Effects of the die geometry on the expansion of polystyrene foams blown with carbon dioxide

This article presents the effects of the die geometry on the expansion ratio of extruded polystyrene foams blown with CO₂. Three groups of interchangeable filamentary dies were used to represent the die parameters. The experimental results reveal that a strong relationship exists between the expansion ratio of the extruded polystyrene foams and the die geometry through its effects on the pressure‐drop rate, the die pressure, the amount of premature cell growth, and the initial shape of the extrudate. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

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

This 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     

Extrusion of poly(ether imide) foams using pressurized CO2: Effects of imposition of supercritical conditions and nanosilica modifiers

Foams of an engineering plastic, poly(ether imide), were extruded using a single screw extruder employing pressurized CO₂ as the blowing agent. The porosity, pore size distributions, and the density of the foams were especially affected by the pressure drop, the pressure loss rate, and temperature at the die. Significant increases in porosity and pore size and corresponding decreases in density were observed when the pressure imposed on CO₂ became greater than the critical pressure values of CO₂ (i.e., the temperature was always greater than the critical temperature of the CO₂ in the extruder and the die). The viscoelastic material functions of the extruded foams depended especially on the density of the foam, with the elastic modulus increasing with density. The incorporation of nanosilica particles in the 0.08–0.6% by weight range increased only the density of the foam and did not provide any benefits in controlling of the nucleation rate and the pore size distribution, presumably due to their poor dispersibility and agglomerated state in the single screw extruder. POLYM. ENG. SCI., 54:2064–2074, 2014. © 2013 Society of Plastics Engineers     

Effect of die temperature on the morphology of microcellular foams

A study on the extrusion of microcellular polystyrene foams at different foaming temperatures was carried out using CO₂ as the foaming agent. The contraction flow in the extrusion die was simulated with FLUENT computational fluid dynamics code at two temperatures (150°C and 175°C) to predict pressure and temperature profiles in the die. The location of nucleation onset was determined based on the pressure profile and equilibrium solubility. The relative importance of pressure and temperature in determining the nucleation rate was compared using calculations based on classical homogeneous nucleation theory. Experimentally, the effects of die temperature (i.e., the foaming temperature) on the pressure profile in the die, cell size, cell density, and cell morphology were investigated at different screw rotation speeds (10 ～ 30 rpm). Experimental results were compared with simulations to gain insight into the foaming process. Although the foaming temperature was found to be less significant than the pressure drop or the pressure drop rate in deciding the cell size and cell density, it affects the cell morphology dramatically. Open and closed cell structures can be generated by changing the foaming temperature. Microcellular foams of PS (with cell sizes smaller than 10 μm and cell densities greater than 10 cells/cm³) are created experimentally when the die temperature is 160°C, the pressure drop through the die is greater than 16 MPa, and the pressure drop rate is higher than 10⁹ Pa/sec.     

Fabrication of polystyrene/nano‐CaCO3 foams with unimodal or bimodal cell structure from extrusion foaming using supercritical carbon dioxide

The article surveyed the fabrication of polystyrene (PS)/nano‐CaCO₃ foams with unimodal or bimodal cellular morphology from extrusion foaming using supercritical carbon dioxide (sc‐CO₂). In order to discover the factors influenced the cell structure of PS/nano‐CaCO₃ foams, the effects of die temperature, die pressure, and nano‐CaCO₃ content on cell size, density, and morphology were investigated detailed. The results showed that the nano‐CaCO₃ content affected the cell size and morphology of PS/nano‐CaCO₃ foams significantly. When the die temperature and pressure was 150°C and 18 MPa, respectively, the foams with 5 wt% nano‐CaCO₃ exhibited the unimodal cellular morphology. As the nano‐CaCO₃ content increased to 20 wt%, a bimodal cell structure of the foams could be obtained. Moreover, it was found that the bimodal structure correlated more strongly with the pressure drop than the foaming temperature. The article revealed that unimodal or bimodal cellular morphology of PS/nano‐CaCO₃ foams could be achieved by changing the extrusion foaming parameters and nano‐CaCO₃ content. POLYM. COMPOS., 37:1864–1873, 2016. © 2015 Society of Plastics Engineers     

Use of Nitrogen as a Blowing Agent for the Production of Fine‐Celled High‐Density Polyethylene Foams

Summary: While many experiments have been performed to examine the effects of administering CO₂ as a blowing agent in the foaming process, very few studies have investigated the use of N₂ for this purpose. In this study, foaming experiments were conducted in extrusion using HDPE as a polymeric material and N₂ as a blowing agent. Talc was used as a nucleating agent, and three different pressure‐drop rates were applied to study the effects of pressure‐drop rates on HDPE foams. The experimental results revealed that the void fraction of high‐density foams blown with N₂ was not affected by the die temperature, contrasting the situation in low‐density foams. Surprisingly, it was the cell density which determined the void fraction of high‐density foams. It was also found that the use of talc significantly increased the cell density and the void fraction of the foams and minimized the role played by the pressure‐drop rate in cell nucleation.

Effect of N₂ content on the cell density of HDPE foams.     

Physical extruder foaming of poly(lactic acid)—processing and foam properties

The article describes extrusion foaming of poly(lactic acid) (PLA) using carbon dioxide in the supercritical state as foaming agent emphasizing the steps required to establish a stable extrusion process. Low melt strength of PLA plays a role in optimizing processing conditions. The tests included PLA grades of different viscosity in addition to a chain extender. Processing at low temperature is possible due to the plasticizing effect of the CO₂ on the PLA melt and a sufficiently low melt temperature is also a prerequisite in production of stable foams due to improved melt strength. Foams were characterized by density, cell structure, crystallinity, and mechanical properties in compression. Low density, microcellular foams with density down to 20–30 kg/m³ were obtained for three different PLA grades. Varying die temperature and pressure drop rate we can explain observed abrupt drops in density with increasing CO₂ content by the interplay between cell nucleation and gas diffusivity at given temperatures. An effect on melt strength similar to using a chain extender is achieved by lowering the melt temperature at the die. Observed variations in sample crystallinity do not correlate with foam density. The PLA foams have good energy absorption capability. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Microcellular extrusion of PLA utilizing solid‐state nucleation in the gas‐saturated pellet extrusion process

In this study, we explore the use of solid‐state nucleation in polymer pellets as a means to create microcellular PLA foams in extrusion. This is achieved by using gas‐saturated PLA pellets as input to the extruder. Foam density, bubble size, and bubble density is reported and compared with microcellular foams created in the gas‐injection extrusion process. PLA pellet gas concentrations between 17 and 29 mg CO₂/g PLA was found to produce quality microcellular foams in this process. Gas concentrations within this range were achieved by varying methods that included partial saturation, desorption from full saturation, and blending saturated with unsaturated pellets. This gas concentration window that produced microcellular foams was found to be independent of the saturation and desorption process used to achieve the desired concentration. We further compare the pressure drop and pressure drop rate of the gas‐saturated pellet extrusion process showing that similar foams can be produced at pressures orders of magnitude lower than the alternative gas‐injection extrusion processes. Investigations into extrusion pressures support the hypothesis that the gas‐saturated pellet extrusion process utilizes solid‐state nucleation in the feed section of the extruder to achieve high bubble density foams. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Foaming behavior of isotactic polypropylene in supercritical CO2 influenced by phase morphology via chain grafting

Polystyrene (PS) and poly(methyl methacrylate) (PMMA) grafted isotactic polypropylene copolymers (iPP-g-PS and iPP-g-PMMA) with well-defined chain structure were synthesized by atom transfer radical polymerization using a branched iPP (iPP-B) as polymerization precursor. The branched and grafted iPP were foamed by using supercritical CO₂ as the blowing agent with a batch method. Compared to linear iPP foam, the iPP-B foams had well-defined close cell structure and increased cell density resulted from increased melt strength. Further incorporating PS and PMMA graft chains into iPP-B decreased the crystal size and increased the crystal density of grafted copolymers. In iPP-g-PS foaming, the enhanced heterogeneous nucleation by crystalline/amorphous interface further decreased the cell size, increased the cell density, and uniformized the cell size distribution. In contrast to this, the iPP-g-PMMA foams exhibited the poor cell morphology, i.e., large amount of unfoamed regions and just a few cells distributed among those unfoamed regions, although the crystal size and crystal density of iPP-g-PMMA were similar to those of iPP-g-PS. It was found that the iPP-g-PMMA exhibited PMMA-rich dispersed phase, which had higher CO₂ solubility and lower nucleation energy barrier than copolymer matrix did. The preferential cell nucleation within the PMMA-rich phase or at its interface with the matrix accounted for the poor cell morphology. The different effect of phase morphology on the foaming behavior of PS and PMMA grafted copolymers is discussed with the classical nucleation theory.     

Measurements and modeling of PS/supercritical CO2 solution viscosities

This 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.     

Extrusion foaming of polystyrene/carbon particles using carbon dioxide and water as co-blowing agents

Carbon particles such as platelet-like graphite (GR), spherically shaped activated carbon (AC), and tubular carbon nanofiber (CNF) were used as additives in extruded polystyrene (PS) foams with carbon dioxide (CO₂) and water as co-blowing agents. It was found that GR is the best additive for improving the thermal insulation performance of CO₂ based foam samples because of GR’s good absorption and reflectivity of infrared (IR) radiation. However, when the GR concentration was higher than 0.5 wt.%, the extruded foams exhibited large bubbles in the center of the foam and the extrusion line became unstable. By adding water carried by AC as a co-blowing agent, it was able to decrease the temperature in the center of the extruded foam, which successfully eliminated the bubble problem and achieved stable foam extrusion with good control of the foam density and cell morphology. Moreover, water carried by AC could also improve the mechanical performance of extruded foams containing CNF or GR. Water was not found in the extruded foams and the presence of water during extrusion did not affect the molecular weight and glass transition temperature of PS. Our results showed that a combination of AC as a water carrier and GR as an absorber and reflector of IR radiation can produce CO₂ based PS foams with good thermal insulation and mechanical properties, particularly with the presence of a small amount of CNF nanoparticles.     

Batch Foam Processing of Polypropylene/Polydimethylsiloxane Blends

Batch foaming processes were employed to prepare plastic foams from polypropylene (PP)/polydimethylsiloxane (PDMS) blends. Various amounts of PDMS were added to a PP matrix, and the resulting blends were batch foamed at different saturation pressures using carbon dioxide (CO₂) as the blowing agent. Ultimately, the blend foams exhibited better cell morphologies and higher cell densities in comparison with those prepared from PP alone. The increased solubility of CO₂ in PDMS made it as a CO₂ reservoir to induce more nucleation. When the PDMS content exceeded a certain level, however, it exerted a negative influence on cell density. Moreover, as the saturation pressure was raised, the cell density of the blend foams increased significantly. It was also noted that the addition of PDMS to the PP matrix generated some very small cells in the larger cell walls.     

Continuous microcellular polystyrene foam extrusion with supercritical CO2

The continuous production of polystyrene microcellular foams with supercritical CO₂ was achieved on a two‐stage single‐screw extruder. Simulations related to the foaming process were accomplished by modeling the phase equilibria with the Sanchez‐Lacombe equation of state and combining the equations of motion, the energy balance, and the Carreau viscosity model to characterize the flow field and pressure distribution in the die. The position of nucleation in the die was determined from the simulation results via a computational fluid dynamics code (FLUENT). Experimental parameters were selected according to the T_g and phase equilibria. The effects of CO₂ concentration and die pressure are explored. Below the solubility limit, higher CO₂ concentrations lead to smaller cell size and greater cell density. With an increase of die pressure, the cell size decreases and the cell density increases.     

Foam extrusion characteristics of thermoplastic resin with fluorocarbon blowing agent. II. Polystyrene foam extrusion

The foam extrusion characteristics of three different grades of polystyrene resin were investigated. For the study, cylindrical dies with various values of length-to-diameter ratio, entrance angle, and reservior-to-capillary diameter ratio were used. Fluorocarbon blowing agents were used, and mixtures of citric acid and sodium bicarbonate were used as nucleating agent. It was found that the die temperature, shear rate, the type and concentration of blowing agent, and die geometry affect the quality of the extruded polystyrene foam. Foam density and open cell fraction were used in determining the quality of extruded foams. We have found that the extrudate swell ratio is correlatable to foam density, independent of the die temperature employed. However, the die temperature has been found to be a very sensitive processing variable governing the quality of extruded foams.     

Measurement of interfacial tension in PS/LDPE melts saturated with supercritical CO2

The effect of supercritical carbon dioxide (scCO₂) on the interfacial tension between a polymer melt pair—polystyrene (PS) and low density polyethylene (LDPE)—was studied using the pendant drop method at temperatures varying from 200 to 240°C and CO₂ pressures up to 18 MPa. The LDPE melt was prepared in a high pressure optical cell and the PS pendant drop was injected into the LDPE melt with a special high pressure syringe. For measurements with scCO₂, the optical cell was first pressurized with scCO₂ and measurements were taken after the saturation of scCO₂ into both polymer melts. Excellent agreement was found with literature data for the same system without using scCO₂. For the polymer pair saturated with scCO₂, it was found that the interfacial tension decreases significantly with increasing CO₂ pressure and appears to level off at higher CO₂ pressures.     

A study of cell nucleation in the extrusion of polypropylene foams

An investigation has been performed of the cell nucleation and initial growth behaviors in the foam processing of polypropylene (PP) in both the linear and branched forms. These materials were foamed in extrusion with the two blowing agents, CO₂ and isopentane. The cell density generally increased with an increased content of the blowing agent, for both CO₂ and isopentane. The effect of processing pressure on the cell density was distinct when CO₂ was used, whereas no pressure effect was observed in the foam processing with isopentane. The cell morphologies for the two PPs were found to be significantly different. A slightly lower nuclei density was observed in the branched PP foams than in the linear PP foams. However, the phenomenon of cell coalescence was observed much less in the branched PP foams. Most cells in the branched PP foams were closed, whereas in the linear PP foams they were connected to each other. The experimental results indicated that the branched structure played an important role in determining the cell morphologies through its effects on the melt strength and/or melt elasticity.     

Solid‐state microcellular polycarbonate foams. I. The steady‐state process space using subcritical carbon dioxide

The process parameters for production of solid‐state microcellular polycarbonate using subcritical CO₂ were explored. Sufficiently long foaming times were used to produce foams, where cell growth had completed, resulting in steady‐state structures. A wide range of foaming temperatures and saturation pressures below the critical pressure of CO₂ were investigated, establishing the steady state process space for this polymer–gas system. Processing conditions are presented that produce polycarbonate foams where both the foam density and the average cell size can be controlled. The process space showed that we could produce foams at a constant density, while varying the cell size by and order of magnitude. At a relative density of 0.5, the average cell size could be varied from 4 to 40 μm. The ability to produce such a family of foams opens the possibility to explore the effect of microstructure, like cell size on the properties of cellular materials. It was found that the minimum foaming temperature for a given concentration of CO₂, determined from the process space, agrees well with the predicted glass transition temperature of the gas–polymer solution. A characterization of the average cell size, cell size distribution, and cell nucleation density for this system is also reported. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

Extruded polystyrene foams with bimodal cell morphology

Extrusion foaming using supercritical carbon dioxide (CO₂) as the blowing agent is an economically and environmentally benign process. However, it is difficult to control the foam morphology and maintain its high thermal insulation comparing to the conventional foams based on fluorocarbon blowing agents. In this study, we demonstrated that polystyrene (PS) foams with the bimodal cell morphology can be produced in the extrusion foaming process using CO₂ and water as co-blowing agents and two particulate additives as nucleation agents. One particulate is able to decrease the water foaming time so both CO₂ and water can induce foaming simultaneously, while the other increases the CO₂ nucleation rate with little effect on the CO₂ foaming time. Our experimental results showed that a dual particulate combination of nanoclay and activated carbon provided the best bimodal structure. The bimodal foams exhibited much better compressive properties and slightly better thermal insulation for PS foams.     

CO2 foaming based on polystyrene/poly(methyl methacrylate) blend and nanoclay

A poly(methyl methacrylate) (PMMA) and nanoclay composite was dispersed into polystyrene (PS) using a twin‐screw extruder. The mixture was then batch foamed with supercritical CO₂. It was found that the cell density of foams based on the blend is higher than that based on the weight average of the two pure polymer components at the same foaming conditions. The cell size decreases and the cell density increases with the increase of the PMMA domain size. One explanation is that the large PMMA domains serve as a CO₂ reservoir and the nucleation in the PS phase is enhanced by the diffusion of CO₂ from the PMMA phase to the PS phase. Very small PMMA domains cannot function as a CO₂ reservoir, and so they are not able to facilitate the nucleation. A much higher cell density and smaller cell size were observed when nanoclay was located at the interface of the PMMA and the PS domains, serving as the heterogeneous nucleating agents. POLYM. ENG. SCI., 47:103–111, 2007. © 2007 Society of Plastics Engineers     

Effect of dispersion method and process variables on the properties of supercritical CO2 foamed polystyrene/graphite nanocomposite foam

In this study, polystyrene/nanographite nanocomposite foams were made by different compounding methods, such as direct compounding, pulverized sonication compounding, and in situ polymerization, to understand the effect of the process variables on the morphology of the nanocomposites and their foam. The foam was made by batch foaming using CO₂ as the blowing agent. Various foaming pressures and temperatures were studied. The results indicated that the cell size decreased and the cell morphology was improved with the advanced dispersion of the nanoparticles. Among the three methods, the in situ polymerization method provided the best dispersion and the resulting nanocomposite foam had the finest cell size and the highest cell density. In addition, adding nanoparticles as a nucleating agent can make foams of similar cell size and cell density at a much lower foaming pressure. This result can be explained by the classical nucleation theory. This discovery could open up a newroute to produce microcellular foams at a low foaming pressure. POLYM. ENG. SCI., 53:2061–2072, 2013. © 2013 Society of Plastics Engineers