Time-temperature cycles

Figure 3.17 Schematic time-temperature cycle for the controlled crystallization of a glass-ceramic body.

This section describes some of the tools available for intelligent development of process cycles, such as the time-temperature cycles used in curing composites. Current industrial practice is typically limited to the use of cure cycles. The cycles are based on a series of autoclave temperature and pressure states so that traditional linear, regulatory process control methods can be used. These recipes may not be the ideal method for process control of batch processes because they do not ... 

Again, the minimum and maximum loading configurations should be studied. Thermocouples will be placed both inside and outside the container at the cool spot location(s), in the steam exhaust line, and in constant-temperature baths outside the chamber. The F0 value will be calculated based on the temperature recorded by the thermocouple inside the container at the coolest area of the load. Upon completion of the cycle, the F0 value will indicate whether the cycle is adequate or if alterations must be made. Following the attainment of the desired time-temperature cycle, cycles are repeated until the user is satisfied with the repeatability aspects of the cycle validation process. Statistical analysis of the F0 values achieved at each repeated cycle may be conducted to verify the consistency of the process and the confidence limits for achieving the desired F0 value. 

The automation of laboratory operations to enhance the process development chemist s armory has also grown as an activity. Equipment is available (e.g., Buchi Syncore8) to aid in the evaluation and optimization of such as time-temperature cycles in a given reaction, or in combinatorial and parallel synthesis endeavors. 

Starch can be enzymically converted in the presence of pigment. The conversion follows a similar time-temperature cycle as in neat starch conversion. The pigment will adsorb a portion of the enzyme adsorption can be minimized by the addition of sodium silicate to the mixture prior to the addition of the enzyme (Vanderbilt process). Even with silicate treatment, a higher quantity of enzyme will be required to reach a specific viscosity target. Other coating components, such as latex and lubricants, have to be added after the conversion. The Vanderbilt process is now rarely used for the preparation of coating binder. 

In case (b) in Fig. 4, some crystallinity is developed at all blend compositions but the amount is somewhat reduced from that in case (a). Blends of a copolyester, KODAK, with PC have shown this type of response (29). In case (c), no crystallinity develops during the particular time-temperature cycle for blends rich in the miscible diluent. Such behavior has been observed for PVF2 blends containing various miscible, oxygen containing pol3nners (12). 

Each resin fiber combination has one or more optimum cure techniques, depending on the proposed environment. The prepreg manufacturer supplies a time-temperature cycle that may have been used to manufacture the test specimens employed in generating preliminary mechanical and physical properties. Frequently, it has been necessary to modify the preliminary cure cycle because of part configuration (thickness) or because of production economics. There are several developments that help to inject science into the otherwise hit-or-miss, time-consuming procedure of cure cycle optimization. They are... 

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