Plastic and nonmetallic article shaping or treating: processes – Direct application of electrical or wave energy to work – Producing or treating porous product
Reexamination Certificate
2001-01-31
2003-09-09
Kuhns, Allan R. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Direct application of electrical or wave energy to work
Producing or treating porous product
C264S045500, C264S045800, C264S048000, C264S051000, C264S102000, C425S00400R, C425S135000
Reexamination Certificate
active
06616886
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the production of a skinless or low skin flexible polyurethane foam slab using a continuous foaming process.
During the continuous production of free rise flexible slabstock polyurethane foam, a dense skin forms on the top surface of the foam slab. The skin varies in thickness depending upon the chemical formulation used to form the foam, the environmental conditions and other processing parameters. The top skin forms on free-rise polyurethane flexible foams formed using various processing equipment, including, inter alia, MAXFOAM™, VARIMAX™, VARIABLE PRESSURE FOAMING (VPF™), FOAM-ONE™, EDGEMASTER™, conventional direct laydown, and CO
2
processing equipment (i.e., Novaflex™, CarDio™, and Beamech CO-2™).
Depending upon the formulation, the top skin that forms on the polyurethane slab has a high density and a thickness from 0.01 to 0.25 inches. For many end use applications, the skin must be cut away and discarded as waste, thus resulting in loss of chemicals that could have produced useful polyurethane foam. Depending upon the exact thickness and density of the top skin formed, the formulation chemicals present in the skin typically are sufficient to have produced 0.1 to 2.5 inches of additional foam height. This skin thus represents a significant yield loss in the foaming process.
Apparatus and methods have been proposed in attempts to eliminate the top skin, but they have not proved successful for forming flexible polyurethane slabstock foam. One common method is to apply a paper or polyethylene sheet to the top surface of the foam during the early stages of the foam forming chemical reaction (PINTOMAX™). Using this PINTOMAX™ process, skin formation may be partly avoided because the sheet forms a barrier between the foam and the atmosphere above the foam. However, the sheet also prevents gases generated during the foaming process (such as CO
2
, methylene chloride, acetone, pentane or other auxiliary blowing agents) from escaping without damaging the foam cell structure and forming splits and voids on the top surface of the slab.
U.S. Pat. No. 3,816,233 discloses a method for forming a polyurethane sheet with different properties on its top and bottom surfaces produced by differentially heating (or cooling) those surfaces during foam expansion. The patent seeks a foam with a “porous skin”. In the preferred embodiment, a top cover sheet placed on the top surface of the foaming mixture is heated with infrared radiation from lamps to temperature from 25° to 100° F. above the temperature of the bottom surface of the foaming mixture. The preferred heating temperature is 175° F. to 200° F. for the heated side and 80° F. to 120° F. for the unheated (or cooled) side. The foam is then compressed to a desired thickness and the cover sheet is removed. The patent thus seeks to form a region of higher density, like a skin on the foam sheet surface, and uses top heating to accomplish this result.
U.S. Pat. No. 3,345,439 describes forming rigid thermoplastic structures. The patent explains that maintaining the temperature of the walls of the structure in which the foam is produced affects the thickness of the skin that is formed on the surface of the material. Lower temperatures produce thicker unexpanded skins. According to the patent, the rising foam is passed between heating platens, and in order to achieve a more gradual expansion, one of the heating platens may be angled such that the heating field has a greater intensity at the leading edge than at the trailing edge. The patent relates only to rigid foams.
Published patent application WO 00/47384 discloses a method and apparatus for producing skinless flexible polyurethane slabstock foam in continuous or molding processes in which the foaming occurs in a closed space with a gas saturated atmosphere. The preferred gas in the gas saturated atmosphere is carbon dioxide, or a mixture of carbon dioxide with other gases. The application suggests it may also be advisable to heat the surface of the enclosure in which foaming occurs to prevent undesired condensation thereon.
Perhaps similar to WO 00/47384, U.S. Pat. 5,804,113 discloses a method for producing slabstock polyurethane foam under controlled pressure and temperature conditions. Free rise expansion of the foaming ingredients takes place within a hermetically enclosed space. The temperature in the enclosed space is controlled by introducing heated air. The air temperature range is from 10° C. to 75° C., preferably 20° C. to 50° C. (Col. 5, lines 28-29). The patent does not discuss foam skin formation or methods for eliminating such skin.
The prior art does not teach or suggest a method and apparatus for preventing or reducing the skin formation on a flexible polyurethane slabstock foam without additional gases or foam-forming ingredients or extraneous films. Nor does the prior art teach or suggest a method and apparatus for preventing or reducing the skin formation on a flexible polyurethane slabstock foam produced on an open conveyor.
SUMMARY OF THE INVENTION
A method for reducing or eliminating a surface skin when producing a flexible polyurethane foam slab includes the steps of: (a) feeding a chemically reactive polyurethane foaming mixture into a region in which foaming expansion occurs, and (b) heating the top surface of the polyurethane foaming mixture during foaming expansion and/or after such expansion. Preferably, heat is applied only as such foaming expansion occurs.
The chemically reactive polyurethane foaming mixture defines an internal temperature as it expands. The internal temperature escalates as the foaming mixture is conveyed away from the feeding zone of the dispensing device. Preferably, the heating is conducted so as to prevent a significant temperature gradient from forming between the internal temperature and the temperature measured at the top surface of the polyurethane foaming mixture. Heating the top surface to a temperature within about±50° F. of the internal temperature of the foaming mixture has produced satisfactory results for most polyurethane foams.
Most preferably, the top surface of the polyurethane foaming mixture is heated to a temperature in the range of about 140° F. to 350° F. Excellent results have been obtained by heating to temperatures in the range of about 230° F. to 260° F. The preferred temperature is dependent upon the chemical formulation, the processing parameters and the ambient conditions during the process. Heating may also be applied in an escalating fashion, such that the top surface of the foaming mixture is heated to a greater degree as expansion occurs and as the foaming mixture is conveyed along a conveying path away from the dispensing device.
The heating may be supplied by one or a combination of radiant heat lamps, radiant heating panels, by heated gas, preferably air, or other heating sources introduced into the region in which foaming expansion occurs, and that are determined to be appropriate for the foaming chemical formula, process conditions and space limitations to maximize the benefit of skin density reduction. The heat may be applied from the region immediately adjacent to the dispensing device and to a region up to approximately 200 feet downstream and away from the dispensing device. For VARIMAX™ foaming equipment, for example, the heat may be applied to the surface of the foaming mixture starting at the trough and extending up to 200 feet downstream from the trough. For conventional foaming equipment in which the dispensing device does not include a trough, the heat may be applied to the surface of the foaming mixture within about twenty feet after the foaming mixture is laid onto a fall plate or conveyor and heating may be continued up to 200 feet downstream from the dispensing device.
The invention further comprises an apparatus for producing a skinless or low skin flexible polyurethane foam slab, which may have a trough or a dispensing device into which a chemically reactive polyurethane foaming mixture is introduced, a fall plate onto whi
Bartsch Theodore J.
Peterson Bruce W.
Connolly Bove & Lodge & Hutz LLP
Foamex L.P.
Kuhns Allan R.
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