A bopp photoelectric reflection film for backlight module and a preparation method thereof

Abstract

The present application relates to the technical field of film, in particular to a BOPP photoelectric reflection film for backlight module and a preparation method thereof. The film comprises a first surface layer, a first isolation layer, a nucleation core layer, a second isolation layer and a second surface layer which are stacked in sequence. The nucleation core layer contains beta crystal nucleation masterbatch, which is prepared from polypropylene resin and supported calcium pimelate beta crystal nucleating agent which is compatibilized by polypropylene, and the beta crystal nucleation masterbatch is only added into the nucleation core layer material during preparation. The film is prepared by five-layer co-extrusion casting, longitudinal stretching, transverse stretching and heat setting. The reflectivity of the film at 550 nm can reach 97.6% to 98.3%, and the average reflectivity at 380 to 780 nm can reach 96.4% to 97.1%, which is suitable for liquid crystal display backlight module.

Description

Technical Field

[0001] This invention relates to the field of thin film technology, and in particular to a BOPP photoelectric reflective film for backlight modules and its preparation method. Background Technology

[0002] As LCD technology advances towards higher resolution and narrower bezels, the performance requirements for the reflective film in direct-lit backlight modules are becoming increasingly stringent. The reflective film not only needs to have a high reflectivity of ≥95% in the 380-780nm visible light band, but also needs to maintain a reflectivity uniformity deviation of ≤2%, a dimensional change rate of ≤1% when heated to 120℃, and a backlight component brightness uniformity of ≥95%, in order to avoid bright spots, dark spots, or color shifts in the displayed image.

[0003] Current research and development of backlight reflective films mainly focuses on improving reflection efficiency, with three main technical approaches: first, adding a large amount of inorganic white filler (such as rutile titanium dioxide) to increase the scattering interface; second, introducing components incompatible with polypropylene (such as polyolefin elastomers) to induce microporous structures; and third, adding β-crystal nucleating agents to promote the formation of polypropylene β-crystals, utilizing the micropores generated by the crystal transformation to improve reflectivity. Among these, the reflective performance of porous white reflective films is highly dependent on the number, size, and uniformity of the internal pores.

[0004] To achieve higher reflectivity, companies often optimize performance by increasing filler content or the proportion of β-crystal nucleating agents. However, these improvements have significant limitations: inorganic fillers or β-crystal nucleating agents have poor compatibility with the polypropylene matrix, easily leading to agglomeration and resulting in localized scattering interface density imbalances, causing reflectivity fluctuations; the fabrication of the pore structure relies on the foaming process, making it difficult to precisely control pore size (often resulting in large pores of 10-50 μm) and distribution uniformity, and the surface layer is prone to random openings due to pore penetration, compromising the integrity of the reflective surface; furthermore, excessive aggregation of the nucleating phase exacerbates the anisotropic shrinkage of the film when heated, causing a dimensional change rate exceeding 1.5% at 120°C, which easily leads to warping after assembly, resulting in uneven backlight output. These problems make it difficult for existing reflective films to meet the performance requirements of high-end backlight modules. Summary of the Invention

[0005] In view of this, the purpose of this invention is to propose a BOPP photoelectric reflective film for backlight modules and its preparation method, so as to achieve controllable distribution of the thickness direction of the micropore induced source film while maintaining the high visible light reflectivity of the BOPP reflective film, and to take into account the uniformity of reflection, thermal dimensional stability and uniformity of backlight brightness.

[0006] To achieve the above objectives, the present invention provides a BOPP photoelectric reflective film for a backlight module, comprising a first surface layer, a first insulating layer, a nucleation core layer, a second insulating layer, and a second surface layer stacked sequentially. The mass ratio of the first surface layer, the first isolation layer, the nucleation core layer, the second isolation layer, and the second surface layer is 10:10:60:10:10; Both the first and second surface layers are formed from surface material, which, by mass parts, comprises 1800 parts polypropylene resin and 150-210 parts inorganic surface-treated rutile titanium dioxide. Both the first and second isolation layers are formed of isolation layer material, which, by mass parts, comprises 1800 parts of polypropylene resin and 90-150 parts of inorganic surface-treated rutile titanium dioxide. The nucleating core layer is formed from nucleating core layer material, which, by mass parts, comprises 6900-7100 parts of polypropylene resin, 1300-1700 parts of inorganic surface-treated rutile titanium dioxide, and 450-750 parts of β-crystal nucleating masterbatch. The β-crystal nucleating masterbatch contains polypropylene resin and polypropylene segmented β-crystal nucleating agent. The polypropylene segmented β-crystal nucleating agent is prepared by sequentially mixing a supported calcium pimecronate β-crystal nucleating agent, high acid value and low viscosity maleic anhydride-grafted polypropylene, low acid value and high viscosity maleic anhydride-grafted polypropylene, and a non-reactive high crystallinity polypropylene wax. The supported calcium pimecronate β-crystal nucleating agent is prepared by reacting nano-calcium carbonate with pimecronate. The β-crystal nucleation masterbatch is contained only in the nucleation core layer.

[0007] Preferably, the mass ratio of the first surface layer, the first isolation layer, the nucleation core layer, the second isolation layer, and the second surface layer is 8-12:8-12:56-64:8-12:8-12.

[0008] Preferably, the surface material is prepared by weight of 1800 parts polypropylene resin, 150-210 parts inorganic surface-treated rutile titanium dioxide, 4 parts antioxidant 1010, 4 parts antioxidant 168 and 6 parts calcium stearate.

[0009] Preferably, the isolation layer material is prepared by weight parts of 1800 parts polypropylene resin, 90-150 parts inorganic surface-treated rutile titanium dioxide, 4 parts antioxidant 1010, 4 parts antioxidant 168 and 6 parts calcium stearate.

[0010] Preferably, the nucleating core material is prepared by means of 6900-7100 parts polypropylene resin, 1300-1700 parts inorganic surface-treated rutile titanium dioxide, 450-750 parts β-crystal nucleating masterbatch, 18-22 parts antioxidant 1010, 18-22 parts antioxidant 168 and 18-22 parts calcium stearate.

[0011] Preferably, by mass parts, the β-crystal nucleating masterbatch is prepared from 820-880 parts of polypropylene resin, 120-180 parts of polypropylene segmented β-crystal nucleating agent, 2-3 parts of antioxidant 1010, 2-3 parts of antioxidant 168, and 4-6 parts of calcium stearate; the polypropylene segmented β-crystal nucleating agent is prepared by sequentially mixing 600 parts of supported calcium pimecrolate β-crystal nucleating agent, 30-42 parts of high acid value, low viscosity maleic anhydride-grafted polypropylene, 38-55 parts of low acid value, high viscosity maleic anhydride-grafted polypropylene, and 20-32 parts of non-reactive, highly crystalline polypropylene wax; the supported calcium pimecrolate β-crystal nucleating agent is prepared by reacting 675-825 parts of nano-calcium carbonate with 170-250 parts of pimecrolate in an ethanol-water system.

[0012] Preferably, the high acid value, low viscosity maleic anhydride-grafted polypropylene has a SAP value of 80-100 and a viscosity of 300-400 cps at 190°C; the low acid value, high viscosity maleic anhydride-grafted polypropylene has a SAP value of 15-20 and a viscosity of 1400-1700 cps at 190°C; and the non-reactive, highly crystalline polypropylene wax has a viscosity of 1300-1700 mPa·s at 170°C.

[0013] In this invention, SAP value refers to saponification value, with units of mgKOH / g. The high SAP value, low viscosity maleic anhydride-grafted polypropylene is used to preferentially wet and interfacially coat the surface of the supported calcium pimecrolate β-crystal nucleating agent; the low SAP value, high viscosity maleic anhydride-grafted polypropylene is used to improve the compatibility and melt shear stability between the coated nucleating agent and the continuous polypropylene phase; the non-reactive, highly crystalline polypropylene wax is used to improve the dispersibility of nucleating agent particles in the polypropylene resin and reduce localized agglomeration.

[0014] Preferably, the inorganic surface-treated rutile titanium dioxide is zirconium aluminum inorganic surface-treated rutile titanium dioxide, and the titanium dioxide content is not less than 95%.

[0015] Furthermore, the present invention also provides a method for preparing a BOPP photoelectric reflective film for a backlight module, comprising the following steps: (1) React nano-calcium carbonate with pimelic acid to obtain a supported calcium pimelic acid β-crystal nucleating agent; (2) The supported calcium pimecrolate β crystal nucleating agent, high acid value low viscosity maleic anhydride grafted polypropylene, low acid value high viscosity maleic anhydride grafted polypropylene and non-reactive high crystallinity polypropylene wax are sequentially mixed to obtain polypropylene segmented β crystal nucleating agent. (3) Mix polypropylene resin, the polypropylene segmented β crystal nucleating agent, antioxidant 1010, antioxidant 168 and calcium stearate and extrude granulate to obtain β crystal nucleating masterbatch. (4) Prepare the surface material, the isolation layer material, and the nucleation core layer material separately; (5) The surface material, isolation layer material and nucleating core layer material are co-extruded and cast in the order of first surface layer, first isolation layer, nucleating core layer, second isolation layer and second surface layer to obtain a cast sheet; (6) The casting is subjected to longitudinal stretching, transverse stretching, heat setting and cooling in sequence to obtain the BOPP photoelectric reflective film for the backlight module.

[0016] Preferably, in step (5), the extrusion temperature of the first and second surface layers is 225°C, the extrusion temperature of the first and second isolation layers is 225°C, the extrusion temperature of the nucleating core layer is 215°C, the die temperature is 225°C, and the melt is cast through the die to an 85°C cooling roller to obtain a casting with a thickness of 1900μm to 2200μm.

[0017] Preferably, in step (6), the cast sheet is preheated at 116-120℃ for 30s, stretched longitudinally by 3.8-4.2 times at 123-127℃, the longitudinally stretched sheet is preheated at 143-147℃ for 20s, stretched transversely by 5.0-5.2 times at 146-150℃, and then heat-set at 158-162℃ for 20-22s. The transverse relaxation during the heat-setting stage is controlled at 3%-5%. After heat-setting, the sheet is cooled to below 60℃ under tension and then wound up.

[0018] The beneficial effects of this invention are: (1) This invention achieves precise and controllable distribution of micropore induction sources in the film thickness direction through in-situ preparation of supported calcium pimecrolate β-crystal nucleating agent, stepwise polypropylene chain segmentation treatment, β-crystal nucleating masterbatch dispersion, and five-layer structure core layer localization design. Data shows that the reflectivity of Example 1 at 550nm reaches 98.1%, and the average reflectivity of 380-780nm is 96.8%, which is more than 1.2% higher than that of the unsegmented comparison. Moreover, the area ratio of β-crystal melting peak is stable at 49.8%, avoiding reflectivity fluctuations caused by nucleating agent agglomeration.

[0019] (2) The multi-layer isolation structure of the present invention effectively blocks the direct contact between the nucleation core layer and the film surface, prevents random openings in the surface layer, and improves the overall performance. In Example 1, the reflectivity uniformity reached 98.2%, the maximum dimensional change rate at 120℃ was only 0.8%, and the brightness uniformity of the backlight component reached 97.1%. Compared with the comparative example that omitted masterbatch or changed the core layer ratio, all indicators were significantly improved, solving the defects of uneven reflection, thermal deformation, and uneven backlight output in the prior art. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0021] The polypropylene resin used in the specific implementation method is Sinopec F280M, with a melt flow rate of 2.8 g / 10 min at 230℃ and 2.16 kg, and an isotactic index of 98%. The nano-calcium carbonate used is Shanghai Maclean Biochemical Technology Co., Ltd.'s C699096 type nano-calcium carbonate, with an average particle size of 50 nm. The pimelic acid used is TCI Chemical Industry Development Co., Ltd.'s P0435 type pimelic acid. The high acid value, low viscosity maleic anhydride-grafted polypropylene AC 597P used is Honeywell AC 597P propylene-maleic anhydride copolymer, with a SAP value of 90 and a viscosity of 350 cps at 190℃. The low acid value, high viscosity maleic anhydride-grafted polypropylene AC 1325P used is Honeywell AC 1325P propylene-maleic anhydride copolymer, with a SAP value of 18 and a viscosity of 1600 cps at 190℃. The non-reactive, highly crystalline polypropylene wax used is Clariant Licocene. PP6502, with a viscosity of 1500 mPa·s at 170℃, is a metallocene-catalyzed high-crystallinity polypropylene wax. The rutile titanium dioxide used is LOMON R-996 zirconium aluminum inorganic surface-treated rutile titanium dioxide from Longbai Group, with a titanium dioxide content of not less than 95%. Example 1:

[0022] Step 1: Dry the nano-calcium carbonate in a 110℃ forced-air drying oven for 6 hours. Add 4000g of deionized water and 800g of anhydrous ethanol to a reaction vessel equipped with a stirrer, reflux condenser, and exhaust pipe. After heating to 60℃, add 750g of the dried nano-calcium carbonate and disperse at 800rpm for 20min. Add 210g of pimelic acid to 1200g of anhydrous ethanol, stir at 70℃ for 30min, and filter through a 100-mesh stainless steel sieve. The resulting pimelic acid-ethanol solution is added dropwise over 60min. In the nano-calcium carbonate dispersion, the system temperature was maintained at 70℃ and the stirring speed at 800rpm during the dropwise addition. The reaction vessel was not sealed, and the generated gas was discharged through the tail gas exhaust pipe. After the dropwise addition was completed, the reaction continued for 180min. After the reaction solution was filtered, the filter cake was washed twice with an ethanol-water solution composed of 500g anhydrous ethanol and 500g deionized water, with 1000g of the solution used for each wash. The washed filter cake was then vacuum dried at 90℃ and an absolute pressure not exceeding 5kPa for 8h to obtain the supported calcium pimecrolate β-crystal nucleating agent. Step 2: Add 600g of the supported calcium pimecrolate β-crystal nucleating agent obtained in Step 1 to a mixer that has been purged with nitrogen three times. Set the mixer temperature to 145℃ and the rotor speed to 60rpm. Add 35g of high acid value, low viscosity maleic anhydride-grafted polypropylene A-C597P and mix for 20min. Raise the mixer temperature to 160℃ and add 45g of low acid value, high viscosity maleic anhydride-grafted polypropylene A-C1325P. Continue mixing for 25min. Raise the mixer temperature to 170℃ and add 25g of non-reactive high crystallinity polypropylene wax. Continue mixing for 15min. After discharge, cool to below 40℃ under nitrogen protection, pulverize and pass through a 300-mesh sieve, controlling the residue to be no more than 1%, to obtain the polypropylene segmented β-crystal nucleating agent. Step 3: Add 850g of polypropylene resin, 150g of the polypropylene segmented β-crystal nucleating agent obtained in Step 2, 2g of antioxidant 1010, 2g of antioxidant 168 and 5g of calcium stearate to a high-speed mixer and mix at 600 rpm for 10 min at room temperature. Add the mixture to a twin-screw extruder and set the temperatures of zones one to six to 170℃, 185℃, 195℃, 205℃, 205℃ and 200℃ respectively. Set the die head temperature to 200℃ and the screw speed to 250 rpm. After the extruded strip is cooled in a 25℃ water bath, it is granulated and dried at 80℃ for 4 h to obtain β-crystal nucleating masterbatch. Step 4: Add 1800g of polypropylene resin, 180g of zirconium aluminum inorganic surface-treated rutile titanium dioxide, 4g of antioxidant 1010, 4g of antioxidant 168 and 6g of calcium stearate to a high-speed mixer and mix at 600rpm for 10min. Add the mixture to a twin-screw extruder and set the temperatures of zones one to six to 180℃, 200℃, 215℃, 225℃, 225℃ and 220℃ respectively. Set the die head temperature to 220℃ and the screw speed to 250rpm. After the extruded strip is cooled in a 25℃ water bath, it is granulated and dried at 80℃ for 4h to obtain the first surface material and the second surface material. Step 5: Add 1800g of polypropylene resin, 120g of zirconium aluminum inorganic surface-treated rutile titanium dioxide, 4g of antioxidant 1010, 4g of antioxidant 168 and 6g of calcium stearate to a high-speed mixer and mix at 600rpm for 10min. Add the mixture to a twin-screw extruder and set the temperatures of zones one to six to 180℃, 200℃, 215℃, 225℃, 225℃ and 220℃ respectively. Set the die head temperature to 220℃ and the screw speed to 250rpm. After the extruded strip is cooled in a 25℃ water bath, it is granulated and dried at 80℃ for 4h to obtain the first isolation layer material and the second isolation layer material. Step 6: Add 7000g of polypropylene resin, 1500g of zirconium aluminum inorganic surface-treated rutile titanium dioxide, 600g of β-crystal nucleation masterbatch obtained in Step 3, 20g of antioxidant 1010, 20g of antioxidant 168 and 20g of calcium stearate to a high-speed mixer and mix at 600rpm for 15min. Add the mixture to a twin-screw extruder. Set the temperatures of zones one to six to 175℃, 195℃, 210℃, 215℃, 215℃ and 210℃ respectively. Set the die head temperature to 210℃ and the screw speed to 220rpm. After the extruded strip is cooled in a 25℃ water bath, it is granulated and dried at 80℃ for 4h to obtain the nucleated core layer material. Step 7: Dry the first and second surface layer materials obtained in Step 4, the first and second isolation layer materials obtained in Step 5, and the nucleation core layer material obtained in Step 6 at 80℃ for 4 hours. Add each layer material to the corresponding extrusion channel of the five-layer co-extrusion casting system and co-extrude them in the order of first surface layer, first isolation layer, nucleation core layer, second isolation layer, and second surface layer. The mass flow ratio of the five layers is controlled at 10:10:60:10:10. The extrusion temperature of the first and second surface layers is set to 225℃, the extrusion temperature of the first and second isolation layers is set to 225℃, the extrusion temperature of the nucleation core layer is set to 215℃, and the die temperature is set to 225℃. The melt is cast through the die to an 85℃ cooling roller. Control the traction speed and extrusion amount to obtain a casting sheet with a thickness of 2000μm. Step 8: Preheat the cast sheet obtained in Step 7 at 118℃ for 30s, stretch it 4 times longitudinally at 125℃, preheat the longitudinally stretched sheet at 145℃ for 20s, stretch it 5 times transversely at 148℃, and then heat set it at 160℃ for 20s. During the heat setting stage, the transverse relaxation amount is controlled at 4%. After heat setting, cool it to below 60℃ under tension and then roll it up to obtain a biaxially stretched polypropylene photoelectric reflective film for the backlight module. Example 2:

[0023] Step 1: Dry the nano-calcium carbonate in a 110℃ forced-air drying oven for 6 hours. Add 3600g of deionized water and 700g of anhydrous ethanol to a reaction vessel equipped with a stirrer, reflux condenser, and exhaust pipe. After heating to 58℃, add 675g of dried nano-calcium carbonate and disperse at 780rpm for 20min. Add 170g of pimelic acid to 1000g of anhydrous ethanol, stir at 68℃ for 30min, and filter through a 100-mesh stainless steel sieve. The resulting pimelic acid-ethanol solution is added dropwise over 55min. The nano-calcium carbonate dispersion was added dropwise, maintaining the system temperature at 68℃ and the stirring speed at 780 rpm. The reaction vessel was not sealed, and the generated gas was discharged through the tail gas exhaust pipe. After the dropwise addition was completed, the reaction continued for 160 min. After the reaction solution was filtered, the filter cake was washed twice with an ethanol-water solution composed of 450 g of anhydrous ethanol and 450 g of deionized water, using 900 g of the solution each time. The washed filter cake was then vacuum dried at 88℃ and an absolute pressure not exceeding 5 kPa for 8 h to obtain the supported calcium pimecrolate β-crystal nucleating agent. Step 2: Add 600g of the supported calcium pimecrolate β-crystal nucleating agent obtained in Step 1 to a mixer that has been purged with nitrogen three times. Set the mixer temperature to 142℃ and the rotor speed to 58rpm. Add 30g of high acid value, low viscosity maleic anhydride-grafted polypropylene A-C597P and mix for 18min. Raise the mixer temperature to 158℃ and add 38g of low acid value, high viscosity maleic anhydride-grafted polypropylene A-C1325P. Continue mixing for 23min. Raise the mixer temperature to 168℃ and add 20g of non-reactive, highly crystalline polypropylene wax. Continue mixing for 14min. After discharge, cool to below 40℃ under nitrogen protection, pulverize and pass through a 300-mesh sieve, controlling the residue to be no more than 1%, to obtain the polypropylene segmented β-crystal nucleating agent. Step 3: Add 880g of polypropylene resin, 120g of polypropylene segmented β-crystal nucleating agent obtained in Step 2, 2g of antioxidant 1010, 2g of antioxidant 168 and 4g of calcium stearate to a high-speed mixer and mix at 600rpm for 10min at room temperature. Add the mixture to a twin-screw extruder and set the temperatures of zones one to six to 170℃, 185℃, 195℃, 205℃, 205℃ and 200℃ respectively. Set the die head temperature to 200℃ and the screw speed to 250rpm. After the extruded strip is cooled in a 25℃ water bath, it is granulated and dried at 80℃ for 4h to obtain β-crystal nucleating masterbatch. Step 4: Add 1800g of polypropylene resin, 150g of zirconium aluminum inorganic surface-treated rutile titanium dioxide, 4g of antioxidant 1010, 4g of antioxidant 168 and 6g of calcium stearate to a high-speed mixer and mix at 600rpm for 10min. Add the mixture to a twin-screw extruder and set the temperatures of zones one to six to 180℃, 200℃, 215℃, 225℃, 225℃ and 220℃ respectively. Set the die head temperature to 220℃ and the screw speed to 250rpm. After the extruded strip is cooled in a 25℃ water bath, it is granulated and dried at 80℃ for 4h to obtain the first surface material and the second surface material. Step 5: Add 1800g of polypropylene resin, 90g of zirconium aluminum inorganic surface-treated rutile titanium dioxide, 4g of antioxidant 1010, 4g of antioxidant 168 and 6g of calcium stearate to a high-speed mixer and mix at 600rpm for 10min. Add the mixture to a twin-screw extruder. Set the temperatures of zones one to six to 180℃, 200℃, 215℃, 225℃, 225℃ and 220℃ respectively. Set the die head temperature to 220℃ and the screw speed to 250rpm. After the extruded strip is cooled in a 25℃ water bath, it is granulated and dried at 80℃ for 4h to obtain the first isolation layer material and the second isolation layer material. Step 6: Add 7100g of polypropylene resin, 1300g of zirconium aluminum inorganic surface-treated rutile titanium dioxide, 450g of β-crystal nucleation masterbatch obtained in Step 3, 18g of antioxidant 1010, 18g of antioxidant 168 and 18g of calcium stearate to a high-speed mixer and mix at 600rpm for 15min. Add the mixture to a twin-screw extruder and set the temperatures of zones one to six to 175℃, 195℃, 210℃, 215℃, 215℃ and 210℃ respectively. Set the die head temperature to 210℃ and the screw speed to 220rpm. After the extruded strip is cooled in a 25℃ water bath, it is granulated and dried at 80℃ for 4h to obtain the nucleation core layer material. Step 7: Dry the first and second surface layer materials obtained in Step 4, the first and second isolation layer materials obtained in Step 5, and the nucleation core layer material obtained in Step 6 at 80℃ for 4 hours. Add each layer material to the corresponding extrusion channel of the five-layer co-extrusion casting system and co-extrude them in the order of first surface layer, first isolation layer, nucleation core layer, second isolation layer, and second surface layer. The mass flow ratio of the five layers is controlled at 8:12:60:12:8. The extrusion temperature of the first and second surface layers is set to 225℃, the extrusion temperature of the first and second isolation layers is set to 225℃, the extrusion temperature of the nucleation core layer is set to 215℃, and the die temperature is set to 225℃. The melt is cast through the die to an 85℃ cooling roller. Control the traction speed and extrusion amount to obtain a casting sheet with a thickness of 1900μm. Step 8: Preheat the cast sheet obtained in Step 7 at 116℃ for 30s, stretch it 3.8 times longitudinally at 123℃, preheat the longitudinally stretched sheet at 143℃ for 20s, stretch it 5 times transversely at 146℃, and then heat set it at 158℃ for 20s. During the heat setting stage, the transverse relaxation amount is controlled at 3%. After heat setting, cool it to below 60℃ under tension and then roll it up to obtain a biaxially oriented polypropylene photoelectric reflective film for the backlight module. Example 3:

[0024] Step 1: Dry the nano-calcium carbonate in a 110℃ forced-air drying oven for 6 hours. Add 4400g of deionized water and 900g of anhydrous ethanol to a reaction vessel equipped with a stirrer, reflux condenser, and exhaust pipe. After heating to 62℃, add 825g of the dried nano-calcium carbonate and disperse at 820rpm for 20min. Add 250g of pimelic acid to 1400g of anhydrous ethanol, stir at 72℃ for 30min, and filter through a 100-mesh stainless steel sieve. The resulting pimelic acid-ethanol solution is added dropwise over 65min. In the nano-calcium carbonate dispersion, the system temperature was maintained at 72℃ and the stirring speed at 820rpm during the dropwise addition. The reaction vessel was not sealed, and the generated gas was discharged through the tail gas exhaust pipe. After the dropwise addition was completed, the reaction continued for 200min. After the reaction solution was filtered, the filter cake was washed twice with an ethanol-water solution composed of 550g anhydrous ethanol and 550g deionized water, with 1100g of the solution used for each wash. The washed filter cake was then vacuum dried at 92℃ and an absolute pressure not exceeding 5kPa for 8h to obtain the supported calcium pimecrolate β-crystal nucleating agent. Step 2: Add 600g of the supported calcium pimecrolate β-crystal nucleating agent obtained in Step 1 to a mixer that has been purged with nitrogen three times. Set the mixer temperature to 148℃ and the rotor speed to 62rpm. Add 42g of high acid value, low viscosity maleic anhydride-grafted polypropylene A-C597P and mix for 22min. Raise the mixer temperature to 162℃ and add 55g of low acid value, high viscosity maleic anhydride-grafted polypropylene A-C1325P. Continue mixing for 27min. Raise the mixer temperature to 172℃ and add 32g of non-reactive, highly crystalline polypropylene wax. Continue mixing for 16min. After discharge, cool to below 40℃ under nitrogen protection, pulverize and pass through a 300-mesh sieve, controlling the residue to be no more than 1%, to obtain the polypropylene segmented β-crystal nucleating agent. Step 3: Add 820g of polypropylene resin, 180g of polypropylene segmented β-crystal nucleating agent obtained in Step 2, 3g of antioxidant 1010, 3g of antioxidant 168 and 6g of calcium stearate to a high-speed mixer and mix at 600rpm for 10min at room temperature. Add the mixture to a twin-screw extruder and set the temperatures of zones one to six to 170℃, 185℃, 195℃, 205℃, 205℃ and 200℃ respectively. Set the die head temperature to 200℃ and the screw speed to 250rpm. After the extruded strip is cooled in a 25℃ water bath, it is granulated and dried at 80℃ for 4h to obtain β-crystal nucleating masterbatch. Step 4: Add 1800g of polypropylene resin, 210g of zirconium aluminum inorganic surface-treated rutile titanium dioxide, 4g of antioxidant 1010, 4g of antioxidant 168 and 6g of calcium stearate to a high-speed mixer and mix at 600rpm for 10min. Add the mixture to a twin-screw extruder and set the temperatures of zones one to six to 180℃, 200℃, 215℃, 225℃, 225℃ and 220℃ respectively. Set the die head temperature to 220℃ and the screw speed to 250rpm. After the extruded strip is cooled in a 25℃ water bath, it is granulated and dried at 80℃ for 4h to obtain the first surface material and the second surface material. Step 5: Add 1800g of polypropylene resin, 150g of zirconium aluminum inorganic surface-treated rutile titanium dioxide, 4g of antioxidant 1010, 4g of antioxidant 168 and 6g of calcium stearate to a high-speed mixer and mix at 600rpm for 10min. Add the mixture to a twin-screw extruder. Set the temperatures of zones one to six to 180℃, 200℃, 215℃, 225℃, 225℃ and 220℃ respectively. Set the die head temperature to 220℃ and the screw speed to 250rpm. After the extruded strip is cooled in a 25℃ water bath, it is granulated and dried at 80℃ for 4h to obtain the first isolation layer material and the second isolation layer material. Step 6: Add 6900g of polypropylene resin, 1700g of zirconium aluminum inorganic surface-treated rutile titanium dioxide, 750g of β-crystal nucleation masterbatch obtained in Step 3, 22g of antioxidant 1010, 22g of antioxidant 168 and 22g of calcium stearate to a high-speed mixer and mix at 600rpm for 15min. Add the mixture to a twin-screw extruder. Set the temperatures of zones one to six to 175℃, 195℃, 210℃, 215℃, 215℃ and 210℃ respectively. Set the die head temperature to 210℃ and the screw speed to 220rpm. After the extruded strip is cooled in a 25℃ water bath, it is granulated and dried at 80℃ for 4h to obtain the nucleated core layer material. Step 7: Dry the first and second surface layer materials obtained in Step 4, the first and second isolation layer materials obtained in Step 5, and the nucleation core layer material obtained in Step 6 at 80℃ for 4 hours. Add each layer material to the corresponding extrusion channel of the five-layer co-extrusion casting system and co-extrude them in the order of first surface layer, first isolation layer, nucleation core layer, second isolation layer, and second surface layer. The mass flow ratio of the five layers is controlled at 12:8:60:8:12. The extrusion temperature of the first and second surface layers is set to 225℃, the extrusion temperature of the first and second isolation layers is set to 225℃, the extrusion temperature of the nucleation core layer is set to 215℃, and the die temperature is set to 225℃. The melt is cast through the die to an 85℃ cooling roller. Control the traction speed and extrusion amount to obtain a casting sheet with a thickness of 2100μm. Step 8: Preheat the cast sheet obtained in Step 7 at 120℃ for 30s, stretch it 4.2 times longitudinally at 127℃, preheat the longitudinally stretched sheet at 147℃ for 20s, stretch it 5 times transversely at 150℃, and then heat set it at 162℃ for 20s. During the heat setting stage, the transverse relaxation amount is controlled at 5%. After heat setting, cool it to below 60℃ under tension and then roll it up to obtain a biaxially stretched polypropylene photoelectric reflective film for the backlight module. Example 4:

[0025] Step 1: Dry the nano-calcium carbonate in a 110℃ forced-air drying oven for 6 hours. Add 4000g of deionized water and 800g of anhydrous ethanol to a reaction vessel equipped with a stirrer, reflux condenser, and exhaust pipe. After heating to 60℃, add 750g of the dried nano-calcium carbonate and disperse at 800rpm for 20min. Add 190g of pimelic acid to 1100g of anhydrous ethanol, stir at 70℃ for 30min, and filter through a 100-mesh stainless steel sieve. The resulting pimelic acid-ethanol solution is added dropwise over 60min. In the nano-calcium carbonate dispersion, the system temperature was maintained at 70℃ and the stirring speed at 800rpm during the dropwise addition. The reaction vessel was not sealed, and the generated gas was discharged through the tail gas exhaust pipe. After the dropwise addition was completed, the reaction continued for 180min. After the reaction solution was filtered, the filter cake was washed twice with an ethanol-water solution composed of 500g anhydrous ethanol and 500g deionized water, with 1000g of the solution used for each wash. The washed filter cake was then vacuum dried at 90℃ and an absolute pressure not exceeding 5kPa for 8h to obtain the supported calcium pimecrolate β-crystal nucleating agent. Step 2: Add 600g of the supported calcium pimecrolate β-crystal nucleating agent obtained in Step 1 to a mixer that has been purged with nitrogen three times. Set the mixer temperature to 145℃ and the rotor speed to 60rpm. Add 32g of high acid value, low viscosity maleic anhydride-grafted polypropylene A-C597P and mix for 20min. Raise the mixer temperature to 160℃ and add 52g of low acid value, high viscosity maleic anhydride-grafted polypropylene A-C1325P. Continue mixing for 25min. Raise the mixer temperature to 170℃ and add 28g of non-reactive high crystallinity polypropylene wax. Continue mixing for 15min. After discharge, cool to below 40℃ under nitrogen protection, pulverize and pass through a 300-mesh sieve, controlling the residue to be no more than 1%, to obtain the polypropylene segmented β-crystal nucleating agent. Step 3: Add 860g of polypropylene resin, 140g of the polypropylene segmented β-crystal nucleating agent obtained in Step 2, 2g of antioxidant 1010, 2g of antioxidant 168 and 5g of calcium stearate to a high-speed mixer and mix at 600rpm for 10min at room temperature. Add the mixture to a twin-screw extruder and set the temperatures of zones one to six to 170℃, 185℃, 195℃, 205℃, 205℃ and 200℃ respectively. Set the die head temperature to 200℃ and the screw speed to 250rpm. After the extruded strip is cooled in a 25℃ water bath, it is granulated and dried at 80℃ for 4h to obtain β-crystal nucleating masterbatch. Step 4: Add 1800g of polypropylene resin, 170g of zirconium aluminum inorganic surface-treated rutile titanium dioxide, 4g of antioxidant 1010, 4g of antioxidant 168 and 6g of calcium stearate to a high-speed mixer and mix at 600rpm for 10min. Add the mixture to a twin-screw extruder and set the temperatures of zones one to six to 180℃, 200℃, 215℃, 225℃, 225℃ and 220℃ respectively. Set the die head temperature to 220℃ and the screw speed to 250rpm. After the extruded strip is cooled in a 25℃ water bath, it is granulated and dried at 80℃ for 4h to obtain the first surface material and the second surface material. Step 5: Add 1800g of polypropylene resin, 110g of zirconium aluminum inorganic surface-treated rutile titanium dioxide, 4g of antioxidant 1010, 4g of antioxidant 168 and 6g of calcium stearate to a high-speed mixer and mix at 600rpm for 10min. Add the mixture to a twin-screw extruder. Set the temperatures of zones one to six to 180℃, 200℃, 215℃, 225℃, 225℃ and 220℃ respectively. Set the die head temperature to 220℃ and the screw speed to 250rpm. After the extruded strip is cooled in a 25℃ water bath, it is granulated and dried at 80℃ for 4h to obtain the first isolation layer material and the second isolation layer material. Step 6: Add 7050g of polypropylene resin, 1450g of zirconium aluminum inorganic surface-treated rutile titanium dioxide, 550g of β-crystal nucleation masterbatch obtained in Step 3, 20g of antioxidant 1010, 20g of antioxidant 168 and 20g of calcium stearate to a high-speed mixer and mix at 600rpm for 15min. Add the mixture to a twin-screw extruder. Set the temperatures of zones one to six to 175℃, 195℃, 210℃, 215℃, 215℃ and 210℃ respectively. Set the die head temperature to 210℃ and the screw speed to 220rpm. After the extruded strip is cooled in a 25℃ water bath, it is granulated and dried at 80℃ for 4h to obtain the nucleated core layer material. Step 7: Dry the first and second surface layer materials obtained in Step 4, the first and second isolation layer materials obtained in Step 5, and the nucleation core layer material obtained in Step 6 at 80℃ for 4 hours. Add each layer material to the corresponding extrusion channel of the five-layer co-extrusion casting system and co-extrude them in the order of first surface layer, first isolation layer, nucleation core layer, second isolation layer, and second surface layer. The mass flow ratio of the five layers is controlled at 10:8:64:8:10. The extrusion temperature of the first and second surface layers is set to 225℃, the extrusion temperature of the first and second isolation layers is set to 225℃, the extrusion temperature of the nucleation core layer is set to 215℃, and the die temperature is set to 225℃. The melt is cast through the die to an 85℃ cooling roller. Control the traction speed and extrusion amount to obtain a casting sheet with a thickness of 2000μm. Step 8: Preheat the cast sheet obtained in Step 7 at 118℃ for 30s, stretch it 4 times longitudinally at 125℃, preheat the longitudinally stretched sheet at 145℃ for 20s, stretch it 5.2 times transversely at 148℃, and then heat set it at 160℃ for 22s. During the heat setting stage, the transverse relaxation amount is controlled at 4%. After heat setting, cool it to below 60℃ under tension and then roll it up to obtain a biaxially stretched polypropylene photoelectric reflective film for the backlight module. Example 5:

[0026] Step 1: Dry the nano-calcium carbonate in a 110℃ forced-air drying oven for 6 hours. Add 4200g of deionized water and 850g of anhydrous ethanol to a reaction vessel equipped with a stirrer, reflux condenser, and exhaust pipe. After heating to 61℃, add 780g of dried nano-calcium carbonate and disperse at 810rpm for 20min. Add 230g of pimelic acid to 1300g of anhydrous ethanol, stir at 71℃ for 30min, and filter through a 100-mesh stainless steel sieve. The resulting pimelic acid-ethanol solution is added dropwise over 62min. During the dropwise addition of nano-calcium carbonate, the system temperature was maintained at 71℃ and the stirring speed at 810 rpm. The reaction vessel was not sealed, and the generated gas was discharged through the tail gas exhaust pipe. After the dropwise addition was completed, the reaction continued for 190 min. After the reaction solution was filtered, the filter cake was washed twice with an ethanol-water solution composed of 520 g anhydrous ethanol and 520 g deionized water, with 1040 g of the solution used for each wash. The washed filter cake was then vacuum dried at 91℃ and an absolute pressure not exceeding 5 kPa for 8 h to obtain the supported calcium pimecrolate β-crystal nucleating agent. Step 2: Add 600g of the supported calcium pimecrolate β-crystal nucleating agent obtained in Step 1 to a mixer that has been purged with nitrogen three times. Set the mixer temperature to 146℃ and the rotor speed to 60rpm. Add 40g of high acid value, low viscosity maleic anhydride-grafted polypropylene A-C597P and mix for 20min. Raise the mixer temperature to 160℃ and add 42g of low acid value, high viscosity maleic anhydride-grafted polypropylene A-C1325P. Continue mixing for 25min. Raise the mixer temperature to 170℃ and add 24g of non-reactive, highly crystalline polypropylene wax. Continue mixing for 15min. After discharge, cool to below 40℃ under nitrogen protection, pulverize and pass through a 300-mesh sieve, controlling the residue to be no more than 1%, to obtain the polypropylene segmented β-crystal nucleating agent. Step 3: Add 830g of polypropylene resin, 170g of the polypropylene segmented β-crystal nucleating agent obtained in Step 2, 3g of antioxidant 1010, 2g of antioxidant 168 and 5g of calcium stearate to a high-speed mixer and mix at 600 rpm for 10 min at room temperature. Add the mixture to a twin-screw extruder and set the temperatures of zones one to six to 170℃, 185℃, 195℃, 205℃, 205℃ and 200℃ respectively. Set the die head temperature to 200℃ and the screw speed to 250 rpm. After the extruded strip is cooled in a 25℃ water bath, it is granulated and dried at 80℃ for 4 h to obtain β-crystal nucleating masterbatch. Step 4: Add 1800g of polypropylene resin, 200g of zirconium aluminum inorganic surface-treated rutile titanium dioxide, 4g of antioxidant 1010, 4g of antioxidant 168 and 6g of calcium stearate to a high-speed mixer and mix at 600rpm for 10min. Add the mixture to a twin-screw extruder and set the temperatures of zones one to six to 180℃, 200℃, 215℃, 225℃, 225℃ and 220℃ respectively. Set the die head temperature to 220℃ and the screw speed to 250rpm. After the extruded strip is cooled in a 25℃ water bath, it is granulated and dried at 80℃ for 4h to obtain the first surface material and the second surface material. Step 5: Add 1800g of polypropylene resin, 140g of zirconium aluminum inorganic surface-treated rutile titanium dioxide, 4g of antioxidant 1010, 4g of antioxidant 168 and 6g of calcium stearate to a high-speed mixer and mix at 600rpm for 10min. Add the mixture to a twin-screw extruder. Set the temperatures of zones one to six to 180℃, 200℃, 215℃, 225℃, 225℃ and 220℃ respectively. Set the die head temperature to 220℃ and the screw speed to 250rpm. After the extruded strip is cooled in a 25℃ water bath, it is granulated and dried at 80℃ for 4h to obtain the first isolation layer material and the second isolation layer material. Step 6: Add 6950g of polypropylene resin, 1600g of zirconium aluminum inorganic surface-treated rutile titanium dioxide, 700g of β-crystal nucleation masterbatch obtained in Step 3, 22g of antioxidant 1010, 20g of antioxidant 168 and 20g of calcium stearate to a high-speed mixer and mix at 600rpm for 15min. Add the mixture to a twin-screw extruder. Set the temperatures of zones one to six to 175℃, 195℃, 210℃, 215℃, 215℃ and 210℃ respectively. Set the die head temperature to 210℃ and the screw speed to 220rpm. After the extruded strip is cooled in a 25℃ water bath, it is granulated and dried at 80℃ for 4h to obtain the nucleated core layer material. Step 7: Dry the first and second surface layer materials obtained in Step 4, the first and second isolation layer materials obtained in Step 5, and the nucleation core layer material obtained in Step 6 at 80℃ for 4 hours. Add each layer material to the corresponding extrusion channel of the five-layer co-extrusion casting system and co-extrude in the order of first surface layer, first isolation layer, nucleation core layer, second isolation layer, and second surface layer. The mass flow ratio of the five layers is controlled at 9:11:60:11:9. The extrusion temperature of the first and second surface layers is set to 225℃, the extrusion temperature of the first and second isolation layers is set to 225℃, the extrusion temperature of the nucleation core layer is set to 215℃, and the die temperature is set to 225℃. The melt is cast through the die to an 85℃ cooling roller. Control the traction speed and extrusion amount to obtain a casting sheet with a thickness of 2200μm. Step 8: Preheat the cast sheet obtained in Step 7 at 119℃ for 30s, stretch it 4.2 times longitudinally at 126℃, preheat the longitudinally stretched sheet at 146℃ for 20s, stretch it 5.2 times transversely at 149℃, and then heat set it at 161℃ for 20s. During the heat setting stage, the transverse relaxation amount is controlled at 4%. After heat setting, cool it to below 60℃ under tension and then wind it up to obtain a biaxially oriented polypropylene photoelectric reflective film for the backlight module.

[0027] Comparative Example 1: The difference from Example 1 is that in step 2, high-acid-value, low-viscosity maleic anhydride-grafted polypropylene A-C597P, low-acid-value, high-viscosity maleic anhydride-grafted polypropylene A-C1325P, and non-reactive high-crystallinity polypropylene wax are not added. Instead, the supported calcium pimecrolate β-crystal nucleating agent obtained in step 1 is directly pulverized and passed through a 300-mesh sieve, with the residue controlled to be no more than 1%, as an untreated polypropylene chain segmentation β-crystal nucleating agent. In step 3, 150g of the untreated polypropylene chain segmentation β-crystal nucleating agent is used instead of 150g of the polypropylene chain segmentation β-crystal nucleating agent. The remaining conditions are the same as in Example 1.

[0028] Comparative Example 2: The difference from Example 1 is that in step 2, the amounts of high-acid-value, low-viscosity maleic anhydride-grafted polypropylene A-C597P, low-acid-value, high-viscosity maleic anhydride-grafted polypropylene A-C1325P, and non-reactive high-crystallinity polypropylene wax added are still 35g, 45g, and 25g, respectively. However, the order of addition is adjusted as follows: first, 25g of non-reactive high-crystallinity polypropylene wax is added at 145℃ and mixed for 20min; then, 45g of low-acid-value, high-viscosity maleic anhydride-grafted polypropylene A-C1325P is added at 160℃ and mixed for 25min; finally, 35g of high-acid-value, low-viscosity maleic anhydride-grafted polypropylene A-C597P is added at 170℃ and mixed for 15min. All other conditions are the same as in Example 1.

[0029] Comparative Example 3: The difference from Example 1 is that in step 2, 45g of low-acid-value, high-viscosity maleic anhydride-grafted polypropylene A-C1325P is not added, and the same amount of 45g of low-acid-value, high-viscosity maleic anhydride-grafted polypropylene A-C1325P is replaced with high-acid-value, low-viscosity maleic anhydride-grafted polypropylene A-C597P. That is, the total amount of high-acid-value, low-viscosity maleic anhydride-grafted polypropylene A-C597P added in step 2 is 80g. All other conditions are the same as in Example 1.

[0030] Comparative Example 4: The difference from Example 1 is that in step 2, 25g of non-reactive high-crystallinity polypropylene wax is not added, and the 25g of non-reactive high-crystallinity polypropylene wax is replaced by an equal mass of low-acid-value, high-viscosity maleic anhydride-grafted polypropylene A-C1325P. That is, the total amount of low-acid-value, high-viscosity maleic anhydride-grafted polypropylene A-C1325P added in step 2 is 70g. The other conditions are the same as in Example 1.

[0031] Comparative Example 5: The difference from Example 1 is that step 3, the preparation of the β-crystal nucleation masterbatch, is not performed; in step 6, instead of adding 600g of the β-crystal nucleation masterbatch, 89g of the polypropylene segmented β-crystal nucleating agent obtained in step 2 and 511g of polypropylene resin are added to maintain the effective amount of the polypropylene segmented β-crystal nucleating agent added in step 6 consistent with that in Example 1. All other conditions are the same as in Example 1.

[0032] Comparative Example 6: The difference from Example 1 is that: in step 6, the amount of β-crystal nucleation masterbatch added is adjusted from 600g to 360g; in step 7, 240g of the β-crystal nucleation masterbatch obtained in step 3 is added to the extrusion channels corresponding to the first surface layer, the first isolation layer, the second isolation layer, and the second surface layer, respectively, with 60g added to each corresponding extrusion channel. An additional 60g is deducted from the amount of polypropylene resin used in the corresponding extrusion channel, so that the total amount of β-crystal nucleation masterbatch added remains 600g, but it is no longer located only in the nucleation core layer. All other conditions are the same as in Example 1.

[0033] Comparative Example 7: The difference from Example 1 is that in step 7, the mass flow ratio of the five layers is adjusted from 10:10:60:10:10 to 5:5:80:5:5, so that the distance between the nucleation core layer and any film surface is less than 15% of the total thickness of the biaxially oriented polypropylene photoelectric reflective film. The remaining conditions are the same as in Example 1.

[0034] Comparative Example 8: The difference from Example 1 is that in step 2, after discharge, the material is cooled to below 40°C under nitrogen protection, crushed, and passed through a 100-mesh sieve instead of a 300-mesh sieve, and the residue is not controlled to be no more than 1%. The other conditions are the same as in Example 1.

[0035] Performance testing: The biaxially oriented polypropylene photoelectric reflective films obtained in Examples 1-5 and Comparative Examples 1-8 were used as test samples. Each sample was cut from a position at least 2m from the end of the film roll, avoiding creases, contamination, broken edges, and areas with obvious thickness anomalies. The cutting directions of the samples used for reflectivity and heating dimensional change rate were marked with both longitudinal and transverse directions. The samples used for backlight assembly testing were cut to 210mm × 130mm. All samples were conditioned for 48 hours in a standard environment of 23℃ and 50% relative humidity according to GB / T2918-2018 before testing. For backlight assembly testing, the same 10.1-inch direct-lit white LED backlight assembly was used. After removing the original reflector, the samples obtained in the examples and comparative examples were installed. The diffuser plate, diffuser sheet, prism sheet, lamp board, power supply, and structural frame remained unchanged. The constant current drive current of the LED was set to 350mA, and the test began after 30 minutes of illumination.

[0036] Characterization of β-crystal thermal behavior: Differential scanning calorimetry (DSC) was performed on the samples obtained in the examples and comparative examples according to GB / T19466.3-2004. 5.0 mg ± 0.2 mg of each sample was cut from the central region of the film and placed in an aluminum crucible, using an empty aluminum crucible as a reference. The nitrogen flow rate was set to 50 mL / min. The test procedure was as follows: heating from 30 °C to 200 °C at a rate of 10 °C / min, holding for 3 min, then cooling to 30 °C at a rate of 10 °C / min, and then heating again to 200 °C at a rate of 10 °C / min. The peak areas of the β-crystal melting peak and the α-crystal melting peak in the second heating curve were recorded. The β-crystal melting peak area ratio was calculated as the ratio of the β-crystal melting peak area to the sum of the β-crystal melting peak areas and the α-crystal melting peak areas. Each sample was tested in triplicate, and the arithmetic mean was taken.

[0037] Spectral reflectance and reflectance uniformity: The reflectance properties of the samples were tested using a spectrophotometer with an integrating sphere according to GB / T3979-2008 and GB / T3978-2008. The inner diameter of the integrating sphere was 150 mm, the test geometry was d / 8° including the specular reflection component, the standard illuminator was D65, the standard observer was 10°, the wavelength range was 380 nm to 780 nm, and the wavelength interval was 10 nm. A calibrated barium sulfate standard white plate was used for calibration before testing. Three 100 mm × 100 mm test pieces were cut from each sample. Tests were conducted at nine points on each piece, including the center point and eight points around the perimeter. The reflectance at 550 nm and the average reflectance from 380 nm to 780 nm were recorded. Reflectance uniformity was calculated by multiplying 100% of the ratio of the minimum to the average reflectance at 550 nm among the nine test points.

[0038] Dimensional change rate under heating: The dimensional change rate under heating of the films obtained in the test examples and comparative examples was determined according to GB / T12027-2004 and in conjunction with the test temperature conditions for heat shrinkage of biaxially oriented polypropylene films in GB / T10003-2008. Five 100mm × 100mm test pieces were cut from each sample. Measurement lines of 100mm length were marked along the center of each test piece in both the longitudinal and transverse directions. The temperature of the hot air circulating oven was stabilized at 120℃±3℃. The samples were placed flat on a metal tray that had reached the test temperature. After heating for 5 minutes, the samples were removed and cooled for 30 minutes at 23℃ and 50% relative humidity. The lengths of the longitudinal and transverse marking lines after heating were measured. The dimensional change rates under heating in both the longitudinal and transverse directions were calculated by multiplying the absolute value of the change in the length of the marking line before and after heating by 100%, with the larger value between the longitudinal and transverse values ​​taken as the larger dimensional change rate under heating.

[0039] Backlight assembly brightness uniformity: Tested according to the optical and photoelectric parameters test method for backlight assemblies for liquid crystal displays in SJ / T11460.6.1-2015. The films obtained in the examples and comparative examples were respectively used as reflective sheets and installed in the same backlight assembly. Other optical films and structural components remained unchanged. Testing was conducted in a dark room with an ambient temperature of 23℃ and a relative humidity of 50%. After the backlight assembly was lit for 30 minutes, a luminance meter was placed in the normal direction of the light-emitting surface, with a measurement distance of 500mm. The brightness of the light-emitting surface was tested at 9 points: the center point and 8 points around the perimeter. The brightness uniformity of the backlight assembly was calculated by multiplying the ratio of the minimum brightness value to the average brightness value among the 9 test points by 100%. Each sample was assembled and tested repeatedly 3 times, and the arithmetic mean was taken.

[0040] Table 1 Performance Test Results

[0041] As shown in Table 1, Examples 1-5 all exhibited high β-crystal melt peak area ratios and visible light reflectance performance. Among them, Example 2, due to its relatively low addition of β-crystal nucleating masterbatch and zirconium aluminum inorganic surface-treated rutile titanium dioxide, achieved a 550nm reflectance of 97.6% and an average 380-780nm reflectance of 96.4%. Example 3, due to its increased intensity of the chain segmentation treatment of the supported calcium pimerate β-crystal nucleating agent, the amount of β-crystal nucleating masterbatch and zirconium aluminum inorganic surface-treated rutile titanium dioxide, saw its β-crystal melt peak area ratio increase to 56.2% and its 550nm reflectance increase to 98.3%. However, its reflectance uniformity and backlight component brightness uniformity were slightly lower than those of Example 1, indicating that a higher nucleation and scattering phase content is beneficial for increasing the number of scattering interfaces, but may also increase local porosity differences. Example 1, under the combined effects of a five-layer mass flow ratio of 10:10:60:10:10, stepwise polypropylene chain segmentation treatment, and masterbatch dispersion, exhibits a reflectivity uniformity of 98.2%, a maximum dimensional change rate of 0.8% at 120℃ heating, and a backlight component brightness uniformity of 97.1%, demonstrating superior overall performance.

[0042] Compared to Example 1, Comparative Example 1 did not undergo polypropylene chain segmentation treatment, Comparative Example 2 changed the order of adding the chain segmentation treatment agent, Comparative Example 5 omitted the preparation of β-crystal nucleating masterbatch, and Comparative Example 8 reduced the sieving fineness. All of these reduced the dispersion stability of the β-crystal nucleating agent in the continuous polypropylene phase, resulting in a simultaneous decrease in the proportion of β-crystal melt peak area, reflectivity uniformity, and backlight component brightness uniformity. Although the total amount of β-crystal nucleating masterbatch added in Comparative Example 6 was the same as in Example 1, it introduced some of the β-crystal nucleating masterbatch into the surface layer and the isolation layer, causing the micropore induction source to no longer concentrate in the nucleation core layer. Although Comparative Example 7 had a higher proportion of β-crystal melt peak area due to the increased proportion of the nucleation core layer, the surface layer and isolation layer were insufficient in thickness, and the reflectivity uniformity and backlight component brightness uniformity decreased to 90.7% and 89.6%, respectively. This indicates that simply increasing the proportion of the nucleating phase cannot simultaneously achieve uniform reflection and dimensional stability.

[0043] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A BOPP photoelectric reflective film for a backlight module, characterized in that, It includes a first surface layer, a first isolation layer, a nucleation core layer, a second isolation layer, and a second surface layer stacked in sequence; The mass ratio of the first surface layer, the first isolation layer, the nucleation core layer, the second isolation layer, and the second surface layer is 8-12:8-12:56-64:8-12:8-12; Both the first and second surface layers are formed from surface material, which, by mass parts, comprises 1800 parts of polypropylene resin and 150-210 parts of inorganic surface-treated rutile titanium dioxide. Both the first and second isolation layers are formed of isolation layer material, which, by mass parts, comprises 1800 parts of polypropylene resin and 90-150 parts of inorganic surface-treated rutile titanium dioxide. The nucleating core layer is formed from nucleating core layer material, which, by mass parts, comprises 6900-7100 parts of polypropylene resin, 1300-1700 parts of inorganic surface-treated rutile titanium dioxide, and 450-750 parts of β-crystal nucleating masterbatch. The β-crystal nucleating masterbatch contains polypropylene resin and polypropylene segmented β-crystal nucleating agent. The polypropylene segmented β-crystal nucleating agent is prepared by sequentially mixing a supported calcium pimecronate β-crystal nucleating agent, high acid value and low viscosity maleic anhydride-grafted polypropylene, low acid value and high viscosity maleic anhydride-grafted polypropylene, and a non-reactive high crystallinity polypropylene wax. The supported calcium pimecronate β-crystal nucleating agent is prepared by reacting nano-calcium carbonate with pimecronate. The β-crystal nucleation masterbatch is contained only in the nucleation core layer.

2. The BOPP photoelectric reflective film for a backlight module according to claim 1, characterized in that, The mass ratio of the first surface layer, the first isolation layer, the nucleation core layer, the second isolation layer, and the second surface layer is 10:10:60:10:

10.

3. The BOPP photo reflective film for back light module according to claim 1, wherein, The surface material is prepared by weight of 1800 parts polypropylene resin, 150-210 parts inorganic surface-treated rutile titanium dioxide, 4 parts antioxidant 1010, 4 parts antioxidant 168 and 6 parts calcium stearate.

4. The BOPP photoelectric reflective film for a backlight module according to claim 1, characterized in that, The isolation layer material is prepared by weight parts of 1800 parts polypropylene resin, 90-150 parts inorganic surface-treated rutile titanium dioxide, 4 parts antioxidant 1010, 4 parts antioxidant 168 and 6 parts calcium stearate.

5. The BOPP photoelectric reflective film for a backlight module according to claim 1, characterized in that, The nucleating core material, by weight, is prepared from 6900-7100 parts of polypropylene resin, 1300-1700 parts of inorganic surface-treated rutile titanium dioxide, 450-750 parts of β-crystal nucleating masterbatch, 18-22 parts of antioxidant 1010, 18-22 parts of antioxidant 168, and 18-22 parts of calcium stearate.

6. The BOPP photo reflective film for back light module according to claim 1, wherein, The β-crystal nucleating masterbatch, by mass fraction, is prepared from 820-880 parts of polypropylene resin, 120-180 parts of polypropylene segmented β-crystal nucleating agent, 2-3 parts of antioxidant 1010, 2-3 parts of antioxidant 168, and 4-6 parts of calcium stearate; the polypropylene segmented β-crystal nucleating agent is prepared by sequentially mixing 600 parts of supported calcium pimecrolate β-crystal nucleating agent, 30-42 parts of high acid value, low viscosity maleic anhydride-grafted polypropylene, 38-55 parts of low acid value, high viscosity maleic anhydride-grafted polypropylene, and 20-32 parts of non-reactive, highly crystalline polypropylene wax; the supported calcium pimecrolate β-crystal nucleating agent is prepared by reacting 675-825 parts of nano-calcium carbonate with 170-250 parts of pimecrolate in an ethanol-water system.

7. The BOPP photoelectric reflective film for a backlight module according to claim 1, characterized in that, The high-acid-value, low-viscosity maleic anhydride-grafted polypropylene has a SAP value of 80-100 and a viscosity of 300-400 cps at 190°C; the low-acid-value, high-viscosity maleic anhydride-grafted polypropylene has a SAP value of 15-20 and a viscosity of 1400-1700 cps at 190°C; the non-reactive, highly crystalline polypropylene wax has a viscosity of 1300-1700 mPa·s at 170°C.

8. The BOPP photoelectric reflective film for a backlight module according to claim 1, characterized in that, The inorganic surface-treated rutile titanium dioxide is zirconium aluminum inorganic surface-treated rutile titanium dioxide, and the titanium dioxide content is not less than 95%.

9. A method for preparing a BOPP photoelectric reflective film for a backlight module, comprising preparing the BOPP photoelectric reflective film for a backlight module as described in any one of claims 1-8, characterized in that, Includes the following steps: (1) React nano-calcium carbonate with pimelic acid to obtain a supported calcium pimelic acid β-crystal nucleating agent; (2) The supported calcium pimecrolate β crystal nucleating agent, high acid value low viscosity maleic anhydride grafted polypropylene, low acid value high viscosity maleic anhydride grafted polypropylene and non-reactive high crystallinity polypropylene wax are sequentially mixed to obtain polypropylene segmented β crystal nucleating agent. (3) Mix polypropylene resin, the polypropylene segmented β crystal nucleating agent, antioxidant 1010, antioxidant 168 and calcium stearate and extrude granulate to obtain β crystal nucleating masterbatch. (4) Prepare the surface material, the isolation layer material, and the nucleation core layer material separately; (5) The surface material, isolation layer material and nucleating core layer material are co-extruded and cast in the order of first surface layer, first isolation layer, nucleating core layer, second isolation layer and second surface layer to obtain a cast sheet; (6) The casting is subjected to longitudinal stretching, transverse stretching, heat setting and cooling in sequence to obtain the BOPP photoelectric reflective film for the backlight module.

10. The method for preparing a BOPP photoelectric reflective film for a backlight module according to claim 9, characterized in that, In step (5), the extrusion temperature of the first and second surface layers is 225°C, the extrusion temperature of the first and second isolation layers is 225°C, the extrusion temperature of the nucleation core layer is 215°C, the die temperature is 225°C, and the melt is cast through the die to an 85°C cooling roller to obtain a casting with a thickness of 1900μm to 2200μm. In step (6), the cast sheet is preheated at 116-120℃ for 30s, stretched longitudinally by 3.8-4.2 times at 123-127℃, the longitudinally stretched sheet is preheated at 143-147℃ for 20s, stretched transversely by 5.0-5.2 times at 146-150℃, and then heat-set at 158-162℃ for 20-22s. The transverse relaxation during the heat-setting stage is controlled at 3%-5%. After heat-setting, the sheet is cooled to below 60℃ under tension and then wound up.

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