This invention relates to a novel process whereby nylon-6 and copolymers of .epsilon.-caprolactam having discreet polyamide segments are prepared having reduced levels of water-extractable materials.
Nylon-6 is an impact resistant, heat resistant thermoplastic resin useful in the production of films, molded and extruded articles and fibers for carpets, textiles and tire cords. Copolymers of .epsilon.-caprolactam have additional uses as surfactants, dispersants and as components in coating compositions.
Nylon-6 is commercially produced by polymerizing .epsilon.-caprolactam at temperatures from about 180.degree. to 300.degree. C. in the presence of water and acetic acid. The polymerization mixture thus formed reaches an equilibrium wherein it contains about 8 to 15 weight percent water-extractable material comprising monomeric caprolactam and oligomers of caprolactam. The presence of this extractable material in the polymer causes inferior properties, e.g., weakness and softness, in the fibers, films and other articles produced therefrom. Thus, to be suitably employed in its aforementioned uses, nylon-6 and copolymers of .epsilon.-caprolactam must contain less than about 3 weight percent extractable material. Accordingly, caprolactam monomers and oligomers must be removed by extraction, vaporization or like procedures.
Generally, the monomers and oligomers are extracted by leaching the crude nylon-6 with water or steam at 95.degree. to 100.degree. C. This process, however, requires a large capital investment for equipment and also consumes substantial quantities of water and energy. In another commercially used process, .epsilon.-caprolactam monomers are vaporized from the nylon under reduced pressure, but this process does not always adequately remove the less volatile oligomers.
Because the aforementioned processes for removing the extractable material substantially increase the production costs of polycaprolactam, a process wherein nylon-6 is prepared having low levels of water-extractable material would substantially reduce production costs and is therefore highly desirable.