Batch Foaming Research Articles

Rubber-lignin-ammonium polyphosphate bio-composite foams: Fabrication, thermomechanical properties and flame retardancy

Bio-composite foams based on Epoxidized Natural Rubber (ENR) filled with lignin (LG) and ammonium polyphosphate (APP) were fabricated via batch foaming. The addition of APP accelerated the foaming process at lower temperatures. Pre-mixing induced ionic and hydrogen bonding between the LG and the APP particles, which reduced crosslinking between LG and ENR. The resulting ENR bio-composite foams with LG/APP exhibited a significant increase in compressive strength (up to 700 %) and modulus (up to 600 %) compared to the ENR foam baseline. Furthermore, the LG/APP foams demonstrated lower thermal conductivity than both the ENR foam baseline and foams containing only LG or APP, attributed to optimal thermal conduction in the solid phase and convection within the pore cells. The combination of APP and LG produced synergistic effects, with phosphorus (from APP) and high carbon content (from LG) enhancing flame-retardant efficiency. This study highlights the potential of these sustainable bio-composite foams for applications requiring enhanced thermal insulation and flame retardancy attributes for insulation and other practical applications.

Synthesis of High-Tg Nonisocyanate Polyurethanes via Reactive Extrusion and Their Batch Foaming

Synthesis of High-<i>T</i>_g Nonisocyanate Polyurethanes via Reactive Extrusion and Their Batch Foaming

Lightweight polypropylene/BaTiO3 composite foams with tunable dielectric constant

Lightweight dielectric foam materials with tunable dielectric constant of 2–1 have broad application prospects in the field of Luneburg lens. Expandable polystyrene (EPS) foam is commonly used for fabricating discrete dielectric layers of Luneburg lens, while its temperature resistance and mechanical properties are poor. EPS foam with dielectric constant of >1.5 is also difficult to prepare because of its low expansion ratio. In this work, choosing polypropylene (PP) with better temperature resistance and mechanical properties as polymer matrix, PP/BaTiO3 composite foams are fabricated by subcritical CO2 batch foaming and steam-chest molding. Through the dual regulation of expansion ratio and mold-opening distance, the dielectric constant of PP/BaTiO3 foams can be tunable from ∼1.19 to ∼1.91, with the density increasing from ∼0.14 to ∼0.59 g/cm3. Compared with EPS foams, PP/BaTiO3 foams have lower density under the similar dielectric constant, indicating the weight reduction of polymer dielectric foam materials by introducing high dielectric fillers.

Second cell growth optimized by heating mode during two-step supercritical CO2 foaming polymer process

The batch foaming of supercritical fluids faces numerous challenges, such as poor controllability due to dynamic temperature changes during depressurization foaming, and prolonged saturation time and heterogeneous temperature distribution during temperature rising foaming, which leads to lower productivity and non-uniform cells. To overcome these challenges, a unique two-step foaming method in combination with microwave enhancement was proposed. The two-step foaming method involved depressurization foaming for cell nucleation and initial growth, followed by temperature rising foaming for subsequent cell growth, producing foams with higher expansion ratio. Meanwhile, a conduction-microwave (C-M) heating mechanism which integrates different heating schemes to enhance heating efficiency and uniformity was innovatively proposed. The effect of the microwave radiation time, power, initial environmental temperature, and content of CO2 in polymer on the foaming by C-M heating was studied. Compared to existing methods, the foaming by C-M heating produces polystyrene (PS) foam with lower density and more uniform cells.

Characterisation of flame retarded recycled PET foams produced by batch foaming

CO2-assisted batch foaming was used to manufacture low-density (ρ = 200–350 kg/m3) microcellular foams from bottle-grade recycled poly(ethylene terephthalate) (rPET) in flame retarded form. The foamability of the PET regrind was enhanced using a reactive chain extender while the flame retardant properties of the rPET foams were improved by incorporating 6% aluminium-tris-(diethylphosphinate) flame retardant (FR). The effects of adding 1% natural montmorillonite (MMT) and polytetrafluoroethylene (PTFE) powder as potential cell nucleating agent and flame retardant synergist were also investigated. As a result of FR addition, the foam structure became less uniform, but the presence of MMT was found to improve uniformity of cellular size and distribution. The applied FR mainly acts in the gas phase as flame inhibitor, but also promotes charring of the foams, as revealed by thermogravimetric analysis (TGA) and pyrolysis combustion flow calorimetry (PCFC). The fine microcellular structure and high cell density allow homogeneous distribution of FR additives, and thus only moderate increase in flammability was observed for the high-porosity (>75%) foams compared to the corresponding bulk materials, as characterized by similar limited oxygen index (LOI) values. In cone calorimeter tests, for the flame retarded foams a 50% reduction in peak heat release rate (PHRR) and a 30% reduction in total heat release (THR) values were measured compared to the FR-free reference. The PTFE addition to the FR formulation was found to increase the time-to-ignition (TTI), reduce PHRR and effective heat of combustion (EHC) while increase charring. The mechanical performance of the flame retarded rPET foams was found to be primarily determined by the apparent density and less affected by the presence of FR additives. Due to strain-induced crystallization occurring during cell growth, the rPET foams are highly crystalline (χ > 25%) which leads to increased thermomechanical resistance compared to unfoamed references.

Open-Celled Foams from Polyethersulfone/Poly(Ethylene Glycol) Blends Using Foam Extrusion.

Polyethersulfone (PESU), as both a pristine polymer and a component of a blend, can be used to obtain highly porous foams through batch foaming. However, batch foaming is limited to a small scale and is a slow process. In our study, we used foam extrusion due to its capacity for large-scale continuous production and deployed carbon dioxide (CO2) and water as physical foaming agents. PESU is a high-temperature thermoplastic polymer that requires processing temperatures of at least 320 °C. To lower the processing temperature and obtain foams with higher porosity, we produced PESU/poly(ethylene glycol) (PEG) blends using material penetration. In this way, without the use of organic solvents or a compounding extruder, a partially miscible PESU/PEG blend was prepared. The thermal and rheological properties of homopolymers and blends were characterized and the CO2 sorption performance of selected blends was evaluated. By using these blends, we were able to significantly reduce the processing temperature required for the extrusion foaming process by approximately 100 °C without changing the duration of processing. This is a significant advancement that makes this process more energy-efficient and sustainable. Additionally, the effects of blend composition, nozzle temperature and foaming agent type were investigated, and we found that higher concentrations of PEG, lower nozzle temperatures, and a combination of CO2 and water as the foaming agent delivered high porosity. The optimum blend process settings provided foams with a porosity of approximately 51% and an average foam cell diameter of 5 µm, which is the lowest yet reported for extruded polymer foams according to the literature.

In situ 3D printing of poly-ether-ether-ketone / poly-ether-imide hierarchical cellular foams containing electromagnetic absorbent

For the expanding application of light-weight integral structures meeting both structural and physical requirements, the development of advanced materials with high performances in multiple functions and their automatically manufacturing processes with mass production potential at low cost has received sustained attention from both academia and industry. Therefore, an in situ 3D printing technology of poly-ether-ether-ketone (PEEK)/ poly-ether-imide (PEI) hierarchical cellular foam containing electromagnetic absorbent was proposed in this paper. Fillers with electromagnetic functions, nano-carbon and Fe 3 O 4 , were introduced into the blend of two high-performance thermoplastics, semicrystalline PEEK and amorphous PEI, and a lightweight nanocomposite foam with functionalized hierarchical cellular structure in micrometer scale was manufactured by combining in situ foaming process into 3D printing. The in situ foaming mechanism of the materials was carefully studied and optimal process parameters were obtained. When the desorption time of materials is 48 h, the 3D printed foams present the smallest cell size and the highest cell density. The foaming behavior of this special material is different from that of batch foaming. Mechanical tests show that the flexural modulus increases with decreasing cell size and increasing cell density. In X-band (8.2–12.4 GHz), the hierarchical cellular structure with gradient layers performs low reflectivity and absorption-dominant EMI shielding properties. The lightweight nanocomposite foam manufactured by the in situ 3D printing technology have great potential for applications such as functional-structural integrated skins in aerospace, protection of high-power instruments and 5 G communication products.

Lightweight and tough PVDF foams via high‐pressure foam injection molding with core‐back operation

AbstractPolyvinylidene fluoride (PVDF) foams prepared by batch foaming have been intensively studied, but it still remains a challenge to fabricate high‐performance PVDF foams for structural applications. Herein, by adding polymethyl methacrylate (PMMA) as the minor component, the blend foams were successfully fabricated using supercritical carbon dioxide (CO2) as a green blowing agent via foam injection molding (FIM) with core‐back operation. The added PMMA could reduce the crystal size of PVDF. Owing to fine cellular structure and small crystal sizes, the injection‐molded blend microcellular foams exhibited superior mechanical properties. Especially, the elongation at break and specific breaking energy of the blend foam with 40% void fraction were separately increased by 184% and 297% compared to that of the PVDF foam. These findings revealed that the lightweight microcellular PVDF foams with improved toughness could be manufactured by the scale‐up and efficient FIM technology, which showed a promising future in automotive and other structural applications.

Effect of chain relaxation on the shrinkage behavior of TPEE foams fabricated with supercritical CO2

A series of branched thermoplastic polyester elastomer (TPEE) samples with different branch densities was prepared through reactive extrusion. The creep recovery experimental results showed that an increase in the branch density led to more deformation in the branched TPEE samples. Batch foaming was conducted to prepare TPEE foams, and the cell morphology was subsequently characterized. The shrinkage behavior of the TPEE foams was investigated using the Kohlrausch–Williams–Watts (KWW) model. This was also used to obtain the relaxation time and breadth of the relaxation time distribution. The vital role of chain relaxation in shrinkage was investigated. The results showed that the relaxation time decreased and the relaxation time distribution became narrower with larger stretching, which led to shrinkage of the TPEE foams. The rapid decrease in the relaxation time and narrowing of the relaxation time distribution with a larger branch density also resulted in shrinkage. Therefore, it is effective to prolong the relaxation time and broaden the relaxation time distribution by increasing the cell density or decreasing the branch density to reduce the shrinkage. Increasing the cell density could maintain both expansion ratio and dimensional stability. However, decreasing the branch density was accompanied by sacrificing the expansion ratio to reduce shrinkage.

Matricial foaming

We designed an innovative pressure vessel for a high throughput study of batch foaming of thermoplastic polymers. With the new pressure vessel, the foaming process can be conducted concurrently on sixteen samples (arranged in a 4 × 4 wells matrix) at four different temperatures and at four different pressure drop rates, at the same saturation pressure. We herein describe the design principle, the working conditions and the first results gathered on poly(ε-caprolactone) samples foamed with carbon dioxide as the blowing agent. We explored the processing variables space: foaming temperature in the range 30–40 °C; pressure drop rate in the range 1–5 MPa/s; saturation pressure fixed at 5 MPa within a single experiment. The expected dependence of density as well as morphology on processing variables proves the suitability of the method for the high throughput study of novel polymers and can be utilized for the fast development of resins suitable for foaming.

Modeling and Experiment for the Diffusion Coefficient of Subcritical Carbon Dioxide in Poly(methyl methacrylate) to Predict Gas Sorption and Desorption.

Several researchers have investigated the phenomenon of polymer–gas mixtures, and a few have proposed diffusion coefficient values instead of a diffusion coefficient model. There is a paucity of studies focused on the continuous change in the diffusion coefficient corresponding to the overall pressure and temperature range of the mixture. In this study, the gas sorption and desorption experiments of poly(methyl methacrylate) (PMMA) were conducted via solid-state batch foaming, and the weight change was measured using the gravimetric method with a magnetic balance. The control parameters were temperature, which ranged from 290 to 370 K, and pressure, which ranged from 2 to 5 MPa in the subcritical regime. Based on the experimental data, the diffusion coefficient of the PMMA was calculated using Fick’s law. After calculating the diffusion coefficient in the range of the experiment, the diffusion coefficient model was employed using the least-squares method. Subsequently, the model was validated by comparing the obtained results with those in the literature, and the overall trend was found to be consistent. Therefore, it was confirmed that there were slight differences between the diffusion coefficient obtained using only Fick’s equation and the value using by a different method.

Effect of viscoelasticity on the foaming behaviour of long-chain branched polypropylene with different branching degrees analysed by using bubble-growth modelling

Long-chain branched polypropylene (LCB-PP) was generated by a melt grafting reaction with trimethylolpropane triacrylate (TMPTA) as the grafting monomer and zinc dimethyldithiocarbamate (ZDMC) or styrene (St) as the co-monomers. High-temperature size exclusion chromatography (HT-SEC) was used to characterize the molecular mass, molecular mass distribution and branching degree of the modified PP. Subsequently, linear and nonlinear viscoelasticity data were obtained by a small amplitude oscillatory shear (SAOS) test and uniaxial extensional viscosity measurement. The pressure balanced bubble-growth (PBB) model was used to calculate the critical coalescence radius (CCR) curves for PP to quantitatively correlate the viscoelasticity and foamability and evaluate the foamability of PP combined with batch foaming. It is shown that with increasing branching degree, the foam shows a better resistance to bubble coalescence, resulting in a more regular bubble structure and a higher expansion ratio. Moreover, viscosity should be controlled over an appropriate range for realising good foamability; otherwise, excessive viscosity can suppress bubble growth and limit the expansion ratio. MPPZ4 modified with 4 wt% TMPTA and 0.4 wt% ZDMC showed the best foamability. A combination of the PBB model and batch foaming experiment is used to quantitatively explain the contribution of LCB to foaming.

Preparation of polyethylene terephthalate foams at different saturation temperatures using dual methods of supercritical batch foaming

Preparation of polyethylene terephthalate foams at different saturation temperatures using dual methods of supercritical batch foaming

Morphological evaluation of microcellular foamed composites developed through gas batch foaming integrating Fused Deposition Modeling (FDM) 3D printing technique

In this article, the development of microcellular structure foams has developed by integrating the two successful and existing technologies, namely CO2 gas batch foaming and Fused Deposition Modeling (FDM) 3D printing technique. It is a novel approach to manufacture complex design porous products for customized applications. The eventual cell morphologies of the extruded 3D printing filament depends on the process parameters pertaining to both microcellular foaming and 3D printing processes. Further, morphological study has been conducted to evaluate the cell morphologies of the 3D printing filament developed through customized FDM setup. During this process, the significance of various process parameters including saturation pressure, saturation time, desorption time, feed rate and extrusion temperature were thoroughly studied. To pursue this study base material used was acrylonitrile butadiene styrene (ABS). The 3D printed filaments consisted of cells with an average cell size in the range of 2.3–276 µm and the average cell density in the range of 4.7 × 104 to 4.3 × 109 cells/cm3. Finally, it has found that by controlling the process parameters different cell morphologies can be developed as per the end application.

Polylactide scaffold fabrication using a novel combination technique of fused deposition modeling and batch foaming: dimensional accuracy and structural properties

A combination technique of fused deposition modeling (FDM) and batch foaming is introduced as a novel technique to fabricate tissue-engineered scaffolds. Three scenarios are implemented to fabricate porous scaffolds for tissue engineering applications. The dimensional accuracy, porosity, and pore size of samples are compared in three scenarios. The first scenario includes solid samples produced by FDM 3D printing at different infill percentages. The findings revealed that the porosity of solid FDM samples can be controlled by adjusting the infill percentage. However, the real porosity was smaller than the nominal porosity. The results indicated that the solid FDM samples had lower dimensional accuracy compared to solid injected samples. The second scenario consists of foamed injected samples, injection-molded samples foamed by batch foaming technique, at different injection pressures. The results showed that a bimodal pore structure containing big pores in the fine range and small pores in the micro range were achieved. The third scenario contains foamed FDM samples, FDM samples foamed using a batch foaming technique. The findings disclosed that the foamed FDM samples had higher dimensional accuracy than the foamed injected samples. Also, the foaming significantly improved the porosity of FDM samples, for instance by 43% at infill percentage of 80%. The FDM/batch foaming combination technology can produce porous tissue-engineered scaffolds with simultaneously macro and micropores, desirable porosity, and excellent dimensional accuracy. This technique has a bright outlook for future scaffolds applications.

Confined batch foaming of semi‐crystalline rubbery elastomers with carbon dioxide using a mold

AbstractConfined foaming of poly(ethylene‐co‐vinyl acetate‐co‐carbon monoxide) using carbon dioxide as a physical blowing agent in a mold with either permeable or impermeable boundaries has been explored as a strategy to control final foam dimensions and morphology. The results are discussed in terms of comparisons to free‐foaming experiments conducted at the same pressure and temperature conditions following the same pressurization and depressurization paths. Foaming experiments were carried out at 30 and 40°C and 100, 200, and 300 bar followed by rapid depressurization of the foaming cell. Confined foaming led to smaller pores with more uniform distributions across the polymer cross‐section. However, bulk foam densities of the foams generated under confinement were higher than those generated under the free‐foaming mode. Surface characteristics and skin layer formation were altered by expansion against both the permeable and impermeable boundaries. Confined foaming promotes uniform pore distribution and overall dimensional uniformity and may impart surface texture but the trade‐off is in the degree to which the bulk foam density can be lowered.

Radiation Grafting Assisted Preparation of Layered Structure Polypropylene Foam with Superthermal Insulation and Hydrophobic Properties via a Supercritical CO2 Batch Foaming Process

A layered porous, high heat insulation, waterproof, and recyclable polypropylene (PP) foam was prepared by radiation grafting assisted supercritical CO₂ foaming. First, dodecafluoroheptyl methacrylate was grafted on PP powder via radiation-induced graft polymerization. Then the grafted product (PP-g) with a grafting rate of 12.8% was blended with PP powder, and subsequently the blends were foamed. The obtained PP/PP-g (10) foam exhibited a highly oriented layered cell structure and large expansion ratio, which largely increased the number of air layers of heat conduction and the length of the conduction path, so an ultralow heat transfer coefficient of 29.3 mW/m·k was achieved. In addition, the raspy surface of the PP/PP-g (10) foam possessed a highly hydrophobic property (water contact angle: 146.7 ± 0.5°). The method provides a new strategy for improving the thermal insulation and hydrophobic performances, which has outstanding potential application in lightweight thermal insulation materials.

Evolution of cell morphology from sub-macroscale to nanoscale in modified thermoplastic polyether ester elastomer via supercritical CO2 foaming

Modified thermoplastic polyether ester elastomer (TPEE) foams with the cell size from sub-macroscale to nanoscale were prepared by solid-phase batch foaming in the presence of supercritical CO2 (scCO2). Multifunctional epoxy chain extender (CE) with the functionality of 9 was incorporated into TPEE to form a moderate crosslinking structure. The rheological property and crystallization behavior of modified TPEE were investigated, and then the cell morphologies of as-prepared TPEE foams were analyzed. It was found that the introduction of branching and crosslinking structure led to an increased melt viscosity and a reduction of crystallinity of TPEE. Moreover, it was identified that the rheological property dominated the foamability at high foaming temperature of 185 °C, and then the fine sub-macroscale cells were seen. Conversely, the change of crystallization behavior mainly affected the evolution of cell morphology at low foaming temperature of 160 °C, and the foams with nano and/or micro cells are obtained. As a result, a series of TPEE foams with the cell morphology from sub-macroscale to nanoscale could be achieved by adjusting the crosslinking degree of modified TPEE and the temperature of supercritical CO2 foaming.

Batch foaming of ultra-high molecular weight polyethylene with supercritical carbon dioxide: Influence of temperature and pressure

This work is aimed at studying the foaming behavior of pure ultra-high molecular weight polyethylene (UHMWPE) with supercritical carbon dioxide (scCO2). The effects of foaming temperature and saturation pressure on the final foam structure were investigated by using two different one-step batch foaming processes. An in-situ high-pressure observing system was used to record the crystal change of UHMWPE film during heating and cooling stages with/without pressurized CO2. The results showed that the cell size and the cell density were affected by the combined effects of crystal, temperature, and pressure. Experimental results with different foaming temperatures showed that higher foaming temperatures led to larger cells and lower cell densities. In the processes of foaming during the heating stage (Proc1) and cooling stage (Proc2), the expansion ratio increased first and then decreased with the increase of temperature. Before obtaining the maximum expansion ratio, compare with Proc2, the cell size and expansion ratio of foams were smaller, and the cell density was higher in Proc1. Experimental results with different saturation pressures showed that higher pressure led to lower cell density and larger average cell diameter in Proc1 due to the reduction of crystals and melt strength. While in Proc2, higher saturation pressure led to higher cell density due to the increase of solubility of CO2, and the cell density decreased as the pressure further increased due to cell coalescence.

Amorphous Polymers' Foaming and Blends with Organic Foaming-Aid Structured Additives in Supercritical CO2, a Way to Fabricate Porous Polymers from Macro to Nano Porosities in Batch or Continuous Processes.

Organic polymers can be made porous via continuous or discontinuous expansion processes in scCO2. The resulting foams properties are controlled by the interplay of three groups of parameters: (i) Chemical, (ii) physico-chemical, and (iii) technological/process that are explained in this paper. The advantages and drawbacks of continuous (extrusion, injection foaming) or discontinuous (batch foaming) foaming processes in scCO2, will be discussed in this article; especially for micro or nano cellular polymers. Indeed, a challenge is to reduce both specific mass (e.g., ρ < 100 kg·m−3) and cell size (e.g., average pore diameter ϕaveragepores < 100 nm). Then a particular system where small “objects” (coreshells CS, block copolymer MAM) are perfectly dispersed at a micrometric to nanometric scale in poly(methyl methacrylate) (PMMA) will be presented. Such “additives”, considered as foaming aids, are aimed at “regulating” the foaming and lowering the pore size and/or density of PMMA based foams. Differences between these additives will be shown. Finally, in a PMMA/20 wt% MAM blend, via a quasi one-step batch foaming, a “porous to nonporous” transition is observed in thick samples. A lower limit of pore size (around 50 nm) seems to arise.