Kinetics of drying of certain lacquers and paints in isothermal conditions using a thermogravimetric analyser

Abstract

The tools used for thermogravimetric analysis exhibit high measuring precision for three measurements: mass changes, temperature and temperature changes. This high measuring precision has contributed to the widespread use of thermogravimetric analysis in various scientific and technological fields: characterization and assessment of the thermal stability of various types of materials, composition analysis, kinetic studies, corrosion studies and industrial process simulation. The furnace of the Mettler Toledo TGA-SDTA851e equipment may be considered a mini-reactor capable of mimicking the conditions existing in an industrial reactor. Thus, 5-mm-diameter dry fir tree wooden discs were used, on which various lacquers and paints available on the market were applied. The experimental tests at constant temperature (25 °C) consisted of sample mass change analysis conducted for a period of 2 h. The findings enabled us to draw the drying curves (material moisture − drying time) and drying rate curves (drying rate − material moisture). The moisture ratio data obtained experimentally were fitted to different models, to determine the best-fit model. The drying rate curves revealed the existence of two drying periods: the constant-rate drying period (very short, lasting approximately 10 min) and the decreasing speed drying period. Thermogravimetric analysis in the air or nitrogen, in dynamic conditions, for the three types of fresh alkyd paint available on the market and the paint layer obtained after complete drying, i.e. natural ageing in laboratory conditions for 8 weeks, revealed a complex thermal decomposition mechanism including a sequence of three to six stages, with varying percentages of mass loss depending on their structure and on the working atmosphere.