Xanthate-modified nanoTiO2 as a novel vulcanization accelerator enhancing mechanical and antibacterial properties of natural rubber

With the emerging of sustainability, the fabrication of effective and eco-friendly agents for rubber industry has attracted extensive attention. In this study, a novel and nontoxic titanium dioxide-based vulcanization accelerator (xanthate-modified nanotitanium dioxide (TDSX)) with excellent antibacterial performance, for the first time, was synthesized under the catalyst of ceric ammonium nitrate. Notably, the thermal stability of xanthate was greatly enhanced after being grafted on titanium dioxide (TiO2) nanoparticles, in which the activation energy was increased from 6.4 to 92.5 kJ/mol, enabling the obtained TDSX with multiple functions, mainly consisting of fabulous vulcanization-promoting effects, reinforcing effects, antibacterial properties, and anti-ultraviolet aging effects for natural rubber (NR). Simultaneously, the TDSX can be effectively and uniformly dispersed in the rubber matrix along with the developed interface interaction between TDSX particles and rubber matrix. Compared to the traditional accelerators 2-mercaptobenzothiazole (M) system, the tensile strength and the tearing strength of NR/TDSX was improved by 26.3 and 40.4%, respectively. Potentially, our work for preparing green vulcanization accelerator can provide a new design strategy for multifunctional high performance elastomer materials.


Introduction
As one of the most strategically important elastic materials, natural rubber displays multitude of applications due to its remarkable properties, especially for outstanding malleability, high strength, and excellent resilience. However, currently, commercial small-molecule vulcanization accelerators generally face the issues of poor thermal stability, weak dispersion, and serious aggregation, finally leading to the environmental problems and unsatisfactory properties of the products [1]. Therefore, nowadays, numerous attempts have been taken to solve the aforementioned shortcomings, where the physical or chemical loading of small molecule additives onto inorganic fillers can be an effective approach to address the related issues [2]. Besides, with the development of sustainability, another typical problem for current vulcanization accelerators is the toxicity since they can generate nitrosamine compounds, which are believed to have a negative effect on the health of human. Thus, the preparation of nontoxic, environmentally friendly, highperformance vulcanization accelerators is rather essential in the rubber industry.
Vulcanization accelerator refers to the substance that can increase the vulcanization speed and lower the vulcanization temperature after adding the rubber compound. Xanthates are generally prepared from the reaction of sodium or potassium hydroxide with an alcohol and carbon disulfide, whose structural formula is R-O-C = S(S)-M (R represents alkyl groups and M represents metal atoms) [3]. Xanthate, an environmentally friendly and efficient vulcanization accelerator, has attracted much attention in the latex industry since it does not generate carcinogens during processing [4]. Lamentedly, it remains a challenge for xanthate to be applied in the rubber industry due to serious self-aggregation and instability to temperature [5]. Therefore, a favorable method to deal with the aforementioned issues is to load xanthate on the filler [6]. For instance, Li et al. [7] used an eco-friendly starch as the carrier for xanthate to decline related drawback.
Natural rubber contamination by bacteria takes place easily that can affect both the quality and the price. Thus, Mourad et al. [8] investigated the incorporation of the prepared Ag-SiO 2 core shell particles in natural rubber matrix and the antibacterial properties of resulting composites. Thus, Xu et al. [9] prepared a functional TiO 2 /Ser (sericite) composite to improve the antibacterial properties of rubber products, such as medical carriers and food packaging.
Due to significant difference in size compared to their bulk materials, TiO 2 nanoparticles have attracted much attention for their high surface-to-volume ratio and adaptability for multiple functions. It is a promising carrier, which can be widely used in rubber composites owing to low cost, absence of toxicity, and fabulous antimicrobial performance [10]. Unfortunately, due to the poor compatibility and serious self-aggregation caused by extremely small size and the hydroxyl on the surface of TiO 2 nanoparticles [11], it is still difficult for TiO 2 nanoparticles to be uniformly dispersed in the rubber matrix. To address these issues, various strategies have been employed to functionalize the surface of TiO 2 nanoparticles for reducing undesirable interactions and favoring desired interactions [12]. Herein, we selected cerium ammonium nitrate, a powerful oxidant [13], to address relevant problems since cerium ammonium nitrate work similar to the Lewis acid catalyst and has powerful oxidation capacity.
To the best of our knowledge, there is no reported study on the preparation of TiO 2 nanoparticle-grafted xanthate vulcanization accelerator, where cerium ammonium nitrate worked as a catalyst. In detail, to solve the problem of xanthate migration and self-assembly of TiO 2 nanoparticles, cerium ammonium nitrate worked as the oxidizing agent of the hydroxyl groups of TiO 2 nanoparticles to improve the reactivity of hydroxyl, ultimately achieving the successful preparation of chemically grafted xanthate on TiO 2 nanoparticles (TDSX). A comparative study on the antibacterial performances and mechanical and thermal properties of NR/TDSX, NR/M, and NR/(M/ TDSX) composites was also carried out. We believe that our study can provide a new approach to fabricate multifunctional vulcanization accelerators with antibacterial properties by simple catalysis of high valence metal ions, which is expected to expand the application range of natural rubber.

Synthesis of TDSX accelerator
TiO 2 nanoparticles was dried in an air-dry oven at 80°C for 1 h. Then, 26 g dried titanium dioxide and 650 mL N,Ndimethylformamide were added in a three-necked flask successively. The mixture was stirred with 3.5 g ceric ammonium nitrate at 80°C for 2 h to activate hydroxyl groups on the surface of TiO 2 nanoparticles. Thereafter, 40 g solid NaOH was added in the three-necked flask at 35°C for 1 h under continuous stirring, followed by 76 g CS 2 that was slowly dripped inside. To obtain the TDSX vulcanization accelerator, the mixture was stirred for 3 h, filtered, and dried under reduced pressure at 40°C for 36 h. The synthesis route of TDSX is show in Figure 1.

Preparation of natural rubber composites
The formulas of natural rubber composites were carried out as presented in Table 1. In the investigation of the optimal proportion of TDSX, TDSX was used as a vulcanization accelerator at loadings of 4, 5, 6, 7, and 8 parts per hundred parts of rubber (phr). Then, the natural rubber composites containing the best ratio of TDSX and the M (fixed at 0.7 phr) curing system were compared.
Natural rubber and other additives were successively compounded with samples on a two-roll mill within 12 min. After homogenization, the composites were subjected to compression at 145°C for the optimum curing time (T90) to generate sulfur crosslinks [14].

Characterization
Fourier transform infrared (FTIR) spectroscopy of untreated TiO 2 and TDSX recorded on a PerkinElmer spectrometer with the highly pure potassium bromide tableting was performed. X-ray photoelectron spectroscopy (XPS) was carried out on an Axis Supra with an aluminum (mono) Kα source (1486.6 eV). Carbon-13 nuclear magnetic resonance ( 13 C-NMR) (600 MHz JNM-ECZ600R spectrometer, JEOL, Japan) was used to elucidate the chemical structure of the material. To determine the hydrophobicity of TDSX and TiO 2 , the static contact angle of a droplet on the surface was measured and analyzed by a Kruss DSA 30 (Germany) contact angle goniometer [15]. The thermogravimetric analysis (TGA) was performed using a Mettler-Toledo TGA/DSC subjected to a heating rate of 10°C/min in a heating range of 25-500°C, in which the activation energies (E a ) of thermal degradation in the main degradation stage were calculated according to the following equations [16]: where W 0 , W t , and W ∞ are the mass of samples at the initial time, mass at a given time, and final sample weight, respectively. α is the degradation rate, R is the gas constant (8.314 J/mol/K), and T is the absolute temperature (K).
The bonding rubber content on the surface of TDSX particles and TiO 2 particles was calculated according to the means suggested in the study by Ahn et al. [17]. The curves of vulcanization were obtained by rotorless rheometer MOR-2000E (Electronics Wuxi Liyuan Chemical Equipment Co., Ltd) at 145°C [18]. Mechanical properties were analyzed using an electronic universal testing machine (WDW-0.5C) provided by Hualong Test Instruments Corporation, Shanghai. The surface and the morphology of tensile fracture were analyzed by a Phenom prox scanning electron microscope (SEM). The abrasion was determined by previously reported methods [19].
To characterize the ability of molecular chains motion, dynamic mechanical analysis (DMA) was performed by a TA DMA Q850 instrument in a tensile mode at a frequency of 5 Hz and a heating rate of 3°C/min [20]. Crosslinking density of vulcanized natural rubber composites was determined by the equilibrium swelling method in toluene and   was calculated according to the classical Flory-Rehner equation [21]. The antibacterial activity was investigated against Escherichia coli (A TCC 25922, Gram-negative) and Staphylococcus aureus (CMCC(B) 26003, Gram-positive) using the flat colony counting method [22]. The sterilized NR/M, NR/TiO 2 , and NR/TDSX samples were placed in the E. coli and S. aureus liquid culture medium for 24 h, and then 50 µL of the liquid bacterial culture medium was taken from each, diluted, and evenly dispersed with a coating rod on nutrient agar. The number of viable bacteria after 24 h of cultivation was calculated to investigate its antibacterial properties [23]. The vulcanized rubber was cut into dumbbellshaped specimens and placed in the UV aging test box. Aging at a temperature of 50°C for 8 days, the radiation intensity is 0.8 W/m 2 [24].

Results and discussion
To figure out the success of reaction and loading efficiency, FTIR, XPS, 13 C-NMR, and TGA were subsequently used to check out the changes. Figure 2(a) shows the FTIR spectrum of TiO 2 nanoparticles and TDSX.
The peak at 3,428 cm −1 is the stretching vibration absorption peak of the hydroxyl group (−OH) caused by water or adsorbed water. The strong band at 1,449 cm −1 corresponds to the characteristic band of the carbonyl group [25]. Compared to TiO 2 , the newly generated peaks on TDSX at 1,006, 1,140, and 883 cm −1 , which were attributed to the absorptions of C-O, C]S, and C-S, respectively, indicated that sodium xanthate has been successfully in situ formed onto the surface of TiO 2 nanoparticles. Thereafter, the related chemical natures of the surface  The characteristic parameters (initial temperature (T 0 ), final temperature (T f ), temperature at maximum decomposition rate (T p ), and E a ) of thermal degradation of sodium isobutyl xanthate and TDSX species were the checked by XPS (Figure 2b). The newly induced peak at 168.2 eV corresponding to S 2p of TDSX was observed. The detailed spectrum of S element (Figure 2c) shows that the S 2p (161.4 and 163.3 eV), which belong to the existence of S element with two kinds of chemical structures [26], were presented in the spectrum of TDSX. The peak at 168.6 eV may be due to the existence of residual CS 2 . Besides, this information was further confirmed by the results of 13 C-NMR (Figure 2d). The signal at 169 ppm is assigned to the O-C(]S)-S in TDSX, while a weak signal at 108 and 41 ppm belonging to the signals of residual DMF solvent is observed. All of the results demonstrate that TDSX has been successfully prepared. TGA was subsequently carried out to illustrate the loading efficiency and related thermal properties [27] (Figure 2e and Table 2). Compared to sodium isobutyl xanthate, the values of T 0 , T p , and T f for TDSX were as expected increased by 53.1, 152.5, and 204.4°C respectively, which were in well accordance with the change of E a since it was dramatically enhanced from 6.4 to 92.5 kJ/mol, suggesting that the thermal stability of xanthate was significantly improved. In addition, according to the weight losses of titanium dioxide and TDSX in the temperature region between 200 and 500°C, the grafting efficiency of TDSX was 9.6 wt%.
Subsequently, a systematic comparison of vulcanization behaviors and mechanical properties for various samples was done (Figure 3a-c and Table 3). T90 is the optimum vulcanization time, and its decrease indicates that the vulcanization speed is accelerated. The difference between minimum torque and maximum torque (MH-ML) characterizes the degree of vulcanization of the rubber [28]. Obviously, the cross-linking process at 145°C shows a significant discrepancy (Figure 3(a)). Although 8 phr of TDSX have a lower T90 and higher MH-ML, it is considered that the flat vulcanization period is conducive to the industrialization of rubber [29], and the best content of TDSX was 7 phr. Figure 3 Since dispersion of filler plays a vital role in mechanical properties of natural rubber products, SEM was then used to check the fracture surfaces of various systems [30] (Figure 4a).
The rubber molecular chains adsorbed on the surface of the nanoparticles appear to slip and straighten orientation during the stretching process, which can effectively enhance the degree of orientation and strengthen  the rubber matrix [31]. The surface of NR/M was smooth, and the tensile strength was low since there are no nanoparticles added to the NR/M. Besides, aggregated TiO 2 particles in large sizes are easily observed in NR/(M/TiO 2 ), illustrating that the dispersion of TiO 2 was not even due to the hydrophilicity caused by hydroxyl groups, while that for TDSX was excellent [32]. The aggregated particles in the rubber matrix are not conducive to the performance of the adsorption efficiency and are very negative to the reinforcing result [33]. Therefore, the tensile strength and the tear strength of NR/(M/TiO 2 ) were even worse than those of NR/M. In addition, the blurry interface between TDSX and natural rubber also proved that the formation of xanthate on the surface of TiO 2 nanoparticles overcame an incompatibility between TiO 2 and rubber and improved interfacial adhesion among the two phases [34]. Besides, TDSX-based natural rubber composite displayed the relatively high bound rubber contents (Figure 4b), meaning more rubber chains are anchored on the surface of TDSX particles. It may be induced by the strong interaction between TDSX and rubber matrix during the vulcanization of rubber chains since TDSX particles could be homogenously dispersed to form enhanced contact area to rubber chains [35].
To quantify the degree of filler-rubber interactions in vulcanizates, equilibrium swelling measurement is used to obtain crosslinking density [36]. The existence of TDSX leads to maximal effective crosslinking density ( Figure  4c), indicating NR/TDSX composite had a most mature cross-linked network [37]. In consequence, both filler network and filler-rubber interaction were enhanced in NR/TDSX. Benefiting from the excellent dispersion and enhanced interaction of TDSX as well as high crosslinking density, the obtained TDSX functionalized natural rubber composite acquired superior mechanical properties as shown in Figure 4d   Furthermore, the dynamic mechanical analysis (DMA) of NR/M and NR/(M/TiO 2 ) composites was conducted to investigate the related vulcanization effects (Figure 5b-d), which displayed variations of the storage modulus (E′) and loss factor (tan δ) as a function of temperature [38]. Generally, a depression in tan δ height indicated the reduction number of mobile polymer chains during the glass transition, and the relative peak height is proportional to the volume of the constrained polymer chains [39]. The peak height of tan δ for NR/TDSX composite further decreased compared to that of NR/M and NR/(M/TiO 2 ) composites, meaning that more rubber chains were constrained in TDSX than those in M and M/TiO 2 . This is in well agreement with the result of bound rubber content.
Besides, the NR/TDSX composite exhibited the highest value of E′ (Figure 5b), which was probably due to the enhanced cross-linking density [40]. In addition, a lower tan δ 60°C value means a lower rolling resistance, which is an important parameter to evaluate the rolling resistance for high-performance rubber production [41]. It was obvious that the rolling resistance value of the NR/ TDSX composites was the lowest as shown in Figure 5(d). Hence, the NR/TDSX composites exhibited remarkable improvements in their static and dynamic mechanical properties, suggesting that the NR/TDSX composites would be rather competitive in potential applications as green tire materials.
Ultimately, thanks to the existing and well dispersion of TiO 2 nanoparticles, the obtained NR/TDSX exhibited outstanding antibacterial behavior by inhibiting the growth of Staphylococcus aureus (Gram-positive) and Escherichia coli (Gram-negative) in contrast to that of NR/M and NR/(M/ TiO 2 ) composites, which can be evaluated by the flat colony counting method [42] (Figure 6). The colonies of S. aureus and E. coli of both NR/M and NR/(M/TiO 2 ) vulcanizates are greater than 500, while NR/TDSX produces almost no colonies, demonstrating that the most efficient antibacterial specimen was NR/TDSX.
With the increase in UV aging time, the tensile strength of composites in the three systems all decreased to varying degrees (Figure 7a). After 8 days of UV aging, the tensile strength retention rates of NR/M, NR/(M/TiO 2 ), and NR/TDSX were 60.7, 64.9, and 85.1%, respectively. Meanwhile, the elongation at break of the NR/M and NR/ (M/TiO 2 ) decreased to 80.9 and 82.1% of their original states (Figure 7b), respectively, while that for the NR/TDSX composite was barely affected under the UV aging, indicating that TDSX can improve the anti-ultraviolet aging performance of natural rubber and resist the damage of mechanical properties caused by ultraviolet aging.

Conclusion
In summary, a successful synthesis of a multifunctional vulcanization accelerator through an in situ method by  the reaction between CS 2 and TiO 2 nanoparticles under the catalyst of ceric ammonium nitrate, was achieved. The enhanced thermal degradation of (TDSX) enabled it to be used as an efficient vulcanization accelerator for rubber at normal vulcanization temperature (145°C). Besides, TDSX can evenly be dispersed in the natural rubber matrix owing to the strong interface interaction among TDSX and natural rubber (NR) matrix. In addition, the NR/TDSX with enhanced mechanical properties showed good antibacterial effect and anti-ultraviolet aging capability. It is believed that new methods to fabricate eco-friendly and multifunctional vulcanization accelerator may open up new horizons for the development of the rubber industry.
Funding information: The authors are grateful for the financial supports by High-Level Talent Program of Hainan Province (2019RC140) and the National Natural Science Foundation of China (51663008).
Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.

Conflict of interest:
The authors state no conflict of interest.