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A Review of CO 2 Applications in the Processing of Polymers
Abstract
The use of supercritical carbon dioxide as a processing solvent for the physical processing of polymeric materials is reviewed. Fundamental properties of CO2/polymer systems are discussed with an emphasis on available data and measurement techniques, the development of theory or models for a particular property, and an evaluation of the current state of understanding for that property. Applications such as impregnation, particle formation, foaming, blending, and injection molding are described in detail including practical operating information for selected topics. The review concludes with some forward-looking discussion on the future of CO2 in polymer processing.
... Several review articles from the available literature have suggested PLA processing using CO2 under elevated pressure [2,5,16,17,19,25,26] and, as mentioned, almost all of them are focused on foaming. However, the present article is the first systematic review of the process mechanisms, fundamental properties, and up-to-date technologies of dry- with methods of PLA processing (Keywords: ("PLA" or "poly(lactic acid)") and ("supercritical CO 2 " or "high-pressure CO 2 " or "dense CO 2 ") and ("drying" or "foaming" or "impregnation" or "particle generation" on 15 November 2022). ...
... However, the present article is the first systematic review of the process mechanisms, fundamental properties, and up-to-date technologies of dry- with methods of PLA processing (Keywords: ("PLA" or "poly(lactic acid)") and ("supercritical CO 2 " or "high-pressure CO 2 " or "dense CO 2 ") and ("drying" or "foaming" or "impregnation" or "particle generation" on 15 November 2022). Several review articles from the available literature have suggested PLA processing using CO 2 under elevated pressure [2,5,16,17,19,25,26] and, as mentioned, almost all of them are focused on foaming. However, the present article is the first systematic review of the process mechanisms, fundamental properties, and up-to-date technologies of drying, foaming, impregnation, and particle generation applied for the processing of PLA using d-CO 2 and sc-CO 2 . ...
... For instance, Trindade Coutinho et al. [128] reported that the SSI process of 3 h enabled 2.6 times higher drug loading into PLA film compared with the conventional soaking method that lasted 10 days. An additional advantage of SSI is that the BCs' load and distribution through a polymer matrix can be easily tuned by changing the process conditions [15,25,129]. When sc-CO 2 leads to PLA plasticization and swelling, enabling the mobility of macromolecular chains and increased space, it favors the sorption of BCs [14,18,25]. ...
This review provides a concise overview of up-to-date developments in the processing of neat poly(lactic acid) (PLA), improvement in its properties, and preparation of advanced materials using a green medium (CO2 under elevated pressure). Pressurized CO2 in the dense and supercritical state is a superior alternative medium to organic solvents, as it is easily available, fully recyclable,
has easily tunable properties, and can be completely removed from the final material without postprocessing steps. This review summarizes the state of the art on PLA drying, impregnation, foaming, and particle generation by the employment of dense and supercritical CO2 for the development of new materials. An analysis of the effect of processing methods on the final material properties was focused on neat PLA and PLA with an addition of natural bioactive components. It was demonstrated
that CO2-assisted processes enable the control of PLA properties, reduce operating times, and require less energy compared to conventional ones. The described environmentally friendly processing techniques and the versatility of PLA were employed for the preparation of foams, aerogels, scaffolds, microparticles, and nanoparticles, as well as bioactive materials. These PLA-based materials can
find application in tissue engineering, drug delivery, active food packaging, compostable packaging, wastewater treatment, or thermal insulation, among others.
... The pore formation process in polymers during foaming with supercritical CO 2 is sketched in figure 2(a). It consist of the following steps [34][35][36]: (a) The polymer is saturated with supercritical CO 2 under constant temperature and pressure conditions until the solubility limit is reached, whereby T glass of the polymer decreases. (b) As the pressure is released, the solubility of CO 2 in the polymer decreases. ...
... Our observations indicate a transition between the formation of nanoporous inner structures combined with surface indentations (for 60-20 µm particles), towards the sole formation of the latter without a foamed core (for 10 µm particles). As CO 2 diffuses through the surface, its concentration in the surrounding matrix is depleted [35]. Therefore it is not available for further nucleation and pore formation in close proximity [35,37,40]. ...
... As CO 2 diffuses through the surface, its concentration in the surrounding matrix is depleted [35]. Therefore it is not available for further nucleation and pore formation in close proximity [35,37,40]. For small particles the CO 2 concentration might be depleted by the preferential void formation close to the surface to an extend that not enough gas is left to form a foamed core [37,40]. ...
Nowadays, titanium dioxide (TiO2) is the most commercially relevant white pigment. Nonetheless, it is widely criticized due to its energy-intensive extraction and costly disposal of harmful by-products. Furthermore, recent studies discuss its potential harm for the environment and the human health. Environment friendly strategies for the replacement of TiO2as a white pigment can be inspired from nature. Here whiteness often originates from broadband light scattering air cavities embedded in materials with refractive indices much lower than that of TiO2. Such natural prototypes can be mimicked by introducing air-filled nano-scale cavities into commonly used polymers. Here, we demonstrate the foaming of initially transparent poly(methyl methacrylate) (PMMA) microspheres with non-toxic, inert, supercritical CO2. The properties of the foamed, white polymeric pigments with light scattering nano-pores are evaluated as possible replacement for TiO2pigments. For that, the inner foam structure of the particles was imaged by phase-contrast X-ray nano-computed tomography (nano-CT), the optical properties were evaluated via spectroscopic measurements, and the mechanical stability was examined by micro compression experiments. Adding a diffusion barrier surrounding the PMMA particles during foaming allows to extend the foaming process towards smaller particles. Finally, we present a basic white paint prototype as exemplary application.
... Besides increasingly intensive research activity, some applications have been successfully applied at industrial scale, such as wood impregnation with antimould agents [2] or the water-free dyeing of textile fibers [3]. Several authors have reviewed the advances and opportunities in scCO 2 -assisted impregnation/deposition, some of them as a part of very comprehensive reviews on supercritical processing of polymers [4][5][6], or focused particularly on impregnation [7,8]. Some reviews on scCO 2 -assisted impregnation applied to specific fields, such as wood protection [9], biomedical and tissue engineering [10], implants [11], textile dyeing [12], and food packaging [13], among others, have also been published in past years. ...
... If the depressurization rate is low enough compared to the polymer relaxation rate, the swollen polymer tends to recover its original dimensions. However, if the plasticized polymer becomes glassy during this step, this process will be interrupted, and the polymer will remain totally or partially swollen [5,6]. ...
... Using a similar approach, Pantic et al. [114,115] studied the incorporation of liposoluble vitamins D 3 and K 3 in alginate aerogel particles. Interestingly, assays were performed not only at supercritical conditions, but also at subcritical temperatures (5,15, and 25°C), due to the thermal sensitivity of these vitamins and the prolonged impregnation times (up to 24 h). The authors observed that impregnation yield was higher and faster at near-and supercritical conditions, but the vitamin stability was higher when using liquid CO 2 . ...
The loading of polymeric carriers with active pharmaceutical and nutraceutical ingredients using supercritical CO2 (scCO2) as solvent and diffusion enhancer has received increasing attention as an alternative for the development of active materials and delivery systems. In this contribution, recent advances in scCO2 impregnation/deposition in the pharmaceutical, biomedical, and nutraceutical fields are reviewed, covering the period 2015-2021. The main physicochemical phenomena underlying the impregnation/deposition process (phase equilibrium, diffusion, plasticization, sorption, adsorption, etc.) are briefly presented and discussed. Applications are reviewed including drug and botanical drug products, medical devices, and dietary supplements. The effects of different process variables on the impregnation efficiency and some relevant properties of the obtained materials are also critically discussed and compared.
... Supercritical carbon dioxide (scCO 2 ) has been mostly used as an available, cheap and non-toxic 33 physical foaming agent in many polymers [6], [7],[8], [9]. Moreover, its critical point is rather "low" (31 34 °C and 7.4 MPa) so it is quite easy to reach the CO 2 critical state and take advantage of the 35 combination of gaseous and liquid properties: good diffusivity and a good solvent capability [10], [11]. ...
... Generally speaking, as observed in various polymers [7],[11], [19], the sorption curves may have 287 different behaviors upon pressure rise. Either the pressure curve is first linear and then it is levelling 288 off (existence of a plateau) or it exhibits a purely linear regime. ...
... Either the pressure curve is first linear and then it is levelling 288 off (existence of a plateau) or it exhibits a purely linear regime. Indeed Tomasko et al. [7] have shown 289 that depending on the pressure range, the CO 2 sorption is not always proportional to CO 2 pressure. 290 This is due to the variations of the Henry's coefficient (> ? ) in Henry's law (Equation 9): 291 = > ? ...
A FTIR (Fourier Transform Infrared) microscope combined to a CO2 high-pressure cell has been used to determine simultaneously the CO2 uptake and the swelling of acrylic polymer systems, precursors of CO2-foamed polymers (PMMA), a triblock copolymer (MAM) and a PMMA/10 wt% MAM blend. Samples were saturated with supercritical CO2 (scCO2) up to high pressures (5, 10, 30 MPa) at 40 °C and 130 °C. The behavior upon pressure increase is indeed different at the two temperatures in the three systems, showing either a linear or a plateau shape. These two regimes are analyzed in view of literature. Addition of 10 wt% MAM copolymer led to a significant increase of CO2 uptake (23% for neat PMMA to 42% for the blend, 40 °C, 30 MPa). Such data are useful to find out, before foaming, the best routes to produce low-density and nanoporous polymer foams, using scCO2 as foaming agent.
... scCO 2 offers mass transport advantages in comparison to other solvents, as the supercritical fluid provides "liquid-like" solubility and surface tension in tandem with "gas-like" diffusivity [19]. In polymer & composites processing, scCO 2 can plasticize polymers, enabling reductions in viscosity and glass transition temperature and enhancing mass transport within the polymer matrix [20,21]. These conditions can help improve the dispersion of fillers and additives within composite systems and enable reduction in processing parameters for high molecular weight polymers [20,21]. ...
... In polymer & composites processing, scCO 2 can plasticize polymers, enabling reductions in viscosity and glass transition temperature and enhancing mass transport within the polymer matrix [20,21]. These conditions can help improve the dispersion of fillers and additives within composite systems and enable reduction in processing parameters for high molecular weight polymers [20,21]. For example, the diffusivity of dimethyl phthalate in PVC is six orders of magnitude higher under supercritical conditions [20]. ...
... These conditions can help improve the dispersion of fillers and additives within composite systems and enable reduction in processing parameters for high molecular weight polymers [20,21]. For example, the diffusivity of dimethyl phthalate in PVC is six orders of magnitude higher under supercritical conditions [20]. scCO 2 is most effective at swelling and plasticizing amorphous polymers, or amorphous portions of a polymer [20,22]. ...
Natural fibers are a lightweight, carbon negative alternative to synthetic reinforcing agents in polymer composites. However, natural fibers typically exhibit lower mechanical performance than glass fibers due to weak interfacial adhesion between plant cells in the fiber and damage to the fibers during extraction from a plant stem. However, improvement of natural fiber mechanical performance could enable their wide-scale incorporation in structural composite applications, significantly reducing composite weight and carbon footprint. This study seeks to develop a novel, cost-effective method to significantly improve natural fiber stiffness via repair of damage caused by extraction and/ or stiffening of the weak cellular interfaces within a natural fiber. Supercritical fluids have been shown to be capable of swelling and plasticizing amorphous polymers, increasing additive absorption. In this work. supercritical-carbon dioxide (scCO2) was used as a solvent to assist with infusion of nanoparticles into flax fibers at pressures ranging from 1200-4000psi. Fiber analysis with Plasma Focused Ion Beam-Scanning Electron Microscopy (PFIB-SEM) showed that nanoparticles were capable of penetrating and bridging openings between cells, suggesting the ability for nanoparticle treatment to assist with crack repair. Additionally, treated fibers contained uniform surface coatings of nanoparticles, potentially reducing fiber porosity and modifying interfacial properties when embedded in a polymer matrix. Overall, this method of nanoparticle reinforcement of natural fibers could enable development of high-performance lightweight, low-carbon footprint composites for transportation or industrial applications.
... Polymer foam is a kind of solid/gas composite material characterized by a polymer matrix replete with numerous tiny foam holes, commonly referred to as porous polymer material. In contrast to unfoamed polymers, polymer foam exhibits many advantages, including low density, lower thermal conductivity, high impact strength, and lower dielectric constant, etc. [1][2][3][4]. Owing to their outstanding functional characteristics and low material cost, polymer foam can be widely used in some fields such as aircraft, automobiles, food packaging, sports equipment, insulation materials, and filter materials [5][6][7][8][9][10][11][12][13][14]. In recent years, the increased demand for polymer foam, coupled with its extensive applications, has driven the rapid development of the polymer foam industry. ...
... Moreover, Supercritical fluids are characterized by their chemical inertness, non-toxicity, and non-flammability [43]. The unique amalgamation of gas-like viscosity and liquid-like density found in the supercritical fluids renders them the outstanding solvents or plasticizers in polymer processing, such as polymer blending, polymer modification, particle production, polymer composites, polymer synthesis, and especially in microcellular foaming [2,[44][45][46][47]]. ...
In comparison with unfoamed polymers, polymer foams find extensive application in various civil and industrial fields such as packaging, sports equipment, absorbents, and automotive components due to their advantages of lightweight, high strength-to-weight ratio, excellent insulation properties, high thermal stability, high impact strength, toughness, and long fatigue life. The preparation of conventional polymer foam typically necessitates the incorporation of chemical foaming agents into the polymer, raising environmental issues, which pave the way for the utilization of supercritical fluids. Supercritical fluids exemplified by supercritical carbon dioxide or supercritical nitrogen, are renowned for their environmentally friendly and non-toxic characteristics, thus offering a viable alternative to conventional chemical foaming agents. Supercritical fluids exhibit gas-like diffusion and liquid-like density, offering excellent plasticization effects on polymer melts. This substantially reduces the melt viscosity, melting point, and glass transition temperature of the polymer, facilitating the preparation of uniformly distributed, smaller-sized, and higher-density microcellular foams. This review first provides an overview of the characteristics of supercritical fluids and commonly used supercritical fluid foaming agents. Subsequently, the dissolution, diffusion, and interactions of supercritical fluids in polymers were discussed, followed by a focused elucidation of the cell nucleation (homogeneous and heterogeneous) and growth (island model and cell model). Finally, the application of supercritical fluids in the foam manufacturing techniques is highlighted, including batch foaming, extrusion foaming, and injection foaming, while emphasizing the challenges that still exist in polymer foaming.
... We utilized the unique characteristics of a COC-CO2 system to realize the fusion of the layer and the assembly of the structure at 120 °C, ~60 °C below the glass transition temperature of the material. This approach [4] leverages the substantial CO2 solubility in COC and the resultant reduction in the glass transition temperature of the polymer, as well as increased polymer chain mobility [31,32] to realize fusion between the layers at a significantly lower temperature. Moreover, guided by thermodynamic analysis [33], a temperature slightly lower than the COC-CO2 bulk glass transition temperature (120 °C) was chosen to bond the structure, utilizing the understanding of a more severe glass transition temperature depression and greater chain mobility at the polymer surface [34]. ...
... We utilized the unique characteristics of a COC-CO 2 system to realize the fusion of the layer and the assembly of the structure at 120 • C,~60 • C below the glass transition temperature of the material. This approach [4] leverages the substantial CO 2 solubility in COC and the resultant reduction in the glass transition temperature of the polymer, as well as increased polymer chain mobility [31,32] to realize fusion between the layers at a significantly lower temperature. Moreover, guided by thermodynamic analysis [33], a temperature slightly lower than the COC-CO 2 bulk glass transition temperature (120 • C) was chosen to bond the structure, utilizing the understanding of a more severe glass transition temperature depression and greater chain mobility at the polymer surface [34]. ...
This paper discusses the fabrication and characterization of cyclic olefin copolymer (COC)-based pseudo-piezoelectric materials (piezoelectrets) with exceptionally high piezoelectric activity, and their potential use in sensing applications. Piezoelectrets that utilize a novel microhoneycomb structure to achieve high piezoelectric sensitivity are carefully engineered and fabricated at a low temperature using a supercritical CO2-assisted assembly. The quasistatic piezoelectric coefficient d33 of the material can reach up to 12,900 pCN−1 when charged at 8000 V. The materials also exhibit excellent thermal stability. The charge build-up in the materials and the actuation behavior of the materials are also investigated. Finally, applications of these materials in pressure sensing and mapping and in wearable sensing are demonstrated.
... The improvement in strength as fiber diameter decreases could be a result of a smaller diameter fiber containing fewer middle lamellae (weak pectins) or fewer defects throughout the fiber, resulting in a delay in crack initiation in comparison to thicker fibers [20,21]. Repair of damage to the fibers induced upon extraction or improvements to the mechanical performance of the middle lamella could improve the overall mechanical properties of a technical fiber Supercritical fluids, including supercritical carbon dioxide (scCO2) have been shown to be effective at swelling and plasticizing amorphous polymers, improving mass transport in the polymer matrix, and reducing polymer viscosity [24,25]. For example, polyvinyl chloride in scCO2 was previously shown to exhibit enhanced diffusivity of dimethyl phthalate, with transport of the additive increasing by over six orders of magnitude under supercritical conditions [24]. ...
... Repair of damage to the fibers induced upon extraction or improvements to the mechanical performance of the middle lamella could improve the overall mechanical properties of a technical fiber Supercritical fluids, including supercritical carbon dioxide (scCO2) have been shown to be effective at swelling and plasticizing amorphous polymers, improving mass transport in the polymer matrix, and reducing polymer viscosity [24,25]. For example, polyvinyl chloride in scCO2 was previously shown to exhibit enhanced diffusivity of dimethyl phthalate, with transport of the additive increasing by over six orders of magnitude under supercritical conditions [24]. In this work, the authors sought to leverage the unique interactions of scCO2 with amorphous polymers to improve technical fiber performance. ...
In recent years, consumer products have been increasingly utilizing sustainable materials to attempt to reduce the product’s carbon footprint. For example, the automotive industry has incorporated a variety of natural fiber polymer composites on vehicles in the last 20 years, including wheat straw in the Ford Flex and flax fibers on the Polestar Precept and the Porsche Cayman GT4 Clubsport. However, natural fibers exhibit lower strength and stiffness in comparison to synthetic reinforcing agents, such as glass fiber. In this work, the authors are developing a technique to improve the mechanical performance of flax fibers for use in structural composites. Supercritical fluids, including supercritical-carbon dioxide (scCO2), have been shown to swell and plasticize amorphous polymers, resulting in increased mass transport and absorption of additives. The weak intercellular region within flax fibers, commonly called the middle lamella, consists mainly of amorphous pectin. In this work, the authors hypothesize that scCO2 could be used to swell amorphous polymers in a fiber’s structure (e.g. pectin) and enable reinforcement with nanoparticles, resulting in fiber performance enhancement. Pectin films were created for proof-of-concept experiments and treated with scCO2 at pressures ranging from 1200-4000psi in the presence of titanium dioxide nanoparticles (TiO2). TiO2 nanoparticles were shown to be able to enter pectin films upon treatment with scCO2 for 24 hours. The same treatment process was used on dew retted, mechanically extracted flax fibers and after treatment for 24 hours, the average tensile strength of the fibers was improved by over 40%. Overall, this method of incorporation of nanoparticles within natural fibers could enable development of low-density, low-carbon footprint polymeric composites for use in structural industrial applications.
... Many applications of scCO 2 are related to its solubility and transport properties in polymeric systems [7,8], including polymerization [9], polymer foaming [10], or even membrane processes coupled with extraction methods [11]. More recent challenges lie in the development of a wide infrastructure for captured CO 2 , that requires the knowledge of the interactions between compressed CO 2 and pipelines, O-rings or liners materials [12]. ...
... RT ∇μ i Fick's law in terms of chemical potential gradient (8) By equating the two expressions and after some algebra one obtains: ...
The sorption and transport of CO2 in two polymers, Matrimid and PDMS, were modelled using data available across the critical region, at various temperatures and up to 18 MPa. The experimental trends show a complex behavior that is affected by the transition from gas-like to liquid-like density of CO2, as well as by the sorption-induced glass transition of the polymer. The Non Equilibrium Thermodynamics (NET-GP) approach for the solubility, coupled to its complementary tool for the permeability, the Standard Transport Model (STM), allows to represent thoroughly the complexity of CO2 sorption and permeation in this operative range with a self-consistent set of parameters. Furthermore, the model offers a deep insight in the swelling induced by CO2 in the different states of the polymers, and allows to decouple the kinetic and thermodynamic contributions to the transport phenomena in a meaningful way. This work takes a step forward in the understanding and simulation of the complex interactions between high pressure, supercritical CO2 and industrially relevant polymeric materials.
... Supercritical carbon dioxide (scCO 2 ) foaming is a novel technique for producing porous polymer materials under high-temperature and high-pressure conditions because of its low cost, wide availability, high diffusion rate, cell adjustability, and non-toxicity [12,13]. Following the saturation period, bubble nucleation, growth, and maturation occur through rapid pressure release, which increases the polymer volume several times [14][15][16]. Moreover, high-pressure scCO 2 foaming can be integrated with continuous extrusion foaming for large-scale fabrication of PA56 foams to improve efficiency, limit waste, and reduce costs. ...
Bio-based polyamide 56 (PA56) foams, characterised by ultrahigh compressive strengths and expansion ratios, were prepared using supercritical CO2 intermittent foaming technology. Chain-extended PA56/PA66 [CE(PA56/PA66)] was synthesised through the use of a chain extender (ADR, epoxy-based). An investigation was conducted into the influence of ADR and PA66 content on the rheological behaviour, mechanical properties, thermal properties, and foaming performance of the CE (PA56/PA66). The non-isothermal crystallisation behaviour of CE(PA56/PA66) was analysed using the Jeziorny, Mo, and Kissinger methods. The results demonstrated that the branched structure considerably improved the viscoelasticity and foaming properties of PA56, thus improving its cellular morphology. The addition of PA66 increased the crystallisation rate of the system and reduced its activation energy. PA66 promotes melt enhancement, crystallisation nucleation, and cell nucleation in the CE(PA56/PA66). At a foaming temperature of 255 °C, the foams produced from the branched PA56 with 20 phr PA66 exhibited smaller cell diameter (140 μm) and higher cell density (4.7 × 10⁶ cells/cm³). The compressive strength of the PA56 foam with 4 phr CE and 20 phr PA66 (0.68 MPa) was 119% higher than that of pure CEPA56 foam. This study provides an effective method for preparing a high expansion ratio and compressive strength of bio-based PA56 foam and an essential theoretical reference for its processing and moulding technology.
Graphical abstract
... Therefore, carbon dioxide can be transformed into a supercritical liquid once entering the reservoir, saving the energy required for its conversion. Thirdly, scCO 2 can be converted into gas form and expelled from the reservoir after fracturing, causing no damage to the rock formation and preventing expansion; thus, it is non-toxic, non-polluting, non-flammable, and recyclable [6]. As an emerging oil and gas production method, scCO 2 fracturing technology exhibits significant advantages in terms of environmental friendliness, efficiency, and adaptability. ...
Supercritical CO2 has wide application in enhancing oil recovery, but the low viscosity of liquid CO2 can lead to issues such as poor proppant-carrying ability and high filtration loss. Therefore, the addition of thickening agents to CO2 is vital. Hydrocarbon polymers, as a class of green and sustainable materials, hold tremendous potential for acting as thickeners in supercritical CO2 systems, and PVAc is one of the best-performing hydrocarbon thickeners. To further improve the viscosity enhancement and solubility of PVAc, here we designed a novel polymer structure, PVAO, by introducing CO2-affine functional groups to PVAc. Molecular dynamics simulations were adopted to analyze viscosity and relevant solubility parameters systematically. We found that PVAO exhibits superior performance, with a viscosity enhancement of 1.5 times that of PVAc in supercritical CO2. While in the meantime, PVAO maintains better solubility characteristics than PVAc. Our findings offer insights for the future design of other high-performance polymers.
... High-pressure fluids demonstrate outstanding penetration capabilities, particularly in a supercritical state. These fluids exhibit dissolving properties akin to liquids, coupled with a gas-like viscosity and diffusion coefficients, rendering them soluble in the majority of polymers [86]. To prevent induced foaming inside the high-pressure vessel during the pressure relief process, the saturation temperature is usually set much lower than the softening temperature of the polymer (typically between 25 • C and 60 • C), and the saturation pressure should not be too high (typically between 3 MPa and 10 MPa). ...
This review introduces an innovative technology termed “Micro-Extrusion Foaming (MEF)”, which amalgamates the merits of physical foaming and 3D printing. It presents a groundbreaking approach to producing porous polymer fibers and parts. Conventional methods for creating porous materials often encounter obstacles such as the extensive use of organic solvents, intricate processing, and suboptimal production efficiency. The MEF technique surmounts these challenges by initially saturating a polymer filament with compressed CO2 or N2, followed by cell nucleation and growth during the molten extrusion process. This technology offers manifold advantages, encompassing an adjustable pore size and porosity, environmental friendliness, high processing efficiency, and compatibility with diverse polymer materials. The review meticulously elucidates the principles and fabrication process integral to MEF, encompassing the creation of porous fibers through the elongational behavior of foamed melts and the generation of porous parts through the stacking of foamed melts. Furthermore, the review explores the varied applications of this technology across diverse fields and imparts insights for future directions and challenges. These include augmenting material performance, refining fabrication processes, and broadening the scope of applications. MEF technology holds immense potential in the realm of porous material preparation, heralding noteworthy advancements and innovations in manufacturing and materials science.
... 20 Generally, dissolving gas affects viscosity, gas diffusivity, and gas-polymer interfacial tension, properties that are important in the foaming process. 21 Unlike CO 2 , the relatively low solubility of N 2 in TPU required that the process be operated at elevated pressure to increase the solubility. 22 While the increase in processing pressure also increases the operation cost, there may also be some advantages. ...
Fabricating low-density elastomeric foams with a homogenous cell structure has been challenging because of foam shrinkage. The low modulus of the elastomers and the high diffusivity of blowing agents lead to foam shrinkage and poor surface quality. Using N 2 as a physical blowing agent (PBA) is an eco-friendly foaming method to overcome foam shrinkage. In this study, PTMEG-MDI/BD-based TPU with a hardness of 85A, 90A, and 95A were foamed using N 2 . Nitrogen as a blowing agent created a homogeneous cell structure with an expansion ratio of more than five times, an average cell size of less than 25 μm, and a cell density of more than 10 ⁹ cells/cm ³ . The shrinkage ratio and cellular morphology of CO 2 - and N 2 -blown TPU foam were compared at their maximum expansion ratio. At the maximum expansion ratio, the N 2 -blown foams exhibited a fine cell structure with a shrinkage ratio of less than 4%. However, the CO 2 -blown TPU foam showed a shrinkage ratio of 12.7% with significant cracks in the sample. The results demonstrate that using N 2 as a blowing agent can achieve an expansion ratio similar to that of CO 2 -blown foam, and the shrinkage problems of elastomeric foams can be significantly eliminated, which may help to maintain the physical properties of the foam.
... Ethylene is a plant hormone that regulates climacteric fruit ripening, and a number of studies have been conducted that analyze the ethylene profile during tomato ripening and its response to adverse environmental conditions like low oxygen, high temperature, etc. [3,5,6]. Among the promising technologies developed to regulate ethylene production or to maintain food quality, the use of CO 2 has attracted much attention due to its easiness to obtain, low cost [7,8], and long history of safe use for producing luscious and ripe fruits to be sold in grocery stores [9][10][11]. Research has found that CO 2 can either promote or inhibit ethylene production. ...
Tomatoes are a perishable and seasonal fruit with a high economic impact. Carbon dioxide (CO2), among several other reagents, is used to extend the shelf-life and preserve the quality of tomatoes during refrigeration or packaging. To obtain insight into CO2 stress during tomato ripening, tomatoes at the late green mature stage were conditioned with one of two CO2 delivery methods: 5% CO2 for 14 days (T1) or 100% CO2 for 3 h (T2). Conventional physical and chemical characterization found that CO2 induced by either T1 or T2 delayed tomato ripening in terms of color change, firmness, and carbohydrate dissolution. However, T1 had longer-lasting effects. Furthermore, ethylene production was suppressed by CO2 in T1, and promoted in T2. These physical observations were further evaluated via RNA-Seq analysis at the whole-genome level, including genes involved in ethylene synthesis, signal transduction, and carotenoid biosynthesis. Transcriptomics analysis revealed that the introduction of CO2 via the T1 method downregulated genes related to fruit ripening; in contrast, T2 upregulated the gene encoding for ACS6, the enzyme responsible for S1 ethylene synthesis, even though there was a large amount of ethylene present, indicating that T1 and T2 regulate tomato ripening via different mechanisms. Quantitative real-time PCR assays (qRT-PCR) were used for validation, which substantiated the RNA-Seq data. The results of the present research provide insight into gene regulation by CO2 during tomato ripening at the whole-genome level.
... The two-component system containing Sc-CO 2 has various practical functions and hence, to have information of phase equilibria of these mixtures is indispensable. So, the SCFs methodology been utilized to numerous industrial practices trade with various polymer such as anti-solvent precipitation, fine particles creation and polymerization (McHugh and Krukonis, 1994;Cooper, 2000;Tomasko et al., 2003). The lab-made examinations have been constantly published by Byun and their team people. ...
The solubility information of fluoro-monomer (meth) acrylate in organic solvents is an important factor affecting their use in numerous engineering practices. This study investigated the phase equilibria of 1H, 1H-perfluorooctyl acrylate (PFOA) and 1H, 1H-perfluorooctyl methacrylate (PFOMA) in supercritical CO2 using optical fiber and contact lenses. The solubility curves investigations were conducted at different temperatures (313.2 to 393.2) K and pressures (3.31 to 16.84) MPa, and the mole fraction of (0.032 to 0.630). Results revealed that the PFOA + SC-CO2 and PFOMA + SC-CO2 systems exhibited a type-I behavior. The RMSD (%) for the PFOA + SC-CO2 [kij = 0.075, ηij = 0.0], and PFOMA + SC-CO2 [kij = 0.075, ηij = 0.0] models using two factors determined at 353.2 K evaluated with the alterable parameters at each T were 5.08 %, and 5.36 %, respectively. The correlation of the experimental response for the PFOA + SC-CO2 and PFOMA + SC-CO2 two component models were examined using Peng-Robinson (PR) equation of state (EOS) involving two parameters (kij, ηij) base on a fluid mixture rule. Additionally, the critical properties (pc, Tc and ω) and vapor pressure of PFOA and PFOMA were assessed using the Joback-Lyderson group impact.
... pressure was 10 MPa for all the materials. Supercritical CO2 can significantly reduce both the bulk and surface glass transition temperature of the polymer, as well as increase the interfacial wetting, diffusion and randomization to forge bonding interface, enabling polymer films easy to be bonded together [53,54]. This approach enables the porous structure to be assembled with the structural features largely preserved. ...
... Carbon dioxide impregnates the polymer matrix to a different degree, depending on various factors, such as experimental conditions and polymer type [74]. Upon depressurization, CO 2 that has already dissolved in the polymer matrix can become supersaturated and nucleate bubbles, which induces foam or minor defects in the polymer structure [105][106][107][108]. Induced bubble formation, growth, and foaming are methods widely used in polymer processing for various applications, such as creating a porous structure into polymers, and for drug loading [32,73,74,109,110]. ...
Surface cleaning of plastic materials of historical value can be challenging due to the high risk of inducing detrimental effects and visual alterations. As a result, recent studies have focused on researching new approaches that might reduce the associated hazards and, at the same time, minimize the environmental impact by employing biodegradable and green materials. In this context, the present work investigates the effects and potential suitability of dense carbon dioxide (CO2) as an alternative and green solvent for cleaning plastic materials of historical value. The results of extensive trials with CO2 in different phases (supercritical, liquid, and vapor) and under various conditions (pressure, temperature, exposure, and depressurization time) are reported for new, transparent, thick poly(methyl methacrylate) (PMMA) samples. The impact of CO2 on the weight, the appearance of the samples (dimensions, color, gloss, and surface texture), and modifications to their physicochemical and mechanical properties were monitored via a multi-analytical approach that included optical microscopy, Raman and attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopies, and micro-indentation (Vickers hardness). Results showed that CO2 induced undesirable and irreversible changes in PMMA samples (i.e., formation of fractures and stress-induced cracking, drastic decrease in the surface hardness of the samples), independent of the conditions used (i.e., temperature, pressure, CO2 phase, and exposure time).
... Moreover, because each polymer system has a unique chemical structure and can display, for instance, different molecular weights (the latter can impact inclusively the melting and/or glass transition temperature), each system requires a rather peculiar processing pathway. Therefore, most of the polymer processing techniques are often based on a multitude of physico-chemical approaches 102,[105][106][107][108][109][110][111][112][113][114][115] and depend on whether polymer chains are being processed in the solid state, solutions or films. ...
Most of the technological applications that shape the development of our society and lifestyle are based on polymers. Whether we are talking about the automotive and aviation industry, packaging and adhesive materials, insulator and optoelectronic applications, medical devices and antibacterial/drug delivery solutions, environment and cosmetics, just to name a few, polymers have their valuable and well-established place. This is inclusively due to their low cost and straightforward processability. The latter is a key to the pathway towards future, novel polymer-based technologies due to its massive impact on the structure-property relationship. Therefore, it is critical to understand which strategies and processing techniques can be improved and further employed to efficiently alter the microstructure of specific polymers and thus, to control and enhance their final properties. In this work, we classify and analyze the most prominent processing techniques that have been recently developed and used in research and industrial laboratories to efficiently tune and manipulate, on nano-, micro- and macro-scales, the molecular arrangements and packing of, especially, conjugated polymers in films, solutions and solid state.
... One of the important factors for most amorphous polymers is the reduction of the glass transition temperature (Tg), as Tg characterizes the phase change from the glass to rubber transition. By principle, the decrease in Tg can be stated as a thermodynamic effect, not a hydrostatic pressure effect, because of the intermolecular interactions between CO 2 and the polymer (Tomasko et al. 2003) During expansion, an increase in Tg is seen with the evaporation of CO 2 at a temperature at which the structure is solidified without further expansion (Sauceau et al. 2011a). With the injection of CO 2 as a plasticizer during the extrusion process, a decrease in glass transition temperature (Tg) with a reduction in viscosity of many polymers occurs without altering their pseudoplastic behavior, which is not present in the final formulations (Chauvet, Fig. 6. ...
Hot-melt extrusion (HME) technology is one of the primary approaches that has been implemented in recent years to overcome poor drug solubility/dissolution issues through the development of solid dispersion systems. Carbon dioxide (CO2) either in supercritical (SupC) or subcritical (SubC) forms has been introduced to HME as a temporary plasticizer, reducing the operating temperature and eventually processing heat-sensitive molecules more efficiently. In this paper, a comprehensive review of CO2-HME processes focused on pharmaceutical polymers and applications is presented. The steps and requirements for the setup of experimental devices are demonstrated, with a detailed influence of CO2 characteristics on HME processes. The most relevant physical and chemical properties of pharmaceutical grade polymers subjected to the CO2- HME process are described. The basic knowledge and main mechanisms of HME process parameters in conjunction with CO2 concentration with regard to process feasibility and final product formation are discussed. HME coupled with CO2 is extensively reviewed to provide a complete understanding of how to optimize the process parameters and conditions to reach optimized characteristics of final outcomes, as well as the sequential relationship between those outcomes (foaming → porosity → milling → tableting). Pharmaceutical applications of CO2-based HME are presented in detailed case studies, including extrusion feasibility, solubility, dissolution rate enhancement, and gastroretentive or floating drug delivery. Finally, the current status of general CO2-based techniques, as well as future perspectives and opportunities for promising applications through the integration of CO2 with HME is presented.
... CO 2 is a gas with low toxicity and is being used in the food [1] and pharmaceutical industries [2]. CO 2 is known to specifically dissolve in polymers, and research on the mixture of polymers and CO 2 has been actively conducted in relation to the polymer process [3] and polymer synthesis [4]. The dissolution of CO 2 in solid [5] or melted polymers [6] has been used in the foaming process. ...
Carbon dioxide (CO2)-assisted polymer compression method is used for plasticizing polymers with subcritical CO2 and then crimping the polymer fibers. Given that this method is based on crimping after plasticization by CO2, it is very important to know the degree of plasticization. In this study, heat treatment was gently applied on raw material fibers to obtain fibers with different degrees of crystallinity without changing the shape of the fibers. Simultaneously, two types of sheets were placed in a pressure vessel to compare the degree of compression and the degree of hardness. Furthermore, a model was used to derive the relative Young’s modulus of porous materials composed of polymer fibers with different degrees of crystallinity. In the model, the amount of strain was calculated according to the Young’s modulus as a function of porosity and reflected in compression. Young’s modulus of porous polymers in the presence of CO2 has been shown to vary significantly with slight differences in crystallinity, indicating that extremely low crystallinity is significant for plasticizing the polymer by CO2.
... Description of DSC thermogram of the PVP and HPMC has been reported in literatures previously [85][86][87][88]. Diffusion of sc-CO 2 into the PVP and HPMC polymers and interaction with their basic sites led to their heavy plasticization and swelling, altering their physical and mechanical properties [89]. The disappearance of the S-VPA endothermic peak at 240.7 • C confirmed the complete entrapment of the S-VPA particles into the structure of the polymer. ...
To improve the bioavailability and reduce the therapeutic dose and side effects of Sodium valproate (S-VPA), nanoparticles of this drug were loaded on the polyvinylpyrrolidone (PVP) and hydroxypropyl methylcellulose (HPMC) polymers via the supercritical solvent impregnation (SSI) process. This process was carried out at different pressures (150–250 bar), temperatures (308–328 K), and impregnation times (60–180 min). The Box–Behnken design (BBD) method was applied to study the effect of operational parameters on the S-VPA/PVP and S-VPA/HPMC loadings and optimize the condition. Maximum loadings on both PVP (1.56%) and HPMC (1.5%) were achieved at 250 bar, 318 K, and 180 min. Moreover, the impregnated samples (S-VPA/PVP and S-VPA/HPMC) were investigated by the FTIR, DSC, SEM, DLS, and XRD analysis. The S-VPA nanoparticles with the mean particle size of 27 nm and 128 nm were uniformly impregnated on the PVP and HPMC polymers, respectively. The results confirmed the lower crystallinity of the impregnated S-VPA nanoparticles, leading to their higher solubility and bioavailability. Finally, Enhanced S-VPA solubility was approved by comparing the dissolution rate of the original and the impregnated S-VPA samples in the optimum conditions.
... Similarly, its ability to promote cement hydration increased with alkali-activated materials, increasing pores. ScCO 2 was used as a foaming agent for polymer foaming due to its unique properties [51,52]. It was generally mixed with a polymer to form a fluid under supercritical conditions, and pores were generated due to the thermodynamic instability when the subsequent pressure was reduced [53]. ...
Geothermal energy is a clean and renewable energy that can be extracted by energy piles. Low density filling materials with relatively high thermal conductivity are ideal for energy pile construction, due to the low cost and high heat exchange efficiency of filling materials. In this study, three types of low density filling materials (Class G foam cement, alkali-activated foam cement, and supercritical CO2 (ScCO2) modified alkali-activated foam cement) were tested to evaluate their performances to serve as candidate filling materials for energy piles. The thermal conductivities of the three materials were measured and showed that the minimum thermal conductivity was 0.22 W·mK⁻¹ (foam cement without slag), and the maximum thermal conductivity was 0.32 W·mK⁻¹ (foam cement with 5 % CO2 modified slag). By analyzing the cement hydration products and distribution of pores, adding slag improved the hydration of cement, which caused a longer foaming time and increased the porosity of samples. ScCO2 modification of slag increased the porosity and the thermal conductivities of the samples. In summary, slag with ScCO2 modification can promote the thermal conductivity of foam cement, which is beneficial to the application of shallow energy piles.
... One of the possible reasons for this behavior is the higher viscosity of pressurized propane (69.180 μPa.s at 50 bar / 65 • C and 95.428 μPa.s at 200 bar / 65 • C) in comparison to CO 2 (56.101 μPa.s at 200 bar / 65 • C) [13]. Besides, it is important to mention that pressurized CO 2 is reported to act as a plasticizer for polymers, reducing its viscosity [63,64]. As mentioned in the previous sections, it was possible to observe through the sapphire window that the high viscosity of the system can affect the mass transfer processes and reduce the collision frequencies between monomer and enzyme molecules, leading to smaller monomer conversion and reaction yield values. ...
Here, the synthesis of polyglobalide (PGl) by enzymatic ring-opening polymerization (e-ROP) is investigated, using pressurized carbon dioxide (CO2), pressurized CO2 + dichloromethane (DCM), and pressurized propane as solvents. Particularly, the effects of phase equilibrium on the course of e-ROP and PGl final properties are discussed. The partition coefficients of CO2, DCM, propane, globalide and PGl were calculated with help of thermodynamic models, providing proper understanding of monomer partitioning in the reaction system. Reactions performed in pure CO2 resulted in monomer conversions of 100%. Besides, when only one liquid phase was present inside the reactor, PGl samples presented low polydispersities and high average molecular weights. When carried out in CO2 + DCM, e-ROP resulted in lower monomer conversions and PGl samples with higher polydispersities and lower average molecular weights. Finally, reactions carried out in pressurized propane (200 bar) produced PGl samples with the highest average molecular weights among the analyzed products.
... This fact should translate to a higher nucleation rate in PLA. Nevertheless, in our study, the PLA film reached a molten viscous state due to the use of a saturation temperature near the PLA melting temperature (154 • C), which could dramatically decrease the viscosity of PLA and enhance cell nucleation [36,37]. Moreover, the differences in the pore average diameter should mainly be a consequence of the different scCO 2 densities and pressure drop used in each study. ...
Microcellular nanocomposite foams functionalized with cinnamaldehyde (Ci) were obtained through two-step supercritical foaming and impregnation processing. PLA nanocomposite foams with different C30B concentrations (1, 2, and 3 wt.%) were obtained by foaming with scCO2 at 25 MPa and 135 °C and impregnated with Ci at 12 MPa and 40 °C. The effect of the C30B content and Ci incorporation on the morphological, structural, thermal, and release properties of the developed foams were investigated. The incorporation of Ci was not influenced by C30B’s addition. The presence of C30B and Ci incorporation reduced the average pore diameter slightly and the crystallinity degree of the foams extensively. Simultaneously, the experimental and theoretical characterization of the Ci release from the PLA nanocomposite foams in EtOH 50% was analyzed. The mechanism of Ci release from the foams was defined as a quasi-Fickian diffusion process that could be successfully described using the Korsmeyer–Peppas model. The active PLA foams presented a higher potential of migration and faster release when compared with that reported in commonly used PLA films, showing that biopolymeric foams could be potentially used as active food packaging to improve the migration of active compounds with low migration potentials in order to improve their biological activity in foods.
... Thus these genetic modifications have been shown to improve the metabolic processes of HCHO oxidation to CO 2 , can be widely applied to obtain any product of interest or eliminate formaldehyde and obtain CO 2 that is widely applied in the processing of different polymers (Tomasko et al., 2003;Wang et al., 2011) ...
Breakthrough in obtaining higher performance in triacylglycerols and obtaining different chemicals, such as alcohols, alkanes, carboxylic acids and fatty acids; commodity chemi cals, such as monoalcohols, diols, carboxylic, and dicarboxylic acids; and biopolymers. Several micro organisms have been genetically manipulated to produce desirable molecules, for application in biofuels, biohydrogen, and chemicals. This chapter describes the advances made in the genetic manipulation for obtaining different biochemical and biofuels, in models of microorgan isms. It also discusses the perspective of using the enzymes involved in the biosynthesis pathway of the different compounds in biocatalysis in order to make efficient the production of compounds of interest.
Enzymatic hydrolysis of semicrystalline poly(ethylene terephthalate) (PET) is hindered by the hydrophobic nature and crystallinity of the substrate, and it highly depends on the available interfacial area between substrate and aqueous phase. While most studies leverage particle size reduction to increase interfacial area, this study investigates the use of supercritical CO2 (scCO2) to increase internal surface area in PET, and its impact on the enzymatic hydrolysis yields. Our work shows that scCO2 pretreatment of semicrystalline PET resulted in up to 2-fold higher terephthalic acid (TPA) yield relative to the untreated counterpart using Humicola insolens cutinase (HiC) enzyme. There is a positive correlation between the total pore surface area in the scCO2-pretreated PET samples and the final TPA yield. In addition, preliminary kinetic studies revealed faster initial production of TPA for scCO2-treated PET relative to untreated PET. ScCO2-treated PET samples showed no significant changes in the crystalline content and thermal properties. However, NMR data indicated that scCO2-treated PET has a slightly higher apparent number-average molecular weight (Mn) relative to that of untreated PET. Overall, scCO2 pretreatment led to increased semicrystalline PET susceptibility to HiC enzyme action, resulting in increased TPA yields.
Polyimide foams (PIFs) are usually synthesized by solution polymerization, followed by chemical foaming to prepare thermosetting foam. In this research, lightweight microcellular thermoplastic polyimide foams (TPIFs) were fabricated via a novel two-step foaming approach using supercritical carbon dioxide (scCO2) as a blowing agent. The poly(amic acid) (PAA) and polyester ammonium salt (PEAS) precursor solutions were synthesized with pyromellitic dianhydride (PMDA) as dianhydride reagents and polyether amine Jeffamine D230 as aliphatic diamine reagent via blending and solution polymerization, respectively. The solution polymerization process demonstrated a higher molecular weight and superior formability than the blending process. The optimum thermal imidization temperature of 200 ◦C was optimized via the thermal and rheological property analysis. The cell morphology and mechanical properties of the TPIFs could be turned by varying the saturation time, foaming pressure, and imidization temperature. At a thermal imidization temperature of 200 ◦C, the TPIFs exhibited a branched structure with a small mean cell diameter (123.78 μm), and a high compressive strength (0.4 MPa) under 10 % strain at high temperature and pressure, which was more than ten times that of the TPIFs with thermal imidization temperature of 130 ◦C. This research provides a feasible method for producing high volume
expansion ratio TPIFs with adjustable microcellular structures and outstanding mechanical properties.
The interest in bio‐based alternatives to classical polyesters such as poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT) is steadily growing to achieve a more sustainable approach to polymer materials. In this study, PBT/poly(butylene furanoate) (PBF) blends are prepared, characterized and extrusion foamed. PBF as a bio‐based polyester offers two advantages. The ecological footprint of the material is reduced, and additionally, it can be used in Diels‐Alder reactions at the blend surface to support fusion of the foamed beads. The blending behavior of the polyesters is investigated using samples prepared in a microcompounder, particularly focused on the miscibility of the blends and transesterification reactions. The blends are thermodynamically immiscible but show a certain degree of transesterification according to nuclear magnetic resonance (NMR) spectroscopy. The morphology of blend beads produced by an extrusion foaming process is analyzed regarding their cell density, cell size distribution, and open‐cell content. It is shown that PBF has a positive effect on the bead foam morphology. The use of a bifunctional linker designed for chemical fusion of the bead surfaces allows to obtaining of molded parts, in contrast to beads containing pure PBT.
Due to the low melt strength, PET foam cells easily grow, and due to the low CO2 concentration in PET, generating more nucleation sites during the foaming process is difficult. As a result, PET foam produced using the supercritical CO2 foaming method possesses a large cell size and a low expansion ratio, which leads to poor mechanical performance. In addition, there has not been much research on how to improve CO2 concentration in PET. In this work, the PC with higher melt strength and CO2 absorption capacity was incorporated into the PET matrix to improve the foaming behaviors of the blend. The results showed that the melt strength and CO2 concentration of PET/PC blends are much higher than that of pure PET. For instance, the melt strength and CO2 concentration of the PET/PC10 blend would be more than twice that of pure PET when the PC content is only 10%wt. By studying the foaming behavior, it was found that high melt strength inhibited the cell overgrowth and collapse of PET/PC foam, resulting in the foam with good cell structure was obtained. In addition, the high CO2 concentration in PET/PC blend is conducive to cell nucleation, thereby increasing the foam expansion ratio during the CO2 foaming process. Finally, the PET/PC blend foam with excellent mechanical properties was obtained. To be specific, the expansion ratio of the PET/PC blend foam was about 9.61, and the average cell size was only 40.38 μm. And the PET/PC blend foam possessed an excellent compressive strength of 7.41 MPa due to the strong interfacial bead cohesion, which showed the application prospect in the field of construction engineering.
As a polycrystalline polymer, isotactic polybutene-1 (iPB-1) will form different crystalline structures when it crystallizes under different conditions. In this work, we found a simple method to manipulate the cell structure of iPB-1 foams by controlling the annealing time of crystalline form II at room temperature and employing supercritical CO2 foaming. As the content of crystalline form II increases, the storage modulus (G′) and complex viscosity (|η * |) increase, and the solubility of CO2 in iPB-1 increases. Crystalline form II has a wider foaming temperature window compared to crystalline form I and exhibits a more uniform cell structure at temperatures between 95 °C and 125 °C. The average cell diameters of crystalline form II and I are 33.9 µm and 11.2 µm at 115 °C and 15 MPa CO2, respectively. A small amount of crystalline form I acts as a heterogeneous nucleation agent during the foaming process, resulting in the formation of a “petal-like” cell structure. During the foaming saturation process, the melted portion of iPB-1 with unstable crystalline form II transforms into crystalline form I′ under high-pressure CO2, while the unmelted portion transforms into crystalline form I. As the pressure increases, the cell structure of iPB-1 undergoes a transition from “petal-like” to bimodal cell structure, ultimately achieving a more uniform structure. Moreover, increasing foaming pressure can also change the cell structure of the foamed material from closed-cell to open-cell. The occurrence of bimodal melting peaks in the foamed samples is beneficial for the adhesion of beads during the molding process.
In this chapter industrial scale applications based on solvent properties of supercritical fluids are described in first sub-chapter. Thermodynamic fundamentals of extraction processes of solids and liquids with supercritical fluids are presented. Different operating modes of extraction units as well as different separation strategies are presented. Energy consumption and design considerations for design of industrial scale extraction units for solids extraction are given. A special part of sub-chapter is devoted to ultrahigh pressure extraction where operating pressure in extraction process is more than 1000 bar. Benefits of ultrahigh pressure extraction are presented in details. Scale-up of different extraction processes which are realized on industrial scale are presented. In the second sub-chapter processes for polymer particles production as well for polymer foaming are presented. Fundamental data for design and scale-up of Crystallization, Rapid expansion of supercritical solution—RESS, Gas anti-solvent—GAS and Particles from gas saturated solution—PGSS™ processes are reviewed.KeywordsSupercritical fluidExtractionSolidLiquidPolymerParticle formationPolymer foamingCrystallizationRESSGASPGSS™
The continuity of conductive network and conductivity performance in double-percolated conductive polymer composites (CPCs) can be improved after static annealing, though, there are often increased viscosities in CPCs when filler contents increased, making annealing process difficult. Herein, the incompatible polycaprolactone (PCL)/polystyrene (PS) systems were designed to double-percolated PCL/PS/multi-wall carbon nanotube (MWCNT) composites, subsequently, thermal and plasticizing effects during carbon dioxide (CO2) annealing were combined to assist phase coalescence. Benefiting from viscosity reduction, phase coarsening, electrical conductivities and electromagnetic interference shielding effectiveness (EMI SE) of PCL/PS/MWCNT composites were promoted even at 5 wt. % filler content. After supercritical CO2 annealing, percolation threshold decreased from 0.50 wt. % to 0.24 wt. %, and EMI SE increased from 31.8 dB to 39.8 dB, which improved 25 %. This work demonstrates phase coarsening of double percolated CPCs is effectively controlled by CO2 annealing conditions, and the final degree of performance improvement is connected to polymer systems.
Polycarbonate-based thermoplastic polyurethane (TPU) is an advanced thermoplastic elastomer that has excellent yellowing, oxidation and corrosion resistances, as well as outstanding mechanical properties. Polycarbonate-based TPU foams have potentially important applications in sport goods, biomedical, and chip polishing fields. In this study, two microcellular foaming strategies including Heating-Foaming (H-foaming) and Cooling-Foaming (C-foaming) with carbon dioxide as blowing agents were proposed to prepare structure-tunable polycarbonate-based TPU foams. Firstly, TPU foams with similar cellular morphology were prepared by H-Foaming and C-Foaming, and they were used to explore the effect of foaming strategy on the cyclic compression properties of the TPU foam. Secondly, TPU foams with the same expansion ratio but with different cell sizes were prepared by H-Foaming, which were further used to explore the effect of cell sizes on the cyclic compression properties of the TPU foam. Thirdly, TPU foams with similar cell size but with different expansion ratios were prepared, and they were used to explore the dependences of the cyclic compression properties on the expansion ratio of TPU foams. It was found that the compression strength of the TPU foam prepared by C-Foaming can be enhanced by more than 15%, in comparing with that of the TPU foam prepared by H-Foaming. Furthermore, it was demonstrated that the expansion ratio of TPU foam is the key structural factor in determining its compression strength. The compression strength reduced from 5.91 to 0.17 MPa with the expansion ratio increasing from 0 to 7.2. Reducing cell size leads to enhanced compression strength but deteriorated compression resilience. Increasing expansion ratio will firstly deteriorate and then benefit the compression resilience.
Dissolving gas in polymer caused them to plasticize and induced crystallization. As a result, the glass transition temperatures (Tg) and melting points (Tm) decreased and crystallinity changed. This study investigated the effect of N2 dissolution on the thermal behavior of polyether-based thermoplastic polyurethanes (TPUs). Although the affinity between the polymers and N2 is not strong, the solubility of N2 in TPU is below 3 wt% at pressures below 10 MPa and temperatures in the range of 190 °C~210 °C. A unique phenomenon is reported. Dissolving N2 in TPU reduced the melting point but decreased crystallinity.
A weight-loss-based test showed that the N2 sorption increased with increased soft segment (SS) content. Annealing with N2 showed an apparent plasticizing effect on TPU with high SS content. The melting peak of imperfect ordered crystals, around 100 °C, completely disappeared. This research contributes to understanding the impact of N2 plasticization on TPU crystallization and the development of nitrogen foaming technology.
Facilitating cell ingrowth and biomineralized deposition inside filaments of 3DP scaffolds are an ideal bone repair strategy. Here, 3D printed PLGA/HA scaffolds with hydroxyapatite content of 50% (P5H5) and 70% (P3H7) were prepared by optimizing 3D printing inks, which exhibited good tailorability and foldability to meet clinical maneuverability. The supercritical CO2 foaming technology further endowed the filaments of P5H5 with a richer interconnected pore structure (P5H5-C). The finite element and computational fluid dynamics simulation analysis indicated that the porosification could effectively reduce the stress concentration at the filament junction and improved the overall permeability of the scaffold. The results of in vitro experiments confirmed that P5H5-C promoted the adsorption of proteins on the surface and inside of filaments, accelerated the release of Ca and P ions, and significantly upregulated osteogenesis (Col I, ALP, and OPN)- and angiogenesis (VEGF)-related gene expression. Subcutaneous ectopic osteogenesis experiments in nude mice further verified that P5H5-C facilitated cell growth inside filaments and biomineralized deposition, as well as significantly upregulated the expression of osteogenesis- and angiogenesis-related genes (Col I, ALP, OCN, and VEGF) and protein secretion (ALP, RUNX2, and VEGF). The porosification of filaments by supercritical CO2 foaming provided a new strategy for accelerating osteogenesis of 3DP implants.
Nanoscale foaming using nucleating agents was modeled based on coarse-grained molecular dynamics simulations to analyze the nucleation and growth of nanocellular foams. Interactions between the polymer chains and nucleating agents, the blowing rate, and the number of nucleating agents were varied, and foam formation was analyzed. Snapshots of the simulated foamed structures were obtained and characteristic parameters—such as the number of foams and the size distribution of the foams—were estimated. In the case of weak attractive interactions between the polymers and nucleating agents, foams formed first at the interfaces of the polymers and nucleating agents, then formed in the polymer matrix. In the case of strong attractive interactions, foams originated only in the polymer matrix. These nucleation-type foams depended on the number and size of the foams, and on their growth and coalescence. Our simulations indicate that the foamed structures can be modulated by the following parameters: interactions between the polymer chains and nucleating agents, the blowing rate, and the number of nucleating agents.
In foaming processes, the blowing agent has a significant influence on the material behaviour and the necessary processing parameters. Low-density polypropylene foam sheets are usually produced with aliphatic hydrocarbons or alkanes as physical blowing agent. Due to the necessary safety precautions and the environmental impact, there is great interest in using alternative blowing agents such as CO 2 . The sole use of CO 2 often leads to corrugation, open cells or surface defects on the foam sheet and therefore requires modifications to the process technology. For this reason, blowing agent mixtures based on CO 2 and organic solvents are used for the production of foam sheets. For developing a process model describing the melt flow in the extrusion die and the formation of cells, specific material data like diffusion coefficients are necessary. For CO 2 and N 2 as sole blowing agent, experimental data exist in the literature. Since no experimental data are available for co-blowing agents such as ethanol at elevated temperatures as they occur in the foam process, these data were calculated using molecular dynamics (MD) simulations. The benefit of MD simulations lies in their ability to reduce the experimental effort and, in particular, to provide data in cases where this data is not available through experimental measurements. The calculated diffusion coefficient values are compared to experimental data from the literature and presented for CO 2 , N 2 and ethanol in polypropylene. The calculated diffusion coefficients of CO 2 and N 2 are compared with literature results and agree well with them. For the ethanol molecules, the diffusion coefficient is compared relative to the both aforementioned ones considered the larger size of the ethanol molecule compared to N 2 and CO 2 . The results of the diffusion coefficients for ethanol are reasonable compared to the values found for the other two molecules.
Currently, the ‘sustainability’ and ‘renewability’ is most important aspects for the environment and for the living beings. In the chemical industry, the solvent plays a key role in the chemical processes. A large number of chemical processes are carried out in the presence of solvents. In many chemical industries, the volatile organic compounds (VOCs) are used as a solvent which have adverse effect on the environment and human health. As VOCs are hazardous, there is a need of replacement of these traditional, volatile organic solvent. Hence, there is a rising interest in non-volatile solvents. The nature of CO2 in its liquid and supercritical state as a solvent is explored. CO2 is abundant in the atmosphere. It can reach its critical states by raising the temperature and pressure greater than its critical value. Supercritical carbon dioxide (scCO2) is readily available. It is cheap, non-flammable, recyclable, and non-toxic. Supercritical fluids (SCFs) have both gaseous and liquid properties, which enable it to penetrate anything and to dissolve materials into their compound, respectively. In addition, supercritical CO2 and H2O form a solvent which is organic in nature. These supercritical fluids can be useful in a number of fields like synthesis of various materials, drug delivery, chromatography, processing of polymers, extraction, purification and separation, biomedical applications, etc. This chapter examines the usage of CO2 as a solvent to generate greener process and to develop different products.
When CO 2 is dissolved into a polymer, viscosity of the polymer is drastically reduced. In this paper, the melt viscosities of low-density polyethylene (LDPE)/supercritical CO 2 and polypropylene (PP)/ supercritical CO 2 solutions were measured using a capillary rheometer equipped at a foaming extruder, where CO 2 was injected into a middle of its barrel and dissolved into the molten polymer. While monitoring the dissolved CO 2 concentration on-line by a Near Infrared spectroscopy, the viscosity measurements were performed by varying the content of CO 2 in the range of 0 to 5.0 wt% and temperature in the range of 150 to 175 O C αfor LDPE and 170 to 200 O C for PP. 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/CO 2 solutions. The experimental results indicated that the viscosity was reduced down to 30 % of the neat polymer by dissolving CO 2 up to 5.0 wt% at temperature 150 O C for LDPE/CO 2 solution and it was depressed down to 40 % by 6.0 wt%CO 2 dissolution at 170 O C for PP/CO 2 solution. A mathematical model is proposed to predict viscosity reduction owing to CO 2 dissolution. The model was developed by combining the Cross-Carreau model with the Doolittle's equation in terms of free volume. Using the Sanchez-Lacombe equation of state and the solubility data measured by a magnetic suspension balance, the free volume fractions of polymer/CO 2 solutions were calculated to accommodate the effects of temperature, pressure and CO 2 content. The developed model can successfully predict the viscosities of LDPE/CO 2 and PP/CO 2 solutions from a PVT data of the neat polymers and CO 2 solubility data.
A modified fluid-lattice theory of fluids considering a finite quasi-lattice coordination number and a constant lattice site volume for all r-mers is developed. The theory is tested against experimental information on specific volumes, vapor pressures, orthobaric densities and heats of vaporization of pure components. Some common polymers and some typical polymer solvents have been chosen for testing the theory. The theoretical treatment has been extended to mixtures and tested against experimental data on volumes of mixing, heats of mixing, and χ interaction parameters of polymer solutions. Results have been compared with those obtained with a two-parameter new Flory theory and with the three-parameter new Huggins theory. Both for pure components and for mixtures, the effect of introducing a non-random quasi-chemical correction is discussed.
An equation of state for associating liquids is presented as a sum of three Helmholtz energy terms: Lennard-Lones (LJ) segment (temperature-dependent hard sphere + dispersion), chain (increment due to chain formation), and association (increment due to association). This equation of state has been developed by extending Wertheim's theory obtained from a resummed cluster expansion. Pure component molecules are characterized by segment diameter, segment-segment interaction energy, for example, Lennard-Jones σ and ε, and chain length expressed as the number of segments. There are also two association parameters, the association energy and volume, characteristic of each site - site pair. The agreement with molecular simulation data is shown to be excellent at all the stages of development for associating spheres, mixtures of associating spheres, and nonassociating chains.
Synopsis In dispersive mixing of immiscible liquids the minimum attainable dropsize is often deduced from the critical value of the Capillary number (the ratio of the shear stress to the interfacial stress) necessary for drop breakup under quasiequilibrium conditions. The critical Capillary number shows a minimum if the viscosity ratio between dispersed and continuous phase is about one. Hence, it is commonly accepted that the finest morphology is obtained if both viscosities match. In practical mixing devices, however, small drops are formed by a transient mechanism of thread breakup during extension rather than by stepwise breakup under equilibrium conditions. For Newtonian liquids, a comparison is made between the dropsizes resulting from a stepwise equilibrium and a transient breakup mechanism. Generally, the transient mechanism yields smaller drops and, more interestingly, a higher viscosity ratio between the dispersed and continuous phases results in a finer morphology, as already indicated by Tjahjadi and Ottino ( 1991). In the present paper the comparison is elaborated over a broad range of the relevant parameters while a compact illustrative presentation of the results is given to stress the possible consequences for practical blend morphologies.
ICI AND LINDE HAVE JOINTLY developed a new carbon dioxide-based fluid, called Washpoint, that the companies say will enable liquid CO 2 to challenge the entrenched position of hydrocarbons and potentially cancer-causing perchloroethylene as solvents in the dry-cleaning market.
This text deals from a fundamental viewpoint with the behaviour of polymers at surfaces and interfaces. Topics covered include the nature and properties of the surface of a polymer melt, the structure of interfaces between different polymers and between polymers and non-polymers, the molecular basis of adhesion, and the properties of polymers at liquid surfaces. Emphasis is placed on the underlying physical principles. Statistical mechanics models of the behaviour of polymers near interfaces are introduced, with the emphasis on theory that is tractable and applicable to experimental situations. Advanced undergraduates, graduate students and research workers in physics, chemistry and materials science with an interest in polymers will find this book of value.
Supercritical fluids are neither gas nor liquid, but can be compressed gradually from low to high density and they are therefore interesting and important as tunable solvents and reaction media in the chemical process industry. By adjusting the density the properties of these fluids can be customised and manipulated for a given process - physical or chemical transformation. Separation and processing using supercritical solvents such as CO2 are currently on-line commercially in the food, essential oils and polymer industries. Many agencies and industries are considering the use of supercritical water for waste remediation. Supercritical fluid chromatography represents another, major analytical application. Significant advances have recently been made in materials processing, ranging from particle formation to the creation of porous materials.
The chapters in this book provide tutorial accounts of topical areas centred around: (1) phase equilibria, thermodynamics and equations of state; (2) critical behaviour, crossover effects; (3) transport and interfacial properties; (4) molecular modelling, computer simulation; (5) reactions, spectroscopy; (6) phase separation kinetics; (7) extractions; (8) applications to polymers, pharmaceuticals, natural materials and chromatography; (9) process scale-up.
The application of on-line rheometry is considered with particular reference to polymer melts plasticized by physical blowing agents. Experimental investigations were performed on polystyrene containing 0-15 wt% of HCFC-142b or HFC-134a or 0-5 wt% of carbon dioxide at various temperatures and shear rates. It was shown that results could be used to indicate the solubility limit and related to the glass transition temperature.
Rheological modelling and data fits were compared indicating that a time/ temperature-composition (i.e. relative concentrations of polymers/blowing agent mixtures) superposition principle was valid. The model was extended to consider effects using a semi-crystalline polymer (polypropylene).
It was concluded that an on-line retum-to-stream process control rheometer could satisfactorily measure and monitor mixtures of polymers and physical blowing agents used in thermoplastic foam extrusion. Appropriate viscosity measurements could be directly linked to a number of critical variables.
Polymerization is the process of converting moeomer(s) to long chain molecules. It is a basic process to produce materials with “microstructural” features. The microstractural consequences of polymerization are reflected in the molecular weight, molecular weight distribution, chain end groups, repeat unit orientation and chain regularity (as in tacticity), monomer sequence distributions (as in copolymers), branching, or crosslinking. The chain microstructure influences the ultimate properties of polymers that find ever increasing use in our everyday life.
In this paper, a new in-line method was used to directly measure the gas solubility in polymer melts during foam extrusion. Metered quantities of polymer and specfic gases were blended in a single screw extruder which is capable of producing polymeric foams of variable density A specially designed optical window and flow restrictor value placed between the die and the end of the extruder were used to generate dynamic solubility data by observing the onset of bubble formation/dissolution at the window through a microscope-CCD camera-monitor/recorder system. Solubility data were determined for systems involving nitrogen. argon and carbon dioxide in polystrene and polethylene terephthalate at different temperatures. Good agreement with literature data oo the PS-CO2 system was obtained.
The conditions that induce the phase separation and the bubble nucleation for the thermoplastic foam extrusion process in which physical foaming agents (PFA) are involved are obviously linked to the solubility parameters: temperature, PFA content, and pressure. However, it has been reported that flow or shear can significantly modify these degassing conditions. An inline detection method based on ultrasonic sensors was used to investigate the influence of the shear on the foaming conditions of polystyrene/ HFC134a mixtures, for PS resins of various melt flow rates. An increase of the degassing pressure at low melt temperature was observed for high viscosity resins. Deviation from solubility data has been attributed to the combined effects of elongational and shears stresses.
The miniaturization of biomedical and biochemical devices for micro-electromechanical systems (BioMEMS) has gained a great deal of attention in recent years. Products include biochips/biosensors, drug delivery devices, tissue scaffolds, and bioreactors. In the past, MEMS devices have been fabricated almost exclusively in silicon, glass or quartz because of the similar technology available in the microelectronics industry. For applications in the biochemistry and biomedical field, polymeric materials are a desirable choice because of their lower cost, good processibility, and biocompatibility. Polymer micro-/nanofabrication techniques, however, are still not well developed. In this paper, we briefly introduce various BioMEMS applications, basic microfluidic principles and functions, and processing techniques for micro-/nano-scale structures.
Supercritical CO2 prevents water pollution in the dyeing of textiles and polymers.
Results are given about a new dyeing process for dyeing synthetic fibre material with disperse dyes. The use of supercritical carbon dioxide as dyeing medium completely avoids water pollution and the need of drying. Laboratory results show excellent levelness and fastnesses on the dyeing of poly(ethylene terephthalate) and polyamides. Especially, the good results are achievable for small lots like textile accessories. The state of technical development is briefly described.
A constant temperature process of generating microcellular polymers is presented. The process employs a sudden pressure drop to induce phase separation in a solution of supercritical carbon dioxide and poly(methyl methacrylate). The method makes use of the glass transition depression due to the presence of diluent in the polymer rather than heating the polymer to above its normal glass transition temperature. Cell sizes and densities can be varied by changing the process conditions such as saturation pressure, temperature, crosslinking and saturation time.
This article describes a strategy for suppressing the coalescence of cells during shaping in continuous processing of microcellular plastics. Microcellular plastics are foamed polymers characterized by a cell density greater than 109 cells/cm3 and fully grown cell size on the order of 10 μm. Since the invention of microcellular plastics in a batch process, research has focused on cost-effective, continuous processing of these materials. The basic approach to the production of microcellular structures is to continuously form a polymer/gas solution, to nucleate a large number of bubbles using rapid pressure drop, to shape a nucleated polymer/gas solution under pressure, and to induce a volume expansion to desired expansion ratio. Successful completion of these steps in extrusion will manufacture microcellular foamed plastics with a high cell population density. The critical issue to be dealt with in this article is how to prevent the deterioration of the nucleated cell-density via cell coalescence, which vigorously occurs in a shear field during shaping while the nucleated cells are growing. An effective strategy for preventing the cell coalescence is established and some critical experiments are then presented which verify the concept of preserving the high cell population density in a shear field.
The objective of this review is to outline the range of research activities being undertaken on supercritical fluid (SCF) science and technology primarily under the Japanese government funded priority research area on supercritical fluids. The research area is organized into four divisions: I. Solution Structures; II. Equilibrium and Transport Properties; III. Separations and Processing; and IV. Reactions. In division I, simulations and spectroscopic experiments are performed to study the interactions in supercritical fluids and clustering dynamics. In division II, properties of refrigerants, triglycerides and fatty acids, alcohols, and coal chemicals are measured and correlated. In division III, applications of SCF for separating and processing natural products, biomass, coal and coal liquids, polymers, and ceramics are explored. In division IV, homogeneous and heterogeneous reactions are being researched which include carboxylations, photo-induced, Fischer-Tropsch, enzyme reactions, and material conversions. New processes for converting waste cellulose and polymers into chemical intermediates and processes for producing thin films and metal oxides are shown to have great promise. Key objectives of the program and research results are extracted and reviewed from publications and government reports covering the past few years of research activity. In this respect, this review concentrates almost exclusively on Japanese research in supercritical fluids and makes no attempt to review numerous supercritical fluid research in other countries.
A novel process to produce microcellular PVC foams with a very homogeneous cell distribution and cell densities ranging from 10⁷ to 10⁹ cells/cm³ is described. Microcellular PVC foams with relative densities (density of foam divided by the density of unfoamed polymer) ranging from 0.15 to 0.94 have been produced. In this paper, experimental results on bubble nucleation and growth are presented. It was found that the bubble nucleation density increases with foaming temperature and eventually reaches a limiting value, while the average bubble diameter is relatively independent of the foaming temperature. A majority of the cell growth was found to occur in the early stages of foaming.
Recent advances in the use of supercritical CO2 (scCO2) in polymer processing (i.e. injection molding and extrusion) have increased the need for rheological data of polymer/scCO2 solutions at high pressures. In this study, a slit die with a sudden contraction was used to investigate the entrance pressure drop as well as the shear and extensional viscosities of a polystyrene (PS) melt and of a PS/scCO2 solution. Dissolution of CO2 into the PS melt was shown to reduce its entrance pressure drop as well as its shear and extensional viscosities. The entrance pressure drop of PS and PS/CO2 was found to be a strong (exponential) function of pressure. The entrance pressure drop as a function of wall shear stress could be fitted with a master curve for all experiments at different temperature, pressure and CO2 concentrations. Shear viscosities of PS as well as PS/CO2 were described using the generalized Cross-Carreau model and the Doolittle equation and different free volume models were compared.
The non-Fickian mass transport behavior observed in polymeric materials is here analyzed through a viscoelastic constitutive equation for the diffusive flux, endowed with concentration-dependent relaxation time and diffusivity. The present model thus overcomes the limitations shown by Neogi's model and by the Cattaneo equation used by Camera-Roda and Sarti. In addition to the Fickian behaviors at both small and high Deborah numbers, the model accounts for case II behavior and anomalous diffusion at intermediate Deborah number, possibly with the presence of a shock wave in the concentration profile which moves at a concentration-dependent velocity. Weight uptake accelerations and overshoots are also accounted for at intermediate Deborah numbers. The dimensionless problem has been numerically solved and the role of the relevant dimensionless parameters is discussed for both sorption and desorption.
A model of continuous-site distribution for gas sorption in glassy polymers is examined with sorption data of CO2 and Ar in polycarbonate. A procedure is presented for determining from a measured isotherm the number of sorption sites in a polymer, an important parameter that previously had to be assumed. With this parameter value and solubility data obtained at zero pressure, the model can reasonably predict sorption isotherms of CO2 in glassy polycarbonate for a wide temperature range. The number of sorption sites and the average site volume evaluated from CO2 sorption isotherms are employed for the prediction of Ar sorption isotherms with zero-pressure solubility data and the independently measured partial molar volume of Ar. A reasonable fit to the measured isotherms of Ar is achieved. With the proposed procedure, the continuous-site model shows several advantages over the conventional dual-mode sorption model. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 883–888, 2000
In situ Fourier transform IR and UV–vis spectroscopy have been used for partitioning of solutes between supercritical (sc) CO2 and polymer phases. Partitioning of deuterated water and azo-dyes between poly(methyl methacrylate) and scCO2 have been measured. Despite the low solubilities of polar azo-dyes in scCO2 the partition coefficient of these dyes is ca. 104. In addition, the degree of partitioning of cosolvents methanol and acetone from scCO2 into cross-linked poly(dimethylsiloxane) as well as the pressure dependence of the partition coefficient have been determined. Specific intermolecular interactions have been identified and related to the cosolvent partitioning between scCO2 and polymer phases.
The affect of talc on cell nucleation of polypropylene (PP) foam was investigated. Branched high-melt-strength PP was used to reduce bubble coalescence during extrusion foam processing. When isopentane was used as a blowing agent, bubble nucleation was dominated by the talc concentration, and the isopentane content did not affect the cell density very much. By contrast, when CO2 was used as a blowing agent, both talc and CO2 concentrations determined the cell-population density. However, the effect of talc on nucleation was significant only when the CO2 content was low. In addition, the maximum achievable cell density was not improved by talc when CO2 was used whereas the maximum cell density was dramatically improved by talc when isopentane was used. The experimental results indicate that the heterogeneous bubble nucleation mechanisms in plastic foam processing with CO2 are fundamentally different from those in the processing with a long-chain blowing agent such as isopentane.
This text deals with the behavior of polymers at surfaces and
interfaces. Topics covered include the nature and properties of the
surface of a polymer melt, the structure of interfaces among different
polymers and between polymers and nonpolymers, the molecular basis of
adhesion and the properties of polymers at liquid surfaces. Emphasis is
placed on the underlying physical principles. It introduces statistical
mechanics models of polymer behavior near interfaces, emphasizing theory
that is applicable to experimental situations. Advanced undergraduates,
graduate students and research workers in physics, chemistry and
materials science with an interest in polymers will find this book of
interest.
New experiments have succeeded in measuring actual rates of nucleation and are revealing the shortcomings of classical nucleation theory, which assumes that the molecular-scale regions of the new phase may be treated using bulk thermodynamics and planar surface free energies. In response to these developments, new theories have been developed that incorporate information about molecular interactions in a more realistic fashion. This article reviews recent experimental and theoretical advances in the study of nucleation of liquids from the vapor and of crystals from the melt, with particular emphasis on phenomena that relate to particle formation in the atmosphere.
The thermodynamic properties of amorphous phases of linear molecular chains are obtained from statistical mechanics by means of a form of the quasi-lattice theory which allows for chain stiffness and the variation of volume with temperature. A second-order transition is predicted for these systems.
This second-order transition has all the qualitative features of the glass transition observed experimentally. It occurs at a temperature which is an increasing function of both chain stiffness and chain length and a decreasing function of free volume.
The molecular ``relaxation times'' are shown to increase rapidly as the second-order transition temperature is approached from above.
To permit quantitative application of the theory and determine the relationship between the second-order transition and the glass transition observed in ``slow'' experiments these two transitions are tentatively identified. By this means quantitative predictions are made concerning the variations of (1) glass temperature with molecular weight, (2) volume with temperature, (3) volume with molecular weight, (4) volume at the glass temperature with the glass temperature for various molecular weights of the same polymer, (5) specific heat vs temperature, and (6) glass temperature with mole fraction of low-molecular weight solvent, since extensive experimental results are available for these properties. These and other theoretical predictions are found to be in excellent agreement with the experimental results.
Statistical associating fluid theory (SAFT) is an equation of state that can be used to calculate the phase behavior of mixtures comprised of components that exhibit wide disparities in molecular size, such as solvent–polymer mixtures. In this paper, we model the phase behavior of a PVAC-PTAN block copolymer composed of a CO2–phobic polyvinyl acetate (PVAC) and a CO2–philic poly(1,1,2,2-tetrahydroperfluorooctyl acrylate) (PTAN) in supercritical carbon dioxide (scCO2) using SAFT. SAFT is a molecular-based equation that is designed to account for effects of molecular association, chain flexibility, repulsive and dispersion interactions. The group contribution approach of Lora et al. was used to obtain the physical SAFT parameters for PVAC and PTAN polymers. PTAN was modeled as a non-associating polymer while PVAC was modeled with two association sites per molecule. Cloud curves of CO2–PVAC, CO2–PTAN and of the PVAC-b-PTAN–CO2 system were predicted, and good agreement was obtained with the experimental data available. Additionally, critical micellar densities (CMD) appear to be successfully predicted for the PVAC-b-PTAN–CO2 system using a criteria based in the variation of osmotic pressure with surfactant concentration. This was made possible by the ability of SAFT to handle long chain and association interactions.
The CO2 sorption isotherms of samples of polystyrene/polycarbonate blends with different compositions which were prepared by casting from 1,4-dioxane solutions were obtained as a function of composition of the polymer blend at 25°C using a Cahn-sorption apparatus. The characterization of the polymer blend system was obtained from the point of thermal behaviour such as differential scanning calorimetry, X-ray diffraction, and 13C CP/MAS n.m.r. From the characterization of the polymer blend, it was confirmed that there was a small amount of crystallinity of polycarbonate. Basically the polymer blend system studied here is heterogeneous (i.e. phase-separated) but it was found that a small amount of polystyrene is partly miscible with polycarbonate and this probably disrupts the crystallization of polycarbonate especially at the higher polystyrene content. CO2 sorption isotherms of the blend system including two homopolymers appeared to obey the dual mode sorption isotherm, which is a characteristic of the glassy state. The change of the CO2 sorption amount for the polymer blend was mainly explained by the amount of the crystallite of polycarbonate, indicating that the effect of the partly miscible region on the CO2 sorption behaviour is very small.
This 'Article' is a BOOK REVIEW, that reviews "Physics of Polymer Surfaces and Interfaces" Ed. I.C. Sanchez; Butterworth-Heinemann, Oxford, 1992; ISBN 0 7506 9214 6. The first 7 chapters of this book present short pedagogical reviews of theories of fluid interfacial phenomena applicable to polymer interfaces. The striking aspect of these chapters, particularly Chapters 1,2,5 and 7, is how much of the classical theory of capillarity has been imported into polymer surface science. For example, the reader will find reviews of interfacial thermodynamics, square gradient theory, density functional theory and capillary wave phenomena. One quickly gains the impression that polymer science and liquid state theory are merging into one subject: the study of complex fluids. One finds no mention of power law dispersion interactions which are perhaps the last significant bridge to cross in this merger and which the work of the past decade (and much earlier in the USSR) has shown to play a dominant role in fluid interfacial phenomena. More traditional polymer physics is presented in Chapters 4 and 6, where lattice techniques and random walk theory are applied to the structure of homogeneous polymers. In a typically individualist chapter, de Gennes discusses the possible significance of chain end adsorption to the mechanical properties of polymer interfaces. In all, these seven theoretical chapters serve as an introduction to the field, rather than as a detailed survey, and most of the technical detail necessary for applications to polymer problems is left to the references.
The second half of the book concentrates on the characterization of polymer surfaces and interfaces. Each chapter gives an introduction to a technique, describing the underlying principles and how it may be applied to studying interfaces and surfaces. As is inevitable with such a diverse range of techniques and authors each place their own emphasis either on the technique or the application. Since in general the underlying theory is not discussed in great detail, i.e. beyond the level found in much of the existing literature on these topics, we feel the book would have gained in interest considerably had there been greater emphasis on the novel applications of the cited techniques. Chapter 8, which discusses interlayer diffusion using neutron reflectivity, presents a good balance between explanation of the technique and its application. Other chapters consider mainly optical methods of characterizing interfaces, including more erudite techniques such as forward recoil spectroscopy, but also scanning angle reflectometry, Fourier transform infra-red spectroscopy and surface light scattering. As a reprieve from these 'conventional experiments' Dean and Webber present an interesting chapter on Monte Carlo simulations of polymer coils at interfaces in particular looking at direct- and electron-energy transfer processes.
Our overall impression is that the book gives a flavour of the diversity of techniques which may be used to probe surfaces and interfaces in general, and polymer interfaces in particular. The book will prove useful to graduate students and researchers wishing to learn more about related and complementary techniques.
[NB: RG loaded this item but is currently (2019) unable to correctly identify citations to it - those listed appear to be citations for the entire issue v.34,no.14 of Polymer.]
The reduction of the interfacial tension at the polystyrene (PS, M{sub n} = 1850)-supercritical CO{sub 2} interface is reported for poly(1,1-dihydroperfluorooctyl acrylate)(PFOA) and the block copolymers PS-b-PFOA(3.7K/27K) and PS-b-poly(dimethylsiloxane) (PDMS) (2K/16K, 500/10K) at 45 C. PS-b-PDMS (2K/16K) lowers the interfacial tension to 0.5 dyn/cm at 45 C and 238 bar, more than that of any of the other copolymers. On the basis of the dynamics of the lowering of the interfacial tension, the apparent diffusion coefficient of PS-b-PDMS (2K/16K) is 8 {times} 10{sup {minus}6} cm{sup 2}/s. The critical micelle concentration of PS-b-PFOA (3.7K/27K) is 9 {times} 10{sup {minus}4} wt %. Whereas both the PDMS- and PFOA-based copolymers studied adsorb on the PS surface, PS-b-PFOA is much more effective in stabilizing the PS emulsions. The difference in stabilization is discussed in terms of the surfactant adsorption and the interactions of the anchor and buoy blocks with CO{sub 2} and the PS surface.
The theory developed by Helfand and Tagami, and by Helfand and Sapse, to predict the interfacial properties between two immiscible polymers is extended to the case of compressible, nonsymmetric mixtures of polymers. Use is made of the previously discussed analogy between the modified diffusion equations describing the statistics of the two polymeric constituents and the equations of motion of a single particle in a plane. Introduction of a new trajectory for the particle helped to circumvent the complexity previously encountered in the direct perturbational calculation. It is pointed out that the presence of finite compressibility of the polymers tends to diminish slightly the interfacial tension, and that this tendency appears more prominent according as the two polymers become physically indistinguishable, when the intersegment interaction is of nonlocal nature. This is approximately the case with the polyethylene/polyvinylacetate or the polydimethylsiloxane/polyvinylacetate pair. Extensive calculations of the thickness of the interface, the interfacial tension, and the behaviors of the density profiles of the polymer segments are performed for fourteen pairs of polymers.
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CO2 can be a good solvent for many compounds when used in its compressed liq- uid or supercritical fluid state. Above its critical temperature and critical pressure (Tc = 31 °C, Pc = 73.8 bar), CO2 has liquid-like densities and gas-like viscosities, which allows for safe commercial and laboratory operating conditions. Many small molecules are readily soluble in CO2, whereas most macromolecules are not. This has prompted development of several classes of small molecule and polymeric surfactants that enable emulsion and dispersion polymerizations as well as other technological processes.
The dyeing behaviors for several types of high-speed and normal speed spun poly (ethylene terephthalate) fibers were compared in supercritical CO2 fluid. At lower temperature and pressure, the high-speed spun fibers, which had inherently larger crystallite sizes and lower birefringence, showed a larger dye uptake than the other fibers. However, when the supercritical conditions were elevated to 125°C and 230 bar, the dye uptake of both types increased markedly and the difference in dye uptake between the fibers became small. This suggests that the swelling of fibers in supercritical CO2 fluid exceeded a certain degree and then the diffusion of dye molecules was promoted. The swelling also promoted the rearrangement of molecular chains and permitting cold crystallization to occur. The modification of fiber structure through the dyeing in supercritical CO2 fluid was serious especially for the fibers whose inherent structure was not so well developed.
The experimental data obtained for the nucleation of microcellular foams are compared with the theoretical model developed in the first part of this paper. Polystyrene (PS) with rubber particles as nucleation sites is used as an exploratory system. Nitrogen is used as a physical blowing agent to nucleate the bubbles. The influence of process variables, such as saturation pressure, foaming temperature, and concentration and size of rubber particles, is discussed. Results indicate that all these variables play important roles during the nucleation process. A nucleation mechanism based on the survival of microvoids against the resisting surface and elastic forces has been modeled to obtain the cell nucleation density. Increase in saturation pressure increase the cell density to a critical pressure. Beyond this critical pressure, there is no increase in bubble number, indicating that all microvoids are activated. The effect of temperature is more complex than the effect of pressure. Increase in concentration of the rubber particles increase the nucleation cell density. In general, the experimental data are well described by the nucleation model presented in Part I.
An extrusion system that can create a polymer/gas solution rapidly for continuous processing of microcellular plastics is presented. Microcellular plastics are characterized by cell densities greater than 10(9) cells/cm(3) and fully grown cells smaller than 10 mu m. Previously these microcellular structures have been produced in a batch process by saturating a polymeric material with an inert gas under high pressure followed by inducing a rapid drop in the gas solubility. The diffusion phenomena encountered in this batch processing is typically slow, resulting in long cycle times. In order to produce microcellular plastics at industrial production rates, a means for the rapid solution formation is developed. The processing time required for completing the solution formation in the system was estimated from experimental data and the dispersive mixing theory based on an order-of-magnitude analysis. A means for promoting high bubble nucleation rates in the gas-saturated polymer via rapid heating is also discussed. The feasibility of the continuous production of microcellular plastics by the rapid polymer/gas solution formation and rapid heating was demonstrated through experiments. The paper includes not only a brief treatment of the basic science of the polymer/gas systems but also the development of an industrially viable technology that fully utilizes the unique properties of microcellular plastics.
Research conducted in the 1990s has demonstrated that one can design compounds which are “CO2-philic”, that is, they exhibit miscibility with CO2 at pressures significantly below that of conventional alkyl-functional analogues. The use of such CO2-philic functional groups in the design of surfactants and chelating agents has allowed successful implementation of CO2 in processes such as emulsion polymerization, heavy metal extraction, and others. However, the high cost of widely used fluorinated CO2-philes can render the economics of a CO2 process unfavorable. Consequently, we have investigated the design of CO2-philes composed of only carbon, hydrogen, and oxygen. Using fundamental data on the thermophysical behavior of CO2 as a guide, we focused on the design and synthesis of functionalized polyethers and copolymers of cyclic ethers and carbon dioxide that were found to be soluble in liquid and supercritical carbon dioxide. By choosing the proper amount of incorporated carbon dioxide, we can generate propylene oxide−CO2 copolymers that exhibit lower miscibility pressures than fluoroethers with the same number of repeat units.
FT-IR and FT-Raman spectroscopy have been applied to elucidate the morphology and structure of poly(ethylene terephthalate) (PET) processed with supercritical CO2 (scCO2). Infrared and Raman spectra of PET samples show an increased degree of crystallinity after being subjected to scCO2. FT-Raman spectroscopy has also been utilized to study the supercritical dyeing of PET samples with azo dyes. These results may aid optimization of `solvent-free' processes for advanced processing of packaging materials and supercritical fluid dyeing of textiles.
The glass transition in polystyrene-ethylene at pressures up to 78 atm has been studied using a Tian-Calvet heat-flow calorimeter. The glass transition temperature Tg decreases with an increase in ethylene pressure, the largest depression being 67°C at 78 atm. The plasticization effect of ethylene is found to be almost the same as that of CO2, and is predicted well by the statistical thermodynamical formulation of the glass transition in polymer-diluent systems. The plasticization of amorphous poly(aryl ether ether ketone), PEEK, with CO2 at 100°C and 100 atm results in induction of about 17% crystallinity in a high molecular weight sample. Overall crystallinity values as high as 44% are observed in the case of a low molecular weight PEEK sample.
This work concerns the effects of the filler size on cell nucleation during the foaming process. The cell density of foams with fillers of two different sizes has been investigated using the foaming process simulator developed previously. It was found that the cell density is strongly affected by the filler size. Foams with a fine filler show a higher cell density at a high saturation pressure but give a lower cell density at a low saturation pressure. At a certain value of the saturation pressure, cell density becomes similar with both fillers. This transition pressure changes with the foaming condition. It goes down with a higher pressure drop rate.
The experimental results have been explained with an analysis off iller particle size distribution. The analysis also recommended a way to select filler size if a high cell density is desired in the foaming process.
Sorption of CO2 in poly(methyl methacrylate) at 35−200 °C and concurrent dilation of the polymer at 35−85 °C over a pressure range up to 50 atm were studied. Dissolution and Flory−Huggins interaction parameters for the gas in the polymer, not only in the rubbery state but also in the glassy state, were estimated by analyzing the sorption data above the glass transition temperature (Tg0, 105 °C). Isothermal glass transition of the polymer/gas system was observed on isotherms of sorption and dilation below Tg0. Partial molar volumes of sorbed CO2 determined from the sorption and dilation isotherms increased with increasing concentration to the glass transition concentration. These isotherms were also analyzed on the basis of extended dual-mode models of sorption and dilation. From obtained parameters of the dual-mode models, nonequilibrium properties such as mean size and number of microvoids for the pure polymer and the CO2-sorbed polymer in the glassy state were evaluated. The mean size, dependent upon CO2 exposure history of the polymer, was in the range of 20−100 A3, and the number of microvoid ((1−18) × 1020 voids/cm3) was dependent upon both temperature and the exposure history.
When treated with compressed CO2, syndiotactic polystyrene (sPS) undergoes a number of solid-solid transitions that do not occur on treatment with liquid solvents. For example, planar mesophase --> beta, alpha --> beta, and gamma --> beta transitions can be brought about under appropriate conditions of temperature and CO2 pressure. In addition, the transitions of glassy sPS to the planar mesomorphic and to the a form, and the gamma --> alpha transition occur at temperatures lower than when the same transitions are effected under ambient pressure. The dissolved CO2 lowers the glass transition and the cold crystallization temperatures of sPS at the rate of -0.92 and -0.58 degrees C/atm, respectively. Crystallization kinetics from the sPS-CO2 solution follow the Avrami equation, but the value of the exponent n is lower than when crystallization is conducted under ambient pressure.
Forced Rayleigh scattering (FRS) has been used to measure the tracer diffusion coefficient of azobenzene in low molecular weight polystyrene (PS) as a function of temperature, and in high molecular weight PS plasticized by CO2 at 35 °C and CO2 pressures from 14 to 85 bar. In contrast to dye diffusion in pure PS and PS plasticized by tricresyl phosphate, where the effect of plasticization by temperature, chain ends, or added diluent can be accounted for by (T − Tg) scaling, dye diffusion in CO2-plasticized PS is enhanced by 2−3 orders of magnitude over values predicted on the basis of Tg depression alone. This enhancement begins at surprisingly low CO2 pressures (<15 bar at 35 °C) and is maintained across the CO2-induced glass transition. A large difference in mobilities of the cis and trans isomers of azobenzene is also observed in the presence of CO2, which is much greater than that seen in the experiments on pure PS. Two additional FRS relaxation modes unrelated to translational diffusion of the dye molecules have been identified in this study: a fast, local relaxation attributed to dye rotation, and a slow relaxation attributed to the dynamic response of the PS/CO2 matrix to a chemical potential driving force associated with the azobenzene isomers.
The heterogeneous (solid/supercritical fluid solution) free-radical polymerization of styrene in supercritical carbon dioxide-swollen poly(chlorotrifluoroethylene) (PCTFE) polymer film (63 mil) has been studied as an approach to polymer blend preparation. Decompression followed by thermal initiation using AIBN or tert-butyl perbenzoate yields polymer blends with the polystyrene trapped inside the matrix polymer. Polymerization prior to decompression yields more extensively modified products. Polystyrene content and the distribution of polystyrene in the blend can be controlled by adjusting the concentration of styrene in the supercritical fluid or by controlling the time that the PCTFE film is in contact with the fluid. The control imparted is consistent with the phase behavior and absorption kinetics of the styrene-CO2-PCTFE system. Transmission electron microscopy and energy dispersive X-ray analysis (EDX) indicate that the polystyrene exists as discrete phase-segregated regions throughout the thickness of the PCTFE film. Thermal analysis of blends and infrared data from extraction experiments indicate that radical grafting reactions do not occur to any significant extent. EDX data indicate that blends with composition gradients of adjustable degrees of severity can be produced by using soaking periods of shorter duration than is required to achieve equilibrium solubility of the monomer in the substrate.