High-haze calcite BOPET (Biaxially Oriented Polyester) composite diffusion film as well as preparation method and application thereof

Abstract

The invention relates to the technical field of optical functional films, and discloses a high-haze calcite BOPET composite diffusion film and a preparation method and application thereof.The gradient micropore-compact layer composite structure design is adopted, functional composition of all layers is optimized, on one hand, in a surface compact layer, low-filling nano calcite is used for constructing a high-compact structure, and on the other hand, the high-haze calcite BOPET composite diffusion film is prepared; the surface light reflection is effectively reduced, the mechanical strength is improved, and meanwhile, the light transmittance loss is avoided; on the other hand, double-particle-size calcite nano-particles and micron-particles are introduced into the bottom microporous layer, a multi-scale scattering network is constructed, efficient light diffusion within the wide spectral range of 380-780 nm is achieved, interface scattering loss is remarkably reduced, and the light transmittance and haze balance of the optical film are greatly improved; meanwhile, integrated forming is achieved through a co-extrusion-two-way stretching process, inorganic / organic interface combination is optimized, the problem of filler agglomeration is effectively solved, and meanwhile the preparation cost is reduced.

Description

Technical Field

[0001] The invention belongs to the technical field of optical functional films, and particularly relates to a high-haze calcite BOPET composite diffusion film and a preparation method and application thereof. Background Art

[0002] With the continuous development of liquid crystal display technology, the requirements for the optical performance of display devices are becoming increasingly higher. As a key optical component in the backlight module of an LCD display, the main function of the diffusion film is to evenly diffuse the light emitted by the backlight source, eliminate the point or line traces of the light source, and improve the brightness uniformity of the display. Currently, the most common diffusion films on the market are mainly composed of a polyethylene terephthalate (PET) substrate and a light diffuser. By adding light-diffusing particles with different refractive indices to the substrate, the light scattering and diffusion effect is achieved.

[0003] In the prior art, the preparation methods of diffusion films mainly include coating and co-extrusion. The coating method is to coat a resin layer containing light-diffusing particles on the surface of a PET substrate. Patent No. CN106908873A discloses a method for preparing a high-haze and high-transmittance diffusion film. This method coats a light-diffusing layer containing diatomaceous earth particles, organic particles, thermosetting acrylic resin and other components on the surface of the base film, and then heats and heats the layer to form the film. The co-extrusion method is to add light-diffusing particles directly to the substrate and prepare the diffusion film by extrusion molding. Patent No. CN111239868A discloses a diffusion film and its production method. The diffusion film includes a built-in high-haze substrate, a light-diffusing layer and an anti-adhesion layer, wherein the light-diffusing layer includes a resin film-forming material, spherical light-diffusing particles and an additive.

[0004] Existing diffuser film technology still faces a series of challenges: there is an inherent contradiction between high haze and high transmittance. Increasing the concentration of light diffusers can improve haze, but it also significantly reduces transmittance. Furthermore, the absorption effect of traditional inorganic particles in the blue light band can cause color coordinate shifts, and the uniformity of the inorganic filler's dispersion in the polymer matrix is ​​difficult to control, easily forming micron-sized aggregates, resulting in uneven light scattering and regional degradation of the film's transmittance. Furthermore, highly filled diffuser films suffer from brittle deterioration due to stress concentration at the inorganic / organic interface, resulting in reduced elongation at break and difficulty meeting flexibility requirements. With the advancement of Mini-LED backlight technology, the coordinated realization of ultra-thinness and high light density penetration performance has become a new requirement, but existing technologies are insufficiently adaptable. In flexible and foldable screen applications, the need to balance high haze and flexibility further highlights the bottleneck of material design. Complex processes and high costs also hinder the large-scale application of high-performance diffuser films. Summary of the Invention

[0005] The present invention aims to address the technical bottlenecks of the above-mentioned prior art, such as the inversion of haze and transmittance, poor weather resistance, and high cost of traditional diffusion films. By adopting a gradient microporous-dense layer composite structure design and optimizing the functional composition of each layer, on the one hand, in the surface dense layer, a high-density structure is constructed using low-filling nanocalcite, which effectively reduces surface light reflection, improves mechanical strength, and avoids transmittance loss. On the other hand, in the bottom microporous layer, double-sized calcite nanoparticles and micron particles are introduced to construct a multi-scale scattering network, achieving efficient light diffusion in a wide spectral range of 380-780 nm, significantly reducing interface scattering losses, and greatly improving the transmittance and haze balance of the optical film. At the same time, through the co-extrusion-biaxial stretching process, the inorganic / organic interface is optimized, effectively solving the problem of filler agglomeration, and reducing the preparation cost.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions: First, the present invention provides a high-haze calcite BOPET composite diffusion membrane, comprising a surface dense layer and a bottom microporous layer with a gradient structure; The surface dense layer has a porosity of <5% and a thickness of 5-10 μm, and an oriented structure with a crystallinity of ≥85% is formed by longitudinal stretching; The porosity of the bottom microporous layer is 20-30%, and closed micropores with a gradient distribution of pore size of 1-8 μm are formed through a transverse stretching synchronous foaming process, and the closed porosity is ≥90%.

[0007] The composition of the surface dense layer and the underlying microporous layer of the gradient structure includes, by mass percentage: Matrix material: 75% to 85% polyethylene terephthalate; Light diffuser: 12% to 20% double-size calcite particles modified with silane coupling agent vapor; Foaming agent: 0.5% to 1.5% chemical foaming agent; Coupling agent: 0.1% to 0.3% silane coupling agent; Antistatic agent: 0.3% to 0.8% non-ionic antistatic agent; Heat stabilizer: 0.2% to 0.5% of a compound of phosphite and hindered phenol, wherein the mass ratio of phosphite to hindered phenol is 1:2.

[0008] The double-size calcite particles are nano-calcite of 100-300 nm and micro-calcite of 3-8 μm, with a mass ratio of 1:2 to 1:5, of which the nano-calcite accounts for 1% to 3% and the micro-calcite accounts for 5% to 9%.

[0009] The present invention optimizes the ratio and dispersibility of dual-scale particles to construct a multi-level scattering system, achieving efficient light diffusion in a wide spectral range of 380-780 nm, while reducing interface scattering losses and significantly improving the transmittance and haze balance of the optical film.

[0010] Nanoparticles (100-300 nm): Their particle size is close to the short wavelength of visible light (380-500 nm). They preferentially scatter blue and violet light through Rayleigh scattering, significantly reducing short-wavelength forward scattering losses and improving light transmittance. Micronized particles (3-8μm): Their particle size matches the long wavelengths of visible light (500-780 nm). They diffuse red and yellow light through Mie scattering, enhancing haze uniformity. By adjusting the mass ratio of nanoparticles to microparticles (1:2 to 1:5), a complementary scattering network is formed, avoiding the scattering blind spots caused by a single particle size.

[0011] Second, the double-size calcite particles of the present invention are prepared by a modified silane coupling agent vapor injection process, and the preparation method comprises the following steps: (1) Calcite pretreatment: The calcite powder is dried to a moisture content of less than 0.3 wt%; (2) Silane coupling agent steam injection: The pretreated calcite powder is added to the twin-screw extruder for extrusion. At the same time, under nitrogen protection, the silane coupling agent is transported to the steam injection device of the melting section of the twin-screw extruder through a precision metering pump. Nitrogen is used as the carrier gas to inject the silane coupling agent into the melting section of the twin-screw extruder. The silane coupling agent is instantly vaporized at 250~255℃ and a nitrogen carrier gas flow rate of 5~8 L / min to form a uniformly dispersed vapor phase. (3) Dynamic grafting: silanol groups react with hydroxyl groups on the surface of calcite to form stable Si-O-Ca covalent bonds and release water molecules at the same time; (4) Vacuum devolatilization: Through a high vacuum devolatilization system, small molecular by-products are forced to be discharged at a pressure of -0.06-0.08 MPa, and finally in-situ grafted calcite is obtained.

[0012] Through the above steps, calcite is modified through a silane coupling agent vapor injection process. Using nitrogen as a carrier gas and precisely controlling the vapor partial pressure, the silane coupling agent is instantly hydrolyzed at a high temperature of 255°C to generate active silanol groups, which undergo a dynamic condensation reaction with the hydroxyl groups on the calcite surface, achieving efficient in-situ grafting of Si-O-Ca covalent bonds. Compared to traditional solvent pre-modification methods, this vapor injection process eliminates the need for organic solvents, eliminating the risk of volatile residues, and the reaction time is only 10 seconds, significantly improving the dispersion uniformity and interfacial bonding strength of the calcite in the PET matrix.

[0013] Preferably, the foaming agent decomposes at 105-115° C. during the transverse stretching stage, and forms a light-scattering microporous structure in cooperation with calcite particles through CO2 / N2 gas generation, thereby enhancing scattering by utilizing the refractive index difference at the gas-solid interface.

[0014] Preferably, the foaming agent is one or more combinations of azodicarbonamide, 4,4'-oxybisbenzenesulfonylhydrazide, p-toluenesulfonylhydrazide, sodium bicarbonate, and 5-phenyltetrazole.

[0015] Preferably, the silane coupling agent is one or more of γ-aminopropyltriethoxysilane KH-550, γ-(2,3-epoxypropyloxy)propyltrimethoxysilane KH-560, and γ-methacryloxypropyltrimethoxysilane KH-570.

[0016] Preferably, the nonionic antistatic agent is one or more combinations of ethylene bisstearamide, glycerol monostearate, stearamide, and erucamide, replacing the traditional quaternary ammonium salt antistatic agent.

[0017] Preferably, the heat stabilizer is a compound of phosphite and hindered phenol, which is used to inhibit calcite-catalyzed thermal oxidation of PET.

[0018] This invention uses a nonionic antistatic agent (ethylene bisstearamide (EBS-75)) to replace traditional quaternary ammonium salt antistatic agents, effectively preventing decomposition and failure during high-temperature extrusion. Furthermore, a phosphite (Irgafos 168) and a hindered phenol (Irganox 1010) are combined to significantly inhibit the thermal oxidation reaction of PET catalyzed by calcite.

[0019] Preferably, the surface hardness of the surface dense layer is ≥3H, and the haze contribution rate is ≤5%; the haze of the bottom microporous layer is ≥85%, and the light transmittance is ≥88%.

[0020] Third, the present invention also provides a method for preparing a high-haze calcite BOPET composite diffusion membrane, comprising the following steps: (1) The double-size calcite particles were premixed with polyethylene terephthalate resin and then added to a twin-screw extruder. Under the protection of nitrogen, the mixture was melt-extruded through the twin-screw extruder with segmented temperature control (230-240℃ / 250-255℃ / 250-255℃ / 240-250℃ / 240-250℃) and the screw speed was 300-350 rpm. (2) After the melt is extruded through the T-die, it is first stretched longitudinally to form a dense surface layer, and then stretched transversely to form a microporous bottom layer; Preferably, in step (2), the longitudinal stretching temperature of the gradient structure is 90-100°C, and the stretching ratio is 3.5-4.0; the transverse stretching temperature is 105-115°C, and the stretching ratio is 3.0-3.5. During the transverse stretching process, the foaming agent decomposes to generate gas to form micropores.

[0021] Fourth, the present invention also provides a backlight module using a high-haze calcite BOPET composite diffusion film.

[0022] Compared with the prior art, the present invention has the following beneficial effects: The dual-size calcite particle collaborative scattering system and gradient pore structure design, with the surface dense layer porosity less than 5% and the bottom microporous layer porosity of 20-30%, break through the bottleneck of the traditional mutual restriction of haze and transmittance, and achieve high-performance collaborative optimization of haze ≥85% and transmittance ≥88%.

[0023] Steam injection dynamic grafting technology innovatively utilizes steam injection (nitrogen carrier gas at 5-8 L / min, 250-255°C) to achieve in-situ grafting of silane coupling agents, significantly improving the dispersion uniformity of calcite. Combined with biaxial stretching and simultaneous foaming technology, a microporous structure with gradient pore size is constructed, effectively reducing production costs while maintaining optical performance, achieving both economical and environmentally friendly results.

[0024] The innovative design of the functional additive system, by replacing traditional quaternary ammonium antistatic agents with alkyl sulfonates, successfully addresses the failure problem during high-temperature processing. Furthermore, the introduction of a stabilizing system combining phosphites and hindered phenols effectively inhibits the catalytic effect of calcite on PET thermal oxidation, thereby ensuring the long-term stability of the material. This design achieves synergistic optimization of haze, transmittance, and mechanical properties, ensuring not only the economic viability of mass production but also the ability to meet the stringent requirements of high-end display devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a SEM image of the dense layer on the surface of the high-haze calcite BOPET composite diffusion membrane prepared in Example 1 of the present invention.

[0026] Figure 2 This is a SEM image of the bottom microporous layer of the high-haze calcite BOPET composite diffusion membrane prepared in Example 1 of the present invention.

[0027] Figure 3 This is a SEM image of the micropores of the bottom layer of the high-haze calcite BOPET composite diffusion membrane prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0028] The present invention will be further described below in conjunction with specific embodiments.

[0029] In the present invention, unless otherwise specified, all equipment and raw materials can be purchased from the market or are commonly used in the industry. The methods in the following embodiments, unless otherwise specified, are all conventional methods in the art.

[0030] A high-haze calcite BOPET composite diffusion membrane, consisting of a gradient-structured surface dense layer and a bottom microporous layer; In some specific embodiments, the surface dense layer has a porosity of <5%, a thickness of 5-10 μm, and an oriented structure with a crystallinity of ≥85% formed by longitudinal stretching; In some specific embodiments, the porosity of the bottom microporous layer is 20-30%, and closed micropores with a gradient distribution of pore sizes of 1-8 μm are formed through a transverse stretching synchronous foaming process, and the closed porosity is ≥90%.

[0031] In some specific embodiments, the composition of the surface dense layer and the bottom microporous layer of the gradient structure includes, by mass percentage: Matrix material: 75% to 85% polyethylene terephthalate; Light diffuser: 12% to 20% double-size calcite particles modified with silane coupling agent vapor; Foaming agent: 0.5% to 1.5% chemical foaming agent; Coupling agent: 0.1% to 0.3% silane coupling agent; Antistatic agent: 0.3% to 0.8% non-ionic antistatic agent; Heat stabilizer: 0.2% to 0.5% of a compound of phosphite and hindered phenol, wherein the mass ratio of phosphite to hindered phenol is 1:2.

[0032] In some specific embodiments, the dual-size calcite particles are nano-calcite with a size of 100-300 nm and micro-calcite with a size of 3-8 μm, and the mass ratio thereof is 1:2 to 1:5.

[0033] In some specific embodiments, the double-size calcite particles are prepared by a modification process of a silane coupling agent vapor injection process, and the preparation method comprises the following steps: (1) Calcite pretreatment: The calcite powder is dried to a moisture content of less than 0.3 wt%; (2) Silane coupling agent steam injection: The pretreated calcite powder is added to the twin-screw extruder for extrusion. At the same time, under nitrogen protection, the silane coupling agent is transported to the steam injection device of the melting section of the twin-screw extruder through a precision metering pump. Nitrogen is used as the carrier gas to inject the silane coupling agent into the melting section of the twin-screw extruder. The silane coupling agent is instantly vaporized at 250~255℃ and a nitrogen carrier gas flow rate of 5~8 L / min to form a uniformly dispersed vapor phase. (3) Dynamic grafting: silanol groups react with hydroxyl groups on the surface of calcite to form stable Si-O-Ca covalent bonds and release water molecules at the same time; (4) Vacuum devolatilization: Through a high vacuum devolatilization system, small molecular by-products are forced to be discharged at a pressure of -0.06-0.08 MPa, and finally in-situ grafted calcite is obtained.

[0034] In some specific embodiments, the foaming agent decomposes at 105-115° C. during the transverse stretching stage, and forms a light-scattering microporous structure in cooperation with the calcite particles through CO 2 / N 2 gas generation.

[0035] In some specific embodiments, a high haze calcite BOPET composite diffusion membrane, the foaming agent is one or more combinations of azodicarbonamide, 4,4'-oxybisbenzenesulfonylhydrazide, p-toluenesulfonylhydrazide, sodium bicarbonate, and 5-phenyltetrazole; And / or, the silane coupling agent is one or more combinations of γ-aminopropyltriethoxysilane KH-550, γ-(2,3-epoxypropyloxy)propyltrimethoxysilane KH-560, and γ-methacryloxypropyltrimethoxysilane KH-570; And / or, the nonionic antistatic agent is one or more combinations of ethylene bisstearamide, glycerol monostearate, stearamide, and erucamide, replacing the traditional quaternary ammonium salt antistatic agent; And / or, the heat stabilizer is a compound of phosphite and hindered phenol, which is used to inhibit calcite-catalyzed thermal oxidation of PET.

[0036] In some specific embodiments, a high-haze calcite BOPET composite diffusion membrane has a surface hardness of the surface dense layer ≥3H and a haze contribution rate ≤5%; the haze of the bottom microporous layer is ≥85% and the transmittance is ≥88%.

[0037] In some specific embodiments, a method for preparing a high-haze calcite BOPET composite diffusion membrane comprises the following steps: (1) The double-size calcite particles were premixed with polyethylene terephthalate resin and then added to a twin-screw extruder. Under the protection of nitrogen, the mixture was melt-extruded through the twin-screw extruder with segmented temperature control (230-240℃ / 250-255℃ / 250-255℃ / 240-250℃ / 240-250℃) and the screw speed was 300-350 rpm. (2) After the melt is extruded through the T-die, it is first stretched longitudinally to form a dense surface layer, and then stretched transversely to form a microporous bottom layer.

[0038] In some specific embodiments, a method for preparing a high-haze calcite BOPET composite diffusion membrane is provided, wherein in step (2), the longitudinal stretching temperature of the gradient structure is 90-100° C., and the stretching ratio is 3.5-4.0; the transverse stretching temperature is 105-115° C., and the stretching ratio is 3.0-3.5, and during the transverse stretching process, the foaming agent decomposes to generate gas to form micropores.

[0039] A backlight module adopts a high-haze calcite BOPET composite diffusion film.

[0040] The present invention is described below by way of specific examples. It should be understood that these examples are intended only to illustrate the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit and scope of the inventive concept, any changes and advantages that can be imagined by those skilled in the art are included in the present invention, and the scope of protection of the present invention is defined by the appended claims and any equivalents thereof.

[0041] Example 1 A high-haze calcite BOPET composite diffusion membrane with a bottom microporous layer having a porosity of 20%; The surface dense layer and bottom microporous layer of the gradient structure include, matrix material: 83% polyethylene terephthalate (PET, intrinsic viscosity 0.65 dl / g); Light diffuser: 15% dual-particle calcite (200nm nanoparticles: 5μm microparticles, mass ratio 1:3) modified with KH-560 steam; Foaming agent: 1.0% azodicarbonamide; Coupling agent: 0.2% γ-(2,3-epoxypropoxy)propyltrimethoxysilane (KH-560) Antistatic agent: 0.5% ethylene bisstearamide Heat stabilizer: 0.3% compound system (phosphite: hindered phenol = 1:2) The double-size calcite particles are prepared by modifying the silane coupling agent vapor injection process, and the preparation method comprises the following steps: (1) Calcite pretreatment: The calcite powder was dried at 105 °C for 4 h to reduce its moisture content to less than 0.3 wt%; (2) Silane coupling agent steam injection: The pretreated calcite powder is added to the twin-screw extruder for extrusion. At the same time, under nitrogen protection, the silane coupling agent is transported to the steam injection device of the melting section of the twin-screw extruder through a precision metering pump. Nitrogen is used as the carrier gas to inject the silane coupling agent into the melting section of the twin-screw extruder. The silane coupling agent is instantly vaporized at 252°C and a nitrogen carrier gas flow rate of 6 L / min to form a uniformly dispersed vapor phase. (3) Dynamic grafting: silanol groups react with hydroxyl groups on the surface of calcite to form stable Si-O-Ca covalent bonds and release water molecules at the same time; (4) Vacuum devolatilization: Through a high vacuum devolatilization system, small molecular by-products are forced to be discharged at a pressure of 0.08 MPa, and finally in-situ grafted calcite is obtained.

[0042] The foaming agent decomposes at 110 °C during the transverse stretching stage, and forms a light-scattering microporous structure in collaboration with calcite particles through CO2 / N2 gas generation.

[0043] A method for preparing a high-haze calcite BOPET composite diffusion membrane comprises the following steps: (1) The double-size calcite particles were premixed with polyethylene terephthalate resin and then added to a twin-screw extruder. Under the protection of nitrogen, the mixture was melt-extruded by the twin-screw extruder with segmented temperature control (230-240℃ / 250-255℃ / 250-255℃ / 240-250℃ / 240-250℃) and a screw speed of 320 rpm. (2) After the melt is extruded through the T-die, it is first stretched longitudinally to form a dense surface layer. The corresponding SEM image is shown in the attached figure. Figure 1 Then, the bottom microporous layer is formed by synchronous foaming through transverse stretching. The corresponding SEM image is shown in the attached figure. Figure 2 shown.

[0044] In step (2), the longitudinal stretching temperature of the gradient structure is 95°C and the stretching ratio is 3.8; the transverse stretching temperature is 110°C and the stretching ratio is 3.2. During the transverse stretching process, the foaming agent decomposes to generate gas to form micropores. The corresponding SEM images are shown in the attached figure. Figure 3 shown.

[0045] In this embodiment, the thickness of the surface dense layer is 8 μm, and the bottom microporous layer is formed by the transverse stretching synchronous foaming process with a pore size of 3 μm.

[0046] Example 2 The only difference between this embodiment and embodiment 1 is that the calcite ratio is 1:2.

[0047] Example 3 The only difference between this embodiment and embodiment 1 is that the calcite ratio is 1:5.

[0048] Example 4 The only difference between this embodiment and embodiment 1 is that the porosity is 25%.

[0049] Example 5 The only difference between this embodiment and embodiment 1 is that the porosity is 30%.

[0050] Example 6 The only difference between this embodiment and embodiment 1 is that the antistatic agent is replaced by 0.5% ethylene bisstearamide.

[0051] Example 7 The only difference between this embodiment and embodiment 1 is the heat stabilizer (phosphite: hindered phenol = 1:3).

[0052] Example 8 The only difference between this embodiment and embodiment 1 is that the longitudinal stretching ratio is 4.0 and the transverse stretching ratio is 3.5.

[0053] Comparative Example 1 The only difference between this comparative example and Example 1 is that there is no gradient structure and the porosity is 15%.

[0054] Comparative Example 2 The only difference between this comparative example and Example 1 is that it contains only micron particles of a single calcite particle size.

[0055] Comparative Example 3 The only difference between this comparative example and Example 1 is that the calcite is not modified (directly mixed).

[0056] Comparative Example 4 The only difference between this comparative example and Example 1 is that a traditional quaternary ammonium salt antistatic agent is used.

[0057] Comparative Example 5 The only difference between this comparative example and Example 1 is that no heat stabilizer is added.

[0058] Comparative Example 6 This comparative example differs from Example 1 only in that the longitudinal stretching ratio is 3.0 (low crystallinity).

[0059] The composite diffusion membranes prepared in the above examples and comparative examples were cut into A4 size and tested for various properties. The results are shown in Table 1. The test items and methods are as follows: Haze contribution rate or transmittance test: Take an A4-sized composite diffusion film to be tested and place it in a haze tester to measure its transmittance and haze value.

[0060] Surface hardness test: According to the national standard method GB / T 6739-2006 "Paints and varnishes - Determination of film hardness by pencil method", the surface hardness of the prepared composite diffusion membrane was tested using a pencil hardness tester.

[0061] Closed Porosity: Based on the national standard method GB / T 10799-2008 “Determination of Volume Percentage of Open and Closed Pores in Rigid Foam Plastics”, the closed porosity of the prepared composite diffusion membrane was tested using a mercury porosimeter.

[0062] Light transmittance attenuation: Based on the national standard method GB / T 15596-2009 "Plastics - Determination of color and performance changes after exposure to sunlight, natural weather or artificial light under glass", the light transmittance attenuation of the prepared composite diffusion film was tested using a UVB-313 lamp with an irradiation intensity of 0.76 W / m² and a cycle of 500 hours.

[0063] Haze retention after folding: Based on the national standard method GB / T 30776-2014 "Test method for flexural resistance of plastic films", MIT folding endurance tester, folding angle: 135°; frequency: 100 times / min; number of cycles: 1000 times; the haze retention rate of the prepared composite diffusion membrane after folding was tested.

[0064] Table 1 Performance test results of a high haze calcite BOPET composite diffusion membrane Haze contribution rate (%) Light transmittance (%) Surface hardness (H) Closed cell rate (%) Transmittance attenuation (%) Haze retention rate after folding (%) Example 1 4.2 89.5 3.2 92 2.1 98.5 Example 2 3.8 90.2 3.1 91 2.3 97.8 Example 3 4.8 88.7 3.3 93 1.9 99.1 Example 4 5.1 87.9 3.0 89 2.5 96.3 Example 5 6.3 86.2 2.8 85 3.2 94.7 Example 6 4.3 89.3 3.2 92 2.2 98.2 Example 7 4.1 89.8 3.2 92 1.8 99.3 Example 8 4.0 90.1 3.5 94 1.7 99.6 Comparative Example 1 12.5 82.4 1.5 70 5.8 85.2 Comparative Example 2 7.2 85.3 3.0 88 3.5 92.4 Comparative Example 3 4.5 83.1 2.5 75 7.2 88.7 Comparative Example 4 4.3 86.7 3.1 90 4.1 89.5 Comparative Example 5 4.4 84.2 3.1 90 9.6 82.3 Comparative Example 6 4.6 87.3 2.0 86 3.8 90.1 As can be seen from the results in Table 1, the high-haze calcite BOPET composite diffusion films prepared by the method of the present invention in Examples 1 to 8 effectively reduce surface light reflection, improve mechanical strength, and avoid the loss of transmittance; at the same time, the film greatly improves the transmittance and haze balance of the optical film.

[0065] Compared to Example 1, Comparative Example 1, which uses a non-gradient structure, has a porosity of 15%, high haze, and significant transmittance loss. This indicates that the non-gradient structure leads to unbalanced performance. However, using low-fill nanocalcite to create a highly dense structure effectively reduces surface light reflection, improves mechanical strength, and avoids transmittance loss.

[0066] Compared to Example 1, Comparative Example 2, which contains only single-sized micron-sized calcite particles, exhibits higher haze and lower transmittance than the dual-sized calcite, confirming the low scattering efficiency of single-sized particles. However, by introducing dual-sized calcite particles—nanoparticles and micron-sized particles—a multiscale scattering network was constructed, achieving efficient light diffusion across a wide spectral range of 380-780 nm. This significantly reduces interfacial scattering losses and significantly improves the transmittance and haze balance of the optical film.

[0067] Compared to Example 1, the closed porosity of the unmodified calcite in Comparative Example 3 was significantly lower, indicating that agglomeration in the unmodified group resulted in uneven micropore distribution, leading to a significant decrease in light transmittance. This demonstrates that the presence of Si-O-Ca bonds can effectively reduce interfacial light loss.

[0068] Compared with Example 5, Comparative Example 4 used a traditional quaternary ammonium antistatic agent, which resulted in ion migration and accelerated aging. However, by replacing the traditional quaternary ammonium antistatic agent with an alkyl sulfonate, the failure problem during high-temperature processing was successfully overcome.

[0069] Compared to Example 6, Comparative Example 5, which lacks the addition of a thermal stabilizer, exhibits significant transmittance degradation, indicating that calcite catalyzes PET chain breakage, leading to an accelerated decrease in transmittance. However, the introduction of a stabilizing system combining phosphite and hindered phenol effectively inhibits the catalytic effect of calcite on PET thermal oxidation, thereby ensuring the long-term stability of the material.

[0070] Compared to Example 7, the longitudinal stretch ratio in Comparative Example 6 was increased to 3.0, while the hardness decreased to 2H, indicating that low crystallinity leads to decreased mechanical strength. However, by adopting a composite structure with a longitudinally stretched surface dense layer and a transversely stretched underlying microporous layer, the surface dense layer successfully achieved a surface hardness exceeding 3H, demonstrating that highly oriented crystallization can effectively enhance the rigidity of the dense layer. Furthermore, the haze contribution of the underlying microporous layer was kept below 5%, effectively decoupling the conflict between hardness and scattering properties.

[0071] Unless otherwise defined, all technical and scientific terms used herein have the same meanings as commonly understood by those skilled in the art to which this disclosure pertains. Unless otherwise specified, the raw materials and equipment used herein are conventional in the art and can be obtained from conventional commercial sources. The methods used herein are conventional in the art, unless otherwise specified.

[0072] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any way. Any simple modification, change and equivalent transformation made to the above embodiment based on the technical essence of the present invention still fall within the scope of protection of the technical solution of the present invention.

Claims

1. A high haze calcite BOPET composite diffusion membrane, characterized in that: It consists of a surface dense layer and a bottom microporous layer with a gradient structure; The porosity of the surface dense layer is less than 5%, the thickness is 5 to 10 μm, and an oriented structure with a crystallinity of ≥85% is formed by longitudinal stretching; the porosity of the bottom microporous layer is 20 to 30%, and closed micropores with a gradient distribution of pore size of 1 to 8 μm are formed by a transverse stretching synchronous foaming process, and the closed porosity is ≥90%.

2. The high haze calcite BOPET composite diffusion membrane according to claim 1, characterized in that: The composition of the surface dense layer and the bottom microporous layer of the gradient structure includes, by mass percentage: Matrix material: 75% to 85% polyethylene terephthalate; Light diffuser: 12% to 20% double-size calcite particles modified with silane coupling agent vapor; Foaming agent: 0.5% to 1.5% chemical foaming agent; Coupling agent: 0.1% to 0.3% silane coupling agent; Antistatic agent: 0.3% to 0.8% non-ionic antistatic agent; Heat stabilizer: 0.2% to 0.5% of a compound of phosphite and hindered phenol, wherein the mass ratio of phosphite to hindered phenol is 1:

2.

3. The high haze calcite BOPET composite diffusion membrane according to claim 2, characterized in that: The double-size calcite particles are nano-calcite with a size of 100-300 nm and micro-calcite with a size of 3-8 μm, and the mass ratio thereof is 1:2 to 1:

5.

4. The high haze calcite BOPET composite diffusion membrane according to claim 2, characterized in that: The double-size calcite particles are prepared by a modified silane coupling agent steam injection process, and the preparation method comprises the following steps: (1) Calcite pretreatment: The calcite powder is dried to a moisture content of less than 0.3 wt%; (2) Silane coupling agent steam injection: The pretreated calcite powder is added to the twin-screw extruder for extrusion. At the same time, under nitrogen protection, the silane coupling agent is transported to the steam injection device of the melting section of the twin-screw extruder through a precision metering pump. Nitrogen is used as the carrier gas to inject the silane coupling agent into the melting section of the twin-screw extruder. The injection temperature of the silane coupling agent is 250~255℃, and the carrier gas flow rate is 5~8 L / min. (3) Dynamic grafting: silanol groups undergo condensation reaction with hydroxyl groups on the surface of calcite; (4) Vacuum devolatilization: Through a high vacuum devolatilization system, small molecular by-products are discharged at a pressure of -0.06-0.08 MPa, and in situ grafted calcite is finally obtained.

5. The high haze calcite BOPET composite diffusion membrane according to claim 2, characterized in that: The decomposition temperature of the foaming agent in the transverse stretching stage is 105-115°C.

6. The high haze calcite BOPET composite diffusion membrane according to claim 2, characterized in that: The foaming agent is one or more combinations of azodicarbonamide, 4,4'-oxybisbenzenesulfonylhydrazide, p-toluenesulfonylhydrazide, sodium bicarbonate, and 5-phenyltetrazole; And / or, the silane coupling agent is one or more combinations of γ-aminopropyltriethoxysilane KH-550, γ-(2,3-epoxypropoxy)propyltrimethoxysilane KH-560, and γ-methacryloxypropyltrimethoxysilane KH-570; And / or, the nonionic antistatic agent is one or more combinations of ethylene bisstearamide, glycerol monostearate, stearamide, and erucamide.

7. The high haze calcite BOPET composite diffusion membrane according to claim 1, characterized in that: The surface hardness of the surface dense layer is ≥3H, and the haze contribution rate is ≤5%; the haze of the bottom microporous layer is ≥85%, and the light transmittance is ≥88%.

8. A method for preparing a high-haze calcite BOPET composite diffusion membrane, characterized in that: The following steps are involved: (1) The double-size calcite particles were premixed with polyethylene terephthalate resin and then added to a twin-screw extruder. Under the protection of nitrogen, the mixture was melt-extruded in a segmented temperature-controlled twin-screw extruder at a screw speed of 300-350 rpm. (2) After the melt is extruded through the T-die, it is first stretched longitudinally to form a dense surface layer, and then stretched transversely to form a microporous layer at the bottom.

9. The method for preparing a high-haze calcite BOPET composite diffusion membrane according to claim 8, characterized in that: In step (2), the longitudinal stretching temperature is 90-100° C., and the stretching ratio is 3.5-4.0; the transverse stretching temperature is 105-115° C., and the stretching ratio is 3.0-3.

5. During the transverse stretching process, the foaming agent decomposes to generate gas to form micropores.

10. A backlight module, characterized in that: The high-haze calcite BOPET composite diffusion membrane according to claim 1 is used.

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