A high-concentration carbon nanotube water-based epoxy composite coating with a hydrophobic shell layer constructed by tannic acid and a preparation method thereof

Abstract

This invention belongs to the field of waterborne nanocomposite materials and anti-corrosion coatings. It relates to a high-concentration carbon nanotube waterborne epoxy composite coating and its preparation method, utilizing tannic acid to construct a hydrophobic shell, to solve problems such as porosity, whitening, and water absorption swelling caused by the application of traditional dispersants in waterborne carbon nanotube coatings. The method first uses polyvinylpyrrolidone (PVP) to ultrasonically disperse multi-walled carbon nanotubes (MWCNTs) in aqueous phase. Tannic acid is then introduced under alkaline conditions for in-situ complexation and self-assembly, constructing a hydrophobic cross-linked film at the PVP / MWCNTs interface to achieve hydrophobic modification, obtaining a stable, bubble-free, high-concentration carbon nanotube modified slurry. This slurry is then compounded with waterborne epoxy resin and various additives, and a curing agent is added to cure it into a film. This invention improves the dispersion stability of MWCNTs and its interfacial compatibility with epoxy groups by constructing a hydrophobic shell through in-situ self-assembly, shielding hydrophilic groups, enhancing the coating's impermeability and long-term anti-corrosion performance. The process is environmentally friendly and has excellent industrial application prospects.

Description

Technical Field

[0001] This invention relates to the field of waterborne nanocomposite coating technology, specifically to a method for preparing a high-concentration multi-walled carbon nanotube waterborne epoxy anti-corrosion composite coating by utilizing tannic acid to construct a hydrophobic crosslinked film layer through in-situ complexation and self-assembly on the surface of polyvinylpyrrolidone (PVP) / carbon nanotubes. Background Technology

[0002] Waterborne epoxy coatings are environmentally friendly and widely used in industrial corrosion protection of metal substrates. Multi-walled carbon nanotubes (MWCNTs) possess extremely high aspect ratios and excellent mechanical strength, making them ideal nanoscale physical reinforcements and corrosion-resistant barrier fillers. Their interlocking structure within the coating significantly extends the lateral penetration path of corrosive media such as water molecules (i.e., the "maze effect"). However, due to strong van der Waals forces, MWCNTs are prone to irreversible aggregation in aqueous phases. Existing technologies mostly employ traditional ionic surfactants (such as sodium dodecylbenzenesulfonate, SDBS) for non-covalent surface modification and dispersion.

[0003] However, when preparing high-concentration (carbon nanotube mass fraction ≥ 1 wt%) waterborne resin coating systems, traditional ionic dispersants present the following technical challenges that are difficult to reconcile: 1) Bubble retention and pinhole disaster: Traditional small-molecule surfactants such as SDBS have extremely high interfacial activity, which easily generates a large number of difficult-to-eliminate microbubbles during the shearing and stirring process of coating compounding. This results in a dense network of pinholes and crater-like shrinkage defects in the cured paint film, completely destroying the continuity and physical shielding performance of the coating. 2) Phase separation and internal whitening: Small-molecule surfactants have poor physical compatibility with macromolecular epoxy groups. In the later stages of film formation, the strongly hydrophilic polar groups easily lock in moisture, forming microscopic water pockets inside the paint film and causing light scattering. This leads to a significant "optical whitening" phenomenon inside the paint film, deteriorating the overall density of the paint film.

[0004] To overcome the foaming drawbacks of traditional ionic dispersants, using nonionic polymers (such as polyvinylpyrrolidone, PVP) to disperse carbon nanotubes through steric hindrance is an emerging option. However, PVP molecular chains themselves have extremely strong hydrophilicity and hygroscopicity. When waterborne epoxy coatings are subjected to prolonged service environments such as humid heat or saline media, the PVP segments inside the coating film are prone to water absorption and swelling, forming microscopic water penetration channels at the interface between carbon nanotubes and the epoxy resin matrix. This leads to a significant decrease in the overall shielding and long-term anti-corrosion performance of the coating. Therefore, how to modify the PVP / carbon nanotube interface to "de-hydrophilize" it while maintaining zero foaming dispersion, and construct a long-lasting hydrophobic shielding layer, is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a high-concentration carbon nanotube waterborne epoxy composite coating and its preparation method, which utilizes tannic acid to construct a hydrophobic film layer in situ on the surface of PVP / carbon nanotubes.

[0006] Technical solution

[0007] A method for preparing a high-concentration carbon nanotube waterborne epoxy composite coating includes the following steps.

[0008] (1) Preparation of hydrophobically modified high-concentration non-foaming slurry: Multi-walled carbon nanotubes (MWCNTs) and non-ionic polymeric dispersant polyvinylpyrrolidone (PVP) were added to pure water at a mass ratio of 1:0.8~1.5, and the mass concentration of MWCNTs was controlled at 1 wt%~1.5 wt%. Using a probe-type ultrasonic disruptor, the slurry was ultrasonically dispersed in an ice-water bath for 30~45 minutes at a power of 200 W~300 W with a pulse mode of 3 seconds on, 1 second off, to obtain a uniform and stable non-ionic carbon nanotube pre-dispersed slurry. The pH of the pre-dispersed slurry was adjusted to 8.0~8.5 using a 2% (w / w) weak alkaline regulator. Subsequently, the slurry was further prepared according to a mass ratio of tannic acid to MWCNTs of 0.3~0.6: 1. Add a 2% (w / w) aqueous solution of tannic acid dropwise to the pre-dispersed slurry and magnetically stir for 1-2 hours at 20℃-25℃. Utilizing the multi-point hydrogen bonding between the dense catechol / pyrogallic acid groups in tannic acid (TA) and the pyrrolidone carbonyl groups on the PVP molecular chain, in-situ self-assembly of TA on the PVP / MWCNTs surface is promoted. Because tannic acid molecules contain numerous hydrophobic aromatic ring structures, after binding with PVP through hydrogen bonds, the aromatic rings align outwards, forming a dense cross-linked film layer on the carbon nanotube surface with a hydrophobic benzene ring framework. This encapsulates the original hydrophilic carbonyl groups in PVP within the inner layer, achieving interface "dehydration" modification. This reaction is carried out at room temperature (20℃-25℃) with magnetic stirring at 400 r / min for 1-2 hours (see Appendix). Figure 1 This yields a modified high-concentration non-foaming slurry.

[0009] (2) Coating base material compounding: Weigh 40-60 parts of waterborne epoxy resin, add 20-30 parts of the above modified high-concentration non-foaming slurry, and add 2.0-3.0 parts of film-forming aid, 1.1-1.7 parts of leveling agent, and 0.3-0.5 parts of defoamer in sequence; disperse mechanically at a speed of 400 rpm-500 rpm for 10-15 minutes under low shear to allow the aid and slurry to flow fully and mix evenly in the resin, and obtain the main paint base material.

[0010] (3) Curing, crosslinking and film formation: Add 60-80 parts of water-based epoxy curing agent to the main paint base material, and continue stirring at 400 rpm for 2-3 minutes until the curing agent is evenly dispersed. Apply the resulting mixed coating to the surface of a metal substrate that has undergone gradient polishing, and allow it to cure naturally at room temperature (20℃~25℃) for 2 days to crosslink and form a film.

[0011] Preferably, the weak alkaline regulator mentioned in step (1) is either dilute ammonia or sodium hydroxide aqueous solution.

[0012] Beneficial effects

[0013] (1) Eliminate foaming from the source and achieve defect-free film formation: This invention relies on the spatial steric repulsion force of PVP to achieve bubble-free monodispersion of high-concentration carbon nanotubes. Under low shear and mild stirring, it solves the problem of interfacial bubble retention in traditional ionic systems. After the paint film is cured, the surface is smooth and there are no macroscopic pinholes or shrinkage defects.

[0014] (2) Interfacial supramolecular complexation and locking to construct a long-lasting hydrophobic and anti-corrosion physical barrier: This invention utilizes the macromolecular supramolecular complexation effect of tannic acid (TA) and PVP to successfully construct a hydrophobic composite film layer that is insensitive to water on the surface of carbon nanotubes. This film layer successfully "de-hydrophilizes" the originally hydrophilic PVP, which not only blocks the capillary permeation path of water along the carbon nanotube-resin interface, but also endows the entire composite coating with resistance to salt water swelling and media penetration, thereby fundamentally improving the long-lasting anti-corrosion life.

[0015] (3) Excellent blending compatibility, eliminating internal phase separation and whitening: The carbon nanotube composite modified with tannic acid has excellent physical compatibility with the two-component waterborne epoxy resin matrix. During the drying process, it does not produce small molecule enrichment areas or water sacs, thus avoiding the chronic problem of "optical whitening" inside the paint film. After film formation, the color is darker, isotropic and continuous and dense. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the anti-corrosion mechanism and three-dimensional barrier network of the hydrophobic shell constructed by in-situ self-assembly of tannic acid on the surface of PVP / carbon nanotubes in this invention (where B5 represents the water-sensitive degradation path of the pure PVP system without modification; C4 and C6 represent the dense cross-linked composite film and the three-dimensional labyrinth anti-corrosion network constructed after in-situ dehydration modification with tannic acid, respectively).

[0017] Figure 2 These are comparative photographs of the surface macroscopic morphology of Comparative Example 1 (a1) and Example 1 (a2) after being cured into films on tinplate sheets.

[0018] Figure 3These are comparative photographs showing the surface macromorphology of Comparative Example 1 (b1) and Example 1 (b2) after being cured into films on Q235 steel plates.

[0019] Figure 4 The Nyquist electrochemical impedance spectroscopy of the cured coating films of Example 1 and Comparative Example 2 in 3.5wt% NaCl solution is shown as a comparison (confirming the improvement of impedance by dehydration modification).

[0020] Figure 5 Tafel polarization curves of the cured coatings of bare Q235 steel plates, Comparative Example 2 and Example 1 are compared (confirming the effect of corrosion kinetic inhibition). Detailed Implementation

[0021] Example 1

[0022] (1) Substrate pretreatment: Take Q235 steel plate and tinplate sheet, and use 400 grit, 800 grit, 1000 grit and 1500 grit sandpaper to finely grind the Q235 steel plate step by step. Then put it into anhydrous ethanol for ultrasonic cleaning to remove the oil stains on the surface, wipe it clean and set it aside.

[0023] (2) Slurry preparation: 1 g of PVP (model K30) was completely dissolved in 98 g of pure water, and 1 g of multi-walled carbon nanotubes (MWCNTs) were added. The beaker was placed in an ice-water bath and continuously dispersed for 40 minutes using a probe-type ultrasonic disruptor (power 270 W, pulse mode: 3 s on, 1 s off) to obtain a stable carbon nanotube / PVP predispersant. Subsequently, 25 g of the pre-dispersion solution (containing 0.25 g of multi-walled carbon nanotubes) was taken, and the pH value was adjusted to 8.3 using 2% dilute ammonia solution. Then, 5 g of 2% tannic acid aqueous solution (containing 0.1 g of tannic acid, at which point the mass ratio of tannic acid to multi-walled carbon nanotubes was 0.4:1, which fully meets the range of claim 1) was added dropwise to the pre-dispersion solution. The mixture was stirred at a constant speed of 400 rpm for 2 hours at 20℃~25℃ to obtain a modified high-concentration carbon nanotube non-foaming slurry (the effective mass concentration of carbon nanotubes was 1 wt%).

[0024] (3) Formulation of the main paint: Weigh 50 g of waterborne epoxy resin, add 25 g of the above modified high-concentration non-foaming slurry, and then add 2.5 g of film-forming agent, 1.4 g of leveling agent and 0.4 g of defoamer in sequence. Use a mechanical mixer to stir and disperse at a constant speed of 400 rpm for 10 minutes to obtain the main paint.

[0025] (4) Mixing and curing: Add 70 g of the matching water-based epoxy curing agent to the main paint and continue to stir gently at 400 rpm for 2.5 minutes. Immediately brush the well-mixed coating evenly onto the pretreated Q235 steel plate and tinplate sheet, and let it cure naturally at 20℃~25℃ for 2 days to form a film. Then, transfer the cured film sample to the subsequent performance test.

[0026] Example 2

[0027] (1) Substrate pretreatment: Same as in Example 1.

[0028] (2) Slurry preparation: 1.2 g of PVP (model K30) was completely dissolved in 97.8 g of pure water, and 1 g of multi-walled carbon nanotubes (MWCNTs) were added. The beaker was placed in an ice-water bath and continuously dispersed for 40 minutes using a probe-type ultrasonic disruptor (power 250 W, pulse mode: 3 s on, 1 s off) to obtain a stable carbon nanotube / PVP pre-dispersion. Then, 25 g of the pre-dispersion (containing 0.25 g of MWCNTs) was taken, and the pH value was adjusted to 8.0 using a 2% sodium hydroxide aqueous solution. 3.75 g of a 2% tannic acid aqueous solution (containing 0.075 g of tannic acid, with a tannic acid to MWCNT mass ratio of 0.3:1) was added dropwise. The mixture was stirred at a constant speed of 400 rpm for 2 hours at 20℃~25℃ to obtain a modified high-concentration carbon nanotube non-foaming slurry.

[0029] (3) Main paint compounding: Weigh 50 g of waterborne epoxy resin, add 25 g of the above modified high-concentration non-foaming slurry, and then add 2.2 g of film-forming agent, 1.2 g of leveling agent and 0.3 g of defoamer in sequence. Use a mechanical mixer to stir and disperse at a constant speed of 400 rpm for 10 minutes.

[0030] (4) Mixed curing: Same as in Example 1.

[0031] Example 3

[0032] (1) Substrate pretreatment: Same as in Example 1.

[0033] (2) Slurry preparation: 1.5 g of PVP (model K30) was completely dissolved in 97.0 g of pure water, and 1.5 g of multi-walled carbon nanotubes (MWCNTs) were added. The beaker was placed in an ice-water bath and continuously dispersed for 45 minutes using a probe-type ultrasonic disruptor (power 300 W, pulse mode: 3 s on, 1 s off) to obtain a high-concentration carbon nanotube / PVP pre-dispersion. Then, 25 g of the pre-dispersion (containing 0.375 g of MWCNTs) was taken, and the pH value was adjusted to 8.5 using a 2% sodium hydroxide aqueous solution. 11.25 g of a 2% tannic acid aqueous solution (containing 0.225 g of tannic acid, with a tannic acid to MWCNT mass ratio of 0.6:1) was precisely added to it. The mixture was stirred at a constant speed of 400 rpm for 2 hours at 20℃~25℃ to obtain a modified high-concentration carbon nanotube non-foaming slurry.

[0034] (3) Main paint compounding: Weigh 50 g of waterborne epoxy resin, add 25 g of the above modified high-concentration non-foaming slurry, and then add 3.0 g of film-forming agent, 1.6 g of leveling agent and 0.5 g of defoamer in sequence. Use a mechanical mixer to stir and disperse at a constant speed of 400 rpm for 10 minutes.

[0035] (4) Mixed curing: Same as in Example 1.

[0036] Comparative Example 1 (Conventional Ionic Dispersant Comparison Group)

[0037] (1) Slurry preparation: Dissolve 1 g sodium dodecylbenzenesulfonate (SDBS) in 98 g pure water and add 1 g MWCNTs. Disperse using the same ultrasonic parameters as in Example 1 (270 W, pulse mode for 40 minutes).

[0038] (2) Compounding and Curing: Take 25 g of the SDBS pre-dispersed slurry and add waterborne epoxy resin, film-forming agent, leveling agent and defoamer strictly according to the same formula and addition amount as in Example 1. Blend and mechanically stir at 400 rpm. During the blending and compounding stage, the system produces dense, thick and unremovable foam due to the violent chemical foaming of small molecule polar groups under shear force, resulting in poor defoaming effect. Subsequently, add 70 g of curing agent and stir evenly. Brush the mixture onto Q235 steel plates and tinplate sheets with the same pretreatment and cure at 20℃~25℃ for 2 days.

[0039] Comparative Example 2 (Pure PVP Dispersed Unmodified Group)

[0040] (1) Slurry preparation: 1 g of PVP was completely dissolved in 98 g of pure water, and 1 g of multi-walled carbon nanotubes (MWCNTs) were added. The mixture was continuously dispersed in an ice-water bath using a probe-type ultrasonic disruptor (270 W, pulse mode for 40 minutes) to obtain a 1 wt% pure PVP carbon nanotube slurry.

[0041] (2) Compounding and curing: Accurately transfer 25 g of the pure PVP slurry, without any tannic acid post-treatment, and directly add 50 g of waterborne epoxy resin, film-forming agent, leveling agent and defoamer according to the same component ratio as in Example 1, and mix mechanically for 10 minutes. Finally, add 70 g of curing agent, brush it onto Q235 steel plate and tinplate sheet with the same pretreatment, and cure at 20℃~25℃ for 2 days.

[0042] Coating performance testing methods and result analysis

[0043] Performance testing standards

[0044] Macroscopic defects and morphology of cured paint film: Pinholes and crater defects on the surface were observed by taking pictures with a digital camera.

[0045] Cross-cut adhesion rating: The cross-cut test and rating are conducted according to standard GB / T 9286-2021.

[0046] Salt water shielding performance: According to standard GB / T 1733-1993, the sample was completely immersed in 3.5 wt% NaCl aqueous solution, and the area of ​​blistering, swelling, whitening and red rust on the surface was recorded after 7 days (168h).

[0047] Electrochemical characterization method: Testing was conducted using an electrochemical workstation (three-electrode system). A Q235 steel plate with a composite coating was used as the working electrode (effective test area 1.0 cm²). 2 The saturated calomel electrode (SCE) was used as the reference electrode, the platinum electrode as the auxiliary electrode, and the test medium was a 3.5 wt% NaCl solution.

[0048] Comparison table of experimental results data Foaming state during compounding stage It exhibits no foaming and has good fluidity. No visible bubbles, smooth. Very few microbubbles, which dissipate easily. Vigorous chemical foaming produces dense, thick foam. It exhibits no foaming and has good fluidity. Macroscopic defects in cured paint film It is flat and smooth, continuous and dense, and free of macroscopic pinholes. Smooth surface, free of pinholes. It is basically flat and has no visible shrinkage cavities. The surface is covered with numerous pinholes and "volcanic craters" that have punctured the paint film. It is flat and smooth, without macroscopic pinholes or shrinkage cavities. Internal morphology of paint film It has a deep, uniform black color, forms a continuous hydrophobic layer, and has no whitish tinge. It is a deep, even black color with no whitening. Deep black, without any whitening. The paint film shows obvious optical whitening and localized whitening. Initially, it is a deep, even black color with no whitening. Cross-cut adhesion rating (level) Level 0 (No peeling at all) Level 0 Level 1 Level 3 (localized, large-scale peeling along the cut surface) Level 0 Performance after soaking in 3.5wt% NaCl for 168 hours It is in perfect condition, with no blistering, swelling, or red rust. Intact, no bubbles. No bubbling, slight edge etching. The pinholes were severely ulcerated, and the entire board was covered with red rust and blisters. The paint film edges are bubbling and bulging.

[0049] In-depth analysis of electrochemical test results

[0050] (1) The decisive influence of in-situ modification of tannic acid on the impedance characteristics of the coating (see Appendix) Figure 4 )

[0051] Appendix Figure 4The electrochemical impedance spectroscopy (Nyquist plot) of Q235 steel in 3.5 wt% NaCl solution is shown, along with the unmodified group (Comparative Example 2, PVP-MWCNTs) and the tannic acid modified group (Example 1, TA / PVP-MWCNTs).

[0052] From the appendix Figure 4 It can be seen that Q235 steel has the smallest capacitive arc radius due to the lack of nano-barrier fillers and epoxy resin coating protection. Both groups of coatings exhibit single capacitive arc characteristics, and the size of the capacitive arc radius directly reflects the density of the film-forming material and its ability to resist media penetration barriers.

[0053] Comparative Example 2 (pure PVP dispersion group) without the introduction of tannic acid has a smaller capacitive arc diameter, and the extrapolated impedance value is only about 7000 Ω•cm. 2 This is because the exposed pyrrolidone carbonyl group in the PVP chain segment has strong hydrophilicity, and water molecules can easily permeate laterally through capillary action along the carbon nanotube interface, resulting in a lower local impedance.

[0054] In contrast, Example 1 (TA / PVP-MWCNTs) modified by in-situ supramolecular self-assembly of tannic acid as described in this invention exhibits a significantly increased capacitive arc radius and a substantial increase in extrapolated impedance to 11000 Ω•cm. 2 This result directly confirms that the polyphenolic structure of tannic acid deeply encapsulates and locks the water-sensitive groups of PVP through strong hydrogen bonds, achieving a "de-hydrophilization" transformation of the interface. The resulting hydrophobic shell, combined with a defect-free, high-concentration carbon nanotube network, forms an excellent nanoscale physical barrier "maze network" inside the coating film, resulting in a substantial improvement in impedance performance.

[0055] (2) Thermodynamic and kinetic inhibition mechanism of coating corrosion protection (see appendix) Figure 5 )

[0056] Appendix Figure 5 The potentiodynamic polarization curves of bare Q235 carbon steel, the unmodified group (Comparative Example 2, PVP-MWCNTs / Epoxy), and the tannic acid modified group (Example 1, TA / PVP-MWCNTs / Epoxy) in 3.5 wt% NaCl solution are shown. Self-corrosion potential E corr With corrosion current density i corr These are two core parameters for evaluating the corrosion resistance of materials: E corr A positive shift indicates a decrease in the system's thermodynamic tendency to corrode, and a smaller i corr This corresponds to a slower metal dissolution kinetic rate.

[0057] Bare Q235 carbon steel has the most negative self-corrosion potential and the strongest thermodynamic corrosion tendency. After introducing PVP-modified carbon nanotube epoxy coating, the self-corrosion potential shifts significantly to the positive side. The composite coating synergistically modified with tannic acid (TA) has the most positive corrosion potential, effectively reducing the thermodynamic driving force for substrate corrosion.

[0058] Comparing the corrosion current densities, it can be seen that the log i value of bare Q235 carbon steel is significantly larger, and the corrosion reaction rate is the fastest. After coating with carbon nanotube modified epoxy coating, the corrosion current density of the system decreases significantly, and the log i of TA / PVP-MWCNTs / Epoxy coating is at the lowest level, and the metal anodic dissolution process is significantly inhibited.

[0059] Analysis of the anodic branching characteristics of the polarization curves shows that the anodic current of the bare steel increases sharply with increasing potential, indicating that the substrate is highly susceptible to active dissolution. Both epoxy coatings significantly compress the anodic current range, with a noticeable anodic response only appearing at high positive potentials. This demonstrates that the composite coating can effectively block the contact between the corrosive medium and the steel substrate through its dense film structure. Compared to a single PVP-dispersed carbon nanotube coating, the tannin-modified system can further optimize the dispersion uniformity of MWCNTs in epoxy resin, reduce internal porosity defects in the coating, and simultaneously, tannic acid can chelate with the steel substrate to form a passivating and corrosion-inhibiting film. Through the synergistic effect of physical shielding and chemical corrosion inhibition, optimal electrochemical corrosion protection performance is achieved.

[0060] Technical effects and underlying mechanism understanding

[0061] By introducing a single variable group (Comparative Example 2), the outstanding technical innovation and underlying physicochemical mechanism of the long-lasting anti-corrosion composite coating based on tannic acid supramolecular network locking described in this invention have been confirmed:

[0062] (1) The decisive role of PVP macromolecules in overcoming foaming defects (Comparison of Example 1 and Comparative Examples 1 and 2)

[0063] Traditional small-molecule surfactants (such as SDBS in Comparative Example 1) exhibit a drastic decrease in interfacial tension and air trapping due to their active polar groups during high-speed shear stirring. This intense chemical foaming leaves numerous penetrating craters and pinholes on the cured paint film surface. See Appendix. Figure 2 With appendix Figure 3The images show a comparison of the macroscopic surface morphology of each experimental group after curing on the substrate. Comparative Example 1 (photos a1, b1) exhibits a surface riddled with pinholes, crater-like shrinkage cavities, and severe brush marks. These defects disrupt the continuity of the paint film, creating rapid penetration channels for saltwater to the metal substrate. In contrast, Examples 1 (photos a2, b2) and Comparative Example 2 utilize the large, long-chain structure of nonionic polymer PVP to provide steric repulsion, replacing ionic surfactants. The cured paint films are smooth, continuous, and dense, completely free of macroscopic pinholes and shrinkage cavities. This confirms that this method is a universally effective way to eliminate bubble defects in coating formulations at the source and ensure the continuity of the anti-corrosion matrix.

[0064] (2) In-situ dehydration modification of tannic acid inhibits interfacial swelling and improves corrosion resistance (Example 1 and Comparative Example 2)

[0065] The core inventiveness of this invention lies in the comparison of salt water resistance between Example 1 and Comparative Example 2: Although Comparative Example 2 (pure PVP group) avoided air bubbles and pinholes, and exhibited excellent initial smoothness and adhesion, it possesses extremely high water sensitivity due to the large number of hydrophilic pyrrolidone groups in the PVP molecular chain. When the coating of Comparative Example 2 was immersed in a 3.5 wt% NaCl solution for an extended period, external water molecules rapidly penetrated laterally along the hydrophilic segments of the PVP in the amorphous resin. In the later stages of immersion, the hydrophilic PVP underwent severe water absorption and swelling, which not only disrupted the original hydrogen bond between the epoxy resin and the metal substrate (leading to a sharp drop in adhesion and blistering), but also resulted in subsequent peeling and blistering of the paint film. To overcome this inherent defect of the pure PVP system, Example 1 of this invention introduced tannic acid (TA) under weakly alkaline conditions. The highly dense catechol / pyrogallic acid groups in the tannic acid structure can spontaneously and with high density, multi-point strong hydrogen bonding complexes and supramolecular self-assembly occur with the pyrrolidone carbonyl groups on the PVP chain. Through this macromolecular cross-linking and locking, tannic acid successfully modifies the originally hydrophilic PVP in situ by "dehydrating" it, deeply encapsulating and hiding the water-sensitive groups within a water-insoluble hydrophobic composite film (as shown in the attached image). Figure 1 (Mechanism shown). This hydrophobic shell acts like a microscopic waterproof gate on the carbon nanotube surface, completely blocking the physical diffusion paths of water molecules and corrosive media along the carbon nanotube interface. (Combined with attached...) Figure 4 , Figure 5The electrochemical data show that, due to the synergistic effect of the high-concentration, defect-free carbon nanotube physical barrier network (maze effect) and the hydrophobic shell of tannic acid, the impedance characteristics of Comparative Example 2 deteriorate significantly over time. In contrast, after long-term continuous high-concentration salt water erosion, the electrochemical polarization and impedance characteristics of Example 1 remain stable at extremely high levels, and the paint film does not swell, whiten, or blister. This invention combines the "non-bubbling, high dispersion" and "long-lasting water-resistant shielding and corrosion protection" of high-concentration carbon nanotubes.

Claims

1. A method for preparing a high-concentration carbon nanotube waterborne epoxy composite coating utilizing tannic acid to construct a hydrophobic shell, characterized in that, Includes the following steps: (1) Preparation of hydrophobically modified carbon nanotube slurry: Multi-walled carbon nanotubes and nonionic polymer dispersant polyvinylpyrrolidone were added to water at a mass ratio of 1:0.8~1.5, and the mass percentage concentration of the multi-walled carbon nanotubes was controlled at 1 wt%~1.5 wt%; ultrasonic dispersion was performed to obtain a uniform and stable nonionic carbon nanotube pre-dispersion; the pH value of the nonionic carbon nanotube pre-dispersion was adjusted to 8.0~8.5 using an alkaline regulator; subsequently, the mass ratio of tannic acid to multi-walled carbon nanotubes was 0.3~0.

6. :

1. Add tannic acid aqueous solution to the pre-dispersion liquid and stir for 1-2 hours at room temperature. Use tannic acid to construct a hydrophobic crosslinked film layer by in-situ complexation and self-assembly on the surface of polyvinylpyrrolidone / carbon nanotubes to obtain modified high-concentration carbon nanotube slurry; (2) Compounding of waterborne epoxy composite coating base: Weigh 40-60 parts of waterborne epoxy resin by weight, add 20-30 parts of the modified high-concentration carbon nanotube slurry, and add 2.0-3.0 parts of film-forming aid, 1.1-1.7 parts of leveling agent, and 0.3-0.5 parts of defoamer in sequence; disperse by mechanical stirring and mix evenly to obtain composite coating base. (3) Curing and crosslinking: Add 60-80 parts of waterborne epoxy curing agent to the composite coating base and mix evenly by mechanical stirring.

2. The method according to claim 1, characterized in that: In step (1), the stirring speed of the stirring reaction at room temperature is 400 rpm.

3. The method according to claim 1, characterized in that: In step (1), the polyvinylpyrrolidone is of type K30.

4. The method according to claim 1, characterized in that: In step (1), the ultrasonic dispersion treatment uses a probe-type ultrasonic disruptor with a power of 200 W to 300 W, and operates in pulse mode, specifically working for 3 seconds and pausing for 1 second. The total ultrasonic dispersion time is 30 to 45 minutes. The temperature of the carbon nanotube pre-dispersion liquid is controlled by an ice-water bath during the ultrasonic dispersion process.

5. The method according to claim 1, characterized in that: In step (1), the weak alkaline regulator is ammonia water with a mass fraction of 2% or sodium hydroxide aqueous solution with a mass fraction of 2%.

6. The method according to claim 1, characterized in that: In step (1), the tannic acid aqueous solution has a mass fraction of 2% and is added dropwise.

7. The method according to claim 1, characterized in that: In step (2), the mechanical stirring and dispersion is performed at a speed of 400 rpm to 500 rpm for 10 to 15 minutes.

8. The method according to claim 1, characterized in that: In step (3), the stirring speed is 400 rpm and the time is 2 to 3 minutes.

9. A high-concentration carbon nanotube waterborne epoxy composite coating, characterized in that, By weight, it includes the following components: Waterborne epoxy resin: 40-60 parts; Modified high-concentration carbon nanotube slurry: 20-30 parts; Film-forming aid: 2.0~3.0 parts; Leveling agent: 1.1~1.7 parts; Defoamer: 0.3~0.5 parts; Water-based epoxy curing agent: 60-80 parts; The modified high-concentration carbon nanotube slurry is a slurry in which a hydrophobic cross-linked film layer is constructed by in-situ complexation and self-assembly of tannic acid on the surface of polyvinylpyrrolidone / multi-walled carbon nanotubes.

10. The application of the high-concentration carbon nanotube waterborne epoxy composite coating as described in claim 9 in the field of metal corrosion protection or conductivity.

Images