Abstract
In recent years, we have developed highly resilient (HR) flexible molded foams with superb durability, resilience and vibration characteristics, and we have reported that these improvements were based on control of cross link degree of polymer, morphology and mobility of hard and soft segments. This paper describes factors that dominantly influence the durability and the mechanism of fatigue by investigation of cross linking by covalent and hydrogen-bonds and the morphology of hard/soft segments. In this study, we use molded foams with different durabilities. These are TDI-based high performance/conventional HR, MDI-based high-performance/conventional HR, and hot molded foams.
On the whole, a small apparent viscosity coefficient, which is calculated from vibration characteristics of the foam, and a higher cross link degree of foam polymer make static and dynamic durability of flexible molded foam better. Mobility of the soft segment, estimated by solid state NMR, also dominates dynamic durability. Accordingly, hydrogen-bonds of the hard segment contribute against creep as a cross link point in static/dynamic durability test, and tight hydrogen-bonds of hot molded foam are durable against rupture or rebonding by water molecules in the wet compression set oven.
The drop in cross link degree of foam polymer was not observed during vibration durability test. Accordingly, the mechanism of fatigue could be the change of state in hydrogen-bonds or tangling of polymer-chains, or relatively macro rupture.
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