Micro and nano cellular amorphous polymers (PMMA, PS) in supercritical CO2 assisted by nanostructured CO2-philic block copolymers - One step foaming process

Microcellular foaming of commodity amorphous polymers, poly(methyl methacrylate) (PMMA), and poly(styrene) (PS) was studied in supercritical CO₂ via a batch one-step process in the presence of block copolymers able to change their foaming behaviour and therefore the porous structures. Triblock (styrene-co-butadiene-co-methylmethacrylate SBM, methylmethacrylate-co-butylacrylate-co-methylmethacrylate MAM) terpolymers were blended to PS or PMMA by extrusion. They showed advantages compared to classical PS-PMMA polymer blends in terms of cell size control and reduction of cell size. Foaming is carried out on bulk injection molded samples which were saturated under high pressures of CO₂ (300 bars) at different temperatures (25° C to 80 °C) and different depressurization rates (pressure drop rates from 150 bar/min to 12 bar/min). Very distinct cellular structures and densities were controlled by varying either the copolymer type or the foaming conditions (T,P). Cell sizes ranged from 0.2 μm to 200 μm, and densities from 0.30 g/cm³ to 1 g/cm³ in the polymers considered. Particularly, when triblock copolymers were able to self organize (nanostructuring) in a polymer matrix, they became phase separated at a nanometer level, presenting nanostructured polymers matrixes. To conclude the study, a possible nanostructuring mechanism is suggested based on the interplay between rubbery and highly CO₂-philic blocks/rigid and less CO₂-philic blocks. It is demonstrated that block copolymer additives are a good pathway towards micro and ultra microcellular supercritical CO₂ foaming of amorphous polymers.     

Ultrahigh compressive strength and heat resistance of polystyrene/polyphenylene oxide foams of supercritical carbon dioxide foaming

Polystyrene (PS) foam materials are lightweight, but suffer from poor compressive strength and heat resistance, among other problems, which limit their application. Herein, a method for preparing PS foam with high compressive strength and high heat resistance using supercritical CO₂ is proposed. PS/polyphenylene oxide (PPO) blends were prepared using a corotating intermeshing twin-screw extruder. The results showed that PPO exhibited excellent molecular-level compatibility with PS, which substantially improved mechanical properties and heat resistance of PS. Foam samples of PS/PPO blends with the same expansion ratio were prepared via batch foaming experiments, and the compressive strength of different foams was determined at different temperatures. At room temperature, the compressive strength of the PS/PPO-30% foam increased by 173% compared with pure PS foam. As the testing temperature increased from 30 to 120°C, the compressive strength of pure PS foams decreased rapidly. Nevertheless, PS/PPO foams maintained high compressive strength at high temperatures.     

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.     

Foaming of polymers with supercritical CO2: An experimental and theoretical study

Microcellular polystyrene (PS) foams and porous structures of the biodegradable poly(d,l-lactic acid) (P_d,lLA) were prepared with the batch foaming technique (pressure quench) using supercritical CO₂ as blowing agent. The effect of pressure, temperature and depressurization rate on the final porous structure was investigated. The results revealed that the size of the pores decreases and their population density increases with pressure increase, or decrease of temperature, and/or increase of the depressurization rate. The results were correlated by combining nucleation theory with NRHB model in order to account for and emphasize the physical mechanism related to nucleation of bubbles inside the supersaturated polymer matrix. A satisfactory agreement between correlations and experimental data was obtained indicating that the nucleation theory yields quantitative correlations when variables such as sorption, degree of plasticization, and surface tension of the system polymer-supercritical fluid are accurately described.     

Cell structure and impact properties of foamed polystyrene in constrained conditions using supercritical carbon dioxide

To obtain cellular with small cell diameter, to control cell structure and to improve impact strength of foaming materials, the quick-heating method was applied for foaming polystyrene (PS) using supercritical CO₂ (Sc-CO₂) as physical blowing agent. Then, changes of cell structure and impact strength in microcellular foamed PS materials under constrained conditions were studied. The effects of foaming processing parameters, such as foaming temperature, saturation pressure and foaming time on the cell structure and impact strength of foamed PS in the constrained conditions were studied. The results showed that the Sc-CO₂ solubility and nucleation density in the constrained conditions were not influenced compared with those under free foaming conditions. However, cells in constrained foaming process are mostly circular and independent with thick cell walls; the phenomenon of cell coalescence and collapse was effectively eliminated under constrained conditions. In addition, cell diameters in constrained foaming process decrease with increase in foaming temperature and increase with increase in the foaming time. Compared with that in free foaming conditions, the cell growth was restrained dramatically under constrained conditions which resulted in smaller cell diameter. Moreover, higher impact strength could be obtained for foamed PS as foaming time was prolonged, foaming temperature was increased or saturation pressure was enhanced.     

The preparation and properties of polystyrene/functionalized graphene nanocomposite foams using supercritical carbon dioxide

In this work, polystyrene (PS)/functionalized graphene nanocomposite foams were prepared using supercritical carbon dioxide. Thermally reduced graphene oxide (TRG) and graphene oxide (GO) were incorporated into the PS. Subsequently, the nanocomposites were foamed with supercritical CO₂. The morphology and properties of the nanocomposites and the nucleation efficiency of functionalized graphene in foaming PS are discussed. Compared with GO, TRG exhibited a higher nucleation efficiency and more effective cell expansion inhibition thanks to its larger surface area and better exfoliated structure. It is suggested that the factors that have a significant influence on the nucleation efficiency of TRG and GO originate from the differences in surface properties and chemical structure. Furthermore, PS/TRG nanocomposites and their nanocomposite foams also possess good electrical properties which enable them to be used as lightweight functional materials.© 2012 Society of Chemical Industry     

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.     

Extrusion of PE/PS blends with supercritical carbon dioxide

The effects of dissolved supercritical carbon dioxide on the viscosity and morphological properties were investigated for polyethylene/polystyrene blends in a twin-screw extruder. The viscosities of the blend/CO₂ solutions were measured using a wedge die mounted on the extruder. A considerable reduction of viscosity was found when CO₂ was dissolved in the blend. It was observed that the dissolution of CO₂ into PE/PS blends, regardless of the CO₂ content used, led to decreased shear thinning behavior resulting in an increase of the power law index from 0.29 to 0.34. The cell structures of foamed PE/PS blends showed a typical dependence of pressure and CO₂ concentration, with higher operating pressures and CO₂ content leading to a smaller cell size. Also, it was noted that the size of the dispersed PS phase in the PE/PS phase blends decreased by increasing the CO₂ concentration, and that the dispersed PS phase domains were highly elongated in the direction normal to the cell radius.     

Effect of poly(butylenes succinate) on the microcellular foaming of polylactide using supercritical carbon dioxide

Microcellular Polylactide (PLA) and PLA/poly(butylenes succinate) (PBS) foams were prepared by batch foaming process with supercritical carbon dioxide. The introduced PBS phase was immiscible with the PLA matrix and separated as domains. The study of CO₂ solubility in PLA and PLA/PBS blends indicated the addition of PBS decreased the gas solubility due to the poor affinity of CO₂ for PBS. The crystallization behavior of PLA was enhanced by small amount of PBS with lower cold crystallization temperature and higher crystallinity. However, separated PBS droplets led to less perfect and small crystallites, which showed greatly effect for the PLA foaming process. The investigation on the foaming conditions dependence indicated the PLA/PBS blends required higher temperature and longer time for the cell growth, which were nucleated around the interface between PLA and PBS. With less CO₂ content in the PLA or PLA/PBS blends after different desorption time, the final cell morphology exhibited more uniform size distribution with bigger average cell size and smaller cell density. Different from the well closed-cell structure for neat PLA foam, the PLA/PBS foam presented open cell structure due to the cell nucleation around the PLA/PBS interface and the lower melt strength of PBS phase.     

Investigation of the nanocellular foaming of polystyrene in supercritical CO2 by adding a CO2‐philic perfluorinated block copolymer

Nanocellular foaming of polystyrene (PS) and a polystyrene copolymer (PS‐b‐PFDA) with fluorinated block (1,1,2,2‐tetrahydroperfluorodecyl acrylate block, PFDA) was studied in supercritical CO₂ (scCO₂) via a one‐step foaming batch process. Atom Transfer Radical Polymerization (ATRP) was used to synthesize all the polymers. Neat PS and PS‐b‐PFDA copolymer samples were produced by extrusion and solid thick plaques were shaped in a hot‐press, and then subsequently foamed in a single‐step foaming process using scCO₂ to analyze the effect of the addition of the fluorinated block copolymer in the foaming behaviour of neat PS. Samples were saturated under high pressures of CO₂ (30 MPa) at low temperatures (e.g., 0°C) followed by a depressurization at a rate of 5 MPa/min. Foamed materials of neat PS and PS‐b‐PFDA copolymer were produced in the same conditions showing that the presence of high CO₂‐philic perfluoro blocks, in the form of submicrometric separated domains in the PS matrix, acts as nucleating agents during the foaming process. The preponderance of the fluorinated blocks in the foaming behavior is evidenced, leading to PS‐b‐PFDA nanocellular foams with cell sizes in the order of 100 nm, and bulk densities about 0.7 g/cm³. The use of fluorinated blocks improve drastically the foam morphology, leading to ultramicro cellular and possibly nanocellular foams with a great homogeneity of the porous structure directly related to the dispersion of highly CO₂‐philic fluorinated blocks in the PS matrix. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Two-step micro cellular foaming of amorphous polymers in supercritical CO2

Microcellular foaming of amorphous rigid polymers, polymethylmethacrylate (PMMA) and polystyrene (PS) was studied in supercritical CO₂ (ScCO₂) in the presence of several types of additives, such as triblock (styrene-co-butadiene-co-methylmethacrylate, SBM and methylmethacrylate-co-butylacrylate-co-methylmethacrylate, MAM) terpolymers. This work is focused in the two-step foaming process, in which the sample is previously saturated under ScCO₂ being expanded in a second step out of the CO₂ vessel (e.g. in a hot oil bath) where foaming is initiated by the change of temperature near or above the glass transition temperature of the glass/polymer glassy system. Samples were saturated under high pressures of CO₂ (300 bar), at room temperature, for 16 h, followed by a quenching at a high depressurization rate (150 bar/min). In the last step, foaming was carried out at different temperatures (from 80 °C to 140 °C) and different foaming times (from 10 s to 120 s). It was found that cellular structures were controlled selecting either the additive type or the foaming conditions. Cell sizes are ranging from 0.3 μm to 300 μm, and densities from 0.50 g/cm³ to 1 g/cm³ depending on the polymers considered.     

Polymeric hydrogels and supercritical fluids: The mechanism of hydrogel foaming

A novel method, the hydrogel foaming, is used in this work for the production of porous, polymer-based materials by processing with supercritical carbon dioxide (CO₂). This method is applied to crystalline hydrophilic polymers that, practically, exhibit no phase transition (melting or glass transition) below thermal decomposition temperature and, due to their crystallinity, do not absorb CO₂. Such polymers are mainly natural (semi)-crystalline polymers (e.g. chitosan, cellulose, etc.) for which the classical polymer foaming method with supercritical carbon dioxide is not applicable. The hydrogel foaming process (similar to classical polymer foaming) is applied to gelatin, chitosan, and gelatin/chitosan blend hydrogels that are physically crosslinked and may also be chemically crosslinked with glutaraldehyde vapour. After the foaming process, water is removed from the gels by mild freeze-drying leading to porous materials. Pore size control can be achieved by controlling different process parameters. Gelatin exhibits solubility in water up to high concentrations and forms thermoreversible hydrogels, rendering it a suitable choice for the investigation of the process mechanism. The mechanism of hydrogel foaming is explored on the basis of X-ray diffraction, calorimetry, rheology, sorption, Raman spectroscopy measurements and theoretical calculations with the NRHB (Non Random Hydrogen Bonding) equation-of-state model. The sorption and Raman spectroscopy measurements suggest that, besides dissolution in water (of the hydrogel), extensive CO₂ sorption by the polymer also occurs. Based on these results, a critical discussion is made and a mechanism for the hydrogel foaming is proposed.     

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     

CO2 foaming of poly(ethylene glycol)/polystyrene blends: Relationship of the blend morphology,CO2 mass transfer,and cellular structure

When polymer blends are foamed by physical foaming agents, such as CO₂ or N₂, not only the morphology and viscosity of the blend polymers but also the solubility and diffusivity of the physical foaming agents in the polymers determine the cellular structure: closed cell or open cell and monomodal or bimodal. The foam of poly(ethylene glycol) (PEG)/polystyrene (PS) blends shows a unique bimodal (large and small) cellular structure, in which the large‐size cells embrace a PEG particle. Depending on the foaming condition, the average size of the large cells ranges from 40 to 500 μm, whereas that of small cells becomes less than 20 μm, which is smaller than that of neat PS foams. The formation mechanism of the cellular structure has been investigated from the viewpoint of the morphology and viscosity of the blend polymer and the mass‐transfer rate of the physical foaming agent in each polymer phase. The solubility and diffusivity of CO₂, which determine the mass‐transfer rate of CO₂ from the matrix to the bubbles, were measured by a gravimetric measurement, that is, a magnetic suspension balance. The solubility and diffusivity of CO₂ in PS differed from those in PEG: the diffusion coefficient of CO₂ in PEG at 110°C was 3.36 × 10^?9 m²/s, and that in PS was 2.38 × 10^?10 m²/s. Henry's constant in PEG was 5600 cm³ (STP)/(kg MPa) at 110°C, and that in PS was 3100 cm³ (STP)/(kg MPa). These differences in the transport properties, morphology of the blend, and CO₂‐induced viscosity depression are the control factors for creating the unique cellular structure in PEG/PS blends. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1899–1906, 2005     

Generation of microcellular foams of PVDF and its blends using supercritical carbon dioxide in a continuous process

Use of supercritical carbon dioxide (scCO₂) as a blowing agent to generate microcellular polymer foams (MPFs) has recently received considerable attention due to environmental concerns associated with conventional organic blowing agents. While such foams derived from amorphous thermoplastics have been previously realized, semicrystalline MPFs have not yet been produced in a continuous scCO₂ process. This work describes the foaming of highly crystalline poly(vinylidene fluoride) (PVDF) and its blends with amorphous polymers during extrusion. Foams composed of neat PVDF and immiscible blends of PVDF with polystyrene exhibit poor cell characteristics, whereas miscible blends of PVDF with poly(methyl methacrylate) (PMMA) yield foams possessing vastly improved morphologies. The results reported herein illustrate the effects of blend composition and scCO₂ solubility on PVDF/PMMA melt viscosity, which decreases markedly with increasing PMMA content and scCO₂ concentration. Morphological characterization of microcellular PVDF/PMMA foams reveals that the cell density increases as the PMMA fraction is increased and the foaming temperature is decreased. This study confirms that novel MPFs derived continuously from semicrystalline polymers in the presence of scCO₂ can be achieved through judicious polymer blending.     

Use of nanoparticles for improving the foaming behaviors of linear PP

This research presents the foaming behaviors of linear polypropylene (PP) and PP/clay nanocomposites blown with supercritical CO₂. The cell nucleation and expansion behaviors of the linear PP and PP‐based nanocomposites at various clay contents during extrusion foaming are studied. The experimental results indicate that the nano‐particles have a positive impact on improving the cell morphology, the cell density and the expansion ratio of the linear PP foams. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

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.     

Temperature influence and CO2 transport in foaming processes of poly(methyl methacrylate)–block copolymer nanocellular and microcellular foams

Fabricated by high-pressure or supercritical CO₂ gas dissolution foaming process, nanocellular and microcellular polymer foams based on poly(methyl methacrylate) (PMMA homopolymer) present a controlled nucleation mechanism by the addition of a methylmethacrylate–butylacrylate–methylmethacrylate block copolymer (MAM), leading to defined nanocellular morphologies templated by the nanostructuration of PMMA/MAM precursor blends. Influence of the CO₂ saturation temperature on the foaming mechanism and on the foam structure has been studied in 90/10 PMMA/MAM blends and also in the neat (amorphous) PMMA or (nanostructured) MAM polymers, in order to understand the role of the MAM nanostructuration in the cell growth and coalescence phenomena. CO₂ uptake and desorption measurements on series of block copolymer/homopolymer blend samples show a competitive behavior of the soft, rubbery, and CO₂-philic block of PBA (poly(butyl acrylate)) domains: fast desorption kinetics but higher initial saturation. This competition nevertheless is strongly influenced by the type of dispersion of PBA (e.g. micellar or lamellar) and a very consequent influence on foaming.CO₂ sorption and desorption were characterized in order to provide a better understanding of the role of the block copolymer on the foaming stages. Poly(butyl acrylate) blocks are shown to have a faster CO₂ diffusion rate than poly(methyl methacrylate) but are more CO₂-philic. Thus gas saturation and cell nucleation (heterogeneous) are more affected by the PBA block while cell coalescence is more affected by the PMMA phases (in the copolymer blocks + in the matrix).     

Preparation, characterization, and supercritical carbon dioxide foaming of polystyrene/graphene oxide composites

Graphene oxide (GO) was prepared by oxidation of graphite using the Hummers method, and was modified by isocyanate to obtain dispersed GO sheets in dimethylformamide. Polystyrene (PS)/GO composites were prepared by solution blending, and their morphologies and properties were characterized. The addition of GO increased the glass transition temperature of the PS/GO composites. The storage modulus and thermal stability of the composites were also improved compared with PS. Foams of PS and PS/GO composites were prepared by supercritical carbon dioxide foaming. The composite foams exhibited slightly higher cell density and smaller cell size compared with the PS foam, indicating the GO sheets can act as heterogeneous nucleation agents.     

Visual observation and numerical studies of N2 vs. CO2 foaming behavior in core‐back foam injection molding

We investigated , by visual observation and numerical calculations , the foaming behavior of polypropylene within a foam injection mold cavity with the environmentally benign physical blowing agents nitrogen (N₂) and carbon dioxide (CO₂) . An 85‐ton core‐back injection‐molding machine with temperature and pressure monitoring systems as well as a high‐pressure view cell was used for the investigation . The experiments showed a prominent difference in bubble nucleation and growth between N₂ and CO₂ injection foaming . Even when the weight concentration of N₂ dissolved in polymer was one‐third that of CO₂ , N₂ injection foaming provided a bubble number density that was 30 times larger and a bubble size that was one‐third smaller compared to CO₂ injection foaming . Classical bubble nucleation and growth models developed for batch foaming were employed to analyze these experimental results . The models reasonably explained the differences in injection foaming behavior between N₂ and CO₂ . It was clearly demonstrated by both experiments and numerical calculations that N₂ provides a higher number of bubbles with a smaller bubble size in foam injection molding compared to CO₂ as a result of the lower solubility of N₂ in the polymer and the larger degree of super‐saturation . POLYM. ENG. SCI., 2011. ©2011 Society of Plastics Engineers