During annealing, molded parts are exposed to elevated temperatures in order to accelerate their final crosslinking, to optimize the physical properties of the material and to remove the remaining volatile components and by-products of the crosslinking reactions. The optimal curing time and the optimal temperature conditions depend on the type of material, the specific recipe, the cross-linking system and the planned use of the molded part. Diffusion and evaporation of volatiles should occur at a temperature greater than the operating temperature of the part. Always ask your supplier for advice!
The tempering is carried out as standard for molded parts with peroxide or bisphenol crosslinkers. However, it can also be used on molded parts with a sulfur-containing crosslinker in order to improve certain physical properties. In some applications, tempering is absolutely necessary.
In the case of silicone moldings with a peroxide-containing crosslinker, the heat treatment removes volatile substances and by-products of the peroxide so that an odorless molding is also created. If the material is developed for contact with food or drinking water, the temperature is absolutely necessary so that the requirements of the standards of FDA, BfR, KTW or WRAS can be met.
Platinum-based crosslinkers for silicone elastomers do not contain peroxide. In order to be able to meet the requirements for contact with foodstuffs, however, these too must be tempered. This removes volatile substances from the material and improves its physical properties, in particular the permanent compression set and stability at high temperatures.
FKM-based compounds are often subjected to two-phase crosslinking in order to optimize the physical properties of the material. Compression set and tear strength are two of the properties that are most often improved by annealing. The initial curing is usually done under pressure in a mold or an autoclave to prevent pore formation or cracking of the part due to outgassing of by-products of the crosslinking reactions.
Secondary hardening is achieved during tempering. Typical tempering cycles can last between 4 and 24 hours. The length of time depends on the type of material and the crosslinker used. Please see below for more information.