Question: I used your test markers to test some film extruded from a masterbatch we are developing, and the results seemed inconclusive. It was hard to tell if the ink was beading or maintaining its integrity. Any comments?
Answer: If the test marker results seem inconclusive, it sounds like the surface energy of the film varies from spot to spot. There are several possible reasons for this.
First, if there are spots of contamination on the surface (fingerprints, transfer of contaminants from a process roll, contact of the sample with a foreign object, etc.), the contaminated spots will definitely have a lower surface energy, which would cause the swath of test ink you applied to show inconsistency in wetting vs. beading.
In the case of a polymer blend, or a polymer modified with multiple additives, it is possible that the blend is not being adequately dispersed in the extruder barrel, causing the various components of the formulation to segregate in the melt. In this case, concentrations of lower surface energy additives – or of lower surface energy polymers in a blend – will naturally bloom to the surface, creating “puddles” of low energy scattered across the film. Blends that include additives or modifiers with incompatible solubility coefficients could have the same problem: Even if they blend in the extruder barrel, they may disassociate before setting in the final film structure.
If the film is surface treated, it is possible that the treatment itself is contributing to the variation in dyne level. This case usually manifests in patterns that can be traced back to the mechanics and geometry of the treater.
To properly investigate any of these causes, it would be best to perform the dyne test using the drawdown method, which allows you to evaluate a relatively large area of film surface all in one pass. Patterns of variation will appear, which will be a valuable clue in determining the root cause of the problem. Also, by testing over a variety of dyne levels, you may be able to determine what might be called surface energy topography: just as an example, you may find that some randomly dispersed areas are testing at only about 32 dynes/cm, whereas the majority of the surface holds at 38. That might suggest blooming of low surface energy partials at those locations. And, in the case of film which has not been surface treated, it may provide a clue to which constituent is not being properly dispersed.
Even by using the simple method of applying the test fluids with cotton swabs, some patterns are easily recognizable. For example, spreading 38 dyne/cm test fluid over and around a fingerprint on a 44 dyne/cm surface will show the print as clearly as a forensics lab photo.