THE EFFECTS OF CURE TEMPERATURE AND TIME ON THE BULK TENSILE AND FRACTURE BEHAVIOR OF A STRUCTURAL ADHESIVE

The effects of thermal cure and cool-down conditions as well as the presence of carrier cloth on the room temperature bulk tensile and Mode-I fracture properties of a rubber-modified structural adhesive have been investigated experimentally. The model adhesives used in this work are Metlbond 1113 (with carrier cloth) and Metlbond 1113-2 (without carrier cloth) solid film modified epoxy resins. The possibility of optimizing the cure conditions via bulk tensile and fracture behavior are determined. Based on a simple differential thermal analysis methodology, and using a single first order kinetic reaction model, variations in the degree of cure of the model adhesives subjected to different cure schedules are predicted. It is shown that for a fixed cool-down condition and cure time duration, both the tensile strength and rigidity values can be maximized over a range of cure temperatures. By increasing the cure time duration, higher peak strength and rigidity values are obtained at lower cure temperatures. The slow cool-down condition is shown to increase the optimum tensile properties. The presence of carrier cloth appears to increase the optimum strength values for the fast cool-down condition and also the extent of void formation during the cure process. The short term viscoelastic tensile stress relaxation behavior of Metlbond 1113 is shown to be affected by the thermal cure history. Using the Modified Bingham model, the relaxation times seem to increase as higher cure temperatures are used. The Mode-I fracture toughness (K(,Ic)) of the model adhesives have been determined using the SEN tension geometry over a range of cure conditions. It is found that the optimum K(,Ic)'s are obtained at low cure temperatures--long cure time conditions in the absence of carrier cloth when the slow cool-down condition is employed. The fracture energy values (G(,Ic)) are found using the small-scale crack tip yielding assumption and the bulk tensile properties. It is found that the optimum G(,Ic)'s are obtained at high temperature - short time cure conditions in the absence of carrier cloth when the slow cool-down condition is used. SEM fractographic and microscopic examinations reveal that changes in cure conditions result in varying degrees of stress whitening ahead of the crack tip in the fracture specimens and along the surface for the tensile specimens. Based on the application of a modified bilinear form of Ramberg-Osgood model, the stress-strain behavior as well as the stress whitening stress levels for Metlbond 1113 are predicted and agree well with the experimental results.