JP7757241B2 - Molded body, its manufacturing method and recycling method using same - Google Patents

Description

The present invention relates to a molded article of polyethylene terephthalate resin, and more specifically to a molded article of polyethylene terephthalate resin used to produce a monomer by chemical decomposition of polyethylene terephthalate, a method for producing the same, and a method for recycling polyethylene terephthalate resin products using the same.

Plastic bottles are widely used in large quantities as containers for a variety of liquids, including drinking water, seasonings, cooking oil, alcoholic beverages, fuel, and detergents, due to their light weight and transparency, which allows for easy viewing of the contents. Of these, polyethylene terephthalate bottles (hereinafter sometimes referred to as PET bottles) account for the majority of such bottles. Previously, used plastic bottles were disposed of by incineration or landfill, but in recent years, the Act on the Promotion of Separate Collection and Recycling of Containers and Packaging came into effect, and along with the collection and recycling of PET bottles, the separate collection and recycling of used plastic containers and packaging has begun.

Methods for recycling these used plastics include material recycling, in which used plastics are remanufactured as raw materials, chemical recycling, in which used plastics are broken down to the monomer level and reused, and thermal recycling, in which used plastics are reused as energy.

Taking used PET bottles as an example, chemical recycling methods have been developed and are beginning to be put into practical use. These methods involve chemically decomposing polyester, recovering oligomers or monomers, and then repolymerizing the polyester. Examples of known methods include washing used PET bottles, crushing them into flakes, adding methanol to decompose them into dimethyl terephthalate, and then hydrolyzing them again to obtain high-purity terephthalic acid (see Patent Document 1, etc.); adding ethylene glycol to polyethylene terephthalate to decompose it into bis(2-hydroxyethyl) terephthalate, and then melt-polycondensing the bis(2-hydroxyethyl) terephthalate to obtain polyethylene terephthalate polymer (see Patent Document 2); and reacting polyethylene terephthalate with water in a high-temperature, high-pressure state known as supercritical or subcritical to hydrolyze it and obtain terephthalic acid (see Patent Document 3). Furthermore, a method of obtaining terephthalic acid by enzymatic hydrolysis of polyethylene terephthalate has also been proposed in recent years (see Patent Document 4).

In addition, a method has been proposed for obtaining monomers by enzymatic hydrolysis of semi-crystalline polyester, in which the polymer is melted at a temperature above its melting point and then rapidly cooled below its crystallization temperature to amorphize it, thereby improving decomposition efficiency (Patent Document 5).

Japanese Patent Application Laid-Open No. 2002-60369 Japanese Patent Application Laid-Open No. 2000-169623 Japanese Patent Application Laid-Open No. 2000-53801 Special table 2016-505650 publication U.S. Patent Publication No. 2019/218360

As mentioned above, chemical recycling methods, in which used polyethylene terephthalate products such as bottles are broken down to the monomer level for reuse, can basically be said to be methods for chemically decomposing polyethylene terephthalate. However, none of these methods have sufficient decomposition efficiency when put into practical use, and there is room for improvement when it comes to practical application. Therefore, a primary object of the present invention is to provide a polyethylene terephthalate resin molded product that has excellent decomposition efficiency and is easy to chemically recycle. Another object of the present invention is to provide a method for producing such a molded product and a method for recycling used polyethylene terephthalate resin products using such a molded product.

Patent Document 5, cited above, discloses that the crystallinity of polyester affects the efficiency of hydrolysis, and furthermore, that decrystallization improves decomposition efficiency. In an attempt to chemically recycle used polyethylene terephthalate resin products, the inventors produced polyethylene terephthalate molded articles with low crystallinity and attempted hydrolysis. Surprisingly, they discovered that even if the crystallinity is too low, the decomposition efficiency decreases. Further investigation led the inventors to discover that the decomposition efficiency of polyethylene terephthalate can be further improved by producing molded articles with a crystallinity correlation parameter greater than a certain level (i.e., a crystallinity smaller than a certain level) and a glass transition temperature greater than a certain level. The present invention is based on this discovery. Specifically, the gist of the present invention is as follows:

[1] A molded article of polyethylene terephthalate resin used to produce a monomer by chemical decomposition of polyethylene terephthalate,
The glass transition temperature (Tg) and the crystallinity correlation parameter (ΔH _Tc1 /ΔH _Tm ) of the molded body measured using a temperature modulated differential scanning calorimeter are
Tg≧70°C
ΔH _Tc1 /ΔH _Tm ≧0.5
A molded body that satisfies the above.
[2] The molded article according to [1], which has a film shape with a thickness of 10 to 350 μm.
[3] The molded article according to [2], which has a specific surface area of 7600 mm ² /g or more.
[4] The molded article according to [1], wherein the polyethylene terephthalate resin is a recycled material obtained by reusing a product made of polyethylene terephthalate.
[5] The molded article according to [4], wherein the polyethylene terephthalate resin has an intrinsic viscosity (IV) of 1.4 or less.
[6] The molded article according to [1], wherein the chemical decomposition is carried out by an enzyme.
[7] A method for producing the molded article according to [1], comprising withdrawing polyethylene terephthalate resin from a molten state at a draft ratio of 5 to 60 to obtain a molded article.
[8] The method according to [7], wherein the polyethylene terephthalate resin is a recycled material obtained by reusing a product made of polyethylene terephthalate.
[9] A method for recycling polyethylene terephthalate resin products, comprising:
A molded article according to [1] is produced from a used polyethylene terephthalate resin product,
Immersing the molded body in a medium containing an enzyme;
producing a monomer or oligomer by chemical decomposition of the polyethylene terephthalate;
A method comprising:

The present invention makes it possible to provide polyethylene terephthalate resin molded articles that are highly decomposable and easy to chemically recycle.

1 is a graph showing the relationship between the glass transition temperature and the decomposition efficiency for each of the molded bodies of Examples 1 to 6 and Comparative Examples 1 to 4. 1 is a graph showing the relationship between the crystallinity correlation parameter and the decomposition efficiency for each of the molded bodies of Examples 1 to 6 and Comparative Examples 1 to 4.

<Molded body>
The molded article according to the present invention is a molded article made of a polyethylene terephthalate resin, and the molded article has a glass transition temperature (Tg) and a crystallinity correlation parameter (ΔH _Tc1 /ΔH _Tm ) measured using a temperature modulated differential scanning calorimeter, which satisfies the following:
Tg≧70°C
ΔH _Tc1 /ΔH _Tm ≧0.5
In the present invention, excellent decomposition efficiency is realized by producing a polyethylene terephthalate resin molded article having a crystallinity correlation parameter of 0.5 or more (a certain amount or more of amorphous parts are present in the polyethylene terephthalate) and a glass transition temperature of 70°C or more (i.e., having a certain degree of crystal orientation). The reason for this is not clear, but is thought to be as follows.

In other words, polyethylene terephthalate, which is known as a semi-crystalline polymer, is known to have both crystalline and amorphous portions in the solid state, but from the perspective of hydrolysis efficiency, a low degree of crystallization is preferable. On the other hand, when attempting to obtain a resin molded product with a low degree of crystallization, there is a limit to how much the surface area of the molded product can be increased. Therefore, by producing a molded product with a glass transition temperature above a certain value (i.e., a molded product that has been stretched to a certain extent), it is thought that it is possible to balance surface area and crystallinity and maximize hydrolysis efficiency.

It is known that polyethylene terephthalate (hereinafter simply referred to as PET) can be rapidly cooled from a molten state to produce a nearly amorphous PET resin, and the glass transition temperature of such amorphous PET resin is approximately 68°C. It is also known that PET undergoes oriented crystallization and increases in crystallinity through stretching or other processes, and that the glass transition temperature of stretched PET film increases to approximately 78°C. In the present invention, a molded product is produced with a glass transition temperature of 70°C or higher. From the perspective of increasing the surface area and improving decomposition efficiency, a glass transition temperature of 73°C or higher is preferred.

In the present invention, the glass transition temperature refers to the extrapolated glass transition onset temperature obtained as the inflection point of a melting endothermic curve measured using a temperature-modulated differential scanning calorimeter (DSC) under the following conditions. A method for obtaining a molded product with a glass transition temperature of 70°C or higher will be described later.

<Measurement conditions>
Atmosphere: Nitrogen atmosphere Measurement temperature range: 0 to 150°C
- Temperature rise rate: 3°C/min - Temperature modulation mode: ±0.48°C/60 seconds (heat only mode)
Sample weight: 3 mg

The molded article of the present invention also has a crystallinity correlation parameter of 0.5 or higher. A higher crystallinity correlation parameter promotes the hydrolysis of PET, improving decomposition efficiency, with a preferred range of 0.5 to 0.7. While a higher crystallinity correlation parameter is preferable from the standpoint of hydrolysis efficiency, if the crystallinity correlation parameter is too high, it becomes difficult to obtain a molded article having the aforementioned glass transition temperature of 70°C or higher.

In the present invention, the crystallinity correlation parameter refers to a value calculated from the following formula using the values of the temperature-rising crystallization enthalpy (ΔH _Tc1 ) and the crystalline melting enthalpy (ΔH _Tm ) obtained from the melting endothermic curve measured using a temperature-modulated differential scanning calorimeter in the same manner as above. This value has a negative correlation with the crystallinity measured by a density gradient method or the like.
Crystallinity correlation parameter (X) = ΔH _Tc1 / ΔH _Tm

From the viewpoint of the hydrolysis efficiency of PET, the molded article of the present invention is preferably in the form of a film. In the case of a film-shaped molded article, its thickness is preferably 10 to 350 μm, more preferably 50 to 200 μm. From the viewpoint of the specific surface area, it can be said that a smaller film thickness is preferable, but in order to efficiently obtain a thin film, a high draft ratio is required when forming the film-like membrane, or a stretching treatment is required after film formation, making it difficult to obtain a molded article having a ΔH _Tc1 /ΔH _Tm of 0.5 or more.

Furthermore, when the molded article is in the form of a film, from the viewpoint of the efficiency of hydrolysis of PET, the specific surface area is preferably 7600 to 160,000 ^{mm 2} /g, and more preferably 7600 to 31,000 mm ² /g. It is thought that the larger the specific surface area of the molded article, the more accelerated the hydrolysis of PET will be. However, to obtain a molded article with a large specific surface area, it is necessary to increase the draft ratio during film formation or to perform a stretching treatment after film formation, which makes it difficult to obtain a molded article with a ΔH _Tc1 /ΔH _Tm of 0.5 or more.

The intrinsic viscosity (IV) of PET is preferably 1.4 or less. The lower the intrinsic viscosity, the smaller the molecular weight, and the faster the depolymerization reaction can proceed to the monomer or oligomer level. On the other hand, if the intrinsic viscosity is too low, molding may be difficult when the PET is cooled from a molten state to form a molded article. Furthermore, if the intrinsic viscosity of PET is too low, crystallization may proceed easily during melt molding, and the crystallinity correlation parameter of the resulting molded article may be less than 0.5.

The PET that makes up the molded product is obtained by polycondensation of two main constituent monomers, ethylene glycol and terephthalic acid, but other monomers may also be copolymerized in addition to these two components as diol or dicarboxylic acid components.

Dicarboxylic acid components that can be used as copolymerization components for PET include, for example, malonic acid, succinic acid, glutaric acid, adipic acid, suberic acid, sebacic acid, dodecanedioic acid, eicosanedioic acid, pimelic acid, azelaic acid, methylmalonic acid, ethylmalonic acid, adamantanedicarboxylic acid, norbornenedicarboxylic acid, cyclohexanedicarboxylic acid, decalindicarboxylic acid, isophthalic acid, phthalic acid, 1,4-naphthalenedicarboxylic acid, 1,5-naphthalenedicarboxylic acid, 2,6-naphthalenedicarboxylic acid, 1,8-naphthalenedicarboxylic acid, 4,4'-diphenyldicarboxylic acid, 4,4'-diphenyletherdicarboxylic acid, 5-sodiumsulfoisophthalic acid, phenylendanedicarboxylic acid, anthracenedicarboxylic acid, phenanthrenedicarboxylic acid, 9,9'-bis(4-carboxyphenyl)fluorene acid, 2,5-furandicarboxylic acid, and ester derivatives thereof.

Diol components used as copolymerization components for PET include, for example, 1,2-propanediol, 1,3-propanediol, butanediol, 2-methyl-1,3-propanediol, hexanediol, neopentyl glycol, cyclohexanedimethanol, cyclohexanediethanol, decahydronaphthalenedimethanol, decahydronaphthalenediethanol, norbornanedimethanol, norbornanediethanol, tricyclodecanedimethanol, tricyclodecanediethanol, tetracyclododecanedimethanol, tetracyclododecanediethanol, decalindimethanol, decalindiethanol, 5-methylol-5-ethyl-2-(1,1-dimethyl 2,2-bis(4-hydroxycyclohexylpropane), 2,2-bis(4-(2-hydroxyethoxy)cyclohexyl)propane, cyclopentanediol, 3-methyl-1,2-cyclopentadiol, 4-cyclopentene-1,3-diol, adamantanediol, paraxylene glycol, bisphenol A, bisphenol S, styrene glycol, trimethylolpropane, pentaerythritol, diethylene glycol, triethylene glycol, and bis-β-hydroxyethyl terephthalate (BHET).

The molded article of the present invention is composed of PET resin, but it may contain resin components other than PET, or other components such as additives. For example, one or more of various additives such as plasticizers, light stabilizers, antioxidants, UV absorbers, flame retardants, colorants, pigments, fillers, release agents, antistatic agents, fragrances, foaming agents, and antibacterial and antifungal agents may be blended into the molded article, provided that they do not impair moldability when obtaining the molded article as a material for manufacturing the molded article, or inhibit the chemical decomposition of PET.

<Method of manufacturing molded body>
The molded article of the present invention can be obtained by feeding PET resin into an extruder and melting it at a temperature of 265°C to 295°C, passing the resin through a filter or gear pump to remove foreign matter and equalize the extrusion rate, and then discharging the molten resin from a T-die onto a casting drum or cooling drum to cool the resin.

In order to obtain a molded article having a glass transition temperature of 70°C or higher, it is preferable to take up the molten resin on a casting drum during film formation so that the draft ratio is in the range of 5 to 60. The draft ratio is expressed as the take-up speed (V)/discharge outlet speed ( _V0 ), and the larger the draft ratio, the more oriented crystallization progresses, resulting in a higher glass transition temperature of the obtained molded article. The discharge outlet speed ( _V0 ) is expressed by the following formula:
Discharge outlet velocity (V ₀ )=discharge amount (W)/density (ρ)/cross-sectional area of discharge outlet (S)

The resulting molded article may then be stretched, if necessary. For example, when the draft ratio during film formation is low, the stretching conditions can be adjusted after film formation so that the resulting molded article has a glass transition temperature of 70°C or higher and a crystallinity correlation parameter of 0.5 or higher.

Stretching may be uniaxial or biaxial. For example, from the standpoint of productivity and flatness, sequential biaxial stretching may be performed in the longitudinal direction, transverse direction, and longitudinal direction. The stretching ratio during stretching is adjusted in relation to the draft ratio described above. Generally, the higher the stretching ratio, the more oriented crystallization progresses, resulting in an increase in glass transition temperature and a decrease in the crystallinity correlation parameter.

Considering chemical recycling, the PET resin used to obtain molded products is preferably recycled. The recycled material may be petroleum-derived PET resin, plant-derived PET resin, or a mixture thereof. Examples of plant-derived PET resins include PET resins that use diethylene glycol derived from bioethanol as a diol component. For example, flakes made from recovered used PET bottles, sorted, crushed, and washed to remove contaminants and foreign matter, can be used. PET bottles are typically manufactured using PET resin with an IV value of approximately 0.8. However, when flakes obtained from used PET bottles are used as the raw material for PET resin, the molded product obtained by remelting the flakes is known to have a lower IV value than the original PET due to hydrolysis of the PET during melt molding. Therefore, using such recycled materials as raw materials for molded products allows for the production of molded products with a desirable intrinsic viscosity (IV = 1.4 or less), resulting in molded products with even better decomposition efficiency.

<Recycling method>
In the present invention, by using the molded article described above, it becomes possible to efficiently recycle PET resin products. For example, a molded article is produced from a used PET resin product as described above, and the obtained molded article is chemically decomposed by a conventionally known method, thereby obtaining the monomers or oligomers that constitute PET from the used PET resin product. By polymerizing the obtained monomers or oligomers, a PET resin product can be produced again from the used PET resin product.

As an example of chemical recycling of PET resin products, flakes obtained by crushing used PET bottles are remelted to produce the molded product of the present invention, which is then immersed in a solvent such as methanol or ethylene glycol, or in a medium containing an enzyme, whereby polyethylene terephthalate is depolymerized to produce terephthalic acid.

PET hydrolysis can be carried out using a raw material composition containing, for example, a molded body, microorganisms capable of expressing and excreting a degrading enzyme, and water. A buffer may be added to the raw material composition to stabilize the pH. From the perspective of enzyme activity, the pH of the raw material composition is preferably 5 to 11.

The depolymerization treatment time is preferably 1 to 24 hours, and more preferably 4 to 16 hours.

To isolate the monomer from the recovered active ingredient, the monomer (terephthalic acid) can be extracted with an organic solvent in which the monomer dissolves, and the monomer that makes up the polyester can be isolated from the extracted monomer solution by distillation, crystallization, etc., or the monomer can be directly subjected to crystallization or distillation. The monomer solution can also be subjected to ion exchange treatment or other processes before extraction, distillation, crystallization, etc.

In the above-mentioned chemical recycling, using a medium containing an enzyme rather than using a solvent such as methanol or ethylene glycol for depolymerization eliminates the need for solvent recovery equipment, allowing for more efficient and inexpensive chemical recycling.

The present invention will now be described in more detail using examples, but the present invention is not limited to these examples.

<Production of Molded Product>
Used PET bottles were crushed and washed to remove contaminants and foreign matter, and then turned into flakes. The flakes were then remelted in an extruder and processed into pellets.
The resulting pellets were fed into an extruder, and the molten resin was extruded onto a casting drum adjusted to 20°C while varying the discharge rate and take-up speed to form a film. Another method was a simultaneous stretching method, in which the film obtained by the casting method was further heated to 100°C and subjected to simultaneous biaxial equal-fold stretching. Ten types of molded articles were obtained in Examples 1 to 6 and Comparative Examples 1 to 4. The thicknesses of the resulting molded articles were as shown in Table 1 below.

The IV value of each molded article was measured using a relative viscometer (Viscotec Y501C, manufactured by Malvern Panalytical) at a temperature of 25°C. The sample was dissolved in a solvent of phenol:tetrachloroethane = 1:1 to prepare a solution. As a result of the measurement, the IV value of each molded article was 0.75.
The weight-average molecular weight of each molded product was measured using a high-speed GPC device (HLC-8320GPC, manufactured by Tosoh Corporation). The sample was a solution dissolved in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP). Chloroform was used as the mobile phase, and a TSKgel Super HM-H column manufactured by Tosoh Corporation was used as the column, and measurements were performed at 40°C. TSKgel standard polystyrene manufactured by Tosoh Corporation was used as the molecular weight standard. As a result of the measurement, the weight-average molecular weight of each molded product was 45,000.

<Measurement of Crystallinity Correlation Parameter>
Approximately 3 mg of each molded product was placed in an aluminum pan (TA Instruments, TZero aluminum pan) and subjected to measurement using a temperature-modulated differential scanning calorimeter (TA Instruments, DSC2500). The measurement conditions were a temperature rise rate of 3°C/min and temperature modulation conditions of ±0.48°C/60 seconds in heat-only mode. Data analysis was performed using TA Instruments' analysis software "TRIOS."
The crystallization enthalpy (ΔH _Tc1 ) and crystalline melting enthalpy (ΔH _Tm ) were calculated from the total heat flow profile obtained from the temperature modulation heating measurement. Regarding the calculation range, ΔH _Tc1 was appropriately set within the range of 145°C or less depending on the peak shape, and ΔH _Tm was fixed at 210 to 270°C.
The crystallinity correlation parameter (X) was calculated from the obtained ΔH _Tc1 and ΔH _Tm according to the following formula.
Crystallinity correlation parameter (X) = ΔH _Tc1 / ΔH _Tm
The crystallinity correlation parameter (X) of each of the obtained molded bodies was as shown in Table 1 below.

<Measurement of Glass Transition Temperature>
For each molded product, a reversing heat flow profile was obtained by temperature-modulated heating measurement in the same manner as described above, and the extrapolated glass transition onset temperature obtained as the inflection point of the obtained profile curve was taken as the glass transition temperature (Tg). More specifically, the above analysis software was used, and the calculation range was set to 60°C as the analysis start temperature and the analysis end temperature according to the profile shape, and TgOnset analysis was performed to calculate the glass transition temperature.
The glass transition temperatures (Tg) of the obtained molded products are shown in Table 1 below.

<Hydrolysis of Molded Product>
Each molded article was cut into a disk shape with a diameter of 14 mm to prepare a sample, which was then immersed in a buffer solution containing the enzyme for 24 hours to hydrolyze the PET. The decomposition efficiency (D) was calculated using the following formula from the total weight ( _Wb ) of the molded article before immersion in the buffer solution containing the enzyme and the total weight (Wa) of the molded article after immersion and drying.
Decomposition efficiency (D)=(W _b −W _a )/W _b ×100

The relationship between the glass transition temperature and decomposition efficiency of each molded product in Examples 1 to 5 and Comparative Examples 1 to 4 is shown in Figure 1. Furthermore, the relationship between the crystallinity correlation parameter and decomposition efficiency of each molded product is shown in Figure 2.

As is clear from FIGS. 1 and 2 and the evaluation results in Table 1, the molded articles (Examples 1 to 6) having a glass transition temperature of 70°C or higher and a crystallinity correlation parameter of 0.5 or higher have excellent PET decomposition efficiency.
On the other hand, it can be seen that the molded product (Comparative Example 1) having a small draft ratio and a glass transition temperature of less than 70°C, even though the crystallinity correlation parameter was 0.5 or more, had a lower PET decomposition efficiency than the molded products of Examples 1 to 6.
Furthermore, it can be seen that molded articles (Comparative Examples 2 to 4) having a glass transition temperature of 70°C or higher but a crystallinity correlation parameter of less than 0.5 have lower PET decomposition efficiency than the molded articles of Examples 1 to 6.

Claims (9)

A molded article of a polyethylene terephthalate resin used to produce a monomer by chemical decomposition of polyethylene terephthalate,
The glass transition temperature (Tg) and the crystallinity correlation parameter (ΔH _Tc1 /ΔH _Tm ) of the molded body measured using a temperature modulated differential scanning calorimeter are
Tg≧70°C
ΔH _Tc1 /ΔH _Tm ≧0.5
(In the formula, ΔH _Tc1 and ΔH _Tm respectively represent the temperature-rising crystallization enthalpy (ΔH Tc1 ) and the crystalline melting enthalpy (ΔH _Tm ) obtained from the melting endothermic curve measured under the following conditions using a temperature-modulated differential scanning calorimeter (DSC ₎ .
<Measurement conditions>
Atmosphere: Nitrogen atmosphere
・Measurement temperature range: 0 to 150℃
Heating rate: 3°C/min
Temperature modulation mode: ±0.48°C/60 seconds (heat only mode)
Sample weight: 3 mg
A molded body that satisfies the above. The molded article according to claim 1, which has a film shape with a thickness of 10 to 350 μm. The molded article according to claim 2, having a specific surface area of 7600 mm ² /g or more. The molded article according to claim 1, wherein the polyethylene terephthalate resin is a recycled material made from a product made of polyethylene terephthalate. The molded article described in claim 4, wherein the polyethylene terephthalate resin has an intrinsic viscosity (IV) of 1.4 or less. The molded article according to claim 1, wherein the chemical decomposition is carried out by an enzyme. A method for producing the molded article described in claim 1, comprising withdrawing polyethylene terephthalate resin from a molten state at a draft ratio of 5 to 60 to obtain a molded article. The method of claim 7, wherein the polyethylene terephthalate resin is a recycled material obtained by reusing products made from polyethylene terephthalate. A method for recycling polyethylene terephthalate resin products, comprising:
A molding according to claim 1 is produced from a used polyethylene terephthalate resin product,
Immersing the molded body in a medium containing an enzyme;
producing a monomer or oligomer by chemical decomposition of the polyethylene terephthalate;
A method comprising: