Application of Silicone Adhesives
It’s normally not that hard to stick different material together, but when sticking silicone extrusions to different substrates, the only thing that can stick to silicone is almost always silicone.
This is where we often find non-silicone based adhesives fail. They generally lack this property unique to silicone and when failure occurs it can lead in-field leakage, warranty claims and not so happy customers.
We often get asked exactly why Silicone materials are resistant to glues and adhesives. It comes down to the fact of it having a ‘Low Surface Energy” in contrast to other materials.
This means that in short those molecules on a surface of Silicone say compared to standard wood glue are energetically less favourable than other materials. This means Silicone is quite resistant in its own right, making it a great sealing material for a range of applications.
The best adhesives to glue silicone to metal or plastic substrates are silicone-based adhesives. Silicone adhesive works by providing a strong, flexible bond between silicone parts along with high elongation.
Silicone rarely fails in application and in the instances it does, it’s quite often due to contamination of the adhesive. So before installing the silicone seals, it’s necessary to clean the application substrates with clean fabric and detergent.
A variety of detergents could achieve this including detergent with isopropyl alcohol. Use of detergents with lotions, waxes, fragrances, oils or creams is not advisable as then can often leave an invisible residue on the surface.
For example. interior grove walls with grease and/or oil present will need special attention for removal of contaminants. Quite often this will require cleaning with a solution of isopropyl alcohol with warm water and a ‘lint-free’ cloth.
Usage of primers is also another way of adding strength to the adhesion. By priming the surface of a silicone rubber, one can enhance the bond ability potential by changing the surface chemistry. Combined with certain adhesives the user of primers can be very effective.
It is noteworthy that primers normally expire within 18 months from the manufacture date, and if not stored as per manufacturer instruction (i.e. exposed to air/moisture), they can expire prematurely and this is noted by the liquid becoming milky.
A common primer is Dow Corning 1200 OS Primer Clear which can be purchased from Bearing Distributors (61 2 9822 5806). It works well with common silicone-based adhesives such as Silastic 747. It’s important to follow the standard process listed below to prime and apply adhesives to achieve a strong adhesion:
Before you commence the job, it’s critical to work out exactly the materials you need. This may include the cleaning fluid, primer, the wet liquid silicone and of course the solid silicone profile piece.
Clean both surfaces, silicone extrusion as well as the frame. Isopropyl alcohol-based detergents works well for cleaning. The premier itself is also a good cleaner!
When both surfaces are cleaned, prime both surfaces (by wiping or spraying). It’s important to let the primer to dry (aka flash) before applying the adhesive.
Apply silicone adhesive onto the supporting surface and lay the silicone extrusion onto the adhesive and press it so the adhesive gets evenly distributed in between.
Clamp the two substrates together for minimum 2 days so the adhesive cures. It’s important not to put too much clamping force; otherwise the sealant would walk away from between substrates. 0.5mm adhesive is the minimum thickness to achieve a strong adhesion.
Once you have removed the clamps, it’s critical to test the strength of the adhesive bond. We recommend the ‘Pull Test’ which is simply pulling the silicone with reasonable strength to see that it holds up. If it comes off there may have been problems with the cleaning or product – if it remains, chances are you have a safe and stable seal.
To learn more about Jehbco’s silicone rubber and suitable adhesives, Contact Us right here.