Enhanced Ozone Aging Resistance of Natural Rubber with 2-Mercaptobenzothiazole as the Constant-Viscosity Agent

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Introduction
Compared with the good characteristics of synthetic rubber in some specifc felds, natural rubber (NR) has more excellent comprehensive performance and is mainly used to prepare tires for cars and airplanes. However, during the period of processing NR from raw rubber to rubber products, with the Mooney viscosity and Wallace plasticity values increased gradually [1][2][3][4], a phenomenon of hardening emerges and makes the procedure of processing and producing difcult, furthermore leading to a negative efect on the serving properties of NR. Moreover, because of the aldehyde groups existing in the molecular chain of NR, the molecular chain will cross-link and harden NR in the course of storage and application [5][6][7].
To solve this issue, the introduction of the constantviscosity agent into NR is an efective method to inhibit the cross-linking caused by an aldehyde group, aiming at preparing an NR of constant stickiness [8][9][10]. In recent years, it has attracted much attention that MBT was a new type of thiol-based constant-viscosity agent for the constantviscosity treatment of NR, which could efectively solve the storage hardening of rubber. Compared with traditional hydroxylamine-based constant-viscosity agents, the new thiol-based constant-viscosity agent has the advantages of low toxicity and good thermal stability [9]. Tus, the application of the thiol-based constant-viscosity agent in the production of NR is necessary and promising. On the other hand, because of the increasing environmental pollution, the concentration of ozone in the air increases quite a lot, a large part of which is derived from the emission of car exhaust in the city. As we all know, it is very easy for ozone to react quickly with double bonds in the main chain of NR [11][12][13]. With the ozone in the air, a gray, hard, and brittle flm is formed on the surface of NR, similar to being sprayed with frost. Under the condition of stress or strain, the flm cracks until the NR is fractured with the extension of aging time. Terefore, the investigation of ozone aging resistance is of great importance to obtain high-performance NR products. Tere were few studies on the ozone aging properties of constant-viscosity NR, among which the samples were remained prepared by the traditional hydroxylamine-based constant-viscosity agents. What's worse, the modifcation efects of those conventional constant-viscosity agents on ozone aging performance are basically adverse. Wang et al. [14] from our research group had successfully developed a high-performance NR with excellent ozone aging resistance, physical and mechanical properties. However, the hardening problem still happened and degraded the quality of this NR during the storage, which immensely impedes the actual application of NR. Consequently, it is of great practical signifcance to investigate the comprehensive performance including ozone aging resistance, high performance, and constant stickiness of NR treated using the constant-viscosity agent.
In this paper, NR with diferent contents of 2-mercaptobenzothiazole (MBT) as the constantviscosity agent was prepared via blending. Te storage hardening value, chemical structure, surface morphology, and mechanical properties of the MBT-treating NR before and after ozone aging were investigated and compared with Mooney viscosity, plasticity value, Fourier transform attenuated total refection infrared spectroscopy, low magnifcation magnifying glass, and tensile test, respectively. It displayed great potential of ozone aging resisting enhancement in highperformance NR.

Experimental Setup
2.1. Materials. Te NR fresh latex was supplied by Hainan Natural Rubber Industry Group Jinlong Processing Branch. Te 2-mercaptobenzothiazole (MBT) was purchased from Shanghai Aladdin Biology Co., Ltd. and dissolved in ammonia water of 2.5 wt%, preparing a constant-viscosity solution of 5 wt%. All the other chemicals were of AR grade and used as received.

Preparation of High-Viscosity NR with Constant-Viscosity
Treatment. First, 6 samples of NR fresh latex were taken and diluted with water until the dry rubber content is about 25 wt %. Ten, relative to the content of dry rubber, the MBT with given amounts of 0, 0.05, 0.1, 0.15, 0.2, and 0.25 wt% was added into the samples, which are named by raw NR: NR/ MBT-0.05, NR/MBT-0.10, NR/MBT-0.15, NR/MBT-0.20, and NR/MBT-0.25, respectively. Second, the activator [15], prepared from a ratio of 1 : 1 between silica and sodium bisulfte, was added, stirred, and placed for a while with the content of 0.1 wt%. To form the clot, a certain amount of biological coagulant was added and stirred evenly. After that, the clot was dehydrated, rinsed, pressed, and dried in an electric blast drying oven at a temperature of 70°C for 7∼15 days. Te normal NR sample for comparison is the NR with a standard of No. 5 produced from the same batch of fresh latex in the factory and solidifed by formic acid with a standard producing process of rubber.

Accelerated Storage of Raw
Rubber. Te raw rubber was evenly made to be thin slices with the thickness of 3.2 to 3.6 mm and placed in a dryer with P 2 O 5 preheated at 60°C in advance. Subsequently, the samples were stored in a drying oven with the constant temperature of 60°C for 6, 12, 24, and 48 h, respectively. Ten, the samples were taken out and cooled to room temperature. Finally, the raw rubber with an accelerated storage was obtained.

Preparation of Vulcanizate.
According to the formula of pure rubber, 100 g of NR, 0.5 g of stearic acid, 6 g of zinc oxide, 0.5 g of accelerator, and 3.5 g of sulfur were mixed in the two-roll mill of JTC-T52, the sample was prepared and placed for a period to be homogenized. Ten, the vulcanization curve was determined. Eventually, the vulcanized rubber was obtained through the processing of vulcanizing in a QLB-D vulcanizing machine.

Characterizations.
Te storage hardening value can be determined by the diference between Mooney viscosity and plasticity values before and after accelerated storage, expressed as follows. ΔMv � Mv-Mv 0 , where Mv is the Mooney viscosity after 48 hours of accelerated storage at a temperature of 60°C and Mv 0 is the Mooney viscosity before storage. ΔP � P-P 0 , where P is the plasticity after accelerated storage for 48 hours at a temperature of 60°C, P 0 is the initial value of plasticity [9]. According to GB/T1232.1-2000, a domestic MV2-90E Mooney viscometer was used to measure the Mooney viscosity of the raw-rubber samples without and with accelerated storage for diferent times. According to GB/T3510-2006, the plasticity values of the samples were measured on the Wallace rapid shape meter.
Te Fourier transform attenuated total refection infrared spectroscopy (ATR-FTIR) test of raw rubber named A∼F with diferent MBT dosages were carried out on a German Bruker TENSOR27 type Fourier transform infrared spectroscopy analyzer with 16 scans and the scanning range of 650∼4000 cm −1 . In addition, the surface of the samples taken was smooth and fat.
Te ozone aging resistance test of vulcanized rubber was conducted referring to GB/T7762-2014. Te vulcanized rubber of raw NR, NR/MBT-0.05, NR/MBT-0.10, NR/MBT-0.15, NR/MBT-0.20, and NR/MBT-0.25 was sampled into a dumbbell type and fxed in the dark for 48 hours. Subsequently, the samples sufered the ozone aging process with the temperature of 40°C, a humidity of 65%, and an ozone concentration of 100 ppm for 24 hours. Te surface morphology of each sample was observed by a low-magnifcation magnifying glass, and the time when the crack began to appear on the surface of the sample was recorded. Te Nikon Digital Sight DS-U3 was used to catch the microscopic digital photograph of each group enduring the ozone aging for 6 h with the magnifcation of 20 times. Ten the number of cracks on the photograph was compared to determine the ozone aging resistance of each sample.
Te mechanical properties test of vulcanizate before and after ozone aging was carried out on the AI-3000 tensile tester with a tensile speed of 500 mm/min, according to GB/ T528-1998. Figures 1 and 2, the Mooney viscosity (Mv 0 ) and initial plastic value (P 0 ) of raw rubber with diferent amounts of MBT were tested at different accelerated storage times. It can be seen from the curve that with the delay of the accelerated storage time, the Mooney viscosity and plasticity values of the raw NR without MBT added, while the samples with MBT added were obviously gentle. It can be concluded that the addition of MBT as the constant-viscosity agent exactly has a constantviscosity efect on the storage of raw rubber. When the content of MBT is 0.05 wt% in the rubber, the change of the values of NR/MBT-0.05 is still large, representing a poor constant-viscosity efect due to the insufciency of MBT. When the dosage of MBTreaches 0.15% or more, the curve is basically gentle, and the change of Mooney viscosity and plasticity before and after accelerated storage is small, indicating that the rubber has achieved a good constantviscosity efect.

Te Constant-Viscosity Efect of MBT on High-Performance NR. As shown in
To confrm the constant-viscosity efect, it is convenient to introduce the concept of storage hardening value and visually see the numerical changes of each group of samples before and after accelerated storage. As shown in Table 1, along with the increasing amount of MBT, the values of ΔMv and ΔP decreased, indicating that the constant-viscosity agent of MBT has a certain constant-viscosity efect during the accelerated storage of rubber. Tis is consistent with the previous results [16], where Yu reported that the storage hardening value can be efectively reduced to below 4 with the MBT content of 0.14 phr, achieving a constant-viscosity efect. For NR/MBT-0.15, NR/MBT-0.20, and NR/MBT-0.25, the ΔMv ≤ 4 and ΔP ≤ 4 indicated that the constant viscosity of the rubber was favorable under these three groups. Tat is, when the amount of MBT as the constantviscosity agent is 0.15% or more, NR/MBT achieves a positive constant-viscosity efect. It has been found that the constant-viscosity efect of MBT on high-performance NR is as same as that of normal NR. When the dosage of MBT reaches 0.15% wt, the reaction between sulfhydryl and aldehyde groups in MBT has reached the optimal efect; that is, the constant viscosity efect has also reached the optimal performance of ozone aging. Increasing the content of MBT has little efect on the number of aldehyde groups.

Infrared Spectroscopy Analysis of Constant Viscosity NR.
Te Fourier transform attenuated the total refection infrared spectrum of raw NR without MBT as shown in Figure 3. Te characteristic peaks normally found in NR are identifed in the fgure and described in Table 2, respectively. Among them, the appearance of two weak peaks of C=O stretching vibration at 1736 cm −1 and C-H stretching vibration at 2725 cm −1 in the aldehyde group can prove the existence of the aldehyde group in the NR. Te protein has an amide structure, which is mainly refected in the characteristic absorption bands of amide I at 1654 cm −1 and amide II at 1541 cm −1 in the infrared [17,18]. Te characteristic absorption band of amide I overlaps with the C=C stretching vibration peak in the NR to be masked. Te characteristic peak at 1542 cm −1 in the fgure is the characteristic absorption band of amide II in protein, which proves the presence of protein in NR. Tere is a large O-H peak at 3300 cm −1 , which may be caused by the incomplete drying of the rubber. Te local infrared spectra at 2800 cm −1 ∼1500 cm −1 of the samples such as raw NR, NR/MBT-0.05, NR/MBT-0.10, NR/ MBT-0.15, NR/MBT-0.20, and NR/MBT-0.25 with and without MBT are shown in Figure 3, respectively. As shown in Figure 3, the addition of MBT would have no efect on the content of the C�C double bond in the six samples, prepared from the same batch of latex. Te infuence of the MBT on the characteristic peak of C�C is negligible. Terefore, this characteristic peak can be used as the base peak. It can be found more intuitively that after adding MBT, the protein amide II absorption band near 1542 cm −1 of NR is obviously enhanced, and the position is shifted. Te introduction of proteases in the production of highperformance NR would promote the decomposition of proteins, while the addition of MBT may cause an inhibitory efect on this process. Terefore, the NR/MBT-0.25 of NR with MBT added is of more protein than the raw NR. Furthermore, the addition of MBT in rubber may bring disadvantages to the solidifying and producing processes, which would show adverse impact on the high performance of rubber to some extent.
Te decrease of C�O and C�C stretching vibration peaks at 1736 cm −1 and 1663 cm −1 ,respectively, was not obvious. As presented in Figure 3, for the aldehyde group, the variation of the two characteristic peaks can be determined based on the area ratio of the C-H characteristic peak at 2725 cm −1 to the C�C characteristic peak at 1663 cm −1 and the aldehyde group at 1736 cm −1 . Te area ratio of the C�O characteristic peak to the C�C characteristic peak at 1663 cm −1 is calculated and compared [19,20].
It was calculated that the addition of MBT weakened the relative intensity of the aldehyde-based characteristic peak in the rubber. It can be speculated that the MBT added would prevent the cross-linking reaction of the aldehyde group during storage by reacting with the aldehyde group in the molecular chain of rubber, thereby inhibiting the occurrence of storage hardening. Of course, the occurrence of storage hardening will also be afected by other factors. Te infuence of the reaction between the thiol group and the aldehyde group in MBT is mainly considered.

Ozone Aging Resistance Analysis of Constant-Viscosity
Vulcanizate. Te NR vulcanizate samples with diferent viscosity agent contents in the samples of raw NR, NR/MBT-0.05, NR/MBT-0.10, NR/MBT-0.15, NR/MBT-0.20, and NR/ MBT-0.25 were subjected to ozone aging resistance test under the temperature of 40°C, humidity of 65%, and ozone concentration of 100 ppm. Te ozone aging resistance of the sample can be compared using the time when the surface of the sample begins to crack, as shown in Table 3. Te highperformance NR without the constant-viscosity agent of MBT begins to crack on the surface after an ozone aging time of 240 min, which is much later than that of normal NR, meaning that the high-performance NR own an obviously better ozone aging resistance by itself. Te addition of a constant-viscosity agent has a certain improvement for the ozone aging resistance of rubber at low contents. For NR/ MBT-0.15 with the MBTcontent of 0.15%, the crack began to appear after aged for 300 minutes, which was the latest, exhibiting the best ozone aging resistance. Ten, with the continuous increase of MBT content, the ozone aging resistance decreased. When the MBT content reached to 0.25%, the ozone aging performance of NR/MBT-0.25 was below that of the raw NR without MBT.
Te microphotograph of the rubber after ozone aging for 6 hours is shown in Figure 4, and the magnifcation is 20 times. Under the same aging time, the cracks in the NR/ MBT-0.15 and NR/MBT-0.20 are shorter, narrower, and fewer than those of the others. It is investigated that the NR/ MBT-0.15 has the best ozone aging resistance. Although the crack in NR/MBT-0.10 is dense, the crack length is short, due to the unconnection of the cracks. Compared with raw NR, the fne cracks in NR/MBT-0.05 are fewer and shorter, indicating that a small amount of MBT added has a certain improvement efect on the ozone aging resistance of rubber. Combined with the occurrence time of cracks [21][22][23], it can be found that the ozone aging resistance in each group of samples is NR/MBT-0.15 > NR/MBT-0.20 > NR/MBT-0.10 > NR/MBT-0.05 > raw NR > NR/MBT-0.25.
Te impact of MBT on ozone aging resistance is a complex process. Te reaction between MBT and NR is connected to the NR molecular chain. Te sulfur atom in MBT has an empty d orbital to capture electrons. Ozone has a strong oxidizing power during the ozone aging, and will attack sulfur atoms preferentially, reaching a stable state after losing electrons [24]. Terefore, the addition of MBT has an advancing efect on ozone-aging performance. Moreover, MBT itself acts as a vulcanization accelerator to promote the cross-linking, which reduces the double bonds in the molecular chain of NR after vulcanization and enhances ozone aging performance. Studies have found that amino acids in the decomposition products of proteins would contribute to the cross-linking of rubber vulcanization, allowing more double bonds to participate in the vulcanization, leaving fewer double bonds and improving the ozone aging performance. Amine groups can also take part in the ozone reaction, forming amine oxygen compounds on the surface to slow the occurrence of ozone aging on the surface of the gel [12,25]. Te excessive addition of MBT will inhibit the breakdown of protein, resulting in the reduction of ozone aging performance for rubber. Terefore, the efect of MBT added is comprehensive on the ozone aging resistance of high-performance NR.

Mechanical Properties of High-Performance Constant-Viscosity NR.
Tere are excellent mechanical properties and high tensile strength of high-performance NR. Before and after the accelerated storage experiments were conducted, the mechanical properties of the high-performance   gels with diferent contents of MBT in sample were performed. As shown in Table 4, the addition of a constantviscosity agent has a little efect on the tensile strength of high-performance NR, while a certain efect of maintaining the performance on the NR before and after accelerated storage. After 24 hours of accelerated storage, the tensile strength of raw NR without MBT declined by 3.59 MPa, but the tensile strength of raw NR with MBT decreased less noticeably. It was demonstrated that the constant-viscosity agent worked during the accelerated storage, so that the tensile properties of the stored sample were well maintained. Similarly, the constant-viscosity efect of MBT could be found from the change of the tensile strength before and after storage with a value of 300%, which may be resulted from the reaction of MBT with NR. Te ozone aging test can be carried out to evaluate the ozone aging resistance based on the cracking time, surface morphology, and the retention of tensile properties for the sample after aging. Before and after ozone aging for 24 h, the mechanical properties of high-performance NR with different contents of MBT added are shown in Table 5

Conclusion
In this paper, the efects of addition and specifc dosages of the constant-viscosity agent on the ozone aging performance of NR were analyzed and investigated briefy. Te data of the accelerated storage experiment showed that when the amount of the constant-viscosity agent reached 0.15% or more, the changes of the processing properties such as Mooney viscosity and Wallace plasticity during storage hardening were successfully solved, achieving a good constant-viscosity efect. Te infrared analysis data confrmed that MBT eliminated the intermolecular crosslinking during storage by reacting with the aldehyde group in advance, thus preventing the occurrence of storage hardening and proving the important role of the aldehyde group in the storage hardening phenomenon. Te initial tensile strength of the NR was not infuenced much by the addition of the constant-viscosity agent and stable at about 28 MPa. However, the tensile strength of the rubber without the constant-viscosity agent was decreased by 3.59 MPa after storage, while the rubber maintained the original high tensile strength after adding the constantviscosity agent. It is found that with the increasing contents of MBT, the ozone aging resistance of rubber shows a process of initial increasing and subsequent decreasing.
With the MBT dosage of 0.15%, the ozone aging resistance of the NR/MBT-0.15 reached the best. Although the efect of MBT added on the ozone aging resistance of NR is comprehensive, this work provided a certain value for the actual production of NR and suggested a potential application in preventing NR hardening and cracking during storage and use.

Data Availability
Te data used to support the fndings of this study are included within the article.

Conflicts of Interest
Te authors declare that they have no conficts of interest.

Authors' Contributions
Juan Zhou wrote the original draft and investigated the study. Peng Deng reviewed and edited the article. Hongxing Gui proposed the methodology and reviewed and edited the article. Junxiao Xu proposed the methodology and performed data curation. Jianhe Liao reviewed and edited the article and supervised the study. Aiwu Ding performed formal analysis and contributed to project administration.