Copolyester from terephthalic acid containing low diethylene glycol

Abstract

Copolyesters made from the direct esterification of terephthalic acid with diols including ethylene glycol, but which contain low diethylene glycol (DEG) content, and processes for making the copolyesters. The copolyesters are characterized by comprising 1.0 wt % or less of DEG without requiring the use of DEG-suppressing additives. The processes are characterized by features including operating at lower pressures and lower EG:TPA feed mole ratios in the first reaction zone, while simultaneously at higher temperatures than typical operation in order to lower incorporation of DEG into the final polymer.

We claim: 
     
         1 . A process for preparing a copolyester, the process comprising:
 (a) introducing terephthalic acid (TPA), ethylene glycol (EG), and 1,4-cyclohexanedimethanol (CHDM) at an EG:TPA molar ratio of 2.3:1 to 2.7:1 into a first reaction zone (Z1);   (b) reacting TPA with EG and CHDM in Z1 at a melt temperature of at least 250° C. and a pressure of up to 40 psi to form a first esterification product comprising copolyester monomers, oligomers, or both and unreacted TPA, EG, and CHDM;   (c) passing the first esterification product to a second reaction zone (Z2);   (d) esterifying the unreacted TPA, EG, and CHDM in the first esterification product in Z2 at a melt temperature of at least 250° C. and a pressure of up to 20 psi to form a second esterification product comprising additional copolyester monomers, oligomers, or both;   (e) passing the second esterification product to a third reaction zone (Z3);   (f) polycondensing the second esterification product in the presence of a polycondensation catalyst in Z3 to form a prepolymerization product comprising copolyester oligomers;   (g) passing the prepolymerization product to one or more reaction zones (ZF); and   (h) polycondensing the prepolymerization product in the presence of the polycondensation catalyst in ZF to form a copolyester comprising 1.0 wt % or less of DEG.   
     
     
         2 . The process of  claim 1 , wherein the reacting step (b) is carried out at a melt temperature of 257 to 265° C. and a pressure of 30 to 40 psi and or wherein the esterifying step (d) is carried out at a melt temperature of 257 to 265° C. 
     
     
         3 . The process of  claim 1 , wherein the copolyester comprises 19 to 96.5 mol % of residues of ethylene glycol and 3.5 to 81 mol % of residues of 1,4-cyclohexanedimethanol, based on the total diol residues in the copolyester; or
 wherein the copolyester comprises 50 to 90 mol % of residues of ethylene glycol and 10 to 50 mol % of residues of 1,4-cyclohexanedimethanol, based on the total diol residues in the copolyester; or   wherein the copolyester comprises 65 to 75 mol % of residues of ethylene glycol and 25 to 35 mol % of residues of 1,4-cyclohexanedimethanol, based on the total diol residues in the copolyester.   
     
     
         4 . The process of  claim 1 , wherein the copolyester has an inherent viscosity of 0.6 to 0.9 dL/g. 
     
     
         5 . The process of  claim 1 , wherein the copolyester comprises 0.9 wt % or less of DEG; or
 wherein the copolyester comprises 0.8 wt % or less of DEG; or wherein the copolyester comprises 0.7 wt % or less of DEG; or wherein the copolyester comprises 0.5 to 0.8 wt % of DEG; or wherein the copolyester comprises 0.5 to 0.7 wt % of DEG; or wherein the copolyester comprises 0.5 to 0.6 wt % of DEG.   
     
     
         6 . A process for preparing a copolyester, the process comprising:
 (a) introducing 1,4-cyclohexanedimethanol (CHDM) and a first portion of fresh ethylene glycol (EG1) into a first reaction zone (Z1);   (b) reacting EG1 and CHDM with a diacid in Z1 to form a first esterification product comprising copolyester monomers, oligomers, or both and unreacted EG1, CHDM, and diacid;   (c) withdrawing from Z1 an esterification recycle stream comprising unreacted EG1, CHDM, and diacid;   (d) passing the esterification recycle stream to a heat exchanger;   (e) heating the esterification recycle stream in the heat exchanger to form a heated esterification recycle stream;   (f) passing the heated esterification recycle stream back to Z1;   (g) introducing fresh diacid comprising terephthalic acid, isophthalic acid, or mixtures thereof (g1) upstream of the heat exchanger into the esterification recycle stream or (g2) downstream of the heat exchanger into Z1;   (h) passing the first esterification product to a second reaction zone (Z2);   (i) introducing a second portion of fresh ethylene glycol (EG2) into Z2;   (j) esterifying the unreacted EG1, CHDM, and diacid in the first esterification product and EG2 in Z2 to form a second esterification product comprising additional copolyester monomers, oligomers, or both;   (k) passing the second esterification product to a third reaction zone (Z3);   (l) introducing a third portion of fresh ethylene glycol (EG3) into Z3;   (m) polycondensing the second esterification product in the presence of a polycondensation catalyst in Z3 to form a prepolymerization product comprising copolyester oligomers;   (n) passing the prepolymerization product to one or more reaction zones (ZF); and   (o) polycondensing the prepolymerization product in the presence of the polycondensation catalyst in ZF to form a copolyester containing 1.0 wt % or less of DEG   wherein the overall molar ratio of EG:TPA introduced into the process ranges from 2.3:1 to 2.71:1 where EG is the sum of EG1, EG2, and EG3 in moles, and   the reacting step (b) in Z1 is carried out at a melt temperature of 250 to 270° C. and a pressure of 25 to 40 psi; and   the esterifying step (j) in Z2 is carried out at a melt temperature of 250 to 270° C. and a pressure of 8 to 20 psi.   
     
     
         7 . The process of  claim 6 , wherein the fresh diacid is introduced (g1) upstream of the heat exchanger into the esterification recycle stream. 
     
     
         8 . The process of  claim 6 , wherein the fresh diacid is introduced (g2) downstream of the heat exchanger into Z1. 
     
     
         9 . The process of  claim 6 , wherein the molar ratio of EG1:TPA ranges from 2.0:1 to 2.69:1, the molar ratio of EG2:TPA ranges from 0.01:1 to 0.15:1, and the molar ratio of EG3:TPA ranges from 0.01:1 to 0.15:1. 
     
     
         10 . The process of  claim 6 , wherein the reacting step (b) in Z1 is carried out at a melt temperature of 257 to 265° C. and a pressure of 30 to 40 psi. 
     
     
         11 . The process of  claim 6 , wherein the esterifying step (j) in Z2 is carried out at a melt temperature of 257 to 265° C. and a pressure of 8 to 20 psi. 
     
     
         12 . The process of  claim 6 , wherein the copolyester comprises:
 (a) a diacid component comprising 100 mol % of residues of terephthalic acid, isophthalic acid, or mixtures thereof; and   (b) a diol component comprising 19 to 96.5 mol % of residues of ethylene glycol and 3.5 to 81 mol % of residues of 1,4-cyclohexanedimethanol,   wherein the diacid component is based on 100 mol % of total diacid residues in the copolyester and the diol component is based on 100 mol % of total diol residues in the copolyester; or   wherein the copolyester comprises:   (a) a diacid component comprising 90 to 100 mol % of residues of terephthalic acid; and   (b) a diol component comprising 50 to 90 mol % of residues of ethylene glycol and 10 to 50 mol % of residues of 1,4-cyclohexanedimethanol,   wherein the diacid component is based on 100 mol % of total diacid residues in the copolyester and the diol component is based on 100 mol % of total diol residues in the copolyester; or   wherein the copolyester comprises:   (a) a diacid component comprising 90 to 100 mol % of residues of terephthalic acid; and   (b) a diol component comprising 65 to 75 mol % of residues of ethylene glycol and 25 to 35 mol % of residues of 1,4-cyclohexanedimethanol,   wherein the diacid component is based on 100 mol % of total diacid residues in the copolyester and the diol component is based on 100 mol % of total diol residues in the copolyester.   
     
     
         13 . The process of  claim 6 , wherein the copolyester has an inherent viscosity of 0.6 to 0.9 dL/g, the inherent viscosity being determined by dilute solution viscosity at 25° C. using a Viscotek Differential or Modified Viscometer according to ASTM D 5225, “Standard Test Method for Making Solution Viscosity of Polymers with a Different Viscometer”. 
     
     
         14 . The process of  claim 6 , wherein the copolyester comprises 0.9 wt % or less of DEG; or wherein the copolyester comprises 0.8 wt % or less of DEG; or wherein the copolyester comprises 0.7 wt % or less of DEG. 
     
     
         15 . The process of  claim 6 , wherein the copolyester comprises 0.5 to 0.8 wt % of DEG; or wherein the copolyester comprises 0.5 to 0.7 wt % of DEG; or wherein the copolyester comprises 0.5 to 0.6 wt % of DEG.