Fillers shear viscosity

Rheological properties of filled polymers can be characterised by the same parameters as any fluid medium, including shear viscosity and its interdependence with applied shear stress and shear rate elongational viscosity under conditions of uniaxial extension and real and imaginary components of a complex dynamic modulus which depend on applied frequency [1]. The presence of fillers in viscoelastic polymers is generally considered to reduce melt elasticity and hence influence dependent phenomena such as die swell [2]. 

Yield stress values can depend strongly on filler concentration, the size and shape of the particles and the nature of the polymer medium. However, in filled polymer melts yield stress is generally considered to be independent of temperature and polymer molecular mass [1]. The method of determining yield stress from flow curves, for example from dynamic characterization undertaken at low frequency, or extrapolation of shear viscosity measurements to zero shear rate, may lead to differences in the magnitude of yield stress determined [35]. 

Fig. 8 The log shear viscosity versus log shear rate plot of 34NBRo-MMT at various filler loadings...

Figure 9.27. Shear viscosity vs. filler concentration. [Adapted, by permission, from, Cheng J, Bigio D I, Briber R M, Antec 97. Conference proceedings, Toronto, April 1997, 162-7.]...

Figure 13.15 shows the effects of and Af on the zero-shear viscosity r o(relative) for dispersions of infinitely rigid anisotropic filler particles. It is seen that, at any given and Af, T o increases more with fibers than it does with platelets. 

Similarly, Table 17.8 shows data for neat HDPE and a composite material, containing rice hulls and Biodac (29% each) as principal fillers. Because the power-law index is lower for the composite material (0.34 compared to 0.44 for the neat HDPE), the effect of filler on viscosity is increased with decrease of the shear rate. At the shear rate of 500 s fillers increase the viscosity by only 35% at the shear rate of 1 s fillers increase the viscosity by 150%, when melts of a composite deck board and the neat HDPE are compared. 

In general, capillary (steady shear) and parallel plate (dynamic shear) viscosity data of cellulose-filled composite materials (HDPE- or PP-based) at high filler... 

Table 17.24 shows data for a series of regrinds all derived from the same composition, that is, HDPE (MFI 0.5) filled with rice hulls and Biodac , 29% of each filler. The storage and loss moduli were plotted against the frequency in double logarithmic coordinates, and the respective slopes are shown in Table 17.24. In all cases the storage modulus and the loss modulus increased with frequency. The same data were analyzed earlier, in Table 17.12, in terms of their zero-shear viscosity.

Figure 10.3. Effect of several variables, viz., pressure, temperature, resin molecular weight, as well as filler and additive concentration on a flow curve - shear viscosity vs.

Some modifications of the melt flow behavior of thermoplastics that can be observed depending on filler concentration are a yield-like behavior (i.e., in these cases, there is no flow until a finite value of the stress is reached), a reduction in die swell, a decrease of the shear rate value where nonlinear flow takes place, and wall slip or nearwall slip flow behavior [14, 27, 46]. Other reported effects of flllers on the rheology of molten polymers are an increase of both the shear thinning behavior and the zero-shear-rate viscosity with the filler loading and a decrease in the dependence of the filler on viscosity near the glass transition temperature [18, 47-49]. 

Finally, shear viscosity is strongly affected by the clay in the blends, especially at high PEN contents. A lubricating effect rather than a filler effect reveals the possibility that the clay is not well dispersed in the polymer blend, and migration of particles in the flow to the wall region can explain the observed reduction in shear viscosity. When MMT clay is mixed with crystallizable polymers such as polyesters, some processing problems arise because the crystallization process is modifled by nucleation effects induced by the nanoparticles. Moreover, these particles also influence the kinetics of transesteriflcation between PET and PEN, besides other factors such as the reaction time and extruder processing temperature. In Reference 72, a quaternary alkyl ammonium compound (Cl8) and MAH were used to modify the surface properties of the clay... 

Clays compatibilized and evenly dispersed in the rubber matrix tend to build networks at low concentration. Rheological measurements allow us to observe the occurrence of the filler networking phenomenon at low filler content (even 4 phr) in rubber matrices such as IR, ENR, SBR, EPR. At zero shear, the viscosity of RCN is thus higher than the one of the neat elastomer. However, OC reduces the steady shear viscosity of RCN, with pronounced shear-thinning behaviour, increasing with the clay content, a higher extent of extrudate, a... 

For filled Newtonian polymer melts of even higher filler volume fractions, a general empirical expression can be used to calculate shear viscosity of the composite (25). 

Some more investigations, such as variation of the concentration of filler and change of the particle size, were made to get further details about the influence of chalk in PVC. It could be shown that the stabilizing effect increases with the content of calcium carbonate in the mixture. But there is a maximum at about 30 phr filler because of the increase in shear viscosity in the used processing machines which leads to an increasing thermomechanical treatment of the material. Another limiting factor is the change of the mechanical behavior from... 

Despite the importance of extensional viscosity studies in filled-poljrmer processing it is unfortunate that the effects of various factors like size distribution of the fillers, nature of the surface, flocculation, etc. as in the case of shear viscosity (section 4.2), has not been studied at all. Only the effect of the shape of the filler on the extensional viscosity has been brought out by Nicodemo et al. [177]. A lot more research in this area is thus warranted. The main reason for the dearth of information in this area is probably due to the absence of a cheap and simple apparatus for the measurement of extensional viscosity. Most of the workers have had to design their own apparatus for determining extensional viscosities depending on the system to be measured and the facilities available. 

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