Preparation of ultra-high molecular weight polyester

#There is a rapidly growing market for high molecular weight polyester resin. For instance, the market for polyethylene terephthalate bottle resin is rapidly expanding. There is also a large demand for polyethylene terephthalate having a high molecular weight for use in making reinforcements for rubber articles, such as tire cords. High molecular weight polyethylene terephthalate resin can also be used in manufacturing trays for frozen food which can be heated in either microwave ovens or convection ovens. There is a demand for higher and higher molecular weight resin in this application. For instance, there is a demand for polyethylene terephthalate tray resin which has an intrinsic viscosity of greater than 1.0 dl/g. However, polyethylene terephthalate resins having intrinsic viscosities of greater than 1.0 are not widely available due to technical difficulties associated with producing such resins utilizing standard commercial polymerization techniques. These difficulties consequently result in such resins which are produced utilizing standard commercial techniques being very expensive.

#High molecular weight polyesters are commonly produced from low molecular weight polyesters of the same composition by solid state polymerization. The low molecular weight polyesters which are used in such solid state polymerizations can be prepared by conventional melt polymerizations. Solid state polymerization is generally considered advantageous in that the handling of high molecular weight ultra-high viscosity molten polymers is eliminated during the solid state polymerization phase. Thermal degradation during the solid state portion of the polymerization is also essentially avoided.

#In melt polymerizations, molecular weight increases by esterification as well as transesterification. During the initial polymerization, esterification predominates. As the molecular weight increases, most of the carboxyl ends are consumed by esterification and water is eliminated. The final polycondensation occurs by transesterification and the elimination of glycol. The attainment of high molecular weight is hindered by reduced diffusion of the glycol polymerization by-product due to increased polymer viscosity.

#This invention is applicable to virtually any polyester which can be solid state polymerized. The most common type of polyesters which will be solid state polymerized using the technique of this invention will have at least about 75 mole percent of their acid moieties being provided by an aromatic dicarboxylic acid, such as terephthalic acid, isophthalic acid, or a naphthalinic dicarboxylic acid (preferably 2,6-) with their diol moieties being provided by glycols such as ethylene glycol, butylene glycol, 1,4-dimethylol cyclohexane and the like or aromatic diols such as hydroquinone and catechol. Such polyesters can also contain other dicarboxylic acids such as adipic acid, isophthalic acid, sebacic acid, and the like. Polyethylene terephthalate (PET), polyethylene isophthalate, polyethylene naphthalate, and polybutylene terephthalate homopolymers are some representative examples of such polyesters that can be solid state polymerized to high molecular weights. Blends of various polyesters can also be polymerized using the process of this invention. For instance, it can be utilized in solid state polymerizing melt blends of polyethylene terephthalate and polyethylene isophthalate. Such blends of polyethylene terephthalate and polyethylene isophthalate have excellent gas barrier properties and are very useful in making packaging materials, such as beverage bottles (see U.S. Pat. No. 4,551,368).

#The polyester prepolymers (starting polyesters) utilized in this invention can be made in any manner but are typically prepared by conventional melt polymerization techniques. Thus, conventional temperatures, catalysts, amounts of catalysts, stabilizers, and the like, are utilized in manners well known in the literature and art in making the polyester prepolymer. These polyester prepolymers have an initial starting IV (intrinsic viscosity) of at least about 0.2 dl/g in a 60:40 phenol:tetrachloroethane solvent system at a temperature of 30.degree. C. and at a concentration of 0.4 g/dl. The polyester prepolymers which are solid state polymerized in accordance with this invention will generally have an original or starting IV of from about 0.2 to about 3.0 dl/g. However, the process of this invention can be used to solid state polymerize polyester prepolymers having intrinsic viscosities of greater than 3.0 dl/g. Preferably, the polyester prepolymer used will have a starting IV which is within the range of about 0.3 to 2.0 dl/g. In most cases, the polyester prepolymers will have an original IV within the range of 0.4 to 0.6 dl/g. Virtually any type of polyester prepolymer can be solid state polymerized using the technique of this invention. Such polyester prepolymers are comprised of one or more diacid components and one or more diol components.

#The diacid component in the polyesters to which this invention pertains are normally alkyl dicarboxylic acids which contain from 4 to 36 carbon atoms, diesters of alkyl dicarboxylic acids which contain from 6 to 38 carbon atoms, aryl dicarboxylic acids which contain from 8 to 20 carbon atoms, diesters of aryl dicarboxylic acids which contain from 10 to 22 carbon atoms, alkyl substituted aryl dicarboxylic acids which contain from 9 to 22 carbon atoms, or diesters of alkyl substituted aryl dicarboxylic acids which contain from 11 to 22 carbon atoms. The preferred alkyl dicarboxylic acids will contain from 4 to 12 carbon atoms. Some representative examples of such alkyl dicarboxylic acids include glutaric acid, adipic acid, pimelic acid, and the like. The preferred diesters of alkyl dicarboxylic acids will contain from 6 to 12 carbon atoms. A representative example of such a diester of an alkyl dicarboxylic acid is azelaic acid. The preferred aryl dicarboxylic acids contain from 8 to 16 carbon atoms. Some representative examples of aryl dicarboxylic acids are terephthalic acid, isophthalic acid, and orthophthalic acid. The preferred diesters of aryl dicarboxylic acids contain from 10 to 18 carbon atoms. Some representative examples of diesters of aryl dicarboxylic acids include diethyl terephthalate, diethyl isophthalate, diethyl orthophthalate, dimethyl naphthalate, diethyl naphthalate and the like. The preferred alkyl substituted aryl dicarboxylic acids contain from 9 to 16 carbon atoms and the preferred diesters of alkyl substituted aryl dicarboxylic acids contain from 11 to 15 carbon atoms.