1 The main components of LSR
LSR is usually composed of a base rubber - polymethyl vinyl siloxane raw rubber, crosslinking agent - polymethyl hydrogen silicone, catalyst - transition metals (e.g. platinum) and other metals. -transition metal (such as platinum, nickel, rhodium, etc.) complexes, etc. According to different applications, other fillers can be added, such as fumed or precipitated silica, iron oxide, titanium dioxide and carbon black. In order to produce transparent grade LSR, can also add silicone resin as a filler.LSR is a class of silicone rubber elastomers obtained by crosslinking agent in the SiH group and the base rubber in the formation of bridges between the Si-Cl=CH2 group.
1.1 Base Gum
Polymerized vinyl silicone raw gum is the base gum of LSR. The molecular weight distribution of the raw rubber of LSR is wide, generally from thousands to 100-200 thousand. This is because components with small molecular weights can reduce viscosity and components with large molecular weights can increase strength. According to the performance of the desired vulcanized rubber, the vinyl content in the raw polymethyl vinyl siloxane rubber should be controlled within a certain range. Vinyl content is too low, the crosslinking density is small, the vulcanized rubber performance is poor; on the contrary, the crosslinking density is too large, the vulcanized rubber becomes brittle, the elongation, aging resistance is not good. Polymethyl vinyl siloxane raw rubber molecular end group for vinyl, is conducive to expanding the mold and improve the tear resistance; Polymethyl vinyl siloxane raw rubber molecular chain and both ends of a certain amount of vinyl, crosslinking accompanied by molecular mode of its own growth, which can further improve the physical and mechanical properties of the vulcanized rubber.
1.2 Crosslinking agent
Poly(methylhydrosiloxane) is the crosslinking agent of LSR, the reactive hydrogen atoms in the molecule that are directly connected to the silicon atoms and the vinyl group in the base rubber - poly(methylvinylsiloxane) raw rubber carry out the addition reaction to make the rubber vulcanized. Usually there are at least three ≡ SiH groups in a molecule, in order to make the vulcanized rubber mesh structure of the flexibility and physical and mechanical properties have been significantly improved. In the preparation of LSR, it is necessary to pay attention to the molar ratio of SiH groups in the crosslinking agent to the silicone vinyl group in the base rubber, and only by matching them can we get the vulcanized rubber with the best performance. Considering the full utilization of vinyl and the loss of silicon-hydrogen bonding, it is generally appropriate to have a slight excess of hydrogen base.
1.3 Catalyst
The complexes of transition metals of group VIII in the periodic table of the elements are almost always catalytic for the addition of ≡SiH and ≡SiCH=CH2, but various forms of platinum and its compounds and complexes are usually used in LSR. Currently, homogeneous catalysts are mainly used, of which the more commonly used are complexes formed by chloroplatinic acid with chain olefins, cycloalkanes, alcohols, aldehydes, and ethers. Because this catalyst has high activity and selectivity, but most of the activity is high, so that the rubber vulcanization is too fast, safe operation time is short. If the metal platinum, platinum catalyst dosage of the minimum should be the total amount of base rubber and crosslinking agent 1 × 10-7. But taking into account the impurity of platinum poisoning due to the system, its actual dosage is generally 1 × 10-6-2 × 10-5, the dosage is too high, not only uneconomical, but also to increase the dosage of inhibitors.
1.4 Inhibitors
Poly(methylvinylsiloxane) raw rubber and fillers, crosslinking agents and catalysts can be mixed after the reaction at room temperature. The mixing and processing of adhesive materials require a certain amount of time, the reactants in the operation of the first curing, you can not get the desired shape and properties. This is especially true for LSR, which requires little or no catalytic reaction before the vulcanization temperature, and rapid catalytic reaction after the vulcanization temperature is reached. The method of inhibiting the reaction is usually the addition of an inhibitor. Inhibitors can form certain forms of complexes with the platinum catalyst, affecting the equilibrium shift of the reaction. Effective inhibitors can be placed with the rubber *** with a considerable period of time, only heated to the vulcanization temperature before decomposition. Inhibitors are divided into two categories, one is added as an additive to the rubber, and platinum role in blocking its activity; the other is made in advance with inhibitory ligands complex (composite catalyst), thereby inhibiting the catalytic activity of platinum. The use of more common is the compatibility of alkyne alcohol compounds, nitrogen-containing compounds, organic peroxides and so on. The general amount added is 1%-5% of the mass of the base rubber.
1.5 Fillers
Silica as a reinforcing filler for LSR, the same as blended silicone rubber, condensed silicone rubber, can make the tensile strength of the LSR increased by about 40 times. For the higher strength requirements, but also has a good fluidity of electronic components potting with adhesive materials or making molds with adhesive materials, silica viscosity is too high. The use of MQ silicone resin that can be dissolved in the base polymer as a filler, can make the viscosity of the LSR does not rise significantly but the strength of a significant increase, and can get a transparent elastomer.
1.6 Other compatibilizers
General silicone rubber compatibilizers are suitable for LSR, appropriate selection of compatibilizers is an important way to improve the performance of vulcanized rubber. For example, alkaline earth metals, rare earth elements and oxides of certain transition metals and octanoates of these metals can significantly improve the heat resistance of LSR; with chloroplatinic acid catalyst or add quartz powder and other flame retardant fillers, can improve the flame retardant properties; titanium dioxide, iron oxide, cobalt blue, chromium yellow, alumina, and other coloring agents can be obtained in different colors of the LSR; add carbon black LSR can be used as semiconductor materials and so on.
2 Vulcanization mechanism
LSR vulcanization mechanism is the same as the ordinary addition of molding silicone rubber, is also a vinyl-containing polydiorganosiloxane as the base polymer, low molecular weight polymethylhydrosiloxane as a crosslinking agent, in the presence of platinum catalyst heated crosslinking into the mesh structure.
2.1 Reaction process
The recounting material is added to a cleaned reaction tank and dissolved by adding toluene, followed by thorough mixing with the base polymer, and the toluene is evaporated under heat and reduced pressure. After cooling, add the calculated amount of crosslinking agent and catalyst, after mixing and defoaming, maintained at 70-80 ℃ for 2-4h, can be vulcanized into transparent silicone rubber.
3 LSR applications and new product development
LSR has excellent electrical insulation properties, aging resistance, high mechanical strength, good elasticity, fast and convenient molding, reaction without by-products, non-toxic and tasteless, the use of a wider range of temperatures, and safety and hygiene, the product can be extended and other advantages, can be made into a series of different forms, different uses, differentiated products; can be used for electronic components, electrical equipment encapsulation or packaging. Electronic components, electrical equipment encapsulation or potting, such as the manufacture of high-voltage electrical products (such as insulators, surge arresters, transformers, insulation jacket), wire and cable terminal joints, etc.; can be used as medical materials, made of artificial gills, artificial lungs and artificial crystals, etc.; can be used as a dental mold materials, used to reproduce cultural relics and artifacts for television, VCR transformer drying treatment agent, fruits and vegetables preservation of freshness gas withering It can be used as dental impression material for reproduction of cultural relics and handicrafts, transformer drying treatment agent for TV and VCR, and air withering film for preservation of fruits and vegetables. With the development of LSR technology, its application will be more and more extensive in the fields of daily chemical industry, human body beauty, medical and health care, sports health care, automobile shock absorption and so on. With the progress of science and technology, more and more new technologies, new products and new applications on the performance of silicone rubber puts forward higher requirements, LSR developed in recent years with special properties are mainly the following.
3.1 Self-lubricating (oil penetration) LSR
Self-lubricating (oil penetration) LSR after vulcanization, silicone oil molecules will slowly precipitate, distributed on the surface of the silicone rubber parts, the formation of many small oil beads. These oil beads can reduce friction and minimize or avoid damage to the parts during the assembly process of the seal. And the surface and the interior in the vulcanization molding 24h will form a dynamic equilibrium, when the surface of the silicone oil is removed, the interior will slowly precipitate silicone oil, to restore the lubricity of the surface.
3.2 Self-adhesive LSR
Self-adhesive liquid LSR can have good adhesion with most substrates without using primer. It not only solves the disadvantage of using primer, but also promotes the development and production of composite parts made of butyl silicone rubber and other materials. Many parts in automobiles are made from composites of silicone rubber and materials such as engineering plastics or steel. Production of such composite parts of the traditional process is first processed separately, and then in the two bonding surface coated with adhesive, and then assembled. This process is labor-intensive, inventory-intensive, difficult to control, and cannot be used to produce composite parts with complex designs. All of these problems can be solved with the use of self-bonding LSR and the advanced two-step LSR/ETP (Engineering Thermoplastics)*** molding process. Currently, self-bonding LSR has been increasingly used in the automotive industry. In addition, there are both self-adhesive and self-lubricating LSR, with their production of composite parts with both self-adhesive LSR and self-lubricating LSR a variety of advantages.
3.3 liquid foaming silicone rubber
Dow Corning Toray Silicone Co., Ltd. disclosed the liquid silicone rubber and foaming silicone rubber preparation method patent. Liquid two organic polysiloxane (containing selective addition of inorganic fillers) and heating expansion of thermoplastic resin hollow particles of powder mix and add a sufficient amount of curing agent, in enough to make the resin powder expansion of the temperature of the material heat treatment, that is, the production of small density and good adiabatic properties of foamed silicone rubber. Japan's Shin-Etsu developed from foaming hot vulcanization of liquid silicone rubber compositions, the compositions at 40 ℃ storage 7 d is still liquid, heating foaming, foaming rate of 1.2 times the Shore A hardness of 35.
3.4 high-strength LSR
Rhodia (Rhodia) Silicone enhancement of Silbione? (registered trademark) LSR products molding Short cycle time, easy demolding, good transparency, 30%-50% higher average tear strength, low compression set, good elasticity, no post vulcanization required Rhodia Silbione? LSR is available in two grades: 100% solid and pure dimethylsilicone elastomers. The transparent Silbione LSR is used for injection molding, while the liquid silicone is ideal for use in healthcare and consumer products such as intravenous systems, precision moldings, tube molding, tableware and children's healthcare products. Currently available for industrial and healthcare applications are 40, 50, 60 and 70 durometer standard and high strength products. In addition, several products are available to meet Pharmacopeia-grade viscosity index standards.Dow Corning's reformulated liquid silicone rubber, Silas-tic 9280-70, has higher heat resistance, faster cure and better mold release. Curing at 150°C for about 5 min forms an elastomer with a hardness of 70. When molded, the compound improves its resistance to compression set by about 35% and increases elongation by about 16% compared to previous formulations. The material increases its tensile strength by 10% when molded.
3.5 Liquid Fluorosilicone Rubber
Liquid Fluorosilicone Rubber has excellent resistance to high temperatures and excellent oil resistance; has a low volumetric swelling rate, can produce direct contact with diesel fuel connectors, in harsh environments can maintain long-term stability; without the need for a second vulcanization, has a low permanent deformation rate; easy to color. It is very suitable for the production of oil-resistant parts for automobiles and so on. There are also high-pressure grade LSR and weathering vulcanized LSR, which are UV and ozone resistant and lightweight. The distinctive feature of weathering LSR is that the vulcanization time of LSR can be freely controlled.