HK1020548A - Process and device for producing foam using carbon dioxide dissolved under pressure - Google Patents

Description

Method and apparatus for producing foam material from carbon dioxide dissolved under pressure

The invention relates to a method and an apparatus for producing foam materials by dissolving carbon dioxide under pressure as a foaming agent, wherein the composition to be foamed is mixed under pressure with preferably liquid carbon dioxide and subsequently foamed under reduced pressure. Liquid starting materials for the preparation of plastics are the most commonly used foaming compositions, which after foaming harden to form a foam as a result of polyaddition or condensation polymerization. The invention is particularly relevant to polyurethane foams.

In the production of polyurethane foams, at least one of the two reactive components (polyisocyanate and a compound containing hydrogen atoms reactive with isocyanate, in particular a polyol) is admixed with a liquid or gaseous blowing agent and subsequently with the other component, and the resulting mixture is introduced either batchwise into a mold or continuously onto a conveyor where it is foamed and hardened.

There are a number of processes for producing foam materials which have found wide application in the art. First, a liquid which is volatile at low temperatures, such as low molecular weight chlorofluorocarbons, methylene chloride, pentane and the like, is used, which volatilizes out of the reaction mixture while it is still liquid to form small bubbles (physical foam production). Alternatively, air may be forced into the reaction mixture or one of the components (mechanical foam production), and finally water may be added as a blowing agent to the polyol component to produce the polyurethane foam, which when mixed with the isocyanate component releases carbon dioxide as the blowing gas by reaction with the isocyanate (chemical foam production).

Since liquid carbon dioxide is compatible with the environment and is industrially hygienic, and also because of its high solubility in polyols, liquid carbon dioxide has been frequently proposed as a blowing agent (GB-A803771, US-A3184419). However, these past proposals have not hitherto become part of the process technology, the obvious reason for this being that it is very difficult to form a homogeneous foam when the reaction mixture has to be reduced from a pressure of 10 to 20 bar. In this connection, the problem is firstly that, immediately after the depressurization, the carbon dioxide evaporates relatively abruptly, which leads to a considerable increase in the volume of the reaction mixture, for example by a factor of 10, which is difficult to control, and secondly that the reaction mixture tends to deform on release of the carbon dioxide, which is the case at pressures below CO₂This may occur at equilibrium vapor pressures of 3 to 6 bar at the relevant temperature, at which point the carbon dioxide is suddenly released like an explosion, with the result that large bubbles or shrinkage cells are contained in the foam.

In order to produce homogeneous foam structures, it is known to introduce uniformly distributed air or nitrogen bubbles as seeds into the liquid reaction mixture, which prevents local supersaturation of physically dissolved or chemically produced blowing agents.

However, when carbon dioxide dissolved by a physical method is used as a blowing agent, there is a problem in that the addition polymerization reaction hardly proceeds during the release of carbon dioxide, and therefore the foam pile obtained after the release of carbon dioxide is still very sensitive to stress due to the shearing force. Shear forces acting on the foam pile can cause the bubbles to collapse, thereby forming large bubbles and causing an uneven foam structure. This is particularly the case when the bubble diameter is so large that the bubble deviates from a spherical shape, where the volume of the bubble in the foam pile occupies a greater proportion of space than would be equivalent to the most dense ball pile. When 0.5 parts by weight of carbon dioxide based on 100 parts by weight of the reaction mixture is released at atmospheric pressure, the carbon dioxide gives a bubble volume corresponding to the packing of the most dense spheres.

In the continuous production of block foam (see Becker/Braun "handbook of plastics" (Kunststoffhandbuch) volume 7, "Polyurethane" (Polyurethane),1993, FIGS. 4.8 and 4.9, P.148) the deposition of foam stacks on the conveyor belt and the distribution over the width of the conveyor belt can be regarded as problematic.

In order to overcome the problem of the deposition and distribution of the foam pile on the conveyor belt, EP-a645226 has suggested: the polyurethane-reactive mixture, which contains the pressurized dissolved carbon dioxide, should first be distributed under pressure over the width of the conveyor belt in a pressure zone called a pressure equalization chamber; the pressure is then relieved in a pressure relief section extending the width of the belt, the pressure relief section being configured as a slit or an array of passage openings extending the width of the belt to provide sufficient flow resistance; and a foaming chamber is provided behind the conveyor belt, which foaming chamber is arranged transversely above the conveyor belt and widens in the direction of flow, from which the bubbles flow out at a flow rate that is matched to the speed of the conveyor belt. The disadvantage of this proposal is that the flow rate of the reaction mixture in the pressure-equalizing chamber with a large volume is too low, coupled with a long residence time of the bubbles in the foaming chamber, a large ratio of the foaming chamber wall to the foaming chamber cross-section, and that it is difficult to prevent a difference between the flow rate at the outlet of the foaming chamber and the speed of the conveyor belt. In particular, the large bubbles produced by the shearing force of the lower boundary wall of the foaming chamber on the foam pile and the large bubbles produced during the transfer from the foaming chamber to the conveyor belt, which are covered by the foam pile, cannot be dissipated by bursting the contained gas into the environment. Conversely, enlarged gas bubbles, for example, generated at the bottom of the foam pile, migrate into the foam pile during the ongoing addition polymerization reaction and are included in the foam material produced.

DE-a4422568, DE-a4425317 and DE-a4425319 have proposed a compact foaming device which neither comprises a pressure equalization chamber with a long forced flow path nor a foaming chamber.

According to the applicant's proposal of the not yet published DE-A4442254, the flow rate of the polyurethane-reactive mixture is effectively reduced by deflection on a backflush surface as it flows out of the decompression zone.

The known foaming devices are always unsatisfactory because of their too short life. The reason for this is either the inevitable solid particles which are entrained in the reaction mixture as a result of the starting materials of the reaction mixture coming from the tanks and pipes, or the polyurethane lumps which separate out from the mixing chamber or from the feed to the foaming apparatus, etc., which block the narrow cross-section of the pressure-reducing zone. The problem of plugging is exacerbated if the reaction mixture contains a homogeneous distribution of solids, such as color pigments, melamine powder or recycled powder, even if the solid particle size is significantly smaller than the size of the depressurization zone.

The object of the invention is a process for the continuous production of slabstock foams by foaming a polyurethane mixture containing carbon dioxide dissolved under pressure, wherein the pressure of the reaction mixture is reduced in the course of passing through a gap-like pressure reduction zone, characterized in that the pressure reduction zone is constructed as one or more coaxial annular gaps and the velocity of the reaction mixture is reduced after passing through the annular gap or gaps, also in the course of passing through the annular gap or gaps, in the annular gap-like speed reduction zone or zones.

The gap-defining surfaces which constitute the pressure-reducing zone are preferably at least temporarily movable relative to each other.

Since the two bounding surfaces of the slot can move relative to each other, any solid particles that may have stuck in the slot are carried out of the slot by the reaction mixture flowing through the slot at high speed at the same time, so that clogging is prevented.

The relative movement of the slot-defining surfaces can be effected periodically with high frequency, for example by transmitting ultrasonic vibrations onto at least one of the defining surfaces.

Relative movement may also be achieved by momentarily opening or closing the aperture.

If the pressure reducing zone is formed by one or more concentric annular slits, relative movement of the slit-defining surfaces can be achieved by rotation of one defining surface relative to the other.

The movement of the slot-defining surfaces relative to each other can be performed continuously, periodically or intermittently.

In the case of rotationally symmetrical slots, the slot-defining surfaces preferably rotate continuously with respect to one another at a rotational speed of 1 to 60 revolutions per minute, most preferably 1 to 10 revolutions per minute.

If the movement is by means of ultrasound excitation, intermittent movement is preferred.

According to another preferred embodiment of the invention, the gap width is adjustable. The pressure-sensitive head, which acts on the gap-width adjusting means in order to keep the pressure of the polyurethane-reactive mixture constant by adjusting the width of the gap before it passes through the decompression zone, i.e. through the gap, is preferably mounted at the outlet of the mixing chamber, on a connecting pipe between the mixing chamber and the foaming device, or in a distribution chamber of the foaming device.

According to a preferred embodiment of the invention, the reaction mixture, after exiting from the pressure reduction zone, is deflected in its direction of flow by at least 90 °, preferably by 90 ° to 150 °, wherein the cross-section of flow after deflection is 5 to 30 times, most preferably 10 to 20 times, greater than the cross-section of flow in the pressure reduction zone. The structure of this deceleration zone, which adjoins the pressure reduction zone, is of course also slit-shaped. The flow direction is dimensioned such that in the deceleration zone there is substantially no release of carbon dioxide due to the damping time of the release.

The size of the pressure reduction zone in the flow direction, i.e. the length of the gap, may be 1 to 20mm, particularly 2 to 10mm, most preferably 3 to 8 mm. The gap width of the pressure reduction zone, i.e.the distance between the gap interfaces, can be 0.1 to 0.5mm, the specific dimensions being dependent on the viscosity of the polyurethane-reaction mixture and the feed rate. When a filler is included in the polyurethane-reactive mixture, the gap preferably has a width of 0.2 to 0.5 mm.

The linear velocity of the reaction mixture flowing through the depressurization zone depends on the following factors: the size of the pressure reduction zone in the flow direction; the width of the gap; the viscosity of the reaction mixture; and the pressure prevailing in the pressure distribution chamber, which must be higher than the saturated vapour pressure of the dissolved carbon dioxide. Typical linear velocities may be 10-25 m/s. For the foaming of reaction mixtures having a high carbon dioxide content, for example containing 3 to 6% by weight of carbon dioxide, a high linear velocity is preferred in the pressure-reduction zone, for example a velocity of 15 to 25m/s, in order to shorten the residence time in the foaming apparatus.

The polyurethane-reactive mixture fed to the foaming device according to the invention is prepared as follows:

aliphatic, cycloaliphatic, araliphatic, aromatic and heterocyclic polyisocyanates, such as those described by W.Siefken in Justus Liebigs Annalen der Chemie,562, P75-136, are preferably used as the isocyanate component.

Aromatic polyisocyanates are preferably used, particularly preferably those which are generally commercially available, such as toluene 2, 4-and 2, 6-diisocyanate and mixtures of any of these isomers ("TDI"); polyphenylpolymethylene polyisocyanates such as those prepared by condensation with aniline-formaldehyde and subsequent phosgenation ("crude MDI"); and polyisocyanates containing carbodiimide groups, urethane groups, allophanate groups, isocyanurate groups, urea groups or biuret groups ("modified polyisocyanates"), in particular those derived from toluene 2, 4-and/or 2, 6-diisocyanate.

Compounds having a molecular weight of generally 60-5000, preferably 100-2000, most preferably 200-800, containing at least two hydrogen atoms capable of reacting with isocyanates may be used as the second component ("polyol component"). It is understood that in addition to the compounds having amino, thiol or carboxyl groups, preferred hydroxyl-containing compounds, especially those having 2 to 8 hydroxyl groups, especially those having a molecular weight of 200-2000, preferably 300-1200, such as polyesters, polyethers, polythioethers, polycondensates, polycarbonates and polyesteramides, which contain at least 2, usually 2 to 8, preferably 2 to 6 hydroxyl groups, are those compounds which are well known in the art for the production of polyurethane foams; polyether polyols are particularly preferred.

Compounds suitable as polyol components are described in EP-B121850 at P.6-9.

In addition, water, other blowing agents, foam stabilizers, catalysts, fillers, color pigments, melamine powder and recycled plastic powder, as well as other auxiliary materials and additives known in the art, can optionally be used to produce the reaction mixture. These known and still usable media are disclosed in EP-B121850 at P.9-11.

According to the invention, water is most preferred as auxiliary blowing agent, preferably in an amount of 1 to 7% by weight of the reaction mixture. Advantageously, the amount of water used is 2 to 5% by weight.

The additives used can be added separately to the mixer unit in which the isocyanate component and the polyol component are mixed, or even to one of the two main components before the isocyanate is mixed with the polyol, wherein, together with the water used and other auxiliary components which may react with the isocyanate, only the polyol component is admixed.

The technical principle of the process for producing polyurethane foams is described in "handbook of plastics" (Kunststoff-Handbuch) by Becker/Braun: "Polyurethanes" (Polyurethanes),1993, P.143-149, and is discussed in particular in P.148 and FIG. 4.9.

The components are preferably mixed in what is known as a low pressure stirred mixing chamber, wherein, according to the invention, the pressure in the mixing chamber is often maintained above the saturated vapour pressure of the dissolved carbon dioxide.

The carbon dioxide is dissolved in one or more of the components, particularly the polyol component, before the components are introduced for mixing.

The amount of dissolved carbon dioxide is preferably 1 to 7% by weight, preferably 2 to 5% by weight, based on the total weight of the reaction mixture.

In addition, air and/or nitrogen may be dispersed or dissolved in the composition to aid in the formation of the bubble nuclei.

The dissolution of carbon dioxide, preferably only in the polyol component, may be achieved in any manner, for example:

a) incorporating gaseous carbon dioxide into a polyol in a vessel containing the polyol by means of a stirrer, the pressure in the vessel being maintained at 15 to 25 bar;

b) mixing liquid carbon dioxide with the polyol at room temperature, for example in a static mixer at a pressure of 70 to 80 bar, and subsequently reducing the pressure to 15 to 25 bar before being introduced into a low-pressure stirred mixing vessel;

c) liquid carbon dioxide cooled to-20 ℃ is mixed with the polyol component at room temperature under a pressure of 15 to 25 bar, wherein the mixing is carried out by dissolving the carbon dioxide into the polyol component before it can volatilize.

It has been found that the preferred process c) can be achieved with particular success since carbon dioxide can be made to have a pronounced tendency to dissolve by means of a high-speed liquid flow stirrer, which is arranged on the polyol line at the point where liquid carbon dioxide is introduced.

The components of the reactive synthetic material, at least one of which has carbon dioxide dissolved therein, are then fed to a stirred mixing hopper and mixed and fed to the foaming apparatus of the invention as they flow out of the mixing hopper.

The invention will be described in more detail below with reference to fig. 1 to 11:

FIG. 1 is a general diagram showing a simplified side view of a process kit for carrying out the invention;

figure 2 shows a first embodiment of the foaming device of the invention;

figures 3 and 4 show an alternative embodiment of detail Z in figure 2;

FIG. 5 shows a practical kit for a rotationally symmetric foaming apparatus;

FIG. 6 shows a foaming device with concentric annular slits;

FIGS. 7, 8 and 9 show detail X (the pressure reducing section) and alternative embodiments of FIG. 6; and

fig. 10 and 11 show a foaming device of the present invention having a plurality of parallel slits.

Fig. 1 is a side view of a conveyor belt 1 in a slabstock foam installation. The lower laminate film 2 is fed onto a conveyor belt 1 and pulled to the right by the conveyor belt at a speed of 3 to 7 m/min. The isocyanate 31 and the polyol 32, which may contain 3 to 7% by weight of dissolved carbon dioxide, and other additives and auxiliary materials 33 are fed under pressure into the mixing device 3. The pressure often maintained in the mixing device should be above the solution vapor pressure of the dissolved carbon dioxide. The reaction mixture from the mixing device 3 is sent to the foam production device 4. The partially foamed reaction mixture supersaturated with carbon dioxide emerges from the foaming device 4 as a foam pile and flows freely up the conveyor belt 1. The foam pile 5 formed in the lower part of the circularly symmetrical foaming device 4 is distributed in free flow over the width of the conveyor belt first, wherein in order to prevent flow in the opposite direction to the conveyor belt run, a substantially vertical baffle 6 is arranged, which is transverse to the conveyor belt. The upper laminate film 8 is fed out at a distance from the outflow point of the foam pile by means of guide rollers embedded in the foam pile, the direction of which coincides with the running direction of the conveyor belt. Further, the side laminate films 10 are sent to both sides of the conveyor belt by guide rollers 9.

The foaming device 4 according to the invention shown in fig. 2 is composed of a rotationally symmetrical outer shell 41 and a rotationally symmetrical central body 42, which enclose a mixture distribution chamber 43 into which the polyurethane-reactive mixture enters through tangentially directed orifices 44. The casing 41 and the central body 42 also constitute a gap 45, which is a decompression zone through which the mixture advances at high speed as it is pushed by the usual pressure. When the mixture flow expands outward in a rotationally symmetrical manner, it impinges on the backflush surface 46, so that the velocity is reduced to a turbulent flow in the deceleration zone 47 by an amount corresponding to the cross-sectional area of the deceleration zone, which is larger than the gap 45. The reactive mixture then foams as the dissolved carbon dioxide is released, where the space in the lower part of the foaming device is filled along the profile 48 of the central body 42 and is free flowing. The central body 42 is provided with a rotary drive so that the slot-defining surface provided by the central body can move relative to the slot-defining surface provided by the housing so that any particulate material deposited in the slot is removed. Means are also provided for axially displacing the central body 42 so that the width of the gap 45 can be adjusted. The deviation of the pressure set in the distribution chamber 43, obtained from the measuring device P side, allows to control the thrust of the central body axial displacement by the control unit C.

Fig. 3 shows an enlarged view of detail Z in fig. 2, which shows the configuration of the reflection surface 46 in the form of a rotationally symmetrical groove in the deceleration zone 47, which serves to guide the deceleration of the turbulence.

Fig. 4 shows an enlarged view of detail Z from fig. 2, which shows a further embodiment, in which the deceleration in the deceleration zone 47 is effected by means of an offset of 135 °.

As shown in fig. 2, the polyurethane-reactive mixture to be foamed slides along the profile 48 of the central body 42, whereby the space below the annular gap-like relief zone is filled. In this case, in particular in large foaming plants, i.e. plants with a high throughput of polyurethane-reactive mixture, there is a risk that already occurring gas bubbles will collapse as the mixture slides off the mold line 48, so that occasionally large shrinkage cavities will be formed, which large shrinkage cavities (see fig. 1) no longer migrate from the foam mass accumulated on the conveyor belt 1 to the surface. In order to prevent such an extensive foaming process from being mixed in, a device can be provided according to the invention for the early generation of a rotationally symmetrical form of the liquid foam (foam pile) which breaks away in a plane extending radially in the flow direction.

Fig. 5 is a perspective view in half section of a foaming device 4 according to the invention, in which a foam pile flow 5 and a foam breaking means 51 are also depicted. The edge 52 of the breaking device 51 facing the foaming device 4 cuts the foam mass substantially radially, whereby a profile line drawn with a dashed line begins. After the foam stream 5 has been thus divided at its axis 53, the larger bubbles formed along the profile 48 of the central body 42 will escape at its surface.

The foaming device of the invention shown in fig. 6 has two concentric decompression zones 45a and 45b in the form of annular slits. The central body 418 is permanently fixed to the housing 41 of the foaming device 4. One of the bounding surfaces of the annular gaps 45a and 45b is formed by a disc 411 which is fixed to a sleeve 413 by 3-6 supports 412. The hub 413 extends beyond the distribution chamber 43 to enable it to rotate about the axis of the central body 418. As for the remaining components, the same numerals refer to the same components as those in fig. 2. The reactive mixture streams exiting annular gaps 45a and 45b undergo a directional deflection as they impinge upon each other, thereby reducing their velocity in their common velocity reduction zone. As an alternative, detail X in fig. 6, shown in fig. 7, shows a rib 46 which replaces the function of the backflush surface.

Fig. 8 shows an alternative to detail X in fig. 6, which shows a deflection in the direction of 135 ° immediately after the decompression zone.

Figure 9 shows an alternative to detail X in figure 6, which is provided with separate deceleration zones 47a and 47 b.

Figures 10 and 11 show a foaming device of the invention with a plurality of concentric decompression zones 45 and corresponding deceleration zones 47. Fig. 10 shows a section B-B in fig. 11. Fig. 11 shows a section a-a in fig. 10. In fig. 10 and 11, components corresponding to those in fig. 6 are designated by the same reference numerals. The relative movement of the two bounding surfaces of the pressure-reducing zone 45 can be produced, for example, by means of an ultrasound crystal 49 mounted on the foaming device housing 41 (and possibly additionally if desired).

Claims (10)

1. A method for the continuous production of slabstock foam by foaming of a polyurethane-reactive mixture containing pressurized dissolved carbon dioxide. Wherein the reactive mixture is depressurized through a slot-shaped depressurization zone, which process is characterized in that the depressurization zone is formed by one or more coaxial annular slots, and the velocity of the reactive mixture, after passing through the one or more annular slots, is reduced in one or more coaxial deceleration zones in the shape of annular slots.

2. The process as claimed in claim 1, characterized in that the direction of flow of the reactive mixture in the depressurization zone and the direction of flow in the velocity reduction zone form an angle of 90 to 150 °.

3. A method according to claim 1 or 2, characterized in that the flow direction in the velocity reduction zone is substantially parallel to the axis of the annular gap.

4. A method according to any one of claims 1-3, characterized in that the gap-defining surfaces of the deceleration zone are movable, at least temporarily, relative to each other.

5. A method according to claim 4, characterized in that the slot-defining surfaces are caused to rotate relative to each other.

6. A method as claimed in claim 4, characterized in that the mutual displacement of the slot-defining surfaces is effected ultrasonically.

7. A method according to claim 4, characterized in that the width of the gap of the pressure-reducing zone is instantaneously narrowed and/or widened.

8. An apparatus for the continuous production of block foam by foaming of a polyurethane-reactive mixture containing carbon dioxide dissolved under pressure, comprising a rotationally symmetrical pressure distribution chamber having one or more coaxial outlet slits, wherein the flow direction of the reaction mixture through the outlet slits forms an angle of 90 to 150 ° with the axis of the pressure distribution chamber, wherein in each case the delimiting surfaces of the slits are always provided by a rotationally symmetrical coaxial central body which is arranged inside the pressure distribution chamber, wherein immediately following the outlet slits, axially parallel annular slits are also provided, the cross section of which is 8 to 20 times greater than the cross section of the outlet slits.

9. The apparatus of claim 8 wherein means are provided for axially moving the central body.

10. Apparatus according to claim 8 or 9, wherein means are provided for rotating the central body.