Method for producing polyester resin by using regenerated bis(2-hydroxyethyl) terephthalate aqueous solution

1 . A process for preparing a polyester resin, which comprises:

(1) depolymerizing waste polyester to prepare recycled bis(2-hydroxyethyl) terephthalate;

(2) mixing the recycled bis(2-hydroxyethyl) terephthalate with water at 60° C. to 110° C. to prepare an aqueous solution of recycled bis(2-hydroxyethyl) terephthalate having a concentration of 50% by weight to 95% by weight, wherein the concentration is the weight of the recycled bis(2-hydroxyethyl) terephthalate based on the total weight of the aqueous solution of recycled bis(2-hydroxyethyl) terephthalate; and

(3) preparing a polyester resin through an esterification reaction and a polycondensation reaction using the aqueous solution of recycled bis(2-hydroxyethyl) terephthalate,

wherein a change in yellow index (ΔYID) calculated by following equation is 6 or less:

ΔYID=YID (day 5)−YID (day 0)

wherein:

YID (day 5) is a yellow index (YID) value measured when recycled bis(2-hydroxyethyl) terephthalate obtained by storing the aqueous solution of recycled bis(2-hydroxyethyl) terephthalate prepared in step (2) at 60° C. to 110° C. for 5 days and then evaporating water is dissolved in ethylene glycol at a concentration of 25% by weight at 120° C. for 1 hour,

YID (day 0) is a yellow index (YID) value measured when recycled bis(2-hydroxyethyl) terephthalate prepared in step (1) is dissolved in ethylene glycol at a concentration of 25% by weight at 120° C. for 1 hour, and

the yellow index (YID) value is calculated using a color analyzer based on transmission data for a solution obtained with Illuminant D65 using Color Flex EZ of Hunterlab at an observer's angle of 2°.

2 . The process for preparing a polyester resin of claim 1 , wherein step (1) comprises:

(1a) subjecting waste polyester to depolymerization by a first glycolysis reaction at a temperature of 180° C. to 200° C. to obtain a first reactant;

(1b) subjecting the first reactant to depolymerization by a second glycolysis reaction at a temperature of 150° C. to 170° C. to obtain a second reactant;

(1c) subjecting the second reactant to ion exchange through an ion-exchange resin to obtain a third reactant;

(1d) removing an unreacted glycol from the third reactant through distillation at a temperature of 150° C. or lower to obtain a fourth reactant; and

(1e) subjecting the fourth reactant to distillation to obtain crude bis(2-hydroxyethyl) terephthalate.

3 . The process for preparing a polyester resin of claim 1 , wherein the recycled bis(2-hydroxyethyl) terephthalate has a peak area fraction of bis(2-hydroxyethyl) terephthalate of 96% or more and a peak area fraction of 2-hydroxyethyl[2-(2-hydroxyethoxy)ethyl] terephthalate of 2.5% or less, when measured by high-performance liquid chromatography (HPLC).

4 . The process for preparing a polyester resin of claim 1 , wherein step (2) comprises:

(2a) mixing the recycled bis(2-hydroxyethyl) terephthalate with water at 60° C. to 110° C.; and

(2b) adjusting the concentration to 50% by weight to 95% by weight through further addition or evaporation of water after the mixing.

5 . The process for preparing a polyester resin of claim 1 , wherein, when recycled bis(2-hydroxyethyl) terephthalate obtained by storing the aqueous solution of recycled bis(2-hydroxyethyl) terephthalate prepared in step (2) at 60° C. to 110° C. for 5 days and then evaporating water is dissolved in ethylene glycol at a concentration of 25% by weight at 120° C. for 1 hour, the yellow index (YID) is measured to be 5 or less.

6 . The process for preparing a polyester resin of claim 1 , wherein, when the aqueous solution of recycled bis(2-hydroxyethyl) terephthalate prepared in step (2) is stored at 60° C. to 110° C. for 5 days, the purity of the recycled BHET is measured to have a difference of 5% or less from the initial purity.

7 . The process for preparing a polyester resin of claim 1 , wherein at least one monomer selected from the group consisting of (a) a dicarboxylic acid or a derivative thereof, (b) ethylene glycol or diethylene glycol, and (c) a diol-based comonomer is further introduced into the esterification reaction.

8 . The process for preparing a polyester resin of claim 7 , wherein the dicarboxylic acid comprises at least one selected from terephthalic acid and isophthalic acid.

9 . The process for preparing a polyester resin of claim 7 , wherein the diol-based comonomer comprises at least one selected from the group consisting of cyclohexanedimethanol, cyclohexanedimethanol derivatives, and isosorbide.

10 . The process for preparing a polyester resin of claim 9 , wherein the cyclohexanedimethanol derivative is 4-(hydroxymethyl)cyclohexylmethyl-4-(hydroxymethyl)cyclohexanecarboxylate or 4-(4-(hydroxymethyl)cyclohexylmethoxymethyl)cyclohexylmethanol.

11 . The process for preparing a polyester resin of claim 7 , wherein the diol-based comonomer further comprises 1,2-propanediol, 1,3-propanediol, 2-methyl-1,3-propanediol, 2-methylene-1,3-propanediol, 2-ethyl-1,3-propanediol, 2-isopropyl-1,3-propanediol, 2,2-dimethyl-1,3-propanediol, 1,4-butanediol, 2,3-butanediol, 3-methyl-1,5-pentanediol, 3-methyl-2,4-pentanediol, 1,6-hexanediol, 1,2-cyclohexanediol, and 1,4-cyclohexanediol.

12 . The process for preparing a polyester resin of claim 1 , wherein the polyester resin has a value obtained by subtracting the b value from the L value of 85 or more when Hunter Lab color space is measured.

13 . A process for preparing an aqueous solution of recycled bis(2-hydroxyethyl) terephthalate, which comprises:

(1) depolymerizing waste polyester to prepare recycled bis(2-hydroxyethyl) terephthalate; and

(2) mixing the recycled bis(2-hydroxyethyl) terephthalate with water at 60° C. to 110° C. to prepare an aqueous solution thereof having a concentration of 50% by weight to 95% by weight, wherein the concentration is the weight of the recycled bis(2-hydroxyethyl) terephthalate based on the total weight of the aqueous solution of recycled bis(2-hydroxyethyl) terephthalate,

wherein a change in yellow index (ΔYID) calculated by following equation is 6 or less:

ΔYID=YID (day 5)−YID (day 0)

wherein:

YID (day 5) is a yellow index (YID) value measured when recycled bis(2-hydroxyethyl) terephthalate obtained by storing the aqueous solution of recycled bis(2-hydroxyethyl) terephthalate prepared in step (2) at 60° C. to 110° C. for 5 days and then evaporating water is dissolved in ethylene glycol at a concentration of 25% by weight at 120° C. for 1 hour,

YID (day 0) is a yellow index (YID) value measured when recycled bis(2-hydroxyethyl) terephthalate prepared in step (1) is dissolved in ethylene glycol at a concentration of 25% by weight at 120° C. for 1 hour, and

the yellow index (YID) value is calculated using a color analyzer based on transmission data for a solution obtained with Illuminant D65 using Color Flex EZ of Hunterlab at an observer's angle of 2°.