1. Field of The Invention
This invention relates to poly-2pyrrolidone compositions containing a small amount of certain polyesters. In another aspect, this invention relates to methods of extruding such compositions.
2. The Prior Art
Poly-2-pyrrolidone is the source of a useful synthetic fiber for the textile industry. The melt-sprinnable white solid polymer is produced by the alkaline-catalyzed polymerization of 2-pyrrolidone in the presence of carbon dioxide (see U.S. Pat. No. 3,721,652). Polypyrrolidone so-produced is melt-spun into filaments by extrusion from multi-hole spinnerets. In meltspinning, the polymer composition is extruded in a molten condition at a melt temperature which is generally greater than about 270.degree. C. The extrusion must be carried out with care because of the tendency of the polymer to thermally degrade and revert to monomer. Degradation produces an unacceptable extrudate containing foam or bubbles. If extrusion is attempted at appreciably lower temperatures to avoid thermal decomposition, fibers of lower tensile strength are produced. Consequently, in order to melt extrude polypyrrolidone efficiently, one may either seek to increase the thermal stability of the polymer, or to improve the extrudability of the polymeric composition.
The unusual propensity of poly-2-pyrrolidone to thermally degrade back to its monomer and the adverse effect on melt spinning or extrusion is recognized to be a serious problem and the prior art has made a number of attempts to mitigate this problem. For example, U.S. Pat. No. 3,052,654 attempts to reduce the degradation during melt extrusion by pre-heating the poly-2-pyrrolidone to remove degradation products, e.g., pyrrolidone. U.S. Pat. No. 3,017,393 teaches that the extrusion problem may be alleviated by washing or treating the polymer with an aqueous organic carboxylic acid solution and U.S. Pat. No. 3,072,615 teaches using dilute aqueous fatty acid.
U.S. Pat. No. 3,009,893 teaches that the problem may be reduced by the incorporation of relatively large amounts of certain substances which apparently function as melting point depressants.
The prior art has also proposed various other additives to mitigate this problem, however, the problem is necessarily empirical in nature and until a given substance has actually been tried, it cannot be predicted whether it will have a beneficial effect.
It has now been discovered that a significant reduction in thermal monomer reversion can be made by the incorporation of very small amounts of certain polyesters. This is particularly surprising since tests using dioctylphthalate showed substantially lower enhancements in thermal stability.
U.S. Pat. No. 3,097,181 broadly suggests blending various filament-forming polymers (e.g., nylon 66, nylon 6, polyesters, acrylics, polyvinyl resins, polyolefins, cellulose triacetite, etc.) with poly-2-pyrrolidone in order to obtain a fiber having a desirable combination of the properties of poly-2-pyrrolidone and other polymer.