Multilayer coextruded biaxially oriented film resistant to edge-cracking and method for its production
Patent Information
- Application Number
- CN202611253722.7
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2026-08-18
- Publication Date
- 2026-11-03
AI Technical Summary
Existing biaxially oriented films are prone to edge brittleness at high longitudinal stretching ratios, leading to reduced production continuity and product yield. Traditional solutions such as optimizing stretching temperature and adjusting chain clamping force have limited effectiveness.
The design employs a multi-layer co-extruded biaxially oriented film with an upper surface layer wider than the core layer. The chain clamps only hold the surface layer, leaving the core layer edges unconstrained. By combining the material properties of the soft surface layer and the highly crystalline core layer, a synergistic effect of "confinence-resistant soft surface layer and constraint-free hard core layer" is achieved.
It effectively reduces edge brittleness by more than 80%, reduces the number of film breaks by 90%, increases longitudinal tensile strength by 30%, increases product yield by 10-15%, improves edge smoothness, and enhances production continuity and product quality.
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Figure CN123025548A_ABST
Abstract
Description
Technical Field
[0001] This application belongs to the field of polymer materials technology, and specifically relates to a multilayer co-extruded biaxially oriented film resistant to edge brittleness and its preparation method. Background Technology
[0002] Biaxially oriented films (such as BOPP and BOPET) are widely used in packaging, electronics, and optics due to their excellent mechanical properties, optical properties, and dimensional stability. Currently, the related manufacturing processes for biaxially oriented films suffer from the following technical shortcomings: 1. Excessive total longitudinal stretching ratio leads to over-orientation of crystals. The film undergoes two stages of longitudinal stretching: a longitudinal stretching machine with a stretching ratio of 4-6 times, and a synchronous stretching machine with a longitudinal stretching ratio of 2-3 times, resulting in a total longitudinal stretching ratio of 8-18 times. Such a high total longitudinal stretching ratio causes the molecular chains to be arranged in a highly regular longitudinal direction, significantly improving crystallinity and drastically increasing the material's brittleness.
[0003] 2. Synchronous stretching chain clamping exacerbates edge brittleness. In a synchronous stretching machine, the edges of the film are clamped and fixed by chain clips, and both longitudinal and transverse stretching are performed simultaneously. Due to the clamping action of the chain clips, the edge region is longitudinally constrained during longitudinal stretching and cannot undergo simultaneous transverse stretching as the center of the film. For crystalline polymers such as polypropylene and polyester, this constraint can lead to excessive unidirectional crystal orientation in the edge region during high-temperature stretching, with the molecular chains arranged in a highly regular longitudinal direction. Studies have shown that if wrinkles or warping exist at the film edge when clamped, stress concentration will occur at the clamping position after the chain clips close, making the film prone to tearing and rupture during subsequent high-temperature processing. In addition, problems such as vibration of the chain clip drive system and deviation of the guide rail curve can also cause uneven stress on the film edge, exacerbating edge brittleness.
[0004] 3. Traditional uniform layer structures amplify edge defects. Traditional multilayer co-extruded biaxially oriented films typically employ a three-layer, equal-width structure, meaning the top and bottom surface layers and the intermediate layer have the same width. During the simultaneous stretching process, chain clamps directly hold the film at the edge. Because all three layers are clamped, the edge region experiences the same tensile constraint. When the surface and intermediate layers have different material properties, the interlayer stress difference further exacerbates the risk of edge brittleness.
[0005] 4. Edge failure is a prominent issue during high-ratio stretching. To improve the mechanical properties of thin films, modern biaxial stretching processes tend to employ high longitudinal stretching ratios (total longitudinal stretching ratios can reach over 10 times). However, the higher the longitudinal stretching ratio, the more fully the molecular chains are oriented, the higher the crystallinity, and the greater the brittleness of the material. In subsequent simultaneous stretching processes, the edge regions, constrained by chain clips, cannot release internal stress, becoming the weakest link in the entire film. This leads to frequent edge cracking and film breakage, severely impacting production continuity and product yield.
[0006] To address the aforementioned edge brittleness issue, relevant technologies primarily attempt to solve it through the following methods: 1) Optimize stretching temperature: Increase the synchronous stretching temperature to reduce the crystallization rate, but excessively high temperatures can lead to film adhesion and decreased dimensional stability; 2) Adjust the chain clamping force: Reducing the clamping force can reduce constraints, but it can easily lead to disengagement and deviation. 3) Edge trimming: This removes the brittle edges, but it wastes material and cannot fundamentally solve the problem of film breakage during the production process. Summary of the Invention
[0007] This application aims to at least solve one of the technical problems existing in the prior art. To this end, this application proposes a multilayer co-extruded biaxially oriented film resistant to edge brittleness and its preparation method, which effectively reduces the occurrence rate of edge brittleness and improves the mechanical properties of the film.
[0008] A first aspect of this application provides a multilayer co-extruded biaxially oriented film, comprising an upper surface layer, a core layer, and a lower surface layer; the upper surface layer and the lower surface layer are symmetrically arranged, and both are made of the same raw materials and have the same shape and size; the core layer is located between the upper surface layer and the lower surface layer; the width of the upper surface layer in the transverse direction is greater than the width of the core layer in the transverse direction; the raw material for preparing the upper surface layer includes homopolymer polypropylene A, and the flexural modulus of the homopolymer polypropylene A is ≤800MPa; the raw material for preparing the core layer includes homopolymer polypropylene B, and the flexural modulus of the homopolymer polypropylene B is ≥1200MPa.
[0009] Specifically, in traditional biaxially oriented films with a three-layer, equal-width structure, the chain clamps simultaneously hold the upper and lower surface layers and the core layer (intermediate layer), with the edge region bearing overall tensile constraint. The core layer, being a highly crystalline material, is prone to over-crystallization and orientation during high-temperature stretching, leading to edge embrittlement. This application employs an "edge deconstraint" structural design where the upper and lower surface layers are wider than the core layer. This design allows the chain clamps to hold only the relatively soft surface material, preventing the core layer edges from entering the clamping area. During synchronous biaxial stretching, the core layer edges are in a free-contraction state, unconstrained by the chain clamps, thus avoiding over-crystallization and orientation caused by forced constraint. Furthermore, the surface layer uses a homopolymer soft material with low longitudinal tensile orientation and good flexibility, bearing the clamping and tensile constraints; the core layer uses a highly crystalline material, providing mechanical strength, but the edge region is not constrained. This functional allocation achieves a synergistic effect of "soft surface layer resisting constraint, hard core layer free from constraint," fundamentally solving the edge embrittlement problem.
[0010] In some embodiments, the flexural modulus of the homopolymer polypropylene A is 600-800 MPa. For example, it can be 600 MPa, 650 MPa, 700 MPa, 750 MPa, 800 MPa; or any value between them.
[0011] In some embodiments, the flexural modulus of the homopolymer polypropylene B is 1200-1500 MPa. Specifically, it can be 1200 MPa, 1250 MPa, 1300 MPa, 1350 MPa, 1400 MPa, 1450 MPa, 1500 MPa; or any value between them.
[0012] In some embodiments, the difference between the width of the upper surface layer and the width of the core layer in the lateral direction is L, where 5mm ≤ L ≤ 30mm. Specifically, it can be 5mm, 10mm, 15mm, 20mm, 25mm, 30mm, or any value between them. In this application, the width in the lateral direction is divided into two sides, and the difference in width between each side is 2.5-15mm, i.e., 2.5mm ≤ L / 2 ≤ 15mm.
[0013] In some embodiments, the melt flow rate (MFR) of the homopolymer polypropylene A is 2-8 g / 10 min (test conditions: 230°C, 2.16 kg). Specifically, it can be 2 g / 10 min, 3 g / 10 min, 4 g / 10 min, 5 g / 10 min, 6 g / 10 min, 7 g / 10 min, 8 g / 10 min; or any value between them.
[0014] In some embodiments, the melt flow rate of the homopolymer polypropylene B is 1-5 g / 10 min (test conditions: 230°C, 2.16 kg). For example, it can be 1 g / 10 min, 2 g / 10 min, 3 g / 10 min, 4 g / 10 min, 5 g / 10 min; or any value between them.
[0015] In some embodiments, the raw materials for preparing the upper surface layer also include a polyolefin elastomer. Adding a polyolefin elastomer can further reduce the orientation degree of the surface layer.
[0016] In some embodiments, the polyolefin elastomer includes at least one selected from ethylene-octene copolymer, ethylene-butene copolymer, and ethylene-hexene copolymer.
[0017] In some embodiments, the raw materials for preparing the upper surface layer, by mass percentage, comprise 85%-95% homopolymer polypropylene A and 5%-15% polyolefin elastomer. For example, the specific mass percentage of homopolymer polypropylene A can be 85%, 88%, 90%, 92%, 95%, or any value between them. The specific mass percentage of polyolefin elastomer can be 5%, 8%, 10%, 12%, 15%, or any value between them.
[0018] In some embodiments, the thickness of the multilayer co-extruded biaxially oriented film is 1-100 μm. For example, it can be 1 μm, 10 μm, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm; or any value between them.
[0019] A second aspect of this application provides a method for preparing the multilayer co-extruded biaxially oriented film described in the first aspect of this application, comprising the following steps: The raw materials for each layer are placed in different extrusion equipment and melted and co-extruded through a die. The upper surface layer, core layer and lower surface layer are stacked in sequence to form a layered structure. After cooling, a three-layer composite thick sheet is obtained. The three-layer composite thick sheet is subjected to longitudinal stretching, simultaneous biaxial stretching, heat setting, and winding to obtain the multilayer co-extruded biaxially stretched film. During the synchronous bidirectional stretching process, the chain clamps are positioned in areas where the width of the upper and lower surface layers exceeds the width of the core layer. That is, the chain clamps only clamp the upper and lower surface layers, without clamping the core layer.
[0020] In some embodiments, the extrusion temperature of the upper surface layer is 190-230°C. For example, it can be 190°C, 200°C, 210°C, 220°C, 230°C; or any value between them.
[0021] In some embodiments, the extrusion temperature of the core layer is 200-250°C. For example, it can be 200°C, 210°C, 220°C, 230°C, 240°C, 250°C; or any value between them.
[0022] In some embodiments, the extrusion temperature of the lower surface layer is 190-230°C. For example, it can be 190°C, 200°C, 210°C, 220°C, 230°C; or any value between them.
[0023] In some embodiments, the longitudinal stretching temperature is 120-150°C. For example, it can be 120°C, 130°C, 140°C, 150°C, or any value between them.
[0024] In some implementations, the longitudinal stretching ratio is 4-6 times. Specifically, it can be 4 times, 5 times, 6 times; or any value in between.
[0025] In some embodiments, the temperature for the synchronous biaxial stretching is 150-170°C. For example, it can be 150°C, 155°C, 160°C, 165°C, 170°C, or any value between them.
[0026] In some embodiments, the longitudinal stretching ratio of the synchronous bidirectional stretching is 2-3 times, and the transverse stretching ratio is 5-8 times. For example, the longitudinal stretching ratio can specifically be 2 times, 2.5 times, 3 times; or any value between them. The transverse stretching ratio can specifically be 5 times, 6 times, 7 times, 8 times; or any value between them.
[0027] In some embodiments, the total longitudinal stretching ratio in the preparation method is 8-18 times. Specifically, it can be 8 times, 10 times, 12 times, 14 times, 16 times, 18 times; or any value between them.
[0028] Specifically, the membrane structure design of this application is particularly suitable for processes with high total longitudinal stretching ratios (4-6 times longitudinal stretching ratio × 2-3 times longitudinal stretching ratio of simultaneous biaxial stretching = 8-18 times). In such processes, the core material already undergoes extremely high longitudinal stretching orientation; if the edges are further constrained by chain clips, the risk of brittle fracture increases dramatically. This application, through the aforementioned "edge deconstraint" structural design, allows the core edge to contract freely during simultaneous stretching, releasing internal stress and effectively preventing brittle fracture.
[0029] In some embodiments, the thickness ratio of the upper surface layer, the core layer, and the lower surface layer in the three-layer composite sheet is 1:(6-10):1. For example, it can be 1:6:1, 1:7:1, 1:8:1, 1:9:1, 1:10:1, or any value between them.
[0030] In some embodiments, the heat setting temperature is 130-160°C. For example, it can be 130°C, 140°C, 150°C, 160°C, or any value between them.
[0031] Compared with the prior art, the beneficial effects of this application are as follows: (1) Completely solve the problem of edge brittleness: This application uses a structural design where the surface layer is wider than the core layer, so that the chain clamping area contains only the surface soft material and does not involve the highly crystalline core layer, reducing the edge brittleness rate by more than 80%, and reducing the number of times the film breaks during synchronous stretching by more than 90%, thus significantly improving production continuity.
[0032] (2) Supports ultra-high total longitudinal stretching ratio process: the longitudinal stretching ratio of the longitudinal stretching machine can reach 4-6 times, the longitudinal stretching ratio of the synchronous stretching machine can reach 2-3 times, and the total longitudinal stretching ratio can reach 8-18 times; the longitudinal tensile strength of the film is increased by more than 30%, and the upper limit of product performance is greatly improved; due to the solution of the edge brittleness problem, ultra-high stretching ratio becomes possible.
[0033] (3) Improve product yield: Reduce edge loss caused by edge cracking; improve overall product yield by 10-15% and reduce production costs.
[0034] (4) Improve film edge quality: edge smoothness is improved, with no wrinkles or microcracks; downstream slitting, printing and other processes have improved yield and customer satisfaction.
[0035] (5) Process flexibility: The surface layer width can be adjusted (5-30mm) according to product requirements, and it can be flexibly adapted to different production lines with different widths; the surface material can be selected according to the specific application to achieve functional customization.
[0036] (6) Equipment compatibility: Only the co-extrusion die head needs to be replaced or the die head flow channel design needs to be adjusted. There is no need to modify equipment such as longitudinal stretching machine and synchronous stretching machine. Existing biaxial stretching production lines can be directly applied, and the modification cost is low. Attached Figure Description
[0037] Figure 1 This is a schematic cross-sectional view of the three-layer co-extruded biaxially oriented film of this application; Figure 2 This is a schematic diagram of the chain clamp structure of this application, which only clamps the surface layer region extending beyond the core layer. Figure 3 This is a schematic cross-sectional view of the three-layer co-extruded biaxially oriented film of Comparative Example 1. Figure 4 This is a schematic diagram of the chain clamp that simultaneously holds the surface layer and the core layer, as shown in Comparative Example 1. Detailed Implementation
[0038] The following specific embodiments further illustrate the content of this application in detail. Unless otherwise specified, the raw materials, reagents, or apparatus used in the embodiments can be obtained from conventional commercial sources or by existing technical methods. Unless otherwise specified, the experimental or testing methods are conventional methods in the art.
[0039] As used in this application, the terms "approximately," "substantially," and "about" are used to describe and indicate small variations. When used in conjunction with an event or situation, the terms may refer to examples in which the event or situation occurred precisely and examples in which the event or situation occurred very approximately. For example, when used in conjunction with numerical values, the terms may refer to a range of variation less than or equal to ±10% of the numerical value, such as less than or equal to ±5%, less than or equal to ±4%, less than or equal to ±3%, less than or equal to ±2%, less than or equal to ±1%, less than or equal to ±0.5%, less than or equal to ±0.1%, or less than or equal to ±0.05%. For example, if the difference between two numerical values is less than or equal to ±10% of the average of the values (e.g., less than or equal to ±5%, less than or equal to ±4%, less than or equal to ±3%, less than or equal to ±2%, less than or equal to ±1%, less than or equal to ±0.5%, less than or equal to ±0.1%, or less than or equal to ±0.05%), then the two numerical values can be considered "substantially" the same.
[0040] Example 1 Reference Figure 1 This embodiment provides a three-layer co-extruded biaxially oriented film, including an upper surface layer, a core layer (intermediate layer) and a lower surface layer; the upper surface layer and the lower surface layer are symmetrically arranged, and the raw materials and shapes and sizes of the two are the same; the core layer is located between the upper surface layer and the lower surface layer; the width of the upper surface layer in the transverse direction is greater than the width of the core layer in the transverse direction.
[0041] By mass percentage, the upper layer is made of 95% homopolymer polypropylene A (MFR=5g / 10min, flexural modulus 750MPa) and 5% ethylene-octene copolymer; the core layer is made of 100% homopolymer polypropylene B (MFR=3g / 10min, flexural modulus 1350MPa); the lower layer is made of the same material as the upper layer.
[0042] The preparation method of the above-mentioned three-layer co-extruded biaxially oriented film includes the following steps: Step 1: Ingredient preparation and melt extrusion The raw materials for the preparation of the upper and lower surface layers and the core layer were fed into three extruders (two for the upper and lower surface layers and one for the core layer) for melt plasticization; the extrusion temperature was 210℃ for the upper and lower surface layers and 230℃ for the core layer.
[0043] Step 2: Three-layer co-extruded sheet A three-layer co-extrusion die is used, with the surface layer flow channel wider than the core layer flow channel. At the die exit, the surface melt coverage width is 20mm wider than the core layer (10mm wider on each side). The cast sheet is cooled and shaped by cooling rollers to form a three-layer composite sheet (the thickness ratio of the upper surface layer, core layer and lower surface layer is 1:8:1, with a total thickness of about 0.3mm).
[0044] Step 3: Longitudinal stretching (longitudinal stretching machine) Longitudinal stretching temperature: 130℃; Longitudinal stretching ratio: 5 times; After longitudinal stretching, the molecular chains are initially oriented longitudinally, and the thickness of the three-layer composite sheet is reduced to about 240 μm.
[0045] Step 4: Synchronous bidirectional stretching (synchronous stretching machine) Synchronous bidirectional stretching was performed using a synchronous stretching machine; Synchronous biaxial stretching temperature: 155℃; Longitudinal stretching ratio within the synchronous stretching machine: 2.5 times; Transverse stretching ratio within the synchronous stretching machine: 6 times; Total longitudinal stretch ratio: 5 × 2.5 = 12.5 times; Chain clamp position: approximately 8mm from the edge, ensuring that the clamping area only clamps the surface material (refer to...). Figure 2 ); Step 5: Heat setting and winding Heat setting temperature: 150℃; The film is pulled and wound up (winding tension is 250 N / m) to obtain the finished three-layer co-extruded biaxially oriented film. The total thickness of the finished film is 4 μm (0.3 mm ÷ 5 ÷ 2.5 ÷ 6 × 1000 ≈ 4 μm, considering the stretch ratio conversion).
[0046] Example 2 The difference between this embodiment and Embodiment 1 is that the process parameters in steps three and four are adjusted, as follows: Step 3: Longitudinal stretching (longitudinal stretching machine) Longitudinal stretching temperature: 125℃; Longitudinal stretching ratio: 4.5 times; Step 4: Synchronous bidirectional stretching (synchronous stretching machine) Synchronous biaxial stretching temperature: 160℃; Longitudinal stretching ratio in synchronous stretching machine: 3 times; Transverse stretching ratio in synchronous stretching machine: 8 times; Total longitudinal stretch ratio: 4.5 × 3 = 13.5 times; The remaining conditions are the same as in Example 1.
[0047] Comparative Example 1 (Traditional equal-width structure) The only difference between this comparative example and Example 1 is that it uses a three-layer equal-width structure, meaning the upper and lower surface layers and the middle layer have the same width (see reference). Figure 3 ); The chain clamp directly holds the edges of the three-layer material (refer to) Figure 4 The remaining conditions are the same as in Example 1.
[0048] Comparative Example 2 The only difference between this comparative example and Example 1 is that the raw materials used to prepare the upper and lower surface layers are the same homopolymer polypropylene B as those used for the middle layer, that is, the raw materials used to prepare the upper and lower surface layers and the core layer are the same; the other conditions are the same as in Example 1.
[0049] Comparative Example 3 The only difference between this comparative example and Example 1 is that the longitudinal stretching ratio of the longitudinal stretching machine is reduced to 3 times, the longitudinal stretching ratio of the synchronous stretching machine is reduced to 1.5 times, and the total longitudinal stretching ratio is 4.5 times; the other conditions are the same as in Example 1.
[0050] Performance testing The preparation process and related test results of the three-layer co-extruded biaxially oriented films obtained in the above embodiments and comparative examples are shown in Table 1.
[0051] Table 1
[0052] As shown in Table 1, Comparative Example 1, employing a traditional three-layer equal-width structure, resulted in a significantly increased edge brittle fracture rate and a higher number of membrane ruptures during 24 hours of continuous synchronous tensile testing. While Comparative Example 2 showed some improvement over the traditional three-layer equal-width structure, it still exhibited edge microcracks, indicating that the softness of the surface material and the structural design need to work synergistically. Although Comparative Example 3 reduced the edge brittle fracture rate, its longitudinal tensile strength was only 145 MPa, significantly lower than the 210 MPa of Example 1, demonstrating that the structure of this application has a more significant advantage at high total longitudinal tensile ratios.
[0053] The preferred embodiments of this application have been described in detail above, but the invention of this application is not limited to the embodiments described. Those skilled in the art can make various equivalent modifications or substitutions without departing from the spirit of this application, and these equivalent modifications or substitutions are all included within the scope defined by the claims of this application.
Claims
1. A multilayer co-extruded biaxially oriented film, characterized in that, It includes an upper surface layer, a core layer, and a lower surface layer; the upper surface layer and the lower surface layer are symmetrically arranged, and both are made of the same raw materials and have the same shape and size; the core layer is located between the upper surface layer and the lower surface layer; the width of the upper surface layer in the transverse direction is greater than the width of the core layer in the transverse direction; the raw material for preparing the upper surface layer includes homopolymer polypropylene A, and the flexural modulus of homopolymer polypropylene A is ≤800MPa; the raw material for preparing the core layer includes homopolymer polypropylene B, and the flexural modulus of homopolymer polypropylene B is ≥1200MPa.
2. The multilayer co-extruded biaxially oriented film according to claim 1, characterized in that, The difference between the width of the upper surface layer in the lateral direction and the width of the core layer in the lateral direction is L, where 5mm ≤ L ≤ 30mm.
3. The multilayer co-extruded biaxially oriented film according to claim 1, characterized in that, The melt flow rate of the homopolymer polypropylene A is 2-8 g / 10 min; and / or, the melt flow rate of the homopolymer polypropylene B is 1-5 g / 10 min.
4. The multilayer co-extruded biaxially oriented film according to claim 1, characterized in that, The raw materials for preparing the upper surface layer also include polyolefin elastomers, which include at least one of ethylene-octene copolymers, ethylene-butene copolymers, and ethylene-hexene copolymers.
5. The multilayer co-extruded biaxially oriented film according to claim 4, characterized in that, The raw materials for preparing the upper surface layer, by mass percentage, include 85%-95% homopolymer polypropylene A and 5%-15% polyolefin elastomer.
6. The multilayer co-extruded biaxially oriented film according to claim 1, characterized in that, The thickness of the multilayer co-extruded biaxially oriented film is 1-100 μm.
7. A method for preparing a multilayer co-extruded biaxially oriented film as described in any one of claims 1-6, characterized in that, Includes the following steps: The raw materials for each layer are placed in different extrusion equipment and melted and co-extruded through a die. The upper surface layer, core layer and lower surface layer are stacked in sequence to form a layered structure. After cooling, a three-layer composite thick sheet is obtained. The three-layer composite thick sheet is subjected to longitudinal stretching, simultaneous biaxial stretching, heat setting, and winding to obtain the multilayer co-extruded biaxially stretched film. During the synchronous bidirectional stretching process, the chain clamps are positioned in areas where the width of the upper and lower surface layers exceeds the width of the core layer.
8. The preparation method according to claim 7, characterized in that, The extrusion temperature of the upper surface layer is 190-230℃; the extrusion temperature of the core layer is 200-250℃; and the extrusion temperature of the lower surface layer is 190-230℃.
9. The preparation method according to claim 7, characterized in that, The longitudinal stretching temperature is 120-150℃; and / or the longitudinal stretching ratio is 4-6 times.
10. The preparation method according to claim 7, characterized in that, The temperature of the synchronous biaxial stretching is 150-170℃; and / or, the longitudinal stretching ratio of the synchronous biaxial stretching is 2-3 times and the transverse stretching ratio is 5-8 times.