Varnishes transition temperature

Substituted nonheat-reactive resins do not form a film and are not reactive by themselves, but are exceUent modifier resins for oleoresinous varnishes and alkyds. Thein high glass-transition temperature and molecular weight provide initial hardness and reduce tack oxygen-initiated cross-linking reactions take place with the unsaturated oils. 

The first synthetic polymers to be used as paint varnishes were acrylic and vinylic resins. Poly(vinyl acetate) (PVAc), commercialized under the name Mowilith by Hoechst and Vinylite by Union Carbide, has been used in conservation as an adhesive since 1932 and in 1937 it was proposed as a picture varnish by Stout and Cross [63]. PVAc was soon rejected as a varnish because, despite its light stability and good solubility in organic solvents, it demonstrated poor optical properties in terms of colour saturation and the tendency to pick up dirt due to its low glass transition temperature. 

Poly(vinyl acetate) is used for adhesives and as a wood glue (40% solution), as a raw material in lacquers and varnishes (dispersions), and as a concrete additive (in the form of a fine, dispersible powder obtained by spray drying). Poly(vinyl acetate) grades that are more resistant to hydrolysis are obtained by copolymerization with vinyl stearate or vinyl pivalate, since the saponification rate is reduced by the bulkier side groups. Pure poly(vinyl pivalate) has too high a glass transition temperature, 78 C, for most poly(vinyl ester) applications. Other copolymers of vinyl acetate are produced with ethylene (see Section 25.2.1) or vinyl chloride (see Section 25.7.5.3). 

Physical properties Tensile strength, hardness, extensibility, glass transition temperature These data (derived from manufacturer s literature or practical experience) indicate the limits of use PVAL coatings form strong films but have little adhesion to picture varnish... 

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