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Silicone rubber is a product obtained by vulcanization (crosslinking) of rubber macromolecules (elastomer) containing silicon in its structure. Synonyms - siloxane, organosilicon, silicone rubbers. The side substituent groups on the elastomer macromolecules may be alkyl, mobile, or even fluorine. The structure of the rubber molecule directly determines the practical properties of the finished composition: hardness, heat resistance, elasticity, cold flow, etc. In addition to the polymer part, rubber contains many more active and inert fillers. For example, silicon dioxide is able to multiply the strength characteristics, and kaolin (clay) is introduced in order to reduce the cost of the composition. Rubber itself is a viscous system with very little tensile strength. Therefore, to give it high physical and mechanical characteristics, cross-linking agents (vulcanizates) are introduced. The finished rubber compound additionally contains dyes, antioxidants, light stabilizers, softeners, etc., which are added in order to impart certain performance characteristics. Characteristics All performance characteristics are somehow related to the presence of silicon in the rubber structure. It allows you to combine the organic and inorganic nature of materials within the system. Here are some key characteristics that make silicones unique:
• Heat resistance. This is the main indicator when choosing silicone rubber. General purpose rubbers cannot be operated at temperatures above 100-150 °C. For silicones, the temperature range of 150-250 °C is a normal working environment. In the extreme temperature range up to 300 °C, siloxane rubbers withstand up to several weeks and even months. The production cycle may provide for the laying of the upper temperature mode of operation, usually this is achieved by the temperature of the 2nd stage of vulcanization by 20-25 ° C higher than the expected values. The only restriction in use is that an airless environment reduces the service life of rubbers; • Cold resistance. Silicone rubbers are not among the lowest temperature rubbers. By introducing modifiers, it is possible to reduce the temperature limit of cold flow. The elasticity of rubber products remains unchanged down to -80 °C. As the temperature drops, the material becomes brittle; • Weather resistance. Silicones do not crack, break or fade when exposed to temperature extremes, ultraviolet and sunlight; • Environmental friendliness and toxicological safety. Silicones do not emit harmful chemical compounds. They are inert and safe for contact with the human body. This feature determines their use in the food industry and medicine; • Anti-adhesive properties. Nothing sticks to the finished silicone. For this reason, they are used as conveyors, for example for moving hot sticky masses and media in the form of conveyor belts.
When choosing silicone for the production of rubber products, it is necessary to determine the key technical characteristics (priority to heat resistance). Since organosilicon polymers are quite expensive plastics. The high price of silicone must be taken into account when calculating the cost of the product. Production technology The production process for producing rubber compounds involves several key production steps:
• Rolling. On two-roll rollers, all components of the recipe are pre-mixed. The mixture is averaged and becomes uniform in composition; • Blending. Next, the raw rubber mixture is sent to a closed rotary mixer (Banbury). This stage allows you to get the most homogeneous composition and finely grind all the components; • Calendering. The finished mixture from the rubber mixer is sent to the calenders. This is a multi-roll system that allows you to stretch and roll out rubber into a thin tape. Then it will be convenient to cut it into pieces; • Pressing (vulcanization). At this stage, not only the final molding of the product takes place, but also chemical crosslinking (vulcanization). The crude mixture becomes a strong and resilient rubber.
These are the main (obligatory steps). In addition to them, there may be several more preparatory operations, their presence depends on the final properties of the product. Areas of use Silicone rubber is quite firmly entrenched in many industries. Their use is especially significant in those tasks where exposure to high temperatures and aggressive environments is expected. Here are the main applications of silicone:
• Electrical engineering. Siloxane polymers are used to produce reliable and resistant cable sheaths. Silicone is resistant to electricity and provides the proper level of insulation for wires. It is especially important to use it in an environment of high humidity and the presence of ultraviolet radiation. In extreme loads, silicone rubber is not subject to aging and wear; • Industrial engineeringoenie. Here the main task of silicones is high-quality and chemically resistant sealing of metal elements. Silicone rubbers can also be used as a durable coating for the transport wheels of mobile structures; • Transport nodes. We are talking about the protective coating of conveyor belts and rolling shafts. In the chemical and food industries, sticky hot materials need to be moved and sometimes shaped into blanks. Silicone does not stick or melt, so it fits perfectly; • Medicine. Organosilicon is a physiologically safe material in contact with the human body. It does not emit toxic compounds and is completely inert. Catheters, tubes, vial caps, etc. are made from it.
Properties and characteristics Silicone rubber has useful properties and characteristics.
Thermal resistance. The conversation is about the ability of silicone to maintain the structure and properties when used in extreme temperatures. So, their operating range varies from -60°C to +180°C. Silicone products of increased heat resistance operate at temperatures from -100 ° C to + 300 ° C. And the melting point of the elastomer under discussion is +480 ° C, however, this parameter depends on the composition of the silicones. chemical inertness. Silicone rubber shows resistance to fresh/marine/boiling water, salt solutions, mild acids and bases, phenols, alcohols, mineral oils, hydrogen peroxide. Note the ability of silicone to interact with aliphatic carbons, which is manifested by pronounced swelling of the elastomer. True, all the properties of the latter are restored after the complete evaporation of aliphatic hydrocarbons. weather resistance. The polymer is highly resistant to precipitation, air, light, UV radiation, ozone, electrical discharges and fields. Resistance to the last two factors allows the use of silicone parts in the electrical industry. electrical properties. At a temperature of + 100 ° C, the electrical insulating performance of silicone rubber exceeds that of all other elastomers, due to which it is used as an electrical insulating material. Physiological safety. Silicone rubber, when properly processed, is a non-toxic material that is widely used in the production of rubber goods for medicine, as well as for pharmaceutical and food industries. anti-adhesive properties. Most grades of silicone rubber have poor adhesion (stickiness). This circumstance allows them to be used in the manufacture of molds, coatings for conveyor belts designed to move sticky and / or hot products, as well as shafts used by the textile industry. Thermal properties. The thermal conductivity of silicone at a temperature of +80°C is ~4*10-4 cal/cm.deg.s, and the coefficient of linear expansion in the temperature range from 0°C to +150°C is ~2*10-4 deg.-1.
It is impossible not to note one more property of silicone - the durability of rubber goods made from it. So, when using silicone products at the optimal temperature regime: from -50 ° C to + 150 ° C, - subject to constant air access, there are no restrictions on their service life. Production Silicone rubber is produced at enterprises equipped with special equipment, the list of which is presented: silicone rubber
The raw materials for their manufacture are mixtures containing a significant number of components, we are talking about / about:
synthetic rubbers; vulcanizing agents: sulfur, peroxides, metal oxides and other substances - in addition, radiation can be a vulcanizing agent; antioxidants, which are introduced, in particular, into silicone rubber in order to slow down the aging process of the final product; neozone D, wax, paraffin and other substances are used as antioxidants; accelerators - components that reduce the flow time during the vulcanization process; regenerated products - products that are obtained during the processing of old rubber goods, as well as waste generated during the production of rubber; pigments - mineral or organic dyes; plasticizers: for example, vaseline, bitumen, stearic acid, paraffin, vegetable oils - improving the elasticity of silicone; fillers that give the final product certain properties.
Silicone rubber is recycled in a variety of ways.
Molding and injection molding. The essence of the indicated method consists in pouring or pressing the rubber mixture into molds and keeping them for a certain time under pressure, the value of which can vary from 40 to 80 kg/cm2. An important condition for the processing of rubber silicone compounds by molding and injection molding is the removal of air from the molds (when they are closed), contrary toIn this case, insufficiently vulcanized areas of brown color may form in the vulcanizate. Injection molding. Silicone mixtures are processed in this way in large-scale production. This technology involves the use of higher temperatures with a much shorter curing process. Rubber products obtained by injection molding are characterized by lower hardness, which can be compensated by introducing a larger amount of filler into the mixture. Extrusion. This method is used for the manufacture of profile parts, tapes, hoses, rods, cable sheaths. Vulcanization takes place in the heating duct, where hot air/steam is supplied, designed to provide a temperature in the range from +250°C to +350°C. As soon as the product acquires a stable shape, the next stage begins, called post-vulcanization (annealing), which can take place in the same place, in the heating duct, provided there is sufficient air supply, or in an oven, in which air circulation must also be provided. Layering. The essence of this method is to apply the rubber mixture in one of the following ways: immersion, spreading with a rekel knife, calendering. The first web of fabric is immersed in an emulsion containing silicone rubber and a solvent. At a temperature of +80°C, the solvent evaporates. Then the temperature rises from +120°C to +250°C, and the rubber is vulcanized on the fabric. The immersion method makes it possible to obtain very thin coatings. The rake-knife spreading technique provides thicker coatings and is also used to apply the silicone layer on one side only. The essence of calendering is to apply the mixture to the fabric with a calender. Vulcanization can take place in a hydraulic press, a heating duct, or in a vulcanizing machine. Calendering differs from the two previous methods in the worst adhesion of the fabric to silicone. However, it provides thick layers of rubber on one or both sides of the surface.
