High-solids coating solvents

Methyl amyl ketone, derived from the crossed aldol condensation of -butyraldehyde and acetone, is used predominandy as a high solids coatings solvent. It is also employed as a replacement for the very toxic 2-ethoxyethyl acetate [111-15-9]. 

MAK is used as a high solids coating solvent (163) and in fragrances. It is available in the United States from Eastman (Kingsport, Tennessee), International Chemical Group (La Mesa, California) (47), and Union Carbide (South Charleston, West Virginia), and was priced at 1.36/kg in October 1994. 

However, this difference would not seem to be large enough to account for the very large differences in solvent losses that have been reported. Another factor may be that high solids coatings may reach a stage where solvent loss is controUed by diffusion rate much eadier than is the case in low solids coatings (8,43). 

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),... 

A related approach to high solids coatings is to incorporate a reactive chemical as the solvent. This allows control of the coating viscosity during application but minimizes the VOCs released because the solvent is cross-linked into the coating (Johnson, 1995 Robinson et al., 1994 Adams, 1996 Rupa Vani et al., 1993 Kuo et al., 1994 Hoffman, 1995). Reactive diluents take many forms, sometimes even as a natural oil as in the case discussed by Dirlikov et al. (1992) and Banov (1994). 

High Solids Coatings, High solids coatings resemble the technology of solvent-free coatings but the compositions contain ca 70% by volume of solid resin and are modified by reactive diluents, low viscosity multiftinctional resins, or backbone structures other than the bisphenol A moiety. 

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). 

Ellis, W.H. (1983) Comparative solvent evaporative mechanisms for conventional and high solids coatings. Journal of Coatings Technology, 55 (695), 63-72. 

The accomplishments in the past decade have been considerable. Much remains to be done. One wonders, however, whether ecological factors will not so promote the use of powders, films, and aqueous and high solids coatings that the incentive for further study of solvents will be greatly curtailed. 

Evaporation Rate of Solvents Evaporation of Solvent-Water Blends Flash Point of Solvents Air Quality Regulations and Solvents Formulation of Solvent Blends Solvents for High-Solids Coatings... 

Rule 66 (later called Rule 442 of the South Coast Air Quality Management District) also limited oven emissions from baked coatings. Incineration, absorption, or recovery of volatiles is needed to meet this restriction. In further regulations, use of high-solids, low-solvent, water-borne, and other types of coatings with low-volatile emissions was encouraged. The evolution of various air pollution regulations has been summarized (74, 75). 

High-solids coatings of the solvent type have been discussed in other sections of this chapter. This field is active, but greatest emphasis appears to be focused on the resin component of high-solids solvent coatings. Nevertheless, proper solvent selection can materially help in the quest for high-solids coatings based on solvents. 

---