The Surface Chemistry of HDGS: The Effect of Modification on the Adhesion of Radiation Curable Coatings.

In order to develop a new class of coating that exhibits good performance, avoiding the use of environmental unfriendly metal pre-treatments, it is necessary to fully understand how the coating interacts with the metal substrates. To this end the adhesion between hot dip galvanised steel (HDGS) and a UV-cured coating based on an epoxy acrylate resin has been investigated. The HDGS surface chemistry is influenced by the various process and production steps, in particular from the skin-pass process, as the HDGS surface produced is not homogeneous. Indeed XPS gives very similar analyses for panels showing good and bad adhesion. ToF-SIMS and AES, giving good spatial resolution, are able to differentiate between panels showing good adhesion and panels showing poor adhesion. A high coverage of zinc oxide on the HDGS surface leads to good adhesion properties. Water-based pre-treatments that increase the zinc oxide coverage, by the removal of a layer of surface segregated AI2O3, improve the adhesion of the resin to the HDGS. Adsorption studies have been employed to understand how the coating interacts with the metal substrate. These studies have revealed that the phosphorus-based adhesion promoters included in the coating strongly modify the surface chemistry of HDGS, dissolving the oxide layer covering the HDGS surface, a mixture of zinc and aluminium oxide, and promoting the formation of a zinc phosphate layer over the metal substrate. The study of the interfacial failure surfaces produced by stripping off the coating from the HDGS panels in a simple peel test revealed that the poor coating/metal adhesion is due to an interfacial failure between the coating and the regions where the original aluminium oxide layer is still present. The failure created in these regions then propagates leading to a cohesive failure within the zinc phosphate layer present at the HDGS/coating interface