Cure window, measurement

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. 

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). 

Isothermal DSC measurements were made with a Perkin Elmer DSC-2C apparatus, modified for UV irradiation (Figure 1). The aluminum sample holder enclosure cover contains two windows, one for the sample and one for the reference compartment. The windows consist of cylindrical quartz cuvettes which have been evacuated in order to prevent moisture condensation. The windows were mounted by using a thermally cured epoxy adhesive. 

Infrared measurements of extent of cure under conditions similar to the TICA experiments were conducted. ATS was cast from methylene chloride solution onto KBr windows and, after vacuum evaporation of all solvent, the KBr windows were put into the Rheometrics RMS environmental chamber and were subjected to a temperature profile under nitrogen identical to the mechanical measurement experiments (2 C/min). The windows were removed one at a time at various temperatures and IR spectra were taken at room temperature. 

In near infrared spectroscopic (NIR) cure rate studies, a small amount (10-15 g) of sample was manually combined using the indicated ratio of materials. After the bubbles had been removed via centrifugation, the mixture (about 1 g) was poured onto a microscope slide prepared with a 1 mm spacer window cut from foam tape with adhesive on both sides. Another slide was placed on top, and a zero-time measurement was made on the Cary 14 spectrophotometer. The absorption at 2.205A was used as a measure of the amount of unreacted epoxy resin. The sample was kept at the appropriate temperature and spectra were measured at the indicated times. 

The nature of the vulcanizing agent and of the rubber (i.e., EPDM compounds with peroxide in this instance) is of prime importance to the kinetics of cure the activation energy and the preexponential factor play major roles, and the cure enthalpy is also of some importance in spite of its low value, whereas the order of the overall reaction is 1. Thus, because of the kinetics of cure, the time of cure varies considerably with the temperature, and thus there is a narrow temperature window through which meaningful data can be obtained. The cure enthalpy is rather low and thus does not intervene much in the process. Nevertheless, DSC experiments, which are based on measuring the enthalpy evolved, are also commonly used for determining the kinetics of cure [12]. 

Charge recombination luminescence was measured in a set-up described in detail elsewhere (75). Stoichiometric mixtures of DGEBF and DDM in aluminium pans were taken to the cure tenq)erature at 15 G/min and cured isothermally under nitrogen in a chamber covered by a quartz window. The sanq>le was intermittently irradiated with a Kulzer Duralex UV-3(X) fibre optic wand for 60 s. After each irradiation the shutter of the photomultiplier was opened with a delay of 5 s, and the initial intensity of emitted light, 7o, was recorded. 

By comparing log ionic conductivity profiles with loss factor measurements, the operator can identify the window in the process during which the material is fluid (and therefore workable), develops molecular weight and crosslinks (which is critical to product performance and appearance), and is completely cured (which identifies the proper time to demould the product or remove jigs). The dielectric analyser can also record the dielectric properties of the resin during cooling. 

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