Polyolefin breathable adjusting film for preservation and preparation method of polyolefin breathable adjusting film

Abstract

The invention discloses a polyolefin breathable adjusting film for preservation and a preparation method of the polyolefin breathable adjusting film. The thin film comprises at least one microporous layer composed of a polyolefin matrix and an inorganic filler, and asymmetric microporous structures with different pore diameters and opening area ratios are formed on the two sides of the thin film, so that differential regulation and control on transmission behaviors of oxygen, carbon dioxide and water vapor are realized. The preparation method sequentially comprises the steps of filling master batch preparation, non-oriented sheet extrusion, two-way stretching and heat setting, and a stable pore structure gradient is formed in the film thickness direction through the cooling rate difference in the extrusion stage, temperature difference stretching in the stretching stage and single-side contact treatment in the heat setting stage. The film can spontaneously adjust the internal microenvironment in fruit and vegetable packaging, improve breathing balance and humidity control, and prolong the fresh-keeping period of fruits and vegetables in the storage and transportation process.

Description

Technical Field

[0001] This invention relates to the field of membrane manufacturing technology, and in particular to a polyolefin breathability regulating film for preservation and its preparation method. Background Technology

[0002] Fruits and vegetables continue to respire after harvest, maintaining their life activities by consuming sugars, organic acids, and other substances in their own tissues, as well as consuming oxygen and releasing carbon dioxide. As storage time increases, if the gas composition and humidity conditions in the external environment are not properly controlled, the internal nutrients of fruits and vegetables will be consumed in large quantities, the respiration intensity will increase, and they will be prone to physiological disorders due to low oxygen damage or high carbon dioxide accumulation, resulting in deterioration in quality such as rotting, dehydration, and wilting.

[0003] Current fruit and vegetable packaging mostly uses conventional polyolefin films such as polyethylene and polypropylene, or simply perforated bags. The permeability and gas selectivity of these films often fail to meet the requirements for long-term preservation. On the one hand, ordinary single-layer films have low oxygen and carbon dioxide permeability, making fruits and vegetables prone to insufficient oxygen and excessive carbon dioxide accumulation in sealed packaging. On the other hand, traditional films lack effective differentiation in the diffusion and permeation of different gases; the transport rates of oxygen and carbon dioxide change roughly synchronously, making it difficult to create a suitable gas composition and humidity balance within the same package, tailored to the different respiration characteristics of various fruits and vegetables. Furthermore, to prevent condensation during refrigerated transport, some packaging materials increase the number of perforations or improve overall moisture permeability to promote moisture expulsion, but this exacerbates water loss and quality decline in fruits and vegetables.

[0004] To improve the shelf life and long-distance transportation performance of fruits and vegetables, various improved packaging materials and modified atmosphere storage solutions have been proposed. These include microporous perforations in films, coating the film surface with specific polymer layers, or using multilayer composite films with high gas permeability. According to public reports, Mitsui Chemicals Co., Ltd. of Japan has developed a packaging film called "AdFresh" to maintain the freshness of fruits and vegetables, achieving a shelf life of several months when used to package grapes. However, the material system and manufacturing process of such products are relatively complex, resulting in high costs and limited practical commercial application, making it difficult to universally replace existing general-purpose packaging materials.

[0005] In summary, the existing technology lacks a breathable regulating film that is based on polyolefins, has a simple structure, controllable microporous structure and macroscopic thickness, and achieves a reasonable match between the permeability characteristics of oxygen, carbon dioxide and water vapor, so that a stable microenvironment suitable for the respiratory metabolism of fruits and vegetables can be formed inside the packaging during long-term storage and transportation, while taking into account mechanical strength and processing adaptability. Summary of the Invention

[0006] The purpose of this invention is to overcome the aforementioned problems in the prior art and provide a polyolefin breathable conditioning film for preservation and its preparation method. By introducing inorganic fillers with specific particle size and content range into the polyolefin matrix, and combining the inorganic filler gradient distribution, the cooling rate difference in the extrusion stage, the temperature difference stretching in the biaxial stretching stage, and the unilateral heat setting process, an asymmetric microporous structure with a limited pore size ratio and open area ratio is constructed in the thickness direction of the film. This ensures the mechanical strength and dimensional stability of the film while creating a suitable microenvironment for respiratory metabolism during the storage and transportation of fruits and vegetables, thereby improving the shelf life and storage quality of fruits and vegetables.

[0007] To achieve the above objectives, a first aspect of the present invention provides a polyolefin breathability regulating film for preservation, the film comprising at least one polyolefin microporous layer, the microporous layer being formed by a polyolefin matrix and inorganic fillers dispersed in the polyolefin matrix to form a microporous structure, the film having a first surface and a second surface disposed opposite to each other, satisfying: (1) The total thickness of the film is 15-50 μm and the overall porosity is 40-70%; (2) The average surface aperture D1 of the first surface is 0.05 to 0.30 μm, and the opening area ratio P1 of the first surface is 5 to 25%. (3) The average surface aperture D2 of the second surface is 0.20 to 0.80 μm, the open area ratio P2 of the second surface is 20 to 60%, and D1 / D2 is 0.3 to 0.7, P1 / P2 is 0.2 to 0.6; (4) Under conditions of 23℃, 50% relative humidity, and a pressure difference of 0.1MPa, the oxygen permeability of the membrane is 4000–12000 cm⁻¹. 3 / (m 2 (24h·0.1MPa), the ratio of carbon dioxide to oxygen transmission rate is 3.0–5.0, and the water vapor transmission rate is 1000–3000 g / (m³). 2 •24h).

[0008] As a further improvement of the present invention, the polyolefin matrix is ​​at least one of polypropylene, linear low-density polyethylene, metallocene polyethylene and high-density polyethylene or a blend thereof.

[0009] As a further improvement of the present invention, the inorganic filler has a mass of 20-60% of the polyolefin matrix, and the inorganic filler is at least one of calcium carbonate, talc and silica, and has a volume average particle size of 0.5-3.0 μm.

[0010] As a further improvement of the present invention, the film comprises three polyolefin microporous layers, each microporous layer having a thickness of 5 to 25 μm, and the total thickness of the film after stacking is 20 to 35 μm; the mass fraction of inorganic filler in each microporous layer is distributed in a gradient along the thickness direction of the film, and the mass fraction of inorganic filler in the microporous layer closer to the second surface is higher than that in the microporous layer closer to the first surface.

[0011] As a further improvement of the present invention, the film has a longitudinal tensile strength of 80-200 MPa and an elongation at break of 50-180%; the film has a heat shrinkage rate of no more than 8% after being treated at 80°C for 30 minutes.

[0012] As a further improvement of the present invention, the film contains 0.05 to 1.0 wt% of an anti-fogging agent and 0.05 to 0.8 wt% of a slip agent. The anti-fogging agent is at least one of glycerol fatty acid ester, sorbitan fatty acid ester and polyoxyethylene fatty acid ester, and the slip agent is an amide or organosilicon slip agent.

[0013] The total thickness of the film is limited to 15–50 μm, and the overall porosity is controlled at 40–70%. Based on this structure, a first surface and a second surface with distinct differences in average pore size and open area ratio are formed on both sides of the film, resulting in an asymmetrical distribution of the microporous structure. The average pore size of the first surface is in the fine pore range of 0.05–0.30 μm, and the open area ratio is maintained at a low level of 5–25%, making this surface a relatively dense microporous layer. Under the low-temperature, high-humidity storage conditions of respiring fruits and vegetables, the water vapor generated by transpiration is less likely to reach local supersaturation after reaching the first surface, nor is it likely to form a continuous water film between the pores, thus significantly reducing the risk of condensation into droplets. Compared with microporous membranes with symmetrical pore structures, this dense surface can maintain a high-humidity environment without visible water droplets, reducing mold growth and fruit peel damage, which is one of the key conditions for achieving long-term storage and transportation stability.

[0014] The second surface has an average pore size of 0.20–0.80 μm and an open area ratio of 20–60%, with a significantly higher degree of pore opening than the first surface, resulting in more unobstructed gas diffusion channels on the outer side of the film. This reduces the diffusion resistance of carbon dioxide, making its dissipation path prioritized over oxygen penetration, resulting in a stable carbon dioxide to oxygen permeability ratio (3.0–5.0) under steady-state conditions. This selectivity does not stem from inherent chemical differences in the polyolefin material, but rather from a mass transfer direction bias caused by the difference in pore size (D1 / D2 = 0.3–0.7) and open area ratio (P1 / P2 = 0.2–0.6) between the first and second surfaces. This creates an independent regulatory mechanism for oxygen and carbon dioxide transport at the interface, allowing carbon dioxide generated during respiration to escape promptly while maintaining sufficient oxygen levels to support normal fruit and vegetable metabolism. If the above ratio deviates from the specified range, for example, tends to 1, the pore structure tends to be symmetrical, making it difficult to achieve the differentiated effect of preferential carbon dioxide dissipation and moderate oxygen entry; if the ratio is too small, the first surface will be too dense, which will inhibit oxygen supply and hinder water vapor dissipation, causing hypoxia damage or humidity accumulation problems.

[0015] Based on the aforementioned asymmetric microporous structure, a combination of 40–70% porosity and 15–50 μm film thickness forms interconnected multi-scale channels, enabling a steady-state oxygen permeability of 4000–12000 cm⁻¹. 3 / (m 2 The vapor transmission rate (VPS) is in the medium to high range (24h), while the water vapor transmission rate can be maintained at 1000–3000 g / (m²). 2 The medium range of 24h allows for dynamic coupling between gas diffusion and water vapor migration within the membrane. Compared to microporous membranes with the same porosity but symmetrical pore structures on both sides, the membrane of this invention more easily maintains suitable oxygen levels, avoids continuous carbon dioxide accumulation, and limits condensation formation during the peak respiration period of fruits and vegetables. This results in a relatively stable microenvironment over a storage and transportation period of several weeks to several months, preventing deterioration behaviors such as rapid oxygen depletion, uncontrolled internal relative humidity, or water droplet condensation covering the fruit surface.

[0016] Through the synergistic combination of the aforementioned film thickness, overall porosity, pore size of the first and second surfaces, open-pore ratio range, and the ratio relationship between the two sides, the film exhibits good gas regulation capabilities without altering the polyolefin material system. It can create a microenvironment conducive to the respiratory and metabolic balance of fruits and vegetables within sealed packaging. Symmetrical microporous membranes deviating from this structural system, or microporous membranes optimized for a single parameter, cannot simultaneously meet the requirements of inhibiting condensation, maintaining oxygen supply, and promoting carbon dioxide dissipation at the same thickness and porosity.

[0017] A second aspect of the present invention provides a method for preparing the polyolefin breathability-regulating film for preservation as described above, comprising the following steps: S1. Preparation of filler masterbatch: Polyolefin resin, inorganic filler and additives are melt-mixed and granulated at 170-230℃ in a certain proportion to obtain polyolefin filler masterbatch containing inorganic filler; S2, Unoriented sheet forming: The filler masterbatch is blended with polyolefin resin and extruded at 170–230°C to obtain unoriented polyolefin sheets with a thickness of 40–200 μm through die forming. S3, Biaxial tension: The unoriented polyolefin sheet is heated to 90-130°C and stretched in the longitudinal direction at a stretch ratio of 2.5-4.5 times, and then stretched in the transverse direction at 100-140°C at a stretch ratio of 2.0-3.5 times to form a through-pore structure. S4. Heat setting and cooling winding: The biaxially stretched film was heat-set at 110–150°C for 3–60 seconds, then cooled to below 40°C and wound up to obtain a polyolefin breathability regulating film with a thickness of 15–50 μm.

[0018] As a further improvement of the present invention, in the heat setting process of step S4, the film is heat-set only with the second surface in contact with the heating roller, and the first surface does not directly contact the heating roller.

[0019] As a further improvement of the present invention, in step S2, after the extruded melt is formed by the die head, the melt corresponding to the first surface side is immediately brought into contact with a cooling roller at a temperature of 10 to 25°C for cooling, while the melt corresponding to the second surface side is cooled by air at a temperature of 20 to 40°C. The linear speed is controlled at 5 to 20 m / min, so that the cooling rate of the first surface side is higher than that of the second surface side.

[0020] As a further improvement of the present invention, in step S3, the unoriented polyolefin sheet is fed into the stretching zone, and the first surface side is heated to 90-110°C through the first heating zone, while the second surface side is heated to a temperature 5-25°C higher than the first surface side through the second heating zone. Under this temperature difference condition, longitudinal stretching is performed at a stretching ratio of 2.5-4.0 times. After the longitudinal stretching is completed, the sheet is directly introduced into the transverse stretching zone without intermediate cooling. In the transverse stretching zone, the second surface side is kept facing the heat source, and transverse stretching is performed at a stretching ratio of 2.0-3.5 times.

[0021] This invention introduces inorganic fillers into a polyolefin matrix and performs melt blending, unoriented sheet extrusion, biaxial stretching, and unilateral heat setting to create a film with a continuous microporous structure. Furthermore, a stable asymmetric microporous structure is constructed along the film's thickness direction through a combination of differential cooling rates, temperature-differential stretching, and unilateral contact heat setting. This structure endows the film with selective permeability to oxygen and carbon dioxide, enabling it to spontaneously regulate gas composition in a sealed packaging environment, maintaining the optimal microenvironment required for fruit and vegetable respiration, thereby significantly improving preservation performance.

[0022] First, in step S1, the filler masterbatch obtained through melt mixing uniformly coats the inorganic filler within the polyolefin melt, forming a stable dispersion. This improves the melt strength of the unoriented sheet, enabling controlled interfacial debonding and micropore growth during subsequent biaxial stretching. The uniformity of the inorganic filler dispersion in the masterbatch directly determines the number and connectivity of the film's pore structure, thus affecting the final gas permeability.

[0023] Secondly, in step S2, the filler masterbatch is blended with polyolefin resin and extruded into an unoriented sheet. Forming with a die ensures uniform thickness and a stable interlayer interface, providing a controllable initial structure for the subsequent stretching process. Furthermore, in the improved embodiment of this invention, differentiated cooling is applied to the extruded sheet: the first surface is rapidly cooled by a cooling roller at 10–25°C, while the second surface is cooled by air at 20–40°C. This significantly increases the cooling rate of the first surface compared to the second surface. This difference in cooling path results in a gradient of crystallinity along the sheet thickness, decreasing from the first surface to the second surface. This makes the structure on the first surface more compact and restricts chain segment movement, while the structure on the second surface is relatively loose, making it more prone to interfacial debonding and pore expansion during subsequent stretching. This crystallinity gradient provides the necessary structural basis for the subsequent construction of asymmetric microporous structures.

[0024] Secondly, biaxial stretching is performed in step S3. Through continuous stretching in both the longitudinal and transverse directions, a large number of interfacial cavities are generated in the polyolefin matrix and gradually expand into a through-hole microporous structure. A further improvement in this invention involves creating a temperature difference stretching environment during the biaxial stretching process, maintaining the first surface at 90–110°C and the second surface at a temperature range 5–25°C higher than the first surface. Under the influence of this temperature difference, the polyolefin segments near the second surface are in a more fluid state, resulting in significantly higher rates of interfacial debonding and pore growth compared to the first surface side. This promotes the preferential growth of larger pores and increases the open area ratio on the second surface side. Furthermore, after longitudinal stretching, the process proceeds directly into the transverse stretching zone without intermediate cooling, preventing the initial pores between the MD layers from being "frozen." These pores can continue to expand along the second surface direction under the high-temperature conditions of TD stretching, further amplifying the differences in pore size and open area ratio between the two surfaces.

[0025] Furthermore, a heat-setting process at 110–150°C is employed in step S4 to lock in the crystalline structure of the film, thereby stabilizing its pore structure. As an improvement, during the heat-setting process, only the second surface contacts the heating roller, while the first surface is suspended and does not contact the heat source. This allows the second surface to receive higher heat and generate a larger chain segment relaxation space, which helps maintain a larger pore structure on the second surface and suppresses heat-setting shrinkage. Meanwhile, the first surface tends to maintain a smaller pore size and a lower porosity due to the lower temperature rise.

[0026] The present invention, by adopting the above technical solution, has the following beneficial effects: 1. By forming distinct microporous structures on both sides of the membrane, oxygen and carbon dioxide exhibit directional differences in their diffusion paths, achieving a synergistic effect of maintaining metabolic needs with high oxygen permeability and preventing carbon dioxide accumulation with faster dissipation speed.

[0027] 2. The first surface of the film maintains a low porosity and fine pore structure, making it difficult for a continuous condensation layer to form on the surface; the second surface maintains a high porosity, which is conducive to the escape of water vapor and respiratory gases. The combination of the two side structures can maintain a stable high humidity environment without excessive condensation during long-term storage, significantly improving the appearance retention and storage stability of fruits and vegetables compared to films with symmetrical pore structures.

[0028] 3. By setting an inorganic filler mass fraction that increases along the thickness direction in the polyolefin microporous layer, and introducing a cooling rate gradient in the unoriented sheet extrusion stage, introducing temperature difference stretching in the biaxial stretching stage and cooperating with unilateral heat setting, this invention organically couples the inorganic filler gradient, crystallinity gradient and stretching-induced pore development direction, so that the side near the second surface preferentially forms a large pore and high porosity region, while the side near the first surface maintains a fine pore and low porosity structure.

[0029] 4. By limiting the inorganic filler mass fraction to 20-60% of the matrix mass and the particle size to 0.5-3.0 μm, and combining it with the above-mentioned gradient structure, directional stretching, and heat setting process, this invention can achieve a microporous structure with a high porosity of 40-70% while still ensuring that the film has a tensile strength of 80-200 MPa in the longitudinal direction and a thermal shrinkage rate of no more than 8%, thus taking into account high air permeability, mechanical strength, and dimensional stability. Detailed Implementation

[0030] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0031] Unless otherwise defined, all scientific and technical terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art.

[0032] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0033] Unless otherwise specified, all materials and reagents used in the following examples are commercially available.

[0034] The present invention will now be described in detail with reference to specific embodiments, which are intended to understand rather than limit the invention.

[0035] Unless otherwise specified, the raw materials used in the following examples are all commercially available industrial products, and their sources and main indicators are as follows: Polypropylene resin PP-1: Homopolymer polypropylene resin, melt flow rate 3.0 g / 10 min (230℃ / 2.16 kg), isotacticity ≥95%, used for biaxially oriented films, purchased from Daqing Petrochemical, China National Petroleum Corporation.

[0036] Linear low-density polyethylene (LLDPE-1): density 0.918 g / cm³ 3 The melt mass flow rate was 2.0 g / 10 min (190℃ / 2.16 kg), and it was purchased from Sinopec Maoming Branch.

[0037] Calcium carbonate CaCO3-1: Light calcium carbonate with surface modified by stearic acid, with a volume average particle size of 1.0 μm and a specific surface area of ​​4.5 m² / g, purchased from a functional powder company in Guangdong.

[0038] Silica SiO2-1: Precipitated silica, with a volume average particle size of 2.0 μm and a specific surface area of ​​200 m². 2 / g, purchased from fumed or precipitated silica manufacturers.

[0039] Anti-fogging agent AF-1: Glyceryl monostearate (GMS), acid value ≤5mgKOH / g, purity ≥95%.

[0040] Slip agent SL-1: Erucamide-based slip agent, melting point 76-82℃, purity ≥98%.

[0041] Slip agent SL-2: Polydimethylsiloxane-based organosilicon slip agent, effective content ≥30wt%.

[0042] Other additives, such as antioxidants and lubricants, are conventional film-grade products and are added at conventional levels.

[0043] Thin film gas permeability test: Oxygen and carbon dioxide transmission rates were determined according to GB / T1038 standard at 23℃, 50% relative humidity, and 0.1MPa pressure difference; water vapor transmission rates were determined according to GB / T1037 standard at 38℃ and 90% relative humidity, and converted to equivalent values ​​at 23℃.

[0044] Micropore structure testing: Field emission scanning electron microscopy (FESEM) was used to image the first and second surfaces of the thin film. Image analysis software was used to statistically analyze the pore size distribution and calculate the average pore size and open area ratio. The overall porosity was determined by the density method: the density of the solid material was calculated based on the theoretical density of the resin and inorganic filler, and the porosity was obtained by comparing it with the measured density of the thin film.

[0045] Mechanical properties: The longitudinal tensile strength and elongation at break were determined at 23℃ according to GB / T13022; the heat shrinkage rate was measured after treatment in a water bath at 80℃ for 30 minutes according to GB / T12027.

[0046] Fruit and vegetable preservation performance test: Using grapes or tomatoes as model fruits and vegetables, packaging tests were conducted under refrigeration conditions of 5℃ and 90% relative humidity, and the shelf life, rot rate, weight loss rate and condensation were recorded. Example 1

[0047] This embodiment discloses a method for preparing a polyolefin breathability regulating film for preservation and the resulting film. The film is a three-layer polypropylene-based microporous structure with an inorganic filler gradient and an asymmetric microporous structure along the thickness direction.

[0048] The thin film structure consists of three microporous layers, with the inner layer L1, the middle layer L2, and the outer layer L3 arranged sequentially from the first surface to the second surface. The formulations of each layer (by mass) are as follows: L1 (dense microporous layer near the first surface): Polypropylene PP-1: 80 parts; LLDPE-1: 10 servings; CaCO3-1: 25 parts (approximately 25 wt% based on total resin content); SiO2-1: 5 parts (approximately 5 wt%); Antifogging agent AF-1: 0.3 parts; Slip agent SL-1: 0.2 parts; Antioxidant: 0.1 parts; L2 (intermediate layer, transitional microporous layer): Polypropylene PP-1: 75 parts; LLDPE-1: 10 servings; CaCO3-1: 35 parts; SiO2-1: 5 parts; Antifogging agent AF-1: 0.3 parts; Slip agent SL-1: 0.2 parts; Antioxidant: 0.1 parts; L3 (the side closest to the second surface, open microporous layer): Polypropylene PP-1: 70 parts; LLDPE-1: 10 servings; CaCO3-1: 45 parts; SiO2-1: 5 parts; Antifogging agent AF-1: 0.3 parts; Slip agent SL-2: 0.3 parts; Antioxidant: 0.1 parts.

[0049] The preparation method of polyolefin breathability-modifying film includes the following steps: Preparation of S1 filler masterbatch: Three filler masterbatches, MB-L1, MB-L2, and MB-L3, were prepared respectively, corresponding to the combinations of resin and inorganic fillers in the formulations of layers L1 to L3: PP-1, LLDPE-1, CaCO3-1, and SiO2-1 were added to a co-rotating twin-screw extruder with a screw diameter of 50 mm in the above proportions. The barrel temperature was set sequentially from the feeding section to the die head as 180℃, 190℃, 200℃, 210℃, and 210℃, respectively. The screw speed was 250 rpm. The extrudate was water-cooled and granulated to obtain filler masterbatch particles with a particle size of 2-3 mm. The mass fraction of inorganic fillers in each masterbatch was approximately 30 wt%, 40 wt%, and 50 wt%, respectively.

[0050] The anti-fogging agent and slip agent are added to the masterbatch or additive resin in a premixed manner to ensure that the total amount of anti-fogging agent in the final film is about 0.6 wt% and the total amount of slip agent is about 0.4 wt%.

[0051] S2 unoriented sheet forming: Three-layer unoriented sheets are formed by co-extruding a flat film extruder using a three-extruder.

[0052] Extruder A (L1 layer): The mass ratio of filler masterbatch MB-L1 to PP-1 is 6:4, and the overall inorganic filler mass fraction after blending is approximately 22wt%. The barrel temperature is 190~220℃, and the die head temperature is 220℃.

[0053] Extruder B (L2 layer): MB-L2 to PP-1 mass ratio is 7:3, inorganic filler is about 28wt%.

[0054] Extruder C (L3 layer): MB-L3 to PP-1 mass ratio is 8:2, inorganic filler is about 35wt%.

[0055] The three melt streams are stacked in the co-extrusion die head in a thickness ratio of 3:4:3 (L1:L2:L3) and extruded through a die head gap of 0.8mm to form a melt curtain.

[0056] After the melt is extruded, the melt closest to the first surface immediately comes into contact with a cooling roller at a temperature of 15°C. The cooling roller has a smooth surface and a linear speed of 10 m / min. The corresponding second surface side is cooled by an air knife at 30°C. By controlling the temperature of the cooling roller, the air temperature, and the linear speed, the cooling rate of the first surface side is significantly higher than that of the second surface side, and a three-layer unoriented sheet with a thickness of approximately 120 μm is wound up.

[0057] S3 bidirectional tension: The resulting unoriented sheet is fed into a biaxial stretching unit for temperature-differential stretching: (1) Longitudinal tension (MD): The temperature on both sides is controlled by the temperature difference between the roller surfaces. The roller surface temperature on the side closer to the first surface is set to 100℃, and the roller surface temperature on the side closer to the second surface is set to 112℃.

[0058] The sheet was longitudinally stretched at a stretch ratio of 3.3 times under this temperature difference condition, and the stretching time was about 5 seconds.

[0059] (2) Lateral stretching (TD): The longitudinally stretched film is not cooled in the intermediate stage and is directly fed into the transverse stretching furnace. The furnace temperature is maintained at 135°C, so that the second surface always faces the infrared heating source, while the first surface faces the relatively cooler support chain.

[0060] Under these conditions, the material is stretched laterally at a magnification of 3.0, with a stretching time of approximately 8 seconds.

[0061] The film thickness after stretching is approximately 22 μm.

[0062] S4 heat setting and cooling winding: The biaxially stretched film is held in a heat-setting zone at 135°C for 20 seconds. During the heat-setting process, only the second surface contacts the heating roller, while the first surface is suspended and does not directly contact the roller surface. After heat setting, the film is cooled to below 30°C by a multi-stage cooling roller and then wound up.

[0063] The final product is a three-layer polyolefin breathability regulating film with a total thickness of 23 μm. Example 2

[0064] This embodiment discloses a method for preparing a polyolefin breathability regulating film for preservation and the resulting film. The film is also a three-layer polyolefin-based microporous structure, but compared with Example 1, the content of inorganic filler, the resin ratio and the interlayer thickness ratio have been adjusted.

[0065] The thin film structure consists of three microporous layers, with the inner layer L1, the middle layer L2, and the outer layer L3 arranged sequentially from the first surface to the second surface. The formulations of each layer (by mass) are as follows: L1 (dense microporous layer near the first surface): Polypropylene PP-1: 70 parts; LLDPE-1: 20 servings; CaCO3-1: 22 parts (approximately 22 wt% based on the total amount of resin); SiO2-1: 3 parts (approximately 3 wt%) Antifogging agent AF-1: 0.3 parts; Slip agent SL-1: 0.2 parts; Antioxidant: 0.1 parts.

[0066] L2 (intermediate layer, transitional microporous layer): Polypropylene PP-1: 68 parts; LLDPE-1: 20 servings; CaCO3-1: 32 parts; SiO2-1: 3 parts; Antifogging agent AF-1: 0.3 parts; Slip agent SL-1: 0.2 parts; Antioxidant: 0.1 parts.

[0067] L3 (the side closest to the second surface, open microporous layer): Polypropylene PP-1: 62 parts; LLDPE-1: 25 servings; CaCO3-1: 42 parts; SiO2-1: 3 parts; Antifogging agent AF-1: 0.3 parts; Slip agent SL-2: 0.3 parts; Antioxidant: 0.1 parts.

[0068] The preparation method of polyolefin breathability-modifying film includes the following steps: S1 Filler Masterbatch Preparation Three filler masterbatches, MB-L1, MB-L2, and MB-L3, were prepared respectively, corresponding to the combinations of resin and inorganic fillers in the formulations of layers L1 to L3: PP-1, LLDPE-1, CaCO3-1, and SiO2-1 are added to a co-rotating twin-screw extruder with a screw diameter of 50 mm in the above proportions. The barrel temperature is set sequentially from the feeding section to the die head as 180℃, 190℃, 200℃, 210℃, and 210℃, and the screw speed is 250 rpm. The extrudate is then water-cooled and granulated to obtain filler masterbatch particles with a particle size of 2-3 mm.

[0069] The mass fractions of inorganic fillers in the three masterbatches are approximately: MB-L1: 25wt%; MB-L2: 35wt%; MB-L3: 45wt%.

[0070] The anti-fogging agent and slip agent are added to the masterbatch or additive resin in a premixed manner, so that the total amount of anti-fogging agent in the final film is about 0.6 wt% and the total amount of slip agent is about 0.4 wt%.

[0071] S2 Unoriented Sheet Molding Three-layer unoriented sheets are formed by co-extruding a flat film extruder using a three-extruder.

[0072] Extruder A (L1 layer): MB-L1 and PP-1 are blended at a mass ratio of 6:4, with approximately 20wt% inorganic filler; barrel temperature is 190-220℃, and die head temperature is 220℃.

[0073] Extruder B (L2 layer): MB-L2 and PP-1 are blended at a mass ratio of 7:3, with approximately 27 wt% inorganic filler.

[0074] Extruder C (L3 layer): MB-L3 and PP-1 are blended at a mass ratio of 8:2, with approximately 36 wt% inorganic filler.

[0075] The three melt streams are stacked in the co-extrusion die head at a thickness ratio of L1:L2:L3=2:5:3, and extruded through a die head gap of 0.9mm to form a melt curtain.

[0076] After the melt is extruded, the side closest to the first surface immediately comes into contact with a cooling roller at a temperature of 12°C; the side corresponding to the second surface is cooled by an air knife at a temperature of 32°C and a linear speed of 8 m / min, so that the cooling rate of the first surface is significantly higher than that of the second surface, resulting in a three-layer unoriented sheet with a thickness of about 150 μm.

[0077] S3 biaxial tensile strength The unoriented sheet is fed into a biaxial stretching unit for the following temperature difference stretching steps: (1) Longitudinal stretching (MD) The roller surface temperature on the side closer to the first surface is set to 95℃, and the roller surface temperature on the side closer to the second surface is set to 110℃.

[0078] Under this temperature difference condition, longitudinal stretching was performed at a stretch ratio of 3.0, with a stretching time of approximately 5 seconds.

[0079] (2) Lateral stretching (TD) The longitudinally stretched film is not subjected to intermediate cooling and is directly fed into the transverse stretching furnace. The furnace temperature is set to 130°C to ensure that one side of the second surface faces the infrared heating source and the first surface faces the cooler chain.

[0080] Under these conditions, the material is stretched laterally at a ratio of 2.6, with a stretching time of approximately 8 seconds.

[0081] The film thickness after stretching is approximately 28 μm.

[0082] S4 Heat setting and cooling winding The biaxially stretched film is held in a heat-setting zone at 130°C for 25 seconds. During the heat-setting process, only the second surface contacts the heating roller, while the first surface is suspended and does not contact the roller surface. It is then cooled to below 30°C by multiple cooling rollers and then wound up.

[0083] The final result is a three-layer polyolefin breathability regulating film with a total thickness of approximately 30 μm. Example 3

[0084] This embodiment discloses a method for preparing a polyolefin breathability-regulating film for preservation and the resulting film.

[0085] The thin film structure consists of three microporous layers, with the inner layer L1, the middle layer L2, and the outer layer L3 arranged sequentially from the first surface to the second surface. The formulations of each layer (by mass) are as follows: L1 (dense microporous layer near the first surface): Polypropylene PP-1: 78 parts; LLDPE-1: 10 servings; Talc-1: 20 parts (average particle size 1.2 μm); SiO2-1: 3 parts; Antifogging agent AF-1: 0.3 parts; Slip agent SL-1: 0.2 parts; Antioxidant: 0.1 parts.

[0086] L2 (intermediate layer, transitional microporous layer): Polypropylene PP-1: 72 parts; LLDPE-1: 10 servings; Talc-1 talc: 32 parts; SiO2-1: 3 parts; Antifogging agent AF-1: 0.3 parts; Slip agent SL-1: 0.2 parts; Antioxidant: 0.1 parts.

[0087] L3 (the side closest to the second surface, open microporous layer): Polypropylene PP-1: 68 parts; LLDPE-1: 12 servings; Talc-1 talc: 42 parts; SiO2-1: 3 parts; Antifogging agent AF-1: 0.3 parts; Slip agent SL-2: 0.3 parts; Antioxidant: 0.1 parts.

[0088] The preparation method of polyolefin breathability-modifying film includes the following steps: S1 Filler Masterbatch Preparation Three filler masterbatches, MB-L1, MB-L2, and MB-L3, were prepared respectively, corresponding to the combination of resin and talc / SiO2 in layers L1 to L3.

[0089] PP-1, LLDPE-1, Talc-1, and SiO2-1 are added to a co-rotating twin-screw extruder with a screw diameter of 50 mm in the above proportions. The barrel temperature is set to 180℃, 190℃, 200℃, 210℃, and 210℃, and the screw speed is 250 rpm. After water cooling and granulation, masterbatch particles with a particle size of 2-3 mm are obtained.

[0090] The mass fraction of inorganic fillers in the three masterbatches is approximately: MB-L1: Approximately 22 wt%; MB-L2: Approximately 35 wt%; MB-L3: Approximately 43 wt%.

[0091] The anti-fogging agent and slip agent are added in a premixed manner, so that the final film contains approximately 0.6 wt% anti-fogging agent and approximately 0.4 wt% slip agent.

[0092] S2 Unoriented Sheet Molding Three-layer unoriented sheets are formed by co-extruding a flat film extruder using a three-extruder.

[0093] Extruder A (L1 layer): MB-L1 and PP-1 are blended at a mass ratio of 6:4, with approximately 20wt% inorganic filler; temperature 190~220℃, die head 220℃.

[0094] Extruder B (L2 layer): MB-L2 and PP-1 are blended in a 7:3 ratio, with approximately 27 wt% inorganic filler.

[0095] Extruder C (L3 layer): MB-L3 and PP-1 are blended at a ratio of 8:2, with approximately 34 wt% inorganic filler.

[0096] The three melt streams are stacked in the die head in a thickness ratio of L1:L2:L3=3:4:3 and extruded through a 0.8mm die gap to form a melt curtain.

[0097] After the melt is extruded, it immediately comes into contact with a cooling roller at a temperature of 15°C on the side closest to the first surface; the second surface is cooled by an air knife at a temperature of 30°C with a linear speed of 9 m / min, resulting in a three-layer unoriented sheet with a thickness of approximately 125 μm.

[0098] S3 biaxial tensile strength (1) Longitudinal stretching (MD) The roller surface temperature difference is controlled as follows: First surface side: 98℃ Second surface side: 112℃ Under this temperature difference, the material is stretched longitudinally at a ratio of 3.2 for approximately 5 seconds.

[0099] (2) Lateral stretching (TD) After longitudinal stretching, it is directly fed into a transverse furnace without cooling. The furnace temperature is 132℃, and the second surface faces the infrared heat source.

[0100] Stretch laterally at a magnification of 2.8 times for approximately 8 seconds.

[0101] The film thickness after stretching is approximately 23 μm.

[0102] S4 heat setting and cooling winding: It is held in the 133°C heat-setting zone for 20 seconds, and heat-setting is performed only by the second surface contact heating roller. It is then cooled to below 30°C by multi-stage cooling rollers and then wound up.

[0103] The final result was a three-layer polyolefin breathability-regulating film with a total thickness of 24 μm.

[0104] The results of the performance tests on the thin films of Examples 1-3 are listed in Table 1.

[0105] Table 1

[0106] Comparative Example 1 This comparative example uses the same formulation and three-layer structure as Example 1, but the difference in cooling rates between the first and second surfaces is eliminated during the unoriented sheet extrusion stage.

[0107] The film formulation is the same as in Example 1, and will not be repeated here.

[0108] S1 Filler Masterbatch Preparation Three filler masterbatches, MB-L1, MB-L2, and MB-L3, were prepared according to the method of Example 1.

[0109] S2 Unoriented Sheet Molding Three-layer unoriented sheets were formed using a three-extruder co-extrusion flat film mill. The formulation was the same as in Example 1, but the process differed as follows: After the melt is extruded, both sides of the melt are simultaneously cooled by a cooling roller at a temperature of 25°C, without using a difference in cooling rate; the linear speed is maintained at 10 m / min.

[0110] The thickness of the unoriented sheet is approximately 120 μm.

[0111] Because the cooling rates are the same on both sides, no crystallinity gradient is formed in the thickness direction of the sheet, making it difficult to form an asymmetric microporous structure later.

[0112] S3 biaxial tensile strength Both longitudinal and transverse tensioning are performed under uniform temperature conditions. Longitudinal stretching roller surface temperature: 105℃ on both sides, stretching ratio 3.3 times.

[0113] Lateral stretching oven temperature: 135℃, the distance between the two sides of the film and the infrared heating source is symmetrical, and the stretching ratio is 3.0 times.

[0114] The film thickness after stretching is approximately 23 μm.

[0115] S4 Heat setting and cooling winding The heat setting temperature is 135℃ and the setting time is 20s. However, the method of alternating contact between the two sides of the setting roller is adopted, that is, the first surface contacts the roller surface for 10s and then the second surface contacts the roller surface for 10s, so that the single-sided heat setting effect is not produced.

[0116] The final result was a three-layer microporous membrane with a total thickness of 24 μm.

[0117] The results of the thin film performance test for Comparative Example 1 are listed in Table 2.

[0118] Table 2

[0119] Comparative Example 2 The difference between this comparative example and Example 1 is that the thermal stretching is omitted in step S3. Step S3 is as follows: The temperature of both sides of the longitudinal stretching roller surface is 105℃ with no temperature difference; the magnification ratio is maintained at 3.3 times.

[0120] The transverse stretching furnace temperature is 135℃, and the distance between the two sides facing the heat source is the same; the multiplier is maintained at 3.0 times.

[0121] The film thickness after stretching is approximately 23 μm.

[0122] The results of the thin film performance test for Comparative Example 2 are listed in Table 3.

[0123] Table 3

[0124] Conclusions: In Examples 1-3, the total film thickness was between 23 and 30 μm, the overall porosity was between 57 and 63%, the average pore size ratio D1 / D2 of the first surface to the second surface was stable between 0.30 and 0.33, the open area ratio P1 / P2 was stable between 0.28 and 0.32, and the corresponding oxygen permeability was maintained between 6200 and 7800 cm⁻¹. 3 / (m 2 (24h·0.1MPa), the ratio of carbon dioxide to oxygen transmission rate is 3.55–3.59, and the water vapor transmission rate is 1600–1850 g / (m³). 2 (24h), while the longitudinal tensile strength reaches 130~145MPa, and the heat shrinkage rate at 80℃ for 30min is controlled below 5.2%, proving that under the premise of meeting the asymmetric microporous structure parameters, it can take into account both air permeability and mechanical strength and dimensional stability.

[0125] In Comparative Example 1, after eliminating the cooling rate difference in the extrusion stage and employing double-sided heat setting, the pore structure on both sides of the film tends to be symmetrical, with D1 / D2 and P1 / P2 increasing to 0.93 and 0.95 respectively. The corresponding CO2 / O2 permeability ratio is only 2.38, failing to achieve preferential carbon dioxide dissipation. In Comparative Example 2, the temperature difference stretching was eliminated in the biaxial stretching stage. Although the cooling gradient and unilateral heat setting were retained, D1 / D2 and P1 / P2 were 0.71 and 0.76 respectively, and the CO2 / O2 permeability ratio dropped to 2.65. This indicates that relying solely on a single process control is insufficient to obtain a stable asymmetric microporous structure and the desired gas selectivity. The above comparative results demonstrate that the combination of cooling rate gradient, temperature difference stretching, and unilateral heat setting has a synergistic effect on forming and locking the pore size and porosity difference between the first and second surfaces. Deviating from this combined process makes it difficult to achieve the desired performance.

[0126] The above description is merely a preferred embodiment of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principles of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A polyolefin breathable regulating film for preservation, characterized in that, the film comprises at least one polyolefin microporous layer, the microporous layer is formed by a polyolefin matrix and an inorganic filler dispersed in the polyolefin matrix to form a microporous structure, the film has a first surface and a second surface arranged oppositely, and satisfies: (1) the total thickness of the film is 15-50 μm, and the overall porosity is 40-70%; (2) the surface average pore size D1 of the first surface is 0.05-0.30 μm, and the open area ratio P1 of the first surface is 5-25%; (3) the surface average pore size D2 of the second surface is 0.20-0.80 μm, the open area ratio P2 of the second surface is 20-60%, and D1 / D2 is 0.3-0.7, and P1 / P2 is 0.2-0.6; (4) The oxygen permeability of the film is 4000-12000 cm 3 / (m 2 ·24h·0.1MPa) at 23°C, 50% relative humidity, and a pressure difference of 0.1 MPa, the ratio of carbon dioxide permeability to oxygen permeability is 3.0-5.0, and the water vapor permeability is 1000-3000 g / (m 2 ·24h).

2. The polyolefin breathable and moisture regulating film for preservation according to claim 1, wherein, the polyolefin matrix is at least one of polypropylene, linear low density polyethylene, metallocene polyethylene and high density polyethylene or a blend thereof.

3. The polyolefin breathable and moisture regulating film for preservation according to claim 1, wherein, The mass of the inorganic filler is 20-60% of the mass of the polyolefin matrix, the inorganic filler is at least one of calcium carbonate, talcum powder and silicon dioxide, and the volume average particle size is 0.5-3.0 μm.

4. The polyolefin breathable and moisture regulating film for preservation according to claim 1, wherein The film comprises three polyolefin microporous layers, each microporous layer has a thickness of 5-25 μm, and the total thickness of the film after lamination is 20-35 μm; the mass fraction of the inorganic filler in each microporous layer is gradiently distributed along the thickness direction of the film, and the mass fraction of the inorganic filler in the microporous layer close to the second surface side is higher than that in the microporous layer close to the first surface side.

5. The polyolefin breathable and moisture regulating film for preservation according to claim 1, wherein, The tensile strength of the film in the longitudinal direction is 80-200 MPa, and the elongation at break is 50-180%; the heat shrinkage rate of the film after being treated at 80℃ for 30 min is not more than 8%.

6. The polyolefin breathable and moisture regulating film for preservation according to claim 1, wherein The film contains 0.05-1.0 wt% of an antifogging agent and 0.05-0.8 wt% of a slip agent, the antifogging agent is at least one of glycerol fatty acid ester, sorbitan fatty acid ester and polyoxyethylene fatty acid ester, and the slip agent is an amide or silicone slip agent.

7. A process for the production of a polyolefin breathable regulating film for the preservation of foodstuffs according to any one of claims 1-6, characterized in that, The method comprises the following steps: S1, preparation of a filled masterbatch: polyolefin resin, inorganic filler and auxiliary agent are mixed and granulated at 170-230℃ to obtain a polyolefin filled masterbatch containing inorganic filler; S2, forming of an unoriented sheet: the filled masterbatch is blended with polyolefin resin, extruded at 170-230℃, and formed into an unoriented polyolefin sheet with a thickness of 40-200 μm through a die; S3, biaxial stretching: the unoriented polyolefin sheet is heated to 90-130℃, stretched in the longitudinal direction at a stretching ratio of 2.5-4.5 times, and then stretched in the transverse direction at a stretching ratio of 2.0-3.5 times at 100-140℃ to form a through microporous structure; S4, heat setting and cooling and winding: the film after biaxial stretching is heat set at 110-150℃ for 3-60 s, then cooled to below 40℃ and wound to obtain a polyolefin breathable regulating film with a thickness of 15-50 μm.

8. A method for preparing a polyolefin breathability-regulating film for preservation according to claim 7, characterized in that, In the heat setting process of step S4, the film is only heat set with the second surface contacting the heating roller, and the first surface does not directly contact the heating roller.

9. A method for preparing a polyolefin breathability-regulating film for preservation according to claim 7, characterized in that, In step S2, after the extrusion melt is shaped through the die, the melt corresponding to the first surface side is immediately contacted with a cooling roller with a temperature of 10-25℃ for cooling, and the melt corresponding to the second surface side is cooled through 20-40℃ air cooling, with the linear speed controlled at 5-20 m / min, so that the cooling rate of the first surface side is higher than that of the second surface side.

10. A method for preparing a polyolefin breathability-regulating film for preservation according to claim 7, characterized in that, In step S3, the unoriented polyolefin sheet is sent into the stretching zone, the first surface side is heated to 90-110℃ through the first heating zone, while the second surface side is heated to a temperature 5-25℃ higher than that of the first surface side through the second heating zone, and longitudinal stretching is performed at a stretching ratio of 2.5-4.0 times under the temperature difference; after the longitudinal stretching, the transverse stretching zone is directly entered without intermediate cooling, the second surface side is kept facing the heat source in the transverse stretching zone, and transverse stretching is performed at a stretching ratio of 2.0-3.5 times.

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