Phenyl silicone rubber is a new type of silicone rubber material developed to meet the increasing performance requirements such as resistance to high and low temperatures. It is formed by introducing phenyl groups to the side chains of polysiloxane and co-ring-opening polymerization modification of methylphenyl tricyclic compound (A3), methyl mixed cyclic compound (DMC), and vinyl cyclic compound (VMC). Its temperature resistance range has expanded from -60°C - 280°C of the existing methyl vinyl silicone rubber to -70°C - 350°C, and the short-term working temperature can reach -110°C - 400°C. It also has properties such as ablation resistance and radiation resistance, and is mainly used in the fields of power, electronic appliances, automobiles, industrial deep refrigeration, aerospace, engines, etc.
When the phenyl content is 5 - 15% (ratio of phenyl to silicon atom), it is called low-phenyl silicone rubber. At this time, the hardening temperature of the rubber drops to -115°C, giving it better low-temperature resistance and high damping. It still has flexible elasticity at -100°C. When the phenyl content is 15 - 25%, it is called medium-phenyl silicone rubber, which has the characteristics of ablation resistance and flame resistance. When the phenyl content is above 30%, it is called high-phenyl silicone rubber, which has excellent radiation resistance.
Medium-phenyl and high-phenyl silicone rubbers are difficult to process and have poor physical and mechanical properties, and their production and application are subject to certain limitations. Medium-phenyl silicone rubber has excellent cold resistance and flame retardancy. Once on fire, it can self-extinguish. High-phenyl silicone rubber has excellent radiation resistance. With the increase of phenyl content, the molecular rigidity increases, and the radiation and flame resistance improve.
Mechanism of action:
Thermal-oxidative aging mechanism
Thermal-oxidative aging is the most common form of aging in silicone rubber aging. Most silicone rubber products are used in high-temperature environments. Due to the combined effects of heat and oxygen, thermal-oxidative aging occurs. During the process of thermal-oxidative aging, heat promotes the oxidation of silicone rubber, and oxygen promotes the thermal degradation of silicone rubber.
The factors affecting the aging of silicone rubber are:
The molecular chain structure and composition of silicone rubber are the main factors determining the level of heat resistance. The main chain of silicone rubber containing only Si-O atoms, due to its high flexibility and easy curling, certain trace impurities (such as water, hydroxyl groups or residual catalysts, etc.) can quickly initiate the degradation of the main chain. The difficulty of degradation depends not only on the structure of the silicone rubber itself, but also on the nature and content of the impurities. The high polarity of the Si-O bond also determines that it is vulnerable to polar attacks and quickly causes the thermal rearrangement and degradation of the main chain. In addition to the above-mentioned thermal rearrangement and degradation of the main chain, there is also the oxidation of side groups, making the reaction more complex.
The processing of silicone rubber almost always uses white carbon black for reinforcement, and the active hydroxyl groups on the surface of white carbon black, the residual adsorbed water on its surface, and the water generated by the condensation of hydroxyl groups all reduce the heat resistance of silicone rubber.
In most cases, the residual catalyst inevitably present during the synthesis of silicone rubber also has a certain degree of impact on the thermal stability of silicone rubber.
There are mainly three ways to improve the thermal-oxidative aging resistance of silicone rubber:
Change the structure of the side chain groups of silicone rubber, such as introducing phenyl groups, to prevent the decomposition of the side chain groups of silicone rubber and cause cross-linking or degradation of the molecular main chain;
Introduce bulky segments, such as decaboranyl, phenylene, and phenoxy groups, into the main chain of silicone rubber to improve the thermal stability of the cross-linking bonds of the vulcanizate;
Add heat-resistant agents, such as ferric oxide and cerium dioxide, to the rubber compound to prevent side chain oxidation cross-linking and main chain cyclization and degradation.
Cold resistance mechanism
Rubber has high elasticity, but at low temperatures, due to the weakened thermal motion of rubber molecules and the freezing of molecular segments, it gradually loses elasticity. The two important processes affecting the cold resistance of rubber are the glass transition and crystallization transition. The cold resistance of silicone rubber vulcanizate is related to the glass transition process and crystallization process. The glass transition temperature Tg of dimethyl silicone rubber and methyl vinyl silicone rubber is about -125°C, but after its vulcanizate is placed at -50°C, it loses elasticity due to strong crystallization, thus limiting its long-term working ability at low temperatures. Replacing methyl groups with phenyl groups can disrupt the regularity of the polydimethylsiloxane molecular chain, thereby greatly reducing the crystallization temperature and crystallinity of the polymer and the crystallinity of the vulcanizate.
From the above analysis, we can see that introducing phenyl groups in the side chain can not only prevent the decomposition of the side chain groups of silicone rubber and cause cross-linking or degradation of the molecular main chain, but also disrupt the regularity of the polydimethylsiloxane molecular chain, thereby greatly reducing the crystallization temperature and crystallinity of the polymer and the crystallinity of the vulcanizate. Therefore, methylphenyl silicone rubber can simultaneously expand the upper limits of low and high temperatures.