Butter packaging composite film and preparation method

Abstract

The invention relates to the technical field of butter packaging materials and discloses a butter packaging composite film and a preparation method thereof. The method comprises the following steps: synthesizing a vinylmethylsiloxane sodium sulfonate monomer; bridging an inorganic two-dimensional nano-platelet hydrotalcite and an organic polypropylene matrix through the vinylmethylsiloxane sodium sulfonate monomer to obtain a bridged composite, and subjecting the bridged composite to a melt granulation process to obtain a low-temperature resistant and oxygen-barrier polypropylene masterbatch; setting a film layer structure, a formula and a dosage of each film layer of the polypropylene composite film, using the low-temperature resistant and oxygen-barrier polypropylene masterbatch as a raw material for a low-temperature toughening and barrier functional layer of the polypropylene composite film, and adopting a co-extrusion blow molding film-forming process to obtain the butter packaging composite film. The film product can meet the low-temperature toughness requirements of frozen butter packaging and also has excellent oxygen barrier properties, thus having practical application value.

Description

Technical Field

[0001] The invention relates to the technical field of butter packaging materials, in particular to a butter packaging composite film and a preparation method thereof. Background Art

[0002] The current market structure of butter packaging materials is as follows: plastic packaging (accounting for about 50%), whose main materials are polypropylene composite film (dominant in the mid- and low-end markets) and polyethylene composite film (mostly used for short-term storage); aluminum foil packaging (accounting for about 25%), which is the core material of the high-end market; others are paper / degradable material packaging (accounting for about 20%), which has outstanding environmental properties.

[0003] Among them, polypropylene composite film presents an intermediate advantage in butter packaging. Compared with aluminum foil packaging, it has lower cost and better flexibility. Compared with polyethylene and paper packaging, it has stronger oxygen barrier properties. In addition, polypropylene is a food-grade plastic, non-toxic, odorless, and chemically stable at low temperatures. It will not migrate harmful substances into butter. Its molecular structure is stable and its oil resistance is better than polyethylene. It can reduce the penetration of fat in butter into the packaging. Therefore, polypropylene composite film is the preferred solution for balancing performance and cost.

[0004] Practice has shown that low-temperature storage is key to maintaining butter quality. Refrigerated storage is only suitable for short-term preservation, while frozen storage can significantly extend the quality of butter. However, polypropylene's glass transition temperature (Tg) is approximately -10°C to 0°C, so it exhibits low-temperature brittleness in a frozen environment (below -18°C), deteriorating its mechanical properties. Molecular chain shrinkage in a frozen environment can also slightly reduce air permeability. However, due to polypropylene's inherently poor oxygen barrier properties, it still cannot meet the technical requirements for oxygen barrier properties for butter packaging materials. Summary of the Invention

[0005] The present invention obtains a butter packaging composite film product by grafting and modifying polypropylene and designing a composite structure. The film product can meet the low-temperature toughness requirements in frozen butter packaging and also has excellent oxygen barrier performance.

[0006] A method for preparing a composite film for butter packaging comprises the following steps:

[0007] Step 1: Synthesize vinylmethylsiloxane sodium sulfonate monomer;

[0008] Step 2: Using vinylmethylsiloxane sodium sulfonate monomer as a bridging agent for compounding inorganic two-dimensional nano-sheet hydrotalcite with an organic polypropylene matrix to prepare a low-temperature resistant oxygen-barrier polypropylene masterbatch, the bridging method is as follows: the monomer undergoes ion exchange with the anions between the hydrotalcite interlayers through sulfonate ions to obtain vinylmethylsiloxane functionalized hydrotalcite, the vinyl groups on the surface of the obtained hydrotalcite undergo free radical graft copolymerization with the tertiary carbon position of the polypropylene main chain in a molten state and under the action of an initiator to obtain a bridging composite, and the obtained bridging composite is subjected to a melt granulation process to obtain the masterbatch;

[0009] Step 3: Set the film layer structure, formula and dosage of the polypropylene composite film, use low-temperature resistant and oxygen-barrier polypropylene masterbatch as the raw material for the low-temperature toughening and barrier functional layer of the composite film, and adopt a co-extrusion blow molding film forming process to prepare a butter packaging composite film.

[0010] Preferably, the preparation method of the vinylmethylsiloxane sodium sulfonate monomer is:

[0011] Under the action of a photoinitiator, 1 molar equivalent of the alkenyl functional group of the vinyl-terminated methylsiloxane monomer and 0.91-0.99 molar equivalents of the thiol functional group of 4-aminothiophenol undergo a thiol-ene click reaction under the action of ultraviolet light to generate a vinylmethylsiloxane amino monomer;

[0012] A vinylmethylsiloxane sodium sulfonate monomer is generated by a ring-opening reaction between 1 molar equivalent of vinylmethylsiloxane amino monomer and 1.01-1.09 molar equivalent of 1,3-propane sultone, followed by a neutralization reaction under the action of sodium hydroxide.

[0013] Preferably, the vinyl-terminated methylsiloxane monomer is one of 1,3-divinyltetramethyldisiloxane, 1,5-divinylhexamethyltrisiloxane, 1,7-divinyloctamethyltetrasiloxane and 1,9-divinyldecamethylpentasiloxane.

[0014] Preferably, the preparation method of the 1,9-divinyldecamethylpentasiloxane is: 1,9-divinyldecamethylpentasiloxane is generated by condensing 1 molar equivalent of silyl chloride groups of 1,5-dichlorohexamethyltrisiloxane with 2.01-2.09 molar equivalents of silyl ethoxy groups of vinyldimethylethoxysilane.

[0015] Preferably, the photoinitiator is one of 2,2-dimethoxy-2-phenylacetophenone, 2-hydroxy-2-methyl-1-phenylpropanone, and 2,2-diethoxyacetophenone.

[0016] Preferably, the process parameters of the screw extruder corresponding to the low-temperature toughening and barrier functional layer in the co-extrusion blow molding process of step three are set as follows: the temperatures of zones 1-3 are 140-160°C, 170-190°C, and 200-210°C, respectively, and the flow channel temperature is 190-200°C.

[0017] A composite film for butter packaging is prepared according to the above method. The product structure of the composite film for butter packaging comprises the following layers arranged in sequence:

[0018] Support layer: The formula is 100wt% polypropylene resin, the amount is 20-30 parts by weight;

[0019] First adhesive layer: The formula is 100wt% maleic anhydride grafted polypropylene resin, the amount is 3-10 parts by weight;

[0020] Low-temperature toughening and barrier functional layer: The formula is 100wt% low-temperature resistant oxygen barrier polypropylene masterbatch, the dosage is 30-50 parts by weight;

[0021] Second adhesive layer: its formula and dosage are the same as those of the first adhesive layer;

[0022] Heat seal layer: The formula is 100wt% metallocene polypropylene resin, the dosage is 20-30 parts by weight.

[0023] Preferably, the formula of the low-temperature resistant and oxygen-barrier polypropylene masterbatch is: 85-95 wt% polypropylene, 3-10 wt% hydrotalcite and 1-5 wt% vinylmethylsiloxane sodium sulfonate monomer.

[0024] Preferably, the hydrotalcite is one of MgAl-LDH, ZnAl-LDH and CaAl-LDH.

[0025] Preferably, the thickness of the butter packaging composite film is 50-150 μm. Beneficial effects

[0026] The present invention designs and synthesizes a vinylmethylsiloxane sodium sulfonate monomer, and uses the vinylmethylsiloxane sodium sulfonate monomer to bridge an inorganic two-dimensional nano-sheet hydrotalcite and an organic polypropylene matrix to prepare a bridged composite, and the bridged composite is subjected to a melt granulation process to prepare a low-temperature resistant oxygen-barrier polypropylene masterbatch;

[0027] The film layer structure, formula and dosage of the polypropylene composite film are set, and the low-temperature resistant and oxygen-barrier polypropylene masterbatch is used as the raw material for the low-temperature toughening and barrier functional layer of the polypropylene composite film. The co-extrusion blow molding film forming process is adopted to prepare the butter packaging composite film. Compared with the conventional polypropylene composite film without the addition of the low-temperature resistant and oxygen-barrier polypropylene masterbatch, it still shows excellent mechanical properties at a low temperature of -18°C, can meet the low-temperature toughness requirements in the frozen butter packaging, and has excellent comprehensive performance and practical application value. DETAILED DESCRIPTION

[0028] The invention is based on the principle of molecular design and uses 4-aminothiophenol as the basic connecting framework. First, a vinyl group containing a siloxane chain is introduced through a thiol-alkenyl click reaction with a vinyl-terminated methylsiloxane monomer, and then a sulfonate ion is introduced through an amino-sultone ring-opening reaction with 1,3-propane sultone to obtain a vinylmethylsiloxane sodium sulfonate monomer.

[0029] Through the bridging effect of vinylmethylsiloxane sodium sulfonate monomer, the effective compounding of inorganic two-dimensional nano-sheet hydrotalcite and organic polypropylene matrix is ​​achieved to produce a new polypropylene masterbatch. This masterbatch contains both silicon-oxygen chains with excellent low-temperature toughness and two-dimensional sheet-like nanomaterial hydrotalcite with excellent barrier properties. Therefore, this masterbatch will exhibit low-temperature resistance and oxygen barrier properties.

[0030] Synthesize vinylmethylsiloxane sodium sulfonate monomer, its chemical structure formula is:

[0031] ;

[0032] The synthesis process of vinylmethylsiloxane sodium sulfonate monomer is as follows:

[0033] Step 1: In the presence of a photoinitiator, a vinyl-terminated methylsiloxane monomer is subjected to a click reaction with 0.95 molar equivalents of the thiol group of 4-aminothiophenol under the action of ultraviolet light to generate a vinylmethylsiloxane amino monomer.

[0034] The photoinitiator is one of 2,2-dimethoxy-2-phenylacetophenone, 2-hydroxy-2-methyl-1-phenylacetone, and 2,2-diethoxyacetophenone; 2,2-dimethoxy-2-phenylacetophenone is selected in this experimental example;

[0035] Step 2: 1 molar equivalent of vinylmethylsiloxane amino monomer and 1.03 molar equivalent of 1,3-propane sultone undergo a ring-opening reaction, followed by a neutralization reaction under the action of sodium hydroxide to generate a vinylmethylsiloxane sodium sulfonate monomer;

[0036] Among them, the vinyl-terminated methylsiloxane monomer can be selected from the following raw materials:

[0037] 1,3-Divinyltetramethyldisiloxane: ;

[0038] 1,5-Divinylhexamethyltrisiloxane: ;

[0039] 1,7-Divinyloctamethyltetrasiloxane: ;

[0040] 1,9-Divinyldecamethylpentasiloxane: ;

[0041] The present invention selects representative 1,3-divinyltetramethyldisiloxane (n=1) and 1,9-divinyldecamethylpentasiloxane (n=4) for research and development experiments:

[0042] R&D Experiment 1: Using the method for synthesizing sodium vinylmethylsiloxane sulfonate monomer, when 1,3-divinyltetramethyldisiloxane is used as the vinyl-terminated methylsiloxane monomer, sodium vinyltetramethyldisiloxane sulfonate monomer is synthesized, and its chemical structure is:

[0043] ;

[0044] The specific preparation steps of vinyl tetramethyl disiloxane sodium sulfonate monomer are as follows:

[0045] Under nitrogen protection, 1.9 g of 1,3-divinyltetramethyldisiloxane (CAS No. 2627-95-4), 0.7 g of 2,2-dimethoxy-2-phenylacetophenone, and 30 mL of anhydrous tetrahydrofuran were added to a three-necked flask and stirred at room temperature until completely dissolved. Then, 10 mL of an anhydrous tetrahydrofuran solution containing 1.2 g of 4-aminothiophenol was added dropwise to the three-necked flask under ultraviolet light irradiation (360 nm, 10 cm). After the addition was complete, stirring and reaction were continued under ultraviolet light irradiation for 60 minutes. The solvent was removed by rotary evaporation, and the mixture was washed with deionized water and dried in vacuo to obtain a vinyltetramethyldisiloxane amino monomer.

[0046] 0.6 g of 1,3-propane sultone and 20 mL of anhydrous tetrahydrofuran were added to a three-necked flask and stirred at room temperature until completely dissolved. Then, 20 mL of anhydrous tetrahydrofuran solution containing 1.5 g of vinyltetramethyldisiloxane amino monomer was added dropwise to the three-necked flask, and the mixture was stirred at room temperature for 30 min. The mixture was heated to 60° C. and stirred for 48 h. The mixture was cooled to room temperature, and 10 mL of deionized water containing 0.4 g of sodium hydroxide was added. The mixture was heated to 50° C. and stirred for 6 h. The mixture was cooled to room temperature, filtered, washed with deionized water, and dried in vacuo to obtain a vinyltetramethyldisiloxane sodium sulfonate monomer.

[0047] The H NMR spectrum of the sodium vinyl tetramethyl disiloxane sulfonate monomer is characterized as follows: 1 H NMR (DMSO-d6, 400MHz) δ: 0.04 (s, 6H), 0.09 (s, 6H), 0.98-1.02 (t, 2H), 1.83-1.91 (m, 2H), 2.78-2.82 (t, 2H), 2.89-2.92 (t, 2H), 3.25 -3.30 (m, 2H), 5.71-5.84 (dd, 2H), 5.99-6.08 (t, 1H), 6.21-6.24 (t, 1H), 6.43-6.45 (d, 2H), 7.17-7.19 (d, 2H).

[0048] R&D Experiment 2: Using the method for synthesizing sodium vinylmethylsiloxane sulfonate monomer, when 1,9-divinyldecamethylpentasiloxane is used as the vinyl-terminated methylsiloxane monomer, sodium vinyldecamethylpentasiloxane sulfonate monomer is synthesized, and its chemical structure is:

[0049] ;

[0050] The specific preparation steps of the vinyl decamethyl pentasiloxane sodium sulfonate monomer are as follows:

[0051] Prepare a vinyl decamethyl pentasiloxane amino monomer. The specific experimental steps are different from those for the vinyl methyl siloxane amino monomer only in that 4.1 g of 1,9-divinyl decamethyl pentasiloxane (prepared as follows) is used instead of 1.9 g of 1,3-divinyltetramethyldisiloxane.

[0052] The specific experimental steps for preparing sodium vinyl decamethyl pentasiloxane sulfonate monomer differ from those for sodium vinyl tetramethyl disiloxane sulfonate monomer only in that 2.7 g of vinyl decamethyl pentasiloxane amino monomer is used to replace 1.5 g of vinyl methyl siloxane amino monomer;

[0053] The H NMR spectrum of the sodium vinyl decamethyl pentasiloxane sulfonate monomer is characterized as follows: 1H NMR (DMSO-d6, 400MHz) δ: 0.01 (s, 6H), 0.05 (s, 6H), 0.07 (s, 6H), 0.12 (s, 6H), 0.16 (s, 6H), 1.04-1.07 (t, 2H), 1.82-1.89 (m, 2H), 2.82-2.85 (t, 2H), 2.92 -2.96(t, 2H), 3.28-3.32(m, 2H), 5.72-5.81(dd, 2H), 6.02-6.08(t, 1H), 6.31-6.34(t, 1H), 6.64-6.66(d, 2H), 7.30-7.32(d, 2H);

[0054] The preparation method of 1,9-divinyldecamethylpentasiloxane is as follows: 1 molar equivalent of 1,5-dichlorohexamethyltrisiloxane silicon chloride group (-Si-Cl) and 2.01 molar equivalent of vinyldimethylethoxysilane silicon ethoxy group (-Si-O-CH2CH3) are condensed to generate 1,9-divinyldecamethylpentasiloxane. The specific preparation steps are as follows: 5.5g of 1,5-dichlorohexamethyltrisiloxane, 30mL of anhydrous tetrahydrofuran and 20mL of Deionized water was added to the three-necked flask and stirred at room temperature for 30 minutes. Then, 50 mL of vinyldimethylethoxysilane solution (prepared from 5.2 g of vinyldimethylethoxysilane, 30 mL of anhydrous tetrahydrofuran, and 20 mL of deionized water) and 1.5 mL of glacial acetic acid were added to the three-necked flask in sequence. The temperature was raised to 60°C and stirred for 8 hours. The mixture was cooled to room temperature, the solvent was removed by rotary evaporation, and the mixture was dried in vacuo to obtain 1,9-divinyldecamethylpentasiloxane. Its hydrogen nuclear magnetic resonance spectrum was characterized as follows: 1 H NMR (DMSO-d6, 400MHz) δ: 0.05 (s, 12H), 0.11 (s, 12H), 0.15 (s, 6H), 5.73-5.84 (dd, 4H), 6.02-6.08 (t, 2H). Example 1:

[0055] Preparation of vinylmethylsiloxane-functionalized hydrotalcite I: Hydrotalcite nanosheets are modified using a vinyltetramethyldisiloxane sulfonate monomer. The sulfonate anions of the vinyltetramethyldisiloxane sulfonate monomer first undergo an ion exchange reaction with the interlayer anions of the hydrotalcite nanosheets, and then electrostatically bind with the metal cations of the layer to obtain vinylmethylsiloxane-functionalized hydrotalcite I.

[0056] Among them, the hydrotalcite nanosheets can be selected from one of MgAl-LDH, ZnAl-LDH, and CaAl-LDH; in this embodiment, MgAl-LDH is selected;

[0057] The specific experimental steps for preparing vinylmethylsiloxane functionalized hydrotalcite I are as follows: 8 g of nano-MgAl-LDH powder (flake diameter 1-4 μm), 60 mL of anhydrous tetrahydrofuran and 40 mL of deionized water are added to a three-necked flask, ultrasonicated for 30 min, heated to 60°C and stirred for 2 h, then 10 mL of anhydrous tetrahydrofuran solution containing 2 g of sodium vinyltetramethyldisiloxane sulfonate monomer is added to the three-necked flask, stirred and reacted at 60°C for 8 h, filtered, washed with anhydrous ethanol and deionized water in turn, and vacuum dried to obtain vinylmethylsiloxane functionalized hydrotalcite I. Example 2:

[0058] Preparation of vinylmethylsiloxane-functionalized hydrotalcite II: Hydrotalcite nanosheets are modified with a vinyl decamethyl pentasiloxane sodium sulfonate monomer. The sulfonate anions of the vinyl decamethyl pentasiloxane sodium sulfonate monomer first undergo an ion exchange reaction with the interlayer anions of the hydrotalcite nanosheets, and then electrostatically bind with the metal cations of the layer to obtain vinylmethylsiloxane-functionalized hydrotalcite II.

[0059] Among them, the hydrotalcite nanosheets can be selected from one of MgAl-LDH, ZnAl-LDH, and CaAl-LDH; in this embodiment, MgAl-LDH is selected;

[0060] The specific experimental steps for preparing vinylmethylsiloxane functionalized hydrotalcite II refer to the preparation experiment of vinylmethylsiloxane functionalized hydrotalcite I, with the only difference being that the vinyltetramethyldisiloxane sodium sulfonate monomer is replaced by the vinyldecamethylpentasiloxane sodium sulfonate monomer. Example 3:

[0061] Preparation of low-temperature resistant and oxygen-barrier polypropylene masterbatch I: Free radicals are generated by the polypropylene resin under the action of a peroxide initiator, and the alkenyl functional groups contained in the vinylmethylsiloxane functionalized hydrotalcite I are coupled with the free radicals in the polypropylene resin to achieve graft modification of the polypropylene resin. The resin is then extruded and granulated using a twin-screw extruder to obtain low-temperature resistant and oxygen-barrier polypropylene masterbatch I.

[0062] The peroxide initiator is one of dicumyl peroxide (DCP), tert-butyl hydroperoxide (TBHP), dibenzoyl peroxide (BPO), and tert-butyl perbenzoate (TBPB); in this embodiment, dicumyl peroxide (DCP) is selected;

[0063] The specific experimental steps for preparing low-temperature resistant and oxygen-barrier polypropylene masterbatch I are as follows: 9 g of polypropylene resin (brand SB815MO), 1 g of vinylmethylsiloxane functionalized hydrotalcite I, and 0.5 g of dicumyl peroxide initiator are added to a twin-screw extruder through a feed port, coupled by high-temperature melt mixing, and then melt-melted, extruded, and pelletized in a twin-screw extruder to obtain low-temperature resistant and oxygen-barrier polypropylene masterbatch I;

[0064] Among them, the process parameters of the twin-screw extruder are set as: preheating temperature 180°C, temperatures of zones 1-6 are 180°C, 185°C, 190°C, 195°C, 210°C, and 200°C respectively, rotation speed is 200 r / min, and melting processing time is 10 min. Example 4:

[0065] Preparation of low-temperature resistant and oxygen-barrier polypropylene masterbatch II: Free radicals are generated by the polypropylene resin under the action of a peroxide initiator. The alkenyl functional groups contained in the vinylmethylsiloxane-functionalized hydrotalcite II are coupled with the free radicals in the polypropylene resin to achieve graft modification of the polypropylene resin. The resulting product is then extruded and granulated using a twin-screw extruder to obtain low-temperature resistant and oxygen-barrier polypropylene masterbatch II.

[0066] The peroxide initiator is one of dicumyl peroxide (DCP), tert-butyl hydroperoxide (TBHP), dibenzoyl peroxide (BPO), and tert-butyl perbenzoate (TBPB); in this embodiment, dicumyl peroxide (DCP) is selected;

[0067] The specific experimental steps for preparing low-temperature resistant and oxygen-barrier polypropylene masterbatch II refer to the preparation experiment of low-temperature resistant and oxygen-barrier polypropylene masterbatch I, with the only difference being that vinylmethylsiloxane functionalized hydrotalcite II is used to replace vinylmethylsiloxane functionalized hydrotalcite I. Embodiment 5:

[0068] A composite film for butter packaging, the product structure of which comprises the following layers arranged in sequence:

[0069] Support layer: The formula is 100wt% polypropylene resin (brand SB815MO), the amount is 25 parts by weight;

[0070] First adhesive layer: 100 wt% maleic anhydride grafted polypropylene resin (brand QB510), 5 parts by weight;

[0071] Low-temperature toughening and barrier functional layer: The formula is 100wt% low-temperature resistant oxygen barrier polypropylene masterbatch, the dosage is 40 parts by weight;

[0072] Second adhesive layer: 100 wt% maleic anhydride grafted polypropylene resin, 5 parts by weight;

[0073] Heat seal layer: The formula is 100wt% metallocene polypropylene resin (brand MR30MC2), the dosage is 25 parts by weight;

[0074] Among them, the low-temperature resistant and oxygen-barrier polypropylene masterbatch is the low-temperature resistant and oxygen-barrier polypropylene masterbatch I or the low-temperature resistant and oxygen-barrier polypropylene masterbatch II. Example 6:

[0075] A method for preparing a composite film for butter packaging comprises the following steps: preparing ingredients according to a recipe for the composite film for butter packaging, feeding raw materials for each layer into the hoppers of five screw extruders of a five-layer co-extrusion film blow molding unit, combining molten resin at the die head through a diverter, extruding through a die head, blowing, and pulling (the blow-up ratio is controlled at 2.8), cooling, and winding to prepare a composite film for butter packaging with a thickness of 100 μm;

[0076] The process parameters of the screw extruder corresponding to the support layer and the heat-sealing layer are set as follows: the temperatures of zones 1-3 are 130°C, 150°C, and 180°C, respectively; the flow channel temperature is 175°C; and the speed is 40 r / min.

[0077] The process parameters of the screw extruder corresponding to the bonding layer were set as follows: the temperatures of zones 1-3 were 130°C, 145°C, and 175°C, respectively, the flow channel temperature was 170°C, and the rotation speed was 30 r / min;

[0078] The process parameters of the screw extruder corresponding to the low-temperature toughening and barrier functional layer are set as follows: the temperatures of zones 1-3 are 150°C, 180°C, and 205°C, respectively; the flow channel temperature is 200°C; and the rotation speed is 60 r / min.

[0079] Among them, when the low-temperature resistant and oxygen-barrier polypropylene masterbatch is the low-temperature resistant and oxygen-barrier polypropylene masterbatch I, the prepared film product is recorded as butter packaging composite film I;

[0080] When the low-temperature resistant and oxygen-barrier polypropylene masterbatch is the low-temperature resistant and oxygen-barrier polypropylene masterbatch II, the prepared film product is recorded as butter packaging composite film II.

[0081] Comparative Example:

[0082] Preparation of conventional polypropylene composite film: Compared with the butter packaging composite film I, the only difference is that polypropylene resin (brand SB815MO) is used to replace the low-temperature resistant and oxygen-barrier polypropylene masterbatch I.

[0083] Performance testing:

[0084] 1. Oxygen barrier performance test: The oxygen permeability of the sample is tested in accordance with the standard GB / T 1038.1-2022 "Plastic film and sheeting gas permeability test method Part 1: Differential pressure method";

[0085] 2. Heat sealing performance test: The samples were heat sealed using an HSG-C heat sealer with a sealing area of ​​15 cm × 1 cm, a heat sealing temperature of 140°C, a heat sealing pressure of 0.2 MPa, and a heat sealing time of 2.0 s.

[0086] The heat seal performance of the heat-sealed samples was tested according to QB / T 2358-1998 "Test Method for Heat Seal Strength of Plastic Film Packaging Bags" at a test speed of 300 mm / min and a clamp spacing of 50 mm. The heat seal strength of the samples was recorded.

[0087] 3. Mechanical properties test:

[0088] (1) 25°C room temperature tensile strength test: According to GB / T 1040.1-2018 "Determination of tensile properties of plastics", a 150 mm × 20 mm sample (sampled along the film blowing direction) was subjected to a tensile test at a tensile rate of 50 mm / min at 25°C, and the longitudinal tensile strength of the sample was recorded;

[0089] (2) -18℃ low temperature tensile test: According to GB / T 1040.1-2018 "Determination of tensile properties of plastics", a 150mm×20mm sample (sampled along the film blowing direction) was placed in a -18℃ refrigerator for 48 hours and then subjected to a tensile test at a tensile rate of 50mm / min. The longitudinal tensile strength of the sample was recorded.

[0090] (3) 25℃ normal temperature impact resistance test: According to GB / T 8809-2015 "Plastic film resistance to pendulum impact test method", a 100mm×100mm sample was subjected to an impact test using a type A punch at 25℃, and the pendulum impact energy of the sample was recorded;

[0091] (4) -18℃ low temperature impact resistance test: According to GB / T 8809-2015 "Plastic film anti-pendulum impact test method", a 100mm×100mm sample was placed in a -18℃ refrigerator for 48 hours and then subjected to an impact test using an A-type punch. The anti-pendulum impact energy of the sample was recorded.

[0092] 4. Hygienic performance test: The hygienic performance of the samples was tested in accordance with GB / T 5009.60-2003 "Analytical method for the hygienic standard of polyethylene, polystyrene and polypropylene moldings for food packaging". The physical and chemical indicators of the experimental results were based on GB / T 5009.71-2003 "Analytical method for the hygienic standard of polypropylene resin for food packaging".

[0093] The above experimental results are shown in Tables 1-3.

[0094] Table 1 Performance test results of butter packaging composite film

[0095]

[0096] Table 2 Performance test results of butter packaging composite film II

[0097]

[0098] Table 3 Performance test results of butter packaging composite film

[0099]

[0100] By comprehensively analyzing the above experimental results, the following conclusions can be drawn:

[0101] (1) The oxygen permeability of the butter packaging composite film product prepared by the present invention is less than 1.5cm 3 / (m 2 •24h•0.1MPa), with good oxygen barrier performance;

[0102] (2) The butter packaging composite film product prepared by the present invention using the independently developed low-temperature resistant and oxygen-barrier polypropylene masterbatch has achieved significant improvement in longitudinal tensile strength and resistance to pendulum impact energy under low temperature (-18°C) conditions compared to conventional polypropylene composite films, showing excellent low-temperature resistance.

[0103] Experimental results show that melt-grafting hydrotalcite nanosheets modified with sodium vinyl decamethyl pentasiloxane sulfonate monomer onto polypropylene significantly improves the performance of the resulting butter packaging composite film. This may be due to the molecular structure of sodium vinyl decamethyl pentasiloxane sulfonate monomer containing more silicon-oxygen bonds and longer silicon-oxygen chains, resulting in superior flexibility and, therefore, a more pronounced modification effect on polypropylene.

[0104] (3) The butter packaging composite film product prepared by the present invention also has excellent heat sealing performance, and its sanitary performance meets the requirements of national standards, and has practical application value.

Claims

1. A method for preparing a composite film for butter packaging, characterized in that: The following steps are involved: Step 1: Synthesize sodium vinylmethylsiloxane sulfonate monomer, the general chemical structure of which is: ； Step 2: Using vinylmethylsiloxane sodium sulfonate monomer as a bridging agent for compounding inorganic two-dimensional nano-sheet hydrotalcite with an organic polypropylene matrix to prepare a low-temperature resistant oxygen-barrier polypropylene masterbatch, the bridging method is as follows: the monomer undergoes ion exchange with the anions between the hydrotalcite interlayers through sulfonate ions to obtain vinylmethylsiloxane functionalized hydrotalcite, the vinyl groups on the surface of the obtained hydrotalcite undergo free radical graft copolymerization with the tertiary carbon position of the polypropylene main chain in a molten state and under the action of an initiator to obtain a bridging composite, and the obtained bridging composite is subjected to a melt granulation process to obtain the masterbatch; Step 3: Setting the film layer structure, formulation and dosage of each film layer of the polypropylene composite film, using low-temperature resistant oxygen barrier polypropylene masterbatch as the raw material for the low-temperature toughening and barrier functional layer of the composite film, and adopting a co-extrusion blow molding film forming process to prepare a butter packaging composite film; The product structure of the butter packaging composite film is the following layers arranged in sequence: Support layer: The formula is 100wt% polypropylene resin, the amount is 20-30 parts by weight; First adhesive layer: The formula is 100wt% maleic anhydride grafted polypropylene resin, the amount is 3-10 parts by weight; Low-temperature toughening and barrier functional layer: The formula is 100wt% low-temperature resistant oxygen barrier polypropylene masterbatch, the dosage is 30-50 parts by weight; Second adhesive layer: its formula and dosage are the same as those of the first adhesive layer; Heat seal layer: The formula is 100wt% metallocene polypropylene resin, the dosage is 20-30 parts by weight.

2. The method for preparing a composite film for butter packaging according to claim 1, wherein: The preparation method of the vinylmethylsiloxane sodium sulfonate monomer is: Under the action of a photoinitiator, 1 molar equivalent of the alkenyl functional group of the vinyl-terminated methylsiloxane monomer and 0.91-0.99 molar equivalents of the thiol functional group of 4-aminothiophenol undergo a thiol-ene click reaction under the action of ultraviolet light to generate a vinylmethylsiloxane amino monomer; A vinylmethylsiloxane sodium sulfonate monomer is generated by a ring-opening reaction between 1 molar equivalent of vinylmethylsiloxane amino monomer and 1.01-1.09 molar equivalent of 1,3-propane sultone, followed by a neutralization reaction under the action of sodium hydroxide.

3. The method for preparing a composite film for butter packaging according to claim 2, wherein: The vinyl-terminated methylsiloxane monomer is one of 1,3-divinyltetramethyldisiloxane, 1,5-divinylhexamethyltrisiloxane, 1,7-divinyloctamethyltetrasiloxane and 1,9-divinyldecamethylpentasiloxane.

4. The method for preparing a composite film for butter packaging according to claim 3, characterized in that: The preparation method of the 1,9-divinyldecamethylpentasiloxane is as follows: 1 molar equivalent of silyl chloride groups of 1,5-dichlorohexamethyltrisiloxane and 2.01-2.09 molar equivalents of silyl ethoxy groups of vinyldimethylethoxysilane are condensed to generate 1,9-divinyldecamethylpentasiloxane.

5. The method for preparing a composite film for butter packaging according to claim 2, characterized in that: The photoinitiator is one of 2,2-dimethoxy-2-phenylacetophenone, 2-hydroxy-2-methyl-1-phenylpropanone, and 2,2-diethoxyacetophenone.

6. The method for preparing a composite film for butter packaging according to claim 1, characterized in that: In the co-extrusion blow molding process of step three, the process parameters of the screw extruder corresponding to the low-temperature toughening and barrier functional layer are set to: the temperatures of zones 1-3 are 140-160°C, 170-190°C, and 200-210°C, respectively, and the flow channel temperature is 190-200°C.

7. A composite film for butter packaging prepared according to the method according to any one of claims 1 to 6, characterized in that: The formula of the low-temperature resistant and oxygen-barrier polypropylene masterbatch is: 85-95wt% polypropylene, 3-10wt% hydrotalcite and 1-5wt% vinylmethylsiloxane sodium sulfonate monomer.

8. The butter packaging composite film according to claim 7, characterized in that: The hydrotalcite is one of MgAl-LDH, ZnAl-LDH and CaAl-LDH.

9. The composite film for butter packaging according to claim 7, characterized in that: The thickness of the butter packaging composite film is 50-150 μm.

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