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

Description

Ref.: 24LG29I Description of the industrial invention entitled "Plant and method for the controlled introduction of a blowing agent into a reagent of a polyurethane mixture” under the name Stemma Srl with headquarters in Cornuda (TV). ***** The present invention relates to a system and a method for the controlled introduction of a blowing agent into a reagent of a polyurethane mixture. In particular, the present invention relates to a system and a method for the controlled introduction of a blowing agent into a reagent, polyol or isocyanate, of a polyurethane mixture suitable for use for the molding of footwear components. The use of polyurethane mixtures for the production of components for is known footwear. Such polyurethane mixtures are used, for example, for production of complete soles or individual components, such as treads and / or midsole. Polyurethane mixtures are also used in the sector footwear, for the production of insoles or internal inserts. Polyurethane mixtures are also used to perform the direct moulding onto the upper of the sole or sole components. As is well known, polyurethane mixtures are obtained from mainly from two reagents, a polyol and an isocyanate, and are the result of two competing reactions: the gelling reaction which provides the urethane group as a synthetic product, responsible of the increase in molecular weight and viscosity of the polymer, and the expansion reaction (“blowing”) which, through the reaction between the molecules of Ref.: 24LG29I water, present in the polyol, and the isocyanate, produces carbon dioxide as chemical blowing agent. The development of polyurethane mixtures has mainly focused on the optimization of the two reactions mentioned above, so as to make them synchronized so that as the viscosity of the polymer increases corresponds to a gradual formation of pores, to avoid the collapse of the mixture, called “shrinking”. In recent times the need for polyurethane soles has increased that are light and able to ensure high comfort (“cushioning”) at the user's foot. To obtain soles with reduced density values, for example lower than 0.5 g / cm , we resort to the use of physical expanding agents, capable of assisting the expansion action carried out by the water, without interfering with the formation of polyurethane. The most commonly used physical blowing agents are hydrocarbons, such as pentane. They undergo a change of state, going from the liquid state to the gaseous, following the absorption of the heat produced by the reaction of polymerization of polyol and isocyanate. An alternative method for obtaining more expanded polyurethane mixtures involves the introduction of carbon dioxide directly into the of the mixing head of the molding machine. This method has the disadvantage, however, that carbon dioxide is not always uniformly dissolved within the polyurethane mixture. There is therefore the risk of large carbon dioxide bubbles remaining trapped inside the reaction mixture. These bubbles will be found Ref.: 24LG29I consequence also in the finished product, once the polyurethane has solidified, with the possibility of giving rise to production waste. An alternative method for introducing carbon dioxide into a polyurethane mixture is, for example, described in EP1726612B1. This patent describes a method in which a certain amount of carbon dioxide carbon dioxide is dissolved under pressure in at least one of the reactants in the mixture polyurethane, preferably in isocyanate, before the reaction occurs of mixing between polyol and isocyanate. This method, compared to the solution that involves the introduction of carbon dioxide carbon directly at the mixing head of the molding machine, allows to obtain a more polyurethane mixture homogeneous in which the carbon dioxide bubbles are uniformly distributed distributed. However, the implementation of this method requires a long time. processing, both to introduce carbon dioxide into the reagent, and to ensure that the carbon dioxide is uniformly dissolved in the reagent to expand. Carbon dioxide is, in fact, introduced under pressure directly inside a tank containing the isocyanate or polyol for a period of time sufficient to dissolve the desired amount of carbon dioxide. Following this injection, a further period of time must be allowed, not negligible, to allow the carbon dioxide to distribute itself evenly uniform in the reagent. The main purpose of the present invention is, therefore, to make a system and method are available for the controlled introduction of a Ref.: 24LG29I blowing agent in a polyurethane mixture reagent, which allow the above-mentioned problems to be resolved. A particular object of the present invention is to provide a system and a method for the controlled introduction of a blowing agent in a reagent of a polyurethane mixture, which allows to obtain in rapid times an intimate mixing of the blowing agent with the reagent to be expand. A further object of the present invention is to make available a system and a method for the controlled introduction of a blowing agent in a reagent of a polyurethane mixture, which allows to obtain polyurethane mixtures expanded without the addition of a blowing agent interferes with the polymerization and chemical expansion reaction, water-driven, polyurethane mixture. Finally, it is a task of the present invention to provide a system and a method for the controlled introduction of a blowing agent in a reagent of a polyurethane mixture, which allows to use the reagent for obtaining low density polyurethane mixtures, less than 0.35 g / cm . The purpose and main tasks described above are achieved with a system in accordance with claim 1 and with a method in accordance with claim 16. To clarify the explanation of the innovative principles of this invention and its advantages over the known art, will be described below below, with the help of the attached drawings, some exemplary realizations applying these principles. In the drawings: Ref.: 24LG29I - Figure 1 shows a schematic view of the components of a system subject of the present invention; - Figure 2A shows a sectional view of a component of the system figure 1; - figures 2B and 2C are figures similar to figure 2A, but relating to different embodiments of the component; - Figure 3 shows a perspective view, partially in section, of a further component of the system in figure 1; - Figure 4 shows a schematic cross-sectional view of the component in Figure 3, taken along the plane of trace IV-IV; - figure 5 shows a view similar to figure 3, but relating to a second embodiment of such component; - Figure 6 shows a schematic cross-sectional view of the component in Figure 5, taken along the VI-VI track plane; - Figure 7 shows a perspective view, partially exploded, of a further embodiment of the component depicted in figures 4-6; - figures 8 and 9 show schematically possible arrangements of the component of figures 4-7, within the system which is the subject of the invention; - figure 10 shows schematically the connection between a second form of realization of the system which is the object of the invention and a circuit of molding. Referring first to figure 1, a plant for the introduction controlled dissolution of a blowing agent in a reagent of a mixture polyurethane, is indicated as a whole with the numerical reference 10. Ref.: 24LG29I As will be clear from the following description, the said system 10 is suitable to introduce a blowing agent into the polyol in a controlled manner or in the isocyanate of the polyurethane mixture. This polyurethane mixture can be a polyurethane foam based polyether or a polyester-based polyurethane foam. As is known, the polyurethane mixture may contain any additives, in addition to the reagents base made of polyol and isocyanate. This polyurethane mixture can be advantageously used in machines designed to perform casting molding, i.e. mold molding open, or by injection, i.e. in a closed mould, of footwear components, such as soles, midsoles, insoles, etc... The polyurethane mixture obtained from reagents, in which one or both are were added with blowing agent by means of the plant and the method of This invention can also be used in machines intended for perform spray molding. A spraying device in which can be used said polyurethane mixture is described in the patent European EP 2346614 in the name of the same applicant. The polyurethane mixture obtained from reagents treated within the plant 10 can, finally, also be advantageously used in machines direct injection molding of soles, or parts of soles, onto the upper. As shown in figure 1, the system 10 comprises a tank 12 for the storage of said reagent. Tank 12 is provided with a conduit exit 14. The system 10 also includes a recirculation pump 16. This recirculation pump recirculation 16 is arranged in correspondence with the outlet duct 14 and has Ref.: 24LG29I a delivery duct 18 connected to a first inlet 22 of an ejector 20. As clearly visible in figure 1, a second inlet 24 of the ejector 20 is connected to a blowing agent supply circuit 26. The system 10 also comprises at least one mixer 30 connected to a outlet duct 28 of ejector 20. As shown in figure 1, the system 10 also includes a conduit recirculation 32 having an inlet 34, connected to an outlet 36 of the mixer 30, and an outlet 38, connected to the storage tank 12. In accordance with the invention, the system 10 comprises a device for power supply 40, placed upstream of the second inlet 24 of the ejector 20, and connected to the blowing agent supply circuit 26. The system 10 is also characterised by the fact that it includes means of control 50 of said feeding device 40 to vary the quantity of blowing agent introduced into the ejector 20 through the second inlet 24. As will be clear from the following description, the system 10 allows a uniform dispersion of the blowing agent within the polyol or of the isocyanate of the polyurethane mixture. As will be described in detail in the following, the blowing agent preferably consists of anhydride carbonic. However, other types of carbon can also be used to advantage. gases, such as nitrogen or argon. Furthermore, the system 10 of the present invention allows to perform the injection of blowing agent continuously for the time necessary to Ref.: 24LG29I achieve two objectives: a uniform dispersion of the blowing agent in the reagent and the nucleation of the blowing agent inside the reagent. In fact, the system which is the object of the invention can work in a continuous cycle and allows, thanks to the presence of the mixer, to disperse evenly uniform the blowing agent inside the polyol or isocyanate. Furthermore, the controlled introduction of the blowing agent into the reagent through the ejector it ensures that, once the point of maximum solubility has been reached of the blowing agent inside the reagent, nucleation is promoted of the blowing agent. This nucleation process leads to the formation of microbubbles of blowing agent which are found to be uniformly distributed in the reagent. Advantageously, the blowing agent introduced into the polyol or in the isocyanate, it will work in synergy with 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 reactants of the mixture polyurethane by the system and method of the present invention, not It replaces the chemical blowing agents normally used, but improves performance. Furthermore, the reagent treated in the plant object of the invention, be it polyol or isocyanate, not only has a lower density than the density of starting point, but also has a uniform density. In this way the reagent mixed with the blowing agent is adequately prepared to be immediately used in the subsequent moulding phases of the Ref.: 24LG29I polyurethane, while optimizing quality and mechanical properties of the final printed product. Advantageously, as shown schematically in figure 1 and figure 10, the system 10 may comprise a first measuring device pressure 52 and a second pressure measuring device 54. The first pressure measuring device 52 is preferably placed at downstream of recirculation pump 16, near the first inlet 22 of the ejector 20. The second pressure measuring device 54, in turn, is preferably placed downstream of mixer 30, in correspondence with the recirculation duct 32. The first pressure measuring device 52 and the second pressure measuring device pressure measurement 54 may each include a pressure switch. Such pressure measuring devices 52, 54 may also consist of a pressure gauge. Advantageously, the control means 50 of the power supply device 40 are configured to operate the device 40 to vary the amount of blowing agent introduced into the ejector 20 based on the processing of the pressure values ​​measured by the first pressure measuring device 52 and from the second pressure measuring device 54, i.e. on the basis of the pressure value of the reagent detected before and after it occurred in it the injection of blowing agent. In this way it is possible to perform a feedback control of the quantity of blowing agent introduced into the reagent on the basis of the fall of Ref.: 24LG29I pressure recorded in the reagent as it passes through the ejector 20 and the mixer 30. As anticipated, the plant 10 includes a storage tank 12 for the reagent, be it polyol or isocyanate. The reagent is in a liquid state inside tank 12. Preferably, the storage tank 12 is equipped with means of heating 42 (see figure 1), configured to heat the reagent therein content. 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 blowing agent and the interaction between reagent and blowing agent. As shown in figure 1, the tank 12 may also comprise means of mixing 44. Such mixing means 44 are preferably placed in rotation by means of a shaft driven by a motor 46, placed outside the tank 12. Tank 12 may also be equipped with a pressure regulator 48, configured to control the reagent pressure inside the tank 12. Preferably, tank 12 has a capacity of between 50 and 250 litres. Even more preferably, tank 12 has a capacity of between 100 and 150 liters. Alternatively the tank can be replaced by a system power supply intended to directly introduce the reagent inside of the plant 10. Ref.: 24LG29I For the purposes of this invention there is complete equivalence between the tank 12 and such a power supply system. The following notes are valid both in the case of tank 12, as well as in the case of the fuel system. For convenience, reference will be made to tank 12. As anticipated, tank 12 is also equipped with an outlet duct 14. This outlet duct 14 is preferably arranged in correspondence with a lower portion of tank 12. Advantageously, the outlet duct 14 can be provided, in the section upstream of the recirculation pump 16, of a shut-off valve 56. As shown in figure 10, downstream of the shut-off valve 56 can a filter 58 must be mounted. This filter 58 has the function of filtering the reagent from any impurities present inside it, so as to guarantee the correct system operation. Preferably, the control means 50 also controls such a valve. interception 56. In this way, in the event that there is any in the reagent the desired amount of blowing agent has been injected, the valve interception 56 can be closed, thus interrupting the flow of reagent inside the system 10. In this case, alternatively or in addition to the closure of the shut-off valve 56, can be the recirculation pump 16 is also stopped. With reference to figures 1 and 10, the recirculation pump 16 has the function of pump the reagent from tank 12 to ejector 20 and mixer 30, to then allow the reagent to return to tank 12, via the duct of recirculation 32. Ref.: 24LG29I The recirculation pump 16 therefore allows for continuous movement of the reagent. This continuous movement ensures that the reagent maintains a uniform temperature which favors the subsequent introduction and mixing the blowing agent into the reagent itself. Advantageously, the control means 50 can also regulate the pump operation 16. In particular, through the means of command 50 it is possible to vary the flow rate of reagent introduced into the ejector 20. Furthermore, as mentioned above, in case the reagent has been injected the desired amount of blowing agent, the control means 50 can also command the stop of the recirculation pump 16. With reference to figures 2A-2B-2C, the ejector 20 may comprise a nozzle convergent 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 clearly visible in figures 2A-2B-2C, a chamber of mixing 64 can be arranged between the converging nozzle 60 and the nozzle divergent 62. The second input 24 of the ejector 20 is preferably in communication with fluid 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 fitting 66. Such fitting 66 can advantageously have variable section, passing from a larger section in proximity of the entrance 24 to a smaller section near the chamber of mixing 64. Ref.: 24LG29I Preferably, said mixing chamber 64 has a cross-section constant. Advantageously, ejector 20 creates a Venturi effect. The narrowing caused by the converging nozzle 60 causes, in fact, a increase in the speed of the fluid passing through the ejector 20 and consequently a decrease in pressure that allows the agent to be sucked out expanding into the mixing chamber 64 through the second inlet 24 and the connection 66. Advantageously, the narrowing of the connection 66 It allows to speed up the introduction of the blowing agent into the reagent. Through the diverging nozzle 62, the mixture formed by the reagent and is slowed down by the blowing agent, thus promoting dispersion of the blowing agent in the reagent. In figures 2A-2B-2C, the inlet of the liquid reagent into the ejector 20, the suction of the blowing agent into the mixing chamber 64 and the liquid reagent outlet from the ejector 20 are schematically represented by the arrows F1, G1 and F2. As visible in figure 2A, the converging nozzle 60 and the diverging nozzle 62 they can have the same length and the same inclination. Alternatively, as shown in figure 2B, the converging nozzle 60 may have a shorter length than the divergent nozzle 62. In this case case, the inclination of the converging nozzle 60 is preferably greater with respect to the inclination of the divergent nozzle 62. In a further embodiment, shown for example in Figure 2C, the converging nozzle 60 can have a greater length than the nozzle Ref.: 24LG29I divergent 62. In this case, the inclination of the convergent nozzle 60 is preferably lower than the inclination of the diverging nozzle 62. With reference to figures 3-7, the mixer 30 is preferably a static mixer. Advantageously, the mixer 30 comprises an outer casing 68 and a mixing element 70, arranged inside said outer casing 68. Said mixing element 70 comprises, in turn, a plurality of lamellar elements 72 arranged in succession along a longitudinal axis L of the outer casing 68 (see figures 4, 6 and 7). The lamellar elements 72 have a cross-section which is substantially coincides with the internal cross-section of the casing 68, so as to ensure that all liquid passing through the mixer comes into contact with the lamellar elements 72 and be mixed. As shown in Figures 3-7, the outer casing 68 is preferably of hollow cylindrical shape. Each lamellar element 72, in turn, may comprise advantageously a first series of slats 74, inclined according to a first direction, and a second series of slats 76, inclined according to a second direction. Preferably, the first series 74 and the second series 76 of slats of each lamellar element 72 are arranged in succession inside of the external casing 68 (see figures 4 and 6). Advantageously, as shown in figure 7, the mixing element 70 It can be made up of a plurality of 72 helical lamellar elements. Also Ref.: 24LG29I in this case, such helical elements 72 are arranged inclined with respect to a longitudinal axis L of the outer casing 68. Furthermore, as clearly visible in figure 7, such lamellar elements 72 can be arranged inside the casing 68 so that adjacent lamellar elements are rotated with respect to the longitudinal axis L. In this arrangement rotated the transverse axes T of adjacent lamellar elements are then among themselves staggered. As shown in figures 3-7, the pitch and number of lamellar elements inside the package may vary. In the embodiment schematically depicted in figures 3 and 4, the lamellar elements are greater in number and more spaced apart from each other, compared to to the embodiment schematically depicted in figures 5 and 6. In an alternative embodiment of the invention, the system 10 may comprising a plurality of mixers 30. As shown schematically in figure 8, the mixers 30 of such plurality they can be arranged in series with each other, so as to form a line of mixing 300. Alternatively, the mixers 30 of such plurality may be arranged in parallel to each other. As shown schematically in figure 9, the mixing lines 300, made up of a plurality of mixers arranged in series, which in turn can be arranged in parallel. In the example shown in figure 9, the system 10 comprises twelve mixers 30. These mixers are arranged so as to form three lines of 300 mixing units each consisting of four separate mixers. Ref.: 24LG29I The three 300 mixing lines are in turn arranged in parallel to each other. In Figures 4, 6, 7-9, the flow into and out of the mixer 30 or the mixing line 300 is schematically represented by the arrows M1 and M2. The function of the mixing element 70 is to create a turbulent flow inside the mixer 30 which promotes mixing between the reagent and the blowing agent. In particular, the function of the blowing element mixing 70 is to promote a uniform dispersion of the agent expanding inside the reagent. Referring to figures 1 and 10, the agent power supply circuit expanding agent 26 may comprise a pressure vessel 80 for the agent expanding and an outlet duct 82 connected to said vessel pressure 80. The pressure vessel 80 is intended to supply the blowing agent to the system 10. Preferably, such vessel 80 contains carbon dioxide in the gaseous state. The blowing agent is preferably kept inside the vessel 80 at a pressure of approximately 200 bar. Alternatively to the pressure vessel 80, the outlet duct 82 may be connected to a separate network (not shown in the attached figures) intended for provide the blowing agent to be introduced into the reagent. As anticipated, other gases, other than carbon dioxide, can be used as blowing agents, such as nitrogen and argon. As shown in the figures, preferably the power supply device 40, by which it is possible to vary the quantity of blowing agent Ref.: 24LG29I introduced into the reagent, it is placed in correspondence with the outlet duct 82. Such a power supply device 40 may be a proportional valve. Alternatively, the power supply device 40 may consist of a compressor or in a pump. Advantageously, a check valve 84 can be arranged in correspondence of the outlet duct 82. This valve 84 has the function of prevent reagent from accidentally flowing into the feed circuit of the blowing agent 26. With reference to figure 10, the system 10 may also comprise a diverter valve 86 placed upstream of the ejector 20. This diverter valve 86 is configured to divert to a second connecting duct 88 the reagent taken from storage tank 12. The control means 50 can also operate such diverter valve 86. With reference to figure 1, the system 10 may also comprise a sensor or detection probe 92, which is positioned at the outlet duct 14 between the recirculation pump 16 and the ejector 20. In particular, sensor 92 can be positioned between the recirculation pump 16 and the first blood pressure measuring device 52. The sensor 92 advantageously has the function of detecting the concentration of the blowing agent inside the reagent. The control means 50 can be operated manually. Preferably, the control means 50 are operated by a control unit computerized control 90. Such a unit can be, for example, a Ref.: 24LG29I programmable logic controller (PLC) connected to said control means 50. As shown schematically in figures 1 and 10, the control unit computerized 90 is also connected to the first measuring device pressure 52 and the second pressure measuring device 54. Advantageously, the control unit 90 is configured to detect in a continuously the pressures recorded by the pressure measuring devices 52, 54. Based on the pressure data detected by devices 52, 54, the unit of control 90, through the control means 50, is able to control the operation of the power supply device 40, in particular it is in able to regulate the flow rate and pressure of the blowing agent introduced in ejector 20. This way it is ensured that the quantity of blowing agent introduced in the reagent is optimal. The control unit 90 can, based on the data detected by the devices 52, 54, always using the control means 50, adjust the flow rate of the pump 16 and can command the closing of the shut-off valve 56 or the stopping of the pump 16. As shown in figure 1, sensor 92 can also be connected to the control unit 90 to which it sends the detected concentration data, so continuously or at a specific time interval. The control unit 90 on the basis of the concentration measurements detected by the sensor 92 will regulate the operation of the control means 50 power supply device 40, the recirculation pump 16 and / or the shut-off valve power supply 56. Ref.: 24LG29I In case the system 10 is equipped with a diverter valve 86, the unit control 90, through the control means 50, can also control such diverter valve 86. In particular, the control unit 90 is able to address via the valve 86 a fraction of the reagent flow rate taken from tank 12 to the second connecting duct 88. Advantageously, valve 86 can direct towards the second conduit of connection 88 the entire flow of reagent taken from tank 12. This This event occurs when the concentration value of the agent expanding agent in the reagent has reached the desired value. Advantageously as shown in figure 10, through the second duct of connection 88 the system 10 can be directly connected to a circuit 100 polyol or isocyanate feed of a machine for the polyurethane molding. In particular, through the second connecting pipe 88 the reagent mixed with the blowing agent can be sent to a tank 112 of collection of polyol or isocyanate intended for feeding, by means of a pump 116, a mixing head 120 of a molding machine polyurethane. As anticipated, the present invention also refers to a method for the controlled introduction of a blowing agent into a reagent of a polyurethane mixture. The method includes the following phases: a) pre-selection of a required concentration value for the agent expanding agent in the reagent; Ref.: 24LG29I b) withdrawal from a storage tank of the reagent to be added with the blowing agent; c) controlled injection of a first quantity of blowing agent into said reagent; d) mixing said blowing agent with said reagent. The method is characterised by the fact that it understands, following the phase of mixing d), a measurement step e) of the concentration of the agent expanding in the reagent and for the fact that it includes a return phase f) at the controlled injection phase c), if the measured concentration value of the blowing agent in the reagent is different from the concentration value preselected. The reagent, therefore, can be subjected to steps c) and d) more than once. until the concentration of blowing agent in the reagent is reaches the preset value. Advantageously, the measurement phase e) can be conducted in a directly or indirectly. Directly, the concentration of the blowing agent in the reagent is measured directly in the reagent taken from the storage tank, e.g. for example via the sensor or probe 92. Indirectly, the concentration of the blowing agent in the reagent can be measured by measuring the density of the mixture polyurethane obtained starting from the reagent added with agent expanding. To detect this density of the polyurethane mixture you can use the well known free cup expansion test. In particular, performing a Ref.: 24LG29I comparison between the density value of the polyurethane mixture obtained starting from from reagents not added with the blowing agent and the density value of the polyurethane mixture obtained starting from at least one reagent added with the blowing agent, it is possible to obtain the concentration of the blowing agent in the reagent. The method may include, following the sampling phase b), a first reagent pressure measurement phase and, following the phase of mixing d), a second phase of measuring the reagent pressure and of the blowing agent mixed together. Advantageously, the agent concentration measurement step expanding in the reagent may include a processing step of the pressure values ​​measured in the first phase of pressure measurement and in the second phase of pressure measurement. Preferably, on the basis of the results of this measurement phase, the first quantity of can be varied blowing agent injected into the reagent. Advantageously, if the measured concentration value of the agent expanding agent in the reagent is equal to the preselected concentration value, the controlled injection phase c) can be stopped and the reagent mixed with the blowing agent can be directly sent to a circuit of 100 polyol or isocyanate feed to a molding machine of polyurethane. Preferably, the blowing agent injection step is performed by injecting carbon dioxide, nitrogen, or argon into the reagent. This method can be advantageously implemented using the 10 system. described above. Ref.: 24LG29I The reagent, which can be polyol or isocyanate, can be taken from the storage tank 12 or from a connected supply circuit at plant 10. The sampling phase b) can be performed using the recirculation pump 16 which can be set to impart a flow rate between 10 and 80 liters / minute, more preferably between 30 and 60 liters / minute. By varying the flow rate value of the reagent, taking into account the fact that the cross-section of the duct 14 is preferably constant, for example equal to 1”, is it is possible to vary the pressure of the reagent in the system. Preferred values ​​of reagent pressure, before the injection phase, are around 8-8.5 bar. The injection phase b) is preferably performed using the ejector 20. Preferably, the injection phase of the blowing agent is performed at a pressure between 3 and 90 bar. Even more preferably, the phase of injection is performed at a pressure of approximately 7-12 bar. The injection pressure of the blowing agent is set according to the reagent pressure detected in the first pressure measurement phase. For example, if in the first phase of pressure measurement, a pressure value equal to approximately 8 bar, the injection phase of the agent expanding may occur at a pressure in the range of 7.5 bar and 8.5 bars. With these pressure values, it was measured in the second phase of pressure measurement, a pressure drop Δp, equal to approximately 1-1.5 bar. By setting these pressure values, together with the flow rate value before indicated, it is possible to increase the concentration of blowing agent in the Ref.: 24LG29I reagent so as to obtain, in the subsequent molding phase, mixtures polyurethanes having a density even lower than 0.35 g / cm . Mixing step c) can be performed using mixer 30. The variation of the first amount of blowing agent injected into the reagent can be operated by means of the power supply device 40. The first phase of pressure measurement and the second phase of pressure measurement pressure can be performed via the first measuring device pressure 52 and the second pressure measuring device 54. From the above, it is now clear how the system and method of the This invention allows the objectives to be achieved advantageously pre-established. In particular, the specific arrangement of an ejector and a mixer as described above, it allows for continuous and continuous injection reduced times a desired amount of blowing agent inside a reagent of a polyurethane mixture. Furthermore, it allows such blowing agent to be evenly distributed inside the reagent which can be immediately ready to use to be used in well-known molding plants, without having to undergo further treatments. The reagent added with blowing agent can be used in the known open-mold or injection molding systems. The reagent, polyol or isocyanate, can also be used in molding for spraying polyurethane mixtures, for example in the method described in EP2346614B1 in the name of the same applicant. Ref.: 24LG29I Of course, the above description of the realizations that apply the innovative principles of the present invention are reported by way of example of such innovative principles and should therefore not be taken as a limitation of the scope of the patent claimed herein. The Mandatory Eng. Luca Marri Della Dragotti & Associates Srl (Registration No. 1527 B)

Claims

Claims 1. System (10) for a controlled introduction of a blowing agent into a reagent of a polyurethane mixture; said system comprising: - a tank (12) for the storage of said reagent, said tank (12) being provided with an outlet duct (14); - a recirculation pump (16), arranged in correspondence with 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 supply circuit (26) of 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 feeding device (40), placed upstream of the second inlet (24) of the ejector (20), and connected to the feeding circuit (26) of the expanding agent; - control means (50) of said feeding device (40) to vary the quantity of expanding agent introduced into the ejector (20) through the second inlet (24).; 2. System (10) according to claim 1, characterised by comprising a first pressure measuring device (52), placed downstream of the recirculation pump (16) in the vicinity of the first inlet (22) of the ejector (20), and a second pressure measuring device (54), placed downstream of said at least one mixer (30) in correspondence with the recirculation duct (32); said control means (50) being configured to vary the quantity of expanding agent introduced into the ejector (20) on the basis of the processing of the pressure values ​​measured by the first pressure measuring device (52) and the second pressure measuring device (54).

3. System (10) according to any of the preceding claims, characterised in that it comprises a sensor or detection probe (92), positioned in correspondence with 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 expanding agent inside the reagent.

4. System (10) according to any of the preceding claims, characterised 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. System (10) according to the previous claim, characterised in that said second inlet (24) is in fluid communication with said mixing chamber (64).

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

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

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

9. Plant (10) according to the previous claim, characterised in that the mixing element (70) comprises a plurality of lamellar elements (72) arranged in succession along a longitudinal axis L of the external casing (68).

10. System (10) according to the preceding claim, characterised in that each lamellar element (72) of said plurality comprises a first series of lamellae (74), inclined according to a first direction, and a second series of lamellae (76), inclined according to a second direction; the first series (74) and the second series (76) of lamellae of each lamellar element (72) being arranged in succession inside the external casing (68).

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

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

13. System (10) according to any of the preceding claims, characterised in that the supply circuit of the expanding agent (26) comprises a pressure vessel (80) for the expanding agent and an outlet duct (82) connected to said vessel (80); said supply device (40) being arranged in correspondence with said outlet duct (82).

14. System (10) according to any of the preceding claims, Ref.: 24LG29I characterised in that said control means (50) are operated by a computerised control unit (90).

15. System (10) according to any of claims 2 and 14, characterised in that said control unit (90) is configured to continuously detect the pressures recorded by the first and second pressure measuring devices (52, 54); on the basis of the pressure data detected by the first and second measuring devices (52, 54), the control unit (90) is able to control the operation of the supply device (40), via the control means (50).

16. A method for the controlled introduction of a blowing agent into a reagent of a polyurethane mixture, said method comprising the steps of: a) pre-selecting a required concentration value for the blowing agent in the reagent; b) withdrawing from a storage tank the reagent to be added with the blowing agent; c) controlled injection of a first quantity of blowing agent into said reagent; d) mixing said blowing agent with said reagent; the method being characterised by comprising, following the mixing step (d), a measurement step (e) of the concentration of the blowing agent in the reagent and by comprising a return step (f) to the controlled injection step (c), if the measured value of the concentration of the blowing agent in the reagent is different from the pre-selected concentration value.

17. A method according to the preceding claim, characterized in that the measurement step e) is carried out directly or indirectly; in the direct manner, the concentration of the blowing agent in the reagent is measured directly in the reagent taken from the storage tank; in the indirect manner, the concentration of the blowing agent in the reagent is measured by measuring the density of the polyurethane mixture obtained starting from the reagent added with blowing agent.

18. A method according to any of claims 16-17, characterised in that it comprises, following the sampling 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 expanding agent mixed together.

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

20. A method according to any of claims 16 to 19, characterised in that based on the results of such measurement step e), the first quantity of blowing agent injected into the reagent is varied.

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