Polyurethane networks, reductions

The moduli of model polyurethane networks clearly show reductions below the values expected for perfect networks, with the reductions increasing with pre-gel intramolecular reactlon(5-7). The reductions can be shown to be too large to come solely from pre-gel loop forma-tion( ), some must occur post-gel. In addition, extrapolation to conditions of zero pre-gel intramolecular reaction, by increasing reactant concentrations, molar masses of reactants or chain stiffness, still leaves a residual proportion of inelastic chains due to gel-gel intramolecular reaction. It is basically a law-of-mass-action effect( ). The numbers of reactive groups on gel molecules are unlimited. Intramolecular reaction occurs, and some of this gives Inelastic chains. Only a small amount of such reaction has a marked effect on the modulus. 

Thus, one can identify at least three factors that increase the longterm strength of Sprut-5M adhesive at the stage of formation of the polyurethane network but which have no further effect after the adhesive cures completely. Actually, in this state of the adhesive there is no indication of the structure that is observed with the orientation of the molecular chains there is an abrupt reduction of the opportunity for macroradicals to recombine and for the submicrocracks to be filled. At this stage of curing, despite some decrease of the long-term strength under a load of 50% of the fracture load, the Sprut-5M adhesive has much better resistance to the effect of constant loads than does the polyester adhesive (see Fig. 3.2, curve 1 (after 102 days) and curve 4). 

Zammarano M, Kramer RH, Harris R, Ohlemiller TJ, Shields JR, Rahatekar SS, Lacerda S, Gilman JW. Flammability reduction of flexible polyurethane foams via carbon nanofiber network formation. Polym. Adv. Technol. 2008 19 588-595. 

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