Plant and method for the controlled introduction of a blowing agent into a reagent of a polyurethane mixture

Abstract

Plant (10) for the controlled introduction of a blowing agent into a reagent of a polyurethane mixture; said plant comprising: - a tank (12) for storing said reagent, said tank (12) being provided with an outlet duct (14); - a recirculation pump (16), provided along said outlet duct (14) and having a delivery duct (18) connected to a first inlet (22) of an ejector (20); a second inlet (24) of said ejector (20) being connected to a circuit (26) supplying the blowing agent; - at least one mixer (30) connected to an outlet duct (28) of said ejector (20); - a recirculation duct (32) having an inlet (34), connected to an outlet (36) of the mixer (30), and an outlet (38), connected to said storage tank (12); - a supply device (40), situated upstream of the second inlet (24) of the ejector (20) and connected to the circuit (26) supplying the blowing agent; - control means (50) for said supply device (40) for varying the amount of blowing agent introduced into the ejector (20) via the second inlet (24).

Description

[0001] Plant and method for the controlled introduction of a blowing agent into a reagent of a polyurethane mixture

[0002] The present invention relates to a plant and a method for the controlled introduction of a blowing agent into a reagent of a polyurethane mixture.

[0003] In particular, the present invention relates to a plant and a method for the controlled introduction of a blowing agent into a reagent, polyol or isocyanate, of a polyurethane mixture suitable for being used for the moulding of footwear components.

[0004] It is known to use polyurethane mixtures for the production of footwear components. Such polyurethane mixtures are used, for example, for the production of complete soles or single components, such as treads and / or mid-soles. Polyurethane mixtures are also used, in the shoe manufacturing sector, for the production of insoles or internal inserts.

[0005] Polyurethane mixtures are likewise used for performing the direct moulding, onto the upper, of the sole or sole components.

[0006] As is well-known, polyurethane mixtures are obtained mainly from two reagents, i.e. a polyol and an isocyanate, and are the result of two mutually competing reactions: the gelling reaction, which provides the urethane group as the synthesis product, responsible for raising the molecular weight and the viscosity of the polymer, and the expansion (blowing) reaction, which, by means of the reaction between the water molecules, present in the polyol, and the isocyanate, produces carbon dioxide as the chemical blowing agent.

[0007] The development of the polyurethane mixtures has focused mainly on optimizing the two aforementioned reactions, synchronizing them so that the increase in the viscosity of the polymer corresponds to the gradual formation of pores, so as to prevent the collapse of the mixture, referred to as “shrinking”.

[0008] In recent times there has been an increasing need to obtain polyurethane soles which are light and able to ensure a high degree of comfort (cushioning) of the user’s foot.

[0009] In order to obtain soles with low density values, for example less than 0.5 g / cm3, physical blowing agents are used, these being able to assist the expansion action performed by water, without interfering with the formation of the polyurethane.

[0010] The physical blowing agents which are most used are hydrocarbons, for example pentane. They undergo a change in state, passing from the liquid state to a gaseous state, following absorption of the heat produced by the polyol and isocyanate polymerization reaction.

[0011] An alternative method for obtaining more highly expanded polyurethane mixtures involves the introduction of carbon dioxide directly in the mixing head of the moulding machine.

[0012] This method has the drawback, however, that the carbon dioxide is not always dissolved uniformly within the polyurethane mixture. There is therefore the risk that large-size carbon dioxide bubbles may remain trapped inside the reaction mixture. These bubbles consequently will also be present in the finished product, once the polyurethane has solidified, resulting in the possibility of a production loss.

[0013] An alternative method for the introduction of carbon dioxide into a polyurethane mixture is described, for example, in EP1726612B1 . This patent describes a method in which a given amount of carbon dioxide is dissolved under pressure in at least one of the reagents of the polyurethane mixture, preferably in the isocyanate, before the mixing reaction between polyol and isocyanate takes place.

[0014] This method, compared to the solution which involves the introduction of the carbon dioxide directly in the mixing head of the moulding machine, allows a more homogeneous polyurethane mixture to be obtained, the bubbles of carbon dioxide being uniformly distributed within said mixture.

[0015] However, the implementation of this method requires long processing times, both in order to introduce the carbon dioxide into the reagent and in order to ensure that the carbon dioxide is uniformly dissolved in the reagent to be expanded.

[0016] The carbon dioxide is, in fact, introduced under pressure directly inside a tank containing the isocyanate or the polyol for a period of time sufficient to allow dissolving of the desired amount of carbon dioxide.

[0017] Following said injection, it is necessary to wait a further - not insignificant - period of time in order to allow the carbon dioxide to be uniformly distributed in the reagent.

[0018] The main object of the present invention is, therefore, to provide a plant and a method for the controlled introduction of a blowing agent into a reagent of a polyurethane mixture, which are able to overcome the aforementioned drawbacks.

[0019] A particular aim of the present invention is to provide a plant and a method for the controlled introduction of a blowing agent into a reagent of a polyurethane mixture, which are able to achieve rapid thorough mixing of the blowing agent with the reagent to be expanded.

[0020] A further aim of the present invention is to provide a plant and a method for the controlled introduction of a blowing agent into a reagent of a polyurethane mixture, which are able to obtain mixing of the expanded polyurethane mixtures without the addition of the blowing agent interfering with the polymerization and chemical expansion reaction of the polyurethane mixture, induced by the water.

[0021] Finally, a aim of the present invention is to provide a plant and a method for the controlled introduction of a blowing agent into a reagent of a polyurethane mixture, whereby the reagent may be used to obtain polyurethane mixtures with a low density, of less than 0.35 g / cm3.

[0022] The object and the main aims described above are achieved with a plant according to Claim 1 and a method according to Claim 16.

[0023] In order to illustrate more clearly the innovative principles of the present invention and its advantages compared to the prior art, some examples of embodiment applying these principles will be described below with the aid of the attached drawings. In the drawings:

[0024] - Figure 1 shows a schematic view of the components of a plant according to the present invention;

[0025] - Figure 2A shows a cross-sectional view of a component of the plant according to Figure 1 ;

[0026] - Figures 2B and 2C are figures similar to Figure 2A, but relating to different embodiments of the component;

[0027] - Figure 3 shows a partially sectioned perspective view of a further component of the plant according to Figure 1 ; - Figure 4 shows a schematic cross-sectional view of the component according to Figure 3, along the plane indicated by IV-IV;

[0028] - Figure 5 shows a view, similar to Figure 3, but relating to a second embodiment of said component;

[0029] - Figure 6 shows a schematic cross-sectional view of the component shown in Figure 5, along the plane indicated by VI-VI;

[0030] - Figure 7 shows a partially exploded perspective view of a further embodiment of the component shown in Figures 3-6;

[0031] - Figures 8 and 9 show in schematic form possible arrangements of the components shown in Figures 3-7, in the plant according to the invention;

[0032] - Figure 10 shows in schematic form the connection between a second embodiment of the plant according to the invention and a moulding circuit.

[0033] With reference firstly to Figure 1 , a plant for the controlled introduction of a blowing agent into a reagent of a polyurethane mixture is denoted overall by the reference number 10.

[0034] As will become clear from the description below, said plant 10 is designed to introduce, in a controlled manner, a blowing agent into the polyol or isocyanate of the polyurethane mixture.

[0035] Said polyurethane mixture may be a polyether-based polyurethane foam or a polyester-based polyurethane foam. In a known manner, the polyurethane mixture may contain possible additives, in addition to the basic reagents consisting of polyol and isocyanate.

[0036] Said polyurethane mixture may be advantageously used in machines intended to perform the moulding by means of casting, i.e. in an open mould, or by means of injection, i.e. in a closed mould, of footwear components, such as soles, mid-soles, insoles, etc.

[0037] The polyurethane mixture obtained from reagents, where a blowing agent was added to one or both of them by means of the plant and the method according to the present invention, may also be used in machines intended to perform moulding by means of spraying. A spraying device in which said polyurethane mixture may be used is described in European patent EP 2346614 in the name of the same Applicant.

[0038] The polyurethane mixture obtained from reagents treated within the plant 10 may, finally, be advantageously used also in machines for the direct-injection moulding of soles, or sole parts, onto an upper.

[0039] As shown in Figure 1 , the plant 10 comprises a tank 12 for storing said reagent. The tank 12 is provided with an outlet duct 14.

[0040] The plant 10 further comprises a recirculation pump 16. Said recirculation pump 16 is arranged along the outlet duct 14 and has a delivery duct 18 connected to a first inlet 22 of an ejector 20.

[0041] As is clearly visible in Figure 1 , a second inlet 24 of the ejector 20 is connected to a circuit 26 for supplying the blowing agent.

[0042] The plant 10 further comprises at least one mixer 30 connected to an outlet duct 28 of the ejector 20.

[0043] As shown in Figure 1 , the plant 10 also comprises a recirculation duct 32 having an inlet 34, connected to an outlet 36 of the mixer 30, and an outlet 38, connected to the storage tank 12.

[0044] In accordance with the invention, the plant 10 comprises a supply device 40 located upstream of the second inlet 24 of the ejector 20 and connected to the circuit 26 supplying the blowing agent. The plant 10 is further characterized in that it comprises control means 50 for said supply device 40 for varying the amount of blowing agent introduced into the ejector 20 via the second inlet 24.

[0045] As will become clear from the description below, the plant 10 allows uniform dispersion of the blowing agent inside the polyol or isocyanate of the polyurethane mixture. As will be described in detail below, the blowing agent preferably consists of carbon dioxide. However, other gases, such as nitrogen or argon, may also be advantageously used.

[0046] Moreover, the plant 10 according to the present invention allows the continuous injection of blowing agent to be performed for the duration required in order to achieve two goals: ensure a uniform dispersion of the blowing agent in the reagent and promote nucleation of the blowing agent inside the reagent.

[0047] In fact, the plant according to the invention may work in a continuous cycle and allow, owing to the presence of the mixer, the blowing agent to be dispersed in a uniform manner inside the polyol or the isocyanate.

[0048] Moreover, the controlled introduction of the blowing agent into the reagent by means of the ejector ensures that, once the point of maximum solubility of the blowing agent within the reagent is reached, nucleation of the blowing agent is promoted. Said nucleation process results in the formation of microbubbles of blowing agent which are uniformly distributed in the reagent.

[0049] Advantageously, the blowing agent introduced into the polyol or the isocyanate will operate in synergy with the carbon dioxide, which acts as chemical blowing agent, produced by the reaction between the water, present in the polyol, and the isocyanate. In other words, the blowing agent introduced into the reagents of the polyurethane mixture by means of the plant and the method according to the present invention does not replace the chemical blowing agents which are normally used, but improves the performance thereof.

[0050] Moreover, the reagent treated in the plant according to the invention, whether it be polyol or isocyanate, not only has a density less than the starting density, but also has a uniform density.

[0051] In this way, the reagent mixed with the blowing agent is adequately prepared so that it may be immediately used during the following polyurethane moulding stages, optimizing both the qualities and the mechanical properties of the moulded end product.

[0052] Advantageously, as shown schematically in Figure 1 and Figure 10, the plant 10 may comprise a first pressure measurement device 52 and a second pressure measurement device 54.

[0053] The first pressure measurement device 52 is preferably situated downstream of the recirculation pump 16, in the vicinity of the first inlet 22 of the ejector 20.

[0054] The second pressure measurement device 54, in turn, is preferably situated downstream of the mixer 30, along the recirculation duct 32.

[0055] The first pressure measurement device 52 and the second pressure measurement device 54 may each comprise a pressure gauge. Said pressure measurement devices 52, 54 may also consist of a manometer.

[0056] Advantageously, the control means 50 for the supply device 40 are configured to operate the device 40 so as to vary the amount of blowing agent introduced into the ejector 20 depending on the processing of the pressure values measured by the first pressure measurement device 52 and by the second pressure measurement device 54, namely depending on the pressure value of the reagent detected before and after the blowing agent has been injected therein.

[0057] In this way it is possible to perform feedback control of the amount of blowing agent introduced into the reagent depending on the drop in pressure detected in the reagent during its passage through the ejector 20 and the mixer 30.

[0058] As already mentioned, the plant 10 comprises a storage tank 12 for the reagent, whether it be polyol or isocyanate. The reagent is in a liquid state inside the tank 12.

[0059] Preferably, the storage tank 12 is provided with heating means 42 (see Figure 1 ) configured to heat the reagent contained therein.

[0060] In this way, the reagent, which is in a viscous liquid state, is kept at the desired temperature, so as to facilitate the subsequent introduction of the blowing agent and the interaction between reagent and blowing agent.

[0061] As shown in Figure 1 , the tank 12 may also comprise mixing means 44. Said mixing means 44 are preferably rotated by means of a shaft driven by a motor 46 located on the outside of the tank 12.

[0062] The tank 12 may also be provided with a pressure regulator 48 configured to control the pressure of the reagent inside the tank 12.

[0063] Preferably, the tank 12 has a capacity of between 50 and 250 litres. Even more preferably, the tank 12 has a capacity of between 100 and 150 litres.

[0064] Alternatively, the tank may be replaced by a supply plant intended to introduce the reagent directly into the plant 10.

[0065] For the purposes of the present invention there is complete equivalence between the tank 12 and such a supply plant. The comments below are applicable both in the case of a tank 12 and in the case of a supply plant.

[0066] For convenience reference will be made to the tank 12.

[0067] As already mentioned, the tank 12 is also provided with an outlet duct 14. Said outlet duct 14 is preferably arranged on a bottom portion of the tank 12.

[0068] Advantageously, the outlet duct 14 may be provided, along the section upstream of the recirculation pump 16, with a shut-off valve 56.

[0069] As shown in Figure 10, downstream of the shut-off valve 56 a filter 58 may be mounted. Said filter 58 has the function of filtering off any impurities present in the reagent so as to ensure the correct operation of the plant.

[0070] Preferably, the control means 50 also control operation of the shut-off valve 56. In this way, in the case where the desired amount of blowing agent has been injected into the reagent, the shut-off valve 56 may be closed, so as to interrupt the flow of reagent inside the plant 10. In this case, as an alternative to or in addition to closing the shut-off valve 56, stoppage of the recirculation pump 16 may also be performed.

[0071] With reference to Figures 1 and 10, the recirculation pump 16 has the function of pumping the reagent from the tank 12 towards the ejector 20 and the mixer 30 and then allowing the reagent to return into the tank 12 via the recirculation duct 32.

[0072] The recirculation pump 16 therefore allows the reagent to be moved continuously. This continuous movement ensures that the reagent maintains a uniform temperature which favours the subsequent introduction and mixing of the blowing agent in the reagent itself.

[0073] Advantageously, the control means 50 may also adjust the operation of the pump 16. In particular, via the control means 50 it is possible to vary the flowrate of reagent introduced into the ejector 20.

[0074] Moreover, as mentioned above, in the case where the desired amount of blowing agent has been injected into the reagent, the control means 50 may also perform stoppage of the recirculation pump 16.

[0075] With reference to Figures 2A-2B-2C, the ejector 20 may comprise a convergent nozzle 60, in fluid communication with the first inlet 22, and a divergent nozzle 62, in fluid communication with the outlet duct 28 of the ejector 20. As is clearly visible in Figures 2A-2B-2C, a mixing chamber 64 may be arranged between the convergent nozzle 60 and the divergent nozzle 62.

[0076] The second inlet 24 of the ejector 20 is preferably in fluid communication with said mixing chamber 64. In particular, the second inlet 24 may be in fluid communication with the mixing chamber 64 by means of a connector 66. Said connector 66 advantageously may have a variable cross-section, with a bigger cross-section in the vicinity of the inlet 24 and smaller cross-section in the vicinity of the mixing chamber 64.

[0077] Preferably, said mixing chamber 64 has a constant cross-section. Advantageously, the ejector 20 creates a Venturi effect.

[0078] The constriction produced by the convergent nozzle 60 causes, in fact, an increase in the speed of the fluid which crosses the ejector 20 and consequently a reduction in pressure which allows the blowing agent to be sucked into the mixing chamber 64 through the second inlet 24 and the connector 66. Advantageously, the constriction of the connector 66 allows the introduction of the blowing agent into the reagent to be accelerated. Via the divergent nozzle 62 the mixture formed by the reagent and the blowing agent is slowed down, so as to favour the dispersion of the blowing agent in the reagent.

[0079] In Figures 2A-2B-2C the inflow of the liquid reagent into the ejector 20, the suction of the blowing agent into the mixing chamber 64 and the outflow of the liquid reagent from the ejector 20 are schematically indicated by the arrows F1 , G1 and F2.

[0080] As can be seen in Figure 2A, the convergent nozzle 60 and the divergent nozzle 62 may have the same length and the same inclination.

[0081] Alternatively, as shown by way of example in Figure 2B, the convergent nozzle 60 may have a length smaller than that of the divergent nozzle 62. In this case, the inclination of the convergent nozzle 60 is preferably greater than the inclination of the divergent nozzle 62.

[0082] In a further embodiment, shown by way of example in Figure 2C, the convergent nozzle 60 may have a length greater than that of the divergent nozzle 62. In this case, the inclination of the convergent nozzle 60 is preferably smaller than the inclination of the divergent nozzle 62.

[0083] With reference to Figures 3-7, the mixer 30 is preferably a static mixer.

[0084] Advantageously, the mixer 30 comprises an outer casing 68 and a mixing element 70 arranged inside said outer casing 68.

[0085] Said mixing element 70 comprises, in turn, a plurality of blade elements 72 arranged in succession along a longitudinal axis L of the outer casing (see Figures 4, 6 and 7).

[0086] The blade elements 72 have a cross-section which substantially coincides with the internal cross-section of the casing 68 so as to ensure that all the liquid which crosses the mixer comes into contact with the blade elements 72 and is mixed.

[0087] As shown in Figures 3-7, the outer casing 68 preferably has a hollow cylindrical shape.

[0088] Each blade element 72, in turn, may comprise advantageously a first series of blades 74, which are inclined along a first direction, and a second series of blades 76, which are inclined along a second direction. Preferably, the first series 74 and the second series 76 of blades of each blade element 72 are arranged in succession inside the outer casing 68 (see Figures 4 and 6).

[0089] Advantageously, as shown in Figure 7, the mixing element 70 may consist of a plurality of helical blade elements 72. In this case also, said blade elements 72 are arranged inclined with respect to a longitudinal axis L of the outer casing 68.

[0090] Moreover, as is clearly visible in Figure 7, said blade elements 72 may be arranged inside the casing 68 so that the adjacent blade elements are rotated with respect to the longitudinal axis L. In this rotated arrangement the transverse axes T of adjacent blade elements are therefore staggered relative to each other.

[0091] As shown in Figures 3-7, the pitch and the number of blade elements inside the casing may vary.

[0092] In the embodiment schematically shown in Figures 3 and 4, the blade elements consist of a greater number and are more spaced from each other than in the embodiment schematically shown in Figures 5 and 6.

[0093] In an alternative embodiment of the invention, the plant 10 may comprise a plurality of mixers 30. As shown schematically in Figure 8, the mixers 30 of said plurality may be arranged in series with each other, so as to form a mixing line 300.

[0094] Alternatively, the mixers 30 of said plurality may be arranged in parallel with each other.

[0095] As shown schematically in Figure 9, the mixing lines 300, which are formed by a plurality of mixers arranged in series, may in turn be arranged in parallel. In the example shown in Figure 9, the plant 10 comprises twelve mixers 30. These mixers are arranged so as to form three mixing lines 300 each formed by four separate mixers.

[0096] The three mixing lines 300 are in turn arranged in parallel with each other.

[0097] In Figures 4, 6, 7-9, the flow into and out of the mixer 30 or the mixing line 300 is schematically indicated by the arrows M1 and M2.

[0098] The function of the mixing element 70 is to create a turbulent flow inside the mixer 30 which favours the mixing together of the reagent and the blowing agent. In particular, the function of the mixing element 70 is to favour a uniform dispersion of the blowing agent inside the reagent.

[0099] With reference to Figures 1 and 10, the circuit for supplying the blowing agent 26 may comprise a pressure vessel 80 for the blowing agent and an outlet duct 82 connected to said pressure vessel 80.

[0100] The pressure vessel 80 is intended to supply the blowing agent to the plant 10. Preferably, said vessel 80 contains carbon dioxide in the gaseous state.

[0101] The blowing agent is preferably kept inside the vessel 80 at a pressure of about 200 bar.

[0102] As an alternative to the pressure vessel 80, the outlet duct 82 may be connected to a separate network (not shown in the attached figures) intended to supply the blowing agent to be introduced into the reagent.

[0103] As already mentioned, gases, other than carbon dioxide, may be used as blowing agents, for example nitrogen and argon.

[0104] As shown in the figures, preferably the supply device 40, by means of which it is possible to vary the amount of blowing agent introduced into the reagent, is arranged along the outlet duct 82.

[0105] Said supply device 40 may be a proportional valve.

[0106] Alternatively, the supply device 40 may consist of a compressor or a pump.

[0107] Advantageously, a non-return valve 84 may be arranged along the outlet duct 82. Said valve 84 has the function of preventing the reagent from flowing accidentally into the circuit 26 supplying the blowing agent.

[0108] With reference to Figure 10, the plant 10 may also comprise a deviator valve 86 situated upstream of the ejector 20. Said deviator valve 86 is configured to deviate towards a second connection duct 88 the reagent drawn off from the storage tank 12.

[0109] The control means 50 may also operate said deviator valve 86.

[0110] With reference to Figure 1 , the plant 10 may also comprise a detection sensor or probe 92 which is positioned along the outlet duct 14 between the recirculation pump 16 and the ejector 20.

[0111] In particular, the sensor 92 may be positioned between the recirculation pump 16 and the first pressure measurement device 52.

[0112] The sensor 92 advantageously has the function of detecting the concentration of the blowing agent inside the reagent.

[0113] The control means 50 may be manually operated.

[0114] Preferably, the control means 50 are operated by means of a computerized control unit 90. Said unit may be, for example, a programmable logic controller (PLC) connected to said control means 50.

[0115] As shown schematically in Figures 1 and 10, the computerized control unit 90 may also be connected to the first pressure measurement device 52 and to the second pressure measurement device 54.

[0116] Advantageously, the control unit 90 is configured to continuously detect the pressures detected by the pressure measurement devices 52, 54. Based on the pressure data detected by the devices 52, 54, the control unit 90, by means of the control means 50, is able to control the operation of the supply device 40, in particular is able to adjust the flowrate and the pressure of the blowing agent introduced into the ejector 20.

[0117] In this way it is ensured that the amount of blowing agent introduced into the reagent is optimal.

[0118] The control unit 90, depending on the data detected by the devices 52, 54 - again via the control means 50 - may adjust the throughput of the pump 16 and close the shut-off valve 56 or stop the pump 16.

[0119] As shown in Figure 1 , the sensor 92 may also be connected to the control unit 90 to which it sends the concentration data detected, either continuously or at a set time interval.

[0120] The control unit 90, depending on the concentration measurements detected by the sensor 92, will adjust via the control means 50 the operation of the supply device 40, the recirculation pump 16 and / or the supply valve 56.

[0121] In the case where the plant 10 is provided with a deviator valve 86, the control unit 90, via the control means 50, may also operate said deviator valve 86.

[0122] In particular, the control unit 90 is able to direct, via the valve 86, a fraction of the reagent flow taken from the tank 12 towards the second connection duct

[0123] 88.

[0124] Advantageously, the valve 86 may direct towards the second connection duct 88 the entire reagent flow taken from the tank 12. This eventuality occurs when the concentration value of the blowing agent in the reagent has reached the desired value.

[0125] Advantageously, as shown in Figure 10, by means of the second connection duct 88, the plant 10 may be directly connected to a polyol or isocyanate supply circuit 100 of a polyurethane moulding machine.

[0126] In particular, by means of the second connection duct 88, the reagent mixed with the blowing agent may be sent to a polyol or isocyanate storage tank 112 intended to supply, by means of a pump 116, a mixing head 120 of a polyurethane moulding machine.

[0127] As already mentioned, the present invention also relates to a method for the controlled introduction of a blowing agent into a reagent of a polyurethane mixture.

[0128] The method comprises the steps of: a) preselection of a required concentration value for the blowing agent in the reagent; b) drawing off from a storage tank the reagent to which the blowing agent is to be added; c) controlled injection of a first amount of blowing agent into said reagent; d) mixing said blowing agent with said reagent.

[0129] The method is characterized in that it comprises, following the mixing step d), a step e) of measuring the concentration of the blowing agent in the reagent and in that it comprises a step f) of reverting to the controlled injection step c) if the measured concentration value of the blowing agent in the reagent is different from the preselected concentration value.

[0130] The reagent, therefore, may be subjected to the steps c) and d) more than once until the concentration of the blowing agent in the reagent reaches the preselected value.

[0131] Advantageously, the measurement step e) may be performed in a direct or indirect manner.

[0132] In the direct manner, the concentration of the blowing agent in the reagent is measured directly in the reagent drawn off from the storage tank, for example by means of the sensor or the probe 92.

[0133] In the indirect manner, the concentration of the blowing agent in the reagent may be measured by means of measurement of the density of the polyurethane mixture obtained from the reagent with added blowing agent.

[0134] In order to detect said density of the polyurethane mixture, it is possible to use the well-known free cup expansion test. In particular, by carrying out a comparison between the density value of the polyurethane mixture obtained from reagents with no blowing agent added and the density value of the polyurethane mixture obtained from at least one reagent with the blowing agent added, it is possible to obtain the concentration of the blowing agent in the reagent.

[0135] The method may comprise, following the drawing-off step b), a first step for measuring the pressure of the reagent and, following the mixing step d), a second step for measuring the pressure of the reagent and the blowing agent which are mixed together. Advantageously, the step of measuring the concentration of the blowing agent in the reagent may comprise a step of processing the pressure values measured during the first pressure measurement step and the second pressure measurement step. Preferably, depending on the results of this measurement step, the first amount of blowing agent injected into the reagent may be varied.

[0136] Advantageously, if the measured concentration value of the blowing agent in the reagent is equal to the preselected concentration value, the controlled injection step c) may be stopped and the reagent mixed with the blowing agent may be directly sent to a polyol or isocyanate supply circuit 100 of a polyurethane moulding machine.

[0137] Preferably, the blowing agent injection step is performed by injecting carbon dioxide, nitrogen or argon into the reagent.

[0138] Advantageously this method may be performed by means of the plant 10 described above.

[0139] The reagent, which may be polyol or isocyanate, may be drawn off from the storage tank 12 or from a supply circuit connected to the plant 10.

[0140] The drawing-off step b) may be performed by means of the recirculation pump 16 which may be set to impart a flowrate of between 10 and 80 litres / minute, more preferably between 30 and 60 litres / minute.

[0141] By varying the reagent flowrate value, considering that the diameter of the duct 14 is preferably constant, for example equal to 1 ”, the pressure of the reagent in the plant may be varied.

[0142] Preferable pressure values of the reagent, before the injection step, are in the region of 8 - 8.5 bar. The injection step b) is preferably performed by means of the ejector 20.

[0143] Preferably, the blowing agent injection step is performed in a pressure range of 3 to 90 bar. Even more preferably, the injection step is performed at a pressure of about 7-12 bar.

[0144] The blowing agent injection pressure is set depending on the pressure of the reagent detected during the first pressure measurement step.

[0145] For example, if in the first pressure measurement step, a pressure value of about 8 bar is detected, the blowing agent injection step may take place at a pressure within a range of between 7.5 bar and 8.5 bar.

[0146] With these pressure values, during the second pressure measurement step, a drop in pressure Ap, equal to about 1 -1.5 bar, was measured.

[0147] By setting these pressure values, together with the aforementioned flowrate value, it is possible to increase the concentration of blowing agent in the reagent so as to obtain, during the following moulding step, polyurethane mixtures having a density of even less than 0.35 g / cm3.

[0148] The mixing step c) may be performed by means of the mixer 30.

[0149] The variation in the first amount of blowing agent injected into the reagent may be performed by means of the supply device 40.

[0150] The first pressure measurement step and the second pressure measurement step may be performed by means of the first pressure measurement device 52 and the second pressure measurement device 54.

[0151] From the above description it is now clear how the plant and method according to the present invention are able to achieve advantageously the predefined objects.

[0152] In particular, the specific arrangement of an ejector and a mixer as described above allows the continuous injection, within a shorter amount of time, of the desired amount of blowing agent inside a reagent of a polyurethane mixture.

[0153] Moreover, said blowing agent is uniformly distributed within the reagent which may be immediately ready for use and be used in the known moulding plants without having to undergo further treatment.

[0154] The reagent with the added blowing agent may be used in the known openmould casting or injection moulding plants.

[0155] The reagent, i.e. polyol or isocyanate, may also be used in the spraymoulding of polyurethane mixtures, for example in the method described in EP2346614B1 in the name of the same Applicant.

[0156] Obviously, the above description of embodiments applying the innovative principles of the present invention is provided by way of example of these innovative principles and must therefore not be regarded as limiting the scope of the rights claimed herein.

Claims

Claims1. Plant (10) for a controlled introduction of a blowing agent into a reagent of a polyurethane mixture; said plant comprising:- a tank (12) for storing said reagent, said tank (1 ) being provided with an outlet duct (14);- a recirculation pump (16), arranged along said outlet duct (14) and having a delivery duct (18) connected to a first inlet (22) of an ejector (20); a second inlet (24) of said ejector (20) being connected to a circuit (26) supplying the blowing agent;- at least one mixer (30) connected to an outlet duct (28) of said ejector (20);- a recirculation duct (32) having an inlet (34), connected to an outlet (36) of said at least one mixer (30), and an outlet (38), connected to said storage tank (12);- a supply device (40), located upstream of the second inlet (24) of the ejector (20) and connected to the circuit (26) supplying the blowing agent;- control means (50) for said supply device (40) for varying the amount of blowing agent introduced into the ejector (20) via the second inlet (24).

2. Plant (10) according to claim 1 , characterized in that it comprises a first pressure measurement device (52) situated downstream of the recirculation pump (16) in the vicinity of the first inlet (22) of the ejector (20), and a second pressure measurement device (54), situated downstream of said at least one mixer (30) along the recirculation duct (32); said control means (50) being configured to vary the amount of blowing agent introduced into the ejector (20) depending on the processing of the pressure values measured by the first pressure measurement device (52) and by the second pressure measurement device (54).

3. Plant (10) according to any one of the preceding claims, characterized in that it comprises a detection sensor or probe (92) positioned along the outlet duct (14) between the recirculation pump (16) and the ejector (20); said sensor (92) having the function of detecting the concentration of the blowing agent inside the reagent.

4. Plant (10) according to any one of the preceding claims, characterized in that said ejector (20) comprises a convergent nozzle (60), in fluid communication with said first inlet (22), and a divergent nozzle (62), in fluid communication with said outlet duct (28); a mixing chamber (64) being arranged between the convergent nozzle (60) and the divergent nozzle (62).

5. Plant (10) according to the preceding claim, characterized in that the said second inlet (24) is in fluid communication with said mixing chamber (64).

6. Plant (10) according to the preceding claim, characterized in that the mixing chamber (64) has a constant cross-section.

7. Plant (10) according to any one of the preceding claims, characterized in that said at least one mixer (30) is a static mixer.

8. Plant (10) according to any one of the preceding claims, characterized in that said at least one mixer (30) comprises an outer casing (68) and a mixing element (70), arranged inside said outer casing (68).

9. Plant (10) according to the preceding claim, characterized in that the mixing element (70) comprises a plurality of blade elements (72) arranged in succession along a longitudinal axis L of the outer casing (68).

10. Plant (10) according to the preceding claim, characterized in that each blade element (72) of said plurality comprises a first series of blades (74), inclined along a first direction, and a second series of blades (76), inclined along a second direction; the first series (74) and the second series (76) ofblades of each blade element (72) being arranged in succession inside the outer casing (68).

11. Plant (10) according to any one of the preceding claims, characterized in that the mixing element (70) comprises a plurality of helical blade elements (72).

12. Plant (10) according to any one of the preceding claims, characterized in that it comprises a plurality of mixers (30), the mixers of said plurality being arranged in series or parallel with each other.

13. Plant (10) according to any one of the preceding claims, characterized in that the circuit (26) supplying the blowing agent comprises a pressure vessel (80) for the blowing agent and an outlet duct (82) connected to said vessel (80); said supply device (40) being arranged along said outlet duct (82).

14. Plant according to any one of the preceding claims, characterized in that said control means (50) are operated by means of a computerized control unit (90).

15. Plant (10) according to any one of Claims 2 and 14, characterized in that said control unit (90) is configured to detect continuously the pressures detected by the first and second pressure measurement devices (52, 54); based on the pressure data detected by the first and second measurement devices (52, 54), the control unit (90) is able to control the operation of the supply device (40) by means of the control means (50).

16. Method for a controlled introduction of a blowing agent into a reagent of a polyurethane mixture, said method comprising the following steps: a) preselection of a required concentration value for the blowing agent in the reagent; b) drawing off from a storage tank the reagent to which the blowing agent is tobe added; c) controlled injection of a first amount of blowing agent into said reagent; d) mixing said blowing agent with said reagent; the method being characterized in that it comprises, following the mixing step d), a step e) of measuring the concentration of the blowing agent in the reagent and in that it comprises a step f) of reverting to the controlled injection step c) if the measured concentration value of the blowing agent in the reagent is different from the preselected concentration value.

17. Method according to the preceding claim, characterized in that the measurement step e) is performed in a direct and indirect manner; in the direct manner, the concentration of the blowing agent in the reagent is measured directly in the reagent drawn off from the storage tank; in the indirect manner, the concentration of the blowing agent in the reagent is measured by means of measurement of the density of the polyurethane mixture obtained from the reagent with added blowing agent.

18. Method according to either one of claims 16 or 17, characterized in that it comprises, following the drawing-off step b), a first step for measuring the pressure of the reagent and, following the mixing step d), a second step for measuring the pressure of the reagent and the blowing agent which are mixed together.

19. Method according to the preceding claim, characterized in that the step e) of measuring the concentration of the blowing agent in the reagent comprises a step of processing the pressure values measured in the first pressure measurement step and in the second pressure measurement step.

20. Method according to any one of claims 16 to 19, characterized in that the first amount of blowing agent injected into the reagent is varied depending onthe results of said measurement step e).

21. Method according to any one of claims 16 to 20, characterized in that, if the measured value of the concentration of the blowing agent in the reagent is equal to the preselected concentration value, the controlled injection step c) is stopped.

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