Effect of glass transition temperature and saturation temperature on the solid‐state microcellular foaming of cyclic olefin copolymer

Effect of glass transition temperature and saturation temperature on the solid‐state microcellular foaming of cyclic olefin copolymer (COC)—including CO₂ solubility, diffusivity, cell nucleation, and foam morphology—were investigated in this article. COCs of low T_g (78°C) and high T_g (158°C) were studied. Solubilities are 20–50% higher in high T_g COC than in the low T_g COC across the saturation temperature range. Diffusivities are about 15% higher on average in high T_g COC for temperatures up to 50°C. A much faster increase of diffusivity beyond 50°C is observed in low T_g COC due to it being in the rubbery state. Under similar gas concentration, high T_g COC starts foaming at a higher temperature. And the foam density decreases faster in low T_g COC with foaming temperature. Also, high T_g COC foams show about two orders of magnitude higher cell nucleation density than the low T_g COC foams. The effect of saturation temperature on microcellular foaming can be viewed as the effect of CO₂ concentration. Nucleation density increases and cell size decreases exponentially with increasing CO₂ concentration. Uniform ultramicrocellular structure with an average cell size of 380 nm was created in high‐T_g COC. A novel hierarchical structure composed of microcells (2.5 μm) and nanocells (cell size 80 nm) on the cell wall was discovered in the very low‐density high‐T_g COC foams. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42226.     

Measurement and prediction of LDPE/CO2 solution viscosity

When CO₂ is dissolved into a polymer, the viscosity of the polymer is drastically reduced. In this paper, the melt viscosities of low‐density polyethylene (LDPE)/supercritical CO₂ solutions were measured with a capillary rheometer equipped at a foaming extruder, where CO₂ was injected into a middle of its barrel and dissolved into the molten LDPE. The viscosity measurements were performed by varying the content of CO₂ in the range of 0 to 5.0 wt% and temperature in the range of 150°C to 175°C, while monitoring the dissolved CO₂ concentration on‐line by Near Infrared spectroscopy. Pressures in the capillary tube were maintained higher than an equilibrium saturation pressure so as to prevent foaming in the tube and to realize single‐phase polymer/CO₂ solutions. By measuring the pressure drop and flow rate of polymer running through the tube, the melt viscosities were calculated. The experimental results indicated that the viscosity of LDPE/CO₂ solution was reduced to 30% of the neat polymer by dissolving CO₂ up to 5.0 wt% at temperature 150°C. A mathematical model was proposed to predict viscosity reduction owing to CO₂ dissolution. The model was developed by combining the Cross‐Carreau model with Doolittle's equation in terms of the free volume concept. With the Sanchez‐Lacombe equation of state and the solubility data measured by a magnetic suspension balance, the free volume fractions of LDPE/CO₂ solutions were calculated to accommodate the effects of temperature, pressure and CO₂ content. The developed model can successfully predict the viscosity of LDPE/CO₂ solutions from PVT data of the neat polymer and CO₂ solubility data.     

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     

Effect of die temperature on the morphology of microcellular foams

A study on the extrusion of microcellular polystyrene foams at different foaming temperatures was carried out using CO₂ as the foaming agent. The contraction flow in the extrusion die was simulated with FLUENT computational fluid dynamics code at two temperatures (150°C and 175°C) to predict pressure and temperature profiles in the die. The location of nucleation onset was determined based on the pressure profile and equilibrium solubility. The relative importance of pressure and temperature in determining the nucleation rate was compared using calculations based on classical homogeneous nucleation theory. Experimentally, the effects of die temperature (i.e., the foaming temperature) on the pressure profile in the die, cell size, cell density, and cell morphology were investigated at different screw rotation speeds (10 ～ 30 rpm). Experimental results were compared with simulations to gain insight into the foaming process. Although the foaming temperature was found to be less significant than the pressure drop or the pressure drop rate in deciding the cell size and cell density, it affects the cell morphology dramatically. Open and closed cell structures can be generated by changing the foaming temperature. Microcellular foams of PS (with cell sizes smaller than 10 μm and cell densities greater than 10 cells/cm³) are created experimentally when the die temperature is 160°C, the pressure drop through the die is greater than 16 MPa, and the pressure drop rate is higher than 10⁹ Pa/sec.     

The sorption behaviors in PLLA-CO<Subscript>2</Subscript> system and its effect on foam morphology

The sorption behavior, physical properties, and foam morphologies in poly(_L-lactic acid) (PLLA)-CO₂ system were studied in this paper. The solubility and diffusion coefficient of CO₂ in PLLA in the range of 0 °C to 45 °C and pressure up to 5.5 MPa were investigated. The diffusion coefficients were analyzed to determine the plasticization glass transition temperature (T _g ) of the PLLA-CO₂ systems. The data of T _g s of PLLA at various CO₂ pressures demonstrated that PLLA-CO₂ system exhibited a retrograde vitrification behavior, which has never been reported on semi-crystalline polymer-gas system by systematic measurement of solubility data. The sorption curves of PLLA at certain temperatures and pressures exhibited a characteristic keen which indicated the rejection of CO₂ from the polymer matrix due to CO₂-induced crystallization. The fundamental understanding of PLLA-CO₂ interactions was utilized to control the CO₂ solubility and crystallinity in PLLA thus the physical properties, in order to develop various unique foam structures.     

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%.     

Foaming of linear isotactic polypropylene based on its non-isothermal crystallization behaviors under compressed CO2

The non-isothermal crystallization behaviors of isotactic polypropylene (iPP) under ambient N₂ and compressed CO₂ (5–50 bar) at cooling rates of 0.2–5.0 °C/min were carefully studied using high-pressure differential scanning calorimeter. The presence of compressed CO₂ had strong plasticization effect on the iPP matrix and retarded the formation of critical size nuclei, which effectively postponed the crystallization peak to lower temperature region. On the basis of these findings, a new foaming strategy was utilized to fabricate iPP foams using the ordinary unmodified linear iPP with supercritical CO₂ as the foaming agent. The foaming temperature range of this strategy was determined to be as wide as 40 °C and the upper and lower temperature limits were 155 and 105 °C, which were determined by the melt strength and crystallization temperature of the iPP specimen under supercritical CO₂, respectively. Due to the acute depression of CO₂ solubility in the iPP matrix during the foaming process, the iPP foams with the bi-modal cell structure were fabricated.     

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     

Study of different effect on foaming process of biodegradable bionolle in supercritical carbon dioxide

Microcellular foaming of biodegradable Bionolle in supercritical CO₂ has been produced. The effects of a series of variable factors, such as saturation temperature, saturation pressure, and depressurization time and step on the foam structures and density, were studied through measurement of density and SEM observation. The experimental results show that higher saturation temperatures lead to an increase in bulk densities; and different depressurization time and step result in different product cell morphology. In addition, at some saturation temperature, the orientation of the cells can be found in the product morphology. XRD experimental results show that the foaming treatment with SC CO₂ increased the crystallinity of Bionolle. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2901–2906, 2006     

Synthesis and Assessment of a CO2‐Switchable Foaming Agent Used in Drilling Fluids for Underbalanced Drilling

In underbalanced drilling, a switchable foam fluid is essential to reduce the drilling cost. A switchable foaming agent was synthesized by carbonyl–amine condensation and characterized by Fourier transform infrared and ¹H nuclear magnetic resonance (NMR) spectroscopy. Thermogravimetric analysis and differential scanning calorimetry showed that the tolerable temperature limit of the surfactant was 128 °C. The effectiveness of CO₂/N₂ switching was confirmed by analysis of the electrical conductivity and surface tension. Utilizing the foaming agent, 3 different foam systems (unstable, stable, and hard) were designed for drilling after formula optimization. Experimentally, the self‐circulation indicated that the foaming fluids still maintained great foaming performance even after multiple cycles. The experiment also indicated that the suspension of the foam systems was 50–90 times that of water and had a significant resistance to salts (NaCl, CaCl₂). Besides, the foam systems found that the suitable foaming temperature was 40–100 °C and that the hard foam system could maintain the foaming performance up to 120 °C. In the oil resistance experiment, the foaming ability of the foam systems decreased obviously above a kerosene content of 5% (w/v), whereas a certain foaming performance still could be ensured below 10% kerosene.     

Solubility of the diarrheic shellfish toxin okadaic acid in supercritical CO2

The solubility of okadaic acid (OA) in supercritical CO₂ was measured using a flow-type apparatus with sequential sampling during dynamic nonrecirculating experiments at saturation conditions. Methanol and water were used as solvent modifiers of CO₂. Collected OA was measured by high-performance liquid chromatography with fluorimetric detection after derivatization with 1-bromoacetylpyrene to obtain the labeled ester of the toxin. Solubility results were obtained with methanol concentrations ranging from 0 to 8.5% volume in the CO₂ density range of 0.495 to 0.913 g/mL at 40, 60, and 73°C. Measured solubility of OA ranged from 0 to 15×10⁻⁶ mol/L, increasing with methanol concentration and fluid density and diminishing with temperature. Experiments with water-modified CO₂ up to 0.3% volume (near saturation) were done at 60°C; solubilities of OA up to 5×10⁻⁶ mol/L were measured. This is the first approach to handle the liposoluble diarrheic shellfish toxins with supercritical CO₂. The study, with pure OA, provides useful information regarding the effects of pressure, temperature, and addition of modifiers on its solubility. Obtained results show that the toxin can be solubilized in this media and potential applications are suggested and being currently investigated.     

Phase equilibrium characteristics of supercritical CO2/poly(ethylene terephthalate) binary system

The solubility of carbon dioxide in poly (ethylene terephthalate) (PET) at high pressure and elevated temperature conditions was investigated for a better understanding of the phase equilibrium characteristics of supercritical CO₂/PET binary system and useful data for the process development of the supercritical fluid dyeing. Based on the principle of pressure decaying, a novel experimental apparatus suitable to high pressure and high temperature measurement was established. The solubilities of CO₂ in PET were measured with the apparatus at temperatures of 110, 120, and 130°C and pressures up to 30.0 MPa. The results show that the solubility of CO₂ in PET increases with the increase of pressure and CO₂ density, respectively, at a constant temperature, whereas it decreases with the increase of temperature at a constant pressure. The Sanchez‐Lacombe equation of state (S‐L EOS) was used to correlate the experimental data. The calculated results are in good agreement with the experimental ones. The average absolute relative derivation (AARD) is less than 3.91%. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Preparation and characterization of polystyrene foams from limonene solutions

The foaming process has been traditionally performed at high temperature because the CO₂ and the polymer should behave as a homogeneous solution. The addition of a solvent could avoid the high working temperature while the homogeneity is ensured. Among the terpene oils, limonene outlines as a good candidate to carry out the dissolution of polystyrene because it respects the green chemistry principle, it is highly soluble in CO₂ and very compatible with the polymer.The sorption of CO₂ is the first step of the foaming process. The presence of the terpene oil enhances the solubility of the gas which is solubilized in the Polystyrene as well as in the limonene. During the foaming process, many parameters can be tuned to customize the foams. In this work, a fractional factorial design of experiment was proposed to determine the effect of pressure, temperature, concentration of the solution, contact time and vent time over the diameter of cells, its standard deviation and the cells density. The proposed foaming process can be simply performed at mild pressure and temperature thanks to the presence of the solvent. The results showed that the most suitable conditions to foam polystyrene from limonene solutions are 90 bar, 30 °C, 0.1 gPS/ml Lim, 240 min contacting and 30 min venting. Finally, the samples were characterized to determine the amount of residual solvent, their glass transition and degradation temperature checking that the foams presented around 5% of solvent traces but did not show any evidence of degradation.     

CO2 foaming of poly(ethylene glycol)/polystyrene blends: Relationship of the blend morphology,CO2 mass transfer,and cellular structure

When polymer blends are foamed by physical foaming agents, such as CO₂ or N₂, not only the morphology and viscosity of the blend polymers but also the solubility and diffusivity of the physical foaming agents in the polymers determine the cellular structure: closed cell or open cell and monomodal or bimodal. The foam of poly(ethylene glycol) (PEG)/polystyrene (PS) blends shows a unique bimodal (large and small) cellular structure, in which the large‐size cells embrace a PEG particle. Depending on the foaming condition, the average size of the large cells ranges from 40 to 500 μm, whereas that of small cells becomes less than 20 μm, which is smaller than that of neat PS foams. The formation mechanism of the cellular structure has been investigated from the viewpoint of the morphology and viscosity of the blend polymer and the mass‐transfer rate of the physical foaming agent in each polymer phase. The solubility and diffusivity of CO₂, which determine the mass‐transfer rate of CO₂ from the matrix to the bubbles, were measured by a gravimetric measurement, that is, a magnetic suspension balance. The solubility and diffusivity of CO₂ in PS differed from those in PEG: the diffusion coefficient of CO₂ in PEG at 110°C was 3.36 × 10^?9 m²/s, and that in PS was 2.38 × 10^?10 m²/s. Henry's constant in PEG was 5600 cm³ (STP)/(kg MPa) at 110°C, and that in PS was 3100 cm³ (STP)/(kg MPa). These differences in the transport properties, morphology of the blend, and CO₂‐induced viscosity depression are the control factors for creating the unique cellular structure in PEG/PS blends. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1899–1906, 2005     

Effect of dispersion method and process variables on the properties of supercritical CO2 foamed polystyrene/graphite nanocomposite foam

In this study, polystyrene/nanographite nanocomposite foams were made by different compounding methods, such as direct compounding, pulverized sonication compounding, and in situ polymerization, to understand the effect of the process variables on the morphology of the nanocomposites and their foam. The foam was made by batch foaming using CO₂ as the blowing agent. Various foaming pressures and temperatures were studied. The results indicated that the cell size decreased and the cell morphology was improved with the advanced dispersion of the nanoparticles. Among the three methods, the in situ polymerization method provided the best dispersion and the resulting nanocomposite foam had the finest cell size and the highest cell density. In addition, adding nanoparticles as a nucleating agent can make foams of similar cell size and cell density at a much lower foaming pressure. This result can be explained by the classical nucleation theory. This discovery could open up a newroute to produce microcellular foams at a low foaming pressure. POLYM. ENG. SCI., 53:2061–2072, 2013. © 2013 Society of Plastics Engineers     

Experimental and modeling studies on the solubility of sub‐ and supercritical carbon dioxide (scCO2) in potato starch and derivatives

The solubility of CO₂ in native potato starch (NPS) and potato starch acetate (SA) at two different temperatures (50°C and 120°C) and various pressures (up to 25 MPa) was determined using a magnetic suspension balance. Within the experimental window, a maximum solubility of 31 mg CO₂/g_sample for NPS and 79.4 mg CO₂/g_sample for SA was obtained. The CO₂ sorption behavior is highly depending on the temperature and pressure. The solubility data were modeled with the Sanchez Lacombe equation of state (S‐L EOS). The swelling (S_w) values, as predicted using the S‐L EOS, were relatively small and a maximum value of 6.1% was obtained for SA at 25 MPa and 120°C. POLYM. ENG. SCI., 2011. © 2010 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     

A Study of the Crystallization, Melting, and Foaming Behaviors of Polylactic Acid in Compressed CO2

The crystallization and melting behaviors of linear polylactic acid (PLA) treated by compressed CO₂ was investigated. The isothermal crystallization test indicated that while PLA exhibited very low crystallization kinetics under atmospheric pressure, CO₂ exposure significantly increased PLA’s crystallization rate; a high crystallinity of 16.5% was achieved after CO₂ treatment for only 1 min at 100 °C and 6.89 MPa. One melting peak could be found in the DSC curve, and this exhibited a slight dependency on treatment times, temperatures, and pressures. PLA samples tended to foam during the gas release process, and a foaming window as a function of time and temperature was established. Based on the foaming window, crystallinity, and cell morphology, it was found that foaming clearly reduced the needed time for PLA’s crystallization equilibrium.     

Open-celled microcellular foaming and the formation of cellular structure by a theoretical pattern in polystyrene

An open-celled structure was produced using polystyrene and supercritical carbon dioxide in a novel batch process. The required processing conditions to achieve open-celled structures were predicted by a theoretical model and confirmed by the experimental data. The theoretical model predicts that at least a saturation pressure of 130 bar and a foaming time between 9 and 58 s are required for this system to produce an open-celled structure. The foaming temperature range has been selected to be higher than the polymer glass transition temperature yet not higher than a temperature limit where the gas starts leaving the system. The experimental results in the batch foaming process verified the model substantially. The SEM pictures showed the presence of pores between the cells, and the mercury porosimetry test results verified the overall open-celled structure. Experimental results also showed that by increasing the saturation pressure and the foaming temperature, there was a drop in the time required for open-celled structure formation. At saturation pressure of 130 bar, foaming temperature of 150 °C and a foaming time of 60 s, open-celled microcellular polystyrene foams were obtained using supercritical CO₂ in the batch process. Based on the results, a schematic diagram, depicting the process of foam structure formation from nucleation to bubble coalescence and gas escape from polymer, was proposed. Theoretical calculations showed that by increasing foaming time, cell size was increased and cell density was reduced and the experimental results verified this prediction.     

Solubility and diffusion coefficient of supercritical-CO2 in polycarbonate and CO2 induced crystallization of polycarbonate

The solubility and diffusion coefficient of supercritical CO₂ in polycarbonate (PC) were measured using a magnetic suspension balance at sorption temperatures that ranged from 75 to 175 °C and at sorption pressures as high as 20 MPa. Above certain threshold pressures, the solubility of CO₂ decreased with time after showing a maximum value at a constant sorption temperature and pressure. This phenomenon indicated the crystallization of PC due to the plasticization effect of dissolved CO₂. A thorough investigation into the dependence of sorption temperature and pressure on the crystallinity of PC showed that the crystallization of PC occurred when the difference between the sorption temperature and the depressed glass transition temperature exceeded 40 °C (T − T_g ≥ 40 °C). Furthermore, the crystallization rate of PC was determined according to Avrami's equation. The crystallization rate increased with the sorption pressure and was at its maximum at a certain temperature under a constant pressure.