Polyurethane elastomers present

Polyurethane elastomers presently exist in three basic groups classified on the basis of their processing characteristics and identified as liquids, millable rubber and thermoplastic elastomers. Their basic chemical building blocks are identical and processing individuality is derived from molecular weight and terminal end-group functionality. 

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... 

Glass Temperature. The glass temperatures for a substantial number of polyurethane elastomers, similar to those discussed herein, were found (1 ) to increase linearly with the concentration of urethane moieties, [U]. For the present elastomers prepared using LHT-240 and TIPA, [U] should be 1.10 and 1.15 moles/kg, respectively. Their glass temperatures should be about —57°C, indicated by the previous data. 

Atomic force microscopy and attenuated total reflection infrared spectroscopy were used to study the changes occurring in the micromorphology of a single strut of flexible polyurethane foam. A mathematical model of the deformation and orientation in the rubbery phase, but which takes account of the harder domains, is presented which may be successfully used to predict the shapes of the stress-strain curves for solid polyurethane elastomers with different hard phase contents. It may also be used for low density polyethylene at different temperatures. Yield and rubber crosslink density are given as explanations of departure from ideal elastic behaviour. 17 refs. 

Castable Polyurethane Elastomers explains the production process of polyurethane components from both the theoretical and practical points of view. It describes the underlying concepts for the raw material supplier recommendations and explains how to achieve application-specific properties in polyurethane. The book explains the production of prepolymers with special focus on health and safety issues. It presents the different types of methods available on both the micro and macro levels and explains the rationale behind choosing the system needed to create a cost-effective, application-specific product. 

Published information on urethane polymerization detail largely concerns thermoset urethane elastomers systems.4 13 In particular, the work of Macosko et. al. is called to attention. The present paper supplements this literature with information on the full course of linear thermoplastic urethane elastomer formation conducted under random melt polymerization conditions in a slightly modified Brabender PlastiCorder reactor. Viscosity and temperature variations with time were continuously recorded and the effects of several relevant polymerization variables - temperature, composition, catalyst, stabilizer, macroglycol acid number, shortstop - are reported. The paper will also be seen to provide additional insight into the nature and behavior of thermoplastic polyurethane elastomers. 

In the present study we utilized Plasticorder torque values (polymer viscosity) on a relative basis, principally to follow the course of thermoplastic polyurethane elastomer polymerization, and the effect of several relevant variables on this polymerization. These torque data, expressed in meter-grams, served our purposes well, and we have not attempted to translate them to absolute rheological units. Such translation, which is a rather complex problem, has been addressed by others using a different rotor type and other polymers in the Plasticorder.14 That study concluded that the shear rate, y, of the Brabender Plasticorder with a somewhat different rotor configuration (roller blade) is in the range of 23 - 228 sec 1 over the rotor speed range of 30 - 200 rpm. 

The Brabender Plasticorder is also a useful device for studying the effect of shear and heat on fully formed polymeric materials. And this is another way that we used it in the present study. The technique employed was to bring the Plasticorder mixing chamber to temperature, then add granules of thermoplastic polyurethane elastomer (at 25°C) to the chamber and record the viscosity-time-temperature pattern that developed. ... 

Pure MDI, having two -NCO groups/mol, is commercialised mainly as 4,4" isomers, but it is possible to use 2,4 and 2,2 isomers. The main applications of pure MDI (especially the 4,4" isomer) are polyurethane elastomers, microcellular elastomers and some flexible foams. The structures of pure MDI isomers are presented in Figure 2.2. 

The present book aims to advance this understanding, by means of a systematic study of the effects of varying chemical composition of model polyurethane elastomers on (a) their physical structure at the important nanometre length-scale, and (b) the resulting mechanical properties of interest. 

The present book is organized into 6 chapters. Chapter 1 describes general aspects on the chemistry of polyurethane elastomers their origins and development, the principles and synthesis mechanisms, as well as general considerations on the main chemical parameters that define such materials, i. e. diisocyanate, macrodiol and chain extender. Selected considerations regarding the reactivity of diisocyanates, the hydrogen bonding and its dynamic and quantum aspects are also discussed in this chapter. 

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. 

Due to its advanced technology and low cost the rubber industry often prefers the use of sulphur-vulcanized polyurethane elastomers even though some of their technical properties, e.g. resistance to thermal degradation, are inferior to the peroxide- and diisocyanate-cured grades which often have short processing times (i.e. they are scorchy) and whose cure may be adversely affected by the presence of moisture in the unvulcanized rubber mix water is present in rubber fillers, e.g. carbon blacks usually contain about 0-5-1% and some non-black fillers such as silicas and clays 2-10%. Also to maximize scorch time it is common practice to quench-cool the rubber after internal mixing by immersion in cold-water tanks or by cold-water spray application to the surface of the hot-milled sheet. 

Polyurethane rubbers have now been used as seal materials for some time on account of their unique ability to combine resistance to swelling in oil with high strengths and high stiffnesses. Their ability, in some classes, to be processed as thermoplastics, is also considered useful as manufacture of the seal can then be automated and hence quality is more reproducible. A limitation in their use has been that they depend upon physical types of crosslinking for their elastic and strength properties and when certain specific temperatures are reached these crosslinks rapidly weaken and the polyurethane elastomer melts and fails. At present most rubber seals are made from vulcanized covalently crosslinked rubbers where crosslinks are based on sulphur or carbon, and these do not melt at elevated temperatures, but instead decompose. 

Special methods are necessary for the production of polyurethane dispersions because of the thermodynamically unstable nature of these two-phase systems. A simple application of the emulsion polymerization techniques for isocyanate polyaddition reactions is not possible, due to the reactivity of the NCO group with water. The extreme water sensitivity of all polyurethane preparation procedures which calls for the complete absence of water is obviously a major problem to be overcome in their preparation. It is surprising, given the basic hydrolytic degradability of polyurethane elastomers, to find that polyurethane latex has good long-term stability when it is a two-phase system. This is in contrast to the situation that applies when it is present in a one-phase system. 

The above technique can be applied to estimate all the groups present in a polyurethane elastomer. 

An Additional Rapid Infrared Method for the Quantitative Analysis of NCO Present in a Polyurethane Elastomer... 

Figure 6.26. Comparative data from DMA (at/= 1 Hz),TSC q= l°C/min) and DSC measurements q = 10°C/min) for a polyurethane based on 2,4-toluene diisocyanate (NCO/OH = 1.2).The thermally stimulated currents spectrum is the only one to present the current signatures of both the soft-segment and hard-segment glass transitions. [Adapted from plots presented by Hsu and coworkers (1999), with permission of Elsevier. Tliis article was pubhshed in Thermochimica Acta, Volume 333, by J.-M. Hsu, D.-L. Yang, and S.-K. Huang, TSC/RMA study on the depolarization transitions of TDI-based polyurethane elastomers with the variation in NCO/OH content, pp. 73-86, Elsevier (1999).]...

At the present time, only castor oil based polyurethane elastomers are commercial, the others are not. None of the polyesters are commercial, although they are particularly promising for developing tropical countries due to the ability to grow the oil seeds locally, and produce the dibasic acid from the oil itself. Some of the current literature on the subject includes work from India, the Peoples Republic of China, and... 

The preparation of polyurethane aqueous dispersions can be accomplished in several ways. The procedure claimed to yield at present the highest quality products involves the reaction of an isocyanate-terminated prepolymer in a water-miscible solvent, e.g., acetone, with a diamine carboxylate or sulfonate. The resultant polyurethane ionomer solution is mixed with water which forms the dispersion and further chain-extends the polymer. Finally, the organic solvent is removed by distillation and can be recycled. As in the case of the thermoplastic polyurethane elastomer adhesives, dispersions can also be prepared with different degrees of crystallinity in the polyurethanes depending on the nature of the prepolymer. 

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