A method for upgrading and recycling waste pbt and a biodegradable polyester prepared by using the method

Abstract

The application provides a resource upgrading method of waste PBT and a biodegradable polyester prepared by using the method. The resource upgrading method provided in the application comprises the following steps: (1) performing reaction pretreatment on the waste PBT; (2) mixing at least one of an aliphatic monomer and an aliphatic ester polymer with the pretreated waste PBT, and performing exchange reaction under the catalysis of a metal catalyst to obtain a prepolymer; and (3) performing reverse reaction of the exchange reaction under a reduced pressure condition, and removing the volatile compounds in the process of the reverse reaction to obtain the biodegradable polyester. The reaction path provided in the application does not need to depolymerize the PBT into small molecular monomers and derivatives, reduces the difficulty and process cost of chemical upgrading and recycling of the PBT, realizes complete utilization of the waste PBT, and has significant environmental and economic benefits.

Description

Technical Field

[0001] This invention belongs to the field of plastic recycling technology, and in particular to a method for upgrading waste PBT to a resource-based form and a biodegradable polyester prepared using this method. Background Technology

[0002] Polybutylene terephthalate (PBT) is a crystalline thermoplastic polyester plastic formed by the condensation polymerization of terephthalic acid and 1,4-butanediol. The product is milky white, translucent to opaque, and possesses excellent properties such as heat resistance, weather resistance, corrosion resistance, good electrical properties, low water absorption, and good gloss. Currently, PBT is widely used in electronics, automotive parts, machinery, and household goods, and is considered one of the five major general-purpose engineering plastics, along with polyphenylene ether, polycarbonate, polyoxymethylene, and polyamide. In 2021, the global consumption of PBT resin was approximately 1.46 million tons, with China accounting for about 50%. However, most PBT products are still consumed as single-use items, with a waste recycling rate of less than 10%. Due to the rigid benzene ring structure and strong hydrophobicity of the PBT molecular chain, the ester bonds in the molecular chain are difficult to hydrolyze, and its degradation period in natural environments can take tens to hundreds of years, thus classifying it as a non-degradable plastic. The conventional landfill and incineration methods for generating large amounts of PBT waste not only occupy valuable land resources, but also cause serious pollution to soil, water, and air, ultimately endangering human health.

[0003] Currently, the resource utilization of waste PBT is mainly achieved through the following two methods: (1) Physical downgrading recycling, which involves reprocessing the recycled waste PBT through physical means such as crushing and melting. Since the processing often leads to the breakage of PBT molecular chains and a decrease in molecular weight, it can only be used to manufacture lower-level products. (2) Chemical upgrading recycling, which involves transforming waste PBT into products with higher added value through chemical reactions. Currently, the chemical upgrading recycling of PBT involves re-depolymerizing it back to the initial terephthalic acid and 1,4-butanediol monomers and their derivatives through reactions such as ammonolysis, alcoholysis, and hydrolysis, so that it can be reused as raw materials. For example, patent application CN 114773668A discloses a method for recycling waste PBT into a biodegradable poly(butylene adipate / terephthalate) (PBAT) polymer, its preparation, and its application. The method involves using waste PBT material as a raw material for alcoholysis to obtain bis(hydroxybutyl) terephthalate (BHBT) monomeric form, followed by transesterification polycondensation of the recycled BHBT with bis(hydroxyhexyl) terephthalate (BHAT) to obtain the biodegradable PBAT polymer. However, this chemical upgrading and conversion process from PBT polymer to monomer is not only complex, but also suffers from complex and inefficient post-separation due to the presence of oligomers with varying degrees of polymerization in the product, resulting in low economic benefits. Compared to new monomer raw materials from chemical sources, it lacks cost advantages. For instance, terephthalic acid is derived from the air oxidation of p-xylene, possessing the advantage of a mature process; 1,4-butanediol is derived from the acetylene-aldehyde method, where formaldehyde and acetylene react to obtain 1,4-butynediol, which is then hydrogenated, offering advantages in low cost and high yield.

[0004] Therefore, in order to overcome the above problems, it is urgent to find a more efficient and low-cost way to chemically upgrade and recycle waste PBT. Summary of the Invention

[0005] In order to solve the problems existing in the prior art, the main objective of this invention is to provide a chemical upgrading and resource recovery method for waste PBT and a biodegradable polyester prepared by the method.

[0006] To achieve the above objectives, in a first aspect, the present invention proposes a method for the resource-based upgrading of discarded PBTs, comprising the following steps:

[0007] (1) Pre-treatment of waste PBT;

[0008] (2) Mix at least one of an aliphatic monomer and an aliphatic ester polymer with the pretreated waste PBT and carry out an exchange reaction under the catalysis of a metal catalyst to obtain a prepolymer. The aliphatic monomer includes an aliphatic diol and an aliphatic diacid, and the aliphatic ester polymer is obtained by reacting the aliphatic diol and the aliphatic diacid. At least one of the aliphatic diol and the aliphatic diacid is a volatile compound.

[0009] (3) The prepolymer is subjected to the reverse reaction of the exchange reaction under reduced pressure, and the volatile compounds are removed during the reverse reaction to obtain a biodegradable polyester.

[0010] In step (2) of the technical solution of this invention, at least one of an aliphatic monomer and an aliphatic ester polymer is mixed with the pretreated waste PBT, and an exchange reaction (including hydroxy-ester exchange reaction (HE), hydroxy-carboxyl exchange reaction (HC), carboxyl-ester exchange reaction (CE), and ester-ester exchange reaction (EE)) is carried out under catalysis to form a prepolymer. The reaction mechanism of the hydroxy-ester exchange reaction (HE) is shown in formula (I):

[0011]

[0012] The reaction mechanism of the hydroxyl-carboxyl exchange reaction (HC) is shown in formula (II):

[0013]

[0014] The reaction mechanism of the carboxyl-ester exchange reaction (CE) is shown in equation (III):

[0015]

[0016] The reaction mechanism of ester-ester exchange (EE) is shown in equation (Ⅳ):

[0017]

[0018] Through the forward reaction of the above exchange reaction, biodegradable aliphatic repeating units can be inserted into the PBT molecular chain to form a prepolymer. Then, through the reverse reaction process of HE or CE in step (3), small molecule alcohols or acids are regenerated, and volatile small molecule alcohols or acids are removed from the reaction system by physical means of high temperature and reduced pressure, thereby obtaining a high molecular weight biodegradable polyester product.

[0019] The reaction route provided in this invention directly realizes the conversion from waste PBT to biodegradable polyester (i.e., polymer-polymer) without going through the polymer-monomer-polymer conversion mode, which greatly simplifies the chemical upgrading and recycling steps.

[0020] In some embodiments, in step (2), at least one of the aliphatic diol and the aliphatic diacid is a volatile compound, including: when the molar ratio of the aliphatic diol to the aliphatic diacid is greater than 1, the aliphatic diol is a volatile compound; or when the molar ratio of the aliphatic diol to the aliphatic diacid is less than 1, the aliphatic diacid is a volatile compound; or both the aliphatic diol and the aliphatic diacid are volatile compounds.

[0021] In some embodiments, the aliphatic monomer further includes an aliphatic anhydride, wherein the aliphatic anhydride is the anhydride corresponding to the aliphatic dicarboxylic acid.

[0022] In the technical solution of this invention, when the aliphatic monomer includes the anhydride corresponding to an aliphatic dicarboxylic acid, the exchange reaction occurring in step (2) also includes a hydroxy-anhydride bond exchange reaction (HA). Specifically, the reaction mechanism of the hydroxy-anhydride bond exchange reaction (HA) is shown in formula (V):

[0023]

[0024] Correspondingly, in step (3), the reverse reaction process of HA can be used to regenerate the corresponding acid anhydride, and the acid anhydride can be removed from the reaction system by physical means of high temperature and reduced pressure.

[0025] In some embodiments, the aliphatic diol includes at least one of ethylene glycol, 1,3-propanediol, 1,4-butanediol, and diethylene glycol; and the aliphatic dicarboxylic acid includes at least one of oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, 2-methylsuccinic acid, 2,2-dimethylsuccinic acid, or the corresponding anhydride.

[0026] In some embodiments, the molar ratio of the repeating unit of the PBT to the aliphatic monomer is 1:100-100:1; and the molar ratio of the repeating unit of the PBT to the repeating unit of the aliphatic ester polymer is 1:100-100:1.

[0027] In some embodiments, in step (2), the amount of the metal catalyst is 0.01-1% of the reaction substrate by mass percentage.

[0028] In some embodiments, the pretreatment in step (1) includes: sorting, cleaning, crushing and drying the waste PBT.

[0029] In some embodiments, in step (2), the reaction temperature for carrying out the exchange reaction is 200-260°C, and the reaction time is 1-48h.

[0030] In some embodiments, in step (3), the reaction temperature for the reverse reaction is 200-280°C, the reaction time is 0.5-48h, and the reaction pressure is 1-1000Pa.

[0031] Secondly, the present invention also proposes a biodegradable polyester, which is prepared by the method described in any of the above-mentioned methods.

[0032] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0033] (1) The reaction route provided in this invention does not require PBT to be depolymerized into monomer molecules, which simplifies the chemical upgrading and recycling process of PBT and achieves efficient utilization of waste PBT while reducing conversion costs.

[0034] (2) The resource upgrading method of the present invention can be implemented directly using existing PBT production equipment, which reduces the difficulty of the recycling process and is conducive to the rapid promotion of the process.

[0035] (3) This invention uses waste PBT to synthesize thermoplastic and fully biodegradable plastics, realizing the one-time conversion of non-degradable PBT plastics into biodegradable plastics, which is of great significance to solving the environmental pollution caused by them. Detailed Implementation

[0036] To better illustrate the purpose, technical solution, and advantages of the present invention, the present invention will be further described below through specific embodiments.

[0037] In both the embodiments and comparative examples of this invention, waste PBT is used as a raw material for recycling. The source of the waste PBT can be any PBT product (such as recycled PBT material). Since waste PBT is used as a raw material for recycling, in the embodiments and comparative examples of this invention, the waste PBT material can be pre-treated before feeding and reacting. The pre-treatment method includes: washing, crushing and drying the waste PBT material.

[0038] Unless otherwise specified, all other chemical substances used in the embodiments and comparative examples of this invention are commercially available products. The number-average molecular weight and molecular weight distribution data were determined by gel permeation chromatography (GPC), with narrow-distribution polystyrene used as the standard.

[0039] Examples 1-9

[0040] The resource recovery and upgrading method for discarded PBT disclosed in Examples 1-9 includes the following steps:

[0041] (1) In a 250 mL reactor, a certain mass of waste PBT, 1,4-butanediol, adipic acid, and stannous chloride (0.05% of the total mass of alcohol and acid) were added. The molar ratios of 1,4-butanediol and adipic acid are shown in Table 1 below. Under normal pressure, the mixture was heated to 240 °C and reacted for 2 hours. During the reaction, moisture was removed by a nitrogen gas flow. The final product was hydroxyl-terminated poly(butylene adipate-co-butylene terephthalate), i.e., PBAT prepolymer. The reaction equation for this process is shown below:

[0042]

[0043] (2) The PBAT prepolymer was placed in a reactor, and the reaction pressure was reduced to 30 Pa by vacuum extraction. The reaction temperature was maintained at 260 °C, and the polymerization reaction was carried out for 4 hours. During the reaction, it was found that the excess low-boiling-point (228 °C) 1,4-butanediol was removed from the system and existed in liquid form. After the reaction was completed, the PBAT product with hydroxyl groups at the end was obtained. The reaction equation for this process is shown in the following formula:

[0044]

[0045] Table 1 shows the molar ratio of the reaction raw materials PBT repeating unit, 1,4-butanediol and adipic acid in Examples 1-9, as well as the number average molecular weight of the PBAT prepolymer obtained in step (1) and the number average molecular weight and molecular weight distribution data of the PBAT product obtained in step (2) as shown in Table 1.

[0046] Table 1. Raw material composition and product information of Examples 1-9

[0047]

[0048]

[0049] Examples 10-18

[0050] The resource recovery and upgrading method for discarded PBT disclosed in Examples 10-18 includes the following steps:

[0051] (1) In a 250 mL reactor, a certain molar ratio of 1,4-butanediol to adipic acid was added, as shown in Table 2, along with stannous chloride (0.05% of the total mass of alcohol and acid). The mixture was heated to 200 °C under normal pressure and reacted for 1 hour, during which time moisture was removed by a nitrogen gas stream to synthesize hydroxyl-terminated polybutylene adipate (PBA). The reaction equation for this process is shown below:

[0052]

[0053] (2) A certain mass of waste PBT was added to react with hydroxyl-terminated PBA. Under normal pressure, the mixture was heated to 240°C and reacted for 1 hour. During the reaction, moisture was removed by a nitrogen gas flow. The final product was hydroxyl-terminated poly(butylene adipate-co-butylene terephthalate), i.e., PBAT prepolymer. The reaction equation for this process is shown below:

[0054]

[0055] (3) The above PBAT prepolymer was placed in a reactor, and the reaction pressure was reduced to 30 Pa by vacuum extraction. The reaction temperature was maintained at 260 °C for 4 hours. During the reaction, it was found that the excess low-boiling-point (228 °C) 1,4-butanediol was removed from the system and existed in liquid form. After the reaction was completed, PBAT product with hydroxyl end groups was obtained. The reaction equation for this process is shown in the following equation:

[0056]

[0057] Table 2 shows the molar ratio of PBT and PBA in Examples 10-18, as well as the number-average molecular weight of the PBAT prepolymer obtained in step (1) and the number-average molecular weight of the PBAT product obtained in step (2).

[0058] Table 2. Raw material composition and product information for Examples 10-18

[0059]

[0060] Example 19

[0061] The resource-based upgrading method for discarded PBT disclosed in this embodiment is basically the same as that in Embodiment 1, except that ethylene glycol is used to replace 1,4-butanediol in Embodiment 1.

[0062] Example 20

[0063] The resource-based upgrading method for discarded PBT disclosed in this embodiment is basically the same as that in Embodiment 1, except that 1,3-propanediol is used in this embodiment to replace 1,4-butanediol in Embodiment 1.

[0064] Table 3 shows the molar ratio of PBT and PBA in Examples 19-20, as well as the number-average molecular weight of the PBAT prepolymer obtained in step (1) and the number-average molecular weight of the PBAT product obtained in step (2).

[0065] Table 3. Raw material composition and product information for Examples 19-20

[0066]

[0067] Example 21

[0068] The resource recovery and upgrading method for discarded PBTs disclosed in this embodiment includes the following steps:

[0069] (1) In a 250 mL reactor, a certain mass of waste PBT, 1,4-butanediol, succinic acid, and stannous chloride (0.05% of the total mass of alcohol and acid) were added. The molar ratio of 1,4-butanediol to succinic acid was 1:1.1. Under normal pressure, the mixture was heated to 240 °C and reacted for 2 hours. During the reaction, moisture was removed by a nitrogen gas flow. The final product was poly(butylene succinate-co-butylene terephthalate) with carboxyl groups, i.e., PBST prepolymer. The reaction equation for this process is shown below:

[0070]

[0071] (2) The above PBST prepolymer was placed in a reactor, and the reaction pressure was reduced to 30 Pa by vacuum extraction. The reaction temperature was maintained at 260 °C, and the polymerization reaction was carried out for 4 hours. During the reaction, it was found that the excess low-boiling-point (236.15 °C, which can sublimate at even lower temperatures) succinic acid was removed from the system and existed in solid form. After the reaction was completed, a PBST product with carboxyl end groups was obtained. The reaction equation for this process is shown below:

[0072]

[0073] Example 22

[0074] The resource-based upgrading method for waste PBT disclosed in this embodiment is basically the same as that in embodiment 21, except that 2,2-dimethylsuccinic acid is used to replace succinic acid in embodiment 21.

[0075] Example 23

[0076] The resource-based upgrading method for discarded PBT disclosed in this embodiment is basically the same as that in embodiment 21, except that succinic anhydride is used to replace succinic acid in embodiment 21 in this embodiment.

[0077] Example 24

[0078] The resource-based upgrading method for waste PBT disclosed in this embodiment is basically the same as that in embodiment 21, except that 2,2-dimethylsuccinic anhydride is used to replace succinic acid in embodiment 21.

[0079] Table 4 shows the molar ratios of the PBT repeating units, aliphatic diols, and aliphatic diacids in Examples 21-24, as well as the number-average molecular weights of the prepolymers obtained in step (1) and the product obtained in step (2).

[0080] Table 4. Raw material composition and product information for Examples 21-24

[0081]

[0082]

[0083] Comparative Examples 1-9

[0084] The resource recovery and upgrading methods for discarded PBTs disclosed in Comparative Examples 1-9 include the following steps:

[0085] (1) In a 250 mL reactor, a certain mass of waste PBT, 1,4-butanediol, adipic acid, and stannous chloride (0.05% of the total mass of alcohol and acid) were added, wherein the molar ratio of 1,4-butanediol to adipic acid was 1:1.1. Under normal pressure, the mixture was heated to 240 °C and reacted for 2 hours, during which moisture was removed by a nitrogen gas flow. Finally, a PBAT prepolymer with carboxyl end groups was obtained. The reaction equation for this process is shown below:

[0086]

[0087] (2) The above PBAT prepolymer was placed in a reactor, and the reaction pressure was reduced to 30 Pa by vacuum extraction. The reaction temperature was maintained at 260 °C, and the polymerization reaction was carried out for 4 hours. During the reaction, it was found that the excess high-boiling-point (330.5 °C, decomposition) adipic acid could not be removed from the system. After the reaction was completed, only low molecular weight PBAT products with carboxyl end groups were obtained. The reaction equation for this process is shown in the following equation:

[0088]

[0089] Table 5 shows the molar ratios of the reaction raw materials PBT repeating units, 1,4-butanediol and adipic acid in Comparative Examples 1-9, as well as the number average molecular weight of the PBAT prepolymer obtained in step (1) and the number average molecular weight and molecular weight distribution of the PBAT product obtained in step (2) as shown in Table 5.

[0090] Table 5. Raw material composition and product information of Comparative Examples 1-9

[0091]

[0092] The above comparative examples show that if the molar amount of volatile monomers (such as 1,4-butanediol) is less than the molar amount of non-volatile monomers (such as adipic acid), it is difficult to remove the excess adipic acid through the reverse process of the carboxyl-ester exchange reaction to achieve the equimolar ratio of alcohol and acid required for high molecular weight. Therefore, only low molecular weight products can be obtained.

[0093] Comparative Examples 10-18

[0094] The resource-based upgrading method for waste PBT disclosed in Comparative Examples 10-18 includes the following steps: adding a certain mass of waste PBT and polybutylene adipate (PBA) to an extruder, heating to 260°C under normal pressure, and extruding to obtain poly(butylene adipate-co-butylene terephthalate), i.e., PBAT product.

[0095] The molar ratios of PBT repeating units and PBA in the reaction raw materials of Comparative Examples 10-18 are shown in Table 5 below. The number-average molecular weights of the PBAT products prepared in each example are shown in Table 5 below.

[0096] Table 5. Raw material composition and product information of Comparative Examples 10-18

[0097]

[0098]

[0099] The above comparative examples show that if the excess volatile monomer components are not removed through the reverse reaction in the exchange of hydroxy-ester, carboxyl-ester, or hydroxy-anhydride bonds, the equimolar ratio of alcohol and acid required for high molecular weight cannot be achieved. Therefore, only low molecular weight products can be obtained under blending conditions.

[0100] Experimental Example 1

[0101] Mechanical property testing: The PBAT products prepared in Examples 1-24 and Comparative Examples 1-18 were injection molded into corresponding test strips according to a unified process. The tensile strength and elongation at break of the samples were measured according to the measurement method specified in ISO 527-2 Plastics Tensile Properties Test Method (tensile rate of 50 mm / min). The test results are shown in Table 6 below.

[0102] Table 6. Mechanical property test results of Examples 1-24 and Comparative Examples 1-18

[0103]

[0104] As shown in Table 6, the mechanical properties of biodegradable plastics can be easily controlled by adjusting the formulation ratio of each raw material and the polymerization method in this invention. Furthermore, the molecular weight and mechanical properties of the products obtained by using excess volatile aliphatic diols (such as 1,4-butanediol) or acids (such as succinic acid), or by directly adding PBA during polymerization, are significantly better than those obtained under the same conditions using non-volatile acids (such as adipic acid) or by blend extrusion.

[0105] Experiment Example 2

[0106] Test of the final aerobic biodegradation capacity of materials under controlled composting conditions (according to test standard: GB / T19277.1-2011 / ISO 14855-1:2005): The sample was mixed with inoculum and placed in a pre-prepared compost container. The dry weight ratio of the inoculum to the dry weight of the material was 6:1, and the volume of the mixture did not exceed 3 / 4 of the compost container's volume. During the test, the carbon dioxide content in the exhaust gas of each compost container was measured periodically using a total organic carbon analyzer, and the ratio of this content to the theoretical release was calculated to obtain the biodegradation rate (%). The test results are shown in Table 7 below.

[0107] Table 7. Results of compost biodegradation capacity test

[0108]

[0109] As shown in Table 7, the present invention can conveniently control the biodegradation rate and biodegradation cycle of biodegradable plastics by adjusting the formulation ratio and polymerization method.

[0110] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A method for resource-based upgrading of discarded PBTs, characterized in that, Includes the following steps: (1) Pre-treatment of waste PBT before reaction; (2) At least one of the aliphatic monomers and aliphatic ester polymers is mixed with the pretreated waste PBT and subjected to an exchange reaction under the catalysis of a metal catalyst to obtain a prepolymer. The aliphatic monomers include aliphatic diols and anhydrides corresponding to aliphatic diacids. The aliphatic ester polymers are obtained by reacting the aliphatic diols and anhydrides corresponding to the aliphatic diacids. The molar ratio of the aliphatic diols to the anhydrides corresponding to the aliphatic diacids is less than 1, and the anhydrides corresponding to the aliphatic diacids are volatile compounds or both the aliphatic diols and the anhydrides corresponding to the aliphatic diacids are volatile compounds. (3) The prepolymer is subjected to the reverse reaction of the exchange reaction under reduced pressure, and the volatile compounds are removed during the reverse reaction to obtain a biodegradable polyester.

2. The method as described in claim 1, characterized in that, The aliphatic diol includes at least one of ethylene glycol, 1,3-propanediol, 1,4-butanediol, and diethylene glycol; and the anhydride corresponding to the aliphatic dicarboxylic acid includes at least one of the anhydrides corresponding to oxalic acid, malonic acid, succinic acid, glutaric acid, 2-methylsuccinic acid, and 2,2-dimethylsuccinic acid.

3. The method as described in claim 1, characterized in that, The molar ratio of the repeating unit of the PBT to the aliphatic monomer is 1:100-100:1; and the molar ratio of the repeating unit of the PBT to the repeating unit of the aliphatic ester polymer is 1:100-100:

1.

4. The method as described in claim 1, characterized in that, In step (2), the amount of the metal catalyst is 0.01-1% of the reaction substrate by mass percentage.

5. The method as described in claim 1, characterized in that, The pretreatment in step (1) includes: classifying, cleaning, crushing and drying the waste PBT.

6. The method as described in claim 1, characterized in that, In step (2), the reaction temperature for the exchange reaction is 200-260℃ and the reaction time is 1-48h.

7. The method as described in claim 1, characterized in that, In step (3), the reaction temperature for the reverse reaction is 200-280℃, the reaction time is 0.5-48h, and the reaction pressure is 1-1000Pa.

8. A biodegradable polyester, characterized in that, The biodegradable polyester is prepared by the method according to any one of claims 1-7.

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