Extrusion Foaming Process Research Articles

Environmentally-friendly solvent pulping and grafting strategies for better foamability of poly(lactic acid)/agricultural waste biocomposites

In this study, the foamability of poly (lactic acid) (PLA) through a continuous extrusion process was enhanced by using an agricultural waste, environmentally friendly pulping methods, and reactive compatibilization of the lignocellulosic filler with matrix in the extrusion flow field. Both applied solvent-pulping methods by using chemicals with low safety and toxicity risks were considerably successful in purifying the micron-sized cellulosic fibers of rice straw (RS). The interface of PLA matrix with RS pulp fibers was modified by applying reactive compatibilizers containing maleic anhydride (MA) and epoxy functional groups. Both compatibilizers had the ability to simultaneously react with the hydroxyl groups of RS pulp fibers and end groups of PLA chains during the melt-compounding process. As a result of RS pulping and PLA/RS pulp compatibilizing reactions, the processability of PLA in an extrusion foaming process was substantially enhanced, in a way that the void fraction and cell density of PLA foam increased more than seven and two times, respectively. The extruded lightweight all-green biocomposite foams can be applied in construction and packaging applications.

Foamability of all‐green polylactide/rice straw pulp biocomposites through continuous extrusion process: Effects of pulping and reactive compatibilization

AbstractIn the present work, the processability of poly(lactic acid) (PLA) in the extrusion foaming process was improved by using rice straw (RS) as an agricultural waste. In order to extrude a sustainable lightweight foam, the soda‐pulping and bleaching modification were performed on the RS particles, which resulted in the attainment of purified micron‐sized cellulosic fibre with higher aspect ratio. The addition of these fibres, with larger interfacial area between filler and matrix, to the PLA melt enhanced the foam void fraction and cell density more than one order of magnitude. The use of a reactive compatibilizer in the biocomposite showed further beneficial effects on the PLA foamability. The used reactive compatibilizer, with the ability of simultaneous reactions with the end groups of PLA macromolecules and hydroxyl groups of the lignocellulosic pulp, in the biocomposite noticeably improved the viscoelastic properties of melt and lengthened the macromolecule relaxation times. As a result, the biocomposite melt with higher melt strength was obtained, which kept the melt integrity during the bubble growth stage, hindered the cell coalescence, and retained a larger volume of gas inside the melt. Contrary to these influences, the compatibilization activity of this additive weakened the heterogeneous nucleation role of the cellulosic fibres. However, by choosing proper pulp loading, the extrusion of a lightweight biodegradable PLA‐based biocomposite foam can be feasible, which can be used in interior construction applications.

Machine learning investigation of polylactic acid bead foam extrusion

AbstractThis study employs machine learning algorithms to analyze the bead foam extrusion process and to assess the impact of processing parameters, specifically focusing on their effects on bead foam density and melt pressure in under water granulation (UWG) for polylactic acid (PLA). These interrelated parameters, influenced by processing parameters such as temperature, screw speed, and blowing agent, possess challenges for traditional empirical methods to capture. The key factors that significantly impact the prediction of melt pressure in UWG are blowing agent, injector pressure, temperature in B‐extruder and die size. Likewise, essential parameters for predicting bead foam density comprise blowing agent, injector pressure, temperature in B‐extruder, die plate temperature, melt temperature in B‐extruder, and melt pressure in B‐extruder. Machine learning (ML) models were employed to forecast bead foam density and melt pressure in UWG using various processing parameters in PLA bead foam extrusion. The random forest model achieved a high coefficient of determination score of 0.96 for predicting melt pressure in UWG. Additionally, the decision tree model demonstrated effective predictions for bead density, with the score: 0.81. These ML models can be applied to diverse materials, leading to more sustainable, efficient processes for bead foam extrusion.

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.

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.

Retraction Note: Synergy of propylene-based copolymer in high-melt-strength polypropylene for extrusion foaming process

Retraction Note: Synergy of propylene-based copolymer in high-melt-strength polypropylene for extrusion foaming process

Partially Bio‐Based Polyester Bead Foams via Extrusion Foaming of Poly(butylene terephthalate)/Poly(butylene furanoate) Blends

AbstractThe interest in bio‐based alternatives to classical polyesters such as poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT) is steadily growing to achieve a more sustainable approach to polymer materials. In this study, PBT/poly(butylene furanoate) (PBF) blends are prepared, characterized and extrusion foamed. PBF as a bio‐based polyester offers two advantages. The ecological footprint of the material is reduced, and additionally, it can be used in Diels‐Alder reactions at the blend surface to support fusion of the foamed beads. The blending behavior of the polyesters is investigated using samples prepared in a microcompounder, particularly focused on the miscibility of the blends and transesterification reactions. The blends are thermodynamically immiscible but show a certain degree of transesterification according to nuclear magnetic resonance (NMR) spectroscopy. The morphology of blend beads produced by an extrusion foaming process is analyzed regarding their cell density, cell size distribution, and open‐cell content. It is shown that PBF has a positive effect on the bead foam morphology. The use of a bifunctional linker designed for chemical fusion of the bead surfaces allows to obtaining of molded parts, in contrast to beads containing pure PBT.

Synergy of propylene-based copolymer in high-melt-strength polypropylene for extrusion foaming process

Synergy of propylene-based copolymer in high-melt-strength polypropylene for extrusion foaming process

Continuous extrusion foaming process of biodegradable nanocomposites based on poly(lactic acid)/carbonaceous nanoparticles with different geometric shapes: An insight into involved physical, chemical and rheological phenomena

AbstractOne‐step extrusion foaming process of biodegradable poly(lactic acid) (PLA)‐based nanocomposites in the presence of chemical foaming agent and chain extender has great complexity to control. In this work, PLA‐based nanocomposites containing different carbonaceous nanoparticles with different geometric shapes were obtained through a co‐rotating twin‐screw extrusion process to get insight into the dominant phenomena, which control the structural and final properties of PLA foams. The reactive extrusion foaming of PLA melt was investigated in the presence of carbon black, carbon nanotubes (CNTs), graphene oxide (GO), and a chain extender additive as well as a chemical foaming agent. Nanoparticles affect the continuous extrusion foaming of PLA melt through several phenomena, including providing bubble heterogeneous nucleation sites, intensifying the chemical decomposition of the foaming agent, improving the PLA structural modification reaction, increasing PLA molecular weight and restricting the dissolved gas removal from the melt. By influencing the involved phenomena in process, the nanoparticles at low levels, especially CNT and GO, increased the void content and cell size of PLA foams. The incorporation of CNT and GO nanofillers at the 0.5 phr level increased the electrical conductivity of PLA foams sharply by 9 and 10 orders of magnitude, resulting in lightweight, biodegradable semi‐conductive foams.

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.

An insight into simultaneous phase transition phenomena and melt-rheology of low-density polyethylene/ethylene–vinyl acetate copolymer/expanded graphite nanocomposites through continuous extrusion foaming process

In the present article, the influences of expanded graphites (EGs) with small, medium, and large aspect ratios (SEG, MEG, and LEG, respectively) on the foamability of low-density polyethylene (LDPE)/ethylene–vinyl acetate copolymer (EVA) blend obtained from continuous extrusion foaming process were studied. The addition of EGs to the PE/EVA blend resulted in foams with smaller cells and higher cell densities. Among the different nanofillers used, EG with larger aspect ratio caused the formation of PE/EVA foams with higher void fraction, smaller cell size and larger cell density. EG nanoplatelets assisted bubble nucleation and acted as an impermeable barrier against the gas removal from the melt, where LEG behaved more successfully due to having a larger surface area with the melt. During the bubble nucleation and growth stages, PE/EVA liquid–liquid and solid–liquid phase separations took place concurrently. The presence of EG nanofillers influenced the PE/EVA phase separation and increased the mutual solubility of the polymeric constituents in each other. Besides, EG nanolayers enhanced the viscoelastic properties of PE/EVA melt, and thus prevented cell wall rupture and gas release from the melt.

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).

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.

Peculiar micro and nano cell morphology of PBT/PTFE nanofibrillated composite foams of supercritical CO2 foaming induced by in-situ formed 3D PTFE nanofiber networks

This study investigates the role of polytetrafluoroethylene (PTFE) nanofibers on the peculiar micro and nano foaming behavior of polybutylene terephthalate (PBT) before and after crystallization. In-situ PBT/PTFE nanofibrillated composites were prepared by melt compounding in torque rheometer. A physical network of high aspect ratio PTFE nanofiber is formed as the result of the fibers splitting and entanglement demonstrated by scanning electron microscope (SEM). In molten PBT, the formation of the nanofiber networks significantly increases the melt viscoelastic properties and promotes cell nucleation. As a result, high expansion ratio foams with average cell size of 13 μm were achieved in the melting PBT, which has some guiding significance for the formation of microcellular foam in extrusion foaming process. In addition, the crystallization process and crystallization morphology were investigated by differential scanning calorimeter (DSC), polarizing microscope (POM) and SEM. The crystallization kinetics of PBT were promoted due to the more crystallization nucleation interface provided by the high specific surface area of PTFE nanofibers. In the crystallization process of PBT/PTFE nanofibrillated composites, the PBT molecular chain first crystallized along the surface of PTFE nanofiber to form shish-kebab crystalline structure, and then the PBT molecular chain away from PTFE nanofibers crystallized. Consequently, the shish-kebab crystalline structure and the two-step crystallization induced formation of the peculiar oriented micro and nano cells. The research could present a facile and flexible method to regulate the cell morphology and expand the application of PBT foam.

Analysis of Bubble Growth in Supercritical CO2 Extrusion Foaming Polyethylene Terephthalate Process Based on Dynamic Flow Simulation.

Bubble growth in the polymer extrusion foaming process occurs under a dynamic melt flow. For non-Newtonian fluids, this work successfully coupled the dynamic melt flow simulation with the bubble growth model to realize bubble growth predictions in an extrusion flow. The initial thermophysical properties and dynamic rheological property distribution at the cross section of the die exit were calculated based on the finite element method. It was found that dynamic rheological properties provided a necessary solution for predicting bubble growth during the supercritical CO2 polyethylene terephthalate (PET) extrusion foaming process. The introduction of initial melt stress could effectively inhibit the rapid growth of bubbles and reduce the stable size of bubbles. However, the initial melt stress was ignored in previous work involving bubble growth predictions because it was not available. The simulation results based on the above theoretical model were consistent with the evolution trends of cell morphology and agreed well with the actual experimental results.

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.

Rheological and foaming behaviors of long-chain branched polyamide 6 with controlled branch length

A series of long-chain branched polyamide 6 samples (LCB PA6s) with well-defined branch length were successfully fabricated via reactive extrusion by tuning various linear PA6s as reactive materials. Their structure-property relationships were systematically investigated. The GPC-rheology method was performed to demonstrate the existence of LCB topological structure and quantitatively determine the relative branch length for three LCB PA6s. Rheological characterization showed that an increase in branch length resulted in the increased zero-shear viscosity and storage modulus at low frequency, reduced loss factor and high strain hardening coefficient under elongational flow, i.e., the enhanced elastic response and melt strength of PA6s. Finally, the melt foamability of the LCB PA6s were verified by batch foaming experiments with supercritical CO2 as the blowing agent. The LCB-Mb 4.5 with the longest branch length and highest melt strength appeared the biggest expansion ratio of 18.2 and broadest foaming window of 70 °C, which was expected to be in production on a large scale through continuous extrusion foaming process.

Thermoplastic foaming with thermo-expandable microcapsules: Mathematical modeling and numerical simulation for extrusion process

A numerical simulation for foaming a melted thermoplastic by extrusion process is conducted, where thermo-expandable microcapsules are used as a blowing agent. Firstly, a growth model for an encapsulated bubble is developed to match experimental results. Then, the growth model is used to numerically solve a 2D non-isothermal die-swell problem, making possible to simulate the extrusion foaming process. In particular, the model is able to predict the foaming location. Numerical results are compared with experimental data obtained by optical microscopy observations and density measurements on extruded samples. The effects of thermoplastic melt rheology on foaming and the die swell induced by foaming are also investigated. This simulation helps to estimate process parameters such as extrusion speed and to predict die swell and sample porosity in whole, to finally optimize the industrial process.

Good extrusion foaming performance of long‐chain branched PET induced by its enhanced crystallization property

AbstractPoly(ethylene terephthalate) (PET) was modified by regulating different contents of branching agent epoxy‐based multifunctional oligomer and chain extender pyromellitic dianhydride in reactive extrusion process. The modified PET with better long‐chain branched (LCB) structure boosted its rheological properties, and its enhancement of melt viscoelasticity resulted in excellent foamability in molten‐state foaming process using supercritical CO2 as blowing agent. More importantly, the branched structures acted as crystal sites to accelerate the crystallization kinetic of LCB PET whether under atmospheric pressure or high‐pressure CO2. The shear and elongation flow inside die further quickly induced the crystallization of LCB PET. The rapidly generated fine crystals could both introduce heterogeneous cell nucleation and suppress CO2 escape, so the cell morphology of LCB PET in continuous extrusion foaming process exhibited a three‐fold increase in cell density and smaller uniform cell size with respect to those of other foam‐grade PET with long‐chain structure.

Improving the flame‐retardant property of bottle‐grade PET foam made by reactive foam extrusion

AbstractUpcycling of low intrinsic viscosity (IV) poly(ethylene terephthalate) (PET) grades, such as bottle‐ or recycled grades, by a reactive foam extrusion process, provides an appropriate alternative to high pricing, high IV grades commonly used for foaming applications. However, the drawback of bottle‐grade PET foams is its flame retardant (FR) performance. In this study, pyromellitic dianhydride was used as a chain extender to foam bottle‐grade PET. The influence of different FRs, containing halogenated (HFR) and four different phosphorous‐based types, on the processability and final foam properties was investigated. HFR showed better processability to achieve proper foams with fine morphology compared to P‐based FRs, where the FR content was adjusted between 2 and 5 wt%. However, HFR exhibited lower FR performance by cone calorimeter testing compared to the P‐based FRs and the commercial reference foam Kerdyn. Nonetheless, all of the FRs can only improve the time to ignition of the neat PET foams while the other values depend on the specific type of FR. In addition, all FR foams have improved mechanical properties more than twice in comparison to the neat PET foam.