A moisture control preservative film for fruits and vegetables and a method for preparing the same

Abstract

The application discloses a kind of humidity control fresh-keeping films for fruits and vegetables and preparation method thereof, belong to polymer fresh-keeping film technical field.A kind of humidity control fresh-keeping film for fruits and vegetables, the humidity control fresh-keeping film includes core layer and the skin layer being covered in the both sides of core layer, by weight parts: the core layer is made of including the following raw materials: PBAT resin 40-60 parts;PLA resin 15-25 parts;Modified MOF functional master batch 15-30 parts;Chain extender 0.5-2 parts;Dispersing lubricant 0.2-1 part;The skin layer is made of including the following weight parts of raw materials:PBAT resin 60-70 parts, PLA resin 30-40 parts, chain extender 0.5-1.5 parts.The PBAT matrix of the core layer provides the channel of moisture fast transmission, and the PLA / PBAT blend of the skin layer plays the role of moderate flow limiting, can effectively extend shelf life to 7-9 days, and fruit is not mildewed, no shriveling.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of polymer preservative film, and particularly relates to a humidity-controllable preservative film for fruits and vegetables and a preparation method thereof. BACKGROUND

[0002] Fresh fruits and vegetables will still have active respiration and transpiration after being picked, and continuously release heat, carbon dioxide and water vapor to the surrounding environment. When they are placed in airtight plastic packaging, the released water vapor will quickly accumulate, causing the relative humidity in the packaging to rise sharply, often exceeding 95%. When the outside temperature drops slightly, the inner wall of the packaging and the surface of the fruits and vegetables will condense fine liquid water droplets due to the temperature difference. High humidity and liquid water create conditions for the rapid reproduction of various molds, yeasts and spoilage bacteria, causing fruits and vegetables to rapidly mold and rot. How to scientifically and effectively manage the moisture in the packaging, both to maintain the necessary high humidity to prevent fruits and vegetables from wilting and to avoid the condensation and accumulation of liquid water, is a core technical problem to extend the shelf life of fruits and vegetables and reduce postharvest losses.

[0003] At present, the mainstream fruit and vegetable preservative packaging on the market mainly includes the following types:

[0004] The first type is the traditional polyethylene (PE) or polypropylene (PP) preservative film: this type of material has excellent water resistance, but the moisture permeability is extremely low, and severe dew condensation often occurs, leading to the rotting of fruits and vegetables.

[0005] The second type is a physically punched film or a microporous film: by punching holes in the film to increase the air and moisture permeability, although the dew condensation problem is solved, but often the pendulum swings to the other extreme, leading to excessive water loss, fruits and vegetables dry and lose weight in a short time, and the open hole structure is easy to let external bacteria invade.

[0006] The third type is a modified film added with inorganic powder (such as calcium carbonate, talc, diatomite, etc.): this method attempts to adjust the moisture permeability through the interface micropores generated by the filler. However, conventional inorganic fillers have poor compatibility with hydrophobic polyolefin or polyester matrix, and it is difficult to achieve uniform dispersion at high filler loading, and the film is easily broken during film blowing, seriously affecting the mechanical strength and transparency of the film. More importantly, these inorganic fillers usually only have one-way moisture absorption or permeability function.

[0007] MOF materials have ultra-high specific surface area and adjustable pore structure, and are theoretically ideal humidity regulators. However, MOF materials usually contain a large number of metal coordination defects and hydrophilic groups, which makes them easily agglomerate in non-polar polymer matrix, forming micron-sized large particles, not only damaging the mechanical properties of the film, but also failing to fully exert the advantages of their nanoscale pores.

[0008] Therefore, how to develop a fresh-keeping film which can realize the nanoscale uniform dispersion of MOF materials in the biodegradable matrix, retain the pore activity to realize dynamic humidity control, and has high strength and high transparency is a technical problem to be solved in the field of functional packaging materials. SUMMARY

[0009] In order to solve the defects in the above technical solutions, the purpose of the present application is to provide a humidity-controlling fresh-keeping film for fruits and vegetables and a preparation method thereof. In order to achieve the above purpose, the present application provides a humidity-controlling fresh-keeping film for fruits and vegetables, which comprises a core layer and a skin layer coated on both sides of the core layer, and the core layer is made of the following raw materials by weight:

[0010] The core layer is made of the following raw materials: PBAT resin 40-60 parts; PLA resin 15-25 parts; modified MOF functional masterbatch 15-30 parts; chain extender 0.5-2 parts; dispersing lubricant 0.2-1 part;

[0011] The skin layer is made of the following raw materials by weight: PBAT resin 60-70 parts, PLA resin 30-40 parts, chain extender 0.5-1.5 parts.

[0012] The preparation of the modified MOF functional masterbatch specifically includes the preparation of modified MOF powder and the preparation of modified MOF functional masterbatch.

[0013] The melt mass flow rate of the PBAT resin is 3-5 g / 10min (190℃, 2.16kg) ;

[0014] The melt mass flow rate of the PLA resin is 3-7 g / 10min (210℃, 2.16kg)

[0015] The chain extender in the core layer and the skin layer is any one of ADR-4368 or ADR-4468;

[0016] The dispersing lubricant in the core layer is erucic acid amide and / or zinc stearate;

[0017] The average particle size of the MOF in the modified MOF functional masterbatch is 100-300 nm, and the specific surface area is >1300 m² / g.

[0018] In a preferred embodiment of the present application, the MOF is ZIF-8;

[0019] The preparation of the modified MOF functional masterbatch specifically includes the preparation of modified MOF powder and the preparation of modified MOF functional masterbatch.

[0020] The preparation of the modified MOF powder comprises the following steps: Step 1, preparation of a polymer solution: in a three-necked flask, 500 mL of xylene is added; heated to 95-100°C; under stirring, maleic anhydride grafted polybutylene succinate is added in batches; keep the temperature and stir for 30-60 minutes until the polymer is completely dissolved to form a clear and transparent but viscous polymer hot solution; keep warm for standby.

[0021] Step 2, preparation of a MOF dispersion: in another container, ZIF-8 nano-powder is added to anhydrous ethanol; high-intensity ultrasonic dispersion is performed using an ultrasonic cell crusher for 30 minutes to prevent particle agglomeration, and a milky white dispersion is obtained.

[0022] Step 3, mixing and reaction: stop the standby of the polymer hot solution in Step 1, cool it to 60°C, then pour the ZIF-8 ethanol dispersion in Step 2 into the reaction kettle containing the polymer hot solution prepared in Step 1 under high-speed stirring; after the dropwise addition is completed, continue to stir at 60°C for 20 minutes; then increase the oil bath temperature to 90°C to make the mixed solvent system in a micro-boiling reflux state; add a catalyst to the system; keep refluxing and stirring for 5 hours.

[0023] Step 4, after the reaction is completed: stop heating, naturally cool to room temperature, centrifuge the slurry, discard the supernatant; and add 80°C hot xylene to the centrifugal precipitate, resuspend and wash with strong stirring for 30 minutes, and heat centrifugation again. Repeat 2 times, and finally dry the solid in an 80°C vacuum oven for 24 hours to prepare the modified MOF powder.

[0024] The polymer maleic anhydride grafted polybutylene succinate in Step 1 of the preparation of the modified MOF powder is prepared by the following method: the maleic anhydride grafted polybutylene succinate is prepared by reactive extrusion, and the specific preparation steps are as follows: weigh the dried PBS resin particles, maleic anhydride monomer, and initiator dicumyl peroxide. Dissolve the maleic anhydride monomer and dicumyl peroxide in a small amount of acetone to form a uniform solution; then place the PBS resin particles in a high-speed mixer, and under low-speed stirring, uniformly spray the above acetone solution on the surface of the PBS particles, mix for 5 minutes, and let stand for 30 minutes to allow the acetone to evaporate, so that the monomer and initiator are uniformly adsorbed on the surface of the resin.

[0025] Then, reactive extrusion is carried out: the pre-mixed material is added to a co-rotating twin-screw extruder to prepare the maleic anhydride grafted polybutylene succinate (PBS-g-MAH) graft.

[0026] The key process parameters of the extrusion are set as follows:

[0027] Temperature setting: Zone 1 (feeding) 130℃, Zone 2-6 (reaction zone) 140℃-150℃-155℃-155℃-150℃, Die head 145℃.

[0028] Screw combination: Two groups of kneading blocks are set in the reaction zone (Zone 3, 4) to provide strong shear force to promote the reaction.

[0029] Vacuum devolatilization: Strong vacuum is opened at the end of the extruder (Zone 7); the last extruded strip is cooled by air-cooled conveyor belt, then cut into particles, and then dried in a vacuum oven at 80℃ for 8 hours to obtain the maleic anhydride grafted polybutylene succinate graft.

[0030] The preparation of the modified MOF functional masterbatch includes the following steps:

[0031] (1) Mix the modified MOF powder, PBAT resin, PBS-g-MAH, chain extender, and zinc stearate in a high-speed mixer at 1500 rpm for 5 minutes.

[0032] (2) After mixing, add it to a co-rotating twin-screw extruder (L / D=40).

[0033] (3) Extrusion process setting: Temperature zoning setting: Zone 1 (feeding) 130℃, Zone 2 145℃, Zone 3 155℃, Zone 4 160℃, Zone 5 155℃, Die head 150℃; Screw speed: 350 rpm.

[0034] Vacuum venting: Strong vacuum is opened at Zone 5 to remove trace amounts of water and small molecular volatiles from the raw materials.

[0035] (4) Extruded strip, air-cooled, cut into particles, and vacuum packaged at 60℃ to obtain the reactive MOF functional masterbatch.

[0036] A method for preparing a humidity-controlling fresh-keeping film for fruits and vegetables includes the following steps: the structure is skin / core / skin, and the thickness ratio is 1:2:1.

[0037] Step a: Dry the core layer and skin layer raw materials separately in a vacuum air dryer at 80℃ for 6 hours, with a moisture content of <50ppm.

[0038] Step b: Add the dried raw materials to a three-layer co-extrusion film blowing machine.

[0039] The core layer extruder (B) has a temperature setting of 150℃-160℃-165℃.

[0040] The skin layer extruder (A / C) has a temperature setting of 160℃-170℃-175℃.

[0041] Die head: temperature 170℃, die gap 1.2mm.

[0042] Step c: the inflation ratio is set to 3.0, the pulling speed is 15 m / min, and the frost line height is controlled at about 300 mm.

[0043] Step d: after winding, the film is aged in a 40℃ oven for 48 hours.

[0044] Advantages of the present application:

[0045] 1. The present application innovatively uses the anhydride groups on the PBS-g-MAH segment to direct esterification and bidentate coordination bonding with the defect sites on the MOF surface in the presence of a catalyst, so that this chemical bonding effect inoculates a layer of PBS molecular brush on the surface of the MOF particles, which is fully compatible with the matrix resin in thermodynamics; and in subsequent melt processing, this layer of molecular brush and the core layer matrix (PBAT / PLA) undergo strong physical entanglement, thereby realizing the nanoscale uniform dispersion of MOF in the polyester matrix and solving the dispersion problem at high filling amounts.

[0046] 2. Traditional PE preservative films are prone to dew condensation and mold due to too high barrier property, and perforated films are prone to fruit dehydration due to too high moisture permeability; the present application utilizes the synergistic effect of the unique gate effect of MOF and the skin-core layer structure; when the relative humidity inside the package is too high, the highly dispersed MOF channels in the core layer open and quickly adsorb excess water vapor to prevent condensation on the inner wall; when the relative humidity decreases due to environmental temperature fluctuations, the water in the channels is slowly released; at the same time, the PBAT matrix of the core layer provides a channel for rapid water transport, and the PLA / PBAT blend of the skin layer plays a moderate flow limiting role, effectively extending the shelf life to 7-9 days without mold and drying of the fruits.

[0047] 3. The anhydride groups on the surface of the MOF functional master batch added in the present application can also undergo secondary reactions with the hydroxyl / carboxyl groups at the ends of PBAT / PLA in the matrix in situ during extrusion to form a chemical crosslinking network of MOF-grafted chains-matrix; this interfacial reinforcement effect makes the film excellent in longitudinal tensile strength even at high filling amounts, and also meets the requirements of packaging film for puncture resistance and load bearing. DETAILED DESCRIPTION

[0048] In order to make the objectives, technical solutions and advantages of the present application clearer, further detailed description will be made to the present application in combination with embodiments, the illustrative embodiments and the description thereof are only used to explain the present application, and do not limit the present application. In addition, for the numerical range in the present application, it should be understood that each intermediate value between the upper limit and the lower limit of the range is also specifically disclosed. Each smaller range between any stated value or stated range and any other stated value or intermediate value in the stated range is also included in the present application. The upper limit and the lower limit of these smaller ranges can be independently included or excluded from the range.

[0049] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present application, preferred methods and materials are described. All documents mentioned in this specification are incorporated by reference to disclose and describe in full the methods and / or materials which are described in the cited documents. In case of conflict between the content of the specification and that of any document incorporated herein by reference, the content of the specification prevails.

[0050] Various modifications and changes can be made to the specific implementation of the present application described in this specification without departing from the scope or spirit of the application. Other implementations of the present application will be apparent to those skilled in the art from consideration of the specification and practice of the application disclosed herein. The specification and examples given are exemplary only.

[0051] As used herein, the terms “comprises”, “comprising”, “includes”, “including”, “has”, “having”, and the like are open-ended terms that are intended to denote the inclusion of elements or steps without precluding the presence or addition of one or more other elements or steps.

[0052] Example 1

[0053] A moisture control fresh-keeping film for fruits and vegetables comprises a core layer and a skin layer coated on both sides of the core layer, and the core layer is made of the following raw materials by weight: PBAT resin 50 parts; PLA resin 20 parts; modified MOF functional masterbatch 22.5 parts; chain extender 1.2 parts; dispersing lubricant 0.6 parts;

[0054] The skin layer is made of the following raw materials by weight: PBAT resin 65 parts, PLA resin 35 parts, chain extender 1.0 part.

[0055] The melt mass flow rate (MFR) of the PBAT resin is 3-5 g / 10 min (190°C, 2.16 kg);

[0056] The melt mass flow rate (MFR) of the PLA resin is 3-7 g / 10 min (210°C, 2.16 kg).

[0057] The chain extender in the core layer and the skin layer is ADR-4368;

[0058] The dispersed lubricant in the core layer is a mixture of erucamide and zinc stearate;

[0059] The average particle size of the MOF in the modified MOF functional master batch is 100-300 nm, and the specific surface area is >1300 m² / g;

[0060] In a preferred embodiment of the present application, the MOF is ZIF-8.

[0061] The ZIF-8 nanopowder is purchased from Nanjing Xianfeng Nanometer Material Technology Co., Ltd.

[0062] The preparation of the modified MOF functional master batch specifically includes the preparation of the modified MOF powder and the preparation of the modified MOF functional master batch.

[0063] The preparation of the modified MOF powder includes the following steps: Step 1, preparation of a polymer solution: in a 1000 mL three-necked flask equipped with mechanical stirring and a condenser, 500 mL of xylene is added; heated to 95-100℃; under stirring, 50g of maleic anhydride grafted polybutylene succinate (PBS-g-MAH) is added in batches; keep the temperature and stir for 50 minutes until the polymer is completely dissolved to form a clear transparent but viscous polymer hot solution; keep warm for standby.

[0064] Step 2, preparation of a MOF dispersion: in another container, 200g of ZIF-8 nanopowder is added to 1500mL of absolute ethanol; high-intensity ultrasonic dispersion is carried out using an ultrasonic cell crusher for 30 minutes to prevent particle agglomeration, and a milky white ZIF-8 dispersion is obtained.

[0065] Step 3, mixing and reaction: stop the warming of the polymer hot solution in Step 1, and cool it to 60℃, then pour the ZIF-8 ethanol dispersion in Step 2 into the reaction kettle containing the polymer hot solution prepared in Step 1 under high-speed stirring (600 rpm); after the dropwise addition is completed, continue to stir at 60℃ for 20 minutes;

[0066] Then the oil bath temperature is raised to 90℃, and the mixed solvent system (ethanol / xylene) is in a micro-boiling reflux state; after the system is stable at 90℃, 1.0g of anhydrous zinc acetate (previously dissolved in a small amount of ethanol) is added as a catalyst; keep refluxing and stirring for 5 hours.

[0067] Step 4, after the reaction is completed: stop heating, and cool naturally to room temperature. At this time, as the temperature decreases and ethanol is present, the polymer chains grafted with ZIF-8 will partially precipitate, precipitating out together with the filler; centrifuge the slurry (8000 rpm), and discard the supernatant; in order to remove the ungrafted free polymer: add 500 mL of hot xylene (80°C) to the centrifuged precipitate, resuspend and wash by vigorous stirring for 30 minutes, and centrifuge again. Repeat 2 times, and finally dry the solid in a vacuum oven at 80°C for 24 hours, to obtain the modified MOF powder.

[0068] The polymer maleic anhydride grafted polybutylene succinate (PBS-g-MAH) in the preparation of the polymer solution in Step 1 in the preparation of the modified MOF powder is prepared by the following method: the maleic anhydride grafted polybutylene succinate (PBS-g-MAH) is prepared by reaction extrusion, and the specific preparation steps are as follows: weigh 100 parts of dried PBS resin particles, weigh 2.0 parts of maleic anhydride monomer, and weigh 0.1 part of initiator dicumyl peroxide. Dissolve the maleic anhydride monomer and dicumyl peroxide in a small amount of acetone (about 5-10 mL) to form a uniform solution; then place the PBS resin particles in a high-speed mixer, and under low-speed stirring, uniformly spray the above acetone solution on the surface of the PBS particles, mix for 5 minutes, and let stand for 30 minutes to allow the acetone to evaporate, so that the monomer and initiator are uniformly adsorbed on the surface of the resin. Then, reaction extrusion is carried out: the pre-mixed material is added to a co-rotating twin-screw extruder (L / D = 40).

[0069] The key process parameters are set as follows:

[0070] Temperature setting: Zone 1 (feeding) 130°C, Zone 2-6 (reaction zone) 140°C-150°C-155°C-155°C-150°C, die head 145°C.

[0071] Screw combination: two groups of kneading blocks are set in the reaction zone (Zone 3, Zone 4) to provide strong shear force to promote the reaction.

[0072] Vacuum devolatilization: strong vacuum (-0.09 MPa) is opened at the end of the extruder (Zone 7) to completely remove unreacted residual monomer MAH and small molecule byproducts; finally, the extruded strip is cooled by air cooling conveyor belt, cut into particles, and then dried in a vacuum oven at 80°C for 8 hours, to obtain the maleic anhydride grafted polybutylene succinate (PBS-g-MAH) graft.

[0073] The preparation of the modified MOF functional master batch includes the following steps: (1) mixing 30 parts of modified MOF powder, 65 parts of PBAT resin, 3 parts of PBS-g-MAH, 1 part of chain extender ADR-4368, and 1 part of zinc stearate in a high-speed mixer at 1500 rpm for 5 minutes;

[0074] (2) After mixing, add a co-rotating twin-screw extruder (L / D=40).

[0075] (3) Extrusion process setting: temperature partition setting: zone 1 (feeding) 130℃, zone 2 145℃, zone 3 155℃, zone 4 160℃, zone 5 155℃, die head 150℃; screw speed: 350 rpm.

[0076] Vacuum exhaust: strong vacuum (-0.08 MPa) is opened in the fifth zone to remove trace amounts of water and small molecular volatiles from the raw materials.

[0077] (4) Extrusion stretching, air cooling, granulation, vacuum packaging at 60℃, and the reactive MOF functional master batch is obtained.

[0078] The total thickness of the preservative film is usually 10-60 μm, preferably 20-40 μm. In the following examples, 30 ± 2 μm is used as an example.

[0079] A method for preparing a humidity-controlling preservative film for fruits and vegetables includes the following steps: the structure is skin layer / core layer / skin layer, and the thickness ratio is 1:2:1.

[0080] Step a: dry the core layer and skin layer raw materials separately in a vacuum air dryer at 80℃ for 6 hours, with a moisture content of <50 ppm.

[0081] Step b: add the dried raw materials to a three-layer co-extrusion blown film machine.

[0082] The core layer extruder (B) has a temperature setting of 150℃-160℃-165℃.

[0083] The skin layer extruder (A / C) has a temperature setting of 160℃-170℃-175℃.

[0084] The die head has a temperature of 170℃ and a die gap of 1.2mm.

[0085] Step c: set the blow ratio to 3.0, the pulling speed to 15m / min, and the frost line height to about 300mm.

[0086] Step d: after winding, the film is aged in a 40℃ oven for 48 hours to eliminate internal stress and complete the reaction.

[0087] Embodiment 2

[0088] A moisture control fresh-keeping film for fruits and vegetables comprises a core layer and a skin layer coated on both sides of the core layer, and the ingredients are as follows in parts by weight:

[0089] The core layer is made of the following raw materials: PBAT resin 60 parts; PLA resin 15 parts; modified MOF functional masterbatch 15 parts; chain extender 0.5 parts; dispersing lubricant 0.2 parts;

[0090] The skin layer is made of the following raw materials in parts by weight: PBAT resin 70 parts, PLA resin 30 parts, chain extender 0.5 parts.

[0091] The chain extender in the core layer and the skin layer is ADR-4368;

[0092] The dispersing lubricant in the core layer is a mixture of erucic acid amide and zinc stearate;

[0093] The average particle size of the MOF in the modified MOF functional masterbatch is 100-300 nm, and the specific surface area is >1300 m² / g.

[0094] In a preferred embodiment of the present application, the MOF is ZIF-8;

[0095] The preparation method of the modified MOF functional masterbatch in Embodiment 2 and the preparation method of the moisture control fresh-keeping film are the same as those in Embodiment 1.

[0096] Embodiment 3

[0097] A moisture control fresh-keeping film for fruits and vegetables comprises a core layer and a skin layer coated on both sides of the core layer, and the ingredients are as follows in parts by weight:

[0098] The core layer is made of the following raw materials: PBAT resin 40 parts; PLA resin 25 parts; modified MOF functional masterbatch 30 parts; chain extender 2 parts; dispersing lubricant 1 part;

[0099] The skin layer is made of the following raw materials in parts by weight: PBAT resin 60 parts, PLA resin 40 parts, chain extender 1.5 parts.

[0100] The chain extender in the core layer and the skin layer is ADR-4468;

[0101] The dispersing lubricant in the core layer is erucic acid amide;

[0102] The average particle size of the MOF in the modified MOF functional masterbatch is 100-300 nm, and the specific surface area is >1300 m² / g.

[0103] In a preferred embodiment of the present application, the MOF is ZIF-8.

[0104] The preparation method of the modified MOF functional master batch and the preparation method of the moisture-controlling fresh-keeping film in Example 3 are the same as those in Example 1.

[0105] Example 4

[0106] A moisture-controlling fresh-keeping film for fruits and vegetables comprises a core layer and a skin layer coated on both sides of the core layer, and the core layer and the skin layer are prepared from the following raw materials by weight:

[0107] The core layer is prepared from the following raw materials by weight: PBAT resin 43 parts, PLA resin 25 parts, modified MOF functional master batch 30 parts, chain extender 1.5 parts, and dispersing lubricant 0.5 parts.

[0108] The skin layer is prepared from the following raw materials by weight: PBAT resin 60 parts, PLA resin 40 parts, and chain extender 1.2 parts.

[0109] The chain extender in the core layer and the skin layer is ADR-4368.

[0110] The dispersing lubricant in the core layer is a mixture of erucic acid amide and zinc stearate.

[0111] The average particle size of the MOF in the modified MOF functional master batch is 100-300 nm, and the specific surface area is >1300 m² / g.

[0112] In a preferred embodiment of the present application, the MOF is ZIF-8.

[0113] The preparation method of the modified MOF functional master batch and the preparation method of the moisture-controlling fresh-keeping film in Example 4 are the same as those in Example 1.

[0114] Example 5

[0115] A moisture-controlling fresh-keeping film for fruits and vegetables comprises a core layer and a skin layer coated on both sides of the core layer, and the core layer and the skin layer are prepared from the following raw materials by weight:

[0116] The core layer is prepared from the following raw materials by weight: PBAT resin 43 parts, PLA resin 25 parts, modified MOF functional master batch 30 parts, chain extender 1.5 parts, and dispersing lubricant 0.5 parts.

[0117] The skin layer is prepared from the following raw materials by weight: PBAT resin 60 parts, PLA resin 40 parts, and chain extender 1.2 parts.

[0118] The chain extender in the core layer and the skin layer is ADR-4368.

[0119] The dispersing lubricant in the core layer is erucic acid amide.

[0120] The average particle size of the MOF in the modified MOF functional master batch is 100-300 nm, and the specific surface area is >1300 m² / g.

[0121] In a preferred embodiment of the present application, the MOF is ZIF-8.

[0122] In a preferred embodiment of the present application, the MOF is ZIF-8.

[0123] Comparative Example 1

[0124] A moisture-controlling fresh-keeping film for fruits and vegetables comprises a core layer and a skin layer coated on both sides of the core layer, and the components and their amounts are as follows by weight:

[0125] The core layer is made of the following raw materials: PBAT resin 40 parts; PLA resin 25 parts; modified MOF functional master batch 30 parts; chain extender 2 parts; dispersing lubricant 1 part.

[0126] The skin layer is made of the following raw materials by weight: PBAT resin 60 parts, PLA resin 40 parts, chain extender 1.5 parts.

[0127] The chain extender in the core layer and the skin layer is ADR-4468.

[0128] The dispersing lubricant in the core layer is erucamide.

[0129] In a preferred embodiment of the present application, the MOF is ZIF-8.

[0130] The difference between Comparative Example 1 and Example 3 is that an equal amount of unmodified ZIF-8 powder is directly added to the core layer without PBS-g-MAH graft modification.

[0131] In addition, the remaining components and their amounts in Comparative Example 1 and the preparation method are the same as those in Example 3.

[0132] Comparative Example 2

[0133] A moisture-controlling fresh-keeping film for fruits and vegetables comprises a core layer and a skin layer coated on both sides of the core layer, and the components and their amounts are as follows by weight:

[0134] The core layer is made of the following raw materials: PBAT resin 40 parts; PLA resin 25 parts; modified MOF functional master batch 30 parts; chain extender 2 parts; dispersing lubricant 1 part.

[0135] The skin layer is made of the following raw materials by weight: PBAT resin 60 parts, PLA resin 40 parts, chain extender 1.5 parts.

[0136] The chain extender in the core layer and the skin layer is ADR-4468;

[0137] The dispersing lubricant in the core layer is erucamide;

[0138] In a preferred embodiment of the present application, the MOF is ZIF-8;

[0139] The difference between Comparative Example 2 and Example 3 is that no modified MOF functional master batch is added in the core layer, and the amount of PBAT resin is correspondingly increased to make up the fraction.

[0140] In addition, the remaining components and their amounts in Comparative Example 2 are the same as those in Example 3.

[0141] Comparative Example 3

[0142] A moisture control fresh-keeping film for fruits and vegetables, comprising a core layer and a skin layer coated on both sides of the core layer, by weight;

[0143] The core layer is made of the following raw materials: PBAT resin 40 parts; PLA resin 25 parts; modified MOF functional master batch 30 parts; chain extender 2 parts; dispersing lubricant 1 part;

[0144] The skin layer is made of the following raw materials by weight: PBAT resin 60 parts, PLA resin 40 parts, chain extender 1.5 parts.

[0145] The chain extender in the core layer and the skin layer is ADR-4468;

[0146] The dispersing lubricant in the core layer is erucamide;

[0147] The average particle size of the MOF in the modified MOF functional master batch is 100-300 nm, and the specific surface area is >1300 m² / g.

[0148] In a preferred embodiment of the present application, the MOF is ZIF-8;

[0149] The difference between Comparative Example 3 and Example 3 is that silane coupling agent KH-550 is used instead of PBS-g-MAH to modify ZIF-8.

[0150] In addition, the remaining components and their amounts in Comparative Example 3 are the same as those in Example 3.

[0151] Comparative Example 4

[0152] A moisture control fresh-keeping film for fruits and vegetables, comprising a core layer and a skin layer coated on both sides of the core layer, by weight;

[0153] The core layer is made of raw materials including: PBAT resin 40 parts; PLA resin 25 parts; unmodified ZIF-8 nano powder 22.5 parts; maleic anhydride grafted polybutylene succinate 7.5 parts; chain extender 2 parts; dispersing lubricant 1 part;

[0154] The skin layer is made of raw materials including: PBAT resin 60 parts, PLA resin 40 parts, chain extender 1.5 parts.

[0155] The chain extender in the core layer and the skin layer is ADR-4468;

[0156] The dispersing lubricant in the core layer is erucic acid amide;

[0157] In a preferred embodiment of the present application, the MOF is ZIF-8;

[0158] The difference between Comparative Example 4 and Example 3 is that:

[0159] The difference between the preparation process is that: Comparative Example 4 does not pre-prepare modified MOF powder and modified MOF functional masterbatch, but uses a “one-step method” melt reaction extrusion process to prepare core layer raw material particles.

[0160] The specific operation of Comparative Example 4 is as follows: all raw materials of the core layer (PBAT resin, PLA resin, unmodified ZIF-8 nano powder, maleic anhydride grafted polybutylene succinate, chain extender, dispersing lubricant) are physically premixed in a high-speed mixer; the above mixture is directly added into a co-rotating twin-screw extruder, trying to complete the grafting reaction of maleic anhydride grafted polybutylene succinate to ZIF-8 and the blending and granulation of all components at the same time in a molten state; the temperature, screw combination and rotation speed and other parameters of the extruder are consistent with the process of preparing “modified MOF functional masterbatch” in Example 1.

[0161] Finally, the core layer particles prepared by the “one-step method” are prepared into the final preservative film together with the skin layer raw materials according to the three-layer co-extrusion film blowing process in Example 1.

[0162] In addition, the remaining components and their amounts in Comparative Example 4 are consistent with Example 3.

[0163] Test Example

[0164] Haze and light transmittance: refer to national standard GB / T 2410 “Determination of light transmittance and haze of transparent plastics”;

[0165] Mechanical property test: refer to national standard GB / T 1040.3 “Determination of tensile properties of plastics”;

[0166] Water vapor transmission: refer to national standard GB / T 1037 "Plastic film and sheet water vapor permeability test method", which needs moderate moisture permeability for fruit and vegetable preservation, such as 150-250g / m²·24h;

[0167] Strawberry preservation good fruit rate: commercially available Hongyan fresh strawberries were selected, and fruits with uniform size, no mechanical damage, and no disease were selected; they were respectively loaded into packaging bags made of the films of examples 1-5 and comparative examples 1-3, and the excess air was discharged and the mouth was heat sealed. Placed in a 4°C refrigerator for cold storage; after 14 days, the number of fruits with mildew, rot and juice leakage was counted, and the good fruit rate was calculated.

[0168] The test results are shown in Table 1:

[0169] Table 1

[0170]

[0171] Data analysis of test results: it can be seen from comparative example 3 and comparative example 1 that 25 parts of MOF filler are added, and the haze of the film directly physically blended in comparative example 1 is as high as 42.2%, and the tensile strength is only 17.5 MPa; this is because the specific surface area of nano ZIF-8 is extremely large, and it is easy to cause secondary agglomeration in the organic matrix, and the interfacial bonding force between the two phases is weak, forming a large number of stress concentration points; and the anhydride groups on the PBS-g-MAH segment in the application are innovatively used to direct the esterification reaction and bidentate coordination bonding with the defect sites on the surface of MOF under the action of liquid phase and catalyst, so that this chemical bonding effect inoculates a layer of PBS molecular brush on the surface of MOF particles, which is thermodynamically fully compatible with the matrix resin; so that in the subsequent melt processing, this layer of molecular brush and the core layer matrix (PBAT / PLA) occur strong physical entanglement, thereby realizing the nanoscale uniform dispersion of MOF in the polyester matrix, solving the dispersion problem under high filling amount; realizing the nanoscale uniform dispersion and strong interfacial bonding of the filler in the matrix.

[0172] It is found that although the silane coupling agent KH-550 is used to replace PBS-g-MAH to modify ZIF-8, it can also improve the dispersibility to some extent, but the silane coupling agent has small molecular weight and can easily penetrate and block the microporous structure of ZIF-8, resulting in loss of moisture absorption activity. The long-chain polymer used in the application can only be anchored on the outer surface of ZIF-8 due to steric hindrance effect, so as to retain the channel activity inside the MOF; so as to have more beneficial ability to extend the shelf life; and also has better mechanical properties to meet the needs of packaging film for puncture resistance and load bearing.

[0173] Comparative Example 3 and Comparative Example 2, the water vapor permeability of Comparative Example 2 is as high as 700 g / m2·24h, and serious dew condensation occurs, which also causes the fruits to lose water and shrivel, so that the good fruit rate is only 15%; while the Examples 1-5 of the present application introduce modified MOF, not only adjust the water vapor permeability to the suitable interval of 175-210, and in the range of Examples 1-5, whether the filler amount is increased or the matrix ratio is adjusted, the film maintains excellent comprehensive performance, which proves that the technical scheme of the present application has excellent robustness and universality in a wide process window.

[0174] The haze of the film prepared in Comparative Example 4 is more than 50%, the light transmittance is less than 50%, and the film presents an opaque turbid state, completely losing the most basic appearance requirement as a packaging material; it is illustrated that in the high-temperature and high-viscosity polymer melt, the agglomeration rate of nano-ZIF-8 particles is much faster than the grafting reaction rate of the molecular chain of maleic anhydride grafted polybutylene succinate; the MOF particles without liquid phase pre-dispersion and surface anchoring quickly form a large number of micron-sized scattering centers under strong shear force, finally making the film lose all transparency.

[0175] And the film of Comparative Example 4 has insufficient longitudinal tensile strength of less than 10 MPa and elongation at break of less than 50%, completely without basic toughness and carrying capacity as a packaging film; again confirming that the process preparation of Comparative Example 4 will cause serious agglomeration of MOF; the water vapor permeability of Comparative Example 4 cannot be measured, which illustrates that the film of Comparative Example 4 completely loses the dynamic humidity control ability; finally, it also leads to the strawberry fresh-keeping good fruit rate to drop to 0%.

[0176] The above specific embodiments further illustrate the purpose, technical scheme and beneficial effects of the present application, and it should be understood that the above description is only a specific embodiment of the present application and is not used to limit the protection scope of the present application, any modification, equivalent replacement, improvement, etc. within the spirit and principles of the present application should be included in the protection scope of the present application.

Claims

1. A moisture-controlling and preservation film for fruits and vegetables, characterized in that, The moisture-controlling and food-preserving film comprises a core layer and a skin layer covering both sides of the core layer, in parts by weight: The core layer is made from the following raw materials: 40-60 parts of PBAT resin; 15-25 parts of PLA resin; 15-30 parts of modified MOF functional masterbatch; 0.5-2 parts of chain extender; and 0.2-1 parts of dispersant lubricant. The skin layer is made from the following raw materials in parts by weight: 60-70 parts PBAT resin, 30-40 parts PLA resin, and 0.5-1.5 parts chain extender; The modified MOF powder used in the modified MOF functional masterbatch was prepared by the following method: Step 1: Preparation of polymer solution: Add 500 mL of xylene to a three-necked flask; heat to 95-100 °C; while stirring, add maleic anhydride-grafted polybutylene succinate in batches; maintain the temperature and stir for 30-60 minutes until the polymer is completely dissolved, forming a clear and transparent polymer hot solution with a certain viscosity; keep warm for later use. Step 2, Preparation of MOF dispersion: In another container, ZIF-8 nanoparticles were added to anhydrous ethanol; high-intensity ultrasonic dispersion was performed for 30 minutes using an ultrasonic cell disruptor to prevent particle agglomeration, resulting in a milky white dispersion. Step 3, Mixing and Reaction: Stop keeping the polymer hot solution in Step 1 at a constant temperature and cool it down to 60°C. Then, pour the ZIF-8 ethanol dispersion from Step 2 into the reaction vessel containing the polymer hot solution prepared in Step 1 in batches under high-speed stirring. After the addition is complete, continue stirring at 60°C for 20 minutes; The oil bath temperature was then raised to 90°C, bringing the mixed solvent system to a gentle reflux state; a catalyst was added to the system; and the reaction was maintained under reflux with stirring for 5 hours. Step 4: After the reaction is complete: stop heating, allow to cool naturally to room temperature, centrifuge the slurry and discard the supernatant; add 80℃ hot xylene to the centrifuged precipitate, stir vigorously to resuspend and wash for 30 minutes, and centrifuge again; repeat twice, and finally place the solid in an 80℃ vacuum oven to dry for 24 hours to obtain the modified MOF powder.

2. The moisture-controlling and preservation film for fruits and vegetables according to claim 1, characterized in that, The moisture-controlled preservation film includes a core layer and a skin layer covering both sides of the core layer. By weight, the core layer is made of the following raw materials: 40 parts PBAT resin; 25 parts PLA resin; 30 parts modified MOF functional masterbatch; and 2 parts chain extender. 1 part of dispersing lubricant; The skin is made from the following raw materials in parts by weight: 60 parts PBAT resin, 40 parts PLA resin, and 1.5 parts chain extender.

3. The moisture-controlling and preservation film for fruits and vegetables according to claim 1, characterized in that, The melt flow rate of the PBAT resin is 3-5 g / 10 min at 190°C and 2.16 kg load. The melt flow rate of the PLA resin is 3-7 g / 10 min at 210°C and 2.16 kg load.

4. The moisture-controlling and preservation film for fruits and vegetables according to claim 1, characterized in that, The chain extender in the core and skin layers is either ADR-4368 or ADR-4468.

5. A moisture-controlling and preservation film for fruits and vegetables according to claim 1, characterized in that, The dispersing lubricant in the core layer is erucamide and / or zinc stearate.

6. A moisture-controlling and preservation film for fruits and vegetables according to claim 1, characterized in that, The MOF in the modified MOF functional masterbatch has an average particle size of 100~300 nm and a specific surface area > 1300 m² / g.

7. A moisture-controlling and preservation film for fruits and vegetables according to claim 1, characterized in that, The preparation of the modified MOF functional masterbatch includes the following steps: (1) Mixing the modified MOF powder, PBAT resin, PBS-g-MAH, chain extender, and zinc stearate in a high-speed mixer at 1500 rpm for 5 minutes; (2) After mixing, add to a co-rotating twin-screw extruder with L / D=40; (3) Extrusion process settings: Temperature zone settings: Zone 1 130℃, Zone 2 145℃, Zone 3 155℃, Zone 4 160℃, Zone 5 155℃, Die head 150℃; Screw speed: 350 rpm; Vacuum exhaust: In Zone 5, a strong vacuum is turned on to remove trace amounts of moisture and small volatile molecules from the raw material. (4) Extrude into strips, air cool, pelletize, and vacuum package at 60℃ to obtain reactive MOF functional masterbatch.

8. A method for preparing a moisture-controlling and preservation film for fruits and vegetables according to any one of claims 1-7, characterized in that, The method for preparing the moisture-controlling and preservation film for fruits and vegetables includes the following steps: Step a: Dry the core layer and skin layer raw materials separately in a vacuum blower at 80℃ for 6 hours, with a moisture content of <50ppm; Step b: Add the dried raw material to the three-layer co-extrusion blown film machine; wherein the core extruder (B) is set to a temperature of 150℃-160℃-165℃; wherein the skin extruders A / C are set to a temperature of 160℃-170℃-175℃; the die head is set to a temperature of 170℃ and a die gap of 1.2mm. Step c: Set the inflation ratio to 3.0, the traction speed to 15m / min, and the frost line height to 300mm; Step d: After winding, cure the film roll in a 40℃ drying oven for 48 hours.

9. A method for preparing a moisture-controlling and preservative film for fruits and vegetables according to claim 8, characterized in that, The moisture-controlling and preservation film for fruits and vegetables has a structure of skin layer / core layer / skin layer with a thickness ratio of 1:2:1.

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