A review of VOC emissions and drying mechanisms for interior paints and coatings.

This report reviews research into the release of volatile organic compounds (VOCs) from paints and coatings from two perspectives: (a) drying and film formation, and (b) voc emission into indoor air. The former has been investigated by the paint industry for some decades, especially in relation to understanding drying mechanisms to assist product formulation and development. The latter is of more recent interest and is directed to predicting and controlling the impact of VOC emissions from paints and coatings on indoor air quality. This requires an understanding of voc emission mechanisms and the review aims to identify knowledge gained from paint drying studies to assist in current VOC emission research. A primary aim is to act as an information resource for industrial and environmental researchers. The review is based on 120 key references published over the last 30 years. The focus is traditional water-based latex paints and solvent-based coatings for use in buildings rather than novel industrial coatings of recent years. The coating process and drying mechanisms are reviewed for each of these. This includes a limited review of volatile paint constituents and their health hazards. Solvent-based coatings form semi-dry films by volatility-limited evaporation, similar to neat solvent, but this process is complicated for multisolvent systems due to compositional changes and molecular interactions. Evaporation rate drops sharply at around 50% of v/v resin content as diffusion becomes the rate-limiting process. "Solidified" coatings may contain 20% v/v solvent. Molar volumes of solvent molecules are then critical to the evaporation process, not solvent volatility. Solvent diffusion coefficients in the resin, solvent concentration and glass transition temperature of the solvent-resin system are important parameters to diffusion-limited evaporation. Drying of water-based latex paints is complicated by latex particle transport and coalescence and the effect of relative humidity on the volatile loss of water c.f. solvent. Latex paints are formulated with specific solvents that will evaporate slowly so that they are retained to assist latex particles to coalesce and form a continuous film, a process that may take several days. Subsequent diffusion-limited evaporation of the coalescing solvent is necessary to form a durable film and may be influenced by a plasticising effect of residual water, an effect not present in solvent-based coatings. voc emission studies have focused on mathematical modelling of the emission process and sink effects of interior surfaces of experimental chambers or building interiors. Many studies have involved only semi-dry films and recent but limited studies have evaluated both wet and semi-dry films and empirical models appropriate to VOC emissions. Theoretical predictions of volatility- and diffusion-limited emission dynamics have been made but none demonstrated experimentally for both processes. Research into volatility-limited emission from several wet sources was able to model the behaviour of non-film forming sources (stain, wax) but not late stage behaviour of a polyurethane coating, probably due to diffusion becoming the predominant mechanism. The report concludes with directions for future research. The relevance of volatility or diffusion to the emission process and its dynamics needs to be clarified, especially when selecting appropriate coating conditions (substrate, thickness, loading ratio, environmental conditions) to simulate the built environment. The role of air velocity to evaporative flux from wet coatings needs better understanding; investigation has been limited but free convection at an evaporating surface may better simulate air velocity at an interior surface than a forced air velocity.