Question: We have determined that a dyne level result of about 42 to 44 qualifies an aluminum surface for our bonding operations. We get this dyne level pretty consistently from our cleaning line, but recently we had a case where the surface wetted out all the way to 50 dynes/cm. It’s hard to imagine that the cleaning process was actually providing that clean of a surface. Any ideas on this?
Answer: For most aluminum alloys, a reading of 42 to 44 dynes/cm reflects a relatively clean surface, essentially free of oils or other problematic contaminants. So, your historical experience that this result predicts good adhesion is not surprising.
As to how you could end up with a reading considerably higher for no obvious reason, I have a few thoughts. First, is it possible the test fluids became contaminated from exposure to oils or other surface contaminants? If you had previously tested samples that failed at the usual dyne levels (in other words, were not clean), then re-used the same test markers – or somehow re-introduced contaminants into bottles of test fluid – their surface tension would be affected. Specifically, it would decrease due to incorporation of the low surface tension contaminant into the reagent. This would cause “higher” readings.
Second, did you perform the test starting at lower dyne levels, which wetted for considerably more than two seconds, then worked upwards until you determined the dyne level which started to bead up in just a couple of seconds? Based on your usual results, I would recommend starting at about 34 to 36 dynes/cm to ensure full wetting at the outset of the test. Finding the transition point from wetting to beading is important: if you start the test at too high a dyne level, spurious wetting can occur, which will invalidate results. This is why we stress that the test is finished as soon as you find the dyne level which beads up in 2 seconds or less. If this is the case, it suggests that your parts actually have a low surface energy, and will likely prove difficult to bond.
Finally, if your cleaning process uses solvents or surfactants (detergents), it is possible that the rinse cycle was ineffective. This would leave a film of low surface tension solvent or surfactant on the part, which could become solubilized in the dyne solutions, dropping their surface tension as soon as they contact the surface. This is the same effect as the first possibility noted above, but from a different cause. The way to test for this possibility would be to hand-rinse the parts and re-test. If the results return to your expected values, that would be an indication that the cleaning solution is not being entirely rinsed from the parts. In that case, the first thing I would look at is the purity of the rinse water. Distilled water has a surface tension of 72 dynes/cm at 20° C – if your rinse water has a lower surface tension, it is contaminated.