KR20250100327A - Method for producing recycled polyester using recycled dimethyl terephthalate (DMT) and recycled ethylene glycol (EG) produced by low-temperature methanolysis depolymerization technology - Google Patents

Abstract

A method for manufacturing recycled polyester using waste polyester,
A raw material introduction step in which recycled dimethyl terephthalate (DMT) and recycled ethylene glycol (EG), manufactured by a low-temperature methanolysis-based depolymerization technology, are introduced into a reactor at a molar ratio of 1.0:1.0 to 2.2; an ester exchange reaction step in which the internal temperature of the reactor is increased to 210 to 260°C and the external temperature to 280 to 320°C for manufacturing recycled bis-2-hydroxyethylene terephthalate (BHET), and 100 to 600 ppm of zinc acetate (ZnAC) as a catalyst is introduced based on the weight of recycled dimethyl terephthalate; 10 to 400 ppm of a metal catalyst selected from titanium (Ti), antimony (Sb), and germanium (Ge) is added to the manufactured recycled bis-2-hydroxyethylene terephthalate (BHET), and trimethyl phosphate (TMP) as a heat stabilizer is added. The present invention provides a method for manufacturing recycled polyester using waste polyester, characterized by comprising a condensation polymerization reaction step of adding 200 to 900 ppm of ethylene glycol (EG) BHET in an amount of 5 to 25 wt% based on the weight of the polymer, and a recycled polyester chip manufacturing step.
According to the method for manufacturing recycled polyester of the present invention, all colorless and colored waste polyester generated domestically can be depolymerized into monomers without distinguishing the grade, and recycled polyester with excellent physical properties can be manufactured.
In addition, the method for manufacturing waste polyester according to the present invention can be used for various purposes as an eco-friendly product that can drastically reduce carbon emissions.

Description

Method for producing recycled polyester using recycled dimethyl terephthalate (DMT) and recycled ethylene glycol (EG) produced by low-temperature methanolysis depolymerization technology

The present invention relates to a method for producing recycled polyethylene terephthalate (hereinafter referred to as “PET”) by subjecting recycled dimethyl terephthalate (hereinafter referred to as “DMT”) obtained by depolymerizing polyester waste and recycled ethylene glycol (hereinafter referred to as “EG”) to an ester exchange reaction and a condensation polymerization reaction.

Recently, with the development of the plastics industry, plastic products have become widespread, and waste disposal has become a major environmental problem.

Plastics are widely used in a variety of fields such as films, beverage bottles, and fibers, and as a result, a large amount is widely used. However, the recovery rate for recycling waste plastics is only about 25% worldwide, and the remaining unrecovered resources are being disposed of through incineration or landfill, which not only causes environmental pollution, but also causes a large economic loss due to the final disposal of usable resources.

According to the Ministry of Environment's Resource Recycling Information System, waste fibers generated from industrial sites nationwide recently reached 1,239 tons per day, which is about six times the amount generated in 2018 (224 tons in 2017). Of the total 1,239 tons of waste fibers generated in 2018, 67 tons were incinerated and 18 tons were landfilled, and it is not even known what happened to the remaining waste fibers.

Over the past 15 years, clothing production has more than doubled, and projections suggest that demand for textiles will increase by 84% over the next 20 years, driven by the growth of the so-called fast fashion industry, which is driving down prices and shorter consumption cycles for clothing products each year.

The severity of waste fiber is less well known than that of waste plastic, and experts and environmental groups agree that the problem of waste fiber must be recognized and solved. In addition, the market share of recycled PET in the PET market, one of the synthetic fibers that accounts for 60% of the entire fiber market, has increased by approximately 6% from 8% in 2007 to 14% in 2017, indicating that the demand for sustainable fibers is steadily increasing.

Since solidified polyester is unstable in heat, it is almost impossible to reuse it after melting it at a temperature higher than its melting point, so the waste must be recovered at a lower temperature. The process for recovering waste polyester includes a process for recovering raw materials such as terephthalic acid (TPA), dimethyl terephthalate (DMT), and ethylene glycol (EG) through depolymerization of waste polyester using a catalyst, and a process for manufacturing bis-2-hydroxyethyl terephthalate (BHET), which is an intermediate product.

The depolymerization pathways that are being applied for industrial recycling of polyester include hydrolysis, glycolysis, methanolysis, and ammonolysis, and various chemical depolymerization methods are widely used, ranging from a combination of these to a composite process that utilizes the advantages of each process.

Glycolysis is a depolymerization reaction that adds glycol as a reactant. The most common example of glycolysis is the process of manufacturing bis(2-hydroxyethyl) terephthalate (BHET) by adding an excessive amount of ethylene glycol, one of the monomer raw materials. Since ethylene glycol, one of the raw materials for the synthesis of polyethylene terephthalate (PET) polymer, is used as a reactant, the products manufactured by the depolymerization reaction have a chemical structure in which ethylene glycol is already bonded to both ends of terephthalate. Therefore, even if only some of the raw materials of the existing condensation polymerization process using terephthalic acid are replaced, very favorable results can be obtained in terms of reaction kinetics.

Glycolysis is usually performed under reflux conditions of the reactant glycol, and there are problems in that the speed of the step of decomposing oligomers into monomers is slow, and even if the reaction time is delayed, the equilibrium between compounds is reached, so the product purity may be low, and it is not easy to separate the final product, the monomer, from the reactant with high purity or high yield.

The methanolysis process is one of the processes that has been widely applied in actual commercial processes, from the past global chemical companies to small and medium-sized plastic industries. Theoretically, dimethyl terephthalate (DMT) is obtained as the final monomer product by the above process, and as the ester exchange reaction progresses, ethylene glycol (EG) corresponding to the equivalent number of moles of decomposed terephthalate can be liberated.

Registered patent No. 10-2462599 discloses a method for producing DMT and EG through a low-temperature methanolysis process that treats waste polyester in a mixture containing methanol, calcium carbonate (K ₂ CO ₃ ) and a polar aprotic solvent.

The present invention aims to provide a method for producing recycled polyester by subjecting recycled DMT and recycled EG obtained in this manner to ester exchange reaction and condensation polymerization reaction.

Registered Patent No. 10-2462599 (announced on November 3, 2022) Publication Patent No. 10-2023-0068533 (Published on May 18, 2023) Publication Patent No. 10-2023-0132222 (Published on September 15, 2023)

The present invention provides a method for performing depolymerization of a polymer including an ester functional group, which enables production under conditions requiring no or relatively low energy consumption from reaction to product purification, thereby greatly reducing energy consumption, and thereby providing a recycled polyester resin having excellent physical properties and appearance through ester exchange reaction and condensation polymerization reaction of recycled dimethyl terephthalate (DMT) and recycled ethylene glycol (EG).

In order to solve the above problem, the present invention provides a method for manufacturing recycled polyester using waste polyester, comprising: a raw material introduction step in which recycled dimethyl terephthalate (DMT) and recycled ethylene glycol (EG), manufactured by a low-temperature methanolysis-based depolymerization technology, are introduced into a reactor at a molar ratio of 1.0:1.0 to 2.2; an ester exchange reaction step in which the internal temperature of the reactor is increased to 210 to 260°C and the external temperature to 280 to 320°C for manufacturing recycled bis-2-hydroxyethylene terephthalate (BHET), and 100 to 600 ppm of zinc acetate (ZnAC) as a catalyst is introduced based on the weight of recycled dimethyl terephthalate; and a metal catalyst selected from titanium (Ti), antimony (Sb), and germanium (Ge) is added to the manufactured recycled bis-2-hydroxyethylene terephthalate (BHET). The present invention provides a method for manufacturing recycled polyester using waste polyester, characterized by comprising a condensation polymerization step of adding 10 to 400 ppm of trimethyl phosphate (TMP) as a heat stabilizer, 200 to 900 ppm of ethylene glycol (EG) in an amount of 5 to 25 wt% based on the weight of manufactured recycled BHET, and a recycled polyester chip manufacturing step.

In one embodiment of the present invention, the ester exchange reaction step may include a first temperature increasing step of increasing the temperature of the introduced raw material to an internal temperature of 150°C and an external temperature of 100 to 180°C within 1 to 2 hours, a second temperature increasing step of increasing the temperature of the internal temperature of the reactor from 150°C to 170 to 200°C at a temperature increasing rate of 0.1 to 3.0°C/min, a first holding step of maintaining the temperature of the internal temperature of the reactor at 170 to 200°C for 30 to 60 minutes, a third temperature increasing step of increasing the temperature of the internal temperature of the reactor from 170 to 200°C to 210 to 260°C at a temperature increasing rate of 0.1 to 3.0°C/min, and a second holding step of maintaining the temperature at 210 to 260°C for 30 to 60 minutes.

In addition, the condensation polymerization reaction step is characterized in that the temperature inside the reactor is increased from 210 to 260°C to 260 to 310°C at a temperature increase rate of 0.1 to 3.0°C/min, and is performed in a vacuum state at a final pressure of 0.1 to 0.5 Torr for 150 to 180 minutes.

On the one hand, the above condensation polymerization reaction step is characterized by having a cold trap of -30.0 to -60.0°C so that unreacted ethylene glycol is condensed and removed internally.

The above-mentioned recycled polyester chip manufacturing step is characterized by manufacturing the recycled polyester inside the reactor by extruding it into water using nitrogen pressure and cutting it.

According to the method for manufacturing recycled polyester of the present invention, all colorless and colored waste polyester generated domestically can be depolymerized into monomers without distinguishing the grade, and recycled polyester with excellent physical properties can be manufactured.

In addition, the method for manufacturing waste polyester according to the present invention can be used for various purposes as an eco-friendly product that can drastically reduce carbon emissions.

Figure 1 is a schematic diagram of a process for manufacturing recycled polyester according to the present invention.

Hereinafter, the present invention will be described in more detail through examples and experimental examples. However, these examples are only intended to help understand the present invention and the scope of the present invention is not limited to these examples in any way.

According to one embodiment of the present invention, a method for producing recycled polyester using waste polyester comprises: a raw material introduction step of introducing recycled dimethyl terephthalate (DMT) and recycled ethylene glycol (EG), which are produced by a low-temperature methanolysis-based depolymerization technology, into a reactor at a molar ratio of 1.0:1.0 to 2.2; an ester exchange reaction step of increasing the internal temperature of the reactor to 210 to 260°C and the external temperature to 280 to 320°C, and introducing 100 to 600 ppm of zinc acetate (ZnAC) as a catalyst based on the weight of recycled dimethyl terephthalate to produce recycled bis-2-hydroxyethylene terephthalate (BHET); The method comprises a condensation polymerization step of reacting the manufactured recycled bis-2-hydroxyethylene terephthalate (BHET) by adding 10 to 400 ppm of a metal catalyst selected from titanium (Ti), antimony (Sb), and germanium (Ge), 200 to 900 ppm of trimethyl phosphate (TMP) as a heat stabilizer, and 5 to 25 wt% of ethylene glycol (EG) based on the weight of the manufactured recycled BHET; and a recycled polyester chip manufacturing step.

First, recycled dimethyl terephthalate (DMT) and recycled ethylene glycol (EG), which are used as raw materials, are obtained by low-temperature methanolysis-based depolymerization. They are recovered by depolymerizing methanol and calcium carbonate at 25℃ using them as catalysts.

The recycled dimethyl terephthalate (DMT) and recycled ethylene glycol (EG) are fed into the reactor at a molar ratio of 1.0:1.0 to 2.2. The composition of the monomers affects the transesterification reaction rate, the purity of bis-2-hydroxyethylene terephthalate (BHET), and the viscosity of the final product. Therefore, if less than 1.0 mol of recycled ethylene glycol is mixed per 1 mol of recycled dimethyl terephthalate (DMT), unreacted residual monomer remains in the transesterification reaction, and if it exceeds 2.2 mol, the reaction time becomes longer and the amount of residual impurities increases, which lowers the intrinsic viscosity of the final product.

After raw materials are fed into the reactor, zinc acetate (ZnAC) as a catalyst can be fed in an amount of 100 to 600 ppm based on the weight of recycled dimethyl terephthalate (DMT). After the raw materials are fed into the reactor, the reactor is sealed, and the internal stirrer is rotated at 60 to 100 rpm. The temperature inside the reactor is sequentially increased from room temperature to 230°C through a temperature-raising step and a holding step, and the ester exchange reaction is performed for 3 to 5 hours.

If the temperature is raised rapidly at the beginning of the ester exchange reaction, a large amount of DMT and EG is released, preventing the reaction from occurring sufficiently. If the temperature increase time is too long, methanol is not extracted sufficiently, causing the viscosity to decrease during the condensation polymerization reaction stage.

In the ester exchange reaction step, the stirrer inside the reactor stirs at 60 to 100 rpm, and the temperature is initially heated to 150°C. If the temperature is increased too quickly, exceeding 3.0°C/min, in the second temperature increase step from 150°C to 170 to 200°C, DMT and EG are discharged at the initial temperature increase point, and the ester exchange reaction cannot occur sufficiently. In addition, if the temperature increase speed is too slow, less than 0.1°C, the processing time increases excessively and methanol is not completely extracted.

Once the second temperature increase is completed, the first maintenance step is performed at an internal temperature of 170 to 200°C for 30 to 60 minutes to ensure that the ester exchange reaction is sufficiently carried out. The degree of completion of the ester exchange reaction can be judged by checking the amount of methanol discharged at this time. When the amount of methanol discharged compared to the input amount of DMT reaches a molar ratio of 1.0:1.8 to 3.0 mol, the ester exchange reaction can be considered complete.

After the first maintenance step is completed, the internal temperature of the reactor is increased from 170 to 200°C to 210 to 260°C for a third temperature increasing step at a temperature increasing rate of 0.1 to 3.0°C/min, and the second maintenance step is performed to maintain the temperature at 210 to 260°C for 30 to 60 minutes, thereby completing the ester exchange reaction. When the temperature is increased to 210 to 260°C, unreacted EG is vaporized and removed to the outside. If methanol is not discharged during the second maintenance time at 210 to 260°C and the vaporization of EG no longer progresses, the ester exchange reaction is completed and recycled bis-2-hydroxyethylene terephthalate (BHET) is formed.

After the ester exchange reaction is completed, a condensation polymerization reaction step using recycled bis-2-hydroxyethylene terephthalate (BHET) is performed. This is a step in which 10 to 400 ppm of a metal catalyst selected from titanium (Ti), antimony (Sb), and germanium (Ge), 200 to 900 ppm of trimethyl phosphate (TMP) as a heat stabilizer, and 5 to 25 wt% of ethylene glycol (EG) are added to the manufactured recycled bis-2-hydroxyethylene terephthalate (BHET) and reacted.

The metal catalyst is for promoting the condensation polymerization reaction and may be at least one selected from TiO ₂ , Sb ₂ O ₃ , and GeO ₂ . It would be more preferable to use GeO ₂ .

It is preferable to add the metal catalyst at 10 to 400 ppm based on bis-2-hydroxyethyl terephthalate at 265°C.

In addition, trimethyl phosphate (TMP) may be added to improve the thermal stability of bis-2-hydroxyethyl terephthalate (BHET). It is preferable to add 200 to 900 ppm of the trimethyl phosphate (TMP) based on bis-2-hydroxyethyl terephthalate (BHET) at 260°C.

On the one hand, in order to promote temperature increase, 5 to 25 wt% of recycled BHET manufactured from ethylene glycol (EG) based on bis-2-hydroxyethyl terephthalate is added and reacted, and in order to prevent color change due to oxidation of the reaction product, nitrogen is continuously supplied into the reactor while oxygen is discharged, and the target intrinsic viscosity (IV) is adjusted to 0.3 to 0.9 dl/g through reactor stirring speed and stirrer torque control.

Since BHET begins to melt at 230℃ or higher, the temperature is increased from 210 to 260℃ to 260 to 310℃, the stirrer is rotated at 30 to 120 rpm, and the condensation polymerization reaction is performed for 150 to 180 minutes while reducing the pressure to a vacuum of 0.1 to 0.5 Torr. At this time, the temperature is increased at a rate of 0.1 to 3.0℃/minute.

In the condensation polymerization step, unreacted EG and EG for temperature elevation are removed as byproducts, and are removed by condensation in a cold trap having a temperature of -30.0 to -60.0°C, thereby preventing deterioration of the properties of the recycled polyester produced. After the temperature reaches 260°C to 310°C, the rotation speed of the stirrer is adjusted to 20 to 100 rpm for 150 to 180 minutes, while controlling the current load of the stirrer and the inherent viscosity (IV) of the recycled polyester produced, thereby completing the condensation polymerization reaction step of the recycled polyester.

When the above condensation polymerization reaction is completed, the vacuum state of the reactor is released, stirring is stopped, high-pressure nitrogen is supplied, and the pressure is used to extrude the reaction product, recycled polyester, into a water bath containing cold water and cut into a certain length to manufacture recycled polyester chips.

Hereinafter, specific examples and experimental examples according to the present invention will be described. The following examples are provided as examples only to help understand the present invention, and the technical scope of the present invention is not limited thereby.

Recycled polyester chips were manufactured using recycled dimethyl terephthalate (DMT) and recycled ethylene glycol (EG) manufactured using a low-temperature methanolysis-based depolymerization technique according to examples and comparative examples of one embodiment of the present invention, and their physical properties were compared.

[Example]

<Example 1>

When recycled DMT and recycled EG were added at a molar ratio of 1:1.3 and the ester exchange reaction was performed under different conditions such as 100 to 600 ppm of zinc acetate (ZnAC), temperature elevation conditions, maintenance conditions, and rotation speed of the stirrer, the reaction was judged to be complete when no more methanol flowed out, and the reaction conditions and manufacturing process status were compared while manufacturing recycled bis-2-hydroxyethylene terephthalate (BHET), which are shown in [Table 1].

[Table 1]

In Example 1-1, where the temperature was rapidly increased from 150℃ to 170~200℃, it was confirmed that a lot of DMT and EG were discharged to the outside during the initial temperature increase, so that the ester exchange reaction did not occur sufficiently. In Example 1-2, where the temperature was increased from 170~200℃ to 210~260℃ at a relatively slow temperature increase rate, it was confirmed that methanol was not extracted 100%, so the viscosity dropped below the target level in the next process, the condensation polymerization stage. It was confirmed that the temperature increase rates of steps 1, 2, and 3 and the 1st and 2nd holding temperatures and times of Example 1-3 were the most desirable.

<Example 2>

Under the conditions of Example 1-3, 200 ppm of GeO ₂ as a metal catalyst, 300 to 900 ppm of trimethyl phosphate (TMP) as a heat stabilizer, and 5 to 25 wt% of ethylene glycol based on the weight of the BHET being manufactured were added to the reactor, and the internal temperature of the reactor was increased to 230°C. In Examples 2-1 and 2-4, the stirrer was not operated, while in Example 2-2, the stirrer was rotated at 60 rpm, and in Example 2-3, the stirrer was rotated at 100 rpm while stirring was performed. While a vacuum was applied inside the reactor at 210 to 260°C, the temperature was increased at different heating rates and times for each example from 260 to 310°C.

Finally, the holding time was varied for each example at a temperature of 260 to 310°C, and the rotation speed of the stirrer was changed from 30 rpm in Example 2-1, 60 rpm in Example 2-2 to 20 rpm after 3 hours of reaction, 100 rpm in Example 2-3 to 50 rpm after 2 hours of reaction, and 100 rpm in Example 2-4 to 90 rpm, 40 rpm, and 30 rpm in steps. The intrinsic viscosity (IV) and color (L value) of the produced recycled polyester chips and the current load of the stirrer were compared with each other, and are shown in [Table 2].

The intrinsic viscosity (IV) of the recycled polyester chips manufactured above was measured by a conventional method, and the color of the recycled polyester chips was measured for the L* value using a colorimeter CR-310 according to the method of ASTM D6290.

[Table 2]

In Example 2-1, the intrinsic viscosity of the final reactant was low, the color was poor, and bubbles were generated in the condensation polymerization reaction stage. In Example 2-2, the reaction time was excessive, so carbonization occurred during the reaction process, and the load of the stirrer increased, so that the current value was high at 0.17 to 0.16, confirming that the process workability was poor. In Example 2-3, the intrinsic viscosity was poor at 0.40, and the color value was also low at 39.84. It is believed that this is due to the carbonization that occurred as a result of adding 300 ppm of trimethyl phosphate (TMP). In Example 2-4, where the initial rotation speed of the stirrer was sequentially reduced from 100 rpm to 30 rpm, the intrinsic viscosity value, color, and current load were the best at 0.09 to 0.11 mA.

Claims (5)

A method for manufacturing recycled polyester using waste polyester,
A raw material input step in which recycled dimethyl terephthalate (DMT) and recycled ethylene glycol (EG), manufactured by a low-temperature methanolysis-based depolymerization technology, are input into a reactor at a molar ratio of 1.0:1.0 to 2.2;
An ester exchange reaction step in which the internal temperature of the reactor is increased to 210 to 260°C and the external temperature is increased to 280 to 320°C for the production of recycled bis-2-hydroxyethylene terephthalate (BHET), and 100 to 600 ppm of zinc acetate (ZnAC) as a catalyst is added based on the weight of recycled dimethyl terephthalate;
A condensation polymerization reaction step in which 10 to 400 ppm of a metal catalyst selected from titanium (Ti), antimony (Sb), and germanium (Ge), 200 to 900 ppm of trimethyl phosphate (TMP) as a heat stabilizer, and 5 to 25 wt% of ethylene glycol (EG) are added to the manufactured recycled bis-2-hydroxyethylene terephthalate (BHET) and reacted;
A method for manufacturing recycled polyester using waste polyester, characterized by comprising a step of manufacturing recycled polyester chips. In the first paragraph,
The above ester exchange reaction step is a first temperature increasing step in which the temperature of the introduced raw materials is increased to an internal temperature of 150°C in the reactor and an external temperature of 100 to 180°C within 1 to 2 hours;
A second temperature increasing step in which the temperature inside the reactor is increased from 150℃ to 170~200℃ at a temperature increasing rate of 0.1~3.0℃/min;
The first maintenance step is to maintain the internal temperature of the reactor at 170 to 200°C for 30 to 60 minutes;
A method for manufacturing recycled polyester using waste polyester, characterized by comprising a third temperature increasing step of increasing the temperature inside a reactor from 170 to 200°C to 210 to 260°C at a temperature increasing rate of 0.1 to 3.0°C/min; and a second maintenance step of maintaining the temperature at 210 to 260°C for 30 to 60 minutes. In the first paragraph,
A method for manufacturing recycled polyester using waste polyester, characterized in that the above condensation polymerization reaction step is performed by heating the internal temperature of the reactor from 210 to 260°C to 260 to 310°C at a heating rate of 0.1 to 3.0°C/min and under vacuum at a final pressure of 0.1 to 0.5 Torr for 150 to 180 minutes. In the first paragraph,
A method for manufacturing recycled polyester using waste polyester, characterized in that the above condensation polymerization reaction step is provided with a cold trap having a temperature of -30.0 to -60.0°C so that unreacted ethylene glycol is condensed and removed internally. In paragraph 1,
The above-mentioned recycled polyester chip manufacturing step is a method for manufacturing recycled polyester using waste polyester, characterized in that the recycled polyester inside the reactor is extruded into water using nitrogen pressure and cut to manufacture the recycled polyester.