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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 |
No |
Yes |
| Reversion resistance |
No |
Yes |
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)
Colour
Electrical and thermal properties
Adhesion
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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