Optimising Coatings Formulations with Enhanced Adhesion for Challenging Environments

This project will be funded by Becker Industrial Coatings, a major international supplier of coil coatings located in the north-west of England.

Adhesion has been the focus of a considerable number of investigations in the past, so many of its secrets have been revealed. However delamination can still occur and this phenomenon can be encountered in the pre-coated metal industry. It is not easy to understand why one polymeric coating based does not stick to a previously applied coating of similar chemistry. Previous work between Becker Industrial Coatings and the University of Surrey has indicated that penetration of the second coating into the first is necessary in order to set up a strong bond between the two systems. This penetration needs only to be of the order of a few tens of nano-meters but never the less it must occur. More recent work has concentrated on understanding the driving forces which enable a single paint formulation to separate into distinct primer and topcoat layers.

Many coatings are thermosetting systems, which means they undergo a reaction to produce an infinite 3D network of chemical bonds or crosslinks. Often the density of crosslinks can prevent the penetration of the second coat into the first resulting in poor adhesion and ultimate delamination. However this is not always the case, sometimes the difference between one coating having good adhesion and another not is as ‘trivial’ as a minor change in the chemical make-up of the backbone of the major polymer in the formulation. Thus delamination is a result of a lack of compatibility which prevents the setting up of an interphase on the nano-scale.

The interphases can be studied preparing samples using microtomy and carrying out analysis by spectroscopic techniques that can analyse areas at high spatial and depth resolution. One of these techniques is time of flight secondary ion mass spectrometry (ToF-SIMS) another is X-ray photoelectron spectroscopy (XPS). The application of these techniques within materials science in general and adhesion investigations in particular has for many years been pioneered by the lead supervisor and his research group at the University of Surrey. In particular we have been very successful in elucidating adhesion mechanisms relevant to the coil coatings industry, in particular poly vinylidene fluoride (PVdF) coatings applied to various primers, where a clear interphase has been identified.

However, it is not just sequentially applied coatings that have interphase issues. Some materials are so incompatible that when mixed in liquid form the rapidly segregate into different phases. Beckers are developing a system that separates into a primer and topcoat after application (known as self-stratifying coatings). Can topcoat and primer show sufficient adhesion despite their inherent incompatibility? If so how? Is an interphase formed and how does it change with formulation?

Finally some systems self-segregate into different zones not in to layers. What types of interphases exist in these systems? Are they robust despite incompatibility? How are they affected by formulation changes?

Answers to some if not all of the questions posed above will be sought through the application of sophisticated surface analytical techniques, including sample preparation by ultra-low angle microtomy, such as ToF-SIMS and XPS to formulated products assembled in the research laboratories of Becker Industrial Coatings by the successful candidate.