Recyclable high-barrier polyolefin film and method for preparing the same

Abstract

The application provides a recyclable high-barrier polyolefin film and a preparation method thereof, belongs to the field of food packaging, and is composed of a polyolefin base material and a crosslinked PVA / PEI composite coating coated on the surface of the base material.The raw materials of the composite coating include the following in parts by weight: 80-95 parts of PVA, 5-20 parts of PEI, 1-1.5 parts of boric acid and 0.2-0.3 parts of glutaraldehyde.The application selects boric acid as a main crosslinking agent, and simultaneously adds a small amount of aldehyde crosslinking agent to further densify the coating network.Through optimization of the crosslinking temperature and the crosslinking process, the barrier property in a high-humidity environment can be improved, and meanwhile, the coating prepared by the application has recyclability, so that the resources can be fully recycled.

Description

Technical Field

[0001] This invention relates to the field of food packaging materials, and more specifically to a recyclable high-barrier polyolefin film and its preparation method. Background Technology

[0002] Polyolefin films are widely used in the packaging industry due to their lightweight, low cost, and excellent mechanical properties. However, pure polyolefin films have poor oxygen barrier properties, which cannot meet the storage requirements of oxygen-sensitive products. In existing technologies, polyvinyl alcohol (PVA) coatings are often used to improve barrier properties.

[0003] JPH08245816A discloses a high oxygen barrier polyolefin film suitable for food packaging. It is formed by directly coating the surface of a polyolefin film with a surface tension ≥35mN / m with a water-based anchoring agent such as PVA and PEI in a mixing ratio of 99 / 1 to 80 / 20, and then drying it at 70-130℃. The barrier properties can be further improved by vapor deposition of metals or inorganic compounds. The film has excellent oxygen barrier properties, moisture resistance and heat sealing strength. It does not contain PVDC components that pollute the environment, has low cost and simple process, and can replace traditional PVDC coated films.

[0004] However, the PVA molecular chain contains a large number of hydrophilic hydroxyl groups, resulting in poor moisture resistance of the coating. It easily absorbs water and swells in high humidity environments, and its oxygen barrier properties significantly deteriorate. Furthermore, while existing crosslinking agents can improve moisture resistance, the crosslinked PVA / PEI coatings are mostly irreversible crosslinked structures, making them difficult to recycle and reuse after disposal, leading to resource waste and environmental pressure. Therefore, developing a modified film that can significantly improve the moisture resistance of the coating and is recyclable has become an urgent need in this field. Summary of the Invention

[0005] This invention provides a recyclable high-barrier polyolefin film and its preparation method. By reversibly coordinating and crosslinking boric acid with the ortho-hydroxyl groups of PVA, and moderately crosslinking with a small amount of glutaraldehyde, the moisture resistance and structural stability of the coating are synergistically improved. At the same time, by utilizing the alkaline reversible hydrolysis characteristics of borate ester bonds, the coating can be efficiently recycled and reused, solving the problems of barrier attenuation and non-recyclability of existing PVA / PEI coatings under high humidity.

[0006] Specifically, the present invention discloses a recyclable high-barrier polyolefin film, which is composed of a polyolefin substrate and a cross-linked PVA / PEI composite coating coated on the surface of the substrate. The raw materials of the composite coating include, by weight, 80-95 parts of PVA, 5-20 parts of PEI, 1-1.5 parts of boric acid, and 0.2-0.3 parts of glutaraldehyde.

[0007] Preferably, the degree of polymerization of the PVA is 300-1500 and the degree of saponification is 88-99.5%; the PEI is branched PEI with a molecular weight between 50,000 and 100,000.

[0008] Preferably, the polyolefin substrate is a BOPP, CPP or PE film, and the surface of the substrate is corona treated with a surface tension ≥38mN / m.

[0009] This invention also discloses a method for preparing a recyclable high-barrier polyolefin barrier membrane, comprising the following steps:

[0010] (1) Preparation of PVA / PEI mixed solution: Mix PVA aqueous solution and PEI aqueous solution in proportion, stir at 300-800 rpm and 20-30℃ for 20-40 min until uniform, and adjust pH to 7-8;

[0011] (2) Pre-crosslinking: Heat the PVA / PEI mixed solution to 40-50℃, add boric acid, stir at constant temperature for 50-75 min, add glutaraldehyde and continue stirring for 10-15 min, let stand to degas for 30-60 min, and obtain the crosslinking coating working solution.

[0012] (3) Coating: The cross-linking coating working liquid is coated onto the surface of the polyolefin substrate by gravure coating method. First, the surface moisture is removed by online hot air drying at 50-80℃ for 1-2 min, and then it is transferred to a constant temperature oven at 35-45℃ for further drying and curing for 6-8 h to obtain a polyolefin barrier film.

[0013] Preferably, the concentrations of both the PVA aqueous solution and the PEI aqueous solution are 5-10 wt%.

[0014] Preferably, in step (3) of the preparation method, the coating speed is 60-80 m / min, and the dry coating amount is controlled to be 0.6-1.6 g / m. 2 .

[0015] This invention also discloses a method for recycling a composite coating of a recyclable high-barrier polyolefin film, comprising the following steps:

[0016] (1) De-crosslinking: Cut the waste barrier membrane into fragments, immerse them in 0.5-1.0 mol / L NaOH aqueous solution, stir at 60-80℃ for 30-60 min to obtain a mixed solution containing PVA and PEI;

[0017] (2) Separation and purification: Filter the mixed solution to remove insoluble impurities, take the filtrate, and neutralize it with hydrochloric acid to pH 7-8;

[0018] (3) Concentration and compounding: The neutralized solution is concentrated to a solid content of 10-15 wt% by rotary evaporation. Boric acid is added according to the original ratio, stirred for 20-40 min and allowed to stand to remove bubbles, to obtain the regenerated coating working solution, which can be reused for substrate coating. Because the amount of glutaraldehyde added is extremely low, no replenishment is required after decrosslinking, and it does not affect the performance of the regenerated coating.

[0019] This invention selects boric acid as the main crosslinking agent, which undergoes reversible coordination with the ortho-hydroxyl groups of PVA to form a stable borate ester crosslinking structure; at the same time, a small amount of glutaraldehyde is added for moderate crosslinking reaction, which further densifies the coating network while serving as a trace auxiliary crosslinking agent, without forming an irreversible crosslinking network and without affecting the coating's alkali solubility and recyclability.

[0020] This invention optimizes the crosslinking temperature and process, which can improve the stability of the coating without making it difficult to hydrolyze and recycle due to excessive crosslinking, thus achieving full recycling of resources.

[0021] Compared with the prior art, the present invention has the following technical advantages:

[0022] (1) The polyolefin film obtained by the present invention can maintain high barrier properties in high humidity environment, and its composite coating can be recycled and reused. It can still maintain high barrier properties after reuse, and has extremely high use value. It can improve the environment while reducing production costs.

[0023] (2) The recycling process of the present invention is simple and does not require toxic catalysts or complex equipment; the preparation process is compatible with existing polyolefin film production lines, has low production costs, and is suitable for large-scale application. Detailed Implementation

[0024] The raw materials used in the embodiments and comparative examples of this invention are as follows:

[0025] The polyolefin substrate is a 25μm BOPP film (surface tension 40mN / m), PVA: degree of polymerization 500, degree of saponification 99.0%, and PEI is a branched PEI with a molecular weight of 70000.

[0026] Example 1

[0027] A recyclable high-barrier polyolefin film, the raw materials by weight are: 92 parts PVA, 8 parts PEI, 1 part boric acid, and 0.2 parts glutaraldehyde.

[0028] Its preparation method includes the following steps:

[0029] (1) Preparation of PVA / PEI mixed solution: Mix 8wt% PVA aqueous solution and 8wt% PEI aqueous solution in proportion, stir at 500 rpm and 25℃ for 30 min until uniform, adjust pH to 7.5 to obtain PVA / PEI mixed solution;

[0030] (2) Pre-crosslinking: Heat the PVA / PEI mixed solution to 45°C, add boric acid, stir at constant temperature for 75 min, add glutaraldehyde, continue stirring for 15 min, let stand to degas for 45 min, and obtain the pre-crosslinking coating working solution.

[0031] (3) Coating: The cross-linking coating working solution is coated onto the surface of the BOPP substrate using a gravure coating method at a speed of 60 m / min and a dry coating amount of 1 g / m. 2 First, the surface moisture is removed by online hot air drying at 70℃ for 1.5 minutes, and then it is transferred to a constant temperature oven at 40℃ for further drying and curing for 7 hours to obtain BOPP barrier film.

[0032] Example 2

[0033] The difference from Example 1 is that the PVA is 85 parts, PEI is 15 parts, boric acid is 1 part, glutaraldehyde is 0.3 parts, the pre-crosslinking temperature is 50°C, and the other raw materials and preparation steps are the same.

[0034] Example 3

[0035] The difference from Example 1 is that the PVA is 88 parts, PEI is 12 parts, boric acid is 1.5 parts, glutaraldehyde is 0.2 parts, the pre-crosslinking temperature is 40°C, and the other raw materials and preparation steps are the same.

[0036] Comparative Example 1

[0037] The difference from Example 1 is that boric acid is used instead of glutaraldehyde in equal amounts, while the other raw materials and preparation steps are the same.

[0038] Comparative Example 2

[0039] The difference from Example 1 is that the pre-crosslinking temperature is 60°C, while the other raw materials and preparation steps are the same.

[0040] Comparative Example 3

[0041] The difference from Example 1 is that the amount of glutaraldehyde added is increased to 0.5 parts, while the other raw materials and preparation steps are the same.

[0042] Recycling and Reuse in Examples 1-3

[0043] De-crosslinking: Take the waste barrier membrane samples from Examples 1-3, immerse them in a 1.0 mol / L NaOH aqueous solution, stir in a water bath at 60 ℃ for 30 min, and observe that the coating is completely dissolved to obtain a mixed solution;

[0044] Separation and purification: Filter the mixed solution to remove insoluble impurities, take the filtrate, and neutralize it with 37% hydrochloric acid to pH 7.5;

[0045] Concentration and compounding: The neutralized solution is concentrated to 10 wt% solid content using a rotary evaporator, boric acid is added according to the original ratio, stirred for 30 min and allowed to stand to remove bubbles, to obtain the regenerated coating working solution;

[0046] Regenerated membrane preparation: Regenerated coated barrier membranes (referred to as Regenerated Examples 1-3) were prepared according to the coating and drying process of Example 1.

[0047] According to the recycling process of Example 1, the cross-linking of Comparative Examples 2 and 3 was decross-linked. Due to the irreversible structure caused by excessive cross-linking / high-temperature cross-linking, the coating could not be completely dissolved and was difficult to recycle.

[0048] Performance testing

[0049] The barrier membranes of Examples 1-3, Comparative Examples 1-3, and Regenerated Examples 1-3 were subjected to performance tests. Before testing, the samples were conditioned for 24 hours in the corresponding temperature and humidity environments. Oxygen permeability was tested according to the standard method of GB / T 19789-2021, with a test temperature of 23.0±0.1℃ and humidity conditions of 60%RH and 85%RH. The tests were performed in triplicate, and the average value was taken. The unit is cm³ / (m²·24h·atm). Water vapor permeability was tested according to the standard method of GB / T 26253-2010, with a test temperature of 40.0±0.5℃ and relative humidity conditions of 90±2%. The tests were performed in triplicate, and the average value was taken. The unit is g / (m²·24h). The heat seal strength was determined using the following method: A defect-free film sample was cut to a width of 15 mm and a length of 100 mm. The coated side was used as the heat seal surface. The sample was heat-sealed at 130℃, 0.2 MPa pressure, and a heat-sealing time of 2 seconds, ensuring an effective heat seal width ≥ 5 mm. The maximum tensile strength of the weld peel was tested using a tensile testing machine at 50 mm / min in a 20℃, 60% RH environment. At least five parallel samples were used in each group, and the arithmetic mean was taken as the result, expressed in N / 15 mm. The results are shown in the table below.

[0050]

[0051] As shown in the table above, the barrier films of Examples 1-3 have significantly better oxygen permeability, water vapor permeability, and heat sealing strength than Comparative Examples 1-3 under medium and high humidity environments.

[0052] In Comparative Example 1, the lack of glutaraldehyde resulted in insufficient overall cross-linking, leading to higher oxygen and water vapor permeability. In Comparative Example 2, the excessively high pre-cross-linking temperature affected the uniformity of the subsequent coating, resulting in decreased barrier properties and heat-sealing strength, making recycling impossible. In Comparative Example 3, further increasing the amount of glutaraldehyde had very limited effect on improving barrier performance and could easily lead to a series of problems such as coating embrittlement, reduced heat-sealing strength, and inability to recycle.

[0053] Compared to Example 1, the oxygen permeability and heat seal strength of Regenerated Example 1 decreased only slightly, demonstrating that the barrier membrane of the present invention has excellent recyclability.

[0054] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A recyclable high barrier polyolefin film, characterized by: It consists of a polyolefin substrate and a cross-linked PVA / PEI composite coating applied to the surface of the substrate. The raw materials of the composite coating include, by weight, 80-95 parts PVA, 5-20 parts PEI, 1-1.5 parts boric acid, and 0.2-0.3 parts glutaraldehyde.

2. The recyclable high-barrier polyolefin film according to claim 1, characterized in that: The PVA has a degree of polymerization of 300-1500 and a degree of saponification of 88-99.5%; the PEI has a molecular weight of 50,000-100,000.

3. The recyclable high-barrier polyolefin film according to claim 1, characterized in that: The polyolefin substrate is a BOPP, CPP or PE film, and the surface of the substrate is corona treated with a surface tension ≥38mN / m.

4. A method for preparing a recyclable high-barrier polyolefin film according to any one of claims 1-3, characterized in that, Includes the following steps: (1) Preparation of PVA / PEI mixed solution: Mix PVA aqueous solution and PEI aqueous solution in proportion, stir at 300-800 rpm and 20-30℃ for 20-40 min until uniform, and adjust pH to 7-8; (2) Pre-crosslinking: Heat the PVA / PEI mixed solution to 40-50℃, add boric acid, stir at constant temperature for 50-75 min, add glutaraldehyde and continue stirring for 10-15 min, let stand to degas for 30-60 min, and obtain the crosslinking coating working solution. (3) Coating: The cross-linking coating working liquid is coated onto the surface of the polyolefin substrate by gravure coating method. First, the surface moisture is removed by online hot air drying at 50-80℃ for 1-2 minutes, and then it is transferred to a constant temperature oven at 35-45℃ for 6-8 hours to continue drying and curing to obtain a polyolefin barrier film.

5. The method for preparing a recyclable high-barrier polyolefin film according to claim 4, characterized in that, The concentrations of both PVA and PEI aqueous solutions were 5-10 wt%.

6. The method for preparing a recyclable high-barrier polyolefin film according to claim 4, characterized in that, The coating speed in step (3) is 60-80 m / min, and the dry coating amount is controlled to be 0.6-1.6 g / m 2 .

7. A method for recycling the coating of a recyclable high-barrier polyolefin film according to any one of claims 1-3, comprising the following steps: (1) De-crosslinking: Cut the waste barrier membrane into fragments, immerse them in 0.5-1.0 mol / L NaOH aqueous solution, stir at 60-80℃ for 30-60 min to obtain a mixed solution containing PVA and PEI; (2) Separation and purification: Filter the mixed solution to remove insoluble impurities, take the filtrate, and neutralize it with hydrochloric acid to pH 7–8; (3) Concentration and compounding: The neutralized solution is concentrated to a solid content of 10-15 wt% by rotary evaporation, boric acid is added according to the original ratio, stirred for 20-40 min and allowed to stand to remove bubbles, and the regenerated coating working solution is obtained, which can be reused for substrate coating.

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