Rtv Silicone Rubber Compounds
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BONDING, SEALING, POTTING/ ENCAPSULATION AND COATING WITH RTV SILICONE RUBBER COMPOUNDS CREATING TOMORROW’S SOLUTIONS Where to Find What Introduction 3 Properties 4 Bonding, Sealing 8 Potting/Encapsulation, Coating 10 Silicone Systems 1 RTV-1 Systems 14 RTV-2 Systems 16 RTV-2 Processing Information 18 WACKER Worldwide 0 WHOEVER IS FAMILIAR WITH WACKER RTV SILICONE RUBBER COMPOUNDS KNOWS THEIR STRENGTHS LOOK FOR A PRODUCT AND FIND IT! Room-temperature-vulcanizing silicone With more than 50 years’ experience in rubber compounds are a traditional silicone technology and an above- mainstay of WACKER SILICONES’ average commitment to R&D, WACKER product portfolio. Our range of ready- SILICONES has long since become to-use products for the numerous indispensable to its customers for their applications in the fields of bonding, technical progress. This would not sealing, potting/encapsulation, coating be possible without ongoing, confidential and mold-making is accordingly dialog – without the trading of knowledge, impressive. Equally impressive, however, experience and ideas that powers a is WACKER SILICONES’ experience in shared undertaking. Our joint undertaking the processing techniques and material right here and now is to find the ideal requirements specific to these industries: product for your application. from DIY-scale right up to industrial production lines. You can always rely on WACKER’s technical support service. 3 BENEFIT FROM QUALITY Thermal Resistance Bonding Properties Mechanical Properties Thermal resistance is just one of the Silicone rubber grades adjusted to be WACKER supplies RTV silicone rubber outstanding material properties that self-adhesive are suitable for many compounds to meet a wide range of characterize silicones. For applications substrates. The bond quality will depend mechanical requirements. Compounds involving temperatures above 150 °C, on the nature of the material to be with a processing viscosity in excess silicones are the only elastomer bonded, the stress, the kind of bond of 10,000 mPa s yield elastomers with candidate. Unlike other elastomers, and possibly the surface treatment. excellent tensile strength, elongation at they can withstand temperatures up The most superior bonds are obtained break and tear strength. Low-viscosity to 180 °C permanently; special heat- on oxidic and siliceous surfaces. products (less than 10,000 mPa s) yield stabilized grades will even withstand Good adhesive strength is also obtained elastomers with mechanical properties brief exposure to temperatures of with a number of plastics. that are generally not as strong. up to 300 °C. For maximum thermal stability, it is important that the silicone In other instances, it may be necessary RTV silicone foams constitute a special rubber cures, or vulcanizes, completely. or advisable to prime the substrate or class of RTV silicones. Although their Thermal stability can be enhanced subject it to flame or plasma treatment mechanical properties are less good on still further by subjecting the silicone prior to bonding. Surfaces must always account of their porous structure, these compound to subsequent, well- be clean and free of grease. Low-boiling foams offer excellent damping and ventilated post-curing, during which solvents such as acetone1 or aliphatic insulation. Their compressibility makes the temperature is raised slowly. hydrocarbons are suitable cleaning them ideal for use in seals. At the other end of the scale, silicones agents. remain flexible down to -50 °C, with Silicone gels are another special class of specialty grades resisting temperatures Release Properties RTV silicones. These very soft materials as low as -110 °C. The surfaces of silicone elastomers (Shore A < 0) have an extremely low show a pronounced release effect when elastic modulus, so that even large Expansion used with inorganic and organic temperature differences cause only little Silicone rubber elastomers have a materials such as gypsum, concrete, thermomechanical stress. Silicone coefficient of linear expansion that is a polyester, epoxide, polyurethane, gels are thus ideal for protecting delicate direct function of filler loading and polyamide, polystyrene, PVC, wax and electronic components. hence of specific gravity. The higher the metal alloys. This effect is exploited density of a rubber, therefore, the lower in the use of silicone rubber compounds is its coefficient of linear expansion. to make molded parts and reproductions. Typical values are in the range of 2·10-4 WACKER supplies pourable, spreadable m/mK and are thus substantially higher and kneadable silicone rubber grades to than for other potting/encapsulation suit a wide variety of applications. compounds. Since silicone rubber undergoes relatively pronounced expan- sion during the curing process, the shrinkage encountered on cooling is likewise pronounced. This “apparent” shrinkage must be taken into account in advance. 1 Please pay attention to the material’s flammability. 4 Permeability to Gas and Water-Vapor Chemical Resistance Radiation Resistance At room temperature, the gas Silicone rubber is resistant to aqueous Silicone rubber can be exposed to high permeability of silicone rubber is about solutions, diluted acids and bases doses of electromagnetic radiation – ten times that of natural rubber. Not until and polar solvents. Solvents such as from the microwave to the UV range – 100–150 °C do the permeability values ketones, esters and hydrocarbons without any noticeable effects. In converge. Under normal conditions, may cause reversible swelling of the terms of radiation resistance and hot- silicone rubber may contain some silicone rubber but do not destroy its air resistance, it is superior to all other 15–20 vol % of dissolved air. Reducing chemical structure. Products with a organic elastomers. This explains the or increasing the gas pressure causes high filler loading and/or that are very use of silicone rubber in such a wide a decrease or increase in the quantity of hard tend to swell less than softer variety of applications, ranging from dissolved gas. Consequently, a sudden compounds. The only way to remove microwave cookers to solar generators. pressure drop will make dissolved air cured silicone rubber from a surface become visible in the form of bubbles. is by destroying the rubber completely Flame Resistance Special care must be taken when with concentrated sulfuric acid, The auto-ignition temperature of cured metering equipment is used, as the use alcoholic alkali solution or the like. silicone rubber is about 430 °C. of compressed air to convey the The resultant flame burns at 750 °C. compound can also cause bubbling. In addition to the usual combustion Similar effects can occur on rapid products, namely carbon dioxide and heating, as the gas solubility decreases water, silicon dioxide is produced with a rise in temperature. The water- and forms an insulating layer of ash vapor permeability, as determined in on the surface. Silicones are flame- accordance with DIN 53122 under resistant – even without the addition climatic conditions D and a film of halogen compounds, which can thickness of 2 mm, is about 20 g/m2 d. generate corrosive and toxic gases in a fire. Generally speaking, their fire performance is better than that of many organic elastomers and casting resins. Environmental Compatibility Electrical Properties Optical Properties Silicones have the same basic chemical The electrical properties of silicone The color and appearance of silicone structure as quartz. The cured rubber rubber remain almost constant over the rubber is determined by the fillers therefore poses no known ecological or temperature range from -45 °C to selected for the compound in question. physiological risks. +180 °C. At room temperature they The light permeability of thin layers of compare well with those of other unfilled materials is almost 100 % in the Thermal Conductivity insulating materials. What makes silicone visible range from 400 to 760 nm. These The thermal conductivity of most rubber elastomers far superior to materials only become opaque in the silicone elastomers is in the range from other materials, however, is the fact that UV range below 200 nm. Their refractive 0.15–0.25 W/(Km) at room temperature. elevated temperatures have no appreci- index nD25 is between 1.410 and 1.404. However, values up to 2.5 W/(Km) may able effect on their insulation resistance, be obtained with specialty grades that dielectric strength or dissipation factor. Storage Stability have a high filler loading. These properties even remain constant SEMICOSIL®, ELASTOSIL® and when the compounds are immersed in WACKER SilGel® silicone rubber grades water. have a shelf life of up to 24 months (the exact time will depend on the grade in Normally, silicone elastomers are question) when stored in the original, electrical insulators. However, they can sealed containers at between 5 °C be rendered electrically conductive by and 30 °C. Storage beyond the date adding specialty fillers. specified does not necessarily mean that the material is unusable, but a Unlike organic elastomers and casting quality check should be performed on resins, RTV silicone rubber shows the most important properties. excellent arc and tracking resistance. Specialty grades can attain values > 3.5 kV (IEC 587) for arc resistance and > 300 s for tracking resistance. 6 DISCOVER DIVERSITY By virtue of their diverse and excellent Property Overview • Superlative dielectric properties that properties, RTV silicone rubber