Silicone rubber (MQ, VMQ, PMQ, PVMQ, FMQ)
Common chemical composition:
– [SiO]X –
Silicone rubber consists of polymers whose main chain is composed of silicon and oxygen atoms (siloxane). It is possible for the lateral chains to be:
- Methyl (MQ);
- Methyl and vinyl (VMQ);
- Methyl and phenyl (PMQ);
- Methyl, vinyl, phenyl (PVMQ);
- Methyl and trifluoromethyl (FMQ).
Most types of silicone rubber contain MQ. It generally has an average molecular weight. It exhibits high heat resistance (>200°C) due to the silicon-oxygen bond and proximity to the methyl groups that stabilize this bond even more. It also performs very well at low temperatures. It is cross-linked through the action of peroxides on the lateral methyl groups. The vulcanizates of this and other types of silicone rubber have very weak mechanical properties but remain very stable under a wide range of temperatures.
VMQ has a similar constitution to that of MQ. The only difference is that some methyl groups are replaced by vinyl groups. It has properties similar to those of MQ. The vinyl units present increase the peroxide cross-linking and result in better resistance to permanent deformation and offers the possibility of producing oil-resistant items.
After MQ, the most common presence is that of PMQ. Unfortunately, phenyl groups tend to lower heat resistance and increase resistance to hydrolysis. An interesting property is good resistance to the cross-linking effect exerted by radiation.
PVMQ consists of elastomers containing fluorine. The introduction of fluorine increases polarity thus increasing incompatibility with hydrocarbon elements, producing excellent resistance to fuels and oils. It can be used with a wide range of temperatures (-50/180°C).
Fields of Application:
Silicone rubber is a special rubber that is expensive and difficult to handle. It tends to make cylinders extremely sticky and causes problems due to low rigidity of the uncured form during extrusion. It is also used in items bonded to fabrics (glass) and metal.
Silicone rubber is used in those items that require:
- High heat resistance;
- Exceptional resistance to low temperatures;
- Ozone, weathering and radiation resistance;
- Non-stick property
For example, it is used in the aircraft industry for tubes, profiles and gaskets; in the automobile industry for O-rings, radiator hoses and spark plug caps; in the healthcare industry for transfusion tubes, fittings for medical equipment; in nuclear power plants for items required to withstand gamma rays and intense heat; in the food industry for conveyor belts and ovens.