High-barrier heat-sealable BOPET (Biaxially Oriented Polyethylene Terephthalate) film and preparation method thereof

Abstract

The invention relates to the field of polyester films, in particular to a high-barrier heat-sealable BOPET film and a preparation method thereof.In the scheme, a combination of a hydrogenated styrene-butadiene copolymer, polyolefin, a vinyl chloride-propylene copolymer and polylactic acid is adopted in a heat-sealing layer, a uniform system can be integrally formed, meanwhile, the heat-sealing performance of the BOPET film is good, and the BOPET film can be widely applied to the field of high-barrier heat-sealable BOPET films. Good barrier performance can also be provided, and the good improvement effect is achieved on oxygen barrier and water vapor barrier tending to be achieved for heat sealing.

Description

Technical Field

[0001] This application relates to the field of polyester films, and in particular to a high-barrier, heat-sealable BOPET film and its preparation method. Background Technology

[0002] Polyester is a widely used thermoplastic. With its excellent thermal stability, transparency, impact resistance, chemical stability, and recyclability, it is widely used in food and non-food packaging materials, building materials, electrical insulation materials, and liquid crystal electronic displays. BOPET film is a film structure obtained directly by biaxially stretching PET film or by co-extruding PET with other materials. It is widely used in the packaging of food, pharmaceuticals, and other materials.

[0003] For food packaging films, heat-sealing films are typically used to heat-seal the system to ensure normal packaging use while also maintaining food hygiene as much as possible. Another key concern for packaging films of food, pharmaceuticals, and other materials is the need for good gas barrier properties. Since PET itself has relatively low barrier properties, an additional barrier layer is usually added to the PET layer. This layer typically contains components such as polyethylene 2,5-furandicarboxylate, ethylene-vinyl alcohol copolymer, or nylon, which can achieve high barrier properties. This utilizes the inherent gas barrier properties of PET to extend the shelf life of the materials inside the packaging bag. Furthermore, to achieve both heat sealing and barrier properties, BOPET films are often designed in an ABCBA format, where C is the polyester film, B is the barrier film, and A is the heat-sealing film, generally using low-melting-point components such as PETG.

[0004] The above solution has a problem: low-melting-point components are usually selected in the sealing area, and their molecular regularity is lower than that of PET molecules. Therefore, the gas barrier properties in this area will be affected. At the same time, the heat sealing process may also weaken the interlayer bonding performance of the composite film, causing the barrier layer and the heat sealing layer to separate and form microchannels. This will also lead to the problem of reduced barrier properties of BOPET film after heat sealing packaging. Summary of the Invention

[0005] To improve the barrier properties of heat-sealable BOPET film and reduce gas leakage in the heat-sealing area without affecting the heat-sealing performance and transparency of the system, this application designs a barrier heat-sealable BOPET film and provides a corresponding preparation method to achieve the effects of high overall gas barrier, high sealing reliability and high transparency.

[0006] First, this application provides a high-barrier, heat-sealable BOPET film, comprising at least a co-extruded heat-sealable layer, a first barrier layer, a core layer, a second barrier layer, and a second surface layer arranged sequentially. The core layer comprises at least polyester, and the first and / or second barrier layers comprise at least one or more of ethylene-vinyl alcohol copolymer, polyethylene 2,5-furandicarboxylate, and nylon. The heat-sealable layer comprises the following components by mass percentage: Hydrogenated styrene-butadiene copolymer 20-30% Polyolefins 1-5% 5-10% vinyl chloride-propylene copolymer Polylactic acid balance; The density of the polyolefin is 0.925–0.935 g / cm³.

[0007] In the above scheme, hydrogenated styrene-butadiene copolymer, vinyl chloride-propylene copolymer and polyolefin components are added to the heat-sealing layer. As a whole, since none of the three polymers have unsaturated bonds, and the hydrogenated styrene-butadiene copolymer has a six-membered ring olefin structure on the branch chain, it can interfere with the formation of the regularly arranged molecular chains in the system, thus resulting in an amorphous state of the system, and therefore will not affect the overall transparency.

[0008] Building upon the above, in terms of heat-sealing performance, a higher-density polyolefin is used overall. This provides good flowability during heat sealing without affecting the heat-sealing performance of PLA, and the molecules are more easily tightly packed, improving overall gas barrier performance. During heat sealing, it does not show any disadvantage compared to a pure polylactic acid system. Furthermore, the hydrogenated styrene-butadiene copolymer provides better gas barrier properties and enhances the system's flexibility and adhesion to the barrier layer. Due to its larger branched chains, it can form corresponding entanglement structures with the adjacent first barrier layer during co-extrusion, thereby improving the connection stability between the first barrier layer and the heat-sealing layer and reducing microchannel formation during biaxial stretching and heat sealing. The vinyl chloride-polypropylene copolymer improves heat-sealing performance, enhancing flowability and adhesion. Furthermore, the introduction of chlorine atoms further improves overall hydrophobicity, reducing water vapor penetration and damage to the heat-sealing layer, thus improving the barrier effect. Overall, the above scheme ensures good oxygen and water vapor barrier performance after heat sealing.

[0009] Preferably, the polyolefin is LLDPE, a linear polyethylene. Due to its linear nature, LLDPE exhibits better performance during heat sealing, higher flexibility, and better transparency without affecting light transmittance. Furthermore, the higher density of LLDPE molecules results in a more regular molecular arrangement, leading to better gas barrier properties.

[0010] Preferably, the first barrier layer comprises the following components by mass percentage: Polyamide 10-30% The balance is polyethylene 2,5-furandicarboxylate.

[0011] In the above scheme, polyethylene 2,5-furandicarboxylate is used as the main barrier material. Its rigid furan structure helps promote close molecular packing, while the polar oxygen atoms enhance the dipole-dipole interactions between molecules, forming a dense hydrogen bond network, further inhibiting the diffusion path of gas molecules. In this scheme, a small amount of polyamide is incorporated into the system. This improves the moisture resistance and enhances the system's barrier performance against water vapor. Simultaneously, the polyamide's rigidity provides better puncture resistance while maintaining barrier properties. Based on these considerations, the preferred polyamide is obtained by condensing aliphatic diamines and aliphatic diacids. It has a chain-like molecular structure and lower light absorption, resulting in better overall processing performance, requiring less high application temperatures, and good compatibility with polyethylene 2,5-furandicarboxylate. Furthermore, its better elongation properties allow for better adhesion between the polyamide and the heat-sealing layer, reducing interlayer separation during heat sealing and minimizing the formation of microchannels.

[0012] Preferably, the first barrier layer further comprises 5-10% by weight of ethylene-vinyl acetate copolymer.

[0013] A small amount of ethylene-vinyl acetate copolymer in the system can improve the compatibility of polyamide and polyethylene 2,5-furandicarboxylate, and also improve the overall processing performance and toughness. Furthermore, it helps to improve the bonding tightness between the first barrier layer and the heat-sealing layer. Based on the above, in the first barrier layer, the molar content of vinyl acetate monomer in the ethylene-vinyl acetate copolymer is 20-28%. The content of vinyl acetate monomer affects the overall sealing performance. If the vinyl acetate content is too high, the system's barrier performance against water vapor will decrease; if the vinyl acetate content is too low, it will fail to achieve the desired effect of improving the compatibility of polyamide and polyethylene 2,5-furandicarboxylate, as well as improving the bonding tightness between the first barrier layer and the heat-sealing layer.

[0014] Preferably, the first barrier layer contains a toughening agent comprising 5-20% of the total content. More preferably, the toughening agent can be a material such as PBS resin, which improves the toughness of the first barrier layer and helps it maintain its toughness during extrusion and stretching, thereby improving yield and the mechanical properties of the film.

[0015] Preferably, the second surface layer contains an opening agent comprising 10-30% of its mass. Since the second surface layers are typically stacked together after processing, adding a small amount of opening agent can improve the ease of separation of the second surface layers, thereby reducing membrane damage during transportation and use.

[0016] In addition, this application also provides a method for preparing the above-mentioned high-barrier heat-sealable BOPET film. Specifically, the BOPET film is prepared by mixing the raw materials of each layer separately, extruding them in different extruders, cooling them to form a co-extruded sheet, and then preparing it by a two-step biaxial stretching method. The extrusion temperature of different layers should meet at least one of the following conditions: a. The extrusion temperature of the heat-sealing layer is 160–200°C; b. The extrusion temperature of the first barrier layer is 220–260°C; c. The extrusion temperature of the core layer is 250–300°C, and / or.

[0017] At the aforementioned processing temperatures, the temperature gradients of each layer are relatively smooth, and the interlayer fusion is good, making delamination and breakage less likely. Preferably, in the biaxial stretching step, the longitudinal stretching ratio is 3.0–3.5, and the transverse stretching ratio is 3.5–4.0. Through the above preparation method, good gas barrier properties and mechanical properties can be provided overall.

[0018] In summary, this application provides a high-barrier, heat-sealable BOPET film. By introducing hydrogenated styrene-butadiene copolymer and vinyl chloride-propylene copolymer into the heat-sealable layer, and adjusting its flowability with a small amount of polyolefin, while the first barrier layer is a combination of polyamide and polyethylene 2,5-furandicarboxylate, with the addition of ethylene-vinyl acetate copolymer to adjust its compatibility, the resulting heat-sealable layer and barrier layer are tightly bonded together. Furthermore, gas leakage at the seal point will not occur due to heat sealing, thus preventing poor airtightness, and gas leakage is also less likely to occur due to separation between the barrier layer and the heat-sealable layer. Detailed Implementation

[0019] The technical solution of this application will be further described through the following specific embodiments.

[0020] The following scheme was used to evaluate the following embodiments: 1. Gas permeability test: Two symmetrical films were cut and heat-sealed in a dry environment using a heat sealer at 110℃, 0.5mPa, and 1s. The heat seal width was 15mm, and the heat seal dividing line was controlled in the middle area of ​​the sample. Subsequently, the oxygen barrier performance of the sample was tested using a C103M gas permeability tester according to GB / T 1038-2000, and the water vapor barrier performance of the sample was tested using a C303M water vapor transmission rate tester according to GB / T 26253-2010.

[0021] 2. Heat sealing reliability: Cut two symmetrical films and heat seal them in a dry environment using a heat sealing machine with parameters of 110℃, 0.5mPa, and 1s. The heat seal width is 15mm. According to EN 415-6 standard, a peel test is performed using a heat seal strength tester at a speed of 300mm / min. The heat seal reliability is evaluated by the force required to pull the heat-sealed area apart.

[0022] Example 1: In this example, a high-barrier, heat-sealable BOPET film was designed and produced. The film, arranged sequentially from one side to the other, consists of a heat-sealable layer, a first barrier layer, a core layer, a second barrier layer, and a second surface layer. Each layer specifically comprises the following components and parameters: The heat-sealing layer, 5 μm thick, comprises the following components by mass percentage: Hydrogenated styrene-butadiene copolymer (Taiwan Polymer 1325EUT) 25% LLDPE (ExxonMobil™ C4LL, density 0.925 g / cm³) 3% Vinyl chloride-propylene copolymer (Shin-Etsu Chemicals, Japan) 8% Polylactic acid (Haizheng REVODE110) Balance; The first and second barrier layers are 5 μm thick and use the same formulation, containing the following components by mass percentage: Polyamide 1212 (DuPont) 20% Ethylene-vinyl acetate copolymer (Tosoh, Japan, EVA634, VA content 26%) 10% Toughening agent (PBS) 10% Poly(2,5-furandicarboxylate) (Hangzhou Tangneng) Balance.

[0023] The core layer is 10 μm thick, and the second surface layer is 5 μm thick. Both layers use the same formulation and contain the following components by weight percentage: Opening agent (fumed silica) 10% Polyester (bright PET) 90%.

[0024] The specific preparation method of the BOPET film in Example 1 is as follows: In extruder A, the raw materials for the core layer and the second surface layer are added according to the mass percentage, and the extrusion temperature is controlled at 270°C. In extruder B, the raw materials for the first barrier layer and the second barrier layer are added according to the mass percentage, and the extrusion temperature is 235°C. In extruder C, the raw materials for the heat-sealing layer are added according to the mass percentage, and the extrusion temperature is 180°C. The above extruders pass through a five-layer distributor in a certain proportion and are cooled on a chiller to form a multi-layer co-extruded sheet. Then, a BOPET 30μm film is produced by a two-step biaxial stretching method (longitudinal stretching ratio of 3.2 and transverse stretching ratio of 3.8).

[0025] It should be noted that the above scheme only provides an example preparation method. Since only three extruders are used in this scheme, for ease of processing, the second surface layer is set to have the same composition as the core layer, and the first barrier layer and the second barrier layer have the same composition to reduce the equipment cost in the production process. In fact, an opening agent may not be added to the core layer, and other components may be used in the second surface layer, such as a polyester-nylon blend system or a polyester-COC blend system. The second barrier layer may also use polyethylene 2,5-furandicarboxylate or polyamide as a separate layer, and a small amount of maleic anhydride-grafted polyethylene or other compatibilizers may be added to improve its adhesion performance with the core layer and the second surface layer. All of the above schemes are within the scope of protection of this application and will not be elaborated here.

[0026] Example 1 was tested and found to have an oxygen permeability of 4.469 cm³ / (m²·24h·0.1MPa), a water vapor permeability of 5.840 g / (m²·24h), and a heat seal peel strength of 4.4 N / 15mm.

[0027] To facilitate the verification of the heat-sealing layer and the first barrier layer, the heat-sealing layer composition as shown in Table 1 and the first barrier layer and second barrier layer compositions as shown in Table 2 were designed, as follows: In this embodiment, heat-sealing layer A1 is the heat-sealing layer used in Example 1, and barrier layer B1 is the first barrier layer and the second barrier layer in Example 1.

[0028] First, the use of hydrogenated styrene-butadiene copolymer was verified. Heat-sealing layers A1 to A5 were selected and orthogonal experiments were conducted with barrier layers B1, B2 and B11, respectively. The results are shown in Table 3.

[0029] The data in Table 3 show that the addition of hydrogenated styrene-butadiene copolymer (CBC) significantly improves the performance of the heat-sealing layer. Correspondingly, the permeability of water vapor and oxygen initially increases and then decreases with increasing CBC content. This may be because excessive CBC addition can easily lead to crystallization and protrusions at the interface, making bonding with the first barrier layer difficult. Furthermore, excessive CBC addition can reduce the heat-sealing performance of the heat-sealing layer and cause a certain degree of graying and reduced transparency in the system. Comparison of Examples 11-15 with other examples demonstrates that EVA can also improve heat-sealing strength and gas barrier properties in the system.

[0030] To further verify the role of the vinyl chloride-propylene copolymer in the system, heat-sealing layers A1, A6 to A8 were selected and combined with barrier layers B1, B12 to B17 in a combination experiment. The results were compared with those of Example 1, and the results are shown in Table 4.

[0031] The experimental data in Table 4 show that the addition of both polyamide and vinyl chloride-propylene copolymer significantly improves the gas barrier properties of the system, especially the water vapor permeability, and also slightly improves the oxygen barrier properties. The combined addition of both significantly improves the water vapor barrier performance. Among various polyamides, PA1212 and PA66, two non-ring-opening polyamides, offer better barrier properties, showing certain advantages in both water vapor and oxygen barrier properties.

[0032] Furthermore, the selection and use of polyolefins in the heat-sealing layer were verified. Heat-sealing layers A9 to A17 were selected and combined with the second barrier layer B1. The results were compared with those of Example 1, and the results are shown in Table 3.

[0033] The experimental data above show that using polyolefins in this application significantly affects the heat-sealing peel strength of the system compared to the method without adding polyolefins. Furthermore, due to the improved heat-sealing performance, it also significantly improves the oxygen and water vapor permeability of the system after heat sealing. If the density of the polyolefin is too low, or if LDPE with certain branches is used, the permeability of small molecules such as oxygen will increase.

[0034] Furthermore, experiments were conducted on the addition and selection of ethylene-vinyl acetate copolymer. Specifically, heat-sealing layer A1 was selected to be combined with barrier layers B1 to B11, and the experimental results are shown in Table 5.

[0035] The experimental data above show that the VA content in EVA significantly affects the permeability and adhesion of the system. Excessive VA content leads to a significant increase in water vapor permeability, while insufficient VA content results in poor heat sealing performance, reduced peel strength, and increased overall permeability.

[0036] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A high barrier, heat sealable BOPET film, characterized in that, At least comprising heat-seal layer, first barrier layer, core layer, second barrier layer and second surface layer which are co-extruded and arranged in sequence, the core layer at least contains polyester, the first barrier layer and / or second barrier layer at least contains one or more of ethylene-vinyl alcohol copolymer, poly(ethylene-2,5-furandicarboxylate) and nylon, the heat-seal layer contains the following components by mass percentage: Hydrogenated styrene-butadiene copolymer 20-30% Polyolefin 1-5% Vinyl chloride-propylene copolymer 5-10% Polylactic acid balance. The density of the polyolefin is 0.925-0.935 g / cm³.

2. A high barrier, heat sealable BOPET film according to claim 1, characterized in that, The polyolefin is LLDPE.

3. A high barrier, heat sealable BOPET film, characterized in that, The first barrier layer contains the following components by mass percentage: Polyamide 10-30% Poly(ethylene-2,5-furandicarboxylate) balance.

4. A high barrier, heat sealable BOPET film according to claim 3, characterized in that, The polyamide is obtained by condensation of aliphatic diamine and aliphatic diacid.

5. A high barrier, heat sealable BOPET film according to claim 3, characterized in that, The first barrier layer also contains ethylene-vinyl acetate copolymer accounting for 5-10% of the mass of the first barrier layer.

6. A high barrier, heat sealable BOPET film according to claim 5, characterized in that, The molar content of vinyl acetate monomer in the ethylene-vinyl acetate copolymer is 20-28%.

7. A high barrier, heat sealable BOPET film according to claim 1, characterized in that, The first barrier layer contains toughening agent accounting for 5-20% of the mass of the first barrier layer.

8. A high barrier, heat sealable BOPET film according to claim 1, characterized in that, The second surface layer contains opening agent accounting for 10-30% of the mass of the second surface layer.

9. Process for the production of high-barrier, heat-sealable BOPET films according to any one of claims 1 to 8, characterized in that, The BOPET film is prepared by mixing the raw materials of each layer respectively, adding them into different extruders for extrusion, then forming a co-extruded cast sheet after cooling, and then performing two-step biaxial stretching, wherein the extrusion temperature of different layers should meet at least one of the following conditions: a. The extrusion temperature of the heat-seal layer is 160-200℃; b. The extrusion temperature of the first barrier layer is 220-260℃; c. The extrusion temperature of the core layer is 250-300℃, and / or.

10. The process for the preparation of high barrier, heat-sealable BOPET films according to claim 9, characterized in that, In the biaxial stretching step, the longitudinal stretching ratio is 3.0-3.5, and the transverse stretching ratio is 3.5-4.0.

Images