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The Hot Isostatic Pressing (HIP) process is based on the combined action of high levels of pressure and temperature. In general, such a process is used for reducing or eliminating microporosities in the component, especially when it is produced by additive manufacturing (AM) or casting. In the present work HIP is used for compacting TI6Al4V-ELI powders by means of a pressurized Argon gas acting at high temperature on a sealed can under vacuum in which Argon is previously inflated; thus, a subsequent Solid-State Foaming (SSF) heat treatment allows to produce a Titanium foam without any melting by exploiting gas entrapment. Samples extracted from billets produced setting different HIP parameters (size of Titanium particles and pressures) have been investigated in this work by means of heat treatments in furnace for the SSF: the temperature was kept constant (1020 °C), but the duration was varied in the range 1 - 6 h; the samples were finally analysed by Light Microscopy. Finally, the Response Surface Method (RSM) was used to determine the conditions able to increase both the size and the percentage area of the pores in order to fully control both the involved processes (HIP and SSF). Experimental results revealed that the porosity determined by the SSF is strongly affected by HIP parameters and by the SSF duration: the highest dimension of Ti particles and the highest level of Argon pressure determined the largest values of porosity, in terms of both percentage and pore dimension. The investigated process for producing porous titanium structures can be properly and efficiently combined with manufacturing processes able to create highly customised parts, not only in terms of geometry but also in terms of porosity. [ABSTRACT FROM AUTHOR]

Powder metallurgy (PM) processes for porous copper and alloys have seen some commercial successes, but PM methods have the disadvantage of relatively low porosity or strength that is compromised by stress-concentrating interparticle bonds. To increase porosity without compromising scalability, a Cu-CuO metal matrix composite powder was utilized to produce additional microscale porosity within the particles by oxide reduction. These Cu-CuO powders were pressed at 1, 2, or 3 GPa, and made porous at 600, 800, or 1000 °C to investigate the effects of pressing and sintering parameters on the overall strength and density. It was found that the formation of porosity is weakly dependent on compaction pressure (maximum 6% difference from 1 GPa to 3 GPa), while the final porosity varied by ~16% overall (~40% for 1 GPa and 600 °C to 24% for 3 GPa and 1000 °C). The strength of the porous Cu was highest after being reduced at 600 °C but also exhibited some flaking at the edges at high strain. The 1 GPa, 600 °C samples have a higher specific strength than wrought Cu annealed at the same temperature, as was demonstrated under uniaxial quasi-static compression as well as split Hopkinson pressure bar impact. [ABSTRACT FROM AUTHOR]

Powder-based methods that are used to make porous metals are relatively simple and scalable, and porosity can be controlled by interparticle spacing as well as the addition of a sacrificial template. A relatively new process based on reducing oxides in a metal matrix has been demonstrated to produce microscale porosity within individual powder particles and thereby may be used to enhance other powder metal techniques. Templating methods require relatively large quantities of powder, but oxide-reduction feedstock powders have only been produced by small-batch ball milling processes (e.g., 10 s of grams). Planetary ball milling is capable of processing larger quantities of powder (e.g., 100 s of grams) but has significantly different milling characteristics. To successfully apply this technique, it was systematically studied in terms of composition, milling conditions, and the addition of stearic acid to control powder size and morphology along with final porosity. It was found that by controlling basic parameters, such as oxide levels and milling time, a relatively high porosity (25%) and powder percentage (99%) can be achieved in Cu-2 mol% CuO with only 0.035 wt% stearic acid and only 90 min of milling. [ABSTRACT FROM AUTHOR]

Powder metallurgy (PM) processes for porous copper and alloys have seen some commercial successes, but PM methods have the disadvantage of relatively low porosity or strength that is compromised by stress-concentrating interparticle bonds. To increase porosity without compromising scalability, a Cu-CuO metal matrix composite powder was utilized to produce additional microscale porosity within the particles by oxide reduction. These Cu-CuO powders were pressed at 1, 2, or 3 GPa, and made porous at 600, 800, or 1000 °C to investigate the effects of pressing and sintering parameters on the overall strength and density. It was found that the formation of porosity is weakly dependent on compaction pressure (maximum 6% difference from 1 GPa to 3 GPa), while the final porosity varied by ~16% overall (~40% for 1 GPa and 600 °C to 24% for 3 GPa and 1000 °C). The strength of the porous Cu was highest after being reduced at 600 °C but also exhibited some flaking at the edges at high strain. The 1 GPa, 600 °C samples have a higher specific strength than wrought Cu annealed at the same temperature, as was demonstrated under uniaxial quasi-static compression as well as split Hopkinson pressure bar impact.

The ability to fabricate dual pore tissue scaffolds remains a challenge with respect to material selection and control over geometry. Herein, a novel method combining additive manufacturing and solid‐state foaming to fabricate dual porous tissue scaffolds is presented. Structures with designed architectures are 3D printed followed by solid‐state foaming resulting in macro‐ and micropores, respectively. The advantage of this approach is its applicability to a wide range of materials with flexibility to control the pore sizes during the design stage and foaming process, respectively. The obtained scaffolds have macro‐ and micropore sizes of 200 and 25 μm, respectively, with a porosity of 71%. The scaffolds are seeded and cultured with MCF7 breast cancer cells to study cell adhesion and growth followed by the efficacy of oleuropein treatment. The results show that the scaffold is able to support attachment, proliferation, and viability of the cells. In addition, the scaffolds do not elicit any toxicity and the MCF7 cells are equally susceptible to the oleuropein treatment when grown on the scaffolds. In summary, a scalable, controllable, and repeatable process for the fabrication of dual pore tissue scaffolds is presented, which can aid in the development of bioartificial organs. [ABSTRACT FROM AUTHOR]

Metal foams possess excellent physical and mechanical properties. This paper reviews the common manufacturing process of metal foams. Various ways used to produce metal foams based on metal properties are described. The manufacturing process follows four primary routes: liquid state, solid state, ion or vapour processing. Liquid-state processing produces porosity to liquid or semi-liquid metals, and solid-state foaming produces metal foams with metal powder as starting material. For ion and vapour processing methods, metals are electro-deposited onto a polymer precursor. The polymer precursor is removed by chemical or heat treatment to produce metal foams. The advantages and limitations of each manufacturing process are also described.

Micro cellular polymer foams find numerous applications such as in filtration, tissue scaffolds, insulation, and catalyst carriers. This study reports on a solvent free approach to fabricate microcellular poly lactic acid foams through a combination of additive manufacturing and microwave foaming. This approach provides the capability to foam polymers in a repeatable and controllable manner with minimal human intervention. Initially, samples are 3D printed, followed by gas saturation, and microwave foaming. The effect of microwave foaming parameters, namely power, temperature and time on the pore morphology was analyzed using a complete parametric study. The results show that microwave foaming can successfully generate porous structure with power and temperature being the main factors affecting the pore morphology. A combination of midrange power and high temperature results in foams with desired properties of small pore size and high porosity. The resulting foams have pore size as small as 120 μm with porosity of 78%. These foams fabricated using a solvent free approach find applications in biomedical field as three dimensional tissue scaffolds for drug testing and bio‐artificial organ development. [ABSTRACT FROM AUTHOR]

This research examined the optimization of the sustainable manufacturing process for polyester-based polymers/Fe3O4 nanocomposite foaming. The foamed structure was achieved by using a solid-state foaming process, where the prepared foams were tested in order to ascertain the optimum foaming parameters with the highest foaming ratios and the lowest foaming densities. The foaming parameters used in this research were the polymer type, nanoparticle percentage, packing pressure, holding time, foaming temperature, and foaming time. Two levels were selected for each factor, and a Taguchi plan was designed to determine the number of experiments required to reach a conclusion. Further characterization techniques, namely, differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD) were used with the original samples to gain a better understanding of their structure and chemical composition. The data analysis showed that regardless of the parameters used, a high foaming ratio resulted in a low density. The introduction of nanoparticles (NPs) to the polymer structure resulted in higher foaming ratios. This increment in foaming ratio was noticeable on Corro-Coat PE Series 7® (CC) polymer more than Jotun Super Durable 2903® (JSD). The optimum parameters to prepare the highest foaming ratios were as follows: CC polymer with 2% NPs, compressed under a pressure of 10 K lbs. for a 3 min holding time and foamed at 290 °C for 15 min in the oven. [ABSTRACT FROM AUTHOR]

This research examined the optimization of the sustainable manufacturing process for polyester-based polymers/Fe3O4 nanocomposite foaming. The foamed structure was achieved by using a solid-state foaming process, where the prepared foams were tested in order to ascertain the optimum foaming parameters with the highest foaming ratios and the lowest foaming densities. The foaming parameters used in this research were the polymer type, nanoparticle percentage, packing pressure, holding time, foaming temperature, and foaming time. Two levels were selected for each factor, and a Taguchi plan was designed to determine the number of experiments required to reach a conclusion. Further characterization techniques, namely, differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD) were used with the original samples to gain a better understanding of their structure and chemical composition. The data analysis showed that regardless of the parameters used, a high foaming ratio resulted in a low density. The introduction of nanoparticles (NPs) to the polymer structure resulted in higher foaming ratios. This increment in foaming ratio was noticeable on Corro-Coat PE Series 7® (CC) polymer more than Jotun Super Durable 2903® (JSD). The optimum parameters to prepare the highest foaming ratios were as follows: CC polymer with 2% NPs, compressed under a pressure of 10 K lbs. for a 3 min holding time and foamed at 290 °C for 15 min in the oven.

Solid-state nanofoaming experiments are conducted on two polymethyl methacrylate (PMMA) grades of markedly different molecular weight using CO2 as the blowing agent. The sensitivity of porosity to foaming time and foaming temperature is measured. Also, the microstructure of the PMMA nanofoams is characterized in terms of cell size and cell nucleation density. A one-dimensional numerical model is developed to predict the growth of spherical, gasfilled voids during the solid-state foaming process. Diffusion of CO2 within the PMMA matrix is sufficiently rapid for the concentration of CO2 to remain almost uniform spatially. The foaming model makes use of experimentally calibrated constitutive laws for the uniaxial stress versus strain response of the PMMA grades as a function of strain rate and temperature, and the effect of dissolved CO2 is accounted for by a shift in the glass transition temperature of the PMMA. The maximum achievable porosity is interpreted in terms of cell wall tearing and comparisons are made between the predictions of the model and nanofoaming measurements; it is deduced that the failure strain of the cell walls is sensitive to cell wall thickness. [ABSTRACT FROM AUTHOR]

In this study, the microcellular thermoplastic polyurethane foam with excellent fluorescent performance and a density of about 0.32 g/cm3 was prepared using solid-state foaming technology. The micro-sized green fluorescent pigment not only endowed thermoplastic polyurethane film with color, but also acted as nucleating agent to influence the cell morphology of thermoplastic polyurethane foam. Laser scanning confocal microscopy testing demonstrated that the characteristic excitation and emission peaks were observed in thermoplastic polyurethane/fluorescent pigment films and foams at 465 and 510 nm, respectively. It is found that the specific intensity of the characteristic emission peak was higher than that of the unfoamed counterpart, resulting from the formation of skin-core structure in thermoplastic polyurethane foam. By adjusting the thickness of unfoamed skin layer, the fluorescent performance of thermoplastic polyurethane / fluorescent pigment foam could be optimized. [ABSTRACT FROM AUTHOR]

In this study, the microcellular thermoplastic polyurethane foam with excellent fluorescent performance and a density of about 0.32 g/cm3 was prepared using solid-state foaming technology. The micro-sized green fluorescent pigment not only endowed thermoplastic polyurethane film with color, but also acted as nucleating agent to influence the cell morphology of thermoplastic polyurethane foam. Laser scanning confocal microscopy testing demonstrated that the characteristic excitation and emission peaks were observed in thermoplastic polyurethane/fluorescent pigment films and foams at 465 and 510 nm, respectively. It is found that the specific intensity of the characteristic emission peak was higher than that of the unfoamed counterpart, resulting from the formation of skin-core structure in thermoplastic polyurethane foam. By adjusting the thickness of unfoamed skin layer, the fluorescent performance of thermoplastic polyurethane / fluorescent pigment foam could be optimized. [ABSTRACT FROM AUTHOR]

To prepare the porous NC-based (nitrocellulose-based) gun propellants, the batch foaming process of using supercritical CO2 as the physical blowing agent is used. The solubilities of CO2 in the single-base propellants and TEGDN (trimethyleneglycol dinitrate) propellants are measured by the gravimetric method, and SEM (scanning electron microscope) is used to observe the morphology of foamed propellants. The result shows that a large amount of CO2 could be dissolved in NC-based propellants. The experimental results also reveal that the energetic plasticizer TEGDN exerts an important influence on the pore structure. The triaxial tensile failure mechanism for solid-state nucleation is used to explain the nucleation of NC-based propellants in the solid state. Since some specific foaming behaviors of NC-based propellants can not be explained by the failure mechanism, a solid-state nucleation mechanism which revises the triaxial tensile failure mechanism is proposed and discussed.

Auxetic epoxy resin foams were produced by solid-state foaming thanks to the use of properly shaped precursors. In fact, a re-entrant hexagonal shape of the precursors is preserved during foaming and results in a foam with a complex structure: a thin macro-structure with the re-entrant geometry filled with foam. The auxetic behavior was observed by using tensile tests at different temperatures (room temperature, 80℃, and 100℃). Indentation tests were also carried out to evaluate the gradient properties across the lines of the thin re-entrant macro-structure. In order to show that the auxetic behavior depended on the internal macro-structure, tests were also performed on foam panels obtained by cylindrical tablets and, therefore, with a standard-hexagonal macro-structure. In conclusion, the auxetic behavior was observed only for the foam panels with re-entrant hexagonal structure at 80℃. In this case, a negative Poisson’s ratio is immediately achieved at small strains and tends to a zero plateau value for longitudinal strains up to 1%. [ABSTRACT FROM AUTHOR]

A novel fabrication process of small pore-size nickel foams has been developed using electroless plating of solid-state foamed immiscible polymer blends. Ethylene acrylic acid (EAA) and polystyrene (PS) were melt-blended with extrusion to obtain a dual phase cocontinuous morphology. Gas saturation and foaming studies were performed to determine appropriate process conditions for foaming of the two-phase material. Open-celled polymer templates were obtained by extracting the PS phase with dichloromethane (DCM). The templates were subsequently used for nickel foam plating in ethanol-based electroless plating solutions. Nickel foams with pore sizes on the level of tens of micro-meters and porosity above 90% were fabricated. It was found that gas concentration and foaming temperature were major process variables significantly affecting the foam porosity. Foaming allowed faster PS extraction and higher porosity of the nickel plating templates. Because of the small pore size, ethanol-based solutions need to be used to ensure the infiltration of plating solutions. The developed process is a bulk method and can be used for large-scale fabrication of small pore-size nickel foams with high porosity. [ABSTRACT FROM AUTHOR]

Highlights: [•] Biodegradable foams have been fabricated by solid-state foaming. [•] Blowing agent mixtures of scCO2 and EL have been used to control foams properties. [•] Adding EL to scCO2 enhances polymer plasticization and foaming. [•] Foaming increases with the decrease of the polymer crystalline structure. [Copyright &y& Elsevier]

Polymer nanofoams have recently attracted significant interest in both industry and academia. The unique nanoscaled porous structure could bring unprecedented material properties that have not been seen in conventional or microcellular polymer foams. It has been hypothesized that nanofoams could have a much higher specific strength and toughness as well as significantly improved thermal resistivity. In this research, we study the fabrication and characterization of polyetherimide nanofoams using a supercritical carbon dioxide foaming process. A process map indicating the conditions to obtain various polyetherimide foam structures, including micro-, micro/nano transition, and nanofoams has been established. Two types of nanofoams were observed, one made with high gas concentrations and the other with high foaming temperatures. The one with high gas concentrations exhibited a higher specific modulus than that of unfoamed polyetherimide. Nanofoams generally showed a higher thermal resistivity than microfoams with similar relative densities. It is found that the equilibrium CO2 concentration in polyetherimide under the supercritical conditions does not fit well to the well-known dual-mode sorption model. A new gas concentration model was developed to describe the CO2 uptake under supercritical conditions. [ABSTRACT FROM PUBLISHER]

Abstract: In this work we investigated the solid-state supercritical CO2 (scCO2) foaming of poly(ɛ-caprolactone) (PCL), a semi-crystalline, biodegradable polyester, and PCL loaded with 5wt% of hydroxyapatite (HA) nano-particles. In order to investigate the effect of the thermal history and eventual residue of the crystalline phase on the pore structure of the foams, samples were subjected to three different cooling protocols from the melt, and subsequently foamed by using scCO2 as blowing agent. The foaming process was performed in the 37–40°C temperature range, melting point of PCL being 60°C. The saturation pressure, in the range from 10 to 20MPa, and the foaming time, from 2 to 900s, were modulated in order to control the final morphology, porosity and pore structure of the foams and, possibly, to amplify the original differences among the different samples. The results of this study demonstrated that by the scCO2 foaming it was possible to produce PCL and PCL-HA foams with homogeneous morphologies at relatively low temperatures. Furthermore, by the appropriate combination of materials properties and foaming parameters, we prepared foams with porosities in the 55–85% range, mean pore size from 40 to 250μm and pore density from 105 to 108 pore/cm3. Finally, we also proposed a two-step depressurization foaming process for the design of bi-modal and highly interconnected foams suitable as scaffolds for tissue engineering. [Copyright &y& Elsevier]

The potential of using dispersive domains in a polymer blend as a bubble nucleating agent was investigated by exploiting its high dispersibility in a matrix polymer in the molten state and its immiscibility in the solid state. In this experiments, polypropylene (PP) was used as the nucleating agent in polystyrene (PS) and poly(methyl methacrylate) (PMMA) foams at the weight fraction of 10, 20, and 30 wt %. PP creates highly dispersed domains in PS and PMMA matrices during the extrusion processing. The high diffusivity of the physical foaming agent, i.e., CO2 in PP, and the high interfacial tension of PP with PS and PMMA could be beneficial for providing preferential bubble nucleation sites. The experimental results of the pressure quench solid-state foaming of PS/PP and PMMA/PP blends verified that the dispersed PP could successfully increase the cell density over 106 cells/cm3 for PS/PP and 107 cells/cm3 for PMMA/PP blend and reduce the cell size to 24 μm for PS/PP and 9 μm for PMMA/PP blends foams. The higher interfacial tension between PP and the matrix polymer created a unique cell morphology where dispersed PP particles were trapped inside cells in the foam. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011 [ABSTRACT FROM AUTHOR]

Solid-state foaming is a relatively new process for producing closed-cell thermoset foams starting from powders. In this study, the possibility of introducing nanoclay into epoxy foams by solid-state foaming was investigated. The nanoclay was found to affect the foaming efficiency and the foam density; even if large nanoclay aggregates are present in the foams, for a small range of filler content (2.5—3 wt%), the mechanical properties are positively influenced, as shown by specific compressive toughness which attains a maximum value. Electron microscope observations provided important information on the dependence of the cell size on the filler percentage, and the presence of a critical size (about 5 μm) of nanoclay aggregates, below which they are coherent with the matrix. [ABSTRACT FROM PUBLISHER]

A new foaming process has been developed for epoxy resins. Uncured epoxy tablets are fabricated by pressing commercial powders in a steel mold at room temperature and used as foam precursors. The tablets foam when heated in a muffle at high temperature. No blowing agent was added because the foaming mechanism depends on the uncured resin boiling point. The foaming temperature is set to be high enough to rapidly produce the resin boiling but not excessive to avoid the thermal degradation. During boiling, the epoxy resin polymerizes and the bubbles freeze in the final structure. Epoxy foams are obtained by heating the compacted tablets in cylindrical copper molds, having internal diameter equal to the tablet diameter. Several process parameters have been changed in the experiment to understand their correlation with the foaming efficiency. However, the foaming ratio (expressed in terms of the ratio between the final and the initial tablet height) is found to be mainly dependent on the initial tablet density. In optimal conditions, the foaming ratio can rise up to 6. Thermal tests have been performed to evaluate the epoxy powder behavior during the cure, whereas mechanical compression tests were used to evaluate the final performances of the foams. [ABSTRACT FROM AUTHOR]

Foaming an epoxy is challenging because the process involves the curing reaction of epoxy and hardener (from monomer to oligomer, to a gel and a final three-dimensional crosslinked network) and the loading of gas phase into the epoxy phase to develop the cellular structure. The latter process needs to be carried out at the optimum curing stage of epoxy to avoid cell coalescence and to allow expansion. The environmental concern regarding the usage of chemical blowing agent also limits the development of epoxy foams. To surmount these challenges, this study proposes a solid-state CO2 foaming of epoxy. Firstly, the resin mixture of diglycidylether of bisphenol-A (DGEBA) epoxy and polyamide hardener is pre-cured to achieve various solid-state sheets (preEs) of specific storage moduli. Secondly, these preEs undergo CO2 absorption using an autoclave. Thirdly, CO2 absorbed preEs are allowed to free-foam/expand in a conventional oven at various temperatures; lastly, the epoxy foams are post-cured. PreE has a distinctive behavior once being heated; the storage modulus is reduced and then increases due to further curing. Epoxy foams in a broad range of densities could be fabricated. PreE with a storage modulus of 4 × 104–1.5 × 105 Pa at 30 °C could be foamed to densities of 0.32–0.45 g/cm3. The cell morphologies were revealed to be star polygon shaped, spherical and irregularly shaped. The research proved that the solid-state CO2-foaming technique can be used to fabricate epoxy foams with controlled density. [ABSTRACT FROM AUTHOR]

This study reports on the production and characterization of highly porous (up to 91%) composite foams for potential bone tissue engineering (BTE) applications. A calcium phosphate-based glass particulate (PGP) filler of the formulation 50P 2 O 5 -40CaO-10TiO 2 mol.%, was incorporated into biodegradable poly(d,l-lactic acid) (PDLLA) at 5, 10, 20, and 30 vol.%. The composites were fabricated by melt compounding (extrusion) and compression molding, and converted into porous structures through solid-state foaming (SSF) using high-pressure gaseous carbon dioxide. The morphological and mechanical properties of neat PDLLA and composites in both nonporous and porous states were examined. Scanning electron microscopy micrographs showed that the PGPs were well dispersed throughout the matrices. The highly porous composite systems exhibited improved compressive strength and Young's modulus (up to >2-fold) and well-interconnected macropores (up to ~78% open pores at 30 vol.% PGP) compared to those of the neat PDLLA foam. The pore size of the composite foams decreased with increasing PGPs content from an average of 920 µm for neat PDLLA foam to 190 µm for PDLLA-30PGP. Furthermore, the experimental data was in line with the Gibson and Ashby model, and effective microstructural changes were confirmed to occur upon 30 vol.% PGP incorporation. Interestingly, the SSF technique allowed for a high incorporation of bioactive particles (up to 30 vol.%-equivalent to ~46 wt.%) while maintaining the morphological and mechanical criteria required for BTE scaffolds. Based on the results, the SSF technique can offer more advantages and flexibility for designing composite foams with tunable characteristics compared to other methods used for the fabrication of BTE scaffolds.

In this study, a single paragraph of acrylonitrile-butadiene-styrene (ABS)/recycled polyethylene terephthalate (R-PET) polymeric foams is prepared using CO2 as a blowing agent. First, the sorption kinetics of subcritical and supercritical CO2 are first studied at saturation temperatures from −20 to 40 °C and a pressure of 10 MPa, in order to estimate the diffusion coefficient and the sorption amount. As the sorption temperature increases, the diffusion coefficient of CO2 increases while the sorption amount decreases. Then, a series of two-step solid-state foaming experiments are conducted. In this process, a specimen is saturated with liquid CO2 and foamed by dipping the sample in a high-temperature medium at 60 to 120 °C. The effects of foaming temperature and depressurization rate on the morphology and structure of ABS/R-PET microcellular foams are examined. The mean cell size and the variation of the cell size distribution increases as the foaming temperature and the depressurization rate increases. [ABSTRACT FROM AUTHOR]