A method for dispersing polylactic acid stereocomplex fibers and its application in a polylactic acid matrix

Abstract

The present invention discloses a method for dispersing polylactic acid stereocomplex fibers, comprising the following steps: (1) obtaining a polylactic acid fiber mat by electrospinning L-lactic acid and D-lactic acid; (2) subjecting the polylactic acid fiber mat to heat treatment to obtain a polylactic acid stereocomplex fiber mat; (3) placing the polylactic acid stereocomplex fiber mat in a mixed solvent of chloroform and petroleum ether and stirring to disperse it, thereby obtaining polylactic acid stereocomplex fibers. The present invention also discloses an application of the polylactic acid stereocomplex fibers in a polylactic acid matrix. By utilizing the solvent resistance of the polylactic acid stereocomplex crystals, the conventional electrospun polylactic acid fiber mat is dispersed into single fibers, enabling them to be uniformly dispersed in the polylactic acid matrix; by utilizing the difference in melting temperature between the polylactic acid stereocomplex crystals and the homogeneous crystals, a polylactic acid-based composite material is prepared through the polylactic acid stereocomplex fibers, avoiding the problem of poor interfacial compatibility and providing a relatively wide hot processing window for the polylactic acid-based fiber reinforced composite material.

Description

Technical Field

[0001] The present invention relates to the technical field of composite fibers, in particular to a dispersion method of polylactic acid stereocomplex fibers and their application in a polylactic acid matrix. Background Art

[0002] With the in-depth concept of sustainable development, it has become an inevitable trend to replace petroleum-based plastics with bio-based plastics. Polylactic acid has become one of the widely used bio-based plastics due to its renewable nature, relatively low production cost, and good mechanical strength. Fiber-reinforced polylactic acid lightweight materials can be used in the fields of automotive interiors, sports industry, electronic appliances, etc. due to their high strength-to-weight ratio. However, traditional fiber fillers as reinforcing phases often have problems with poor interfacial compatibility with polylactic acid. In response to this problem, scholars at home and abroad have proposed using polylactic acid to make reinforcing fibers, and by adjusting the differences in optical purity, molecular weight, and crystallinity between the polylactic acid fiber filler and the polylactic acid matrix, it is possible to achieve the composite and processing molding with polylactic acid while maintaining the fiber structure and properties. Based on this, broadening the thermal processing window of this polylactic acid-based fiber self-reinforced composite material and reducing the processing difficulty have become key issues.

[0003] The monomer lactic acid molecule of polylactic acid has optical activity due to its chiral structure, which enables polylactic acid to be divided into L-polylactic acid, D-polylactic acid, and racemic polylactic acid. Among them, L-polylactic acid and D-polylactic acid can be highly crystalline, and their melting temperatures are in the range of 170 - 180°C. When L-polylactic acid and D-polylactic acid are mixed, stereocomplex crystal forms and their respective homogeneous crystal forms can be generated. The stereocomplex crystal has solvent resistance and higher thermal stability, its melting temperature can reach about 230°C, and the stereocomplex crystal has an inducing effect on the generation of the polylactic acid homogeneous crystal.

[0004] Electrospinning technology can process polymers into nanofibers with diameters in the nanometer range. It is one of the simplest and most cost-effective methods for producing nanofibers currently, and has scalability for large-scale production. The nanoscale size makes electrospun fibers have higher fiber mechanical strength compared with traditional microfibers. On the one hand, as the fiber diameter decreases, the degree of defects on the fiber is lower. On the other hand, it has a larger specific surface area, resulting in a larger interfacial area between the fiber and the polymer matrix, improving the interfacial interaction between the fiber and the matrix. In addition, it has been confirmed that through electrospinning technology, the molecular chains of L-polylactic acid and D-polylactic acid can be pre-aligned, which enables polylactic acid to almost completely form stereocomplex crystal forms without homogeneous crystal forms during subsequent cold crystallization treatment, resulting in polylactic acid nanofibers with a melting temperature as high as 230 °C. These undoubtedly endow electrospun polylactic acid nanofibers with greater application potential. However, electrospun nanofibers can usually only be collected as non-woven mats with three-dimensional connectivity, making it difficult to disperse them in the polymer matrix, undoubtedly limiting the application potential of electrospun nanofibers. Summary of the Invention

[0005] The present invention provides a method for dispersing stereocomplex fibers of polylactic acid and its application in a polylactic acid matrix to solve the above technical deficiencies, enabling the stereocomplex fibers of polylactic acid to be dispersed in the polymer matrix.

[0006] The present invention discloses a method for dispersing stereocomplex fibers of polylactic acid, comprising the following steps:

[0007] (1) Electrospinning L-polylactic acid and D-polylactic acid to obtain a polylactic acid fiber mat;

[0008] (2) Heat-treating the polylactic acid fiber mat to obtain a stereocomplex fiber mat of polylactic acid;

[0009] (3) Placing the stereocomplex fiber mat of polylactic acid in a mixed solvent of chloroform and petroleum ether and stirring to disperse it to obtain stereocomplex fibers of polylactic acid.

[0010] The volume ratio of chloroform to petroleum ether in the mixed solvent is 1:1.5 - 2.5.

[0011] In the step (1), the intrinsic viscosities of L-polylactic acid and D-polylactic acid are both 1.5 - 2.2 dl / g.

[0012] In the step (1), the amounts of L-polylactic acid and D-polylactic acid used are equal.

[0013] In the step (2), the heat treatment temperature is 90 - 130 °C, and the duration is 3 - 5 h.

[0014] In step (3), the stirring and dispersion time is 15 - 36 h, and the stirring speed is 1000 - 1800 rpm.

[0015] In step (3), the mass percentage of the polylactic acid stereocomplex fiber mat in the mixed solvent is less than or equal to 10%.

[0016] An application of polylactic acid stereocomplex fiber in a polylactic acid matrix. Polylactic acid particles and a polylactic acid stereocomplex fiber mat are placed in a mixed solvent of chloroform and petroleum ether and stirred. Then, the precipitate is collected by centrifugation to obtain a blank material with polylactic acid stereocomplex fiber dispersed in the polylactic acid matrix. The blank material is hot-pressed at 200 °C and 10 Mpa to obtain a fiber-reinforced polylactic acid composite material.

[0017] Among them, the volume ratio of chloroform to petroleum ether in the mixed solvent is 1:1.5 - 2.5, the mass percentage of polylactic acid particles in the mixed solvent is less than or equal to 10%, and the mass percentage of the polylactic acid stereocomplex fiber mat in the mixed solvent is less than or equal to 1.5%.

[0018] A dispersion method of a polylactic acid stereocomplex fiber and its application in a polylactic acid matrix obtained by the present invention has the beneficial effects as follows:

[0019] Utilize the solvent resistance of the polylactic acid stereocomplex crystal to disperse the conventional polylactic acid electrospun fiber mat into single fibers, enabling it to be evenly dispersed in the polylactic acid matrix. At the same time, utilize the melting temperature difference between the polylactic acid stereocomplex crystal and the homogeneous crystal to prepare a polylactic acid-based composite material through the polylactic acid stereocomplex fiber, avoiding the problem of poor interfacial compatibility and providing a relatively wide hot processing window for the polylactic acid-based fiber-reinforced composite material. Description of the Drawings

[0020] Figure 1 It is the SEM photograph of the polylactic acid stereocomplex fiber mat of Example 1 of the present invention;

[0021] Figure 2 It is the heating curve collected by a differential scanning calorimeter at 10 °C / min for the polylactic acid stereocomplex fiber mat of Example 1 of the present invention;

[0022] Figure 3 It is the SEM photograph of the polylactic acid stereocomplex fiber after dispersion in the mixed solvent in Example 1 of the present invention;

[0023] Figure 4 It is the SEM photograph of the polylactic acid stereocomplex fiber after dispersion in the solvent in Comparative Example 1 of the present invention;

[0024] Figure 5 It is the SEM photograph of the polylactic acid stereocomplex fiber after dispersion in the solvent in Comparative Example 2 of the present invention;

[0025] Figure 6 SEM photograph of the polylactic acid stereocomplex fiber of Comparative Example 3 of the present invention after being dispersed in a solvent;

[0026] Figure 7 SEM photograph of the dispersion of fibers in the fiber-reinforced polylactic acid composite material of Example 2 of the present invention;

[0027] Figure 8 Stress-strain statistical graph measured for the fiber-reinforced polylactic acid composite material of Example 2 of the present invention in the tensile mode of a universal material testing machine. Detailed Description of the Invention

[0028] To further elaborate on the technical means and effects adopted by the present invention to achieve the predetermined invention purpose, the following, in conjunction with the accompanying drawings and preferred embodiments, details the specific implementation manners, structures, features, and effects according to the present invention as follows.

[0029] Example 1:

[0030] The present invention discloses a method for dispersing polylactic acid stereocomplex fibers, comprising the following steps:

[0031] (1) Dissolve 0.5 g of L-polylactic acid (intrinsic viscosity 1.7 dl / g) and 0.5 g of D-polylactic acid (intrinsic viscosity 1.8 dl / g) in 10 mL of hexafluoroisopropanol (at a mass percentage of 10%), and stir at room temperature until the solution is clear and transparent to obtain a spinning precursor solution;

[0032] (2) Place the spinning precursor solution in (1) in an electrospinning device, generate polylactic acid as-spun fibers through the action of a high-voltage electrostatic field and the rapid evaporation of hexafluoroisopropanol, and collect the polylactic acid as-spun fibers into a polylactic acid fiber mat through a cylindrical collector;

[0033] (3) Heat-treat (anneal) the polylactic acid fiber mat in (2) at 100 °C for 4 h, and simultaneously evaporate the residual hexafluoroisopropanol to obtain a polylactic acid stereocomplex fiber mat;

[0034] (4) Place the polylactic acid stereocomplex fiber mat obtained in (3) in a mixed solvent with a volume ratio of chloroform to petroleum ether of 4:6 at a mass percentage of 2%, and stir vigorously at 1500 rpm at room temperature for 20 h to disperse the fibers, collect the precipitate by centrifugation, and obtain polylactic acid stereocomplex fibers after drying.

[0035] As Figure 1 shown, it can be seen that the fiber size is about 250 nm; the heating curve of the polylactic acid stereocomplex fiber mat collected by a differential scanning calorimeter is as Figure 2As shown, it can be seen that the melting temperature of the fiber is 228 °C, which is about 60 °C higher than that of the homogeneous crystal of polylactic acid; the morphology of the dispersed polylactic acid stereocomplex fiber is as Figure 3 shown, and it can be seen that the structure of the fiber is maintained.

[0036] Comparative Example 1:

[0037] The difference in the dispersion method of this comparative example from that of Example 1 lies in: different mixed solvents.

[0038] Specifically, the mixed solvent of this comparative example is toluene and petroleum ether with a volume ratio of 4:6, and other steps are the same as those in Example 1 to obtain polylactic acid stereocomplex fibers.

[0039] As Figure 4 shown, the structure of the dispersed polylactic acid stereocomplex fiber is severely damaged.

[0040] Comparative Example 2:

[0041] The difference in the dispersion method of this comparative example from that of Example 1 lies in: different volume ratios of the mixed solvent.

[0042] Specifically, the mixed solvent of this comparative example is chloroform and petroleum ether with a volume ratio of 8:2, and other steps are the same as those in Example 1 to obtain polylactic acid stereocomplex fibers.

[0043] As Figure 5 shown, the structure of the dispersed polylactic acid stereocomplex fiber is severely damaged.

[0044] Comparative Example 3:

[0045] The difference in the dispersion method of this comparative example from that of Example 1 lies in: the mixed solvent is replaced with a single chloroform, and other steps are the same as those in Example 1 to obtain polylactic acid stereocomplex fibers.

[0046] As Figure 6 shown, the structure of the dispersed polylactic acid stereocomplex fiber is severely damaged.

[0047] Comparative Example 4:

[0048] The difference in the dispersion method of this comparative example from that of Example 1 lies in: the mixed solvent is replaced with a single petroleum ether, and other steps are the same as those in Example 1 to obtain polylactic acid stereocomplex fibers.

[0049] The test results show that the polylactic acid stereocomplex fiber cannot be dispersed in petroleum ether.

[0050] Example 2:

[0051] The application of the polylactic acid stereocomplex fiber in this example includes the following steps:

[0052] (1) 0.5 g of L-polylactic acid (intrinsic viscosity 1.7 dl / g) and 0.5 g of D-polylactic acid (intrinsic viscosity 1.8 dl / g) were dissolved in 10 mL of hexafluoroisopropanol (10% by mass), and stirred at room temperature until the solution became clear and transparent to obtain a spinning precursor solution.

[0053] (2) The spinning precursor solution in (1) is placed in an electrospinning device to produce polylactic acid pre-spun fibers through the action of a high-voltage electrostatic field and rapid volatilization of hexafluoroisopropanol, and the polylactic acid pre-spun fibers are collected into a polylactic acid fiber mat through a cylindrical collector.

[0054] (3) The polylactic acid fiber mat in (2) was heat treated (annealed) at 100° C. for 4 h, and residual hexafluoroisopropanol was evaporated to obtain a polylactic acid stereocomposite fiber mat.

[0055] (4) At room temperature, commercial polylactic acid particles with a relative solvent mass percentage of 10% and the polylactic acid stereocomposite fiber mat obtained in (3) with a mass percentage of 0.3% are placed in a mixed solvent of chloroform and petroleum ether in a volume ratio of 4:6, and stirred at 1500 rpm for 20 hours to fully mix the fibers with the polylactic acid matrix in advance. The precipitate is collected by centrifugation, dried, and then hot-pressed at 200°C and 10 MPa to obtain a fiber-reinforced polylactic acid composite material.

[0056] The dispersion of the polylactic acid stereocomposite fibers in the fiber-reinforced polylactic acid composite material obtained in this embodiment is as follows: Figure 7 As shown in Figure 2, it can be seen that the fibers are evenly dispersed in the polylactic acid matrix; the stress-strain statistics are shown in Figure 2. Figure 8 As shown, it can be seen that compared with pure polylactic acid, both stress and strain are improved, indicating that the toughness is improved.

[0057] Embodiment 3:

[0058] The method for dispersing the polylactic acid stereocomposite fibers of this embodiment comprises the following steps:

[0059] (1) 0.5 g of L-polylactic acid (intrinsic viscosity 1.7 dl / g) and 0.5 g of D-polylactic acid (intrinsic viscosity 1.8 dl / g) were dissolved in 10 mL of hexafluoroisopropanol (10% by mass), and stirred at room temperature until the solution became clear and transparent to obtain a spinning precursor solution.

[0060] (2) The spinning precursor solution in (1) is placed in an electrospinning device to produce polylactic acid pre-spun fibers through the action of a high-voltage electrostatic field and rapid volatilization of hexafluoroisopropanol, and the polylactic acid pre-spun fibers are collected into a polylactic acid fiber mat through a cylindrical collector.

[0061] (3) Anneal the polylactic acid fiber mat in (2) at 130 °C for 4 h, and at the same time evaporate the residual hexafluoroisopropanol to obtain a polylactic acid stereocomplex fiber mat.

[0062] (4) At room temperature, place 10% by mass of commercial polylactic acid particles and the polylactic acid stereocomplex fiber mat obtained in (3) in a mixed solvent with a volume ratio of chloroform to petroleum ether of 4:6 at a mass percentage of 0.3%, and stir at 1500 rpm for 20 h to allow the fibers to be fully pre-mixed with the polylactic acid matrix. Collect the precipitate by centrifugation, and after drying, hot press it into a polylactic acid self-reinforced composite material.

[0063] Example 4:

[0064] The method for dispersing the polylactic acid stereocomplex fiber in this example includes the following steps:

[0065] (1) Dissolve 0.5 g of L-polylactic acid (intrinsic viscosity 1.7 dl / g) and 0.5 g of D-polylactic acid (intrinsic viscosity 1.8 dl / g) in 10 mL of hexafluoroisopropanol (at a mass percentage of 10%), and stir at room temperature until the solution is clear and transparent to obtain a spinning precursor solution.

[0066] (2) Place the spinning precursor solution in (1) in an electrospinning device, generate polylactic acid as-spun fibers through the action of a high-voltage electrostatic field and the rapid volatilization of hexafluoroisopropanol, and collect the polylactic acid as-spun fibers into a polylactic acid fiber mat through a cylindrical collector.

[0067] (3) Anneal the polylactic acid fiber mat in (2) at 130 °C for 4 h, and at the same time evaporate the residual hexafluoroisopropanol to obtain a polylactic acid stereocomplex fiber mat.

[0068] (4) Place the polylactic acid stereocomplex fiber mat obtained in (3) in a mixed solvent with a volume ratio of chloroform to petroleum ether of 3:7 at a mass percentage of 1%, and stir at 1500 rpm at room temperature for 28 h to disperse the fibers. Collect the precipitate by centrifugation. After drying, polylactic acid stereocomplex fibers are obtained.

[0069] Example 5:

[0070] The method for dispersing the polylactic acid stereocomplex fiber in this example includes the following steps:

[0071] (1) Dissolve 0.5 g of L-polylactic acid (intrinsic viscosity 1.7 dl / g) and 0.5 g of D-polylactic acid (intrinsic viscosity 1.8 dl / g) in 10 mL of hexafluoroisopropanol (at a mass percentage of 10%), and stir at room temperature until the solution is clear and transparent to obtain a spinning precursor solution.

[0072] (2) Place the spinning precursor solution in (1) into an electrospinning device. Through the action of a high-voltage electrostatic field and the rapid evaporation of hexafluoroisopropanol, primary polylactic acid fibers are produced, and the primary polylactic acid fibers are collected into a polylactic acid fiber mat by a cylindrical collector.

[0073] (3) Anneal the polylactic acid fiber mat in (2) at 130 °C for 4 h, and at the same time evaporate the residual hexafluoroisopropanol to obtain a polylactic acid stereocomplex fiber mat.

[0074] (4) At room temperature, place 10% by mass of commercial polylactic acid particles and the polylactic acid stereocomplex fiber mat obtained in (3) in a mixed solvent with a volume ratio of chloroform to petroleum ether of 3:7.5 at a mass percentage of 0.3%, and stir at 1500 rpm for 28 h to allow the fibers and the polylactic acid matrix to be fully pre-mixed in advance. Collect the precipitate by centrifugation, and after drying, hot press it into a fiber-reinforced polylactic acid composite material.

[0075] As described above, it is only a preferred embodiment of the present invention, and there is no limitation in any form to the present invention. Although the present invention has been disclosed as above with a preferred embodiment, it is not intended to limit the present invention. Any person skilled in the art, within the scope of the technical solution of the present invention, can make some changes or modifications to equivalent embodiments by using the above-disclosed technical content. However, as long as it does not depart from the content of the technical solution of the present invention, any simplified modification, equivalent change and modification made to the above embodiments based on the technical essence of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. Application of a polylactic acid stereocomplex fiber in a polylactic acid matrix, characterized in that, Dispersion of polylactic acid stereocomplex fibers includes the following steps: (1) Electrospinning L-polylactic acid and D-polylactic acid to obtain a polylactic acid fiber mat; (2) Heat-treating the polylactic acid fiber mat to obtain a polylactic acid stereocomplex fiber mat; (3) Placing the polylactic acid stereocomplex fiber mat in a mixed solvent 1 of chloroform and petroleum ether and stirring to disperse, obtaining polylactic acid stereocomplex fibers; Placing polylactic acid particles and the polylactic acid stereocomplex fiber mat in a mixed solvent 2 of chloroform and petroleum ether and stirring, then centrifuging to collect the precipitate, obtaining a blank with polylactic acid stereocomplex fibers dispersed in a polylactic acid matrix, and hot-pressing and molding the blank at 200 °C and 10 Mpa to obtain a fiber-reinforced polylactic acid composite material.

2. The application of a polylactic acid stereocomplex fiber according to claim 1 in a polylactic acid matrix, characterized in that, The volume ratio of chloroform to petroleum ether in the mixed solvent 2 is 1:1.5 - 2.5, the mass percentage content of polylactic acid particles in the mixed solvent is less than or equal to 10%, and the mass percentage content of the polylactic acid stereocomplex fiber mat in the mixed solvent is less than or equal to 1.5%.

3. The application of a polylactic acid stereocomplex fiber according to claim 1 in a polylactic acid matrix, characterized in that, The volume ratio of chloroform to petroleum ether in the mixed solvent 1 is 1:1.5 - 2.

5.

4. Use of a polylactic acid stereocomplex fiber according to claim 1 in a polylactic acid matrix, characterized in that, In the step (1), the intrinsic viscosities of L-polylactic acid and D-polylactic acid are both 1.5 - 2.2 dl / g.

5. Use of a polylactic acid stereocomplex fiber according to claim 1 in a polylactic acid matrix, characterized in that, In the step (1), the dosages of L-polylactic acid and D-polylactic acid are equal.

6. The application of a polylactic acid stereocomplex fiber according to claim 1 in a polylactic acid matrix, characterized in that, In the step (2), the heat-treatment temperature is 90 - 130 °C and the duration is 3 - 5 h.

7. Use of a polylactic acid stereocomplex fiber according to claim 1 in a polylactic acid matrix, characterized in that, In the step (3), the stirring and dispersing duration is 15 - 36 h and the stirring speed is 1000 - 1800 rpm.

8. Use of a polylactic acid stereocomplex fiber according to claim 1 in a polylactic acid matrix, characterized in that, In the step (3), the mass percentage content of the polylactic acid stereocomplex fiber mat in the mixed solvent is less than or equal to 10%.

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