ECC ‘Coatings for Plastics' Program in Berlin Examines Adhesion Issues
A key issue in the coating of plastics is of course adhesion - and so it was no surprise that the first day of the conference was almost totally devoted to substrates and their pre-treatment. Adhesion, especially to thermoplastic polyolefins (TPOs), remains a substantial problem for waterborne coatings.
NCPOs for TPOsPaul Swan (Eastman) presented new non-chlorinated polyolefin adhesion promoters (NCPOs) which, in primers, and especially when blended with traditional CPOs, lead to very good adhesion of the coatings to TPO substrates and impart excellent petrol resistance to coatings systems.
Kathrin Lehmann (Degussa/Tego Chemie Service) presented new Gemini surfactants that can perform the balancing act of serving as highly effective anti-foam agents for waterborne coating formulations and simultaneously enhancing wetting of substrates and adhesion of waterborne coatings.
Lasting Chemical ActivationThree speakers proposed direct chemical modification of the substrate surface: Dr. Jürgen Nagel (Institute for Polymer Research in Dresden) reported on research into functionalizing the surface of plastics by means of a reaction inside the injection mold as a way to make the surface more reactive. Rosanna Telesca (Ciba), on the other hand, presented a plasma activation by which substrates can be coated with a layer of photoinitiator molecules during activation that react according to their nature with UV coatings to afford very good adhesion. In a very similar vein, Dr. Pere Castell-Muixi (Technical University of Eindhoven) grafts benzophenone onto PP substrates to provide good adhesion for UV powder coatings. The advantage of all three techniques is that the substrate remains lastingly active, in contrast to the case for pure plasma methods and flame treatment.
3D Plasma SourcesThe more physical surface-pretreatment regimes were addressed by Bernd Stedry (Henkel), who presented cleaning methods involving high-pressure water jets; by Dr. Hildegard Sung-Spitzl (Iplas), who demonstrated new plasma sources that are particularly notable for rapid plasma activation of 3D parts; and by Jörn Birkel (Clean-Lasersysteme), who showed that laser evaporation constitutes a sufficiently quick method of activating the substrate's surface.
Simulation Simplifies Process DevelopmentJosé Gamero (Lactec) reported on highly efficient coating techniques that utilize "pigging" and are conducive to particularly fast, economical color changes. Ulrich Strohbeck (Fraunhofer IPA Stuttgart) demonstrated simulation techniques that can substantially simplify the development of methods for coating plastics. Among other things, given the correct approach, the conductivity of the still liquid coating can be exploited to dissipate electric charges, a fact that permits more effective electrostatic application, even to plastics that act as insulators.
Class A Surfaces With Coated FilmsA highly interesting alternative method of coating substrates directly was presented by Dr. Winfried Kreis (BASF Coatings). A thin, chromophoric film is covered with a clearcoat, shaped and UV-cured. Only then is it back-molded with the plastic substrate. The advantage of this method is that Class A high-gloss surfaces can be achieved, and there is no problem with reproducibility or homogeneity of difficult hues, such as brilliant silver or emerald black.
Sol-gel coating systems for plastics were discussed by Dr. Stefan Gödicke (Nano-X). These coatings are notable for their extreme mar resistance. Karl-Heinz Fischer (Dr. Schoch AG) gave an account of coatings on PP that do not require any pre-treatment - however, after his paper, the audience was still trying to work out how this feat can be achieved chemically.
High-Throughput Adhesion TestsDr. Radislav Potyrailo (General Electric) reported on his research into the use of combinatorial high-throughput experiments (HTE) to develop new coatings systems featuring improved adhesion. Miniaturized, automated formulation and adhesion tests have been developed that are much, much faster than conventional methods. Many highly promising, new starting formulations have been found in this way.
Finally, three speakers presented new coatings raw materials: Stefan Smeets (Surface Specialties UCB) described efficient binders for UV coatings; Dirk Mestach (Akzo Nobel Resins) demonstrated novel, surfactant-free acrylic dispersions for aqueous formulations; and Frank Kother (BykChemie) showed how waxes may be used efficiently for matting and texturizing plastic coatings.
Plastics Substrates Developing Too Quickly?A distinctive element of the ECC was again the group discussions on specific expert questions that had been submitted in advance, especially on the subject of coatings adhesion. For example, there was a discussion about how current test methods do not yield any information about the underlying mechanisms of adhesion, with the result that a good or bad performance provides no insight into the reasons for adhesion or non-adhesion.
There are fundamental studies that could provide such information but they suffer from the problem that "almost every year" the automotive sector alone introduces another substrate variant that often requires a complete review of coating systems - this is happening too quickly to enable important issues of industrially relevant substrates to be researched. A possible solution would be to make greater use of HTE, perhaps combined with simulations and followed by exact analyses of selected systems, both good ones and bad ones.