Cure window

The agreement between the experimental and calculated values of Ce, is excellent. The data shown in Figure 2 are for a constant bake time of 17 minutes. The upper and lower limits on define a cure window. The cure window for the low solids coating is 50 C. The model was further tested by measuring extents of reaction and temperature profiles for samples attached to different parts of a car body which passed through a pilot plant oven. This simulation tested the model under conditions where the substrate temperatures were far from constant. As shown in Table II, the agreement between the experimental and calculated values of Ce is again excellent. 

Values of ACe decrease with increasing Zm- since the car body temperature profiles become more uniform. Another way to increase car body temperature uniformity (and thus decrease ACe ) is to increase the bake time. For example, increasing the bake time from 17 minutes to 25 minutes essentially compensates for a decrease in cure window of 50 C to 35 C. 

Figure 7. Relative oven throughput versus paint cure window for values of Z -n of 0.25 (Q), 0 2 (C]) 0 15 (A), and 0.10 min-1 ( 7.

A network structure model has been developed from which a parameter that correlates well with physical measures of paint cure can be calculated. This model together with a kinetic model of crosslinking as a function of time and temperature has been used to evaluate the cure response of enamels in automotive assembly bake ovens. It is found that cure quality (as measured by the number and severity of under and overbakes) is good for a conventional low solids enamel. These results are in agreement with physical test results. Use of paints with narrower cure windows is predicted to result in numerous, severe under and over bakes. Optimization studies using SIMPLEX revealed that narrow cure window paints can be acceptably cured only if the bake time is increased or if the minimum heating rate on the car body is increased. 

Differences in Network Structure. Network formation depends on the kinetics of the various crosslinking reactions and on the number of functional groups on the polymer and crosslinker (32). Polymers and crosslinkers with low functionality are less efficient at building network structure than those with high functionality. Miller and Macosko (32) have derived a network structure theory which has been adapted to calculate "elastically effective" crosslink densities (4-6.8.9). This parameter has been found to correlate well with physical measures of cure < 6.8). There is a range of crosslink densities for which acceptable physical properties are obtained. The range of bake conditions which yield crosslink densities within this range define a cure window (8. 9). 

Bauer and Dickie (60) devised the concept of a cure window to explain in practical terms how to control film morphology. The cure window is the range of temperatures and cure times over which acceptable properties are obtained. Too low a temperature will produce green properties, and too high a temperature leads to decomposition. 

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