Closed-cell polymers

Thermal insulation is available over a wide range of temperatures, from near absolute zero (-273 C) ( 59.4°F) to perhaps 3,(1()0°C (5,432°F). Applications include residential and commercial buildings, high- or low-temperature industrial processes, ground and air vehicles, and shipping containers. The materials and systems in use can be broadly characterized as air-filled fibrous or porous, cellular solids, closed-cell polymer foams containing a gas other than air, evacuated powder-filled panels, or reflective foil systems. 

The effect of gas compression on the uniaxial compression stress-strain curve of closed-cell polymer foams was analysed. The elastic contribution of cell faces to the compressive stress-strain curve is predicted quantitatively, and the effect on the initial Young s modulus is said to be large. The polymer contribution was analysed using a tetrakaidecahedral cell model. It is demonstrated that the cell faces contribute linearly to the Young s modulus, but compressive yielding involves non-linear viscoelastic deformation. 3 refs. 

Gas compression in closed-cell polymer foams was analysed, and the effect on the uniaxial compression stress-strain curve predicted. Results were compared with experimental data for a foams with a range of cell sizes, and the heat transfer conditions inferred from the best fit with the simulations. The lateral expansion of the foam must be considered in the simulation, so in subsidiary experiments Poisson s ratio was measured at high compressive strains. 13 refs. 

A method for the prediction of cushion curves for polymer packaging foams from a single impact stress-strain curve is proposed. The method is valid if there is a master curve for the increasing stress part of the stress-strain curve. For closed-cell polymer foams that deform by yielding... 

Surprisingly, the surfactant concentration was found to be more important than the phase volume in determining the final cellular structure a material prepared from a HIPE of as much as 97% internal phase volume and 5% surfactant still gave a closed-cell polymer [129]. 

B. Reconstruction of Closed-cell Polymer Foam Structure 179... 

Mills, N.J., and H.X. Zhu. 1999. The high strain compression of closed-cell polymer foams. 

To perform optimally, the char, or similar barrier should be continuous, coherent, adherent and oxidation-resistant. It should be a good thermal insulator (which implies closed-cell character) and it should have low permeability to gases, to liquid pyrolysate, and to molten polymer. Moreover, the char must be formed in a timely manner before the polymer is extensively pyrolyzed. 

Gibov et al. (9) showed that combustion vapors and air could penetrate through a typical char layer. Capillarity served to bring molten polymer to the surface where it could pyrolyze and burn. One answer to this problem is obviously to create a closed cell foam. Gibov et al. showed that the incorporation of boric acid and ammonium phosphate helped minimize penetrability of the char (Fig. 1). 

Japon, S., Leterrier, Y. and Manson, J.-A. E., Recycling of polyethylene terephthalate) into closed-cell foams, Polym. Eng. Sci., 40, 1942 (2000). 

Foams are commercially produced several ways. Some polymerization processes produce their own foam. Polyurethanes, for example, are very exothermic. When they are formed, if a little water is present, CO2 will be a by-product. As the polymer forms, the CO2 will cause closed cell foam. As another example, a blowing agent can be injected into the molten polymer. The agent will later decompose, giving off a gas when the polymer is heated to melting. Epoxy resins are expanded into foams this way. 

Foamed polymers. Thermosets and thermoplastics formed into low density, cellular materials containing bubbles of gas. Rigid foams have their gas bubbles in closed cells, inhibiting flexibility flexible foams have the bubbles in open cells, permitting the gas to escape as the foam is flexed. 

An alternative means of generating a polyimide foam with pore sizes in the nanometer regime has been developed [80-90]. This approach involves the use of block copolymers composed of a high temperature, high Tg polymer and a second component which can undergo clean thermal decomposition with the evolution of gaseous by-products to foam a closed-cell, porous structure (Fig. 7). 

Model (redrawn from a.5) of the cell air and the polymer stmcture acting in parallel in a compressed closed-cell foam... 

When a closed cell foam is uniaxially compressed, it can be assumed that the compressive stress is a sum of the stresses taken by the polymer structure and that taken by the cell gas. For a foam with zero lateral expansion when uniaxially compressed, and isothermal gas compression, the latter contribution (Tg is given by (295) ... 

At strains >10%, when the polymer structure has begun to collapse, gas loss, by diffusion through the cell faces of closed cell foams, may contribute to the creep. The effect of this on the creep of LDPE and EVA foams was determined (266). The foam diffusivity for air was predicted from the polymer permeability P and the foam density p using ... 

Details are given of the fabrication of foams with uniform closed-cell structures. LDPE was used as polymer feedstock. Thermal analysis was performed using DSC and morphologies were examined using SEM. 12 refs. 

Journal of Applied Polymer Science 83, No.2, 10th Jan.2002, p.357-66 MORPHOLOGY AND PHYSICAL PROPERTIES OF CLOSED CELL MICROCELLULAR ETHYLENE-OCTENE COPOLYMER EFFECT OF PRECIPITATED SILICA FILLER AND BLOWING AGENT Nayak N C Tripathy D K Indian Institute of Technology... 

Journal of Polymer Science Polymer Physics Edition 38, No.7, 1st April 2000, p.993-1004 PREDICTION OF THE RADIATION TERM IN THE THERMAL CONDUCTIVITY OF CROSSLINKED CLOSED CELL POLYOLEFIN... 

The benefits of the Sioplas moisture crosslinking process for the manufacture of crosslinked PE foam and the end-uses of such foams are discussed. The process involves grafting of alkoxysilanes onto ethylene homo- or copolymers to provide moisture crosslinkable polymers suitable for the manufacture of flexible, rigid, open-ceUed, closed-cell or very low density foams. 11 refs. 

Journal of Applied Polymer Science 75, No.l, 3rd Jan.2000, p. 156-66 MECHANICAL CHARACTERISATION OF CLOSED-CELL POLYOLEFIN FOAMS... 

Journal of Applied Polymer Science 73, No. 14, 29th Sept. 1999, p.2825-35 ANOMALOUS THICKNESS INCREASE IN CROSSLINKED CLOSED CELL POLYOLEFIN FOAMS DURING HEAT TREATMENTS Rodriguez-Perez M A Ahnanza O De Saja J A Valladolid,Universidad Columbia,University... 

Patent Number US 5817705 A 19981006 SHORT TIME FRAME PROCESS FOR PRODUCING EXTRUDED CLOSED CELL LOW DENSITY PROPYLENE POLYMER FOAMS... 

Journal of Polymer Science Polymer Physics Edition 36, No. 14, Oct. 1998, p.2587-96 THERMAL EXPANSION OF CROSSLINKED CLOSED-CELL POLYETHYLENE FOAMS... 

Cellular Polymers IV. Conference proceedings.. Shawbury, 5th-6th June, 1997, paper 15. 6124 DEFORMATION MECHANISMS EM LDPE CLOSED CELL FOAMS... 

Patent Number US 5554661 A 19960910 CLOSED CELL, LOW DENSITY ETHYLENIC POLYMER FOAM PRODUCED WITH INORGANIC HALOGEN-FREE BLOWING AGENTS... 

The concentration of the surfactant in the monomer phase was found to be critical to the formation of a stable polymer foam [129,130]. At least 4% surfactant, relative to the total oil phase, was required for PolyHIPE formation, whereas formulations containing above 80% resulted in the formation of an unconnected or closed-cell material. Surfactant levels between 20 and 50% were deemed to be optimum at all internal phase volumes. Additionally, Litt et aL [131] demonstrated that block copolymer surfactants can be used to prepare water-in-styrene HIPEs. From these, highly porous uncrosslinked polystyrene PolyHIPE materials were synthesised. 

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