A novel double-layer composite film and application thereof

Abstract

The application belongs to the technical field of bio-based packaging materials, and particularly relates to a novel double-layer composite film and application thereof. The novel double-layer composite film comprises the following steps: reacting palmitic anhydride and carboxymethyl cellulose to obtain carboxymethyl cellulose palmitate; adding a pH-sensitive natural dye indicator to obtain carboxymethyl cellulose palmitate nanoemulsion containing a pH-sensitive substance; then loading a natural polymer material to prepare a film material; finally adding the film material to the carboxymethyl cellulose palmitate to prepare a composite film, thereby obtaining the novel double-layer composite film. The double-layer composite film can be self-emulsified without adding a surfactant, and has the ability to load functional indicator (pH-sensitive color indicator) substances; meanwhile, the barrier performance of the film material, such as a chitosan film, to water vapor and oxygen is improved, and the double-layer composite film has a very promising application in food or fruit preservation. The method provided by the application is not only green in raw materials, but also uses a flow casting method, which can be applied to industrial production and has a good application prospect.

Description

Technical Field

[0001] This invention belongs to the field of bio-based packaging materials technology, and more specifically, relates to a novel double-layer composite film and its application. Background Technology

[0002] The widespread use of petroleum-based plastics in packaging materials has led to environmental problems. With increasing environmental awareness, biodegradable bio-based materials have become a hot topic in the packaging industry. Chitosan, one of the most abundant polysaccharides in nature, has attracted much attention for its excellent film-forming ability in food packaging. As people's living standards continue to improve, they are paying more and more attention to food safety, thus raising the requirements for food packaging films. Food packaging is crucial for maintaining the integrity of food and protecting it from biological and chemical damage. However, chitosan films have insufficient water vapor barrier capacity, which is detrimental to extending the shelf life of food. Food spoilage detection requires complex testing procedures and instruments; packaging films that can respond to food spoilage offer a simpler approach. Adding active substances to the chitosan film matrix can further broaden the functionality of composite films. Currently, most methods for adding natural active substances to chitosan films employ nano-encapsulation, such as adding natural pigments. These pigments change color when the environment changes; currently, widely used pigments include anthocyanins, curcumin, and carotene.

[0003] Cellulose is one of the most abundant renewable resources, and its structure, with numerous hydrogen bonds, endows it with hydrophilicity. Palm oil, a bio-based raw material with a triglyceride structure, possesses excellent hydrophobicity. Given the current demands of food packaging, it is essential to develop a fully bio-based, environmentally friendly packaging film with good barrier properties and color responsiveness. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this application provides a novel bilayer composite membrane. This application employs two strategies: First, carboxymethyl cellulose grafted with palm oil is used to prepare amphiphilic fibers for loading curcumin, improving the dispersibility of curcumin in chitosan solution. Since curcumin undergoes deprotonation in an alkaline environment, causing a change in light absorption wavelength and a reddish color, the addition of curcumin enables the chitosan membrane to exhibit color-responsive properties. Second, cellulose palmitate is prepared by grafting palm oil with cellulose, and the chitosan membrane and cellulose palmitate membrane are then combined to form a bilayer composite membrane. The addition of long carbon chains provides hydrophobicity to the material, and the addition of the cellulose palmitate membrane significantly improves the barrier properties of the chitosan membrane. Furthermore, this is the first time that a fully bio-based bilayer membrane has been prepared using cellulose ester and chitosan.

[0005] Therefore, the primary objective of this application is to provide a novel bilayer composite membrane preparation method for chitosan (curcumin@carboxymethyl cellulose palmitate)-cellulose palmitate. Compared to other chitosan composite membrane preparation methods, we use fully bio-based materials, which aligns with environmentally friendly development trends. Furthermore, the introduction of cellulose palmitate significantly improves the barrier properties of the chitosan membrane.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A novel method for preparing a bilayer composite membrane includes the following steps:

[0008] S1. Palm oil is used as a raw material to prepare palmitoleic acid, and then palmitoleic acid is further prepared into palmitoleic anhydride;

[0009] S2: The palm oil anhydride obtained in step S1 is reacted with carboxymethyl cellulose. After the reaction is completed and purified, carboxymethyl cellulose palmitate is obtained.

[0010] S3: Add the carboxymethyl cellulose palmitate obtained in step S2 to a pH-sensitive natural dye indicator to obtain a pH-sensitive carboxymethyl cellulose palmitate nanoemulsion.

[0011] S4: Add natural polymer materials to the pH-sensitive carboxymethyl cellulose palmitate nanoemulsion obtained in step S3 to prepare a film;

[0012] S5: Cellulose palmitate is formed into a film, and then the film-forming material obtained in step S4 is added on top of it to prepare a composite film, thus obtaining the novel bilayer composite film.

[0013] The preparation of cellulose palmitate in step S5 is the same as in steps S1 and S2, except that carboxymethyl cellulose can be replaced with cellulose.

[0014] Preferably, the pH-sensitive natural dye indicator in step S3 is any one or more of anthocyanins, curcumin, carotene, and shikonin.

[0015] Preferably, the natural polymer material in step S4 is any one or more of agarose, chitosan, starch, cellulose, pectin, gelatin, corn protein, and alginate.

[0016] Preferably, curcumin is added in step S3. The curcumin addition process involves dissolving carboxymethyl cellulose palmitate and curcumin separately, then mixing and homogenizing them. The mass ratio of carboxymethyl cellulose palmitate to curcumin is 5-15:1. This invention utilizes curcumin to impart color responsiveness to the chitosan membrane. When food with high nitrogen content spoils, volatile alkaline nitrogen is produced. When exposed to an alkaline environment, curcumin changes from yellow to red, thus providing intelligent indication.

[0017] Preferably, chitosan is added in step S4, and the mass fraction of chitosan is 2-5 wt%.

[0018] Preferably, the molar ratio of carboxymethyl cellulose and palm oil anhydride in step S2 is 1:3 to 6.

[0019] Preferably, in step S2, p-toluenesulfonic acid is used as a catalyst and dimethyl sulfoxide is used as a cosolvent for dissolution; wherein, the mass ratio of palm oil anhydride to p-toluenesulfonic acid is 12:1, and the mass ratio of carboxymethyl cellulose to dimethyl sulfoxide is 1:10.

[0020] Preferably, the film formation in step S5 is carried out by casting.

[0021] This application also claims protection for the novel bilayer composite membrane obtained by the preparation method described above.

[0022] Compared with the prior art, the beneficial effects of the present invention are:

[0023] The long carbon chain used in this invention comes from biomass resources, palm oil. The reaction raw materials are renewable and do not pollute the environment. The waste liquid treatment and recycling during the preparation process is simple and conducive to industrial production.

[0024] The palm oil anhydride synthesized in this invention has the characteristic of high reactivity compared to palm oil itself. Unmodified palm oil cannot react with carboxymethyl cellulose, and the reaction process is friendly. Compared with other hydrophobic modifications of carboxymethyl cellulose, it is more green and chemical.

[0025] The carboxymethyl cellulose palmitate synthesized in this invention can self-emulsify without the addition of additional surfactants and has the ability to load hydrophobic drugs, thereby improving the dispersibility of curcumin solution in chitosan solution.

[0026] The cellulose palmitate synthesized in this invention is hydrophobic, which greatly improves the water vapor and oxygen barrier properties of chitosan membranes, making it very promising for the preservation of some fruits.

[0027] This invention uses green raw materials and employs a casting method, which can be applied to industrial production for large-scale preparation. This method is simple to prepare a fully bio-based two-layer membrane structure. Attached Figure Description

[0028] Figure 1 Infrared images of Examples 4, 5, 6 and Comparative Example 1.

[0029] Figure 2 The images shown are SEM images of Examples 4, 5, and 6.

[0030] Figure 3 The mechanical properties diagrams are for Examples 4, 5, and 6.

[0031] Figure 4 Hygroscopicity and water solubility diagrams for Examples 4, 5, and 6.

[0032] Figure 5 This is a diagram illustrating the color responsiveness of Example 6. Detailed Implementation

[0033] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments and figures. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the 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. Where specific conditions are not specified in the embodiments, preparation is carried out according to conventional conditions or conditions recommended by the manufacturer. Reagents or instruments used where the manufacturer is not specified are all conventional products that can be purchased commercially.

[0034] Example 1

[0035] Weigh out 60g of palm oil, 50ml of anhydrous ethanol, and 11g of... Dissolve 50 ml of deionized water in NaOH, mix well, and react at 70 °C for 1.5 h. Adjust the pH to 3 with phosphoric acid to stop the reaction. Dilute with deionized water at 80 °C four times, and remove water by rotary evaporation for 2 h to obtain palmitoleic acid. Weigh palmitoleic acid, add 50 wt% 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDI) salt, and react at 70 °C for 20 min. Centrifuge at 8000 rpm for 5 min while hot. Add 33.3 wt% 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDI) salt a second time, and centrifuge at 8000 rpm for 5 min while hot to obtain palmitoleic anhydride. Weigh 3 g of carboxymethyl cellulose, 2 g of p-toluenesulfonic acid, and 90 ml of dimethyl sulfoxide. After the carboxymethyl cellulose is completely dissolved, add 24 g of palmitoleic anhydride, and react at 110 °C for 2.5 h. Wash with dichloromethane, centrifuge to purify the product, and finally obtain the purified carboxymethyl cellulose palmitate sample.

[0036] Example 2

[0037] Weigh 60g of palm oil, 50ml of anhydrous ethanol, and dissolve 11g of NaOH in 50ml of deionized water. Mix well and react at 70℃ for 1.5h. Adjust the pH to 3 with phosphoric acid to stop the reaction. Dilute with deionized water at 80℃ four times, and remove water by rotary evaporation for 2h to obtain palmitoleic acid. Weigh the palmitoleic acid, add 50wt% 1-ethyl-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDI), and react at 70℃ for 20min. While hot, centrifuge at 8000rpm for 5min. Add 33.3wt% 1-ethyl-(3- Dimethylaminopropylcarbodiimide hydrochloride (EDI) salt was centrifuged at 8000 rpm for 5 min while hot to obtain palmitole anhydride; 3.4 g of microcrystalline cellulose, 11.18 g of DBU, and 30 g of dimethyl sulfoxide were weighed and dissolved in a carbon dioxide switched solvent system. After the microcrystalline cellulose solution was completely clear and transparent, 30 g of palmitole anhydride was added dropwise using a constant pressure dropping funnel. The reaction was carried out at 60 °C for 4 h. After the reaction was completed, the solution was washed and purified with anhydrous ethanol to obtain cellulose palmitate.

[0038] Example 3

[0039] Weigh 20 mg of carboxymethyl cellulose palmitate prepared in Example 1 and dissolve it in deionized water. Dissolve 2 mg of curcumin in dichloromethane. Mix the two solutions and homogenize for 7 min. Remove dichloromethane by rotary evaporation. Remove the precipitate with an organic nylon filter to obtain curcumin@carboxymethyl cellulose palmitate emulsion.

[0040] Example 4

[0041] Weigh 1g of the cellulose palmitate prepared in Example 2, dissolve it in 10ml of tetrahydrofuran, and spread it evenly on a smooth glass plate by casting. Dry it into a film at 30°C and 40-50% humidity.

[0042] Example 5

[0043] Prepare a 3wt% chitosan solution using a 1% acetic acid aqueous solution, and add 60% glycerol. Add a curcumin@carboxymethyl cellulose palmitate solution to the chitosan solution, stir to disperse evenly, and spread it evenly on a smooth glass plate using a casting method. Dry it into a film at a temperature of 30℃ and an ambient humidity of 40-50%.

[0044] Example 6

[0045] A 3 wt% chitosan solution was prepared using a 1% acetic acid aqueous solution, and 60% glycerol was added. The curcumin@carboxymethyl cellulose palmitate solution prepared in Example 3 was added to the chitosan solution and stirred until evenly dispersed. A cellulose palmitate film of Example 4 was prepared using a casting method. After complete drying, a chitosan (curcumin@carboxymethyl cellulose palmitate) solution was spread evenly on top of the film using the same casting method. The film was dried at 30°C and 40-50% humidity for 24 hours to obtain a chitosan (curcumin@carboxymethyl cellulose palmitate)-cellulose palmitate bilayer composite film.

[0046] Comparative Example 1

[0047] Prepare a 3wt% chitosan solution using a 1% acetic acid aqueous solution, add 60% glycerol, and spread the solution evenly on a smooth glass plate using a casting method. Dry the solution at 30℃ and 40-50% humidity to form a film.

[0048] Test methods and conditions:

[0049] The infrared spectra of chitosan membrane, cellulose palmitate membrane, chitosan (curcumin@carboxymethyl cellulose palmitate) membrane, and chitosan (curcumin@carboxymethyl cellulose palmitate)-cellulose palmitate membrane were measured using a Nicolet IS10 spectrometer (Thermo Fisher, USA).

[0050] SEM images of cellulose palmitate membranes, chitosan (curcumin@carboxymethyl cellulose palmitate) membranes, and chitosan (curcumin@carboxymethyl cellulose palmitate)-cellulose palmitate membranes were obtained using a scanning electron microscope (EVO MA15, ZEISS). The EDX scan of the chitosan (curcumin@carboxymethyl cellulose palmitate)-cellulose palmitate membrane is a potassium (K) distribution map. K was added to the chitosan (curcumin@carboxymethyl cellulose palmitate) membrane in the bilayer composite membrane to obtain a cross-sectional EDX scan of the bilayer composite membrane.

[0051] The mechanical properties of cellulose palmitate membrane, chitosan (curcumin@carboxymethyl cellulose palmitate) membrane, and chitosan (curcumin@carboxymethyl cellulose palmitate)-cellulose palmitate membrane were determined using an electronic universal testing machine equipped with a 2500N pressure sensor.

[0052] The samples were cut into small pieces for testing water solubility and hygroscopicity. For the water solubility test of cellulose palmitate membranes, chitosan (curcumin@carboxymethylcellulose palmitate) membranes, and chitosan (curcumin@carboxymethylcellulose palmitate)-cellulose palmitate membranes, the samples were dried at 100°C for 24 hours to reach a constant weight (m0). The dried samples were then placed in 50 mL of water at room temperature for 24 hours with occasional slight shaking. The dissolved samples were then dried at 100°C to reach a constant weight (m1) and calculated using formula (1). For the hygroscopicity test of cellulose palmitate membranes, chitosan (curcumin@carboxymethylcellulose palmitate) membranes, and chitosan (curcumin@carboxymethylcellulose palmitate)-cellulose palmitate membranes, the membranes were dried at 105°C for 24 hours and weighed (m0). Then, they were stored at room temperature in an environment with a humidity of 70-80% for 24 hours and weighed again (m1).

[0053]

[0054] The water vapor barrier properties of cellulose palmitate membrane, chitosan (curcumin@carboxymethyl cellulose palmitate) membrane, and chitosan (curcumin@carboxymethyl cellulose palmitate)-cellulose palmitate membrane were tested using GB / T 1037 with a Jinan Langguang (C360) instrument.

[0055] The oxygen barrier properties of cellulose palmitate membranes, chitosan (curcumin@carboxymethyl cellulose palmitate) membranes, and chitosan (curcumin@carboxymethyl cellulose palmitate)-cellulose palmitate membranes were tested in Burgger, Germany, according to GB / T 1038.

[0056] The color response of cellulose palmitate membranes, chitosan (curcumin@carboxymethyl cellulose palmitate) membranes, and chitosan (curcumin@carboxymethyl cellulose palmitate)-cellulose palmitate membranes is characterized by a combination of the following four graphs (e.g. Figure 5 ):

[0057] (a) Color changes of curcumin@carboxymethyl cellulose palmitate lyophilized powder dissolved in phosphate buffer at different pH values, at a concentration of 1 mg / ml;

[0058] (b) Color changes of chitosan (curcumin@carboxymethyl cellulose palmitate)-cellulose palmitate membrane after soaking in buffer solutions of different pH values ​​for 5 minutes;

[0059] (c) Ultraviolet spectra of different solutions in Figure a;

[0060] (d) Color changes and sensitivity of chitosan (curcumin@carboxymethyl cellulose palmitate)-cellulose palmitate membranes at 1 cm above 0.8 mol / L ammonia water at different times (RGB).

[0061] Analysis of measurement results

[0062] Figure 1 The images show the FTIR spectra of chitosan membrane (CS), cellulose palmitate membrane (MCCPOA), chitosan (curcumin@carboxymethyl cellulose palmitate) membrane CS(Cur@CMCPOA), and chitosan (curcumin@carboxymethyl cellulose palmitate)-cellulose palmitate membrane (CS(Cur@CMCPOA)-MCCPOA).

[0063] The molecular interactions within the composite thin film were analyzed using FT-IR spectroscopy, and the results are as follows: Figure 1 As shown, the acetyl-CO peak (amide I) of CS appears at 1631 cm⁻¹. -1 The NH tensile vibration (amide II) peak appears at 1596 cm⁻¹. -1 The peak of the C=O ester bond in MCCPOA appears at 1740 cm⁻¹. -1 The presence of this marker indicates that palm oil was successfully grafted onto microcrystalline cellulose; the 1631 cm⁻¹ on CS(Cur@CMCPOA) demonstrates this. -1 and 1596cm -1 The peaks are those of amide I and amide II in CS, and the peak of the benzene ring on curcumin is also located at 1596 cm⁻¹. -1 1743cm on CS (Cur@CMCPOA) -1 The peaks are ester groups on CMCPOA and 1753 cm⁻¹ on CS(Cur@CMCPOA-MCCPOA). -1 and 1738cm -1 The corresponding ester peaks are on MCCPOA and CMCPOA, respectively; the 1630 cm⁻¹ peak on CS(Cur@CMCPOA-MCCPOA) is also present. -1 and 1594cm -1 The peaks are those of amide I and amide II in CS, and the peak of the benzene ring on curcumin is also located at 1594 cm⁻¹. -1 The asymmetric tensile vibration of CO on CS is 1631 cm⁻¹ from CS(Cur@CMCPOA). -1 and 1596cm -1 The offset is 1630cm -1 and 1594cm -1 This indicates that there is a hydrogen bond interaction between CS and Cur@CMCPOA, Cur@CMCPOA.

[0064] Figure 2SEM images of cellulose palmitate membrane (MCCPOA), chitosan (curcumin@carboxymethyl cellulose palmitate) membrane CS (Cur@CMCPOA), and chitosan (curcumin@carboxymethyl cellulose palmitate)-cellulose palmitate membrane (CS(Cur@CMCPOA)-MCCPOA). In Figure (a), a and b are scanning electron microscope (SEM) images of the lower surface of the MCCPOA film with different covariates, and c is a cross-sectional SEM image of the MCCPOA film; in Figure (a), d and e are SEM images of the lower surface of the CS(Cur@CMCPOA) film with different covariates, and f is a cross-sectional SEM image of the CS(Cur@CMCPOA) film; in Figure (b), a and b are SEM images of the lower surface of the MCCPOA film with different covariates in the CS(Cur@CMCPOA)-MCCPOA composite film, c is a cross-sectional SEM image of the CS(Cur@CMCPOA)-MCCPOA composite film; d and e are SEM images of the lower surface of the CS(Cur@CMCPOA) film with different covariates in the CS(Cur@CMCPOA)-MCCPOA composite film; f is a cross-sectional EDX scan of the CS(Cur@CMCPOA)-MCCPOA composite film. Scanning electron microscopy (SEM) results show that the surfaces of the MCPOA film, CS(Cur@CMCPOA) film, and CS(Cur@CMCPOA)-MCCPOA film are dense and smooth. Cross-sectional SEM images of the CS(Cur@CMCPOA)-MCCPOA film confirm the successful fabrication of a bilayer composite film. EDI distribution indicates good fusion between the two layers, demonstrating the successful fabrication of the bilayer composite film using a casting process.

[0065] Figure 3 This chart shows the mechanical properties of three membranes: cellulose palmitate membrane (MCCPOA), chitosan (curcumin@carboxymethyl cellulose palmitate) membrane (CS(Cur@CMCPOA), and chitosan (curcumin@carboxymethyl cellulose palmitate)-cellulose palmitate membrane (CS(Cur@CMCPOA)-MCCPOA). The chart shows that the stress of the MCCPOA-CS(Cur@CMCPOA3) membrane is 10.7 MPa higher than that of the CS(Cur@CMCPOA3) membrane, and the elongation at break is 30.55% higher. This indicates that the bilayer structure is beneficial for improving stress and elongation at break. Figure 2 The good fusion of the two-layer structure in the middle can also indirectly prove this.

[0066] Figure 4The graphs show the hygroscopicity and water solubility of cellulose palmitate membrane (MCCPOA), chitosan (curcumin@carboxymethyl cellulose palmitate) membrane CS(Cur@CMCPOA), and chitosan (curcumin@carboxymethyl cellulose palmitate)-cellulose palmitate membrane (CS(Cur@CMCPOA)-MCCPOA). The water solubility of MCCPOA-CS(Cur@CMCPOA3) is 13.05% lower than that of CS(Cur@CMCPOA3), and the hygroscopicity of MCCPOA-CS(Cur@CMCPOA3) is 5.85% lower than that of CS(Cur@CMCPOA3). These results indicate that the introduction of MCCPOA is beneficial for improving the water solubility and hygroscopicity of the membrane.

[0067] Table 1 shows the water vapor barrier properties and oxygen barrier properties of cellulose palmitate membrane (MCCPOA), chitosan (curcumin@carboxymethyl cellulose palmitate) membrane CS(Cur@CMCPOA), and chitosan (curcumin@carboxymethyl cellulose palmitate)-cellulose palmitate membrane (CS(Cur@CMCPOA)-MCCPOA). As shown in Table 1, the oxygen barrier performance of CS(Cur@CMCPOA)-MCCPOA is 1.83 × 10⁻⁶. -17 The water vapor barrier performance is 2.19×10. -13 In comparison, the oxygen and water vapor barrier properties of the CS(Cur@CMCPOA3) membrane were significantly improved, indicating that the introduction of MCCPOA enhanced the oxygen and water vapor barrier properties of the CS(Cur@CMCPOA)-MCCPOA membrane, surpassing most current research findings. The barrier performance results suggest that incorporating MCCPOA membranes to prepare a bilayer chitosan composite membrane structure helps overcome the poor barrier properties of chitosan membranes, broadening the application prospects of chitosan membranes in the packaging field.

[0068] Table 1

[0069]

[0070] Figure 5 It is the color responsiveness of chitosan (curcumin@carboxymethyl cellulose palmitate)-cellulose palmitate membrane (CS(Cur@CMCPOA)-MCCPOA) membrane. Figure 5Figure a shows the color change of Cur@CMCPOA lyophilized powder of the same concentration (1 mg / ml) dissolved in phosphate buffer at different pH values. The figure shows that the solution color changes from yellow to red at pH 11 and 12, indicating that the curcumin emulsion does indeed exhibit a red shift in color under alkaline conditions. Figure c shows the UV spectra of different solutions in figure a. The figure reveals a change in UV absorption wavelength at pH 11 and 12, which further confirms the reason for the color change in the curcumin emulsion. Figure 5 Figure b shows the color change of the MCCPOA-CS (Cur@CMCPOA3) membrane after immersing it in buffer solutions of different pH values ​​for 5 minutes. The results show that MCCPOA-CS (Cur@CMCPOA3) also exhibits color responsiveness; the membrane color changed from yellow to red at pH 11 and 12. Figure d shows the sensitivity of the MCCPOA-CS (Cur@CMCPOA3) membrane at different times at a distance of 1 cm above 0.8 mol / L ammonia solution, calculated using RGB. A sensitivity greater than or equal to 25% indicates that it possesses color responsiveness suitable for use as a smart preservation film. The figure shows that the sensitivity of the MCCPOA-CS (Cur@CMCPOA3) membrane is sufficient.

[0071] The above embodiments of the present invention are merely examples to clearly illustrate the technical solutions of the present invention, and are not intended to limit the specific implementation of the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the claims of the present invention should be included within the protection scope of the claims of the present invention.

Claims

1. A method for preparing a bilayer composite membrane, characterized in that, Includes the following steps: S1. Palm oil is used as a raw material to prepare palmitoleic acid, and then palmitoleic acid is further prepared into palmitoleic anhydride; S2: The palm oil anhydride obtained in step S1 is reacted with carboxymethyl cellulose. After the reaction is completed and purified, carboxymethyl cellulose palmitate is obtained. S3: Add the carboxymethyl cellulose palmitate obtained in step S2 to a pH-sensitive natural dye indicator to obtain a carboxymethyl cellulose palmitate nanoemulsion containing a pH-sensitive natural dye indicator. S4: Chitosan is added to the carboxymethyl cellulose palmitate nanoemulsion containing pH-sensitive natural dye indicator obtained in step S3 to prepare a film-forming material. S5: Cellulose palmitate is formed into a film, and then the film-forming material obtained in step S4 is applied on it to prepare a composite film, that is, the double-layer composite film is obtained. The preparation of cellulose palmitate in step S5 is the same as in steps S1 and S2, except that carboxymethyl cellulose is replaced with cellulose.

2. The preparation method according to claim 1, characterized in that, In step S3, the pH-sensitive natural dye indicator is any one or more of anthocyanins, curcumin, and shikonin.

3. The preparation method according to claim 1, characterized in that, In step S3, curcumin is added. The curcumin is added by dissolving carboxymethyl cellulose palmitate and curcumin separately, and then mixing and homogenizing them. The mass ratio of carboxymethyl cellulose palmitate to curcumin is 5~15:

1.

4. The preparation method according to claim 1, characterized in that, In step S4, the mass fraction of chitosan is 2-5 wt%.

5. The preparation method according to claim 1, characterized in that, The molar ratio of carboxymethyl cellulose and palm oil anhydride in step S2 is 1:3~6.

6. The preparation method according to claim 1, characterized in that, In step S2, p-toluenesulfonic acid is used as a catalyst and dimethyl sulfoxide is used as a cosolvent for dissolution; wherein, the mass ratio of palm oil anhydride to p-toluenesulfonic acid is 12:1, and the mass ratio of carboxymethyl cellulose to dimethyl sulfoxide is 1:

10.

7. The preparation method according to claim 1, characterized in that, In step S5, the film formation process is carried out using the casting method.

8. The bilayer composite membrane obtained by any of the preparation methods described in claims 1 to 7.

9. The application of the double-layer composite film according to claim 8 in food packaging or fruit preservation.

Images