Showing papers in "Cellular Polymers in 2005"

Mechanical Performance of Auxetic Polyurethane Foam for Antivibration Glove Applications

Abstract: In this study the static and dynamic characteristics of conventional open cell polyurethane (PU), of auxetic (negative Poisson’s ratio) and of iso-density foams were analysed. The specimens were produced from conventional gray open-cells polyurethane foam with 30-35 pores/inch and 0.0027 g/cm 3 density, by means of process which has been previously defi ned by the authors. Poisson’s ratio measurements were performed under quasi-static conditions using an MTS 858 servo-hydraulic test machine and a video image acquisition system. For the auxetic foams the results suggested similar behaviour to that previously reported in the literature, with signifi cant increases in stiffness during compressive loading, and a signifi cant dependence of the Poisson’s ratio on the applied strain. Transmissibility tests, performed in accordance with the ISO 13753 procedure for antivibration glove materials, suggested a strong dependence of the transmissibility on the foam manufacturing parameters. Within the frequency range from 10 to 31.5 Hz the transmissibility was found to be greater than 1, while it was less than 1 at all frequencies greater than 31.5 Hz. The transmissibility results were similar to the mean values for 80 resilient materials tested by Koton et. al., but were higher than the fi ve best materials (not all polymeric) identifi ed by the same researchers. In this study it has been suggested that the resilient behaviour of glove isolation materials should also be evaluated in terms of the indentation characteristics. A simple, linear elastic, Finite Element simulation was therefore performed, and the indentation results suggested that auxetic foams offer a signifi cant decrease in compressive stresses with respect to conventional PU foams.

Design of an Artificial Intervertebral Disc Exhibiting a Negative Poisson's Ratio:

Abstract: An artificial intervertebral disc exhibiting an anisotropic negative Poisson's ratio has been designed and characterized in the laboratory. This disc prosthesis incorporates negative Poisson's rati...

Effect of Processing Parameters on Cellular Structures and Mechanical Properties of PMMA Microcellular Foams

Abstract: In this study, the effect of processing parameters on the cellular morphologies and mechanical properties of PMMA microcellular foams is investigated. Microcellular closed cell Poly(methyl-methacry...

Constrained solid-state foaming of microcellular panels

Abstract: This paperpresentsa novel constrained foaming process to produce microcellular panels. A systematic study of various process variables is conducted using a two.stage sliding-level design of experiment approach. The resulting microcellular sheets have thicknesses in the 5 - 15 mm range and densities reductions as low as 92%. It is shown that an unfoamed integral skin of desired thickness can be produced with a microcellular core at various densities. These microcellular panels are envisioned for use in advanced panel systems and sandwich structures, such as in housing construction and other load-bearing applications.

Influence of mould temperature on the thickness of a skin layer and impact strength in the microcellular injection moulding process

Abstract: The thickness of a skin layer on the parts made by a microcellular injection moulding process may influence its properties, including impact and tensile strength, density and sound transmission. Th...

Critical Issues in Extrusion Foaming of Plastic/Woodfiber Composites:

Abstract: Foaming of wood-fiber/plastic composites (WPC) with a fine-celled structure can offer benefits such as improved ductility and impact strength, lowered material cost, and lowered weight, which can i...

Characterization of EVA Residues from the Shoe Industry and Post-Consumer Urban-Waste Polyethylenes:

Abstract: This paper presents a characterization of a crosslinked EVA residue (EVA-c) from expanded sheets used in the shoe industry and post-consumer urban-waste polyethylenes regarding their molecular (FTIR), mechanical (tensile and impact tests), morphological(SEM), thermal(DSC, TGA)and dynamic-mechanical(DMTA) properties. For comparison, the properties of the EVA -c and recycled polyethylenes are compared to respective virgin polymers. The recycled polyethylenes generally presented similar properties to the virgin ones. On the other hand, some EVA-c properties differed from virgin ones since it has a high degree of crosslinking and it therefore has a higher tensile modulus and lower elongation at break, notched Izod impact strength and hardness. Additionally, crosslinking was also found to modify the thermal properties (TGA and DSC) of EVA-c.

The effect of clay content on PMMA-clay nanocomposite foams

Abstract: In this study the CO 2 sorption at 45 °C in PMMA nanocomposite films containing 2 wt.% of nanoclay has been measured using an in-situ gravimetric technique. The films examined were prepared by compression moulding material obtained by dry-blend and solvent co-precipitation techniques. The CO 2 diffusion coefficients were found to be higher for the dry-blended nanocomposite due to the larger agglomerations of the organoclay agglomerations, which prevented the polymer chains from fully wetting and intercalating the clay particles. The T g -p profile for PMMA nanocomposite containing 2 wt.% nanoclay in the presence of CO 2 was also measured using high-pressure DSC. The glass transition phase envelope was shifted vertically by approximately 10 °C when compared to the value reported in the literature for neat PMMA. This result suggests that the nanoclay affects the plasticization behaviour of PMMA under high-pressure CO 2 conditions. The cellular morphologies obtained for these PMMA nanocomposite foams produced by batch processing with subcritical CO 2 are strongly dependent upon the clay content and the dispersion of the nanoclay in the material. In the case of intercalated nanocomposites, most clay particles exist as agglomerated stacks of silicate sheets. On foaming the cells tend to form around the clay particles causing either irregular-shaped cells or layers to be produced. As a result, the cell density increases and the mean cell size decreases in the foamed nanocomposite on increasing the nanoclay content. Accordingly, the resulting cell structures are highly non-uniform and show large variations in cellular morphologies throughout the foam.

Effect of weld lines on the injection moulding of structural foams. I. Foam morphology

Abstract: The effect of weld lines on the morphology of injection moulded structural polymer foams was studied. In this first part, the effect of processing conditions on density and morphology of high density polyethylene foams isreported. In particular, the effect of density reduction and injection speed was investigated. The results indicate that for lower density foams, skin thickness decreases substantially along flow path and cells collapse is present. At the weld line, local reduction of skin thickness and cell size is related to the formation of a V-notch prior the final expansion and to higher pressures generated by the difficulty to expulse the entrapped air inside the mold. As of the effect of injection speed, cell size and skin thickness differences between samples with or without weld lines were observed and explained by time and gravity effects.

Polyethylene-Kevlar Composite Foams III: Torsion Properties:

Abstract: High density closed-cell Kevlar-polyethylene composite foams (17 - 30% void fraction) were prepared by compression molding and characterized via torsion rectangular tests in order to determine the effect of thin unfoamed skins and Kevlar content on shear modulus. It was found that structural foam models gave better results than uniform foam models indicating that thin skins have an important effect on the shear modulus of polymer foams. The normalized modulus of our composite structural foams can be predicted by a sandwich model in combination with Berlin 's approach and Rosen 's model for the aspect ratio of the fibres.

Correlation of decompression time and foaming temperature on the cell density of foamed polystyrene

Abstract: In this paper, the correlation between the foaming temperature and the decompression rate (decompression time) of the cell density that is the number of cells per unit volume remaining in the foame...

Do nanoparticles really assist in nucleation of fine cells in polyolefin foams

Abstract: Nanotechnology is an enabling technology that is extremely large to cover in one technical paper. It touches various technology disciplines and covers several applications. The purpose of this paper is to investigate theeffect of nanoclay on nucleation of polypropylene foams. Particularly, the goal is to understand the ability of nanocomposites in producing finer cells compared to talc in polypropylene.

Compressive Deformation and Energy Absorption Characteristics of Conductive Carbon Black Reinforced Microcellular EPDM Vulcanizates

Abstract: Compressive stress-strain properties of unfilled and conductive carbon black (VulcanXC 72) filled oil extended EPDM (keltan 7341A) microcellular vulcanizates were studied as a function of blowing a...

Effect of Foaming Temperature Control on Cell Size and Cell Density of Microcellular Plastics

Abstract: In this study, noticing foaming temperature as a factor, which induces thermodynamic instability for cell nucleation of Microcellular plastics, the effect of control method of foaming temperature on cell size and cell density - that is number per unit volume of foamed plastics - were investigated. Generally, foaming by using batch process is carried out as follows. First, blowing agent is soaked into plastics until saturation under high pressure and soaking temperature. After plastics were saturated with blowing agent, pressure is released rapidly and then temperature is raised to foaming temperature and cells are nucleated and grown. Finally, rapid cooling controls cell growth. In this case, two methods can be considered for the control of foaming temperature. One is the elevated temperature method in which temperature is raised to foaming temperature and cells are grown after decompression in the foaming process. The other is the constant temperature method in which the temperature is already kept at foaming temperature before decompression. That is, it is the method of performing soaking and foaming at the same temperature. Polymethylmethacrylate (PMMA) resins were foamed under foaming conditions which the same foaming magnification is produced by both methods and cell size and cell density of foamed PMMA were investigated. As results, in case of production of the foamed plastics having the same foam magnification, it turned out that cell density of foamed plastics becomes large and average cell size becomes small but the maximum cell size becomes large by the elevated temperature method. On the other hand, although the maximum cell size becomes small, average cell size becomes large by the constant temperature method.

Prediction of cushion curves for closed cell polyethylene-based foams. Part I. Modelling

Abstract: The possibility of prediction of the cushioning performance of closed cell foams under compressive impacts has been analyzed, using simple theoretical models. Several corrections have been introduc...

Study on the Aging Rules of Rigid Polyurethane Foams at Indoor Storage and Different Hygrothermal Conditions

Abstract: The aging rules of rigid polyurethane foam (PUR) at indoor storage and different hygrothermal conditions have been studied. Four parameters, which are mass, dimension, compressive strength and compressive modulus were tested. At indoor storage, mass, dimension and compressive strength vary slowly with an increase in aging time, while compressive modulus decreases quickly. PUR is sensitive to relative humidity (RH) verified by accelerated hygrothermal aging, and hydrolysis of ester group is the main reason resulting in the decrease of compressive properties. The filling with fire retardant and glass beads had some effect on hygrothermal aging properties of PUR. The addition of fire retardant increased compressive strength with aging time in the total trend, but it made dimension stability worse. The addition of glass beads slightly improved hygrothermal aging properties.

Mass transport of cell gases in carbon dioxide blown PET (polyethylene terephthtalate) foam insulation

Abstract: Polyethylene terephthalate (PET) foam is a possible replacement option for polyurethane (PUR) foam as insulation material in district heating pipes. In this study, the diffusion coefficients and activation energies of cell gases in carbon dioxide blown PET foam (densities 148-157 kg.m -3 ) were determined at temperatures between 23 °C and 90 C. The foam thermal ageing due to the mass transport of air into and carbon dioxide out of the foam was about ten times slower in PET foam than in PUR foam. The thermal conductivities of the PET foam boards were detennined in a heat flow meter apparatus. The contribution to the foam thermal conductivity due to conduction in the solid polymer and radiation within the cell voids was determined to 17 mW.m -1 .K -1 at 20 °C. This is higher than the value estimated for PUR foam in district heating pipes. 12 m W.m -1 .K -1 . This contribution can probably be reduced by developing low density PET foam and reducing the cell size.

Microcellular Foams Prepared by Exposing (CO2 + H2O) Laden Polymers to Microwaves:

Abstract: Polymers conditioned with compressed CO 2 and H 2 O underwent rapid foaming when exposed to microwaves using a conventional oven. The rapidly oscillating water molecule dipoles generated enough heat to allow nucleation and growth of closed cells. Microcellular foams with cells less than 10 μm were produced from polymers such as poly(methyl methacrylate) and polycarbonate. The technique provides a fast foaming process whereby the cell morphology can be easily controlled by adjusting the microwaves intensity and flux, and exposure time.