1. Field of the Invention
This invention relates to a process for the hydrolysis of adiponitrile and production of nylon 6,6. More specifically, the invention relates to the catalytic hydrolysis of adiponitrile at low catalyst levels.
2. Description of Related Art
Polyamides are currently produced commercially from dicarboxylic acids and diamines. For example, nylon 6,6 is commercially produced from adipic acid and hexamethylenediamine. Alternate routes to these materials have been explored within the art. For example, the art has taught processes to produce nylon 6,6 from adiponitrile and hexamethylenediamine. U.S. Pat. No. 2,245,129 discloses a process to produce polyamides by reacting adiponitrile, hexamethylenediamine, and water at high temperatures and pressures. A modified process to react adiponitrile with hexamethylenediamine and water was disclosed in U.S. Pat. No. 3,847,876. Catalysts were introduced into the processes to promote this reaction as disclosed by U.S. Pat. Nos. 4,490,521; 4,542,205; 4,603,192; 4,725,666; 4,749,776; 4,436,898; and 4,528,362. U.S. Pat. No. 4,501,881 discloses a process to form polyamides from adiponitrile, hexamethylenediamine, adipic acid and water. A significant shortcoming of these before mentioned processes to produce polyamides from adiponitrile and hexamethylenediamine is the production of significant levels of the dimer of the aliphatic diamine. In the case of hexamethylenediamine, the dimer product would be bis(hexamethylene)triamine (BHMT). As taught in U.S. Pat. No. 4,739,035 and U.S. Pat. No. 5,627,257 the diamine dimerization is promoted by the high temperatures and pressures required by the processes. These as formed diamine dimers or triamines serve as crosslinkers for the linear polyamide chain. The resultant gel content has been found to lead to significant product quality deterioration.
This shortcoming was partially overcome through a two step process disclosed in U.S. Pat. No. 4,739,035. The first step involved the hydrolysis of adiponitrile with water, catalysts and from 0 to 10 weight percent of the total hexamethylenediamine needed for the reaction. The second step consisted of adding the remainder of the hexamethylenediamine followed by polymerization. Such a two step process provided triamine levels in the 560 to 1,300 ppm levels versus the 1,420 to 1,610 ppm levels found through the teachings of other background art.
This shortcoming was more fully overcome through a further two step process disclosed in U.S. Pat. No. 5,627,257. The first step consisted of the nearly full hydrolysis of the adiponitrile to adipic acid utilizing a catalyst and a cocatalyst. The cocatalyst was described as a saturated aliphatic or aromatic dicarboxylic acid. The second step consisted of the addition of at least an equimolar amount of diamine followed by polymerization. Through use of this process, they were able to achieve triamine levels between 500 and 600 ppm. A shortcoming of this process was the substantial amounts of catalyst utilized. The level of the catalyst exemplified ranged from about 1 to 1.5 weight percent based on the adiponitrile level. For example, the patent teaches the use of 5 grams of orthophosphorous acid and 1.8 grams of calcium hypophosphite catalyst in the hydrolysis of 487 grams of adiponitrile, (see U.S. Pat. No. 5,627,257, Example 1).