WO2024244847A1 - Freeze-stretched thin film and preparation process therefor - Google Patents

Abstract

Disclosed in the present invention are a freeze-stretched thin film and a preparation process therefor. The freeze-stretched thin film sequentially comprises a corona layer, a secondary surface layer, a middle layer, a secondary inner layer and a heat seal layer, wherein the raw materials of the corona layer comprise a polymer of olefin A, and the melting point of the polymer of olefin A is greater than that of polyethylene; the raw materials of the secondary surface layer comprise a polymer of olefin A and an olefin A-ethylene copolymer; the raw materials of the middle layer comprise polyethylene and an olefin A-ethylene copolymer, and the content of the olefin A-ethylene copolymer in the middle layer is lower than that of the olefin A-ethylene copolymer in the secondary surface layer; the raw materials of the secondary inner layer comprise polyethylene; and the raw materials of the heat seal layer comprise metallocene polyethylene. The freeze-stretched thin film not only has good cold resistance, body-fitting effect, tensile strength, impact strength and puncture resistance, but also is inexpensive, recyclable and has good environmental protection performance.

Description

A frozen stretched film and its preparation process Technical Field

The invention belongs to the technical field of frozen films, and in particular relates to a frozen stretched film and a preparation process thereof.

Background Art

Frozen film is a packaging material used to package frozen foods, such as glutinous rice balls, dumplings, seafood, etc. This application has high requirements for the cold resistance, body-fitting effect and strength of the frozen film. The packaging bag produced cannot be brittle under freezing conditions and can fit tightly on the packaged items in the packaged state. Since the frozen film has excellent puncture resistance (needle puncture) and body-fitting effect, it is also suitable for the packaging of electronic products and medical devices.

At present, most of the freezing films on the market are five-layer co-extruded materials of PA (polyamide)/adhesive layer/PE/PE/PE (polyethylene). Among them, PA material is in the outermost layer of the film material because of its good gas barrier and moisture resistance, and PE is in the inner layer of the film material because of its good heat sealing performance.

However, since PA and PE are two completely different substances, they cannot enter the next recycling system (such as granulation) after the first use, and can only be discarded or incinerated, causing environmental pollution. Therefore, this structure is not environmentally friendly, and its materials cannot be recycled, which does not comply with relevant national policies. In addition, the price of PA is more expensive than polyolefins, which does not meet market competition requirements.

Summary of the invention

The purpose of the invention is to provide a freeze-stretched film and a preparation process thereof, wherein the freeze-stretched film not only has good cold resistance, body-fitting effect, tensile strength, impact strength and puncture resistance, but also is low in price, can be recycled and has good environmental performance.

In order to achieve the above-mentioned object of the invention, the technical solution of the present invention is:

A frozen stretch film comprises a corona layer, a sub-surface layer, an intermediate layer, a sub-inner layer and a heat-sealing layer in sequence, wherein:

The raw material of the corona layer includes a polymer of olefin A, and the melting point of the polymer of olefin A is greater than the melting point of polyethylene;

The raw materials of the sub-surface layer include polymers of olefin A and olefin A-ethylene copolymers;

The raw materials of the intermediate layer include polyethylene and olefin A-ethylene copolymer, and the content of olefin A-ethylene copolymer in the intermediate layer is lower than that in the sub-surface layer;

The material of the secondary inner layer includes polyethylene;

The raw material of the heat sealing layer includes metallocene polyethylene.

In the present invention, the raw materials of each layer are olefin polymers, the raw materials have the same properties, are easy to recycle in the later stage, and have good environmental performance; and the price of olefin is lower than that of PA, so the preparation cost of the film is greatly reduced.

In the present invention, olefin A can be any olefin except ethylene, as long as the melting point of the polymer of olefin A is greater than the melting point of polyethylene, so that there is a temperature difference between the corona layer and the heat sealing layer to facilitate bag making.

In the present invention, the sub-surface layer contains not only the same olefin A polymer as the corona layer, but also olefin A-ethylene copolymer, and the middle layer contains not only the same polyethylene as the sub-inner layer, but also olefin A-ethylene copolymer, which forms a transition between the corona layer and the sub-inner layer. The olefin A-ethylene copolymer has excellent compatibility with the olefin A polymer and polyethylene. This excellent compatibility allows the molecules of the blended materials to be entangled, effectively improving the interlayer bonding force. Therefore, the corona layer and the sub-inner layer can be tightly bonded without the use of an adhesive, so that the film has excellent cold resistance (can be quickly frozen at -40°C and stored in an environment of -18°C), puncture resistance and drop resistance (high impact strength).

Preferably, in the above-mentioned freeze-stretched film, the olefin A is propylene. There is a temperature difference of 50° C. between the melting points of polypropylene and polyethylene, so during heat sealing, the heat sealing layer melts and adheres at the heat sealing temperature, while the corona layer does not melt.

Preferably, in the above-mentioned freeze-stretched film, the polymer of olefin A is homopolypropylene. Compared with other types of polypropylene, homopolypropylene has higher strength.

Preferably, in the above-mentioned freeze-stretched film, the content of olefin A in the olefin A-ethylene copolymer is higher than that of ethylene; for example, Vistamaxx, a product of Mobil Corporation of the United States, has an ethylene content of about 15% and a propylene content of about 85%.

Thus, since the content of olefin A-ethylene copolymer in the sub-surface layer is higher than that in the middle layer, in general, the content of olefin A in the sub-surface layer is higher, and has a higher compatibility with the corona layer; while the content of ethylene in the middle layer is higher, and has a higher compatibility with the sub-inner layer. In this case, the content of olefin A decreases from the corona layer to the heat-sealing layer, while the content of ethylene increases, and the bonding force between the film layers is stronger.

Preferably, in the above-mentioned freeze-stretched film, the content of olefin A-ethylene copolymer in the sub-surface layer is 40-80% by mass percentage;

In the intermediate layer, the content of olefin A-ethylene copolymer is 30-70%. Preferably, in the above-mentioned freeze-stretched film, in the intermediate layer and the secondary inner layer, the polyethylene is at least one of low-density polyethylene and metallocene high-density polyethylene.

Preferably, in the above-mentioned freeze-stretched film, the thicknesses of the corona layer, sub-surface layer, middle layer, sub-inner layer and heat-sealing layer are 5%-50%, 10%-50%, 10%-50%, 10%-50% and 5%-50% of the film thickness respectively.

Preferably, the thickness of the above-mentioned freeze-stretched film is 80-160 microns.

As further preferred, the thickness of the above-mentioned freeze-stretched film is 100-120 microns.

The present invention also provides a process for preparing the above-mentioned frozen stretched film, which comprises the following steps:

(1) Using the preset raw materials, a five-layer composite film is obtained in a five-layer co-extrusion film blowing machine;

Among them, the extrusion temperature of the A extruder used for extruding the corona layer is: 160°C in zone 1, 180°C in zone 2, 200°C in zone 3, and 220°C in zone 4;

The extrusion temperatures of the B extruder used for extruding the secondary outer layer are: 120°C in zone 1, 160°C in zone 2, 180°C in zone 3, and 190°C in zone 4;

The extrusion temperatures of the C extruder used to extrude the middle layer are: 100°C in zone 1, 150°C in zone 2, 180°C in zone 3, and 190°C in zone 4;

The extrusion temperatures of the D extruder used for extruding the secondary inner layer are: 110°C in zone 1, 160°C in zone 2, 180°C in zone 3, and 190°C in zone 4;

The extrusion temperatures of the E extruder used for extruding the heat seal layer are: 110°C in zone 1, 150°C in zone 2, 170°C in zone 3, and 180°C in zone 4;

(2) The five-layer composite film is subjected to thickness measurement, corona treatment, winding, plasticization, cross-linking, and slitting to obtain the freeze-stretched film.

In the present invention, the five-layer composite film is also subjected to electron beam cross-linking treatment. The electron beam cross-linking treatment can change the molecular entanglement mode between the film layers and convert the branched structure into a network cross-linked structure, thereby not only further improving the puncture resistance and drop resistance of the film, but also making the film softer (high tensile strength), better in body adhesion, and without knife sticking phenomenon.

The cross-linking step is carried out in a scanning box, in which an electron beam bombards the film to achieve the purpose of cross-linking. The electron beam voltage is 0.5 MeV, the beam current is 50 mA, and the vacuum degree of the scanning box is less than 2×10 ^-4 .

Compared with the prior art, the beneficial effects of the present invention are as follows:

(1) In the present invention, the raw materials of each layer are polymers of olefins, and the raw materials have the same properties, which is convenient for subsequent recycling and has good environmental performance. In addition, the price of olefins is lower than that of PA, which greatly reduces the preparation cost of the film.

(2) In the present invention, the sub-surface layer contains not only the same olefin A polymer as the corona layer but also olefin A-ethylene copolymer, and the middle layer contains not only the same polyethylene as the sub-inner layer but also olefin A-ethylene copolymer, thus forming a transition between the corona layer and the sub-inner layer. The olefin A-ethylene copolymer has excellent compatibility with both the olefin A polymer and the polyethylene. This excellent compatibility allows the molecules of the blended materials to be entangled, effectively improving the interlayer bonding force. Therefore, the corona layer and the sub-inner layer can be tightly bonded without the use of an adhesive, so that the film has excellent cold resistance, puncture resistance and drop resistance (high impact strength).

(3) The preparation method of the present invention includes the step of subjecting the five-layer composite film to electron beam cross-linking treatment. The electron beam cross-linking treatment can change the molecular entanglement mode between the film layers and transform the branched structure into a network cross-linked structure, thereby not only further improving the puncture resistance and drop resistance of the film, but also making the film softer (high tensile strength), better in body adhesion and without knife sticking.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic structural diagram of a freeze-stretched film according to the present invention;

FIG. 2 is a flow chart of the preparation process of the freeze-stretched film of the present invention.

DETAILED DESCRIPTION

The technical solution of the present invention is further described in detail below in conjunction with the accompanying drawings and specific implementation methods.

Example 1

As shown in FIG1 , a frozen stretched film according to this embodiment includes a corona layer, a sub-surface layer, an intermediate layer, a sub-inner layer and a heat-sealing layer in sequence; in terms of mass percentage, the raw material compositions of each layer are respectively:

Corona layer: isotactic homopolymer polypropylene (Singapore, TPC, FS3028; without anti-sticking agent and slip agent; TM (melt point) = 163 ° C, MI = 3.5) 100%;

Second outer layer: isotactic homopolymer polypropylene 20% + propylene-ethylene copolymer (U.S., Mobil, Vistamaxx; TM (melt point) = 59°C, MI = 5.0) 80%;

Middle layer: Metallocene high-density polyethylene (UK, INEOS, F4330; does not contain anti-sticking agent and slip agent. Density 0.938, MI=4.0) 70% + propylene-ethylene copolymer 30%;

Second inner layer: low-density polyethylene (USA, Dow, ELITE AT6201; TM (melt point) = 106 ° C, MI = 0.85) 100%;

Heat sealing layer: Metallocene polyethylene (Japan, SP0510, MLLDPE; TM (melt point) = 98°C, MI = 1.2) 97% + synthetic silica (Anpaise, CHAB-10, active ingredients account for 10%) 3%.

The preparation process of the freeze-stretched film is shown in FIG2 , and comprises the following steps:

(1) Using the preset raw materials, a five-layer composite film is obtained in a five-layer co-extrusion film blowing machine;

Among them, the extrusion temperature of the A extruder used for extruding the corona layer is: 160°C in zone 1, 180°C in zone 2, 200°C in zone 3, and 220°C in zone 4;

The extrusion temperatures of the B extruder used for extruding the secondary outer layer are: 120°C in zone 1, 160°C in zone 2, 180°C in zone 3, and 190°C in zone 4;

The extrusion temperatures of the C extruder used to extrude the middle layer are: 100°C in zone 1, 150°C in zone 2, 180°C in zone 3, and 190°C in zone 4;

The extrusion temperatures of the D extruder used for extruding the secondary inner layer are: 110°C in zone 1, 160°C in zone 2, 180°C in zone 3, and 190°C in zone 4;

The extrusion temperatures of the E extruder used for extruding the heat seal layer are: 110°C in zone 1, 150°C in zone 2, 170°C in zone 3, and 180°C in zone 4;

(2) The five-layer composite film is subjected to thickness measurement, corona treatment, winding, plasticization, cross-linking, and slitting to obtain the freeze-stretched film;

The cross-linking step is carried out in a scanning box, in which an electron beam bombards the film to achieve the purpose of cross-linking, the electron beam voltage is 0.5 Mev, the beam current is 50 mA, and the vacuum degree of the scanning box is less than 2×10 ^-4 .

The other steps are performed according to methods known in the prior art.

The thickness of the frozen stretched film prepared in this embodiment is 110 microns, wherein the thickness of the corona layer is 10 microns, the thickness of the subsurface layer is 25 microns, the thickness of the middle layer is 25 microns, the thickness of the subinner layer is 25 microns, and the thickness of the heat sealing layer is 15 microns.

Example

This embodiment is a frozen stretched film, and its structure and preparation process are the same as those of embodiment 1, except that the raw material composition of each layer of the film is:

Corona layer: isotactic homopolymer polypropylene (Korea, Samsung, HF429; TM (melt point) = 162 ° C, MI = 8.0) 100%;

Second outer layer: isotactic homopolymer polypropylene 20% + propylene-ethylene copolymer (U.S., Mobil, Vistamaxx; TM (melt point) = 59°C, MI = 5.0) 80%;

Middle layer: low-density polyethylene (USA, Dow, ELITE AT6201; TM (melt point) = 106°C, MI = 0.85) 70% + propylene-ethylene copolymer 30%;

Second inner layer: low-density polyethylene (Mobil, 151BW; does not contain anti-sticking agent and lubricant, density 0.933, MI = 3.0) 50% + metallocene high-density polyethylene (Dow Chemical, 2047G; does not contain anti-sticking agent and lubricant, density 0.917, MI = 2.3) 50%;

Heat sealing layer: Metallocene polyethylene (Japan, SP0510, MLLDPE; TM (melt point) = 98°C, MI = 1.2) 95% + synthetic silica (Anpaise, CHAB-10, active ingredients account for 10%) 5%.

Example

This embodiment is a frozen stretched film, and its structure and preparation process are the same as those of Example 1, except that the content of propylene-ethylene copolymer in the sub-outer layer is 40%, and the content of propylene-ethylene copolymer in the middle layer is 70%.

The structure and preparation process of the frozen stretched film of this comparative example are basically the same as those of Example 1, except that the content of the propylene-ethylene copolymer in the sub-outer layer is 10%, and the content of the propylene-ethylene copolymer in the middle layer is 90%.

The structure and preparation process of the freeze-stretched film of this comparative example are basically the same as those of Example 1, except that the preparation process does not include a cross-linking step.

The freeze-stretched films prepared in Examples 1-3 and Comparative Examples 1-2 were taken and various physical properties of the films were tested. The test results are shown in Table 1.

As can be seen from Table 1, compared with Comparative Examples 1-2, the films of Examples 1-3 have higher temperature resistance (increased by 5-7°C), so when the films are cut, they are less likely to stick to the knife; at the same time, the films of Examples 1-3 have better cold resistance, tensile strength, impact strength, puncture resistance and body-fitting effect. In Comparative Example 1, since the content of propylene-ethylene polymer in the sub-surface layer is less than that in the middle layer, it does not play a due transitional role between the corona layer and the sub-inner layer, resulting in a decrease in its various physical properties. Comparative Example 2 is due to the lack of cross-linking treatment in the preparation process, and the lack of bonding between the film layers, which also leads to a decrease in its various physical properties.

Claims (10)

A frozen stretch film, comprising a corona layer, a sub-surface layer, an intermediate layer, a sub-inner layer and a heat-sealing layer in sequence, characterized in that: The raw material of the corona layer includes a polymer of olefin A, and the melting point of the polymer of olefin A is greater than the melting point of polyethylene; The raw materials of the sub-surface layer include polymers of olefin A and olefin A-ethylene copolymers; The raw materials of the intermediate layer include polyethylene and olefin A-ethylene copolymer, and the content of olefin A-ethylene copolymer in the intermediate layer is lower than that in the sub-surface layer; The material of the secondary inner layer includes polyethylene; The raw material of the heat sealing layer includes metallocene polyethylene. The freeze-stretched film according to claim 1, characterized in that the olefin A is propylene. The freeze-stretched film according to claim 2, characterized in that the polymer of olefin A is homopolypropylene. The freeze-stretched film according to claim 1, characterized in that the content of olefin A in the olefin A-ethylene copolymer is higher than the content of ethylene. The freeze-stretched film according to claim 1, characterized in that, in the sub-surface layer, the content of olefin A-ethylene copolymer is 40-80% by mass percentage; In the intermediate layer, the content of olefin A-ethylene copolymer is 30-70%. The frozen stretched film according to claim 1, characterized in that in the middle layer and the sub-inner layer, the polyethylene is at least one of low-density polyethylene and metallocene high-density polyethylene. The frozen stretch film according to any one of claims 1 to 6, characterized in that the thicknesses of the corona layer, the sub-surface layer, the middle layer, the sub-inner layer and the heat-sealing layer are 5%-50%, 10%-50%, 10%-50%, 10%-50% and 5%-50% of the film thickness, respectively. The freeze-stretched film according to claim 7, characterized in that the film thickness is 80-160 microns. The freeze-stretched film according to claim 8, characterized in that the film thickness is 100-120 microns. The process for preparing a frozen stretched film according to any one of claims 1 to 9, characterized in that it comprises the following steps: (1) Using the preset raw materials, a five-layer composite film is obtained in a five-layer co-extrusion film blowing machine; Among them, the extrusion temperature of the A extruder used for extruding the corona layer is: 160°C in zone 1, 180°C in zone 2, 200°C in zone 3, and 220°C in zone 4; The extrusion temperatures of the B extruder used for extruding the secondary outer layer are: 120°C in zone 1, 160°C in zone 2, 180°C in zone 3, and 190°C in zone 4; The extrusion temperatures of the C extruder used to extrude the middle layer are: 100°C in zone 1, 150°C in zone 2, 180°C in zone 3, and 190°C in zone 4; The extrusion temperatures of the D extruder used for extruding the secondary inner layer are: 110°C in zone 1, 160°C in zone 2, 180°C in zone 3, and 190°C in zone 4; The extrusion temperatures of the E extruder used for extruding the heat seal layer are: 110°C in zone 1, 150°C in zone 2, 170°C in zone 3, and 180°C in zone 4; (2) The five-layer composite film is subjected to thickness measurement, corona treatment, winding, plasticization, cross-linking, and slitting to obtain the freeze-stretched film.