Depolymerization method and application of polybutylene terephthalate

Abstract

The invention belongs to the technical field of copolymerization modification of degradable polyester, and relates to a depolymerization method and application of polybutylene terephthalate. During depolymerization, polybutylene terephthalate, dimethyl carbonate and dihydric alcohol are mixed and then subjected to a depolymerization reaction, and an intermediate ester is prepared; the dihydric alcohol is dihydric alcohol which does not form a ring with dimethyl carbonate in the depolymerization reaction process; the molar weight of the dimethyl carbonate is 10-30% of the molar weight of a repetitive unit of the polybutylene terephthalate; the sum of the molar weight of a 1, 4-butanediol unit in the polybutylene terephthalate and the molar weight of the added dihydric alcohol is a, the sum of the molar weight of a terephthalic acid unit in the polybutylene terephthalate and the molar weight of the added dimethyl carbonate is b, and the ratio of b to a is (1.2-1.5): 1; during application, the prepared intermediate ester is subjected to transesterification under the condition of negative pressure to obtain the copolyester. The depolymerization method is simple and easy to implement; the depolymerization product can be directly applied.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of degradable polyester copolymer modification, and relates to a polybutylene terephthalate depolymerization method and application. BACKGROUND

[0002] High-value recycling of waste polyesters has become a research hotspot. As an important engineering plastic, polybutylene terephthalate is widely used in electronic appliances, automobile parts and other fields, and its waste amount is increasing year by year. Therefore, recycling and reuse of polybutylene terephthalate is of great significance.

[0003] The existing recycling methods of polybutylene terephthalate mainly include physical and chemical recycling. The physical recycling method is through cleaning, crushing, and remelting processing steps, but this method is only suitable for high-quality polybutylene terephthalate, and usually has the problems of serious thermal degradation and performance degradation. The chemical recycling method is to depolymerize polybutylene terephthalate into small molecules, and then reuse the small molecules.

[0004] In terms of chemical recycling, a variety of technical solutions have been proposed. For example, patent CN115582385B discloses a polybutylene terephthalate waste recycling method, which uses methanol as a depolymerization agent to decompose polybutylene terephthalate into regenerated dimethyl terephthalate (DMT) under supercritical conditions, and uses the regenerated dimethyl terephthalate as raw material to prepare regenerated polyester. The depolymerization reaction temperature is 280-360℃, and the reaction time is 40min. Although this technical solution can complete depolymerization in a short time, the reaction temperature and pressure are very harsh, which is difficult to scale up.

[0005] The other method uses diols to depolymerize polybutylene terephthalate, but it is inevitable to generate 1,4-butanediol which is easy to dehydrate into a ring to generate tetrahydrofuran under heat and pressure conditions. Especially when 1,4-butanediol is used as the diol, a large amount of added 1,4-butanediol causes the tetrahydrofuran cyclization reaction to be more intense under pressure conditions, which seriously affects the depolymerization efficiency and the separation of the product. In addition, the isomer diol (in addition to 1,4-butanediol), when participating in the depolymerization of polybutylene terephthalate, generates product components that are complex, including isomer terephthalate, so that secondary treatment is needed for purification, such as secondary ester exchange by methanol to obtain DMT. For example, the patent CN118880483A adopts this method, first dissolving the waste polybutylene terephthalate in a solvent and a cosolvent, then adding diols and a catalyst to the dissolved waste polybutylene terephthalate to depolymerize, obtaining a depolymerization solution containing monomers, then adding methanol to the depolymerization solution to prepare a DMT mixed solution by ester exchange reaction, and finally obtaining high-purity DMT after recrystallization, filtration and refining processing. The patent uses methanol for secondary ester exchange, resulting in a long preparation process, and the liquid phase components after ester exchange separation are complex, including dissolved DMT, methanol, diols, tetrahydrofuran, water, etc., which is difficult to separate and recover.

[0006] In recent years, dimethyl carbonate has been used in polyester depolymerization research due to its low toxicity, biodegradability and mild reaction characteristics. For example, the patent CN119320321A discloses a technology for depolymerizing PET using dimethyl carbonate. Although polybutylene terephthalate and PET have similar structures, dimethyl carbonate can theoretically depolymerize polybutylene terephthalate, but unlike the PET system which directly obtains DMT and ethylene carbonate, when using dimethyl carbonate alone to depolymerize polybutylene terephthalate, the thermodynamic ring formation of dimethyl carbonate with 1,4-butanediol is difficult, resulting in a variety of diol products and complex product components, which are difficult to utilize directly.

[0007] Therefore, it is of great significance to study a polybutylene terephthalate depolymerization method and application to solve the above problems. SUMMARY

[0008] The purpose of the present application is to solve the problems in the prior art and provide a polybutylene terephthalate depolymerization method and application.

[0009] To achieve the above-mentioned purpose, the technical scheme adopted by the present application is as follows:

[0010] A polybutylene terephthalate depolymerization method, which comprises the following steps: mixing polybutylene terephthalate, dimethyl carbonate and diols to perform a depolymerization reaction, and obtaining an intermediate ester compound.

[0011] The dihydric alcohol is a dihydric alcohol that does not form a ring with the dimethyl carbonate during the depolymerization reaction;

[0012] The molar amount of the dimethyl carbonate is 10 to 30% of the molar amount of the repeating unit of the polybutylene terephthalate;

[0013] The sum of the molar amount of the 1,4-butanediol unit in the polybutylene terephthalate and the molar amount of the added dihydric alcohol is a, and the sum of the molar amount of the terephthalic acid unit in the polybutylene terephthalate and the molar amount of the added dimethyl carbonate is b, and b:a = 1.2 to 1.5:1.

[0014] If the dimethyl carbonate is used alone to depolymerize the polybutylene terephthalate to obtain an intermediate ester, the following problems occur:

[0015] (1) The molecular weight distribution of the intermediate ester is too wide, and the reasons are as follows:

[0016] The boiling point of the dimethyl carbonate is relatively low, which is 90℃, and the depolymerization reaction of the dimethyl carbonate and the polybutylene terephthalate mainly relies on the transesterification, and the transesterification temperature is 160 to 200℃, which makes part of the dimethyl carbonate vaporize in the reaction kettle. Due to the existence of the reaction kettle pressure, the gas phase and the liquid phase simultaneously depolymerize the polybutylene terephthalate, and the depolymerization speeds of the gas phase and the liquid phase are not synchronized, the depolymerization rate of the liquid phase is faster, and the depolymerization rate of the gas phase is slower, thereby causing the molecular weight distribution of the intermediate ester to be too wide, and the intermediate ester with a narrow molecular weight distribution cannot be formed.

[0017] (2) The intermediate ester cannot be polymerized, and the reasons are as follows:

[0018] In the early stage of depolymerization, the transesterification reaction occurs between the dimethyl carbonate and the polybutylene terephthalate, and the intermediate ester with different polymerization degrees and with a chain end of a carbonate group is generated. In the late stage of depolymerization, the transesterification occurs between the chain end carbonate group and the ester bond in the chain, and the carbonate group (as an acid unit) is linked into the chain.

[0019] Under the condition of vacuum polycondensation, the secondary transesterification reaction occurs between the intermediate esters with a chain end of a carbonate group, the chain end carbonate groups react with each other to generate new ester bonds and release dimethyl carbonate, the reaction selectively consumes the chain end carbonate groups, and further changes the molar ratio of the acid unit to the alcohol unit to the ideal 1:1. However, the carbonate groups in the chain cannot be removed by the transesterification reaction, and the acid unit in the system is always excessive. The gradual polycondensation reaction requires that the acid-alcohol ratio be close to 1:1, otherwise the polymerization degree (molecular weight) cannot be improved.

[0020] If the dihydric alcohol is used alone to depolymerize the polybutylene terephthalate to obtain an intermediate ester, the following problems occur:

[0021] (a) The molecular weight distribution of the intermediate ester is too wide, due to the following reasons:

[0022] The depolymerization reaction system is a heterogeneous reaction system, and the polybutylene terephthalate is in a solid state and cannot be dissolved in the diol, which is in a liquid state. This insufficient contact between the solid-liquid two phases leads to uneven reaction rate: the polybutylene terephthalate on the surface rapidly depolymerizes to form low-molecular-weight intermediate esters, while the polybutylene terephthalate inside slowly depolymerizes to form high-molecular-weight intermediate esters due to limited mass transfer.

[0023] (b) Low depolymerization efficiency.

[0024] The use of dimethyl carbonate and diol to co-depolymerize polybutylene terephthalate can solve the above problems, and the specific analysis is as follows:

[0025] For problem (1), the present application reduces the amount of dimethyl carbonate used, so less dimethyl carbonate vaporizes, the amount of dimethyl carbonate participating in the depolymerization reaction in the gas phase is significantly reduced, the difference between the gas-liquid phase depolymerization is greatly reduced, the imbalance in the proportion of intermediate esters of different molecular weights produced by rapid depolymerization in the liquid phase and slow depolymerization in the gas phase is significantly improved, thereby effectively avoiding the problem of too wide molecular weight distribution of intermediate esters caused by the asynchronization of gas-liquid phase depolymerization, and intermediate esters with narrower molecular weight distribution and more uniform performance can be obtained.

[0026] For problem (a), in the early stage of depolymerization, due to the higher reactivity of dimethyl carbonate than diol, dimethyl carbonate preferentially reacts with polybutylene terephthalate to form intermediate esters with carbonate group chain ends, at this time the molecular weight distribution of the intermediate esters is relatively wide, i.e. the intermediate esters with carbonate group chain ends are composed of low-molecular-weight intermediate esters with carbonate group chain ends and high-molecular-weight intermediate esters with carbonate group chain ends. In the later stage of depolymerization, the intermediate esters with carbonate group chain ends will undergo transesterification with diol. Since the low-molecular-weight intermediate esters with carbonate group chain ends have shorter chain length and more end groups, they will preferentially undergo transesterification with diol to increase the molecular weight, and the final intermediate esters have a higher overall molecular weight.

[0027] For problem (2), since the present application uses dimethyl carbonate and diol to co-depolymerize polybutylene terephthalate, only the amount of diol needs to be controlled to be close to the number of carbonate groups in the chain that cannot be removed by transesterification, so as to ensure that the acid-alcohol ratio of the system is close to 1:1; In order to avoid the side reaction of forming a ring between diol and dimethyl carbonate, diol is selected to be a diol that does not form a ring with dimethyl carbonate during the depolymerization reaction.

[0028] For problem (b), the overall depolymerization efficiency will be significantly improved because the reaction activity of dimethyl carbonate is higher than that of diols.

[0029] The end group of the intermediate esterification product obtained by depolymerization is methoxy or hydroxyl, the methoxy group is derived from dimethyl carbonate, and the hydroxyl group is derived from polybutylene terephthalate and diols. When b > a is controlled, the excess of diacid units in the reaction system can be ensured, so that the end group of the intermediate esterification product is basically composed of methoxy groups. When the ratio is too large, that is, the addition amount of diols is low and the addition amount of dimethyl carbonate is high, it is easy to cause excessive depolymerization of polybutylene terephthalate, most of the diols participate in the depolymerization of polybutylene terephthalate, and the participation in the molecular chain homogenization is limited, and a large amount of oligomerization esterification product still exists. If the ratio is too low, that is, the addition amount of diols is high and the addition amount of dimethyl carbonate is low, it is easy to cause the molecular chain to be unable to be effectively depolymerized into an intermediate esterification product with a carbonate unit end group, that is, the number of reaction sites for subsequent transesterification is reduced, and it is difficult to obtain a copolyester with high molecular weight.

[0030] The depolymerization reaction mechanism is shown in the following figure:

[0031] ;

[0032] The intermediate esterification product with a methoxy end group can be transesterified to prepare a copolyester, as described in the reference (Facilesustainable synthesis of polyester-polycarbonate and effects of the carbonateon thermal, mechanical, and transparency properties. ACS SustainableChemistry & Engineering, 11(43), 15754-15764.).

[0033] As a preferred technical solution:

[0034] The polybutylene terephthalate depolymerization method described above has a depolymerization reaction temperature of 150-220℃, a time of 2-4h, and a pressure of 0.1-0.3MPa. In this temperature range, the problem of incomplete depolymerization of polybutylene terephthalate molecular chain due to insufficient reaction activity caused by too low temperature can be avoided, and the problem of uneven depolymerization caused by too high pressure due to too high temperature can be avoided, which affects the end group composition.

[0035] The polybutylene terephthalate is prepared by the method as described above, and the diol is one or more of 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, diethylene glycol, 1,4-cyclohexanedimethanol, isosorbide and 2,2,4,4-tetramethylcyclobutanediol.

[0036] The polybutylene terephthalate is prepared by the method as described above, and before mixing the polybutylene terephthalate, dimethyl carbonate and diol, the polybutylene terephthalate is cleaned, crushed and dried, and the purpose of drying is to prevent the hydrolysis side reaction of dimethyl carbonate during the depolymerization reaction.

[0037] The polybutylene terephthalate is prepared by the method as described above, and the intermediate ester has a polymerization degree of 5-10, a molecular weight distribution index of 3-3.5, and a terminal hydroxyl group content of 2-15 mg KOH / g. -1 .

[0038] The application further provides a preparation method of a copolyester, which comprises subjecting the intermediate ester to an ester exchange reaction under negative pressure to obtain the copolyester (PBCT), wherein the intermediate ester is prepared by the method as described above.

[0039] During the ester exchange reaction, the terminal groups of the intermediate ester react with each other, i.e., the terminal methoxyl groups and the terminal dimethyl carbonate are subjected to an ester exchange reaction, and the dimethyl carbonate is removed, thereby realizing the molecular chain growth. The ester exchange reaction mechanism is shown in the following figure:

[0040] .

[0041] As a preferred technical solution:

[0042] The preparation method of the copolyester as described above, the ester exchange reaction is carried out at a temperature of 200-260℃, a time of 2-6h and a pressure of 1-100Pa, and the reaction temperature in this range avoids the problem that a too low reaction temperature causes the final polycondensation reaction to be unable to proceed, thereby avoiding the problem that a too high reaction temperature causes the thermal degradation side reaction in the final polycondensation reaction to be enhanced, thereby affecting the color of the product.

[0043] The preparation method of the copolyester as described above, the copolyester has a number average molecular weight of 3×10 4 ~1×10 5 g / mol, a breaking strength of 25-50MPa, an elongation at break of 90-1500%, and a 90-day biodegradation rate of 50-90%.

[0044] Beneficial effects:

[0045] (1) The depolymerization method of the polybutylene terephthalate can obtain intermediate ester with narrower molecular weight distribution and more uniform performance by using dimethyl carbonate and dihydric alcohol to co-depolymerize the polybutylene terephthalate.

[0046] (2) The depolymerization method of the polybutylene terephthalate is simple and easy to implement, and realizes resource utilization of waste polybutylene terephthalate.

[0047] (3) The application of the polybutylene terephthalate, the intermediate ester can be directly applied, and the operation is simple. BRIEF DESCRIPTION OF DRAWINGS

[0048] Figure 1 The infrared spectrum of the intermediate ester in Example 1 and the copolyester in Example 7;

[0049] Figure 2 The nuclear magnetic resonance spectrum of the copolyester in Example 7. DETAILED DESCRIPTION

[0050] The application will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the application and not to limit the scope of the application. In addition, it should be understood that those skilled in the art can make various modifications or modifications to the application after reading the content taught by the application, and these equivalent forms also fall within the scope defined by the appended claims of the application.

[0051] The performance indicators in the examples and comparative examples of the application involve the following test methods:

[0052] (1) Test of number average molecular weight and molecular weight distribution index: the number average molecular weight of the copolyester is tested by gel permeation chromatography (GPC), and specifically, a solution (2 mg / mL) of the copolyester in the examples is dissolved in hexafluoroisopropanol solvent, and a polystyrene standard (manufacturer: Scrstandard, model: SCPS) is used for calibration before testing.

[0053] (2) End hydroxyl content: the end hydroxyl value of the copolyester is determined by using an automatic potentiometric titrator, and specifically, reference is made to GB / T12008.7-2010.

[0054] (3) Test of degradable performance: the copolyester sample material is mixed with the compost inoculum and placed in a composting container under the conditions of oxygen concentration of 10%, temperature (58±2C) and humidity (50~55%), and the composting is carried out, the final release amount of CO2 after 90 days of degradation of the material is measured, and the biodegradation rate of the material is represented by the ratio of the actual CO2 release amount to the theoretical maximum release amount. The reference is cellulose with a particle size of less than 20 μm.

[0055] (4) Breaking strength, breaking elongation: the copolyester is processed into 150x10x4mm dumbbell-shaped tensile samples according to GB / T 1040.1-2006 by injection molding, and after heat stress elimination at room temperature for 2 weeks, the breaking strength and breaking elongation performance test is carried out, and the tensile rate is 50mm / min -1 .

[0056] (5) The molar amount n0 of the repeating unit of the polybutylene terephthalate is m0 / N 重复 , wherein m0 is the mass (g) of the polybutylene terephthalate, and N 重复 is the molar mass of the repeating unit of the polybutylene terephthalate, and the value is 220g / mol.

[0057] (6) The molar amount n1 of the 1,4-butanediol unit in the polybutylene terephthalate is n2=n0, wherein n2 is the molar amount of the terephthalic acid unit in the polybutylene terephthalate.

[0058] Example 1

[0059] A depolymerization method of polybutylene terephthalate, the specific steps are as follows:

[0060] (1) Preparation of raw materials:

[0061] Polybutylene terephthalate: the degree of polymerization is 200;

[0062] Dimethyl carbonate;

[0063] Diol: 1,4-butanediol;

[0064] (2) After mixing the polybutylene terephthalate, dimethyl carbonate and diol, the depolymerization reaction is carried out at 150℃, 0.1MPa for 4h to prepare an intermediate ester; the infrared spectrum of the intermediate ester is shown in Figure 1 ;

[0065] The molar amount of dimethyl carbonate is 10% of the molar amount of the repeating unit of the polybutylene terephthalate; let the sum of the molar amount of the 1,4-butanediol unit in the polybutylene terephthalate and the molar amount of the added diol be a, and the sum of the molar amount of the terephthalic acid unit in the polybutylene terephthalate and the molar amount of the added dimethyl carbonate be b, b:a=1.5:1.

[0066] The final prepared intermediate ester has a degree of polymerization of 5, a molecular weight distribution index of 3.1, and a hydroxyl end group content of 2mgKOH / g -1 .

[0067] Comparative Example 1

[0068] A method for depolymerizing polybutylene terephthalate, which is substantially the same as that of Example 1, except that the molar amount of dimethyl carbonate in step (2) is 8% of the molar amount of the repeating unit of polybutylene terephthalate.

[0069] The intermediate ester compound thus obtained has a polymerization degree of 10, a molecular weight distribution index of 4.0, and a terminal hydroxyl group content of 30 mgKOH / g -1 .

[0070] Comparing Example 1 with Comparative Example 1, it can be found that the intermediate ester compound of Comparative Example 1 has a wider molecular weight distribution and a higher terminal hydroxyl group content. This is because the amount of dimethyl carbonate added is too small to effectively break the molecular chain of polybutylene terephthalate, resulting in a higher polymerization degree and a wider molecular weight distribution of the intermediate ester compound. At the same time, it also leads to the fact that part of the butanediol units are not effectively reacted, resulting in a higher terminal hydroxyl group content of the intermediate ester compound.

[0071] Comparative Example 2

[0072] A method for depolymerizing polybutylene terephthalate, which is substantially the same as that of Example 1, except that the molar amount of dimethyl carbonate in step (2) is 35% of the molar amount of the repeating unit of polybutylene terephthalate.

[0073] The intermediate ester compound thus obtained has a polymerization degree of 3, a molecular weight distribution index of 4.5, and a terminal hydroxyl group content of 2.0 mgKOH / g -1 .

[0074] Comparing Example 1 with Comparative Example 2, it can be found that the intermediate ester compound of Comparative Example 2 has a wider molecular weight distribution. This is because the amount of dimethyl carbonate added is too high, resulting in part of the polybutylene terephthalate being directly depolymerized into monomer DMT, and the average polymerization degree of the intermediate ester compound being lower, and the molecular weight distribution being wider.

[0075] Comparative Example 3

[0076] A method for depolymerizing polybutylene terephthalate, which is substantially the same as that of Example 1, except that b:a = 1.1:1 in step (2).

[0077] The intermediate ester compound thus obtained has a polymerization degree of 5, a molecular weight distribution index of 4.2, and a terminal hydroxyl group content of 52 mgKOH / g -1 .

[0078] Comparing Example 1 with Comparative Example 3, it can be found that the intermediate ester compound of Comparative Example 3 has a higher terminal hydroxyl group content. This is because the total molar amount of diacid units in the system is lower than the total molar amount of diol units, and the proportion of excess diacid units is low, which cannot achieve the complete reaction of the hydroxyl groups, resulting in a higher terminal hydroxyl group content of the intermediate ester compound.

[0079] Comparative Example 4

[0080] A method for depolymerizing polybutylene terephthalate, which is basically the same as Example 1, except that in step (2), b:a = 1.6:1.

[0081] The final intermediate esterification product has a polymerization degree of 5, a molecular weight distribution index of 4.2, and an end hydroxyl group content of 2.3 mgKOH g -1 .

[0082] Comparing Comparative Example 4 and Example 1, it can be found that the molecular weight distribution of the intermediate esterification product of Comparative Example 4 is wider, because under the premise of the same amount of added dimethyl carbonate, b:a = 1.6:1 indicates that the total molar amount of diacid units in the system greatly exceeds the total molar amount of diol units, resulting in limited homogenization of diols and a wider molecular weight distribution.

[0083] Comparative Example 5

[0084] A method for depolymerizing polybutylene terephthalate, which is basically the same as Example 1, except that no diol is added during preparation, and the molar amount of dimethyl carbonate is the sum of the molar amounts of dimethyl carbonate and diol in Example 1.

[0085] The final intermediate esterification product has a polymerization degree of 2, a molecular weight distribution index of 4, and an end hydroxyl group content of 2.4 mgKOH g -1 .

[0086] Comparing Comparative Example 5 and Example 1, it can be found that the polymerization degree of the intermediate esterification product of Comparative Example 5 is very low, because no diol is added, and at this amount of dimethyl carbonate, polybutylene terephthalate is directly depolymerized into small molecule monomers and monomer esterification products of butanediol, resulting in low polymerization degree of the intermediate esterification product.

[0087] Comparative Example 6

[0088] A method for depolymerizing polybutylene terephthalate, which is basically the same as Example 1, except that no dimethyl carbonate is added during preparation, and the molar amount of diol is the sum of the molar amounts of dimethyl carbonate and diol in Example 1.

[0089] The final intermediate esterification product has a polymerization degree of 2, a molecular weight distribution index of 4.5, and an end hydroxyl group content of 300 mgKOH g -1 .

[0090] Comparative Example 6 and Example 1 are compared, it can be found that the intermediate esterification product of Comparative Example 6 has a lower degree of polymerization and a very high content of terminal hydroxyl groups, because no dimethyl carbonate is added, the dihydric alcohol directly depolymerizes to form terephthalic acid dihydroxybutyl ester monomer and a small amount of oligomers, resulting in a lower degree of polymerization of the intermediate esterification product and a very high content of terminal hydroxyl groups.

[0091] Example 2

[0092] A method for depolymerizing polybutylene terephthalate, the specific steps are as follows:

[0093] (1) Preparation of raw materials:

[0094] Polybutylene terephthalate: degree of polymerization is 150;

[0095] Dimethyl carbonate;

[0096] Dihydric alcohol: 1,5-pentanediol;

[0097] (2) After mixing polybutylene terephthalate, dimethyl carbonate and dihydric alcohol, depolymerization reaction is carried out at 160°C and 0.15MPa for 3h to prepare an intermediate esterification product;

[0098] The molar amount of dimethyl carbonate is 30% of the molar amount of the repeating unit of polybutylene terephthalate; let the sum of the molar amount of 1,4-butanediol unit in polybutylene terephthalate and the molar amount of added dihydric alcohol be a, and the sum of the molar amount of terephthalic acid unit in polybutylene terephthalate and the molar amount of added dimethyl carbonate be b, b:a=1.3:1.

[0099] The finally prepared intermediate esterification product has a degree of polymerization of 5, a molecular weight distribution index of 3.5, and a terminal hydroxyl group content of 2.5mgKOH / g -1 .

[0100] Example 3

[0101] A method for depolymerizing polybutylene terephthalate, the specific steps are as follows:

[0102] (1) Preparation of raw materials:

[0103] Polybutylene terephthalate: degree of polymerization is 250;

[0104] Dimethyl carbonate;

[0105] Dihydric alcohol: 1,6-hexanediol;

[0106] (2) After mixing polybutylene terephthalate, dimethyl carbonate and dihydric alcohol, depolymerization reaction is carried out at 180°C and 0.2MPa for 2h to prepare an intermediate esterification product;

[0107] The molar amount of dimethyl carbonate is 15% of the molar amount of the repeating unit of polybutylene terephthalate; let the sum of the molar amount of 1,4-butanediol unit in polybutylene terephthalate and the molar amount of added dihydric alcohol be a, and the sum of the molar amount of terephthalic acid unit in polybutylene terephthalate and the molar amount of added dimethyl carbonate be b, b:a = 1.35:1.

[0108] The polymerization degree of the finally prepared intermediate ester is 8, the molecular weight distribution index is 3, and the terminal hydroxyl group content is 2.7 mgKOH / g -1 .

[0109] Example 4

[0110] A method for depolymerizing polybutylene terephthalate, the specific steps being as follows:

[0111] (1) Preparation of raw materials:

[0112] Polybutylene terephthalate: polymerization degree is 200;

[0113] Dimethyl carbonate;

[0114] Dihydric alcohol: diethylene glycol;

[0115] (2) After mixing polybutylene terephthalate, dimethyl carbonate and dihydric alcohol, depolymerization reaction is carried out at 200°C and 0.25 MPa for 2.5 h to prepare an intermediate ester;

[0116] The molar amount of dimethyl carbonate is 20% of the molar amount of the repeating unit of polybutylene terephthalate; let the sum of the molar amount of 1,4-butanediol unit in polybutylene terephthalate and the molar amount of added dihydric alcohol be a, and the sum of the molar amount of terephthalic acid unit in polybutylene terephthalate and the molar amount of added dimethyl carbonate be b, b:a = 1.2:1.

[0117] The polymerization degree of the finally prepared intermediate ester is 7, the molecular weight distribution index is 3.3, and the terminal hydroxyl group content is 7 mgKOH / g -1 .

[0118] Example 5

[0119] A method for depolymerizing polybutylene terephthalate, the specific steps being as follows:

[0120] (1) Preparation of raw materials:

[0121] Polybutylene terephthalate: polymerization degree is 200;

[0122] Dimethyl carbonate;

[0123] Diol: 1,4-cyclohexanedimethanol

[0124] (2) The intermediate ester compound was prepared by mixing polybutylene terephthalate, dimethyl carbonate and diol, and then performing depolymerization reaction at 210°C and 0.28 MPa for 2.25 h;

[0125] The molar amount of dimethyl carbonate was 25% of the molar amount of the repeating unit of polybutylene terephthalate; let the sum of the molar amount of 1,4-butanediol unit in polybutylene terephthalate and the molar amount of added diol be a, and the sum of the molar amount of terephthalic acid unit in polybutylene terephthalate and the molar amount of added dimethyl carbonate be b, b:a = 1.25:1.

[0126] The polymerization degree of the finally prepared intermediate ester compound was 10, the molecular weight distribution index was 3.35, and the end hydroxyl group content was 15 mgKOH / g -1 .

[0127] Example 6

[0128] A method for depolymerizing polybutylene terephthalate, the specific steps are as follows:

[0129] (1) Preparation of raw materials:

[0130] Polybutylene terephthalate: polymerization degree 200;

[0131] Dimethyl carbonate;

[0132] Diol: isosorbide and 2,2,4,4-tetramethylcyclobutane diol in a molar ratio of 1:1;

[0133] (2) The intermediate ester compound was prepared by mixing polybutylene terephthalate, dimethyl carbonate and diol, and then performing depolymerization reaction at 220°C and 0.3 MPa for 2 h;

[0134] The molar amount of dimethyl carbonate was 20% of the molar amount of the repeating unit of polybutylene terephthalate; let the sum of the molar amount of 1,4-butanediol unit in polybutylene terephthalate and the molar amount of added diol be a, and the sum of the molar amount of terephthalic acid unit in polybutylene terephthalate and the molar amount of added dimethyl carbonate be b, b:a = 1.4:1.

[0135] The polymerization degree of the finally prepared intermediate ester compound was 9, the molecular weight distribution index was 3.4, and the end hydroxyl group content was 9 mgKOH / g -1 .

[0136] Example 7

[0137] A method for preparing a copolyester, the specific process is as follows:

[0138] The intermediate ester of Example 1 was subjected to transesterification at 200°C, 1 Pa for 6h to obtain a copolyester, the infrared spectrum of which is shown in Figure 1 , and the nuclear magnetic resonance spectrum of which is shown in Figure 2 .

[0139] The number average molecular weight of the copolyester finally obtained was 1 x 10 5 g / mol, the breaking strength was 30 MPa, the elongation at break was 800%, and the biodegradation rate was 85% in 90 days.

[0140] Comparative Example 7

[0141] A method for preparing a copolyester, which is basically the same as Example 7, except that the intermediate ester of Example 1 is replaced by the intermediate ester of Comparative Example 1.

[0142] The number average molecular weight of the copolyester finally obtained was 2.5 x 10 4 g / mol, the breaking strength was 15 MPa, and the elongation at break was 600%.

[0143] Comparing Comparative Example 7 with Example 7, it can be found that the molecular weight of Comparative Example 7 is reduced, and the mechanical properties are poor. This is because the intermediate ester of Comparative Example 1 has a high degree of polymerization, so that the end group sites available for transesterification are reduced, the difficulty of transesterification is increased, the molecular weight is difficult to increase, and thus the mechanical properties of the comparative example are poor.

[0144] Comparative Example 8

[0145] A method for preparing a copolyester, which is basically the same as Example 7, except that the intermediate ester of Example 1 is replaced by the intermediate ester of Comparative Example 2.

[0146] The number average molecular weight of the copolyester finally obtained was 1 x 10 4 g / mol, the breaking strength was 8 MPa, and the elongation at break was 40%.

[0147] Comparing Comparative Example 8 with Example 7, it can be found that the molecular weight of Comparative Example 8 is greatly reduced, and the mechanical properties are extremely poor. This is because the intermediate ester of Comparative Example 2 has a very low degree of polymerization, and due to excessive depolymerization, there is a part of DMT monomers produced by depolymerization. During the re-polymerization process, the reaction molar ratio is unbalanced due to vacuum devolatilization, so that a copolyester with high molecular weight cannot be obtained, and thus the mechanical properties of the comparative example are poor.

[0148] Comparative Example 9

[0149] A method for preparing a copolyester, which is basically the same as Example 7, except that the intermediate ester of Example 1 is replaced by the intermediate ester of Comparative Example 3.

[0150] The final copolyester had a number average molecular weight of 6 × 10⁻⁶. 4 g / mol, with a breaking strength of 25 MPa and an elongation at break of 650%.

[0151] Comparing Comparative Example 9 with Example 7, it can be found that the molecular weight of Comparative Example 9 is reduced and its mechanical properties are poor. This is because the increase in the amount of diol added to the intermediate esterification leads to an increase in the number of hydroxyl groups at the end group, and the reaction cannot proceed completely in the form of acid unit transesterification. The reaction becomes more difficult, resulting in a slight decrease in molecular weight, which in turn leads to poor mechanical properties of the comparative example.

[0152] Comparative Example 10

[0153] A method for preparing a copolyester is basically the same as in Example 7, except that the intermediate esterified product of Example 1 is replaced with the intermediate esterified product of Comparative Example 4.

[0154] The final copolyester had a number average molecular weight of 8 × 10⁻⁶. 4 g / mol, with a breaking strength of 28 MPa and an elongation at break of 700%.

[0155] Comparing Comparative Example 10 with Example 7, it can be found that Comparative Example 10 has lower molecular weight and mechanical properties. This is because the reduction in the relative content of diol leads to limited homogenization effect of intermediate ester, wider molecular weight distribution, and a small amount of small molecule monomers produced by depolymerization remain, which increases the difficulty of polymerization and thus results in poor mechanical properties of the comparative example.

[0156] Comparative Example 11

[0157] A method for preparing a copolyester is basically the same as in Example 7, except that the intermediate ester of Example 1 is replaced with the intermediate ester of Comparative Example 5.

[0158] The final copolyester had a number average molecular weight of 8 × 10⁻⁶. 3 g / mol, with a breaking strength of 2 MPa and an elongation at break of 60%.

[0159] Comparing Comparative Example 11 with Example 7, it can be found that the molecular weight of Comparative Example 11 is significantly reduced and its mechanical properties are extremely poor. This is because the degree of polymerization of the intermediate ester of Comparative Example 11 is extremely low, and due to excessive depolymerization, there are DMT monomers generated by depolymerization. During the repolymerization process, the reaction molar ratio will be unbalanced due to vacuum devolatilization, making it impossible to obtain a high molecular weight copolyester, which in turn leads to the poor mechanical properties of the comparative example.

[0160] Comparative Example 12

[0161] A method for preparing a copolyester, substantially the same as Example 7, except that the intermediate ester of Example 1 is replaced by the intermediate ester of Comparative Example 6.

[0162] The number average molecular weight of the copolyester finally obtained is 3 x 10 4 g / mol, the breaking strength is 40 MPa, the breaking elongation is 300%, and the biodegradation rate in 90 days is 0%.

[0163] Comparing Comparative Example 12 with Example 7, it can be found that the biodegradation rate of Comparative Example 12 is 0, because Comparative Example 11 does not use dimethyl carbonate for depolymerization, but 1,4-butanediol for depolymerization, and the re-polymerization thereof results in polybutylene terephthalate, which is not biodegradable.

[0164] Example 8

[0165] A method for preparing a copolyester, the specific process being as follows:

[0166] The intermediate ester of Example 2 is subjected to ester exchange reaction at 210°C and 10 Pa for 5 h to obtain the copolyester.

[0167] The number average molecular weight of the copolyester finally obtained is 7.2 x 10 4 g / mol, the breaking strength is 42 MPa, the breaking elongation is 100%, and the biodegradation rate in 90 days is 70%.

[0168] Example 9

[0169] A method for preparing a copolyester, the specific process being as follows:

[0170] The intermediate ester of Example 3 is subjected to ester exchange reaction at 220°C and 50 Pa for 4 h to obtain the copolyester.

[0171] The number average molecular weight of the copolyester finally obtained is 4.7 x 10 4 g / mol, the breaking strength is 37 MPa, the breaking elongation is 150%, and the biodegradation rate in 90 days is 60%.

[0172] Example 10

[0173] A method for preparing a copolyester, the specific process being as follows:

[0174] The intermediate ester of Example 4 is subjected to ester exchange reaction at 230°C and 60 Pa for 3 h to obtain the copolyester.

[0175] The number average molecular weight of the copolyester finally obtained is 1 x 10 5 g / mol, the breaking strength is 25 MPa, the breaking elongation is 1500%, and the biodegradation rate in 90 days is 90%.

[0176] Example 11

[0177] A method for preparing a copolyester is as follows:

[0178] The intermediate ester of Example 5 is subjected to transesterification at 240°C and 80 Pa for 2.5 hours to obtain a copolyester.

[0179] The copolyester thus obtained has a number average molecular weight of 8 x 10 4 g / mol, a breaking strength of 29 MPa, an elongation at break of 400%, and a biodegradation rate of 85% in 90 days.

[0180] Example 12

[0181] A method for preparing a copolyester is as follows:

[0182] The intermediate ester of Example 6 is subjected to transesterification at 260°C and 100 Pa for 2 hours to obtain a copolyester.

[0183] The copolyester thus obtained has a number average molecular weight of 3 x 10 4 g / mol, a breaking strength of 50 MPa, an elongation at break of 90%, and a biodegradation rate of 50% in 90 days.

Claims

1. A method for depolymerization of polybutylene terephthalate, characterized by, polybutylene terephthalate, dimethyl carbonate and a diol are mixed and then subjected to a depolymerization reaction to obtain an intermediate ester compound; the diol is a diol that does not form a ring with dimethyl carbonate during the depolymerization reaction; the molar amount of dimethyl carbonate is 10-30% of the molar amount of the repeating unit of the polybutylene terephthalate; the sum of the molar amount of the 1,4-butanediol unit in the polybutylene terephthalate and the molar amount of the added diol is a, and the sum of the molar amount of the terephthalic acid unit in the polybutylene terephthalate and the molar amount of the added dimethyl carbonate is b, and b:a = 1.2-1.5:

1.

2. The method of depolymerization of polybutylene terephthalate according to claim 1, characterized in that, the temperature of the depolymerization reaction is 150-220°C, the time is 2-4 hours, and the pressure is 0.1-0.3 MPa.

3. The method of depolymerization of polybutylene terephthalate according to claim 1, characterized in that, the diol is one or more of 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, diethylene glycol, 1,4-cyclohexanedimethanol, isosorbide, and 2,2,4,4-tetramethylcyclobutanediol.

4. The method for depolymerizing polybutylene terephthalate according to any one of claims 1 to 3, characterized in that, The intermediate ester has a degree of polymerization of 5 to 10, a molecular weight distribution index of 3 to 3.5, and a terminal hydroxyl group content of 2 to 15 mg KOH / g -1 .

5. A process for the preparation of a copolyester, characterized in that, The intermediate ester compound is subjected to a transesterification reaction under a negative pressure to obtain the copolyester, wherein the intermediate ester compound is obtained by the depolymerization method of any one of claims 1-4.

6. A process for the preparation of a copolyester according to claim 5, characterized in that, the temperature of the transesterification reaction is 200-260°C, the time is 2-6 hours, and the pressure is 1-100 Pa.

7. A process for the preparation of a copolyester according to claim 5 or 6, characterised in that, The number average molecular weight of the copolyester is 3 x 10 4 1 x 10 5 g / mol, the breaking strength is 25-50 MPa, the elongation at break is 90-1500%, and the biodegradation rate in 90 days is 50-90%.

Images