Typically, when a polymer is prepared by techniques such as solution polymerization, it is present in relatively dilute concentrations for ease of handling. Subsequent processing techniques to which the polymer solution will eventually be subjected such as dry-spinning, however, necessitate that the polymer be concentrated.
For example, when halogenated aromatic polyesters, such as the condensation products of 4,4'-isopropylidene-2,2', 6,6'-tetrachlorodiphenol or 4,4'-isopropylidene-2,2',6,6'-tetrabromodiphenol with isophthalic acid and/or terephthalic acid or the ester forming derivatives thereof, are prepared by the solution polymerization technique as described, for example, in U.S. Pat. No. 3,234,167, and as described herein, the final polymer solution after neutralization and removal of the acid acceptor, contains approximately 10% by weight polymer and about 3 to 5%, by weight, water in methylene chloride. This solution is too dilute for direct spinning so the polymer solution must be concentrated to about 20%, by weight, polymer.
A continuous method for neutralizing excess tertiary amine acid acceptor is disclosed in U.S. Pat. No. 4,322,521, entitled "Improved Process for Producing Halogenated Aromatic Polyesters," by Albert G. Williams. A continuous countercurrent extraction method for removing tertiary amine hydrochloride from the polymer solution is disclosed in U.S. Pat. No. 4,360,662, entitled "Continuous Countercurrent Extraction Process for Removing Water-Soluble Impurities From Water Immiscible Polymer Solutions" and filed concurrently herewith, by Albert G. Williams. The disclosures of each of the above-identified patent applications are hereby incorporated by reference.
It would be advantageous to develop a continuous method for concentrating halogenated aromatic polyester solutions, and polymer solutions generally, so that the entire process for preparing such polymer solutions from polymerization through spinning could be carried out on a continuous basis and in a manner sufficient to avoid the cost of precipitation, drying, solvent recovery and handling thereof.
One problem involved in attempting to concentrate a polymer solution, such as a 10% polymer solution of a halogenated aromatic polyester, to a more concentrated solution, such as a 20% solution, is the formation of polymer gels or "skins" on the surface of the polymer due to the poor diffusion/evaporation balance which exists therein. Also, such a polymer solution has a high viscosity (a 10% solution has a viscosity of from about 50 to about 100 poise, and a 20% polymer solution has a viscosity of from about 1000 to about 3000 poise) and notoriously low heat transfer coefficients thereby requiring large equipment and high capital outlay.
The search has continued for continuous methods for concentrating a polymer solution while avoiding the above-discussed problems of the prior art. The present invention was made as a result of this search.