Epoxy resin-based curing agent and method for producing the same

Abstract

The present disclosure provides an epoxy resin-based curing agent and a preparation method thereof, the preparation method comprising: 1) preparing an epoxy resin-based curing agent by Mannich reaction of nonyl phenol, a formaldehyde source and m-xylylenediamine; or 2) preparing an epoxy resin-based curing agent by Mannich reaction of nonyl phenol, phenol, a formaldehyde source and m-xylylenediamine; or 3) preparing an epoxy resin-based curing agent by Mannich reaction of nonyl phenol, cashew phenol, a formaldehyde source and m-xylylenediamine. The epoxy resin-based curing agent of the present disclosure not only greatly reduces the volatility and toxicity of free m-xylylenediamine, but also reduces the free water and volatile content through controllable dehydration, greatly improves the construction environment, and improves the water resistance and long-term service reliability of the cured product under harsh corrosion or water immersion working conditions.

Description

Technical Field

[0001] This disclosure relates to the field of polymer materials, and in particular to epoxy resin-based curing agents and their preparation methods. Background Technology

[0002] Port terminals, cross-sea bridges, offshore platforms, submarine pipelines, and hydraulic structures are often in environments that are constantly damp, intermittently submerged, or completely submerged. Stable hydration layers and fouling films frequently exist on their surfaces. These hydration layers, composed of adsorbed water, dissolved salt ions, and fine particles, are thin but highly continuous, making it easy for resins to "slip" at the interface or become isolated by the water film. This makes it difficult for conventional epoxy materials to achieve effective wetting and interfacial contact.

[0003] In addition to water film barriers, the seawater environment also presents factors such as salt ion migration, localized alkaline environments caused by cathodic protection, attached biofilms, and corrosion products. These factors further reduce interface cleanliness and increase interface uncertainty. Especially under low-temperature conditions, resin viscosity increases, diffusion and penetration capabilities decrease, and the curing reaction rate slows down, narrowing the underwater construction window and making interface defects more likely to be retained and evolve into peeling or seepage channels during subsequent service.

[0004] Existing underwater epoxy systems mostly achieve short-term bonding by increasing resin viscosity, introducing hydrophilic / water-absorbing components, or using external pressure drainage. However, while high-viscosity or paste-like systems can reduce the risk of being washed away by water flow, their wetting and penetration capabilities are often insufficient, making it difficult to enter the micropores and rough textures of the substrate to form an effective mechanical bond. Secondly, hydrophilic / water-absorbing components can absorb moisture and create a local drainage effect in the short term, but this is easily accompanied by excessive water absorption and swelling, plasticization of the cured product, reduction in modulus, and stress concentration at the interface. Thirdly, relying on external pressure drainage places high demands on construction equipment and working conditions, making it unsuitable for complex geometries, confined spaces, or underwater emergency repairs.

[0005] Therefore, achieving an underwater curing system that is "wettable, drainable, rapidly network-forming, and long-term water-resistant" without significantly increasing system complexity or relying on functional fillers is a key requirement in this field. Curing agent molecular design is considered an important pathway to achieving these goals: on the one hand, it requires reducing interfacial energy and weakening the continuity of the hydration layer through hydrophobic segments; on the other hand, it requires ensuring the curing reaction proceeds fully through reactive amine hydrogen structures and improving wet interfacial penetration and network toughness through compliant molecular segments. Summary of the Invention

[0006] The Mannich reaction couples the nonylphenol structure, formaldehyde source, and amine structure at the molecular level. In this invention, nonylphenol is used as the phenol source, formaldehyde as the aldehyde source, and m-phenylenediamine as the amine source to prepare a Mannich base curing agent. The resulting curing agent molecule typically contains a benzylamine structure, a tertiary / secondary amine structure, and a nonyl hydrophobic side chain. These structural features provide both hydrophobic drainage and wetting penetration capabilities at wet or underwater interfaces, and form a cross-linked network during the reaction with epoxy resin, thereby improving wet bonding reliability and water resistance stability.

[0007] The purpose of this invention is to provide a two-component epoxy resin curing system and its preparation method that can cure in humid and underwater environments, thereby solving the problems of insufficient interfacial adhesion, incomplete curing, short application window, and poor long-term water resistance stability of existing underwater epoxy materials under low temperature and hydration layer conditions. A further objective is to make the curing system simple in raw materials, controllable in the preparation process, and convenient for on-site in-situ repair, batch preparation, and standardized packaging.

[0008] The embodiments of this disclosure provide a method for preparing an epoxy resin-based curing agent, comprising:

[0009] 1) The epoxy resin-based curing agent is prepared by nonylphenol, formaldehyde source, and m-phenylenediamine via the Mannich reaction; or

[0010] 2) The epoxy resin-based curing agent is prepared by a Mannich reaction of nonylphenol, phenol, formaldehyde source, and m-phenylenediamine; or

[0011] 3) The epoxy resin-based curing agent is prepared by nonylphenol, cashew phenol, formaldehyde source and m-phenylenediamine through Mannich reaction.

[0012] In some embodiments, the formaldehyde source includes one or more of formaldehyde aqueous solution, paraformaldehyde, and triformaldehyde.

[0013] In some embodiments, in 1), the molar ratio of nonylphenol: formaldehyde source: m-phenylenediamine is (0.8-1.2):(0.8-1.2):(0.9-1.2).

[0014] In some embodiments, the epoxy resin-based curing agent prepared by nonylphenol, a formaldehyde source, and m-phenylenediamine via a Mannich reaction comprises:

[0015] Nonylphenol is added to the reactor and stirred. Nitrogen gas is introduced to replace the air. The temperature is raised to 75-85℃ to completely melt it and form a homogeneous phase. Then, m-phenylenediamine is added in batches at 75-85℃ and stirring is continued to obtain a homogeneous premixed phase.

[0016] Paraformaldehyde was added to the premixed phase in batches. The reaction temperature was controlled at 85-95℃ during the addition process. After the addition was completed, the temperature was raised to 100-105℃ and the reaction was stirred for 2-4 hours.

[0017] After the reaction is complete, maintain the temperature at 100-105℃, first lower it to 48-52 kPa and hold for 5-15 min, then lower it to 19-21 kPa and hold for 15-25 min, and finally lower it to 8-10 kPa and hold for 40-60 min, removing the water until the change in system mass within 30 min is <0.1% and the Karl Fischer moisture content is ≤0.5%.

[0018] In some embodiments, in 2), the molar ratio of nonylphenol: phenol: formaldehyde source: m-phenylenediamine is (0.8-1.2): (0.8-1.2): (2-2.2): (1.8-2.2).

[0019] In some embodiments, the epoxy resin-based curing agent prepared by nonylphenol, phenol, formaldehyde source and m-phenylenediamine via the Mannich reaction comprises:

[0020] Nonylphenol and phenol are added to the reactor, stirred, and nitrogen is introduced to replace the air; the temperature is raised to 75-85℃ to completely melt them and form a homogeneous phase, and then m-phenylenediamine is added in batches at 75-85℃, and stirring is continued to obtain a homogeneous premixed phase;

[0021] Paraformaldehyde was added to the premixed phase in batches. The reaction temperature was controlled at 85-95℃ during the addition process. After the addition was completed, the temperature was raised to 100-105℃ and the reaction was stirred for 2-4 hours.

[0022] After the reaction is complete, maintain the temperature at 100-105℃, first lower it to 48-52 kPa and hold for 5-15 min, then lower it to 19-21 kPa and hold for 15-25 min, and finally lower it to 8-10 kPa and hold for 40-60 min, removing the water until the change in system mass within 30 min is <0.1% and the Karl Fischer moisture content is ≤0.5%.

[0023] In some embodiments, in 3), the molar ratio of nonylphenol: cashew phenol: formaldehyde source: m-phenylenediamine is (0.8-1.2): (0.8-1.2): (2-2.2): (1.8-2.2).

[0024] In some embodiments, the epoxy resin-based curing agent prepared by nonylphenol, cashew nut phenol, formaldehyde source and m-phenylenediamine via the Mannich reaction comprises:

[0025] Nonylphenol and cashew phenol are added to the reactor, stirred, and nitrogen is introduced to replace the air; the temperature is raised to 75-85℃ to completely melt them and form a homogeneous phase, and then m-phenylenediamine is added in batches at 75-85℃, and stirring is continued to obtain a homogeneous premixed phase;

[0026] Paraformaldehyde was added to the premixed phase in batches. The reaction temperature was controlled at 85-95℃ during the addition process. After the addition was completed, the temperature was raised to 100-105℃ and the reaction was stirred for 2-4 hours.

[0027] After the reaction is complete, maintain the temperature at 100-105℃, first lower it to 48-52 kPa and hold for 5-15 min, then lower it to 19-21 kPa and hold for 15-25 min, and finally lower it to 8-10 kPa and hold for 40-60 min, removing the water until the change in system mass within 30 min is <0.1% and the Karl Fischer moisture content is ≤0.5%.

[0028] Another embodiment of this disclosure provides an epoxy resin-based curing agent system, the epoxy resin-based curing agent system comprising an epoxy resin-based curing agent and an epoxy resin component obtained according to the above preparation method.

[0029] In some embodiments, the epoxy resin component includes one or both of bisphenol A type epoxy resin and bisphenol F type epoxy resin; the epoxy resin component further includes an active diluent, which includes one or more of monofunctional glycidyl ether or polyfunctional glycidyl ether.

[0030] The main advantages of this invention include: 1) This invention uses m-phenylenediamine (MXDA) as the sole amine source and achieves "semi-Mannich substitution" by controlling the degree of reaction, forming a rigid aromatic ring skeleton with asymmetric reactivity. In contrast, existing technologies mostly use fully substituted Mannich bases (with impaired crosslinking density) or unmodified volatile small molecule polyamines; 2) Some embodiments of this invention use para-nonyl-substituted phenol as the sole phenol source, ensuring that Mannich substitution mainly occurs at the ortho position and retains the phenolic hydroxyl site. This not only improves the enrichment of hydrophobic side groups at the substitution end at the molecular level, but also the free phenolic hydroxyl groups play a significant synergistic catalytic role in the epoxy ring-opening of the unsubstituted highly active amino group, greatly improving the low-temperature curing ability; 3) The molecule of this invention exhibits a unique asymmetric structure at both ends. One end has a "nonyl hydrophobic segment + phenolic hydroxyl catalytic / hydrogen bond site + sterically hindered secondary amine", while the other end retains a "highly active primary amine and rigid aromatic ring". This "asymmetric active and rigid-hydrophobic coupling" structural unit retains the excellent chemical resistance and anti-corrosion barrier properties of m-phenylenediamine, while effectively overcoming its easy moisture absorption and "blushing" defect. It is more suitable for achieving strong adhesion and curing into a network in heavy-duty anti-corrosion coatings or low-temperature and humid interfaces; 4) Some embodiments of this invention limit the phenol source, aldehyde source and amine source to "nonylphenol + formaldehyde source + m-phenylenediamine", and achieve half-substitution through precise control, making the crosslinking functionality (precisely retaining 3 active hydrogens), reactivity gradient, hydrophobic nonyl density and substitution site (mainly ortho-position) of the curing agent more controllable; 5) Through semi-Mannich modification and raw material ratio design, not only is the volatility and toxicity of free m-phenylenediamine significantly reduced, but the free water and volatile content is also reduced through controlled dehydration, which greatly improves the construction environment and enhances the water resistance stability and long-term service reliability of the cured product under harsh anti-corrosion or water immersion conditions. Attached Figure Description

[0031] The above and other features, advantages, and aspects of the embodiments of this disclosure will become more apparent from the accompanying drawings and the following detailed description. Throughout the drawings, the same or similar reference numerals denote the same or similar elements. It should be understood that the drawings are schematic, and elements are not necessarily drawn to scale.

[0032] Figure 1 This is a schematic diagram of the chemical reaction synthesis of the Mannich base curing agent according to an embodiment of the present invention;

[0033] Figure 2 The stress and strain test results are for Embodiments 5 and 6 of the present invention;

[0034] Figure 3 This is a schematic diagram of the chemical reaction synthesis of the Mannich base curing agent according to another embodiment of the present invention;

[0035] Figure 4The stress and strain test results are for Embodiments 12 and 13 of the present invention;

[0036] Figure 5 This is a schematic diagram of the chemical reaction synthesis of the Mannich base curing agent according to another embodiment of the present invention;

[0037] Figure 6 The stress and strain test results are for Examples 19 and 20 of the present invention. Detailed Implementation

[0038] Embodiments of this disclosure will now be described in more detail with reference to the accompanying drawings. While some embodiments of this disclosure are shown in the drawings, it should be understood that this disclosure can be implemented in various forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of this disclosure. It should be understood that the accompanying drawings and embodiments of this disclosure are for illustrative purposes only and are not intended to limit the scope of protection of this disclosure.

[0039] It should be understood that the various steps described in the method embodiments of this disclosure can be performed in sequence and / or in parallel. Furthermore, method embodiments may include additional steps and / or omit the steps shown. The scope of this disclosure is not limited in this respect.

[0040] The term "comprising" and its variations as used herein are open-ended inclusions, meaning "including but not limited to". The term "based on" means "at least partially based on". The term "one embodiment" means "at least one embodiment"; the term "another embodiment" means "at least one additional embodiment"; the term "some embodiments" means "at least some embodiments". Definitions of other terms will be given in the description below.

[0041] Unless otherwise specified, the term "parts" in this disclosure refers to parts by mass; "underwater curing" refers to the curing and maintenance of the sample under immersion conditions in water (fresh water or artificial seawater). The test temperature is assumed to be 25±2℃.

[0042] This invention provides an epoxy resin-based curing agent or curing system, comprising component A and component B. Component A is an epoxy resin matrix, selected from one or both of bisphenol A type epoxy resin and bisphenol F type epoxy resin. To adjust the initial viscosity, improve low-temperature workability, and enhance penetration into rough interfaces, component A may further include 0-30 parts by weight of an active diluent, preferably a monofunctional or polyfunctional glycidyl ether, such as butyl glycidyl ether, phenyl glycidyl ether, fatty alcohol glycidyl ether, or combinations thereof; the above examples are for illustrative purposes only, and specific types can be selected based on compatibility and application window.

[0043] In some embodiments, component B is a Mannich base curing agent. Figure 1 A schematic diagram of the chemical reaction synthesis of the Mannich base curing agent is shown, which is prepared by nonylphenol, formaldehyde source, and m-phenylenediamine through the Mannich reaction. In some embodiments, the formaldehyde source is an aqueous formaldehyde solution, paraformaldehyde, or trioxyformaldehyde; the molar ratio of nonylphenol:formaldehyde source:m-phenylenediamine is (0.8-1.2):(0.8-1.2):(0.9-1.2). By adjusting the amount of formaldehyde source added, the feeding method, and the degree of dehydration, the viscosity and free small molecule content of the curing agent can be controlled while ensuring the content of reactive amine hydrogen, thereby obtaining component B suitable for underwater mixing and construction.

[0044] In some embodiments, during use, the dosage of component A and component B is calculated according to the principle of matching epoxy equivalent with active hydrogen equivalent, and then mixed evenly to obtain an epoxy system that can be applied in humid or underwater environments. The mixing process can employ low-speed stirring combined with wall scraping to reduce air bubbles; short-term degassing can be performed if necessary. For double-tube packaging or static mixing pipe construction, component A and component B can be pre-packaged separately and mixed online during use to improve underwater construction efficiency and mix ratio stability.

[0045] This curing agent system utilizes the hydrophobic drainage and interfacial energy regulation of the nonylphenol fragment, along with the wetting-penetration ability of m-phenylenediamine, to weaken the obstacle of the stable hydration layer on the substrate surface to wetting and adhesion, and to form a continuous cross-linked network under room temperature or low temperature conditions.

[0046] In some embodiments, the curing agent system of this application does not contain functional fillers; however, without changing the core curing system of this invention (i.e., component A epoxy resin and component B Mannich base curing agent) and its curing mechanism, inert fillers, thixotropic modifiers, defoamers, pigments, or anti-settling agents may be selectively added according to workability requirements. For example, to improve the anti-sagging ability of vertical or overhead surface construction, a small amount of thixotropic modifier may be selectively added; to reduce curing shrinkage or improve flowability, inert fillers may be selectively added. The types and amounts of the above optional additives do not constitute a limitation on this application.

[0047] Raw materials and equipment

[0048] (1) Nonylphenol (NP), industrial grade, moisture ≤0.2%. Phenol (PhOH), industrial grade. Cardanol, industrial grade. Formaldehyde source can be 37% formaldehyde aqueous solution, trioxymethylene or paraformaldehyde (when measured as paraformaldehyde, it is calculated as CH2O equivalent). m-Phenylenediamine, nominal amine value is about 136.2 mgKOH / g.

[0049] (2) Epoxy resin: Bisphenol A type epoxy resin (E51 or equivalent, epoxy equivalent EEW=185-195 g / eq); Bisphenol F type epoxy resin (EEW=170-180 g / eq). Reactive diluent (optional): Butyl glycidyl ether (BGE) or phenyl glycidyl ether (PGE), added in amounts of 0-30 parts, used to adjust low-temperature viscosity and underwater spreading.

[0050] (3) Equipment: 1 L four-necked flask or jacketed reactor (equipped with mechanical stirrer, temperature probe, reflux condenser, dropping funnel / solid feed port), nitrogen protection device, vacuum pump (can be stabilized to 5-20 kPa absolute pressure), Karl Fischer moisture analyzer, rotational viscometer (25℃), infrared spectrometer (FT-IR), and nuclear magnetic resonance (1H NMR) for structural confirmation when necessary.

[0051] Example 1 Preparation of component B (Mannich base curing agent) (paraformaldehyde as formaldehyde source).

[0052] (1) Feeding ratio: The molar ratio of nonylphenol: formaldehyde equivalent: m-phenylenediamine is (0.9-1.1):(0.9-1.1):(0.9-1.2). Taking the preparation of about 200g of curing agent as an example: weigh 119.57 g of nonylphenol, 16.98 g of paraformaldehyde, and 73.91 g of m-phenylenediamine. This ratio allows one of the two primary amines of m-phenylenediamine to undergo the Mannich reaction, and the prepared semi-Mannich base takes into account both reactivity and storage viscosity.

[0053] (2) Premixing and heating: Nonylphenol was added to the reactor, mechanical stirring was started (300 rpm), and a small amount of nitrogen gas (50-100 mL / min) was introduced to replace the air; the temperature was raised to 78-82℃ to completely melt it and form a homogeneous phase. Then, m-phenylenediamine was added in three portions at 78-82℃ (about 25 g each time, 5 min apart), the stirring speed was increased to 400 rpm, and stirring was continued for 15 min to obtain a homogeneous “NP-MXDA” premixed phase.

[0054] (3) Batching and reaction: Paraformaldehyde was added in 6 batches (approximately 2.5 g per batch, with an interval of 5 min). During the addition process, the reactor temperature was controlled at 85-95℃ to avoid local aggregation that could lead to instantaneous heat release and agglomeration. After all the paraformaldehyde was added, the temperature was raised to 100-105℃ and stirred at 450 rpm for 3 h. Samples were taken every 1 h during the reaction, and FT-IR was used to track the enhancement of CN absorption related to benzylamine and the changes in the free amine-related peaks (3300-3500 cm⁻¹). -1 Morphological changes in the NH / OH region, 1200-1350 cm -1 CN region enhancement, etc.

[0055] (4) Vacuum Removal: After the reaction is complete, maintain the reactor temperature at 100-105℃ and gradually remove by-product water and volatiles by vacuum: first reduce the pressure to 48-52 kPa and hold for 10 min, then reduce it to 19-21 kPa and hold for 20 min, and finally reduce it to 8-10 kPa and hold for 40-60 min. If foam rises during the removal process, the vacuum can be broken intermittently or the stirring speed can be reduced to 250 rpm. The removal should be stopped when the change in system mass within 30 min is <0.1% and the Karl Fischer moisture content is ≤0.5%.

[0056] (5) Cooling and filtration: Remove the vacuum and purge with nitrogen to backpressure. After cooling to 58-62℃, filter through a 100-mesh stainless steel mesh to obtain component B curing agent B-1. The obtained curing agent is a pale yellow transparent to semi-transparent viscous liquid with a viscosity of 3000-8000 mPa·s at 25℃; the amine value is 304.5 mgKOH / g (related to the degree of substitution). The active hydrogen equivalent (AHEW) is converted from the amine value and used for subsequent proportioning calculations.

[0057] (6) Structural description: The obtained curing agent has a rigid skeleton of m-phenylenediamine and an asymmetric molecular structure. One end amino group is introduced into the structure via a semi-Mannich reaction: —NH—CH2—Ar(OH)—C9H 19 The structural unit has an unreacted free primary amine (—CH2NH2) at one end; Ar is a 4-nonyl-substituted benzene ring, and the Mannich substitution site is mainly located at the ortho position (2 and / or 6 position) of the phenolic hydroxyl group. The substituted end is mainly a secondary amine and allows for a small amount of further substitution to form tertiary amines or bi-terminal substituted byproducts.

[0058] Recommended specifications for curing agents are shown in Table 1:

[0059] Table 1

[0060] Appearance Visual inspection (25℃) Pale yellow, transparent to semi-transparent, with no obvious layering. Moisture (%) Karl Fischer ≤0.5 Viscosity (mPa·s) 25℃, rotational viscometer 3000-8000 Amine value (mgKOH / g) GB / T 12008.2 or equivalent 304.5 Free formaldehyde (%) Acetylacetone method or equivalent ≤0.05

[0061] Example 2 Preparation of component A (epoxy resin matrix).

[0062] (1) Resin ratio: NPEF-170 bisphenol F type epoxy resin and AGE epoxy reactive diluent are mixed at a mass ratio of 9:1.

[0063] (2) Mixing conditions: AGE epoxy reactive diluent and bisphenol F epoxy resin were added to a covered container and preheated in a 50°C constant temperature water bath for 10 min to reduce viscosity; stirring (200 rpm) was turned on and mixed for 20 min, with the wall scraped twice; then the temperature was lowered to 30°C and vacuum degassed at -0.08 MPa for 8-10 min to obtain component A A-1.

[0064] A / B ratio calculation, mixing and workability determination

[0065] (1) Equivalent ratio: Matched according to "epoxy equivalent = active hydrogen equivalent". The mixed EEW of A-1 is 170.6 g / eq, and the AHEW of B-1 is converted by amine value (amine value of 304.5 mgKOH / g corresponds to AHEW of about 122.86 g / eq). Then the theoretical mass ratio B / A≈(A mass / EEW)*AHEW / A mass = AHEW / EEW≈122.86 / 170.6, that is, A:B≈100:72.

[0066] (2) Mixing process: Weigh A-1 and B-1 at a mass ratio of 100:72 and place them in a 1 L cup; stir with a low-speed paddle (300 rpm) for 1 min, scrape the wall, and then stir at 350 rpm for 1 min, with the total mixing time controlled at 2.5 min. Record the workable time (25℃) immediately after mixing: the workable time is 45 min when the viscosity of the system increases significantly and it can no longer self-level.

[0067] 3) Underwater gel / initial curing: Place the mixed adhesive into a polyethylene bag or dispensing device and let it stand in water at 20°C; the initial curing criterion is that the surface is no longer sticky to the touch, and the initial curing time is 1.5 hours. Complete curing in water at 20°C requires 7-14 days.

[0068] Preparation and testing methods for underwater bonding specimens (in accordance with GB / T 7124-2008)

[0069] (1) Steel-to-steel lap shearing: Q235 or 304 stainless steel test pieces with dimensions of 100 mm × 25 mm × 2 mm were selected; they were sanded unidirectionally with 80# sandpaper to Ra approximately 2-5 µm, degreased with ethanol, and then soaked in water for 30 min before use. The lap length was 12.5 mm, and the adhesive layer thickness was 0.20 ± 0.05 mm (controlled with a metal probe). After assembly, the entire piece was immediately immersed in water at 20℃ for curing.

[0070] (2) Tension / compression: When preparing the casting, pour the mixed rubber into the polytetrafluoroethylene mold, let it stand in the air for 10 minutes to remove bubbles, and then immerse it in water to cure; the tension is in accordance with GB / T 2567 (dumbbell type), and the compression is in accordance with GB / T 2569.

[0071] (3) Water absorption rate and water retention rate: The casting body was cut into 50 mm × 10 mm × 4 mm test strips, vacuum dried at 60℃ to a constant weight m0, and then immersed in water at 20℃; the strips were removed, dried, and weighed m1 at 1 d, 7 d, and 21 d. Water absorption rate = (m1 - m0) / m0 × 100%. Water retention rate is expressed as shear strength after immersion / dry shear strength × 100%.

[0072] Examples 3-8

[0073] Example 3 (Underwater Shear Bonding): The curing agent was prepared according to Example 1, and the resin matrix was prepared according to Example 2; steel-to-steel lap joint samples were prepared by mixing A:B=100:72, and the shear strength was tested after curing in water at 20℃ for 14 days. The test results are shown in Table 2.

[0074] Example 4 (Air Shear Bonding): The curing agent was prepared according to Example 1, and the resin matrix was prepared according to Example 2; steel-to-steel lap joint samples were prepared by mixing A:B=100:72 and curing them in a dry environment at 20℃ for 14 days. The shear strength was then tested, and the test results are shown in Table 2.

[0075] Example 5 (In-situ Low-Temperature Underwater Curing): The process of preparing the Mannich base epoxy resin system in Example 3 was repeated. The prepared epoxy resin system was poured into a dumbbell-shaped mold under complete water immersion at 20°C as described above. After in-situ curing for 14 days, a tensile test was performed. The test results are as follows. Figure 2 As shown.

[0076] Example 6 (Room Temperature Curing): The prepared Mannich base epoxy resin system was repeated in Example 5 and poured into a dumbbell-shaped mold under a drying environment at 20°C. After in-situ curing for 14 days, a tensile test was performed. The test results are as follows: Figure 2 As shown.

[0077] Example 7 (Underwater positive tensile bonding): As described in Example 3, Mannich base epoxy resin curing agent was prepared. A Q235 steel block with a handle was selected, with a bonding size of 45mm × 45mm. The surface was sanded unidirectionally with 80# sandpaper to a Ra of approximately 2-5 µm. After degreasing with ethanol, the surface was immersed in water for 30 min for later use. The adhesive layer thickness was 0.20 ± 0.05 mm (controlled with a metal probe). Immediately after assembly, the entire assembly was immersed in water at 20℃ for curing for 14 days. A pull-out test was then conducted to check the bond strength. The test results are shown in Table 2.

[0078] Example 8 (drying and positive tension bonding): Same as Example 7, the bonded steel block with handle was cured in a dry environment at 20℃. After 14 days, a pull-out test was conducted to test the bonding strength. The test results are shown in Table 2.

[0079] Table 2

[0080] 3 0.34 4 1.51 7 2.7 8 10.5

[0081] In some embodiments, the Mannich reaction can couple nonylphenol, phenol, formaldehyde source, and amine structure at the molecular level. Some embodiments of the present invention use nonylphenol and phenol as phenol sources, formaldehyde source as aldehyde source, and m-phenylenediamine as amine source to prepare Mannich base curing agents. The resulting curing agent molecules typically contain benzylamine structures, tertiary / secondary amine structures, and nonyl hydrophobic side chains. These structural features provide both hydrophobic drainage and wetting penetration capabilities at wet or underwater interfaces, and form a cross-linked network during the reaction with epoxy resin, thereby improving wet bonding reliability and water resistance stability. This curing agent can solve the problems of insufficient interfacial adhesion, incomplete curing, short application window, and poor long-term water resistance stability of existing underwater epoxy materials under low temperature and hydration layer conditions.

[0082] In some embodiments, component B is a Mannich base curing agent. Figure 3 A schematic diagram of the chemical reaction synthesis of Mannich base curing agent is shown, which is prepared by nonylphenol, phenol, formaldehyde source, and m-phenylenediamine through the Mannich reaction. In some embodiments, the formaldehyde source is an aqueous formaldehyde solution, paraformaldehyde, or triformaldehyde; the molar ratio of nonylphenol:phenol:formaldehyde source:m-phenylenediamine is (0.8-1.2):(0.8-1.2):(2-2.2):(1.8-2.2). By adjusting the amount of formaldehyde source, the feeding method, and the degree of dehydration, the viscosity and free small molecule content of the curing agent can be controlled while ensuring the content of reactive amine hydrogen, thereby obtaining component B suitable for underwater mixing and construction.

[0083] By employing this curing agent, some embodiments of the present invention limit the phenol source to a compound combination of "50% mol nonylphenol + 50% mol phenol". This utilizes the low steric hindrance and high reactivity of phenol to ensure the efficient conversion of the Mannich reaction, while introducing an appropriate amount of hydrophobic long chains through nonylphenol, achieving a precise balance between high catalytic activity and excellent hydrophobic properties of the curing system at the molecular level. Furthermore, some embodiments of the present invention exhibit a unique asymmetric structure, with the substitution end possessing "nonyl / phenyl hydrophobic segments + phenolic hydroxyl hydrogen bonding sites + sterically hindered secondary amines", and the free end retaining "highly active primary amines and rigid aromatic rings". This "asymmetric activity and rigid-hydrophobic coupling" structural unit not only effectively overcomes the hygroscopic "whitening" defect of amines, but is also more suitable for effectively dissipating interfacial moisture in a completely immersive environment, achieving deep wetting penetration and underwater curing into a network. Furthermore, some embodiments of the present invention strictly limit the phenolic source, aldehyde source, and amine source to "an equimolar mixture of nonylphenol / phenol + formaldehyde source + m-phenylenediamine," making the crosslinking functionality of the curing agent (precisely retaining 3 active hydrogen curing sites), reactivity gradient, hydrophobic nonyl density, and substitution sites more controllable. In addition, through semi-Mannich modification and raw material ratio design, not only is the volatility and toxicity of free m-phenylenediamine significantly reduced, but the content of free water and small molecule volatiles is also reduced through controllable dehydration, greatly improving the retention rate of mechanical properties and long-term service reliability of the cured product under harsh conditions such as complete water immersion.

[0084] This curing agent system utilizes the hydrophobic drainage and interfacial energy regulation of the nonylphenol fragment, the high reactivity and high rigidity of phenol, and the wetting-penetration ability of m-phenylenediamine to weaken the obstacle of the stable hydration layer on the substrate surface to wetting and adhesion, and form a continuous cross-linked network under room temperature or low temperature conditions.

[0085] Example 9: Preparation of Component B (Mannich base curing agent) (using paraformaldehyde as the formaldehyde source)

[0086] (1) Feeding ratio: The molar ratio of nonylphenol: phenol: formaldehyde equivalent: m-phenylenediamine is (0.9-1.1): (0.9-1.1): (2-2.2): (1.8-2.2). Taking the preparation of about 200g of curing agent as an example: weigh 72.14 g of nonylphenol, 30.81 g of phenol, 20.5 g of paraformaldehyde, and 89.18 g of m-phenylenediamine. This ratio allows one of the two primary amines of m-phenylenediamine to undergo the Mannich reaction, and the prepared semi-Mannich base balances reactivity and storage viscosity.

[0087] (2) Premixing and heating: Nonylphenol and phenol were added to the reactor, and mechanical stirring was started (300 rpm). A small amount of nitrogen gas (75 mL / min) was introduced to replace the air. The temperature was raised to 78-82℃ to completely melt and form a homogeneous phase. Then, m-phenylenediamine was added in three portions at 78-82℃ (about 30 g each time, 5 min apart). The stirring speed was increased to 400 rpm and stirring was continued for 15 min to obtain a homogeneous premixed phase.

[0088] (3) Batching and reaction: Paraformaldehyde was added in 6 batches (approximately 3.1 g per batch, with an interval of 5 min). During the addition process, the reactor temperature was controlled at 85-95℃ to avoid local aggregation that could lead to instantaneous heat release and agglomeration. After all the paraformaldehyde was added, the temperature was raised to 100℃ and stirred at 450 rpm for 3 h. Samples were taken every 1 h during the reaction, and FT-IR was used to track the enhancement of CN absorption related to benzylamine and the changes in the free amine-related peaks (3300-3500 cm⁻¹). -1 Morphological changes in the NH / OH region, 1200-1350 cm -1 CN region enhancement, etc.

[0089] (4) Vacuum Removal: After the reaction is complete, maintain the reactor temperature at 100-105℃ and gradually remove the by-product water and volatiles by vacuum: first reduce the pressure to 48-52 kPa and hold for 10 min, then reduce it to 19-21 kPa and hold for 20 min, and finally reduce it to 8-10 kPa and hold for 50 min. If foam rises during the removal process, the vacuum can be broken intermittently or the stirring speed can be reduced to 250 rpm. The removal should be stopped when the change in system mass within 30 min is <0.1% and the Karl Fischer moisture content is ≤0.5%.

[0090] (5) Cooling and filtration: Remove the vacuum and purge with nitrogen to backpressure. After cooling to 58-62℃, filter through a 100-mesh stainless steel mesh to obtain component B curing agent B-2. The obtained curing agent is a pale yellow transparent to semi-transparent viscous liquid with a viscosity of 3000-8000 mPa·s at 25℃; the amine value is generally 367.4 mgKOH / g (related to the degree of substitution). The active hydrogen equivalent (AHEW) is converted from the amine value and used for subsequent proportioning calculations.

[0091] (6) Structural description: The obtained curing agent has an asymmetric structure with m-phenylenediamine as the backbone. One end of the amino group is introduced into the —NH—CH2—Ar(OH)—R structural unit through a half-Mannig reaction, while the other end retains the unreacted free primary amino group (—CH2NH2); where Ar is a benzene ring and R is a nonyl group located at the para position (—C9H 19The α- or β-hydrogen atom (-H) corresponds to the equimolar cocondensation segment of nonylphenol and phenol; the Mannich substitution site is mainly located at the ortho position of the phenolic hydroxyl group (position 2 and / or position 6). The substitution ends are mainly secondary amines, and a small amount of further substitution is allowed to form tertiary amine structures or byproducts in which both ends of the amino group are substituted.

[0092] Recommended specifications for curing agents are shown in Table 3:

[0093] Appearance Visual inspection (25℃) Pale yellow, transparent to semi-transparent, with no obvious layering. Moisture (%) Karl Fischer ≤0.5 Viscosity (mPa·s) 25℃, rotational viscometer 4000-10000 Amine value (mgKOH / g) GB / T 12008.2 or equivalent 367.4 Free formaldehyde (%) Acetylacetone method or equivalent ≤0.05

[0094] Component A was prepared according to Example 2.

[0095] A / B ratio calculation, mixing and workability determination

[0096] (1) Equivalent ratio: It is recommended to match according to "epoxy equivalent = active hydrogen equivalent". Take the mixed EEW of A-1 as 170.6 g / eq (example value, needs to be corrected according to actual resin data), and convert the AHEW of B-2 from the amine value (example: amine value of 367.4 mgKOH / g corresponds to AHEW of about 101.82 g / eq). Then the theoretical mass ratio B / A≈(A mass / EEW)*AHEW / A mass = AHEW / EEW≈101.82 / 170.6, that is, A:B≈100:60. In order to take into account early underwater curing, it can be finely adjusted within ±10% and verified by gel time, exothermic peak and early strength.

[0097] (2) Mixing process: Weigh A-1 and B-2 at a mass ratio of 100:60 and place them in a 1 L cup; stir with a low-speed paddle (300 rpm) for 1 min, scrape the wall, and then stir at 350 rpm for 1 min. The total mixing time should be controlled within 2-3 min. Record the workable time (25℃) immediately after mixing: the endpoint is when the viscosity of the system increases significantly and it can no longer self-level. The typical workable time is 45 min.

[0098] (3) Underwater gel / initial curing: The mixed adhesive is placed in a polyethylene bag or dispensing device and left to stand in water at 20°C; the initial curing criterion is that the surface is no longer sticky to the touch, and the typical initial curing time is 1.5 h. Complete curing in water at 20°C usually takes 7-14 days.

[0099] The preparation and testing methods for underwater bonding samples shall conform to GB / T 7124-2008, as described above.

[0100] Examples 10-15

[0101] Example 10 (Underwater Shear Bonding): The curing agent was prepared according to Example 9, and the resin matrix was prepared according to Example 2; the mixture was prepared according to the ratio of A:B=100:60, and steel-steel lap joint samples were prepared according to the above steps. After curing in water at 20°C for 14 days, the shear strength was tested. The test results are shown in Table 4.

[0102] Example 11 (Air Shear Bonding): The curing agent was prepared according to Example 9, and the resin matrix was prepared according to Example 2; the mixture was prepared according to A:B=100:60, and steel-steel lap joint samples were prepared according to the above steps. After curing in a dry environment at 20°C for 14 days, the shear strength was tested. The test results are shown in Table 4.

[0103] Example 12 (In-situ Low-Temperature Underwater Curing): The process of preparing the Mannich base epoxy resin system in Example 10 was repeated. The prepared epoxy resin system was poured into a dumbbell-shaped mold under a completely immersed water environment at 20°C. After in-situ curing for 14 days, a tensile test was performed. The test results are as follows: Figure 4 As shown.

[0104] Example 13 (Room Temperature Curing): The prepared Mannich base epoxy resin system was repeated in Example 12 and poured into a dumbbell-shaped mold under a drying environment at 20°C. After in-situ curing for 14 days, a tensile test was performed. The test results are as follows: Figure 4 As shown.

[0105] Example 14 (Underwater positive tensile bonding): Similar to Example 10 above, a Mannich base epoxy resin curing agent was prepared. A Q235 steel block with a handle was selected, with a bonding size of 45mm × 45mm. The surface was sanded unidirectionally with 80# sandpaper to a Ra of approximately 2-5 µm. After degreasing with ethanol, the surface was immersed in water for 30 min for later use. The adhesive layer thickness was 0.20 ± 0.05 mm (controlled with a metal probe). Immediately after assembly, the entire assembly was immersed in water at 20℃ for curing for 14 days. A pull-out test was then conducted to check the bond strength. The test results are shown in Table 4.

[0106] Example 15 (drying and positive tension bonding): Same as Example 14, the bonded steel block with handle was cured in a dry environment at 20°C. After 14 days, a pull-out test was conducted to test the bonding strength. The test results are shown in Table 4.

[0107] Table 4

[0108] 10 0.84 11 2.41 14 6.5 15 9.5

[0109] In some embodiments, the Mannich reaction can couple nonylphenol, cashew nutshellone structures, formaldehyde sources, and amine structures at the molecular level. In some embodiments of the present invention, nonylphenol and cashew nutshellone are used as the phenol source, formaldehyde as the aldehyde source, and m-phenylenediamine as the amine source to prepare the Mannich base curing agent. The resulting curing agent molecule typically contains a benzylamine structure, a tertiary / secondary amine structure, and a nonyl hydrophobic side chain. These structural features can simultaneously provide hydrophobic drainage and wetting penetration capabilities at wet or underwater interfaces, and form a cross-linked network during the reaction with epoxy resin, thereby improving wet bonding reliability and water resistance stability.

[0110] To address the problems of insufficient interfacial adhesion, incomplete curing, short application window, and poor long-term water resistance of existing underwater epoxy materials under low-temperature and hydration layer conditions, this embodiment of the invention provides a curing system comprising component A and component B. Component A is an epoxy resin matrix, selected from bisphenol A type epoxy resin, bisphenol F type epoxy resin, or any mixture thereof. To adjust the initial viscosity, improve low-temperature workability, and enhance penetration into rough interfaces, component A may further include 0-30 parts by weight of an active diluent, preferably a monofunctional or polyfunctional glycidyl ether, such as butyl glycidyl ether, phenyl glycidyl ether, fatty alcohol glycidyl ether, or combinations thereof; the above examples are for illustrative purposes only, and specific types can be selected based on compatibility and application window.

[0111] In some embodiments, component B is a Mannich base curing agent. Figure 5 A schematic diagram of the chemical reaction synthesis of Mannich base curing agent is shown, which is prepared by nonylphenol, cashew nut shell powder, formaldehyde source, and m-phenylenediamine through the Mannich reaction. In some embodiments, the formaldehyde source is an aqueous formaldehyde solution, paraformaldehyde, or trioxymethylene; the molar ratio of nonylphenol: cashew nut shell powder: formaldehyde source: m-phenylenediamine is (0.8-1.2):(0.8-1.2):(2-2.2):(1.8-2.2). By adjusting the amount of formaldehyde source, the feeding method, and the degree of dehydration, the viscosity and free small molecule content of the curing agent can be controlled while ensuring the content of reactive amine hydrogen, thereby obtaining component B suitable for underwater mixing and construction.

[0112] Some embodiments of this invention use m-phenylenediamine as the sole amine source, achieving "semi-Mannich substitution" through precise control of the reaction degree, forming a rigid aromatic ring-containing skeleton with asymmetric reactivity. Existing technologies mostly use fully substituted Mannich bases (leading to severely compromised crosslinking density) or unmodified volatile small-molecule polyamines. Some embodiments of this invention creatively limit the phenolic source to a compound combination of "50% mol nonylphenol + 50% mol cashew nut shell powder," ensuring that Mannich substitution mainly occurs at the ortho position while retaining the phenolic hydroxyl site. Through the steric hindrance of the nonyl group and the synergistic effect of the C15 unsaturated long straight chain of cashew nut shell powder, the enrichment degree of hydrophobic flexible side groups at the substitution end and the interfacial wetting properties are greatly improved at the molecular level. Some embodiments of the present invention exhibit a unique asymmetric molecular structure, with the substitution end possessing "nonyl / pentadecanyl dual hydrophobic segments + phenolic hydroxyl hydrogen bonding sites + sterically hindered secondary amines," and the free end retaining "highly active primary amines and rigid aromatic rings." This "asymmetric activity and rigid-flexible hydrophobic coupling" structural unit not only effectively overcomes the defects of amines' easy hygroscopic whitening, but is also more suitable for effectively dissipating interfacial moisture in humid or even completely submerged environments, achieving deep wetting penetration and underwater curing into a network. Some embodiments of the present invention strictly limit the phenolic source, aldehyde source, and amine source to "an equimolar mixture of nonylphenol / cashew phenol + formaldehyde source + m-phenylenediamine," making the crosslinking functionality (precisely retaining 3 active hydrogen curing sites), reactivity gradient, dual composite hydrophobic chain density, and substitution sites (mainly ortho-position) of the curing agent more controllable. Furthermore, through semi-Mannich modification and raw material ratio design, not only was the volatility and toxicity of free m-phenylenediamine significantly reduced, but the content of free water and small molecule volatiles was also reduced through controlled dehydration, which significantly improved the mechanical property retention rate, water resistance stability and long-term service reliability of the cured product under harsh working conditions such as complete water immersion.

[0113] This curing agent system utilizes the hydrophobic drainage and interfacial energy regulation effects of nonylphenol and cashew phenol fragments, along with the wetting-penetration ability of m-phenylenediamine, to weaken the obstacle of the stable hydration layer on the substrate surface to wetting and adhesion, and to form a continuous cross-linked network under room temperature or low temperature conditions.

[0114] Example 16 Preparation of Component B (Mannich base curing agent) (paraformaldehyde as formaldehyde source)

[0115] (1) Feeding ratio: The molar ratio of nonylphenol: cashew phenol: formaldehyde equivalent: m-phenylenediamine is (0.8-1.2):(0.8-1.2):(2-2.2):(1.8-2.2). Taking the preparation of about 200g of curing agent as an example: weigh 53.96g of nonylphenol, 73.46g of cashew phenol, 15.32g of paraformaldehyde, and 66.70g of m-phenylenediamine. This ratio allows one of the two primary amines of m-phenylenediamine to undergo the Mannich reaction, and the prepared semi-Mannich base balances reactivity and storage viscosity.

[0116] (2) Premixing and heating: Nonylphenol and cashew phenol were added to the reactor, and mechanical stirring was started (300 rpm). A small amount of nitrogen gas (75 mL / min) was introduced to replace the air. The temperature was raised to 78-82℃ to completely melt and form a homogeneous phase. Then, m-phenylenediamine was added in three portions at 78-82℃ (about 22 g each time, 5 min apart). The stirring speed was increased to 400 rpm and stirring was continued for 15 min to obtain a homogeneous premixed phase.

[0117] (3) Batching and reaction: Paraformaldehyde was added in 6 batches (approximately 2.8 g per batch, with an interval of 5 min). During the addition process, the reactor temperature was controlled at 85-95℃ to avoid local aggregation that could lead to instantaneous heat release and agglomeration. After all the paraformaldehyde was added, the temperature was raised to 100℃ and stirred at 450 rpm for 3 h. Samples were taken every 1 h during the reaction, and FT-IR was used to track the enhancement of CN absorption related to benzylamine and the changes in the free amine-related peaks (3300-3500 cm⁻¹). -1 Morphological changes in the NH / OH region, 1200-1350 cm -1 CN region enhancement, etc.

[0118] (4) Vacuum Removal: After the reaction is complete, maintain the reactor temperature at 100-105℃ and gradually remove the by-product water and volatiles by vacuum: first reduce the pressure to 48-52 kPa and hold for 10 min, then reduce it to 19-21 kPa and hold for 20 min, and finally reduce it to 8-10 kPa and hold for 50 min. If foam rises during the removal process, the vacuum can be broken intermittently or the stirring speed can be reduced to 250 rpm. The removal should be stopped when the change in system mass within 30 min is <0.1% and the Karl Fischer moisture content is ≤0.5%.

[0119] (5) Cooling and filtration: Remove the vacuum and purge with nitrogen to backpressure. After cooling to 60°C, filter through a 100-mesh stainless steel mesh to obtain component B curing agent B-3. The obtained curing agent is a yellowish-brown transparent to semi-transparent viscous liquid with a viscosity of 1000-5000 mPa·s at 25°C. The amine value is generally 274.8 mgKOH / g (related to the degree of substitution). The active hydrogen equivalent (AHEW) is converted from the amine value and used for subsequent proportioning calculations.

[0120] (6) Structural description: The obtained curing agent has an asymmetric structure with m-phenylenediamine as the backbone. One end of the amino group is introduced into the —NH—CH2—Ar(OH)—R structural unit through a half-Mannig reaction, while the other end retains the unreacted free primary amino group (—CH2NH2); where Ar is a benzene ring and R is a nonyl group (—C9H 19The chain consists of a 15-carbon unsaturated straight chain, corresponding to equimolar cocondensation segments of nonylphenol (side chain mainly located at the para position) and cashew phenol (side chain located at the meta position), respectively; the Mannich substitution sites are mainly located at the ortho position (2 and / or 6 position) of the phenolic hydroxyl group. The substitution ends are mainly secondary amines, and a small amount of di-substitution is allowed to form tertiary amine end groups and a very small amount of byproducts in which both di-terminal amino groups are substituted.

[0121] Recommended specifications for curing agents are shown in Table 5:

[0122] Table 5

[0123] Appearance Visual inspection (25℃) Translucent yellowish-brown, without obvious layering Moisture (%) Karl Fischer ≤0.5 Viscosity (mPa·s) 25℃, rotational viscometer 1000-5000 Amine value (mgKOH / g) GB / T 12008.2 or equivalent 274.8 Free formaldehyde (%) Acetylacetone method or equivalent ≤0.05

[0124] Component A was prepared according to Example 2.

[0125] A / B ratio calculation, mixing and workability determination

[0126] (1) Equivalent ratio: Matched according to "epoxy equivalent = active hydrogen equivalent". The mixed EEW of A-1 is 170.6 g / eq, and the AHEW of B-3 is converted by amine value (amine value of 274.8 mgKOH / g corresponds to AHEW of about 136.13 g / eq). Then the theoretical mass ratio B / A≈(A mass / EEW)*AHEW / A mass = AHEW / EEW≈136.13 / 170.6, that is, A:B≈100:80. In order to take into account early underwater curing, it can be finely adjusted within ±10% and verified by gel time, exothermic peak and early strength.

[0127] (2) Mixing process: Weigh A-1 and B-3 at a mass ratio of 100:80 and place them in a 1 L cup; stir with a low-speed paddle (300 rpm) for 1 min, scrape the wall, and then stir at 350 rpm for 1 min. The total mixing time should be controlled within 2-3 min. Record the workable time (25℃) immediately after mixing: the endpoint is when the viscosity of the system increases significantly and it can no longer self-level. The typical workable time is 45 min.

[0128] (3) Underwater gel / initial curing: The mixed adhesive is placed in a polyethylene bag or dispensing device and left to stand in water at 20°C; the initial curing criterion is that the surface is no longer sticky to the touch, and the typical initial curing time is 1.5 h. Complete curing in water at 20°C usually takes 7-14 days.

[0129] The preparation and testing methods for underwater bonding samples shall conform to GB / T 7124-2008, as described above.

[0130] Examples 17-22

[0131] Example 17 (Underwater Shear Bonding): The curing agent was prepared according to Example 16, and the resin matrix was prepared according to Example 2; the mixture was prepared according to A:B=100:80, and steel-to-steel lap joint samples were prepared according to the above description. After curing in water at 20°C for 14 days, the shear strength was tested, and the test results are shown in Table 6.

[0132] Example 18 (Air Shear Bonding): The curing agent was prepared according to Example 16, and the resin matrix was prepared according to Example 2; the mixture was prepared according to A:B=100:80, and steel-steel lap joint samples were prepared according to the above description. After curing in a dry environment at 20°C for 14 days, the shear strength was tested, and the test results are shown in Table 6.

[0133] Example 19 (In-situ Low-Temperature Underwater Curing): The process of preparing the Mannich base epoxy resin system in Example 17 was repeated. The prepared epoxy resin system was poured into a dumbbell-shaped mold under a completely immersed water environment at 20°C. After in-situ curing for 14 days, a tensile test was performed. The test results are as follows: Figure 6 As shown.

[0134] Example 20 (Room Temperature Curing): Repeating Example 19, the prepared Mannich base epoxy resin system was poured into a dumbbell-shaped mold under a 20°C drying environment as described above. After in-situ curing for 14 days, a tensile test was performed. The test results are as follows: Figure 6 As shown.

[0135] Example 21 (Underwater positive tensile bonding): Similar to Example 17 above, a Mannich base epoxy resin curing agent was prepared. A Q235 steel block with a handle was selected, with a bonding size of 45mm × 45mm. The surface was sanded unidirectionally with 80# sandpaper to a Ra of approximately 2-5 µm. After degreasing with ethanol, the surface was immersed in water for 30 min for later use. The adhesive layer thickness was 0.20 ± 0.05 mm (controlled with a metal probe). Immediately after assembly, the entire assembly was immersed in water at 20℃ for curing for 14 days. A pull-out test was then conducted to check the bond strength. The test results are shown in Table 6.

[0136] Example 22 (drying and positive tension bonding): Same as Example 21, the bonded steel block with handle was cured in a dry environment at 20°C. After 14 days, a pull-out test was conducted to test the bonding strength. The test results are shown in Table 6.

[0137] Table 6

[0138] 17 2.17 18 3.2 21 6.7 22 7.5

[0139] Depend on Figure 2 , Figure 4 and Figure 6It is known that the rigid molecular structure of phenol can greatly improve the tensile strength of the curing agent system. However, compared with the system containing some cashew nut shell powder and pure nonylphenol, its fracture toughness is lower, and the overall appearance is "hard and brittle". The system containing some cashew nut shell powder has both strength and toughness and exhibits good resistance to underwater conditions compared with the system containing some phenol and pure nonylphenol.

[0140] Although the subject matter has been described using language specific to structural features and / or methodological logic, it should be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or actions described above. Rather, the specific features and actions described above are merely illustrative examples of implementing the claims.

Claims

1. A method for preparing an epoxy resin-based curing agent, characterized in that, include: 1) The epoxy resin-based curing agent is prepared by nonylphenol, formaldehyde source, and m-phenylenediamine via the Mannich reaction; or 2) The epoxy resin-based curing agent is prepared by a Mannich reaction of nonylphenol, phenol, formaldehyde source, and m-phenylenediamine; or 3) The epoxy resin-based curing agent is prepared by nonylphenol, cashew phenol, formaldehyde source and m-phenylenediamine through Mannich reaction.

2. The preparation method according to claim 1, characterized in that, The formaldehyde source includes one or more of formaldehyde aqueous solution, paraformaldehyde, and triformaldehyde.

3. The preparation method according to claim 1, characterized in that, In 1), the molar ratio of nonylphenol: formaldehyde source: m-phenylenediamine is (0.8-1.2):(0.8-1.2):(0.9-1.2).

4. The preparation method according to claim 3, characterized in that, The epoxy resin-based curing agent prepared by nonylphenol, formaldehyde source and m-phenylenediamine via Mannich reaction comprises: Nonylphenol is added to the reactor and stirred. Nitrogen gas is introduced to replace the air. The temperature is raised to 75-85℃ to completely melt it and form a homogeneous phase. Then, m-phenylenediamine is added in batches at 75-85℃ and stirring is continued to obtain a homogeneous premixed phase. Paraformaldehyde was added to the premixed phase in batches. The reaction temperature was controlled at 85-95℃ during the addition process. After the addition was completed, the temperature was raised to 100-105℃ and the reaction was stirred for 2-4 hours. After the reaction is complete, maintain the temperature at 100-105℃, first lower it to 48-52 kPa and hold for 5-15 min, then lower it to 19-21 kPa and hold for 15-25 min, and finally lower it to 8-10 kPa and hold for 40-60 min, removing the system until the mass change within 30 min is <0.1% and the Karl Fischer moisture content is ≤0.5%.

5. The preparation method according to claim 1, characterized in that, In 2), the molar ratio of nonylphenol: phenol: formaldehyde source: m-phenylenediamine is (0.8-1.2): (0.8-1.2): (2-2.2): (1.8-2.2).

6. The preparation method according to claim 5, characterized in that, The epoxy resin-based curing agent prepared by nonylphenol, phenol, formaldehyde source and m-phenylenediamine via the Mannich reaction comprises: Nonylphenol and phenol are added to the reactor, stirred, and nitrogen is introduced to replace the air; the temperature is raised to 75-85℃ to completely melt them and form a homogeneous phase, and then m-phenylenediamine is added in batches at 75-85℃, and stirring is continued to obtain a homogeneous premixed phase; Paraformaldehyde was added to the premixed phase in batches. The reaction temperature was controlled at 85-95℃ during the addition process. After the addition was completed, the temperature was raised to 100-105℃ and the reaction was stirred for 2-4 hours. After the reaction is complete, maintain the temperature at 100-105℃, first lower it to 48-52 kPa and hold for 5-15 min, then lower it to 19-21 kPa and hold for 15-25 min, and finally lower it to 8-10 kPa and hold for 40-60 min, removing the system until the mass change within 30 min is <0.1% and the Karl Fischer moisture content is ≤0.5%.

7. The preparation method according to claim 1, characterized in that, In 3), the molar ratio of nonylphenol: cashew phenol: formaldehyde source: m-phenylenediamine is (0.8-1.2): (0.8-1.2): (2-2.2): (1.8-2.2).

8. The preparation method according to claim 7, characterized in that, The epoxy resin-based curing agent prepared by nonylphenol, cashew nut phenol, formaldehyde source and m-phenylenediamine via Mannich reaction comprises: Nonylphenol and cashew phenol are added to the reactor, stirred, and nitrogen is introduced to replace the air; the temperature is raised to 75-85℃ to completely melt them and form a homogeneous phase, and then m-phenylenediamine is added in batches at 75-85℃, and stirring is continued to obtain a homogeneous premixed phase; Paraformaldehyde was added to the premixed phase in batches. The reaction temperature was controlled at 85-95℃ during the addition process. After the addition was completed, the temperature was raised to 100-105℃ and the reaction was stirred for 2-4 hours. After the reaction is complete, maintain the temperature at 100-105℃, first lower it to 48-52 kPa and hold for 5-15 min, then lower it to 19-21 kPa and hold for 15-25 min, and finally lower it to 8-10 kPa and hold for 40-60 min, removing the system until the mass change within 30 min is <0.1% and the Karl Fischer moisture content is ≤0.5%.

9. An epoxy resin-based curing agent system, characterized in that, The epoxy resin-based curing agent system comprises an epoxy resin-based curing agent and an epoxy resin component obtained by the preparation method according to any one of claims 1 to 8.

10. The epoxy resin-based curing agent system according to claim 9, characterized in that, The epoxy resin component includes one or both of bisphenol A type epoxy resin and bisphenol F type epoxy resin; the epoxy resin component also includes an active diluent, which includes one or more of monofunctional glycidyl ether or polyfunctional glycidyl ether.

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