Patrick C. Lee

Effect of Talc Content on the Volume Expansion Ratio of Extruded PP Foams

This paper elucidates the effects of cell density on the volume expansion behavior of polypropylene (PP) foams blown with butane in extrusion. The cell density was controlled by varying the talc content, and foam expansion was observed at a fixed blowing agent content while varying the melt and die temperatures. As observed in our previous studies, the curve of the final expansion ratio of PP foam versus temperature showed a typical mountain shape for each talc content, confirming that the two governing expansion mechanisms were gas loss and stiffening of melt. As the talc content increased, the expansion curve skewed towards the lower temperature, which showed that the expanded foams with a high talc content were more susceptible to gas loss at elevated temperatures. This indicates that the processing temperature should be decreased to have a large expansion ratio from the extruded PP foams at a high talc content. In other words, the optimum temperature to maximize expansion decreased at a higher talc content. It is believed that the shift of the expansion curves was caused by the promoted expansion rate of extruded foams with a greater talc content because of the reduced diffusion distance of gas molecules to the nearest stabilized nucleus. On the other hand, the increased cell density at a high talc content increased the number of cell layers in the cross section of the extruded foam, and thereby the gas loss was localized to the cells on the surface which acted favorable for the final expansion ratio to a certain degree.

Polymer-polymer interfacial slip in multilayered films

Significant slip can occur in the flow of a blend of two immiscible polymers due to reduced entanglements at their interface. The slip is of practical importance because of its effect on morphology and adhesion in, for example, disordered two-phase blends or multilayer films. Interfacial slip was quantified using two polymer pairs each with closely matched viscosity and elasticity but different miscibility (χ): polypropylene (PP)/polystyrene (PS) χ=0.04 and polyethylene (PE)/fluoropolymer (FP) χ≅0.1. To control the amount of interfacial area, we prepared alternating layers by coextrusion. The number of layers of PP/PS ranged from 20 to 640 while that for PE/FP was 80. Nominal viscosity of the multilayer samples was measured with three types of rheometers: an in-line slit-die rheometer, rotational parallel-disks, and sliding plate. Good agreement was found between the three methods. The nominal viscosity as well as shear normal stresses of the multilayer samples decreased with the number of layers. From th...

Fabrication of poly (ϵ-caprolactone) microfiber scaffolds with varying topography and mechanical properties for stem cell-based tissue engineering applications

Highly porous poly (ϵ-caprolactone) microfiber scaffolds can be fabricated using electrospinning for tissue engineering applications. Melt electrospinning produces such scaffolds by direct deposition of a polymer melt instead of dissolving the polymer in a solvent as performed during solution electrospinning. The objective of this study was to investigate the significant parameters associated with the melt electrospinning process that influence fiber diameter and scaffold morphology, including processing temperature, collection distance, applied, voltage and nozzle size. The mechanical properties of these microfiber scaffolds varied with microfiber diameter. Additionally, the porosity of scaffolds was determined by combining experimental data with mathematical modeling. To test the cytocompatability of these fibrous scaffolds, we seeded neural progenitors derived from murine R1 embryonic stem cell lines onto these scaffolds, where they could survive, migrate, and differentiate into neurons; demonstrating the potential of these melt electrospun scaffolds for tissue engineering applications.

Effects of CO2 and Talc Contents on Foaming Behavior of Recyclable High-melt-strength PP

This article presents an experimental study on the foaming behavior of recyclable high-melt-strength (HMS) branched polypropylene (PP) with CO2 as a blowing agent. The foamability of branched HMS PP has been evaluated using a tandem foaming extruder system. The effects of CO2 and nucleating agent contents on the final foam characteristics have been thoroughly investigated. Low density (i.e., 12-14-fold), fine-celled (i.e., 107-109 cells/cm3) PP foams were successfully produced using a small amount of talc (i.e., 0.8 wt%) and 5 wt% CO2.

Improvement of Cell Opening by Maintaining a High Temperature Difference in the Surface and Core of a Foam Extrudate

This article presents an extrusion-based, open-cell foaming process using thermoplastic polymers such as polystyrene (PS) and polycarbonate (PC) with supercritical CO2. Our previous studies have indicated that a cell opening can be promoted by inducing: (i) a nonhomogeneous melt structure by cross-linking, polymer blending, or filler compounding, (ii) cell-wall thinning by a high volume expansion ratio while maintaining soft cell walls, (iii) cell-wall thinning by a high cell-population density, and (iv) plasticization of the soft region of the cell walls with a secondary blowing agent. Until now, the foam extrudate temperature across the cross-section was maintained uniformly for the simplicity of the experiments. In this study, the significant temperature difference between the core and surface of the foam extrudate was induced by surface cooling method. This method increased the chance of cell opening by: (i) increasing the core temperature of the foam extrudate and thereby softening the cell walls, and (ii) decreasing the foam surface temperature to prevent gas loss and thereby increasing the internal gas pressure within the cells. The effects of CO2 content, surface quenching, die geometry, and temperature on foam morphologies were investigated. Low-density, microcellular, open-cell foams were successfully produced. The large intercellular pores were observed from micrographs for both PS and PC foams at optimum processing conditions.

Extruded open-celled ldpe-based foams using non-homogeneous melt structure

This paper presents an open-celled foaming extrusion process with low-density polyethylene (LDPE) and LDPE/polystyrene (PS) blends. The basic strategy for achieving a high open-cell content was to induce a hard/soft melt structure with crosslinking and to foam this non-homogeneous melt structure. The hard sections formed by crosslinking assist in maintaining the shape of each cell and the overall foam structure, while the soft sections easily open up the cell walls during cell growth. Since too hard a polymer matrix would adversely affect cell opening, an optimum amount of crosslinking was observed for cell opening. The effect of the processing temperature on cell opening was also investigated in this study. A large expansion ratio of foam at a low temperature was favorable for cell opening with thin cell walls, but too low a temperature was not desirable because of the increased melt strength. This optimum processing temperature for cell opening was experimentally verified at various crosslinking agent contents. Blending with a small amount of PS turned out to be effective for cell opening; it yielded a higher cell density, which caused thinner cell wall thickness. Optimizing the material compositions and processing temperature successfully achieved a high open-cell content up to 99%.

Fabrication and characterization of melt-blended polylactide-chitin composites and their foams

This study details the fabrication and foaming of melt-blended polylactide (PLA) and chitin composites. The chitin used for compounding was as-received, as chitin nanowhiskers and as chitin nanowhiskers with a compatibilizing agent. The chitin nanowhiskers were produced by an acid-hydrolysis technique and their morphology was examined with transmission electron microscopy. The composite morphology was characterized with scanning electron microscopy and was related to the observed thermal, rheological, and mechanical behaviors of the composites. Chitin was found to decrease the thermal stability of the composites. Addition of chitin was also found to reduce the viscosity of the composites, which is believed to be because of the hydrolysis of PLA during melt blending of chitin in suspension. The stiffness of the composites was found to increase with increasing chitin content while the strength was found to decrease. Porous PLA—chitin composites were produced by a two-step batch-foaming technique, and the expansion behavior was correlated with the visco-elastic observations. The statistical significance of chitin type and composition dependence on the mechanical properties and foam morphologies were evaluated.

A Study on the Foaming Behaviors of PP Resins with Talc as Nucleating Agent

This paper elucidates the effects of nucleating agents on the foaming behavior of polypropylene (PP) foams blown with butane in extrusion. The cell density was controlled by varying the talc content, and foam expansion was observed at a fixed blowing-agent content while varying the melt and die temperatures. As observed in our previous studies, the curve of the final expansion ratio of PP foam versus temperature showed a typical mountain shape for each talc content, indicating the two governing expansion mechanisms which are gas loss and stiffening of melt. As the talc content increased, the expansion curve skewed towards the lower temperature, which showed that the expanded foams of high talc content were more susceptible to gas loss. In order to analyze this change, the early-stage expansion behavior of extruded PP foams was investigated using a CCD camera. The expansion-profile images captured at the die exit show that the expansion rate of extruded foams was observed to be faster with a higher talc content because of the reduced diffusion distance of gas molecules to the nearest stabilized cell. The higher growth rate promoted the formation of an initial hump in the expansion profile which is known to be detrimental to large expansion. In order to decrease the expansion rate and thereby remove the initial hump, the temperature had to be further decreased, and consequently, the optimum temperature to maximize expansion decreased at a higher talc content. On the other hand, the increased cell density with a high talc content increased the number of cell layers in the cross section of the extruded foam, and consequently the gas loss was localized to the cells on the surface which acted favorably for the final expansion ratio to a certain degree.

Polymer-polymer interfacial slip by direct visualization and by stress reduction

We studied polymer-polymer interfacial slip in bilayer films of highly immiscible (interaction parameter, χ≅0.1) polyethylene and fluoropolymer from medium to higher shear stresses (10–200 kPa) using both visualization and stress reduction. We found good agreement between results from the two methods as well as with previous studies using multilayers by Lee et al. [J. Rheol. 53, 893–915 (2009)] and visualization of flow in a transparent capillary by Migler et al. [J. Rheol. 45, 565–581 (2001)]. We observed two power-law regions: Vslip∝τ6.2 with a transition to Vslip∝τ1.8 at 50 kPa. This is in contrast to the theory of Brochard-Wyart and de Gennes [C. R. Acad. Sci., Ser. II: Mec., Phys., Chim., Sci. Terre Univers 317, 13–17 (1993)], which predicts a transition from infinite slope to a slope of one at a critical stress.

Strategies for achieving microcellular LDPE foams in extrusion

This paper describes the fundamental process design for achieving microcellular foams using low-density polyethylene (LDPE) in extrusion. Microcellular foams are classified as foams with cell densities larger than 109 cells/cm3 and cell sizes in the order of 10 micrometers. Supercritical CO2 was used as a blowing agent in microcellular foaming due to its high volatility, which greatly increases thermodynamic instability. Our previous studies have indicated that microcellular foams cannot be produced from pure LDPE in a conventional microcellular extrusion system because of the high activation energy for cell nucleation. To increase the cell-nuclei density, an attempt was made at reducing the free energy for bubble nucleation by heterogeneous cell-nucleation. LDPE blends, with a small amount of polystyrene (PS) and/or a nucleating agent, were employed to induce heterogeneous cell-nucleating spots. The amount of the PS phase was varied to determine the optimum content. Furthermore, the melt strength of LDPE was increased by crosslinking. Microcellular LDPE foams have been successfully obtained in extrusion and the materials and processing windows have been clearly identified. The amount of injected CO2 was varied in order to investigate its effect on the cell-population density.