JP3015061B2 - Polyethylene terephthalate and method for producing the same - Google Patents

Description

Description: TECHNICAL FIELD OF THE INVENTION The present invention relates to polyethylene terephthalate used for forming bottles and the like, and more particularly, to polyethylene terephthalate which is less likely to cause mold contamination during molding.

Technical background of the invention and its problems Conventionally, various resins have been employed as materials for containers such as seasonings, oils, beverages, cosmetics, and detergents depending on the type of filling content and the purpose of use. .

Among these, polyethylene terephthalate is excellent in mechanical strength, heat resistance, transparency and gas barrier properties, and is particularly suitable as a material for beverage filling containers such as juices, soft drinks, and carbonated drinks.

Such a polyethylene terephthalate is supplied to a molding machine such as an injection molding machine to form a preform for a hollow molded body, and the preform is inserted into a mold having a predetermined shape, stretch-blown, and then heat-treated (heat set). And then molded into a hollow molded container.

However, conventional polyethylene terephthalate has
Oligomers such as cyclic trimers were included, and mold contamination was likely to occur due to the adhesion of the oligomers to the inner surface of the mold.

Such mold stains may cause surface roughening and whitening of the obtained bottle. If the bottle is whitened,
The bottle must be discarded. For this reason, when a bottle is molded using a conventionally known polyethylene terephthalate, mold stains must be frequently removed, and there is a serious problem that productivity of the bottle is significantly reduced.

In view of the current situation as described above, the present inventors have intensively studied to obtain polyethylene terephthalate that does not easily cause mold contamination during molding.The main cause of mold contamination during molding is that the molding of polyethylene terephthalate is difficult. It has been found that a large amount of cyclic trimer is sometimes generated and the amount of oligomer such as cyclic trimer contained in polyethylene terephthalate increases.

The present inventors have further studied based on the above findings, and when performing blow molding using polyethylene terephthalate as a raw material left for a specific period or more in an air atmosphere where the temperature and humidity are within a specific range, the metal molding is performed at the time of molding. The present inventors have found that mold contamination is unlikely to occur, and have completed the present invention.

JP-A-59-25815 discloses a method of treating polyethylene terephthalate powder with heated steam at 110 ° C. or higher in order to crystallize polyethylene terephthalate prior to solid-phase polycondensation of polyethylene terephthalate. It has been disclosed.

In JP-A-59-219328, the intrinsic viscosity is at least 0.4 dl / g or more, and the density is 1.35 g / cm ³ or less.
A step of conditioning a polyester having ethylene terephthalate units as a main repeating unit so that the moisture content becomes at least 0.2% by weight, a step of pre-crystallizing at a temperature of 140 ° C or higher, and a temperature of 180 ° C or higher and 240 ° C or lower. And a method for producing a high-polymerization degree polyester, which comprises a step of performing solid-phase polymerization under an inert gas atmosphere or under reduced pressure.

Object of the Invention The present invention is to solve the problems associated with the prior art as described above, when molded using a mold as a raw material of molded products such as beverage filling containers, during molding It is an object of the present invention to provide a polyethylene terephthalate in which the amount of cyclic trimers generated in a mold is small, and mold contamination is less likely to occur.

SUMMARY OF THE INVENTION The polyethylene terephthalate according to the present invention has a density of
1.38 g / cm ³ or more, the content of the cyclic trimer is 0.40% by weight or less, and the water absorption is 2,000 ppm or more.

Here, the water absorption in the present invention is a value corresponding to the amount of water from which water adhering to the surface of polyethylene terephthalate has been removed, and the percentage of the water contained in the polyethylene terephthalate permeated into the polyethylene terephthalate is included. means.

Further, such polyethylene terephthalate has a cyclic trimer content of 0.40% by weight or less and a water absorption rate of
Polyethylene terephthalate in the range of 50 to 1000 ppm is stored in an air atmosphere at a relative humidity of 50 to 80% and a temperature of 0 to 50 ° C.
Leave for more than a day or 80-95% relative humidity, 0 temperature
It can be produced by allowing the polyethylene terephthalate to absorb water by leaving it in an air atmosphere at 〜50 ° C. for 30 days or more.

Further, the polyethylene terephthalate according to the present invention has a relative humidity of 20% or more and a water content of 2000 ppm or more at a moisture content of water existing in the chip, that is, a moisture content of water present on both the surface and the inside of the polyethylene terephthalate. By leaving the film in an air atmosphere of 0 ° C. or more for 20 days or more, the polyethylene terephthalate can be produced by absorbing water.

That is, by leaving polyethylene terephthalate for a long period of time as described above, moisture on the surface of polyethylene terephthalate gradually penetrates inside polyethylene terephthalate, and polyethylene terephthalate having the above-mentioned water absorption is obtained. The above treatment corresponds to making polyethylene terephthalate absorb water.

DETAILED DESCRIPTION OF THE INVENTION Hereinafter, the polyethylene terephthalate according to the present invention and a method for producing the same will be specifically described.

The polyethylene terephthalate according to the present invention has a specific intrinsic viscosity as described later, has a cyclic trimer content of 0.40% by weight or less, and has a water absorption of 2000 ppm or more.

Then, such polyethylene terephthalate is manufactured by being left in a specific atmosphere for a specific time as described above.

The thus obtained polyethylene terephthalate having a water absorption of 2,000 ppm or more is dried under vacuum or dried under air flow, and then the water content in the chip is reduced by 50%.
When heated at 300 ° C. for 8 minutes at a temperature of not more than ppm, the amount of cyclic trimer generated upon heating is not more than 0.1 ppm.

Therefore, such polyethylene terephthalate is
This is supplied to a molding machine such as an injection molding machine to form a preform for a hollow molded body, this preform is inserted into a mold having a predetermined shape, stretch blow-molded, and then heat-set to form a hollow molded container. In this case, the cyclic trimer hardly adheres to both the mold for preform molding and the mold for stretch blow molding, and mold contamination due to such cyclic trimer adhesion hardly occurs. .

When the polyethylene terephthalate according to the present invention is used as a raw material for molded articles such as bottles, the reason for the decrease in the amount of cyclic trimer that causes mold fouling is estimated as follows.

That is, the water contained in the polyethylene terephthalate deactivates the catalyst (the polycondensation reaction catalyst used in the production of the polyethylene terephthalate) contained in the polyethylene terephthalate as well as the water, so that the catalyst is deactivated. Although the amount of active catalyst occupying decreases, the active catalyst occupying in the catalyst almost disappears when the water absorption of polyethylene terephthalate is 2000 ppm or more, and therefore, even if polyethylene terephthalate is charged into a mold and heated during molding, It is considered that the polycondensation reaction of the monomer remaining in the polyethylene terephthalate hardly proceeds, and therefore, a cyclic trimer hardly occurs during the heat molding.

In the present invention, the amount of the cyclic trimer contained in the polyethylene terephthalate molded article is measured as follows.

That is, a predetermined amount of polyethylene terephthalate is dissolved in ortho-chlorophenol, reprecipitated with tetrahydrofuran, filtered to remove linear polyethylene terephthalate, and then the obtained filtrate is subjected to liquid chromatography (LC7A manufactured by Shimadzu Corporation). Determine the amount of cyclic trimer contained in the supplied polyethylene terephthalate, divide this value by the amount of polyethylene terephthalate used for measurement,
Let it be the amount (% by weight) of the cyclic trimer contained in polyethylene terephthalate.

The polyethylene terephthalate according to the present invention as described above can be produced, for example, as follows.

The polyethylene terephthalate according to the present invention is produced using terephthalic acid or an ester-forming derivative thereof and ethylene glycol or an ester-forming derivative thereof as raw materials.
Up to mol% of other dicarboxylic acids and / or other glycols may be copolymerized.

As the dicarboxylic acid used for copolycondensation other than terephthalic acid, specifically, phthalic acid, isophthalic acid, naphthalene dicarboxylic acid, diphenyl dicarboxylic acid, aromatic dicarboxylic acids such as diphenoxyethane dicarboxylic acid,
Examples thereof include aliphatic dicarboxylic acids such as adipic acid, sebacic acid, azelaic acid and decanedicarboxylic acid, and alicyclic dicarboxylic acids such as cyclohexanedicarboxylic acid.

As glycols used for copolycondensation other than ethylene glycol, specifically, trimethylene glycol,
Aliphatic glycols such as propylene glycol, tetramethylene glycol, neopentyl glycol, hexamethylene glycol, dodecamethylene glycol, alicyclic glycols such as cyclohexanedimethanol, bisphenols, hydroquinone, 2,2-bis (4-β-
Aromatic diols such as (hydroxyethoxyphenyl) propane;

A raw material containing terephthalic acid or an ester-forming derivative thereof and ethylene glycol or an ester-forming derivative thereof as described above is esterified in the presence of an esterification catalyst, and then liquid-phased in the presence of a polymerization catalyst. After the polymerization, solid-phase polymerization is performed.

As a method for producing the polyethylene terephthalate according to the present invention, any of a batch method and a continuous method may be adopted. An example of a preferable production method will be described below. Specifically, first, terephthalic acid or its ester-forming property is described. A slurry containing the derivative and ethylene glycol or an ester-forming derivative thereof is prepared.

Such a slurry contains 1.02 to 1.4 mol, preferably 1.03 to 1.3 mol, of ethylene glycol or its ester-forming derivative per 1 mol of terephthalic acid or its ester-forming derivative. This slurry is continuously supplied to the esterification reaction step.

The esterification reaction is carried out using a device in which at least two esterification reactors are connected in series, under conditions where ethylene glycol is refluxed, while removing water generated by the reaction outside the system by a rectification column. . The reaction conditions for performing the esterification reaction are such that the temperature of the first-stage esterification reaction is usually 240 to 270 ° C, preferably 245 to 265 ° C, and the pressure is usually 0.2 to 3 kg / cm ² G, preferably 0.5 to 265 ° C. 2 kg / cm ² , and the temperature of the final esterification reaction is usually 250 to 280 ° C., preferably 255 to 275 ° C., and the pressure is usually 0 to 1.5 kg / cm ² G, preferably 0 to 1.3 kg / cm ² G.

Therefore, when the esterification reaction is carried out in two stages, the first-stage and second-stage esterification reaction conditions are respectively in the above ranges, and when the esterification reaction is carried out in three or more stages, the second stage The reaction conditions for the esterification reaction from the first stage to the stage immediately before the final stage are those between the above-mentioned first-stage reaction conditions and final-stage reaction conditions.

For example, when the esterification reaction is performed in three stages, the reaction temperature of the second stage esterification reaction is usually 245
To 275 ° C, preferably 250 to 270 ° C, and the pressure is usually 0 to 2
kg / cm ² G and preferably 0.2~1.5kg / cm ² G. The conversion of these esterification reactions, at each stage,
Although there is no particular limitation, it is preferable that the increase and the degree of the esterification reaction rate in each stage are smoothly distributed, and the esterification reaction product in the final stage is usually
It is desirable to reach at least 90%, preferably at least 93%.

A low-order condensate is obtained by these esterification steps,
The number average molecular weight of this low-order condensate is usually from 500 to 5,000.

Such an esterification reaction can be carried out without adding any additives other than terephthalic acid and ethylene glycol, and can be carried out in the presence of a polycondensation catalyst described later. Tertiary amines such as tri-n-butylamine and benzyldimethylamine, quaternary ammonium hydroxides such as tetraethylammonium hydroxide, tetra-n-butylammonium hydroxide and trimethylbenzylammonium hydroxide and lithium carbonate, sodium carbonate, carbonate The addition of a small amount of a basic compound such as potassium or sodium acetate is preferred because the ratio of dioxyethylene terephthalate component units in the main chain of polyethylene terephthalate can be kept at a relatively low level.

Next, the obtained lower-order condensate is heated to a temperature equal to or higher than the melting point of the obtained polyethylene terephthalate under reduced pressure in the presence of a polycondensation catalyst, and the resulting glycol is distilled out of the system to undergo polycondensation. It is supplied to the liquid phase condensation polymerization step.

Such a polycondensation reaction in the liquid phase may be performed in one stage or may be performed in a plurality of stages. When the reaction is carried out in a plurality of stages, the polycondensation reaction conditions are such that the reaction temperature of the first stage polycondensation is usually from 250 to 290 ° C, preferably from 260 to 280 ° C, and the pressure is usually from 500 to 20 Torr, preferably from 200 to Torr. 30 Torr, and the temperature of the final polycondensation reaction is usually 265 to 300
° C preferably 275-295 ° C, pressure is usually 10-0.1Tor
r Preferably it is 5 to 0.5 Torr.

When the polycondensation reaction is carried out in two stages, the first stage and second stage polycondensation reaction conditions are respectively in the above ranges, and when the polycondensation reaction is carried out in three or more stages, the second stage starts from the second stage. The reaction conditions of the polycondensation reaction up to the first stage of the final stage are conditions between the above-mentioned reaction conditions of the first stage and the final stage.

For example, when the polycondensation reaction is performed in three stages, the reaction temperature of the second stage polycondensation reaction is usually 260 to 295 ° C.
It is preferably 270-285 ° C. and the pressure is usually 50-2 Torr
Preferably it is in the range of 40 to 5 Torr. The intrinsic viscosity [η] reached in each of these polycondensation reaction steps is not particularly limited, but it is preferable that the degree of increase in the intrinsic viscosity in each stage is smoothly distributed, and furthermore, the final stage polycondensation The intrinsic viscosity [η] of polyethylene terephthalate obtained from the reactor is usually 0.50 to 0.80 dl / g, preferably 0.1 to 0.8 dl / g.
It is desirably in the range of 55 to 0.75 dl / g.

In this specification, the intrinsic viscosity is calculated from the solution viscosity measured at 25 ° C. after heating and dissolving 1.2 g of polyethylene terephthalate in 15 cc of ortho-chlorophenol.

The polycondensation reaction is preferably performed in the presence of a catalyst and a stabilizer. As a catalyst, germanium dioxide, germanium tetraethoxide, germanium tetra n-
A germanium compound such as butoxide, an antimony catalyst such as antimony trioxide, and a titanium catalyst such as titanium tetrabutoxide can be used. Among these catalysts, it is preferable to use a germanium dioxide compound because the resulting polyethylene terephthalate has excellent hue and transparency. As the stabilizer, trimethyl phosphate, triethyl phosphate, tri-n
-Butyl phosphate, trioctyl phosphate, triphenyl phosphate, phosphates such as tricresyl phosphate, triphenyl phosphite, trisdodecyl phosphite, phosphites such as tris nonyl phenyl phosphite, methyl acid phosphate, Acidic phosphates such as isopropyl acid phosphate, butyl acid phosphate, dibutyl phosphate, monobutyl phosphate, dioctyl phosphate, and phosphorus compounds such as phosphoric acid and polyphosphoric acid are used. The proportion of these catalysts or stabilizers used is usually 0.0005 to 0.2% by weight, preferably 0.001 to 0.05% by weight of the metal in the catalyst, based on the weight of the mixture of terephthalic acid and ethylene glycol.
%, And in the case of a stabilizer, the weight of phosphorus atoms in the stabilizer is usually 0.001 to 0.1% by weight, preferably 0.002 to 0.02% by weight. These catalysts and stabilizers can be supplied at the stage of the esterification reaction step or in the first step of the polycondensation reaction step.
It can also be supplied to the reactor at the stage.

Polyethylene terephthalate used in the present invention,
As described above, a dicarboxylic acid other than terephthalic acid and a diol other than ethylene glycol may be contained in an amount of 20 mol% or less, but the polyethylene terephthalate particularly preferably used in the present invention has a general formula [I] The content of the ethylene terephthalate component unit (a) represented by the formula is in the range of 95.0 to 99.0 mol%, and the general formula [I
I] The content of the dioxyethylene terephthalate component unit (b) represented by the formula (1) is preferably in the range of 1.0 to 5.0 mol%.

The polyethylene terephthalate obtained in such a liquid phase polycondensation step is usually formed into particles (chips) by a melt extrusion molding method.

Such granular polyethylene terephthalate is usually
It is desirable to have an average particle size of 2.0 to 5.0 mm, preferably 2.2 to 4.0 mm.

The density of the polyethylene terephthalate obtained from the final polycondensation reactor is usually 1.33 to 1.35 g / cm ³ .

The particulate polyethylene terephthalate obtained as described above is supplied to a solid phase polycondensation step.

The granular polyethylene terephthalate supplied to the solid phase polymerization step may be preliminarily heated to a temperature lower than the temperature at which solid phase polycondensation is performed, and then preliminarily crystallized, and then supplied to the solid phase polycondensation step.

The pre-crystallization step can be carried out by heating the granular polyethylene terephthalate in a dry state, usually at a temperature of 120 to 200 ° C., preferably 130 to 180 ° C. for 1 minute to 4 hours. The reaction can also be carried out by heating usually at a temperature of 120 to 200 ° C. for 1 minute or more under an inert atmosphere.

The solid-phase polycondensation step in which the above-mentioned granular polyethylene terephthalate is supplied comprises at least one step, and the condensation temperature is usually 190 to 230 ° C, preferably 195 to 225 ° C,
The pressure is usually 1 kg / cm ² G to 10 Torr, preferably normal pressure to 10
The solid-phase polycondensation reaction is carried out under an atmosphere of an inert gas such as nitrogen gas, argon gas, or carbon dioxide gas at 0 Torr. Among these inert gases, nitrogen gas is preferred.

The polyethylene terephthalate thus obtained has an intrinsic viscosity of 0.7 to 1.5 dl / g, preferably 0.72 to 1.5 dl / g.
g is desirable.

The density of polyethylene terephthalate is usually 1.
It is desirably 38 g / cm ³ or more, preferably 1.39 g / cm ³ or more.

In the present invention, those having an amount of the cyclic trimer of 0.4% by weight or less, preferably 0.35% by weight or less are used.

The polyethylene terephthalate of the present invention has a water absorption of 2000 ppm or more.

As described above, in polyethylene terephthalate having a high water absorption, an increase in the cyclic trimer in the molding process when molding as a raw material of a molded article such as a bottle is significantly suppressed. This can be confirmed, for example, by measuring the increase in the amount of the cyclic trimer after heating and melting polyethylene terephthalate to a temperature of 290 ° C. to form a stepped square plate. Thus, polyethylene terephthalate having a water absorption of 2000 ppm or more, specifically, at a temperature of 290
The amount y (% by weight) of the cyclic trimer after being heated and melted at ° C. to form a stepped square plate can be suppressed to a range of y ≦ −0.20x + 0.20.

In the above formula, x is the concentration (% by weight) of the cyclic trimer before forming the stepped square plate.

In the present specification, a stepped square plate is formed from granular polyethylene terephthalate by the following method, and the amount of cyclic trimer increase y (% by weight) after forming the square plate in this manner is as follows. Is measured.

That is, 2 kg of granular polyethylene terephthalate, whose cyclic trimer content was previously measured (measured value X%), was heated at a temperature of 140 ° C.
Dry using a tray-type dryer for at least 16 hours at a temperature of 10 ° C. and a pressure of 10 torr to reduce the water content of the granular polyethylene terephthalate to 50 ppm or less.

Next, the dried granular polyethylene terephthalate was injection-molded using a M-70A injection molding machine manufactured by Meiki Seisakusho under the conditions of a cylinder temperature of 290 ° C. and a mold cooling water temperature of 15 ° C. To obtain a molded product.

The injection molding of the stepped square plate-like molded product is performed by supplying the dried granular polyethylene terephthalate to the injection molding machine from a hopper so that the measuring time is 12 seconds and the injection time is 60 seconds. The residence time of the molten resin in the molding machine is about 72 seconds. The weight per one stepped square plate-shaped molded product is 75 g, and the cyclic trimer measurement sample is prepared using any one of the eleventh to fifteenth samples after the start of injection molding.

The stepped square plate-like molded product 1 has a shape as shown in FIG. 1, the thickness of the part A is about 6.5 mm, the thickness of the part B is about 5 mm, and the thickness of the part C is It is about 4mm. Using this part C, the amount of increase in cyclic trimer of the molded product is examined.

Next, the formed plate-like molded product having a thickness of 4 mm is cut into chips, and the amount of the cyclic trimer is measured as a sample for measuring the cyclic trimer.

In addition, the polyethylene terephthalate according to the present invention has a small increase in acetaldehyde at the time of molding, so that a molded article such as a bottle having a small amount of acetaldehyde can be obtained. When polyethylene terephthalate having a high acetaldehyde content is used as a molded product,
This may cause an unpleasant odor or an unpleasant odor, or change the flavor and aroma of the contents. When a photographic film is produced from polyethylene terephthalate having a high acetaldehyde content, fogging is likely to occur.

Effect of the Invention The polyethylene terephthalate according to the present invention has an intrinsic viscosity of 0.7 to 1.5 dl / g when dissolved in an ortho-chlorophenol solution and measured at a liquid temperature of 25 ° C., and has a cyclic trimer content of Is 0.40% by weight or less and the water absorption is 2000pp
m or more, the amount of cyclic trimers generated during molding is small, and the total amount of cyclic trimers contained in polyethylene terephthalate during molding is small, so that mold contamination is less likely to occur during molding.

Therefore, the polyethylene terephthalate according to the present invention can improve the productivity of molded articles such as bottles since the mold does not need to be frequently cleaned when producing molded articles, and furthermore, the whitening of the obtained bottles can be improved. Can be prevented.

Hereinafter, the present invention will be described with reference to examples, but the present invention is not limited to these examples.

Example 1 Granular polyethylene terephthalate having an intrinsic viscosity of 0.80 dl / g, a density of 1.40 g / cm ³ and a cyclic trimer content of 0.33% by weight was heated to a temperature of 40 ° C. at a relative humidity of 60%. The polyethylene terephthalate was allowed to absorb water by being left in the set thermo-hygrostat for 40 days.

The water on the surface of the polyethylene terephthalate particles left for 40 days was removed by a filter paper until the filter paper became free of water. Thereafter, the proportion of water contained in the polyethylene terephthalate, that is, the water absorption was 4,500 ppm.

This polyethylene terephthalate is injected with an injection molding machine for 200
C., and the amount of the cyclic trimer in the obtained injection molded plate was measured by the method described above. As a result, the amount of the cyclic trimer was 0.34% by weight.

Examples 2 to 5 The same granular polyethylene terephthalate as in Example 1 was left in a thermo-hygrostat set at the relative humidity and temperature shown in Table 1 for the time shown in Table 1 to obtain the polyethylene terephthalate of Examples 2 to 5. Was.

Table 1 also shows the water absorption in the polyethylene terephthalate and the amount of the cyclic trimer measured in the same manner as in Example 1.

The polyethylene terephthalate of Examples 1 to 5 was used as a raw material to repeatedly form a preform by an injection molding machine, then stretch-blow molded the preform, and then heat-set to form a bottle. The molded bottle was of excellent quality with little whitening.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a stepped square plate-like molded product.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-59-219328 (JP, A) JP-A-1-306421 (JP, A) JP-A-58-144573 (JP, A) 86797 (JP, A) JP-A-59-25815 (JP, A) JP-A-55-13715 (JP, A) JP-A-55-89330 (JP, A) JP-A-2-298512 (JP, A) JP-A-1-304912 (JP, A) JP-B-42-26532 (JP, B1) (58) Fields investigated (Int. Cl. ⁷ , DB name) C08G 63/00-63/91

Claims (4)

(57) [Claims] (1) a density of 1.38 g / cm ³ or more, a cyclic trimer content of 0.40% by weight or less, and a water absorption of 2000 pp
m or more, and dried polyethylene terephthalate is heated and melted at 290 ° C. by an injection molding machine to form a stepped square plate. −0.20x + 0.20 (where x is the concentration (% by weight) of the cyclic trimer before forming the stepped square plate) polyethylene terephthalate. (2) a density of 1.38 g / cm ³ or more, a cyclic trimer content of 0.40% by weight or less, and a water absorption of 50 to 1000 pp;
2. The polyethylene terephthalate according to claim 1, wherein the polyethylene terephthalate in the range of m is left in an air atmosphere at a relative humidity of 50 to 80% and a temperature of 0 to 50 ° C. for 50 days or more to allow the polyethylene terephthalate to absorb water. A method for producing polyethylene terephthalate. (3) a density of 1.38 g / cm ³ or more, a cyclic trimer content of 0.40% by weight or less, and a water absorption of 50 to 1000 pp;
2. The polyethylene terephthalate according to claim 1, wherein the polyethylene terephthalate in the range of m is left in an air atmosphere at a relative humidity of 80 to 95% and a temperature of 0 to 50 ° C. for 30 days or more to absorb water. A method for producing polyethylene terephthalate. 4. A polyethylene terephthalate having a density of 1.38 g / cm ³ or more, a cyclic trimer content of 0.40% by weight or less, and a water content of 2000 ppm or more of water present on the surface and inside thereof, The method for producing polyethylene terephthalate according to claim 1, wherein the polyethylene terephthalate is allowed to stand in an air atmosphere having a humidity of 20% or more and a temperature of 0 ° C or more for 20 days or more to absorb water.