Dwell-time reduction

The subsequent records in Figure 12 are for saxitoxin derivatives bearing a 21-sulfo group (Bl, 2), an 11-a-hydroxysulfate (GTX2, 3), and both substituents (Cl, 4). Note the difference in the dwell times of the blocking events, in particular the great reduction in dwell time associated with the presence of the 21-sulfo group. 

The modeling of residual stress development during cure can be used to optimize the processing conditions to reduce or control residual stresses. The current process model is used next to assess the effects of several processing conditions on residual stresses. Reduced cure temperature, longer dwell times, slower cool down rate, and the use of novel cure cycles are all feasible for the reduction of residual stresses. 

The MRC cycle calls for a 182°C cure temperature. The effect of cure temperature on residual stress was investigated by curing specimens at four other cure temperatures (171, 165, 160, and 149°C) while holding the dwell time (4 hours) constant. In Figure 8.18 the dimensionless curvature for these specimens is plotted versus the cure temperature. The curvature is reduced as the cure temperature is decreased with significant reduction in curvature obtained for dwell temperatures of 165°C or less. The final curvature as predicted by the viscoelastic process model is overlaid with the experimental data in Figure 8.18 and is shown to capture the trend. 

Figure 5. The effect of reduction potential of a caustic hydrazine solution on copper and nickel undercut. The potentials are referenced to the saturated calomel electrode. The temperature was 80 C and the dwell time 600 s.

In some systems, increasing the dwell time —that is, the time over which the full load is applied to the surface—results in an increase in the size of the indentation and hence an apparent reduction in the hardness of the material. This effect is called indentation creep. 

Corrosion effects in the thinner cladding, operating at the reduced system pressure and for the shorter dwell time, are expected to be less Important than in the enriched reactor. The lower corrosion rate and shorter time, more than offset the reduction in thickness of cladding in terms of Hp pick-up. Fig. 8 shows the -expected variation of Hg content with dwell time for... 

Polyesters, unlike polyamides, have little tendency to char when held in the molten state for a long time. Instead, they decline in viscosity and molecular weight. To avoid reduction in properties, polyesters should be exposed to the lowest possible application temperatures and the shortest dwell time in the melt equipment. Failure to prevent hydrolysis of the hot melt can result in very dramatic deterioration in the properties of the ultimate bonds. 

Hot Melts Using PVC/vinyl acetate copolymers will permit the processing of hot melts at lower temperatures due to viscosity reduction of the melt itself. If the hot melt can be processed at lower temperatures, color shifts due to long dwell times in the molten state can be reduced in some cases. Residence time of the molten plastisol in the heated tank should be carefully monitored. 

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