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Silicones

Srcoll

Dispensing media

Silicones

Silicones are used as functional materials when temperature resistance coupled with high elasticity is required. 

Safely built on sand

Due to their chemical nature, the class of silicones occupies a special position in the field of adhesives, since they are not based on the element carbon, but on silicon. They can be used in a wide range of applications. Their health safety is particularly appreciated in cosmetics, the food sector and medical applications. Silicone products are also suitable as sealants in the construction sector or as additives in paints and cleaning agents. 

The automotive industry appreciates their high temperature resistance and flexibility even at low temperatures as well as their hydrophobicity in demanding sealing applications in the field of sensors and control units. Due to their aging resistance and optical clarity, silicones are sought-after functional materials in LED and display production (optical bonding).

All silicones, both in sealing applications and in full electronic encapsulation, are characterized by their broad temperature spectrum (-50 °C + 200 °C). Typically, silicones achieve a tensile shear strength of up to 5 MPa with high flexibility (elongation at break of up to 100 percent). Silicones are good electrical insulators and are therefore used in the field of full electronic encapsulation. Like all elastomers, silicones tend to swell when exposed to the appropriate media. Unlike epoxies and polyurethanes, however, this process is reversible. After evaporation of the solvent, the material is unchanged. In contrast to polyurethane-based materials, exposure to UV light is easily tolerated, especially in outdoor applications. The curing mechanisms are manifold.

Our Duplex Plus static mixing head allows precise bead application. Due to the endless piston technology of the eccentric screw, a short back-twist pulse can promote thread breakage of the adhesive bead on the component.

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Applications and requirements

1/2C polycondensation curing silicones

For silicones, the abbreviation RTV (e.g. 1C RTV silicone) is often used. RTV stands for room temperature curing or room temperature vulcanization. The best-known subclass is represented by the 1C RTV moisture-curing silicones, which are frequently used as sealants in the sanitary sector. These cure to elastomers under atmospheric humidity and the elimination of a by-product that escapes from the material during the curing reaction and is responsible for the characteristic odor (typically acetic acid or alcohol).

Consistent exclusion of moisture in the area of the dispensing components and material preparation is necessary to ensure trouble-free operation. Mixing is not necessary with these systems; they can be applied directly from the dispensing pump to the component. 

Dispensing pumps

In contrast to polyaddition materials, a by-product is formed during the curing reaction. This is released into the environment. Likewise, moisture must be able to enter the bond so that the reaction can take place. In metal bonding, corrosion protection must be taken into account with acetic acid 1C RTV silicones. In the case of plastics, the release product can lead to stress cracks in the material under certain circumstances.

1/2C polyaddition curing silicones

If no release product can be tolerated or the ingress of moisture is not possible, the class of polyaddition-curing silicones can be used with comparable end properties. Depending on the system, these cure already at room temperature (2C systems) or at elevated temperature (1C systems). Curing times in the range of minutes can be achieved by increasing the temperature.

Compared to the relatively robust systems that react under polycondensation, the platinum-crosslinking polyaddition systems are sensitive to catalyst poisons such as sulfur (e.g. from cable connections or flux from the soldering process). Various heavy metals as well as amines (e.g. hardeners from neighboring epoxy processes) then no longer cure or cure only very slowly. In our machine concepts, we consistently avoid all sources of risk for catalyst poisons. This is often done in close coordination with your material supplier.

A large field of application for the addition-curing RTV silicones is the full encapsulation of electronic components and control units. For these self-leveling, low-viscosity systems, material preparation under vacuum is required, as silicones can absorb a great deal of gas. This is easily achieved with our MPS preparation systems. In addition to room-temperature and heat-curing systems, UV-curing systems are also available on the market, which are made to react by targeted irradiation with UV light of a defined wavelength according to the data sheet.

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