Cure temperatures, regimes

Since solvent evaporation and imidization in themselves are not destructive processes, the most crucial temperature regime lies between 150 °C and 250 °C. Here solvent removal and maximum imidization occurs simultaneously causing tremendous shrinkage and the creation of maximum stress in the polymer film. At this point it is not unusual to observe cracking problems in the polymer film, depending on the inherent mechanical properties of the partially cured poly-... 

Major influences on the kinetic curve of a cement include the phase composition of the clinker, the particle size distribution of the cement and the RH and temperature regimes during curing. Other influences include the w/c ratio, the content and distribution of admixtures, including gypsum, the reactivities of individual clinker phases and probably others, such as the microstructures of the clinker and of the cement particles. 

Upon cooling, after a 2 hour cure, both materials exhibit linear stress-temperature profiles. This indicates that the glass transition temperature is at or above the cure temperature, and that measurements have been made in the glassy elastic regime. The glassy-state Ea can be calculated from the slopes of these curves. For the polyimide it is 0.13 MPa/°C and for the BCB it is 0.16 MPa/°C. Note that the polyimide bears a higher cumulative stress at room temperature because of the stress induced by solvent evaporation, in spite of its lower Ea. 

The proposed synergetic model allows to estimate temperature boundaries of curing process reahzation and to select its optimal temperature regime. For this let us estimate system adaptabihty measure by curing temperature j as follows... 

The same process continues beyond optimum or maximum cure state if cure is allowed to continue ( overcure ). Again, as seen in Figure 6, the cure state now drops from the maximum to attain a lower plateau. This decline is faster and more pronounced as cure temperature increases. The window of time for near-optimum cure narrows, and the regime of overcure grows. 

The proposed synergetic model allows temperature boundaries of realisation of the curing process to be estimated and its optimal temperature regime to be selected. For this let us estimate the system adaptivity measure by curing temperature A as... 

In Figure 3.43 the dependence of the spectral dimension of microgels on is adduced for the system 2DPP+HCE/DDM. As one can see, the value d = 1.0 is reached at 363 K, which corresponds to a linear macromolecule [35] and at < 363 K linearly connected chains do not form in the curing process. At cuT corresponding to the transition to stationary temperature regime... 

Curing is the process of exposing plates pasted positive and negative to a regime of (a) controlled time (minimum 32h), (b) temperature (30-35°C), and (c) relative humidity (>90%). This process converts the free lead into lead oxide, using oxygen from the surrounding air. The plates are allowed to cure for a minimum of 32 h. Care is also taken to ensure that the maximum temperature of the plate does not exceed 60°C. The cured plates are then parted. 

Figures 1 and 2 show an increase in during cure brings about a decrease in 0 = T — in the isothermal regime. Assuming the Arrhenius dependence of the rate constants on T, one can get a good superposition of the kinetic cilrves in the region well above and a considerable retardation of the reaction gt reaction temperatures near or below (Fig. 2).

Most of the early work on GAP decomposition and combustion was based on global decomposition pathways [74]. The physiochemical processes involved in the combustion of a cured GAP strand is schematically illustrated in Fig. 2. The entire combustion-wave structure can be segmented into three regions solid-phase, near-surface two phase, and gas-phase regimes. In the solid phase, the extent of chemical reactions is usually negligible due to the low temperature and short residence time. Thermal decomposition and ensued... 

Note that the curves shift for the two sets of samples prepared on two different days. This indicates the limits of sample reproducibility due to ambient temperature fluctuations that affected the kinetics and final epoxy conversion. Therefore, different preparation series, and hence aging regimes, may be compared only qualitatively. In future work, curing conditions will have to be controlled even more precisely. 

Elastomer-modified epoxy resin systems with more complexity to their preparation scheme have been demonstrated. Two examples suffice. Shelley and Clarke (9 ) instruct that a vulcanization procedure can be successfully employed to Improve elevated temperature properties in the cured resin mass. This step occurs subsequent to the esterification regime. It can be practiced with impunity at low rubber contents (7.5-10%) without gelation or Indeed very much viscosity Increase. Peroxides appear to be preferred over sulfur/sulfur donor systems. Table VII displays an example of this procedure with a solid DGEBA resin. 

FIGURE 28 Comparison of the [Jp t) - /J curves as a function of the reduced retardation time, t/a-T, of fully cured Epon 828/DDS (O), 1001/DDS (0), 1004/DDS (A), and 1007/DDS (V). The shift factors and reference temperature for Epon 828/DDS are identical to the values used in Fig. 8. For the other three elastomers, horizontal shifts of -0.72, -1.18, and -1.5 have been applied to superimpose the data in the short time regime associated with segmental relaxation. 

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