A packaging box based on spontaneous modified atmosphere and dynamic balance of micro-environment humidity based on bidirectional concentration difference

Abstract

The application relates to the technical field of food preservation packaging, and particularly provides a packaging box based on spontaneous modified atmosphere packaging and dynamic humidity balance of a microenvironment, which comprises a box body, a box cover and at least one microporous membrane assembly, the microporous membrane assembly comprises a nanocomposite membrane composed of a nanometer microporous membrane hydrophobic layer and a supporting layer, and the hydrophobic layer has selective permeation characteristics of 'gas permeation but water impermeation'. The packaging box utilizes the oxygen and carbon dioxide bidirectional concentration difference formed by the respiration of fresh food to drive gas exchange, realizes spontaneous modified atmosphere packaging, and simultaneously utilizes the 'gas permeation but water impermeation' characteristics of the hydrophobic layer to drive bidirectional migration of water vapor according to humidity difference, so that dynamic humidity balance of the microenvironment is realized. The application can realize accurate regulation and control of gas components and dynamic humidity balance of the microenvironment simultaneously without active aeration equipment, and is suitable for fresh fruit and vegetable and seafood preservation transportation.

Description

Technical Field

[0001] This invention belongs to the field of food preservation and packaging technology, and involves the cross-application of modified atmosphere storage containers, humidity control packaging, microporous membrane separation materials and packaging box structures. Specifically, it relates to a packaging box based on a spontaneous modified atmosphere and dynamic balance of microenvironment humidity driven by bidirectional concentration difference. Background Technology

[0002] Fresh fruits and vegetables continue to respire after harvest. Respiration refers to the process by which fruits and vegetables, using oxygen as a substrate, oxidize and decompose organic matter into carbon dioxide and water under the catalysis of enzymes, releasing energy. Its intensity is usually expressed as the respiration rate, which is the amount of oxygen absorbed or carbon dioxide released by 1 kg of fruits and vegetables in 1 hour, measured in mg (mL) / h·kg (fresh weight). The result of respiration is the continuous consumption of oxygen in the packaging environment and the accumulation of carbon dioxide.

[0003] When the oxygen concentration in the packaging environment is too low, fruits and vegetables are forced to switch to anaerobic respiration. Incomplete oxidation of organic matter produces harmful metabolites such as ethanol and acetaldehyde, accelerating tissue aging and decay, causing irreversible quality deterioration. Conversely, when the carbon dioxide concentration exceeds the tolerance threshold of fruits and vegetables, it will cause carbon dioxide damage. Typical symptoms include browning of the skin or internal tissues, collapse, dehydration, wilting, and even the appearance of cavities. Studies have shown that the suitable concentration range for most modified atmosphere packaging of fruits and vegetables is 2%–5% oxygen and 3%–8% carbon dioxide, combined with nitrogen, which can achieve a longer shelf life under low temperature conditions of 6–8℃.

[0004] Taking sweet cherries as an example, relevant experiments show that 5% oxygen + 8% carbon dioxide is the optimal controlled atmosphere storage condition. Under this condition, the respiration intensity of cherries is significantly reduced, and the rot rate and browning index are greatly reduced. However, when the carbon dioxide concentration increases to 10%, it may have exceeded the optimal concentration, and there is a risk of carbon dioxide damage.

[0005] Besides respiration, transpiration is also a significant factor in the postharvest deterioration of fruit and vegetable quality. Continuous transpiration and water loss after harvest will lead to wilting, weight loss, and a decrease in commercial value. In low-temperature environments, water vapor inside the packaging condenses into condensate, which accumulates on the surface of the fruits and vegetables and at the bottom of the packaging, providing favorable conditions for microbial growth.

[0006] The spoilage mechanism of seafood and other aquatic products differs from that of fruits and vegetables. Modified atmosphere packaging (high carbon dioxide + low oxygen) can effectively inhibit the growth of aerobic bacteria and delay spoilage. However, carbon dioxide absorption by the fish meat can lead to a decrease in pH and exudation issues, which need to be considered in the design of the modified atmosphere packaging solution. The aforementioned post-harvest physiological characteristics dictate that the preservation packaging of fresh food must simultaneously address the two core issues of gas composition control and humidity balance.

[0007] Currently, modified atmosphere packaging technology for fresh food can be mainly divided into the following four categories: (1) Ordinary sealed packaging: This method isolates fruits and vegetables from the outside world through a completely sealed packaging. Because the respiration of the fruits and vegetables inside the packaging continuously consumes oxygen and releases carbon dioxide, the carbon dioxide concentration in the sealed environment continuously accumulates, and oxygen is gradually depleted, eventually leading to carbon dioxide poisoning and hypoxia (anaerobic respiration). Simultaneously, transpiration causes the fruits and vegetables to wilt, and the condensation of water vapor upon cooling leads to the growth of microorganisms. This method cannot achieve gas exchange and humidity control, and is only suitable for very short-term transportation scenarios.

[0008] (2) Modified Atmosphere Packaging (MAP): This method uses specialized modified atmosphere packaging equipment to replace the gas inside the packaging with a pre-mixed gas (e.g., low oxygen, high carbon dioxide, nitrogen balance) to achieve a precise initial modified atmosphere environment. The "Test Method for Gas Permeability of Plastic Films and Sheets" (GB / T 1038) specifies the test method for the gas permeability of modified atmosphere packaging materials, providing a standard basis for the evaluation of active modified atmosphere packaging. However, active modified atmosphere packaging has the following shortcomings: ① It requires specialized modified atmosphere packaging equipment, which is costly; ② It is not suitable for small-to-medium scale transportation and direct sales from the production site; ③ The gas ratio needs to be precisely adjusted according to different product categories, making operation complex; ④ The gas concentration inside the packaging continuously changes with respiration, making it difficult to maintain a stable initial modified atmosphere environment over a long period.

[0009] (3) Passive modified atmosphere packaging: Gas exchange is achieved by creating holes in the packaging or using breathable materials, which utilizes the natural diffusion of gases inside and outside the packaging. The simplest solution is to make holes in the packaging box wall, but the permeability of this solution cannot be precisely controlled, and there is no humidity control function. The control accuracy is low and it is impossible to maintain a stable modified atmosphere environment.

[0010] (4) Wrapping with plastic wrap: This is a simple preservation method commonly used in direct sales from the place of origin. It achieves a certain degree of gas exchange by wrapping a breathable film around the outer layer of fruits and vegetables. However, this method has defects such as uneven air permeability, prominent problems of condensation accumulation, and inability to accurately control the amount of gas exchange.

[0011] Microporous membranes are thin films made from hydrophobic polymer materials through processes such as stretching or spinning. Their pore size can be precisely controlled to the submicron level (100–500 nm). The core characteristic of nanoporous membranes is "air permeability but water impermeability": due to their strong hydrophobicity (water contact angle of approximately 110°–120°), liquid water cannot pass through the submicron-sized pores due to its high surface tension, while gaseous water molecules and other gases can pass freely. This selective permeability allows nanoporous membranes to effectively block liquid water permeation while achieving gas exchange, providing a material basis for resolving the contradiction between gas control and humidity balance in modified atmosphere packaging.

[0012] The gas permeation mechanisms of nanoporous membranes mainly include the following three types: ① Diffusion – under the drive of concentration gradient, gas molecules diffuse from the high concentration side to the low concentration side, following Fick's law; ② Pressure difference driven flow – when there is a slight pressure difference inside and outside the packaging, gas permeates through the pores under convection; ③ Molecular sieve effect – based on the selective separation of gas molecules by pore size.

[0013] In practical applications, nanoporous membranes are often combined with other materials (such as nonwoven fabrics and polymer coatings) to balance permeability, selectivity, and mechanical strength. Among these, the composite structure of a hydrophobic layer of the nanoporous membrane and a polyester nonwoven fabric support layer is a typical current technical approach. The hydrophobic layer provides selective permeability, while the support layer provides mechanical strength and installation adaptability. Furthermore, nanomembranes used in food packaging must comply with the relevant requirements of the National Food Safety Standard for Plastic Materials and Products for Food Contact (GB 4806.7-2016).

[0014] A search revealed the following related solutions in existing patent technologies: Patent 1 (CN224000209U) discloses a self-regulating atmosphere patch with a nucleoporous membrane for preserving fruits and vegetables, and the fruit and vegetable box itself. Its structure is a three-layer composite structure: a nucleoporous membrane layer + an inner protective layer (nylon mesh) + an outer protective layer (nylon mesh). This solution utilizes the uniform and precise micropores of the nucleoporous membrane to achieve self-regulating atmosphere based on the gas concentration difference across the micropores. The key shortcomings of this patent are: ① It only achieves gas composition regulation, lacking a humidity balance mechanism, and cannot solve the problems of condensation accumulation and evaporation loss inside the packaging; ② The nucleoporous membrane uses a passive diffusion mechanism, and the permeability rate cannot be dynamically adjusted, so it cannot adaptively adjust the permeation rate when the gas concentration inside the packaging changes; ③ The nucleoporous membrane requires a particle accelerator for preparation, resulting in high costs and hindering large-scale promotion; ④ The patch installation method is only suitable for thin-walled boxes, with poor compatibility with commonly used thick-walled packaging boxes such as corrugated cardboard boxes.

[0015] Patent 2 (CN121180594A) discloses a two-way breathable valve that can be installed in fruit and vegetable packaging boxes. It proposes a five-component modular design for the two-way breathable valve, using a nano-functional membrane (polyethylene-based nanocomposite material) as the core for breathability. This solution utilizes the "bionic intelligence" characteristics of the nano-functional membrane to passively adjust the permeability when the concentration of ethylene or carbon dioxide inside the packaging increases. The key shortcomings of this patent are: ① It lacks a clear description of the "two-way concentration difference driven" mechanism, making the driving force for breathability adjustment unclear and unable to achieve precise gas exchange based on the two-way concentration difference; ② Humidity control is a passive material-level characteristic, not an active balancing mechanism, and cannot dynamically adjust the water vapor permeability according to the humidity difference between the inside and outside of the packaging; ③ The polyethylene-based material has a limited temperature resistance range (-20~80℃), and its performance may be limited in low-temperature environments during cold chain transportation or in high-temperature seasons; ④ It does not involve a selective design of "breathable but not water-permeable," and cannot simultaneously solve the problems of gas exchange and condensate barrier.

[0016] Patent 3 (CN101698436B) discloses a breathable window for packaging, proposing a breathable window based on a microporous ultrafine fiber non-woven membrane, using a single-layer PP (polypropylene) membrane with a pore size of 0.01–10 μm and a thickness of 0.5–0.6 mm. This solution utilizes a three-dimensional labyrinth structure, allowing gas to pass through and exchange within the labyrinthine pores, achieving bidirectional ventilation. The key shortcomings of this patent are: ① The single-layer membrane structure lacks a composite design, making it impossible to balance breathability selectivity, mechanical strength, and installation adaptability; ② The hydrophobicity is a passive physical barrier, not an active humidity control, and cannot dynamically balance water vapor content based on the humidity difference between the inside and outside of the packaging; ③ The breathability is fixed and cannot be adjusted, failing to adapt to the differentiated needs of different types of fruits and vegetables with different respiration intensities for modified atmosphere conditions; ④ The PP material has limited temperature resistance, resulting in insufficient performance stability under extreme temperature conditions.

[0017] In summary, existing modified atmosphere packaging technologies have the following common drawbacks: (1) Separation of gas regulation and humidity balance. Existing technologies either focus only on gas composition regulation (such as patent 1) or treat humidity regulation as an incidental passive property of materials (such as patents 2 and 3), neither of which has established a synergistic mechanism between gas exchange and humidity balance. In actual fresh food preservation scenarios, the gas concentration change caused by respiration and the humidity change caused by transpiration are two synchronous and interrelated processes. Separating these processes will inevitably lead to limited preservation effects.

[0018] (2) Insufficient air permeability control precision and lack of dynamic response capability. Existing passive modified atmosphere solutions (such as perforated or simple breathable membranes) have fixed and unadjustable air permeability, and cannot dynamically respond to real-time changes in gas concentration inside the packaging. Although active modified atmosphere solutions can achieve precise control, they are expensive and complex to operate, making them unsuitable for small and medium-sized transportation scenarios.

[0019] (3) The selective design of "breathable but not water-permeable" has not been achieved. Most existing solutions have not resolved the contradiction between gas exchange and liquid water barrier. When water vapor evaporates inside the packaging and condenses upon cooling, the existing breathable structure cannot effectively block the condensate from passing through, leading to the growth of microorganisms and contamination of the outside of the packaging.

[0020] (4) The concentration difference driving mechanism is not fully utilized. Although some existing technologies mention the concentration difference in some solutions (such as Patent 1), none of them utilize the bidirectional concentration difference inside and outside the packaging as a systematic driving force. That is, they do not consider the concentration difference of oxygen and carbon dioxide in both directions to drive gas exchange at the same time, nor do they incorporate the humidity difference as the driving force for bidirectional migration of water vapor into the overall controlled atmosphere balance design.

[0021] Therefore, there is an urgent need for a packaging box that can achieve spontaneous controlled atmosphere and dynamic balance of microenvironment humidity based on bidirectional concentration difference, so as to solve the two core problems of precise control of gas composition and dynamic balance of humidity at the same time, and adapt to the differentiated preservation needs of different types of fresh food. Summary of the Invention

[0022] In view of the deficiencies of the prior art disclosed in the background section, the technical problems to be solved by the present invention include: (1) The problem of the separation between gas control and humidity balance. Existing modified atmosphere packaging technology either only focuses on the control of gas composition or treats humidity control as an incidental passive property of materials, failing to establish a synergistic mechanism between gas exchange and humidity balance. In actual fresh food preservation scenarios, the gas concentration change caused by respiration and the humidity change caused by transpiration are two processes that occur simultaneously and are related to each other.

[0023] (2) Problems of insufficient air permeability control precision and lack of dynamic response capability. The existing passive modified atmosphere solution has a fixed and unadjustable air permeability, and cannot dynamically respond to the real-time changes in gas concentration inside the packaging; although the active modified atmosphere solution can achieve precise control, the equipment cost is high and the operation is complicated, which is not suitable for small and medium-sized transportation and direct sales from the place of origin.

[0024] (3) The contradiction between gas exchange and liquid water barrier. Existing breathable structures do not achieve the selective design of "breathable but impermeable to water", and cannot effectively block liquid water from passing through while allowing free gas exchange, resulting in condensation accumulation, microbial growth and external packaging contamination.

[0025] (4) The problem of the concentration difference driving mechanism not being fully utilized. Existing technologies do not utilize the bidirectional concentration difference inside and outside the packaging as a systematic driving force, that is, they do not consider the concentration difference of oxygen and carbon dioxide in both directions to drive gas exchange, nor do they incorporate the humidity difference as a driving force for water vapor migration into the overall controlled atmosphere balance design.

[0026] To solve the above-mentioned technical problems, the present invention provides the following technical solution: a packaging box based on a two-way concentration difference for spontaneous controlled atmosphere and dynamic balance of microenvironment humidity, comprising a box body, a box lid and at least one microporous membrane assembly, wherein the microporous membrane assembly is installed on the side wall of the box body.

[0027] The microporous membrane assembly includes a nanocomposite membrane, which is formed by bonding a hydrophobic layer of a nanoporous membrane and a support layer together through hot pressing or adhesive bonding. The nanocomposite membrane and the adhesive material together constitute the microporous membrane assembly.

[0028] The microporous membrane assembly is installed on the side wall of the box, serving as the sole channel for gas exchange between the inside and outside of the packaging. A sealed design ensures that all gas exchange occurs through the nanocomposite membrane, preventing "short-circuit" exchanges that occur directly through installation gaps or other paths without the membrane's selective permeability.

[0029] The nanoporous hydrophobic membrane layer has micropores that allow gas molecules to pass through. The pore size is smaller than the diameter of a liquid water droplet but larger than the kinetic diameter of gas molecules and water vapor molecules, giving the nanoporous hydrophobic membrane layer selective permeability—"breathable but impermeable to water." The pore size of the nanoporous hydrophobic membrane layer is 0.1–0.5 μm, the porosity is 60%–90%, and the water contact angle is 110°–120°. The support layer is a non-woven fabric layer or a mesh layer, specifically a polyester non-woven fabric layer, providing mechanical support, puncture resistance, and cushioning.

[0030] This invention utilizes the oxygen and carbon dioxide concentration differences created by the respiration of fresh food, which consumes oxygen and releases carbon dioxide, to drive the diffusion of oxygen from the outside to the inside of the packaging and carbon dioxide from the inside to the outside, respectively, achieving spontaneous modified atmosphere packaging based on bidirectional concentration differences. The gas exchange mechanism is based on Fick's first law of diffusion: J = -D·(dC / dx), where J is the diffusion flux, D is the diffusion coefficient, and dC / dx is the concentration gradient. In modified atmosphere packaging applications, the relationship between gas concentration C and partial pressure P is C = P / RT, therefore the diffusion flux can be expressed as J = -D·(dP) / (RT·dx).

[0031] Once the packaging is sealed, the respiration of fresh food continuously consumes oxygen and releases carbon dioxide, creating the following bidirectional concentration difference-driven mechanism: (1) Low oxygen concentration inside and high oxygen concentration outside: The oxygen concentration inside the packaging is lower than that of the outside atmosphere (approximately 20.9%) due to respiration consumption. Driven by the concentration difference, oxygen diffuses from the outside into the packaging through the nanocomposite membrane. (2) High carbon dioxide concentration inside and low carbon dioxide concentration outside: The carbon dioxide concentration inside the packaging is higher than that of the outside atmosphere (approximately 0.04%) due to respiration release. Driven by the concentration difference, carbon dioxide diffuses from the packaging into the outside through the nanocomposite membrane. The above-mentioned inward diffusion of oxygen and outward diffusion of carbon dioxide occur simultaneously, constituting a "bidirectional concentration difference-driven" gas exchange mechanism. This mechanism requires no external power or active gas filling equipment and relies entirely on the spontaneous gas exchange driven by the gas concentration difference inside and outside the packaging.

[0032] In the initial stage after sealing, the oxygen concentration continuously decreases while the carbon dioxide concentration continuously increases, leading to a widening concentration difference between the inside and outside of the packaging and a corresponding increase in the diffusion rate. When the diffusion rate equals the respiration rate—that is, when the oxygen consumption of the fruits and vegetables equals the amount of oxygen entering through the membrane and the carbon dioxide production of the fruits and vegetables equals the amount of carbon dioxide expelled through the membrane—the gas concentration inside the packaging reaches a dynamic equilibrium, forming a steady-state controlled atmosphere. This dynamic equilibrium process is coupled with the relationship between the respiration rate of fruits and vegetables and the oxygen / carbon dioxide concentration described by the Michaelis-Menten kinetic model, constituting a self-regulating "reaction-diffusion" system.

[0033] This invention also utilizes the "breathable but impermeable" properties of the nanoporous membrane hydrophobic layer to drive the bidirectional migration of water vapor based on the humidity difference between the inside and outside of the packaging, thereby achieving a dynamic balance of humidity in the microenvironment. The principle is as follows: The strong hydrophobicity of nanoparticles prevents liquid water from passing through submicron-sized pores (pore size 0.1-0.5μm) due to high surface tension, while gaseous water molecules and other gas molecules can pass through freely. Therefore: (1) Water vapor drainage mechanism when humidity is too high: When the humidity inside the packaging is higher than that outside, excess water vapor (gas) can diffuse out through the nanoporous membrane, preventing excessive humidity and condensation accumulation inside the packaging, and avoiding the growth of microorganisms; (2) Water retention mechanism when humidity is too low: When the humidity inside the packaging is lower than the appropriate range, the nanoporous membrane blocks liquid water from passing through, slowing down the rapid loss of water, and maintaining a moderate humidity environment inside the packaging in conjunction with the micro-positive pressure effect, preventing wilting caused by water loss due to transpiration of fruits and vegetables (the water content of fruits and vegetables is 65%-96%, and a loss of 5% of water results in wilting and loss of freshness). The above mechanism achieves two-way dynamic regulation of humidity: it prevents condensation and microbial growth caused by excessive humidity, and also prevents wilting and loss of freshness caused by excessively low humidity, thereby achieving a dynamic balance of humidity in the microenvironment.

[0034] During the sealed operation of the packaging box, the following factors together generate and maintain a slightly positive pressure state inside the packaging: (1) Respiratory heat generation effect: Fresh food releases heat through respiration.

[0035] (1) Heat accumulation during respiration leads to a slightly higher temperature inside the sealed packaging than the external environment temperature. The gas expands due to heat, causing a slight increase in internal pressure. (2) Evaporation and moisture generation effect: Fresh food continuously evaporates and generates water vapor, increasing the partial pressure of water vapor inside the packaging and causing a slight increase in total internal pressure. The maintenance of micro-positive pressure has the following promoting effects on the atmosphere and humidity balance function of the present invention: (1) Promote carbon dioxide discharge: Under micro-positive pressure, the pressure inside the packaging is slightly higher than that outside. Under the pressure difference, carbon dioxide is promoted to diffuse out from the inside, forming a synergistic effect with the concentration difference; (2) Enhance the selectivity of "breathable but not water-permeable": Micro-positive pressure promotes gas molecules to diffuse to the outside through nanopores, but liquid water cannot penetrate into the packaging in reverse due to the hydrophobic effect of nanopores and high surface tension; (3) Maintain moderate humidity: Micro-positive pressure, combined with the water-blocking properties of the nanopore hydrophobic layer, slows down the rapid loss of water vapor inside the packaging, maintains a moderate humidity environment, and prevents fruits and vegetables from wilting; (4) Prevent reverse penetration of external polluting gases: Under micro-positive pressure, the pressure inside the packaging is slightly higher than that outside. External polluting gases (such as ethylene, odorous gases, etc.) cannot penetrate into the packaging in reverse, thus improving food safety.

[0036] This invention also provides a method for preserving fresh food using the aforementioned packaging box, comprising the following steps: placing the fresh food inside the box and sealing the box with the lid; utilizing the respiration of the fresh food to consume oxygen and release carbon dioxide, creating a concentration difference between the inside and outside of the packaging for oxygen (lower concentration inside, higher concentration outside) and carbon dioxide (higher concentration inside, lower concentration outside), driving oxygen to diffuse from the outside into the packaging and carbon dioxide to diffuse from the inside into the outside, causing the gas concentration inside the packaging to spontaneously tend towards dynamic equilibrium; simultaneously utilizing the "air-permeable but water-impermeable" properties of the nanoporous membrane hydrophobic layer, driving bidirectional migration of water vapor based on the humidity difference between the inside and outside of the packaging: when the humidity inside the packaging is higher than that outside, water vapor diffuses out to the outside in gaseous form through the microporous membrane; when the humidity inside the packaging is lower than a suitable range, the microporous membrane blocks liquid water from passing through, slowing down the rapid loss of moisture.

[0037] Compared with the prior art, the beneficial effects of the present invention are: 1. Self-generating modified atmosphere packaging, eliminating the need for active inflation equipment. This invention is based on a two-way concentration difference-driven gas exchange mechanism, achieving self-generating modified atmosphere packaging without external power or active inflation equipment. It is energy-saving and low-cost, suitable for small- to medium-scale transportation and direct sales from the production site. Compared to active modified atmosphere packaging, which requires specialized modified atmosphere equipment and complex operations, the self-generating modified atmosphere solution of this invention significantly reduces the barrier to entry and operating costs.

[0038] 2. Precise Controlled Atmosphere Achieved Through Bidirectional Concentration Difference. This invention utilizes the concentration difference between oxygen and carbon dioxide in two directions to drive gas exchange, achieving higher gas exchange efficiency and control precision compared to unidirectional diffusion or passive perforation methods. When the gas concentration inside the packaging deviates from the equilibrium point, the concentration difference automatically increases, and the diffusion rate increases accordingly, forming a negative feedback regulation mechanism to achieve a spontaneous tendency towards a steady-state controlled atmosphere. This "reaction-diffusion" self-regulating system can maintain stable controlled atmosphere conditions without human intervention.

[0039] 3. The nanoporous membrane achieves dynamic humidity balance by being "breathable but water-impermeable." The hydrophobic layer of the nanoporous membrane allows gas to pass freely while blocking liquid water from permeating. This prevents condensation and microbial growth caused by excessive humidity inside the packaging, while also preventing wilting and loss of freshness due to excessively low humidity, thus achieving a dynamic balance of humidity in the microenvironment. Compared to existing technologies that separate gas control and humidity balance, this invention achieves both gas exchange and humidity balance through a single functional component, resulting in a simple structure and synergistic functionality.

[0040] 4. Slight positive pressure maintains and enhances food safety. Thermodynamics from respiration and moisture production from evaporation work together to maintain a slightly positive pressure within the packaging, effectively preventing external pollutants (such as ethylene and odorous gases) from back-permeating into the packaging, thus improving food safety and hygiene. Simultaneously, the slightly positive pressure and concentration gradient create a synergistic effect, promoting carbon dioxide removal and oxygen replenishment, enhancing the efficiency of controlled atmosphere balance.

[0041] 5. The composite membrane structure balances functionality and mechanical strength. The nanoporous membrane provides selective breathability, while the support layer provides mechanical support and puncture resistance. The combination of these two layers achieves synergistic performance between the functional and supportive layers. Compared to single-layer membrane structures, the composite membrane structure of this invention significantly improves mechanical strength and installation adaptability while maintaining selective breathability.

[0042] 6. The present invention also provides a method for preserving fresh food using the above-mentioned packaging box, which expands the application scenarios of the packaging box and provides a complete technical solution for the preservation of fresh food from packaging structure to usage method. Attached Figure Description

[0043] Figure 1 This is a schematic diagram of the overall structure of the packaging box of the present invention; Figure 2 This is a cross-sectional schematic diagram of the packaging box body of the present invention; Figure 3 For the present invention Figure 2 Enlarged schematic diagram of the structure at point A in the middle; Figure 4 This is a schematic diagram of the structural composition of the microporous membrane assembly of the present invention.

[0044] In the diagram: 1. Box body; 11. Side wall; 2. Microporous membrane assembly; 21. Hydrophobic layer of nanoporous membrane; 211. Micropores; 22. Support layer; 3. Box cover. Detailed Implementation

[0045] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below through embodiments and in conjunction with the accompanying drawings. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. Example

[0046] I. Overall structure of the packaging box: like Figure 1 and Figure 3 As shown, the packaging box in this embodiment includes a box body 1, a box lid 3, and a microporous membrane assembly 2.

[0047] The box body 1 and the lid 3 together form a sealed cavity for containing fresh food. Box body 1 is an insulated foam box (such as an EPS foam box or an aluminum foil insulated foam box), with a wall thickness of 25–30 mm and external dimensions of 40 × 30 × 25 cm. It is suitable for the preservation and transportation of fruits such as cherries, with a fruit capacity of 5–10 kg. The density of box body 1 is 18–25 kg / m³.

[0048] The lid 3 is a sleeve-type structure. The edge of the lid 3 is provided with a flange, which is embedded in the groove on the upper edge of the box body 1 to form a fitting sealing structure.

[0049] Microporous membrane modules 2 are installed on the side walls 11 of the box body 1, serving as the sole channel for gas exchange between the inside and outside of the packaging. The microporous membrane modules 2 are installed on the upper part of both side walls 11 of the box body 1, near the lid 3, to avoid direct contact with the goods inside the box. In this embodiment, four microporous membrane modules 2 are used, two on each side wall 11, with each microporous membrane module 2 having an opening area of ​​500 mm². The total opening area of ​​the four microporous membrane modules 2 is 2000 mm², which can meet the gas exchange requirements of cherries at a low temperature of 6–8°C.

[0050] II. Microporous membrane module structure: like Figure 4 As shown, the microporous membrane module 2 has a bilayer composite structure: The hydrophobic layer 21 of the nanoporous membrane is a hydrophobic polymer microporous membrane (such as polytetrafluoroethylene microporous membrane, PVDF microporous membrane, PU microporous membrane, or PI microporous membrane), with a pore size of 0.1–0.5 μm, a porosity of 60%–90%, and a water contact angle of approximately 110°–120°. The micropores 211 of the PTFE microporous membrane are uniformly distributed, with pore sizes between liquid water droplets (greater than 1 μm) and gas molecules / water vapor molecules (less than 0.5 nm), achieving selective permeability ("breathable but impermeable to water") based on a physical sieving mechanism. The air permeability of the PTFE microporous membrane is 1000–15000 cc / min / cm²@7kPa (the specific permeability can be adjusted according to the type of fresh food and the pore area), and it can withstand splashes of water from any direction without leakage. The operating temperature range of the PTFE material is -200 to +260°C, and the overall temperature resistance range of the composite membrane is -40 to +180°C.

[0051] Support layer 22: This is a polyester nonwoven fabric layer that provides mechanical support, puncture resistance, and cushioning. When the support layer 22 is combined with the PTFE hydrophobic layer 21, it protects the PTFE hydrophobic layer 21 from mechanical damage and provides a cushioning bonding surface during installation, ensuring the airtightness between the microporous membrane assembly 2 and the housing 1.

[0052] The PTFE microporous membrane and the support layer 22 are bonded together via a hot-pressing composite process. Hot-pressing is a purely thermal bonding technology that does not require adhesives. After hot-pressing, a hot-melt bond is formed between the PTFE microporous membrane and the support layer 22, eliminating the need for adhesives and preventing clogging of the PTFE microporous membrane, thus ensuring air permeability. The microporous membrane assembly 2 is composed of the aforementioned nanocomposite membrane and adhesive material, and is fixedly installed on the side wall 11 of the housing 1 using the adhesive material.

[0053] III. The working process of controlled atmosphere driven by two-way concentration difference: Once the container is sealed, the respiration of the fresh food inside continues to consume oxygen and release carbon dioxide. This results in the following bidirectional concentration difference: Driven by the oxygen concentration difference between the inside and outside: The oxygen concentration inside chamber 1 continuously decreases due to respiration, falling below the oxygen concentration in the outside atmosphere (approximately 20.9%). Driven by this concentration difference, oxygen diffuses from the outside into chamber 1 through the micropores 211 of the PTFE hydrophobic layer 21. According to Fick's first law of diffusion: J = -D·(dC / dx), the greater the oxygen concentration difference, the greater the diffusion flux, and the faster the rate at which oxygen replenishes into chamber 1.

[0054] Driven by the concentration difference between the inside and outside of carbon dioxide: The carbon dioxide concentration inside chamber 1 continuously rises due to respiration, exceeding the concentration of carbon dioxide in the outside atmosphere (approximately 0.04%). Driven by this concentration difference, carbon dioxide diffuses from inside chamber 1 to the outside through the micropores 211 of the PTFE hydrophobic layer 21. The kinetic diameter of carbon dioxide is 0.330 nm, smaller than that of oxygen (0.346 nm), making its diffusion easier and its permeability greater.

[0055] The simultaneous inward diffusion of oxygen and outward diffusion of carbon dioxide constitute a "two-way concentration difference driven" gas exchange mechanism. This mechanism requires no external power or active aeration equipment, relying entirely on the spontaneous gas exchange driven by the gas concentration difference inside and outside the packaging.

[0056] In the initial stage after sealing, the oxygen concentration continuously decreases while the carbon dioxide concentration continuously increases, leading to a widening concentration difference between the inside and outside of chamber 1 and a corresponding increase in the diffusion rate. When the diffusion rate equals the respiration rate—that is, when the oxygen consumption of the fresh food equals the amount of oxygen entering through the membrane and the carbon dioxide production of the fresh food equals the amount of carbon dioxide escaping through the membrane—the gas concentration inside chamber 1 reaches a dynamic equilibrium, forming a steady-state controlled atmosphere environment. This dynamic equilibrium process constitutes a self-regulating "reaction-diffusion" system that can maintain stable controlled atmosphere conditions without human intervention.

[0057] Taking cherries as an example, cherries are a carbon dioxide-tolerant variety, allowing a minimum oxygen concentration of 2%–5% and a maximum carbon dioxide concentration of 10%–15%, making them suitable for controlled atmosphere environments with high carbon dioxide and low oxygen. In this embodiment, the total open area of ​​the four microporous membrane components 2 is 2000 mm², and the microporous membrane permeability is 10000 cc / min / cm²@7 kPa, which can stabilize the oxygen concentration in the chamber 1 within a suitable range of 2%–5% and the carbon dioxide concentration within a suitable range of 8%–12%, meeting the controlled atmosphere requirements for cherry preservation.

[0058] IV. The working process of dynamic humidity balance: Fruits and vegetables have a water content of 65%–96%, and continuously transpire water vapor after harvest. During the sealed operation of the packaging box, dynamic humidity balance is maintained through the following mechanisms: The process of releasing water vapor when humidity is too high: When the humidity inside chamber 1 is higher than that outside, excess water vapor (gaseous) diffuses out to the outside through the microporous membrane driven by the concentration difference. Due to the "air-permeable but water-impermeable" properties of the nanoporous membrane, gaseous water molecules can freely pass through the micropores 211. Therefore, excess water vapor inside the chamber is released in gaseous form, preventing excessive humidity and condensation accumulation, and avoiding the growth of microorganisms.

[0059] Water retention process when humidity is too low: When the humidity inside container 1 is below the suitable range, the nanoporous membrane's ability to block liquid water slows down the rapid loss of moisture. Simultaneously, the micro-positive pressure effect, combined with the water-blocking properties of the nanoporous membrane, maintains a moderate humidity environment inside container 1, preventing wilting caused by water loss through transpiration in fruits and vegetables. Fruits and vegetables wilt and lose freshness when they lose 5% of their water content.

[0060] The above-mentioned drainage and water retention processes automatically switch according to the humidity difference between the inside and outside of the chamber 1, realizing bidirectional dynamic adjustment of the microenvironment humidity without manual intervention.

[0061] V. The working process of maintaining a slight positive pressure: During the sealing operation of the packaging box, the following factors together generate and maintain a slight positive pressure state inside box 1: Respiratory heat generation effect: Fresh food releases heat through respiration. The accumulation of respiratory heat causes the temperature inside the sealed box 1 to be slightly higher than the external ambient temperature. The gas inside the box 1 expands due to heat, causing a slight increase in internal pressure.

[0062] Evaporation and moisture generation effect: Fresh food continuously evaporates and produces water vapor, increasing the partial pressure of water vapor inside the box 1 and slightly increasing the total internal pressure.

[0063] Maintaining a slight positive pressure has the following beneficial effects on the air conditioning and humidity balancing function of this invention: ① Promote carbon dioxide discharge: Under a slightly positive pressure, the pressure inside chamber 1 is slightly higher than that outside. Driven by the pressure difference, carbon dioxide diffuses and is discharged from the inside to the outside, forming a synergistic effect with the concentration difference. ② Enhance the selectivity of "breathable but impermeable": micro-positive pressure drives gas molecules to diffuse to the outside through the nanoporous membrane, but liquid water cannot leak due to hydrophobicity and high surface tension; ③ Maintain moderate humidity: The micro-positive pressure combined with the water-blocking properties of the nanoporous membrane slows down the rapid loss of water vapor inside the chamber 1, thus maintaining a moderate humidity environment; ④ Preventing reverse permeation of external pollutants: Under a slightly positive pressure, the pressure inside chamber 1 is slightly higher than that outside, preventing external pollutants (such as ethylene, odorous gases, etc.) from permeating into chamber 1 and improving food safety. Example

[0064] This embodiment is basically the same as embodiment 1 in structure and working principle, the only difference being the adjustment of the parameters of the microporous membrane component 2 to adapt to the preservation and transportation of seafood products.

[0065] Unlike fruits and vegetables, the spoilage mechanism of aquatic products is mainly based on the proliferation of aerobic bacteria. Modified atmosphere packaging (MAP) follows the principle of high carbon dioxide and low oxygen, which can effectively inhibit the growth of aerobic bacteria. However, carbon dioxide absorption by the fish can lead to a decrease in pH and exudation problems, so the carbon dioxide concentration must be controlled to avoid being too high in the MAP design. Seafood spoils faster than fruits and vegetables, requiring a larger permeable surface area to accelerate gas exchange.

[0066] In view of the above characteristics, the parameters in this embodiment are adjusted as follows: (1) Specifications of box 1: EPS foam box with a wall thickness of 30-40mm (thickened type) to meet the higher requirements of seafood transportation for heat preservation performance. The density of box 1 is 20-25 kg / m³, and the external dimensions are 50×35×30cm.

[0067] (2) Number and opening area of ​​microporous membrane components 2: There are 6 microporous membrane components 2, with 3 installed on the upper part of each of the two side walls 11 of the box body 1. The opening area of ​​each microporous membrane component 2 is 500 mm², and the total opening area is 3000 mm², which is 50% more than the 2000 mm² in Example 1, in order to accelerate the carbon dioxide discharge rate and avoid the problem of the fish meat pH dropping and exudate due to excessive carbon dioxide concentration in the box body 1.

[0068] (3) Nanoporous membrane parameters: The pore size is 0.3-0.5 μm, which is higher than the 0.1-0.5 μm used for cherry transportation in Example 1, so as to improve the air permeability and meet the rapid gas exchange requirements of seafood in combination with the increased total open area.

[0069] The working process of this embodiment is the same as that of embodiment 1. It achieves a dynamic balance between spontaneous atmosphere control and microenvironment humidity based on bidirectional concentration difference driving, which will not be described in detail here. Example

[0070] This embodiment is basically the same as the one in terms of structure and working principle as that in embodiment 1. The difference is that the membrane parameters are adjusted for fruits and vegetables with different respiratory characteristics.

[0071] For fruits and vegetables with low respiration rates (such as apples), the number of microporous membrane components 2 can be reduced or the total open area can be reduced (such as a total open area of ​​1200-1600 mm²) to avoid excessively rapid gas exchange leading to excessively high oxygen concentration and excessively low carbon dioxide concentration, thus losing the modified atmosphere effect.

[0072] For leafy vegetables with high respiration rates, the number of microporous membrane modules 2 can be increased or the total open area can be increased (e.g., a total open area of ​​2400–3600 mm²) to meet the higher gas exchange requirements.

[0073] The specific number and total open area of ​​the microporous membrane module 2 can be calculated and determined according to the type of fresh food, the amount of fruit, and the storage temperature, based on the balance between respiration rate and diffusion rate.

[0074] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A packaging box based on a spontaneous controlled atmosphere and dynamic balance of microenvironment humidity based on a two-way concentration difference, comprising a box body (1), a box lid (3), and at least one microporous membrane assembly (2), wherein the microporous membrane assembly (2) is mounted on the side wall (11) of the box body (1), characterized in that: The microporous membrane assembly (2) includes a nanocomposite membrane, which is composed of a nanoporous membrane hydrophobic layer (21) and a support layer (22). The nanoporous membrane hydrophobic layer (21) has micropores (211) for gas molecules to pass through. The pore size of the nanoporous hydrophobic membrane (21) is smaller than the diameter of liquid water droplets and larger than the dynamic diameter of gas molecules and water vapor molecules, giving the nanoporous hydrophobic membrane (21) a selective permeability characteristic of "breathable but impermeable to water". The packaging box utilizes the oxygen and carbon dioxide concentration differences inside and outside the packaging formed by the respiration of fresh food consuming oxygen and releasing carbon dioxide. These differences drive oxygen to diffuse from the outside to the inside of the packaging and carbon dioxide to diffuse from the inside of the packaging to the outside, respectively, thus achieving spontaneous modified atmosphere based on bidirectional concentration differences. The packaging box also utilizes the "breathable but not water-permeable" properties of the nanoporous membrane hydrophobic layer (21) to drive the bidirectional migration of water vapor based on the humidity difference between the inside and outside of the packaging, thereby achieving a dynamic balance of microenvironment humidity.

2. The packaging box according to claim 1, characterized in that: The pore size of the nanoporous hydrophobic membrane (21) is 0.1-0.5 μm, the porosity is 60%-90%, and the water contact angle is 110°-120°.

3. The packaging box according to claim 1, characterized in that: The support layer (22) is a non-woven fabric layer or a mesh layer, and the nanoporous membrane hydrophobic layer (21) is fixed together with the support layer (22) by a hot-pressing composite process or an adhesive process.

4. The packaging box according to claim 1, characterized in that: The hydrophobic layer (21) of the nanoporous membrane is a hydrophobic polymer microporous membrane.

5. The packaging box according to claim 1, characterized in that: The microporous membrane assembly (2) serves as the only channel for gas exchange between the inside and outside of the packaging, and a sealing structure is provided between the box body (1) and the box cover (3).

6. The packaging box according to claim 1, characterized in that: The box (1) is a thermal insulation foam box with a wall thickness of 25-40 mm and a density of 18-25 kg / m³.

7. The packaging box according to claim 1, characterized in that: The microporous membrane assembly (2) is installed on the upper part of the two side walls (11) of the box body (1), near the box cover (3).

8. The packaging box according to claim 1, characterized in that: The number of microporous membrane modules (2) is 2 to 8, the opening area of ​​each microporous membrane module (2) is 400 to 600 mm², and the total opening area is 800 to 4800 mm².

9. The packaging box according to claim 1, characterized in that: The air permeability of the hydrophobic layer (21) of the nanoporous membrane is 1000~15000cc / min / cm²@7kPa, and the temperature resistance range of the nanocomposite membrane is -40~+180°C.

10. A method for preserving fresh food using the packaging box described in any one of claims 1 to 9, characterized in that, Includes the following steps: S1. Place the fresh food inside the box (1) and cover the box with the lid (3) to seal the box. S2. By utilizing the respiration of fresh food to consume oxygen and release carbon dioxide, a concentration difference of low oxygen concentration inside and high carbon dioxide concentration outside the packaging is formed inside and outside the packaging. This drives oxygen to diffuse from the outside to the inside of the packaging and carbon dioxide to diffuse from the inside of the packaging to the outside, so that the gas concentration inside the packaging spontaneously tends to a dynamic equilibrium. S3. Simultaneously, utilizing the "breathable but not water-permeable" characteristics of the nanoporous membrane hydrophobic layer (21), water vapor is driven to migrate bidirectionally based on the humidity difference between the inside and outside of the packaging: when the humidity inside the packaging is higher than that outside, water vapor diffuses out to the outside in gaseous form through the microporous membrane; when the humidity inside the packaging is lower than the appropriate range, the microporous membrane blocks liquid water from passing through, slowing down the excessive loss of moisture.

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