Cure kinetics of flame-sprayed thermoset coatings

Thermoset powder coatings are traditionally cured by an isothermal heat treatment in an oven. This paper studies thermoset deposition by flame spraying, which is an alternative technology with potential for depositing these coatings. However, the flame-spray process is non-isothermal and the temperature of the coating oscillates greatly during deposition due to the scanning action of the spray gun. As a result, the process conditions needed to provide a satisfactory amount of cure cannot be defined simply by a given time at a specified temperature, as it is in conventional isothermal baking. The results in this study show that the rate of curing varies drastically during the spraying operation and that a mathematical model is needed for the prediction of the degree of cure. This research provides a model that can quantify the cure degree by determining the Arrhenius constants of the polymer and integrating over the temperature-time profile of the coating during deposition. The latter thermal profile may be obtained by real-time measurements or by theoretical calculations. The model is applied to a combined thermal spraying and infrared irradiance process and validated by differential scanning calorimetry. The work demonstrates that process parameters have a major influence and shows, for example, that very thick metal substrates can suppress curing due to a heat sink effect. The results also indicate that conventional epoxies cannot be fully cured during flame spraying and need additional thermal energy, as in infrared irradiation. The research develops a quantitative model to predict the degree of cure that takes into account the polymer formulation and flame-spraying conditions. It provides a methodology that could be used for the process control of flame-sprayed thermoset coatings in an industrial context.