Cure gradient

Finally, that work was summarized by Thostenson and Chou, who showed that both numerical and experimental results indicated that volumetric heating due to microwaves promoted an inside-out cure of the thick laminates and dramatically reduced the overall processing time [115]. Under conventional thermal conditions, to reduce thermal gradients, thick laminates were processed at lower cure temperature and heated with slow heating rates, resulting in excessive cure times. Outside-in curing of the autoclave processed composite resulted in visible matrix cracks, while cracks could not be seen in the microwave-processed composite. The formation of cure gradients within the two composites cured under both micro-wave and conventional conditions are presented in Fig. 31. 

Fig. 31 a,b. Formation of cure gradients with two laminates during a conventional cures b microwave cures. Reprinted from (2001) Polym Compos 22 197 [115] with permission... 

Although cure gradients existed in both composites treated under miaowave and thermal conditions, differences in the solidification behavior were notified. In the conventionally processed composite, the outside-in cure gradients were most significant during the early stages of the cure cycle, and the maximum cure rate for this epoxy resin system occurred at the... 

Figure 56 Formation of cure gradients with two laminates during (a) conventional and (b) microwave cures. Reproduced with permission from Thostenson, E. T. Chou, T.-W. Polym. Composites 200, 22,197. " ...

In order to get experimental information about the concentration and temperature range where phase separation occurs, it is required to perform a series of isothermal curing experiments varying these parameters [49]. By interconnecting individual phase separation points, say 10% solvent at 40 °C is needed while 17% solvent has to be employed at 100 °C,a phase separation fine is constructed. In order to obtain the temperature and concentration ranges where phase separation occurs with a very few experiments, a gradient oven made out of aluminum, schematically shown in Fig. 8, was constructed. 

Plotted in Fig. 10 are phase separation lines, which were obtained with the gradient oven for epoxies cured in the presence of the above solvents [88]. At the left side of these lines, no phase separation occurs and the materials stay transparent. The right side of these phase separation lines gives the temperature and composition ranges where phase separation occurs. The onset of the phase separation, which gives one single point on the phase separation line, can easily be detected for each concentration, as the samples become opaque as a consequence of the formation of liquid domains in the pm-range. 

There has been extensive work in understanding the origin and level of stresses that develop with coating systems due to curing and/or a thermal gradient. For solventless cured epoxy systems, curing stresses are small (6-8) these can be neglected in the present study since the concern is with solventless cured epoxy systems. 

Cobalt(II) chloride was dissolved in poly(amide acid)/ N,N-dimethylacetamide solutions. Solvent cast films were prepared and subsequently dried and cured in static air, forced air or inert gas ovens with controlled humidity. The resulting structures contain a near surface gradient of cobalt oxide and also residual cobalt(II) chloride dispersed throughout the bul)c of the film. Two properties of these films, surface resistivity and bullc thermal stability, are substantially reduced compared with the nonmodified condensation polyimide films. In an attempt to recover the high thermal stability characteristic of polyimide films but retain the decreased surface resistivity solvent extraction of the thermally imidized films has been pursued. 

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