DE29622866U1 - High-pressure mixing device for the production of foamed polyurethane foam - Google Patents

Description

PCT/EP 96/04070 G 45368 AH/co

High pressure mixing device for Production of foamed polyurethane foam

The present invention relates to the production of foamed polyurethane foams and, in particular, relates to a high pressure mixing apparatus for producing polyurethane foams from a reaction mixture of chemical components comprising a polyol, an isocyanate, additives and a very low boiling inert blowing agent, such as liquid CO ₂ , which are mixed in the liquid state.

Typical self-cleaning high pressure mixing devices for the production of foamed polyurethane material are described in several previous patents, e.g. in US-A-3,706,515, US-A-4,175,874, US-A-4,332,335 and US-A-4,440,500.

Generally, a high pressure mixing device comprises a cylindrical mixing chamber into which the chemical components supplied under high pressure are injected; the components are thoroughly mixed together due to the violent collision of the jets, resulting in a liquid mixture which can be discharged through a discharge channel. Typical arrangements of mixing chamber and discharge channel are shown in the above-mentioned patents. Mixing devices of this type have generally been proposed and found to be effective for producing liquid polyurethane mixtures which foam due to the CO ₂ generation by the chemical reaction which takes place between the polyol and water within the cavity of a mold.

The pre-expansion technique, more commonly known as frothing, is also well known for the manufacture of polyurethane foams; according to this technique, a non-reactive, low boiling inert gas, or blowing agent, is suitably added and mixed with the reactive polyurethane components, and the resulting mixture is foamed by releasing the pressure as it is poured into the mold cavity, so as to cause rapid foaming or pre-expansion of the polyurethane mixture into a very dense, creamy form before the chemical reaction between the components and the polymerization phase starts.

In the past, various attempts have been made to produce pre-expanded polyurethane foams by foaming with low-boiling blowing agents; devices of this type are described, for example, in US-A-3,184,419, US-A-4,115,299, US-A-4,418,041 and US-A-5,120,770.

The main problem with the mechanical mixing systems according to US-A-3 184 419 concerns the cleaning of the rotor and the mixing chamber, which must necessarily be done by washing with solvents, with the resulting risks of environmental pollution, which are no longer acceptable in modern factories.

In contrast to the above example, in order to overcome certain disadvantages, a foaming method was proposed in US-A-4 115 299 and US-A-4 418 041, using a high pressure mixing device, wherein the blowing agent, after mixing the polyurethane components, is injected into the outlet channel at a low pressure, at a pressure lower than that theoretically existing inside the mixing chamber. In the case of using a very low boiling blowing agent, such as CO ₂ , which requires a high pressure to be mixed in the liquid state, e.g. between 5 and 20 bar,

Problems arise from the uncontrolled expansion of the mixture, resulting in cast foams of very low quality. Liquid CO ₂ is very difficult to control during foaming due to rapid evaporation within the mixture, causing a very strong jet of foamed material at the outlet of the mixing device.

Although pre-expanding polyurethane foams with an inert gas is a well-known technique and has also been proposed for high-pressure mixing devices, it has not been possible to successfully apply this technique, especially in the manufacture of molded articles, due to the practical impossibility of obtaining an adequately controlled release of the blowing agent during the pre-expansion or foaming phase. Foaming polyurethane foam with a self-cleaning high-pressure mixing device therefore represents a problem that is still unsolved.

It would therefore be desirable to have available a high pressure mixing device suitable for applying the foaming technique with very low boiling point propellants, in particular with liquid CO ₂ /, which allows control during the release of the liquid propellant and rapid settling of the foamed mixture on leaving the mixing chamber, and also to have a high pressure mixing device of the self-cleaning type which has a simple structure and which is particularly suitable for the production of molded articles with repetitive and very short operating cycles.

In the search for a solution to this problem, we have investigated various designs of mixing devices that could allow the mixing of a very low boiling propellant in a liquid state, while at the same time providing good stabilization of the flow of the propellant emerging from the mixing device.

foamed material; in particular, we have considered, without success, experiments with devices of the type described in EP-A-0 070 486 and US-A-5 277 567.

Both of the above-mentioned documents show a Z-shaped design of the flow path between a mixing chamber and an outlet channel for completely different purposes and applications than in the present invention.

In particular, EP-A-0 070 486 shows the use of a throttle body at the outlet of the mixing chamber, which is arranged tangentially with respect to the outlet channel, in order to generate a rotary movement in the mixture at a very short distance from the mixing chamber, which causes a rapid settling of the liquid mixture in the outlet channel.

On the other hand, US-A-5 277 567 proposes a simple Z-shaped design of mixing and settling chambers and the outlet channel, without any throttling body, the specific object being to provide a method and a device for coating cast articles with a layer of paint directly inside the mold. In addition to the various fields of application, this second document considers it disadvantageous to create a throttling at the outlet of the mixing chamber, which should therefore be avoided. Taking this and the results of some tests into account, the mixing devices of the types described above were found to be completely unsuitable for the foaming technique.

Despite this negative result, in the search for a new design for a high pressure mixing device that would make it suitable for the foaming technique in the manufacture of molded articles made of polyurethane foam, attempts were nevertheless made to test solutions using simple flow diversions, which did not produce particularly successful results.

It was in fact found that simple L- or Z-shaped designs, as proposed in the prior art, did not allow sufficient pressure to be generated in the mixing chamber to keep the CO ₂ in the liquid phase due to the high volatility of the liquid CO ₂ . In practice, the CO ₂ gasified very quickly, causing the mixture in the liquid state to be expelled with great force to the outside, with a significant loss of propellant; the advantages of the pre-expansion technique were therefore largely negated.

The first results were therefore discouraging and led to the conclusion that the conventionally known high-pressure mixing devices were completely unsuitable for foaming polyurethane mixtures.

The reasons for this are the lack of any control of the release of CO ₂ during the foaming step, which results in an excessively violent expulsion of the mixture from the mixing chamber; this is probably the reason for bubble formation in the foam and excessive turbulence of the foaming mixture when leaving the outlet channel.

However, starting from conflicting points of view in the prior art, it was found after various tests that by appropriately throttling the mixing chamber to maintain optimal pressure values sufficient to keep the CO ₂ , or more generally the low-boiling blowing agent, in the liquid phase during mixing, in combination with a controlled release of the blowing agent into the mixture during foaming, immediately after and near the mixing chamber outlet, it was possible not only to mix the blowing agent in an appropriate manner and to stabilize the flow of the mixture, but also to obtain a high quality of the foamed foams, which

have a homogeneous cell structure without bubbles and are suitable for casting into moulds.

The invention relates to a high-pressure mixing device, in particular for the production of foamed polyurethane foams by mixing reactive polyurethane components with a low-boiling blowing agent, the device comprising: a body defining a mixing chamber (10) with a longitudinal axis in which the polyurethane components and the blowing agent are mixed at a first pressure (Pl) sufficient to keep the blowing agent in the liquid phase; wherein the mixing chamber (10) is connected to an outlet channel which is oriented differently with respect to the mixing chamber (10) of the mixing device, and wherein throttling means (26) are provided on the outlet side of the mixing chamber (10), which mixing chamber (10) is connected to the outlet channel (19) via a central frothing chamber (18), in combination with a dynamic pressure generating means which is present between the frothing chamber (18) and the outlet channel (19) in order to generate a dynamic pressure in the mixture within the frothing chamber (18) with a value (P2) which is lower than the pressure (Pi) within the mixing chamber (10) of the mixing device.

The foaming of a polyurethane mixture with a liquid blowing agent, especially with liquid CO ₂ , is critical due to the large increase in foam volume in a very short time, which can lead to ten times the volume of the liquid mixture leaving the mixing chamber.

The innovative aspect of the invention lies in the way of controlling the pressure conditions inside the mixing chamber and in the mixture immediately after leaving the mixing chamber, so that the mixture begins to foam, remaining in a semi-liquid state for a short period of time without any significant gas release from the propellant, which is then released in a gradual and controlled manner.

while the mixture expands along a stabilization path. This therefore results in the production of expanded polyurethane foam of the open cell or closed cell type which is substantially free from bubbles and defects and has a good cell structure, and is particularly suitable for the manufacture of molded articles, insulation or structural parts with flexible or rigid polyurethane foams of the required quality.

The invention therefore allows the application of the foaming technique in high pressure mixing devices of the self-cleaning type, which completely eliminate the use of solvents and the use of parts that have to be replaced or discarded.

The invention and preferred embodiments are described in more detail below and in the drawings, in which: ■

Figure 1 is a schematic view of a first embodiment of a device according to the invention;

Figure 2 is a schematic view of a second embodiment of a device according to the invention;

Figure 3 is a cross-sectional view of a preferred embodiment of a device according to the invention.

With reference to Figure 1 we will describe the general features of a mixing apparatus as well as the process according to the invention for the production of pre-expanded or foamed polyurethane foams using a low boiling inert blowing agent, e.g. liquid CO ₂ .

The device according to the invention essentially comprises a mixing chamber 10, e.g. of cylindrical shape, having a longitudinal axis which is vertically aligned in Figure 1, and

has a very short length, ranging between 2 to 2.5 diameters of the chamber or less , into which chamber the reactive chemical components, such as a polyol and an isocyanate, contained in tanks A and B, together with the respective additives, are injected through nozzles 11 and 12. Components A and B and the additives are introduced into chamber 10 under high pressure in stoichiometrically measured amounts by means of respective volumetric pumps 13 and 14. Reference numeral 15 shows the supply of a low boiling, inert blowing agent, in particular liquid CO ₂ , which is injected into the line for supplying one of the components, e.g. into the polyol stream, in amounts ranging between 1 and 10% by weight of the polyol.

In order to ensure that mixing in the chamber 10 is carried out under pressure-controlled conditions sufficient to maintain the propellant in the liquid state, e.g. at a pressure of between 5 and 20 bar or more, which is above the partial pressure of the gas dissolved in the mixture at that temperature, the outlet 17 of the mixing chamber 10, which opens into a subsequent, central foaming chamber 18, is partially closed by interposing a throttle body which can be formed by any suitable means as described below.

The central chamber 18, in which the initial pre-expansion or foaming of the mixture takes place under pressure-controlled conditions, consists essentially of a cylindrical bore having a considerable length and a diameter equal to or greater than that of the mixing chamber 10; the longitudinal axis of the chamber 18 is further arranged at an angle, for example perpendicular to the longitudinal axis of the mixing chamber 10, the central chamber 18 opening into an outlet channel 19 having a cylindrical bore of larger diameter than the mixing chamber 10 and the foaming chamber 18, in order to discharge the foamed mixture by calming the flow

to expand; the longitudinal axis of the channel 19 is in turn arranged at an angle, for example perpendicular to the longitudinal axis of the central chamber 18, and it extends over a considerable length.
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According to the present invention, it is necessary that the central chamber 18, also called foaming chamber, has a considerable length that optimizes the controlled release of the propellant; more precisely, it is necessary that inside the central chamber 18 there takes place a partial foaming step of the mixture under pressure-controlled conditions in such a way that near and immediately behind the outlet 17 of the mixing chamber there is created a back pressure P2 that is lower than the pressure Pl inside the mixing chamber 10, for example a pressure P2 that is in the range between 0.2 and 2 bar and is sufficient to initially keep the mixture in a semi-liquid state in order to prevent an uncontrolled or sudden explosion of the mixture.

Through tests carried out, it has been found that good results can be obtained by throttling the flow at the outlet of the mixing chamber 10 so that mixing is carried out under a high pressure sufficient to keep the CO ₂ or the propellant in the liquid state, and by creating a low back pressure P2 at or near the outlet opening 17 of the mixing chamber 10. By throttling and suitably creating a back pressure downstream of the throttled outlet of the mixing chamber along the flow path of the mixture which begins to foam, it is possible to obtain a gradual and controlled release of the propellant within a foaming chamber 18 extending between the mixing chamber 10 and an outlet channel 19 of the device; As a result, not only can the release of pressure be controlled and the foamed mixture calmed or stabilized, but also, by varying and suitably adjusting the length in the longitudinal direction and the diameter of the chambers 10, 18 and the

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channel 19, depending on the required flow rate, the factors that cause bubble or void formation are eliminated; therefore it is possible to obtain a smooth discharge of the foamed mixture, without turbulence, which was not achievable with conventional high pressure mixing devices.

According to the invention, the device has been provided with means for throttling the outlet of the mixing chamber, in combination with means for diverting the mixture from the chamber 10 to the chamber 18 and to the channel 19, which means are suitable for generating in the chamber 18 a back pressure P 2 necessary to prevent a high discharge speed of the flow and to calm the foaming mixture. These means can be implemented in any suitable form in order to produce in their combination the mixing of the propellant in liquid form and the generation of the necessary back pressure gradient at the outlet of the mixing chamber, which bring about a gradual release of the propellant and a substantial stabilization of the foaming mixture.

In the example of Figure 1, the means for generating the dynamic pressure P2 inside the chamber 18 have been obtained by, for example, providing the chamber 18 with a suitable length for generating the necessary pressure drop which, together with the pressure drop caused by the deflection of the flow at the inlet and outlet of the foaming chamber 18, generates the dynamic pressure P2 required downstream of the throttling.

From tests and experiments it has been found that in cases where the foaming chamber 18 opens directly into the outlet channel 19, as in the example of Figure 1, in order to generate the back pressure P2 such that bubbles or larger voids in the foam are substantially eliminated, it is necessary that the chamber 18 should have a length equal to or three times greater than its diameter; also lengths of the foaming chamber

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e.g. between 2.5 and 3 diameters are within the scope of the present invention, although they may give less successful results.

Based on the above explanations, it is obvious that the concept underlying the foaming process according to the invention is to create pressure drops downstream of a mixing chamber restriction sufficient to keep the mixture exiting the mixing chamber and beginning to foam in a substantially semi-liquid state so as to avoid rapid gas release of the blowing agent at the outlet of the mixing chamber, thereby preventing the formation of gases which would otherwise be present in the foam and the molded articles.

Figure 2 shows a flow path configuration substantially similar to that of Figure 1, except that the foaming chamber 18 is now of shorter length and its outlet 20 opens between the outlet channel 19 and a rear closed end chamber 21 defining a dead zone for the accumulation of foaming material, which in this example is located at the rear end and axially aligned with the outlet channel 19, but could be positioned differently if required.

The formation of a closed end zone 21 for the mixture at the outlet end of the foaming chamber 18, between the latter and the outlet channel 19, leads to a further attenuation of the mixture flow, with the result that the quality of this foam is improved due to the total absence of voids or bubbles and the consequent better utilization of the propellant. In the case of Figure 2, it was found from tests carried out that the foam produced had a comparatively lower density than that obtained with the design of the device according to Figure 1, and a reduction in the amount of propellant of between 15 and 20% was possible;

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this can be explained by the fact that the mixture filling the closed chamber 21 is essentially free of turbulence, eliminating the effect of the solid wall at the point where the flow is diverted, as shown in the example of Figure 1, and forming a liquid surface which is a continuation of the mixing chamber wall or a liquid buffer at the bottom of the outlet channel which tends to further dampen the mixture.

Figure 3 of the accompanying drawings shows a practical embodiment of a high-pressure mixing device of the self-cleaning type, on the basis of which the schematic functional diagrams of Figures 1 and 2 can be carried out.

For the sake of clarity, the same reference numerals as in Figure 1 or in Figure 2 have been used in Figure 3 to designate similar or equivalent parts.

In the example of Figure 3, the device has a body 22 formed from several parts, among which is a small mixing chamber 10 of substantially cylindrical shape, having a cleaning piston 23 sliding therein, connected to the piston part 24 of a first hydraulic actuation cylinder 25. The mixing chamber 10 opens into a central frothing chamber 18 at right angles thereto, having a suitable length and a diameter larger than that of the mixing chamber 10; a second mixing piston 26 slides within the chamber 18 and is connected to the piston part 27 of a second hydraulic actuation cylinder 28. Figure 3 further shows means for adjusting the stroke of the piston 26, which for example consist of an adjustable body 29 which can be operated by screws so as to fix the retracted position of the piston 27 in order to adjust or vary the degree of throttling at the outlet opening of the mixing chamber 10.

The middle or foaming chamber 18 in turn leads into a wider outlet channel 19 which is arranged at an angle, e.g. vertically, with respect to this chamber and has a slidable third cleaning piston 30 which is connected to the piston part 31 of a third hydraulic actuating cylinder 32.

All hydraulic cylinders can be actuated in a suitable sequence to move the respective cleaning pistons between a retracted position as shown in Figure 3, in which the mixing chamber 10 opens into the frothing chamber 18 and the frothing chamber 18 opens into the discharge channel 19, and an advanced position to close the device; in this position the cleaning piston 23 acts first and expels residual mixture within the chamber 10 into the frothing chamber 18; then the cleaning piston 26 is actuated to expel residual mixture from the frothing chamber 18 into the discharge channel 19, and finally the cleaning piston 30 is actuated to expel residual mixture within the discharge channel 19, the sequence being reversed when the device is opened.

In particular, in the embodiment according to Figure 3, the cleaning piston 30 of the discharge channel 19 can be moved between a fully advanced position in which it closes the outlet of the frothing chamber 18 and the channel 19 and a first retracted position in which the tip of the piston 30 is flush with the outer surface of the frothing chamber 18, as in the example of Figure 1, or between the previously mentioned advanced position and a second retracted position behind the previous one in which the tip of the piston 30 and its guide bore form the closed chamber 21 according to the example of Figure 2; for example, the piston 30 can be retracted with respect to the chamber 18 between 0.5 and one time the diameter of the piston itself or less. It is obvious that in the second case the frothing chamber 18 can also be three diameters shorter and

the actuating cylinder 32 can be provided with a suitable adjustable stopping device, such as that indicated by the reference numeral 29 for the actuating cylinder 28.
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In the example of Figure 3, the longitudinal axes of both chambers 10 and 18 as well as of the outlet channel 19 lie in the same plane; however, another three-dimensional position or arrangement of the chambers 10, 18 and the channel 19 can also be provided, which results in larger or smaller deflection angles of the flow than those shown, in the general case also deflection angles other than 90°. In all these cases, by means of the embodiment shown in Figure 3, it is possible to use a high-pressure mixing head for the production of polyurethane mixtures using the foaming technique, which is completely self-cleaning, has a compact structure and is extremely functional in use.

Tests and experiments were carried out with the device according to the present invention designed according to both examples of Figure 1 and Figure 2, and the results were compared with those of conventional systems.

example 1

In this comparative example, a conventional device was used which was modified according to the example of Figure 1, except that the central chamber 18 in this case had a length of less than three diameters. Furthermore, the mixing chamber 10 opened freely, without any restriction, into the chamber 18.

The mixing chamber 10 had a diameter of 12 mm and essentially the same length;
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the foaming chamber 18 had a diameter of 16 mm and a length of 32 mm;

the outlet channel 19 had a diameter of 30 mm and a length of 90 mm.

A mixture of polyurethane components in conventional formulation was used, using liquid CO ₂ as a blowing agent in an amount of 5 parts by weight with respect to the polyol of the reaction mixture.

The pumps 13 and 14 were adjusted to a total flow rate of the device of 400 g per second, whereby a pressure Pl close to atmospheric pressure was measured inside the mixing chamber 10, which is totally unsuitable for mixing the propellant in the liquid state; no dampening effect on the flow was achieved at the outlet of the chamber 10.

The foamed mixture emerging from the device was poured directly into the open cavity of a mold.

The results were completely negative in the sense that there was a turbulent outflow of a partially foamed and partially still liquid mixture, which caused a noticeable dispersion of the CO ₂ .

The resulting foamed mixture resulted in the formation of a foam of inconsistent quality, with irregular and large voids.
30

Example 2

The same device as that described in Example 1 was used, whereby according to the invention, throttling means were provided at the outlet of the mixing chamber 10 by the cleaning piston 26 of the foaming chamber 18, as shown in

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Figure 1, until a homogeneous flow of well foamed mixture was obtained at the outlet, the mixture being poured into the mould cavity in a uniform and smooth manner. Mixing within the chamber 10 was carried out at a pressure of about 17 bar, a pressure of about 1.5 bar being measured within the foaming chamber 18 near the outlet of the mixing chamber 10.

The foam had a density of about 27 kg/m ³ with a homogeneous and fine cell structure and without any major voids. The quality of the foam was found to be commercially acceptable.

Example 3

Finally, further in accordance with the invention, a device as in the previous example was used, wherein the piston 30 of the outlet channel was further retracted, so as to form a closed chamber 21 as in the example of Figure 2.

The test was repeated with the same formulation of the mixture as in the previous examples.

A pressure P2 of about 1.3 bar was measured inside the foaming chamber 18, and a pressure of about 17 bar inside the mixing chamber.

The foamed mixture came out in a very smooth and controlled manner, as in the previous example, obtaining in this case a foam of improved quality with a density of about 26 kg/m ³ and a homogeneous cellular structure completely free of holes and voids.

At the end of the tests, unexpected results were found - which were not achieved with conventional designs of the known high-pressure mixing devices.

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were not available - by proceeding according to the present invention; the foaming technique can therefore be advantageously used in high pressure mixing devices for the production of foamed polyurethane foams of improved quality by controlling the release of the blowing agent in a suitable manner. By creating a back pressure in a foaming chamber according to the invention, it was therefore possible to obtain foamed polyurethane foams of satisfactory quality suitable for any application, e.g. for casting foam bodies for cushions and car seats, for forming thermal insulation for refrigerators, for forming panels or for many other similar applications.

Claims (6)

- 18 - Claims 1. High-pressure mixing device, in particular for producing foamed polyurethane foams by mixing reactive polyurethane components with a low-boiling blowing agent, the device comprising: a body having a mixing chamber (10) with a longitudinal axis in which the polyurethane components and the blowing agent are mixed at a first pressure (Pl) sufficient to keep the blowing agent in the liquid state; wherein the mixing chamber (10) is connected to an outlet channel arranged at an angle thereto, and wherein throttle means (26) are provided at the outlet of the mixing chamber (10), characterized in that the mixing chamber (10) is connected to the outlet channel (19) via an intermediate foaming chamber (18), and that dynamic pressure generating means are provided between the foaming chamber (18) and the outlet channel (19) for generating a dynamic pressure in the mixture within the foaming chamber (18) with a value (P2) which is lower than the pressure (P1) within the mixing chamber (10) of the mixing device. 2. Device according to claim 1, in which the foaming chamber (18) has a cylindrical shape and the dynamic pressure generating means have a length of the foaming chamber (18) which is at least 2.5 to 3 times greater than the diameter of the foaming chamber (18). 3. Device according to claim 2, in which the means for generating the dynamic pressure comprise a flow deflection of the foaming mixture between the foaming chamber (18) and the outlet channel (19). 4. Device according to claim 2, which has a closed end zone (21) for collecting foaming mixture at the deflection - 19 - point of flow between the frothing chamber (18) and the outlet channel (19). 5. Apparatus according to claim 1, in which the mixing chamber (10), the frothing chamber (18) and the outlet channel (19) each comprise a slidable cleaning member (23, 26, 30) connected to a hydraulic actuator (25, 28, 32) for movement between a retracted position and an advanced, closed position for the apparatus, the throttling means for the outlet of the mixing chamber (10) comprising the cleaning member (26) of the frothing chamber (18) and adjustable stop means (29) arranged behind the cleaning member (26) for the frothing chamber (18) for varying the throttling of the mixing chamber outlet (17). 6. Apparatus according to claim 4, in which the cleaning member (30) for the outlet channel (19) is movable within a guide bore extending rearwardly with respect to the frothing chamber (18), the closed end zone (21) for the accumulation of the frothing mixture being provided by retracting the tip portion of the cleaning member (39) into the guide bore.