Room temperature vulcanized silicone rubber products processing.silicone bibs uk sales

　　Silicone Rubber is a polymer elastic material with both inorganic and organic properties. Its molecular main chain is alternately composed of silicon atoms and oxygen atoms (—Si—O—Si—), and the side chains are connected with silicon atoms. Hydrocarbon or substituted hydrocarbon organic group, this group can be methyl, unsaturated vinyl (the mole fraction generally does not exceed 01005) or other organic groups, this low unsaturation molecular structure makes silicone rubber The products have excellent heat aging resistance and weathering resistance, and are resistant to ultraviolet and ozone erosion. The flexibility of the molecular chain is large, and the interaction force between the molecular chains is weak. These structural characteristics make the vulcanizate soft and elastic, but the physical properties are poor.

　　Silicone rubber was developed in the 1940s, and the earliest researched variety abroad was dimethyl silicone rubber. Around 1944, it was put into production by American DowCorning Company and General Electric Company respectively. In the early 1960s, my country successfully researched and put it into industrial production. In addition to China, the countries that produce silicone rubber now include the United States, Britain, Japan, Russia and Germany, with more than 1,000 varieties and brands.

　　3. Room temperature vulcanized silicone rubber

　　Room temperature vulcanization (also known as condensation vulcanization) silicone rubber refers to silicone rubber products that can be vulcanized at room temperature without heating. Its molecular structure is characterized by the presence of active functional groups such as hydroxyl or acetoxy at both ends of the molecular main chain. Under certain conditions, these functional groups undergo condensation reaction to form a cross-linked structure and become an elastomer.

　　3.1 Varieties and characteristics

　　RTV silicone rubber has a relatively low molecular weight and is usually a viscous liquid. According to its vulcanization mechanism and use process, it can be divided into one-component RTV silicone rubber and two-component RTV silicone rubber.

　　3.1.1 One-component RTV silicone rubber

　　One-component RTV silicone rubber is a hydroxyl-terminated low molecular weight silicone rubber mixed with a reinforcing agent, dried to remove water, and then a crosslinking agent (containing a hydrolyzable multifunctional siloxane) is added. At this time, kneading The glue has become a polymer containing multifunctional end groups, which is encapsulated in a closed container. When extruded, it contacts with moisture in the air, so that the functional groups in the glue are hydrolyzed to form unstable hydroxyl groups, which are then condensed and cross-linked to form elastomers.

　　One-component RTV silicone rubber can be divided into deacidification type and non-deacidification type according to the type of crosslinking agent. The former is widely used, and the crosslinking agent used is acetoxy siloxane (such as methyltriacetoxysilane or methoxytriacetoxysilane), which releases by-product acetic acid during the vulcanization process, which is corrosive to metals effect. There are many types of non-deacidification condensation vulcanization types, including the dealcohol condensation vulcanization type with alkoxy (such as methyltriethoxysilane) as the crosslinking agent. This reaction only depends on the action of moisture in the air, and the vulcanization is slow. It is necessary to add vulcanization accelerators of alkyl titanates, and alcohols are released during vulcanization, which has no corrosive effect, and is most suitable for electrical insulation products. In addition, there are deamination condensation vulcanization with silazane as crosslinking agent (organic amine is released during vulcanization, odor, and corrosion to copper) and deoxime vulcanization with acetone oxime and butanone oxime as crosslinking agent, Deamidation vulcanization and deketone vulcanization with fast vulcanization speed, etc.

　　According to the modulus of the product, it can be divided into low modulus (deamidation type), medium modulus (suitable for building sealant) and high modulus (dealcoholization type); according to the practical performance of the product, it can be divided into general type and special type There are two types of products, among which special types include flame retardant type, surface paintable type, mildew-proof type and pollution-resistant type.

　　One-component RTV silicone rubber has good adhesion to a variety of materials (such as metal, glass, ceramics, etc.), and is particularly convenient to use, generally without weighing, stirring, and defoaming operations. Vulcanization starts from the surface and progresses gradually to the depths. One-component RTV silicone rubber is mainly used as an adhesive and as a sealing and gap-filling material in the construction industry.

　　3.1.2 Two-component RTV silicone rubber

　　The vulcanization of this type of rubber is caused by the interaction between the hydroxyl groups of the raw rubber under the action of catalysts (organotin salts, such as dibutyltin dilaurate, stannous octoate, etc.) and crosslinking agents (alkoxysilanes, such as ethyl orthosilicate). It can be divided into deethanol condensation vulcanization, dehydrogenation condensation vulcanization, dehydration condensation vulcanization and dehydroxyamine condensation vulcanization, etc. The dealcoholization type is the most common. Two-component RTV silicone rubber is usually made by mixing raw rubber, filler and crosslinking agent into one component, raw rubber, filler and catalyst into another component, and then mixing the two components by metering when using. The vulcanization time of the two-component mainly depends on the amount of catalyst. The larger the amount, the faster the vulcanization. In addition, the higher the ambient temperature, the faster the vulcanization; there is no internal stress during vulcanization, no shrinkage, and no expansion. During vulcanization, the condensation reaction proceeds simultaneously inside and on the surface, and there is no difficulty in deep vulcanization of thick products. It has no adhesion to other materials, and needs to be primed with a surface treatment agent when bonding with other materials. Two-component RTV silicone rubber can be used as molding and potting materials.

　　The room temperature vulcanized silicone rubber can be made into different viscosities according to the requirements of use. Generally, there are fluid grades, medium consistency grades and consistency grades. The fluid grade compound has fluidity and is suitable for pouring and spray gun operation; if a lower viscosity compound is required (when pouring into a narrow gap), methyltriethoxysilane or its oligomer can be infiltrated into the compound. Diluted with methylsilicone 201. The viscosity of the medium-consistency rubber compound is just enough to flow fully without completely dripping, and a product with a smooth surface can be obtained, which is suitable for gluing and dipping. Consistency grades have a putty-like consistency and can be handled by hand, scraper or caulking knife, or applied to a variety of fabrics by calendering

　　In recent years, room temperature vulcanized silicone rubber has been very active in improving research. With the expansion of application areas, there have been high adhesion, high strength, high elongation, low modulus, flame retardant, oil resistance and fast curing. New varieties.

　　3.2 Cooperation

　　3.2.1 Vulcanizing agent

　　One-component RTV silicone rubber mainly relies on moisture in the air for cross-linking reaction, and the rubber compound should be stored in a closed state before use; in two-component RTV silicone rubber (except addition reaction system), the silicone rubber containing terminal hydroxyl Commonly used vulcanizing agents are silicates (such as ethyl orthosilicate) and titanates (such as n-butyl titanate); the catalysts mainly use organic tin salts, such as dibutyltin dilaurate, stannous octoate Wait. The vulcanization rate can be changed by adjusting the amount of vulcanizing agent and catalyst. The amount of vulcanizing agent is generally 1-10 parts; the amount of catalyst is generally 0.5-5 parts. Tests have shown that the catalyst dibutyltin dilaurate has a corrosive effect on copper. Dibutyltin oxide [(C4H9)2SnO] or dioctyltin oxide [(C8H17)2SnO] and tetraethyl orthosilicate [Si(OC2H5)4] The reflux product was used as a vulcanization system, and the vulcanizate was stored in contact with copper for 1 year and no corrosion was found.

　　3.2.2 Reinforcing filler

　　RTV silicone rubber must also be added with silica as a reinforcing agent, otherwise the strength is lower than that of thermal vulcanization. The combination method is the same as that of the hot vulcanization type.

　　3.3 Processing

　　3.3.1 One-component RTV silicone rubber

　　One-component RTV silicone rubber must be stored in a closed container isolated from water and air, and can generally be used within a few months. There is no need to add catalyst during use, as long as the rubber compound is extruded from the airtight container to contact the air, so it is very convenient to use; it can be processed in a short time by molding, extrusion or other methods, and then exposed to the air for a certain period of time. The material is vulcanized to become an elastomer.

　　One-component RTV silicone rubber is an adhesive.

　　When used for bonding, no surface treatment agent is required, that is, it has good bonding properties to glass, ceramics, metal, wood, plastic and vulcanized silicone rubber. Because this rubber is exposed to moisture in the air at room temperature and starts to vulcanize from the surface, and then gradually vulcanizes inward through the diffusion of moisture. Over-thick products require a long time for internal vulcanization, so there is a certain limit to the thickness of the product (or the depth of the seal). The thickness should generally not exceed 10mm.

　　If more than 10mm is required, multiple construction methods can be used.

　　The humidity of the air has a decisive influence on the vulcanization speed. The higher the humidity, the faster the vulcanization. When the climate is relatively dry and the humidity is very small, water can be sprayed to increase the moisture in the air, so that it can achieve the actual required vulcanization speed.

　　3.3.2 Two-component RTV silicone rubber

　　The two-component room temperature vulcanized silicone rubber should be stored in a cool and dry place, avoid direct sunlight, if the storage time exceeds 4 months, it should be inspected, and the performance will remain unchanged before continuing to use.

　　(1) Addition of catalyst

　　Add the catalyst to the RTV silicone rubber compound of liquid or medium consistency, and stir it by hand to disperse it. After mixing evenly, place the compound in an airtight container to vacuumize and maintain it at 0.67～2.67kPa for 3～5min. to remove air bubbles. When thick grades of rubber are used, a mixer, kneader or mixer can be used to mix the catalyst into the compound. The catalyst can be measured by weighing method or volumetric method. Since the amount of catalyst is generally only 0.5 to 5 parts, attention should be paid to mixing evenly. The room temperature vulcanized silicone rubber is gradually cross-linked and cured after mixing with the catalyst, so it should be prepared according to the required amount. If there is any leftover, it can be stored in a low temperature place (such as in the refrigerator) to prolong the use time.

　　(2) Coating of fabrics

　　The room temperature vulcanized silicone rubber can be added with catalyst in the following methods, and coated on various fabrics by gluing or calendering, without diluting with solvent to make glue.

　　The methods of adding the catalyst during gluing are: 1. Add it to the rubber compound before gluing; 2. Add it to the other side of the glue-coated fabric, and let the catalyst penetrate through the cloth layer to cross-link the rubber; 3. Before gluing, add it to the fabric to be coated glue surface. The first method limits the operation time to be within the pot life of the compound, otherwise it will be cured and cannot be applied; the operation time of the latter two methods is not limited by the pot life of the compound.

　　(3) Dissolution of rubber compound

　　Common aromatic solvents, such as toluene or xylene, can be used to dissolve the rubber compound to prepare a room temperature vulcanized dimethyl silicone rubber glue.

　　This cement can be dip-coated onto fabrics by the universal dipping method.

　　(4) Bonding

　　RTV silicone rubber can be made into adhesives for bonding various materials. When used for various vulcanized silicone rubber and its bonding with metal or non-metal (such as glass, glass fiber reinforced plastic, polyethylene, polyester, etc.), the adhesive is made up of two groups of A and B. Component A is a paste room temperature vulcanized silicone rubber containing an appropriate amount of reinforcing filler, a small amount of titanium dioxide and iron oxide, and component B is a vulcanization system composed of various vulcanizing agents (ethyl orthosilicate, butyl titanate, etc. ) and catalyst (dibutyltin dilaurate, etc.). Before use, the two components are fully mixed according to the mass ratio of 9:1. The active period of the adhesive is 40min (20℃, relative humidity is 65%). If you want to extend the active period, you can reduce the amount of catalyst, but the amount should not be less than 1 part, otherwise the adhesion will be poor. Excessive amount of catalyst will reduce the heat resistance of vulcanizate.

　　The bonding process is completed with or without pressure at room temperature. The surface of the adherend should be removed from dirt and cleaned with acetone or toluene; then a layer of surface treatment agent should be applied to the metal or non-metal surface and dried at room temperature for 1 to 2 hours (the specific time should depend on the temperature and humidity at that time. After setting), you can apply adhesive for bonding. The surface treated with surface treatment agent does not affect the adhesion effect when gluing within 1 week.

　　(5) Vulcanization

　　The vulcanization of two-component RTV silicone rubber is realized by adding a liquid catalyst. The curing time varies with the amount of vulcanizing agent and catalyst, from ten minutes to 24h; increasing or decreasing the temperature can also shorten or prolong the curing time.

　　Room temperature vulcanization silicone rubber products generally do not require secondary vulcanization in an oven. However, due to the generation of trace volatile substances (such as ethanol) during the vulcanization process, the volatile substances are not easy to escape when the silicone products are vulcanized. Vulcanization method, that is, pouring or filling with a thickness of 10-15mm each time. After losing fluidity, it is placed for 30 minutes, and then pouring or filling is continued. If the use temperature of thick products is higher than 150 °C, it is best to heat treatment at 100 °C after vulcanization at room temperature to drive off volatile substances and improve the heat resistance of the products.

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