High-hydrophobicity flexible starch film based on lignin-metal chelate coating and preparation method of high-hydrophobicity flexible starch film

Abstract

The invention provides a high-hydrophobicity flexible starch film based on a lignin-metal chelate coating and a preparation method of the high-hydrophobicity flexible starch film. The preparation method comprises the following steps: (1) separating lignin from black liquor by adopting a water precipitation method; (2) preparing a starch film solution; (3) preparing a lignin / metal ion chelate; and (4) loading the lignin chelate on the surface by a tape casting method to prepare the hydrophobic flexible starch film. The method comprises the following steps: dissolving formic acid lignin obtained by separation through different solvents, dispersing the formic acid lignin into deionized water for self-assembly, then preparing a chelate with metal ions, casting the chelate to the surface of a starch film solution containing a certain water content for loading, and drying to prepare the high-hydrophobicity flexible starch film. The flexibility and hydrophobicity of the starch film prepared by the method are remarkably improved, the application performance of the starch film is greatly improved, and technical support is provided for simple and green preparation of the high-hydrophobicity flexible starch film and development and application of lignin.

Description

Technical Field

[0001] The present invention relates to the technical field of bio-based new materials, and specifically to a preparation method for a highly hydrophobic flexible starch film by surface coating with a high phenolic hydroxyl group-active lignin-metal ion chelate, which can be applied to fields such as packaging, agricultural film covering, and biomedical materials. Background Art

[0002] Traditional starch films have problems such as low hydrophobicity (contact angle < 90°) and low mechanical strength. In existing technologies, the hydrophobic performance is mostly enhanced by blending with hydrophobic polymers (such as polyurethane), but the improvement amplitude is limited and sometimes the original biodegradability of starch may be sacrificed. Traditional synthetic polymer polyvinyl alcohol and plasticizers (such as glycerol, sorbitol, etc.) can be used to improve the flexibility and plasticity of starch films, but they do not enhance the hydrophobic performance of starch films. Lignin, as a natural polyphenol polymer, has abundant phenolic hydroxyl groups and strong chelating ability, can form a dense hydrophobic network with metal ions, and can achieve high hydrophobicity of starch films by coating lignin-metal ion chelates on the substrate surface.

[0003] However, the application technology of directionally coating lignin / metal ion chelates on the surface of flexible starch films has not been fully developed. Different from the traditional method of enhancing hydrophobicity by dip coating on the fabric surface, starch films containing a large number of hydroxyl functional groups have high hydrophilicity and will damage their own structure when contacting with aqueous lignin / metal ion chelates. And if the starch film is formed by blending the starch solution with lignin / metal ion chelates, it is not easy to improve the flexibility and hydrophobic performance of the starch film at the same time. Therefore, it is of great significance for this field to provide a technology for constructing a rough structure on the surface of starch films through surface loading to improve the hydrophobic performance of starch films. Summary of the Invention

[0004] In view of the above deficiencies of the prior art, the present invention provides a highly hydrophobic flexible starch film based on a lignin-metal chelate coating and a preparation method thereof, including the following steps: (1) separating lignin from black liquor by the water precipitation method; (2) preparing a starch film solution; (3) preparing a lignin / metal ion chelate; (4) preparing a hydrophobic flexible starch film by surface loading of the lignin chelate by the casting method. The present invention dissolves the separated formic acid lignin, forms a chelate with the metal ion ferric, and then loads it onto the surface of a starch solution with a water content of 85 - 89% by the casting method, and prepares a highly hydrophobic flexible starch film after drying. The flexibility and hydrophobic performance of the starch film prepared by this method are significantly improved, greatly improving the application performance of the starch film, providing technical support for the simple and green preparation of highly hydrophobic flexible starch films and the development and application of lignin, and the preparation process is simple and controllable and is conducive to popularization.

[0005] The specific technical solution of the present invention is as follows: A highly hydrophobic flexible starch film based on a lignin-metal chelate coating, comprising a starch film and a lignin / metal ion chelate coated on its surface, wherein the amount of the lignin / metal ion chelate is 1-5% of the absolute dry weight of the starch film; the lignin is lignin formate, and the metal ion is Fe 3+ or Al 3+ , and the mass ratio of lignin to metal ion is 0.25~1:1; the metal ion is derived from one or more of ferric chloride, ferric sulfate, aluminum chloride, and aluminum sulfate.

[0006] The thickness of the obtained flexible starch film after drying is 0.06-0.08 mm, the maximum breaking strength can reach 34.2 MPa, the maximum elongation at break can reach 95%, the maximum initial contact angle can reach 128.09°, and the maximum contact angle can reach 98.24° after 180 s.

[0007] The inventor also provides a preparation method of the above-mentioned highly hydrophobic flexible starch film based on a lignin-metal chelate coating, comprising the following steps: (1) Lignin separation: Lignin with a high content of phenolic hydroxyl groups is separated from formic acid pulping black liquor, and it is redissolved with an organic solvent to obtain a lignin solution; (2) Preparation of starch film: A certain amount of chitosan and glycerol are added to the starch dispersion, and the film is formed in a petri dish by the casting method to obtain a starch film. The thickness range of the obtained starch film is 0.45~0.48 mm, and the moisture content is 85~89%; (3) Preparation of lignin / metal ion chelate; (4) The lignin / metal ion chelate is loaded on the surface of the starch film by the casting method, and a highly hydrophobic flexible starch film is prepared after drying.

[0008] Furthermore, the method for separating lignin formate from formic acid pulping black liquor in step (1) specifically adopts the water precipitation method. The volume ratio of formic acid pulping black liquor to the added deionized water is 6~10:1. The precipitate is obtained by centrifugal separation, and then lignin with a high content of phenolic hydroxyl groups is prepared by the freeze-drying method. Then it is redissolved with an organic solvent, and lignin nanoparticles are prepared by dialysis with deionized water.

[0009] The organic solvent used is selected from one of tetrahydrofuran (THF), acetone (ACE), and 60% ethanol / water (EtOH) system. When redissolving, the dissolution concentration of lignin in the above organic solvent is not limited, with the purpose of completely dissolving lignin. The lignin nanoparticles prepared by this method, after being measured by the Folin-Ciocalteu reagent method (when the lignin concentration is 1 mg / mL during measurement), are found to have a significantly increased content of phenolic hydroxyl groups. The content of phenolic hydroxyl groups after treatment with the tetrahydrofuran system is 6.8 mmol / g, that after treatment with the acetone system is 5.8 mmol / g, and that after treatment with 60% ethanol is 5.6 mmol / g. The increase in the content of phenolic hydroxyl groups enhances the activity of lignin, making it easier to form chelates with metal ions and generate hydrophobic properties. At the same time, it also increases the bonding strength of hydrogen bonds formed between lignin and starch molecules through hydroxyl groups, making the combination more compact. After chelating with iron ions and then measuring, it is found that the content of phenolic hydroxyl groups in lignin dissolved in the tetrahydrofuran system is 0.64 mmol / g, that dissolved in acetone is 0.74 mmol / g, and that dissolved in 60% ethanol system is 0.79 mmol / g. After chelating with aluminum ions and then measuring, it is found that the content of phenolic hydroxyl groups in lignin dissolved in the tetrahydrofuran system is 0.65 mmol / g, that dissolved in acetone is 0.76 mmol / g, and that dissolved in 60% ethanol system is 0.81 mmol / g. This shows that the chelation effect of lignin with iron is the best under the tetrahydrofuran system, and the content of phenolic hydroxyl groups after chelation is the lowest. At the same time, it shows that the self-assembly behavior of lignin is inconsistent under different solvents, which will also affect the chelation of lignin with iron ions to a certain extent. Similarly, the chelation of the same lignin with aluminum ions is also affected by the lignin solvent dispersion system. The self-assembly behavior of lignin dissolved in different solvents is different, and the content of phenolic hydroxyl groups on the surface is different, which affects the chelation with aluminum ions. Therefore, it will have an impact on chelation. However, the technical solution of this application can still select aluminum ions as an alternative technical ion.

[0010] Comparing with the content of phenolic hydroxyl groups in formic acid lignin obtained by the ordinary method, it is found that the content range of phenolic hydroxyl groups in the patent (CN116949810A) and the article (Preparation and hydrophobic property research of formic acid lignin nanoparticles) is between 1.11 - 2.81 mmol / g, which is much lower than the content of phenolic hydroxyl groups obtained by the water precipitation method described in this application. The inventor further analyzed that the degradation products of hemicellulose are soluble in water, and at the same time, lignin can be precipitated and separated by centrifugation. Compared with the method of evaporating and concentrating lignin, the water precipitation method reduces the temperature during lignin separation, which can reduce the condensation of lignin to a certain extent and protect the free phenolic hydroxyl groups of lignin.

[0011] Further, the starch used in step (2) can be selected from cereal starches such as corn starch, potato starch, wheat starch, or starches from other sources; in this application, corn starch is further preferably used, and the starch concentration in the starch dispersion is 3 wt%. The starch dispersion is gelatinized at 90°C for half an hour, and then a chitosan solution (concentration 1 wt%) accounting for 3.5 - 10% of the dry starch mass and 5 - 10% glycerol are added. More preferably, a chitosan solution (concentration 1 wt%) accounting for 5% of the dry starch mass and 10% glycerol are used.

[0012] Since both chitosan and glycerol contain hydrophilic hydroxyl groups, while ensuring the flexibility of the starch film is improved, the hydrophilic property of the starch film can be enhanced, and within the above addition range, the cost of the final starch film can be further reduced. At the same time, a tight biopolymer network structure can be formed between chitosan and starch, so that after lignin is added, through the hydrogen bond action between the tight hydroxyl groups, the number of exposed hydrophilic hydroxyl groups is reduced, improving the hydrophobic property of the starch film. The addition of glycerol can endow the starch film with flexibility, improve the elongation at break of the starch film, and enhance the application performance of the starch film.

[0013] In the lignin / metal ion chelate in step (3), the metal ion is Fe 3+ or Al 3+ ; the metal ion is derived from corresponding soluble salts such as ferric chloride, ferric sulfate, aluminum chloride, aluminum sulfate, etc.

[0014] During preparation, first, the metal ion is prepared into an aqueous metal ion solution, and then mixed according to the mass ratio of formic acid lignin to metal ion of 1:1, heated and reacted at 90°C for 1 - 3 h to fully chelate. After the reaction, ultrasonic dispersion is carried out for 30 s, and finally a dispersion of the chelate is obtained, and the concentration of the final chelate dispersion is controlled at 0.20 - 0.25% (w / w) by dilution or concentration.

[0015] In step (4), the lignin / metal ion chelate is loaded on the surface of the starch film by the casting method, so that the loading amount of the lignin / metal ion chelate accounts for 1 - 5% (w / w) of the dry starch amount in the starch film. During the casting process, the corresponding amount of the lignin chelate dispersion is loaded on the surface of the starch film to uniformly cover it. Air-dry at room temperature to obtain a hydrophobic flexible starch film with a thickness of 0.06 - 0.08 mm.

[0016] Finally, a starch film with a uniform surface rough structure is obtained, making the starch film have uniform hydrophobicity, flexibility, and mechanical properties. After testing, its maximum tensile strength can reach 34.2 MPa, the maximum elongation at break can reach 95%, the maximum initial contact angle can reach 128.09°, and the maximum contact angle after 180 s can reach 98.24°.

[0017] In summary, the beneficial effects of adopting the technical solution of the present invention are as follows: When the formic acid lignin / metal ion chelate dispersion flows on the surface of the starch film, the water content of the starch film is 85-89%. After the dispersion flows, it will not flow back and forth or flow to the bottom of the petri dish. Thus, it is ensured that after drying, the formic acid lignin / metal ion chelate can be evenly dispersed on the surface of the starch film, forming a uniform rough structure, enhancing the hydrophobic property of the starch film while retaining the flexibility of the starch film. Description of the Drawings

[0018] Figure 1 It is a dynamic contact angle diagram of starch films coated with different dosages of lignin / metal ion chelates. Among them, the vertical coordinate "Dynamic contact angle" is the dynamic contact angle; CS-CHI-GI is the blank starch film, with an initial contact angle of 100° and a contact angle of 53.9° at the end of 180 s; -n (n = 1, 2, 3, 4) is the addition amount of the chelate relative to the absolutely dry starch film, corresponding to Examples 1-4 respectively. Detailed Embodiments

[0019] The following will further describe the implementation scheme of the present invention through examples. It is necessary to point out that the following examples are only used to further illustrate the present invention, and the implementation manner of the present invention is not limited thereto. Some non-essential adjustments and improvements made by those familiar with the relevant fields according to the above invention scheme still fall within the protection scope of the present invention.

[0020] The formic acid pulping black liquor described in the following examples is derived from the existing technology of poplar formic acid pulping black liquor.

[0021] Example 1 A preparation method of a highly hydrophobic and flexible starch film based on a lignin-ferric metal ion chelate coating, the specific steps are as follows: (1) Separation of lignin: The method for separating formic acid lignin from formic acid pulping black liquor adopts the water precipitation method: the volume ratio of formic acid black liquor to added deionized water is 1:10. After centrifugal separation (centrifugal speed 3000 rpm, centrifugal time 10 min), a precipitate is obtained, and lignin with a high phenolic hydroxyl content is prepared by the freeze-drying method (freeze-drying time 48 h, temperature -60 °C).

[0022] (2) Preparation of starch solution: Corn starch is dispersed in water with a concentration of 3 wt%. It is gelatinized at 90 °C for half an hour, and then a chitosan solution (1 wt%) accounting for 5% of the mass of the absolutely dry starch film and 10% of glycerol are added. The starch film solution is poured into a petri dish to form a film, so that the thickness of the starch film is 0.45-0.48 mm and the water content is 85-89%.

[0023] (3) Preparation of lignin / metal ion chelate: Before chelation, the solvent systems used for dissolving formic acid lignin to prepare nanoparticles were tetrahydrofuran (THF), acetone (ACE), and 60% ethanol / water (EtOH) system respectively. First, metal ions were prepared into an aqueous metal ion solution, and then mixed according to the mass ratio of formic acid lignin to metal ions of 1:1, and heated at 90 °C for 1 - 3 h for sufficient chelation. After the reaction, ultrasonic dispersion was carried out for 30 s, and finally a dispersion of the chelate was obtained, and the concentration of the final chelate dispersion was controlled at 0.21% (w / w) by dilution or concentration; the metal ion Fe 3+ was derived from soluble iron salts such as ferric chloride or ferric sulfate.

[0024] (4) Loading the chelate on the surface of the starch film by the casting method: The addition amount of the lignin / metal ion chelate was 1% of the mass of the dry starch film, and it was loaded onto the surface of the starch film in step (1) by the casting method. After air-drying at room temperature, a flexible starch film was prepared, and the thickness of the starch film was 0.06 - 0.08 mm.

[0025] (5) Using an optical contact angle measuring instrument to test the water contact angle on the surface of the starch film. The test times were selected as 0 s, 30 s, 60 s, 90 s, 120 s, 150 s, and 180 s, and the corresponding contact angles were measured respectively. The average value of the three tests was used as the test result, and other properties of the flexible starch film were also tested. The results are as Figure 1 shown in a. Generally speaking, the hydrophobic properties of the starch films cast by the lignin casting method obtained from the three solvent dispersion systems are not very different.

[0026] FAL in the figure THF / FeⅢ-1, FAL ACE / FeⅢ-1 and FAL EtOH / FeⅢ-1 respectively represent the chelation of lignin dispersed with metal ion FeⅢ under different dissolution systems (THF: tetrahydrofuran; ACE: acetone; EtOH: 60% ethanol / water system), and -n (n = 1, 2, 3, 4) is the addition amount of the chelate relative to the dry starch film, corresponding to Examples 1 - 4 respectively.

[0027] At the addition amount of 1% of the mass of the dry starch film for the lignin / metal ion chelate, among the three films of FAL THF / FeⅢ-1, FAL ACE / FeⅢ-1 and FAL EtOH / FeⅢ-1, the one with the best contact angle performance is FAL EtOH / FeⅢ-1, and the ones with the best strength and flexibility are FAL THF / FeⅢ-1. The test data of the flexible starch films are shown in Table 1: the maximum breaking strength of the three films can reach 8.2 MPa, the maximum elongation at break can reach 95%, the maximum initial contact angle can reach 119.8°, and the maximum contact angle after 180 s can reach 72.92°.

[0028] Table 1 Breaking strength, elongation at break, initial contact angle and contact angle at 180 s of starch films Sample Breaking strength (MPa) Elongation at break (%) Initial contact angle (°) Contact angle after 180 s (°) <![CDATA[FAL THF / FeⅢ-1]]> 8.2 95 107.65 72.92 <![CDATA[FAL THF / FeⅢ-2]]> 18 32 113.74 67.5 <![CDATA[FAL THF / FeⅢ-3]]> 23 17 120.2 96.65 <![CDATA[FAL THF / FeⅢ-4]]> 22.5 4.6 121.34 98.24 <![CDATA[FAL ACE / FeⅢ-1]]> 6.7 35 116.41 70.36 <![CDATA[FAL ACE / FeⅢ-2]]> 3 14 122.26 65.95 <![CDATA[FAL ACE / FeⅢ-3]]> 16 8 111.78 84.28 <![CDATA[FAL ACE / FeⅢ-4]]> 34 2 102.94 81.66 <![CDATA[FAL EtOH / FeⅢ-1]]> 5.1 36 119.8 71.11 <![CDATA[FAL EtOH / FeⅢ-2]]> 6.7 43 128.09 78.95 <![CDATA[FAL EtOH / FeⅢ-3]]> 14 4 104.46 63.15 <![CDATA[FAL EtOH / FeⅢ-4]]> 28 6.8 106.17 76.22 CS-CHI-GI 8.2 77.6 100.03 53.5 。

[0029] Comparative Example 1 Referring to Example 1, a control sample CS-CHI-GI was prepared with the same addition amounts of CS (starch), CHI (chitosan) and GI (glycerol). The difference was that steps (3) and (4) were not carried out, that is, the surface loading of the lignin / metal ion chelate was not performed, and it was prepared by air drying under the same room temperature conditions.

[0030] As can be seen from Table 1, compared with the control blank film CS-CHI-GI, after adding 1% of the lignin / metal ion chelate, the contact angles of the obtained starch films were significantly improved. As Figure 1 shown in a, at 180 s, although the contact angles of the treated starch films decreased, the differences between them were small, but they were all significantly higher than the contact angle of the blank film.

[0031] Example 2

[0032] Implemented according to Example 1, the difference was that in step (2), the addition amount of the chitosan solution (1 wt% concentration) accounted for 5% of the mass of the dry starch film, and the addition amount of glycerol was 5%; in step (4), the addition amount of the lignin / metal iron ion chelate was 2 wt% of the dry starch film.

[0033] As can be seen from Table 1, compared with the comparative example, after adding 2% of the lignin / metal ion chelate, the contact angles of the obtained starch films were significantly improved. As Figure 1 shown in b, the surface contact angle of the starch film loaded with FAL EtOH / FeⅢ-2 was the highest, greater than that of FAL THF / FeⅢ-2 and FAL ACE / FeⅢ-2. At 180 s, the contact angle of the starch film with the lignin chelate surface loaded decreased significantly, and the contact angle of the starch film with the lignin chelate surface loaded dissolved by tetrahydrofuran and acetone was lower than that of the starch film with the lignin surface loaded dissolved in the 60% ethanol system. This may be because the particles of lignin dispersed after separation in the 60% ethanol system were smaller, and larger roughness could be formed at a lower dosage, so the effect of improving the hydrophobic angle was better than the other two systems.

[0034] Example 3

[0035] The method is implemented in accordance with Example 1, except that in step (2), the amount of chitosan solution (1 wt% concentration) added is 7.5% of the mass of the absolute dry starch film, and the amount of glycerol added is 10%; and in step (4), the amount of lignin / metal iron ion chelate added is 3 wt% of the absolute dry starch film.

[0036] As can be seen from the table, compared with the comparative example, after adding 3% of lignin / metal ion chelate, the contact angle of the obtained starch film was significantly improved. Figure 1 As shown in c, at 30s, the contact angle of the starch film loaded on the surface of the chelate formed by lignin in the 60% ethanol system decreased significantly, while the contact angle of the starch film loaded on the surface of the chelate formed by lignin dissolved in tetrahydrofuran and acetone decreased relatively slowly. Among them, the water contact angle of the starch film after the cast of tetrahydrofuran dissolved lignin was still over 110° at 180s. As can be seen from Table 1, the surface cast FAL THF The contact angle of the starch film obtained after separation of / FeⅢ-3 was significantly higher than the hydrophobic angle of the starch film loaded with lignin obtained by the other two systems. This may be because the particles formed after the lignin separated by tetrahydrofuran is dispersed are larger, and the chelate surface layer formed after the dosage is increased has better roughness at this dosage.

[0037] Example 4

[0038] The method is implemented in accordance with Example 1, except that in step (2), the amount of chitosan solution (1 wt% concentration) added is 10% of the mass of the absolute dry starch film, and the amount of glycerol added is 10%; and in step (4), the amount of lignin / metal iron ion chelate added is 4 wt% of the absolute dry starch film.

[0039] As shown in Table 1, compared with the comparative example, after adding 4% of lignin / metal ion chelate, the contact angle of the obtained starch film was significantly improved. Figure 1 As shown in Figure d, at 30s, the contact angle of the starch film loaded with lignin chelate by the casting method tended to decrease slowly, especially the contact angle of the starch film coated with lignin dissolved in tetrahydrofuran and acetone, and the contact angle of the starch film coated with lignin dissolved in 60% ethanol system decreased most significantly. Among them, the water contact angle of the starch film coated with lignin dissolved in tetrahydrofuran was still over 110° at 180s. Table 1 shows that the surface casting FAL THF The contact angle of the starch film obtained after / FeⅢ-4 separation was significantly higher than the hydrophobic angle of the starch film loaded with lignin obtained by the other two systems. This is because after the lignin obtained by the tetrahydrofuran separation system chelates with metal ions, the surface roughness of the starch film is greater after surface casting loading at this dosage, which is more conducive to improving the hydrophobicity of the starch film.

[0040] Preparation Method of Highly Hydrophobic Flexible Starch Film Based on Lignin-Metal Aluminum Ion Chelate Coating (1)Separation of lignin: The method for separating formic acid lignin from formic acid pulping black liquor adopts the water precipitation method. The volume ratio of formic acid black liquor to added deionized water is 1:10. After centrifugal separation (centrifugal speed 3000 rpm, centrifugal time 10 min), precipitation is obtained, and lignin with a high content of phenolic hydroxyl groups is prepared by freeze-drying method (freeze-drying time 48 h, temperature -60 °C).

[0041] (2)Preparation of starch solution: Corn starch is dispersed in water at a concentration of 3 wt%. It is gelatinized at 90 °C for half an hour, and then a chitosan solution (1 wt% concentration) accounting for 5% of the mass of the absolute dry starch film and 10% glycerol are added. The starch film solution is poured into a petri dish and air-dried to form a film, making the thickness of the starch film 0.45 - 0.48 mm and the water content 85 - 89%.

[0042] (3)Preparation of lignin / metal ion chelate: Before chelation, the solvent systems used for dissolving formic acid lignin to prepare nanoparticles are tetrahydrofuran (THF), acetone (ACE), and 60% ethanol / water (EtOH) system respectively.

[0043] First, metal ions are prepared into an aqueous metal ion solution, and then they are mixed according to the mass ratio of formic acid lignin to metal ions of 1:1, heated and reacted at 90 °C for 1 h to fully chelate. After the reaction, ultrasonic dispersion is carried out for 30 s, and finally a dispersion of the chelate is obtained, and the concentration of the final dispersion of the chelate is controlled at 0.21% (w / w) by dilution or concentration; the metal ion Al 3+ is derived from soluble aluminum salts such as aluminum chloride or aluminum sulfate.

[0044] (4)Coating of chelate on the surface of starch film by casting method: The addition amount of lignin / metal ion chelate is 1% of the mass of the absolute dry starch film, and it is loaded onto the surface of the starch film by the casting method. After air-drying at room temperature, a flexible starch film is prepared, and the thickness of the starch film is 0.06 - 0.08 mm.

[0045] (5)Use an optical contact angle measuring instrument to test the water contact angle on the surface of the starch film. Select the test times as 0 s, 30 s, 60 s, 90 s, 120 s, 150 s, and 180 s, and measure their corresponding contact angles respectively. Take the average value of three tests as the test result, and at the same time test other properties of the flexible starch film. The results are shown in Table 2. Table 2 shows the breaking strength, elongation at break, initial contact angle, and contact angle at 180 s of the starch film obtained after loading lignin / Al III chelate; -n (n = 1, 2, 3, 4) in the table is the addition amount of the chelate relative to the absolute dry starch film, corresponding to Examples 5 - 8 respectively.

[0046] Among them, FAL THF / Al Ⅲ-1, FAL ACE / Al Ⅲ-1 and FAL EtOH / Al Ⅲ-1 respectively represent the chelation of lignin dispersed with metal ion Al Ⅲ under different dissolution systems (THF: tetrahydrofuran; ACE: acetone; EtOH: 60% ethanol / water system), and the addition amount is 1% of the mass of the dry starch film. Its maximum breaking strength can reach 8 MPa, the maximum elongation at break can reach 93%, the maximum initial contact angle can reach 118.49°, and the contact angle can reach 73.19° after 180 s.

[0047] Table 2 Breaking strength, elongation at break, initial contact angle and contact angle at 180 s of starch films Sample Breaking strength (MPa) Elongation at break (%) Initial contact angle (°) Contact angle after 180 s (°) <![CDATA[FAL THF / Al Ⅲ-1]]> 8.0 93 108.69 72.89 <![CDATA[FAL THF / Al Ⅲ-2]]> 18.3 12 114.78 68.59 <![CDATA[FAL THF / Al Ⅲ-3]]> 22.7 17.8 121.62 97.58 <![CDATA[FAL THF / Al Ⅲ-4]]> 22.1 4.8 122.74 97.43 <![CDATA[FAL ACE / Al Ⅲ-1]]> 6.8 36 118.49 71.63 <![CDATA[FAL ACE / Al Ⅲ-2]]> 3.1 15 121.61 66.36 <![CDATA[FAL ACE / Al Ⅲ-3]]> 16.5 8.3 112.38 85.86 <![CDATA[FAL ACE / Al Ⅲ-4]]> 34.2 2.2 101.94 87.86 <![CDATA[FAL EtOH / Al Ⅲ-1]]> 5.3 35.9 118.38 73.19 <![CDATA[FAL EtOH / Al Ⅲ-2]]> 6.9 43.1 127.19 79.67 <![CDATA[FAL EtOH / Al Ⅲ-3]]> 13.9 4.3 105.66 64.65 <![CDATA[FAL EtOH / Al Ⅲ-4]]> 28.1 6.9 107.57 77.32 CS-CHI-GI 8.2 77.6 100.03 53.5 As can be seen from Table 2, compared with the CS-CHI-GI film of Comparative Example 1, after adding 1% of the lignin / metal ion chelate, the contact angles of the obtained starch films are all significantly improved. At 180 s, the contact angle of the treated starch film decreases significantly, but it is significantly improved compared with the blank sample.

[0048] Example 6 Implemented according to Example 5, the difference is that in step (2), the addition amount of the chitosan solution (1 wt% concentration) accounts for 5% of the dry starch film, and the addition amount of glycerol is 5 wt%; in step (4), the addition amount of the lignin / metal aluminum ion chelate is 2 wt% of the dry starch film.

[0049] The thickness of the starch film is 0.06 - 0.08 mm, the maximum breaking strength can reach 18.3 MPa, and the maximum elongation at break can reach 43.1%. As shown in Table 2, the maximum initial contact angle can reach 127.19°, and the contact angle can reach 79.67° after 180 s. Compared with the control CS-CHI-GI film, after adding 2% of the lignin / metal ion chelate, the contact angles of the obtained starch films are all significantly improved. At 180 s, the contact angle of the treated starch film decreases significantly, but it is significantly improved compared with the blank sample.

[0050] Example 7

[0051] Implemented according to Example 5, the difference is that in step (2), the addition amount of the chitosan solution (1 wt% concentration) accounts for 7.5% of the dry starch film, and the addition amount of glycerol is 10 wt%; in step (4), the addition amount of the lignin / metal aluminum ion chelate is 3 wt% of the dry starch film.

[0052] The thickness of the starch film is 0.06 - 0.08 mm, the maximum breaking strength can reach 22.7 MPa, and the maximum elongation at break can reach 17.8%. As shown in Table 2, the maximum initial contact angle can reach 121.62°, and the maximum contact angle can reach 97.58° after 180 s. Compared with the control sample CS-CHI-GI, after adding 3% of the lignin / metal ion chelate, the contact angles of the obtained starch films are all significantly improved. At 180 s, the contact angle of the treated starch film decreases significantly, but it is significantly improved compared with the blank sample.

[0053] Example 8

[0054] Implemented according to Example 5, the difference is that the addition amount of the chitosan solution (1wt% concentration) in step (2) accounts for 10% of the dry starch film, and the addition amount of glycerol is 10wt%; in step (4), the addition amount of the lignin / metal aluminum ion chelate is 4wt% of the dry starch film.

[0055] After air-drying at room temperature, a flexible starch film is prepared. The thickness of the starch film is 0.06 - 0.08 mm, the maximum breaking strength can reach 34.2 MPa, and the maximum elongation at break can reach 6.9%. As can be seen from Table 2, compared with the control sample CS-CHI-GI, after adding 4% of the lignin / metal ion chelate, the initial contact angles of the obtained starch films are all significantly improved. At 180 s, the contact angle of the treated starch film decreases significantly, but it is significantly improved compared with the blank sample.

[0056] The above embodiments are the preferred embodiments of the present invention, mainly showing and describing the main features and basic principles of the present invention. However, the implementation embodiments of the present invention are not limited by the above embodiments. Without departing from the spirit and scope of the present invention, any modifications, changes, substitutions, combinations, and simplifications made by those skilled in the relevant fields and technologies should be regarded as within the scope of the present invention.

Claims

1. A highly hydrophobic flexible starch film based on a lignin-metal chelate coating, comprising a starch film and a lignin / metal ion chelate coated on its surface, characterized in that, Wherein the dosage of the lignin / metal ion chelate is 1-5% of the absolute dry weight of the starch film; the lignin is formic acid lignin, and the metal ion is Fe 3+ or Al 3+ , and the mass ratio of lignin to metal ion is 0.25-1:1; the metal ion is derived from one or more of ferric chloride, ferric sulfate, aluminum chloride, and aluminum sulfate.

2. The highly hydrophobic flexible starch film based on a lignin-metal chelate coating according to claim 1, wherein The thickness of the dried flexible starch film is 0.06 - 0.08 mm, the highest breaking strength can reach 34.2 MPa, the highest elongation at break can reach 95%, the highest initial contact angle can reach 128.09°, and the highest contact angle after 180 s can reach 98.24°.

3. The preparation method of the highly hydrophobic flexible starch film based on the lignin-metal chelate coating according to claim 1, characterized in that, It includes the following steps: (1) Lignin separation: Lignin with a high content of phenolic hydroxyl groups is separated from formic acid pulping black liquor, and it is redissolved with an organic solvent to obtain a lignin solution; (2) Preparation of starch film: A certain amount of chitosan and glycerol are added to the starch dispersion, and the film is formed in a petri dish by the casting method to obtain a starch film. The thickness range of the obtained starch film is 0.45 - 0.48 mm, and the water content is 85 - 89%; (3) Preparation of lignin / metal ion chelate; (4) The lignin / metal ion chelate is loaded on the surface of the starch film by the casting method, and a highly hydrophobic flexible starch film is prepared after drying.

4. The preparation method of the highly hydrophobic flexible starch film based on the lignin-metal chelate coating according to claim 3, characterized in that, In step (1), the method for separating formic acid lignin from formic acid pulping black liquor specifically adopts the water precipitation method. The volume ratio of formic acid pulping black liquor to the added deionized water is 6 - 10:

1. The precipitate is obtained by centrifugal separation, and then lignin with a high content of phenolic hydroxyl groups is prepared by the freeze-drying method; then it is redissolved with an organic solvent, and lignin nanoparticles are prepared by dialysis with deionized water; the organic solvent used is selected from one of tetrahydrofuran, acetone, and 60% ethanol / water system.

5. The preparation method of the highly hydrophobic flexible starch film based on the lignin-metal chelate coating according to claim 3, characterized in that, The starch used in step (2) is selected from corn starch, potato starch, wheat starch or starch from other sources; the starch concentration in the starch dispersion is 3 wt%; the starch dispersion is gelatinized at 90 °C for half an hour, and then a 1 wt% chitosan solution accounting for 3.5 - 10% of the absolute dry starch mass and 5 - 10% of glycerol are added.

6. The preparation method of the highly hydrophobic flexible starch film based on the lignin-metal chelate coating according to claim 5, characterized in that, The starch used in step (2) is selected from corn starch. After the starch dispersion is gelatinized, a 1 wt% chitosan solution accounting for 5% of the absolute dry starch mass and 10% of glycerol are added.

7. The preparation method of the highly hydrophobic flexible starch film based on the lignin-metal chelate coating according to claim 3, characterized in that, In the lignin / metal ion chelate in step (3), the metal ion is Fe 3+ or Al 3+ ; The metal ion is derived from corresponding soluble salts such as ferric chloride, ferric sulfate, aluminum chloride, aluminum sulfate, etc.

8. The preparation method of the highly hydrophobic flexible starch film based on the lignin-metal chelate coating according to claim 7, characterized in that, When preparing the lignin / metal ion chelate in step (3), first, the metal ion is prepared into an aqueous metal ion solution, and then it is mixed according to the mass ratio of formic acid lignin to metal ion of 1:1, and heated and reacted at 90 °C for 1 - 3 h to fully chelate. After the reaction, it is ultrasonically dispersed for 30 s, and finally, a dispersion of the chelate is obtained, and the concentration of the final dispersion of the chelate is controlled at 0.20 - 0.25 wt% by dilution or concentration.

9. The preparation method of the highly hydrophobic flexible starch film based on the lignin-metal chelate coating according to claim 7, characterized in that, In step (4), the lignin / metal ion chelate is loaded on the surface of the starch film by the casting method, and the loading amount of the lignin / metal ion chelate accounts for 1 - 5% of the weight of the absolute dry starch in the starch film; during the casting process, the corresponding amount of the lignin chelate dispersion is loaded on the surface of the starch film to uniformly cover it. Air-dry at room temperature to obtain a hydrophobic flexible starch film with a thickness of 0.06 - 0.08 mm.

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