Eric Hansen
Abstract
This paper presents the theoretical basis for differing physical properties of polymeric coatings applied from solutions of varied solvent composition; examples from the literature which show this effect for different resin and solvent systems; results of experimental work on films of poly (vinylacetates); and, emphasizes the importance of this effect for polymers used as coatings, adhesives or consolidants In the field of conservation.
The generally accepted conformation of a polymer In solution Is the “random chain.” The chain conformation will be extended In solvent mixtures that are thermodynamically favorable, “good” solvents, or retracted In solvent mixtures that are thermodynamically unfavorable, or “poor” solvents. These effects can persist in the dried films obtained from solutions of polymers, as has been shown for cellulose acetate, poly (vinylacetate) and poly (methylmethacrylate).
Free films obtained from casting different molecular weight PVAC (AYAA, AYAF and AYAT) were characterized by tensile fracture at a six month interval. Films cast from solutions of acetone or an acetone/ethanol mixture developed higher strength and lower elongation, after a six month time interval, in comparison to films cast from chloroform solutions which developed lower strength and higher elongation. Films cast from toluene solutions, by contrast, were shown to be plasticized from the retention of a significant amount of solvent, as demonstrated by gas-chromatographic and thermal analysis.
The dependence on solvent “type” of the physical properties of polymers is greatest for polar solvents and polar polymers. This class of polymers includes acetates, acrylics and their copolymers. Other properties that can be affected are adhesion to a substrate, hardness, refractive index and longevity (aging). Experimental work illustrating this effect can offer solution compositions for optimum physical properties and performance of polymers, explain failures caused by incorrectly expected polymer performance and also offer guidelines for solvent replacement based upon considerations of toxicity.