Stoichiometric mixtures of DGEBA (diglycidyl ether of bisphenol A)/DDS (diaminodiphenyl sulfone) and DGEBA/mPDA (meta phenylene diamine) have been isothermally cured by electromagnetic radiation and conventional heating using thin film sample configurations. Fourier transform infrared spectroscopy (FTIR) was used to measure the extent of cure. Thermal mechanical analysis (TMA) was used to determine the glass transition temperatures directly from the cured thin film samples. Well-defined glass transitions were observed in the TMA thermcgraph for both thermal and microwave cured samples. Significant increases in the reaction rates have been observed in the microwave cured DGEBA/DDS samples. Only slight increases in the reaction rates have been observed in the microwave cured DGEBA/mPDA samples. Higher glass transition temperatures were obtained in microwave cured samples compared to those of thermally cured ones after gelation. The magnitude of increases of glass transition temperature is much larger for the DGEBA/DDS system than 'DGEBA/mPDA system. The microwave radiation effect was much more significant in DGEBA/DDS system than in DGEBA/mPDA system. DiBenedetto's model was used to fit the experimental Tg data of both thermal and microwave cured epoxy resins.
Stoichimetric mixtures of a diglycidyl ether of bisphenol A (DGEBA)/ diaminodiphenyl sulfone (DDS) and a DGEBA/meta phenylene diamine (mPDA) were cured using both microwave and thermal energy. Fourier transform infrared (FTIR) was used for the measurement of the extent of cure and thermal mechanical analysis (TMA) was used for the determination of the glass transition temperature (Tg). The cure kinetics of the DGEBA/mPDA and DGEBA/DDS systems were described by an autocatalytic kinetic model up to vitrification in both the microwave and thermal cure. For the DGEBA/mPDA system, the reaction rate constants of the primary amine‐epoxy reaction are equal to those of the secondary amine‐epoxy reaction, and the etherification reaction is negligible for both microwave and thermal cure. For the DGEBA/DDS system, the reaction rate constants of the primary amine‐epoxy reaction are greater than those of the secondary amine‐epoxy reaction and the etherification reaction is only negligible at low cure temperatures for both microwave and thermal cure. Microwave radiation decreases the reaction rate constant ratio of the secondary amine‐epoxy reaction to the primary amine‐epxy reaction and the ratio of the etherification reaction to the primary amine‐epoxy reaction. Tg data were fitted to the DiBenedetto model. A master curve and a time‐temperature‐transformation (TTT) diagram were constructed. The vitrification time is shorter in microwave cure than in thermal cure, especially at higher isothermal cure temperatures. For the DGEBA/mPDA system, the minimum vitrification time is two to five times shorter in the microwave cure than in the thermal cure. For the DGEBA/DDS system, the minimum vitrification time is 44 times shorter in the microwave cure than in the thermal cure.
Dielectric properties of poly(ethylene terephthalate) (PET) were measured over a frequency range of 10 KHz to 2.45 GHz and a temperature range of 20 to 110°C. Relaxation peaks were identified at 1) fixed frequency with variable temperatures, and 2) fixed temperature with variable frequencies. The crystallinity of poly (ethylene terephthalate) was measured using differential scanning calorimetry (DSC). Relationships between crystallinity, dielectric properties, and location of the dielectric relaxation peak on the frequency and temperature scales were studied for poly(ethylene terephthalate). Also, the dielectric loss factor decreases with increased crystallinity at 2.45 GHz and 4 GHz within the temperature range studied.
Polymers and polymer composites have been processed In a cylindrical resonant microwave applicator at a frequency of 2.45GHz. Stoichlometric mixtures of two epoxy/amine systems, DGEBA (Diglycidyl Ether of Bisphenol A)/DDS (4,4'-Diaminodiphenyl Sulfone) and DGEBA/mPDA(m-Phenylene Diamine), were microwave and thermally cured Isothermally using a thin film technique. FTIR was used to determine the extent of cure. Increased reaction rates were observed In microwave cure when compared to those of thermal cure. The rate Increase due to microwave effects was much greater for the DGEBA/DDS system than for DGEBA/mPDA. Also, crossply and unidirectional 24-ply and 72-ply graphite/epoxy laminates(AS4/3501-6 prepreg, Hercules Corp.). were processed using microwave radiation. The flexural properties of the microwave processed composites were strongly dependent on the resonant heating mode. Comparable flexural properties were obtained for the unpressurized microwave processed composites and the pressurized autoclave processed composites. Proper controlled-hybrid modes are required to process composites of high mechanical properties. The procedures for obtaining these controlled-hybrid modes are described.
Microwave processing has been investigated as an alternative to conventional thermal method in processing polymer matrix composite materials. The main advantages of microwave processing over thermal processing are that: 1) microwave heating is volumetric, direct, selective, instantaneous, and controllable which offers advantages such as fast heating and minimization of temperature excursion; 2) microwave radiation can provide many desirable features in polymer and composite processing, such as enhanced polymerization rates and glass transition temperatures of thermosets, improved mechanical properties of composite materials, and increased adhesion between graphite fibers and matrix.Microwave heating has been used in food processing, drying, material processing, waste treatment, and organic synthesis. This paper summarizes the current status of microwave technology for the processing of polymer matrix composite materials. The discussion will be focused on the use and development of batch and continuous techniques using tunable single mode resonant microwave cavities for processing polymer composites.
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