Page 670 - Handbook of Thermal Analysis of Construction Materials

638 Chapter 16 - Paints and Coatings

The DTA curve for the wet wastes shows an exotherm effect in the
range of 160–790°C, which corresponds to a major weight loss in the TG
curve between 170° and 350°C. A sharp exotherm is observed in the DTA
curve of the dry wastes in the initial stage for the first weight loss (below
200°C), which may be caused by oxidation of the low-boiling components
of the painting material. [12] A second exotherm is also observed between
240° and 550°C due to another weight loss, followed by an endotherm
which was attributed to the decarbonation of calcium carbonate (44.6%
weight loss). From the results, Sebastian reported that the DTA curves
showed that the thermal behavior of the mixture depends on the decompo-
sition of the raw-paint waste, the main component. It was concluded that the
decomposition process, the final temperature, the corresponding weight
loss, and the calorific value curves provide valuable data on the suitable
mode of preparation of wastes prior to their combustion.
Thin film analysis (TFA) is another technique that has been applied
to the study of the drying of curable and latex-based coatings. [13] This novel
approach, based on a simple force-sensing device capable of carrying
various probes, measures quantitative changes in mechanical and rheologi-
cal properties of drying films in-situ on a test panel. By imposing a thermal
gradient along a heating block under the sample, measurements on a series
of temperatures can be obtained in a single experiment. The technique has
been used by Bahra, et al., [13] to monitor changes in the viscosity during film
drying of household paints and automotive basecoat films. Figure 3
displays the viscosity measurements obtained with the TFA for two types
of household paints, a water-based emulsion and a white spirit-based gloss
paint. The viscosity values for both paints remains low for an initial period
less than four minutes, after which a sharp increase is observed. The initial
period is longer for the gloss paint, which was attributed to the greater
resistance to brushmarking experienced with this type of paint.
Bahra, et al., [13] also used TFA to monitor the buildup of the scratch
resistance of coatings. The reported results can be related to the extent-of-
cure in cross-linking resins and to phenomena such as vitrification. Figure
4 shows an example of scratch resistance of a drying film for an alkyd-resin
gloss paint at two different measuring speeds (made lengthwise at a constant
temperature) as a function of time. The scratch resistance builds up sooner
when the needle is moved with a fast stroke rather than with a slow stroke.
Measurements from scratches along the x-direction were also performed at
temperatures between 20° and 60°C after 0, 15, and 45 minutes. The results
showed a general increase in the scratch resistance with time (Fig. 5), which
indicated an increase in the degree of curing with temperature and time. The