High-solids coatings drying

The main requirements for a coating are that it is easy to apply (i.e., low viscosity) and dries to a non-tacky finish within a reasonable time. Originally, this equated to the solution and evaporation of the solvent. However, because of environmental concerns, formulations have tended to shift the concentration of solvents from 80% to less than 20% (known as high solids coatings). Many industrial processes use polymer powders (of approximately 40 pm diameter),... 

Solvents are selected such that some will escape relatively quickly from paint films to prevent excessive flow, while others will escape slowly to provide film leveling and adhesion. With typical alkyd coatings, the first 30% of solvent has been observed to evaporate as quickly as the neat solvents, essentially at a constant rate which is dependent on volatility. Later stage evaporation occurred several times more slowly and was rate-controlled by solvent diffusion to the surface of the paint film. The transition point between this behavior was defined as the resin solids content at which the evaporation rate due to volatility equaled that due to diffusion. Transition points have been observed to typically occur at a resin solids content of 40-50% v/v. Thus alkyd paints, normally formulated at 27-40% v/v resin solids, generally exhibit rapid initial solvent-release driven by volatility while high solids coatings (usually 65-75% v/v resin solids) dry solely by a diffusion-controlled process with negligible influence by solvent volatility (Ellis, 1983). 

Because of this barrier to fundamental studies involving oligomers, one cannot provide examples of research on the drying of high-solids coatings. This situation must be corrected, for high solids offer the most promise in the resolution of ecological and economic problems. 

Most resins used as binders for solventbased or high-solid coatings are solids or viscous liquids that need to be diluted to lower viscosity for easy processing into paint, ease of applicability of the paint and better film formation thereafter due to evaporation. Thus, even though solvents are not an integral part of dry coating films, they are important components of paints and coatings. 

Fig.12. Comparison between a conventional high solid alkyd coating ( ) and an alkyd based on a hyperbranched aliphatic polyester ( ). Drying time as a function of molar mass [123]...

Pettersson and Sorensen have described a number of different thermoset resin structures based on hyperbranched aliphatic polyesters [123]. Their results can best be exemplified by a study on hyperbranched alkyd coating resins. A comparative study was performed between an alkyd resin based on a hyperbranched aliphatic polyester and a conventional high solid alkyd, which is a less branched structure. The hyperbranched resin had a substantially lower viscosity than the conventional resin of comparable molecular weight, that is, less solvent was needed to obtain a suitable application viscosity. The hyperbranched resin also exhibited much shorter drying times than the conventional resin, although the oil content was similar. These achievements would not have been possible without a change in architecture of the backbone structure of the resins (Figs. 12,13). 

Polyester coating. A high solids polyester baking enamel (Cargill 5776) was prepared and spray-applied to yield a 50 jum dry film thickness (Table 4). 

Recently, hot melt PSA systems have been introduced and radiation curable PSA systems are at the commercial development stage. High solids (50%-70% by wt.) nonaqueous dispersion acrylic PSA systems have also been reported(1). Unlike the hot melt and radiation cured systems which require new capital outlay in coating head and/or curing (drying) equipment, BFG has developed PSA systems, based on Hycar 2100R reactive acrylic liquid polymers and isocyanate terminated prepolymer, which can be processed at 80% solids (by wt.) with equipment presently used in the PSA industry, namely, the reverse roll and knife-over-roll coater. 

High-solids epoxy coatings can be applied at dry film thickness (dft) between 200 xm and 1000 xm in one or two coats. 

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