Process for the hydrolytic preparation of a prepolymer amide from a C12 -aminocarboxylic acid lactam
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a process for the preparation of a prepolymer amide, having a degree of polymerization within the range from 5 to 20 (number average) from a C 12  -aminocarboxylic acid lactam by hydrolytic polymerization at an elevated temperature and elevated pressure. 
     2. Discussion of the Background 
     The preparation of prepolymer amides from aminocarboxylic acid lactams at an elevated temperature and under elevated pressure is known in principle. 
     Thus, Japanese Published Specification No. 59/164,327 describes a method of preparation of polycondensates from lactams having 7 to 12 members within the ring in a temperature range from 220° to 280° C. and under pressure ranging up to 10 bar. This choice of parameters leads to unsatisfactory results with respect to the space/time yield with C 12  -aminocarboxylic acid polycondensates. 
     Further, a process is known from Japanese Published Specification 60/41,647 in which temperatures in the range from 295° to 370° C. and a water content from 2 to 50 percent by weight are used. 
     SUMMARY OF THE INVENTION 
     Accordingly, the object of the present invention is to provide a method for the preparation of a prepolymer amide from a C 12  -aminocarboxylic acid lactam in which the temperature and water content of the reaction mixture are related to one another in such a way that the prepolymer can be produced economically. 
     This object and other objects which will become apparent from the specification have been achieved by the process of the present invention which comprises hydrolytically polymerizing a lactam/water mixture of a C 12  -aminocarboxylic acid lactam at a temperature of about 240°-360° C. and a steam pressure of about 30-180 bar, and in which the water content of the lactam/water mixture is in the range of about 7.5-30% by weight, to produce a prepolymer amide having a number average degree of polymerization of 5-20. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The chemical process for preparing a prepolymer amide from aminocarboxylic acid lactams is known in all chemical and technical detail (see Ullmanns Enzyklopadie d. techn. Chemie (Ullmann&#39;s Encyclopedia of Industrial Chemistry), 4th edition, volume 19, Chapter 2.2 and Chaper 3). 
     According to the invention, a steam pressure in the range from 30 to 180 bar, preferably 60 to 150 bar, a temperature in the range from 240° to 360° C., preferably 280° to 355° C., and a water content of the lactam/water mixture in the range from 7.5 to 30 percent by weight, preferably 10 to 25 percent by weight, are used. In a more preferred process, a steam pressure in the range from 45 to 100 bar, a temperature from 260° C. to an upper limit the boundary line of which is linear in a plot of temperature vs. water content, and is characterized by the following parameters: 
     340° C.--3.0 percent by weight of water, 
     325° C.--10.0 percent by weight of water, 
     310° C.--20.0 percent by weight of water, 
     300° C.--30.0 percent by weight of water, 
     and a water content in the range from 7.5 to 30 percent by weight are chosen. 
     The starting monomer may be pure C 12  -aminocarboxylic acid lactam or up to 6.5 percent by weight of this monomer can be replaced by caprolactam and/or hexamethylenediamine and dodecanedicarboxylic acid. 
     Using the process according to the invention it now becomes possible to achieve an economic space/time yield within the predetermined pressure range. 
     Surprisingly, it has been found that in spite of the pressure increasing with increasing water content the reaction times cannot be reduced below a certain limiting value and the latter is achieved at a comparatively modest water content (see Table I). 
     The process according to the invention unexpectedly permits a marked improvement in the space/time yield. Thus, in the prior art, a reaction time of about 6 to 8 hours is required for almost quantitative conversion. In the process according to the invention the time can be lowered to between 10 minutes and 3.5 hours depending on the balancing of the parameters. 
     Other features of the invention will become apparent during the course of the following descriptions of exemplary embodiments which are given for illustration of the invention and are not intended to be limiting thereof. 
    
    
     EXAMPLES 
     Experiments 1 to 6 and A to C (corresponding to Japanese Published Specification 60/41,647) were carried out in a high-pressure autoclave with a 1000 ml nominal volume, equipped with a stirrer, electric heater, temperature and pressure measuring device, and for nitrogen and vacuum connection and addition and sampling devices. After the aluryl-lactam pellets had been introduced, the reactor was flushed with very pure nitrogen, evacuated (5 mbar) and heated, and the hydrolytic polymerization was then started by rapid addition of water which had been distilled under N 2  and was thermostatically kept at the reaction temperature. 
     The samples taken under pressure at the reaction temperature were rapidly cooled to room temperature. The water content was determined gravimetrically from the weight loss on drying (50° C., 100 mbar). The number-average degree of polymerization (p n ) was determined by end group analysis (potentiometric) titration of the material dissolved in m-cresol or by indicator titration of the material dissolved in boiling benzyl alcohol). The lauryl-lactam content was determined by gas chromatography. 
     The space/time yield was calculated assuming that the partial volumes of water and lauryl-lactam were additive at the reaction temperature. 
     The experimental parameters and results are shown in Table I. 
     
                                           TABLE I                                 
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                                    Water                                 
                                    content of                            
                                          Residual      Space-time        
Charge            Reaction                                                
                         Maximum                                          
                               Reaction                                   
                                    sample                                
                                          Lauryl-lactam                   
                                                        yieldnt           
 Experiment No.                                                           
       Lauryl-lactam (g)                                                  
               Water (g)                                                  
                   temperature (°C.)                               
                          pressure (bar)                                  
                                time (min)                                
                                     (% by) (weight)                      
                                           of the dried sample (% by      
                                          weight)                         
                                                      ##STR1##            
                                                         ##STR2##         
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A     728.5    38.5                                                       
                  355     59    22   2.9  0.55       18.3                 
                                                        18                
1     676.0    75.5                                                       
                  355    1.3    15.5                                      
                                     7.5  0.60       12.1                 
                                                        24                
2     577.5   144.5                                                       
                  355    144    12  15.5  0.43       8.5                  
                                                        27                
3     486.5   208.5                                                       
                  355    167    12  24.9  0.53       7.4                  
                                                        23                
B     402.0   268.0                                                       
                  355    170    13.5                                      
                                    35.0  0.67       6.2                  
                                                        17                
4     618.0   154.5                                                       
                  318    102    40  16.9  0.48       9.8                  
                                                        8.5               
C     447.0   298.0                                                       
                  318    109   45   37.2  0.60       7.5                  
                                                        5.5               
5     736.0    82.0                                                       
                  278     51   298   8.5  0.50       17.2                 
                                                        1.32              
6     648.0   162.0                                                       
                  278     62   211  18.5  0.52       12.3                 
                                                        1.65              
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     In experiments D to I the procedure followed was in accordance with Japanese Published Specification 60/41,641, in that, using two reciprocating diaphragm metering pumps, water and liquid lauryl-lactam, after having been heated to the reaction temperature, were combined at the top of an electrically heated reaction tube of 10.2 liters volume, equipped with static mixer elements. After having passed through the reaction tube, the melt was cooled to 210° C. and discharged via a pressure-retaining valve (130 bar) into a vessel kept at a constant pressure of 20 bar. A sampling device permitted melt under pressure to be withdrawn between the exit of the reaction tube and the cooling zone. The samples were cooled rapidly to room temperature and were analyzed, as described above, to determine the contents of water and residual lauryl-lactam, and the number-average degree of polymerization. 
     The residence time was calculated from the volume of the tubular reactor and the stream passing through, assuming that the partial volumes of water and lauryl-lactam were additive at the reaction temperature. 
     The experimental parameters and results are shown in Table II. 
     
                                           TABLE II                                
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                               Water                                      
                               content of                                 
                                     Residual   Space-time                
Charge            Reaction                                                
                         Residence                                        
                               sample                                     
                                     Lauryl-lactam content                
                                                yield                     
 Experiment No.                                                           
       Lauryl-lactam (kg/h)                                               
               Water (kg/h)                                               
                   temperature (°C.)                               
                          time (min)                                      
                                (% by) (weight)                           
                                      of the dried sample (% by           
                                                 ##STR3##                 
                                                    ##STR4##              
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A      7.892  0.733Z                                                      
                  318    56     8.0  1.0        14.0                      
                                                   6.8                    
E      9.788  2.076                                                       
                  318    40    16.8  0.8        9.6                       
                                                   8.5                    
F      6.294  3.858                                                       
                  318    45    37.3  0.9        7.4                       
                                                   5.5                    
G     11.055  1.027                                                       
                  318    40     8.2  5.1        -- --                     
H      9.788  2.076                                                       
                  318    40    16.8  0.8        -- --                     
I      7.091  4.346                                                       
                  318    40    37.5  1.6        -- --                     
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     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.