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Technical Express

What is silicones?

Silicones, known chemically as polyorganosiloxanes, have a structure similar to that of organically modified quartz, with a backbone consisting of alternating silicon and oxygen atoms.Their properties can be modified by attaching different organic groups to the backbone. Nearly all silicone products are derived from the following three types of raw materials:

Silicone fluids are linear polymers whose chains contain between 2 and well over 1,000 silicon atoms, each of which is linked to the next by an oxygen atom. Unlike mineral oils, silicone fluids change very little in viscosity over a wide temperature range. Another characteristic is their high wetting power, which contributes to such special properties as water repellency and antifoaming action. Another important property of silicone fluids is that they are inherently inert towards other substrates.

The constituents of silicone rubbers are long-chain polysiloxanes, catalysts, crosslinking agents and various fillers, such as fumed silica, quartz, chalk and kaolin, as well as other additives such as pigments, adhesion promoters, and the like.

They are converted to the elastomeric (rubbery) state by vulcanization, ie, crosslinking of the chains.

According to the type of vulcanization (crosslinking agent and temperature) and the viscosity of the base polymers silicone rubbers are classified as follows:

RTV-1 silicone rubbers are one-component, ready-to-use, room-temperature vulcanizing systems. They comprise polydimethylsiloxanes, crosslinking agents, fillers, and auxiliaries.After application, crosslinking is initiated by contact with atmospheric moisture and proceeds with the elimination of by-products.It therefore starts at the surface with the formation of a skin and gradually extends inwards.

RTV-2 silicone rubbers are two-component, pourable, spreadable or kneadable compounds that cure to highly flexible silicone vulcanizates on addition of the crosslinking agent.
There are two types of vulcanization:

Addition curing
Condensation curing
According to the type of vulcanization, RTV-2 silicone rubbers exhibit the following characteristic differences:

Condensation curing
Addition curing
Catalyst Organotin compound
Organoplatinum compound
Shrinkage Approx.
0.5 - 1.5%
< 0.1
Curing acceleration
Rapid curing agent
Additives, temperature
Sensitivity to inhibition
Reversion resistance

A whole range of potting compounds from flexible silicone gel to silicone rubber with Shore A 80 is available.

Properties such as:
Rheology (flow behaviour)
Reactivity (pot life, curing time)
Electrical and thermal properties
can be widely varied and optimized to suit any application.

Liquid silicone rubbers (LSR)

Liquid silicone rubbers are of the HTV type. They are two-component systems that can be pumped, cure much more rapidly than HTV silicone rubbers and do not liberate by-products on curing.

As the name suggests, HTV silicone rubbers are vulcanized at high temperatures, in the presence of organic-peroxide curing agents.

Silicone resins

The class of silicone resins extends from relatively low-molecular intermediates to high-molecular, densely crosslinked resins of a wide variety of structures.

Reactive silicone resins offer great scope for copolymerization with organic resins, eg, polyesters.

Crosslinking is generally carried out at elevated temperatures over a long period of time, during which a temporary thermoplastic phase occurs.

Silicone resins are the basis of silicone masonry protection agents. The high heat resistance and outstanding range of properties of silicone resins are utilized in the electrical industry in binders for glass fabrics and sealants for light bulbs or in impregnants for electric motors.


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