Batch Foaming Process Research Articles

Facile preparation of open-cellular porous poly (l-lactic acid) scaffold by supercritical carbon dioxide foaming for potential tissue engineering applications

Porous poly (l-lactic acid) (PLLA) scaffold has been widely used as substitutes for tissue engineering. However, PLLA is intrinsically difficult to be produced for low-density and high porosity by physical foaming procedure, due to its weak melt strength, slow crystallization kinetic, and low heterogeneous nucleation efficiency in bulk. For the sake of enhancing the PLLA’s crystallization and heterogeneous nucleation in supercritical CO2 (ScCO2) foaming, a pressure-induced flow (PIF) method was applied on neat PLLA. As a result, the PLLA’s crystallinity and its spherulite size increased from original 15.3% and 5.0nm to 42.1% and 28.5nm, respectively. High-strength and low-density PLLA foams with uniform and controlled cellular morphology were feasibly produced, varying the foaming temperature from 100 to 140°C in a ScCO2 batch foaming process. The closed cellular structure was transformed to high-interconnected porous structure, when reducing the foaming temperature. The open-cellular PLLA scaffolds foams, with a relatively high open pore content of 77.3%, prepared using this approach can achieve a high porosity up to 92.5%, and exhibit excellent compressive stress. To evaluate its tissue engineering application, long-term culture of mouse embryonic fibroblast cells (MEFs) demonstrated that the selected open-cellular PLLA scaffold provided prominent advantages ranging from enhanced cell adhesion and proliferation to facilitated nutrient transport. The combination of PIF and CO2 foaming process can produce a great promising PLLA scaffold for tissue engineering with desirable open-cellular structure, high-strength, low-cost and easy production capacity.

Preparation of polymer foams with a gradient of cell size: Further exploring the nucleation effect of porous inorganic materials in polymer foaming

Polymeric foams with gradient of cell size have shown significant potential in many applications, but, the fabrication of such foams is difficult. By producing blowing agent concentration or temperature gradient inside the sample, some recently developed methods have successfully prepared gradient polymer foams, whereas, precise control system and/or delicate operation are highly needed. Herein, we reported a simple method to obtain cell size gradient in polymer foams by a batch foaming of the polymers on a substrate with nanopore structure. Anodized aluminum oxide (AAO) film modified with a fluorinated silane was used as the substrate. Polymers including polystyrene, polymethyl methacrylate, and polylactic acid were compressed onto the modified AAO film, and then foamed by a batch foaming process using scCO2 as blowing agent. SEM characterizations on the foam morphology revealed that for all the samples, the size of cell near the substrate was smaller and gradually increased along the distance from AAO surface. It is also found that the gradient structure varied with the polymer type. Specifically, polylactic acid foams showed a perfect gradient of cell size from ∼13μm to ∼40μm in a thickness of more than 1.0mm, providing significant potentials in many applications. Moreover, by choosing the substrates with nanopores of different size and/or changing the foaming condition, the foam morphologies including cell size, cell density, and cell size gradient range can be readily tuned.

Complex cellular structure evolution of polystyrene/poly (ethylene terephthalate glycol-modified) foam using a two-step depressurization batch foaming process

In order to decrease the cell size and maintain very high volume expansion ratio simultaneously, a methodology for the preparation of complex cellular structure (CCS) in polystyrene/poly(ethylene terephthalate glycol-modified) (PS/PETG) blend using two-step depressurization pressure batch foaming process was proposed. First, the optimum foaming temperature for PS and PS/PETG blend, respectively, were confirmed by one-step depressurization foaming process. Then, the CCS in PS and PS/PETG blending foam were fabricated by two-step depressurization foaming process at the optimum foaming temperature. The rheological properties of PS and PS/PETG blend were tested by dynamic rotational rheometer. The dispersion morphologies and foam morphologies were observed by scanning electron microscope. The lowest densities of PS and PS/PETG blending foams were obtained at the temperature of 136℃. The interfaces between PS and PETG could act as nucleation sites for the cell nucleation, which were helpful to fabricate the CCS. The CCS could be controlled by tuning the degree of the first-step depressurization and the holding time. The results showed that the large cells could be beneficial to decrease the foam density and the presence of small cells was beneficial to increase the cell number.

Expanded Linear Low-Density Polyethylene Beads: Fabrication, Melt Strength, and Foam Morphology

A linear low-density polyethylene copolymerized with 1-hexene (H-LLDPE) was synthesized by gas-phase polymerization, using the metallocene catalyst systems. The H-LLDPE exhibited longer ethylene average sequence length, narrower molecular weight distribution, and higher melt strength, compared with linear low-density polyethylene copolymerized with 1-butene (B-LLDPE). The expanded polyethylene (EPE) beads were prepared by a batch foaming process using above two LLDPEs as base resins. Both ethylene average sequence length and comonomer content were measured by solid-state nuclear magnetic resonance (NMR). Influences of melt strength and molecular structure on the cellular morphology of EPE are investigated using scanning electron microscopy (SEM) and a melt strength meter, respectively. The influence of saturation temperature and pressure on cell density and morphology are examined by homogeneous and heterogeneous nucleation theory. Results indicate that, with higher melt strength and longer ethylene average sequence length, the demonstrated H-LLDPE facilitates the entire batch-foaming process and yields improved cellular structure of EPE beads over that of B-LLDPE.

Microcellular Foaming of Biodegradable PLA/PPC Composite Using Supercritical CO<sub>2</sub>

Poly (lactic acid) (PLA)/poly (propylene carbonate) (PPC) composite foams were microcellular foamed with CO2 through a batch foaming process. The influences of PPC contents, foaming temperature, and saturation pressure on the cell structure and foam density were investigated. The biodegradable PLA/PPC composite foam showed a controlled structure of microcellular and nanocellular. With an increase in saturation temperature and pressure, the cell size was increasing and both the cell density and foam density were decreased simultaneously.

Effect of Mixing Intensity on Foaming Behavior of LLDPE/HDPE Blends in Thermal Induced Batch Process

ABSTRACTLinear low density polyethylene (LLDPE) and high density polyethylene (HDPE) (with different shear thinning behaviors) were melt blended at two different compositions (10 and 25 wt%. of HDPE) and three different rotational speeds. Batch foaming process was performed over a wide range of temperatures. Bulk density measurement, cell structure evaluation, differential scanning calorimetry (DSC), and oscillatory rheology tests were performed. Results indicated the deterioration of foamability for those blended at the highest selected speed of 120 rpm. The blends mixed at a speed of 60 rpm, exhibited a slightly lower expansion ratio compared with those of mixed at a speed of 10 rpm with the same trend over the foaming temperature. For blends with 25 wt%. HDPE, proper cellular structures were obtained through a broader range of foaming temperature.

Morphologies and electromagnetic interference shielding performances of microcellular epoxy/multi-wall carbon nanotube nanocomposite foams

Microcellular epoxy/multi-wall carbon nanotube (EP/MWCNT) composite foams loaded with 0.5, 1.0, 2.0 and 3.0 wt% of multi-walled carbon nanotubes (MWCNTs) were prepared by a batch foaming process with supercritical carbon dioxide (scCO2). The morphologies of EP/MWCNT composite foams were analyzed by scanning electron microscopy (SEM). It was found that there was a synergic effect between the formation of microcellular structure and the addition of MWCNTs on improving the performances of EP/MWCNT composite foams. The addition of MWCNTs promoted the formation of microcellular structure, and the growth of cells in turn induced the redistribution of MWCNTs. Furthermore, the electromagnetic interference (EMI) shielding effectiveness (SE) of EP/MWCNT solid and foamed composites was investigated. The results indicated that the foaming improved the specific EMI SE from 5.2 to 21.3 dB cm3/g at the MWCNT content of 1.0 wt%. The introduction of MWCNTs and microcellular structure synergistically enhanced the EMI shielding performance of epoxy-based composites.

Enhanced electrical conductivity in graphene-filled polycarbonate nanocomposites by microcellular foaming with sc-CO2

Electrically conductive polycarbonate (PC) foams containing a low concentration of graphene nanoplatelets (0.5 wt.%) were produced with variable range of expansion ratio by applying a high-pressure batch foaming process using sc-CO2. The structure of the foams was assessed by means of SEM, AFM and WAXS, and the electrical conductivity was measured in the foam growing direction. Results showed that electrical conductivity of PC composite foams remarkably increased when compared to that of non-foamed PC composite, with both the electrical conductivity and the main cell size of the foams being directly affected by the resultant expansion ratio of the foam. This interesting result could be explained by the development of an interconnected graphene nanoparticle network composed by increasingly well-dispersed and reoriented graphene nanoplatelets, which was developed into the solid fraction of the foam upon foaming by sudden depressurising of the plasticised CO2-saturated PC preform. Some evidences of morphological changes in the graphene nanoplatelets after foaming were obtained by analysing variations in graphene’s (0 0 2) diffraction plane, whose intensity decreased with foaming. A reduction of the average number of layers in the graphene nanoplatelets was also measured, both evidences indicating that improved dispersion of graphene nanoparticles existed in the PC composite foams. As a result, foams with a proper combination of low density and enhanced electrical conductivity could be produced, enabling them to be used in applications such as electromagnetic interference shielding.

The effect of pressurized and fast stabilization on one step batch foaming process for the investigation of cell structure formation

Control of cell structure during pressure drop is essential for the investigation of cell formation mechanism. In this work, a one step batch foaming setup was totally redesigned to stabilize foam structure at initial stages of cell forming during and after depressurization as a novel idea. The saturation pressure, duration of pressure drop and foaming temperatures were 18.5MPa, 100ms and 70, 90, 110°C, respectively. The shortest stabilized foaming time was 5ms and the longest was 5000ms. For comparison purposes non-stabilized samples were also produced. It was concluded that the effect of stabilization on foam morphology is dependent on the foaming temperature. Foam structure at foaming temperatures of 70°C did not change with change of foaming time significantly. Foam structure at foaming temperatures of 110°C for the shortest and the longest foaming times changed from 90μm to 170μm and 7E+06cell/cm3 to 8E+06cell/cm3.

Structural properties of batch foamed acrylonitrile butadiene styrene/nanoclay nanocomposites

Present study investigates the effect of adding nanoclay particles and foaming conditions on cellular properties of ABS/Nanoclay nanocomposite foams. For this purpose, first nanocomposite components are mixed using a twin-screw extruder. The amount of added nanoclay is considered 2 and 4 percentage by weight. Also, poly methyl methacrylate (PMMA) with the percentages of 2 and 4 wt% are used as the compatibilizer. Distribution of nanoclay particles in ABS matrix is examined using X-ray diffraction test. Then, nanocomposite samples are produced using injection molding process and foamed using batch foaming process. CO2 gas is used as the physical blowing agent of the foaming process. The saturation time of samples and the effect of adding of nanoclay on CO2 sorption/desorption rate was assessed by gravimetric method prior to foaming experiments. Then, nanocomposite samples are foamed under different process conditions according to a Taguchi L9 orthogonal array. The obtained results reveal that adding nanoclay increases the cell density and decreases both foam density and cell size of ABS/nanoclay composite foams compared to the pure ABS foams. Results show that the structural properties of nanocomposite foams depend on both processing parameters and percentage of nanoclay in the nanocomposite specimens.

Experimental and simulation study of the physical foaming process using high-pressure CO2

The simultaneous simulation of nucleation and growth of multiple bubbles was performed for a batch foaming process of low-density polyethylene (LDPE), polypropylene (PP) and polystyrene (PS) using CO2 as a foaming agent. The simulation model involved a Blander–Katz bubble nucleation rate equation and a set of bubble growth rate equations that account for the momentum balance across the bubble interface, the diffusion of dissolved gas molecules in a polymer, and the overall mass balance of the gas in bubbles and in the polymer. The present study considered the overlapping of the concentration boundary layers. The simulation results could predict the experimental results of the bubble nucleation and bubble growth behaviors of the polymers in a fairly quantitative manner. Moreover, the effects of the physical properties on bubble nucleation and bubble growth dynamics were demonstrated in a series of sensitivity studies. The effect of interfacial tension and the diffusion coefficient strongly affected the bubble nucleation and growth rates, respectively.

Formation mechanism and tuning for bi-modal cell structure in polystyrene foams by synergistic effect of temperature rising and depressurization with supercritical CO2

Polystyrene (PS) foams are fabricated using supercritical carbon dioxide (Sc-CO2) as a physical foaming agent in a batch foaming process. It is demonstrated that a bi-modal cell structure (BMCS) is obtained by the synergistic effect of temperature rising and depressurization using a varying-temperature mode (VTM). The formation mechanism for the BMCS is revealed. Cell nucleation occurs in the supercritical CO2-saturated polymer both in temperature rising and depressurizing stages. The nuclei formed in the former stage develop into large cells, whereas the ones formed in the latter stage evolve into small cells. The effects of foaming temperature, saturation temperature, second saturation time and foaming pressure in the VTM on the cellular structure of PS foams were investigated. The results showed that no BMCS is formed when the foaming temperature is only 10°C higher than saturation temperature. By manipulating the foaming parameters, the BMCS PS foams with a wide range of mean diameters (50.0–713.7μm) and densities (2.1×103–3.6×106cells/cm3) for the large cells, and a wide range of foam density (0.09–0.73g/cm3) are obtained.

Thermal Properties and Morphology Characteristics of a PP Mono-Filament and its Foam

Abstract The polypropylene (PP) mono-filament is an alternative material that can be used for insulation, packaging, and polymer composite applications. In this study, PP mono-filaments were prepared by extrusion free-fall at temperatures of 190, 200, and 210°C through a micro extrusion die with a diameter of 900 μm. The free-fall speeds of the filament were controlled by a gear pump driving at speeds ranging from 12.5 to 20.0 min−1, and the extrudates were cooled at room temperature. The PP mono-filaments were subsequently foamed by a batch foaming process with sc-CO2 as the blowing agent at foaming temperatures of 165, 170, 175 and 180°C. The thermal properties and crystallinity of the prepared filaments and their foams were investigated using a DSC. The foam morphology and isotropic foam properties were characterized by SEM micrographs. The results show that the various morphologies of the PP mono-filament foams deviated from the isotropic foam behavior and had isotropic foam index values in the range of 1.1 to 2.3. In addition, the PP mono-filament that was foamed at 175°C expanded in the longitudinal direction and exhibited the highest expansion ratio, but its degree of crystallinity decreased compared to the mono-filaments. PP mono-filament foam represented a narrow crystallization temperature range and crystallization time than mono-filament and tended to a faster crystalline growth.

Melt foamability of reactive extrusion‐modified poly(ethylene terephthalate) with pyromellitic dianhydride using supercritical carbon dioxide as blowing agent

By reactive extrusion with pyromellitic dianhydride (PMDA), foamable poly(ethylene terephthalate) (PET) was obtained, which achieved a maximum intrinsic viscosity of 1.36 dL/g with PMDA content 0.8 wt%. Dynamic shear rheological properties were measured to characterize the structure evolution of modified PET. And the Avrami analysis was extended for the non‐isothermal crystallization process of modified PET, which relates to cell stabilization in the melt foaming process. Based on the batch foaming process with supercritical carbon dioxide as blowing agent, broad foaming temperature windows were obtained for PETs modified with 0.8 and 0.5 wt% PMDA, in which PET foams with the expansion ratio between 10 and 50 times, the cell diameter between 15 and 37 μm, and the cell density between 6.2 × 108 and 1.6 × 109 cells/cm3 were controllably produced. POLYM. ENG. SCI., 55:1528–1535, 2015. © 2014 Society of Plastics Engineers

Investigation on the foaming behaviors of NC-based gun propellants

Abstract To prepare the porous NC-based (nitrocellulose-based) gun propellants, the batch foaming process of using supercritical CO 2 as the physical blowing agent is used. The solubilities of CO 2 in the single-base propellants and TEGDN (trimethyleneglycol dinitrate) propellants are measured by the gravimetric method, and SEM (scanning electron microscope) is used to observe the morphology of foamed propellants. The result shows that a large amount of CO 2 could be dissolved in NC-based propellants. The experimental results also reveal that the energetic plasticizer TEGDN exerts an important influence on the pore structure. The triaxial tensile failure mechanism for solid-state nucleation is used to explain the nucleation of NC-based propellants in the solid state. Since some specific foaming behaviors of NC-based propellants can not be explained by the failure mechanism, a solid-state nucleation mechanism which revises the triaxial tensile failure mechanism is proposed and discussed.

Effects of enhanced compatibility by transesterification on the cell morphology of poly(lactic acid)/ polycarbonate blends using supercritical carbon dioxide

Poly(lactic acid)/polycarbonate blends were prepared by melt mixing. The transesterification reaction between poly(lactic acid) and polycarbonate was promoted by using tetrabutyl titanate as a catalyst. For poly(lactic acid)/polycarbonate (weight ratio of 75/25) blend with catalyst content up to 0.5 wt%, polycarbonate particles finely dispersed in the poly(lactic acid) matrix and the adhesion between the phases were improved, due to the enhanced compatibility by transesterification reaction. The blends were foamed by using batch-foaming process with CO2 as the blowing agent. Cell density of poly(lactic acid)/polycarbonate blend increased at low-catalyst content, while decreased at high-catalyst content, which was due to the changes of interfacial properties of blend phases through transesterification reaction and crystallinity of polycarbonate component. Cell types of poly(lactic acid)/polycarbonate blends were transferred from submicro-sized and even nanoscale cells to microscale cells by the deceased crystallinity of poly(lactic acid) component, which was caused by the increased degree of transesterification reaction and temperature. The declined viscosity of poly(lactic acid)/polycarbonate blend because of degradation during blend processing led to large cell size and low-cell density. However, the improvement of elasticity and viscosity of poly(lactic acid)/polycarbonate blend by transesterification reaction could decrease the cell size.

Ultrasound‐induced nucleation in microcellular polymers

ABSTRACTIn this study, microcellular Acrylonitrile–Butadiene–Styrene foams with high cell density and expansion ratio has been manufactured using ultrasound‐induced nucleation technique in solid‐state batch foaming process. Influence of sonication time, sonication frequency, and ultrasound power were found very crucial in designing of cellular morphology. The initial 10 s of ultrasound exposure was found to influence the foam morphology critically. Longer periods of ultrasound exposure developed foams with lower average cell size as compared to foams developed with lesser ultrasound exposure time. Higher sonication power resulted in foams with uniform morphology and higher cell densities as compared to foams developed with lower sonication intensities. Finally, the ultrasonic frequency was also found to influence the morphology intensely. Low frequency sonication resulted in foams with uniform cell distribution, whereas high frequency sonication developed bimodal microcellular type of microstructure. The results coherently demonstrate that with the advent of ultrasonic waves, the energy barrier for cell nucleation swiftly decreases which enhances the cell density in the final foamed product. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40742.

Effects of saturation temperature/pressure on melting behavior and cell structure of expanded polypropylene bead

The expanded polypropylene beads with microcellular structure were prepared by an autoclave-based batch foaming process using non-supercritical CO2 as the foaming agent. Herein, smaller polypropylene micropellets (∼0.6 mm) were obtained from three kinds of polypropylene resin by underwater micro-pelletizer system. The results from differential scanning calorimetry indicated that the double melting peaks of beads moved to higher temperature with increasing saturation temperature and pressure. The kind of comonomer did not exert noticeable influence on the double melting behaviors. The X-ray diffraction of expanded polypropylene bead revealed that characteristic peaks of α-type crystal did not change compared with that of original polypropylene micropellets. The relationship between cell morphology and saturation temperature/pressure of expanded polypropylene beads are discussed preliminarily by scanning electron microscope.

Preparation and Characteristics of Foamed NC‐Based Propellants

AbstractFoaming properties of the three NC‐based (nitrocellulose‐based) propellants, namely, single‐base propellant, NG (nitroglycerine) propellant and TEGDN (triethylene glycol dinitrate) propellant were investigated in the batch foaming process by using supercritical CO2 as the physical foaming agent. Burning characteristics of the foamed NC‐based propellants were also investigated in this work. For this study, the CO2 desorption of the three NC‐based propellants were plotted by the gravimetric method. The morphology and burning characteristics of these foamed NC‐based propellants were characterized by scanning electron microscope (SEM) and closed vessel experiment. The test data revealed that the energetic plasticizer has a considerable effect on the pore formation in the NC matrix although it has little effect on the CO2 solubility in the NC‐based propellants. Moreover, the SEM images showed the foaming temperature also plays an important role in the pore parameters of foamed propellants. Furthermore, the data of closed vessel experiment indicated that the burning characteristics of foamed NC‐based propellants largely depend on the pore parameters, and the porous structure of foamed propellants would considerably increase the mass conversion rate.

Fabrication and Characterization of PLA/PHBV-Chitin Nanocomposites and Their Foams

This paper examines the effect of biobased chitin nanowhisker fillers on the thermal, rheological, physical, mechanical and morphological properties of biobased polylactic acid (PLA) and PLA/polyhydroxybutyrate-co-valerate (PHBV) blended nanocomposites as well as the physical, mechanical and morphological properties of porous PLA and PLA/PHBV nanocomposite foams. Solid nanocomposites of PLA, PLA/PHBV and chitin nanowhiskers were manufactured through melt blending while porous nanocomposites foams were fabricated through a batch foaming process with the aid of CO2 as blowing agent. It was found that by incorporating small quantities of chitin nanowhiskers (<2 wt%) the mechanical properties of solid specimens are improved while strength and expandability of the foam can be significantly improved, yielding a homogenously distributed cell morphology with average cell size of 1.5 μm.