Thermoplastic elastomers viscosity

An entirely new concept was iatroduced iato mbber technology with the idea of "castable" elastomers, ie, the use of Hquid, low molecular-weight polymers that could be linked together (chain-extended) and cross-linked iato mbbery networks. This was an appealing idea because it avoided the use of heavy machinery to masticate and mix a high viscosity mbber prior to mol ding and vulcanization. In this development three types of polymers have played a dominant role, ie, polyurethanes, polysulftdes, and thermoplastic elastomers. 

FIGURE 38.11 Log of complex viscosity (17 ) of the acrylonitrile butadiene mbber/waste NBR/styrene-co-acrylonitrile (NBR/w-NBR/SAN)-based thermoplastic elastomer (TPE) compositions as a function of w-NBR content at 100 rad/s. (Reprinted from Anandhan, S., De, P.P., De, S.K., Swayajith, S., and Bhowmick, A.K., Kautsch. Gummi Kunst., 11, 1, 2004. With permission.)... 

This comprises a thermoplastic elastomer based on a polyolefin and a rubber. The polyolefin is a PP homo- or copolymer having a specified weight-average molec.wt. and elongational viscosity (measured at a temperature of 170C, a rate of elongation of 0.03 1/s and at a time of 10 s). 

An integrally monlded composite article comprised of (I) a nonfoam layer formed from a thermoplastic elastomer powder composition (A) and (II) a foam layer formed from a foamable composition comprised of (i) (B) a thermoplastic synthetic resin powder, and (C) a heat decomposable foaming agent and (D) a liquid coating agent, wherein the thermoplastic elastomer powder (A) is comprised of a composition of an ethylene-alpha-olefm copolymer mbber and a polyolefin resin or thermoplastic elastomer powder comprised of a partially crosslinked composition of an ethylene-alpha-olefm copolymer mbber and a polyolefin resin, the thermoplastic elastomer powder having a complex dynamic viscosity at 250 deg C and a freqnency of 1 radian/sec of not more than 1.5x1,000,000 poise and having a Newtonian viscosity index n, calculated by a specific formula. 

The dynamic melt viscosity measurements of select star blocks and a similar triblock were carried out on a rheometric mechanical spectrometer, RMS. Circular molded samples of 2 cm diameter and -1.5 mm thickness were subjected to forced sinusoidal oscillations. Dynamic viscosities were recorded in the frequency range of 0.01-100 rad/s at 180 °C. Figure 10 shows the complex viscosities of two select star blocks and a similar linear triblock. The plots showed characteristic behavior of thermoplastic elastomers, i.e., absence of Newtonian behavior even in the low frequency region. The complex viscosity of the star block... 

Although styrene-diene diblock copolymers are used in some applications, particularly in the area of viscosity index improvement (VII) additives for motor oil, styrenic block copolymers are most often used as thermoplastic elastomers. In these applications the styrene blocks phase separate, crosslinking the rubber blocks in a thermally reversible fashion. The simplest structure capable of exhibiting this behavior is a linear styrene-diene-styrene triblock. The most obvious way to produce such a molecule is by sequential polymeriza-... 

Figure 21.4 Corrected melt viscosity as a function of shear stress and temperature for the three block copolymers studied. Reproduced with permission from Legge, Holden and Schroeder, Thermoplastic Elastomers A Comprehensive Review, Hanser Verlag, Munich, 1987...

As emphasized above, in contrast to common thermoplastics, thermoplastic elastomers contain a very soft phase (with Tg around —50°C), which is in a liquid state at room temperature and is characterized by a viscosity closer to that of low-molecular-weight liquids rather than a solid amorphous polymer. In this respect it seems useful to recall that the molecular weight of the PTMG and PEG used is 1000, i.e. one is dealing with typical oligomer systems. For this reason it looks reasonable to accept that such a liquid will be characterized by a negligibly small microhardness, in the equation ... 

Summarizing, it can be concluded that a relatively sharp (within 2-4% of deformation) drop in H is observed for copolymers of PBT but in comparison with homo-PBT this transition occurs at much higher deformations (between 25 and 30%). This difference as well as the following increase and decrease of H are related to the structural peculiarities of thermoplastic elastomers - the presence of a soft amorphous phase which first deforms and the existence of a physical network. The very low H values obtained for PEE are related to the fact that the PBT crystallites are floating in an amorphous matrix characterized by a low viscosity. 

Duvdevani(40) have been directed at modification of ionomer properties by employing polar additives to specifically interact or plasticize the ionic interactions. This plasticization process is necessary to achieve the processability of thermoplastic elastomers based on S-EPDM. Crystalline polar plasticizers such as zinc stearate can markedly affect ionic associations in S-EPDM. For example, low levels of metal stearate can enhance the melt flow of S-EPDM at elevated temperatures and yet improve the tensile properties of this ionomer at ambient temperatures. Above its crystalline melting point, ca. 120°C, zinc stearate is effective at solvating the ionic groups, thus lowering the melt viscosity of the ionomer. At ambient temperatures the crystalline additive acts as a reinforcing filler. 

Figure 12 Apparent elongational viscosity versus shear rate for Santoprene thermoplastic elastomers at 204°C...

Thermoplastic elastomers (TPEs) with blocks of polydiene rubber are subject to degradation at the carbon-carbon double-bond sites and require proper stabilization. In SIS block copolymers, chain scission is the predominant degradation mechanism. In an SIS block copolymer, the addition of a more effective stabilizer, AO-3, alone or blended with a secondary antioxidant, PS-1, can provide a significantly superior performance over AO-1 alone or with PS-1. Resistance to discoloration after static oven aging at 80°C (176°F) is improved dramatically (Fig. 5). Viscosity stabilization (melt flow index stability) (Fig. 6) is also improved drastically using AO-3/PS-1. 

The viscosity of the curable molding compounds (before curing is completed) is usually very low (at 70-120 °C) - around 1-50 Pa s. Thermoplastic melts are between 200 Pa s and 800 Pa s (see Table 10, Sect. Processing of thermoplastic elastomers ). In contrast to injection molding of thermoplastics, duroplastics form flashes and skins the subsequent removal of which is expensive. 

The flow behavior of block copolymers differs from that of the parent homopolymers. Let us first examine the temperature dependence of the viscosity rj for the thermoplastic elastomers. Below the glass transition temperature of polystyrene (about 110 C) the triblock material has a viscosity intermediate between that of the parent homopolymers, as shown in Figure 4.22. This is normal and expected. However, at a temperature where flow is well developed in the polystyrene, 140 C, an inversion occurs, the block copolymer assuming the higher viscosity (Holden et a/., 1969b). The reason for this inversion lies in the difficulty of pulling styrene blocks out of their normal phase and into and through the polybutadiene phase, and vice versa. Motions of this type are required for viscous flow, and... 

The material properties needed for simple mold filling simulation are listed in Table 11.3. The material properties essential for advanced filling, packing, and cooling simulations are listed in Tables 11.4 through 11.6. The requirements are essentially the same for thermoplastics and thermoplastic elastomers, while in the case of reactive materials, such as thermosets, the main differences are the reactive polymer viscosity in place of melt viscosity data and reaction kinetics data. 

Unfortunately, for block copolymer thermoplastic elastomers, any trials to accommodate the viscosity changes by carbon blacks into theoretical considerations, or to fit the equations cited onto the rheological behavior of black-filled systems have not been conducted up to now. Of course, such trials will be relatively complex and time-consuming work because thermoplastic elastomer shows intricate properties due to its unique microstructure. 

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