Band 12, Heft 1

An in-line rheometer previously described was evaluated for use in the upgrading of post-consumer, recycled HDPE. Feed resin of variable melt index was brought to a preset, uniform viscosity by peroxide induced crosslinking in a twin-screw extruder. Closed loop control was demonstrated using the rheometer as a viscosity sensor. Set-point tracking was achieved using PID-IMC, Dahlin and minimum variance (MV) controllers. The Dahlin controller showed the best load rejection.

Morphology development was investigated during blending of linear low density polyethylene (LLDPE) and polystyrene (PS) (the minor phase) in a tangential counter-rotating twin screw extruder, with and without the use of 5 wt.-% Kraton styrene-ethylene/butylene-styrene (SEBS) triblock copolymers as compatibilizing agents. Dispersive mixing in the Non-Intermeshing Twin Screw Extruder (NITSE) is improved with an increase of percent drag flow up to 50 %; the screws configuration includes reverse flight elements and/or cylinders. Experiments have indicated that the combined effect of the reverse flight elements and cylinders increases the reduction rate of droplets due to a longer time in the feed screw where the most significant changes of phase morphology occur. Screw stagger has been shown to have a minor effect on the phase scale of the final dispersion. The more significant reduction of the phase scale is achieved by increasing screw speed under constant flow conditions.

The in situ compatibilization of poly(butylene terephthalate) (PBT) with liquid crystalline polymer (LCP) via catalyzed transesterification reaction was examined through thermal, rheological, morphological and mechanical studies. The addition of catalyst, dibutyltindilaurate, was found to promote the transesterification between PBT and LCP. Maintenance of high vacuum environment was required to produce the high molecular weight copolymers in the reactor, which, acting at the interface, diminished the size of the dispersed phase (LCP) and enhanced interfacial adhesion that led to an early increase of the flexural strength. Flexural strength was reduced after long time reaction due to the chain scission. Transesterification confirmed by NMR spectroscopic analysis played a major role in the interchange process.

The influence of processing temperature on the morphological and mechanical properties of polycarbonate (PC) blended with a thermotropic liquid crystalline polymer (TLCP) has been studied. At a barrel temperature of 310°C, the TLCP of the blends exhibits the high orientation level and fiber alignment which correspond to maximum tensile strength. This indicates that the processing temperature might result in the change of the viscosity ratios of the dispersed and continuous phase and is one of the main processing parameters.

The effect of drawing and annealing on structure development and resulting properties of bacterial poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (P(3HB-co-3HV)) fibers have been investigated. Melt spun and subsequently drawn and annealed fibers were characterized by wide-angle and small-angle X-ray scattering techniques. WAXS studies revealed that these fibers possess bimodal chain orientation with two populations of crystals: in the first population the chain axes are oriented primarily along the fiber axis and in the second population these chain axes are oriented in the transverse direction to the fiber axis. In the latter crystals, a- and b-axes are randomly distributed around their c-axes. At the annealing temperature of 100° C, the relative population of the crystals oriented normal to the fiber axis was found to be large and their proportion decreases with the increase of annealing temperature. SAXS patterns were found to accompany the changes observed in the WAXS patterns. The tensile strength of these fibers improved by the increase of the proportion of the transversely oriented chains.

Air rings, used today in film blowing systems, have been developed down the years through trial and error experiments. They represent the limiting factor concerning the efficiency of film blowing lines with respect to throughput and take-off speed. Numerical simulation of the cooling process in film blowing provides a tool for the design and optimization of future cooling systems. Individual process and geometric parameters can be changed interactively with the effects on flow and heat transfer processes being analysed and applied to similar situations. The knowledge acquired regarding the physical interactions in the cooling of blown film makes a targeted optimization possible. The purpose of this paper is to present the foundamentals for calculating turbulent air flows in blown film extrusion and, additionally, a model for complete FE-simulation of the flow and heat transfer processes. Furthermore, the achieved simulation results will be discussed.

The relevance of enhancement of crystallisation kinetics by effect of shear flow and rheology during polymer solidification to the phenomena taking place during the injection moulding process has been shown by means of simulations performed on the basis of Lord and Williams [1, 2] model and its recent extensions [3 to 5]. A key point of the simulation was the solidification criterion based on a critical crystallization index; on the basis of simple calorimetric and rheological tests in the limit of zero shear rate, values of a few percent were given to the solidification crystallinity value. Many experimental features of pressure history both in the runner and in the cavity are recovered by model predictions, if both effects mentioned above are properly accounted for. However, predictions for gate sealing time, although improved, still have a sensitivity to gate thickness lower than that shown by the experiments. Similar conclusions, obviously, regard also the mass entering in the mould during holding. Another mechanism which sums up to shear stresses to accelerate thin gate solidification has still to be identified.

In view of optimizing the industrial compression molding process of thermoplastic composites, the rheological and microstructural behavior of a polypropylene/glass fiber composite is investigated in model squeeze-flow geometries. The overall stress / strain behavior of the material at various compression rates is recorded and the induced orientation of the fibers is investigated by means of a special electron microscopic characterization method. By contrast to pure polypropylene, it is shown that in the high speed range, the macroscopic flow process is controlled by both the viscous extension and the relative sliding of parallel fibrous layers, the latter becoming unstable when the flow undergoes a rapid transition from divergent to convergent. Under high pressure, voids are dissolved in the polymer melt. In the case of non-isothermal compression, the rheology of the composite is not significantly affected by the cooling of the surface layers. The multilayer plug-flow model based on the sliding mechanism of viscous layers is found to reproduce correctly the experimental stress/strain behavior.

The poly-(ethylene-b-propylene) was nucleated for a study of the structure modifications as well as the improvement of the mechanical properties, particularly the impact resistance. The nucleation was accomplished by two various red pigments: a linear trans-quinacridone dye (as ß-phase nucleator) and a dimethyl quinacridone resulting in the formation of a pure a-phase structure. Such procedure allowed the comparison of both the crystal structure and the morphology of the copolymer. For the ß-nucleated copolymer a maximum of the k-value (fraction of ß-phase) was found for a concentration of the nucleating agent of 5 × 10 −4 wt.-%. By means of the WAXS measurements (in the transmission and reflection mode) an important structure gradient was detected in the cross-section of the injection moulded samples. This structure gradient was also observed by optical microscopy, where an agreement between optical observation of the spherulites and the k-value distribution on the cross-section of the samples was observed. For both nucleating agents (linear transquinacridone and dimethyl quinacridone) a dependence of the mechanical properties of the copolymer (modulus of elasticity and brittleness by impact resistance) on the pigment concentration was observed. For the ß-phase nucleated copolymer the maximum of the k-value of the hexagonal crystal structure corresponds to the highest ductility by impact test.

Short term shearing at low degrees of supercooling with subsequent monitoring of the crystallization process of iso-tactic polypropylenes leads to highly oriented surface layers and fine grained layers that reach further into the core of the solidified samples. This process is highly dependent on the content of long polymer chains or nucleating agents. The present paper shows a first experimental evidence for these facts together with theoretical considerations. If long polymer chains are present, as it is the case in reactor grades of iPP, shear induced crystallization is more sensitive to shear treatment. As a consequence, the orientation of the surface layers is much higher in these materials. Additionally, the surface layers – both, highly oriented and fine grained – are more distinct and, therefore, allow an easier quantitative determination. If the materials contain nucleating agents, highly oriented layers occur in materials containing long polymer chains as well as in rheology controlled grades. A first theoretical description of the possible role of nucleating agents is given.