Method for improving heat resistance of toughened polylactic acid

Abstract

The invention discloses a method for improving heat resistance of toughened polylactic acid, and belongs to the technical field of polylactic acid material modification.The method comprises the steps that a, a toughened polylactic acid material is provided, the toughened polylactic acid material comprises an MBS / PLA composite material, and the content of MBS in the composite material is 5-15 wt%; and b, placing the toughened polylactic acid material in a crosslinking solution to carry out a crosslinking reaction, and constructing a crosslinked polylactic acid interface layer on the surface of the toughened polylactic acid material through the crosslinking reaction. According to the method, the heat resistance of the toughened polylactic acid composite material can be greatly improved, the heat distortion temperature HDT of the toughened polylactic acid composite material is remarkably improved, meanwhile, the basic toughness of the composite material is kept, the heat resistance improving method of the toughened polylactic acid can be regulated and controlled according to performance requirements, and flexibility is provided for meeting the requirements of different application scenes of the toughened polylactic acid.

Description

Technical Field

[0001] The present invention relates to the technical field of polylactic acid material modification, and in particular to a method for improving the heat resistance of toughened polylactic acid. Background Art

[0002] Polylactic acid (PLA) is a polyester polymer, mainly composed of lactic acid molecules. Polylactic acid has excellent biodegradability and biocompatibility. However, due to the molecular structure of PLA, it has high rigidity and brittleness, resulting in poor overall toughness. In terms of mechanical properties, the unnotched simply supported beam impact strength of PLA is only 16-18 kJ / m 2 , and in terms of heat resistance, its heat load deflection temperature (HDT) is only about 54°C. Therefore, compared with petroleum-based plastics (such as polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), etc.), PLA is obviously inferior in toughness and heat resistance. These shortcomings have greatly limited the application of PLA.

[0003] To overcome the toughness and heat resistance limitations of PLA, commonly used modification methods include blending, cross-linking, copolymerization, and grafting. While blending can improve PLA's toughness, it can easily lead to phase separation, gelation, or weak interfacial bonding, thereby reducing the composite's performance, especially its heat resistance. In contrast, cross-linking involves introducing a cross-linking agent or irradiating PLA to cross-link with other monomers, forming a three-dimensional network polymer structure. While cross-linking can improve the material's thermal stability and degradation resistance, it also presents challenges such as processing difficulties and recyclability, and the improvement in PLA's toughness is limited.

[0004] Huiliang Zhang et al. (Journal of Applied Polymer Science, 2012, 125(S2):E550-E561) studied the toughening modification of polylactic acid (PLA) with methyl methacrylate-butadiene-styrene (MBS). They found that when the MBS content reached 10% (mass fraction), the impact strength of the blend was significantly improved. However, because the core of MBS's core-shell structure is composed of butadiene-styrene copolymer, which has low thermal stability and high-temperature resistance, the properties of the core when MBS is added to PLA may limit the overall thermal stability of the MBS / PLA composite, resulting in an inability to significantly improve the heat load deformation temperature of PLA.

[0005] Chinese patent literature CN115926404A discloses a method for preparing a polylactic acid cross-linked modified material with high strength, good heat resistance and degradability, by N-(2-hydroxyethyl)-N / -2-propylene thiourea cross-linked modified polylactic acid, introducing thiourea on the polylactic acid molecular chain, improving the heat resistance of polylactic acid. Thiourea cross-linking mainly forms a three-dimensional network structure through the cross-linking reaction between the thiourea groups, but this network structure has certain rigidity, and may cause an increase in the overall rigidity of the material, rather than a raising of toughness. In addition, the formation of the thiourea cross-linked structure in polylactic acid may cause the local solidification of the polylactic acid molecular chain or the aggregation of cross-linking points, thereby affecting the ductility and fracture toughness of the material. The formation of cross-linking points may limit the movement and deformation of the molecular chain, making the material become more brittle and hard. Therefore, this method cannot improve its toughness while improving the heat resistance of PLA.

[0006] Chinese patent document CN112094488A discloses a high-toughness, high-heat-resistant polylactic acid (PLA) composite material and its preparation method, wherein the toughening agent used is a copolymer of butylene adipate and butylene terephthalate (PBAT), and the heat-resistant modifier is a copolymer (MG) of methyl methacrylate (MMA) and glycidyl methacrylate (GMA). The notched impact strength of the high-toughness, high-heat-resistant polylactic acid composite material described in the document is from 86 J / m 2 Increased to 257J / m 2 , the glass transition temperature increased from 62.9°C to 77.56°C. The toughening agent PBAT and the heat-resistant modifier MG used in this paper require precise proportioning and mixing, which requires specific equipment and processing conditions, as well as complex preparation processes. Furthermore, these high-performance additives are expensive. These factors increase production costs and reduce efficiency, thus limiting the commercialization of polylactic acid in certain applications.

[0007] Since the above-mentioned blending modification and cross-linking modification methods cannot achieve a synergistic improvement in the toughness and heat resistance of the PLA material, it is also impossible to improve the heat resistance of the toughened and modified polylactic acid while maintaining its basic toughness. Summary of the Invention

[0008] In response to the above-mentioned problems existing in the prior art, the present invention combines blending toughening with subsequent dynamic in-situ surface cross-linking technology to construct a gradient cross-linked interface layer on the surface of the toughened polylactic acid matrix, thereby synergistically improving the toughness and heat resistance of the toughened polylactic acid composite material.

[0009] One of the purposes of the present invention is to provide a method for improving the heat resistance of toughened polylactic acid, the method comprising the following steps:

[0010] Step a: providing a toughened polylactic acid material, wherein the toughened polylactic acid material comprises an MBS / PLA composite material, wherein the content of MBS in the composite material is 5 to 15 wt%;

[0011] Step b: placing the toughened polylactic acid material in a cross-linking solution to carry out a cross-linking reaction, and constructing a cross-linked polylactic acid interface layer on the surface of the toughened polylactic acid material through the cross-linking reaction.

[0012] In some embodiments of the present invention, the interface layer has a gradient cross-linking structure in which the cross-linking density gradually decreases from the surface of the toughened polylactic acid material toward the inside.

[0013] In some embodiments of the present invention, the method for improving the heat resistance of toughened polylactic acid includes: controlling the gradient crosslinking structure of the interface layer by regulating the reaction time, reaction temperature and / or ratio of the crosslinking solution.

[0014] In some embodiments of the present invention, the crosslinking solution includes dicumyl peroxide (DCP), vinyltriethoxysilane, and dibutyltin dilaurate, and the weight ratio of the three is: 10-20 parts of dicumyl peroxide (DCP), 80-90 parts of vinyltriethoxysilane, and 0.04-0.12 parts of dibutyltin dilaurate. In other embodiments of the present invention, the weight ratio of the three is: 12-18 parts of dicumyl peroxide (DCP), 82-88 parts of vinyltriethoxysilane, and 0.04-0.12 parts of dibutyltin dilaurate.

[0015] In some embodiments of the present invention, the reaction temperature of the cross-linking reaction ranges from 45 to 75°C or from 45 to 70°C.

[0016] In some embodiments of the present invention, the reaction time of the cross-linking reaction is 24 to 72 hours or 24 to 48 hours.

[0017] In some embodiments of the present invention, the weight ratio is: 12 to 18 parts of dicumyl peroxide (DCP), 82 to 88 parts of vinyltriethoxysilane, and 0.04 to 0.12 parts of dibutyltin dilaurate.

[0018] In some embodiments of the present invention, the content of MBS in the MBS / PLA composite material is 8 to 12 wt %.

[0019] In some embodiments of the present invention, the toughened polylactic acid material having the cross-linked polylactic acid interface layer has an unnotched simply supported beam impact strength higher than 60 kJ / m 2 , HDT is higher than 58℃.

[0020] Another object of the present invention is to provide a toughened polylactic acid material, which is prepared according to the method for improving the heat resistance of toughened polylactic acid described in the present invention.

[0021] Compared with the prior art, the present invention has the following beneficial technical effects:

[0022] 1. Significant toughness foundation: The toughened polylactic acid material of the present invention uses a 5-15wt% MBS / PLA composite material as the base material. The MBS toughening agent with a core-shell structure effectively utilizes its "soft core and hard shell" properties and good compatibility with the PLA matrix, greatly improving the basic toughness of the toughened polylactic acid.

[0023] 2. The MBS / PLA composite material prepared according to the method of the present invention has both good toughness and heat resistance. Specifically, the present invention can greatly improve the heat resistance of the MBS / PLA composite material by constructing a cross-linked interface layer through a cross-linking reaction on the surface of the toughened polylactic acid MBS / PLA composite material, and its heat deformation temperature HDT is significantly improved, while basically maintaining the basic toughness of the MBS / PLA composite material. For example, in a specific embodiment of the present invention, the HDT value of the MBS / PLA composite material after surface cross-linking treatment is significantly higher than that of the uncross-linked MBS / PLA composite material, and the HDT can reach up to 111.38°C, an increase of more than 100%; the impact strength of the MBS / PLA composite material after surface cross-linking treatment is maintained at 60kJ / m 2 Above, the highest reaches 93.98kJ / m 2 Compared with the uncrosslinked MBS / PLA composite material, the impact strength of the MBS / PLA composite material after surface crosslinking treatment does not decrease significantly, and even improves in some specific embodiments.

[0024] 3. In the present invention, the cross-linked interface layer constructed on the surface of the toughened polylactic acid MBS / PLA composite material can effectively regulate the structure of the surface gradient cross-linked layer (such as cross-linking depth and density) by regulating the reaction conditions of the cross-linking reaction (such as changing the reaction temperature, reaction time, and catalyst dosage in the embodiment), thereby achieving gradient control of the heat resistance (especially HDT) of the MBS / PLA composite material. Therefore, the present invention provides flexibility for improving the heat resistance of toughened polylactic acid to meet the needs of different application scenarios of toughened polylactic acid. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 Schematic diagram of the cross-sectional structural mechanism of the MBS / PLA composite material prepared by the method for improving the heat resistance of the toughened polylactic acid composite material of the present invention.

[0026] Figure 2These are the infrared spectra of the surface of the MBS / PLA composite material spline prepared in Example 4 of the present invention, the infrared spectra of the surface of the PLA spline prepared in Comparative Example 1, and the infrared spectra of the surface of the MBS / PLA composite material spline prepared in Comparative Example 2.

[0027] Figure 3 This is an optical microscopic image of a cross-section of the MBS / PLA composite material strip prepared in Example 4 of the present invention, showing a cross-linked interface layer.

[0028] Figure 4 This is an optical microscopic image of the surface cross-section of the PLA strip prepared in Comparative Example 3, showing the cross-linked interface layer.

[0029] Figure 5a This is the AFM morphology image of the surface of the MBS / PLA composite material specimen prepared in Comparative Example 2.

[0030] Figure 5b This is an AFM morphology image of the surface of the MBS / PLA composite material specimen prepared in Example 4 of the present invention. DETAILED DESCRIPTION

[0031] Various aspects of the present invention will be described in detail below in conjunction with specific implementation methods and exemplary embodiments. These specific descriptions and exemplary embodiments are only used to illustrate the present invention and do not constitute any limitation on the scope of protection of the present invention.

[0032] In the present invention's method for improving the heat resistance of toughened polylactic acid, the toughened polylactic acid material comprises an MBS / PLA composite. The toughening agent MBS has a core-shell structure, combining a soft core composed of a butadiene-styrene copolymer with a hard shell composed of polymethyl methacrylate (PMMA). This core-shell design imparts the MBS material with exceptional rigidity and modulus, thereby enhancing the toughness of the PLA composite. The MBS toughening agent content in the composite is 5 to 15 wt%, meeting the composite's basic toughness requirements. In an exemplary embodiment of the present invention, the MBS content in the composite is 8 to 12 wt%.

[0033] The 5-15 wt% MBS / PLA composite material used in the present invention can be prepared according to methods known in the prior art, or can be prepared by the following steps, including:

[0034] Step 1: Dry the raw material PLA and toughening agent MBS in a vacuum oven at 60-80°C for 10-14 hours until the moisture content is less than 300 ppm.

[0035] Step 2: Weigh 85-95 wt% of dried PLA and 5-15 wt% of MBS, place both in a high-speed mixer at 32,000 rpm and blend for 2-10 minutes. The resulting mixture is then added to a twin-screw extruder for mixing, extrusion, and pelletization. In some embodiments, the temperatures of the twin-screw extruder from the feed inlet to the discharge outlet are set to 145-155°C, 165-175°C, and 175-185°C, respectively. The main engine speed is 30-50 rpm, and the feed screw speed is 10-30 rpm.

[0036] Step 3: Dry the obtained pellets in a vacuum oven at 60-80°C for 6-10 hours until the moisture content is less than 300 ppm.

[0037] Step 4: Add the dried pellets to an injection molding machine for injection molding to produce an MBS / PLA composite product containing 5-15 wt% MBS. In some embodiments, the injection zone temperature of the injection molding machine is set to 180-190°C, the die zone temperature is set to 35°C, the injection pressure is 0.38-0.42 MPa, the holding pressure is 0.18-0.22 MPa, and the holding time is 15-30 seconds.

[0038] The toughened polylactic acid material described in the present invention can be a toughened polylactic acid product with any size and shape, and can be prepared by injection molding. In the specific embodiments and comparative examples of the present invention, in order to facilitate performance testing, the size of the toughened polylactic acid material is the same as the size of the performance test standard sample.

[0039] In the method for improving the heat resistance of toughened polylactic acid of the present invention, the toughened polylactic acid material is placed in a cross-linking solution to undergo a chemical cross-linking reaction, and a cross-linked polylactic acid interface layer is constructed on the surface of the toughened polylactic acid material. Figure 1 Schematically shows the cross-sectional structural mechanism diagram of the MBS / PLA composite material prepared according to the method of the present invention.

[0040] The cross-linking solution used for the cross-linking reaction in the present invention includes a cross-linking agent, a coupling agent and a catalyst. In an exemplary embodiment, the cross-linking agent is dicumyl peroxide (DCP), the coupling agent is vinyltriethoxysilane, and the catalyst is dibutyltin dilaurate. The weight ratio of the three is: 12 to 18 parts of dicumyl peroxide (DCP), 82 to 88 parts of vinyltriethoxysilane, and 0.04 to 0.12 parts of dibutyltin dilaurate. The DCP in the cross-linking solution triggers a free radical reaction, vinyltriethoxysilane participates in the formation of a cross-linked network as a coupling agent, and dibutyltin dilaurate is accelerated as a catalyst. The present invention can regulate the ratio of the cross-linking solution as needed, especially the catalyst content therein, and then regulate the cross-linking reaction, control the cross-linking depth and cross-linking density of the cross-linked interface layer, and achieve the desired heat resistance and toughness.

[0041] In the method for improving the heat resistance of toughened polylactic acid of the present invention, the reaction temperature range of the cross-linking reaction for constructing a cross-linked interface layer on the surface of the toughened polylactic acid material is 45 to 75°C. In some embodiments of the present invention, the reaction temperature range is 45 to 60°C. The reaction time of the cross-linking reaction is 24 to 72 hours. In an exemplary embodiment of the present invention, the reaction time of the cross-linking reaction is 24 to 48 hours. The present invention can regulate the cross-linking reaction by controlling the reaction temperature and reaction time of the cross-linking reaction according to the performance requirements for the toughened polylactic acid, and then control the gradient structure of the cross-linked interface layer (such as cross-linking depth and cross-linking density), so that the toughened polylactic acid material can achieve the desired heat resistance and toughness.

[0042] The present invention is further described in detail below through seven exemplary embodiments and three comparative examples of the present invention.

[0043] Example 1

[0044] In the method for improving the heat resistance of toughened polylactic acid in this embodiment, the toughened polylactic acid material is an MBS / PLA composite material, wherein the MBS content is 8 wt %. The method of this embodiment includes the following steps:

[0045] a. Provide MBS / PLA composite material specimens

[0046] The preparation of MBS / PLA composite material specimens includes the following steps:

[0047] Step 1: Dry the raw material PLA and toughening agent MBS in a vacuum oven at 60°C for 12 hours until the moisture content is less than 300 ppm.

[0048] Step 2: According to the ratio of 92wt% PLA and 8wt% MBS, 92g of PLA and 8g of MBS were weighed and blended in a high-speed mixer at 32000rpm for 5min. The resulting mixture was added to a twin-screw extruder for mixing, extrusion, and pelletization. The temperatures of the twin-screw extruder from the feed port to the discharge port were set to 150°C, 170°C, and 180°C, respectively. The main engine speed was 40rpm, and the feed screw speed was 20rpm.

[0049] Step 3: The pellets were dried at 60°C for 8 h until the moisture content was less than 300 ppm.

[0050] Step 4: The dried pellets were added to an injection molding machine for injection molding. The temperature of the injection zone of the injection molding machine was set to 180-190°C, the temperature of the module zone was set to 35°C, the injection pressure was 0.4 MPa, the holding pressure was 0.2 MPa, and the pressure was maintained for 20 seconds to obtain several splines of MBS / PLA composite materials containing 8 wt% MBS. The size of each spline was 4 mmх10 mmх80 mm.

[0051] b. Construction of a cross-linked interface layer on the surface of MBS / PLA composite strips

[0052] Step 1: Place the injection-molded strips in a cross-linking solution for reaction at 45°C in a sealed container for 48 hours. The cross-linking solution ratio is:

[0053] Dicumyl peroxide (DCP) Vinyltriethoxysilane Dibutyltin dilaurate 14.4g 85.6g 0.12g

[0054] Step 2: After the cross-linking reaction, the sample strips were taken out from the cross-linking solution, washed with deionized water, dried in a vacuum oven at 60°C for 30 minutes, and finally taken out and sealed for storage.

[0055] Example 2

[0056] In the method for improving the heat resistance of toughened polylactic acid in this embodiment, the toughened polylactic acid material is an MBS / PLA composite material, wherein the MBS content is 8 wt %. The method of this embodiment includes the following steps:

[0057] a. Provide MBS / PLA composite material specimens

[0058] Step 1: Dry the raw material PLA and toughening agent MBS in a vacuum oven at 60°C for 12 hours until the moisture content is less than 300 ppm.

[0059] Step 2: According to the ratio of 92wt% PLA and 8wt% MBS, 92g of PLA and 8g of MBS were weighed and blended in a high-speed mixer at 32000rpm for 5min. The resulting mixture was added to a twin-screw extruder for mixing, extrusion, and pelletization. The temperatures of the twin-screw extruder from the feed port to the discharge port were set to 150°C, 170°C, and 180°C, respectively. The main engine speed was 40rpm, and the feed screw speed was 20rpm.

[0060] Step 3: The pellets were dried at 60°C for 8 h until the moisture content was less than 300 ppm.

[0061] Step 4: The dried pellets were added to an injection molding machine for injection molding. The temperature of the injection zone of the injection molding machine was set to 180-190°C, the temperature of the module zone was set to 35°C, the injection pressure was 0.4 MPa, the holding pressure was 0.2 MPa, and the pressure was maintained for 20 seconds to obtain several splines of MBS / PLA composite materials with 8 wt% MBS, each spline having a size of 4 mmх10 mmх80 mm.

[0062] b. Construction of a cross-linked interface layer on the surface of MBS / PLA composite strips

[0063] Step 1: Place the injection molded strip in a crosslinking solution for reaction at 60°C in a sealed container for 36 hours. The crosslinking solution ratio is:

[0064] Dicumyl peroxide (DCP) Vinyltriethoxysilane Dibutyltin dilaurate 14.4g 85.6g 0.08g

[0065] Step 2: After the cross-linking reaction, the sample strips were taken out from the cross-linking solution, washed with deionized water, dried in a vacuum oven at 60°C for 30 minutes, and finally taken out and sealed for storage.

[0066] Example 3

[0067] In the method for improving the heat resistance of toughened polylactic acid in this embodiment, the toughened polylactic acid material is an MBS / PLA composite material, wherein the MBS content is 12 wt %. The method of this embodiment includes the following steps:

[0068] a. Provide MBS / PLA composite material specimens

[0069] The preparation of MBS / PLA composite material specimens includes the following steps:

[0070] Step 1: Dry the raw material PLA and toughening agent MBS in a vacuum oven at 60°C for 12 hours until the moisture content is less than 300 ppm.

[0071] Step 2: According to the ratio of 88 wt% PLA and 12 wt% MBS, 88 g of PLA and 12 g of MBS were weighed and blended in a high-speed mixer at 32,000 rpm for 5 minutes. The resulting mixture was added to a twin-screw extruder for mixing, extrusion, and pelletization. The temperatures of the twin-screw extruder from the feed port to the discharge port were set to 150° C., 170° C., and 180° C., respectively. The main engine speed was 40 rpm, and the feed screw speed was 20 rpm.

[0072] Step 3: The pellets were dried at 60°C for 8 h until the moisture content was less than 300 ppm.

[0073] Step 4: The dried pellets were added to an injection molding machine for injection molding. The temperature of the injection zone of the injection molding machine was set to 180-190°C, the temperature of the module zone was set to 35°C, the injection pressure was 0.4 MPa, the holding pressure was 0.2 MPa, and the pressure was maintained for 20 seconds to obtain several splines of MBS / PLA composite materials with 12 wt% MBS. The size of each spline was 4 mmх10 mmх80 mm.

[0074] b. Construction of a cross-linked interface layer on the surface of MBS / PLA composite strips

[0075] Step 1: Place the injection molded strip in a crosslinking solution for reaction at 75°C in a sealed container for 24 hours. The crosslinking solution ratio is:

[0076] Dicumyl peroxide (DCP) Vinyltriethoxysilane Dibutyltin dilaurate 14.4g 85.6g 0.04g

[0077] Step 2: After the cross-linking reaction, the sample strips were taken out from the cross-linking solution, washed with deionized water, dried in a vacuum oven at 60°C for 30 minutes, and finally taken out and sealed for storage.

[0078] Example 4

[0079] In the method for improving the heat resistance of toughened polylactic acid in this embodiment, the toughened polylactic acid material is an MBS / PLA composite material, wherein the MBS content is 10 wt %. The method of this embodiment includes the following steps:

[0080] a. Provide MBS / PLA composite material specimens

[0081] The preparation of MBS / PLA composite material specimens includes the following steps:

[0082] Step 1: Dry the raw material PLA and toughening agent MBS in a vacuum oven at 60°C for 12 hours until the moisture content is less than 300 ppm.

[0083] Step 2: According to the ratio of 90wt% PLA and 10wt% MBS, 90g of PLA and 10g of MBS were weighed and blended in a high-speed mixer at 32000rpm for 5 minutes. The resulting mixture was added to a twin-screw extruder for mixing, extrusion, and pelletization. The temperatures of the twin-screw extruder from the feed port to the discharge port were set to 150°C, 170°C, and 180°C, respectively. The main engine speed was 40rpm, and the feed screw speed was 20rpm.

[0084] Step 3: The pellets were dried at 60°C for 8 h until the moisture content was less than 300 ppm.

[0085] Step 4: The dried pellets were added to an injection molding machine for injection molding. The temperature of the injection zone of the injection molding machine was set to 180-190°C, the temperature of the module zone was set to 35°C, the injection pressure was 0.4 MPa, the holding pressure was 0.2 MPa, and the pressure was maintained for 20 seconds to obtain several splines of MBS / PLA composite materials with 10 wt% MBS. The size of each spline was 4 mmх10 mmх80 mm.

[0086] b. Construction of a cross-linked interface layer on the surface of MBS / PLA composite strips

[0087] Step 1: Place the injection molded strip in a crosslinking solution for reaction at 75°C in a sealed container for 48 hours. The crosslinking solution ratio is:

[0088] Dicumyl peroxide (DCP) Vinyltriethoxysilane Dibutyltin dilaurate 14.4g 85.6g 0.04g

[0089] Step 2: After the cross-linking reaction, the sample strips were taken out from the cross-linking solution, washed with deionized water, dried in a vacuum oven at 60°C for 30 minutes, and finally taken out and sealed for storage.

[0090] Example 5

[0091] In the method for improving the heat resistance of toughened polylactic acid in this embodiment, the toughened polylactic acid material is an MBS / PLA composite material, wherein the MBS content is 10 wt %. The method of this embodiment includes the following steps:

[0092] a. Provide MBS / PLA composite material specimens

[0093] The preparation of MBS / PLA composite material specimens includes the following steps:

[0094] Step 1: Dry the raw material PLA and toughening agent MBS in a vacuum oven at 60°C for 12 hours until the moisture content is less than 300 ppm.

[0095] Step 2: According to the ratio of 90wt% PLA and 10wt% MBS, 90g of PLA and 10g of MBS were weighed and blended in a high-speed mixer at 32000rpm for 5 minutes. The resulting mixture was added to a twin-screw extruder for mixing, extrusion, and pelletization. The temperatures of the twin-screw extruder from the feed port to the discharge port were set to 150°C, 170°C, and 180°C, respectively. The main engine speed was 40rpm, and the feed screw speed was 20rpm.

[0096] Step 3: The pellets were dried at 60°C for 8 h until the moisture content was less than 300 ppm.

[0097] Step 4: The dried pellets were added to an injection molding machine for injection molding. The temperature of the injection zone of the injection molding machine was set to 180-190°C, the temperature of the module zone was set to 35°C, the injection pressure was 0.4 MPa, the holding pressure was 0.2 MPa, and the pressure was maintained for 20 seconds to obtain several splines of MBS / PLA composite materials with 10 wt% MBS. The size of each spline was 4 mmх10 mmх80 mm.

[0098] b. Construction of a cross-linked interface layer on the surface of MBS / PLA composite strips

[0099] Step 1: Place the injection molded strip in a crosslinking solution for reaction at 75°C in a sealed container for 48 hours. The crosslinking solution ratio is:

[0100] Dicumyl peroxide (DCP) Vinyltriethoxysilane Dibutyltin dilaurate 12g 88g 0.04g

[0101] Step 2: After the cross-linking reaction, the sample strips were taken out from the cross-linking solution, washed with deionized water, dried in a vacuum oven at 60°C for 30 minutes, and finally taken out and sealed for storage.

[0102] Example 6

[0103] In the method for improving the heat resistance of toughened polylactic acid in this embodiment, the toughened polylactic acid material is an MBS / PLA composite material, wherein the MBS content is 10 wt %. The method of this embodiment includes the following steps:

[0104] a. Provide MBS / PLA composite material specimens

[0105] The preparation of MBS / PLA composite material specimens includes the following steps:

[0106] Step 1: Dry the raw material PLA and toughening agent MBS in a vacuum oven at 60°C for 12 hours until the moisture content is less than 300 ppm.

[0107] Step 2: According to the ratio of 90wt% PLA and 10wt% MBS, 90g of PLA and 10g of MBS were weighed and blended in a high-speed mixer at 32000rpm for 5 minutes. The resulting mixture was added to a twin-screw extruder for mixing, extrusion, and pelletization. The temperatures of the twin-screw extruder from the feed port to the discharge port were set to 150°C, 170°C, and 180°C, respectively. The main engine speed was 40rpm, and the feed screw speed was 20rpm.

[0108] Step 3: The pellets were dried at 60°C for 8 h until the moisture content was less than 300 ppm.

[0109] Step 4: The dried pellets were added to an injection molding machine for injection molding. The temperature of the injection zone of the injection molding machine was set to 180-190°C, the temperature of the module zone was set to 35°C, the injection pressure was 0.4 MPa, the holding pressure was 0.2 MPa, and the pressure was maintained for 20 seconds to obtain several splines of MBS / PLA composite materials with 10 wt% MBS. The size of each spline was 4 mmх10 mmх80 mm.

[0110] b. Construction of a cross-linked interface layer on the surface of MBS / PLA composite strips

[0111] Step 1: Place the injection molded strip in a crosslinking solution for reaction at 75°C in a sealed container for 48 hours. The crosslinking solution ratio is:

[0112] Dicumyl peroxide (DCP) Vinyltriethoxysilane Dibutyltin dilaurate 18g 82g 0.04g

[0113] Step 2: After the cross-linking reaction, the sample strips were taken out from the cross-linking solution, washed with deionized water, dried in a vacuum oven at 60°C for 30 minutes, and finally taken out and sealed for storage.

[0114] Example 7

[0115] In the method for improving the heat resistance of toughened polylactic acid in this embodiment, the toughened polylactic acid material is an MBS / PLA composite material, wherein the MBS content is 10 wt %. The method of this embodiment includes the following steps:

[0116] a. Provide MBS / PLA composite material specimens

[0117] The preparation of MBS / PLA composite material specimens includes the following steps:

[0118] Step 1: Dry the raw material PLA and toughening agent MBS in a vacuum oven at 60°C for 12 hours until the moisture content is less than 300 ppm.

[0119] Step 2: According to the ratio of 90wt% PLA and 10wt% MBS, 90g of PLA and 10g of MBS were weighed and blended in a high-speed mixer at 32000rpm for 5 minutes. The resulting mixture was added to a twin-screw extruder for mixing, extrusion, and pelletization. The temperatures of the twin-screw extruder from the feed port to the discharge port were set to 150°C, 170°C, and 180°C, respectively. The main engine speed was 40rpm, and the feed screw speed was 20rpm.

[0120] Step 3: Dry the obtained pellets at 60°C for 8 hours until the moisture content is less than 300 ppm.

[0121] Step 4: The dried pellets were added to an injection molding machine for injection molding. The temperature of the injection zone of the injection molding machine was set to 180-190°C, the temperature of the module zone was set to 35°C, the injection pressure was 0.4 MPa, the holding pressure was 0.2 MPa, and the pressure was maintained for 20 seconds to obtain several splines of MBS / PLA composite materials with 10 wt% MBS. The size of each spline was 4 mmх10 mmх80 mm.

[0122] b. Construction of a cross-linked interface layer on the surface of MBS / PLA composite strips

[0123] Step 1: Place the injection molded strip in a crosslinking solution for reaction at 70°C in a sealed container for 48 hours. The crosslinking solution ratio is:

[0124] Dicumyl peroxide (DCP) Vinyltriethoxysilane Dibutyltin dilaurate 14.4g 85.6g 0.04g

[0125] Step 2: After the cross-linking reaction, the sample strips were taken out from the cross-linking solution, washed with deionized water, dried in a vacuum oven at 60°C for 30 minutes, and finally taken out and sealed for storage.

[0126] Comparative Example 1

[0127] In Comparative Example 1, polylactic acid was used as a raw material and the same injection molding process as in Examples 1-7 was used to prepare polylactic acid strips. The specific steps were as follows:

[0128] Step 1: Dry the raw PLA pellets in a vacuum oven at 60°C for 12 h until the moisture content is less than 300 ppm.

[0129] Step 2: The dried pellets were added to an injection molding machine for injection molding. The injection zone temperature of the injection molding machine was set to 180-190°C, the die zone temperature was set to 35°C, the injection pressure was 0.4 MPa, the holding pressure was 0.2 MPa, and the pressure was maintained for 20 seconds. Several pure PLA strips were obtained. Each strip had a size of 4 mm x 10 mm x 80 mm. The mass of each strip was 4 g.

[0130] Comparative Example 2

[0131] In Comparative Example 2, the same MBS / PLA composite material strips as those in Example 4 were prepared, containing 10 wt % MBS. The difference from Example 4 was that the strips in Comparative Example 2 were not subjected to surface cross-linking treatment.

[0132] Comparative Example 3

[0133] Using polylactic acid as a raw material, a polylactic acid strip was prepared using the same molding process as in Example 1-7, and the strip was subjected to surface cross-linking treatment as follows:

[0134] Step 1: Take 100g of raw PLA and dry it in a vacuum oven at 60°C for 12h until the moisture content is less than 300ppm.

[0135] Step 2: Add the dried pellets to the injection molding machine for injection molding. The temperature of the injection zone of the injection molding machine is set to 180-190°C, the temperature of the module zone is set to 35°C, the injection pressure is 0.4 MPa, the holding pressure is 0.2 MPa, and the pressure is maintained for 20 seconds to obtain several pure PLA splines, each with a size of 4 mmх10 mmх80 mm.

[0136] Step 3: Place the injection molded strips in a crosslinking solution for reaction at 75°C in a sealed container for 48 hours. The crosslinking solution ratio is:

[0137] Dicumyl peroxide (DCP) Vinyltriethoxysilane Dibutyltin dilaurate 14.4g 85.6g 0.04g

[0138] Step 4: After the cross-linking reaction, the specimens were taken out from the cross-linking solution, washed with deionized water, dried in a vacuum oven at 60°C for 30 minutes, and finally taken out and sealed for storage.

[0139] The properties of the strips prepared in Examples 1-7 and Comparative Examples 1-3 will be tested below, and the microscopic morphology of the cross-linked interface layer will be observed and analyzed.

[0140] 1. Impact strength and heat deformation temperature HDT test

[0141] The unnotched simply supported beam impact strength and heat distortion temperature (HDT) of the specimens prepared in Examples 1-7 and Comparative Examples 1-3 were tested. The impact strength was tested in accordance with GB / T 1043.2-2018, using a 4J pendulum for 10 repeated tests of the same example or comparative example, and the results were averaged. The heat distortion temperature (HDT) was tested in accordance with GB / T 1634.2-2019, using a 0.45 MPa load for the same example or comparative example, and the results were averaged three times. The test results are shown in Table 1 below.

[0142] Table 1. Toughness and heat resistance test results

[0143]

[0144] As can be seen from Table 1, in Examples 1-7 of the present invention, by constructing a cross-linked interface layer on the surface of the MBS / PLA composite material, the bulk toughness of the MBS / PLA composite material is effectively maintained while the heat resistance is greatly improved.

[0145] As for toughness, the results in Table 1 show that the unnotched impact strength of the MBS / PLA composites prepared in Examples 1-7 is maintained at 60 kJ / m 2 Among them, Example 3 even reached 93.98 kJ / m 2 , significantly higher than the pure PLA material prepared in Comparative Example 2. Furthermore, compared to the MBS / PLA composite prepared in Comparative Example 2 without surface cross-linking, the impact strength did not significantly decrease, and even improved under certain conditions. This is due to the fact that cross-linking occurs primarily on the surface, without destroying the toughened structure created by the MBS within.

[0146] With regard to heat resistance, the results in Table 1 show that in Examples 1-7 of the present invention, by constructing a cross-linked interface layer on the surface of the MBS / PLA composite material, the HDT values of the MBS / PLA composite materials were all higher than 58°C, which was significantly higher than the MBS / PLA composite material of Comparative Example 2 without cross-linking treatment (HDT of 53.4°C). Specifically, the HDT of the MBS / PLA composite material prepared in Example 4 was as high as 111.38°C, an improvement of more than 100%.

[0147] Furthermore, Table 1 shows that the performance enhancement of the MBS / PLA composite using the method of the present invention is controllable. Specifically, by adjusting the surface crosslinking reaction conditions (e.g., by varying the reaction temperature, reaction time, catalyst dosage, etc.), the structure of the gradient crosslinked layer on the surface of the MBS / PLA composite (e.g., crosslinking depth and density) can be effectively regulated, depending on the application requirements. This allows for gradient control of the composite's ultimate performance (particularly HDT). Furthermore, by adjusting the MBS content in the MBS / PLA composite, the basic toughness of the composite can be regulated, achieving tunable toughness, depending on the application requirements. This provides flexibility for meeting the needs of different application scenarios.

[0148] 2. Microscopic morphology observation of the cross-linked interface layer

[0149] In order to further explore the effect of the cross-linked interface layer on the material properties, the cross-sections of the MBS / PLA composite material prepared in Example 4 (surface cross-linked) and the comparative example 3 (pure PLA material surface cross-linked) were observed using an optical microscope. The corresponding optical microscopic images are shown in Figure 2. Figure 3 and Figure 4 shown.

[0150] Depend on Figure 3 and Figure 4 Observation results showed that in Comparative Example 3, a surface cross-linked layer with an average thickness of 37.58 μm formed on the surface of the pure PLA material, beneath which lay the transparent PLA matrix. In stark contrast, the MBS / PLA composite material of Example 4 formed a thicker surface cross-linked layer with an average thickness of 87.22 μm. The opaque white matrix beneath this cross-linked layer was the MBS / PLA composite material.

[0151] Calculations show that the thickness of the cross-linked layer of the MBS / PLA composite material of Example 4 is approximately 2.32 times that of the cross-linked layer of the pure PLA material of Comparative Example 3. This significant difference in thickness is primarily attributed to the unique chemical structure of MBS (methyl methacrylate-butadiene-styrene copolymer). The unsaturated double bonds (derived from butadiene units) and other reactive functional groups present in the MBS molecular chain have higher reactivity under the same cross-linking conditions compared to the relatively chemically stable ester bonds in the PLA molecular chain. Therefore, under the same cross-linking treatment conditions, the presence of MBS in the PLA matrix can participate in or promote deeper and wider cross-linking reactions, ultimately resulting in the formation of a cross-linked layer on the surface of the MBS / PLA composite material that is much thicker than that of the pure PLA material.

[0152] 3. Elastic modulus test of cross-linked interface layer

[0153] The Young's modulus of the sample prepared in Example 4 was tested using an atomic force microscope (AFM). The test results showed that the Young's modulus of the cross-linked interface layer of the sample in Example 4 was as high as 3.949 GPa, which was significantly higher than the Young's modulus of the uncross-linked MBS / PLA composite matrix in the sample (0.778 GPa), and the former was about 5.1 times that of the latter. This shows that the MBS / PLA composite material greatly improves the stiffness and hardness of the surface layer of the MBS / PLA composite material by constructing a cross-linked interface layer through a surface cross-linking reaction. The increase in Young's modulus means that the ability of the surface layer to resist elastic deformation is significantly enhanced, which usually leads to better wear resistance and scratch resistance. The internal uncross-linked MBS / PLA matrix material maintains flexibility. The uncross-linked MBS / PLA matrix has a lower Young's modulus, which is consistent with its desired high toughness (ability to absorb impact energy) characteristic as a toughening material, forming a performance gradient structure, and the material presents a structural feature of being hard on the outside and tough on the inside. Although Young's modulus itself is not directly equivalent to heat resistance (the ability of a material to maintain its properties at high temperatures), the highly cross-linked network structure can usually limit the movement of molecular chains at high temperatures, thereby improving the thermal stability of the material.

[0154] 4. Infrared spectroscopy analysis of the cross-linked interface layer surface

[0155] The samples obtained from Example 4, Comparative Example 1 and Comparative Example 2 were analyzed by infrared spectroscopy, and the obtained infrared spectra were as follows: Figure 2 As shown. Figure 2 It can be seen that compared with the pure PLA material (without surface cross-linking treatment) in Comparative Example 1, the infrared spectrum of the MBS / PLA composite material after adding 10 wt% MBS in Comparative Example 2 is at 2996 cm -1 and 2960cm -1The stretching vibration peak intensity of CH3 at 1750cm -1 The stretching vibration peak intensity of C=O at 2996 cm was enhanced. This indicates that the addition of toughening agent MBS leads to an increase in the number of CH3 and C=O groups in the mixture. However, compared with the comparative example 1 before surface crosslinking treatment, the infrared spectrum of the example 4 after crosslinking treatment has an increase at 2996 cm -1 and 2960cm -1 The stretching vibration peak intensity of CH3 at 1750cm -1 The stretching vibration peak intensities of C=O at the α-H bond are weakened. This may be because the cross-linking reaction leads to the formation of a part of the cross-linked structure, so that some CH3 and C=O groups are incorporated into the cross-linked structure and no longer exist in the material in a free state.

[0156] 5. Observation of the surface morphology of the cross-linked interface layer

[0157] In order to further explore the effect of the cross-linked interface layer on the surface of the material, an atomic force microscope (AFM) was used to scan the 80 μm × 80 μm area on the surface of the materials prepared in Comparative Example 2 and Example 4, and a height sensor was used to observe the sample surface. The three-dimensional micromorphology of Comparative Example 2 and Example 4 was observed as follows: Figure 5a and Figure 5b As shown. Figure 5a It can be observed that the surface of the MBS / PLA composite material of Comparative Example 2 (without crosslinking) has an array of concave and convex morphologies, which may be due to the uneven dispersion of MBS in the composite material or the aggregation of particles. In the absence of a crosslinking layer, the MBS particles form some tiny depressions or protrusions on the surface of the material, resulting in an uneven surface. Figure 5b It can be observed that the surface concave-convex morphology of the material of Example 4 after surface cross-linking treatment becomes significantly denser, and the array-like concave-convex morphology before cross-linking disappears. This is because the cross-linking agent DCP triggers the cross-linking reaction of the polylactic acid molecules in the cross-linking reaction, forming a dense network cross-linked structure; and the presence of the cross-linked layer fills the original concave or convex area, making the material surface smoother.

[0158] The surface of the MBS / PLA composite material with a cross-linked polylactic acid interface layer forms a dense network cross-linked structure. The existence of this structure brings multiple advantages. First, the formation of the cross-linked structure effectively increases the number of cross-linking points and cross-linking chains between the molecular chains of the material, thereby improving the material's ability to resist external stress. Second, the cross-linked structure exhibits good heat resistance under high temperature conditions. The presence of cross-linking points and cross-linking chains increases the steric hindrance between molecules, restricts the movement of the molecular chains, and improves the thermal stability and heat resistance of the material. This enables the MBS / PLA composite material with a cross-linked polylactic acid interface layer to maintain structural stability and mechanical properties in high temperature environments.

[0159] In summary, the present invention successfully achieves a synergistic improvement in the toughness and heat resistance of the PLA composite material by constructing a cross-linked polylactic acid interface layer on the surface of the blended modified toughened polylactic acid composite material. The mechanism is as follows: Figure 1 As shown, the formation of the cross-linked polylactic acid interface layer significantly improves the heat resistance of the MBS / PLA composite material while maintaining its original toughness. The method of the present invention provides a broader application field for polylactic acid composite materials, especially in fields requiring both toughness and heat resistance. On the other hand, the unnotched impact strength data and HDT data in Table 1 show that the method for improving the heat resistance of the toughened polylactic acid composite material proposed in the present invention is controllable and adjustable, and can be adjusted according to actual application requirements, thereby meeting the requirements of different application fields for the mechanical properties and heat resistance of polylactic acid composite materials.

[0160] It should be noted that, in this document, the terms "comprises," "includes," or any other variations thereof are intended to encompass non-exclusive inclusion, such that a process, method, article, or apparatus comprising a series of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such process, method, article, or apparatus. In the absence of further limitations, the elements defined by the term "comprises" do not exclude the presence of other identical elements in the process, method, article, or apparatus comprising the elements.

[0161] The above embodiments are only used to illustrate the technical solutions of the present invention and cannot be used to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in this field should understand that: without departing from the essence and scope of the present invention, the technical solutions described in the above embodiments can still be modified, or some of the technical features therein can be replaced by equivalents; and these modifications or equivalent replacements still fall within the scope covered by the present invention.

Claims

1. A method for improving the heat resistance of toughened polylactic acid, characterized in that: The method comprises the following steps: Step a: providing a toughened polylactic acid material, wherein the toughened polylactic acid material comprises an MBS / PLA composite material, wherein the content of MBS in the composite material is 5 to 15 wt%; Step b: placing the toughened polylactic acid material in a cross-linking solution to carry out a cross-linking reaction, and constructing a cross-linked polylactic acid interface layer on the surface of the toughened polylactic acid material through the cross-linking reaction.

2. The method for improving the heat resistance of toughened polylactic acid according to claim 1, wherein: The interface layer has a gradient cross-linking structure in which the cross-linking density gradually decreases from the surface of the toughened polylactic acid material toward the inside.

3. The method for improving the heat resistance of toughened polylactic acid according to claim 2, wherein: The method comprises: controlling the gradient cross-linking structure of the interface layer by regulating the reaction time, reaction temperature and / or the ratio of the cross-linking solution.

4. The method for improving the heat resistance of toughened polylactic acid according to claim 1, wherein: The cross-linking solution includes dicumyl peroxide (DCP), vinyltriethoxysilane, and dibutyltin dilaurate, and the weight ratio of the three is: 10-20 parts of dicumyl peroxide (DCP), 80-90 parts of vinyltriethoxysilane, and 0.04-0.12 parts of dibutyltin dilaurate.

5. The method for improving the heat resistance of toughened polylactic acid according to claim 1, wherein: The reaction temperature of the cross-linking reaction ranges from 45 to 75°C.

6. The method for improving the heat resistance of toughened polylactic acid according to claim 1, wherein: The cross-linking reaction takes 24 to 72 hours.

7. The method for improving the heat resistance of toughened polylactic acid according to claim 4, wherein: The weight ratio is: 12 to 18 parts of dicumyl peroxide (DCP), 82 to 88 parts of vinyltriethoxysilane, and 0.04 to 0.12 parts of dibutyltin dilaurate.

8. The method for improving the heat resistance of toughened polylactic acid according to claim 1, wherein: The content of MBS in the MBS / PLA composite material is 8-12 wt%.

9. The method for improving the heat resistance of toughened polylactic acid according to claim 6, wherein: The reaction time of the cross-linking reaction is 24 to 48 hours.

10. The method for improving the heat resistance of toughened polylactic acid according to claim 5, wherein: The temperature range of the cross-linking reaction is 45-70°C.

11. The method for improving the heat resistance of toughened polylactic acid according to claim 1, wherein: The unnotched simply supported beam impact strength of the toughened polylactic acid material having the cross-linked polylactic acid interface layer is higher than 60 kJ / m 2 , HDT is higher than 58℃.

12. A toughened polylactic acid material prepared according to the method according to any one of claims 1 to 11.

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