Polypropylene normal stress difference

Fig. 9. First normal stress differences of polypropylene of different molecular weight and distribution (30) see Table 4 for key. To convert N /m to...

Various investigations have considered the effects of titanate treatments on melt rheology of filled thermoplastics [17,41]. Figure 10, for example, shows that with polypropylene filled with 50% by weight of calcium carbonate, the inclusion of isopropyl triisostearoyl titanate dispersion aid decreases melt viscosity but increases first normal stress difference. This suggests that the shear flow of the polymer is promoted by the presence of titanate treatment, and is consistent with the view that these additives provide ineffective coupling between filler particles and polymer matrix [42]. 

Fig. 10. Viscosity and first normal stress difference vs. shear stress for polypropylene (at 200 °C) filled with calcium carbonate (50 wt%) with and without a titanate coupling agent (TTS ) (O, ) pure polypropylene (PP) (A,A) PP/CaC03=50 50 (by wt.) ( , ) PP/CaC03=50 50 with TTS (1 wt%). The open symbols were obtained from a cone and plate instrument and the closed symbols from a slit/capillary rheometer.

In addition to familiar phenomena such as these, new phenomena arise because of the synergism between the properties of the polymer and those of the particle filler. One such synergistic effect is enhanced shear thinning, which occurs because the shear rate experienced by the polymer confined between two particles can be much larger than the overall imposed shear rate (Khan and Prud homme 1987). Another general observation is that the filled melt is often effectively less elastic than the polymer alone, evidently because the filler enhances the viscosity more than it does the first normal stress difference Ni (Han 1981 Han et al. 1981). Thus, Fig, 6-38 shows that at fixed shear stress the first normal stress difference Nj for polypropylene decreases upon addition of CaC03 particles. ... 

Figure 6.38 Viscosity (open symbols) and first normal stress difference (closed symbols) as a function of shear stress for neat polypropylene melt (Q. )> the same melt filled with 50% by weight CaC03 particles of size 2.5 fx, (A, A), and the filled melt with a titanate coupling agent ( , ). (From Han et al. 1981, reprinted with permission from the Society of Plastics Engineers.)...

A systematic study of the basic rheological properties for a wide variety of polypropylene melts has been made by Minoshima et al. [89]. These authors measured shear viscosities at low shear rates in a Rheomatrics mechanical spectrophotometer and at high rates in an Instron capillary rheometer. The principal normal stress difference, Ni, was measured in the mechanical spectrophotometer with a cone and plate device. The elongational viscosity, of special importance to fiber formation, was measured in an apparatus built by Ide and White [90],... 

FIGURE 3.7 Principal normal stress difference as a function of shear rate for polypropylene at 180°C. (From Minoshima, W. White, J.L. Spruiell, J.E. Polym. Eng. Sci., 1980, 20, 1166. With permission.)... 

Van Oene. Levitt et al. [25] have observed that polypropylene droplets were elongated perpendicular to the flow direction in a polystyrene matrix that was highly elashc. The extent of particle stretching in the perpendicular direction of flow was found to be proportional to the normal stress differences between the phases. Particle contraction was observed upon cessation of the shearing action, which confirms the role of tire elasticity on deformation. 

Figure 7.5(a) Variation of primary normal stress difference with shear stress for polypropylene at 200°C filled with calcium carbonate. (Reprinted from Refs 77, 81 with kind permission from Academic Press Inc., New York, USA.)... 

Figure 24 The viscosity (dotted line) and first normal-stress difference coefficient (1st NSDC, solid line) as a function of shear rate for a polypropylene melt at 240° C, [15],...

The first difference of normal stresses (tr, t) may serve as an indirect index of the highly elastic properties of polymeric systems [199]. C. D. Han [200] related (ru with the residual pressure at outlet Pt)dt. Han, who observed its reduction in polypropylene filled with calcium carbonate [201], concluded that filling decreases the normal stresses. Note that addition of fibrous fillers, vice versa, somewhat increases Pexi, [180]. 

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