Silicone rubber is odorless and non-toxic, not afraid of high temperature and resistant to severe cold. It has good strength and elasticity at 300 degrees Celsius and minus 90 degrees Celsius. Silicone rubber also has good electrical insulation, oxygen resistance, aging resistance, light resistance and aging resistance, mildew resistance, chemical stability, etc. Since the advent of high-temperature vulcanized dimethyl silicone rubber, the rapid development of silicone rubber has made silicone rubber widely used.

Structural characteristics and excellent properties of silicone rubber

1. High molecular bond energy:

The C-C bond energy is 348 KJ/mol; the Si-O bond energy is as high as 444 KJ/mol; and the ultraviolet (300 nm) energy is only 400 KJ/mol. The molecular chain is helical, the bond angle of siloxane is larger (130°-160°), and the intermolecular force is small.

2. The unique properties of silicone rubber:

Excellent weather resistance, UV, O2, O3, salt spray, etc.;

Low surface energy, excellent hydrophobicity and hydrophobic migration;

Excellent insulating properties, stable dielectric properties in a wide temperature and frequency range. Arc resistance and leakage resistance are very good;

It has good high temperature resistance and can be used continuously at 180-200℃. Although the strength of silicone rubber at room temperature is only half of that of natural rubber or some synthetic rubbers, it can still maintain a certain degree of flexibility, resilience and surface hardness in a high temperature environment above 200 °C, and has no obvious mechanical properties. Variety;

Good elasticity, good low temperature flexibility, can be used at -50 ℃. The glass transition temperature of silicone rubber is generally -70~-50℃, and the special formula can reach -100℃, indicating its excellent low temperature performance. This is of great significance to the lines used in the aviation and aerospace industries.

Good flame retardant performance, almost no toxic and harmful gases are released when burning;

It can be made into a transparent product, and defects such as air bubbles or impurities can be easily detected by visual inspection.

Compared with general-purpose rubber, silicone rubber has relatively simple compounding components for all three types of silicone rubber, as is thermal vulcanization type. In addition to raw rubber, the compounding agents mainly include reinforcing agents, vulcanizing agents and some special auxiliaries. Generally, only 5 to 6 components are needed to form a practical formula.

Silicone rubber formulation design should consider the following points:

(1) Silicone rubber is a raw rubber with high saturation, and usually cannot be vulcanized with sulfur, but hot vulcanized. Thermal vulcanization uses organic peroxide as vulcanizing agent, so the compound must not contain active substances that can interact with peroxide decomposition products, otherwise it will affect vulcanization.

(2) Silicone rubber products are generally used at high temperatures, and their compounding agents should be stable at high temperatures. For this reason, inorganic oxides are usually used as reinforcing agents.

(3) Silicone rubber can easily cause the cleavage and rearrangement of siloxane bonds under the action of a small amount of polar chemical reagents such as acids or bases, resulting in a reduction in the heat resistance of silicone rubber. Therefore, they must be considered when selecting compounding agents. The acidity and alkalinity of the vulcanizate should also be considered, so as not to affect the performance of the vulcanizate.

Choice of raw rubber

When designing the formula, raw rubber with different characteristics should be selected according to the performance and use conditions of the product. For general silicone rubber products, vinyl silicone rubber can be used when the operating temperature is within the range of -70°C to 250°C. Rubber; when the product requires high and low temperature resistance as well as fuel or solvent resistance, fluorosilicone rubber should be used.

Vulcanizing agent

The vulcanizing agents used for thermal vulcanization of silicone rubber mainly include organic peroxides, aliphatic azo compounds, inorganic compounds, high-energy rays, etc. Among them, organic peroxides are the most commonly used. This is because organic peroxides are generally stable at room temperature, but can rapidly decompose to generate free radicals at higher vulcanization temperatures, thereby crosslinking silicone rubber.

Peroxides can be divided into two categories according to their activity. One type is general-purpose type, that is, with high activity, which can vulcanize all kinds of silicone rubber; the other type is vinyl-specific type, that is, low activity, which can only vulcanize vinyl-containing silicone rubber.

When the rubber compound is stored at room temperature, the vulcanizing agent will volatilize, and it is best to use the molecular sieve as a carrier. The vulcanizing agent DTBP does not react with air or carbon black, and can be used in the manufacture of conductive rubber and products with difficult molding operations. The vulcanizing agent DBPMH is similar to DTBP, but it is not volatile at room temperature, and its decomposition products are very volatile, which can shorten the second-stage vulcanization time. The vulcanizing agent DCP does not volatilize at room temperature, and has the characteristics of a vinyl-only type. At the same time, the decomposition product has low volatility, and can be used for vulcanization in situations where the external pressure is small. Vulcanizing agent TBPB is used in the manufacture of sponge products.

The amount of peroxide used is affected by a number of factors. For example, raw rubber variety, filler type and amount, processing technology, etc. Generally speaking, as long as the desired crosslinking can be achieved, the vulcanizing agent should be kept as little as possible. However, the actual dosage is much higher than the theoretical dosage, because the influence of various processing factors must be considered, such as uneven mixing, loss of peroxide in the storage of the rubber compound, and resistance of air and other compounding agents during vulcanization.