Polyurethane, tensile stresses

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. 

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. 

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... 

Polyurethanes are among the better adhesives for cryogenic applications. Room temperature-curing polyurethane adhesives in current use provide higher ultimate shear and tensile stress and higher peel and shock properties at -253°C than the earlier polyurethanes. This situation is the inverse of what happens to most structural adhesives. The polyurethane adhesives increase in strength at -253°C, but become weaker at ambient and higher temperatures, as shown in Table 10.3. ... 

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.)...

In 2007, Cao and Jana at Akron tethered Cloisite 30B clay particles onto SMP polyurethanes [70]. The polyurethanes were synthesized from aromatic diisocyanates, a crystalline polyester polyol, and poly(e-caprolactone) (PCL) diol. The monomer ratios were cleverly balanced between an excess ratio of isocyanate in the polymer system and the pendant alcohol groups (-CH2CH2OH) on the quaternary ammonium ions fi om the Cloisite 30B. The team observed an increase in the rubbery modulus at 100°C (Tm-I- 50°C) of nearly one order of magnitude, from 4—5 MPa without loading to more than 20 MPa with loading. In the carefully crafted study, the authors determined that at high (5%) clay content, a more rapid relaxation of induced tensile stress reduces the recovery force of SMPs. In addition,... 

Fig. 9 Polyurethane network creep rate spectra obtained at tensile stresses of 0.2,1.0, or 5.0 MPa, and the mechanical loss spectral contour (1 Hz) [19]...

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. 

Excessive shear strain should generally be avoided in adhesive joints because cohesive failure is likely to occur once shear deformation leads to a buildup of tensile stress at the end of the overlap bond area. In practice, it has been found that strain in assemblies with rubbery moisture curing one-component polyurethane adhesives should be limited to values of shear strain tany below 0.5. Specific viscoelastic pressure sensitive tapes may even tolerate shear strains of tany up to 2. The maximum tolerable strain of structural adhesive joints using adhesives with a highly cross-linking density, however, is typically more than one order of magnitude lower than that. 

Stress decay (relaxation) measurements of propellant binders are a way to obtain insight into the network structure of binder systems (29). In addition, high hysteretical losses appear to be associated with good tensile properties. Figure 5 shows a normalized stress-decay vs. time plot of a polyurethane elastomer. If the reference stress, [Pg.105]

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. 

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