Introduction to the types and characteristics of room temperature vulcanized silicone rubber

Room temperature vulcanized silicone rubber (RTV) is a new type of silicone elastomer that came out in the 1960s. The most notable feature of this rubber is that it can be cured in situ without heating at room temperature, such as pressing, and it is extremely convenient to use. Therefore, as soon as it came out, it quickly became an important part of the whole silicone product. RTV silicone rubber is now widely used as adhesives, sealants, protective coatings, potting and molding materials, and has its uses in various industries.
RTV silicone rubber is known as liquid silicone rubber because of its low molecular weight. Its physical form is usually a flowable fluid or a viscous paste, and its viscosity is between 100 and 1,000,000 centistokes. According to the requirements of use, the pre-vulcanization compound can be made into a self-leveling pouring material or a non-flowing but scratchable putty. The filler used in room temperature vulcanized silicone rubber is similar to high temperature vulcanized silicone rubber, and it is reinforced with white carbon black, so that the vulcanized rubber has a breaking strength of 10-60 kg/cm2. Adding different additives can make the rubber compound have different specific gravity, hardness, strength, fluidity and thixotropy, and make the vulcanizate have various special properties such as flame retardant, electrical conductivity, thermal conductivity, and ablation resistance.
RTV silicone rubber can be divided into one-component and two-component RTV silicone rubber according to its packaging method, and can be divided into condensation type and addition type according to the vulcanization mechanism. Therefore, RTV silicone rubber can be divided into three types according to the composition, vulcanization mechanism and use process, namely, one-component RTV silicone rubber, two-component condensation type RTV silicone rubber and two-component addition type RTV silicone rubber. .
The raw rubber of one-component and two-component condensation type RTV silicone rubber is α,ω-dihydroxy polysiloxane; the addition type RTV silicone rubber contains alkenyl and hydrogen side groups (or end groups) Polysiloxane is also known as low temperature vulcanized silicone rubber (LTV) because it can achieve good curing effect at a temperature slightly higher than room temperature (50-150 ℃) during curing.
These three series of RTV silicone rubber have their own advantages and disadvantages: the advantage of one-component RTV silicone rubber is that it is easy to use, but the deep curing speed is difficult; the advantage of two-component RTV silicone rubber is that it does not emit heat during curing. , The shrinkage rate is small, no expansion, no internal stress, curing can be carried out at the same time inside and on the surface, and deep vulcanization is possible; the vulcanization time of addition type RTV silicone rubber is mainly determined by the temperature, so the vulcanization can be controlled by adjusting the temperature. speed.
1.One-component RTV silicone rubber
The vulcanization reaction of one-component RTV silicone rubber is triggered by moisture in the air. The commonly used cross-linking agent is methyltriacetoxysilane, its Si-O-C bond is easily hydrolyzed, the acetoxy group is combined with the hydrogen group in the water to form acetic acid, and the hydroxyl group in the water is moved to the original acetoxy group. position, it becomes trihydroxymethylsilane. Trihydroxymethylsilane is extremely unstable, and is easily condensed with linear organosilicon whose end groups are hydroxyl groups to form a cross-chain structure. Usually, the raw silicone rubber containing silanol end groups and various compounding agents such as fillers, catalysts, and cross-linking agents are put into a sealed hose, extruded from the container when used, and vulcanized with the help of moisture in the air. Elastomer, while releasing low molecular weight.
In addition to methyltriacetoxysilane, the crosslinking agent can also be a silane containing an alkoxy group, an oxime group, an amine group, an amide group, and a ketone group. When it is cross-linked with an alkoxy group, it releases alcohol, which is called dealcoholized one-component RTV silicone rubber. Depending on the agent, the one-component RTV silicone rubber can be deacidified, deoxime, dealcoholized, deamined, deamidated and deketone, etc., but deacidification is currently the most widely used. 
The vulcanization time of one-component RTV silicone rubber depends on the vulcanization system, temperature, humidity and thickness of the silicone rubber layer. Increasing the temperature and humidity of the environment can speed up the vulcanization process. Under typical environmental conditions, generally after 15 to 30 minutes, the surface of the silicone rubber can be non-sticky, and the adhesive layer with a thickness of 0.3 cm can be cured within a day. The depth and strength of the cure will gradually build up over three weeks.
One-component RTV silicone rubber has excellent electrical properties and chemical inertness, as well as heat resistance, natural aging resistance, flame resistance, moisture resistance, breathability and other properties. They can maintain elasticity for a long time in the range of -60 ~ 200 ℃. It does not absorb or release heat during curing, has a small shrinkage rate after curing, and has good adhesion to materials. Therefore, it is mainly used as an adhesive and sealant, and other applications include form-in-place gaskets, protective coatings and caulking materials. Many one-part silicone rubber adhesive formulations exhibit self-bonding properties on a variety of materials such as most metals, glass, ceramics, and concrete, such as bare aluminum, with shear strengths up to 200 psi 2. The tear strength can reach 20 lbs.ft./inch2 (0.35 joules/cm2). When bonding is difficult, a primer can be applied to the substrate to improve the bond strength. The primer can be a reactive silane monomer or resin, which, when cured on the substrate, forms a layer of modified suitable on silicone-bonded surfaces.
Although the one-component room temperature vulcanized silicone rubber is easy to use, because its vulcanization is based on the moisture in the atmosphere, the thickness of the vulcanized rubber is limited, and it can only be used in applications requiring a thickness of less than 6 mm.
The vulcanization reaction of one-component RTV silicone rubber is gradually carried out from the surface to the depth. The thicker the adhesive layer, the slower the curing. When the deep part needs to be cured quickly, the step-by-step vulcanization method can be used, and some rubber compound can be added each time, and then the material can be added after vulcanization, which can reduce the total vulcanization time. The addition of magnesium oxide accelerates the vulcanization of deep layers.
The vulcanization reaction of one-component RTV silicone rubber is to vulcanize it into an elastomer by interacting with moisture in the air. With different chain agents, one-component RTV silicone rubber can be deacidified, deoxime, dealcoholized, deamined, deamidated and deketone. The vulcanization time of one-component RTV silicone rubber depends on the vulcanization system, temperature, humidity and thickness of the silicone rubber layer. Increasing the temperature and humidity of the environment can speed up the vulcanization process. Under typical environmental conditions, after 15 to 30 minutes, the surface of the silicone rubber can be tack-free, and the adhesive layer with a thickness of 0.3 cm can be cured within a day. The depth and strength of the cure will gradually build up over three weeks.
One-component RTV silicone rubber has excellent electrical properties and chemical inertness, as well as heat resistance, natural aging resistance, flame resistance, moisture resistance, breathability and other properties. They can maintain elasticity for a long time in the range of -60~200℃. It does not absorb or release heat during curing, has a small shrinkage rate after curing, and has good adhesion to materials. Therefore, it is mainly used as an adhesive and sealant, and other applications include form-in-place gaskets, protective coatings and caulking materials. Many one-part silicone rubber adhesive formulations exhibit self-bonding properties on a variety of materials such as most metals, glasses, ceramics, and concrete. When bonding is difficult, a primer can be applied to the substrate to improve the bonding strength. The primer can be reactive silane monomers or resins. When they are cured on the substrate, a modified layer suitable for Silicone bonded surfaces. Although the one-component room temperature vulcanized silicone rubber is easy to use, because its vulcanization depends on the moisture in the atmosphere, the thickness of the vulcanized rubber is limited, and it can only be used in applications requiring a thickness of less than 6 mm. The vulcanization reaction of one-component room temperature vulcanized silicone rubber is gradually carried out from the surface to the depth. The thicker the adhesive layer, the slower the curing. When the deep part needs to be cured quickly, the step-by-step vulcanization method can be used, and some rubber compound can be added each time, and then the material can be added after vulcanization, which can reduce the total vulcanization time. The addition of magnesium oxide accelerates the vulcanization of deep layers.
2.Two-component condensation type RTV silicone rubber
Two-component condensation type RTV silicone rubber is the most common RTV silicone rubber. The raw rubber is usually hydroxyl-terminated polysiloxane, which is combined with other compounding agents and catalysts to form a rubber compound.
The vulcanization reaction of two-component RTV silicone rubber is not initiated by moisture in the air, but by a catalyst. Usually, the raw silica gel, filler, and cross-linking agent are packaged as one component, and the catalyst is packaged separately as another component, or other combination methods are used, but the catalyst and the cross-linking agent must be packaged separately. Regardless of the packaging method, curing begins only when the two components are thoroughly mixed together. The commonly used cross-linking agent is ethyl orthosilicate, and the catalyst is dibutyltin dilaurate. Appropriate fillers and additives are added according to the desired properties of the final product. In recent years, many countries have banned the addition of dibutyltin in food bags and blood plasma bags because dibutyltin dilaurate is a medium-toxic substance, which has basically been replaced by low-toxic octyltin.
The vulcanization time of the two-component condensation type RTV silicone rubber mainly depends on the type, amount and temperature of the catalyst. The higher the amount of catalyst, the faster the vulcanization, and the shorter the rest time. At room temperature, the shelf time is generally a few hours. To prolong the shelf time of the rubber compound, the method of cooling can be used. Two-component condensation type RTV silicone rafter adhesive takes about a day to fully cure at room temperature, but only 1 hour at 150°C. The curing speed can be significantly increased by the synergistic effect of using the accelerator γ-aminopropyltriethoxysilane.
Two-component RTV silicone rubber can maintain elasticity for a long time in the temperature range of 65~250℃, and has excellent electrical properties and chemical stability, water resistance, ozone resistance, weather resistance, and simple usage, strong process applicability , therefore, widely used as potting and molding material. Various electronic and electrical components are coated and potted with RTV silicone rubber, which can protect against moisture (anti-corrosion, shock, etc.). It can improve performance and stability parameters. Two-component RTV silicone rubber is especially suitable for deep potting. Sealing material and has a faster vulcanization time, which is superior to one-component room temperature vulcanized silicone rubber.
The vulcanization reaction of two-component RTV silicone rubber is not initiated by moisture in the air, but by a catalyst. Usually, the rubber compound and the catalyst are packaged separately as a component. Curing begins to occur only when the two components are thoroughly mixed together. The vulcanization time of the two-component condensation type RTV silicone rubber mainly depends on the type, amount and temperature of the catalyst. The higher the amount of catalyst, the faster the vulcanization, and the shorter the rest time. At room temperature, the shelf time is generally a few hours. To prolong the shelf time of the rubber compound, the method of cooling can be used. Two-component condensation type RTV silicone rafter adhesive takes about a day to fully cure at room temperature, but only 1 hour at 150°C. The speed of curing can be significantly increased by the use of accelerators for synergistic effects.
Two-component RTV silicone rubber can maintain elasticity for a long time in the temperature range of 65~250 °C, and has excellent electrical properties and chemical stability. , therefore, widely used as potting and molding material. Various electronic and electrical components are coated and potted with RTV silicone rubber, which can protect against moisture (anti-corrosion, shock, etc.). It can improve performance and stability parameters. Two-component RTV silicone rubber is especially suitable for deep potting. It is better than the one-component RTV silicone rubber and has a faster vulcanization time. The two-component RTV silicone rubber has excellent anti-stick properties after vulcanization, and the shrinkage rate during vulcanization is extremely small. Therefore, it is suitable for making soft molds for casting epoxy resin, polyester resin, polystyrene, polyurethane, vinyl plastic, paraffin, low melting point alloy, etc. In addition, using two-component room temperature vulcanized silicone rubber Its high simulation performance can replicate various exquisite patterns on cultural relics. Two-component room temperature vulcanized silicone rubber should be paid attention to when using: first weigh the rubber and catalyst separately, and then mix them in proportion. The mixing process should be carefully operated to avoid To minimize the amount of entrained gas. After the rubber compound is mixed evenly (the color is uniform), the bubbles can be removed by standing or decompression (vacuum degree of 700 mm Hg). After all the bubbles are discharged, at room temperature or at the specified temperature It will be vulcanized into silicone rubber after being placed under it for a certain period of time.
3.Two-component addition type RTV silicone rubber 
Two-component addition type RTV silicone rubber is divided into elastic silicone gel and silicone rubber. The former has lower strength and the latter has higher strength. Their vulcanization mechanism is based on the addition reaction (hydrosilation reaction) between the vinyl group (or propylene group) on the end group of the silicone raw rubber and the silicon hydrogen group on the cross-linking agent molecule. In this reaction, no by-products are released. Since no low-molecular substances are released during the cross-linking process, the addition type RTV silicone rubber does not shrink during the vulcanization process. This type of vulcanizate is non-toxic, high mechanical strength, has excellent hydrolytic stability (even under high pressure steam), good low compression set, low flammability, deep vulcanization, and vulcanization speed can be controlled by temperature, etc. Therefore, it is a type of silicone rubber that is vigorously developed at home and abroad. Two-component RTV silicone rubber can maintain elasticity for a long time in the temperature range of 65~250℃, and has excellent electrical properties and chemical stability, water resistance, ozone resistance, weather resistance, and simple usage, strong process applicability , therefore, widely used as potting and molding material. Various electronic and electrical components are coated and potted with RTV silicone rubber, which can protect against moisture (anti-corrosion, shock, etc.). It can improve performance and stability parameters. Two-component RTV silicone rubber is especially suitable for deep potting. Sealing material and has a faster vulcanization time, which is superior to one-component room temperature vulcanized silicone rubber.
Two-component RTV silicone rubber has excellent anti-sticking properties after vulcanization, and the shrinkage rate is very small during vulcanization. Therefore, it is suitable for making soft molds for casting epoxy resin, polyester resin, polystyrene, Molds for polyurethane, vinyl, paraffin, low melting point alloys, etc. In addition, a variety of exquisite patterns can be reproduced using the high simulation performance of two-component RTV silicone rubber. For example, in the reproduction of cultural relics, it can be used to replicate ancient bronzes, and in the production of artificial leather, it can be used to replicate the skin patterns of animals such as snakes, pythons, crocodiles, and pangolins, which has the effect of mixing the fake with the real.
Two-component RTV silicone rubber should pay attention to several specific issues when using it: first, weigh the base material, crosslinking agent and catalyst separately, and then mix them in proportion. Usually the two components should be provided in different colors, so that the mixing of the two components can be visually observed, and the mixing process should be performed carefully to minimize the amount of entrained gas. After the rubber compound is mixed evenly (the color is uniform), the bubbles can be removed by standing or decompression (vacuum degree of 700 mm Hg). eraser.
In addition to the methyl group, the side groups on the main chain of the two-component room temperature vulcanized silicone rubber siloxane can be replaced with other groups such as phenyl, trifluoropropyl, cyanoethyl, etc. to improve its low temperature resistance and heat resistance. , radiation resistance or solvent resistance. At the same time, heat-resistant, flame-retardant, heat-conductive, and conductive additives can be added as required to prepare a silicone rubber with ablation resistance, flame-retardant, heat-conducting and electrical-conductivity properties.
(1) Methyl RTV Silicone Rubber Methyl RTV silicone rubber is an old variety of general-purpose silicone rubber, which has the advantages of water resistance, ozone resistance, arc resistance, corona resistance and weathering resistance. It can be used in a temperature range of 60 to 200 °C. Therefore, it is widely used as a filling and sealing material for electronic and electrical components, and a moisture-proof, shock-proof, high- and low-temperature resistant filling and sealing material for instruments and meters. It can also be used to make molds for casting polyester resin, epoxy resin and low melting point alloy parts. Can also be used as dental impression material. Coated with methyl RTV silicone rubber on cotton cloth and paper bags, it can be made into conveyor belts and packaging bags for conveying viscous items. Rubber Technology Network
(2) Methyl Biphenyl RTV Silicone Rubber Methyl biphenyl RTV silicone rubber has a wider temperature range than methyl RTV silicone rubber in addition to the excellent properties of methyl RTV silicone rubber (- 100～250℃). The low-phenyl RTV silicone rubber (108-1) with a phenyl content of 2.5 to 5% can maintain elasticity at a low temperature of -120 °C, and is currently the best low-temperature performance among silicone rubbers; ~20% room temperature glue (108-2) has good radiation resistance, ablation resistance and self-extinguishing properties. If a certain amount of heat-resistant additives such as Fe2O3 is added to it, the thermal aging performance can be improved. It is suitable for 250 Use or do ablation-resistant putty coatings and encapsulation materials at high temperatures above ℃.
 
Like other room temperature glues, methyl phenyl room temperature glue can be used for dipping, impression and release. If you want to increase the adhesion with other materials, you must perform surface treatment on the material to be adhered before using the material. The steps of surface treatment are as follows: clean the surface of the material with acetone solvent 1 to 2 times, and then treat it with a surface agent 1 ～2 times, bake in a 60 ℃ oven for several minutes, at this time, a layer of film with little stickiness is formed on the surface of the material, and the glue can be applied.
(3) Methyl block RTV silicone rubber Methyl block RTV silicone rubber is a modified variety of methyl RTV silicone rubber. It is made of polydimethylsiloxane (107 glue) with hydroxyl end cap and A copolymer of methyltriethoxysilane oligomer (molecular weight 3-5). Under the catalysis of dibutyltin dilaurate, the hydroxyl group in polydimethylsiloxane and the ethoxy group in polymethyltriethoxysilane are condensed to form a polymer with a three-way structure. Compared with methyl room temperature vulcanized silicone rubber, it has higher mechanical strength and adhesive force, and can be used for a long time in the temperature range of 70 to 200 °C.
Methyl block RTV silicone rubber has the properties of shockproof, moistureproof, waterproof, breathable, ozone resistance, weathering resistance, weak acid and weak alkali resistance. It has good electrical insulation, good adhesion, and low cost. Therefore, it can be widely used in potting, coating, stamping, demolding, releasing drug carrier and other occasions. The electronic components encapsulated with methyl block room temperature glue have the functions of shockproof, moistureproof, sealing, insulating and stabilizing various parameters. Applying methyl block room temperature glue directly to the speaker can reduce and eliminate the intermediate frequency points of the speaker. After vulcanization, the resonance frequency performance of the speaker can be reduced by about 20 Hz. After mixing a certain amount of additives into the methyl block room temperature adhesive, it can be used as a paper release agent. After coating a thin layer of methyl block room temperature glue on the conveyor belt of candy and biscuits in the food industry, the anti-stick properties of the canvas can be improved, thereby improving the appearance of the food and improving the utilization rate of raw materials.
Adding an appropriate amount of fumed silica into the methyl block room temperature adhesive can be used to install window glass, curtain wall, window frame, joints of prefabricated panels, and expansion joints of airport runways. In addition, it can also be used as an adhesive for magnetic cores and templates in electronic computer memory, and as an adhesive for conductive silicone rubber and non-conductive silicone rubber. Treating fabrics with methyl-block RTV silicone rubber improves fabric hand, softness, and flex abrasion resistance.
(4) Room temperature vulcanized silicone rubber.
Room temperature vulcanized nitrile silicone rubber is poly-β-nitrile ethyl methyl siloxane. In addition to the light resistance, ozone resistance, moisture resistance, high and low temperature resistance and excellent electrical insulation properties of silicone rubber, room temperature vulcanized silicone rubber is mainly characterized by resistance to non-ferrous metals. It has good resistance to polar solvents such as aliphatic and aromatic solvents, and its oil resistance is similar to that of ordinary oil-resistant nitrile rubber. It can be used as sealing filling for oil-contaminated parts and oil-resistant electronic components.
(5) RTV fluorosilicone rubber RTV fluorosilicone rubber is poly-γ-trifluoropropyl methyl siloxane. Its main features are fuel oil resistance, solvent resistance and high temperature degradation resistance, as well as good extrusion out performance. Mainly used for sealing and caulking of integral fuel tanks of supersonic aircraft, bonding and fixing of fluorosilicone rubber gaskets and gaskets; bonding of silicone rubber and fluorosilicone rubber, as well as fuel oil resistance in chemical engineering and general industry; solvent-resistant parts of bonding.
(6) Room temperature vulcanized phenylene silicone rubber Room temperature vulcanized phenylene silicone rubber is a silicone (biphenyl) siloxane polymer, and its outstanding advantage is that it has excellent resistance to high-energy rays. The test proves that after being irradiated with 1x109 Roentgen γ-ray or 1x1018 neutrons/cm2, the rubber elasticity can still be maintained, which is 10 to 15 times larger than that of room temperature vulcanized methyl silicone rubber and 5 to 5 larger than room temperature vulcanized phenyl silicone rubber. 10 times.

Room temperature vulcanized phenylene silicone rubber can be used in the atomic energy industry, nuclear power plant and space flight as a high temperature and radiation resistant bonding and sealing material and an insulating protective layer for motors.