Foaming of linear isotactic polypropylene based on its non-isothermal crystallization behaviors under compressed CO2

The non-isothermal crystallization behaviors of isotactic polypropylene (iPP) under ambient N₂ and compressed CO₂ (5–50 bar) at cooling rates of 0.2–5.0 °C/min were carefully studied using high-pressure differential scanning calorimeter. The presence of compressed CO₂ had strong plasticization effect on the iPP matrix and retarded the formation of critical size nuclei, which effectively postponed the crystallization peak to lower temperature region. On the basis of these findings, a new foaming strategy was utilized to fabricate iPP foams using the ordinary unmodified linear iPP with supercritical CO₂ as the foaming agent. The foaming temperature range of this strategy was determined to be as wide as 40 °C and the upper and lower temperature limits were 155 and 105 °C, which were determined by the melt strength and crystallization temperature of the iPP specimen under supercritical CO₂, respectively. Due to the acute depression of CO₂ solubility in the iPP matrix during the foaming process, the iPP foams with the bi-modal cell structure were fabricated.     

Preparation of low density carbon foams by foaming molten sucrose using an aluminium nitrate blowing agent

Low density carbon foams have been prepared by thermo-foaming of molten sucrose using aluminium nitrate as a blowing agent to produce solid organic foams followed by dehydration and carbonization. Gas bubbles are generated in the molten sucrose due to water vapour produced by the acid catalysed condensation between sucrose hydroxyl groups and NO_x gases produced by the thermal decomposition of the aluminium nitrate. Higher melt viscosity achieved by cross-linking of the condensation products of sucrose through co-ordination of the aluminium ions with the hydroxyl groups stabilizes the bubbles against coalescence and rupture. The foam volume, foaming time and setting time depend on the aluminium nitrate concentrations. The carbon obtained by the pyrolysis of the solid organic foams has turbostratic graphite structure. The foams produced have an interconnected near-spherical cellular structure. The carbon foams prepared at aluminium nitrate concentrations in the range of 0.5–4 wt.% have a density and average cell size in the ranges of 0.085–0.053 g/cc and 1.55–0.83 mm, respectively. The alumina (～0.17–1.34 wt.%) produced from the aluminium nitrate is concentrated more on the surface of cell walls, ligaments, and struts.     

Polycarbonate foams with tailor-made cellular structures by controlling the dissolution temperature in a two-step supercritical carbon dioxide foaming process

Closed-cell polycarbonate foams were prepared using a two-step foaming process, which consisted of the initial dissolution of supercritical CO₂ (scCO₂) into PC foaming precursors and their later expansion by heating using a double contact restriction method. The effects of the parameters of both CO₂ dissolution and heating stages on the cellular structure characteristics as well as on the physical aging of PC in the obtained foams were investigated. A higher amount of CO₂ was dissolved in PC with increasing the dissolution temperature from 80 to 100 °C, with similar CO₂ desorption trends and diffusion coefficients being found for both conditions. PC foams displayed an isotropic-like microcellular structure at a dissolution temperature of 80 °C. It was shown that it is possible to reduce their density while keeping their microcellular structure with increasing the heating time. On contrary, when dissolving CO₂ at 100 °C and later expanding, PC foams presented a cellular morphology with bigger cells and with an increasingly higher cell elongation in the vertical growth direction with increasing the heating time. Comparatively, PC foams obtained by dissolving CO₂ at 100 °C presented a more marked physical aging after CO₂ dissolution and foaming, although this effect could be reduced and ultimately suppressed with increasing the heating time.     

The influence of process parameters on in situ inorganic foaming of alkali-bonded SiC based foams

Silicon carbide (SiC) foams were developed with a low temperature process by using an inorganic alkali aluminosilicates binder, also known as geopolymer. The foaming agent was the metallic silicon present as impurity in the SiC powder. Si⁰ in the alkaline solution led to gas evolution that induced the foaming of the slurries. The binder was a geopolymeric resin with atomic ratio Si/Al = 2 and potassium as alkaline cation, classified as (K)poly(silalate-siloxo). The geopolymeric resin was prepared using metakaolin as aluminosilicatic raw powder, while the alkali aqueous solution was KOH/K₂SiO₃. Metakaolin in alkaline conditions dissolved and re-precipitated to form geopolymeric nano-particulates that acted as a glue to stick together SiC particles (90 wt.%). Process parameters such as water addition, mixing time and curing temperature were correlated to the foam structure. The formation of prolate pores induced anisotropy in the compressive strength. The foams were studied by dilatometric analysis in inert and oxidative atmospheres up to 1200 °C.     

Preparation of a novel BN/SiC composite porous structure

A kind of BN/SiC open cell ceramic foams were fabricated from complex co-polymeric precursors of polycarbosilane and tris(methylamino)borane [B(NHCH₃)₃] using a high pressure pyrolysis foaming technique. The as-fabricated foams exhibit cell sizes ranging from 1 to 5 mm with bulk densities varying from 0.44 to 0.73 g/cm³, depending on the proportion of the starting materials. Studies on microstructure and properties of the porous material shown that addition of BN into SiC can improve dramatically its oxidation resistance during 800–1100 °C and compression strength which was generally about a 5–10 times higher than that of a pure SiC foam.     

Preparation and characterization of polystyrene foams from limonene solutions

The foaming process has been traditionally performed at high temperature because the CO₂ and the polymer should behave as a homogeneous solution. The addition of a solvent could avoid the high working temperature while the homogeneity is ensured. Among the terpene oils, limonene outlines as a good candidate to carry out the dissolution of polystyrene because it respects the green chemistry principle, it is highly soluble in CO₂ and very compatible with the polymer.The sorption of CO₂ is the first step of the foaming process. The presence of the terpene oil enhances the solubility of the gas which is solubilized in the Polystyrene as well as in the limonene. During the foaming process, many parameters can be tuned to customize the foams. In this work, a fractional factorial design of experiment was proposed to determine the effect of pressure, temperature, concentration of the solution, contact time and vent time over the diameter of cells, its standard deviation and the cells density. The proposed foaming process can be simply performed at mild pressure and temperature thanks to the presence of the solvent. The results showed that the most suitable conditions to foam polystyrene from limonene solutions are 90 bar, 30 °C, 0.1 gPS/ml Lim, 240 min contacting and 30 min venting. Finally, the samples were characterized to determine the amount of residual solvent, their glass transition and degradation temperature checking that the foams presented around 5% of solvent traces but did not show any evidence of degradation.     

Polypropylene/carbon nanotube nano/microcellular structures with high dielectric permittivity,low dielectric loss,and low percolation threshold

Nano/microcellular polypropylene/multiwalled carbon nanotube (MWCNT) composites exhibiting higher electrical conductivity, lower electrical percolation, higher dielectric permittivity, and lower dielectric loss are reported. Nanocomposite foams with relative densities (ρ_R) of 1.0–0.1, cell sizes of 70 nm–70 μm, and cell densities of 3 × 10⁷–2 × 10¹⁴ cells cm⁻³ are achieved, providing a platform to assess the evolution of electrical properties with foaming degree. The electrical percolation threshold decreases more than fivefold, from 0.50 down to 0.09 vol.%, as the volume expansion increases through foaming. The electrical conductivity increases up to two orders of magnitude in the nanocellular nanocomposites (1.0 > ρ_R > ∼0.6). In the proper microcellular range (ρ_R ≈ 0.45), the introduction of cellular structure decreases the dielectric loss up to five orders of magnitude, while the decrease in dielectric permittivity is only 2–4 times. Thus, microcellular composites containing only ∼0.34 vol.% MWCNT present a frequency-independent high dielectric permittivity (∼30) and very low dielectric loss (∼0.06). The improvements in such properties are correlated to the microstructural evolution caused by foaming action (biaxial stretching) and volume exclusion. High conductivity foams have applications in electromagnetic shielding and high dielectric foams can be developed for charge storage applications.     

Integrated process of supercritical CO2-assisted melt polycondensation modification and foaming of poly(ethylene terephthalate)

An integrated process of melt polycondensation modification and foaming of poly(ethylene terephthalate) (PET) was performed in a high pressure vessel assisted by supercritical carbon dioxide (scCO₂). ScCO₂ was firstly employed to sweep PET melt, i.e., high pressure CO₂ continuously flows through the vessel at a fixed flow rate to remove small molecules for higher molecular weight PET, then this modified PET melt was directly foamed through a rapid depressurization process using scCO₂ as blowing agent. In this integrated process, PET with high melt strength after polycondensation modification could be foamed directly in molten state. Therefore, re-molten process of solid modified PET pellets was canceled to avoid its degradation and CO₂ saturation time could be saved in foaming process, thus processing time could be shortened and energy efficiency could be improved. The influences of scCO₂ sweeping treatment time, pressure and flow rate on properties of the modified PETs and cell morphologies of the foamed PETs were investigated respectively. The results showed that CO₂ sweeping treatment could effectively enhance PET melt polycondensation modification process, which was superior to that of N₂ treatment. PET foams with average cell diameter of 32–62 μm and cell density of 1 × 10⁷ to 4 × 10⁷ cells/cm³ have been obtained in the integrated process. Compared with the process of only foaming modified PET by scCO₂ or performing scCO₂ assisted modified PET further melt polycondensation process and scCO₂ foaming process separately, this integrated process produced better cell morphology.     

Preparation of carbon foams with enhanced oxidation resistance by foaming molten sucrose using a boric acid blowing agent

Boric acid was used as a blowing agent as well as a boron precursor for the preparation of boron-doped carbon foams from molten sucrose. The H⁺ generated, due to the formation of a complex between sucrose and boric acid, catalyzes the –OH to –OH condensation reaction leading to the polymerization and the foaming of the molten sucrose. The char yield of the solid organic foams increased from 24 to 39 wt.% when the boric acid concentration increased from 0 to 8 wt.%, due to the formation of the B–O–C cross-links between sucrose polymer by B–OH to C–OH condensation. The inductively coupled plasma analysis showed the presence of 0.44–3.4 wt.% boron in the carbon foams. The density and compressive strength decreased and cell size increased with boric acid concentration. The room temperature thermal conductivity of the boron-doped carbon foams was in the range of 0.057–0.043 W m⁻¹ K⁻¹. The weight loss studies by dynamic and isothermal heating showed the increased oxidation resistance with boron concentration.     

Preparation of Si3N4 ceramic foams by simultaneously using egg white protein and fish collagen

Fish collagen, a kind of fibrous protein, and egg white protein were selected as foaming agent to prepare ceramic foams by protein foaming method. Ceramic foams with open porosity of 84.8–86.9%, average pore size of 216–266 μm and compressive strength of 8.7–13.7 MPa were fabricated. Studies of fish collagen addition on the influence of open porosity, pore size distribution and mechanical property of ceramic foams were investigated. In comparison with single addition of 8 wt% egg white protein, the combinational addition of 2 wt% fish collagen and 6 wt% egg white protein results in 23% increase in average pore size. In addition, the introduction of fish collagen decreases the count of small pores. Moreover, with the introduction of fish collagen, pores become regular in shape.     

Effects of heating rate on the structure and properties of SiOC ceramic foams derived from silicone resin

Silicon oxycarbide ceramic foams were fabricated in a single step manufacturing process using in situ foaming of SiOC powders loaded silicone resin. The effects of heating rate on the porosity, compressive strength and microstructure of the ceramic foams were investigated. The porosity (total and open) increased firstly and then decreased with increasing heating rate. It was possible to control the total and open porosity of ceramic foams within a range of 81.9–88.2% and 62.4–72.5% respectively, by adjusting the heating rate from 0.25 °C/min to 3 °C/min while keeping the silicone resin content at 90 vol%. However, the compressive strength decreased with increasing the heating rate progressively, and the average compressive strength of the foams was in the range of 1.0–2.3 MPa. Micrographs indicated that the ceramic foams which cross-linked at a heating rate less than 1 °C/min had a well-defined open-cell and regular pore structure.     

Onset coarsening/coalescence of cobalt oxides in the form of nanoplates versus equi-axed micron particles

The change of specific surface area and pore size distribution coupled with N₂ adsorption–desorption hysteresis isotherm, in particular that typical to cylindrical pores, were used to determine the onset coarsening/coalescence in the temperature range of 500–800 °C for Co(OH)₂ derived Co₃O₄ nanoplates and 700–1000 °C for CoO-derived Co₃O₄ powders (backtransformed to CoO above 900 °C) which are equi-axed in shape and microns in size. The vigorous onset coarsening/coalescence of the nanoplates and equi-axed micron particles was found to occur within minutes having apparent activation energy of 37 ± 7 kJ/mol (based on t_0.7, i.e. time for 70% surface area reduction) and 113 ± 8 kJ/mol (based on t_0.3), respectively. The surface area reduction process of the nanoplates was found to be controlled by (1 1 1)-specific coalescence besides a coarsening–repacking process more common to the equi-axed particles. The present static experimental results of coarsening–coalescence of the Co₃O₄ (below 900 °C) or CoO particles (above 900 °C) supports our previous supposition that CoO and Co₃O₄ nanocondensates could readily assemble as nanochain aggregates and further coalesce into a close packed manner below 1000 °C by the radiant heating effect in a dynamic laser ablation process.     

Particle-stabilized ultra-low density zirconia toughened alumina foams

Zirconia toughened alumina (ZTA) is one of the leading engineering ceramics; it is used in a wide range of components and products in applications for which high strength, high toughness, and high temperature stability are needed. The particle-stabilized direct foaming method has lately become a subject of particular interest. Nevertheless only a few studies on combining ZTA ceramics and particle-stabilized direct foaming have been reported. Therefore, in this study, ultra-low density ZTA foams having single strut wall thickness, cell size ranging from 80 μm to 200 μm, and above 90% porosity were successfully fabricated via the particle-stabilized direct foaming method. Valeric acid was used as particle surface modifier to render the particles partially hydrophobic, which stabilized the air/water interface of the ZTA foams. The sintered foams maintained compressive strength up to 8 MPa with porosity of 90%.     

Temperature influence and CO2 transport in foaming processes of poly(methyl methacrylate)–block copolymer nanocellular and microcellular foams

Fabricated by high-pressure or supercritical CO₂ gas dissolution foaming process, nanocellular and microcellular polymer foams based on poly(methyl methacrylate) (PMMA homopolymer) present a controlled nucleation mechanism by the addition of a methylmethacrylate–butylacrylate–methylmethacrylate block copolymer (MAM), leading to defined nanocellular morphologies templated by the nanostructuration of PMMA/MAM precursor blends. Influence of the CO₂ saturation temperature on the foaming mechanism and on the foam structure has been studied in 90/10 PMMA/MAM blends and also in the neat (amorphous) PMMA or (nanostructured) MAM polymers, in order to understand the role of the MAM nanostructuration in the cell growth and coalescence phenomena. CO₂ uptake and desorption measurements on series of block copolymer/homopolymer blend samples show a competitive behavior of the soft, rubbery, and CO₂-philic block of PBA (poly(butyl acrylate)) domains: fast desorption kinetics but higher initial saturation. This competition nevertheless is strongly influenced by the type of dispersion of PBA (e.g. micellar or lamellar) and a very consequent influence on foaming.CO₂ sorption and desorption were characterized in order to provide a better understanding of the role of the block copolymer on the foaming stages. Poly(butyl acrylate) blocks are shown to have a faster CO₂ diffusion rate than poly(methyl methacrylate) but are more CO₂-philic. Thus gas saturation and cell nucleation (heterogeneous) are more affected by the PBA block while cell coalescence is more affected by the PMMA phases (in the copolymer blocks + in the matrix).     

The use of egg shells to produce Cathode Ray Tube (CRT) glass foams

Cleaned Cathode Ray Tube (CRT) (panel and funnel) waste glasses produced from dismantling TV and PC colour kinescopes were used to prepare glass foams by a simple and economic processing route, consisting of a direct heating of glass powders at relatively low temperatures (600–800 °C). This study reports on the feasibility of producing glass foams using waste egg shells as an alternative calcium carbonate-based (95 wt%) foaming agent derived from food industry. The foaming process was found to depend on a combination of composition, processing temperature and mixture of raw materials (glass wastes). Hot stage microscopy (HSM), X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize foams and evaluate the foaming ability and the sintering process. The experimental compositions allowed producing well sintered glass foams with suitable properties for some functional applications with environmental benefits such as: (1) reduced energy consumption because of the low heat treatment temperatures used; and (2) materials produced exclusively from residues.     

Novel ‘inorganic gel casting’ process for the manufacturing of glass foams

A new technique for the production of glass foams was developed, based on alkali activation and gel casting. The alkali activation of soda-lime waste glass powders allowed for the obtainment of well-dispersed concentrated suspensions, undergoing gelification by treatment at low temperature (75 °C). An extensive direct foaming was achieved by mechanical stirring of partially gelified suspensions, comprising also a surfactant. The suspensions were carefully studied in terms of rheological behavior, so that the final microstructure (total amount of porosity, cell size) can be directly correlated with the degree of gelification.A sintering treatment, at 700–800 °C, was finally applied to stabilize the foams, in terms of leaching of alkaline ions. Considering the high overall porosity (88–93%), the newly obtained foams exhibited a remarkable compressive strength, in the range of 1.7–4.8 MPa.     

Influence of the glass–calcium carbonate mixture's characteristics on the foaming process and the properties of the foam glass

We prepared foam glasses from cathode-ray-tube panel glass and CaCO₃ as a foaming agent. We investigated the influences of powder preparation, CaCO₃ concentration and foaming temperature and time on the density, porosity and homogeneity of the foam glasses. The results show that the decomposition kinetics of CaCO₃ has a strong influence on the foaming process. The decomposition temperature can be modified by varying the milling time of the glass–CaCO₃ mixture and thus for a specific CaCO₃ concentration an optimum milling time exists, at which a minimum in density and a homogeneous closed porosity are obtained. Under the optimum preparation conditions the samples exhibit a density of 260 kg/m³. The thermal conductivity of the foam glass was measured to be 50–53 mW/(m K). The observed dependence of the foaming process on the decomposition kinetics of the foaming agent can be applied as a universal rule for foaming processes based on thermal decomposition.     

Development of a strategy for the foaming of polystyrene dissolutions in scCO2

Interfacial tension (IFT) is a key physicochemical parameter that plays an important role during the foaming process of polymers. The pendant droplet method is a useful technique to determine the IFT, which has been studied at mild working conditions (0–9 MPa, 303.15–313.15 K). In this work, limonene and cymene were used as solvent to prepare polymer dissolutions and observe their influence on the experimental IFT measurements. Also, the effect of CO₂ on the glass transition of polystyrene and polystyrene dissolutions was studied in order to select the most suitable temperature to carry out the experiments. The behaviour of the polystyrene dissolution in the presence of CO₂ can be considered similar to molten polymer. A controlled foaming of the polystyrene-solvent mixtures can be easily carried out at moderate temperature and pressures by exploiting the advantages that provide the solvent, obtaining completely “dried” PS foams.     

New consolidation process inspired from making steamed bread to prepare Si3N4 foams by protein foaming method

A new consolidation process had been developed for preparing Si₃N₄ ceramic foams by using protein foaming method, which was inspired from the preparation of steamed bread. The main advantage of this consolidation process was no crack development during foamed slurry consolidation process. By using this new consolidation, Si₃N₄ ceramic foams with open porosities of 79.6–87.3% and compressive strength of 2.5–22 MPa were prepared. Protein addition and solid content on mechanical properties and pore structures of the as-prepared ceramic foams were investigated. Results indicated that the open porosity decreases with protein addition and solid content while compressive strength increased with solid content. With the increase of solid content, pores of the ceramic foams became regular in shape and uniform in size while both size and number of windows on the walls decreased.     

Foaming strategies for bioabsorbable polymers in supercritical fluid mixtures. Part II. Foaming of poly(ɛ-caprolactone-co-lactide) in carbon dioxide and carbon dioxide + acetone fluid mixtures and formation of tubular foams via solution extrusion

This paper reports on the foaming of poly(ɛ-caprolactone-co-lactide) in carbon dioxide and carbon dioxide + acetone mixtures. Experiments were carried out in specially designed molds with porous metal surfaces and fluid circulation features to generate foams with uniform dimensions at 60, 70 and 80 °C at pressures in the range 7–28 MPa. Depending upon the conditions, foams with pores in the range from 5 to 200 μm were generated. Adding acetone to carbon dioxide improved the uniformity of the pores compared to foams formed by carbon dioxide alone. In addition, a unique high-pressure solution extrusion system was designed and used to form porous tubular constructs by piston-extrusion of a solution from a high-pressure dissolution chamber through an annular die into a second chamber maintained at controlled pressure/temperature and fluid conditions. Long uniform porous tubular constructs with 6 mm ID and 1 mm wall thickness were generated with glassy polymers like poly(methyl methacrylate) by extruding solutions composed of 50 wt% polymer + 50 wt% acetone, or 25 wt% polymer + 10% acetone + 65% carbon dioxide at 70 °C and 28 MPa. Pores were in the 50 μm range. The feasibility of forming similar tubular constructs were demonstrated with poly(ɛ-caprolactone-co-lactide) as well. Tubular foams of the copolymer with interconnected pores with pore sizes in the 50 μm range were generated by extrusion of the copolymer solution composed of 25 wt% polymer + 10 wt% acetone + 65 wt% carbon dioxide at 70 °C and 28 MPa. Reducing the acetone content in the solution led to a reduction of pore sizes. Comparisons with the foaming behavior of the homopolymer poly(ɛ-caprolactone) that were carried out in the molds with porous metal plates show that the foaming behavior of the copolymer is more akin to the foaming behavior of the caprolactone homopolymer component.