Preparation method of modified high-strength and high-toughness polylactic acid

Abstract

This invention discloses a method for preparing modified high-strength and high-toughness polylactic acid (PLA), comprising: S1, mixing boric acid and hydroxyl silicone oil and then performing a polymerization reaction to obtain a precursor polymer matrix with a silky luster; S2, mixing the polymer matrix prepared in the above step with oleic acid, and performing a thorough blending and plasticizing reaction in a Hacker torque rheometer to obtain an SSG polymer with a dynamic B-O crosslinking structure; S3, melt-blending the SSG polymer prepared in the above step with PLA in a twin-cone screw extruder, and uniformly dispersing the mixture to obtain a PLA / SSG blend, i.e., the high-strength and high-toughness PLA exhibiting strain rate hardening energy absorption effect. According to this invention, it is possible to significantly improve the elongation at break and impact energy absorption of PLA with almost no reduction in its ultimate tensile strength, which has broad application prospects in the field of additive manufacturing.

Description

Technical Field

[0001] This invention relates to the field of polymer technology, and in particular to a method for preparing modified high-strength and tough polylactic acid. Background Technology

[0002] Polylactic acid (PLA) is a biodegradable and environmentally friendly material polymerized from lactide monomers derived from renewable starch. It exhibits excellent biodegradability, being completely degraded into CO2 and H2O by microorganisms in nature. However, the inherent brittleness of PLA limits its further industrial applications. Facing increasingly demanding service environments, improving the static toughness and dynamic energy absorption properties of PLA is urgently needed. In recent years, researchers have achieved a simple, effective, and cost-efficient toughening method by melt-blending PLA with other polymers at high temperatures, and this method can, to a certain extent, achieve the final properties of customized PLA-based composites. For example, Zhang et al. (Materials For Mechanical Engineering 2017, 41, 57-60) melt-blended polybutylene adipate (PBAT) with PLA and found that the addition of PBAT significantly improved the static tensile toughness of the PLA / PBAT mixture. Studies by Dang Haichun (Chemical Industry and Engineering Progress 2021, 40, 3380-3388) et al. showed that using star-shaped polyurethane (SPUR3) as an additive phase can improve the dynamic fracture toughness of PLA to some extent. However, the improvement in toughness is often accompanied by a decrease in strength. In their research (Journal of Applied Polymer Science 2019, 136, 8-16), a PLA / polyhydroxyalkanoate (PHA) blend was successfully prepared, with an elongation at break nearly 40 times that of pure PLA. Unfortunately, due to the significantly lower inherent strength of PHA compared to PLA, the strength and modulus of the final blend were significantly reduced. Similarly, Chinese patents CN113402864A and CN113831711A disclose specific solutions for toughening PLA using polyamide thermoplastic elastomers and polyester elastomer particles, respectively. These solutions also fail to achieve a balance between strength and toughness. Summary of the Invention

[0003] To address the shortcomings of existing technologies, the present invention aims to provide a method for preparing modified high-strength and high-toughness polylactic acid (PLA), which can significantly improve its elongation at break and impact energy absorption with almost no reduction in the ultimate tensile strength of PLA, and has broad application prospects in the field of additive manufacturing. To achieve the above-mentioned objectives and other advantages of the present invention, a method for preparing modified high-strength and high-toughness PLA is provided, comprising:

[0004] S1. Boric acid and hydroxyl silicone oil are mixed and then subjected to a polymerization reaction to obtain a precursor polymer matrix with a silky luster.

[0005] S2. The polymer matrix prepared in the above steps is mixed with oleic acid and fully blended and plasticized in a Hacker torque rheometer to obtain an SSG polymer with a dynamic BO crosslinking structure.

[0006] S3. The SSG polymer prepared in the above steps is melt-blended with PLA in a twin-cone screw, and after uniform dispersion, PLA / SSG blend is obtained, which is the high-strength and tough polylactic acid with strain rate hardening energy absorption effect.

[0007] Preferably, the polymerization temperature in step S1 is 200°C, the mixing ratio of boric acid and hydroxyl silicone oil is 80 mg / mL, and to ensure thorough mixing, it needs to be stirred once every 20 minutes for more than 2 hours until the reactants show a silky luster.

[0008] Preferably, the mass fraction of oleic acid in step S2 is 1%, and it needs to be thoroughly stirred in a Hacker torque rheometer to ensure that the plasticizing reaction time is more than 30 minutes.

[0009] Preferably, in step S3, the mass ratio of SSG polymer to PLA is 1:99 to 30:70, the mixing temperature is 200℃, and the melt blending time is not less than 10 min to ensure that SSG is uniformly dispersed in the PLA matrix.

[0010] Compared with the prior art, the beneficial effects of this invention are:

[0011] (1) The PLA / SSG blend involved in this invention can significantly improve the elongation at break and impact energy absorption of PLA without reducing the ultimate tensile strength, and has broad application prospects.

[0012] (2) The PLA / SSG blend preparation process involved in this invention has the advantages of simple operation, easy implementation, industrialization and low cost;

[0013] (3) The PLA / SSG blend involved in this invention has excellent processing performance and can be widely used in the fields of additive manufacturing and injection molding. Attached Figure Description

[0014] Figure 1 Fourier transform infrared spectra of the high-strength and high-toughness PLA / SSG copolymer, pure PLA, and pure SSG prepared by the modified high-strength and high-toughness polylactic acid according to the present invention.

[0015] Figure 2The image shows the liquid nitrogen brittle fracture morphology of the high-strength and high-toughness PLA / SSG copolymer with an SSG content of 2% according to the preparation method of the modified high-strength and high-toughness polylactic acid of the present invention.

[0016] Figure 3 The figure illustrates the application of the high-strength and high-toughness PLA / SSG copolymer prepared by the modified high-strength and high-toughness polylactic acid according to the present invention in continuous fiber additive manufacturing. Detailed Implementation

[0017] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0018] Description of the raw materials and their sources involved in this invention:

[0019] Boric acid: chemical formula: H3BO3, purchased from Shanghai Maclean Biochemical Co., Ltd.

[0020] Hydroxyl silicone oil: Structural formula: HO[(CH3)2SiO] n H was purchased from Shanghai Maclean Biochemical Co., Ltd.

[0021] Oleic acid: Chemical formula: C 18 H 34 O2, purchased from Shanghai Maclean Biochemical Co., Ltd.

[0022] Polylactic acid: Grade: 4032D, purchased from NatureWorks Ltd., USA

[0023] Reference Figure 1 A method for preparing modified high-strength and high-toughness polylactic acid (PLA) includes: preparing a shear-hardening polymer (SSG) with a dynamic BO crosslinking structure based on polymerization and plasticizing reactions; secondly, melt-blending the polymer with a PLA matrix; and finally, preparing a PLA / SSG blend by injection molding. Compared with pure PLA, the PLA / SSG blend disclosed in this invention exhibits significantly improved elongation at break and impact energy absorption efficiency while almost without reducing the ultimate tensile strength.

[0024] This invention relates to experimental specifications:

[0025] The tensile properties test specifications for high-strength and high-toughness PLA / SSG blends were performed according to GB / T 1040.2-2022. The specimen type was 1BA, injection molded using a micro-injection molding machine. During the test, the loading rate was 1 mm / min, and the sampling frequency was set to 20 Hz. Specimen conditioning was performed according to GB / T 2918-1998, with a temperature of 23±2℃ and a relative humidity of 50%±10%. Each specimen was tested at least five times.

[0026] The impact energy absorption effect of PLA / SSG blends was evaluated using an impact bending test. The specimens were 80 mm × 10 mm × 4 mm in size and were injection molded using a micro-injection molding machine. Specimen conditioning followed GB / T2918-1998, with an experimental temperature of 23 ± 2℃ and a relative humidity of 50% ± 10%. Each specimen was tested at least five times.

[0027] The processing performance of PLA / SSG blends was evaluated by continuous fiber 3D printing. The printing nozzle temperature was 200℃, the printing line width was 1.2mm, the layer height was 0.35mm, the experimental ambient temperature was 23±2℃, and the relative humidity was 50%±10%.

[0028] The SSG polymers with dynamic BO crosslinking structures used in Examples 1-5, i.e., shear-hardening adhesives, were prepared based on the following method:

[0029] Mix 80g of boric acid and 1L of hydroxyl silicone oil and carry out a polymerization reaction at normal pressure and high temperature of 200℃ for more than 2 hours. At the same time, to ensure the reaction is complete, it needs to be stirred once every 20 minutes to obtain reactant A with a large amount of silky luster.

[0030] Oleic acid was added to reactant A at a mass fraction of 1%. The mixture was stirred thoroughly for at least 30 minutes under normal pressure using a Hacker torque rheometer to induce a plasticizing reaction, ultimately yielding the shear-hardening polymer SSG. At this stage, SSG exhibits a significant strain rate hardening effect, displaying a rubbery state under slow loading and rapidly transitioning to a glassy state under rapid stretching.

[0031] Example 1

[0032] Polylactic acid (PLA) granules were continuously dried in a forced-air drying oven at 80°C for 6 hours to ensure complete evaporation of moisture. Then, 9.9g of dried PLA granules and 0.1g of SSG polymer were weighed, mixed evenly, and added to a twin-tapered screw. The mixture was then fully melt-blended at 60 rpm and 200°C for 8 minutes until torque equilibrium was achieved, resulting in a PLA / SSG blend. Subsequently, the twin-tapered screw outlet was connected to a micro-injection molding machine, and the PLA / SSG melt was injected into the mold through an injection gun. To ensure smooth demolding, the mold temperature was set to 35°C, the injection pressure to 0.5 MPa, and the holding time to 30 seconds. After injection molding, the mold was opened, and standard tensile / impact bending test specimens were removed.

[0033] Example 2

[0034] Polylactic acid (PLA) granules were continuously dried in a forced-air drying oven at 80°C for 6 hours to ensure complete evaporation of moisture. Then, 9.8g of dried PLA granules and 0.2g of SSG polymer were weighed, mixed evenly, and added to a twin-tapered screw. The mixture was then fully melt-blended at 60 rpm and 200°C for 8 minutes until torque equilibrium was achieved, yielding a PLA / SSG blend. Subsequently, the twin-tapered screw outlet was connected to a micro-injection molding machine, and the PLA / SSG melt was injected into the mold through an injection gun. To ensure smooth demolding, the mold temperature was set to 35°C, the injection pressure to 0.5 MPa, and the holding time to 30 seconds. After injection molding, the mold was opened, and standard tensile / impact bending test specimens were removed.

[0035] Example 3

[0036] Polylactic acid (PLA) granules were continuously dried in a forced-air drying oven at 80°C for 6 hours to ensure complete evaporation of moisture. Then, 9.5g of dried PLA granules and 0.5g of SSG polymer were weighed, mixed evenly, and added to a twin-tapered screw. The mixture was then fully melt-blended at 60 rpm and 200°C for 8 minutes until torque equilibrium was achieved, resulting in a PLA / SSG blend. Subsequently, the twin-tapered screw outlet was connected to a micro-injection molding machine, and the PLA / SSG melt was injected into the mold through an injection gun. To ensure smooth demolding, the mold temperature was set to 35°C, the injection pressure to 0.5 MPa, and the holding time to 30 seconds. After injection molding, the mold was opened, and standard tensile / impact bending test specimens were removed.

[0037] Example 4

[0038] Polylactic acid (PLA) granules were continuously dried in a forced-air drying oven at 80°C for 6 hours to ensure complete evaporation of moisture. Then, 9g of dried PLA granules and 1g of SSG polymer were weighed, mixed evenly, and added to a twin-tapered screw. The mixture was then fully melt-blended at 60 rpm and 200°C for 8 minutes until torque equilibrium was achieved, resulting in a PLA / SSG blend. Subsequently, the twin-tapered screw outlet was connected to a micro-injection molding machine, and the PLA / SSG melt was injected into the mold through an injection gun. To ensure smooth demolding, the mold temperature was set to 35°C, the injection pressure to 0.5 MPa, and the holding time to 30 seconds. After injection molding, the mold was opened, and standard tensile / notched impact test specimens were removed.

[0039] Example 5

[0040] Polylactic acid (PLA) granules were continuously dried in a forced-air drying oven at 80°C for 6 hours to ensure complete evaporation of moisture. Then, 7g of dried PLA granules and 3g of SSG polymer were weighed, mixed evenly, and added to a twin-tapered screw. The mixture was then fully melt-blended at 60 rpm and 200°C for 8 minutes until torque equilibrium was achieved, yielding a PLA / SSG blend. Subsequently, the twin-tapered screw outlet was connected to a micro-injection molding machine, and the PLA / SSG melt was injected into the mold through an injection gun. To ensure smooth demolding, the mold temperature was set to 35°C, the injection pressure to 0.5 MPa, and the holding time to 30 seconds. After injection molding, the mold was opened, and standard tensile / impact bending test specimens were removed.

[0041] Comparative Example 1

[0042] Polylactic acid (PLA) granules were continuously dried in a forced-air drying oven at 80°C for 6 hours to ensure complete evaporation of moisture. Then, 10g of dried PLA granules were weighed and added to a twin-tapered screw compressor. The mixture was then fully melt-blended at 60 rpm and 200°C for 8 minutes until torque equilibrium was achieved. The melt-blending process was then completed. Subsequently, the twin-tapered screw compressor outlet was connected to a micro-injection molding machine, and the PLA melt was injected into the mold through the injection gun. To ensure smooth demolding, the mold temperature was set to 35°C, the injection pressure to 0.5 MPa, and the holding time to 30 seconds. After injection molding, the mold was opened, and standard tensile / impact bending test specimens were removed.

[0043] The Fourier transform infrared spectra of Comparative Example 1, Examples 2 and 5, and the shear-hardening adhesive are as follows: Figure 1 As shown, it can be observed that with the increase of SSG content (0% to 100%), the infrared spectrum at 1753 cm⁻¹... -1 The peak intensities of the carbonyl stretching vibration peak (C=O), representing PLA content, show a gradually decreasing trend, while those at 787 cm⁻¹ show a decreasing trend. -1The methyl symmetric stretching vibration peak (CH3-Si-CH3), representing the SSG content, shows a gradually increasing trend, indicating that SSG is uniformly dispersed in the PLA matrix. Furthermore, comparing the four absorbance curves, no new absorption peaks were observed, suggesting that no chemical reaction occurred between SSG and PLA during the melt blending process.

[0044] The brittle fracture section of liquid nitrogen in Example 2 is as follows: Figure 2 As shown, the microstructure of the PLA / SSG blend exhibits a typical "island structure," where the circular "islands" represent dispersed submicron SSG particles, and the continuous phase is the PLA matrix. This structure reflects the incompatibility between SSG and PLA, verifying the... Figure 1 The FTIR results show that, after mixing SSG and PLA, an oil-water-like emulsion was formed, and this phase-separated microstructure was maintained after curing. Further measurements indicate that when the SSG / PLA mixing ratio is 98 / 2, the diameter of the dispersed phase (SSG) in the mixture is approximately 470 nm.

[0045] Table 1. Quasi-static tensile and dynamic impact bending test results of Examples 1-5 and Comparative Example 1

[0046]

[0047] As shown in Table 1, when the SSG content is around 2% (Example 2), the static tensile strength of the PLA / SSG blend reaches its maximum. Compared with pure PLA in Comparative Example 1, the tensile strength remains almost unchanged, while the elongation at break increases by nearly 40 times. When the SSG content is around 10% (Example 4), the dynamic impact energy absorption of the PLA / SSG blend reaches its maximum, with an increase of 330%. Furthermore, the study observed that with the increase of SSG content, the elongation at break and impact energy absorption of the PLA / SSG blend both show a trend of increasing and then decreasing. This is because, on the one hand, agglomeration is prone to occur when the SSG volume fraction is high, forming stress concentration zones that induce crack initiation; on the other hand, agglomeration weakens the deflection and energy absorption effect of the interface on cracks, causing cracks to propagate directly in the SSG phase, resulting in premature fracture of the specimen.

[0048] Figure 3 This study showcases some molded products of PLA / SSG blends in continuous fiber 3D printing. The printing process was smooth, and the product quality was satisfactory, demonstrating its excellent machinability.

[0049] In summary, the high-strength and high-toughness PLA / SSG blends involved in this invention can significantly improve the elongation at break (40 times) and impact energy absorption (330%) of PLA without reducing the ultimate tensile strength. Moreover, the preparation process is simple, with excellent processing performance and low cost. By designing the addition of different proportions of shear-hardening adhesive SSG, the tensile strength, elongation at break, and impact energy absorption properties of the blend can be adjusted, thus possessing customization capabilities.

[0050] The number of devices and processing scale described herein are for the purpose of simplifying the description of the invention, and applications, modifications and variations thereof will be apparent to those skilled in the art.

[0051] Although embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. For those skilled in the art, other modifications can be easily made. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details and illustrations shown and described herein.

Claims

1. A method for preparing a modified high-strength and high-toughness polylactic acid, characterized by, The method comprises the following steps: The PLA particles are continuously dried in a blast drying oven at 80°C for 6 hours to ensure sufficient evaporation of moisture; then 9.9 g of dried PLA particles and 0.1 g of SSG polymer are weighed and uniformly mixed, and then fed into a twin-screw extruder, and fully melt-blended at a rotation speed of 60 rpm and a temperature of 200°C for 8 minutes until the torque is balanced, and then the melt-blended product is obtained; then the outlet of the twin-screw extruder is connected to a micro-injection molding machine, and the PLA / SSG melt is injected into a mold through an injection gun; in order to ensure smooth demolding, the temperature of the injection mold is set to 35°C, the injection pressure is 0.5 MPa, and the holding time is 30 seconds; after the injection is completed, the mold is opened and the standard tensile / impact bending test piece is taken out; The PLA is a brand: 4032D; the SSG polymer is prepared based on the following method: 80 g of boric acid and 1 L of hydroxyl silicone oil are mixed, and a polymerization reaction is carried out at normal pressure and high temperature of 200°C for more than 2 hours; in order to ensure sufficient reaction, the mixture needs to be stirred every 20 minutes; thus, a reaction product A with a large amount of silk luster is obtained; 1% of oleic acid is added to the reaction product A in terms of mass fraction, and a plasticizing reaction is carried out by fully stirring the mixture for more than 30 minutes under normal pressure using a HAAKE torque rheometer; finally, a shear-hardening gel polymer SSG is obtained; at this time, the reaction product SSG exhibits obvious strain rate hardening effect, shows a rubbery state under slow loading, and quickly enters a glassy state under rapid stretching.

2. A method for preparing a modified high-strength and high-toughness polylactic acid, characterized by, The method comprises the following steps: The PLA particles are continuously dried in a blast drying oven at 80°C for 6 hours to ensure sufficient evaporation of moisture; then 9.9 g of dried PLA particles and 0.1 g of SSG polymer are weighed and uniformly mixed, and then fed into a twin-screw extruder, and fully melt-blended at a rotation speed of 60 rpm and a temperature of 200°C for 8 minutes until the torque is balanced, and then the melt-blended product is obtained; then the outlet of the twin-screw extruder is connected to a micro-injection molding machine, and the PLA / SSG melt is injected into a mold through an injection gun; in order to ensure smooth demolding, the temperature of the injection mold is set to 35°C, the injection pressure is 0.5 MPa, and the holding time is 30 seconds; after the injection is completed, the mold is opened and the standard tensile / impact bending test piece is taken out; The PLA is a brand: 4032D; the SSG polymer is prepared based on the following method: 80 g of boric acid and 1 L of hydroxyl silicone oil are mixed, and a polymerization reaction is carried out at normal pressure and high temperature of 200°C for more than 2 hours; in order to ensure sufficient reaction, the mixture needs to be stirred every 20 minutes; thus, a reaction product A with a large amount of silk luster is obtained; 1% of oleic acid is added to the reaction product A in terms of mass fraction, and a plasticizing reaction is carried out by fully stirring the mixture for more than 30 minutes under normal pressure using a HAAKE torque rheometer; finally, a shear-hardening gel polymer SSG is obtained; at this time, the reaction product SSG exhibits obvious strain rate hardening effect, shows a rubbery state under slow loading, and quickly enters a glassy state under rapid stretching.

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