Classification and characteristics of silicone rubber

Classification and characteristics of silicone rubber

1.1 Classification

Silicone rubber can be divided into three types according to its curing mechanism: hot curing type, room temperature curing type and addition reaction type.

1.2 Characteristics

(1) High and low temperature resistance

Among all rubbers, silicone rubber has the widest working temperature range (- 100~350 ℃). For example, properly matched vinyl silicone rubber or low phenyl silicone rubber can still retain elasticity after being aged for thousands of hours at 250 ℃ or hundreds of hours at 300 ℃ in hot air; The low phenyl silicone rubber vulcanizate can remain elastic after being aged for tens of hours at 350 ℃ in hot air. Its glass transition temperature is – 140 ℃, and its vulcanizate is still elastic at – 70~100 ℃. When silicone rubber is used as the thermal protective coating on the inner wall of the rocket nozzle, it can withstand thousands of degrees of high temperature instantaneously. See Table 1 for the continuous service life of silicone rubber under high temperature.

(2) Ozone aging resistance, oxygen aging resistance, light aging resistance and weather aging resistance

The performance of silicone rubber vulcanizate has no significant change after being exposed to outdoor sun for several years under free state. The comparison of ozone aging resistance between silicone rubber and other rubber is shown in Table 2.

(3) Electrical insulation performance

The electrical insulation performance of silicone rubber vulcanizate changes little when affected with moisture, frequency change or temperature rise, and the silicon dioxide generated after combustion is still an insulator. In addition, there are few carbon atoms in the molecular structure of silicone rubber, and carbon black is not used as filler, so it is not easy to scorch during arc discharge, and it is very reliable in high-voltage applications. Its corona resistance and arc resistance are excellent. Its corona resistance life is 1000 times that of PTFE and arc resistance life is 20 times that of fluororubber.

(4) Special surface properties and physiological inertia

The surface energy of silicone rubber is smaller than that of most organic materials, and it has low moisture absorption. The water absorption rate after long-term immersion in water is only about 1%. Its physical properties do not decline, and its mildew resistance is good. It does not bond with many materials, and it can play an isolation role. Silicone rubber is tasteless, non-toxic, has no adverse effect on human body, has slight reaction with body tissues, and has excellent physiological inertia and physiological aging.

(5) High permeability

Compared with other polymer materials, silicone rubber has good air permeability. The permeability of nitrogen, oxygen and air at room temperature is 30-40 times higher than NR; It is selective for gas permeation, for example, the permeability of carbon dioxide is about 5 times that of oxygen.

(6) Biomedical performance

The characteristics of the molecular structure of silicone rubber make it have excellent biomedical properties, which has been proved by the successful application of a large number of animal and human experiments.

2 Heat vulcanized silicone rubber

Heat vulcanized silicone rubber refers to silicone rubber with high relative molecular weight (400000-600000). The organic peroxide is used as the curing agent. After heating, the organic peroxide is decomposed to produce free radicals, and cross-linked with the organic side groups of the rubber to obtain the vulcanizate.

2.1 Variety and characteristics

Thermal vulcanized silicone rubber is the earliest type of rubber, which has developed into many varieties. It can be divided into the following 7 types according to chemical composition:

(1) Dimethyl silicone rubber

Dimethylsiloxane rubber is the oldest variety of silicone rubber.

It can keep good elasticity in the temperature range of – 60~250 ℃. Due to the shortcomings of low vulcanization activity, poor process performance, thick wall products are easy to foam during secondary vulcanization, and high temperature compression deformation, at present, except for a small amount used for fabric coating, it has been replaced by methyl vinyl silicone rubber.

(2) Methyl vinyl silicone rubber

Methylvinyl polysiloxane rubber (abbreviated as vinyl silicone rubber) is copolymerized by dimethylsiloxane and a small amount of vinyl siloxane. The vinyl mole fraction is generally 0.001~0.003. The introduction of a small amount of unsaturated vinyl improved the vulcanization process and the properties of the finished product, especially the heat aging resistance and high temperature compression resistance. In the production of silicone rubber, methyl vinyl silicone rubber has the largest output, the widest application, and the most varieties and brands. In addition to a large number of general-purpose rubber compounds, all kinds of special silicone rubber and silicone rubber with processing characteristics (such as high-strength silicone rubber, low compression permanent deformation silicone rubber, conductive silicone rubber, thermal conductive silicone rubber, non two-stage vulcanized silicone rubber, granular silicone rubber, etc.) are also processed based on it.

(3) Methyl vinyl phenyl silicone rubber

Methylvinyl phenyl polysiloxane rubber (abbreviated as phenyl silicone rubber) is made by introducing diphenylsiloxane chain (or methyl phenyl siloxane chain) into the molecular chain of vinyl silicone rubber. When the phenyl mole fraction is 0.05 ～ 0.10, it is generally referred to as low phenyl silicone rubber. At this time, the hardening temperature of the rubber is reduced to the lowest value (- 115 ℃), making it have the best low temperature resistance, and it still has elasticity below – 100 ℃. With the increase of phenyl mole fraction, the rigidity of molecular chain also increases, and its crystallization temperature rises instead.

When the phenyl mole fraction is 0.15~0.25, it is collectively referred to as medium phenyl silicone rubber, which has flame resistance characteristics. When the phenyl mole fraction is above 0130, it is collectively referred to as high phenyl silicone rubber, which has excellent radiation resistance. Phenyl silicone rubber is used in places where low temperature resistance, ablation resistance, high energy radiation resistance and heat insulation are required. The production and application of medium phenyl silicone rubber and high phenyl silicone rubber are limited due to their difficult processing and poor physical properties.

(4) Methyl vinyl trifluoropropyl silicone rubber

Methylvinyl trifluoropropyl polysiloxane rubber (methyl vinyl trifluoropropyl polysiloxane rubber for short) is introduced into the molecular chain of vinyl silicone rubber (generally trifluoropropyl), which has excellent oil resistance and solvent resistance. For example, for aliphatic, aromatic and chlorinated hydrocarbon solvents, petroleum based fuel oils, lubricating oils, hydraulic oils and some synthetic oils, their operating temperature ranges from – 50 ℃ to 250 ℃, and they are stable at room and high temperatures.

(5) Phenylene silicone rubber and phenylene ether silicone rubber

Phenylene polysiloxane rubber and phenylene ether silicone rubber are new varieties of silicone rubber containing phenylene or phenyl ether chain segments in the molecular chain. They are developed to meet the requirements of nuclear power plants and navigation technology. Their main characteristics are high tensile strength and resistance to γ X-ray, high temperature resistance (above 300 ℃), but cold resistance is not as good as low phenyl silicone rubber.

(6) Nitrile silicone rubber

Nitrile silicone rubber mainly introduces methyl – β－ Nitrile ethyl siloxane chain link or methyl – γ－ An elastomer of cyanopropyl siloxane chain link, its main characteristics are similar to those of fluorosilicone rubber, that is, it is oil resistant, solvent resistant and has good low temperature resistance. However, due to the factors that cause nitrile hydrolysis under polymerization conditions, the repeatability of raw rubber is poor, and its application and development are limited.

(7) Silicone boron rubber

Boron silicone rubber is a new type of silicone rubber with decaborane cage structure on the main chain of the molecule. It has high heat aging resistance and can work at 400 ℃ for a long time. It can work continuously for several hours at 420~480 ℃, while it can remain elastic at – 54 ℃. It is suitable for sealing materials in high-speed aircraft and spacecraft. At the end of the 1960s, there was a series of brands of silicone boron rubber in the United States, but few reports were made after the 1970s. The main reason may be that the processing properties of the rubber compound and the elasticity of the vulcanizate are very poor, and the synthesis of carbon boron is very complex, toxic, and expensive.

The hot vulcanized silicone rubber is sold in the form of raw rubber or vulcanizate, and is generally prepared into rubber compounds with various characteristics for users to choose. According to different characteristics, it can be divided into the following categories:

(1) Universal type (general strength type). It is composed of vinyl silicone rubber and reinforcing agent. The physical properties of vulcanizate are medium. It is a kind of compound with the largest amount and the strongest versatility.

(2) High strength type. Vinyl silicone rubber or low phenyl silicone rubber shall be used. Vapor phase white carbon black with large specific surface area or modified white carbon black shall be used as reinforcing agent, and comprehensive improvement measures such as adding appropriate processing aids and special additives shall be taken to improve the cross-linking structure and tear strength.

(3) High temperature resistant type. Silicone rubber with high temperature resistance of 300~350 ℃ can be prepared by using vinyl silicone rubber or low phenyl silicone rubber, the type of reinforcing agent and heat resistant additive.

(4) Low temperature type. The low phenyl silicone rubber is used. The brittleness temperature reaches – 120 ℃, and it still has elasticity at – 90 ℃.

(5) Low compression permanent deformation type. Vinyl silicone rubber is mainly used, and the special organic peroxide of vinyl is used as the curing agent. When the compression ratio is 30%, the permanent deformation after 24~72h compression at 150 ℃ is 7.0%~15% (20%~30% for ordinary silicone rubber).

(6) Wire and cable type. It mainly adopts vinyl silicone rubber, and uses fumed silica with good electrical insulation performance as the reinforcing agent, which has good extrusion process performance.

(7) Oil and solvent resistant. Ammonia silicone rubber is mainly used, which is generally divided into two categories: general purpose type and high-strength type.

(8) Flame retardant. The rubber compound composed of vinyl silicone rubber and halogen-containing or platinum compounds as flame retardant has good flame resistance.

(9) Conductive type. Silicone rubber with volume resistivity of 2.0 ～ 100 Ω• cm can be obtained by using vinyl silicone rubber, acetylene carbon black or metal powder as filler, and high temperature vulcanization or addition vulcanization.

(10) Heat shrinkable. When thermoplastic materials with certain melting temperature or softening temperature are added to vinyl silicone rubber, the thermal shrinkage of silicone rubber compound can reach 35%~50%.

No two-stage vulcanization type. The vinyl silicone rubber with high vinyl content was prepared by controlling the pH value of raw rubber and compounding agent and adding special additives.

(11) Sponge type. By adding organic foaming agents such as nitroso compounds, azo and diazo compounds into vinyl silicone rubber, a sponge with uniform foaming can be prepared.

In addition, thermal conductive silicone rubber, fluorescent silicone rubber and medical grade compound are also sold abroad.

With the continuous development of the use of silicone rubber, the varieties and brands of rubber materials are increasing. Too many brands will cause confusion in production, storage, transportation and sales. Some factories have correspondingly divided multiple varieties into several typical base rubbers and several characteristic additives (including pigments, vulcanizing agents, etc.) for sale. Users can obtain the final product by mixing them according to certain formula and mixing technology as required.

This method not only makes the variety simple and clear, but also has large production batch, stable quality, low cost and improved competitiveness.

2.2 Cooperation

The compounding agent of heat vulcanized silicone rubber mainly includes reinforcing agent, vulcanizing agent and some special additives. Generally, only 5-6 components are needed to form a practical formula. The following points should be considered in the formulation design:

(1) Silicone rubber is raw rubber with high saturation, which cannot be vulcanized with sulfur. Organic peroxide should be used as the vulcanizing agent. Therefore, the rubber compound must not contain active substances that can react with the decomposition products of peroxide (such as carbon black produced by tank process, some organic accelerants and antioxidants), otherwise the vulcanization will be affected.

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

(3) Silicone rubber is easy to cause the cleavage and rearrangement of siloxane bond under the action of trace acid or alkali and other chemical reagents, which leads to the reduction of heat resistance of silicone rubber. Therefore, the acid-base property and the acidity of peroxide decomposition products must be taken into account when selecting the compounding agent, so as not to affect the performance of the vulcanizate.

2.2.1 Selection of raw rubber

Vinyl silicone rubber can be used for silicone rubber products with general service temperature requirements (- 70~250 ℃); When the use temperature of the product is required to be high (- 90~300 ℃), low phenyl silicone rubber can be used; When the products are required to be resistant to high and low temperatures and fuel or solvent, fluorosilicone rubber shall be used.

2.2.2 Vulcanizing agent and curing mechanism

(1) Vulcanizing agent

Vulcanizing agents used for heat vulcanized silicone rubber mainly include organic peroxides, aliphatic azo compounds, inorganic compounds and high-energy rays, among which organic peroxides are the most commonly used. This is because organic peroxides are generally stable at room temperature, but can quickly decompose to produce free radicals at a higher curing temperature, thus making silicone rubber cross-linked. Common curing agents for silicone rubber are shown in Table 3.

These peroxides can be divided into two categories according to their activity: one is universal type with high activity, which can vulcanize all kinds of silicone rubber; The other type is vinyl special type. Because of its low activity, it can only vulcanize silicone rubber containing vinyl.

The amount of peroxide is affected by many factors, such as raw rubber type, filler type and amount, processing technology, etc. As long as the required crosslinking degree can be achieved, curing agent should be used as little as possible. For vinyl silicone rubber (vinyl mole fraction is 0.0015) molding compound, the common dosage range of various peroxides is as follows (calculated by 100 phr of raw rubber): BP 0.5~1 phr; 1~2 copies of DCBP; 1~2 copies of DTBP; DCP 0.5~1; 0.5~1 portion of DBPMH; 0.5~1 TBPB.

The amount of peroxide should decrease with the increase of vinyl content. The amount of peroxide in the rubber slurry, extruded products and adhesives should be greater than that in the molding compound. In some cases, the use of two kinds of peroxides together can reduce the amount of curing agent, properly reduce the curing temperature and improve the curing effect.