Cross-link picture varnish

Poly(butyl methacrylate) (PBMA) began to be used as a picture varnish in the early 1930s. It encountered a considerable success because of its resistance to yellowing, adequate flexibility, no dirt pick-up and good solubility in nonpolar hydrocarbon solvents. Products based on PBMA, such as Elvacite 2044 and Elvacite 2045 by Du Pont, were abandoned when it was discovered that under light exposure they cross-link to an unexpected extent becoming insoluble [64]. From this point of view acrylic copolymers based on methyl and ethyl acrylates/methacrylates show a much better long-term stability. 

Feller, R. L. (1971a). Early smdies on the cross-linking of polymers. In R. L. EeHer, N. Stolow, E. H. Jones (Eds.), On picture varnishes and their solvents (pp. 195-201). Case Western Resawe University. 

Paraloid B-67, a PiBMA (Chiantore and Lazzari, 1996), is used as a picture varnish (Figure A3.14). Although it cross-links (Feller, 1976), especially under severe UV exposure (Chiantore and Lazzari, 2001), it appears to be peculiarly stable and of Class A standard (Feller, 1983). However, this stability appears to derive from an unknown composition, which creates an extended induction period before cross-linking starts. It is soluble in less polar solvents than B-72. When applied to marble, 15% rapidly becomes insoluble, which increases on accelerated ageing to 70% (Favaro et al., 2006). 

---