A new purification method for 2-mercaptobenzothiazole and its application as vulcanization accelerator

In this work, a new purification method for 2-Mercaptobenzothiazole (MBT) and its application as a vulcanization accelerator was reported, and the properties of MBT samples were comprehensively investigated. According to the chemical equation of this purified reaction, the byproduct of the CO2 method was easy to treat. Results showed that the melting point and yield of experimental CAMBT samples were increased to 173.6 °C and 93.11%, respectively. The chemical structure and crystal absorption peak of all MBT samples were also measured by the FTIR and XRD analysis. Thermogravimetric analysis was used to study the residue of resin in MBT samples. Element content confirmed the existence of C, N, H, and S elements, and there were no other elements in the products. What is more important, the mechanical properties of experimental CDMBT@NR and SAMBT@NR samples were better than that of UMBT@NR. Thus, this paper offers a new purification method for MBT and investigates its application as a vulcanization accelerator.


Introduction
Natural rubber (NR) with many attractive properties (i.e., outstanding resilience, anti-fatigue property, and low cost) is widely used in automobile tires and rubber products [1,2]. In addition, NR is an elastomeric crosslinked material comprising an elastomer and additives [3,4]. However, NR with sticky and not durable property will begin to deteriorate in a few days [5,6]. Therefore, vulcanization additives, protective additives, processing additives, adhesive additives, and special functional additives are used to give rubber products high strength, high elasticity, corrosion resistance, aging resistance, and ease of processing properties [7,8]. Among these five categories of additives, vulcanization additives are an essential factor in rubber mechanical properties [9]. To the best of our knowledge, the vulcanization of rubber is a chemical process, which forms a three-dimensional network [10,11]. Moreover, the degree of cross-linking is the decisive factor in the properties of the natural rubber system. Therefore, various vulcanization additives and types are used to obtain better mechanical and end-use properties, which makes the elastomers vulcanize at a low temperature and a faster rate [12,13].
Vulcanization additives, especially accelerators, have been widely used in rubber vulcanization in the last decades. 2-Mercaptobenzothiazole is one of the most widely used vulcanization accelerators, which could short the vulcanization time and reduce the vulcanization temperature [13][14][15]. In addition, vulcanization accelerators can improve the mechanical properties of natural rubber products [16]. It is worth noting that the purity of MBT is the most critical factor for the effect of vulcanization. There are two commonly used purified methods including the acid-base method and the organic solvent method. The acid-base method is to dissolve the unpurified 2-Mercaptobenzothiazole (UMBT) with sodium hydroxide solution to make the MBT-sodium salt dissolved in the solution, and then use dilute sulfuric acid to adjust the pH of the solution and filter out the Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
insoluble [17]. Adding sulfuric acid to the filtrate to precipitate MBT, washing and drying the precipitated MBT is the acid-base method. As for organic solvent method, the first step is also to dissolve the UMBT with sodium hydroxide solution to form an aqueous solution of MBT-sodium salt. After filtering the insoluble matter, the MBT-sodium salt in the filtrate is extracted with an organic solvent [18]. After separation, the MBT-resin is dissolved in the organic solvent and MBT-sodium salt is collected. Comparing the two processes, the acid-base method MBT generally has a higher yield, stable color, and less MBT content in the resin, but the amount of waste water is large, being difficult to be treated. As for the solvent method MBT, the product contains some resin impurities, and its purity is about 1% lower than that of the acid-base method MBT.
In this work, a new purification method using CO 2 for 2-Mercaptobenzothiazole is reported. CO 2 dissolves in water to form a weak acid, which can precipitate the dissolved UMBT. Moreover, the byproduct is easy to handle, including ammonium bicarbonate and ammonium carbonate. This work will provide a new purified method for UMBT in a green, suitable, and sustainable way.

Experimental
2.1. Materials 2-Mercaptobenzothiazole (MBT) was provided by XINCE Co., Ltd, China. All chemical reagents, including ammonia, sulfuric acid, sulfur, zinc oxide, and other reagents were obtained from Sinopharm Chemical Reagent Co., Ltd, China, and directly used without further purification.

Purification methods of MBT
The work was carried out in a high-pressure reactor (HYX-0.5G; Dalian Forth Instrument Factory, China). Briefly, unpurified 2-Mercaptobenzothiazole (UMBT) and ammonia were added to the reactor to dissolve UMBT. After that, sulfuric acid solution and CO 2 were added in the reactor to purify MBT, respectively. The purified MBT using sulfuric acid and CO 2 was named SAMBT and CDMBT, respectively. The chemical equation of the different purification methods was shown in figure 1.

Cure characteristics
Mixings of different vulcanization systems were carried out using a laboratory two-roll mill according to the method described by the American Society for Testing and Materials (ASTM) D-3184-80. An Oscillating Disk Rheometer (ALPHA ODR 2000) were used to measure the cure characteristics of the different vulcanization systems at 170°C according to the ISO 3417 method. The respective cure times as measured by TC90, scorch times, torque, and cure rates were determined from the rheograph.

Crosslink density determination
The specimens (10 × 10 × 1 mm 3 ) were immersed in toluene in closed beaker for 72 h. After wiping the surface of the samples, they were quickly weighed (w 1 ) and dried at 40°C for 48 h and weighed again (w 2 ). The volume fraction of the samples was calculated by formula (1): Where, v is the volume fraction of polymer, r r and r s are the rubber and solvent density. The crosslink density was calculated by following Flory-Rhener equation (2): Where, V 0 is the molar volume of solvent (106 cm 3 /mol for toluene), and x is constant (0.39 for toluene).

Characterization
The yield of all MBT samples were calculated to the formula (3): In this formula, m 2 and m 1 are the weight of MBT samples before and after purified, respectively. All MBT samples were determined by a Thermo Nicolet FTIR spectrometer (Nicolet 6700, USA) in the wavenumber range of 400-4000 cm −1 with a resolution of 4 cm −1 to analyze the chemical structure. The x-ray Diffraction (XRD) patterns of the samples were carried out on an x-ray Diffractometer (D8 ADVANCE, Germany). A fully automatic melting point instrument (MPA100, USA) was used to determine the melting point of MBT samples.
The thermal stability of all MBT samples was measured by a thermogravimetric analyzer (TGA, STA449F5, Germany). The onset degradation temperature (T o ) and the maximum degradation temperature (T max ) were obtained from TG curves. The tensile strength of all NR samples was performed on a CMT6503 universal testing machine (MTS SYSTEMS, China) with a stretching rate of 4 mm min −1 . The element content of MBT samples was measured by an element analyzer (Elementar Analysensysteme GmbH, Germany) to analyze the content of C, H, S, and N.

Results and discussion
3.1. Properties of purified MBT samples As we mentioned above, a new purification method using CO 2 for MBT is reported. The by-products produced during the reaction were environmentally friendly. The properties of MBT samples prepared by different purified methods were shown in table 1. The melting point of UMBT was 172.6 ± 0.1°C, which was lower than that of CDMBT (NO. 3-5) and SAMBT (NO. [8][9][10]. This result could be caused by the removal of resin in the purified process. In addition, the yield of SAMBT in purified process was higher than that of CDMBT, which may be caused by the strong acidity effect. However, the purified process using CO 2 is more environmentally friendly. The sulfate products may be produced during the production process, which was difficult to handle. Therefore, the purified method using CO 2 is a more suitable and sustainable method for UMBT purification.

Chemical structure analysis
As shown in figure 2(a), FTIR spectra is used to analyze the chemical structure of UMBT and purified MBT samples. The peak around 3028-2960 cm −1 was attributed to the aromatic hydrogen stretching vibration, and the peak at 2884 cm −1 corresponds to the S-H stretching vibration in all MBT samples [19]. The peak at 1587 cm −1 and 1492 cm −1 was caused by the C=C stretching vibration in figure 2(b) [20]. In addition, the presence of 1430 cm −1 and 657 cm −1 of all samples was due to C=N-S and C-S groups [20].

XRD analysis
As shown in figure 3, XRD spectra are used to analyze all MBT samples. The peak at 13.5°, 16°, 20°corresponds to characteristic MBT reflections [21][22][23]. Thus, MBT is successfully purified using CO 2 and sulfuric acid to fabricate CDMBT and SAMBT. Though the FTIR spectra and XRD pattern of CDMBT were similar to SAMBT, the preparation process of CDMBT is more environmentally friendly. Therefore, the purified method using CO 2 is more suitable for this reported reaction. Figures 4(a) and (b) are the Thermogravimetric (TG) analysis and the corresponding DTG curves of all MBT samples. The weight loss of UMBT was higher than that of CDMBT and SAMBT, which was caused by the presence of resin [24,25]. Moreover, T o and T max of all MBT samples could be calculated according to the TG images. Generally, the higher T o , the better the thermal stability of all MBT samples. Therefore, the sequence of the thermal stability of all MBT samples was UMBT > CDMBT ∼ SAMBT. The relatively higher thermal stability of UMBT was caused by the existence of resin. Figure 5 is the element content of all MBT samples. As expected, the element content of C, N, S, and H using different purified methods was the same, indicating that purified methods had no obvious influence on all MBT  samples [26,27]. Therefore, the purified method using CO 2 and sulfuric acid is suitable for this reported reaction.

Vulcanization effect of MBT
Rubber compound films were prepared using the formulations as shown in    UMBT@NR. The tensile strength and elongation at the break of NR film are shown in figure 6. The mechanical property of CDMBT@NR and SAMBT@NR was higher than that of UMBT@NR, which was caused by higher purity. Therefore, the purified method using CO 2 and sulfuric acid is suitable for this reported reaction to fabricate NR film with the good mechanical properties.

Conclusion
In summary, this paper reported a new purification method for 2-Mercaptobenzothiazole (MBT) and its application as a vulcanization accelerator, and comprehensively studied the properties of all MBT samples. The CO 2 method was easy to treat and the byproduct was easy to handle. Compared with that of contrast samples, the melting point and yield of experimental CAMBT samples increased to 173.6 and 93.11%, respectively. FTIR and XRD analyses were used to study the chemical structure and crystal absorption peak of all MBT samples. Element content proved the existence of C, N, H, and S elements, and TG analysis confirmed the existence of resin. In particular, the mechanical properties of experimental CDMBT@NR and SAMBT@NR samples were better than that of UMBT@NR. Therefore, this paper provides a new purification method for unpurified MBT and investigates its application as a vulcanization accelerator.  ODRT is the Oscillating Disc Rheometer Torque and calculated using the formula: 0.9 * (MH -ML) + ML. Cure rate is calculated using the formula 100/ (t90 -t2) (%/min).