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Chemical reactions on the "finished" silicone

Whereas hydrolysing the appropriate silanes yields "finished" silicone fluids and silicone resins, the components of a silicone rubber still have to be cross-linked with each other (vulcanised or cured).

There are three different types of crosslinking reactions:

  • Peroxide (-initiated) curing (where polymer contains vinyl groups)
  • Platinum catalysed addition curing (where polymer contains vinyl groups and crosslinking agent contains Si-H groups)
  • Tin catalysed condensation curing (between a, w - dihydroxypolydimethylsiloxanes and silicic acid esters)

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.

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.

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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).

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Condensation curing (RTV-2 rubber)

Typical catalysts for condensation curing are dibutyltin dilaurate and dibutyltin octoate. They catalyse the reaction between a, w - 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.

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Condensation curing (RTV-1 rubber)

RTV-1 silicone rubbers are one component products that are free-flowing or paste like in consistency. They react with atmospheric humidity to form flexible rubbers (RTV-1=Room Temperature Vulcanizing, 1-component). By virtue of their outstanding properties, these silicone rubbers are ideal for many sealing, bonding and coating applications.

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During the manufacturing process, terminal OH groups of the polysiloxane react with the crosslinking agent, generating curable products. The reaction itself takes place on exposure to atmospheric moisture and is accompanied by the liberation of hydrolosis products. This reaction, which is also referred to as vulcanization, starts with the formation of a skin at the surface of the rubber and continues gradually towards the inside.

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 Cross linking 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