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.
3.3. Thermal properties
Fig. 7 shows the TGA thermograms of the alkyd coatings with
varying percentage of DPE. There was no significant difference in
the thermograms, except when the films were heated up to 600 ◦C,
Fig. 4. FT-IR spectra of high solids alkyd coating films of (a) ALK 1 (0% DPE) and (b)
ALK 6 (60% DPE) with and without driers at varying hours of drying time showing
disappearance of unsaturation.
ALK 1 films degraded 7–8% higher than ALK 9. This difference in
weight loss between the two films may be due to the presence of
comparatively higher branched structure in ALK 9.
3.4. Mechanical properties
Table 3 shows the mechanical properties of high solids alkyd
films. All the films showed typical alkyd drying behavior. However,
films of ALK 9 (100% DPE) showed superior pencil and scratch hardness
values after 7 days of maturation. All the films passed in the
bending test when tested on conical mandrel.