Many models for predicting volatile organic compounds (VOC) emissions from latex paints have been developed. Earlier models were developed for solvent-borne paints, particularly since these paints evaporate rapidly and can be modeled with simple decay models. However, paint has changed in the past fifty years, and a transition has been made towards water-borne paints. These paints were introduced for indoor applications because they lacked the health hazards and odors of their solvent-borne counterparts. These paints also have organic modifiers, therefore it is very important to predict how these modifiers evaporate from the coated material. New mechanistic models that can predict slow emitting VOCs over long periods of time are not available. An improved ability to predict VOC emissions from latex paints could lead to improved understanding, better policy-making and promotion of environmental regulations that benefit both the consumer and producers of architectural coatings. This research improves on existing models used to estimate VOC emissions off-gassed from latex paints. The developed two layer model (2LM) has a layer for paint and substrate material, and accounts for mass transfer at the paint layer, and diffusion transport between paint and material layers. The model provides a semi-mechanistic way to predict paint drying and VOC emissions from coatings on a variety of substrates. The model only requires the estimation of one parameter (the paint layer diffusion coefficient), unlike other models available that require multiple parameter estimations. This model is robust in the sense that it could be used to predict VOC emissions from paint, as well as predicting the variation of the internal VOC distribution on both paint and material layers with time. The model was tested and validated with empirical data collected from previous controlled chamber experiments, and also with data collected from short evaporation experiments. Critical paint components like polymer and pigment composition and its relation to VOC fate and transport after paint application, both initially and over long periods of time, were explored. Modeling results indicated that the diffusion coefficient of 2,2,4-trimethyl-1,3-pentadediol monoisobutyrate (TMPD-MIB) in the paint layer does not depend on the thickness of the wet paint film, but it depends on the pigment volume concentration (PVC) of the paint. Additionally, a constant diffusion coefficient used in the 2LM was successful for modeling emissions of TMPD-MIB from low pigment volume concentration (LPVC) paints, but it failed to capture the physical mechanisms of the drying film for high pigment volume concentration (HPVC) paints. A major finding from this research was that a detailed gas phase analysis of mass transport for TMPD-MIB would have negligible effects on the predicted overall evaporation rate. Therefore, the entire wet and dry emissions processes are likely dominated by diffusion processes. / text

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Rheology modifiers such as hydrophobically-modified ethoxylated urethane (HEUR)thickeners are included in waterborne latex coatings to optimize shear-rate dependent viscosity and other rheological properties. While these HEUR polymers are commonly used in industry, the complex chemical interactions that contribute to rheological properties are still not completely understood. Prior work in this area has focused on understanding latex-HEUR and latex-surfactant-HEUR interactions that affect rheological properties. Additionally, studies have been previously conducted to understand the relaxation mechanisms of complex interactions present in HEUR-thickened waterborne latex coatings under various dynamic conditions. The objective of this work is to extend the experimental work to fully-formulated coatings and determine the effects of additional ingredients in a fully-formulated system. Coating formulations were prepared with a target 90 KU (Kreb Units) viscosity, having 0.23wt% HEUR. The pigment volume concentration (PVC) and non-volatiles by volume (NVV) were kept constant at 19.87% and 30.47%, respectively. An analysis of phase stability (presence or absence of syneresis), flow sweep (10-2 to 103 s-1), oscillatory strain (10-2 to 102 %), and oscillatory frequency (10-2 to 102 Hz) data was carried out in an attempt to determine connections among these properties. Furthermore, brief comparisons were made with previous results on latex-HEUR and latex-HEUR-surfactant systems that utilized the same HEUR thickener and latex used in this study. In the fully-formulated system, 0.23wt% HEUR was found to be in excess of what is needed to saturate latex surfaces. This HEUR level is less than half of the level needed to saturate latex surfaces in simpler latex-HEUR systems in previous studies. Fully-formulated coatings, in addition to having TiO2 and other ingredients are more crowded than the previous systems. It appeared that a depletion flocculation mechanism dominated at low surfactant concentrations for fully-formulated systems in this study as evident from syneresis; large HEUR aggregates appear to build enough osmotic pressure to drive aggregation of latex and pigment particles resulting in depletion flocculation. At increasing surfactant levels, the depletion flocculation mechanism was negated allowing the associative HEUR bridge networks to dominate and stabilize the system. Phase stability for fully-formulated systems in this study were associated with Newtonian viscosity plateaus on flow sweeps, strain hardening on oscillatory strain sweeps, and formation of high frequency moduli plateaus in frequency sweeps. Further increase of surfactant concentration appeared to disrupt the stable latex-HEUR network due to competitive adsorption of surfactant on latex particles, resulting in syneresis from bridging flocculation. Possible correlations between phase stability and high relaxation times were seen, although further analysis of relaxation time data and simulations will need to be carried out to better understand the behavior of HEUR in fully-formulated systems.