Processing and production of hot vulcanized silicone rubber products.silicone baby bottle brush manufacturer

　　There are countless silicone materials and processes, and the most popular processes and types of rubber are still hot vulcanized silicone rubber products. Nowadays, many products in different industries are replaced by silicone rubber materials, and the factors of substitution are very clear. , mainly due to product characteristics and various changes and unique performance deployment, and silicone rubber product materials can be divided into various functions and processes for actual deployment, so do you know about process coordination and production processing?

　　Thermal vulcanization silicone rubber refers to silicone rubber with high relative molecular weight (400,000 to 600,000). The organic peroxide is used as a vulcanizing agent, and the organic peroxide is decomposed by heating to generate free radicals, which form cross-links with the organic side groups of the rubber to obtain a vulcanizate.

　　Varieties and Characteristics

　　Thermal vulcanization silicone rubber is the earliest type of rubber, and has developed many varieties. It is divided into the following 7 types according to different chemical compositions:

　　(1) Dimethyl silicone rubber

　　Dimethyl silicone rubber (polydimethylsiloxanerubber), referred to as methyl silicone rubber, is the oldest variety of silicone rubber. It can maintain good elasticity in the temperature range of -60 ~ 250 ℃. Due to the shortcomings of low vulcanization activity, poor process performance, easy foaming of thick-walled products during secondary vulcanization, and large compression deformation at high temperature, it has been replaced by methyl vinyl silicone rubber except for a small amount of fabric coating.

　　(2) Methyl vinyl silicone rubber

　　Methyl vinyl silicone rubber (methylvinylpolysiloxanerubber), referred to as vinyl silicone rubber, is a copolymer of dimethyl siloxane and a small amount of vinyl siloxane. The introduction of a small amount of unsaturated vinyl greatly improves the vulcanization process and finished product properties, especially heat aging resistance and high temperature compression deformation resistance. In the production of silicone rubber, methyl vinyl silicone rubber has the largest output, the widest application, and the most varieties and grades. Silicone rubber, low compression set silicone rubber, conductive silicone rubber, thermally conductive silicone rubber, silicone rubber without two-stage vulcanization, granular silicone rubber, etc.) are also processed and matched based on it.

　　(3) Methyl vinyl phenyl silicone rubber

　　Methylvinylphenylpolysiloxanerubber (Methylvinylphenylpolysiloxanerubber) is referred to as phenyl silicone rubber, which is made by introducing diphenylsiloxane segments (or methylphenylsiloxane segments) into the molecular chain of vinyl silicone rubber. completed. When the phenyl molar fraction is 0.05 to 0.10, it is collectively referred to as low phenyl silicone rubber. At this time, the hardening temperature of the rubber is reduced to the lowest value (-115 °C), so that it has the best low temperature resistance performance, at -100 °C The following is still elastic. With the increase of the phenyl molar fraction, the rigidity of the molecular chain also increases, and the crystallization temperature increases instead.

　　When the mole fraction of phenyl is between 0.15 and 0.25, it is collectively referred to as medium phenyl silicone rubber, which has the characteristics of flame resistance. When the phenyl molar 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 applications requiring low temperature resistance, ablation resistance, high energy radiation resistance, heat insulation, etc. The production and application of medium and high phenyl silicone rubbers are limited due to their difficult processing and poor physical properties.

　　(4) Methyl vinyl trifluoropropyl silicone rubber

　　Methylvinylrtrifluoropropylpolysiloxanerubber, referred to as fluorosilicone rubber, is a fluoroalkyl group (usually trifluoropropyl) introduced into the molecular chain of vinyl silicone rubber, which has excellent oil resistance, Solvent resistance. For example, for aliphatic, aromatic and chlorinated hydrocarbon solvents, various petroleum-based fuel oils, lubricating oils, hydraulic oils and some synthetic oils, the operating temperature range is -50 ~ 250 ° C, at room temperature and high temperature Good stability.

　　(5) Phenylene silicone rubber and phenylene ether silicone rubber

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

　　(6) Nitrile silicone rubber

　　Nitrile silicone rubber (nitrilsiliconerubber) is mainly an elastomer containing methyl-β-nitrile ethyl silicone link or methyl-γ-nitrile propyl silicone link in the molecular chain. Its main characteristics are similar to fluorine. Silicone rubber is similar in that it is oil and solvent resistant and has good low temperature resistance. However, due to the factors that cause the hydrolysis of the nitrile group under the polymerization conditions, the repeatability of the raw rubber is poor, and its application development is limited to a certain extent.

　　(7) Silicon boron rubber

　　Silicon boron rubber (boronsiliconerubber) is a new type of silicone rubber containing a decaborane cage structure on the main chain of the molecule. Work for several hours while still retaining elasticity at -54°C. It is suitable for sealing material in high-speed aircraft and spacecraft. In the late 1960s, the United States had a series of commercial grades of silicon boron rubber, but it was rarely reported after the 1970s. The main reason may be that the process performance of the rubber compound and the elasticity of the vulcanizate are very poor, and the synthesis of carbon and boron rubber is very complicated and toxic. large and expensive.

　　Hot vulcanized silicone rubber products are sold in the form of raw rubber or vulcanized rubber. Generally, they are formulated into rubber compounds with various characteristics for users to choose. According to different characteristics, they 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, and it is a kind of rubber compound with the largest amount and the strongest versatility.

　　(2) High strength type. Use vinyl silicone rubber or low phenyl silicone rubber, use fumed silica or modified silica with larger specific surface area as reinforcing agent, and add appropriate processing aids and special additives to synthesize Improve the performance of the product, improve the cross-linking structure, and improve the tear strength.

　　(3) High temperature resistant type. Using vinyl silicone rubber or low phenyl silicone rubber, the type of reinforcing agent and heat-resistant additives can be properly selected to obtain silicone rubber resistant to high temperature of 300-350 °C.

　　(4) Low temperature type. Using low phenyl silicone rubber, the brittle temperature can reach -120℃, and it is still elastic at -90℃.

　　(5) Low compression set type. Vinyl silicone rubber is mainly used, and organic peroxide specially used for vinyl is used as vulcanizing agent. When the compression rate is 30%, the permanent deformation after compressing at 150 ° C for 24-72 hours is 7.0%-15% (common silicone rubber). 20% to 30%).

　　(6) Wire and cable type. Vinyl silicone rubber is mainly used, and fumed silica with good electrical insulation performance is used as reinforcing agent, which has good extrusion process performance.

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

　　(8) Flame retardant type. Using vinyl silicone rubber, adding halogen-containing or platinum compounds as flame retardants, it has good flame resistance.

　　(9) Conductive type: vinyl silicone rubber is used, acetylene carbon black or metal powder is used as filler, and high temperature vulcanization or addition vulcanization method is selected to obtain silicone rubber with a volume resistivity of 2.0 to 100Ω·cm.

　　(10) Heat shrinkable type. A thermoplastic material with a certain melting temperature or softening temperature is added to the vinyl silicone rubber, and the thermal shrinkage rate of the silicone rubber compound can reach 35% to 50%.

　　No secondary vulcanization type. It is prepared by using vinyl silicone rubber with high vinyl mass fraction, by controlling the pH value of raw rubber and compounding agent, and adding special additives.

　　(11) Sponge type. Adding organic foaming agents such as nitroso compounds, azo and diazo compounds to vinyl silicone rubber can make sponges with uniform foaming.

　　In addition, there are thermal conductive silicone rubber, fluorescent silicone rubber and medical grade silicone rubber products sold abroad.

　　With the continuous development of the application of silica gel, the varieties and grades of rubber materials are increasing day by day, and too many grades will cause confusion in production, storage, transportation and sales. Some silicone rubbers have been correspondingly divided into several typical base rubbers and several characteristic additives (including pigments, vulcanizing agents, etc.) for sale. Users can mix them according to certain formulas and mixing techniques according to their needs. The final silicone product. This method not only makes the variety simple and clear, but also has large production batch, stable quality, lower cost and improved competitiveness.

　　2. Cooperation

　　The compounding agent of hot vulcanized silicone rubber mainly includes reinforcing agent, vulcanizing agent and some special auxiliaries. Generally, only 5 to 6 components are needed to form a practical formula. The formulation design should consider the following points:

　　(1) Silicone rubber is raw rubber with high degree of saturation. Usually, it cannot be vulcanized with sulfur. Organic peroxides should be used as vulcanizing agents. Therefore, the rubber compound must not contain active substances (such as grooves) that can interact with peroxide decomposition products. carbon black, some organic accelerators and antioxidants, etc.), otherwise it will affect the vulcanization.

　　(2) Silicone rubber products are generally used at high temperatures, and their compounding agents should be stable at high temperatures, and 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 chemical reagents such as trace acids or alkalis, resulting in the reduction of heat resistance of silicone rubber. Therefore, the acidity and alkalinity of the compounding agent and the acidity of the peroxide decomposition product must be considered when selecting the compounding agent, so as not to affect the performance of the vulcanizate.

　　The choice of raw rubber

　　For silicone rubber products with general operating temperature requirements (-70 ~ 250 ℃), vinyl silicone rubber can be used; when the operating temperature of the products is high (-90 ~ 300 ℃), low phenyl silicone rubber can be used ; When the product requires high and low temperature resistance and fuel or solvent resistance, fluorosilicone rubber should be used.

　　Curing agent and curing mechanism

　　Vulcanizing agent

　　The vulcanizing agents used for thermal vulcanization of 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 rapidly decompose to generate free radicals at higher vulcanization temperatures, thereby causing crosslinking of silicone rubber. Commonly used vulcanizing agents for silicone rubber are shown in Table 3.

　　These peroxides can be divided into two categories according to their activity level: one is general-purpose type, with high activity, which can vulcanize all kinds of silicone rubber; the other is vinyl-specific type, because of its low activity, Can only vulcanize vinyl-containing silicone rubber.

　　The amount of peroxide is affected by various factors such as the variety of raw rubber, the type and amount of filler, and the processing technology. As long as the required degree of crosslinking can be achieved, the vulcanizing agent should be used as little as possible. For vinyl silicone rubber (the vinyl molar fraction is 0.0015), the commonly used dosage ranges of various peroxides are as follows (based on 100 parts of raw rubber): BP 0.5 to 1 part; DCBP 1 to 2 parts ; DTBP 1-2 parts; DCP 0.5-1 part; DBPMH 0.5-1 part; TBPB 0.5-1 part.

　　As the mass fraction of vinyl increases, the amount of peroxide should decrease. The amount of peroxide in glue, extruded product compound and adhesive compound should be larger than that in molding compound. In some cases, two kinds of peroxides are used together, which can reduce the amount of vulcanizing agent, and can appropriately reduce the vulcanization temperature and improve the vulcanization effect.

　　Vulcanization mechanism

　　When the silicone rubber is vulcanized with peroxide, the crosslinking of the peroxide to the silicone rubber is carried out by a free radical reaction between two activated methyl groups or vinyl groups. The crosslinking of dimethyl silicone rubber is carried out according to the following reaction formula:

　　The crosslinking of vinyl silicone rubber is carried out according to the following reaction formula:

　　In addition to the above-mentioned peroxide vulcanization, silicone rubber can also be vulcanized by radiation with high-energy rays. Radiation vulcanization is also carried out by a free radical mechanism. When the molar fraction of vinyl in the raw rubber is high (0.01) or used in combination with other rubbers, sulfur vulcanization can also be used, but the performance is extremely poor.

　　Reinforcing agents and related mechanisms

　　The strength of unreinforced silicone rubber vulcanizate is very low, only about 0.3MPa, and has no practical use value. Adding appropriate reinforcing agent can make the strength of silicone rubber vulcanizate reach 3.9-9.8MPa, 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 adverse effects of peroxide vulcanization (especially with substances with acid and alkali) on silicone rubber.

　　The reinforcing fillers for silicone rubber can be divided into reinforcing fillers and weakly reinforcing fillers according to their reinforcing effects. The particle size of the former is 10-50nm and the specific surface area is 70-400m2·g-1 , the reinforcement effect is better; the latter particle size is 300 ~ 1000nm, the specific surface area is below 30m2·g-1, the reinforcement effect is poor.

　　Reinforcing filler

　　The reinforcing filler of silicone rubber mainly refers to synthetic silica, also known as silica. Silica can be divided into fumed silica and precipitation silica. 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, and can be used in combination with other reinforcing agents or weak reinforcing agents. Prepare rubber compounds for different application requirements.

　　Compared with the silicone rubber compound reinforced with fumed silica, the mechanical strength of the compound reinforced with precipitated silica is slightly lower, and the dielectric properties (especially the dielectric properties after moisture) are poor, but It has good heat aging resistance and low cost of mixing rubber. When the mechanical strength of the product is not high, the precipitated silica can be used alone or in combination with the fumed silica.

　　Using treated white carbon black as a reinforcing agent, the mechanical strength of the rubber compound is high, the performance of the mixing and re-melting process is good, and the transparency of the vulcanizate is also good, so it is widely used in medical products. In addition, this compound has good adhesion and good solubility, and can be used for bonding and making glue.

　　Weak reinforcing filler

　　Weak reinforcing fillers can also be called inert fillers, which have little reinforcing effect on silicone rubber. They are generally not used alone in silicone rubber, but are used in combination with silica to adjust the hardness of silicone rubber and improve the rubber compound. Process performance and oil resistance and solvent resistance of vulcanizate, reduce the cost of rubber compound. Commonly used weak reinforcing agents are diatomite, quartz powder, zinc oxide, ferric oxide, titanium dioxide, zirconium silicate and calcium carbonate.

　　The dosage and physical properties of commonly used reinforcing agents for silicone rubber are shown in Table 4.

　　Auxiliary

　　(1) Structure control agent

　　The silicone rubber compound reinforced with fumed silica will change during storage.

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