Preparation of carbon foams with supercritical toluene

Carbon foams with pore sizes of 10–50 μm were prepared with mesophase pitch and toluene as the carbonaceous precursor and supercritical agent, respectively. Results revealed that the light pitch components and dissolved toluene in pitch significantly affected the pore structures of resultant carbon foams. The amount of toluene dissolved in molten pitch is greatly dependent on the foaming conditions, such as the ratio of toluene to pitch, foaming temperature, foaming pressure and saturation time. Carbon foams with hierarchical porous structures are obtained by controlling the amount of light pitch components.     

Carbon foams with high compressive strength derived from mixtures of mesocarbon microbeads and mesophase pitch

Carbon foam with relatively high compressive strength and suitable thermal conductivity was prepared from mixtures of mesocarbon microbeads (MCMBs) and mesophase pitch, followed by foaming, carbonization and graphitization. The influence of addition amount of MCMB on the properties of as-prepared carbon foams was investigated in detail. Results showed that addition of MCMBs into mesophase pitch could significantly reduce the amount and length of cracks in carbon foams, which results in increase of compressive strength of carbon foams. Carbon foam with high compressive strength of 23.7 MPa and suitable thermal conductivity of 43.7 W/mK, was obtained by adding 50% MCMBs into mesophase pitch, followed by foaming, carbonization and graphitization.     

Extruded polystyrene foams with bimodal cell morphology

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

Effects of pitch fluoride on the thermal conductivity of carbon foam derived from mesophase pitch

Carbon foams with high thermal conductivity were obtained from mixtures of mesophase pitch and pitch fluoride. The addition of pitch fluoride in mesophase pitch could significantly increase the specific thermal conductivity of as-prepared carbon foams. After graphitization at 2873 K, the specific thermal conductivity of carbon foams increased from 82 up to 155.4 (W/mK)/(g/cm³) when the content of pitch fluoride was 3% in the raw material.     

Preparation of mesophase-pitch-based carbon foams at low pressures

A simple method for preparing the mesophase-pitch-based carbon foams at low pressures through prolonging the soaking time in the preparation process of the mesophase pitch was disclosed. The physical properties, morphologies and the crystal structure of the as-obtained foams were investigated. Bulk density of the resultant carbon foams cover a range 0.514-0.624 g/cm³, under the preparation pressure range 0.5-2 MPa. The SEM micrographs revealed that the thermal shrinkage of the graphitized foams derived from the higher softening point mesophase pitch was less than that of the foam from the lower softening point; Optical micrographs showed that higher softening point mesophase pitch derived carbon foams exhibited better orientation and less microcracks in both junctions and ligaments; The XRD results revealed that higher softening point pitch derived graphitized foams exhibited smaller interlayer spacing and larger crystallite size. The properties of the carbon foam can be severely affected by the properties of the precursor, so it is critical to tailor the properties of the pitch precursor so as to obtain high performance and low cost products.     

泡沫炭在自发泡过程中的成核行为研究

结合中间相沥青的热性能和微型气泡的受力行为,通过比较不同原料体系所制泡沫炭的孔结构差异,研究了泡沫炭在自发泡过程中的成核行为。结果表明:1)中间相沥青在发泡过程中最初生成的气体将成为泡核,随后生成的微型气泡将在表面能及气泡内外压差的驱动下,在泡核周围逐步进行兼并、聚集和膨胀,并在表面张力作用下成为球形气泡;2)自发泡过程中,中间相沥青的热性能是影响成核行为和泡沫炭孔结构均一性的主要因素。     

中间相沥青基泡沫炭的研究及应用

中间相沥青基泡沫炭具有低密度、高强度、高导热、抗冲击、耐高温、抗氧化等诸多特点,因而具有广泛的应用前景.目前,中间相沥青基泡沫炭最常用的制备方法是自发泡法.本文详细综述了中间相沥青基泡沫炭制备过程中的影响因素,也概述了近几年来国内外对泡沫炭的改性研究及应用.     

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     

石油系中间相沥青基泡沫炭的制备与结构研究

分别以中间相含量为100％和30％的两种石油系中间相沥青为前驱体制备泡沫炭，利用SEM、偏光显微镜观察泡沫炭的孔和孔壁等微观形貌，利用XRD分析石墨化泡沫炭的微晶结构。考察不同中间相含量前驱体的发泡性能，同时探讨了焦化手段对泡沫炭结构和性能的影响。结果表明，两种前驱体均能制备得到孔结构均匀、开孔率高的泡沫炭；100％中间相含量的前驱体相对更适合发泡；通过延长焦化时间可以优化泡沫炭结构、提高石墨化度，进而提高泡沫炭产品性能。     

聚醚酰亚胺均相成核发泡行为研究

以超临界CO₂为发泡剂,采用间歇式发泡技术制备了聚醚酰亚胺（PEI）微孔泡沫。通过改变发泡温度、发泡压力与样品浸泡时间等工艺条件,研究了PEI的均相成核发泡行为。实验还通过二次降压法制备了具有复合泡孔结构的PEI微孔泡沫材料。结果表明,复合泡孔结构提高了PEI微孔泡沫的发泡倍率,第一次压力降ΔP₁与第二次保压时间Δt₂是影响复合泡孔结构参数的重要影响因素。     

Formation mechanism of carbon foams derived from mesophase pitch

Carbon foams were prepared from mesophase pitch using foaming, carbonization and graphitization processes. The physical and chemical properties of the mesophase pitch during thermal treatment were studied by Fourier transform infrared spectroscopy, thermogravimetry, mass spectroscopy, rheometry and scanning electron microscopy. The results suggest that gases released from the pitch dissolve, saturate, nucleate and grow in the molten pitch during foaming. Then the resultant bubbles coalesced with the neighboring bubbles driven by the surface tension of the molten pitch. This coalescence generates a shear stress to force aromatic planes of the pitch to arrange regularly and paralleled to the axis of a ligament. The growth of bubbles stopped when the pitch became semi-coke at a temperature above 733 K. The viscosity and surface tension of the molten pitch are major factors that influence the growth of bubbles. After carbonization at 1073 K and graphitization at 2873 K, the well aligned aromatic planes in the foams evolve into highly aligned graphitic structures.     

Foaming behavior of microcellular foam polypropylene/modified nano calcium carbonate composites

A new process was used to prepare microcellular foams with supercritical carbon dioxide as the physical foaming agent in a batch. The foaming temperature range of the new process was about five times broader than that of the conventional one. Characterization of the cellular structure of the original polypropylene (PP) and PP/nano‐CaCO₃ (nanocomposites) foams was conducted to reveal the effects of the blend composition and processing conditions. The results show that the cellular structure of the PP foams was more sensitive to the foaming temperature and saturation pressure variations than that of the nanocomposite foams. Uniform cells of PP foams are achieved only at a temperature of 154°C. Also, the low pressure of 20 MPa led to very small cells and a low cell density. The competition between the cell growth and cell nucleation played important role in the foam density and was directly related to the foaming temperature. Decreasing the infiltration temperature depressed the initial foaming temperature, and this resulted in significantly larger cells and a lower cell density. A short foaming time led to a skin–core structure; this indicated that a decrease in the cell size was found from skin to core, but the skin–core structure gradually disappeared with increasing foaming time. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Carbon foam derived from various precursors

A series of carbon foams was developed by using low-cost precursors, such as coal, coal tar pitch and petroleum pitch. The properties of the resultant carbon foams cover a wide range, e.g., bulk density, 0.32-0.67 g/cm³, compressive strength, 2.5-18.7 MPa, isotropic and anisotropic microstructure, etc. The investigation of foaming mechanism and the relationship between properties and structure indicate that the fluidity and dilatation of the foaming precursors significantly affect the foaming performance and foam structure. Raw coal samples were foamed directly without pretreatment in this work. However, for the pitch based foaming precursor, a thermal pretreatment is necessary to adjust its thermoplastic properties to meet the foaming requirement. The mechanical strength of carbon foam is found to be related to not only the foam cell structure, but also the fluidity and anisotropic domain size of the foaming precursors. In addition, the micro and mesopore structure in carbon foam matrix was investigated by gas adsorption and it was found that it also affects the strength of carbon foam and is related to the fluidity of foaming precursor.     

Development of CO2 for polymer foam applications

Polymer foams created using supercritical carbon dioxide as a processing solvent have garnered much interest in recent years. The properties of supercritical CO₂ make it ideally suited to replace organic solvents such as chlorofluorocarbons that are being phased out for environmental reasons. A number of processes and products are already being used in industry, and we provide several examples of these developments. Yet, despite all that is now known about polymer/CO₂ systems, a complete understanding of the foaming process, particularly nucleation, is lacking. In this article, we give an overview of the state of the art in polymer foaming with CO₂ and highlight the challenges that remain to be overcome.     

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

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

聚醚砜超临界CO2发泡行为研究

以超临界CO₂为发泡剂,采用间歇发泡法制备了聚醚砜（PES）泡沫。采用旋转流变仪和扫描电子显微镜分析表征了PES发泡的均相成核行为,继而分别以滑石粉和二氧化硅（SiO₂）作为异相成核剂,探究了PES发泡过程中的异相成核行为。结果表明,间歇发泡法制备PES泡沫的发泡区间为200~230 ℃;最佳浸泡压力为20 MPa;最佳浸泡时间为3 h;未改性PES的泡孔直径均可在10 μm以下,泡孔密度在10⁹~10¹⁰ 个/cm³之间,但泡孔壁较厚;SiO₂相对于Talc,表现出更显著的异相成核作用;在210 ℃、3 h、20 MPa的发泡条件下,添加0.9 %（质量分数,下同）的SiO₂,可得到泡孔直径为0.77 μm,泡孔密度为7.14×10¹¹ 个/cm³的PES微孔泡沫。     

超临界CO2微孔发泡塑料的研究进展

利用超临界CO2发泡制备微孔发泡塑料是当前的研究热点,具有发泡效率高、绿色环保、对制品的机械性能及外观影响小等优点.综述了超临界CO2微孔发泡塑料的研究进展.介绍了超临界CO2的三种发泡机理:均相成核、非均相成核与外力场加强成核机理.概述了间歇式、半连续式和连续式发泡工艺,阐述了发泡过程中防止泡孔黏结、破裂的方法及发泡...     

Foam processing and cellular structure of polylactide-based nanocomposites

Via a batch process in an autoclave, the foam processing of neat polylactide (PLA) and two different types of PLA-based nanocomposites (PLACNs) has been conducted using supercritical carbon dioxide (CO₂) as a foaming agent. The cellular structures obtained from various ranges of foaming temperature-CO₂ pressure were investigated by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The incorporation with nano-clay induced heterogeneous nucleation because of a lower activation energy barrier compared with homogeneous nucleation as revealed by the characterization of the interfacial tension between bubble and matrix. The grown cells having diameter of ∼200 nm were localized along the dispersed nano-clay particles in the cell wall. The dispersed nano-clay particles acted as nucleating sites for cell formation and the cell growth occurs on the surfaces of the clays. The PLACNs provided excellent nanocomposite foams having high cell density from microcellular to nanocellular.     

Carbon foam derived from pitches modified with mineral acids by a low pressure foaming process

Carbon foams with an anisotropic texture and high mechanical strength were produced using precursors obtained after thermo-oxidation treatment of commercial coal-tar pitch with H₂SO₄ and HNO₃. The investigations of the relation between the properties of the precursor and the structure of obtained foam indicate, that the composition and softening point of the pitch precursor significantly affect the foaming process, foam structure and foam mechanical strength. The composition and properties of the modified pitches allow foam formation at relatively low pressure and fast heating rate during the foaming process without a stabilization treatment. The foaming process of pitch-based carbon foams, pre-treatment of the precursors, and the properties of resultant foams are discussed in this paper.     

Graphene nanoribbons (GNRs) by unzipping MWCNTs for the improvement of PMMA microcellular foams

This work presents the cellular microstructures and properties of PMMA/graphene nanoribbons (GNRs) microcellular foams. GNRs were obtained by oxidative unzipping multiwalled carbon nanotubes and solvent thermal reduction in dimethylformamide (DMF), then they were mixed with PMMA to fabricate PMMA/GNRs nanocomposites by solution blending. Subsequently, supercritical carbon dioxide (scCO₂) as a friendly foaming agent was applied to fabricate PMMA/GNRs microcellular foam by a batch foaming in a special mold. The morphology of cell structure was analyzed by scanning electron microscopy and image software, showing that the addition of a smaller content of GNRs caused a fine cellular structure with a higher cell density (～3 × 10¹¹ cells/cm³) and smaller cell sizes (～1 μm) due to their remarkable heterogeneous nucleation effect. The mechanical testing of PMMA/GNRs microcellular foams demonstrated that the obtained GNRs also could be used as a reinforcing filler to increase the mechanical properties of PMMA foams. An improvement in the compressive strength of ～80% (about 39% increase standardized by specific compressive strength) was achieved by 1.5 wt % GNRs addition, and the thermal stability of PMMA/GNRs foams was enhanced too. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45182.