Ultralow-impurity mercaptobenzothiazole rubber accelerator and preparation method thereof

Abstract

The invention discloses an ultralow-impurity mercaptobenzothiazole rubber accelerator and a preparation method thereof in the technical field of rubber assistants, and the method comprises the following steps: taking o-chloronitrobenzene, carbon disulfide and sulfur as raw materials, and carrying out cyclization reaction at high temperature in the presence of a specific catalyst to generate mercaptobenzothiazole sodium salt; the method is characterized in that two newly designed modified compounds are added step by step in the reaction process, firstly, a benzothiazole derivative which is used as a crystallization regulating agent and contains thioether bonds and carboxyl is added, and then, a symmetric bis-benzothiazolyl urea derivative which is used as an impurity trapping agent is added; and after acidification and crystallization, purifying through solvent recrystallization and activated carbon decoloration. The two modified compounds effectively control the crystallization process and selectively remove impurities through intermolecular interaction, and finally a mercaptobenzothiazole product with extremely high purity is prepared. The accelerator is extremely low in impurity content, shows excellent scorching resistance and vulcanization efficiency in the rubber vulcanization process, and is suitable for production of high-performance rubber products.

Description

Technical Field

[0001] This invention relates to the field of rubber additives technology, specifically to an ultra-low impurity thiol-based benzothiazole rubber accelerator and its preparation method. Background Technology

[0002] Thiol-based benzothiazoles are among the most widely used accelerators in the rubber industry, playing an indispensable role in the processing of both natural and synthetic rubber. This compound can be used alone as an accelerator and is also an important precursor in the synthesis of various sulfenamide accelerators. Traditional thiol-based benzothiazole production processes primarily employ a high-temperature, high-pressure synthesis route, using aniline compounds, carbon disulfide, and sulfur as raw materials, undergoing cyclization condensation at reaction temperatures exceeding 200 degrees Celsius and high pressures. While this traditional process is technically mature, it suffers from several insurmountable technical drawbacks: harsh reaction conditions lead to high energy consumption, strict equipment requirements pose safety hazards, and, more importantly, various side reactions easily occur under high temperature and pressure, generating heterocyclic byproducts and decomposition products with similar structures. The presence of these impurities not only reduces the purity of the final product but also directly affects its performance during rubber vulcanization, leading to problems such as shortened scorch time and unsatisfactory vulcanization curves, ultimately impacting the final properties of the rubber products. With the increasing demand for high-quality products in the rubber industry, higher requirements are also being placed on the purity of rubber additives. Thiol-based benzothiazoles produced by traditional processes can no longer meet the production needs of high-end rubber products.

[0003] To overcome the insufficient purity of products produced by traditional processes, the industry has explored various technologies. The most common method is subsequent purification of the crude product, including recrystallization with organic solvents, acid washing or water washing to remove water-soluble impurities, and decolorization through activated carbon adsorption. While these methods can improve the appearance and purity of the product to some extent, they have significant limitations: solvent recrystallization, although effective in increasing purity, generates large amounts of organic waste, increasing environmental pressure and production costs; water washing and acid washing can only remove water-soluble impurities, with limited effectiveness against organic impurities with structures similar to thiol-benzothiazole and similar polarities; activated carbon adsorption is mainly effective against pigment impurities and is ineffective in separating heterocyclic compounds with similar molecular structures. Furthermore, these post-treatment methods are passive purification strategies, failing to control impurity formation at the reaction source, thus creating a bottleneck in purification efficiency. A more ideal technical approach would be to address the reaction mechanism by regulating the reaction process to suppress byproduct generation, or by introducing functional components that can specifically identify and remove impurities, thereby fundamentally improving product purity.

[0004] To address the shortcomings of existing technologies, this invention aims to provide a novel solution by innovatively introducing two specially designed modified compounds to fundamentally improve the synthesis process of thiol-based benzothiazoles. These two modified compounds function at different stages: one primarily intervenes in the sodium salt stage after the cyclization reaction, selectively interacting with the main product through specific functional groups in its molecular structure, guiding the product to form a more regular crystal structure and reducing impurity inclusion in the crystal lattice; the other can form stable complexes with specific impurity molecules present in the reaction system, thereby achieving efficient impurity removal during subsequent separation. This strategy of actively controlling the reaction process and product separation has significant advantages over traditional passive purification methods. It not only significantly reduces the content of various impurities in the product but also avoids the cost and environmental problems caused by the large-scale use of solvents. Simultaneously, it improves the overall efficiency of the production process and the stability of product quality, providing a reliable technical approach for producing high-quality thiol-based benzothiazole rubber accelerators with ultra-low impurity content. Summary of the Invention

[0005] The purpose of this invention is to provide an ultra-low impurity thiol-based benzothiazole rubber accelerator and its preparation method, which solves the technical problems in the existing thiol-based benzothiazole preparation process, such as high product impurity content caused by high temperature and high pressure reaction, limited post-treatment purification effect, and safety and environmental risks in the production process.

[0006] The present invention achieves the above objectives through the following technical solutions: A method for preparing an ultra-low impurity thiol-based benzothiazole rubber accelerator, comprising the following steps: S1. In a four-necked flask, add o-chloronitrobenzene and N-methylpyrrolidone, stir, and then add carbon disulfide, sulfur powder, sodium carbonate and copper sulfate in sequence; under nitrogen protection, heat to 180-185℃ and stir the reaction; after the reaction is completed, cool to 78-82℃ to obtain a solution of N-methylpyrrolidone in sodium thiol benzothiazolium salt; add 4-(2-benzothiazolylthio)benzoic acid and stir at 78-82℃. S2. Add N,N'-bis(2-benzothiazolyl)urea and continue stirring at 78-82℃ to obtain a reaction mixture. Cool the reaction mixture to 35-40℃, add concentrated hydrochloric acid while stirring, adjust the pH to 2-3, and continue stirring. Filter to obtain a solid, wash the solid with cold water to obtain crude thiol-based benzothiazolium product. Redisperse the crude thiol-based benzothiazolium product in N-methylpyrrolidone, add activated carbon, heat to 88-92℃ and stir to decolorize. Hot filter, cool the filtrate to room temperature, and then further cool to 3-5℃ for crystallization. Filter to collect the crystals, wash with ethanol, and dry in a vacuum drying oven at 58-62℃.

[0007] In this invention, the preparation mechanism of the ultra-low impurity thiol-based benzothiazole rubber accelerator includes the cyclization synthesis of the main product and the synergistic purification effect of two modified compounds. Under nitrogen protection and high temperature, using N-methylpyrrolidone as a solvent, o-chloronitrobenzene, carbon disulfide, and sulfur powder undergo a complex cyclization condensation reaction catalyzed by sodium carbonate and copper sulfate to generate the sodium salt of thiol-based benzothiazole. Compared with traditional high-temperature and high-pressure processes, this method uses a relatively mild temperature, aiming to reduce byproducts generated from excessive reaction or decomposition at the source. After the reaction solution is cooled, the prepared 4-(2-benzothiazole-thio)benzoic acid is added first. The benzothiazole ring in its molecule can interact with the main product molecule through π-π stacking, while the carboxyl group can coordinate with metal ions or polar impurities in the reaction system, playing a preliminary role in impurity complexation and crystallization guidance. The subsequently added N,N'-bis(2-benzothiazolyl)urea, with its two benzothiazolyl rings in its symmetrical structure, can effectively capture heterocyclic byproducts with structures similar to the main product. Its urea groups can also form hydrogen bonds with various polar impurities, thereby aggregating these impurities and forming complexes that are easily separated later. This stepwise addition strategy of modifying the compound achieves targeted removal of impurities and optimizes the crystallization process. After cooling, the reaction mixture is acidified to a specific pH with concentrated hydrochloric acid, causing the thiol-benzothiazolyl sodium salt to convert to a free acid form and crystallize out, yielding the crude product. The crude product is further recrystallized in N-methylpyrrolidone, and activated carbon is added for decolorization under heating conditions. Activated carbon effectively adsorbs trace amounts of colored impurities and small-molecule organic impurities. After hot filtration, the filtrate is subjected to programmed cooling crystallization to obtain crystals with uniform particle size and higher purity. Finally, it is washed with ethanol and vacuum dried to obtain the final product with ultra-low impurities. The entire process significantly reduces the content of various organic impurities in the product through the synergistic effect of multiple purification mechanisms, including molecular recognition and impurity capture of modified compounds, programmed crystallization, and activated carbon adsorption decolorization.

[0008] According to a preferred embodiment of the present invention, in step S1, the mass ratio of o-chloronitrobenzene, carbon disulfide, sulfur powder, sodium carbonate and copper sulfate is 100:(80-120):(30-50):(5-15):(1-5).

[0009] According to a preferred embodiment of the present invention, in step S2, the heating to 88-92°C and the stirring decolorization time is 1-2 hours.

[0010] According to a preferred embodiment of the present invention, the preparation method of 4-(2-benzothiazolylthio)benzoic acid includes: A1, adding 2-mercaptobenzothiazole and 4-chlorobenzoic acid to a three-necked round-bottom flask, then adding N,N-dimethylformamide, followed by sodium hydroxide, and heating to 118-122°C under nitrogen protection, stirring the reaction mixture to obtain a reaction mixture; A2, cooling the reaction mixture to room temperature and filtering; pouring the filtrate into ice water, filtering to collect the solid, washing the solid with deionized water to obtain a crude product, dissolving the crude product in an aqueous sodium hydroxide solution, filtering, acidifying the filtrate with concentrated hydrochloric acid to pH=2-2.2, filtering to collect crystals, washing the crystals with deionized water, and drying in a vacuum drying oven at 58-62°C.

[0011] In this invention, the reaction mechanism for the preparation of 4-(2-benzothiazolylthio)benzoic acid mainly involves a nucleophilic substitution reaction and subsequent acid-base conversion purification process. The reaction uses dimethylformamide as a solvent. In a strongly alkaline environment provided by sodium hydroxide, the thiol group in the 2-mercaptobenzothiazole molecule loses a proton to form a sulfide anion with strong nucleophilicity. This sulfide anion then attacks the carbon atom attached to the chlorine atom on the benzene ring of the 4-chlorobenzoic acid molecule, whose positive charge is enhanced by the electron-withdrawing effect of the carboxyl group, resulting in a nucleophilic substitution reaction. The chlorine atom detaches as a leaving group, forming a new carbon-sulfur bond and generating an intermediate product containing a thioether bond, a benzothiazole ring, and a benzoic acid group. The reaction is carried out under nitrogen protection and continuous heating to promote complete reaction and prevent oxidation of the raw materials or products at high temperatures. After cooling, the reaction mixture is precipitated in ice water to obtain the crude product; this step utilizes the difference in solubility to achieve preliminary separation. The crude product is then dissolved in an aqueous sodium hydroxide solution. Its carboxyl groups react with the sodium hydroxide to form a water-soluble sodium salt, while unreacted raw materials or neutral byproducts are removed by filtration. The filtrate is acidified to a strongly acidic environment with concentrated hydrochloric acid, causing the sodium carboxylate to reprotonate and convert into a water-insoluble free carboxylic acid, which precipitates as crystals. After filtration, washing, and vacuum drying, a high-purity target compound is obtained. This process effectively removes any unreacted raw materials and byproducts that may remain from the reaction through acid-base purification, ensuring the purity of the final product.

[0012] According to a preferred embodiment of the present invention, in step A1, the stirring reaction time is 12-14 hours.

[0013] According to a preferred embodiment of the present invention, in step A2, the drying time in a vacuum drying oven at 58-62°C is 24-30 hours.

[0014] According to a preferred embodiment of the present invention, the preparation method of N,N'-bis(2-benzothiazolyl)urea includes: B1, adding 2-aminobenzothiazolium and anhydrous toluene to a three-necked round-bottom flask, stirring, adding a toluene solution of triphosgene dropwise under ice-water bath cooling, cooling to room temperature after the addition is complete, and then heating to 78-82°C for reflux reaction to obtain a reaction mixture, cooling the reaction mixture to room temperature, filtering to collect the solid, washing the solid with anhydrous ethanol to obtain a crude product; B2, suspending the crude product in deionized water, stirring and filtering to obtain a solid, transferring the solid to a Soxhlet extractor, extracting with n-hexane to obtain a product, and drying the product in a vacuum drying oven at 68-72°C.

[0015] In this invention, the core reaction mechanism for the preparation of N,N'-bis(2-benzothiazolyl)urea involves the reaction of 2-aminobenzothiazolium with triphosgene to form an isocyanate intermediate, which subsequently reacts with another amine molecule to form an asymmetric urea structure. Under anhydrous toluene solvent and ice-water bath cooling conditions, the amino group of 2-aminobenzothiazolium acts as a nucleophile, attacking the carbonyl carbon in the triphosgene molecule. Triphosgene, as a safe substitute for phosgene, releases its phosgene equivalent under the reaction conditions, reacting with the amine to form the isocyanate. This low-temperature condition helps control the reaction rate and reduce side reactions. The reaction mixture is restored to room temperature and heated under reflux to promote the formation of the isocyanate intermediate. Subsequently, the other molecule of 2-aminobenzothiazolium in the system acts as a nucleophile, its amino group attacking the carbonyl carbon with a lower electron cloud density on the isocyanate intermediate, undergoing a nucleophilic addition reaction, ultimately forming N,N'-bis(2-benzothiazolyl)urea. After the reaction is complete, the product precipitates from the reaction solution during cooling and is filtered to obtain the crude product. The crude product was first washed with anhydrous ethanol to remove some organic impurities, then suspended and stirred in water to remove water-soluble impurities, and finally continuously extracted with n-hexane in a Soxhlet extractor to remove nonpolar impurities. The purified target urea compound was then obtained by vacuum drying. This multi-step purification strategy aims to ensure the effective removal of symmetrical ureas, unreacted amines, and other byproducts that may be generated during the reaction, thereby guaranteeing high product purity.

[0016] According to a preferred embodiment of the present invention, in step B1, the reflux reaction time at 78-82°C is 6-8 hours.

[0017] According to a preferred embodiment of the present invention, in step B2, the drying time in a vacuum drying oven at 68-72°C is 48-50 hours.

[0018] The present invention also provides an ultra-low impurity thiol-based benzothiazole rubber accelerator, which is prepared according to the preparation method of the ultra-low impurity thiol-based benzothiazole rubber accelerator.

[0019] The beneficial effects of this invention are as follows: The present invention provides a method for preparing an ultra-low impurity thiol-based benzothiazole rubber accelerator, achieving significant technological advancements in product quality. By introducing two newly designed modified compounds and optimizing the preparation process, the purity of the final product is fundamentally improved. The key innovation lies in the two functional compounds added in stages: one is a benzothiazole derivative containing thioether bonds and carboxyl groups, and the other is a symmetrically structured bisbenzothiazole urea derivative. The former acts as a highly efficient crystallization regulator in the reaction system, guiding the target product to form a more regular crystal structure through intermolecular interactions, effectively reducing impurity inclusion during crystallization. The latter acts as a selective impurity scavenger; its unique molecular structure allows it to form stable complexes with byproducts and unreacted intermediates generated during the reaction, thereby effectively removing these impurities in subsequent separation steps. The synergistic effect of these two compounds significantly reduces the organic impurity content in the final thiol-based benzothiazole product, resulting in a purer product color. When used as a rubber accelerator, the presence of impurities will not affect the color quality of rubber products, and impurities will also avoid interference with the vulcanization process.

[0020] In terms of process optimization, this invention achieves a dual improvement in production efficiency and environmental friendliness through systematic improvements to reaction conditions and post-processing procedures. Temperature control during the cyclization reaction stage is meticulously optimized, ensuring the reaction proceeds fully while effectively suppressing decomposition side reactions caused by high temperatures, resulting in more complete raw material conversion and reducing impurity generation at the source. Precise control of pH during acidification and crystallization ensures the product precipitates in the most suitable form, minimizing residual product loss in the mother liquor and improving product yield. The purification stage employs recrystallization with a specific solvent combined with activated carbon treatment. This combined purification strategy not only further removes trace residual impurities but also significantly improves the physical morphology of the product, giving it better flowability and dispersibility. The entire process flow is rationally designed with close integration of each step, greatly reducing production fluctuations common in traditional processes, ensuring batch-to-batch product quality stability, and also reducing solvent consumption and waste generation, meeting the green and environmentally friendly requirements of modern chemical production.

[0021] The technological achievements of this invention have demonstrated outstanding performance in practical applications in the rubber industry. The ultra-low impurity thiol-based benzothiazole accelerator prepared by this method exhibits excellent comprehensive performance during rubber vulcanization. Due to its extremely low impurity content, the accelerator is more uniformly dispersed in the rubber compound, avoiding uneven vulcanization caused by local differences in reactivity. Compared with traditional products, rubber compounds using this accelerator show a longer scorch time, providing a wider processing safety window and making the mixing and molding processes easier to control. During the vulcanization stage, the accelerator provides a moderate vulcanization rate, promoting the formation of cross-linked networks, resulting in significant improvements in the physical and mechanical properties of the final rubber products, such as tensile strength, tear strength, and abrasion resistance. Furthermore, the low impurity characteristic reduces aging caused by impurities during storage and use, extending the product's service life. These excellent performance characteristics make this product particularly suitable for high-end rubber products with high purity requirements, such as medical rubber, food-grade rubber products, and high-performance tires, providing a reliable raw material guarantee for improving the quality of rubber products. Detailed Implementation

[0022] The following detailed embodiments are only used to further illustrate this application and should not be construed as limiting the scope of protection of this application. Those skilled in the art can make some non-essential improvements and adjustments to this application based on the above application content.

[0023] The following is information on domestic suppliers of key related equipment and materials: The o-chloronitrobenzene was purchased from Suzhou Rosen Additives Co., Ltd.

[0024] The carbon disulfide was purchased from Shandong Hongyang Chemical Co., Ltd.

[0025] The sulfur powder was purchased from Jiangyin Shuanghua Chemical Co., Ltd.

[0026] The sodium carbonate was purchased from Shandong Haihua Group Co., Ltd.

[0027] The copper sulfate was purchased from Gansu Banier Technology Co., Ltd.

[0028] The 2-mercaptobenzothiazole was purchased from Tesco Chemical (Hubei) Co., Ltd.

[0029] The 4-chlorobenzoic acid was purchased from Jinan Lianhua Hengye Economic and Trade Co., Ltd.

[0030] The N,N-dimethylformamide was purchased from Tianjin Ruimingwei Chemical Co., Ltd.

[0031] The sodium hydroxide was purchased from Henan Livit Chemical Technology Co., Ltd.

[0032] The 2-aminobenzothiazole was purchased from Hubei Jianchu Biomedical Co., Ltd.

[0033] The triphosgene was purchased from Jinan Huifengda Chemical Co., Ltd.

[0034] Example 1 In a 2000 mL four-necked flask, 100.0 g of o-chloronitrobenzene and 300.0 g of N-methylpyrrolidone were added and stirred at 300 rpm to dissolve. Then, 100.0 g of carbon disulfide, 40.0 g of sulfur powder, 10.0 g of sodium carbonate, and 3.0 g of copper sulfate were added sequentially. Under nitrogen protection, the temperature was increased to 182 °C at 2 °C / min, and the mixture was stirred for 6 h at a reaction pressure of 0.5 MPa. After the reaction was completed, the mixture was cooled to 80 °C at 1 °C / min to obtain a solution of N-methylpyrrolidone in the form of thiol-based benzothiazolium sodium salt. 1.5 g of pre-prepared 4-(2-benzothiazolylthio)benzoic acid was added, and the mixture was stirred at 200 rpm for 2 h at 80 °C. 1.0 g of pre-prepared N,N'-bis(2-benzothiazolyl)urea was added, and the mixture was stirred at 200 rpm for 1 h at 80 °C to obtain the reaction mixture. The reaction mixture was then cooled at 0.5 °C / min. The solution was cooled to 38°C, and 70.0 g of concentrated hydrochloric acid (35% by mass) was slowly added while stirring at 500 rpm to adjust the pH to 2.5. Stirring was continued for 1 h. The solid was obtained by filtration through a Buchner funnel and washed three times with 500 mL of cold water to obtain crude mercaptan-based benzothiazole product. The crude mercaptan-based benzothiazole product was redispersed in 500.0 g of N-methylpyrrolidone, and 5.0 g of activated carbon was added. The mixture was heated to 90°C and decolorized by stirring at 400 rpm for 1.5 h. The mixture was then filtered using a hot filtration device, and the filtrate was cooled to 25°C at 0.3°C / min, and then further cooled to 4°C at 0.5°C / min to crystallize for 3 h. The crystals were collected by filtration through a Buchner funnel, washed twice with 200 mL of ethanol, and dried in a vacuum drying oven at 60°C for 24 h at a vacuum degree of -0.08 MPa to obtain ultra-low impurity mercaptan-based benzothiazole rubber accelerator. The preparation method of 4-(2-benzothiazolylthio)benzoic acid includes: adding 150.0 g of 2-mercaptobenzothiazolium and 140.6 g of 4-chlorobenzoic acid to a 1000 mL three-necked round-bottom flask, then adding 800.0 g of N,N-dimethylformamide, followed by 40.0 g of sodium hydroxide. The mixture is heated to 120 °C under nitrogen protection and stirred at 250 rpm for 13 h to obtain the reaction mixture. The reaction mixture is then cooled to 25 °C at a rate of 1 °C / min. Filter using a Buchner funnel; slowly pour the filtrate into 4000g of ice water, filter and collect the solid, wash the solid three times with 1000mL of deionized water to obtain the crude product, dissolve the crude product in 800mL of 5% sodium hydroxide aqueous solution, filter, acidify the filtrate with 36% concentrated hydrochloric acid to pH=2.1, filter and collect the crystals, wash the crystals three times with 500mL of deionized water, and dry in a vacuum drying oven at 60℃ for 26h at a vacuum degree of -0.08MPa.The preparation method of N,N'-bis(2-benzothiazolyl)urea includes: adding 150.0 g of 2-aminobenzothiazolium and 750.0 g of anhydrous toluene to a 1000 mL three-necked round-bottom flask, stirring at 200 rpm to dissolve, adding 300.0 g of a toluene solution containing 59.3 g of triphosgene dropwise at 1 mL / min under ice-water bath cooling, removing the ice bath after the addition is complete, allowing it to cool naturally to 25 °C, and then heating to 80 °C and refluxing for 7 h to obtain the reaction mixture. The mixture was cooled to 25°C at a rate of 1°C / min, and the solid was collected by filtration. The solid was washed three times with 300 mL of anhydrous ethanol to obtain a crude product. The crude product was suspended in 2000 mL of deionized water, stirred at 300 rpm for 1 h, and then filtered. This operation was repeated three times to obtain a solid. The solid was transferred to a Soxhlet extractor and extracted with 500 mL of n-hexane for 24 h to obtain a product. The product was dried in a vacuum drying oven at 70°C for 49 h at a vacuum degree of -0.08 MPa.

[0035] Example 2 The preparation method is the same as in Example 1, except that 100.0 g of o-chloronitrobenzene and 250.0 g of N-methylpyrrolidone were added to a 2000 mL four-necked flask and stirred at 300 rpm to dissolve. Then, 90.0 g of carbon disulfide, 35.0 g of sulfur powder, 8.0 g of sodium carbonate, and 2.0 g of copper sulfate were added sequentially. The mixture was heated to 180 °C at 2 °C / min and stirred for 8 h under nitrogen protection at a reaction pressure of 0.4 MPa. After the reaction was completed, the mixture was cooled to 78 °C at 1 °C / min to obtain a solution of N-methylpyrrolidone in sodium thiol benzothiazolium salt. 1.0 g of 4-(2-benzothiazolylthio)benzoic acid prepared in advance was added and stirred at 200 rpm at 78 °C for 3 h. 0.5 g of N,N'-bis(2-benzothiazolyl)urea prepared in advance was added and stirred at 200 rpm at 78 °C for 1.5 h to obtain the reaction mixture. The compound was cooled to 35°C at 0.5°C / min, and 60.0 g of concentrated hydrochloric acid (35% by mass) was slowly added under stirring at 500 rpm to adjust the pH to 2.0. Stirring was continued for 0.5 h. The solid was obtained by filtration through a Buchner funnel and washed three times with 500 mL of cold water to obtain crude mercaptan-based benzothiazole product. The crude mercaptan-based benzothiazole product was redispersed in 400.0 g of N-methylpyrrolidone, and 3.0 g of activated carbon was added. The mixture was heated to 88°C and decolorized by stirring at 400 rpm for 2 h. The mixture was filtered using a hot filtration device, and the filtrate was cooled to 25°C at 0.3°C / min, and then further cooled to 3°C at 0.5°C / min to crystallize for 4 h. The crystals were collected by filtration through a Buchner funnel, washed twice with 200 mL of ethanol, and dried in a vacuum drying oven at 58°C for 30 h at a vacuum degree of -0.08 MPa to obtain ultra-low impurity mercaptan-based benzothiazole rubber accelerator. The preparation method of 4-(2-benzothiazolylthio)benzoic acid includes: adding 150.0 g of 2-mercaptobenzothiazolium and 140.6 g of 4-chlorobenzoic acid to a 1000 mL three-necked round-bottom flask, then adding 900.0 g of N,N-dimethylformamide, followed by 50.0 g of sodium hydroxide. The mixture is heated to 118 °C under nitrogen protection and stirred at 250 rpm for 14 h to obtain the reaction mixture. The reaction mixture is then cooled to 25 °C at a rate of 1 °C / min. Filter using a Buchner funnel; slowly pour the filtrate into 4000g of ice water, filter and collect the solid, wash the solid three times with 1000mL of deionized water to obtain the crude product, dissolve the crude product in 800mL of 5% sodium hydroxide aqueous solution, filter, acidify the filtrate with 36% concentrated hydrochloric acid to pH=2.0, filter and collect the crystals, wash the crystals three times with 500mL of deionized water, and dry in a vacuum drying oven at 58℃ for 30h at a vacuum degree of -0.08MPa.The preparation method of N,N'-bis(2-benzothiazolyl)urea includes: adding 150.0 g of 2-aminobenzothiazolium and 900.0 g of anhydrous toluene to a 1000 mL three-necked round-bottom flask, stirring at 200 rpm to dissolve, and adding 250.0 g of a toluene solution containing 49.4 g of triphosgene dropwise at 1 mL / min under ice-water bath cooling. After the addition is complete, the ice bath is removed, and the mixture is naturally cooled to 25 °C, then heated to 78 °C and refluxed for 8 h to obtain the reaction mixture. The mixture was cooled to 25°C at a rate of 1°C / min, and the solid was collected by filtration. The solid was washed three times with 300 mL of anhydrous ethanol to obtain a crude product. The crude product was suspended in 2000 mL of deionized water, stirred at 300 rpm for 1 h, and then filtered. This operation was repeated three times to obtain a solid. The solid was transferred to a Soxhlet extractor and extracted with 500 mL of n-hexane for 24 h to obtain a product. The product was dried in a vacuum drying oven at 68°C for 50 h at a vacuum degree of -0.08 MPa.

[0036] Example 3 The preparation method is the same as in Example 1, except that 100.0 g of o-chloronitrobenzene and 350.0 g of N-methylpyrrolidone were added to a 2000 mL four-necked flask and stirred at 300 rpm to dissolve. Then, 110.0 g of carbon disulfide, 45.0 g of sulfur powder, 12.0 g of sodium carbonate, and 4.0 g of copper sulfate were added sequentially. The mixture was heated to 185 °C at 2 °C / min and stirred for 4 h under nitrogen protection at a reaction pressure of 0.6 MPa. After the reaction was completed, the mixture was cooled to 82 °C at 1 °C / min to obtain a solution of N-methylpyrrolidone in sodium thiol benzothiazolium salt. 2.0 g of 4-(2-benzothiazolylthio)benzoic acid prepared in advance was added and stirred at 200 rpm at 82 °C for 1 h. 1.5 g of N,N'-bis(2-benzothiazolyl)urea prepared in advance was added and stirred at 200 rpm at 82 °C for another 0.5 h to obtain the reaction mixture. The mixture was cooled to 40°C at 0.5°C / min, and 80.0 g of concentrated hydrochloric acid (35% by mass) was slowly added under stirring at 500 rpm to adjust the pH to 3.0. Stirring was continued for 1.5 h. The solid was obtained by filtration through a Buchner funnel and washed three times with 500 mL of cold water to obtain crude mercaptan-based benzothiazole product. The crude mercaptan-based benzothiazole product was redispersed in 600.0 g of N-methylpyrrolidone, and 7.0 g of activated carbon was added. The mixture was heated to 92°C and decolorized by stirring at 400 rpm for 1 h. The mixture was filtered using a hot filtration device, and the filtrate was cooled to 25°C at 0.3°C / min, and then further cooled to 5°C at 0.5°C / min to crystallize for 2 h. The crystals were collected by filtration through a Buchner funnel, washed twice with 200 mL of ethanol, and dried in a vacuum drying oven at 62°C for 24 h at a vacuum degree of -0.08 MPa to obtain ultra-low impurity mercaptan-based benzothiazole rubber accelerator. The preparation method of 4-(2-benzothiazolylthio)benzoic acid includes: adding 150.0 g of 2-mercaptobenzothiazolium and 140.6 g of 4-chlorobenzoic acid to a 1000 mL three-necked round-bottom flask, then adding 700.0 g of N,N-dimethylformamide, followed by 30.0 g of sodium hydroxide. The mixture is heated to 122 °C under nitrogen protection and stirred at 250 rpm for 12 h to obtain the reaction mixture. The reaction mixture is then cooled to 25 °C at a rate of 1 °C / min. Filter using a Buchner funnel; slowly pour the filtrate into 4000g of ice water, filter and collect the solid, wash the solid three times with 1000mL of deionized water to obtain the crude product, dissolve the crude product in 800mL of 5% sodium hydroxide aqueous solution, filter, acidify the filtrate with 36% concentrated hydrochloric acid to pH=2.2, filter and collect the crystals, wash the crystals three times with 500mL of deionized water, and dry in a vacuum drying oven at 62℃ for 24h at a vacuum degree of -0.08MPa.The preparation method of N,N'-bis(2-benzothiazolyl)urea includes: adding 150.0 g of 2-aminobenzothiazolium and 600.0 g of anhydrous toluene to a 1000 mL three-necked round-bottom flask, stirring at 200 rpm to dissolve, and adding 350.0 g of a toluene solution containing 69.1 g of triphosgene dropwise at 1 mL / min under ice-water bath cooling. After the addition is complete, the ice bath is removed, and the mixture is naturally cooled to 25 °C, then heated to 82 °C and refluxed for 6 h to obtain the reaction mixture. The mixture was cooled to 25°C at a rate of 1°C / min, and the solid was collected by filtration. The solid was washed three times with 300 mL of anhydrous ethanol to obtain a crude product. The crude product was suspended in 2000 mL of deionized water, stirred at 300 rpm for 1 h, and then filtered. This operation was repeated three times to obtain a solid. The solid was transferred to a Soxhlet extractor and extracted with 500 mL of n-hexane for 24 h to obtain a product. The product was dried in a vacuum drying oven at 72°C for 48 h at a vacuum degree of -0.08 MPa.

[0037] Comparative Example 1 The preparation method is the same as in Example 1, except that 100.0 g of o-chloronitrobenzene and 300.0 g of N-methylpyrrolidone were added to a 2000 mL four-necked flask and stirred at 300 rpm to dissolve. Then, 100.0 g of carbon disulfide, 40.0 g of sulfur powder, 10.0 g of sodium carbonate, and 3.0 g of copper sulfate were added sequentially. The mixture was heated to 182 °C at 2 °C / min and stirred for 6 h under nitrogen protection at a reaction pressure of 0.5 MPa. After the reaction was completed, the mixture was cooled to 80 °C at 1 °C / min to obtain a solution of N-methylpyrrolidone in sodium thiol benzothiazole. The reaction mixture was cooled to 38 °C at 0.5 °C / min, and 70 g of concentrated hydrochloric acid (35% by mass) was slowly added while stirring at 500 rpm. Adjust the pH to 2.5 and continue stirring for 1 hour; filter using a Buchner funnel to obtain the solid, wash the solid three times with 500 mL of cold water to obtain crude mercaptobenzothiazole product; redisperse the crude mercaptobenzothiazole product in 500.0 g of N-methylpyrrolidone, add 5.0 g of activated carbon, heat to 90 °C and stir at 400 rpm for 1.5 hours to decolorize; filter using a hot filter, cool the filtrate to 25 °C at 0.3 °C / min, and then further cool to 4 °C at 0.5 °C / min to crystallize for 3 hours; collect the crystals by filtering using a Buchner funnel, wash twice with 200 mL of ethanol, and dry in a vacuum drying oven at 60 °C for 24 hours at a vacuum degree of -0.08 MPa to obtain mercaptobenzothiazole rubber accelerator.

[0038] Comparative Example 2 The preparation method is the same as in Example 1, except that 100.0 g of o-chloronitrobenzene and 300.0 g of N-methylpyrrolidone were added to a 2000 mL four-necked flask and stirred at 300 rpm to dissolve. Then, 100.0 g of carbon disulfide, 40.0 g of sulfur powder, 10.0 g of sodium carbonate, and 3.0 g of copper sulfate were added sequentially. The mixture was heated to 182 °C at 2 °C / min and stirred for 6 h under nitrogen protection at a reaction pressure of 0.5 MPa. After the reaction was completed, the mixture was cooled to 80 °C at 1 °C / min to obtain a solution of N-methylpyrrolidone in sodium thiol benzothiazolium salt. 1.5 g of pre-prepared 4-(2-benzothiazolylthio)benzoic acid was added and stirred at 200 rpm at 80 °C for 2 h. The reaction mixture was then cooled to 38 °C at 0.5 °C / min and stirred at 500 rpm. 70.0 g of concentrated hydrochloric acid (35% by mass) was slowly added under stirring to adjust the pH to 2.5, and stirring was continued for 1 h. The solid was obtained by filtration through a Buchner funnel and washed three times with 500 mL of cold water to obtain crude mercaptobenzothiazole product. The crude mercaptobenzothiazole product was redispersed in 500.0 g of N-methylpyrrolidone, and 5.0 g of activated carbon was added. The mixture was heated to 90 °C and stirred at 400 rpm for 1.5 h for decolorization. The mixture was filtered using a hot filtration device, and the filtrate was cooled to 25 °C at 0.3 °C / min, and then further cooled to 4 °C at 0.5 °C / min for 3 h for crystallization. The crystals were collected by filtration through a Buchner funnel, washed twice with 200 mL of ethanol, and dried in a vacuum drying oven at 60 °C for 24 h at a vacuum degree of -0.08 MPa to obtain mercaptobenzothiazole rubber accelerator.

[0039] Comparative Example 3 The preparation method is the same as in Example 1, except that 100.0 g of o-chloronitrobenzene and 300.0 g of N-methylpyrrolidone were added to a 2000 mL four-necked flask and stirred at 300 rpm to dissolve. Then, 100.0 g of carbon disulfide, 40.0 g of sulfur powder, 10.0 g of sodium carbonate, and 3.0 g of copper sulfate were added sequentially. The mixture was heated to 182 °C at 2 °C / min and stirred for 6 h under nitrogen protection at a reaction pressure of 0.5 MPa. After the reaction was completed, the mixture was cooled to 80 °C at 1 °C / min to obtain a solution of N-methylpyrrolidone in sodium thiol benzothiazolyl salt. 1.0 g of pre-prepared N,N'-bis(2-benzothiazolyl)urea was added, and the mixture was stirred at 200 rpm at 80 °C for 1 h to obtain a reaction mixture. The reaction mixture was cooled to 38 °C at 0.5 °C / min and then stirred at 5... 70.0 g of concentrated hydrochloric acid (35% by mass) was slowly added under stirring at 00 rpm to adjust the pH to 2.5, and stirring was continued for 1 h. The solid was obtained by filtration through a Buchner funnel and washed three times with 500 mL of cold water to obtain crude mercaptobenzothiazole product. The crude mercaptobenzothiazole product was redispersed in 500.0 g of N-methylpyrrolidone, and 5.0 g of activated carbon was added. The mixture was heated to 90 °C and decolorized by stirring at 400 rpm for 1.5 h. The mixture was filtered using a hot filtration device, and the filtrate was cooled to 25 °C at 0.3 °C / min, and then further cooled to 4 °C at 0.5 °C / min to crystallize for 3 h. The crystals were collected by filtration through a Buchner funnel, washed twice with 200 mL of ethanol, and dried in a vacuum drying oven at 60 °C for 24 h at a vacuum degree of -0.08 MPa to obtain mercaptobenzothiazole rubber accelerator.

[0040] Performance testing and results analysis According to existing national and industry standards, the performance of the ultra-low impurity thiol-based benzothiazole rubber accelerators prepared in Examples 1-3 and Comparative Examples 1-3 was tested using the following methods: Accelerator purity was tested using high-performance liquid chromatography (HPLC) with a C18 reversed-phase column. Isocrates were performed using a methanol-water (70:30 v / v) mixture as the mobile phase. The detection wavelength was 254 nm, the column temperature was maintained at 35°C, the flow rate was set to 1.0 mL / min, and the injection volume was 10 μL. Quantitative analysis was performed using the external standard method. For the weight loss test, 2.00 g of sample was accurately weighed and placed in a pre-weighed weighing bottle. The sample was dried in a vacuum drying oven at 105±2°C for 2 h, then cooled in a desiccator for 30 min before weighing. For the ash content test, 1.000 g of sample was accurately weighed and placed in a pre-weighed porcelain crucible. The ash content was measured using an electric... After slow carbonization in the furnace until no smoke is produced, the sample is transferred to a muffle furnace and calcined at 750±25℃ for 3 hours. After removal, it is placed in a desiccator to cool for 45 minutes and weighed for calculation. The initial melting point test is performed using the capillary method. The sample is ground into powder and placed in a capillary tube with an inner diameter of 1.0 mm. The sample is then placed in a melting point apparatus and heated at a rate of 1℃ / min. For rubber application performance testing, 100 parts of natural rubber, 5 parts of zinc oxide, 2 parts of stearic acid, 0.8 parts of accelerator sample, and 2.5 parts of sulfur are mixed on an open mill according to the standard formula. The mixture is passed through a thin mill 6 times and sheeted. The scorch time and positive vulcanization time are measured using a vulcanizer at 150±1℃. The mixed rubber is then vulcanized on a flat vulcanizing machine to the positive vulcanization time to prepare standard test pieces. The tensile strength and elongation at break are measured using an electronic tensile testing machine at a tensile speed of 500 mm / min.

[0041] Table 1: Performance test results of each embodiment and comparative example As shown in Table 1, a comparative analysis of the performance test results of Examples 1-3 and Comparative Examples 1-3 clearly demonstrates that the present invention successfully solves the three major technical problems in the existing preparation process of thiol-based benzothiazoles. Regarding the issue of high impurity content in the product, the purity of Examples 1-3 all reached over 99.5%, while Comparative Example 1 was only 96.83%, and Comparative Examples 2 and 3 were 98.42% and 97.95%, respectively. This fully demonstrates that the synergistic effect of the two modified compounds can effectively remove impurities. Simultaneously, the heating loss of the Examples (0.16-0.18%) and ash content (0.07-0.09%) were significantly lower than those of Comparative Example 1 (0.36% and 0.27%), indicating that the present invention fundamentally reduces impurity inclusion and inorganic residues through the crystallization-directing effect of 4-(2-benzothiazole-thio)benzoic acid and the impurity-capturing effect of N,N'-bis(2-benzothiazole-)urea. Regarding the issue of limited post-processing purification effect, the initial melting point of the example reached 179.1-179.3℃, which is significantly higher than that of the comparative example (177.6-178.3℃). This indicates that the product has a more complete crystal structure and higher purity. This is due to the two modified compounds added stepwise playing their roles at different stages, forming a multiple purification mechanism and breaking through the limitations of traditional single purification technology. Regarding the mitigation of safety and environmental risks in the production process, the examples demonstrated a longer scorch time of 22.1-22.5 minutes and a shorter positive vulcanization time of 8.6-8.8 minutes in rubber application tests, representing a significant improvement over Comparative Example 1's 18.2 minutes and 10.5 minutes. This proves that the present invention, through optimizing reaction conditions and adding modified compounds, achieves the preparation of high-quality products under relatively mild conditions, avoiding the decomposition side reactions of traditional high-temperature and high-pressure processes, while improving the safety and efficiency of the vulcanization process. Furthermore, the tensile strength (26.5-26.8 MPa) and elongation at break (578-585%) of the products in the examples are superior to those in the comparative example, further verifying the positive impact of low impurity content on the performance of the final product and achieving full-process optimization from production to application performance.

Claims

1. A process for the preparation of an ultra-low impurity mercaptobenzothiazole rubber accelerator characterized by the steps of Comprising: S1, in a four-port flask, adding o-chloronitrobenzene and N-methylpyrrolidone, stirring, then adding carbon disulfide, sulfur powder, sodium carbonate and copper sulfate in turn; Under the protection of nitrogen, heating to 180-185 DEG C, stirring reaction; after the reaction is completed, cooling to 78-82 DEG C, to obtain mercaptobenzothiazole sodium salt N-methylpyrrolidone solution; adding 4-(2-benzothiazolylthio) benzoic acid, stirring at 78-82 DEG C; S2, adding N,N'-bis (2-benzothiazolyl) urea, continue to stir at 78-82 DEG C, to obtain the reaction mixture; the reaction mixture is cooled to 35-40 DEG C, under stirring, adding concentrated hydrochloric acid, adjusting pH to 2-3, continue to stir; Filtering, to obtain solid, the solid is washed with cold water, to obtain crude mercaptobenzothiazole product; the crude mercaptobenzothiazole product is redispersed in N-methylpyrrolidone, adding activated carbon, heating to 88-92 DEG C, stirring decolorization; hot filtration, the filtrate is cooled to room temperature, then further cooled to 3-5 DEG C crystallization; filtering to collect crystals, washing with ethanol, drying in a vacuum drying oven at 58-62 DEG C.

2. The process for the preparation of ultra-low impurity mercaptobenzothiazole rubber accelerator according to claim 1, characterized in that, In step S1, the mass ratio of o-chloronitrobenzene, carbon disulfide, sulfur powder, sodium carbonate and copper sulfate is 100: (80-120): (30-50): (5-15): (1-5).

3. The process for the preparation of ultra-low impurity mercaptobenzothiazole rubber accelerator as claimed in claim 1, wherein, In step S2, the time of heating to 88-92 DEG C, stirring decolorization is 1-2h.

4. The process for the preparation of ultra-low impurity mercaptobenzothiazole rubber accelerator as claimed in claim 1, wherein, The preparation method of the 4-(2-benzothiazolylthio) benzoic acid comprises the following steps: A1, adding 2-mercaptobenzothiazole and 4-chlorobenzoic acid in a three-port round-bottom flask, then adding N,N-dimethylformamide, and then adding sodium hydroxide; under the protection of nitrogen, heating to 118-122 DEG C, stirring reaction, to obtain a reaction mixture; A2, cooling the reaction mixture to room temperature, filtering; pouring the filtrate into ice water, filtering to collect solid, washing the solid with deionized water, to obtain a crude product; dissolving the crude product in sodium hydroxide aqueous solution, filtering, and acidifying the filtrate with concentrated hydrochloric acid to pH=2-2.2; filtering to collect crystals, washing the crystals with deionized water, and drying in a vacuum drying oven at 58-62 DEG C.

5. The process for the preparation of ultra-low impurity mercaptobenzothiazole rubber accelerator as claimed in claim 4, wherein, In step A1, the time of stirring reaction is 12-14h.

6. The process for the preparation of ultra-low impurities mercaptobenzothiazole rubber accelerator as claimed in claim 4, wherein, In step A2, the time of drying in a vacuum drying oven at 58-62 DEG C is 24-30h.

7. The process for the preparation of ultra-low impurities mercaptobenzothiazole rubber accelerator as claimed in claim 1, wherein, The preparation method of the N,N'-bis (2-benzothiazolyl) urea comprises the following steps: B1, adding 2-aminobenzothiazole and anhydrous toluene in a three-port round-bottom flask, stirring, and adding triphosgene toluene solution dropwise under ice water bath cooling; after the dropwise addition is completed, cooling to room temperature, and then heating to 78-82 DEG C, refluxing reaction, to obtain a reaction mixture; cooling the reaction mixture to room temperature, filtering to collect solid, and washing the solid with anhydrous ethanol, to obtain a crude product; B2, suspending the crude product in deionized water, stirring, and filtering, to obtain solid; transferring the solid to a soxhlet extractor, extracting with n-hexane, to obtain a product; and drying the product in a vacuum drying oven at 68-72 DEG C.

8. The process for the preparation of ultra-low impurities mercaptobenzothiazole rubber accelerator as claimed in claim 7, wherein, In step B1, the time of heating to 78-82 DEG C, refluxing reaction is 6-8h.

9. The process for the preparation of ultra-low impurity mercaptobenzothiazole rubber accelerator as claimed in claim 7, wherein, In Step B2, the drying time in a vacuum oven at 68-72°C is 48-50 h.

10. An ultra-low impurity mercaptobenzothiazole rubber accelerator characterized in that, The ultra-low impurity mercaptobenzothiazole rubber accelerator is prepared according to the method of claim 1-9.

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