4.8 Development and Application of a Mathematical Model of the Drying Process of Coatings in Conservation
The Getty Conservation Institute
Period of Activity: 1987-1990
Solvent retention in coatings can have a significant influence on their mechanical properties and their aging behavior. A better understanding of the kinetics involved was necessary.
Taketomo, A. G., "A Materials Science Bibliography of Relevant Polyvinyl Acetate Literature," In-house annotated bibliography, 82 citations, September 22, 1987.
ABSTRACT-This bibliography was prepared in support of GCI's materials science evaluation of poly(vinyl acetate) polymer films poly(vinyl acetate) polymer films ;and coatings. The databases searched include: AATA Chem Abstracts; Dissertation Abstracts; and NTIS. Literature concentrated upon were:
a.) instrumental methods of analysis
b.) physical properties testing
c.) degradative mechanisms
Abstracts have been rewritten to reflect the research interests of the GCI.
Hansen, E., "The Effects of the Solvent Used in Casting on the Physical Properties of Poly(vinylacetate)," Paper presented at WAAC Conference, Yosemite, California, November 5, 1988.
ABSTRACT-Applying a polymeric film (coating or adhesive) by solvent evaporation (drying) of a solution can result in films of different properties depending upon the thermodynamic state of the polymer in solution. "Good" solvents extend the polymer chain in solution and "poor" solvents cause retraction. A larger conformation (hydrodynamic volume) promotes interchain interactions while a smaller conformation promotes intrachain interactions. These changes in polymer conformation persist in the film after solvent evaporation. Examples of this effect from the literature include ethylcellulose films and poly(vinylacetate) films. Experimental data are presented to show that this effect persists through time by testing films 1.5 and 6 months after casting. Films of polyvinylacetate (AYAT, AYAF, and AYAA ) were tested by tensile fracture for mechanical properties, gas chromatographic analysis for retained solvent, and by thermal analysis for determining the glass transition temperature (Tg). Films from poor solvents (acetone and acetone-ethanol-water ) resulted in higher Tg values and brittle, stronger films in comparison to films from good solvents (chloroform, toluene) with a resulting lower Tg and significantly greater elongation. Retained solvent is greatest for polar polymers and polar solvents, and thus is an important consideration in the field of conservation where acetates and acrylics are widely used as coatings, consolidants, and adhesives. Other properties affected are adhesion to a substrate, refractive index, hardness, and longevity (aging). Substitution of one solvent by another solvent based only upon workability of the solution or considerations of toxicity of the solvent may result in unexpected and undesirable physical properties.
Hansen, E., "The Effect of Solvent 'Type' Used for the Application of Polymer Films on the Resultant Physical Properties: I. Poly(vinyl acetate)," Paper presented at the 17th Annual Meeting of the American Institute for Conservation, Cincinnati, Ohio, June 1989.
Hansen, E., M. Derrick, M. Schilling, and R. Garcia, "The Effects of Solution Application on Some Mechanical and Physical Properties of Thermoplastic Amorphous Polymers Used in Conservation: Poly(vinyl acetates)," Journal of the American Institute for Conservation, Vol. 30, pp. 203-213, 1991.
Hansen, E. F., and A. G. Taketomo, "A Technical Note on the Casting of Unsupported Polymeric Films," Studies in Conservation, 34, 1989, pp. 147-152.
ABSTRACT-In conducting research on the physical properties and durability of coating materials used in conservation, difficulty was encountered in efforts to make uniformly thick, coherent, freestanding films for testing purposes. An efficient, rapid casting technique has been developed utilizing reusable molds. The technique is applicable to the preparation of films from a wide range of materials.
Hansen, E. F., M. R. Derrick, M. R. Schilling, and R. Garcia, "The Effects of Solution Application on Some Mechanical and Physical Properties of Thermoplastic Amorphous Polymers Used in Conservation: Poly (vinyl acetate)," Paper presented at the 17th Annual Meeting of the American Institute for Conservation, Cincinnati, Ohio, June 1989.
ABSTRACT-Quantitative effects on the tensile properties of solution-cast poly(vinylacetate) films result from using solvents of different quality. Higher strength and lower elongation are evident in polymer films of poly(vinylacetate) AYAT cast from a solution of acetone or an acetone/ethanol/water mixture, while lower strength and greater elongation are evident in films cast from solution of chloroform or toluene after drying for 180 days. Films of AYAT cast from a toluene solution retained a significant amount (>4%) of solvent, while films cast from the other three solvents retained little solvent (>0.3%). For all polymer films, the glass transition temperature (Tg) was elevated greater that 18 °C above the Tg of the bulk polymer. The differences in the tensile properties of the films with little retained solvent is explained on the basis of the thermodynamic quality of the solvents, i.e., the physical properties of the polymer film in the dry state may be affected by the conformation of the polymer chain in solution. Chloroform is a "good" solvent for poly-(vinylacetate) and the polymer chain is relatively extended in this solvent, in comparison to the "poorer" and more polar solvent, acetone. The affect of solvent quality on the polymer morphology in the dried film was also demonstrated with Fourier Transform Infrared Spectroscopy. Changes in spectral band intensity within a series of different molecular weight poly(vinyl-acetates) can be discerned when these are deposited on KBr from chloroform solution, but not when deposited from an acetone solution.
Hansen, E. F., M. R. Derrick, M. R. Schilling, and R. Garcia, "The Effects of Solution Application on Some Mechanical and Physical Properties of Thermoplastic Amorphous Polymers Used in Conservation: Poly (vinyl acetate)," Journal of the American Institute for Conservation, Vol. 30, 1991, pp. 203-213.
ABSTRACT-See preceding citation.
Hansen, E. F., "The Effects of Solvent Quality on Some Properties of Thermoplastic Amorphous Polymers Used in Conservation," Materials Issues in Art and Archaeology IV, Vol. 352, 1995, Symposium held May 16-21, 1994, Cancun, Mexico, Materials Research Society, Pittsburgh, Pennsylvania, pp. 807-812.
ABSTRACT - This paper reviews the effects of solvent quality on the physical properties of "amorphous" polymers applied from solution, with a focus on recent testing of materials used on the conservation of art objects. Because the solvent composition or quality affects the shape and orientation of polymer molecules in solution, the nature of the partly crystalline dry film can also be affected. Thus the physical and optical properties of a number of polymers have been shown to vary when deposited from solutions of different quality. This phenomenon is specifically related to the desired performance of a coating, adhesive or consolidant used for the conservation of an art object. The effect of "good" or "poor" quality solvents on the physical properties of solution-cast films of poly(vinylacetate) and Acryloid B-72 are discussed. In addition to polymer specific variations due to solvent quality on the glass transition temperature and the tensile properties (strength and elongation), the effects of quantitative amounts of retained solvent on these properties are considered.