As any professional who works with adhesives knows, surface preparation is one of the most vital elements in achieving a high bond strength and optimising durability and lifespan of bonded joints. Different substrate materials will require different types of adhesives for bonding and different methods of surface preparation. Some surfaces may require nothing more than having the dust brushed away, while others may require elaborate surface preparation procedures prior to bonding.
Whatever materials you work with, it’s a good idea to equip yourself with a comprehensive knowledge of surface preparation. Even when not much surface preparation is required, going the extra few steps may make a big difference in the consistency and effectiveness of your adhesive bond.
Removing intermediate surfaces
When considering surface preparation, first think about the materials you want to bond together. Do you want to bond paint to an oxide layer, or do you really want to bond the steel that is under the paint to the aluminium that is under that aluminium oxide layer? Do you want to bond that mold release agent to mold release agent or bond the pieces of polycarbonate under that mold release agent? In summary, use the surface treatment necessary to remove what is covering the material to be bonded.
Even when it appears easy to just apply adhesive epoxy directly on top of the paint, it is important to bear in mind that the strength of the joint is only going to be as good as the strength of the paint – i.e., if the paint peels off, then the bond will fail. For this reason, it is a very good idea to remove the paint and bond directly to the material underneath.
Rust/oxide layer removal
Removing rust mainly requires a good rust cleaner with oxalic acid. Try out a few brands and see what you like best. Make sure to use rubber gloves and goggles, as this stuff can be pretty abrasive.
Use a wire brush and the cleaner to scrub away the rust.
Depending on the extent of the damage caused by the rust and the adhesive you’re using, you may have to sand (or preferably abrade) the area down to a smooth surface before applying the adhesive.
It’s a good idea after all this sanding and brushing to give your metal surface a good clean off with a clean cloth and some acetone or isopropanol. These act as a good degreaser and evaporate to leave a clean, dry, contamination-free surface to bond to.
Many metals form an invisible oxide layer which can on occasion be quite weak. It is a good idea to abrade and degrease all metals before bonding, if possible. Use carborundum paper to abrade metal (similar to sand paper but more robust and less messy). If you have a lot to abrade, using a grit blaster is a good idea, so as not to end up with arms like an Olympic shot-putter.
Okay, we all know how to sand, more or less, but there are a few tricks you can apply to improve your sanding kung fu…
Some general tips for sanding:
Use high-quality paper. It tends to last longer and be more effective.
Always start with the big, chunky, coarse papers, and work your way down to the finer grain papers. The coarser papers will even out the surface while the finer papers will smooth it down.
Tap the dust off of your sanding paper now and then to keep the grit from building up and scratching the surface or impeding the paper’s progress.
Some tips for abrading metal:
Before abrading, remove grease from metal surfaces with acetone or isopropanol. This prevents you from embedding the grime any further.
Use carborundum paper rather than sandpaper, starting with a rough grade to remove the bulk of the oxide layer and then a fine grade to get a good finish.
When dry abrading, don’t push down on the paper. Apply a light touch and let the paper do the work.
For a more consistent surface, don’t just sand in one direction, use an even, circular motion to give the adhesive a good surface to “key” into.
When wet abrading, apply a little bit of pressure and short strokes. Use plenty of water and don’t let the surface dry while sanding. It is important to dry the metal straightaway afterward to prevent rusting.
Of course, you need a clean surface. If you’ve got a layer of dirt on both surfaces, all you’re doing is sticking dirt to dirt.
The main task at hand is just removing any dust, grease or mould, so anything that gets the job done is fine. Know your surfaces and how to clean them. However, on some surfaces you may not be able to visibly see dirt or contamination – there may be traces of release agent if the component has been moulded or silicone-based cleaning agents which can be present on glass surfaces. Using an isopropanol wipe before bonding will set your mind at ease.
In order for an adhesive to effectively bond to a substrate, it must be able to “wet” the surface. Some surface treatments affect the surface energy. In non-technical terms it is the difference between water on a freshly waxed car which beads up and rolls off or water on an old unwaxed car that wets the entire car. Methods to increase wetability include plasma treatment, corona discharge and product-specific primers.
See the video here.
For further help and advice, please contact Permabond.