Acid- and base-catalyzed epoxy-phenol thermosets from catechol, resorcinol and hydroquinone: A structure-property study (vol 138, e50995, 2021)

In order to better understand the design rules of epoxy-phenol thermosets we will report on the chemistry and (thermo)mechanical properties of cured epoxy-phenol thermoset films. Ortho-, meta- and para-isomers of dihydroxybenzene (DHB) were reacted with the diglycidyl ether of bisphenol A (DGEBA) in the presence of an acid catalyst or triphenylphosphine (PPh3). The glass transition temperatures (T-g) of the cross-linked films decreases in the order of meta- (T-g = 115 degrees C) > ortho- (T-g = 102 degrees C) > para-DHB (T-g = 96 degrees C) as measured by differential scanning calorimetry. Uniaxial tensile testing of cross-linked films showed excellent stress-strain behavior. The average ultimate strength values ranged from 65 to 82 MPa and the average values of the strain-at-break ranged from 4.8% to 6.9% at 25 degrees C for all cross-linked films. When a PPh3 was used, the network properties were profoundly different. The base catalyzed thermoset of DGEBA and meta-DHB shows a T-g of 85 degrees C, which is 30 degrees C lower than the T-g of the acid-catalyzed analog. Tensile films appear to be more ductile, as they exhibit a strain-at-break of 20%. The results of this study confirm that simple dihydroxybenzene hardeners can be used to prepare cross-linked films with excellent thermomechanical properties.