A porous drag-reducing coating and a method of making the same

Abstract

The present application belongs to the technical field of marine drag reduction materials, and particularly relates to a porous drag reduction coating and a preparation method thereof. The method comprises the following steps: coating a mixed solution of a natural polymer, cellulose nanocrystals, two-dimensional nanomaterials and water on a substrate, then immersing the substrate in a metal salt solution for a certain period of time to form a crosslinked coating, taking out the substrate, processing the substrate by using a directional freezing casting technology, immersing the substrate in an organic-inorganic mixed solution for a certain period of time, taking out the substrate, removing inorganic salts in the substrate, obtaining a porous crosslinked coating, and hot-pressing the porous crosslinked coating to obtain a porous drag reduction coating. The organic-inorganic mixed solution is obtained by mixing a silane coupling agent, an inorganic precursor, an inorganic salt, a gluey polymer and an organic solvent. The porous drag reduction coating obtained finally has the characteristics of rigidity and softness, a rough surface, good mechanical stability, obvious micro-eddy current and excellent drag reduction performance.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of marine drag reduction materials, and particularly relates to a porous drag reduction coating and a preparation method thereof. BACKGROUND

[0002] Water vehicles such as ships and underwater vessels are affected by frictional resistance during underwater movement, and the speed is reduced and the energy consumption is obviously increased. At present, super-hydrophobic drag reduction and flexible drag reduction are common drag reduction technologies in the field. Super-hydrophobic drag reduction usually needs to construct a rigid surface with rough structure and low interaction force with water, but the rigid rough super-hydrophobic surface is easy to wear and tear, and the drag reduction stability is poor. The flexible drag reduction technology represented by dolphin skin has less stringent restrictions on the flow field than the rigid super-hydrophobic surface, but once the external flow field is too strong, the flexible surface will be permanently deformed and cannot be restored, thereby losing the drag reduction effect. Based on the problems in the prior art, there is no good solution. SUMMARY

[0003] The purpose of the present application is to provide a porous drag reduction coating with rough structure and rigid-flexible combination and a preparation method thereof to solve the above problems existing in the prior art.

[0004] To achieve the above purpose, the technical scheme of the present application is as follows:

[0005] In the first aspect, the present application provides a preparation method of a porous drag reduction coating, which comprises the following steps:

[0006] A mixed solution of natural polymer, cellulose nanocrystal, two-dimensional nanomaterial and water is coated on a substrate, and then the substrate is immersed in a metal salt solution for a certain time and taken out, so as to obtain a cross-linked coating coordinated with metal ions;

[0007] The cross-linked coating coordinated with metal ions is treated by directional freeze casting technology to obtain a cross-linked coating with vertical pore structure,

[0008] The organic-inorganic mixed solution of the cross-linked coating with vertical pore structure is immersed in an organic-inorganic mixed solution for a certain time, and then taken out and the inorganic salt is removed, so as to obtain a porous cross-linked coating; the organic-inorganic mixed solution is obtained by mixing silane coupling agent, inorganic precursor, inorganic salt, adhesive polymer and organic solvent;

[0009] The porous cross-linked coating is heat-pressed to obtain a porous drag reduction coating.

[0010] The natural polymer is at least one of agar, sodium alginate and chitosan;

[0011] The cellulose nanocrystal is at least one of sulfonated cellulose nanocrystal, aminated cellulose nanocrystal, carboxylated cellulose nanocrystal and phosphated cellulose nanocrystal.

[0012] The two-dimensional nanomaterial is at least one of graphene oxide, molybdenum disulfide, boron nitride;

[0013] The metal salt is at least one of calcium chloride, ferric chloride, cerium nitrate, neodymium chloride.

[0014] The mass fraction of the natural polymer, the cellulose nanocrystal and the two-dimensional nanomaterial in the mixed solution is 0.5-10wt%, 0.1-10wt% and 0.01-5wt% respectively.

[0015] The mass fraction of the metal salt in the metal salt solution is 0.01-10wt%.

[0016] The silane coupling agent is at least one of vinyltriethoxysilane, methacryloyloxypropyltrimethoxysilane and gamma-mercaptopropyltriethoxysilane.

[0017] The inorganic precursor is at least one of tetraethyl orthosilicate and tetrabutyl titanate.

[0018] The inorganic salt is at least one of sodium chloride, magnesium chloride and sodium sulfate.

[0019] The adhesive polymer is at least one of polyurethane, amino organosilicon resin and fluorocarbon resin.

[0020] The organic solvent is at least one of methanol and ethanol.

[0021] The mass fraction of the silane coupling agent, the inorganic precursor, the inorganic salt and the adhesive polymer in the organic-inorganic mixed solution is 5-10wt%, 0.5-15wt%, 0.5-10wt% and 0.5-5wt% respectively.

[0022] The hot-pressing temperature is 60-120℃, and the hot-pressing duration is 0.5-24h.

[0023] The processing of the cross-linked coating coordinated with metal ions by using the directional freezing casting technology refers to freezing for 0.5-24h in an environment of-60℃ to-4℃, and then freeze-drying for 6-72h.

[0024] The organic-inorganic mixed solution further comprises an additive with a mass fraction of 0.5-10wt%, and the additive is at least one of montmorillonite, diatomite and halloysite.

[0025] The reaction temperature for obtaining the organic-inorganic mixed solution is 5-65℃.

[0026] In the second aspect, the application provides a porous drag-reducing coating prepared by the preparation method.

[0027] Compared with the prior art, the present application has the following advantages:

[0028] The present application provides a preparation method of a porous drag-reducing coating. A cross-linked coating with vertical pore structure is first prepared by metal ion coordination and directional freeze casting technology. On this basis, a porous cross-linked coating with surface porous structure is further constructed by in-situ filling of non-polar materials and inorganic salt template technology. The final porous drag-reducing coating has the characteristics of rigidity and flexibility, good mechanical stability, and significant micro-eddy current, and excellent drag-reducing property. DETAILED DESCRIPTION

[0029] The present application will be further described in detail below in combination with specific embodiments and schemes.

[0030] A preparation method of a porous drag-reducing coating, the preparation method comprising:

[0031] coating a mixture of a natural polymer, cellulose nanocrystals, and a two-dimensional nanomaterial and water on a substrate, immersing the substrate in a metal salt solution for a certain period of time, and then taking out the substrate to obtain a cross-linked coating coordinated with metal ions;

[0032] treating the cross-linked coating coordinated with metal ions by directional freeze casting technology to obtain a cross-linked coating with vertical pore structure,

[0033] immersing the cross-linked coating with vertical pore structure in an organic-inorganic mixed solution for a certain period of time, removing the inorganic salt therefrom, and then obtaining a porous cross-linked coating; the organic-inorganic mixed solution is obtained by mixing a silane coupling agent, an inorganic precursor, an inorganic salt, a gluey polymer, and an organic solvent;

[0034] heat-pressing the porous cross-linked coating to obtain a porous drag-reducing coating.

[0035] Optionally, the natural polymer is at least one of agar, sodium alginate, and chitosan.

[0036] The cellulose nanocrystals are at least one of sulfonated cellulose nanocrystals, aminated cellulose nanocrystals, carboxylated cellulose nanocrystals, and phosphated cellulose nanocrystals.

[0037] Optionally, the two-dimensional nanomaterial is at least one of graphene oxide, molybdenum disulfide, and boron nitride.

[0038] The metal salt is at least one of calcium chloride, ferric chloride, cerium nitrate, and neodymium chloride.

[0039] Optionally, the mass fraction of the natural polymer, the cellulose nanocrystals, and the two-dimensional nanomaterial in the mixture is 0.5-10 wt%, 0.1-10 wt%, and 0.01-5 wt%, respectively.

[0040] The mass fraction of the metal salt in the metal salt solution is 0.01-10wt%;

[0041] Optionally, the silane coupling agent is at least one of vinyltriethoxysilane, methacryloyloxypropyltrimethoxysilane and gamma-mercaptopropyltriethoxysilane.

[0042] The inorganic precursor is at least one of tetraethyl orthosilicate and tetrabutyl titanate.

[0043] The inorganic salt is at least one of sodium chloride, magnesium chloride and sodium sulfate.

[0044] The adhesive polymer is at least one of polyurethane, amino silicone resin and fluorocarbon resin.

[0045] The organic solvent is at least one of methanol and ethanol.

[0046] Optionally, the mass fractions of the silane coupling agent, the inorganic precursor, the inorganic salt and the adhesive polymer in the organic-inorganic mixed solution are 5-10wt%, 0.5-15wt%, 0.5-10wt% and 0.5-5wt% respectively.

[0047] Optionally, the hot-pressing temperature is 60-120℃, and the hot-pressing duration is 0.5-24h.

[0048] Optionally, the processing of the cross-linked coating coordinated with metal ions by the directional freezing casting technology refers to freezing for 0.5-24h in an environment of-60℃ to-4℃, and then freeze-drying for 6-72h.

[0049] Optionally, the organic-inorganic mixed solution further comprises an additive with a mass fraction of 0.5-10wt%, and the additive is at least one of montmorillonite, diatomite and halloysite.

[0050] Optionally, the reaction temperature for obtaining the organic-inorganic mixed solution is 5-65℃.

[0051] A porous drag-reducing coating prepared by the preparation method.

[0052] The principle of the application is explained as follows:

[0053] First, the mixture of natural polymer, cellulose nanocrystal, two-dimensional nanomaterial and water is coated on the substrate, and then the coated substrate is immersed in a metal salt solution for a certain time. The metal ions in the metal salt solution form a coordination with the abundant hydroxyl groups in the cellulose nanocrystal. By introducing metal coordination crosslinking, a crosslinked coating is formed. Then, the crosslinked coating with metal ion coordination is treated by directional freeze casting technology to obtain a crosslinked coating with vertical pore structure and vertical orientation structure. The principle of directional freeze casting technology is that the crosslinked coating with metal ion coordination is placed in a temperature field to cool down. Water will gradually freeze along the temperature gradient. The ice crystal column produced by freezing will squeeze, displace and embed the skeleton of the crosslinked coating with metal ion coordination between the ice crystal columns. Then, the crosslinked coating with metal ion coordination is placed in a freeze dryer and dried. As the pressure gradually decreases at room temperature, the ice crystal column begins to change from solid phase to liquid phase and then to gas phase. After the ice crystal column template is removed, the vertical pore structure is formed at the original position of the ice crystal column, and the skeleton of the crosslinked coating with metal ion coordination arranged by the ice crystal column is retained. Finally, a crosslinked coating with two-dimensional nanosheet vertical orientation structure is obtained to improve the structural stability of the crosslinked coating.

[0054] The crosslinked coating with vertical pore structure is immersed in an organic-inorganic mixed solution. Since the organic-inorganic mixed solution is obtained by mixing silane coupling agent, inorganic precursor, inorganic salt, adhesive polymer and organic solvent, the hydrolysis and condensation reaction of the silane coupling agent and the inorganic precursor generates inorganic materials. The hydrolysis and condensation reaction occurs in the vertical pore structure of the crosslinked coating, realizing in-situ filling of the inorganic materials. At the same time, the adhesive polymer enters the gap between the inorganic particles and the vertical pore structure, playing the role of “rivet” and improving the interfacial compatibility between the inorganic materials and the crosslinked coating. The inorganic materials have various active groups, which can form hydrogen bonds with the inorganic salt. After removing the inorganic salt, a pore structure is formed at the original position of the inorganic salt, and a porous crosslinked coating is obtained. Finally, a final porous drag-reducing coating is obtained by hot pressing technology. When in use, the water flow generates a downstream micro-vortex in the pore structure on the surface of the porous drag-reducing coating, causing the boundary layer to slip and thus significantly reducing the frictional resistance.

[0055] Example 1:

[0056] A method for preparing a porous drag-reducing coating, which is performed according to the following steps in sequence:

[0057] Step one, first add natural polymer, cellulose nanocrystal, two-dimensional nanomaterial into deionized water, stir at 5℃ for 24h to obtain a mixed solution; evenly coat the mixed solution on a substrate (coating thickness is 100µm), immerse in a metal salt solution after standing at room temperature for 1min, the immersion temperature is 5℃, the immersion time is 0.1h, to obtain a metal ion coordinated cross-linked coating; the mass fraction of natural polymer, cellulose nanocrystal, two-dimensional nanomaterial in the mixed solution is 0.5wt%, 10wt%, 5wt% respectively; the natural polymer is agar; the cellulose nanocrystal is sulfonated cellulose nanocrystal; the two-dimensional nanomaterial is graphene oxide nanosheet; the mass fraction of metal salt in the metal salt solution is 10wt%; the metal salt is calcium chloride;

[0058] Step two, take out the metal ion coordinated cross-linked coating prepared in step one, clean with deionized water, and then process it by directional ice crystal freeze casting technology to construct a cross-linked coating with vertical pore structure; the directional ice crystal freeze casting can be: first place the sample in the freezing chamber of the freeze casting machine for unidirectional freezing, gradually cool to-60℃, the freezing time of the whole unidirectional freezing process is 0.5h; then place it in a freeze dryer for 6h of freeze drying;

[0059] Step three, mix silane coupling agent, inorganic precursor, inorganic salt (particle size is 0.1µm), additive (particle size is 0.1µm), adhesive polymer, organic solvent, stir at 5℃ for 0.5h to obtain an organic-inorganic mixed solution; the mass fraction of silane coupling agent, inorganic precursor, inorganic salt, additive, adhesive polymer in the organic-inorganic mixed solution is 0.5wt%, 0.5wt%, 0.5wt%, 0.5wt%, 0.5wt% respectively; the silane coupling agent is vinyltriethoxysilane; the inorganic precursor is tetraethyl orthosilicate; the inorganic salt is sodium chloride; the additive is montmorillonite; the adhesive polymer is polyurethane; the organic solvent is methanol;

[0060] Step four, take out the metal ion coordinated cross-linked coating prepared in step two, thaw at room temperature, then immerse in the organic-inorganic mixed solution prepared in step three for 0.5h to obtain an inorganic material in-situ filled cross-linked coating;

[0061] Step five, take out the inorganic material in-situ filled cross-linked coating prepared in step four, immerse in deionized water for 1h to remove the inorganic salt template, to obtain a porous cross-linked coating with a porous surface structure;

[0062] Step six, heat press the porous cross-linked coating prepared in step five at 60℃ for 24h to finally obtain a porous drag-reducing coating.

[0063] After water tunnel test, the drag reduction rate of the coating is 9.7%.

[0064] Example 2:

[0065] A method for preparing a porous drag-reducing coating, which is performed in the following order:

[0066] Step one, add natural polymer, cellulose nanocrystal, two-dimensional nanomaterial into deionized water, stir at 95℃ for 0.5h to obtain a mixed solution; uniformly coat the mixed solution on a substrate (coating thickness is 2cm), immerse in a metal salt solution after standing at room temperature for 10min, the immersion temperature is 95℃, the immersion time is 24h, to obtain a metal ion coordinated cross-linked coating; the mass fraction of natural polymer, cellulose nanocrystal, two-dimensional nanomaterial in the mixed solution is 10wt%, 0.1wt%, 0.01wt% respectively; the natural polymer is sodium alginate; the cellulose nanocrystal is amino cellulose nanocrystal; the two-dimensional nanomaterial is molybdenum disulfide nanosheet; the mass fraction of metal salt in the metal salt solution is 0.01wt%; the metal salt is ferric chloride;

[0067] Step two, take out the metal ion coordinated cross-linked coating prepared in step one, clean with deionized water, and then process it by directional ice crystal freeze casting technology to construct a cross-linked coating with vertical pore structure; the directional ice crystal freeze casting can be: first place the sample in the freeze chamber of the freeze casting machine for unidirectional freezing, gradually cool to-60℃, the freezing time of the whole unidirectional freezing process is 10h; then place it in a freeze dryer for 20h;

[0068] Step three, mix silane coupling agent, inorganic precursor, inorganic salt (particle size is 100µm), additive (particle size is 50µm), adhesive polymer, organic solvent, stir at 65℃ for 24h to obtain an organic-inorganic mixed solution; the mass fraction of silane coupling agent, inorganic precursor, inorganic salt, additive, adhesive polymer in the organic-inorganic mixed solution is 10wt%, 15wt%, 10wt%, 10wt%, 5wt% respectively; the silane coupling agent is methacryloxypropyltrimethoxysilane; the inorganic precursor is tetrabutyl titanate; the inorganic salt is magnesium chloride; the additive is diatomite; the adhesive polymer is amino organosilicon resin; the organic solvent is ethanol;

[0069] Step four, take out the metal ion coordinated cross-linked coating prepared in step two, thaw at room temperature, then immerse in the organic-inorganic mixed solution prepared in step three for 24h to obtain an inorganic material in-situ filled cross-linked coating;

[0070] Step five, take out the inorganic material in-situ filled cross-linked coating prepared in step four, immerse in deionized water for 56h to remove the inorganic salt template, to obtain a porous cross-linked coating with a porous structure on the surface;

[0071] Step six, heat press the porous cross-linked coating prepared in step five at 120℃ for 0.5h to finally obtain a porous drag-reducing coating.

[0072] The coating drag reduction rate is 8.9% through the water tunnel test.

[0073] Example 3:

[0074] A preparation method of a porous drag reduction coating is performed according to the following steps in sequence:

[0075] Step one, natural polymer, cellulose nanocrystal, and two-dimensional nanomaterial are added to deionized water, and stirred at 65℃ for 14h to obtain a mixed solution; the mixed solution is uniformly coated (coating thickness is 1000µm) on a substrate, and after standing at room temperature for 5min, it is immersed in a metal salt solution, the immersion temperature is 75℃, and the immersion time is 10h, to obtain a metal ion coordinated cross-linked coating; the mass fraction of natural polymer, cellulose nanocrystal, and two-dimensional nanomaterial in the mixed solution is 3wt%, 4wt%, and 1wt% respectively; the natural polymer is chitosan; the cellulose nanocrystal is carboxylated cellulose nanocrystal; the two-dimensional nanomaterial is boron nitride nanosheet; the mass fraction of metal salt in the metal salt solution is 2wt%; the metal salt is neodymium chloride;

[0076] Step two, the metal ion coordinated cross-linked coating prepared in step one is taken out, washed with deionized water, and then treated by directional ice crystal freeze casting technology to construct a cross-linked coating with vertical pore structure; the directional ice crystal freeze casting can be: first, place the sample in the freezing chamber of the freeze casting machine for unidirectional freezing, gradually cool to-30℃, and the freezing time of the whole unidirectional freezing process is 24h; then, place it in a freeze dryer for 72h of freeze drying;

[0077] Step three, mix silane coupling agent, inorganic precursor, inorganic salt (particle size is 50µm), additive (particle size is 20µm), adhesive polymer, and organic solvent, and stir at 45℃ for 12h to obtain an organic-inorganic mixed solution; the mass fraction of silane coupling agent, inorganic precursor, inorganic salt, additive, and adhesive polymer in the organic-inorganic mixed solution is 3wt%, 3wt%, 5wt%, 6wt%, and 3wt% respectively; the silane coupling agent is γ-mercaptopropyl triethoxysilane; the inorganic precursor is tetraethyl orthosilicate; the inorganic salt is sodium sulfate; the additive is halloysite; the adhesive polymer is fluorocarbon resin; and the organic solvent is ethanol;

[0078] Step four, take out the metal ion coordinated cross-linked coating prepared in step two, thaw at room temperature, and then immerse in the organic-inorganic mixed solution prepared in step three for 8h to obtain an inorganic material in-situ filled cross-linked coating;

[0079] Step five, take out the inorganic material in-situ filled cross-linked coating prepared in step four, immerse in deionized water for 36h to remove the inorganic salt template, and obtain a porous cross-linked coating with a porous structure on the surface;

[0080] Step six, the porous cross-linked coating prepared in step five is hot-pressed at 80℃ for 12h, and finally a porous drag-reducing coating is obtained.

[0081] The coating drag-reduction rate is 10.5% through water tunnel test.

[0082] Example 4:

[0083] A method for preparing a porous drag-reducing coating is performed according to the following steps in turn:

[0084] Step one, natural polymer, cellulose nanocrystal, and two-dimensional nanomaterial are added to deionized water, and a mixed solution is obtained after stirring at 45℃ for 8h; the mixed solution is uniformly coated (coating thickness is 1500µm) on a substrate, and after standing at room temperature for 8min, it is immersed in a metal salt solution, with an immersion temperature of 55℃ and an immersion time of 15h, to obtain a metal ion coordinated cross-linked coating; the mass fractions of natural polymer, cellulose nanocrystal, and two-dimensional nanomaterial in the mixed solution are 5wt%, 5wt%, and 3.5wt% respectively; the natural polymer is agar; the cellulose nanocrystal is phosphated cellulose nanocrystal; the two-dimensional nanomaterial is boron nitride nanosheet; the mass fraction of metal salt in the metal salt solution is 5wt%; the metal salt is neodymium chloride;

[0085] Step two, the metal ion coordinated cross-linked coating prepared in step one is taken out, washed with deionized water, and then treated by directional ice crystal freeze casting technology to construct a cross-linked coating with vertical pore structure; the directional ice crystal freeze casting can be: first, place the sample in the freezing chamber of the freeze casting machine for unidirectional freezing, gradually cool to -50℃, and the freezing time of the whole unidirectional freezing process is 12h; then place it in a freeze dryer for 60h of freeze drying;

[0086] Step three, mix silane coupling agent, inorganic precursor, inorganic salt (particle size is 50µm), additive (particle size is 20µm), adhesive polymer, and organic solvent, and stir at 65℃ for 24h to obtain an organic-inorganic mixed solution; the mass fractions of silane coupling agent, inorganic precursor, inorganic salt, additive, and adhesive polymer in the organic-inorganic mixed solution are 5wt%, 5wt%, 10wt%, 5wt%, and 5wt% respectively; the silane coupling agent is vinyltriethoxysilane; the inorganic precursor is tetraethyl orthosilicate; the inorganic salt is sodium sulfate; the additive is halloysite; the adhesive polymer is fluorocarbon resin; and the organic solvent is ethanol;

[0087] Step four, take out the metal ion coordinated cross-linked coating prepared in step two, thaw at room temperature, and then immerse it in the organic-inorganic mixed solution prepared in step three for 10h to obtain an inorganic material in-situ filled cross-linked coating;

[0088] Step five, the cross-linked coating filled with in-situ inorganic material prepared in step four is taken out and immersed in deionized water for 48 h to remove the inorganic salt template, and a porous cross-linked coating with a porous structure on the surface is obtained;

[0089] Step six, the porous cross-linked coating prepared in step five is hot-pressed at 90°C for 10 h, and finally a porous drag-reducing coating is obtained.

[0090] The water tunnel test shows that the drag reduction rate of the coating is 12.4%.

[0091] Comparative Example 1:

[0092] This comparative example is basically the same as Example 4, except that:

[0093] In step one, the concentration of the metal salt solution is 0 wt%, lacking the coordination of metal ions, and finally no cross-linked coating can be obtained, and the drag reduction performance test cannot be performed.

[0094] Comparative Example 2:

[0095] This comparative example is basically the same as Example 4, except that:

[0096] Lacking the ice crystal freezing casting technology of step two, it is impossible to obtain a vertical pore structure and to realize the in-situ filling of inorganic material in step four. The final coating has poor strength and is easy to break, and the drag reduction performance test cannot be performed.

[0097] Comparative Example 3:

[0098] This comparative example is basically the same as Example 4, except that:

[0099] Lacking steps three and four, it is impossible to realize the in-situ filling of inorganic material in step four. The final coating has poor strength and is easy to break, and the drag reduction performance test cannot be performed.

[0100] Comparative Example 4:

[0101] This comparative example is basically the same as Example 4, except that:

[0102] Lacking the inorganic salt template dissolution of step five, the final coating has a dense surface, no porous structure, and no micro-eddy current effect. The water tunnel test shows that the drag reduction rate of the coating is 0.4%.

[0103] Comparative Example 5:

[0104] This comparative example is basically the same as Example 4, except that:

[0105] Lacking the hot-pressing process of step six, the final coating has a large layer spacing and a loose structure. There is a material shedding phenomenon during the water tunnel drag reduction test.

[0106] By comparing example 4 and comparative examples 1-5, it can be seen that the porous drag-reducing coating prepared by the application has the characteristics of rigidity and flexibility and surface roughness, good mechanical stability, obvious micro-vortex and excellent drag-reducing performance.

Claims

1. A method for preparing a porous drag-reducing coating, characterized in that: The preparation method comprises: A mixture of natural polymers, cellulose nanocrystals, two-dimensional nanomaterials and water is coated on a substrate, immersed in a metal salt solution for a certain period of time and then removed to obtain a cross-linked coating coordinated by metal ions; The cross-linked coating with vertical pore structure is obtained by treating the metal ion coordinated cross-linked coating with directional freeze casting technology. The cross-linked coating having a vertical pore structure is immersed in an organic-inorganic mixed solution for a certain period of time, and then taken out to remove the inorganic salt therein to obtain a porous cross-linked coating; the organic-inorganic mixed solution is obtained by mixing a silane coupling agent, an inorganic precursor, an inorganic salt, an adhesive polymer, and an organic solvent; hot-pressing the porous cross-linked coating to obtain a porous drag-reducing coating; The natural polymer is at least one of agar, sodium alginate, and chitosan; The cellulose nanocrystals are at least one of sulfonated cellulose nanocrystals, amination-modified cellulose nanocrystals, carboxylation-modified cellulose nanocrystals, and phosphorylation-modified cellulose nanocrystals; The two-dimensional nanomaterial is at least one of graphene oxide, molybdenum disulfide, and boron nitride; The metal salt is at least one of calcium chloride, ferric chloride, cerium nitrate, and neodymium chloride; The mass fractions of natural polymer, cellulose nanocrystal and two-dimensional nanomaterial in the mixed solution are 0.5-10wt%, 0.1-10wt% and 0.01-5wt% respectively; The mass fraction of the metal salt in the metal salt solution is 0.01-10wt%; The silane coupling agent is at least one of vinyl triethoxysilane, methacryloxypropyl trimethoxysilane, and γ-mercaptopropyl triethoxysilane; The inorganic precursor is at least one of tetraethyl orthosilicate and tetrabutyl titanate; The inorganic salt is at least one of sodium chloride, magnesium chloride and sodium sulfate; The adhesive polymer is at least one of polyurethane, amino silicone resin, and fluorocarbon resin; The organic solvent is at least one of methanol and ethanol; The mass fractions of the silane coupling agent, inorganic precursor, inorganic salt and adhesive polymer in the organic-inorganic mixed solution are 5-10wt%, 0.5-15wt%, 0.5-10wt% and 0.5-5wt% respectively; The hot pressing temperature is 60-120°C.

2. The method for preparing a porous drag-reducing coating according to claim 1, characterized in that: The hot pressing time is 0.5-24h.

3. The method for preparing a porous drag-reducing coating according to claim 1, wherein: The treatment of the metal ion coordinated cross-linked coating by using the directional freeze casting technology includes: freezing at -60°C to -4°C for 0.5-24 hours, and then freeze-drying for 6-72 hours.

4. The method for preparing a porous drag-reducing coating according to claim 1, wherein: The organic-inorganic mixed liquid further comprises an additive with a mass fraction of 0.5-10 wt %, and the additive is at least one of montmorillonite, diatomaceous earth, and halloysite.

5. The method for preparing a porous drag-reducing coating according to claim 1, wherein: The mixing temperature of the organic-inorganic mixed liquid is 5-65°C.

6. A porous drag-reducing coating, characterized in that: The porous drag-reducing coating is prepared by the preparation method according to any one of claims 1 to 5.