TopForward 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.

TopForward 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).


TopForward 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 %).

TopForward 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.

TopForward 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