Frozen-stretched film and process for its production

Abstract

The application discloses a frozen-stretched film and a preparation process thereof. The frozen-stretched film comprises, in sequence, a corona layer, a subsurface layer, an intermediate layer, a subsurface inner layer and a heat-sealing layer. The raw material of the corona layer comprises a polymer of olefin A, and the melting point of the polymer of olefin A is higher than that of polyethylene. The raw material of the subsurface layer comprises the polymer of olefin A and an olefin A-ethylene copolymer. The raw material of the intermediate layer comprises polyethylene and the olefin A-ethylene copolymer, and the content of the olefin A-ethylene copolymer in the intermediate layer is lower than that in the subsurface layer. The raw material of the subsurface inner layer comprises polyethylene. The raw material of the heat-sealing layer comprises a metallocene polyethylene. The frozen-stretched film has good cold resistance, a good skin effect, tensile strength, impact strength and puncture resistance, is low in price, can be recycled and has good environmental protection performance.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of frozen film, and particularly relates to a frozen stretch film and a preparation process thereof. BACKGROUND

[0002] The frozen film is a packaging material for packaging frozen food such as Tangyuan, dumplings and seafood. The frozen film has high requirements for cold resistance, body effect and strength in such use occasions. The packaging bag made of the frozen film cannot be brittle in the frozen condition, and can be tightly attached to the packaged objects in the packaging state. Since the frozen film has excellent puncture resistance (needle puncture) and body effect, it is also suitable for packaging electronic products and medical devices.

[0003] At present, the commercially available frozen film is a PA (polyamide) / adhesive layer / PE / PE / PE (polyethylene) five-layer co-extrusion material. The PA material is located at the outermost layer of the film material due to its good gas barrier property and moisture resistance. The PE is located at the inner layer of the film material due to its good heat sealing property.

[0004] However, since PA and PE are two completely different substances, they cannot enter the next cycle system (such as granulation) after the first use is completed, and can only be discarded or incinerated to cause environmental pollution. Therefore, the structure is a non-environmental protection structure, and the material cannot be recycled. In addition, the price of PA is higher than that of polyolefin, which does not meet the market competition requirements. SUMMARY

[0005] The present application provides a frozen stretch film and a preparation process thereof. The frozen stretch film has good cold resistance, body effect, tensile strength, impact strength and puncture resistance, and is low in price, can be recycled and has good environmental protection performance.

[0006] To achieve the above-mentioned application purposes, the technical scheme of the present application is as follows:

[0007] A frozen stretch film comprises, in sequence, a corona layer, a subsurface layer, an intermediate layer, a sub-inner layer and a heat sealing layer, wherein,

[0008] The raw material of the corona layer comprises a polymer of an olefin A, and the melting point of the polymer of the olefin A is higher than that of polyethylene;

[0009] The raw material of the subsurface layer comprises a polymer of an olefin A and an olefin A-ethylene copolymer;

[0010] The raw material of the intermediate layer comprises polyethylene and an olefin A-ethylene copolymer, and the content of the olefin A-ethylene copolymer in the intermediate layer is lower than that in the subsurface layer;

[0011] The raw material of the sub-inner layer comprises polyethylene;

[0012] The raw material of the heat-seal layer comprises a metallocene polyethylene.

[0013] In the present application, the raw material of each layer is an olefin polymer, and the raw materials have the same properties, which is convenient for recycling in later stage and has good environmental protection performance; and the price of the olefin is lower than that of PA, so that the preparation cost of the film is greatly reduced.

[0014] In the present application, the olefin A can be any olefin except ethylene, as long as the melting point of the polymer of the olefin A is higher than that of polyethylene, so that there is a temperature difference between the corona layer and the heat-seal layer to facilitate bag making.

[0015] In the present application, the sub-surface layer not only contains the polymer of the same olefin A as the corona layer, but also contains an olefin A-ethylene copolymer, and the intermediate layer not only contains the same polyethylene as the sub-inner layer, but also contains an 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 polymer of the olefin A and polyethylene. This excellent compatibility causes the entanglement between the molecules of the blended material, effectively improving the interlayer bonding force. Therefore, the corona layer and the sub-inner layer can be tightly bonded without using an adhesive, so that the film has excellent cold resistance (can be frozen at a medium speed at-40℃ and stored in an environment at-18℃), puncture resistance and drop resistance (high impact strength).

[0016] As a preferred, in the above-mentioned frozen stretched film, the olefin A is propylene. The melting point of polypropylene and polyethylene has a temperature difference of 50℃, so that during heat sealing, the heat-seal layer melts and bonds at the heat-seal temperature, while the corona layer does not melt.

[0017] As a preferred, in the above-mentioned frozen stretched film, the polymer of the olefin A is homopolypropylene. Compared with other types of polypropylene, the homopolypropylene has higher strength.

[0018] As a preferred, in the above-mentioned frozen stretched film, the content of the olefin A in the olefin A-ethylene copolymer is higher than that of ethylene; for example, the product Vistamaxx of the American Mobil Company, in which the content of ethylene is about 15% and the content of propylene is about 85%.

[0019] Therefore, the content of the olefin A in the sub-surface layer is higher than that in the intermediate layer, and the content of ethylene in the intermediate layer is higher than that in the sub-inner layer. In this case, from the corona layer to the heat-seal layer, the content of the olefin A decreases, and the content of ethylene increases, so that the bonding force between the layers is stronger.

[0020] As preferred, in the above-mentioned frozen-stretched film, the content of the olefin A-ethylene copolymer in the sub-surface layer is 40-80% by mass;

[0021] The content of the olefin A-ethylene copolymer in the intermediate layer is 30-70%. As preferred, in the above-mentioned frozen-stretched film, the polyethylene in the intermediate layer and the sub-inner layer is at least one of low-density polyethylene and metallocene high-density polyethylene.

[0022] As preferred, in the above-mentioned frozen-stretched film, the thickness of the corona layer, the sub-surface layer, the intermediate layer, the sub-inner layer and the heat-seal layer is 5%-50%, 10%-50%, 10%-50%, 10%-50% and 5%-50% of the film thickness in turn.

[0023] As preferred, the thickness of the above-mentioned frozen-stretched film is 80-160 microns.

[0024] As further preferred, the thickness of the above-mentioned frozen-stretched film is 100-120 microns.

[0025] The present application also provides a preparation process of the above-mentioned frozen-stretched film, which comprises the following steps:

[0026] (1) using preset raw materials to obtain a five-layer composite film in a five-layer co-extrusion film blowing machine;

[0027] The extrusion temperature of the A extruder for extruding the corona layer is: 160℃ in zone 1, 180℃ in zone 2, 200℃ in zone 3 and 220℃ in zone 4.

[0028] The extrusion temperature of the B extruder for extruding the sub-outer layer is: 120℃ in zone 1, 160℃ in zone 2, 180℃ in zone 3 and 190℃ in zone 4.

[0029] The extrusion temperature of the C extruder for extruding the intermediate layer is: 100℃ in zone 1, 150℃ in zone 2, 180℃ in zone 3 and 190℃ in zone 4.

[0030] The extrusion temperature of the D extruder for extruding the sub-inner layer is: 110℃ in zone 1, 160℃ in zone 2, 180℃ in zone 3 and 190℃ in zone 4.

[0031] The extrusion temperature of the E extruder for extruding the heat-seal layer is: 110℃ in zone 1, 150℃ in zone 2, 170℃ in zone 3 and 180℃ in zone 4.

[0032] (2) after the five-layer composite film is measured in thickness, subjected to corona treatment, wound, plasticized, crosslinked and slitted, the above-mentioned frozen-stretched film is obtained.

[0033] In the present application, the five-layer composite film is also subjected to electron beam cross-linking treatment, which can change the way of molecular entanglement between the film layers, convert the branched chain structure into a network cross-linking structure, so as to not only further improve the puncture resistance and drop resistance of the film, but also make the film more flexible (high tensile strength), better in skin effect, and free of the phenomenon of sticking to the knife.

[0034] The cross-linking step is carried out in a scanning box, in which the film is bombarded by an electron beam to achieve the cross-linking purpose. 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 .

[0035] Compared with the prior art, the present application has the following beneficial effects:

[0036] (1) In the present application, the raw materials of each layer are all polyolefin polymers, which have the same properties and are easy to recycle and have good environmental protection performance. The price of polyolefin is lower than that of PA, so the preparation cost of the film is greatly reduced.

[0037] (2) In the present application, the sub-layer contains not only the polyolefin A polymer same as the corona layer, but also an olefin A-ethylene copolymer, and the intermediate layer contains not only the polyethylene same as the sub-inner layer, but also an 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 polyolefin polymer and polyethylene. This excellent compatibility makes the molecules of the blended material entangle, effectively improving the interlayer bonding force, so that the corona layer and the sub-inner layer can be tightly bonded without using an adhesive, and the film has excellent cold resistance, puncture resistance and drop resistance (high impact strength).

[0038] (3) The preparation method of the present application includes a step of subjecting the five-layer composite film to electron beam cross-linking treatment, which can change the way of molecular entanglement between the film layers, convert the branched chain structure into a network cross-linking structure, so as to not only further improve the puncture resistance and drop resistance of the film, but also make the film more flexible (high tensile strength), better in skin effect, and free of the phenomenon of sticking to the knife. BRIEF DESCRIPTION OF DRAWINGS

[0039] Figure 1 Fig. 1 is a structural schematic diagram of the frozen stretched film of the present application;

[0040] Figure 2 Fig. 2 is a preparation process flow chart of the frozen stretched film of the present application. DETAILED DESCRIPTION

[0041] The technical solutions of the present application will be further described in detail below in combination with the drawings and specific embodiments.

[0042] Example 1

[0043] As shown in the table below, the present example is a frozen-stretched film, which comprises, in order, a corona layer, an outer sub-layer, an intermediate layer, an inner sub-layer, and a heat-seal layer; the raw material composition of each layer is as follows, in terms of mass percentage: Figure 1 Corona layer: isotactic homopolymer polypropylene (Singapore, TPC, FS3028; without anti-adhesion agent and slip agent; TM (melt point) = 163°C, MI = 3.5) 100%;

[0044] Outer sub-layer: isotactic homopolymer polypropylene 20% + propylene-ethylene copolymer (USA, Mobil, Vistamaxx; TM (melt point) = 59°C, MI = 5.0) 80%;

[0045] Intermediate layer: metallocene high-density polyethylene (UK, Ineos, F4330; without anti-adhesion agent and slip agent. Density 0.938, MI = 4.0) 70% + propylene-ethylene copolymer 30%;

[0046] Inner sub-layer: low-density polyethylene (USA, Dow, ELITE AT6201; TM (melt point) = 106°C, MI = 0.85) 100%;

[0047] Heat-seal layer: metallocene polyethylene (Japan, SP0510, MLLDPE; TM (melt point) = 98°C, MI = 1.2) 97% + synthetic silica (Anpianse, CHAB-10, active ingredient accounting for 10%) 3%.

[0048] The preparation process of the frozen-stretched film is shown in the table below, which comprises the following steps:

[0049] Figure 2

[0050] (1) Using the preset raw materials, a five-layer composite film is obtained in a five-layer co-extrusion film blowing machine;

[0051] The extrusion temperature of the A extruder for extruding the corona layer is as follows: Zone 1 160°C, Zone 2 180°C, Zone 3 200°C, Zone 4 220°C;

[0052] The extrusion temperature of the B extruder for extruding the outer sub-layer is as follows: Zone 1 120°C, Zone 2 160°C, Zone 3 180°C, Zone 4 190°C;

[0053] The extrusion temperature of the C extruder for extruding the intermediate layer is as follows: Zone 1 100°C, Zone 2 150°C, Zone 3 180°C, Zone 4 190°C;

[0054] ​​The extrusion temperature of the D extruder for extruding the sub-inner layer is: 110°C for zone 1, 160°C for zone 2, 180°C for zone 3, and 190°C for zone 4.

[0055] The extrusion temperature of the E extruder for extruding the heat-seal layer is: 110°C for zone 1, 150°C for zone 2, 170°C for zone 3, and 180°C for zone 4.

[0056] (2) The five-layer composite film is subjected to thickness measurement, corona treatment, winding, plasticization, crosslinking, and slitting to obtain the frozen-stretched film.

[0057] The crosslinking step is performed in a scanning box, in which the film is bombarded by an electron beam to achieve crosslinking. 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 x 10 -4 .

[0058] The other steps are performed according to the methods known in the art.

[0059] The frozen-stretched film prepared in this example has a thickness of 110 microns, in which the thickness of the corona layer is 10 microns, the thickness of the sub-surface layer is 25 microns, the thickness of the intermediate layer is 25 microns, the thickness of the sub-inner layer is 25 microns, and the thickness of the heat-seal layer is 15 microns.

[0060] Example 2

[0061] This example is a frozen-stretched film, which has the same structure and preparation process as Example 1, except that the raw material composition of each layer of the film is as follows:

[0062] Corona layer: isotactic homopolymer polypropylene (South Korea, Samsung, HF429; TM (melt point) = 162°C, MI = 8.0) 100%;

[0063] Sub-surface layer: isotactic homopolymer polypropylene 20% + propylene-ethylene copolymer (USA, Mobil, Vistamaxx; TM (melt point) = 59°C, MI = 5.0) 80%;

[0064] Intermediate layer: low-density polyethylene (USA, Dow, ELITE AT6201; TM (melt point) = 106°C, MI = 0.85) 70% + propylene-ethylene copolymer 30%;

[0065] Sub-inner layer: low-density polyethylene (USA, Mobil, 151BW; without anti-adhesion agent and slip agent, density 0.933, MI = 3.0) 50% + metallocene high-density polyethylene (USA, Dow Chemical, 2047G; without anti-adhesion agent and slip agent, density 0.917, MI = 2.3) 50%;

[0066] Heat-seal layer: metallocene polyethylene (Japan, SP0510, MLLDPE; TM (melt point) = 98°C, MI = 1.2) 95% + synthetic silica (Anpiaque, CHAB-10, active ingredient 10%) 5%.

[0067] Example 3

[0068] The frozen-stretched film of this example has the same structure and preparation process as Example 1, except that the content of propylene-ethylene copolymer in the secondary outer layer is 40%, and the content of propylene-ethylene copolymer in the intermediate layer is 70%.

[0069] Comparative Example 1

[0070] The frozen-stretched film of this example has the same structure and preparation process as Example 1, except that the content of propylene-ethylene copolymer in the secondary outer layer is 10%, and the content of propylene-ethylene copolymer in the intermediate layer is 90%.

[0071] Comparative Example 2

[0072] The frozen-stretched film of this example has the same structure and preparation process as Example 1, except that the preparation process does not include a cross-linking step.

[0073] The frozen-stretched films prepared in Examples 1-3 and Comparative Examples 1-2 were tested for various physical properties, and the results are shown in Table 1.

[0074] Table 1

[0075]

[0076] 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), and are less likely to stick to the knife during slitting operation. At the same time, the films of Examples 1-3 have better cold resistance, tensile strength, impact strength, puncture resistance, and skin effect. In Comparative Example 1, the content of propylene-ethylene copolymer in the secondary outer layer is less than that in the intermediate layer, and the secondary outer layer does not play its due transitional role between the corona layer and the secondary inner layer, resulting in a decrease in various physical properties. In Comparative Example 2, the cross-linking treatment is not performed in the preparation process, and the interlayer bonding force is insufficient, also resulting in a decrease in various physical properties.

Claims

1. A frozen-stretched film, comprising, in order, a corona layer, a subsurface layer, an intermediate layer, a sub-inner layer and a heat-seal layer, characterized in that, the raw material of the corona layer comprises a polymer of an olefin A, and the melting point of the polymer of the olefin A is higher than that of polyethylene; the raw material of the subsurface layer comprises a polymer of the olefin A and an olefin A-ethylene copolymer; the raw material of the intermediate layer comprises polyethylene and the olefin A-ethylene copolymer, and the content of the olefin A-ethylene copolymer in the intermediate layer is lower than that in the subsurface layer; the raw material of the sub-inner layer comprises polyethylene; and the raw material of the heat-seal layer comprises metallocene polyethylene. The olefin A is propylene. The polymer of the olefin A is homopolymer polypropylene. The content of the olefin A in the olefin A-ethylene copolymer is higher than that of ethylene. The content of the olefin A-ethylene copolymer in the subsurface layer is 40-80% by mass percentage. The content of the olefin A-ethylene copolymer in the intermediate layer is 30-70%. The polyethylene in the intermediate layer and the sub-inner layer is at least one of low-density polyethylene and metallocene high-density polyethylene. The thicknesses of the corona layer, the subsurface layer, the intermediate layer, the sub-inner layer and the heat-seal layer are 5-50%, 10-50%, 10-50%, 10-50% and 5-50% of the thickness of the film, respectively. The thickness of the film is 80-160 microns. The thickness of the film is 100-120 microns. The film is obtained by the following steps: (1) using preset raw materials to obtain a five-layer composite film in a five-layer co-extrusion film blowing machine; wherein the extrusion temperature of an A extruder 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 temperature of a B extruder for extruding the subsurface layer is 120°C in zone 1, 160°C in zone 2, 180°C in zone 3 and 190°C in zone 4; the extrusion temperature of a C extruder for extruding the intermediate layer is 100°C in zone 1, 150°C in zone 2, 180°C in zone 3 and 190°C in zone 4; the extrusion temperature of a D extruder for extruding the sub-inner layer is 110°C in zone 1, 160°C in zone 2, 180°C in zone 3 and 190°C in zone 4; and the extrusion temperature of an E extruder for extruding the heat-seal layer is 110°C in zone 1, 150°C in zone 2, 170°C in zone 3 and 180°C in zone 4; and (2) obtaining the frozen-stretched film by measuring the thickness, corona treatment, winding, plasticizing, crosslinking and slitting of the five-layer composite film. ​ ​ ​ ​ ​ 2. The frozen-stretched film according to claim 1, wherein ​ 3. The frozen-stretched film according to claim 2, wherein ​ 4. The frozen-stretched film according to claim 1, wherein ​ 5. The frozen-stretched film according to claim 1, wherein ​ ​ 6. The frozen-stretched film according to claim 1, wherein ​ 7. The frozen-stretched film according to any one of claims 1 to 6, wherein ​ 8. The frozen-stretched film according to claim 7, wherein ​ 9. The frozen-stretched film according to claim 8, wherein ​ 10. The process for producing a frozen-stretched film according to any one of claims 1 to 9, wherein ​ ​ ​ ​ ​ ​ ​ ​

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