Renée Stein, Connie B. Roth, Elly Stewart Davis, and Olivia L. F. Boyd
Abstract
Since its introduction to the field of heritage conservation by Stephen Koob in 1986, Acryloid B-72 has been used extensively for coating, consolidating, and adhering a wide range of materials. Revisited in 2018 for the AIC Objects Specialty Group, the recommended formulations of Acryloid B-72 include small amounts (0.1 weight % or 1 teaspoon) of hydrophobic fumed silica. This addition is stated to improve rheological and working properties, such as flow, film formation, and evaporation rate. Fumed silica is a commercially produced, low density, high surface area particulate agglomerate of silica nanoparticles. The resin and fumed silica mixture is, therefore, a polymer-nanoparticle composite. Since the mid-1990’s research in polymer physics has demonstrated how adding tiny amounts of nanoparticles can cause large improvements in polymer properties resulting from the high interfacial area between polymer and nanoparticles. One of the characteristic features of polymer-nanoparticle composites is the increased strength imparted by very small amounts of nanoparticles. Research undertaken by the Physics Department and Carlos Museum at Emory University quantified the increase in strength relative to the amount of fumed silica in Acryloid B-72 mixtures. Recalling Koob’s original tests with glass slides, we built an apparatus to measure the weight tolerance of joins made to glass rods with different formulations of Acryloid B-72 and fumed silica. The resulting data demonstrate the appreciable increase in strength, a near doubling, accomplished by adding fumed silica to the resin and suggest an optimal percentage for maximum strength. Further testing evaluated the sheer strength of joined ceramic sherds, comparing neat resin, Koob’s mixture, and the optimal percentage derived from strength testing. Practical application reflects the capacity to use lower resin concentrations, allowing better penetration into cracks and voids, while still accomplishing joint strength due to the behavior of the polymer-nanoparticle composite.
In addition to summarizing the strength testing results for B-72 and fumed silica mixtures, this presentation considers the collaboration between student, professor, and conservators that enabled the research. An undergraduate physics major undertook the strength testing as an honors thesis project. The research of the faculty advisor focuses on soft matter physics, including how interfaces between components in polymer systems affect the physical properties and system dynamics. Her research group of graduate and undergraduate students develops experimental methods to understand the behavior of polymers and study the effects of temperature, mechanical forces, and other influences, such as particle interfaces. Conservators at Emory University’s Michael C. Carlos Museum provided insight into field practice and offered input on experimental design. Conservators also evaluated the experimental results for their practical impact on application and use of the polymer-nanoparticle composites, conducting trials with mock-ups and artifacts. This sort of fundamental characterization of treatment materials can be difficult to accomplish in small conservation labs that are principally tasked with preventive collections care and exhibition-driven object interventions. Recognizing the opportunity of collaboration and developing the research as a student project are productive strategies. This project was also useful preparation for the student, who went on to pursue graduate work in materials science.