A pH-responsive self-separating fully-bio-based degradable lunch box material and a preparation method thereof, and a degradable lunch box

Abstract

This invention discloses a pH-responsive, self-separating, fully biodegradable lunchbox material and its preparation method, belonging to the field of biodegradable lunchbox technology. The pH-responsive, self-separating, fully biodegradable lunchbox material comprises: a starch base layer; a polylactic acid (PLA) barrier layer; and a dynamic covalent bond interface layer with a thickness of 20-50 μm located between the starch base layer and the PLA barrier layer. The dynamic covalent bond interface layer is formed by dynamic cross-linking of hydroxyl groups in starch molecules with carboxyl groups at the ends of PLA chains through borate ester bonds (B–O), wherein boric acid serves as the cross-linking medium, and the cross-linking density gradually increases from 0.3 mol / kg on the starch base layer side to 0.5 mol / kg on the PLA barrier layer side. This pH-responsive, self-separating, fully biodegradable lunchbox material helps to achieve high sealing performance during its service life, rapid self-separation after disposal, and visualization of the degradation process.

Description

Technical Field

[0001] This invention relates to the field of biodegradable lunchbox technology, specifically to a pH-responsive, self-separating, fully bio-based biodegradable lunchbox material and its preparation method, as well as a biodegradable lunchbox. Background Technology

[0002] Traditional biodegradable lunch boxes often employ a multi-layered structure constructed from thermoplastic starch (TPS) and polylactic acid (PLA) to balance cost, rigidity, and barrier properties. However, these materials still face three major technical bottlenecks in practical applications: poor interfacial compatibility and insufficient mechanical properties: Due to the high hydrophilic hydroxyl content of starch and the hydrophobic nature of PLA, the significant difference in interfacial polarity results in a lack of effective chemical bonding, leading to interlayer bonding strength generally below 0.1 MPa. When containing hot soup or oily liquids, interlayer delamination easily occurs, with measured leakage rates exceeding 60%, failing to meet the airtightness requirements of food packaging. Difficult to separate after disposal and low degradation efficiency: Existing products mostly rely on physical co-extrusion or hot-pressing lamination, without a designed controllable dissociation mechanism. While starch components can degrade relatively quickly in composting or soil environments (30–60 days), PLA has a degradation cycle of over 180 days. Furthermore, due to interlayer adhesion, the degradation process is not coordinated, often producing flaky or fibrous residues, posing a risk of microplastic release. The degradation process is not visible, and users lack awareness: Current biodegradable lunch boxes lack intuitive indicators of the degradation progress. Consumers cannot determine whether the material has been fully mineralized, leading to low willingness to sort and dispose of food properly, or even mistakenly disposing of it in non-recyclable waste, thus diminishing the environmental value of biodegradable materials. Summary of the Invention

[0003] As described in the prior art, one of the objectives of this invention is to provide a pH-responsive, self-separating, fully biodegradable lunchbox material that combines high sealing performance during its service life, rapid self-separation after disposal, and visualization of the degradation process.

[0004] The second objective of this invention is to provide a method for preparing a pH-responsive, self-separating, fully biodegradable lunchbox material, which can successfully prepare the biodegradable lunchbox material.

[0005] The third objective of this invention is to provide a pH-responsive, self-separating, fully biodegradable lunchbox.

[0006] One of the objectives of this invention is achieved through the following technical solution: A pH-responsive, self-separating, fully biodegradable lunchbox material comprising: Starch base layer; Polylactic acid barrier layer; A dynamic covalent interface layer with a thickness of 20-50 μm is located between the starch base layer and the polylactic acid barrier layer. The dynamic covalent bond interface layer is formed by the dynamic cross-linking of hydroxyl groups in starch molecules and carboxyl groups at the ends of polylactic acid chains through borate ester bonds (B–O), with boric acid as the cross-linking medium. The cross-linking density increases from 0.3 mol / kg on the starch base layer side to 0.5 mol / kg on the polylactic acid barrier layer side.

[0007] Furthermore, the dynamic covalent bond interface layer is a pH-responsive self-separating adhesive layer, or a pH-responsive self-separating adhesive layer is further set between the starch base layer and the polylactic acid barrier layer. The pH-responsive self-separating adhesive layer is a chitosan-boron ester dynamic bonding network with a thickness of 20-50 μm and a crosslinking density of 0.35-0.45 mol / kg. The bonding strength of the pH-responsive self-separating adhesive layer decreases by more than 90% within 24 hours when pH>7.5. The degree of deacetylation of chitosan is ≥85%, and the molecular weight is 50,000-100,000 g / mol.

[0008] Furthermore, the polylactic acid barrier layer is a PLA-anthocyanin block copolymer film, wherein anthocyanins are grafted onto PLA segments via enzyme catalysis, with a grafting rate of 2-5 wt%, and the anthocyanins are cyanidin-3-O-glucoside.

[0009] Furthermore, the molar ratio of starch hydroxyl groups in the starch base layer to terminal carboxyl groups in the polylactic acid barrier layer is 1:1.2-1:1.5, and the borate ester bond is formed by reacting in an ethanol-water mixed solution containing 1.5–2.0 wt% boric acid at 80–85 °C for 2–3 hours.

[0010] The second objective of this invention is achieved by the following technical solution: A method for preparing a pH-responsive, self-separating, fully biodegradable lunchbox material includes the following steps: S1. Construction of the dynamic covalent bond interface layer: Thermoplastic starch film (thickness 0.3–0.5 mm) and carboxyl-terminated polylactic acid (PLA-COOH, COOH content ≥80 μmol / g) film (thickness 0.15–0.25 mm) were stacked together. The films were then sequentially immersed in an ethanol-water mixed solution with boric acid concentration ranging from 1.5–2.0 wt% for 10–20 min at 80–85 °C. The ethanol volume fraction in the solution was 70–80%. A dynamic crosslinking layer of borate ester bonds (B–O) with a thickness of 20–50 μm was formed in situ at the interface between the two films. The crosslinking density increased gradually from the starch side (0.3 mol / kg) to the PLA side (0.5 mol / kg). After removal, the films were vacuum dried at 50 °C to constant weight, yielding a thermoplastic starch film with a gradient crosslinking interface and a polylactic acid film with a gradient crosslinking interface, respectively. S2, Multi-layer lamination and hot pressing - vacuum forming: Thermoplastic starch film with gradient cross-linking interface and polylactic acid film with gradient cross-linking interface are stacked sequentially according to the desired structure. The stacked film is placed in a flat vulcanizing machine and hot-pressed at 150–160℃ and 0.5–1.0MPa pressure for 5–10 minutes to fully bond the layers and complete the interface cross-linking. After cooling to room temperature, a multilayer sheet is obtained, which is a biodegradable lunch box material.

[0011] Furthermore, when a color indication function is required, the preparation of a PLA-anthocyanin copolymer film is included after step S1: Polylactic acid (PLA, Mn=40000–60000) and cyanidin-3-O-glucoside (3–5 wt% of PLA) were dissolved in dichloromethane, and lipase Novozym435 (2–3 wt% of PLA) was added. The mixture was stirred and reacted at 70–75 °C for 30 min–1 h under nitrogen protection. After the reaction was completed, the enzyme was removed by filtration, and the solvent was removed by rotary evaporation. The obtained PLA-anthocyanin block copolymer was pressed into tablets and cast into films to obtain PLA-anthocyanin copolymer films with a grafting rate of 2–5 wt%.

[0012] Furthermore, when enhanced interlayer controllable separation performance is required, after step S1, a pH-responsive self-separating adhesive layer is formed: Chitosan with a degree of deacetylation ≥85% and a molecular weight of 50,000–100,000 is dissolved in a 2wt% aqueous acetic acid solution to prepare a solution with a solid content of 3–5wt%. Boric acid is added to make the final concentration 0.3–0.5 mol / kg, and the solution is stirred evenly to obtain an adhesive coating liquid. The coating liquid is applied to the surface of a thermoplastic starch film or a carboxyl-terminated polylactic acid film by roller coating or spraying, with a coating amount of 3–5 g / m². The film is then dried at 40°C to form a pH-responsive self-separating adhesive layer.

[0013] The third objective of this invention is achieved by the following technical solution: A pH-responsive, self-separating, fully biodegradable lunchbox is produced by placing the biodegradable lunchbox material in a vacuum forming mold and vacuum forming it into a lunchbox shape at 150–160℃ and a vacuum degree of 0.06–0.08 MPa. After natural cooling, the lunchbox is demolded to obtain the biodegradable lunchbox. The lunchbox comprises: The inner PLA barrier layer has a thickness of 0.15-0.25mm; Intermediate dynamic covalent bond interface layer, 20-50μm thick; The outer starch base layer is 0.3-0.5 mm thick. During the service life of the lunchbox, the pH is <6 and the bonding strength between the inner layers is >0.8MPa. In the waste composting environment, the pH is >7.5, and the lunchbox automatically separates within 24 hours, with an overall degradation cycle of <90 days.

[0014] Furthermore, the lunchbox also includes a pH-responsive self-separating adhesive layer disposed between the starch base layer and the polylactic acid barrier layer.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) The pH-responsive, self-separating, fully biodegradable lunchbox material provided by this invention has the following beneficial effects: 1. Dynamic covalent bond IPN: Starch hydroxyl groups and PLA terminal carboxyl groups form BO bonds under boric acid catalysis, with an interfacial crosslinking density of 0.3-0.5 mol / kg and a gradient distribution (0.3 on the starch side → 0.5 mol / kg on the PLA side), ensuring stress transfer efficiency; 2. Self-healing mechanism: BO bonds can be reversibly broken and rearranged under microwave heating (>80℃), repairing microcracks within 10 minutes and reducing leakage rate from 45% to <1%; 3. Intelligent indication: Anthocyanins are covalently grafted onto PLA, and color develops when pH>7. After esterase cleavage, the color disappears, realizing "color disappearance = degradation complete"; 4. Self-separating adhesive layer: The chitosan-boronate network's bonding strength decreases from 1.2 MPa to <0.05 MPa within 24 hours under ammonia conditions during composting (pH>7.5), resulting in automatic delamination. During its service life (containing food with pH 5-6), the lunchbox of this invention exhibits a bonding strength >1.2 MPa and a leakage rate <1%; after disposal, the degradation rate is >88% within 90 days; after interlayer separation, the starch layer degrades in <30 days, the PLA layer in <90 days, and there are no interlayer microplastic residues.

[0016] (2) Boron ester bonds (BO) form a stable cross-linked network during processing, giving the lunchbox high strength and self-healing ability. In the alkaline environment of composting (pH>7.5), BO bonds hydrolyze rapidly, achieving automatic separation between layers and independent and efficient degradation of each component. At the same time, PLA-anthocyanin copolymer gives the lunchbox a color indicator function, and the disappearance of color indicates that degradation is complete, which improves public participation and regulatory convenience. Detailed Implementation

[0017] The present invention will now be further described in conjunction with specific embodiments. It should be noted that, without conflict, the embodiments or technical features described below can be arbitrarily combined to form new embodiments.

[0018] Example 1: This embodiment provides a pH-responsive, self-separating, fully biodegradable lunchbox material, comprising: Starch base layer; Polylactic acid barrier layer; A dynamic covalent interface layer with a thickness of 20-50 μm is located between the starch base layer and the polylactic acid barrier layer. The dynamic covalent interface layer is formed by the dynamic crosslinking of hydroxyl groups in starch molecules with carboxyl groups at the ends of polylactic acid (PLA) chains via borate ester bonds (B–O). Boric acid acts as the crosslinking medium, with the crosslinking density gradually increasing from 0.3 mol / kg on the starch substrate side to 0.5 mol / kg on the PLA barrier layer side. This gradient crosslinking structure is achieved by pretreating the composite sheet in a 1.5 wt% boric acid-ethanol solution for 10 min, followed by a further reaction in a 2.0 wt% boric acid-ethanol solution for 15 min. Utilizing the diffusion kinetics and reactivity differences of boric acid towards the PLA side, a crosslinking density distribution increasing from the inside out is formed. By controlling the concentration gradient impregnation process of the boric acid solution, the borate ester crosslinking density in the interface layer gradually increases from 0.3 mol / kg on the starch side to 0.5 mol / kg on the PLA side.

[0019] This embodiment also provides a method for preparing a pH-responsive, self-separating, fully biodegradable lunchbox material, including the following steps: S1. Construction of the dynamic covalent bond interface layer: Thermoplastic starch film (thickness 0.3–0.5 mm) and carboxyl-terminated polylactic acid (PLA-COOH, COOH content ≥80 μmol / g) film (thickness 0.15–0.25 mm) were stacked together. The films were then sequentially immersed in an ethanol-water mixed solution with boric acid concentration ranging from 1.5–2.0 wt% for 10–20 min at 80–85 °C. The ethanol volume fraction in the solution was 70–80%. A dynamic crosslinking layer of borate ester bonds (B–O) with a thickness of 20–50 μm was formed in situ at the interface between the two films. The crosslinking density increased gradually from the starch side (0.3 mol / kg) to the PLA side (0.5 mol / kg). After removal, the films were vacuum dried at 50 °C to constant weight, yielding a thermoplastic starch film with a gradient crosslinking interface and a polylactic acid film with a gradient crosslinking interface, respectively. S2, Multi-layer lamination and hot pressing - vacuum forming: Thermoplastic starch film with gradient cross-linking interface and polylactic acid film with gradient cross-linking interface are stacked sequentially according to the desired structure. The stacked film is placed in a flat vulcanizing machine and hot-pressed at 150–160℃ and 0.5–1.0MPa pressure for 5–10 minutes to fully bond the layers and complete the interface cross-linking. After cooling to room temperature, a multilayer sheet is obtained, which is a biodegradable lunch box material.

[0020] Example 2: This embodiment provides a pH-responsive, self-separating, fully biodegradable lunchbox material. Unlike Embodiment 1, the polylactic acid barrier layer is a PLA-anthocyanin block copolymer, in which anthocyanins are grafted onto PLA segments via enzymatic catalysis at a grafting rate of 2-5 wt%. The material exhibits color loss and a degradation rate ≥85% after 21 days of burial. In this embodiment, the PLA-anthocyanin copolymer is prepared as follows: Polylactic acid (PLA, Mn=40000) and cyanidin-3-O-glucoside (3–5 wt% of PLA) were dissolved in dichloromethane. Lipase Novozym435 (2–3 wt% of PLA) was added, and the mixture was stirred at 70–75 °C for 30 min–1 h under nitrogen protection. After the reaction, the enzyme was removed by filtration, and the solvent was removed by rotary evaporation. The resulting PLA-anthocyanin block copolymer was pressed into sheets and cast into films to obtain PLA-anthocyanin copolymer films with a grafting rate of 3 wt%. The obtained PLA-anthocyanin copolymer was deep blue at pH > 7.5, and the color disappeared after 21 days in compost.

[0021] Anthocyanins (natural pigments) are grafted onto PLA segments via enzymatic catalysis to form PLA-anthocyanin-PLA block copolymers. Anthocyanins act as both chromophores and "enzyme cleavage switches." They exhibit color at pH > 7 and are cleaved by microbial esterases, resulting in colorless degradation products. This achieves an intelligent indication function of "color disappearance = degradation complete."

[0022] The method for preparing pH-responsive, self-separating, fully biodegradable lunchbox material provided in this embodiment includes the following steps: S1. Construction of the dynamic covalent bond interface layer: A thermoplastic starch film (thickness 0.3–0.5 mm) and a carboxyl-terminated polylactic acid (PLA-COOH, COOH content ≥80 μmol / g) film (thickness 0.15–0.25 mm) are stacked together. The films are then sequentially immersed in an ethanol-water mixed solution with a boric acid concentration ranging from 1.5–2.0 wt% to high concentration, and reacted at 80–85 °C for 10–20 min. The ethanol volume fraction in the solution is 70–80%. A 20–50 μm thick dynamic crosslinking layer of borate ester bonds (B–O) is formed in situ at the interface between the two films, with the crosslinking density increasing gradually from the starch side (0.3 mol / kg) to the PLA side (0.5 mol / kg). After removal, the films are vacuum dried at 50 °C to constant weight, yielding a thermoplastic starch film with a gradient crosslinking interface and a polylactic acid film with a gradient crosslinking interface, respectively. S2: When a color indication function is required, after step S1, the preparation of a PLA-anthocyanin copolymer film is also included. Polylactic acid (PLA, Mn=40000) and cyanidin-3-O-glucoside (3–5 wt% of PLA) were dissolved in dichloromethane, and lipase Novozym435 (2–3 wt% of PLA) was added. The mixture was stirred and reacted at 70–75 °C for 30 min–1 h under nitrogen protection. After the reaction was completed, the enzyme was removed by filtration, and the solvent was removed by rotary evaporation. The obtained PLA-anthocyanin block copolymer was pressed into tablets and cast into films to obtain PLA-anthocyanin copolymer films with a grafting rate of 3 wt%. S3, Multi-layer lamination and hot pressing - vacuum forming: Thermoplastic starch film with gradient cross-linking interface and polylactic acid film with gradient cross-linking interface are stacked sequentially according to the desired structure. The stacked film is placed in a flat vulcanizing machine and hot-pressed at 150–160℃ and 0.5–1.0MPa pressure for 5–10 minutes to fully bond the layers and complete the interface cross-linking. After cooling to room temperature, a multilayer sheet is obtained, which is a biodegradable lunch box material.

[0023] Example 3: This embodiment provides a pH-responsive self-separating, fully biodegradable lunchbox material. Unlike Embodiment 2, a pH-responsive self-separating adhesive layer is further provided between the starch base layer and the polylactic acid barrier layer. The pH-responsive self-separating adhesive layer is a chitosan-boronate dynamic bonding network with a thickness of 20-50 μm and a crosslinking density of 0.35-0.45 mol / kg. The bonding strength of the pH-responsive self-separating adhesive layer decreases by more than 90% within 24 hours when pH>7.5. The degree of deacetylation of chitosan is ≥85%, and the molecular weight is 50,000-100,000 g / mol.

[0024] This embodiment also provides a method for preparing a pH-responsive, self-separating, fully biodegradable lunchbox material, including the following steps: S1. Construction of the dynamic covalent bond interface layer: Thermoplastic starch film (thickness 0.3–0.5 mm) and carboxyl-terminated polylactic acid (PLA-COOH, COOH content ≥80 μmol / g) film (thickness 0.15–0.25 mm) were stacked together. The films were then sequentially immersed in an ethanol-water mixed solution with boric acid concentration ranging from 1.5–2.0 wt% for 10–20 min at 80–85 °C. The ethanol volume fraction in the solution was 70–80%. A dynamic crosslinking layer of borate ester bonds (B–O) with a thickness of 20–50 μm was formed in situ at the interface between the two films. The crosslinking density increased gradually from the starch side (0.3 mol / kg) to the PLA side (0.5 mol / kg). After removal, the films were vacuum dried at 50 °C to constant weight, yielding a thermoplastic starch film with a gradient crosslinking interface and a polylactic acid film with a gradient crosslinking interface, respectively. S2. When a color indication function is required, after step S1, the preparation of a PLA-anthocyanin copolymer film is also included: Polylactic acid (PLA, Mn=40000) and cyanidin-3-O-glucoside (3–5 wt% of PLA) were dissolved in dichloromethane, and lipase Novozym435 (2–3 wt% of PLA) was added. The mixture was stirred and reacted at 70–75 °C for 30 min–1 h under nitrogen protection. After the reaction, the enzyme was removed by filtration, and the solvent was removed by rotary evaporation. The resulting PLA-anthocyanin block copolymer was pressed into tablets and cast into films to obtain PLA-anthocyanin copolymer films with a grafting rate of 3 wt%. The anthocyanin used in this step is preferably cyanidin-3-O-glucoside. S3. When it is necessary to enhance the controllable separation performance between layers, after step S1, a pH-responsive self-separating adhesive layer is also formed: Chitosan with a degree of deacetylation of 90% and a molecular weight of 75,000 was dissolved in a 2 wt% aqueous acetic acid solution to prepare a solution with a solid content of 3–5 wt%. Boric acid was added to make the final concentration 0.4 mol / kg, and the solution was stirred evenly to obtain an adhesive coating liquid. The coating liquid was applied to the surface of a thermoplastic starch film or a carboxyl-terminated polylactic acid film by roller coating or spraying, with a coating amount of 3 g / m². The film was then dried at 40°C to form a pH-responsive self-separating adhesive layer. S4. Multi-layer lamination and hot pressing - vacuum forming: Thermoplastic starch film with gradient cross-linking interface and polylactic acid film with gradient cross-linking interface are stacked sequentially according to the desired structure. The stacked film is placed in a flat vulcanizing machine and hot-pressed at 150–160℃ and 0.5–1.0MPa for 5–10 minutes to ensure that the layers are fully bonded and the interface cross-linking is completed. After cooling to room temperature, a multilayer sheet is obtained, which is a biodegradable lunch box material. When it contains a pH-responsive self-separating adhesive layer, it can be combined with a dynamic covalent bond interface layer to form a multifunctional interface, or set as an independent intermediate layer. In this embodiment, the pH-responsive self-separating adhesive layer employs dynamic bonding of chitosan and borate ester. During service life (pH<6): BO bonds are stable, with a binding strength of 0.8-1.2 MPa, preventing leakage.

[0025] After disposal (compost pH > 7.5): Ammonia gas breaks the borate ester bonds, and the interlayer bonding strength decreases to < 0.05 MPa within 24 hours, resulting in automatic delamination.

[0026] Comparative Example 1 Interface-free layer: The difference between this comparative example and Example 1 is that a dynamic covalent bond interface layer is not constructed. Instead, the starch substrate layer and the PLA barrier layer are directly hot-pressed together to verify the bonding strength during interface-free modification.

[0027] Preparation steps: Corn starch (hydroxyl content 12.5 mmol / g) was hot-pressed into a starch base layer sheet with a thickness of 0.4 mm, and carboxyl-terminated PLA (COOH content 85 μmol / g, Mn=50000) was hot-pressed into a PLA barrier layer sheet with a thickness of 0.2 mm. The two layers were directly hot-pressed together at 170℃ and 5 MPa pressure for 5 minutes without adding boric acid catalyst or performing alcohol solution interface reaction treatment, resulting in a control sample without interface layer modification. The obtained sample showed no obvious interface transition layer between the layers, and the starch and PLA had a clear physical contact interface.

[0028] Comparative Example 2 Physical blending: The difference between this comparative example and Example 1 is that a simple physical blending method is used instead of the dynamic covalent bond interface crosslinking technology of the present invention, in order to verify the difference between physical blending and chemical crosslinking in terms of interface bonding performance.

[0029] Preparation steps: Corn starch (hydroxyl content 12.5 mmol / g) and carboxyl-terminated PLA (COOH content 85 μmol / g, Mn=50000) were directly added to a mixer at a mass ratio of 2:1 and physically blended at 170℃ and 60 rpm for 10 minutes without adding boric acid catalyst or performing esterification reaction in alcohol solution. The blend was hot-pressed into a sheet with a thickness of 0.6 mm. In this material, starch and PLA are in a macroscopically separated state with no oriented interfacial layer structure.

[0030] Comparative Example 3: Same as Example 1, but the crosslinking density of borate ester was uniformly controlled at 0.4 mol / kg with no gradient distribution.

[0031] This comparative example is basically the same as Example 1, except that a uniform crosslinking strategy is used when constructing the dynamic covalent bond interface layer. That is, a constant concentration of boric acid solution (1.8 wt%) is used throughout the interface layer to keep the crosslinking density at 0.4 mol / kg, so as not to form a gradient structure that increases from the inside to the outside. The remaining preparation steps are the same as in Example 1.

[0032] Comparative Example 4 This comparative example is used to verify whether anthocyanins must be grafted via enzyme catalysis to achieve intelligent color indication function.

[0033] Polylactic acid (PLA, Mn=50,000) was directly mixed with cyanidin-3-O-glucoside (5wt% of PLA mass), and melt-blended at 160–170℃ using a twin-screw extruder. After granulation, the mixture was cast into a film to obtain a physically mixed PLA-anthocyanin film without chemical bonds.

[0034] The remaining preparation steps are the same as in Example 2, that is, the film is used as the inner layer and combined with the starch base layer containing the gradient cross-linking interface, and then hot-pressed into a lunch box.

[0035] Experimental data Experimental Example 1 Experimental methods: Interface bonding strength was tested according to GB / T2790-1995 "Adhesives - Test Method for Peel Strength at 180 Degrees", with a peel rate of 100 mm / min. Leakage rate test: The lunchbox was filled with 95℃ hot water, inverted for 24 hours, and the mass of the leaked liquid was weighed. The self-healing function was tested repeatedly after microwave heating (900W, 85℃). The test results are shown in Table 1.

[0036] Table 1 Performance Comparison of Dynamic Covalent Bond Interface Layer

[0037] Results analysis: Advantages of gradient crosslinking: Gradient crosslinked samples have a 47.1% higher bonding strength and an 86.3% lower leakage rate than uniformly crosslinked samples. This is because the high crosslinking density side (PLA side) provides strong anchoring, while the low crosslinking density side (starch side) provides stress buffering, preventing interfacial brittle fracture.

[0038] Self-healing effect: Microwave repair of the sample in Example 1 further reduced the leakage rate of the sample by 61.9%, and the dynamic rearrangement of BO bonds effectively healed the microcracks. The repair efficiency decreased by <5% with the number of cycles.

[0039] Critical thickness: The interface layer of 20-50μm is the key technology. If it is too thin (<20μm), a continuous network cannot be formed. If it is too thick (>50μm), it increases the cost and reduces transparency.

[0040] Experiment Example 2 Experimental Methods: The same lunchbox sample underwent 10 consecutive microwave repair cycles. Before each repair, artificial scratches were created. After repair, the interfacial bonding strength and leakage rate were tested. Color stability ΔE was measured using a spectrophotometer according to CIELab standards. The results are shown in Table 2.

[0041] Table 2 Cyclic stability of self-healing function

[0042] Results analysis: Cyclic durability: After 10 repairs, the bond strength decreased by only 8.0%, and the leakage rate was still <2%, proving that the dynamic rearrangement of BO bonds is highly reversible and that the molecular chains did not undergo irreversible breakage.

[0043] Color stability: ΔE increased from 2.8 to 3.5, but remained below the human eye recognition threshold (ΔE=5), indicating that the anthocyanin covalent grafting was strong and that microwave treatment did not damage the chromophores.

[0044] Economic efficiency: The system can cover the reuse of lunch boxes in 10 repair cycles (such as recycling and cleaning takeout boxes), with an energy consumption of only 0.015kWh per repair, which is cost-effective.

[0045] Conclusion: The dynamic cross-linking design of BO bonds gives the lunch box the potential for reusability, significantly reducing the environmental burden of a single use, which is in line with the concept of circular economy.

[0046] Experimental Example 3 Experimental Methods: Multilayer materials containing the adhesive layer were placed in buffer solutions with different pH values ​​to simulate food environments (pH 5.5), the initial composting stage (pH 7.0), and the active composting stage (pH 7.8). Binding strength was tested as described above, and stratification time was determined by visually observing the time required for the binding strength to be <0.05 MPa. The degradation rate increase factor = degradation rate of a single layer after self-separation / degradation rate of the multilayer material without self-separation. The results are shown in Table 3.

[0047] Table 3 Performance of the intelligent self-separating adhesive layer under different pH conditions

[0048] Results analysis: pH response threshold: The adhesive layer begins to dissociate when pH>7.0, responds rapidly when pH>7.5, and its strength decreases by more than 90% after 24 hours, consistent with the characteristics of the chitosan-boronate network.

[0049] Self-separation driving force: Ammonia (NH3) reacts with borate ester bonds to generate ammonium borate, which catalyzes the hydrolysis of BO bonds. The separation time is only 18 hours, which is much shorter than the manual stripping cycle of existing technologies (>30 days).

[0050] Degradation synergistic effect: After stratification, the starch layer has an increased specific surface area, which increases the degradation rate by 5.8 times. The PLA layer is also freed from interfacial constraints, and the enzyme accessibility is improved. The overall degradation cycle is shortened from >180 days to <90 days.

[0051] Conclusion: The self-separating adhesive layer is an intelligent switch connecting "performance during service life" and "post-disposal treatment", achieving the technical effect of "no leakage when in use and quick separation when discarded".

[0052] Experiment Example 4 Experimental Methods: Lunch box samples were buried in industrial compost (58℃, 65% humidity). Color parameters (Lab) and degradation rate (weight loss method) were measured periodically. Anthocyanins were degraded by esterases through covalent bond cleavage, and the residual amount was determined by HPLC. ΔE calculation: ΔE = [(ΔL)² + (Δa*)² + (Δb*)²]^(1 / 2). The results are shown in Table 4.

[0053] Table 4. Verification of PLA-Anthocyanin Intelligent Indication Function

[0054] Results analysis: Necessity of covalent grafting: Anthocyanins in physically mixed samples are easily leached out, with a ΔE of 8.2 after 21 days, rendering the indicator function ineffective; in covalently grafted samples, the ΔE is 2.5, and the color disappearance is synchronized with the degradation process.

[0055] Anthocyanins have a dual function: they act as chromophores (initially a*=32, appearing blue) and as "enzyme cleavage switches." Esterases preferentially cleave the anthocyanin-PLA ester bond, generating signal molecules that accelerate overall degradation.

[0056] Grafting rate optimization: A grafting rate of 3.5wt% yields the best overall effect, with high color saturation (significant change in ΔE) and no impact on PLA crystallinity (crystallization in DSC test decreased slightly from 25% to 23%).

[0057] The above embodiments are merely preferred embodiments of the present invention and should not be construed as limiting the scope of protection of the present invention. Any non-substantial changes and substitutions made by those skilled in the art based on the present invention shall fall within the scope of protection claimed by the present invention.

Claims

1. A pH-responsive, self-separating, fully biodegradable lunchbox material, characterized in that, Include: Starch base layer; Polylactic acid barrier layer; A dynamic covalent interface layer with a thickness of 20-50 μm is located between the starch base layer and the polylactic acid barrier layer. The dynamic covalent interface layer is formed by the dynamic cross-linking of hydroxyl groups in starch molecules and carboxyl groups at the ends of polylactic acid chains through borate ester bonds, wherein boric acid serves as the cross-linking medium, and the cross-linking density gradually increases from 0.3 mol / kg on the starch base layer side to 0.5 mol / kg on the polylactic acid barrier layer side.

2. The pH-responsive, self-separating, fully biodegradable lunchbox material as described in claim 1, characterized in that, The dynamic covalent bond interface layer is a pH-responsive self-separating adhesive layer, or a further pH-responsive self-separating adhesive layer is provided between the starch base layer and the polylactic acid barrier layer. The pH-responsive self-separating adhesive layer is a chitosan-boron ester dynamic bonding network with a thickness of 20-50 μm and a crosslinking density of 0.35-0.45 mol / kg. The bonding strength of the pH-responsive self-separating adhesive layer decreases by more than 90% within 24 hours when pH>7.

5. The degree of deacetylation of the chitosan is ≥85%, and the molecular weight is 50,000-100,000 g / mol.

3. The pH-responsive, self-separating, fully biodegradable lunchbox material as described in claim 1, characterized in that, The polylactic acid barrier layer is a PLA-anthocyanin block copolymer film, wherein anthocyanins are grafted onto PLA segments via enzyme catalysis, with a grafting rate of 2-5 wt%, and the anthocyanins are cyanidin-3-O-glucoside.

4. The pH-responsive, self-separating, fully biodegradable lunchbox material as described in claim 1, characterized in that, The molar ratio of starch hydroxyl groups in the starch base layer to terminal carboxyl groups in the polylactic acid barrier layer is 1:1.2-1:1.5, and the borate ester bond is formed by reacting in an ethanol-water mixed solution containing 1.5–2.0 wt% boric acid at 80–85°C for 2–3 hours.

5. The method for preparing the biodegradable lunchbox material according to any one of claims 2 to 4, characterized in that, Includes the following steps: S1. Construction of the dynamic covalent bond interface layer: Thermoplastic starch film and carboxyl-terminated polylactic acid (PLA) film were stacked and then sequentially immersed in an ethanol-water mixed solution with boric acid concentration ranging from 1.5–2.0 wt% to high concentration. The mixture was then immersed at 80–85 °C for 10–20 min, with an ethanol volume fraction of 70–80%. A dynamic cross-linked layer of borate ester bonds with a thickness of 20–50 μm was formed in situ at the interface between the two films, with the cross-linking density increasing in a gradient from the starch side to the PLA side. After removal, the film was vacuum dried at 50 °C to constant weight, yielding thermoplastic starch film and polylactic acid film with gradient cross-linked interfaces, respectively. S2, Multi-layer lamination and hot pressing - vacuum forming: The thermoplastic starch film with gradient cross-linking interface and the polylactic acid film with gradient cross-linking interface are stacked sequentially according to the desired structure. The stacked film is placed in a flat vulcanizing machine and hot-pressed at 150–160°C and 0.5–1.0 MPa for 5–10 minutes to ensure that each layer is fully bonded and the interface cross-linking is completed. After cooling to room temperature, a multilayer sheet is obtained, which is the biodegradable lunch box material.

6. The method for preparing the biodegradable lunchbox material as described in claim 5, characterized in that, When a color indication function is required, the preparation of a PLA-anthocyanin copolymer film is included after step S1: Polylactic acid and cyanidin-3-O-glucoside were dissolved in dichloromethane, and lipase Novozym435 was added. The mixture was stirred and reacted at 70–75 °C for 30 min–1 h under nitrogen protection. After the reaction was completed, the enzyme was removed by filtration, and the solvent was removed by rotary evaporation. The obtained PLA-anthocyanin block copolymer was pressed into a sheet and cast into a film to obtain the PLA-anthocyanin copolymer film with a grafting rate of 2–5 wt%.

7. The method for preparing the biodegradable lunchbox material as described in claim 5, characterized in that, When enhanced interlayer controllable separation performance is required, the pH-responsive self-separating adhesive layer is further formed after step S1: Chitosan with a degree of deacetylation ≥85% and a molecular weight of 50,000–100,000 is dissolved in a 2wt% aqueous acetic acid solution to prepare a solution with a solid content of 3–5wt%. Boric acid is added to make the final concentration 0.3–0.5 mol / kg, and the solution is stirred evenly to obtain an adhesive coating liquid. The coating liquid is applied to the surface of the thermoplastic starch film or the surface of the carboxyl-terminated polylactic acid film by roller coating or spraying, with a coating amount of 3–5 g / m². The film is then dried at 40°C to form the pH-responsive self-separating adhesive layer.

8. A pH-responsive, self-separating, fully biodegradable lunchbox, characterized in that, The biodegradable lunchbox material according to any one of claims 1-4 is placed in a vacuum forming mold and vacuum-formed into a lunchbox shape at 150–160°C and a vacuum degree of 0.06–0.08 MPa. After natural cooling, it is demolded to obtain the biodegradable lunchbox, which comprises: The inner PLA barrier layer has a thickness of 0.15-0.25mm; Intermediate dynamic covalent bond interface layer, 20-50μm thick; The outer starch base layer is 0.3-0.5 mm thick. During the service life of the lunchbox, the pH is <6 and the bonding strength between the inner layers is >0.8MPa. In the waste composting environment, the pH is >7.5, and the lunchbox automatically separates within 24 hours, with an overall degradation cycle of <90 days.

9. A pH-responsive, self-separating, fully biodegradable lunchbox as described in claim 8, characterized in that, The lunchbox also includes a pH-responsive self-separating adhesive layer disposed between the starch base layer and the polylactic acid barrier layer.

Images