Abstract:
It has been suggested that a high strength polycaproamide fiber having excess number of carboxyl end groups over amino end groups can be produced by melt spinning a polymer prepared by polymerizing e-caprolactam and reacting the polymer with a dibasic carboxylic acid containing at least six carbon atoms. However, with incorporation of this dicarboxylic acid reactant in continuous operation, serious problems have been encountered in melt-spinning of the polymer due to the frequent occurrence of &#34;nubs&#34; or enlarged places in the extruded polyamide filament. It has now been found that the occurrence of said nubs in the fiber can be greatly reduced by utilizing certain cyclic tetracarboxylic acid dianhydrides and reacting the dianhydride at the end of the polymerization. Moreover, dyeing properties of the fiber may be more readily controlled.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of U.S. application Ser. No. 622,273, filed Oct. 10, 1975. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a process for the melt-spinning of a filamentary structure from a synthetic polyamide polymer. More particularly, it is concerned with a continuous process for the formation of high strength polycaproamide filament, yarn or the like by melt-spinning a synthetic linear fiber-forming polycaproamide having excess number of carboxyl and groups over amino end groups. 
     U.S. Pat. No. 2,174,527 discloses that dibasic dicarboxylic acids when employed in excess in a diacid-diamine polyamide, serve to terminate the polymer, thereby minimizing further increase in polymer molecular weight. 
     U.S. Pat. No. 2,241,321 teaches polymerization of caprolactam in the presence of a diamine, followed by addition of sebacic acid and completion of the polymerization. The sebacic acid apparently acts as a chain-extending agent. 
     More recently, U.S. Pat. No. 3,386,967 discloses that a high strength polycaproamide yarn having an excess number of carboxyl end groups over amino end groups may be produced from a high molecular weight polymer prepared by polymerizing e-caprolactam and reacting the polymer with a dibasic carboxylic acid containing at least six carbon atoms. Because polycondensation of the polymer in the presence of a dibasic carboxylic acid is relatively slow, the acid is desirably included at the start of the polymerization reaction. Even so, equilibrium is approached in the polymerization reaction mixture at number average molecular weight not above about 20,000 under usual polymerization conditions, and special measures are required to carry the reaction further. One particularly useful method of accomplishing the required further reaction is to remove volatile by-products of the polymerization such as water by flowing an inert gas across the reaction mixture surface, desirably followed by application of vacuum. 
     Although the process of U.S. Pat. No. 3,386,967 constitutes a major contribution to this art, we have found that in continuous operation of the process on a commercial basis, particularly at maximum production rates, serious problems have been encountered in melt spinning due to the frequent occurrence of &#34;nubs&#34; in the fiber. The term &#34;nubs&#34; is conventionally applied and is used herein to mean enlarged sections of filament no more than several filament diameters in length. Nubs may be formed by a foreign, nonorientable substance which interferes with normal fiber stretch in a short section, resulting in an enlargement. Foreign substances which are believed to have contributed to nubs in the present instance include carbonized polymer and gels formed in the polymer. Gels appear to be the chief cause, i.e., the nubs are probably created by non-orientable gel from cross-linked polymer. Thermal degradation of the polymer may be an important causative factor. 
     The reactions in thermal degradation of polyamides containing dicarboxylic acid additives are not entirely understood. It is likely that thermal degradation produces a decomposition product which serves to form cross-links between amide groups and adjacent polymer chains. The decomposition reaction proceeds slowly, finally building up a three-dimensional network of molecules which may be called polymer gel and which eventually reaches the stage where it forms an infusible coating on the walls of the equipment. 
     A serious difficulty which arises from the formation of this polymer gel on the interior walls is that from time to time pieces break off and get into the flowing polymer stream where they produce damage to the spinning equipment. 
     The greatest difficulty, however, is caused by polymer gel which has progressed to the three-dimensional structural stage, but which has not yet reached the stage of being infusible. This kind of polymer gel is readily carried with the stream of flowing polymer. Being still molten or at least softened, it passes through the pump and even through the filter medium to show up either as discontinuities or as viscosity differences in the spun filament. When these filaments are later drawn, these defects may cause breaks in the filaments which either cause the whole thread to break or else form nubs which go through to be counted as quality defects in the final yarns. 
     SUMMARY OF THE INVENTION 
     It is an object of this invention to avoid the above difficulties by minimizing gel formations in the molten polyamide. Another object is to avoid accumulation of polymer gel on the walls of the reactor, in the pump, or in the filtering medium when melt-spinning the polyamide. A further object is to improve the uniformity and quality of filaments or fibers formed from the molten polymer; in particular to minimize nub formation in the filaments. Other objects will become apparent from the disclosure and the appended claims. 
     These objects are accomplished by the present invention which provides an improvement in the process for the formation of polycaproamide fiber from a fiber-forming polycaproamide polymer having excess number of carboxyl end groups over amino end groups, involving melt-extruding the polymer through an orifice into a quenching medium and thereafter stretching the resulting filament, the improvement comprising: 
     a. forming a molten polymerization mixture at 240°-290° C. from e-caprolactam; 
     b. smoothly stirring said polymerization mixture while flowing over the surface thereof an inert gas at a flow rate of at least 1 unit volume of said gas, measured at standard temperature and pressure, per hour per each unit volume of said polymerization mixture, until the total primary amino group plus carboxyl group analysis of the resulting hot water washed and dried polymer is between 95 and 135 milliequivalents per kilogram of polymer; and 
     c. reacting said polymer at 250°-290° C. with about 0.06-0.7 mol, preferably about 0.1-0.4 mol, per 100 mols of lactam in the polymer, of a cyclic tetracarboxylic acid dianhydride selected from the group consisting of pyromellitic dianhydride; 3,3&#39;,4,4&#39;-benzophenone tetracarboxylic dianhydride; 1,4,5,8-naphthalenetetracarboxylic dianhydride; bicyclo[2.2.2.[oct-7-ene-2,3,5,6,tetracarboxylic dianhydride; tetrahydrofuran-2,3,4,5-tetracarboxylic dianhydride; and phenylenebis-(3-butane-1,2-dicarboxylic anhydride) until the primary amino group analysis of the polymer is between 5 and 35 milliequivalents, preferably between 5 and 30 milliequivalents, per kilogram of polymer and the carboxyl group analysis of the polymer is between 30 and 90 milliequivalents per kilogram of polymer, thereby reducing the occurrence of nubs in the fiber. 
     As indicated hereinabove, the present invention may be considered an improvement over the disclosure of U.S. Pat. No. 3,386,967 which is incorporated herein by reference. 
     The dianhydrides employed in the present invention are known compounds which are commercially available. In general, the dianhydrides are characterized as being thermally stable and non-volatile under the conditions of polymerization. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will now be further described in the following specific examples which are to be regarded solely as illustrative and not as restricting the scope of the invention. All parts and percentages are by weight unless otherwise indicated. 
     EXAMPLE 1 
     Over a period of about 3 weeks, about 100 parts per hour of e-caprolactam was continuously fed to a stirred reactor operating at a temperature of 265° C., together with steam at a pressure of about 50 p.s.i.g. to form a prepolymer melt. A trace amount of copper compound soluble in the reaction mixture was incorporated in the reaction mixture as heat stabilizer. 
     Polymerization was continuously accomplished in a series of three stirred reactors operated at conditions described in U.S. Pat. No. 3,109,835 to Apostle, i.e., the water and part of the unreacted lactam was continuously removed from the prepolymer melt by smoothly stirring the reaction melt at about atmospheric pressure at a temperature of about 265° C. while sweeping the surface of the smoothly stirred reaction mixture with dry nitrogen gas at a rate of about 3-4 unit volumes of gas measured at standard temperature and pressure (STP) per hour per unit volume of the reaction mixture, for approximately 4 hours. The final reactor may be operated at reduced pressure if desired. 
     The resulting polymer was continuously extruded into a warm water bath and chopped into pellets 1/10 by 1/10 inch in size. The pellets were then hot water washed at 100° C. to reduce the content of hot water soluble constituents to about 1-2%; and the pellets were dried to less than 0.1% moisture. The polymer was a white solid having a relative viscosity of about 55 to 60 as determined by concentration of 11 grams of polymer in 100 milliliters of 90 percent formic acid at 25° C. (ASTMD-789-62T). The total primary amino group plus carboxyl group analysis of the polymer was 105-115 milliequivalents per kilogram of polymer. The polycaproamide pellets were melted at 260°-285° C. and reacted for 15 minutes with about 0.2 mol of pyromellitic dianhydride per 100 moles of lactam in the polymer, then melt-extruded under a pressure of about 1500 p.s.i.g. through a 70-orifice spinnerette to produce 3900-denier fiber. The fiber was then collected and drawn about 3 times its extruded length to produce a 1300-denier yarn. For convenience, this yarn hereinafter will be called Yarn A. The polymer of Yarn A had a total primary amino group plus carboxyl group analysis of 107 milliequivalents per kilogram of polymer and a primary amino group analysis of 30 milliequivalents per kilogram of polymer. 
     In order to provide for comparative testing of polycaproamide made with a dicarboxylic acid additive, Yarn B having about the same total primary amino group plus carboxyl group analysis and primary amino group analysis per kilogram of polymer as Yarn A was prepared by the general procedure of this example except that sebacic acid at a concentration of 0.2 mol percent of the lactam in the polymerization reaction mixture was fed to the prepolymer reactor with the e-caprolactam. Yarn A and Yarn B both showed a good tensile strength of about 9.5 grams per denier. Yarn A and Yarn B were also tested for the number of nubs per pound as shown in Example 2. 
     EXAMPLE 2 
     This example outlines the method used for locating, identifying and calculating the nubs per pound in Yarn A and Yarn B as prepared in Example 1. In this method a nub is defined as an enlarged place in a filament which is no more than several filament diameters in length. This method may be used for either monofilament or multifilament yarns; however, it is not applicable to most types of crimped yarn. 
     In accordance with the test, the 1300-denier yarn is drawn directly from the package by means of an air aspirator and is passed through an opening of known width, specifically, 0.006 inch in width. Such an opening is conveniently provided by use of a ceramic cleaner gap, which is well-known in the art. The presence of a nub is detected when it stops the yarn passage through the opening. The filaments are separated and the cause of the yarn stopping identified as a nub or as the twisted end of a broken filament. For representative results, about 10 pounds of yarn is passed through the gap and the number of nubs counted. Table I below shows the results of testing on Yarn A and Yarn B. 
     
                       Table I______________________________________Determination of Nubs per Pound            Average            Nub Count perSample           Pound of Yarn______________________________________Yarn A            4.2Yarn B           14.4______________________________________ 
    
     The individual standard deviation in this test was 1.8 nubs per pound of yarn, so that the standard deviation of the average for the 10-pound sample was about 0.57 nubs per pound. Accordingly, the difference between the two averages, i.e., 10.2 nubs per pound, was statistically significant at the 99.9% level of significance. 
     EXAMPLE 3 
     A yarn was produced according to the procedure of Example 1 except that the polymerization reaction was initiated with 5 parts of aminocaproic acid instead of steam and 0.2 mol of 3,3&#39;,4,4&#39;-benzophenone tetracarboxylic dianhydride was used instead of the pyromellitic dianhydride. The polymer of the resulting yarn had a total primary amino group plus carboxyl group analysis of 106 milliequivalents per kilogram of polymer and a primary amino group analysis of 30 milliequivalents per kilogram of polymer. The average nub count of the yarn was 4.0 per pound of yarn. 
     EXAMPLE 4 
     A yarn was produced according to the procedure of Example 1 except that the polycaproamide pellets were melted at 260°-280° C. and reacted for 2.3 minutes with 0.2 mol of 1,4,5,8-naphthalenetetracarboxylic dianhydride per 100 mols of lactam in the polymer. The polymer in the resulting yarn had a total primary amino group plus carboxyl group analysis of 105 milliequivalents per kilogram of polymer and a primary amino group analysis of 28 milliequivalents per kilogram of polymer. The average nub count of the yarn was 2.8 per pound of yarn. 
     EXAMPLE 5 
     A yarn was produced according to the procedure of Example 1 except that the polycaproamide pellets were melted at about 270° C. and reacted for 1.6 minutes with 0.2 mol of bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride per 100 mols of lactam in the polymer. The polymer in the resulting yarn had a total primary amino group plus carboxyl group analysis of 107 milliequivalents per kilogram and a primary amino group analysis of 34 milliequivalents per kilogram of polymer. The average nub count of the yarn was 3.1 per pound of yarn. 
     EXAMPLE 6 
     A yarn was produced according to the procedure of Example 1 except that the polycaproamide pellets were melted at about 265° C. and reacted for 3 minutes with 0.3 mol of tetrahydrofuran-2,3,4,5-tetracarboxylic dianhydride per 100 mols of lactam in the polymer. The polymer in the resulting yarn had a total primary amino group plus carboxyl group analysis of 78 milliequivalents per kilogram of polymer and a primary amino group analysis of 18 milliequivalents per kilogram of polymer. The average nub count of the yarn was 2.9 per pound of yarn. 
     EXAMPLE 7 
     A yarn was produced according to the procedure of Example 1 except that the polycaproamide pellets were melted at about 260° C. and reacted for 5 minutes with 0.14 mol of phenylenebis-(3-butane-1,2-dicarboxylic anhydride) per 100 mols of lactam in the polymer. The polymer in the resulting yarn had a total primary amino group plus carboxyl group analysis of 75 milliequivalents per kilogram and a primary amino group analysis of 10 milliequivalents per kilogram of polymer. The average nub count of the yarn was 3 per pound of yarn. 
     EXAMPLE 8 
     In further tests, a polycaprolactam polymer having a relative viscosity of 23, as determined by the above-described test, a primary amino group analysis of 91 milliequivalents per kilogram of polymer and a carboxyl group analysis of 36 milliequivalents per kilogram of polymer was melted at about 270° C. and reacted for 15 minutes with 0.34 mol of pyromellitic dianhydride per 100 mols of lactam. The resulting polymer had a relative viscosity of 50 by the above-described test. In other runs it was shown that the relative viscosity could be further increased by increasing the concentration of the dianhydride. Thus, considerable flexibility can be built into the process by varying the relative viscosity of the polymer, or by using a polymer with an excess of amine ends, or by adding various amounts of the dianhydride. 
     EXAMPLE 9 
     In further tests, a polycaprolactam polymer having a relative viscosity of 50.6, as determined by the above-described test, a primary amine group analysis of 50 milliequivalents per kilogram of polymer, and a carboxyl group analysis of 50 milliequivalents per kilogram of polymer was melted at 260° C. and agitated with 0.088 mol of 3,3,4,4&#39;-benzophenonetetracarboxylic dianhydride per 100 mols of lactam in the polymer. After 1, 2, 5 and 10 minutes of agitation, samples of the polymer were withdrawn from the agitated melt, and the samples were analyzed for relative viscosity as determined by the above-described test, and primary amine groups. Results are shown in the following table: 
     
         ______________________________________                   Primary Amine Groups,Agitation Time,        Relative   Milliequivalents PerMinutes      Viscosity  Kilogram of Polymer______________________________________1            72.9       16.32            71.2       16.95            71.1       17.810           69.2       18.2______________________________________ 
    
     These results show that the reaction between the polymer and cyclic tetracarboxylic dianhydride is relatively fast, i.e., a reaction time of 1 minute was ample. In other tests, reaction times up to 30 minutes were found suitable for use in the present invention. 
     EXAMPLE 10 
     The procedure of Example 9 was followed except that the 3,3&#39;,4,4&#39;-benzophenonetetracarboxylic dianhydride was replaced with 0.2 mol of phenylene-bis-(3-butane-1,2-dicarboxylic anhydride). Polymer products were analyzed as in Example 9, and results are shown in the following table: 
     
         ______________________________________                   Primary Amine Groups,Agitation Time,        Relative   Milliequivalents PerMinutes      Viscosity  Kilogram of Polymer______________________________________1            67.9       5.42            70.0       5.05            68.7       5.110           69.9       6.7______________________________________ 
    
     In similar tests, the number of primary amine groups was increased to about 22 milliequivalents per kilogram of polymer when the addition of this cyclic tetracarboxylic acid dianhydride was reduced to 0.07 mol per 100 mols of lactam in the polymer.