Production of polystyrene microcellular foam plastics and a comparison of late‐ and quick‐heating

A batchwise process for the production of microcellular plastics was studied in the polystyrene–nitrogen system. The effects of saturation temperature, saturation pressure, and late‐ and quick‐heating on the microcellular structure were investigated by considering the solubility of the gas in the polymer. It was found that the mean cell diameter was reduced and the cell number density increased with increase in the gas solubility. Variation in the saturation temperature showed that the cell number density had a minimum and the mean cell diameter had a maximum at about 350 K, which was related to the minimum solubility of nitrogen in polystyrene. The long heating time at 393 K of a solution saturated under 25 MPa increased the cell diameter, reduced the cell number density, and gave a maximum volume expansion ratio at about 300 s. Further heating caused the cell size and volume expansion ratio to be decreased, which might be caused by diffusion of the gas out of the polymer sample. The effect of the saturation temperature under high saturation pressure on the cell number density was qualitatively well predicted by the nucleation theory. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 2383–2395, 2000     

动态发泡工艺参数对PS微孔塑料泡孔结构的影响

以超临界CO2为发泡剂,用振动诱导发泡模拟装置研究了微孔塑料动态成型过程中气体饱和压力、压力释放速率、温度、气体饱和时间、稳态剪切速率、振动等工艺参数对聚苯乙烯（PS）微孔塑料泡孔结构的影响。研究发现,PS微孔塑料试样的泡孔结构随着气体饱和压力和压力释放速率的提高而得到改善,而温度、气体饱和时间、稳态剪切速率则存在一个最佳的操作范围,在此范围内制得的PS微孔塑料试样泡孔密度最大,泡孔尺寸最小。在稳态剪切速率一定的情况下,通过施加振动可以进一步改善泡孔结构．     

Production of microcellular foam plastics by supercritical carbon dioxide

The production of microcellular plastic was studied in the polymethyl metacrylate (PMMA)-supercritical carbon dioxide and polycarbonate (PC)-supercritical carbon dioxide systems. The test pieces of PMMA and PC were put into a saturation vessel of which temperature and pressure were kept constant. Supercritical carbon dioxide at temperature between 303K and 393K and pressure between 100 bar and 250 bar was used as a foaming agent. After saturation of carbon dioxide, the pressure was quickly released to atmospheric pressure. The samples were immediately taken out from the vessel and heated in an oil bath. The fractured part of the sample was used for microstructure analysis with SEM. The effect of the saturation temperature, pressure of sorption and the foaming time on the cell mean size and cell density of the foam was investigated by considering the solubility of carbon dioxide in PMMA and PC. The foam morphologies of the foamed plastics were affected by solubility of carbon dioxide, which was directly related to saturation temperature and pressure. The cell density increased and, consequently, the cell size decreased with the solubility of carbon dioxide. The foaming time can be used a controlling factor to obtain the desired foam structure and the volume expansion ratio.     

Open‐celled microcellular thermoplastic foam

A theoritical model of the production of open‐cell microcellular foam is presented. This model allows the prediction of the conditions necessary to produce these materials. Experiments verify the model quite well. The results of the batch processing experiments indicate the processing parameters that promote the development of open‐celled microcellular polystryene foam. A saturation pressure of 17.2 MPa (2500 psig) provides the nucleation density necessary to form an impinged structure with microcellular bubble density. A foaming temperature of 200°C promotes the formation of both internal and surface porosity. A scaled time between 1 and 2.7 seconds develops a foam structure that intrudes a large volume. Samples foamed at 200°C for 1 and 2 seconds possess pores less than 1 μm in diameter. These samples represent scaled times of 1 and 2 seconds. Therefore, to produce open‐celled microcellular polystyrene foam with batch processing, samples should be saturated at approximately 17.2 MPa (2500 psig) and foamed for a scaled time between 1 and 2 seconds.     

新型多孔聚酯纤维的制备

介绍了一种制备发泡聚酯纤维的方法,并研究了处理工艺与多孔结构的影响。通过光学显微镜结果分析及计算,研究了加压压力、加压时间、发泡温度及发泡时问等参数对纤维中气泡密度的影响。结果表明,在其他工艺条件不变的情况下,发泡聚酯纤维中气泡密度分别随着压力、加压时间、发泡温度和发泡时间的增加而增大。其中发泡时间大于10 s后,时间对气泡密度无明显影响。     

Preparation and morphology characterization of microcellular styrene‐co‐acrylonitrile (SAN) foam processed in supercritical CO2

Microcellular polymeric foam structures have been generated using a pressure‐induced phase separation in concentrated mixtures of supercritical CO₂ and styrene‐co‐acrylonitrile (SAN). The process typically generates a microcellular core structure encased by a non‐porous skin. Pore growth occurs through two mechanisms: diffusion of CO₂ from polymer‐rich regions into the pores and also through CO₂ gas expansion. The effects of saturation pressure, temperature and swelling time on the cell size, cell density and bulk density of the porous materials have been studied. Higher CO₂ pressures (hence, higher fluid density) provided more CO₂ molecules for foaming, generated lower interfacial tension and viscosity in the polymer matrix, and thus produced lower cell size but higher cell densities. This trend was similar to what was observed in swelling time series. While the average cell size increased with increasing temperature, the cell density decreased. The trend of bulk density was similar to that of cell size. © 2000 Society of Chemical Industry     

Microcellular foaming of low‐density polyethylene using nano‐CaCo3 as a nucleating agent

In this study, microcellular foaming of low‐density polyethylene (LDPE) using nano‐calcium carbonate (nano‐CaCO₃) were carried out. Nanocomposite samples were prepared in different content in range of 0.5–7 phr nano‐CaCO₃ using a twin screw extruder. X‐ray diffraction and scanning electron microscopy (SEM) were used to characterize of LDPE/nano‐CaCO3 nanocomposites. The foaming was carried out by a batch process in compression molding with azodicarbonamide (ADCA) as a chemical blowing agent. The cell structure of the foams was examined with SEM, density and gel content of different samples were measured to compare difference between nanocomposite microcellular foam and microcellular foam without nanomaterials. The results showed that the samples containing 5 phr nano‐CaCO₃ showed microcellular foam with the lowest mean cell diameter 27 μm and largest cell density 8 × 10⁸ cells/cm³ in compared other samples. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Investigation on microstructure and properties of foamed (wood fiber)/(recycled polypropylene) composites

Microcellular foamed (wood fiber)‐reinforced recycled polypropylene composites (MFWPCs) were prepared by an injection molding process where azodicarbonamide was used as a chemical foaming agent. The influence of injection parameters (injection temperature, dwell pressure) on the microcellular structure (cell diameter and cell density) and the mechanical properties of the MFWPCs were investigated. The results indicated that when the melting temperature was 180°C and the dwell pressure was 12.5 MPa, a uniformly distributed microcellular structure of MFWPCs was obtained. Compared with solid wood plastic composites, the density of the MFWPCs decreased by 24.5%, and its impact strength of MFWPCs increased by 53%, because the propagation direction of the crack changed to the “skip” or “bifurcation” mechanism as a result of the microcellular structure, and the surrounding matrix of this structure made it easy to produce forced high‐elastic deformation. The toughening mechanism of the microcellular structure was analyzed. J. VINYL ADDIT. TECHNOL., 2012. © 2012 Society of Plastics Engineers     

Microcellular extrusion‐foaming of polylactide with chain‐extender

The effects of adding epoxy‐functionalized chain‐extender (CE) and processing conditions such as die temperature on the cell morphology, volume expansion ratio (VER), open cell content (OCC), and crystallization of microcellular extruded polylactide (PLA) were investigated. When compared with pure PLA, the addition of talc decreased the average cell size and increased the cell density. Moreover, the simultaneous addition of talc and CE led to denser and more uniform cell structure up until 1.0% CE content. In general, the volume expansion ratio and open cell content of all the formulations decreased with the die temperature. The addition of talc decreased both VER and OCC, whereas the addition of CE increased both. The die temperature did not affect the degree of crystallinity significantly. The addition of talc increased the crystallinity, but the addition of CE decreased it. Finally, the molecular weight of PLA increased significantly with the addition of CE. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Effects of Process Variables on Microcellular Structure and Crystallization of Polypropylene Foams with Supercritical CO2 as the Foaming Agent—A Study of Microcellular Foaming of Polypropylene

The effects of process variables on the microcellular structure and crystallization of foamed polypropylene (PP) with supercritical CO₂ as the foaming agent were investigated in this article. The cell size increased and the cell density reduced with increased foaming temperature. Differently, both the cell diameter and cell density increased as saturation pressure increased. DSC curves showed that the melting peak was broadened when supercritical CO₂ foaming PP. Furthermore, the width at half-height of the melting peak increased, the melting peak moved to higher temperature, and the melting point and crystallinity enhanced as the foaming temperature lowered and the saturation pressure enhanced.     

Generation of microcellular polymeric foams using supercritical carbon dioxide. II: Cell growth and skin formation

We have generated microcellular polymeric foam structures using a pressure induced phase separation in concentrated mixtures of supercritical CO₂ and poly(methyl methacrylate). The process typically generates a microcellular core structure encased by a nonporous skin, the thickness of which decreases with increasing saturation pressure. This trend can be described by a model for skin formation that is based on the diffusion rate of gas out of the sample. Significant density reductions on the order of 30 to 70% can be achieved by changing the pressure and temperature conditions in the foaming process. There are several ways in which the saturation pressure affects the average cell size, with the net effect that cell size decreases sharply with increasing pressure above 2000 psi, leveling out at higher pressures. Cell size increases with increasing temperature from 40°C to 70°C. A model for cell growth, based on a cell model of Aremanesh and Advani, modified to include the effect of CO₂ on model parameters, reproduces these trends.     

A simple method for preparation of polymer microcellular foams by in situ generation of supercritical carbon dioxide from dry ice

In this work, poly(methyl methacrylate) (PMMA) and PMMA/nanoclay nanocomposite microcellular foams were successfully prepared using a simple method based on in situ generation of supercritical carbon dioxide (CO₂) from dry ice. The method was compared with conventional methods exempted from high pressure pump and a separate CO₂ tank. Effect of various processing conditions such as saturation temperature and pressure and clay concentration on cellular morphology and hardness of the prepared microcellular foams was examined. State of the clay dispersion in the prepared PMMA/clay nanocomposites was characterized using X-ray diffraction and transmission electron microscopy techniques. Field emission scanning electron microscopy was used to study cellular morphology of the prepared foams. It was observed that elevation of saturation temperature from 85 to 105 °C at constant saturation pressure increased cell density and decreased average cell size of the prepared PMMA foams. Furthermore, an increase in saturation pressure from 120 to 180 bar resulted in a reduction in average cell diameter and an increase in cell density of the prepared PMMA foams. On the basis of the gathered results, optimum conditions for preparation of PMMA microcellular foams were determined and applied for preparation of PMMA/nanoclay microcellular foams. It was shown that incorporation of clay into the polymer matrix resulted in a finer and more uniform cellular morphology in the final microcellular foams. It was also observed that incorporation of nanoclay into the prepared foams, up to 3 wt%, led to a moderate increase in the foam hardness.     

Microcellular injection molding of in situ modified poly(ethylene terephthalate) with supercritical nitrogen

The microcellular injection molding (commercially known as MuCell) of in situ polymerization‐modified PET (m‐PET) was performed using supercritical nitrogen as the physical blowing agent. Based on the design of experiment matrices, the influence of operating conditions on the mechanical properties of molded samples was studied systematically for two kinds of m‐PETs, namely, n‐m‐PET and m‐m‐PET synthesized using pentaerythritol and pyromellitic dianhydride (PMDA) as modifying monomers, respectively. Optimal conditions for injection molding were obtained by analyzing the signal‐to‐noise (S/N) ratio of the tensile strength of the molded samples. The specific mechanical properties, especially the impact strength, of the microcellular samples under those optimal conditions increased significantly. Scanning electron microscope analyses showed a uniform cell structure in the molded specimens with an average cell size of around 35 µm. The m‐m‐PET modified with PMDA generated a slightly finer cell structure and a higher cell density than the n‐m‐PET. POLYM. ENG. SCI., 54:2739–2745, 2014. © 2013 Society of Plastics Engineers     

Microcellular extrusion of PLA utilizing solid‐state nucleation in the gas‐saturated pellet extrusion process

In this study, we explore the use of solid‐state nucleation in polymer pellets as a means to create microcellular PLA foams in extrusion. This is achieved by using gas‐saturated PLA pellets as input to the extruder. Foam density, bubble size, and bubble density is reported and compared with microcellular foams created in the gas‐injection extrusion process. PLA pellet gas concentrations between 17 and 29 mg CO₂/g PLA was found to produce quality microcellular foams in this process. Gas concentrations within this range were achieved by varying methods that included partial saturation, desorption from full saturation, and blending saturated with unsaturated pellets. This gas concentration window that produced microcellular foams was found to be independent of the saturation and desorption process used to achieve the desired concentration. We further compare the pressure drop and pressure drop rate of the gas‐saturated pellet extrusion process showing that similar foams can be produced at pressures orders of magnitude lower than the alternative gas‐injection extrusion processes. Investigations into extrusion pressures support the hypothesis that the gas‐saturated pellet extrusion process utilizes solid‐state nucleation in the feed section of the extruder to achieve high bubble density foams. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

氯化聚乙烯/废旧轮胎胶粉共混发泡性能研究

采用模压法进行发泡,以氯化聚乙烯（CM）为基体,研究了胶粉用量、硫化温度、硫化助剂对CM发泡体的泡体性能、泡孔结构的影响。结果表明,CM发泡体随着胶粉用量的增加,发泡密度逐渐增加,泡孔体积及发泡倍率逐渐减少。综合CM发泡体的泡体性能及泡孔结构,当胶粉用量为10份,170℃下硫化7min,使用TCHC作硫化助剂时,可得到发泡效果好,断面均匀分布,泡孔结构明显的CM发泡体。     

Low density microcellular foam processing in extrusion using CO2

A continuous extrusion process for the manufacture of low-density microcellular polymers is presented. Microcellular polymers are foamed plastics characterized by a cell density greater than 10⁹ cells/cm³ and a fully grown cell size on the order of 10 μm. Previous research on continuous processing of microcellular polymers has focused on control of microcell nucleation in extrusion. This paper presents an effective means for control of cell growth to achieve a desired expansion ratio with CO₂ as a blowing agent in microcellular foam processing. Also, a strategy to prevent deterioration of the cell-population density via cell coalescence during expansion is presented. Promotion of a desired volume expansion ratio and prevention of cell coalescence in microcellular foam processing were experimentally verified. By tailoring the extrusion processing parameters, microcellular HIPS foams with a cell density of 10¹⁰ cells/cm³ and a controlled expansion ratio in the range of 1.5 to 23 were obtained.     

Effects of the properties of blowing agents on the processing and performance of extruded starch acetate

The extrusion of polysaccharide‐based polymers, such as starch acetate, is quite different from that of ordinary synthetic polymers. To understand how the physiochemical properties of blowing agents affect plasticization and expansion processes, starch acetate was extruded with water, ethanol, and ethyl acetate. The studied properties and factors were the evaporation rate, surface tension, boiling point, solubility index, latent heat of vaporization of blowing agents, extrusion temperature, and nucleating‐ and blowing‐agent concentrations. The properties of the blowing agents and operating conditions affected the solubility of the matrix polymer, the nucleation process, and cell growth, which affected the foam density and specific volume. A high temperature increased the cell density and specific volume when water and ethanol were used because a high temperature increased the solubility of starch acetate in water and ethanol and promoted nucleation. Ethyl acetate already had high solvency to starch acetate and a high evaporation rate. A high temperature reduced the melting strength, thereby reducing the cell density and specific volume. Water evaporation was greater, despite a high latent heat of evaporation (hr) and boiling point, than the average volumes of ethanol and ethyl acetate that evaporated. The blowing‐agent efficiency was a function of the solvency, blowing‐agent evaporation rate, and operating conditions. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1880–1890, 2005     

Solid‐state microcellular polycarbonate foams. II. The effect of cell size on tensile properties

The effect of cell size on the tensile behavior of high‐relative‐density microcellular polycarbonate foams is investigated. Microcellular PC foams were produced in a way that allowed the average cell size to be varied, while the foam density was held constant. The polycarbonate‐CO₂ system offers an order of magnitude variation in the average cell size at a given density, allowing the tensile properties of microcellular polycarbonate to be investigated as a function of cell size. It was found that the tensile modulus, tensile strength, elongation to break, and toughness are not significantly affected when the average cell size is varied from 2.8 to 37.1 μm, and the nominal relative density is held constant at 0.5. This result is significant for solid‐state processing of microcellular polycarbonate foams of the type produced here, for it shows that regardless of the processing conditions and regardless of the average cell size, if two foams have the same density then they will also have the same tensile properties. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

玉米秸秆颗粒热解活化过程的热力学和动力学研究

利用热重分析研究了玉米秸秆颗粒制备活性炭的热解和活化过程.热解采用氮气为保护气体,升温速率 K/min,终温723 K,恒温时间30 min.热解过程主要发生在40～620 K之间,热解炭材微孔结构和机械强度良好.活化采用CO2为活化剂,升温速率10 K/min,终温1073 K,恒温时间20 min,活化反应主要发生在90～1070 K之间,得到的活性炭具有较大的比表面积(404.6 m²/g)和优良的吸附性能.通过对热解和活化过程的热重及差热变化综合分析,确定了两段热化学反应的活化能和频率因子.了解了热解和活化过程的热力学和动力学特性,为制备优质的生物质颗粒活性炭提供理论指导作用.     

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.