Seal against fuel with fluorosilicone

The unique properties of fluorosilicone (FMVQ) can solve issues in applications where both a wide operating temperature and resistance to fuel and other solvents is necessary. For these types of applications, such as when you need to seal against fuel, tradtitional silicone adhesives fall short.

Silicone adhesives are often an excellent choice for bonding or sealing against a number of fluids. They can even handle being immersed in said fluid during long persionds of time. But there are certin fluids that will make silicone swell, or even deteriorate it. Among these fluids are for example gasoline, organic solvents and fuels as well as silicone fluids. For these harsh conditions, the solution to the problem is fluorosilicone.

Chemical compatibility of fluorosilicone

Just like regular silicone elastomers, fluorosilicones offer a similarly wide operation temperature, oxidative stability and remarkable flexibility. They are also highly resistant to heat, ozone and sunlight (UV).

In other words, fluorosilicone works just lika a regular silicone (dimethyl silcione) would, but with increased resistance to fluids such as gasoline, diesel, acetone and alcohol. Additionally, fluorosilicone does not swell in regular silicone fluid and vice versa.

In the picture below, the abilities of fluorosilicones are displayed compared to a regular silicone. Details of the same dimensions have been immersed in jet fuel. After a certain time, the samples are taken out of the fuel and weighed to see how much they have absorbed. The difference in swelling is quite substantial – the winner being fluorosilicone.

Swelling of regular silicone compared to fluorosilicone in jet fuel (source: Avantor Nusil).

Important to note, however, is that fluorosilicone is less resistant than regular silicone against polar fluids. Polar fluids include ketones, aldehydes, amines and break fluids that are not based on petroleum.

Fluorosilicone chemistry

The chemical difference between a regular silicone and a fluorosilicone is that the fluorosilicone has trifluoropropyl-groups (-CH2CH2CF3) covalently bonded to the main polymer chains. Flourosilicones can have different levels of these side groups depending on the desired properties. There are for example products with 100 mol% fluorosilicone and 50 mol% fluorosilicone. Sometimes, you can also run across the term FMVQ which esentially means fluorosilicone.

When to pick fluorosilicone

Fluorosilicone can be used to seal against fuel, gasoline, diesel, non-polar solvents and silicone fluids. They can be used as adhesives, sealants, moulded silicone parts or protective coatings that may come in contact with these types of fluids.

Common uses for fluorosilicone are within the automotive and aviation industries. A quite common application is the sealing of fuel tanks. Often in automotive applications, the combination of fuel and high temperature resistance is necessary in order to maintain the properties that would make you choose silicone to begin with. Tests show that fluorosilicones are superior to regular silicones for these types of challenges that you may encounter when you need to seal against fuel.

The biggest drawback of using a fluorosilicone is the price which is substantially higher than for a regular silicone adhesive. Fluorosilicone is therefore notheing you will want to choose unless there is a need for it.

Product recommendations

Nusil FS-3730 is a 1-component moisture curing fluorosilicone adhesive.

Nusil CF1-3510 is a 2-component, heat curing fluorosilicone.

There are many more options so please contact us for advice!

Also check out Nusil’s selector guide and fluorosilicone flyer with tons of product options to choose from.