Phenomenological kinetic modelling of the esterification of rosin and polyols

The kinetic study of the esterification of rosin acids and two polyhydric alcohols, glycerol and pentaerythritol, has been undertaken, using both a physical study of the solubility of the polyols in rosin and a statistical study using regression techniques for selecting the most appropriate kinetic model for each system. Frozen samples were observed by optical microscopy and by SEM and, while two phases at relatively low concentrations were observed for glycerol, a single phase was observed for pentaerythritol, even at high concentrations of the polyol. Afterwards, solubility curves with respect to temperature of glycerol in two media (rosin and a mixture rosin:triester) were obtained through an especially designed experimental set-up, observing a low solubility of the polyol at 240 °C and a medium solubility at 260–270 °C, in sigmoidal-shaped solubility curve. On the other hand, non-linear regression techniques were employed to select a kinetic model for each esterification system. For pentaerythritol a second order kinetic model was obtained based on the information from trends of the initial reaction rate and the kinetic constants derived from statistical regression using the integral method as well as microscopic observations. The same tools led, in the rosin–glycerol system, to a two-stage kinetic model: pseudo-first order for initial times and higher glycerol concentration, second-order for longer reaction time values and lower glycerol concentration, with a critical glycerol concentration where the biphasic system turned into a monophasic one. The mass balance for the polyol includes its stripping due to water and turpentine evaporation in the case of the most volatile polyol, glycerol. All tools, both physical-microscopy and statistical-regression led to the same values of solubility of glycerol in rosin.

► SEM observations and statistical analysis are used to discriminate kinetic models.
► Glycerol–rosin reacting system evolves from a biphasic to a monophasic system.
► Pentaerythritol–rosin reacting system is monophasic.
► Pentaerythritol reacts at a lower reaction rate due to steric hindrances.