WO2003035731A1

WO2003035731A1 - Method for the production of polyurethane foam

Info

Publication number: WO2003035731A1
Application number: PCT/EP2002/011399
Authority: WO
Inventors: Hans-Michael Sulzbach; Reiner Raffel; Herbert Steilen; Jürgen Wirth
Original assignee: Hennecke Gmbh
Priority date: 2001-10-24
Filing date: 2002-10-11
Publication date: 2003-05-01
Also published as: DE10151855C2; DE10151855A1

Abstract

Disclosed is a method for the production of polyurethane foam by mixing an isocyanate component and a polyol component in a low pressure mixing unit in the presence of a dissolved foaming agent and air and/or nitrogen as a bubble nucleator. The bubble nucleator is dissolved by fine dispersal and dissolution at a pressure of 2-20 bars in the polyol component before introduction into the mixer.

Description

Process for the production of polyurethane foam

The present invention relates to a process for the production of polyurethane foam, in particular a process for the continuous production of polyurethane block foam, or for the production of heat-insulating housing foams, e.g. of refrigerator housings.

The mechanical and thermal properties of polyurethane foams are largely determined by the size and uniformity of the foam bubbles

(Cells). Fine-cell foams are sought for many applications. The linear cell number density achievable according to the prior art is 25 cells / cm, in extreme cases also up to 30 cells / cm.

The foam structure is generally formed in two to three stages before the curing of the polyurethane mass by first forming microbubbles, which act as bubble nuclei, in a first stage; in a second stage, dissolved gases diffuse into the existing bubble nuclei, the dissolved gases generally being dissolved in the reaction mass in the form of low-boiling liquids which evaporate due to the onset of the exothermic polyol-polyisocyanate reaction; and, in a third stage, over time with the second stage, carbon dioxide generated by the reaction of water with isocyanate, which diffuses further into the foam bubbles. Gases with very low solubility in the polyol, the isocyanate or the polyol / isocyanate mixture, such as air, nitrogen and noble gases, are particularly suitable as bubble nucleating agents.

The achievable cell structure of the finished polyurethane foam is essentially determined by the number and uniformity of the germ bubbles formed in the first stage, since the foaming agents themselves diffuse into the already existing bubble germs before a sufficiently large oversaturation can be achieved, which leads to the formation of new germ bubbles leads. So it is necessary to make a foam one Produce density of 100 kg / m ³ with a cell number density of 25 / cm, in the first stage to produce 200,000 bubble nuclei / cm ³ polyurethane-isocyanate mixture; a foam of 30 kg / cm ^{3 of the} same cell number density initially requires 750,000 bubble nuclei / cm ³ .

A number of publications deal with the problem of bubble nuclei production, often called "gas loading" in the professional world.

According to DE-A 28 36 286, air is dispersed in a polyol partial stream by means of mechanical dispersing apparatus with volumes comparable to the partial stream volume, the liquid foam obtained, the remaining polyol component and the isocyanate component being fed to an agitator mixer.

According to EP-A 565 974, it is proposed to supply the polyurethane components to a mixing head with adjustable injection pressure and adjustable mixing chamber pressure, the gas content of the isocyanate being continuously increased or decreased immediately before entering the mixing head. The isocyanate flows as a thin film through a flow chamber designed as a centrifuge with several coaxial annular chambers. The gas pressure in the flow chamber is controlled in the sense of an increase or decrease in the gas content of the isocyanate. The

Bubble nuclei are generated while the isocyanate is being injected into the mixing chamber. A disadvantage of this process is that, due to the relatively small mass transfer area of the isocyanate film in the centrifuge, it is not possible to bring enough gas into solution in the relatively short flow time. Maximum line count densities of 90 / inch = 35 / cm with a foam density of about 50 kg / m ^{3 are} described.

The object of the present invention is to provide a process for the production of polyurethane foam which allows the number of cells in the foam to be controlled within wide limits. A further object of the present invention is to continuously introduce comparatively large amounts of gases suitable as bubble nucleating agents into the polyol / polyisocyanate mixture.

Another object of the present invention is to provide a polyurethane

To provide foam with an extremely fine cell structure.

A further object of the present invention is to provide a process for the production of fine-celled polyurethane foam which is completely in the low pressure range, i.e. in the pressure range below 25 bar.

In addition, it has been shown that the invention solves a further problem in the production of polyurethane block foam, namely the processing of solid, powdery fillers, e.g. the processing of solid flame retardants, such as melamine, in polyurethane foams. The powdery

Fillers are usually either premixed in a storage container with one of the polyurethane reactive components, usually the polyol, and then fed continuously to the foaming process, or else mixed online with the polyol component. In particular in the case of on-line powder metering, air which is located in the interstices between the grains is inevitably introduced into the liquid flow. This air has two harmful effects on the foaming process. On the one hand pinholes and cavities are created and on the other hand this air leads to cracks in the foam block. Cracks occur because air remains on the surface of the powder particles, which prevents the particles from being completely wetted in liquid. Such poorly wetted particles are the starting point for crack formation. In the art, therefore, the off-line premixing of the polyol component with the filler is preferred, the premixes being treated over a prolonged period, preferably under a slight vacuum, until complete wetting. The object of the present invention is therefore also a process for producing filler-containing polyurethane foams by on-line metering of the fillers, complete wetting being ensured by the polyol component.

The present invention relates to a process for the production of polyurethane foam by mixing an isocyanate component and a polyol component in a mixing unit in the presence of a dissolved foaming agent and air and / or nitrogen as the bubble nucleating agent, which is characterized in that the bubble nucleating agent is formed by fine dispersion and pressure increase in at least one of the components is dissolved before being introduced into the mixer. The fine dispersion of the bubble nucleating agent is preferably carried out in a high-speed disperser based on the rotor / stator principle. The relative speed between the rotor and stator is preferably 15 to 40 m / sec. The shear gap between the rotor and stator can be between 0.3 and 1 mm. Dispersers with several, concentrically alternating rotor and

Stator.

The bubble nucleating gas is preferably dispersed and dissolved in the polyol component. The invention is described below with reference to this preferred embodiment. However, the statements apply accordingly if the bubble nucleating agent dispersion and dissolution takes place in the isocyanate component or in both components.

According to the invention, an agitator mixing head, a static mixer or a friction mixer according to the low-pressure process, ie. H. in the pressure range up to

25 bar. An agitator mixing head is particularly preferred.

The disperser preferably consists of an axial inlet space for the polyol bubble nucleating agent mixture, a first rotor, a first stator, a last rotor, a concentric collecting space for the fine dispersion and one

Outlet. A second hotor and a second stator can be provided between the first stator and the last rotor.

The rotors and stators preferably have through-slots which are extended in the radial-axial plane, the ratio of the tangential slot width and the radial length of the slots of the rotors preferably being between 1: 3 and 1:10.

The operating conditions of the disperser are preferably selected such that a pressure of 2 to 20 bar is built up behind the disperser due to the centrifugal forces generated by the disperser. To maintain the pressure, a throttle valve is provided in front of the inlet of the polyol component loaded with gas into the mixing unit, which preferably allows pressure control.

More preferably, the bubble nucleating agent is predispersed with the polyol component before the polyol component is introduced into the fine dispersant, for example by static mixing elements provided in the poly line or by porous introduction elements for the bubble nucleating agent into the polyol component, for example sintered bodies, sieve structures or glass frits. The pre-dispersion ensures that the formation of larger gas spaces on the suction side of the disperser, which reduces the effect of the dispersant, is avoided.

It is essential that the pressure generated by the throttle valve limits the delivery capacity of the disperser in such a way that the pressure on its suction side does not drop to such an extent that agglomeration and separation of gas bubbles with the formation of a vortex in the axial inlet space of the disperser occurs ,

As an alternative or in addition to the throttle valve upstream of the mixing unit inlet, a metering pump, ie a pump with essentially pre-pressure-independent pressure, is preferably installed behind the disperser to limit its delivery rate. pending delivery, provided. In this case, the polyol delivery pump for delivery from the storage container is preferably not provided as a metering pump, but rather as a simple delivery pump with an increased delivery rate compared to the metering pump provided behind the disperser, the delivery to the disperser being limited by a pressure-controlled overflow valve and the overflow Amount of polyol is returned to the reservoir.

A pressure which is at least 1.5 times, particularly preferably 2 to 3 times, the solution equilibrium pressure of the bubble nucleating gas is preferably maintained behind the metering pump.

The suction effect desired per se in commercial rotor / stator dispersers which are used for dispersing or mixing incompressible liquid or solid / liquid components can be further reduced by providing a stator as the first element in the flow direction. This reliably avoids the formation of droplets which is undesirable for gas dispersion.

According to a further preferred embodiment, the radial flow cross section of the multi-stage rotor / stator disperser is designed such that it is in

Direction of passage expanded, for example by increasing the number of through slots, so that under the effect of the also increasing centrifugal force, a decrease in pressure takes place in such a way that gas bubbles comminuted up to the mechanical dividing limit expand and further comminution in the subsequent shear gap becomes accessible.

Due to the increased pressure generated behind the last rotor stage, the undissolved gas has bubble diameters due to the compression, which are far below the bubble diameters that can be achieved by mechanical division, and dissolves completely completely relatively quickly. In industrial plants where the

Distance between the units is 5 to 10 m or more, the sufficient time for the complete dissolution of the bubble nucleating gas up to the mixing head entrance. Otherwise the complete dissolution of the gas can be guaranteed by appropriately larger pipe cross sections.

The metered supply of the bubble nucleating gas for predispersion is preferably carried out in an amount of 5 to 10 Nl / 100 kg of polyol. The pressure upstream of the mixing chamber inlet which can be set by the throttle valve and the rotor speed of the fine disperser is preferably selected such that the solution partial pressure of the bubble nucleating agent in the polyol component is at most 70%, preferably between 20 and 50%, of the prevailing pressure.

After the polyol containing the bubble nucleating agent has been introduced into the low-pressure mixing unit, usually an agitator mixing head, the pressure on the internal agitator pressure is reduced, usually to 1 bar or slightly below, so that the polyol is 30 to 150% oversaturated, with spontaneous bubbles - nucleation takes place.

The actual foaming agent, water and or low-boiling liquids, such as pentane or methylene chloride, are preferably introduced directly into the low-pressure mixing unit and mixed there simultaneously with polyol and isocyanate. If carbon dioxide is used as an additional foaming agent, this is preferably introduced directly behind the fine disperser or behind the metering pump if a metering pump is provided behind the disperser.

Fillers can preferably be added to the polyol via a mixing screw before the bubble nucleating gas is introduced into the polyol. The fillers are completely wetted in the rotor / stator disperser with dissolution of adhering gas.

The high and effective bubble nucleating gas dispersion according to the invention with subsequent solution in the polyol or isocyanate component is not due The low pressure process is limited, although the fine dispersion of the gas by means of rotor / stator dispersers unfolds its superiority over other gas loading processes, especially in the low pressure process. Although the method has been described particularly with regard to the low pressure method, it is also applicable to the high pressure method using a countercurrent injection mixing head. In this case, the metering pump located behind the disperser would generate the required pressure of 100 to 250 bar, which is maintained by the injection nozzle integrated in the mixing head. In the case of discontinuous high-pressure processes, the component is recirculated to the storage container in a known manner during the interruption of production. The gas supply for predispersion is preferably switched off during the interruption.

The invention also relates to a device suitable for carrying out the method according to the invention, the storage containers for the isocyanate and polyol components, a mixing head for mixing the components, metering pumps for metered conveying of the components to the mixing head, and connecting lines from the storage containers via the pumps to the mixing head In the connecting line between the polyol pump and the mixing head, means for metered introduction of a gaseous bubble nucleating agent with predispersion in the polyol component and a fast-running disperser connected on the suction side, based on the rotor / stator principle, and a throttle valve are provided directly in front of the agitator mixing head inlet are.

The mixing head is preferably designed as an agitator mixing head for the low-pressure process.

According to the invention, it is possible to provide a foaming agent-containing polyol-Z-polyisocyanate mixture at the mixing head outlet which is up to 2 million

Contains bladder germs per cm ^{3 of} mixture. This makes it possible to To produce foams with a density of 100 kg / m ³ with a cell number density of up to 60 / cm or with a density of 30 kg / m ³ with a cell number density of up to 38 / cm. Polyurethane foams according to the invention with a density of 50 kg / m ³ have up to 45 cells / cm and foams with a density of 80 kg / m ³ up to 54 cells / cm.

With a foam density of 30 kg / m ^{3, the} cell number density is preferably at least 30 / cm; at a foam density of 100 kg / m ³ at least 50 / cm (each without fillers).

The invention is explained in more detail with reference to the following figures:

1 shows a polyol tank 1, an isocyanate tank 2, associated metering pumps 3 and 4 and an agitator mixing head 5 which are connected by corresponding lines. According to the invention, as indicated by arrow 6, the bubble nucleating gas is introduced into the poly line, predispersed via a static mixer 7, and then introduced into the rotor / stator disperser 8. The pressure built up by the disperser is held by the throttle 9 at the mixing chamber entrance and is preferably regulated by a pressure measurement 10. Furthermore, a sight glass 11 can be provided in the line for visual or electro-optical control of the clarity of the polyol solution. In the event that the polyol appears milky in the sight glass, either the admission pressure at the throttle 9 can be increased and / or the rotor speed of the disperser 8 can be increased.

FIG. 2 shows an embodiment of the invention in which a metering pump 3b is provided behind the disperser 8 to limit the delivery rate of the disperser 8. The polyol pump 3a is not calibrated. Their delivery rate is about 10% above the metering pump 3b. Excess pumped polyol is returned to the tank 1 through the overflow valve 13. The same reference numerals, also in the other figures, denote the same elements as in FIG. 1 or FIG. 2. In the case of the embodiment of the invention as a high-pressure process, the mixing head 5 is designed as a countercurrent injection mixing head. Recirculation lines from the mixing head to the storage container are provided for the discontinuous high-pressure process. The metering pump 3b is designed as a high-pressure metering pump. The function of the throttle valve 9 is assumed by the injection nozzle at the inlet of the high-pressure mixing head.

3 shows an embodiment of the invention for the production of filler-containing polyurethane foam. The filler is mixed with the polyol from a filler reservoir 20 via a filler metering unit 21 by means of a mixer 22, which is preferably designed as a screw mixer. Pump 3a is designed as a metering pump and delivers a defined polyol flow to the mixer 22. Metering pump 3b serves to limit the delivery rate of the fine disperser.

4 shows an embodiment of the invention using carbon dioxide as a further foaming agent. The carbon dioxide 30 is preferably introduced behind the disperser 8 and metering pump 3b by means of metering pump 31 into the pressurized poly line and mixed with the polyol via an additionally provided static mixer 32. As a further foaming agent, water can be introduced into the mixer via one of the additive feed lines 12. Other additives can be the usual additives used in polyurethane foam production, such as surface-active substances, catalysts, etc. In this case, the mixer 5 has a pressure relief element 51 at the outlet, which maintains a pressure in the mixer which keeps the carbon dioxide in solution until the mixer is discharged.

Claims

Patentansprtiche

1. A process for the production of polyurethane foam by mixing an isocyanate component and a polyol component in a mixing unit in the presence of a dissolved foaming agent and air and / or nitrogen as bubble nucleating agent, characterized in that the bubble nucleating agent by fine dispersion and pressure increase in at least one of the components before introduction is dissolved in the mixer.

2. The method according to claim 1, characterized in that the bubble nucleating agent is first predispersed in the component in a static mixer and then finely dispersed and at least partially dissolved in a fast-running disperser based on the rotor / stator principle with pressure increase becomes.

3. The method according to claim 1 or 2, characterized in that a preferably controllable throttle valve is provided to maintain the pressure of the component up to the mixing chamber inlet before the mixing chamber inlet.

4. The method according to any one of claims 1 to 3, characterized in that the bubble nucleating agent is dispersed / dissolved in the component in such an amount that the solution partial pressure of the bubble nucleating agent in the component is a maximum of 70% of the increased pressure.

5. The method according to claim 4, characterized in that the solution partial pressure is 20 to 50% of the increased pressure.

6. The method according to any one of claims 1 to 5, characterized in that the increased pressure is generated by the dispersant.

7. The method according to any one of claims 1 to 6, characterized in that a metering pump is provided following the disperser to limit its delivery rate.

8. The method according to any one of claims 1 to 7, characterized in that water and / or low-boiling liquids, such as pentane or methylene chloride, are fed to the low-pressure mixing unit as foaming agents.

9. ^{• The} method according to any one of claims 1 to 8, characterized in that carbon dioxide is used as an additional foaming agent.

10. The method according to claim 9, characterized in that the carbon dioxide is introduced into the polyol component after the fine dispersion of the Blaseπkeimmittels and pressure increase.

11. The method according to any one of claims 1 to 10, characterized in that the polyol component contains fillers.

12. The method according to claim 9, characterized in that the filler is mixed with the polyol before predispersion of the bubble nucleating agent.

13. Device for performing the method according to one of claims 1 to 12, containing storage containers for the isocyanate and the polyol component, a mixing head for mixing the components, metering pumps for metered delivery of the components to the mixing head and

Connecting lines from the storage containers via the pumps to the mixing head, in at least one of the connecting lines between the feed pump and mixing head means for the metered introduction of a gaseous bubble nucleating agent, a static mixer for predispersion of the bubble nucleating agent in the component and a suction line connected in the conveying direction, quickly ongoing, on the rotor / stator principle based disperser and a throttle valve are provided immediately in front of the mixing head inlet.

14. The apparatus according to claim 13, characterized in that an additional metering pump is provided after the disperser according to the rotor / stator principle.

15. Polyurethane foam with a density of 30 to 100 kg / m ³ and a linear cell number density of at least 30 to 50 cm ^-1 .