Fabrication of highly expanded thermoplastic polyurethane foams using microcellular injection molding and gas‐laden pellets

Thermoplastic polyurethane (TPU) is one of the most widely used and versatile thermoplastic materials. TPU foams have been extensively applied in various industries including the furniture, automotive, sportswear, and packaging industries. In this study, two methods of producing highly expanded TPU injection molded foam were investigated: (1) microcellular injection molding (MIM) with N₂ as a blowing agent, and (2) a novel gas‐laden pellet/MIM combined process with N₂ and CO₂ as co‐blowing agents. Two designs of experiments (DOEs) were performed to learn the influences of key processing parameters and to optimize foam quality. By using N₂ and CO₂ as co‐blowing agents, a bulk density as low as 0.20 g/cm³ was successfully achieved with a hysteresis compression loss of 24.4%. POLYM. ENG. SCI., 55:2643–2652, 2015. © 2015 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 new microcellular injection molding process for polycarbonate using water as the physical blowing agent

This article presents a new process for producing microcellular injection molded plastic parts using water as the physical blowing agent and micro‐scaled particles as the cell nucleating agents. Distilled water with dissolved salt were fed through the hopper of an injection molding machine at a preset rate and mixed with polycarbonate (PC) in the machine barrel. Microcellular PC tensile bars were then injection molded with different shot volumes, water/salt solution feed rates, and salt concentrations. Tiny salt crystals of 10–20 μm recrystallized during molding acted as nucleating agents in the PC foamed parts. The surface roughness, mechanical properties, and microstructure of the solid and foamed parts were measured and compared with microcellular injection molded parts using supercritical fluid (SCF) nitrogen as the physical blowing agent. At a similar weight reduction of about 10%, the water foamed PC parts have a smooth surface comparable to that of solid injection molded parts. They also possess similar, if not better, mechanical properties compared to SCF nitrogen foamed PC parts. Without the nucleating agent, PC/water foamed parts exhibit much larger and fewer bubbles within the molded parts. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

Novel foaming method to fabricate microcellular injection molded polycarbonate parts using sodium chloride and active carbon as nucleating agents

Microcellular injection molding can fabricate lightweight, dimensionally stable plastic parts while using less material and energy. This article investigates a new process using water vapor as the physical blowing agent and comparing two kinds of nucleating agents, namely, cubic sodium chloride (NaCl) and non‐uniform active carbon (AC). The effects of different nucleating agents on the surface roughness, mechanical properties, and microstructure of solid and foamed parts were characterized. Compared with typical microcellular injection molded parts, water vapor‐foamed polycarbonate (PC)/NaCl had a smooth surface comparable to that of solid parts, whereas foamed PC/AC had desirable specific mechanical properties as well as an attractive average weight reduction of 16.4 wt%. Low density and non‐uniform AC particles, used as a nucleating agent and reinforcement, improved the microcellular structure. Based on PC molecular weight measurement, the melt processing and water vapor‐foaming processes did induce a slight amount of thermal degradation and hydrolytic degradation, respectively. POLYM. ENG. SCI., 55:1634–1642, 2015. © 2014 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     

Foaming of EPDM with water as blowing agent in injection molding

Chemical foaming of elastomers is state of the art and preferred to the more complex systems engineering of physical foaming, yet, many commonly used chemical blowing agents often are hazardous. In current investigations, we introduced water bound to carrying substances (silica, carbon black) into elastomer compounds. A stable, reproducible foaming process can be implemented using water as physical blowing agent. In first tests, the average cell diameters in injection molded elastomer parts exceed the average cell diameters of chemically foamed parts. Yet, varied amounts of blowing agent can reduce the cell diameters. Furthermore, nucleating agents and water carriers are being examined to reduce cell diameters and reach cellular structures and mechanical properties of chemically foamed parts. In conclusion, foaming of elastomers with water is a promising. Yet, further examinations have to cover the effect mechanism of foaming and vulcanization as well as continuous processing and compounding. Rear end of an EPDM part foamed with water carried on silica in injection molding process (mold temperature 195 °C, breathing mold opening 2 mm) © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43613.     

Influence of crystallization on microcellular foaming behavior of polyamide 6 in a supercritical CO2-assisted route

Supercritical CO₂ as a blowing agent has attracted increasing interest in the preparation of microcellular polyamide 6 (PA6) foams. In this work, we developed the supercritical CO₂-assisted method to prepare a series of different microcellular PA6 foams by controlling its crystallization properties in two steps and carefully investigated the corresponding crystallization properties of modified PA6 and foams using various techniques. Initially, a multifunctional epoxy-based chain extender (CE) was used to produce high-melt strength-modified PA6 with improved foaming ability; then, the resulting PA6 was foamed to prepare the microcellular foams of PA6 using supercritical CO₂ as a blowing agent in a batch foaming route. The CE effectively enhanced the melt strength of PA6, and CE usage was optimized to obtain a threshold of high branching without crosslinking. The number of crystals was also adjusted by the saturation temperature. Furthermore, these crystals that formed during the saturation process served as high-efficiency bubble nucleating agents and then limited the growth of bubbles at the same time. The microcellular foams of PA6 were successfully obtained with a cell size of 10.0 μm, and cell density of 2.0 × 10⁹ cells/cm³ at the saturation temperature of 225°C.     

Controlling sandwich‐structure of PET microcellular foams using coupling of CO2 diffusion and induced crystallization

Controlling sandwich‐structure of poly(ethylene terephthalate) (PET) microcellular foams using coupling of CO₂ diffusion and CO₂‐induced crystallization is presented in this article. The intrinsic kinetics of CO₂‐induced crystallization of amorphous PET at 25°C and different CO₂ pressures were detected using in situ high‐pressure Fourier transform infrared spectroscopy and correlated by Avrami equation. Sorption of CO₂ in PET was measured using magnetic suspension balance and the diffusivity determined by Fick's second law. A model coupling CO₂ diffusion in and CO₂‐induced crystallization of PET was proposed to calculate the CO₂ concentration as well as crystallinity distributions in PET sheet at different saturation times. It was revealed that a sandwich crystallization structure could be built in PET sheet, based on which a solid‐state foaming process was used to manipulate the sandwich‐structure of PET microcellular foams with two microcellular or even ultra‐microcellular foamed crystalline layers outside and a microcellular foamed amorphous layer inside. © 2011 American Institute of Chemical Engineers AIChE J, 58: 2512–2523, 2012     

N2‐filled hollow glass beads as novel gas carriers for microcellular polyethylene

N₂‐filled hollow glass beads (HGB) were first used as novel gas carriers to prepare microcellular polymers by compression molding. Dicumyl peroxide was acted as crosslink agent to control the produced microcellular structure of low density polyethylene (LDPE)/HGB. The effect of temperature, pressure and the content of gel on the embryo‐foaming, and final‐foaming structure are investigated. Scanning electronic microscopy shows that the average cell size of microcellular LDPE ranges from 0.1 to 10 μm, and the foam density is about 10⁹–10¹¹ cells/cm³. A clear correlation is established between preserving desirable micromorphologies of microcellular LDPE in different processing stage and tuning processing factors. The pertinent foaming mechanism of microcellular materials foamed with HGB is proposed. Because of the good mechanical strength, low density, weak water‐absorption, and excellent heat insulate ability, microcellular LDPE has great potential application in energy building materials. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

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     

Label behavior property attached on microcellular foamed parts

In comparison with the conventional foaming process, microcellular foaming by injection molding has the advantage of forming small bubbles of consistent size. Because of the reduction in the cycle time, the removal of sink marks, scale reliability, and weight lightening, microcellular foaming by injection molding is widely applied to electrical products, such as automotive parts, office automation equipment, and laptops. When microcellular foaming by injection molding is used with a resin such as polycarbonate, acrylonitrile butadiene styrene, or PC/ABS, microbubbles form. This enables the manufacture of cell phones, notebooks, and personal digital assistants (PDAs), which are impossible to produce with the conventional foaming technique because these products require a thin wall. For most thin‐wall products, spray and labeling processes are added. Therefore, research into the spray and labeling characteristics of injected foamed parts should come before applications. In this article, we analyze the swelling phenomenon that results from labeling on microcellular foamed parts. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 289–293, 2005     

Effects of processing parameters and thermal history on microcellular foaming behaviors of PEEK using supercritical CO2

In this study, we mainly investigate the solid‐state foaming of polyether ether ketone (PEEK) with different crystallinities using supercritical CO₂ as a physical blowing agent. The gaseous mass‐transfer and thermophysical behaviors were studied. By altering the parameters of the foaming process, microcellular foams with different cell morphologies were prepared. The effect of crystallization on the cell morphology was also investigated in detail. The results indicate that the crystallization restricts gas diffusion in the material, and the thermophysical behaviors of the saturated PEEK sample with low crystallinity presents two cold crystallization peaks. The cell density decreases and the cell size increases as the saturation pressure increases. The cell density of the microcellular foams prepared under 20 MPa is 1.23 × 10¹⁰ cells/cm³, which is almost 10 times compares to that under 8 MPa. The cell size increases as the foaming time extends or the foaming temperature increases. It is interesting that the cell morphology with a bimodal cell‐size distribution is generated when the samples are foamed at temperatures higher than 320°C for a sufficient time. Additionally, nanocellular foams can be obtained from a highly crystallized PEEK after the decrystallization process. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42576.     

Microcellular foams of glass–fiber reinforced poly(phenylene sulfide) composites generated using supercritical carbon dioxide

Microcellular foaming of poly(phenylene sulfide) (PPS) and its glass–fiber (GF) reinforced composites using supercritical CO₂ as a blowing agent presents a promising approach to produce novel cellular materials with tailored microstructures. This study investigated the effects of the material composition and process conditions on the foaming behaviors and final morphologies of the microcellular foamed PPS and PPS/GF composites. The rheological and thermal properties as well as the saturation and desorption behaviors of CO₂ in the pure PPS and PPS/GF composites were also detailedly discussed. The results show that microcellular foams with various relative densities, cell sizes, cell‐size distributions, and cell densities can be attained by tailoring the fiber content and key process parameters. At low foaming temperatures below the cold crystallization temperature, the microcellular foamed PPS and PPS/GF composites both present a unimodal cell‐size distribution. At elevated temperatures, the generated crystalline superstructures including spherulites in the polymer matrix and transcrystals around the GF will cause a secondary heterogeneous cell nucleation. This leads to the observations of bimodal and trimodal cell‐size distributions in the pure PPS and the PPS/GF composites, respectively. The mechanisms for the solid‐state foaming behaviors of pure PPS and PPS/GF composites have been illustrated by establishing theoretical models. POLYM. COMPOS., 37:2527–2540, 2016. © 2015 Society of Plastics Engineers     

Effect of supercritical nitrogen content on the weld lines and mechanical properties of microcellular injection molded double gate PP/Al parts

Injection molding products made of aluminum flakes and polymer blends exhibit a distinctive esthetic effect. However, during the filling process, the melt flows in different directions converge and collide, resulting in the flop effect of the aluminum flake and consequent weld line formation. Herein, microcellular injection molding (MIM) was employed to fabricate polypropylene/aluminum flakes (PP/Al) composite foamed parts with distinct weld lines using supercritical nitrogen (scN₂) as the physical blowing agent. The scN₂ content has a significant effect on cell diameter and cell density. When the scN₂ content was 0.6%, the weld line width of the foamed part was 13.03 μm, while it was 30.41 μm for the solid counterpart due to the expansion and rupture of cells in the flow front during filling. Moreover, the orientation of Al flakes was mostly along the flow direction for the foamed parts, while it was generally aligned perpendicular to the flow direction for solid parts in the weld line region. In addition, the flexural modulus of foamed parts was increased by 29% compared with the solid parts, although the tensile strength was reduced by 18% due to the alignment of Al flakes and the stress concentration on the cell walls. Therefore, this work provides insight into the improvement of flexural property and the mitigation of weld lines for injection molded composite parts using MIM.     

The cell forming process of microcellular injection‐molded parts

In this article, we studied the cell forming process of microcellular injection‐molded parts. Using a modified injection molding machine equipped with a Mucell^® SCF delivery system, microcellular‐foamed acrylonitrile–butadiene–styrene parts with different shot sizes were molded. The cell structure on the fractured surfaces along the direction both vertical and parallel to melt flow in the molded parts was examined. The results showed that a regular spherical cells region and a distorted ellipsoidal cells region exist in the molded parts simultaneously. The length of the distorted cells region along the melt flow direction in the molded parts remained basically unchanged for different shot sizes and it is about 195 mm away from the flow front in this study's conditions. The cell formation mechanism was analyzed, two cell forming processes in microcellular injection molding, the “foam during filling” process and the “foam after filling” process, were proposed. It was also found that the melt pressure in the filling stage is the dominant factor affecting the cell forming process, and there is a critical melt pressure value in the filling stage, 20.9 MPa, as the dividing line of the two cell forming processes in this study. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40365.     

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.     

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.     

Development and processing of flame retardant cellulose acetate compounds for foaming applications

Cellulose acetate (CA) is a bio-based polymeric material suitable to replace foamed polystyrene (PS) boards in applications for building insulation. Foam boards can be produced by extrusion foaming with physical blowing agents. In addition, the high heat deflection temperature and good mechanical properties (e.g., tensile and compression strength) of CA make it suitable for the injection molding of technical parts. In general, flame retardancy of foamed products is often required in building or electronic applications. This article presents the effects of various flame retardant (FR) additives, process settings, and the calibration of the foam board on flammability, foam morphology, and mechanical properties of extruded CA boards. Different formulations of FR additives and foaming agents were investigated regarding density and morphology of the foamed boards. Furthermore, investigations on foam behavior for foam injection molding with physical blowing agents were conducted. The foamed parts were characterized with regard to their flammability. © 2019 The Authors. Journal of Applied Polymer Science published by Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48863.     

Integrated process of supercritical CO2-assisted melt polycondensation modification and foaming of poly(ethylene terephthalate)

An integrated process of melt polycondensation modification and foaming of poly(ethylene terephthalate) (PET) was performed in a high pressure vessel assisted by supercritical carbon dioxide (scCO₂). ScCO₂ was firstly employed to sweep PET melt, i.e., high pressure CO₂ continuously flows through the vessel at a fixed flow rate to remove small molecules for higher molecular weight PET, then this modified PET melt was directly foamed through a rapid depressurization process using scCO₂ as blowing agent. In this integrated process, PET with high melt strength after polycondensation modification could be foamed directly in molten state. Therefore, re-molten process of solid modified PET pellets was canceled to avoid its degradation and CO₂ saturation time could be saved in foaming process, thus processing time could be shortened and energy efficiency could be improved. The influences of scCO₂ sweeping treatment time, pressure and flow rate on properties of the modified PETs and cell morphologies of the foamed PETs were investigated respectively. The results showed that CO₂ sweeping treatment could effectively enhance PET melt polycondensation modification process, which was superior to that of N₂ treatment. PET foams with average cell diameter of 32–62 μm and cell density of 1 × 10⁷ to 4 × 10⁷ cells/cm³ have been obtained in the integrated process. Compared with the process of only foaming modified PET by scCO₂ or performing scCO₂ assisted modified PET further melt polycondensation process and scCO₂ foaming process separately, this integrated process produced better cell morphology.     

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