Preparation method of lignin-silica reinforced composite membrane and composite membrane

Abstract

The present invention discloses a preparation method of a lignin-silica reinforced composite film and the composite film. In the present invention, maleic anhydride-grafted modified lignin is used as a bio-based toughening agent, surface-treated silica nanoparticles are used as nano-fillers, and zirconium ions (Zr 4+ ) are used as cross-linking agents. By virtue of the high valence state and strong coordination ability of zirconium ions, multi-tooth coordination bonds can be formed simultaneously with the carboxyl groups (-COOH) of maleic anhydride-modified lignin and the hydroxyl groups (-OH) on the surface of nano-silica, constructing a three-dimensional dynamic cross-linking network, significantly enhancing its synergistic toughening effect, and greatly improving the mechanical properties of the composite film, solving the problems of poor dispersion and stability of existing silica fillers during dispersion in the matrix through surface modification and grafting, and limited performance improvement, etc.

Description

Technical Field

[0001] The present invention belongs to the technical field of polymer materials, and more specifically, relates to a preparation method of a lignin-silica reinforced composite film and the composite film. Background Art

[0002] In recent years, with the increasing prominence of environmental problems and people's attention to sustainable development, the research and application of bio-based degradable materials have received more and more attention. As an important bio-based degradable polymer, PLA has good biocompatibility, biodegradability and processing properties, and shows great application potential in the fields of packaging, agriculture, medical treatment, etc. However, the mechanical properties of PLA, especially its brittleness and low impact strength, limit its application in more fields.

[0003] Currently, the main methods to enhance the performance of PLA include adding toughening agents, fillers, etc. Commonly used fillers include inorganic nanomaterials, cellulose, lignin, etc. The uniform dispersion of fillers can effectively improve the strength, stiffness and toughness of polymers. At the same time, good dispersion helps to evenly distribute stress and reduce local stress concentration. On the premise that the addition amount of fillers in the polymer matrix is limited, how to improve the dispersion of fillers is an important factor in improving the performance of polymers. Currently, the surface of fillers is often modified by chemical or physical methods to improve the compatibility with polymers, thereby improving the dispersion of fillers in the matrix. However, the modification process is complex, costly, and the modification effect is affected by the types of fillers and modifiers. In addition, when using multiple fillers to enhance together, only the synergistic effect between the fillers and the polymer matrix is often considered, ignoring the mutual synergistic effect between different types of fillers, resulting in limited enhancement effect of the fillers.

[0004] In the preparation method of a reaction blending toughened polylactic acid starch composite in CN117511155B, components such as modified silica, cellulose ether, toughening agent, etc. are used to compound with polylactic acid and starch to obtain a composite material. The lignin organic molecular structure is introduced into the modified silica, which improves the dispersion and compatibility of silica in polymer materials. At the same time, synergistic effects can be generated between multiple components to further improve the product performance, thereby endowing the reaction blending toughened polylactic acid starch composite with better mechanical properties and stability. This patent uses lignin to improve the dispersion and compatibility of silica, solves the problems of agglomeration and uneven particles of silica in the polymer matrix, thereby improving the performance of the polymer. However, the dispersion of silica is limited by the dispersion of lignin and also by the functional group density, and there are still disadvantages of insufficient dispersion degree and poor stability. Moreover, lignin and silica are connected by covalent bonds of carboxyl and hydroxyl groups. Although the interfacial binding force is enhanced, the bond energy is high and irreversible, which may lead to a lack of energy dissipation pathways when the material is stressed, and the performance improvement is limited. Summary of the Invention

[0005] The present invention provides a lignin-silica reinforced composite film and a preparation method thereof to overcome the problems of poor dispersion and stability of existing silica fillers during dispersion in a matrix through surface modification and grafting, as well as limited performance improvement.

[0006] The present invention is achieved through the following technical solutions:

[0007] A preparation method of a lignin-silica reinforced composite film, the steps including:

[0008] S1. Adding maleic anhydride-modified lignin and coupling agent-treated silica nanoparticles with a mass ratio of 1-2:1 to a solvent, mixing and dispersing evenly.

[0009] S2. Adding a zirconium isopropoxide Zr(OCH2CH2CH3)4 solution to the dispersion liquid in S1, reacting under a protective gas atmosphere to obtain a pre-crosslinked modified lignin / SiO2 mixture, wherein the addition amount of the zirconium alkoxide is 4.5-5 times the weight of the maleic anhydride-modified lignin;

[0010] S3. Adding the pre-crosslinked modified lignin / SiO2 mixture to a polymer matrix, mixing evenly, and then co-extrusion casting to obtain a composite film.

[0011] Further, the preparation steps of the maleic anhydride-modified lignin include:

[0012] Dispersing lignin powder in a solvent, adding maleic anhydride, reacting under a protective gas atmosphere, and washing and drying to obtain maleic anhydride-modified lignin.

[0013] Further, the addition amount of the maleic anhydride is 50-80% of the mass of the lignin.

[0014] Further, the temperature of the reaction is 25-35 °C, and the time is 1-2.5 h.

[0015] Further, the polymer matrix includes one or more of polylactic acid, polycaprolactone, polyhydroxyalkanoate, polybutylene succinate, and thermoplastic starch.

[0016] Further, the content of the pre-crosslinked modified lignin / SiO2 mixture in the composite film is 15-25 wt%.

[0017] Further, the composite film is also added with additives such as a plasticizer and an antioxidant.

[0018] A composite film, including the lignin-silica reinforced composite film prepared by the above method.

[0019] Further, the composite film includes a hydrophilic layer, a reinforcing layer, and a substrate layer, wherein the hydrophilic layer is a PLA film containing 5-8 wt% of a hydrophilic modifier, the reinforcing layer is a lignin-silica reinforcing composite film, and the substrate layer is paper.

[0020] Further, the thickness of the hydrophilic layer is 10-15 μm, and the thickness of the reinforcing layer is 20-30 μm.

[0021] Further, the hydrophilic modifier is polyvinyl alcohol (PVA).

[0022] Compared with the prior art, the beneficial effects are as follows:

[0023] In the present invention, lignin and silica are used to synergistically enhance the performance of the polymer matrix. Lignin plays a major role in energy dissipation, while nano-silica improves strength and restricts crack propagation. Zirconium ion crosslinking synergistically enhances the synergistic effect of lignin and silica, thereby significantly improving the toughness of the composite film. Specifically, zirconium ions have a high valence state and strong coordination ability, and can simultaneously form multidentate coordination bonds with the carboxyl groups (-COOH) of maleic anhydride-modified lignin and the hydroxyl groups (-OH) on the surface of nano-silica, constructing a three-dimensional dynamic crosslinking network. This network structure has the following characteristics:

[0024] (1) Reversibility: The coordination bonds of Zr 4+ can be broken and recombined under stress, and the toughness of the material is significantly improved through the energy dissipation mechanism.

[0025] (2) High-density crosslinking: The strong coordination ability of Zr 4+ results in a crosslinking point density much higher than that of traditional covalent bond grafting, enhancing the interfacial binding force.

[0026] After the reaction of maleic anhydride with lignin in the present invention, a large number of polar groups such as carboxyl groups (-COOH) are introduced into the lignin molecular chain. These polar groups can interact with the ester groups (-COO-) on the polymer molecular chain, such as through hydrogen bonding, thereby significantly improving the interfacial compatibility between PLA and modified lignin. The enhanced compatibility reduces stress concentration at the interface and improves the overall strength and toughness of the material. The present invention uses a coupling agent to improve the interfacial compatibility between nano-silica and the PLA matrix, prevent nano-silica agglomeration, and make it uniformly dispersed in the matrix. The uniformly dispersed nano-silica can act as a reinforcing phase to improve the strength and stiffness of the film. Specific Embodiments

[0027] The following is further explained and illustrated with reference to embodiments, but the specific embodiments do not limit the present invention in any form. Unless otherwise specified, the methods and equipment used in the embodiments are conventional methods and equipment in the art, and the raw materials used are all commercially available conventional raw materials.

[0028] In the preparation processes of the following examples and comparative examples, the average molecular weight of the PLA used is 20,000; the rotation speed of the high-speed mixer is 800 rpm, and the mixing time is 15 min; the screw rotation speed of the twin-screw extruder is 200 pm, and the extrusion temperatures in zones 1-12 are 120, 170, 180, 185, 180, 160, 175, 185, 175, 180, 160, 175 °C respectively, the head temperature is 165 °C, the screw rotation speed is 200 rpm, and the feeding rotation speed is 8 rpm. Among them, vacuum degassing systems are equipped in zones 6 and 11.

[0029] Example 1

[0030] This example provides a method for preparing a lignin-silica reinforced composite film, and the steps include:

[0031] S1. Prepare surface-treated silica nanoparticles;

[0032] Add 20 wt% of silica nanoparticles with a particle size of 10-50 nm to 80 wt% of absolute ethanol, and ultrasonically disperse for 30 minutes to prepare a nanoparticle dispersion; then dissolve 10 wt% of KH-550 silane coupling agent in 90 wt% of absolute ethanol to prepare a coupling agent solution; slowly add the coupling agent solution to the nanoparticle dispersion, stir with a magnetic stirrer at room temperature for 60 minutes, and then heat in a water bath to 80 °C and stir for 120 minutes at a rotation speed of 500 rpm; after the reaction is completed, cool the mixture to room temperature, filter, wash with absolute ethanol 3 times, and then vacuum dry at 60 °C for 12 hours to obtain surface-treated silica nanoparticles.

[0033] S2. Prepare maleic anhydride-modified lignin;

[0034] Add 15 wt% of lignin powder to 85 wt% of N,N-dimethylacetamide (DMAc), stir with a magnetic stirrer at 300 rpm at room temperature for 2 hours until completely dispersed; then add maleic anhydride with a mass ratio of 50% of lignin, and react at 130 °C under nitrogen protection for 4 hours. After the reaction is completed, cool the mixture to room temperature, filter, wash repeatedly with deionized water until neutral (pH = 7), and then vacuum dry at 60 °C for 24 hours to obtain maleic anhydride-modified lignin.

[0035] S3. Prepare a pre-crosslinked modified lignin / SiO2 mixture;

[0036] 15 parts of maleic anhydride modified lignin and 8 parts of 10 - 50 nm silica nanoparticles after surface treatment with silane coupling agent were added to DMAc. After ultrasonic dispersion for 30 minutes, a solution of zirconium isopropoxide (Zr(OCH2CH2CH3)4) 4.5 times the amount of maleic anhydride modified lignin was added. At room temperature, under nitrogen protection, it was stirred with a magnetic stirrer at a speed of 300 rpm for 2 hours. After the reaction, the mixture was dried in vacuo at 40 °C for 24 hours to obtain a pre - crosslinked modified lignin / SiO2 enhancer mixture.

[0037] S4. Prepare the reinforced composite material;

[0038] 77.8 parts of PLA, 20 parts of the pre - crosslinked modified lignin / SiO2 mixture, 2 parts of triethyl citrate and 0.2 part of antioxidant 1010 were mixed and extruded to obtain a masterbatch of the reinforced composite material.

[0039] S5. Prepare the multi - layer composite film;

[0040] The PLA hydrophilic layer masterbatch containing 5% PVA and the masterbatch of the reinforced composite material were respectively fed into a multi - layer co - extrusion casting machine. The extrusion temperature was 210 °C, the die head temperature was 200 °C, the cooling roll temperature was 25 °C, and the drawing speed was 100 m / min to prepare a double - layer composite film. The double - layer composite film was laminated with a paper substrate to obtain a multi - layer composite film, where the paper thickness was 200 μm, the hydrophilic layer thickness was 15 μm, and the reinforced layer thickness was 29 μm.

[0041] Weighed 5 grams of lignin before modification and lignin after modification respectively, added them into 100 ml of different solvents (methanol, ethanol, acetone, toluene, n - hexane), stirred at room temperature for 24 hours, and observed the dissolution situation. The results are shown in Table 1 below:

[0042] Table 1

[0043]

[0044] It can be seen from the solubility test that the solubility of the modified lignin in polar solvents (methanol, ethanol, acetone) is higher than that of the lignin before modification, while the solubility in non - polar solvents (toluene, n - hexane) changes little or even slightly decreases. This is because the grafting rate of maleic anhydride is relatively high, and the introduced carboxyl groups increase the polarity of lignin, resulting in an increase in its solubility in polar solvents and a slight decrease in its solubility in non - polar solvents.

[0045] The prepared pre - crosslinked modified lignin / SiO2 enhancer mixture was characterized by XPS, and the data are shown in Table 2 below:

[0046] Table 2

[0047]

[0048] Example 2

[0049] The process of this example is basically the same as that of Example 1, except that in the pre-crosslinked modified lignin / SiO2 mixture, there are 10 parts of maleic anhydride modified lignin, 8 parts of surface-treated silica nanoparticles, and the addition amount of zirconium isopropoxide (Zr(OCH2CH2CH3)4) solution is 5 times that of maleic anhydride modified lignin.

[0050] Example 3

[0051] The process of this example is basically the same as that of Example 1, except that in the pre-crosslinked modified lignin / SiO2 mixture, there are 20 parts of maleic anhydride modified lignin, 10 parts of surface-treated silica nanoparticles, and the addition amount of zirconium isopropoxide (Zr(OCH2CH2CH3)4) solution is 4.5 times that of maleic anhydride modified lignin.

[0052] Example 4

[0053] The process of this example is basically the same as that of Example 1, except that in the pre-crosslinked modified lignin / SiO2 mixture, there are 10 parts of maleic anhydride modified lignin, 5 parts of surface-treated silica nanoparticles, and the addition amount of zirconium isopropoxide (Zr(OCH2CH2CH3)4) solution is 5 times that of maleic anhydride modified lignin.

[0054] Comparative Example 1

[0055] The process of this comparative example is basically the same as that of Example 1, except that the raw materials of the reinforced composite film include 77.8 parts of PLA, 20 parts of surface-treated silica nanoparticles, 2 parts of triethyl citrate, and 0.2 part of antioxidant 1010, which are mixed, extruded, and cast to prepare the reinforced composite film.

[0056] Comparative Example 2

[0057] The process of this comparative example is basically the same as that of Example 1, except that the raw materials of the reinforced composite film include 77.8 parts of PLA, 20 parts of maleic anhydride modified lignin, 2 parts of triethyl citrate, and 0.2 part of antioxidant 1010, which are mixed.

[0058] Comparative Example 3

[0059] The process of this comparative example is basically the same as that of Example 1, except that the raw materials of the reinforced composite film include 77.8 parts of PLA, 11 parts of maleic anhydride modified lignin, 9 parts of surface-treated silica nanoparticles, 2 parts of triethyl citrate, and 0.2 part of antioxidant 1010, which are mixed.

[0060] Comparative Example 4

[0061] The process of this comparative example is basically the same as that of Example 1, except that the raw materials of the enhanced composite film include a mixture of 77.8 parts of PLA, 20 parts of PBAT, 2 parts of triethyl citrate and 0.2 part of antioxidant 1010.

[0062] Comparative Example 5

[0063] The process of this comparative example is basically the same as that of Example 1, except that in the pre-crosslinked modified lignin / SiO2 mixture, there are 15 parts of maleic anhydride modified lignin, 8 parts of surface-treated silica nanoparticles, and the addition amount of zirconium isopropoxide (Zr(OCH2CH2CH3)4) solution is 4 times that of maleic anhydride modified lignin.

[0064] Comparative Example 6

[0065] The process of this comparative example is basically the same as that of Example 1, except that in the pre-crosslinked modified lignin / SiO2 mixture, there are 15 parts of maleic anhydride modified lignin, 8 parts of surface-treated silica nanoparticles, and the addition amount of zirconium isopropoxide (Zr(OCH2CH2CH3)4) solution is 6 times that of maleic anhydride modified lignin.

[0066] Comparative Example 7

[0067] In this comparative example, 77.8 parts of PLA, 20 parts of modified lignin / SiO2 mixture, 2 parts of triethyl citrate and 0.2 part of antioxidant 1010 are mixed and extruded to obtain the enhanced composite material. The modified lignin / SiO2 mixture consists of 10 parts of maleic anhydride modified lignin and 8 parts of surface-treated silica nanoparticles. No zirconium isopropoxide (Zr(OCH2CH2CH3)4) solution is added, and the modified lignin and silica are only mixed and then directly dried.

[0068] The enhanced composite material masterbatch in Example 1 is made into a single-layer enhanced composite film by extrusion casting. Using pure PLA as the control group, the performance tests of the multi-layer composite films prepared in Example 1 and the comparative examples are carried out respectively. Among them:

[0069] Tensile strength, elongation at break: GB / T 1040.3-2006 (Test method for tensile properties of plastics)

[0070] Impact strength: GB / T 1043-2008 (Test method for impact properties of plastics)

[0071] Tear strength: GB / T 1044-2008 (Test method for tear strength of plastics)

[0072] Water absorption rate: GB / T 1033-2008 (Test method for water absorption of plastics)

[0073] The test results are shown in Table 3 below:

[0074] Table 3

[0075]

[0076] As can be seen from Table 1 above, compared with using only lignin or silica as fillers, the present invention uses lignin and silica to synergistically enhance the properties of the polymer matrix. Also, from the performance comparison among pure PLA, Comparative Example 7, and Example 1, it can be seen that lignin and SiO2 can significantly improve the performance without adding zirconium isopropoxide (Zr(OCH2CH2CH3)4) solution. Moreover, after dynamically crosslinking lignin and SiO2 with zirconium isopropoxide (Zr(OCH2CH2CH3)4) solution, the strength and toughness are further improved.

[0077] Obviously, the above-mentioned embodiments of the present invention are merely examples for clearly illustrating the present invention, rather than limitations on the implementation manners of the present invention. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all implementation manners here. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention shall be included within the protection scope of the claims of the present invention.

Claims

1. A method for preparing a lignin-silica reinforced composite membrane, characterized in that the steps Comprising: S1. Add maleic anhydride-modified lignin and silica nanoparticles treated with a coupling agent with a mass ratio of 1-2:1 to a solvent, mix well, and disperse evenly. S2. Add zirconium isopropoxide solution to the dispersion in S1. Under the atmosphere of a protective gas, stir and react at 25-35 °C for 1-2.5 h, and obtain a pre-crosslinked modified lignin / SiO2 mixture after drying, wherein the addition amount of zirconium isopropoxide is 4.5-5 times the weight of maleic anhydride-modified lignin. S3. Add the pre-crosslinked modified lignin / SiO2 mixture to the polymer matrix, mix well, and prepare a composite membrane. The polymer matrix includes one or more of polylactic acid, polycaprolactone, polyhydroxyalkanoate, and polybutylene succinate.

2. The preparation method of the lignin-silica reinforced composite membrane according to claim 1, characterized in that The preparation steps of the maleic anhydride-modified lignin include: Disperse lignin powder in a solvent, add maleic anhydride, react under the atmosphere of a protective gas, and obtain maleic anhydride-modified lignin after washing and drying.

3. The preparation method of the lignin-silica reinforced composite film according to claim 2, characterized in that, The addition amount of the maleic anhydride is 50-80% of the mass of lignin.

4. The preparation method of the lignin-silica reinforced composite membrane according to claim 1, characterized in that, The content of the pre-crosslinked modified lignin / SiO2 mixture in the composite membrane is 15-25 wt%.

5. A composite film, characterized in that, Comprising the lignin-silica reinforced composite membrane prepared by the method according to any one of claims 1-4.

6. The composite film according to claim 5, wherein Comprising a hydrophilic layer, a reinforcing layer, and a substrate layer, wherein the hydrophilic layer is a PLA membrane containing 5-8 wt% of a hydrophilic modifier, the reinforcing layer is a lignin-silica reinforced composite membrane, and the substrate layer is paper.

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