A method for preparing a modified chitosan degradable composite fresh-keeping film

Abstract

The present invention relates to a method for preparing a modified chitosan degradable composite cling film. Chitosan is dissolved in an aqueous glacial acetic acid solution to obtain a chitosan solution; the chitosan solution is mixed with a triglyceride additive solution and a palmitic acid additive solution and stirred to prepare a film solution; the film solution is ultrasonically degassed and evenly coated onto a flat container; the film solution is dried and separated from the flat container to obtain a modified chitosan composite cling film. The modified chitosan composite cling film simultaneously has excellent swelling, mechanical properties, and water barrier properties. Intermolecular hydrogen bonds are formed between triglycerides, chitosan, and palmitic acid, making the film structure denser and significantly improving the mechanical properties of the composite cling film, resulting in excellent tensile strength and elongation at break. At the same time, the water barrier properties of the cling film are improved, effectively preventing nutrient loss and extending the shelf life of fruits and vegetables. The film meets the requirements of GB / T 10457-2021 "General Rules for Quality of Plastic Self-Adhesive Cling Films for Food Use."

Description

Technical Field

[0001] The invention relates to the field of environmental technology, belongs to biodegradable materials, and particularly relates to a modified chitosan degradable composite fresh-keeping film and a preparation method thereof. Background Art

[0002] With increasing attention to environmental protection and food quality and safety, the shortcomings of traditional polymer packaging, such as non-biodegradability, non-recyclability, difficulty in disposal, and environmental pollution, have become increasingly apparent. This has led researchers to focus on biodegradable polymer materials as food packaging and preservation materials. Some carbohydrate polymers, such as starch, cellulose derivatives, chitosan, and pectin, are biodegradable and edible, environmentally friendly and non-toxic. They have been used as packaging materials to extend the shelf life of food and are considered promising biodegradable packaging materials for industrial development.

[0003] Among the numerous carbohydrate polymers, chitosan is a copolymer of chitin and natural carbohydrates in a low-acetyl-substituted form, composed of β-(1-4)-2-acetylamino-D-glucose and β-(1-4)-2-amino-D-glucose units. It exhibits excellent film-forming properties, permeability, and adsorption, and is easily amenable to structural modification and modification, making it a highly effective natural polymer functional membrane material. Chitosan membranes are widely used in the food industry due to their simple, environmentally friendly, and safe preparation process, which does not involve the use of toxic substances. Furthermore, chitosan is the second most abundant polysaccharide in the world and can be obtained from abundant renewable resources, primarily waste from the shellfish industry, at a low cost. Therefore, it has great commercial potential. However, chitosan is a hydrophilic polymer, and composite membranes prepared with it have poor moisture barrier properties compared to commercially available polyethylene and polyvinyl chloride membranes. This tendency to absorb moisture and dissolve in high humidity is a major limitation to its widespread application, necessitating the modification of chitosan.

[0004] Chitosan molecules contain two active groups, carboxyl and amino, offering a variety of modification possibilities, including cross-linking, copolymerization, blending, ionization, or the formation of coordination complexes between amino groups on the chitosan molecular chain and transition metal ions. Chitosan membranes are typically prepared using chitosan as the membrane matrix, with the addition of polysaccharides (such as starch, pectin, cellulose, and its derivatives), proteins (such as myosin, collagen, and casein), lipids (such as glycerol, plant essential oils, and stearic acid), or nanomaterials (such as nano-titanium dioxide and nano-zinc oxide) to enhance the membrane's mechanical, barrier, antibacterial, and antioxidant properties. Currently, the most common chitosan membrane-forming solvent is glacial acetic acid. Chitosan monomers are cross-linked through intramolecular and intermolecular hydrogen bonds, van der Waals forces, and physical entanglement, resulting in a uniform dispersion in the glacial acetic acid solution, forming a three-dimensional network structure. When chitosan is compounded with various substances, active groups such as hydroxyl and amino groups on the molecule can form hydrogen bonds, electrostatic interactions, and hydrophobic interactions with hydroxyl, acetyl, and carboxyl groups on the additives, resulting in a tighter composite membrane structure. This structure, in turn, influences performance, enhancing the mechanical properties, water barrier properties, and antimicrobial activity of single chitosan membranes. The uniformly mixed and degassed membrane solution, when dried in an oven, forms a biodegradable composite membrane with excellent gas barrier and antibacterial properties, improving food safety and extending shelf life. However, current technologies still struggle to balance swelling, water barrier properties, and mechanical properties. Summary of the Invention

[0005] In order to solve the problems of the existing technology, the purpose is to adjust the addition amount of different components, use the PBAT index in GB / T10457-2021 "General Quality Rules for Plastic Self-adhesive Cling Film for Food" as the evaluation standard, and explore the optimal film-forming ratio to improve the poor water barrier properties of single chitosan film, easy moisture absorption and dissolution in high humidity environment, poor mechanical properties, and the inability of modified composite film to take into account multiple properties.

[0006] In order to improve the above problems, the present invention blends and modifies chitosan matrix with palmitic acid and triglyceride, degasses and dries the membrane liquid to obtain a biodegradable composite film with excellent swelling, water barrier and mechanical properties.

[0007] The present invention provides a method for preparing a modified chitosan-based degradable composite fresh-keeping film, comprising the following steps:

[0008] (1) dissolving chitosan in an aqueous solution of glacial acetic acid to obtain a chitosan solution;

[0009] (2) mixing the chitosan solution, triglyceride additive solution, and palmitic acid additive solution and stirring them uniformly to prepare a membrane solution;

[0010] (3) The membrane liquid is ultrasonically degassed and then evenly coated on a flat container; the membrane liquid is dried and separated from the flat container to obtain a modified chitosan composite fresh-keeping film.

[0011] In the step (1), the preferred ratio of chitosan to glacial acetic acid aqueous solution is (0.5-2.0) g:100 mL.

[0012] In the step (1), chitosan is preferably dissolved in a glacial acetic acid aqueous solution with a volume fraction of 1-3%.

[0013] In the step (2), the preferred ratio of triglyceride to ethylene glycol is (50-150) mg:100 mL, and the preferred ratio of triglyceride additive to chitosan dissolving solution is (3.0-6.0) mL:100 mL.

[0014] In the step (2), the preferred usage ratio of palmitic acid to ethanol is (1-3) g:30 mL, and the usage ratio of palmitic acid additive to chitosan dissolving solution is (0.3-0.6) mL:100 mL.

[0015] In the step (2), the preferred stirring speed is 600-800 r / min and the stirring time is 0.5-1.5 h.

[0016] In step (3), the membrane liquid is preferably ultrasonically degassed for 0.5-1.5 h and at an ultrasonic frequency of 35-45 kHz.

[0017] In the step (3), the membrane liquid is preferably dried at a temperature of 30-55° C. and for a drying time of 15-25 h.

[0018] Compared with the prior art, the above technical solution proposed by the present invention can achieve the following beneficial effects:

[0019] (1) It can be seen from the data in the table below that the new preparation method provided by the present invention can make the modified chitosan composite cling film have excellent swelling, mechanical properties and water barrier properties. (2) Triglycerides can form intermolecular hydrogen bonds with chitosan and palmitic acid, making the film structure denser, significantly improving the mechanical properties of the composite cling film, having excellent tensile strength and elongation at break, making the cling film more resilient, and at the same time making the cling film have a lower water vapor permeability, improving the water barrier properties of the cling film, and can effectively prevent the loss of nutrients and extend the shelf life of fruits and vegetables. It meets the requirements of GB / T10457-2021 "General Rules for Quality of Plastic Self-Adhesive Cling Film for Food". (3) Palmitic acid molecules contain hydrophilic groups (-COOH) and have a large swelling degree in water. This makes the present invention have a swelling degree superior to the existing technology level while having good mechanical properties, providing favorable conditions for degradation reactions by light or microorganisms. This shows that the polymer chains in the composite cling film are more easily degraded into small molecular weight compounds, which can effectively shorten the degradation cycle of the cling film. (4) The process of the present invention is simple, does not involve toxic substances in the production process, is green and safe, and is suitable for promotion of industrial production.

[0020] Table 1 Performance comparison of the preservative film prepared by this method and the prior art

[0021]

[0022] DETAILED DESCRIPTION

[0023] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0024] Example 1

[0025] 1. Dissolve chitosan in a 1% by volume glacial acetic acid aqueous solution to obtain a chitosan solution, wherein the ratio of chitosan to glacial acetic acid aqueous solution is 0.5 g:100 mL.

[0026] 2. Add triglyceride additive solution and palmitic acid additive solution to the chitosan solution and stir to obtain a membrane solution at a stirring speed of 600 r / min and a stirring time of 0.5 h. The triglyceride additive solution is a mixture of triglyceride and ethylene glycol, wherein the ratio of triglyceride to ethylene glycol is 50 mg:100 mL, and the ratio of triglyceride additive solution to chitosan solution is 3.0 mL:100 mL. The palmitic acid additive solution is a mixture of palmitic acid and ethanol, wherein the ratio of palmitic acid to ethanol is 1 g:30 mL, and the ratio of palmitic acid additive solution to chitosan solution is 0.3 mL:100 mL.

[0027] 3. Ultrasonic degassing of the mixed membrane solution was performed and then evenly applied to a flat container. After drying, the membrane solution was separated from the flat container to obtain a composite membrane. The membrane solution was ultrasonically degassed for 0.5 h at an ultrasonic frequency of 35 kHz. The membrane solution was dried at 30°C for 15 h.

[0028] Example 2

[0029] 1. Dissolve chitosan in a 3% by volume glacial acetic acid aqueous solution to obtain a chitosan solution, wherein the amount ratio of chitosan to glacial acetic acid aqueous solution is 2.0 g:100 mL.

[0030] 2. Add 6.0 mL of triglyceride additive solution and 0.6 mL of palmitic acid additive solution to the chitosan solution and stir to obtain a membrane solution at a stirring speed of 800 r / min for 1.5 hours. The triglyceride additive solution is a mixture of triglyceride and ethylene glycol, wherein the ratio of triglyceride to ethylene glycol is 150 mg:100 mL, and the ratio of triglyceride additive solution to chitosan solution is 6.0 mL:100 mL. The palmitic acid additive solution is a mixture of palmitic acid and ethanol, wherein the ratio of palmitic acid to ethanol is 3 g:30 mL, and the ratio of palmitic acid additive solution to chitosan solution is 0.6 mL:100 mL.

[0031] 3. Ultrasonic degassing of the mixed membrane solution was performed and then evenly applied to a flat container. After drying, the membrane solution was separated from the flat container to obtain a composite membrane. The membrane solution was ultrasonically degassed for 1.5 hours at an ultrasonic frequency of 45 kHz. The membrane solution was dried at a temperature of 55°C for 25 hours.

[0032] Example 3

[0033] 1. Dissolve chitosan in a 2% by volume glacial acetic acid aqueous solution to obtain a chitosan solution, wherein the amount ratio of chitosan to glacial acetic acid aqueous solution is 1 g:100 mL.

[0034] 2. Add triglyceride additive solution and palmitic acid additive solution to the chitosan solution and stir to obtain a membrane solution at a stirring speed of 700 r / min and a stirring time of 1.0 h. The triglyceride additive solution is a mixture of triglyceride and ethylene glycol, wherein the ratio of triglyceride to ethylene glycol is 100 mg:100 mL, and the ratio of triglyceride additive solution to chitosan solution is 5.0 mL:100 mL. The palmitic acid additive solution is a mixture of palmitic acid and ethanol, wherein the ratio of palmitic acid to ethanol is 2 g:30 mL, and the ratio of palmitic acid additive solution to chitosan solution is 0.5 mL:100 mL.

[0035] 3. Ultrasonic degassing of the mixed membrane solution was performed, followed by transfer of a predetermined amount to a flat container. After drying, the membrane solution was separated from the flat container to obtain a composite membrane. The membrane solution was ultrasonically degassed for 1 hour at a frequency of 40 kHz. The membrane solution was dried at a temperature of 40°C for 20 hours.

[0036] Performance test of chitosan composite cling film: (1) Determination of swelling degree: After the cling film is dried to constant weight, it is placed in a beaker and covered with 50 mL of distilled water with plastic wrap. After 24 hours, the wet film is taken out and weighed. The swelling degree is calculated using the difference in mass between the two times. (2) Determination of transparency: The transmittance is measured at 800 nm using a UV-visible spectrophotometer. (3) Determination of water vapor permeability: Dry anhydrous calcium chloride is placed in a weighing bottle to provide an environment with a relative humidity of 0%. The weighing bottle mouth is sealed with a dry film and weighed. Then, a certain vapor pressure difference is maintained on both sides of the composite film. After 24 hours, it is weighed again. The water vapor permeability is calculated using the difference in mass between the two times. (4) Determination of mechanical properties: The tensile strength and elongation at break of the film are measured using a universal material testing machine. The initial clamping distance is 5 cm and the extension speed is 1 mm / s.

[0037] Table 2 Comparison of different properties between the embodiment and the comparative example

[0038]

[0039] Comparing each example with several other types of composite cling films listed in Table 1 reveals that the chitosan / gallic acid composite film exhibits superior mechanical properties but poor water barrier and swelling performance. While the chitosan / pectin composite film exhibits excellent water barrier and swelling performance, its mechanical properties do not meet national standards. While meeting the mechanical performance requirements of the PBAT index in GB / T 10457-2021, "General Specification for Quality of Plastic Self-Adhesive Cling Films for Food Use," each example exhibits balanced and excellent swelling, water barrier, and mechanical properties. This is likely due to hydrogen bonding between chitosan, palmitic acid, and triglyceride molecules, which creates a tighter intermolecular connection and, in turn, a denser film structure, effectively inhibiting water diffusion and improving mechanical and water barrier properties. This also confirms the potential of triglycerides as plasticizers. In addition, the palmitic acid molecule contains a hydrophilic group (-COOH) and has a large degree of swelling in water. Therefore, each embodiment has excellent mechanical properties and water barrier properties while having a better swelling degree, which is beneficial to its subsequent light or biodegradation reaction and shortens the degradation cycle. The reason why the palmitic acid addition amount in Example 2 is the largest and the swelling degree is reduced may be that the increase in the amount of triglyceride added increases the intermolecular force, and the solvent molecules are blocked from entering the interior of the film molecules, which leads to a decrease in swelling degree. In summary, after the synergistic modification of palmitic acid and triglycerides, the various properties of the chitosan composite fresh-keeping film are significantly improved, which is in line with the broad prospects for industrial production.

[0040] The technical solutions disclosed and proposed by the present invention can be implemented by those skilled in the art by drawing on the content of this document and appropriately changing the conditions, routes, and other aspects. Although the methods and preparation techniques of the present invention have been described through preferred embodiments, it is obvious that those skilled in the art can modify or recombine the methods and technical routes described herein without departing from the content, spirit, and scope of the present invention to achieve the ultimate preparation technology. It is particularly important to point out that all similar substitutions and modifications that are obvious to those skilled in the art are considered to be included in the spirit, scope, and content of the present invention.

Claims

1. A method for preparing a modified chitosan-based degradable composite fresh-keeping film, characterized in that: The following steps are involved: (1) dissolving chitosan in an aqueous solution of glacial acetic acid to obtain a chitosan solution; (2) mixing the chitosan solution, triglyceride additive solution, and palmitic acid additive solution and stirring them uniformly to prepare a membrane solution; (3) The membrane liquid is ultrasonically degassed and then evenly coated on a flat container; the membrane liquid is dried and separated from the flat container to obtain a modified chitosan composite fresh-keeping film.

2. The method for preparing the modified chitosan-based degradable composite fresh-keeping film according to claim 1, wherein: In the step (1), the ratio of chitosan to glacial acetic acid aqueous solution is (0.5-2.0) g:100 mL.

3. The method for preparing the modified chitosan-based degradable composite fresh-keeping film according to claim 1, wherein: In the step (1), chitosan is dissolved in a glacial acetic acid aqueous solution with a volume fraction of 1-3%.

4. The method for preparing the modified chitosan-based degradable composite fresh-keeping film according to claim 1, wherein: In the step (2), the triglyceride additive solution is a mixture of triglyceride and ethylene glycol, the dosage ratio of triglyceride to ethylene glycol is (50-150) mg:100 mL, and the dosage ratio of the triglyceride additive solution to the chitosan dissolving solution is (3.0-6.0) mL:100 mL.

5. The method for preparing the modified chitosan-based degradable composite fresh-keeping film according to claim 1, wherein: In the step (2), the palmitic acid additive is a mixture of palmitic acid and ethanol, the dosage ratio of palmitic acid to ethanol is (1-3) g:30 mL, and the dosage ratio of palmitic acid additive to chitosan dissolving solution is (0.3-0.6) mL:100 mL.

6. The method for preparing the modified chitosan-based degradable composite fresh-keeping film according to claim 1, wherein: In the step (2), the stirring speed is 600-800 r / min and the stirring time is 0.5-1.5 h.

7. The method for preparing the modified chitosan-based degradable composite fresh-keeping film according to claim 1, wherein: In the step (3), the membrane liquid ultrasonic degassing time is 0.5-1.5h, and the ultrasonic frequency is 35-45kHz.

8. The method for preparing the modified chitosan-based degradable composite fresh-keeping film according to claim 1, wherein: In the step (3), the membrane liquid is dried at a temperature of 30-55° C. and for a time of 15-25 h.

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