Loss tangent, polyurethanes

Commonly used materials for cable insulation are poly(vinyl chloride) (PVC) compounds, polyamides, polyethylenes, polypropylenes, polyurethanes, and fluoropolymers. PVC compounds possess high dielectric and mechanical strength, flexibiUty, and resistance to flame, water, and abrasion. Polyethylene and polypropylene are used for high speed appHcations that require a low dielectric constant and low loss tangent. At low temperatures, these materials are stiff but bendable without breaking. They are also resistant to moisture, chemical attack, heat, and abrasion. Table 14 gives the mechanical and electrical properties of materials used for cable insulation. 

A more selective approach consists in trying to influence the kinetics of formation of at least one network in this case, the two networks are formed more or less simultaneously, and the resulting morphology and properties can be expected to vary to some extent without changing the overall composition. The same system as previously studied, PUR/PAc, has been utilized in order to prepare a series of in situ simultaneous IPNs (SIM IPNs), by acting essentially on two synthesis parameters the temperature of the reaction medium and the amount of the polyurethane catalyst. Note that the term simultaneous refers to the onset of the reactions and not necessarily to the process. The kinetics of the two reactions are followed by Fourier transform infra-red (FTIR) spectroscopy as described earlier (7,8). In this contribution, the dynamic mechanical properties, especially the loss tangent behavior, have been examined with the aim to correlate the preceding synthesis parameters to the shape and temperature of the transitions of the IPNs. 

Figure 4. The large broad "maximum" in the loss tangent isotherm of 73-19 polyurethane adhesive at 40 C is attributed to hard segment development. This peak is not observed above 50 C because the hard segment glass transition temperature has been exceeded.

Polyurethane PU (cross-linked) 283 8.86 101.6 238 [151] ar,s of the softening dispersion from rubber to glass from dynamic mechanical data taken over the frequency range of 45-6000 Hz and the T-range of -16-39 °C. The loss tangent exhibits a broad maximum resembling the behavior of PIB. 

Fig. 3.4. Temperature dependence of the loss tangent (tan S) of different diisocyanate-based polyurethanes. PU type Capa 225/diisocyanate/1A BDO molar ratio 1 2 6 1 (from Barikanf 1986).

Figure 3.4 compares the loss tangents (tan 5) of different diisocyanate-based polyurethanes. For each PU three relaxation peaks are observed present in the low temperature range and designated as the a, jS and y peaks. The overall effects of these diisocyanate structures are significant, with the major phase changes occurring at about —15 to —33°C (Table 3.11). 

Even without an analytical expression to describe the shape of H, it is clear that increasing steepness of H in the transition zone as portrayed in Fig. 12-11 will be accompanied by a compression of the transition from rubberlike to glasslike consistency into a narrower region of logarithmic time scale. Plots of both transient and dynamic moduli and compliances, as exemplified in Chapter 2, rise and fall with steeper slopes. Perhaps the most sensitive index of the sharpness of the transition is the loss tangent, which is plotted in Fig. 12-12 for several prototypes the polyurethane rubber, poly( -octyl methacrylate), poly(vinyl acetate), and Hevea rubber. Here the frequency scale has been arbitrarily selected to make the maxima coincide. The sharpness in the loss maximum correlates with the slope of H in the transition zone. The shape emphasizes the failure of the modified Rouse theory to provide a detailed description of the properties in the transition zone, since it predicts tan 5 = 1 independent of frequency in this region. The drop in tan 5 at high... 

FIG. 12-12. Loss tangent in the transition zone plotted against frequency for (1) polyurethane rubber, (2) poly( -octyl methacrylate), (3) polyfvinyl acetate), (4) Hevea rubber. 

Figure 2.17. Typical DMA curves of polyurethane elastomers tensile storage modulus, E, (left) and loss tangent, tan 6 = E"/E, (right). Measurements were performed at a frequency of 1 s The PEUU curves are typical for weakly phase-segregated elastomer, while PEU curves are typical for strongly phase-separated elastomer with percolated hard phase [27]...

Results of the loss tangent, tan 6, at low frequencies, obtained for a SEES copolymer and carbon nanotube (CNT)/polyurethane (PUR) nanocomposites, are analysed. The hindering effect of nanostructures (i.e. ordered domains or nanoparticles) on the motion of polymer chains is revealed by the occurrence of a mechanical relaxation at low frequencies. In the case of SEES copolymer, the effect of domains orientation and extender oil on a characteristic time associated with the relaxation is investigated. For CNT/PUR nanocomposites, the variation of the frequency at which the relaxation takes place, with CNT concentration and temperature, reveals that the interactions between the nanotubes and the polymer chains are improved with temperature... 

Fig. 17. Dependence of the dielectric permeability e and of the tangent of dielectric loss angle tg8 on the apparent density of rigid oligomeric foams. Epoxide - PE-8 (1) and PE-9 (2) polyurethane - PPU-204 (3), PPU-305 A (4) andPPU-307 (5) )...

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