Abstract:
The present invention is directed to an agitator-free mixing head and a method for agitator-free production of a reaction mixture from an isocyanate and a higher-viscous polyol formulation, wherein both constituents are fed to a mixing zone and mixed with each other therein, whereby a polyol formulation with a viscosity of at least 3000 mPa.s is fed at low pressure into the mixing zone and at least two flows of low-viscosity constituents (isocyanate and optionally low-viscous polyol) are injected into the flow of high-viscous polyol formulation at high pressure.

Description:
FIELD OF THE INVENTION 
     The invention relates to a process for the production, without a stirring apparatus, of a polyurethane-forming reaction mixture from an isocyanate and a polyol formulation, wherein these two components are fed to a mixing zone and are mixed with one another therein, and wherein at least one of the components has a viscosity greater than 3000 mPa.s, preferably greater than 5000 mPa.s. 
     BACKGROUND OF THE INVENTION 
     New fields of application require the processing of polyol formulations of higher viscosity. The use of components of higher viscosity offers many advantages. In general the viscosity of polyether polyols, and in particular polyester polyols, increases distinctly with increasing functionality. In order to achieve high degrees of cross-linking, and thus a better mechanical property level, the use of relatively larger quantities and constituents of polyol components of high functionality is desirable. 
     As high-viscosity component, the isocyanate component is also used in the form of a prepolymer. 
     Satisfactory mixing in a self-cleaning mixing chamber comprising no stirring apparatus still presents difficulties to the extent to which the viscosity of such components increases. Page 175, last paragraph of the “Kunststoff-Handbuch”, Vol. 7, “Polyurethanes” (published by Carl Hanser Verlag, Munich 1993), states: “Whereas the miscibility limit was previously reached at 1500 mPa.s, it is currently possible to handle viscosities exceeding 2000 mPa.s in accordance with the injection method”. In reality, however, it has not yet been possible to process reaction components with viscosities above 3000 mPa.s in accordance with the counterflow high-pressure injection method. The resultant foam contains striations and the physical values fall rapidly. It has therefore been necessary to use mixing heads comprising stirring apparatus. 
     It was also known (“Kunststoff-Handbuch, p. 125) to reduce the viscosity at an elevated processing temperature to 2000 to 3000 mPa.s. However, the increased component temperature causes the reactivity of the systems to increase such that the foaming process can no longer be controlled. 
     Where the following observations relate to the polyol component as high-viscosity component, this is by way of example. The observations apply correspondingly when an isocyanate component is used as high-viscosity component. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a process and a mixing head with which it is possible, without using a stirring apparatus, to produce a reaction mixture of the type referred to in the introduction which reacts to form a high quality product. 
     This object is achieved in that a polyol formulation having a viscosity of at least 3000 mPa.s is fed into the mixing zone at low pressure and in that at least two streams of low-viscosity components (isocyanate and optionally low-viscosity polyol) are injected at high pressure into the stream of high-viscosity polyol formulation. 
     Surprisingly, this results in a good mixing of components of higher viscosity. The invention makes use of the fact that, as a result of the splitting of the higher-viscosity component into two or more sub-streams, although a shift occurs in its energy, the energy is increased overall. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 a  and  1   b  illustrate a mixing head according to the invention with an outlet having the form of a slot die. 
     FIG. 2 illustrates a mixing head according to the invention with an outlet in the form of a spoon- or fan nozzle. 
     FIGS. 3 a  and  3   b  illustrate a mixing head according to the invention for batchwise operation. 
     FIGS. 4 a,    4   b  and  4   c  illustrate an alternative embodiment of a mixing head according to the invention for batchwise operation. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Preferably the cross-sectional area of the stream of the polyol formulation is 10 to 100 times greater than the sum of the cross-sectional areas of the isocyanate streams. 
     Optionally in cooperation with preferred processing pressures of 50 to 300 bar for isocyanate and less than 20 bar for the polyol formulation, good results are achieved. A pressure of between 70 and 250 bar is preferred and a pressure of between 100 and 200 bar is particularly preferred. 
     Preferably, the sub-streams of the isocyanate component are injected in a cross-flow. 
     Here it should be noted that relatively good results are still attainable in the case of not too great a deviation from the cross-flow input. 
     It is also advantageous for the injection jet not to be directed towards the central axis of the mixing zone since an at least approximately tangential input direction provides for a good degree of mixing as a result of additional turbulence. Normally the input locations will be uniformly distributed over the periphery of the mixing zone and optionally only the low-viscosity component will be supplied at the end side. 
     In the case of the classical counterflow injection principle with component viscosities of &lt;2000 mPa.s, the following energy distribution is used: 
     
       
         ( m   Poly   ×p   Poly )+( m   Iso   ×P   Iso )= E   
       
     
     With an output of 4 kg/min polyol (viscosity 4000 mPa.s) and 6 kg/min isocyanate (viscosity 50 mPa.s), thus a total output of 10 kg/min at injection pressures of 100 bar, the following energy distribution occurs: 
     
       
         (4×100)+(6×100) 
       
     
     energy of polyol 400+energy of isocyanate 600=1000 
     Conversely, in the new process the polyol is supplied at 10 bar and the isocyanate is injected in a cross-flow at 180 bar in two sub-streams oppositely directed to one another: 
     
       
         ( m   Poly   ×p   Poly )+[( m   ISO  /2× p   Iso )+( m   Iso /2× p   Iso )]= E    
       
     
     4×10+(6/2×180)+(6/2×180) 
     Energy of polyol 40 +energy of isocyanate 1080 =1120 
     In accordance with an alternative embodiment of the new process, all the streams are introduced into the mixing zone in the same plane. 
     It is also possible to supply only the higher-viscosity constituent of the polyol formulation at low pressure and, separately therefrom, to inject low-viscosity constituents, as well as the isocyanate, at high pressure into the stream of the high-viscosity constituent. 
     The following example can be considered here: 4 kg/min polyol is processed, of which ⅔ is of high viscosity and ⅓ is of low viscosity. The high-viscosity constituent is input in perpendicular manner into the mixing zone at a pressure of 10 bar. The low-viscosity constituent and 6 kg/min isocyanate are injected transversely in a total of three sub-streams, offset by 120° , at 150 bar. The associated energy calculation, in which “poly H” is high-viscosity polyol and “poly N” is low-viscosity polyol, is as follows: 
     
       
         [(⅓ m   PolyH   ×p   Poly )+(⅔ m   PolyN   ×p   Poly )]+[( m   Iso /2× p   Iso )+( m   Iso /2× p   Iso )]= E    
       
     
     (1.33×10)+(2.66×150)+(6/2×150)+(6/2×150) 
     Energy of polyol 413 + energy of isocyanate 900=1313 
     When the polyol formulation is split into high-viscosity and low-viscosity constituents, the high-viscosity constituent can also be introduced in perpendicular manner into the mixing chamber, while the low-viscosity constituent is injected in one stream and the total quantity of isocyanate in another stream in opposition to one another: 
     
       
         [(⅓ m   PolyH   ×p   Poly )+(⅔ m   PolyN   ×p   Poly )]+[( m Iso ×p   Iso )= E   
       
     
     (1.33×10)+(2.66×150)+(6×150) 
     Energy of polyol  413 + energy of isocyanate 900=1313 
     Naturally, both the isocyanate and/or the low-viscosity polyol can also be injected in a plurality of sub-streams. 
     A mixing head with no stirring apparatus, comprising a mixing chamber with inlet openings for the components, is preferentially suited to the implementation of the new process. 
     The novelty is to be considered to consist in an inlet opening for the higher-viscosity polyol formulation and at least two inlet openings for low-viscosity components (isocyanate and optionally low-viscosity polyol), where the cross-sectional area of the inlet opening for the higher-viscosity polyol formulation is 10 to 100 times greater than the sum of the cross-sectional areas of the inlet openings for the low-viscosity component(s). 
     Such mixing heads are self-cleaning and in principle—depending upon their control facilities—permit continuous operation, for example for downstream belt systems, or also batchwise operation for the filling of moulding tools or other cavities to be filled. 
     The invention permits the production of reaction mixtures using polyol components with viscosities exceeding 3000 mPa.s, in particular of between 10,000 and 50,000 mPa.s. 
     A limitation of the viscosity of the polyol component is imposed by the required conveying means. Gear pumps, screw-spindle pumps or also worm conveyors are suitable conveying means at up to 100,000 mPa.s. 
     In the following the invention will be explained in detail making reference to the attached Figures. 
     The mixing head  1  according to the invention as shown in FIG. 1 a  contains a supply pipe  11  for the high-viscosity polyol and inlet openings  12  for the isocyanate. The inlet openings  12  have sealing cones  15  which are movable against prestressed springs and which open when isocyanate is supplied (arrow  14 ) at the required pressure. The mixing zone  13 , which is not sharply defined in terms of geometry, is in the form of a simple flow-through pipe into which the injection nozzles  12  for the isocyanate open. The mixing zone  13  contains no turbulence-producing built-in members of any kind. The injection nozzles  12  for the isocyanate are preferably not oriented in the direction of the axis of the mixing zone  13  but rotated in the plane perpendicular to the axis in such manner that, as a result of the injection of the isocyanate, the high-viscosity polyol stream undergoes a rotary movement in the mixing zone  13  in order to assist the mixing. The outlet  2  of the mixing head  1  has the form of a slot die. FIG. 1 b  shows a section perpendicular to the drawing plane of FIG. 1 a  through the mixing head outlet  2 . 
     FIG. 2 shows a mixing head  1  as in FIG. 1 a,  but with the outlet  2  in the form of a fan nozzle. 
     The mixing head  1  according to FIG. 3 a  is suitable for intermittent batchwise operation for the production of foam mouldings. The mixing head has a sealing piston  20  which is movable hydraulically in the axial direction (arrow  21 ). In order to fill the mould, the sealing piston  20  is moved back (as indicated by the arrow  21 ) so that the inlet  11  for the high-viscosity polyol and the injection nozzle  12  for the isocyanate are freed for entry into the mixing zone  13 . From the mixing zone  13 , the mixture passes into the outlet pipe  2 , through which the mixture is introduced into the mould. When filling of the mould is complete, the sealing piston  20  is returned to the position shown, the inlets  11  and  12  to the mixing zone  13  being blocked. At the same time, the inlets  11  and  12  are connected via grooves  28  and  29  in the sealing piston  20  with the recirculation pipes  24  and  25  via which the high-viscosity polyol and the isocyanate are fed back into their respective storage containers. The recirculating stream is maintained during breaks between batches, as is customary in polyurethane production technology. The mixing head also has a hydraulically operated cleaning piston  26  which, at the end of a batch, once the sealing piston  20  has moved into the sealing position, ejects the mixture remaining in the outlet pipe  2  from the outlet pipe  2 . FIG. 3 b  shows a section A—A through FIG. 3 a.  In the drawing shown by way of example three injection nozzles are provided for introducing the isocyanate. Variations in the construction of the mixing head are possible and can readily be deduced by the person skilled in the art from Becker/Braun, “Kunststoff-Handbuch”, Vol. 7, Polyurethanes, p. 177-182 ( 1993) . Essential to the invention is the large cross-section of the supply pipe for the high-viscosity polyol as compared with the isocyanate inlet openings, with correspondingly large cross-sections of the recirculation pipes, so that the high-viscosity polyol can be conveyed in a substantially pressure-free manner. 
     FIG. 4 a  shows an alternative embodiment of the mixing head according to the invention for intermittent batchwise operation, in which the turning angles for the high-viscosity polyol are kept small. The same reference symbols indicate the same elements as in FIG. 3 a.  FIG. 4 b  shows a section B—B through FIG. 4 a.  FIG. 4 c  shows a representation corresponding to FIG. 4 a  in which the sealing piston  20  has been moved into the sealing position. On introduction into the mixing zone  13 , the high-viscosity polyol stream supplied via the supply pipe  11  is turned through only 20 to 35°. The introduction of the isocyanate (four inlet openings  12   a  and  12   b  arranged in pairs opposite one another) is effected perpendicular to the polyol stream. Here the isocyanate supply pipes  14   a  and  14   b  are offset in pairs in such manner that the polyol stream in the introduction planes is made to rotate in the opposite direction. Recirculation of the isocyanate at the end of a batch is effected via grooves  29  in the sealing piston  20 . Recirculation of the high-viscosity polyol is effected by means of a bore  28  through the sealing piston  20 , which bore produces the connection to the recirculation pipe  24  in the sealing position.