Apparatus and process for producing polymeric foam

Details Apparatus and process for producing polymeric foam Apparatus and process for producing polymeric foam

1999-07-09

2001-06-05

Cooney, Jr., John M. (Department: 1711)

Gas and liquid contact apparatus

Fluid distribution

Valved

C261SDIG002, C264S045900, C264S051000, C264S211000, C264S211210, C366S158500, C366S182100, C366S192000, C366S336000, C425S190000, C425S00400R, C425S00400R, C521S079000, C521S130000, C521S133000, C521S155000

Reexamination Certificate

active

06241220

ABSTRACT:

The present invention relates to an apparatus and process for producing polymeric foam an din particular, but not exclusively, to an apparatus and process for the continuous production of polymeric foam.
The preparation of polymeric foams conventionally requires the homogenous mixing of several components which react to form a foamed polymer. 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 polymer.
By discharging a layer of the mixture onto a moving conveyor (with a movable base and with adjustable opposed side walls) the components of the mixture react and continuously produce a polymeric foam block.
More recently, the continuous production of low 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 vaporisation 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.
In published WO96/00644, it is proposed to expand a foamable reactive mixture into a large number of individual flows at shear rates of above 500/sec. This is sought to be achieved by passing the reactive mixture through one or more separate fine meshed screens having a screen size from 0.025 to 0.3 mm. The “screens” envisaged in WO96/00644 comprise apertured discs or nets, ie. having simple direct paths therethrough. The various embodiments shown by way of example in WO96/00644 all use separate, multiple fine-meshed screens which are held apart axially by respective spacers so as to have a clear space there between. This appears to have a likely disadvantage that, due to the individual pressure drops across these screens, some frothing could take place in the spaces between the screens which would be highly undesirable in practice. A further disadvantage is that the fine mesh screens can only permit a limited pressure drop without deformation or damage to the screen.
In published WO-A-9616782, it is proposed that a reactive mixture containing carbon dioxide dissolved under pressure should be expanded suddenly so as to produce high rates of shear in the mixture. This is to be achieved by passing the mixture through at least one opening extending to less than 1 mm in at least one direction. The at least one opening can be a fine-mesh sieve, a perforated plate, a slotted grid or an extended slit. A further perforated plate, acting as a stabilizing sieve can be placed downstream of the aforementioned at least one opening. Thus, it is known from WO-A-9616782 to provide a permeable discharge head for use in the production of polymeric foams, through which a reaction mixture is discharged with a controlled pressure drop for formation of a froth, the discharge head comprising plural individual meshes.
UK 1 575 049 is concerned with filter mediums for removing contaminants from molten polymers in the formation of synthetic fibres. It proposes a filter medium structure wherein layers of sintered metal fibres are disposed upstream and downstream of a woven screen member. However, this patent is not concerned with the production of such foams from their constituent reactive chemicals and it does not propose to use the filter medium for the purpose of controlling the pressure drop at a discharge head for the production of such polymeric foams.
The method and apparatus proposed in WO96/02377 are similar to those of WO96/00644 but use a “screening stack” (Siebpackung) to repeatedly divide a reactive mixture into a plurality of individual flows whilst impressing multi-alternating shear rates with maximum values of at least 200/sec. The “screen packing” of WO96/02377 is constituted by a “granular mass” of regular or irregular organic or inorganic filling bodies, with the preferred filling bodies being spheres WO96/00644 also mentions that the “screen packing” might comprise sintered metal plates or sintered glass plates, ie plates made of sintered metal or glass particles. The disadvantages of sintered particle plates are discussed herein, after example 7.
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 the 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-0645226, WO96/00644 and WO96/02377 are all based on a three step process of:
(1) premixing under pressure at least one of the reactants and liquid CO
2
;
(2) mixing the component containing the carbon dioxide with the further reactive components or with the second reactive component under pressure; and then
(3) passing the mixed reactive components containing the CO
2
through a pressure reducing device.
The latter systems all rely on having fully mixed “reactive mixtures” upstream of the fine-mesh screen(s) or screen packing, 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.
It is an object of the present invention to provide an improved apparatus and process for producing polymeric foam of uniformity high quality.
In accordance with a first aspect of the present invention, there is provided an apparatus for producing a polymeric foam, wherein foam components are brought together under pressure and are passed through a permeable discharge head where the pressure is reduced, the permeable discharge head comprising plural individual woven wire meshes sintered together to form a composite element, preferably of permeability coefficient between 1×10
−6
and 200×10
−6

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