Silicone knowledge of high temperature vulcanized silicone rubber

- Mar 28, 2018-

The vulcanization of high-temperature vulcanized silicone rubber is generally divided into two stages. In the first stage, silicon raw rubber, reinforcing agents, additives, vulcanizing agents, and structure control agents are mixed, and then the kneaded material is pressurized in a metal mold. Heat forming and curing, the pressure is about 50 kg/cm2, the temperature is 120 ~ 130 °C, the time is 10 ~ 30 minutes, the second stage is to remove the silicone rubber from the mold, put the oven, in the 200 ~ 250 Roasting at °C for several hours to 24 hours causes further vulcanization of the rubber while decomposing and volatilizing the organic peroxide.

The reinforcing fillers of silicone rubber are various types of silica, which can increase the strength of the vulcanizate by a factor of ten. The addition of various additives mainly reduces the cost of the rubber, improves the properties of the rubber, and imparts various special properties such as flame retardant and electrical conductivity to the vulcanizate. The cross-linking agents are various organic peroxides such as benzoyl peroxide, benzoyl 2,4-dichloroperoxide, dicumyl peroxide, 2,5-dimethyl-2,5-bis Tetrabutyl peroxy hexane etc. The structural control agent is added to prevent the compounding compound from being placed for a long time, resulting in "structured" hardening of the compound and difficult curing, and methyl hydroxy silicone oil or diphenyl dihydroxy silane may be used as a structure control agent. .

High-temperature vulcanized silicone rubber is a high-molecular-weight (typically 400-800,000) polyorganosiloxane (ie, raw rubber) with reinforcing fillers and other various additives. The organic peroxide is used as a vulcanizing agent. (Molded, extruded, calendered) or injection molded and cross-linked into rubber at high temperatures. This kind of rubber is generally abbreviated as silicone rubber.

Silicone rubber classification

The pendant groups on the silicone rubber backbone may be methyl, ethyl, vinyl, phenyl, trifluoropropyl, and the like. The most commonly used is methyl, and other groups can also be introduced to improve processability and other properties. Therefore, depending on the nature of the side groups and rubber formulations, various types of silicone rubber can be obtained, which can be generally divided into the following types: general type (containing methyl and vinyl), high temperature and low temperature type (including Phenyl, methyl and vinyl), low compression set (methyl and vinyl), low shrinkage (devolatization) and solvent resistance (fluorosilicone rubber). Here are some important types of silicone rubber.

First, dimethyl silicone rubber

Dimethyl silicone rubber is one of the earliest commercially available silicone rubbers. It can maintain good elasticity in the range of -60 to 200°C, good aging resistance, excellent electrical insulation properties, moisture resistance, shock resistance, and physiological inertness. characteristic.

 

Dimethyl silicone rubber is mainly used for fabric coating, but also can be made into a variety of extrusion and rolling products for the electromechanical, aerospace, automotive and medical industries. However, due to the low vulcanization activity of dimethylsilicone rubber, it is difficult to vulcanize when it is used to manufacture thick products, and the internal layer is prone to blistering and the permanent deformation is high at high temperature. Therefore, it has been replaced by methyl vinyl silicone rubber.

Second, methyl vinyl silicone rubber

Since the introduction of a small amount of vinyl in the macromolecular structure of the silicone rubber can greatly improve the vulcanization processability of the silicone rubber, most of the currently used silicone rubbers contain a vinyl group. Methyl vinyl silicone rubber is the most common type of silicone rubber and currently dominates the production of silicone rubber at home and abroad. Because it introduces a partially unsaturated vinyl on the side chain, its processability and physical and mechanical properties are superior to those of the two base silicone rubbers. In addition to its general characteristics of dimethyl silicone rubber, it also has a wide range of use temperature, can maintain a good elasticity in a range of 60 ~ 260 °C, it is easier to cure than dimethyl silicone rubber, with a smaller compression permanent Deformation, better resistance to solvent swelling and stability against high-pressure steam, and excellent cold resistance, etc., but also because of the use of less active peroxide for curing, thereby reducing the bubble generated during vulcanization and poor rubber stability weakness. Therefore, the use of methyl vinyl silicone rubber can be made of larger thickness products.

Methyl vinyl silicone rubber is the most widely used type of silicone rubber. In recent years, a variety of high-performance and special-purpose silicone rubbers have emerged, mostly based on vinyl silicone rubber, such as high-strength silicone rubber, low Permanently deformed silicone rubber, no post-vulcanized silicone rubber, heat-resistant conductive silicone rubber, and medical silicone rubber. Methyl vinyl silicone rubber is widely used in the aviation industry as a protective layer for gaskets, sealing materials, and fragile and shock-proof components. In the electrical industry, it can be used as an advanced insulating material for electronic components, dynamic sealing rings for high temperature potentiometers, etc. Underground long-distance communication equipment seals; In medical, because of the small physiological response of methyl vinyl silicone rubber to the human body, non-toxic, it is used as a surgical plastic, artificial heart valves, blood vessels and so on.

Third, fluorine silicone rubber

Fluorosilicone rubber refers to γ-trifluoropropylmethylpolysiloxane. Fluorosilicone rubber has excellent chemical resistance, solvent resistance and lubricating oil resistance. The silicone rubber has a low expansion rate in a non-polar solvent, good cold resistance and thermal stability, and good resistance to ignition, so it is in the aircraft. , rockets, missiles, spaceflight, petrochemicals used as contact with fuel oil and lubricating oil hoses, gaskets, seals, fuel tank villages, etc., can also be used to manufacture corrosion-resistant clothes, gloves and coatings, adhesives, etc. .

Fourth, methyl phenyl vinyl silicone rubber

Methylphenyl vinyl silicone rubber is a product obtained by introducing a methylphenylsilyloxy chain segment or a diphenylsilyloxy segment into the molecular chain of methylvinyl silicone rubber.

The introduction of phenyl groups on the side groups of polysiloxanes, due to the disruption of the regularity of the dimethylsiloxane structure, greatly reduces the crystallization temperature of the polymers and extends the low temperature application range of the polymer materials. Therefore, the methylphenyl vinyl silicone rubber has excellent compressive permanent deformation, a wide temperature range, oxidation resistance, weather resistance, shock resistance, moisture resistance, and good electrical insulation properties in addition to the methyl vinyl silicone rubber. Resistance to low temperature, ablation and radiation resistance. These properties vary with the phenyl content of the molecular chain. Generally, the phenyl content (the ratio of phenyl to silicon atoms) is 5 to 10%. It is said to be low-phenyl silicone rubber, which has unique cold resistance properties and can maintain rubber elasticity at -70 to -100°C. It is the best low-temperature performance of all rubbers, plus it has both methyl vinyl silicon. The advantages of rubber and its low cost make it a great alternative to methyl vinyl silicone rubber. Phenyl content of 20 to 40% is called phenyl silicone rubber. It has excellent flame resistance and can self-extinguish in the event of fire. Phenyl content of 40 to 50% is called high-phenyl silicone rubber, it has excellent radiation resistance, resistance to γ-ray 1xI08 Roentgen. In general, as the phenyl content increases, the rigidity of the silicone rubber molecular chain gradually increases, and the low temperature resistance of the silicone rubber gradually decreases, but as the phenyl content increases, the flame resistance and the anti-radiation of the vulcanized rubber increase. Illumination.

Methylphenyl vinyl silicone rubber is one of the important materials in the aerospace industry, cutting-edge technology and other sectors of the national economy. It can be used to make various molded and extruded products for use as a cold-resistant rubber in the aviation industry and for ablation resistance. Heat-resistant aging or radiation resistant seals, pads, tubes, rods, etc.