A polypropylene packaging film and a method for preparing the same

Abstract

The application relates to a polypropylene packaging film and a preparation method thereof. The method is characterized in that: first, maleic anhydride grafted polypropylene is reacted with 1,6-hexanediamine, and residual primary amine type amino functionalized polypropylene is obtained through washing, drying and primary amine determination; then, the polypropylene is reacted with another batch of maleic anhydride grafted polypropylene, and 1,6-hexanediamine is supplemented, and a step-by-step coupling polypropylene master batch is prepared under the condition that the ratio of total primary amine equivalent to anhydride equivalent is 0.300:1, and the non-extracted primary amine equivalent accounts for 20.0%-35.0% of the total primary amine equivalent. The master batch is mixed with a propylene-ethylene-butene terpolymer and a propylene-based elastomer to form a heat-sealing layer material, and is double-layer co-extrusion cast with a polypropylene core layer material. The obtained high-performance polypropylene packaging film has the advantages of low heat-sealing initial temperature, wide stable heat-sealing interval, high temperature sealing retention capacity and continuous processing stability.

Description

Technical Field

[0001] This invention belongs to the field of polypropylene film material technology, specifically relating to a polypropylene packaging film and its preparation method. Background Technology

[0002] Polypropylene packaging film is characterized by low density, good transparency and moisture resistance, and thermoplastic processing. It typically employs a layered structure consisting of a high-melting-point core layer and a low-melting-point heat-sealing layer. With the increasing demands on sealing cycle time and temperature tolerance in high-speed packaging equipment, high-performance polypropylene packaging film not only requires a lower heat-sealing initiation temperature but also needs to maintain heat-sealing strength, thermal adhesion, and edge thickness over a wide temperature range, while preventing gelation, crystallization, or pressure fluctuations in the heat-sealing layer during continuous extrusion.

[0003] To lower the heat-sealing initiation temperature, existing technologies typically increase the proportion of random copolymer polypropylene, terpolymers, or propylene-based elastomers. Patent CN114734699A discloses a cast polypropylene heat-sealing layer comprising an ethylene-propylene-1-butene random copolymer, a metallocene propylene-1-butene copolymer, and a propylene-based elastomer; Patent CN115991034B discloses a polypropylene composite film comprising composition A, comprising homopolymer polypropylene, propylene impact copolymer, polyolefin elastomer, and a slip agent, and composition B, comprising random polypropylene, a slip agent, and optionally a polyolefin elastomer; Patent CN114989532B discloses a heat-sealing material obtained by vulcanized extrusion granulation of copolymer polypropylene, low-density polyethylene, elastomer, polyoleamide, vulcanizing agent, and antioxidant.

[0004] Adding low-melting-point copolymers or elastomers facilitates bonding of the heat-sealable layer at lower temperatures, but increasing their proportion makes the heat-sealable layer more prone to flow at higher temperatures and causes thinner sealing edges. Simply increasing high-melting-point polypropylene or improving the degree of crosslinking may raise the heat-seal initiation temperature and increase gelation and extrusion fluctuations. Although existing methods of coupling anhydride-grafted polypropylene with diamines can improve the melt strength of polypropylene, small-molecule diamines may still cause uneven bonding distribution during localized rapid reactions. Therefore, existing methods still need to address the contradiction between low-temperature sealing initiation and high-temperature sealing reliability, as well as the stability of continuous processing. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to improve the sealing cohesion and edge thickness retention during high-temperature heat sealing while maintaining a low heat-sealing initiation temperature of polypropylene packaging film, and to reduce gelation, crystal points and pressure fluctuations during continuous extrusion.

[0006] To solve the above-mentioned technical problems, the present invention provides a method for preparing polypropylene packaging film, comprising steps S1 to S4.

[0007] In step S1, maleic anhydride-grafted polypropylene is melt-plasticized and then molten 1,6-hexanediamine is added. After the reaction, devolatilization, extrusion and pelleting are performed. The resulting pellets are washed and dried. The total accessible primary amine value and the primary amine value converted from extractable free 1,6-hexanediamine are measured separately. The difference between the two is taken as the non-extractable primary amine value of residual primary amine type aminofunctionalized polypropylene.

[0008] In step S2, maleic anhydride-grafted polypropylene that did not participate in the reaction in step S1 is melt-plasticized. After forming a continuous melt, residual primary amine-type amino-functionalized polypropylene is added through side feeding. The reaction is carried out for 30-90 seconds. Then, molten 1,6-hexanediamine is added through the downstream liquid metering port, and the reaction continues for 60-180 seconds. Based on the anhydride equivalent contained in the maleic anhydride-grafted polypropylene that did not participate in the reaction in step S1, the ratio of total primary amine equivalent to anhydride equivalent in the feed is 0.300:1. The non-extractable primary amine equivalent provided by the residual primary amine-type amino-functionalized polypropylene accounts for 20.0%-35.0% of the total primary amine equivalent in the feed, and the remaining primary amine equivalent is provided by 1,6-hexanediamine. After the reaction, an antioxidant is added, and vacuum devolatilization and melt filtration are performed. The mixture is then extruded, cooled, and pelletized to obtain stepwise coupling polypropylene masterbatch.

[0009] In step S3, heat-sealing layer material and core layer material are prepared; based on the total mass of heat-sealing layer resin, the heat-sealing layer resin consists of 11.0% step-coupled polypropylene masterbatch and 89.0% of the remaining resin, which consists of propylene-ethylene-butene terpolymer and propylene-based elastomer in a mass ratio of 80:20; the core layer material includes film-grade homopolymer polypropylene and film-grade random copolymer polypropylene.

[0010] In step S4, the heat-sealing layer material and the core layer material are co-extruded and cast in a double layer to stack the heat-sealing layer and the core layer, thereby obtaining a polypropylene packaging film.

[0011] In step S1, the reactive anhydride mass fraction of maleic anhydride-grafted polypropylene can be 1.00%, the melt flow rate can be 35.0 g / 10 min, the molar ratio of 1,6-hexanediamine molecules to anhydride can be 1.25:1, the reaction temperature can be 185℃, and the reaction time can be 90 s. After devolatilization and pelleting, it can be washed three times with a 50% ethanol aqueous solution at 60℃, then washed once with purified water, and dried for 8 hours at 80℃ and a vacuum gauge pressure of -0.08 MPa.

[0012] The total accessible primary amine value of the residual primary amine-type aminofunctionalized polypropylene obtained in step S1 can be 0.084 mmol / g, the primary amine value converted from extractable free 1,6-hexanediamine can be 0.004 mmol / g, the non-extractable primary amine value can be 0.080 mmol / g, and the extractable free 1,6-hexanediamine can account for 4.8% of the total accessible primary amine equivalent.

[0013] In step S2, the non-extractable primary amine equivalent provided by the residual primary amine type amino-functionalized polypropylene can account for 25.0% of the total primary amine equivalent in the feed; after adding the residual primary amine type amino-functionalized polypropylene, the reaction can proceed at 195℃ for 60s, and after adding 1,6-hexanediamine, the reaction can continue for 120s; the vacuum gauge pressure for vacuum devolatilization can be -0.08MPa, the devolatilization time can be 45s, and a 100-mesh filter can be used for melt filtration.

[0014] The fractional coupling polypropylene masterbatch obtained in step S2 has a melt flow rate of 3.5 to 5.8 g / 10 min at 230°C and 2.16 kg, and a hot xylene insoluble content of 0.7% to 1.7%. It can also be continuously extruded into strips after being remelted at 200°C.

[0015] In step S3, for every 100 parts by weight of heat-sealing resin, 0.08 parts by weight of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], 0.08 parts by weight of tris(2,4-di-tert-butylphenyl) phosphite, 0.08 parts by weight of erucamide and 0.18 parts by weight of film-grade anti-sticking silica may be added, and the median particle size of the film-grade anti-sticking silica may be 3.5 μm; based on the total mass of the core layer resin, the core layer material may be composed of 92.0% film-grade homopolymer polypropylene and 8.0% film-grade random copolymer polypropylene.

[0016] In step S4, the extrusion temperature of the heat-sealing layer can be 180–220°C, the extrusion temperature of the core layer can be 200–240°C, the temperature of the distributor and die head can be 200–230°C, and the temperature of the cooling roller can be 20–40°C. The total thickness of the resulting polypropylene packaging film can be 25–60 μm, and the thickness of the heat-sealing layer can be 3–8 μm.

[0017] The present invention also provides a polypropylene packaging film prepared by the above-described method. The total thickness of the polypropylene packaging film can be 35 μm, wherein the thickness of the heat-sealing layer can be 5 μm and the thickness of the core layer can be 30 μm.

[0018] Beneficial effects: (1) First, residual primary amine-type amino-functionalized polypropylene is prepared. After washing, the non-extractable primary amine value is measured, and the subsequent feed amount is calculated based on this value, so that the amount of reactive macromolecular amine is based on a uniform equivalent standard. When preparing the masterbatch, the material is first brought into contact with maleic anhydride-grafted polypropylene that has not participated in the previous reaction, and then 1,6-hexanediamine is added. This helps to reduce the gelation and uneven dispersion caused by the local concentrated reaction of small molecule diamines, and improves the stability of the masterbatch during repeated melting and filtration processing.

[0019] (2) The heat-sealing layer is formed by a low-melting-point propylene-ethylene-butene terpolymer and a propylene-based elastomer to form a low-temperature bonding phase, and a limited proportion of step-coupled polypropylene masterbatch is added. The low-melting-point resin maintains chain segment movement and interfacial bonding at a lower temperature, and the step-coupled polypropylene masterbatch helps the heat-sealing layer maintain melt cohesion and reduce edge thinning after the temperature rises, thus taking into account the cooling heat-sealing strength, hot adhesion strength and stable heat-sealing range.

[0020] (3) The non-extractable primary amine equivalent provided by the residual primary amine type amino-functionalized polypropylene accounts for 20.0% to 35.0% of the total primary amine equivalent in the feed, and the remaining primary amine equivalent is made up by 1,6-hexanediamine. This ratio can achieve a balance between low-temperature initiation, high-temperature sealing and continuous extrusion stability.

[0021] (4) The step-coupled polypropylene masterbatch, terpolymer, propylene-based elastomer and polypropylene core layer are all based on polyolefins, which are suitable for melt co-extrusion and interlayer continuous molding. With the addition of antioxidants, erucamide and anti-sticking silica in limited amounts, it is possible to control film adhesion, dynamic friction coefficient, haze and crystal points while improving the sealing and retention ability at high temperature. Detailed Implementation

[0022] Raw materials, measurement standards and testing instructions: The reactive anhydride mass fraction of maleic anhydride-grafted polypropylene used in this section was 1.00%, the melt flow rate was 35.0 g / 10 min, and the melt flow rate was measured at 230℃ and 2.16 kg. The purity of the 1,6-hexanediamine used was not less than 99.0%. Pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and tris(2,4-di-tert-butylphenyl) phosphite were used as hindered phenolic antioxidants and phosphite antioxidants, respectively.

[0023] The anhydride equivalent is calculated based on the mass of maleic anhydride-grafted polypropylene, the mass fraction of reactive anhydride, and a molar mass of maleic anhydride of 98.06 g / mol. The primary amine equivalent is calculated based on the amount of substance of the primary amine group; 1 mol of 1,6-hexanediamine is considered as 2 mol of primary amine equivalent. The primary amine equivalent substitution ratio is the percentage of non-extractable primary amine equivalent provided by amino-functionalized polypropylene to the total primary amine equivalent in the feed.

[0024] Melt flow rate was determined according to GB / T 3682.1-2018. Unless otherwise specified, all listed masses are actual feed masses, and theoretical equivalents are used to determine the feed ratio. Melt flow rate, hot xylene insoluble content, and non-extractable primary amine value are used to characterize the melt flowability, gelation level, and primary amine equivalent required for subsequent feeds, respectively.

[0025] During the double-layer co-extrusion casting process, the extrusion temperature of the heat-sealing layer is 180–220℃, the extrusion temperature of the core layer is 200–240℃, the temperature of the distributor and die head is 200–230℃, and the temperature of the cooling roller is 20–40℃. The total thickness of the resulting polypropylene packaging film is 25–60 μm, and the thickness of the heat-sealing layer is 3–8 μm.

[0026] Preparation Example 1: Residual Primary Amine Type Amino-functionalized Polypropylene Weigh 6000g of maleic anhydride-grafted polypropylene and dry it at 80℃ for 3 hours. Weigh 88.9g of 1,6-hexanediamine and heat it to 60℃ while maintaining it in a molten state. The maleic anhydride-grafted polypropylene used contains 60.0g of reactive anhydride, corresponding to 0.612mol of anhydride; the amount of 1,6-hexanediamine is 0.765mol, and the molar ratio of diamine molecules to anhydride is 1.25:1.

[0027] A co-rotating twin-screw extruder with a sealed liquid metering port, a side feed port, and two-stage vacuum devolatilization ports was used. The temperatures of each zone were set sequentially to 175℃, 180℃, 185℃, 185℃, and 180℃, and the screw speed was set to 180 r / min. Dried maleic anhydride-grafted polypropylene was added through the main feed port. After it formed a continuous melt, molten 1,6-hexanediamine was added through the liquid metering port, and the material reacted at 185℃ for 90 s. The material then passed sequentially through two vacuum devolatilization zones with vacuum gauge pressures of -0.07 MPa and -0.09 MPa, respectively, for a total devolatilization time of 60 s. After extrusion, water cooling, and pelleting, the material was washed with a 50% ethanol aqueous solution (twice the pellet weight) at 60℃ for 30 min with stirring, repeated three times; then washed again with purified water (twice the pellet weight) at 60℃ for 30 min. The washed granules were dried at 80℃ and under a vacuum pressure of -0.08MPa for 8 hours.

[0028] Take 2.000 g of dried granules, add 80 mL of xylene, and dissolve at 130 °C. Add 40 mL of glacial acetic acid to form a titration medium, and perform potentiometric titration with a 0.0100 mol / L perchloric acid-glacial acetic acid standard solution. Use the same mass of unaminated maleic anhydride-grafted polypropylene as a blank to obtain the total approximate primary amine value. Take another 5.000 g of dried granules, extract with anhydrous ethanol using a Soxhlet extract for 6 h, and determine the free 1,6-hexanediamine in the extract using phthalaldehyde derivatization high performance liquid chromatography, and convert it to a primary amine value.

[0029] The total accessible primary amine value was 0.084 mmol / g, and the primary amine value converted from extractable free 1,6-hexanediamine was 0.004 mmol / g, with a difference of 0.080 mmol / g. Subsequent feeds were fed with a non-extractable primary amine value of 0.080 mmol / g. Extractable free 1,6-hexanediamine accounted for 4.8% of the total accessible primary amine equivalent. The melt flow rate of the resulting pellets was 24.6 g / 10 min, yielding amino-functionalized polypropylene with residual primary amines.

[0030] Preparation Example 2: Stepwise coupling polypropylene masterbatch with a primary amine equivalent substitution ratio of 20.0% The fresh maleic anhydride-grafted polypropylene referred to in this preparation example refers to maleic anhydride-grafted polypropylene that did not participate in the amination reaction of Preparation Example 1 and has a reactive anhydride mass fraction of 1.00%.

[0031] 4644.8 g of fresh maleic anhydride-grafted polypropylene and 355.2 g of the amino-functionalized polypropylene obtained in Preparation Example 1 were weighed and dried at 80 °C for 3 h and 2 h, respectively. 6.60 g of 1,6-hexanediamine was weighed and heated to 60 °C. The fresh maleic anhydride-grafted polypropylene provided 473.67 mmol of anhydride equivalent; the amino-functionalized polypropylene provided 28.42 mmol of non-extractable primary amine equivalent; and the 1,6-hexanediamine provided 113.59 mmol of primary amine equivalent. The ratio of total primary amine equivalent to anhydride equivalent was 0.300:1, and the primary amine equivalent provided by the amino-functionalized polypropylene accounted for 20.0% of the total primary amine equivalent.

[0032] The temperatures of each zone of the co-rotating twin-screw extruder were set sequentially to 180℃, 190℃, 195℃, 200℃, and 195℃, with the screw speed set to 160 r / min. Fresh maleic anhydride-grafted polypropylene was first added through the main feed port. After complete plasticization, amino-functionalized polypropylene was added through the side feed port, and the mixture was reacted at 195℃ for 60 seconds. Subsequently, 1,6-hexanediamine, maintained at 60℃, was added through the downstream liquid metering port, and the reaction continued for 120 seconds. After the reaction, 2.5 g of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and 2.5 g of tris(2,4-di-tert-butylphenyl) phosphite were added, and the mixture was devolatilized under a vacuum gauge pressure of -0.08 MPa for 45 seconds. After being filtered through a 100-mesh melt filter, the mixture was extruded, cooled, and pelletized.

[0033] The melt flow rate of the obtained masterbatch was 5.8 g / 10 min. 1.000 g of the masterbatch was added to 100 mL of xylene and refluxed at 135 °C for 1 h. The mixture was then filtered while hot, and the insoluble residue was dried at 105 °C to constant weight, with a hot xylene insoluble content of 0.7%. After remelting the masterbatch at 200 °C, it could be continuously extruded into strips to obtain a stepwise coupling polypropylene masterbatch with a primary amine equivalent substitution ratio of 20.0%.

[0034] Preparation Example 3: Stepwise Coupling Polypropylene Masterbatch with a Primary Amine Equivalent Substitution Ratio of 25.0% Except for changing the amount of fresh maleic anhydride-grafted polypropylene to 4563.7 g, the amount of amino-functionalized polypropylene obtained in Preparation Example 1 to 436.3 g, and the amount of 1,6-hexanediamine to 6.08 g, the other raw materials, order of addition, and preparation conditions were the same as in Preparation Example 2. Fresh maleic anhydride-grafted polypropylene provided 465.40 mmol of anhydride equivalent; amino-functionalized polypropylene and 1,6-hexanediamine provided 34.90 mmol and 104.64 mmol of primary amine equivalent, respectively. The ratio of total primary amine equivalent to anhydride equivalent was 0.300:1, and the primary amine equivalent provided by amino-functionalized polypropylene accounted for 25.0% of the total primary amine equivalent.

[0035] The obtained masterbatch has a melt flow rate of 4.9 g / 10 min and a hot xylene insoluble content of 0.9%. After remelting at 200 °C, it can be continuously extruded into strips to obtain a stepwise coupling polypropylene masterbatch with a primary amine equivalent substitution ratio of 25.0%.

[0036] Preparation Example 4: Stepwise Coupling Polypropylene Masterbatch with a Primary Amine Equivalent Substitution Ratio of 30.0% Except for changing the amount of fresh maleic anhydride-grafted polypropylene to 4485.4 g, the amount of amino-functionalized polypropylene obtained in Preparation Example 1 to 514.6 g, and the amount of 1,6-hexanediamine to 5.58 g, the other raw materials, addition order, and preparation conditions were the same as in Preparation Example 2. The ratio of total primary amine equivalent to anhydride equivalent was 0.300:1, and the primary amine equivalent provided by the amino-functionalized polypropylene accounted for 30.0% of the total primary amine equivalent.

[0037] The obtained masterbatch has a melt flow rate of 4.1 g / 10 min and a hot xylene insoluble content of 1.2%. After remelting at 200 °C, it can be continuously extruded into strips to obtain a stepwise coupling polypropylene masterbatch with a primary amine equivalent substitution ratio of 30.0%.

[0038] Preparation Example 5: Stepwise coupling polypropylene masterbatch with a primary amine equivalent substitution ratio of 35.0% Except for changing the amount of fresh maleic anhydride-grafted polypropylene to 4409.8 g, the amount of amino-functionalized polypropylene obtained in Preparation Example 1 to 590.2 g, and the amount of 1,6-hexanediamine to 5.09 g, the other raw materials, addition order, and preparation conditions were the same as in Preparation Example 2. The ratio of total primary amine equivalent to anhydride equivalent was 0.300:1, and the primary amine equivalent provided by the amino-functionalized polypropylene accounted for 35.0% of the total primary amine equivalent.

[0039] The obtained masterbatch has a melt flow rate of 3.5 g / 10 min and a hot xylene insoluble content of 1.7%. After remelting at 200 °C, it can be continuously extruded into strips to obtain a stepwise coupling polypropylene masterbatch with a primary amine equivalent substitution ratio of 35.0%.

[0040] Preparation Example 6: Stepwise coupling polypropylene masterbatch with a primary amine equivalent substitution ratio of 15.0% Except for changing the amount of fresh maleic anhydride-grafted polypropylene to 4728.7 g, the amount of amino-functionalized polypropylene obtained in Preparation Example 1 to 271.3 g, and the amount of 1,6-hexanediamine to 7.14 g, the other raw materials, addition order, and preparation conditions were the same as in Preparation Example 2. The ratio of total primary amine equivalent to anhydride equivalent was 0.300:1, and the primary amine equivalent provided by the amino-functionalized polypropylene accounted for 15.0% of the total primary amine equivalent.

[0041] The obtained masterbatch has a melt flow rate of 6.7 g / 10 min and a hot xylene insoluble content of 0.5%. After remelting at 200 °C, it can be continuously extruded into strips to obtain a stepwise coupling polypropylene masterbatch with a primary amine equivalent substitution ratio of 15.0%.

[0042] Preparation Example 7: Stepwise Coupling Polypropylene Masterbatch with a Primary Amine Equivalent Substitution Ratio of 40.0% Except for changing the amount of fresh maleic anhydride-grafted polypropylene to 4336.6 g, the amount of amino-functionalized polypropylene obtained in Preparation Example 1 to 663.4 g, and the amount of 1,6-hexanediamine to 4.63 g, the other raw materials, order of addition, and preparation conditions were the same as in Preparation Example 2. The ratio of total primary amine equivalent to anhydride equivalent was 0.300:1, and the primary amine equivalent provided by the amino-functionalized polypropylene accounted for 40.0% of the total primary amine equivalent.

[0043] The resulting masterbatch had a melt flow rate of 2.7 g / 10 min and a hot xylene insoluble content of 2.6%. The masterbatch could be remelted and continuously extruded at 200 °C, but a small number of discontinuous bumps appeared on the surface of the extruder strip, resulting in a stepwise coupling polypropylene masterbatch with a primary amine equivalent substitution ratio of 40.0%.

[0044] Preparation Example 8: Small molecule diamine coupled polypropylene 4563.7 g of fresh maleic anhydride-grafted polypropylene was weighed and dried at 80 °C for 3 h. 6.08 g of 1,6-hexanediamine was weighed and heated to 60 °C. Using the co-rotating twin-screw extruder used in Preparation Example 2, the fresh maleic anhydride-grafted polypropylene was first fully plasticized, and then all of the 1,6-hexanediamine was added. The reaction was carried out at 195 °C for 120 s. The amino-functionalized polypropylene obtained in Preparation Example 1 was not added in this preparation example. Subsequently, 2.5 g of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and 2.5 g of tris(2,4-di-tert-butylphenyl) phosphite were added, and the mixture was devolatilized under a vacuum gauge pressure of -0.08 MPa for 45 s. After being filtered through a 100-mesh melt filter, the mixture was extruded, cooled, and pelletized.

[0045] The resulting material had a melt flow rate of 8.6 g / 10 min and a thermal xylene insoluble content of 0.7%, yielding small-molecule diamine-coupled polypropylene. The primary amine equivalent provided by 1,6-hexanediamine in Preparation Example 8 was consistent with the primary amine equivalent provided by 1,6-hexanediamine in Preparation Example 3.

[0046] Preparation Example 9: One-time co-feed polypropylene material 4563.7 g of fresh maleic anhydride-grafted polypropylene and 436.3 g of the amino-functionalized polypropylene obtained in Preparation Example 1 were weighed and dried at 80 °C for 3 h and 2 h, respectively, and then mixed for 5 min. 6.08 g of 1,6-hexanediamine was weighed and heated to 60 °C. The types and amounts of each raw material, the ratio of total primary amine equivalent to acid anhydride equivalent, and the primary amine equivalent substitution ratio were the same as in Preparation Example 3.

[0047] The mixture of two types of polypropylene was fed into the co-rotating twin-screw extruder used in Preparation Example 2 through the main feed port in one go. All of the 1,6-hexanediamine was added at the first liquid metering port when the material began to form a continuous melt. There was no separate side feeding for the amino-functionalized polypropylene and no 60-second pre-reaction stage. After the material reacted at 195°C for 180 seconds, 2.5 g of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and 2.5 g of tris(2,4-di-tert-butylphenyl) phosphite were added. The mixture was devolatilized for 45 seconds under a vacuum gauge pressure of -0.08 MPa. After being filtered through a 100-mesh melt filter, the mixture was extruded, cooled, and pelletized.

[0048] The resulting material had a melt flow rate of 2.9 g / 10 min and a thermal xylene insoluble content of 3.4%. The material could be remelted, but dispersed gel bumps were present in the extruder, resulting in a single-feed polypropylene material.

[0049] Example 1: Step (1): Select a propylene-ethylene-butene terpolymer with a melting peak temperature of 95℃ and a melt flow rate of 7.0 g / 10 min, and a propylene-based elastomer with a melting peak temperature of 72℃ and a melt flow rate of 8.0 g / 10 min. Weigh 550 g of the stepwise coupling polypropylene masterbatch from Preparation Example 2, 3560 g of the propylene-ethylene-butene terpolymer, and 890 g of the propylene-based elastomer. Add 4.0 g of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], 4.0 g of tris(2,4-di-tert-butylphenyl) phosphite, 4.0 g of erucamide, and 9.0 g of film-grade anti-sticking silica with a median particle size of 3.5 μm. Mix at 60 r / min for 5 min to obtain the heat-sealing layer material. The stepwise coupling polypropylene masterbatch accounts for 11.0% of the total mass of the heat-sealing resin, and the mass ratio of the terpolymer to the propylene-based elastomer is 80:20.

[0050] Step (2): Weigh 27600g of film-grade homopolymer polypropylene with a melt flow rate of 3.1g / 10min and a melting peak temperature of 162℃ and 2400g of film-grade random copolymer polypropylene with a melt flow rate of 6.0g / 10min and a melting peak temperature of 145℃, mix for 5min to obtain the core layer material.

[0051] Step (3): Film is produced using a double-layer co-extrusion casting machine. The temperatures of each zone of the heat-sealing extruder are set to 185℃, 195℃, 205℃, and 210℃, and the screw speed is set to 70 r / min. The temperatures of each zone of the core extruder are set to 200℃, 215℃, 225℃, and 230℃, and the screw speed is set to 90 r / min. The distributor temperature is set to 220℃, the die temperature to 225℃, the cooling roller temperature to 30℃, and the casting line speed to 16 m / min. The heat-sealing layer thickness is set to 5 μm and the core layer thickness to 30 μm by adjusting the metering pump speeds of the two extruders. After winding, a polypropylene packaging film with a total thickness of 35 μm is obtained.

[0052] Example 2: Except for replacing the stepwise coupling polypropylene masterbatch in Preparation Example 2 with 550g of the masterbatch prepared in Preparation Example 3, the other raw materials, amounts and preparation conditions were the same as in Example 1, and a polypropylene packaging film was obtained.

[0053] Example 3: Except for replacing the stepwise coupling polypropylene masterbatch in Preparation Example 2 with 550g of the masterbatch prepared in Preparation Example 4, the other raw materials, amounts, and preparation conditions were the same as in Example 1, and a polypropylene packaging film was obtained.

[0054] Example 4: Except for replacing the stepwise coupling polypropylene masterbatch in Preparation Example 2 with 550g of the masterbatch prepared in Preparation Example 5, the other raw materials, amounts and preparation conditions were the same as in Example 1, and a polypropylene packaging film was obtained.

[0055] Comparative Example 1: Linear Polypropylene Baseline The stepwise coupling polypropylene masterbatch prepared in Example 3 was replaced with linear homopolymer polypropylene (550g) with a melt flow rate of 4.8g / 10min and a melt peak temperature of 161℃. Except for the addition of 0.28g pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate] and 0.28g tris(2,4-di-tert-butylphenyl) phosphite to the heat-sealing layer, the other raw materials, amounts, and preparation conditions were the same as in Example 2, resulting in a polypropylene packaging film. After the addition, the antioxidant mass of the heat-sealing layer was consistent with that in Example 2, and the resin ratio remained unchanged.

[0056] Comparative Example 2: Physical mixtures composed of the same materials Except for replacing 550g of the stepwise coupling polypropylene masterbatch from Example 3 with 502.1g of the small-molecule diamine-coupled polypropylene obtained in Preparation Example 8 and 47.9g of the amino-functionalized polypropylene obtained in Preparation Example 1 as a physical premix, the remaining raw materials, amounts, and preparation conditions were the same as in Example 2. The two materials were mixed only during the preparation of the heat-sealing layer material and did not react sequentially in the independent masterbatch preparation stage, resulting in a polypropylene packaging film. The mass fraction of amino-functionalized polypropylene in this premix was consistent with the corresponding mass fraction in the masterbatch of Preparation Example 3.

[0057] Comparative Example 3: Single co-feeding of the same reactants Except for replacing 550g of the stepwise coupling polypropylene masterbatch from Preparation Example 3 with 550g of the one-time co-feed polypropylene material obtained in Preparation Example 9, the other raw materials, amounts, and preparation conditions were the same as in Example 2, resulting in a polypropylene packaging film. Preparation Example 9 and Preparation Example 3 had the same reaction raw materials, amounts of each raw material, and total primary amine / anhydride equivalent ratio; only the independent side-feeding and pre-reaction stages of the amino-functionalized polypropylene were omitted.

[0058] Comparative Example 4: The primary amine equivalent substitution rate was 15.0%. Except for replacing the stepwise coupling polypropylene masterbatch obtained in Example 3 with 550g of the primary amine equivalent of 15.0% obtained in Example 6, the other raw materials, amounts and preparation conditions were the same as in Example 2, and a polypropylene packaging film was obtained.

[0059] Comparative Example 5: The primary amine equivalent substitution rate was 40.0%. Except for replacing the stepwise coupling polypropylene masterbatch obtained in Example 3 with 550g of the primary amine equivalent of 40.0% obtained in Example 7, the other raw materials, amounts and preparation conditions were the same as in Example 2, and a polypropylene packaging film was obtained.

[0060] Performance testing: Each example and comparative example of polypropylene packaging film was prepared in 3 batches independently. The film rolls were tested after being placed at 23±2℃ and 50±5% relative humidity for 48 hours according to GB / T 2918-2018. Unless otherwise specified, 5 parallel samples were set for each batch, and the intra-batch average was calculated first, and then the results in the table were calculated using the intra-batch average of the 3 batches.

[0061] 1. Cooling heat seal strength, heat seal initiation temperature, and stable heat seal range Two heat-sealing layers of polypropylene packaging film were placed opposite each other and heat-sealed under a heat-sealing pressure of 0.20 MPa and a heat-sealing time of 0.50 s. The heat-sealing temperature was increased from 90℃ to 145℃, with test points set at 5℃ intervals. After heat-sealing, the samples were placed at 23±2℃ for 24 hours, and then cut into strips with a width of 15 mm according to QB / T 2358-1998. The strips were then peeled 180° at a clamp separation speed of 300 mm / min, and the average force of the stable peeled section was recorded as the cooling heat-sealing strength.

[0062] The lowest set temperature at which the cooling heat seal strength first reaches 3.0 N / 15 mm is recorded as the heat seal initiation temperature. A continuous stable heat seal range must simultaneously meet the following conditions: the cooling heat seal strength is not less than 80% of the maximum cooling heat seal strength in this group; there is no melt-through or continuous shrinkage at the seal edge; and the combined thickness of the two layers at the thinnest point of the seal edge is not less than 55% of the combined thickness of the unsealed two layers. The heat seal initiation temperature and stable heat seal range were determined based on all test points from 90 to 145℃. Tables 1 and 2 list the results under representative temperatures.

[0063] Table 1. Low-temperature heat-sealing performance of various polypropylene packaging films ; Table 2. High-temperature heat-sealing performance of various polypropylene packaging films ; 2. Heat adhesion strength and edge sealing thickness retention rate The hot-tack strength was determined according to GB / T 34445-2017. The heat-sealing pressure and heat-sealing time were set to 0.20 MPa and 0.50 s, respectively, and the maximum separation force at 110℃ and 140℃ was recorded. For the edge-sealing thickness retention rate, after heat-sealing at 140℃ and cooling for 24 h, five cross-sections were taken at equal intervals along the middle of the edge. The total thickness of the double layer at the thinnest point of each cross-section was measured using an optical microscope. The edge-sealing thickness retention rate at 140℃ was calculated as the ratio of the total thickness of the double layer at the thinnest point to the total thickness of the unsealed double layer. The results are shown in Table 3.

[0064] Table 3. Thermal adhesion strength and edge sealing thickness retention of various polypropylene packaging films ; 3. Haze, coefficient of kinetic friction, and anti-adhesion properties Haze was measured according to GB / T 2410-2008 using an integrating sphere transmission haze meter and a D65 light source. Five samples were taken at equal intervals along the transverse direction for each batch of membranes. The coefficient of dynamic friction was measured according to GB / T 10006-2021. With the heat-sealing layers of two membranes facing each other, a slider with a mass of 200g and a contact size of 63mm × 63mm was used at a moving speed of 100mm / min. Anti-adhesion was measured according to GB / T39935-2021, and the separation force was reported for membrane samples with the heat-sealing layers facing each other. The results are shown in Table 4.

[0065] Table 4 Optical and surface properties of various polypropylene packaging films ; 4. Processing stability Each film sample was continuously extruded for 60 minutes at the same extrusion temperature, screw speed, and casting speed. The die head pressure and main engine torque of the heat-sealing extruder were recorded every 30 seconds, and the coefficient of variation was calculated. After continuous film formation, the 100-mesh metal filter of the heat-sealing extruder was removed, and the soluble polymer was washed away with xylene at 135℃. The filter was then dried at 105℃ to constant weight, and the residual amount of the filter was calculated based on the amount of heat-sealing material passing through. Additionally, 5m³ of film from each batch was taken... 2 The number of gels and crystal points with a maximum size of not less than 0.20 mm was counted under transmitted light conditions. The results are shown in Table 5.

[0066] Table 5 Processing stability of various polypropylene packaging films ; Results analysis: The difference between Example 2 and Comparative Example 1 is that Example 2 added a stepwise coupling polypropylene masterbatch with a primary amine equivalent substitution ratio of 25.0%, while Comparative Example 1 added the same mass of linear polypropylene. The heat-sealing initiation temperatures of Example 2 and Comparative Example 1 were 95°C and 100°C, respectively. The heat-sealing strengths at 140°C were 7.1 N / 15 mm and 4.7 N / 15 mm, respectively, and the edge-sealing thickness retention rates at 140°C were 73% and 51%, respectively. This indicates that the stepwise coupling polypropylene masterbatch can improve high-temperature sealing cohesion and edge-sealing thickness retention while maintaining low-temperature sealing capability.

[0067] The difference between Example 2 and Comparative Example 2 is that in Example 2, the masterbatch was prepared by first adding amino-functionalized polypropylene and then adding 1,6-hexanediamine, while in Comparative Example 2, physical mixing was only performed during the heat-sealing layer preparation stage. The 140°C cooling heat-sealing strength of Example 2 and Comparative Example 2 were 7.1 N / 15 mm and 5.4 N / 15 mm, respectively; the 140°C hot tack strength were 3.4 N / 15 mm and 2.3 N / 15 mm, respectively; and the filter screen residue was 15 mg / kg and 32 mg / kg, respectively. This indicates that the masterbatch formed by the stepwise reaction has good high-temperature sealing retention and filtration processability.

[0068] Example 2 and Comparative Example 3 used the same reactants and amounts, with a total primary amine equivalent to anhydride equivalent ratio of 0.300:1 and a primary amine equivalent substitution rate of 25.0% in both examples. The only difference was the feeding sequence during masterbatch preparation. The head pressure variation coefficients for Example 2 and Comparative Example 3 were 2.5% and 6.7%, respectively, and the number of gels and crystal points were 1.8 per m. 2 and 8.7 per m 2 The heat-sealing strengths at 140℃ were 7.1 N / 15 mm and 4.9 N / 15 mm, respectively, indicating that independent side feeding and the pre-reaction stage are beneficial to reducing local gelation and uneven dispersion.

[0069] The primary amine equivalent substitution ratios in Example 1 and Comparative Example 4 were 20.0% and 15.0%, respectively. Both had a heat-sealing initiation temperature of 100°C, and their heat-sealing strengths at 140°C were 6.5 N / 15 mm and 5.4 N / 15 mm, respectively. The upper limits of their stable heat-sealing ranges were 140°C and 135°C, respectively. This indicates that increasing the primary amine equivalent substitution ratio from 15.0% to 20.0% improved the high-temperature end sealing retention capability without increasing the low-temperature initiation temperature.

[0070] The primary amine equivalent substitution ratios in Example 4 and Comparative Example 5 were 35.0% and 40.0%, respectively. When the ratio was increased to 40.0%, the heat-sealing strength at 140°C increased from 7.9 N / 15 mm to 8.1 N / 15 mm, but the heat-sealing initiation temperature increased from 100°C to 105°C, the anti-adhesion separation force increased from 0.73 N / 100 mm to 1.26 N / 100 mm, and the number of gels and crystal points increased from 3.1 per m. 2 Increased to 5.9 per m 2 This indicates that an excessively high primary amine equivalent substitution ratio is not conducive to balancing low-temperature sealing, film surface condition, and continuous processing stability.

[0071] In Examples 1 to 4, when the primary amine equivalent substitution ratio increased from 20.0% to 35.0%, the 140°C cooling heat seal strength increased from 6.5 N / 15 mm to 7.9 N / 15 mm, the 140°C hot tack strength increased from 3.0 N / 15 mm to 3.9 N / 15 mm, and the edge sealing thickness retention rate increased from 67% to 80%. Simultaneously, haze, dynamic friction coefficient, and extrusion fluctuations gradually increased. A primary amine equivalent substitution ratio of 25.0% resulted in a lower heat seal initiation temperature, while ratios of 30.0% to 35.0% resulted in higher high-temperature end sealing strength and thickness retention rate.

[0072] The aforementioned changes may be related to the fact that the inter-chain linkages formed by stepwise coupling of polypropylene reduce heat seal layer loss during high-temperature heat sealing. Excessive primary amine equivalent substitution or elimination of sequential feeding can lead to increased localized multi-chain linkages and gelation, potentially limiting low-temperature interdiffusion of chain segments and increasing extrusion fluctuations.

Claims

1. A method for preparing a polypropylene packaging film, characterized in that, Includes the following steps: S1. After the maleic anhydride-grafted polypropylene is melt-plasticized, molten 1,6-hexanediamine is added. After the reaction, the mixture is devolatilized, extruded, and pelletized. The resulting pellets are washed and dried. The total accessible primary amine value and the primary amine value converted from extractable free 1,6-hexanediamine are determined separately. The difference between the two is taken as the non-extractable primary amine value of the residual primary amine type aminofunctionalized polypropylene. S2. The maleic anhydride-grafted polypropylene that did not participate in the reaction in step S1 is melted and plasticized. After forming a continuous melt, the residual primary amine-type amino-functionalized polypropylene is added through side feeding. The reaction is carried out for 30-90 seconds. Then, molten 1,6-hexanediamine is added through the downstream liquid metering port, and the reaction is continued for 60-180 seconds. In this process, based on the anhydride equivalent of the maleic anhydride-grafted polypropylene that did not participate in step S1, the ratio of the total primary amine equivalent to the anhydride equivalent in the feed is 0.300:

1. The non-extractable primary amine equivalent provided by the residual primary amine-type amino-functionalized polypropylene accounts for 20.0% to 35.0% of the total primary amine equivalent in the feed, and the remaining primary amine equivalent is provided by 1,6-hexanediamine. After the reaction, an antioxidant is added, and vacuum devolatilization and melt filtration are performed. The mixture is then extruded, cooled, and pelletized to obtain stepwise coupling polypropylene masterbatch. S3. Prepare the heat-sealing layer material and the core layer material; based on the total mass of the heat-sealing layer resin, the heat-sealing layer resin consists of 11.0% stepwise coupling polypropylene masterbatch and 89.0% of the remaining resin, wherein the remaining resin consists of a propylene-ethylene-butene terpolymer and a propylene-based elastomer in a mass ratio of 80:20; the core layer material comprises film-grade homopolymer polypropylene and film-grade random copolymer polypropylene. S4. The heat-sealing layer material and the core layer material are co-extruded and cast in a double layer to stack the heat-sealing layer and the core layer, thereby obtaining a polypropylene packaging film.

2. The preparation method according to claim 1, characterized in that, In step S1, the reactive anhydride mass fraction of the maleic anhydride-grafted polypropylene is 1.00%, the melt mass flow rate is 35.0 g / 10 min, the molar ratio of 1,6-hexanediamine molecules to anhydride is 1.25:1, the reaction temperature is 185℃, and the reaction time is 90 s. After devolatilization and pelleting, it is washed three times with a 50% ethanol aqueous solution at 60℃, then washed once with purified water, and dried for 8 h at 80℃ and a vacuum gauge pressure of -0.08 MPa.

3. The preparation method according to claim 1, characterized in that, The total accessible primary amine value of the residual primary amine-type aminofunctionalized polypropylene obtained in step S1 was 0.084 mmol / g, the primary amine value converted from extractable free 1,6-hexanediamine was 0.004 mmol / g, the non-extractable primary amine value was 0.080 mmol / g, and the extractable free 1,6-hexanediamine accounted for 4.8% of the total accessible primary amine equivalent.

4. The preparation method according to claim 1, characterized in that, In step S2, the non-extractable primary amine equivalent provided by the residual primary amine type amino-functionalized polypropylene accounts for 25.0% of the total primary amine equivalent in the feed. After adding the residual primary amine type amino-functionalized polypropylene, the reaction is carried out at 195°C for 60s, and after adding 1,6-hexanediamine, the reaction continues for 120s. The vacuum gauge pressure for vacuum devolatilization is -0.08MPa, the devolatilization time is 45s, and the melt filtration uses a 100-mesh filter.

5. The preparation method according to claim 1, characterized in that, The melt mass flow rate of the fractional coupling polypropylene masterbatch obtained in step S2 at 230℃ and 2.16kg is 3.5 to 5.8 g / 10 min, and the hot xylene insoluble content is 0.7% to 1.7%.

6. The preparation method according to claim 1, characterized in that, In step S3, for every 100 parts by weight of heat-sealing resin, 0.08 parts by weight of pentaerythritol tetrakis[β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], 0.08 parts by weight of tris(2,4-di-tert-butylphenyl) phosphite, 0.08 parts by weight of erucamide and 0.18 parts by weight of film-grade anti-sticking silica are added, wherein the median particle size of the film-grade anti-sticking silica is 3.5 μm; based on the total mass of the core layer resin, the core layer material consists of 92.0% film-grade homopolymer polypropylene and 8.0% film-grade random copolymer polypropylene.

7. The preparation method according to claim 1, characterized in that, In step S4, the extrusion temperature of the heat-sealing layer is 180-220°C, the extrusion temperature of the core layer is 200-240°C, the temperature of the distributor and die head is 200-230°C, and the temperature of the cooling roller is 20-40°C.

8. The preparation method according to claim 1, characterized in that, The total thickness of the resulting polypropylene packaging film is 25–60 μm, and the thickness of the heat-sealing layer is 3–8 μm.

9. A polypropylene packaging film, characterized in that, The polypropylene packaging film is prepared by the preparation method according to any one of claims 1 to 8.

10. The polypropylene packaging film according to claim 9, characterized in that, The total thickness of the polypropylene packaging film is 35 μm, of which the thickness of the heat-sealing layer is 5 μm and the thickness of the core layer is 30 μm.