Hydrolysis-resistant thermoplastic molding composition comprising high molecular weight polybutylene terephthalatepolyester and a dicarboxylic acid salt
BACKGROUND OF THE INVENTION 
     This invention relates to hydrolysis-resistant, thermoplastic molding compositions made of high-molecular weight, linear polybutylene terephthalate. 
     Polybutylene terephthalates are excellent materials for the manufacture of films, sheets, molded parts, etc. When used to manufacture molded parts by the injection-molding or extrusion methods, in particular, polybutylene terephthalate must have a viscosity number of &gt;120 cm 3  /g. 
     Polybutylene terephthalates are customarily produced by transesterification or esterification and subsequent polycondensation of terephthalic acid or its polyester-forming derivatives and an alkanediol in the presence of catalysts. According to these processes, conducted in the melt, viscosities of the required magnitude, for example, for injection-molding and extrusion purposes, cannot be attained since, because of the thermal load, decomposition reactions occur as early as in the polycondensation stage so that the desired high viscosity cannot be reached. Another limitation on the viscosity of the polyesters in the melt condensation process results from the viscosity of the melt. The latter increases with progressive polycondensation, making it difficult or even impossible for the volatile, low-molecular weight reaction products to escape, thereby inhibiting the polycondensation progress. Furthermore, technical difficulties are encountered in discharging the high-viscosity melt from the reaction vessel (DOS No. 2,117,748). 
     If necessary, a solid-phase recondensation is carried out (British Pat. No. 1,066,162; U.S. Pat. No. 3,405,098). This is done by bringing the polyester, in granulated or powdered form, to temperatures ranging from 10°-60° C. below the melting point. In this temperature range, the polybutylene terephthalate is treated under an inert gas stream or under vacuum until the desired, high viscosity has been achieved. The recondensation can be effected either discontinuously, e.g., in a tumbler dryer (DOS No. 2,117,748), or continuously, for example in a fluidized-bed procedure (DOS No. 1,804,551) or in a screw-type reactor (DOS No. 2,162,618). A basic disadvantage of the solid-phase recondensation is derived from the long reaction period at elevated temperature; as a result, although a polybutylene terephthalate of the desired viscosity is obtained, it is already damaged to such an extent that it is greatly decomposed during melt processing. Molded components made of such a material are brittle and prone to breakage. 
     Therefore, it has been proposed to utilize polybutylene terephthalate in ground form to reduce the recondensation period; however, this process is not economical. Furthermore, there is a danger of caking of the powder (DOS&#39;s No. 2,152,245 and 2,117,748). German Pat. Nos. 3,033,468 and 3,033,469 propose a method wherein recondensation is conducted in the presence of alkanediols in two stages. This process has the drawback that it consumes a large amount of time. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of this invention to avoid or ameliorate the aforedescribed disadvantages while maintaining the desired good properties of polybutylene terephthalate. 
     It is another object of this invention to provide a method of accomplishing this while also providing a product having very good hydrolysis resistance. 
     These objects have been attained by adding to a polybutylene terephthalate, 0.1-10 mole %, based on its content of dicarboxylic acid component, of a salt of Formula (A) 
     
         [(.sup.- O.sub.3 S).sub.x R--CO.sub.2.sup.-).sub.2 ]M,     (A) 
    
     wherein 
     M=an alkali or alkaline earth metal cation selected in kind and number to provide electrical neutrality, 
     R=an aromatic, cycloaliphatic, or aliphatic residue of 2-20 carbon atoms in the carbon chain, and 
     x=0 or 1. 
     DETAILED DISCUSSION 
     The molding compositions of this invention preferably contain 0.5-2.5 mole % of salts of Formula (A) based on the number of moles of the dicarboxylic acid component in the polybutylene terephthalate. 
     Suitable aromatic-containing salts of Formula (A) include alkali and alkaline earth metal salts of, for example, 5-sulfoisophthalic acid (SIPS)--e.g., Na 3  [SIPS], K 3  [SIPS], NaK 2  [SIPS], NaMg[SIPS], NaCa[SIPS], KCa[SIPS]--as well as the corresponding salts of sulfo-substituted phthalic and terephthalic acids and also the non-sulfo-substituted iso- and terephthalic acids, as well as unsubstituted phthalic acid. Sulfo-substitution in other positions is also included. Suitable aliphatic dicarboxylic acids are those of alkyl groups of 2-20 carbon atoms in the carbon chain, for example, oxalic acid, succinic acid, adipic acid, dodecanedioic acid, etc. and their sulfo-substituted counterparts. Suitable cycloaliphatic salts are the hexahydro derivatives corresponding to the aromatic salts discussed above, e.g., cyclohexyl derivatives based on tetraline. 
     Preferred salts are the alkali and alkaline earth metal salts of isophthalic acid and 5-sulfoisophthalic acid. Besides single salts, it is also possible to use mixed salts or salt mixtures. 
     Suitable polyesters primarily include homopolybutylene terephthalate. 
     Up to 20 mole %, preferably 5-15 mole %, of the terephthalic acid in polybutylene terephthalate can be replaced by aromatic, cycloaliphatic, or aliphatic dicarboxylic acids. Examples of suitable dicarboxylic acids include isophthalic acid, phthalic acid, cyclohexane-1,4-dicarboxylic acid, adipic acid, sebacic acid, azelaic acid, decanedicarboxylic acid, or similar compounds. Up to 20 mole %, preferably 5-15 mole % of the 1,4-butanediol can be replaced by other diols, e.g., ethylene glycol, 1,3-propanediol, 1,6-hexanediol, neopentyl glycol, 1,4-dimethylolcyclohexane, 1,12-dodecanediol, or similar compounds. See, e.g., British Pat. No. 1,320,520 whose disclosure is incorporated by reference herein. 
     The polybutylene terephthalate employed according to this invention can be produced in the usual way by transesterification or esterification and subsequent polycondensation of terephthalic acid or its polyester-forming derivatives, as well as the corresponding alkanediol in the presence of catalysts (Sorensen and Campbell, Preparative Methods of Polymer Chemistry, Interscience Publishers Inc., NY, 1961, pp. 111-127; &#34;Kunststoff-Handbuch&#34; [Plastics Manual], Vol. VIII, C. Hanser Verlag, Munich, 1973; or Journal of Polymer Science, Part A 1, 4: 1851-1859, 1966, all of whose disclosures are incorporated by reference herein. Molecular weights are usually in the range of 30 000-60 000 g/mol (weight average). 
     The timing of the addition of the salts of Formula (A) is uncritical. The addition can take place at any desired point during the preparation of the polyesters. It is also easily possible to introduce the salts into the polyester as late as during ancillary processing (recondensation, addition of auxiliary agents and additives etc.). 
     Customary additives and auxiliary agents, such as pigments, processing aids, fillers and reinforcements, hydrolysis-, thermal-, or UV-stabilizers, can also be incorporated during manufacture or during subsequent preparation of the finished, treated molding compositions. 
     After addition of the salts of Formula (A), the molding compositions of this invention exhibit excellent recondensation qualities. For example, using them, recondensation proceeds very rapidly, producing products of a high viscosity number (&gt;120 cm 3  /g), e.g., as required especially for the manufacture of molded articles according to the injection-molding or extrusion processes. Furthermore, the molding compositions of this invention are distinguished by a superior hydrolysis resistance; in other words, molded parts produced from them have a very good hot-water stability, e.g., values of hydrolysis as defined below of about 0.5-20%. This fact is surprising insofar as copolyalkylene terephthalates containing monosodium-5-sulfoisophthalate as the co-component, are said to have a pronounced susceptibility to hydrolysis (Amer. Dyest. Rep. 57: 36 [1970]; &#34;Makromol. Chem.&#34; [Macromolecular Chemistry] 176: 2839 [1975]). 
     Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The following preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever. In the following examples, all temperatures are set forth uncorrected in degrees Celsius; unless otherwise indicated, all parts and percentages are by weight. 
     The viscosity number (J) was determined according to DIN [German Industrial Standard] 16 779, part 2, on solutions of 0.5 g of polybutylene terephthalate in 100 ml of phenol/o-dichlorobenzene (50/50 parts by weight) at 25° C. 
     The hydrolysis stability of the copolyesters is characterized by the reduction in viscosity number (J) after a 24-hour treatment of the finely ground material with 130° C. hot water in a pressure vessel. 
     The decrease in viscosity is defined as percentage reduction of the viscosity number (J) due to the hydrolysis: ##EQU1## (J o  =starting viscosity number; J Hydr . =viscosity number after hydrolysis test). 
     Experiments denoted by letters are not in accordance with the invention. 
    
    
     EXAMPLES 
     I. Preparation of Molding Compositions of Invention 
     A. Experiments 1-6 and 13-18, Respectively 
     In a 100-liter reactor, 200 moles of a mixture of dimethyl terephthalate and 5-sulfoisophthalic acid trisodium salt (content listed in tables) is transesterified with 27 kg (300 moles) of 1,4-butanediol and 15 g of titanium tetraisopropylate as the catalyst in a weak nitrogen stream, first for 3 hours at 160° C., then for 2 hours at 200° C. under agitation until the methanol quantity theoretically to be expected has been removed by distillation. 
     Thereupon, during the course of 2 hours, a water-jet vacuum is applied; within another 1.5 hours, the reaction mixture is then heated under stirring to 250° C. These conditions are maintained for 30 minutes; subsequently, a vacuum is applied of &lt;1 mbar, and the reaction mixture is polycondensed to a viscosity number (J) of 95-120 cm 3  /g. After eliminating the vacuum with N 2 , the melt is discharged in rod shape, cooled in water, granulated, and dried. The output of polyester is about 45 kg of granulated material. If desired, the granulated material is furthermore ground into a powder. 
     B. Experiment 7 
     In a discontinuously operating laboratory kneader, 1 mol-% (based on the dicarboxylic acid component) of 5-sulfoisophthalic acid trisodium salt is mixed in the melt with polybutylene terephthalate (J: 110 cm 3  /g) at 250° C. Processing of the melt takes place according to (A). 
     C. Experiments 8-12 
     The polyesters are produced according to (A), except that the salts indicated in Table 2 are utilized in place of 5-sulfoisophthalic acid trisodium salt. The products are used as powders. 
     D. Experiments A through C 
     The polyesters are prepared according to (A)--but without the addition of a salt according to this invention. 
     E. Experiments D through F 
     For comparison, a polyethylene terephthalate is produced according to (A). The catalyst for the transesterification stage is 15 g of Mn acetate and 3.3 g of Co acetate; and 43 g of triphenyl phosphate and 1.1 g of GeO 2  (dissolved in ethylene glycol) are used as the catalysts for the polycondensation stage. As contrasted to the mode of operation according to (A), the transesterification is conducted in the first temperature stage at 180° C., and the polycondensation is effected at 280° C. 
     II. Recondensation 
     Recondensation of the products for Experiments 1, 2, and (A) takes place in a 500-l tumbler dryer in a water-jet vacuum at 210° C. The products for Experiments 3-7 and (B), finely ground in a laboratory mill, are recondensed in a rotary evaporator under an oil pump vacuum at 210° C. 
     
                       TABLE 1                                                     
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Recondensation                                                            
                     J [cm.sup.3 /g]                                      
                     after a Recondensation                               
       Content of Na.sub.3 [SIPS]*                                        
                     Period of [h]                                        
Experiment                                                                
         [mol-%]         0       6     24                                 
______________________________________                                    
Granulate                                                                 
1        1.0              98     178   276                                
2        2.5              95     179   252                                
A        0                95     156   220                                
Powder                                                                    
3        0.5             120     338   482                                
4        1.0             120     330   490                                
5        2.5             112     258   418                                
6        5.0              95     214   348                                
7        1.0             106     258   354                                
B        0               118     190   280                                
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 *[SIPS] = 5Sulfoisophthalate                                             
 
    
     
                       TABLE 2                                                     
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Recondensation                                                            
                     J [cm.sup.3 /g] after a                              
Salt                 Recondensation Period                                
Ex-                 Content  of [h]                                       
periment            [mol-%]  0     6     24                               
______________________________________                                    
 8     NaK.sub.2 [SIPS]*                                                  
                    1.0      118   266   370                              
 9     NaCa[SIPS]*  1.0      120   275   422                              
10     Na.sub.2 Isophthalate                                              
                    1.0      115   232   378                              
11     Na.sub.2 Isophthalate                                              
                    2.5      113   248   408                              
12     Na.sub.2 Oxalate                                                   
                    1.0      120   278   486                              
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 *[SIPS] = 5Sulfoisophthalate                                             
 
    
     
                       TABLE 3                                                     
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Recondensation and Hydrolysis Stability                                   
Content of    J [cm.sup.3 /g] after a Recon-                              
                               Decrease in                                
Na.sub.3 [SIPS]*                                                          
              densation Period of [h]                                     
                               Viscosity                                  
Experiment                                                                
        [mol-%]   0      6      24     [%]                                
______________________________________                                    
D       0         95     122    149    65                                 
E       1.0       79     --     --     62                                 
F       2.5       78     120    150    62                                 
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 *[SIPS] = 5Sulfoisophthalate                                             
 
    
     
                       TABLE 4                                                     
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Hydrolysis Stability                                                      
          Content of            Decrease in                               
          Na.sub.3 [SIPS]*                                                
                       J.sub.o  Viscosity                                 
Experiment                                                                
          [mol-%]      [cm.sup.3 /g]                                      
                                [%]                                       
______________________________________                                    
13        0.5          158**    ***                                       
14        1.0          98       12                                        
15        1.0          162**     3                                        
16        2.5          95       11                                        
17        2.5          178**     8                                        
18        2.5          242**     7                                        
C         0            109      45                                        
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 *[SIPS] = 5Sulfoisophthalate                                             
 **Recondensed at 210° C. under a waterjet vacuum.                 
 ***Viscosity number increases.                                           
 
    
     The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples. From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.