Buying that premium tube of threadlocking adhesive is no guarantee of achieving an unbreakable bond. Before adhesive is even applied proper surface preparation should take place. Unless surfaces are free of contaminants like rust, water, oil, dirt and dust, the quality and lifetime of the bonding joint will be adversely affected.
It all comes down to something called ‘surface energy’. Ensuring that your substrate has a higher surface energy than the adhesive you are applying will create a better bond. Dirt and other contaminants reduce the surface energy of the substrate, so using cleaners and degreasers is an effective way to increase the strength of your bond.
1. Imagine water on the bonnet of a car
A good example to use to explain the concept of surface energy is that of a waxed car bonnet. A bonnet that has been waxed after washing has lower surface energy than water, so water just rolls off the bonnet surface like a ball. This is called ‘minimal wetting’. A car that has not been waxed has much less friction and higher surface energy, so that when it comes into contact with the water, the water spreads out and fills the contact area.
The objective of surface preparation is to increase the wetting process and give your adhesive a greater area in which to expand into.
2. Cleaning & degreasing the substrate
While it might look clean enough with the naked eye, your substrate can hold contaminants from machining, handling, protecting, manufacturing and facility surroundings. It is estimated that even a thin film of oil will reduce adhesive performance by as much as 60%. Applying a cleaner, isopropyl alcohol or a degreasing solvent such as Loctite 7063 will remove dirt and debris. It may be necessary to immerse the substrate in a fresh bath of solvent, rinsing and then drying before applying the adhesive.
3. Roughing up the substrate
We all know that we should sand a surface before painting it. The same applies to adhesion and surface preparation. Giving the substrate a rough abrasive workover with sandpaper or a wire brush will remove heavy deposits as well as creating a surface area with greater peaks and valleys, which has the effect of giving the adhesive more to grip to and increasing adhesive properties. Degreasing should take place before and after the abrasion process.
4. Applying a primer
Once the part is clean, dry and rough, it might still have a low surface energy. Some materials are notoriously hard to bond, like polyethylene, polypropylene, polytetrafluoroethylene (PTFE) and thermoplastic rubber materials. If that is the case, applying a polyolefin primer such as Loctite 770 can increase the surface energy of these substrates so adhesives bond more effectively. Apply by spraying, brushing or dipping at ambient temperature.
If you have any questions about surface preparation or need any help with applying adhesive, please do not hesitate to contact our technical team.
