For the Curing and Modification of Silicones
One of the most important methods to cure and modify silicone is the hydrosilylation reaction under the use platinum-based catalysts.
During the curing process a vinyl component reacts with a crosslinker with multiple Si-H groups to yield an elastic polymer. The use of platinum as a homogeneous catalyst facilitates the reaction at a very low platinum loading.
The well-known Karstedt’s catalyst is the most used hydrosilylation catalyst, as it is highly active, easy to handle and safe to use. The Ashby’s catalyst is more suitable for high temperature curing, commonly used in high temperature extruded silicone systems. The Lamoreaux’ catalyst on the other hand is for example applied for silicone rubbers with improved fire-retardant properties. Lastly there is the hydrosilylation catalyst (MeCp)PtMe3. In contrast to the other three catalysts, it is activated by UV.
Heraeus Precious Metals offers all mentioned hydrosilylation catalysts, with the Karstedt’s catalyst being the most requested and important one.
Heraeus Precious Metals is a major supplier of Karstedt’s catalysts for the silicones market. Our range of Karstedt’s catalysts provide curing characteristics for every application. The Karstedt’s catalyst can be shipped in various dilutions with solvents to meet customer needs. Heraeus’ catalysts are produced with a narrow variance, which minimizes catalyst waste while delivering constant activity designed for a steady process. The homogeneous Karstedt’s catalysts is typically employed in very low concentrations (30–100ppm) and remains in the silicones. Where homogeneous catalysts are not an option, Heraeus also offers a broad range of suitable heterogeneous catalysts.
Heraeus offers customized formulations of the Karstedt’s catalyst tailored to the customer needs. Possible modifications include the activity, viscosity, solvent and platinum content. Further information is included in the brochure about the different Karstedt’s catalysts and their customization .
| Low Turbidity | Super Cure | Heat Resistant | |
|---|---|---|---|
| Advantage | Avoid filtration and platinum losses after dilution with silicone polymer | Faster curing time at room temperature | High resistance to heat and stress |
| Shelf life | Standard shelf life | Less shelf life | Longer shelf life |
| Application Example | Transparent silicones | Dental applications | Paper release coating |
As an alternative to thermal curing, UV curing with a photoactive catalyst can be utilized for the hydrosilylation reaction. This method is especially useful for the crosslinking of thermally sensitive polymers because no additional heat is required. Moreover, the inclusion of heat-sensitive parts such as electronic parts is enabled. Heraeus offers the UV activated catalyst (MeCp)PtMe3. Applications for UV curing are for example adhesives or dental applications. Benefits include improved curing time for liquid silicone elastomers and a more energy-efficient process because no heating ovens are required.
For the hydrosilylation of two component room temperature vulcanizing systems, it is recommended to use platinum catalysts at the 5-50 ppm range on total formulation weight. Further modification is possible by utilizing moderators depending on reaction conditions.
Poisoning occurs when a compound irreversibly reacts with the catalyst, stopping the hydrosilylation reaction. Therefore, to ensure the best performance, the following compounds should be avoided:
To best preserve the catalyst and its reactivity the following aspects should be considered when it comes to storage. Preferably it is stored under 30 °C and in the dark to prevent decomposition from UV light. The higher the platinum concentration, the colder it should be stored. To avoid oxidation by O2, the catalyst solution should be stored under argon.
The curing process is very fast and requires slowing down to gain processing time. Therefore, inhibitors such as ethynoles are used. The curing is stopped at room temperature completely which ensures a slow curing at temperatures over 60 °C. A moderator such as Vinyl-D2 or Vinyl-D4 is added to the catalyst to slow down the reaction at room temperature and enable a fast cure at high temperatures.
