Main components of 1 LSR
LSR is usually composed of base rubber-polymethylvinylsiloxane raw rubber, crosslinking agent-polymethylhydrosiloxane, and catalyst-transition metal complex (such as platinum, nickel, rhodium, etc.). ), and other fillers can be added according to different purposes, such as white carbon black, iron oxide, titanium dioxide, carbon black, etc. In order to prepare transparent LSR, silicone resin can also be added as filler. LSR is a kind of silicone rubber elastomer bridged by SiH group in crosslinking agent and Si-Cl=CH2 group in base rubber.
1. 1 basic adhesive
Polysiloxane raw rubber is the basic rubber of LSR. The molecular weight distribution of LSR raw rubber is wide, generally from several thousand to10-200 thousand. Because components with small molecular weight can reduce viscosity, components with large molecular weight can improve strength. According to the properties of the required vulcanizate, the vinyl content in polymethylvinyl siloxane raw rubber should be controlled within a certain range. The vinyl content is too low, the crosslinking density is low, and the vulcanizate performance is poor; On the contrary, if the crosslinking density is too high, the vulcanizate will become brittle, and the elongation and aging resistance will be poor. When the molecular terminal group of polymethylvinyl siloxane raw rubber is vinyl, it is beneficial to expand the mold and improve the tear resistance; When there is a certain amount of vinyl between the molecular chains and both ends of polymethylvinyl siloxane raw rubber, the molecular module itself will increase during crosslinking, which can further improve the physical and mechanical properties of vulcanized rubber.
1.2 crosslinking agent
Polymethylhydrosiloxane is a cross-linking agent of LSR. The active hydrogen atoms directly connected with silicon atoms in its molecules react with vinyl groups in the raw rubber of polymethylvinylsiloxane, which is the basic rubber, to vulcanize the raw rubber. Usually, there are at least three ≡SiH groups in one molecule, which can obviously improve the flexibility and physical and mechanical properties of vulcanized rubber network structure. When preparing LSR, we should pay attention to the molar ratio of hydrosilyl groups in crosslinking agent to hydrosilyl groups in base glue. Only by matching them can we get the best vulcanizate. Considering the full utilization of vinyl groups and the loss of silicon hydrogen bonds, it is generally appropriate to have a little more hydrogen groups.
1.3 catalyst
The complexes of transition metals in Group VIII of the Periodic Table of Elements have addition catalysis for almost all≡ ≡SiH and≡ ≡SiCH=CH2, but LSR usually uses various forms of platinum and its compounds and complexes. At present, homogeneous catalysts are mainly used, among which the complexes of chloroplatinic acid with olefins, cycloalkanes, alcohols, aldehydes and ethers are widely used. Because of the high activity and selectivity of this catalyst, most of them have high activity, which makes the rubber compound vulcanize too fast and the safe operation time is short. In terms of metallic platinum, the minimum dosage of platinum catalyst should be 1× 10-7 of the total amount of base glue and crosslinking agent. However, considering the platinum poisoning caused by impure system, the actual dosage is generally/kloc-0 /×10-6-2×10-5, which is too high and uneconomical, thus increasing the dosage of inhibitors.
1.4 inhibitor
The raw rubber of polymethylvinyl siloxane can react at room temperature after being mixed with filler, crosslinking agent and catalyst. However, it takes some time to mix and process the rubber mixture. If the reactants are solidified in advance during operation, the required shape and properties will not be obtained. This is especially true for LSR, so it is required that almost no catalytic reaction is carried out before the vulcanization temperature, and then the catalytic reaction is carried out quickly after reaching the vulcanization temperature. The method to inhibit the reaction is usually to add inhibitors. Inhibitors can form some kind of complex with platinum catalyst, which affects the movement of reaction equilibrium. Effective inhibitors can be placed with the rubber compound for a long time and will not decompose until they are heated to the vulcanization temperature. Inhibitors can be divided into two types. One is added to rubber compound as additive, which reacts with platinum to prevent its activity. The other is to prepare complexes (composite catalysts) with inhibitory ligands in advance, thus inhibiting the catalytic activity of platinum. Commonly used are alkynol compounds, nitrogen-containing compounds and organic peroxides with good compatibility. Generally, the addition amount is 1%-5% of the base rubber.
1.5 filler
As the reinforcing filler of LSR, silica can improve the tensile strength of LSR by about 40 times like mixed silicone rubber and condensed silicone rubber. The viscosity of silica is too high for compounds used to package electronic components or to manufacture molds with high strength requirements and good fluidity. Using MQ silicone resin soluble in basic polymer as filler, the viscosity of LSR will not increase significantly, but the strength can be improved significantly, and transparent elastomer can be obtained.
1.6 Other ingredients
Generally speaking, the compounding agent of silicone rubber is suitable for LSR, and reasonable selection of compounding agent is an important way to improve the properties of vulcanized rubber. For example, oxides of alkaline earth metals, rare earth elements and some transition metals and their octoate salts can significantly improve the heat resistance of LSR; Using chloroplatinic acid catalyst or adding flame retardant filler such as quartz powder can improve the flame retardant performance; Different colors of LSR; Can be obtained from colorants such as titanium dioxide, iron oxide, cobalt blue, chrome yellow and alumina. LSR added with carbon black can be used as semiconductor material and so on.
2 vulcanization mechanism
The vulcanization mechanism of LSR is the same as that of ordinary addition silicone rubber. Polydi-silicone siloxane containing vinyl group was used as the basic polymer, and polymethyl hydrogen siloxane with low molecular weight was used as the crosslinking agent, and the network structure was crosslinked by heating in the presence of platinum catalyst.
2. 1 reaction process
Adding the above materials into a clean reaction tank, dissolving with toluene, fully mixing with the basic polymer, heating and depressurizing, and evaporating the toluene. After cooling, the calculated amount of cross-linking agent and catalyst are added, evenly mixed and defoamed, and then vulcanized at 70-80 DEG C for 2-4 hours.
Application and new product development of 3 LSR
LSR has the advantages of excellent electrical insulation, aging resistance, high mechanical strength, good elasticity, rapid and convenient molding, no by-products, no toxicity and tasteless, wide application temperature, safety and hygiene, and strong product extensibility. , can be made into different forms, different uses of serialized and differentiated products; It can be used for packaging or potting electronic components and electrical equipment, such as manufacturing high-voltage power electrical products (such as insulators, lightning arresters, transformer insulation sheaths), wire and cable terminal connectors, etc. It can be used as medical materials to make artificial gills, artificial lungs, artificial lenses, etc. It can be used as dental impression material, for copying cultural relics and handicrafts, as transformer anti-drying treatment agent for TV sets and video recorders, and as fresh-keeping modified atmosphere film for fruits and vegetables. With the development of LSR technology, it will be more and more widely used in daily chemical industry, human beauty, medical care, sports health care, automobile shock absorption and other fields. With the progress of science and technology, more and more new technologies, new products and new applications put forward higher requirements for the properties of silicone rubber. In recent years, LSR with special properties has been developed as follows.
3. 1 Self-lubricating (oil leakage) LSR
After the self-lubricating (oiling) LSR is vulcanized, 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 and reduce or avoid the damage of parts during the assembly of sealing rings. Moreover, after vulcanization molding for 24 hours, the surface and the interior will form a dynamic balance. When the silicone oil on the surface is removed, the silicone oil will slowly precipitate from the inside and restore the lubricity of the surface.
3.2 self-adhesive LSR
Self-adhesive liquid LSR has good adhesion to most substrates without primer. It not only solves the shortcomings of primer coating, but also promotes the research and development and production of silicone rubber and other composite parts. Many parts in automobiles are composed of silicone rubber and engineering plastics or steel. The traditional process of producing such composite parts is to process them separately, then coat adhesive on their bonding surfaces and assemble them. This process not only consumes manpower and occupies inventory, but also is difficult to control the quality, and it can not be used to produce complex composite parts. All these problems can be solved by using self-adhesive LSR and advanced two-step LSR/ETP (engineering thermoplastic) * * * molding process. At present, self-adhesive LSR has been widely used in automobile industry. In addition, there are LSR with self-adhesive and self-lubricating functions, and the composite parts produced by them have all the advantages of self-adhesive LSR and self-lubricating LSR.
3.3 Liquid Foamed Silicone Rubber
Japanese Dow Corning Toray silicone Co., Ltd. disclosed the patent of preparation method of liquid silicone rubber and foamed silicone rubber. Liquid diorganopolysiloxane (containing optional inorganic filler) is mixed with thermal expansion thermoplastic resin hollow particle powder, and enough curing agent is added, and the material is heat-treated at a temperature sufficient to expand the resin powder, thus obtaining foamed silicone rubber with low density and good thermal insulation. Shinetsu Company of Japan has developed a foamed hot vulcanized liquid silicone rubber composition. After storage at 40℃ for 7 days, the composition was still liquid and heated to foam. The foaming ratio is 65,438 0.2 times, and the Shore A hardness is 35.
3.4 high-intensity LSR
Enhanced Silbione of rhodia silicone? (registered trademark) LSR products have short molding cycle, easy demoulding, good transparency, high average tear strength of 30%-50%, small compression deformation, good elasticity and no need for post-vulcanization. Rhodia silbey Aung? There are two kinds of LSR: one is 100% solid, and the other is pure dimethyl siloxane elastomer. Transparent silicon creature? LSR is used for injection molding, and liquid silica gel is very suitable for health care products and consumer goods, such as intravenous injection systems, precision modules, pipe molding, tableware and children's health care products. At present, the products used in industry and medical care include 40, 50, 60 and 70 hardness standard products and high-strength products. In addition, some products can meet the viscosity index standard of USP. The modified liquid silicone rubber Silas-tic 9280-70 of Dow Corning Company has higher heat resistance, faster curing speed and better demoulding property. Cured at 150℃ for about 5 min to form an elastomer with hardness of 70. When the rubber compound is molded, compared with the previous formula, its compression deformation resistance can be improved by about 35%, and its elongation can be improved by about 65438 06%. After the material is molded, its tensile strength can be increased by 10%.
3.5 liquid fluorosilicone rubber
Liquid fluorosilicone rubber has excellent high temperature resistance and oil resistance; The volume expansion rate is low, which can produce connectors that are in direct contact with diesel oil and maintain long-term stability in harsh environment; No need for secondary vulcanization, low permanent deformation rate; Easy to color. It is very suitable for producing oil-resistant parts such as automobiles. In addition, there are high-pressure LSR and standby sulfide LSR. High pressure LSR has the advantages of ultraviolet resistance, ozone resistance and light weight. The remarkable feature of LSR is that the curing time of LSR can be controlled freely.