxane, some of the methyl groups are replaced by vinyl groups.
— PVMQ: Some of the methyl groups of a VMQ are replaced by phenyl groups.
— FVMQ: Some of the methyl groups of a VMQ are replaced by fluor propyl substituents.
An important feature for classifying silicones is their viscosity, which is used to differentiate between liquid and solid silicone rubber. Solid silicone rubber, also known as HTV (= high-temperature vulcanized), is characterized by polymers with relatively high molecular weight and long polymer chains. In liquid silicone rubber the polymer chains are shorter, and this results in better flow properties. The molecular weight of the individual polymers is lower than in solid rubber.