The strength of unreinforced silicone rubber vulcanizate is very low, only about 0.3MPa, and has no practical use value. The strength of silicone rubber vulcanizate can reach 3.9-9.8MPa by using appropriate reinforcing agent, which is extremely important to improve the performance of silicone rubber and prolong the service life of products. The selection of silicone rubber reinforcing fillers should take into account the high temperature use of silicone rubber and the use of peroxide vulcanization, especially the adverse effects of acidic and alkaline substances on silicone rubber.
Reinforcing fillers for silicone rubber can be divided into reinforcing fillers and non-reinforcing fillers according to their reinforcing effects. The diameter of the former is 10-50nm, and the specific surface area is 70-400m²/g. The effect is better; the latter is 300-10000nm, the specific surface area is below 30m²/g, and the reinforcement effect is poor.
1. Reinforcing filler
(1) Types and characteristics of silica, the reinforcing filler used in silicone rubber mainly refers to synthetic silica, also known as silica. Silica is divided into fumed silica and precipitated silica.
a. The size, specific surface area, surface properties, and structure of fumed silica particles are related to factors such as the ratio of raw gas, combustion speed, and residence time of SiO2 cores in the combustion chamber.
The finer the fumed silica particles, the larger its specific surface area, and the better the reinforcement effect, but the worse the operating performance. On the contrary, its particles are thicker and its specific surface area is smaller, so the reinforcement effect is poor, but its operability is better. Fumed silica is one of the most commonly used reinforcing agents for silicone rubber. The vulcanizate reinforced by it has high mechanical strength and good electrical properties. Fumed silica can be used in combination with other reinforcing agents or weak reinforcing agents to prepare rubber compounds with different application requirements.
b. Precipitated silica
The properties of precipitated silica are affected by precipitation conditions such as acidity, temperature, etc. Compared with the silicone rubber compound reinforced with fumed silica, the compound reinforced with precipitated silica has slightly lower mechanical strength and poorer dielectric properties, especially after being exposed to moisture, but is resistant to aging. The performance is better and the cost of the compound is much lower. When the mechanical strength of the product is not high, precipitated silica can be used or it can be used in combination with fumed silica. Silica can be treated with suitable compounds to make it a surface hydrophobic substance. There are two main treatment methods: liquid phase method and gas phase method. The conditions of the liquid phase method are easy to control, the product quality is stable, and the treatment effect is good, but the process is complicated and the solvent needs to be recovered; the gas phase method has a simple treatment process, but the quality of the product is not stable enough and the treatment effect is poor.
(2) The reinforcement mechanism of silica and the reinforcement mechanism of surface chemical silica to silicone rubber. It is considered that there are the following two.
a. The rubber is adsorbed by the filler particles. The filler particles adsorb the polymer, so that the rubber molecular segments are directly fixed in the vicinity of the filler particles or oriented along the surface of the filler or retained by the filler aggregates.
b. The combination of rubber and filler particles. The combination of filler particles and polymer segments results in effective cross-linking and polymer entanglement of filler particles.
Based on the above two functions, silica can play a reinforcing effect on silicone rubber.
Due to the different production methods of silica, its acidity and alkalinity are different. The fumed silica is acidic, and the precipitated silica is alkaline. The pH of the purest HCL-free fumed silica is 6, which is due to the dissociation of the hydroxyl groups on the surface of the silica to produce H+ in water. When the pH value is lower than 4.6, it is caused by the retained HCL caused by high temperature hydrolysis.