Two-step Foaming Process Research Articles

Tuning anisotropic properties of epoxy/multi-walled carbon nanotube composites via two-step foaming

Epoxy (EP) composite foams with 1 wt% of multi-walled carbon nanotube (MWCNT) were prepared through a two-step foaming process, which involved first limited foaming the prepreg in a mold and then foaming the resultant foam again. The evolution of nanotubes’ arrangement and cell morphology during different second-step foaming (free-foaming or limited-foaming) process and its effect on compressive, electrical and electromagnetic interference (EMI) shielding properties of EP/MWCNT foams in both vertical and horizontal directions were carefully investigated. Differently interesting combination of anisotropy and isotropy in compressive strength, electrical resistivity, permittivity and EMI SE were facilely realized. For example, Limited-Free foam with a density of 0.401 g/cm3 exhibited a significant anisotropy in electrical resistivity (ρ∥/ρ⊥ = 2.93×10−3) but an isotropy in compressive strength (σ⊥/σ∥ = 1), permittivity (ε′∥/ε′⊥ = 1) and EMI shielding effectiveness (SE∥/SE⊥ = 1), while Limited-Limited foam with a density of 0.304 g/cm3 showed an isotropy in electrical resistivity (ρ∥/ρ⊥ = 0.96), permittivity (ε′∥/ε′⊥ = 0.99) and EMI SE (SE∥/SE⊥ = 0.99) but an obvious anisotropy in compressive strength (σ⊥/σ∥ = 0.74). In addition, the relationship between microsturcture and properties of composite foams was established and the mechanism of tuning anisotropic properties was also investigated. This study paves a way to the development of conductive composite foams with designed isotropic/anisotropic properties.

Influence of foaming-induced crystallization behavior on the structure of poly(butylene adipate-co-terephthalate) supercritical foamed beads

The crystallization behavior during foaming directly affects the foaming properties. For crystalline polymers, there is no consensus on the influence of the crystallization behavior during foaming process on the stabilization of the cell structure. In this work, PBAT foamed bead and unfoamed pellets were prepared by controlling the saturated temperatures in supercritical CO2, soaking step (one or two) and the depressurization rate, respectively. Double melting peaks were observed in the DSC curve of supercritical CO2 foamed PBAT beads. By comparing the outgassing rates we find that the stretching-induced crystallization caused by the rapid expansion of the gas during foaming plays an important role in the stabilization of the cells. Although the crystalline perfection or crystal size at this time is much smaller than that of the crystalline grains formed during static cooling, the rapid crystallization is effective in stabilizing the cell structure of the foamed pores. Compared to normal supercritical foaming processes, the two-step foaming process of soaking CO2 at high temperatures followed by foaming at low temperatures results in an increase in cell size and expansion ratio. At high temperatures, more CO2 diffuses into the PBAT pellets, increasing the instantaneous gas concentration in the pellets for foaming, and the rapid stretching produces stretching-induced crystallization that raises the average size of the cells, further increasing the expansion multiplicity of individual cells. The average cell size of foam beads rises from 37.8 to 48.8 µm and the expansion ratio also increases to 8.6 with saturated temperature increasing form 95 to 105°C. The two-step soaking foaming method is a more efficient way of manufacturing industrial foamed beads, allowing for the preparation of better foam beads at low temperatures.

Fabrication, Processing, Properties, and Applications of Closed-Cell Aluminum Foams: A Review.

Closed-cell aluminum foams have many excellent properties, such as low density, high specific strength, great energy absorption, good sound absorption, electromagnetic shielding, heat and flame insulation, etc. As a new kind of material, closed-cell aluminum foams have been used in lightweight structures, traffic collision protections, sound absorption walls, building decorations, and many other places. In this paper, the recent progress of closed-cell aluminum foams, on fabrication techniques, including the melt foaming method, gas injection foaming method, and powder metallurgy foaming method, and on processing techniques, including powder metallurgy foaming process, two-step foaming process, cast foaming process, gas injection foaming process, mold pressing process, and integral foaming process, are summarized. Properties and applications of closed-cell aluminum foams are discussed based on the mechanical properties and physical properties separately. Special focuses are made on the newly developed cast-forming process for complex 3D parts and the improvement of mechanical properties by the development of small pore size foam fabrication and modification of cell wall microstructures.

Novel Composite Planks Made of Shape Memory Polyurethane Foaming Material with Two-Step Foaming Process.

In this study, shape memory polyurethane (SMP) foaming material is used as the main material that is incorporated with carbon fiber woven fabrics via two-step foaming method, forming sandwich-structured composite planks. The process is simple and efficient and facilitates any composition as required. The emphasis of this study is protection performances, involving puncture resistance, buffer absorption, and electromagnetic wave shielding effectiveness. The proposed soft PU foam composite planks consist of the top and bottom PU foam layers and an interlayer of carbon fiber woven fabric. Meanwhile, PU foam is incorporated with carbon staple fibers and an aluminized PET film for reinforcement requirements and electromagnetic wave shielding effectiveness, respectively. Based on the test results, the two-step foaming process can provide the PU foam composite planks with excellent buffer absorption, puncture resistance, and electromagnetic wave shielding effectiveness; therefore, the proposed composite planks contribute a novel structure composition to SMP, enabling it to be used as a protective composite. In addition, the composites contain conductive material and thus exhibit a greater diversity of functions.

Evolution of anisotropic bubbles and transition of the mechanical and electrical properties during a non-continuous two-step foaming of epoxy/carbon nanofiber composites

A non-continuous two-step foaming process was reported to prepare microcellular epoxy/carbon nanofiber (EP/CNF) composite foams. This approach involved firstly limited-foaming epoxy tablets in a mold with a fixed cavity to obtain preformed foams with designed anisotropic cellular structure and following by free-foaming them in a heating oven. The evolution of cell morphology, rearrangement of carbon nanofibers, and changes of the mechanical and electrical properties in epoxy composite foams during the two-step foaming process were systematically investigated. Although the anisotropic cellular structure tended to become isotropic after reheating, the final composite foams (Two-step foam) still exhibited an anisotropy in the electrically conductive and mechanical properties. This result was attributed to that the further growth of anisotropic bubbles cannot completely disrupt the vertical alignment of CNF. The oriented CNF, in turn, significantly hindered the expansion in that direction, leading to an anisotropic expansion of the preformed foams during reheating, which could inevitably affect the evolution of cell morphology. In addition, the two-step foaming process effectively eliminated the negative effect of compressed gas occurring during the limited-foaming process, enhancing the glass transition temperature nearly to that of the foams prepared through the free-foaming process. This work opens a potential avenue to realign the nanofillers and prepare composite foams with tunable properties.

Two-step foaming process for production of PMMA nanocellular polymer foams via ultra-high pressure and rapid depressurization

A two-step foaming process was developed that uses different temperatures in each step and combines ultra-high saturation pressure (Ps > 50 MPa) and rapid depressurization (> 1.25 GPa/s) for production of nanocellular polymer foams. The effect of process conditions were investigated using polymethylmethacrylate (PMMA) and carbon dioxide. Foams with small cells (< 100 nm) and a large cell nucleation density (N0 > 1015 nuclei cm−3) were obtained, which were of a finer structure than those made by previous one-step foaming process. N0 tended to increase even at Ps =100 MPa. A fine cell structure (30.5 nm ± 10.4 nm) was observed with scanning transmission electron microscopy (STEM), and was confirmed when two-step process conditions were used. The process and advantages of ultra-high pressure are discussed with regard to porosity, cell size and N0.

Two-step foaming process combined with hot-rolling in fabrication of an aluminium foam sandwich panel

The present study addresses a new aluminium foam sandwich (AFS) manufacturing process by simplifying the technological processes and reducing costs by use of a two-step foaming process and hot-rolling. The interface between the coversheet and precursor core was bonded tightly by four passes of hot-rolling at 480 °C to realise simultaneous metallurgical bonding and foaming. The addition of Mg and pre-heated SiC makes the foaming process more stable and increases the apparent viscosity of the melt and decreases the adverse effects of gravity drainage and capillary action. The optimum foaming conditions occurred at a temperature of 720 °C for between 270 s and 390 s.

Effects of Modification and Heat Treatment on Microstructure and Mechanical Property of Two-Step Foamed ZL111 Alloy Foam

ZL111 alloy foams with a porosity of 80% and an average pore diameter of 3.5 mm were fabricated using a two-step foaming process, and the effects of modification and heat treatment on their microstructure and mechanical property were studied. The results indicates that by Y&amp;Sr modification, most of the eutectic silicon in the ZL111 alloy foam is transformed from plate-like into dot-like forms, and the average size of evenly distributed α-Al grain is reduced from 80~100μm to 30~40μm, which is more efficient than separate Y or Sr modifications. By combining Y and Sr modification and T6 heat treatment, the α-Al grain size of ZL111 alloy foam maintains its previous modified effect, eutectic silicon remains spherical and well-distributed, and CuAl2and Al9FeMg3Si5are dispersed homogeneously at the α-Al grain boundary. The Y&amp;Sr modification and T6 heat treatment also significantly improved the compressive property of ZL111 alloy foam, when we compared them with the untreated ZL111 alloy foam. The compressive strength rises from 13.3 MPa to 22.6 MPa, the densification strain improves from 59.3% to 76.9%, and the energy absorption capacity increases from 4.87 MJ/m3to 13.77 MJ/m3.

Foaming of Transient Polymer Hydrogels.

Foams made with polymer hydrogels can be used in a variety of applications, such as scaffolds for biomedical applications or decontamination processes. However, from a practical point of view, it is difficult to introduce bubbles into viscous or viscoelastic fluids and to produce large volumes of hydrogel foams. In the present article, we investigate the foaming process of poly(vinyl alcohol) (PVA)/borax transient hydrogels, where PVA chains reversibly bind to borax molecules. In a previous article, we showed that foams obtained with PVA/borax mixtures are highly stable because of both high interfacial and bulk viscosities and can be used to quickly absorb liquids, which make them suitable for detergency or decontamination processes. To produce these foams, we use a two-step foaming process which consists in first shearing a PVA solution to obtain a PVA foam and second adding borax to the PVA foam under continuous shearing. The obtained PVA/borax foams are stable for weeks. In this study, we observe a shear-induced collapse of the foams for formulations containing a low borax/PVA ratio, whereas they remain stable under shear for high PVA/borax ratios. Using scaling arguments, we find that the shear-induced collapse of the foams and bubbles is obtained below a critical ratio, NE/NB = 15, of the number of entanglements per chain, NE, and the number of borax per chain, NB. Rheology measurements show that the samples present a shear-thickening behavior that increases with the borax concentration. We suggest that during the foaming process when the shearing rate is of the order of 100 s–1, the viscosity of these samples diverges, leading to a viscous to fragile transition. To mimic the fast stretching of the PVA/borax thin films during the foaming process, we study the stretching of individual PVA/borax catenoid-shaped thin films at high stretching rates. We observe that the films containing low PVA/borax ratios do not minimize their surface area unlike what is theoretically expected for standard surfactant films. Moreover, the films tend to be unstable and fracture because the PVA/borax network does not have time to rearrange and relax stresses for high stretching rates.

Highly anisotropic crosslinked HDPE foams with a controlled anisotropy ratio: Production and characterization of the cellular structure and mechanical properties

In this paper a two-step foaming process was used to produce highly anisotropic foams with controlled anisotropy ratio. Anisotropic cellular materials are of great interest due to the enhanced mechanical properties that they show in the anisotropy direction. Therefore, the production of foams with high and controlled anisotropy ratios is an essential topic when it comes to design materials with good mechanical performance. In this work, high density polyethylene (HDPE) foams with anisotropy ratios larger than 3 have been produced. A foaming process consisting of two consecutive heating steps: pre-foaming and foaming, has been employed. Foaming kinetics at four different pre-foaming temperatures (145°C, 155°C, 170°C and 180°C) and at a constant foaming temperature (180°C) have been performed. Results showed that the pre-foaming temperature is the key parameter that allows controlling the anisotropy ratio of the final foam. A relationship between the pre-foaming temperature, the number of cells and the anisotropy ratio has been established. Low pre-foaming temperatures lead to low numbers of cells, which eventually results in high anisotropy ratios. Mechanical properties in compression were measured and compared with the anisotropy of the cellular structure, being that the elastic modulus increases with the anisotropy ratio.

Mold-Filling Ability of Aluminum Alloy Melt during the Two-Step Foaming Process

For the two-step foaming method, one of the most cost-effective ways to fabricate three-dimensional shaped aluminum alloy foams with dense outer surface skin, it is crucial to describe and predict the mold-filling behavior of the shaped aluminum alloy foams with a favorable pore-distribution accurately. In this paper, a mold-filling model for semi-solid aluminum alloy foams was initially established and subsequently employed to predict the filling height, which represents the mold-filling ability of semi-solid aluminum alloy foams in a specially designed tube-like mold. Our results indicate that the proposed model can be applied to characterize the mold-filling property of aluminum alloy melts in a quantitative manner. Theoretically, our findings actually provide a guideline for mass-production of the shaped aluminum alloy foams by using the two-step foaming process.

Effects of graphene nanoplatelets on the morphology of polycarbonate–graphene composite foams prepared by supercritical carbon dioxide two-step foaming

Low density polycarbonate foams containing different amounts of graphene nanoplatelets with variable cellular morphologies were prepared using a supercritical carbon dioxide two-step foaming process, which consisted of the dissolution of supercritical CO2 into moulded foam precursors and their later expansion by double contact restricted foaming. The effects of the processing conditions and graphene content on the cellular morphology of the obtained foams were investigated, showing that the addition of increasingly higher amounts of graphene nanoplatelets resulted in foams with increasingly smaller cell sizes and higher cell densities, due on the one hand to their effectiveness as cell nucleating agents and on the other to their platelet-like geometry, which limited CO2 loss during foaming due to a barrier effect mechanism. Especially significant was the addition of 5wt.% graphene nanoplatelets, as the high concentration of graphene limited CO2 escape and cell coalescence during expansion, enabling to obtain highly expanded microcellular foams.

Fabrication and characterization of anorthite foam ceramics having low thermal conductivity

Porous anorthite ceramics with low thermal conductivity were successfully prepared using fly ash and gypsum by direct foaming and slip casting method. Effects of dispersant and foaming process on the performance of the porous materials were investigated. The results show adiabatic anorthite ceramics with the highest open porosity (94%) and the lowest thermal conductivity (0.042 W/m K) can be fabricated by adding 10% content of gypsum, 0.8% content of SHMP and using two-step foaming process. High porosity and small pore size are the main factors resulting in the low thermal conductivity. Meanwhile, the thermal conductivity can be predicted with the proportionality coefficient χ obtained by fitting the experimental data based on previous Gong's model. The a and b values in the expression of the proportionality coefficient χ were further discussed. The results show that they are affected by the pore size and distribution, and then the thermal conductivity will change accordingly.

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.

Two-step micro cellular foaming of amorphous polymers in supercritical CO 2

Microcellular foaming of amorphous rigid polymers, polymethylmethacrylate (PMMA) and polystyrene (PS) was studied in supercritical CO 2 (ScCO 2) in the presence of several types of additives, such as triblock (styrene-co-butadiene-co-methylmethacrylate, SBM and methylmethacrylate-co-butylacrylate-co-methylmethacrylate, MAM) terpolymers. This work is focused in the two-step foaming process, in which the sample is previously saturated under ScCO 2 being expanded in a second step out of the CO 2 vessel (e.g. in a hot oil bath) where foaming is initiated by the change of temperature near or above the glass transition temperature of the glass/polymer glassy system. Samples were saturated under high pressures of CO 2 (300 bar), at room temperature, for 16 h, followed by a quenching at a high depressurization rate (150 bar/min). In the last step, foaming was carried out at different temperatures (from 80 °C to 140 °C) and different foaming times (from 10 s to 120 s). It was found that cellular structures were controlled selecting either the additive type or the foaming conditions. Cell sizes are ranging from 0.3 μm to 300 μm, and densities from 0.50 g/cm 3 to 1 g/cm 3 depending on the polymers considered.

Batch foaming of SAN/clay nanocomposites with scCO2: A very tunable way of controlling the cellular morphology

This paper aims at elucidating some important parameters affecting the cellular morphology of poly(styrene-co-acrylonitrile) (SAN)/clay nanocomposite foams prepared with the supercritical CO2 technology. Prior to foaming experiments, the SAN/CO2 system has first been studied. The effect of nanoclay on CO2 sorption/desorption rate into/from SAN is assessed with a gravimetric method. Ideal saturation conditions are then deduced in view of the foaming process. Nanocomposites foaming has first been performed with the one-step foaming process, also called depressurization foaming. Foams with different cellular morphology have been obtained depending on nanoclay dispersion level and foaming conditions. While foaming at low temperature (40 °C) leads to foams with the highest cell density (∼1012–1014 cells/cm3), the foam expansion is restricted (d∼0.7–0.8 g/cm3). This drawback has been overcome with the use of the two-step foaming process, also called solid-state foaming, where foam expansion occurs during sample dipping in a hot oil bath (d∼0.1–0.5 g/cm3). Different foaming parameters have been varied, and some schemes have been drawn to summarize the characteristics of the foams prepared – cell size, cell density, foam density – depending on both the foaming conditions and nanoclay addition. This result thus illustrates the huge flexibility of the supercritical CO2 batch foaming process for tuning the foam cellular morphology.

Preparation of Porous Titanium by a Novel Foaming Process and MG63 Cell Behavior &lt;i&gt;In Vitro&lt;/i&gt;

A novel two-step foaming process has been developed to prepare the porous titanium implant. By using H2O2 and stearic acid as foaming reagent ordinally, a foamed structure with an open, interconnected pore morphology was obtained. The mechanical property was determined by compressive test. In vitro study was conducted to evaluate the ability of the porous titanium to support the growth and differentiation of Human osteosarcoma cell line MG63. The results show that the porous titanium has better interconnection compared to that obtained by traditional slurry foaming and its compressive strength and Young’s modulus were approximate 23.6 MPa and 2.1 GPa, respectively. Cell culture experiment results indicate that the porous titanium has good biocompatibility and acid-alkali treatment facilitates the adherence and proliferation of cells.