Silicone rubber – its structure determines its properties
Silicone rubber differs from other elastomers in that it is not made up of purely organic compounds. Unlike the carbon polymer chain of conventional elastomers, the silicone polymer chain consists of alternately arranged silicon and oxygen atoms (siloxanes). Silicone elastomers contain reinforcing substances and fillers, the nature and amount of which have a significant influence on the mechanical and chemical properties of the vulcanisates produced by curing.
Production of silicone rubber
Silicone rubber is produced by polycondensation, the starting point for which is dimethyldichlorosilane. 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 fluoropropyl 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 vulcanised), is characterised 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.
Material advantages of silicone elastomers
Silicone rubber differs from other elastomers through its unique range of properties. Of particular interest are the characteristic features that arise directly from the polymer and filler structure rather than from additives or a special surface treatment.
Silicone elastomers offer:
- Good thermal stability up to 260°C
- Good low-temperature resistance down to -100°C
- Flexibility and elasticity virtually independent of temperature
- Outstanding weathering and UV resistance
- Long service life even under adverse mechanical and chemical conditions
- Electrical insulating capability
- Odourless and tasteless
- Simple and flexible to manufacture and handle
In comparison to other elastomers, silicone rubber is exceptionally pure and is therefore especially suitable for use in the food and medical sectors.
Storing silicone rubber – points to bear in mind
Silicone rubber can be stored for up to twelve months under the correct conditions. However, the Mooney viscosity can increase over time because the polymer and filler seek to establish a strong interaction, although this is reversible. Silicone rubber that has been stored for too long can usually be made processable again by re-plasticising on a roll mill. In difficult cases it may also be necessary to add a masticating aid.
General instructions for storing silicone rubber:
- Store in closed containers at temperatures from 0 to 30°C
- Do not expose to direct sunlight
- Store solid rubber separately from organic rubber and crosslinking chemicals
- When opening containers of liquid rubber, avoid getting dirt onto the surface of the material
Curing of silicone rubber
Silicones are divided into two main groups, depending on the crosslinking mechanism:
- addition-curing silicones and
- condensation-curing silicones.
Addition-curing silicone rubber
Addition-curing silicones consist of a one- or two-component system. Two-component silicones are mixed shortly before processing and, depending on the formulation, can cure (vulcanise) even at room temperature, without generating reaction heat. They cure to form silicone elastomers, which are non-toxic and offer high elasticity, relatively high thermal stability, excellent dimensional accuracy when fully cured and in-mould release properties.<br/> One-component systems are ready for processing without additional mixing, but they have a limited storage life.
Peroxide curing of silicone rubber
Peroxide curing involves the use of organic peroxides. At elevated temperature these break down into highly reactive radicals, which bring about the chemical crosslinking of the polymer chains. This process results in a highly elastic, three-dimensional network.
Processing options for silicone rubber
Which method is used for processing silicone elastomers is determined by such factors as: the desired mechanical, chemical and physical properties of the vulcanisates, whether the silicone rubber is in liquid or solid form, whether the geometry of the parts is simple or complex, and in what volumes the parts are to be produced. While injection moulding is recommended for complex part geometries and large volumes, for example, extrusion is more suitable for continuous products.
Press-forming methods, in which solid silicone rubber is vulcanised in moulds under pressure and at high temperatures, can be used for a wide variety of applications. Compression moulding and transfer moulding are the main methods. The vulcanising time depends on the temperature of the starting materials, the mould temperature and the thickness of the mouldings.
In extrusion the rubber is pressed through a shaping die and then vulcanised. The necessary pressure is achieved by means of a screw, in which the starting material is homogenised, deaerated and compacted. Hoses, round cords and flat belts are just some of the products made by extrusion.
Where large volumes are involved and consistently high product quality is required, injection moulding is the method of choice. Injection moulding delivers vulcanisates with good dimensional accuracy and with virtually no post-processing required. For that reason, production cycles can be much shorter than in the previously mentioned methods. The higher mould and machine costs are generally offset by the high productivity.
Possible applications for silicone elastomers
The range of uses for silicones is immense. As these elastomers are biologically neutral, they have become established in almost every area of daily life.
Silicones in the automotive industry
In the automotive sector, silicone rubber is predominantly found in the form of seals and gaskets, including:
- Engine seals
- Cylinder head gaskets
- Radiator seals
- Radiator seals
- Engine Mount
- Cable insulation
- Spark plug boots
- Exhaust pipe mounts
- Vibration absorbers
- Turbocharger hoses
However, it is also used to make hoses (such as turbocharger and radiator hoses), vacuum diaphragms, torsional vibration dampers and sheaths for ignition cables.
Silicones in the construction industry
In the building industry, silicones are used as sealants for filling gaps and also as coating compounds for fabrics or for producing mould-making and potting compounds.
Silicones in the medical and pharmaceutical industry
Dentistry uses liquid silicone rubbers to produce exact impressions and precision models of teeth and jaws. Silicones are also used for fillings and implants.
In orthopaedics, silicone elastomers are used to make liners or inner sockets for prostheses and to manufacture exoprostheses.
Silicones also serve as the basis for:
- Ventilation bags
- Cardiac pacemakers
- Cardiac valves
Other uses of silicone elastomers
Silicone rubber is the commonest material for coating rollers. The fact that it is easy to handle is an important advantage here. Minor defects can usually be repaired without having to remove the roller, while in the event of more significant damage the entire cover can be replaced quickly and inexpensively. Silicone rubber foams easily, so it is also used as a starting material for foams such as sponge rubber foams.