Foaming Process Research Articles

Mild Fabrication of Polymeric Porous/Nonporous Micro‐Composite Structures via Enhancing the Photothermal Conversion of Ultrafast Laser

AbstractLocalized foaming of polymers is promising to fabricate polymeric composite structures for applications in biomedical, aerospace, automotive, etc. However, the severe pyrolysis and carbonization of polymer chains during localized foaming may degrade the bulk strength. Herein, a mild localized foaming strategy is proposed to fabricate poly(lactic acid) (PLA) micro‐foams within dense PLA matrices using ultrafast lasers. The PLA substrates doped with graphene (Gr) and azodicarbonamide (AC) are developed for mild foaming. A picosecond laser is applied to pattern PLA micro‐foams on the PLA/Gr/AC substrates to fabricate porous/nonporous micro‐composite structures. The thermal behavior of the PLA/Gr/AC substrates is characterized to evaluate their capability in photothermal conversion and mild foaming during ultrafast laser pulses. To optimize the laser foaming process, the microstructure of the laser‐foamed PLA processed with different laser parameters is studied. It demonstrates that the oxidation of the functional groups dominates the side reactions on the PLA chains with slight variation in molecular weight during laser foaming, indicating that the PLA chains keep almost intact and mild localized foaming is realized. Furthermore, the developed technology is demonstrated for micro‐patterning and texturing, material toughness programming, and shape memory programming, showing promising applications for smart materials, metamaterials, micro‐robotics, and biological scaffolds.

Effect of calcium silicate on foaming and anti‐shrinkage behavior of thermoplastic polyamide elastomers: Revelation of the open‐cell mechanism

AbstractThermoplastic polyamide elastomer (TPAE)/calcium silicate (CaSiO3) open‐cell foam with low shrinkage and high expansion ratio was prepared by using supercritical CO2 as a blowing agent. Adding CaSiO3 led to increased melt viscoelasticity and reduced crystallinity of TPAE. The primary mechanism of CaSiO3 promoting TPAE foam to form the open‐cell structure was discussed. The interaction between CaSiO3 and TPAE is fragile, making CaSiO3 easy to fall off the cell wall and form the open‐cell structure during foaming process. The increasing CaSiO3 content leads to increased cell density and thinned cell walls, thus increasing the tensile stress on cell wall. On the other hand, the agglomeration of CaSiO3 increases particle size, making it easier to fall off from the adhesion of TPAE matrix. The two aspects work together to improve the open‐cell content of TPAE foam up to 93.28%. The open‐cell structure is conducive to increasing the gas exchange rate and significantly decreasing foam shrinkage ratio. Finally, TPAE foam with a shrinkage ratio of only 2.68% and an expansion ratio of 18.77 times was obtained. In addition, open‐cell structure improves the adsorption performance of TPAE/CaSiO3 foam by 500%.

On the Potential of Manufacturing Multi-Material Components With Micro/Nanocellular Structures Via the Hybrid Process of Electromagnetic Forming Injection Foaming

Abstract Multimaterial design with a combination of solid and foam structures offers a promising avenue for reducing component weight while enhancing their functionalities. However, the complexity of multistage manufacturing processes poses significant challenges to adopting such approaches. To address these challenges, this paper introduces an innovative concept known as Electromagnetic Forming Injection Foaming (EFIF), which integrates injection molding, forming, and foaming processes into a single hybrid process. This process begins with a simultaneous filling-forming phase, followed by supercritical fluid (SCF) assisted foaming controlled by electromagnetic forming. Through a series of experimental and analytical studies, this work investigates the feasibility and effectiveness of EFIF. First, the impact of pressure drop rate and pressure drop on cell size and density is examined through a specialized experimental setup enabling performing injection, forming, and foaming processes in a single operation. The potential influence of electromagnetic forming on foam injection molding is explored through experiments focusing on the effects of a polymer layer between sheet metal blank and the electromagnetic coils. Additionally, an analytical study evaluates the EFIF process by calculating expected pressure drop rates under different processing conditions and their influence on cell nucleation rates. The results showed the possibility of achieving pressure drop rates up to 1.5 × 105 bar/sec, resulting in nucleation rates up to 1.77 × 109 nuclei/cm3sec. Overall, this paper highlights the potential of EFIF to merge existing technologies into a scalable solution for manufacturing multimaterial components with micro- to nanocellular polymer foams.

Mutual and Thermal Diffusivities in Binary Mixtures of Polystyrene with Dissolved N2 or CO2 by Dynamic Light Scattering

In physical foaming processes of thermoplastics, a liquid mixture consisting of a molten polymer with a dissolved blowing agent is extruded through a die or injected into a mold. The morphology of the foam matrix strongly depends on the mutual diffusion coefficient D11 of the liquid polymer-blowing agent mixture. For a better understanding of the underlying physical mechanisms during the foaming process and the development of corresponding models, accurate knowledge of D11 is needed. This work reports on the simultaneous measurement of D11 and the thermal diffusivity a of liquid mixtures consisting of polystyrene melts with dissolved nitrogen (N2) or carbon dioxide (CO2) at mass fractions wsolute up to 0.003 or 0.02 and temperatures T between (433 and 533) K using dynamic light scattering (DLS). The determined D11 range is between (1 and 4) × 109 m2⋅s−1 and are slightly larger for the mixtures containing N2 at a given T and wsolute. D11 could be determined with an average expanded relative uncertainty of 18%. Considering all investigated state points and the achieved experimental uncertainties, both D11 and a are independent on the amount of dissolved gas, despite relatively large mole fractions of the dissolved blowing agents xsolute of 0.997 and 0.93.

Polybutylene adipate terephthalate/polylactic acid interface enhanced compatibilization and its bead-foaming characteristics

Bead foaming technique is regarded as a highly promising method for preparing foams with complex geometries and high expansion ratios. The biodegradability of poly(butylene adipate-co-terephthalate) (PBAT) has garnered significant attention in the field of foam materials. However, due to inherent disadvantages such as low melt strength and low modulus, PBAT faces challenges during bead foaming. In this study, a small amount of polylactic acid (PLA) was incorporated into PBAT. Utilizing the differential melting points of PLA and PBAT, PLA served as physical cross-linking points. The epoxy-based chain extender ADR4370S was used as a chain extender and compatibilizer. By varying its content, the compatibility and foaming performance of the PBAT/PLA blend were regulated. Finally, the foaming process employed supercritical carbon dioxide (scCO2) impregnation followed by heating to address the hydrolysis issue of the PBAT/PLA blend during bead foaming. The results demonstrated that the introduction of ADR could initiate reactions between its epoxy groups and PBAT and PLA, resulting in grafting and chain extension. When the ADR content reached 0.6 wt%, the cell structure evolved from a bimodal to a uniform cell structure, with a minimum average cell size of 12.3 μm and a maximum foaming ratio of 10.3 times.

Tuning cellular structure in a previously developed microcellular acrylonitrile butadiene styrene/thermoplastic polyurethane blend foams

AbstractThis study investigates the cell structure control in 50% thermoplastic polyurethane (TPU) and 50% acrylonitrile butadiene styrene (ABS) blend foam using CO2 as a physical blowing agent, focusing on the effects of variable foaming parameters on the microstructure. Samples measuring 25 × 25 × 1 mm were produced and analyzed for foam structure. The foaming process involved saturating the samples with CO2 gas at pressures of 4, 5.5, and 7 MPa, followed by rapid pressure release and immersion in a hot glycerol bath. The foaming parameters included varied temperatures (80, 90, and 120°C) and times (5–80 s). Scanning electron microscope (SEM) analysis provided data on cell size and density. Results indicated that increasing the saturation pressure enhanced CO2 uptake in the ABS/TPU blend, with the CO2 uptake rate peaking early in the process. Higher foaming temperatures and extended foaming times led to increased cell size, cell density, and expansion ratio. These findings highlight the significant role of process parameters in controlling the cell structure of ABS/TPU blend foams, offering valuable insights into optimizing foam properties for industrial applications.Highlights Optimization of foam parameters leads to cell structure control in ABS/TPU composite foams for industrial applications. Increasing saturation pressure significantly boosts CO2 uptake in ABS/TPU composite foams. Increasing the foaming temperature and duration leads to larger cell sizes, higher cell density, and greater expansion ratios in ABS/TPU composite foams.

Innovative reinforcement method for metal foam cell wall using CNTs

Carbon nanotubes (CNTs) and their composites are gaining popularity due to their exceptional strength qualities. It is well known that adding CNTs to metal foam composites boosts compressive strength. On the other hand CNT addition is still a costly process due to high cost of the CNTs. This study presents a novel and cost-effective solution by selectively adding CNTs to the structurally weakest regions of aluminum foam materials produced via powder metallurgy, employing a newly developed focused multi-step additive method. The cell borders of aluminum foam are strengthened with multiple spherical layers of CNTs, using a transfer method by initially coating the space holders used at the foaming process. The strength increase effect of this CNT addition method was compared with the widely known aluminum foam production parameters via a 4-parameter design of experiment (DOE) study. Compressive strength values of the samples were evaluated using a constant speed compression test acc. to ISO13314. The compacting pressure, CNT concentration, sintering temperature, and sintering period were chosen as DOE parameters, and 78% of the interactions effecting on final compressive strength could be explained with the model. As a result, it was established that, compared to the other parameters, sintering duration had the highest influence on compressive strength. But besides It has also been shown that adding 0.53% CNT by weight only to the cell border regions increases overall strength by 9%. This weight-strength increase ratio is compared with similar studies in the literature and found to be providing a production cost advantage due to the lower amount of CNT addition requirement for the comparable weight relative strength increase. Focused strength increase method has potential to enable controlled failure of foam materials by selectively strengthening strength critical areas of a component.

Methodology for Liquid Foam Templating of Hydrogel Foams: A Rheological and Tomographic Characterization

AbstractHydrogel foams are widely used in many applications such as biomaterials, cosmetics, foods, or agriculture. However, controlling precisely foam morphology (bubble size or shape, connectivity, wall and strut thicknesses, homogeneity) is required to optimize their properties. Therefore, a method is proposed here for generating, controlling, and characterizing the morphology of hydrogel foams from liquid foam templates: Using the example of Alginate‐CaHPO4‐based hydrogel foams, a highly controllable foaming process is provided by bubbling nitrogen through nozzles into the solution, which produces hydrogel foams with millimeter‐sized bubbles. A rheological characterization protocol of the foam's constituent material is first implemented and highlights the impact of the initial liquid foam properties and of the competition between the solidification kinetics and the foam aging mechanisms on the resulting morphology. X‐ray tomographic characterization performed on solidifying and solidified samples then demonstrates that by controlling the temporal evolution of the foam via its formulation, it is possible to tune the final morphology of the alginate foams. This method can be adapted to other hydrogel or polymer formulations, foam characteristics and length scales, as soon as solidification processes happen on timescales shorter than foam destabilization mechanisms.

Development of Dynamic Four-Dimensional Printing Technology for Patterned Structures by Applying Microcellular Foaming Process.

Four-dimensional (4D) printing adds the dimension of time to 3D-printed specimens, causing movement when external stimuli are applied. This movement enables applications across various fields, including the soft robotics, aerospace, apparel, and automotive industries. Traditionally, 4D printing has utilized special materials such as shape-memory polymers (SMPs) or shape-memory alloys (SMAs) to achieve this movement. This study explores a novel approach to 4D printing by applying microcellular foaming processes (MCPs) to 3D printing. This study primarily aims to design and fabricate patterned specimens using common materials, such as PLA, through 3D printing and to analyze their dynamic behavior under various foaming conditions. To demonstrate the potential applications of this technology, the degree of bending was measured by controlling the patterning and foaming conditions. The 3D-printed specimens with microcellular foaming exhibited predictable deformations owing to the asymmetric expansion caused by differential gas saturation. The results confirm that 4D printing can be realized using conventional materials without the need for smart materials and can introduce foaming processes as a new external stimulus. This study highlights the potential of combining 3D printing with microcellular foaming for advanced 4D printing applications.

Compatibilizing and foaming of PC/PMMA composites with nano‐cellular structures in the presence of transesterification catalyst

AbstractCompatibility of polycarbonate (PC) and polymethyl methacrylate (PMMA) alloys was improved by using a transesterification catalyst (SnCl2·2H2O). Modified PC/PMMA alloys exhibit single Tg, and their initial island phase existing in the SEM were transformed into uniform surface. Besides, the transmittance of the modified alloys was increased from original 40% to 85%. Moreover, PC/PMMA alloys and PC foams with micro‐cellular and nano‐cellular structures were prepared by solid‐state CO2 foaming in the presence of transesterification catalyst. Distinctively, there are obvious nano‐cellular structures existing in the PC samples, but no related nanostructures were found in PMMA samples, after treated by same amount of catalyst and foaming process for pure PC and PMMA matrix. Furthermore, the effects of foaming temperature and segment structure on their foaming behavior were also studied. Additionally, a uniaxial stress experiment was conducted at a specific temperature to simulate the biaxial stress during the foaming process for discovering the mechanism of nanopore formation. Therefore, the concept of nano‐cellular structures will point out a direction for the development of high‐performance, heat insulation PC materials of the next generation.Highlights Transesterification catalysts enhanced compatibility between PC and PMMA. Nanopore structures were successfully constructed in PC foams. Segment stretching was the main reason for the formation of nanopores.

Waste ramie fiber/EVA composites with wideband sound-absorbing performance with mixing and foaming process

Increasing noise pollution and ramie fibers waste resource problems need to be solved urgently. In order to resolve the problems, the porous sound-absorbing composites with high sound-absorbing performance were prepared by using waste ramie fibers as reinforcement materials, vinyl acetate copolymer (EVA) as matrix materials, azodicarbonamide(AC) as foaming agent, ZnO as auxiliary foaming agent, and dicumyl peroxide (DCP) as cross-linking agent, which were mixed in a two-stick mixer according to a certain ratio and then hot-pressing process. The process conditions were optimized using single factor analysis. Under the optimal process conditions, the sound-absorbing composites had a maximum sound absorption coefficient (SAC) of 0.72, an average SAC of 0.37, and a noise reduction coefficient (NRC) of 0.41. The sound absorption performance grade reached level III, and the sound absorption performance was excellent in the frequency range of 600 ∼ 6300 Hz. The introduction of foaming agents made the composites with more pores, and the average SAC of foamed composites increased by 346 %(compared with the unfoamed composites). Then, the sound-absorbing mechanism was analyzed. This study presents a novel approach to the recycling and reusing of the waste ramie fibers. It also provides a theoretical and experimental basis for the development of sound-absorbing composites with wider sound absorption band, larger (sound absorption coefficient) SAC and wider application, which is of great significance for sound absorption in the building fields.

Development of a biocompatible radiotherapy spacer using 3D printing and microcellular foaming process for enhanced prostate cancer treatment

In this study, a new active spacer was developed using three-dimensional (3D) printing technology and microcellular foaming process to overcome the limitations of current spacers used in prostate cancer treatment. In prostate cancer treatment, a spacer is inserted between the prostate and rectum to increase the distance between the two organs so as to reduce the radiation dose to the rectum. Radiotherapy spacer is widely used in particle therapy, such as proton and carbon beams, and X-ray radiation therapies, including intensity-modulated radiotherapy and stereotactic body radiation therapy. However, existing spacers have been reported to cause significant side effects. Therefore, this study introduces a 3D printing technique using polycaprolactone to develop a spacer that provides customized treatment and minimizes side effects. This technique utilizes the volume expansion that occurs when a 3D spacer printed to fit a patient’s organs undergoes foaming through a supercritical carbon dioxide (scCO2)-assisted microcellular foaming process. Additionally, the use of scCO2 allows simultaneous gas absorption and sterilization, thereby reducing the number of process steps. This study confirms the potential of the newly developed spacer to provide effective and safe radiotherapy for prostate cancer, reduce patient discomfort, and minimize rectal side effects during radiation treatment.

Influence of foaming additives on the porous structure and properties of lightweight geopolymers based on ash–slag waste

Lightweight geopolymer is a promising thermal insulation material that conserves energy used for heating and cooling buildings. This study investigates foaming agents that influence the porous structure and properties of geopolymer foams based on ash–slag waste from thermal power plant using a 12 M sodium hydroxide solution, waterglass, and foaming agents (30 % hydrogen peroxide solution, aluminum, silicon, and zinc powders). The study demonstrates the feasibility of foaming geopolymers by water evaporation without foaming additives, where the best density of 949 kg/m3 and compressive strength of 4.2 MPa were achieved in samples foamed by microwave radiation, but such properties cannot attribute geopolymers as lightweight. Foaming with H2O2 is hard to synchronize with geopolymerization reactions. Aluminum and silicon powders lead to rapid reactions in alkaline conditions, resulting in uneven pore sizes. Zinc powder was the most effective foaming agent, producing materials with a density of 331 kg/m3 and compressive strength of 1.17 MPa, and it also integrates into the geopolymer gel structure. The foaming process should occur at room temperature. Elevated temperatures increase curing rates and result in incomplete geopolymerization, preventing uniform pore formation. Synthesized geopolymers are suitable for building insulation, basements, underground pipelines, and utility networks.

Solubility of supercritical CO2 in polystyrene

Expanded polystyrene (ePS) plays an important role in the food packaging industry. However, the foaming process is environmentally unfriendly. A sustainable alternative is dissolving supercritical CO2 (scCO2) in the polystyrene (PS) matrix. Most studies so far were performed at temperatures above the PS glass transition temperature; however, a more general temperature window is desirable. In this work, the solubility of scCO2 in polystyrene was measured at 323 K, 343 K, 363 K and 383 K and pressure up to 130 bar using a magnetic suspension balance (MSB). It was concluded that the solubility of CO2 in PS decreases with temperature and increases with pressure. The Sanchez-Lacombe Equation of State was utilized to estimate the degree of swelling. The model developed was able to derive the experimentally determined solubilities after correction for the swelling. The interaction parameter, k12, turned out to be only a function of temperature. With these results the solubility and swelling of PS in scCO2 can be more accurately assessed for different temperatures and pressures.

Easily manufacture lightweight EPDM/POE/KB foam with high electromagnetic shielding efficiency and high absorption coefficient through chemical foaming

Utilizing foam-structured composite materials for electromagnetic shielding offers several advantages, including lightweight properties, low filler load, and a high absorption coefficient. However, their application in electromagnetic shielding is often limited by relatively lower mechanical performance. This study employed melt blending and chemical foaming processes to fabricate a lightweight EPDM/POE/KB composite foam with favorable mechanical characteristics, heat resistance, and electromagnetic absorbing performance. Significantly, the foam, with a density of 0.195 g/cm³, demonstrates an absorption-dominated shielding mechanism (absorption coefficient of 0.71) and good electromagnetic shielding performance of 20.57 dB in the X-band. The honeycomb structure formed within the material contributes to the multiple reflections of electromagnetic waves. Additionally, the composite foam exhibits excellent flexibility, with a fracture elongation greater than 150 %, alongside tensile and compressive strengths of 1.46 MPa and 0.92 MPa, respectively, demonstrating satisfactory mechanical performance. Furthermore, the composite foam also possesses good thermal stability. The study provides a low-cost and effective method for preparing lightweight, high-absorption coefficient, and excellent mechanical performance electromagnetic shielding foam.

Comparative Study of the Foaming Behavior of Ethylene-Vinyl Acetate Copolymer Foams Fabricated Using Chemical and Physical Foaming Processes.

Ethylene-vinyl acetate copolymer (EVA), a crucial elastomeric resin, finds extensive application in the footwear industry. Conventional chemical foaming agents, including azodicarbonamide and 4,4'-oxybis(benzenesulfonyl hydrazide), have been identified as environmentally problematic. Hence, this study explores the potential of physical foaming of EVA using supercritical nitrogen as a sustainable alternative, garnering considerable interest in both academia and industry. The EVA formulations and processing parameters were optimized and EVA foams with densities between 0.15 and 0.25 g/cm3 were produced. Key findings demonstrate that physical foaming not only reduces environmental impact but also enhances product quality by a uniform cell structure with small cell size (50-100 μm), a wide foaming temperature window (120-180 °C), and lower energy consumption. The research further elucidates the mechanisms of cell nucleation and growth within the crosslinked EVA network, highlighting the critical role of blowing agent dispersion and localized crosslinking around nucleated cells in defining the foam's cellular morphology. These findings offer valuable insights for producing EVA foams with a more controllable cellular structure, utilizing physical foaming techniques.

Effect of the heat treatment on the compression of Cu20Sn matrix syntactic foams reinforced with Fe-hollow spheres

This work analyzes the manufacturing process, phase transformations and compression behavior of un-treated and heat-treated syntactic foams consisting of a Cu20Sn alloy matrix reinforced with Fe-hollow spheres. Manufacturing process included the infiltration of the molten alloy into preforms of spheres sintered at 1000 °C during 1 h, obtaining syntactic foams with porosities near 52 %. These foams were heat-treated at 750 °C for 1 h, followed by quenching in water, which led to phase transformations from ε to β. This fact improved the compressive properties of the foams, increasing compression strengths from 195 to 243 MPa. Interfaces were not found between matrix and the Fe-hollow spheres.

Current Trends in the Use of Biomass in the Manufacture of Rigid Polyurethane Foams: A Review

This paper discusses methods of using biomass from the agriculture, forestry, food and aquaculture industries as potential raw materials for bio-polyols and as fillers in the production of rigid polyurethane (RPUR) foams. Various aspects of obtaining bio-polyols are discussed, as well as the impact of replacing petrochemical polyols with bio-polyols on the properties of foams. Special attention is paid to the conversion of vegetable oils and lignin. Another important aspect of the research is the use of biomass as foam fillers. Chemical and physical modifications are discussed, and important factors, such as the type and origin of biomass, particle size and amount, affecting the foaming process, microstructure and properties of RPUR foams are identified. The advantages and disadvantages of using biomass in foam production are described. It is found that bio-polyols can replace (at least partially) petrochemical polyols while maintaining the high insulation and strength of foams. In the case of the use of biomass as fillers, it is found that the shaping of their properties is largely dependent on the specific characteristics of the filler particles. This requires further research into process optimization but allows for the fine-tuning of RPUR foam properties to meet specific requirements.

Investigating the Foaming Process of a Thermoplastic Elastomer (TPE) Using Rheology and Image Analysis

Investigating the Foaming Process of a Thermoplastic Elastomer (TPE) Using Rheology and Image Analysis

Non-destructive Evaluation of Cavitation Erosion Behavior of Alumina-based Ceramic Samples

Numerous industrial parts, devices, and processes are designed to withstand the conditions that lead to cavitation erosion. Metallic, ceramic, and composite materials used for these conditions must achieve specific mechanical characteristics required to resist cavitation erosion. When molten metal or alloy flows and comes into contact with refractory material or coated furnace linings, cavitation erosion can occur. This phenomenon is particularly expected in metallurgy, especially in casting operations. Alumina-based refractories, specifically low cement castable (ALCC), are often used in furnace lining applications due to their superior properties, such as high refractoriness, thermal stability, and mechanical characteristics. Mullite is another refractory material frequently used in foundry lining applications. It can be utilized as a coating in casting processes, such as the Lost Foam process, which is a novel method for producing high-quality, cost-effective castings. These two refractory materials were chosen to study their behavior under cavitation conditions. An ultrasonic vibratory test with a stationary specimen (ASTM G-32) was used for experimental cavitation determination. The results of mass loss and surface morphological parameters of degradation revealed that ALCC samples eroded predominantly at the surface, while the mullite samples exhibited more significant degradation by depth.