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Experiments conducted on several different elastomer-modified epoxy systems indicated that the high fracture energy of most structural adhesives is achieved through crack-tip deformation processes that are viscoelastic. It is essential therefore that the fracture behavior of such materials be determined as a function of temperature and loading history. The linear viscoelastic properties of the model systems were functions of formulation and thermal history hut when these parameters were controlled the behavior was thermo-rheologically simple over a wide range of conditions. The fracture behavior was also dependent on formulation and thermal history although the effects of history were quite small in the range ofconditions studied here. The fracture behavior at various temperatures and loading rates could be characterized to a first approximation by a master cyrve of fracture energy us. reduced time-to-failure. This characterization makes it possible to compare the properties of different formulations and to predict their fracture behavior over a wide range of conditions.

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Dynamic mechanical properties have been investigated for interpenetrating-network systems based on polyol-cured polyurethanes (PU) and 2 to 1 n-butyl acrylate-n-butyl methacrylate (Ac) networks. The systems were formed simultaneously (SIN) from all of the precursors and reactants for both networks in the same vessel, and sequentially (SIPN) by swelling a precured PU with the reactants that will form the Ac network. If the Ac network is formed after gelation of the PU, the IPNs are transparent and appear to have single T (tan 6,,,=) between those of the homonetworks; visible-phase separation occurs if the Ac is intentionally polymerized prior to PU gelation. Damping curves were lower and broader and the T (tan &, , , , ) and rubber moduli were higher for the SIN than for the SIPN systems. Up to 65 percent Ac, the T (tan 6,,,) data for both SIN and SIPN fit the Gordon-Taylor equation if a T (tan 6,,,) for the Ac homonetwork 7°C higher than observed is used, suggesting a higher crosslink density for the Ac network under these conditions. The differences in properties of the SIN and SIPN are assumed to be dependent on sample homogeneity and upon the presence of a tin catalyst in the SIN preparation. This can result in limited Ac-network formation and consequent phase separation before PU gelation has occurred, and the catalyst may also increase the extent of interaction, such as grafting or hydrogen-bond formation between the networks.