Double electric layer regulation based core-shell structure drag reduction and corrosion prevention composite coating, preparation method and application thereof

Abstract

This invention provides a core-shell structure drag-reducing and corrosion-resistant composite coating based on double-layer regulation, its preparation method, and its application. The composite coating comprises a first coating applied to a substrate surface and a second coating applied to the surface of the first coating. The first coating comprises a polysiloxane; the second coating comprises nano-titanium dioxide with a surface-modified modifier, the modifier being a quaternary ammonium salt compound (A) having a long-chain alkyl group. Compound (A) is adsorbed onto the titanium dioxide surface through quaternary ammonium groups to form an organic coating layer. The surface roughness Ra of the composite coating is <0.06 μm. The composite coating provided by this invention achieves integrated synergy of drag reduction and corrosion protection functions, overcoming the defects of poor interface compatibility and mutual functional constraints caused by functional superposition in existing technologies.

Description

Technical Field

[0001] This invention relates to a core-shell structure drag-reducing and corrosion-resistant composite coating based on double-layer regulation, its preparation method and application, belonging to the field of underwater drag-reducing and corrosion-resistant technology. Background Technology

[0002] Underwater drag reduction and corrosion prevention technologies directly impact the economic efficiency, safety, and environmental performance of marine equipment, and are becoming increasingly important in the context of deep-sea development and the shipping industry. Ships and marine structures are exposed to harsh seawater environments for extended periods; biofouling and electrochemical corrosion significantly increase surface roughness, leading to a 10%–50% increase in drag, which in turn increases fuel consumption and carbon emissions. Furthermore, corrosion causes approximately 3% of global GDP loss annually, seriously threatening structural safety and service life. Coating protection, as a key means combining drag reduction and corrosion prevention, is driving the development of non-toxic and environmentally friendly nano-coatings and intelligent release systems, especially given the increasingly stringent regulations on antifouling agents by the International Maritime Organization (IMO). Therefore, strengthening basic research on coatings is a crucial path to achieving cost reduction, efficiency improvement, and green development.

[0003] Current research on drag-reducing and corrosion-resistant coatings mainly revolves around three major directions: organic-inorganic nanocomposites, surface micro / nano structure regulation, and intelligent functional integration. The mainstream approach involves embedding nanoparticles such as titanium dioxide and silica into polydimethylsiloxane (PDMS), epoxy resin, or polyurethane matrices, constructing low surface energy micro / nano structures through spraying or sol-gel processes to achieve synergistic enhancement of superhydrophobic properties and corrosion barriers. Furthermore, the development of fluorine-free waterborne systems responds to the demands of environmental protection and sustainable development. Multifunctional integration is becoming a trend, integrating corrosion resistance, drag reduction, anti-icing, antifouling, and even conductivity into a single coating through molecular structure design to adapt to extreme working conditions such as deep sea and polar regions.

[0004] Reference 1 describes how the microstructure of a superhydrophobic coating (KH550@SiO2 / STA@TiO2 / waterborne polyurethane) reduces the contact area between the fluid and solid interfaces, thereby reducing the penetration of corrosive media into the liquid and lowering flow resistance. Results show that the coating reduces drag by 27% and the corrosion current by four orders of magnitude.

[0005] Reference 2 describes the encapsulation of a pH-responsive cobalt-based metal-organic framework (Co-MOF) loaded with benzotriazole (BTA) within a layered silica shell, further modified with a fluorine-free, low-surface-energy HDTMS layer. Results show that this coating maintains a high impedance modulus (>10) in a 3.5 wt% NaCl solution. 9 Ω·cm 2 The maximum drag reduction rate can reach 31.5%.

[0006] Reference 3 describes a multi-level rough superhydrophobic coating, PVDF-PDMS-PF@μ / nSiO2 (PPPSS), prepared by cold spraying SiO2 modified with perfluorodecyltrimethoxysilane (PFDT) and polyvinylidene fluoride (PVDF) and polydimethylsiloxane (PDMS). Experimental results show that the |Z| of the PPPSS coating... 0.01 Hz It is three orders of magnitude higher than the bare substrate. Flow test results show that the coating achieves a maximum drag reduction of 34.5%.

[0007] Inspired by the smooth, compliant skin properties of dolphins and the common antifouling and drag-reducing smooth liquid-filled porous surfaces (SLIPs), reference 4 describes the preparation of a smooth liquid surface (SLLS) by smartly anchoring a linear long-chain polymer (HPSM) with good compliance and lubrication properties onto the interface. The drag reduction rate of the coating was measured using a rotational rheometer, and the results showed that the best drag reduction rate of this coating was 10%.

[0008] Reference 5 proposes a strategy combining microstructure with benzodiazepine (BTA) to construct a sharkskin-like shield structure, thereby improving drag reduction and wear resistance. Results show that, compared to a smooth pure epoxy coating, the Texture-BTA-3% coating improves corrosion resistance by 30% and drag reduction by approximately 10%.

[0009] Reference 6 describes the preparation of a superhydrophobic coating with a thickness of approximately 13.5 micrometers by spraying hexadecyltrimethoxysilane (HDTMS) modified SiO2 nanoparticles onto the surface of H13 steel with a laser-etched fin-like microstructure. Rheometer testing showed that this superhydrophobic coating achieved a drag reduction rate of up to 23.3% and a corrosion inhibition rate of 95.16%.

[0010] Cited literature 1: Deng Y, Li Z, Yin Z, et al. A facile method for constructing scalable and low-cost superhydrophobic coating with anti-corrosion and drag-reduction properties[J]. Industrial Crops and Products, 2024, 216(000):11.

[0011] Citation 2: Zha Q, Deng Y, Yin Z, et al. Diatom-inspired pH-responsive MOF coatings for active corrosion protection and sustained drag reduction in dynamic marine environments[J]. Chemical Engineering Journal, 2010, 523(000): 12.

[0012] Citation 3: Zhao Y, Zhang P, Gu X, et al. Preparation of PVDF-PDMS-SiO2 multi-stage rough superhydrophobic coating with excellent anti-corrosion and drag reduction performance via one-step cold spraying[J]. Surface & Coatings Technology, 2023, 471.

[0013] Citation 4: Zhang J, Feng M, Wu X, et al. A bio-inspired liquid-like smooth copolymer coating with superior biofouling resistance and drag reduction[J]. Surface & Coatings Technology, 2024, 494.

[0014] Citation 5: Qin L, Lu S, Liu J, et al. Bionic non-smooth epoxy resin coating with corrosion inhibitor for drag-reduction and durability[J]. Progress in Organic Coatings, 2022.

[0015] Cited literature 6: He Z, Li S, Su Summary of the Invention

[0016] The problem the invention aims to solve

[0017] Existing underwater drag-reducing and corrosion-resistant coating technologies are mainly divided into three categories: biomimetic superhydrophobic coatings, hydrophilic / hydrogel coatings, and silicone fouling-releasing coatings. They generally suffer from the following technical shortcomings:

[0018] Biomimetic superhydrophobic coatings suffer from poor gas layer stability, limited pressure resistance, and easy failure of anti-corrosion function; hydrophilic / hydrogel coatings, as mentioned in the above-cited literature, suffer from weak mechanical properties, poor adhesion to the substrate, and mutual constraints between drag reduction and anti-corrosion functions; organosilicon fouling release coatings suffer from poor impermeability, low mechanical strength, and difficulty in meeting long-term anti-corrosion requirements when used alone.

[0019] Furthermore, existing technologies generally suffer from poor synergy between drag reduction and corrosion prevention functions, making it difficult to achieve effective unification of the two from a single material structure. Surface functional structures lack stability under complex service environments such as dynamic water flow, high pressure, and long-term immersion, resulting in rapid functional decay. Moreover, they fail to effectively utilize the key microstructure of the solid / liquid interface double layer and lack a synergistic design approach to simultaneously achieve electrochemical corrosion prevention and fluid drag reduction by regulating the behavior of the double layer.

[0020] Based on the above problems, the present invention needs to provide a composite coating material that can actively regulate the double electric layer structure of the solid / liquid interface and has both high stability drag reduction performance and long-term physical / electrochemical shielding performance, so as to overcome the defects of poor functional synergy, insufficient interface stability and ineffective utilization of microscopic action mechanisms in the prior art.

[0021] Solution for solving the problem

[0022] This invention first provides a core-shell structure drag-reducing and corrosion-resistant composite coating based on double-layer electrical modulation, comprising a first coating applied to the surface of a substrate and a second coating applied to the surface of the first coating, wherein...

[0023] The first coating comprises polysiloxane;

[0024] The second coating includes nano-titanium dioxide with a modified surface, wherein the modified material includes a quaternary ammonium salt compound (A) having a long-chain alkyl group, and the compound (A) is adsorbed onto the surface of the titanium dioxide to form an organic coating layer through the quaternary ammonium group;

[0025] The surface roughness Ra of the composite coating is less than 0.06 μm.

[0026] According to the composite coating of the present invention, the long-chain alkyl group includes C12-C20 alkyl groups.

[0027] According to the composite coating of the present invention, the thickness of the first coating is 2~8μm; and the thickness of the second coating is 5~15μm.

[0028] According to the composite coating of the present invention, the thickness of the organic coating layer is 5~20 nm.

[0029] According to the composite coating of the present invention, the polysiloxane comprises polydimethylsiloxane; and the compound (A) comprises an ammonium halide salt having an alkyl group having a C12-C20 group.

[0030] The present invention also provides a method for preparing a composite coating according to the present invention, comprising the following steps:

[0031] S1:

[0032] S1a: Compound (A) and nano-titanium dioxide are mixed in a solvent and reacted to obtain nano-titanium dioxide modified by compound (A);

[0033] S1b: A composite solution is obtained by dispersing nano-titanium dioxide modified with compound (A) in a solvent;

[0034] S2: The polysiloxane prepolymer and curing agent are coated on the substrate surface and pre-cured to form a substrate adhesion layer;

[0035] S3: The composite solution is applied to the surface of the substrate adhesion layer and cured to form a composite coating.

[0036] According to the preparation method of the present invention, in step S1a, the amount of compound (A) used during the reaction is 10-50% by mass of nano-titanium dioxide.

[0037] The solvent includes alcohol solvents;

[0038] The reaction temperature is 30~70℃, and the reaction time is 1~3h.

[0039] According to the preparation method of the present invention, in step S1b, the content of the nano-titanium dioxide modified by compound (A) in the composite solution is 0.05% to 0.4% by mass.

[0040] The solvent includes alcohol solvents.

[0041] According to the preparation method of the present invention, in step S2, the pre-curing temperature is 60~80℃ and the pre-curing time is 10~60min.

[0042] According to the preparation method of the present invention, in step S3, the curing temperature is 60~80℃ and the curing time is 1~3h.

[0043] The present invention also provides an application of the composite coating described in the present invention on ships or marine structures.

[0044] The effects of the invention

[0045] By implementing the above technical solution, the present invention can achieve the following technical effects:

[0046] The organic-inorganic synergistic composite coating provided by this invention uses polysiloxane as a flexible matrix and titanium dioxide nanoparticles as a rigid framework. A uniform modification layer of long-chain alkyl quaternary ammonium salt compound (A) is constructed on the surface of the titanium dioxide nanoparticles. This actively weakens turbulent shear stress through electroviscous effects, while simultaneously electrostatically repelling chloride ions to block the electrochemical corrosion pathway. This charge-driven mechanism eliminates excessive dependence on the morphology of micro / nano structures, fundamentally solving the bottleneck of performance degradation caused by mechanical wear. Therefore, this design achieves a paradigm shift from "structure-dependent" to "material intrinsic performance-driven," providing a more reliable protection solution for marine equipment.

[0047] The organic-inorganic synergistic composite coating provided by this invention achieves integrated synergy of drag reduction and corrosion protection functions, overcoming the defects of poor interface compatibility and mutual functional constraints caused by functional superposition in existing technologies. It significantly improves the stability of the interface functional structure under complex service environments, overcoming the defects of easy failure of the gas layer or hydration layer and rapid decay of drag reduction performance in existing technologies. For the first time, the microscopic mechanism of the double electric layer is systematically introduced into the design of drag-reducing and corrosion-resistant materials, realizing the synergistic regulation of electrochemical corrosion protection and fluid drag reduction. Attached Figure Description

[0048] Figure 1 Figure 'a' shows a simulation diagram of the fluid-solid interface interaction in the TiO2-H2O composite model. Figure 1 Figure b shows a simulation diagram of the fluid-solid interface interaction in the TiO2@CTAB-H2O composite model;

[0049] Figure 2 Image 'a' in the diagram shows a TEM image of TiO2 nanoparticles. Figure 2 b in the image shows a TEM image of TiO2@CTAB. Figure 2 c in the figure shows the infrared spectra of TiO2, CTAB, and TiO2@CTAB;

[0050] Figure 3 Figure a shows a scanning electron microscope image of the TiO2-PDMS coating obtained in Comparative Example 3. Figure 3 Figure b shows a scanning electron microscope image of the TiO2@CTAB-PDMS coating prepared in Example 1. Figure 3 c~g in the figure show the energy dispersive spectroscopy (EDS) spectra of C, O, N, Ti and Br of the TiO2@CTAB-PDMS coating prepared in Example 1, respectively.

[0051] Figure 4 Figure 'a' shows the surface morphology test results of Comparative Example 1. Figure 4 b in the figure shows the surface morphology test results of Example 1. Figure 4 c in the figure shows the surface morphology test results of Comparative Example 4. Figure 4 The 'd' in the figure shows the surface morphology test results of Comparative Example 5. Figure 4 The figure 'e' shows the surface morphology test results of Comparative Example 6;

[0052] Figure 5 Figure 'a' shows the XPS full spectrum scan of the TiO2@CTAB composite particles. Figure 5 Figures b to f show the XPS elemental spectra of C 1s, O 1s, Ti 2p, N 1s, and Br 3d for TiO2@CTAB composite particles;

[0053] Figure 6 Figure 'a' shows the static contact angle test results for Comparative Example 1. Figure 6 b in the figure shows the static contact angle test results of Example 1. Figure 6 Figure 'c' shows the static contact angle test results for Comparative Example 4. Figure 6 The value of 'd' in the figure shows the static contact angle test results of Comparative Example 5. Figure 6 The value of 'e' in the figure shows the static contact angle test results of Comparative Example 6;

[0054] Figure 7 Figures a and b show the torque curves and drag reduction test results of the coatings obtained in Example 1 and Comparative Examples 1-3, respectively.

[0055] Figure 8 The drag reduction rate test results of the coatings prepared in Example 1 and Comparative Examples 4-6 are shown in the figure;

[0056] Figure 9 Figures a through d show the polarization curves, Nyquist plots, phase angle plots, and Bode plots of the coatings obtained in Example 1 and Comparative Examples 1 through 3 in 3.5% NaCl solution, respectively. Detailed Implementation

[0057] The present invention will now be described in detail. The descriptions of the technical features described below are based on representative embodiments and specific examples of the present invention, but the present invention is not limited to these embodiments and specific examples. It should be noted that:

[0058] In this specification, the range of values ​​referred to as "value A to value B" refers to the range including the endpoint values ​​A and B.

[0059] In this specification, the numerical range indicated by "above" or "below" refers to the numerical range that includes the stated number.

[0060] In this specification, the word "may" has two meanings: to perform a certain process and not to perform a certain process.

[0061] In this specification, the terms "optional" or "optional" are used to indicate the use or omission of certain substances, components, procedures, application conditions, etc.

[0062] In this specification, "room temperature" or "room temperature" refers to an indoor ambient temperature of "23±2℃", and unless otherwise specified, the "viscosity" of this invention refers to the viscosity at this temperature.

[0063] All unit names used in this manual are international standard unit names, and unless otherwise stated, the "%" indicates weight or mass percentage.

[0064] In this specification, the term "substantially" is used to indicate that the standard deviation from the theoretical model or theoretical data is within a range of 5%, preferably 3%, and more preferably 1%.

[0065] In this specification, the terms “comprising” and / or “including” are used to indicate the presence of features, steps, operations, devices, components and / or combinations thereof.

[0066] In this specification, references to "some specific / preferred embodiments," "other specific / preferred embodiments," "implementation," etc., refer to specific elements (e.g., features, structures, properties, and / or characteristics) related to that embodiment, which are included in at least one of the embodiments described herein and may or may not be present in other embodiments. Furthermore, it should be understood that these elements may be combined in any suitable manner in various embodiments.

[0067] This invention primarily provides a drag-reducing and corrosion-resistant composite coating based on double-layer electrical modulation of a core-shell structure, its preparation method, and its application. This invention is based on the following insights:

[0068] As mentioned earlier, underwater vehicles, ships, marine engineering equipment, and pipelines operate in marine or freshwater environments for extended periods. Their surfaces face two major challenges: increased energy consumption due to increased fluid friction resistance and material failure caused by corrosive media. Developing surface materials that combine efficient drag reduction with long-term corrosion protection is crucial for improving equipment energy efficiency and extending service life. Currently, mainstream underwater drag-reducing and corrosion-resistant coatings are mainly classified into three categories: biomimetic superhydrophobic coatings, hydrophilic / hydrogel coatings, and silicone fouling-releasing coatings. Specifically:

[0069] 1. Biomimetic superhydrophobic coating

[0070] Biomimetic superhydrophobic coatings reduce drag by constructing micro / nano-rough structures on the material surface and modifying it with low surface energy materials. This is achieved by using the air layer trapped at the solid / liquid interface to form a "plastic gas layer," transforming solid-liquid friction into gas-liquid internal friction. However, this type of coating has significant shortcomings in practical applications: First, under conditions of hydrostatic pressure changes, long-term water flow shear force, or high pressure, the trapped air layer is unstable and prone to escape or collapse. Once the gas layer disappears, the surface becomes completely wetted, resulting in the loss of drag reduction and, in fact, the micro / nano structure increasing frictional resistance. Second, the coating's corrosion resistance depends on the physical isolation effect of the gas layer. When the coating is partially damaged or the gas layer fails, the micro / nano structure interface easily becomes a capillary channel for corrosive media, exacerbating pitting corrosion and making it difficult to simultaneously meet the dual requirements of long-term corrosion protection and stable drag reduction.

[0071] 2. Hydrophilic / hydrogel coating

[0072] Hydrophilic / hydrogel coatings utilize surface hydrophilic groups (such as polyethylene glycol and zwitterionic polymers) to bind water molecules and form a dense hydrated layer, transforming solid / solid friction into solid / liquid / solid friction to reduce shear force. However, this type of coating has the following problems: First, the mechanical properties of hydrogel materials differ significantly from those of metal substrates, making them prone to peeling and wear under high-flow-rate scouring, resulting in limited service life; second, the coating is prone to internal stress during water absorption and swelling, leading to cracking, while water molecules and corrosive ions (such as chloride ions) can penetrate along the hydrated layer to the substrate interface, causing interfacial corrosion; third, there is an inherent contradiction between the drag reduction function's dependence on moisture and the corrosion protection function's requirement to block moisture, making it difficult for existing technologies to construct a high-impedance physical shielding layer while maintaining high hydrophilicity.

[0073] 3. Silicone-based fouling release coating

[0074] Organosilicon fouling release coatings use organosilicon materials such as polydimethylsiloxane as the matrix. Utilizing their low surface energy and low elastic modulus, they make it difficult for marine organisms to adhere firmly and also provide a certain drag reduction effect. However, these coatings have poor impermeability; oxygen and corrosive ions can easily penetrate the coating and reach the metal substrate, making it difficult to meet long-term corrosion protection requirements when used alone. Furthermore, they have low mechanical strength and poor scratch resistance, making them easily damaged during construction and service. Their drag reduction performance is also easily affected by surface contamination, resulting in insufficient functional stability.

[0075] A comprehensive analysis of the existing technologies reveals the following common technical deficiencies in the field of underwater drag reduction and corrosion protection coatings:

[0076] First, there is poor functional synergy. Existing technologies often combine drag reduction and corrosion protection as independent functions, lacking a design approach that achieves functional synergy from the perspective of a single material structure. This results in weak interfacial bonding between the drag reduction layer and the corrosion protection layer, or the realization of one function at the expense of weakening another.

[0077] Second, the stability of the interface structure is insufficient. Whether it is the maintenance of the gas layer of the superhydrophobic coating or the maintenance of the hydration layer of the hydrogel coating, the existing technologies cannot guarantee the stability of the surface functional structure under complex service environments such as dynamic water flow, high pressure and long-term immersion. The functional decay rate is faster than the degradation rate of the material itself.

[0078] Third, the microscopic mechanisms of action have not been effectively utilized. Existing technologies pay little attention to the key microstructure of the solid / liquid interface double layer, which determines the interfacial charge distribution, ion transport, and near-wall fluid slip behavior. They have failed to achieve the synergistic effect of electrochemical corrosion prevention (inhibiting charge transfer) and fluid drag reduction (regulating interfacial slip) by actively controlling the double layer structure.

[0079] To address the shortcomings of the existing technology, this invention proposes the following technical problems to be solved and technical solutions to those problems:

[0080] First, this invention addresses the problems of poor surface functional structure stability and susceptibility to failure under high pressure and dynamic water flow environments in existing underwater drag-reducing coatings. Existing superhydrophobic coatings rely on an air layer that is difficult to maintain long-term under water flow shear and hydrostatic pressure, while the hydration layer of hydrogel coatings is easily peeled off under scouring, leading to a rapid decline in drag-reducing performance. This invention aims to construct a stable core-shell structure and utilize shell structural design and double-layer regulation to form a novel drag-reducing mechanism that does not rely on the physical existence of a trapped air layer or hydration layer, thereby improving the stability of the interface structure under complex service environments.

[0081] Secondly, this invention addresses the inherent contradiction between the difficulty of achieving efficient drag reduction and long-term corrosion protection in existing drag-reduction materials. In current technologies, achieving drag reduction often comes at the cost of weakened physical shielding or electrochemical corrosion protection, or the use of multi-layered composite structures leads to weak interfacial bonding and a high risk of interlayer failure. This invention aims to achieve a unified drag reduction and corrosion protection function through an integrated core-shell structure design, where the core layer and shell layer respectively assume different roles such as mechanical support, physical shielding, and interfacial function regulation, thus avoiding interfacial compatibility issues caused by functional overlap.

[0082] Third, this invention addresses the problem that existing technologies do not fully utilize the microscopic mechanisms of the electric double layer at the solid / liquid interface and lack means to actively regulate interfacial charge and fluid slip behavior. Existing coatings are rarely designed from the perspective of electrochemical and hydrodynamic coupling, failing to utilize the regulatory effects of the electric double layer structure on ion transport, charge transfer, and near-wall fluid slip. This invention aims to actively regulate the electric double layer structure at the solid / liquid interface through the synergistic design of material chemical composition and core-shell structure. It utilizes the charge repulsion effect of the electric double layer to inhibit the adsorption and penetration of corrosive ions, while simultaneously using the interfacial slip effect induced by the electric double layer to reduce fluid friction resistance, thereby achieving an integrated function of drag reduction and corrosion prevention based on the electric double layer effect.

[0083] Electric double layer drag reduction is a cutting-edge technology that actively reduces drag by controlling the charge distribution at the solid-liquid interface. Its core principle lies in using surfactants or ionized groups to form a dense nanoscale adsorption layer on the wall surface, reconstructing traditional no-slip boundary conditions. When cationic molecules anchor to a solid surface, their hydrophilic head groups attract counterions in the solution, constructing an electrochemical electric double layer (EDL) at the interface, generating a significant electroviscous effect: under shear stress, the relaxation motion of ions within the EDL consumes turbulent pulsation energy, suppressing the generation of near-wall vortices; simultaneously, the electrodynamic slip induced by the charged surface reduces the fluid velocity gradient and wall shear stress. Compared to superhydrophobic drag reduction relying on micro / nano structures, the electric double layer mechanism does not require fragile or rough structures; the electrostatic shielding effect of the EDL repels corrosive ions, achieving an integrated drag reduction and corrosion protection function.

[0084] Through research, the inventors designed a TiO2-polysiloxane composite coating modified with a long-chain alkyl quaternary ammonium salt compound (A). Taking CTAB (hexadecyltrimethylammonium bromide) as the long-chain alkyl quaternary ammonium salt compound (A) as an example, CTAB forms a stable organic coating layer on the surface of TiO2 nanoparticles through electrostatic adsorption, effectively inhibiting the aggregation of TiO2 nanoparticles. The polysiloxane then achieves uniform dispersion on the substrate, constructing a dense and smooth coating structure. Molecular dynamics simulations show that the synergistic effect of CTAB's "interface anchoring-water structure regulation-slip enhancement" weakens interfacial hydrogen bonding, increasing the slip length and achieving excellent drag reduction. Furthermore, the synergistic protective system formed by the dense physical barrier constructed by the long chain of CTAB, the electrostatic repulsion effect of the quaternary ammonium salt head group, and the enhanced interfacial stability enhances corrosion resistance. This amphiphilic molecular interface modification strategy achieves the integrated function of drag reduction and corrosion prevention.

[0085] <First Aspect>

[0086] A first aspect of the present invention provides a core-shell structure drag-reducing and corrosion-resistant composite coating based on double-layer regulation, comprising a first coating applied to the surface of a substrate and a second coating applied to the surface of the first coating, wherein the first coating comprises polysiloxane; the second coating comprises nano-titanium dioxide (TiO2) modified by a modifier, wherein the modifier comprises a quaternary ammonium salt compound (A) having a long-chain alkyl group, and the compound (A) is adsorbed onto the surface of the titanium dioxide through the quaternary ammonium group to form an organic coating layer.

[0087] In this invention, the surface roughness Ra of the composite coating is less than 0.06 μm, preferably less than 0.05 μm, and more preferably less than 0.03 μm.

[0088] (Matrix)

[0089] There is no particular limitation on the type of substrate used in this invention. It can be a commonly used material for ships or marine structures, and examples include steel, aluminum, aluminum alloys, etc.

[0090] (First coating)

[0091] The first coating of the present invention is mainly an adhesive layer, which is used to form a flexible substrate adhesion layer on the surface of the substrate, so that the second coating can be effectively adhered. In the present invention, the first coating mainly includes polysiloxane.

[0092] In some specific implementations, the polysiloxane may include polydimethylsiloxane (PDMS), etc.

[0093] There is no particular limitation on the thickness of the first coating, as long as it can achieve effective adhesion of the second coating. Preferably, the thickness of the first coating can be 2~8μm, such as 3μm, 5μm, 7μm, etc.

[0094] (Second coating)

[0095] The second coating of the present invention is a modified TiO2 coating, wherein the modified TiO2 has a core-shell structure. Specifically, a quaternary ammonium salt compound (A) with long-chain alkyl groups forms a stable organic coating layer on the surface of titanium dioxide (TiO2) through electrostatic adsorption of quaternary ammonium groups. This coating layer can effectively inhibit the aggregation of TiO2 nanoparticles, so that the second coating is uniformly dispersed on the surface of the first coating. The second coating is mainly attached to the surface of the first coating through physical action.

[0096] In this invention, the second coating can form an electric double layer effect in the liquid medium, which is the core mechanism for achieving low frictional resistance, thereby effectively reducing drag underwater. Specifically, taking CTAB as a quaternary ammonium salt compound (A) with a long-chain alkyl group as an example, the molecular dynamics simulation diagram of the TiO2@CTAB coating is as follows. Figure 1 As shown, where Figure 1 In the diagram, 'a' and 'b' represent the fluid-solid interface interaction simulations of the TiO2-H2O composite model and the TiO2@CTAB-H2O composite model, respectively. Combined with... Figure 1 The microscopic mechanism by which TiO2@CTAB coatings reduce drag through interface structure modulation is analyzed:

[0097] like Figure 1 As shown in Figure a, in the TiO2-H2O system, a hydrogen bond network forms between water molecules and surface hydroxyl groups, enhancing interfacial adhesion and thus restricting the sliding of water molecules, increasing flow resistance. In contrast, the TiO2@CTAB-H2O system ( Figure 1 In b), the CTAB molecule uses a positively charged hydrophilic head group (-N). +(CH3)3) Anchored to the TiO2 surface, the hydrophobic alkyl chains extend outward to form an ordered molecular layer. This structure significantly modulates the distribution and motion of water molecules at the interface: the CTAB head group attracts and orders water molecules through electrostatic interactions, forming a stable hydrated layer; simultaneously, the hydrophobic chain coverage reduces direct hydrogen bonding between water molecules and the TiO2 surface, lowering the interfacial adhesion energy barrier. This synergistic mechanism of "interfacial anchoring-water molecule structure regulation-slip enhancement" effectively improves the diffusion and mobility of water molecules in the near-wall region, increases the slip length, and thus reduces flow shear resistance. Furthermore, the CTAB molecular layer induces an alternating ordered-disorder water molecule structure in the interfacial region, which helps to smooth the near-wall velocity profile, suppress turbulent bursts, and ultimately exhibits a significant drag reduction effect in macroscopic flow. These results demonstrate that efficient drag reduction can be achieved by rationally designing the molecular interface to regulate the water structure and dynamic behavior.

[0098] Furthermore, in terms of corrosion protection, the modification of TiO2 surfaces with quaternary ammonium salt compounds (A) containing long-chain alkyl groups introduces multiple protective mechanisms. These long-chain alkyl groups form a dense protective layer at the interface, effectively hindering water molecules and dissolved corrosive ions (such as Cl-). - The quaternary ammonium salt compound (A) with long-chain alkyl groups transports substances to the substrate, delaying the enrichment process of corrosive media in the interfacial region. Simultaneously, the positive charge on the head group of the quaternary ammonium salt compound (A) creates a local electrostatic potential field at the interface, repelling anionic corrosive media and inhibiting their adsorption and migration on the electrode surface. Furthermore, the quaternary ammonium salt compound (A) with long-chain alkyl groups enhances the coating's adhesion stability through the interaction between its head group and the TiO2 surface, and the arrangement of the hydrophobic chains also reduces the activity and permeability of the interfacial water layer. This synergistic mechanism of "physical barrier-electrostatic repulsion-interfacial stabilization" not only improves the coating's barrier performance against corrosive media but also helps maintain the integrity of the interfacial structure and long-term protective capability in dynamic fluid environments.

[0099] In summary, TiO2 coatings modified with long-chain alkyl quaternary ammonium salt compounds (A) reduce drag by regulating the interfacial water structure and weakening hydrogen bonding, and improve corrosion resistance through the synergistic effect of hydrophobic physical barriers and electrostatic repulsion.

[0100] In some specific embodiments, the long-chain alkyl quaternary ammonium salt compound (A) includes C12-C20 alkyl groups, preferably C14-C20 alkyl groups, and more preferably C16-C18 alkyl groups.

[0101] In some preferred embodiments, the compound (A) may include an ammonium halide salt having an alkyl group of C12-C20, and as a preferred embodiment, the compound (A) may be hexadecyltrimethylammonium bromide.

[0102] In some specific implementations, the particle size of the nano-titanium dioxide can be 50~200nm, preferably 50~100nm.

[0103] In some specific implementations, the thickness of the organic coating layer can be 5~20nm, for example, 8nm, 10nm, 12nm, 15nm, etc.

[0104] In some specific implementations, the thickness of the second coating can be 5~15μm, for example, 8μm, 10μm, 12μm, 14μm, etc.

[0105] In some preferred embodiments, the composite coating of the present invention is effective at a shear rate of 100 s⁻¹. -1 At that time, the maximum drag reduction rate can be 42%; when aluminum is used as the substrate, the corrosion current density of the substrate can be reduced by about three orders of magnitude (3.404 μA·cm). -2 Reduced to 3.27×10 -3 μA·cm -2 The charge transfer resistance can be improved by four orders of magnitude (4.392 × 10⁻⁶). 4 Ω·cm 2 Increased to 7.573×10 8 Ω·cm 2 The bacterial coverage of the TiO2@CTAB-PDMS composite coating surface was 0.12%, which was 99% lower than that of the aluminum substrate surface. Molecular dynamics simulations showed that the synergistic effect of CTAB's "interface anchoring-water structure regulation-slip enhancement" could weaken the interfacial hydrogen bonding, resulting in a 150% increase in slip length.

[0106] <Second aspect>

[0107] A second aspect of the present invention provides a method for preparing a composite coating according to the first aspect, comprising the following steps:

[0108] S1:

[0109] S1a. Compound (A) and nano-titanium dioxide were mixed in a solvent and reacted to obtain nano-titanium dioxide modified with compound (A);

[0110] S1b. The nano-titanium dioxide modified by compound (A) is dispersed in a solvent to obtain a composite solution;

[0111] S2: The polysiloxane prepolymer and curing agent are coated on the substrate surface and pre-cured to form a substrate adhesion layer;

[0112] S3: The composite solution is applied to the surface of the substrate adhesion layer and cured to form a composite coating.

[0113] Steps S1 and S2 can be performed separately, simultaneously, or in any order.

[0114] (Step S1)

[0115] Step S1 of the present invention includes step S1a, which is a modification step of nano-titanium dioxide particles. The nano-titanium dioxide particles are modified by a surface modification method using compound (A). Specifically, compound (A) and nano-titanium dioxide are mixed in a solvent and reacted to obtain nano-titanium dioxide modified by compound (A).

[0116] The type of compound (A) is the same as that described in the first aspect, and will not be repeated here.

[0117] In some specific implementations, to further improve the uniformity of the reaction, the mixing can be performed by first dissolving compound (A) in a solvent, stirring until completely dissolved, then adding titanium dioxide, and finally ultrasonically dispersing at 100-500W for 5-20 minutes. The dissolution temperature can be 30-70°C during the dissolution step.

[0118] In some specific embodiments, the reaction temperature can be 30~70℃, for example, 35℃, 40℃, 45℃, 50℃, 60℃, etc.; the reaction time is 1~3h, for example, 1.5h, 2h, 2.5h, etc. Preferably, the reaction can be carried out under stirring conditions, and the stirring can be carried out using stirring methods commonly used in the art, and the stirring rate can be 300~500rpm.

[0119] In some specific embodiments, the amount of compound (A) can be 10-50% by mass of the nano-titanium dioxide, for example, 15%, 20%, 25%, 30%, 35%, 40%, 45%, etc. By controlling the amount of the quaternary ammonium salt compound (A) with long-chain alkyl groups within the above range, the following effects can be achieved: First, the quaternary ammonium salt cationic head groups are adsorbed onto the surface of nano-titanium dioxide through electrostatic interaction, forming a dual stabilizing mechanism of steric hindrance and electrostatic repulsion, effectively inhibiting the agglomeration of nanoparticles and ensuring uniform dispersion in the spin-coating solution during the preparation process, thereby uniformly dispersing on the surface of the first coating; Second, the long-chain alkyl groups extend outward, constructing a low surface energy hydrophobic layer on the coating surface, reducing the adhesion of water droplets and oil stains, thereby synergistically improving drag reduction and corrosion resistance; In addition, an appropriate amount of compound (A) can also improve the interfacial compatibility between TiO2 and the first coating and substrate, especially the aluminum alloy substrate, promoting uniform film formation.

[0120] If the amount of compound (A) is less than 10% by mass, the surface coverage of nano-titanium dioxide will be insufficient, and the particles will easily agglomerate. After spin coating, the coating will show uneven particle accumulation and increased surface roughness, resulting in a decrease in drag reduction effect and microscopic defects in the anti-corrosion barrier. If the amount of compound (A) is greater than 50% by mass, the excessive free quaternary ammonium salt cannot be completely anchored to the TiO2 surface. It will precipitate to the coating surface or form an independent phase during the drying process, causing increased film brittleness, reduced adhesion, and even weakening hydrophobicity and corrosion resistance due to the exposure of hydrophilic head groups.

[0121] The present invention does not particularly limit the type of solvent, as long as it can fully dissolve the solvent. For example, it can be a commonly used alcohol solvent in the art, such as ethanol and isopropanol.

[0122] In some specific embodiments, the reaction step may be followed by a post-processing step, specifically, solid-liquid separation, washing, drying, and grinding. The present invention does not particularly limit the solid-liquid separation, washing, drying, and grinding steps, and they can be selected as needed. Specifically, the reacted solution can be centrifuged (6000~10000 r / min, 5~20 min) to collect the precipitate, washed 2~5 times with a solvent to remove residual compound (A), and then vacuum dried at 50~70℃ for 10~20 h before grinding to obtain TiO2 composite powder modified with compound (A).

[0123] The nano-titanium dioxide can be commercially available or prepared. The present invention does not particularly limit the preparation method of the nano-titanium dioxide, and methods commonly used in the art can be used.

[0124] Furthermore, step S1 of the present invention also includes step S1b, which is the preparation of a composite solution containing nano-titanium dioxide modified by compound (A).

[0125] In some specific embodiments, the content of the nano-titanium dioxide modified by compound (A) in the composite solution is 0.05% to 0.4% by mass, for example, it can be 0.1% by mass, 0.2% by mass, 0.3% by mass, etc.

[0126] When the content of nano-titanium dioxide modified by compound (A) is controlled within the above range, a uniformly dispersed and dense nano-titanium dioxide thin film structure modified by compound (A) can be formed on the surface of the substrate adhesion layer. This thin film structure has a low surface roughness and can achieve a dual improvement in drag reduction and corrosion resistance.

[0127] The present invention does not particularly limit the type of solvent used to form the above-mentioned composite solution, as long as it can sufficiently disperse the nano-titanium dioxide modified by compound (A). For example, it can be a commonly used alcohol solvent in the art, such as ethanol or isopropanol. In some specific embodiments, the dispersion can be performed by ultrasonic treatment (300~500W) for 5~20 minutes.

[0128] (Step S2)

[0129] Step S2 of the present invention is the pre-curing of the first coating. Specifically, the polysiloxane prepolymer and the curing agent are coated on the substrate surface and pre-cured to form a substrate adhesion layer.

[0130] The types of polysiloxanes are the same as those described in the first aspect, and will not be repeated here.

[0131] The present invention does not particularly limit the type and amount of the curing agent, and can select it as needed.

[0132] In some specific embodiments, the amount of the polysiloxane prepolymer can be 50~100 mg / cm³. 2 For example, it can be 60mg / cm³ 2 70mg / cm 2 80mg / cm 2 90mg / cm 2 wait.

[0133] In some specific implementations, the pre-curing temperature can be 60~80℃, for example, 65℃, 70℃, 75℃, etc.; the pre-curing time can be 10~60min, for example, 20min, 30min, 40min, 50min, etc. By controlling the pre-curing conditions within the above range, the first coating can be formed into a semi-cured state that is not fully cured. Applying the second coating in this state allows the TiO2 coating modified by compound (A) to form better on the surface of the first coating, avoiding the embedding of TiO2 modified by compound (A) into the first coating, thus preventing the effective improvement of drag reduction and corrosion protection.

[0134] (Step S3)

[0135] Step S3 of the present invention is the preparation of the composite coating. Specifically, the composite solution obtained in S1b above is coated on the surface of the substrate adhesion layer and cured to form a composite coating.

[0136] In some specific implementations, the curing temperature is 60~80℃, for example, it can be 65℃, 70℃, 75℃, etc.; the curing time is 1~3h, for example, it can be 1.5h, 2h, 2.5h, etc.

[0137] In some specific implementations, the amount of the composite solution used can be 50~200 μL / cm³. 2 For example, it can be 80 μL / cm 2 100μL / cm 2 120μL / cm 2 150μL / cm 2 180μL / cm 2 wait.

[0138] Example

[0139] The embodiments of the present invention will be described in detail below with reference to examples. However, those skilled in the art will understand that the following examples are for illustrative purposes only and should not be considered as limiting the scope of the invention. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer are followed. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0140] Example 1

[0141] (1) Titanium dioxide nanoparticles were prepared by the sol-gel method:

[0142] First, 90 mL of anhydrous ethanol, 5 mL of deionized water, and 1.0 mL of concentrated hydrochloric acid were mixed in a 250 mL beaker and magnetically stirred for 10 min to prepare an acidic alcohol-water reaction medium. Separately, 20 mL of tetrabutyl titanate and 30 mL of anhydrous ethanol were mixed in a dry beaker and stirred until homogeneous to prepare a titanium source precursor solution. The acidic medium was transferred to a 250 mL three-necked flask and placed in a 25°C water bath with vigorous stirring. Simultaneously, the titanium source solution was added dropwise through a constant-pressure dropping funnel at a rate of approximately 2 drops / s over approximately 60 min. After the addition was complete, the reaction was continued at 25°C for approximately 7 h with stirring to obtain a translucent to milky white titanium dioxide sol. This sol was sealed and aged at room temperature for 24 h, then dried at 60°C for 24 h to obtain a white solid. Finally, it was ground into nanoparticles using an agate mortar and pestle and stored in a sealed container.

[0143] (2) Preparation of CTAB-modified titanium dioxide nanoparticles by surface modification method:

[0144] 0.20 g of CTAB was dissolved in 100 mL of anhydrous ethanol preheated to 50°C and stirred until completely dissolved. Then, 1.00 g of self-made titanium dioxide powder was added, and the mixture was first ultrasonically dispersed at 300 W for 10 min, followed by stirring at 50°C and 400 rpm for 2 h. After the reaction was complete, the precipitate was collected by centrifugation (8000 rpm, 10 min) and washed three times with anhydrous ethanol to remove residual CTAB. The final sample was vacuum dried at 60°C for 12 h, ground, and then TiO2@CTAB composite powder was obtained and sealed for later use.

[0145] (3) The TiO2@CTAB-PDMS composite coating was prepared by a layered method:

[0146] First, PDMS prepolymer and curing agent were mixed uniformly at a mass ratio of 10:1 to obtain a mixture. Approximately 2.2g of the mixture was scraped onto a clean substrate surface (the substrate was an aluminum sheet with a diameter of 60mm and a thickness of 1mm), and pre-cured at 70℃ for 30min to form a substrate adhesion layer. Subsequently, TiO2@CTAB powder was dispersed in isopropanol at a ratio of 0.1wt%, and after preliminary mixing by mechanical stirring, it was ultrasonically treated at 400W power for 10min to form a homogeneous composite solution. Finally, 5mL of this solution was dipped onto the pre-cured PDMS substrate, placed on a horizontal platform, and cured at 70℃ for 2h to obtain a TiO2@CTAB-PDMS composite coating.

[0147] Comparative Example 1

[0148] An uncoated aluminum substrate was used as Comparative Example 1.

[0149] Comparative Example 2

[0150] The difference from Example 1 is that TiO2@CTAB coating is not used to obtain a PDMS coating.

[0151] Comparative Example 3

[0152] The difference from Example 1 is that CTAB was not used to modify TiO2, resulting in a TiO2-PDMS coating.

[0153] Comparative Example 4

[0154] The difference from Example 1 is that TiO2@CTAB powder is dispersed in isopropanol at a ratio of 0.5 wt%, while the remaining steps are the same as in Example 1.

[0155] Comparative Example 5

[0156] The difference from Example 1 is that TiO2@CTAB powder is dispersed in isopropanol at a ratio of 1 wt%, while the remaining steps are the same as in Example 1.

[0157] Comparative Example 6

[0158] The difference from Example 1 is that TiO2@CTAB powder is dispersed in isopropanol at a ratio of 1.5 wt%, while the remaining steps are the same as in Example 1.

[0159] Performance testing

[0160] 1. Surface morphology and chemical composition testing:

[0161] (1) Transmission electron microscopy (TEM) test: The TiO2 and TiO2@CTAB particles prepared in Example 1 were tested using transmission electron microscopy (TEM) and the results were obtained respectively. Figure 2 a and b in the example.

[0162] Depend on Figure 2 As can be seen from a, the unmodified TiO2 is irregularly spherical or ellipsoidal with a particle size of about 50~200nm. The lattice stripes are clear, and the interplanar spacing of 0.35 nm corresponds to the (101) crystal plane of the anatase phase. However, there is obvious agglomeration between the particles.

[0163] Depend on Figure 2 As shown in b, an amorphous organic coating layer of approximately 10 nm thickness is visible on the surface of the TiO2@CTAB particles. The lattice fringes are blurred, the particle boundaries become rounded, and the interparticle spacing is significantly increased. This indicates that CTAB molecules form a stable steric hindrance layer on the TiO2 surface through electrostatic adsorption, effectively suppressing hard aggregation between nanoparticles. This difference in microstructure directly confirms the successful anchoring of CTAB, providing a structural basis for subsequent uniform dispersion and interfacial double-layer modulation in PDMS matrices.

[0164] (2) Fourier Transmission Infrared Spectroscopy (FT-IR) Test: The Fourier Transmission Infrared Spectroscopy (FT-IR) test was performed on TiO2, CTAB and TiO2@CTAB obtained in Example 1. The test results are as follows: Figure 2 As shown in c in the figure.

[0165] Figure 2 In section c, the infrared absorption spectrum of TiO2 nanoparticles is shown in curve a. It can be seen that at 3400 cm⁻¹... -1 The characteristic peaks appearing correspond to the stretching vibrations of surface hydroxyl groups (Ti-OH) and adsorbed water, indicating that there are a large number of hydrophilic hydroxyl groups on the TiO2 surface; at 1632 cm⁻¹ -1 The characteristic peak at the position corresponds to the HOH bending vibration of adsorbed water molecules, and is located at 3400 cm⁻¹. -1 The peaks together confirm the presence of physically adsorbed water on the surface. (At 700-510 cm⁻¹) -1The characteristic peaks appearing at the specified position correspond to Ti-O-Ti lattice vibrations, which is the characteristic lattice breathing mode of anatase TiO2 nanoparticles, directly confirming the existence of the TiO2 crystal phase structure. Curves b and c represent the infrared absorption spectra of CTAB and TiO2@CTAB, respectively. It can be seen that the TiO2 surface at 3400 cm⁻¹... -1 Broad and strong absorption peaks appeared at all positions, representing hydrogen-bonded hydroxyl groups, indicating the presence of a certain number of hydroxyl groups on the surface of the nanoparticles before and after modification. The infrared characteristic peaks of hexadecyltrimethylammonium bromide (CTAB) mainly originate from its long-chain alkyl and quaternary ammonium salt structures, with a peak at 2920 cm⁻¹. -1 (CH3 / CH2 asymmetric stretching) and 2853 cm -1 The (CH3 / CH2 symmetric stretching) peak is the strongest characteristic peak, corresponding to the methyl and methylene vibrations of the hexadecyl chain; at 960 cm⁻¹ -1 The characteristic peak appearing at the position corresponds to the quaternary ammonium group -N + The CN stretching vibration of (CH3)3 directly reflects the presence of the trimethylammonium cation structure in the CTAB molecule; at 911 cm⁻¹ -1 The characteristic peaks appearing at the specified positions correspond to the deformation vibrations of the methyl group in the quaternary ammonium group. This indicates that CTA + It adsorbs onto the TiO2 surface through electrostatic interaction, forming an interfacial composite structure, but the quaternary ammonium salt core structure remains intact.

[0166] (3) Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) tests: The TiO2-PDMS coating obtained in Comparative Example 3 and the TiO2@CTAB-PDMS coating prepared in Example 1 were tested by scanning electron microscopy (SEM). The test results are as follows: Figure 3 As shown in a and b, energy dispersive spectroscopy (EDS) was performed on the TiO2@CTAB-PDMS coating prepared in Example 1. The test results are as follows: Figure 3 As shown in c~g in the diagram.

[0167] Depend on Figure 3 As can be seen from a, when the PDMS surface is coated with only pure TiO2, it exhibits a porous and loose particle agglomeration structure. Although the anatase nanoparticles maintain an irregular spherical shape, they are severely agglomerated, forming secondary particles with a particle size of 1~2μm. The coating surface has high roughness and microcracks and pore defects, resulting in poor compactness.

[0168] Depend on Figure 3As shown in b, the TiO2@CTAB coating on the PDMS surface exhibits significantly improved particle dispersion uniformity and reduced agglomeration due to the formation of an organic coating layer of approximately 5-10 nm thick by CTAB molecules on the TiO2 particle surface. This results in a smoother, denser surface and a marked reduction in cracks. This indicates that CTAB effectively inhibits nanoparticle agglomeration through electrostatic adsorption and steric hindrance, enhancing the interparticle interface bonding force and thus constructing a denser thin film structure. This provides a crucial structural basis for the coating's long-term corrosion resistance and drag reduction performance.

[0169] Figure 3 The c~g values ​​represent the distribution characteristics of five elements—C, O, Ti, N, and Br—on the particle surface. Figure 3 The 'c' in the image represents the surface scan of carbon (C). It can be seen that the C signal exhibits a halo-like distribution around the Ti core, with a significant increase in intensity at the particle edges. This directly confirms the presence of long-chain hexadecyl (-C) groups in CTAB. 16 H 33 Successfully coated onto the TiO2 surface. Figure 3 In the image, d represents the surface distribution scan of the O element. It can be seen that the signal covers the entire particle region and has uniform intensity. It includes TiO2 lattice oxygen (Ti-O-Ti), as well as surface adsorbed hydroxyl groups (Ti-OH) and oxygen-containing functional groups that may be introduced by CTAB. Figure 3 In the image, 'e' represents the distribution of Ti elements. It can be seen that the signal intensity is concentrated in the core region of the particles, forming a discrete array of bright spots, which corresponds to the framework position of TiO2 nanoparticles with a particle size of approximately 50–200 nm, consistent with the SEM morphology. Figure 3 f and g in the image are surface scan images of N and Br elements. It can be seen that the signal intensities of N and Br elements are relatively weak, but their spatial distribution highly overlaps with that of C, concentrating around the periphery of the particles, corresponding to the quaternary ammonium cation head group (-N). + The discrete, speckled distribution of CTAB (CH3)3 indicates that CTAB forms island-like adsorption or monolayer modification on the surface of nanoparticles. Br, as a counter ion, exhibits a distribution pattern completely co-located with N, further verifying the integrity of the CTAB molecular structure and its electrostatic adsorption mechanism. Comprehensive analysis shows that the distribution regions of C, N, and Br highly overlap and are all enriched at the boundaries of TiO2 particles, while the Ti signal is encapsulated in the core, confirming that CTAB forms a stable and uniform organic modification layer on the TiO2 surface, rather than a simple physical mixture. This spatial correlation of elements provides direct structural evidence for organic-inorganic interface composites.

[0170] (4) Surface roughness Ra test: The three-dimensional morphology and surface roughness Ra of the coating surfaces obtained in Example 1 and Comparative Examples 1, 4-6 were tested using a white light interferometer. The results are as follows: Figure 4 As shown, where, Figure 4 In the figures a~e, they correspond to Comparative Example 1, Example 1, Comparative Example 4, Comparative Example 5 and Comparative Example 6, respectively.

[0171] Depend on Figure 4 As can be seen from 'a', the original matrix surface is smooth but chemically homogeneous, with a surface roughness Ra of 0.144 μm. Such a structure lacks the ability to actively regulate the near-wall fluid velocity profile. When underwater, the fluid molecules interact strongly with the wall, resulting in typical no-slip boundary conditions and high viscous frictional resistance.

[0172] Depend on Figure 4 As shown in b, the coating surface obtained in Example 1 formed sparse and uniformly dispersed nanoparticles. The hexadecyltrimethylammonium bromide (CTAB) modification effectively suppressed particle aggregation, constructing a binary composite structure combining a micron-scale matrix and nanoscale protrusions on the substrate surface, with a surface roughness Ra as low as 0.021 μm. The sparsely protruding TiO2@CTAB nanoparticles act as rigid micro-bearings (non-rotating), locally raising the effective shear surface between the fluid and the solid wall. This structure significantly reduces the actual contact area between the fluid and the underlying solid substrate, causing the fluid to flow mainly above the particle tips rather than directly contacting the substrate surface. The CTAB modified on the particle surface constitutes a low-surface-energy hydrophobic molecular brush. This molecular layer significantly reduces the solid-liquid interface energy and may weaken the binding of interfacial water molecules or promote the formation of a layer of quasi-free water molecules through its hydrophobic properties. These effects work synergistically to weaken the adhesion between fluid molecules and the solid surface.

[0173] Depend on Figure 4 c and Figure 4 As can be seen from d in Comparative Examples 4 and 5, the excessively high particle density of the coating surface leads to a sharp increase in surface roughness Ra to 0.06–0.66 μm, forming a continuous, highly undulating peak-valley structure. When underwater, these large-scale rough elements penetrate or significantly disturb the laminar sublayer near the wall, inducing flow separation, generating intense turbulent bursts and secondary flow motions. The generation and dissipation of this additional turbulent kinetic energy completely offset or even mask the potential lubrication effect that nanoparticles may bring, instead significantly increasing frictional resistance.

[0174] Depend on Figure 4 As can be seen from 'e' in Comparative Example 6, the surface roughness Ra of the coating obtained by the severe agglomeration of particles forms large-scale irregular protrusions, which increases to 0.157 μm, even higher than the surface roughness of the original substrate. The size of the irregular protrusions formed is much larger than the thickness of the laminar sublayer. When underwater, this structure will directly penetrate and severely disrupt the stability of the laminar sublayer, becoming a powerful turbulence generator and causing severe energy loss.

[0175] 2. X-ray photoelectron spectroscopy (XPS) test: The TiO2@CTAB composite particles prepared in Example 1 were subjected to X-ray photoelectron spectroscopy (XPS) test, and the test results are as follows: Figure 5 As shown.

[0176] Figure 5 In the image, 'a' represents the full X-ray photoelectron spectroscopy (XPS) spectrum of the TiO2@CTAB composite particles. The spectrum shows characteristic peaks of Ti 2p (approximately 459 eV) and O 1s (approximately 530 eV), indicating the lattice structure of the TiO2 substrate. The C1s (approximately 285 eV) peak originates from the long-chain alkyl group of CTAB, and the N 1s (approximately 402 eV) peak corresponds to the quaternary ammonium cation (-N...). + The (CH3)3 and Br3d (approximately 68 eV) peaks, acting as counterions, together confirm that CTAB molecules are completely adsorbed on the TiO2 surface.

[0177] Figure 5 Figures b through f show the results of high-resolution spectral analysis of elements C, O, Ti, N, and Br, respectively, after peak fitting using the XPSPEAK program. All high-resolution XPS spectra were corrected for binding energy using the C 1s characteristic peak of externally contaminated carbon (binding energy set at 284.8 eV) as a reference. Figure 5 As shown in b, the three sub-peaks within the C 1s element peak (at 284.5 eV, 285.7 eV, and 288.7 eV, respectively) correspond to different chemical bonding modes of carbon (CC / CH, CN, and C=O). Figure 5 As shown in c, the O 1s element peak produces three sub-peaks at 529.7 eV, 531.4 eV, and 532.7 eV, respectively, which represent the characteristic peaks of the Ti-O-Ti, Ti-OH, and CO chemical bonds. The Ti 2p3 / 2 and Ti 2p1 / 2 peaks ( Figure 5 d) in the XPS spectrum shows a double peak at energies of 458.13 eV and 463.86 eV. Figure 5 The value of e in the figure shows that the N 1s peak can be split into three sub-peaks, located at 398.9 eV and 402 eV. These sub-peaks correspond to different chemical bonding modes of the N 1s element (-C=NH and -N). + (CH3)3), indicating that an adsorption film of CTAB was formed on the titanium dioxide surface. From Figure 5 As can be seen from f, Br 3d5 / 2 and Br 3d3 / 2 exhibit two sub-peaks at 67.6 eV and 68.8 eV, respectively, in the Br 3d elemental image. The coexistence of these elemental peaks indicates that CTAB forms a stable organic modification layer on the surface of TiO2 particles through electrostatic interaction, effectively realizing the construction of an organic-inorganic composite interface.

[0178] 3. Static contact angle test: Static contact angle tests were performed on the coating surfaces obtained in Example 1 and Comparative Examples 1, 4-6. The results are as follows: Figure 6 As shown, where, Figure 6 In the figures, a~e correspond to Comparative Example 1, Example 1, Comparative Example 4, Comparative Example 5, and Comparative Example 6, respectively. The specific testing method was as follows: The surface wettability of the samples was characterized using an optical contact angle meter (Dataphysics OCA 25) at room temperature (25±1℃) and relative humidity of 40%-60%. Before testing, the samples were ultrasonically cleaned sequentially with anhydrous ethanol and deionized water, and then dried with nitrogen. Using deionized water as the test solution, 5 μL droplets were vertically added to the sample surface using a microsyringe. After the droplets stabilized, the Young-Laplace fitting method was used to capture and calculate the droplet profile angle. At least five different locations were randomly selected on each sample surface for measurement, and the results are expressed as "mean ± standard deviation".

[0179] Depend on Figure 6 As can be seen from a in Figure 1, Comparative Example 1 exhibits a weak hydrophilic contact angle of approximately 79.9° due to the obvious groove-like microstructure on its surface and the influence of surface hydroxyl groups.

[0180] Depend on Figure 6 As can be seen from b, the contact angle of the surface of Example 1 was significantly increased to 123.2° due to the introduction of the TiO2@CTAB-PDMS composite coating. This transformation is due to a dual synergistic mechanism: on the one hand, TiO2 nanoparticles construct a micro-nano rough structure on the PDMS surface, which traps air in the grooves to form an air cushion. The water droplets actually contact the composite interface between air and solid rather than the pure solid surface, which is consistent with the Cassie-Baxter wetting model. On the other hand, CTAB molecules are adsorbed on the TiO2 surface through their positively charged quaternary ammonium groups, while the hydrophobic long-chain alkyl groups extend outward to form a low surface energy molecular modification layer on the surface, thereby achieving a synergistic effect of physical rough structure and chemical hydrophobic modification.

[0181] Depend on Figure 6 As can be seen from c~e in Example 1, the surface contact angles of Comparative Examples 4~6 show a gradual decreasing trend. This is mainly because the excessive filler content causes the nanoparticles to agglomerate, which destroys the optimal uniformity of the micro-nano rough structure. Some areas change from the Cassie-Baxter state to the Wenzel wetting state. At the same time, the excessive addition also interferes with the orderly arrangement of CTAB, and some hydrophilic TiO2 surfaces are exposed. Since Comparative Examples 4~6 cannot achieve a balance between dispersibility and rough structure, the hydrophobic properties eventually decrease.

[0182] 4. Drag Reduction Performance Test: The drag reduction effect of the substrate control group (Comparative Example 1), PDMS coating (Comparative Example 2), TiO2 coating (Comparative Example 3), and TiO2@CTAB-PDMS coating (Example 1) was evaluated using an Anton Paar MCR302 rheometer (Germany). The test results are as follows: Figure 7 As shown in a and b in the figure.

[0183] The specific test method is as follows: a 60 mm diameter substrate is fixed to the base plate with double-sided tape, and the temperature is maintained at 25 ± 0.1 ℃. Deionized water is injected between the sample surface and the upper rotor. The torque is measured by a sensor connected to the rotor, and the shear rate during rotation is calculated using the following formula (1). ):

[0184] (1)

[0185] In formula (1), The value represents the average linear velocity of the rotor surface, d represents the thickness of the liquid layer, and the unit of shear rate is s. -1 .

[0186] The drag reduction rate (DR%) can be calculated using the following formula:

[0187] (2)

[0188] In formula (2), T0 represents the torque measured before coating treatment, while T C This indicates the torque measured after the coating treatment. The unit of measurement for torque is millinewton-meter (mN·m).

[0189] Depend on Figure 7 As can be seen from 'a', the torque values ​​of all four samples monotonically increase with increasing shear rate. The substrate (Comparative Example 1) exhibits the highest torque, followed by the PDMS coating (Comparative Example 2), the TiO2-PDMS coating (Comparative Example 3), while the TiO2@CTAB-PDMS composite coating (Example 1) consistently shows the lowest torque value. Furthermore, the PDMS coating did not demonstrate any drag reduction effect within the test range; instead, it increased frictional resistance.

[0190] Depend on Figure 7 As can be seen from b in the figure, the PDMS coating (Comparative Example 2) is effective across the entire shear rate range (100-1000 s). -1Within the range of -10% to -3%, all exhibited increased drag, with the curve fluctuating within a small range and showing no obvious regularity with shear rate. The average increase in drag was approximately -4%. This indicates that the PDMS coating actually increased the system drag. The TiO2-PDMS coating (Comparative Example 3) showed a clear drag reduction effect. With increasing shear rate, the drag reduction rate of the TiO2-PDMS coating showed a trend of first increasing and then decreasing. At low shear rates (100-300 s⁻¹), the drag reduction was significantly reduced. -1 Under these conditions, the drag reduction rate continues to increase, reaching a maximum of 12%. With increasing shear rate, within the range of 300-800 s⁻¹... -1 The drag reduction rate of the TiO2-PDMS coating generally shows a decreasing trend, with a minimum drag reduction rate of 4%, followed by a decrease at higher shear rates (800-1000 s⁻¹). -1 The drag reduction ratio showed a slight increase under all conditions. This indicates that the addition of TiO2 nanoparticles effectively suppressed the drag-increasing effect of pure PDMS, thereby improving the drag reduction performance of the coating. The TiO2@CTAB-PDMS composite coating (Example 1) exhibited the best drag reduction performance under all conditions, with the drag reduction ratio initially decreasing and then stabilizing with increasing shear rate. Its initial drag reduction value (100s) was... -1 The percentage was 42%. As the shear rate increased, in the range of 200-400 s... -1 The drag reduction rate in this range is 15-24%. This is followed by higher shear rates (700-1000 s⁻¹). -1 The percentage has slightly decreased, but it remains stable in the relatively high range of 11-15%, which is significantly better than the other two coatings used in the same period.

[0191] PDMS has low surface energy and poor interfacial compatibility with aqueous test fluids, increasing the complexity of near-wall flow. Under shear stress, friction between PDMS polymer chains and fluid molecules intensifies, generating additional viscous energy dissipation. The introduction of TiO2 nanoparticles fundamentally transforms the hydrodynamic behavior of the coating. The embedded rigid nanoparticles alter the local rheological properties of the PDMS matrix and construct a surface roughness structure with specific dimensions. At higher shear rates, this roughness structure can induce the formation of micro-eddies in the near-wall region, optimizing boundary layer momentum exchange and thus transforming fluid drag into hydrodynamic support that favors slip, achieving a shift from drag increase to drag reduction. Further modification with CTAB optimized the drag reduction performance of the system from multiple dimensions: First, CTAB improved the dispersion of TiO2 in the PDMS matrix, resulting in a more uniform and ordered surface rough structure; second, the amphiphilic molecules modified with CTAB self-assembled at the solid-liquid interface, forming a flexible molecular brush. Its hydrophilic ends capture water molecules to form a hydration layer, while its hydrophobic ends anchor to the coating surface, jointly constructing a stable slip boundary layer and significantly reducing interfacial frictional resistance. Finally, the TiO2@CTAB-PDMS composite coating exhibited drag reduction performance at a shear rate of 100 s⁻¹. -1 The drag reduction rate is highest at the highest speed, reaching 42%. In summary, by introducing rigid TiO2 particles into the PDMS matrix and synergistically modifying the CTAB interface molecules, a systematic improvement in the drag reduction performance of the coating over a wide shear rate range was achieved.

[0192] The samples obtained in Example 1 and Comparative Examples 4-6 were tested in the range of 2000-14000 s using the method described above. -1 The drag reduction performance within the shear rate range is as follows: Figure 8 As shown.

[0193] Depend on Figure 8 It can be seen that Example 1 exhibits the best drag reduction performance at 2000s. -1 The drag reduction rate reaches 23% at 4000 s⁻¹, and increases with shear rate to 4000 s⁻¹. -1 Increased to 25%, in the 6000~10000s -1 The range stabilized at a peak of 26%, then at 12000s. -1 The time dropped slightly to 25%, 14000s -1 The drag reduction rate further decreased to 24%, and remained in the high drag reduction range of 23% to 26%, indicating that the micro-nano rough structure and low surface energy modification of Example 1 achieved the best balance, which can effectively capture and stabilize the gas film and maintain good gas film lubrication effect even at high shear rates.

[0194] Comparative Example 4 exhibits a positive drag reduction rate across the entire shear rate range, but the value is low, reaching only around 2000 s. -1The initial value was 6%, which gradually decreased to 4000 s as the shear rate increased. -1 6% at the time, then in the 6000~10000s -1 The range remained stable at 5%, between 12000 and 14000 seconds. -1 When the concentration is further reduced to 5%, it exhibits weak drag reduction characteristics overall. This may be due to the excessive surface roughness and poor gas film stability at high addition levels, which makes it easily destroyed under high shear.

[0195] Comparative Examples 5 and 6 both exhibited negative drag reduction rates under all test conditions, indicating increased drag. The drag reduction rate of Comparative Example 5 fluctuated between -5% and -6%, while that of Comparative Example 6 ranged from -7% to -9%. Furthermore, the negative values ​​tended to increase with increasing shear rate. This suggests that excessive TiO2@CTAB addition led to severe agglomeration of nanoparticles, uneven surface roughness distribution, and some areas transitioning to a Wenzel wetted state. The gas film could not exist stably, and the increased solid-liquid contact area and turbulent disturbances due to surface protrusions resulted in frictional resistance exceeding that of a smooth surface, thus creating a drag-increasing effect.

[0196] 5. Electrochemical Performance Testing: All electrochemical tests were conducted in a constant-temperature (25±1°C) 3.5wt% NaCl solution without stirring. The pH was 6.8±0.2, and the sample was immersed for 24 hours to simulate a long-term corrosive environment. Polarization curves and electrochemical impedance spectroscopy (EIS) tests were performed using a three-electrode system: the working electrode was the sample, the reference electrode was a saturated calomel electrode (SCE), and the auxiliary electrode was a platinum sheet. The test frequency range was 10 Hz. 5 Hz to 10 -2 The frequency was Hz, and the AC disturbance amplitude was 10 mV. Polarization curves, Nyquist plots, phase angle plots, and Bode plots of the aluminum matrix control group (Comparative Example 1), PDMS (Comparative Example 2), TiO2-PDMS (Comparative Example 3), and TiO2@CTAB-PDMS (Example 1) in 3.5% NaCl solution were tested according to the above method, and are shown below. Figure 9 As shown in a~d in the diagram.

[0197] Figure 9 In the figure, 'a' represents the polarization curve of the sample in 3.5% NaCl solution. Electrochemical parameters obtained by Tafel extrapolation (based on the extrapolation intersection of the linear regions of the anodic and cathodic polarization curves) indicate that the corrosion potential (E) of the aluminum substrate... corr =-0.665V) is the most negative, corrosion current density (I corr =3.404 μA·cm 2The highest value indicates poor thermodynamic stability and strong corrosion tendency. The Ecorr values ​​of PDMS, TiO2-PDMS, and TiO2@CTAB-PDMS coatings all show a certain negative shift (to -0.684V, -0.682V, and -0.714V, respectively), but I... corr Significantly reduced, especially the I of TiO2@CTAB-PDMS corr (3.27×10) -3 μA·cm 2 The charge transfer resistance (R) was reduced by approximately three orders of magnitude compared to the control group, indicating that while the coating may promote the cathodic reaction, it kinetically greatly inhibits the corrosion process through its barrier effect. ct The resistance to electron transfer at the corrosion interface can be estimated using the capacitive arc diameter in the low-frequency region of the Nyquist plot. ct With corrosion current density (I corr Satisfying relation (I) corr ∝1 / R ct The higher the Rct value, the lower the corrosion rate.

[0198] Figure 9 In the diagram, b represents the Nyquist plot. It can be seen that the TiO2@CTAB-PDMS coating has the largest capacitive arc radius, followed by the TiO2-PDMS coating and the PDMS coating, while the aluminum substrate has the smallest. This indicates that the TiO2@CTAB-PDMS coating is dense and effectively blocks electrolyte penetration and charge transfer. Although the control group has the smallest capacitive arc radius, its corrosion current is relatively high, reflecting defects in the natural oxide film and significant localized corrosion. The PDMS capacitive arc radius is slightly larger than the control group, indicating a limited barrier effect, as micropores or interface defects facilitate charge transfer.

[0199] Figure 9 In the figure, 'c' represents the phase angle plot. It can be seen that the phase angle of TiO2@CTAB-PDMS is close to -80° over a wide frequency range, and the curve is relatively flat, indicating that the coating exhibits near-ideal capacitance behavior, with a uniform interface and strong charge storage capacity. The control group has a lower peak phase angle and narrower bandwidth, reflecting poor capacitance characteristics of the natural oxide film and weaker protective effect.

[0200] Figure 9 In the diagram, 'd' represents the Bode plot, which shows that TiO2@CTAB-PDMS exhibits the highest impedance modulus in the low-frequency region (e.g., 0.01Hz), significantly higher than other samples, indicating its superior protective performance. The control group showed the lowest low-frequency impedance, consistent with the high corrosion current.

[0201] It should be noted that although the technical solution of the present invention has been described with specific examples, those skilled in the art will understand that the present invention should not be limited thereto.

[0202] The various embodiments of the present invention have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen to best explain the principles, practical application, or technical improvements to the embodiments in the market, or to enable others skilled in the art to understand the embodiments disclosed herein.

Claims

1. A double electric layer based core-shell structure drag reduction and corrosion protection composite coating, characterized in that, It includes a first coating applied to the surface of a substrate and a second coating applied to the surface of the first coating, wherein, The first coating comprises polysiloxane; The second coating includes nano-titanium dioxide with a modified surface, wherein the modified material includes a quaternary ammonium salt compound (A) having a long-chain alkyl group, and the compound (A) is adsorbed onto the surface of the titanium dioxide to form an organic coating layer through the quaternary ammonium group; The surface roughness Ra of the composite coating is less than 0.06 μm.

2. The composite coating of claim 1, wherein, The long-chain alkyl group includes C12-C20 alkyl groups.

3. The composite coating according to claim 1 or 2, characterized in that, The thickness of the first coating is 2~8μm; the thickness of the second coating is 5~15μm; and the thickness of the organic coating layer is 5~20nm.

4. The composite coating according to any one of claims 1 to 3, characterized in that, The polysiloxane includes polydimethylsiloxane; the compound (A) includes an ammonium halide salt having an alkyl group of C12-C20.

5. A method for preparing a composite coating according to any one of claims 1 to 4, characterized in that, Includes the following steps: S1： S1a: Compound (A) and nano-titanium dioxide are mixed in a solvent and reacted to obtain nano-titanium dioxide modified by compound (A); S1b: A composite solution is obtained by dispersing nano-titanium dioxide modified with compound (A) in a solvent; S2: The polysiloxane prepolymer and curing agent are coated on the substrate surface and pre-cured to form a substrate adhesion layer; S3: The composite solution is applied to the surface of the substrate adhesion layer and cured to form a composite coating.

6. The preparation method according to claim 5, characterized in that, In step S1a, during the reaction, the amount of compound (A) used is 10-50% by mass of the nano-titanium dioxide. The solvent includes alcohol solvents; The reaction temperature is 30~70℃, and the reaction time is 1~3h.

7. The preparation method according to claim 5 or 6, characterized in that, In step S1b, the content of the nano-titanium dioxide modified by compound (A) in the composite solution is 0.05% to 0.4% by mass. The solvent includes alcohol solvents.

8. The preparation method according to any one of claims 5 to 7, characterized in that, In step S2, the pre-curing temperature is 60~80℃, and the pre-curing time is 10~60min.

9. The preparation method according to any one of claims 5 to 8, characterized in that, In step S3, the curing temperature is 60~80℃ and the curing time is 1~3h.

10. The application of a composite coating according to any one of claims 1 to 4 on a ship or marine structure.

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