Environment-friendly degradable polyolefin heat-shrinkable film and preparation process thereof

Abstract

This invention proposes an environmentally friendly and biodegradable polyolefin heat shrinkable film and its preparation process, comprising a three-layer co-extruded outer layer, a core layer, and an inner surface layer; the outer and inner surface layers are composed of the following raw materials by weight: 65-75 parts of linear low-density polyethylene A, 15-25 parts of polypropylene carbonate, 10-15 parts of chitosan, 2-5 parts of bamboo fiber A, 2-6 parts of maleic anhydride-grafted polyethylene, 1-3 parts of graphene oxide, 0.5-1.0 parts of opening agent, and 1-2 parts of slip agent; the core layer comprises 60-70 parts of linear low-density polyethylene B, 15-20 parts of ethyl acetate, 10-15 parts of starch-grafted modified acrylic monomer, 2-6 parts of calcium phosphate ceramic, 5-10 parts of bamboo fiber B, 2-5 parts of compatibilizer, and 0.5-1 part of antioxidant. The environmentally friendly and biodegradable polyolefin heat shrink film provided by this invention can simultaneously improve the heat shrinkage rate, mechanical properties and degradation efficiency of traditional polyolefin heat shrink film, thereby achieving performance improvement of biodegradable heat shrink film and controllable degradation.

Description

Technical Field

[0001] This invention relates to the field of packaging materials technology, specifically to an environmentally friendly and biodegradable polyolefin heat shrink film and its preparation process. Background Technology

[0002] Heat shrink film is a thermoplastic film that shrinks significantly when heated. It is manufactured by stretching or blowing, instilling internal stress into the film. When used, heating releases this stress, allowing the film to return to its original size and tightly adhere to the packaged item. It is widely used in bulk packaging for food, beverages, daily chemicals, building materials, machinery, and other products. Heat shrink film offers advantages such as attractive appearance, tight adhesion to goods, moisture and water resistance, good transparency, and excellent protection, making it one of the fastest-growing products in the packaging industry in recent years.

[0003] Currently, most commercially available heat shrink films are made from traditional petroleum-based plastics, such as polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), and polyester (PET). Taking POF (multilayer co-extruded polyolefin) heat shrink film as an example, it is typically made using linear low-density polyethylene (LLDPE) as the middle layer and copolymer polypropylene as the inner and outer layers, through a multilayer co-extrusion process. These traditional plastic materials are extremely difficult to degrade in the natural environment, causing serious "white pollution" problems after disposal. Traditional polyolefin plastics are difficult to degrade in the natural environment, breaking down into microplastics that flow into soil and water bodies, seriously impacting the ecological environment and biological health. According to statistics, the use of agricultural plastic film in my country accounts for more than half of the total agricultural plastic film use. While its large-scale use brings economic value, it also causes serious "white pollution," with residual film being difficult to recycle and degrade, directly affecting the soil's regeneration capacity. Therefore, developing biodegradable heat shrink films has become an important research direction in the field of packaging materials.

[0004] In recent years, research on biodegradable heat-shrinkable films has mainly focused on the following directions: First, replacing traditional polyolefins with fully biodegradable polyester materials such as polylactic acid (PLA), polybutylene terephthalate adipate copolyester (PBAT), and polybutylene succinate (PBS); Second, adding photosensitizers or degradation promoters such as starch to polyolefins to achieve partial degradation of polyolefins; Third, introducing biodegradable functional layers while maintaining mechanical properties through multilayer co-extrusion structure design.

[0005] However, existing technologies still have the following shortcomings: fully biodegradable polyester materials such as PLA have poor toughness and high brittleness, while PBAT has problems with poor crystallinity and low melt strength, and the heat shrinkage rate is often difficult to reach the level of traditional polyolefin heat shrink film; in the scheme of adding photosensitizers or starch to polyolefins, it is difficult to accurately control the degradation induction period of photosensitizers, while the large polarity difference and poor compatibility between starch and polyolefins lead to a significant decrease in mechanical properties.

[0006] Therefore, developing an environmentally friendly and biodegradable heat shrink film that retains the excellent mechanical and shrinkage properties of traditional polyolefin heat shrink film while also possessing controllable degradation capabilities is of significant practical importance and market value. Summary of the Invention

[0007] The purpose of this invention is to provide an environmentally friendly and biodegradable polyolefin heat shrink film, which solves the technical problem of how to simultaneously improve the heat shrinkage rate, mechanical properties and degradation efficiency of traditional polyolefin heat shrink film. Through systematic formulation design and multi-layer co-extrusion structure optimization, the waste after use is rapidly degraded in the natural environment, thereby improving the performance of the biodegradable heat shrink film and making degradation controllable.

[0008] An environmentally friendly and biodegradable polyolefin heat shrink film, comprising a three-layer co-extruded outer layer, a core layer, and an inner layer;

[0009] The outer and inner outer layers are composed of the following raw materials in parts by weight: 65-75 parts of linear low-density polyethylene A, 15-25 parts of polypropylene carbonate, 10-15 parts of chitosan, 2-5 parts of bamboo fiber A, 2-6 parts of maleic anhydride-grafted polyethylene, 1-3 parts of graphene oxide, 0.5-1.0 parts of opening agent, and 1-2 parts of slip agent;

[0010] The core layer is composed of the following raw materials by weight: 60-70 parts of linear low-density polyethylene B, 15-20 parts of ethyl acetate, 10-15 parts of starch-grafted modified acrylic monomer, 2-6 parts of calcium phosphate ceramic, 5-10 parts of bamboo fiber B, 2-5 parts of compatibilizer, and 0.5-1 part of antioxidant.

[0011] The grafting rate of the starch-grafted modified acrylic monomer is 15-35%.

[0012] By weight, (polypropylene carbonate + chitosan) / bamboo fiber A = 8-15.

[0013] By weight, (graphene oxide + starch-grafted modified acrylic monomer) / calcium phosphate ceramic = 3-8.

[0014] By weight, linear low-density polyethylene B / (ethyl acetate + bamboo fiber B) = 2-3.

[0015] The opening agent is any one of synthetic silica, talc, diatomaceous earth, and calcium carbonate;

[0016] The slip agent is any one of erucamide, calcium stearate, or organosilicon.

[0017] The compatibilizer is any one of acrylic acid-grafted polyolefin resin, glycidyl methacrylate (GMA)-grafted polymer, oxazoline-grafted polystyrene (RPS), and modified polyacrylate, and the grafting rate is 0.8% to 1.5%.

[0018] The addition of this compatibilizer can effectively reduce the interfacial tension between the starch phase and the polyolefin phase, improve the compatibility between the components, and enhance the overall mechanical properties of the film.

[0019] The antioxidant is a compound of hindered phenolic antioxidants and phosphite antioxidants in a mass ratio of 1:1 to 1:3.

[0020] The thickness of the heat-shrinkable film is 20-80 μm, and the thickness ratio of the three layers is outer layer: core layer: inner layer = 1:2:2.

[0021] A process for preparing an environmentally friendly and biodegradable polyolefin heat shrink film includes the following steps:

[0022] Step S1: Graft maleic anhydride onto polyethylene;

[0023] Step S2: Grafting starch onto modified acrylic monomers;

[0024] Step S3: Weigh the raw materials of each component of the outer surface layer, inner surface layer and core layer according to the formula, and mix them evenly separately;

[0025] Each component raw material was vacuum dried at 50–60°C for 4–8 hours to remove moisture, with the moisture content controlled to be <0.05%.

[0026] Step S4: The mixed three-layer raw materials are fed into the three extruders of the three-layer co-extrusion blown film unit, melted and plasticized, and then co-extruded through the three-layer co-extrusion die to obtain a cylindrical film preform with an A / B / A three-layer structure.

[0027] Step S5: Biaxially stretch the preform, controlling the blow-up ratio to be 2.5 to 3.0, the longitudinal stretching ratio to be 3 to 5 times, and the stretching temperature to be 100 to 120°C;

[0028] The biaxial stretching is carried out in a stretching oven. After cooling and setting, the thickness of the film is 20-80 μm, and the thickness ratio of the three layers is outer layer: core layer: inner layer = 1:2-4:1.

[0029] Step S6: Cool the stretched film to room temperature for setting;

[0030] Step S7: After cooling and setting, the film is subjected to corona treatment, edge trimming, and winding to obtain the finished heat shrink film.

[0031] After cooling and setting, the film is subjected to corona treatment to increase its surface tension (treatment value ≥ 38 dynes), then trimmed, flattened, and wound up to the set diameter to obtain the environmentally friendly and biodegradable polyolefin heat shrink film.

[0032] In step S2, the preparation method of starch-grafted modified acrylic monomer is as follows:

[0033] Step S21: Mix natural starch (corn starch, potato starch or cassava starch) with a sodium hydroxide solution of 1-3% by mass at a mass ratio of 1:2 to 1:4, and stir and gelatinize at 60-70°C for 2-3 hours;

[0034] Step S22: After adjusting the pH to neutral, add ammonium persulfate initiator and acrylic acid monomer, and perform graft polymerization at 60-70℃ for 2-3 hours. After washing, drying, pulverizing and sieving, the grafted modified starch is obtained.

[0035] Step S23: The amount of acrylic monomer added is 15-20% of the starch mass, and the amount of ammonium persulfate initiator added is 0.5-1.0% of the starch mass.

[0036] By grafting modification, hydrophilic and biocompatible polyacrylic acid segments are introduced into the starch molecular chain, which improves the interfacial compatibility between starch and polyolefin matrix and provides reactive active sites for PBAT composites.

[0037] In step S4, the process parameters for the three-layer co-extrusion are as follows: outer layer extruder temperature 160–180℃, core layer extruder temperature 180–200℃, inner surface layer extruder temperature 160–180℃, die temperature 170–180℃, screw speed 50–60 rpm for the outer layer, 60–80 rpm for the core layer, and 50–70 rpm for the inner surface layer; the die gap is 1.0–1.5 mm. The three-layer co-extrusion technology is existing technology and will not be described in detail here.

[0038] In the overall preparation process, parameters such as temperature, screw speed, blow-up ratio, stretching ratio, and cooling temperature in each stage can be adjusted within a certain range according to the actual equipment conditions and raw material specifications to achieve the best film performance.

[0039] The technical details not described in this solution are based on the conventional understanding of those skilled in the art and can be implemented in conjunction with existing technologies, and will not be elaborated further here.

[0040] The beneficial effects of this invention are as follows:

[0041] (1) By designing a three-layer co-extrusion A / B / A structure and introducing a multi-compatibilizer system, the degradable components are uniformly dispersed in the polyolefin matrix. The longitudinal heat shrinkage rate of the film is more than 65%, the transverse heat shrinkage rate is ≥59%, the tensile strength and elongation at break meet or even exceed the requirements of GB / T13519 standard, and the haze is less than 10%. While significantly improving the degradation performance, it maintains the excellent performance of traditional polyolefin heat shrink film. The preparation process of this invention can be achieved using conventional three-layer co-extrusion blown film equipment. No special or high-cost equipment modification is required. The processing window is wide and it is easy to realize industrial production.

[0042] (2) Modified starch, PBAT and other bio-based and biodegradable materials are used to partially replace petrochemical-based polyolefins, which reduces dependence on non-renewable resources and lowers carbon emissions. The degradation products are mainly carbon dioxide, water and biomass residue, which will not cause secondary pollution to the environment and are in line with the development concept of circular economy and green packaging.

[0043] (3) In this scheme, the addition of polypropylene carbonate, chitosan, and bamboo fiber to the polyolefin heat shrink film achieves the following synergistic effects:

[0044] First, polypropylene carbonate acts as a biodegradable adhesive. Bamboo fiber also forms a "strong combination" with ethyl acetate (PLA). When bamboo fiber is used to reinforce PLA to make composite materials, it can not only degrade itself, but its hydrophilicity can also attract water, accelerate the hydrolysis of PLA matrix, and make the overall degradation rate of composite materials faster than that of pure PLA.

[0045] Secondly, bamboo fiber acts like "steel bars" to enhance stiffness and tensile strength, polypropylene carbonate provides basic toughness and ductility, and chitosan acts as an "interface agent" to improve the compatibility of the two, making the structure more robust and potentially synergistically improving impact resistance.

[0046] Third, chitosan can significantly improve the heat resistance of polypropylene carbonate and effectively inhibit the thermal degradation of polypropylene carbonate during processing, making production more stable. In addition, the combination of polypropylene carbonate and chitosan can also endow the material with good bioactivity and biocompatibility. Chitosan can also bring broad-spectrum antibacterial effects, endow shrink film with natural active packaging function, and extend the shelf life of food.

[0047] (4) The addition of graphene oxide, starch-grafted modified acrylic monomer, and calcium phosphate ceramics to this scheme produces the following multi-point synergistic effects:

[0048] First, starch grafted with acrylic monomers can act as a "bridge". The polyethylene-like part in the grafted polymer can entangle with the polyethylene matrix chain segment, while the polar carboxyl group (-COOH) can form strong interactions such as hydrogen bonds and ionic bonds with the oxygen-containing functional groups of graphene oxide and calcium phosphate, thus constructing a stable "organic-inorganic" mixed three-dimensional network, effectively avoiding interface defects and stress concentration.

[0049] Secondly, starch grafts can encapsulate graphene oxide like a "surfactant," effectively preventing its aggregation by utilizing steric hindrance. Calcium phosphate particles, acting as "spacers," can also play a similar role, achieving uniform dispersion of graphene oxide.

[0050] Third, graphene oxide significantly enhances the elastic modulus, starch grafts improve mechanical and rheological properties through compatibilization, while calcium phosphate can enhance rigidity and heat resistance. This system is expected to produce composite films with both high strength and high toughness. Graphene oxide itself has certain antibacterial properties. When it is combined with calcium phosphate, it can produce a photothermal synergistic antibacterial effect, which can efficiently kill bacteria under light irradiation.

[0051] Fourth, graphene oxide, as a stress concentrate, can induce silver crevice and shear bands in the matrix to absorb energy. At the same time, the strong interfacial bonding can efficiently transfer and transmit stress, while the uniformly dispersed calcium phosphate particles can also hinder crack propagation.

[0052] This multi-component system achieves comprehensive improvement in mechanical properties, barrier properties, functional characteristics and processability through the synergistic effect between components, and develops new packaging materials with high strength, high toughness, high barrier properties, antistatic properties and even bioactivity.

[0053] (5) In this scheme, linear low-density polyethylene (LLDPE), ethyl acetate (PLA) and bamboo fiber are mixed together, and with the help of a compatibilizer, the following synergistic technical effects are achieved:

[0054] One method is to add bamboo fiber to low-density polyethylene (LDPE), which can significantly improve tensile strength and flexural strength.

[0055] Secondly, as an excellent elastomer, LLDPE can effectively compensate for the brittleness of PLA and bamboo fiber systems and improve the impact resistance of materials. This method of physical toughening through LLDPE can avoid the decrease in material strength caused by adding too many small molecule plasticizers (such as glycerin).

[0056] Third, LLDPE can improve the melt flow of PLA, making it easier to mold through processes such as extrusion and injection molding; the water absorption rate of the composite material is lower than that of pure PLA, which helps to improve the dimensional stability of the product in certain humid environments.

[0057] Fourth, linear low-density polyethylene (LLDPE) is compatible with ethyl acetate, which is itself a biodegradable material, thus improving the overall biodegradability of the material.

[0058] (6) Poor compatibility between polyolefins and polar materials such as starch and PBAT is a key issue restricting the performance of blended systems. This invention improves compatibility through multiple means: First, grafted modified starch is used to introduce polyacrylic acid segments into the starch molecular chain. There is a certain compatibility between polyacrylic acid and polyolefin segments, which can serve as an interface bridge. Second, maleic anhydride-grafted polyethylene (PEgMAH) and maleic anhydride-grafted polyolefin elastomer (POEgMAH) are added as reactive compatibilizers. The maleic anhydride groups can react with the hydroxyl and carboxyl groups of starch and PBAT to form covalent bonds at the interface, which greatly reduces the interfacial tension.

[0059] (7) The final heat shrink film has a longitudinal heat shrinkage rate of ≥65%, a transverse heat shrinkage rate of ≥59%, an elongation at break of ≥200%, and a degradation weight loss rate of ≥68% in a simulated natural environment for 180 days. Attached Figure Description

[0060] Figure 1 This is a photograph of the shrink film prepared in test group 1 in Example 1 of the present invention.

[0061] Figure 2 This is a photograph of the shrink film prepared in test group A in Example 2 of the present invention. Detailed Implementation

[0062] To more clearly illustrate the technical features of this solution, the following detailed implementation method will be used to explain the solution.

[0063] Example 1

[0064] An environmentally friendly and biodegradable polyolefin heat shrink film, comprising a three-layer co-extruded outer layer, a core layer, and an inner layer;

[0065] The outer and inner outer layers are composed of the following raw materials by weight: 65 parts of linear low-density polyethylene A, 15-25 parts of polypropylene carbonate, 10-15 parts of chitosan, 2-5 parts of bamboo fiber A, 2 parts of maleic anhydride grafted polyethylene, 1 part of graphene oxide, 0.5 parts of opening agent, and 1 part of slip agent.

[0066] The core layer is composed of the following raw materials by weight: 60 parts of linear low-density polyethylene B, 15 parts of ethyl acetate, 10 parts of starch-grafted modified acrylic monomer, 2 parts of calcium phosphate ceramic, 5 parts of bamboo fiber B, 2 parts of compatibilizer, and 0.5 parts of antioxidant.

[0067] The grafting rate of starch-grafted modified acrylic monomers is 15-35%.

[0068] By weight, (polypropylene carbonate + chitosan) / bamboo fiber A = 8-15.

[0069] The opening agent is diatomaceous earth;

[0070] The slip agent is calcium stearate;

[0071] The compatibilizer is an acrylic-grafted polyolefin resin with a grafting rate of 0.8%–1.5%.

[0072] The addition of this compatibilizer can effectively reduce the interfacial tension between the starch phase and the polyolefin phase, improve the compatibility between the components, and enhance the overall mechanical properties of the film.

[0073] The antioxidant is a compound of hindered phenolic antioxidants and phosphite antioxidants in a 1:1 mass ratio.

[0074] The heat shrink film has a thickness of 20μm, and the thickness ratio of the three layers is outer layer: core layer: inner layer = 1:2:2.

[0075] In this Example 1, based on the above parameters and data, the following three sets of experiments were designed to verify the synergistic effect between polypropylene carbonate, chitosan, and bamboo fiber A.

[0076] Experimental Group 1: 15 parts polypropylene carbonate, 15 parts chitosan, 2 parts bamboo fiber A, and other components as above. (See attached image) Figure 1 ;

[0077] Experimental Group 2: 20 parts polypropylene carbonate, 13 parts chitosan, 3 parts bamboo fiber A, and the remaining components are as above;

[0078] Experimental Group 3: 25 parts polypropylene carbonate, 11 parts chitosan, 4 parts bamboo fiber A, and the remaining components are as above; traditional polyethylene heat shrink film is used as a comparative example. Table 1 shows the mechanical properties of the shrink film in each experimental group of Example 1, and Table 2 shows the optical properties and degradation properties of the shrink film in each experimental group of Example 1.

[0079] Table 1 shows the mechanical properties of the shrink film in each test group of Example 1.

[0080] experiment Longitudinal heat shrinkage rate / % Transverse heat shrinkage rate / % Longitudinal tensile strength / MPa Transverse tensile strength / MPa Longitudinal elongation at break / % Transverse elongation at break / % Experimental group 1 68.5 62.3 26.4 22.8 385 412 Experimental group 2 65.2 59.8 28.9 24.7 420 445 Experimental group 3 71.3 64.1 30.1 25.8 350 380

[0081] As can be seen from Table 1, in experimental groups 1-3, the longitudinal and transverse thermal shrinkage rates generally decreased first and then increased. The longitudinal and transverse elongation at break also showed a similar trend. This indicates that there are optimal parameter data for polypropylene carbonate, chitosan, and bamboo fiber A. The longitudinal and transverse tensile strengths continuously increase because the bamboo fiber content increases, and the chitosan and bamboo fiber work together to improve the overall longitudinal and transverse tensile strengths. As the chitosan content decreases, the rate of increase in longitudinal and transverse tensile strengths also decreases.

[0082] Table 2 shows the optical and degradation properties of the shrink film in each experimental group in Example 1 of this embodiment.

[0083] experiment Haze / % Weight loss rate after 30 days of degradation / % Degradation weight loss rate after 60 days / % 90-day weight loss rate of degradation / % Weight loss rate after 180 days of degradation / % Molecular weight reduction rate after 180 days of degradation / % Experimental group 1 8.2 12.3 28.5 46.2 72.6 78.4 Experimental group 2 9.5 15.6 32.1 51.8 81.3 85.6 Experimental group 3 7.6 10.8 25.4 42.6 68.9 73.2

[0084] Referring to Table 2, the data on haze, degradation weight loss rate, and molecular weight reduction rate after 180 days of degradation show that haze first increases and then decreases. As the number of degradation days increases, the weight loss rate also increases continuously, with the highest weight loss rate in experimental group 2. The molecular weight reduction rate after 180 days of degradation first increases and then decreases. This indicates that the optical and degradation properties of shrink film can be adjusted by changing the composition of polypropylene carbonate, chitosan, and bamboo fiber.

[0085] The testing standards for each performance aspect in this solution are as follows:

[0086] (1) Heat shrinkage rate: According to GB / T13519 standard, the longitudinal and transverse shrinkage rates were measured after heating in an oven at 120℃ for 10 seconds.

[0087] (2) Tensile strength and elongation at break: determined in accordance with GB / T1040.3 standard.

[0088] (3) Degradation performance: Degradation experiments were conducted in simulated natural soil environment and sunlight conditions in accordance with GB / T41010-2021 standard. The weight loss rate and molecular weight reduction rate were recorded at 15 days, 30 days, 60 days, 90 days and 180 days respectively.

[0089] (4) Optical performance: Haze was measured according to ASTM D1003 standard.

[0090] Example 2

[0091] Based on Experiment 1 of Example 1, Example 2 is introduced.

[0092] By weight, (graphene oxide + starch-grafted modified acrylic monomer) / calcium phosphate ceramic = 3-8.

[0093] In this Example 2, based on the above parameters and data, the following three sets of experiments were designed to verify the synergistic effect between graphene oxide, starch-grafted modified acrylic monomer, and calcium phosphate ceramic.

[0094] Experimental Group A: 10 parts starch-grafted modified acrylic monomer, 2 parts calcium phosphate ceramic, 1 part graphene oxide; other components not mentioned above, see [link to relevant documentation]. Figure 2 ;

[0095] Experimental Group B: 12 ​​parts of starch-grafted modified acrylic monomer, 4 parts of calcium phosphate ceramic, 2 parts of graphene oxide, and other components not involved are as above;

[0096] Experimental Group C: 15 parts of starch-grafted modified acrylic monomer, 6 parts of calcium phosphate ceramic, 3 parts of graphene oxide, and other components not mentioned above; Table 3 shows the mechanical properties of the shrink film under each experimental group in Example 1, and Table 4 shows the optical properties and degradation properties of the shrink film under each experimental group in Example 1.

[0097] Table 3 shows the mechanical properties of the shrink film in each test group of Example 2.

[0098] experiment Longitudinal heat shrinkage rate / % Transverse heat shrinkage rate / % Longitudinal tensile strength / MPa Transverse tensile strength / MPa Longitudinal elongation at break / % Transverse elongation at break / % Experimental group A 68.8 62.1 28.5 23.9 398 435 Experimental group B 67.6 59.4 29.6 25.2 425 452 Experimental group C 71.0 63.8 31.5 26.3 382 402

[0099] As can be seen from Table 3, in experimental groups A, B, and C, the longitudinal and transverse thermal shrinkage rates generally decreased first and then increased. The longitudinal and transverse elongation at break also showed a similar trend. This indicates that there are optimal parameter data for graphene oxide, starch-grafted modified acrylic monomer, and calcium phosphate ceramics. The longitudinal and transverse tensile strengths continuously increase, which is due to the interaction between calcium phosphate ceramics and graphene oxide, resulting in an increase in graphene oxide content, which improves the overall longitudinal and transverse tensile strengths.

[0100] Since the starch graft can encapsulate graphene oxide like a "surfactant", it helps to increase dispersibility. As can be seen from the test group data under the same performance in Table 3, the normalization is more obvious than that in Example 1, which indicates that the overall performance is more stable and uniform.

[0101] Table 4 shows the optical and degradation properties of the shrink film in each experimental group in Example 2 of this embodiment.

[0102] experiment Haze / % Weight loss rate after 30 days of degradation / % Degradation weight loss rate after 60 days / % 90-day weight loss rate of degradation / % Weight loss rate after 180 days of degradation / % Molecular weight reduction rate after 180 days of degradation / % Experimental group A 8.0 13.5 28.8 46.8 72.9 78.9 Experimental group B 9.1 15.4 32.3 52.0 81.8 85.8 Experimental group C 7.2 11.5 26.7 43.9 72.7 74.5

[0103] Referring to Table 4, the data on haze, degradation weight loss rate, and molecular weight reduction rate after 180 days of degradation show that haze first increases and then decreases. As the number of degradation days increases, the weight loss rate also increases continuously, with the highest weight loss rate in experimental group 2. The molecular weight reduction rate after 180 days of degradation first increases and then decreases. Overall, the data are generally better than those of Example 1, and the normalization is more obvious. This indicates that the optical and degradation properties of the shrink film can be adjusted by changing the composition of starch-grafted modified acrylic monomer, calcium phosphate ceramic, and graphene oxide.

[0104] Example 3

[0105] Based on Experiment 1 of Example 1, Example 3 is introduced.

[0106] By weight, linear low-density polyethylene B / (ethyl acetate + bamboo fiber B) = 2-3.

[0107] In this Example 3, based on the above parameters and data, the following three sets of experiments were designed to verify the synergistic effect between linear low-density polyethylene B, ethyl acetate, and bamboo fiber B.

[0108] Experimental Group X: 60 parts of linear low-density polyethylene B, 20 parts of ethyl acetate, 5 parts of bamboo fiber B, and other components not involved are as above;

[0109] Experimental group Y: 65 parts of linear low-density polyethylene B, 18 parts of ethyl acetate, 8 parts of bamboo fiber B, and other components not involved are as above;

[0110] Experimental Group Z: 70 parts of linear low-density polyethylene B, 15 parts of ethyl acetate, 10 parts of bamboo fiber B, and other components not mentioned above; Table 5 shows the mechanical properties of the shrink film in each experimental group of Example 3, and Table 6 shows the optical properties and degradation properties of the shrink film in each experimental group of Example 3.

[0111] Table 5 shows the mechanical properties of the shrink film in each test group of Example 3.

[0112] experiment Longitudinal heat shrinkage rate / % Transverse heat shrinkage rate / % Longitudinal tensile strength / MPa Transverse tensile strength / MPa Longitudinal elongation at break / % Transverse elongation at break / % Experimental group X 68.2 62.5 26.8 23.2 387 414 Experimental group Y 65.4 59.4 29.4 24.8 421 447 Experimental group Z 71.2 64.0 31.2 26.2 358 387

[0113] As can be seen from Table 1, in the experimental group XZ, ​​the longitudinal heat shrinkage rate and the transverse heat shrinkage rate generally decreased first and then increased. The longitudinal elongation at break and the transverse elongation at break also showed a similar trend. The longitudinal tensile strength and the transverse tensile strength increased continuously. This is because the bamboo fiber content increased continuously, and the addition of ethyl acetate improved the overall longitudinal tensile strength and the transverse tensile strength.

[0114] Table 6 shows the optical and degradation properties of the shrink film in each experimental group of Example 3.

[0115] experiment Haze / % Weight loss rate after 30 days of degradation / % Degradation weight loss rate after 60 days / % 90-day weight loss rate of degradation / % Weight loss rate after 180 days of degradation / % Molecular weight reduction rate after 180 days of degradation / % Experimental group X 8.2 12.5 28.6 46.2 72.5 78.4 Experimental group Y 9.5 14.7 31.8 51.2 80.4 85.1 Experimental group Z 7.6 10.2 24.8 42.1 68.1 72.6

[0116] Referring to Table 2, the data on haze, degradation weight loss rate, and molecular weight reduction rate after 180 days of degradation show that haze first increases and then decreases. As the number of degradation days increases, the weight loss rate also continuously increases, with the highest weight loss rate observed in experimental group 2. The molecular weight reduction rate after 180 days of degradation first increases and then decreases. Compared to the data in Example 1, the overall data for experimental group XY shows a decrease, which is due to the decrease in ethyl acetate content, thus affecting its degradation performance. This also indicates that the optical and degradation properties of the shrink film can be adjusted by modifying the composition of linear low-density polyethylene B, ethyl acetate, and bamboo fiber B.

[0117] Example 4

[0118] This Example 4 applies to Examples 1-3 described above.

[0119] A process for preparing an environmentally friendly and biodegradable polyolefin heat shrink film includes the following steps:

[0120] Step S1: Graft maleic anhydride onto polyethylene;

[0121] Step S2: Grafting starch onto modified acrylic monomers;

[0122] Step S3: Weigh the raw materials of each component of the outer surface layer, inner surface layer and core layer according to the formula, and mix them evenly separately;

[0123] Each component raw material was vacuum dried at 50–60°C for 4–8 hours to remove moisture, with the moisture content controlled to be <0.05%.

[0124] Step S4: The mixed three-layer raw materials are fed into the three extruders of the three-layer co-extrusion blown film unit, melted and plasticized, and then co-extruded through the three-layer co-extrusion die to obtain a cylindrical film preform with an A / B / A three-layer structure.

[0125] Step S5: Biaxially stretch the preform, controlling the blow-up ratio to be 2.5 to 3.0, the longitudinal stretching ratio to be 3 to 5 times, and the stretching temperature to be 100 to 120°C;

[0126] The biaxial stretching is carried out in a stretching oven. After cooling and setting, the thickness of the film is 20-80 μm, and the thickness ratio of the three layers is outer layer: core layer: inner layer = 1:2:2.

[0127] Step S6: Cool the stretched film to room temperature for setting;

[0128] Step S7: After cooling and setting, the film is subjected to corona treatment, edge trimming, and winding to obtain the finished heat shrink film; this is an existing technical operation and will not be described in detail here.

[0129] After cooling and setting, the film is subjected to corona treatment to increase its surface tension (treatment value ≥ 38 dynes), then trimmed, flattened, and wound up to the set diameter to obtain the environmentally friendly and biodegradable polyolefin heat shrink film.

[0130] In step S2, the preparation method of starch-grafted modified acrylic monomer is as follows:

[0131] Step S21: Mix natural starch (corn starch is used in this example) with sodium hydroxide solution with a mass fraction of 1-3% at a mass ratio of 1:2 to 1:4, and stir and gelatinize at 60-70°C for 2-3 hours;

[0132] Step S22: After adjusting the pH to neutral, add ammonium persulfate initiator and acrylic acid monomer, and perform graft polymerization at 60-70℃ for 2-3 hours. After washing, drying, pulverizing and sieving, the grafted modified starch is obtained.

[0133] Step S23: The amount of acrylic acid monomer added is 15-20% of the starch mass, and the amount of ammonium persulfate initiator added is 0.5-1.0% of the starch mass.

[0134] By grafting modification, hydrophilic and biocompatible polyacrylic acid segments are introduced into the starch molecular chain, which improves the interfacial compatibility between starch and polyolefin matrix and provides reactive active sites for PBAT composites.

[0135] In step S4, the process parameters for the three-layer co-extrusion are as follows: outer layer extruder temperature 160-180℃, core layer extruder temperature 180-200℃, inner layer extruder temperature 160-180℃, die temperature 170-180℃, screw speed 50-60 rpm for the outer layer, 60-80 rpm for the core layer, and 50-70 rpm for the inner layer; the die gap is 1.0-1.5 mm.

[0136] In the overall preparation process, parameters such as temperature, screw speed, blow-up ratio, stretching ratio, and cooling temperature in each stage can be adjusted within a certain range according to the actual equipment conditions and raw material specifications to achieve the best film performance.

[0137] The environmentally friendly biodegradable polyolefin heat shrink films prepared in Examples 1-3 of this invention exhibit excellent heat shrinkage performance. Their longitudinal shrinkage rate exceeds 65%, and their transverse shrinkage rate both exceed 59%, reaching or even surpassing the level of traditional polyethylene heat shrink films (GB / T13519 standard requires a transverse shrinkage rate of 0-55% and a longitudinal shrinkage rate of 50-80%). In terms of mechanical properties, both tensile strength and elongation at break meet the requirements of GB / T13519 standard, with Example 3 showing the best overall performance. Regarding optical performance, the haze is below 10%, superior to the 12% level of traditional PE heat shrink films.

[0138] In terms of degradation performance, the degradation effect of the product of this invention is significantly better than that of traditional PE heat shrink film. Under simulated natural environmental conditions, the film of Example 2 achieved a weight loss rate of 81.8% and a molecular weight decrease rate of 85.8% within 180 days, indicating that the polyolefin macromolecules have undergone significant breakage and degradation. In contrast, the traditional PE heat shrink film has a weight loss rate of less than 5% under the same conditions, and basically does not degrade. From the degradation rate analysis, significant degradation begins within 30 days, and more than 30% has been degraded by 60 days, showing that the photobiological synergistic degradation mechanism of this invention has good degradation initiation and sustainability.

[0139] It should be noted that the technical features not mentioned in the above embodiments of this solution can be implemented based on the minimum value of the relevant parameter range to reduce experimental costs; the detailed material composition not mentioned, which has little impact on the experimental results, can be implemented based on materials commonly used by those skilled in the art and at the lowest market price; the data in the above table are all average values ​​of the tests, which are within the error range and are sufficient to illustrate the performance variation pattern, and will not be described in detail here.

[0140] Furthermore, the linear low-density polyethylene A and linear low-density polyethylene B, as well as bamboo fiber A and bamboo fiber B mentioned in this plan, have the same composition. This plan only distinguishes them by classifying them as A and B, and it has no impact on the implementation of the plan.

[0141] The technical features of this invention not described can be implemented by or using existing technology, and will not be repeated here. Of course, the above description is not a limitation of this invention, and this invention is not limited to the examples above. Any changes, modifications, additions or substitutions made by those skilled in the art within the scope of this invention should also be within the protection scope of this invention.

Claims

1. An environmentally friendly and biodegradable polyolefin heat shrink film, characterized in that, It includes a three-layer co-extruded outer layer, core layer, and inner surface layer; The outer and inner outer layers are composed of the following raw materials in parts by weight: 65-75 parts of linear low-density polyethylene A, 15-25 parts of polypropylene carbonate, 10-15 parts of chitosan, 2-5 parts of bamboo fiber A, 2-6 parts of maleic anhydride-grafted polyethylene, 1-3 parts of graphene oxide, 0.5-1.0 parts of opening agent, and 1-2 parts of slip agent; The core layer is composed of the following raw materials by weight: 60-70 parts of linear low-density polyethylene B, 15-20 parts of ethyl acetate, 10-15 parts of starch-grafted modified acrylic monomer, 2-6 parts of calcium phosphate ceramic, 5-10 parts of bamboo fiber B, 2-5 parts of compatibilizer, and 0.5-1 part of antioxidant.

2. The environmentally friendly and biodegradable polyolefin heat-shrinkable film according to claim 1, characterized in that, The grafting rate of the starch-grafted modified acrylic monomer is 15-35%.

3. The environmentally friendly and biodegradable polyolefin heat-shrinkable film according to claim 1, characterized in that, By weight, (polypropylene carbonate + chitosan) / bamboo fiber A = 8-15.

4. The environmentally friendly and biodegradable polyolefin heat-shrinkable film according to claim 1, characterized in that, By weight, (graphene oxide + starch-grafted modified acrylic monomer) / calcium phosphate ceramic = 3-8.

5. The environmentally friendly and biodegradable polyolefin heat-shrinkable film according to claim 1, characterized in that, By weight, linear low-density polyethylene B / (ethyl acetate + bamboo fiber B) = 2-3.

6. The environmentally friendly and biodegradable polyolefin heat shrink film according to claim 1, characterized in that, The opening agent is any one of synthetic silica, talc, diatomaceous earth, and calcium carbonate; The slip agent is any one of erucamide, calcium stearate, or organosilicon. The compatibilizer is any one of acrylic acid-grafted polyolefin resin, glycidyl methacrylate (GMA)-grafted polymer, oxazoline-grafted polystyrene (RPS), and modified polyacrylate, and the grafting rate is 0.8% to 1.5%. The antioxidant is a compound of hindered phenolic antioxidants and phosphite antioxidants in a mass ratio of 1:1 to 1:

3.

7. The environmentally friendly and biodegradable polyolefin heat-shrinkable film according to claim 1, characterized in that, The thickness of the heat-shrinkable film is 20-80 μm, and the thickness ratio of the three layers is outer layer: core layer: inner layer = 1:2:

2.

8. A process for preparing an environmentally friendly and biodegradable polyolefin heat-shrinkable film according to any one of claims 1 to 7, characterized in that, Includes the following steps: Step S1: Graft maleic anhydride onto polyethylene; Step S2: Grafting starch onto modified acrylic monomers; Step S3: Weigh the raw materials of each component of the outer surface layer, inner surface layer and core layer according to the formula, and mix them evenly separately; Step S4: The mixed three-layer raw materials are fed into the three extruders of the three-layer co-extrusion blown film unit, melted and plasticized, and then co-extruded through the three-layer co-extrusion die to obtain a cylindrical film preform with an A / B / A three-layer structure. Step S5: Biaxially stretch the preform, controlling the blow-up ratio to be 2.5 to 3.0, the longitudinal stretching ratio to be 3 to 5 times, and the stretching temperature to be 100 to 120°C; Step S6: Cool the stretched film to room temperature for setting; Step S7: After cooling and setting, the film is subjected to corona treatment, edge trimming, and winding to obtain the finished heat shrink film.

9. The preparation process of the environmentally friendly and biodegradable polyolefin heat shrink film according to claim 8, characterized in that, In step S2, the preparation method of starch-grafted modified acrylic monomer is as follows: Step S21: Mix natural starch with a sodium hydroxide solution of 1-3% by mass at a mass ratio of 1:2 to 1:4, and stir and gelatinize at 60-70°C for 2-3 hours; Step S22: After adjusting the pH to neutral, add ammonium persulfate initiator and acrylic acid monomer, and perform graft polymerization at 60-70℃ for 2-3 hours. After washing, drying, pulverizing and sieving, the grafted modified starch is obtained. Step S23: The amount of acrylic monomer added is 15-20% of the starch mass, and the amount of ammonium persulfate initiator added is 0.5-1.0% of the starch mass.

10. The preparation process of the environmentally friendly and biodegradable polyolefin heat-shrinkable film according to claim 8, characterized in that, In step S4, the process parameters for the three-layer co-extrusion are as follows: outer layer extruder temperature 160-180℃, core layer extruder temperature 180-200℃, inner surface layer extruder temperature 160-180℃, die temperature 170-180℃, screw speed 50-60 rpm for the outer layer, 60-80 rpm for the core layer, and 50-70 rpm for the inner surface layer; the die gap is 1.0-1.5 mm.

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