Transparent coatings used over painted wood include varnishes and waxes. Oils may be added to the list of transparent coating materials used to finish unpainted wood. Varnishes are transparent or translucent top coatings that give a gloss (Perry, 1804). They are similar to glazes but generally unpigmented and are used to protect the finished work against moisture, pollutant gases, dust and handling. Transparent finishes increase the sheen and deepen the colour of the surface by filling pits and surface irregularities thus reflecting more light back to the eye rather than scattering it, that is they provide optical saturation. Historically varnishes were based on natural resins but waxes and oils have also been used in combination with them in various varnish formulations, as well as on their own, to enhance both bare wood and finished surfaces. The twentieth century has seen a large range of synthetic thermoplastic and thermosetting polymer materials added to the list of transparent surface coatings available for wood. For a review of traditional and modern coatings commonly found on furniture see Mills and White (1987). For coatings commonly used for conservation purposes, see Horie (1987). The chemistry of these materials is further discussed by Ash and Ash (1982), Kolesky (1995) and Bentley (1998).
To form a continuous thin film, coatings, like adhesives, need to be in liquid form at some stage in their application. This requirement can be met in various ways. Thermoplastic finishes found on furniture are relatively hard at room temperature but during their application heat can be used to make them soft or fluid. Materials such as waxes, wax-resin mixtures and shellac can be made sufficiently fluid by the vigorous motion of a hand-held rubbing tool (friction polishing) or by the motion of a lathe.
A category of thermoplastic materials, important to coating and adhesive technology but of minor use in plastics manufacturing, is materials that dissolve in solvent and harden again without undergoing any change as the solvent evaporates. This category, known as solvent release coatings, includes traditional spirit varnishes made from clear hard solvent-soluble polymers called resins and solvents such as ethanol. Two commonly used solvent release varnishes are shellac in ethanol and methyl methacrylate in petroleum benzine. Other thermoplastic resin types commonly used in the preparation of solvent release varnishes in the fabrication or conservation of furniture include the polycyclohexanones (e.g. ketone N and MS2A), poly(vinyl acetate), poly(vinyl butyral), and a range of acrylics.
Thermoplastic resin varnishes are usually easy to prepare and are sufficiently durable for interior use, having good abrasion resistance but only moderate resistance to water and other solvents. The fact that such coatings are susceptible to solvent action makes it comparatively easy to repair surface damage such as scratches and abrasions. Ageing of the coating may restrict solubility to some extent as crosslinking may occur in natural resin coatings, but they are generally resistant to this process due to lack of reactive sites on the polymer chains. Consequently they generally remain relatively soluble even after ageing. Virtually all of the solvent leaves the coating once the film has reached full hardness, though this may take weeks or months (Martens, 1968). The process of film formation and the life of the film are extensively discussed by Feller et al. (1985). Some solvent release coatings, for example oil/resin varnishes which contain a drying, oil have an additional film forming process involving oxygen where network polymerization takes place.
Thermosetting varnishes harden by irreversible chemical reaction which usually results in greater polymerization and crosslinking than occurs with thermoplastics. A general name for this class of coating is reactive, because setting takes place as the result of chemical reaction. In cases where oxygen is directly involved in the setting reaction, as with drying oils, the term oxidative may be used. True inter-molecular networking, or crosslinking requires a minimum of two double bonds or reactive sites on one chain and three on another (Kolesky, 1995). Important thermosetting finishes include fixed oil varnishes, oriental lacquer (urushi) and modern two-component catalysed poly mers such as epoxies, polyesters, polyure-thanes and some acrylics.
Traditional finishes in this class, known as fixed oil varnishes or oil/resin varnishes, include resin(s), an oil and a solvent. Such mixtures often require heat or pressure to coax the components into a solution. Typical reactive varnishes include copal and linseed oil in turpentine or alkyd resin and tung oil in a petroleum distillate. Modern polyurethane and alkyd varnishes are chemically modified drying oils which also harden by oxidation. The drying oils such as poppy, linseed and nut oils and mixtures of these oils with natural resins have been in common use at least since the fourteenth century (Gettens and Stout, 1966). The curing of oil varnishes is a function of solvent release and the oxidative polymerization of the oil component.
Thermosetting polymer coatings are harder and tougher than thermoplastics, have improved resistance to moisture, solvents and staining and thus yield greater protection for objects in use. Their lower molecular weight in solution allows for high solid content applications. However, they have generally greater potential long-term reactivity tending toward yellowing and their resistance to solvents makes them more difficult to repair and very difficult to remove independently of other coatings.
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