Three-Step Cure Cycles

Mechanical testing of the three-step cure specimens indicated that no sacrifice in properties resulted from the modification of the process cycle. The retainment of mechanical properties (transverse strength and modulus) coupled with the reduction in dimensionless curvature for the three-step cure cycles investigated provides another suitable cure cycle modification for reduction of residual stresses in composite materials. Overall processing time has not been increased beyond that specified in the MRC cycle. Thus, with no increase in process time and comparable mechanical properties, the residual stresses have been reduced by more than 20 percent in comparison to the MRC cycle baseline data. 

The methodology presented is generic and can be applied to other types of materials, laminate constructions, and processing methods. In all cases a characterization of the cure state is needed first. As a result, models must be developed to predict the mechanical response as a function of the cure state. Mechanics models can then be developed for the specific application of interest. 

(1969) Twenty-Fourth Annual Technical Conference, SPI, Inc., Paper No. 2-d 

Harper, B.D., Weitsman, Y. AIAA/ASME/ASCE/AHS 22nd Structures, Structural Dynamics and Materials Conference, Atlanta, Georgia, April 6-8 (1981) AIAA-81-0580, p. 325-332 

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Figure 8.20 Three-step cure cycles and MRC cycle for IM6/3100...

Figure 8.21 Dimensionless curvature (tf) for MRC and three-step cure cycles...

These three cure systems have in common the need for a two-step cure cycle to generate the best cured properties. The first step is the appHcation of heat and pressure in a mold to shape the article (press cure). The second step is a high temperature oven cycle at atmospheric pressure to obtain the final cured properties. 

The prepared phthalonitrile prepolymers were polymerized under MW irradiation using a 3-step cure cycle. The prepolymer sample (I, II, IIIC, or IVC) (0.5 g) in a small Pyrex reaction vial was placed in MW reactor. The curing was carried out with a preprogrammed three step cycles at 260°C for 30 min, at 280°C for 30 min and at 300°C for 30 min. The temperature of the reaction was monitored using a shielded thermocouple (ATC-300) inserted directly into the corresponding reaction chamber. 

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