Varnishes and Surface Coatings: Special Considerations
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Author: Richard C. Wolbers
Date: Submitted September, 1997
Compiler: Wendy Samet
VARNISHING ACRYLIC EMULSION PAINTINGS[edit | edit source]
Selecting appropriate materials for the varnishing of acrylic emulsion paintings presents particular problems. Reversibility is questionable as even after aging, acrylic emulsion paints are at least partially soluble in solvents such as xylene. This is not only because of the solubility of the emulsified acrylic resins themselves, but because of the solubility of the additives used to formulate these paints. In addition, the final formed film can be porous due to the nature of the drying of an emulsion paint.
Acrylic emulsion paints contain a wide variety of materials which may make wetting the surface of the acrylic painting quite different from wetting a more traditional oil paint surface.
The typical emulsion-type paint used in artist's tube colors is derived either through the direct emulsification of a pre-existing or pre-formed polymer into an aqueous phase, or through the in situ polymerization of a monomer dispersion in water. In either case, constructing these paints requires that they contain a number of non-volatile ingredients not found in artists oil paints and which greatly affect the nature of emulsion paint film and its affinity for varnish. A typical artist's acrylic emulsion tube paint might include the following ingredients (given with their typical weight percentages):
Polymer 40% Opaque Pigments 20% Extenders 15% Colloids 1.2% Plasticizers 1% Modifiers 1% Preservatives .5% Anti-Corrosives .1% Buffers .1% Fungicides .5% Coalescing Agents .5% Defoamers .5% Thickeners .5% Water 10% Surfactants 2.6%
(Martens 1968, 515)
Typical polymers used in these kinds of preparations are poly(alkylmethacrylate)s, poly(vinyl acetate)s, or their copolymers. Surfactants can be anionic, cationic, nonionic, or blends of these. Typical modifiers are aldehydes, mercaptans, or chlorinated hydrocarbons. Protective colloids include poly(vinyl alcohol)s and cellulosic ethers. Common buffering agents include citrate, acetate, phosphate, and carbonate salts. Glycols are often used as coalescing agents. Plasticizers can be a range of low volatility solvents (Martens 1968, 526–7).
It should be clear from the range of nonvolatile materials that comprise these paint preparations, that an equally broad range of solubility parameters may be associated with the various fractions included in these paints. The hydrocarbon and aromatic solvent blends that are typical of the delivery solvents for varnishes currently in use in conservation could, at least partially, solubilize a number of the constituents in these preparations. Modifiers, surfactants, protective colloids, plasticizers, freeze-thaw stabilizers, and even the biocides present could be extracted and/or redeposited from films rewetted by solvent during varnish application. These soluble components can also mix with and become incorporated into the varnish affecting its aging and stability characteristics. The loss of plasticizers in particular may be problematic on films under the low but constantly applied stress normally present in a fabric supported painting.
Different components of the emulsion may collect in specific locations within the dried acrylic emulsion film. Surfactants, used to disperse pigments in either the aqueous or polymer phases, and to emulsify the polymers into water tend to collect at the air/paint surface boundary on drying. Consequently, the surfaces of these films can be highly charged or concentrated in specific surfactants. Both the wetting and adhesion of subsequent materials can be adversely affected by this accumulation of surfactant on the dried paint film surface.
The appearance of many of these paint surfaces can be intimately tied to the presence, and specific amounts, of surfactants present. Surfactants that are present on pigment surfaces as dispersal aids, can be extracted with hydrocarbon or aromatic solvents. Their loss may result in greater disparities between the refractive indices of pigment and polymer binder, with a concomitant loss of saturation in the appearance of specific colorants.
Although the individual micelles of dispersed materials in emulsion paints theoretically coalesce into a uniform paint film, the process may rarely reach completion in real paints (Martens 1968, 20). Porosity is also suggested by the relatively high pigment/volume concentrations of these paints. In fact, “visual vestiges of the original individual latex globules may be found months and years after the coating is laid down. The nonvolatile ingredients that are not soluble in the globule substance are trapped in the inter-sticial cells between the globules when the volatiles escape…. Cure is a relative term. What may be an adequately cured film for gently handling may still be raw in respect to solvent attack, for example” (Martens 1968, 20–1). This porosity makes the physical inclusion of any applied material, including varnishes, a less likely event and makes the reversibility of any varnish questionable.
References[edit | edit source]
- Martens, C.R., ed. 1968. Technology of paints, varnishes and lacquers. New York: Reinhold Book Corp.
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