Shear flow compounding

Fig. 7.24 Breakup of a droplet of 1 mm diameter in simple shear flow of Newtonian fluids with viscosity ratio of 0.14, just above the critical capillary number. [Reprinted by permission from H. E.H. Meijer and J. M. H. Janssen, Mixing of Immiscible Fluids, in Mixing and Compounding of Polymers, I. Manas-Zloczower and Z. Tadmor, Eds., Hanser, Munich (1994).]...

The effect of additives and modifiers on product properties will not be discussed here because it is beyond the scope of this textbook. We must emphasize, however, that the ultimate objective of compounding additives and modifiers in polymer matrices is to obtain specific multicomponent and multiphase structures and morphologies needed to obtain certain desired product properties. We will only discuss their effects on the compounded systems rheology and, mainly, the shear flow viscosity, and their effects on compounding equipment and processes. 

The addition of a very small amount of styrene-isoprene-styrene (SIS) triblock compatibilizer, introduced as a compounded pellet with PS 685, suppresses the shear flow-induced coalescence appreciably, as seen by comparing Fig. 11.35 with Fig. 11.37. On the other hand, there is no effect of this very small amount of SIS on the dispersion rate. 

Thus, within the shear rate range in question, the following material behavior can be derived from the apparent flow curve, as confirmed for aU clay-ceramic extrusion compounds investigated to date Bingham body with shear flow (Qs) and core flow (Qk), plus an additional slippage fraction (Qg) in the flow proflle. 

Lee et al. [2007] studied the rheological behavior of poly(ethylene-co-vinyl acetate) (EVAc 40 wt% VAc) and its CPNC with < 10 wt% C30B the tests were conducted under steady-state and small oscillatory shear flow. The samples were prepared by melt compounding at 110 C for 25 min, which resulted in a high degree of dispersion. The flow behavior was quite regular, well described by the Carreau-Yasuda equation [Carreau, 1968,1972 Yasuda, 1979] ... 

Continuous Mixers FCM Buss Kneader Uniform mix, geared for higher productivity, requires free-flowing preblend. Low heat/shear history compounds possible. 

There was little discussion of this perspective during the next 25 years, and only in the 1960s was there renewed attention. In 1962, Zakharenko et al. [Zl] in Moscow reported shear flow measurements of rubber-carbon black compounds. In 1972, Vinogradov et al. [V8], also in Moscow, reported similar results for other rubber-carbon black compounds and indicated the occurence of yield values. At the same time similar behavior was reported for talc-polypropylene compounds by Chapman and Lee [C8] of Shell and for titanium dioxide-polyethylene compounds by Minagawa and White [M29]. 

From about 1980, there have been extensive investigations of the shear viscosity of rubber-carbon black compounds and related filled polymer melts. Yield values in polystyrene-carbon black compounds in shear flow were found by Lobe and vhiite [L15] in 1979 and by Tanaka and White [Tl] in 1980 for polystyrene with calcium carbonate and titanium dioxide as well as carbon black. From 1982, White and coworkers found yield values in compounds containing butadiene-styrene copolymer [Ml, M37, S12, S18, T7, W29], polyiso-prene [M33, M37, S12, S18], polychloroprene [S18], and ethylene-propylene terpolymer [OlO, S18]. Typical shear viscosity-shear stress data for rubber-carbon black compounds are shown in Figs. 5(a) and (b). White et al. [S12, S18, W28] fit these data with both Eq. (56) and die expression... 

Lobe and White [L15] studied stress relaxation following imposed strains in polystyrene-carbon black compounds and found that the stresses did not decay to zero, but to a finite value of stress roughly equal to the yield value of Eqs. (56) and (57). Montes et al. [M37] have found similar effects in rubber-carbon black compounds. This is shown in Fig. 7. Montes et al. found similar effects in stress relaxation following shear flow. 

FIGURE 8 Comparison of rheological model of Eqs. (70)-(72) with experiment on rubber-carbon black compound, (a) Steady shear viscosity, (b) Transient, (c) Shear-rest-shear flow behavior. 

The instruments of Turner and Moore [T12] and Montes et al. [M38] are pressurized by an external reservoir. This allows shear flow measurements to be carried out at constant shear rates and conditions for the development of slippage in individual compounds to be determined. 

Montes S, White JL, Nakajima N (1988) Rheological behavior of rubber Carbon Black compounds in various shear flow histories. J. Non-Newtonian Fluid Mech 28 183-212... 

However, even without external mechaifical stress imposed by shear flow, reorganization of the agglomerates occurs. In dairy science, the separation of milk into curd and whey and the conversion of amorphous starch into a crystalline state with expulsion of physically bonded water are known examples. Silica, aluminum oxide and latex represent industrially produced compounds that show reorgaiuzation. The reason for reorgaiuzation of silica is the same polymerization reaction which is responsible for gelation. After the gel network has formed, there are still uncondensed silanol groups. A zoom of two solid silica particles is shown in Fig. 2. 

Figure 11.3 gives the phase morphology of as-blended 30/70 PMMA/PS specimens that were subjected to shear flow. In the as-blended 30/70 PMMA/PS specimens, the minor component PMMA (the gray/white areas) forms the discrete phase and the major component PS (the dark areas) forms the continuous phase, where both large and small drops are present (see Figure 11.3a). The small-size PMMA drops must have resulted from the breakup of the large dispersed PMMA phase during the compounding in a twin-screw extruder. Note that the state of dispersion in as-blended 30/70 PMMA/PS specimen is reversed from that in as-blended 70/30 PMMA/PS specimen (compare Figure 11.3a with Figure 11.1a). When an as-blended 30/70 PMMA/PS specimen was...

It is not uncommon for the presence of bulk nanofiller particles in a conventional polymer system to lead to a reduction in melt spiimability or even provoke the formation of aggregates, owing to instabilities, such as localisation and phase segregation. However, in this work, because the layered-silicate had already been intercalated and partially exfoliated via compounding prior to melt-spinning, the resultant particles would exhibit an improved aspect ratio and anisotropy. This, in effect, should lead to an enhanced mesoscopic reorientation ability of the silicate platelets in the shear flow direction, which, in turn, could promote the realignment of the polymer chain (Giannelis et al., 1999). 

Step 2. If no seasonal or special cause can be found in compound, scorch conditions may be to look at mold closure rates. From Equation 16C.7 above and by examination of Figure 16C.4, we can see that the heat build-up during shear flow is proportional to the square of the shear rate. Check to see if final mold closure rates have been accelerated, which would cause a major increase in heat build-up during flow. If this is the case, return to the older, slower closure rates if possible. 

It is obvious that in rubber processing there will be slip, because the surface of processing equipment in contact with gum rubbers and compounds is usually shiny. This is in contrast to plastic processing equipment, in which the surfaces become coated with a thin layer of the degraded material. With plastics this fact indicates the velocity of the melt at the metal interface is zero, i.e., laminar shear flow. A study of slip is multi-faceted, because there are many types of slip, a steady slip, slip with a lubricated layer, slip-stick, slip involving a fracture at the interface or ruhhing like a dynamic friction measurement. 

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