GB2301595A - Vertical production of foamed polyurethane slab-stocks - Google Patents

Abstract

A process and an apparatus for upwardly production of foamed polyurethane slab-stocks. The reactive polyurethane components are mixed, under pressure, with a low-boiling blowing agent kept in the liquid state; frothing of the mixture is started by continuously flowing the same into a frothing device (10) comprising a pressure equalizing chamber (13) and a pressure-drop zone (17, 31, 32) axially extending in the flow direction, which emerges into a frothing cavity (19) at the bottom of an upwardly diverging and upwardly extending expansion enclosure (20) provided with moving wall means (22) for taking off the expanding foam.

Description

VERTICAL PRODUCTION OF FOAMED POLYURETHANE SLA3-STOCKS The present invention relates to a process and an apparatus for the continuous production of polyurethane foam slab-stocks using, in a novel and advantageous manner, the innovative principles of a frothing device which is the subject matter of a previous European Patent Application EP A 0,645,226.

In the continuous production of pcyurethane foam slab-stocks, both of the flexible and rigid type, horizontally extending plants are conventionally used, sai plants being characterised by large overall dimensions anc a high daily output, since a normal polyurethane foaming process usually requires high travel speeds of the conveyor and a consequently long polymerization path for the foam, so as to avoid the undesirable phenomenon whereby the mixture still in the liquid state, flows back underneath the expanding polyurethane foam or in the initial polymerization phase; conventional plants of this kind are described for example in US-A-3,325,823 and US-A-3,786,122.

In order to overcome this drawback and with the object of providing a plant having smaller dimensions and a smaller throughput, so as to permit better results and a more economical management, the European Patent Application EP-A0,645,226 proposes a particular frothing process and a system for the continuous production of polyurethane foam slabstocks, which use a special frothing device whereby it is possible to mix the reactive polyurethane components with a low-boiling blowing agent, in particular CO2 or some other chemically inert blowing agent, under pressure controlled conditions, such that the blowing agent is kept in a liquid state before frothing takes place.In particular, according to this document, frothing of the polyurethane mixture is performed by means of a continuous process comprising the steps of: preparing a mixture of reactive polyurethane components and a low-boiling blowing agent in a liquid state; feeding the resulting mixture into a pressure-equalizing chamber and distributing the mixture over an enlarged pressure-drop zone axially extending in the flow direction of the same mixture to provide a back-pressure up-stream the pressure-drop zone to keep the blowing agent at the liquid state, and starting the frothing by gradually releasing the pressure and the blowing agent in the mixture while flowing through said pressure-drop zone and into a frothing cavity of a lay-down device to distribute the foam onto a substrate moving along a horizontal path.

In the past, machines for the continuous production of polyurethane foams based on conventional water containing formulations have also been proposed where the polyurethane foam was produced and drawn upwards at relatively low speed and throughput, so as to reduce the overall dimensions of the entire plant. Examples thereof can be found in various prior documents, for example in French Patent Application 2,023,961, or in the European Patent Application EP-A-0,058,553. Of the various solutions proposed, few of them have been applied in practice.

In particular, both in the Patent Application FR-A 2,023,961 and in the European Application EP-A-0,0;8,553, the polyurethane mixture obtained by a conventional mixing device, is fed in the liquid state into a through provided at the bottom of a foaming space, where the mixture is expanded by gas generated by the chemical reaction between the polyurethane components and water, polymerizing as it is drawn upwards; the height of the foaming space and the speed with which the foaming polyurethane material is drawn up, as well as the length of the entire polymerization pat, therefore depend on the mixing and on the same foaming conditions.

In general, these prior documents propose a process and an apparatus for conventional polyurethane mIxtures, in which expansion of the foam is obtained exclusively owing to the effect of the carbon dioxide generated by the chemical reaction between water and isocyanate. Therefore, the height of the foaming space required in order to obtain a complete expansion and polymerization of the foam is still undesirably excessive; moreover, the characteristics of the polyurethane material and of the plant itself, strongly depend on the technology used.In particular care should be taken to reduce pressure on the foaming mass, at the beginning of the rising step, to avoid any negative influence on =he structure of the cells, since foaming for chemical reaction between polyurethane components is subjected to changes in rate due to many uncontrollable or variable factors. None of these documents therefore suggests or proposes a process and a plant which are suitable for the frothing technology, in the continuous production of polyurethane foam slab-stocks.

From experimental tests, it has now been discovered that by appropriately applying the frothing technology in the upward production of foamed polyurethane slab-stocks, it is possible to improve foam structure, as well as to reduce the overall dimension of the apparatus and simplify the same, since expansion takes place during upwardly directed frothing of the mixture and before chemical reaction starts; negative influence of the pressure in the expanding mass is therefore less critical and substantially reduced.

Therefore, an object of the present invention is to provide a process and a plant for the continuous production of polyurethane foam slab-stocks, which advantageously use the frothing technology and a special foam expansion device designed to allow mixing under pressure of the polyurethane components and an inert blowing agent in the liquid state, as well as to control the release of the blowing agent, avoiding turbulent evaporization of the blowing agent by keeping conditions suitable for promoting frothing of the mixture, such that a controlled expansion of the foam takes place in a comparatively small space.

According to a first aspect of the invention, a process for the continuous production of polyurethane foam slab-stocks has been provided, comprising the steps of: preparing a mixture of reactive polyurethane components and a low-boiling blowing agent in a liquid state; feeding the resulting mixture into a pressure-equalizing chamber and distributing said mixture over an enlarged pressure drop zone axially extending in the flow direction of the same mixture to provide a back-pressure up-stream of the pressure crop zone to keep the blowing agent at a liquid state. Froth--.g is then started by gradually releasing the pressure and tne blowing agent in the mixture while flowing through the pressure-drop zone and into a frothing cavity, the mixture being frothed into a progressively expanding polyurethane mater~21 by means of the blowing agent inside an open top expansion enclosure that extends upwardly from the frothing cavity, and then upwardly taking-off the expanding material from t:e expansion enclosure while still frothing and the chemical reaction between the polyurethane components takes place.

According to a second aspect of the invention, an apparatus suitable for the continuous section of polyurethane foam slab-stocks has been provided. Such apparatus includes a mixing device for mixing polyurethane chemical components and a liquid blowing agent, sh as CO2, under pressure controlled conditions. The mixing device is connected by a feeding line to a frothing device, .he frothing device comprising a mixture distributing an pressure equalizing chamber opening onto a pressure-drop cone axially extending in the flow direction to provide a back-pressure in the pressure equalizing chamber to keep the blowing agent in a liquid state, and to gradually release the blowing agent into the polyurethane mixture within a frothing cavity. The apparatus further comprises in combination an open top expansion enclosure having upwardly extending walls; a frothing cavity between the pressure-drop zone of the frothing device and a diverging portion at the bottom of the expansion enclosure means; and an upwardly extending foam taking-off system comprising sheet material sliding on the upwardly extending walls and moving together with the foaming material inside the expansion enclosure.

The process and apparatus according to the present invention will be illustrated in greater detail hereinbelow with reference to the accompanying drawings, in which: Figure 1 is a longitudinal sectional view through a plant according to the invention; Figure 2 is a longitudinal sectional view of the plant, along a plane perpendicular to that of Figure 1; Figure 3 is a cross-sectional perspective view, on a larger scale, of the frothing device of Figure 1; Figure 4 is a cross-sectional view of a modified frothing device, Figures 5 to 8 show different embodiments of the pressure-drop zone of the frothing device for the plant of Figure 1.

The present invention is based on the advantageous use of the frothing technology in an upwardly directed continuous production of polyurethane foam slab-stocks. Such slab-stock product occurs by means of a frothing device, as developed and described in a prior European Patent Application EP-A-0,645,226 of the same Applicant, which has been applied to an upwardly directed production of polyurethane foam slabstocks so as to improve the foaming conditions in that the frothing of the mixture occurs in the same direction in which the gaseous blowing agent is released, inside the foaming material, all of which occurs within an extremely short space, as the polyurethane foam moves and is drawn up at low speed from the bottom of an expansion enclosure.In this way, not only are the frothing conditions and expansion c r the foam improved, but the heightwise dimensions of the entre foaming plant can also be reduced, compared to conventional ones.

As shown in Figures 1 and 2, the plant substantially comprises a frothing device 10 provided at the bottom of an expansion enclosure comprising a path 11 for the polyurethane foam 12, which extends vertically and in which polymerization occurs.

The frothing device 10, in the exartple shown, consists of a tubular distribution bar 13 comprising a longitudinal chamber 14 connected by ducts 15 to tne outlet of a mixing apparatus 16 of any suitable type i which the chemically reactive polyurethane components, such as a polyol and isocyanate, together with any additives, and an inert, low- boiling blowing agent, for example liquid CD2 at liquid state are mixed under pressure controlled conitions. The blowing agent is subsequently released in the form of gas for causing the frothing and the expansion of the polyurethane mixture, prior to the chemical reaction between the polyurethane components and polyermization or the foam material takes place.

The chamber 14 of the tubular bar 14 opens towards a frothing cavity 19 through a pressure-drop zone 17 axially extending in the flow direction. Chamber 14 has the function of equalizing the pressure and homogeneously distributing the mixture over the area of the pressure-drop zone 17, substantially over the entire width of the blocks of polyurethane material to be produced, in the example shown.

In particular, as shown in Figure 1 and in the larger-scale view of Figure 3, the distribution bar 13 of the frothing device has, on one side, a pressure-drop zone 17 in the form of a narrow longitudinal slot which emerges inside a short horizontal duct 18 in turn leading into an upwardly directed frothing cavity 19. Cavity 19 has a funnel shaped section 19A protruding within the bottom of an open-top expansion enclosure for example, as shown in Figure 1, having upwardly diverging side walls 20 which extend heightwise from the diverging section 19A of the frothing cavity.

As stated above the diverging section 19A of the frothing cavity partially penetrates between the side walls 20 which define the bottom of side walls 26 and provide entry into an open top expansion enclosure extending vertically, inside which frothing and expansion of polyurethane mixture can occur in a controlled manner while the expanding material moves, being drawn upwardly from the bottom.

In particular, the expansion enclosure and the path 11 for polymerization of the foam are delimited, on the sides, by moving surfaces which accompany and laterally contain the polyurethane foam 12 as it moves towards an upper cutting zone 21.

As shown in Figures 1 and 2 of the drawings, each of the moving surfaces which accompany the polyurethane foam is defined by a sheet material 22 in strip form, for example a paper strip or plastic film, which is unwound from a lower roller 23 and is rewound onto an upper roller 24. The rollers 23 and 24 are suitably power actuated and controlled so that the speed of each individual strip of material 22 corresponds to the speed at which the polyurethane foam 12 is drawn or travels upwardly. The strip material 22 also contains the frothing mixture during the frothing step.

Upward drawing of the polyurethane foam 12 must therefore be carried out at a speed which is related to the speed of expansion and rising of the mixture inside the frothing cavity 19, 19A and the expansion enclosure. Such upward drawings obtained by means of at least two conveyor belts 25 as shown in Figure 2 arranged on two opposite sides of the path 21. Rigid side walls 26 are provided on the other two sides, with walls 26 extending upwardly from the top edge of the walls 20, along the path 11, over a sufficient space to allow dimensional and structural stabilisatlon of the polyurethane foam 12 during polymerization.

As shown, the polyurethane foam 12 is drawn upwards, beyond the rollers 24 for rewinding the strip 22, towards the top of the apparatus and a transverse cutting device, schematically shown at 21, where individual slab-stocks 27 of expanded polyurethane foam are cut and removed from the top of the apparatus.

Figure 3 of the drawings shows, on a large scale, a perspective cross-sectional view of a first embodiment of the frothing apparatus 10, forming a substantial part of the plant.

As can be noted from Figure 3, an elongated slot defines a pressure-drop zone. That slot is delimited by parallel flat surfaces which are substantially extending in the direction of flow. Further slot defining pressure-drop zone 17 is provided or positioned to be in a plane parallel to and laterally extending at a distance from the longitudinal axis of the frothing cavity 19 and the upwardly extending polymerization path 11. The length in the flow direction and the thickness of the slot 17 must be such as to generate, inside the chamber 14 of the frothing device and the mixing device 16, a back-pressure sufficient to keep the blowing agent in a liquid state.Purely by way of example, the slot defining pressure-drop zone 17 may have an opening thickness perpendicular to the flow direction of between 1 and 10 tenths of a millimetre, a length in the direction of flow of between about 10 and 30 mm, as well as a width in a cross machine direction of the expansion enclosure 20, for example of about 2 metres and in any case equal to the width, in the same plane, of the polyurethane material 12.

In general term, the shape, the cross sectional area and the geometrical dimensions of the pressure-drop zone 17 are dependent upon several parameters. More precisely in the case of Figure 3, the geometrical features of the slot defining pressure-drop zone 17 to obtain a pressure P upstream the same, that is in the pressure equalizing chamber 14 of the frothing device 10, suitable to keep the blowing agent at the liquid state, for example at a pressure of 5 Bar or higher, and to provide laminar flow and required shearing conditions through the same slot, at a flow velocity considerably below the critical value or Reynolds number, for example at a velocity of about 5 to 10 m/s, may be calculated by the following formula Q 1 P = -------- -------- K in which L P = pressure (Bar) in the chamber 14 of the frothing device; Q = output (lt/min);; L = extension (mm) of the slot in the longitudinal direction of the chamber 14; 1 = length (mm) of the slot in the flow direction; S = height or thickness (mm) of the slot; K = 0,03 4 1,2 depending on viscosity and specific gravity of the chemical components obtained by experimental tests.

In Figures 1 and 3 the slot defining pressure drop zone 17 is arranged in a plane parallel to the axis of the vertical path 11; however, it could aslo be differently oriented or form any angle with said plane.

Again from Figure 3 it can be noted that the slot defining pressure-drop zone 17 leads into a first horizontal duct 18 of the frothing cavity, delimited by an external surface of the said distribution bar 13, by a side wall 28 set back with respect to the slot defining pressure-drop zone 17 and by an upper horizontal wall 29 which extends from rear to beyond the aforementioned slot.

The upwardly directed part of the frothing cavity 19 is in turn defined by a bottom wall 30, which preferably forms an extension of the upper surface of the distribution bar 13, as well as by diverging side walls 19A and by end walls 19B.

It is important to note that the walls 19A and 19B penetrate into the bottom of the expansion enclosure 20 which extends upwardly over a distance sufficient to allow the complete expansion of the polyurethane mixture under the effect of CO2 or the blowing agent which is gradually released in the gaseous state while the polyurethane components are not yet reacting chemically with one another.

Since the polyurethane mixture is instantly frothed upon release of the CO2, and assuming the appearance of a dense cream which fills the cavity 19, the froth will expand in a homogeneous manner in the same vertical take-off direction of the polyurethane material 12. The cellular structure obtained in the polyurethane foam is fine and homogeneous, without large bubbles or cavities. Such frothing and expansion is accomplished in a relatively small expansion space and with very low take-off speed.

Therefore, both the speed of the strip material 22 which are moving together with the polyurethane foam 12 during its upward movement, and the speed of the conveyor belts 25, must be suitably related to the expansion speed of the polyurethane foam while frothing. Such speeds must be substantially constant so as to avoid formation of polyurethane foams having too high a density or a lack of homogeneity owing to the presence of holes or cavities.

The working conditions of the various moving parts of the apparatus must, therefore, be suitably related to one another, keeping the speeds at a constant and optimum value, via suitable adjusting and control means (not shown), determining in each case the most appropriate speeds by means of suitable tests, in accordance with the estimated expansion velocity and rising of the polyurethane foam during frothing.

Tests were carried out using an apparats according to the example of Figures 1 and 2 for the production of slabstocks of frothed polyurethane material having a rectangular section of 2 x 1.2 m widthwise. The apparats extended heightwise over a distance of about 5 m, including the upper zone for cutting and removal of the blocks, and as provided with two conveyors 25 which were operated at a constant speed of 0.9 m/minute. Frothing and expansion of the foam was performed along a vertical section of about 1 m, znrresponding to a time of about 1.2 minutes; the distribution bar 13 had a slot with a thickness of 5/laths of a millimetre and was delimited by parallel walls which extended in the direction of flow over 30 mm.

Strips of kraft paper were also used for defining the moving surfaces 22 which accompany the polyurethane foam 12 along the polymerization path 11.

A polyurethane mixture formulated in three component parts was used: PART A Throughput 24 /minute Polyether polyol 3,500 mw 100.00 Water 2. SC Ammino catalyst 0.two Silicone 1.20 PART B Throughput 12.5 kg/minute 80:20 TDI 55.00 PART C Throughput 0.05 kg/minute Stannous octoate based catalyst 0.22 The aforementioned quantities are defined as parts by weight of polyol.

The part A of the polyurethane mixture was moreover saturated with a certain quantity of carbon dioxide in the liquid state, in sufficient quantities to reach a pressure of about 10 bar inside the storing tank.

After starting up of the apparatus and determining the optimum speed for the conveyor belts, the polyurethane mixture frothed regularly, completing its expansion at the end of the enclosure 20, as it continued its upward travel along the polymerization path 11, drawn along by the conveyors 25.

At the end of its upward travel, the polyermized polyurethane material was cut into blocks of about 1.5 m height, obtaining a polyurethane foam with homogeneous density, measured at 15 kg/m3 The structure of the cells was fine and uniform and did not contain any voids or bubbles of large dimensions, therefore proving to be of a commercially acceptable quality.

The tests were repeated several times, achieving substantially homogeneous results which have demonstrated the major advantages which can be obtained with the method and apparatus according to the present invention.

Figure 4 shows a possible modificatlon for the frothing device, which in certain cases enables im?rovement of the working of the plant and the features of the polyurethane foam. The same reference numbers are used in 'lure 4 as those of the previous figures to indicate similar or equivalent parts.

The apparatus shown in Figure 4 differs from that shown in Figure 3 mainly owing to the fact that =e wall 28 which closes off the end of the duct 18, now forms a continuation of a lateral surface of the slot defining the pressure-drop zone 17. This continuity of the su- aces tends to reduce the cavitation phenomena which causes turbulence into the polyurethane mixture emerging from the slot defining the pressure-drop zone 17 of the distribution bar 13, thus improving the quality of the froth and polyurethane foam.

It has been stated that the drop-pressur zone 17 in the example of Figures 1 to 4 is in the form of a restricted slot extending in the flow direction of the mixture and for the entire length of the pressure equalizing chamber 14 of the frothing device.

Figure 5 of the drawings schematically represents a front view of the distribution bar 13, facing the outlet of the slot 17. The remaining Figures 6, 7 and 8 show similar views for different solutions and different shapes of the appertures defining the pressure-drop zone.

In Figure 6 the pressure drop zone is defined, for example, by four side by side set of arranged short slots 31, extending in the longitudinal direction of the bar 13.

Conversely, Figure 7 shows the slots 31 arranged transversely in bar 13, while Figure 8 shows a pressure-drop zone provided by a set of tubular holes with a circular cross section.

Although a rectangular shape of the bar 13 and a slot configuration of the pressure-drop zone have provided better results, nevertheless different shapes of the chamber 14 and pressure-drop zone of the frothing device, as well as different shapes and disposition of the apertures may be provided within the principle of the present invention.

From the above description and illustrations it is therefore obvious that the invention relates to a new combination of a vertical plant for the continuous production of slab-stocks of rigid or flexible polyurethane foams, using a device which allows an initial frothing phase, moving on upward in direction, before chemical reaction takes place thus improving the entire process and the quality of the foamed material.

Claims (13)

1. A process for the continuous prction of polyurethane foam, comprising the steps of: preparing a mixture of reactive polyurethane components and a low-boiling blowing agent in a liid state; feeding the resulting mixture into a pressure-equalizing chamber and distributing the mixture to flow over an englarged pressure-drop zone extending in the flow directs of the mixture to provide a back-pressure up-stream of the pressuredrop zone to keep the blowing agent at a liquid state; starting frothing of the mixture by gradually releasing the pressure and the blowing agent in the mixture while flowing the mixture through the pressure-drop zone and into a frothing cavity, the frothing mixture being progressively eanded by the blowing agent, reacting the polyurethane components inside an open top expansion enclosure extending upwardly from the frothing cavity, and drawing the expanding material upwardly from the bottom of the expansion enclosure while the frothing and chemical reaction between the polyurethane components takes place.

2. A process according to Claim 1, including the additional step of laterally diverting the frothing mixture towards an upwardly diverging portion of the frothing cavity.

3. A plant suitable for the continuous production of polyurethane foam slab-stocks comprising a mixing device for mixing polyurethane chemical components and a liquid blowing agent, such as CO2, under pressure controlled conditions, is connected by a feeding line, through which a resulting polyurethane mixtures moves, to a frothing device, said frothing device comprising a mixture distributing and pressure equalizing chamber opening onto a pressure-drop zone extending in a flow direction for the polyurethane mixture to provide a back-pressure in said pressure equalizing chamber to keep the blowing agent in a liquid state, and to gradually release the blowing agent into the polyurethane mixture within a frothing cavity, said plant further including an open top expansion enclosure defining by upwardly extending side walls; said frothing cavity being provided between said pressure-drop zone of the frothing device and a diverging portion at the bottom of said expansion enclosure; and a foam take-off system comprising sheet material moving on at least an opposing pair of said side walls together with the foaming material inside said expansion enclosure.

4. A plant according to Claim 3, in which said pressure-drop zone is in the form of a slot aperture longitudinally extending from said pressure equalizing chamber.

5. A plant according to Claim 4, in which said slot aperture extends laterally in a plane parallel to the longitudinal axis of said expansion enclosure.

6. A plant according to Claim 4, further including a guide surface in said frothing cavity that extends outwardly from one side of said slot aperture.

7. A plant according to Claim 3, in which said pressure-drop zone comprises a plurality of pressure-drop apertures.

8. A plant according to Claim 7, in which said plurality of pressure-drop apertures comprises at least one set of side by side spaced apart pressure-drop slots located along at least a portion of said pressure equalizing chamber.

9. A plant according to Claim 8, wherein said plurality of pressure-drop apertures extend in a direction parallel to an axial direction of said pressure equalizing chamber.

10. A plant according to Claim 8, in witch said plurality of pressure-drop apertures extend transversely to an axial direction of said pressure equalizing chamber.

11. A plant according to Claim 3, in which said pressure-drop zone comprises at least one set of spaced apart, side by side tubular openings.

12. A plant according to Claim 3, in which said pressure equalizing and mixture distributing chamber, and said pressure-drop zone, are elongated in at least one crossdirection of said frothing cavity.

13. A plant for the continuous production of polyurethane foam slab-stocks as hereinbefore described with reference to the accompanying drawings.