Capillary extrusion

To evaluate extensively the processability of PVC/EPR and to compare it with that of PVC having roughly the same molecular weight, we carried out a set of measurements by a capillary extrusion rheometer and Brabender plastograph. Other runs were carried out by an industrial size extruder. 

In the vast literature on melt flow instabilities in capillary extrusion, the most misleading information is the report that the material of construction of the capillary die has no effect on the flow curve of linear polyethylene, or, in particular, on the instability region [32, 68] - see a quotation by Tordella cited in Sect. 3. Experiments using screw-threaded dies have further led people to believe that the slip (at the flow discontinuity transition) with linear polyethylene therefore appears not to result from adhesive breakdown at the polymer-die in-... 

As discussed in more detail below, recent experiments convincingly showed that the flow oscillation in capillary extrusion of LPE is interfacial in nature due to a reversible coil-stretch transition at the melt/die wall boundary. Pressure oscillation phenomenon has also been reported in extrusion of other polymer melts. In particular, there are well-defined oscillations in controlled-rate capillary flow of PB that were found to arise from the same interfacial molecular instability [62]. 

Liang J. Z., Ness J. N. (1998). The melt die-swell behaviour during capillary extrusion of LDPE/PP blends. Polym. Test, 17,179-189... 

Shih, C.K. (1979) Capillary extrusion and mold flow characteristics of an incompatible blend of two elastomers, in Science and Technology of Polymer Processh (eds Nam P. Suh and N.-H. Sung), MIT Press, Cambridge, Massachusetts, 528. 

A number of experimental methods have been applied to measure the melt viscosity of polymers (53-55,65), but capillary extrusion techniques probably are generally preferred. Rotational methods are also used, and some permit the measurement of normal stress effects resulting from elasticity as well as of viscosity. Slit rheometers can also be used to measure normal stress (66). Oscillatory shear measurements are useful for measuring the elasticity of poljmier melts (57,58). Controlled stress methods have also been applied (59). Squeeze film flow has also been proposed as a geometry suitable for processibility testing of polymer melts... 

Capillary extrusion has the possibility to produce a large spectrum of molecular weights, on the transversal section of the extrudate because of the complex profile of the shear stresses. 

Quintana et al. (95) prepared two types of PECT copolyesters with 25 and 30% of CHDM units and containing small amounts of pen-taerithrytol branching agent. Then, they studied the rheological features and flow-induced crystallization of PECT copolymers by capillary extrusion experiments at 180 °C. They observed that copolymers with the lower content in CHDM were able to crystallize in these conditions whereas linear or branched PECT with 30 %-mole of CHDM units did not crystallize. 

In addition to viscosity measurements, the capillary rheometer (Gottfert Rheograph 2002) was used to investigate the effect of shear rate on the morphology of the blends prepared earlier in the corotating twin-screw extruder (A see Tables 2 and 3) with an output of 5.4kg/h. In these capillary extrusion experiments, the shear rate was varied from 150 to 8000see by adjusting the speed of the piston. The strand was allowed to fall freely into the water bath without extension. 

The morphologies of the injection-molded samples are consistent with the capillary extrusion results reported in Part I. 

Nakajima et al. [101] studied the viscoelastic behavior of butadiene-acrylonitrile copol)rmer filled with carbon black. Capillary extrusion measurements with an Instron and dynamic oscillatory measurements with a Rheovibron suggested the occurrence of strain hardening in filled elastomer due to tensile extension causing structural changes in the carbon black filled elastomer. It is possible fliat the structure built by the carbon black in the elastomer increasingly jams against... 

Fig. 3.12 Temperature profiles for power-law fluid during capillary extrusion at different capillary wall temperatures [88].

Important here are the results in inert atmospheres that show no degradation for rubber mastication above 140°C (see Fig. 3.5 [8]), and for polystyrene extrusion above 180°C [18]. The approximate agreement between the minimum degradation temperature calculated and observed for several polymers is thus likely fortuitous. In mastication, the polymer mass is nearly homogeneous. This is not the case on capillary extrusion. In addition, convection plays a different role in capillary extrusion and in mastication experiments. 

Sdireiber, H. P., A study of time dqtoideace of viscosity in capillary extrusion of polyethylene, J. AppL Potym. Sd., 4, 38-44 (1960). 

Schreiber, H. P. and Rudin, A., Some elastic effects in the capillary extrusion of polyethylenes,/. Appl. Polym. ScL, 3, 122-124 (1960). 

Capillary rheometer - Because the material experiences both pressure and shear in the capillary extrusion, the compaction by extrusion is expected to be more effective than... 

Stress relaxation, capillary extrusion, dynamic and tensile stress-strain were measured [1]. The results of stress relaxation are presented in Figure 7.1a for the gum rubbers and Figure 7.1b for the compounds. 

The capillary extrusion data [5] are shown in Figures 7.2a and 7.2b. As seen in these figures, the relative differences between the gum rubbers remain in the filled compounds. Moreover, the magnitudes of the differences are not diminished in the presence of carbon black. 

Figure 7.5 (a) Viscosities of gum rubber B obtained by capillary extrusion, dynamic oscillatory and tensile stress-strain measurements. 

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