A new method for estimating the cellular structure of plastic foams based on dielectric anisotropy

The characterization of the cellular structure of plastic foams has long been of importance to investigate their physical properties. We have been developing an instrument that measures the orientation of seat materials based on their dielectric anisotropy. It is demonstrated by electromagnetic theory that composite materials with an island-sea structure show a macroscopic anisotropy of dielectric constant because of the shape of islands, even if both the islands and sea are dielectrically isotropic themselves. The orientation based on the dielectric anisotropy caused by the shape of cells was investigated for the polystyrene by using our microwave cavity resonator method. From this data, a better manufacturing condition to minimize the thermal shrinkage was derived. The manufacturing condition was taken as extrusion rate, foaming temperature and die gap. The shape of cells of polystyrene foams was estimated using the derived equations, which express the maximum and minimum dielectric constant of whole composite materials as a function of the eccentricity of their ellipsoidal islands and the volume fraction and the dielectric constant of each materials in the islands-sea structure.     

Tailoring the structure of cellular vitreous carbon foams

Cellular vitreous carbon (CVC) foams have been prepared from tannin-based resin modified by isocyanates of different natures, added in different amounts, and according to different mixing methods. We show that low amounts of isocyanate allow considerable changes in the porous structures of the resultant CVC foams. The latter are now more homogeneous, present thicker cell walls, porous struts, and their cells are less connected with each others. Additionally, average cell size and density can be independently controlled, whereas they were intimately related to each other in the first generation of tannin-based CVC foams. The accessible range of bulk density is also much wider (0.04–0.3 g cm^?3) than before. CVC foams modified with isocyanate present extremely low thermal conductivity (less than 0.1 W m^?1 K^?1), whereas their mechanical properties have been significantly improved (up to 8.4 and 140 MPa for compressive strength and elastic modulus, respectively). Modification by isocyanate thus opened a new route towards CVC foams of controlled porosity and properties.     

Cell structure and dynamic properties of injection molded polypropylene foams

The cell structure and properties of branched and linear polypropylene (PP) foams containing organically modified nanoclay and maleic anhydride grafted polypropylene (PPMA) have been thoroughly investigated. X-ray diffraction (XRD) and melt rheometry were used to identify the structure and linear viscoelastic properties of the nanocomposites, as well as the effectiveness of two different compatibilizers. These nanocomposites were used in injection molding to investigate their foamability and the influence of experimental conditions such as chemical foaming agent concentration, shot size, back pressure, injection speed, as well as melt temperature and different injection methods on the resulting cell structure of the foams. Quite different results were obtained with the linear and the branched PP. While the foamability of the branched PP was intrinsically good, that of the linear one could largely be improved by modifying its rheological properties and favoring nucleation through the addition of nanoclay. The effect of cell structure on the dynamic mechanical properties of the foams was also investigated using dynamic mechanical analysis (DMA). POLYM. ENG. SCI., 47:1070–1081, 2007. © 2007 Society of Plastics Engineers     

聚丙烯/蒙脱土纳米复合材料研究

本文综述了聚丙烯／蒙脱土纳米复合材料的特点、制备方法,并对复合材料的结构表征方法作了介绍。     

纳米复合抗菌丙纶性能研究

将聚丙烯 ,纳米陶瓷粒子 ,沸石混合造粒制得抗菌母粒 ,聚丙烯切片与抗菌母粒共混熔融纺丝 ,得到纳米复合抗菌丙纶。测试了纤维的抗菌性能、热性能、力学性能 ,并对纳米粒子及纤维进行了扫描电镜分析。结果表明 :纳米抗菌剂最佳含量在 0 .8%左右 ,纤维抑菌率达 90 %以上 ,且耐久性好。纤维结晶度下降 ,而熔点提高。纳米抗菌剂在纤维中有少量凝聚 ,纤维断裂强度略有降低 ,但能够满足加工及服用要求     

Effect of clay dispersion on the cell structure of LDPE/clay nanocomposite foams

In this study, the effect of clay and its dispersion state on the cell morphology and foaming behavior of chemically crosslinked polyethylene (PE) foams were examined. In addition, the effect of foaming process on the clay morphology was also considered. It was shown that the morphology of the clay before the foaming process and its compatibility with PE matrix play a major role in determining the final foam properties. A PE‐g‐MA compatibilizer was used to increase the melt intercalation of PE onto the clay galleries and to improve clay dispersion in the PE matrix. The uniform dispersion of clay provided greater and well‐ dispersed nucleation sites. This led to smaller cell size, narrower cell size distribution, and higher cell density, and lower foam density. During the foaming process, intercalated clays were delaminated due to the rapid polymer melt expansion that inhibited gas release and increased foam expansion ratio. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Foam processing and cellular structure of polypropylene/clay nanocomposites

Polypropylene (PP)/clay nanocomposites (PPCNs) were autoclave‐foamed in a batch process. Foaming was performed using supercritical CO₂ at 10 MPa, within the temperature range from 130.6°C to 143.4°C, i.e., below the melting temperature of either PPCNs or maleic anhydride‐modified PP (PP‐MA) matrix without clay. The foamed PP‐MA and PPCN2 (prepared at 130.6°C and containing 2 wt% clay) show closed cell structures with pentagonal and/or hexagonal faces, while foams of PPCN4 and PPCN7.5 (prepared at 143.4°C, 4 and 7.5 wt% clay) had spherical cells. Scanning electron microscopy confirmed that foamed PPCNs had high cell density of 10⁷–10⁸ cells/mL, cell sizes in the range of 30–120 μm, cell wall thicknesses of 5–15 μm, and low densities of 0.05–0.3 g/mL. Interestingly, transmission electron microscopic observations of the PPCNs' cell structure showed biaxial flow‐induced alignment of clay particles along the cell boundary. In this paper, the correlation between foam structure and rheological properties of the PPCNs is also discussed.     

Synthesis of polystyrene-carbon nanofibers nanocomposite foams

In this study, the use of carbon nanofibers (CNFs) as nucleating agents to produce polystyrene nanocomposite foams was demonstrated. With the addition of CNFs, microcellular foams with uniform cell size distributions were obtained. Compared to nanoclay and single-walled carbon nanotubes (SWCNTs), CNFs exhibit substantially higher nucleation efficiency in the foaming process. The underlying mechanism is semi-quantitatively explained by the classical nucleation theory. The homogeneous fiber distribution and favorable surface and geometrical characteristics of CNFs make them ideal nucleating agents.     

Effects of silica particle size on the structure and properties of polypropylene/silica composites foams

In this study, we prepared organically modified silica materials with various particle sizes, in the ranges of micrometer and nanometer, and their polypropylene (PP) composites. The PP micro/nanocomposites were then molded through conventional and microcellular injection molding processes. The effects of silica particle sizes on the structure, mechanical and rheological properties were investigated. The results showed that PP/silica nanocomposites provide better tensile strength than that of foamed nanocomposites. The addition of silica also increased the tensile strength of the nanocomposites, but decreased the tensile strength of microcomposites. Therefore, the tensile strength of PP/silica nanocomposites is better than that of PP/silica microcomposites. The silica particles helped the nanocomposites to develop small cells in the foaming process. Rheological results indicated an increase in the viscosity with the addition of nano silica and micro silica to PP. The viscosity increase for the silica nanocomposites was found greater than that of microcomposites at the same filler content.     

热氧老化对硅橡胶发泡材料泡孔结构与性能的影响

采用热氧加速老化的方法研究了硅橡胶发泡材料（SRF）的老化性能,探究了不同老化时间对SRF材料的泡孔结构和性能的影响规律。结果表明,不同密度SRF的泡孔结构不同,密度较大的SRF泡孔尺寸较小,泡孔壁较厚,闭孔比例较大。在热氧老化过程中,随着老化时间的延长,泡孔结构逐渐遭到破坏;当老化时间相同时,密度越大的SRF泡孔结构遭到的破坏情况越严重。此外,结合SRF的泡孔微观结构和性能来看,SRF泡孔结构的破坏会导致其压缩永久变形率和应力损失率升高。     

Extrusion of polypropylene foams with hydrocerol and isopentane

An experimental investigation was conducted to research the cell nucleation behavior in the extrusion foam processing of polypropylene (PP) using hydrocerol and isopentane. While the hydrocerol and isopentane are considered to function as the nucleating agent which determines the cell-population density and as the blowing agent to control the volume expansion ratio respectively, both agents affected the cell-population density. In addition, synergistic effects of these agents on the cell density were observed. In foam processing with hydrocerol, a higher cell-population density was noted at lower processing pressures and at higher polymer flow rates. This phenomenon is of interest since the cell density, in general, increases as the processing pressure increases when only a physical blowing agent is used in the foam processing. The experimental results indicate that the nucleation in the foam processing with hydrocerol is governed by a heterogeneous nucleation mechanism; also, the quality of the mixing of the polymer and the agents, as well as the amount of gas lost during the plastication of the pellets in the barrel, exerts a strong influence on the resulting cell density.     

Synthesis and processing of PMMA carbon nanotube nanocomposite foams

Poly(methyl methacrylate) (PMMA) multi-walled carbon nanotubes (MWCNTs) nanocomposites were synthesized by several methods using both pristine and surface functionalized carbon nanotubes (CNTs). Fourier transform infrared (FTIR) spectroscopy was used to characterize the presence and types of functional groups in functionalized MWCNTs, while the dispersion of MWCNTs in PMMA was characterized using scanning electron microscopy (SEM). The prepared nanocomposites were foamed using carbon dioxide (CO₂) as the foaming agent. The cell morphology was observed by SEM, and the cell size and cell density were calculated via image analysis. It was found that both the synthesis methods and CNTs surface functionalization affect the MWCNTs dispersion in the polymer matrix, which in turn profoundly influences the cell nucleation mechanism and cell morphology. The MWCNTs are efficient heterogeneous nucleation agents leading to increased cell density at low particle concentrations. A mixed mode of nucleation mechanism was observed in nanocomposite foams in which polymer rich and particle rich region co-exist due to insufficient particle dispersion. This leads to a bimodal cell size distribution. Uniform dispersion of MWCNTs can be achieved via synergistic combination of improving synthesis methodology and CNTs surface functionalization. Foams from these nanocomposites exhibit single modal cell size distribution and remarkably increased cell density and reduced cell size. An increase in cell density of ∼70 times and reduction of cell size of ∼80% was observed in nanocomposite foam with 1% MWCNTs.     

Influence of the dispersion of Nanoclays on the cellular structure of foams based on polystyrene

In the present work blends of polystyrene (PS) with sepiolites have been produced using a melt extrusion process. The dispersion degree of the sepiolites in the PS has been analyzed by dynamic shear rheology and X-ray micro-computed tomography. Sepiolites treated with quaternary ammonium salts (O-QASEP) are better dispersed in the PS matrix than natural sepiolites (N-SEP) or sepiolites organo-modified with silane groups (O-SGSEP). A percolated network is obtained when using 6.0 wt% of O-QASEP, 8.0 wt% of N-SEP and 10.0 wt% of O-SGSEP. It has been shown that multiple extrusion processes have a negative effect on the polymer architecture. They produce a reduction in the length of the polymeric chains, and they do not lead to a better dispersion of the particles in the polymer matrix. Foams have been produced using a gas dissolution foaming process, where a strong effect of the dispersion degree on the cellular structure of the different foams was found. The effects on the cellular structure obtained by using different types of sepiolites, different contents of sepiolites and different extrusion conditions have been analyzed. The foams produced with the formulations containing O-QASEP present the lowest cell size and the most homogeneous cellular structures.     

Electrical conductivity of polyurethane/MWCNT nanocomposite foams

Polymeric foams with electrical conductivity represent a novel and very interesting class of materials rather sporadically studied. In this study, the feasibility to prepare electrically conductive Rigid Polyurethane (PUR) foams at various densities using multiwall carbon nanotubes (MWCNT) at varying contents was investigated. The produced PUR/MWCNT foams exhibited electrical conductivity over a wide range of densities and nanofiller contents. The effect of these two parameters on the electrical conductivity of the final foam system was studied. To explain the behavior, Statistical Percolation laws were employed. Model parameters were elaborated for both cases showing that the percolation model can adequately describe the behavior. The foaming process is analyzed in further detail to assist in the explanation and understanding of theexperimental observations. Finally, a material design map is proposed for the preparation of electrically conductive foams. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

Fabrication of microcellular polymer/graphene nanocomposite foams

Graphene has recently gained revolutionary aspirations because of its remarkable electronic, thermal and mechanical properties. These unique properties make it promising for preparing multifunctional nanocomposites. In recent years, polymer foams based on graphene have also received increasing attention in both the scientific and industrial communities. This review presents an overview of polymer/graphene nanocomposite foams discussing the production of graphene, the polymer functionalization of graphene and different polymer/graphene foams with different properties. One of the most promising avenues is to fabricate tough and lightweight materials with superior electrical and electromagnetic interference shielding properties. Copyright © 2012 Society of Chemical Industry     

Broad-band electrical conductivity of carbon nanofibre-reinforced polypropylene foams

The influence of foaming a semi-crystalline polymer reinforced with different concentrations of carbon nanofibres (0–20 wt.%) on the formation of an electrically conductive network was studied at room temperature using an impedance analyzer over a wide interval of frequencies (from 10⁻² to 10⁶ Hz). Composites were prepared by melt-compounding using a twin-screw extruder, and later chemically foamed. Although composite materials displayed lower conductivities than expected, assuming a percolative behavior, foaming promoted a tunnel-like conduction at lower CNF concentrations than in the solids. At higher CNF concentrations, no great improvements were achieved as tunneling conduction decreased with increasing local crystallinity. Foams showed electrical conduction characteristics typical of a conductive random-distributed fibre-like system, while the behavior of the solids was closer to a system of spherical particles, related to CNF aggregation. The anisotropic cellular structure of the 20 wt.% CNF composite foamed by a physical foaming process disrupted the preferential in-plane CNF orientation attained during solid preparation, with these foams showing higher through-plane conductivity and more isotropic electrical properties than the chemically-foamed ones. It has been demonstrated that foaming PP–CNF composites resulted in the formation of a conductive network at lower CNF concentrations than in the solids, with foams showing the potential for use in conductive high-performance lightweight composite systems.     

A Taguchi analysis on structural properties of polypropylene microcellular nanocomposite foams containing Fe2O3 nanoparticles in batch process

Polypropylene (PP) as a thermoplastic polymer has been foamed using batch foaming process. CO₂ is used as the blowing agent of the foaming. Ferrous oxide nanoparticles (nano Fe₂O₃) are also added as reinforcement. Effect of different parameters including nanoparticle weight percentage, foaming temperature and time on the structural properties of PP/nano Fe₂O₃ nanocomposites is investigated using Taguchi approach. Scanning electron microscope results depict that an appropriate microcellular structure is obtained with the cell density of 10⁹ cells/cm³ and almost 1 μm of cell size. Analysis of variance results indicated that foaming temperature is the most significant parameter on the structural properties. Cell density and expansion ratio are decreased by increasing foaming temperature. This phenomenon could be due to the reducing melt strength of polymer/gas mixture. It was also inferred that adding 2 wt% of nanoparticles leads to 80% improvement in cell density while cell size and expansion ratio was decreased.     

Preparation and insulation property studies of thermoplastic PMMA-silica nanocomposite foams

The comparative studies for the effect of vinyl-modified silica (VMS) and raw silica (RS) particles on the cell structure, insulation (dielectric and thermal transport) properties, and thermal stability of thermoplastic PMMA-silica nanocomposite (PSN) foams are described. The VMS particles were synthesized by the conventional acid-catalyzed sol-gel reactions of tetraethyl orthosilicate (TEOS) in the presence of 3-(trimethoxysilyl)propyl methacrylate (MSMA) molecules. The as-prepared VMS particles were then characterized through fourier transform infrared (FTIR), solid-state ¹³C-nuclear magnetic resonance (¹³C-NMR) and ²⁹Si-NMR spectroscopy. Subsequently, the PSN materials were prepared via in-situ bulk polymerization. The dispersion of silica particles in PMMA matrix was observed by transmission electron microscopy (TEM) studies. Gel permeation chromatography (GPC) was used to determine the molecular weights of as-prepared samples. The PSN materials were used to produce foams by a batch process in an autoclave using nitrogen as foaming agent. The effect of VMS and RS particles on the cell structure, insulation properties and thermal stability of PSN foams were investigated by scanning electron microscopy (SEM), LCR meter, Transient plane source (TPS) technique and thermal gravimetric analysis (TGA), respectively. The better dispersion capability of VMS particles in PSN foams was found to lead enhanced nucleation efficiency, thermal stability and decreased dielectric constant (ε′), dielectric loss (ε″) and thermal conductivity (k). POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Thermal and mechanical properties of a polypropylene nanocomposite

An experimental polypropylene (PP) nanocomposite, containing approximately 4 wt % of an organophilic montmorillonite clay, was prepared and characterized, and its properties were compared with those of talc‐filled (20–40 wt %) compositions. Weight reduction, with maintained or even improved flexural and tensile moduli, especially at temperatures up to 70°C, was a major driving force behind this work. By a comparison with the analytical data from a nylon 6 (PA‐6) nanocomposite, it was found that the PP nanocomposite contained well‐dispersed, intercalated clay particles; however, X‐ray diffraction, transmission electron microscopy, dynamic mechanical analysis, and permeability measurements confirmed that exfoliation of the clay in PP was largely absent. The increased glass‐transition temperature (T_g) of a PA‐6 nanocomposite, which possessed fully exfoliated particles, indicated the molecular character of the matrix–particle interaction, whereas the PP nanocomposite exhibited simple matrix–filler interactions with no increase in T_g. The PP nanocomposite exhibited a weight reduction of approximately 12% in comparison with the 20% talc‐filled PP, while maintaining comparable stiffness. Undoubtedly, considerable advantages may be available if a fully exfoliated PP nanocomposite is fabricated; however, with the materials available, a combination of talc, or alternative reinforcements, and nanocomposite filler particles may provide optimum performance. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1639–1647, 2003     

抗冲聚丙烯结构与性能研究

对部分国内外抗冲聚丙烯(PP)产品进行了微观形态和结构分析，研究其对材料宏观力学性能的影响。实验结果表明：抗冲PP是一个含有PP均聚物、丙烯与乙烯-丙烯两嵌段共聚物、乙丙橡胶(EPR)、聚乙烯均聚物等的多相体系。EPR的分子序列结构对聚合物抗冲击性能起主要作用。在序列结构中，丙烯、乙烯单体在分子链上的位置交换越频繁，抗冲击性能越得到提高。丙烯序列平均长度的增大对抗冲击性能有一定的削弱作用。