Silicone rubber can be divided into two categories: kneading silicone rubber and liquid silicone rubber according to its commodity form. Compounding silicone rubber, also commonly known as thermally vulcanized silicone rubber, is composed of polyorganosiloxane with a linear high degree of polymerization (5000-10000 siloxane structural units) with reinforcing fillers, fillers and imparting various properties. The base material prepared with the additive of , when used, is mixed with a vulcanizing agent, and is formed and vulcanized to make a product, which belongs to free radical polymerization.

Liquid silicone rubber is different from mixed rubber. It is based on linear polyorganosiloxane with moderate degree of polymerization, and is a self-leveling or thixotropic base polymer prepared by adding cross-linking agent, reinforcing filler and other compounding agents. Elastomers are vulcanized and cross-linked with moisture in the air as an initiator or under the heating of a catalyst. Liquid silicone rubber can be divided into two systems: condensation type and addition type according to its vulcanization mechanism.

1. Condensed one-component liquid silicone rubber

  Condensation type one-component liquid silicone rubber is made of hydroxyl-terminated polydimethylsiloxane (viscosity 3000-100000 mPa.s) as the base polymer (commonly known as base rubber) and cross-linking agent, catalyst, filler and other characteristic additives. After mixing evenly in a moisture-isolating mixer, it is packaged in a sealed cylinder or hose, extruded under pressure during use, and vulcanized into an elastomer when exposed to moisture in the air, which neither absorbs heat nor does it in the process. exothermic. The curing time depends on the temperature, humidity and thickness of the silicone layer of the curing system.

  Condensed one-component liquid silicone rubber is widely used as adhesive, sealant and coating material due to its adhesiveness. The one-component liquid silicone rubber used as an adhesive should have good adhesion and no corrosion to the substrate after vulcanization, as well as good physical and mechanical properties and electrical properties. Therefore, it is best to use non-corrosion De-alcoholized or de-acetone-type one-component silicone rubber. When choosing a one-component silicone rubber sealant, the main consideration is the modulus after complete vulcanization, the adhesion to the contact substrate, dynamic durability, surface weather resistance, corrosion to the substrate, and pollution to the surrounding area. Among them, the modulus is a parameter that characterizes the mobility of the sealant, which is measured by the elongation stress of the sealant, which is also the grade mark of the sealant. The characteristics of typical varieties are described as follows:

  (1) One-component high modulus type: it is the most widely used variety, and its curing system includes deacetic acid type, deketoxime type, dealcohol type;

  (2) One-component low-modulus type: the adhesiveness is worse than the above-mentioned varieties, and it must be primed on the surface of the contact substrate when used;

  (3) Sealant for plastics: dealcoholization curing system, used for seam sealing of transparent plastic sheets instead of inorganic glass;

  (4) Martin grease type: This sealant is only cured on the surface, so it can be regarded as a very low modulus type, which is used for the joints of difficult-to-bond and brittle substrates such as plexiglass and plastics;

  (5) Structural sealant: It is used for the bonding of glass curtain walls of high-rise buildings, and its adhesion, durability, fatigue resistance, curing speed, etc. have extremely high requirements to adapt to the wind pressure resistance and earthquake resistance of components. and prevent the risk of damage or falling of the glass in extreme cases.


2. Condensed two-component liquid silicone rubber

  Condensed two-component liquid silicone rubber is one component of base rubber, filler and cross-linking agent, and the catalyst is another component. The two are mixed in a certain proportion, and the vulcanization can be carried out at the same time inside and on the surface. The crosslinking agent is usually ethyl orthosilicate and methyltriethoxysilane, and the catalyst is usually an organotin compound. The vulcanization speed is related to the type, dosage and ambient temperature of the crosslinking agent, catalyst, and the vulcanizing machine and its dosage remain unchanged. Increasing the temperature and increasing the ambient humidity can speed up the vulcanization speed and shorten the vulcanization time. According to the type of crosslinking agent, it can be divided into dealcohol condensation, deoxime condensation, dehydration condensation and dehydrogenation condensation.

Its applications are mainly divided into four categories:
1. Electrical and electronic equipment casting packaging, insulation sealing
2. Reproduction of cultural relics and handicrafts, models for imprinting artificial leather surfaces, dental impressions
3. Foam for flame retardant, heat insulation and buffering
4. Sealant for caulking

3. Addition liquid silicone rubber

Addition type liquid silica gel is directly vulcanized into elastomers or gels of various cross-linking densities from liquid state at room temperature or heated under the action of platinum catalyst. The vulcanization rate can be freely adjusted by the catalyst type and inhibitor. However, in the process of storage and vulcanization, it is not allowed to contact organic compounds containing N, P, S and other elements and heavy metal ion compounds containing Sn, Ph, Hg, Bi, As and other heavy metal ions, which will poison the catalyst and hinder the vulcanization.

Addition type liquid silicone rubber does not produce by-products during the cross-linking and vulcanization process, the shrinkage rate is extremely small, it can be deeply cured, and it has no corrosion to the contacted materials. It is the preferred filling material in the electrical and electronic industry. It can be used as a mold for making shoes, with high strength and good tear resistance; it can be used as a flame retardant treatment agent for transformers in TV sets and video recorders, and modified atmosphere films for fresh-keeping fruits and vegetables.