  Chemical
Reactions on the "Finished" Silicone -
Crosslinking and Modification of Silicones
Whereas hydrolyzing the appropriate silanes yields "finished" silicone
fluids and silicone resins, the components of a silicone
rubber still
have to be crosslinked with each other (vulcanized or cured). There
are three different types of crosslinking reactions:
Aside from the necessary reagents
and reaction conditions, addition curing and condensation curing
also require a suitable catalyst. A platinum catalyst is needed
for addition, and a tin catalyst for condensation curing systems.
In contrast, peroxide-initiated curing does not require a catalyst.
Addition
Curing (RTV-2 rubber)
As already mentioned, addition curing functions
by attaching Si-H groups to double bonds. Salts or complexes of platinum,
palladium or rhodium may serve as catalyst. If platinum-olefin complexes
are used, curing will take place at room temperature. Platinum complexes
containing nitrogen are used for effecting addition curing at elevated
temperatures (e.g. Pt-complexes with pyridine, benzonitrile or benzotriazole).
Peroxide
Curing (HTV rubber)
To carry out peroxide curing, it is first necessary
to generate free radicals. This can be done either with heat or with
radiation. Different organic peroxides may serve as free-radical
generators for initiating this type of curing. More selective crosslinking
and thus superior vulcanizates can be obtained by incorporating vinyl
groups into the polymer (0.5 – 1.0 mol %).
Condensation
Curing (RTV-2 rubber)
Typical catalysts for condensation curing are
dibutyltin dilaurate and dibutyltin dioctoate. They catalyze the
reaction between α, ω-dihydroxypolydimethylsiloxanes
and silicic acid esters. Water has a strong accelerating effect on
the rate of reaction. The rate of reaction also depends on the crosslinking
agent (its functionality, concentration and chemical structure) and
on the type of catalyst.
Unlike organic latexes and rubbers, no sulfur
is used for curing silicone rubbers.
Differences
Between Condensation and Addition Curing RTV-2 Rubbers
 |
|
| Condensation
curing |
Addition curing |
Blending ratio of silicone rubber and catalyst variable within
limits |
Blending ratio of the two components is fixed |
Crosslinker agent and catalyst are both contained in the catalyst |
Crosslinking agent (H-siloxane) in rubber component 1, catalyst
(platinum complex) in rubber component 2 |
Curing impaired only by lack of water |
Curing impaired by various substances (sulfur compounds etc) |
Curing rate largely independent of temperature |
Curing rate heavily dependent on temperature |
Chemical shrinkage due to release of alcohol |
Practically no shrinkage |
Release products (alcohol) may cause reversion from
80°C and above |
No reversion possible |
Long pot life and hence long curing times |
Where pot life is long, curing can be accelerated by exposure
to elevated temperatures |
|
