CN114905817A

CN114905817A - Heavy-package composite film and preparation method thereof

Info

Publication number: CN114905817A
Application number: CN202110183015.6A
Authority: CN
Inventors: 李�瑞; 姜艳峰; 吴双; 张明强; 杨国兴; 赵兴龙; 安彦杰; 葛腾杰; 张德英; 王世华
Original assignee: Petrochina Co Ltd
Current assignee: Petrochina Co Ltd
Priority date: 2021-02-08
Filing date: 2021-02-08
Publication date: 2022-08-16

Abstract

The invention discloses a heavy packaging composite film and a preparation method thereof, wherein the heavy packaging composite film comprises the following components: an outer layer, a core layer and an inner layer; the heavy packaging composite film comprises an outer layer, a core layer and an inner layer, wherein the outer layer, the core layer and the inner layer are 5-25 wt%, 50-90 wt% and 5-25 wt% based on the total weight of the heavy packaging composite film; the outer layer comprises 84-94 wt% of metallocene medium density polyethylene, 5-15 wt% of linear low density polyethylene and 1-5 wt% of auxiliary agent; the core layer comprises 95-99 wt% of metallocene medium density polyethylene and 1-5 wt% of auxiliary agent; the inner layer comprises 84-94 wt% of metallocene medium density polyethylene, 5-15 wt% of linear low density polyethylene and 1-5 wt% of auxiliary agent. The heavy-package composite film produced by the layered formula has the advantages of short production flow, one-time forming, batch production, high-speed packaging, quick transformation and adjustment, high strength, difficult damage, no pollution, convenient recycling, stiffness, smoothness, attractive appearance, easiness in bag stacking, slippage prevention, material and labor cost saving and the like.

Description

Heavy-package composite film and preparation method thereof

Technical Field

The invention relates to a film and a preparation method thereof, in particular to a heavy packaging composite film and a preparation method thereof, and specifically relates to a three-layer co-extrusion puncture-resistant polyethylene heavy packaging film and a preparation method thereof.

Background

The continuous production of blow-molded film products for Form-Fill-Seal (FFS) bags is a heavy packaging film technology developed in recent years in China, and is mainly used for automatic packaging lines of large-scale synthetic resin products such as polyethylene, polypropylene and the like. In the packing process, a plurality of working procedures and continuous and automatic completion of the working process are realized, so that the packing capacity can reach 1700-1900 bags/hour generally, and the requirement of high-speed packing of an ultra-large synthetic resin device is met. The traditional packaging of synthetic resin granules uses a composite woven bag with an opening at the top, and the packaging process is complicated due to multiple processes, so the packaging capacity of a single packaging line is usually limited to 1400 bags/hour or less, namely the maximum packaging amount per line year is about 1150 ten thousand, the traditional packaging method can only be applied to a polymer production device with the yield of 10 ten thousand tons/year or less, and if the traditional packaging method is applied to a polymer packaging line with the yield of more than 20 ten thousand tons/year, the packaging efficiency is low and the cost is high.

From the production scale of synthetic resin plants, the average level of new plants in the world of 1990 is between 8 and 10 ten thousand tons/year, and the scale of the new plants after 2000 is at least 20 ten thousand tons/year, and the new plants are further developed toward centralization and large-scale production, and the juice scale of the new plants or the proposed plants is generally more than 40 ten thousand tons. Therefore, with the rapid increase of the scale of the synthetic resin device, the traditional composite film woven bag is far from meeting the requirements of the current large-scale industrial production. The blow molding film product for the FFS bag has excellent comprehensive performance in the aspects of product packaging, storage, transportation, moisture prevention, appearance and the like, conforms to the requirements and trends of international product packaging, is popular with users, and is bound to gradually replace the traditional composite film textile bag to become a main product in heavy packaging industry.

The main raw materials of the blown film product for the FFS bag are LLDPE (linear low density polyethylene) and LDPE (low density polyethylene). Meanwhile, in order to increase the strength of the bag or improve the barrier property and the weather resistance (different use environments such as high temperature or low temperature) of the bag and facilitate various purposes such as automatic opening, filling, sealing and the like of a packaging machine in the operation process, various modifiers such as an opening agent, a barrier agent, an antistatic agent and the like are respectively added, and generally account for less than 3 percent of the total amount of raw materials.

The production of the film for the FFS bag can be a tube film method and a flat film method according to the process form. The flat membrane method has the characteristics of high production capacity, high film strength, high printing efficiency and the like, but the flat membrane method needs to cut off leftover materials, has complex working procedures and large workload of operation and maintenance, and is not widely adopted. The tube-film method has high orientation degree, the width and thickness of the film can be adjusted by changing the blow-up ratio, the production flexibility is higher, the equipment investment is less, the problem of cutting off the edge materials does not exist, the utilization rate of raw materials is high, the newly developed rotary machine head and a computer control system can enable the film to have uniform thickness and smooth roll shape, and in addition, the stability and the cooling efficiency of the film blowing process are greatly improved by a plurality of air-out bubble film cooling systems and stabilizing technologies, so the tube-film method (namely the film blowing process) becomes the best production method for producing the film products for the FFS bags in China.

At present, the film blowing technology for FFS bags is divided into single-layer extrusion technology and multi-layer extrusion technology according to different product forms. The production principles are basically the same, and mainly the equipment is distinguished. The single-layer film extruder mainly adopts a large single-screw extruder and is provided with a single die head, the three-layer co-extrusion equipment is divided into three small extruders and is provided with a three-layer laminated film blowing head capable of being automatically adjusted, and other equipment is the same. Because the packaging of synthetic resin products not only needs to ensure the strength, but also needs to comprehensively consider the properties of heat sealing, printing, stiffness, stacking and the like, three-layer co-extruded films are generally selected. The main reasons are: the three-layer co-extrusion film can decompose the quality index, and different film layers can be utilized to meet different quality index items, so that the requirement of meeting the overall quality index is met. If the formula with good viscosity, antistatic property and good printing and coloring performance is selected on the outer layer; the middle layer is selected with a formula with high strength and high stiffness; the inner layer is selected from a formula with good heat sealability and good sealing property, and the like, so as to meet different requirements of users. Therefore, in the production of synthetic resin packaging films, a three-layer coextrusion film blowing process is generally adopted at present.

At present, the domestic heavy packaging film is mainly produced by using blended resin and various additives as raw materials, and the method has the disadvantages of high production difficulty, high cost and uneven product quality due to various types of blended resin and additives. In addition, when the heavy packaging film is produced by blending, the mechanical strength and the heat sealing strength are low, the stiffness and the thermal deformation are poor, so that various accidents such as low filling speed, bag body breakage and the like often occur, and the automatic packaging production line is difficult to maintain rapid production in serious cases.

Taking the currently widely applied film blowing technology for three-layer co-extrusion type FFS bags as an example, the raw materials of metallocene LLDPE, LDPE, HDPE and other additives are manually broken and then added into respective raw material storage tanks, and are sucked into an automatic quality batching system by a vacuum suction device. The batcher in the mass batching system mixes the materials according to the requirement and adds the materials into the corresponding extruder, and the plasticizing extrusion treatment is carried out under the condition that the average extrusion temperature is about 200 ℃. Three extruders at different angles supply three different ingredients respectively and send the ingredients to the die head, and at an operating temperature, a core layer, an inner layer and an outer layer of the film are formed respectively and extruded by the die head through a manual screen changing mechanism. And then the film bubble is blown up by cooling air with the temperature controlled below 25, the outside of the film bubble is cooled by a cooling air ring, the inside of the film bubble is cooled by a plurality of cooling air, and hot air is pumped out from the upper part of the film bubble. The bubble can be controlled to be stabilized under a certain air pressure by adjusting the air inflow of the cooling air of the bubble, so that the set blow-up ratio is achieved.

The main performance index test of the blown film product for the FFS bag is formulated according to the characteristics of a packing material and the requirements of transportation, storage and the like, and the main test items comprise dart impact strength, tensile breaking strength, breaking elongation, yield strength, friction coefficient and the like. Since the blown film for the FFS bag is a novel heavy packaging film product, no unified national standard exists at present, and specific indexes can be established by negotiation between users and manufacturers according to transportation and storage environments.

The quality of the blown film product for FFS bags depends mainly on the raw material formulation and equipment performance: from the aspect of formula analysis, the self quality, mutual proportioning, addition proportion of additives and the like of the raw materials LLDPE and LDPE have obvious influence on all properties. Through comparative analysis, not only the raw materials of different brands have influence on the product quality and the production control process, but also the raw materials of the same brand of different production plants have great influence on the product quality. However, the thickness of the film produced by domestic equipment is mainly a positive deviation, although on average within + 8%, but locally actually exceeds this criterion. The purpose of increasing the positive deviation is to increase the strength and other performance indexes by increasing the thickness. In addition, the sale of the film is mainly based on tonnage calculation price, but not based on length or number, so that film blowing production enterprises do not mind the positive deviation of the film thickness in normal indexes, and users of film products have great opinions. With the increasing number of membrane manufacturing enterprises and the increasing desire of users of membrane products to reduce costs, it is believed that the market competition of such products will be more intense and the low quality membrane products will be difficult to stand on the market.

Due to the influence of various factors such as production equipment, process technology, applied raw materials and the like of the three-layer co-extrusion heavy packaging film, in the existing preparation method, the quality index of the product usually pays attention to various physical and mechanical performance indexes of the film, such as tensile strength, tearing strength, label falling impact strength and the like, the index of the thickness uniformity of the heavy packaging film product is rarely paid attention to, and even in the industry standard of 'multi-layer co-extrusion heavy packaging film and bag for packaging' recently issued by the state, the realization of the index of how to ensure the film thickness uniformity is not involved. At present, all domestic heavy packaging film production enterprises carry out qualification judgment on the quality of products by adopting a sampling inspection method, but if the uniformity of the thickness of the film is not high, the condition is easy to occur, and even though all detection indexes of the film are qualified, the product is likely to have quality accidents of bag breaking and material leakage in the post-packaging application due to the existence of some extremely thin point positions, so that the heavy quality compensation risk of the heavy packaging film production enterprises is caused.

None of the documents currently published on repackaging films relate to the uniformity of the thickness of the repackaging film. For example, the disclosures in the patent publications relate to the improvement of mechanical properties of heavy packaging film products by selecting appropriate linear low density polyethylene materials, and the disclosures in the patent publications of heat sealability publication number mention the improvement of mechanical properties of single layer heavy packaging films by using ethylene-vinyl acetate copolymer, metallocene linear low density polyethylene and ultra low density polyethylene. The patent literature relates to a three-layer co-extrusion heavy packaging film and a preparation method thereof, and the mechanical property, the heat sealing property and the surface friction property of the three-layer co-extrusion heavy packaging film are improved by selecting proper one, medium density polyethylene and ethylene-vinyl acetate copolymer raw materials. The patent literature relates to a two-layer packaging film and a preparation method thereof, and the mechanical property of the film is also improved by selecting proper raw materials.

In the existing production method, the following various factors may cause unevenness in film thickness. Firstly, because various raw material components, especially low proportion components, have certain deviation from the theoretical proportion in the actual adding amount, the difference of the bubble components in different areas can be caused, and the difference finally causes the thickness difference of the bubble in different areas. The second is due to local drag differences in the melt channel, which may be due to local build-up in the channel, which can lead to deviations in the thickness of the film bubble in certain areas. The third situation is that the non-uniformity of the cooling temperature distribution of the film bubble can cause the thickness of different areas of the film bubble to deviate during the longitudinal and transverse stretching thinning process of the film bubble. Therefore, there is an urgent need in the art to develop a method capable of overcoming the defects of the existing preparation method, ensuring stable and uniform thickness of the film bubble in the preparation process, and adjusting the composition of each film of the inner layer, the middle layer and the outer layer of the three-layer co-extruded heavy packaging film corresponding to the three groups of melts respectively, which is based on the reason that the raw material ratios are combined in a blending manner to cause uneven mixing, each composition and ratio cause differences in the processing process, and finally cause various accidents such as package breaking, insufficient heat sealing strength, irregular sealing temperature and time and the like, so that the package breaking rate after the product is packaged is high, thereby not only causing great loss to specific product users at the downstream, but also bringing great pressure to the normal production of the automatic packaging line.

The special metallocene heavy packaging film resin used at present is a linear low-density polyethylene resin, has narrow molecular weight distribution, contains a certain amount of short branched chains, needs to be added with LDPE to improve the processability when the processability of a product is poor, has low tensile and falling-standard strength, and needs to be added with medium-high density polyethylene to improve the strength. The market lacks a puncture-resistant medium-density polyethylene heavy-duty packaging film resin with wide molecular weight distribution, unique short branched chain structure and a small amount of long branched chain structure, and the film prepared by processing the resin has good mechanical property and processability, good comprehensive performance, uniform quality and low production cost.

Disclosure of Invention

The heavy packaging composite film has the advantages of high-speed packaging, quick transformation and adjustment, high strength, difficulty in damage, no pollution, convenience in recycling, stiffness, smoothness, attractiveness, easiness in stacking and packaging, slippage prevention, material and labor cost saving and the like.

The invention provides a heavy packaging composite film, which comprises three layers: the outer layer, the core layer and the inner layer; the formula of the components is as follows: the weight ratio of the outer layer to the core layer to the inner layer is 5-25 wt% to 50-90 wt% to 5-25 wt%, preferably 10-20 wt% to 60-80 wt% to 10-20 wt%, more preferably 15 wt% to 70 wt% to 15 wt%, based on the total weight of the heavy packaging composite film.

The outer layer comprises 84-94 wt% of metallocene medium density polyethylene, 5-15 wt% of linear low density polyethylene and 1-5 wt% of auxiliary agent; the core layer comprises 95-99 wt% of metallocene medium density polyethylene and 1-5 wt% of an auxiliary agent; the inner layer comprises 84-94 wt% of metallocene medium density polyethylene, 5-15 wt% of linear low density polyethylene and 1-5 wt% of auxiliary agent; ensures that the three-layer heavy-packaging composite film has better puncture resistance and tensile strength.

The auxiliary agent of the invention is one or more of PPA, slipping agent, opening agent or antistatic agent.

The metallocene medium density polyethylene is prepared by the polymerization reaction of ethylene and 1-hexene in a single reactor under the action of hydrogen, inert gas and a wide distribution metallocene catalyst, wherein the melt index of the metallocene medium density polyethylene is 0.3-2.0g/10min, and the density is 0.928-0.938g/cm ³ . The relative molecular mass range of the metallocene medium density polyethylene is 6000-280000, preferably 50000-240000; wherein the number average molecular weight range is 6000-60000, preferably 30000-50000; the weight-average molecular mass range is 80000-280000, preferably 100000-240000; the relative molecular mass distribution is 3.0 to 7.0, preferably 4.0 to 6.0.

The metallocene medium density polyethylene of the invention has a density of from 0.928 to 0.938g/cm ³ The linear low density polyethylene has a density of 0.916 to 0.926g/cm ³ 。

The melt index of the metallocene medium density polyethylene is 0.5-1.5g/10min, and the melt index of the linear low density polyethylene is 0.5-1.5g/10 min.

The melt flow ratio of the metallocene medium density polyethylene is 20-70, preferably 35-60; the processing torque is 30-60 Nm; preferably 35-55; compared with the traditional metallocene products on the market, the processing torque is greatly reduced by 50-90 Nm; the larger the melt flow ratio of the product is, the wider the molecular weight distribution is, the smaller the processing torque of the product is, the less the processing energy consumption of the product is, the better the stability and the product quality in the processing process are, and the better the comprehensive performance of the product is.

The films produced by processing the metallocene medium density polyethylene of the invention have unique characteristics: due to the special wide molecular weight distribution, the unique short branched chain structure and a small amount of long branched chain structure, the tensile strength is high, the puncture resistance is good, and the standard falling impact strength is improved by 20 percent compared with the traditional metallocene LLDPE on the market.

The comonomer in the preparation process of the metallocene medium density polyethylene is 1-hexene which is used for adjusting the density of the polyethylene; hydrogen is used to regulate the molecular weight of the polyethylene; the inert gas is nitrogen.

The molecular chain of the metallocene medium density polyethylene of the invention has a unique structure: the metallocene medium density polyethylene has the following molecular chain structure:

wherein: - (CH) ₂ )m-CH ₃ Represents a long chain branch; m is greater than or equal to 100; n is 8000 or more.

The branching distribution of the butyl short branched chains in the polyethylene molecular chains is in discontinuous change, the long branched chains are randomly distributed on the main chains of the polyethylene molecules, and the main chains of the polyethylene molecules are different in length. The comonomer inserted into the main chain of the polyethylene molecule is 1-hexene, the branching distribution of the comonomer in the molecular chain is discontinuously changed, the average distance between branching points is 20-25 ethylene sequence lengths, namely the average sequence length of ethylene between the branching points is 20-25. The long-chain branches are randomly distributed on a polyethylene molecular main chain, the branching distribution of the long-chain branches in a molecular chain is randomly changed, the average distance between the branching points is 500-5000 ethylene sequence lengths, namely the average sequence length of ethylene between the branching points is 500-5000.

The metallocene medium density polyethylene of the invention has obvious non-uniformity of molecular chain structure, and the non-uniformity of the molecular chain structure comprises non-uniformity caused by different molecular weight and distribution and non-uniformity caused by different branched chain length, content and distribution, and has great influence on comprehensive information of polyethylene products. The resin has wide molecular weight distribution, unique short branched chains and a few long branched chain structures, so that a plurality of unfolded branched chains pass through an amorphous area between crystal layers and enter other crystal areas, a large number of frenulum molecules exist between the crystal layers, and the mechanical property of the polymer is improved; meanwhile, the branched chains contain more long-chain branched chains, so that the rheological property is improved, and the processing property is improved. In addition, the special structure has certain influence on the crystal size, the transparency of the product is also improved to a certain extent, and the comprehensive performance of the product is greatly improved.

The wide distribution metallocene catalyst used in the preparation of the metallocene medium density polyethylene of the present invention consists of a chromium compound supported on a carrier. The zirconium compound is a zirconium chloride compound and is modified by titanium, chromium and the like; the support is an inorganic oxide such as silica, alumina, zirconium chloride or even thoria.

The polymerization reaction in the preparation process of the metallocene medium density polyethylene of the present invention includes slurry polymerization, gas phase polymerization and solution polymerization. When the polymerization is gas-phase polymerization, the polymerization reaction temperature is 80-90 ℃, and preferably 83-86 ℃; the polymerization pressure is 1.8-2.5 MPa, preferably 2.0-2.3 MPa; the circulating gas velocity is 0.60-0.82 m/s, preferably 0.64-0.74 m/s; the retention time is 1-8 h, preferably 4-6 h.

In the polymerization reaction in the preparation process of the metallocene medium density polyethylene, the molar ratio of 1-hexene to ethylene is 0.01: 1-0.09: 1, preferably 0.03: 1-0.06: 1; the molar ratio of hydrogen to ethylene is 0.001:1 to 0.01:1, preferably 0.003:1 to 0.006: 1.

In the resin composition, wear-resistant metallocene medium density polyethylene is mainly adopted, and the molecular chain of the wear-resistant metallocene medium density polyethylene has a unique structure: wide molecular weight distribution, unique short branched chain and small amount of long branched chain structure. The resin with wide molecular weight distribution, wherein the relative molecular weight low part can reduce the apparent viscosity of the melt, play a role in internal plasticization, improve the shear thinning sensitivity and improve the processability of the product. A small amount of long-chain branched structures can also improve the shear thinning sensitivity and the processing performance of the product. The heterogeneity caused by the difference of molecular weight and distribution and the heterogeneity caused by the difference of branch length and content and distribution determine the crystal shape of the product, and the unique crystal shape makes the resin show unique physical properties. The unique molecular chain structure ensures that the crystal form formed in the crystallization process is beneficial to improving the optical performance and the processing performance of the product, so that the relative molecular mass distribution of the resin is 3.0-7.0, the melt flow ratio is 20-50, the processing torque is 30-60Nm, and the falling standard impact strength of the film processed by the resin is improved by 20%.

The invention also provides a preparation method of the heavy packaging composite film, which comprises the steps of preparing the heavy packaging composite film by using a blown film technology through an FFS bag, using a three-layer co-extrusion heavy packaging film blowing machine, feeding the mixture into a mass batcher from a stock bin, mixing the raw materials by the mass batcher according to the proportion of the heavy packaging composite film, feeding the mixture into 3 extruders arranged at different angles, sending the mixture to a die head through a net changing mechanism to respectively form a core layer and inner and outer layers of the heavy packaging composite film, blowing cooling air by a fan to blow and expand a film tube into film bubbles, cooling the film bubbles by an external cooling air ring and a film bubble internal cooling system, processing the film bubbles by a drafting device and a corona processor, and then feeding the film bubbles into a winding device to be wound and cut to obtain the heavy packaging composite film.

Compared with the prior art, the heavy packaging composite film can be widely applied to the production in the field of heavy packaging films, such as FFS packaging, large goods packaging and the like, the thickness is reduced from 0.16mm to 0.12mm on the premise of meeting the performance index of the FFS film, and the cost is greatly reduced. The heavy packaging film produced by the layered formula has the advantages of short production flow, one-time forming, batch production, high-speed packaging, quick transformation and adjustment, high strength, difficult damage, no pollution, convenient recycling, stiffness, attractiveness, easiness in stacking, slippage prevention, material and labor cost saving and the like.

Detailed Description

The present invention is described in detail below by way of examples, it should be noted that the examples are only for the purpose of further illustrating the present invention and should not be construed as limiting the scope of the present invention, and that the skilled person may make some insubstantial modifications and adaptations to the invention described above.

The main performance indexes and test standards used in the present invention are as follows:

TABLE 1 main Performance indices and test standards of FFS film products

Example 1:

the total amount of the raw materials is 100kg, the film thickness is 0.12mm, and the components of the heavy packaging composite film of the embodiment are shown in the following table 2:

TABLE 2

After the three-layer co-extrusion film blowing production line is extruded and molded according to the production process, the product is sampled and analyzed, the mechanical properties of the obtained product all meet the standard, and the puncture resistance and the film strength are obviously improved compared with other 0.16mm heavy-duty packaging products after the thickness is reduced to 0.12 mm.

The main performance indexes and test standards of the FFS film product of this example are shown in table 3 below:

TABLE 3

Example 2: the total amount of the raw materials is 100kg, the film thickness is 0.14mm, and the components of the heavy packaging composite film of the embodiment are shown in the following table 4:

TABLE 4

After the three-layer co-extrusion film blowing production line is extruded and molded according to the production process, the product is sampled and analyzed, the mechanical properties of the obtained product all meet the standard, and the puncture resistance and the film strength are obviously improved compared with other 0.16mm heavy-duty packaging products after the thickness is reduced to 0.14 mm.

The main performance indexes and test standards of the FFS film product of this example are shown in table 5 below:

TABLE 5

Example 3: the total amount of the raw materials is 100kg, the film thickness is 0.16mm, and the components of the heavy packaging composite film of the embodiment are shown in the following table 6:

TABLE 6

After the three-layer co-extrusion film blowing production line is extruded and molded according to the production process, the product is sampled and analyzed, the mechanical properties of the obtained product all meet the standard, and the puncture resistance and the film strength are obviously improved compared with other 0.16mm heavy-duty packaging products after the thickness is 0.16 mm.

The main performance indexes and test standards of the FFS film product of this example are shown in table 7 below:

TABLE 7

Comparative example: the total amount of the raw materials is 100kg, the film thickness is 0.16mm, and the components of the heavy packaging composite film of the embodiment are shown in the following table 8:

TABLE 8

After the three-layer co-extrusion film blowing production line is extruded and molded according to the production process, the product is sampled and analyzed, and the mechanical properties of the obtained product all meet the standard.

The major performance indicators and test standards for the FFS film product of this comparative example are shown in table 9 below:

TABLE 9

Compared with the prior art, the heavy packaging film composition disclosed by the embodiment of the invention can be widely applied to the production in the field of heavy packaging films, such as FFS packaging, large goods packaging and the like, the thickness is reduced from 0.16mm to 0.12mm on the premise of meeting the performance index of an FFS film product, the performances of the product in the embodiment are still better than those of the product in the traditional comparative example, the tensile strength is improved by 15-40% compared with the comparative example, the standard drop impact strength is improved by 30-50%, and the comprehensive performance of the product is remarkably improved, so that the thickness of the film can be greatly reduced on the premise of meeting the performance requirement, and the cost is reduced.

The present invention is capable of other embodiments, and various changes and modifications may be made by one skilled in the art without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

1. A heavy-duty composite film, comprising: an outer layer, a core layer and an inner layer; the heavy packaging composite film comprises an outer layer, a core layer and an inner layer, wherein the outer layer, the core layer and the inner layer are 5-25 wt%, 50-90 wt% and 5-25 wt% based on the total weight of the heavy packaging composite film;

the outer layer comprises 84-94 wt% of metallocene medium density polyethylene, 5-15 wt% of linear low density polyethylene and 1-5 wt% of auxiliary agent;

the core layer comprises 95-99 wt% of metallocene medium density polyethylene and 1-5 wt% of an auxiliary agent;

the inner layer comprises 84-94 wt% of metallocene medium density polyethylene, 5-15 wt% of linear low density polyethylene and 1-5 wt% of auxiliary agent.

2. The heavy packaging composite film of claim 1, wherein the outer layer, core layer and inner layer are 5-20 wt%, 60-90 wt% and 5-20 wt%.

3. The heavy-duty composite film of claim 2, wherein the ratio of the outer layer to the core layer to the inner layer is from 15 wt% to 70 wt% to 15 wt%.

4. The repackaged composite film of claim 1 further comprising a processing aid, wherein the aid is at least one of PPA, a slip agent, an opening agent, and an antistatic agent.

5. The repackaged composite film of claim 1, wherein the metallocene medium density polyethylene has a density of 0.928 to 0.938g/cm ³ The linear low density polyethylene has a density of 0.916 to 0.926g/cm ³ 。

6. The heavy-duty composite film according to claim 1, wherein said metallocene medium density polyethylene has a melt index of 0.5 to 1.5g/10min, and said linear low density polyethylene has a melt index of 0.5 to 1.5g/10 min.

7. The repackaged composite film of claim 1, wherein the metallocene medium density polyethylene has a relative molecular mass of 6000 to 280000, a number average molecular mass of 6000 to 60000, a weight average molecular mass of 80000 to 280000, and a relative molecular mass distribution of 3.0 to 7.0.

8. The repackaged composite film of claim 7, wherein said metallocene medium density polyethylene has a relative molecular mass of 50000 to 240000, a number average molecular mass of 30000 to 50000, a weight average molecular mass of 100000 to 240000, and a relative molecular mass distribution of 4.0 to 6.0.

9. The heavy packaging composite film of claim 1, wherein the total thickness of the heavy packaging composite film is from 0.12mm to 0.16 mm.

10. A preparation method of a heavy packaging composite film according to any one of claims 1-9, characterized in that the film is prepared by a blown film technology process through FFS bags, a three-layer co-extrusion heavy packaging film blow molding machine is used, the raw materials enter a mass proportioning device from a storage bin, the mass proportioning device mixes the raw materials and then supplies the raw materials to 3 extruders placed at different angles, the raw materials are sent to a die head through a screen changing mechanism to respectively form a core layer, an inner layer and an outer layer of the heavy packaging composite film, cooling air is blown in by a fan to blow and expand a film tube into film bubbles, the film bubbles are cooled by an external cooling air ring and a film bubble internal cooling system, and then the film bubbles enter a rolling device to be rolled and cut after being processed by a drafting device and a corona processor, so that the heavy packaging composite film is obtained.