A method for preparing a high-rigidity biaxially stretched polypropylene film

Abstract

This invention relates to the field of polymer materials technology, specifically to a method for preparing a high-stiffness biaxially oriented polypropylene film. The method uses low melt flow rate homopolymer polypropylene as the reaction donor, and prepares a modified masterbatch through styrene-assisted glycidyl methacrylate melt grafting, subsequent addition of antioxidants, pre-reaction with adipic acid, and downstream branching reaction with triglycidyl isocyanurate. The modified masterbatch is then melt-blended at 9%-13% with biaxially oriented polypropylene matrix resin, 2%-3% with polypropylene modifier resin, and additives to obtain a final blend. Finally, the film is obtained through casting, longitudinal stretching of 4.8-5.2 times, transverse stretching of 8.5-9.5 times, heat setting, and winding. The resulting film exhibits high tensile modulus of elasticity, low heat shrinkage, and low fisheye count.

Description

Technical Field

[0001] This invention relates to the field of polymer materials technology, and in particular to a method for preparing a high-stiffness biaxially oriented polypropylene film. Background Technology

[0002] Biaxially oriented polypropylene (BOPP) film is widely used in packaging, electrical engineering, and tape industries due to its excellent optical properties, high barrier properties, and mechanical properties. Market demands for BOPP film performance are increasing, especially in applications requiring high stiffness and high modulus to ensure packaging rigidity and high-speed filling stability. Therefore, effectively improving the longitudinal and transverse tensile modulus of elasticity (i.e., a key indicator of stiffness) of the film has become a technological focus.

[0003] To improve the stiffness of BOPP films, existing technologies mainly focus on improving the resin raw materials and stretching process. A common approach is to directly use highly crystalline, high-rigidity homopolymer polypropylene as the raw material. This type of resin has a higher flexural modulus, which theoretically can impart higher initial stiffness to the film. However, such high-rigidity resins usually have insufficient melt strength. During biaxial stretching, the rapid deformation occurring simultaneously in the longitudinal and transverse directions can easily lead to uneven local thickness or even film breakage, severely limiting production speed and yield. This presents a fundamental contradiction between improving stiffness and ensuring the stability of the stretching process.

[0004] Another approach is to use polypropylene resin with a wide molecular weight distribution, where the high molecular weight fraction provides melt strength, which is beneficial for tensile stability. However, an excessively wide molecular weight distribution can cause the low molecular weight components in the film to migrate or relax during subsequent heat treatment or use, resulting in a higher heat shrinkage rate and affecting the dimensional stability of the film during subsequent processing (such as printing and lamination) and final use. This presents a new challenge in balancing improving processing stability with controlling heat shrinkage performance.

[0005] Modifying polypropylene through chemical reactions, such as introducing long-chain branched structures, is an effective way to improve melt strength. Glycidyl methacrylate (GMA) grafting is a common method. However, when introducing polar monomers like GMA onto the polypropylene molecular chain, the grafting reaction competes with the β-fracture degradation reaction of the polypropylene chain. If not properly controlled, excessive chain scission not only reduces the molecular weight and weakens the modification effect, but also significantly increases gel points and fisheye defects in the film due to the generation of large amounts of oligomers. Published literature indicates that adding styrene (St) as a comonomer can improve the grafting efficiency of GMA and inhibit polypropylene chain scission, but this only solves the local grafting efficiency problem. When attempting to enhance the effect by increasing the modification ratio, issues such as the uniformity of the modified point distribution, the control of the branched structure, and the compatibility between high-content modified resins and the base resin become prominent, often leading to a sharp increase in defects such as fisheyes and gels, failing to meet the appearance requirements of high-end films.

[0006] In summary, existing technologies, in pursuing high stiffness BOPP films, often fall into a dilemma of balancing performance and processability, regardless of whether they employ high-rigidity resins, resins with wide molecular weight distributions, or perform single-reaction modification: simply increasing rigidity or melt strength may come at the cost of sacrificing tensile stability or increasing thermal shrinkage; while deep modification aimed at reducing thermal shrinkage easily introduces too many defects, compromising the film's uniformity. Therefore, developing a preparation method that can synergistically improve film stiffness (high tensile modulus), significantly reduce thermal shrinkage, and simultaneously ensure extremely low fisheye count and excellent biaxial tensile processing stability has become a pressing technical challenge in this field. Summary of the Invention

[0007] In view of this, the purpose of this invention is to propose a method for preparing high stiffness biaxially oriented polypropylene film, so as to solve the contradiction problem in the existing high stiffness BOPP film preparation technology, which is difficult to achieve simultaneously by using high rigidity resin, wide molecular weight distribution resin or simple reaction modification, resulting in the difficulty of improving film stiffness, reducing heat shrinkage rate and processing stability (low defects).

[0008] To achieve the above objectives, the present invention provides a method for preparing a high-stiffness biaxially oriented polypropylene film, comprising the following steps: S1. Using polypropylene with a melt flow rate lower than that of the polypropylene matrix resin for biaxially oriented polypropylene film as the donor resin, the polypropylene donor resin is subjected to a melt grafting reaction with glycidyl methacrylate in the presence of styrene and a free radical initiator, and an antioxidant is added in the later stage of the melt grafting reaction to obtain a high molecular weight polypropylene epoxy reaction site masterbatch. S2. The high molecular weight polypropylene epoxy reaction site masterbatch is subjected to a pre-melting reaction with a portion of adipic acid and catalyst. After the pre-melt enters the post-reaction zone, a branching reaction side feed consisting of high molecular weight polypropylene epoxy reaction site masterbatch, the remaining adipic acid and triglycidyl isocyanurate is added to carry out the branching reaction and obtain the modified masterbatch. S3. The biaxially oriented polypropylene film is melt-blended with polypropylene matrix resin, the modified masterbatch and polypropylene conditioning resin to obtain a final blend for biaxially oriented polypropylene film. S4. The final mixture for biaxially oriented polypropylene film is melt-cast to form a casting sheet, and the casting sheet is sequentially stretched longitudinally, stretched laterally, and heat-set to obtain a high-stiffness biaxially oriented polypropylene film. The modified masterbatch contains 9%-13% of the total mass of the final mixture for biaxially oriented polypropylene film, and the polypropylene conditioning resin contains 2%-3% of the total mass. The longitudinal stretching ratio is 4.8-5.2 times, and the transverse stretching ratio is 8.5-9.5 times.

[0009] Preferably, the polypropylene donor resin is homopolymer polypropylene, which has a melt flow rate of 0.8 g / 10 min at 230°C and a load of 2.16 kg, and a density of 0.90 g / cm³. 3 It has a melting point of 162℃, a Vicat softening temperature of 153℃, and a flexural modulus of 1400MPa.

[0010] Preferably, the polypropylene matrix resin used for the biaxially oriented polypropylene film is homopolymer polypropylene, which has a melt flow rate of 8 g / 10 min at 230°C and a load of 2.16 kg, and a density of 0.90 g / cm³. 3 The polypropylene resin has a melting point of 160℃, a Vicat softening temperature of 153℃, and a flexural modulus of 1350MPa. The melt flow rate of the polypropylene resin at 230℃ and a load of 2.16kg is 2g / 10min, and its density is 0.90g / cm³. 3 It has a melting point of 164℃, a Vicat softening temperature of 160℃, and a flexural modulus of 1750MPa.

[0011] Preferably, in step S1, 11,000-13,000 parts by weight of polypropylene donor resin, 330-650 parts by weight of glycidyl methacrylate, 165-325 parts by weight of styrene and 11-26 parts by weight of free radical initiator are mixed to obtain high molecular weight polypropylene graft premix; the free radical initiator is dicumyl peroxide.

[0012] Preferably, in step S1, the high molecular weight polypropylene grafted premix is ​​added to a co-rotating twin-screw extruder for melt grafting reaction. The co-rotating twin-screw extruder has a length-to-diameter ratio of 40:1, and the temperatures from the feed end to the die head are 160-170℃, 170-180℃, 180-190℃, 185-195℃, 185-195℃, and 180-190℃ respectively. The screw speed is 110-130 rpm, the average residence time of the material is 85-95 s, and nitrogen is introduced from the feed section to the third temperature zone at a flow rate of 0.8 L / min-1.2 L / min.

[0013] Preferably, in step S1, after the material enters the fifth temperature zone, 33-39 parts of hindered phenolic antioxidant and 22-26 parts of phosphite antioxidant are added through the side feed port; a vacuum de-volume port is set from the fifth temperature zone to the front of the machine head, and the absolute pressure is controlled to be below 20 kPa.

[0014] Preferably, in step S2, 11,000-13,000 parts by weight of high molecular weight polypropylene epoxy reaction site masterbatch, 11-26 parts by weight of adipic acid, and 4-9 parts by weight of catalyst are added from the main feed port of the twin-screw extruder for pre-reaction of the front-end melt; the catalyst is triphenylphosphine; separately, 500 parts by weight of high molecular weight polypropylene epoxy reaction site masterbatch, 22-52 parts by weight of adipic acid, and 11-26 parts by weight of triglycidyl isocyanurate are premixed for 10 minutes at 25°C and 55-65 rpm to obtain 533-578 parts by weight of branched reaction side feed; the front-end melt enters the fifth... During the temperature zone, the branching reaction side feed is added through the side feed port; the length-to-diameter ratio of the twin-screw extruder is 40:1, the first four temperature zones are 172-178℃, 178-183℃, 183-188℃ and 183-188℃ respectively, the screw speed is 85-95 rpm, and the average residence time of the front melt pre-reaction is 55-65s; the temperature from the fifth temperature zone to the die head is 188-192℃, 192-198℃ and 198-202℃ respectively, the average residence time of the branching reaction is 65-75s, and vacuum devolatilization is performed before the die head at an absolute pressure of less than 20kPa.

[0015] Preferably, in step S3, 83,450-88,550 parts by weight of biaxially oriented polypropylene film-grade polypropylene matrix resin, 9,000-13,000 parts of modified masterbatch, 2,000-3,000 parts of polypropylene conditioning resin, 140-160 parts of hindered phenolic antioxidant, 90-110 parts of phosphite antioxidant, and 220-280 parts of calcium stearate are added to a twin-screw extruder; the temperatures of each zone of the twin-screw extruder are 188-192℃, 208-212℃, 222-228℃, 228-232℃, 228-232℃, and 222-228℃ respectively, the screw speed is 150-170 rpm, and the average residence time of the material is 75-85 s.

[0016] Preferably, in step S4, the final mixture for biaxially oriented polypropylene film is added to a single-screw casting extruder. The barrel temperature is sequentially set to 218-222℃, 232-238℃, 242-248℃, and 242-248℃. The T-die temperature is 242-248℃. After the melt is filtered through a 120-mesh metal filter, it is cast onto a cooling roller at 33-38℃. The traction speed is adjusted to make the thickness of the cast sheet 850-950μm.

[0017] Preferably, in step S4, the casting is preheated at 116-120°C for 18-22 seconds, longitudinally stretched at 123-127°C, then preheated at 156-160°C for 14-16 seconds, and transversely stretched at 160-164°C; subsequently heat-set at 162-166°C for 16-20 seconds, and 3-5% of the tensile tension is released in the transverse direction, and finally cooled to below 40°C.

[0018] The beneficial effects of this invention are: (1) This invention uses high molecular weight polypropylene with low melt flow rate as the reaction donor, combined with styrene-assisted GMA grafting, post-addition of antioxidants, segmented flexible pre-anchoring of adipic acid, and downstream branching reaction of triglycidyl isocyanurate. The resulting modified masterbatch is then introduced into the matrix resin in a low proportion of 9%-13% with polypropylene conditioning resin, forming a complete reaction and blending control system. This scheme significantly improves the longitudinal and transverse tensile modulus of the film and reduces the thermal shrinkage rate without significantly increasing fisheye defects, achieving a balance between high stiffness, low thermal shrinkage, and excellent processing stability.

[0019] (2) The segmented and spatially distributed reaction design of this invention has a clear synergistic effect. Styrene assists in ensuring the effective grafting of GMA onto high molecular weight chains and reduces chain breakage (the performance of Comparative Example 2 decreased across the board after the removal of styrene); the antioxidant is added in the later stage, which not only protects the free radical reaction in the grafting stage, but also ensures the stability of subsequent processing (the addition of antioxidant in Comparative Example 3 in advance reduced crystallinity and modulus); adipic acid participates in the reaction in two steps, realizing the pre-anchoring of flexible segments and subsequent efficient cross-linking, avoiding the surge in fisheyes caused by uneven reaction (the one-time addition in Comparative Example 4 increased the number of fisheyes to 31.5 / m). 2 Finally, the blending of a low proportion of modified masterbatch with a small amount of polypropylene conditioning resin creates a composite structure in the matrix with a moderately long-chain branched network and rigid reinforcing points, which synergistically improves the melt strength and the orientation retention ability of the final film, thereby obtaining an optimized structure with high modulus and low shrinkage during biaxial stretching. 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] Raw material source and parameters: The polypropylene matrix resin used for biaxially oriented polypropylene film is LyondellBasell's Moplen HP420M, a homopolymer polypropylene with a melt flow rate of 8 g / 10 min at 230℃ / 2.16 kg load and a density of 0.90 g / cm³. 3 It has a melting point of 160℃, a Vicat softening temperature of 153℃, and a flexural modulus of 1350MPa.

[0022] The high molecular weight polypropylene donor resin selected was LyondellBasell's Moplen HP556E, a homopolymer polypropylene with a melt flow rate of 0.8 g / 10 min at 230℃ / 2.16 kg load and a density of 0.90 g / cm³.3 It has a melting point of 162℃, a Vicat softening temperature of 153℃, and a flexural modulus of 1400MPa.

[0023] The polypropylene conditioning resin selected was LyondellBasell's Adstif HA622H, which had a melt flow rate of 2 g / 10 min at 230℃ and a load of 2.16 kg, and a density of 0.90 g / cm³. 3 It has a melting point of 164℃, a Vicat softening temperature of 160℃, and a flexural modulus of 1750MPa.

[0024] The hindered phenolic antioxidant selected was BASF's IRGANOX 1010 as the primary antioxidant.

[0025] The phosphite antioxidant selected was BASF's IRGAFOS168, used as a processing stabilizer.

[0026] The calcium stearate used is Sigma-Aldrich's Vetec series product number V900174. Example 1:

[0027] Step 1: Add 12000g of high molecular weight polypropylene donor resin, 480g of glycidyl methacrylate, 240g of styrene and 18g of dicumyl peroxide to a low-speed mixer and mix for 20 minutes at 25℃ and 60rpm to obtain high molecular weight polypropylene grafted premix. Step 2: Add the high molecular weight polypropylene grafted premix obtained in Step 1 to a conventional co-rotating twin-screw extruder with an L / D ratio of 40:1. Set the temperatures from the feed end to the die head to 165℃, 175℃, 185℃, 190℃, 190℃ and 185℃ respectively. Set the screw speed to 120 rpm and the average residence time of the material to 90 s. Introduce nitrogen from the feed section to the third temperature zone at a flow rate of 1 L / min. After the material enters the fifth temperature zone, add 36 g of hindered phenolic antioxidant and 24 g of phosphite antioxidant through the side feed port. Set a vacuum devolatilization port from the fifth temperature zone to the die head. Control the absolute pressure to below 20 kPa. After the melt is filtered through an 80-mesh metal filter, it is drawn into strips, water-cooled, pelletized, and then vacuum-dried at 80℃ for 3 h to obtain high molecular weight polypropylene epoxy reaction site masterbatch. Step 3: Take 12000g of high molecular weight polypropylene epoxy reaction site masterbatch, 18g of adipic acid, and 6g of triphenylphosphine, and add them through the main feed port of a twin-screw extruder. The length-to-diameter ratio of the extruder is 40:1. The first four temperature zones are set to 175℃, 180℃, 185℃, and 185℃ respectively, with a screw speed of 90 rpm and an average residence time of 60s. Separately take 500g of high molecular weight polypropylene epoxy reaction site masterbatch, 36g of adipic acid, and 18g of triglycidyl isocyanurate. Premixed for 10 min at 25℃ and 60 rpm, 554 g of branching reaction side feed was obtained. When the current section melt entered the fifth temperature zone, 554 g of branching reaction side feed was added through the side feed port. The temperatures from the fifth temperature zone to the die head were set to 190℃, 195℃ and 200℃ respectively, with an average residence time of 70 s. Vacuum devolatilization was carried out in front of the die head at an absolute pressure of less than 20 kPa. After being drawn, water-cooled and pelletized, the melt was vacuum dried at 80℃ for 3 h to obtain the modified masterbatch. Step 4: Add 86,000g of biaxially oriented polypropylene film matrix resin, 11,000g of modified masterbatch, 2,500g of polypropylene conditioning resin, 150g of hindered phenolic antioxidant, 100g of phosphite antioxidant, and 250g of calcium stearate to a twin-screw extruder. Set the temperatures of each zone of the extruder to 190℃, 210℃, 225℃, 230℃, 230℃, and 225℃ respectively, the screw speed to 160rpm, and the average residence time of the material to 80s. After melt mixing, filtration through an 80-mesh metal screen, stretching, water cooling, and pelletizing, the final mixture for biaxially oriented polypropylene film is obtained. Step 5: Add 100,000g of the final mixture for biaxially oriented polypropylene film to a single-screw casting extruder. Set the barrel temperature sequentially to 220℃, 235℃, 245℃, and 245℃, and the T-die temperature to 245℃. After filtering the melt through a 120-mesh metal screen, cast it onto a 35℃ cooling roller. Adjust the traction speed to achieve a cast film thickness of 900μm. Preheat the resulting cast film at 118℃ for 20s, then stretch it longitudinally at 125℃ with a longitudinal stretching ratio of 5 times. Preheat it again at 158℃ for 15s, then stretch it transversely at 162℃ with a transverse stretching ratio of 9 times. Subsequently, heat set it at 164℃ for 18s, releasing 4% of the tensile tension in the transverse direction. Finally, cool it to below 40℃, trim the edges, and wind it up to obtain a high-stiffness biaxially oriented polypropylene film. Example 2:

[0028] Step 1: Add 11,000g of high molecular weight polypropylene donor resin, 330g of glycidyl methacrylate, 165g of styrene and 11g of dicumyl peroxide to a low-speed mixer and mix for 18 minutes at 23°C and 55 rpm to obtain high molecular weight polypropylene grafted premix. Step 2: Add the high molecular weight polypropylene grafted premix obtained in Step 1 to a conventional co-rotating twin-screw extruder with an L / D ratio of 40:1. Set the temperatures from the feed end to the die head to 160℃, 170℃, 180℃, 185℃, 185℃, and 180℃ respectively. Set the screw speed to 110 rpm and the average residence time of the material to 95 s. Introduce nitrogen from the feed section to the third temperature zone at a flow rate of 0.8 L / min. After the material enters the fifth temperature zone, add 33 g of hindered phenolic antioxidant and 22 g of phosphite antioxidant through the side feed port. Set a vacuum devolatilization port from the fifth temperature zone to the die head and control the absolute pressure to below 20 kPa. After the melt is filtered through an 80-mesh metal filter, it is drawn into strips, water-cooled, pelletized, and then vacuum-dried at 80℃ for 3 h to obtain high molecular weight polypropylene epoxy reaction site masterbatch. Step 3: Take 11,000g of high molecular weight polypropylene epoxy reaction site masterbatch, 11g of adipic acid, and 4g of triphenylphosphine, and add them through the main feed port of a twin-screw extruder. The length-to-diameter ratio of the extruder is 40:1. The first four temperature zones are set to 172℃, 178℃, 183℃, and 183℃ respectively, the screw speed is 85 rpm, and the average residence time is 65s. Separately take 500g of high molecular weight polypropylene epoxy reaction site masterbatch, 22g of adipic acid, and 11g of triglycidyl isocyanurate. Premixed at 25℃ and 55rpm for 10min, 533g of branching reaction side feed was obtained. When the current section melt entered the fifth temperature zone, 533g of branching reaction side feed was added through the side feed port. The temperatures from the fifth temperature zone to the die head were set to 188℃, 192℃ and 198℃ respectively, with an average residence time of 75s. Vacuum devolatilization was carried out in front of the die head at an absolute pressure of less than 20kPa. After being drawn, water-cooled and pelletized, the melt was vacuum-dried at 80℃ for 3h to obtain the modified masterbatch. Step 4: Add 88,550g of biaxially oriented polypropylene film matrix resin, 9,000g of modified masterbatch, 2,000g of polypropylene conditioning resin, 140g of hindered phenolic antioxidant, 90g of phosphite antioxidant, and 220g of calcium stearate to a twin-screw extruder. Set the temperatures of each zone of the extruder to 188℃, 208℃, 222℃, 228℃, 228℃, and 222℃ respectively, with a screw speed of 150rpm and an average material residence time of 85s. After melt mixing, filtration through an 80-mesh metal screen, stretching, water cooling, and pelletizing, the final mixture for biaxially oriented polypropylene film is obtained. Step 5: Add 100,000g of the final mixture for biaxially oriented polypropylene film to a single-screw casting extruder. Set the barrel temperature sequentially to 218℃, 232℃, 242℃, and 242℃, and the T-die temperature to 242℃. After filtering the melt through a 120-mesh metal screen, cast it onto a 33℃ cooling roller. Adjust the traction speed to achieve a cast film thickness of 850μm. Preheat the resulting cast film at 116℃ for 18s, then stretch it longitudinally at 123℃ with a longitudinal stretching ratio of 4.8 times. Next, preheat it at 156℃ for 14s, then stretch it transversely at 160℃ with a transverse stretching ratio of 8.5 times. Subsequently, heat-set it at 162℃ for 16s, releasing 3% of the tensile tension in the transverse direction. Finally, cool it to below 40℃, trim the edges, and wind it up to obtain a high-stiffness biaxially oriented polypropylene film. Example 3:

[0029] Step 1: Add 13000g of high molecular weight polypropylene donor resin, 650g of glycidyl methacrylate, 325g of styrene and 26g of dicumyl peroxide to a low-speed mixer and mix for 22 minutes at 27℃ and 65rpm to obtain high molecular weight polypropylene grafted premix. Step 2: Add the high molecular weight polypropylene grafted premix obtained in Step 1 to a conventional co-rotating twin-screw extruder with an L / D ratio of 40:1. Set the temperatures from the feed end to the die head to 170℃, 180℃, 190℃, 195℃, 195℃, and 190℃ respectively. Set the screw speed to 130 rpm and the average residence time of the material to 85 s. Introduce nitrogen from the feed section to the third temperature zone at a flow rate of 1.2 L / min. After the material enters the fifth temperature zone, add 39 g of hindered phenolic antioxidant and 26 g of phosphite antioxidant through the side feed port. Set a vacuum devolatilization port from the fifth temperature zone to the die head and control the absolute pressure to below 20 kPa. After the melt is filtered through an 80-mesh metal filter, it is drawn into strips, water-cooled, pelletized, and then vacuum-dried at 80℃ for 3 h to obtain high molecular weight polypropylene epoxy reaction site masterbatch. Step 3: Take 13000g of high molecular weight polypropylene epoxy reaction site masterbatch, 26g of adipic acid, and 9g of triphenylphosphine, and add them through the main feed port of a twin-screw extruder. The length-to-diameter ratio of the extruder is 40:1. The first four temperature zones are set to 178℃, 183℃, 188℃, and 188℃ respectively, with a screw speed of 95 rpm and an average residence time of 55s. Separately take 500g of high molecular weight polypropylene epoxy reaction site masterbatch, 52g of adipic acid, and 26g of triglycidyl isocyanurate. Premixed for 10 min at 25℃ and 65 rpm, 578 g of branching reaction side feed was obtained. When the current section melt entered the fifth temperature zone, 578 g of branching reaction side feed was added through the side feed port. The temperatures from the fifth temperature zone to the die head were set to 192℃, 198℃ and 202℃ respectively, with an average residence time of 65 s. Vacuum devolatilization was carried out in front of the die head at an absolute pressure of less than 20 kPa. After being drawn, water-cooled and pelletized, the melt was vacuum-dried at 80℃ for 3 h to obtain the modified masterbatch. Step 4: Add 83450g of biaxially oriented polypropylene film matrix resin, 13000g of modified masterbatch, 3000g of polypropylene conditioning resin, 160g of hindered phenolic antioxidant, 110g of phosphite antioxidant, and 280g of calcium stearate to a twin-screw extruder. Set the temperatures of each zone of the extruder to 192℃, 212℃, 228℃, 232℃, 232℃, and 228℃ respectively, the screw speed to 170rpm, and the average residence time of the material to 75s. After melt mixing, filtration through an 80-mesh metal screen, stretching, water cooling, and pelletizing, the final mixture for biaxially oriented polypropylene film is obtained. Step 5: Add 100,000g of the final mixture for biaxially oriented polypropylene film to a single-screw casting extruder. Set the barrel temperature sequentially to 222℃, 238℃, 248℃, and 248℃, and the T-die temperature to 248℃. After filtering the melt through a 120-mesh metal screen, cast it onto a 38℃ cooling roller. Adjust the traction speed to achieve a cast film thickness of 950μm. Preheat the resulting cast film at 120℃ for 22s, then stretch it longitudinally at 127℃ with a longitudinal stretching ratio of 5.2 times. Preheat it again at 160℃ for 16s, then stretch it transversely at 164℃ with a transverse stretching ratio of 9.5 times. Subsequently, heat-set it at 166℃ for 20s, releasing 5% of the tensile tension in the transverse direction. Finally, cool it to below 40℃, trim the edges, and wind it up to obtain a high-stiffness biaxially oriented polypropylene film. Example 4:

[0030] Step 1: Add 11500g of high molecular weight polypropylene donor resin, 460g of glycidyl methacrylate, 230g of styrene and 17g of dicumyl peroxide to a low-speed mixer and mix for 20 minutes at 24℃ and 60rpm to obtain high molecular weight polypropylene grafted premix. Step 2: Add the high molecular weight polypropylene grafted premix obtained in Step 1 to a conventional co-rotating twin-screw extruder with an L / D ratio of 40:1. Set the temperatures from the feed end to the die head to 165℃, 175℃, 185℃, 190℃, 190℃ and 185℃ respectively. Set the screw speed to 115 rpm and the average residence time of the material to 90 s. Introduce nitrogen from the feed section to the third temperature zone at a flow rate of 1 L / min. After the material enters the fifth temperature zone, add 35 g of hindered phenolic antioxidant and 23 g of phosphite antioxidant through the side feed port. Set a vacuum devolatilization port from the fifth temperature zone to the die head. Control the absolute pressure to below 20 kPa. After the melt is filtered through an 80-mesh metal filter, it is drawn into strips, water-cooled, pelletized, and then vacuum-dried at 80℃ for 3 h to obtain high molecular weight polypropylene epoxy reaction site masterbatch. Step 3: Take 11500g of high molecular weight polypropylene epoxy reaction site masterbatch, 17g of adipic acid, and 6g of triphenylphosphine, and add them through the main feed port of a twin-screw extruder. The length-to-diameter ratio of the extruder is 40:1. The first four temperature zones are set to 175℃, 180℃, 185℃, and 185℃ respectively. The screw speed is 90rpm, and the average residence time is 60s. Separately take 500g of high molecular weight polypropylene epoxy reaction site masterbatch, 35g of adipic acid, and 17g of triglycidyl isocyanurate. Premixed for 10 min at 25℃ and 60 rpm, 552 g of branching reaction side feed was obtained. When the current section melt entered the fifth temperature zone, 552 g of branching reaction side feed was added through the side feed port. The temperatures from the fifth temperature zone to the die head were set to 190℃, 195℃ and 200℃ respectively, with an average residence time of 70 s. Vacuum devolatilization was carried out in front of the die head at an absolute pressure of less than 20 kPa. After being drawn, water-cooled and pelletized, the melt was vacuum-dried at 80℃ for 3 h to obtain the modified masterbatch. Step 4: Add 86,000g of biaxially oriented polypropylene film matrix resin, 10,500g of modified masterbatch, 3,000g of polypropylene conditioning resin, 150g of hindered phenolic antioxidant, 100g of phosphite antioxidant, and 250g of calcium stearate to a twin-screw extruder. Set the temperatures of each zone of the extruder to 190℃, 210℃, 225℃, 230℃, 230℃, and 225℃ respectively, the screw speed to 160rpm, and the average residence time of the material to 80s. After melt mixing, filtration through an 80-mesh metal screen, stretching, water cooling, and pelletizing, the final mixture for biaxially oriented polypropylene film is obtained. Step 5: Add 100,000g of the final mixture for biaxially oriented polypropylene film to a single-screw casting extruder. Set the barrel temperature sequentially to 220℃, 235℃, 245℃, and 245℃, and the T-die temperature to 245℃. After filtering the melt through a 120-mesh metal screen, cast it onto a 35℃ cooling roller. Adjust the traction speed to achieve a cast film thickness of 900μm. Preheat the resulting cast film at 118℃ for 20s, then stretch it longitudinally at 125℃ with a longitudinal stretching ratio of 5 times. Preheat it again at 158℃ for 15s, then stretch it transversely at 162℃ with a transverse stretching ratio of 9 times. Subsequently, heat set it at 164℃ for 18s, releasing 4% of the tensile tension in the transverse direction. Finally, cool it to below 40℃, trim the edges, and wind it up to obtain a high-stiffness biaxially oriented polypropylene film. Example 5:

[0031] Step 1: Add 12500g of high molecular weight polypropylene donor resin, 560g of glycidyl methacrylate, 280g of styrene and 22g of dicumyl peroxide to a low-speed mixer and mix for 21 minutes at 26℃ and 60rpm to obtain high molecular weight polypropylene grafted premix. Step 2: Add the high molecular weight polypropylene grafted premix obtained in Step 1 to a conventional co-rotating twin-screw extruder with an L / D ratio of 40:1. Set the temperatures from the feed end to the die head to 168℃, 178℃, 188℃, 193℃, 193℃, and 188℃ respectively. Set the screw speed to 125 rpm and the average residence time of the material to 88 s. Introduce nitrogen from the feed section to the third temperature zone at a flow rate of 1.1 L / min. After the material enters the fifth temperature zone, add 38 g of hindered phenolic antioxidant and 25 g of phosphite antioxidant through the side feed port. Set a vacuum devolatilization port from the fifth temperature zone to the die head and control the absolute pressure to below 20 kPa. After the melt is filtered through an 80-mesh metal filter, it is drawn into strips, water-cooled, pelletized, and then vacuum-dried at 80℃ for 3 h to obtain high molecular weight polypropylene epoxy reaction site masterbatch. Step 3: Take 12500g of high molecular weight polypropylene epoxy reaction site masterbatch, 22g of adipic acid, and 7g of triphenylphosphine, and add them through the main feed port of a twin-screw extruder. The length-to-diameter ratio of the extruder is 40:1. The first four temperature zones are set to 176℃, 181℃, 186℃, and 186℃ respectively, the screw speed is 92 rpm, and the average residence time is 58s. Separately take 500g of high molecular weight polypropylene epoxy reaction site masterbatch, 44g of adipic acid, and 22g of triglycidyl isocyanurate. Premixed for 10 min at 25℃ and 60 rpm, 566 g of branching reaction side feed was obtained. When the current section melt entered the fifth temperature zone, 566 g of branching reaction side feed was added through the side feed port. The temperatures from the fifth temperature zone to the die head were set to 191℃, 196℃ and 201℃ respectively, with an average residence time of 68 s. Vacuum devolatilization was carried out in front of the die head at an absolute pressure of less than 20 kPa. After being drawn, water-cooled and pelletized, the melt was vacuum-dried at 80℃ for 3 h to obtain the modified masterbatch. Step 4: Add 85,000g of biaxially oriented polypropylene film matrix resin, 12,500g of modified masterbatch, 2,000g of polypropylene conditioning resin, 150g of hindered phenolic antioxidant, 100g of phosphite antioxidant, and 250g of calcium stearate to a twin-screw extruder. Set the temperatures of each zone of the extruder to 190℃, 210℃, 225℃, 230℃, 230℃, and 225℃ respectively, the screw speed to 160rpm, and the average residence time of the material to 80s. After melt mixing, filtration through an 80-mesh metal screen, stretching, water cooling, and pelletizing, the final mixture for biaxially oriented polypropylene film is obtained. Step 5: Add 100,000g of the final mixture for biaxially oriented polypropylene film to a single-screw casting extruder. Set the barrel temperature sequentially to 221℃, 236℃, 246℃, and 246℃, and the T-die temperature to 246℃. After filtering the melt through a 120-mesh metal screen, cast it onto a 36℃ cooling roller. Adjust the traction speed to achieve a cast film thickness of 920μm. Preheat the resulting cast film at 119℃ for 20s, then stretch it longitudinally at 126℃ with a longitudinal stretching ratio of 5.1 times. Preheat it again at 159℃ for 15s, then stretch it transversely at 163℃ with a transverse stretching ratio of 9.2 times. Subsequently, heat-set it at 165℃ for 18s, releasing 4% of the tensile tension in the transverse direction. Finally, cool it to below 40℃, trim the edges, and wind it up to obtain a high-stiffness biaxially oriented polypropylene film. Example 6:

[0032] Step 1: Add 12000g of high molecular weight polypropylene donor resin, 420g of glycidyl methacrylate, 210g of styrene and 15g of dicumyl peroxide to a low-speed mixer and mix for 19 minutes at 25℃ and 58rpm to obtain high molecular weight polypropylene grafted premix. Step 2: Add the high molecular weight polypropylene grafted premix obtained in Step 1 to a conventional co-rotating twin-screw extruder with an L / D ratio of 40:1. Set the temperatures from the feed end to the die head to 165℃, 175℃, 185℃, 190℃, 190℃ and 185℃ respectively. Set the screw speed to 120 rpm and the average residence time of the material to 90 s. Introduce nitrogen from the feed section to the third temperature zone at a flow rate of 1 L / min. After the material enters the fifth temperature zone, add 36 g of hindered phenolic antioxidant and 24 g of phosphite antioxidant through the side feed port. Set a vacuum devolatilization port from the fifth temperature zone to the die head. Control the absolute pressure to below 20 kPa. After the melt is filtered through an 80-mesh metal filter, it is drawn into strips, water-cooled, pelletized, and then vacuum-dried at 80℃ for 3 h to obtain high molecular weight polypropylene epoxy reaction site masterbatch. Step 3: Take 12000g of high molecular weight polypropylene epoxy reaction site masterbatch, 15g of adipic acid, and 5g of triphenylphosphine, and add them through the main feed port of a twin-screw extruder. The length-to-diameter ratio of the extruder is 40:1. The first four temperature zones are set to 174℃, 180℃, 185℃, and 185℃ respectively. The screw speed is 88 rpm, and the average residence time is 62s. Separately take 500g of high molecular weight polypropylene epoxy reaction site masterbatch, 30g of adipic acid, and 15g of triglycidyl isocyanurate. Premixed at 25℃ and 58rpm for 10min, 545g of branching reaction side feed was obtained. When the current section melt entered the fifth temperature zone, 545g of branching reaction side feed was added through the side feed port. The temperatures from the fifth temperature zone to the die head were set to 190℃, 195℃ and 200℃ respectively, with an average residence time of 70s. Vacuum devolatilization was carried out in front of the die head at an absolute pressure of less than 20kPa. After being drawn, water-cooled and pelletized, the melt was vacuum-dried at 80℃ for 3h to obtain the modified masterbatch. Step 4: Add 86,500g of biaxially oriented polypropylene film matrix resin, 10,000g of modified masterbatch, 3,000g of polypropylene conditioning resin, 150g of hindered phenolic antioxidant, 100g of phosphite antioxidant, and 250g of calcium stearate to a twin-screw extruder. Set the temperatures of each zone of the extruder to 191℃, 211℃, 226℃, 231℃, 231℃, and 226℃ respectively, the screw speed to 165rpm, and the average residence time of the material to 78s. After melt mixing, filtration through an 80-mesh metal screen, stretching, water cooling, and pelletizing, the final mixture for biaxially oriented polypropylene film is obtained. Step 5: Add 100,000g of the final mixture for biaxially oriented polypropylene film to a single-screw casting extruder. Set the barrel temperature sequentially to 222℃, 238℃, 248℃, and 248℃, and the T-die temperature to 248℃. After filtering the melt through a 120-mesh metal screen, cast it onto a 37℃ cooling roller. Adjust the traction speed to achieve a cast film thickness of 880μm. Preheat the resulting cast film at 120℃ for 21s, then stretch it longitudinally at 127℃ with a longitudinal stretching ratio of 5 times. Preheat it again at 160℃ for 16s, then stretch it transversely at 164℃ with a transverse stretching ratio of 9 times. Subsequently, heat set it at 166℃ for 19s, releasing 4% of the tensile tension in the transverse direction. Finally, cool it to below 40℃, trim the edges, and wind it up to obtain a high-stiffness biaxially oriented polypropylene film.

[0033] Comparative Example 1: The difference from Example 1 is that in step one, 12,000g of high molecular weight polypropylene donor resin is replaced with 12,000g of biaxially oriented polypropylene film matrix resin, and the modified masterbatch is prepared by continuing to use the replaced material in steps two and three, with the other conditions being the same as in Example 1.

[0034] Comparative Example 2: The difference from Example 1 is that in step one, 240g of styrene is not added, and the amount of high molecular weight polypropylene donor resin added is adjusted from 12000g to 12240g to keep the total amount of premix added in step one unchanged. The other conditions are the same as in Example 1.

[0035] Comparative Example 3: The difference from Example 1 is as follows: In step one, 36g of hindered phenolic antioxidant and 24g of phosphite antioxidant are added simultaneously with the addition of 12000g of high molecular weight polypropylene donor resin, 480g of glycidyl methacrylate, 240g of styrene and 18g of dicumyl peroxide; In step two, hindered phenolic antioxidant and phosphite antioxidant are no longer added through the side feed port after the material enters the fifth temperature zone, and the other conditions are the same as in Example 1.

[0036] Comparative Example 4: The difference from Example 1 is that in step three, 18g of adipic acid is not added to the main feed port; 500g of high molecular weight polypropylene epoxy reaction site masterbatch, 54g of adipic acid and 18g of triglycidyl isocyanurate are taken separately and premixed for 10min at 25℃ and 60rpm to obtain 572g of branching reaction side feed; when the current section melt enters the fifth temperature zone, 572g of branching reaction side feed is added through the side feed port, and the other conditions are the same as in Example 1.

[0037] Comparative Example 5: The difference from Example 1 is that in step three, 18g of triglycidyl isocyanurate is not added to the branching reaction side feed, and the high molecular weight polypropylene epoxy reaction site masterbatch in the branching reaction side feed is adjusted from 500g to 518g to keep the total amount of branching reaction side feed unchanged. The other conditions are the same as in Example 1.

[0038] Comparative Example 6: The difference from Example 1 is that in step three, 18g of adipic acid is not added to the main feed port, and the amount of high molecular weight polypropylene epoxy reaction site masterbatch added to the main feed port is adjusted from 12000g to 12018g; 36g of adipic acid is not added to the branching reaction side feed, and the amount of high molecular weight polypropylene epoxy reaction site masterbatch in the branching reaction side feed is adjusted from 500g to 536g, so as to keep the total amount of material added to the main feed port and the side feed port unchanged. The other conditions are the same as in Example 1.

[0039] Comparative Example 7: The difference from Example 1 is that in step four, 86,000g of biaxially oriented polypropylene film is replaced with 86,000g of modified masterbatch instead of polypropylene matrix resin, so that the total amount of modified masterbatch added in step four is adjusted from 11,000g to 97,000g. The other conditions are the same as in Example 1.

[0040] Comparative Example 8: The difference from Example 1 is that in step four, 2500g of polypropylene conditioning resin is not added, and the amount of polypropylene matrix resin for biaxially oriented polypropylene film is adjusted from 86000g to 88500g to keep the total mass of the final mixture for biaxially oriented polypropylene film unchanged. The other conditions are the same as in Example 1.

[0041] Performance testing: Melt mass flow rate: The melt mass flow rate of the biaxially oriented polypropylene film-specific final mix granules obtained in Examples 1-6 and Comparative Examples 1-8 was determined according to GB / T 3682.1-2018. The test was conducted using a mass method at a test temperature of 230℃ and a nominal load of 2.16 kg. After the sample was added to the melt flow rate tester barrel and preheated for 5 min, the initial extrudate was discarded. Subsequently, the extrudate was collected and weighed at the same cutting time intervals. Each sample was tested in triplicate, and the arithmetic mean was taken as the melt mass flow rate of that sample.

[0042] Differential scanning calorimetry (DSC): The crystallization peak temperature and crystallinity of the films obtained in Examples 1-6 and Comparative Examples 1-8 were determined according to GB / T 19466.3-2004. For each sample, 5-8 mg of sample was cut from the middle of the film and placed in an aluminum crucible. The test was conducted under nitrogen flow rate of 50 mL / min. The heating program was as follows: from 30 °C to 200 °C at 10 °C / min and held for 3 min, then decreased to 30 °C at 10 °C / min, and then increased to 200 °C at 10 °C / min. The peak temperature of the exothermic crystallization peak on the cooling curve was taken as the crystallization peak temperature. The crystallinity was calculated using the enthalpy of fusion from the second heating curve. The enthalpy of fusion for complete crystallization of polypropylene was taken as 209 J / g. Each sample was tested in triplicate, and the arithmetic mean was taken.

[0043] Thickness and fisheye count: Film thickness was determined according to GB / T 6672-2001. Ten locations were evenly selected along the transverse direction of each sample, and a mechanical thickness gauge with an accuracy of 1 μm was used for measurement. The arithmetic mean was taken as the thickness. The fisheye count was determined according to the appearance inspection method of GB / T10003-2008, combined with the large-area transmitted light counting method. Five consecutive slices with an area of ​​2.0 m² were taken along the longitudinal direction of each sample. 2 The flat film has a total testing area of ​​10.0 m². 2 The sample was placed on a transmitted light observation stage with an illuminance of 1000 lx and an observation distance of 300 mm. The number of gel dots, fisheye dots, and obvious unmelted particles with a diameter of not less than 0.3 mm was counted and converted into the number of fisheye dots per square meter. The total count of the total tested area for each sample was taken as the number of fisheye dots for that batch of samples.

[0044] Tensile modulus of elasticity: The longitudinal and transverse tensile moduli of elasticity of the film were determined according to GB / T 1040.3-2006. The specimens were long strips, 150 mm long and 15 mm wide, with a clamp spacing of 100 mm and a testing speed of 250 mm / min. The long side of the longitudinal specimen was parallel to the casting and traction direction, and the long side of the transverse specimen was perpendicular to the casting and traction direction. Five specimens were tested in each direction. Invalid data due to breakage or obvious slippage at the clamps were discarded, and the arithmetic mean of the valid data was taken.

[0045] Bending stiffness: Referring to the general principle of bending stiffness determination in GB / T 23144-2023, an electronic bending stiffness tester was used to determine the longitudinal and transverse bending stiffness of the film. For each sample, longitudinal and transverse specimens, each 38 mm wide and 80 mm long, were cut and placed in an environment of 23℃ and 50% relative humidity for 24 hours before testing. The test employed a two-point bending method, with a clamping length of 10 mm, a bending angle of 15°, and a bending speed of 5° / s. Five specimens were tested in each direction, and the arithmetic mean was taken as the bending stiffness.

[0046] Heat shrinkage rate: The heat shrinkage rate of the film was determined according to GB / T 10003-2008. Five 100mm × 100mm square specimens were cut for each sample. 100mm measurement lines were marked along both the longitudinal and transverse directions in the center area of ​​each specimen. After measuring the initial length, the specimens were laid flat on a smooth glass plate sprinkled with a small amount of talcum powder and placed in a forced-air oven at 120℃ for 120s. After removal, they were cooled at 23℃ for 30min, and the lengths of the longitudinal and transverse marking lines were measured again. The heat shrinkage rate was calculated as the percentage of the difference between the initial length and the length after heating to the initial length. The longitudinal and transverse measurements were recorded separately, and the arithmetic mean of the five specimens was taken.

[0047] Table 1 Performance Test Results

[0048] As shown in Table 1, compared with Comparative Example 1, which replaced the high molecular weight polypropylene donor resin with ordinary biaxially oriented polypropylene film matrix resin, Example 1 achieved a melt mass flow rate of 6.35 g / 10 min and a fisheye count of only 6.3 / m. 2 Under these conditions, the longitudinal tensile modulus and transverse tensile modulus increased to 2628 MPa and 4875 MPa, respectively, while the longitudinal thermal shrinkage rate and transverse thermal shrinkage rate decreased to 1.6% and 0.7%, respectively. This indicates that using high molecular weight polypropylene with a low melt flow rate as a reaction site carrier is more beneficial for improving orientation retention while maintaining film processing stability.

[0049] After removing styrene in Comparative Example 2, the final melt flow rate increased to 7.64 g / 10 min, the crystallization peak temperature and crystallinity were 119.0 °C and 50.3%, respectively, and the longitudinal and transverse tensile modulus and thermal shrinkage were significantly worse than those in Example 1. This indicates that styrene-assisted grafting has a positive effect on improving the effective introduction of glycidyl methacrylate and reducing polypropylene chain degradation.

[0050] In Comparative Example 3, although the number of fish eyes did not increase significantly after the antioxidant was added to the grafted premix in advance, its crystallization peak temperature, crystallinity and tensile modulus were all lower than those in Example 1, indicating that adding the antioxidant in the later stage is more conducive to balancing the free radical grafting reaction and processing stability.

[0051] In Comparative Example 4, after adding adipic acid in a single side feeding, the number of fish eyes increased to 31.5 / m². 2This indicates that without pre-anchoring with flexible front-end, uneven reaction distribution is likely to occur in the material. After removing triglycidyl isocyanurate in Comparative Example 5 and adipic acid in Comparative Example 6, the longitudinal heat shrinkage rates were 2.6% and 3.1%, respectively, and the transverse heat shrinkage rates were 1.6% and 2.0%, respectively, both higher than those in Example 1. This shows that the segmented combination of adipic acid and triglycidyl isocyanurate is more effective in reducing orientation shrinkage.

[0052] Comparative Example 7, using a high proportion of modified masterbatch as the film matrix, showed a lower thermal shrinkage rate but an increased fisheye count of 88.4 per m. 2 Furthermore, the longitudinal and transverse tensile elastic moduli are lower than those of Example 1, indicating that the modified branched structure is not suitable for direct use as the main resin of biaxially oriented films in a continuous high-proportion manner.

[0053] In Comparative Example 8, after removing the polypropylene modulating resin, the longitudinal and transverse tensile moduli were 2490 MPa and 4615 MPa, respectively, which were lower than those in Example 1, indicating that the polypropylene modulating resin and the low-proportion modified masterbatch have a synergistic stiffening effect.

[0054] The number of fish eyes in Examples 1-6 was no higher than 9.7 per m. 2 The longitudinal tensile modulus of elasticity is 2515-2760 MPa, and the transverse tensile modulus of elasticity is 4645-5155 MPa. The longitudinal thermal shrinkage rate is 1.1%-2.0%, and the transverse thermal shrinkage rate is 0.5%-1.1%, demonstrating good high stiffness, low thermal shrinkage, and biaxial tensile processing stability. In particular, Example 3 maintained a low fisheye count and achieved the highest longitudinal and transverse tensile modulus of elasticity even with a higher amount of modified masterbatch and a higher draw ratio, indicating that the combined scheme of segmented grafting, segmented branching, and low-proportion introduction of the present invention can achieve comprehensive effects that are difficult to achieve with single-component substitution.

[0055] 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 method for preparing a high-stiffness biaxially oriented polypropylene film, characterized in that, Includes the following steps: S1. Using polypropylene with a melt flow rate lower than that of the polypropylene matrix resin for biaxially oriented polypropylene film as the donor resin, the polypropylene donor resin is subjected to a melt grafting reaction with glycidyl methacrylate in the presence of styrene and a free radical initiator, and an antioxidant is added in the later stage of the melt grafting reaction to obtain a high molecular weight polypropylene epoxy reaction site masterbatch. S2. The high molecular weight polypropylene epoxy reaction site masterbatch is subjected to a pre-melting reaction with a portion of adipic acid and catalyst. After the current section melt enters the subsequent reaction zone, a branching reaction side feed consisting of high molecular weight polypropylene epoxy reaction site masterbatch, the remaining adipic acid and triglycidyl isocyanurate is added to carry out the branching reaction and obtain the modified masterbatch. S3. The biaxially oriented polypropylene film is melt-blended with polypropylene matrix resin, the modified masterbatch and polypropylene conditioning resin to obtain a final blend for biaxially oriented polypropylene film. S4. The final mixture for biaxially oriented polypropylene film is melt-cast to form a casting sheet, and the casting sheet is sequentially stretched longitudinally, stretched laterally, and heat-set to obtain a high-stiffness biaxially oriented polypropylene film. The modified masterbatch contains 9%-13% of the total mass of the final mixture for biaxially oriented polypropylene film, and the polypropylene conditioning resin contains 2%-3% of the total mass. The longitudinal stretching ratio is 4.8-5.2 times, and the transverse stretching ratio is 8.5-9.5 times.

2. The method for preparing a high-stiffness biaxially oriented polypropylene film according to claim 1, characterized in that, The polypropylene donor resin is homopolymer polypropylene, which has a melt flow rate of 0.8 g / 10 min at 230°C and a load of 2.16 kg, and a density of 0.90 g / cm³. 3 It has a melting point of 162℃, a Vicat softening temperature of 153℃, and a flexural modulus of 1400MPa.

3. The method for preparing a high-stiffness biaxially oriented polypropylene film according to claim 1, characterized in that, The biaxially oriented polypropylene film uses homopolymer polypropylene as its matrix resin, which has a melt flow rate of 8 g / 10 min at 230°C and a load of 2.16 kg, and a density of 0.90 g / cm³. 3 The polypropylene resin has a melting point of 160℃, a Vicat softening temperature of 153℃, and a flexural modulus of 1350MPa. The melt flow rate of the polypropylene resin at 230℃ and a load of 2.16kg is 2g / 10min, and its density is 0.90g / cm³. 3 It has a melting point of 164℃, a Vicat softening temperature of 160℃, and a flexural modulus of 1750MPa.

4. The method for preparing a high-stiffness biaxially oriented polypropylene film according to claim 1, characterized in that, In step S1, 11,000-13,000 parts by weight of polypropylene donor resin, 330-650 parts by weight of glycidyl methacrylate, 165-325 parts by weight of styrene and 11-26 parts by weight of free radical initiator are mixed to obtain high molecular weight polypropylene graft premix; the free radical initiator is dicumyl peroxide.

5. The method for preparing a high-stiffness biaxially oriented polypropylene film according to claim 4, characterized in that, In step S1, the high molecular weight polypropylene grafted premix is ​​added to a co-rotating twin-screw extruder for melt grafting reaction. The co-rotating twin-screw extruder has a length-to-diameter ratio of 40:1, and the temperatures from the feed end to the die head are 160-170℃, 170-180℃, 180-190℃, 185-195℃, 185-195℃, and 180-190℃ respectively. The screw speed is 110-130 rpm, the average residence time of the material is 85-95 s, and nitrogen is introduced from the feed section to the third temperature zone at a flow rate of 0.8 L / min-1.2 L / min.

6. The method for preparing a high-stiffness biaxially oriented polypropylene film according to claim 5, characterized in that, In step S1, after the material enters the fifth temperature zone, 33-39 parts of hindered phenolic antioxidant and 22-26 parts of phosphite antioxidant are added through the side feed port; a vacuum de-volume port is set from the fifth temperature zone to the front of the machine head, and the absolute pressure is controlled to be below 20 kPa.

7. The method for preparing a high-stiffness biaxially oriented polypropylene film according to claim 1, characterized in that, In step S2, 11,000-13,000 parts by weight of high molecular weight polypropylene epoxy reaction site masterbatch, 11-26 parts by weight of adipic acid, and 4-9 parts by weight of catalyst are added from the main feed port of a twin-screw extruder for pre-melting reaction in the front section; the catalyst is triphenylphosphine; separately, 500 parts by weight of high molecular weight polypropylene epoxy reaction site masterbatch, 22-52 parts by weight of adipic acid, and 11-26 parts by weight of triglycidyl isocyanurate are premixed for 10 minutes at 25°C and 55-65 rpm to obtain 533-578 parts by weight of branched reaction side feedstock; the front section melt enters the fifth temperature zone. During this process, the branching reaction side feed is added through the side feed port; the length-to-diameter ratio of the twin-screw extruder is 40:1, the first four temperature zones are 172-178℃, 178-183℃, 183-188℃ and 183-188℃ respectively, the screw speed is 85-95 rpm, and the average residence time of the front melt pre-reaction is 55-65s; the temperatures from the fifth temperature zone to the die head are 188-192℃, 192-198℃ and 198-202℃ respectively, the average residence time of the branching reaction is 65-75s, and vacuum devolatilization is performed before the die head at an absolute pressure of less than 20kPa.

8. The method for preparing a high-stiffness biaxially oriented polypropylene film according to claim 1, characterized in that, In step S3, by weight, 83,450-88,550 parts of biaxially oriented polypropylene film matrix resin, 9,000-13,000 parts of modified masterbatch, 2,000-3,000 parts of polypropylene conditioning resin, 140-160 parts of hindered phenolic antioxidant, 90-110 parts of phosphite antioxidant, and 220-280 parts of calcium stearate are added to a twin-screw extruder; the temperatures of each zone of the twin-screw extruder are 188-192℃, 208-212℃, 222-228℃, 228-232℃, 228-232℃, and 222-228℃ respectively, the screw speed is 150-170 rpm, and the average residence time of the material is 75-85 s.

9. The method for preparing a high-stiffness biaxially oriented polypropylene film according to claim 1, characterized in that, In step S4, the final mixture for biaxially oriented polypropylene film is added to a single-screw casting extruder. The barrel temperature is successively set to 218-222℃, 232-238℃, 242-248℃, and 242-248℃. The T-die temperature is 242-248℃. After the melt is filtered through a 120-mesh metal filter, it is cast onto a cooling roller at 33-38℃. The traction speed is adjusted to make the thickness of the cast sheet 850-950μm.

10. The method for preparing a high-stiffness biaxially oriented polypropylene film according to claim 9, characterized in that, In step S4, the casting is preheated at 116-120℃ for 18-22s, and then longitudinally stretched at 123-127℃; then preheated at 156-160℃ for 14-16s, and then transversely stretched at 160-164℃; subsequently heat-set at 162-166℃ for 16-20s, and 3-5% of the tensile tension is released in the transverse direction, and finally cooled to below 40℃.

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