Polyurethanes tensile properties

Hydrolysis studies compared a polycarbonate urethane with a poly(tetramethyl-ene adipate) urethane and a polyether urethane based on PTMEG. After 2 weeks in 80°C water, the polycarbonate urethane had the best retention of tensile properties [92], Polycarbonates can hydrolyze, although the mechanism of hydrolysis is not acid-catalyzed, as in the case of the polyesters. Polycarbonate polyurethanes have better hydrolysis resistance than do standard adipate polyurethanes, by virtue of the highest retention of tensile properties. It is interesting to note in the study that the PTMEG-based urethanes, renowned for excellent hydrolysis resistance, had lower retention of physical properties than did the polycarbonate urethanes. 

It is speculated that the water does not diffuse as easily into the polycarbonate as into the PTMEG urethane, making the polycarbonate more resistant to plasticization by water and subsequent hydrolysis. The tensile properties of the PTMEG polyurethane eventually returned to nearly the original tensile properties, if the material was allowed to dry at room temperature for 2 weeks. This observation lends credence to the idea that the PTMEG urethane was plasticized by water. 

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

The equilibrium shear modulus of two similar polyurethane elastomers is shown to depend on both the concentration of elastically active chains, vc, and topological interactions between such chains (trapped entanglements). The elastomers were carefully prepared in different ways from the same amounts of toluene-2,4-diisocyanate, a polypropylene oxide) (PPO) triol, a dihydroxy-terminated PPO, and a monohydroxy PPO in small amount. Provided the network junctions do not fluctuate significantly, the modulus of both elastomers can be expressed as c( 1 + ve/vc)RT, the average value of vth>c being 0.61. The quantity vc equals TeG ax/RT, where TeG ax is the contribution of the topological interactions to the modulus. Both vc and Te were calculated from the sol fraction and the initial formulation. Discussed briefly is the dependence of the ultimate tensile properties on extension rate. 

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]

It is important to note that tensile strength is controlled by density, of course, but tensile strength is otherwise a molecular phenomenon. Molecular weight, cross-linking, and other chemistiy level factors affect tensile properties. We should note that, with the exception of composites, the polyurethanes we will design based... 

In most cases, the factors that affect tensile properties also affect the compressive strength of a polyurethane. This is most easily depicted in a graph such as Figure 3.12 that shows compressive strengths of foams. 

Lactic acid is an example. It is conveniently added to the polyol and upon reaction with the isocyanate, it is inserted into the backbone. Upon exposure to fluids in the body, the polymer is cut at the insertion site. The reduction in tensile properties is dramatic. Although the fragments are suspect, the result is by definition degradable. A number of researchers at W.R. Grace prepared biodegradable hydrophilic polyurethanes by a similar technique but the product was never available commercially. 

Tensile properties are used in the evaluation of batches of material as well as in determining the aging effects on polyurethane. Currently, the time taken for the preparation and stabilization of the samples does not make this test ideal for a batch-to-batch real-time quality control tool. Testing is normally carried out to one of the following standards ... 

In the evaluation of the effect of chemicals on polyurethanes, there are two main test methods used. The first is the change in tensile properties, and the second is the absorption into the material. The following test methods deal with the resistance to liquids. 

Y. I. Tien and K. H. Wei, High-tensile-property layered silicates/polyurethane nanocomposites by using reactive silicates as pseudo chain extenders, Macromolecules 34, 9045-9252 (2001). 

Table IV. Tensile Properties of Polymyrcene-Modified, Polyurethane Resins...

The tensile properties of polyester-based thermoplastic polyurethanes were studied as a function of the time of exposure of the plastics to water, methanol, methanol-water, methanol-Isooctane, and methanol-water-lsooctane. The resulting decrease In the tensile properties of the plastics was attributed to reaction of the plastics with water and methanol. As Indicated by the decrease In properties, reaction with methanol Is Initially faster, but the reaction rate with water Increases with time — presumably because of the autocatalytic nature of the reaction. Nuclear magnetic resonance spectroscopy Indicated that the reaction mechanisms with methanol and water were transesterification and hydrolysis, respectively. 

Aging of Tensile Bars. Polyurethane tensile samples were aged In methanol, water, an equal volume mixture of the two, and other mixtures containing Isooctane. While only graphical results of polymer A tensile properties are presented In this section, similar results were obtained with polymer B. Tensile properties of both polymers A and B are listed In Table I. 

The efficacy of polyurethane and styrene butadiene rubber (SBR) as binders for ground rubber prepared from waste tires was compared to a formulation of a compound developed without binder. Without binder, the effect of both sulfur and accelerator content on tensile properties are studied, as well as the effect of ageing on these properties [29]. The suggested uses of the unbound product include rubber blocks, and ballast mats for railway applications. 

In general, the contributions of both the soft and hard segments in the polyurethanes can be correlated with the properties observed. The soft rubbery block primarily affects resiliency, wear, tear, compression set and low temperature properties, while the hard block affects hardness, modulus and tensile properties [11]. 

Polyurethane elastomers are known for their high elongation, tensile strength and modulus properties. The combination of these properties provides toughness and durability in fabricated parts. Cast elastomers extended with butanediol can maintain these tensile properties when the use temperature is about 80 C. When these elastomers are subjected to higher temperatures, reduction in the tensile properties are observed due to the weakening of physical bonds in the elastomer. 

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