Foaming Extruder Research Articles

Morphology and Compressive Properties of Extruded Polyethylene Terephthalate Foam.

The cell structure and compressive properties of extruded polyethylene terephthalate (PET) foam with different densities were studied. The die of the PET foaming extruder is a special multi-hole breaker plate, which results in a honeycomb-shaped foam block. The SEM analysis showed that the aspect ratio and cell wall thickness of the strand border is greater than that of the strand body. The cells are elongated and stronger in the extruding direction, and the foam anisotropy of the structure and compressive properties decrease with increasing density. The compression results show typical stress-strain curves even though the extruded PET foam is composed of multiple foamed strands. The compression properties of PET foam vary in each of the three directions, with the best performing direction (i.e., extrusion direction) showing stretch-dominated structures, while the other two directions show bending-dominated structures. Foam mechanics models based on both rectangular and elongated Kelvin cell geometries were considered to predict the compressive properties of PET foams in terms of relative density, structure anisotropy, and the properties of the raw polymer. The results show that the modulus and strength anisotropy of PET foam can be reasonably predicted by the rectangular cell model, but more accurate predictions were obtained with an appropriately assumed elongated Kelvin model.

Dual mechanism of microenvironmental pH modulation and foam melt extrusion to enhance performance of HPMCAS based amorphous solid dispersion

Hydroxypropyl methylcellulose acetate succinate (HPMCAS) is an excellent polymeric carrier for melt extrusion amorphous solid dispersion. However, its pH-dependent solubility limits its application, especially for narrow absorption window drugs. The current study proposed a novel dual approach of foam extrusion and microenvironmental pH modulation to overcome this limitation. Sodium bicarbonate was used as a blowing agent and the remaining sodium carbonate acted as an internal pH modifier. Compared with conventional extrusion, foam extrusion dramatically lowered the extrudate physical strength (breaking force and hardness decreased by 20-fold; breaking energy and deformation energy decreased by >30-fold). Milling efficiency of foam extrudate was largely improved compared with that of conventional extrudates, demonstrating smaller particle size, larger specific surface area, and ability to pass through a smaller milling screen. The foam extrudate could generate a supersaturation concentration up to 8-fold higher than the solubility of the pure drug. It also significantly enhanced drug dissolution in a two-step biorelevant medium (p < 0.05). This novel approach improved both manufacturing processability and dissolution of HPMCAS-based solid dispersions.

Effect about Axial Vibration on Solution Viscosities of PS/Supercritical CO&lt;sub&gt;2&lt;/sub&gt;

In this paper, a technology of determining the melt viscosity of a polystyrene(PS)/supercritical CO2solution using a linear capillary tube die mounted on a foaming dynamic extruder is presented. CO2was injected into the dynamic extrusion barrel. Single-phase PS/CO2solutions were formed using a microcellular extrusion system and phase separation was prevented by maintaining a high pressure in the capillary tube die. Bymeasuring the pressure drop through the die, the viscosity of PS/CO2, solutions was determined. The dynamic extruder foaming apparatus was developed and using it to study the effects of CO2content and vibration on viscosity of PS/CO2solution. The blow agents and vibration can plastic the polymer viscosity. A theoretical model based on the generalized Cross-Carreau model was proposed to describe the shear-thinning behavior of PS/CO, solutions at various shear rates. The zeroshearviscosity was modeled using a generalized Arrhenius equation to accommodate the effects of temperature, pressure, CO2content, and vibration.

A novel strategy for the preparation of long chain branching polypropylene and the investigation on foamability and rheology

A novel strategy for introducing long chain branches (LCB) on commercial polypropylene (PP) was described using an extender agent, poly (hexamethylendiamine-guanidine hydrochloride) (PHGH), to couple with glycidyl methacrylate-grafted PP (PP-GMA). The presence of LCB was confirmed by GPC and melt strength measurements in two modified PP samples, MPP-1 and MPP-2. The foamability of LCB PPs and commercially linear PP (EPS) was investigated by foam extrudate. The results showed that the foamability of LCB PPs was significantly improved. To find out the difference in foamability of EPS and LCB PPs, some rheological properties were investigated. In the dynamic shear measurement, it was found that the elastic response of LCB PPs at low frequencies was distinctly enhanced in comparison with that of EPS, implying a presence of a long relaxation time mode that was not revealed in linear PP. The elongational rheology results showed that the strain hardening behaviors of LCB PPs were still intense even at higher strain rate of 1 s −1; moreover, MPP-2 could sustain larger elongational stress and deformation, which helps to improve the foamability. In step-shear stress relaxation measurements, the fast and slow relaxation processes were observed over the entire ranges of strains, indicating the presence of two different relaxation time modes. Also, the strain dependence of slow damping function became weaker because of the introduction of LCB on linear PP. In addition, it was also found that the rheological behaviors characterizing the long relaxation time mode were further enhanced with the increase of the concentration of PHGH in the melt grafting reactions.

Visualization of initial expansion behavior of butane‐blown low‐density polyethylene foam at extrusion die exit

AbstractA charge‐coupled device (CCD) camera was used to observe n‐butane‐blown low‐density polyethylene (LDPE) foams at the extrusion die exit. The effects of butane content, nucleating‐agent (talc) content, aging modifier (glycerol monostearate), die temperature, and die geometry on the initial expansion behavior of the foam extrudate were studied. A transparent section of the foam extrudate was found at certain die temperatures. The reasons for the existence of this transparent section and its effect on the processing conditions and material formulations are discussed. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers

Effect of die geometry on foaming behaviors of high‐melt‐strength polypropylene with CO2

AbstractThis article reports on a systematic study that was conducted to investigate the effects of die geometry (i.e., pressure and pressure drop rate) on the cell nucleation and growth behaviors of noncrosslinked high‐melt‐strength (HMS) polypropylene (PP) foams blown with supercritical CO2. The experimental results showed that the cellular morphologies of PP foams were sensitive to the die geometry. Furthermore, the initial expansion behavior of the foam extrudate at the die exit was recorded using a high‐speed CCD camera. This enabled us to achieve a more thorough understanding of the effect of die geometry on both the initial expansion behavior and the final cellular morphology of HMS PP foams. The effect of die temperature on cell morphology was also studied. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci 2008

Improvement of Cell Opening by Maintaining a High Temperature Difference in the Surface and Core of a Foam Extrudate

This article presents an extrusion-based, open-cell foaming process using thermoplastic polymers such as polystyrene (PS) and polycarbonate (PC) with supercritical CO2. Our previous studies have indicated that a cell opening can be promoted by inducing: (i) a nonhomogeneous melt structure by cross-linking, polymer blending, or filler compounding, (ii) cell-wall thinning by a high volume expansion ratio while maintaining soft cell walls, (iii) cell-wall thinning by a high cell-population density, and (iv) plasticization of the soft region of the cell walls with a secondary blowing agent. Until now, the foam extrudate temperature across the cross-section was maintained uniformly for the simplicity of the experiments. In this study, the significant temperature difference between the core and surface of the foam extrudate was induced by surface cooling method. This method increased the chance of cell opening by: (i) increasing the core temperature of the foam extrudate and thereby softening the cell walls, and (ii) decreasing the foam surface temperature to prevent gas loss and thereby increasing the internal gas pressure within the cells. The effects of CO2 content, surface quenching, die geometry, and temperature on foam morphologies were investigated. Low-density, microcellular, open-cell foams were successfully produced. The large intercellular pores were observed from micrographs for both PS and PC foams at optimum processing conditions.

Effects of CO2 and Talc Contents on Foaming Behavior of Recyclable High-melt-strength PP

This article presents an experimental study on the foaming behavior of recyclable high-melt-strength (HMS) branched polypropylene (PP) with CO2 as a blowing agent. The foamability of branched HMS PP has been evaluated using a tandem foaming extruder system. The effects of CO2 and nucleating agent contents on the final foam characteristics have been thoroughly investigated. Low density (i.e., 12-14-fold), fine-celled (i.e., 107-109 cells/cm3) PP foams were successfully produced using a small amount of talc (i.e., 0.8 wt%) and 5 wt% CO2.

Measurement and prediction of LDPE/CO2 solution viscosity

AbstractWhen CO2 is dissolved into a polymer, the viscosity of the polymer is drastically reduced. In this paper, the melt viscosities of low‐density polyethylene (LDPE)/supercritical CO2 solutions were measured with a capillary rheometer equipped at a foaming extruder, where CO2 was injected into a middle of its barrel and dissolved into the molten LDPE. The viscosity measurements were performed by varying the content of CO2 in the range of 0 to 5.0 wt% and temperature in the range of 150°C to 175°C, while monitoring the dissolved CO2 concentration on‐line by Near Infrared spectroscopy. Pressures in the capillary tube were maintained higher than an equilibrium saturation pressure so as to prevent foaming in the tube and to realize single‐phase polymer/CO2 solutions. By measuring the pressure drop and flow rate of polymer running through the tube, the melt viscosities were calculated. The experimental results indicated that the viscosity of LDPE/CO2 solution was reduced to 30% of the neat polymer by dissolving CO2 up to 5.0 wt% at temperature 150°C. A mathematical model was proposed to predict viscosity reduction owing to CO2 dissolution. The model was developed by combining the Cross‐Carreau model with Doolittle's equation in terms of the free volume concept. With the Sanchez‐Lacombe equation of state and the solubility data measured by a magnetic suspension balance, the free volume fractions of LDPE/CO2 solutions were calculated to accommodate the effects of temperature, pressure and CO2 content. The developed model can successfully predict the viscosity of LDPE/CO2 solutions from PVT data of the neat polymer and CO2 solubility data.

In-process density control of extruded foam PVC using wavelet packet analysis of ultrasound waves

Ultrasound waves (UT) propagating at a frequency of 2.25 MHz through the melt flow inside an extruder die were used to monitor the density of the foam extrudate. Acoustic properties of the polymer melt, manifested by the behavior of the ultrasound waves inside the die, were empirically correlated to the density of the extruded foam. Wavelet packet analysis was used to filter the ultrasound echo signals. A three layer multi-layer perceptron artificial neural network was used to correlate the response of the ultrasound sensor to the measured foam density. The density of the extruded foam was varied by varying the processing conditions over a wide practical range. The experimental work shows a strong correlation between the acoustic properties of the polymer melt inside the extrusion die and the resulting foam density. A strong correlation was obtained by using the filtered ultrasound waves using wavelet packet analysis.

Dissolution of Atmospheric Gases in Polystyrene Melt in Single and Twin-Screw Foaming Extruders

Dissolution and solubility characteristics of carbon dioxide in polystyrene melt were studied in both single screw and twin-screw foaming extruders. The effects of the main processing conditions on the gas dissolution behavior were also studied. Solubility data obtained from the twin-screw extrusion experiments showed better consistency compared with those obtained from the single screw experiments, indicating the importance of enhanced mixing in affecting the gas dissolution and solubility behavior during an extrusion foaming process. Gas dissolution inside a single screw foaming extruder was studied for three atmospheric gases (carbon dioxide, nitrogen and argon) in polystyrene melts. The results were compared with those obtained in a twin-screw extruder for the carbon dioxide-polystyrene system in an effort to elucidate mechanisms for the complex process of gas dissolution inside a foaming extruder.

Polymeric Foaming Simulation for Extrusion Processes

A numerical simulation for polymeric foaming extrusion processes was conducted. Combining classical nucleation rate and bubble growth models with a non-Newtonian fluid model of a flow, a simultaneous bubble nucleation and growth behavior in a flow field was simulated. Simulation results were compared with the experimental data obtained by visual observations at a foaming extruder, where a polypropylene resin was physically foamed. The effects of physical parameters in foaming model on bubble size and number density calculation were intensively examined by sensitivity analysis.

Effects of Process Conditions on the Dissolution of Carbon Dioxide in PS in Foaming Extruders

Abstract The effects of the main processing conditions on the dissolution behavior of CO2 in polystyrene melt were studied using a novel in-line method in connection with both single and twin-screw foaming extruders. Apparent solubility data obtained from the twin-screw extrusion experiments showed better consistency compared with those obtained from the single screw experiments, indicating the importance of enhanced mixing in affecting the gas dissolution and solubility behavior during an extrusion foaming process. Gas dissolution inside a twin-screw foaming extruder was further studied for CO2 in the polystyrene melt. The measured gas up-take was compared with the values calculated assuming equilibrium conditions in the gas injection zone in an effort to elucidate possible mechanisms for gas dissolution inside a foaming extruder during the actual foaming process.

Development of a Dilatometer for Measurement of the PVT Properties of a Polymer/CO2 Solution Using a Foaming Extruder and a Gear Pump

This paper presents an innovative dilatometer that can measure the pressure-volume-temperature (PVT) properties of polymer/CO2 solutions in a molten state. The basic rationale of the design is to determine the density (or equivalently, the specific volume) of a polymer/CO2 solution by separately measuring the mass and volume flow rates of the solution flowing in an extruder at each temperature and pressure. A positive-displacement gear pump mounted on an extruder is used to measure the volume flow rate of the solution. A single-phase polymer/CO2 solution is formed by injecting a metered amount of CO2 into a polymer melt and completely dissolving it in the melt using a foam extrusion line. The temperature of solution was precisely controlled and homogenized by using the second extruder in a tandem system and a heat exchanger with a static mixer. The pressure was controlled by the rotational speed of the screw in the second extruder. In order to reduce leakage across the gear pump, the difference between the upstream and downstream pressures was minimized using a variable resistance valve attached downstream of the gear pump. The mass flow rate was measured by directly collecting the extruded polymer melt for a fixed time after degassing CO2. A critical set of experiments was carried out to verify the functions of the system using pure polymer melts with known PVT data. Finally, the system was used to measure the specific volume of PS/CO2 solutions as a function of CO2 concentration, temperature, and pressure.

In-Line Density Monitoring of Rigid PVC Foam during Extrusion Process

Abstract An ultrasound-based sensor has been investigated to monitor the density of rigid PVC foam, in-line, during the extrusion process. A 2.25 MHz ultrasound transducer was used to measure the acoustic properties; i.e. speed of sound, acoustic impedance, and attenuation, in the polymer melt prior to exiting the extrusion die. A special transducer adapter and wave re ector were incorporated in the design of the extrusion die. The density of PVC foam was varied by varying the blowing agent content in the resin compound prior to extrusion. A mathematical derivation of the behavior of ultrasound waves in the melt is presented and used to calculate the attenuation coefficient of the polymer melt at various foam densities. An empirical relation was derived between the calculated attenuation coefficient of the polymer melt and the measured density of the foam extrudate. This relation can be used to estimate the foam density based on the acoustic measurement of the melt in the extrusion die. Further work is proposed to normalize for the effect of variations in the melt temperature and pressure on the ultrasound measurements.

On‐line NIR sensing of CO2 concentration for polymer extrusion foaming processes

AbstractAn on‐line sensor using near infrared (NIR) spectroscopy is developed for monitoring CO2 concentration in polymeric extrusion foaming processes. NIR absorption spectra are acquired by a probe installed at the foaming extruder die. The calibration curve relating the absorbance spectrum at 2019 nm to the dissolved gas concentration is derived so as to infer dissolved CO2 gas concentration on‐line from measured NIR spectra. Experimental results show the developed on‐line NIR sensor can successfully estimate dissolved CO2 concentration in the molten polymer and illustrate that the developed NIR sensing technique is among the more promising methods for quality control of polymeric extrusion foaming processes.

Measurements and modeling of PS/supercritical CO2 solution viscosities

AbstractThis paper presents a technology to determine the melt viscosity of a PS/super‐critical CO2 solution using a linear capillary tube die mounted on a foaming extruder. CO2 was injected into the extrusion barrel and the content of CO2 was varied in the range of O to 4 wt% using a positive displacement pump. Single‐phase PS/CO2 solutions were formed using a microcellular extrusion system and phase separation was prevented by maintaining a high pressure in the capillary tube die. By measuring the pressure drop through the die, the viscosity of PS/CO2 solutions was determined. The experimental results indicate that the PS/CO2 solution viscosity is a senstive function of shear rate, temperature, pressure, and CO2 content. A theoretical model based on the generalized Cross‐Carreau model was proposed to describe the shear‐thinning behavior of PS/CO2 solutions at various shear rates. The zero‐shear viscosity was modeled using a generalized Arrhenius equation to accommo‐date the effects of temperature, pressure, and CO2 content. Finally, the solubility of CO2 has been estimated by monitoring the pressure drop and the absolute pressure in the capillary die.