Silicone rubber formula design

Compared with general-purpose rubber, silicone rubber has a relatively simple compounding component of all three types of silicone rubber, as well as a hot-vulcanized type. In addition to raw rubber, the compounding agent mainly includes a reinforcing agent, a vulcanizing agent and some special auxiliaries, and generally only needs 5 to 6 components to form a practical formula. The silicone rubber formulation design should take into account the following points.

(1) Silicone rubber is a high-saturation raw rubber, which is usually not vulcanizable with sulfur, but is heat vulcanized. Thermal vulcanization uses organic peroxides as vulcanizing agents. Therefore, the rubber compounds must not contain active substances that can interact with the decomposition products of peroxides, otherwise they will affect vulcanization.

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

(3) Silicone rubber can easily cause cracking and rearrangement of siloxane bonds under the action of a trace amount of polar chemicals such as acid or alkali, resulting in a decrease in heat resistance of silicone rubber. Therefore, it is necessary to consider them when selecting compounding agents. The acidity and alkalinity should also take into account the acidity of the peroxide decomposition products so as not to affect the properties of the vulcanizate.

(1) Selection of raw rubber

When designing the formula, the raw rubber with different characteristics should be selected according to the performance and use conditions of the product. For general silicone rubber products, the temperature is in the range of -70 ° C ~ 250 ° C, vinyl silicone rubber can be used; when the product temperature requirements are high (-90 ~ 300 ° C), low phenyl silicon can be used Rubber; fluorosilicone rubber should be used when the product is required to withstand high and low temperatures and is resistant to fuel or solvent.

(2) vulcanizing agent

The vulcanizing agents used for the heat-vulcanized silicone rubber mainly include organic peroxides, aliphatic azo compounds, inorganic compounds, high-energy rays, and the like, and the most commonly used ones are organic peroxides. This is because organic peroxides are generally relatively stable at room temperature, but are rapidly decomposed to generate radicals at higher vulcanization temperatures, thereby crosslinking the silicone rubber.

Peroxides can be classified into two categories according to their activity levels. One type is general-purpose type, that is, it has high activity and can vulcanize various silicone rubbers; the other type is vinyl-specific type, that is, it has low activity and can only vulcanize vinyl-containing silicone rubber.

In addition to the above general differences between the two types of peroxides, each of the peroxides has its own characteristics. Vulcanizing agent BP is the most commonly used vulcanizing agent for molded products. It has high vulcanization speed and high production efficiency, but it is not suitable for the production of thick products. The vulcanizing agent DCBP is not volatile due to its product, and bubbles are generated when it is not pressurized during vulcanization. It is particularly suitable for continuous vulcanization of hot air extruded from the product, but its decomposition temperature is low, causing scorch, and the rubber storage time is short. The vulcanizing agent BP and DCBP are both crystalline powders, which are explosive and safe to handle and suitable for dispersion. Generally, they are used in a paste of silicone oil or silicone rubber, and the content is generally 50%. The vulcanizing agent has a boiling point of 110 ° C and is extremely volatile. The vulcanizing agent volatilizes when the compound is stored at room temperature, and is preferably used in the form of a 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 articles that are difficult to mold. The vulcanizing agent DBPMH is similar to DTBP, but it does not volatilize at room temperature, and its decomposition products are highly volatile, which can shorten the second-stage vulcanization time. The vulcanizing agent DCP does not volatilize at room temperature, has the characteristics of a vinyl-specific type, and has low volatility of decomposition products, and can be used for vulcanization in a case where the external pressure is small. The vulcanizing agent TBPB is used to make sponge products.

The amount of peroxide used is affected by a number of factors. For example, raw rubber varieties, filler types and dosages, processing techniques, and the like. In general, the vulcanizing agent should be as small as possible as long as the desired crosslinking is achieved. However, the actual amount is much higher than the theoretical amount, because it must take into account the influence of various processing factors, such as uneven mixing, peroxide loss in the storage of the rubber, and resistance of air and other compounding agents during vulcanization. For vinyl silicone rubber molded products, the usual ranges of various peroxides are as follows: vulcanizing agent BP0.5~1; vulcanizing agent DCBP1~2; vulcanizing agent DTBP1~2; vulcanizing agent DCP0.5~ 1; vulcanizing agent DBPMH 0.5 ~ 1; vulcanizing agent TBPB 0.5 ~ 1. As the vinyl content increases, the amount of peroxide should be reduced. The amount of peroxyl content in the rubber, extruded product and adhesive compound should be higher than that in the molding compound. In some cases, the use of two peroxides can reduce the amount of vulcanizing agent, and can appropriately lower the vulcanization temperature and improve the vulcanization effect.

(3) Reinforcing agent

The unreinforced silicone rubber vulcanizate has a very low strength of only about 0.3 MPa and has no practical use value. The strength of the silicone rubber vulcanizate can be 3.9 to 9.8 MPa by using a suitable reinforcing agent, which is extremely important for improving the performance of the silicone rubber and prolonging the service life of the product. The choice of silicone rubber reinforcing fillers should take into account the high temperature use of silicone rubber and the vulcanization with peroxides, especially the acid-alkaline substances adversely affect silicone rubber.

The reinforcing filler for silicone rubber can be divided into reinforcing fillers and non-reinforcing fillers according to the reinforcing effect. The former has a diameter of 10 to 50 nm and a specific surface area of 70 to 400 m 2 /g. The effect is better; the latter is 300-10000 nm, the specific surface area is below 30 m2/g, and the reinforcing effect is poor.

1 reinforcing filler

(1) The type and characteristics of silica, the reinforcing filler used in silicone rubber mainly refers to synthetic silica, also known as white carbon black. White carbon black is divided into gas phase white carbon and precipitated white carbon black.

The size, specific surface area, surface properties, structure, etc. of the gas phase white carbon black particles are related to the ratio of the raw material gas, the burning speed, and the residence time of the SiO2 core in the combustion chamber.

The finer the gas phase white carbon black particles, the larger the specific surface area, the better the reinforcing effect, but the worse the handling performance. On the contrary, its particles are coarser, the specific surface area is smaller, and the reinforcing effect is worse, but the operability is better.

Vapor white carbon black is one of the most commonly used reinforcing agents for silicone rubber. The vulcanized rubber which is reinforced by it has high mechanical strength and good electrical properties. The fumed silica can be used in combination with other reinforcing agents or weak reinforcing agents to prepare the compound for different use requirements.

b. precipitated white carbon black

The performance of precipitated silica is affected by precipitation conditions such as acidity, temperature, and the like.

Compared with the silicone rubber compound reinforced with fumed silica, the rubber reinforced with precipitated silica is slightly lower in mechanical strength, and the dielectric properties, especially the dielectric properties after damp, are poor, but heat aging The performance is better and the cost of the rubber compound is much lower. When the mechanical strength of the article is not critical, precipitated silica may be used or used in combination with fumed silica.

Silica can be treated with a suitable compound to form a surface hydrophobic material. The treatment methods mainly include liquid phase method and gas phase method. The conditions of the liquid phase method are easy to control, the product quality is stable, the treatment effect is good, but the process is complicated, the solvent needs to be recovered; the gas phase treatment process is simple, but the quality of the product is not stable enough, and the treatment effect is poor.

The substance used as the surface treatment agent is, in principle, a substance which can react with the hydroxyl group on the surface of the silica, and there are the following.

1) Alcohol 2) Chlorosilane 3) Alkoxysilane 4) Hexamethyldisiloxane 5) Silazane.

(2) Reinforcing mechanism of white carbon black and surface chemistry The reinforcing mechanism of silica black to silicone rubber is considered to be the following two.

a. The rubber is adsorbed by the filler particles to adsorb the polymer, so that the rubber molecular segment is directly fixed in the vicinity of the filler particles or oriented along the surface of the filler or retained by the filler aggregate.

b. Rubber and filler particle bonding The filler particles combine with the polymer segment to produce effective crosslinking and the polymer entangles the filler particles.

Based on the above two effects, the silica is reinforced by the silicone rubber.

Silica is different in acidity and alkalinity due to its different production methods. The fumed silica is acidic and the precipitated silica is alkaline. The purest HCL-free fumed silica has a pH of 6, which is due to the H+ generated by the dissociation of hydroxyl groups on the surface of silica. When the pH is lower than 4.6, it is caused by HCL remaining in high temperature hydrolysis.

(3) Method for determining physical and chemical properties of silica

The physicochemical properties of silica directly reflect the quality of the product, so accurate measurement is very important for different use requirements. At present, the indicators of foreign manufacturers are not the same, but some recognized important indicators must be measured. The most important indicators are: indicators that reflect their primary structure, such as particle size and dispersion, specific surface area; indicators that reflect their secondary structure, such as oil absorption values; indicators that reflect their surface chemistry, such as various hydroxyl groups on the surface The concentration and so on.

Since the particle size and the particle size distribution differ depending on the formation conditions and the particle growth, the particle diameter of the silica is not uniform, and the particle diameter which is usually referred to has only a statistical average.

b Determination of Specific Surface Area The specific surface area is an indicator of the size of the external surface area of the powder material. For a porous powder material, the specific surface area is the sum of the surface area and the external surface area within the pore.

In general, the particle size of the powder material is inversely proportional to its specific surface area, so the measurement of the specific surface product can qualitatively reflect the particle size of the powder. Since the electron microscope is not available in all industrial units, the particle size of the powder cannot be obtained, and thus the measurement of the specific surface area has important practical application value.

Determination of surface hydroxyl groups There are silanol groups on the surface of silica, and many applications of silica are directly related to such groups. Therefore, it is important to quantitatively determine the surface hydroxyl groups.

The data of the hydroxyl group on the surface of silica often includes total hydroxyl groups, adjacent hydroxyl groups, and isolated hydroxyl groups. The latter two are bound to the surface of silica in the form of Si-OH, collectively referred to as bound hydroxyl; the total hydroxyl is the sum of the hydroxyl groups bound to the hydroxyl molecules adsorbed on the surface of the silica, these hydroxyl groups The data can be determined under different conditions. The measurement conditions are:

1) The hydroxyl group measured by direct sampling in the white carbon black bag is the total hydroxyl amount;
2) The hydroxyl group measured after drying the silica at 110 ° C for 3 hours is a binding hydroxyl group;
3) The hydroxyl group measured after drying the silica at 600 ° C for 3 hours is an isolated hydroxyl group;
4) The difference between the combined hydroxyl group and the spacer is an adjacent hydroxyl group.

d Determination of secondary structure It is generally believed that the degree of secondary structure directly affects the reinforcing behavior of the filler, so it is also important to determine the secondary structure. However, there is still no good measurement method. At present, there are two widely used methods: one is to determine the apparent specific volume under compression; the other is to determine the oil absorption value.

2 weak reinforcing filler

Weakly reinforcing fillers, also known as inert fillers, have only a small reinforcing effect on silicone rubber. They are generally not used alone in silicone rubber, but instead act with silica to adjust the hardness of silicone rubber and improve it. The process performance of the rubber compound and the oil resistance and solvent resistance of the vulcanized rubber reduce the cost of the rubber compound.

Commonly used weak reinforcing agents are diatomaceous earth, quartz powder, zinc oxide, titanium dioxide, zirconium silicate and calcium carbonate.