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Silicone guide

In 1980, Silmid began trading as a supplier of silicone products into the aviation market. Since then, our portfolio has extended to a number of other chemistries, but silicones still make up a large part of our business.

Silicone products come in all sorts of colours, sizes and varieties, but to find the right product can sometimes be difficult. The below guide gives a quick illustration of the questions you should ask before selecting a product.

1 Part vs 2 Part

1 part silicones are standalone products that cure with no additional support. This is through Room Temperature Vulcanisation (RTV), meaning the product will begin to harden when exposed to temperatures of around 20°C. These products benefit from ease of use, as they require no mixing.

However, when depth is an issue, a 2 part product can be used. These products cure with the help of a catalyst or curing agent, meaning the cure spreads evenly throughout the silicone resulting in an even cure. This is often referred to as encapsulating or potting.

 

The above diagram shows a potting application. If a one part product was used, a full cure would only happen at point A, with uncured silicone underneath. Using, and correctly mixing, a 2 part product avoids this issue and ensures a full, even cure.

Acetoxy v Neutral Cure

With 1 part silicones, there are different cure systems available, categorised by the chemical given off during the curing process.

Acetoxy silicones give off acetic acid when they cure. This results in an unpleasant vinegar like smell, but can also lead to issues with the substrate depending on the material used. They are still heavily used due to cost effectiveness.

Neutral cure silicones do not give off any odour, but more importantly do not produce any corrosive chemicals during the cure that can damage components or the substrate. This is especially important in electrical applications. Neutral cure silicones can be further broken down into oxime cure (giving off Methyl Ethyl Ketoxime) and Alkoxy (giving off alcohol).

Flow Properties

The difference between flowing (self-levelling) and non-flowing silicones is determined by their viscosity. Lower viscosity products tend to have better flow properties and are useful in hard to reach applications. Non flowable silicones are thicker, but are ideal for vertical applications where you do not want the silicone to run.

Dow and Momentive both have a range of silicones to meet these requirements and more. A basic overview of these products is included here, with reference to ancillary product available to improve adhesion, speed up cure and clean up excess product.