Sealing with Silicone Resins - Automotive Industry
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Silicones are a group of synthetic polymers with a remarkable range of properties. Silicone-based resins have exceptionally high thermal resistance and elasticity and are usually nonhazardous, physiologically speaking. Depending on the application and requirements, it is possible to use 1K or 2K resins, known as RTV-1 and RTV-2 silicone.
Silicone rubbers (or SR) are special types of polymer bonds that have an intermediate position between organic and inorganic bonds. Silicones, technically also known as poly(organo)siloxanes, or siloxanes for short, are therefore like hybrids with a range of properties that in this form cannot be attained by any comparable plastic.
Silicones can be used in a wide variety of areas: Since usually they do not represent a health hazard, they are used in the pharmaceutical and medical and cosmetics industries as well as in the manufacture of textiles and kitchenware. Silicones are frequently used as sealants and fillers in the construction industry. In their liquid form they are used as an ingredient in massage oils, cleaning agents, paints, test fluids and special printing inks, among others.
Due to their exceptional electrical insulation properties, silicone-based casting resins play a key role as insulators in electronics manufacturing, such as in transformers or in the form of thermally conductive adhesives. Their excellent hydrophobic properties and their high elasticity make them attractive for challenging sealing applications in the automotive industry and in lighting technology. For this purpose they are used among other things to seal motors, sensors and enclosures. In addition, many silicone materials have excellent optical transparency and thus very good light transmission properties. Since they do not tend to go yellow with age or under the effects of heat and light, they are frequently used for the encapsulation of LEDs and for the bonding of displays and touchscreens. Even in medical equipment these highly physiologically compatible materials are used as dispensing materials for pacemakers or for manufacturing sleep apnea masks.
One characteristic of cured silicone casting resins is their high thermal resistance in a temperature range from approx. -40 to > 250°C. For this reason, they are frequently used when PU cannot meet the temperature requirements. In addition, the materials are characterized by high resistance to weathering and low heat generation during the curing reaction. Compared to epoxy resins and polyurethane, however, users often have to dig deeper into their wallets to obtain this exceptional set of properties.
Silicone materials have a characteristically low hardness level. This can negatively impact the particular application. Even the substantially higher gas and water vapor permeability compared to epoxies and polyurethanes as well as the less favorable adhesion to certain substrates must be taken into consideration for particular applications. Silicone also tends to swell upon contact with solvents such as gasoline, ethyl alcohol and toluene. This reaction, however, is reversible, which means that the material returns to its original form after the solvent completely evaporates.
Due to their fast cross-linking at low temperatures, UV cured silicones are often used in electrical and electronics manufacturing. 1K and 2K silicone systems, which cure at room temperature and are thus known as RTV-1 and RTV-2 silicones (RTV = room temperature vulcanizing), are also frequently used. RTV-1 systems cure to the end product through humidity. This reaction is also described as condensation curing. To ensure thorough curing, these silicones are used only for thin layers. Curing in closed systems without incoming air is not possible. In such cases RTV-2 systems need to be used.
RTV-2 silicones can either cure through a condensation or additive reaction, which means through humidity or by adding an appropriate hardener. Higher layer thicknesses can be achieved with 2K systems. Curing of the material also depends less on exterior conditions than in the case of RTV-1 silicones. Systematically raising the temperature speeds up curing dramatically. However, in the case of RTV-2 systems you always have to make sure to mix the components in precisely the right ratio. If not enough hardener is added, the material will not cure completely. The resin will remain soft and sticky, and the mechanical stability will be significantly reduced. Overdosage leads to shortened pot life in addition to severely compromising the mechanical stability of the material bond.
Particularly in the case of condensation-cured silicone resins it is important to process the material in an environment completely free of humidity. If this is ignored, it will cause an early curing reaction and the silicone material will be rendered unusable even before the actual dispensing procedure. To prevent this we recommend preparing and feeding the material in a vacuum.
- Scheugenpflug offers its proven LiquiPrep LP804 material preparation and feed system for situations like this. This system can of course also be used to reliably prepare addition curing or UV-cured silicone.
Learn more about the LiquiPrep LP804
- The CNCell silicone dispensing system is the right choice for precise material application with repeat accuracy even in the case of short cycle times. Its modular construction allows for fast cell adjustment to changing processes and batch sizes.
Learn more about the CNCell
- If the silicone casting resin has to be applied in a vacuum, different vacuum potting systems are available depending on the cycle time and workpiece size
Learn more about the vacuum potting systems
TIP: Silicone fluids also defy gravity to coat surfaces completely. It is therefore important to always process silicone on separate systems and ideally in its own space.