Fig. 5 shows the DSC thermographs for the various high solids alkyd coating compositions. Fig. 5(a)–(d) depicts the DSC thermograms of films having different DPE content from third day (72 h) to ninth day (216 h) of drying time. The thermograms reveal important changes in the curing behavior of the films as oxidation proceeds. In the beginning of the drying process, two
distinct exothermic peaks were observed in the DSC thermograms which merge into one broad peak with increase in drying
time (Fig. 5(a) and (b)). The two separate peaks in the beginning of drying process indicate the presence of two different routes
of decomposition, i.e. peroxide decomposition/radical propagation reactions and recombination/cross-linking of fatty acid chains
arising in absence of oxygen. Mallegol also reported a similar mechanism for oxidative drying of conventional alkyd-based varnishes [25]. As shown in Fig. 5(c) and (d), the heat of reaction decreases with time for coating films of ALK 1 (0% DPE) whereas in the case of ALK 9 (100% DPE), the heat of reaction increases in the initial stage up to 24 h and then decreases. It indicates faster cross-linking for ALK 1 (0% DPE) at the initial stages of drying when compared to ALK 9 (100% DPE). Fig. 6 illustrates the plot between heat of reaction derived from DSC exotherms and number of hours of drying. The graph shows that the coating samples attain a constant heat of reaction after 72 h of drying. In the initial 72 h of drying, ALK 1 coating film shows rapid decrease whereas heat of reaction of ALK 9 remains almost same. This suggests that in the case of ALK 9 films the cross-linking in the initial 72 h of drying takes place at a controlled rate.