Cure, oven optimization

In order to answer these questions, the kinetic and network structure models were used in conjunction with a nonlinear least squares optimization program (SIMPLEX) to determine cure response in "optimized ovens ". Ovens were optimized in two different ways. In the first the bake time was fixed and oven air temperatures were adjusted so that the crosslink densities were as close as possible to the optimum value. In the second, oven air temperatures were varied to minimize the bake time subject to the constraint that all parts of the car be acceptably cured. Air temperatures were optimized for each of the different paints as a function of different sets of minimum and maximum heating rate constants. 

In general the SIMPLEX program adjusts variables subject to supplied constraints to minimize a given object function. The object function must be an explicit function of the variables. In the case where paint cure uniformity was optimized, the oven was divided into three zones and the variables were the three oven... 

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. 

Filament-Winding. This requires a mandrel to shape the desired finished product. Continuous filament or woven tape is fed through a liquid resin bath to impregnate it, and then wound onto the mandrel in a calculated pattern to optimize the final properties (Table 15.27). The assembly is oven-cured. A collapsible mandrel can then be removed from the plastic product or the mandrel can be left as a part of the finished product. These are the strongest plastic products ever made. Typical products are pipes, tanks, and pressure bottles. Other suggested products... 

A postcure or additional curing step, usually done out of the mold simply by placing the part in an air-circulating oven, will complete the cure. The part will then have achieved optimal properties, and further reaction will not be probable. Thermoset compounders generally can recommend postcuring cycles where appropriate. 

In the vast majority of tape and label applications which use a release liner, the adhesive is applied to the release liner and then nipped to the backing. This method does not optimize adhesion to the backing, but does prevent its exposure to heat (from hot adhesive or subsequent solvent/water removal). The release liner is normally quite stiff because of its paper or PET support layer and is more resistant to heat than many backings (most liners have already been exposed to the oven cure of the silicone layer). In addition, the silicone layer is mirror smooth which yields a correspondingly smooth exposed adhesive layer for the tape or label. 

---