A puncture-resistant thinning stretch film and its preparation method

Abstract

The present invention relates to a puncture-resistant thinning stretch film, which is composed of five layers of materials. Among them, the raw materials of the second layer, the third layer and the fourth layer include 40 to 50 parts of copolymerized linear low-density polyethylene, 20 to 30 parts of ethylene-methyl acrylate copolymer, 3 to 10 parts of ultra-low-density polyethylene, 1 to 2 parts of maleic anhydride, 0.1 to 0.25 parts of benzoyl peroxide and 1 to 5 parts of active nano-calcium carbonate, by weight. Copolymerized linear low-density polyethylene, ethylene-methyl acrylate copolymer and ultra-low-density polyethylene, as ethylene polymers, undergo cross-linking under the action of benzoyl peroxide to form network-like cross-linked structures of varying degrees, thereby improving the puncture resistance of the film; in addition to playing a role in strengthening and toughening, active nano-calcium carbonate can also increase the melt flow rate, thereby facilitating the thinning of the film without affecting its transparency; ultra-low-density polyethylene can reduce the thickness of the film while maintaining the toughness of the co-extruded film and can also provide self-adhesion for the film layer.

Description

Technical Field

[0001] The present invention relates to the technical field of packaging materials, and in particular to a puncture-resistant thinned stretch film and a preparation method thereof. Background Art

[0002] Stretch film, also known as stretch film, offers excellent tensile strength, tear resistance, and transparency, along with good self-adhesiveness and high elongation. Its exceptional wrapping force and retractability allow products to be compactly and securely bundled into a single unit, preventing them from collapsing during transport. Consequently, it is widely used in the packaging industry. Currently, stretch film is primarily used during the transportation of various products to stabilize, protect, and identify them. Therefore, it must possess high puncture resistance, good shrinkage and shrinkage stress resistance, and high transparency. With the continuous development of society, the demand for stretch film packaging is increasing. Consequently, achieving environmentally friendly packaging materials has become a key priority, and reducing the use of stretch film by thinning it is a viable option. However, thinning the film can reduce tensile strength, potentially leading to overload fracture. It also reduces puncture resistance, making it susceptible to scratches and ruptures on sharp objects, and can also cause the material to neck during stretching. This phenomenon is usually improved by increasing the resin's melt flow rate (MFR). However, MFR is an inverse measure of polyethylene molecular chain length or its average size. As MFR increases, the tensile strength, tear strength, stress cracking resistance, heat resistance, weather resistance, and impact strength of the stretch film decrease. Therefore, how to achieve thinner stretch film while maintaining excellent tensile strength, puncture resistance, and high transparency has become a bottleneck in the stretch film industry.

[0003] Chinese patent CN109181195B discloses an ultrathin film and its production method. Using maleic anhydride-grafted low-density polyethylene as a base material, it incorporates metallocenes, polyisobutylene, graphene oxide, polyisoprene, and polyurethane. This approach reduces film production costs while improving machinability and adhesion. It also increases the film's temperature adaptability and promotes self-adhesion. The resulting film has a thickness of 5-10 μm, but its mechanical strength is somewhat reduced. Furthermore, Chinese patent CN112677603A discloses a stretch film and its production method. The film consists of seven layers. An ethylene-propylene elastomer is added to the seventh layer to impart higher tensile properties. Furthermore, carbon nanotubes pretreated with a silane coupling agent are added to each layer to improve the film's anisotropy. Optimizing the extrusion equipment and production process yields a thinner film with high mechanical properties. However, this stretch film uses a mixture of metallocene polyethylene, PBAT, and PLA. Due to the significant differences in the melting points of these raw materials, long-term production can lead to the accumulation of char on the die lip, significantly impacting film quality and increasing the number of downtimes for cleaning. Furthermore, the film produced using carbon nanotubes has poor transparency, hindering identification of packaged items and increasing the chance of delivery errors.

[0004] Therefore, it is of positive practical significance to produce a stretch film that is ultra-thin but also has high mechanical strength, puncture resistance and transparency. Summary of the Invention

[0005] In view of the shortcomings of the existing technology, the first purpose of the present invention is to provide a puncture-resistant thinned stretch film to solve the problems in the existing technology such as low puncture resistance after thinning of the stretch film, decreased mechanical strength, and a significant decrease in light transmittance of the stretch film after toughening and modification.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0007] The present invention provides a puncture-resistant thinned stretch film, which is composed of five layers of material: from the outside to the inside of the stretch film, there are the first layer, the second layer, the third layer, the fourth layer, and the fifth layer; the second layer, the third layer, and the fourth layer are made of the same material;

[0008] Among them, the raw materials of the second layer, the third layer and the fourth layer include 40 to 50 parts of copolymerized linear low-density polyethylene, 20 to 30 parts of ethylene-methyl acrylate copolymer, 3 to 10 parts of ultra-low-density polyethylene, 1 to 2 parts of maleic anhydride, 0.1 to 0.25 parts of benzoyl peroxide and 1 to 5 parts of active nano-calcium carbonate, calculated by weight.

[0009] The present invention proposes a puncture-resistant, thinned stretch film. Benzoyl peroxide, activated nano-calcium carbonate, maleic anhydride, and ultra-low-density polyethylene (ULDPE) in the middle three layers work together to form a toughening system. The copolymerized linear low-density polyethylene (LLDPE), ethylene-methyl acrylate copolymer, and ULDPE, as ethylene polymers, undergo cross-linking under the action of benzoyl peroxide, forming a network of crosslinked structures to varying degrees, thereby improving the film's puncture resistance. The activated nano-calcium carbonate not only strengthens and toughens the film but also increases its melt flow rate, facilitating thinning without compromising transparency. The ULDPE maintains the toughness of the co-extruded film while reducing film thickness and providing self-adhesiveness. The combined effects of these components allow the stretch film to maintain excellent puncture resistance, tensile strength, and transparency even under thinning conditions.

[0010] Optionally, the raw materials for the first layer include 70 to 80 parts by weight of copolymerized linear low-density polyethylene, 20 to 30 parts by weight of metallocene polyethylene and 2 to 5 parts by weight of polyisobutylene.

[0011] According to the above description, the first layer of raw materials mainly provides the film with self-adhesiveness.

[0012] Optionally, the raw material for the fifth layer includes, by weight, 60 to 80 parts of copolymerized linear low-density polyethylene, 5 to 40 parts of low-density polyethylene, and 0.1 to 0.5 parts of a fluoropolymer processing aid.

[0013] According to the above description, the fluorine-containing polymer processing aid added to the fifth layer raw material can effectively improve the film processing performance, play a traction role during film processing, make the product smooth and defect-free, which is conducive to the thinning of the stretch film and avoids the problem of char accumulation on the lip of the mold during long-term production.

[0014] Optionally, the active nano calcium carbonate has a spherical shape, an average particle size of 20 to 60 nm, and a BET specific surface area of ​​30 to 36 m 2 / g, the transparency is basically transparent.

[0015] According to the above description, the active nano-calcium carbonate can enhance the toughness of the system, and due to its substantially transparent nature, its addition will not affect the transparency of the stretch film.

[0016] Optionally, the mass ratio of raw materials of the first to fifth layers is 15-20:60-70:15-20.

[0017] Another aspect of the present invention provides a method for preparing a puncture-resistant thinned stretch film, comprising the following steps:

[0018] S1. Melt blending of raw materials: Weigh the raw materials according to the raw material ratio of the first to fifth layers, add the raw materials of each layer into the corresponding screw extruder for melt blending to obtain a uniform melt, and then extrude it into a distributor through the screw, and the distributor distributes the melt according to the mass ratio of each layer;

[0019] S2, cooling and solidification: the distributed melt is fed into the extrusion die for five-layer co-extrusion, with a die gap of 30-80 μm, and then cooled and solidified by rollers to form a film;

[0020] S3. Winding: The wound film is pulled and then wound. The storage temperature of the film roll is 10-40℃.

[0021] Optionally, the first layer and the fifth layer are melt-plasticized by a single-screw extruder, and the second layer, the third layer and the fourth layer are melt-plasticized by a twin-screw extruder; the twin-screw extruder and the distributor are connected by a melt pump.

[0022] Optionally, the heating temperature of the screw extruder is divided into 8 sections: zone I is 220-240°C, zone II is 200-220°C, zone III is 160-170°C, zone IV is 170-180°C, zone V is 180-190°C,

[0023] Zone VI is 190-200℃, Zone VII is 200-220℃, and Zone VIII is 220-240℃.

[0024] Optionally, the heating temperature of the melt pump is 220-240°C; the heating temperature of the screw extruder die head is 220-240°C.

[0025] According to the above description, by controlling parameters such as the melting temperature, the final cross-linking of PE can be achieved between the mold and the cooling roller, so that the melt flow rate of the resin changes from large to small, which is conducive to the uniform dispersion of active nano-calcium carbonate. DETAILED DESCRIPTION

[0026] The invention is further described in detail below with reference to specific embodiments.

[0027] The present invention provides a puncture-resistant thinned stretch film, which is composed of five layers of material: from the outside to the inside of the stretch film, there are the first layer, the second layer, the third layer, the fourth layer, and the fifth layer; the second layer, the third layer, and the fourth layer are made of the same material;

[0028] Among them, the raw materials of the second layer, the third layer and the fourth layer include, by weight, 40 to 50 parts of copolymerized linear low-density polyethylene (copolymerized LLDPE), 20 to 30 parts of ethylene-methyl acrylate copolymer (EVA), 3 to 10 parts of ultra-low-density polyethylene (ULLDPE), 1 to 2 parts of maleic anhydride (MAH), 0.1 to 0.25 parts of benzoyl peroxide (BPO) and 1 to 5 parts of active nano-calcium carbonate.

[0029] Preferably, the mass fraction of methyl acrylate in the ethylene-methyl acrylate copolymer is 10% to 20%, and the melt flow rate is 3 to 5 g / 10 min.

[0030] Preferably, the ultra-low density polyethylene is DowDuPont 4213.

[0031] Optionally, the raw materials for the first layer include 70 to 80 parts by weight of copolymerized linear low-density polyethylene, 20 to 30 parts of metallocene polyethylene (mLLDPE) and 2 to 5 parts of polyisobutylene (PIB).

[0032] Preferably, the metallocene polyethylene has a melt index of 4 to 8 g / 10 min and a density of 0.91 to 0.92 g / cm 3 , the relative molecular mass distribution is 2.0~5.0.

[0033] Optionally, the raw material for the fifth layer includes, by weight, 60 to 80 parts of copolymerized linear low-density polyethylene, 5 to 40 parts of low-density polyethylene (LDPE), and 0.1 to 0.5 parts of a fluoropolymer processing aid.

[0034] Preferably, the fluoropolymer processing aid is 3M Telema polymer processing aid FX5911.

[0035] Optionally, the active nano calcium carbonate has a spherical shape, an average particle size of 20 to 60 nm, and a BET specific surface area of ​​30 to 36 m 2 / g, the transparency is basically transparent.

[0036] Optionally, the mass ratio of raw materials of the first to fifth layers is 15-20:60-70:15-20.

[0037] Preferably, the mass ratio of raw materials of the first to fifth layers is 20:60:20.

[0038] It is prepared by the following method:

[0039] S1. Melt blending of raw materials: Weigh the raw materials according to the raw material ratio of the first to fifth layers, add the raw materials of each layer into the corresponding screw extruder for melt blending to obtain a uniform melt, and then extrude it into a distributor through the screw, and the distributor distributes the melt according to the mass ratio of each layer.

[0040] S2. Cooling and solidification: The distributed melt is fed into an extrusion die for five-layer co-extrusion with a die gap of 30 to 80 μm, and then cooled and solidified by a roller to form a film.

[0041] Preferably, the die gap of the die head is 30 μm.

[0042] S3. Winding: The wound film is pulled and then wound. The storage temperature of the film roll is 10-40℃.

[0043] Optionally, the first layer and the fifth layer are melt-plasticized by a single-screw extruder, and the second layer, the third layer and the fourth layer are melt-plasticized by a twin-screw extruder; the twin-screw extruder and the distributor are connected by a melt pump.

[0044] Optionally, the heating temperature of the screw extruder is divided into 8 sections: zone I is 220-240°C, zone II is 200-220°C, zone III is 160-170°C, zone IV is 170-180°C, zone V is 180-190°C, zone VI is 190-200°C, zone VII is 200-220°C, and zone VIII is 220-240°C.

[0045] Optionally, the heating temperature of the melt pump is 220-240°C; the heating temperature of the screw extruder die head is 220-240°C.

[0046] Example 1:

[0047] A puncture-resistant thinned stretch film comprising the following components in parts by weight:

[0048] The raw materials for the first layer include 70 parts of C6 LLDPE, 30 parts of mLLDPE, and 2 parts of PIB; the raw materials for the second, third, and fourth layers include 50 parts of C6 LLDPE, 20 parts of EVA, 3 parts of ULLDPE, 2 parts of MAH, 0.25 parts of BPO, and 5 parts of active nano-calcium carbonate; the raw materials for the fifth layer include 80 parts of C6LLDPE, 5 parts of LDPE, and 0.5 parts of fluoropolymer processing aid.

[0049] It is prepared by the following method:

[0050] S1. Melt blending of raw materials: Weigh the raw materials according to the raw material ratio of the first to fifth layers, add the raw materials of each layer into the corresponding screw extruder for melt blending to obtain a uniform melt, and then extrude it into a distributor through the screw. The distributor distributes the melt according to the raw material mass ratio of the first layer: the second layer, the third layer and the fourth layer: the fifth layer of 20:60:20.

[0051] S2. Cooling and solidification: The distributed melt is fed into an extrusion die for five-layer co-extrusion with a die gap of 30 μm, and then passed through a forming roller and a cooling roller for cooling and solidification to form a film.

[0052] S3. Winding: After the film is formed, the stretch film is pulled by the traction roller and then sent to the winding roller for winding. The storage temperature of the film roll is 10-40℃.

[0053] The first layer and the fifth layer are melted and plasticized by a single-screw extruder, and the second layer, the third layer and the fourth layer are melted and plasticized by a twin-screw extruder; the twin-screw extruder and the distributor are connected by a melt pump.

[0054] The heating temperature of the screw extruder is divided into 8 sections: zone I is 220-240°C, zone II is 200-220°C, zone III is 160-170°C, zone IV is 170-180°C, zone V is 180-190°C, zone VI is 190-200°C, zone VII is 200-220°C, and zone VIII is 220-240°C.

[0055] The heating temperature of the melt pump is 220-240°C; the heating temperature of the screw extruder die head is 220-240°C.

[0056] Example 2:

[0057] The difference between this embodiment and embodiment 1 is that the addition amount of each component is different.

[0058] A puncture-resistant thinned stretch film comprising the following components in parts by weight:

[0059] The raw materials for the first layer include 70 parts of C6 LLDPE, 30 parts of mLLDPE, and 2 parts of PIB; the raw materials for the second, third, and fourth layers include 40 parts of C6 LLDPE, 30 parts of EVA, 10 parts of ULLDPE, 2 parts of MAH, 0.25 parts of BPO, and 5 parts of active nano-calcium carbonate; the raw materials for the fifth layer include 60 parts of C6LLDPE, 5 parts of LDPE, and 0.5 parts of fluoropolymer processing aid.

[0060] Comparative Example 1

[0061] The difference between this comparative example and Example 2 is that the raw materials of the second layer, the third layer and the fourth layer do not contain MAH, BPO and active nano-calcium carbonate.

[0062] A puncture-resistant thinned stretch film comprising the following components in parts by weight:

[0063] The raw materials for the first layer include 70 parts of C6 LLDPE, 30 parts of mLLDPE, and 2 parts of PIB; the raw materials for the second, third, and fourth layers include 40 parts of C6 LLDPE, 30 parts of EVA, and 10 parts of ULLDPE; the raw materials for the fifth layer include 60 parts of C6 LLDPE, 5 parts of LDPE, and 0.5 parts of fluoropolymer processing aid.

[0064] Comparative Example 2

[0065] This comparative example differs from Example 2 in that the raw materials of the second layer, the third layer and the fourth layer do not contain ULLDPE.

[0066] A puncture-resistant thinned stretch film comprising the following components in parts by weight:

[0067] The raw materials for the first layer include 70 parts of C6 LLDPE, 30 parts of mLLDPE, and 2 parts of PIB; the raw materials for the second, third, and fourth layers include 40 parts of C6 LLDPE, 30 parts of EVA, 2 parts of MAH, 0.25 parts of BPO, and 5 parts of active nano-calcium carbonate; the raw materials for the fifth layer include 60 parts of C6 LLDPE, 5 parts of LDPE, and 0.5 parts of fluoropolymer processing aid.

[0068] Comparative Example 3

[0069] The difference between this comparative example and Example 2 is that the raw materials of the second layer, the third layer and the fourth layer do not contain ULLDPE, MAH, BPO and active nano-calcium carbonate.

[0070] A puncture-resistant thinned stretch film comprising the following components in parts by weight:

[0071] The raw materials for the first layer include 70 parts of C6 LLDPE, 30 parts of mLLDPE, and 2 parts of PIB; the raw materials for the second, third, and fourth layers include 40 parts of C6 LLDPE and 30 parts of EVA; the raw materials for the fifth layer include 60 parts of C6 LLDPE, 5 parts of LDPE, and 0.5 parts of fluoropolymer processing aid.

[0072] Comparative Example 4

[0073] The difference between this comparative example and Example 2 is that the raw material of the fifth layer does not contain a fluorine-containing polymer processing aid.

[0074] A puncture-resistant thinned stretch film comprising the following components in parts by weight:

[0075] The raw materials for the first layer include 70 parts of C6 LLDPE, 30 parts of mLLDPE, and 2 parts of PIB; the raw materials for the second, third, and fourth layers include 40 parts of C6 LLDPE, 30 parts of EVA, 10 parts of ULLDPE, 2 parts of MAH, 0.25 parts of BPO, and 5 parts of active nano-calcium carbonate; the raw materials for the fifth layer include 60 parts of C6LLDPE and 5 parts of LDPE.

[0076] The performance evaluation test was carried out on the stretch films obtained in each embodiment and comparative example. The specific test results are shown in Table 1.

[0077] The test standards for each item are as follows:

[0078] Thickness: GB / T6672-2001 Plastic film and sheeting - Determination of thickness - Mechanical measurement method;

[0079] Tensile strength and elongation at break: Tests were conducted in accordance with GB / T 1040.3-2006, Plastics—Determination of Tensile Properties—Part 3: Film and Sheeting—Test Conditions, as specified in BB / T 0024-2018 Stretch Film for Transport Packaging. 15 x 1.5 cm specimens were cut for testing, with a clamp distance of 50 mm and a speed of 250 mm / min.

[0080] Puncture resistance test: The test is carried out in accordance with the specifications in Appendix C of BB / T 0024-2018 Stretch Wrap Film for Transport Packaging;

[0081] Light transmittance: Determined according to GB / T 2410-2008 Transparent Plastics Light Transmittance and Haze Test Method.

[0082] Table 1 Performance test results

[0083]

[0084]

[0085] As can be seen from the above table, from Example 2 and Comparative Example 1, the tensile strength, elongation at break, and rupture force tests of Example 2 are generally better than those of Comparative Example 1. This is because the ethylene polymer in Example 2 is cross-linked under the action of the initiator, and the spherical active nano-calcium carbonate plays a role in strengthening and toughening. The light transmittance of Example 2 does not change significantly compared with that of Comparative Example 1. This is because the spherical active nano-calcium carbonate used in the present invention is substantially transparent and is uniformly dispersed in the film, and its addition has no effect on the light transmittance of the film.

[0086] From the test results of Example 2 and Comparative Examples 2 and 3, it can be seen that the elongation at break and the rupture force test results of Example 2 are better than those of Comparative Example 2. This is attributed to the fact that the ultra-low density polyethylene used in Example 2 can not only provide self-adhesion for the film layer, but also maintain the toughness of the co-extruded film. In addition, the melting points of the ultra-low density polyethylene and the linear low density polyethylene are close, the heating range is controllable, and there is no crystal point. The reason why the elongation at break and the rupture force test data of Example 2 are better than those of Comparative Example 3 is that Comparative Example 3 lacks the toughening system composed of ultra-low density polyethylene, maleic anhydride, benzoyl peroxide and active nano-calcium carbonate.

[0087] Comparing Example 2 with Comparative Example 4, it can be seen that the tensile strength, elongation at break, rupture force, and light transmittance of Example 2 are generally better than those of Comparative Example 4. This is because Comparative Example 4 lacks a fluoropolymer processing aid, resulting in very poor film fluidity, obvious edge effect, uneven thickness, stress defects starting from the thinnest parts, and long-term production of char accumulation at the lip of the mold, thereby affecting the light transmittance of the film.

[0088] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A puncture-resistant, thinned stretch film, comprising five layers of material, characterized in that: From the outside to the inside of the stretch film, there are the first layer, the second layer, the third layer, the fourth layer, and the fifth layer in order; the second layer, the third layer, and the fourth layer are made of the same material; The raw materials of the second layer, the third layer and the fourth layer include, by weight, 40-50 parts of copolymerized linear low-density polyethylene, 20-30 parts of ethylene-methyl acrylate copolymer, 3-10 parts of ultra-low-density polyethylene, 1-2 parts of maleic anhydride, 0.1-0.25 parts of benzoyl peroxide and 1-5 parts of active nano-calcium carbonate; In parts by weight, the raw materials of the first layer include 70-80 parts of copolymerized linear low-density polyethylene, 20-30 parts of metallocene polyethylene and 2-5 parts of polyisobutylene; In parts by weight, the raw materials for the fifth layer include 60-80 parts of copolymerized linear low-density polyethylene, 5-40 parts of low-density polyethylene and 0.1-0.5 parts of a fluorine-containing polymer processing aid.

2. The puncture-resistant thinned stretch film according to claim 1, characterized in that: The active nano-calcium carbonate has a spherical morphology, an average particle size of 20-60 nm, and a BET specific surface area of ​​30-36 m 2 / g, the transparency is basically transparent.

3. The puncture-resistant thinned stretch film according to claim 1, characterized in that: The mass ratio of the raw materials of the first layer, the sum of the second to fourth layers, and the fifth layer is 15-20:60-70:15-20.

4. A method for preparing the puncture-resistant thinned stretch film according to any one of claims 1 to 3, characterized in that: The following steps are involved: S1. Melt blending of raw materials: Weigh the raw materials according to the raw material ratio of the first to fifth layers, add the raw materials of each layer into the corresponding screw extruder for melt blending to obtain a uniform melt, and then extrude it into a distributor through the screw, and the distributor distributes the melt according to the mass ratio of each layer; S2, cooling and solidification: the distributed melt is fed into the extrusion die for five-layer co-extrusion, with a die gap of 30~80μm, and then cooled and solidified by rollers to form a film; S3. Winding: The film is pulled and then wound after forming. The storage temperature of the film roll is 10~40℃.

5. The method for preparing the puncture-resistant thinned stretch film according to claim 4, wherein: The first layer and the fifth layer are melted and plasticized by a single-screw extruder, and the second layer, the third layer and the fourth layer are melted and plasticized by a twin-screw extruder; the twin-screw extruder and the distributor are connected by a melt pump.

6. The method for preparing the puncture-resistant thinned stretch film according to claim 4, wherein: The heating temperature of the screw extruder is divided into 8 sections: zone I is 220-240°C, zone II is 200-220°C, zone III is 160-170°C, zone IV is 170-180°C, zone V is 180-190°C, zone VI is 190-200°C, zone VII is 200-220°C, and zone VIII is 220-240°C.

7. The method for preparing the puncture-resistant thinned stretch film according to claim 5, wherein: The heating temperature of the melt pump is 220-240°C; the heating temperature of the screw extruder die head is 220-240°C.

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