Novel injection molding foaming approaches using gas‐laden pellets with N2, CO2, and N2 + CO2 as the blowing agents

A novel method of producing injection molded parts with a foamed structure has been developed. It has been named supercritical fluid‐laden pellet injection molding foaming technology (SIFT). Compared with conventional microcellular foaming technologies, it lowers equipment costs without sacrificing the production rate, making it a good candidate for mass producing foamed injection molded parts. Both N₂ and CO₂ can be suitably used in this process as the physical blowing agent. However, due to their distinct physical properties, it is necessary to understand the influence of their differences over the process and the outcomes. Comparisons were made in this study between using CO₂ and N₂ as the blowing agents in terms of the part morphologies, as well as the shelf life and gas desorption process of the gas‐laden pellets. After gaining a good understanding of the SIFT process and the gas‐laden pellets, a novel foam injection molding approach combining the SIFT process with microcellular injection molding was proposed in this study. Both N₂ and CO₂ can be introduced into the same foaming process as the coblowing agents in a two‐step manner. Using an optimal content ratio for the blowing agents, as well as the proper sequence of introducing the gases, foamed parts with a much better morphology can be produced by taking advantage of the benefits of both blowing agents. In this study, the theoretical background is discussed and experimental results show that this combined approach leads to significant improvements in foam cell morphology for low density polyethylene, polypropylene, and high impact polystyrene using two different mold geometries. POLYM. ENG. SCI., 54:899–913, 2014. © 2013 Society of Plastics Engineers     

Visual observation and numerical studies of N2 vs. CO2 foaming behavior in core‐back foam injection molding

We investigated , by visual observation and numerical calculations , the foaming behavior of polypropylene within a foam injection mold cavity with the environmentally benign physical blowing agents nitrogen (N₂) and carbon dioxide (CO₂) . An 85‐ton core‐back injection‐molding machine with temperature and pressure monitoring systems as well as a high‐pressure view cell was used for the investigation . The experiments showed a prominent difference in bubble nucleation and growth between N₂ and CO₂ injection foaming . Even when the weight concentration of N₂ dissolved in polymer was one‐third that of CO₂ , N₂ injection foaming provided a bubble number density that was 30 times larger and a bubble size that was one‐third smaller compared to CO₂ injection foaming . Classical bubble nucleation and growth models developed for batch foaming were employed to analyze these experimental results . The models reasonably explained the differences in injection foaming behavior between N₂ and CO₂ . It was clearly demonstrated by both experiments and numerical calculations that N₂ provides a higher number of bubbles with a smaller bubble size in foam injection molding compared to CO₂ as a result of the lower solubility of N₂ in the polymer and the larger degree of super‐saturation . POLYM. ENG. SCI., 2011. ©2011 Society of Plastics Engineers     

A novel thermoplastic polyurethane scaffold fabrication method based on injection foaming with water and supercritical carbon dioxide as coblowing agents

Injection foaming is an method for mass producing lightweight, foamed plastic components with excellent dimensional stability while using less material and energy. In this study, a novel injection foaming method employing supercritical CO₂ (scCO₂) and water as coblowing agents was developed to produce thermoplastic polyurethane (TPU) components with a uniform porous structure and no solid skin. Various characterization techniques were employed to investigate the cell morphology, crystallization behavior, and static and dynamic mechanical properties of solid injection molded samples, foamed samples using CO₂ or water as a single blowing agent, and foamed samples using both CO₂ and water as coblowing agents. When compared with CO₂ foamed samples, samples produced by the coblowing method exhibited much more uniform cell morphologies without a noticeable reduction in mechanical properties. Moreover, these TPU samples had almost no skin layer, which permitted the free transport of nutrients and waste throughout the samples. Such a mass‐produced, skin‐free structure is desirable in tissue engineering. In this study, the biocompatibility of the scaffolds was confirmed and the effect of these blowing agents on the TPU foaming behavior was studied. POLYM. ENG. SCI., 54:2947–2957, 2014. © 2014 Society of Plastics Engineers     

A new microcellular foam injection‐molding technology using non‐supercritical fluid physical blowing agents

A new foam injection‐molding technology was developed to produce microcellular foams without using supercritical fluid (SCF) pump units. In this technology, physical blowing agents (PBA), such as nitrogen (N₂) and carbon dioxide (CO₂), do not need to be brought to their SCF state. PBAs are delivered directly from their gas cylinders into the molten polymer through an injector valve, which can be controlled by a specially designed screw configuration and operation sequence. The excess PBA is discharged from the molten polymer through a venting vessel. Alternatively, additional PBA is introduced through the venting vessel when the polymer is not saturated with PBA. The amount of gas delivered into the molten polymer is controlled by the gas dosing time of the injector valve, the secondary reducing pressure of the gas cylinder and the outlet (back) pressure of the venting vessel. Microcellular polypropylene foams were prepared using the developed foam injection‐molding technology with 2–6 MPa CO₂ or 2–8 MPa N₂. High expansion foams with an average cell size of less than 25 μm were prepared. The developed technology dispels arguments for the necessity to pressurize N₂ or CO₂ to the SCF to prepare microcellular foams. POLYM. ENG. SCI., 57:105–113, 2017. © 2016 Society of Plastics Engineers     

Generation of low‐density high‐performance poly(arylene ether sulfone) foams using a benign processing technique

In this study, a benign process was used to successfully produce low density foam from poly(arylene ether sulfone) (PAES). Both carbon dioxide (CO₂) and water as well as nitrogen and water were used as physical blowing agents in a one‐step batch process. A large amount of blowing agents (up to 7.5%) was able to diffuse into the PAES resin in a 2‐h saturation time. Utilizing water and CO₂ as the blowing agents yielded foam with better properties than nitrogen and water because both the water and CO₂ are plasticizers for the PAES resin. PAES foam produced from CO₂ and water had a large reduction in foam density (～80%) and a cell size of ～50 μm, while maintaining a primarily closed cell structure. The small cell size and closed cell structure enhanced the mechanical properties of the foam when compared with the PAES foam produced from nitrogen and water. The tensile, compressive, and notched izod impact properties of the PAES foams were examined, and the compressive properties were compared to commercially available structural foams. With reduced compression strength of 39 MPa and reduced compression modulus of 913 MPa, the PAES foam is comparable to polyetherimide and poly(vinylchloride) structural foams. POLYM. ENG. SCI., 2009. © 2008 Society of Plastics Engineers     

Microcellular foaming of poly(phenylene sulfide)/poly(ether sulfones) blends using supercritical carbon dioxide

Microcellular foaming of poly(phenylene sulfide)/poly(ether sulfones) (PPS/PES) blends presents a promising approach to produce high‐performance cellular materials with tailored microstructures and enhanced properties. This study investigated the effects of multiphase blend composition and process conditions on the foaming behaviors and final cellular morphology, as well as the dynamic mechanical properties of the solid and microcellular foamed PPS/PES blends. The microcellular materials were prepared via a batch‐foam processing, using the environment‐friendly supercritical CO₂ (scCO₂) as a blowing agent. The saturation and desorption behaviors of CO₂ in PPS/PES blends for various blend ratios (10 : 0, 8 : 2, 6 : 4, 5 : 5, 4 : 6, 2 : 8, and 0 : 10) were also elaborately discussed. The experimental results indicated that the foaming behaviors of PPS/PES blends are closely related to the blend morphology, crystallinity, and the mass‐transfer rate of the CO₂ in each polymer phase. The mechanisms for the foaming behaviors of PPS/PES blends have been illustrated by establishing theoretical models. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42634.     

Comparative study on foaming process of thermoplastic polyester and polyether polyurethane with supercritical CO2 as foaming agent

ABSTRACT

The aim of this study was the comparison of beads foaming process and synergistic effect of foaming process parameters on melting crystalline characteristics between thermoplastic polyester and polyether polyurethane(TPU) via batch process using supercritical CO₂(scCO₂) as blowing agent. Method was based on the generation of expandable TPU foam beads (ETPU) prepared by self-designed autoclave foaming apparatus. The surface morphology, cell structure, and crystalline melting characteristics of the ETPU depending on the processing parameters, such as saturation pressure (P_f), saturation temperature (T_f), and saturation time (t_f), were investigated. Results demonstrated both ETPU exhibited fine closed cell morphology with polygons structure and would induce the disappearance of multiple endothermic peaks during the foam process. As any of those three process parameters increased, the HS content, high crystalline melting and E_r were improved, while with regard to cell structure, P_f demonstrated the opposite effect compared with T_f and t_f. Comparing with polyether TPU, polyester TPU possess a larger cell density and a clear bimodal cell structure could be found under certain condition, while beads foaming process had a greater impact on polyether TPU, which had a low density and the appropriate t_f is 2h.     

Poly(ethylene terephthalate) foams: Correlation between the polymer properties and the foaming process

The foamability of two food‐grade, high‐molecular‐weight poly(ethylene terephthalate)s (PETs) was investigated. Sorption tests were performed to determine the solubility and diffusivity of N₂ and CO₂ in molten polymers at 250°C with a magnetic suspension balance. Pressure‐volume‐temperature (pVT) data were also measured and used in the context of the Sanchez–Lacombe equation of state to predict the sorption isotherms. The thermal properties, in terms of the glass‐transition, melting, and crystallization temperatures, were measured by differential scanning calorimetry analysis on the two high‐molecular‐weight PETs and, for comparison, on a bottle‐grade PET. The rheological properties were measured to asses the improvement of the high‐molecular‐weight PET with respect to the bottle‐grade one. Expansion tests were performed on the two high‐molecular‐weight grades and bottle‐grade PETs with a batch foaming process with N₂, CO₂, and an 80–20 wt % N₂–CO₂ mixture used as blowing agents. The whole processing window was explored in terms of temperature, pressure drop rate, and saturation pressure. The results of the foaming experiments were correlated to gas sorption and the thermal and rheological properties of the polymers in the molten state. The results proved the feasibility of foam processing these two high‐molecular‐weight grades, which gave, when compared to the bottle grade at specific foaming conditions, very low densities and fine morphologies. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Extruded polystyrene foams with bimodal cell morphology

Extrusion foaming using supercritical carbon dioxide (CO₂) as the blowing agent is an economically and environmentally benign process. However, it is difficult to control the foam morphology and maintain its high thermal insulation comparing to the conventional foams based on fluorocarbon blowing agents. In this study, we demonstrated that polystyrene (PS) foams with the bimodal cell morphology can be produced in the extrusion foaming process using CO₂ and water as co-blowing agents and two particulate additives as nucleation agents. One particulate is able to decrease the water foaming time so both CO₂ and water can induce foaming simultaneously, while the other increases the CO₂ nucleation rate with little effect on the CO₂ foaming time. Our experimental results showed that a dual particulate combination of nanoclay and activated carbon provided the best bimodal structure. The bimodal foams exhibited much better compressive properties and slightly better thermal insulation for PS foams.     

The synergy of supercritical CO2 and supercritical N2 in foaming of polystyrene for cell nucleation

This study examines the foaming behaviour of polystyrene (PS) blown with supercritical CO₂–N₂ blends. This is achieved by observing their foaming processes in situ using a visualization system within a high-temperature/high-pressure view-cell. Through analyzing the cell nucleation and growth processes, the foaming mechanisms of PS blown with supercritical CO₂–N₂ blends have been studied. It was observed that the 75% CO₂–25% N₂ blend yielded the highest cell densities over a wide processing temperature window, which indicates the high nucleating power of supercritical N₂ and the high foam expanding ability of supercritical CO₂ would produce synergistic effects with that ratio in batch foaming. Also, the presence of supercritical CO₂ increased the solubility of supercritical N₂ in PS, so the concentration of dissolved supercritical N₂ was higher than the prediction by the simple mixing rule. The additional supercritical N₂ further increased the cell nucleation performance. These results provide valuable directions to identify the optimal supercritical CO₂–N₂ composition for the foaming of PS to replace the hazardous blowing agents which are commonly used despite their high flammability or ozone depleting characteristics.     

Climate‐friendly polyurethane blowing agent based on a carbon dioxide adduct from palmitic acid grafted polyethyleneimine

To explore a new blowing agent for polyurethanes (PUs), palmitic acid was grafted onto a branched polyethyleneimine (bPEI; weight‐average molecular weight = 25,000 Da) via N,N′‐carbonyldiimidazole condensation to form a hydrophobically modified bPEI [palmitic acid grafted branched polyethyleneimine (C₁₆–bPEI)] with a grafting rate of 12%. A CO₂ adduct of C₁₆–bPEI, which trapped 16.8% CO₂ in it, was synthesized from C₁₆–bPEI. The long alkyl chain grafting improved the dispersibility of the CO₂ adduct in the PU raw materials and favored a homogeneous release of CO₂ to blow PUs during the exothermic foaming process. The preliminary results show that the foams possessed a density of 72.0 kg/m³ and a compressive strength of 246 kPa; this matched the required values of foams for the thermal insulation of underground steel pipes. This new blowing agent emitted nothing but CO₂ to the atmosphere, so it will not promote ozone depletion and will avoid global warming problems that are associated with traditional blowing agents such as chlorofluorocarbons and hydrochloroflourocarbons. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43874.     

Carbon dioxide adduct from polypropylene glycol grafted polyethyleneimine as a climate-friendly blowing agent for polyurethane foams

A novel CO₂ adduct has been synthesised from a branched polyethyleneimine with polypropylene glycol (PPG) side chains and can serve as an alternative to traditional polyurethane blowing agents, such as hydrochloroflourocarbons and hydrofluorocarbons, which cause ozone depletion and/or global warming. The CO₂ adduction trapped 13.8 wt% of CO₂, forming alkylammonium carbamates in the main chains. Therefore, the prepared blowing agent is amphophilic, and can form micellae-like spheres in the mixture of polyurethane raw materials called the white component. Once this mixture is blended with isocyanate or the black component like in conventional foaming processes, the consequent exothermic polymerisation drives the release of the captured CO₂ from the micelles, serving as the foaming gas. Meanwhile, the blowing agent gradually restores its original polyamine structure, whose bulky PPG side chains can sterically inhibit the reaction between the restored amine groups and the isocyanate groups in the growing polyurethane chains. The resulting foam displays uniform cellular morphology with a much lower density than the control sample blown by trances of water from the raw materials. This is the first report using thermally instable CO₂ adduct to blow polyurethanes, which could pave the way for the next generation of climate-friendly polyurethane blowing agents.     

Ni‐Al2O3/Ni‐foam catalyst with enhanced heat transfer for hydrogenation of CO2 to methane

Monolithic Ni‐Al₂O₃/Ni‐foam catalyst is developed by modified wet chemical etching of Ni‐foam, being highly active/selective and stable in strongly exothermic CO₂ methanation process. The as‐prepared catalysts are characterized by x‐ray diffraction scanning electron microscopy, inductively coupled plasma atomic emission spectrometry, and H₂‐temperature programmed reduction‐mass spectrometry. The results indicate that modified wet chemical etching method is working efficiently for one‐step creating and firmly embedding NiO‐Al₂O₃ composite catalyst layer (～2 μm) into the Ni‐foam struts. High CO₂ conversion of 90% and high CH₄ selectivity of >99.9% can be obtained and maintained for a feed of H₂/CO₂ (molar ratio of 4/1) at 320°C and 0.1 MPa with a gas hourly space velocity of 5000 h^?1, throughout entire 1200 h test over 10.2 mL such monolithic catalysts. Computational fluid dynamics calculation and experimental measurement consistently confirm a dramatic reduction of “hotspot” temperature due to enhanced heat transfer. © 2015 American Institute of Chemical Engineers AIChE J, 61: 4323–4331, 2015     

Experimental study and PC-SAFT simulations of sorption equilibria in polystyrene

The knowledge of sorption equilibria of blowing agents in polystyrene (PS) is necessary for the optimization of PS foam production. The sorption equilibria were studied experimentally using a gravimetric apparatus and simulated by the perturbed-chain statistical associating fluid theory (PC-SAFT) equation of state. As chlorinated and fluorinated hydrocarbons are being phased out, alternative blowing agents are important. We place emphasis on n-pentane, as sorption data for the PS+n-pentane system are scarce in the literature. The measured n-pentane and iso-pentane sorption isotherms were used to evaluate the PC-SAFT binary interaction parameters. The sorption of CO₂, N₂ and He in PS was also studied. Cosorption of pentanes in PS was predicted and comparison of the results with our experimental data proved good performance of the PC-SAFT model. The industrially interesting sorption enhancement and inhibition effects were studied using both experimental and simulated ternary data.     

Supercritical CO2–assisted synthesis and characterization of a polystyrene/thermal polyurethane blend

A blend of polystyrene and thermal polyurethane (PS/TPU) was prepared using supercritical (SC) CO₂ as a substrate‐swelling agent and monomer/initiator carrier. The SC CO₂/styrene/TPU ternary system was studied. Virgin TPU and synthesized blends were characterized through differential scanning calorimetry, infrared spectroscopy, rheometric measurements, and SEM. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 2016–2020, 2005     

Structure optimization of polycaprolactone foams by using mixtures of CO2 and N2 as blowing agents

The foaming process of poly(?‐caprolactone) (PCL) with carbon dioxide and nitrogen has been investigated in this work from a theoretical and experimental point of view. CO₂ and N₂ showed very different behavior, as foaming agents for PCL. This was due to the different transport, chemical, and physical properties of the polymer/gas mixture that led to different foam morphology in terms of density, cell number density, and cell size. The lowest density (0.03 g/cm³) was obtained with CO₂, but the highest number of cells with N₂ (although with a higher density, (0.2 g/cm³). Foam with a low‐density microcellular structure, was obtained when a mixture of the two gases was employed. POLYM. ENG. SCI., 45:432–441, 2005. © 2005 Society of Plastics Engineers     

Simulation of liquid physical blowing agents for forming rigid urethane foams

A computer‐based simulation for rigid polyurethane foam‐forming reactions was compared with experimental data for six blowing agents including methyl formate and C5‐C6 hydrocarbons. Evaporation of blowing agent was modeled as an overall mass transfer coefficient times the difference in activity of the blowing agent in the gas foam cells versus the resin walls of the cells. Successful modeling hinged upon use of a mass transfer coefficient that decreased to near zero as the foam resin approached its gel point. Modeling on density agreed with experimental measurements. The fitted parameters allowed for interpretations of the final disposition of the blowing agent, especially, if the blowing agent successfully led to larger foam cells versus being entrapped in the resin. The only component‐specific fitted parameters used in the modeling was the activity coefficient that was lower for methyl formate than the value used for hydrocarbons. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42454.     

Enhancing cell nucleation of thermoplastic polyolefin foam blown with nitrogen

More than ever before, thermoplastic polyolefin (TPO) is actively used for interior and exterior applications in the automotive industry. Foamed TPO parts use far less material than their solid counterparts and, thereby, reduce material cost, weight, and fuel usage. However, foamed TPO is not yet in mainstream use because the appropriate foaming technology is not yet well developed. The literature reports the use of carbon dioxide (CO₂) as a blowing agent for TPO; 1 - 3 there is, however, little research on the use of nitrogen (N₂) as a blowing agent, despite nitrogen's numerous advantages. In this study, various talc contents were added to a TPO matrix consisting of polypropylene blended with a metallocene‐based polyolefin elastomer. The effect of talc on the TPO foams blown with N₂ was studied with a batch foaming simulation system. The simulated results were compared actual foam extrusion results. The influence of the N₂ content and processing conditions on the cell nucleation behavior is discussed. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

A study of cell nucleation in the extrusion of polypropylene foams

An investigation has been performed of the cell nucleation and initial growth behaviors in the foam processing of polypropylene (PP) in both the linear and branched forms. These materials were foamed in extrusion with the two blowing agents, CO₂ and isopentane. The cell density generally increased with an increased content of the blowing agent, for both CO₂ and isopentane. The effect of processing pressure on the cell density was distinct when CO₂ was used, whereas no pressure effect was observed in the foam processing with isopentane. The cell morphologies for the two PPs were found to be significantly different. A slightly lower nuclei density was observed in the branched PP foams than in the linear PP foams. However, the phenomenon of cell coalescence was observed much less in the branched PP foams. Most cells in the branched PP foams were closed, whereas in the linear PP foams they were connected to each other. The experimental results indicated that the branched structure played an important role in determining the cell morphologies through its effects on the melt strength and/or melt elasticity.     

Mixed matrix membranes based on 6FDA polyimide with silica and zeolite microsphere dispersed phases

Mixed matrix membranes (MMMs) prepared with 6FDA‐DAM polymer using ordered mesoporous silica MCM‐41 spheres (MSSs), Grignard surface functionalized MSSs (Mg‐MSSs) and hollow zeolite spheres are studied to evaluate the effects of surface modification on performance. Performance near or above the so‐called permeability‐selectivity trade‐off curve was achieved for the H₂/CH₄, CO₂/N₂, CO₂/CH_4, and O₂/N₂ systems. Two loadings (8 wt % and 16 wt %) of MSSs were tested using both constant volume and Wicke–Kallenbach sweep gas permeation systems. Besides single gas H₂, CO₂, O₂, N₂, and CH₄ tests, mixed gas (50/50 vol %) selectivities were obtained for H₂/CH₄, CO₂/N₂, CO₂/CH₄, and O₂/N₂ and found to show enhancements vs. single gases for CO₂ including cases. Mg‐MSS/6FDA‐DAM was the best performing MMM with H₂/CH₄, CO₂/N₂, CO₂/CH₄, and O₂/N₂ separation selectivities of 21.8 (794 Barrer of H₂), 24.4 (1214 Barrer of CO₂), 31.5 (1245 Barrer of CO₂), and 4.3 (178 Barrer of O₂), respectively. © 2015 American Institute of Chemical Engineers AIChE J, 61: 4481–4490, 2015