Natural straw modified PLA composite material as well as preparation method and application thereof

Abstract

The invention relates to a natural straw modified PLA composite material as well as a preparation method and application thereof, and belongs to the technical field of composite materials. The antibacterial ultraviolet light absorber with the antibacterial function is prepared and grafted to the surface of the natural straw, the natural straw filler is endowed with the antibacterial function and the ultraviolet protection capacity, ultraviolet aging defect sites caused by introduction of the natural straw into the composite material are specially protected, and the antibacterial function and the ultraviolet protection capacity of the composite material are greatly improved. According to the present invention, the anti-ultraviolet aging performance of the composite material is substantially improved, the problem that the weatherability of the composite material is reduced due to the introduction of the porous straw is effectively overcome, and the fat long chain in the antibacterial ultraviolet light absorber can further perform sufficient physical winding and diffusion with the hydrophobic PLA molecular chain so as to substantially improve the compatibility between the straw and the PLA, such that the anti-ultraviolet aging performance of the composite material is substantially improved, and the anti-ultraviolet aging performance of the composite material is substantially improved; the overall performance of the composite material is enhanced.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of composite materials, and particularly relates to a natural straw modified PLA composite material and a preparation method and application thereof. BACKGROUND

[0002] As a biobased degradable plastic derived from renewable resources such as corn and cassava, polylactic acid (PLA) is considered as one of the ideal alternatives to traditional petroleum-based plastics due to its good biocompatibility, processing performance and complete compostability, and has shown broad application prospects in packaging, textiles, medical devices and other fields.

[0003] In order to reduce the cost of PLA and improve its brittleness, and further improve its environmental properties, researchers have focused on abundant agricultural waste-natural straw. The natural straw fiber is used as a reinforcing filler to blend with PLA to prepare a composite material, which is an effective way to realize the high-value utilization of waste and develop full biobased materials. However, the natural straw has a porous and hydrophilic physical structure. When blended with PLA, these pores form channels for ultraviolet light, oxygen and water vapor to penetrate into the interior of the material. PLA is sensitive to ultraviolet light, and the introduction of straw makes the aging factor more easily invade and initiate and accelerate the synergistic degradation of light-oxygen-water in the weak interface area, forming a vicious cycle of "interface failure-new channel formation", which significantly amplifies and accelerates the overall ultraviolet aging process of the composite material, and makes its weather resistance worse than that of pure PLA. In order to solve the above technical defects, the application provides a natural straw modified PLA composite material and a preparation method and application thereof. SUMMARY

[0004] The purpose of the application is to provide a natural straw modified PLA composite material and a preparation method and application thereof, which are used to solve the problems mentioned in the background.

[0005] The synthesis of benzotriazole ultraviolet absorbers can adopt two synthesis routes:

[0006] The purpose of the application can be achieved by the following technical solutions:

[0007] A preparation method of a natural straw modified PLA composite material, comprising the following steps:

[0008] In the first step, 3-amino-5-(tert-butyl)-4-hydroxybenzoic acid is subjected to a diazotization reaction with nitrite, and the obtained product is subjected to a coupling reaction with 4-aminopyridine to obtain an ultraviolet absorber precursor;

[0009] The preparation process is as follows: 4-amino pyridine, water is mixed in a reaction vessel under ice water bath condition, open stirring after adding dropwise 10% mass fraction sodium hydroxide aqueous solution to adjust the pH of the system to 8-9 to obtain coupling liquid for standby, under ice water bath condition, 3-amino-5-(tert-butyl)-4-hydroxybenzoic acid and 20-30% mass fraction concentrated hydrochloric acid are mixed in another reaction vessel, open stirring after adding dropwise 20-30% mass fraction nitrite solution to the system, the dropwise adding time is 2h, the system temperature is controlled to-5-0℃ during dropwise adding, after dropwise adding, continue to react for 10-20min, then add urea to the system, filter out the filter residue, and drop the obtained filtrate into the coupling liquid, the dropwise adding time is 2h, continue stirring during dropwise adding and control the system temperature to-5-0℃, after dropwise adding, continue to react for 10-20min, after the reaction is completed, the solid is separated by filtration, and the obtained solid is washed with deionized water and dried to obtain the ultraviolet absorber precursor.

[0010]

[0011] The second step is to oxidize and close the ring of the ultraviolet absorber precursor to obtain the ultraviolet absorber.

[0012] The preparation process is as follows: the ultraviolet absorber precursor, anhydrous copper sulfate, and glacial acetic acid are mixed in a reaction vessel, stirring is started, and then the temperature is raised to 100-120℃, and the reaction is carried out for 2-3h, during which oxygen is continuously introduced, after the reaction is completed, the reaction liquid is poured into deionized water for precipitation, the solid is separated by filtration, and then the obtained solid is recrystallized to obtain the ultraviolet absorber.

[0013]

[0014] The third step is to carry out quaternary ammonium salt reaction of the ultraviolet absorber with brominated aliphatic hydrocarbon to obtain the antibacterial ultraviolet absorber.

[0015] The preparation process is as follows: the ultraviolet absorber, brominated aliphatic hydrocarbon, and acetonitrile are mixed in a reaction vessel, stirring is started, and then the temperature of the system is raised to 40-60℃, and the reaction is carried out at 40-60℃ for 12-24h, then the reaction liquid is poured into petroleum ether for precipitation, the solid is separated by filtration, and then the obtained solid is washed with deionized water and dried to obtain the antibacterial ultraviolet absorber.

[0016]

[0017] The fourth step is to wash, crush, and sieve the natural straw, and then carry out alkali treatment, after the alkali treatment is completed, the solid is separated by filtration and washed with deionized water and dried to obtain the alkali-treated straw.

[0018] The alkali treatment process is as follows: the sieved natural straw is added to a sodium hydroxide aqueous solution with a mass fraction of 4-8%, and the mixture is stirred and reacted at a temperature of 60-80℃ for 2-4 hours.

[0019] Step 5: Acylation modification of alkalized straw using an antibacterial ultraviolet absorber to obtain modified straw:

[0020] The preparation process is as follows: 2-morpholinoethanesulfonic acid is dissolved in deionized water. After stirring, 10% sodium hydroxide aqueous solution is added dropwise to adjust the pH of the system to 5.5-6.5. Then, an antibacterial ultraviolet absorber, 4-dimethylaminopyridine, 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride, dimethyl sulfoxide, and alkalized straw are added to the system. The reaction is then carried out at a temperature of 40-50℃ for 12-18 hours. After the reaction is completed, the solid is separated by filtration and the obtained solid is washed with anhydrous ethanol and deionized water in sequence and then dried to obtain modified straw.

[0021] Step 6: Mix the modified straw, PLA masterbatch, and antioxidant, and then melt-extrude and granulate the mixture using a twin-screw extruder to obtain a natural straw modified PLA composite material.

[0022] Furthermore, the nitrite is sodium nitrite.

[0023] Furthermore, the bromoaliphatic hydrocarbon is one of lauryl bromide and myristyl bromide.

[0024] Furthermore, the natural straw is one of wheat straw, rice straw, or corn straw.

[0025] Furthermore, the antioxidant is one or more of antioxidant 1010, antioxidant 1035, and antioxidant 168.

[0026] Furthermore, the mass ratio of modified straw, PLA masterbatch, and antioxidant used in the preparation of natural straw modified PLA composite material is 100:8-12:0.2-0.5, and the extrusion temperature during the preparation process is 170-180℃.

[0027] A natural straw-modified PLA composite material, which is prepared by any of the above steps.

[0028] An application of a natural straw-modified PLA composite material, specifically its application in the field of packaging materials.

[0029] The beneficial effects of this invention are:

[0030] 1) This invention prepares an antibacterial ultraviolet absorber and grafts the ultraviolet absorber onto the surface of natural straw, endowing the natural straw filler with antibacterial function and ultraviolet protection capability. This invention integrates the antibacterial agent and the ultraviolet absorber at the molecular level. It can not only regulate the substituents through the nitrogen atom of the pyridine ring, so that the coupling reaction in the first step can stably occur at the ortho position of the amino group, but also use the strong electron-withdrawing effect of the pyridine quaternary ammonium salt to reduce the LUMO orbital energy level of the pyridotriazole structure, so that the absorption spectrum of the ultraviolet absorber will redshift and broaden the absorption spectrum of the ultraviolet absorber.

[0031] 2) This invention grafts ultraviolet absorbers onto the surface of natural straw, providing special protection for the ultraviolet aging defect sites in the composite material caused by the introduction of natural straw. This greatly improves the ultraviolet aging resistance of the composite material and effectively overcomes the problem of decreased weather resistance of the composite material caused by the introduction of porous straw.

[0032] 3) This invention grafts an antibacterial agent onto the surface of natural straw. The hydrophilicity of the natural straw allows it to adsorb water molecules from the environment, promoting the ionization of pyridyl quaternary ammonium salts and continuously releasing positively charged antibacterial cations. This endows the composite material with efficient and long-lasting contact antibacterial properties. Moreover, the long aliphatic chains in the antibacterial agent can also undergo sufficient physical entanglement and diffusion with the hydrophobic PLA molecular chains, greatly improving the compatibility between the straw and PLA phases and enhancing the overall performance of the composite material. Detailed Implementation

[0033] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0034] It should be understood that the expression “one or more of…” individually includes each of the objects described after the expression, as well as various different combinations of two or more of the described objects, unless otherwise understood from the context and usage. The expression “and / or” combined with three or more described objects should be understood to have the same meaning, unless otherwise understood from the context.

[0035] It should be understood that the use of “including,” “having,” or “containing,” including its grammatical synonyms, should generally be understood as open-ended and non-restrictive, for example, not excluding other unstated elements or steps, unless otherwise specifically stated or understood from the context.

[0036] It should be understood that the order of the steps or the order in which certain actions are performed is not important as long as the invention remains operational. Furthermore, two or more steps or actions can be performed simultaneously.

[0037] Furthermore, the numerical ranges and parameters used to define the present invention are approximate values, and the relevant values ​​in the specific embodiments have been presented as precisely as possible. However, any value inevitably contains standard deviations due to individual test methods. Therefore, unless explicitly stated otherwise, it should be understood that all ranges, quantities, values, and percentages used in this disclosure are modified with the word "approximately". Here, "approximately" generally means that the actual value is within plus or minus 10%, 5%, 1%, or 0.5% of a specific value or range.

[0038] Example 1

[0039] A method for preparing a natural straw-modified PLA composite material includes the following steps:

[0040] Step 1: Under ice-water bath conditions, 2.8 parts by mass of 4-aminopyridine and 200 parts by mass of water were mixed in a reaction vessel. After stirring, 10% sodium hydroxide aqueous solution was added dropwise to adjust the pH of the system to 8 to obtain a coupling solution for later use. Under ice-water bath conditions, 6 parts by mass of 3-amino-5-(tert-butyl)-4-hydroxybenzoic acid and 42 parts by mass of 30% concentrated hydrochloric acid were mixed in another reaction vessel. After stirring, 10 parts by mass of 20% sodium nitrite solution were added dropwise to the system over 2 hours, with the system temperature controlled at -5°C during the addition. After the addition was completed, the reaction was continued for 10 minutes. Then, 0.5 parts by mass of urea were added to the system, the residue was removed by filtration, and the resulting filtrate was added dropwise to the coupling solution over 2 hours, with continuous stirring and the system temperature controlled at -5°C during the addition. After the addition was completed, the reaction was continued at this temperature for 10 minutes. After the reaction was completed, the solid was separated by filtration, and the resulting solid was washed with deionized water and dried to obtain the ultraviolet absorber precursor.

[0041] Step 2: According to the mass fraction, mix 6 parts of the ultraviolet absorber precursor, 0.6 parts of anhydrous copper sulfate, and 60 parts of glacial acetic acid in a reaction vessel. After stirring, react for 3 hours at 100°C, during which oxygen is continuously introduced. After the reaction is completed, pour the reaction solution into deionized water to precipitate, filter to separate the solid, and then recrystallize the obtained solid to obtain the ultraviolet absorber.

[0042] Step 3: According to the mass fraction, mix 3 parts of ultraviolet absorber, 2.9 parts of lauryl bromide, and 40 parts of acetonitrile in a reaction vessel. After stirring, raise the system temperature to 40°C and react at 40°C for 24 hours. Then, pour the reaction solution into petroleum ether to precipitate. After filtering to separate the solid, wash the obtained solid with deionized water and dry it to obtain the antibacterial ultraviolet absorber.

[0043] Step 4: Wash, crush, and sieve the wheat straw according to the mass fraction. Add 20 parts of the sieved wheat straw to 120 parts of 8% sodium hydroxide aqueous solution. After stirring, react at 60℃ for 4 hours. Then filter to separate the solid and wash the obtained solid with deionized water and dry to obtain alkalized straw.

[0044] Step 5: Dissolve 6 parts of 2-morpholine ethanesulfonic acid in 294 parts of deionized water by mass. After stirring, add 10% sodium hydroxide aqueous solution to adjust the pH of the system to 5.5. Then add 3 parts of antibacterial ultraviolet absorber, 0.3 parts of 4-dimethylaminopyridine, 1.5 parts of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride, 60 parts of dimethyl sulfoxide, and 15 parts of alkalized straw to the system. Then react at 40℃ for 18 hours. After the reaction is completed, filter to separate the solid and wash the obtained solid with anhydrous ethanol and deionized water in sequence and then dry to obtain modified straw.

[0045] Step 6: By weight, mix 8 parts modified straw, 100 parts PLA masterbatch, and 0.2 parts antioxidant 1010, and then melt-extrude and granulate the mixture using a twin-screw extruder at a temperature of 170℃ to obtain natural straw modified PLA composite material.

[0046] A natural straw-modified PLA composite material, which is prepared by the above steps.

[0047] An application of a natural straw-modified PLA composite material, specifically its application in the field of packaging materials.

[0048] Example 2

[0049] A method for preparing a natural straw-modified PLA composite material includes the following steps:

[0050] Step 1: Under ice-water bath conditions, mix 3.2 parts by weight of 4-aminopyridine and 200 parts by weight of water in a reaction vessel. After stirring, add 10% sodium hydroxide aqueous solution to adjust the pH of the system to 8.5 to obtain the coupling solution for later use. Under ice-water bath conditions, mix 7 parts by weight of 3-amino-5-(tert-butyl)-4-hydroxybenzoic acid and 63 parts by weight of 25% concentrated hydrochloric acid in another reaction vessel. After stirring, add 9.4 parts by weight of 25% sodium nitrite to the system. The solution was added dropwise over 2 hours, with the system temperature controlled at -2.5°C during the dropwise addition. After the dropwise addition was complete, the reaction continued for 15 minutes. Then, 0.75 parts of urea were added to the system, the filter was used to remove the residue, and the resulting filtrate was added dropwise to the coupling solution over 2 hours, with continuous stirring and the system temperature controlled at -2.5°C during the dropwise addition. After the dropwise addition was complete, the reaction was continued at this temperature for 15 minutes. After the reaction was completed, the solid was separated by filtration, and the resulting solid was washed with deionized water and dried to obtain the ultraviolet absorber precursor.

[0051] Step 2: According to the mass fraction, mix 7 parts of the ultraviolet absorber precursor, 0.7 parts of anhydrous copper sulfate, and 70 parts of glacial acetic acid in a reaction vessel. After stirring, react for 2.5 hours at a temperature of 110°C, during which oxygen is continuously introduced. After the reaction is completed, pour the reaction solution into deionized water to precipitate, filter to separate the solid, and then recrystallize the obtained solid to obtain the ultraviolet absorber.

[0052] Step 3: Mix 3.5 parts of ultraviolet absorber, 3.5 parts of lauryl bromide, and 50 parts of acetonitrile in a reaction vessel by mass. After stirring, raise the temperature of the system to 50°C and react at 50°C for 18 hours. Then, pour the reaction solution into petroleum ether to precipitate. After filtering to separate the solid, wash the obtained solid with deionized water and dry it to obtain the antibacterial ultraviolet absorber.

[0053] Step 4: According to the mass fraction, wash, crush and sieve the rice straw, and add 20 parts of the sieved rice straw to 160 parts of 6% sodium hydroxide aqueous solution. After stirring, react at 70℃ for 3 hours. Then filter to separate the solid and wash the obtained solid with deionized water and dry to obtain alkalized straw.

[0054] Step 5: Dissolve 6 parts of 2-morpholine ethanesulfonic acid in 294 parts of deionized water by mass. After stirring, add 10% sodium hydroxide aqueous solution to adjust the pH of the system to 6. Then add 3.5 parts of antibacterial ultraviolet absorber, 0.35 parts of 4-dimethylaminopyridine, 1.75 parts of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride, 70 parts of dimethyl sulfoxide, and 15 parts of alkalized straw to the system. Then react at 45℃ for 15 hours. After the reaction is completed, filter to separate the solid and wash the obtained solid with anhydrous ethanol and deionized water in sequence and then dry to obtain modified straw.

[0055] Step 6: By weight, mix 10 parts modified straw, 100 parts PLA masterbatch, 0.3 parts antioxidant 1035, and 0.1 parts antioxidant 168, and then melt-extrude and granulate the mixture using a twin-screw extruder at a temperature of 175℃ to obtain natural straw modified PLA composite material.

[0056] A natural straw-modified PLA composite material, which is prepared by the above steps.

[0057] An application of a natural straw-modified PLA composite material, specifically its application in the field of packaging materials.

[0058] Example 3

[0059] A method for preparing a natural straw-modified PLA composite material includes the following steps:

[0060] Step 1: Under ice-water bath conditions, 3.6 parts by mass of 4-aminopyridine and 200 parts by mass of water were mixed in a reaction vessel. After stirring, 10% sodium hydroxide aqueous solution was added dropwise to adjust the pH of the system to 9 to obtain a coupling solution for later use. Under ice-water bath conditions, 8 parts by mass of 3-amino-5-(tert-butyl)-4-hydroxybenzoic acid and 80 parts by mass of 20% concentrated hydrochloric acid were mixed in another reaction vessel. After stirring, 9 parts by mass of 30% sodium nitrite solution were added dropwise to the system over 2 hours, with the system temperature controlled at 0°C during the addition. After the addition was completed, the reaction was continued for 20 minutes. Then, 1 part by mass of urea was added to the system, the residue was removed by filtration, and the resulting filtrate was added dropwise to the coupling solution over 2 hours, with continuous stirring and the system temperature controlled at 0°C during the addition. After the addition was completed, the reaction was continued for 20 minutes. After the reaction was completed, the solid was separated by filtration, and the obtained solid was washed with deionized water and dried to obtain the ultraviolet absorber precursor.

[0061] Step 2: According to the mass fraction, mix 8 parts of the ultraviolet absorber precursor, 0.8 parts of anhydrous copper sulfate, and 80 parts of glacial acetic acid in a reaction vessel. After stirring, react for 2 hours at a temperature of 120°C, during which oxygen is continuously introduced. After the reaction is completed, pour the reaction solution into deionized water to precipitate, filter to separate the solid, and then recrystallize the obtained solid to obtain the ultraviolet absorber.

[0062] Step 3: According to the mass fraction, mix 4 parts of ultraviolet absorber, 4.3 parts of myristyl bromide, and 60 parts of acetonitrile in a reaction vessel. After stirring, raise the system temperature to 60°C and react at 60°C for 12 hours. Then, pour the reaction solution into petroleum ether to precipitate. After filtering to separate the solid, wash the obtained solid with deionized water and dry it to obtain the antibacterial ultraviolet absorber.

[0063] Step 4: Wash, crush, and sieve the corn stalks according to the mass fraction. Add 20 parts of the sieved corn stalks to 200 parts of a 4% sodium hydroxide aqueous solution. After stirring, react at 80℃ for 2 hours. Then filter to separate the solids and wash the obtained solids with deionized water and dry them to obtain alkalized straw.

[0064] Step 5: Dissolve 6 parts of 2-morpholine ethanesulfonic acid in 294 parts of deionized water by mass. After stirring, add 10% sodium hydroxide aqueous solution to adjust the pH of the system to 6.5. Then add 4 parts of antibacterial ultraviolet absorber, 0.4 parts of 4-dimethylaminopyridine, 2 parts of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride, 80 parts of dimethyl sulfoxide, and 15 parts of alkalized straw to the system. Then react at 50℃ for 12 hours. After the reaction is completed, filter to separate the solid and wash the obtained solid with anhydrous ethanol and deionized water in sequence and then dry to obtain modified straw.

[0065] Step 6: By weight, mix 12 parts modified straw, 100 parts PLA masterbatch, 0.35 parts antioxidant 1010, and 0.15 parts antioxidant 168, and then melt-extrude and granulate the mixture using a twin-screw extruder at a temperature of 180℃ to obtain natural straw modified PLA composite material.

[0066] A natural straw-modified PLA composite material, which is prepared by the above steps.

[0067] An application of a natural straw-modified PLA composite material, specifically its application in the field of packaging materials.

[0068] Comparative Example 1

[0069] The difference between this comparative example and Example 1 is that, instead of preparing an antibacterial ultraviolet absorber, a commercially available ultraviolet absorber, UV-327, was used to replace the antibacterial ultraviolet absorber, and the mixture was melt-extruded and blended.

[0070] A method for preparing a natural straw-modified PLA composite material includes the following steps:

[0071] By weight, 8 parts modified straw, 100 parts PLA masterbatch, 0.2 parts antioxidant 1010, and 3 parts ultraviolet absorber UV-327 were mixed and melt-extruded and granulated using a twin-screw extruder at a temperature of 170℃ to obtain natural straw modified PLA composite material.

[0072] A natural straw-modified PLA composite material, which is prepared by the above steps.

[0073] An application of a natural straw-modified PLA composite material, specifically its application in the field of packaging materials.

[0074] Comparative Example 2

[0075] The difference between this comparative example and Example 1 is that, instead of preparing an antibacterial ultraviolet absorber, a commercially available antibacterial agent, hexadecyltrimethylammonium bromide, is used to replace the antibacterial ultraviolet absorber, and the mixture is blended by melt extrusion.

[0076] A method for preparing a natural straw-modified PLA composite material includes the following steps:

[0077] By mass percentage, 8 parts modified straw, 100 parts PLA masterbatch, 0.2 parts antioxidant 1010, and 3 parts hexadecyltrimethylammonium bromide were mixed and melt-extruded and granulated using a twin-screw extruder at a temperature of 170°C to obtain natural straw modified PLA composite material.

[0078] A natural straw-modified PLA composite material, which is prepared by the above steps.

[0079] An application of a natural straw-modified PLA composite material, specifically its application in the field of packaging materials.

[0080] Experimental Example 1

[0081] The performance of the natural straw-modified PLA composites in Examples 1-3 and Comparative Examples 1-2 was tested. The tensile strength of each component sample in Examples 1-3 and Comparative Examples 1-2 was tested according to the national standard GB / T 1040.2-2022 "Determination of Tensile Properties of Plastics". The tensile strength of each component sample in Examples 1-3 and Comparative Example 1 was subjected to a 500-hour ultraviolet accelerated aging test according to GB / T 16422.3-2022 "Laboratory Light Source Exposure Test Method for Plastics". The tensile strength of the aged samples was then tested, and the tensile strength retention rate was calculated as the tensile strength of the aged sample divided by the initial tensile strength. The antibacterial properties of each component sample in Examples 1-3 and Comparative Example 2 against Escherichia coli and Staphylococcus aureus were tested according to the national standard GB / T 31402-2015 "Test Method for Antibacterial Properties of Plastic Surfaces". The test results are shown in Table 1.

[0082] Table 1

[0083] Item Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Tensile strength / MPa 54.62 56.42 58.74 48.21 44.36 Tensile strength retention rate / % 85.3 86.5 85.6 81.2 \ Bacteriostatic rate of E. coli / % 99.3 99.9 99.9 \ 96.7 Bacteriostatic rate of S. aureus / % 99.9 99.9 99.9 \ 98.4

[0084] This article uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention, including the best mode, and also to enable any person skilled in the art to practice the present invention, including manufacturing and using any device or system, and implementing any combined method. It should be noted that for those skilled in the art, several improvements and modifications can be made to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

Claims

1. A method for preparing a natural straw-modified PLA composite material, characterized in that, Includes the following steps: The first step involves reacting 3-amino-5-(tert-butyl)-4-hydroxybenzoic acid with nitrite in a diazotization reaction, and then coupling the product with 4-aminopyridine to obtain a precursor for the ultraviolet absorber. The second step is to oxidize the UV absorber precursor to obtain the UV absorber; The third step involves reacting the ultraviolet absorber with a bromoaliphatic hydrocarbon to obtain an antibacterial ultraviolet absorber; Step 4: After washing, crushing and sieving the natural straw, it is subjected to alkali treatment. After the alkali treatment is completed, the solid is filtered to separate the solid and the obtained solid is washed with deionized water and dried to obtain alkalized straw. Step 5: Acylation modification of alkalized straw using an antibacterial ultraviolet absorber to obtain modified straw: Step 6: The modified straw, PLA masterbatch, and antioxidant are blended, extruded, and granulated to obtain natural straw modified PLA composite material.

2. The method for preparing a natural straw-modified PLA composite material according to claim 1, characterized in that, The nitrite is sodium nitrite, the brominated aliphatic hydrocarbon is one of lauryl bromide and myristyl bromide, the natural straw is one of wheat straw, rice straw and corn straw, and the antioxidant is one or more of antioxidant 1010, antioxidant 1035 and antioxidant 168.

3. The method for preparing a natural straw-modified PLA composite material according to claim 1, characterized in that, The first step of the preparation process is as follows: Under ice-water bath conditions, 4-aminopyridine and water were mixed in a reaction vessel, and the pH of the system was adjusted to 8-9 to obtain a coupling solution for later use. Under ice-water bath conditions, 3-amino-5-(tert-butyl)-4-hydroxybenzoic acid and 20-30% concentrated hydrochloric acid were mixed in another reaction vessel. After stirring, 20-30% nitrite solution was added dropwise to the system. During the addition, the system temperature was controlled at -5 to 0°C. After the addition was completed, the reaction was continued for 10-20 minutes. Then, urea was added to the system, the residue was removed by filtration, and the resulting filtrate was added dropwise to the coupling solution. During the addition, the system temperature was continuously stirred and controlled at -5 to 0°C. After the addition was completed, the reaction was continued at this temperature for 10-20 minutes to obtain a precursor for ultraviolet absorbers.

4. The method for preparing a natural straw-modified PLA composite material according to claim 1, characterized in that, The second step of the preparation process is as follows: The UV absorber precursor, anhydrous copper sulfate, and glacial acetic acid are mixed in a reaction vessel. After stirring, the mixture is reacted at 100–120°C for 2–3 hours to obtain the UV absorber, with oxygen continuously introduced during the reaction.

5. The method for preparing a natural straw-modified PLA composite material according to claim 1, characterized in that, The third step of the preparation process is as follows: An antibacterial ultraviolet absorber, a brominated aliphatic hydrocarbon, and acetonitrile are mixed in a reaction vessel. After stirring, the system temperature is raised to 40–60°C and reacted at 40–60°C for 12–24 hours to obtain the antibacterial ultraviolet absorber.

6. The method for preparing a natural straw-modified PLA composite material according to claim 1, characterized in that, The fourth step, alkaline treatment, is as follows: The sieved natural straw is added to a sodium hydroxide aqueous solution with a mass fraction of 4-8%, and the mixture is stirred and reacted at a temperature of 60-80℃ for 2-4 hours.

7. The method for preparing a natural straw-modified PLA composite material according to claim 1, characterized in that, The preparation process in step five is as follows: 2-morpholinoethanesulfonic acid was dissolved in deionized water, and the pH of the system was adjusted to 5.5-6.5 after stirring. Then, an antibacterial ultraviolet absorber, 4-dimethylaminopyridine, 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride, dimethyl sulfoxide, and alkalized straw were added to the system. The mixture was then reacted at 40-50℃ for 12-18 hours to obtain modified straw.

8. The method for preparing a natural straw-modified PLA composite material according to claim 1, characterized in that, The mass ratio of modified straw, PLA masterbatch, and antioxidant used in the preparation of natural straw modified PLA composite material is 100:8-12:0.2-0.5, and the extrusion temperature during the preparation process is 170-180℃.

9. The natural straw-modified PLA composite material obtained by the preparation method according to claim 1.

10. The application of the natural straw modified PLA composite material according to claim 9 in the field of packaging materials.

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