Under revision.
Weathering Durability
The work I do on weathering durability is focused on polymeric coatings, but there is no reason why the modeling approaches should not apply to any material that suffers the slings and arrows of an aggressive environment. Like all researchers, I try to find my own niche and thus try to avoid duplicating what others do, whilst trying to do something useful and worthwhile. In all of this, I must acknowledge the contribution of Dr. Brian Hinderliter, my students and post-docs who have been very tolerant and have done a lot of work. My other colleagues are very kind about my questions and often put me right.
The two parts to my attempts to understand the durability of polymeric coatings are:
(i) modeling that actually attempts to predict (in an actuarial sort of way) service lifetime, i.e. how the properties of coatings deteriorate in natural and artificial environments,
(ii) measurements of the relaxation properties of polymers as they degrade.
The modeling uses the recognition that a huge number of aggressive molecules, photons and other events arrive at random times and random places across an exposed material. This means that Monte Carlo simulations of these processes, or the use of the statistics of random processes, via the Central Limit Theorem, can be used to gauge how damage accumulates at the surface (they get rougher, and change chemically) or within the bulk of a material. The extent of damage can be linked to physical properties via some very well known (venerable) ideas, e.g. the Griffith fracture criterion, Bennett and Porteus work on the reflectance of rough surface etc. In principle, the Monte Carlo approach can be as exact as our knowledge of the composition of our material and its degradation mechanisms. Unfortunately, even our modern computers struggle to produce results within reasonable periods so we have tended to focus on the Central Limit Theorem approach to derive some simple algebraic models of our approach. In the algebra, we have no ambitions about capturing all the details, but we hope to provide some simple, but useful, guidance based on physics and chemistry (no arbitrary curve fitting) for those who want to understand the parameters that determine deterioration and how to compare their candidate materials. In this work, we have derived very simple relations for how gloss loss, contact angle, fracture strength, yellowing and corrosion protection diminish with exposure that are based the assumption that weathering induced flaws determine the failure; we have not yet modeled how polymer network properties change under environmental assault.
Our experimental work has focused on how mechanical properties and surface topography change during weathering. There are a number of very good scientists who use spectroscopy and microscopy to study the chemistry of degradation so we have focused on understanding how changes occur in the parameters that are required in the models above, i.e. surface roughness, mechanical properties etc. This has lead to the rediscovery of polymer physical aging, particularly how it changes mechanical properties as a polymer is being degraded. Thirty to forty years ago, physical aging was said to be important during exposure, but everybody’s attention wandered. More recently, we have looked at how surface roughness generated by degradation relaxes during those periods when the aggressive species is absent. This has proved to be sensitive to how the glass transition temperature is related to the exposure temperature and has shown, in two polymers, how the glass transition near the surface of a crosslinked polymer is much lower than in the bulk. This may be due to the normal inhibition of crosslinking near the surface of a coating and/or the polymer molecules may have more freedom near the surface, as they do in thermoplastics.
Layered Double Hydroxides, LDH
One of the major problems with applying all the wonderful findings on potential nanocomposites is that they are very difficult to use in high concentrations or in bulk. Dispersing them is a serious impediment because there is a huge number of particles involved, with huge surface energy to overcome. If we find ways to synthesize nanocomposites, in situ, on larger particles that are already dispersed in the system, then we might get further and gain greater advantage. To this end, we have been depositing LDH material on the surface of pigment particles. We are certainly successful in the synthesis and we are measuring the outcome, at present.
History of Paint Technology
Occasionally, I get involved with art historians and conservators, primarily when the field wants someone to discuss old paint technology. I was not involved when the old masters and even modern artists used their paints, but I can understand the materials that they used and I can explain how material variation occurred and the consequences. It has been fascinating learn how artists view their media and to see how subtleties in material composition determine how paint performs and changes over much longer periods than the average DIYer or contractor would worry about. My other advantage is that there has been paint research at NDSU since 1905, so the library has an enviable archive of old books and journals to peruse when people call for help. For its own sake, although it may not be a glamorous topic, it is interesting how external events (wars, shortages), polymer science, pigment development and analytical instrumentation all lead to the current state of paint technology. It is a humbling experience for a hard scientist to discover how much art historians know.