Biaxially-oriented low-temperature coating-free film and preparation method thereof

Abstract

The invention belongs to the technical field of two-way stretching coating-free films, and particularly relates to a two-way stretching low-temperature coating-free film and a preparation method thereof.The two-way stretching low-temperature coating-free film comprises a surface layer, a middle layer and a bottom layer, and the total thickness of the coating-free film is 13-25 micrometers; the thickness of the surface layer is 1.3-2.5 microns, the surface layer accounts for 10%-15% of the total thickness of the coating-free film, the mass ratio of the surface layer is 12%-18%, and the surface layer is a BOPP coating-free film subjected to plasma treatment and is used for providing adhesive force; the thickness of the middle layer is 10.4-18.75 microns, the middle layer accounts for 80% of the total thickness of the coating-free film, the mass ratio of the middle layer is 65-75%, and the middle layer is a high-strength BOPP base material and is used for ensuring the mechanical performance of the coating-free film; the thickness of the bottom layer is 1.3-3.75 micrometers, the bottom layer accounts for 10%-15% of the total thickness of the coating-free film, and the mass of the bottom layer accounts for 10%-15%. The coating-free film can be directly printed or compounded in a low-temperature environment of 50-80 DEG C, prime coat treatment is not needed, compared with a traditional process, the production period is greatly shortened, the production efficiency is improved, and the coating-free film is particularly suitable for heat-sensitive product packaging.

Description

Technical Field

[0001] The present invention relates to the technical field of biaxially oriented coating-free films, in particular to a biaxially oriented low-temperature coating-free film and a preparation method thereof. Background Art

[0002] Biaxially oriented polypropylene (BOPP) coating-free films are widely used in food, pharmaceutical, and daily chemical packaging, as well as in label printing, due to their high transparency, excellent mechanical strength, and chemical stability. However, conventional BOPP coating-free films often require a primer treatment at temperatures of 120-150°C before printing or lamination to enhance the surface's adhesion to inks and composite materials. This process has numerous drawbacks: First, high-temperature treatment and primer coating increase production energy consumption and equipment investment, leading to increased costs; second, primer materials often contain volatile organic compounds (VOCs), which are emitted into the atmosphere during production, causing environmental pollution; third, the complex process extends production cycles and reduces production efficiency, and high-temperature treatment may affect the quality of temperature-sensitive products. To address these issues, existing technologies have attempted improvements. For example, Chinese patent CN108820856B discloses a coating-free, biaxially oriented polypropylene cold-seal base film and its preparation method. This technology achieves coating-free cold-seal functionality by adding specific additives to the surface layer, simplifying some processes. However, this technology primarily focuses on cold-seal performance and still suffers from insufficient ink bonding at low temperatures. Its low-temperature printing compatibility still needs improvement, and it cannot meet the packaging market's demand for efficient, environmentally friendly, and multifunctional coating-free film materials.

[0003] Therefore, it is of great significance to develop a BOPP coating-free film that can be directly printed or laminated at low temperature and does not require a primer. Summary of the Invention

[0004] In view of the deficiencies in the prior art, the present invention provides a biaxially oriented low-temperature coating-free film and a preparation method thereof to solve the problems raised in the above background technology.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solutions: a biaxially oriented low-temperature coating-free film and a preparation method thereof, comprising a surface layer, an intermediate layer and a bottom layer, the total thickness of the coating-free film being 13-25 microns; the surface layer has a thickness of 1.3-2.5 microns, accounting for 10%-15% of the total thickness of the coating-free film and 12%-18% by mass, and is a plasma-treated BOPP coating-free film used to provide adhesion; the intermediate layer has a thickness of 10.4-18.75 microns, accounting for 80% of the total thickness of the coating-free film and 65%-75% by mass, and is a high-strength BOPP substrate used to ensure the mechanical properties of the coating-free film; the bottom layer has a thickness of 1.3-3.75 microns, accounting for 10%-15% of the total thickness of the coating-free film and 10%-15% by mass.

[0006] As a preferred technical solution of the present invention, the surface layer is introduced with active groups by plasma treatment; the middle layer is prepared by adding 5% by weight of ethylene-propylene copolymer and 0.5% of sorbitol nucleating agent to homopolymerized polypropylene resin; the bottom layer is added with 3% by weight of quaternary ammonium salt antistatic agent when antistatic treatment is performed, and the surface resistance after antistatic treatment is ≤1×10 9 Ω, add glycerol fatty acid ester anti-fog agent accounting for 4% by weight during anti-fog treatment.

[0007] As a preferred technical solution of the present invention, the toughening agent of the middle layer is ethylene-propylene copolymer, and the nucleating agent is a sorbitol derivative; the antistatic agent of the bottom layer is a quaternary ammonium salt, and the antifogging agent is a glycerol fatty acid ester.

[0008] As a preferred technical solution of the present invention, the following steps are included: Raw material preparation: The surface layer material is 100 parts by mass of BOPP resin, with 2-5 parts by mass of silicone coupling agent and 1-3 parts by mass of acrylate monomer added; the middle layer material is 100 parts by mass of homopolypropylene resin, with 3-8 parts by mass of ethylene-propylene copolymer and 0.3-1 parts by mass of sorbitol nucleating agent added; the bottom layer material is added with 2-5 parts by mass of quaternary ammonium salt antistatic agent or 3-6 parts by mass of glycerol fatty acid ester antifogging agent according to functional requirements; Extrusion molding: The three layers of raw materials are added to the corresponding extruders respectively, with the surface layer extruder temperature at 180-190°C, the middle layer at 190-200°C, and the bottom layer at 185-195°C. The extrusion is carried out through a multi-layer co-extrusion die head, and the mass ratio of the extrusion amount of the surface layer, the middle layer and the bottom layer is controlled to be 1:(6-8):1; by adjusting the die head temperature distribution and the screw speed, the film extrusion speed is controlled at 10-13m / min, and the thickness tolerance is controlled at ±0.5 microns; Cooling the cast sheet: The film is rapidly cooled by a chill roller with a surface temperature of 10-15°C, and the cooling medium is circulating water to form a cast sheet; The cast sheet is transported at a constant speed by a traction device, and the extruder screw speed is monitored and feedback adjusted in real time using an infrared thickness gauge to ensure the uniformity of the cast sheet thickness; Biaxial Stretching: The cast sheet first enters the longitudinal stretching machine, where it is heated to 105-115°C by preheating rollers to impart a certain degree of mobility to the molecular chains. It is then stretched longitudinally by stretching rollers at a draw ratio of 3-5x. During this stretching process, the molecular chains are oriented longitudinally, achieving an orientation degree of over 85%. The stretched film is heat-set by heat-setting rollers at 125-135°C to stabilize the molecular chain orientation. After longitudinal stretching, the film enters the transverse stretching machine, where it is secured to the chain with clamps at an inlet temperature of 150-160°C. As the chain moves, the film is subjected to transverse tensile forces in the stretching zone, stretching it transversely at a draw ratio of 6-8x, achieving full transverse orientation of the molecular chains and achieving an orientation degree of over 88%. After stretching, the film is heat-set at 160-170°C to ensure dimensional stability.

[0009] Post-treatment: To further enhance the printability of the film's surface, the stretched film undergoes corona treatment. The film is passed through a corona treatment device at 2-4kW for 2-4 seconds, generating polar groups on the film's surface. This increases the surface tension from 38mN / m to 42-45mN / m, enhancing its ink-binding ability. Winding and Packaging: The corona-treated film is wound by a winding device at a tension of 12-20N. During the winding process, tension fluctuations are strictly controlled within a ±1N range to prevent wrinkles. After winding, the film is packaged in 20-micron-thick polyester film for moisture- and dust-proof packaging, facilitating storage and transportation.

[0010] As a preferred technical solution of the present invention, in the step of cooling the cast sheet, the cast sheet is transported at a constant speed by a traction device, and an infrared thickness gauge is used to monitor and feedback-adjust the extruder screw speed in real time to ensure the uniformity of the cast sheet thickness.

[0011] As a preferred technical solution of the present invention, in the biaxial stretching step, the longitudinal stretching makes the molecular chain orientation degree along the longitudinal direction of the coating-free film reach more than 85%, and the transverse stretching makes the molecular chain orientation degree along the transverse direction of the coating-free film reach more than 88%, thereby improving the tensile strength of the coating-free film.

[0012] As a preferred technical solution of the present invention, in the post-treatment step, the corona treatment increases the surface tension of the coating-free film from 38 mN / m to 42-45 mN / m, thereby improving the bonding force between the coating-free film surface and the ink.

[0013] As a preferred technical solution of the present invention, in the winding and packaging step, a polyester coating-free film with a thickness of 20 microns is used to package the coating-free film, and the tension fluctuation during the winding process is controlled at ±1N to prevent the coating-free film from wrinkling.

[0014] Compared with the prior art, the present invention provides a biaxially oriented low-temperature coating-free film and a preparation method thereof, which has the following beneficial effects: The invented coating-free film can be directly printed at low temperatures of 50-80°C through plasma treatment and optimization of raw material formula, without the need for primer treatment. Compared with traditional processes, it greatly shortens the production cycle and improves production efficiency. It is especially suitable for packaging heat-sensitive products. DETAILED DESCRIPTION

[0015] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention. Example

[0016] The present invention provides the following technical solutions: a biaxially oriented low-temperature coating-free film and a preparation method thereof, comprising a surface layer, an intermediate layer and a bottom layer, wherein the total thickness of the coating-free film is 13-25 microns; the surface layer is 1.3-2.5 microns thick, accounting for 10%-15% of the total thickness of the coating-free film and 12%-18% by mass; the BOPP coating-free film is surface modified by a plasma treatment process, wherein the plasma treatment uses an argon / oxygen mixed gas with a power of 1-3kW and a treatment time of 1-2s, and active groups are introduced on the surface of the coating-free film to increase the surface tension of the coating-free film to 42-45mN / m, thereby enhancing the bonding strength with inks and composite materials and ensuring printing and laminating effects at low temperatures; the intermediate layer is 10.4-18.75 microns thick, accounting for 80% of the total thickness of the coating-free film and 65%-75% by mass, and is prepared by adding 5% by mass of ethylene- It is made of propylene copolymer and 0.5% sorbitol nucleating agent, which is used to ensure the mechanical properties of the coating-free film, such as tensile strength and puncture resistance; the bottom layer thickness is 1.3-3.75 microns, accounting for 10%-15% of the total thickness of the coating-free film and 10%-15% by mass. It can be functionally treated according to actual application requirements, such as adding 3%-5% by mass of quaternary ammonium salt antistatic agent to achieve antistatic function; or adding 4%-6% by mass of glycerol fatty acid ester as an anti-fog agent to give the coating-free film anti-fog properties.

[0017] Specifically, the following steps are included: Raw material preparation: The surface layer material is 100 parts by mass of BOPP resin, with 2-5 parts by mass of silicone coupling agent and 1-3 parts by mass of acrylate monomer added; the middle layer material is 100 parts by mass of homopolypropylene resin, with 3-8 parts by mass of ethylene-propylene copolymer and 0.3-1 parts by mass of sorbitol nucleating agent added; the bottom layer material is added with 2-5 parts by mass of quaternary ammonium salt antistatic agent or 3-6 parts by mass of glycerol fatty acid ester antifogging agent according to functional requirements; Extrusion molding: The three layers of raw materials are added to the corresponding extruders respectively, with the surface layer extruder temperature at 180-190°C, the middle layer at 190-200°C, and the bottom layer at 185-195°C. The extrusion is carried out through a multi-layer co-extrusion die head, and the mass ratio of the extrusion amount of the surface layer, the middle layer and the bottom layer is controlled to be 1:(6-8):1; by adjusting the die head temperature distribution and the screw speed, the film extrusion speed is controlled at 10-13m / min, and the thickness tolerance is controlled at ±0.5 microns; Cooling the cast sheet: The film is rapidly cooled by a chill roller with a surface temperature of 10-15°C, and the cooling medium is circulating water to form a cast sheet; The cast sheet is transported at a constant speed by a traction device, and the extruder screw speed is monitored and feedback adjusted in real time using an infrared thickness gauge to ensure the uniformity of the cast sheet thickness; Biaxial stretching: For longitudinal stretching, the sheet is preheated to 105-115°C, stretched at a stretch ratio of 3-5 times, and heat-set at 125-135°C; for transverse stretching, the sheet is stretched at a stretch ratio of 6-8 times at 150-160°C, and heat-set at 160-170°C. Post-processing: The non-coating film is subjected to corona treatment with a power of 2-4kW and a time of 2-4s, reeled with a tension of 12-20N, and packaged with polyester non-coating film for moisture and dust proofing.

[0018] Specifically, during the cooling step of the cast sheet, the cast sheet is transported at a constant speed by a traction device, and an infrared thickness gauge is used to monitor and feedback-adjust the extruder screw speed in real time to ensure the uniformity of the cast sheet thickness.

[0019] Specifically, in the biaxial stretching step, the longitudinal stretching causes the molecular chain orientation degree along the longitudinal direction of the coating-free film to reach more than 85%, and the transverse stretching causes the molecular chain orientation degree along the transverse direction of the coating-free film to reach more than 88%, thereby improving the tensile strength of the coating-free film.

[0020] Specifically, in the post-treatment step, the corona treatment increases the surface tension of the coating-free film from 38 mN / m to 42-45 mN / m, thereby improving the bonding strength between the coating-free film surface and the ink.

[0021] Specifically, in the winding and packaging step, a polyester non-coating film with a thickness of 20 microns is used to package the non-coating film. During the winding process, the tension fluctuation is controlled within ±1N to prevent the non-coating film from wrinkling.

[0022] Example 1 Raw material preparation Surface layer raw materials: Take 100 parts by mass of BOPP resin, add 3 parts by mass of silicone coupling agent and 2 parts by mass of acrylic ester monomer, and mix in a high-speed blender for 10 minutes to evenly disperse the additives in the resin.

[0023] Middle layer raw materials: 100 parts by mass of homopolymer polypropylene resin, 5 parts by mass of ethylene-propylene copolymer, and 0.5 parts by mass of sorbitol nucleating agent were added to a mixing device and stirred for 15 minutes to ensure that all components were evenly mixed.

[0024] Bottom layer raw material: 3 parts by mass of a quaternary ammonium salt antistatic agent were added to 100 parts by mass of BOPP resin and stirred for 8 minutes to obtain a bottom layer raw material.

[0025] Extrusion The surface material was fed into an extruder and the temperature settings were: feeding section 185°C, compression section 188°C, metering section 190°C, and die head temperature 190°C.

[0026] The middle layer raw materials were added to the main extruder, and the temperature settings were: feeding section 195°C, compression section 198°C, metering section 200°C, and die head temperature 200°C.

[0027] The bottom layer raw materials were added to another extruder, and the temperature was controlled as follows: 190°C in the feeding section, 193°C in the compression section, 195°C in the metering section, and 195°C in the die head.

[0028] Start the three-layer co-extrusion die head, control the extrusion mass ratio of the surface layer, middle layer and bottom layer to 1:7:1, adjust the die head temperature distribution and screw speed, so that the film extrusion speed is stable at 12m / min, the thickness is controlled at 20 microns, and the thickness tolerance is ±0.5 microns.

[0029] Cooling the casting After extruding from the die, the film immediately enters a chilled roller cooling system with a surface temperature of 12°C and circulating water as the cooling medium. Driven by a traction device, the film is conveyed at a constant speed. An infrared thickness gauge monitors the film's thickness in real time, providing feedback to adjust the extruder screw speed to ensure uniform film thickness.

[0030] Biaxial stretch Longitudinal stretching: The cast sheet enters the longitudinal stretching machine, is heated to 110°C by the preheating roller, and then is longitudinally stretched by the stretching roller at a stretching ratio of 4 times. The stretched coating-free film is heat-set under the heat-setting roller at 130°C.

[0031] Transverse stretching: After longitudinal stretching and shaping, the coating-free film enters the transverse stretching machine with an inlet temperature of 155°C. It is transversely stretched at a stretching ratio of 7 times in the stretching area and then heat-set at a temperature of 165°C.

[0032] Post-processing The stretched non-coating film was subjected to corona treatment with a corona treatment power of 3 kW and a treatment time of 3 s, thereby increasing the surface tension of the non-coating film to 42 mN / m.

[0033] The corona-treated non-coating film is wound at a tension of 15N. During the winding process, the tension fluctuation is controlled within ±1N and the film is packaged with a 20-micron thick polyester non-coating film.

[0034] Example 2 Raw material preparation Surface layer raw materials: Weigh 100 parts by mass of BOPP resin, add 4 parts by mass of silicone coupling agent and 2.5 parts by mass of acrylic acid ester monomer, and stir for 12 minutes.

[0035] Intermediate layer raw materials: 100 parts by mass of homopolypropylene resin, 6 parts by mass of ethylene-propylene copolymer, and 0.7 parts by mass of sorbitol nucleating agent were fully mixed and stirred for 16 minutes.

[0036] Bottom layer raw materials: Add 4 parts by mass of glycerol fatty acid ester antifogging agent to 100 parts by mass of BOPP resin and stir for 10 minutes until uniform.

[0037] Extrusion The temperature settings of the surface material extruder are: feeding section 182°C, compression section 187°C, metering section 192°C, and die head temperature 192°C.

[0038] The temperatures of the middle layer raw material extruder were as follows: feeding section 192°C, compression section 197°C, metering section 202°C, and die head temperature 202°C.

[0039] The bottom layer raw material extruder temperature was: feeding section 187°C, compression section 192°C, metering section 197°C, and die head temperature 197°C.

[0040] The film was extruded through a three-layer co-extrusion die head, with the mass ratio of each layer being controlled at 1:6.5:1. The parameters were adjusted to achieve a film extrusion speed of 11.8 m / min, a thickness of 18 μm, and a thickness tolerance of ±0.48 μm.

[0041] Cooling the casting The diaphragm is rapidly cooled by chilled rollers with a surface temperature of 13°C. The cooling medium is circulating water. The thickness is monitored and adjusted in real time during the traction device conveying the casting.

[0042] Biaxial stretch Longitudinal stretching: The cast sheet was preheated to 112°C, stretched at a stretching ratio of 4.3 times, and heat-set at 132°C.

[0043] Transverse stretching: Transverse stretching at a stretch ratio of 7.3 times at 157°C and heat setting at 167°C.

[0044] Post-processing The corona treatment power is 3.3kW and the time is 3.3s, which increases the surface tension of the non-coating film to 43mN / m.

[0045] Rewinding with a tension of 16.2N, with tension fluctuation controlled to ±0.8N, and polyester coating-free packaging.

[0046] Example 3 Raw material preparation Surface layer materials: 100 parts by mass of BOPP resin, 2 parts by mass of silicone coupling agent and 1 part by mass of acrylate monomer were selected and mixed and stirred for 10 minutes.

[0047] Middle layer raw materials: 100 parts by mass of homopolypropylene resin, 3 parts by mass of ethylene-propylene copolymer, and 0.3 parts by mass of sorbitol nucleating agent were placed in a mixer and stirred for 14 minutes.

[0048] Bottom layer raw materials: Add 2 parts by mass of quaternary ammonium salt antistatic agent to 100 parts by mass of BOPP resin and stir for 8 minutes.

[0049] Extrusion The surface extruder temperature was set as follows: feeding section 180°C, compression section 185°C, metering section 190°C, and die head temperature 190°C.

[0050] The middle layer extruder temperature was: feeding section 190°C, compression section 195°C, metering section 200°C, and die head temperature 200°C.

[0051] The bottom extruder temperature was: feeding section 185°C, compression section 190°C, metering section 195°C, and die head temperature 195°C.

[0052] Extruded through a three-layer co-extrusion die, the mass ratio of the surface layer, middle layer and bottom layer was maintained at 1:6:1. The die and screw parameters were adjusted to make the film extrusion speed 10m / min, the thickness controlled at 13 microns, and the thickness tolerance ±0.5 microns.

[0053] Cooling the casting The diaphragm is rapidly cooled by chilled rollers with a surface temperature of 10°C. The cooling medium is circulating water. During the transportation process of the traction device, the thickness of the casting is monitored and adjusted in real time using an infrared thickness gauge.

[0054] Biaxial stretch Longitudinal stretching: The cast sheet was preheated to 105°C, stretched at a draw ratio of 3 times, and then heat-set at 125°C.

[0055] Transverse stretching: Transverse stretching is performed at a stretching ratio of 6 times at 150°C, and then heat-set at 160°C.

[0056] Post-processing The corona treatment power is 2kW and the treatment time is 2s, which increases the surface tension of the non-coating film to 42mN / m.

[0057] The product is wound with a tension of 12N, with the tension fluctuation controlled within ±1N, and packaged with 20-micron thick polyester coating-free film.

[0058] Example 4 Raw material preparation Surface layer raw materials: Take 100 parts by mass of BOPP resin, add 5 parts by mass of silicone coupling agent and 3 parts by mass of acrylic ester monomer, and stir thoroughly for 15 minutes.

[0059] Middle layer raw materials: 100 parts by mass of homopolymer polypropylene resin, 8 parts by mass of ethylene-propylene copolymer, and 1 part by mass of sorbitol nucleating agent were mixed uniformly and stirred for 20 minutes.

[0060] Bottom layer raw materials: Add 5 parts by mass of quaternary ammonium salt antistatic agent to 100 parts by mass of BOPP resin and stir for 12 minutes until completely dispersed.

[0061] Extrusion The surface extruder temperature was: feeding section 190°C, compression section 195°C, metering section 200°C, and die head temperature 200°C.

[0062] The temperature of the middle layer extruder was as follows: feeding section 200°C, compression section 205°C, metering section 210°C, and die head temperature 210°C.

[0063] The bottom extruder temperature was: feeding section 195°C, compression section 200°C, metering section 205°C, and die head temperature 205°C.

[0064] The film was extruded through a three-layer co-extrusion die head, with the mass ratio of each layer being controlled to be 1:8:1. The parameters were adjusted to achieve a film extrusion speed of 13 m / min, a thickness of 25 μm, and a thickness tolerance of ±0.5 μm.

[0065] Cooling the casting The diaphragm is cooled by chilled rollers with a surface temperature of 15°C. The cooling medium is circulating water. The thickness is monitored and adjusted in real time during transportation by the traction device.

[0066] Biaxial stretch Longitudinal stretching: The cast sheet was heated to 115°C, stretched at a stretching ratio of 5 times, and heat-set at 135°C.

[0067] Transverse stretching: Transverse stretching at 8 times the stretching ratio at 160°C, heat setting at 170°C.

[0068] Post-processing The corona treatment power is 4kW and the time is 4s. The surface tension of the non-coating film is increased to 45mN / m.

[0069] The product is wound with a tension of 20N, with tension fluctuation controlled at ±1N, and packaged with 20-micron thick polyester coating-free film.

[0070] Example 5 Raw material preparation Surface layer material: 100 parts by mass of BOPP resin, 3.5 parts by mass of silicone coupling agent and 2 parts by mass of acrylate monomer were added, and stirred for 11 minutes.

[0071] Intermediate layer raw materials: 100 parts by mass of homopolypropylene resin, 5 parts by mass of ethylene-propylene copolymer, and 0.6 parts by mass of sorbitol nucleating agent were mixed and stirred for 15 minutes.

[0072] Bottom layer raw materials: Add 3 parts by mass of glycerol fatty acid ester antifogging agent to 100 parts by mass of BOPP resin and stir for 9 minutes.

[0073] Extrusion Surface extruder temperature: feeding section 183°C, compression section 188°C, metering section 193°C, die head temperature 193°C.

[0074] The middle layer extruder temperature was as follows: feeding section 193°C, compression section 198°C, metering section 203°C, and die head temperature 203°C.

[0075] The bottom extruder temperature was: feeding section 188°C, compression section 193°C, metering section 198°C, and die head temperature 198°C.

[0076] Extruded through a three-layer co-extrusion die, the mass ratio of the surface layer, middle layer and bottom layer was maintained at 1:7:1, and the parameters were adjusted to make the film extrusion speed 11m / min, thickness 16 microns, and thickness tolerance ±0.4 microns.

[0077] Cooling the casting The diaphragm is rapidly cooled by chilled rollers with a surface temperature of 11°C. The cooling medium is circulating water. The thickness is monitored and adjusted by an infrared thickness gauge during the conveying process of the traction device.

[0078] Biaxial stretch Longitudinal stretching: The cast sheet was preheated to 108°C, stretched at a stretching ratio of 3.5 times, and heat-set at 128°C.

[0079] Transverse stretching: Transverse stretching at a stretch ratio of 6.5 times at 153°C, and heat setting at 163°C.

[0080] Post-processing The corona treatment power is 2.5kW, the time is 2.5s, and the surface tension of the non-coating film is increased to 42mN / m.

[0081] The product is wound with a tension of 13N, with a tension fluctuation controlled at ±0.8N, and is packaged with a 20-micron thick polyester coating-free film.

[0082] Example 6 Raw material preparation Surface layer raw materials: Weigh 100 parts by mass of BOPP resin, add 4.5 parts by mass of silicone coupling agent and 2.8 parts by mass of acrylic ester monomer, and stir for 13 minutes.

[0083] Intermediate layer raw materials: 100 parts by mass of homopolypropylene resin, 7 parts by mass of ethylene-propylene copolymer, and 0.8 parts by mass of sorbitol nucleating agent were fully mixed and stirred for 17 minutes.

[0084] Bottom layer raw materials: Add 5 parts by mass of glycerol fatty acid ester antifogging agent to 100 parts by mass of BOPP resin and stir for 11 minutes.

[0085] Extrusion Surface extruder temperature: feeding section 187°C, compression section 192°C, metering section 197°C, die head temperature 197°C.

[0086] The middle layer extruder temperature was: feeding section 197°C, compression section 202°C, metering section 207°C, and die head temperature 207°C.

[0087] The bottom extruder temperature was: feeding section 192°C, compression section 197°C, metering section 202°C, and die head temperature 202°C.

[0088] The film was extruded through a three-layer co-extrusion die head, with the mass ratio of each layer being controlled at 1:7.5:1. The parameters were adjusted to achieve a film extrusion speed of 12.5 m / min, a thickness of 22 μm, and a thickness tolerance of ±0.45 μm.

[0089] Cooling the casting The diaphragm is cooled by chilled rollers with a surface temperature of 14°C. The cooling medium is circulating water. The thickness is monitored and adjusted in real time during transportation by the traction device.

[0090] Biaxial stretch Longitudinal stretching: The cast sheet was heated to 113°C, stretched at a stretching ratio of 4.5 times, and heat-set at 133°C.

[0091] Transverse stretching: Transverse stretching at 7.5 times the stretching ratio at 158°C, heat setting at 168°C.

[0092] Post-processing The corona treatment power is 3.8kW, the time is 3.8s, and the surface tension of the non-coating film is increased to 44mN / m.

[0093] The product is wound with a tension of 18N, with a tension fluctuation controlled to ±0.9N, and packaged with a 20-micron thick polyester coating-free film.

[0094] Comparative Example 1 The traditional BOPP coating-free film preparation process is adopted, and primer treatment is performed before lamination.

[0095] Raw material preparation: Ordinary BOPP resin is used to prepare the coating-free layers without adding special additives.

[0096] Extrusion molding, cooling casting and biaxial stretching: The process is basically the same as that in Example 1, but no special raw material addition and treatment of the surface layer are performed.

[0097] Post-processing: The stretched coating-free film is primed with a traditional primer material with a coating thickness of 0.5 microns, and then composited.

[0098] Performance Testing The performance of the coating-free films prepared in Examples 1-6 and Comparative Example 1 was tested, and the test results are shown in the following table: The test results show that the biaxially oriented low-temperature coating-free films prepared in Examples 1-6 of the present invention are superior to the traditional BOPP coating-free film of Comparative Example 1 in terms of low-temperature composite adaptability, transparency, surface tension, ink adhesion, composite strength, and mechanical properties, fully demonstrating the technical advantages and stability of the present invention.

[0099] Finally, it should be noted that the above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art will be able to modify the technical solutions described in the aforementioned embodiments or substitute equivalents for some of the technical features therein. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A biaxially oriented low-temperature coating-free film and a preparation method thereof, comprising a surface layer, an intermediate layer and a bottom layer, characterized in that: The total thickness of the coating-free film is 13-25 microns; the surface layer is 1.3-2.5 microns thick, accounting for 10%-15% of the total thickness of the coating-free film and 12%-18% by mass. It is a plasma-treated BOPP coating-free film used to provide adhesion; the middle layer is 10.4-18.75 microns thick, accounting for 80% of the total thickness of the coating-free film and 65%-75% by mass. It is a high-strength BOPP substrate used to ensure the mechanical properties of the coating-free film; the bottom layer is 1.3-3.75 microns thick, accounting for 10%-15% of the total thickness of the coating-free film and 10%-15% by mass.

2. The biaxially oriented low-temperature coating-free film and the preparation method thereof according to claim 1, characterized in that: The surface layer is treated with plasma to introduce active groups; The middle layer is prepared by adding 5% by weight of ethylene-propylene copolymer and 0.5% by weight of sorbitol nucleating agent to homopolymer polypropylene resin; when the bottom layer is subjected to antistatic treatment, 3% by weight of quaternary ammonium salt antistatic agent is added, and when the bottom layer is subjected to anti-fog treatment, 4% by weight of glycerol fatty acid ester antifog agent is added.

3. The biaxially oriented low-temperature coating-free film and the preparation method thereof according to claim 2, characterized in that: The toughening agent of the middle layer is ethylene-propylene copolymer, and the nucleating agent is a sorbitol derivative; the antistatic agent of the bottom layer is a quaternary ammonium salt, and the antifogging agent is glycerol fatty acid ester.

4. The biaxially oriented low-temperature coating-free film and the preparation method thereof according to claim 3, characterized in that: The following steps are involved: Raw material preparation: The surface layer material is 100 parts by mass of BOPP resin, with 2-5 parts by mass of silicone coupling agent and 1-3 parts by mass of acrylate monomer added; the middle layer material is 100 parts by mass of homopolypropylene resin, with 3-8 parts by mass of ethylene-propylene copolymer and 0.3-1 parts by mass of sorbitol nucleating agent added; the bottom layer material is added with 2-5 parts by mass of quaternary ammonium salt antistatic agent or 3-6 parts by mass of glycerol fatty acid ester antifogging agent according to functional requirements; Extrusion molding: The three layers of raw materials are added to the corresponding extruders respectively, with the surface layer extruder temperature at 180-190°C, the middle layer at 190-200°C, and the bottom layer at 185-195°C. The extrusion is carried out through a multi-layer co-extrusion die head, and the mass ratio of the extrusion amount of the surface layer, the middle layer and the bottom layer is controlled to be 1:(6-8):1; by adjusting the die head temperature distribution and the screw speed, the film extrusion speed is controlled at 10-13m / min, and the thickness tolerance is controlled at ±0.5 microns; Cooling the cast sheet: The film is rapidly cooled by a chill roller with a surface temperature of 10-15°C, and the cooling medium is circulating water to form a cast sheet; The cast sheet is transported at a constant speed by a traction device, and the extruder screw speed is monitored and feedback adjusted in real time using an infrared thickness gauge to ensure the uniformity of the cast sheet thickness; Biaxial stretching: For longitudinal stretching, the sheet is preheated to 105-115°C, stretched at a stretch ratio of 3-5 times, and heat-set at 125-135°C; for transverse stretching, the sheet is stretched at a stretch ratio of 6-8 times at 150-160°C, and heat-set at 160-170°C. Post-processing: The non-coating film is subjected to corona treatment with a power of 2-4kW and a time of 2-4s, reeled with a tension of 12-20N, and packaged with polyester non-coating film for moisture and dust proofing.

5. The biaxially oriented low-temperature coating-free film and the preparation method thereof according to claim 4, characterized in that: During the step of cooling the cast sheet, the cast sheet is transported at a constant speed by a traction device, and the speed of the extruder screw is monitored and adjusted in real time by an infrared thickness gauge to ensure uniform thickness of the cast sheet.

6. The biaxially oriented low-temperature coating-free film and the preparation method thereof according to claim 4, characterized in that: In the biaxial stretching step, the longitudinal stretching makes the molecular chain orientation degree along the longitudinal direction of the coating-free film reach more than 85%, and the transverse stretching makes the molecular chain orientation degree along the transverse direction of the coating-free film reach more than 88%, thereby improving the tensile strength of the coating-free film.

7. The biaxially oriented low-temperature coating-free film and the preparation method thereof according to claim 4, characterized in that: In the post-treatment step, the corona treatment increases the surface tension of the coating-free film from 38 mN / m to 42-45 mN / m, thereby improving the bonding force between the surface of the coating-free film and the ink.

8. The biaxially oriented low-temperature coating-free film and the preparation method thereof according to claim 4, characterized in that: In the winding and packaging step, a polyester coating-free film with a thickness of 20 microns is used to package the coating-free film. During the winding process, the tension fluctuation is controlled within ±1N to prevent the coating-free film from wrinkling.

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