Preparation method and application of multi-response polymer fresh water absorption pad of sandwich-like structure

Abstract

The invention belongs to the technical field of intelligent controlled-release antibacterial packaging, and particularly relates to a preparation method and application of a multi-response polymer fresh water absorption pad of a sandwich-like structure. The preparation method comprises the following steps: firstly, preparing a hydrophilic inner-layer spinning solution, a humidity response middle-layer spinning solution and a pH response outer-layer spinning solution; the hydrophilic inner layer spinning solution, the humidity response middle layer spinning solution and the pH response outer layer spinning solution are subjected to continuous electrostatic spinning to prepare a composite membrane of a three-layer nanofiber structure, namely the multi-response polymer fresh water absorption pad of a sandwich-like structure. Through the layered design, the water absorption pad integrates the functions of one-way moisture transmission, antibiosis and corrosion prevention, the preservation period of food is prolonged, self-adaptive adjustment can be achieved according to the humidity and pH change in the storage environment, the dynamic requirements of different food storage conditions are met, and the more accurate preservation effect is provided. And meanwhile, various food packaging requirements can be met, food loss is effectively reduced, economic benefits are improved, and the application prospect is wide.

Description

Technical Field

[0001] The present invention belongs to the technical field of intelligent controlled-release antibacterial packaging, and specifically relates to a preparation method and application of a multi-responsive polymer fresh water absorbent pad with a "sandwich" structure. Background Art

[0002] With the improvement of living standards, consumers have increasingly stringent requirements for food quality, especially in the field of fresh fruits, vegetables and meat preservation. How to effectively extend the shelf life of food, reduce spoilage and maintain its nutritional content and sensory quality has become a key issue that the food industry needs to solve urgently. Among them, moisture content is a key factor in determining food quality and stability. Fresh foods (such as meat, aquatic products, vegetables and fruits) are prone to release liquid during storage and transportation. Excessive moisture provides an ideal water activity environment for microorganisms such as bacteria and molds, thereby accelerating food spoilage. In addition, excess moisture can cause fruits and vegetables to soften, meat tissue to become sticky or water loss, directly affecting the appearance and taste of food, thereby reducing consumer acceptance. More importantly, nutrients in food (such as vitamin C, vitamin E and unsaturated fatty acids) are more susceptible to oxidative damage in a humid environment. The increase in moisture will enhance the activity of oxidants (such as oxygen, free radicals), thereby accelerating the degradation of nutrients, further affecting the shelf life and nutritional value of food.

[0003] To address this problem, fresh food absorbent pads have been introduced on the market as a common solution. Commercially available fresh food absorbent pads are usually made of cellulose or other highly absorbent materials, have good water absorption capacity, can effectively absorb excess liquid, but have no fresh-keeping and antibacterial effects. For example, patent CN219970627U discloses a polymer fresh food absorbent pad that can quickly absorb the moisture generated during the storage of fresh food, prevent liquid from overflowing, and keep the food clean and beautiful. However, this type of absorbent pad still has some limitations. Its water absorption and retention capacity is limited, especially when processing foods with high liquid content (such as meat and aquatic products), once the absorbent pad reaches a saturated state, the liquid may seep back to the surface of the food, which accelerates corruption. There are also literature reports that antibacterial materials are added to the absorbent pad to achieve antibacterial effects, but the active materials in the absorbent pad loaded with antibacterial materials are mainly released from the polymer surface through a diffusion mechanism, and the release power mainly depends on the concentration gradient. This single release mode cannot fully match the food preservation needs and lacks the ability to dynamically adjust the release rate according to changes in the storage environment, resulting in low utilization of antibacterial agents and may cause drug resistance problems. Summary of the invention

[0004] In view of the shortcomings of the prior art, the present invention aims to solve the technical problems that the existing fresh food absorbent pads have limited water absorption and water holding capacity, and cannot be dynamically adjusted according to the quality changes of fresh products during storage. The present invention provides a preparation method and application of a multi-responsive polymer fresh food absorbent pad based on a "sandwich" structure. The prepared multi-responsive polymer fresh food absorbent pad with a "sandwich" structure is intended to achieve moisture management and intelligent active substance release. Through layered design, the absorbent pad integrates unidirectional moisture transmission, antibacterial and antiseptic functions, which not only extends the shelf life of food, but also can be adaptively adjusted according to the humidity and pH changes in the storage environment to meet the dynamic needs of different food storage conditions and provide a more accurate preservation effect. At present, there is no technical disclosure that can integrate absorbent pads and precise release of biological antibacterial materials. In addition, the asymmetric wettability design of the "sandwich" structure ensures unidirectional moisture transmission, making moisture management more efficient, thereby further improving the preservation performance of food.

[0005] The present invention achieves the above technical objectives through the following technical means:

[0006] A method for preparing a multi-responsive polymer fresh water absorbent pad having a "sandwich" structure, comprising the following steps:

[0007] Step 1, preparation of hydrophilic inner layer spinning solution;

[0008] Polyvinyl alcohol (PVA) is dissolved in deionized water, heated in a water bath and stirred until completely dissolved to obtain a PVA solution; sodium alginate (SA) is dissolved in deionized water, stirred at room temperature until completely dissolved to obtain a SA solution;

[0009] The PVA solution and the SA solution are mixed to obtain a mixed solution; the mixed solution is then placed in an oven to stand, and the mixed solution after standing is subjected to ultrasonic dispersion for degassing treatment, and a hydrophilic inner layer spinning solution is obtained after the treatment;

[0010] Furthermore, in step one, the dosage ratio of polyvinyl alcohol to deionized water is 0.8-1.4 g:10 mL; the dosage ratio of sodium alginate to deionized water is 0.02-0.05 g:10 mL; the temperature of the water bath heating is 70-90° C.; and the volume ratio of the PVA solution to the SA solution is 4-6:1.

[0011] Furthermore, in step 1, the temperature of the oven is 35-50° C., and the standing time is 8 hours; the ultrasonic dispersion is performed by ultrasonic cell crusher with a power of 50-60W, a working time interval of 1s, and a processing time of 0.5-1h.

[0012] Step 2: Preparation of humidity responsive layer spinning solution;

[0013] Ethylene-vinyl alcohol copolymer (EVOH) is dissolved in glacial acetic acid and stirred in a water bath until completely dissolved to obtain an EVOH solution; essential oil is dissolved in an ethanol solution to obtain a mixed solution, which is recorded as solution A; the EVOH solution is cooled to room temperature and mixed with solution A to obtain a mixed solution, which is recorded as solution B, and the mixed solution B is ultrasonically dispersed to obtain a solution that is a humidity response layer spinning solution;

[0014] Furthermore, in step 2, the mass fraction of the EVOH solution is 10-12wt%, the volume concentration of glacial acetic acid is 50%; the mass fraction of solution A is 1.5-3wt%; the temperature of the water bath heating is 80-90°C;

[0015] Furthermore, in step 2, the essential oil described in step 2 is any one or more of oregano essential oil, cinnamon essential oil, eugenol essential oil, clove essential oil, fennel essential oil or thyme essential oil; the volume concentration of the ethanol solution is 50%; the volume ratio of the solution A and the EVOH solution is 1:10-15.

[0016] Furthermore, the ultrasonic dispersion in step 2 is performed by ultrasonic treatment using an ultrasonic cell pulverizer with a power of 50 to 60 W, a working time interval of 1 s, and a treatment time of 0.5 to 1 h.

[0017] Step 3: preparing pH response layer spinning solution;

[0018] Dissolve corn alcohol-soluble protein (Zein) in glacial acetic acid, and stir at room temperature until it is completely dissolved to obtain a Zein solution; mix the Zein solution with the solution A obtained in step 2, and the obtained mixed solution is recorded as solution C, and ultrasonic dispersion treatment is performed on solution C, and the treated solution is the pH response layer spinning solution;

[0019] Furthermore, in step three, the mass fraction of the Zein solution is 15-30wt%, and the volume concentration of glacial acetic acid is 50%; the volume ratio of the solution A and the Zein solution is 1:5-10; the ultrasonic dispersion treatment is performed using an ultrasonic cell crusher with a power of 50-60W, a working time interval of 1s, and a treatment time of 0.5-1h.

[0020] Step 4: Preparation of a multi-responsive polymer fresh water absorbent pad with a "sandwich" structure;

[0021] The hydrophilic inner layer spinning solution, the humidity responsive middle layer spinning solution and the pH responsive outer layer spinning solution are spun in sequence by electrospinning to prepare a composite membrane with a three-layer nanofiber structure, that is, a multi-responsive polymer fresh water absorbent pad with a "sandwich" structure; wherein the electrospinning can be carried out layer by layer in a forward or reverse order, that is, the inner layer is first spun with the hydrophilic inner layer spinning solution, and then the inner layer is spun with the humidity responsive middle layer spinning solution, and finally the outer layer is spun with the pH responsive outer layer spinning solution; or the outer layer is first spun with the pH responsive outer layer spinning solution, and then the inner layer is spun with the humidity responsive middle layer spinning solution, and finally the inner layer is spun with the hydrophilic inner layer spinning solution.

[0022] The specific steps are as follows:

[0023] First, the hydrophilic inner layer spinning solution prepared in step 1 is loaded into a syringe and installed on an electrospinning machine; the spinning high voltage power supply voltage and the spinning solution advancement rate are set to perform electrospinning; the spun nanofibers are received by oil paper on a rotating drum, and a hydrophilic inner layer fiber membrane is obtained after the solution advancement is completed;

[0024] After the spinning of the hydrophilic inner layer is completed, the humidity-responsive middle layer spinning solution prepared in step 2 is loaded into a syringe, the spinning high-voltage power supply voltage and the spinning solution advancement rate are set, and the humidity-responsive layer electrostatic spinning is performed; the hydrophilic inner layer fiber membrane is used to receive the spun nanofibers, and the nanofibers are spun on the surface of the hydrophilic inner layer fiber membrane to form a humidity-responsive middle layer membrane; after the spinning is completed, a double-layer fiber membrane including a hydrophilic inner layer fiber membrane and a humidity-responsive middle layer membrane is obtained;

[0025] After the double-layer fiber membrane is completed, the pH-responsive outer layer spinning solution prepared in step three is loaded into a syringe, the spinning high-voltage power supply voltage and the spinning solution advancement rate are set, and the pH-responsive outer layer electrospinning is performed; the double-layer fiber membrane is used to receive the spun nanofibers, which are spun on the surface of the double-layer fiber membrane to form a pH-responsive outer layer membrane; after spinning is completed, a multi-responsive polymer fresh water-absorbing pad with a "sandwich" structure is obtained.

[0026] Furthermore, in step four, the high-voltage power supply voltage for electrospinning the hydrophilic inner layer spinning solution is 20-23 kV, and the propulsion rate is 0.5-0.7 mL / h; the high-voltage power supply voltage for electrospinning the humidity-responsive middle layer spinning solution is 10-15 kV, and the propulsion rate is 0.3-0.5 mL / h; the high-voltage power supply voltage for electrospinning the pH-responsive outer layer spinning solution is 25-29 kV, and the propulsion rate is 0.7 mL / h; the electrospinning needles are all 20G, the spinning temperature is 20°C, and the relative humidity is 40%.

[0027] The prepared "sandwich" structured multi-responsive polymer fresh water absorbent pad has a three-layer structure, which, from inside to outside, consists of a hydrophilic inner fiber membrane (water-absorbing inner layer), a humidity-responsive middle layer membrane (humidity-responsive layer), and a pH-responsive outer layer membrane (pH-responsive layer).

[0028] The multi-responsive polymer fresh water absorbent pad based on the prepared "sandwich" structure is used for the preservation of fresh food.

[0029] Furthermore, the fresh food described in step 4 includes fresh meat (including pork, beef, chicken, fish), aquatic products (including shrimp, crab, shellfish and other seafood), fruits and vegetables (strawberries, cherries, watermelons, tomatoes, cucumbers, lettuce, etc.), poultry eggs (eggs) and processed foods (cured meats, sliced ​​fruits and pre-made salads, etc.).

[0030] The prepared multi-responsive polymer fresh water absorbent pad with a "sandwich" structure can also be used to prepare packaging materials, including packaging boxes and packaging bags.

[0031] The multi-response polymer fresh water-absorbing pad of the "sandwich" structure prepared by the present invention is composed of an inner layer, an intermediate layer and an outer layer. The inner layer is a hydrophilic inner layer fiber membrane, which is composed of a highly absorbent material, is in direct contact with food, can quickly absorb surface moisture, and directs moisture into the intermediate layer through its strong hydrophilicity, reduces local humidity, prevents liquid back-seepage, and inhibits microbial growth. The intermediate layer is a humidity-responsive intermediate layer membrane, which is composed of a humidity-responsive polymer (ethylene / vinyl alcohol copolymer) and a biomass antibacterial material. Under conditions of high humidity, the ethylene / vinyl alcohol copolymer will deform in this humidity range, reducing the release resistance of the polymer to active substances such as essential oils, promoting the accelerated release of antibacterial agents, inhibiting the further reproduction of microorganisms, and extending the shelf life of food. The outer layer is a pH-responsive outer layer membrane, which is composed of a pH-responsive polymer (zein) and a biomass antibacterial material. Zein can sense pH changes caused by microbial activity or chemical reactions in a food storage environment, trigger the structural adjustment of molecular chains, further enhance the release of antibacterial agents, and provide additional protection in the early stage of food deterioration, thereby extending the edible period of food. In addition, the unique asymmetric wettability of the sandwich structure ensures the unidirectional transmission of water and the intelligent release of antimicrobial agents, effectively regulates the moisture content in food, and achieves the precise release of biomass antibacterial materials, further optimizing the preservation effect.

[0032] Beneficial effects of the present invention:

[0033] (1) The present invention utilizes an asymmetric wettability design of a three-layer structure to enable the absorbent pad to have a one-way moisture transfer function, which can quickly absorb excess liquid on the surface of food and lock it inside the absorbent pad to prevent the liquid from seeping back to the surface of the food. This design not only keeps the surface of the food dry, but also inhibits the growth of microorganisms, thereby effectively extending the shelf life of the food; at the same time, it maintains good sensory quality and enhances the consumer experience.

[0034] (2) The present invention introduces a humidity-responsive polymer ethylene-vinyl alcohol copolymer (EVOH) into the middle layer. The polar hydroxyl group in its chemical structure gives it a high affinity for water molecules. When the ambient humidity increases, water molecules combine with the hydroxyl groups in EVOH through hydrogen bonds, prompting EVOH to absorb moisture. This water absorption process causes changes in the physical properties of the material. The molecular chains of EVOH relax and expand, thereby increasing its permeability and promoting the release of the loaded antimicrobial agent. In addition, the glass transition temperature of EVOH is greatly affected by humidity. Under dry conditions, EVOH is in a hard and brittle glass state, but when the humidity increases, the absorbed moisture reduces the interaction between the molecular chains, causing EVOH to gradually become soft and transform from a glass state to a rubber state. This structural change further reduces the polymer's resistance to the release of active substances, triggering the release of antimicrobial agents, thereby effectively inhibiting the growth of microorganisms. The design can intelligently respond to humidity fluctuations and changes in the moisture content of the food itself during storage and sales, inhibiting the rapid reproduction of microorganisms. The outer layer uses a pH-responsive polymer, zein, which is rich in polar and charged amino acids, especially acidic amino acids with carboxyl side chains. When the pH environment changes, these carboxyl groups are ionized into negative charges (–COO-), which enhances the electrostatic repulsion between molecules and weakens the interaction between molecular chains. The weakening of this interaction triggers structural changes in the Zein molecular chain and accelerates the release rate of antimicrobial ingredients. This mechanism can promote the release of antimicrobial ingredients in the early stages of food spoilage, provide additional protection for food, extend the edible period of food, and realize intelligent antimicrobial and antiseptic functions.

[0035] (3) The present invention cleverly combines moisture management, humidity response and pH response functions through a "sandwich" structure, achieving efficient coordination of the functions of each layer; this design enables the absorbent pad to automatically adjust its performance according to different storage conditions, meet various food packaging needs, effectively reduce food loss, and improve economic benefits, especially showing broad application prospects in the high-end food market.

[0036] (4) The present invention adopts electrostatic spinning technology, which is simple and easy to operate and has good prospects for industrial production. At the same time, the absorbent pad can not only be generated in the form of nanofibers, but can also be directly spun onto a variety of existing packaging materials such as packaging boxes and packaging bags, and can be used in combination with other fresh-keeping technologies such as modified atmosphere packaging to provide a variety of solutions for food preservation; it has various forms, is convenient and has long-term fresh-keeping performance, showing a wide range of application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 Schematic diagram of the multi-responsive polymer fresh water absorbent pad with a "sandwich" structure prepared in the present invention.

[0038] Figure 2 This is a static water drop contact angle diagram of the front and back sides of the fresh food absorbent pad prepared in Example 1 of the present invention.

[0039] Figure 3 Graphs showing the release rates of the essential oil loaded in the humidity-responsive middle layer in Example 2 of the present invention under 30% RH and 95% RH environments, respectively.

[0040] Figure 4 Graphs showing the release rates of the essential oil loaded in the pH-responsive outer layer in Example 3 of the present invention under pH 5.5 and pH 6.5 environments, respectively.

[0041] Figure 5 This is a diagram showing the changes in TVB-N during the actual application of fresh cold meat in Example 3 of the present invention. DETAILED DESCRIPTION

[0042] 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 in conjunction with specific embodiments and with reference to the accompanying drawings.

[0043] like Figure 1 As shown, the multi-responsive polymer fresh water absorbent pad of the present invention has a "sandwich" structure, which, from the inside to the outside, comprises a hydrophilic inner fiber membrane (water-absorbing inner layer), a humidity-responsive middle membrane (humidity-responsive layer) and a pH-responsive outer membrane (pH-responsive layer).

[0044] The inner layer is made of highly absorbent material, which is in direct contact with food. It can quickly absorb surface moisture and direct moisture into the middle layer through its strong hydrophilicity, reducing local humidity, preventing liquid back-seepage, and inhibiting microbial growth. The middle layer is composed of humidity-responsive polymers (ethylene / vinyl alcohol copolymers) and biomass antibacterial materials. Microorganisms reproduce rapidly under high humidity conditions. For example, the active growth humidity of mold is above 75% RH, and bacteria are more likely to reproduce above 80% RH. Ethylene / vinyl alcohol copolymers will deform in this humidity range, reducing the release resistance of the polymer to active substances such as essential oils, promoting the accelerated release of antibacterial agents, inhibiting further reproduction of microorganisms, and extending the shelf life of food. The outer layer is composed of pH-responsive polymers (zein) and biomass antibacterial materials. Zein can sense pH changes caused by microbial activity or chemical reactions in the food storage environment, triggering structural adjustments of molecular chains, further enhancing the release of antibacterial agents, and providing additional protection at the early stage of food deterioration, thereby extending the edible period of food. In addition, the unique asymmetric wettability of the sandwich structure ensures the unidirectional transmission of water and the intelligent release of antimicrobial agents, effectively regulates the moisture content in food, and achieves the precise release of biomass antibacterial materials, further optimizing the preservation effect.

[0045] Embodiment 1:

[0046] A method for preparing a multi-responsive polymer fresh water absorbent pad having a "sandwich" structure, comprising the following steps:

[0047] Step 1, preparation of hydrophilic inner layer spinning solution;

[0048] Take 0.8g PVA and dissolve it in 10mL deionized water, heat it in a water bath at 70℃ and stir until it is completely dissolved; take 0.02g SA and dissolve it in 10mL deionized water, stir it at room temperature until it is completely dissolved; blend the above two solutions in a volume ratio of 4:1, place the mixed solution in a vacuum oven for 8h and deaerate; place the deaerated mixed solution in an ultrasonic cell crusher for ultrasonic dispersion, the ultrasonic treatment power is 50W, the working time interval is 1s, and the treatment time is 0.5h, so as to obtain a hydrophilic inner layer spinning solution.

[0049] Step 2: Preparation of humidity responsive layer spinning solution;

[0050] Take an appropriate amount of EVOH and dissolve it in glacial acetic acid, heat it in a water bath at 80℃ and stir until it is completely dissolved to obtain a 10wt% EVOH solution; take an appropriate amount of oregano essential oil and dissolve it in a 50% ethanol solution to obtain a 1.5wt% oregano essential oil ethanol solution. Cool the EVOH solution to room temperature, and blend the oregano essential oil ethanol solution and the EVOH solution in a volume ratio of 1:10. Use an ultrasonic cell crusher to ultrasonically disperse the solution, the ultrasonic treatment power is 50W, the working time interval is 1s, and the treatment time is 0.5h. The solution obtained is the humidity response layer spinning solution.

[0051] Step 3: preparing pH response layer spinning solution;

[0052] Take an appropriate amount of Zein and dissolve it in glacial acetic acid with a volume concentration of 50%, stir it at room temperature until it is completely dissolved, and obtain a 15wt% Zein solution; blend the Zein solution with the oregano essential oil ethanol solution in a volume ratio of 1:5. Use an ultrasonic cell crusher to perform ultrasonic dispersion treatment on the solution, with an ultrasonic treatment power of 50W, a working time interval of 1s, and a treatment time of 0.5h. The obtained solution is the pH response layer spinning solution.

[0053] Step 4: Preparation of a multi-responsive polymer fresh water absorbent pad with a "sandwich" structure;

[0054] The fresh food absorbent pad consists of a hydrophilic inner layer, a humidity responsive middle layer and a pH responsive outer layer.

[0055] First, the hydrophilic inner layer spinning solution prepared in step 1 is loaded into a syringe and installed on an electrospinning machine; the high voltage power supply voltage is set to 20 kV and the propulsion rate is set to 0.5 mL / h to perform electrospinning; the nanofibers spun out are received by oil paper on a rotating drum, and a hydrophilic inner layer fiber membrane is obtained after the solution propulsion is completed;

[0056] After the spinning of the hydrophilic inner layer is completed, the humidity-responsive middle layer spinning solution prepared in step 2 is loaded into the syringe, the high-voltage power supply voltage is set to 10 kV, and the propulsion rate is set to 0.3 mL / h to perform humidity-responsive layer electrospinning; the hydrophilic inner layer fiber membrane is used to receive the spun nanofibers, and the nanofibers are spun on the surface of the hydrophilic inner layer fiber membrane to form a humidity-responsive middle layer membrane; after the spinning is completed, a double-layer fiber membrane including a hydrophilic inner layer fiber membrane and a humidity-responsive middle layer membrane is obtained;

[0057] After the double-layer fiber membrane is completed, the pH-responsive outer layer spinning solution prepared in step 3 is loaded into a syringe, the high-voltage power supply voltage is set to 25 kV, the propulsion rate is 0.7 mL / h, and the pH-responsive outer layer electrospinning is performed; the double-layer fiber membrane is used to receive the spun nanofibers, and the nanofibers are spun on the surface of the double-layer fiber membrane to form a pH-responsive outer layer membrane; after the spinning is completed, a nanofiber film composed of a hydrophilic inner layer membrane, a humidity-responsive middle layer membrane, and a pH-responsive outer layer membrane is obtained, which is a multi-responsive polymer fresh water absorbent pad of a "sandwich" structure. The multi-responsive polymer fresh water absorbent pad prepared in Example 1 is cut into a 15 cm × 15 cm rectangle for standby use.

[0058] In order to verify the dynamic water transport behavior of the "sandwich" structure fresh water absorbent pad, a syringe was used to drop 100 μL droplets on the hydrophilic inner layer and pH responsive outer layer of the multi-responsive polymer fresh water absorbent pad, and its static diffusion was observed. Figure 2 As shown in the figure, the droplets spread rapidly on the inner surface due to its good hydrophilicity and hygroscopicity, without penetration or dripping. On the outer layer, due to the hydrophobicity of Zein, the droplets did not spread and maintained their original shape. This shows that the prepared fresh water absorbent pad has asymmetric wettability, and the presence of water droplets does not affect the structural morphology of the water absorbent pad.

[0059] Embodiment 2:

[0060] A method for preparing a multi-responsive polymer fresh water absorbent pad having a "sandwich" structure, comprising the following steps:

[0061] Step 1: Preparation of hydrophilic inner layer spinning solution

[0062] Take 1.4g PVA and dissolve it in 10mL deionized water, heat it in a water bath at 90℃ and stir until it is completely dissolved; take 0.05g SA and dissolve it in 10mL deionized water, stir it at room temperature until it is completely dissolved; blend the above two solutions in a volume ratio of 6:1, place the mixed solution in a vacuum oven and let it stand for 8h and degas; place the degassed mixed solution in an ultrasonic cell crusher for ultrasonic dispersion, with an ultrasonic treatment power of 60W, a working time interval of 1s, and a treatment time of 1h. The hydrophilic inner layer spinning solution is obtained.

[0063] Step 2: Preparation of Humidity Response Layer Spinning Solution

[0064] Take an appropriate amount of EVOH and dissolve it in glacial acetic acid, heat it in a water bath at 80℃ and stir it until it is completely dissolved to obtain a 12wt% EVOH solution; take an appropriate amount of cinnamon essential oil and dissolve it in a 50% ethanol solution to obtain a 3wt% cinnamon essential oil ethanol solution. Cool the EVOH solution to room temperature, and blend the cinnamon essential oil ethanol solution and the EVOH solution in a volume ratio of 1:15. Use an ultrasonic cell crusher to ultrasonically disperse the solution, the ultrasonic treatment power is 60W, the working time interval is 1s, and the treatment time is 1h. The solution obtained is the humidity response layer spinning solution.

[0065] Step 3: Preparation of pH responsive layer spinning solution

[0066] Take an appropriate amount of Zein and dissolve it in 50% glacial acetic acid. Stir it at room temperature until it is completely dissolved to obtain a 30wt% Zein solution. Blend the Zein solution with the cinnamon essential oil ethanol solution in a volume ratio of 1:10. Use an ultrasonic cell crusher to perform ultrasonic dispersion treatment on the solution. The ultrasonic treatment power is 60W, the working time interval is 1s, and the treatment time is 1h. The obtained solution is the pH response layer spinning solution.

[0067] Step 4: Preparation of a multi-responsive polymer fresh water absorbent pad with a "sandwich" structure

[0068] The fresh food absorbent pad consists of a hydrophilic inner layer, a humidity responsive middle layer and a pH responsive outer layer.

[0069] First, the hydrophilic inner layer spinning solution prepared in step 1 is loaded into a syringe and installed on an electrospinning machine; the high voltage power supply voltage is set to 23 kV and the propulsion rate is set to 0.7 mL / h to perform electrospinning; the nanofibers spun out are received by oil paper on a rotating drum, and a hydrophilic inner layer fiber membrane is obtained after the solution propulsion is completed;

[0070] After the spinning of the hydrophilic inner layer is completed, the humidity-responsive middle layer spinning solution prepared in step 2 is loaded into the syringe, the high-voltage power supply voltage is set to 15 kV, and the propulsion rate is set to 0.5 mL / h to perform humidity-responsive layer electrospinning; the hydrophilic inner layer fiber membrane is used to receive the spun nanofibers, and the nanofibers are spun on the surface of the hydrophilic inner layer fiber membrane to form a humidity-responsive middle layer membrane; after the spinning is completed, a double-layer fiber membrane including a hydrophilic inner layer fiber membrane and a humidity-responsive middle layer membrane is obtained;

[0071] After the double-layer fiber membrane is completed, the pH-responsive outer layer spinning solution prepared in step three is loaded into a syringe, the high-voltage power supply voltage is set to 29 kV, and the propulsion rate is 0.7 mL / h to perform pH-responsive outer layer electrospinning; the double-layer fiber membrane is used to receive the spun nanofibers, which are spun on the surface of the double-layer fiber membrane to form a pH-responsive outer layer membrane; after spinning, a nanofiber film composed of a hydrophilic inner layer membrane, a humidity-responsive middle layer membrane and a pH-responsive outer layer membrane is obtained, which is a multi-responsive polymer fresh water-absorbing pad with a "sandwich" structure.

[0072] In order to verify the humidity responsiveness of the "sandwich" structure fresh water absorbent pad, the humidity responsive layer was prepared separately using electrospinning technology, and the cumulative release rate of essential oil under different humidity conditions (30% RH and 95% RH) in an ambient temperature of 25°C was measured using UV-visible spectrophotometry. Figure 3 As shown in the figure, with the increase of relative humidity (RH), the chain mobility of EVOH also increases, which reduces the diffusion resistance of essential oils and thus improves the diffusion capacity of essential oils. Under 90% RH conditions, the essential oil release rate of nanofibers is significantly higher than that under 30% RH conditions, indicating that the increase in humidity effectively enhances the release performance of essential oils.

[0073] Embodiment 3:

[0074] A method for preparing a multi-responsive polymer fresh water absorbent pad having a "sandwich" structure, comprising the following steps:

[0075] Step 1: Preparation of hydrophilic inner layer spinning solution

[0076] Take 1g PVA and dissolve it in 10mL deionized water, heat it in a water bath at 85℃ and stir until it is completely dissolved; take 0.035g SA and dissolve it in 10mL deionized water, stir it at room temperature until it is completely dissolved; blend the above two solutions in a volume ratio of 5:1, place the mixed solution in a vacuum oven and let it stand for 8h and degas; place the degassed mixed solution in an ultrasonic cell crusher for ultrasonic dispersion, the ultrasonic treatment power is 55W, the working time interval is 1s, and the treatment time is 0.5h. The hydrophilic inner layer spinning solution is obtained.

[0077] Step 2: Preparation of Humidity Response Layer Spinning Solution

[0078] Take an appropriate amount of EVOH and dissolve it in glacial acetic acid, heat it in a water bath at 85℃ and stir until it is completely dissolved to obtain an 11wt% EVOH solution; take an appropriate amount of cinnamon essential oil and dissolve it in a 50% ethanol solution to obtain a 2wt% cinnamon essential oil ethanol solution. Cool the EVOH solution to room temperature, and blend the cinnamon essential oil ethanol solution and the EVOH solution in a volume ratio of 1:13. Use an ultrasonic cell crusher to ultrasonically disperse the solution, the ultrasonic treatment power is 55W, the working time interval is 1s, and the treatment time is 0.5h. The solution obtained is the humidity response layer spinning solution.

[0079] Step 3: Preparation of pH responsive layer spinning solution

[0080] Take an appropriate amount of Zein and dissolve it in 50% glacial acetic acid. Stir it at room temperature until it is completely dissolved to obtain a 25wt% Zein solution. Blend the Zein solution with the cinnamon essential oil ethanol solution in a volume ratio of 1:7. Use an ultrasonic cell crusher to perform ultrasonic dispersion treatment on the solution. The ultrasonic treatment power is 55W, the working time interval is 1s, and the treatment time is 0.5h. The obtained solution is the pH response layer spinning solution.

[0081] Step 4: Preparation of a "sandwich" structure fresh water absorbent pad

[0082] Step 4, preparation of a multi-responsive polymer fresh water absorbent pad with a "sandwich" structure; first, the hydrophilic inner layer spinning solution prepared in step 1 is loaded into a syringe and installed on an electrospinning machine; the high voltage power supply voltage is set to 22 kV and the propulsion rate is set to 0.6 mL / h to perform electrospinning; the oil paper on the drum is used to receive the spun nanofibers, and the hydrophilic inner layer fiber membrane is obtained after the solution propulsion is completed;

[0083] After the spinning of the hydrophilic inner layer is completed, the humidity-responsive middle layer spinning solution prepared in step 2 is loaded into the syringe, the high-voltage power supply voltage is set to 13 kV, and the propulsion rate is set to 0.4 mL / h to perform humidity-responsive layer electrospinning; the hydrophilic inner layer fiber membrane is used to receive the spun nanofibers, and the nanofibers are spun on the surface of the hydrophilic inner layer fiber membrane to form a humidity-responsive middle layer membrane; after the spinning is completed, a double-layer fiber membrane including a hydrophilic inner layer fiber membrane and a humidity-responsive middle layer membrane is obtained;

[0084] After the double-layer fiber membrane is completed, the pH-responsive outer layer spinning solution prepared in step three is loaded into a syringe, the high-voltage power supply voltage is set to 28 kV, and the propulsion rate is 0.7 mL / h to perform pH-responsive outer layer electrospinning; the double-layer fiber membrane is used to receive the spun nanofibers, which are spun on the surface of the double-layer fiber membrane to form a pH-responsive outer layer membrane; after spinning, a nanofiber film composed of a hydrophilic inner layer membrane, a humidity-responsive middle layer membrane and a pH-responsive outer layer membrane is obtained, which is a multi-responsive polymer fresh water-absorbing pad with a "sandwich" structure.

[0085] In order to verify the pH responsiveness of the "sandwich" structure fresh water absorbent pad, the pH responsive layer was prepared separately using electrospinning technology, and the cumulative release rate of essential oil under different pH conditions (pH 5.5 and pH 6.5) at 25°C was measured using UV-visible spectrophotometry. Figure 4 As shown in the figure, in these two buffer solutions with different pH values, the release trend of essential oils is similar, the release amount gradually increases and finally reaches equilibrium. As the pH value increases, the equilibrium time decreases and the final release rate indicates that the prepared nanofiber membrane has significant pH responsiveness.

[0086] At the same time, fresh meat that is susceptible to microbial infection and corruption was taken as the research object to evaluate the preservation effect of the multi-responsive polymer fresh food absorbent pad (taking the product of Example 3 as an example).

[0087] The fresh meat (pork) was sealed in a sterile packaging box with a multi-responsive polymer fresh absorbent pad and stored in a constant temperature and humidity chamber at 4°C and 75% RH to simulate the cold chain storage environment.

[0088] The experiment used fresh absorbent paper purchased from Fufengyuan Company as a control, focusing on testing the total volatile base nitrogen (TVB-N) content of fresh meat during storage as an indicator for evaluating the comprehensive performance of the multi-responsive polymer fresh absorbent pad. Figure 5 As shown in the figure, the TVB-N value of the fresh meat stored using ordinary absorbent pads exceeded the safety upper limit on the 4th day, indicating that the meat had spoiled and lost its sale and edible value; while the TVB-N value of the fresh meat stored using the multi-responsive polymer absorbent pad exceeded the threshold on the 10th day, proving that the multi-responsive polymer fresh absorbent pad with a "sandwich" structure prepared in the project has the effect of extending the shelf life of food.

[0089] Note: The above embodiments are only used to illustrate the present invention and are not intended to limit the technical solutions described in the present invention; therefore, although this specification has described the present invention in detail with reference to the above embodiments, a person skilled in the art should understand that the present invention can still be modified or replaced by equivalents; and all technical solutions and improvements thereof that do not depart from the spirit and scope of the present invention should be included in the scope of the claims of the present invention.

Claims

1. A method for preparing a multi-responsive polymer fresh water absorbent pad with a "sandwich" structure, comprising the following steps: Step 1, preparation of hydrophilic inner layer spinning solution; Dissolve polyvinyl alcohol in deionized water, heat in a water bath and stir until completely dissolved to obtain a PVA solution; dissolve sodium alginate in deionized water, stir at room temperature until completely dissolved to obtain a SA solution; The PVA solution and the SA solution are mixed to obtain a mixed solution; the mixed solution is then placed in an oven to stand, and the mixed solution after standing is subjected to ultrasonic dispersion for degassing treatment, and a hydrophilic inner layer spinning solution is obtained after the treatment; Step 2: Preparation of humidity responsive layer spinning solution; The ethylene-vinyl alcohol copolymer is dissolved in glacial acetic acid and stirred in a water bath until completely dissolved to obtain an EVOH solution; the essential oil is dissolved in an ethanol solution to obtain a mixed solution, which is recorded as solution A; the EVOH solution is cooled to room temperature and mixed with solution A to obtain a mixed solution, which is recorded as solution B; the mixed solution B is ultrasonically dispersed to obtain a solution that is a humidity response layer spinning solution; Step 3: preparing pH response layer spinning solution; Dissolve zein in glacial acetic acid, and stir at room temperature until it is completely dissolved to obtain a Zein solution; mix the Zein solution with the solution A obtained in step 2, and the obtained mixed solution is recorded as solution C, and ultrasonic dispersion treatment is performed on solution C, and the treated solution is the pH response layer spinning solution; Step 4: Preparation of a multi-responsive polymer fresh water absorbent pad with a "sandwich" structure; The hydrophilic inner layer spinning solution, the humidity responsive middle layer spinning solution and the pH responsive outer layer spinning solution are sequentially spun by electrospinning to prepare a composite membrane with a three-layer nanofiber structure, i.e., a multi-responsive polymer fresh water absorbent pad with a "sandwich" structure. The electrospinning can be carried out layer by layer in a forward or reverse order, that is, the inner layer is first spun with a hydrophilic inner layer spinning solution, and then the middle layer is spun with a humidity responsive middle layer spinning solution, and finally the outer layer is spun with a pH responsive outer layer spinning solution; or the outer layer is first spun with a pH responsive outer layer spinning solution, and then the middle layer is spun with a humidity responsive middle layer spinning solution, and finally the inner layer is spun with a hydrophilic inner layer spinning solution.

2. The method for preparing a multi-responsive polymer fresh water absorbent pad with a "sandwich" structure according to claim 1, characterized in that: In step 1, the dosage ratio of polyvinyl alcohol to deionized water is 0.8-1.4 g:10 mL; the dosage ratio of sodium alginate to deionized water is 0.02-0.05 g:10 mL; the water bath heating temperature is 70-90° C.; the volume ratio of the PVA solution to the SA solution is 4-6:

1.

3. The method for preparing a multi-responsive polymer fresh water absorbent pad with a "sandwich" structure according to claim 1, characterized in that: The temperature of the oven in step 1 is 35-50° C., and the standing time is 8 hours; the ultrasonic dispersion is performed by using an ultrasonic cell crusher with an ultrasonic treatment power of 50-60W, a working time interval of 1s, and a treatment time of 0.5-1h.

4. The method for preparing a multi-responsive polymer fresh water absorbent pad with a "sandwich" structure according to claim 1, characterized in that: In step 2, the mass fraction of the EVOH solution is 10-12wt%, the volume concentration of glacial acetic acid is 50%; the mass fraction of solution A is 1.5-3wt%; and the temperature of the water bath heating is 80-90°C.

5. The method for preparing a multi-responsive polymer fresh water absorbent pad with a "sandwich" structure according to claim 1, characterized in that: Further in step 2, the essential oil described in step 2 is any one or more of oregano essential oil, cinnamon essential oil, euphorbia essential oil, clove essential oil, fennel essential oil or thyme essential oil; the volume concentration of the ethanol solution is 50%; the volume ratio of the solution A and the EVOH solution is 1:10-15; the ultrasonic dispersion is performed by ultrasonic cell pulverizer with a power of 50-60W, a working time interval of 1s, and a processing time of 0.5-1h.

6. The method for preparing a multi-responsive polymer fresh water absorbent pad with a "sandwich" structure according to claim 1, characterized in that: In step 3, the mass fraction of the Zein solution is 15-30wt%, and the volume concentration of glacial acetic acid is 50%; the volume ratio of the solution A and the Zein solution is 1:5-10; the ultrasonic dispersion treatment is performed using an ultrasonic cell crusher with a power of 50-60W, a working time interval of 1s, and a treatment time of 0.5-1h.

7. The method for preparing a multi-responsive polymer fresh water absorbent pad with a "sandwich" structure according to claim 1, characterized in that: The preparation steps of the multi-responsive polymer fresh water absorbent pad with a "sandwich" structure in step 4 are as follows: First, the hydrophilic inner layer spinning solution prepared in step 1 is loaded into a syringe and installed on an electrospinning machine; the spinning high voltage power supply voltage and the spinning solution advancement rate are set to perform electrospinning; the spun nanofibers are received by oil paper on a rotating drum, and a hydrophilic inner layer fiber membrane is obtained after the solution advancement is completed; After the spinning of the hydrophilic inner layer is completed, the humidity-responsive middle layer spinning solution prepared in step 2 is loaded into the syringe, the spinning high-voltage power supply voltage and the spinning solution advancement rate are set, and the humidity-responsive layer electrospinning is performed; the hydrophilic inner layer fiber membrane is used to receive the spun nanofibers, and the nanofibers are spun on the surface of the hydrophilic inner layer fiber membrane to form a humidity-responsive middle layer membrane; After spinning, a double-layer fiber membrane including a hydrophilic inner fiber membrane and a humidity-responsive middle layer membrane is obtained; After the double-layer fiber membrane is completed, the pH-responsive outer layer spinning solution prepared in step 3 is loaded into a syringe, the spinning high-voltage power supply voltage and the spinning solution propulsion rate are set, and the pH-responsive outer layer electrospinning is performed; the double-layer fiber membrane is used to receive the spun nanofibers, and the nanofibers are spun on the surface of the double-layer fiber membrane to form a pH-responsive outer layer membrane; after the spinning is completed, a multi-responsive polymer fresh water-absorbing pad with a "sandwich" structure is obtained; Among them, the high-voltage power supply voltage for electrospinning of the hydrophilic inner layer spinning solution is 20-23kV, and the propulsion rate is 0.5-0.7mL / h; the high-voltage power supply voltage for electrospinning of the humidity-responsive middle layer spinning solution is 10-15kV, and the propulsion rate is 0.3-0.5mL / h; the high-voltage power supply voltage for electrospinning of the pH-responsive outer layer spinning solution is 25-29kV, and the propulsion rate is 0.7mL / h; the electrospinning needles are all 20G, the spinning temperature is 20°C, and the relative humidity is 40%.

8. A method for preparing a multi-responsive polymer fresh water absorbent pad having a "sandwich" structure according to any one of claims 1 to 7, characterized in that: The multi-responsive polymer fresh water absorbent pad has a three-layer structure, which comprises, from inside to outside, a hydrophilic inner fiber membrane, a humidity responsive middle membrane, and a pH responsive outer membrane.

9. Use of the multi-responsive polymer fresh food absorbent pad with a "sandwich" structure according to claim 8 for preserving fresh food.

10. Use of a multi-responsive polymer fresh water absorbent pad with a "sandwich" structure as claimed in claim 8 for preparing packaging materials, characterized in that: The packaging materials include packaging boxes and packaging bags.

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