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     

Solid‐state microcellular polycarbonate foams. I. The steady‐state process space using subcritical carbon dioxide

The process parameters for production of solid‐state microcellular polycarbonate using subcritical CO₂ were explored. Sufficiently long foaming times were used to produce foams, where cell growth had completed, resulting in steady‐state structures. A wide range of foaming temperatures and saturation pressures below the critical pressure of CO₂ were investigated, establishing the steady state process space for this polymer–gas system. Processing conditions are presented that produce polycarbonate foams where both the foam density and the average cell size can be controlled. The process space showed that we could produce foams at a constant density, while varying the cell size by and order of magnitude. At a relative density of 0.5, the average cell size could be varied from 4 to 40 μm. The ability to produce such a family of foams opens the possibility to explore the effect of microstructure, like cell size on the properties of cellular materials. It was found that the minimum foaming temperature for a given concentration of CO₂, determined from the process space, agrees well with the predicted glass transition temperature of the gas–polymer solution. A characterization of the average cell size, cell size distribution, and cell nucleation density for this system is also reported. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

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     

Temperature influence and CO2 transport in foaming processes of poly(methyl methacrylate)–block copolymer nanocellular and microcellular foams

Fabricated by high-pressure or supercritical CO₂ gas dissolution foaming process, nanocellular and microcellular polymer foams based on poly(methyl methacrylate) (PMMA homopolymer) present a controlled nucleation mechanism by the addition of a methylmethacrylate–butylacrylate–methylmethacrylate block copolymer (MAM), leading to defined nanocellular morphologies templated by the nanostructuration of PMMA/MAM precursor blends. Influence of the CO₂ saturation temperature on the foaming mechanism and on the foam structure has been studied in 90/10 PMMA/MAM blends and also in the neat (amorphous) PMMA or (nanostructured) MAM polymers, in order to understand the role of the MAM nanostructuration in the cell growth and coalescence phenomena. CO₂ uptake and desorption measurements on series of block copolymer/homopolymer blend samples show a competitive behavior of the soft, rubbery, and CO₂-philic block of PBA (poly(butyl acrylate)) domains: fast desorption kinetics but higher initial saturation. This competition nevertheless is strongly influenced by the type of dispersion of PBA (e.g. micellar or lamellar) and a very consequent influence on foaming.CO₂ sorption and desorption were characterized in order to provide a better understanding of the role of the block copolymer on the foaming stages. Poly(butyl acrylate) blocks are shown to have a faster CO₂ diffusion rate than poly(methyl methacrylate) but are more CO₂-philic. Thus gas saturation and cell nucleation (heterogeneous) are more affected by the PBA block while cell coalescence is more affected by the PMMA phases (in the copolymer blocks + in the matrix).     

Microstructure and Properties of Microcellular Poly (phenylene sulfide) Foams by Mucell Injection Molding

A series of microcellular poly (phenylene sulfide) (PPS) foams were prepared by Mucell injection molding. The cell structure, mechanical properties, crystallization behavior and dielectric property of microcellular PPS foams were systemically investigated. The results showed that the longer the length of flow passage of injection mold, the larger cell size of microcellular PPS foams. The injection parameter of shot size played an important role in relative density of microcellular PPS foams. When the relative density of microcellular PPS foam reached to 0.658, the tensile strength, flexural strength and impact strength of PPS foam materials achieved 10.82 MPa, 52.99 MPa and 0.305 J/cm², respectively. Meanwhile, with the relative density decreasing, the dielectric constant of PPS foam materials reduced, while the volume resistivity of its uprated.     

Microcellular extrusion of PLA utilizing solid‐state nucleation in the gas‐saturated pellet extrusion process

In this study, we explore the use of solid‐state nucleation in polymer pellets as a means to create microcellular PLA foams in extrusion. This is achieved by using gas‐saturated PLA pellets as input to the extruder. Foam density, bubble size, and bubble density is reported and compared with microcellular foams created in the gas‐injection extrusion process. PLA pellet gas concentrations between 17 and 29 mg CO₂/g PLA was found to produce quality microcellular foams in this process. Gas concentrations within this range were achieved by varying methods that included partial saturation, desorption from full saturation, and blending saturated with unsaturated pellets. This gas concentration window that produced microcellular foams was found to be independent of the saturation and desorption process used to achieve the desired concentration. We further compare the pressure drop and pressure drop rate of the gas‐saturated pellet extrusion process showing that similar foams can be produced at pressures orders of magnitude lower than the alternative gas‐injection extrusion processes. Investigations into extrusion pressures support the hypothesis that the gas‐saturated pellet extrusion process utilizes solid‐state nucleation in the feed section of the extruder to achieve high bubble density foams. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Microcellular foaming of low‐density polyethylene using nano‐CaCo3 as a nucleating agent

In this study, microcellular foaming of low‐density polyethylene (LDPE) using nano‐calcium carbonate (nano‐CaCO₃) were carried out. Nanocomposite samples were prepared in different content in range of 0.5–7 phr nano‐CaCO₃ using a twin screw extruder. X‐ray diffraction and scanning electron microscopy (SEM) were used to characterize of LDPE/nano‐CaCO3 nanocomposites. The foaming was carried out by a batch process in compression molding with azodicarbonamide (ADCA) as a chemical blowing agent. The cell structure of the foams was examined with SEM, density and gel content of different samples were measured to compare difference between nanocomposite microcellular foam and microcellular foam without nanomaterials. The results showed that the samples containing 5 phr nano‐CaCO₃ showed microcellular foam with the lowest mean cell diameter 27 μm and largest cell density 8 × 10⁸ cells/cm³ in compared other samples. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Cell morphology and mechanical properties of microcellular mucell® injection molded polyetherimide and polyetherimide/fillers composite foams

Microcellular polyetherimide (PEI) foams were prepared by microcellular injection molding using supercritical nitrogen (SC‐N₂) as foaming agent. The effects of four different processing parameters including shot size, injection speed, SC‐N₂ content, and mold temperature on cell morphology and material properties were studied. Meanwhile, multiwalled carbon nanotube (MWCNT), nano‐montmorillonoid (NMMT), and talcum powder (Talc) were introduced into PEI matrix as heterogeneous nucleation agents in order to further improve the cell morphology and mechanical properties of microcellular PEI foams. The results showed that the processing parameters had great influence on cell morphology. The lowest cell size can reach to 18.2 μm by optimizing the parameters of microcellular injection molding. Moreover, MWCNT can remarkably improve the cell morphology of microcellular PEI foams. It was worth mentioning that when the MWCNT content was 1 wt %, the microcellular PEI/MWCNT foams displayed optimum mechanical properties and the cell size decreased by 28.3% compared with microcellular PEI foams prepared by the same processing parameters. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 4171–4181, 2013     

Solid‐state microcellular high temperature vulcanized (HTV) silicone rubber foam with carbon dioxide

A series of microcellular high temperature vulcanized (HTV) silicone rubber foams were prepared using CO₂ as a physical blowing agent. Rheological properties, gas diffusive behavior, and foaming parameters of silicone rubber were investigated. The results show that saturation pressure has a significant effect on the diffusivity of CO₂ in HTV silicone rubber matrix. The gas concentration and diffusivity increase from 2.45 wt % to 3.24 wt %, and from 1.62 × 10^?5 cm²/s to 7.83 × 10^?5 cm²/s as the saturation pressure increases from 2 MPa to 5 MPa, respectively. The value of the gas diffusivity in HTV silicone rubber is almost 1000 times higher than that of the gas diffusivity in polyetherimide (PEI) matrix. Additionally, microcellular HTV silicone rubber foams with the smallest cell diameter of 9.8 μm and cell density exceeding 10⁸ cells/cm³ are achieved. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134 , 44807.     

Rheological properties of injection molded LDPE and mPE foams

The rheological properties of molten LDPE and mPE foams have been measured for small‐amplitude oscillatory shear flow. The foam samples were prepared by injection molding, and the effect of injection conditions on the resultant cell structure is discussed. In all cases cellular foams with closed cells were obtained with cell densities in the range of 4 × 10⁵ ? 7 × 10⁶ cells/cm³ and cell diameters in the range of 30–110 μm. The viscoelastic behavior of the foams is shown to be well described by the emulsion model proposed by Palierne (1) without using any fitting parameter. The linear viscoelastic properties of LDPE and mPE foams depend only on the properties of the polymer matrix and of the gas volume fraction. The Palierne model is also used to predict the linear properties of microcellular foams. Polym. Eng. Sci. 44:2158–2164, 2004. © 2004 Society of Plastics Engineers.     

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     

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     

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     

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.     

Preparation of open‐cell foams from polymer blends by supercritical CO2 and their efficient oil‐absorbing performance

This letter reports on the hydrophobicity and oleophilicity of open‐cell foams from polymer blends prepared by supercritical CO₂. A typical bulk density of the foam is measured to be 0.05 g/cm³. The contact angle of the foam with water is determined to be 139.2°. The foam can selectively absorb the diesel from water with the uptake capacity of 17.0 g/g. The foams are technologically promising for application of oil spill cleanup. © 2016 American Institute of Chemical Engineers AIChE J, 62: 4182–4185, 2016     

Controlling the foam morphology of supercritical CO2-processed poly(methyl methacrylate) with CO2-philic hybrid nanoparticles

Highly CO₂-philic nanoparticles, octatrimethylsiloxy polyhedral oligomeric silsesquioxanes (POSS) are used to increase the affinity of poly(methyl methacrylate) (PMMA) to CO₂ in supercritical carbon dioxide (scCO₂) foaming, thus to improve its foaming performance and the foam morphology. PMMA and PMMA-POSS composite foams were produced based on the two-factorial design, at the upper and lower experimental conditions of pressure, temperature, processing time, and venting rate. The foams of PMMA-5% POSS composites exhibited smaller average pore sizes and higher pore densities than neat PMMA and PMMA-0.5% POSS composites. The smallest average pore diameter (0.3 μm) and the highest pore density (6.33 × 10¹² cm⁻³) were obtained with this composite processed at 35°C, 32 MPa, for 24 h and depressurized with fast-venting rate (0.4 MPa/s). ScCO₂ processing decreased the density of the polymer by more than 50%.     

A simple method for preparation of polymer microcellular foams by in situ generation of supercritical carbon dioxide from dry ice

In this work, poly(methyl methacrylate) (PMMA) and PMMA/nanoclay nanocomposite microcellular foams were successfully prepared using a simple method based on in situ generation of supercritical carbon dioxide (CO₂) from dry ice. The method was compared with conventional methods exempted from high pressure pump and a separate CO₂ tank. Effect of various processing conditions such as saturation temperature and pressure and clay concentration on cellular morphology and hardness of the prepared microcellular foams was examined. State of the clay dispersion in the prepared PMMA/clay nanocomposites was characterized using X-ray diffraction and transmission electron microscopy techniques. Field emission scanning electron microscopy was used to study cellular morphology of the prepared foams. It was observed that elevation of saturation temperature from 85 to 105 °C at constant saturation pressure increased cell density and decreased average cell size of the prepared PMMA foams. Furthermore, an increase in saturation pressure from 120 to 180 bar resulted in a reduction in average cell diameter and an increase in cell density of the prepared PMMA foams. On the basis of the gathered results, optimum conditions for preparation of PMMA microcellular foams were determined and applied for preparation of PMMA/nanoclay microcellular foams. It was shown that incorporation of clay into the polymer matrix resulted in a finer and more uniform cellular morphology in the final microcellular foams. It was also observed that incorporation of nanoclay into the prepared foams, up to 3 wt%, led to a moderate increase in the foam hardness.     

Nanoplatelet reinforcement of cavity cell walls in polymer foams using carbon dioxide supercritical fluid

Reinforcing the cavity cell walls of polymer foams using nanoparticles can offer a new era for the property‐structure‐processing field in the development of functionalized ultra‐light components and devices manufactured from foam. When the nanoparticles are exfoliated in polymers, the viscosity substantially increases and thus mixing or foaming usually becomes almost impossible. We use CO₂ supercritical fluid (CO₂ SCF) for the mixing and foaming of poly(ethylene‐vinyl acetate) copolymer (EVA) with montmorillonite (MMT) nanoplatelets. The in situ evaporation of CO₂ induces robust cavity cells of the EVA/MMT nanocomposite foam in a stable form of spherical shapes, which are seldom achieved by other methods. As the bubble grows and becomes stabilized in CO₂ SCF, the exfoliated MMT nanoparticles are aligned at the cell walls by the Gibbs adsorption principle to minimize the surface energy at the gas–liquid interface and increase the rupture strength of the cavity walls. It is demonstrated that the developed methodology can be successfully used for foaming EVA containing high vinyl acetate (VA) content (>40%). Since EVA is too soft to construct cell walls of foam using conventional methods, the applicability of the developed methodology is extensively broadened for superior adhesion and compatibility with other materials. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46615.     

Preparation of microcellular poly(ethylene‐co‐octene) rubber foam with supercritical carbon dioxide

In the past 3 decades, there has been great advancement in the preparation of microcellular thermoplastic polymer foams. However, little attention has been paid to thermoplastic elastomers. In this study, microcellular poly(ethylene‐co‐octene) (PEOc) rubber foams with a cell density of 2.9 × 10¹⁰ cells/cm³ and a cell size of 1.9 μm were successfully prepared with carbon dioxide as the physical blowing agent with a batch foaming process. The microcellular PEOc foams exhibited a well‐defined, closed‐cell structure, a uniform cell size distribution, and the formation of unfoamed skin at low foaming temperatures. Their difference from thermoplastic foam was from obvious volume recovery in the atmosphere because of the elasticity of the polymer matrix. We investigated the effect of the molecular weight on the cell growth process by changing the foaming conditions, and two important effect factors on the cell growth, that is, the polymer matrix modulus/melt viscoelastic properties and gas diffusion coefficient, were assessed. With increasing molecular weight, the matrix modulus and melt viscosity tended to increase, whereas the gas solubility and diffusion coefficient decreased. The increase in the matrix modulus and melt viscosity tended to decrease the cell size and stabilize the cell structure at high foaming temperatures, whereas the increase in the gas diffusion coefficient facilitated cell growth at the beginning and limited cell growth because most of the gas diffused out of the polymer matrix during the long foaming times or at high foaming temperatures. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Preparation and morphology characterization of microcellular styrene‐co‐acrylonitrile (SAN) foam processed in supercritical CO2

Microcellular polymeric foam structures have been generated using a pressure‐induced phase separation in concentrated mixtures of supercritical CO₂ and styrene‐co‐acrylonitrile (SAN). The process typically generates a microcellular core structure encased by a non‐porous skin. Pore growth occurs through two mechanisms: diffusion of CO₂ from polymer‐rich regions into the pores and also through CO₂ gas expansion. The effects of saturation pressure, temperature and swelling time on the cell size, cell density and bulk density of the porous materials have been studied. Higher CO₂ pressures (hence, higher fluid density) provided more CO₂ molecules for foaming, generated lower interfacial tension and viscosity in the polymer matrix, and thus produced lower cell size but higher cell densities. This trend was similar to what was observed in swelling time series. While the average cell size increased with increasing temperature, the cell density decreased. The trend of bulk density was similar to that of cell size. © 2000 Society of Chemical Industry