David Thomas and I could be career siblings. We both have technical backgrounds in materials and polymer science. We both spent time in industry. And this the first time for both of us at an annual AIC meeting. Add in my pre-conservation R&D work on adhesives and the photochemical reactions of polymers, and it’s not surprising that I was chomping at the bit to hear his talk “Stability of Polyvinyl Butyral Polymers with Light Exposure”.
David explained how he, Matthew Clarke, and Blythe McCarthy were looking at the possibility of using polyvinyl butyral (PVB) as an adhesive material for use in consolidating fragments of wall paintings from the Kizil Cave Complex in Xinjiang Province, China. These wall paintings are generally matte in appearance and are comprised of gypsum on mud plaster support. More specifically, sixteen fragile fragments had been leant to the Freer Gallery from the Smithsonian American Art Museum.
PVB was identified as a potential match to the consolidation needs of the wall painting fragments for a few reasons. It darkens to a matte surface comparable or better than similar materials, it is easily applied and removed using ethanol, and it has been shown to be an effective consolidant for wood and bone materials. This left the question: how does PVB age under light exposure?
Photo-aging of PVB might sound familiar. David pointed out that Robert Feller published results on the photodegradation of PVB materials in 2007 (http://www.sciencedirect.com/science/article/pii/S0141391005005057), digging into the chemical mechanisms of PVB degradation. The goal here, David emphasized, was not to look at the degradation mechanism itself but to investigate a more practical comparison of similar consolidation materials in actual use.
I found David at his most compelling when digging into the compositions and manufacturers of various grades of PVB. The types of PVB available vary primarily in their relative amounts of butyral, alcohol, and acetate content, a consequence of the polymer manufacturing process. He had a clear wealth of knowledge to offer here, and I wish he had presented even more (but that might just be the polymer scientist in me, your mileage may vary).
David and coworkers cast test films of a number of these grades of PVB, as well as control films of pure polyvinyl acetate (the industrial PVB precursor) and the ubiquitous Acryloid B-72 acrylic polymer, which were all then exposed to UVA (long wavelength UV) irradiation. At the same time, coworkers aged the same films by exposure to a weatherometer’s Xe lamp at the National Gallery of Art. They then tracked changes in color, mass loss, and IR spectra.
Trends in weight loss during aging of the films seemed to correspond with the polyvinyl alcohol content of the PVB: more alcohol led to more weight loss (ie, Mowital B30H degraded more quickly than Butvar B-98). The controls of B-72 and polyvinyl acetate showed no weight loss, and no appreciable discoloration. And by using a fiber optic-coupled FTIR to track the C=O carbonyl signal, an indicator of light-induced oxidative degradation, David showed that indeed the most rapidly degrading B30H showed increased C=O during aging compared to B-98, while the stable B-72 showed comparatively little C=O change and thus less degradation. In all cases, light-exposed PVB could be easily removed with ethanol, showing that no cross-linking reactions were occurring.
Comparison of chemistry induced by light sources as different as a weatherometer’s Xe arc lamp and comparatively narrow-band UVA lamp is a delicate affair. So I was disappointed that the talk did not include how the team evaluated light irradiance and dosage. It begs the question of how much photo-chemical energy was actually imparted to each sample. This also begged the question of heat. A Xe arc lamp in a small weatherometer chamber could reasonably be expected to induce a significant amount of heating compared to UVA bulbs. As David highlighted the comparatively high temperatures of Feller’s prior PVB work compared to the work presented here, it makes it difficult to determine how similar or different these varied results might be.
The data showing PVB applied and aged on Plaster of Paris mock-up materials looked promising. David offered some reasonable guesses as to why PVB might be more promising in real object application than on thin films. For example, more opaque materials would shield PVB from full light exposure by acting as a competitive absorber. And the color changes in PVB could be relatively minor compared to the colors of the treated object.
I still would have loved to have seen more about how closely the thin films and mock-ups behaved and had time to ask some of my more nagging questions about light and heat levels.
There are a few broader take-aways from David’s talk that I know I will be keeping in mind for myself:
- It never hurts to take an extra look at the sourcing and composition of “trade name” products. As vague as they might be, a product’s MSDS/SDS could provide a useful starting point to sleuth out the components in new products or find changes in existing ones.
- Mindfulness in equipment specs are crucial for useful comparisons of test cases. Perhaps even more crucial is carefully reporting those specs to simplify and aid others in their own comparisons.
- Don’t take old studies at simple face value. David made a good case for trying them again closer to home and closer to your specific application of interest. You never know what you might find.