An essential work of fracture study of the toughness of thermoset polyester coatings

The fracture toughness of a range of thermoset polyester paints with different cross-link densities has been studied, using the essential work of fracture (EWF) method. The glass transition temperature, T-g, of each of the materials was measured using differential scanning calorimetry, and found to lie between 8 and 46 degrees C. EWF tests were performed on the paint films at a range of temperatures around the measured glass transition temperature of each material. The essential work of fracture, w(e), at T-g was found to decrease with increasing cross-link density from around 20 kJ/m(2) at a cross-link density of 0.4 x 10(-3) mol/cm(3) to around 5 kJ/m(2) for cross-link densities of approximately 1 x 10(-3) mol/cm(3) or higher. A maximum in the essential work of fracture was observed at around T-g when w(e) was plotted versus temperature, which could be attributed to the effect of an alpha-relaxation at a molecular level. The polyesters were found to be visco-elastic, and the applicability of the EWF test to the study of these visco-elastic thermoset materials is discussed. (C) 2014 The Authors. Published by Elsevier B.V.