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Simultaneous tracking of hardness, reactant conversion, solids concentration, and glass transition temperature in thermoset polyurethane coatings

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Abstract

In this work, the curing and hardness evolution of a two-component polyurethane (PU) coating in four different environments, three of which were solvent evaporation-suppressed conditions, were studied. In contrast to previous studies, the simultaneous use of Fourier-transform infrared spectroscopy, gravimetric analysis, and pendulum hardness allowed a transient mapping of the degree of isocyanate conversion, solids concentration, and coating hardness. Furthermore, to explore in more detail the coupling of the underlying mechanisms, the evolution in the average coating glass transition temperature was estimated by dynamic mechanical analysis, and the data was simulated using the so-called Kelley–Bueche equation. For the curing conditions investigated, the final coating hardness differed by a factor of two, with the lowest values obtained for the evaporation-suppressed conditions. Due to the isocyanate groups reaching full conversion for all four series, the reason for the lower hardness was attributed entirely to the plasticizing effect of residual solvent. Using a Kα value of 0.687 in the Kelley–Bueche equation, the coating glass transition temperature as a function of the PU volume fraction could be successfully simulated and was found to increase from about 282 K at a volume fraction of 0.79 to 319 K at one of 0.93. In addition, when the experimental temperature was lower than the coating glass transition temperature, a proportional increase in the pendulum hardness with the reciprocal loss factor was seen. The effects of catalyst concentration in the coating were also investigated, and this parameter was found to have a strong influence on both the surface conversion, the solids concentration, and the coating hardness. A too fast curing rate shortens the time to vitrification, after which the solvent evaporation rate becomes diffusion-controlled and very low, leading to higher residual solvent contents and significantly lower hardness values. The results obtained provide guidelines for how to optimize ventilation conditions during the curing of solvent-based, thermoset PU coatings.

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Acknowledgments

Financial support from the Hempel Foundation to CoaST (The Hempel Foundation Coatings Science and Technology Centre) is gratefully acknowledged.

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Authors and Affiliations

  1. CoaST, Department of Chemical and Biochemical Engineering, Technical University of Denmark (DTU), Building 229, 2800, Kgs. Lyngby, Denmark

    Ting Wang, Claus Erik Weinell, Kim Dam-Johansen & Søren Kiil

  2. Hempel A/S, Lundtoftegårdsvej 91, 2800, Kgs. Lyngby, Denmark

    Juan José Segura & Erik Graversen

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  1. Ting Wang
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  2. Juan José Segura
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Wang, T., Segura, J.J., Graversen, E. et al. Simultaneous tracking of hardness, reactant conversion, solids concentration, and glass transition temperature in thermoset polyurethane coatings. J Coat Technol Res 18, 349–359 (2021). https://doi.org/10.1007/s11998-020-00407-3

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