Basic knowledge about silicone rubber

Basic knowledge about silicone rubber

We all know that silicone rubber products are made from mixed silicone rubber through high temperature vulcanization. So how is the compounded silicone rubber made? What are the basic knowledge of silicone raw materials that we, as salesmen, must understand? Let me take you to the world of silicone rubber today, I believe it will benefit you a lot! The following are some relevant information I have collected for your reference!

First, let me briefly talk about the formation of compounded silicone rubber:

The first is to prepare raw rubber, white carbon black, and silicone oil according to the requirements of the rubber compound.

The second is cooking. Cook the kneaded product in the vacuum kneader.

The third is to use an open mixer to grind the boiled rubber into a roll

The fourth is after the roll of glue is cooled (usually 3-4 hours), the glue is filtered clean in the glue strainer.

Isn't it simple? But we need to understand the relevant ingredients and characteristics of the raw materials in detail, which requires us to spend a little thought to ask the master or collect information to understand these things more deeply.

So, let's take you in-depth understanding of them! In order to be straightforward, I will make a point-by-point statement!

1. What is silicone rubber and how is it classified?

Silica gel is a kind of highly active adsorption material. It is an amorphous substance. It contains polysiloxane, silicone oil, white carbon black (silica), coupling agent and filler, etc. The main component is silica. Its chemical The molecular formula is mSiO2·nH2O. It is insoluble in water and any solvents, non-toxic, tasteless, chemically stable, and does not react with any substances except strong alkalis and hydrofluoric acid. Various types of silica gel form different microporous structures due to their different manufacturing methods. The chemical composition and physical structure of silica gel determine that it has many other similar materials that are difficult to replace: high adsorption performance, good thermal stability, stable chemical properties, high mechanical strength, etc.

Classification of silicone rubber:

According to its vulcanization characteristics, silicone rubber can be divided into two types: hot vulcanized silicone rubber and room temperature vulcanized silicone rubber. According to the different performance and use, it can be divided into general type, ultra low temperature resistant type, ultra high temperature resistant type, high strength type, oil resistant type, medical type and so on. According to the different monomers used, it can be divided into methyl vinyl silicone rubber, methyl phenyl vinyl silicone rubber, fluorosilicone, nitrile silicone rubber and so on.

(1) Dimethyl silicone rubber (referred to as methyl silicone rubber):

The preparation of high-molecular-weight linear dimethyl polysiloxane rubber requires high-purity raw materials. In order to ensure the purity of the raw materials, the industry usually firstly purifies the dimethyl bismuth with a content of 99.5% or more. Chlorosilane is hydrolyzed and condensed in ethanol-water medium under acid catalysis, and the bifunctional siloxane tetramer, namely octamethylcyclotetrasiloxane, is separated, and then the tetracyclic body is exposed to the catalyst. , The formation of high molecular linear dimethyl polysiloxane. The formation reaction of dimethyl silicone rubber can be expressed by the following formula:

Dimethyl silicone rubber is a colorless and transparent elastomer, which is usually vulcanized with organic peroxides with higher activity. Vulcanized rubber can be used in the range of -60~+250℃. Dimethyl silicone rubber has low vulcanization activity and large high-temperature compression permanent deformation. It is not suitable for thick products. It is difficult to vulcanize thick products, and the inner layer is also easy to foam. Since methyl vinyl silicone rubber with a small amount of vinyl has better performance, dimethyl silicone rubber has been gradually replaced by methyl vinyl silicone rubber. Other types of silicone rubbers produced and used today, in addition to dimethyl siloxane structural units, also contain more or less other bifunctional siloxane structural units, but their preparation method is similar to that of dimethyl siloxane. There is no essential difference in the preparation method of rubber. The preparation method is generally to hydrolyze and condense a certain bifunctional silicon monomer required under the conditions that are conducive to the formation of the ring body, and then add octamethyl in the required proportion. Cyclotetrasiloxane is prepared by reacting together under the action of a catalyst.

(2) Methyl vinyl silicone rubber (vinyl silicone rubber for short):

Its structural formula can be expressed as:

Because this kind of rubber contains a small amount of vinyl side chains, it is easier to vulcanize than methyl silicone rubber, so that there are more types of peroxides available for vulcanization, and the amount of peroxides can be greatly reduced. The use of silicone rubber containing a small amount of vinyl and dimethyl silicone rubber can significantly improve the compression set resistance. The low compression set reflects that it has better supportability as a seal at high temperatures. It is one of the necessary requirements for O-rings and gaskets. Methyl vinyl silicone rubber has good process performance and is easy to operate. It can be made into thick products and the surface of extruded and calendered semi-finished products is smooth. It is a commonly used silicone rubber at present.

(3) Methyl phenyl vinyl silicone rubber (referred to as phenyl silicone rubber):

This kind of rubber is obtained by introducing diphenyl siloxane chain links or methyl phenyl siloxane chain links into the molecular chain of vinyl silicone rubber. Its molecular structure can be expressed as follows:

According to the phenyl content (phenyl: silicon atom) in silicone rubber, it can be divided into low phenyl, medium phenyl and high phenyl silicone rubber. When the rubber crystallizes or is close to the glass transition point, or the two conditions overlap, it will cause the rubber to appear rigid. The introduction of an appropriate amount of large-volume groups can damage the regularity of the polymer chain, which can reduce the crystallization temperature of the polymer. At the same time, the introduction of large-volume groups can change the force between the polymer molecules, so the glass can also be changed.化温度。 Transition temperature. Low phenyl silicone rubber (C6H5/Si=6~11%) has excellent low temperature resistance due to the above reasons, and has nothing to do with the type of phenyl monomer used. The brittleness temperature of vulcanized rubber is -120℃, which is the best rubber with low temperature performance today. Low phenyl silicone rubber has the advantages of vinyl silicone rubber, and the cost is not very high, so it has a tendency to replace vinyl silicone rubber. When the phenyl content is greatly increased, the rigidity of the molecular chain will increase, resulting in a decrease in cold resistance and elasticity, but the ablation resistance and radiation resistance will be improved, and the phenyl content will reach C6H5/Si=20~34 % Is middle phenyl silicone rubber with ablation resistance, high phenyl silicone rubber (C6H5/Si=35~50%) has excellent radiation resistance.

(4) Fluorosilicone, nitrile silicone rubber:

Fluorosilicone rubber is a type of silicone rubber with fluoroalkyl groups introduced into the side chain. Commonly used fluorosilicone rubber is fluorosilicone rubber containing methyl, trifluoropropyl and vinyl. Its structure can be expressed as follows:

Fluorosilicone has good heat resistance and excellent oil and solvent resistance, such as aliphatic hydrocarbons, aromatic hydrocarbons, chlorinated hydrocarbons, petroleum-based various fuel oils, lubricating oils, hydraulic oils and some synthetic oils at room temperature And the stability at high temperature is better, which is beyond the reach of pure silicone rubber. Fluorosilicone rubber has good low temperature performance, which is a great improvement for pure fluoroelastomer. The temperature range of fluorosilicone rubber containing trifluoropropyl group to maintain elasticity is generally -50℃~+200℃, and its resistance to high and low temperature is worse than that of vinyl silicone rubber, and it will produce toxic gas when heated to above 300℃. In terms of electrical insulation properties, it is much worse than vinyl silicone rubber. Adding an appropriate amount of low-viscosity hydroxyfluorosilicone oil to the fluorosilicone rubber compound, heat treatment of the rubber compound, and adding a small amount of vinyl silicone rubber can significantly improve the process performance and help solve the problems of rubber sticking to rollers and serious storage structure. , Can extend the effective life of the rubber compound. When the methyl phenyl silicone chain link is introduced into the above fluorosilicone rubber, it will help to improve the low temperature resistance performance, and the processing performance is good.

Nitrile silicone rubber is a type of silicone rubber with a nitrile alkyl group (usually β-nitrile ethyl or γ-nitrile propyl) introduced into the side chain. The introduction of polar nitrile groups improves the oil resistance and solvent durability of silicone rubber, but its heat resistance, electrical insulation and processability are reduced. The structural formula of silicone rubber containing methyl, nitrile alkyl and vinyl groups can be expressed as follows:

The type and content of nitrile alkyl groups have a greater impact on the performance of nitrile silicone rubber. For example, silicone rubber containing 7.5% molar γ-nitrile propyl has similar cold resistance to low phenyl silicone rubber, but has lower oil resistance. The base silicone rubber is better. When the content of γ-cyanopropyl group increases to 33~50% moles, the cold resistance is significantly reduced, the oil resistance is improved, and the heat resistance is 200°C. If β-nitrile ethyl is used instead of γ-nitrile propyl, the heat resistance of nitrile silicone rubber can be further improved.

(5) Phenylene and phenyl ether silicone rubber:

Phenylene silicone rubber is a type of silicone rubber in which phenylene groups are introduced into the main chain of polysiloxane. Its structure can be expressed as:

Due to the introduction of phenylene groups, the radiation resistance of silicone rubber is greatly improved. At the same time, the presence of aromatic rings increases the rigidity of the molecular chain, reduces flexibility, increases the glass transition temperature, and decreases cold resistance, while the tensile strength is Increased. Phenylene silicone rubber has excellent high temperature resistance, radiation resistance, high temperature resistance up to 250 ~ 300 ℃, and has good dielectric properties, moisture and mildew resistance and water vapor resistance. In the raw rubber composition of phenylene silicone rubber, it is suitable when the phenylene content is 60%, the phenyl content is 30%, and the methyl content is 10% (the vinyl content is 0.6%). In this case, the vulcanized rubber Has a good comprehensive performance.

The disadvantage of phenylene silicone rubber is that its low temperature performance is poor, and its brittleness temperature is -25℃, which affects its application in some aspects. The low temperature performance of phenylene silicone rubber is much better than that of phenylene silicone rubber. -64~70℃.

Phenylene oxide silicone rubber is a polysiloxane with phenyl ether and phenylene groups introduced into the molecular backbone. Its molecular structure can be expressed as:

Phenylene ether based silicone rubber has good mechanical properties, and the general tensile strength can reach 150~180 kg/cm (that is, 14.7~17.7Mpa is much higher than the strength of vinyl silicone rubber. At the same time, it has excellent radiation resistance and is better than phenylene. Silicone rubber. It can withstand long-term hot air aging at 250°C, and still has high strength after aging. Although the low-temperature performance of phenylene oxide silicone rubber is worse than that of vinyl silicone rubber, it is far superior to phenylene silicone rubber. Its The dielectric properties are close to those of vinyl silicone rubber, but the phenylene ether based silicone rubber has poor oil resistance. It is neither resistant to non-polar petroleum-based oils nor to polar synthetic oils (such as 4109 diester synthetic lubricating oil, phosphoric acid). Performance of ester hydraulic oil. In short, compared with vinyl silicone rubber, phenylene ether based silicone rubber has higher strength and radiation resistance, similar high temperature resistance and dielectric properties, and poor low temperature performance, oil resistance and elasticity. 。Phenyl ether based silicone rubber has good processing performance and can be used to manufacture special requirements of model products and extruded products.

What are the molding conditions from raw rubber to compound rubber?

During the mixing process, due to the low intermolecular cohesion of the raw silicone rubber, the high-reinforcement and high-temperature-resistant silica filler has become the most important compounding agent for silicone rubber. When selecting other compounding agents, nursery rhymes need to consider the requirements for high-temperature resistance. , That is, under the application conditions of silicone rubber, they must not be volatilized, decomposed, carbonized or discolored, etc. In order to maintain the heat resistance of the raw silicone rubber and reduce the breakage of the silicon bond by acid and alkali attack, it is necessary to prevent external acid when compounding , The alkali is brought in, and the acidic substances produced by the decomposition of the peroxide vulcanizing agent must be removed in time!

Put the mixed raw rubber and other ingredients into a vacuum kneader for cooking. At first the temperature is not high. During the stirring process, the temperature is slowly increased by friction. When it reaches 155-160 degrees, it will be more than half of the temperature. Just a few hours,

Put the boiled glue in an open mill to grind it into a roll. Just be careful not to get dirty. The number of rolls only needs 2-3 laps.

After smoothing, it needs to be cooled for about 3-4 hours, and then the glue is filtered through the match net, the purpose is to filter out some solid materials in the glue

3. What is silicone oil? What are the types of silicone oil? What role does it play in silicone raw materials?

Silicone oil is a kind of polyorganosiloxane with chain structure of different polymerization degree. It is obtained by hydrolyzing dimethyldichlorosilane with water to obtain the primary polycondensation ring body. The ring body is cracked and rectified to obtain the low ring body, and then the ring body, the capping agent and the catalyst are put together to obtain each A mixture of different polymerization degrees can be obtained by vacuum distillation to remove low-boiling substances. The most commonly used silicone oil, all organic groups are methyl, called methyl silicone oil. The organic group can also use other organic groups instead of part of the methyl groups to improve certain properties of the silicone oil and apply to various applications. Other common groups are hydrogen, ethyl, phenyl, chlorophenyl, trifluoropropyl and so on. In recent years, organically modified silicone oil has been developed rapidly, and many organically modified silicone oils with special properties have appeared. Silicone oil is generally colorless (or light yellow), odorless, non-toxic, and non-volatile liquid. Silicone oil is insoluble in water, methanol, glycol and-ethoxyethanol. It is miscible with benzene, dimethyl ether, methyl ethyl ketone, carbon tetrachloride or kerosene. It is slightly soluble in acetone, dioxane, ethanol and alcohol. . It has a small vapor pressure, a higher flash point and ignition point, and a lower freezing point. As the number of segments n is different, the molecular weight increases and the viscosity also increases. The solid silicone oil can have a variety of different viscosities, from 0.65 centistokes to millions of centistokes. If you want to make low-viscosity silicone oil, you can use acid clay as a catalyst and telomerize at 180℃, or use sulfuric acid as a catalyst to telomerize at a low temperature to produce high-viscosity silicone oil or viscous materials. Alkaline catalyst. According to the chemical structure, silicone oil is divided into methyl silicone oil, ethyl silicone oil, phenyl silicone oil, methyl hydrogen silicone oil, methyl phenyl silicone oil, methyl chlorophenyl silicone oil, methyl ethoxy silicone oil, and methyl trifluoropropane. Base silicone oil, methyl vinyl silicone oil, methyl hydroxy silicone oil, ethyl hydrogen silicone oil, hydroxy hydrogen silicone oil, cyanide silicone oil, etc.; from the point of use, there are damping silicone oil, diffusion pump silicone oil, hydraulic oil, insulating oil, Heat transfer oil, brake oil, etc. Silicone oil has excellent heat resistance, electrical insulation, weather resistance, hydrophobicity, physiological inertia and small surface tension. In addition, it also has a low viscosity-temperature coefficient and high compression resistance). Some varieties also have radiation resistance. Performance.

Silicone oil mainly plays an anti-aging effect in the raw material of silicone.

4. Types and characteristics of white carbon black

There are mainly two kinds of gas phase method and precipitation method

Gas phase method: mostly starting from silicon tetrachloride, after mixing with chlorine and oxygen (air), at a high temperature above 1000 degrees, hydrogen is first combusted with oxygen to produce water, and then both react with silicon tetrachloride to hydrolyze to produce fine powder After coalescence, capture and deacidification, white carbon black products are obtained.

Advantages of the gas phase method: pure height, less sioh, high reinforcement rate, hot air vulcanization, high vulcanizate transparency, good electrical properties, airtightness, cushion resistance and dynamic fatigue resistance

Gas phase method use: car parts, wires, cables, medical food, high-strength, high-transparent silicone rubber products, gaskets

Precipitation method: starting from water glass, adding hydrochloric acid or sulfuric acid under stirring to neutralize the reaction to obtain SIO2 precipitation, which is filtered, dried, and ground into high-fineness white carbon black

It can also be obtained from alkaline earth metal silicic acid through acid decomposition reaction.

Advantages of the precipitation method: vulcanized rubber has good resilience, compression set, good swelling resistance and processing performance, low price, and the rubber is not easy to structure, and can only be used for filling and reinforcing rubber and plastic products.

Application of precipitation method: general molded products, rubber rollers, oil-resistant gasket materials

5. What are the main moldings of mixed silicone rubber?

Mainly include: compression molding, transfer molding, injection molding, extrusion molding

6. How many release agents are there? What function does it have?

There are pure silicone oil release agents, solution release agents, emulsion release agents, silicone paste release agents, spray release agents, and curable release agents

The main function of the release agent?

The main function is to prevent or reduce mechanical damage when the molded product is ejected from the mold.

7. The difference between silicone and rubber

We usually mistakenly think that silicone is a kind of rubber, but this is wrong.

The main components of the raw materials of silica gel are extracted from silica (such as silicone oil, white carbon black), while the raw materials of rubber are extracted from resin. The cost of silica gel is more expensive than rubber and is more environmentally friendly.

8. The prospects of silicone rubber products

Today, when the topic of environmental protection and health is getting more and more attention, coupled with the wide application of silicone rubber products, I believe that in the near future,

More and more silicone products will be developed to challenge different fields to meet the needs of the market, and will be more and more popular among Chinese and the world!