Method for preparing environmentally friendly plastic bag

Abstract

The present invention relates to the field of plastic film technology, and in particular to a method for preparing an environmentally friendly plastic bag, comprising the following steps: S1, mixing polyethyleneimine, ammonium polyphosphate, and a first solvent, performing a primary reaction, cooling to room temperature, filtering, washing, and drying to obtain a solid phase; S2, mixing tannic acid and a second solvent, adding tris(hydroxymethyl)aminomethane, adjusting the pH to 7-9, then adding hydrogen peroxide, performing a secondary reaction, adding the solid phase, performing a tertiary reaction, filtering, washing, and drying to obtain a heat-resistant enhancer; S3, placing modified polylactic acid, the heat-resistant enhancer, and an auxiliary agent in a high-speed mixer, mixing until uniform, adding the mixture to a twin-screw extruder, granulating, and then blowing the film with a blower to obtain an environmentally friendly plastic bag. The environmentally friendly plastic bag provided by the present invention has excellent mechanical properties, thermal stability, and UV resistance.

Description

Technical Field

[0001] The invention belongs to the technical field of plastic films, and in particular relates to a method for preparing an environmentally friendly plastic bag. Background Art

[0002] The rapid development of the plastics industry has led to the widespread use of plastic products, especially plastic bags, bringing significant convenience to human life. Plastic bags are widely used due to their low price, light weight, large capacity, and easy storage. However, existing plastic bags remain in the environment for a long time after use and disposal, leading to the accumulation of large amounts of plastic bag waste, which poses a serious environmental threat. Polylactic acid (PLA) is a biodegradable material made from renewable plant resources (such as corn and cassava) through fermentation and polymerization processes. In the natural environment, PLA plastic bags are gradually decomposed into carbon dioxide and water by microorganisms (such as bacteria and fungi). These products are common substances in nature and can participate in the material cycle of the ecosystem. Unlike traditional plastic bags, they do not accumulate in the environment over time, causing white pollution and harming soil, water, and other ecological environments. However, the glass transition temperature of PLA is generally around 50°C-60°C, which means that PLA plastic bags are prone to deformation at or above this temperature range. For example, when used to package hot food or other items, they may soften and collapse, affecting the packaging's performance and integrity. When stored in high-temperature environments for long periods of time, polylactic acid plastic bags may also experience accelerated degradation and reduced mechanical properties, shortening their service life. Therefore, their heat resistance is limited.

[0003] For example, CN117510999A discloses a fully biodegradable, environmentally friendly plastic bag. Its formula includes corn starch, alginic acid, sodium alginate, a plasticizer, and an antioxidant. The weight percentages of each component are: 20-30 parts corn starch, 15-20 parts alginic acid, 15-20 parts sodium alginate, 1-2 parts plasticizer, and 0.5-1 part antioxidant. A method for preparing the fully biodegradable, environmentally friendly plastic bag includes the following steps: Step 1: mixing; Step 2: granulation; Step 3: film blowing; Step 4: pattern printing; and Step 5: winding. In Step 1, the weighed raw materials are placed inside a stirring structure. However, the heat resistance of the plastic bag in this application still needs to be improved.

[0004] For example, CN115926404A discloses a heat-resistant polylactic acid fiber and yarn prepared by cross-linking modified polylactic acid, which relates to the field of polylactic acid material technology. The polylactic acid composite material includes polylactic acid and thiourea-modified polylactic acid, and then the polylactic acid composite material is used to prepare heat-resistant polylactic acid fiber and heat-resistant yarn. The cross-linking modification method of the application is used to prepare thiourea-modified polylactic acid, and then the polylactic acid composite material is prepared from polylactic acid and thiourea-modified polylactic acid. This not only retains the biodegradability of polylactic acid but also improves the heat resistance of polylactic acid. However, thiourea itself has certain toxicity and is a moderately toxic substance. In the process of modifying polylactic acid, even if thiourea participates in the chemical reaction and is incorporated into the molecular structure of polylactic acid, the risk of a small amount of free thiourea being released during subsequent use cannot be completely ruled out. In particular, in some application scenarios with food contact or high safety requirements, this potential toxicity risk will limit the scope of use of the modified polylactic acid material and may pose a potential threat to human health.

[0005] As disclosed in CN102086299A, a transparent crystalline polylactic acid plastic is composed of the following raw materials in parts by weight: 100 parts of polylactic acid, 0.1-5 parts of crystallization nucleating agents, 0.5-20 parts of plasticizers, and 0.5-0 parts of other auxiliary agents. By adding crystallization nucleating agents and plasticizers, the crystallinity, crystallization rate, and heat resistance of the plastic are significantly improved, and its crystal nucleus size is significantly reduced, so that the transparent crystalline polylactic acid plastic has the characteristics of high transparency, high heat resistance, and high tensile strength. Although the nucleating agent can improve the crystallinity of polylactic acid and thus promote heat resistance to a certain extent, it may also cause uneven changes in the mechanical properties of the material. On the one hand, too fast a crystallization rate and too many crystal nuclei may make the crystal size distribution of polylactic acid uneven, and the interfaces between small grains increase. When stressed, these interfaces easily become stress concentration points, causing the toughness of the material to decline, showing as a deterioration in impact resistance, and the product is prone to brittle fracture. On the other hand, in some cases, the addition of nucleating agents may not significantly improve properties such as tensile strength, and may even lead to a decrease in overall mechanical properties due to compatibility issues between the nucleating agent and the polylactic acid matrix.

[0006] In view of this, the present invention is proposed. Summary of the Invention

[0007] The object of the present invention is to provide a method for preparing an environmentally friendly plastic bag. The environmentally friendly plastic bag provided by the present invention has excellent mechanical properties, thermal stability and UV resistance.

[0008] In order to achieve the above object, the present invention provides the following technical solutions:

[0009] An embodiment of the present invention provides a method for preparing an environmentally friendly plastic bag, comprising the following steps:

[0010] S1, mixing polyethyleneimine, ammonium polyphosphate, and a first solvent, performing a reaction, cooling to room temperature, filtering, washing, and drying to obtain a solid phase;

[0011] S2, mixing tannic acid and a second solvent, adding tris(hydroxymethyl)aminomethane and adjusting the pH to 7-9, then adding hydrogen peroxide, and after a secondary reaction, adding a solid phase to carry out a tertiary reaction, filtering, washing, and drying to obtain a heat-resistant enhancer;

[0012] S3, putting the modified polylactic acid, heat-resistant enhancer, and additives into a high-speed mixer, mixing them until uniform, and then adding them into a twin-screw extruder for granulation, and then blowing the film through a blower to obtain an environmentally friendly plastic bag.

[0013] In this application, polyethyleneimine and ammonium polyphosphate are first bonded through acid-base interaction. This is primarily due to the fact that polyethyleneimine is a cationic polymer with numerous amino groups in its molecular chain. These amino groups can be protonated, giving the polyethyleneimine a positive charge. Ammonium polyphosphate, on the other hand, is a salt compound composed of phosphate and ammonium groups. In aqueous solution or a suitable reaction system, the phosphate groups in the ammonium polyphosphate carry a negative charge. The positively charged polyethyleneimine and the negatively charged ammonium polyphosphate attract each other through electrostatic interaction, forming an ionic bond. Tannic acid is then oxidized, primarily to form quinone derivatives by oxidizing the phenolic hydroxyl groups in the tannic acid. Finally, the carbonyl groups in the quinone derivatives formed by the tannic acid react with the amino groups in the combined polyethyleneimine and ammonium polyphosphate, resulting in the grafting of the polyethyleneimine, ammonium polyphosphate, and tannic acid. This is primarily due to the polarity of the carbonyl group, which carries a partial positive charge and is therefore susceptible to attack by nucleophiles. The nitrogen atom of the amino group has a lone pair of electrons, making it a good nucleophile. The amino group attacks the carbonyl carbon atom of the quinone derivative, causing a nucleophilic addition reaction and forming a new carbon-nitrogen bond.

[0014] The heat-resistant enhancer provided by the present invention significantly improves the heat resistance and UV resistance of plastic bags. In addition, by grafting polyethyleneimine, ammonium polyphosphate, and tannic acid together, the present invention avoids the defects of being difficult to disperse and prone to phase separation when blended in polylactic acid.

[0015] When the three are grafted, they are connected by chemical bonds, forming a unified molecular structure. This grafted product has better compatibility in the matrix material and can be dispersed more evenly, avoiding phase separation problems and making the material's performance more stable and uniform. In addition, the grafted structure brings ammonium polyphosphate (APP), polyethyleneimine (PEI), and tannic acid (TA) closer together and interacts more closely, allowing them to more effectively exert a synergistic effect. More specifically, ammonium polyphosphate (APP) provides a phosphorus source for carbonization, polyethyleneimine (PEI) provides a nitrogen source to promote carbonization and gas dilution, and tannic acid (TA) strengthens the carbon layer through its own carbonization. After grafting, these effects can be carried out more efficiently and synergistically, forming a denser and more stable carbon layer, improving the flame retardant effect. In terms of improving the material's mechanical properties, the cross-linking effect of polyethyleneimine (PEI), the interaction between tannic acid (TA) and the matrix, and the effect of ammonium polyphosphate (APP) on crystallization can synergistically enhance the material's overall mechanical properties. However, in a blend, this synergistic effect can be limited by factors such as uneven dispersion. It should also be noted that the molecular structure of the grafted product is relatively stable, making it less susceptible to component migration or separation during use. In a blend, however, because the components are simply physically mixed, they may experience relative movement or precipitation when affected by external environmental factors (such as temperature and solvent), leading to a decrease in material performance. The grafted structure ensures that ammonium polyphosphate (APP), polyethyleneimine (PEI), and tannic acid (TA) maintain a relatively stable ratio and interaction under varying environmental conditions, resulting in more stable and reliable material performance.

[0016] Specifically, ammonium polyphosphate (AMP) is a common intumescent flame retardant for improving heat resistance. At high temperatures, AMP decomposes to produce phosphoric acid, polyphosphoric acid, and other phosphorus-containing compounds. These compounds promote the dehydration and carbonization of polylactic acid, forming a dense char layer. Polyethyleneimine molecules contain a large number of nitrogen atoms. During combustion, nitrogen produces non-combustible gases, diluting the oxygen concentration and promoting char formation. This synergistic effect with AMP makes the char layer more stable and dense. Tannic acid, a polyphenolic compound, can also undergo dehydration and condensation reactions at high temperatures to form a char layer. When these three agents work together, AMP provides a phosphorus source for char formation, while PEI provides a nitrogen source to promote char formation and gas dilution. Tannic acid further enhances the thickness and strength of the char layer through its own carbonization. This char layer effectively prevents heat transfer to the polylactic acid matrix, slowing the thermal degradation of the polylactic acid and thereby improving its heat resistance. Furthermore, PEI contains a large number of amino groups, making it highly reactive. It can chemically react with carboxyl or hydroxyl groups on the polylactic acid (PLA) molecular chains, forming chemical bonds and creating crosslinks between the PLA chains. The crosslinks restrict the movement of the PLA chains, improving the material's rigidity and heat-deformation resistance. Tannic acid molecules contain multiple phenolic hydroxyl groups, which form quinone structures that react with the amino groups in polyethyleneimine, further enhancing the crosslinking effect. Tannic acid also forms a network structure with the PLA chains through hydrogen bonding and other interactions. This, in conjunction with the crosslinking effect of polyethyleneimine, enhances the heat resistance of the PLA. Furthermore, ammonium polyphosphate can act as a nucleating agent, promoting the crystallization of PLA and resulting in a larger number of fine crystals. Increased crystallinity enhances the material's heat resistance because the molecular chains in the crystalline regions are more regularly aligned, leading to stronger intermolecular forces and requiring higher temperatures for melting. The interactions between polyethyleneimine and tannic acid and the PLA chains may also affect the movement and alignment of the PLA chains, thereby affecting the crystallization process. Furthermore, from a thermal degradation perspective, the decomposition products of ammonium polyphosphate can capture free radicals generated by the thermal degradation of polylactic acid, terminating the free radical chain reaction and thereby inhibiting the thermal degradation of polylactic acid. Polyethyleneimine and tannic acid also possess certain antioxidant properties. They react with the reactive oxygen species produced during the thermal degradation of polylactic acid, preventing oxidative degradation. These three ingredients work together to comprehensively inhibit the thermal and oxidative degradation of polylactic acid, improving its thermal stability and, consequently, its heat resistance.

[0017] Tannic acid, a polyphenolic compound, contains multiple conjugated groups in its molecular structure, such as benzene rings and phenolic hydroxyl groups. These conjugated structures absorb UV light, converting its energy into heat or other forms of energy and releasing it, thereby reducing direct UV exposure and damage to the polylactic acid (PLA) molecular chains. Ammonium polyphosphate can also scatter and shield UV light to a certain extent. Its crystal structure and dispersion within the PLA matrix can alter the UV transmission path, reducing the intensity of UV radiation penetrating deep into the PLA. Although polyethyleneimine itself has a relatively weak UV absorption capacity, it interacts with tannic acid and ammonium polyphosphate, promoting their uniform dispersion within the PLA matrix. This allows the tannic acid's UV absorption and the ammonium polyphosphate's scattering and shielding properties to be more effective. The three synergistically enhance UV absorption and shielding, reducing UV damage to the PLA. Furthermore, UV exposure can generate free radicals in the PLA molecular chains, triggering oxidative degradation reactions and degrading the material's properties. Tannic acid has strong antioxidant properties. The phenolic hydroxyl groups in its molecules act as hydrogen donors, reacting with free radicals and converting them into more stable species, thereby terminating the free radical chain reaction. The amino groups in polyethyleneimine are nucleophilic and can also react with and capture free radicals. The phosphorus-containing compounds produced by the decomposition of ammonium polyphosphate in a hot and oxidative environment can bind to free radicals and inhibit their activity. The three components work together to form a more comprehensive free radical scavenging system. Tannic acid directly provides hydrogen atoms to neutralize free radicals, while the amino groups in polyethyleneimine assist in capturing free radicals. The decomposition products of ammonium polyphosphate further stabilize free radicals, comprehensively preventing UV-induced oxidative degradation of polylactic acid and synergistically improving its UV resistance. Furthermore, polyethyleneimine contains numerous amino groups that react with carboxyl or hydroxyl groups on the polylactic acid molecular chains, forming chemical bonds and creating crosslinks between the polylactic acid chains. This crosslinking structure restricts the movement of the polylactic acid chains, enhancing the material's structural stability and making it more resistant to chain breakage and degradation caused by UV exposure. The quinone structures formed by tannic acid can further react with the amino groups in polyethyleneimine, strengthening the crosslinking effect. Ammonium polyphosphate acts as a physical crosslinking point within the polylactic acid matrix, reinforcing the material's network structure. The three elements work synergistically, creating a more stable polylactic acid molecular network through chemical and physical crosslinking. This effectively improves the structural stability of polylactic acid plastics under ultraviolet radiation, thereby enhancing their UV resistance.

[0018] In a preferred embodiment, in step S1, the molecular weight of the polyethyleneimine is 1000-5000 Da.

[0019] In a preferred embodiment, in step S1, the first solvent is a mixture of ethanol and water, and the volume ratio of ethanol to water is (70-90):10.

[0020] In a preferred embodiment, in step S1, the mass ratio of the polyethyleneimine, ammonium polyphosphate and the first solvent is 1:(1.5-3):(20-50).

[0021] In a preferred embodiment, in step S1, the conditions of the primary reaction are: under nitrogen atmosphere, at 80-100° C. for 2-6 hours.

[0022] In a preferred embodiment, in step S2, the second solvent is a mixture of ethanol and water, and the volume ratio of the ethanol to water is (100-120):10.

[0023] In a preferred embodiment, in step S2, the mass ratio of the tannic acid, the second solvent, tris(hydroxymethyl)aminomethane, hydrogen peroxide and the solid phase is 10:(20-40):(3-6):(10-15):(18-30).

[0024] In a preferred embodiment, in step S2, the conditions for the secondary reaction are: stirring until the solution turns reddish brown.

[0025] In a preferred embodiment, in step S2, the conditions for the three reactions are: reaction at 20-40° C. for 4-8 hours.

[0026] In a preferred embodiment, in step S3, the modified polylactic acid is prepared by mixing maleic anhydride, an initiator and polylactic acid, and then melt-extruding the mixture to obtain the modified polylactic acid.

[0027] In a preferred embodiment, based on 100 parts by mass of the polylactic acid, the mass of the maleic anhydride is 1-4 parts, and the mass of the initiator is 0.1-0.4 parts.

[0028] In a preferred embodiment, the initiator is one or more of dibenzoyl peroxide, di-t-amyl peroxide, diisopropyl benzene peroxide, bis(tert-butylperoxyisopropyl)benzene, 2,5-di-tert-butylperoxy-2-5-dimethylhexane, tert-butyl peroxide isopropyl benzene, di-tert-butyl peroxide or diisopropylbenzene hydroperoxide.

[0029] The present invention utilizes maleic anhydride to graft-modify polylactic acid to obtain modified polylactic acid, wherein the anhydride groups in the modified polylactic acid can form chemical bonds with amino groups, hydroxyl groups, and the like in a functional enhancer when blended, thereby enhancing the binding force and compatibility between the two, and further improving the performance stability of the formed environmentally friendly plastic bag, especially the mechanical properties and thermal stability.

[0030] Specifically, in terms of improving compatibility, pure polylactic acid is a non-polar polymer, while ammonium polyphosphate, polyethyleneimine and tannic acid have certain polarity, and their compatibility is poor, and phase separation is easy to occur when blended. Since the modified polylactic acid is grafted with polar maleic anhydride groups, it can react chemically or form hydrogen bonds with polar groups such as the hydroxyl groups on the surface of ammonium polyphosphate, the amino groups of polyethyleneimine and the phenolic hydroxyl groups of tannic acid, thereby reducing the interfacial tension and enhancing the compatibility between the components, making the blending system more uniform and stable. In terms of improving mechanical properties, good compatibility helps to better transfer stress between the components. In a pure polylactic acid blending system, due to the existence of phase separation, stress concentration is obvious, resulting in poor mechanical properties of the material. In the modified PLA blend system, the maleic anhydride grafted chain segments act as bridges, strengthening the interfacial bonding between the PLA matrix and ammonium polyphosphate, polyethyleneimine, and tannic acid. When the material is subjected to external forces, stress is more effectively transferred and dispersed between the components, thereby improving the material's mechanical properties such as tensile strength, flexural strength, and impact toughness. Regarding thermal stability, ammonium polyphosphate is a commonly used flame retardant. In the modified PLA blend system, due to improved compatibility, ammonium polyphosphate can be more evenly dispersed in the PLA matrix, fully exerting its flame retardant effect. At the same time, polyethyleneimine and tannic acid may also synergize with ammonium polyphosphate to form a more stable char structure, hindering the transfer of heat and oxygen, further improving the material's thermal stability. In contrast, in pure PLA blend systems, due to the poor compatibility of the components and the uneven dispersion of flame retardants such as ammonium polyphosphate, this can lead to poor flame retardancy and relatively low thermal stability. In terms of improved flame retardancy, the more evenly dispersed ammonium polyphosphate (AMP) in the modified PLA blend system and its enhanced synergistic effect with other components enable more effective flame retardancy. The thermal decomposition of AMP, which produces phosphoric acid, metaphosphoric acid, and other substances, promotes the dehydration and carbonization of the PLA, forming a dense char layer that isolates heat and oxygen and inhibits combustion. Furthermore, polyethyleneimine and tannic acid also form nitrogen- and carbon-containing flame-retardant layers during combustion, further enhancing flame retardancy. In contrast, in pure PLA blends, the uneven dispersion of AMP compromises the flame retardant effect, resulting in relatively poor flame retardancy.

[0031] In a preferred embodiment, in step S3, the auxiliary agent includes one or more of a stabilizer and a coupling agent.

[0032] In a preferred embodiment, in step S3, the stabilizer is one or more of sodium alginate, zinc stearate, and barium stearate.

[0033] In a preferred embodiment, in step S3, the coupling agent is one or more of KH550, KH560, and KH570.

[0034] In a preferred embodiment, the mass ratio of the modified polylactic acid, the heat-resistant enhancer, and the auxiliary agent is 100:(22-45):(10-20).

[0035] In a preferred embodiment, the extrusion temperature of the twin-screw extruder is 180-190°C.

[0036] Compared with the prior art, the advantages and beneficial effects of the present invention are:

[0037] 1. The heat-resistant enhancer provided by the present invention significantly improves the heat resistance and UV resistance of plastic bags. In addition, by grafting polyethyleneimine, ammonium polyphosphate, and tannic acid together, the present invention avoids the defects of being difficult to disperse and prone to phase separation when blended in polylactic acid.

[0038] 2. The present invention uses maleic anhydride to graft-modify polylactic acid to obtain modified polylactic acid, wherein the anhydride groups in the modified polylactic acid can form chemical bonds with amino groups, hydroxyl groups and other groups in the functional enhancer when blended, thereby enhancing the binding force and compatibility between the two, and further improving the performance stability of the formed environmentally friendly plastic bag, especially the mechanical properties and thermal stability. DETAILED DESCRIPTION

[0039] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0040] Unless otherwise specified, all reagents used in the present invention are commercially available.

[0041] Polyethyleneimine, molecular weight 4000 Da, was purchased from Shanghai MacLean Biochemical Technology Co., Ltd.;

[0042] Ammonium polyphosphate, n=40, was purchased from Shanghai MacLean Biochemical Technology Co., Ltd.;

[0043] Polylactic acid, 4032D, was purchased from Hubei Tesco Chemical Co., Ltd.

[0044] Example 1

[0045] This embodiment provides a method for preparing an environmentally friendly plastic bag, comprising the following steps:

[0046] S1, by weight, mixed 1 part polyethyleneimine, 2 parts ammonium polyphosphate, and 40 parts of a first solvent, reacted at 90°C for 4 hours under a nitrogen atmosphere, where the first solvent was a mixture of ethanol and water in a volume ratio of ethanol to water of 80:10. The mixture was cooled to room temperature, filtered, washed, and vacuum-dried to obtain a solid phase;

[0047] S2, by weight, 10 parts of tannic acid and 30 parts of a second solvent were mixed, 5 parts of tris(hydroxymethyl)aminomethane were added, and the pH was adjusted to 8. The second solvent was a mixture of ethanol and water, with the volume ratio of ethanol to water being 100:10. 13 parts of 30% hydrogen peroxide were then added and stirred until the tannic acid was completely dissolved to form a reddish-brown solution. 20 parts of the solid phase in S1 were then added and reacted at 30°C for 6 hours. The mixture was filtered, washed, and vacuum-dried to obtain a heat-resistant enhancer.

[0048] S3, mixing 2 parts by mass of maleic anhydride, 0.2 parts by mass of dibenzoyl peroxide and 100 parts by mass of polylactic acid, and then melt-extruding the mixture at 180° C. to obtain modified polylactic acid;

[0049] By mass, 100 parts of modified polylactic acid, 30 parts of heat-resistant enhancer, and 10 parts of sodium alginate were put into a high mixer and mixed until uniform, and then added to a twin-screw extruder for granulation, wherein the temperature settings of each zone of the screw were: the temperature of the first zone of the twin-screw extruder was 175°C, the temperature of the second zone was 178°C, the temperature of the third zone was 185°C, the temperature of the fourth zone was 195°C, the temperature of the head was 185°C, the residence time was 1.5min, the pressure was 10MPa, and then the film was blown by a blower to obtain an environmentally friendly plastic bag.

[0050] Example 2

[0051] This embodiment provides a method for preparing an environmentally friendly plastic bag, comprising the following steps:

[0052] S1, by weight, mixed 1 part polyethyleneimine, 3 parts ammonium polyphosphate, and 50 parts of a first solvent, reacted at 90°C for 4 hours under a nitrogen atmosphere, where the first solvent was a mixture of ethanol and water in a volume ratio of ethanol to water of 80:10. The mixture was cooled to room temperature, filtered, washed, and vacuum-dried to obtain a solid phase;

[0053] S2, by weight, 10 parts of tannic acid and 35 parts of a second solvent were mixed, 6 parts of tris(hydroxymethyl)aminomethane were added, and the pH was adjusted to 8. The second solvent was a mixture of ethanol and water, with the volume ratio of ethanol to water being 100:10. 15 parts of 30% hydrogen peroxide were then added, and the mixture was stirred until the tannic acid was completely dissolved to form a reddish-brown solution. 25 parts of the solid phase in S1 were then added, and the mixture was reacted at 30°C for 6 hours. The mixture was filtered, washed, and vacuum-dried to obtain a heat-resistant enhancer.

[0054] S3, mixing 2 parts by mass of maleic anhydride, 0.2 parts by mass of dibenzoyl peroxide and 100 parts by mass of polylactic acid, and then melt-extruding the mixture at 180° C. to obtain modified polylactic acid;

[0055] By mass, 100 parts of modified polylactic acid, 38 parts of heat-resistant enhancer, and 16 parts of sodium alginate were put into a high mixer and mixed until uniform, and then added to a twin-screw extruder for granulation, wherein the temperature settings of each zone of the screw are: the temperature of zone 1 of the twin-screw extruder is 175°C, the temperature of zone 2 is 178°C, the temperature of zone 3 is 185°C, the temperature of zone 4 is 195°C, the temperature of the die head is 185°C, the residence time is 1.5min, the pressure is 10MPa, and then the film is blown by a blower to obtain an environmentally friendly plastic bag.

[0056] Comparative Example 1

[0057] This comparative example provides a method for preparing an environmentally friendly plastic bag, comprising the following steps:

[0058] S1, by weight, mixed 1 part polyethyleneimine, 2 parts ammonium polyphosphate, and 40 parts of a first solvent, reacted at 90°C for 4 hours under a nitrogen atmosphere, where the first solvent was a mixture of ethanol and water in a volume ratio of ethanol to water of 80:10. The mixture was cooled to room temperature, filtered, washed, and vacuum-dried to obtain a solid phase;

[0059] S2, mixing 2 parts by mass of maleic anhydride, 0.2 parts by mass of dibenzoyl peroxide and 100 parts by mass of polylactic acid, and then melt-extruding the mixture at 180° C. to obtain modified polylactic acid;

[0060] By mass, 100 parts of modified polylactic acid, 30 parts of solid phase, and 10 parts of sodium alginate were placed in a high mixer and mixed until uniform, and then added to a twin-screw extruder for granulation, wherein the temperature settings of each zone of the screw were: the temperature of zone 1 of the twin-screw extruder was 175°C, the temperature of zone 2 was 178°C, the temperature of zone 3 was 185°C, the temperature of zone 4 was 195°C, the temperature of the die head was 185°C, the residence time was 1.5min, the pressure was 10MPa, and then the film was blown by a blower to obtain an environmentally friendly plastic bag.

[0061] Comparative Example 2

[0062] This comparative example provides a method for preparing an environmentally friendly plastic bag, comprising the following steps:

[0063] S1, mixing 10 parts of tannic acid and 30 parts of a second solvent, by mass, adding 5-tris(hydroxymethyl)aminomethane and adjusting the pH to 8, wherein the second solvent is a mixture of ethanol and water, with the volume ratio of ethanol to water being 100:10, then adding 13 parts of 30% hydrogen peroxide, stirring until the tannic acid is completely dissolved to form a reddish-brown solution, filtering, washing, and vacuum drying to obtain a heat-resistant enhancer;

[0064] S2, mixing 2 parts by mass of maleic anhydride, 0.2 parts by mass of dibenzoyl peroxide and 100 parts by mass of polylactic acid, and then melt-extruding the mixture at 180° C. to obtain modified polylactic acid;

[0065] By mass, 100 parts of modified polylactic acid, 30 parts of heat-resistant enhancer, and 10 parts of sodium alginate were put into a high mixer and mixed until uniform, and then added to a twin-screw extruder for granulation, wherein the temperature settings of each zone of the screw were: the temperature of the first zone of the twin-screw extruder was 175°C, the temperature of the second zone was 178°C, the temperature of the third zone was 185°C, the temperature of the fourth zone was 195°C, the temperature of the head was 185°C, the residence time was 1.5min, the pressure was 10MPa, and then the film was blown by a blower to obtain an environmentally friendly plastic bag.

[0066] Comparative Example 3

[0067] The difference between this comparative example and Example 1 is that steps S1 and S2 are not performed, and the heat-resistant enhancer in S3 is a mixture of polyethyleneimine, ammonium polyphosphate, and tannic acid in a mass ratio of 1:2:10.

[0068] Comparative Example 4

[0069] The difference between this comparative example and Example 1 is that steps S1 and S2 are not performed, and the heat-resistant enhancer in S3 is a mixture of polyethyleneimine and ammonium polyphosphate in a mass ratio of 1:2.

[0070] Comparative Example 5

[0071] The difference between this comparative example and Example 1 is that steps S1 and S2 are not performed, and the heat-resistant enhancer in S3 is a mixture of polyethyleneimine and tannic acid in a mass ratio of 1:10.

[0072] Comparative Example 6

[0073] The difference between this comparative example and Example 1 is that steps S1 and S2 are not performed, and the heat-resistant enhancer in S3 is a mixture of ammonium polyphosphate and tannic acid in a mass ratio of 2:10.

[0074] Comparative Example 7

[0075] The difference between this comparative example and Example 1 is that the modified polylactic acid in S3 is not modified but is replaced with ordinary polylactic acid.

[0076] Performance Testing

[0077] 1. The performance of the plastic bags obtained in the embodiment and the comparative example was tested using the melt stretching mode of the rheometer. The thickness of the plastic bags involved in the test was uniformly 2.0 mm, and the stretching rate was 15 mm / min. The results are shown in Table 1.

[0078] 2. The plastic bags obtained in the embodiment and the comparative example were placed in a closed space, and UVA-340 was used to provide an ultraviolet environment with a radiation intensity of 0.7 W / m 2 The temperature is 50℃, the holding time is 4h, and the percentage ratio of the tensile strength after constant temperature aging to the tensile strength without constant temperature aging is tested. The results are shown in Table 1.

[0079] Table 1 Performance test results

[0080] Tensile strength / MPa Elongation at break / % Percentage ratio of tensile strength after constant temperature to tensile strength without constant temperature / % Percentage ratio of tensile strength after constant temperature aging to tensile strength without constant temperature aging / % Example 1 45.65 314.58 90.12 86.85 Example 2 47.35 246.84 92.25 89.36 Comparative Example 1 39.46 185.56 80.24 78.65 Comparative Example 2 38.29 192.25 79.25 75.20 Comparative Example 3 27.54 236.58 73.28 69.59 Comparative Example 4 34.53 228.25 76.12 72.36 Comparative Example 5 35.24 216.54 75.65 72.13 Comparative Example 6 36.68 202.54 75.24 72.81 Comparative Example 7 42.55 174.54 85.75 83.29

[0081] From the above performance test results, it can be seen that the environmentally friendly plastic bags obtained in Examples 1-2 have excellent mechanical properties, thermal stability and UV resistance, especially the comprehensive performance of Example 2 is the most outstanding, which is mainly due to the synergistic effect of polyethyleneimine, ammonium polyphosphate, and tannic acid grafted together with modified polylactic acid.

[0082] However, the comparative example does not adopt the necessary technical solutions, resulting in its performance being significantly worse than that of the embodiment in the corresponding performance tests. This further proves the irreplaceable nature of the specific technical solutions of the present application in achieving technical effects and solving technical problems.

[0083] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A method for preparing an environmentally friendly plastic bag, characterized in that: The following steps are involved: Step S1, mixing polyethyleneimine, ammonium polyphosphate, and a first solvent, performing a reaction, cooling to room temperature, filtering, washing, and drying to obtain a solid phase; Step S2, mixing tannic acid and a second solvent, adding tris(hydroxymethyl)aminomethane and adjusting the pH to 7-9, then adding hydrogen peroxide, and after a secondary reaction, adding a solid phase to carry out a tertiary reaction, filtering, washing, and drying to obtain a heat-resistant enhancer; Step S3: Place the modified polylactic acid, heat-resistant enhancer, and additives in a high-speed mixer, mix until uniform, and then add the mixture to a twin-screw extruder for granulation. Then, use a blower to blow the mixture into a film to obtain an environmentally friendly plastic bag. In step S1, the mass ratio of the polyethyleneimine, ammonium polyphosphate and the first solvent is 1: (1.5-3): (20-50); In step S1, the conditions of the primary reaction are: under nitrogen atmosphere, at 80-100° C. for 2-6 hours; The mass ratio of the tannic acid, the second solvent, tris(hydroxymethyl)aminomethane, hydrogen peroxide and the solid phase is 10:(20-40):(3-6):(10-15):(18-30); In step S2, the conditions for the secondary reaction are: stirring until the solution turns reddish brown; In step S2, the conditions for the three reactions are: reacting at 20-40° C. for 4-8 hours; in step S3, the preparation method of the modified polylactic acid is: mixing maleic anhydride, an initiator and polylactic acid and then melt-extruding to obtain the modified polylactic acid.

2. The method for preparing an environmentally friendly plastic bag according to claim 1, characterized in that: In step S1, the molecular weight of the polyethyleneimine is 1000-5000 Da; The first solvent is a mixture of ethanol and water, and the volume ratio of the ethanol to water is (70-90):

10.

3. The method for preparing an environmentally friendly plastic bag according to claim 1, characterized in that: In step S2, the second solvent is a mixture of ethanol and water, and the volume ratio of the ethanol to water is (100-120):

10.

4. The method for preparing an environmentally friendly plastic bag according to claim 3, characterized in that: Based on 100 parts by mass of the polylactic acid, the mass of the maleic anhydride is 1-4 parts, and the mass of the initiator is 0.1-0.4 parts; The initiator is one or more of dibenzoyl peroxide, di-t-amyl peroxide, diisopropyl benzene peroxide, bis(tert-butyl peroxide isopropyl)benzene, 2,5-di-tert-butyl peroxide-2-5-dimethylhexane, tert-butyl peroxide isopropyl benzene or diisopropyl benzene hydroperoxide.

5. The method for preparing an environmentally friendly plastic bag according to claim 1, characterized in that: In step S3, the auxiliary agent includes one or more of a stabilizer and a coupling agent; The stabilizer is one or more of sodium alginate, zinc stearate, and barium stearate; The coupling agent is one or more of KH550, KH560, and KH570.

6. The method for preparing an environmentally friendly plastic bag according to claim 1, characterized in that: The mass ratio of the modified polylactic acid, the heat-resistant reinforcing agent and the auxiliary agent is 100:(22-45):(10-20).