A high-opacity pearlescent biaxially oriented polypropylene film and its preparation method

Abstract

This invention relates to the field of polypropylene film preparation technology, specifically to a high-opacity pearlescent biaxially oriented polypropylene film and its preparation method. The method uses a surface resin as the outer layer and pearlescent core granules as the core layer, and obtains the film through three-layer co-extrusion, longitudinal stretching, transverse stretching, heat setting, and springback. The pearlescent core granules first undergo targeted modification of polypropylene through reactive extrusion, sequentially introducing a first auxiliary agent, 4-methylstyrene and 4-n-octylstyrene, divinylbenzene, and a second auxiliary agent, and finally are blended and granulated with pearlescent filler. This invention can improve the micropore uniformity and pore wall stability during the transverse high-ratio stretching stage, improve whiteness, haze, and opacity with a moderate amount of pearlescent filler, and maintain good transverse tensile strength, tear resistance, and processing stability.

Description

Technical Field

[0001] This invention relates to the field of polypropylene film preparation technology, specifically to a high-opacity pearlescent biaxially oriented polypropylene film and its preparation method. Background Technology

[0002] Pearlescent biaxially oriented polypropylene (Biaxially oriented polypropylene) film is widely used in high-end cigarette packaging, daily chemical wraparound labels, advertising synthetic paper, and self-adhesive facing materials due to its low density, high gloss, good opacity, and recyclability. Current technologies typically incorporate pearlescent fillers such as calcium carbonate and mica titanium into the polypropylene core layer. During the biaxial stretching process, the filler debonds from the polypropylene matrix, forming micropores that scatter light to achieve the covering and pearlescent effect.

[0003] However, with the increasing demand for lightweight films and high opacity, simply increasing the filler content will significantly reduce melt flowability, increase extrusion difficulty, and weaken tensile strength and tear resistance. Ordinary linear isotactic polypropylene, after forming initial micropores through longitudinal stretching, lacks sufficient strain hardening capacity during the transverse high-ratio stretching stage. This makes it difficult to provide uniform support for the micropore walls, easily leading to premature interface debonding, pore wall collapse, and localized microcracks. Consequently, problems such as light transmission spots, uneven haze, and transverse tensile film breakage occur, affecting product yield and batch stability.

[0004] Existing methods using general-purpose branched modifiers or high melt strength polypropylene blends can improve melt strength to some extent, but these are mostly monolithic random modifications, which can easily lead to gelation, decreased cleanliness, or poor processing fluidity. Furthermore, they are difficult to match the phased requirements of pore formation followed by pore stabilization during biaxial stretching. Therefore, how to achieve refined and targeted structural design of the pearlescent core layer polypropylene continuous phase without significantly increasing filler content or compromising mechanical properties and processing stability, in order to simultaneously improve pore uniformity and pore wall support stability, is a pressing technical problem in this field. Summary of the Invention

[0005] In view of this, the purpose of this invention is to provide a high-opacity pearlescent biaxially oriented polypropylene film and its preparation method, so as to provide a biaxially oriented polypropylene film that improves the micropore uniformity and pore wall stability of the BOPP pearlescent film in the transverse high-ratio stretching stage without significantly increasing the amount of calcium carbonate filler, thereby taking into account low light transmittance, high haze, high whiteness, transverse tensile strength, tear resistance and continuous stretching stability.

[0006] To achieve the above objectives, the present invention provides a high-coverage pearlescent biaxially oriented polypropylene film, wherein the biaxially oriented polypropylene film is obtained by three-layer co-extrusion and stretching, with a surface resin as the outer layer and pearlescent core granules as the core layer. The thickness of the biaxially oriented polypropylene film is 34-42 μm.

[0007] The surface resin is SABIC PP520L, with a melt flow rate of 10 g / 10 min and a density of 905 kg / m³. 3 .

[0008] The preparation steps of the pearlescent core layer granules are as follows: (1) Preparation of modified polypropylene resin: Pearl core matrix resin is fed into a twin-screw extruder through the main feed port. After the melt is completely plasticized, the first additive, 4-methylstyrene and 4-n-octylstyrene, divinylbenzene and the second additive are added in sequence. After melting, a melt is formed. The resulting melt is water-cooled, stretched, air-dried and pelletized to obtain modified polypropylene resin. (2) Preparation of pearlescent core layer granules: Pearlescent core matrix resin, modified polypropylene resin, pearlescent filler, primary antioxidant, and secondary antioxidant are mixed and fed into a twin-screw extruder for melt blending. The resulting extrudate is then water-cooled, stretched, and pelletized to obtain pearlescent core granules.

[0009] Preferably, the weight ratio of the pearlescent core matrix resin, the first auxiliary agent, 4-methylstyrene and 4-n-octylstyrene, divinylbenzene and the second auxiliary agent in step (1) is 9600:324-336:35-45:3.5-4.5:2-4:127-138.

[0010] Preferably, in step (1), the addition zones of the first auxiliary agent, 4-methylstyrene and 4-n-octylstyrene, divinylbenzene and the second auxiliary agent are respectively zone 4, zone 6, zone 7 and zone 8 of the twin-screw extruder.

[0011] Preferably, in step (1), the first additive is a mixture of pearlescent core matrix resin powder, trimethylolpropane triacrylate, dicumyl peroxide, and tetramethylthiuram disulfide in a weight ratio of 300:12-18:4-6:8-12.

[0012] Preferably, in step (1), the second additive is a mixture of pearlescent core layer matrix resin powder, maleic anhydride, and dicumyl peroxide in a weight ratio of 100:25-35:2-3.

[0013] Preferably, in step (1), the temperatures of zones 1 to 9 of the twin-screw extruder reaction are set sequentially to 168-172℃, 178-182℃, 183-187℃, 188-192℃, 193-197℃, 198-202℃, 198-202℃, 193-197℃ and 188-192℃; the die head temperature is set to 188-192℃, and the screw speed is set to 200-240 r / min.

[0014] Preferably, the freezing temperature in step (1) is -20°C.

[0015] Preferably, the pearlescent core matrix resin in step (1) needs to be dried before use, with a drying temperature of 80°C and a drying time of 4 hours.

[0016] Preferably, the pearlescent core matrix resin in step (1) is SABIC PP5212P, and the melt flow rate is 3g / 10min.

[0017] Preferably, the pearlescent core matrix resin powder is obtained by freezing and pulverizing dried pearlescent core matrix resin; the particle size of the pearlescent core matrix resin powder is 20-40 mesh.

[0018] Preferably, the weight ratio of the pearlescent core matrix resin, modified polypropylene resin, pearlescent filler, primary antioxidant, and secondary antioxidant in step (2) is 638-678:130-150:190-210:1:1.

[0019] Preferably, the pearlescent filler described in step (2) needs to be dried before use, with a drying temperature of 105°C and a drying time of 3 hours.

[0020] Preferably, the pearlescent filler in step (2) is ultrafine calcium carbonate powder, model Omyafilm BOPP 400-FL.

[0021] Preferably, in step (2), the temperatures of the first to eighth zones of the twin-screw extruder barrel are set to 183-187℃, 188-192℃, 193-197℃, 198-202℃, 203-207℃, 208-212℃, 208-212℃ and 203-207℃ respectively, and the screw speed is set to 170-190 r / min.

[0022] Furthermore, the present invention also provides a method for preparing a high-opacity pearlescent biaxially oriented polypropylene film, comprising the following steps: Using the pearlescent core layer granules as the core layer material and the surface resin as the upper and lower surface layer materials, a three-layer co-extrusion is performed to obtain a three-layer cast sheet; the three-layer cast sheet is stretched, heat-set, and spring-rebounded to obtain a high-opacity pearlescent biaxially oriented polypropylene film.

[0023] Preferably, the weight ratio of the core layer material, the upper surface layer material, and the lower surface layer material is 44:3:3.

[0024] Preferably, the thickness of the three-layer casting is 1550-1650 μm.

[0025] Preferably, the temperature of the three-layer co-extrusion die head is controlled at 225-235℃, and the temperature of the casting roll is controlled at 43-47℃.

[0026] Preferably, the stretching process involves preheating the three-layer cast sheet at 128-134℃ for 18-22 seconds and then stretching it longitudinally to a ratio of 4.6-5.0 times. The longitudinally stretched sheet is then fed into a transverse stretching machine and gradually stretched transversely to a ratio of 8.6-9.0 times at zoned temperatures of 154-158℃, 156-160℃, 158-162℃, and 160-164℃.

[0027] Preferably, the heat setting temperature is 164-168℃ and the time is 4-6s.

[0028] Preferably, the rebound rate is 3.5%-4.5%.

[0029] The beneficial effects of this invention are: Compared with existing technologies, this invention no longer relies on increasing the amount of pearlescent filler to achieve a masking effect. Instead, it addresses the problem of microporous structure instability during high-ratio transverse stretching by performing site-specific molecular chain modification on the continuous phase polypropylene of the pearlescent core layer, thus enabling the film to possess both high masking capacity and processing stability. First, this invention employs an oxidation-induced reactive extrusion method to construct sparse, long-branched nodes within the high molecular weight polypropylene segments, while retaining the flow-regulating effect of the low molecular weight components. This allows the core layer granules to possess both good processing flowability and a stronger strain-hardening response under lateral stretching. Consequently, the forming micropore walls receive continuous support, reducing pore wall connectivity, collapse, and localized tearing, thus lowering the risk of transverse tensile film breakage. Secondly, by adding aromatic monomers, divinylbenzene and maleic anhydride in a partitioned and sequential manner, local aromatic active regions, moderately rigid locking structures and fixed-point polar grafting structures are formed in sequence near the branching nodes. This changes the interface debonding between the filler and the polypropylene matrix from premature disorder to delayed, concentrated and controllable debonding, thereby improving the micropore size and distribution uniformity and enhancing the integrity of the pore walls. In summary, with a low amount of pearlescent filler, the film of the present invention can still achieve high whiteness, low light transmittance and high haze, while maintaining good longitudinal and transverse tensile strength, transverse tear resistance and batch stability. It is suitable for applications such as dairy product wraparound labels, daily chemical white opaque label films and advertising synthetic paper base films. Detailed Implementation

[0030] 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.

[0031] The raw materials used in this embodiment and their sources are as follows: Pearlescent core layer matrix resin: SABIC PP5212P, SABIC, melt flow rate 3 g / 10 min, suitable for co-extrusion core layer of biaxially oriented polypropylene film; Surface layer resin: SABIC PP520L, SABIC PP520L, melt flow rate 10 g / 10 min, density 905 kg / m³. 3 Pearl filler: Omyafilm BOPP 400-FL, ultrafine calcium carbonate powder; Tetramethylthiuram disulfide: Sigma-Aldrich, T24201, purity 97%; Primary antioxidant: Irganox 1010 manufactured by BASF; Secondary antioxidant: Irgafos 168 manufactured by BASF.

[0032] Example 1: A method for preparing a high-opacity pearlescent biaxially oriented polypropylene film, the specific steps of which are as follows: S1 Raw Material Pretreatment: 10000g of pearlescent core matrix resin was dried in an 80℃ hot air circulating oven for 4 hours; 2000g of pearlescent filler was dried in a 105℃ forced air oven for 3 hours; and 30g of maleic anhydride was dried in a 50℃ vacuum drying oven for 2 hours. 400g of the dried pearlescent core matrix resin was frozen at -20℃ for 2 hours and then pulverized to 20-40 mesh to obtain powder. 300g of this powder was then mixed with 15g of tris(hydroxymethyl) Propane triacrylate, 5g dicumyl peroxide, and 10g tetramethylthiuram disulfide were mixed at high speed below 25°C for 5 minutes to obtain the first auxiliary agent; then 100g of the powder was mixed with 30g maleic anhydride and 2g dicumyl peroxide at below 25°C for 3 minutes to obtain the second auxiliary agent; 4-methylstyrene, divinylbenzene, and 4-n-octylstyrene were placed in light-proof, sealed, and nitrogen-protected storage bottles for later use at below 25°C. Preparation of modified polypropylene resin using S2: A conventional parallel co-rotating twin-screw reactive extruder with an aspect ratio of 40 was used. The temperatures of barrel zones 1 to 9 were set sequentially to 170℃, 180℃, 185℃, 190℃, 195℃, 200℃, 200℃, 195℃, and 190℃, respectively. The die head temperature was set to 190℃, the screw speed was set to 220 r / min, and the total feed rate was controlled at 10.1 kg / h. First, 9600 g of pearlescent core matrix resin was continuously added through the main feed port. After the melt was fully plasticized from the end of zone 3 to the beginning of zone 4, 330 g of the first additive was continuously added through the side feed port of zone 4. Subsequently, 40 g of 4-methylstyrene and 4 g of... 4-n-Octylstyrene was continuously added through the liquid injection port in zone 6, 3g of divinylbenzene was continuously added through the liquid injection port in zone 7, and 132g of the second auxiliary agent was continuously added through the side feed port in zone 8; a vacuum of -0.08MPa was opened in zone 9 to exhaust the gas, and the melt was subjected to conventional water cooling, stripping, air drying and pelletizing to obtain modified polypropylene resin. S3 Preparation of Pearlized Core Layer Granules: 6580g of pearlized core layer matrix resin, 1400g of modified polypropylene resin, 2000g of pearlized filler, 10g of primary antioxidant, and 10g of secondary antioxidant were added to a conventional high-speed mixer and mixed for 8 minutes. The mixture was then fed into a conventional parallel co-rotating twin-screw extruder for granulation. The temperatures of zones 1 to 8 of the twin-screw extruder barrel were set sequentially to 185℃, 190℃, 195℃, 200℃, 205℃, 210℃, 210℃, and 205℃, respectively. The screw speed was set to 180 r / min, and a -0.08MPa vacuum was activated in the penultimate zone. After conventional water cooling, stretching, and pelletizing, the extrudate was obtained as pearlized core layer granules. S4 Preparation of Three-Layer Co-Extruded Cast Sheets: 1200g of surface resin was dried at 80℃ for 2 hours and set aside. 8800g of pearlescent core layer granules were used as core layer material. A / B / A three-layer co-extrusion was carried out according to the mass ratio of 600g of upper surface layer, 8800g of core layer, and 600g of lower surface layer, where A is surface resin and B is pearlescent core layer granules B. The co-extrusion die head temperature was controlled at 225℃-235℃, the casting roll temperature was controlled at 45℃, and the traction speed was adjusted to obtain three-layer cast sheets. S5: The above three-layer casting sheet is preheated at 128℃-132℃ for 20s and then longitudinally stretched with a longitudinal stretching ratio of 4.8 times. The longitudinally stretched sheet is then fed into a transverse stretching machine and gradually stretched transversely to 8.8 times at zoned temperatures of 156℃, 158℃, 160℃ and 162℃. Subsequently, it is heat-set at 166℃ for 5s and relaxed with a shrinkage rate of 4% to obtain a high-coverage pearlescent biaxially oriented polypropylene film.

[0033] Example 2: A method for preparing a high-opacity pearlescent biaxially oriented polypropylene film, the specific steps of which are as follows: S1 Raw Material Pretreatment: 10000g of pearlescent core layer matrix resin was dried in an 80℃ hot air circulating oven for 4 hours; 1900g of pearlescent filler was dried in a 105℃ forced air oven for 3 hours; and 25g of maleic anhydride was dried in a 50℃ vacuum drying oven for 2 hours. 400g of the dried pearlescent core layer matrix resin was frozen at -20℃ for 2 hours and then pulverized to 20-40 mesh to obtain powder. 300g of this powder was mixed with 12g of trimethylolpropane triacrylate, 4g of dicumyl peroxide, and 8g of tetramethylthiuram disulfide at high speed below 25℃ for 5 minutes to obtain the first additive. Another 100g of this powder was mixed with 25g of maleic anhydride and 2g of dicumyl peroxide at below 25℃ for 3 minutes to obtain the second additive. 35g of... 4-Methylstyrene, 2g of divinylbenzene, and 4-n-octylstyrene were placed in light-proof, sealed, and nitrogen-protected storage bottles and kept at 25°C or below for later use.

[0034] Preparation of modified polypropylene resin using S2: A conventional parallel co-rotating twin-screw reactive extruder with an aspect ratio of 40 was used. The temperatures of barrel zones 1 to 9 were set sequentially to 168℃, 178℃, 183℃, 188℃, 193℃, 198℃, 198℃, 193℃, and 188℃, respectively. The die head temperature was set to 188℃, the screw speed was set to 200 r / min, and the total feed rate was controlled at 10.0 kg / h. First, 9600 g of pearlescent core matrix resin was continuously added through the main feed port. After the melt was fully plasticized from the end of zone 3 to the beginning of zone 4, 324 g of the first auxiliary agent was continuously added through the side feed port of zone 4. Subsequently, 35 g of 4-methylstyrene and 3.5 g of... 4-n-Octylstyrene was continuously added through the liquid injection port in zone 6, 2g of divinylbenzene was continuously added through the liquid injection port in zone 7, and 127g of the second auxiliary agent was continuously added through the side feed port in zone 8; a vacuum of -0.08MPa was opened in zone 9 to exhaust the gas, and the melt was subjected to conventional water cooling, stripping, air drying and pelletizing to obtain modified polypropylene resin. S3 Preparation of Pearlized Core Layer Granules: 6380g of pearlized core layer matrix resin, 1500g of modified polypropylene resin, 2100g of pearlized filler, 10g of primary antioxidant, and 10g of secondary antioxidant were added to a conventional high-speed mixer and mixed for 8 minutes. The mixture was then fed into a conventional parallel co-rotating twin-screw extruder for granulation. The temperatures of zones 1 to 8 of the twin-screw extruder barrel were set sequentially to 183℃, 188℃, 193℃, 198℃, 203℃, 208℃, 208℃, and 203℃, respectively. The screw speed was set to 170 r / min, and a -0.08MPa vacuum was activated in the penultimate zone. After conventional water cooling, stranding, and pelletizing, the pearlized core layer granules were obtained. S4 Preparation of Three-Layer Co-Extruded Cast Sheets: 1200g of surface resin was dried at 80℃ for 2 hours and set aside. 8800g of pearlescent core layer granules were used as core layer material. A / B / A three-layer co-extrusion was carried out according to the mass ratio of 600g of upper surface layer, 8800g of core layer, and 600g of lower surface layer. The upper and lower surface layers were surface resin, and the core layer was pearlescent core layer granules. The co-extrusion die head temperature was controlled at 225℃, the casting roll temperature was controlled at 43℃, and the traction speed was adjusted to obtain three-layer cast sheets. S5 Biaxial Stretching and Heat Setting: The above three-layer cast sheets are preheated at 128℃ for 18s and then longitudinally stretched to a ratio of 4.6 times. The longitudinally stretched sheets are then fed into a transverse stretching machine and gradually stretched to a ratio of 8.6 times at zoned temperatures of 154℃, 156℃, 158℃ and 160℃. Subsequently, they are heat-set at 164℃ for 4s and relaxed with a shrinkage rate of 3.5% to obtain a high-coverage pearlescent biaxially oriented polypropylene film.

[0035] Example 3: A method for preparing a high-opacity pearlescent biaxially oriented polypropylene film, the specific steps of which are as follows: S1 Raw Material Pretreatment: 10000g of pearlescent core layer matrix resin was dried in an 80℃ hot air circulating oven for 4 hours; 2100g of pearlescent filler was dried in a 105℃ forced-air oven for 3 hours; and 35g of maleic anhydride was dried in a 50℃ vacuum drying oven for 2 hours. 400g of the dried pearlescent core layer matrix resin was frozen at -20℃ for 2 hours and then pulverized to 20-40 mesh to obtain powder. 300g of this powder was mixed with 18g of trimethylolpropane triacrylate, 6g of dicumyl peroxide, and 12g of tetramethylthiuram disulfide at high speed below 25℃ for 5 minutes to obtain the first additive. 100g of the powder was then mixed with 35g of maleic anhydride and 3g of dicumyl peroxide at below 25℃ for 3 minutes to obtain the second additive. 45g of the remaining powder was also mixed... 4-Methylstyrene, 4g of divinylbenzene, and 4-n-octylstyrene were placed in light-proof, sealed, and nitrogen-protected storage bottles and kept at 25°C or below for later use.

[0036] Preparation of modified polypropylene resin using S2: A conventional parallel co-rotating twin-screw reactive extruder with an aspect ratio of 40 was used. The temperatures of barrel zones 1 to 9 were set sequentially to 172℃, 182℃, 187℃, 192℃, 197℃, 202℃, 202℃, 197℃, and 192℃, respectively. The die head temperature was set to 192℃, the screw speed was set to 240 r / min, and the total feed rate was controlled at 10.2 kg / h. First, 9600 g of pearlescent core matrix resin was continuously added through the main feed port. After the melt was fully plasticized from the end of zone 3 to the beginning of zone 4, 336 g of the first additive was continuously added through the side feed port of zone 4. Subsequently, 45 g of 4-methylstyrene and 4.5 g of... 4-n-Octylstyrene was continuously added through the liquid injection port in zone 6, 4g of divinylbenzene was continuously added through the liquid injection port in zone 7, and 138g of the second auxiliary agent was continuously added through the side feed port in zone 8; a vacuum of -0.08MPa was opened in zone 9 to exhaust the gas, and the melt was subjected to conventional water cooling, stripping, air drying and pelletizing to obtain modified polypropylene resin. S3 Preparation of Pearlized Core Layer Granules: 6780g of pearlized core layer matrix resin, 1300g of modified polypropylene resin, 1900g of pearlized filler, 10g of primary antioxidant, and 10g of secondary antioxidant were added to a conventional high-speed mixer and mixed for 8 minutes. The mixture was then fed into a conventional parallel co-rotating twin-screw extruder for granulation. The temperatures of zones 1 to 8 of the twin-screw extruder barrel were set to 187℃, 192℃, 197℃, 202℃, 207℃, 212℃, 212℃, and 207℃, respectively. The screw speed was set to 190 r / min, and a -0.08MPa vacuum was activated in the second-to-last zone. After conventional water cooling, stretching, and pelletizing, the pearlized core layer granules were obtained. S4 Preparation of Three-Layer Co-Extruded Cast Sheets: 1200g of surface resin was dried at 80℃ for 2 hours and then set aside. 8800g of pearlescent core layer granules were used as the core layer material. A / B / A three-layer co-extrusion was carried out according to the mass ratio of 600g of upper surface layer, 8800g of core layer, and 600g of lower surface layer. The upper and lower surface layers were surface resins, and the core layer was pearlescent core layer granules. The co-extrusion die head temperature was controlled at 235℃, the casting roll temperature was controlled at 47℃, and the traction speed was adjusted to obtain the three-layer cast sheets. S5 Biaxial Stretching and Heat Setting: The above three-layer cast sheets are preheated at 134℃ for 22s and then longitudinally stretched to a ratio of 5.0. The longitudinally stretched sheets are then fed into a transverse stretching machine and gradually stretched to a ratio of 9.0 at zoned temperatures of 158℃, 160℃, 162℃ and 164℃. Subsequently, they are heat-set at 168℃ for 6s and relaxed with a shrinkage rate of 4.5% to obtain a high-coverage pearlescent biaxially oriented polypropylene film.

[0037] Comparative Example 1: The difference from Example 1 is that in step S1, the first additive is changed to consist of 325g of pearlescent core layer matrix resin powder and 5g of dicumyl peroxide, and trimethylolpropane triacrylate and tetramethylthiuram disulfide are not added; the other conditions are the same as in Example 1.

[0038] Comparative Example 2: The difference from Example 1 is that in step S2, 4-methylstyrene is added from the liquid injection port in zone 4 instead of zone 6; the other conditions are the same as in Example 1.

[0039] Comparative Example 3: The difference from Example 1 is that in step S2, the second auxiliary agent is changed from being fed from the 8th zone side inlet to being added simultaneously with 4-methylstyrene in the 6th zone, while divinylbenzene is still added in the 7th zone; the other conditions are the same as in Example 1.

[0040] Comparative Example 4: The difference from Example 1 is that in step S2, 4-n-octylstyrene is replaced with an equal amount of 4-methylstyrene, and the other conditions are the same as in Example 1.

[0041] Comparative Example 5: The difference from Example 1 is that in step S2, divinylbenzene is added from the liquid injection port in zone 7 instead of 4-methylstyrene, and is added simultaneously from the liquid injection port in zone 6; the other conditions are the same as in Example 1.

[0042] Comparative Example 6: The difference from Example 1 is that in step S2, 4-n-octylstyrene, 4-methylstyrene, divinylbenzene and maleic anhydride are added together, while the other conditions are the same as in Example 1; Comparative Example 7: The difference from Example 1 is that in step S2, 4-n-octylstyrene, 4-methylstyrene, and divinylbenzene are all replaced with equal amounts of maleic anhydride, while the other conditions are the same as in Example 1; Performance testing The films obtained in Examples 1-3 and Comparative Examples 1-7 were cut off by removing 50 mm from each side, and samples were then taken from the stable central region. All film samples were placed at 23°C and 50% relative humidity for 40 hours before testing. Melt mass flow rate: The test was conducted in accordance with GB / T 3682.1-2018. The granules obtained in step S2 of each example and comparative example were dried in an oven at 80℃ for 2 hours. The melt mass flow rate was measured using a melt mass flow rate meter at 230℃ and a load of 2.16 kg. The preheating time was set to 300 s. Each sample was tested three times consecutively, and the melt mass flow rate was recorded. The unit is g / 10 min. Film thickness: The test was conducted in accordance with GB / T 6672-2001. Film samples obtained from each example and comparative example were taken, and 10 positions were evenly selected in the width direction of the film and measured at least 100 mm apart in the length direction. A thickness gauge with a graduation value of 0.001 mm was used for the test, and the thickness at each point was recorded and the average value was calculated. Whiteness: The test was conducted in accordance with GB / T 2913-1982. The films obtained from each example and comparative example were cut into flat samples of not less than 50mm×50mm. Whiteness was tested using a whiteness meter. Five different positions were tested for each sample, avoiding obvious creases, edges and local defects. The whiteness values ​​were recorded and the average value was calculated. Transmittance and haze: The test was conducted in accordance with GB / T 2410-2008. The films obtained from each example and comparative example were cut into 50mm×50mm samples. The transmittance and haze were measured by a haze meter. Before the test, the zero point and standard plate were calibrated according to the instrument requirements. Five samples were tested for each sample, and each sample was tested once. The average value was taken. Tensile properties: Tested according to GB / T 1040.3-2006. Films obtained from each example and comparative example were cut into specimens along both the longitudinal and transverse directions. The specimen width was 15 mm, the initial distance between the clamps was 50 mm, and the test speed was set to 500 mm / min. Five specimens were tested in each direction, and the tensile strength and elongation at break were recorded, with the average value taken. Tear resistance: The test was conducted in accordance with GB / T 16578.2-2009. Films obtained from each example and comparative example were cut into 63mm×76mm samples along the longitudinal and transverse directions. After pre-cutting according to the standard requirements, Elemandorf pendulum tear tester was used. Ten samples were tested in each direction, and the longitudinal tear strength and transverse tear strength were recorded and the average value was taken. The test results are shown in Table 1.

[0043] Table 1 Performance Test Results

[0044] Data analysis: The film of this invention can still achieve high whiteness, low light transmittance, high haze, and stable lateral tensile and tear resistance even at a relatively thin thickness. It is suitable for dairy product wrap labels, daily chemical opaque label films, and advertising synthetic paper base films, and has strong adaptability.

[0045] Example 1 and Comparative Example 1 show that after removing trimethylolpropane triacrylate and tetramethylthiuram disulfide from the first additive, the melt flow rate increased significantly, while whiteness, light transmittance, transverse tensile strength, and tear resistance all deteriorated. This is because, without branching nodes and free radical regulation, the polypropylene main chain is more prone to breakage, the pore walls formed by transverse stretching lack sufficient support, and the microporous structure is difficult to maintain stably.

[0046] Examples 1 and Comparative Examples 2 and 3 show that simply changing the addition zone of 4-methylstyrene or maleic anhydride leads to a simultaneous decrease in whiteness, transmittance, and transverse mechanical properties. The effect is more pronounced when maleic anhydride is added earlier and in the same zone as 4-methylstyrene. This is because 4-methylstyrene has difficulty forming a stable local aromatic active zone near the branching node; the simultaneous forward movement of maleic anhydride enhances random grafting and chain breakage, causing premature and disordered interfacial debonding, which in turn disrupts the micropore formation window.

[0047] Example 1 and Comparative Example 4 show that after replacing all of 4-n-octylstyrene with an equal amount of 4-methylstyrene, the melt flow rate did not continuously change in a favorable direction, and whiteness, transmittance, transverse elongation at break, and tear resistance did not improve synchronously. This indicates that the small amount of 4-n-octylstyrene in Zone 6 is not simply a dilution of 4-methylstyrene, but rather provides a smooth chain segment transition at the same introduction point, preventing the system from becoming overly rigid after subsequent divinylbenzene locking, thereby reducing transverse tensile stress concentration and maintaining pore wall continuity.

[0048] Example 1 and Comparative Example 5 show that when divinylbenzene is moved to the sixth region and added simultaneously with 4-methylstyrene, the transverse tensile strength, elongation at break, and tear resistance of the film all decrease significantly, and the optical properties are not improved. This is because if both are added simultaneously, flexible occupancy and rigid locking cannot be completed step-by-step, easily forming overly hard regions around the branching nodes, disrupting the continuity of the micropore walls. This demonstrates a temporal synergy between the two, making the technical logic clearer.

[0049] Examples 1 and Comparative Examples 6 and 7 show that adding 4-n-octylstyrene, 4-methylstyrene, divinylbenzene, and maleic anhydride in the same region all at once, or directly replacing the aromatic monomers and divinylbenzene with maleic anhydride, significantly degrades the optical and transverse mechanical properties, and abnormally increases the melt flow rate. This is because the above methods disrupt the progressive relationship of first forming a local aromatic active region, then local rigid locking, and finally the delayed introduction of maleic anhydride, causing maleic anhydride to lose its site-specific grafting basis, and the interface regulation degenerates into disordered debonding.

[0050] 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, the steps can be implemented in any order, 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 high-opacity pearlescent biaxially oriented polypropylene film, characterized in that, The biaxially oriented polypropylene film is obtained by three-layer co-extrusion and stretching, with a surface resin as the outer layer and pearlescent core granules as the core layer. The preparation steps of the pearlescent core layer granules are as follows: (1) Preparation of modified polypropylene resin: After the pearlescent core matrix resin is plasticized in a twin-screw extruder, the first auxiliary agent, 4-methylstyrene and 4-n-octylstyrene, divinylbenzene and the second auxiliary agent are added in sequence to obtain the modified polypropylene resin. (2) Preparation of pearlescent core layer granules: Pearl core matrix resin, modified polypropylene resin, pearl filler, main antioxidant, and auxiliary antioxidant are mixed and fed into a twin-screw extruder for melt blending, and then pelletized to obtain pearl core granules. In step (1), the addition zones of the first auxiliary agent, 4-methylstyrene and 4-n-octylstyrene, divinylbenzene and the second auxiliary agent are zone 4, zone 6, zone 7 and zone 8 of the twin-screw extruder, respectively. The weight ratio of the pearlescent core matrix resin, the first additive, 4-methylstyrene, 4-n-octylstyrene, divinylbenzene, and the second additive in step (1) is 9600:324-336:35-45:3.5-4.5:2-4:127-138.

2. The high-opacity pearlescent biaxially oriented polypropylene film according to claim 1, characterized in that, Step (1) The first additive is a mixture of pearlescent core matrix resin powder, trimethylolpropane triacrylate, dicumyl peroxide and tetramethylthiuram disulfide in a weight ratio of 300:12-18:4-6:8-12.

3. The high-opacity pearlescent biaxially oriented polypropylene film according to claim 1, characterized in that, Step (1) The second additive is a mixture of pearlescent core layer matrix resin powder, maleic anhydride and dicumyl peroxide in a weight ratio of 100:25-35:2-3.

4. The high-opacity pearlescent biaxially oriented polypropylene film according to claim 1, characterized in that, In step (1), the temperatures of zones 1 to 9 of the twin-screw extruder reaction are set sequentially to 168-172℃, 178-182℃, 183-187℃, 188-192℃, 193-197℃, 198-202℃, 198-202℃, 193-197℃ and 188-192℃; the die head temperature is set to 188-192℃, and the screw speed is set to 200-240 r / min.

5. The high-opacity pearlescent biaxially oriented polypropylene film according to claim 1, characterized in that, The weight ratio of the pearlescent core matrix resin, modified polypropylene resin, pearlescent filler, primary antioxidant, and secondary antioxidant in step (2) is 638-678:130-150:190-210:1:

1.

6. The high-opacity pearlescent biaxially oriented polypropylene film according to claim 1, characterized in that, In step (2), the temperatures of the first to eighth zones of the twin-screw extruder barrel are set to 183-187℃, 188-192℃, 193-197℃, 198-202℃, 203-207℃, 208-212℃, 208-212℃ and 203-207℃ respectively, and the screw speed is set to 170-190 r / min.

7. A method for preparing a high-opacity pearlescent biaxially oriented polypropylene film, used to prepare the high-opacity pearlescent biaxially oriented polypropylene film as described in any one of claims 1-6, characterized in that, Includes the following steps: Using the pearlescent core layer granules as the core layer material and the surface resin as the upper and lower surface layer materials, a three-layer co-extrusion is performed to obtain a three-layer cast sheet. The three-layer castings are stretched, heat-set, and spring-rebounded to obtain a high-coverage pearlescent biaxially oriented polypropylene film.

8. The preparation method according to claim 7, characterized in that, The weight ratio of the core layer material, the upper surface layer material, and the lower surface layer material is 44:3:

3.

9. The preparation method according to claim 7, characterized in that, The stretching process is as follows: the three-layer cast sheet is preheated at 128-134℃ for 18-22s and then longitudinally stretched, with a longitudinal stretching ratio of 4.6-5.0 times; the longitudinally stretched sheet is then fed into a transverse stretching machine and gradually stretched transversely to 8.6-9.0 times at zoned temperatures of 154-158℃, 156-160℃, 158-162℃ and 160-164℃.

10. The preparation method according to claim 7, characterized in that, The high-opacity pearlescent biaxially oriented polypropylene film obtained by the preparation method has a thickness of 34-42 μm.

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