Control of viscosity and polycaproamide degradation during vacuum polycondensation

Abstract

Control of polycaproamide degradation during vacuum polycondensation in a polymer finisher is achieved by maintaining a partial pressure of water above the polymer melt above about 10 mm. Hg, preferably above about 30 mm. Hg, absolute pressure. Degradation is minimized to within 3, preferably 2 units of the theoretical difference between carboxyl and amine ends. The water extractables content of the polymer is also controlled to below about 3.5% by weight, preferably 2.5%; viscosity increase of the polymer melt levels out after less than 4 hours to less than 10 FAV units per hour, by means of the invention.

What is claimed is: 
     
       1. In a process for the production of polycaproamide shaped articles from molten anhydrous nascent polymer comprising: a. continuously polymerizing ε-caprolactam at an elevated pressure and temperature in the presence of small amounts of water and a terminating agent to form a prepolymer melt, then   b. continuously removing most of the water and part of unreacted lactam from said prepolymer melt by exposure to vacuum in a vessel providing said prepolymer melt with large surface area-to-volume ratio, residence time in said vessel being about 10 minutes, .Iadd.wherein said polymer is not contacted with a gas following said polymerization, .Iaddend.then   c. continuously transferring said melt to a surface renewal device also having large surface area-to-volume ratio, which promotes completion of the polymerization reaction by polycondensation, and removes residual amounts of vaporizable materials, at a temperature between about 225°C. and about 300°C., and a residence time of more than 1 hour, then   d. continuously extruding said melt, and finally   e. cooling said extruded polymer to form uniformly shaped articles the improvement .[.comprising.]. .Iadd.consisting essentially of the combination of .Iaddend.   carrying out step (c) at an absolute pressure of between about 31 mm. and about 150 mm. of Hg.,   subjecting said melt in step (c) to be sweeping flow of steam, and   maintaining the partial pressure of said steam in the vapor above said melt at above about 30 mm Hg.   .[.so that.]. .Iadd.keeping .Iaddend.the extractables content of said .[.polycaproamide.]. .Iadd.polycaprolactam .Iaddend.at the end of said polymerization .[.is.]. below 3.5% by weight,   .Iadd.minimizing .Iaddend.thermal degradation of the polymer in the melt .[.is minimized.]. to within 3 units of the theoretical difference of ends, and   .Iadd.controlling .Iaddend.the rate viscosity increase of the polymer melt .Iadd.to .Iaddend.level.[.s.]. out after less than four hours to less than 10 FAV units per hour.   
     
     
       2. The process of claim 1 wherein said viscosity increase is less than 8 FAV units after less than 3 hours, and thermal degradation is minimized to within 3 units of the theoretical difference of ends. 
     
     
       3. The process of claim 1 wherein said partial pressure of steam is maintained by sweeping steam overhead and counter to the flow of said polymer melt.