WO1997011824A1

WO1997011824A1 - Process and high-pressure mixing apparatus for the production of frothed polyurethane foams

Info

Publication number: WO1997011824A1
Application number: PCT/EP1996/004070
Authority: WO
Inventors: Carlo Fiorentini
Original assignee: Afros SpA
Current assignee: Afros SpA
Priority date: 1995-09-25
Filing date: 1996-09-18
Publication date: 1997-04-03
Anticipated expiration: 1998-03-25
Also published as: ITMI951975A0; IT1277651B1; ITMI951975A1; DE29622866U1

Abstract

A process and a high-pressure mixing apparatus for the production of a frothed polyurethane foam. Two reactive chemical components and a low-boiling blowing agent (CO2) are mixed into a high pressure mixing apparatus comprising a mixing chamber (10) connected to a discharge duct (19); the flow of the mixture is throttled to generate a pressure inside the mixing chamber (10) which is sufficient to maintain the blowing agent at a liquid state. The mixture is thereupon frothed by releasing the blowing agent along a frothing chamber (18) extending between the mixing chamber (10) and the discharge duct (19); a back-pressure is generated into the frothing mixture by extending the frothing chamber (18) for a substantial length and by changing the flow direction of the frothing mixture while flowing from the frothing chamber (18) into the discharge duct (19).

Description

PROCESS AND HIGH-PRESSURE MIXING APPARATUS FOR THE PRODUC¬ TION OF FROTHED POLYURETHANE FOAMS

The present invention relates to the production of frothed polyurethane foams, and more precisely it refers to a 5. process and to a high-pressure mixing apparatus for the production of polyurethane foams from a reactive mixture of chemical components comprising a polyol, an isocyanate, additives and a very low-boiling inert blowing agent, such as liguid C0₂, which is mixed at a liquid state.

10. Typical self-cleaning high-pressure mixing apparatus for the production of foamed polyurethane material are descri¬ bed in several prior patents, for example in US-A- 3,706,515, US-A-4,175,874, US-A-4,332,335 and US-A- 4,440,500.

15. In general a high-pressure mixing apparatus comprises a cylindrical mixing chamber into which the reactive chemical components supplied at high pressure are injected; the components are thoroughly mixed together owing to the violent inpingiment between the jets, resulting in a

20. liquid mixture which may be poured through a discharge duct. Typical arrangements of the mixing chamber and the discharge duct are shown in the aforementioned patents. The mixing apparatus of this type in general have been proposed and proved to be efficient for the preparation of

25. liquid polyurethane mixtures, which foam due to C0₂ gener- ated by the chemical reaction which takes place between the polyol and water (chemical C0₂) inside the cavity of a mould.

The pre-expansion technique, more commonly referred to as 5. "frothing", is also well known in the production of polyu¬ rethane 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 frothed by releasing the pressure io. when pouring into the cavity of the mould, so as to cause rapid frothing or pre-expansion of the polyurethane mixture in the form of a very dense cream, prior to the start of the chemical reaction between the components, and the polymerization phase.

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

20. The greatest problem in a mechanical mixing system accord¬ ing to US-A-3,184,419 relates to cleaning of the rotor and the mixing chamber, which must be necessarily performed by washing with solvents, with the consequent risks of envir¬ onmental pollution no longer acceptable in the modern

₂₅. plants. In contrast with the above example, in order to overcome certain drawbacks, US-A-4,115,299 and US-A-4,418,041 propo¬ se a frothing method by which high pressure mixing appara¬ tus is used, whereby the blowing agent is injected at a low

5. pressure into the discharge duct, after mixing of the polyurethane components, at a pressure lower than that theoretically existing inside the mixing chamber. Therefo¬ re, in the case of use of a very low-boiling blowing agent such as C0₂ which, in order to be mixed at a liquid state io. requires a high pressure, for example of between 5 and 20 bars, problems arise about the uncontrolled expansion of the mixture which causes moulded foams of very low quali¬ ty. Liquid C0₂ is very critical to control during frothing due to rapid vaporization inside the mixture which causes

15. violent jet of frothed material at the outlet of the mixing device.

Although the pre-expansion of polyurethane foams with an inert gas is a well-known technique and has also been proposed with high-pressure mixing apparatus, hitherto it

20. has been impossible to use it with success, in particular in the production of moulded articles, owing to the practi¬ cal impossibility of obtaining an adequately controlled release of the blowing agent during the pre-expansion or frothing phase. Therefore frothing a polyurethane foam by a

25. self-cleaning and high pressure mixing apparatus represents a problem which is still unresolved. It would therefore be desirable to have a process and a high-pressure mixing apparatus suitable for use of the frothing technique with very low-boiling blowing agents, in particular liquid C0₂, which allow control during the

5. release of the liquid propellant and rapid settling of the frothing mixture upon leaving the mixing chamber, as well as a high-pressure mixing apparatus of self-cleaning type which have a simple structure and is particularly suitable for the production of moulded articles with repetitive and io. extremely short operative cycles.

In the search for a solution to this problem, we have attempted to use a different configuration of the mixing apparatus which might allow the mixing of a very low- boiling blowing agent at a liquid state, providing at the 15. same time a good flow stabilization of the frothing mate¬ rial outcoming the mixing apparatus; in particular, we have unsuccessfully considered to experiment apparatus of the type described in EP-A-0,070,486 and US-A-5,277,567.

Both the documents illustrate a Z-shaped configuration of 20. the flow path between a mixing chamber and a discharge duct for totally different purposes and applications compared to the present invention.

In particular, EP-A-0,070,486, illustrates the use of a throttling member at the outlet of the mixing chamber,

25. which is tangentially arranged in respect to the discharge duct for generating a rotary movement in the mixture at a very short space from the mixing chamber, which causes a rapid settling of the liquid mixture into the discharge duct.

5. On the other hand, US-A-5,277,567 proposes a simple Z- shaped configuration of the mixing and quieting chambers and the discharge duct, in the absence of any throttling member, with the specific object of providing a method and an apparatus for coating moulded articles with lacquer, io. directly inside the shaping mould. Apart from different applications, this second document considers disadvanta¬ geous to provide a throttling at the outlet of the mixing chamber, which therefore must be avoided. Considering the above and the results of some tests, the mixing apparatus

15. of the above referred types have been therefore considered entirely unsuitable for the frothing technique.

Despite this negative consideration, in the search for a new process and a new configuration for the high-pressure mixing apparatus, making them suitable for the frothing 20. technique in the production of moulded articles of polyu¬ rethane foams, it was nevertheless attempted to test solu¬ tions which could use simple flow deviations with not particularly successful results.

It was established, in fact, that a simple L- or Z-shaped 25. configurations, as proposed in the prior art, on account of the high volatility of the liquid C0₂, did not allow the generation, inside the mixing chamber, of the pressure conditions suitable for keeping the C0₂ in the liquid state. In practice the C0₂ gassified rapidly, causing 5. mixture still in the liquid state to be violently ejected outwards, with a considerable loss of blowing agent; the advantages of the pre-expansion technique were thus vani- fied.

The first results were therefore discouraging and led to io. the conclusion that the traditionally known high-pressure mixing apparatus were entirely unsuitable for the frothing of polyurethane mixtures.

The reasons for this lay in the absence of any control for the release of the C0₂ during the frothing step, which 15. resulted in an excessively violent ejection of the mixture from the mixing chamber, this being presumably the cause of bubbles formation in the foam and excessive turbulence of the frothing mixture upon leaving the discharge duct.

Starting from contradicting opinions of the prior art, 20. after various attempts, however, it was established that by properly performing a throttling of the mixing chamber in order to maintain optimum pressure values sufficient for keeping the C0₂, or more generally the low-boiling blowing agent in the liquid state during mixing, in combination 25. with a controlled release of the same blowing agent into the mixture during frothing, immediately after and in the vicinity of the mixing chamber outlet, not only was it possible to adequately mix the blowing agent and stabilize the flow of the mixture, but also a good quality of the 5. frothed foam was obtained, having a homogeneous cellular structure, devoid of bubbles and in a suitable condition for moulding.

According to a first aspect, the invention therefore rela¬ tes to a process for the production of a frothed polyuret-

10. hane foam mixing at least two reactive chemical components and a low boiling blowing agent at a liquid state, by high pressure mixing apparatus comprising a cylindrical mixing chamber (10) connected to a discharge duct (19), comprising the steps of:

15. - forming a reactive mixture by mixing the chemical compo¬ nents and the blowing agent under pressure controlled conditions by throttling the flow of the resulting mixture at the outlet of the mixing chamber (10) to generate in the same mixing chamber (10) a pressure sufficient to maintain

20. the blowing agent at a liquid state;

- frothing the resulting mixture by releasing the blowing agent along a frothing chamber (18) provided between the mixing chamber (10) and the discharge duct (19); and gene¬ rating a back-pressure (P2) into the frothing mixture, by

25. changing the flow direction of the frothed mixture at a substantial length from the mixing chamber (10) which will maximize the releasing of the blowing agent and the quie- ting of the frothing mixture while flowing from the frot¬ hing chamber (18) into the discharge duct (19).

According to another aspect, the invention relates to a high-pressure mixing apparatus, in particular for the

5. preparation of a frothed polyurethane foam by mixing reac¬ tive polyurethane components with a low-boiling blowing agent, the apparatus comprising: a body defining a mixing chamber (10) having a longitudinal axis, in which the polyurethane components and the blowing agent are mixed at io. a first pressure value (Pl) sufficient to keep the blowing agent in a liquid state; wherein the mixing chamber (10) is connected to a discharge duct differently oriented with respect to the same mixing chamber (10) of the mixing apparatus, and wherein throttling means (26) are provided

15. at the outlet side of the mixing chamber (10), said mixing chamber (10) being connected to the discharge duct (19) via an intermediate frothing chamber (18), in combination with back-pressure generating means provided between the frot¬ hing chamber (18) and the discharge duct (19) for genera-

20. ting a back-pressure in the mixture inside the frothing chamber (18) having a value (P2) lower than the pressure (Pl) inside the mixing chamber (10) of the mixing device.

Frothing a polyurethane mixture by a liquid blowing agent, particularly liquid C0₂, is critical due to large increa-

25. sing of the foam volume in a very short time, which tends to reach up to ten times the volume of the liquid mixture leaving the mixing chamber.

The innovative aspect of the invention therefore lies in control mode of the pressure conditions inside the mixing chamber and of the mixture immediately after leaving the

5. same mixing chamber, so that the mixture starts to froth by remaining for a short period of time in a semi-liquid state, without there being substantial gassification of the blowing agent which is subsequently released in a gradual and controlled mode as it expands along a stabilization io. path. This therefore results in the production of a frot¬ hed polyurethane foam of the open or closed cell type which is substantially devoid of bubbles and defects, having a good cell structure, particularly suitable for the produc¬ tion of moulded articles, for insulations or structural

15. parts with flexible or rigid polyurethane foams having the required qualities.

The invention therefore allows application of the frothing technique to high-pressure mixing apparatus at self-clea¬ ning type, which totally eliminate the use of solvents and 20. parts to be removed, replaced or thrown away.

The invention, as well as a preferred embodiment thereof, will be explained in greater detail hereinbelow, and in the drawings, in which:

- Figure 1 is a schematic view showing a first configura- 25. tion of an apparatus according to the invention; - Figure 2 is a schematic view showing a second configura¬ tion of an apparatus according to the invention;

- Figure 3 is a cross-sectional view of a preferred embodi¬ ment of an apparatus according to the invention.

₅. With reference to Figure 1, we shall describe the general features of a mixing apparatus as well as those of the process according to the invention, for the production of pre-expanded or frothed polyurethane foams using a low- boiling, inert, blowing agent, for example liquid C0₂.

10. The apparatus according to the invention substantially comprises a mixing chamber 10, for example cylindrical in shape, having a longitudinal axis vertically directed in Figure 1, and a very short length, ranging between 2 to 2.5 diameters of the same chamber or less, into which the

15. reactive chemical components, such as for example a polyol and an isocyanate contained in the tanks A and B, together with the respective additives, are injected into the mixing chamber via nozzles 11 and 12. The components A and B and the additives are supplied at high pressure into the cham-

20. ber 10 in stoichiometrically metered quantities by means of respective volumetric pumps 13 and 14. Reference 15 deno¬ tes, moreover, the supply of a low-boiling, inert, blowing agent, in particular liquid C0₂, which is injected into the duct for supplying one of the components, for example into

25. the stream of polyol, in quantities of between 1 and 10 per cent by weight of the said polyol. In order to ensure that mixing inside the chamber 10 is performed in a pressure controlled condition, sufficient to keep the blowing agent in the liquid state, for example at a pressure of between 5 and 20 bar, or greater, which is

5. above the partial pressure of the gas dissolved at the temperature of the mixture, the outlet 17 of the mixing chamber 10 which opens into a successive intermediate frothing chamber 18 is partially restricted by arranging in between a throttling member which can be provided by any io. suitable means, as explained further below.

The intermediate chamber 18, inside which initial pre- expansion or frothing of the mixture occurs under pressure controlled conditions, substantially consists of a cylin¬ drical bore having a substantial length and a diameter

15. equal to or greater than that of the mixing chamber (10); the longitudinal axis of the chamber 18 is moreover angu¬ larly arranged, for example perpendicular to the longitudi¬ nal axis of the chamber 10, said intermediate chamber 18 opening out into a discharge duct 19 consisting of a cylin-

20. drical bore of larger diameter than the mixing chamber 10, and the frothing chamber 18, to allow the frothing mixture to expand by smoothing the output flow; the longitudinal axis of the duct 19 is in turn arranged at an angle, for example perpendicular to the longitudinal axis of the

25. intermediate chamber 18, extending over a substantial length. According to the present invention, it is necessary that the intermediate chamber 18, also referred to as frothing chamber, will have a substantial length which will maximise the controlled releasing of the blowing agent; more preci-

5. sely it is necessary that inside the intermediate chamber 18, a partial frothing step of the mixture under pressure controlled conditions takes place, so as to generate in the vicinity and immediately after the outlet 17 of the mixing chamber, a back-pressure P2 lower than the pressure Pl io. existing inside the mixing chamber 10, for example a pres¬ sure P2 ranging between 0.2 and 2 bar sufficient to keep initially the mixture in a semi-liquid state to prevent uncontrolled or sudden explosion of the said mixture.

From tests carried out it has been found that good results

15. are obtainable by suitably throttling the flow at the outlet of the mixing chamber 10 so that mixing is performed under a high pressure sufficient to keep the C0₂ or the blowing agent in liquid state, and by generating a low back-pressure P2, at or in the vicinity of the outlet

20. opening 17 of the said mixing chamber 10. By throttling and suitably generating a back-pressure downstream of the throttled outlet of the mixing chamber, along the flow path of the mixture which is starting to froth, it is possible to obtain a gradual and controlled release of the blowing

25. agent inside a frothing chamber 18 extending between the mixing chamber 10 and a discharge duct 19 of the apparatus; as a result, not only the release of the pressure can be controlled and the frothing mixture quieted or stabilized, but, by varying and suitably adapting the longitudinal length and the diameter of the chambers 10, 18 and the duct 19, depending on the required throughput, the factors 5. causing bubbles formation or cavities are eliminated; therefore it is possible to obtain gentle discharging of a frothed mixture, without turbulence, something which cannot be obtained with conventional high-pressure mixing appara¬ tus.

10. According to the invention, the apparatus has therefore been provided with means for throttling the outlet of the mixing chamber, in combination with means for deviation of the mixture from the chamber 10 to chamber 18 and to the duct 19 suitable for generating inside the chamber 18 a

15. back-pressure P2 necessary for preventing high output speed of the flow and for quieting the frothing mixture. These means may be realized in any suitable form for allowing, in their combination, the mixing of the blowing agent in liquid form and generation of the required back-pressure

20. gradient at the outlet of the mixing chamber, causing a gradual release of the blowing agent and a substantial stabilization of the frothing mixture.

In the example of Figure 1 the means for generating the back-pressure P2 inside the chamber 18 have been obtained

25. by extending, for example, said chamber 18 with a length suitable for providing the necessary pressure drop which. added to the pressure drop caused by the deviation of the flow at the inlet and at the outlet sides of the frothing chamber 18, generates the back-pressure P2 required down¬ stream of the throttlement.

₅. From tests and experiments carried out it has been found that in the case where the frothing chamber 18 opens directly into the discharge duct 19, according to the example in Figure 1, to generate a back-pressure P2 such that formation of bubbles or large cavities in the foam are io. substantially eliminated, it is required that the chamber 18 should have a length equal to or three times greater than its diameter; lengths of the frothing chamber which are comprised, for example between 2.5 and 3 diameters, even though they may provide less successful results, are

₁₅. nevertheless included within the invention.

On the basis of that stated above, therefore, it is obvious that the concept underlying the frothing process according to the invention consists in generating pressure losses downstream a mixing chamber restriction, sufficient to keep

₂0. the mixture which is emerging from the mixing chamber and starting to froth, in a substantially semi-liquid condi¬ tion, such as to avoid rapid gassification of the blowing agent at the outlet of the said mixing chamber, preventing formation of bubbles which otherwise would be present in

₂5. the foam and in the moulded articles. Figure 2 shows a configuration of the flow path substan¬ tially similar to that of Figure 1, with the difference that now the frothing chamber 18 is shorter in length having its outlet 20 which opens out between the discharge 5. duct 19 and a rear dead-end chamber 21 defining a dead zone for accumulation of the frothing mixture, which in this example is arranged at the rear end and axially ali¬ gned to the discharge duct 19 but, if required, could also be differently positioned.

io. The creation of a dead end zone 21 for the mixture at the outlet end of the frothing chamber 18, between the latter and the discharge duct 19, tends to dampen further the mixture flow, with the result of improving the quality of the foam owing to the total absence of cavities or bubbles

15. and the consequent better use of the blowing agent. In the case of Fig. 2, from tests carried out it was established that the foam produced had a density comparatively lower than that obtained with the configuration of the apparatus according to Figure 1, and a reduction of between 15 and

20. 20% in the amount of blowing agent was possible; this may be explained by the fact the mixture substantially devoid of turbulence, which fills the dead chamber 21, eliminates the effect of the rigid wall at the point where the flow is deviated, as shown in the example in Figure 1, forming a

25. liquid surface which constitutes a continuation of the mixing chamber wall or a liquid cushioning at the bottom of the discharge duct which tends to further dampen the mixtu- re .

Figure 3 of the accompanying drawings shows a practical embodiment of a high-pressure mixing apparatus, of the self-cleaning type, on the basis of which both the functio- 5. nal diagram of Figure 1 and that of Figure 2 can be perfor¬ med.

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

io. In the example in Figure 3 the apparatus has a body 22 formed by several parts, comprising a small mixing chamber 10 of substantially cylindrical shape, which has sliding inside it a cleaning plunger 23 connected to the piston member 24 of a first hydraulic actuating cylinder 25. The

15. mixing chamber 10 opens out at right angles into an inter¬ mediate frothing chamber 18, of suitable length, having a diameter greater than that of the mixing chamber 10; a second cleaning plunger 26 slides inside the chamber 18, being connected to the piston member 27 of a second hydrau-

20. lie actuating cylinder 28. Figure 3 shows moreover means for adjusting the stroke of the plunger 26, for example consisting of an adjustable stop 29 which can be operated by screwing so as to define the retracted position of the piston 27, in order to adjust or vary the throttling degree

25. of the outlet opening of the mixing chamber 10. The intermediate or frothing chamber 18 in turn leads into a wide discharge duct 19 arranged angularly, i.e. perpendi¬ cularly with respect to the said chamber, having sliding inside a third cleaning plunger 30 connected to the piston 5. member 31 of a third hydraulic actuating cylinder 32.

All the hydraulic cylinders can be operated in an appro¬ priate sequence to move the respective cleaning plunger between a retracted position, shown in Figure 3, where the mixing chamber 10 opens out into the frothing chamber 18 io. and where the frothing chamber 18 opens out into the di¬ scharge duct 19, and an advanced position to close the apparatus; in this position the cleaning plunger 23 acts first of all, expelling the residual mixture inside the chamber 10 towards the frothing chamber 18; then the dea¬ ls, ning plunger 26 is actuated to expel the residual mixture from the frothing chamber 18 into the discharge duct 19 and, finally, the cleaning plunger 30 acts to expel the residual mixture inside the discharge duct 19, operating in the reverse sequence during opening of the apparatus.

20. In particular, in the solution according to Figure 3, the cleaning plunger 30 of the discharge duct 19 may be moved between a totally advanced position, where it closes the outlet of the frothing chamber 18 and duct 19 and a first retracted position where the tip of the plunger 30 is

25. arranged flush with the surface of the frothing chamber 18, in accordance with the example of Figure l, or between the aforementioned advanced position and a second retracted position back to the preceding one, in which the tip of said plunger 30 and its guide hole define the dead chamber 21 in accordance with the example of Figure 2; by way of

5. example, retraction of the plunger 30 with respect to the chamber 18 may be between 0.5 to 1 times the diameter of the plunger itself, or less. It is obvious that in the second case the frothing chamber 18 may also be shorter by three diameters and the actuating cylinder 32 may be provi- lo. ded with a suitable adjustable stop device, for example such as that indicated by 29 for the actuating cylinder 28.

In the example of Figure 3, the two chambers 10 and 18, as well as the discharge duct 19, have their longitudinal axes lying on a same plane; however, a different tridimentional

15. configuration or arrangement of the chambers 10, 18 and duct 19 may also be envisaged, providing angles of devia¬ tion of the flow which are greater or smaller than those shown, in general also different from 90°. In all these cases, by means of the solution shown in Figure 3, it is

20. possible to use a high-pressure mixing head for the produc¬ tion of polyurethane mixtures with the frothing technique, which is totally self-cleaning, has a compact structure and is extremely functional in use.

Tests and experiments have been carried out with the appa-

25. ratus according to the invention configured in accordance with both the example of Figure 1 and that of Figure 2, comparing the results with those of conventional systems.

Example 1

According to this comparative example, a conventional apparatus has been used, modified in accordance with the 5. example of Figure 1, except for the intermediate chamber 18 which in this case had a length less then three diameters. The mixing chamber 10 opened moreover freely, without any throttling, into the chamber 18.

The mixing chamber 10 had a diameter of 12 mm and a sub- io. stantially corresponding length; the frothing chamber 18 had a diameter of 16 mm and a length of 32 mm; the discharge duct 19 had a diameter of 30 mm and a length of 90 mm.

15. A mixture of polyurethane components of conventional formu¬ lation was used, adopting liquid C0₂ as the blowing agent, in a quantity of five parts by weight with respect to the polyol of the reactive mixture.

The pumps 13 and 14 were preset for an overall throughput 20. of the apparatus of 400 g per second, measuring a pressure Pl close to atmospheric pressure inside the mixing chamber 10, entirely unsuitable for mixing, in the liquid state, of the blowing agent; no damping effect of the flow was obtai- ned at the outlet of the chamber 10.

The frothed mixture which emerged from the apparatus was directly poured into the open cavity of a mould.

The results were completely negative in that there was a 5. turbulent outflow of a partially frothed and partially still liquid mixture which caused a notable dispersion of the C0₂.

The resultant frothed mixture led to the formation of a foam of inconsistent quality, with irregular cells and io. large cavities.

Example 2

Using the same apparatus as that described in Example l, according to the invention a throttling means was provided at the outlet of the mixing chamber 10, by advancing the

15. cleaning plunger 26 of the frothing chamber 18, in accor¬ dance with the diagram of Figure 1, until a homogeneous flow of a well-frothed mixture was obtained at the outlet, said mixture being poured in a gentle and regular manner into the mould cavity. Mixing inside the chamber 10 was

20. performed at a pressure of about 17 bar, measuring a pres¬ sure of about 1.5 Bar inside the frothing chamber 18 near the outlet of the mixing chamber 10. The foam had a density of about 27 kg/m³, with a homoge¬ neous and fine cell structure, devoid of any large cavi¬ ties. The quality of the foam was considered to be of a commercially acceptable quality.

5. Exam le 3

Still in accordance with the invention, finally, an appara¬ tus in accordance with the preceding example was used, having the plunger 30 of the discharge duct retracted further, so as to form a dead chamber 21 in accordance with 10. the example of Figure 2.

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

A pressure P2 of about 1.3 bar was measured inside the frothing chamber 18, and a pressure again of about 17 bar 15. was measured inside the mixing chamber.

The frothed mixture emerged in a very gentle and controlled manner, as in the previous example, obtaining in this case a foam of improved quality, having a density of about 26 kg/m³, and a homogeneous cell structure totally devoid of 20. voids and cavities.

At the end of the tests, unexpected results have been obtained - not possible with conventional configurations of the known high-pressure mixing devices - by acting in accordance with the invention, the frothing technique can therefore be advantageously used in high-pressure mixing for the production of frothed, polyurethane foams having superior quality, by suitably controlling the release of

5. the blowing agent. By applying a back-pressure in a frot¬ hing chamber according to the invention, it was therefore possible to obtain frothed polyurethane foams of satisfac¬ tory quality, suitable for many applications, for example for moulding padding used in cushions and car seats, for io. forming heat insulations for refrigerators, for forming panels, or for many other similar applications.

Claims

1. Process for the production of a frothed polyurethane foam mixing at least two reactive chemical components and a low boiling blowing agent at a liquid state, by high pres-

5. sure mixing apparatus comprising a cylindrical mixing chamber (10) connected to a discharge duct (19), comprising the steps of:

- forming a reactive mixture by mixing the chemical compo¬ nents and the blowing agent under pressure controlled io. conditions by throttling the flow of the resulting mixture at the outlet of the mixing chamber (10) to generate in the same mixing chamber (10) a pressure sufficient to maintain the blowing agent at a liquid state;

- frothing the resulting mixture by releasing the blowing 15. agent along a frothing chamber (18) provided between the mixing chamber (10) and the discharge duct (19); and gene¬ rating a back-pressure (P2) into the frothing mixture, by changing the flow direction of the frothed mixture at a substantial length from the mixing chamber (10) which will 20. maximize the releasing of the blowing agent and the quie¬ ting of the frothing mixture while flowing from the frot¬ hing chamber (18) into the discharge duct (19).

2. Process according to Claim 1, in which said blowing agent comprises carbon dioxide.

₂₅. 3. Process according to Claim 1 in which the blowing agent is comprised between 1 and 10 per cent by weight of a chemical component.

4. Process according to Claim 1, further comprising the step of accumulating the frothing mixture in a dead zone

5. (21) downstream of the frothing chamber (18).

5. Process according to Claim , in which the frothing mixture is accumulated in a dead zone (21) provided in a deviation point for the mixture between the frothing chamber (18) and the discharge duct (19).

io. 6. Process according to Claim 5, in which the accumulation of the mixture in the dead zone (21) is provided at the rear side of the discharge duct (19).

7. Process according to the preceding claim 1, in which the pressure (Pl) inside the mixing chamber (10) is comprised

15. between 5 and 20 bar.

8. Process according to the preceding Claim 1, in which the back-pressure (P2) in the frothing mixture, is greater than 0.2 bar.

9. Process according to Claim 8, in which the back-pressure 20. (P2) in the frothing mixture, is between 0.2 and 2 bar.

10. High-pressure mixing apparatus, in particular for the preparation of a frothed polyurethane foam by mixing reac¬ tive polyurethane components with a low-boiling blowing agent, the apparatus comprising: a body defining a mixing chamber (10) having a longitudinal axis, in which the

₅. polyurethane components and the blowing agent are mixed at a first pressure value (Pl) sufficient to keep the blowing agent in a liquid state; wherein the mixing chamber (10) is connected to a discharge duct differently oriented with respect to the same mixing chamber (10) of the mixing io. apparatus, and wherein throttling means (26) are provided at the outlet side of the mixing chamber (10), said mixing chamber (10) being connected to the discharge duct (19) via an intermediate frothing chamber (18) , in combination with back-pressure generating means provided between the frot-

15. hing chamber (18) and the discharge duct (19) for genera¬ ting a back-pressure in the mixture inside the frothing chamber (18) having a value (P2) lower than the pressure (Pl) inside the mixing chamber (10) of the mixing device.

11. Apparatus according to Claim 10, in which said frothing 20. chamber (18) is of cylindrical form, said back-pressure generating means comprising a length of the frothing chamber (18) at least 2.5 to 3 times greater than the diameter of the same frothing chamber (18).

12. Apparatus according to Claim 11, in which the means for 25. generating the back-pressure comprises a flow deviation for the frothing mixture, between said frothing chamber (18) and discharge duct (19).

13. Apparatus according to Claim 11, comprising a dead end zone (21) for accumulating the frothing mixture, at the deviation point for the flow between the frothing chamber

5. (18) and the discharge duct (19).

14. Apparatus according to Claim 1, in which said mixing chamber (10), said frothing chamber (18) and the discharge duct (19) each comprises a sliding cleaning member (23, 26, 30) connected to a hydraulic actuator (25, 28, 32) to be io. moved between a retracted open position and an advanced closed position for the apparatus, said 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) rearwardly arranged to the

15. cleaning member (26) of the frothing chamber (18) for changing the throttling of the mixing chamber outlet (17).

15. Apparatus according to Claim 13, in which the cleaning member (30) for the discharge duct (19) is movable inside a guide bore rearwardly extending in respect to the frothing

20. chamber (18), said dead end zone (21) for the frothing mixture accumulation being provided by retracting the tip portion of the cleaning member (39) into said guide hole.