WO1996030428A1

WO1996030428A1 - Process for preparing polyesters

Info

Publication number: WO1996030428A1
Application number: PCT/US1996/003739
Authority: WO
Inventors: Randy Steven Beavers; Karen Lynn Carman; Michael Lynn Cassell; Joseph Franklin Knight; Marc Alan Strand; Sara Stanley Wells
Original assignee: Eastman Chemical Company
Priority date: 1995-03-27
Filing date: 1996-03-20
Publication date: 1996-10-03
Also published as: ZA962456B; AU5316396A; CO4560369A1; IL117668A0; TR199600215A2; AR000405A1

Abstract

Disclosed is a process for improving the rate of solid state polymerization of ethylene terephthalate polymers or copolymers comprising copolymerizing terephthalic acid or dimethyl terephthalate and ethylene glycol with 2-10 mol % diethylene glycol and 0.1-10 mol % cyclohexanedimethanol to form a prepolymer, forming solid particles from the prepolymer, and solid state polymerizing the particles at a temperature between the glass transition temperature and melting point of the particles until a predetermined I.V. is reached.

Description

PROCESS FOR PREPARING POLYESTERS

This invention relates to an improved process for making high molecular weight poly(ethylene terephthalate) copolyesters in a solid phase polymerization process. The improved process involves the use of small amounts of 1,4—cyclohexanedimethanol and diethylene glycol in the copolyester composition which provides for significantly improved rates of inherent viscosity buildup.

Background of the Invention

Poly(ethylene terephthalate) (PET) polymers are useful in a wide variety of applications including fibers, molding plastics, sheeting, films and the like. For some applications such as tire cord fibers, beverage and food bottles or containers and food trays, it is necessary to use very high molecular weight poly(ethylene terephthalate) polymers. The melt viscosity of such high molecular weight PET type polymers is so high that it is not feasible or practical to make them by conventional melt phase processes. Thus, it is necessary to make low molecular weight polymers in a melt phase process, to isolate the prepolymer in the form of a pellet, powder or granule, to crystallize the polymer, and to complete the molecular weight buildup in a solid phase polycondensation reactor at a temperature above the glass transition point, but less than the melting point of the polymer. The solid state polycondensation rate of such polymers is very important economically in industrial processes. It was surprising to find that the rate of inherent viscosity (I.V.) buildup in the solid phase polycondensation reaction could be significantly improved if small amounts of 28 PCΪ7US96/03739

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1,4—cyclohexanedimethanol and diethylene glycol were included in the PET type polymers. Branching agents, such as trimellitic anhydride, may also be present in the copolymers. Patents of which we are aware include the following:

U.S. 4,446,303 discloses a process for making high molecular weight PET and modified PET which involves one or more recrystallizations of the polymer incident to solid—state polycondensation reactions. These recrystallizations restore the high polycondensation activity experienced in the early stages of the reaction.

U.S. 4,983,711 discloses copolyesters containing residues of terephthalic acid, ethylene glycol,

1,4—cyclohexanedimethanol, and a trifunctional monomer which are useful in manufacturing rigid containers by extrusion blow molding processes.

U.S. 4,314,928 relates to PET polymers containing up to 10 mol % of certain specified branched or unbranched C to C₁₀ diols which are subjected to a solid phase post—condensation reaction to provide polymers with a fast crystallization rate. The patent is not concerned with diethylene glycol containing copolymers.

U.S. 4,849,497 describes the preparation of PET in which solid state polymerization rates are claimed to be improved by utilizing polyester prepolymers which are in the form of porous pills. U.S. 4,917,845, U.S. 4,957,945 and U.S. 4,792,573 describe high molecular weight polyester resin prepared by rapid solid state polymerization from polyester prepolymer in the form of porous pills. U.S. 4,217,440 describes a method for making branched polyesters. The patent does not involve solid phase polycondensation techniques.

U.S. 3,544,523 describes the polycondensation of PET polymer in the solid state using anticaking agents such as talc to prevent sticking during polycondensation.

U.S. 4,876,326 and U.S 4,755,587 describe the solid state polymerization of polyester polymers in the form of porous pills, giving high molecular weight products in a short time.

European Patent Application 269,583 and Italy 1,199,166 describe a fixed bed solid state polycondensation of granular polyesters after a crystallization treatment with carbon dioxide.

U.S 4,234,708 describes branched PET polymers containing branching agents such as pentaerythritol.

U.S. 4,238,593 describes PET production by solid state polymerization of prepolymer having an optimum carboxyl content. U.S. 4,150,215 describes the preparation of catalyst—free high molecular weight PET by solid—state polymerization of ground low molecular weight polymer. U.S. 4,165,420 describes the solid state polymerization of polyester prepolymer using discrete spherical beads of prepolymer prepared by a spray congealing process.

U.S. 4,154,920 describes a thermally stable PET for carbonated beverage containers obtained by solid—state polymerization of prepolymer. U.S. 4,092,458 describes the preparation of polyester pellets from flakes of waste polyester film giving easily handled material for solid phase polymerization.

U.S. 4,069,194 describes the solid state polymerization of linear polyesters by moving a mixture of polyester granules and spherical glass particles through a heated reactor.

U.S. 3,853,382 describes the solid state polymerization of polyesters, claiming that the rate of polymerization is increased by the presence of an aromatic diisocyanate.

Japan Patent 18116 (1974) describes the solid phase polymerization of particulated aromatic polyesters in a rotating reactor. U.S. 3,960,817 describes polyester polycondensation in the solid phase by heating the polymer in contact with an inert gas under vacuum.

Description of the Invention According to the present invention, there is provided a process for improving the rate of solid state polymerization of ethylene terephthalate polymers or copolymers comprising copolymerizing at least 80 mol % terephthalic acid or dimethyl terephthalate and at least 80 mol % ethylene glycol with 2—10 mol % diethylene glycol and 0.1—10 mol % cyclohexanedimethanol to form a prepolymer having an I.V. of 0.30 to 0.70, forming solid particles from said prepolymer, and solid state polymerizing said particles at a temperature between the glass transition temperature and melting point of said particles until a predetermined I.V. is reached. In the solid phase polycondensation of PET prepolymers, the rate of change in I.V. units (dL/g) has been observed to be 0.015 dL/g per hour at 215°C. It has now been discovered that the rate of I.V. increase can be substantially improved by incorporating small amounts of diethylene glycol and 1,4—cyclohexane¬ dimethanol in the PET polymer. The copolyesters can have an I.V. build—up rate twice as fast as that observed with unmodified PET polymers. Such rate increases have a significant economic impact on the manufacturing costs of high molecular weight PET polymers. In general, the polymers of this invention are made by first making a prepolymer using melt phase polycondensation techniques. These melt phase procedures are well known to those skilled in the art. A wide range of metal based catalysts may be used and may include catalysts based on titanium, manganese, antimony, cobalt, germanium, tin and the like or mixtures of these materials. The prepolymers generally have I.V. values of 0.30 dL/g to 0.70 dL/g.

To obtain high molecular weight polymer (such as those having I.V. values in the range of 0.70 dL/g to 1.50 dL/g, it is generally necessary to use solid—state polymerization techniques. To achieve this, the prepolymers are granulated or pelletized by conventional methods and these particles are then heated at 180°C to 230°C under vacuum or with an inert gas passing through the reactor bed to remove ethylene glycol that diffuses out of the pellets or granules.

It was found that diethylene glycol and 1,4—cyclohexanedimethanol could be incorporated in PET polymers to provide solid state polymerization rate increases. Diethylene glycol concentrations in the copolyesters generally range from 1.5 to 10 mol % while preferred concentrations are 2 to 7 mol %.

Similarly, concentrations of 1,4—cyclohexane¬ dimethanol include 0.1 to 10 mol % with a preferred range of 0.5 to 5 mol %. Higher concentrations of diethylene glycol and 1,4—cyclohexanedimethanol are not desirable because this decreases the softening point of the polymer which makes it difficult to solid state the copolymers without the pellets sticking in the solid state polymerization bed. Either the cis, trans or cis trans isomer mixtures of 1,4—cyclohexanedimethanol may be used. It is also possible to use the 1,3— or 1,2—cyclohexanedimethanol isomers instead of 1,4—cyclohexanedimethanol. In addition to diethylene glycol and cyclohexanedimethanol, it is also possible to have small amounts of other glycols present such as 1,4—butanediol, 1,6-hexanediol, neopentyl glycol and the like. In such cases, the amount of comonomer glycol must be limited so that the crystallinity of the PET copolyester is not eliminated. In making the precursor polymer, one may use terephthalic acid and esterify it with ethylene glycol, 1,4—cyclohexanedimethanol and diethylene glycol prior to the melt phase polycondensation reaction. It is also possible to start with a dialkyl terephthalate ester such as dimethyl terephthalate and to transesterify it with the glycol moieties prior to the melt phase polycondensation reaction. Small amounts of other dibasic acids or their esters may be used if desired. For example, up to 10 mol % of other acids such as succinic, adipic, suberic, isophthalic, naphthalene— dicarboxylic, cyclohexanedicarboxylic and the like may be used.

Both the melt phase and solid state portions of this process may be conducted either batchwise or in a continuous manner. In addition to the linear copolyesters described above, branched copolyesters containing up to 1.0 mol % of suitable branching agents may be made by the process of this invention. Useful branching agents include tricarboxylic acids or ester forming derivatives thereof such as trimellitic (1,2,4—benzenetricarboxylie) acid and anhydride, hemimellitic (1,2,3—benzenetricarboxylic) acid and anhydride, trimesic (1,3,5-benzenetricarboxylic) acid and tricarballyic (1,2,3-propanetricarboxylic) acid. Generally, any tricarboxylic residue containing 6 to 9 carbon atoms may be used. The trifunctional residue also may be derived from an aliphatic triol containing 3 to 8 carbon atoms such as glycerin, trimethylolethane and trimethylolpropane. The amount of the trifunctional monomer residue present in the copolyester preferably is in the range of 0.05 to 0.25 mol %. The preferred trifunctional monomer residues are residues of benzenetricarboxylic acids (including anhydrides) , especially trimellitic acid or anhydride.

Although the copolyesters of this invention are frequently used as produced in the form of fibers or in molded or extruded shapes, additives frequently used in polymers may also be present. These additives may include dyes, colorants, pigments, fillers, stabilizers, molding agents and the like.

The following examples will further illustrate the invention.

Example 1 - Branched PET Copolyester Containing 3.1 Mol % of 1.4-Cyclohexanedimethanol and 5.3 Mol % of Diethylene Glvcol — To a 500 mL single-neck flask are charged 96.81 g (0.50 mole) of dimethyl terephthalate, 58.28 g (0.94 mole) of ethylene glycol, 2.52 g (0.018 mole) of a 30/70 cis trans mixture of 1,4—cyclo— hexanedimethanol, 1.33 g (0.013 mole) of diethylene glycol, 0.19 g (0.001 mole) of trimellitic anhydride, 0.17 mL of a solution of titanium tetraisopropoxide in 1-butanol which is 1.12 wt vol % titanium, 1.50 mL of a solution of manganese acetate in acetic acid and ethylene glycol which is 0.49 wt/vol % manganese,

1.80 mL of a solution of antimony acetate in ethylene glycol which is 1.23 wt/vol % antimony, and 1.50 mL of a solution of cobalt acetate in ethylene glycol which is 0.49 wt/vol % cobalt. The flask is fitted with a metal stirrer, nitrogen inlet and volatiles outlet. The flask is immersed in a Belmont metal bath. The mixture is heated with stirring under nitrogen at 200°C for 60 minutes. The reaction temperature is increased to 210°C and the mixture is heated with stirring under nitrogen for 120 minutes. Via the nitrogen inlet,

0.12 mL of a solution of polyethylene glycol phosphate which is 8.0 wt/vol % phosphorus is added and the reaction temperature is increased to 280°C. When the reaction temperature reaches 280°C, the nitrogen purge is replaced with <0.5 mm Hg pressure and polycondensation is carried out for 60 minutes. The viscous copolymer crystallizes upon cooling under nitrogen. The polymer has an inherent viscosity (I.V.) of 0.54 dL/g. Example 2 — PET Copolyester Containing 3.7 Mol % of 1.4—Cyclohexane Dimethanol and 4.2 Mol % of Diethylene Glycol — To a 500 mL single neck flask are charged 97.00 g (0.50 mole) of dimethyl terephthalate, 58.28 g (0.94 mole) of ethylene glycol, 2.52 g (0.018 mole) of a 30/70 cis/trans mixture of 1,4—cyclohexanedimethanol,

2.33 g (0.022 mole) diethylene glycol, 0.17 mL of a solution of titanium tetraisopropoxide in 1—butanol which is 1.12 wt/vol % titanium, 1.50 mL of a solution of manganese acetate in acetic acid and ethylene glycol which is 0.49 wt/vol % manganese, 1.80 mL of a solution of antimony acetate in ethylene glycol which is 1.23 wt/vol % antimony, and 1.50 mL of a solution of cobalt acetate in ethylene glycol which is 0.49 wt/vol % cobalt. After ester exchange, 0.12 L of a solution of polyethylene glycol phosphate which is 8.0 wt/vol % phosphorus is added and the reaction temperature is increased to 280°C. The reaction conditions are the same as given in example number one. The sample has an I.V. of 0.58 dL/g. Example 3 - PET Copolyester Containing 3.5 Mol % of 1.4—Cvclohexane Dimethanol and 6.4 Mol % of Diethylene Glvcol - To a 500 mL single neck flask are charged 97.00 g (0.50 mole) of dimethyl terephthalate, 58.28 g (0.94 mole) of ethylene glycol, 2.52 g (0.018 mole) of a 30/70 cis/trans mixture of 1,4—cyclohexanedimethanol, 4.30 g (0.041 mole) diethylene glycol, 0.17 mL of a solution of titanium tetraisopropoxide in 1—butanol which is 1.12 wt/vol % titanium, 1.50 mL of a solution of manganese acetate in acetic acid and ethylene glycol which is 0.49 wt/vol % manganese, 1.80 L of a solution of antimony acetate in ethylene glycol which is 1.23 wt/vol % antimony, and 1.50 mL of a solution of cobalt acetate in ethylene glycol which is 0.49 wt/vol % cobalt. After ester exchange, 0.12 mL of a solution of polyethylene glycol phosphate which is 8.0 wt/vol % phosphorus is added and the reaction temperature is increased to 280°C. The reaction conditions are the same as given in example number one. The sample has an I.V. of 0.58 dL/g after melt phase polymerization.

Example 4 — Branched PET Copolyester Containing 0.7 Mol % of 1.4-Cyclohexanedimethanol and 5.7 Mol % of Diethylene Glycol - To a 500 mL single neck flask are charged 96.80 g (0.50 mole) of dimethyl terephthalate, 58.90 g (0.95 mole) of ethylene glycol, 0.36 g (0.003 mole) of a 30/70 cis/trans mixture of 1,4—cyclohexanedimethanol, 2.39 g (0.023 mole) diethylene glycol, 0.19 g (0.001 mole) trimellitic anhydride, 0.17 mL of a solution of titanium tetraisopropoxide in 1-butanol which is 1.12 wt/vol % titanium, 1.50 mL of a solution of manganese acetate in acetic acid and ethylene glycol which is 0.49 wt/vol % manganese, 1.80 L of a solution of antimony acetate in ethylene glycol which is 1.23 wt/vol % antimony, and 1.50 mL of a solution of cobalt acetate in ethylene glycol which is 0.49 wt/vol % cobalt. After ester exchange, 0.12 mL of a solution of polyethylene glycol phosphate which is 8.0 wt/vol % phosphorus is added and the reaction temperature is increased to 280°C. The reaction conditions are the same as given in example number one. The sample has an I.V. of 0.57 dl/g after melt phase polymerization.

Example 5 - PET Copolyester Containing 3.6 Mol % of 1.4—Cvclohexane Dimethanol and 1.4 Mol % of Diethylene Glycol — To a 500 mL single neck flask are charged

97.00 g (0.50 mole) of dimethyl terephthalate, 59.80 g (0.97 mole) of ethylene glycol, 2.52 g (0.018 mole) of a 30/70 cis/trans mixture of 1,4—cyclohexanedimethanol, 0.17 mL of a solution of titanium tetraisopropoxide in 1—butanol which is 1.12 wt/vol % titanium, 1.50 mL of a solution of manganese acetate in acetic acid and ethylene glycol which is 0.49 wt/vol % manganese, 1.80 mL of a solution of antimony acetate in ethylene glycol which is 1.23 wt/vol % antimony, and 1.50 mL of a solution of cobalt acetate in ethylene glycol which is 0.49 wt/vol % cobalt. After ester exchange, 0.12 mL of a solution of polyethylene glycol phosphate which is 8.0 wt/vol % phosphorus is added and the reaction temperature is increased to 280°C. It should be noted that no additional diethylene glycol was added to this sample. The diethylene glycol present was formed in si tu during the melt polymerization. The reaction conditions are the same as given in example number one. The sample has an I.V. of 0.51 dl/g after melt phase polymerization.

Laboratory Solid State Polycondensation of Polymers from Examples 1—5 - Polymers from Examples 1-5 are ground through a 3 mm screen and sieved in a No. 30 mesh screen (0.0232 inch opening) to remove fines and give a uniform particle size. The copolymers are solid state polymerized for 24 hours at 205° or 215°C in a glass laboratory static bed solid state unit using either 1,3—butanediol or diethyl succinate as the heat exchange solvents. A nitrogen gas flow of 4 scfh (standard cubic feet per hour) is passed through the sample particle bed. Samples are taken after 0, 1, 2, 3, 4, 5, 6, 9, 12, 15, 18, 21 and 24 hours of solid state polycondensation. The inherent viscosity is measured on these samples to determine the rate of I.V. increase per hour. The results are summarized in Table 1. These results clearly show the effect of diethylene glycol on the solid state polymerization rate of unbranched PET copolymers prepared in the laboratory. Increasing the diethylene glycol concentration from 1.4 mol % (Example 5) to 6.4 mol % (Example 3) gave a 44.4% increase in solid state rate of 205°C and a 41.9% increase in rate of 215°C.

The copolyesters in this report were characterized by the following methods: Inherent Viscosity (I.V.) — a 0.5 g sample of polymer was dissolved in a 60/40 mixture of phenol tetrachloroethane. The I.V. was determined at 25°C.

Mol % Glycols — The mol % glycols were determined by gas chromatography. Mol % Brancher — The concentration of trimellitic anhydride (TMA) was determined by liquid chromatography.

TABLE 1

Effect of CHDM and DEG Concentration on the

Solid State Rate of PET Synthesized and

Solid State Polymerized in the Laboratory

P I.V^a CHDM^b DEG^C TMA^d Rate e^β Rate §^e

Example dl/q Mol * Mol % Wt % 205°C 215°C

1 0.537 3.1 5.3 0.18 0.035 0.039

2 0.577 3.7 4.2 0.00 0.021 0.024

3 0.580 3.5 6.4 0.00 0.027 0.031

4 0.566 0.7 5.7 0.18 0.035 0.039

5 0.510 3.6 1.4 0.00 0.015 0.018 ^aPrecursor inherent viscosity ^bl,4-Cyclohexanedimethanol ^cDiethylene glycol ^dTrimellitic anhydride ^eChange in I.V. units per hour

Examples 6-14 - Preparation of PET Copolyesters in a Continuous Melt Phase Reactor Process - Copolyesters containing 0.0 to 7.9 mol % of 1,4—cyclohexane¬ dimethanol, 1.0 to 8.4 mol % of diethylene glycol and 0.20±0.03 mol % of trimellitic anhydride are prepared in a continuous reactor. To the first ester exchange reactor is continuously fed molten dimethyl terephthalate, ethylene glycol, and the catalyst components in ethylene glycol. Ester interchange begins in the first reactor which is maintained at 190—210°C under a positive pressure of 40 psi. As methanol is removed from the reactor, the reaction mixture is forced by differential pressure through additional ester exchange reactors and the temperature is increased up to 250°C, while the pressure is reduced to 7 psi. At this point, an ethylene glycol ester of trimellitic acid and the additional catalysts are added to the product stream and the mixture of monomer and low molecular weight oligomers are pumped into a prepolymer reactor. The temperature is 255°C and the pressure is reduced to

250 mm Hg. As the copolymer is continuously moved into a second prepolymer reactor, the temperature is increased to 270°C and the pressure is decreased to 80 mm Hg. The molten prepolymer is pumped into a finishing reactor where melt phase polycondensation is completed at 280°C using a pressure of <4 mm Hg pressure. The viscous copolymer is extruded through a rod die into a water bath where the material is quenched before being pelletized.

Copolymers from Examples 6—14 are screened to obtain a pellet diameter between 0.079 inches and 0.132 inches. The copolymers are solid state polymerized for 24 hours at 205° or 215°C in a glass laboratory static bed solid state unit using 1,3—butanediol or diethyl succinate as the heat exchange solvents. A nitrogen gas flow of 4 scfh is passed through the sample particle bed. Samples are taken after 0, 1, 2, 3, 4, 5, 6, 9, 12, 15, 18, 21 and 24 hours of solid state time. The inherent viscosity is measured on these samples to determine the rate of I.V. increase per hour. The results are summarized in

Table 2. These results show that copolymerizing small concentrations of diethylene glycol or 1,4—cyclohexane¬ dimethanol substantially increases the solid state polymerization rate of the PET copolymers.

TABLE 2

Effect of DEG and CHDM Concentration on the

Solid State Rate of PET Prepared in a

Continuous Reactor and Solid Stated in the Lab

P I.V^a CHDM^b DEG^C TMA^d Rate §^β Rate §^e

Example dl/g Mol % Mol % Wt % 205^βC 215^βC

6 0.59 3.7 1.9 0.22 0.015 0.021

7 0.58 3.5 7.4 0.21 0.023 0.032

3 0.61 7.9 3.3 0.21 0.023 0.032

9 0.63 3.5 4.5 0.18 0.022 0.037

10 0.62 0.0 8.4 0.23 0.024 0.031

11 0.59 7.2 7.7 0.21 0.031 0.044

12 0.57 6.3 1.8 0.20 0.013 0.020

13 0.68 3.6 4.4 0.18 0.023 0.032

14 0.61 0.0 1.0 0.20 0.010 0.015 ^aPrecursor inherent viscosity l,4—Cyclohexanedimethanol ^cDiethylene glycol ^dTrimellitic anhydride ^eChange in I.V. units per hour Examples 15-19 — Preparation of PET Copolyesters in a Continuous Melt Phase Process Followed bv Pilot Plant Solid State Polycondensation — Prepolymers were prepared via a continuous melt phase reactor process as described above. The prepolymers were solid state polymerized in a pilot plant process. To a double coned dryer is charged ~300 lb of pellets, that is jacketed with oil for heating. The dryer is rotated on a shaft and heated at 205°C with 4 scfm (standard cubic feet per minute) of dry nitrogen passing through the reactor to remove the ethylene glycol after it is liberated from the surface of the pellet. Polymer pellets are removed every 2 hours to determine the rate of I.V. change. The results are tabulated in Table 3. These results show that increasing the diethylene concentration from 1.9 mol % to 8.4 mol % and solid stating the samples in a larger scale pilot plant process gives a 54% increase in solid state polycondensation rate (Examples 15 and 19).

TABLE 3

Effect of DEG and CHDM Concentration on the

Solid State Rate of Polymer Synthesized in a

Continuous Reactor and Solid Stated in a Batch Process

P I.V^a CHDM^b DEG^C TMA^d Rate _^e

Example dl/g Mol % MPl % Wt 205°C

15 0.632 3.5 1.9 0.22 0.006

16 0.615 7.1 3.9 0.22 0.009

17 0.580 7.7 7.2 0.21 0.011

18 0.615 0.0 8.4 0.24 0.013

19 0.625 0.0 8.5 0.23 0.013 ^aPrecursor inherent viscosity ^bl,4-Cyclohexanedimethanol ^cDiethylene glycol ^dTrimellitic anhydride ^βChange in I.V. units per hour

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

Claims

CLAIMSWe claim:

1. Process for improving the rate of solid state polymerization of ethylene terephthalate polymers or copolymers comprising copolymerizing at least 80 mol % terephthalic acid or dimethyl terephthalate and at least 80 mol % ethylene glycol with 2—10 mol % diethylene glycol and 0.1-10 mol % cyclohexanedimethanol to form a prepolymer having an I.V. of 0.30 to 0.70, forming solid particles from said prepolymer, and solid state polymerizing said particles at a temperature between the glass transition temperature and melting point of said particles until a predetermined I.V. is reached.

2. Process according to Claim 1 wherein said cyclo¬ hexanedimethanol is 1,4—cyclohexanedimethanol.

3. Process according to Claim 1 wherein said prepolymer has repeat units from at least 90 mol % terephthalic acid, at least 90 mol % ethylene glycol, 2—7 mol % diethylene glycol and 0.5—5 mol % 1,4—cyclohexanedimethanol.