Band 7, Heft 3

Banbury mixers are widely used in polymer processing. The flow field in the chamber of an internal mixer is difficult to analyze due to the complex geometry and transient character of the flow. In this work, a fluìd dynamics analysis package – FIDAP, using the finite element method was employed to simulate the flow patterns in a BB-2 type Banbury mixer. A 3D flow analysis was carried out for the whole mixing chamber. The problem of time dependent flow boundaries was solved by selecting a number of sequential geometries to represent a complete mixing cycle. The flow field was characterized in terms of velocity profiles, pressure contours, shear stresses generated and a parameter λ quantifying the elongational flow components. The last two parameters are the most important ones in analyzing mixing efficiency. The influence of processing conditions (rpm, speed ratio, and initial relative position of the two rotors) on the flow characteristics was analyzed. The flow results from the 3D model were also compared with results previously obtained from a 2D model.

An improved flow model for simulating flow in batch and continuous twin rotor mixers and extruders is presented. The model adopts an Eulerian point of view from the stationary barrel surface. It incorporates Couette flow analysis, which accounts for the curvature effects in the non-intermeshing region, and correction factors for treatment of flow near the bounding sidewalls of a batch mixer. A method for tracking the motion of fluid particles is presented.

To enhance the material properties of polypropylene/rubber blends the rubber phase is crosslinked. In this study an organosilane crosslinking system is used which creates stable Si-O-Si bonds between the rubber chains and produces a polyfunctional network structure with a high crosslinking density. This organosilane system allows in principle to prepare blends which are statically or dynamically crosslinked. Results concerning the morphology of blends prepared under different compounding parameters are presented. Particular attention was directed to the interrelationship of rheological properties of the blend components and the morphological structure of the created blends. In the closing chapter of this paper the material properties of blends, including EPM or EPDM in a silane-crosslinked mode are compared with those of only physically mixed PP/EPDM-blends.

Blends of poly(ethylene terephthalate) (PET) and polypropylene (PP) with several liquid crystalline polymers (LCPs) were successfully generated using a novel blending method, referred to as the dual-extruder mixing method. In this method, the matrix and LCP polymers were plasticated separately in two single-screw extruders. The melt streams were then joined together and blended in a Kenics static mixer containing 18 mixing elements. Strands (rods) and sheets of the blends were extruded using the above method. First, blends of thermoplastics with LCPs were successfully made using this method in which there is little or no overlap of the nominal processing temperatures. Second, the morphology of extruded strands from the dual-extruder mixing method was seen to consist of LCP fibrillar structures which were truly continuous running the length of the extrudate and further devoid of any skin-core type structure. Third, LCP fibril formation was seen to occur even when the LCP concentration was only 4 wt.-% in the blend. By contrast blends from the single-screw extrusion exhibited a distinct skin-core fibril-droplet type of structure.

This paper describes a device which was developed to measure the degree of instability during blown film processing. Two types of data analysis are developed which can be used to analyze the instability data and the relative merits of each are discussed. The paper also describes how the device was used to measure the effect of adding two stabilizers (2 MI LDPE and 0.25 MI LDPE) at different concentrations to a 1 MI LLDPE during blown film processing. The process stability improved with higher concentrations of either LDPE with the 0.25 MI LDPE having the stronger effect. These results are consistent with the belief that long chain branching in polyethylenes plays a major role in stabilizing the blown film processing of polyethylenes.

Traditionally, modeling of the film blowing process consisted mainly of balances of the forces on a single layer of thin film and a mathematically defined freeze-line. In this publication we develop an alternative approach for the theoretical analysis for film blowing of crystalline polymer melts by treating the film as two layers, crystallized and amorphous. Therefore, a two-phase constitutive model is presented for the process simulation of the tubular film blowing of crystalline polymer melts. An upper convected Maxwell equation of state with a single relaxation time and the Oldroyd derivative is applied to the amorphous or liquid-like phase while a perfect plastic-elastic model with yield is used to describe the deformations of the crystallized phase. The model interpretations and predictions are obtained and found to be in qualitative agreement with published results. For polyethylene the model appears to help explain the interactions of the processing conditions and fluid-mechanics in the film blowing process of crystalline polymers.

A thick sheet analysis of deformation is used to simulate the process of plug-assist forming. The formulation is based on the total Lagrangian description and the equations are solved by the finite element method. The results are in agreement with experimental data available in the literature for sheet thickness distribution and plug force required as a function of deflection. The analysis provides the complete stress and strain fields and may be used to locate regions where fracture may initiate.

Optimization of the thickness distribution in extrusion blow molded parts is crucial to minimize cycle times and resin usage. Closed-loop parison programming can be cycle-to-cycle or in-cycle. Corrective action occurs between cycles for the case of cycle-to-cycle control. However, this control scheme does not correct for high frequency disturbances that enter the system, such as periodic flowrate changes, noise from the gap position transducer, uneven melting and compression of the polymer and uneven inflation. Corrective action occurs during the cycle for in-cycle control. This allows for regulation of the high frequency disturbances. Deterministic in-cycle model identification was performed by obtaining the impulse responses of the bottle thickness distribution to pulses in the die gap input during the extrusion cycle. The response was found to be time variant along the individual cycle. Stochastic identification was also performed in order to ascertain a noise model for the system. A first order ARMA noise model was found to adequately describe the process. Control simulations were performed to determine the feasibility of in-cycle control. A minimum variance controller with a constraint for minimizing of manipulated variable movement was found to adequately regulate both a deterministic and a stochastic disturbance input during the extrusion cycle.

An experimental study of iPP injection molding is carried out. Pressure distribution during both filling and post filling stages, mass entering the mold during each stage of the process, morphology and crystallinity distribution of the final object are analysed. Interesting features are evidenced and discussed in relation to gate geometry and filling flow rate. Evidence of flow crystallization inside the cavity is also reported.

An experimental study on the use of regrind in rotomolded polyethylene parts is described. We investigated the introduction of 15, 30, and 100% reground polyethylene into the rotational molding operations. The number of regrind cycles varied from one to four. Rotational molding was done in air and nitrogen inner gas, respectively. The mechanical properties of the parts were investigated, including low temperature dart impact test, tensile test, and Izod impact test. Generally it was found that nitrogen atmospheres substantially improved the impact properties of rotomolded parts made with virgin polyethylene. With parts containing 15% and 30% regrind, the impact properties of boxes made in nitrogen are substantially higher than those molded in air. Also, adjustment of oven time to slight undercure for 15% regrind containing parts rotomolded in air gave good impact properties.