Polylactic acid plasticizer and polylactic acid film preparation process

Abstract

The invention discloses a polylactic acid plasticizer and a polylactic acid film preparation process, and belongs to the field of biomass conversion materials. The bio-based plasticizer is prepared from cardanol and (Z, Z)-9, 12-octadecane dienoic acid dimer as raw materials through the steps of chlorohydrination, epoxy group reconstruction, etherification and the like, an acidic molecular sieve and KOH / ZSM-5 are preferably selected as catalysts, the feed ratio and reaction conditions are optimized, and the yield is high; in the preparation of the polylactic acid film, the compatibility of the plasticizer and polylactic acid is remarkably improved through oxygen plasma modification of the plasticizer, premixing to form a swelling layer, ultrasonic dispersion and regulation and control of the bio-based compatilizer, the tensile strength, elongation at break, water absorption and degradation rate of the obtained film are all at a higher level, and the film has excellent mechanical properties, water resistance and biodegradability; the invention is suitable for packaging and medical fields.

Description

TECHNICAL FIELD

[0001] The application relates to a polylactic acid plasticizer and a polylactic acid film preparation process and belongs to the field of biomass conversion materials. BACKGROUND

[0002] As a biodegradable polymer material, polylactic acid (PLA) has good mechanical properties and biocompatibility and has wide application prospects in the fields of packaging and medical treatment. However, the strong rigidity and high crystallinity of the PLA molecular chain result in large brittleness, low elongation at break and poor processability of the PLA, which limits the practical application of the PLA. In order to improve the flexibility and processability of the PLA, a plasticizer needs to be added to reduce the intermolecular force and improve the chain segment movement ability.

[0003] Most of the existing PLA plasticizers are petroleum-based compounds such as phthalate esters, which have the problems of poor biocompatibility and non-degradability and are contrary to the environment-friendly characteristics of the PLA. Although the biobased plasticizers such as citric acid esters and glycerol derivatives are degradable, the compatibility with the PLA is insufficient, and the plasticizers are prone to migration, which leads to the decrease of the mechanical properties of the material with time. Meanwhile, the low efficiency of the catalyst and the many side reactions in the preparation process of the traditional plasticizers further limit the application of the plasticizers.

[0004] In addition, in the preparation process of the PLA film, the poor interfacial compatibility and uneven dispersion of the plasticizer and the matrix result in the fluctuation of the mechanical properties of the film and the decrease of the water resistance. Therefore, the development of a biobased plasticizer with high compatibility and high efficiency and a matching high-performance PLA film preparation process has become the key to solving the above problems. SUMMARY

[0005] The application aims to overcome the problems of the insufficient biobased source, poor compatibility and low preparation efficiency of the existing PLA plasticizers and the difficulty in balancing the mechanical properties and degradability of the PLA film and provides a cashew phenol-dimer acid-based composite plasticizer and a preparation method thereof and a preparation process of a high-performance polylactic acid film.

[0006] The application aims to overcome the problems of the insufficient biobased source, poor compatibility and low preparation efficiency of the existing PLA plasticizers and the difficulty in balancing the mechanical properties and degradability of the PLA film and provides a cashew phenol-dimer acid-based composite plasticizer and a preparation method thereof and a preparation process of a high-performance polylactic acid film. The preparation of the plasticizer comprises the following steps: S1: reacting (Z,Z)-9,12-octadecadienoic acid dimer and epichlorohydrin under the catalysis of a solid acid catalyst to generate a chlorohydrin intermediate; S2: After cooling, NaOH solution is added to the reaction system to perform elimination-cyclization process to reconstruct the epoxy group, and an intermediate crude product of the compound of formula II is prepared by reaction; S3: The intermediate crude product of the compound of formula II is filtered to remove salt, and after adjusting the reaction system to neutral, water and unreacted epichlorohydrin in the system are removed to obtain the compound of formula II; S4: The compound of formula II is subjected to etherification reaction with cardanol under the catalysis of a solid base catalyst, and after removing trace catalyst and byproducts, a cardanol-dimer acid-based composite plasticizer, i.e., the compound of formula I, is prepared.

[0007] As preferred, the solid acid catalyst is at least one of acidic molecular sieve, AlCl3 / SiO2, FeCl3 / montmorillonite, strongly acidic cation exchange resin, and hydrochloric acid modified rectorite, and the feeding ratio of the solid acid catalyst to the (Z,Z)-9,12-octadecadiene acid dimer is 0.04:1.

[0008] Further, the solid acid catalyst is selected from the group consisting of acidic molecular sieve, AlCl3 / SiO2, FeCl3 / montmorillonite, strongly acidic cation exchange resin, and hydrochloric acid modified rectorite, and the product yield of the solid acid catalyst is higher than that of AlCl3 / SiO2, FeCl3 / montmorillonite, strongly acidic cation exchange resin, and hydrochloric acid modified rectorite.

[0009] Further, the acid sites of the acidic molecular sieve (such as zeolite molecular sieve) are mainly medium-strength Brønsted acid and Lewis acid, the acid strength is moderate, which can effectively reduce the ring-opening energy barrier of the epoxy group, promote the reaction, avoid excessive ring-opening of epichlorohydrin, self-polymerization or decomposition of the dimer acid caused by excessive acid strength, and generate non-target byproducts to reduce the yield of the intermediate, and the uniform microporous and mesoporous structure of the molecular sieve can limit the excessive aggregation of the macromolecular dimer acid, enrich the small molecular epichlorohydrin in the pore channel, increase the local contact concentration of the two, and accelerate the target reaction.

[0010] As preferred, the solid base catalyst is at least one of KOH / ZSM-5, magnesium oxide, calcium oxide, and NaOH solution modified saponite, and the feeding ratio of the solid base catalyst to the (Z,Z)-9,12-octadecadiene acid dimer is 0.04:1.

[0011] Further, the solid base catalyst is selected from the group consisting of KOH / ZSM-5, magnesium oxide, calcium oxide, and NaOH solution modified saponite, and the product yield of the solid base catalyst is higher than that of magnesium oxide, calcium oxide, and NaOH solution modified saponite.

[0012] Further, ZSM-5 molecular sieve has a uniform microporous structure and a large specific surface area. The regular channel has a "shape-selective catalysis" effect on the reaction of cardanol and the compound of formula II in the S4 step. The channel of ZSM-5 can limit the disordered aggregation of the two, so that they contact the active sites in a specific orientation, improve the intermolecular effective collision efficiency, and the generated cardanol-dimer acid-based plasticizer can smoothly diffuse out of the channel, avoiding further side reactions such as ring-opening polymerization of the epoxy group and oxidation of the phenolic hydroxyl group in the channel. In contrast, magnesium oxide and calcium oxide have a blocky or granular amorphous structure, no channel restriction, free diffusion of reactants and products, and easy occurrence of side reactions such as self-polymerization of cardanol, excessive ring-opening of the epoxy group of the compound of formula II, and the like, resulting in a reduced yield.

[0013] As preferred, in step S1, the feeding ratio of the (Z,Z)-9,12-octadecadiene acid dimer to the epichlorohydrin is 1:4-12, the temperature is increased to 110-115°C after mixing, and the temperature is decreased to 90°C after adding the solid acid catalyst.

[0014] Further, when the feeding ratio of the (Z,Z)-9,12-octadecadiene acid dimer to the epichlorohydrin is 1:8, the product yield is better.

[0015] Further, the dimer is a long-chain fatty acid dimer with high viscosity, and the epichlorohydrin has the functions of a reactant and a solvent. When the feeding ratio is 1:8, the excess epichlorohydrin can effectively reduce the viscosity of the system, so that the dimer and the solid acid catalyst are more uniformly dispersed, the contact efficiency of the reactants and the active sites of the catalyst is improved, the mass transfer and the reaction are promoted, and increasing the amount of epichlorohydrin can promote the reaction to proceed in the forward direction, thereby improving the conversion rate of the dimer.

[0016] Further, when the feeding ratio exceeds 1:8, the epichlorohydrin is excessive, the epichlorohydrin may be polymerized by itself or further react with the product to generate non-target by-products, consume raw materials, and reduce the purity of the target intermediate; at the same time, the unreacted epichlorohydrin needs to be removed in step S3, and too much excess epichlorohydrin will increase the energy consumption of distillation, and the residual epichlorohydrin may interfere with the reaction in the subsequent elimination-cyclization step, resulting in a reduced final yield.

[0017] As preferred, in step S2, the amount of the NaOH solution added is 2.4-4 ep, and the NaOH solution is selected to be a 30% mass fraction solution, and the temperature during the addition is 45-55°C.

[0018] Further, in the process, the role of NaOH is to provide an alkaline environment, neutralize the HCl generated by the reaction, and promote the elimination-cyclization as a nucleophile. When the addition amount of NaOH solution is 2.8 eq, it can ensure that the elimination-cyclization reaction proceeds sufficiently, and avoid side reactions caused by excessive amount, forming an optimal balance between reaction efficiency and product stability, and obtaining a better yield.

[0019] Further, the use of 2.8 eq of NaOH solution can ensure that both active sites of the chlorohydrin intermediate are fully reacted, avoiding the residual raw materials due to insufficient base, thereby improving the conversion rate. If the amount of NaOH is less than 2.8 eq, the insufficient alkalinity will lead to incomplete elimination reaction, and part of the chlorohydrin intermediate will not be converted to epoxy group, and the residual chlorohydrin structure will be washed away or other side reactions will occur in subsequent processing, directly reducing the yield. If the alkalinity of the system is too strong, it will cause the non-selective ring opening of the generated compound of formula II, generating diol byproducts, destroying the target structure, and excessive NaOH may react with unreacted epichlorohydrin in the system, causing hydrolysis or polymerization, consuming raw materials and generating impurities, increasing the difficulty of subsequent separation, and reducing the final yield.

[0020] As a preferred, in step S4, the reaction temperature is 110-125℃.

[0021] A polylactic acid film preparation process, the main components of the polylactic acid film are composed of polylactic acid and cashew phenol-dimer acid-based composite plasticizer, and the auxiliary materials are composed of bio-based compatibilizer, antioxidant 1010, multifunctional crosslinking agent and organic tin catalyst; The preparation of the polylactic acid film comprises the following steps: T1: 45℃, the polylactic acid raw material is placed in a DZF-200 vacuum drying oven for vacuum drying for 24h; T2: The bio-based plasticizer is treated by oxygen plasma, the power is 200W, and the time is 5-10min, and the surface of the bio-based plasticizer is introduced with polar groups such as hydroxyl and carboxyl groups; T3: The bio-based plasticizer and PLA particles are pre-mixed at 70-90℃, the bio-based plasticizer is penetrated into the interior of the PLA particles by using the fluidity of the bio-based plasticizer to form a "swelling layer", the temperature is increased to 120-130℃, the bio-based compatibilizer is added, the rotation speed is set to 300-500rpm, and the bio-based compatibilizer is mixed for 5-10min, so that the bio-based compatibilizer is evenly coated on the surface of the bio-based plasticizer and the PLA particles, and then the temperature is increased to 145-150℃, the rotation speed is increased to 800-1000rpm, the multifunctional crosslinking agent and the organic tin catalyst, antioxidant 1010 are added, and the melt blending is mixed for 15-20min; T4: Ultrasonic vibration is introduced during melt blending, the power is 350-500W, and the cavitation effect of ultrasonic is used to break the agglomerates of bio-based plasticizer and refine the size of dispersed phase; T5: The compounded material is transported to a pressure molding machine for hot-pressing molding at a temperature of 100-150°C for 5 min. T6: The sheet is naturally cooled to room temperature, and the sample is cut for testing.

[0022] Further, the plasticizer penetrates into the interior of the PLA particles to form a "swelling layer", expanding the interface of the compatibilizer.

[0023] Further, the acetylated lignin or starch-g-poly(lactic acid) is uniformly coated on the surface of the PLA / plasticizer particles through high-speed shearing (300-500 rpm) to form a stable interface layer.

[0024] Further, 350-500 W ultrasonic vibration is introduced in the melt blending stage to break the plasticizer agglomerates through cavitation effect, and to promote the stretching and adsorption of the molecular chains of the compatibilizer, further refining the size of the dispersed phase.

[0025] Further, the ester groups or grafted PLA segments of the compatibilizer can undergo secondary reaction with a crosslinking agent (such as triethyl citrate) to form a "compatibilization-crosslinking" synergistic network, which not only enhances the mechanical properties, but also avoids the increase in brittleness caused by excessive crosslinking As a preferred embodiment, the bio-based compatibilizer is at least one of lignin derivatives and starch graft copolymer, wherein the lignin derivatives are acetylated lignin, and the starch graft copolymer is starch-g-poly(lactic acid).

[0026] Further, the bio-based compatibilizer enhances the interface bonding between PLA and plasticizer through chemical bridging or physical entanglement, while avoiding the introduction of non-degradable petroleum-based components.

[0027] Further, acetylated lignin is a natural phenolic polymer, and the ester groups of acetylated lignin are similar to the ester bond structure of the PLA backbone, which forms a chemical bond with the PLA molecular chain through ester exchange reaction, enhancing the interface bonding, and the benzene ring structure of lignin can be used as a physical crosslinking point to improve the elastic modulus and tensile strength of the material.

[0028] Further, starch is a hydrophilic polysaccharide, and through grafting of PLA segments (starch-g-poly(lactic acid)), the hydrophilic hydroxyl groups are converted into hydrophobic PLA chains, realizing the modification of the amphiphilic interface, the grafted PLA segments form physical entanglement with the PLA matrix, enhancing the interface bonding, and the starch particles are uniformly dispersed in the PLA matrix through the "bridging effect" of the PLA segments, inhibiting phase separation As a preferred embodiment, the multi-functional crosslinking agent is at least one of diisocyanate and triethyl citrate, and the organotin catalyst is dibutyltin dilaurate.

[0029] Further, when triethyl citrate is selected as the multifunctional crosslinking agent, the mechanical properties are better, but the water absorption and degradation rate are better.

[0030] Further, triethyl citrate, as a bio-based ester compound, has better compatibility with cardanol-dimer acid-based plasticizer and acetylated lignin, can be combined with the terminal hydroxyl and carboxyl groups of the PLA molecular chain through ester exchange reaction, can be uniformly dispersed in the system to form a crosslinking network with moderate density, can enhance the force between molecular chains and reduce molecular slip when stressed, thereby improving the tensile strength and elastic modulus, at the same time, the flexible alkyl chain can act as a molecular buffer to allow the molecular chain to stretch moderately when stressed, thereby avoiding rigid rupture, and therefore the elongation at break is higher; the ester bond crosslinking network formed by the ester bond has stronger polarity and moderate crosslinking density, and water molecules are more easily permeable, resulting in slightly higher water absorption; the ester bond formed is consistent with the main chain structure of PLA, and the crosslinking is more stable, so the degradation rate is relatively slow.

[0031] Further, the isocyanate group of diisocyanate has very high reactivity and is easily reacted with trace amounts of water or hydroxyl groups in the system to form a locally dense crosslinking zone, resulting in stress concentration and reducing the overall mechanical properties; diisocyanate forms urethane bonds by reacting with the hydroxyl groups of PLA through -NCO, and the molecular structure contains hydrophobic methylene and aromatic ring structures, and the crosslinking density is higher, the hydrophobic structure reduces the polar interaction with water molecules, and the high-density crosslinking network can physically block the penetration channels of water molecules, so the water absorption is lower; the urethane bond formed is more easily broken than the ester bond formed by triethyl citrate under the action of esterase or hydrolase secreted by microorganisms, the -NH- group in the urethane bond is easily recognized and attacked by enzymes, resulting in faster collapse of the crosslinking network, and at the same time, the rigid microzones formed by locally dense crosslinking are more easily attached by microorganisms at the initial stage of degradation, accelerating the decomposition of the overall material, so the degradation rate is higher at 360 hours.

[0032] As a preferred, the addition amount of the bio-based plasticizer is 15-25% of the addition amount of the PLA particles.

[0033] In summary, compared with the prior art, the present application has the following advantages: 1. In the scheme of the present application, the bio-based plasticizer has excellent performance, the raw materials are natural phenolic cardanol and fatty acid dimers, which are 100% biobased and can be completely degraded to meet environmental protection requirements; at the same time, the molecular structure contains ester groups and ether bonds, which are similar to the ester bond structure of PLA and have high compatibility, and the polar groups are introduced through plasma treatment to significantly improve the interfacial bonding force with PLA.

[0034] 2、The PLA film prepared in the scheme has excellent comprehensive performance, the synergistic effect of the plasticizer and the compatibilizer is outstanding, the mechanical property is far superior to that of pure PLA and traditional plasticizing systems; the performance can be adjusted by selecting a crosslinking agent, triethyl citrate improves the mechanical property, diisocyanate optimizes water resistance and degradability, and different application scenarios can be adapted; the steps of pre-mixing and swelling, ultrasonic dispersion and the like ensure uniform dispersion of the plasticizer and avoid phase separation, the process is efficient and stable, and the stability is high in batch production.

[0035] 3、The scheme in the application uses bio-based raw materials and recyclable catalysts in the whole process, no toxic and harmful substances are discharged, the degradation rate of the film is relatively high within 360 hours, and the whole life cycle of preparation, use and degradation is environmentally friendly. BRIEF DESCRIPTION OF DRAWINGS

[0036] Figure 1 The synthesis route of the compound of formula I in the application is as follows: Figure 2 The preparation flowchart of the polylactic acid film in the application is as follows. DETAILED DESCRIPTION

[0037] The technical scheme of the application will be further specifically described below through specific examples, but the application is not limited to these examples.

[0038] Example 1 (Z,Z)-9,12-octadecadienoic acid dimer (50.0 g, 0.089 mol) and epichlorohydrin (65.9 g, 0.71 mol, molar ratio 1:8) were put into a four-necked flask, stirred and slowly heated to 115°C (heating rate 5°C / min), 2 g of zeolite molecular sieve was added as a catalyst, the temperature was kept for 30 min to make the catalyst fully dispersed, then the temperature was lowered to 90°C, and the reaction was continued for 1.5 h, the acid value change was monitored every 15 min by sampling, and a chlorohydrin intermediate was generated.

[0039] The temperature of the reaction system was lowered to 52°C, and 24.7 g (2.8 eq, 0.18 mol) of NaOH solution (30% w / w) was slowly added through a constant pressure dropping funnel, the dropping speed was controlled at 1-2 drops / s, a water bath was used to keep the temperature in the range of 50-55°C during the dropping process, and after the dropping was completed, the reaction was continued for 4 h to obtain an intermediate crude product of the compound of formula II.

[0040] After the reaction was completed, the reaction mixture was cooled to room temperature, the reaction mixture was filtered, the filter cake was washed with 100 mL of toluene for three times, the organic phases were combined, washed with 10% dilute hydrochloric acid solution until pH=6-7, and then washed with deionized water until neutral; the organic phase was transferred to a distillation flask, excess epichlorohydrin was recovered by normal pressure distillation, and residual water and low boiling substances were removed by reduced pressure distillation (120°C / 2 mmHg) to obtain a light yellow compound of formula II (the yield is about 92%).

[0041] Into a four-necked flask, 27 g of cardanol and 30.0 g of the compound of formula II prepared above were added, 3.2 g of KOH / ZSM-5 catalyst (loading 10 wt%) was added, and the temperature was slowly raised to 120°C (under nitrogen protection). The reaction was carried out at this temperature for 5 h, and samples were taken every 1 h to monitor the formation of ether bonds by infrared spectroscopy. After the reaction was completed, the temperature was cooled to 60°C, 50 mL of toluene was added for dilution, the solid catalyst was removed by filtration, the filtrate was washed with deionized water three times in turn, and then washed with saturated brine once. The organic phase was dried with anhydrous sodium sulfate for 2 h, filtered, and then toluene was removed by distillation under reduced pressure. Finally, the product was degassed at 130°C / 0.5 mmHg for 30 min to obtain a light brown bio-based plasticizer (the yield was about 88%).

[0042] Example 2 Material preparation: PLA 100 g, plasticizer 20 g, acetylated lignin 5 g, triethyl citrate 1 g, dibutyltin dilaurate 0.1 g, antioxidant 1010 0.2 g Preparation steps: The polylactic acid particles were placed in a DZF-200 vacuum drying oven and dried at 45°C under a vacuum degree of ≤0.095 MPa for 24 h to reduce the moisture content to ≤0.02%. The cardanol-dimer acid-based composite plasticizer was placed in a plasma treatment device, oxygen (purity ≥99.9%) was introduced, the power was set to 200 W, and the treatment time was 5-10 min to introduce polar groups such as hydroxyl (-OH) and carboxyl (-COOH) on the surface of the plasticizer.

[0043] The dried PLA particles and the surface-modified bio-based plasticizer were added to a high-speed mixer and mixed at 70-90°C and a rotation speed of 200-300 rpm for 10 min to allow the plasticizer to penetrate into the interior of the PLA particles and form a "swelling layer". The temperature was raised to 120-130°C, acetylated lignin was added, and the rotation speed was increased to 300-500 rpm for mixing for 5-10 min to allow the compatibilizer to uniformly coat the surface of the PLA / plasticizer particles. The temperature was further raised to 145-150°C, the rotation speed was increased to 800-1000 rpm, and triethyl citrate, dibutyltin dilaurate, and antioxidant 1010 were added in turn, and the melt blending was carried out for 15-20 min.

[0044] During the melt blending process, an ultrasonic vibration device was used to introduce ultrasonic waves with a power of 350-500 W, a frequency of 20-40 kHz, and an action time of 10-15 min to break the plasticizer agglomerates and refine the size of the dispersed phase by ultrasonic cavitation effect.

[0045] The mixed material was transported to a pressure molding machine, and hot-pressed in a mold at 100-150°C and a pressure of 10-15 MPa for 5 minutes to form a film with uniform thickness (mold parameters: length x width = 150 mm x 150 mm, thickness 0.1-0.5 mm). The hot-pressed sheet was naturally cooled to room temperature (23±2°C), and a sample with a size of 100 mm x 100 mm was cut by a sample cutting machine and placed in a dryer for standby.

[0046] Example 3 The prepared polylactic acid film samples were subjected to tensile test (referring to GB / T 1040.2-2022), water resistance test, and degradation test, respectively.

[0047] The mechanical property test data of the polylactic acid film obtained in Example 2 were measured as follows: tensile strength (MPa) = 45.84; elongation at break (%) = 457.9; elastic modulus (MPa) = 1411.4.

[0048] The water resistance test data of the polylactic acid film obtained in Example 2 were measured as follows: 24-hour water absorption rate (%) = 0.324.

[0049] The degradation performance test data of the polylactic acid film obtained in Example 2 were measured as follows: 360-hour degradation rate (%) = 21.5.

[0050] Example 4 This example is a comparative example of Example 1 According to the procedure of Example 1, the solid acid catalyst was replaced by AlCl3 / SiO2, FeCl3 / montmorillonite, strongly acidic cation exchange resin, and hydrochloric acid modified rectorite, respectively, and other conditions were unchanged. The yield was counted, and the product yield was all < 88%, among which the yield using strongly acidic cation exchange resin was the lowest ≤ 62%.

[0051] Example 5 This example is a comparative example of Example 1 According to the procedure of Example 1, the solid base catalyst was replaced by magnesium oxide, calcium oxide, and NaOH solution modified soapstone, and other conditions were unchanged. The yield was counted, and the product yield was all < 88%, among which the yield using calcium oxide was the lowest ≤ 71.5%.

[0052] Example 6 This example is a comparative example of Example 1 According to the procedure of Example 1, the feeding ratio of (Z, Z)-9, 12-octadecadienoic acid dimer to epichlorohydrin was adjusted to 1:4, 1:5, 1:6, 1:7, 1:9, 1:10, 1:11, 1:12, respectively, and other conditions were unchanged. When the feeding ratio was 1:9, 1:10, 1:11, the yield of the product was ≥88%. When the feeding ratio was 1:4, 1:5, 1:6, 1:7, 1:12, the yield of the product was < 88%. Among them, the yield was the lowest ≤ 66.5% when the feeding ratio was 1:4.

[0053] Example 7 This example is a comparative example of Example 1 According to the procedure of Example 1, the amount of NaOH solution added was adjusted to 2.4 eq, 2.5 eq, 3 eq, 3.5 eq, 3.8 eq, 4 eq, respectively, and other conditions were unchanged. When the amount of NaOH solution added was 2.4 eq, 2.5 eq, the yield of the product was ≤ 80%. When the amount of NaOH solution added was > 3 eq, the yield of the product was 85% ~ 90%. Among them, after the reaction was completed when the amount of NaOH solution added was > 3 eq, a large amount of NaOH remained. More 10% dilute hydrochloric acid solution needed to be added in the neutralization step, and the difficulty of post-processing increased, while the production cost did not increase in proportion to the yield.

[0054] Example 8 This example is a comparative example of Example 2 According to the procedure of Example 2, the multi-functional crosslinking agent was replaced by diisocyanate, and other conditions were unchanged. The prepared polylactic acid film samples were subjected to tensile test (referring to GB / T 1040.2-2022), water resistance test, and degradation test, respectively.

[0055] The mechanical property test data of the obtained polylactic acid film were measured as follows: tensile strength (MPa) = 42.13; elongation at break (%) = 380.1; elastic modulus (MPa) = 1127.0.

[0056] The water resistance test data of the obtained polylactic acid film were measured as follows: 24-hour water absorption rate (%) = 0.215.

[0057] The degradation performance test data of the obtained polylactic acid film were measured as follows: 360-hour degradation rate (%) = 27.

[0058] Example 9 This example is a comparative example of Example 2 According to the procedure of Example 2, the amount of bio-based plasticizer added was adjusted to 15 g, 17.5 g, 22.5 g, 25 g, respectively, and other conditions were unchanged. When the amount of the bio-based plasticizer is less than 18%, the tensile strength (MPa) of the obtained polylactic acid film is less than or equal to 35, the elongation at break (%) is less than or equal to 330, the elastic modulus (MPa) is less than or equal to 1000, and the degradation rate in 360 hours (%) is less than or equal to 12. When the amount of the bio-based plasticizer is more than 20%, the tensile strength (MPa) of the obtained polylactic acid film is less than or equal to 38.1, the elongation at break (%) is less than or equal to 427, the elastic modulus (MPa) is less than or equal to 1000, the degradation rate in 360 hours (%) is less than or equal to 26.4, and the water absorption rate in 24 hours (%) is less than or equal to 0.397. It is concluded that when the amount of the bio-based plasticizer is reduced, the mechanical properties of the obtained polylactic acid film are reduced, and the degradation rate is also reduced; when the amount of the bio-based plasticizer is excessive, the tensile strength and the elongation at break of the obtained polylactic acid film are better, but the elastic modulus is reduced, and the degradation rate in 360 hours and the water absorption rate in 24 hours are greatly improved.

[0059] The embodiments of the present application are not limited to the above-described examples, and various changes and improvements can be made to the present application in form and details by those skilled in the art without departing from the spirit and scope of the present application, and these are considered to fall within the protection scope of the present application.

Claims

1. A polylactic acid plasticizer and a polylactic acid film preparation process, characterized by comprising the following steps: The bio-based plasticizer is a cashew phenol-dimer acid-based composite plasticizer, and the main raw material is composed of cashew phenol and (Z,Z)-9,12-octadecadiene acid dimer, and the auxiliary material is composed of epichlorohydrin, solid acid catalyst, NaOH and solid base catalyst; ​ The preparation of the plasticizer comprises the following steps: S1: (Z,Z)-9,12-octadecadiene acid dimer is reacted with epichlorohydrin under the catalysis of a solid acid catalyst to generate a chlorohydrin intermediate; S2: After cooling, NaOH solution is added to the reaction system to perform an elimination-cyclization process to reconstruct the epoxy group, and an intermediate crude product of the compound of formula II is prepared by reaction; S3: The intermediate crude product of the compound of formula II is filtered to remove salt, the reaction system is adjusted to be neutral, and then water and unreacted epichlorohydrin in the system are removed to obtain the compound of formula II; S4: The compound of formula II is subjected to etherification reaction with cashew phenol under the catalysis of a solid base catalyst, and after removing trace catalyst and by-products, the cashew phenol-dimer acid-based composite plasticizer, i.e., the compound of formula I, is prepared.

2. The polylactic acid plasticizer according to claim 1, characterized by: The solid acid catalyst is at least one of acidic molecular sieve, AlCl3 / SiO2, FeCl3 / montmorillonite, strong acid cation exchange resin and hydrochloric acid modified rectorite, and the feeding ratio of the solid acid catalyst to the (Z,Z)-9,12-octadecadiene acid dimer is 0.04:

1.

3. The polylactic acid plasticizer of claim 1, wherein: The solid base catalyst is at least one of KOH / ZSM-5, magnesium oxide, calcium oxide and NaOH solution modified saponite, and the feeding ratio of the solid base catalyst to the (Z,Z)-9,12-octadecadiene acid dimer is 0.04:

1.

4. The polylactic acid plasticizer of claim 1, wherein: In step S1, the feeding ratio of the (Z,Z)-9,12-octadecadiene acid dimer to the epichlorohydrin is 1:4-12, the temperature is increased to 110-115℃ after mixing, and the temperature is decreased to 90℃ after adding the solid acid catalyst.

5. The polylactic acid plasticizer of claim 1, wherein: In step S2, the addition amount of the NaOH solution is 2.4-4 ep, and the NaOH solution is a 30% mass fraction solution, and the temperature is 45-55℃ during the addition.

6. The polylactic acid plasticizer of claim 1, wherein: In step S4, the reaction temperature is 110-125℃.

7. A process for the production of a polylactic acid film, characterized by: The main component of the polylactic acid film is composed of polylactic acid and cashew phenol-dimer acid-based composite plasticizer, and the auxiliary material is composed of bio-based compatibilizer, antioxidant 1010, multifunctional crosslinking agent and organic tin catalyst; The preparation of the polylactic acid film comprises the following steps: T1: The polylactic acid raw material is placed in a DZF-200 vacuum drying box and vacuum dried for 24 h at 45℃; T2: The bio-based plasticizer is subjected to oxygen plasma treatment at a power of 200 W for 5-10 min to introduce polar groups such as hydroxyl and carboxyl groups on the surface of the bio-based plasticizer; T3: Pre-mixing the bio-based plasticizer with PLA particles at 70~90℃, using the flowability of the bio-based plasticizer to penetrate into the interior of the PLA particles to form a "swelling layer", heating to 120~130℃, adding a bio-based compatibilizer, setting the rotation speed to 300~500rpm for 5~10min to make the bio-based compatibilizer evenly coated on the surface of the bio-based plasticizer and PLA particles, then heating to 145~150℃, increasing the rotation speed to 800~1000rpm, adding a multi-functional crosslinking agent and an organic tin catalyst, antioxidant 1010, melt blending for 15~20min; T4: Introducing ultrasonic vibration during melt blending, power 350~500W, using the cavitation effect of ultrasound to break the agglomerates of the bio-based plasticizer and refine the size of the dispersed phase; T5: Transporting the mixed material to a pressure forming machine for hot press forming, temperature 100℃~150℃, time 5min; T6: Naturally cooling the sheet to room temperature, cutting samples for testing.

8. The process for preparing a polylactic acid film according to claim 7, wherein: The bio-based compatibilizer is at least one of a lignin derivative and a starch graft copolymer, wherein the lignin derivative is acetylated lignin and the starch graft copolymer is starch-g-poly(lactic acid).

9. The process for preparing a polylactic acid film according to claim 7, wherein: The multi-functional crosslinking agent is at least one of diisocyanate and triethyl citrate, and the organic tin catalyst is dibutyl tin dilaurate.

10. The process for preparing a polylactic acid film according to claim 7, wherein: The addition amount of the bio-based plasticizer is 15~25% of the addition amount of the PLA particles.

Images