ObjectiveThe purpose of this study was to investigate the degree of conversion (DC) and mechanical properties of a microhybrid Filtek Z250 (3M ESPE) resin composite after aging.MethodThe specimens were fabricated using circular molds to investigate Vickers microhardness (Vickers hardness number [VHN]) and DC, and were prepared according to ISO 4049 for flexural strength testing. The initial DC (%) of discs was recorded using attenuated total reflectance-Fourier transforming infrared spectroscopy. The initial VHN of the specimens was measured using a microhardness tester under a load of 300 g for 15 seconds and the flexural strength test was carried out with a universal testing machine (crosshead speed, 0.5 mm/min). The specimens were then subjected to thermocycling in 5°C and 55°C water baths. Properties were assessed after 1,000–10,000 cycles of thermocycling. The surfaces were evaluated using scanning electron microscopy (SEM). Data were analyzed using 1-way analysis of variance followed by the Tukey honest significant difference post hoc test.ResultsStatistical analysis showed that DC tended to increase up to 4,000 cycles, with no significant changes. VHN and flexural strength values significantly decreased upon thermal cycling when compared to baseline (p < 0.05). However, there was no significant difference between initial and post-thermocycling VHN results at 1,000 cycles. SEM images after aging showed deteriorative changes in the resin composite surfaces.ConclusionsThe Z250 microhybrid resin composite showed reduced surface microhardness and flexural strength and increased DC after thermocycling.
ABSTRACT:The use of carbon/epoxy-novolac composites as advanced ablative materials for insulation of exit cone of solid-propellant rocket nozzles are studied. In this article, three types of carbon fabrics are used and their composites are prepared by use of impregnation and hand lay-up methods. To study the thermal stability and ablation behavior, these composites are tested by thermal tests such as thermogravimetric analysis (TGA) and oxyacetylene standard flame tests; the latter test is one of the most important standard tests of ablative materials. The test apparatus is made according to American standard, ASTM-E-285-80, and over 33 polymeric composites and 3 steel specimens were carried out according to its standards. It is found that the composites that are made up of C-9750 fabric (high-strength carbon fabric) in comparison with steel and the other types of carbon fabric specimens have the highest thermal stability and the best ablation behavior.
Epoxy-Novolac/Anhsydride cure kinetics have been studied by differential scanning calorimetry under isothermal conditions.The system used in this study was Epoxy Novolac resin (DEN431), nadic methyl anhydride as hardener and benzyl dimethyl amine as accelerator. Kinetic parameters including the reaction order, activation energy and kinetic rate constants were investigated. The cure reaction was described with the catalyst concentration and the normalized kinetic model was developed for it. It is shown that, the cure reaction was dependent on the cure temperature and catalyst concentration and proceeded through an autocatalytic kinetic mechanism. The curing kinetic constants and the cure activation energies were obtained by the Arrhenius kinetic model. The suggested kinetic model with a diffusion term was successfully used to describe and predict the cure kinetic of Epoxy Novolac resin compositions as a function of the catalyst content and temperature.KEY WORDS: epoxy novolac resin, nadic methyl anhydride, benzyl dimethyl amine, cure, kinetic, isothermal DSC.
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