A method for recycling thermoplastic polyurethane capable of multiple cycles and value-added utilization

Abstract

The present invention discloses a method for recycling thermoplastic polyurethane that can be recycled multiple times and has increased value. The method comprises the following steps: placing a thermoplastic polyurethane containing oxime in a vacuum, heating the reaction at 80-200°C for 1-20 hours to remove the oxime, and obtaining a thermosetting polyurethane; dissolving the removed oxime in a solvent to obtain an oxime solution when necessary, placing the thermosetting polyurethane in the oxime solution, and performing a reverse reaction at 50-150°C for 10 minutes and 10 hours. After the reaction is completed, the solvent is removed to obtain the thermoplastic polyurethane. The present invention changes the properties of the polyurethane material by removing and returning the oxime. Removing the oxime component from the molecule can cause the remaining components to undergo a cross-linking reaction, so that the thermoplastic material is converted into a thermosetting polyurethane that does not contain oxime and has better mechanical properties. Returning the oxime to the thermosetting polyurethane can cause it to return to the thermoplastic material, thereby realizing value-added recycling of the thermoplastic material, obtaining a thermosetting polyurethane with better mechanical properties, and improving economic value.

Description

Technical Field

[0001] The present invention belongs to the technical field of thermoplastic material recycling, and more particularly relates to a method for recycling thermoplastic polyurethane capable of multiple cycles and value-added recycling. Background Art

[0002] Plastics are inextricably linked to our lives. It's reported that global plastic production will reach 400 million tons annually in 2022 (Plastics Europe. Plastics — The Facts 2023. Available online: https: / / plasticseurope.org / knowledge-hub / plastics-the-facts-2023 / ). Thermoplastics are the primary form of plastics, accounting for over 80% of total plastic production. This massive amount of plastic also places a heavy burden on the environment, creating challenges in waste disposal.

[0003] The current method of recycling thermoplastic materials is mainly through melt reprocessing. This method achieves the goal of reuse by heating the waste to a molten state and reshaping it with the help of mechanical force. However, during the reprocessing process, the material will be affected by factors such as high temperature, shear force, oxygen and moisture, resulting in inevitable and difficult to quantify side reactions, including molecular chain breakage, hydrolysis and oxidation, which will reduce the performance and economic value of the material and lead to product degradation. In addition, degradation has a cumulative effect. After 2-3 times of degradation and recycling, the performance of the product is severely degraded and difficult to reuse, resulting in a limited number of recycling times.

[0004] Currently, people are trying to achieve the transformation of thermoplastic materials into thermosetting materials. However, these transformations often only achieve a short-term property change, resulting in a temporary material that cannot permanently maintain the transformed form; or they usually require grafting other cross-linking agents to obtain a stable thermosetting material. Summary of the Invention

[0005] The object of the present invention is to provide a method for recycling thermoplastic polyurethane in multiple cycles and value-added manner, wherein the thermoplastic polyurethane can be recycled into thermosetting polyurethane with higher mechanical properties, and the value-added recycling can be carried out in multiple cycles without obvious performance degradation after multiple cycles.

[0006] The present invention provides the following technical solutions:

[0007] A method for recycling thermoplastic polyurethane for multiple cycles and value-added use, comprising the following steps:

[0008] Step (1), value-added recovery: dioxime, diol and diisocyanate are mixed and reacted to generate thermoplastic polyurethane containing oxime, and then the thermoplastic polyurethane containing oxime is placed in a vacuum environment for heating reaction, and the oxime is removed to obtain thermosetting polyurethane;

[0009] You can also perform the following steps as needed:

[0010] Step (2), reverse conversion: dissolving oxime in a solvent to obtain an oxime solution, placing the thermosetting polyurethane in the oxime solution for reverse reaction, and removing the solvent after the reaction is completed to obtain the thermoplastic polyurethane again.

[0011] The recycling object of the present invention is thermoplastic materials, especially thermoplastic polyurethane materials, and the target product of recycling is thermosetting polyurethane with higher performance.

[0012] Preferably, the oxime-containing thermoplastic polyurethane contains a large number of dioxime-urethane groups and conventional urethane groups; the dioxime is one or more of dimethylglyoxime, glyoxime, and 1,4-benzoquinone dioxime; the diol is one or more of polytetramethylene glycol, polypropylene glycol, polyethylene glycol, and polycaprolactone diol; and the diisocyanate is one or more of diphenylmethane diisocyanate, toluene-2,4-diisocyanate, and hexamethylene diisocyanate. The molar ratio of dioxime to diol is 0.1:1 to 10:1, and the molar amount of diisocyanate is the sum of the molar amounts of dioxime and diol. The reaction temperature is 50-120°C, and the reaction time is 1-10 hours.

[0013] Preferably, in step (1), the vacuum degree is 10-1000 Pa, the heating temperature is 80-200° C., and the heating time is 1-20 h.

[0014] In the value-added recovery reaction of step (1), after the oxime component in the thermoplastic polyurethane is removed, the remaining components undergo a cross-linking reaction and are converted into a thermosetting polyurethane that does not contain oxime. Specifically, the principle of the value-added recovery reaction is as follows: under heating and vacuum conditions, the dynamic properties of the dioxime-urethane group in the thermoplastic polyurethane are activated, and cracking occurs to produce dioxime and isocyanate-terminated polyurethane prepolymer. The generated dioxime has the ability to sublimate in vacuum and is separated from the main material, causing the reaction to continue in the cracking direction, producing more isocyanate-terminated polyurethane prepolymer. The terminal isocyanate group of the isocyanate-terminated polyurethane prepolymer reacts with the ordinary urethane group in the molecule to form a trifurcated cross-linking site - allophanate group, thereby completing the conversion to thermosetting polyurethane.

[0015] The removed oxime can be recovered by condensation and used as a raw material for synthesizing new thermoplastic polyurethane after recovery; it can also be used as a reaction reagent to return the thermosetting polyurethane obtained by value-added recovery to thermoplastic polyurethane.

[0016] Preferably, the solvent in step (2) is one of ethyl acetate, butyl acetate, tetrahydrofuran, N,N-dimethylformamide, N-methylpyrrolidone, 1,4-dioxane, and dimethyl sulfoxide, and its mass is 1-5 times that of oxime.

[0017] Preferably, the reaction temperature of the reverse reaction in step (2) is 50-150° C., the reaction time is 10 min-10 h, and the reaction is carried out in a sealed container at normal pressure.

[0018] Preferably, in step (2), the solvent is removed by rotary evaporation at a temperature of 25-150° C. for a time of 30 min-2 h.

[0019] In the reverse conversion reaction of step (2), the thermosetting polyurethane can return to a thermoplastic state after obtaining oxime. Specifically, the principle of the reverse conversion reaction is that the thermal reversibility of the allophanate group in the thermosetting polyurethane is activated, and reversible cleavage is performed to produce an isocyanate-terminated polyurethane prepolymer. The isocyanate-terminated polyurethane prepolymer reacts with the dioxime to return to a thermoplastic polyurethane.

[0020] Compared with the current recycling method, the method provided by the present invention has the following advantages:

[0021] (1) The method of the present invention regulates the material components of thermoplastic polyurethane, and changes the properties of the polyurethane material by removing and returning oxime. Removing the oxime component from the molecule can cause the remaining components to undergo a cross-linking reaction, so that the thermoplastic material is converted into a thermosetting polyurethane that does not contain oxime and has better mechanical properties. Returning the oxime to the thermosetting polyurethane can make it return to the thermoplastic material. The obtained thermoplastic material is a linear, non-cross-linked molecule in the traditional sense and can be kept usable at room temperature. The method of the present invention can realize the value-added recovery of thermoplastic materials, obtain thermosetting polyurethane with better mechanical properties, and improve economic value.

[0022] (2) The method of the present invention can be recycled multiple times for value-added recovery, and the performance of the product obtained in each value-added recovery does not significantly decay. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 Schematic diagram of the synthesis reaction of oxime-containing thermoplastic polyurethane;

[0024] Figure 2 Schematic diagram of the reaction for value-added recovery of oxime-containing thermoplastic polyurethane into thermosetting polyurethane;

[0025] Figure 3 Schematic diagram of the reaction for reverse conversion of thermosetting polyurethane into oxime-containing thermoplastic polyurethane;

[0026] Figure 4A schematic diagram of the comprehensive reaction for multiple cycle value-added recycling;

[0027] Figure 5 Schematic diagram of the value-added recovery of oxime-containing thermoplastic polyurethane into thermosetting polyurethane in the embodiment, wherein (a) is thermoplastic polyurethane, and (b) is thermosetting polyurethane;

[0028] Figure 6 Schematic diagram of the dissolution experiment of the initial oxime-containing thermoplastic polyurethane and the thermosetting polyurethane after value-added recovery in the examples, wherein (a) thermoplastic polyurethane, (b) thermosetting polyurethane;

[0029] Figure 7 The NMR spectra of dimethylglyoxime separated from the main material and deposited on the top of the bottle during value-added recovery in the embodiment and the pure dimethylglyoxime raw material;

[0030] Figure 8 Schematic diagram of the reverse conversion of thermosetting polyurethane into oxime-containing thermoplastic polyurethane in the embodiment, wherein (a) thermosetting polyurethane, (b) thermoplastic polyurethane;

[0031] Figure 9 1 is the stress-strain curve of the thermosetting polyurethane that has undergone three cycles of value-added recovery in the embodiment. DETAILED DESCRIPTION

[0032] As mentioned above, the present invention provides a method for recycling thermoplastic polyurethane for multiple cycles and value-added use, the method comprising the following steps:

[0033] Step (1), placing the thermoplastic polyurethane containing oxime in an environment with a vacuum degree of 10-1000 Pa and heating it to 80-200° C. for reaction for 1-20 hours, and obtaining a thermosetting polyurethane after removing the oxime;

[0034] Step (2), dissolving oxime in a solvent to obtain an oxime solution, placing the thermosetting polyurethane in the oxime solution and performing a reverse reaction at 50-150° C. for 10 min-10 h, and removing the solvent after the reaction is completed to obtain a thermoplastic polyurethane; the solvent is one of ethyl acetate, butyl acetate, tetrahydrofuran, N,N-dimethylformamide, N-methylpyrrolidone, 1,4-dioxane, and dimethyl sulfoxide;

[0035] The thermoplastic polyurethane containing oxime is prepared by the following method:

[0036] A dioxime, a diol and a diisocyanate are mixed and reacted to generate a thermoplastic polyurethane containing an oxime, wherein the dioxime is one or more of dimethylglyoxime, ethylenedioxime and 1,4-benzoquinonedioxime, the diol is one or more of polytetramethylene glycol, polypropylene glycol, polyethylene glycol and polycaprolactone diol, and the diisocyanate is one or more of diphenylmethane diisocyanate, toluene-2,4-diisocyanate and 1,6-hexamethylene diisocyanate.

[0037] The technical principle of the method for recycling thermoplastic polyurethane for multiple cycles and value-added use provided by the present invention is as follows: Figure 1-Figure 4 As shown, there are respectively a schematic diagram of the synthesis reaction of oxime-containing thermoplastic polyurethane, a schematic diagram of the value-added recycling reaction, a schematic diagram of the reverse conversion reaction, and a schematic diagram of the comprehensive reaction of multiple-cycle value-added recycling; the method provided by the present invention has low raw material prices, simple material preparation and recycling processes, can achieve value-added recycling, obtain thermosetting polyurethane with better mechanical properties, and improve economic value; in addition, value-added recycling can be carried out multiple times without obvious performance degradation.

[0038] The present invention will be further described below with reference to specific examples. It should be noted that the following examples are only intended to deepen the understanding of the present invention and are not intended to limit the scope of the present invention.

[0039] Example 1

[0040] In this embodiment, dimethylglyoxime is selected as the dibasic oxime, polytetramethylene glycol is selected as the diol, and diphenylhexane diisocyanate is selected as the dibasic isocyanate to prepare thermoplastic polyurethane containing oxime.

[0041] Raw materials: dimethylglyoxime, polytetramethylene glycol (M n = 1000 g / mol) was purchased from Aladdin Reagent Platform; diphenylhexane diisocyanate was purchased from Wanhua Chemical Group Co., Ltd.; N,N-dimethylformamide was purchased from Sinopharm Chemical Reagent Co., Ltd.

[0042] Synthesis of Oxime-Containing Thermoplastic Polyurethane: Dissolve polytetramethylene glycol (40 g, 0.04 mol) and dimethylglyoxime (4.64 g, 0.04 mol) in N,N-dimethylformamide (180 mL). Add diphenylmethane diisocyanate (20 g, 0.08 mol) and allow to react at 70°C for 6 h. After completion of the reaction, remove the N,N-dimethylformamide by rotary evaporation to obtain the oxime-containing thermoplastic polyurethane.

[0043] Value-added recovery of oxime-containing thermoplastic polyurethane to thermosetting polyurethane: Place the oxime-containing thermoplastic polyurethane on a PTFE plate and place it at the bottom of a vacuum reactor (such as Figure 5As shown). Under vacuum conditions (150Pa), heating (140℃) the bottom of the reactor for 5h, thermosetting polyurethane can be obtained. Figure 5 The red box in (b) shows the deposited diacetyl oxime.

[0044] The thermoplastic / thermosetting conversion was confirmed by dissolution experiments. The material was placed in N,N-dimethylformamide solvent. The results were as follows: Figure 6 As shown, Figure 6 (a) Thermoplastic polyurethane is soluble, Figure 6 The thermosetting polyurethane obtained after value-added recovery in (b) did not dissolve, confirming the formation of a cross-linked structure. In addition, the dimethylglyoxime deposited on the top of the reactor was collected and its recovery rate was calculated by weighing to be 98%. The nuclear magnetic resonance spectrum confirmed the chemical structure of the recovered dimethylglyoxime and its high purity (such as Figure 7 shown).

[0045] Reverse conversion of thermosetting polyurethane to oxime-containing thermoplastic polyurethane: dissolve the above collected oxime in N,N-dimethylformamide and add the above obtained thermosetting polyurethane. Figure 8 As shown in (a), the thermosetting polyurethane at this time only swells but cannot dissolve. After reacting at 130℃ for 15 minutes, the thermosetting polyurethane is completely dissolved. Figure 8 As shown in (b); after rotary evaporation to remove N,N-dimethylformamide, a new thermoplastic polyurethane is obtained.

[0046] Test Example 1: Multiple-cycle value-added recycling

[0047] The newly obtained thermoplastic polyurethane in Example 1 is subjected to value-added recovery-reverse conversion-value-added recovery (the method is the same as that in Example 1) to obtain thermosetting polyurethane with three cycles of value-added recovery. Figure 9 As shown in the figure, the mechanical properties test shows that: (1) compared with the initial material, the mechanical properties of the material are significantly enhanced after value-added recycling; (2) after multiple recycling cycles, its mechanical properties do not significantly decay.

[0048] The above embodiments are merely illustrative and not intended to limit the embodiments. Those skilled in the art will appreciate that other variations or modifications may be made based on the above description. Any modifications or changes made to the present invention are within the scope of the present invention.

Claims

1. A method for recycling thermoplastic polyurethane for multiple cycles and value-added use, characterized in that: The method comprises the following steps: Value-added recovery: dioxime, diol and diisocyanate are mixed and reacted to generate thermoplastic polyurethane containing oxime, and then the thermoplastic polyurethane containing oxime is placed in a vacuum and heated at 80-200℃ for 1-20h to remove the oxime to obtain thermosetting polyurethane. The removed oxime is recovered by condensation; Reverse conversion: dissolve the removed oxime in a solvent to obtain an oxime solution, place the thermosetting polyurethane in the oxime solution, and carry out a reverse reaction at a temperature of 50-150°C for 10 minutes to 10 hours. After the reaction is completed, remove the solvent to obtain a thermoplastic polyurethane.

2. The method according to claim 1, characterized in that The vacuum degree in step (1) is 10-1000 Pa.

3. The method according to claim 1, characterized in that The dioxime is one or more of dimethylglyoxime, glyoxime, and 1,4-benzoquinone dioxime; the diol is one or more of polytetramethylene glycol, polypropylene glycol, polyethylene glycol, and polycaprolactone glycol; and the diisocyanate is one or more of diphenylmethane diisocyanate, toluene-2,4-diisocyanate, and 1,6-hexamethylene diisocyanate.

4. The method according to claim 3, characterized in that The molar ratio of dibasic oxime to diol is 0.1-10:1, and the molar weight of diisocyanate is the sum of the molar weights of dibasic oxime and diol.

5. The method according to claim 1, wherein The solvent is one of ethyl acetate, butyl acetate, tetrahydrofuran, N,N-dimethylformamide, N-methylpyrrolidone, 1,4-dioxane, and dimethyl sulfoxide.

6. The method according to claim 1, characterized in that The mass ratio of the solvent to the oxime is 1-5:

1.

7. The method according to claim 1, characterized in that The reverse reaction is carried out under closed and normal pressure conditions.

8. The method according to claim 1, characterized in that The solvent is removed by rotary evaporation at a temperature of 25-150° C. for 30 min-2 h.

9. The method according to claim 1, characterized in that In the value-added recovery step, the thermoreversibility of the dioxime-urethane groups in the thermoplastic polyurethane is activated under heating and vacuum conditions, and reversible cleavage is performed to produce dioxime and isocyanate-terminated polyurethane prepolymers. The terminal isocyanate groups of the isocyanate-terminated polyurethane prepolymer react with the ordinary urethane groups in the molecule to form trifurcated crosslinking sites - allophanate groups, thereby completing the conversion to thermosetting polyurethane. In the reverse conversion step, the thermal reversibility of the allophanate groups in the thermosetting polyurethane is activated and reversibly cleaved to produce an isocyanate-terminated polyurethane prepolymer, which reacts with the dioxime to return to the thermoplastic polyurethane.

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