Biodegradable shrink film as well as preparation method and application thereof

Abstract

The invention relates to the technical field of polyester degradable plastics, and discloses a biodegradable shrink film as well as a preparation method and application thereof. The preparation method of the biodegradable shrink film comprises the following steps: reacting (R)-3-hydroxybutyric acid with 1, 4-dibromobutane to obtain a dihydric alcohol monomer; the preparation method comprises the following steps: copolymerizing a dihydric alcohol monomer and L-lactide to obtain modified polylactic acid; carrying out copolymerization on the geraniol-based compound and L-lactide to obtain a bio-based plasticizer; modifying the bio-based plasticizer with allyl trimethyl ammonium chloride to obtain a modified bio-based plasticizer; the biodegradable shrink film is prepared by taking modified polylactic acid, poly (butylene adipate-co-terephthalate), a modified bio-based plasticizer, a functional filler and palm wax as raw materials. The biodegradable shrink film has the advantages of flexibility, shrinkage performance, lasting antibacterial performance and good barrier performance, and can be used in food packaging materials.

Description

Technical Field

[0001] This invention relates to the field of polyester biodegradable plastics technology, specifically to a biodegradable shrink film, its preparation method, and its application. Background Technology

[0002] In recent years, the demand for plastic packaging films has been increasing, especially heat shrink films, which not only provide secure and attractive packaging but also offer good protection. Commonly used heat shrink films include PVC, PE, PP, PVDC, OPS, and PET. Because heat shrink films made from these materials are non-degradable and pose a significant environmental hazard, various biodegradable materials have been developed, such as polylactic acid (PLA) and polybutylene terephthalate (PBAT). PLA is a fully biodegradable polyester material with high strength and thermal stability; however, PLA's brittleness greatly limits its applications. PBAT has good toughness but poor strength and rigidity, making it complementary to PLA in terms of performance. Therefore, PBAT can be used as a modifier for PLA and blended with it to prepare biodegradable heat shrink films. However, when using PBAT and PLA in combination, their compatibility needs to be addressed; without compatibilizers, the resulting product has poor mechanical properties. Furthermore, when using PLA heat shrink films in food packaging, their antibacterial and barrier properties must be considered.

[0003] To address the above issues, Chinese patent application CN115926410A discloses a biodegradable antibacterial heat shrink film and its processing technology, while Chinese patent CN115891367B discloses a PBAT-PLA composite material and its application in a biodegradable heat shrink film. However, neither Chinese patent CN115891367B nor Chinese patent application CN115926410A can simultaneously solve the problems of poor compatibility between PBAT and PLA, nor the poor antibacterial and barrier properties of polylactic acid heat shrink film. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a method for preparing a biodegradable shrink film, comprising the following steps:

[0005] Step 1: Preparation of modified polylactic acid and modified bio-based plasticizer;

[0006] The preparation of modified polylactic acid includes:

[0007] (R)-3-hydroxybutyric acid reacts with 1,4-dibromobutane to obtain a diol monomer; the diol monomer is copolymerized with L-lactide to obtain modified polylactic acid.

[0008] The preparation of modified bio-based plasticizers includes:

[0009] 3-Mercapto-1,2-propanediol was reacted with modified geraniol to obtain geraniol-based compounds; the geraniol-based compounds were copolymerized with L-lactide to obtain bio-based plasticizers; the bio-based plasticizers were modified with allyltrimethylammonium chloride to obtain modified bio-based plasticizers.

[0010] Step 2: Mix modified polylactic acid, polybutylene terephthalate-adipate, modified bio-based plasticizer, functional filler, and palm wax evenly, melt extrude, cast into a sheet using a die, wind the cast sheet, stretch biaxially, cool and set, and then wind up to obtain a biodegradable shrink film.

[0011] Preferably, in step one, the method for preparing the modified polylactic acid specifically includes:

[0012] (R)-3-hydroxybutyric acid and 1,4-dibromobutane were dissolved in N,N-dimethylformamide. The mixture was stirred at 23-27°C for 20-40 min under a nitrogen atmosphere. Potassium carbonate was then added, and stirring was continued for another 20-40 min. N,N-dimethylformamide was then added, and the mixture was heated to 77-83°C. The temperature was maintained, and stirring was continued for 16-20 h. The mixture was purified to obtain a diol monomer. The mass ratio of (R)-3-hydroxybutyric acid, 1,4-dibromobutane, and potassium carbonate was (8.8-19.4):(8.2-16.4):(11.6-25.5).

[0013] L-lactide, diol monomer, and stannous octoate were mixed in a mass ratio of (10-15):(0.8-1.4):(0.01-0.03) and subjected to ring-opening polymerization at 175-185℃ for 2.5-3.5 h under a nitrogen atmosphere. The mixture was then purified to obtain modified polylactic acid.

[0014] In the above process, (R)-3-hydroxybutyrate (3HB) is a monomer unit of poly(3-hydroxybutyrate) (PHB). Due to its excellent biodegradability and structural compatibility with PLA, it can be used for polylactic acid modification. In this invention, (R)-3-hydroxybutyric acid is reacted with 1,4-dibromobutane to obtain a linear diol monomer containing two 3HB units. Then, the diol monomer is copolymerized with L-lactide to introduce flexible 3HB segments into the polylactic acid backbone. The flexible 3HB segments disrupt the regular stacking of polylactic acid segments, thereby hindering crystal formation and increasing chain fluidity. The introduction of 3HB can improve the flexibility and heat shrinkage properties of polylactic acid and promote biodegradation.

[0015] Preferably, the method for preparing the geraniol compound in step one specifically includes:

[0016] Geraniol, thionyl chloride, and pyridine were mixed in a mass ratio of (7.7-15.4):(6.5-13):(0.1-0.2), reacted at 23-27℃ for 5-7 h, and purified to obtain modified geraniol.

[0017] 3-Mercapto-1,2-propanediol was added to N,N-dimethylformamide and stirred. Then, modified geraniol was added dropwise at 1-5°C, and stirring was continued for 2-3 hours. The mixture was then reacted at 23-27°C for 8-12 hours and purified to obtain geraniol-based compounds. The mass ratio of 3-mercapto-1,2-propanediol, N,N-dimethylformamide, and modified geraniol was (5.4-10.8):(150-200):(8.6-17.2).

[0018] In the above process, geraniol and thionyl chloride react to convert the hydroxyl group into chlorine, resulting in modified geraniol; the chlorine in the modified geraniol reacts with the hydroxyl group at one end of 3-mercapto-1,2-propanediol to obtain geraniol-based compounds.

[0019] Preferably, in step one, the method for preparing the modified bio-based plasticizer specifically includes:

[0020] Geraniol-based compounds and L-lactide were added to chloroform at a molar ratio of (1-2):(2-4), stirred and dissolved to achieve a L-lactide concentration of 1 mol / L in the mixture. The mixture was heated to 58-62℃, and then 1,8-diazabicyclo[5.4.0]undec-7-ene was added. The mixture was refluxed for 50-70 min, and the reaction was terminated with 1 mol / L hydrochloric acid aqueous solution. The mixture was purified to obtain a bio-based plasticizer. The amount of 1,8-diazabicyclo[5.4.0]undec-7-ene was 5.2% of the mass of L-lactide.

[0021] The bio-based plasticizer was added to ethanol, sonicated, and heated to 60-70℃. Then, allyltrimethylammonium chloride and a 0.5wt% azobisisobutyronitrile / ethanol mixed solution were added, and the mixture was stirred for 3-5 hours. After purification, the modified bio-based plasticizer was obtained. The mass ratio of the bio-based plasticizer, allyltrimethylammonium chloride, and azobisisobutyronitrile / ethanol mixed solution was (5.1-7.7):(1.4-2):(15-20).

[0022] In the above process, under the catalysis of 1,8-diazabicyclo[5.4.0]undec-7-ene, geraniol-based compounds copolymerize with L-lactide to form low-molecular-weight polylactic acid compounds (molecular weight around 500), which is the bio-based plasticizer of this invention. Geraniol is a natural terpene derived from plants and has certain antibacterial effects. Furthermore, due to its relatively long aliphatic chain, geraniol can be used as a plasticizer for polylactic acid to improve the flexibility of polylactic acid materials. The bio-based plasticizer formed by copolymerizing geraniol-based compounds with L-lactide contains polylactic acid units, which can interact well with polylactic acid. Good compatibility allows polylactic acid (PLA) materials to maintain excellent transparency. Next, the carbon-carbon double bond of allyltrimethylammonium chloride reacts with the thiol group of the bio-based plasticizer, introducing an antibacterial quaternary ammonium salt structure into the bio-based plasticizer and improving the antibacterial properties of PLA. Furthermore, the anion of allyltrimethylammonium chloride can form an ionic dipole interaction with the carbocation on the polybutylene terephthalate (PBAT) ester group. Since the bio-based plasticizer contains PLA units, the modified bio-based plasticizer of this invention can also serve as a compatibilizer for PLA and PBAT, improving their compatibility.

[0023] Preferably, in step two, the biodegradable shrink film contains the following components by weight: 50-70 parts modified polylactic acid, 30-40 parts polybutylene terephthalate-adipate, 10-18 parts modified bio-based plasticizer, 4.8-9.9 parts functional filler, and 1-3 parts palm wax.

[0024] In the above process, palm wax is used as a bio-based lubricant; polybutylene terephthalate (PET) is mixed with modified polylactic acid (PLA) to improve the toughness of PLA; modified bio-based plasticizers function as plasticizers, compatibilizers, and antibacterial agents; functional fillers endow the polymer matrix with long-lasting antibacterial ability and improve the mechanical properties (strength and toughness) of the polymer matrix. In addition, functional fillers can fill the gaps in the polymer macromolecular network, making it more difficult for water molecules to penetrate and improving barrier performance.

[0025] Preferably, in step two, the melt extrusion temperature is 168-172℃; the biaxial stretching condition is: extrusion temperature is 90-100℃.

[0026] Preferably, the thickness of the biodegradable shrink film is 25-50 μm.

[0027] Preferably, the functional filler in step two is prepared by the following steps:

[0028] Step S1: Gallic acid, γ-CD-MOF, and ethanol are mixed in a mass ratio of (2.5-4.5):(2.5-4.5):(200-300), stirred at 24-26℃ for 20-30 h, and purified to obtain the loaded modified γ-CD-MOF.

[0029] Step S2: Add the modified γ-CD-MOF to N,N-dimethylformamide, sonicate, and then add 5wt% ethylene glycol diglycidyl ether / N,N-dimethylformamide mixture dropwise. React at 70-80℃ for 2-3 hours, purify, and obtain the functional filler. The mass ratio of the modified γ-CD-MOF, N,N-dimethylformamide, and ethylene glycol diglycidyl ether / N,N-dimethylformamide mixture is (3-5):(100-120):(16-36).

[0030] Further, in step S1, the preparation method of the γ-CD-MOF specifically includes:

[0031] Deionized water, γ-cyclodextrin, and potassium hydroxide were mixed and stirred for 5-15 min, then sonicated for 10-20 min. The reaction mixture was filtered, methanol was added to the filtrate, and the mixture was kept in a water bath at 48-52℃ for 25-35 min. Then, 8 mg / mL polyethylene glycol 2000 / methanol mixture was added to the above mixture, stirred for 20-40 min, and allowed to stand for 1.5-2.5 h for purification to obtain γ-CD-MOF. The mass ratio of deionized water, γ-cyclodextrin, potassium hydroxide, methanol, and polyethylene glycol 2000 / methanol mixture was (100-200):(3.2-6.4):(1.1-2.2):(40-60):(160-300).

[0032] In the above process, cyclodextrin is a natural, biodegradable cyclic oligosaccharide with uniform cavities and a large number of hydroxyl groups. γ-CD-MOF was prepared using γ-cyclodextrin as a raw material. γ-CD-MOF is a bio-based MOF material with high specific surface area, richer pores, and high structural stability. It can be used to load the natural active substance gallic acid. Gallic acid is a natural polyphenol compound with strong antioxidant activity and antibacterial properties. Loading gallic acid in γ-CD-MOF can achieve the slow release of gallic acid. Then, the epoxy group at one end of ethylene glycol diglycidyl ether reacts with the hydroxyl groups on the surface of the loaded and modified γ-CD-MOF, introducing flexible ether chains and epoxy groups on the surface of the loaded and modified γ-CD-MOF. The epoxy groups can react with the hydroxyl groups in modified polylactic acid and polybutylene terephthalate, improving the dispersibility of functional fillers in the polymer matrix.

[0033] The biodegradable shrink film prepared by the aforementioned method is described.

[0034] Application of the biodegradable shrink film in food packaging materials.

[0035] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0036] 1. This invention uses geraniol-based compounds and L-lactide to copolymerize and prepare a bio-based plasticizer, which is then modified with allyltrimethylammonium chloride to obtain a modified bio-based plasticizer. The modified bio-based plasticizer of this invention can be used as a plasticizer for biodegradable shrink film materials to improve the flexibility and shrinkage performance of biodegradable shrink film materials, and can also be used as a compatibilizer and antibacterial agent, thereby giving biodegradable shrink film better flexibility, shrinkage performance and antibacterial performance.

[0037] 2. In this invention, gallic acid is loaded into γ-CD-MOF, and then flexible ether chains and epoxy groups are introduced onto the surface of the loaded and modified γ-CD-MOF with ethylene glycol to obtain a functional filler. The functional filler of this invention can be uniformly dispersed in the polymer matrix and improve the flexibility, shrinkage performance, long-lasting antibacterial performance and barrier performance of biodegradable shrink film.

[0038] 3. In this invention, (R)-3-hydroxybutyrate (3HB) segments are introduced into the polylactic acid backbone to obtain modified polylactic acid. The introduction of flexible 3HB segments can improve the flexibility and heat shrinkage properties of polylactic acid and promote biodegradation.

[0039] 4. The shrink film prepared by this invention using a variety of natural biodegradable materials has excellent degradation performance, is green, safe and environmentally friendly, and can be used in food packaging materials. Attached Figure Description

[0040] Figure 1 This is a comparison chart of the elongation at break of the biodegradable shrink films prepared in Examples 2-4 and Comparative Examples 1-5 of the present invention.

[0041] Figure 2 This is a comparison chart of the antibacterial rate tests of the biodegradable shrink films prepared in Examples 2-4 and Comparative Examples 1-5 of the present invention;

[0042] Figure 3 This is a schematic diagram illustrating the synthesis of the bio-based plasticizer of the present invention;

[0043] Figure 4 This is a schematic diagram illustrating the synthesis of the geraniol-based compound of the present invention;

[0044] Figure 5 This is a schematic diagram illustrating the synthesis of the modified geraniol of the present invention. Detailed Implementation

[0045] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0046] Example 1

[0047] This embodiment discloses a method for preparing a functional filler, including the following steps:

[0048] Step S1: Mix 150g deionized water, 4.8g γ-cyclodextrin and 1.7g potassium hydroxide and stir for 10min, then sonicate for 15min. Filter the reaction mixture through a 0.45μm filter membrane. Add 50g methanol to the filtrate and keep it in a 50℃ water bath for 30min. Then add 230g of 8mg / mL polyethylene glycol 2000 / methanol mixture to the above mixture, stir for 30min, let stand for 2h, then soak the obtained precipitate in dichloromethane for 3 days, then neutralize with 10% acetic acid aqueous solution. Finally, vacuum dry the precipitate at 45℃ to obtain γ-CD-MOF.

[0049] 3.5 g gallic acid and 3.5 g γ-CD-MOF were added to 250 g ethanol, stirred at 25 °C for 25 h, centrifuged, and the resulting precipitate was washed twice with ethanol and then dried under vacuum at 45 °C to obtain the loaded modified γ-CD-MOF.

[0050] Step S2: Add 4g of the modified γ-CD-MOF to 110g of N,N-dimethylformamide, sonicate for 40min, then add 26g of a 5wt% ethylene glycol diglycidyl ether / N,N-dimethylformamide mixture dropwise over 30min. React at 75℃ for 2.5h. After the reaction is complete, centrifuge, wash the precipitate twice with ethanol, and then vacuum dry at 45℃ to obtain the functional filler.

[0051] Example 2

[0052] This embodiment discloses a method for preparing a biodegradable shrink film, including the following steps:

[0053] Step 1: Preparation of modified polylactic acid and modified bio-based plasticizer;

[0054] The preparation of modified polylactic acid includes:

[0055] 8.8 g of (R)-3-hydroxybutyric acid and 8.2 g of 1,4-dibromobutane were dissolved in 60 g of N,N-dimethylformamide. The mixture was stirred at 23 °C for 40 min under a nitrogen atmosphere. Then, 11.6 g of potassium carbonate was added, and the mixture was stirred for another 20 min. Next, 60 g of N,N-dimethylformamide was added, and the mixture was heated to 77 °C and kept at a constant temperature for 20 h. The reaction mixture was filtered through a diatomaceous earth filter, and the solvent was removed by rotary evaporation at 60 °C and 0.2 kPa to obtain a crude product. The crude product was purified by column chromatography to obtain a diol monomer. The eluent used in the column chromatography was a mixture of equal volumes of hexane and ethyl acetate.

[0056] 10 g of L-lactide was vacuum dried at 60 °C for 1 h, then mixed with 0.8 g of diol monomer and 0.01 g of stannous octoate, and the mixture was vacuum dried for another 1 h. Then, ring-opening polymerization was carried out at 175 °C for 3.5 h under a nitrogen atmosphere. The reaction mixture was dissolved in chloroform, and the product was precipitated with methanol. The product was filtered and the white precipitate was collected. The white precipitate was washed with methanol and then vacuum dried at 40 °C to obtain modified polylactic acid.

[0057] The preparation of modified bio-based plasticizers includes:

[0058] 7.7g geraniol and 6.5g thionyl chloride were mixed, and then 0.1g pyridine was added. The mixture was reacted at 23°C for 7 hours. After the reaction was completed, excess thionyl chloride was removed by distillation to obtain modified geraniol.

[0059] 5.4 g of 3-mercapto-1,2-propanediol was added to 150 g of N,N-dimethylformamide and stirred for 20 min. Then, 8.6 g of modified geraniol was added dropwise while stirring at 1 °C, and the addition was completed within 20 min. Stirring was continued for 3 h, and then the reaction was carried out at 23 °C for 12 h. After the reaction was completed, the solvent was removed by rotary evaporation to obtain the geraniol-based compound.

[0060] Geraniol-based compounds and L-lactide were added to chloroform at a molar ratio of 1:2 and stirred to dissolve, so that the concentration of L-lactide in the mixture was 1 mol / L. The mixture was heated to 58°C, and then 1,8-diazabicyclo[5.4.0]undec-7-ene was added. The amount of 1,8-diazabicyclo[5.4.0]undec-7-ene was 5.2% of the mass of L-lactide. After reflux for 70 min, the reaction was terminated with 1 mol / L hydrochloric acid aqueous solution. The resulting product mixture was washed three times each with 1 mol / L hydrochloric acid aqueous solution, saturated sodium bicarbonate aqueous solution, and deionized water. Then it was dried with anhydrous sodium sulfate, and the chloroform was removed by rotary evaporation. Finally, it was dried in a vacuum drying oven at 60°C for 24 h to obtain a bio-based plasticizer.

[0061] Add 5.1g of bio-based plasticizer to ethanol, sonicate for 20min, heat to 60℃, then add 1.4g of allyltrimethylammonium chloride and 15g of 0.5wt% azobisisobutyronitrile / ethanol mixed solution, stir and react for 5h. After the reaction is completed, evaporate the solvent to obtain the modified bio-based plasticizer.

[0062] Step 2: By weight, take 50 parts of modified polylactic acid, 30 parts of polybutylene terephthalate-adipate, 10 parts of modified bio-based plasticizer, 4.8 parts of functional filler, and 1 part of palm wax, mix them evenly, melt extrude, cast the film through a die, wind the film, biaxially stretch it, cool and set it, and then roll it up to obtain a biodegradable shrink film with a thickness of 35 μm; wherein, the melt extrusion temperature is 168℃; the biaxial stretching condition is: extrusion temperature is 90℃.

[0063] Example 3

[0064] This embodiment discloses a method for preparing a biodegradable shrink film, including the following steps:

[0065] Step 1: Preparation of modified polylactic acid and modified bio-based plasticizer;

[0066] The preparation of modified polylactic acid includes:

[0067] 19.4 g of (R)-3-hydroxybutyric acid and 16.4 g of 1,4-dibromobutane were dissolved in 80 g of N,N-dimethylformamide. The mixture was stirred at 27 °C for 20 min under a nitrogen atmosphere. Then, 25.5 g of potassium carbonate was added, and stirring was continued for another 20 min. Next, 80 g of N,N-dimethylformamide was added, and the mixture was heated to 83 °C and kept at a constant temperature while stirring for 16 h. The reaction mixture was filtered through a diatomaceous earth filter, and the solvent was removed by rotary evaporation at 60 °C and 0.2 kPa to obtain the crude product. The crude product was purified by column chromatography to obtain the diol monomer. The eluent used in the column chromatography was a mixture of equal volumes of hexane and ethyl acetate.

[0068] 15g of L-lactide was vacuum dried at 60℃ for 1h, then mixed with 1.4g of diol monomer and 0.03g of stannous octoate, and the mixture was vacuum dried for another 1h. Then, ring-opening polymerization was carried out at 185℃ for 2.5h under a nitrogen atmosphere. The reaction mixture was dissolved in chloroform, and the product was precipitated with methanol. The product was filtered and the white precipitate was collected. The white precipitate was washed with methanol and then vacuum dried at 40℃ to obtain modified polylactic acid.

[0069] The preparation of modified bio-based plasticizers includes:

[0070] 15.4g geraniol and 13g thionyl chloride were mixed, and then 0.2g pyridine was added. The mixture was reacted at 27°C for 5 hours. After the reaction was completed, excess thionyl chloride was removed by distillation to obtain modified geraniol.

[0071] 10.8 g of 3-mercapto-1,2-propanediol was added to 200 g of N,N-dimethylformamide and stirred for 40 min. Then, 17.2 g of modified geraniol was added dropwise while stirring at 5 °C, and the addition was completed within 30 min. Stirring was continued for 2 h, and then the reaction was carried out at 27 °C for 8 h. After the reaction was completed, the solvent was removed by rotary evaporation to obtain the geraniol-based compound.

[0072] Geraniol-based compounds and L-lactide were added to chloroform at a molar ratio of 2:4 and stirred to dissolve, so that the concentration of L-lactide in the mixture was 1 mol / L. The mixture was heated to 62°C, and then 1,8-diazabicyclo[5.4.0]undec-7-ene was added. The amount of 1,8-diazabicyclo[5.4.0]undec-7-ene was 5.2% of the mass of L-lactide. After reflux for 50 min, the reaction was terminated with 1 mol / L hydrochloric acid aqueous solution. The resulting product mixture was washed three times each with 1 mol / L hydrochloric acid aqueous solution, saturated sodium bicarbonate aqueous solution, and deionized water. Then it was dried with anhydrous sodium sulfate, and the chloroform was removed by rotary evaporation. Finally, it was dried in a vacuum drying oven at 60°C for 24 h to obtain a bio-based plasticizer.

[0073] 7.7g of bio-based plasticizer was added to ethanol, ultrasonically treated for 40min, heated to 70℃, and then 2g of allyltrimethylammonium chloride and 20g of 0.5wt% azobisisobutyronitrile / ethanol mixed solution were added. The mixture was stirred and reacted for 3h. After the reaction was completed, the solvent was evaporated to obtain the modified bio-based plasticizer.

[0074] Step 2: By weight, take 70 parts of modified polylactic acid, 40 parts of polybutylene terephthalate-adipate, 18 parts of modified bio-based plasticizer, 9.9 parts of functional filler, and 3 parts of palm wax, mix them evenly, melt extrude, cast the film through a die, wind the cast film, biaxially stretch it, cool and set it, and then roll it up to obtain a biodegradable shrink film with a thickness of 35 μm; wherein, the melt extrusion temperature is 172℃; the biaxial stretching condition is: extrusion temperature is 100℃.

[0075] Example 4

[0076] This embodiment discloses a method for preparing a biodegradable shrink film, including the following steps:

[0077] Step 1: Preparation of modified polylactic acid and modified bio-based plasticizer;

[0078] The preparation of modified polylactic acid includes:

[0079] 14.1 g of (R)-3-hydroxybutyric acid and 12.3 g of 1,4-dibromobutane were dissolved in 70 g of N,N-dimethylformamide. The mixture was stirred at 25 °C for 30 min under a nitrogen atmosphere. Then, 18.6 g of potassium carbonate was added, and the mixture was stirred for another 30 min. Another 70 g of N,N-dimethylformamide was added, and the mixture was heated to 80 °C and kept at that temperature for 18 h. The reaction mixture was filtered through a diatomaceous earth filter, and the solvent was removed by rotary evaporation at 60 °C and 0.2 kPa to obtain a crude product. The crude product was purified by column chromatography to obtain a diol monomer. The eluent used in the column chromatography was a mixture of equal volumes of hexane and ethyl acetate.

[0080] 12.5 g of L-lactide was vacuum dried at 60 °C for 1 h, then mixed with 1.1 g of diol monomer and 0.02 g of stannous octoate, and the mixture was vacuum dried for another 1 h. Then, ring-opening polymerization was carried out at 180 °C for 3 h under a nitrogen atmosphere. The reaction mixture was dissolved in chloroform, and the product was precipitated with methanol. The product was filtered and the white precipitate was collected. The white precipitate was washed with methanol and then vacuum dried at 40 °C to obtain modified polylactic acid.

[0081] The preparation of modified bio-based plasticizers includes:

[0082] 11.6 g geraniol and 9.8 g thionyl chloride were mixed, and then 0.1 g pyridine was added. The mixture was reacted at 25 °C for 6 h. After the reaction was completed, excess thionyl chloride was removed by distillation to obtain modified geraniol.

[0083] 8.1 g of 3-mercapto-1,2-propanediol was added to 175 g of N,N-dimethylformamide and stirred for 30 min. Then, 12.9 g of modified geraniol was added dropwise while stirring at 3 °C over 25 min. The mixture was stirred for 2.5 h and then reacted at 25 °C for 10 h. After the reaction was completed, the solvent was removed by rotary evaporation to obtain the geraniol-based compound.

[0084] Geraniol-based compounds and L-lactide were added to chloroform at a molar ratio of 1.5:3 and stirred to dissolve, so that the concentration of L-lactide in the mixture was 1 mol / L. The mixture was heated to 60°C, and then 1,8-diazabicyclo[5.4.0]undec-7-ene was added. The amount of 1,8-diazabicyclo[5.4.0]undec-7-ene was 5.2% of the mass of L-lactide. After reflux for 60 min, the reaction was terminated with 1 mol / L hydrochloric acid aqueous solution. The resulting product mixture was washed three times each with 1 mol / L hydrochloric acid aqueous solution, saturated sodium bicarbonate aqueous solution, and deionized water. Then it was dried with anhydrous sodium sulfate, and the chloroform was removed by rotary evaporation. Finally, it was dried in a vacuum drying oven at 60°C for 24 h to obtain a bio-based plasticizer.

[0085] 6.4g of bio-based plasticizer was added to ethanol, sonicated for 30min, heated to 65℃, and then 1.7g of allyltrimethylammonium chloride and 17.5g of 0.5wt% azobisisobutyronitrile / ethanol mixed solution were added. The mixture was stirred and reacted for 4h. After the reaction was completed, the solvent was evaporated to obtain the modified bio-based plasticizer.

[0086] Step 2: By weight, take 60 parts of modified polylactic acid, 35 parts of polybutylene terephthalate-adipate, 14 parts of modified bio-based plasticizer, 7.4 parts of functional filler, and 2 parts of palm wax, mix them evenly, melt extrude, cast the film through a die, wind the cast film, biaxially stretch it, cool and set it, and then roll it up to obtain a biodegradable shrink film with a thickness of 35 μm; wherein, the melt extrusion temperature is 170℃; the biaxial stretching condition is: extrusion temperature is 95℃.

[0087] The functional fillers used in Examples 2-4 above are the functional fillers prepared in Example 1.

[0088] Comparative Example 1

[0089] Compared with Example 4, Comparative Example 1 used a loaded modified γ-CD-MOF instead of the functional filler in the process of preparing the biodegradable shrink film, while other conditions remained unchanged.

[0090] Comparative Example 2

[0091] Compared with Example 4, Comparative Example 2 used polylactic acid instead of modified polylactic acid in the process of preparing the biodegradable shrink film, while other conditions remained unchanged.

[0092] Comparative Example 3

[0093] Compared with Example 4, Comparative Example 3 used a bio-based plasticizer instead of a modified bio-based plasticizer in the process of preparing the biodegradable shrink film, while other conditions remained unchanged.

[0094] Comparative Example 4

[0095] Compared with Example 4, Comparative Example 4 did not add modified bio-based plasticizers during the preparation of the biodegradable shrink film, and all other conditions remained unchanged.

[0096] Comparative Example 5

[0097] Compared with Example 4, Comparative Example 5 did not add functional fillers during the preparation of the biodegradable shrink film, and all other conditions remained unchanged.

[0098] In the above examples and comparative examples, palm wax, with a melting point of 80-86℃ and a saponification value of 78-95, was sourced from Shandong Guohua Chemical Co., Ltd.; gallic acid, CAS No. 149-91-7, appearing as white or light brown needle-like crystals or powder, with a density of 1.694 g / cm3, was sourced from Shandong Guohua Chemical Co., Ltd.; polyethylene glycol 2000, with a density of 1.27 g / cm3, was sourced from Jinan Honghaoyuan Chemical Technology Co., Ltd.; geraniol, CAS No. 106-24-1, appearing as a liquid, was sourced from Hunan Honghua Jinjia Ecological Environment Engineering Co., Ltd.; L-lactide, CAS No. 4511-42-6, color: off-white solid, melting point: 92-94℃ (lit.), was sourced from Wuhan Shuer Biotechnology Co., Ltd.; γ-cyclodextrin, CAS No. 17465-86-0, appearance: powder, color: white, was sourced from Jiangsu Aofu Biotechnology Co., Ltd.

[0099] Experimental Example

[0100] The biodegradable shrink films prepared in Examples 2-4 and Comparative Examples 1-5 were subjected to performance tests.

[0101] Test 1, Flexibility Test: Measured using a universal testing machine according to ISO 527-3. Cut the film sample into strips of 50mm × 20mm and stretch them at a speed of 30mm / min. At least 10 samples should be tested in each group.

[0102] Test 2: Heat Shrinkage Performance Test: Cut the film into 5cm × 5cm squares and immerse them in 98±0.5℃ hot water without load for 10 seconds to perform heat shrinkage. Then immerse them in 25℃±0.5℃ water for 10 seconds. After removing the samples from the water, measure the dimensions of the film in the longitudinal and transverse directions, and calculate the heat shrinkage rate according to the following formula:

[0103] Heat shrinkage rate = (length before shrinkage - length after shrinkage) / length before shrinkage × 100%.

[0104] Test 3. Antibacterial performance test: The test method refers to the standard QB / T2591-2003 "Test method and antibacterial effect of antibacterial plastics". Escherichia coli (ATCC8099) and Staphylococcus aureus (ATCC6538) were inoculated into each group of samples and cultured for 24 h at (37±1)℃ and relative humidity >90%. The antibacterial rate was calculated based on the actual number of viable bacteria recovered.

[0105] The test results are shown in Table 1:

[0106] Table 1

[0107]

[0108]

[0109] As shown in Table 1, the biodegradable shrink films obtained in Examples 2-4 of this invention possess excellent flexibility, shrinkage performance, and antibacterial properties. A comparison of Comparative Examples 1 and 5 with Example 4 reveals that the functional filler of this invention can improve the flexibility, shrinkage performance, and antibacterial properties of the biodegradable shrink film. By introducing flexible ether chains and epoxy groups onto the surface of the modified γ-CD-MOF, the epoxy groups can react with the hydroxyl groups in the modified polylactic acid and polybutylene terephthalate, improving the dispersibility of the functional filler in the polymer matrix and thus improving the overall performance of the biodegradable shrink film. A comparison of Comparative Example 2 and Example 4 shows that introducing (R)-3-hydroxybutyrate (3HB) segments into the polylactic acid... Modified polylactic acid (PLA) is obtained from the acid backbone. The flexible 3HB segments disrupt the regular stacking of PLA segments, thereby hindering crystal formation and increasing chain fluidity, thus improving the flexibility and heat shrinkage properties of PLA. As can be seen from the comparison between Comparative Examples 3-4 and Example 4, geraniol-based compounds have antibacterial and plasticizing effects on polymers. Due to the presence of quaternary ammonium salt structure and PLA structure, the modified bio-based plasticizer of the present invention also exhibits the role of a compatibilizer. Therefore, the modified bio-based plasticizer of the present invention endows biodegradable shrink film with better flexibility, shrinkage properties and antibacterial properties.

[0110] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A process for the preparation of a biodegradable shrink film, characterized in that, The method comprises the following steps: Step one, preparing modified polylactic acid and modified bio-based plasticizer; Preparation of modified polylactic acid comprises: (R)-3-hydroxybutyric acid and 1,4-dibromobutane are reacted to obtain a diol monomer; the diol monomer is copolymerized with L-lactide to obtain modified polylactic acid; Preparation of modified bio-based plasticizer comprises: 3-mercapto-1,2-propanediol is reacted with modified geraniol to obtain a geraniol-based compound; The geraniol-based compound is copolymerized with L-lactide to obtain a bio-based plasticizer; the bio-based plasticizer is modified by allyltrimethylammonium chloride to obtain a modified bio-based plasticizer; Step two, the modified polylactic acid, polybutylene adipate terephthalate, modified bio-based plasticizer, functional filler and palm wax are uniformly mixed, melt extruded, die cast, wound, bidirectionally stretched, cooled and shaped, and wound to obtain a biodegradable shrinkable film.

2. The method of producing a biodegradable shrink film according to claim 1, characterized by, In step one, the preparation method of the modified polylactic acid specifically comprises: (R)-3-hydroxybutyric acid and 1,4-dibromobutane are dissolved in N,N-dimethylformamide, stirred at 23-27℃ for 20-40min under nitrogen atmosphere, then potassium carbonate is added, and stirring is continued for 20-40min, then N,N-dimethylformamide is added, heated to 77-83℃, and kept stirring for 16-20h, then purified to obtain a diol monomer; wherein the mass ratio of (R)-3-hydroxybutyric acid, 1,4-dibromobutane and potassium carbonate is (8.8-19.4):(8.2-16.4):(11.6-25.5); L-lactide, diol monomer and stannous octoate are mixed in a mass ratio of (10-15):(0.8-1.4):(0.01-0.03), ring-opening polymerization is carried out at 175-185℃ for 2.5-3.5h under nitrogen atmosphere, and then purified to obtain modified polylactic acid.

3. The method of producing a biodegradable shrink film according to claim 1, characterized by, In step one, the preparation method of the geraniol-based compound specifically comprises: Geraniol, thionyl chloride and pyridine are mixed in a mass ratio of (7.7-15.4):(6.5-13):(0.1-0.2), reacted at 23-27℃ for 5-7h, and then purified to obtain modified geraniol; 3-mercapto-1,2-propanediol is added to N,N-dimethylformamide and stirred, then modified geraniol is added dropwise at 1-5℃, and stirring is continued for 2-3h, then reacted at 23-27℃ for 8-12h, and then purified to obtain a geraniol-based compound; wherein the mass ratio of 3-mercapto-1,2-propanediol, N,N-dimethylformamide and modified geraniol is (5.4-10.8):(150-200):(8.6-17.2).

4. The method of producing a biodegradable shrink film according to claim 1, characterized by, In step one, the preparation method of the modified bio-based plasticizer specifically comprises: Geraniol-based compounds and L-lactide were added to chloroform at a molar ratio of (1-2):(2-4), stirred and dissolved to make the concentration of L-lactide in the mixture 1 mol / L. The mixture was heated to 58-62℃, and then 1,8-diazabicyclo[5.4.0]undec-7-ene was added. The mixture was refluxed for 50-70 min, and the reaction was terminated with 1 mol / L hydrochloric acid aqueous solution. After purification, a bio-based plasticizer was obtained. The bio-based plasticizer was added to ethanol, sonicated, and heated to 60-70℃. Then, allyltrimethylammonium chloride and a 0.5wt% azobisisobutyronitrile / ethanol mixed solution were added, and the mixture was stirred for 3-5 hours. After purification, the modified bio-based plasticizer was obtained. The mass ratio of the bio-based plasticizer, allyltrimethylammonium chloride, and azobisisobutyronitrile / ethanol mixed solution was (5.1-7.7):(1.4-2):(15-20).

5. The method of producing a biodegradable shrink film according to claim 1, characterized by, In step two, the biodegradable shrink film contains the following components by weight: 50-70 parts modified polylactic acid, 30-40 parts polybutylene terephthalate adipate, 10-18 parts modified bio-based plasticizer, 4.8-9.9 parts functional filler, and 1-3 parts palm wax.

6. The method of producing a biodegradable shrink film according to claim 1, characterized by, In step two, the melt extrusion temperature is 168-172℃; the biaxial stretching condition is: extrusion temperature is 90-100℃.

7. The method of producing a biodegradable shrink film according to claim 1, characterized by, The functional filler in step two is prepared by the following steps: Step S1: Gallic acid, γ-CD-MOF, and ethanol are mixed in a mass ratio of (2.5-4.5):(2.5-4.5):(200-300), stirred at 24-26℃ for 20-30 h, and purified to obtain the loaded modified γ-CD-MOF. Step S2: Add the modified γ-CD-MOF to N,N-dimethylformamide, sonicate, and then add 5wt% ethylene glycol diglycidyl ether / N,N-dimethylformamide mixture dropwise. React at 70-80℃ for 2-3 hours, purify, and obtain the functional filler. The mass ratio of the modified γ-CD-MOF, N,N-dimethylformamide, and ethylene glycol diglycidyl ether / N,N-dimethylformamide mixture is (3-5):(100-120):(16-36).

8. The method of producing a biodegradable shrink film according to claim 7, characterized by, In step S1, the preparation method of the γ-CD-MOF specifically includes: Deionized water, γ-cyclodextrin, and potassium hydroxide were mixed and stirred for 5-15 min, then sonicated for 10-20 min. The reaction mixture was filtered, methanol was added to the filtrate, and the mixture was kept in a water bath at 48-52℃ for 25-35 min. Then, 8 mg / mL polyethylene glycol 2000 / methanol mixture was added to the above mixture, stirred for 20-40 min, and allowed to stand for 1.5-2.5 h for purification to obtain γ-CD-MOF. The mass ratio of deionized water, γ-cyclodextrin, potassium hydroxide, methanol, and polyethylene glycol 2000 / methanol mixture was (100-200):(3.2-6.4):(1.1-2.2):(40-60):(160-300).

9. A biodegradable shrink film prepared by the method for preparing a biodegradable shrink film according to any one of claims 1-8.

10. The application of the biodegradable shrink film according to claim 9 in food packaging materials.

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