A partnership in coatings

Materials World magazine
,
29 Oct 2018

Stuart Lyon, AkzoNobel Professor of Corrosion Control in the Corrosion and Protection Centre at the University of Manchester, UK, and Simon Gibbon, AkzoNobel Corrosion Community of Practise, discuss their strategic research partnership.

Almost every object that we encounter is made suitable for purpose or given a function through coatings. In 2014, the British Coatings Federation reported that over 10,000 people were employed in the UK in manufacturing coatings, that the UK was a net exporter of paint, and that the coatings industry directly contributed over £11bln to the economy, supporting UK manufacturing and construction sectors worth around £150bln. Overall, coatings underpin the built material world by creating surfaces that have desirable properties, such as aesthetic, protection, conduction and insulation.

Organic polymeric coatings are particularly important, as they are effective, versatile and generally inexpensive. They can be described as any mixture of film-forming materials plus pigments, solvents and other additives, which when applied to a surface and cured or dried, yields a thin film whose primary function is to protect the surface of an object from the environment. However, this simple statement belies great materials complexity.

Protective coatings are essentially polymer-reinforced particulate composites that are generally formulated using a cross-linked polymer resin, solvents and other additives, and contain often nanoscale, functional solid components such as corrosion inhibitors, structural fillers and decorative, colourful pigments.

The cost of corrosion damage to the UK economy was estimated in 1971, by TP Hoar in A survey of corrosion and protection in the United Kingdom (Hoar report), at between 3-4% of GNP per annum while a more recent survey by IOM3 in 1999 put this figure at between 2-3% of GNP. The Corrosion and Protection Centre was set up in Manchester over 40 years ago as a direct outcome of the Hoar report to educate and raise awareness about corrosion science and engineering and to research corrosion protection solutions in support of industry.

AkzoNobel is a Dutch multinational protective and decorative coating manufacturer. The organisation supports considerable UK manufacturing, with a recent investment in Ashington resulting in an advanced and environmentally friendly paint factory.

The University of Manchester and International Paint – the protective coatings business unit of AkzoNobel – have worked together for over 30 years on a range of individual research projects. However, in 2011, AkzoNobel decided on a more strategic approach and, due to its academic depth in corrosion and electrochemical science, capabilities in state-of-the-art materials analysis and broad science base, selected University of Manchester as its global corrosion protection strategic research partner. The partnership has grown to involve 14 academics across three schools at Manchester, running 21 current projects including nine new PhD starts in 2017. The research resides within TRL1-5 and utilises advanced analysis and microscopy tools, including tomography, to understand and improve formulated products and product testing regimes.

Setting up a solution

The recently announced – funded for ~£5m over five years – prosperity partnership between The Engineering and Physical Sciences Research Council, AkzoNobel and the Universities of Manchester and Sheffield, will enable for the first time, a fundamental mechanistic understanding of how the performance of protective organic coatings arises – essentially it will tell us how paint works. Success will allow industry to side-step the current trial and error approaches and to incorporate digital design into the development of paints and similar nanocomposite materials, resulting in the confidence to use sustainable materials, comply with legislative and customer drivers and maintain and extend performance in more extreme environments.

How do paints work?

Paints are colloquially assumed to prevent the environment from getting to the substrate. However, this is generally untrue. Practical corrosion protective coatings are generally permeable to water and oxygen – the main reactants causing corrosion damage – and barely suppress the corrosion mechanism. Here we should remember that corrosion is an electrochemical process involving the transfer of charge – indeed, corrosion of zinc is what drives dry cell batteries. Overall, polymer coatings are likely to provide corrosion protection in at least four ways – form a partial barrier to corrosive species from the external medium, limit ionic charge transfer between anode and cathode areas on the metal substrate, act as matrices for dispersed corrosion inhibitive pigments or contain pathways for the migration of species from such pigments to the metallic interface.

Unfortunately, we neither understand the detailed mechanism of coating failure nor how active species from functional additives are transported within the polymer to and from interfaces. So, the ability to develop more effective protective coatings is limited by the inability to predict their service performance. Overall, the process complexities in the breakdown in corrosion protection that lead to coating failure are poorly understood, and meaningful lifetime models are not available. Consequently, product development is carried out on a largely empirical basis and often driven by comparative performance testing, such as salt-spray testing, that bears little resemblance to practical environmental conditions.

Overall methodology

Our approach to understanding the detailed mechanisms of paint failure relies on the application of advanced analytical tools. While such methods may be routine in other areas of materials science, we have pioneered the application of many of these methods into the domain of protective organic coatings. Starting in 2012, we have tackled several common-sense hypotheses for which little or no prior experimental evidence exists. Our overall aim using this approach was to confirm which of these are valid, and therefore may be valuable in coatings design, and which have no validity and thus may be discarded.

In paint research it is common to use model polymers to study performance in the laboratory. Unfortunately, due to differences in structure and chemistries, these results do not translate reliably into commercially formulated systems. We therefore decided to start with real systems and simplify them by stripping out complicating factors in the formulation. In this way our results can be tied directly back to products and applications of interest. Our research has looked at unfilled lacquer coatings – those that contain no particulate additives – where the key application is in coatings for food and beverage cans, and filled coatings containing solid pigments using, as an example, coatings for strip steel.

Epoxy resins have many of the characteristics required for effective corrosion protective coatings – high volume resistivity, excellent adhesion to surfaces and resistance to alkaline hydrolysis. A well-accepted hypothesis, consistent with the macroscopic behaviour but never tested experimentally, is that the gradual development of defects and/or damage within polymer, due to water ingress, eventually creates a percolating framework connecting the environment to the substrate, resulting in substrate corrosion.

Polymeric nanostructure

Water uptake into polymers can be studied quantitatively by mass change and by vibrational infrared (IR) spectroscopy. However, these methods provide averaged measurements with conventional IR microscopy limited by diffraction to a spatial resolution of around two microns. The University of Manchester obtained the first instrument in Europe able to break this diffraction limit.

This uses a tuneable laser to illuminate a specimen area of 2x2µm while the beam-specimen interaction is probed locally using an atomic force microscope tip. The effective spatial resolution of the measurement corresponds to the sample tip contact area. In this way we were able to map water at a spatial resolution of around 20nm and demonstrated, for the first time, the heterogeneous nature of water absorption. We have gone on to explore the detailed nature of the nanostructure and nanochemistry in epoxy polymers.

Corrosion performance

Corrosion protective coatings formulations include active ingredients – corrosion inhibitors – to protect against gradual environmental deterioration, and adventitious mechanical damage, during service.

In order to limit the corrosion reaction at the anode or cathode, a minimum quantity of active species needs to be released from the coating. However, if too much is leached at an early stage then it will be rapidly exhausted, and the overall lifetime of the coating will be reduced. This is an important balance to get right, and detailed understanding of these processes is critical in coatings design. We have shown how the development of a percolating network of active particles influences leaching kinetics. Thus, we can say that the optimal pigment volume concentration lies at, or just below, the critical 3D percolation threshold – around 20% by volume.

Damage accumulation in coatings

Studying how paints degrade and how corrosion progresses under paint is fundamentally a problem of studying a buried interface. Historically this has been a challenge problem when considering clear lacquers, and indeed an impossible one for filled paints with no transparency. Although optical or analytical electron microscopy has great value, they are post-hoc destructive methods. Moreover, they do not provide the full 3D picture of the connectivity and clustering of particles, which is critical in understanding ingress of environment and egress of active corrosion inhibitors.

There are currently two techniques whereby the internal structure of materials can be visualised in 3D. The first method essentially puts the mechanism for sample preparation into the analytical chamber of a scanning electron microscope. The initially prepared surface is imaged, whereupon a thin slice, typically in the range 50–5nm, is removed by focused ion-beam etching or by ultramicrotomy using a diamond knife. The newly revealed surface is then imaged again. By repeating this process for hundreds or thousands of times, serial sectioning, the images can be stacked together and reconstructed as a 3D visualisation.

Although this is a destructive method, imaging can be combined with those analytical tools available and, in principle, at the available resolution of the microscope. However, only relatively small volumes can be examined. The other method uses X-ray computerised tomography and here larger volumes can be examined, limited by the photon energy and brightness of the X-ray source. The non-destructive nature of the image capture allows examination of changes in materials as a function of time, something that we call 4D imaging. Bench-top instruments are available, but synchrotron light sources are needed for rapid imaging in the time domain.

X-ray 4D tomography allows us to take snapshots of the internal coating structure as a function of environmental exposure time. We show clearly how dissolution of the active inhibitor particles creates cavities within the paint allowing increased access of the environment to the metal substrate. Direct measurement of the consumption of the active inhibitor, combined with knowledge of the 3D connectivity of the particles, allows performance models for protective coatings to be developed and validated.