The photo-stability of acrylic tri-block copolymer blends for the consolidation of cultural heritage

A variety of adhesives are used for the conservation of paintings. These materials include natural adhesives such as animal glues, waxes, and gums which may chemically degrade over time resulting in unwanted discoloration and mechanical failure. Synthetic adhesives have been introduced to address these concerns. However, most consolidating adhesives have not been specifically formulated and tested to meet the high demands of conservation, ultimately resulting in undesirable physical and mechanical properties. Additionally, some synthetic adhesives are less stable and may cross-link making it difficult to remove years after application. This paper investigates the photo-stability of commercially available tri-block acrylic copolymers (PMMA-PnBA-PMMA) to assess their potential long-term serviceability as consolidants for flaking paint. These copolymers were combined with synthetic low molecular weight resins to reduce viscosity of the adhesive and provide tack. The polymer blends underwent accelerated aging under simulated indoor conditions. Blends were analysed for degradation using Fourier-transform infrared spectroscopy and size exclusion chromatography. Upon irradiation, acrylic copolymers (PMMA-PnBA-PMMA) undergo shifts to higher and lower molecular weight, suggesting that degradation occurs by polymer cross-linking and chain scission. Furthermore, the acrylic copolymer degradation rate was influenced by the type of low molecular weight resin. However, with the addition of a hindered amine light stabilizer these blends exhibited minimal changes in molecular weight. Lastly, the peel strength of the blends were investigated and shown to have comparable peel strength to a popular commercial material, Paraloid™ B-72, often used in consolidation thus showing their promise for use within cultural heritage.