Stock material or miscellaneous articles – Structurally defined web or sheet – Including variation in thickness
Reexamination Certificate
1998-09-16
2002-05-07
Copenheaver, Blaine (Department: 1771)
Stock material or miscellaneous articles
Structurally defined web or sheet
Including variation in thickness
C428S308400, C428S317900, C428S317100, C428S920000, C428S921000, C264S045100, C264S045300, C264S048000
Reexamination Certificate
active
06383608
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to a method and apparatus for producing a foam or cellular, plastic product, particularly a foamed plastic product having enhanced fire resistance, and products produced by that method and apparatus.
BACKGROUND DISCUSSION
Foam plastics can be prepared by a variety of methods including, for example, an expansion process which includes expanding a fluid polymer phase to a low density cellular state and then preserving this state. A variety of manufacturing processes have been developed to achieve the steps required in an expansion process which typically include creating cells in the fluid or plastic phase, causing the cells to grow to a desired volume and stabilizing the cellular structure by physical and/or chemical means. The growth of the cells depends on the pressure differential between the cells and the environment and growth can be achieved either (a) by lowering the pressure of the external environment (decompression) or (b) by increasing the internal pressure in the cells (pressure generation) or (c) a combination of both.
Foamable compositions in which the pressure within the cells is increased relative to that of the surrounding environment have generally been called expandable formulations. Polystyrene is one of many materials used in the formation of a foam plastic in an expandable formulation process coupled with a physical stabilization process. One technique features the polymerizing of a styrene monomer in the presence of a blowing agent so that the blowing agent is entrapped in a polymerized bead. Typical blowing agents used in such processes are the various isomeric pentanes and hexanes, halocarbons, and mixtures of these materials.
The fabrication of these expandable particles into a finished cellular-plastic article is generally carried out in two steps. In the first step, the particles are expanded by means of steam, hot water, or hot air into low density replicas of the original material, called prefoamed or preexpanded beads. After proper aging, enough of these prefoamed beads are placed in a mold to just fill it; the filled mold is then exposed to steam. This second expansion of the beads causes them to flow into the spaces between beads and fuse together, forming an integral molded piece. Stabilization of the cellular structure is accomplished by cooling the molded article while it is still in the mold. The density of the cellular article can be adjusted by varying the density of the prefoamed particles.
Polyvinyl chloride and polyethylene are just a few examples of other materials, usually in beaded form, that are used in an expansion formulation process coupled with a physical stabilization process (e.g., cooling).
Another common cellular foam often used in an expansion formulation process is polyurethane. However, the cell growth of polyurethane is often controlled, at least for the most part, by a chemical stabilization process rather than relying solely on a physical stabilization process. The general method of producing cellular polyurethane is to mix a polyfunctional isocyanate with a hydroxyl-containing polymer along with the catalyst necessary to control the rate and type of reaction and typically other additives to control the surface chemistry of the process and to adjust the conditions and reactants such that the exotherm of the reaction causes expansion of the foam considerably.
Other processes for forming cellular products include decompression processes such as extrusion and injection molding. Again, either physical or chemical methods may be used to stabilize products of the decompression process. Provided below are just some examples of forming cellular product formations which include the use of decompression expansion techniques with polystyrene used as a polymer example.
Extruded polystyrene—A solution of blowing agent in molten polymer is formed in an extrusion under pressure and then released such that the blowing agent vaporizes and causes the polymer to expand. As an alternative to introducing a solution of blowing agent, polystyrene beads or pellets containing pentane blowing agent has been used particularly for forming low density, extruded foam sheets.
Injection Molding Polystyrene—Polystyrene granules containing dissolved liquid or gaseous blowing agents are used as feed in a conventional injection-molding process. With close control of time and temperature in the mold and use of vented molds, high density cellular polystyrene molding can be obtained.
Another example of a plastic foam formation process is found in a reaction injection molding or RIM process wherein polyurethane structural foam products are formed by metering into a temperature controlled mold a polyol and isocyanate to generally fill 20% to 60% of the mold depending on the density of the structural foam parts. When the reaction mixture then expands to fill the mold cavity, it forms a component part with an integral, solid skin and a microcellular core.
The foregoing represents just a few examples of the numerous processes used in the industry to form foam plastic products.
Coupled with the expanded use of foam bodies in a variety of fields, there has also been activity in the development of foam component structures having some degree of fire resistance. The prior art techniques include, for instance, providing a higher density, thickened outer layer or skin (e.g., see U.S. Pat. No. 4,191,722 with its thick-skinned roofing panels said to improve fire resistance). Another technique involves protecting the foam core with a fire retardant outer covering such as in U.S. Pat. No. 3,991,252, with protective fire retardant Gypsum layer. U.S. Pat. No. 4,136,215 describes the layering of a thin, continuous layer of inert particulate material under a still curing thermosetting foam layer (e.g., clay, sand, etc.) such that the particulate material penetrates to some extent and adheres to the foam material.
The prior art also describes the introduction of fire resistant material into the original precursors or ingredients being used in the formation of foam material used in products. These flame retardant additives which retard the surface spread of flames include, for example, halogenated materials, antimony trioxide, alumina trihydrate, borates and phosphates (see, for example, U.S. Pat. No. 4,366,204 directed at polyurethane and polyisocyanate rigid foam panels). The introduction of the flame retardant material in with the original precursors does facilitate a wide dispersion of the flame retardant material. However, in large quantities, the flame retardant material can preclude proper formation of the final product or degrade the quality of the product (particularly for situations where the required flame retardation is high as the higher quantity of flame retardant material can create problems in preparing quality products with certain desired characteristics or starting materials). The inventors have determined, for example, that while the initial dispersement of flame additives (e.g., boron) in with reactive foam mix precursors in a die mold allows for acceptable products for car manufacturing, when dealing with sufficient flame retardant additives to satisfy building code levels proper fusing and foam formation did not occur. This was also true with respect to the initial mixing of expandable plastic beads in with fire retardant material as the amount of flame additives needed to satisfy building code levels or standardized association requirements (e.g., ASTM E 108-90—“Standard Test Methods for Fire Tests of Roof Coverings” and ANSI/UL (Underwriters Lab, Inc.) 790—DEVELOPMENTAL SPREAD OF FLAME TEST, which are incorporated herein by reference in their entirety), would result in unacceptable products. That is, the blending of fire retardant materials with expanded plastic beads prior to the molding process failed to produce an acceptable fire retardant foam product either due to the product not properly forming or the failure to satisfy the requirements in, for example, the Class A level of the aforementioned
Burkett William
Carnahan Marvin
Copenheaver Blaine
Ruddock Ula C.
Smith , Gambrell & Russell, LLP
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