Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
1998-08-17
2001-08-07
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...
C521S130000, C521S155000
Reexamination Certificate
active
06271275
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the production of polyurethane foam and, more particularly, to methods and apparatus for on-site production of polyurethane foam having application in packaging and other industries.
BACKGROUND OF THE INVENTION
Polyurethane foams are widely used in a variety of applications, including the packaging industry, in which polyurethane foams are used for cushioning fragile articles for shipping and handling. Various processes for producing polyurethane foams are known in the art. In general, a polyol-containing precursor and an isocyanate-containing precursor are brought together and mixed in the presence of a catalyst to cause a reaction which leads to curing and solidification of the mixture. A gas is introduced into the mixture prior to solidification so that foaming of the mixture occurs.
A desirable objective in mixing the precursors is to achieve sufficient mixing so that the resultant polyurethane foam is substantially uniform and has the desired density for the intended application. Chemical and/or mechanical mixing techniques have been used for aiding the mixing of the polyol and isocyanate precursors. For instance, chemical blowing agents such as hydrocarbons, fluorocarbons, chlorofluorocarbons, and the like, have been used for introducing gas into the precursors to promote a foaming action which also facilitates mixing of the components. However, such chemical agents are costly, and some pose environmental and health hazards.
Consequently, mechanical mixing techniques have been developed for introducing gas into the precursors and for mixing the two precursors. For example, U.S. Pat. No. 5,472,990, issued to Craig et al., describes a method of producing polyurethane foam in which a polyol precursor is mixed with air in a dynamic mixer, and the polyol/air compound is then mixed with an isocyanate precursor in a static mixer. The mixture is discharged from the exit of the static mixer into the workpiece or site where polyurethane foam is needed.
Processes such as the one described in the Craig patent are effective for producing uniform polyurethane foams. However, a major drawback to all such processes in which the two precursors are mixed within a through-flow device, such as a dynamic or static mixer, is that polyurethane begins to form instantly inside the device as the two components mix and begin to react. As a result, unless steps are taken to prevent build-up of polyurethane within the mixing device, the device will eventually become clogged and will cease to function properly. This build-up of polyurethane is conventionally overcome by frequent maintenance of the mixing devices to keep them unclogged and working, including flushing of the devices with a solvent to dissolve the polyurethane deposits from the insides of the devices. However, these maintenance and flushing procedures take time away from more-productive activities. Additionally, the flushing systems add electromechanical complexity and, consequently, add to the cost of a dispensing system with no corresponding gain in efficiency or usefulness. Furthermore, the solvents are costly, and their use and disposal can pose health and environmental problems.
Thus, there has been a need for a method of producing polyurethane foam in which hazardous mixing agents and/or solvents are not required, and which alleviates the problem of mixing devices being clogged by polyurethane deposits.
SUMMARY OF THE INVENTION
In contrast to the prior methods employing mixing chambers as noted above, the present invention provides methods and apparatus for producing polyurethane foam in which the precursors are mixed without the assistance of any mixing chamber or other through-flow device that might become clogged over time. Furthermore, the methods and apparatus of the invention do not require the use of any hazardous chemical mixing agents or solvents.
To these ends, a preferred embodiment of a method in accordance with the invention comprises dispersing bubbles of a gas in a first liquid precursor to form a froth, directing a stream of the froth into a free space at a first predetermined rate of flow, and directing a stream of a second precursor at a second predetermined rate of flow into the free space so as to contact and mingle with the stream of froth, whereby the two precursors mix without the assistance of a through-flow mixing chamber or device and react to form a polyurethane foam.
The gas is preferably air, although carbon dioxide, nitrogen, or any other safe gas may be used. The first precursor which is frothed preferably comprises a polyol-containing precursor (also referred to herein as a “B” component), and the second precursor preferably comprises an isocyanate-containing precursor (also referred to herein as an “A” component). Alternatively, however, the “A” component may be frothed and the “B” component mixed in liquid form with the frothed “A” component. Furthermore, another alternative is to froth both the “A” and “B” components and mix the two froth streams. The invention takes advantage of the finding that a polyol-containing precursor readily mixes sufficiently with an isocyanate-containing precursor to produce a substantially uniform polyurethane foam, without mechanical intervention and without any substantial impingement forces needing to be exerted, as long as one or both of the precursors is formed into a froth prior to being mixed with the other precursor.
In accordance with one embodiment of the invention, the two streams of precursors are directed against a target so as to impinge on the target in close proximity to each other. The precursors then fall together under the influence of gravity and mix as they fall. The streams may be directed to impinge on opposite sides of the target or on the same side of the target. After impingement, the precursors may fall into an open container for foam-in-place packaging, into a mold to produce a shaped article, into a bag to be used as a cushioning device, or into any other site where polyurethane foam is needed.
Various techniques may be employed for generating the froth of the first precursor. In accordance with one preferred embodiment of the invention, bubbles of a gas such as air are introduced into the first precursor within a through-flow device to form a froth, and the froth is then dispensed through a nozzle into the free space to contact and mingle with the second precursor. Advantageously, the liquid first precursor is passed over a porous sintered member through which a gas such as air is passed. The sintered element creates air bubbles which are entrained in the liquid precursor flowing over the sintered member. Thus, froth is created on an as-needed basis. The sintered member advantageously can be incorporated into the dispenser which directs the froth into the free space, so that froth is created immediately upstream of the dispenser exit. Thus, the froth is created on demand and virtually all of the froth that is created is used immediately. Therefore, there is substantially no inventory of froth created which can decay back into liquid form.
However, other techniques for generating the froth can be used. For instance, in another preferred embodiment of the invention, polyol-containing “B” component is heated to about 55°-85° C. and is supplied under pressure to a dispenser, from which the heated “B” component is dispensed into the free space. Air which is naturally dissolved in the “B” component is liberated as the pressure of the “B” component drops to atmospheric pressure at the exit of the dispenser nozzle, and the liberated air forms air bubbles in the “B” component, thus creating a froth.
The invention thus provides methods and apparatus for producing polyurethane foam without costly and hazardous chemical agents and solvents, and without the need for mixing the precursors within any permanent through-flow mixing device. Additionally, it has been found that the process is operable over a wide range of flow rates, in contrast to processes employing a static mixer which typically is tuned f
Cooney Jr. John M.
Lagaly Thomas C.
Sealed Air Corp. (US)
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