Effect of heat treatment on Pinus pinaster rosin: A study of physico chemical changes and influence on the quality of rosin linseed oil varnish

The aim of this study was to get a better understanding of the formulation aspect of a new linseed oil - rosin varnish inspired by the antique violin coating, using heat-treated and non-treated rosin. To do so, the heat treatment effect on rosin was studied on both its thermal properties and some of the varnish properties. The rosin without heat treatment was assigned as the reference. The temperature treatments were: 180 degrees C, 200 degrees C and 250 degrees C. The thermal properties were studied using TMA, DSC, ATG and HPLC. A change due to heat treatment was observed, especially on the softening point and the glass transition, which increase with the temperature. Chemical changes were observed, with the apparition of dehydrogenated and oxidised forms of abietane-based acid, which are not present in the reference rosin. As expected, the amount of turpentine decreases with the heat treatment. The varnish properties are also impacted, with a decrease of brightness and solvent resistance with the temperature. A PCA was made in order to have a statistical analysis of the results. Through this, a clear separation of the four groups was graphically observed, with different representative variables. The variables' influence on each other was also clearly identified, as well as the impact of the chemical changes on thermal and varnish properties.