Chemical Crosslinking Principle
Uncured silicone rubber must be vulcanized to transform it into elastomeric materials. Crosslinking is performed in three different ways.
- Platinum-catalyzed addition curing
- Peroxide Curing
- Condensation Curing
In each of these three classes, specific curing agents, side groups on the polymer and catalysts are used in the formulation of the silicone rubbers.
During platinum-catalyzed addition curing, the crosslinkerʼs Si-H groups react with the vinyl groups of the polymer to form a three-dimensional network. WACKER offers both one-part silicone rubber grades already containing platinum catalyst and two-part grades in which the crosslinker and platinum-catalyst are kept separate until mixed. Independently of the intended application, grades are available that already cure at room temperature as well as grades that only vulcanize at elevated temperature. In addition, WACKER also offers a range of grades in which a specific catalyst can be activated solely by irradiation with UV light.
The principle of addition-curing is found in several product types:
- Solid silicone rubber (HTV)
- Liquid silicones (LSR)
- 2-part silicone rubber (RTV-2)
- Silicone gels
- UV-curable silicone rubbers
- No byproducts released, as a result the silicone rubber cures without shrinkage and without releasing substances that could affect the odor or taste.
- Rapid curing, with pot life and crosslinking rate regulated via the temperature, or, in the case of UV-curable silicones, via the radiation dose.
Small amounts of catalyst poisons, in particular amine and sulfur-containing compounds, may cause inhibition.
Peroxide curing involves the use of organic peroxides. At elevated temperatures, these decompose to form highly reactive radicals that chemically crosslink the polymer chains. The result is a highly elastic, three-dimensional network. WACKER offers peroxide curing agents as pastes.
The principle of peroxide curing is used for solid silicone rubbers (HTV).
- A tried-and-tested curing method
- Many years of expertise and ongoing development together with customers
- Manufacturing process ensures a constant high quality
- Compounds are relatively insensitive to catalyst poisons
Some peroxide curing agents (ELASTOSIL® AUX curing agent C1 and ELASTOSIL® AUX curing agent C6) are inhibited by atmospheric oxygen. Surfaces that come into contact with air during vulcanization do not vulcanize completely and show tackiness.
In condensation-curing, the terminal hydroxyl groups of the polymer react with a siloxane curing agent, releasing small, volatile compounds such as alcohol, acetic acid and amine. In contrast to addition-curing silicone rubbers, this release leads to a material shrinkage of about 3 vol. %. Condensation-curing vulcanization is catalyzed by tin or organotitanium compounds in the presence of small amounts of water. Vulcanization always takes place at room temperature. WACKER offers both ready-to-use one-part silicone rubbers (RTV-1) that vulcanize when exposed to atmospheric humidity, and fast-curing two-part grades.
The principle of addition-curing is found in the following two product types:
- 2-part silicone rubber (RTV-2)
- Silicone rubber (RTV-1)
- Silicone rubber grades are largely insensitive to inhibition by other substances.
- Permit robust adhesives and sealants
RTV-1 silicone rubber compounds need atmospheric humidity for curing – this can significantly increase the vulcanization rate in dry environments.
Condensation-curing RTV-2 silicone rubber grades must not be heated to temperatures exceeding 90°C until fully cured, since reversion of the curing may otherwise occur.