Abstract:
Solutions of polyamide alkyl esters are stabilized by incorporation of a select acidic compound, which inhibits premature imidization of the ester.

Description:
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__________________________________________________________________________Viscosity Behavior Of SolutionsExampleorComparative  Acidic       Viscosity (cs) - DaysExample No.  Compound Amine               0  1  2  3  4  5  6  7  8__________________________________________________________________________CE-1   None     None               445                  447                     447                        448      454                                    456CE-2   None     NMM 447                  454                     469                        488      582                                    6641      O-Phthalic Acid           NMM 447                  450      455                              457CE-3   Octanoic Acid           NMM 448                  454      513                              543CE-4   PMDA-ODA NMM 449                  455                     471                        491      581  Amic AcidCE-5   Octanoic Acid           NMM 449                  456      511   585CE-6   Succinic Anhy.           NMM 452                  461      520   595CE-7   Succinamic Acid           NMM 452                  458      511   5852      O-Phthalic Anhy.           NMM 449                  451      463   473CE-8   None     NMM 449                  456      521   601CE-9   None     NBA 494                  526      545                              549      562CE-10  None     None               454                  457      463                              466      474CE-11  Succinamic Acid           NBA 487                  490      5073      Succinic Anhy.           NBA 472                  476      483                              485      489CE-12  Succinic Acid           NBA 487                  493      506CE-13  Octanoic Acid           NBA 488                  494      506CE-14  Gallic Acid           NBA 494                  502      521                              527CE-15  None     None               467                  470      478CE-16  None     NMM 466                  482      589CE-17  Succinamic Acid           NMM 470                  487      625CE-18  Succinic Anhy.           NMM 467                  500      715CE-19  Succinic Acid           NMM 471         581CE-20  Octanoic Acid           NMM 466         586CE-21  Gallic Acid           NMM 472         596CE-22  None     None               472                  474      479   483.5CE-23  None     NMM 475                  490      650   GEL4      Citric Acid           NMM 478                  478      490   505.25      α-Tartaric Acid           NMM 474                  477      506   539.36      Oxalic Acid           NMM 470                  470      472   475.77      Maleic Anhy.           NMM 474                  480      462   470.8__________________________________________________________________________ *Note: Anhy. represents anhydride.