Low temperature dairy plastic packaging film and method of making same

Abstract

The application relates to the technical field of low-temperature dairy product plastic packaging film research and development, and discloses a low-temperature dairy product plastic packaging film and a preparation method thereof, which comprises the following steps: preparing an epoxy-based plasticizing functional monomer or a multi-ester-based plasticizer; 5-20 parts by weight of polypropylene resin and 1-5 parts by weight of the epoxy-based plasticizing functional monomer or the multi-ester-based plasticizer are firstly blended, melted and extruded through a double-screw extruder, then the particles with an average particle size of 1-5 mm are prepared by using a granulator, and finally the low-temperature-resistant polypropylene film with a film thickness of 120-180 mu m is prepared by using a casting machine. The application provides a method for preparing the low-temperature-resistant polypropylene film, and a dairy product plastic packaging film with excellent low-temperature resistance and mechanical properties and meeting safety standards is prepared.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of low-temperature dairy plastic packaging film research and development, in particular to a low-temperature dairy plastic packaging film and a preparation method thereof. BACKGROUND

[0002] Low-temperature dairy products are beverages or foods processed and packaged after fresh milk is pasteurized, and are widely favored by consumers due to their richer nutritional value, more unique taste and fresher ingredients. Low-temperature dairy products require cold storage in a low-temperature environment at each link of transportation, sales, storage, etc. to prevent the inactivation of nutrients, which has higher requirements for their packaging materials. The packaging materials commonly used for dairy products on the market are plastic packaging films made of polyethylene, polypropylene, polystyrene and polyethylene terephthalate. However, traditional plastics are prone to brittleness and poor mechanical properties at low temperatures, which limits their actual use in the packaging field of low-temperature dairy products.

[0003] Through searching the prior art, it is found that the low-temperature resistance of traditional plastics can be significantly improved by modifying the traditional plastics, such as introducing low-temperature resistant additives. Patent No. CN104448572A discloses a low-temperature dairy plastic packaging material, which can improve the impact resistance and low-temperature resistance of the packaging material by introducing non-woven fabric broken material, salt, tetradecane diacid and other substances into polypropylene plastic.

[0004] In addition, by introducing a plasticizer into a traditional polymer, the plasticizer molecules are inserted between the polymer molecular chains, so as to increase the free activity space of the polymer molecules, weaken the force between the molecular chains, increase the movement ability of the polymer molecular chains, and reduce the glass transition temperature, thereby enhancing the plasticity and low-temperature resistance of the polymer. SUMMARY

[0005] The present application provides a low-temperature dairy plastic packaging film and a preparation method thereof, which comprises the following steps: first, synthesizing a novel plasticizing functional monomer and two novel plasticizers, and then using the ester group structure and alkyl chain structure contained therein to modify the low-temperature resistance of polypropylene resin, so as to prepare a low-temperature dairy plastic packaging film with excellent low-temperature resistance and mechanical properties.

[0006] A preparation method of a low-temperature dairy plastic packaging film, comprising the following steps:

[0007] Step 1: preparing an epoxy-based plasticizing functional monomer or a multi-ester group structure type plasticizer;

[0008] The preparation method of the epoxy-based plasticizing functional monomer is: using hexanoic anhydride as an acylating reagent, and potassium carbonate as a catalyst, esterification reaction is carried out between the phenolic hydroxyl functional group of cardanol and the acid anhydride group of hexanoic anhydride to generate cardanol-based hexanoate; under the catalysis of an organic protonic acid, using hydrogen peroxide as an oxidizing agent, epoxidation reaction is carried out on the alkenyl functional group of the cardanol-based hexanoate to generate the epoxy-based plasticizing functional monomer;

[0009] Step two: preparing a low-temperature-resistant polypropylene film: 5-20 parts by weight of polypropylene resin and 1-5 parts by weight of the epoxy-based plasticizing functional monomer or the multi-ester-based structural plasticizer are first blended, melted and extruded through a double-screw extruder, then particles with an average particle size of 1-5 mm are prepared by using a pelletizer, and finally a low-temperature-resistant polypropylene film with a film thickness of 120-180 μm is prepared by using a casting machine.

[0010] Preferably, the temperature of the double-screw extruder is set to 160-180 °C, and the rotation speed is set to 20-50 r / min; the temperature of the casting machine is set to 165-185 °C, and the rotation speed is set to 40-70 r / min.

[0011] Preferably, the multi-ester-based structural plasticizer includes trimellitic acid type plasticizers and / or malic acid type plasticizers.

[0012] Preferably, the preparation method of the trimellitic acid type plasticizer is: under the catalysis of a phase transfer catalyst, esterification reaction is carried out between the trimellitic anhydride and the epoxy-based plasticizing functional monomer to generate cardanol-based hexanoate-trimellitate; using acetic anhydride as an acylating reagent, acetylation reaction is carried out between the hydroxyl functional group of the cardanol-based hexanoate-trimellitate and the acid anhydride group of the acetic anhydride to generate the trimellitic acid type plasticizer.

[0013] Preferably, the preparation method of the malic acid type plasticizer is: under the catalysis of a phase transfer catalyst, esterification reaction is carried out between the carboxyl functional group of DL-malic acid and the epoxy functional group of the epoxy-based plasticizing functional monomer to generate cardanol-based hexanoate-malate; using acetic anhydride as an acylating reagent, acetylation reaction is carried out between the hydroxyl functional group of the cardanol-based hexanoate-malate and the acid anhydride group of the acetic anhydride to generate the malic acid type plasticizer.

[0014] Preferably, the phase transfer catalyst is selected from one of tetrabutylammonium bromide, tetraethylammonium bromide, tetrabutylammonium chloride and trioctylmethylammonium chloride.

[0015] Preferably, the organic protonic acid is selected from formic acid or acetic acid.

[0016] The low-temperature dairy plastic packaging film prepared by the method is prepared by using polypropylene resin as a film raw material, through a blending-extrusion-casting process, using an epoxy-based plasticizing functional monomer as a low-temperature resistant modification reagent to prepare a low-temperature resistant polypropylene film I, using trimellitic acid plasticizer as a low-temperature resistant modification reagent to prepare a low-temperature resistant polypropylene film II, and using malic acid plasticizer as a low-temperature resistant modification reagent to prepare a low-temperature resistant polypropylene film III.

[0017] Preferably, the glass transition temperature of the low-temperature resistant polypropylene film II is-18℃ to-25℃, the low-temperature elongation at break is 350% to 500%, and the low-temperature notched impact strength is 4 to 6kJ / m 2 .

[0018] Preferably, the glass transition temperature of the low-temperature resistant polypropylene film III is-25℃ to-32℃, the low-temperature elongation at break is 550% to 700%, and the low-temperature notched impact strength is 6.5 to 10kJ / m 2 .

[0019] The low-temperature dairy plastic packaging film prepared by the method is applied to low-temperature dairy packaging.

[0020] Beneficial effects

[0021] The application first synthesizes an epoxy-based plasticizing functional monomer by using esterification and epoxidation, and then synthesizes trimellitic acid plasticizer by using trimellitic anhydride as an epoxy ring-opening reagent and synthesizes malic acid plasticizer by using DL-malic acid as an epoxy ring-opening reagent through epoxy ring-opening esterification and acetylation reactions.

[0022] The low-temperature resistant polypropylene film I is prepared by using the epoxy-based plasticizing functional monomer as a low-temperature resistant modification reagent, the low-temperature resistant polypropylene film II is prepared by using the trimellitic acid plasticizer as a low-temperature resistant modification reagent, and the low-temperature resistant polypropylene film III is prepared by using the malic acid plasticizer as a low-temperature resistant modification reagent.

[0023] It is found through experiments that the three kinds of low-temperature resistant polypropylene films prepared by the application have improved elongation at break and notched impact strength under low-temperature (-18℃) conditions compared with unmodified polypropylene films, and the low-temperature elongation at break and the low-temperature notched impact strength of the low-temperature resistant polypropylene film II and the low-temperature resistant polypropylene film III are significantly improved.

[0024] The glass transition temperature of the low-temperature resistant polypropylene film prepared by the application is reduced to-10℃ to-30℃, and the low-temperature resistant performance of the base polypropylene film is improved compared with the glass transition temperature-3.6℃ of the unmodified polypropylene film.

[0025] Through the experimental results of the health performance, it can be known that the low-temperature resistant polypropylene films prepared by the application have no safety hazards, and can be used in dairy product packaging. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 Chemical reaction formula for synthesizing cashew phenol-based hexanoate;

[0027] Figure 2 Chemical structural formula of the epoxy-based plasticizing functional monomer;

[0028] Figure 3 Chemical structural formula of cashew phenol-based hexanoate-trimellitate;

[0029] Figure 4 Chemical structural formula of the trimellitate-based plasticizer;

[0030] Figure 5 Chemical structural formula of cashew phenol-based hexanoate-malate;

[0031] Figure 6 Chemical structural formula of the malate-based plasticizer;

[0032] Figure 7 Performance experimental results of the low-temperature resistant polypropylene films and the polypropylene films. DETAILED DESCRIPTION

[0033] Experimental Example 1:

[0034] The epoxy-based plasticizing functional monomer is synthesized, and the preparation mechanism is as follows:

[0035] Step 1: hexanoic anhydride is used as an acylating reagent, and esterification reaction occurs between the phenolic hydroxyl functional group of cashew phenol and the acid anhydride group of hexanoic anhydride catalyzed by potassium carbonate, to generate cashew phenol-based hexanoate, and the chemical reaction formula is as shown in Figure 1

[0036] Step 2: hydrogen peroxide is used as an oxidizing agent, and epoxidation reaction occurs to the alkenyl functional group of cashew phenol-based hexanoate catalyzed by an organic protonic acid, to generate an epoxy-based plasticizing functional monomer, and the chemical structural formula is as shown in Figure 2

[0037] Among them, the organic protonic acid catalyst is selected to use formic acid or acetic acid; the experimental example preferably uses formic acid;

[0038] ​​The specific experimental steps for synthesizing the epoxy-based plasticizing functional monomer are as follows: 6.1 g of cardanol and 0.5 g of potassium carbonate are added to a three-necked flask, and under the action of mechanical stirring, the temperature is slowly raised to 60 DEG C for dissolution. 4.6 g of hexanoic anhydride is added to the flask, and stirring is maintained at 60 DEG C for 3 h. After cooling, the product is washed to neutral with 5 wt% sodium bicarbonate solution and deionized water, dried over anhydrous magnesium sulfate, filtered, and distilled under reduced pressure to obtain cardanol-based hexanoate. Then, 4.0 g of cardanol-based hexanoate and 1.5 g of formic acid are added to a three-necked flask, and the temperature is raised to 40 DEG C for stirring for 15 min. 8.5 mL of 30 wt% hydrogen peroxide is added dropwise to the three-necked flask within 1 h, and the temperature is raised to 60 DEG C for stirring for 6 h. The product is washed to neutral with deionized water and distilled under reduced pressure to obtain the epoxy-based plasticizing functional monomer.

[0039] The proton nuclear magnetic resonance spectrum of the epoxy-based plasticizing functional monomer is as follows: 1 H NMR (CDCI3, 400 MHz) δ: 0.87-0.91 (m, 6H), 1.26-1.38 (m, 20H), 1.55-1.63 (m, 8H), 2.53-2.55 (t, 2H), 2.62-2.64 (t, 2H), 3.16-3.20 (m, 2H, -CH(O)CH-), 6.84-7.29 (m, 4H, Ar-H);

[0040] The epoxy-based plasticizing functional monomer is tested by using a Varion EL III type element analyzer, and the test results are as follows: the measured value of C element is 77.92% (theoretical value 77.84%), and the measured value of H element is 10.78% (theoretical value 10.65%);

[0041] According to the above characterization results, it is proved that the epoxy-based plasticizing functional monomer is successfully synthesized.

[0042] Experimental Example Two

[0043] The preparation mechanism for synthesizing the trimellitic acid type plasticizer is as follows:

[0044] First step: esterification reaction occurs between trimellitic anhydride and the epoxy-based plasticizing functional monomer catalyzed by a phase transfer catalyst to generate cardanol-based hexanoate-trimellitate, and the chemical structural formula is as shown in Figure 3

[0045] Among them, the phase transfer catalyst is selected from one of tetrabutylammonium bromide, tetraethylammonium bromide, tetrabutylammonium chloride, and trioctylmethylammonium chloride; and tetrabutylammonium chloride is preferably used in the experimental example;

[0046] Second step: acetylation reaction occurs between the hydroxyl functional group of the cardanol-based hexanoate-trimellitate and the acid anhydride group of acetic anhydride to generate the trimellitic acid type plasticizer, and the chemical structural formula is as shown in​Figure 4 as shown in the following formula:

[0047] The specific experimental steps for synthesizing the trimellitate plasticizer are as follows: 6.2 g of epoxy-based plasticizing functional monomer, 1.0 g of trimellitic anhydride, and 0.1 g of tetrabutylammonium chloride are added to a three-necked flask, and the system is slowly heated to 160°C for reaction under mechanical stirring. The acid value of the system is measured every 0.5 h, and when the acid value no longer decreases, the reaction is terminated. Deionized water is used for washing, and vacuum distillation is performed to obtain cardanol hexanoate-trimellitate; then 7.2 g of cardanol hexanoate-trimellitate and 2.1 g of acetic anhydride are added to a three-necked flask, and the system is heated to 140°C for stirring reaction for 3 h. Vacuum distillation is performed at -0.1 MPa and 120°C, the system is cooled, washed with deionized water until neutral, and rotary evaporated to obtain the trimellitate plasticizer;

[0048] The acid value is determined according to GB / T 1668-2008 “Determination of Acid Value and Acidity of Plasticizers”;

[0049] The hydrogen nuclear magnetic resonance spectrum of the trimellitate plasticizer is as follows: 1 H NMR (CDCI3, 400 MHz) δ: 0.87-0.92 (m, 18H), 1.24-1.60 (m, 72H), 1.71-1.97 (m, 12H), 2.07-2.10 (m, 9H, -COOCH3), 2.53-2.67 (m, 12H), 4.71-4.77 (m, 6H, -COO(CH)-), 6.84-7.35 (m, 12H), 8.03-8.39 (m, 3H);

[0050] The trimellitate plasticizer is tested by using a Varion EL III type element analyzer, and the test results are as follows: the measured value of C element is 72.71% (theoretical value 72.58%), and the measured value of H element is 9.25% (theoretical value 9.10%);

[0051] According to the above characterization results, it is proved that the trimellitate plasticizer is successfully synthesized.

[0052] Experimental Example Three:

[0053] The preparation mechanism of the malic acid type plasticizer is as follows:

[0054] In the first step, the carboxyl functional group of DL-malic acid is esterified with the epoxy functional group of the epoxy-based plasticizing functional monomer under the catalysis of a phase transfer catalyst to generate cardanol hexanoate-malate, and the chemical structural formula is as shown in the following formula: Figure 5 as shown in the following formula:

[0055] The phase transfer catalyst is selected from one of tetrabutylammonium bromide, tetraethylammonium bromide, tetrabutylammonium chloride and trioctylmethylammonium chloride; and the experiment example also preferably uses tetrabutylammonium chloride;

[0056] The second step: using acetic anhydride as an acylating agent, acetylation reaction occurs between the hydroxyl functional groups of the cardanol-based hexanoate-malate and the acid anhydride groups of the acetic anhydride, to generate a malic acid type plasticizer, and the chemical structural formula is as shown in Figure 6 ;

[0057] The specific experimental steps for synthesizing the malic acid type plasticizer are as follows: 6.2 g of the epoxy-based plasticizing functional monomer, 0.7 g of trimellitic anhydride and 0.1 g of tetrabutylammonium chloride are added to a three-necked flask, and under the action of mechanical stirring, the system is slowly heated to 130℃ for reaction, and the acid value of the system is measured every 0.5 h; when the acid value no longer decreases, the reaction is terminated; deionized water is used for washing, and vacuum distillation is performed to obtain the cardanol-based hexanoate-malate; then 4.8 g of the cardanol-based hexanoate-malate and 2.1 g of acetic anhydride are added to a three-necked flask, and the system is heated to 140℃ for stirring reaction for 3 h; vacuum distillation is performed under the conditions of-0.1 MPa and 120℃, and the system is cooled, washed with deionized water until neutral, and rotary evaporated to obtain the malic acid type plasticizer;

[0058] The acid value is determined according to GB / T 1668-2008 “Determination of Acid Value and Acidity of Plasticizers”;

[0059] The nuclear magnetic resonance hydrogen spectrum of the malic acid type plasticizer is characterized as follows: 1 H NMR (CDCl3, 400MHz) δ: 0.87-0.92 (m, 12H), 1.24-1.60 (m, 48H), 1.4-1.92 (m, 8H), 2.04-2.08 (m, 6H, -COOCH3), 2.54-2.67 (m, 8H), 2.75-2.86 (m, 2H), 4.73-4.81 (m, 4H, -COO(CH)-), 5.97-5.99 (t, 1H), 6.84-7.35 (m, 8H);

[0060] The malic acid type plasticizer is tested by using a Varion ELⅢ type element analyzer, and the test results are as follows: the measured value of C element is 70.42% (the theoretical value is 70.30%), and the measured value of H element is 9.28% (the theoretical value is 9.22%);

[0061] According to the above characterization results, it is proved that the malic acid type plasticizer is successfully synthesized.

[0062] Example One:

[0063] Preparation of low-temperature resistant polypropylene film I: 10 g of polypropylene resin and 1 g of epoxy-based plasticizing functional monomer were added to a double-screw extruder preheated to 150°C, blended, melted and extruded by the double-screw extruder, and cut by a pelletizer to obtain plasticized polypropylene particles of 1-5 mm, which were then directly added to a casting machine preheated to 150°C to be cast into a film (with a film thickness controlled at 150 μm), thereby obtaining a low-temperature resistant polypropylene film I.

[0064] The polypropylene resin is a polypropylene resin with a trade name of C1608 from Sinopec Beijing Yanshan Petrochemical Co., Ltd.

[0065] The process parameters of the double-screw extruder were set as follows: the temperatures of zones 1-7 were 165°C, 170°C, 175°C, 180°C, 180°C, 175°C and 170°C, and the rotation speed was 30 r / min.

[0066] The process parameters of the casting machine were set as follows: the temperatures of zones 1-7 were 165°C, 170°C, 180°C, 185°C, 185°C, 175°C and 170°C, and the rotation speed was 60 r / min.

[0067] Example Two:

[0068] A low-temperature resistant polypropylene film II was prepared by using a malic acid type plasticizer to replace the epoxy-based plasticizing functional monomer in Example One according to the same preparation procedure and experimental conditions.

[0069] Example Three:

[0070] A low-temperature resistant polypropylene film III was prepared by using a malic acid type plasticizer to replace the epoxy-based plasticizing functional monomer in Example One according to the same preparation procedure and experimental conditions.

[0071] Comparative Example:

[0072] Preparation of polypropylene film: 10 g of polypropylene resin was added to a double-screw extruder preheated to 150°C, blended, melted and extruded by the double-screw extruder, and cut by a pelletizer to obtain polypropylene particles of 1-5 mm, which were then directly added to a casting machine preheated to 150°C to be cast into a film (with a film thickness controlled at 150 μm), thereby obtaining a polypropylene film.

[0073] The process parameters of the polypropylene resin, the double-screw extruder and the casting machine were the same as in Example One.

[0074] Performance Test:

[0075] The low-temperature resistant polypropylene films in Examples One to Three and the polypropylene film in the comparative example were subjected to the following performance tests, and the specific test methods were as follows:

[0076] I. Mechanical property test:

[0077] (1) The elongation at break of the film sample was tested at 25℃ using an Instron 5565 universal tensile testing machine, and the specific test steps were as follows: the film sample of 30mm x 5mm was fixed on the tensile testing machine, and the tensile test was carried out at a tensile rate of 5mm / min (the test temperature was 25℃), and the room temperature elongation at break of the film sample was recorded;

[0078] The notched impact strength of the film sample was tested at 25℃ using a QYJ1251 type notched sample machine and a ZBC-4B liquid crystal type plastic pendulum impact testing machine, and the specific test steps were as follows: the film sample of 30mm x 5mm was notched on the notched sample machine, the notch depth was 2mm, and the notch tip curvature radius was 0.25mm, and it was fixed on the pendulum impact testing machine for testing (the test temperature was 25℃), and the room temperature notched impact strength of the film sample was recorded;

[0079] (2) After the film sample of 30mm x 5mm was placed in a-18℃ refrigerator for 48h, a part of the sample was fixed on the Instron 5565 universal tensile testing machine, and the tensile test was carried out at a tensile rate of 5mm / min, and another part of the sample was tested for notched impact strength using a QYJ1251 type notched sample machine and a ZBC-4B liquid crystal type plastic pendulum impact testing machine;

[0080] The above experimental results are shown in Table 1 below.

[0081] II. Low temperature resistance test:

[0082] The glass transition temperature of the film sample was tested using a Q2000 type differential scanning calorimeter, and the low temperature resistance of the film sample was analyzed by the glass transition temperature of the film sample;

[0083] The specific test steps of the glass transition temperature were as follows: 5-8mg of the film sample was heated from room temperature to 180℃ at a rate of 10℃ / min, and was kept at 180℃ for 5min to eliminate thermal history, then was cooled to-50℃ at a rate of 10℃ / min, and finally was heated to 180℃ at a rate of 10℃ / min, and the glass transition temperature of the film sample was recorded;

[0084] The above experimental results are shown in Table 1 below.

[0085] III. Hygienic property test:

[0086] The health performance test method of the film sample refers to GB / T 5009.60-2003 "Analysis Method of Health Standard of Polyethylene, Polystyrene and Polypropylene Formed Products for Food Packaging", and the physical and chemical indexes of the experimental results are taken as the standard of GB / T 5009.71-2003 "Analysis Method of Health Standard of Polypropylene Resin for Food Packaging";

[0087] The above experimental results are shown in Table 2 below.

[0088] Table 1 Performance experimental results of each low-temperature-resistant polypropylene film and polypropylene film

[0089]

[0090] Note: Elongation at break (%) = (length of film sample at break - initial length of film sample) / initial length of film sample; Notched impact strength (kJ / m 2 ) = {film sample fracture energy absorbed / (film sample thickness x residual width of film sample notch bottom)} x 10 3 ; Elongation at break change rate (%) = (room temperature elongation at break - low temperature elongation at break) / room temperature elongation at break; Notched impact strength change rate (%) = (room temperature notched impact strength - low temperature notched impact strength) / room temperature notched impact strength;

[0091] Table 2 Health performance experimental results of each low-temperature-resistant polypropylene film and polypropylene film

[0092]

[0093]

[0094] According to the experimental data in Table 1, the following Figure 7 can be obtained by analyzing Figure 7 and Table 2:

[0095] The three low-temperature-resistant polypropylene films prepared by the present application have improved elongation at break and notched impact strength at low temperature (-18℃) compared with unmodified polypropylene films; among them, the low-temperature-resistant polypropylene film II and the low-temperature-resistant polypropylene film III have significantly improved low-temperature elongation at break and low-temperature notched impact strength;

[0096] The glass transition temperature of the low-temperature-resistant polypropylene films prepared by the present application is reduced to -10℃ to -30℃, which is higher than the glass transition temperature of unmodified polypropylene films (-3.6℃), thereby improving the low-temperature resistance of the base polypropylene film.

[0097] According to the health performance experimental results, the low-temperature-resistant polypropylene films prepared by the present application have no safety hazards and can be used in dairy product packaging.

Claims

1. A method for preparing a low-temperature dairy product plastic packaging film, characterized in that, Includes the following steps: Step 1: Prepare a polyester-based structural plasticizer, which includes trimellitic acid-type plasticizers and / or malic acid-type plasticizers; The preparation method of the trimellitic acid type plasticizer is as follows: under the catalysis of a phase transfer catalyst, trimellitic anhydride and epoxy plasticizing functional monomer undergo esterification reaction to generate cashew phenol hexanoate-triptyl ... The preparation method of the malic acid type plasticizer is as follows: under the catalysis of a phase transfer catalyst, the carboxyl functional group of DL-malic acid undergoes an esterification reaction with the epoxy functional group of the epoxy plasticizing functional monomer to generate cashew phenol hexanoate-malate; using acetic anhydride as an acylation reagent, the hydroxyl functional group of cashew phenol hexanoate-malate undergoes an acetylation reaction with the anhydride group of acetic anhydride to generate the malic acid type plasticizer; The preparation method of the epoxy-plasticizing functional monomer is as follows: using hexanoic anhydride as an acylation reagent, potassium carbonate catalyzes the esterification reaction between the phenolic hydroxyl functional group of cashew phenol and the anhydride group of hexanoic anhydride to generate cashew phenol hexanoate; under the catalysis of organic protic acid, hydrogen peroxide is used as an oxidant to cause the alkenyl functional group of cashew phenol hexanoate to undergo an epoxidation reaction to generate the epoxy-plasticizing functional monomer. Step 2: Preparation of low-temperature resistant polypropylene film: 5-20 parts by weight of polypropylene resin and 1-5 parts by weight of polyester-based structural plasticizer are first blended, melted and extruded by a twin-screw extruder, then pelletized into particles with an average particle size of 1-5 mm by a pelletizer, and finally cast into a low-temperature resistant polypropylene film with a film thickness of 120-180 μm by a casting machine. The temperature of the twin-screw extruder is set to 160~180℃ and the rotation speed is set to 20~50r / min; the temperature of the casting machine is set to 165℃~185℃ and the rotation speed is set to 40~70r / min.

2. The method for preparing a low-temperature dairy product plastic packaging film according to claim 1, characterized in that, The phase transfer catalyst is selected from one of tetrabutylammonium bromide, tetraethylammonium bromide, tetrabutylammonium chloride, and trioctylmethylammonium chloride.

3. The method for preparing a low-temperature dairy product plastic packaging film according to claim 1, characterized in that, The organic protic acid is selected from formic acid or acetic acid.

4. A low-temperature dairy product plastic packaging film prepared by the method according to any one of claims 1-3, characterized in that, The low-temperature dairy product plastic packaging film uses polypropylene resin as the film raw material. Through a blending-extrusion-casting process, a low-temperature resistant polypropylene film II is prepared using trimellitic acid-type plasticizer as a low-temperature resistant modifier, and a low-temperature resistant polypropylene film III is prepared using malic acid-type plasticizer as a low-temperature resistant modifier.

5. The low-temperature dairy product plastic packaging film according to claim 4, characterized in that, The low-temperature resistant polypropylene film II has a glass transition temperature of -18℃ to -25℃, a low-temperature elongation at break of 350% to 500%, and a low-temperature notched impact strength of 4 to 6 kJ / m. 2 .

6. The low-temperature dairy product plastic packaging film according to claim 4, characterized in that, The low-temperature resistant polypropylene film III has a glass transition temperature of -25℃ to -32℃, a low-temperature elongation at break of 550% to 700%, and a low-temperature notched impact strength of 6.5 to 10 kJ / m. 2 .

7. The application of the low-temperature dairy product plastic packaging film according to claim 4 in the packaging of low-temperature dairy products.

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