Foaming Process Research Articles

Biobased Immiscible Polylactic Acid (PLA): Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) Blends: Impact of Rheological and Non-isothermal Crystallization on the Bead Foaming Behavior

Nowadays, bead foams are of great interest due to their high lightweight potential. The processing of such foams strongly depends on the crystallization and rheological behavior of the polymers used. By blending polymers, these properties can be tailored to obtain beaded foams with low density, small cell size and high cell density. As a bio-based polymer, PLA is of great interest due to its renewable carbon source. PLA suffers from its low thermal and rheological properties, which can be compensated by using blends. The correlation between the PLA/PHBV ratio and the rheological as well as the crystallization behavior was investigated. The use of PHBV as a minor phase significantly changes the rheological properties and increases the crystallization behavior of PLA. These findings were applied to the foam extrusion process to obtain low density bead foams. Bead foams with densities below 100 kg/m3, mean cell sizes below 50 µm and cell densities of 1 × 107 cells/cm3 were obtained.

Fabrication of thermally resistant and mechanically flexible polyimide foams with anisotropic pore structures for thermal insulation and irradiation tolerance applications

Flexible polyimide foams (PIFs) demonstrate a great potential for applications in the fields of thermal insulation, flame retardancy and irradiation tolerance. In this work, mechanically flexible PIFs with anisotropic pore structure were prepared by adopting the copolymerization strategy and microwave-assisted foaming process. The copolymerization reaction was conducted to synthesize polyester ammonium salt (PEAS) precursors using 3,3′,4,4′-benzophenone tetracarboxylic acid dianhydride (BTDA) as dianhydride, 4,4′-diaminodiphenyl ether (ODA) and 4,4′-diaminodiphenyl sulfone (DDS) as the copolymerized diamines; afterwards, PEAS powders with particle sizes ranging from 100 to 300 μm were obtained by drying, crushing and sieving treatments, which were used as the feedstock for “bottom-up” microwave-assisted foaming and thermal imidization process, thereby leading to the formation of PIFs with regular three-dimensional near-spherical pores in vertical direction (parallel to the pore growth direction) and the aligned ellipsoidal strip-like pore structure in horizontal direction. The directional growth of foam pores resulted in anisotropic mechanical flexibility and thermal insulation performance. Moreover, PIFs possessed excellent thermal stability and flame retardancy with the initial thermal degradation temperature higher than 545 °C and limited oxygen index up to 58.8%. The PIFs exhibited exceptional thermal insulation performance with thermal conductivity values ranging from 0.0276 to 0.0517 W/(m·K) between 25 and 300 °C. Additionally, the PIFs demonstrated excellent irradiation tolerance, with the mechanical and thermal stability retention rate being above 96% after the irradiation experiment (radiation dosage: 1.15 × 105 Gy) using 60Co source. Thus, lightweight PIFs with superior high temperature thermal insulation performance, flame retardancy and irradiation tolerance were successfully prepared, which showed potential applications in aerospace, medical and nuclear power sectors.

Spongy polyelectolyte hydrogel for efficient Solar-Driven interfacial evaporation with high salt resistance and compression resistance

The hydrogel-based evaporator for interface evaporation provides a method for sustainable freshwater production. However, salt accumulation on the surface of the steam generator affects light absorption and reduces steam generation efficiency. Achieving a high-performance, salt-resistant steam generator remains challenging. Here, sponge-like polyelectrolyte composite hydrogel-based solar steam generator (MPS) is reported. The steam generator is designed with an interpenetrating network topology and is made by in-situ polymerizing sodium polyacrylate (P(SA)) in a sponge-like polyvinyl alcohol (PVA) hydrogel framework, using molybdenum disulfide (MoS2) as a photothermal conversion material. Through the foaming process, MPS achieves self-floating and good water transportation performance, which enhances the thermal localization effect on the hydrogel surface and improves energy utilization efficiency (92.95 %), enabling efficient evaporation (3.22 kg m−2h−1). Meanwhile, due to the coupling of P(SA), MPS exhibits excellent salt resistance, maintaining a long-term evaporation rate of 2.8 kg m−2h−1 in high concentration saltwater (20 wt%). The porous structure and interconnected polymer chains enhance the mechanical performance of MPS, making it highly compressible and convenient for collecting freshwater marine and wilderness environments. The preparation strategy of MPS provides a feasible method to simultaneously improve the performance and durability of the interface steam generator in practical applications.

Effects of reactive diluent on processing, structure, and properties of epoxy foams and their sandwich composites

AbstractFoam sandwich composite (FSC) has recently been widely used in transportation, aerospace, shipping, and construction because of its lightweight, high specific strength, stiffness, and fatigue resistance characteristics. This study aimed to prepare, characterize, and test FSC with different diluent addition amounts. Benzyl glycidyl ester was added to the epoxy foam (EF) as a reactive diluent. Then, the EF was introduced into the three‐dimensional hollow composite (3DHC) by vacuum introduction process to prepare the FSC. Fourier transform infrared spectroscopy and thermogravimetric analysis demonstrated that increased diluent content enhanced SiN bond formation, augmenting thermal stability in resin molecules. The effects of different diluents on the mechanical properties of FSC showed that the mechanical properties of FSC were the best when the diluent content was 5 wt%. Compared with 3DHC, the flat compressive strength, flat tensile strength, and shear strength were increased by 7.2, 6.4, and 2.9 times, respectively.Highlights Diluent improves foaming agent dispersion by lowering resin viscosity. Diluent makes the EP easier to be introduced into 3DHC. FSC demonstrates a 723% increase in flat compression strength.

Foaming behaviors of poly (butylene adipate-co-terephthalate) with methyl methacrylate-co-glycidyl methacrylate as chain extender

As an important biodegradable polymer, how to achieve stable cell morphology and high cell density in the foam of poly(butylene adipate-co-terephthalate) (PBAT) is challenging due to its low mechanical and melt strength. To address this issue, an innovative methyl methacrylate-co-glycidyl methacrylate (MG) copolymer was designed and used as a chain extender due to its strong affinity with carbon dioxide (CO2) during the supercritical CO2 foaming process. The branching reaction between the carboxyl end groups of PBAT and the epoxy groups in MG enhanced the molecular weight, melt strength, and mechanical properties of PBAT. Therefore, the introduction of MG resulted in a reduced cell size, higher cell density, stronger compression stress for the foamed PBAT. The results prove that the MG copolymer is an effective chain extender for PBAT, which is beneficial to improve the foaming process and properties of PBAT for its application as package cushioning material.

An energy-efficient solution to sludge drying and combustion process through Camellia oleifera shells amended foaming

Foaming pretreatment has been proven effective in promoting sludge drying, however, the variation in sludge properties significantly influences the foaming efficiency. Inspired by foam stabilizer of solid particles, Camellia oleifera shells (COS) was screened out from various biomasses as an additive incorporated with the CaO for promoting the sludge foaming. For the introduction of COS, this study analyzed the drying behaviors of foamed sludge, quantified the surface cracks information, characterized the combustion performance, and evaluated the energy consumption. The results indicated that 46.72–50.10% of time could be saved in foaming the sludge to 0.70 g/mL by addition of 3.0 wt% COS. Compared with the original sludge (OS), the 0.70 g/mL foamed sludge saved 47.43% of time for sludge drying at 80 °C, and this value further increased to 53.14% with 3.0 wt% COS addition. Combining the multifractal spectra and drying kinetics analysis, the foaming promoted the formation of complex surface cracks in the warm-up period, while COS further improved the complexity of cracks in the constant rate period, and the shrinkage of isolated sludge blocks in the falling rate period, thus enhanced the moisture diffusion and heat transfer. Furthermore, the appropriate porous structure and additional volatile matters promoted the combustion performance. The 0.90 g/mL foamed sludge with COS presented the lowest activation energy of 180.362 kJ/mol in combustion. Overall, compared with OS, the 0.70 g/mL foamed sludge with COS saved 40.65% energy consumption during the foaming, drying and combustion processes, providing an energy-efficient solution for the sludge treatment and disposal.

Morphology and properties of PC/ABS notebook computer shell prepared by a combined in-mold decoration and microcellular injection molding process

In this paper, the PC/ABS notebook computer shells with PET film were prepared using a combined in-mold decoration and microcellular injection molding (IMD/MIM) process. Then the surface quality, morphology, impact toughness and wear resistance were investigated. The results shown that the surface quality is improved by IMD/MIM technology. The three dimensional surface roughness value (Sa) was about 1/3 of that of MIM parts. The cross-section of IMD/MIM samples have three layers. The first layer is a PET film decoration layer with about 132 µm thickness, followed by skin layer (about 22 µm) and foam layer. The range of thickness of the foam layer for the different specimens is about 7803 µm–7820 µm.The cell diameter is between 0.532 µm and 0.638 µm, which is about 2 times than that of MIM parts. Compared with MIM parts, the shape of the bubbles of IMD/MIM parts is ellipsoid with thinner wall. Moreover, the impact toughness of IMD/MIM parts are higher. The best impact toughness is 21.02 KJ/m2 within the scope of this experiment, which is 74% higher than the corresponding MIM part under the same foaming process. This may due to the PET film and micropores could block the initiation of the tip crack. Meanwhile, the IMD/MIM parts have excellent wear resistance property. In most cases, the wear rate of IMD/MIM parts is about 36.6% of MIM part under the same condition.

Incorporating Nanoparticles in Porous Foam Templating for Enhanced Sensitivity of Capacitive Pressure Sensors

AbstractCapacitive pressure sensors based on porous foams have been demonstrated for various biomedical applications (0–10 kPa). Many different methods for fabricating porous foams have been reported. In this work, for the first time, the incorporation of silica nanoparticles are reported into the templating process of porous foams fabricated through a combination of particle and emulsion templating, in order to enhance the formation of smaller microstructures in polydimethylsiloxane foams. The foams are coated with graphene, and pressure sensors developed using these foams showed increased sensitivity, up to 4.08 kPa−1. The incorporation of nanoparticles also improves the linearity of the sensitivity, giving a linear sensitivity for the pressure sensors over a pressure range of 0–6 kPa. Further, these pressure sensors have a low limit of detection of ≈13 Pa. These results indicate that incorporation of suitable nanoparticles in the templating of foams is a promising strategy for developing foam‐based pressure sensors with high and linear sensitivity.

The Effects of a Crosslinking Agent on the Microrheological Properties and Cellular Structure of Silicone Rubber Foam Prepared via a Green Process.

Chemical foaming technology is widely used in the preparation of silicone rubber foam and is attributable to its one-step molding capability and eco-friendly production processes. The microrheological properties of silicone rubber play a pivotal role during the foaming process. In this study, Rheolaser Lab (Formulaction, Toulouse, France) was used to conduct in situ examinations for the influence of a crosslinking agent on the microrheological properties of silicone rubber foam for the first time. This study monitors the entire reaction process of silicone rubber foam from liquid to solid, as well as the matching of crosslinking and foaming reactions. Various parameters, including solid-liquid balance, elasticity index, and macroscopic viscosity index, are measured to analyze the microrheological properties of silicone rubber foam. The results show that the silicone rubber foam exhibits good microrheological properties, thereby demonstrating excellent performance at a crosslinking agent content of 2%. Through adjusting the experimental conditions, a sustainable and efficient approach was proposed for better cellular structure control in the industrial preparation of silicone rubber foam.

Pore Structure and Deformation Correlation of an Aluminum Foam Sandwich Subject to Three-Point Bending.

An Al-Si matrix foam sandwich (AFS) with 6063 Al alloy cover sheets was fabricated by hot rolling combined with melt foaming. A foamable AlSiMg1/SiCp matrix precursor was prepared by the melting route. Hot rolling at 480 °C was carried out to obtain a mechanical bonding interface between the cover sheet and the foamable precursor. Meanwhile, the pore structure of the AFS was deeply affected by the foaming temperature and foaming time during the foaming process. Different pore growth mechanics of the crack-like pore disappearance mechanism (CDM) and pore active expansion mechanism (AEM) were concluded based on the pressure difference in pores inside and outside. Three bending tests were applied to three types of AFSs with different pore structures to evaluate the relation between pore structures and AFS mechanical properties. The bending property of the AFS with fewer layers of pores is like that of a dense material. The bending property of the AFS with a pore size in the range of 0~1 mm presents a typical sandwich shear failure mode. The AFS with a uniform pore structure, in which the shapes of the pores are predominately polygons and the pore diameter is concentrated in the range of 0.5~3 mm, processes a good energy absorption capacity, and the bending stress-strain curve fluctuates greatly after the first stress drop.

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.

A Novel In-Line Measurement and Analysis Method of Bubble Growth-Dependent Strain and Deformation Rates during Foaming.

Bubble growth processes are highly influenced by the elongational viscosity of the blowing agent-loaded polymer melt. Therefore, the elongational viscosity is an important parameter for the development of new polymers for foaming applications, as well as for the prediction of bubble growth processes. Thus, knowledge of the initial expansion and deformation behavior in dependency on the polymer, the blowing agent concentration, and the process conditions is necessary. This study presents a novel method for the in-line observation and analysis of the initial expansion and deformation behavior within the bead foam extrusion process. For this purpose, nitrogen as the blowing agent was injected into the polymer melt (PS and PLA) during the extrusion process. The in-line observation system consists of a borescope equipped with a camera, which was integrated into the water box of an underwater pelletizer. The camera is controlled by a developed trigger by means of angular step signal analysis of a rotary encoder on the cutter shaft of the underwater pelletizer. Thus, images can be taken at any time during the foaming process depending on the cutter position to the die outlet. It is shown that the developed method provides reliable results and that the differences of the initial expansion and deformation behavior during bubble growth can be analyzed in-line in dependency on real foaming process conditions and the type of polymer used.

Improvisation in wicking property of flexible polyurethane foams by adding bamboo and gelatin fillers

PU foam is one of the softest, most easily accessible, and affordable mattress materials. In PU foam, wider pores and a more open-cell structure have both advantages and drawbacks. Despite having fewer cells PU foam does not control airflow very effectively and commonly faces criticism for causing body heat and for retaining moisture. A warm, humid climate because of stored body moisture and sweat functions as a haven for bacteria and germs and encourages the proliferation of dust mites contributing to excessive odour and allergic reactions. This study aims to investigate the impact of bamboo and gelatin filler dosage or loading on the wicking property of the foam by using a discontinuous foaming process. Foams with gelatin and bamboo fillers were prepared at 2, 4, and 8% by weight concentrations, respectively, along with a standard 32-density foam without filler. A comparative study was conducted on the effect of increasing filler content on the wicking and mechanical properties of the foam. The wicking and porosity levels of bamboo-filled foam were higher than standard foam and gelatin-loaded foam. With increasing concentrations of bamboo filler, a rapid increase in foam wicking by 7–10 mm was noted. However, an increase in gelatin filler concentration resulted in an increase in foam porosity and elongation with a slight improvement in wicking ability. Improvisation in the wicking property of the foam was observed with dosage increment of bamboo filler. According to this research findings, the bamboo powder can be utilized as a filler to enhance wicking properties in flexible polyurethane foam.

Calculating diffusion coefficients from molecular dynamics simulations for modelling foam processes of polypropylene: Investigating different process conditions and blowing agents

The choice of blowing agent is very important in foaming processes. In an earlier work the use of molecular dynamics simulations for the estimation of the diffusion coefficients of different blowing agents was discussed at elevated temperatures above the melting point. Therein it was shown that these simulations can give valuable information on blowing agent diffusion in polypropylene. The aim of the recent work is to expand this discussion to the simulation of different influences like pressure, blowing agent mixtures and blowing agents with higher molecular weight at temperatures above the melting point of PP. First a pressure range applied on the combined system of polymer and blowing agent, reasonable for the foam processing is discussed and their influence on the diffusion coefficients for CO2 in polypropylene is evaluated via MD simulations. The next step is the investigation of the influence of mixing a blowing agent with a co-blowing agent. The last part is the presentation of diffusion coefficients for different organic solvents, namely 2-propanol, acetone, ethyl acetate and butyl acetate which could be used as potential co-blowing agents. For both, the change in pressure and the mixing of blowing agents for the systems in question only small influences on the diffusion coefficients was discovered. The results of the diffusion coefficients for the different organic solvents are compared to each other and with former results, considered different molecular structure, size and polarities and are found to be reasonable.

The Application of Box-Behnken Design for Investigating the Supercritical CO2 Foaming Process: A Case Study of Thermoplastic Polyurethane 85A.

Thermoplastic polyurethane (TPU) is a versatile polymer with unique characteristics such as flexibility, rigidity, elasticity, and adjustable properties by controlling its soft and hard segments. To properly design and understand the TPU foaming process through supercritical CO2, a design of experiments approach, the Box-Behnken design (BBD) was adopted using commercial TPU 85A as the model compound. The effect of saturation pressure, saturation temperature, and immersion time on the mean pore size and expansion ratio were investigated. The design space for the production of TPU foam was shown, and the significance of process parameters was confirmed using the analysis of variance (ANOVA). In addition, extrapolation foaming experiments were designed and validated the feasibility of the response surface model developed via BBD. It was found that the pore size of TPU 85A foam could be controlled within 13 to 60 μm, and a stable expansion ratio could be designed up to six.

Computational modeling and simulation of temperature field evolution during the chemical foaming of epoxy foams

AbstractThe evolution and control of the temperature field during the chemical foaming of epoxy resin are of paramount importance because the foam structure results from the competition of resin crosslinking and foaming, both of which are highly dependent on temperature distributions. Herein, the epoxy foams, consisting of diglycidyl ether of bisphenol A, glycidyl amine‐type epoxy resin, and 4,4′‐diamino diphenyl sulfone hardener, are prepared using azodicarbonamide and 4,4′‐oxydibenzenesulfonyl hydrazide as a chemical foaming agent (CFA). Kinetic models for heat release and CFA decomposition are established using the auto‐catalytic and n‐th order models, respectively. By integrating the transient flow of resin during the foaming process, numerical simulations of the temperature field evolution within the self‐expanding geometry are conducted to investigate the effects of foaming temperature, heat transfer coefficient, and mold diameter on spatial temperature distributions. A comparison of the kinetic parameters of epoxy curing and CFA decomposition at various foaming temperatures (433, 443, and 453 K) reveals that the acceleration of the curing rate is consistent with that of the decomposition rate as the foaming temperature increases, then the foam structure remains largely unchanged across different foaming temperatures. However, local overheating is unavoidable for the foams at 443 and 453 K. This study offers a method for optimizing the processing parameters in preparing epoxy foams.Highlights Auto‐catalytic model for predicting heat released rate is well established. The decomposition kinetics of the chemical foaming agent is described by n‐th model. Nonisothermal simulation is implemented to study the self‐expanded process of epoxy foam. The foam structure is predicted by comparing the kinetic between the curing and expansion processes.

Kraft lignin-derived multi-porous carbon toward sustainable electro-Fenton treatment of emerging contaminants

Reclaiming biomass waste to carbon cathode is promising to construct a sustainable electrochemical system. Here, we developed a novel thermal foaming process to reform kraft lignin to a free-standing and multi-porous carbon foam, which can be used as carbon cathode for electro-Fenton treatment of emerging contaminants (ECs). Benefiting from the excellent physicochemical and electrochemical properties with multi-porous and graphitic structure, the synthesized carbon foam cathode enhanced the electrocatalytic generation of H2O2 and •OH, and subsequently promoted the mass transfer and interactions among reactants (i.e., O2, H2O2, Fe2+, •OH, natural organic matters, and ECs). Eventually, an efficient degradation of ECs was achieved most likely via the multi-step •OH addition, cleavage, and ring opening reactions. Overall, the outcomes of this work provide a sustainable and applicable design and strategy to upgrade lignin-containing biomass to carbon cathode for efficient ECs remediation.

Preparation of ZrP2O7-CePO4 composite porous ceramics and their excellent thermal insulation and wave-transmission performance for supersonic aircraft

In this study, a two-step method combining the direct foaming process with the control of solid content and heat treatment temperature was employed to prepare ZrP2O7-CePO4 composite porous materials with varying porosities. The results indicate that an increase in solid content and heat treatment temperature leads to a reduction in porosity and average pore size while simultaneously improving the mechanical properties. The porosities, compressive strengths, thermal conductivities, ε and Tanδ ranged from 63.2 to 85.3 vol%, 19.43 to 0.99 MPa, 0.089 to 0.088 W/(m·K), 1.64 to 2.72, and 0.88 × 10−3 to 1.99 × 10−3, respectively. Electromagnetic simulations have validated that ZrP2O7-CePO4 composite porous ceramics, when used as the core layer material for A-sandwich antenna radomes, exhibit excellent transmittance properties. This suggests the potential application of these materials for supersonic aircraft antenna radomes.

Solvent-Free Method for Masking the Bitter Taste of Azithromycin Dihydrate Using Supercritical Fluid Technology

ABSTRACT Introduction and purpose The unpleasant extremely bitter taste of the orally administered broad-spectrum antibiotic azithromycin decreases patient compliance, especially in pediatrics. This issue can be overcome by decreasing drug interaction with the tasting buds using insoluble polymers at salivary pH (6.8 -7.4), like the cationic polymer Eudragit EPO. Supercritical fluid technology is a green synthesis method for preparing pharmaceutical preparations that replace organic solvents with safe supercritical CO2. This study aimed to mask the bitter taste of azithromycin using the supercritical fluid method and a pH-sensitive Eudragit EPO polymer. Methods: A foaming process was investigated for preparing a formulation (TEST), which comprises treating the polymer with supercritical carbon dioxide (CO2) fluid to prepare a taste-masked dosage form without employing organic solvents or flavors. Results: The use of the supercritical technique at 40 °C and 10 MPa for two hours allowed the manufacturing of solvent-free polymeric foam with azithromycin dispersions; the average calculated percentage of apparent volume change was 62.5 ± 5.9% with an average pore diameter of 34.879 Å. The formulated sample showed low drug release in simulated salivary fluid while keeping its crystalline nature. Moreover, clinical studies on healthy subjects showed that the formula successfully masked azithromycin's bitter taste. Conclusions: Overall, it has been shown herein that the supercritical fluid technology foaming method is promising in masking the bitter taste of bitter ingredients.

A new modelling method for open cell foam structure based on centroidal and capacity constraint power diagram

To efficiently establish a finite element model that is highly compatible with the intricate microstructure of foam materials, this study carries out simulation and experimental investigations into open cell foams and explores their compression behavior assuming isotropy. Using an in-house algorithm, we optimize the traditional power diagram model, which is widely used in foam finite element simulation. Specifically, a parameterized geometric foam model is developed based on the centroidal and capacity constrained power diagram (CCCPD), which accurately reflects the internal meso-structure of foam materials. The optimized foam model can restore the typical deformation process of foams under compressive load, thus addressing the limitations of existing foam models. The accuracy of the optimized model is validated by conducting uniaxial Quasi-static compression experiments on foamed thermoplastic polyurethane (TPU) material and comparing the experimental results of the 3D printing model with the simulation results of the optimized model. The improved foam model serves as a basis for future research on open cell foam materials.