Addition liquid silicone rubber (LSR) is a higher-grade variety of silicone rubber. Compared with condensation liquid silicone rubber, it has no by-products during the vulcanization process, extremely small shrinkage, and can penetrate deep into the skin. It has the advantages of vulcanization and its sealing performance at high temperatures is better than that of the condensation type. In addition, LSR also has the outstanding advantages of simple process and low cost. This is because liquid silicone rubber has a small molecular weight, low viscosity, and is easy to process and form. It can eliminate mixing, preforming, post-finishing and other processes, is easy to automate, saves energy and labor, and has a short production cycle and high efficiency. Therefore, although the raw material price of LSR is slightly higher than that of ordinary silicone rubber, the total cost is lower than that of ordinary silicone rubber, especially when manufacturing small products, its superiority in this aspect is even more evident.
1 The main components of LSR
LSR is usually composed of base glue-polymethylvinylsiloxane raw rubber and cross-linking agent-polymethylhydrogensiloxane , Catalyst - composed of complexes of transition metals (such as platinum, nickel, rhodium, etc.). According to different uses, other fillers can also be added, such as gas phase method or precipitation method silica, iron oxide, titanium dioxide and carbon black. wait. In order to prepare transparent grade LSR, silicone resin can also be added as a filler. LSR is a type of silicone rubber elastomer obtained by forming a bridge between the SiH group in the cross-linking agent and the Si-Cl=CH2 group in the base rubber.
1.1 Basic glue
Polyvinyl siloxane raw rubber is the basic glue of LSR. LSR's raw rubber has a wide molecular weight distribution, generally ranging from thousands to 100,000-200,000. Because components with small molecular weight can reduce viscosity, components with large molecular weight can increase strength. According to the required properties of the vulcanized rubber, the vinyl content in polymethylvinylsiloxane raw rubber should be controlled within a certain range. If the vinyl content is too low, the cross-linking density will be small, and the vulcanized rubber will have poor performance; on the contrary, if the cross-linking density is too high, the vulcanized rubber will become brittle, and the elongation and aging resistance will be poor. When the terminal group of the polymethylvinylsiloxane rubber molecule is vinyl, it is beneficial to mold expansion and improves tear resistance; there is a certain amount of vinyl between the polymethylvinylsiloxane rubber molecular chains and at both ends. When cross-linking is accompanied by the growth of the molecular model itself, this can further improve the physical and mechanical properties of the vulcanized rubber.
1.2 Cross-linking agent
Polymethylhydrogensiloxane is the cross-linking agent of LSR. The active hydrogen atoms in its molecule are directly connected to the silicon atoms and the basic glue—— The vinyl groups in the polymethylvinylsiloxane raw rubber undergo an addition reaction to vulcanize the raw rubber. Usually there are at least 3 or more ≡SiH groups in a molecule, so that the flexibility and physical and mechanical properties of the vulcanized rubber network structure can be significantly improved. When preparing LSR, attention should be paid to the molar ratio of the silicone group in the cross-linking agent to the silicone group in the base rubber. Only by matching them can the vulcanized rubber with the best performance be obtained. Considering the full utilization of vinyl groups and the loss of silicon-hydrogen bonds, it is generally appropriate to have a slight excess of hydrogen groups.
1.3 Catalysts
The complexes of transition metals in Group VIII of the periodic table almost have addition catalytic effects on ≡SiH and ≡SiCH=CH2, but in LSR Platinum, its compounds and complexes, are commonly used in various forms. At present, homogeneous catalysts are mainly used, among which the most commonly used are complexes formed by chloroplatinic acid and alkenes, cycloalkanes, alcohols, aldehydes, ethers, etc. Because this kind of catalyst has high activity and selectivity, most of them have high activity, which makes the rubber vulcanization too fast and the safe operation time is short. If calculated in terms of metallic platinum, the minimum amount of platinum catalyst should be 1×10-7 of the total amount of base glue and cross-linking agent. However, considering the situation of platinum poisoning due to the impurity of the system, the actual dosage is generally 1×10-6-2×10-5. The dosage is too high, which is uneconomical and increases the dosage of inhibitors.
1.4 Inhibitors
Polymethylvinylsiloxane raw rubber can react at room temperature after being mixed with fillers, cross-linking agents and catalysts. The mixing and processing of rubber materials takes a certain amount of time. If the reactants are solidified in advance during operation, the required shape and properties will not be obtained.
This is even more required for LSR, so it is required that there is almost no catalytic reaction before the vulcanization temperature, and the catalytic reaction is quickly carried out after reaching the vulcanization temperature. The reaction is usually inhibited by the addition of inhibitors. Inhibitors can form a certain form of complex with the platinum catalyst, affecting the movement of the reaction equilibrium. Effective inhibitors can be placed with the rubber compound for a long time and will only decompose when heated to the vulcanization temperature. Inhibitors are divided into two categories. One type is added to the rubber as an additive to interact with platinum to block its activity; the other type is a complex (composite catalyst) with inhibitory ligands made in advance to inhibit the activity of platinum. Catalytic activity. The more commonly used ones are acetylenic alcohol compounds with good compatibility, nitrogen-containing compounds, organic peroxides, etc. The general amount added is 1-5% of the base glue quality.
1.5 Filler
As a reinforcing filler for LSR, silica black can increase the tensile strength of LSR by about 40 times, just like mixing silicone rubber and condensation silicone rubber. For rubber materials for potting electronic components or making molds that have high strength requirements and good fluidity, the viscosity of white carbon black is too high. Using MQ-type silicone resin that is soluble in the base polymer as filler can make the viscosity of LSR not increase significantly but the strength can be significantly improved, and a transparent elastomer can be obtained.
1.6 Other compounding agents
General silicone rubber compounding agents are suitable for LSR. Appropriate selection of compounding agents is an important way to improve the performance of vulcanized rubber. For example, oxides of alkaline earth metals, rare earth elements and certain transition metals and octoates of these metals can significantly improve the heat resistance of LSR; using chloroplatinic acid catalysts or adding flame retardant fillers such as quartz powder can improve the flame retardancy. Performance; titanium dioxide, iron oxide, cobalt blue, chrome yellow, aluminum oxide and other colorants can produce LSR of different colors; LSR added with carbon black can be used as semiconductor materials, etc.
2 Vulcanization mechanism
The vulcanization mechanism of LSR is the same as that of ordinary addition silicone rubber. It also uses vinyl-containing polydiorganosiloxane as the base polymer and low molecular weight. Polymethylhydrogensiloxane is used as a cross-linking agent and is heated and cross-linked into a network structure in the presence of a platinum catalyst.
2.1 Reaction process
Describe how the material is added to a clean reaction tank and dissolved in toluene, then thoroughly mixed with the base polymer, heated and decompressed, and toluene is evaporated. After cooling, add the calculated amount of cross-linking agent and catalyst. After mixing and defoaming, maintain it at 70-80°C for 2-4 hours to vulcanize into transparent silicone rubber.
3 Application and new product development of LSR
LSR has excellent electrical insulation properties, aging resistance, high mechanical strength, good elasticity, fast and convenient molding, and no by-products in the reaction. It is non-toxic and tasteless, has a wide operating temperature, is safe and hygienic, and has strong product extensibility. It can be made into serialized and differentiated products of different shapes and uses; it can be used for packaging or potting of electronic components and electrical equipment, such as Manufacture of high-voltage power electrical products (such as insulators, lightning arresters, insulating jackets of transformers), terminal joints of wires and cables, etc.; can be used as medical materials to make artificial gills, artificial lungs, and intraocular lenses; can be used as dental impression materials , used for copying cultural relics and handicrafts, used in transformer drying treatment agents for televisions and video recorders, and air withering films for keeping fruits and vegetables fresh, etc. With the development of LSR technology, its applications in daily chemicals, human beauty, medical and health, sports health, automobile shock absorption and other fields will become more and more widespread. With the advancement of science and technology, more and more new technologies, new products and new applications have put forward higher requirements for the performance of silicone rubber. The following types of LSR with special properties have been developed in recent years.
3.1 Self-lubricating (oil-bleeding) LSR
After vulcanization of self-lubricating (oil-bleeding) LSR, silicone oil molecules will slowly precipitate and distribute on the surface of silicone rubber parts, forming Many tiny oil beads. These oil beads can reduce friction during the assembly process of the seal ring and reduce or avoid damage to the parts. Moreover, the surface and the interior will form a dynamic balance after 24 hours of vulcanization and molding. When the silicone oil on the surface is removed, the silicone oil will slowly precipitate from the interior to restore the lubricity of the surface.
3.2 Self-adhesive LSR
Self-adhesive LSR can have good adhesion to most substrates without using primer. It not only solves the shortcomings of using primers, but also promotes the development and production of composite parts of silicone rubber and other materials. Many parts in cars are made of silicone rubber and engineering plastics or steel and other materials. The traditional process for producing such composite parts is to process them separately, then apply adhesive to the bonding surfaces of the two, and then assemble them. This process is not only labor-intensive, takes up inventory, and difficult to control quality, but also cannot be used to produce composite parts with complex designs. All of these problems can be solved using self-adhesive LSR and using an advanced two-step LSR/ETP (Engineering Thermoplastic) molding process. Currently, self-adhesive LSR has been increasingly widely used in the automotive industry. In addition, there are also LSRs that have both self-adhesive and self-lubricating properties. The composite parts produced with them have the advantages of both self-adhesive LSR and self-lubricating LSR.
3.3 Liquid foamed silicone rubber
Japan’s Dow Corning Toray Silicone Co., Ltd. has disclosed patents for the preparation methods of liquid silicone rubber and foamed silicone rubber. Mix liquid diorganopolysiloxane (containing optionally added inorganic fillers) with heat-expanded thermoplastic resin hollow particle powder, add a sufficient amount of curing agent, and heat-treat the material at a temperature sufficient to expand the resin powder, i.e. Foamed silicone rubber with low density and good thermal insulation properties is produced. Japan's Shin-Etsu Company has developed a self-foaming thermally vulcanized liquid silicone rubber composition. The composition is still liquid after being stored at 40°C for 7 days. It foams when heated, with a foaming rate of 1.2 times and a Shore A hardness of 35.
3.4 High-strength LSR
Rhodia Silicone Company’s reinforced Silbione? (registered trademark) LSR products have a short molding cycle, easy demoulding, and good transparency. The average tear strength is high by 30-50, the compression deformation is small, the elasticity is good, and no post-vulcanization is required. Rhodia Silbione? LSR comes in two varieties: one is 100 solid and the other is pure dimethyl silicone elastomer. Clear Silbione? LSR is used for injection molding, and liquid silicone is ideal for making health care and consumer products such as intravenous systems, precision molds, tube forming, tableware and children's health care products. The products currently used in industry and health care are 40, 50, 60 and 70 hardness standard products and high strength products. Additionally, several products are available that meet USP grade viscosity index standards. Dow Corning's reformed liquid silicone rubber Silas-tic 9280-70 has higher heat resistance, faster curing, and better demoulding. Cure at 150°C for approximately 5 minutes to form an elastomer with a hardness of 70. When molded, the compound improves compression set resistance by approximately 35% and elongation by approximately 16% compared to previous formulations. The material's tensile strength can be increased by 10 when molded.
3.5 Liquid fluorosilicone rubber
Liquid fluorosilicone rubber has excellent high temperature resistance and excellent oil resistance; it has a low volume swelling rate and can produce products in direct contact with diesel. Connectors can maintain long-term stability in harsh environments; no need for secondary vulcanization, low permanent deformation rate; and easy to color. Ideal for producing oil-resistant parts for automobiles, etc. In addition, there are high-pressure grade LSR and weather-cured LSR. High-voltage LSR has the advantages of UV resistance, ozone resistance, and light weight. The distinctive feature of seasonal vulcanization LSR is that the vulcanization time of LSR can be freely controlled.