The feasibility of shaping a nucleated polymer/gas solution represents a significant advancement for microcellular plastics process technology. Through proper design of the foaming die, nucleated solution flows can be shaped to arbitary dimensions while maintaining the functional independence of cell nucleation, cell growth and shaping. To maintain funcational independence, stringent pressure and temperature design specifications, which supersede those of conventional foam processing, must be met by the foaming die design. As a means of aiding the design process, a model is developed for predicting pressure losses and flow rates of nucleated polymer/gas solutions. A comparison of the model predictions and the actual foaming die design performance shows good agreement for limited data. These relatively simple models capture the major physics of the complicated two-phase flow field and provide a sound base from which scale-up of the foaming die concept can be achieved. The nucleated polymer/gas solution flow models predict highly nonlinear volumetric flow rates contrasting constant flow rates predicted for the neat polymer flow. In addition, a convenient method for classifying nucleated polymer/gas solution flow is presented based on a dimensionless ratio of the characteristic flow rate to the characteristic gas diffusion rate.

Microcellular sheet extrusion system process design models for shaping and cell growth control

High-pressure rheological behavior of polymer melts containing dissolved carbon dioxide (CO 2 ) at concentrations up to 6 wt % were investigated using a high-pressure extrusion slit die rheometer. In particular, the steady shear viscosity of poly(methyl methacrylate), polypropylene, low-density polyethylene, and poly(vinylidene fluoride) with dissolved CO 2 were measured for shear rates ranging from 1 to 500 s -1 and under pressure conditions up to 30 MPa. The viscosity of all samples revealed a reduction in the presence of CO 2 with its extent dependent on CO 2 concentration, pressure, and the polymer used. Two types of viscoelastic scaling models were developed to predict the effects of both CO 2 concentration and pressure on the viscosity of the polymer melts. The first approach utilized a set of equations analogous to the Williams-Landel-Ferry equation for melts between the glass-transition temperature (T g ) and T g + 100 °C, whereas the second approach used equations of the Arrhenius form for melts more than 100 °C above T g . The combination of these traditional viscoelastic scaling models with predictions for T g depression by a diluent (Chow model) were used to estimate the observed effects of dissolved CO 2 on polymer melt rheology. In this approach, the only parameters involved are physical properties of the pure polymer melt that are either available in the existing literature or can be measured under atmospheric conditions in the absence of CO 2 . The ability of the proposed scaling models to accurately predict the viscosity of polymer melts with dissolved high-pressure CO 2 were examined for each of the polymer systems.

High-pressure rheology and viscoelastic scaling predictions of polymer melts containing liquid and supercritical carbon dioxide

The density and morphology of closed-cell high-density foams were investigated with four different molecular weights of high-density polyethylene (HDPE). The characterization of polyethylene via rheological methods was used to determine its influence on foam density and morphology. We found that foaming grade decreased with increasing molecular weight and increased with blowing agent content. The average cell size was also a strong function of molecular weight and blowing agent content. Increasing both the molecular weight and amount of blowing agent decreased the cell size. Cell size also increased for our lowest molecular weight HDPE but decreased for the others. Cell density also increased with increasing HDPE molecular weight. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2111–2119, 2003

High‐density polyethylene foams. I. Polymer and foam characterization

Foamed polymers are future materials with their wide application spheres. They can be used to enhance appearance as insulating structures or to save material costs. This study addresses all of these and concerns a new technology to produce foamed tubes and films. An entire blown film extrusion line was built up to fulfil the final target, biaxially oriented foam film with closed cell, microcellular foam structure. Extrusion conditions, which in general means temperature, pressure and flow control of viscoelastic material, is discussed and examined experimentally to define the dominating characteristics. Screw and die design together with foam stabilisation is taken under stringent examination, resulting a three layer blown film die focused for foamed extrudates. The effects of chemical foam nucleating agents and physical and chemical foaming are examined during the study concluding in formation of cell embryos by chemical foaming agents and expanding them into low density foam with physically compressed nitrogen gas. The effect of processing parameters in foam structure is examined and adapted according to the polypropylene. The main characteristics of polypropylene were defined by rheological measurements to promote foaming studies in extrusion. Sharp melting point and linear structure of polypropylene caused troubles in keeping the foam structure closed during expansion, but was also partly resolved with new bimodal type polypropylene owing to high melt strength properties. Some applications, leading to new commercialised innovations in foam films, will also be discussed in greater detail.

A new technology to manufacture polypropylene foam sheet and biaxially oriented foam film

Rheology of Thermoplastic Foam Extrusion Process

Several rheological aspects of thermoplastic foam extrusion are described by a phenomenological model of the flow in an extrusion die. The macroscopic effects of the phase change, from a homogeneous polymer melt that contains a blowing agent, to a foam, are described in terms of two dimensionless parameters. α is defined as the ratio of the pressure gradient in the melt phase to the average pressure gradient in the foam phase. Θ represents the ratio of the pressure drop in the melt phase to the pressure drop in the foam phase. The position at which the phase change initiates, the exit contribution to the Bagley ends pressure correction, and the true wall shear stress in the melt phase are related to α and Θ. The quality of the foam produced is discussed in terms of the extrusion conditions and their effect on the foaming position in the die. It is demonstrated that the ends pressure correction cannot be neglected on the basis of a long die alone. Asymptotic conditions for which the phase change has negligible effect on the melt flow rate are predicted. α is calculated from the data of Han and Villamizar, who measured the pressure distribution in the die and observed the phase change directly. The prediction of α from theory is complicated by the lack of suitable constitutive relations for the foam phase. Since Θ only requires knowledge of the pressure at which the phase change initiates, it is relatively easy to evaluate.

A. M. Kraynik

Papers

Rheological aspects of thermoplastic foam extrusion