Biodegradable PHA / PCL composite material and preparation method thereof

Abstract

The invention discloses a biodegradable PHA / PCL (polyhydroxyalkanoate / polycaprolactone) composite material and a preparation method thereof, and belongs to the technical field of polymer composition.The preparation method comprises the steps that nano cellulose crystals are subjected to quaternary ammonium salt modification and then chemically crosslinked with chitosan through a reverse emulsification method to obtain chitosan / nano cellulose crystal microspheres, and then the chitosan / nano cellulose crystal microspheres are subjected to coordination self-assembly through tannic acid and zinc ions to obtain the biodegradable PHA / PCL composite material. Preparing zinc tannate composite powder; finally, PHA, PCL, the chitosan / nano cellulose crystal microspheres and the zinc tannate composite powder are jointly dissolved in 1, 2-dichloroethane, toluene-2, 4-diisocyanate, glycerol and butyltin dilaurate are added, casting and curing are conducted, and the PHA / PCL composite material is obtained. The material is endowed with excellent mechanical property, antibacterial property, oxidation resistance and water-resistant stability while all components are biodegradable.

Description

Technical Field

[0001] This invention belongs to the field of polymer composite technology, specifically relating to a biodegradable PHA / PCL composite material and its preparation method. Background Technology

[0002] Biodegradable polymer materials are a research hotspot in the field of materials science. PHA / PCL composite materials, as a typical example, are a new type of polymer material composed of two fully biodegradable aliphatic polyesters, polyhydroxyalkanoate (PHA) and polycaprolactone (PCL), through processes such as melt blending. The core feature is that it retains the biodegradability of both (it can be decomposed into water and carbon dioxide in the natural environment or in organisms and return to nature). At the same time, the precise matching of key indicators such as mechanical properties and degradation rate can be achieved through component ratio control and process optimization.

[0003] The initial purpose of designing this composite material was to address the performance deficiencies of single biodegradable materials and achieve "complementary advantages and synergistic performance" to expand application scenarios. PHA alone has shortcomings such as high crystallinity, high brittleness, and poor thermal stability, while PCL alone has problems such as weak bioactivity, slow degradation rate, and low mechanical strength. The combination of the two can make up for the defects of PCL by utilizing the bioactivity and faster degradation rate of PHA, while improving the shortcomings of PHA by leveraging the flexibility and processability of PCL. This composite material has already shown important application value in fields such as medical health, packaging and daily necessities, and agricultural environmental protection.

[0004] Chinese invention patent application CN110564122A discloses a biodegradable antibacterial film and its preparation. The method uses polycaprolactone and polyhydroxyalkanoates as the main matrix, adds the green plasticizer tributyl acetylacetonate and the antibacterial agent carboxymethyl chitosan. After mixing and stirring, the film is extruded and granulated by a twin-screw extruder, and then hot-pressed and vulcanized in a flat plate. The resulting film has good biodegradability and antibacterial properties and is suitable for environmentally friendly packaging and other fields.

[0005] However, this method directly adds hydrophilic carboxymethyl chitosan to the hydrophobic polymer matrix. The two have a large difference in polarity and poor compatibility. This introduces a large number of micro-stress concentration points inside the material. When subjected to external force, cracks are very likely to initiate and propagate from these weak interfaces, leading to a decline in the mechanical properties of the material. Summary of the Invention

[0006] The purpose of this invention is to provide a biodegradable PHA / PCL composite material and its preparation method. Through chemical modification and structural design, this invention solves the problems of weak interfacial bonding and decreased mechanical properties caused by the direct addition of hydrophilic antibacterial agents in the aforementioned comparative patents. First, quaternary ammonium salts are used to modify nanocellulose crystals. Then, chitosan / nanocellulose crystal microspheres are obtained through reverse emulsification and dynamic covalent cross-linking with chitosan. These microspheres are then combined with zinc tannate powder as a multi-active reinforcing phase in subsequent polymerization. They form chemical bonds with chain extenders and cross-linking agents in the matrix, thereby improving the interfacial bonding and water resistance stability.

[0007] In addition, quaternary ammonium salt nanocellulose crystals and chitosan endow the material with antibacterial properties, zinc tannate provides antioxidant protection, dynamic imine bonds and coordination bonds enhance the material's toughness, and all components are biodegradable, achieving a multifunctional combination of reinforcement, toughening, antibacterial and antioxidant properties.

[0008] The objective of this invention can be achieved through the following technical solutions: A method for preparing a biodegradable PHA / PCL composite material includes the following steps: Step 1: After modifying the nanocellulose crystals with quaternary ammonium salt, they are cross-linked with chitosan through reverse emulsification and chemical cross-linking to obtain chitosan / nanocellulose crystal microspheres; zinc tannate composite powder is prepared by coordination self-assembly.

[0009] Step 3: PHA, PCL, chitosan / nanocellulose microspheres and zinc tannate composite powder are dissolved together in 1,2-dichloroethane, and toluene-2,4-diisocyanate, glycerol and butyltin dilaurate are added. After casting and curing, PHA / PCL composite material is obtained.

[0010] Furthermore, the specific preparation steps for chitosan / cellulose nanospheres are as follows: Chitosan powder and quaternary ammonium salt modified nanocellulose crystals were added to a 2 wt% acetic acid aqueous solution and ultrasonically dispersed to prepare a chitosan / nanocellulose crystal mixed solution, which was used as the aqueous phase. Petroleum ether, Span 80, and Tween 60 were stirred at 40-50℃ and 600-700rpm for 30-50 minutes to obtain an oil phase. Then, a chitosan / nanocellulose crystal mixed solution was added, and stirring was continued for 1-2 hours. Then, terephthalaldehyde was added at 50-60℃, and stirring was continued for 1-2 hours. Ammonia was added dropwise to adjust the pH to 9-10. The mixture was centrifuged, and the solid was collected. It was washed alternately with deionized water and ethanol until the supernatant was neutral. The mixture was then freeze-dried to constant weight to obtain chitosan / nanocellulose crystal microspheres.

[0011] Furthermore, the ratio of chitosan powder, quaternary ammonium salt modified nanocellulose crystals, and 2wt% acetic acid aqueous solution is 8-10g: 3.2-4.6g: 400-500mL.

[0012] Furthermore, the ratio of petroleum ether, Span 80, Tween 60, chitosan / nanocellulose crystal mixed solution and terephthalaldehyde is 1000-1200mL: 57.6-64.9g: 2.4-3.6g: 200-300mL: 10-12g.

[0013] Furthermore, the specific preparation steps for quaternary ammonium salt modified nanocellulose crystals are as follows: Nanocellulose crystals were added to distilled water and ultrasonically pulverized for 15-25 minutes to obtain a nanocellulose crystal suspension. The pH was adjusted to 10 with sodium hydroxide solution, and then 65wt% 2,3-epoxypropyltrimethylammonium chloride solution was added. The mixture was stirred at 60-70℃ for 3.5-4.5 hours and at room temperature for 15-20 hours. The mixture was then transferred to a dialysis bag and dialyzed in distilled water for 7-10 days. Finally, it was freeze-dried to constant weight to obtain quaternary ammonium salt modified nanocellulose crystals.

[0014] Furthermore, the ratio of nanocellulose crystals, distilled water, and 65wt% 2,3-epoxypropyltrimethylammonium chloride solution is 4-6g: 400-700mL: 7.6-8.2mL.

[0015] Furthermore, the specific preparation steps of the zinc tannate composite powder are as follows: Tannic acid was dissolved in deionized water, and 1 mol / L zinc sulfate aqueous solution was slowly added while stirring. The pH was adjusted to 5-5.5 with sodium hydroxide solution. After stirring for 2-3 hours, the mixture was centrifuged, and the precipitate was washed with deionized water 3-5 times and freeze-dried to constant weight to obtain zinc tannic acid composite powder.

[0016] Furthermore, the ratio of tannic acid, deionized water, and 1 mol / L zinc sulfate aqueous solution is 6-8 g: 60-90 mL: 12-15 mL.

[0017] Furthermore, the specific preparation steps of the PHA / PCL composite material are as follows: PHA and PCL were mixed evenly to obtain a mixed resin. The mixed resin was dissolved in 1,2-dichloroethane, and chitosan / nanocellulose microspheres, zinc tannate composite powder, toluene-2,4-diisocyanate, glycerol and butyltin dilaurate were added sequentially. Under nitrogen protection, the mixture was stirred at 70-80℃ for 4-6 hours. The reaction solution was cast into a polytetrafluoroethylene mold, and the mold was placed in a vacuum drying oven and cured at 25-40℃ and 0.1-0.15MPa for 48-72 hours. The mold was then demolded to obtain the PHA / PCL composite material.

[0018] Furthermore, the content of PHA in the mixed resin is 20-40 wt%.

[0019] Furthermore, the ratio of the mixed resin, 1,2-dichloroethane, chitosan / nanocellulose microspheres, zinc tannate composite powder, toluene-2,4-diisocyanate, glycerol, and butyltin dilaurate is 100g: 250-350mL: 8-10g: 3-5g: 14.5-16.5g: 3.3-4.6g: 0.5-0.8g.

[0020] The beneficial effects of this invention are: 1. This invention prepares a multifunctional biodegradable composite material through structural design. Firstly, quaternary ammonium salt-modified nanocellulose crystals serve as a rigid framework, forming microspheres with chitosan. Toluene-2,4-diisocyanate, a chain extender, is used to construct covalent "molecular bridges" with the active groups of each component, achieving strong interfacial bonding. This effectively blocks water penetration and prevents filler detachment, allowing the material to maintain long-term mechanical stability in humid environments. The imine bonds in the chitosan microspheres act as environmental response switches. When the material enters a biological environment, these bonds reversibly break, triggering microsphere softening and pore formation, initiating an ordered degradation process. This achieves controllability in the biodegradation process, ensuring the material maintains its performance throughout its service life and degrades orderly in biological environments. Secondly, quaternary ammonium salt-modified cellulose and chitosan provide long-lasting antibacterial properties, while the zinc tannate complex imparts antioxidant properties. This material integrates structural strength, antibacterial properties, antioxidant properties, and controllable degradation performance.

[0021] 2. The chitosan / nanocellulose crystal microspheres of this invention achieve efficient stress transfer through the synergistic effect of nanocellulose crystals and chitosan, enhancing the toughness and crack resistance of the composite material. They also fix the antibacterial functional groups of quaternary ammonium salt and chitosan, achieving long-lasting contact antibacterial effect. The dynamic imine bond cross-linked network structure endows the material with pH response potential, controlling the overall biodegradation rhythm of the material. The microsphere structure, by embedding the highly hydrophilic nanocellulose crystals within the cross-linked chitosan network, achieves physical encapsulation and surface chemical passivation of the hydrophilic components, effectively improving the compatibility and binding stability with the hydrophobic polymer matrix.

[0022] 3. The zinc tannin composite powder in this invention is constructed through molecular self-assembly. The catechol groups in tannin form a coordination structure with zinc ions, which not only achieves the slow release of active ingredients but also endows the material with antioxidant properties. Zinc ions and organic antibacterial components constitute dual antibacterial properties, and the active groups on the surface can serve as cross-linking points to strengthen interfacial bonding and improve mechanical properties. Detailed Implementation

[0023] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments in the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0024] Example 1: A method for preparing a biodegradable PHA / PCL composite material, comprising the following steps: S1: Add 4g of nanocellulose crystals to 400mL of distilled water and sonicate for 15min to obtain a nanocellulose crystal suspension. Adjust the pH to 10 with sodium hydroxide solution, then add 7.6mL of 65wt% 2,3-epoxypropyltrimethylammonium chloride solution. Stir at 60℃ for 3.5h, then at room temperature for 15h. Transfer to a dialysis bag and dialyze in distilled water for 7 days. Freeze dry to constant weight to obtain quaternary ammonium salt modified nanocellulose crystals.

[0025] Under alkaline conditions, the hydroxyl groups on the surface of nanocellulose are activated into more nucleophilic oxygen anions, which attack the epoxy ring of glycidyltrimethylammonium chloride, initiating epoxy ring opening and forming stable ether bonds through nucleophilic substitution reactions, thus covalently grafting quaternary ammonium salt groups onto the surface of nanocellulose crystals.

[0026] S2: 8g of chitosan powder and 3.2g of quaternary ammonium salt modified nanocellulose crystals were added to 400mL of 2wt% acetic acid aqueous solution and ultrasonically dispersed to prepare a chitosan / nanocellulose crystal mixed solution, which was used as the aqueous phase. 1000mL of petroleum ether, 57.6g of Span 80 and 2.4g of Tween 60 were stirred at 40℃ and 600rpm for 30min to obtain the oil phase. Then, 200mL of the chitosan / nanocellulose crystal mixed solution was added and stirred for another 1h. Then, 10g of terephthalaldehyde was added at 50℃ and stirred for another 1h. Ammonia water was added dropwise to adjust the pH to 9. The mixture was centrifuged, and the solid was collected. It was washed alternately with deionized water and ethanol until the supernatant was neutral. It was then freeze-dried to constant weight to obtain chitosan / nanocellulose crystal microspheres.

[0027] By using reverse emulsification and chemical crosslinking, the amino groups on the chitosan molecular chain undergo a nucleophilic addition-elimination reaction with the aldehyde groups of phthalaldehyde in the crosslinking agent, generating dynamic covalent imine bonds (-CH=N-, i.e., Schiff base bonds). This reaction connects the chitosan chains to form a three-dimensional network structure, which is then used to coat the nanocellulose crystals, resulting in chitosan / nanocellulose crystal microspheres.

[0028] S3: Dissolve 6g of tannic acid in 60mL of deionized water, slowly add 12mL of 1mol / L zinc sulfate aqueous solution while stirring, adjust the pH to 5 with sodium hydroxide solution, stir for 2h, centrifuge, wash the precipitate 3 times with deionized water, freeze dry at -80℃ for 24h to obtain zinc tannic acid composite powder.

[0029] Zinc tannate composite powder was constructed by self-assembly. Under weakly acidic conditions, the catechol structural units in the tannic acid molecule partially dissociated to generate phenolic oxygen anions, which combined with zinc ions in the solution through coordination bonds to form stable Zn-O coordination centers. Multiple coordination sites initiated molecular cross-linking, driving self-assembly and forming zinc tannate composite particles.

[0030] S4: Mix 20g PHA and 80g PCL evenly to obtain a mixed resin; dissolve 100g of the mixed resin in 250mL of 1,2-dichloroethane, and add 8g chitosan / nanocellulose microspheres, 3g zinc tannate composite powder, 14.5g chain extender toluene-2,4-diisocyanate, 3.3g crosslinking agent glycerol, and 0.5g catalyst butyltin dilaurate in sequence. Stir at 70℃ for 4h under nitrogen protection. Cast the reaction solution into a polytetrafluoroethylene mold, place the mold in a vacuum drying oven, and cure at 25℃ and 0.1MPa for 48h to completely remove the solvent 1,2-dichloroethane. Demold to obtain the PHA / PCL composite material.

[0031] Example 2: A method for preparing a biodegradable PHA / PCL composite material, comprising the following steps: S1: Add 5g of nanocellulose crystals to 550mL of distilled water and sonicate for 20min to obtain a nanocellulose crystal suspension. Adjust the pH to 10 with sodium hydroxide solution, then add 7.9mL of 65wt% 2,3-epoxypropyltrimethylammonium chloride solution. Stir at 65℃ for 4h, then stir at room temperature for 17.5h. Transfer to a dialysis bag and dialyze in distilled water for 8.5 days. Freeze dry to constant weight to obtain quaternary ammonium salt modified nanocellulose crystals.

[0032] S2: 9g of chitosan powder and 3.9g of quaternary ammonium salt modified nanocellulose crystals were added to 450mL of 2wt% acetic acid aqueous solution and ultrasonically dispersed to prepare a chitosan / nanocellulose crystal mixed solution, which was used as the aqueous phase. 1100mL of petroleum ether, 61.3g of Span 80 and 3.0g of Tween 60 were stirred at 45℃ and 650rpm for 40min to obtain the oil phase. Then, 250mL of the chitosan / nanocellulose crystal mixed solution was added and the mixture was stirred for another 1.5h. Then, 11g of terephthalaldehyde was added at 55℃ and the mixture was stirred for another 1.5h. Ammonia water was added dropwise to adjust the pH to 9.5. The mixture was centrifuged, and the solid was collected. It was washed alternately with deionized water and ethanol until the supernatant was neutral. The solid was then freeze-dried to constant weight to obtain chitosan / nanocellulose crystal microspheres.

[0033] S3: Dissolve 7g of tannic acid in 75mL of deionized water, slowly add 13.5mL of 1mol / L zinc sulfate aqueous solution while stirring, adjust the pH to 5.25 with sodium hydroxide solution, stir for 2.5h, centrifuge, wash the precipitate 4 times with deionized water, freeze dry at -80℃ for 26h to obtain zinc tannic acid composite powder.

[0034] S4: Mix 30g PHA and 70g PCL evenly to obtain a mixed resin; dissolve 100g of the mixed resin in 300mL of 1,2-dichloroethane, and add 9g chitosan / nanocellulose microspheres, 4g zinc tannate composite powder, 15.5g chain extender toluene-2,4-diisocyanate, 3.95g crosslinking agent glycerol, and 0.65g catalyst butyltin dilaurate in sequence. Stir at 75℃ for 5h under nitrogen protection. Cast the reaction solution into a polytetrafluoroethylene mold, place the mold in a vacuum drying oven, and cure at 32.5℃ and 0.125MPa for 60h to completely remove the solvent 1,2-dichloroethane. Demold to obtain the PHA / PCL composite material.

[0035] Example 3: A method for preparing a biodegradable PHA / PCL composite material, comprising the following steps: S1: Add 6g of nanocellulose crystals to 700mL of distilled water and sonicate for 25min to obtain a nanocellulose crystal suspension. Adjust the pH to 10 with sodium hydroxide solution, then add 8.2mL of 65wt% 2,3-epoxypropyltrimethylammonium chloride solution. Stir at 70℃ for 4.5h, then at room temperature for 20h. Transfer to a dialysis bag and dialyze in distilled water for 10 days. Freeze dry to constant weight to obtain quaternary ammonium salt modified nanocellulose crystals.

[0036] S2: 10g of chitosan powder and 4.6g of quaternary ammonium salt modified nanocellulose crystals were added to 500mL of 2wt% acetic acid aqueous solution and ultrasonically dispersed to prepare a chitosan / nanocellulose crystal mixed solution, which was used as the aqueous phase. 1200mL of petroleum ether, 64.9g of Span 80 and 3.6g of Tween 60 were stirred at 50℃ and 700rpm for 50min to obtain the oil phase. Then, 300mL of the chitosan / nanocellulose crystal mixed solution was added and stirred for 2h. Then, 12g of terephthalaldehyde was added at 60℃ and stirred for another 2h. After that, ammonia water was added dropwise to adjust the pH to 10. After centrifugation, the solid was collected and washed alternately with deionized water and ethanol until the supernatant was neutral. The solid was then freeze-dried to constant weight to obtain chitosan / nanocellulose crystal microspheres.

[0037] S3: Dissolve 8g of tannic acid in 90mL of deionized water, slowly add 15mL of 1mol / L zinc sulfate aqueous solution while stirring, adjust the pH to 5.5 with sodium hydroxide solution, stir for 3h, centrifuge, wash the precipitate 5 times with deionized water, freeze dry at -80℃ for 28h to obtain zinc tannic acid composite powder.

[0038] S4: Mix 40g PHA and 60g PCL evenly to obtain a mixed resin; dissolve 100g of the mixed resin in 350mL of 1,2-dichloroethane, and add 10g chitosan / nanocellulose microspheres, 5g zinc tannate composite powder, 16.5g chain extender toluene-2,4-diisocyanate, 4.6g crosslinking agent glycerol, and 0.8g catalyst butyltin dilaurate in sequence. Stir at 80℃ for 6h under nitrogen protection. Cast the reaction solution into a polytetrafluoroethylene mold, place the mold in a vacuum drying oven, and cure at 40℃ and 0.15MPa for 72h to completely remove the solvent 1,2-dichloroethane. Demold to obtain the PHA / PCL composite material.

[0039] In Examples 1-3, the nanocellulose crystals were selected from Guilin Qihong Technology Co., Ltd., and were obtained from cotton through sulfuric acid hydrolysis, with a diameter of 4-10 nm and a length of 100-500 nm; 2,3-epoxypropyltrimethylammonium chloride was selected from Nantong Runfeng Petrochemical Co., Ltd., CAS number 3033-77-0; chitosan powder was selected from Xi'an Lavia Biotechnology Co., Ltd., CAS number 9012-76-4; petroleum ether was selected from Jinan Zekuan Chemical Co., Ltd., CAS number 8032-32-4; ​​and terephthalaldehyde was selected from Shandong Xuchen Chemical Technology Co., Ltd., CAS number 623- 27-8; Tannic acid was selected from Guangzhou Wanbo Environmental Protection Technology Co., Ltd., CAS No. 1401-55-4; PHA is polyhydroxyalkanoate, selected from Wuhan Shuer Biotechnology Co., Ltd.; PCL is polycaprolactone, selected from Hubei Shineng Chemical Technology Co., Ltd., CAS No. 24980-41-4; 1,2-dichloroethane was selected from Zibo Yujin Trading Co., Ltd., CAS No. 107-06-2; Toluene-2,4-diisocyanate was selected from Shanghai Aladdin Biochemical Technology Co., Ltd., CAS No. 26471-62-5, molecular weight 174.16; the remaining raw materials are all commercially available products.

[0040] Comparative Example 1: The difference from Example 1 is that step S1 is omitted, and the quaternary ammonium salt modified nanocellulose crystals in step S2 are replaced with commercially available nanocellulose crystals. The remaining steps remain unchanged, and PHA / PCL composite material is prepared.

[0041] Comparative Example 2: The difference from Example 1 is that step S2 is omitted. The quaternary ammonium salt modified nanocellulose crystal powder obtained in step S1 is mixed with chitosan powder by physical dry method to obtain chitosan / nanocellulose crystal mixed powder. The chitosan / nanocellulose crystal composite powder in step S4 is replaced with chitosan / nanocellulose crystal mixed powder. The remaining steps remain unchanged to prepare PHA / PCL composite material.

[0042] Comparative Example 3: The difference from Example 1 is that step S3 is omitted. Tannic acid powder and zinc sulfate monohydrate powder are physically dry mixed at a mass ratio of 4:1.21 to obtain tannic acid / zinc sulfate mixed powder. In step S4, the zinc tannic acid composite powder is replaced with an equal mass of tannic acid / zinc sulfate mixed powder. The remaining steps remain unchanged to prepare the PHA / PCL composite material.

[0043] Samples were prepared according to the corresponding testing standards, and the performance of the PHA / PCL composite materials prepared in Examples 1-3 and Comparative Examples 1-3 was tested. Tensile strength and elongation at break: Referring to GB / T 1040.3-2006 "Determination of tensile properties of plastics - Part 3: Test conditions for films and sheets", PHA / PCL composite materials were prepared into type 1A dumbbell specimens and tested using an RGM-4100 universal testing machine with a test spacing of 50 mm and a test rate of 2 mm / min. High tensile strength indicates strong load-bearing capacity of the material; high elongation at break indicates good toughness and resistance to brittle fracture.

[0044] Water absorption rate: Refer to GB / T 1034-2008 "Determination of water absorption of plastics", cut the composite film into 50mm×50mm square film pieces, weigh them and record the weight as m0, completely immerse them in distilled water, soak for 24 hours, take them out, wipe the surface moisture with filter paper and weigh them immediately and record the weight as m1. Water absorption rate = (m1-m0) / m0×100. The smaller the value, the better. A low water absorption rate means that water is not easy to penetrate and the interface is dense.

[0045] Degradation rate: The PHA / PCL composite membrane was cut into 4cm×4cm pieces, dried to constant weight, weighed m0 using an electronic balance, and buried in soil rich in microorganisms in a forest to a depth of about 20cm. To maintain soil moisture, deionized water was added periodically to the buried area. After the 12th week, the membrane was taken out, rinsed and dried, and weighed m. The degradation rate was calculated using the formula W=(m0-m) / m0. Ideally, biodegradable materials should maintain structural stability during their service life and degrade smoothly after disposal.

[0046] Antioxidant: 0.5g of the composite membrane was immersed in an ethanol / water solution, and the antioxidant components were extracted by ultrasonication. The supernatant was mixed with DPPH ethanol solution and allowed to stand in the dark for 30 minutes. The absorbance of the mixture was measured at a wavelength of 517nm, and the absorbance of the blank DPPH solution was measured at the same time. DPPH free radical scavenging rate = [1 - (absorbance of the mixture / absorbance of the blank DPPH solution)] × 100. The higher the free radical scavenging rate, the stronger the material's ability to capture free radicals and delay its own oxidative aging.

[0047] Antibacterial rate: Referring to GB / T 31402-2023 "Determination of antibacterial activity of plastics and other non-porous materials", the concentration of Staphylococcus aureus and Escherichia coli bacterial solutions was (5-10)×10. 5 A CFU / mL dilution solution was evenly spread on the surface of an agar plate. A 50mm × 50mm film with a thickness not exceeding 5mm, prepared from the sample, and the same material without antibacterial agent were then placed tightly against the inoculated agar surface. The plate was incubated at 37℃ and relative humidity (RH > 90%) for 24 hours. The number of viable bacteria in the sample was tested, and the antibacterial rate was calculated. The higher the antibacterial rate, the stronger the antibacterial effect of the material. The antibacterial rate formula is: (number of colonies in the control sample - number of colonies in the test sample) / number of colonies in the control sample × 100%.

[0048] The results are shown in Table 1: Table 1. Performance Test Results of PHA / PCL Composite Material

[0049] As can be seen from Table 1, the biodegradable PHA / PCL composite materials prepared in Examples 1-3 of the present invention are significantly better than those in Comparative Examples 1-3, exhibiting superior mechanical properties, lower water absorption, more controllable degradation behavior, and higher antibacterial and antioxidant activity.

[0050] In Comparative Example 1, the tensile strength, elongation at break, water absorption, and degradation rate all deteriorated significantly. This may be due to the lack of a quaternary ammonium salt modification step. The unmodified nanocellulose crystal surface is rich in hydrophilic hydroxyl groups, which have a severe polarity mismatch and interfacial incompatibility with the hydrophobic PHA / PCL matrix. This results in extremely weak interfacial bonding between the filler and the matrix, which cannot effectively transfer stress and significantly reduces mechanical properties. At the same time, the hydrophilic surface easily adsorbs and guides water molecules into the interface, causing interfacial plasticization and swelling stress, leading to increased water absorption, damage to the wet structure, accelerated microbial erosion of the interface, and premature instability of the material.

[0051] In Comparative Example 2, the tensile strength, elongation at break, water absorption, and degradation rate all deteriorated. This may be due to the lack of structured construction of microspheres. Simple physical mixing of powders cannot form a three-dimensional cross-linked network, resulting in uneven dispersion and easy agglomeration of the filler in the matrix. Furthermore, the limited contact area with the matrix and sparse bonding points weaken the reinforcing and toughening effects of the filler, leading to a decrease in the toughness of the material. At the same time, unstructured fillers are more likely to form permeation channels in the matrix, and water is more likely to diffuse along the weak bonding interface between the filler and the matrix, resulting in an increased water absorption rate. The loose structure also makes the material less robust and more easily decomposed by microorganisms and water in the soil environment.

[0052] In Comparative Example 3, both the free radical scavenging rate and antibacterial rate were significantly deteriorated. This may be due to the lack of a coordination assembly process. During the composite material processing, the tannic acid molecules and zinc sulfate powder, which are physically dry mixed, cannot form a stable hybrid structure with zinc ions. Zinc ions are prone to irregular dissolution, aggregation, or local enrichment, resulting in uncontrollable release behavior. This makes it impossible to maintain long-lasting antibacterial and antioxidant properties, and it is difficult to achieve uniform dispersion in the matrix. It is also impossible to produce a long-lasting antibacterial synergistic effect with chitosan and quaternary ammonium salt nanocellulose crystals, which significantly weakens the antibacterial and antioxidant properties of the material and makes it lack durability.

[0053] 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 variations can be made to these embodiments without departing from the principles and spirit of the invention.

Claims

1. A method for preparing a biodegradable PHA / PCL composite material, characterized in that, Includes the following steps: Step 1: After modifying the nanocellulose crystals with quaternary ammonium salt, they are cross-linked with chitosan through reverse emulsification and chemical cross-linking to obtain chitosan / nanocellulose crystal microspheres; Step 2: PHA, PCL, chitosan / nanocellulose microspheres and zinc tannate composite powder are dissolved together in 1,2-dichloroethane, and toluene-2,4-diisocyanate, glycerol and butyltin dilaurate are added. After casting and curing, PHA / PCL composite material is obtained.

2. The method for preparing a biodegradable PHA / PCL composite material according to claim 1, characterized in that, The specific preparation steps of the chitosan / cellulose nanospheres are as follows: Chitosan powder and quaternary ammonium salt modified nanocellulose crystals were added to a 2 wt% acetic acid aqueous solution and ultrasonically dispersed to prepare a chitosan / nanocellulose crystal mixed solution, which was used as the aqueous phase. Petroleum ether, Span 80, and Tween 60 were stirred at 40-50℃ and 600-700rpm for 30-50 minutes to obtain an oil phase. Then, a chitosan / nanocellulose crystal mixed solution was added, and stirring was continued for 1-2 hours. Then, terephthalaldehyde was added at 50-60℃, and stirring was continued for 1-2 hours. Ammonia was added dropwise to adjust the pH to 9-10. The mixture was centrifuged, and the solid was collected. It was washed alternately with deionized water and ethanol until the supernatant was neutral. The mixture was then freeze-dried to constant weight to obtain chitosan / nanocellulose crystal microspheres.

3. The method for preparing a biodegradable PHA / PCL composite material according to claim 2, characterized in that, The ratio of chitosan powder, quaternary ammonium salt modified nanocellulose crystals, and 2wt% acetic acid aqueous solution is 8-10g: 3.2-4.6g: 400-500mL; the ratio of petroleum ether, Span 80, Tween 60, chitosan / nanocellulose crystal mixed solution, and terephthalaldehyde is 1000-1200mL: 57.6-64.9g: 2.4-3.6g: 200-300mL: 10-12g.

4. The method for preparing a biodegradable PHA / PCL composite material according to claim 2, characterized in that, The specific preparation steps for the quaternary ammonium salt modified nanocellulose crystals are as follows: Nanocellulose crystals were added to distilled water and ultrasonically pulverized for 15-25 minutes to obtain a nanocellulose crystal suspension. The pH was adjusted to 10 with sodium hydroxide solution, and then 65wt% 2,3-epoxypropyltrimethylammonium chloride solution was added. The mixture was stirred at 60-70℃ for 3.5-4.5 hours and at room temperature for 15-20 hours. The mixture was then transferred to a dialysis bag and dialyzed in distilled water for 7-10 days. Finally, it was freeze-dried to constant weight to obtain quaternary ammonium salt modified nanocellulose crystals.

5. The method for preparing a biodegradable PHA / PCL composite material according to claim 4, characterized in that, The ratio of the nanocellulose crystals, distilled water, and 65wt% 2,3-epoxypropyltrimethylammonium chloride solution is 4-6g: 400-700mL: 7.6-8.2mL.

6. The method for preparing a biodegradable PHA / PCL composite material according to claim 1, characterized in that, The zinc tannate composite powder is prepared by the following steps: Tannic acid was dissolved in deionized water, and 1 mol / L zinc sulfate aqueous solution was slowly added while stirring. The pH was adjusted to 5-5.5 with sodium hydroxide solution. After stirring for 2-3 hours, the mixture was centrifuged, and the precipitate was washed with deionized water 3-5 times and freeze-dried to constant weight to obtain zinc tannic acid composite powder.

7. The method for preparing a biodegradable PHA / PCL composite material according to claim 6, characterized in that, The ratio of tannic acid, deionized water, and 1 mol / L zinc sulfate aqueous solution is 6-8 g: 60-90 mL: 12-15 mL.

8. The method for preparing a biodegradable PHA / PCL composite material according to claim 1, characterized in that, The specific preparation steps of the PHA / PCL composite material are as follows: PHA and PCL were mixed evenly to obtain a mixed resin. The mixed resin was dissolved in 1,2-dichloroethane, and chitosan / nanocellulose microspheres, zinc tannate composite powder, toluene-2,4-diisocyanate, glycerol and butyltin dilaurate were added sequentially. Under nitrogen protection, the mixture was stirred at 70-80℃ for 4-6 hours. The reaction solution was cast into a polytetrafluoroethylene mold, and the mold was placed in a vacuum drying oven and cured at 25-40℃ and 0.1-0.15MPa for 48-72 hours. The mold was then demolded to obtain the PHA / PCL composite material.

9. The method for preparing a biodegradable PHA / PCL composite material according to claim 8, characterized in that, The ratio of the mixed resin, 1,2-dichloroethane, chitosan / nanocellulose microspheres, zinc tannate composite powder, toluene-2,4-diisocyanate, glycerol, and butyltin dilaurate is 100g: 250-350mL: 8-10g: 3-5g: 14.5-16.5g: 3.3-4.6g: 0.5-0.8g; The content of PHA in the mixed resin is 20-40 wt%.

10. A biodegradable PHA / PCL composite material, prepared according to any one of claims 1-9.

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