Foam Extrusion Research Articles

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.

Supercritical CO2‐assisted extrusion foaming: A suitable process to produce very lightweight acrylic polymer micro foams

AbstractA strategy of CO2‐assisted extrusion foaming of PMMA‐based materials was established to minimize both foam density and porosities dimension. First a highly CO2‐philic block copolymer (MAM: PMMA‐PBA‐PMMA) was added in PMMA in order to improve CO2 saturation before foaming. Then the extruding conditions were optimized to maximize CO2 uptake and prevent coalescence. The extruding temperature reduction led to an increase of pressure in the barrel, favorable to cell size reduction. With the combination of material formulation and extruding strategy, very lightweight homogeneous foams with small porosities have been produced. Lightest PMMA micro foams (ρ = 0.06 g cm−3) are demonstrated with 7 wt% CO2 at 130°C and lightest blend micro foams (ρ = 0.04 g cm−3) are obtained at lower temperature (110°C, 7.7 wt% CO2). If MAM allows a reduction of Tfoaming, it also allows a much better cell homogeneity, an increase in cell density (e.g., from 3.6 107 cells cm−3 to 2 to 6 108 cells cm−3) and an overall decrease in cell size (from 100 to 40 μm). These acrylic foams produced through scCO2‐assisted extrusion has a much lower density than those ever produced in batch (ρ ≥ 0.2 g cm−3).

Sustainable Wheat Protein Biofoams: Dry Upscalable Extrusion at Low Temperature.

Glycerol-plasticized wheat gluten was explored for producing soft high-density biofoams using dry upscalable extrusion (avoiding purposely added water). The largest pore size was obtained when using the food grade ammonium bicarbonate (ABC) as blowing agent, also resulting in the highest saline liquid uptake. Foams were, however, also obtained without adding a blowing agent, possibly due to a rapid moisture uptake by the dried protein powder when fed to the extruder. ABC's low decomposition temperature enabled extrusion of the material at a temperature as low as 70 °C, well below the protein aggregation temperature. Sodium bicarbonate (SBC), the most common food-grade blowing agent, did not yield the same high foam qualities. SBC's alkalinity, and the need to use a higher processing temperature (120 °C), resulted in high protein cross-linking and aggregation. The results show the potential of an energy-efficient and industrially upscalable low-temperature foam extrusion process for competitive production of sustainable biofoams using inexpensive and readily available protein obtained from industrial biomass (wheat gluten).

TPV Foaming by CO2 Extrusion: Processing and Modelling.

This work focuses on the extrusion foaming under CO2 of commercial TPV and how the process influences the final morphology of the foam. Moreover, numerical modelling of the cell growth of the extrusion foaming is developed. The results show how a precise control on the saturation pressure, die geometry, temperature and nucleation can provide a homogeneous foam having a low density (<500 kg/m3). This work demonstrates that an optimum of CO2 content must be determined to control the coalescence phenomenon that appears for high levels of CO2. This is explained by longer residence times in the die (time of growth under confinement) and an early nucleation (expansion on the die destabilizes the polymer flow). Finally, this work proposes a model to predict the influence of CO2 on the flow (plasticizing effect) and a global model to simulate the extrusion process and foaming inside and outside the die. For well-chosen nucleation parameters, the model predicts the final mean radius of the cell foam as well as final foam density.

Fundamental influences of propylene‐based elastomer on the foaming properties of high melt strength polypropylene based on extrusion foaming

AbstractHigh melt strength polypropylene (HMS‐PP)/propylene‐based elastomer (PBE) microcellular foams were prepared by the extrusion foam method. The fundamental mechanism of PBE improving the foaming properties of HMS‐PP was investigated. PBE 6102, with a low melt flow rate and high ethylene content, enables HMS‐PP/PBE blends to form a microphase‐separated structure and maintain good melt strength and pronounced strain hardening, which would promote cell nucleation and hold cell structure at relatively high temperatures. Furthermore, PBE 6102 postpones HMS‐PP crystallization and decreases the optimum foaming temperature, which helps balance melt elasticity and viscosity near the foaming window to avoid cell rupturing and merging. The introduction of PBE 6102 does not reduce the crystallization rate of HMS‐PP nor the crystallinity of the foamed samples, fixing the cell structure to prevent collapse during the cell solidification stage. Microcellular HMS‐PP/PBE 6102 foams with a high expansion ratio, small cell size, and large cell count were successfully fabricated.

Influence of Carbon Dioxide on the Glass Transition of Styrenic and Vinyl Pyridine Polymers: Comparison of Calorimetric, Creep, and Rheological Experiments

The glass transition of amorphous polymers determines the mobility of polymer chains and the time scale of relaxation processes. The glass transition temperature is reduced by the presence of low molecular weight molecules, e.g., dissolved gases or organic solvents. The quantitative knowledge of reduction of the glass transition temperature caused by the addition of carbon dioxide in a polymer melt is highly relevant for foam extrusion. However, measurement of the reduction of glass transition temperature caused by gas loading has to be performed under elevated pressure which implies high experimental efforts. In this work, we discuss and compare three methods for determination of the influence of carbon dioxide on thermal properties of amorphous polymers, i.e., calorimetric measurements, creep tests, and rheological experiments. The advantages and disadvantages of these methods are elucidated. Furthermore, the influence of molecular structure of the styrenic and vinylpyridine polymers on the glass transition temperature is discussed. Polystyrene generally shows the highest reduction of glass transition temperature. Poly(2-vinylpyridine) and poly(4-vinylpyridine) show a slightly less pronounced behaviour in comparison to polystyrene because of the lower polarity of polystyrene. Poly(α-methyl styrene) is associated with a different dependence of glass transition temperature on gas loading in calorimetric and rheological experiments.

Supercritical CO 2 foaming of open‐cell polypropylene/ethylene propylene diene monomer composite foams with oriented cellular structures for water treatment

Efficient treatment of tiny media in wastewater is a global challenge. Herein, an ingenious design to fabricate the highly open-cell polypropylene (PP)/ethylene propylene diene monomer (EPDM) composite foams for water treatment by using supercritical CO2 (scCO2) extrusion foaming coupling with post-stretching is presented. The tensile strain of high melt strength PP is fully utilized in an extensional flow field during foaming. Meanwhile, EPDM is added into PP matrix for obtaining the highly open-cell and oriented porous structures. Interestingly, a variety of structural transformations (like uniform, hollow and oriented cellular structures) can be achieved during extrusion foaming. Furthermore, the effects of different cellular structures on their selective filtration properties are studied. More interestingly, the as-prepared composite foam strips, with the highly open-cell content (~94.4%) and orientation degree (~89.6%), exhibit excellent selective filtration performance, which can successfully remove waste with particle size larger than 1 μm (its filtration efficiency:~ 99.52%). Water droplets can easily pass through the lengthy foam strips in seconds, but tiny particles (like oil droplets or glass microbeads) will be blocked. This work provides a new pathway to prepare highly open-cell composite foams with anisotropic porous structures for water treatment.

Preparation and Properties of HGBTPU Composite Foam for Football Protection

Football is a very dangerous sport. In order to reduce the injury of athletes, we must have corresponding protective articles, and the materials used are very important. Foam material has attracted the attention of all circles for its light weight, high strength, strong absorption impact load, and good heat insulation performance. However, the mechanical properties of foam materials will decrease with the decrease of density, which limits their application in sports, especially in some sports occasions, such as motorcycle competition, football, and other sports protection. In this paper, HGB/TPU foam materials are mainly studied. The material not only has good shock absorption performance but also has light weight and good mechanical properties. It can be used as protective equipment for football. In this paper, HGB/TPU composite foam materials were prepared by extrusion foaming method using TPU and HGB as raw materials. The effects of HGB surface modification, foaming agent content, and HGB content on the cellular structure and mechanical properties of composite foams were also investigated. The experimental results show that when the foaming method is extrusion foaming, the content of HGB is 3% and the content of foaming agent is 1.5%. It has better resistance to external forces, which helps to protect athletes from serious injuries.

Microcellular foam of styrene–isobutylene–styrene copolymer withN2using polypropylene as a crystallization nucleating and shrinkage reducing agent

AbstractStyrene‐isobutylene‐styrene copolymer (SIBS) is a thermoplastic elastomer with excellent chemical stability, biocompatibility, and low gas permeability. SIBS is a good candidate with a high melt viscosity and a high storage modulus to develop new lightweight elastomeric products. Foam injection molding with core‐back operation is an efficient method to prepare SIBS foams. However, it is challenging to prepare a microcellular foam from neat SIBS by melt processing, such as foam extrusion or foam injection molding, because the hard segments cannot play a role in bubble nucleation sites in the molten state. Furthermore, a significant degree of shrinkage occurs after foaming. By introducing a semicrystalline polymer such as polypropylene (PP), the foamability can be improved in foam injection molding processes. By adjusting the foaming temperature to the crystallization temperature of PP, PP crystals provide bubble nucleation sites and increase the viscosity to suppress bubble growth. Microcellular foams with high cell density and small cell size were achieved at 108cells/cm3and approximately 13 μm. PP can also impede the shrinkage of SIBS foams.

Facile one-step method to manufacture polypropylene bead foams with outstanding thermal insulation and mechanical properties via supercritical CO2 extrusion foaming

Expanded polypropylene (EPP) foam, is widely used in the field of packaging, automotive and toys, etc., and its preparation process is mainly obtained by semi-continuous autoclave bead foaming. Because foamed beads can be fabricated into complex 3D components and have excellent structural controllability. In contrast, a novel method for continuous production of expanded PP foamed beads is developed via supercritical CO2 (scCO2) extrusion foaming coupling with a wind cutting device, then the real-time cell growth and pelletizing processes of the extruded bead foams are revealed by using a high-definition camera for the first time. Interestingly, the processes of solubilization, compounding, pelletizing and foaming can happen in a one-step process through the above method, which can greatly shorten the production time, save energy consumption, and promote structural diversification. Besides, the transformation of foamed beads, like spherical, cylindrical and dumbbell-shaped, can be achieved by adjusting the pelletizing speeds based on the Barus effect. Furthermore, the steam-chest molding of the extruded foamed beads into foamed product shows that it is possible to sinter the beads at a low steam pressure (2.0 bar). In addition, the as-prepared foamed products can withstand constantly treading by an adult, and the surface of the foamed product rises only ~7.5 °C after prolonged heating, illustrating the outstanding mechanical and thermal insulation properties. Finally, it can be imagined that the related technical route has important scientific significance for the development of other polymeric bead foaming systems.

Lightweight Polypropylene/Polylactic Acid Composite Foams with Controllable Hollow Radially Gradient Porous Structures for Oil/Water Separation

The structural characteristics of the polymer composite foams play an important role in their properties. Herein, the polypropylene/polylactic acid (PP/PLA) composite foams with the controllable hollow radially gradient porous structures are designed and prepared by the supercritical CO2-assisted foam extrusion coupled with postdrawing auxiliary technology. The above structures consist of the dense skin layer, hollow-core, closed-cell, and open-cell. Under the constant extrusion foaming temperature and pressure, the outer diameter of the as-prepared foams can be adjusted from about 9.8 mm to 5.7 mm, while the inner diameter changes from about 5.5 mm to 1.7 mm, by adding the PLA to regulate the viscoelasticity and thermodynamic differences of the polymeric melt. Besides, the formation mechanism of the hollow radially gradient porous structures is theoretically analyzed. The as-prepared PP/PLA composite foams exhibit outstanding oil/water separation performance, which show potential application in wastewater treatment.

Extrusion foaming of linear and branched polypropylenes – input of the thermomechanical analysis of pressure drop in the die

Abstract This paper aims at a better understanding of the polypropylene (PP) physical extrusion foaming process with the objective of obtaining the lowest possible foam density. Two branched PPs were compared to the corresponding linear ones. Their shear and elongation viscosities were measured as well as their crystalline properties. Trials were conducted in a single screw extruder equipped with a gear pump and a static mixer cooler to adjust the melt temperature at the final die. The effect of decreasing this temperature on the PP foamability and on the pressure drop in the die was analyzed. The foam density of branched PPs varies from high to low values while decreasing the foaming temperature. In the same processing conditions, the foam density of linear PPs does not decrease so much, as already evidenced in the literature. The foamability transition coincides with an increase of the pressure drop in the die. The originality of the work lies in the thermomechanical analysis of the polymer flow in the die which allows the identification of the relevant physical phenomena for a good foamability. The comparison of the experimental pressure drops in the die and the computed ones with the identified purely viscous behavior points out the influence of the foaming temperature and of the PP structure. At high foaming temperature the discrepancy between experimental measurements and the computed pressure drops remains limited. It increases when decreasing the foaming temperature, but the mismatch is much more important for branched PPs than for linear ones. This difference is analyzed as a combination of the activation energy of the viscosity, the elongational viscosity in the convergent geometry of the die which is much more important for branched PPs than for linear ones, and the onset of crystallization which occurs at higher temperature for branched PPs than for linear PPs.

Flame retardancy of PET foams manufactured from bottle waste

Supercritical CO2-assisted extrusion technique was used to produce flame retarded foams from recycled poly(ethylene terephthalate) (rPET). Synergistic combination of aluminium-alkylphosphinate flame retardant (FR) and natural montmorillonite (MMT) was utilised to provide adequate flame retardancy at moderate loadings, i.e. with less than 9% of additives. Addition of FR was found to increase the rate of degradation during production (compounding and foam extrusion), which was effectively compensated by chain extender (CE) addition that ensured the melt strength required to obtain proper foam structure. The effects of the FR content and the CE addition on the viscosity of the rPET compounds and on the morphological, thermal, flammability and mechanical properties of the foam products were comprehensively investigated. When compared to injection moulded bulk materials of identical compositions, the highly porous structure of foams was found to increase flammability according to UL94 tests and LOI measurements but has not been shown to be detrimental to heat release rates as measured by cone calorimetry. It was concluded that with well-balanced composition, i.e. 8% FR + 1% MMT + 1% CE, low-density foams (with porosities higher than 70%) of uniform microcellular structure and prominent flame retardant characteristics (such as V0 rating according to UL94 standard, LOI of 28.5% and by 50% reduced peak of heat release rate and by 30% reduced total heat emission) can be manufactured even from rPET.

Calculating diffusion coefficients from molecular dynamics simulations for foam extrusion modelling of polypropylene with CO2, N2 and ethanol

In foaming processes, the blowing agent has a significant influence on the material behaviour and the necessary processing parameters. Low-density polypropylene foam sheets are usually produced with aliphatic hydrocarbons or alkanes as physical blowing agent. Due to the necessary safety precautions and the environmental impact, there is great interest in using alternative blowing agents such as CO2. The sole use of CO2 often leads to corrugation, open cells or surface defects on the foam sheet and therefore requires modifications to the process technology. For this reason, blowing agent mixtures based on CO2 and organic solvents are used for the production of foam sheets. For developing a process model describing the melt flow in the extrusion die and the formation of cells, specific material data like diffusion coefficients are necessary. For CO2 and N2 as sole blowing agent, experimental data exist in the literature. Since no experimental data are available for co-blowing agents such as ethanol at elevated temperatures as they occur in the foam process, these data were calculated using molecular dynamics (MD) simulations. The benefit of MD simulations lies in their ability to reduce the experimental effort and, in particular, to provide data in cases where this data is not available through experimental measurements. The calculated diffusion coefficient values are compared to experimental data from the literature and presented for CO2, N2 and ethanol in polypropylene. The calculated diffusion coefficients of CO2 and N2 are compared with literature results and agree well with them. For the ethanol molecules, the diffusion coefficient is compared relative to the both aforementioned ones considered the larger size of the ethanol molecule compared to N2 and CO2. The results of the diffusion coefficients for ethanol are reasonable compared to the values found for the other two molecules.

Expanded Beads of High Melt Strength Polypropylene Moldable at Low Steam Pressure by Foam Extrusion.

We explore the foam extrusion of expanded polypropylene with a long chain branched random co-polypropylene to make its production process simpler and cheaper. The results show that the presence of long chain branches infer high melt strength and, hence, a wide foamability window. We explored the entire window of foaming conditions (namely, temperature and pressure) by means of an ad-hoc extrusion pilot line design. It is shown that the density of the beads can be varied from 20 to 100 kg/m3 using CO2 and isobutane as a blowing agent. The foamed beads were molded by steam-chest molding using moderate steam pressures of 0.3 to 0.35 MPa independently of the closed cell content. A characterization of the mechanical properties was performed on the molded parts. The steam molding pressure for sintering expanded polypropylene beads with a long chain branched random co-polypropylene is lower than the one usually needed for standard polypropylene beads by extrusion. The energy saving for the sintering makes the entire manufacturing processes cost efficient and can trigger new applications.

Preparation of Well-Compatibilized PP/PC Blends and Foams Thereof

The performance of polypropylene-poly(ethylene brassylate) block and graft copolymers and a polypropylene-polycaprolactone graft copolymer as compatibilizers for polypropylene-rich polypropylene/bisphenol A polycarbonate (PP/PC, 80/20 wt/wt) blends was elucidated. The copolymers were synthesized either by metal-catalyzed ring-opening polymerization or transesterification of a presynthesized polyester, initiated by hydroxyl-functionalized PPs, which themselves were obtained by catalytic routes or reactive extrusion, respectively. Spectroscopic fingerprints of the copolymers from liquid-state nuclear magnetic resonance (NMR) in combination with scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), dynamic mechanical thermal analysis (DMTA), and rheology analyses of the blends indicated that the compatibilizers spontaneously organize at the interface of the two immiscible polymers leading to the formation of uniform, stable, nanophase morphologies. The effect of the compatibilizers on the performance of the PP/PC blends was evaluated, and well-compatibilized PP/PC blends showed improved melt strength and strain hardening when compared to pure PP. This was verified by the successful foam extrusion using isobutane as a blowing agent of well-compatibilized PP/PC blends to low-density PP-based foams, for which normally long-chain branched PP is required.

Modeling flow and cell formation in foam sheet extrusion of polystyrene with CO2 and co‐blowing agents. Part I: Material model

AbstractIn foam extrusion, process parameters, material properties, and the blowing agent have an influence on the resulting foam properties. For safety and environmental reasons, carbon dioxide (CO2) has gained importance as a physical blowing agent for the production of low‐density polystyrene foam sheets. The sole use of CO2 often leads to corrugation, open cell structures, or surface defects on the foam sheet. As an alternative, blowing agent mixtures based on CO2 and organic solvents such as ethanol, acetone, or ethyl acetate can be used, changing solubility and flow behavior of the gas‐loaded melt. Modeling of the foaming process in the extrusion die could help to reduce experimental effort and accelerate the development of novel blowing agent mixtures. A model approach to describe the melt behavior of polystyrene loaded with various blowing agent mixtures in the extrusion die is developed. Part I of the article describes the modeling of material properties, that is, rheological behavior by a Carreau‐WLF approach with shift factors for temperature, pressure, and blowing agent effects on the glass transition temperature. Solubility behavior is modeled by a combined Henry solubility coefficient approach, showing good agreement with experimental data. Based on the material model, a process model is developed in Part II of this work.

Modeling flow and cell formation in foam sheet extrusion of polystyrene with CO2 and co‐blowing agents. Part II: Process model

AbstractIn foam extrusion, process parameters, material properties, and the blowing agent have an influence on the resulting foam properties. For safety and environmental reasons, carbon dioxide (CO2) has gained importance as physical blowing agent for the production of low‐density polystyrene foam sheets. The sole use of CO2 often leads to corrugation, open cell structures, or surface defects on the foam sheet. As an alternative, blowing agent mixtures based on CO2 and organic solvents such as ethanol, acetone, or ethyl acetate can be used, changing solubility and flow behavior of the gas‐loaded melt. A model approach for describing foam extrusion of polystyrene with various blowing agent mixtures in an annular gap die is developed. Part I of the paper describes the modeling of material properties. In Part II, the process model including nucleation and cell formation in the flow field is developed and applied to a foam sheet extrusion process. Based on the material model, melt flow and formation of cells are modeled by a step‐wise calculation along the die, showing good agreement with experimental data. Dimensionless numbers are used to describe the foaming process and a parameter study based on these dimensionless numbers is presented.

Development and investigation of the applicability of microstructural models for polymeric low density foams directly obtained from computed tomography data

Nowadays, there are several methods to obtain simulation models for foams which consider important microstructural features. This research study presents a method to obtain low density foam models directly from computed tomography (CT) data. Finite element meshes are created from CT measurement results of a polypropylene extrusion foam with two different densities. Sensitivity studies with regard to the tension behaviour are performed with the microstructural models. The study shows that the tension behaviour highly depends on the examined area of the foam because the microstructure and density vary through the foam. Furthermore, the simulation results are validated with experimental results. The validation shows that the tension behaviour of the investigated foams characterised by the simulation approach is in good agreement with the experimentally observed behaviour and that specific microstructural characteristics (e.g. anisotropic cell shapes) are captured in the model.

A study on fabrication of nanocomposite polyethylene foam through extrusion foaming procedure

Foaming a polymer not only turns it into a lightweight material but also gives some special properties to it. However, the most important issue is controlling the foaming process to achieve a desirable structure with high cell density and low relative density. In the present study, the extrusion foaming process of polyethylene was studied through stepwise amendments. An innovative extrusion system was designed and implemented to produce extrusion foams under different material and process conditions using N2as blowing agent. In the first step, the final cooling condition was investigated. The air-cooling condition led to a higher cell density/lower cell size compared to the water-cooling condition although a higher relative density was obtained. In the second step, the effects of the addition of talc and the synergetic effect of talc/nanoclay at different contents were investigated in detail. The hybrid of talc/nanoclay had a noticeably improving effect on the cellular structure. In the third step, the effects of processing parameters including the die temperature and screw speed were studied on the foam properties. Finally, up to 49.4% decrease in the relative density of samples was observed, also cell densities up to 2.5 × 104cell/cm3and cell sizes as small as 280 µm were achieved.