Stabilization of polyamide alkyl ester solutions
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This invention relates to a method of stabilizing the viscosity of a solution of a polyamide alkyl ester solution. The invention also relates to compositions whose viscosity has been stabilized that are comprised of a polyamide alkyl ester. 
     2. Description of the Prior Art 
     Polyimide coatings are known in the art, as shown, for example, in U.S. Pat. No. 4,467,000, issued to Economy, et al. on Aug. 21, 1984. Polyimide materials have been recently investigated by the semiconductor industry for use as insulators in multilevel interconnection systems. Polyimide insulating coatings having particularly desirable planarization characteristics have been prepared from polyamide alkyl esters formed from a pyromellitic alkyl diester and a para-linked aromatic diamine, as disclosed in U.S. Pat. No. 4,612,210, issued to Hofer, et al. on Sep. 16, 1986. 
     Use of solutions containing a polyamide alkyl ester presents a problem, however, in that these solutions may gel prematurely, leading to a reduced shelf life of only a few weeks and to formation of non-uniform coatings. There is a need in the art, therefore, for a method for stabilizing over time the viscosity of a solution of a polyamide alkyl ester. 
     The prior art does not, however, provide such a viscosity stabilization technique suitable for these purposes. Reference is made, for example, to U.S. Pat. No. 4,757,098, issued to Merrem, et al. on Jul. 12, 1988, which discloses a method of stabilizing a polyamide ester polymer precursor against non-photoinduced crosslinking, by adding to a solution of the precursor a chelate complex-former, such as EDTA. 
     SUMMARY OF THE INVENTION 
     Against this background, it has now been discovered, according to the invention, that gelation of such solutions occurs because of amine impurities that are in the solution. Presence of these amines may result from the synthesis of the ester; see, for example, U.S. Pat. No. 4,612,210, cited above, where N-methylmorpholine is used as an acid scavenger. Or, an alkyl amine, such as methyl amine, may be introduced as a residual impurity in the solvent employed, commonly, an amide-type solvent, e.g. N-methyl-2-pyrrolidone. It is now believed, according to the invention, that these amines serve to prematurely catalyze imidization of the ester precursor to the corresponding polyimide. The solution gels to form globular precipitates, producing the undesired results noted above, i.e. significantly reduced shelf life and the eventual formation of coatings with unsatisfactory characteristics, such as non-uniformity, dewetting from the substrate and so forth. 
     In order to overcome this, in accordance with the invention, an effective amount of an acidic compound is incorporated into the solution of the polyamide alkyl ester. A small amount, generally less than 0.1 percent by weight of the total solution, of a select acidic compound is employed to neutralize the amine impurities, but without adversely affecting the properties of the resulting polyimide product. 
     Typically, the solution of the polyamide alkyl ester, comprises as the solvent for the ester, a conventional amide-type solvent, such as N-methyl-2-pyrrolidone. It is noted that various techniques have been developed for treating pyrrolidone for purification purposes. For example, in U.S. Pat. No. 2,806,856, issued to Robinson on Sep. 17, 1957, pyrrolidone is heated to a temperature below the distillation temperature, and is treated first with a carboxylic acid anhydride and then with a non-volatile, strong base, such as sodium or potassium hydroxide. And, in U.S. Pat. No. 3,140,294, issued to Kolyer on Jul. 7, 1964, pyrrolidone is treated first with concentrated sulfuric acid, then with activated carbon; the activated carbon is removed, and the treated pyrrolidone is distilled to remove additional impurities. 
     Amide-type solvents have also been stabilized against degradation and discoloration effects of heat and light, by incorporation of an acyl chloride or an acid anhydride. See U.S. Pat. No. 3,393,170, issued to Koblitz, et al. on Jul. 16, 1968. 
     None of these prior approaches, teaches a method for stabilizing the viscosity of a solution of a polyamide alkyl ester to inhibit premature imidization. 
    
    
     DETAILED DESCRIPTION 
     In accordance with the invention, any acidic compound which is effective in neutralizing the amine impurities in the solution may be employed. Generally, the acidic compound is an organic acid or its corresponding anhydride. Generally, it has been found that the acidic compound employed should have a pKa&lt; about 3. Preferably, the acidic compound is a carboxylic or dicarboxylic acid or its corresponding anhydride having a pKa&lt; about 3. A particularly preferred group of acidic compounds for use in accordance with the invention, includes o-phthalic acid, o-phthalic anhydride, citric acid, α-tartaric acid, oxalic acid and maleic anhydride. 
     The amount of acidic compound incorporated into the solution can vary widely, but it should be employed in an amount which is sufficient to react with substantially all the amine impurities present in the solution. Preferably, the acidic compound is used in excess of that necessary to neutralize all the amine impurities. Generally, the amount of acidic compound incorporated into the solution ranges from about 0.01 percent to about 0.1 percent by weight, based on the weight of the total solution. 
     Preferably, the polyamide alkyl ester is formed from a pyromellitic alkyl diester or a biphenylene dianhydride-derived alkyl diester and a para-linked aromatic diamine. Such diesters are commonly obtained by alcoholosis of pyromellitic dianhydride (PMDA) or biphenylene dianhydride (BPDA), preferred compounds including the dimethyl ester, the diethyl ester and the dipropyl ester. Examples of suitable para-linked aromatic diamines include p-phenylene diamine; 4,4&#39;-diamino phenyl sulfone; 4,4&#39;-diamino biphenyl; bis(4&#39;-amino phenoxy)-1,4-benzene; 3,3&#39;,5,5&#39;-tetramethyl benzidine; and 4,4&#39;-diamino octafluoro benzidine. A preferred embodiment of the invention employs poly(4,4&#39;-phenoxyphenylene pyromellitamide ethyl diester) (PMDA-ODA ethyl diester). A particularly suitable method for preparing such polyimide alkyl esters is disclosed in U.S. Pat. No. 4,612,210, the entire disclosure of which is incorporated herein by reference. 
     Examples of suitable solvents are amide-type solvents including amides, lactams and ureas, such as 
     N,N-dimethyl-formamide; N,N-dimethylacetamide; 
     N-methyl-2-pyrrolidone; 
     N-cyclohexyl-2-pyrrolidone; N,N-diethylacetamide; 
     N,N-diethyl-formamide; 
     N,N-dimethyl-γ-hydroxybutyramide; 
     N,N-dimethylmethoxyacetamide; N,N-dibutylformamide; 
     N-methylpropionamide; 2-piperidone; 
     N-methyl-2-piperidone; N-ethyl-2-pyrrolidone; 
     N-isopropyl-2-pyrrolidone; 5-methyl-2-pyrrolidone; and tetramethylurea. 
     Other suitable solvents include lactones, such as γ-butyrolactone, furfural alcohol and dimethylsulfoxide (DMSO). A preferred embodiment of the invention employs N-methyl-2-pyrrolidone. The amount of solvent employed can vary over wide limits, but in most situations varies from about 75 to about 95 percent, and preferably from about 80 to about 90 percent, of total solution. 
     In carrying out the method of the invention, an effective amount of the acidic compound is incorporated into a solution of the polyamide alkyl ester in a suitable solvent. This may include adding the acidic compound, after the polyamide alkyl ester has been dissolved in the solvent; or, preferably, adding the acidic compound to the solvent before it is mixed with the ester to form the solution. By first adding the acidic compound to the solvent, neutralization of any amine impurities in the solvent can take place prior to admixture with the ester. 
     Compositions of the invention, which include an acidic compound, exhibit a relatively stable viscosity over a period of time, thus satisfactorily achieving extended shelf life without gelling. Further, the acidic compound does not adversely affect the properties of the polyimide coating which is produced from the composition. No specific, additional storage conditions are required after incorporation of the acidic compound to achieve the stabilizing effect of the invention. 
     The following examples are provided to illustrate the invention. 
     EXAMPLES AND COMPARATIVE EXAMPLES 
     Solutions were prepared containing 17 percent by weight of a polyamide alkyl ester, poly (4,4&#39;-phenoxy-phenylene pyromellitamide ethyl diester), 74.7 percent by weight of N-methyl-2-pyrrolidone and 8.3 percent by weight of N-cyclohexyl-2-pyrrolidone. The solutions were then doped with 20 ppm of an amine, either N-methylmorpholine (NMM) or n-butylamine (NBA). Then, 100 ppm (0.01 percent by weight) of an acidic compound was added to the solution. 
     The viscosity of each solution was monitored using a size 400 Cannon-Fenske Routine Viscometer in a water bath held at 35° C., in accordance with standard techniques. (See ASTM D 445 and D 2515.) The results are summarized in TABLE I below. In general, stabilization occurred if the viscosity of the solution remained below about 500 centistokes (cs) over time. 
     
                                           TABLE I                                 
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Viscosity Behavior Of Solutions                                           
Example                                                                   
or                                                                        
Comparative                                                               
       Acidic       Viscosity (cs) - Days                                 
Example No.                                                               
       Compound Amine                                                     
                    0  1  2  3  4  5  6  7  8                             
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CE-1   None     None                                                      
                    445                                                   
                       447                                                
                          447                                             
                             448      454                                 
                                         456                              
CE-2   None     NMM 447                                                   
                       454                                                
                          469                                             
                             488      582                                 
                                         664                              
1      O-Phthalic Acid                                                    
                NMM 447                                                   
                       450      455                                       
                                   457                                    
CE-3   Octanoic Acid                                                      
                NMM 448                                                   
                       454      513                                       
                                   543                                    
CE-4   PMDA-ODA NMM 449                                                   
                       455                                                
                          471                                             
                             491      581                                 
       Amic Acid                                                          
CE-5   Octanoic Acid                                                      
                NMM 449                                                   
                       456      511   585                                 
CE-6   Succinic Anhy.                                                     
                NMM 452                                                   
                       461      520   595                                 
CE-7   Succinamic Acid                                                    
                NMM 452                                                   
                       458      511   585                                 
2      O-Phthalic Anhy.                                                   
                NMM 449                                                   
                       451      463   473                                 
CE-8   None     NMM 449                                                   
                       456      521   601                                 
CE-9   None     NBA 494                                                   
                       526      545                                       
                                   549      562                           
CE-10  None     None                                                      
                    454                                                   
                       457      463                                       
                                   466      474                           
CE-11  Succinamic Acid                                                    
                NBA 487                                                   
                       490      507                                       
3      Succinic Anhy.                                                     
                NBA 472                                                   
                       476      483                                       
                                   485      489                           
CE-12  Succinic Acid                                                      
                NBA 487                                                   
                       493      506                                       
CE-13  Octanoic Acid                                                      
                NBA 488                                                   
                       494      506                                       
CE-14  Gallic Acid                                                        
                NBA 494                                                   
                       502      521                                       
                                   527                                    
CE-15  None     None                                                      
                    467                                                   
                       470      478                                       
CE-16  None     NMM 466                                                   
                       482      589                                       
CE-17  Succinamic Acid                                                    
                NMM 470                                                   
                       487      625                                       
CE-18  Succinic Anhy.                                                     
                NMM 467                                                   
                       500      715                                       
CE-19  Succinic Acid                                                      
                NMM 471         581                                       
CE-20  Octanoic Acid                                                      
                NMM 466         586                                       
CE-21  Gallic Acid                                                        
                NMM 472         596                                       
CE-22  None     None                                                      
                    472                                                   
                       474      479   483.5                               
CE-23  None     NMM 475                                                   
                       490      650   GEL                                 
4      Citric Acid                                                        
                NMM 478                                                   
                       478      490   505.2                               
5      α-Tartaric Acid                                              
                NMM 474                                                   
                       477      506   539.3                               
6      Oxalic Acid                                                        
                NMM 470                                                   
                       470      472   475.7                               
7      Maleic Anhy.                                                       
                NMM 474                                                   
                       480      462   470.8                               
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 *Note: Anhy. represents anhydride.