Extrusion of polystyrene nanocomposite foams with supercritical CO2

Intercalated and exfoliated polystyrene/nano‐clay composites were prepared by mechanical blending and in situ polymerization respectively. The composites were then foamed by using CO₂ as the foaming agent in an extrusion foaming process. The resulting foam structure is compared with that of pure polystyrene and polystyrene/talc composite. At a screw rotation speed of 10 rpm and a die temperature of 200°C, the addition of a small amount (i.e., 5 wt%) of intercalated nano‐clay greatly reduces cell size from 25.3 to 11.1 μm and increases cell density from 2.7 × 10⁷ to 2.8 × 10⁸ cells/cm³. Once exfoliated, the nanocomposite exhibits the highest cell density (1.5 × 10⁹ cells/cm³) and smallest cell size (4.9 μm) at the same particle concentration. Compared with polystyrene foams, the nanocomposite foams exhibit higher tensile modulus, improved fire retardance, and better barrier property. Combining nanocomposites and the extrusion foaming process provides a new technique for the design and control of cell structure in microcellular foams.     

Continuous microcellular polystyrene foam extrusion with supercritical CO2

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

Diffusion and desorption of CO2 in foamed polystyrene film

Based on the existence of the pores in foamed polystyrene (PS), foamed‐non‐Fickian diffusion (FNFD) model was proposed, for the first time, to regress the desorption data obtained by gravimetric method. Results showed that FNFD model could accurately describe the diffusion behavior of CO₂ out of foamed PS, and well predict the solubility of CO₂ in foamed PS. The characterization of scanning electron microscopy indicated that there were abundant pores in the foamed PS, and the pores store most of CO₂, which would diffuse in the pores, adsorb to the wall of the pores, penetrate across walls of the pores, diffuse in the matrix of PS, and desorb out of PS. The mass of CO₂ in the pores of foamed PS was expressed as a function of foaming pressure and temperature according to foaming kinetics. Results showed that the values calculated by this function agreed well with the values obtained from the FNFD model. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45645.     

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

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

超临界CO2管道减压过程中的热力学特性

基于工业规模CO₂管道（长258 m,内径233 mm）实验装置开展了3组不同泄放口径的超临界CO₂的泄放实验,测量了CO₂减压过程中管内介质压力变化以及介质与管壁的温度分布,分析了减压过程中CO₂相态、密度变化及管壁内外传热过程。研究表明,超临界CO₂泄放导致管内介质压力、温度及管壁温度均下降,最终趋于稳定,介质由超临界相变为气液两相最终变为气相。初始阶段的温降幅度最大,对流换热强度最大。距离泄放端越远,管内顶部和底部介质的温降幅度越大,对流换热强度越小,在泄漏口附近的对流换热最为剧烈。随着泄放口径的变大,泄放时间和管道内介质与管道的换热时间都变小,且沿着管道方向的管道内流体和管壁的温度梯度变大,对流换热强度也变大。     

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

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

Effect of SWCNT dispersion on epoxy nanocomposite properties

In nanocomposites containing single wall carbon nanotubes (SWCNTs), the final properties strongly depend on the dispersion quality of these fillers. Various methods have been used to improve the dispersion of nanofillers; however, one of the most effective ways is to functionalize carbon nanotubes (CNTs) with covalent and noncovalent functional groups. In this work, the dispersion of SWCNTs in an epoxy system was studied by using surfactants, acid (COOH), and ester groups (PGE)‐modified CNTs. Rheological and scanning electron microscopy analysis showed that functionalization of CNTs helped in improving the dispersion of fillers in the epoxy matrix. Systems with surfactant modified SWCNTs (1 wt%) exhibited the highest storage modulus at low frequencies after 5‐min sonication. This behavior is associated to a stronger network of fillers as a result of a good dispersion. However, longer sonication times lowered the storage modulus, corresponding to a degradation of the tubes. The effect of the dispersion quality on mechanical properties was also studied using a three‐point bending set‐up. POLYM. COMPOS.,, 2012. © 2012 Society of Plastics Engineers     

Synthesis and properties of polystyrene/graphite nanocomposites

In this paper, graphite/polystyrene nanocomposite is synthesized by in situ polymerization of styrene in a tetrahydrofuran (THF) solution system of potassium (K)-THF-graphite intercalation compound (GIC). K-THF-GIC has proved to initiate polymerization of styrene by the anionic mechanism. Due to the interfacial interaction between the graphite nanolayers and the polymer, the composites exhibit higher glass transition temperature and higher thermal stability when compared to polystyrene. The percolation threshold in the conductivity of the composites is lesser than 8.2 wt% and the dielectric constant can reach as high as 136.     

Effect of nanoparticles orientation on morphology of polymeric nanocomposite foams: preparation of foamed nanocomposite fibers by supercritical carbon dioxide

ABSTRACT

Polymeric foams have received increasing attention in both academic and industrial communities. Using of nanoparticles as heterogeneous nucleation agent has been verified as one of the most valid means to enhance cell nucleation and improve cell morphology. However, few researches have been conducted to investigate the effect of the nanoparticles’ spatial orientation on their nucleation efficiency. In this work, to study the influence of the orientation of nanoparticles on their performance in improving morphology of polymeric foam, thermoplastic polyurethane (TPU) composite fibers with different nanoparticles (carbon nanotubes, graphene and SiO₂) were prepared by using different traction speeds. The different traction speeds lead to different orientation state of the nanoparticles which then resulted different nucleation effect. It was found that carbon nanotubes (CNTs) were easily oriented and aligned along the fiber length direction under the high traction speed, while graphene and SiO₂ nanoparticles did not show orientation under the traction speed in this study. As a result, the foam of TPU/CNTs composite fibers from high traction speed exhibited a much smaller cell size and higher cell density compared to the foams of the fibers from low traction speeds, while TPU/graphene, and TPU/SiO₂ composite fibers with different traction speeds showed almost similar cell size and size density after foaming, indicating that the orientated nanoparticles possessed higher heterogeneous nucleation efficiency. To our best knowledge, this work, for the first time, demonstrated the high nucleation effect of the aligned nanoparticles, which hopefully open a new path for improving the cell morphology of polymeric foam materials.     

Regeneration of nanoporous crystalline syndiotactic polystyrene by supercritical CO2

Guest desorption procedures for s‐PS clathrate samples, leading to formation and regeneration of the nanoporous δ phase, are compared. An extraction procedure, based on supercritical carbon dioxide, allows an easy and fast recovery of the guest molecules operating under relatively mild conditions (90–200 bar, 40°C) and generates a completely empty δ form, also starting from the most stable s‐PS clathrate forms. In agreement with a previously proposed crystal structure of the nanoporous δ form, X‐ray diffraction patterns of δ form powders obtained by this procedure do not show the peak, which is reduced but still apparent in samples extracted with previous procedures based on boiling solvents. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2077–2082, 1999     

Foaming of PCL/clay nanocomposites with supercritical CO2 mixtures: The effect of nanocomposite fabrication route on the clay dispersion and the final porous structure

Supercritical fluids have been established as alternative foaming agents in various polymers as well as nanocomposite systems. Most recently, supercritical carbon dioxide (scCO₂) has also been used in some studies as a medium of clay dispersion in the polymer matrix providing a solvent-free fabrication route for nanocomposites. In this work, this latter route was followed for the development of porous poly(ɿ-caprolactone) (PCL)/clay nanocomposites after pressure quench. Similarly, PCL/clay nanocomposites were also prepared using the solvent casting and melt blending methods and were then processed with scCO₂ with the batch foaming technique (isothermal pressure quench) to produce their porous counterparts. Poor clay dispersion and non-uniform porous structures were observed when pure CO₂ was used as a dispersion medium for nanocomposite preparation and as a blowing agent, respectively. On the contrary, polymer intercalation and more uniform cell structures were produced when CO₂⿿ethanol mixtures were used as blowing agents.     

The synergy of supercritical CO2 and supercritical N2 in foaming of polystyrene for cell nucleation

This study examines the foaming behaviour of polystyrene (PS) blown with supercritical CO₂–N₂ blends. This is achieved by observing their foaming processes in situ using a visualization system within a high-temperature/high-pressure view-cell. Through analyzing the cell nucleation and growth processes, the foaming mechanisms of PS blown with supercritical CO₂–N₂ blends have been studied. It was observed that the 75% CO₂–25% N₂ blend yielded the highest cell densities over a wide processing temperature window, which indicates the high nucleating power of supercritical N₂ and the high foam expanding ability of supercritical CO₂ would produce synergistic effects with that ratio in batch foaming. Also, the presence of supercritical CO₂ increased the solubility of supercritical N₂ in PS, so the concentration of dissolved supercritical N₂ was higher than the prediction by the simple mixing rule. The additional supercritical N₂ further increased the cell nucleation performance. These results provide valuable directions to identify the optimal supercritical CO₂–N₂ composition for the foaming of PS to replace the hazardous blowing agents which are commonly used despite their high flammability or ozone depleting characteristics.     

Synthesis and rheological properties of polystyrene/layered silicate nanocomposite

Exfoliated polystyrene (PS)/organo-modified montmorillonite (MMT) nanocomposites were synthesized through in situ free radical bulk polymerization by dispersing a modified reactive organophilic MMT layered silicate in styrene monomer. The original MMT was modified by a mixture of two commercial cationic surfactants, [2-(acryloyloxy)ethyl](4-benzoylbenzyl)dimethylammonium bromide (ADAB) and cetyltrimethylammonium bromide (CTAB), with the former containing a polymerizable vinyl group. The exfoliating and intercalating structures were probed by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Comparing with pure PS, the nanocomposites show much higher decomposition temperature, higher dynamic modulus, stronger shear thinning behavior and a smaller die swell ratio. The leveling-off of the storage modulus at low frequencies in the oscillatory shear measurements, as well as the observed yield like behavior, implies that the formation of a percolating nanoclay network is the origin of the enhanced viscoelasticity in these composites.     

Microstructure and thermophysical properties of graphite foam/glass composites

Mesophase pitch based graphite foams with different thermal properties and cell structures were infiltrated with glass by pressureless infiltration to prepare potential alternative composites for cooling electronics. Microstructure, thermal diffusivity and coefficient of thermal expansion (CTE) of the obtained composites were investigated. It was demonstrated that there was excellent wettability of the graphite foam by molten glass, and the foam framework was retained well after infiltration, which could facilitate good heat transfer throughout the composites. The highest thermal diffusivity of the composites reached 202.80 mm²/s with a density of 3.81 g/cm³. And its CTE value was 4.53 ppm/K, much lower than the corresponding calculated result (7.46 ppm/K) based on a simple “rule of mixtures” without considering the space limitations of the graphite foams. Thus, the mechanical interlocking within the space limitations of the graphite network played a crucial role in limiting the thermal expansion of the glass. The CTEs of the graphite foam/glass composites varied from 4.53 to 7.40 ppm/K depending on the graphite foam density which varied from 0.82 to 0.48 g/cm³. The CTEs were a good match to those of semiconductor chips and packaging materials.     

Evaluation of interfacial layer properties in the polystyrene/silica nanocomposite

Processing conditions and final mechanical properties of polymer nanocomposites are affected by their interfacial layers behavior. However, it is impossible to determine directly the properties of these layers by dynamic rheometry tests. In this work, the interfacial layers properties are evaluated for polystyrene containing silica nanoparticles by the concept of glass‐transition temperature shift. The samples were prepared via solution‐mixing method and dynamic rheometry was used to determine the viscoelastic behavior of filled polymers in the melt state. This initial step showed that addition of silica particles increased the glass‐transition temperature. By preference, decrease in the filler particle size lead to a drastic increase in the glass‐transition temperature and interfacial layer volume fraction due to relatively high surface area of the small filler particles. Then, in the next step, the viscoelastic properties of interfacial layer have been evaluated on the basis of the properties of neat polystyrene using temperature‐frequency superposition law. For this purpose, the shift factor was calculated from the glass‐transition temperature of the sample with maximum filler content. Finally, the effect of immobilized interfacial layer on the viscoelastic properties of the polymer nanocomposite samples has been estimated. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Study of structural,electrical, and dielectric properties of polystyrene/foliated graphite nanocomposite developed via in situ polymerization

Present article reports the investigations of the electrical and dielectric properties of polystyrene/foliated graphite (PS/FG) nanocomposites. The homogenous embedded structure of FG within the polymer matrix has been confirmed by scanning electron microscopy (SEM). The electrical conductivity of the composite was found to exhibit insulator–conductor transition at a very low percolation threshold of FG. A nonlinear to linear transition in the current–voltage characteristics of the composites was observed when the composite undergoes insulator–conductor transition. The frequency dependence of dielectric constant, dissipation factor, and ac conductivity has also been analyzed using percolation theory. D ‐shore hardness of the nanocomposite was also tested to observe the strength of the composites. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Using supercritical carbon dioxide in preparing carbon nanotube nanocomposite: Improved dispersion and mechanical properties

Improvements in carbon nanotube (CNT) dispersion and subsequent mechanical properties of CNT/poly(phenylsulfone) (PPSF) composites were obtained by applying the supercritical CO₂ (scCO₂)‐aided melt‐blending technique that has been used in our laboratory for nanoclay/polymer composite preparation. The preparation process relied on rapid expansion of the CNTs followed by melt blending using a single‐screw extruder. Scanning electronic microscopy results revealed that the CNTs exposed to scCO₂ at certain pressures, temperatures, exposure time, and depressurization rates have a more dispersed structure. Microscopy results showed improved CNT dispersion in the polymer matrix and more uniform networks formed with the use of scCO₂, which indicated that CO₂‐expanded CNTs are easier to disperse into the polymer matrix during the blending procedure. The CNT/PPSF composites prepared with scCO₂‐aided melt blending and conventional melt blending showed similar tensile strength and elongation at break. The Young's modulus of the composite prepared by means of conventional direct melt blending failed to increase beyond the addition of 1 wt% CNT, but the scCO₂‐aided melt‐blending method provided continuous improvements in Young's modulus up to the addition of 7 wt% CNT. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

Application of organic nonsolvent in the process of solution-enhanced dispersion by supercritical CO2 to prepare puerarin fine particles

An organic nonsolvent, dichloromethane was employed in the process of solution-enhanced dispersion by supercritical CO₂ (SEDS) to prepare fine particles of puerarin. A 2³ factorial experiment was designed to investigate and identify the relative significance of the processing parameters on the surface morphology, particle size and particle size distribution of the products. The effect of the nonsolvent/solvent ratio was found to be dominant in the results regarding particle size. Increasing the nonsolvent content of the puerarin solution decreased the particle size significantly. After optimization, the resulting puerarin nanoparticles exhibited a good spherical shape, a smooth surface and a narrow particle size distribution, with a mean particle size of 0.19 μm. After SEDS processing, the measurements of high performance liquid chromatography (HPLC), ultraviolet (UV) and mass spectrometry (MS) indicated there was no change in the chemical composition of puerarin particles; Fourier transform infrared (FTIR) spectroscopy measurement found the minor structural changes occurred on a molecular level; X-ray powder diffraction (XRPD) analysis revealed that the physical state of puerarin shifted into an amorphous form; and a significant increase in the dissolution rate of puerarin nanoparticles was observed. The SEDS process combined with the addition of dichloromethane could produce puerarin nanoparticles without contamination.     

叶黄素浸膏的超临界CO2流体萃取及其皂化工艺研究

采用水平因子正交试验法研究了超临界CO2流体萃取万寿菊花中叶黄素的主要工艺参数:萃取压力、萃取温度、分离Ⅰ压力、分离Ⅰ温度、分离Ⅱ温度对叶黄素浸膏的萃出量和萃取率的影响规律.结果表明:增加萃取釜的温度和压力以及在萃取釜中加入提携剂对萃取物的出率影响最大,升高萃取釜的温度在一定范围内有利于萃取,超过此温度极限将对萃取过程产生负影响.用萃取出的叶黄素浸膏进行皂化和分离提纯,可得到含量在85%以上的叶黄素单体.