Stress-strain, tensile polyurethanes

Stress-Strain Values Polyurethane stress-strain properties were measured with the Scott Tensile Tester operated at a jaw separation rate of 20 inches per minute. Test pieces were microdumbbells died out of 25 mil thick compression-molded sheets. The test pieces were pre-conditioned for 24 hours at 50% relative humidity and 25°C, and then tested in this environment. 

In practice, up to 90% of polyurethanes are used in compression, a few percent in torsion, and very little in tension. There is considerable data on the tensile stress against tensile strain (elongation) for polyurethanes. Most polyurethane specification sheets provide this data. Figure 7.3 and Figure 7.4 show typical stress-strain curves for both polyester and polyether polyurethanes. 

Tests such as the tensile strength and tear strength tests evaluate polyurethanes to destruction. When polyurethanes are used in a practical situation, the aim is for them to have as long a life as needed in the application. Stress, strain, and shear are applied to the polyurethane at various frequencies and at different temperatures. There may also be dynamic variations on top of a static load, for example, vibrations on a loaded isolation pad. 

Both series of polyurethanes were prepared using a prepolymer technique in which reactants were mixed at 70 °C/1 hour, cast into molds at 105 °C/2 hours, and cured at 80 °C/14 hours. The BD/MDI hard segment contents ranged from 0% (transparent, colorless homopolyurethanes) to 30% w/w (opaque, white copolyurethanes). All elastomers were characterized using DSC, dynamic mechanical, and tensile stress-strain measurements. 

The mechanical properties of SWNT reinforced polyurethane (PU) electrospun nanofibers were studied by Sen et al. (89). Stress-strain analysis showed that the tensile strength of the PU/SWNT nanofiber membrane was enhanced by 46% compared to pure PU nanofiber membrane. This value of tensile strength was further increased by 104% for ester functionalized PU/SWNT membranes because of the improved SWNT dispersion and the enhanced PU-SWNT interfacial interaction. 

The tensile stress-strain deformation pattern for polyurethane elastomers is similar to those of other elastomers, and Fig. 13.1 shows typical curves for urethane elastomers of different hardness. Typically, for elastomers, the shape of the curve changes with increasing deformation so that elastic behaviour over the full stress-strain range cannot be defined simply by Young s modulus. Figure 13.2 shows a stress-strain curve at low strain values. This curve can be described by the general equation... 

Fig. 13.2. Tensile stress-strain curve for a urethane elastomer at low strain. (Source P. Wright A. P. C. Gumming, Solid Polyurethane Elastomers. Maclaren Sons, London, 1969.)...

K. C. Frisch, Topologically Interpenetrating Polymer Networks, Pure Appl. Chem. 43, 229 (1975). Topological Interpenetration. Stress/strain behavior, tensile strength, Tg. Polyurethane SINs. 

K. C. Frisch, D. Klempner, S. K. Mukherjee, and H. L. Frisch, Stress-Strain Properties and Thermal Resistance of Polyurethane-Polyepoxide, Interpenetrating Polymer Networks, J. Appl. Polym. Sci. 18(3), 689 (1974). Polyurethane/Epoxy SIN. Tensile strength. Heat resistance. 

Fig. 2 Stress-strain curves for various segmented polyurethane-urea copol3rniers indicating the dependence of tensile behavior on the soft segment molecular weight. (1) PTMO-2000-MDI-31-DCA, (2) PTMO-IOOO-MDI-31-DCA, and (3) PTMO-650-MDI-31-DCA.

The relationship between the tensile properties of the hybrid materials and their Cloisite 30B content was studied. The stress-strain parameters of the polyurethane samples and their derived nanocomposites are shown in Table 1. For all clay systems, the tensile stress increased in comparison to the unmodified polyurethanes. 

Stress Relaxation Polyurethane networks were also polymerized in a mold with a cylindrical cavity. Uniform rings were cut from the cylinders and weighed. The cross sectional area was then derived from the sample diameter and polymer density and approximated 0.05 cm2. Stress relaxation was measured at several strains between 10 and 43% with an Instron tensile tester. During stress relaxation, the samples were immersed in dioxane and swelled to equilibrium. 

Additional work was performed by Gardner (Refs 92 99) on proplnts for future missions in space. A PBAA AP/Al proplnt, an aluminized double-base proplnt, and a polyurethane—AP/Al proplnt were studied as a function of Co radiation, with a dose rate of 2.54 x 10 R/hr and total doses ranging up to 1.5 x 10 R. The effects were noted on tests with the burning rate, tensile stress, elongation modules and hardness of the three materials. The PBAA proplnt withstood 1.5 X lO R. On the tensile strength, the double base and polyurethane decrease significantly at a dose of 4 x 10 R. On elongation, the double base decreased on stress at max strain after 10 R, while the polyurethane pro-pint decreased on modulus and hardness after a dose of 10 R. Estimates of the radiation effects on polymers are listed in Table 19... 

Fig. 7 (a) Picture of a mechanochromic elastomer made by integrating C120H-RG into a thermoplastic polyurethane backbone in the unstretched state, (b) Picture of the same material in the stretched state. Both pictures were taken under illumination with ultraviolet light, (c) Ratio of monomer to excimer emission 7m//e (circles) and tensile stress (solid line) under a triangular strain cycle between 0% and 500% at a frequency of 0.0125 Hz. Adapted with permission from [41]. Copyright 2006 American Chemical Society... 

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