Single-phase polyurethane elastomers

Elastic Modulus, Network Structure, and Ultimate Tensile Properties of Single-Phase Polyurethane Elastomers... 

Studies have been made of the elastic (time-independent) properties of single-phase polyurethane elastomers, including those prepared from a diisocyanate, a triol, and a diol, such as dihydroxy-terminated poly (propylene oxide) (1,2), and also from dihydroxy-terminated polymers and a triisocyanate (3,4,5). In this paper, equilibrium stress-strain data for three polyurethane elastomers, carefully prepared and studied some years ago (6), are presented along with their shear moduli. For two of these elastomers, primarily, consideration is given to the contributions to the modulus of elastically active chains and topological interactions between such chains. Toward this end, the concentration of active chains, vc, is calculated from the sol fraction and the initial formulation which consisted of a diisocyanate, a triol, a dihydroxy-terminated polyether, and a small amount of monohydroxy polyether. As all active junctions are trifunctional, their concentration always... 

Table 6.5.1 contains data values for Q, Cp for three heterogeneous elastomers with urethane urea hard segments and a single-phase polyurethane PUE-TL. It is seen that pre-liminaiy strain e = 350% does not practically affect the equilibrium swelling and the equilibrium concentration of this material at swelling in DEHP and HP-470. 

Atomic force microscopy and attenuated total reflection infrared spectroscopy were used to study the changes occurring in the micromorphology of a single strut of flexible polyurethane foam. A mathematical model of the deformation and orientation in the rubbery phase, but which takes account of the harder domains, is presented which may be successfully used to predict the shapes of the stress-strain curves for solid polyurethane elastomers with different hard phase contents. It may also be used for low density polyethylene at different temperatures. Yield and rubber crosslink density are given as explanations of departure from ideal elastic behaviour. 17 refs. 

Thermoplastic elastomers, e.g., polyurethane elastomers, have an entirely different structure that is heterogenic. They have an elastomeric matrix and phase-separated hard blocks, which act as embedded physical crosslinking sites. The hard blocks can be softened at elevated temperatures to obtain a single-phase melt that is easily processed. Upon cooling, the two-phase nature is recovered and the material becomes a solid again. 

There is another group of polyurethanes that is chemically crosslinked with a crosslinker, either triol or polyamine or polyisocyanate. They are single-phase elastomers, and they display lower strengths than the thermoplastic urethanes. However, their properties are less temperature sensitive, and elastic recovery is generally considerably better (permanent set is smaller) than in TPUs. Their strength can be improved by adding proper fillers. Such systems are called cast systems since they are processed by casting... 

The efficiency of the theory is demonstrated for the single-phase crosslinked polyurethane elastomers. At low degrees of swelling of materials in liqttids, equation [6.6.6] can be transformed with simplification into the expression ... 

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