Tensile properties, polyurethane elastomers

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. 

Much work has been done on the incorporation of castor oil into polyurethane formulations, including flexible foams [64], rigid foams [65], and elastomers [66]. Castor oil derivatives have also been investigated, by the isolation of methyl ricinoleate from castor oil, in a fashion similar to that used for the preparation of biodiesel. The methyl ricinoleate is then transesterified to a synthetic triol, and the chain simultaneously extended by homo-polymerization to provide polyols of 1,000, 000 molecular weight. Polyurethane elastomers were then prepared by reaction with MDl. It was determined that lower hardness and tensile/elongation properties could be related to the formation of cyclization products that are common to polyester polyols, or could be due to monomer dehydration, which is a known side reaction of ricinoleic acid [67]. Both side reactions limit the growth of polyol molecular weight. 

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]

A recent commercial blend of ABS contains thermoplastic polyurethane elastomer as the main blend component. The blend was introduced in 1990 by Dow Chemical Co., under the trade name Prevail . These blends characteristically exhibit low modulus (340 to 1000 MPa) and high impact strength at low temperamres, e.g. notched Izod values of 370 to 1500 J/m at -29°C. The TPU component of the blend imparts high toughness and also allows paintabihty without a primer. ABS component imparts heat resistance (for paint ovens) and good tensile strength in the blend. The blend is projected to find applications in the automotive markets, particularly as paintable, soft bumper fascias. Typical properties of commercial ABS/TPU blends are shown in Table 15.6. 

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. 

Non-chain extended polyurethane elastomers and their tensile properties... 

The tensile properties at room temperature and at intervals up to 240°C of the two series of polyurethane elastomers studied are given in Table 3.27 (block ratio 1 2 1 and BDO chain-extended) and in Table 3.28 (block ratio 1 3 2 and BDO/CHDM chain-extended). In both these series the percentage of free isocyanate calculated to be present in the elastomer on first casting is varied from 0 to 50% in 5% steps. 

Low temperature Low temperature environments affect the properties of polyurethane elastomers, but no degradation occurs and the effect is completely reversible. An increase in Young s modulus occurs below with a corresponding increase in hardness, tensile strength, tear strength and torsional stiffness, and a decrease in resilience. 

Generally, the polyol will have either a polyester or a polyether backbone. The polyurethane elastomers based on an ester backbone generally have better abrasion resistance, tensile strength, tear strength, and oil resistance. On the other hand, polyurethane elastomers based on a polyether backbone generally possess better low-temperature properties, resiliency, and resistance to hydrolysis. 

In this chapter, we reviewed some of the recent developments in modeling and predicting the mechanical properties of polyurethane elastomers as a function of their formulation. Based on the knowledge of the formulation and processing history, one can roughly predict the degree of the microphase separation between the hard and soft phases. That, in turn, enables the construction of constitutive micromechanical models for the calculation of elastic, viscoelastic, and nonlinear tensile and compressive properties. 

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