A preparation method of a cross-linked composite oil-absorbing film based on multi-stage surface-modified halloysite nanotubes

Abstract

The invention discloses a preparation method of a cross-linked composite oil-absorbing film based on multi-stage surface-modified halloysite nanotubes, comprising the steps of: modifying the halloysite nanotubes with an aminosilane coupling agent to obtain amino-modified halloysite nanotubes; utilizing a substitution reaction between an amino group and a chlorine atom to graft 3-chloropropylene glycol onto the surface of the halloysite nanotubes to obtain hydroxyl-rich halloysite nanotubes; utilizing a ring-opening reaction between a hydroxyl group and an epoxy group to covalently hydrophobically modify the hydroxyl-rich halloysite nanotubes with a glycidyl ether of a long alkyl chain and generate new hydroxyl groups to obtain functionalized hydrophobic halloysite nanotubes; then adding the halloysite nanotubes to a mixed system of polyvinylidene fluoride and dialdehyde polyethylene glycol, applying the film by blade coating, and immersing the film in an acid solution to promote a cross-linking reaction between the hydroxyl group and the dialdehyde polyethylene glycol aldehyde group, washing, and drying to obtain a cross-linked composite oil-absorbing film. The present invention improves the durability and hydrophobic and oleophilic properties of the halloysite nanotube-based composite film.

Description

Technical Field

[0001] The invention relates to the technical field of polyvinyl alcohol composite films, in particular to a preparation method of a cross-linked composite oil-absorbing film based on multi-stage surface-modified halloysite nanotubes. Background Art

[0002] Halloysite nanotubes (HNTs), due to their unique hollow tubular structure, high aspect ratio, and surface modifiability, are often used as functional fillers in polymer membranes for oil-water separation. However, the inherent hydrophilicity of natural HNT surfaces limits their direct application, requiring hydrophobic modification through the use of low-surface-energy materials.

[0003] Current hydrophobic treatment methods are mostly limited to coating the surface with hydrophobic materials, a physical modification that often faces the problem of the hydrophobic layer easily falling off. Alternatively, hydrophobic modifiers are covalently grafted onto the HNT surface. However, the insufficient hydroxyl density on the surface of natural HNTs results in a low grafting efficiency, making it difficult to achieve the desired hydrophobic strengthening effect. More critically, due to the limited number of active sites on the HNT surface, the interfacial interaction with the polymer matrix is ​​weak. Traditional physical blending processes make it difficult to achieve uniform dispersion of HNTs in the matrix, resulting in localized stress concentrations in the composite membrane, which can easily lead to structural collapse during long-term use, severely limiting the practical durability of HNT-based composite membrane materials. Summary of the Invention

[0004] In response to the above-mentioned defects in the prior art, the present invention provides a method for preparing a cross-linked composite oil-absorbing membrane based on multi-stage surface-modified halloysite nanotubes, with the aim of improving the covalent hydrophobic modification effect of the halloysite nanotube surface and its interfacial interaction with the polymer matrix, thereby enhancing the oil-water separation effect of the composite membrane and solving the durability problem.

[0005] The technical solution of the present invention is as follows: a method for preparing a cross-linked composite oil-absorbing film based on multi-stage surface-modified halloysite nanotubes, comprising the following steps:

[0006] (1) Halloysite nanotubes are modified by an aminosilane coupling agent to obtain amino-modified halloysite nanotubes;

[0007] (2) Utilizing the substitution reaction between amino groups and chlorine atoms, 3-MCPD was grafted onto the surface of amination-treated halloysite nanotubes to obtain hydroxylation-rich halloysite nanotubes;

[0008] (3) through a ring-opening reaction between hydroxyl groups and epoxy groups, the hydroxyl-rich halloysite nanotubes are covalently hydrophobically modified with a long alkyl chain glycidyl ether to generate new hydroxyl groups, thereby obtaining functionalized hydrophobic halloysite nanotubes;

[0009] (4) Functionalized hydrophobic halloysite nanotubes were added to a mixed system of polyvinylidene fluoride and dialdehyde polyethylene glycol, and the film was scraped and immersed in an acid solution to promote the cross-linking reaction between the hydroxyl group and the dialdehyde polyethylene glycol aldehyde group. After washing and drying, a cross-linked composite oil-absorbing film based on multi-level surface-modified halloysite nanotubes was obtained.

[0010] Furthermore, the step (1) specifically comprises dispersing the halloysite nanotubes in ethanol, uniformly dispersing by ultrasonication, adding an aminosilane coupling agent, and reacting under reflux at 80-90° C. for 10-16 hours, filtering, washing, and drying to obtain the amino-treated halloysite nanotubes.

[0011] Furthermore, in step (1), the mass ratio of the halloysite nanotubes to the aminosilane coupling agent is 1:(0.1-0.5), and the aminosilane coupling agent is one of 3-aminopropyltrimethoxysilane and 3-aminopropyltriethoxysilane.

[0012] Furthermore, the step (2) specifically comprises ultrasonically dispersing the amino-treated halloysite nanotubes in acetonitrile, adding 3-chloropropanediol, an acid-binding agent and a first catalyst under nitrogen protection, stirring and refluxing the mixture at 70-80° C. for 8-12 hours, filtering, washing and drying to obtain hydroxylated halloysite nanotubes.

[0013] Furthermore, in step (2), the mass ratio of the amination-modified halloysite nanotubes, 3-chloropropylene glycol, the acid-binding agent and the first catalyst is 1: (0.3-0.7): (0.5-1): (0.05-0.1).

[0014] Furthermore, the acid binding agent is one of triethylamine and pyridine, and the first catalyst is 4-dimethylaminopyridine.

[0015] Furthermore, the step (3) is specifically to ultrasonically disperse the hydroxylated halloysite nanotubes in ethanol, sequentially add a long alkyl chain glycidyl ether and a second catalyst, perform hydrophobic modification at 50-70° C. for 10-12 hours, filter, wash, and dry to obtain functionalized hydrophobic halloysite nanotubes.

[0016] Furthermore, the mass ratio of the hydroxylated halloysite nanotubes, the long alkyl chain glycidyl ether and the second catalyst is 1: (1-2): (0.1-0.2), the long alkyl chain glycidyl ether is one of dodecyl glycidyl ether, tetradecyl glycidyl ether and hexadecyl glycidyl ether, and the second catalyst is one of formic acid and acetic acid.

[0017] Furthermore, the step (4) specifically comprises sequentially stirring and dissolving polyvinylidene fluoride and dialdehyde polyethylene glycol in N , NFunctionalized hydrophobic halloysite nanotubes were added to dimethylformamide, and ultrasonic dispersion was performed to obtain a homogeneous casting solution. After degassing, the solution was coated by blade to form a film, and then immersed in a hydrochloric acid aqueous solution with a concentration of 0.5 to 1 mol / L at 40 to 60°C for a cross-linking reaction for 6 to 8 hours. The film was washed and dried to obtain a cross-linked composite oil-absorbing membrane based on multi-level surface-modified halloysite nanotubes.

[0018] Furthermore, the mass ratio of the polyvinylidene fluoride, dialdehyde polyethylene glycol and functionalized hydrophobic halloysite nanotubes is (3-5): (0.3-0.6):1.

[0019] The present invention first pre-treats halloysite nanotubes (HNTs) by amino treatment to introduce active amino groups. 3-MCPD is then grafted onto the surface of the aminated halloysite nanotubes via a substitution reaction to enrich the surface hydroxyl groups. Long alkyl chain glycidyl ethers are then grafted onto the surface of the aminated halloysite nanotubes via a covalent bonding reaction, achieving efficient hydrophobic modification while also introducing new active hydroxyl groups. Finally, the functionalized hydrophobic halloysite nanotubes are dispersed in a mixture of polyvinylidene fluoride and dialdehyde polyethylene glycol, uniformly dispersed by ultrasonication, coated into a film, and immersed in an acid solution for a cross-linking reaction. This process utilizes the hydroxyl groups generated during the hydrophobic modification as cross-linking active sites, and uses dialdehyde polyethylene glycol to achieve covalent cross-linking between the nanofillers, significantly improving interfacial interaction forces, membrane structural stability, and interfacial pore structure. Ultimately, a cross-linked composite membrane with excellent structural stability and hydrophobic and lipophilic properties is obtained.

[0020] The advantages of the present invention compared with the prior art are:

[0021] (1) The present invention adopts a three-stage synergistic modification process of "amination → hydroxyl enrichment → covalent hydrophobic modification" to effectively covalently graft long alkyl chain hydrophobic groups on the surface of halloysite nanotubes while introducing new active hydroxyl groups, which not only achieves efficient hydrophobic modification but also provides active sites for improving its interfacial compatibility with the polymer matrix.

[0022] (2) The present invention constructs a three-dimensional covalent cross-linked network of halloysite nanotubes in the polyvinylidene fluoride system, firmly encapsulating the hydrophobic resin matrix in the three-dimensional cross-linked structure of the filler, significantly improving the interfacial interaction and the structural stability of the membrane, while optimizing the interfacial pore structure.

[0023] (3) The present invention is based on the multi-stage synergistic hydrophobic modification of halloysite nanotubes and the construction of a three-dimensional cross-linked structure of halloysite nanotubes in a polymer matrix, which effectively improves the structural stability and hydrophobic and lipophilic properties of the composite membrane. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a SEM image of the cross-linked composite oil-absorbing film based on multi-level surface-modified halloysite nanotubes prepared in Example 1.

[0025] Figure 2 The contact angle of the cross-linked composite oil-absorbing film based on multi-level surface-modified halloysite nanotubes prepared in Example 1 to water changes with the number of cycles.

[0026] Figure 3 The adsorption amount of silicone oil by the cross-linked composite oil-absorbing film based on multi-level surface-modified halloysite nanotubes prepared in Example 1 changes with the number of cycles. DETAILED DESCRIPTION

[0027] The present invention will be further described below with reference to the examples, but are not intended to limit the present invention.

[0028] Example 1

[0029] A method for preparing a cross-linked composite oil-absorbing film based on multi-stage surface-modified halloysite nanotubes comprises the following steps:

[0030] (1) Disperse 1 g of HNTs (Xianfeng Nano, CAS No.: 1332-58-7, Product No.: 103763) in ethanol and disperse them evenly by ultrasonication. Add 0.1 g of 3-aminopropyltrimethoxysilane and reflux at 80 °C for 16 h. Filter, wash, and dry to obtain amino-treated halloysite nanotubes.

[0031] (2) 1 g of the prepared amino-treated halloysite nanotubes was ultrasonically dispersed in acetonitrile. Under nitrogen protection, 0.3 g of 3-chloropropanediol, 0.5 g of triethylamine and 0.05 g of 4-dimethylaminopyridine were added in sequence. The mixture was refluxed at 70 °C for 12 h. The mixture was filtered, washed and dried to obtain hydroxylated halloysite nanotubes.

[0032] (3) 1 g of the prepared hydroxylated halloysite nanotubes was ultrasonically dispersed in ethanol, and 1 g of dodecyl glycidyl ether and 0.1 g of formic acid were added in sequence. The mixture was stirred at 50 °C for 12 h, filtered, washed, and dried to obtain functionalized hydrophobic HNTs.

[0033] (4) Stir and dissolve 3g of polyvinylidene fluoride and 0.3g of dialdehyde polyethylene glycol in N, N -dimethylformamide, 1g of the prepared functionalized hydrophobic halloysite nanotubes was added, ultrasonically dispersed to obtain a homogeneous casting solution, and then the film was formed by scraping after degassing. The film sample was then immersed in a 0.5mol / L hydrochloric acid aqueous solution at 40°C for 8h, washed and dried to obtain a cross-linked composite oil-absorbing film based on multi-level surface modified halloysite nanotubes, as shown in the SEM image. Figure 1 As shown, the cross-linked composite membrane has a rich pore structure.

[0034] The performance experiments of the cross-linked composite oil-absorbing membrane based on multi-stage surface-modified halloysite nanotubes prepared in Example 1 are as follows:

[0035] Water contact angle test: A contact angle tester (model SDC-350H) was used to characterize the hydrophobicity of the material. The sample was adhered to a glass slide and 5 μl of water was added to the sample surface using a syringe. Five different locations of each sample were measured and the average value was taken. The results are listed in Table 1.

[0036] Oil absorption capacity test: First, the sample was placed in a 60℃ oven for 2 hours and then weighed. The initial weight was recorded as M 0 The sample was then placed in a mixture of silicone oil and water. After 15 minutes, the sample was taken out and placed on a metal filter. After 3 minutes of static dripping, the sample was placed in a 60°C oven for drying for 4 hours and weighed. The weight was recorded as M t Each sample was measured three times and the average value was taken. The oil absorption capacity (AR) of the sample was calculated according to the following formula. The results are listed in Table 1.

[0037] .

[0038] Reusability: The oil adsorbed by the sample was squeezed out by mechanical squeezing and ethanol washing. Each oil-water separation and each squeezing-washing was regarded as one cycle. The changes of the sample water contact angle and oil absorption capacity with the squeezing times were recorded to judge the reusability of the sample. The results of Example 1 are shown in the figure. Figure 2 and Figure 3 As shown in the graph, the cross-linked composite oil-absorbing membrane based on multi-stage surface-modified halloysite nanotubes exhibits only a slight decrease in water contact angle and oil absorption capacity with increasing cycles, while maintaining its excellent hydrophobic properties and oil absorption capacity. This demonstrates that the modification technology employed in this invention effectively enhances the durability of the HNT-based composite membrane. Specifically, the effective covalent hydrophobic modification of the HNTs and the covalent chemical cross-linking between the HNTs effectively improve the structural and performance stability of the composite membrane.

[0039] Example 2

[0040] A method for preparing a cross-linked composite oil-absorbing film based on multi-stage surface-modified halloysite nanotubes comprises the following steps:

[0041] (1) Disperse 1 g of HNTs (Xianfeng Nano, CAS No.: 1332-58-7, Product No.: 103763) in ethanol and disperse them evenly by ultrasonication. Add 0.3 g of 3-aminopropyltriethoxysilane and reflux at 85 °C for 13 h. Filter, wash and dry to obtain amino-treated halloysite nanotubes.

[0042] (2) 1 g of the prepared amino-treated halloysite nanotubes was ultrasonically dispersed in acetonitrile. Under nitrogen protection, 0.5 g of 3-chloropropanediol, 0.75 g of pyridine and 0.075 g of 4-dimethylaminopyridine were added in sequence. The mixture was refluxed at 75 °C for 10 h. The mixture was filtered, washed and dried to obtain hydroxylated halloysite nanotubes.

[0043] (3) 1 g of the prepared hydroxylated halloysite nanotubes was ultrasonically dispersed in ethanol, and 1.5 g of tetradecyl glycidyl ether and 0.15 g of acetic acid were added in sequence. The mixture was stirred at 60 °C for 11 h, filtered, washed, and dried to obtain functionalized hydrophobic HNTs.

[0044] (4) Stir and dissolve 4g of polyvinylidene fluoride and 0.45g of dialdehyde polyethylene glycol in N, N -dimethylformamide, 1 g of the prepared functionalized hydrophobic halloysite nanotubes was added, ultrasonically dispersed to obtain a homogeneous casting solution, and after degassing, the solution was coated by blade to form a film. The film sample was then immersed in a 0.75 mol / L hydrochloric acid aqueous solution at 50°C for 7 h, washed, and dried to obtain a cross-linked composite oil-absorbing film based on multi-stage surface-modified halloysite nanotubes. The water contact angle and oil absorption capacity of the film were measured according to the relevant performance experiments of the product in Example 1, and the results are listed in Table 1.

[0045] Example 3

[0046] A method for preparing a cross-linked composite oil-absorbing film based on multi-stage surface-modified halloysite nanotubes comprises the following steps:

[0047] (1) Disperse 1 g of HNTs (Xianfeng Nano, CAS No.: 1332-58-7, Product No.: 103763) in ethanol and disperse them evenly by ultrasonication. Add 0.5 g of 3-aminopropyltriethoxysilane and reflux at 90 °C for 10 h. Filter, wash and dry to obtain amino-treated halloysite nanotubes.

[0048] (2) 1 g of the prepared amino-treated halloysite nanotubes was ultrasonically dispersed in acetonitrile. Under nitrogen protection, 0.7 g of 3-chloropropanediol, 1 g of triethylamine and 0.1 g of 4-dimethylaminopyridine were added in sequence. The mixture was refluxed at 80 °C for 8 h, filtered, washed and dried to obtain hydroxylated halloysite nanotubes.

[0049] (3) 1 g of the prepared hydroxylated halloysite nanotubes was ultrasonically dispersed in an ethanol solution, and 2 g of hexadecyl glycidyl ether and 0.2 g of formic acid were added in sequence. The mixture was stirred at 70 °C for 10 h, filtered, washed, and dried to obtain functionalized hydrophobic HNTs.

[0050] (4) Stir and dissolve 5g of polyvinylidene fluoride and 0.6g of dialdehyde polyethylene glycol in N, N 1 g of the prepared functionalized hydrophobic halloysite nanotubes was added to dimethylformamide, and ultrasonic dispersion was performed to obtain a homogeneous casting solution. After degassing, the solution was coated by blade to form a film. The film sample was then immersed in a 1 mol / L hydrochloric acid aqueous solution at 60°C for 6 h, washed, and dried to obtain a cross-linked composite oil-absorbing film based on multi-stage surface-modified halloysite nanotubes. The water contact angle and oil absorption capacity of the film were measured according to the relevant performance experiments of the product in Example 1, and the results are listed in Table 1.

[0051] Comparative Example 1

[0052] 3g of polyvinylidene fluoride and 0.3g of dialdehyde polyethylene glycol were stirred and dissolved in N, N 1 g of original HNTs was added to dimethylformamide and ultrasonically dispersed to obtain a homogeneous casting solution. After degassing, the solution was coated with a knife to form a film. The film sample was then immersed in a 0.5 mol / L hydrochloric acid aqueous solution at 40°C for 8 h, washed, and dried to obtain a composite oil-absorbing film. The water contact angle and oil absorption capacity of the composite film were measured according to the relevant performance test of the product in Example 1. The results are listed in Table 1.

[0053] Comparative Example 2

[0054] (1) 1 g of original HNTs was ultrasonically dispersed in ethanol solution, and 1 g of dodecyl glycidyl ether and 0.1 g of formic acid were added in sequence. The mixture was stirred at 50 °C for 12 h, filtered, washed, and dried to obtain modified halloysite nanotubes.

[0055] (2) Stir and dissolve 3g of polyvinylidene fluoride and 0.3g of dialdehyde polyethylene glycol in N, N 1 g of the prepared modified halloysite nanotubes was added to dimethylformamide and ultrasonically dispersed to obtain a homogeneous casting solution. After degassing, the solution was coated by blade to form a film. The film sample was then immersed in a 0.5 mol / L hydrochloric acid aqueous solution at 40°C for 8 h, washed, and dried to obtain a composite oil-absorbing film. The water contact angle and oil absorption capacity of the composite film were measured according to the relevant performance experiments of the product in Example 1. The results are listed in Table 1.

[0056] Comparative Example 3

[0057] Dissolve 3g of polyvinylidene fluoride in N, N 1 g of the functionalized hydrophobic HNTs (prepared according to steps (1), (2) and (3) in Example 1) was added to dimethylformamide, and ultrasonic dispersion was uniformly obtained to obtain a homogeneous casting solution. After degassing, the solution was coated by blade to form a film. The film-formed sample was then immersed in a 0.5 mol / L hydrochloric acid aqueous solution at 40°C for 8 h, washed and dried to obtain a composite oil-absorbing film. The water contact angle and oil absorption capacity of the composite film were measured according to the relevant performance experiments of the product in Example 1. The results are listed in Table 1.

[0058] Table 1 Water contact angle and silicone oil adsorption amount of the composite oil-absorbing films prepared in Examples 1-3 and Comparative Examples 1-3

[0059]

[0060] In the examples of the present invention, the hydroxylation-enriched modification effectively increases the surface active site density of HNTs, enabling a high density of long-chain hydrophobic molecules to be grafted via stable covalent bonds. Furthermore, the crosslinking of the HNT nanotubular structure forms a micro-nano composite roughened interface, endowing the composite membrane with excellent hydrophobic properties, achieving a water contact angle of up to 143.8°. This multi-level structural design, combined with the pore network, enables the cross-linked composite oil-absorbing membrane produced in the examples to exhibit excellent oil-water separation performance, with an adsorption capacity for silicone oil reaching 13.8 g / g.

[0061] In contrast, Comparative Example 1 uses unmodified HNTs, which are unevenly dispersed in the resin matrix due to the limited number of surface active groups, and the crosslinking density with dialdehyde polyethylene glycol is insufficient. Comparative Example 2 directly hydrophobically modifies the HNTs, lacking a hydroxylation pretreatment step, resulting in a limited amount of dodecyl glycidyl ether grafted and the number of newly formed hydroxyl groups, which in turn affects the degree of crosslinking and interfacial interaction. Comparative Example 3 hydroxylation and covalent hydrophobic modification are performed on the HNTs, but a crosslinking network is not introduced into the matrix, making it impossible for covalent bonds to form between the HNTs, and the porosity is greatly reduced. It can be seen that the composite membranes prepared in all comparative examples are not comparable to the composite oil-absorbing membranes obtained in the examples in terms of hydrophobicity, interfacial interaction, and porosity, which also leads to a significant decrease in their hydrophobicity and silicone oil adsorption capacity.

Claims

1. A method for preparing a cross-linked composite oil-absorbing film based on multi-stage surface-modified halloysite nanotubes, characterized in that: The following steps are involved: (1) Halloysite nanotubes are modified by an aminosilane coupling agent to obtain amino-modified halloysite nanotubes; (2) Utilizing the substitution reaction between amino groups and chlorine atoms, 3-MCPD was grafted onto the surface of amination-treated halloysite nanotubes to obtain hydroxylation-rich halloysite nanotubes; (3) through a ring-opening reaction between hydroxyl groups and epoxy groups, the hydroxyl-rich halloysite nanotubes are covalently hydrophobically modified with a long alkyl chain glycidyl ether to generate new hydroxyl groups, thereby obtaining functionalized hydrophobic halloysite nanotubes; (4) Functionalized hydrophobic halloysite nanotubes were added to a mixed system of polyvinylidene fluoride and dialdehyde polyethylene glycol, and the film was scraped and immersed in an acid solution to promote the cross-linking reaction between the hydroxyl group and the dialdehyde polyethylene glycol aldehyde group. After washing and drying, a cross-linked composite oil-absorbing film based on multi-level surface-modified halloysite nanotubes was obtained.

2. The method for preparing a cross-linked composite oil-absorbing film based on multi-stage surface-modified halloysite nanotubes according to claim 1, wherein the step (1) specifically comprises dispersing the halloysite nanotubes in ethanol, uniformly dispersing them by ultrasonication, adding an aminosilane coupling agent, and reacting under reflux at 80 to 90° C. for 10 to 16 hours, filtering, washing, and drying to obtain the amino-modified halloysite nanotubes.

3. The method for preparing a cross-linked composite oil-absorbing film based on multi-stage surface-modified halloysite nanotubes according to claim 2, wherein the mass ratio of the halloysite nanotubes to the aminosilane coupling agent in step (1) is 1:(0.1-0.5), and the aminosilane coupling agent is one of 3-aminopropyltrimethoxysilane and 3-aminopropyltriethoxysilane.

4. The method for preparing a cross-linked composite oil-absorbing film based on multi-stage surface-modified halloysite nanotubes according to claim 1, wherein the step (2) specifically comprises ultrasonically dispersing the amino-modified halloysite nanotubes in acetonitrile, adding 3-chloropropanediol, an acid-binding agent and a first catalyst under nitrogen protection, stirring and refluxing the mixture at 70-80° C. for 8-12 hours, filtering, washing and drying to obtain hydroxylated halloysite nanotubes.

5. The method for preparing a cross-linked composite oil-absorbing film based on multi-stage surface-modified halloysite nanotubes according to claim 4, wherein the mass ratio of the amination-modified halloysite nanotubes, 3-MCPD, acid-binding agent and first catalyst in step (2) is 1: (0.3-0.7): (0.5-1): (0.05-0.1). 6 . The method for preparing a cross-linked composite oil-absorbing film based on multi-stage surface-modified halloysite nanotubes according to claim 5 , wherein the acid-binding agent is one of triethylamine and pyridine, and the first catalyst is 4-dimethylaminopyridine.

7. The method for preparing a cross-linked composite oil-absorbing membrane based on multi-stage surface-modified halloysite nanotubes according to claim 1, wherein the step (3) is specifically to ultrasonically disperse the hydroxylated halloysite nanotubes in ethanol, sequentially add a long alkyl chain glycidyl ether and a second catalyst, perform hydrophobic modification at 50-70°C for 10-12 hours, filter, wash, and dry to obtain functionalized hydrophobic halloysite nanotubes.

8. The method for preparing a cross-linked composite oil-absorbing membrane based on multi-stage surface-modified halloysite nanotubes according to claim 7, wherein the mass ratio of the hydroxylated-rich halloysite nanotubes, the long alkyl chain glycidyl ether, and the second catalyst is 1:(1-2):(0.1-0.2), the long alkyl chain glycidyl ether is one of dodecyl glycidyl ether, tetradecyl glycidyl ether, and hexadecyl glycidyl ether, and the second catalyst is one of formic acid and acetic acid.

9. The method for preparing a cross-linked composite oil-absorbing film based on multi-level surface-modified halloysite nanotubes according to claim 1, wherein the step (4) specifically comprises sequentially stirring and dissolving polyvinylidene fluoride and dialdehyde polyethylene glycol in N , N Functionalized hydrophobic halloysite nanotubes were added to dimethylformamide, and ultrasonic dispersion was performed to obtain a homogeneous casting solution. After degassing, the solution was coated by blade to form a film, and then immersed in a hydrochloric acid aqueous solution with a concentration of 0.5 to 1 mol / L at 40 to 60°C for a cross-linking reaction for 6 to 8 hours. The film was washed and dried to obtain a cross-linked composite oil-absorbing membrane based on multi-level surface-modified halloysite nanotubes.

10. The method for preparing a cross-linked composite oil-absorbing membrane based on multi-stage surface-modified halloysite nanotubes according to claim 9, wherein the mass ratio of the polyvinylidene fluoride, bisaldehyde polyethylene glycol and functionalized hydrophobic halloysite nanotubes is (3-5): (0.3-0.6):1.

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