Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
2000-08-25
2001-12-04
Cooney, Jr., John M. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Cellular products or processes of preparing a cellular...
C264S045900, C264S050000, C264S051000, C264S176100, C264S211000, C264S211210, C425S00400R, C425S00400R, C425S190000, C521S079000, C521S080000, C521S155000, C521S917000
Reexamination Certificate
active
06326413
ABSTRACT:
The present invention relates to a process and apparatus for producing polyurethane foam and in particular to an apparatus and process for the continuous and non-continuous production of polyurethane foam using a gas as the auxiliary blowing agent.
The preparation of polyurethane foams conventionally requires the mixing of several components which cures to form a foamed mass. Typically, the components include a polyol, an isocyanate, a catalyst or catalysts, a surfactant, activators and water. When these components are mixed together in the correct proportions, the water reacts with the isocyanate to produce carbon dioxide for expansion of the polyurethane.
By discharging a layer of the mixture onto a moving conveyor (with a movable base and with adjustable opposed sidewalls) the components of the mixture react and continuously produce a polymeric foam block.
More recently, the continuous production of low density and soft high density foams has involved the use of a relatively inert, low boiling point liquid which is mixed under pressure in a liquid state with the other chemical components in order to provide an auxiliary blowing/frothing action on the mixture before the mixture expands by production of carbon dioxide from the isocyanate/water reaction. The mixture is discharged onto the conveyor and the vaporised low boiling point liquid expands the reaction mixture which then subsequently expands by the chemical production of carbon dioxide from the reaction mixture to form a polymeric foam.
Previously, suitable blowing/frothing agents have included various chlorofluorocarbons (CFCs). Although CFCs have the desired inertness and relatively low boiling point, their use has recently been discouraged on environmental considerations since it is believed that CFCs contribute to depletion of the ozone layer. A suitable replacement for CFCs is carbon dioxide but, since carbon dioxide vaporises at a much lower temperature than . . . CFCs, and indeed must be pressurised in order to exist as a liquid, relatively high pressures have had to be maintained throughout the apparatus and method.
However, unless the expansion of the carbon dioxide occurs under controlled conditions, some of it can be lost and the efficiency of the foam expansion is reduced and poor quality foam may be produced, with non-uniform cell structure and voids or “pin-holes”.
The apparatus described in EP-A-0645226 attempts to discharge the reaction mixture under controlled conditions by discharging the mixture through an elongate pressure-drop zone to initiate frothing, flowing the frothing mixture along a frothing cavity and through an outlet aperture and subsequently discharging the frothing mixture onto a substrate. It also discloses that relatively large amounts of nucleating gases are added to assist in bubble formation.
In published WO96/00644 and WO96/02377 it is proposed to expand a single phase homogeneous foamable reactive mixture containing dissolved liquid CO
2
into a large number of individual flows at shear rates above 500 per second.
The “reactive mixture” which is used in the techniques disclosed and claimed in both WO96/00644 and WO96/02377 is obtained, using at least two reactive components and carbon dioxide as expanding agent, by mixing at least one of the reactive components with carbon dioxide under pressure, thereby producing a mixture containing liquid carbon dioxide, and subsequently mixing the resultant mixture with the other reactive components to form a single phase homogeneous foamable reactive mixture. The latter sequential mixing steps are typically carried out in a conventional static mixer and a rotary mixing head, respectively. As is well known, conventional “static mixers” incorporate a plurality of angled vanes disposed within an internal chamber such as to promote turbulence and mixing of component fluids passed therethrough.
Thus, the method and apparatus of EP-A-0645226, WO96/00644 and WO96/02377 are all based on a three step process of:
(1) pre-mixing under pressure at least one of the reactants and liquid CO
2
;
(2) mixing the component containing the liquid carbon dioxide with the further reactive components or with the second reactive component under pressure; and then
(3) passing the mixed single phase homogeneous reactive components containing the liquid CO
2
through a pressure reducing device.
The latter systems all rely on having fully mixed single phase homogeneous “reactive mixtures” upstream of the fine-mesh screen(s) or screen packaging, since insufficient or insufficiently uniform mixing takes place within the screen apertures themselves.
Other processes using liquid CO
2
require the addition of a higher than normal quantity of supplementary nucleating gases. We have discovered that using our permeable discharge head described hereinafter largely eliminates these supplementary nucleating gases.
Numerous patents have been issued on the use of liquid CO
2
in polyurethane foam. U.S. Pat. No. 4,906,672 refers to methods of adding and dissolving gaseous CO
2
under pressure to the foaming reactants but does not disclose a method of controlling the final pressure reduction and expansion.
The problems of processing with a non-homogeneous combination of foam reactants and CO
2
prior to the discharge device are described in WO96/00644. The discharge device covered by this patent requires “that there is still no formation of gas bubbles prior to passage through the screen”
Published WO96/00644, U.S. Pat. Nos. 3,108,976 and 3,256,218 all propose the use of multiple plain perforated plates for the purpose of achieving the final discharge of foam reactants containing the blowing agent under controlled pressure and controlled cell expansion. WO96/00644 refers to the use of carbon dioxide in the liquid phase whilst U.S. Pat. Nos. 3,108,976 and 3,256,218 refers to CO
2
in both the gaseous and liquid phase. For the reason given in WO96/00644 when using the known multiple plain perforated plates in the discharge device, the quality of foam produced is not of commercially acceptable quality unless a homogeneous mixture, ie. a mixture existing in a single phase (liquid) state, of foam reactants and carbon dioxide is present prior to passage through the discharge device.
It is an object of the present invention to provide a process and apparatus which will give good polyurethane foam using a gaseous auxiliary blowing agent, particularly gaseous CO
2
.
There is described in our own earlier publication WO-A-9702938 a process and apparatus for producing polyurethane foam, wherein reactive foam components and a low-boiling point frothing agent are brought together, under sufficient pressure to maintain the frothing agent in a liquid state, and then passed through a permeable discharge head from which the resulting mixture is discharged, with the pressure being reduced and froth being formed, the permeable discharge head comprising at least one diffuser element having a structure which repeatedly divides and mixes flow paths in three dimensions within the axial thickness of the diffuser element, considered in the direction of flow therethrough, the diffuser element comprising two or more layers of wire mesh which have been sintered together such that contact points of the mesh layers are fused together by the sintering process.
Surprisingly, it has been found that the use of our special permeable discharge head, in the form of plural individual wire meshes sintered together to form at least one composite element which repeatedly mixes the flows as they pass therethrough, enables the production of good quality foam, when the foam components and gaseous CO
2
blowing agent are arranged always to remain in a non-homogeneous state prior to expansion. This therefore means that it is not necessary to ensure fully dissolving of the gaseous CO
2
(ie. it is not necessary to have or achieve a homogeneous mixture prior to expansion) and the CO
2
can be inserted only a small distance from the discharge head, corresponding to only a short time, before it reaches the discharge head.
In accordan
Blackwell James B.
Blackwell John J.
Blackwell Stephen W.
Buckley Geoffrey
Beamech Group Limited
Christie Parker & Hale LLP
Cooney Jr. John M.
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