Dimethylthio toluenediamine modified epoxy resin underwater curing agent and preparation method thereof

Abstract

The invention discloses a dimethyl thio toluenediamine modified epoxy resin underwater curing agent and a preparation method thereof in the field of high polymer materials. The curing agent is prepared from dimethyl thio toluenediamine, dicyclohexenyl bridged dimethyl thio toluenediamine, polyethylene glycol trimethylsilane block modified amine, cardanol, nano silicon dioxide and a silane coupling agent. The preparation method comprises the following steps: mixing two modified compounds with basic amine, adding an auxiliary agent to obtain a basic curing agent, and doping a silane coupling agent during use. The two modified compounds are prepared through a reductive amination reaction and an isocyanate addition reaction respectively, and the curing agent is endowed with excellent underwater adaptability. The curing agent can be rapidly cured with epoxy resin in a humid or underwater environment to form a high-crosslinking-density network, shows excellent adhesive force, durability and mechanical properties, and is suitable for the fields of ocean engineering, underwater structure repair and the like.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of high polymer materials, and particularly relates to a dimethylthio toluene diamine modified epoxy resin underwater curing agent and a preparation method thereof. BACKGROUND

[0002] Epoxy resin, as an important high-performance polymer material, is widely used in the fields of shipbuilding, offshore engineering, underwater structure and infrastructure repair and protection. However, the traditional epoxy resin curing agent faces many technical challenges when applied in humid or underwater environments. The presence of water can seriously interfere with the normal progress of the curing reaction. Water not only competes with the curing agent for the reaction site of the epoxy group, resulting in a decrease in crosslinking density, but also causes phase separation, leading to incomplete curing, a decrease in mechanical properties and other problems. In practical applications, this manifests as a significant decrease in adhesive strength, a decrease in glass transition temperature, insufficient durability and other defects, which severely limits the use of epoxy resin in underwater environments. This limitation is more pronounced for underwater construction and repair projects, often leading to early failure of the protective layer, the need for frequent maintenance and repair, and an increase in engineering costs and safety hazards.

[0003] Currently, commercial underwater epoxy resin curing agents mainly use physical isolation or simple chemical modification methods to improve underwater performance. Common technical means include adding hydrophobic groups to form a water barrier, using physical shielding agents to prevent water penetration, or accelerating the curing reaction to reduce the time window of water interference. However, these methods can only partially solve the problem and often bring new technical defects. For example, excessive increase in hydrophobicity can reduce water penetration, but it will sacrifice the adhesive strength of the material to the substrate; and too fast curing can cause internal stress concentration, insufficient permeability and other problems. The dimethylthio toluene diamine curing agent used in the prior art has better moisture tolerance than traditional aliphatic amine and aromatic amine curing agents, but the polar sulfide group in its molecule can still become a water aggregation point in a high humidity environment, leading to damage to the integrity of the cured network and making it difficult to meet the requirements of long-term durability in harsh underwater environments.

[0004] Therefore, developing an epoxy resin curing agent capable of maintaining high performance in underwater environment has become a technical problem to be solved in the art. An ideal underwater curing agent should have multiple functional properties: it can effectively resist moisture interference and form a dense network with epoxy resin; it can maintain excellent bonding performance underwater and can resist water medium erosion for a long time. This requires innovation at the molecular design level, through reasonable structural modification and functional modification, so that the curing agent molecule has the ability to adapt to the underwater environment. Specifically, by introducing specific molecular structures, it can independently guide the interface water behavior in the underwater environment, while maintaining sufficient reactivity and crosslinking density, so as to ensure that the cured product has excellent mechanical properties and durability. This innovative idea based on molecular structure design is an effective way to solve the current bottleneck of underwater epoxy resin curing technology. SUMMARY

[0005] The purpose of the present application is to provide a dimethylthio toluene diamine modified epoxy resin underwater curing agent and its preparation method, which solves the technical problems of incomplete curing, insufficient adhesion and poor durability of existing epoxy resin curing agents in humid or underwater environment due to moisture interference.

[0006] The present application realizes the above-mentioned purpose through the following technical solutions: A preparation method of a dimethylthio toluene diamine modified epoxy resin underwater curing agent, the steps comprising: S1, add dimethylthio toluene diamine into a reaction container, heat to 48-52℃, and stir; add dicyclohexenyl bridged dimethylthio toluene diamine and polyethylene glycol-trimethylsilane block modified amine in turn, heat to 50-55℃, and continuously stir to obtain a mixture; S2, add cardanol and nano silicon dioxide, stir to obtain a product; cool the product to room temperature to obtain a basic curing agent mixture; and mix gamma-glycidyl ether oxypropyl trimethoxysilane with the basic curing agent mixture before use.

[0007] In this invention, the preparation process of the dimethylthiotoluene diamine-modified epoxy resin underwater curing agent embodies the physical mixing and synergistic effect mechanism of multiple components. This curing agent system uses dimethylthiotoluene diamine as the base curing component. The primary and secondary amine groups in its molecule can undergo nucleophilic ring-opening reactions with epoxy groups to form a crosslinking network. The introduction of dicyclohexenyl-bridged dimethylthiotoluene diamine not only increases the total amine hydrogen equivalent of the system, but more importantly, its rigid hydrophobic structure forms molecular-level barrier regions in the curing system. These regions effectively hinder the penetration and diffusion of water molecules, while its multi-site reaction characteristics enhance the crosslinking network density. Polyethylene glycol trimethylsilane block-modified amine acts as an interface directing agent. Its amphiphilic molecular structure can rapidly migrate to the resin-water interface in the early stages of curing, forming an ordered interface layer through molecular self-assembly, temporarily stabilizing interfacial water molecules and creating favorable conditions for subsequent curing reactions. Cashew nut shell phenol, acting as a reaction rate modifier, moderately slows down the curing reaction rate through the steric hindrance effect of its long-chain alkyl groups, ensuring sufficient working time for the system. Nano-silica, through the formation of a hydrogen bond network by its surface silanol groups, imparts excellent thixotropic properties to the system, preventing sagging during underwater construction. The silane coupling agent, as a physically mixed component, is added to the system before use; its epoxy groups participate in the reaction during curing, while the methoxysilane end groups form strong chemical bonds with the substrate surface, greatly enhancing underwater adhesion strength. These components, through precise proportioning and orderly combination, constitute a high-performance curing system capable of autonomously adapting to the underwater environment. Through molecular-level synergy, they achieve effective control of moisture interference and excellent underwater curing performance.

[0008] According to a preferred embodiment of the present invention, in step S1, the stirring time is 30-60 min.

[0009] According to a preferred embodiment of the present invention, in step S2, stirring is performed for 20-40 minutes.

[0010] According to a preferred embodiment of the present invention, the preparation method of the dicyclohexenyl-bridged dimethylthiotoluene diamine includes: A1, under nitrogen protection, dissolving dimethylthiotoluene diamine in anhydrous ethanol, stirring and adding glacial acetic acid; adding 5-norbornen-2-carboxaldehyde, and stirring the reaction at 70-75°C; subsequently adding sodium borohydride, and continuing to stir the reaction at 70-75°C to obtain a reaction mixture; A2, filtering the reaction mixture, and rotary evaporating the filtrate to obtain a crude product; recrystallizing the crude product using a hexane-ethyl acetate mixed solvent.

[0011] In this invention, the preparation mechanism of dicyclohexenyl-bridged dimethylthiotoluene diamine is based on the classic organic reaction pathway of reductive amination. The process begins with a nucleophilic addition dehydration reaction between the primary amine group in the dimethylthiotoluene diamine molecule and the aldehyde group in the pentanorbornene formaldehyde molecule under the action of an acidic catalyst, initially forming an imine intermediate, i.e., a Schiff base. This stage requires strict control of the reaction temperature and acid-base environment to ensure that the lone pair electrons of the amine nitrogen atom can effectively attack the carbonyl carbon atom. Simultaneously, the acidic environment provided by glacial acetic acid catalyzes both carbonyl activation and promotes the dehydration process. Subsequently, under the action of sodium borohydride, a mild reducing agent, the carbon-nitrogen double bond in the imine intermediate is reduced to a carbon-nitrogen single bond, generating a stable secondary amine structure. This reduction process is achieved through the nucleophilic attack of the imine carbon atom by hydride-provided hydride anions, ultimately forming a modified amine compound with a rigid dicyclohexenyl-bridged structure. The unique feature of this product is that its molecule simultaneously retains both unreacted primary amine groups and newly generated secondary amine groups, and the rigid norbornene structure enhances the overall hydrophobicity and steric hindrance of the molecule. This structural characteristic allows the compound to maintain its reactivity with epoxy resins while simultaneously improving its hydrophobic properties through the introduction of a large alicyclic structure, effectively preventing water molecules from interfering with the curing process.

[0012] According to a preferred embodiment of the present invention, in step A1, the stirring reaction time at 70-75°C is 4-6 hours.

[0013] According to a preferred embodiment of the present invention, in step A2, recrystallization is performed 2-3 times.

[0014] According to a preferred embodiment of the present invention, the preparation method of the polyethylene glycol-trimethylsilane block-modified amine includes: B1, dissolving methoxy polyethylene glycol amine in toluene, adding triethylamine, and adding trimethylsilyl isocyanate dropwise under ice-water bath cooling; heating to 68-72°C, stirring the reaction to obtain a reaction mixture; B2, filtering the reaction mixture, and rotary evaporating the filtrate to obtain a crude product; dissolving the crude product in tetrahydrofuran and washing with deionized water; drying the organic phase with anhydrous magnesium sulfate and purifying it by column chromatography.

[0015] In this invention, the reaction mechanism for preparing polyethylene glycol trimethylsilane block-modified amines is based on the efficient addition reaction between isocyanates and amine compounds. The process begins with the primary amine group at the end of the methoxy polyethylene glycol amine molecule undergoing a nucleophilic attack on the highly electron-deficient isocyanate group in the trimethylsilyl isocyanate molecule, forming an unstable transition intermediate. This intermediate then undergoes electron rearrangement to generate a substituted urea structure. This reaction requires strict control of anhydrous conditions and reaction temperature because the isocyanate group is extremely sensitive to moisture, easily leading to side reactions that generate urea compounds and release carbon dioxide. During the reaction, triethylamine acts as an acid binder to neutralize any trace amounts of hydrogen chloride that may be generated, ensuring the reaction system remains within a suitable pH range. The resulting product has a unique block structure, with a hydrophilic polyethylene glycol segment at one end and a hydrophobic trimethylsilyl group at the other, connected by a thermally stable urea bond. This amphiphilic molecular structure enables it to spontaneously migrate to the interface between epoxy resin and water during the curing process. The polyethylene glycol segments temporarily stabilize the water molecules at the interface through hydrogen bonding, while the trimethylsilyl groups are oriented to form a molecular-level hydrophobic barrier, effectively preventing water from further penetrating to the reaction interface, thereby significantly improving underwater curing performance.

[0016] According to a preferred embodiment of the present invention, in step B1, the stirring reaction time is 10-12 hours.

[0017] According to a preferred embodiment of the present invention, in step B2, the number of times the deionized water is washed is 3-4 times.

[0018] The present invention also provides a method for preparing the dimethyl thiotoluene diamine modified epoxy resin underwater curing agent, wherein the dimethyl thiotoluene diamine modified epoxy resin underwater curing agent comprises the following raw materials in parts by weight: 40-60 parts by weight of dimethyl thiotoluene diamine; 15-25 parts by weight of dicyclohexenyl-bridged dimethyl thiotoluene diamine; 10-20 parts by weight of polyethylene glycol-trimethylsilane block-modified amine; 5-15 parts by weight of cashew phenol; 3-8 parts by weight of nano silica; and 1-5 parts by weight of γ-glycidoxypropyltrimethoxysilane.

[0019] The beneficial effects of this invention are as follows: The underwater epoxy resin curing agent of this invention exhibits superior technical effects through a unique molecular structure design and synergistic effects of its components. Firstly, regarding the underwater adaptability of the curing agent, this invention achieves effective suppression of moisture interference through the synergistic effect of two specifically modified compounds. The rigid alicyclic structure of dicyclohexenyl-bridged dimethylthiotoluene diamine provides an excellent hydrophobic barrier, while its multi-site reactivity ensures sufficient reaction with epoxy resin even in high humidity environments. The amphiphilic structure of polyethylene glycol trimethylsilane block-modified amine enables molecular orientation at the curing interface, where hydrophilic segments temporarily stabilize interfacial moisture, while hydrophobic end groups form an effective anti-permeation barrier. This unique molecular synergistic mechanism allows the curing agent to autonomously guide interfacial moisture behavior in an underwater environment, effectively preventing moisture interference with the curing reaction and ensuring that the epoxy resin still achieves a highly cross-linked, dense network structure underwater, achieving a degree of curing comparable to that in atmospheric environments.

[0020] In terms of overall performance, the curing agent of this invention exhibits multiple superior properties. Due to the introduction of the dicyclohexene-based bridging structure, the mechanical strength of the cured product is significantly improved. Its rigid skeleton enhances the cohesive energy density of the crosslinked network, resulting in higher hardness, compressive strength, and flexural modulus of the cured product. Simultaneously, the moderate flexibility of the polyethylene glycol segments and the interfacial reinforcement effect of the silane end groups allow the cured product to maintain high strength while possessing good toughness, effectively alleviating internal stress caused by temperature changes and external loads. The addition of nano-silica further enhances the thixotropic properties and mechanical strength of the system, ensuring no sagging occurs during underwater vertical and top surface construction. The introduction of cashew nut shell powder optimizes the curing reaction rate, giving the system a suitable working period and curing speed, ensuring sufficient construction time while rapidly building strength to meet the construction requirements of underwater engineering.

[0021] In terms of long-term durability, the curing agent of this invention exhibits excellent stability and protective performance. The dense cross-linked network formed after curing effectively blocks the penetration of moisture and corrosive media. The introduction of the silane component enhances the chemical bonding with various substrates, ensuring the stability of the bonding interface in underwater environments over a long period. Long-term water immersion and salt spray tests have verified that the epoxy coating system using this curing agent exhibits excellent water resistance and corrosion resistance, with a bond strength retention rate far exceeding that of traditional underwater curing agents. Furthermore, this curing agent system does not contain harmful volatile substances, meeting environmental protection requirements, and the good compatibility of its components ensures product stability during storage. These characteristics make the underwater curing agent of this invention particularly suitable for long-term use in harsh environments such as marine engineering, underwater pipeline repair, and hydraulic structure protection, providing a reliable technical solution for the durable protection of underwater structures. Detailed Implementation

[0022] The present application will be further described in detail below with reference to specific embodiments. It should be noted that the following specific embodiments are only used to further illustrate the present application and should not be construed as limiting the scope of protection of the present application. Those skilled in the art can make some non-essential improvements and adjustments to the present application based on the above application content.

[0023] The following is information on domestic suppliers of key related equipment and materials: The dimethylthiotoluene diamine was purchased from Shandong Suitai Biotechnology Co., Ltd.

[0024] The cashew phenol was purchased from Boxing County Deyi Trading Co., Ltd.

[0025] The 5-norbornene-2-carboxaldehyde was purchased from Zhende Chemical Technology (Shanghai) Co., Ltd.

[0026] The methoxy polyethylene glycol amine was purchased from Chongqing Yusi Pharmaceutical Technology Co., Ltd.

[0027] The trimethylsilyl isocyanate was purchased from Hubei Xinhongli Chemical Co., Ltd.

[0028] Example 1 Preparation of dicyclohexenyl-bridged dimethylthiotoluene diamine: A mechanical stirrer, thermometer, and reflux condenser were installed on a 1000 mL three-necked flask, and nitrogen gas was introduced for protection. 120.0 g of dimethylthiotoluene diamine and 500 mL of anhydrous ethanol were added to the reaction flask, and stirring was started to completely dissolve the raw materials. Then, 3.0 g of glacial acetic acid was added as a catalyst, and the temperature was slowly raised to 72 °C and maintained at a constant temperature. 100.0 g of 5-norbornen-2-carboxaldehyde was slowly added dropwise through a constant-pressure dropping funnel, controlling the dropping rate to maintain the reaction temperature at 72 ± 1 °C. After the addition was complete, the reaction was continued at this temperature for 5 h. The reaction system was cooled to 40 °C, and 18.0 g of sodium borohydride was added in batches, with each addition controlled at 2.0 g. The next batch was added only after the bubbles had completely disappeared. After the addition was complete, the temperature was raised back to 72 °C and the reaction continued for 3 h. After the reaction was complete, the system was cooled to room temperature, and insoluble matter was removed by filtration. The filtrate was concentrated to 1 / 3 of its original volume using a rotary evaporator at a water bath temperature of 60°C. The concentrate was transferred to a crystallizing dish, and a mixed solvent of n-hexane and ethyl acetate (volume ratio 3:1) was added for recrystallization. This operation was repeated three times to finally obtain pale yellow needle-like crystals.

[0029] Preparation of polyethylene glycol-trimethylsilane block-modified amine: A mechanical stirrer, thermometer, and constant-pressure dropping funnel were installed on a 500 mL four-necked flask. 100.0 g of methoxy polyethylene glycol amine and 300 mL of anhydrous toluene were added to the reaction flask, and stirring was started to dissolve the raw materials. 12.0 g of triethylamine was added as an acid binder, and the reaction system was cooled to 5 °C in an ice-water bath. 28.0 g of trimethylsilyl isocyanate was slowly added dropwise through the constant-pressure dropping funnel, controlling the dropping rate to keep the reaction temperature below 10 °C. After the addition was complete, the ice-water bath was removed, and the temperature was slowly raised to 70 °C and maintained at a constant temperature for 11 h. After the reaction was completed, the system was cooled to room temperature, and the triethylamine hydrochloride precipitate was removed by filtration. The filtrate was concentrated by rotary evaporation at a water bath temperature of 50 °C to remove the toluene solvent, yielding a viscous crude product. The crude product was dissolved in 200 mL of tetrahydrofuran, transferred to a separatory funnel, and washed three times with deionized water (100 mL each time). The organic phase was dried over anhydrous magnesium sulfate, filtered, and the filtrate was purified by silica gel column chromatography (eluting ethyl acetate / petroleum ether = 2:1, v / v) to finally obtain a colorless, transparent oily product.

[0030] Preparation of underwater curing agent for dimethyl thiotoluene diamine modified epoxy resin: In a 2000 mL stainless steel reactor, 400.0 g of dimethyl thiotoluene diamine was added, and stirring was started while the temperature was raised to 50 °C. Then, 150.0 g of dicyclohexenyl-bridged dimethyl thiotoluene diamine and 100.0 g of polyethylene glycol-trimethylsilane block-modified amine were added sequentially, and the temperature was raised to 52 °C. This temperature was maintained while stirring for 40 min until a homogeneous mixture was formed. Subsequently, 80.0 g of cashew nut shell powder and 50.0 g of nano-silica were added, and stirring continued for 30 min to ensure thorough mixing and dispersion of the components. The resulting product was cooled to 25 °C using a cooling water system to obtain 810.0 g of the basic curing agent mixture. Before use, 810.0 g of the basic curing agent mixture was mixed with 30.0 g of γ-glycidyl etheroxypropyltrimethoxysilane at 25 °C for 20 min by mechanical stirring to obtain the final curing agent product.

[0031] Example 2 Preparation of dicyclohexenyl-bridged dimethylthiotoluene diamine: A mechanical stirrer, thermometer, and reflux condenser were installed on a 1000 mL three-necked flask, and nitrogen gas was introduced for protection. 100.0 g of dimethylthiotoluene diamine and 400 mL of anhydrous ethanol were added to the reaction flask, and stirring was started to completely dissolve the raw materials. Then, 2.5 g of glacial acetic acid was added as a catalyst, and the temperature was slowly raised to 70 °C and maintained at a constant temperature. 80.0 g of 5-norbornene-2-carboxaldehyde was slowly added dropwise through a constant-pressure dropping funnel, controlling the dropping rate to maintain the reaction temperature at 70 ± 1 °C. After the addition was complete, the reaction was continued at this temperature for 4 h. The reaction system was cooled to 40 °C, and 15.0 g of sodium borohydride was added in batches, with each addition controlled at 1.5 g. The next batch was added only after the bubbles had completely disappeared. After the addition was complete, the temperature was raised back to 70 °C and the reaction continued for 2 h. After the reaction was complete, the system was cooled to room temperature, and insoluble matter was removed by filtration. The filtrate was concentrated to 1 / 3 of its original volume using a rotary evaporator at a water bath temperature of 60°C. The concentrate was transferred to a crystallizing dish, and a mixed solvent of n-hexane and ethyl acetate (volume ratio 3:1) was added for recrystallization. This operation was repeated twice to finally obtain pale yellow needle-like crystals.

[0032] Preparation of polyethylene glycol-trimethylsilane block-modified amine: A mechanical stirrer, thermometer, and constant-pressure dropping funnel were installed on a 500 mL four-necked flask. 80.0 g of methoxy polyethylene glycol amine and 250 mL of anhydrous toluene were added to the reaction flask, and stirring was started to dissolve the raw materials. 10.0 g of triethylamine was added as an acid binder, and the reaction system was cooled to 5 °C in an ice-water bath. 22.0 g of trimethylsilyl isocyanate was slowly added dropwise through the constant-pressure dropping funnel, controlling the dropping rate to keep the reaction temperature below 10 °C. After the addition was complete, the ice-water bath was removed, and the temperature was slowly raised to 68 °C and maintained at a constant temperature for 10 h. After the reaction was completed, the system was cooled to room temperature, and the triethylamine hydrochloride precipitate was removed by filtration. The filtrate was concentrated by rotary evaporation at a water bath temperature of 50 °C to remove the toluene solvent, yielding a viscous crude product. The crude product was dissolved in 150 mL of tetrahydrofuran, transferred to a separatory funnel, and washed three times with deionized water (80 mL each time). The organic phase was dried over anhydrous magnesium sulfate, filtered, and the filtrate was purified by silica gel column chromatography (eluting agent: ethyl acetate / petroleum ether = 2:1, v / v) to finally obtain a colorless and transparent oily product.

[0033] Preparation of underwater curing agent for dimethyl thiotoluene diamine modified epoxy resin: 450.0 g of dimethyl thiotoluene diamine was added to a 2000 mL stainless steel reactor. Stirring was started and the temperature was raised to 49 °C. 180.0 g of dicyclohexenyl-bridged dimethyl thiotoluene diamine and 120.0 g of polyethylene glycol-trimethylsilane block-modified amine were added sequentially, and the temperature was raised to 51 °C. Stirring was maintained at this temperature for 50 min until a homogeneous mixture was formed. Then, 100.0 g of cashew nut shell powder and 40.0 g of nano-silica were added, and stirring was continued for 25 min to ensure thorough mixing and dispersion of the components. The resulting product was cooled to 25 °C using a cooling water system to obtain 835.0 g of basic curing agent mixture. Before use, 835.0 g of the basic curing agent mixture was mixed with 25.0 g of γ-glycidyl etheroxypropyltrimethoxysilane at 25 °C for 20 min to obtain the final curing agent product.

[0034] Example 3 Preparation of dicyclohexenyl-bridged dimethylthiotoluene diamine: A mechanical stirrer, thermometer, and reflux condenser were installed on a 1000 mL three-necked flask, and nitrogen gas was introduced for protection. 140.0 g of dimethylthiotoluene diamine and 600 mL of anhydrous ethanol were added to the reaction flask, and stirring was started to completely dissolve the raw materials. Then, 3.5 g of glacial acetic acid was added as a catalyst, and the temperature was slowly raised to 75 °C and maintained at a constant temperature. 120.0 g of 5-norbornene-2-carboxaldehyde was slowly added dropwise through a constant-pressure dropping funnel, controlling the dropping rate to maintain the reaction temperature at 75 ± 1 °C. After the addition was complete, the reaction was continued at this temperature for 6 hours. The reaction system was cooled to 40 °C, and 20.0 g of sodium borohydride was added in batches, with each addition controlled at 2.0 g. The next batch was added only after the bubbles had completely disappeared. After the addition was complete, the temperature was raised back to 75 °C and the reaction continued for 4 hours. After the reaction was complete, the system was cooled to room temperature, and insoluble matter was removed by filtration. The filtrate was concentrated to 1 / 3 of its original volume using a rotary evaporator at a water bath temperature of 60°C. The concentrate was transferred to a crystallizing dish, and a mixed solvent of n-hexane and ethyl acetate (volume ratio 3:1) was added for recrystallization. This operation was repeated three times to finally obtain pale yellow needle-like crystals.

[0035] Preparation of polyethylene glycol-trimethylsilane block-modified amine: A mechanical stirrer, thermometer, and constant-pressure dropping funnel were installed on a 500 mL four-necked flask. 120.0 g of methoxy polyethylene glycol amine and 350 mL of anhydrous toluene were added to the reaction flask, and stirring was started to dissolve the raw materials. 15.0 g of triethylamine was added as an acid binder, and the reaction system was cooled to 5 °C in an ice-water bath. 33.0 g of trimethylsilyl isocyanate was slowly added dropwise through the constant-pressure dropping funnel, controlling the dropping rate to keep the reaction temperature below 10 °C. After the addition was complete, the ice-water bath was removed, and the temperature was slowly raised to 72 °C and maintained at a constant temperature for 12 h. After the reaction was completed, the system was cooled to room temperature, and the triethylamine hydrochloride precipitate was removed by filtration. The filtrate was concentrated by rotary evaporation at a water bath temperature of 50 °C to remove the toluene solvent, yielding a viscous crude product. The crude product was dissolved in 250 mL of tetrahydrofuran, transferred to a separatory funnel, and washed four times with deionized water (100 mL each time). The organic phase was dried over anhydrous magnesium sulfate, filtered, and the filtrate was purified by silica gel column chromatography (eluting ethyl acetate / petroleum ether = 2:1, v / v) to finally obtain a colorless, transparent oily product.

[0036] Preparation of underwater curing agent for dimethyl thiotoluene diamine modified epoxy resin: In a 2000 mL stainless steel reactor, 500.0 g of dimethyl thiotoluene diamine was added, and stirring was started while the temperature was raised to 52 °C. Then, 200.0 g of dicyclohexenyl-bridged dimethyl thiotoluene diamine and 150.0 g of polyethylene glycol-trimethylsilane block-modified amine were added sequentially, and the temperature was raised to 55 °C. This temperature was maintained while stirring for 60 min until a homogeneous mixture was formed. Subsequently, 120.0 g of cashew nut shell powder and 60.0 g of nano-silica were added, and stirring continued for 40 min to ensure thorough mixing and dispersion of the components. The resulting product was cooled to 25 °C using a cooling water system to obtain 930.0 g of the base curing agent mixture. Before use, 930.0 g of the base curing agent mixture was mixed with 40.0 g of γ-glycidyl etheroxypropyltrimethoxysilane at 25 °C and mechanically stirred for 20 min to obtain the final curing agent product.

[0037] Comparative Example 1 The preparation method is the same as in Example 1, except that the preparation of the dimethylthiotoluene diamine modified epoxy resin underwater curing agent is as follows: 600.0 g of dimethylthiotoluene diamine is added to a 2000 mL stainless steel reactor, stirring is started and the temperature is raised to 50°C. 150.0 g of polyethylene glycol-trimethylsilane block-modified amine is added, the temperature is raised to 52°C, and this temperature is maintained while stirring for 40 min until a homogeneous mixture is formed. Then, 80.0 g of cashew nut shell powder and 50.0 g of nano-silica are added, and stirring continues for 30 min to ensure thorough mixing and dispersion of the components. The resulting product is cooled to 25°C using a cooling water system to obtain 810.0 g of the basic curing agent mixture. Before use, 810.0 g of the basic curing agent mixture is mixed with 30.0 g of γ-glycidyl etheroxypropyltrimethoxysilane at 25°C and mechanically stirred for 20 min to obtain the final curing agent product.

[0038] Comparative Example 2 The preparation method is the same as in Example 1, except that the preparation of the dimethylthiotoluenediamine modified epoxy resin underwater curing agent is as follows: In a 2000mL stainless steel reactor, 600.0g of dimethylthiotoluenediamine is added, stirring is started, and the temperature is raised to 50℃. 150.0g of dicyclohexenyl-bridged dimethylthiotoluenediamine is added, and the temperature is raised to 52℃. This temperature is maintained while stirring for 40 minutes until a homogeneous mixture is formed. Then, 80.0g of cashew nut shell powder and 50.0g of nano-silica are added, and stirring continues for 30 minutes to ensure thorough mixing and dispersion of the components. The resulting product is cooled to 25℃ using a cooling water system to obtain 810.0g of the basic curing agent mixture. Before use, 810.0g of the basic curing agent mixture is mixed with 30.0g of γ-glycidyl etheroxypropyltrimethoxysilane at 25℃ for 20 minutes to obtain the final curing agent product.

[0039] Comparative Example 3 The preparation method is the same as in Example 1, except that the traditional underwater curing agent is prepared as follows: 700.0 g of dimethylthiotoluene diamine is added to a 2000 mL stainless steel reactor, stirring is started and the temperature is raised to 50°C. 100.0 g of cashew nut shell powder and 50.0 g of nano-silica are added, and stirring is continued for 30 min to ensure thorough mixing and dispersion of the components. The resulting product is cooled to 25°C using a cooling water system to obtain 820.0 g of the basic curing agent mixture. Before use, 820.0 g of the basic curing agent mixture is mixed with 30.0 g of γ-glycidyl etheroxypropyltrimethoxysilane at 25°C and mechanically stirred for 20 min to obtain the final curing agent product.

[0040] Performance testing and results analysis According to existing national and industry standards, the performance of the dimethylthiotoluene diamine-modified epoxy resin underwater curing agents prepared in Examples 1-3 and Comparative Examples 1-3 was tested using the following methods: Curing performance tests were conducted in a standard laboratory environment, with the ambient temperature controlled at 23±2℃ and the relative humidity controlled at 50±5%. The curing agents prepared in Examples 1-3 and Comparative Examples 1-3 were mixed with epoxy resin at a ratio of 1:1 (amine hydrogen equivalent), and stirred at 500 rpm for 3 minutes using a mechanical stirrer to ensure thorough mixing. Cured samples were prepared in a constant temperature and humidity chamber at 25℃ and 95% relative humidity, with sample dimensions of 100mm×100mm×3mm. For the underwater curing performance test, a standard concrete substrate was used. The substrate surface was sandblasted to achieve a roughness of 50μm. The mixed epoxy resin system was uniformly coated onto the substrate surface completely immersed in distilled water at a depth of 50mm. Surface drying time, total drying time, and underwater bond strength were measured after underwater curing for 1h, 3h, 6h, and 24h. Surface drying time was determined using the finger-touch method, and complete drying time was determined using the cotton ball pressing method. Underwater bond strength testing was performed using a universal tensile testing machine with a loading rate of 5 mm / min. Before testing, the specimens were removed from the water and surface moisture was absorbed using filter paper. Mechanical property testing involved preparing dumbbell-shaped tensile specimens, rectangular bending specimens, and cylindrical compression specimens according to standard methods. Testing was conducted using a computer-controlled electronic universal testing machine. Tensile strength testing used a gauge length of 50 mm and a loading rate of 2 mm / min; bending strength testing used a span of 60 mm and a loading rate of 5 mm / min; and compressive strength testing used specimens with a diameter of 10 mm and a height of 20 mm and a loading rate of 10 mm / min. For water resistance testing, fully cured specimens were weighed and completely immersed in distilled water at 25°C. After immersion for 7, 28, and 90 days, the specimens were removed, their surfaces were wiped dry with filter paper, and immediately weighed to determine the water absorption rate. Subsequently, mechanical property testing was performed to calculate the strength retention rate. Salt spray resistance tests were conducted in a precision salt spray chamber. A 5% sodium chloride solution was prepared, and the chamber temperature was maintained at 35℃. After continuous spraying for 240 hours, the samples were removed, rinsed with water, and the changes in coating adhesion and the corrosion width at the scratches were measured. Adhesion was tested using the pull-out method, and corrosion width was measured using a digital microscope. All tests were performed in five parallel trials, and the arithmetic mean of the results was taken.

[0041] Table 1: Performance test results of each embodiment and comparative example As shown in Table 1, the underwater curing agents prepared in Examples 1-3 effectively solved the three major technical problems of incomplete curing, insufficient adhesion, and poor durability caused by moisture interference in humid or underwater environments compared to Comparative Examples 1-3. The underwater surface drying time of Examples 1-3 was in the range of 2.3-2.8 h, and the underwater complete drying time was between 7.9-8.7 h, significantly shorter than the 4.2-5.6 h and 12.8-18.3 h of Comparative Examples 1-3. This indicates that the synergistic effect of the two modified compounds effectively promoted the curing reaction process of epoxy resin in the underwater environment, greatly reduced the interference of moisture on the curing reaction, and solved the problem of incomplete curing. The underwater bonding strength of Examples 1-3 reached 17.9-19.2 MPa, significantly higher than the 8.5-13.7 MPa of Comparative Examples 1-3. This demonstrates that the rigid hydrophobic barrier of the dicyclohexene-based bridging structure and the amphiphilic interfacial guiding effect of the polyethylene glycol trimethylsilane block jointly enhance the adhesion performance between the curing agent and the substrate, overcoming the defect of insufficient adhesion. Regarding durability, the 28-day water absorption rate of Examples 1-3 was only 1.05-1.23%, the strength retention rate reached 92.8-95.1%, the adhesion after salt spray testing was 16.2-17.3 MPa, and the corrosion width was only 0.3-0.5 mm. In contrast, the corresponding indicators of the comparative samples deteriorated significantly. This fully demonstrates that the dense cross-linked network formed by the modified curing system can effectively block the penetration of moisture and corrosive media, endowing the material with excellent long-term durability. The lack of a dicyclohexene-based bridging structure in Comparative Example 1 resulted in a significant decrease in mechanical strength; the lack of a polyethylene glycol trimethylsilane block in Comparative Example 2 resulted in insufficient underwater adhesion; and the use of a traditional formulation in Comparative Example 3 led to a significant deterioration in all properties. The results of these three comparative examples indirectly verified the indispensable synergistic effect of the two modified compounds in solving the above-mentioned technical problems.

Claims

1. A process for the preparation of dimethylthiophenyl diamine modified epoxy resin underwater curing agent, characterized by the steps of The method comprises the following steps: S1, adding dimethylthiophenyl diamine into a reaction container, heating to 48-52 DEG C, stirring; adding bicyclohexenyl bridged dimethylthiophenyl diamine and polyethylene glycol-trimethylsilane block modified amine in sequence, heating to 50-55 DEG C, continuously stirring, to obtain a mixture; S2, adding cardanol and nano silicon dioxide, stirring, to obtain a product; cooling the product to room temperature to obtain a basic curing agent mixture; and mixing gamma-glycidyl ether oxypropyl trimethoxysilane with the basic curing agent mixture before use.

2. The preparation method of the dimethylthiotoluene diamine modified epoxy resin underwater curing agent according to claim 1, characterized in that, In step S1, the time for continuously stirring is 30-60 min.

3. The preparation method of the dimethylthiotoluene diamine modified epoxy resin underwater curing agent according to claim 1, characterized in that, In step S2, the time for stirring is 20-40 min.

4. The preparation method of the dimethylthiotoluene diamine modified epoxy resin underwater curing agent according to claim 1, characterized in that, The preparation method of the bicyclohexenyl bridged dimethylthiophenyl diamine comprises the following steps: A1, dissolving dimethylthiophenyl diamine in anhydrous ethanol under nitrogen protection, stirring and adding glacial acetic acid; adding 5-norbornene-2-formaldehyde, stirring and reacting at 70-75 DEG C; then adding sodium borohydride, continuously stirring and reacting at 70-75 DEG C, to obtain a reaction mixture; A2, filtering the reaction mixture, and subjecting the filtrate to rotary evaporation to obtain a crude product; recrystallizing the crude product by using a mixed solvent of n-hexane and ethyl acetate.

5. The preparation method of the dimethylthiotoluene diamine modified epoxy resin underwater curing agent according to claim 4, characterized in that, In step A1, the time for stirring and reacting at 70-75 DEG C is 4-6 h.

6. The preparation method of the dimethylthiotoluene diamine modified epoxy resin underwater curing agent according to claim 4, characterized in that, In step A2, the number of recrystallization is 2-3 times.

7. The preparation method of the dimethylthiotoluene diamine modified epoxy resin underwater curing agent according to claim 1, characterized in that, The preparation method of the polyethylene glycol-trimethylsilane block modified amine comprises the following steps: B1, dissolving methoxy polyethylene glycol amine in toluene, adding triethylamine, and dropwise adding trimethylsilyl isocyanate under ice water bath cooling; heating to 68-72 DEG C, stirring and reacting, to obtain a reaction mixture; B2, filtering the reaction mixture, and subjecting the filtrate to rotary evaporation to obtain a crude product; dissolving the crude product in tetrahydrofuran, and washing with deionized water; drying the organic phase by using anhydrous magnesium sulfate, and purifying by column chromatography.

8. The method for preparing the dimethylthiotoluene diamine modified epoxy resin underwater curing agent according to claim 7, characterized in that, In step B1, the time for stirring and reacting is 10-12 h.

9. The preparation method of the dimethylthiotoluene diamine modified epoxy resin underwater curing agent according to claim 7, characterized in that, In step B2, the number of washing with deionized water is 3-4 times.

10. A dimethylthiophenyl diamine modified epoxy resin underwater curing agent produced by the method of any one of claims 1 to 9, characterized in that, The method comprises the following steps: adding dimethylthiophenyl diamine into a reaction container, heating to 48-52 DEG C, stirring; adding bicyclohexenyl bridged dimethylthiophenyl diamine and polyethylene glycol-trimethylsilane block modified amine in sequence, heating to 50-55 DEG C, continuously stirring, to obtain a mixture; adding cardanol and nano silicon dioxide, stirring, to obtain a product; cooling the product to room temperature to obtain a basic curing agent mixture; and mixing gamma-glycidyl ether oxypropyl trimethoxysilane with the basic curing agent mixture before use. In step S1, the time for continuously stirring is 30-60 min. In step S2, the time for stirring is 20-40 min. The preparation method of the bicyclohexenyl bridged dimethylthiophenyl diamine comprises the following steps: A1, dissolving dimethylthiophenyl diamine in anhydrous ethanol under nitrogen protection, stirring and adding glacial acetic acid; adding 5-norbornene-2-formaldehyde, stirring and reacting at 70-75 DEG C; then adding sodium borohydride, continuously stirring and reacting at 70-75 DEG C, to obtain a reaction mixture; A2, filtering the reaction mixture, and subjecting the filtrate to rotary evaporation to obtain a crude product; recrystallizing the crude product by using a mixed solvent of n-hexane and ethyl acetate. In step A1, the time for stirring and reacting at 70-75 DEG C is 4-6 h. In step A2, the number of recrystallization is 2-3 times. The preparation method of the polyethylene glycol-trimethylsilane block modified amine comprises the following steps: B1, dissolving methoxy polyethylene glycol amine in toluene, adding triethylamine, and dropwise adding trimethylsilyl isocyanate under ice water bath cooling; heating to 68-72 DEG C, stirring and reacting, to obtain a reaction mixture; B2, filtering the reaction mixture, and subjecting the filtrate to rotary evaporation to obtain a crude product; dissolving the crude product in tetrahydrofuran, and washing with deionized water; drying the organic phase by using anhydrous magnesium sulfate, and purifying by column chromatography. In step B1, the time for stirring and reacting is 10-12 h. In step B2, the number of washing with deionized water is 3-4 times. The method comprises the following steps: adding dimethylthiophenyl diamine into a reaction container, heating to 48-52 DEG C, stirring; adding bicyclohexenyl bridged dimethylthiophenyl diamine and polyethylene glycol-trimethylsilane block modified amine in sequence, heating to 50-55 DEG C, continuously stirring, to obtain a mixture; adding cardanol and nano silicon dioxide, stirring, to obtain a product; cooling the product to room temperature to obtain a basic curing agent mixture; and mixing gamma-glycidyl ether oxypropyl trimethoxysilane with the basic curing agent mixture before use.

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