MICROWAVE RADIATION CURING OF EPOXY/DIAMINE SYSTEMS

Abstract

Microwave radiation and thermal heating were performed on three different epoxy/amine systems namely Diglycidylether of bisphenol-A (DGEBA)/4,4- 'Diaminodiphenylsulfone(DDS), DGEBA/4,4' -Diaminodiphenylmethane (DDM) and DGEBA/m-Phenylenediamine (mPDA) These processed samples were analysed by thermogravimetry analysis (TGA) and differential scanning calorimetry (DSC). The appropriate temperature scanning range of the three epoxy/amine systems, for DSC analysis, was determined using TGA. An optimal scan rate was chosen to scan all processed samples by DSC The "curable" thermal temperatures were selected from DSC plots of freshly prepared epoxy/amine samples. The temperatures selected for thermal curing ranges from 80°C to 180°C. Samples for microwave curing were irradiated between 200W and 600W microwave power. The temperature-time profile showed that both microwave and thermally processed epoxy/amine samples followed three stages of cure reaction Microwave irradiated epoxy/amine samples consistently gave off significantly greater heat of reaction. The exothermic heat of reactions of thermally cured samples depended directly on the thermal temperature. Microwave and thermally cured samples were compared in terms of specimen temperatures, percentage cure and glass transition temperatures. It was found that percentage cure for both systems increases with glass transition temperature. When the glass transition temperature became comparable with the thermal cure temperature, percentage cure reaches the apparent limiting values (also termed limiting conversion), ie. when the reaction becomes diffusion controlled. A similar reaction, howbeit at a higher rate, was observed for microwaved samples when the glass transition temperatures of the system reached almost constant values. However, its percentage cure (for DGEBA/DDS samples) were comparatively lower than those which were thermally cured. The study also found that the glass transition temperature of microwave cured DGEBA/DDS was lower than under thermal curing. Notwithstanding this, the glass transition temperature for both DGEBA/DDM and DGEBN/mPDA systems were comparable with thermal systems. The lower temperature in microwaved DGEBA/DDS systems was attributed to the entrapment of the functional group in the cross-linking network. The reaction rate of microwave curing of epoxy/amine has been found to be strongly dependent on the curing agent used. TPT diagrams for the three epoxy/amine systems were attempted here. A significantly reduced cure time was able to achieve the characteristic gelation and vitrification states.