How to improve the aging resistance of silicone rubber?

Silicone rubber refers to a rubber in which the main chain consists of alternating silicon and oxygen atoms, and two organic groups are usually attached to the silicon atoms. Ordinary silicone rubber is mainly composed of silicone chain links containing methyl groups and a small amount of vinyl groups.

Thermal aging is the most important form of aging of silicone rubber. The use environment of silicone rubber is in high temperature air, and the combination of heat and oxygen causes thermal aging. In the process of thermal oxygen aging, high temperature promotes the oxidation of silicone rubber, and oxygen promotes the thermal degradation of silicone rubber.

The structural changes of silicone rubber can be divided into two categories in the thermal and oxygen aging process: one is the degradation of molecular chains caused by the thermal and oxygen aging reaction; the other is the cross-linking between molecular chains. Molecular chain rupture causes rubber to become sticky after aging; cross-linked rubber will harden after aging, which will reduce the mechanical properties of rubber and eventually lead to failure. When the air heats up, the vulcanized silicone rubber is cross-linked, and the drop in elongation at break is much greater than the tensile strength.

According to the thermal aging mechanism of silicone rubber vulcanizates, there are several ways to improve their thermal aging properties, as follows:

1. Change the composition of the side chain of the silicone rubber, such as introducing phenyl groups, etc., to prevent the cross-linking or decomposition of the molecular chain of the silicone rubber due to the decomposition of the side chain. This is because the phenyl group is not easily oxidized, and steric hindrance is formed in the silicon chain, so the organic silicon chain is more difficult to decompose. And because of its greater conjugation effect and steric hindrance effect, the thermal decomposition temperature is higher.

2. By introducing large-volume segments into the molecular chain of silicone rubber, the thermal stability of the cross-linked bonds of the vulcanizates, such as carbon, decyloxy, benzene, phenyl ether and cyclic diimino groups, is improved.

3. Heat-resistant additives are added to the rubber compound to prevent side chain oxidative cross-linking and main chain cyclization decomposition, such as iron oxide, dioxane, and hexaphenyl cyclotrisilane.

Add metal oxide antioxidant

The oxidative aging mechanism of silicone rubber is mainly the side chain oxidation reaction caused by free radicals. If this chain reaction is prevented, the oxidation process will stop. Fe2O3 is the most commonly used antioxidant, and the addition amount is usually 3 to 5 parts. Related research reports that the composite metal oxide prepared by the liquid phase co-precipitation method can significantly improve the heat resistance of silicone rubber. Studies have shown that iron-tin composite metal oxide is more effective in improving the heat resistance of silicone rubber than a single iron oxide and tin oxide. This is because the crystal structure of the prepared composite metal oxide has changed, and their respective antioxidant effects have a synergistic effect. It can inhibit oxidation and improve the heat resistance of silicone rubber.

Add silazane

Silazane can significantly improve the heat resistance of silicone rubber. The mechanism is: Silazane can effectively inhibit the degradation of the main chain, eliminate trace moisture in the silicone rubber, and make the heat resistance of silicone rubber up to 350°C. The higher the apparent activation energy and hydrolytic stability of silazane, the better the thermal stability of silicone rubber. The commonly used ones are: hexamethyldisilazane, hexamethylcyclotrisilazane, etc.

Add silicone

Silicone resin is a highly cross-linked polysiloxane with a network structure and is a thermosetting resin. Its main chain is composed of silicon-oxygen bonds and has the same structure as quartz and glass, and has excellent thermal oxidation stability. Heating at 205°C for 42 hours, the weight loss of silicone resin is only 2% to 8%, and after heating at 350°C for 24 hours, the weight loss is less than 20%.