Degradable film blowing material and processing technology thereof

Abstract

The invention discloses a degradable film blowing material and a processing technology thereof, and relates to the technical field of degradable film blowing materials, the degradable film blowing material comprises the following preparation raw materials by mass: 50-70 parts of modified polylactic acid, 20-50 parts of PBAT resin, 5-20 parts of modified starch, 3-10 parts of nano calcium carbonate, 2-5 parts of talcum powder, 1-5 parts of a dispersant, 1-5 parts of a plasticizer, 0.1-1.0 part of an antioxidant and 0.5-1.5 parts of a lubricant. The processing method comprises the following steps: S1, drying modified polylactic acid and PBAT resin, and plasticizing modified starch; s2, mixing the preparation raw materials of the degradable film blowing material, extruding, and granulating to obtain degradable master batches; and S3, carrying out film blowing molding on the degradable master batch to obtain the degradable film blowing material. The degradable film blowing material is relatively high in tearing strength and notch impact strength, and still keeps a relatively high degradation rate.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of degradable blown film materials, and particularly relates to a degradable blown film material and a processing technology thereof. BACKGROUND

[0002] Plastic products have brought about serious environmental pollution problems while providing great convenience for people's life and production. The degradable plastic has significant environmental advantages because it can be quickly degraded after being discarded and is friendly to the environment. The market demand for blown film masterbatch, which is a key raw material for film production, is increasing day by day, especially in many fields such as agriculture, industry and construction.

[0003] At present, patent document CN114369339A discloses a production technology of low-cost biodegradable material and application thereof. The patent effectively improves the high-temperature and humidity resistance of the material by adding modifiers and stabilizers, and prolongs the service life of the material. At the same time, the introduction of nano-plant cellulose not only enhances the mechanical properties of the material, but also maintains good optical properties. The final plastic can be completely degraded into carbon dioxide and water, achieving the environmental protection goal. However, the patent scheme mainly solves the technical problem of degradability, but there is still a certain limitation in improving the strength of the material. SUMMARY

[0004] The application is to overcome the above technical problems, and therefore provides a degradable blown film material and a processing technology thereof. The degradable blown film material of the application improves the mechanical strength by modifying polylactic acid, so that the tear strength and notched impact strength of the prepared degradable blown film material are high, and the degradable blown film material still maintains a high degradation rate.

[0005] The application solves the above technical problems through the following technical scheme.

[0006] A degradable blown film material comprises the following mass parts of preparation raw materials: 50-70 parts of modified polylactic acid, 20-50 parts of PBAT resin (polybutylene adipate terephthalate), 5-20 parts of modified starch, 3-10 parts of nano calcium carbonate, 2-5 parts of talc powder, 1-5 parts of dispersant, 1-5 parts of plasticizer, 0.1-1.0 parts of antioxidant and 0.5-1.5 parts of lubricant.

[0007] Preferably, the degradable blown film material comprises the following mass parts of preparation raw materials: 50-60 parts of modified polylactic acid, 30-45 parts of PBAT resin, 8-15 parts of modified starch, 5-8 parts of nano calcium carbonate, 2-5 parts of talc powder, 1-3 parts of dispersant, 1-3 parts of plasticizer, 0.1-0.5 parts of antioxidant and 0.5-1 part of lubricant.

[0008] In some preferred embodiments, the melting temperature of the PBAT resin is 110-120℃.

[0009] In some preferred embodiments, the PBAT resin has an elongation at break ≥ 500%.

[0010] In some preferred embodiments, the PBAT resin has a tensile strength at break of 18-21 MPa.

[0011] In some preferred embodiments, the modified starch is sodium octenyl succinate starch (CAS No. 66829-29-6) or hydroxypropyl starch (CAS No. 9049-76-7).

[0012] In some preferred embodiments, the nano calcium carbonate has a D50 ≤ 50 nm, preferably the nano calcium carbonate has a D50 = 15-40 nm.

[0013] In some preferred embodiments, the nano calcium carbonate has a specific surface area ≥ 40 m 2 / g.

[0014] The modified polylactic acid comprises the following mass parts of raw materials for preparation: 100 parts of polylactic acid, 3-10 parts of bismaleimide-furan derivative, 0.1-0.8 parts of aluminum chloride, 5-15 parts of epoxy-functionalized polybutadiene, 0.1-0.3 parts of antioxidant, and 0.5-1 parts of lubricant.

[0015] Preferably, the modified polylactic acid comprises the following mass parts of raw materials for preparation: 100 parts of polylactic acid, 3-5 parts of bismaleimide-furan derivative, 0.1-0.5 parts of aluminum chloride, 5-10 parts of epoxy-functionalized polybutadiene, 0.1-0.3 parts of antioxidant, and 0.5-1 parts of lubricant.

[0016] Among them, polylactic acid itself is biodegradable, but has the problem of large brittleness, and needs to be modified to improve toughness. The epoxy groups of epoxy-functionalized polybutadiene interact with the end groups of polylactic acid, improve compatibility, avoid phase separation, and also improve the toughness of polylactic acid. The furan groups and maleimide groups in the bismaleimide-furan derivative undergo reversible Diels-Alder reaction under heating conditions, forming a dynamic crosslinking network, giving the material self-repairing ability and reworkability. In addition, aluminum chloride as a Lewis catalyst can reduce the activation energy of the reverse Diels-Alder reaction.

[0017] The preparation method of the modified polylactic acid comprises the following steps: mixing epoxy-functionalized polybutadiene and bismaleimide-furan derivative, then adding aluminum oxide, reacting at 55-75°C for 2h to form a pre-crosslinked network; extruding, granulating polylactic acid, pre-crosslinked network, antioxidant and lubricant to obtain modified polylactic acid.

[0018] In some preferred embodiments, the Mw of the polylactic acid is 50,000 to 200,000, and more preferably 60,000 to 95,000.

[0019] In some preferred embodiments, the bismaleimide-furan derivative is N , N '-(4-methyl-1,3-phenylene)bismaleimide (CAS No.: 6422-83-9) or N , N' -1,3-Benzenebismaleimide (CAS No.: 3006-93-7).

[0020] In some preferred embodiments, the Mn of the epoxy-functionalized polybutadiene is 3000~4000.

[0021] In some preferred embodiments, the dispersant is polyethylene wax (CAS No.: 9002-88-4), polypropylene wax (CAS No.: 9003-07-0), or stearamide (CAS No.: 124-26-5).

[0022] In some preferred embodiments, the plasticizer is tributyl citrate (CAS No.: 77-94-1), epoxidized soybean oil (CAS No.: 8013-07-8), or polypropylene glycol.

[0023] In some preferred embodiments, the antioxidant is antioxidant 1010 (CAS No.: 6683-19-8), antioxidant 1076 (CAS No.: 2082-79-3), or antioxidant DLTP (CAS No.: 123-28-4).

[0024] In some preferred embodiments, the lubricant is zinc stearate, calcium stearate, or ethylene bis-stearamide (CAS No.: 110-30-5).

[0025] This invention also discloses a processing technology for the aforementioned degradable blown film material, comprising the following steps: S1. Dry the modified polylactic acid and PBAT resin to a moisture content ≤0.02%, and then plasticize the modified starch; S2. The raw materials for preparing the degradable blown film material are mixed, extruded, and granulated to obtain the degradable masterbatch; S3. The degradable masterbatch is blown into a film to obtain a degradable blown film material.

[0026] The plasticizing treatment involves stirring modified starch and glycerol at 55-65°C for 20-50 minutes, wherein the mass ratio of modified starch to glycerol is 1:0.3-0.8.

[0027] In some preferred embodiments, the mixing step involves mixing the S1-treated modified polylactic acid, PBAT resin, modified starch, nano-calcium carbonate, talc, plasticizer, and antioxidant in a high-speed mixer. In some preferred embodiments, the extrusion setup includes an extruder temperature of 160-180°C and a screw speed of 200-300 rpm.

[0028] In some preferred embodiments, the blown film forming is performed using a blown film machine.

[0029] In some preferred embodiments, the temperature settings for blown film forming are: zone 1 temperature 150~160℃, zone 2 temperature 160~170℃, zone 3 temperature 170~180℃, and die head temperature 175~185℃.

[0030] In some preferred embodiments, the blow-up ratio of the blown film forming is 2.5 to 3.5.

[0031] In some preferred embodiments, the winding tension after blown film forming is 10~20N.

[0032] The biodegradable blown film material of this invention can be applied in agriculture, industry, construction, 3D printing, outdoor products and other fields.

[0033] The agricultural sector includes mulch film and greenhouse film; the industrial sector includes protective film for electronic products and protective film for automotive surfaces; the construction sector includes temporary protective film for buildings, waterproof and moisture-proof film, and building formwork film; and the outdoor sector includes tents, waterproof tarpaulins, and outdoor tablecloths.

[0034] Based on common knowledge in the field, the above-mentioned preferred conditions can be combined arbitrarily to obtain various preferred embodiments of the present invention.

[0035] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention modifies polylactic acid (PLA) and adds a dynamic crosslinking agent, bismaleimide-furan derivative, to endow the material with self-healing capabilities and reprocessability. Epoxy-functionalized polybutadiene combines toughening and compatibility-improving functions, significantly enhancing the tear strength and impact strength of PLA. The main components of this invention, such as PLA, PBAT resin, and modified starch, are all biodegradable materials, and non-toxic, environmentally friendly additives are selected to ensure the material's environmental and safety performance.

[0036] 2. The processing technology of the present invention is simple, and the equipment used is all conventional equipment. Without increasing the cost of equipment, the existing formula method is improved. Therefore, the processing technology of the present invention is more suitable for industrial production.

[0037] 3. The longitudinal tear strength of the degradable blown film material of the present invention is ≥135 kN / m, and in some preferred embodiments, the longitudinal tear strength can be 137~145 kN / m; the notched impact strength of the degradable blown film material is ≥36 kJ / m. 2 In some preferred embodiments, the value can be 36~42 kJ / m³. 2 The biodegradable blown film material has a compost degradation rate of ≥85% after 90 days, and in some preferred embodiments it can be 85~92%. Detailed Implementation

[0038] To facilitate understanding of the present invention, the present invention will be described more fully and in detail below with reference to preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0039] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0040] The "range" disclosed in this invention is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be combined arbitrarily; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for specific parameters, it is understood that ranges of 60-110 and 80-120 are also expected. Furthermore, if minimum range values ​​1 and 2 are listed, and if maximum range values ​​3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this invention, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" have been listed herein; "0-5" is merely a shortened representation of these numerical combinations. Furthermore, when a parameter is described as an integer greater than or equal to 2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0041] Unless otherwise specified, all embodiments and optional embodiments of the present invention can be combined with each other to form new technical solutions.

[0042] Unless otherwise specified, all technical features and optional technical features of this invention can be combined to form new technical solutions.

[0043] Unless otherwise specified, all steps of the present invention may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order; for example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0044] Unless otherwise specified, the terms "comprising" and "including" as used in this invention can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0045] Unless otherwise specified, the term "or" is inclusive in this invention. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0046] The raw material information used in the following examples is as follows: Polylactic acid (D,L-polylactic acid) with a Mw of 60,000 to 95,000 was purchased from Shanghai Yuanye Biotechnology. PBAT resin was purchased from Wuhan Haishan Technology Co., Ltd. The PBAT resin has a melting temperature of 110~120℃, a melt index (190℃, 2.16kg) ≤5g / 10min, an elongation at break ≥500%, a tensile strength at break of 18~21MPa, and a Shore D hardness of 39~41. Nano-calcium carbonate was purchased from Liangjiang Chemical Co., Ltd., with a specific surface area >40m². 2 / g, D50=15~40nm, bulk density 0.66g / mL; The epoxy-functionalized polybutadiene was purchased from Jiangshan Chemical (Shanghai) Co., Ltd. as EPOLEAD PB4700, with Mn of 3100, epoxy equivalent of 165 g / eq, viscosity of 6300 mPa·s, and acid value of 0.07 KOH mg / g. The melt index of polyethylene wax is 40 g / 10 min, and the particle size is 1000 mesh; the melt index of polypropylene wax is 35 g / 10 min. This includes, but is not limited to, the models from the aforementioned manufacturers.

[0047] Example 1 The degradable blown film material of this embodiment includes the following raw materials in parts by weight: 60 parts modified polylactic acid, 45 parts PBAT resin, 10.5 parts modified starch (sodium octenyl succinate starch), 7.6 parts nano calcium carbonate, 2.5 parts talc, 2.0 parts dispersant (polyethylene wax), 1.7 parts plasticizer (tributyl citrate), 0.39 parts antioxidant (antioxidant 1010), and 0.5 parts lubricant (zinc stearate).

[0048] Modified polylactic acid is prepared from the following raw materials in parts by weight: 100 parts polylactic acid, 3.5 parts bismaleimide-furan derivative, 0.2 parts aluminum chloride, 5.5 parts epoxy-functionalized polybutadiene, 0.26 parts antioxidant and 0.94 parts lubricant.

[0049] The preparation method of modified polylactic acid includes: mixing epoxy-functionalized polybutadiene and bismaleimide-furan derivative, adding alumina, and reacting at 70°C for 2 hours to form a pre-crosslinked network; extruding and granulating polylactic acid, pre-crosslinked network, antioxidant and lubricant to obtain modified polylactic acid; The processing technology of the biodegradable blown film material in this embodiment is as follows: S1. The modified polylactic acid and PBAT resin are dried to a moisture content of 0.01%, and the modified starch is plasticized. The plasticization process involves stirring the modified starch and glycerol at 60°C for 30 minutes, wherein the mass ratio of modified starch to glycerol is 1:0.5. S2. The modified polylactic acid, PBAT resin, modified starch, nano-calcium carbonate, talc, plasticizer and antioxidant treated in S1 are mixed in a high-speed mixer; the raw materials for preparing the degradable blown film material are mixed and then extruded and granulated to obtain the degradable masterbatch; the extrusion settings are: extruder temperature 170℃ and screw speed 250rpm; S3. The degradable masterbatch is blown into film using a blown film machine. The temperature settings are: Zone 1 temperature 150℃, Zone 2 temperature 160℃, Zone 3 temperature 170℃, Die head temperature 185℃, and the blow-up ratio is 3.2 to obtain the degradable blown film material.

[0050] Example 2 The difference between this embodiment and Embodiment 1 is as follows: The degradable blown film material of this embodiment includes the following raw materials in parts by weight: 50 parts modified polylactic acid, 30 parts PBAT resin, 14.2 parts modified starch, 5.7 parts nano calcium carbonate, 4.8 parts talc, 2.8 parts dispersant, 2.6 parts plasticizer, 0.16 parts antioxidant and 0.8 parts lubricant.

[0051] The other raw materials, steps, and parameters are the same as in Example 1.

[0052] Example 3 The difference between this embodiment and Embodiment 1 is as follows: The degradable blown film material of this embodiment includes the following raw materials in parts by weight: 55 parts modified polylactic acid, 38 parts PBAT resin, 8.1 parts modified starch (hydroxypropyl starch), 6.3 parts nano calcium carbonate, 3.4 parts talc, 1.7 parts dispersant, 3.0 parts plasticizer (stearamide), 0.27 parts antioxidant (antioxidant 1076) and 0.51 parts lubricant (ethylene bis-stearamide).

[0053] The other raw materials, steps, and parameters are the same as in Example 1.

[0054] Example 4 The difference between this embodiment and Embodiment 1 is as follows: The degradable blown film material of this embodiment includes the following raw materials in parts by weight: 70 parts modified polylactic acid, 30 parts PBAT resin, 10 parts modified starch, 7.3 parts nano calcium carbonate, 3.0 parts talc, 2.2 parts dispersant, 1.5 parts plasticizer, 0.42 parts antioxidant and 0.67 parts lubricant.

[0055] The other raw materials, steps, and parameters are the same as in Example 1.

[0056] Example 5 The difference between this embodiment and Embodiment 1 is as follows: Modified polylactic acid is prepared from the following raw materials in parts by weight: 100 parts polylactic acid, 4.5 parts bismaleimide-furan derivative, 0.3 parts aluminum chloride, 7 parts epoxy-functionalized polybutadiene, 0.13 parts antioxidant and 0.75 parts lubricant.

[0057] The other raw materials, steps, and parameters are the same as in Example 1.

[0058] Example 6 The difference between this embodiment and Embodiment 1 is as follows: Modified polylactic acid is prepared from the following raw materials in parts by weight: 100 parts polylactic acid, 13 parts bismaleimide-furan derivative, 0.35 parts aluminum chloride, 5 parts epoxy-functionalized polybutadiene, 0.25 parts antioxidant and 0.8 parts lubricant.

[0059] The other raw materials, steps, and parameters are the same as in Example 1.

[0060] Example 7 The difference between this comparative example and Example 1 is as follows: Modified polylactic acid is prepared from the following raw materials in parts by weight: 100 parts polylactic acid, 3.5 parts bismaleimide-furan derivative, 0.2 parts aluminum chloride, 18.2 parts epoxy-functionalized polybutadiene, 0.15 parts antioxidant and 0.34 parts lubricant.

[0061] The other raw materials, steps, and parameters are the same as in Example 1.

[0062] Comparative Example 1 The polylactic acid in this comparative example was unmodified. The difference between this comparative example and Example 1 is as follows: The degradable blown film material of this embodiment includes the following raw materials in parts by weight: 60 parts polylactic acid, 45 parts PBAT resin, 10.5 parts modified starch, 7.6 parts nano calcium carbonate, 2.5 parts talc, 2.0 parts dispersant, 1.7 parts plasticizer, 0.39 parts antioxidant and 0.3 parts lubricant.

[0063] The other raw materials, steps, and parameters are the same as in Example 1.

[0064] Comparative Example 2 The modified polylactic acid in this comparative example does not contain epoxy-functionalized polybutadiene. The difference between this comparative example and Example 1 is as follows: Modified polylactic acid comprises the following raw materials in parts by weight: 100 parts polylactic acid, 5 parts bismaleimide-furan derivative, 0.2 parts aluminum chloride, 0.3 parts antioxidant and 0.71 parts lubricant.

[0065] The other raw materials, steps, and parameters are the same as in Example 1.

[0066] Comparative Example 3 The modified polylactic acid in this comparative example does not contain bismaleimide-furan derivatives in its raw materials. The difference between this comparative example and Example 1 is as follows: Modified polylactic acid comprises the following raw materials in parts by weight: 100 parts polylactic acid, 0.2 parts aluminum chloride, 7.0 parts epoxy-functionalized polybutadiene, 0.25 parts antioxidant and 0.8 parts lubricant.

[0067] The other raw materials, steps, and parameters are the same as in Example 1.

[0068] Test case The mechanical properties and degradation properties of the degradable blown film materials prepared in the above embodiments and comparative examples were tested, and the test results are shown in Table 1.

[0069] The longitudinal tear strength test method refers to GB / T 16578.1; the notched impact strength test method refers to GB / T1843; and the 90-day compost degradation test method refers to GB / T19277.1.

[0070]

[0071] Unless otherwise specified, all raw materials, reagents, instruments, and equipment used in this invention can be purchased commercially or prepared using existing methods. The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of this invention. It should be understood that the above descriptions are merely specific embodiments of this invention and are not intended to limit the invention. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of this invention should be included within the scope of protection of this invention.

Claims

1. A degradable blown film material, characterized in that, The preparation materials include the following parts by weight: 50-70 parts modified polylactic acid, 20-50 parts PBAT resin, 5-20 parts modified starch, 3-10 parts nano calcium carbonate, 2-5 parts talc, 1-5 parts dispersant, 1-5 parts plasticizer, 0.1-1.0 parts antioxidant and 0.5-1.5 parts lubricant.

2. The degradable blown film material as described in claim 1, characterized in that, At least one of the following conditions a to f must be satisfied: a. The melting temperature of the PBAT resin is 110~120℃; b. The elongation at break of the PBAT resin is ≥500%; c. The tensile strength at break of the PBAT resin is 18~21 MPa; d. The modified starch is sodium octenyl succinate starch or hydroxypropyl starch; e. The D50 of the nano-calcium carbonate is ≤50nm; f. The specific surface area of ​​the nano-calcium carbonate is ≥40m². 2 / g.

3. The degradable blown film material as described in claim 1, characterized in that, The modified polylactic acid comprises the following raw materials in parts by weight: 100 parts polylactic acid, 3-5 parts bismaleimide-furan derivative, 0.1-0.5 parts aluminum chloride, 5-10 parts epoxy-functionalized polybutadiene, 0.1-0.3 parts antioxidant and 0.5-1 parts lubricant.

4. The degradable blown film material as described in claim 3, characterized in that, The preparation method of the modified polylactic acid includes the following steps: epoxy-functionalized polybutadiene and bismaleimide-furan derivative are mixed and then added to alumina, and reacted at 55~75℃ for 2h to form a pre-crosslinked network; polylactic acid, pre-crosslinked network, antioxidant and lubricant are mixed, extruded and granulated to obtain modified polylactic acid.

5. The degradable blown film material as described in claim 4, characterized in that, At least one of the following conditions a to c must be met: a. The Mw of the polylactic acid is 50,000 to 200,000; b. The bismaleimide-furan derivative is N , N -(4-methyl-1,3-phenylene)bismaleimide or N , N' -1,3-phenylenebismaleimide; c. The Mn of the epoxy-functionalized polybutadiene is 3000~4000.

6. The degradable blown film material as described in claim 1 or 3, characterized in that, At least one of the following conditions a to d must be met: a. The dispersant is polyethylene wax, polypropylene wax, or stearamide; b. The plasticizer is tributyl citrate, epoxidized soybean oil, or polypropylene glycol; c. The antioxidant is antioxidant 1010, antioxidant 1076, or antioxidant DLTP; d. The lubricant is zinc stearate, calcium stearate or ethylene bis-stearamide.

7. The processing technology of the degradable blown film material according to any one of claims 1 to 5, characterized in that, Includes the following steps: S1. Dry the modified polylactic acid and PBAT resin to a moisture content ≤0.02%, and then plasticize the modified starch; S2. The raw materials for preparing the degradable blown film material are mixed, extruded, and granulated to obtain the degradable masterbatch; S3. The degradable masterbatch is blown into a film to obtain a degradable blown film material.

8. The processing technology of the degradable blown film material as described in claim 7, characterized in that, The plasticizing treatment involves stirring modified starch and glycerol at 55-65°C for 20-50 minutes, wherein the mass ratio of modified starch to glycerol is 1:0.3-0.

8.

9. The processing technology of the degradable blown film material as described in claim 7, characterized in that, At least one of the following conditions a to b must be met: a. The mixing step involves mixing the S1-treated modified polylactic acid, PBAT resin, modified starch, nano-calcium carbonate, talc, plasticizer, and antioxidant in a high-speed mixer. b. The extrusion setup includes an extruder temperature of 160~180℃ and a screw speed of 200~300rpm.

10. The processing technology of the degradable blown film material as described in claim 7, characterized in that, At least one of the following conditions a to c must be met: a. Temperature settings for blown film forming: Zone 1 temperature 150~160℃, Zone 2 temperature 160~170℃, Zone 3 temperature 170~180℃, Die head temperature 175~185℃; b. The blow-up ratio of the blown film forming is 2.5~3.5; c. The winding tension after the blown film is formed is 10~20N.

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