bisphenol-A Type Research Articles

Positron lifetime study of electron-irradiated epoxy resins

Two bisphenol-A type epoxy resins were irradiated by electron beam and studied afterwards by positron lifetime spectroscopy. An interesting result is that despite the considerable amount of free-radicals no inhibition of positronium formation was observed in the two epoxies. Nevertheless several serious differences were detected between the studied polymers. The results suggest that the radiation-resistant properties of epoxies depend strongly on the amount of the curing agent.

Internal stress of epoxide resin modified with spiro ortho-ester type resin

Bisphenol-A type epoxide resin modified with various amounts of spiro ortho-ester (SOE) resin was cured with an imidazole. Internal stress of the modified systems decreased with increasing fraction of SOE resin in the cured resins. In particular, a drastic reduction of the internal stress was observed in the systems modified with more than 33 mol% SOE resin. In addition, heterogeneous structure was observed with modifier content over 33 mol%, and the elastic modulus of these systems decreased step-wise with increasing ambient temperature. On the other hand, the systems in which the modifier content was less than 20 mol% had homogeneous structure and thus the modulus was considerably higher than that of the former systems. Consequently, it was concluded that the drastic reduction of the internal stress in the systems modified with more than 33 mol% SOE resin depends on the low elastic modulus caused by the formation of heterogeneous structure.

Heat of reaction and curing of epoxy resin

The heat of reaction and kinetics of curing of diglycidyl ether of bisphenol-A (DGEBA) type of epoxy resin with catalytic amounts of ethylmethylimidazole (EMI) have been studied by differential power-compensated calorimetry as a part of the program for the study of process monitoring for composite materials. The results were compared with those from 1∶1 and 1∶2 molar mixtures of DGEBA and EMI. A method of determination of heat of reaction from dynamic thermoanalytical instruments was given according to basic thermodynamic principles. The complicated mechanism, possibly involving initial ionic formation, has also been observed in other measurements, such as by time-domain dielectric spectroscopy. The behavior of commercially available DGEBA resin versus purified monomeric DGEBA were compared. The melting point of purified monomeric DGEBA crystals is 41.4 °C with a heat of fusion of 81 J/g. The melt of DGEBA is difficult to crystallize upon cooling. The glass transition of purified DGEBA monomer occurs around −22 °C with aΔCp of 0.60 J/K/g.

Mechanical relaxation properties of spiro-type epoxide resins cured with acid anhydrides

Low-temperature relaxation behaviour has been investigated for a bisphenol-A type and three spiro-type epoxide resins cured with acid anhydrides. In the spiro-type resin systems, one new relaxation, denoted here as the β′ relaxation, is clearly observed near room temperature. It is suggested that the β′ relaxation is related to the motion of p-phenylene group adjacent to the spiro ring. The impact strength of epoxide resin systems that show the β′ relaxation is considerably higher than that of other cured systems and is proportional to the intensity of the β′ relaxation. Consequently, it is concluded that the existence of the room-temperature relaxation such as the β′ relaxation is associated with increased toughness of cured epoxide resins.

The temperature‐dependence of some mechanical properties of a cured epoxy resin system

AbstractThe tensile mechanical properties and fracture toughness of a Bisphenol‐A type difunctional epoxy resin, cured with different amounts of metaphenylene diamine, using two cure cycles, were determined over a range of temperature. The tensile modulus in the glassy state was seen to be predominantly related to intermolecular packing, while in the rubbery state crosslink density was the important factor. Yielding appeared to be due to an increase in free volume as a result of dilatation during the tensile test and was related to a critical shear stress. The large strain properties like tensile strength, elongation‐to‐break, and toughness showed a more complex dependence on chemical structure, molecular architecture, intermolecular packing, and crosslink density. The roles played by the relaxation processes in determining mechanical properties are highlighted.