Polyhexamethylene adipamide molding compositions not subject to discoloration when conditioned with water
It has been known for many years to stabilize polyamides against thermooxidative damage and attack by ultraviolet light by means of phenolic antioxidants (cf. Kunststoffhandbuch, Polyamide, vol. VI, 1966, published by Carl Hanser Verlag, Munchen, pp. 240 and 241). However, this process suffers from the drawback that the polyhexamethylene adipamide molding compositions thus stabilized tend to discolor toward red or reddish brown when conditioned with water or steam. Conditioning of polyhexamethylene adipamide is necessary in order to achieve the degree of toughness required in industry. 
     It is an object of the present invention to prevent said undesirable discolaration of polyhexamethylene adipamide toward red or reddish brown as has hitherto occurred on conditioning thereof. 
     According to the invention, this object is surprisingly achieved by adding to the molten polyhexamethylene adipamide from 0.001 to 0.5%, by weight of the polyhexamethylene adipamide of a compound whose chain transfer constant for styrene at 100°C is greater than 1.5 and is preferably from 8 to 30. The addition of such compounds in accordance with the invention effectively prevents the red to brown discoloration of polyhexamethylene adipamide when conditioned with water without impairing the action of the phenolic antioxidants added. 
     A suitable chain transfer agent to be added according to the present invention is, for example, n-dodecylmercaptan. Examples of other chain transfer agents are t.-butyl dodecylmercaptan and those described in Polymer Handbook, I. Brandrup and E. H. Immergut, Interscience Publishers, New York, 1966, Tables II - 128 to II - 133. 
     The molding compositions of the invention may contain, as phenolic antioxidants, any of the compounds of this kind which are conventionally used in polyhexamethylene adipamide. Examples thereof are 2,6-di-t.-butyl-p-cresol and N,N&#39;-bis-[3-(3,5-di-t.-butyl-4-hydroxyphenyl)-propionyl]-hexamethylenediamine. Such phenolic antioxidants are used in conventional amounts, for example in amounts of from 0.1 to 0.8%, by weight of the polyhexamethylene adipamide. 
     The chain transfer agents to be added in accordance with the present invention are fed to the molten polyhexamethylene adipamide on completion of the polycondensation of the latter. 
     The present invention is illustrated below with reference to an Example, in which the parts are by weight and the percentages are by weight of the polymer product. Color measurements were carried out on granulated polyhexamethylene adipamide in accordance with DIN 5033. 
     The measurements were run with a recording spectrophotometer DMG 25 by Zeiss against BaSO 4  =  100%, gloss included, using standard light source C/2° observer. 
     The test samples were placed in black powder bowls to a height of 15 mm, three such filled bowls being tested for each sample. In the case of the luminance factor Y the margin or error is ± 0.3 and in the case of the color coordinates x and y it is ± 2 × 10 -   4 . 
     The perforated impact resistance was measured on standard test specimens according to DIN 53,453, this being regarded as a characteristic mechanical property for testing the aging behavior. The standard specimens measuring 4 × 6 × 50 mm were provided with a 3-mm hole at the center and were held in air at 140°C for 30 days. After 3, 10, 20 and 30 days, 10 specimens were taken from each sample and the perforated impact resistance was determined as described in &#34;Kunststoffe&#34; 57 (1967), pp. 825 to 828. 
    
    
     EXAMPLE 1 
     200 parts of polyhexamethylene adipamide granules (K value 73.1) were mechanically blended with 1 part of 2,6-di-t.-butyl-p-cresol and the mixture was melted in a twin-shaft kneader heated at 270°C and then extruded therefrom. The extrudate was granulated, dried and injection molded to standard specimens conforming to DIN 53,453. The granules were subjected to conditioning (3 days at 80°C using distilled water), and their color was measured according to DIN 5033 before and after conditioning. The standard specimens were used for determining the perforated impact resistance. 
     EXAMPLE 2 
     200 parts of polyhexamethylene adipamide granules (K value 73.1) were mechanically blended with 1 part of 2,6-di-t.-butyl-p-cresol and 0.05 part of n-dodecylmercaptan. Further treatment was carried out as described in Example 1. 
     
                                           TABLE 1                                 
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Colorimetric data of Examples 1 and 2                                     
Granules                                                                  
       Untreated            Conditioned (3 days, 80°C, H.sub.2 O)  
       Color coordinates x, y                                             
                            Color coordinates x, y                        
       Luminance factor Y   Luminance factor Y                            
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Example 1                                                                 
        x    y    Y   Color  x    y    Y     Color                        
       0.3324                                                             
            0.3447                                                        
                 51.9                                                     
                     pale yellow                                          
                            0.3745                                        
                                 0.3531                                   
                                      40.4                                
                                          marked reddish brown            
Example 2                                                                 
       0.3342                                                             
            0.3419                                                        
                 57.1                                                     
                     pale yellow                                          
                            0.3498                                        
                                 0.3526                                   
                                      49.9                                
                                          pale yellow                     
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                       TABLE 2                                                     
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Stabilization of polyamide-6,6 of Examples 1 and 2                        
Molding   Perforated impact resistance in cmkg/cm.sup.2 after             
composition of                                                            
           0        3        10    20    30 days                          
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Example 1  60       15.7     3.8   1.4   0.8                              
Example 2  60.8     16.0     4.1   0.8   0.7                              
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