Polyurethane elastomers moduli

Some viscoelasticity results have been reported for bimodal PDMS [120], using a Rheovibron (an instrument for measuring the dynamic tensile moduli of polymers). Also, measurements have been made on permanent set for PDMS networks in compressive cyclic deformations [121]. There appeared to be less permanent set or "creep" in the case of the bimodal elastomers. This is consistent in a general way with some early results for polyurethane elastomers [122], Specifically, cyclic elongation measurements on unimodal and bimodal networks indicated that the bimodal ones survived many more cycles before the occurrence of fatigue failure. The number of cycles to failure was found to be approximately an order of magnitude higher for the bimodal networks, at the same modulus at 10% deformation [5] ... 

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. 

Studies have been made of the elastic (time-independent) properties of single-phase polyurethane elastomers, including those prepared from a diisocyanate, a triol, and a diol, such as dihydroxy-terminated poly (propylene oxide) (1,2), and also from dihydroxy-terminated polymers and a triisocyanate (3,4,5). In this paper, equilibrium stress-strain data for three polyurethane elastomers, carefully prepared and studied some years ago (6), are presented along with their shear moduli. For two of these elastomers, primarily, consideration is given to the contributions to the modulus of elastically active chains and topological interactions between such chains. Toward this end, the concentration of active chains, vc, is calculated from the sol fraction and the initial formulation which consisted of a diisocyanate, a triol, a dihydroxy-terminated polyether, and a small amount of monohydroxy polyether. As all active junctions are trifunctional, their concentration always... 

The higher MW or Polymers 4 and 5 (vs. 1, 2, and 3) is also reflected in the higher 300% modulus values of the former polymers. The 300% modulus value has been shown to increase with thermoplastic polyurethane elastomer number average molecular weight, Mn.17... 

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. 

One particular form of thermoplastic polyurethane elastomer is the elastic fiber known as Spandex. Several commercial materials of this type have been introduced, which include Lycra (Du Pont), Dorlastan (Bayer) Spanzelle (Courtaulds), and Vyrene (U.S. Rubber). Spandex fibers have higher modulus, tensile strength, and resistance to oxidation, and are able to produce finer deniers than natural rubber. They have enabled lighter-weight garments to be produced. Staple fiber blends of Spandex fiber with non-elastic fibers have also been introduced. 

Later advances include short, glass-fiber reinforced, high-modulus (flexural modulus greater than 300,000 psi, i.e., 2070 MPa) polyurethane elastomers produced by the reinforced RIM process. These reinforced high-modulus polyurethane elastomers are considered for automotive door panels, trunk lids, and fender applications. 

DMA is an analysis technique used to determine the dynamic properties of the elastomers [13, 14]. Dynamic properties of the elastomeric materials are important because they influence the performance of certain parts such as wheels and tyres. This method determines the storage modulus G (elastic behaviour), loss modulus G (energy dissipation), tan 8, loss compliance ]" and glass transition temperature (Tg) values. The Tg of the soft segment can determine the low temperature behaviour of polyurethane elastomers. This is not only influenced by the nature of the soft... 

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. 

MAXIMUM TEMPERATURE ( 7) TO WHICH STORAGE MODULUS (log F) VALUES OF POLYURETHANE ELASTOMER WILL REMAIN CONSTANT ... 

The way in which the chemical structure of the various chain extenders of Table 3.16 influences the thermal stability of polyurethane elastomers based on the molecule Capa 225/CHDI/chain extender in the molar ratio 1 2 1, respectively, is given in the following figures and tables. For example, Fig. 3.7 shows the dynamic mechanical thermal properties of a series of the polyurethane elastomers in which the variable is the chain extender. The temperature at which the value of the storage modulus (log E ) changes significantly is considered to indicate the limit of thermal stability of the polyurethane elastomers. 

It will thus be obvious that both covalent and polar crosslinks contribute to the crosslink network and hence modulus of polyurethane elastomers. The dependence of this modulus or temperature can be divided into contributions from a covalently linked network conforming to the statistical theory of rubber elasticity, and contributions from secondary crosslinks which are assumed to have a temperature dependence governed by the Arrhenius law in which the modulus of elasticity is represented by the equation ... 

In urethanes the presence of these strong physical-chemical bonds helps to explain some of the properties of solid polyurethane elastomers. High modulus in tension and compression can be achieved without extending the elastomeric macromolecular structure with reinforcing filler. This means that elastic materials can be obtained with a level of hardness not possible with conventional rubbers. 

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. 13.6. Tensile strength versus Young s modulus for a range of materials. (Courtesy of P. Wright A. P. C. Camming, Solid Polyurethane Elastomers, Maclaren and Sons, London, 1969, and Baier.)...

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. 

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