Relaxation 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. 

Elastomeric Fibers. Elastomeric fibers are polyurethanes combiaed with other nonelastic fibers to produce fabrics with controlled elasticity (see Fibers, elastomeric). Processing chemicals must be carefully selected to protect all fibers present ia the blend. Prior to scouriag, the fabrics are normally steamed to relax uneven tensions placed on the fibers duriag weaving. Scouriag, which is used to remove lubricants and siting, is normally conducted with aqueous solutions of synthetic detergents and tetrasodium pyrophosphate, with aqueous emulsions of perchloroethylene or with mineral spidts and sodium pyrophosphate. 

Figure 28 Variation of relaxation time (/ ) with the percent polyurethane rubber in XNBR-AU blends.

Polyurethane networks were prepared from polyoxypropylene (POP) triols(Union Carbide Niax Polyols) after removal of water by azeotropic distillation with benzene. For Niax LHT 240, the number-average molecular weight determined by VPO was 710 and the number-average functionality fn, calculated from Mjj and the content of OH groupSj determined by using excess phenyl isocyanate and titration of unreacted phenyl isocyanate with dibutylamine, was 2.78 the content of residual water was 0.02 wt.-%. For the Niax LG-56, 1 =2630, fn=2.78, and the content of H2O was 0.02wt.-%. The triols were reacted with recrystallized 4,4"-diphenylmethane diisocyanate in the presence of 0.002 wt.-% dibutyltin dilaurate under exclusion of moisture at 80 C for 7 days. The molar ratio r0H = [OH]/ [NCO] varied between 1.0 and 1.8. For dry samples, the stress-strain dependences were measured at 60 C in nitrogen atmosphere. The relaxation was sufficiently fast and no extrapolation to infinite time was necessary. 

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]

Poisoning with cyanide may occur in a variety of ways accidental or intentional poisoning with cyanide salts, which are used in industry or in laboratories as a result of exposure to hydrogen cyanide in fires when polyurethane foam burns from sodium nitroprusside, which is used therapeutically as a muscle relaxant and produces cyanide as an intermediate product and from the natural product amygdalin, which is found in apricot stones, for example. 

In addition, from thermal and thermomechanical measurements, it is found that typical epoxy-amine networks exhibit one glass transition temperature, Tg, and one sharp well-defined relaxation peak. The same techniques were used for crosslinked polyurethanes based on triol and diisocyanate or diol and triisocyanate (Andrady and Sefcik, 1983). Similar conclusions to those found for epoxy-amine networks were attained. 

Reaction-induced phase separation is certainly also the reason for which an inhomogeneous structure is observed for photocured polyurethane acrylate networks based on polypropylene oxide (Barbeau et al., 1999). TEM analysis demonstrates the presence of inhomogeneities on the length scale of 10-200 nm, mostly constituted by clusters of small hard units (the diacrylated diisocyanate) connected by polyacrylate chains. In addition, a suborganization of the reacted diisocyanate hard segments inside the polyurethane acrylate matrix is revealed by SAXS measurements. Post-reaction increases the crosslink density inside the hard domains. The bimodal shape of the dynamic mechanical relaxation spectra corroborates the presence of a two-phase structure. 

Polyurethanes under tension are very "notch" sensitive and when used as a spring, a nick will propagate and cause failure. Under long-term tension, polyurethanes will suffer from creep (strain relaxation) and set. 

Softer materials normally have better compression set results than the harder grades. This is due to the chains being able to move more readily over each other. Compression set values can also be improved by introducing some degree of permanent cross-linking at the prepolymer stage. If polyurethane is placed under a constant stress, it will creep slowly over the course of time due to strain relaxation. 

Hygroscopic Hysteresis hydroxyl content of 1.0 g of polyol. A material that absorbs moisture readily. The ability of polyurethane to absorb and dissipate energy due to successive deformation and relaxation. A measurement of the area between the deformation and relaxation stress-strain curves. 

Extensive dynamic mechanical property studies have been carried out on hydrogen-bonded (81) and nonhydrogen-bonded (60,82) polyurethanes. Several secondary relaxations were found in addition to the major hard- and soft-segment transitions. Molecular mechanisms could... 

The systematic dynamic mechanical study reported here was undertaken to gain additional insight into the molecular relaxation processes which occur in TDI polyurethanes as well as to test the generality of findings developed from investigations of other chemical classes of segmented polyurethanes. 

TDI Polyurethanes. Two 2,4-T-lP samples with different hard-segment concentrations were studied and found to display a broad a-relaxation maximum, also characterized by a decline in storage modulus of about two and one half orders of magnitude, as shown in Figure 5, a plot of storage and loss modulus vs. temperature at 11 Hz. The position... 

Figure 13. Apparent activation energy of the as relaxation in 2,6-T-2P polyurethanes as a function of hard-segment content from the first (filled points) and second determination (open points)...

The a Relaxation of 2,4-T-lP. Only the 2,4-T-lP block polyurethanes exhibited a single major a relaxation which had a strongly composition-dependent position. Similar behavior was observed by See-fried and co-workers (10) for polyurethanes containing a TDI/BD hard segment and a 2100-Mn polycaprolactone hard segment. The extreme... 

Only the 2,6-T-2P polyurethanes exhibited a relaxation associated with crystallization and later melting of the soft segments. The position of the crystallization maximum was estimated from the location of tan(8)m lx and was somewhat difficult to characterize accurately because of extreme overlapping of the a9 and ac relaxations, as evident in Figure 10. Melting occurred over the region 2°-7°C in all samples. Phase segregation in 2,6-T-2P samples was well developed to the point that the... 

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