Modified lignin reinforced and toughened polylactic acid composite transparent film and preparation method thereof

Abstract

The invention provides a modified lignin reinforced and toughened polylactic acid composite transparent film and a preparation method thereof.The preparation method comprises the steps that firstly, lignosulfonate and maleic anhydride are subjected to graft modification under the condition that an initiator exists, then zinc salt is added for continuous reaction, after the time is up, a reaction product is precipitated through salting-out, and then the modified lignin reinforced and toughened polylactic acid composite transparent film is obtained after separation, washing and drying; the maleic anhydride modified lignosulfonic acid metal salt is obtained; and mixing the maleic anhydride modified lignosulfonic acid metal salt with polylactic acid according to a certain ratio, and carrying out melt blending and film forming to prepare the composite transparent film. According to the preparation method, maleic anhydride and zinc salt are used for synergistically modifying lignosulfonate, the modified maleic anhydride modified lignosulfonate metal salt is mixed with polylactic acid, the composite transparent film with high light transmittance and high mechanical performance is prepared, and the whole preparation process is environmentally friendly and has excellent industrial conversion prospects.

Description

Technical Field

[0001] The present invention belongs to the technical field of polylactic acid-based thin film materials, and relates to a modified lignin-reinforced and toughened polylactic acid composite transparent thin film and a preparation method thereof. Specifically, it relates to the synergistic modification of lignosulfonate by maleic anhydride and zinc salt, and the use of the modified lignosulfonate to reinforce and toughen the polylactic acid composite transparent thin film. Background Art

[0002] Plastic films have good transparency, moisture resistance, high mechanical strength, dimensional stability, light weight, etc., and are widely used in food, medical, building materials, plastic chemical industry, etc. However, most plastic films are disposable products, which are discarded immediately after use. They have low value, are difficult to recycle and degrade after being discarded, causing continuous pollution to the atmosphere, soil, ocean, etc., and becoming the main sources of "white pollution" and marine microplastics. Polylactic acid (PLA) is a polymer material formed by fermenting lactic acid from starch-containing agricultural and sideline products such as corn, potatoes, sugarcane, and beets, and then polymerizing it. It is usually prepared by ring-opening polymerization of lactide. It is the first industrialized biodegradable polymer material, which can be completely degraded into water and carbon dioxide under specific conditions, and is an ideal substitute for non-degradable petroleum-based plastics.

[0003] PLA has good biocompatibility, renewability, thermoplasticity, mechanical properties, etc., and has a wide range of applications in biomedicine, industry and agriculture, food packaging, textile industry. However, the poor toughness and high brittleness of PLA greatly limit its application in areas with high mechanical property requirements. In the prior art, the toughening modification methods of PLA usually include flexible molecular chain graft copolymerization and physical blending modification. For example, Chen et al. prepared imidazolium-based polyionic liquid-b-PLA copolymer (ILA) to improve the brittleness of PLA through comprehensive methods such as continuous monomer feeding copolymerization, quaternization reaction, ion exchange, and interpolymer blending (International Journal of Biological Macromolecules, 2022, 221: 1512-1526). However, this method has problems such as complex process and high cost. Chinese invention patent CN115746011B provides an isosorbide isooctanoate bio-based plasticizer and its preparation method and application. A large amount of organic solvents are used in the synthesis process of this method, and the addition amount is relatively large, which is likely to cause environmental problems and plasticizer migration problems. Zabidi et al. added polybutylene succinate (PBS) to the PLA system to improve the toughness of the composite material (International Journal of Biological Macromolecules, 2023, 251: 126212), but on the other hand, it sacrifices the strength and transparency of the composite material.

[0004] Therefore, there is an urgent need for a toughened polylactic acid composite material with a simpler preparation process, lower cost, and more environmental friendliness to achieve high-demand and high-value application expansion and have good market prospects. Summary of the Invention

[0005] In order to solve the above problems in the prior art, the present invention provides a modified lignin-reinforced and toughened polylactic acid composite transparent film and a preparation method thereof. The preparation method uses maleic anhydride and zinc salt to synergistically modify lignosulfonate, and mixes the modified maleic anhydride-modified lignosulfonate metal salt with polylactic acid to prepare a composite transparent film with high light transmittance and high mechanical properties. The entire preparation process is environmentally friendly and has excellent industrial transformation prospects.

[0006] To achieve the above object, the present invention is realized by a technical solution composed of the following technical measures.

[0007] The present invention provides a preparation method for a modified lignin-reinforced and toughened polylactic acid composite transparent film, which mainly includes the following steps:

[0008] (1) By weight, mix the raw materials mainly including the following components as a mixture:

[0009] 100 parts of polylactic acid,

[0010] 0.1 - 2 parts of maleic anhydride-modified lignosulfonate metal salt,

[0011] The preparation method of the maleic anhydride-modified lignosulfonate metal salt is as follows: graft-modify lignosulfonate with maleic anhydride at 35 - 60 °C for at least 4 h under the condition of an initiator, then add zinc salt and continue to react for at least 2 h. After the time reaches, salt out and precipitate the reaction product, then separate, wash, and dry to obtain the maleic anhydride-modified lignosulfonate metal salt; wherein, the mass ratio of the lignosulfonate to maleic anhydride is 1:(0.6 - 1.4), and the addition amount of the zinc salt is 20 - 100 wt% of the mass percentage of the lignosulfonate.

[0012] (2) Melt-blend and form the mixture obtained in step (1) to prepare a composite transparent film.

[0013] In this article, the polylactic acid (PLA) in step (1) is selected as a conventional polylactic acid industrial raw material, and further preferably selected as the conventional polylactic acid raw material model in the polylactic acid-based transparent film products recorded in the prior art. Those skilled in the art can select a suitable polylactic acid grade according to specific needs and process requirements.

[0014] In one of the technical solutions, the PLA selected in step (1) includes any one or more of PLA110 / PLA290 from Changchun Institute of Applied Chemistry, NatureWorks 4032D / 4060D, Fengyuan FY802, Haizheng Bio REVODE110, and Total L175.

[0015] In this article, the lignosulfonate described in step (1) is a class of compounds formed by the reaction of lignin with sulfite, usually a by-product of sulfite pulping. Based on the requirement of low cost, commercially available raw materials such as sodium lignosulfonate, calcium lignosulfonate, magnesium lignosulfonate, potassium lignosulfonate, and ammonium lignosulfonate can be selected.

[0016] In this article, in step (1), the lignosulfonate is graft-modified with maleic anhydride at 35 - 60°C for at least 4 h in the presence of an initiator. The initiator selected is to promote the graft reaction between maleic anhydride and lignosulfonate. Therefore, those skilled in the art can select a suitable initiator according to chemical common sense, especially the initiators commonly used in the graft modification reaction of maleic anhydride in the prior art.

[0017] In one of the technical solutions, the initiator selected in step (1) includes any one of ammonium persulfate (APS), potassium persulfate (PPS), sodium persulfate (SPS), and ammonium persulfate hydrogen (AMBN); the addition amount of the initiator is 0.5 - 6 wt% of the mass of maleic anhydride.

[0018] In this article, in step (1), the lignosulfonate is graft-modified with maleic anhydride at 35 - 60°C for at least 4 h in the presence of an initiator. For the specific graft modification steps / conditions, those skilled in the art can adapt them according to the actual reaction scale and on-site conditions. It should be noted that the graft modification of maleic anhydride is a conventional modification method in the prior art, and those skilled in the art can directly know the specific graft modification steps / conditions according to the common knowledge in the art.

[0019] To better illustrate the present invention and provide a reference technical solution, in step (1), the graft modification of lignosulfonate with maleic anhydride under the condition of an initiator at 35-60 °C for at least 4 h is specifically carried out by dissolving lignosulfonate and maleic anhydride in deionized water respectively to obtain an aqueous lignosulfonate solution and an aqueous maleic anhydride solution with a mass concentration of 0.05-0.2 g / mL. An initiator is added to the aqueous lignosulfonate solution, and then the aqueous maleic anhydride solution is added dropwise at a dropping rate of 20-50 drops / min with stirring at 35-60 °C, and the stirring reaction continues for at least 4 h. It should be emphasized that although this technical solution is adopted in the following embodiments, this does not mean that this is the only / limiting process method for the graft modification of lignosulfonate with maleic anhydride. Those skilled in the art should fully realize that it can be adapted and adjusted according to the reaction scale and site conditions based on the common knowledge in the art.

[0020] In one technical solution, the zinc salt described in step (1) includes at least one of zinc chloride, zinc sulfate, zinc acetate, and zinc nitrate.

[0021] In one technical solution, in order to make the maleic anhydride-modified lignosulfonic acid metal salt disperse better in polylactic acid, the particle size of the maleic anhydride-modified lignosulfonic acid metal salt described in step (1) is preferably not higher than 100 μm. It can be obtained by routinely grinding and sieving the dried product.

[0022] In one technical solution, before the mixture described in step (2) undergoes melt blending, it can be optionally pre-treated by heat drying to remove moisture and ash in the mixture. Specifically, the mixture is dried at a temperature of 50-80 °C for 6-12 h.

[0023] In one technical solution, additives commonly used in the processing and molding of polylactic acid can also be added to the mixture described in step (1) to further expand the functions / assist the process of the product. For the specific selection of additives, those skilled in the art can refer to the existing technology or literature, such as processing additives / functional additives such as antioxidants, lubricants, flame retardants, anti-aging agents, heat stabilizers, plasticizers, and antibacterial agents. It should be noted that the mixture described in step (1) may or may not include the additives commonly used in the processing and molding of polylactic acid. In the following preferred technical solutions and specific embodiments, in order to minimize the influencing factors in the comparative experiments, no additives are added to the mixture, but this does not mean that appropriate additives cannot be added to the mixture. However, it should be noted that the selection and addition of the above additives should not affect the dispersion effect of the maleic anhydride-modified lignosulfonic acid metal salt in the polylactic acid matrix.

[0024] In this article, the mixture described in step (2) is processed through melt blending and film forming to obtain a composite film product. The melt blending is a conventional process for poly(lactic acid)-based composite materials using poly(lactic acid) as the matrix raw material, such as melt kneading and melt extrusion. The specific process steps / parameters can be directly referred to the existing technical records / existing process methods of poly(lactic acid) raw materials during melt blending.

[0025] In one technical solution, the mixture described in step (2) is successively processed through melt blending and film forming. The melt blending is carried out by melt kneading using a kneader. The specific process parameters are: temperature is 170 - 190 °C, time is 5 - 8 min, and rotation speed is 40 - 50 rpm.

[0026] In this article, the mixture described in step (2) is successively processed through melt blending and film forming to obtain a composite film product. The film forming is a conventional process for preparing poly(lactic acid) films, such as hot pressing film forming and blow molding film forming. The specific process steps / parameters can be directly referred to the existing technical records / existing process methods of poly(lactic acid) during film forming.

[0027] In one technical solution, the mixture described in step (2) is successively processed through melt blending and film forming. The film forming is carried out by hot pressing using a flat vulcanizing machine. The specific process parameters are: hot pressing pressure is 3 - 5 MPa, cold pressing pressure is 2 - 3 MPa, upper plate temperature is 170 - 200 °C, lower plate temperature is 170 - 200 °C. After preheating for 30 - 120 s, hot pressing is carried out for 3 - 5 min, and cold pressing is carried out for 1 - 3 min.

[0028] It should be noted that during the research on the relevant topics of the technical solution of the present invention, the inventor retrieved that lignin is a highly potential reinforcing filler. Lignin is an amorphous polymer formed by connecting three kinds of phenylpropane units through carbon-carbon bonds and ether bonds. Its molecular structure is rich in active functional groups such as benzene rings, aliphatic hydroxyl groups, and phenolic hydroxyl groups, and its structure can be modified by various means. Lignin mainly comes from the by-products of the pulp and paper industry and biorefining industry. It is the second largest biomass resource in the plant kingdom after cellulose in terms of reserves. Most of it is used as fuel, and only 5% of industrial lignin is used to manufacture high-value-added products such as additives, dispersants, and surfactants, which not only causes waste of resources but also leads to secondary pollution due to combustion. However, when lignin is used as a filler in polymers, due to its highly variable and complex molecular structure, it is easy to form aggregates in the polymer matrix, which reduces the toughness and transparency of the polymer to a certain extent.

[0029] Based on this, the inventors carried out chemical modification on lignin in order to solve the above problems. During the experimental exploration process, in order to endow the composite transparent film with good antibacterial properties at the same time, during the experiment of preparing maleic anhydride and zinc chloride modified lignosulfonate by hydrothermal method, on the one hand, it was unexpectedly found that the reaction product could be quickly crystallized and precipitated by weak base salting out, and on the other hand, it was pleasantly discovered that through the synergistic modification of maleic anhydride and zinc chloride, the light transmittance was improved without affecting the mechanical properties of the composite film. In the prior art, when maleic anhydride modified lignosulfonate is used to precipitate the reaction product, organic solvents are usually selected to precipitate the reaction product, or strong acids are selected to precipitate the reaction product, or other more complex methods are used to precipitate the product. There is no record in any existing literature that its precipitation preparation is based on the principle of salting out. Introducing maleic anhydride can improve the flexibility of the modified lignin molecular chain and reduce the color of lignin itself. Introducing zinc can improve the thermal stability of the modified lignin and play a synergistic antibacterial and antioxidant role. At the same time, combining the synergistic modification of maleic anhydride and zinc salt on lignosulfonate greatly improves the toughness and transparency of the polylactic acid-based film, and also endows the polylactic acid-based film with certain antibacterial and antioxidant properties.

[0030] Based on the above inventive points, in one of the technical solutions, the salting out to precipitate the reaction product in step (1) is specifically carried out by adding a weak base to salt out and precipitate the reaction product. For example, any one of sodium percarbonate, sodium bicarbonate, sodium sulfite and ammonia water, which are conventional weak bases for salting out, is added to neutralize and precipitate the reaction product.

[0031] In one of the preferred technical solutions, the salting out to precipitate the reaction product in step (1) is specifically carried out by adding a weak base to salt out and precipitate the reaction product, and the weak base is added until the pH of the reaction solution is 1.5 - 7; more preferably, the weak base is added until the pH of the reaction solution is 2.5 - 3.

[0032] In one of the preferred technical solutions, the salting out to precipitate the reaction product in step (1) is to configure the weak base into a saturated aqueous solution of the weak base, and the reaction product is neutralized and precipitated by adding the saturated aqueous solution of the weak base.

[0033] In this article, the mixing, separation, washing, and drying all follow the conventional principles in chemical engineering processes, and those skilled in the art can perform specific operations according to common knowledge.

[0034] The present invention has the following beneficial effects:

[0035] 1. The present invention provides a method for preparing a modified lignin-reinforced and toughened polylactic acid composite transparent film. This preparation method uses maleic anhydride and zinc salt to synergistically modify lignosulfonate, increase the carboxyl content of lignin, and introduce zinc ions at the terminal carboxyl group. In the modified maleic anhydride-modified lignosulfonic acid metal salt, the overall molecular structure bridges lignin and zinc ions with a fatty long chain. The zinc ions further bridge the fatty long chain and lignin, forming a larger network structure, increasing the molecular flexibility of the reaction product and reducing the color of lignin itself. It can significantly improve the toughness of polylactic acid and enhance the transparency of the composite transparent film.

[0036] 2. The maleic anhydride-modified lignosulfonic acid metal salt provided by the present invention has good antibacterial and antioxidant properties. By modifying the structure of lignosulfonate and introducing zinc ions into the macromolecular chain of maleic anhydride-modified lignin, the antibacterial and antioxidant properties of the polylactic acid composite transparent film are improved.

[0037] 3. The preparation process of the present invention is simple, with mild conditions, easy to operate, and can be mass-produced. The raw materials used are all environmentally degradable raw materials or low-toxic raw materials. The raw materials in the reaction process have less consumption, can be recycled and reused, and no toxic by-products are generated, belonging to an environmentally friendly method. Description of the Drawings

[0038] Figure 1 These are the physical pictures of sodium lignosulfonate, maleic anhydride-modified sodium lignosulfonate, and maleic anhydride-modified lignosulfonic acid metal salt, which are the raw materials or intermediate products used in Comparative Examples 2-3 and Example 1 of the present invention. Among them, SL is sodium lignosulfonate, ML is the intermediate product maleic anhydride-modified sodium lignosulfonate in Comparative Example 2, and M6LZn is the intermediate product maleic anhydride-modified lignosulfonic acid metal salt in Example 1.

[0039] Figure 2 This is the transparency comparison test chart of the comparative samples prepared in Comparative Examples 1-3 of the present invention and the sample prepared in Example 1.

[0040] Figure 3 This is the ultraviolet-visible spectrum comparison chart of the comparative samples prepared in Comparative Examples 1-3 of the present invention and the sample prepared in Example 1 in the wavelength range of 200-800 nm. Detailed Embodiments

[0041] To further understand the present invention, the preferred embodiments of the present invention will be described below in conjunction with examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, rather than limiting the claims of the invention. Those skilled in the art can draw on the content of this article and appropriately modify the process parameters to achieve. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art, and they are all considered to be included in the present invention. The methods and applications of the present invention have been described through preferred embodiments, and those related can obviously make changes or appropriate modifications and combinations to the methods and applications described herein without departing from the content, spirit and scope of the present invention to implement and apply the technology of the present invention. Although it is believed that those of ordinary skill in the art are fully aware of the following terms, the following definitions are still stated to help illustrate the subject matter disclosed by the present invention.

[0042] The present invention provides a method for preparing a modified lignin-reinforced and toughened polylactic acid composite transparent film, which mainly includes the following steps:

[0043] (1) By weight, mix the raw materials mainly including the following components as a mixture:

[0044] 100 parts of polylactic acid,

[0045] 0.1 to 2 parts of maleic anhydride-modified lignosulfonate metal salt,

[0046] The preparation method of the maleic anhydride-modified lignosulfonate metal salt is as follows: graft-modify lignosulfonate with maleic anhydride at 35 to 60 °C for at least 4 h under the condition of an initiator, and then add a zinc salt and continue to react for at least 2 h. After the time is up, salt out and precipitate the reaction product, then separate, wash and dry to obtain the maleic anhydride-modified lignosulfonate metal salt; wherein, the mass ratio of the lignosulfonate to maleic anhydride is 1:(0.6 to 1.4), and the addition amount of the zinc salt is 20 to 100 wt% of the mass percentage of the lignosulfonate;

[0047] (2) Melt-blend and form the mixture obtained in step (1) to prepare a composite transparent film.

[0048] In this article, the polylactic acid (PLA) described in step (1) is selected from conventional polylactic acid industrial raw materials, and is further preferably selected from the conventional polylactic acid raw material models in the polylactic acid-based transparent film products recorded in the prior art. Those skilled in the art can select a suitable polylactic acid grade according to specific needs and process requirements.

[0049] In one of the embodiments, the PLA selected in step (1) includes any one or more of PLA110 / PLA290 from Changchun Institute of Applied Chemistry, NatureWorks 4032D / 4060D, Fengyuan FY802, Haizheng Bio REVODE110, and Total L175.

[0050] In this text, the lignosulfonate described in step (1) is a class of compounds formed by the reaction of lignin with sulfite, usually a by-product of sulfite pulping. In one of the embodiments, based on the requirement of low cost, commercially available raw materials such as sodium lignosulfonate, calcium lignosulfonate, magnesium lignosulfonate, potassium lignosulfonate, and ammonium lignosulfonate can be selected.

[0051] In this text, in step (1), the lignosulfonate is graft-modified with maleic anhydride at 35 - 60 °C for at least 4 h in the presence of an initiator. The selected initiator promotes the graft reaction between maleic anhydride and lignosulfonate. Therefore, those skilled in the art can select a suitable initiator according to chemical common sense, especially the initiators commonly used in the graft modification reaction of maleic anhydride in the prior art.

[0052] In one of the embodiments, the initiator selected in step (1) includes any one of ammonium persulfate (APS), potassium persulfate (PPS), sodium persulfate (SPS), and ammonium persulfate hydrogen (AMBN); the addition amount of the initiator is 0.5 - 6 wt% of the mass of maleic anhydride.

[0053] In this text, in step (1), the lignosulfonate is graft-modified with maleic anhydride at 35 - 60 °C for at least 4 h in the presence of an initiator. For the specific graft modification steps / conditions, those skilled in the art can adapt them according to the actual reaction scale and on-site conditions. It should be noted that the graft modification of maleic anhydride is a conventional modification method in the prior art, and those skilled in the art can directly know the specific graft modification steps / conditions according to the common general knowledge in the art.

[0054] To better illustrate the present invention and provide a reference implementation method, in step (1), the graft modification of lignosulfonate with maleic anhydride under the condition of an initiator at 35-60 °C for at least 4 h is specifically to dissolve lignosulfonate and maleic anhydride in deionized water respectively to obtain an aqueous lignosulfonate solution and an aqueous maleic anhydride solution with a mass concentration of 0.05-0.2 g / mL. An initiator is added to the aqueous lignosulfonate solution, and then the aqueous maleic anhydride solution is added dropwise at a dropping rate of 20-50 drops / min with stirring at 35-60 °C, and the stirring reaction continues for at least 4 h. It should be emphasized that although this technical solution is adopted in the following examples, this does not mean that this is the only / limiting process method for the graft modification of lignosulfonate with maleic anhydride. Those skilled in the art should fully realize that it can be adapted and adjusted according to the reaction scale and on-site conditions based on the common general knowledge in the art.

[0055] In one implementation method, the zinc salt described in step (1) is selected from at least one of zinc chloride, zinc sulfate, zinc acetate, and zinc nitrate.

[0056] In one implementation method, in order to make the maleic anhydride-modified lignosulfonate metal salt disperse better in polylactic acid, the particle size of the maleic anhydride-modified lignosulfonate metal salt described in step (1) is preferably not higher than 100 μm. It can be obtained by routinely grinding and sieving the dried product.

[0057] In one implementation method, before the mixture described in step (2) undergoes melt blending, it can be optionally pre-treated by heat drying to remove moisture and ash in the mixture. Specifically, the mixture is dried at a temperature of 50-80 °C for 6-12 h.

[0058] In one implementation method, additives commonly used in the processing and molding of polylactic acid can also be added to the mixture described in step (1) to further expand the functions / assist the process of the product. For the specific selection of additives, those skilled in the art can refer to the prior art or existing literature, such as processing additives / functional additives such as antioxidants, lubricants, flame retardants, anti-aging agents, heat stabilizers, plasticizers, and antibacterial agents. It should be noted that the mixture described in step (1) may or may not include additives commonly used in the processing and molding of polylactic acid; in the following preferred technical solutions and specific implementation methods, in order to minimize the influencing factors in the comparative experiments, no additives are added to the mixture, but this does not mean that appropriate additives cannot be added to the mixture. However, it should be noted that the selection and addition of the above additives should not affect the dispersion effect of the maleic anhydride-modified lignosulfonate metal salt in the polylactic acid matrix.

[0059] In one embodiment, the amount of the maleic anhydride-modified lignosulfonate metal salt in step (1) is 0.1 to 2 parts, such as 0.1 part, 0.2 part, 0.3 part, 0.4 part, 0.5 part, 0.6 part, 0.7 part, 0.8 part, 0.9 part, 1.0 part, 1.1 parts, 1.2 parts, 1.3 parts, 1.4 parts, 1.5 parts, 1.6 parts, 1.7 parts, 1.8 parts, 1.9 parts, 2.0 parts, or any range or point value therebetween.

[0060] In one embodiment, the mass ratio of the lignosulfonate to maleic anhydride in step (1) is 1:(0.6 to 1.4), such as 1:0.6, 1:0.7, 1:0.8, 1:0.9, 1:1.0, 1:1.1, 1:1.2, 1:1.3, 1:1.4, or any range or point value therebetween.

[0061] In one embodiment, the addition amount of the zinc salt in step (1) is 20 to 100 wt% of the mass percentage of the lignosulfonate, such as 20 wt%, 25 wt%, 30 wt%, 35 wt%, 40 wt%, 45 wt%, 50 wt%, 55 wt%, 60 wt%, 65 wt%, 70 wt%, 75 wt%, 80 wt%, 85 wt%, 90 wt%, 95 wt%, 100 wt%, or any range or point value therebetween.

[0062] In this article, the mixture in step (2) is subjected to melt blending and film forming to prepare a composite film product, wherein the melt blending is a conventional process method of polylactic acid-based composite materials with polylactic acid as the matrix raw material, such as melt kneading, melt extrusion, etc. The specific process steps / process parameters can be directly referred to the existing technical records / existing process methods of polylactic acid raw materials during melt blending.

[0063] In one embodiment, the mixture in step (2) is successively subjected to melt blending and film forming, wherein the melt blending is carried out by melt kneading treatment using a kneader. The specific process parameters are: temperature is 170 to 190 °C, time is 5 to 8 min, and rotation speed is 40 to 50 rpm.

[0064] In this article, the mixture in step (2) is successively subjected to melt blending and film forming to prepare a composite film product, wherein the film forming is a conventional process method for preparing polylactic acid films, such as hot pressing film forming, blow molding film forming. The specific process steps / process parameters can be directly referred to the existing technical records / existing process methods of polylactic acid during film forming.

[0065] In one of the embodiments, the mixture in step (2) is successively subjected to melt blending and film forming, wherein the film forming is carried out by hot pressing with a flat vulcanizing machine. The specific process parameters are as follows: the hot pressing pressure is 3 to 5 MPa, the cold pressing pressure is 2 to 3 MPa, the temperature of the upper plate is 170 to 200 °C, the temperature of the lower plate is 170 to 200 °C. After preheating for 30 to 120 s, hot pressing is carried out for 3 to 5 min, and cold pressing is carried out for 1 to 3 min.

[0066] It should be noted that during the research on the relevant topics of the technical solution of the present invention, the inventor retrieved that lignin is a highly potential reinforcing filler. Lignin is an amorphous polymer formed by connecting three kinds of phenylpropane units through carbon-carbon bonds and ether bonds. Its molecular structure is rich in active functional groups such as benzene rings, aliphatic hydroxyl groups and phenolic hydroxyl groups, and its structure can be modified by various means. Lignin mainly comes from the by-products of the pulp and paper making and biorefining industries. It is the second largest biomass resource in the plant kingdom after cellulose in terms of reserves. Most of it is used as fuel, and only 5% of industrial lignin is used to manufacture high-value-added products such as additives, dispersants and surfactants, which not only causes waste of resources, but also causes secondary pollution due to combustion. However, when lignin is used as a filler in polymers, due to its highly variable and complex molecular structure, it is easy to form aggregates in the polymer matrix, which reduces the toughness and transparency of the polymer to a certain extent.

[0067] Based on this, the inventor carried out chemical modification on lignin in order to solve the above problems. During the experimental exploration process, in order to endow the composite transparent film with good antibacterial properties at the same time, in the experiment of preparing maleic anhydride and zinc chloride modified lignosulfonate by hydrothermal method, on the one hand, it was unexpectedly found that the reaction product could be quickly crystallized and precipitated by weak base salting out, and on the other hand, it was pleasantly discovered that through the synergistic modification of maleic anhydride and zinc chloride, the light transmittance of the composite film was improved without affecting its mechanical properties. In the prior art, when maleic anhydride modified lignosulfonate is used, organic solvents are usually selected to precipitate the reaction product or strong acids are used to precipitate the reaction product, or other more complex methods are used to precipitate the product. There is no record in any existing literature that its precipitation preparation is carried out based on the principle of salting out. Introducing maleic anhydride can improve the flexibility of the modified lignin molecular chain and reduce the color of lignin itself. Introducing zinc can improve the thermal stability of the modified lignin and play a synergistic antibacterial and antioxidant role. At the same time, combining the synergistic modification of maleic anhydride and zinc salt on lignosulfonate greatly improves the toughness and transparency of the polylactic acid-based film, and also endows the polylactic acid-based film with certain antibacterial and antioxidant properties.

[0068] Based on the above inventive points, in one embodiment, the reaction product is precipitated by salting out specifically by adding a weak base. For example, the reaction product is precipitated by neutralization by adding any one of sodium percarbonate, sodium bicarbonate, sodium sulfite, and ammonia water, which are conventional weak bases for salting out.

[0069] In one preferred embodiment, the reaction product is precipitated by salting out specifically by adding a weak base in step (1), and the weak base is added until the pH of the reaction solution is 1.5 to 7; more preferably, the weak base is added until the pH of the reaction solution is 2.5 to 3.

[0070] In one preferred embodiment, the reaction product is precipitated by salting out in step (1) by preparing the weak base into a saturated aqueous solution of the weak base and carrying out neutralization precipitation of the reaction product by adding the saturated aqueous solution of the weak base.

[0071] In this text, the mixing, separation, washing, and drying all follow the conventional principles in chemical engineering, and those skilled in the art can perform specific operations according to common general knowledge.

[0072] The present application will be further explained in detail below with reference to the examples. However, those skilled in the art should understand that these examples are provided only for the purpose of illustration and are not intended to limit the present application.

[0073] Examples

[0074] The embodiments of the present application will be described in detail below in conjunction with the examples. However, those skilled in the art will understand that the following examples are only used to illustrate the present application and should not be regarded as limiting the scope of the present application. For those not specified in the examples, they are carried out under conventional conditions or conditions recommended by the manufacturer. Those reagents or instruments not specified by the manufacturer can be obtained as conventional products through commercial purchase. The present application should not be construed as being limited to the specific examples described.

[0075] 1. Raw materials

[0076] Polylactic acid (PLA), grade PLA110, density 1.2 - 1.3 g / cm 3 , Changchun Institute of Applied Chemistry, Chinese Academy of Sciences;

[0077] Sodium lignosulfonate (SL), Hefei BASF Biotechnology Co., Ltd.;

[0078] Maleic anhydride (MA), Shanghai Titan Technology Co., Ltd.;

[0079] Zinc chloride (ZnCl2), Chengdu Kelong Chemical Co., Ltd.;

[0080] Ammonium persulfate, Chengdu Kelong Chemical Co., Ltd.;

[0081] Anhydrous sodium carbonate (Na2CO3), Chengdu Kelong Chemical Co., Ltd.

[0082] 2. Test methods

[0083] Mechanical properties: Tested according to standard ISO 527, using an Instron 5567 universal material testing machine under a load-bearing capacity of 1 kN, with a tensile rate of 50 mm / min. The average value of the test results of 5 samples is taken as the final result.

[0084] Ultraviolet-visible spectrum: The ultraviolet-visible absorption spectrum of the composite transparent film sample was tested using a UV-3600 ultraviolet-visible-near-infrared spectrophotometer.

[0085] 3. Preparation method

[0086] (1) By weight, mix the raw materials mainly including the following components as a mixture:

[0087] 100 parts of polylactic acid,

[0088] 0.1 - 2 parts of maleic anhydride-modified lignosulfonic acid metal salt,

[0089] The preparation method of the maleic anhydride-modified lignosulfonic acid metal salt is as follows: Dissolve sodium lignosulfonate and maleic anhydride in deionized water respectively to obtain an aqueous solution of sodium lignosulfonate with a mass concentration of 0.2 g / mL and an aqueous solution of maleic anhydride. Add the initiator ammonium persulfate to the aqueous solution of sodium lignosulfonate, and then add the aqueous solution of maleic anhydride dropwise at a dropping rate of 30 drops / min with stirring at 40°C. Continue stirring and reacting for 4 h, then add zinc chloride and continue stirring and reacting for 2 h. After the time is up, salt out and precipitate the reaction product with saturated sodium carbonate aqueous solution, then separate, wash, and dry to obtain the maleic anhydride-modified lignosulfonic acid metal salt;

[0090] Among them, the masses of the sodium lignosulfonate and maleic anhydride are 6 - 7 g respectively, the addition amount of the initiator ammonium persulfate is 0.24 - 0.28 g, and the addition amount of zinc chloride is 20 - 100 wt% of the mass percentage of sodium lignosulfonate;

[0091] The maleic anhydride-modified lignosulfonic acid metal salt is ground and sieved so that its particle size is not higher than 100 μm;

[0092] (2) The mixture obtained in step (1) is successively subjected to melt mixing and hot pressing into a film to prepare a composite transparent film with a thickness of about 0.1 mm;

[0093] Among them, melt mixing is carried out using a mixer, and the specific process parameters are: temperature is 180 °C, time is 5 min, and rotation speed is 50 rpm;

[0094] Among them, hot pressing and forming is carried out using a flat vulcanizing machine, and the specific process parameters are: hot pressing pressure is 5 MPa, cold pressing pressure is 3 MPa, upper plate temperature is 190 °C, lower plate temperature is 190 °C. After preheating for 60 s, hot pressing is carried out for 5 min and cold pressing is carried out for 2 min.

[0095] Example 1

[0096] Example 1 is carried out according to the steps of the above "3. Preparation method". Among them, the masses of sodium lignosulfonate and maleic anhydride are 6 g respectively, the addition amount of initiator ammonium persulfate is 0.24 g, the addition amount of zinc chloride is 2 g, and the maleic anhydride modified lignosulfonic acid metal salt in the mixture is 1 part. A composite transparent film is prepared as a sample, denoted as PLA / M6LZn.

[0097] After testing, the composite transparent film prepared in this example has a tensile strength of 72.41 MPa, an elongation at break of 27.53%, and a light transmittance of 73.60% under the condition of a light wave wavelength of 550 nm.

[0098] Example 2

[0099] Example 2 is carried out according to the steps of the above "3. Preparation method". Among them, the masses of sodium lignosulfonate and maleic anhydride are 6 g respectively, the addition amount of initiator ammonium persulfate is 0.24 g, the addition amount of zinc chloride is 2 g, and the maleic anhydride modified lignosulfonic acid metal salt in the mixture is 0.5 part. A composite transparent film is prepared as a sample.

[0100] After testing, the composite transparent film prepared in this example has a tensile strength of 68.39 MPa, an elongation at break of 45.08%, and a light transmittance of 76.45% under the condition of a light wave wavelength of 550 nm.

[0101] Example 3

[0102] Example 3 is carried out according to the steps of the above "3. Preparation method". Among them, the masses of sodium lignosulfonate and maleic anhydride are 6 g respectively, the addition amount of initiator ammonium persulfate is 0.24 g, the addition amount of zinc chloride is 3 g, and the maleic anhydride modified lignosulfonic acid metal salt in the mixture is 1 part. A composite transparent film is prepared as a sample.

[0103] After testing, the composite transparent film prepared in this example has a tensile strength of 71.04 MPa, an elongation at break of 29.54%, and a light transmittance of 73.86% under the condition of a light wave wavelength of 550 nm.

[0104] Example 4

[0105] Example 4 was carried out according to the steps of the above-mentioned "3. Preparation method", where the mass of sodium lignosulfonate was 6 g, the mass of maleic anhydride was 7 g, the addition amount of initiator ammonium persulfate was 0.28 g, the addition amount of zinc chloride was 2 g, and the maleic anhydride-modified lignosulfonic acid metal salt in the mixture was 1 part, and a composite transparent film was prepared as a sample.

[0106] After testing, for the composite transparent film prepared in this example, the tensile strength was 68.52 MPa, the elongation at break was 27.66%, and the light transmittance under the condition of a light wave wavelength of 550 nm was 74.05%.

[0107] Comparative Example 1

[0108] Comparative Example 1 was carried out by referring to the steps of the above-mentioned "3. Preparation method", but maleic anhydride-modified lignosulfonic acid metal salt was not added. That is, 100 parts of polylactic acid was directly used as the mixture and a transparent film was prepared as a comparative sample according to the method of step (2), denoted as PLA.

[0109] After testing, for the transparent film prepared in this comparative example, the tensile strength was 55.07 MPa, the elongation at break was 8.94%, and the light transmittance under the condition of a light wave wavelength of 550 nm was 82.58%.

[0110] Comparative Example 2

[0111] Comparative Example 2 was carried out by referring to the steps of the above-mentioned "3. Preparation method", but maleic anhydride-modified lignosulfonic acid metal salt was replaced with sodium lignosulfonate. That is, 100 parts of polylactic acid was mixed with 1 part of sodium lignosulfonate for preparation as the mixture, and then a composite transparent film was prepared as a comparative sample according to the method of step (2), denoted as PLA / SL.

[0112] After testing, for the transparent film prepared in this comparative example, the tensile strength was 51.38 MPa, the elongation at break was 5.65%, and the light transmittance under the condition of a light wave wavelength of 550 nm was 68.41%.

[0113] Comparative Example 3

[0114] Comparative Example 3 was carried out according to the method of Example 1, but zinc chloride was not added. That is, after continuously stirring and reacting for 4 h as described in step (1), the reaction product was directly neutralized with saturated sodium carbonate aqueous solution, precipitated, separated, washed, and dried to obtain maleic anhydride-modified sodium lignosulfonate. 100 parts of polylactic acid was mixed with 1 part of maleic anhydride-modified lignosulfonic acid for preparation as the mixture, and then a composite transparent film was prepared as a comparative sample according to the method of step (2), denoted as PLA / ML.

[0115] After testing, the transparent film prepared in this comparative example has a tensile strength of 66.49 MPa, an elongation at break of 14.8%, and a light transmittance of 70.74% under the condition of a light wave wavelength of 550 nm.

[0116] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications made without departing from the spirit and principle of the present invention shall be equivalent replacement methods and are all included in the protection scope of the present invention.

Claims

1. A method for preparing a modified lignin reinforced and toughened polylactic acid composite transparent film, characterized in that The main steps include: (1) The raw materials mainly including the following components are mixed and prepared as a mixed material by weight: 100 parts of polylactic acid, 0.1-2 parts of maleic anhydride modified lignin sulfonate metal salt, The preparation method of the maleic anhydride modified lignin sulfonate metal salt is as follows: lignin sulfonate is grafted with maleic anhydride at 35-60° C. in the presence of an initiator for at least 4 hours, and then zinc salt is added to continue the reaction for at least 2 hours. After the time is up, the reaction product is salted out and precipitated, and then separated, washed and dried to obtain the maleic anhydride modified lignin sulfonate metal salt; wherein the mass ratio of the lignin sulfonate to the maleic anhydride is 1:(0.6-1.4), and the amount of the zinc salt added is 20-100wt% of the mass percentage of the lignin sulfonate; (2) The mixed material obtained in step (1) is subjected to melt blending and film forming to prepare a composite transparent film.

2. The preparation method according to claim 1, characterized in that: The lignin sulfonate in step (1) includes any one of sodium lignin sulfonate, calcium lignin sulfonate, magnesium lignin sulfonate, potassium lignin sulfonate and ammonium lignin sulfonate.

3. The preparation method according to claim 1, characterized in that: The initiator in step (1) includes any one of ammonium persulfate, potassium persulfate, sodium persulfate, and ammonium hydrogen persulfate; the added amount of the initiator is 0.5 to 6 wt % of the mass of maleic anhydride.

4. The preparation method according to claim 1, characterized in that: In step (1), lignin sulfonate is grafted with maleic anhydride in the presence of an initiator at 35-60° C. for at least 4 hours. Specifically, lignin sulfonate and maleic anhydride are dissolved in deionized water to prepare lignin sulfonate aqueous solution and maleic anhydride aqueous solution with a mass concentration of 0.05-0.2 g / mL, respectively. An initiator is added to the lignin sulfonate aqueous solution, and then maleic anhydride aqueous solution is added at 35-60° C. with stirring at a drop rate of 20-50 drops / min, and the stirring reaction is continued for at least 4 hours.

5. The preparation method according to claim 1, characterized in that: The zinc salt in step (1) includes at least one of zinc chloride, zinc sulfate, zinc acetate and zinc nitrate.

6. The preparation method according to claim 1, characterized in that: The mixture in step (1) also includes additives commonly used in the processing and molding of polylactic acid.

7. The preparation method according to claim 1, characterized in that: The mixture in step (2) is sequentially subjected to melt blending and film forming, wherein the melt blending is carried out by using an internal mixer for melt mixing, and the specific process parameters are: temperature of 170-190° C., time of 5-8 min, and rotation speed of 40-50 rpm.

8. The preparation method according to claim 1, characterized in that: The mixed material in step (2) is sequentially melt blended and film-formed, wherein the film-forming is performed by hot pressing film-forming treatment using a flat vulcanizing machine, and the specific process parameters are: hot pressing pressure of 3-5 MPa, cold pressing pressure of 2-3 MPa, upper plate temperature of 170-200°C, lower plate temperature of 170-200°C, preheating for 30-120 seconds, hot pressing for 3-5 minutes, and cold pressing for 1-3 minutes.

9. The preparation method according to claim 1, characterized in that: The salting out of the reaction product in step (1) is specifically carried out by adding a weak base to salt out the reaction product, and the weak base is added until the pH of the reaction solution is 1.5-7.

10. The composite transparent film prepared by the method for preparing the modified lignin reinforced and toughened polylactic acid composite transparent film as claimed in claim 1.

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