Volume 13, Issue 1

Modeling solids conveyance in a polymer extruder is difficult because it is a highly complex process. The solids are subjected to pressure and temperature gradients from the hopper through a heated section, where it is melted into a liquid over a finite distance. The polymer material possesses shear strength, which causes an anisotropic stress distribution to exist between the down channel pressure and the normal stresses on the flights, barrel and core. A new formulation incorporating anisotropic stress states into an isotropic model is presented. Assumptions regarding the direction of the driving force applied by the screw have been examined in addition to coupling its direction with the orientation of the barrel friction force. These assumptions allow the new model to provide increased nonlinearity and flexibility to accurately predict extruder output over wide variations with discharge pressure. A new parameter, the skew angle, is introduced in the proposed model. Predictions of extruder output for varying discharge pressures and barrel temperatures show excellent agreement with experimental data.

In the paper, a characterization of autothermal and conventional extrusion processes of low density polyethylene (LDPE) in the typical single screw extruder equipped with a classical screw was presented. It was shown that autothermal (adiabatic) extrusion process is characterized by better effectivity factors than a conventional extrusion process carried out in the same basic extrusion conditions. In both processes the mass flow rate, total extruder power, total energy consumption per unit mass, extruder energy efficiency, extrusion velocity, temperature distribution along the plasticating system and extruder head, were compared. The attempt to present shear stress and flow rate distribution during laminar and isothermal flow of the non-Newtonian fluid through the circular channel (extrusion die) was undertaken. Appropriate dependences of experimental factors on the rotational speed and angular frequency of the screw were made. Appropriate conclusions were drawn.

The chemical modification of poly(ethylene-co-vinyl acetate) with a primary aliphatic n-alcohol in a modular co-rotating twin screw extruder was investigated. The purpose was to build a theoretical model, able to predict the extent of conversion along the screw axis. First, the influence of operating conditions (throughput, screw speed, barrel temperature and initial reactant concentrations) on the EVA copolymer conversion was experimentally examined. Then, the extent of conversion was computed by coupling flow simulation and reaction kinetics. Theoretical and experimental results were found in good agreement, what allows the prediction of the reaction extent as a function of the processing conditions.

An oxygen acceptive catalyser Fe/FeO mixture was added to the high density polyethylene (HDPE) at different amounts and the stabilisation effects of this additive on the structure of HDPE was investigated. It was shown that the mechanical behaviour and the temperature stability of HDPE was significantly improved with the application of the additive. The experimental results were found to be in accordance with analytical approaches.

The pumping characteristics of several types of screw elements for an intermeshing co-rotating twin screw extruder (ICoTSE) were experimentally studied using molten polystyrene (PS) as a typical non-Newtonian fluid in “LABOTEX30α”. Here, the throughput was regarded as the remainder of drag flow minus pressure flow, based on the previous theoretical and experimental studies with regard to either single or twin-screw extruders. Therefore, the relationship between the throughput and the pressure gradient for a given screw geometry was approximately expressed as a linear dimensionless relation containing two so-called pumping-characteristic parameters which depend on the element geometry and degree of departure of the used fluid from the Newtonian fluid. The pumping-characteristic parameters for two right-hand flighted screw elements (RF) with leads of 0.938D and 0.703D, a neutral kneading disk (NK) and a blister ring (BR) were experimentally determined, and the RF with a lead of 0.938D was taken as an example to observe the effect of the clearance between it and the barrel wall on its pumping-characteristic. The results show that the linear relationship between the dimensionless throughput and the dimensionless pressure gradient for each type of the screw elements used here is a good approximation, and the clearance in the range used here greatly affects the pumping characteristic of the RF. Moreover, the degree of filling and the pressure profile in two kinds of screw configurations used for water-injection foaming devolatilization, which consist of elements with different diameters but with the same dimensionless geometry as those used in this work, were predicted and discussed by using the determined pumping-characteristic parameters under the conditions of given throughput of molten PS and screw speed.

Processability and rheological properties of the blends of poly(ethylene-Co-acrylic acid) (PEA) and epoxidised natural rubber (ENR) have been studied by a Monsanto Processibility Tester (MPT). The blends were found to be miscible during reactive processing due to chemical interaction via esterification reaction at higher proportion of PEA. The rheological properties were studied at three different temperatures (120°C, 130°C and 150°C), five different shear rates in the range from 122s −1 to 2452s −1 and at three different shear stresses (i.e., 400, 500 and 600 kPa). The melt viscosities of the blends at 120°C and 130°C show a positive deviation at lower shear rates and shear stress and at higher proportions of PEA and a negative deviation at higher shear rates and shear stress and lower proportion of PEA, where as at higher temperature (i.e. at 150°C) of melt processing the viscosities show positive deviation (at all shear rates for all the blends) with respect to that calculated by additivity rule. This variation in melt viscosities has been explained on the basis of chemical reaction via esterification leading to grafted structure i.e., PEA-g-ENR.

This paper presents the results of an experimental study of the unstable capillary flow of HDPE/PA 11 blends with special emphasis on the pressure oscillation region. The main result is the finding that addition of only 1 % by weight of the PA 11 component to HDPE completely suppresses the oscillation phenomenon normally found with the latter polymer when the measurement is carried out in a rheometer with a plane capillary entrance. In comparative experiments with a capillary rheometer with a conical (120°) entrance, the pressure oscillation effect disappeared at a PA 11 content of 5%. In compounds containing 1, 3 and 7 % PA 11, distinct pressure oscillations were recorded in the latter case. The stress and elongation at break of the extrudates change in a monotonic fashion with the composition. SEM photographs and DSC measurements show that the two components are basically immiscible.

The melt flow properties of LDPE/HDPE blends in capillary extrusion have been investigated. It is found that the End pressure losses (ΔP end ) increase as a form of non-linear function with increasing the wall shear stress (τ w ) and the ΔP end value of the sample is the minimum at the weight percentage content of HDPE (Φ HD ) being 50% when the τ w is fixed. The blend melt flow basically obeys the power law and the sensitivity of the melt viscosity (η w ) on the wall shear rate (γ w ) is weakened with increasing the ϕ HD . Synergistic effect is obvious for the blend melts under experimental conditions. When the τ w is 42.5KPa, the η w value achieves the maximum at the ϕ HD = 20%. In addition, the relationship between the η w and the test temperature approximately accords with the Arrhenius equation as the τ w is fixed.

The effect of temperature on the rheological response under elongation of two low density polyethylenes (LDPE 1 and LDPE 2), linear low density polyethylene (LLDPE) and their blends of different compositions was observed by using a constant strain rate elongational rheometer. In addition, blends were tested in the oscillatory shear test, and the data measured at 160, 190, and 230°C indicate the larger level of intermolecular interactions in a blend than in the pure polymers. Elongational viscosity measured at temperatures of 140, 150, 170 and 180°C revealed a much greater sensitivity for LLDPE (greater strain hardening with increasing temperature) to changing tenperature than is observed for LDPE. This observatuion, coupled with the effect of melt morphology on elongational rheology, has been used to explain the synergism in elongational viscosity exhibited by LDP-E-rich blends.

Lamellar microstructures can decrease gas and vapor permeability by increasing the diffusive path or the so-called “tortuosity” in plastic films. In this research, phlogopite mica flakes were used as oxygen barrier materials in films produced in a blown film extrusion line. Both high and low density polyethylenes as well as their blends were used as matrices with mica concentrations not exceeding 10-wt.-%. A decrease in oxygen permeability of both low density and high density polyethylene films was achieved with increasing volume fraction of mica. Film morphologies indicated strong flow-induced orientation of the flakes in overlapping, discontinuous layers that were reasonably well oriented parallel to the surface of the films. Melt shear viscosity and melt strength of the mica compounds and tensile properties of films were also examined and correlated with processability. Overall, it was found that, by contrast to the LDPE compounds, optimization of the barrier properties of the HDPE films was a compromise between the desired reduction in permeability and the loss in processability and film ductility. The experimentally determined elastic modulus values were found to be in relatively good agreement with predictions from flake reinforcement theories. A predictive model for the analysis of the permeability results, assuming that both mica flakes and the crystalline domains of the films were impermeable to oxygen, was able to describe the permeability results in spite of several simplifying assumptions.

Fibrous polymer materials formed by LDPE melt spraying with a gas flow during extrusion were exposed to thermally stimulated depolarization. The analysis of thermally stimulated current spectra and the sample charge state has shown that the fibers possess the spontaneous polarizing charge. IR-spectroscopy has proved the material production to be accompanied by vinylene, vinyl, transvinylene and other nonsaturated groups formation. The latters are, probably, behaving as neutral trapping centers during their interaction with charge carriers formed at melt dispersion by the compressed gas flow and its contact with metal parts of equipment. The kinetics of these processes can be monitored by regulating technological regimes of fibrous material formation. Intensified action on the polymer melt might rise the spontaneous polarizing charge whose density can reach 10 −10 to 10 −12 C/cm 2 . Charge of the materials under study is rather stable. It does not fall below 40% upon a 160-days storage in humid atmosphere. It appears thus that the technique of fibrous material production by polymer melt spraying is an electret method.

In many areas of plastics processing the temperature is a decisive process control parameter. Infrared sensors that detect the heat radiation from molten plastic have a response time in the range of milliseconds. Thus in many processes they offer the only possibility to measure time dependent temperatures with high accuracy.

A comparative study of crystallization and orientation development in melt spinning is presented for four polyolefins: high density polyethylene (PE), polypropylene (PP), polybutylene (PB-1), and poly-4-methylpentene-1 (P4MP). The purpose was to compare the behavior of two relatively unknown polyolefins PB-1 and P4MP with the much better known and characterized PE and PP. Melt spinning was carried out at different temperatures. Melt spun filaments were characterized by wide angle x-ray diffraction (WAXS), birefringence and differential scanning calorimetry (DSC) measurements. Polymorphism (multiple crystalline states) is noted in the filaments of polybutylene which exhibits aging from one form to another as well as in polypropylene. Careful attention is given to heat transfer during cooling. The orientation of flexible chains in the melt state is well known to be determined by the stress field. The crystalline orientation of the solidified crystalline filaments as a function of spinline stress is found by WAXS to be highest for the P4MP and lowest for the PE. Birefringences of the melt spun PE fiber are however higher than PP or PB-1 and P4MP which had the lowest values when comparisons are made at the same spinline stress. However if intrinsic birefringence was used, the data order the same as the WAXS results.

The focus of this study is to investigate the sintering mechanism of polyethylene. Experiments were conducted to observe the high temperature sintering behaviors of low density polyethylene (LDPE) and linear low density polyethylene (LLDPE). It was found that the sintering of polyethylene follows Frenke's model and is basically a Newtonian viscous flow. The sintering rate decreases as the viscosity of the resin increases. By increasing the temperature, one can shorten the time needed to complete the coalescence process. Experimental investigation will provide an improved understanding of the sintering of semi-crystalline polymers.

Data relating to the influence of several fillers on flow instabilities in capillary extrusion of two grades of high density polyethylene (HDPE) are presented. Special attention is paid to the pressure oscillation region of the flow curve. First we show that a certain level of flow stabilisation can be achieved with carbon fibres, while carbon black and CaCO 3 have a minor influence only on the oscillation effect when varying the filling between 5% and 10%. The temperature used was 200° C and the capillary entrance was plane (180°). We also show that a complete disappearance of pressure oscillations is obtained when using a different HDPE grade filled with 7% fumed silica or carbon black (also different grade). Compounds containing 7% talc did not stabilise the flow. The same was true when the filling was reduced to 3%. In the latter experiments a capillary rheometer with a conical entrance (120°) was used. The temperature was 160°C. Data obtained by frequency analysis of the fluctuating pressure transducer signal mounted on the rheometric equipment, operating in both cases at constant piston speed, are also presented.

Two injection moulding simulation codes (C-MOLD of AC Technology and VIp of Eindhoven University) are compared with experimental results regarding cavity pressure and product shrinkage. It turns out that the cavity pressure profiles are only moderately well predicted by both codes. The shrinkage predictions can be considered as a post processing step with temperature and pressure fields as an input (the so-called thermoelastic model). It is shown that the shrinkage predictions agree well with the measurements using the thermoelastic model with actual pressure profiles as an input. Direct simulation of product shrinkages resulted in an overprediction of shrinkage.