Why do some silicone rubber products need to undergo secondary vulcanization after molding?

Silicone rubber products are believed to be familiar to everyone. Some products can be directly used after being molded, while others require secondary vulcanization (also known as secondary sulfurization). Actually, after molding, it looks good to the naked eye and can be used normally. Why do we need to undergo secondary vulcanization?

Firstly, it should be noted that not all silicone rubber products require secondary vulcanization of silicone. However, depending on the formula of the raw materials, some can be directly molded in one go, while some special materials such as fluororubber products and EPDM silicone rubber products require secondary vulcanization. This is because during the primary vulcanization of these materials, the peroxides inside will decompose and undergo high polymer reactions, producing some low molecular weight compounds (such as benzene, benzoic acid, etc.). These low molecular weight compounds mixed with rubber will reduce the mechanical properties and lifespan of silicone rubber, and are also toxic. This is very unfavorable for silicone kitchenware, so silicone rubber product manufacturers often use secondary vulcanization to decompose and evaporate low molecular weight compounds.
Secondly, after the first vulcanization is completed, the silicone molecules may not be fully crosslinked (vulcanization is insufficient). Secondary vulcanization of silicone rubber can make it more fully and evenly vulcanized, thereby improving the mechanical properties and durability of the rubber.
Thirdly, it can be subjected to silicone secondary vulcanization, such as the EPD silicone rubber products widely used in automobiles, which can remove odors and improve the comfort of car passengers after secondary vulcanization.

Fourthly, in order to reduce costs and increase production capacity, silicone product manufacturers often shorten the first silicone vulcanization time and then extend the second vulcanization time to ensure that the products are fully vulcanized while also improving their competitiveness.