Supercritical Carbon Dioxide Foaming Research Articles

Highly Durable Superhydrophobic Polymer Foams Fabricated by Extrusion and Supercritical CO2 Foaming for Selective Oil Absorption.

The severe water contamination caused by oil leakage is calling for low-cost and high-performance absorbent materials for selective oil removal. In this study, a scalable green method was proposed to produce polypropylene (PP)/poly(tetrafluoroethylene) (PTFE) composite foams via conventional processing techniques including twin-screw extrusion and supercritical carbon dioxide foaming. To produce the superhydrophobic foam, micro- and nanosized PTFE particles were melt blended with PP and subsequently foamed. Ascribed to the nanofibrillation of microsized PTFE during processing, the fabricated foam exhibited a special highly porous structure with PTFE nanofibrils and nanoparticles uniformly distributed on the pore surfaces within the PP matrix, which resulted in a remarkably high water contact angle of 156.8° and a low contact angle hysteresis of 1.9°. Unlike traditional surface-modified superhydrophobic absorbers, the foams prepared are entirely superhydrophobic, which means that they remain superhydrophobic when being fractured or cut. Moreover, they are highly durable and maintained the superhydrophobicity when subjected to ultrasonication and mechanical sanding. When used in selective oil absorption, the durable foams exhibited excellent absorption efficiency and high stability in repetitive and long-term use. These advantages make the PP/PTFE foam a promising superabsorbent material for water remediation.

Enhancing the EMI shielding of natural rubber-based supercritical CO2 foams by exploiting their porous morphology and CNT segregated networks.

Natural rubber/carbon nanotubes composite foams (F-NR/CNTs) with high electrical conductivity and excellent electromagnetic interference (EMI) performance were developed through a multi-step process including: (a) CNTs assembled on natural rubber latex particles, (b) pre-crosslinking of natural rubber, (c) supercritical carbon dioxide foaming of pre-crosslinked composite samples and (d) post-crosslinking of foamed composite samples. A closed-cell porous structure and a segregated CNT network are clearly observed in the resulting foams. Due to this morphology, F-NR/CNTs exhibit low density, good mechanical properties, and high electrical conductivity. Owing to the multiple radiation reflections and scattering between the cell-matrix interfaces, the composite foams presented an excellent specific shielding effectiveness (SSE) of 312.69 dB cm2 g-1 for F-NR/CNTs containing 6.4 wt% of CNTs, which is significantly higher than those already published for rubber composites containing comparable filler content. Furthermore, the analysis of EMI SE highlights that absorption efficiency is more significant than reflection efficiency, implying that most of the incident electromagnetic radiation is dissipated in the form of heat. This work provides the fundamentals for the design of innovative light weight and efficient EMI shielding foams characterized by a three-dimensional segregated CNT network with huge potential for use in the electronics and aerospace industries.

Formation Mechanism and Tuning for Bimodal Open-Celled Structure of Cellulose Acetate Foams Prepared by Supercritical CO2 Foaming and Poly(ethylene glycol) Leaching

The cellulose acetate/poly(ethylene glycol) (CA/PEG) blend with a weight ratio of 50/50 was solution-blended followed by supercritical carbon dioxide foaming and the PEG leaching process. Interesti...

Strategy to Enhance Conductivity of Polystyrene/Graphene Composite Foams via Supercritical Carbon Dioxide Foaming Process

Composites combining polystyrene (PS) and thermally reduced graphene with different size of sheets were fabricated by melting blending. Composites added large and small size of graphene (graphene mixture) exhibited a higher electronic conductivity. To reduce the weight of composites, PS/graphene composite foams were prepared by scCO2 foaming process. As a result, composites included different size of graphene showed a more uniform bubbles, higher cell density and smaller cell diameter because of synergy in graphene sheets. The foams with graphene mixture also exhibited high cell density in high saturation temperature due to their higher viscosity and glass transition temperature analyzed by high-pressure rheometer and differential scanning calorimeter. For conductivity of foams, the introduced bubbles exhibited a good enhancement of electrical conductivity and percolation threshold decreased from 0.34 vol.% to 0.16 vol.%. It could be found that conductivity increased by raising pressure and decreased with the increase temperature.

Merits of the Addition of PTFE Micropowder in Supercritical Carbon Dioxide Foaming of Polypropylene: Ultrahigh Cell Density, High Tensile Strength, and Good Sound Insulation

Ultrahigh-cell-density polypropylene (PP) foam was prepared by supercritical carbon dioxide (scCO2) foaming in the presence of porous polytetrafluoroethylene (PTFE) micropowder. The voids of PTFE m...

A new promising nucleating agent for polymer foaming: effects of hollow molecular-sieve particles on polypropylene supercritical CO2 microcellular foaming.

Because polypropylene (PP) foam normally exhibits nonuniform cell size and cracked cellular structure, a narrow cell-size distribution and a well-defined morphology are always the focus of PP foaming technology. In this work, hollow molecular-sieve (MS) particles were applied as a potential nucleating agent in supercritical carbon dioxide (scCO2) foaming of PP. It was observed that the addition of MS particles largely narrowed the cell-size distribution. The resultant PP/MS foams exhibited significant concurrent enhancement in their cell density and mechanical properties: the cell density increased remarkably, by approximately 10 times, and the tensile strength increased from 6.1 MPa to 12.6 MPa. The hollow-structure MS particles resulted in a higher heterogeneous nucleation efficiency in the PP foaming process. We believe that the trapping of CO2 in the hollow holes of MS particles largely increased the solubility CO2 in PP and a number of gas cavities were formed. The existence of gas cavities reduced the energy barrier of heterogeneous nucleation, favoring the formation of a well-defined cellular structure. Additionally, the regular-hexagon shape of the cells might endow the PP foam with better mechanical properties compared with a circular cell shape.

Poly(L-lactide)-grafted bioglass/poly(lactide-co-glycolide) scaffolds with supercritical CO2 foaming reprocessing for bone tissue engineering

The bioglass particles/poly(lactide-co-glycolide)(BG/PLGA) scaffold has been extensively explored for biomedical applications due to its excellent advantages of mechanical property and controllable degradation rate. In our previous studies, the BG nanoparticle surface-grafted with poly(L-lactide)(PLLA) could substantially improve the phase compatibility between the polymer matrix and the inorganic phase and the biocompatibility of the scaffolds. However, using the traditional preparation methods to prepare the composite scaffold can barely achieve a high porosity and porous connectivity. In this work, the PLLA-grafted bioglass/PLGA(g-BG/PLGA) scaffolds were prepared by supercritical carbon dioxide foaming(Sc-CO2) with before or after particulate leaching(PL) method(Sc-CO2-PL or PL-Sc-CO2 method, PL/Sc-CO2 methods) and their applications in bone replacement and tissue engineering were investigated. The porosities of the g-BG/PLGA scaffolds prepared by the PL/Sc-CO2 methods were higher than 90%, and their mechanical properties had similar values with human cancellous bone. The proliferations of osteoblasts on the scaffolds were dependent on different preparation methods. The PL/Sc-CO2 methods significantly increased the proliferations of the cells. Computed tomography(CT) three-dimensional(3D) reconstruction tomographies of the implantation study for repairing calvarium defects of rabbits demonstrated that the calvarium defects were almost completely filled by the osteotylus in PL/Sc-CO2 method group at 12 week post-surgery, while there was little callus formation in PL method group and untreated control group. These results indicate that the g-BG/PLGA scaffolds prepared by the PL/Sc-CO2 methods exhibit rapid mineralization and osteoconductivity and are the optimal composites for bone repair.

The influence of in situ synthesized nanoparticles on microstructure and compression properties of polymer foams during supercritical carbon dioxide foaming

ABSTRACTThe influence of in situ synthesized nanoparticles on the microstructure and compression properties of polymer foams during supercritical carbon dioxide foaming has been investigated. The in situ synthesized Ag nanoparticles were chosen to be heterogeneous nucleating agent. For achieving our target, the influence of the nanoparticle size on the cell structure and the nucleation mechanism has been detailed discussed firstly. The results show that the in situ synthesized nanoparticles can be heterogeneous nucleation agent to improve the cell density of the PMMA‐based foams. The particle size is able to reduce to the critical size of heterogeneous nucleation agent and, then, can highly improve the cell density of the foams. The Ag nanoparticles with average size of 2.2 nm led to 85% increase in compressive strength of the foams. The improvement of strength of the polymer matrix and the microstructure of the foams can lead to the remarkable increase in the mechanical properties of the foams. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44629.

Facile preparation of lightweight high-strength biodegradable polymer/multi-walled carbon nanotubes nanocomposite foams for electromagnetic interference shielding

Lightweight conductive polymer composites (CPCs) have been considered as the most promising alternatives to metal-based shields for electromagnetic interference (EMI) shielding application but still face non-degradation issues. We report a facile, inexpensive and green method to implement lightweight biodegradable poly (l-lactic acid) (PLLA)-multiwalled carbon nanotubes (MWCNTs) nanocomposite foams using a combinatorial technology of pressure-induced flow (PIF) processing and supercritical carbon dioxide (Sc–CO2) foaming. Such low-density (∼0.3 g/cm3), low thickness (∼2.5 mm), high compressive strength (∼54 MPa g−1 cm3)) and highly conductive (∼3.4 S m−1) PLLA-MWCNT nanocomposite foams were first reported as an EMI shielding material: it presents high performance EMI shielding with a remarkable effectiveness and a corresponding average specific EMI SE of ∼23 dB and ∼77 dB g−1 cm3, respectively, with less reflection in the measured X-band frequency region. Considering the simple, low-cost and eco-friendly fabrication process, the lightweight, high-strength and highly conductive biodegradable polymer composite foams are expected to be used as high-performance EMI shielding materials in areas such as electronics, automobiles and packaging.

Preparation and Density Control of PMMA Microcellular Foams via Supercritical Carbon Dioxide Foaming

Microcellular polymeric foam is a new class of materials which has been widely used in many industries. The foaming of polymethyl metacrylate (PMMA) using supercritical carbon dioxide (ScCO2) which is inexpensive and environmental friendly has been studied to better understand the foaming process. The pieces of PMMA are put into a saturation vessel of which temperature and pressure are kept constant. Supercritical carbon dioxide (ScCO2) at temperature between 65 °C and 105 °C and pressure between 8 MPa and 16 MPa is used as a foaming agent. After saturation of carbon dioxide, rapid decompression of ScCO2 saturated PMMA yields expanded microcellular foams. The densities of foamed PMMA materials are tested by true density analyzer, while the microstructures of a variety of density foamed PMMA materials are characterized by scaning electron microscopy (SEM). The cell size and cell density are calculated via image analysis. The effect of the process condition on the cell morphologies and mass density of the foam is investigated by considering the solubility of carbon dioxide in PMMA. The relationship between the mass density of foamed PMMA (ρ) and foaming temperature (T) and pressure (P) are respectively certained quantificationally.

Strategy of fabrication of controlled thermosetting gel based on soybean oil towards supercritical carbon dioxide foaming

To produce soybean oil based foam plastics in terms of the supercritical carbon dioxide foaming technique, the B-stage gel formed by the radical copolymerization of acrylated epoxidized soybean oil (AESO) and styrene (St) was prepared and studied. Accordingly, a dual initiator system consisting of cyclohexanone peroxide (CP) and di-tert-butyl peroxide (DTBP) catalyzed by cobalt octoate (CoOct) was introduced, which provided the gel with (i) good permeability and sufficient thermal stability for saturation in the supercritical carbon dioxide and (ii) polymerizability for freezing the depressurization induced cells during post cure at higher temperatures. The relationships among the reaction processes, structural evolvement and properties variation were characterized and analyzed. Additionally, the experimental results were verified by a numerical simulation. It is hoped that the outcome of this work would not only deepen the understanding of the intermediate status of thermosets, but also help to determine the optimal conditions for fabricating thermosetting foams by supercritical carbon dioxide foaming.

Starch-based polymer–IL composites formed by compression moulding and supercritical fluid foaming for self-supported conductive materials

In this work, blends of starch and poly-e-caprolactone (PCL) doped with different concentrations of 1-butyl-3-methylimidazolium acetate ([BMIM]Ac) or 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) were studied. The blends were characterized by mechanical analysis, infra-red spectroscopy (FTIR), differential scanning calorimetry (DSC) and dielectric relaxation spectroscopy (DRS), evaluating the IL doping effect. The samples were subjected to supercritical carbon dioxide foaming and the morphology of the structures was assessed. DSC shows a single glass transition and melting endotherm for foamed and unfoamed samples, having no effect upon IL doping, and DRS shows increased molecular mobility for blends with higher IL concentrations, and some hindrance for lower ones. The conductivity for SPCL doped with 30% [BMIM]Cl, before and after foaming, is comparable to the conductivity of the IL but exhibits more stable conductivity values, opening doors for applications as self-supported conductive materials.

Mesoporous silica particles grafted with polystyrene brushes as a nucleation agent for polystyrene supercritical carbon dioxide foaming

ABSTRACTIn this study, spherical ordered mesoporous silica (s‐OMS) was applied as a new type of nucleating agent in polystyrene (PS) foaming with supercritical CO2 as a blowing agent. These s‐OMS particles were modified by the selective grafting of PS brushes on the outside surface, by which the mesoporous structure inside particles could be maintained. Transmission electron microscopy, X‐ray diffraction, thermogravimetric analysis, Fourier transform infrared spectroscopy, and Brunauer–Emmett–Teller surface area analysis were used to characterize the structure of the original and modified particles; these indicated that the PS brushes were grafted on the outside surface and the inside porous structure were maintained. PS/s‐OMS–PS composites were prepared by a solution blending method, and the s‐OMS–PS particles could have been well dispersed in the PS matrix because of the surface modification. Subsequently, PS and composite microcellular foams were prepared by a batch foaming process, and the morphology characterization on these foams showed that the s‐OMS particles exhibited an excellent heterogeneous effect on PS foaming. The heterogeneous effect became more significant when the foaming temperature or saturation pressure was low. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 4308–4317, 2013

Multiwalled carbon nanotubes grafted with polyamidoamine (PAMAM) dendrimers and their influence on polystyrene supercritical carbon dioxide foaming

In this study, two types of multiwalled carbon nanotubes (MCNTs) are used for the preparation of polystyrene (PS)/MCNT composites. For each type of MCNT, two surface modifications, i.e., grafted with PS brushes (MCNTs-PS) and grafted with polyamidoamine (PAMAM) dendrimers and subsequent PS brushes (MCNTs-PAMAM-PS), are applied. The effects of aspect ratio and surface modification of the MCNTs on their dispersion in the PS matrix, as well as the dynamic mechanical properties and supercritical carbon dioxide (scCO2) foaming of the composites, are investigated. The results indicate that the MCNTs-PS have a more uniform dispersion and a higher dynamic mechanical property reinforcement compared to MCNTs-NH2 and MCNTS-PAMAM-PS. For the heterogeneous nucleation of MCNTs in PS scCO2 microcellular foaming, long MCNTs show a small nucleation effect, while the nucleation effect of the short MCNTs is much more significant. Furthermore, the introduction of PAMAM dendrimers on the surface of MCNTs can improve the heterogeneous nucleation efficiency.

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

We investigated the nucleation effect of well exfoliated and dispersed graphene oxide(GO) sheets on polystyrene(PS) supercritical carbon dioxide foaming. To get PS/GO composites with well exfoliated GO sheets, a method based on latex concept was employed. The characterizations based on X-ray diffraction and transmission electron microscope demonstrated the full exfoliation and uniform dispersion of GO sheets in polymer matrix. Thermal stability and dynamic mechanical properties of the compound materials were investigated by a series of measurements such as differential scanning calorimetry analysis, dynamic mechanical analysis and thermogravimetry analysis, which suggested the higher thermal stability and glass transition temperature of the composites. The supercritical carbon dioxide foaming of pure PS and composites was carried out in a batch foaming system, the results showed that the well exfoliated GO sheets could be a high efficient nucleation agent. For the pure PS foam and PS/GO composite foams produced under the foaming condition of 120 °C and 13.8 MPa, the average foam cell size was reduced from 7.1 um to 4.3 um and the cell density was increased from 4.52 × 109 to 1.84 × 1010 cells/cm3 by around 5.0 wt.% GO sheets.

Preparation and characterization of PMMA graded microporous foams via one-step supercritical carbon dioxide foaming

Supercritical carbon dioxide (ScCO2) foaming which is inexpensive and environmental friendly has been widely used to prepare polymer-based microporous materials. In this paper, PMMA graded microporous materials were foamed by PMMA matrix after an unstable saturation process which was done under supercritical condition of 28MPa and 50 °C. The scanning electron microscopy (SEM) was utilized to observe the morphology of the graded foam. A gas adsorption model was proposed to predict the graded gas concentration in the different region of the polymer matrix. The SEM results showed that the solid and foam region of the graded foam can be connected without laminated layers. With the increasing thickness position of the graded microporous foam, the cell size increased from 3.4 to 27.5 μm, while the cell density decreased from 1.04 × 109 to 1.96 × 107 cells/cm3. It also found that the gradient microporous structure of the foam came from graded gas concentration which was obtained in the initial saturation process.

Cellular biocomposites from polylactide and microfibrillated cellulose

This paper describes the production of “green” microfibrillated cellulose-reinforced polylactide cellular biocomposites using a wet mixing technique combined with supercritical carbon dioxide foaming. The effect of composition on the morphology, density and compression modulus of foams was investigated for different processing parameters, along with the use of a chain extender to modify the melt elasticity. Foams with mean densities ranging from 0.18 to 0.32 g/cm3 were obtained for the neat polylactide and polylactide/5 wt% microfibrillated cellulose, respectively, and there was a corresponding increase in compressive modulus from 25 to 47 MPa. The addition of the chain extender is argued to compensate the molar mass loss induced by the different processing steps, promoting more uniform foam structures and allowing a density reduction of up to 75%.

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.

Fabrication of functionally graded porous polymer via supercritical CO2 foaming

A fabrication process to produce functionally graded porous polymer via supercritical carbon dioxide (ScCO2) foaming is reported in this paper. It utilizes a partial gas saturation technique to obtain non-equilibrium gas concentration profiles in thermoplastic polymer. Once foamed the polymer material obtains a graded foam–solid-foam structure with varying pore size distributions. This functionally graded material fabrication method was studied with polymethyl methacrylate (PMMA) under a ScCO2 saturation condition. A diffusion model was developed to estimate the gas diffusion coefficient and to predict the gas concentration profiles inside the polymer samples. Scanning electron microscopy images were used to analyze the effects of partial saturation on the graded porous structure. Mechanical properties of the foamed samples were characterized using a three-point bending test. It was found that the gas concentration profiles resulted from partial saturation corresponded well to the graded structure after foaming. The proportion of the foam and solid regions inside the polymer sample can be manipulated by controlling the partial saturation profile. The test results also suggested that the graded foam structure could be optimized to achieve desirable mechanical properties.

Scaffold for tissue engineering fabricated by non-isothermal supercritical carbon dioxide foaming of a highly crystalline polyester

Porous scaffolds of a random co-polymer of ω-pentadecalactone (PDL) and ε-caprolactone (CL) (poly(PDL–CL)), synthesized by biocatalysis, were fabricated by supercritical carbon dioxide (scCO2) foaming. The co-polymer, containing 31mol.% CL units, is highly crystalline (Tm=82°C, ΔHm=105Jg−1) thanks to the ability of the two monomer units to co-crystallize. The co-polymer can be successfully foamed upon homogeneous absorption of scCO2 at T>Tm. The effect of soaking time, depressurization rate and cooling rate on scaffold porosity, pore size distribution and pore interconnectivity was investigated by micro X-ray computed tomography. Scaffolds with a porosity in the range 42–76% and an average pore size of 100–375μm were successfully obtained by adjusting the main foaming parameters. Process conditions in the range investigated did not affect the degree of crystallinity of poly(PDL–CL) scaffolds. A preliminary study of the mechanical properties of the scaffolds revealed that poly(PDL–CL) foams may find application in the regeneration of cartilage tissue.