Production method of super-thin ultra-high molecular weight polypropylene bopp film

Abstract

This invention discloses a method for producing ultra-strong and ultra-thin ultra-high molecular weight polypropylene (BOPP) film, comprising the following steps: (1) isotactic polypropylene, ultra-high molecular weight polypropylene, compatibilizer and antioxidant are blended and melt-extruded, and cooled and shaped to form a cast sheet; (2) the cast sheet is subjected to intermittent biaxial stretching, which means that the stretching is paused at least once during the stretching process, and the film is moved to an environment with a lower temperature than the stretching temperature and kept for a period of time to release the stress accumulated in the molecular chains during stretching, and then the stretching is continued to obtain the BOPP base film; (3) the surface of the BOPP base film is subjected to corona treatment; (4) a chlorinated polypropylene modified acrylic resin primer is coated on the surface of the treated BOPP base film, and the finished film is obtained after drying. This invention achieves effective reduction of film thickness and simultaneous improvement of tensile strength, and endows the film with excellent initial adhesion and water resistance stability.

Description

Technical Field

[0001] This invention relates to the field of BOPP film technology, specifically a method for producing ultra-strong, ultra-thin, ultra-high molecular weight polypropylene BOPP film. Background Technology

[0002] Biaxially oriented polypropylene (BOPP) film is widely used in food packaging, electronic devices, medical protective equipment, and capacitor films due to its low density, excellent mechanical properties, good chemical stability, and low cost. With the increasing demands for lightweight and high performance from downstream industries, thinning and strengthening BOPP film has become a core direction for industry development.

[0003] However, existing BOPP films face two major technical problems in practical applications:

[0004] First, the main chain of polypropylene molecules is composed of saturated alkanes, lacking polar groups and having low surface energy, which makes subsequent processing such as printing, lamination, and metallization difficult, and results in poor coating adhesion. To improve surface adhesion, chlorinated polypropylene primers are often used industrially. Chlorinated polypropylene, with its polyolefin main chain structure similar to that of PP substrate, can form a compatible anchoring layer at the interface, improving adhesion. However, the film structure of a single chlorinated polypropylene system is relatively loose, with limited cohesive strength and insufficient water resistance, making it prone to interfacial failure in humid environments. If pure acrylic resin is used as a primer, although the film is dense, its compatibility with the PP substrate is poor, resulting in poor adhesion.

[0005] Second, to achieve film thinning, the biaxial stretching ratio needs to be increased. Although the introduction of ultra-high molecular weight polypropylene (UHMWPP) can significantly improve the mechanical strength of the film by forming more extended chain crystals during stretching due to its ultra-long molecular chains, the entanglement density of UHMWPP molecular chains is extremely high. Under high stretching ratio, it is difficult to respond to macroscopic strain and untangle in time, which easily leads to stress concentration and film breakage, thus limiting the further increase of the stretching ratio and the thinning of the film. Summary of the Invention

[0006] In view of the above-mentioned deficiencies of the prior art, the purpose of the present invention is to provide a method for producing an ultra-strong and ultra-thin ultra-high molecular weight polypropylene BOPP film.

[0007] To solve the above problems, the technical solution of the present invention is: a method for producing ultra-strong and ultra-thin ultra-high molecular weight polypropylene (BOPP) film, comprising the following steps:

[0008] (1) Isotactic polypropylene, ultra-high molecular weight polypropylene, compatibilizer and antioxidant are blended and then melt-extruded, cooled and shaped to form a cast sheet;

[0009] (2) Intermittent biaxial stretching of the casting film. Intermittent biaxial stretching means that the stretching is paused at least once during the stretching process. During the pause, the film is moved to an environment with a lower temperature than the stretching temperature and kept for a period of time to release the stress accumulated in the molecular chains during stretching. Then the stretching is continued to obtain the BOPP base film.

[0010] (3) Corona treatment is performed on the surface of the BOPP base film;

[0011] (4) A chlorinated polypropylene modified acrylic resin primer is coated on the surface of the treated BOPP base film, and the finished film is obtained after drying.

[0012] Furthermore, the components of the cast sheet are as follows by mass ratio: 92.7% isotactic polypropylene, 5% ultra-high molecular weight polypropylene, 2% compatibilizer and 0.3% antioxidant.

[0013] Furthermore, in step (2), the stretching temperature is 160°C, the ambient temperature during the pause is 120°C, and the pause duration is 10s.

[0014] Furthermore, in step (2), the stretching ratio is paused when it reaches 90% of the ultimate stretching ratio of the casting under continuous stretching conditions.

[0015] Furthermore, in step (4), the primer comprises chlorinated polypropylene and hydroxyethyl acrylate.

[0016] Further, the preparation method of the primer is as follows: 70 parts of xylene are added to a container equipped with a mechanical stirrer, thermometer, reflux condenser and dropping funnel, nitrogen protection is applied, the temperature is raised to 60°C, 4 parts of chlorinated polypropylene are added, and the mixture is stirred until completely dissolved. The temperature is then raised to 130°C, and the pre-mixed acrylic monomer mixture is added dropwise over 3 hours through the dropping funnel. After the addition is complete, the reaction is kept at the temperature for 1 hour. 0.5 parts of di-tert-butyl peroxide are added to improve the conversion rate. The temperature is lowered to 60°C, 0.4 parts of N,N-dimethylethanolamine are added for neutralization, the solid content is adjusted to 25% with xylene, and the mixture is filtered to obtain the chlorinated polypropylene modified acrylic primer.

[0017] The beneficial effects of this invention are: by using an intermittent stretching process of "stretching-pause-restretching", the necessary conformational adjustment and untangling time are provided for the ultra-long molecular chains of ultra-high molecular weight polypropylene, effectively releasing the local stress concentration under high stretch ratio, significantly improving the maximum stretch ratio of the blend system containing ultra-high molecular weight polypropylene, and achieving effective reduction of film thickness and simultaneous improvement of tensile strength.

[0018] A chlorinated polypropylene modified acrylic primer is coated on the surface of a pre-formed BOPP base film. The compatibility anchoring effect formed by the migration of chlorinated polypropylene segments to the PP interface, and the dense film-forming network constructed by the acrylic copolymer, synergistically improve the bonding strength between the primer and the base film and the cohesive strength of the primer itself, giving the film excellent initial adhesion and water resistance stability. Detailed Implementation

[0019] To provide a more intuitive and complete understanding of the technical solution of this invention, the following non-limiting features are described:

[0020] Example 1: A method for producing ultra-strong, ultra-thin, ultra-high molecular weight polypropylene (BOPP) film, comprising the following steps:

[0021] (1) Isotactic polypropylene, ultra-high molecular weight polypropylene, compatibilizer and antioxidant are blended and then melt-extruded, cooled and shaped to form a cast sheet;

[0022] (2) Intermittent biaxial stretching of the casting film. Intermittent biaxial stretching means that the stretching is paused at least once during the stretching process. During the pause, the film is moved to an environment with a lower temperature than the stretching temperature and kept for a period of time to release the stress accumulated in the molecular chains during stretching. Then the stretching is continued to obtain the BOPP base film.

[0023] (3) Corona treatment is performed on the surface of the BOPP base film;

[0024] (4) A chlorinated polypropylene modified acrylic resin primer is coated on the surface of the treated BOPP base film, and the finished film is obtained after drying.

[0025] The components of the cast sheet, by mass ratio, are: 92.7% isotactic polypropylene, 5% ultra-high molecular weight polypropylene, 2% compatibilizer, and 0.3% antioxidant. The compatibilizer is PP-g-MAH, and the antioxidant is antioxidant 1010. Step (1) specifically involves weighing 92.7% isotactic polypropylene and 5% ultra-high molecular weight polypropylene (viscosity-average molecular weight 1.5×10⁻⁶) by mass percentage. 6 The mixture consists of 5% g / mol PP-g-MAH, 2% PP-g-MAH, and 0.3% antioxidant 1010. The above components are premixed in a high-speed mixer for 10 min, then fed into a twin-screw extruder for melting and plasticizing. The mixture is then extruded through a T-die to form a melt sheet. The sheet is cooled and shaped by cold roller casting to obtain a cast sheet.

[0026] In step (2), the stretching temperature is 160℃, the ambient temperature during the pause is 120℃, and the pause duration is 10s. Specifically, step (2) involves placing the casting into a synchronous biaxial stretching machine with a stretching chamber temperature of 160℃. After preheating for 60s to eliminate the temperature gradient, an intermittent "stretch-pause-restretch" mode is adopted: the casting is synchronously biaxially stretched at a constant stretching rate to 5.2×5.2 times and then paused. The sample is then quickly transferred to a 120℃ annealing chamber and held in this environment for 10s to release the stress accumulated in the molecular chains during stretching. During this process, the stretching force is monitored to decrease from 128 N to 89 N, and the molecular chain relaxation is approximately 85%. After the pause, the sample is moved back to the 160℃ stretching chamber and stretched at the same rate to 5.9×5.9 times to obtain the reinforced BOPP base film.

[0027] In step (2), the process is paused when the stretching ratio reaches 90% of the ultimate stretching ratio of the casting under continuous stretching conditions.

[0028] In step (4), the primer contains chlorinated polypropylene and hydroxyethyl acrylate.

[0029] The preparation method of the primer is as follows: 70 parts of xylene are added to a container equipped with a mechanical stirrer, thermometer, reflux condenser and dropping funnel. Nitrogen gas is purged and the temperature is raised to 60°C. 4 parts of chlorinated polypropylene are added and stirred until completely dissolved. The temperature is then raised to 130°C. A pre-mixed acrylic monomer mixture (8 parts of methyl methacrylate, 10 parts of styrene, 12 parts of isooctyl acrylate, 6 parts of hydroxyethyl acrylate, 0.5 parts of acrylic acid, and 0.8 parts of di-tert-butyl peroxide) is added dropwise over 3 hours through a dropping funnel. After the addition is complete, the reaction is maintained at this temperature for 1 hour. 0.5 parts of di-tert-butyl peroxide are added to improve the conversion rate. The temperature is lowered to 60°C, and 0.4 parts of N,N-dimethylethanolamine are added for neutralization. The solid content is adjusted to 25% with xylene. The mixture is then filtered to obtain the chlorinated polypropylene modified acrylic primer.

[0030] Example 2: The difference between Example 2 and Example 1 is that the ambient temperature during the pause in step (2) is 100°C and the pause duration is 5s.

[0031] Example 3: The difference between Example 3 and Example 1 is that the ambient temperature during the pause in step (2) is 140°C and the pause duration is 20 s.

[0032] Example 4: The difference between Example 4 and Example 1 is that the mass fraction of hydroxyethyl acrylate in the primer is 4 parts.

[0033] Example 5: The difference between Example 5 and Example 1 is that the mass fraction of hydroxyethyl acrylate in the primer is 8 parts.

[0034] Comparative Example 1 (One-Step Stretching Method): The same casting formulation and preheating conditions as Example 1 were used, except that a conventional one-step continuous stretching method was used.

[0035] Comparative Example 2 (pure chlorinated polypropylene primer): The difference from Example 1 is that pure chlorinated polypropylene primer (without acrylate copolymer component) is coated on the BOPP base film obtained in Example 1.

[0036] Comparative Example 3 (pure acrylic primer): The difference from Example 1 is that a pure acrylic resin primer without chlorinated polypropylene is coated on the BOPP base film obtained in Example 1.

[0037] The performance test results of each embodiment and comparative example are shown in the table below:

[0038] sample Tensile strength / MPa Initial adhesion / level State after 10 days of immersion in water Adhesion grade after 10 days of immersion in water Example 1 242 0 No foaming 0-1 Example 2 215 0 No foaming 1 Example 3 228 0 No foaming 0-1 Example 4 240 0 No foaming 1 Example 5 235 0 Slight bubbling 1 Comparative Example 1 168 - - - Comparative Example 2 241 1 Slight bubbling 2 Comparative Example 3 239 3 Noticeable bubbling 4

[0039] As can be seen from the comparison between Example 1 and Comparative Example 1, the intermittent stretching process significantly improved the tensile strength, indicating that the intermittent relaxation effectively overcame the high stretch ratio limitation and achieved the enhancement and thinning of the film.

[0040] As can be seen from the comparison between Example 1 and Comparative Examples 2 and 3, the chlorinated polypropylene modified acrylic primer has both interfacial compatibility and film density, and its adhesion and water resistance are better than those of the single-component system.

[0041] This invention utilizes an intermittent stretching process of "stretching-pause-re-stretching" to provide the necessary conformational adjustment and deentanglement time for the ultra-long molecular chains of ultra-high molecular weight polypropylene, effectively releasing local stress concentration under high stretch ratios, significantly improving the maximum stretch ratio of the blend system containing ultra-high molecular weight polypropylene, and achieving effective reduction in film thickness and simultaneous improvement in tensile strength.

[0042] During the intermittent stretching process, the molecular chains spontaneously move to untangle and eliminate local stress. As the interval time increases, the local stress is gradually eliminated, and the draw ratio of BOPP is greatly improved. However, during the intermittent process, molecular chains in a partially molten state continuously move into the crystal lattice to form new crystals, which is not conducive to further stretching of BOPP. High intermittent temperatures are conducive to the untangling of molecular chains but not to the formation of new crystals, requiring longer interval times to achieve higher draw ratios, while low intermittent temperatures have the opposite effect. Therefore, to obtain high draw ratio BOPP through intermittent stretching, careful attention must be paid to the matching of interval time and temperature.

[0043] A chlorinated polypropylene modified acrylic primer is coated on the surface of a pre-formed BOPP base film. The compatibility anchoring effect formed by the migration of chlorinated polypropylene segments to the PP interface, and the dense film-forming network constructed by the acrylic copolymer, synergistically improve the bonding strength between the primer and the base film and the cohesive strength of the primer itself, giving the film excellent initial adhesion and water resistance stability.

Claims

1. A process for the production of superstrong ultrathin biaxially oriented polypropylene film of ultra-high molecular weight, characterized in that Includes the following steps: (1) Isotactic polypropylene, ultra-high molecular weight polypropylene, compatibilizer and antioxidant are blended and then melt-extruded, cooled and shaped to form a cast sheet; (2) Intermittent biaxial stretching of the casting film. Intermittent biaxial stretching means that the stretching is paused at least once during the stretching process. During the pause, the film is moved to an environment with a lower temperature than the stretching temperature and kept for a period of time to release the stress accumulated in the molecular chains during stretching. Then the stretching is continued to obtain the BOPP base film. (3) Corona treatment is performed on the surface of the BOPP base film; (4) A chlorinated polypropylene modified acrylic resin primer is coated on the surface of the treated BOPP base film, and the finished film is obtained after drying.

2. The process for production of ultra strong ultra thin UHMWPP BOPP film as claimed in claim 1 wherein: The components of the cast sheet by mass ratio are: 92.7% isotactic polypropylene, 5% ultra-high molecular weight polypropylene, 2% compatibilizer and 0.3% antioxidant.

3. The process for production of ultra strong ultra thin UHMWPP BOPP film as claimed in claim 1 wherein: In step (2), the stretching temperature is 160℃, the ambient temperature during the pause is 120℃, and the pause duration is 10s.

4. The process for production of ultra strong ultra thin UHMWPP BOPP film as claimed in claim 1 wherein: In step (2), the process is paused when the stretching ratio reaches 90% of the ultimate stretching ratio of the casting under continuous stretching conditions.

5. The process for production of ultra strong ultra thin UHMWPP BOPP film as claimed in claim 1 wherein: In step (4), the primer contains chlorinated polypropylene and hydroxyethyl acrylate.

6. The process for production of ultra strong ultra thin UHMWPP BOPP film as claimed in claim 5 wherein: The preparation method of the primer is as follows: 70 parts of xylene are added to a container equipped with a mechanical stirrer, thermometer, reflux condenser and dropping funnel. Nitrogen gas is purged and the temperature is raised to 60°C. 4 parts of chlorinated polypropylene are added and stirred until completely dissolved. The temperature is then raised to 130°C. The pre-mixed acrylic monomer mixture is added dropwise over 3 hours through the dropping funnel. After the addition is complete, the reaction is maintained at this temperature for 1 hour. 0.5 parts of di-tert-butyl peroxide are added to improve the conversion rate. The temperature is lowered to 60°C, and 0.4 parts of N,N-dimethylethanolamine are added for neutralization. The solid content is adjusted to 25% with xylene. The mixture is then filtered to obtain the chlorinated polypropylene modified acrylic primer.