Dyne levels are a great indicator of your chance for adhesion success. But dyne level does not guarantee adhesion. A few factors affect adhesion results: coating and material compatibility; surface energy; process variables; material variables and ultimately bonding requirements such as entanglement.
Coating and material compatibility is the first requirement for achieving adhesion success. Coating suppliers do an excellent job of identifying compatibility of their formulations with various surfaces so this information is readily available. Along with material compatibility will come recommendations for target surface energy for the surface that will receive the coating. Usually surface dyne levels are recommended to be approximately ten dynes greater than the coating’s surface energy.
Most engineered plastics have inert, non-porous surfaces with low surface energy. Atmospheric plasma and flame surface treaters are used to increase the surface energy of these plastics. Each of these technologies has its own treatment signature. In general, they remove organic contaminates, oxidize the surface, and create bonding sites to promote adhesion. It is a common practice to measure dyne level before and after surface treatment. This is a good idea as it will help you verify your starting dyne level and the effect of the surface treater on the dyne level. However there are a few inherent problems with dyne level readings.
Dyne pens and dyne solutions are used to help provide insight as to the current state of a surface in terms of surface energy and wettability. Dyne pens offer an easy and efficient tool to measure dyne level, but they can become contaminated from contact with surfaces. It is advised that dyne pens be used more as a pass or fail test than relied upon for a very specific reading. Testing techniques using dyne solutions rather than pens can provide more accurate results.
Dyne level gives some indication that you have a chance at success. But it does not guarantee it. It is not unheard of to see different adhesion results from materials that display the same dyne levels. So how can this occur? Some common reasons are variables in the process and equipment used to apply the ink, coating or adhesive. Operating variables such as material temperature can play a major impact on adhesion results. Surface migration of additives in the material can play a role as well.
Ultimately good adhesion will require entanglement at a molecular level between the material surface and the coating. Entanglement is dependent upon the variables discussed above; including material and coating compatibility, surface wettability, the availability of functional bonding sites, and control of process parameters.