Polyurethane elastomers, structure

Thermoplastic Polyurethane Elastomer Structure—Thermal Response Relations... 

The possession of these unique properties can, in part, be explained by the way the polyurethane elastomer structure differs from that of conventional elastomers. An elastomer is usually described as a material characterized by the property of high elasticity. This means it has the ability to stretch to a great extent under load and to recover almost completely when the load is released. This property of elasticity is a result of a particular molecular structure, the general features of which are exhibited by all elastomers, although there are many variations in detail. In simple terms this structure consists of long linear flexible molecular chains which are joined together at intervals by crosslinks which may be in the form of... 

It is possible to make some general comments on the relative hydrolytic stability of different polyurethane elastomer structures. The three principal linear polyol series used in synthesizing urethane elastomers can, for hydrolytic stability, be ranked in the following order ... 

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

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]

Table 1. Dependence of the kinetic parameters of the reaction, the structural parameters of the network and the physico-mechanical properties of polyurethane elastomers on the molar fraction of OH groups in the monofunctional reagent21...

The largest segment of the CASE family of polyurethanes are elastomers. Cast polyurethane elastomers reached a new dimension when high pressure impingement mixing led to reaction injection molding (RIM). This technology is used widely in the automotive industry, and reinforced versions (RRIM) and structural molded parts (SRIM) have been added in more recent years. 

Soft blocks are composed of linear, dihydroxy poly ethers or polyesters with molecular weights between 600 and 3000. In a typical polymerization of a thermoplastic polyurethane elastomer, the macroglycol is end capped with the full amount of aromatic diisocyanate required in the final composition. Subsequently, the end-capped prepolymer and excess diisocyanate mixture reacts further with the required stoichiometric amount of monomeric diol to complete the reaction. The diol links the prepolymer segments together while excess diol and diisocyanate form short hard-block sements, leading to the (AB)n structure illustrated in Figure 1. Block lengths in (AB)n polymers are frequently much shorter than those in anionically synthesized ABA block copolymers. 

Polyurethanes.—The photo-oxidation of polyurethane elastomers based on aromatic isocyanate units has been studied and found to involve two principal reactions. The first is a photo-Fries rearrangement with light wavelengths <330 nm and the second is a photoinduced hydroperoxidation.Several hydroperoxide structures were identified [e.g., (5)—(8)] and their importance examined." " Type (5) hydroperoxides are the most photolabile and are... 

Thermoplastic elastomers, e.g., polyurethane elastomers, have an entirely different structure that is heterogenic. They have an elastomeric matrix and phase-separated hard blocks, which act as embedded physical crosslinking sites. The hard blocks can be softened at elevated temperatures to obtain a single-phase melt that is easily processed. Upon cooling, the two-phase nature is recovered and the material becomes a solid again. 

Investigations of the Structure-Property Relationships for RIM Polyurethane Elastomers... 

Skarja GA, Woodhouse KA (2000) Structure-property relationships of degradable polyurethane elastomers containing an amino acid-based chain extender. J Appl Polym Sci 75 (12) 1522-1534... 

The Structure of the Hard Segments in MDI/ diol/PTMA Polyurethane Elastomers... 

SEGMENTED POLYURETHANE ELASTOMERS SCHEME OF THE CHAIN STRUCTURE ... 

Figure 5. Scheme of primary chain structure of segmented polyurethane elastomers and chain segment organization. 

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. 

Nuclear Magnetic Resonance Spectroscopy. Like IR spectroscopy, NMR spectroscopy requires little sample preparation, and provides extremely detailed information on the composition of many resins. The only limitation is that the sample must be soluble in a deuterated solvent (e.g., deuterated chloroform, tetrahydro-furan, dimethylformamide). Commercial pulse Fourier transform NMR spectrometers with superconducting magnets (field strength 4-14 Tesla) allow routine measurement of high-resolution H- and C-NMR spectra. Two-dimensional NMR techniques and other multipulse techniques (e.g., distortionless enhancement of polarization transfer, DEPT) can also be used [10.16]. These methods are employed to analyze complicated structures. C-NMR spectroscopy is particularly suitable for the qualitative analysis of individual resins in binders, quantiative evaluations are more readily obtained by H-NMR spectroscopy. Comprehensive information on NMR measurements and the assignment of the resonance lines are given in the literature, e.g., for branched polyesters [10.17], alkyd resins [10.18], polyacrylates [10.19], polyurethane elastomers [10.20], fatty acids [10.21], cycloaliphatic diisocyanates [10.22], and epoxy resins [10.23]. 

In the absence of a blowing agent and with conditions favoring the formation of linear chains, thermoplastic polyurethane elastomers can be formed. Materials with different properties can be obtained by altering the ratio of hard to soft blocks, and it is also observed that, in many cases, the hard segments can crystallize. This can give rise to phase-separated structures, shown schematieally in Figure 2.6, which behave as thermoplastic elastomers (see also Section 15.6). 

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