Priming of surfaces to achieve improved adhesion has been employed in industry for many years and is of particular importance in the use of silicone adhesive sealants and 2-part RTV silicone rubbers. Silicone adhesives and some 2-part rubbers have built in adhesion promoters - with these products the use of a primer will only be required for use with substrates that are exceptionally difficult to bond to. Most 2-part silicone rubbers will not have any adhesive qualities and the use of a primer will be essential if adhesion is required.
Due to the large variety of substrates any recommendations regarding adhesion and the use of primers can only be taken as a guide, we always recommend thorough testing for adhesion of any system prior to production.
Silicone primers are normally supplied in the form of quick drying solvent solutions and are available in bottle or can packages.
A good primer must have certain properties and sometimes perform several functions:
- It must be easy to apply
- It must dry quickly to a uniform dry film (less than 30 minutes)
- The improvement in adhesion must be demonstrable
- Adhesion to the substrate should not be affected over the temperature range of the rubber
It is essential that a uniform, thin coating of the selected primer is applied and allowed to dry completely before bringing it into contact with the silicone elastomer system.
All silicone primers are supplied as solutions in a wide variety of organic solvents. Some of these solvents present a greater hazard to the user than others. Full details of the solvents and any other potentially hazardous components are given in the individual product Safety Data Sheets, which are available on request.
NB: Always handle primers in well-ventilated areas away from any potential sources of ignition.
- Select the primer of choice or contact CHT for advice on the best primer for a specific application.
- Degrease all surfaces to which the primer and silicone rubber are to be applied. Care should be taken to avoid high boiling point solvent fractions, which often leave undesirable residues.
- Allow the surface to dry or wipe dry with a clean, felt-free rag.
- If applying by brush, apply sparingly and clean brushes in a suitable solvent after use.
- Apply primer to the degreased dry surface to be bonded using single light applications.
- If possible, avoid multiple applications - a thin uniform film of primer is more effective.
- After sufficient surface coverage has been achieved, close the primer container.
- Leave primed articles to dry and cure for between 20 and 60 minutes depending on ambient temperature conditions (the use of elevated temperatures is not recommended). See product data for details.
- If using a one-component silicone sealant, apply it to one primed surface and immediately bring the other primed (or un-primed if desired) surface into contact. Apply gentle pressure to attain close contact, whilst leaving sufficient sealant (normally a minimum of 1mm) to produce a good joint.
- If, for example, adhesion between two substrates employs a 2-part silicone rubber, catalyse and degas the chosen rubber during the primer drying cycle and then pour the rubber in the normal manner.