Showing papers in "Cellular Polymers in 2000"

Negative Poisson's ratio foam as seat cushion material

Abstract: Negative Poisson's ratio foam was used in the development of seat cushions which exert reduced peak pressure upon the skin of seated persons. Foam processing techniques were scaled up. A longer processing time was required for cushion size samples in comparison with small samples. Pressure distributions on a seated subject were measured using a pressure-sensitive array. Seated pressure distribution became more favourable with decreasing sample density for both conventional and re-entrant foam blocks. Foam thickness played a small role in the seated pressure performance of foam cushions. Re-entrant foam at densities of between 2 and 4 Ib/ft 3 (0.032 to 0.064 g/cm 3 ) performed better (lower maximum seating pressure) than conventional foam samples of comparable density.

Modelling the Indentation of low density polymer foams

Abstract: The response of open-cell polyurethane (PU) foams was measured in compression, simple shear and combinations thereof. The data was fitted with Ogden's strain energy function for compressible hyperelastic solids, using one or two moduli. The Ogden function is also compared with the predictions of a micromechanics model. Finite Element Analysis (FEA) was used to predict the foam response for both plane strain indentation and the axisymmetric Indentation Force Deflection (IFD) test. Experimental deformation patterns and pressure distributions validate the predictions. The predicted relative indentation forces are about 20% less than the experimental values. Test piece geometry is shown to affect the IFD values, and interface friction to affect the deformation pattern.

Solid-State Microcellular Acrylonitrile-Butadiene-Styrene Foams

Abstract: SUMMARY Microcellular ABS foams are a novel familv of materials with the ~otential to significantly reduce material costs in a number of applications that currently use solid polymer. ABS foams were produced using carbon dioxide in a solid-state process. Solubility and diffusivity of C02 in ABS was measured, and the latter was found to depend significantly on the gas concentration. The useful range of process-space for ABS-CO, was characterized. Closed cell ABS foams were produced with densities ranging from 1.03 g/cm3 (almost completely solid) to 0.09 c~/cm~. It was determined that there are many different processing conditions that can produce microcellular ABS foams that have the same density. The cell nucleation density was of the order of 10" cells per cm3, and the average cell sizes observed ranged from 0.5 pm to 5.6 pm.

A novel stress-induced nucleation and foaming process and its applications in making homogeneous foams, anisotropic foams, and multilayered foams

Abstract: A new foaming process is described whereby cell nucleation and growth are induced by applying mechanical, compressive stress to a polymer-gas solution in the rubbery state. As opposed to the conventional techniques, the stress-induced nucleation has a very short induction time and occurs at relatively lower temperatures, and foams with high cell density and small cells can be made by this method. And because of the control over the nucleation and cell growth events, selective foaming can be performed to give not only homogeneous foams but also specialty foams such as anisotropic foams and image foams. At high stress levels, cell coalescence leads to multilayered foams with or without cells in the polymeric layers, and with discontinuous gaps in-between the layers.

Impact strength of high relative density solid state carbon dioxide blown crystallizable poly(ethylene terephthalate) microcellular foams

Abstract: SUMMARY In this paper, processing conditions for producing high relative density microcellular CPET foams using C02as a blowing agent are described. Starting with solid CPET, foams with relative densities between 0.5 to 1.0 were produced. Results of instrumented impact testsconducted at varioustemperatures ranging from room temperature to -40°C are presented. The CPET foams exhibit excellent impact properties in the range of temperatures explored.

Design of a novel Extrusion system for Manufacturing Microcellular Polymer

Abstract: The described system for the production of microcellular hollow fibre is based on the thermic extrusion of polymers under addition of CO 2 as a blowing agent by means of pressure build-up and drop-off. The design of a novel extrusion system is presented according to this concept, which involves a modular configured co-rotating twin screw extruder, a gear melt pump and a designed die. The gear melt pump works as barrier between the extruder and the extruding head and allows to adjust the pressure over a wide range. The pressure level reached is dependent on the screw speed of the gear melt pump and the resistance of the polymer melt in the extruding head. To initiate the cell nucleation a die was used to lower the pressure. Microcellular foams have been successfully produced using this system.

Layered foams : A new class of cellular polymers

Abstract: A novel process is described for making integral multilayered polymers with the layers separated by discontinuous narrow gaps containing air and/or a blowing agent. The two step process involves first introducing a low degree of entanglement among some of the polymer chains and then breaking apart these entanglements using a blowing agent to give the discontinuous gaps that resemble elongated, narrow cells. The layer density can be controlled within a wide range, typically 10 to 2000 layers/mm, while the gap between the layers is < 100 nanometers. These layered materials, called nanolayered polymers or layered foams, are mechanically strong and have excellent thermal and electrical, and, possibly, sound insulation properties. The nanolayering process is complimentary to the process described recently (Handa and Zhang, Cellular Polymers 19, (2000) 1) to make microlayered polymers.

Principal correlation of PVC melt elongational properties with foam cell morphology

Abstract: A lab-scale method developed in this work offers new possibilities to polymer foam manufacturers to significantly reduce the number of experiments required on production extruders to optimise PVC foam formulations. Rheotens elongation experiments were applied to analyse PVC foam formulations without a blowing agent with regard to their melt elongational properties such as melt strength and elongation at break. The uni-axial state of deformation represented by these experiments offers an approximation of the melt behaviour in a multi-axial deformation such as foam extrusion. The evaluated foam formulations were foamed on an extruder under identical processing conditions and the resulting cell morphologies were investigated by Scanning Electron Microscopy. Image analysis of the obtained SEM-images confirmed a principal correlation between the melt elongational properties and the foam cell morphology. It appears, that the PVC melt strength mainly influences the mean cell size and the cell size distribution. In addition, the fraction of open cells is decreasing with increasing melt strength.

Cell-gas composition : An important factor in the evaluation of long-term thermal conductivity in closed-cell foamed plastics

Abstract: In addition to the initial thermal conductivity value, the thermal conductivity of closed-cell foamed plastics over long periods of time depends to a large degree on the gas exchange processes between the blowing agent and the ambient air. It is very important to know and to show whether the cell gas remains in the foam for a short time or, to a large extent, for the whole lifetime of the foam material. A lifetime of between c.20 to 50 years is assumed for thermal insulation materials in construction, and it will be shown that blowing agents remain to a large extent in the foam material over such periods. The method most extensively applied for measuring cell-gas composition is to take a sample using a gas-tight syringe and then analyse by gas chromatography. In addition to the measuring method, possible sources of sampling errors and measuring uncertainty are indicated. In order to show the relationship between thermal conductivity and cell-gas composition, thermal conductivity curves for PUR rigid foam materials blown with CFC 11 over a period of 30 years, and PUR foam materials blown with HCFC 141b or pentane over a period of 9 years are presented, with the corresponding measured values for cell-gas composition. Cell-gas measurement continues to be applied as proof of the blowing agent used for consumers and official bodies, and also for compliance with regulations.

Morphology and physical properties of closed-cell microcellular ethylene-octene copolymer : Effect of precipitated CaCO3 filler and blowing agent

Abstract: The morphology of the microcellular ethylene-octene copolymer (Engage®) of both unfilled and precipitated calcium carbonate filled compounds has been studied from SEM photomicrographs with variation of blowing agent and precipitated CaCO 3 filler loading. The average cell size, maximum cell size and cell density varies with variation of blowing agent and filler loading. Physical properties like relative density, hardness, tensile strength, elongation at break, modulus, tear strength decreases with blowing agent concentration. The elastic nature of closed cells reduces the hysteresis loss compared to solid compounds. Stress relaxation behaviour is independent of blowing agent loading, i.e. density of closed cell microcellular vulcanisates. Theoretically flaw sizes are found to be about 2.4 times larger than maximum cell sizes observed from SEM photomicrographs.

Seating pressure distribution using slow-recovery polyurethane foams

Abstract: A 100 mm thickness of Confor slow-recovery foam caused a 10% lowering of skin pressure under the pelvic ischial tuberosities of a static seated subject compared with open cell polyurethane (PU) seating foams. The reasons for the difference were microstructural (semi-closed cells compared with open cells), and mechanical (a lower shear modulus than the open-cell foams, averaged over a range of compressive strain). There was slightly more load spreading below an indenter for an open-cell PU foam, which correlates with slightly higher seating pressure maxima. The pressure distribution property of the Confor foam cannot be explained by its rate dependent polymer viscoelasticity or cell gas flow; these factors lead to higher pressure peaks at short times.

Endurance of polyurethane automotive seating foams under varying temperature and humidity conditions

Abstract: The long-term comfort and durability properties of the flexible moulded foams used in many forms of transportation seating are a subject of increasing interest to foam moulders and seat producers. In previous contributions we have demonstrated the broad range of performance that can be expected from the well established and finely tuned chemistries (Hot Cure, TDI HR, MDI HR and TM-20 HR) currently used to make moulded seating foams all around the world. Existing and proposed new specifications for load bearing and other fatigue losses have necessitated a new look at the fundamental understanding of why foam properties change with temperature and humidity: the mechano-sorptive properties. The polymer morphology of foam specimen representative for the commercially available types has been determined by a combination of spectroscopic techniques: DMS, FT-IR, SAXS and TEM. A rationale is presented for the formation of the observed morphologies during reactive processing based on the theory of spinodal decomposition and a morphological picture is proposed encompassing the measured morphological parameters. To get to the details of how temperature and moisture effect the mechanical response of a polyurethane network, we studied compression sets over a wide range of conditions and correlated those data with eventual morphology changes obtained from spectroscopic studies and with traditional dynamic fatigue tests conducted in similar conditions. Additional insight into the reorganization of the polymer networks was gained from detailed analysis of the foam's dynamic mechanical performance under the influence of transient moisture conditions. Specifically the energy loss associated with moisture induced network reorganization has been measured.

Rigidity characterisation of flexible foams by falling dart rebound tests

Abstract: Low-energy instrumented falling dart impact techniques have been applied to characterise the rigidity of several series of polyolefinic flexible foams, at relatively high strain rates. Rebound tests were specially designed in order to determine the elastic modulus of the foams by application of a theoretical model that describes the indentation phenomena of a hemispherical element on a sample. A potential relationship between values of the determined elastic modulus and foam density was found and, as expected, the foams based in HDPE showed the higher values of the elastic modulus of all the studied foams, whereas the samples containing EVA resulted in the lower values. Moreover, the elastic modulus determined using hemispherical dart headstocks (indenters) with different diameter resulted in a constant value, as the indentation model applied establishes. The results presented in this paper show the utility of this test to characterise mechanically flexible foams, because it is sensible to identify small differences of rigidity due to variable density, foaming process and chemical composition.

Rigid polyurethane foams for buoyancy applications : Hydraulic resistance

Abstract: Polyurethane foams used in floatation applications must have a high `hydraulic resistance', which is the resistance to penetration of water into the foam cells at elevated hydraulic pressures. Experiments are carried out to study the effect of different surfactants on the structure and hydraulic resistance of rigid polyurethane foams with densities in the range 100-200 kg/m3. The hydraulic resistance is evaluated in terms of the weight of water absorbed per unit weight of the foam at a fixed hydraulic pressure, and the cell and strut sizes of the foams are measured by microscopy and image analysis. The hydraulic resistance of the foams increases with the increase in density of the foams. The compression strength of the foam is significantly higher than the corresponding hydraulic pressure the foams can withstand, indicating that the hydraulic resistance is determined by the existence and strength of cell windows. The water absorbed by the foams increases with the hydraulic pressure. Among the different surfactants used, the foams made with Tegostab B8404, Atsurf 3800 or Cresmer 3845 surfactants have a low loss in buoyancy (less than 20%) at 3 bar hydraulic pressure. These foams also have smaller cell windows compared to foams with lower hydraulic resistance.

Optimisation of the production of foamed rubber goods with a lab scale method

Abstract: The development of foamed rubber articles is a very complex process which involves a large number of experiments. The target of these developments is usually the optimisation of the cell structure and surface appearance. These are the most important properties that determine the performance of the finished rubber article under service conditions. The experiments described here show that it is important to investigate the decomposition reaction of the blowing agent and the crosslinking reaction. It will be shown that these reactions interfere strongly with each other. This leads to the conclusion that it is necessary to choose the blowing agents and the crosslinking agents very carefully. This paper describes experiments with a formulation based on ethylene propylene diene rubber (EPDM) to investigate the different interactions caused by the ingredients in this formulation. These experiments were performed on a lab scale. In addition, experiments based on chloroprene rubber (CR) are described. These experiments prove the same interdependences of crosslinking and decomposition reaction. Furthermore, experiments will be described which show how the results can be transferred from the lab scale to a factory scale trial.

Rotational moulding of expanded polyethylene by pellets

Abstract: An innovative method of rotational moulding expanded polyethylene by pellets was reported. Experiments were carried out in a laboratory scale uniaxial machine, which was capable of measuring in-mould air temperature in the cycle. The polymeric material used was linear low-density polyethylene. The foaming material used is endo-thermal chemical blowing agent. Before moulding, the polyethylene material and the chemical-foaming agent were first melt blended by a single-screw extruder and were then cut into pellets of 2-3 mm in diameter. Pellets were then dryblended with pure LLDPE powder by a mixer for moulding. Characterization of moulded part properties was performed after moulding. An optical microscopy equipped with an image analysis system was also employed to determine the bubble distribution in foamed parts. The purpose of this study is to access the mouldability of rotational moulding expanded polyethylene by pellets, and to investigate how the foaming agent and processing conditions can influence the moulding process and the final product quality. It was found that the rotational moulding of expanded parts by pellets saves the cost of powder grinding as well as improves the impact property of moulded parts, but is counteracted by uneven inner surfaces.

Effects of replacement of butadiene-acrylonitrile rubber in butadiene-acrylonitrile rubber/poly(vinyl chloride) blend micro cellular soles by natural rubber, styrene-butadiene rubber, and high styrene resin

Abstract: Natural rubber, styrene butadiene rubber and high styrene resin were used to replace part of nitrile rubber in a 50/50 NBR/PVC (butadiene-acrylonitrile rubber/poly(vinyl chloride) blend. The ternary blend based microcellular soles show improved technical properties while decreasing the cost of the MC soles. About 15 parts of NBR can be replaced by NR, SBR or HSR for cost reduction without much deterioration in properties.