We often get asked a number of questions relating to silicone sticking to other surfaces and substrates. As simple as it may seem, the answer is often more difficult depending on the types of materials involved in the adhesion process. Here we will try to explain the basics of silicone and its adhesive properties to other surfaces.
Once cured, silicone can be extremely difficult to stick to surfaces. Silicone rubber will almost always need some form of adhesive to enable them to fix to a surface and in many ways can be related to Teflon by the way it repels things sticking to it, hence the reason Teflon® is use for frying pan coatings.
There are many different adhesives that can be used from double sided tape all the way to uncured silicone. Different surfaces such as glass, wood, metal and plastic do possess different adhesion properties i.e. good or bad or somewhere in between.
Many customers are puzzled why they experience problems when trying to glue silicone to some other substrate. Here we try to explain some fundamentals that need to be understood if you need silicone to stick to another substrate.
Firstly you need to understand that silicone does not stick to anything other than the adhesive system or the PSA (pressure sensitive adhesive). This means silicone does not interface directly with wood or metal or any other material, instead it interfaces with the PSA itself. The diagram below explains this.
Pressure Sensitive Adhesive is what it says it is – an adhesive which is sensitive to pressure. If the adhesive strip/sheet is applied to a surface and pressure is applied to it, it will stick. The trouble is that as stated before, different substrates will stick better than others.
This is the problem! There is a numerical value and unit that depicts how readily or not any particular substance will accept glue or PSA. There are many different types of glue/adhesive systems some of which utilise a chemical reaction to create a bond, PSA relies purely on “mechanical bonding”. Surface energy or interface energy, can be measured and if a surface has a low value then it will be difficult to adhere to. It will be no surprise that silicone has a low surface energy value therefore creating a non stick surface.
For example: If you take a sheet of silicone and pour on some water, the water will just run off and will not “wet out” as they say.
Looking at the chart; silicone lies around the same vicinity as Teflon® (PTFE) which as mentioned earlier is what they coat onto non-stick frying pans. Surface energy is measured in dynes per centimeter. The dyne level is the actual reading of the critical surface tension. The chart below compares the relative surface energy of commonly used substrates. Silicone comes right at the bottom with a value of 24 along with Teflon®.
Although a problem, the Alchemists have been at work to help out as usual. By means of the addition of various chemicals to the materials surface, the surface energy of Silicone Engineering’s silicone can be increased somewhat in order to allow at least adequate adhesion. It is difficult for just anyone to achieve this level of adhesion as these chemicals are not too readily available and they need to be used within strict application and operational guide lines.
So to answer the original question, silicone is not any more difficult to adhere to one surface than it is to another, it is the ability or not, due to surface energy, of any material to adhere to the PSA.
If they take silicone and try to stick it to two different materials say EVA and Nylon, one would get the impression that silicone sticks to Nylon better than EVA however, it is the Nylon’s ability to adhere to the PSA that is better than EVA’s ability to adhere to the PSA.
What does silicone not adhere/stick to? What does stick to silicone rubber?
Due to the low surface energy of cured silicones, it is almost impossible to get anything to easily stick to silicone. Because of this special glues and surface preparations are needed to bond cured silicones to another material.
What glues work with silicone?
Once cured Silicones can be notoriously difficult to adhere, here are a few tried and tested methods below.
- RTV’s (Room-Temperature-Vulcanization silicone)
- Special acrylics/PSA (Pressure sensitive adhesives)
- Heat vulcanized silicone glue
If a customer has an adhesion problem with our silicone, Silicone Engineering first ask is the silicone coming away from the adhesive or is the silicone along with adhesive coming away from the other surface. As with silicone, other surfaces can be difficult to adhere to, look at the list in the Low Surface Energy column above. Trying to stick PSA backed silicone sheet to aluminium would produce a good bond, but using the same sheet and trying to stick it to Teflon, the bond would not be very good at all.
Silicone does not have a problem with sticking to other materials but it is the PSA that might!