The post-vulcanization process, also known as “post-curing” ) is a subsequent heat treatment performed on the vulcanizates (rubber products) after the vulcanization phase.
During the post-vulcanization process the products are exposed to elevated temperatures in order to accelerate the vulcanization, optimize the physical properties of the material and remove residual volatile substances, unwanted impurities or by-products formed during the vulcanization process. The optimal time and temperature conditions of the post-vulcanization may vary and strongly depend on the type of material, the compound formulation, the cross-linking system (vulcanization) and the application of the product. The migration and evaporation of the remaining volatile substances in the product should be carried out at temperatures higher than the operating temperatures of the environment and the system into which the product will be installed. Please contact the compound supplier regarding the post-vulcanization conditions.
The post-vulcanization is usually performed on the products made from compounds that vulcanize with peroxide or bisphenol. However, it can also be performed on the products made from sulphur compounds, in order to improve certain physical properties. In some cases, the post-vulcanization is prescribed and thus absolutely mandatory.
At VMQ peroxide silicone compounds the post-vulcanization process removes from the products the residual volatile substances, unwanted impurities and by-products of peroxide reactions. This ensures that the products become flawless and clean, without any smell and taste. If the final product is intended to come into contact with food and water, the post-vulcanization is absolutely mandatory in order to meet the requirements of FDA, BfR, KTW or WRAS norms.
VMQ platinum compounds, however, do not contain any peroxides, but in order to meet the requirements of the regulations governing the contact with food and water, the final products should also be post-vulcanized, as this is the only way to remove residual volatile substances and unwanted impurities and thus simultaneously improve some physical properties, especially the resistance to compression set and high-temperature stability.
Compounds based on FKM polymers require a two-stage vulcanization cycle in order to optimize and achieve the final physical properties of the material. Tensile strength and resistance to compression set are two of the properties that are most often improved with the post-vulcanization process. The beginning of the vulcanization process usually takes place under pressure and at an elevated temperature in a tool or autoclave in order to prevent the porosity of product due to gases formation as a result of by-products of the cross-linking reactions. During the post-vulcanization process a secondary vulcanization process is performed. Typical post-vulcanization cycles can last between 4 and 24 hours, depending on the type of material and the crosslinking agent used. You can find more information below.
The majority of physical properties are improved in the first two to four hours. Further improvements in physical properties increase more slowly, therefore long-lasting post-vulcanization processes are required.
The initial increase of the temperature in the oven as a function of time depends on the thickness of the products and should be adjusted accordingly. Products thicker than 4 mm require additional post-vulcanization time. Adequate airflow around each individual product is crucial, therefore it should be ensured that the objects are not stacked on top of each other or overlap during the process. In the case of post-vulcanization of longer extruded profiles make sure that the overlap surface is as small as possible.
During the post-vulcanization a sufficient fresh air supply should also be ensured. In case of poor ventilation the high concentrations of vapours may occur, preventing proper out-gassing and resulting in a risk of fire or explosion. Poor out-gassing and high vapour concentrations in the oven may also lead to a decrease in tensile strength, elongation and resistance to compression set in conjunction with the adhesive surface of the product. Yellowing may also be expected at transparent and coloured silicone compounds. To avoid the aforementioned risks we recommend that the fresh air flow is 120 l/min per kilogram of the compound in a post-vulcanization oven.
Never insert the products made of different types of materials together in the same post-vulcanization oven, even if the post-vulcanization conditions are the same. Ideally you should have separate ovens for individual material types in order to completely avoid cross-contamination due to out-gassing.
If you doubt in the thoroughness of the post-vulcanization process of products from VMQ silicone compound intended for food contact, you shall measure the percentage change in product weight and compare it with the percentage change in weight of the reference sample that was completely and appropriately post-vulcanized. Please note that all volatile substances are never completely removed and that some volatile substances still remain after the post-vulcanization process. At silicone products, a certain proportion of volatile substances will slowly evaporate for a long time at elevated temperatures. The volatile matter content remaining at 200 °C after four hours is low enough for the products to pass most of the tests and requirements set out in the food and water contact standards.
Typical time and temperature conditions of post-vulcanization are as follows:
Certain exceptions apply, for example, to metal-rubber products, where excessive temperatures during post-vulcanization (above 210 °C) may change the properties of the metal (when the whole assembly should be elastic) and adhesion of rubber to the metal. In such case the post-vulcanization conditions should be changed accordingly.