How Can The Level Of Adhesion Be Improved?
A number of steps must be taken to ensure good bonding between the adhesive and substrate. The strength of an adhesive bond can be improved by:
- Selecting an appropriate adhesive (for the chemistry of the substrate surface)
- Ensuring adequate preparation and pre-treatment of substrate
- Carrying out bonding process properly
- Using an adhesion promoter or primer
The most common cause of bond failure is due to the contamination of the bonding surfaces. To avoid this, the substrate surface must be cleaned thoroughly before they are bonded. Potential surface contaminants include oil, greases, and dirt. Ingredients in the rubber compounds such as waxes, plasticisers and process aids can also lead to contamination and ultimately weaking of the bond.
Adhesive primers are a surface coating which are often used to pre-treat the substrate surfaces before the adhesive is applied. Primers are used to increase the strength of the adhesive bond and can be used to help adhesives bond to different surfaces. Pre-treating the substrate surface often improves the adhesive’s ability to wet out the surface. This can help the adhesive to bond to metals, plastics, or materials with low surface energy which it would not otherwise create a strong bond with. Primers can also be used to protect corrosion sensitive surfaces.
Primers are especially useful for use on porous surfaces that otherwise tend to absorb the adhesive. Adhesive primers are also used to strengthen the bond for products designed for environments where the temperature is consistently below zero degrees.
Primers are often formulated to be used on a specific substrate material or in conjunction with a particular adhesive. Adhesive primers are available as a paint or a spray which can be easily applied to the substrate material. The primer should be applied to the surfaces as evenly as possible. Most primers dry within 1 – 3 hours. The substrate must be properly cleaned and dried before the application of the primer to avoid contaminants.
How to Prepare a Surface for Bonding
To successfully remove all potential surface contaminants the Degrease-Abrade-Degrease procedure should be followed. The first degrease removes inherent surface contaminants from the surface. The substrate can then be abraded, either mechanically or chemically. This is an essential step as it increases the surface area of the surface and provides a mechanical key for interlocking. Finally, a second degrease is required to remove any contaminants left by the abradant.
The primer or adhesive must then be applied to the substate within a short time window, to minimise the contamination from the environment or air.