Method for recycling polylactic acid waste

Abstract

The invention discloses a polylactic acid waste recovery method, and belongs to the field of polylactic acid recovery. The method comprises the following steps: putting the polylactic acid waste into a high-low temperature alternating damp-heat test chamber for carrying out damp-heat reaction or heating and hydrolyzing in an aqueous solution to obtain low-molecular-weight polylactic acid waste; the method comprises the following steps: carrying out cracking reaction on low-molecular-weight polylactic acid waste and a catalyst 1 to obtain crude lactide; purifying the crude lactide with a solvent to obtain high-purity lactide; and the high-purity lactide reacts with an initiator and a catalyst 2 to generate new polylactic acid.

Description

Technical Field

[0001] The invention relates to the field of polylactic acid recycling, and in particular to a method for recycling polylactic acid waste. Background Art

[0002] Compared with traditional petroleum-based plastics, the most widely used biodegradable plastic, polylactic acid, can be decomposed into carbon dioxide and water in the environment. But its natural degradation takes quite a long time, and PLA only passes international biodegradation standards in controlled industrial environments at temperatures above 50°C. More importantly, it is a carbon-emitting process. Sustainable development of the polymer industry must include effective waste management and recycling pathways. Although mechanical recycling should be the first choice where appropriate, the need to mix with at least 50% of virgin material and the deterioration of physical properties after repeated reprocessing limit its scope of application. When mechanical recycling is no longer possible, chemical recycling to produce new PLA is preferable to biomass-derived virgin PLA. About half of the costs in the PLA production process come from lactic acid production, which includes the production of biomass, an economic factor that further promotes the closed loop of PLA. In addition, biodegradable polymers are recyclable, and the EU's strategy to achieve 100% recyclable or reusable plastics by 2030 is another driving force. Therefore, it is very necessary to develop a new recycling method to upgrade polylactic acid plastic waste into high-value-added, highly chemically selective products, especially closed-loop recycling into new polylactic acid. Summary of the invention

[0003] In view of the deficiencies of the prior art, the present invention proposes a method for recycling polylactic acid waste.

[0004] The purpose of the present invention can be achieved through the following technical solutions:

[0005] A method for recycling polylactic acid waste comprises the following steps:

[0006] Placing the polylactic acid waste in a high-low temperature alternating wet heat test box for wet heat reaction or heating and hydrolyzing in an aqueous solution to obtain low molecular weight polylactic acid waste;

[0007] Low molecular weight polylactic acid waste is subjected to cracking reaction with catalyst 1 to obtain crude lactide;

[0008] Purifying the crude lactide with a solvent to obtain high-purity lactide;

[0009] High-purity lactide reacts with initiator and catalyst 2 to generate new polylactic acid.

[0010] Optionally, the temperature of the high and low temperature alternating humidity test box is 40-90° C.; and the humidity is 20-100%.

[0011] Optionally, the molecular weight of the low molecular weight polylactic acid waste is not more than 5000.

[0012] Optionally, the catalyst 1 or catalyst 2 is one or more of zinc lactate, zinc oxide, zinc powder, diethyl zinc, tin lactate, tin oxide, stannous lactate, stannous octoate, stannous chloride, tin powder, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD).

[0013] Optionally, the solvent is one or more of toluene and ethyl acetate.

[0014] Optionally, the initiator is one or more of ethanol, 1,4-butanediol, and benzyl alcohol.

[0015] Optionally, the precipitant is one or more of acetone, heptane, and methanol.

[0016] Optionally, the catalyst 1 accounts for 0.01%-0.05% by mass relative to the polylactic acid.

[0017] Optionally, the mass proportion of the initiator relative to the lactide is 0.01%-0.05%.

[0018] Beneficial effects of the present invention:

[0019] Through the treatment method of the present invention, polylactic acid waste or waste polylactic acid products can be cracked into lactide, and then purified to generate new polylactic acid. The recycling method of the present invention can form a closed-loop recycling of polylactic acid waste or waste products, and has high application value in terms of environmental protection and enterprise production cost reduction. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The present invention will be further described below in conjunction with the accompanying drawings.

[0021] Figure 1 A flowchart of the recycling method of the present application;

[0022] Figure 2 The figure is a reaction principle diagram of the recovery method of the present application. DETAILED DESCRIPTION

[0023] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0024] Embodiment 1:

[0025] Polylactic acid straws are recycled by heat degradation in a high and low temperature alternating heat and humidity test chamber. The degradation process is as follows:

[0026] The specific operation process is as follows:

[0027] A. Degradation: Take an appropriate amount of commercial straws and put them into a high-low temperature alternating humidity test chamber (temperature: 90°C; humidity: 90) and take them out after three days. Figure 2 The molecular weight of Guanghua Weiye PLA straw after hydrolysis for three days is shown in Table 1.

[0028] Table 1 Molecular weight of commercial straws and high and low temperature alternating humidity test chamber after hydrolysis for three days

[0029]

[0030] Figure 2 Schematic diagram of polylactic acid hydrolysis

[0031] B. Cracking: Dry the low molecular weight straw in a vacuum oven (40°C, 6h) and crush it into powder with a mortar. Add 400g of straw powder and 2g of stannous lactate into a 1000mL round-bottom flask and stir in a fume hood at 200°C for 3h. After the reaction, sample gas phase detection showed that the lactide conversion rate was 81%, of which L-lactide was 82%, Meso-lactide was 4.6%, and D-lactide was 0.5%.

[0032] C. Purification: The lactide solid obtained in step B was collected, and recrystallized three times by adding toluene. After drying at 40° C. for 6 h, white needle-shaped lactide was obtained with a purity of 99.1%.

[0033] D. Polymerization: Weigh 230 g of the lactide purified in step C into a 1000 mL volumetric flask, melt it at 100°C in a fume hood, add 0.01 g of ethanol and 0.1 g of stannous octoate, and react at 180°C for 3 h under N2 atmosphere.

[0034] E. Purification: The polymer was dissolved in CHCl3 and precipitated in a mixture of acetone and methanol (10 wt% methanol). The precipitation was repeated four times to remove stannous lactate and unreacted monomers. The purified polymer was dried in a fume hood overnight and then dried under vacuum (60°C, 24h). After completion, a sample was taken for GPC testing, and the Mn was 120,000; the Mw was 180,000; and the PDI was 1.6.

[0035] Embodiment 2:

[0036] Polylactic acid particles are recycled by heating and hydrolysis. The degradation process is as follows:

[0037] The specific operation process is as follows:

[0038] A. Degradation: 200g of commercial polylactic acid particles were added to a round-bottom flask containing 200g of deionized water, and refluxed in a fume hood under magnetic stirring at 140°C for 5 days before being taken out. The sample was then placed in a vacuum drying oven (40°C, 6h) and tested by GPC. The test results are shown in Table 2.

[0039] Table 2 Molecular weight of commercial polylactic acid particles and deionized water after five days of hydrolysis

[0040]

[0041]

[0042] B. Dehydration: The oligomer aqueous solution after the reaction in step A was reacted at 170°C under reduced pressure for 2 hours to remove free water and part of the bound water. After the reaction, a sample was taken for GPC detection, and the Mn was 4301; the Mw was 6425; and the PD was 1.5.

[0043] C. Cracking: Add 1.5 g of stannous octoate to the oligomer after the reaction in step B in a 1000 mL round-bottom flask, and stir the reaction at 190°C in a fume hood for 3 hours. After the reaction, sample gas phase detection showed that the lactide conversion rate was 83%, of which L-lactide was 90%, Meso-lactide was 7.6%, and D-lactide was 2.4%.

[0044] D. Purification: The lactide solid collected in step C was added to ethyl acetate for recrystallization three times and dried at 40°C for 6 hours to obtain white needle-shaped lactide with a purity of 99.3%.

[0045] E. Polymerization: Weigh 230 g of the lactide purified in step C into a 1000 mL volumetric flask, melt it at 100°C in a fume hood, add 0.05 g of ethanol and 0.1 g of stannous octoate, and react at 180°C in a N2 environment for 3 h.

[0046] F. Purification: The polymer was dissolved in CHCl3 and precipitated in a mixture of heptane and methanol (10 mol% methanol). The precipitation was repeated four times to remove stannous octoate and unreacted monomers. The purified polymer was dried in a fume hood overnight and then dried under vacuum (60°C, 24h). After completion, a sample was taken for GPC testing, and the Mn was 100,000; the Mw was 160,000; and the PDI was 1.6.

[0047] Embodiment 3:

[0048] Polylactic acid particles are efficiently recycled under the catalyst. The degradation process is as follows:

[0049] The specific operation process is as follows:

[0050] A. Degradation: 200g of commercial polylactic acid particles were added to a round-bottom flask containing 150g of deionized water, and refluxed in a fume hood under magnetic stirring at 140°C for 7h in the presence of 5g of diphenyl phosphate catalyst, and then taken out. The sample was placed in a vacuum drying oven (40°C, 6h) and then tested by GPC. The test results are shown in Table 3.

[0051] Table 3 Molecular weight of commercial polylactic acid particles and deionized water after 7 hours under catalyst

[0052]

[0053] B. Dehydration: The oligomer aqueous solution after the reaction in step A was reacted at 170°C under reduced pressure for 2 hours to remove free water and part of the bound water. After the reaction, a sample was taken for GPC detection, and the Mn was 3621; the Mw was 4297; and the PD was 2.3.

[0054] C. Cracking: Add 10g of stannous octoate to the oligomer after the reaction in step B into a 1000mL round-bottom flask, and stir and react at 190°C in a fume hood for 3h. After the reaction, sample gas phase detection showed that the lactide conversion rate was 79%, of which L-lactide was 92%, Meso-lactide was 6.8%, and D-lactide was 3.2%.

[0055] D. Purification: The lactide solid collected in step C was added to ethyl acetate for recrystallization three times and dried at 40°C for 6 hours to obtain white needle-shaped lactide with a purity of 99.1%.

[0056] E. Polymerization: Weigh 230 g of the lactide purified in step C into a 1000 mL volumetric flask, melt it at 150°C in a fume hood, add 0.05 g of 1,4-butanediol and 0.1 g of stannous octoate, and react at 180°C in a N2 environment for 5 h.

[0057] F. Purification: The polymer was dissolved in CHCl3 and precipitated in a mixture of heptane and methanol (10 mol% methanol). The precipitation was repeated four times to remove stannous octoate and unreacted monomers. The purified polymer was dried in a fume hood overnight and then dried under vacuum (60°C, 24h). After completion, a sample was taken for GPC testing, and the Mn was 94,000; the Mw was 170,000; and the PDI was 1.8.

[0058] In the description of this specification, the description with reference to the terms "one embodiment", "example", "specific example", etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0059] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments, and the above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, and these changes and improvements all fall within the scope of the present invention to be protected.

Claims

1. A method for recycling polylactic acid waste, characterized in that: The following steps are involved: Placing the polylactic acid waste in a high-low temperature alternating wet heat test box for wet heat reaction or heating and hydrolyzing in an aqueous solution to obtain low molecular weight polylactic acid waste; Low molecular weight polylactic acid waste is subjected to cracking reaction with catalyst 1 to obtain crude lactide; Purifying the crude lactide with a solvent to obtain high-purity lactide; High-purity lactide reacts with initiator and catalyst 2 to generate new polylactic acid.

2. The method for recycling polylactic acid waste according to claim 1, characterized in that: The temperature of the high and low temperature alternating damp heat test box is 40-90° C.; the humidity is 20-100%.

3. The method for recycling polylactic acid waste according to claim 1, characterized in that: The molecular weight of the low molecular weight polylactic acid waste is no more than 5000.

4. The method for recycling polylactic acid waste according to claim 1, characterized in that: The catalyst 1 or catalyst 2 is one or more of zinc lactate, zinc oxide, zinc powder, diethyl zinc, tin lactate, tin oxide, stannous lactate, stannous octoate, stannous chloride, tin powder, 1,8-diazabicyclo[5.4.0]undec-7-ene, 1,5,7-triazabicyclo[4.4.0]dec-5-ene, and diphenyl phosphate.

5. The method for recycling polylactic acid waste according to claim 1, characterized in that: The solvent is one or more of toluene and ethyl acetate.

6. The method for recycling polylactic acid waste according to claim 1, characterized in that: The initiator is one or more of ethanol, 1,4-butanediol and benzyl alcohol.

7. The method for recycling polylactic acid waste according to claim 1, characterized in that: The precipitant is one or more of acetone, heptane and methanol.

8. The method for recycling polylactic acid waste according to claim 1, characterized in that: The catalyst 1 accounts for 0.01%-0.05% of the mass of the polylactic acid.

9. The method for recycling polylactic acid waste according to claim 1, characterized in that: The mass proportion of the initiator to the lactide is 0.01%-0.05%.

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