Method for producing polyester resin

Abstract

To provide a method for producing a recycled polyester resin which contains a recycled PET bottle flake as a raw material and has good color tone.SOLUTION: A flake obtained by heat treating a recycled PET bottle flake at 150 to 210°C, and then removing a yellowed flake is used as a raw material.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a method for producing a polyester resin, and more particularly to a method for producing a recycled polyester resin having good color tone. [Background technology]

[0002] Due to its excellent properties, polyester is used in a wide variety of fields, including fibers and films, and in recent years, environmentally friendly materials made from recycled raw materials have been attracting attention. In particular, collecting polyester resin that has been molded and used to make PET beverage bottles, films, etc., i.e., recovered polyester products, and using the flakes as raw materials, has attracted a lot of attention as a resource recycling method.

[0003] On the other hand, recycled polyester resin made from recycled used polyester products has a thermal history not only during molding of the recycled polyester resin, but also during the chemical reactions and molding processes leading up to the primary product, and during the process of recycling raw materials after collection (crushing, washing, melt molding, etc. of collected bottles). For this reason, polystyrene and nitrogen-based compounds contained as impurities promote yellowing due to the thermal history, and the deterioration of the color tone of the resulting recycled polyester resin is a problem. In particular, when depolymerization and repolymerization are performed to obtain recycled polyester resin, the thermal history is even greater, so the deterioration of color tone is significant. Such color deterioration reduces the appeal of products regardless of their use, such as fibers and films, and it was necessary to improve it. For this reason, various methods have been disclosed.

[0004] Patent Document 1 proposes a method for obtaining a raw material of any color tone by crushing the recovered molded product, and then repeating a step of removing the yellowing on the surface of the molded product, a step of removing the yellowed part, and a step of measuring the color tone of the surface of the molded product. As a method for removing the yellowing on the surface, physical methods such as polishing, drilling, cutting, grinding, etc. are mentioned. In addition, as a chemical method, a method of immersing in an oxidizing solution or a reducing solution or a method of pouring it over are mentioned.

[0005] Patent Document 2 proposes a method of adding a specific amount of a pigment such as a benzoxazole-based fluorescent whitening agent to a resin composition consisting of recycled polyester resin and a thermoplastic resin other than polyester. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] JP 2015-061751 A [Patent Document 2] JP 2004-359914 A Summary of the Invention [Problem to be solved by the invention]

[0007] In the proposal of Patent Document 1, due to the shape of the recycled PET bottle flakes used as the raw material in the present invention, it is difficult to remove the yellowing by physical methods, and there are problems with removing the yellowing by chemical methods, such as the need for costly and special equipment for immersion and pouring. In addition, the proposal of Patent Document 2 has problems with the use of unnecessary additives due to the characteristics of the recycled polyester resin to be obtained, which increases the production cost, gives an unintended white color, and increases impurities when recycling is considered.

[0008] The present invention provides a method for producing recycled polyester resin with excellent color tone by removing flakes that turn yellow during the production process of recycled polyester resin from recycled PET bottle flakes, without requiring special equipment or using unnecessary additives. [Means for solving the problem]

[0009] The above object can be achieved by the following configuration. A method for producing polyester resin in which PET bottle flakes are heat-treated at 150-210°C, yellowed flakes are removed, and then the flakes are melted and turned into chips. Effect of the Invention

[0010] According to the present invention, it is possible to produce recycled polyester resin with excellent color tone by removing flakes that turn yellow during the production process of recycled polyester resin from recycled PET bottle flakes, without requiring special equipment or using unnecessary additives. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] The method for producing a polyester resin of the present invention involves melting recycled PET bottle flakes as a raw material and then forming them into chips, and heat-treating the flakes at 150 to 210°C, after which yellowed flakes are removed and used to produce a polyester resin.

[0012] The above-mentioned "recycled PET bottle flakes" can be obtained, for example, by cleaning and crushing PET bottles that have been on the market once and then collected after use. A typical example is PET beverage bottles collected at supermarkets.

[0013] The chipping process after melting may be a process called "repelletization" in which the flakes are simply melted and molded and solidified, or a process called "depolymerization" in which the flakes are fed together with a glycol component to a depolymerization process to obtain oligomers, and the oligomers are then subjected to a polycondensation reaction to obtain PET polymer that is then chipped.

[0014] The color tone improving effect is more remarkable in the case of depolymerization. In addition, in the case of depolymerization, the intrinsic viscosity of the polyester resin can be set arbitrarily, and since any additive can be added in the polycondensation process, various properties can be imparted to the polyester resin.

[0015] The PET bottle flakes used as the raw material in the present invention are heat-treated at 150 to 210°C, and then the yellowed flakes are removed. If the heat treatment temperature is less than 150°C, the flakes are not given enough heat history and the yellowing of the flakes does not proceed sufficiently. As a result, yellowed flakes remain in the chipping process after melting. The heat treatment temperature is more preferably 180°C or higher. On the other hand, if the heat treatment temperature is higher than 210°C, most of the flakes will yellow, and the amount that can be used as raw material will be reduced (the loss rate increases and the manufacturing cost increases).

[0016] Although there are no limitations on the type of heat source or the duration of the heat treatment, it is preferable to use a vacuum dryer and perform the heat treatment under vacuum at 150 to 210° C. for 2.5 to 3.5 hours.

[0017] In the present invention, the color tone b value of the flakes to be removed, i.e., the flakes yellowed after heat treatment, is preferably 12.5 or more. The color tone b value of the yellowed flakes to be removed is more preferably 8.0 or more. If it is 8.0 or more, both the raw material loss rate and the color tone of the polyester resin are good. From the viewpoint of further improving the color tone, the color tone b value of the yellowed flakes to be removed is preferably 5.0 or more, but on the other hand, the raw material loss rate increases and the manufacturing cost increases.

[0018] There is no particular restriction on the method for removing flakes in the present invention. The flakes may be manually selected and removed by measuring the color b value, or an existing device that automatically measures the color b value and removes flakes with a specific color b value using a CCD camera or the like may be used.

[0019] One of the reasons why the color tone of polyester resin deteriorates is that the nitrogen-based compounds contained in the raw material PET bottle flakes promote yellowing when subjected to heat history. In the present invention, the flakes that turn yellow, i.e., the nitrogen-based compounds, are removed in advance by heat treatment, so that the nitrogen atom content of the resulting polyester resin can be reduced to 10 ppm or less. By reducing the nitrogen atom content of the polyester resin obtained by the production method of the present invention to 10 ppm or less, yellowing due to heat history in the molding process can be suppressed, and color tone can be improved. EXAMPLES

[0020] The present invention will be described in more detail below with reference to examples. The physical properties in the examples were measured by the following methods.

[0021] (1) Color tone b value The Hunter value (b value) was measured using a color difference meter (SM Color Computer, model: SM-T45, manufactured by Suga Test Instruments Co., Ltd.), and the b values ​​of the flakes and chips were measured. The b value of the obtained chip was judged as to whether it was suitable for practical use according to the following criteria. 〇(Excellent): Less than 11.0 △ (Practical): 11.0 or more and less than 12.0 × (Not practical): 12.0 or higher

[0022] (2)Nitrogen atom content The sample was weighed into a platinum boat and introduced into an analyzer (Mitsubishi Chemical Corporation, trace nitrogen analyzer, model: ND-100) for thermal decomposition and oxidation, and the generated nitric oxide was measured by chemiluminescence. The concentration was automatically calculated based on a calibration curve prepared in advance using a pyridine standard solution.

[0023] (3) Raw material loss rate The weight of the PET bottle flakes removed after the heat treatment relative to the total weight of the PET bottle flakes before the heat treatment was used as the raw material loss rate, and the usability was judged according to the following criteria. 〇 (Excellent): Less than 1.00% △ (Practical): 1.00% or more and less than 2.00% × (Not practical): 2.00% or more

[0024] Example 1 (Heat treatment process) 600.0 g of PET bottle flakes were placed in a vacuum dryer and heat-treated at 180°C for 3 hours.

[0025] (Flake removal process) The color tone of the heat-treated flakes was measured, and flakes with a b value of 8.0 or more were manually selected and removed. The weight of the flakes removed at this time was 4.0 g, and the raw material loss rate was a good 0.67%.

[0026] (Depolymerization process) 596.0 g of the heat-treated flakes, from which flakes with a b value of 8.0 or more had been removed, were added together with 103.7 g of ethylene glycol to a stirred autoclave equipped with a reflux condenser, which had previously been charged with 596.0 g of bis-2-hydroxyethyl terephthalate prepared by any method. Then, a depolymerization reaction was carried out at a reaction temperature of 240°C.

[0027] (purification process) The resulting reaction product was purified by passing it through a transfer pipe equipped with a nonwoven fabric filter with a filtration accuracy of 10 μm at a temperature of 240° C. and transferred to a polycondensation reaction device. At this time, there was no clogging of the filter or pipe, and the transfer was good.

[0028] (Polycondensation process) The purified reaction product was stirred at 30 rpm while controlling the temperature of the reaction product to 240°C, and to the resulting polyethylene terephthalate, 2,600 ppm by weight, calculated as sulfur atoms, of 5-sodium sulfoisophthalic acid dimethyl ethylene glycol adduct, 1.0% by weight of polyethylene glycol having an average molecular weight of 2,000, 307 ppm by weight of antimony glycolate, 320 ppm by weight of phosphoric acid, 1,600 ppm by weight of lithium acetate tetrahydrate, 900 ppm by weight of pentaerythritol tetrakis[3-(3',5'-di-tert-butyl-4'-hydroxyphenyl)propionate], and 0.07% by weight of titanium oxide were added.

[0029] Thereafter, the reaction system was gradually heated to 285°C, and the pressure was lowered to 40 Pa to carry out the polycondensation reaction. The time to reach the final temperature and the final pressure were both 90 minutes. When the predetermined stirring torque was reached, the reaction system was purged with nitrogen to return to normal pressure to stop the polycondensation reaction, and the mixture was discharged into cold water in the form of strands and immediately cut to obtain polymer chips.

[0030] As shown in Table 1, the nitrogen atom content of the obtained chips was 6 ppm and the color tone b value was 10.7, which was good.

[0031] Example 2 The heat treatment step and the flake removal step were carried out in the same manner as in Example 1. Thereafter, repellentization was carried out in the following manner.

[0032] (Repelletizing process) After removing flakes with a b value of 8.0 or more, 596.0 g of the heat-treated flakes were fed into a twin-screw kneader, where they were melt-mixed and discharged at a set temperature of 280°C and a discharge rate of 2.0 kg / h. The discharged strands were cooled with cold water and immediately cut to obtain polymer chips.

[0033] As shown in Table 1, the raw material loss rate was a good 0.67%, and the nitrogen atom content of the obtained chips was 6 ppm and the color tone b value was 8.6, both of which were good.

[0034] Example 3 The same procedure as in Example 1 was repeated, except that the heat treatment temperature of the PET bottle flakes was changed to 210° C. The raw material loss rate was 1.01%, which was acceptable for practical use, and the nitrogen atom content of the obtained chips was 4 ppm and the color tone b value was 9.8, which was good.

[0035] Example 4 The same procedure as in Example 2 was repeated, except that the heat treatment temperature of the PET bottle flakes was changed to 210° C. The raw material loss rate was 1.01%, which was acceptable for practical use, and the nitrogen atom content of the obtained chips was 4 ppm and the color tone b value was 8.5, which was good.

[0036] Example 5 The same procedure as in Example 1 was repeated, except that the heat treatment temperature of the PET bottle flakes was changed to 150° C. The raw material loss rate was a good 0.56%, and the nitrogen atom content of the obtained chips was 8 ppm and the color tone b value was 11.3, which was acceptable for practical use.

[0037] Example 6 The same procedure as in Example 2 was repeated, except that the heat treatment temperature of the PET bottle flakes was changed to 150° C. The raw material loss rate was a good 0.56%, and the nitrogen atom content of the obtained chips was 8 ppm and the color tone b value was 8.7, both of which were good.

[0038] Example 7 Except for changing the color tone b value of the flakes to be removed after the heat treatment to 5.0 or more, the same procedure as in Example 1 was carried out. The raw material loss rate was 1.93%, which was acceptable for practical use, and the nitrogen atom content of the obtained chips was 3 ppm, and the color tone b value was 9.2, which was good.

[0039] Example 8 Except for changing the color tone b value of the flakes to be removed after the heat treatment to 12.5 or more, the same procedure as in Example 1 was carried out. The raw material loss rate was a good 0.48%, and the nitrogen atom content of the obtained chips was 9 ppm and the color tone b value was 11.8, which was acceptable for practical use.

[0040] Comparative Example 1 The same procedure as in Example 1 was repeated, except that the heat treatment temperature of the PET bottle flakes was changed to 230° C. The nitrogen atom content of the obtained chips was 3 ppm, and the color tone b value was 9.3, which was good, but the raw material loss rate was 3.67%, which was not practical.

[0041] Comparative Example 2 The same procedure as in Example 1 was repeated, except that the heat treatment temperature of the PET bottle flakes was changed to 130° C. The raw material loss rate was a good 0.52%, but the nitrogen atom content of the obtained chips was 18 ppm and the color tone b value was 16.5, both of which were not suitable for practical use.

[0042] Comparative Example 3 The same procedure as in Example 2 was repeated, except that the heat treatment temperature of the PET bottle flakes was changed to 230° C. The nitrogen atom content of the obtained chips was 4 ppm, and the color tone b value was 8.8, which was good, but the raw material loss rate was 3.67%, which was not practical.

[0043] Comparative Example 4 The same procedure as in Example 2 was repeated, except that the heat treatment temperature of the PET bottle flakes was changed to 130° C. The raw material loss rate was a good 0.52% by weight, but the nitrogen atom content of the obtained chips was 19 ppm and the color tone b value was 12.5, which was not suitable for practical use.

[0044] [Table 1]

Claims

1. This is a method for producing polyester resin in which PET bottle flakes are heat treated at 150-210°C, yellowed flakes are removed, and then the flakes are melted and turned into chips.

2. 2. The method for producing a polyester resin according to claim 1, wherein the flakes from which the yellowed flakes have been removed are supplied to an esterification step together with a glycol component, depolymerized to obtain an oligomer, and the oligomer is again polycondensed to obtain a polyester, which is melted and chipped.

3. The method for producing a polyester resin according to claim 1 or 2, wherein the yellowed flakes have a color tone b value of 12.5 or more.

4. 2. A polyester resin obtained by the process according to claim 1, having a nitrogen atom content of 10 ppm or less.

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