Microcellular structure of a thin polycarbonate sheet prepared by compression molding

Microcellular thin polycarbonate sheets have been prepared by compression molding with the cell size in the range of 2∼20 microns, and cell density larger than 10⁸ cells/cm³. The effect of processing parameters on the microcellular polycarbonate structure has been investigated. The cell size decreases with increasing foaming time till 8 minute and then increases. Besides this parameter slightly decreases with increasing foaming pressure, but increases with increasing temperature. The variation of cell density is contrary to that of cell size, and the foam density decreases with increasing foaming pressure and foaming temperature and displays a variable trend with increasing foaming time under different foaming pressures.     

Foaming of trans‐polyisoprene using N2 as the blowing agent

Foaming of trans‐1,4‐polyisoprene (TPI) polymer was carried out through a batch process using nitrogen (N₂) as the blowing agent. TPI vulcanizates having varying crosslink densities were prepared by varying crosslinking agent content and curing time. The vulcanizates were then saturated with N₂ inside a pressure vessel at a pressure of 14 MPa and varying temperatures for 5 hours before effecting the foaming by rapidly quenching the pressure. The effects of varying the crosslinking agent content, silica filler content, and precuring time of the vulcanizates and the effects of varying the gas saturation temperature of foaming on the cell characteristics and physical properties of the foam prepared were investigated. The cells of the TPI foams had a spherical, closed structure. The density, expansion ratio, cell size, cell density, and tensile properties of the foams varied with varying crosslink density of the TPI vulcanizates as well as the saturation temperature of foaming. The important effects of crosslink density and saturation temperature on the N₂ solubility in the TPI matrix and thus on the foam expansion were discussed. The silica filler was found to be acting as a cell nucleating agent and reinforcing filler for the TPI foams.     

Dynamic behavior of nucleation in supercritical N2 foaming of polystyrene-aluminum oxide nanocomposite

In this study, dynamic behaviour of nucleation was investigated during foaming process of polystyrene in presence of nano aluminium oxide. Nano aluminium oxide played a role of a bubble nucleating agent within polystyrene matrix. Foaming process was visually observed in conjunction with supercritical N₂. Furthermore, the effect of nano Al₂O₃ compositions on the growth rate was studied. Also, final density per unit area and the average size in the latest growth steps were assessed. The obtained data were compared with foaming process for unfilled polystyrene under the same conditions. The results demonstrated that the final sizes of bubbles as well as the average cell density of the foam were reduced by using of nucleating agent. The growth rate of bubbles was also decreased by increasing the nano Al₂O₃ particles content. In addition, influence of temperature on foam density of nanocomposite specimens was greater than unfilled polystyrene foam. In presence of nano particle, the cell density was uniformly distributed in nanocomposites specimens. The article is published in the original.     

A novel method to prepare microcellular poly(vinyl alcohol) foam based on thermal processing and supercritical fluid

Combining the thermal processing and supercritical fluid technology develops a novel preparation method of microcellular poly(vinyl alcohol) (PVA). Water, as the plasticizer in system, can form the hydrogen bonding with pendant hydroxyl of PVA and weaken its strong intermolecular and intramolecular forces to realize the thermal processing. Supercritical carbon dioxide (sc‐CO₂) can easily dissolve into water‐plasticized PVA (WPVA) because of the destruction of crystal region caused by water, and the enhanced sc‐CO₂ solubility can greatly improve the foamability of WPVA. The porous structure generates through the saturation of sc‐CO₂ in WPVA sample and followed by pressure drop‐induced phase separation. The foaming behavior of WPVA was studied as a function of saturation pressure, foaming temperature, and saturation time. The cell density, cell size, and distribution of the obtained foam can be controlled by tuning processing conditions. The results revealed that the cell size decreased, and its distribution narrowed with saturation pressure increasing, or decrease of foaming temperature. But excessively increasing the saturation time generated a negative effect on the foaming behavior owing to the deteriorated plasticization effect resulted from the loss of water. Copyright © 2016 John Wiley & Sons, Ltd.     

Supercritical CO<Subscript>2</Subscript> assisted preparation of open-cell foams of linear low-density polyethylene and linear low-density polyethylene/carbon nanotube composites

The open-cell structure foams of linear low-density polyethylene (LLDPE) and linear low-density polyethylene (LLDPE)/multi-wall carbon nanotubes (MWCNTs) composites are prepared by using supercritical carbon dioxide (sc-CO₂) as a foaming agent. The effects of processing parameters (foaming temperature, saturation pressure, and depressurization rate) and the addition of MWCNTs on the evolution of cell opening are studied systematically. For LLDPE foaming, the foaming temperature and saturation pressure are two key factors for preparing open-cell foams. An increase in temperature and pressure promotes both the cell wall thinning and cell rupture, because a high temperature results in a decrease in the viscosity of the polymer, and a high pressure leads to a larger amount of cell nucleation. Moreover, for the given temperature and pressure, the high pressurization rate results in a high pressure gradient, favoring cell rupture. For LLDPE/MWCNTs foaming, the addition of MWCNTs not only promotes the cell heterogeneous nucleation, but also prevents the cell collapse during cell opening, which is critical to achieve the open-cell structures with small cell size and high cell density.     

Effect of Supercritical Carbon Dioxide Conditions on PVDF/PVP Microcellular Foams

Supercritical carbon dioxide (ScCO₂) was used as a physical foaming agent to prepare poly(vinylidene f luoride)/poly(N-vinyl pyrrolidone) (PVDF/PVP) microstructure material. The effects of foaming conditions including saturation pressure, foaming temperature and foaming time on PVDF/PVP foams morphology, thermal and electrical behavior were systematically investigated by scanning electron microscope, differential scanning calorimeter and broadband dielectric spectrometer. Small cell and low cell density were achieved at low pressure of 12 MPa, as increasing saturation pressure, the average cell size increased first, and then decreased. The competition between the cell growth and cell nucleation played an important role in average cell size, which was directly related to ScCO₂ processing conditions. With increasing foaming temperature, cell size was increased and cell density was decreased, in a nearly linear manner. The variation of foaming time was considered to be closely related to the time for cells to grow. Thus, the results revealed that the average cell size enhanced with extending foaming time. The thermal properties of PVDF/PVP composites are slightly inf luenced by foaming parameters, and the dielectric constant of PVDF/PVP composite foams decreased with increasing volume expansion ratio.     

Lightweight lignocellulosic foams for thermal insulation

Foams are mainly composed of dispersed gas trapped in a liquid or solid phase making them lightweight and thermally insulating materials. Additionally, they are applicable for large surfaces, which makes them attractive for thermal insulation. State-of-the-art thermally insulating foams are made of synthetic polymeric materials such as polystyrene. This work focuses on generating foam from surfactants and renewable lignocellulosic materials for thermally insulating stealth material. The effect of two surfactants (sodium dodecyl sulphate (SDS) and polysorbate (T80)), two cellulosic materials (bleached pulp and nanocellulose), and lignin on the foaming and stability of foam was investigated using experimental design and response surface methodology. The volume-optimized foams determined using experimental design were further studied with optical microscopy and infrared imaging. The results of experimental design, bubble structure of foams, and observations of their thermal conductivity showed that bleached pulp foam made using SDS as surfactant produced the highest foam volume, best stability, and good thermal insulation. Lignin did not improve the foaming or thermal insulation properties of the foam, but it was found to improve the structural stability of foam and brought natural brown color to the foam. Both wet and dry lignocellulosic foams provided thermal insulation comparable to dry polystyrene foam.

Graphical abstract

     

Use of Pd-Pt loaded graphene aerogel on nickel foam in direct ethanol fuel cell

A size customized binder-free bimetallic Pd-Pt loaded graphene aerogel deposited on nickel foam plate (Pd-Pt/GA/NFP) was prepared and used as an electrode for an alkaline direct ethanol fuel cell (DEFC) under room temperature. The effect of fuel concentration and metal composition on the output power density of the DEFC was systematically investigated. Under the optimum fuel concentration, the cell could achieve a value of 3.6 mW cm⁻² at room temperature for the graphene electrode with Pd/Pt ratio approaching 1:1. Such results demonstrated the possibility of producing a size customized metal loaded GA/NFP electrode for fuel cell with high performance.     

The influence of the pore size, the foaming temperature and the viscosity of the continuous phase on the properties of foams produced by membrane foaming

Membrane foaming is a new method of foaming. To enlarge the knowledge about the influencing factors and to know how to vary the structure of the resulting foam, different factors were evaluated. A whey protein solution with 10% protein was foamed as a model solution by means of a tubular cross-flow filtration membrane. The pore size of the membrane was varied. The smaller the pore size, the smaller the bubbles produced. As a result, the foam firmness increases and less drainage was observed when smaller pore sizes were applied.

An important factor is that the added amount of gas must be stabilised as completely as possible in the foam. In order to achieve this, both the process and the product parameters were varied. Raising the foaming temperature increased the quantity of stabilised gas. The whey proteins then diffuse faster to the bubble surfaces and stabilise these by unfolding and networking reactions to prevent the coalescence of the bubbles.

The product parameter viscosity was found to influence the foaming result in such a way that up to a viscosity of 40 mPa s the incorporated gas bubbles are stabilised by the higher viscosity. At viscosities higher than 40 mPa s it is difficult to incorporate in the bubbles, and the foam structure becomes coarser due to increased coalescence at the pores of the membrane. The foam stability is enhanced with higher viscosities.     

Dynamics of Foaming of Polystyrene Particles

September 24, 2006 Summary: In this work, we address the industrially relevant problem of the foaming of expandable polystyrene (PS) impregnated by pentane as a traditional down-stream processing in the suspension polymerization of styrene. Once the polystyrene foam is formed by means of a proper foaming agent, e.g., pentane or fluoro- or chloro-hydrocarbons, the blowing agent diffuses out from the cellular structure. Environmental efforts call for the reduced consumption of blowing agents. The dynamics of foaming of polystyrene particles was recorded video-microscopically in our laboratory as the sequence of images of expanding particle located in the small pressure cell placed under the microscope with sufficient depth of focus. The amount of pentane sorbed in PS was controlled by the length of the impregnation period and was determined independently by gravimetric measurements. Strong dependence of the structure of the produced foam and of the foaming dynamics on the amount of sorbed pentane, temperature and particle size is reported and explanations for some observed foaming phenomena are provided.     

Physicochemical control of foam properties

This article summarizes our recent understanding on how various essential foam properties could be controlled (viz. modified in a desired way) using appropriate surfactants, polymers, particles and their mixtures as foaming agents. In particular, we consider the effects of these agents on the foaminess of solutions and suspensions (foam volume and bubble size after foaming); foam stability to liquid drainage, bubble coalescence and bubble Ostwald ripening; foam rheological properties and bubble size in sheared foams. We discuss multiple, often non-trivial links between these foam properties and, on this basis, we summarize the mechanisms that allow one to use appropriate foaming agents for controlling these properties. The specific roles of the surface adsorption layers and of the bulk properties of the foaming solutions are clearly separated. Multiple examples are given, and some open questions are discussed. Where appropriate, similarities with the emulsions are noticed.     

偶氮复合发泡剂对交联聚氯乙烯泡沫结构和性能的影响

采用异氰酸酯和环氧树脂作为交联剂,偶氮二异丁腈（AIBN）和偶氮二甲酰胺（AC）作为发泡剂制备了交联聚氯乙烯泡沫材料。 通过扫描电子显微镜分析了两种发泡剂的粒径、比例以及用量对泡孔结构的影响。 结果表明,AIBN的粒径对泡沫的泡孔结构影响不大,但其用量增加导致泡沫密度减小;AC的粒径对泡孔结构产生一定的影响,最优的粒径范围为8～20 μm,并且随着用量增加,泡孔尺寸变小,但对密度影响不大。 采用DSC研究了AIBN和AC在预混料中的分解情况,提出了两种发泡剂影响泡沫的泡孔结构的机理。 对泡沫材料力学性能的分析表明,泡沫的强度随密度的增加而增加,剪切变形则在60～80 kg/m3密度范围内出现最大值。     

Supercritical CO_2 Assisted Preparation of Open-cell Foams of Linear Low-density Polyethylene and Linear Low-density Polyethylene/Carbon Nanotube Composites

The open-cell structure foams of linear low-density polyethylene(LLDPE) and linear low-density polyethylene(LLDPE)/multi-wall carbon nanotubes(MWCNTs) composites are prepared by using supercritical carbon dioxide(sc-CO_2)as a foaming agent. The effects of processing parameters(foaming temperature, saturation pressure, and depressurization rate) and the addition of MWCNTs on the evolution of cell opening are studied systematically. For LLDPE foaming, the foaming temperature and saturation pressure are two key factors for preparing open-cell foams. An increase in temperature and pressure promotes both the cell wall thinning and cell rupture, because a high temperature results in a decrease in the viscosity of the polymer, and a high pressure leads to a larger amount of cell nucleation. Moreover, for the given temperature and pressure, the high pressurization rate results in a high pressure gradient, favoring cell rupture. For LLDPE/MWCNTs foaming, the addition of MWCNTs not only promotes the cell heterogeneous nucleation, but also prevents the cell collapse during cell opening, which is critical to achieve the open-cell structures with small cell size and high cell density.     

Fabrication of open‐porous PCL/PLA tissue engineering scaffolds and the relationship of foaming process,morphology, and mechanical behavior

Tissue engineering scaffolds should provide a suitable porous structure and proper mechanical strength, which is beneficial for the delivery of growth factor and regulation of cells. In this study, the open‐porous polycaprolactone (PCL)/poly (lactic acid) (PLA) tissue engineering scaffolds with suitable porous scale were fabricated using different ratios of PCL/PLA blends. At the same time, the relationship of foaming process, morphology, and mechanical behavior in the optimized batch microcellular foaming process were studied based on the single‐factor experiment method. The porous structures and mechanical strength of the scaffolds were optimized by adjusting foaming parameters, including the temperature, pressure, and CO₂ dissolution time. The results indicated that the foaming parameters influence the cell morphology, further determine the mechanical behavior of PCL/PLA blends. When the PCL content is high, with the increase of temperature and time, the cell diameter and the elastic modulus increased, and the tensile strength and elastic modulus increased with the increase of the average cell size, and decreased as the increase of the cell density. While when the PLA content was high, the cell diameter showed the same trend, and the tensile strength and elastic modulus were higher, and the elongation at break was lower, and tensile strength and elastic modulus decreased with the increase of the average cell size and increased with the increase of cell density. This work successfully fabricated optimized porous PCL/PLA scaffolds with excellent suitable mechanical properties, pore sizes, and high interconnectivity, indicating the effectiveness of modulating the batch foaming process parameters.     

The Influences of Polymers and Temperature on the Foam Properties of 3‐Dodecylalkoxyl‐2‐hydroxylpropyl Trimethylammonium Chloride

The foam performances of 3‐dodecoxy‐2‐hydroxypropyl trimethylammonium chloride (C₁₂TAC) have been determined in the existence of different relative amount of polymer. The experimental results show that the foaming ability of the mixture systems of the C₁₂TAC/PEG and C₁₂TAC/PVP is stronger than that of the surfactant solutions in the absence of polymer, and with the increase of relative amount of polymer both foaming efficiency and foam stability of the surfactant solutions are evidently enhanced. For the aqueous solution of the surfactant, effect of temperature on foaming properties has also been examined. The results show that both the foaming ability and stability of the foams of the surfactant solutions are highest (or strongest) at 30°C.     

Effect of Surface-modified Clay on the Thermal Stability and Insulation of Polyorganosiloxane Foam

Polyorganosiloxane foam(SIF) nanocomposites reinforced with vinyl-modified montmorillonite(Mt-V) and hydrox-yl-modified montmorillonite(Mt-OH) were prepared through cross-linking and foaming. The effects of modified Mt on the density, pore morphology, and thermal and compressive properties of the prepared polyorganosiloxane foams were investi-gated. The structure of the polyorganosiloxane foam was studied by solid-state nuclear magnetic resonance analysis. Clay dispersion in polyorganosiloxane nanocomposites and pore morphology were investigated by X-ray diffraction and scan-ning electron microscopy analyses. The thermal and mechanical properties of the prepared materials were also evaluated by differential scanning calorimeter, thermogravimetric analysis, thermal diffusivity and compressive strength. The results show that Mt-V exhibits improved cell structure, thermal insulation, and crush compressive than Mt-OH. The addition of modified Mt reduces the density, cell size, and thermal conductivity but increases the high-temperature resistance and com-pressive strength of the nanocomposite. The amount of the residues of SIF/Mt-OH nanocomposites increases by 9% com-pared with that of the pure SIF. Furthermore, SIF/Mt-V decreases the thermal conductivity to 0.014 W/mK and the cell size to 98 μm. Those properties give the material potential application value in the aerospace and construction industry.     

Effect of water on foaming properties of diglycerol fatty acid ester-oil systems

We have studied the effect of added water on the nonaqueous foaming properties of diglycerol fatty acid ester nonionic surfactant systems. Diglycerol monomyristate (designated as DGM) could not foam in nonpolar oils squalane and hexadecane at normal room temperature. Nevertheless, addition of a small amount of water induces a dramatic change in foaming properties. Both the foamability and foam stability increases with the amount of added water within the studied concentration range. Phase behavior study showed that in the dilute regions there is dispersion of solid surfactant in the aforementioned oils in the DGM systems. The particle size of the dispersed solid phase was found to be several tens of microns in the water free system, and hence it tends to coagulate and precipitate. In the case of shorter alkyl chain length, diglycerol monolaurate (DGL) surfactant-oil systems, dispersion of lamellar liquid crystal (Lalpha) is observed at room temperature, and the poor foaming properties were attributed to the large particle size of the liquid crystal. In both the DGL and DGM-oil systems, we observed a tendency of the particle size to decrease with the increasing concentration of added water. At higher temperature, the solid surfactant transforms to lamellar liquid crystal phase, and foaming is improved in the DGM/squalane system. Foams are stable for several minutes. Judging from the foaming test and particle size distribution data it can be concluded that the poor foaming in the diglycerol fatty acid esters-oil systems may possibly be due to bigger particle size, which causes precipitation. Addition of water results in the dispersion of smaller particles and improves the foaming behavior.     

Poly(lactic acid)/organoclay nanocomposites: Thermal,rheological properties and foam processing

In this study, polymer nanocomposites based on poly(lactic acid) (PLA) and organically modified layered silicates (organoclay) were prepared by melt mixing in an internal mixer. The exfoliation of organoclay could be attributed to the interaction between the organoclay and PLA molecules and shearing force during mixing. The exfoliated organoclay layers acted as nucleating agents at low content and as the organoclay content increased they became physical hindrance to the chain mobility of PLA. The thermal dynamic mechanical moduli of nanocomposites were also improved by the exfoliation of organoclay; however, the improvement was reduced at high organoclay content. The dynamic rheological studies show that the nanocomposites have higher viscosity and more pronounced elastic properties than pure PLA. Both storage and loss moduli increased with silicate loading at all frequencies and showed nonterminal behavior at low frequencies. The nanocomposites and PLA were then foamed by using the mixture of CO₂ and N₂ as blowing agent in a batch foaming process. Compared with PLA foam, the nanocomposite foams exhibited reduced cell size and increased cell density at very low organoclay content. With the increase of organoclay content, the cell size was decreased and both cell density and foam density were increased. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 689–698, 2005     

Microcellular foams from polyethersulfone and polyphenylsulfone: Preparation and mechanical properties

Polymeric foams having microcellular structures were successfully prepared from some high-performance thermoplastics, specifically polyethersulfone and polyphenylsulfone. A two-stage batch foaming process was used and the resulting materials had average cell sizes in the range 2-13 μm, and cell densities the order of 10¹⁰-10¹¹ cells/cm³. The foam densities (relative to those of the unfoamed polymers) were in the range 0.90-0.35. Average cell sizes increased with foaming temperature and foaming time; on the other hand, cell densities and relative foam densities decreased slightly with foaming temperature but remained almost constant with foaming time. Experimental values of Young’s modulus in compression and the elastic collapse strength were higher than theoretically predicated at high relative densities, but the discrepancies became small at lower densities. In contrast, Young’s moduli in tension were in very good agreement with theory, but the relative strengths were somewhat lower than predicated.     

Towards novel super‐elastic foams based on isoperene rubber: Preparation and characterization

A novel and conventional closed cell polyisoprene rubber (IR) foams were produced by a single step limited‐expansion and two step unlimited‐expansion foaming process, respectively. The effect of 3 to 12 part per hundred rubber (phr) of azodicarbonamide (ADC) foaming agent on their structure and properties of developed novel foams were studied. In developed novel foams, the density was strangely independent of ADC content; however, the cell sizes conversely related to ADC content and it decreased by 60% (555‐330 μm) and the internal cell pressure build up from 1 to 3.7 atm, which was related to pressure‐free foaming method. The both reasons of compressed gas trapped inside cells and constant density not only caused unique enhancement in novel foams mechanical properties as hardness and modulus but also improved their dynamic properties as hysteresis and elasticity. Results of conventional IR foams showed that, their foam density as well as dynamic and mechanical properties sharply decreased with increasing ADC content from 3 to 12 phr. For clear expression, in samples with 12 phr of ADC, novel developed foams have more foam density (180%), more hardness (240%), more modulus (290%), and smaller cell size (75%) than conventional foams. Finally, novel developed foams were super‐elastic material with no hysteresis and no plastic deformation while conventional foams had 40% hysteresis and 10% plastic deformation under the same compression conditions.