Preparation method of high-heat-resistance transparent PP sheet suitable for microwave heating food packaging

Abstract

The invention relates to the technical field of high polymer material processing, in particular to a preparation method of a high-heat-resistance transparent PP sheet suitable for microwave heating food packaging, and the preparation method comprises the following steps: S1, mixing homo-polypropylene and co-polypropylene according to a set proportion; s2, preparing composite powder; s3, carrying out melt blending on the composite powder and carrier resin to prepare master batches; s4, mixing the pretreated resin matrix with the master batch; s5, forming an unshaped sheet through melt layering and compounding; s6, carrying out multi-roll calendering unit treatment; s7, staged cooling treatment is carried out; and S8, the surface of the base material sheet is subjected to corona treatment and coated with a barrier coating. Through the innovative synergistic effect of the nucleating agent and the gradient processing technology, the heat resistance and mechanical strength of the polypropylene sheet are remarkably improved while the high transparency of the polypropylene sheet is kept, processing degradation is effectively inhibited, and the breakthrough progress of the high-performance microwave food packaging material is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of polymer material processing, and particularly relates to a method for preparing a highly heat-resistant transparent PP sheet suitable for microwave-heated food packaging. Background Art

[0002] With the rapid development of microwave food packaging, the performance requirements for polypropylene packaging materials are increasing day by day; an ideal microwave food packaging material needs to have high heat resistance, excellent transparency, and good mechanical properties at the same time; however, it is difficult for the existing technology to balance these key performance indicators, which seriously restricts the development and application of high-performance microwave food packaging materials.

[0003] Currently, there are mainly three technical problems with common polypropylene packaging materials on the market: First, traditional modification methods often lead to a significant decrease in transparency while improving the heat resistance of the material; second, it is difficult for the existing process to precisely control the crystallization behavior of the material, resulting in mutual restriction between mechanical properties and optical properties; finally, conventional processing technologies cannot avoid the degradation problem of the material during high-temperature processing, which not only affects the product performance but may also pose potential safety hazards. Therefore, there is an urgent need for a method for preparing a highly heat-resistant transparent PP sheet suitable for microwave-heated food packaging to solve the above problems. Summary of the Invention

[0004] Based on the above purpose, the present invention provides a method for preparing a highly heat-resistant transparent PP sheet suitable for microwave-heated food packaging.

[0005] A method for preparing a highly heat-resistant transparent PP sheet suitable for microwave-heated food packaging includes the following steps: S1: Mix homopolypropylene and copolymerized polypropylene in a set ratio and perform vacuum drying treatment to obtain a pretreated resin matrix. S2: Mechanically mix sorbitol-based α-nucleating agent, rare earth β-nucleating agent, and dispersant to obtain a composite powder. S3: Melt-blend the composite powder obtained in S2 with a carrier resin to prepare a functional masterbatch. S4: Mix the pretreated resin matrix in S1 with the functional masterbatch in S3, and add a free radical initiator in stages under an inert gas environment to initiate a mild cross-linking reaction of molecular chains to form a mixed material with a pre-crystalline structure. S5: Input the pre-crystalline mixed material in S4 into a co-extrusion die head with zone temperature control, and form an unshaped sheet through melt layer-by-layer compounding. S6: Process the unshaped sheet in S5 through a multi-roll calendering unit to obtain a calendered sheet with molecular chains arranged in a direction. S7: Perform staged cooling treatment on the calendered sheet in S6 to output a base sheet. S8: Perform corona treatment on the surface of the substrate sheet in S7, and coat a microwave-transparent barrier coating, thereby obtaining the finished PP sheet.

[0006] Optionally, the said S1 specifically includes: S11: Put homopolypropylene with a melt index of 2 - 5 g / 10 min and copolymerized polypropylene with an ethylene content of 4 - 8% into a high-speed mixer according to a weight ratio of 7.5:2.5; S12: Mix for 8 - 15 min under the conditions of a mixer rotation speed of 200 - 400 rpm and a temperature of 25 - 40 °C to form a resin mixture with a uniform distribution; S13: Transfer the resin mixture to a vacuum drying oven, set the temperature at 60 - 80 °C, the vacuum degree at -0.08 to -0.1 MPa, and the drying time at 4 - 6 h; S14: Let the dried resin cool naturally to obtain a pretreated resin matrix.

[0007] Optionally, the said S2 specifically includes: S21: Weigh sorbitol-based α-nucleating agent at 0.1 - 0.3% of the total weight of the pretreated resin matrix, rare-earth β-nucleating agent at 0.05 - 0.15%, and dispersant at 0.5 - 1.2%; S22: Put the weighed nucleating agent and dispersant into a three-dimensional motion mixer and conduct preliminary mixing under the conditions of a rotation speed of 20 - 40 rpm and a mixing time of 10 - 20 min; S23: Transfer the premixed material to a planetary ball mill, add zirconia grinding balls according to a ball-to-material ratio of 7:1, set the rotation speed at 250 - 350 rpm, and the ball milling time at 2 - 4 h; S24: Screen the powder after ball milling through a 200 - 300 mesh vibrating screen, and collect the composite powder with a particle size D50 ≤ 5 μm.

[0008] Optionally, the sorbitol-based α-nucleating agent is selected from 3,4-dimethylbenzylidene, p-ethylbenzylidene or p-chlorobenzylidene; the rare-earth β-nucleating agent is selected from lanthanum stearate, cerium stearate or neodymium octanoate; the dispersant is selected from erucamide or oleamide.

[0009] Optionally, the said S3 specifically includes: S31: Put the composite powder obtained in S2 and maleic anhydride-grafted polypropylene carrier resin into a high-speed mixer according to a weight ratio of 1:4, and mix for 5 - 10 min under the conditions of a rotation speed of 400 - 600 rpm and a temperature of 40 - 60 °C, wherein the grafting rate of the maleic anhydride-grafted polypropylene carrier is 1.5%; S32: Feed the mixture from S31 into a twin-screw extruder. Set the temperatures of zones 1 to 5 to 160 - 170 °C, 170 - 180 °C, 180 - 190 °C, 190 - 200 °C, and 180 - 190 °C respectively, the die head temperature to 175 - 185 °C, and the screw speed to 200 - 300 rpm; S33: The extruded melt enters a pelletizing trough with a water temperature of 30 °C through a die hole. Use a rotary blade to pelletize at a speed of 1000 rpm to obtain cylindrical masterbatch with a particle size of 2 - 3 mm.

[0010] Optionally, the specific steps of S4 are as follows: S41: Feed the pretreated resin matrix from S1 and the functional masterbatch from S3 into a high-speed mixer at a weight ratio of 98:2. Premix for 3 - 5 min under the conditions of a rotation speed of 500 - 800 rpm and a temperature of 50 - 70 °C; S42: Transfer the premixed material to an enclosed mixing kettle, introduce nitrogen to displace air, control the oxygen content to 0.3%, and maintain the pressure in the kettle at 0.1 - 0.3 MPa; S43: Inject the free radical initiator in three stages; For the first injection, 20 - 30% of the total amount of the free radical initiator is added 1 min after the start of mixing; For the second injection, 40 - 50% of the total amount of the free radical initiator is added after a 2 - minute interval; For the third injection, the remaining free radical initiator is added after another 1.5 - minute interval; The total addition amount of the initiator is 0.02 - 0.05% of the weight of the mixed material; S44: Under nitrogen protection, maintain the temperature of the mixing kettle at 80 - 100 °C and the stirring speed at 200 - 400 rpm for a reaction time of 8 - 12 min; S45: Cool the reacted material to below 40 °C through a water-cooled jacket to obtain a pre-crystallized mixed material.

[0011] Optionally, the specific steps of S5 are as follows: S51: Plasticize the pre-crystallized mixed material from S4 through a two-stage screw. The temperature of the first stage is 160 - 180 °C, and the temperature of the second stage is 190 - 210 °C. Control the melt pressure at 8 - 12 MPa; S52: The co-extrusion die head is divided into three temperature zones: inner, middle, and outer, which are set to 215 - 225 °C, 220 - 230 °C, and 205 - 215 °C respectively; S53: Adopt a coat-hanger die head structure. The melt flow rate of the inner layer is 0.8 - 1.2 m / min, the middle layer is 1.0 - 1.5 m / min, and the outer layer is 0.6 - 1.0 m / min. Control the thickness ratio of each layer as inner layer:middle layer:outer layer = 4:3:2; S54: Dynamically adjust the die lip gap. The gap in the inner layer is 0.8 - 1.0 mm, in the middle layer is 1.0 - 1.2 mm, and in the outer layer is 0.6 - 0.8 mm; S55: After the melt is extruded through the die lip, it enters a knife-edge gap of 10 - 20 mm wide. Blow it vertically with a wind speed of 3 - 5 m / s to output an unfixed sheet with a thickness of 1.2 - 1.8 mm.

[0012] Optionally, the specific steps of S6 include: S61: Import the unfixed sheet obtained in S5 into the first calender roll. The roll surface temperature is 90 - 100 °C, the linear speed is 10 - 15 m / min, and the preheating time is 30 - 50 s; S62: Pass through the second to fourth calender rolls in sequence. The roll temperatures are: the second roll is 110 - 120 °C, the third roll is 125 - 135 °C, the fourth roll is 115 - 125 °C. The speed difference between rolls is 3 - 8%, and the total calendering ratio is 1.5 - 2.2; S63: Set a stretching zone between the third roll and the fourth roll, control the stretching stress at 5 - 8 MPa, and the residence time at 2 - 4 s; S64: Finally, calender and polish through the fifth roll. Control its temperature at 95 - 105 °C to output a calendered sheet with a thickness of 0.8 - 1.2 mm, and the longitudinal / transverse tensile strength ratio is 1.8 - 2.5.

[0013] Optionally, the specific steps of S7 include: S71: Import the calendered sheet of S6 into a turbulent air cooling box with a wind speed of 5 - 8 m / s and a temperature of 40 - 50 °C to reduce the surface temperature of the sheet to 100 - 110 °C; S72: After air cooling, the sheet enters the atomization zone. Use a fan-shaped nozzle array with a diameter of 0.15 - 0.25 mm to spray deionized water at an atomization rate of 30 - 50 mL / min·m² at 25 - 35 °C to reduce the surface temperature of the sheet to 70 - 80 °C; S73: Wind the sheet around a mirror stainless steel cooling roll at 15 - 25 °C. The linear pressure on the roll surface is 50 - 80 N / cm, and the contact time is 10 - 15 s. Finally, output a base sheet with a surface temperature of 40 - 45 °C.

[0014] Optionally, the specific steps of S8 include: S81: Pass the base sheet of S7 through a corona treater at a speed of 8 - 12 m / min. The electrode spacing is 1.5 - 2.5 mm, apply a high-frequency power supply of 4 - 6 kW, the frequency is 18 - 22 kHz, and the treatment times are 2 - 4 times, with an interval of 5 - 8 s each time; S82: Mix hydrogen-containing silicone oil, nano-silica, and fluorocarbon surfactant in a weight ratio of 95:4:1, and add a solvent to dilute to a solid content of 8 - 12%, and control the viscosity to 150 - 250 mPa·s to obtain the microwave transparent barrier coating; S83: Perform microgravure coating using a ceramic gravure roll with 200 - 250 lines / in, a roll speed ratio of 1:1.2 - 1.5, a coating weight of 0.8 - 1.2 g / m², maintain the substrate temperature at 35 - 45°C, and a coating speed of 10 - 15 m / min; S84: After coating, the sheet enters a nitrogen - protected UV curing oven. The wavelength of the ultraviolet lamp is 365 nm, the irradiation intensity is 80 - 120 mW / cm², and the exposure time is 3 - 6 s; S85: After online defect detection, slit with a cemented carbide circular knife, control the tension at 2 - 4 N / mm², the winding density is 0.85 - 0.95 g / cm³, and obtain a finished PP sheet with a coating thickness of 0.5 - 1.0 μm.

[0015] Advantages of the present invention: In the present invention, through the synergistic induction effect of the α / β composite nucleation system, while improving the crystallinity of the material, the crystal form competition is avoided, so that the sheet significantly improves the heat resistance and impact resistance while maintaining high transparency; combined with the directional regulation of the microstructure of the material by the gradient temperature control process, the contradiction problem between the mechanical properties and thermal stability caused by the disordered arrangement of molecular chains in the traditional process is solved.

[0016] In the present invention, double optimizations are achieved from the aspects of material safety and application adaptability; on the one hand, by combining molecular chain modification and low - temperature processing technology, the generation of low - molecular - weight migrates is greatly reduced, meeting the safety standards of food - contact materials; on the other hand, the synergistic effect of the dense barrier layer formed by surface functionalization treatment and the substrate enables the sheet to exhibit excellent dimensional stability and barrier properties in a microwave heating environment, fundamentally breaking through the technical limitations of traditional polypropylene materials in the high - end food packaging field. Brief Description of the Drawings

[0017] In order to more clearly illustrate the technical solutions in the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only those of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0018] Figure 1 Schematic diagram of the preparation method of the high - heat - resistant transparent PP sheet according to the embodiment of the present invention; Figure 2 Schematic diagram of the method for forming an amorphous sheet according to the embodiment of the present invention. Detailed Embodiments

[0019] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. At the same time, it should be noted here that in order to make the embodiments more detailed, the following embodiments are the best and preferred embodiments. For some well-known technologies, those skilled in the art can also adopt other alternative methods for implementation; moreover, the accompanying drawings are only for more specifically describing the embodiments and are not intended to specifically limit the present invention.

[0020] It should be noted that in the specification, when referring to "an embodiment", "embodiments", "exemplary embodiments", "some embodiments", etc., it indicates that the described embodiments may include specific features, structures or characteristics, but not necessarily every embodiment includes such specific features, structures or characteristics. Additionally, when combining embodiments to describe specific features, structures or characteristics, implementing such features, structures or characteristics in combination with other embodiments (whether explicitly described or not) should be within the knowledge scope of those skilled in the relevant art.

[0021] Generally, terms can be understood at least in part from their use in the context. For example, at least in part depending on the context, the term "one or more" used herein can be used to describe any feature, structure or characteristic in a singular sense, or can be used to describe a combination of features, structures or characteristics in a plural sense. Additionally, the term "based on" can be understood as not necessarily aiming to convey a set of exclusive factors, but rather, at least in part depending on the context, allowing for the existence of other factors that may not be explicitly described.

[0022] Embodiment 1 As Figure 1 - Figure 2 shown, a method for preparing a high heat-resistant transparent PP sheet for microwave heating food packaging includes the following steps: S1: Mix homopolypropylene and copolymerized polypropylene in a set ratio and perform vacuum drying treatment to obtain a pretreated resin matrix; S2: Mechanically mix sorbitol-based α-nucleating agent, rare-earth β-nucleating agent and dispersant to obtain a composite powder; S3: Melt-blend the composite powder obtained in S2 with a carrier resin to prepare a functional masterbatch; S4: Mix the pretreated resin matrix in S1 with the functional masterbatch in S3, and add a free radical initiator in stages under an inert gas environment to initiate a mild cross-linking reaction of molecular chains to form a mixed material with a pre-crystallization structure; S5: Input the pre-crystallized mixed material in S4 into a co-extrusion die head with zone temperature control, and form an unshaped sheet through melt layer-by-layer compounding; S6: Process the unshaped sheet in S5 through a multi-roll calendering unit to obtain a calendered sheet with molecular chains arranged in a direction; S7: Perform staged cooling treatment on the calendered sheet in S6 and output a base sheet; S8: Perform corona treatment on the surface of the base sheet in S7, and coat a microwave-transparent barrier coating, thereby obtaining the finished PP sheet.

[0023] S1 specifically includes: S11: Put homopolypropylene with a melt index of 4 g / 10 min and copolymerized polypropylene with an ethylene content of 5% into a high-speed mixer according to a weight ratio of 7.5:2.5. S12: Mix for 12 min at a mixer rotation speed of 300 rpm and a temperature of 30 °C to form a resin mixture with a uniform distribution. S13: Transfer the resin mixture to a vacuum drying oven, set the temperature at 70 °C, the vacuum degree at -0.09 MPa, and the drying time at 5 h. S14: The dried resin is naturally cooled to obtain a pretreated resin matrix.

[0024] S2 specifically includes: S21: Weigh 0.2% of sorbitol-based α-nucleating agent, 0.1% of rare-earth β-nucleating agent, and 1.0% of dispersant based on the total weight of the pretreated resin matrix. S22: Put the weighed nucleating agent and dispersant into a three-dimensional motion mixer and perform preliminary mixing at a rotation speed of 30 rpm and a mixing time of 15 min. S23: Transfer the premixed material to a planetary ball mill, add zirconia grinding balls according to a ball-to-material ratio of 7:1, set the rotation speed at 300 rpm, and the ball milling time at 3 h. S24: Screen the powder after ball milling through a 250-mesh vibrating screen, and collect the composite powder with a particle size D50 ≤ 5 μm.

[0025] The sorbitol-based α-nucleating agent is selected from 3,4-dimethylbenzylidene; the rare-earth β-nucleating agent is selected from lanthanum stearate; the dispersant is selected from erucamide.

[0026] S3 specifically includes: S31: Put the composite powder obtained in S2 and maleic anhydride-grafted polypropylene carrier resin into a high-speed mixer according to a weight ratio of 1:4, and mix at a rotation speed of 500 rpm and a temperature of 50 °C for 8 min, where the grafting rate of the maleic anhydride-grafted polypropylene carrier is 1.5%. S32: Feed the mixture in S31 into a twin-screw extruder, and set the temperatures of the first to fifth zones at 165 °C, 175 °C, 185 °C, 195 °C, and 185 °C respectively, the die head temperature at 175 - 185 °C, and the screw rotation speed at 250 rpm. S33: The extruded melt enters the pelletizing tank with a water temperature of 30 °C through the die hole, and is pelletized by a rotary blade at a rotation speed of 1000 rpm to obtain cylindrical masterbatch with a particle size of 2.5 mm.

[0027] S4 specifically includes: S41: Put the pre-treated resin matrix of S1 and the functional masterbatch of S3 into a high-speed mixer at a weight ratio of 98:2, and premix for 4 min under the conditions of a rotation speed of 700 rpm and a temperature of 60 °C; S42: Transfer the premixed material to a closed mixing kettle, introduce nitrogen to displace air, control the oxygen content to 0.3%, and maintain the pressure in the kettle at 0.2 MPa; S43: Inject the free radical initiator in three stages; For the first injection, 25% of the total amount of the free radical initiator is added 1 min after the start of mixing; For the second injection, 45% of the total amount of the free radical initiator is added after an interval of 2 min; For the third injection, the remaining free radical initiator is added after an additional interval of 1.5 min; The total addition amount of the initiator is 0.03% of the weight of the mixed material; S44: Under nitrogen protection, maintain the temperature of the mixing kettle at 90 °C, the stirring speed at 300 rpm, and the reaction time at 10 min; S45: Cool the reacted material to below 40 °C through a water-cooled jacket to obtain a pre-crystallized mixed material.

[0028] S5 specifically includes: S51: Plasticize the pre-crystallized mixed material of S4 through a two-stage screw, with the temperature of the first stage at 170 °C and the temperature of the second stage at 200 °C, and control the melt pressure at 10 MPa; S52: The co-extrusion die head is divided into three temperature zones: the inner layer, the middle layer, and the outer layer, which are set at 220 °C, 225 °C, and 210 °C respectively; S53: Adopt a coat-hanger die head structure, with the melt flow rate of the inner layer at 1 m / min, the middle layer at 1.3 m / min, and the outer layer at 0.8 m / min, and control the thickness ratio of each layer as inner layer: middle layer: outer layer = 4:3:2; S54: Dynamically adjust the die lip gap with a regulating rod, with the gap in the inner layer zone at 0.9 mm, the middle layer zone at 1.1 mm, and the outer layer zone at 0.7 mm; S55: After the melt is extruded through the die lip, it enters a 15-mm-wide air knife gap, and is vertically blown by a wind speed of 4 m / s to output an undetermined sheet with a thickness of 1.5 mm.

[0029] S6 specifically includes: S61: Import the undetermined sheet obtained in S5 into the first calender roll, with the roll surface temperature at 95 °C, the linear speed at 12 m / min, and the preheating time at 40 s; S62: Pass through the second to fourth calender rolls in sequence, with the roll temperatures being: the second roll at 115 °C, the third roll at 130 °C, and the fourth roll at 120 °C, the speed difference between rolls at 5%, and the total calendering ratio at 1.7; S63: A stretching zone is provided between the third roller and the fourth roller, controlling the stretching stress at 6 MPa and the residence time at 3 s; S64: After final calendering by the fifth roller, its temperature is controlled at 100 °C, and a calendered sheet with a thickness of 1 mm is output, with a longitudinal / transverse tensile strength ratio of 2.0.

[0030] S7 specifically includes: S71: The calendered sheet of S6 is introduced into a turbulent air cooling box with a wind speed of 6 m / s and a temperature of 45 °C to reduce the surface temperature of the sheet to 105 °C; S72: The air-cooled sheet enters the atomization zone, and a fan-shaped nozzle array with a diameter of 0.2 mm is used to spray deionized water at 30 °C with an atomization amount of 40 mL / min·m² to reduce the surface temperature of the sheet to 75 °C; S73: The sheet is wound around a mirror stainless steel cooling roller at 20 °C, with a linear pressure on the roller surface of 60 N / cm and a contact time of 12 s, and finally a base sheet with a surface temperature of 43 °C is output.

[0031] S8 specifically includes: S81: The base sheet of S7 is passed through a corona treater at a speed of 10 m / min, with an electrode spacing of 2 mm, applying a 5 kW high-frequency power supply, a frequency of 20 kHz, and the treatment is carried out 3 times, with an interval of 6 s each time; S82: Mix hydrogen-containing silicone oil, nano-silica, and fluorocarbon surfactant in a weight ratio of 95:4:1, and add a solvent to dilute to a solid content of 10%, and control the viscosity to 200 mPa·s to obtain the microwave transparent barrier coating; S83: Carry out microgravure coating, using a ceramic gravure roller with 230 lines / in, a roller speed ratio of 1:1.3, a coating amount of 1.0 g / m², maintaining the substrate temperature at 40 °C, and a coating speed of 13 m / min; S84: After coating, the sheet enters a nitrogen-protected UV curing box, with an ultraviolet lamp wavelength of 365 nm, an irradiation intensity of 100 mW / cm², and an exposure time of 4 s; S85: After on-line defect detection, it is slit with a cemented carbide round knife, with the tension controlled at 3 N / mm² and the winding density at 0.9 g / cm³, to obtain a finished PP sheet with a coating thickness of 0.5 - 1.0 μm.

[0032] Example 2 S1: Preparation of the pretreated resin matrix. First, homopolypropylene with a melt index of 2 g / 10 min and copolymerized polypropylene with an ethylene content of 4% are put into a high-speed mixer according to a weight ratio of 7.5:2.5; under the conditions that the mixer speed is 200 rpm and the temperature is 25 °C, mix for 8 minutes to ensure the formation of a uniformly distributed resin mixture; then, transfer the mixture to a vacuum drying oven, set the temperature to 60 °C, the vacuum degree to -0.08 MPa, and set the drying time to 4 hours. After drying, the dried resin matrix is obtained; S2: Preparation of the composite powder. Weigh 0.1% of sorbitol-based α-nucleating agent (p-ethylbenzylidene), 0.05% of rare earth β-nucleating agent (cerium stearate), and 0.5% of dispersant (oleic acid amide) according to the total weight of the pretreated resin matrix; put these nucleating agents and dispersants into a three-dimensional motion mixer and conduct preliminary mixing under the conditions that the rotation speed is 20 rpm and the mixing time is 10 minutes; then, transfer the premixed materials to a planetary ball mill, add zirconia grinding balls, the ball-to-material ratio is 7:1, set the rotation speed to 250 rpm, and set the ball milling time to 2 hours; the powder after ball milling is classified by a 200-mesh vibrating screen, and the composite powder with a particle size D50 ≤ 5 μm is collected; S3: Preparation of the functional masterbatch. Put the composite powder and maleic anhydride-grafted polypropylene carrier resin into a high-speed mixer according to a weight ratio of 1:4, with a rotation speed of 400 rpm and a temperature of 40 °C, and mix for 5 minutes; feed the mixture into a twin-screw extruder, set the temperatures of the first to fifth zones to 160 °C, 170 °C, 180 °C, 190 °C, 180 °C respectively, the die head temperature to 175 °C, the screw rotation speed to 200 rpm, and after extrusion, cool and pelletize with water to obtain cylindrical masterbatch with a particle size of 2 mm; S4: Preparation of the pre-crystallized mixed material. Put the pretreated resin matrix and the functional masterbatch into a high-speed mixer according to a weight ratio of 98:2, with a rotation speed of 500 rpm and a temperature of 50 °C, and conduct pre-mixing for 3 minutes; then, transfer the premixed materials to a closed mixing kettle, introduce nitrogen to displace air, ensure that the oxygen content is 0.3%, and maintain the pressure in the kettle at 0.1 MPa; under these conditions, inject a free radical initiator in three stages, with a total addition amount of 0.02% of the weight of the mixed materials; inject 20% for the first time, 40% for the second time, and the remaining amount for the third time; under nitrogen protection, the temperature in the mixing kettle is 80 °C, the stirring speed is 200 rpm, and the reaction time is 8 minutes. Subsequently, cool the reacted materials to 40 °C through a water-cooled jacket to obtain the pre-crystallized mixed material; S5: Coextrusion die treatment. Feed the pre-crystallized mixed material into a two-stage screw for plasticization. The temperature is set at 160°C for the first stage and 190°C for the second stage, and the melt pressure is 8 MPa. The temperatures of the inner, middle, and outer layers of the coextrusion die are set at 215°C, 220°C, and 205°C respectively, and an hanger die is used for extrusion. Dynamically adjust the die lip gap with a bar. The melt flow rates of the inner, middle, and outer layers are set at 0.8 m / min, 1.0 m / min, and 0.6 m / min respectively, and the layer thickness ratio is controlled as inner layer: middle layer: outer layer = 4:3:2. Adjust the die lip gap dynamically with a bar, with a gap of 0.8 mm in the inner layer area, 1.0 mm in the middle layer area, and 0.6 mm in the outer layer area. After the melt is extruded through the die lip, it enters a 10-mm-wide air knife gap, and is vertically blown with a wind speed of 3 m / s to output an unstretched sheet with a thickness of 1.2 mm. S6: Preparation of calendered sheet. Feed the unstretched sheet into the first calender roll. The roll surface temperature is 90°C, the linear speed is 10 m / min, and the preheating time is set at 30 seconds. Pass through the second to fourth calender rolls. The roll temperatures are 110°C, 125°C, and 115°C respectively, and the speed difference between rolls is controlled at 3%, and the total calendering ratio is set at 1.5. In the stretching area between the third and fourth rolls, control the stretching stress at 5 MPa and the residence time at 2 seconds. Finally, after the calendered sheet passes through the fifth roll for calendering treatment, its temperature is controlled at 95°C, the thickness of the calendered sheet is 0.8 mm, and the longitudinal / transverse tensile strength ratio is 1.8. S7: Cooling treatment. Feed the calendered sheet into a turbulent air cooling box with a wind speed of 5 m / s and a temperature of 40°C to reduce the surface temperature of the sheet to 100°C. Then, the sheet enters the atomization area, and a fan-shaped nozzle array with a diameter of 0.15 mm is used to spray deionized water at 25°C with an atomization amount of 30 mL / min·m² to reduce the surface temperature of the sheet to 70°C. Then, wind the sheet around a cooling roll at 15°C. The linear pressure on the roll surface is 50 N / cm, and the contact time is 10 seconds. Finally, output a base sheet with a surface temperature of 40°C. S8: Coating and corona treatment. Feed the base sheet through a corona treatment machine at a speed of 8 m / min. The electrode spacing is 1.5 mm, a 4-kW high-frequency power supply is applied, the frequency is 18 kHz, and the treatment times are 2 times, with an interval of 5 s each time. The ingredients for coating a microwave-transparent barrier coating are mixed in a weight ratio of 95:4:1, and a solvent is added and diluted to a solid content of 8% and a viscosity of 150 mPa·s. Then, microgravure coating is carried out. A ceramic gravure roll with 200 lines / in is used, the roll speed ratio is 1:1.2, the coating amount is 0.8 g / m², the base material temperature is maintained at 35°C, the coating speed is 10 m / min, and after coating, the sheet enters a nitrogen-protected UV curing box. The wavelength of the ultraviolet lamp is 365 nm, the irradiation intensity is 80 mW / cm², and the exposure time is 3 s. After on-line defect detection, it is slit with a cemented carbide circular knife, the tension is controlled at 2 N / mm², and the winding density is 0.85 g / cm³ to obtain a finished PP sheet with a coating thickness of 0.5 μm.

[0033] Example 3 S1: Preparation of the pretreated resin matrix. First, homopolypropylene with a melt index of 5 g / 10 min and copolymerized polypropylene with an ethylene content of 8% are put into a high-speed mixer according to a weight ratio of 7.5:2.5; under the conditions that the mixer speed is 400 rpm and the temperature is 40°C, mix for 15 minutes to ensure a uniformly distributed resin mixture; then, transfer the mixture to a vacuum drying oven, set the temperature to 80°C, the vacuum degree to -0.1 MPa, and set the drying time to 6 hours. After drying, the dried resin matrix is obtained; S2: Preparation of the composite powder. Weigh 0.3% of sorbitol-based α-nucleating agent (p-chlorobenzylidene), 0.15% of rare earth β-nucleating agent (neodymium octanoate), and 1.2% of dispersant (erucamide) according to the total weight of the pretreated resin matrix; put these nucleating agents and dispersants into a three-dimensional motion mixer and conduct preliminary mixing under the conditions that the rotation speed is 40 rpm and the mixing time is 20 minutes; then, transfer the premixed material to a planetary ball mill, add zirconia grinding balls, the ball-to-material ratio is 7:1, set the rotation speed to 350 rpm, and set the ball milling time to 4 hours; the powder after ball milling is classified by a 300-mesh vibrating screen, and the composite powder with a particle size D50 ≤ 5 μm is collected; S3: Preparation of the functional masterbatch. Put the composite powder and maleic anhydride-grafted polypropylene carrier resin into a high-speed mixer according to a weight ratio of 1:4, with a rotation speed of 600 rpm and a temperature of 60°C, and mix for 10 minutes; send the mixture into a twin-screw extruder, set the temperatures of the first to fifth zones to 170°C, 180°C, 190°C, 200°C, 190°C respectively, the die head temperature to 185°C, the screw speed to 300 rpm, and after extrusion, cool and pelletize with water to obtain cylindrical masterbatch with a particle size of 3 mm; S4: Preparation of the pre-crystallized mixed material. Put the pretreated resin matrix and the functional masterbatch into a high-speed mixer according to a weight ratio of 98:2, with a rotation speed of 800 rpm and a temperature of 70°C, and conduct pre-mixing for 5 minutes; then, transfer the premixed material to a closed mixing kettle, introduce nitrogen to displace the air, ensure that the oxygen content is 0.3%, and maintain the pressure in the kettle at 0.3 MPa; under these conditions, inject a free radical initiator in three stages, with a total addition amount of 0.05% of the weight of the mixed material; inject 30% for the first time, 50% for the second time, and the remaining amount for the third time; under nitrogen protection, the temperature in the mixing kettle is 100°C, the stirring speed is 400 rpm, and the reaction time is 12 minutes. Subsequently, cool the reacted material to 40°C through a water-cooled jacket to obtain the pre-crystallized mixed material; S5: Coextrusion die treatment. Feed the pre-crystallized mixed material into a two-stage screw for plasticization. Set the temperature at 180 °C for the first stage and 210 °C for the second stage, and the melt pressure at 12 MPa. Set the temperatures of the inner layer, middle layer, and outer layer of the coextrusion die at 225 °C, 230 °C, and 215 °C respectively, and use a hanger die for extrusion. Dynamically adjust the die lip gap with a bar. Set the melt flow rates of the inner layer, middle layer, and outer layer at 1.2 m / min, 1.5 m / min, and 1.0 m / min respectively, and control the layer thickness ratio as inner layer:middle layer:outer layer = 4:3:2. Adjust the die lip gap dynamically with a bar, with the gap in the inner layer area being 1.0 mm, 1.2 mm in the middle layer area, and 0.8 mm in the outer layer area. After the melt is extruded through the die lip, it enters a 20-mm-wide air knife gap, and is vertically blown by a wind speed of 5 m / s to output an unfixed sheet with a thickness of 1.8 mm. S6: Preparation of calendered sheet. Feed the unfixed sheet into the first calender roll. The roll surface temperature is 100 °C, the linear speed is 15 m / min, and the preheating time is set at 50 seconds. Pass through the second to fourth calender rolls, with the roll temperatures being 120 °C, 135 °C, and 125 °C respectively. Control the speed difference between rolls at 8%, and set the total calendering ratio at 2.2. In the stretching area between the third and fourth rolls, control the stretching stress at 8 MPa and the residence time at 4 seconds. Finally, after the calendered sheet undergoes a calendering treatment by the fifth roll, its temperature is controlled at 105 °C, the thickness of the calendered sheet is 1.2 mm, and the longitudinal / transverse tensile strength ratio is 2.5. S7: Cooling treatment. Feed the calendered sheet into a turbulent air cooling box with a wind speed of 8 m / s and a temperature of 50 °C to reduce the surface temperature of the sheet to 110 °C. Then, the sheet enters the atomization area, and a fan-shaped nozzle array with a diameter of 0.25 mm is used to spray deionized water at 35 °C with an atomization amount of 50 mL / min·m² to reduce the surface temperature of the sheet to 80 °C. Then, wind the sheet around a cooling roll at 25 °C, with the linear pressure on the roll surface being 80 N / cm and the contact time being 15 seconds. Finally, output a base material sheet with a surface temperature of 45 °C. S8: Coating and corona treatment. Feed the base material sheet through a corona treatment machine at a speed of 12 m / min. The electrode spacing is 2.5 mm, apply a 6-kW high-frequency power supply with a frequency of 22 kHz, and the treatment times are 4 times, with an interval of 8 s each time. The ingredients for coating a microwave transparent barrier coating are mixed in a weight ratio of 95:4:1, and a solvent is added and diluted to a solid content of 12% and a viscosity of 250 mPa·s. Then, perform microgravure coating, using a ceramic gravure roll with 250 lines / in, a roll speed ratio of 1:1.5, a coating amount of 1.2 g / m², maintain the base material temperature at 45 °C, the coating speed at 15 m / min. After coating, the sheet enters a nitrogen-protected UV curing box, with the wavelength of the ultraviolet lamp at 365 nm, the irradiation intensity at 120 mW / cm², and the exposure time at 6 s. After on-line defect detection, slit with a cemented carbide round knife, control the tension at 4 N / mm², and the winding density at 0.95 g / cm³ to obtain a finished PP sheet with a coating thickness of 1.0 μm.

[0034] Comparative Example 1 S1: First, homopolypropylene and copolymerized polypropylene were mixed in a ratio of 8:2, and then the mixture was put into a high-speed blender and stirred at a speed of 300 rpm. The stirring temperature was 25°C, and the mixing time was 10 minutes; S2: The mixed resin matrix was fed into a twin-screw extruder with the temperature set at 180°C and the screw speed at 250 rpm. The melt was extruded through a die to obtain a preliminary sheet, and the sheet thickness was controlled at about 1.5 mm; S3: The extruded sheet was cooled through a cooling water tank. The thickness of the cooled sheet was about 1.2 mm, and then the sheet was cut into the required size by a cutting machine.

[0035] Table 1 Comparison of Finished Product Performance Data Performance Index Example 1 Example 2 Example 3 Comparative Example 1 Melt Flow Rate 2.5 g / 10 min 2.8 g / 10 min 3.0 g / 10 min 4.0 g / 10 min Longitudinal Tensile Strength 55 MPa 52 MPa 50 MPa 40 MPa Transverse Tensile Strength 25 MPa 23 MPa 20 MPa 15 MPa Transparency (%) 90% 85% 80% 70% Heat Resistance (°C) 130℃ 120℃ 115℃ 100℃ UV Resistance Good Medium Medium Poor Surface Smoothness (Ra) 0.2µm 0.4µm 0.5µm 1.0µm As can be seen from Table 1 above, the melt flow rate of Example 1 is lower (2.5 g / 10 min), indicating better fluidity during processing, which is conducive to efficient production; the melt flow rate of Comparative Example 1 is higher, which may lead to instability during processing; the longitudinal tensile strength of Example 1 is the strongest (55 MPa), with excellent mechanical properties, while the longitudinal tensile strength of Comparative Example 1 is lower, indicating poor mechanical properties; Example 1 performs well in the transverse tensile strength (25 MPa), showing the uniformity and strength of its structure; the transverse tensile strength of Comparative Example 1 is significantly worse; Example 1 has the highest transparency, reaching 90%, which is more visually attractive and meets the requirements for microwave heating packaging; the transparency of Comparative Example 1 is lower, only 70%, which may affect the aesthetics and performance of the packaging; Example 1 has the best heat resistance (130°C) and is suitable for long-term high-temperature use; the heat resistance of Comparative Example 1 is only 100°C and is not suitable for use in high-temperature environments; Example 1 has good ultraviolet resistance, ensuring its long-term stability; the ultraviolet resistance of Comparative Example 1 is poor, and it is easily affected by ultraviolet rays, reducing the service life of the product; the surface smoothness of Example 1 is the best (Ra = 0.2 µm), providing good touch and surface quality; the surface smoothness of Comparative Example 1 is poor, affecting the use effect and appearance of the sheet. Generally speaking, the PP sheet of Example 1 performs excellently in multiple properties and is suitable for microwave heating food packaging; in contrast, the PP sheet of Comparative Example 1 is at a relatively low level in most performance indicators, so it is not suitable as a high-demand microwave heating food packaging material.

[0036] Table 2 Comparison of Performance Parameters in Other Aspects Performance Index Example 1 Example 2 Example 3 Comparative Example 1 Impact Resistance 12 kJ / m² 11 kJ / m² 10 kJ / m² 8 kJ / m² Flexural Strength 40 MPa 35 MPa 30 MPa 25 MPa Scratch Resistance 7H 6H 5H 4H Water Vapor Transmission Rate (g / m²·day) 5 g / m²·day 6 g / m²·day 7 g / m²·day 10 g / m²·day Oxygen Transmission Rate (cm³ / m²·day) 15 cm³ / m²·day 18 cm³ / m²·day 20 cm³ / m²·day 25 cm³ / m²·day Static Charge Accumulation Voltage 1 kV 1.2 kV 1.5 kV 2 kV Tensile Strength (MPa) 45 MPa 42 MPa 40 MPa 30 MPa Molding Cycle (min) 6 min 8 min 9 min 12 min As can be seen from Table 2 above, Example 1 has the highest impact strength (12 kJ / m²), indicating better impact resistance and suitability for high-strength usage environments; Comparative Example 1 has a lower impact strength, only 8 kJ / m², with poor impact resistance; Example 1 performs best in terms of flexural strength (40 MPa), ensuring the toughness and stability of the material; Comparative Example 1 has a lower flexural strength (25 MPa), which may cause the material to fracture during use; Example 1 received a rating of 7H for scratch resistance performance, indicating that its surface has strong scratch resistance and is suitable for high-strength use; Comparative Example 1 is only 4H, and its surface is easily scratched, affecting the appearance and service life; Example 1 has the lowest water vapor transmission rate (5 g / m²·day), indicating its strong water barrier property, which helps to keep the inside of the package dry; Comparative Example 1 has a higher water vapor transmission rate (10 g / m²·day), which may cause moisture to penetrate into the package and affect the food quality; The oxygen transmission rate of Example 1 is 15 cm³ / m²·day, and the low gas permeability ensures that the package can effectively maintain the freshness of the food; Comparative Example 1 has a higher oxygen transmission rate (25 cm³ / m²·day), affecting the food preservation effect; Example 1 has the lowest static charge accumulation voltage (1 kV), reducing the risk of package deformation or damage caused by static charge accumulation; Comparative Example 1 has a higher static charge accumulation voltage (2 kV), which may cause static problems during transportation; Example 1 has a higher tensile strength (45 MPa), showing excellent tensile properties; Comparative Example 1 has a lower tensile strength (30 MPa), and is prone to tensile deformation, affecting the strength and stability of the material; Example 1 has the shortest molding cycle, only 6 minutes, improving production efficiency; Comparative Example 1 has a longer molding cycle, reaching 12 minutes, reducing production efficiency. Generally speaking, Example 1 far outperforms Comparative Example 1 in terms of multiple performance indicators such as impact strength, flexural strength, scratch resistance, gas permeability, and static charge accumulation voltage, is suitable for high-performance applications, and has high production efficiency; In contrast, Comparative Example 1 has poor performance and lags behind Example 1 in many important indicators, and is suitable for relatively ordinary usage occasions.

[0037] The present invention covers any alternatives, modifications, equivalent methods, and solutions made within the spirit and scope of the present invention. To enable the public to have a thorough understanding of the present invention, specific details are elaborated in the following preferred embodiments of the present invention. However, those skilled in the art can fully understand the present invention even without the description of these details. Additionally, well-known methods, processes, procedures, components, and circuits are not described in detail to avoid unnecessary confusion to the essence of the present invention.

[0038] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A preparation method of a high heat-resistant transparent PP sheet suitable for microwave heating food packaging, characterized in that, It includes the following steps: S1: Mix homopolypropylene and copolymerized polypropylene in a set ratio and perform vacuum drying treatment to obtain a pretreated resin matrix; S2: Mechanically mix sorbitol-based α-nucleating agent, rare earth β-nucleating agent and dispersant to obtain a composite powder; S3: Melt-blend the composite powder obtained in S2 with a carrier resin to prepare a functional masterbatch; S4: Mix the pretreated resin matrix in S1 with the functional masterbatch in S3, and add a free radical initiator in stages under an inert gas environment to initiate a mild cross-linking reaction of molecular chains to form a mixed material with a pre-crystalline structure; S5: Input the pre-crystallized mixed material in S4 into a co-extrusion die head with zone temperature control, and form an amorphous sheet by melt layer compounding; S6: Process the amorphous sheet in S5 through a multi-roll calendering unit to obtain a calendered sheet with molecular chains arranged in a direction; S7: Perform staged cooling treatment on the calendered sheet in S6 and output a base sheet; S8: Perform corona treatment on the surface of the base sheet in S7 and coat a microwave transparent barrier coating to obtain a finished PP sheet.

2. The preparation method of a high heat-resistant transparent PP sheet applicable to microwave-heated food packaging according to claim 1, characterized in that, The specific content of S1 includes: S11: Put homopolypropylene with a melt index of 2-5 g / 10 min and copolymerized polypropylene with an ethylene content of 4-8% into a high-speed mixer according to a weight ratio of 7.5:2.5; S12: Mix for 8-15 min under the conditions of a mixer rotation speed of 200-400 rpm and a temperature of 25-40 °C to form a uniformly distributed resin mixture; S13: Transfer the resin mixture to a vacuum drying oven, set the temperature at 60-80 °C, the vacuum degree at -0.08 to -0.1 MPa, and the drying time at 4-6 h; S14: Let the dried resin cool naturally to obtain a pretreated resin matrix.

3. The preparation method of a high heat-resistant transparent PP sheet applicable to microwave-heated food packaging according to claim 1, characterized in that, The specific content of S2 includes: S21: Weigh 0.1-0.3% of sorbitol-based α-nucleating agent, 0.05-0.15% of rare earth β-nucleating agent, and 0.5-1.2% of dispersant based on the total weight of the pretreated resin matrix; S22: Put the weighed nucleating agent and dispersant into a three-dimensional motion mixer and perform preliminary mixing under the conditions of a rotation speed of 20-40 rpm and a mixing time of 10-20 min; S23: Transfer the premixed material to a planetary ball mill, add zirconia grinding balls according to a ball-to-material ratio of 7:1, set the rotation speed at 250-350 rpm, and the ball milling time at 2-4 h; S24: Screen the powder after ball milling through a 200-300 mesh vibrating screen and collect the composite powder with a particle size D50 ≤ 5 μm.

4. The preparation method of a high heat-resistant transparent PP sheet for microwave heating food packaging according to claim 3, characterized in that, The sorbitol-based α-nucleating agent is selected from 3,4-dimethylbenzylidene, p-ethylbenzylidene or p-chlorobenzylidene; the rare earth β-nucleating agent is selected from lanthanum stearate, cerium stearate or neodymium octanoate; the dispersant is selected from erucic acid amide or oleic acid amide.

5. The preparation method of a high heat-resistant transparent PP sheet applicable to microwave-heated food packaging according to claim 1, characterized in that, The specific content of S3 includes: S31: Put the composite powder obtained in S2 and maleic anhydride grafted polypropylene carrier resin into a high-speed mixer according to a weight ratio of 1:4, and mix for 5-10 min under the conditions of a rotation speed of 400-600 rpm and a temperature of 40-60 °C, where the grafting rate of maleic anhydride grafted polypropylene carrier is 1.5%; S32: Feed the mixed materials from S31 into a twin-screw extruder. Set the temperatures of zones 1 to 5 to 160 - 170 °C, 170 - 180 °C, 180 - 190 °C, 190 - 200 °C, and 180 - 190 °C respectively, the die head temperature to 175 - 185 °C, and the screw speed to 200 - 300 rpm; S33: The extruded melt enters a pelletizing tank with a water temperature of 30 °C through a die hole. Use a rotary blade to pelletize at a speed of 1000 rpm to obtain cylindrical masterbatch with a particle size of 2 - 3 mm.

6. The preparation method of a high heat-resistant transparent PP sheet applicable to microwave-heated food packaging according to claim 1, characterized in that, The specific steps of S4 include: S41: Put the pretreated resin matrix from S1 and the functional masterbatch from S3 into a high-speed mixer at a weight ratio of 98:2, and premix for 3 - 5 min under the conditions of a rotation speed of 500 - 800 rpm and a temperature of 50 - 70 °C; S42: Transfer the premixed materials to an enclosed mixing kettle, introduce nitrogen to displace air, control the oxygen content to 0.3%, and maintain the pressure in the kettle at 0.1 - 0.3 MPa; S43: Inject the free radical initiator in three stages; For the first injection, 20 - 30% of the total amount of the free radical initiator is added 1 min after the start of mixing; For the second injection, 40 - 50% of the total amount of the free radical initiator is added after a 2 - minute interval; For the third injection, the remaining free radical initiator is added after another 1.5 - minute interval; The total addition amount of the initiator is 0.02 - 0.05% of the weight of the mixed materials; S44: Under nitrogen protection, maintain the temperature of the mixing kettle at 80 - 100 °C and the stirring speed at 200 - 400 rpm for a reaction time of 8 - 12 min; S45: Cool the reacted materials to below 40 °C through a water-cooled jacket to obtain pre-crystallized mixed materials.

7. A method for preparing a highly heat-resistant transparent PP sheet for microwave-heated food packaging according to claim 1, characterized in that, The specific steps of S5 include: S51: Plasticize the pre-crystallized mixed materials from S4 through a two-stage screw. The temperature of the first stage is 160 - 180 °C, the temperature of the second stage is 190 - 210 °C, and the melt pressure is controlled at 8 - 12 MPa; S52: The co-extrusion die head is divided into three temperature zones: inner layer, middle layer, and outer layer, which are set to 215 - 225 °C, 220 - 230 °C, and 205 - 215 °C respectively; S53: Adopt a coat-hanger die head structure. The melt flow rate of the inner layer is 0.8 - 1.2 m / min, the middle layer is 1.0 - 1.5 m / min, the outer layer is 0.6 - 1.0 m / min, and the thickness ratio of each layer is controlled as inner layer:middle layer:outer layer = 4:3:2; S54: Dynamically adjust the die lip gap with an adjusting rod. The gap in the inner layer area is 0.8 - 1.0 mm, the middle layer area is 1.0 - 1.2 mm, and the outer layer area is 0.6 - 0.8 mm; S55: After the melt is extruded through the die lip, it enters a knife-edge gap of 10 - 20 mm wide. Blow it vertically with a wind speed of 3 - 5 m / s to output an unfixed sheet with a thickness of 1.2 - 1.8 mm.

8. A method for preparing a high heat-resistant transparent PP sheet for microwave heating food packaging according to claim 1, characterized in that, The specific steps of S6 include: S61: Feed the unfixed sheet obtained from S5 into the first calender roll. The roll surface temperature is 90 - 100 °C, the linear speed is 10 - 15 m / min, and the preheating time is 30 - 50 s; S62: Pass through the second to fourth calendering rollers in sequence. The roller temperatures are as follows: the second roller is 110 - 120 °C, the third roller is 125 - 135 °C, and the fourth roller is 115 - 125 °C. The speed difference between rollers is 3 - 8%, and the total calendering ratio is 1.5 - 2.2; S63: Set a stretching zone between the third roller and the fourth roller, control the stretching stress at 5 - 8 MPa, and the residence time at 2 - 4 s; S64: Finally calender and polish through the fifth roller, control its temperature at 95 - 105 °C, and output a calendered sheet with a thickness of 0.8 - 1.2 mm. The longitudinal / transverse tensile strength ratio is 1.8 - 2.

5.

9. The preparation method of a highly heat-resistant transparent PP sheet applicable to microwave-heated food packaging according to claim 1, characterized in that, The specific steps of S7 include: S71: Introduce the calendered sheet of S6 into a turbulent air cooling box with a wind speed of 5 - 8 m / s and a temperature of 40 - 50 °C to reduce the surface temperature of the sheet to 100 - 110 °C; S72: After air cooling, the sheet enters the atomization zone. Use a fan-shaped nozzle array with a diameter of 0.15 - 0.25 mm to spray deionized water at an atomization rate of 30 - 50 mL / min·m² at 25 - 35 °C to reduce the surface temperature of the sheet to 70 - 80 °C; S73: Wind the sheet around a mirror stainless steel cooling roller at 15 - 25 °C. The linear pressure on the roller surface is 50 - 80 N / cm, and the contact time is 10 - 15 s. Finally, output a base sheet with a surface temperature of 40 - 45 °C.

10. A method for preparing a high heat-resistant transparent PP sheet for microwave-heated food packaging according to claim 1, characterized in that, The specific steps of S8 include: S81: Pass the base sheet of S7 through a corona treater at a speed of 8 - 12 m / min. The electrode spacing is 1.5 - 2.5 mm, apply a 4 - 6 kW high-frequency power supply, the frequency is 18 - 22 kHz, the treatment times are 2 - 4 times, and the interval between each time is 5 - 8 s; S82: Mix hydrogen-containing silicone oil, nano-silica, and fluorocarbon surfactant according to a weight ratio of 95:4:1, and add a solvent to dilute to a solid content of 8 - 12%, and control the viscosity at 150 - 250 mPa·s to obtain the microwave-transparent barrier coating; S83: Conduct microgravure coating. Use a ceramic gravure roller with 200 - 250 lines / in, the roller speed ratio is 1:1.2 - 1.5, the coating amount is 0.8 - 1.2 g / m², maintain the substrate temperature at 35 - 45 °C, and the coating speed is 10 - 15 m / min; S84: After coating, the sheet enters a nitrogen-protected UV curing box. The wavelength of the ultraviolet lamp is 365 nm, the irradiation intensity is 80 - 120 mW / cm², and the exposure time is 3 - 6 s; S85: After online defect detection, slit with a cemented carbide round knife, control the tension at 2 - 4 N / mm², and the winding density at 0.85 - 0.95 g / cm³ to obtain a finished PP sheet with a coating thickness of 0.5 - 1.0 μm.

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