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Understanding room temperature vulcanized silicone rubber

Room temperature vulcanized silicone rubber (RTV) is a new type of silicone elastomer introduced in the 1960s. The most significant feature of this rubber is that it can be cured in place without heating and pressure at room temperature, which is extremely convenient to use. Therefore, it became an important part of the whole silicone product as soon as it came out. Now room temperature vulcanized silicone rubber has been widely used as adhesives, sealants, protective coatings, potting and molding materials, and has its uses in various industries. Room temperature vulcanized 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 with a viscosity between 100 and 1000000 centistokes. According to the requirements of use, the rubber material before vulcanization can be formulated into a self-leveling pouring material or a non-flowing but scrapable putty. The filler used for room temperature vulcanized silicone rubber is similar to the high temperature vulcanized silicone rubber. It is reinforced with white carbon black to make the vulcanizate have a breaking strength of 10-60 kg / cm 2. Adding different additives can make the rubber material have different specific gravity, hardness, strength, fluidity and thixotropy, and make the vulcanizate have various special properties such as flame retardancy, electrical conductivity, thermal conductivity, and ablation resistance.

Room temperature vulcanized silicone rubber can be divided into one-component and two-component room temperature vulcanized silicone rubber according to its packaging method. According to the vulcanization mechanism, it can be divided into condensation type and addition molding. Therefore, room temperature vulcanized silicone rubber can be divided into three types according to the composition, vulcanization mechanism and application process, that is, one-component room temperature vulcanized silicone rubber, two-component condensation room temperature vulcanized silicone rubber, and two-component addition-temperature room temperature vulcanized silicone rubber. .

The raw rubbers of the one-component and two-component condensation type room temperature vulcanized silicone rubbers are α, ω-dihydroxy polysiloxane; the addition molding room temperature vulcanized silicone rubber contains alkenyl groups and hydrogen side groups (or end groups). Polysiloxane, 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 ° C) during curing.

These three series of room temperature vulcanized silicone rubber have their own advantages and disadvantages: the advantage of one-component room temperature vulcanized silicone rubber is that it is easy to use, but the deep curing speed is difficult; the advantage of two-component room temperature vulcanized silicone rubber is that it does not release heat during curing , Shrinkage rate is small, no expansion, no internal stress, curing can be carried out at the same time inside and on the surface, and can be vulcanized at the same time; vulcanization time of room temperature vulcanized silicone rubber is mainly determined by temperature, so the temperature can be controlled by adjusting the temperature speed.

First, single-component room temperature vulcanized silicone rubber
The curing reaction of one-component room temperature vulcanized silicone rubber is initiated by the moisture in the air. The commonly used cross-linking agent is methyltriacetoxysilane. Its Si-OC bond is easily hydrolyzed. Acetoxy groups are combined with hydrogen groups in water to form acetic acid, and the hydroxyl groups in water are transferred to the original acetoxy groups. In position, it becomes trihydroxymethylsilane. Trihydroxymethylsilane is extremely unstable and easily condenses with linear silicones whose terminal groups are hydroxyl groups to form a cross-linked structure. Usually, the silicone rubber containing silanol end groups and various compounding agents such as fillers, catalysts, and cross-linking agents are packed into sealed hoses. When used, they are extruded from a container and vulcanized with the help of moisture in the air. Elastomer, release low molecular weight at the same time. In addition to the methyl triacetoxysilane, the cross-linking agent may be a silane containing an alkoxy group, an oxime group, an amine group, an amide group, or a ketone group. When crosslinked with alkoxy group, the alcohol is released, which is called dealcoholized one-component room temperature vulcanized silicone rubber. When crosslinked with oxime group, oxime is generated, which is called deoxidized room temperature vulcanized silicone rubber. Different types of agents, one-component room temperature vulcanized silicone rubber can be deacidified, deoxidized, dealcoholized, deamined, deamidated, and deketoned. One.

The curing time of one-component room temperature vulcanized silicone rubber depends on the vulcanization system, temperature, humidity and the 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 may not be tacky, and the 0.3 cm thick adhesive layer can be cured within one day. The depth and strength of curing will gradually increase in about three weeks.

One-component room temperature vulcanized silicone rubber has excellent electrical properties and chemical inertness, as well as heat resistance, natural aging resistance, flame resistance, humidity resistance, and breathability. They can remain elastic for a long time in the range of -60 to 200 ° C. It does not absorb heat or heat when cured, has a small shrinkage after curing, and has good adhesion to materials. Therefore, it is mainly used as an adhesive and sealant. Other applications include in-situ molding of gaskets, protective coatings and caulking materials. Many formulations of one-component silicone rubber adhesives exhibit automatic bonding properties on a variety of materials such as most metals, glass, ceramics, and coagulation, such as bare aluminum with a shear strength of 200 psi 2. Tear strength up to 20 lbs / ft2 (0.35 J / cm2). When adhesion is difficult, a primer can be applied on the substrate to improve the bonding strength. The primer can be a reactive silane monomer or resin. When they are cured on the substrate, a modified layer of suitable On silicone bonded surfaces.

Although the one-component room temperature vulcanized silicone rubber is convenient to use, the thickness of the vulcanizate is limited due to its vulcanization depending on the moisture in the atmosphere, and it can only be used in situations where the thickness is less than 6 mm.

The curing reaction of the one-component room temperature vulcanized silicone rubber proceeds gradually from the surface to the deeper. The thicker the rubber layer, the slower the curing. When deep curing is also required, layered pouring and stepwise vulcanization can be adopted. Some rubber compounds can be added each time. After vulcanization, the material can be added, which can reduce the total vulcanization time. The addition of magnesium oxide can accelerate the vulcanization of deep glue.

Two-component room temperature vulcanized silicone rubber Two-component room temperature vulcanized silicone rubber can be further divided into condensation type and addition molding.

Two-component condensation type room temperature vulcanized silicone rubber
Two-component condensation type room temperature vulcanized silicone rubber is the most common type of room temperature vulcanized silicone rubber. The raw rubber is usually a hydroxyl-terminated polysiloxane, which is then combined with other additives and catalysts to form a rubber compound. The viscosity can range from 100 centistokes to one million centistokes. The vulcanization reaction of two-component room temperature vulcanized silicone rubber is not initiated by the moisture in the air, but is initiated by a catalyst. Generally, the silicone rubber, filler, and cross-linking agent are packaged as one component, and the catalyst is packaged as another component alone or in other combinations, but the catalyst and the cross-linking agent must be packaged separately. Regardless of the packaging method, curing only begins when the two components are completely mixed together. A common cross-linking agent is ethyl orthosilicate, and the catalyst is dibutyltin dilaurate. Add appropriate fillers and additives according to the properties of the desired final product. In recent years, because dibutyltin dilaurate is a medium-toxic substance, the addition of dibutyltin to food bags and plasma bags has been banned, and has basically been replaced by low-toxic octyltin.

The curing time of the two-component condensation type room temperature vulcanized silicone rubber mainly depends on the type, amount and temperature of the catalyst. The more catalyst used, the faster the sulfidation, and the shorter the standing time. At room temperature, the shelf time is generally a few hours. If you want to extend the shelf time of the rubber, you can use the cooling method. It takes about one day for the two-component condensation room-temperature vulcanizate to cure completely at room temperature, but it only takes one hour at 150 ° C. The synergistic effect by using the accelerator γ-aminopropyltriethoxysilane can significantly increase its curing speed. Two-component room temperature vulcanized silicone rubber can maintain elasticity for a long time in a temperature range of 65-250 ° C, and has excellent electrical properties and chemical stability. It is resistant to water, ozone, and weathering. In addition, the method is simple and the process is applicable. Therefore, it is widely used as a potting and molding material. Various electronic and electrical components are coated with room temperature vulcanized silicone rubber. After being potted, they can protect against moisture (anti-corrosion, shock, etc.). It can improve performance and stability parameters. Two-component room temperature vulcanized silicone rubber is particularly suitable for deep-water irrigation. The sealing material has a faster curing time, which is superior to the one-component room temperature vulcanized silicone rubber.

Two-component room temperature vulcanized silicone rubber has excellent anti-blocking properties after vulcanization, and has very small shrinkage during vulcanization. Therefore, it is suitable for making soft molds, casting epoxy resin, polyester resin, polystyrene, Molds for polyurethane, vinyl plastic, paraffin, low melting alloys, etc. In addition, the high simulation performance of the two-component room temperature vulcanized silicone rubber can reproduce various delicate patterns. For example, in the reproduction of cultural relics, it can be used to reproduce ancient bronzes, and in the production of artificial leather, it can be used to reproduce the skin patterns of animals such as snakes, pythons, crocodiles, and pangolins.

When using two-component room temperature vulcanized silicone rubber, several specific problems should be paid attention to when using it: firstly weigh the base material, crosslinker and catalyst separately, and then mix them in proportion. Generally, 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 handled carefully to minimize the amount of entrained gas. After the rubber is mixed (uniform color), the air bubbles can be removed by standing or decompressing (700 mm Hg vacuum). After all the air bubbles have been discharged, they can be vulcanized into silicon at room temperature or at a specified temperature for a certain period of time. Rubber.

In addition to the methyl group, the side groups of the main component 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-conducting, and conductive additives can be added as needed to produce a silicone rubber with ablation-resistant, flame-retardant, heat-conducting, and conductive properties.

Methyl room temperature vulcanized silicone rubber
Methyl room temperature vulcanized 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 ～ 200 ℃. Therefore, it is widely used as a potting and sealing material for electronic and electrical components, and a moisture-proof, shock-proof, high and low temperature resistant potting and sealing material for instruments and meters. It can also be used to make molds for casting polyester resin, epoxy resin and low melting alloy parts. Can also be used as dental impression materials. Methyl room temperature vulcanized silicone rubber is coated on cotton cloth and paper bags, which can be made into conveyor belts and packaging bags for conveying viscous goods.

2.Methylbisphenyl room temperature vulcanized silicone rubber
In addition to the excellent properties of methyl room temperature vulcanized silicone rubber, methyl bisphenyl room temperature vulcanized silicone rubber has a wider operating temperature range (-100 to 250 ° C) than methyl room temperature vulcanized silicone rubber. Low phenyl room temperature vulcanized silicone rubber (108-1) with a phenyl content of 2.5 to 5% can maintain elasticity at a low temperature of -120 ° C. It is currently the best low temperature performance of silicone rubber; the phenyl content is 10 -20% room temperature adhesive (108-2) has good resistance to radiation, ablation and self-extinguishing. If a certain amount of heat-resistant additives such as Fe2O3 are added to it, it can improve the heat aging performance, suitable for 250 Use or do ablation-resistant putty coatings and encapsulation materials at high temperatures above ℃.

Methylphenyl room temperature glue, like other room temperature glues, can be used for impregnation, impression and film 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 surface treatment steps are as follows: clean the surface of the material with acetone solvent 1 or 2 times, and then treat with ～ 2 times, bake in an oven at 60 ℃ for several minutes. At this time, a film with little stickiness is formed on the surface of the material, and the glue can be applied.

3. Methyl block room temperature vulcanized silicone rubber
Methyl block room temperature vulcanized silicone rubber is a modified variety of methyl room temperature vulcanized silicone rubber. It is a hydroxyl-terminated polydimethylsiloxane (107 rubber) and methyltriethoxysilane oligomer ( Interpolymers with a molecular weight of 3 to 5). Under the catalysis of dibutyltin dilaurate, the hydroxyl group in polydimethylsiloxane and the ethoxy group in polymethyltriethoxysilane are condensed to form a three-way structured polymer. It has higher mechanical strength and adhesion than methyl room temperature vulcanized silicone rubber, and can be used for a long time in a temperature range of 70 to 200 ° C.

Methyl block room temperature vulcanized silicone rubber has shock resistance, moisture resistance, waterproof, breathable, ozone resistance, weather resistance, weak acid and alkali resistance. It has good electrical insulation properties, good adhesion, and low cost. Therefore, it can be widely used in potting, coating, stamping, demoulding, drug carrier release and other occasions. The electronic components potted with methyl block room temperature adhesive have the functions of anti-vibration, moisture-proof, sealing, insulation, and stabilization of various parameters. The methyl block room temperature glue is directly applied to the speaker, which can reduce and eliminate the intermediate frequency points of the speaker. After curing, the resonance frequency performance of the speaker can be reduced by about 20 Hz. It can be used as a paper release agent after mixing a certain amount of additives in methyl block room temperature glue. After coating a thin layer of methyl block room temperature glue on the candy and biscuit conveyor belt of the food industry, the anti-sticking performance of the canvas can be improved, thereby improving the appearance of the food and increasing the utilization rate of raw materials.

Adding a proper amount of fumed silica to the methyl block room temperature adhesive can be used to install the joints of window glass, curtain wall, window frame, prefabricated panels, and expansion joints of airport runways. In addition, it can also be used as an adhesive for magnetic cores and stencils in electronic computer memory, and it can also be used as an adhesive for conductive silicone rubber and non-conductive silicone rubber. Treating the fabric with methyl block room temperature vulcanized silicone rubber can improve the fabric's feel, softness and resistance to abrasion.

4. Room temperature vulcanized silicone rubber
Room temperature vulcanized silicone rubber is poly β-nitrile ethyl methyl siloxane. In addition to room temperature vulcanized silicone rubber, it has the light resistance, ozone resistance, moisture resistance, high and low temperature resistance, and excellent electrical insulation properties. Soluble solvents such as aliphatic and aromatic solvents have good performance, and their oil resistance is similar to that of ordinary oil-resistant nitrile rubber, which can be used as a sealed filling for oil-contaminated parts and oil-resistant electronic components.

5. Room temperature vulcanized fluorosilicone rubber
Room temperature vulcanized fluorosilicone rubber is polyγ-trifluoropropylmethylsiloxane. Its main features are resistance to fuel oil, solvents, high temperature and degradation resistance, and good extrusion properties. It is mainly used for sealing and caulking of the entire fuel tank of supersonic aircraft, bonding and fixing of fluorosilicone rubber gaskets, gaskets; bonding of silicone rubber and fluorosilicone rubber, and resistance to fuel oil 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 silicon benzene (biphenyl) siloxane polymer, and its outstanding advantage is that it has excellent resistance to high-energy rays. The test proves that after being exposed to 1x109 Roentgen γ-rays or 1xl018 neutrons / cm2, the rubber elasticity is still maintained, which is 10 to 15 times larger than room temperature vulcanized methyl silicone rubber and 5 to 5 times 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 plants, and space flight as high temperature and radiation resistant adhesive sealing materials and motor insulation and protection layers.

Third, addition molding room temperature vulcanized silicone rubber
Two-component addition molding room temperature vulcanized silicone rubber has 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 (hydrosilication reaction) of the vinyl (or propylene) group on the silicone rubber end group and the hydrosilyl group on the crosslinker molecule.

In this reaction, a hydride-functional polysiloxane is used as a cross-linking agent (vulcanizing agent) chloroplatinic acid or other soluble platinum compound is used as a catalyst. The vulcanization reaction proceeds at room temperature. Does not emit by-products. Because no low-molecular matter is released during the cross-linking process, the addition molding room temperature vulcanized silicone rubber does not shrink during the vulcanization process. This type of vulcanization