Elsevier

Polymer

Volume 188, 3 February 2020, 122143
Polymer

Antioxidation behavior of bonded primary-secondary antioxidant/styrene-butadiene rubber composite: Experimental and molecular simulation investigations

https://doi.org/10.1016/j.polymer.2019.122143Get rights and content

Highlights

  • A new bonded primary-secondary antioxidant MBC was prepared by thiol-ene reaction.

  • Antioxidant MBC presented well-dispersion and low migration in SBR matrix.

  • Antioxidant MBC greatly enhanced the thermo-oxidative resistance of SBR composite.

  • The effects of MBC on aging of SBR were deeply recognized by molecular simulation.

Abstract

In this work, a novel bonded primary-secondary antioxidant named as MBC was synthesized successfully via thiol-ene addition reaction of primary antioxidant N-(4-anilino phenyl) maleic imide (MC) with secondary antioxidant 2-mercaptobenzimidazole (MB). Then, the antioxidation behavior of MBC in styrene-butadiene rubber (SBR) matrix was systematically investigated using experiments and molecular simulation. The results showed that MBC diminishes the negative impact of MC on the cross-linking density of SBR composite. MBC presented better dispersion and lower migration than MC in the SBR matrix. Furthermore, MBC containing aromatic amino and thioether groups not only capture peroxy radical but also decompose hydroperoxide. Hence, compared with MB/SBR or MB/MC/SBR composite, MBC/SBR composite exhibited outstanding mechanical properties and good thermo-oxidative stabilization during the aging process, indicating that MBC could be a promising antioxidant for the preparation of high-performance SBR composite. Moreover, molecular simulation can provide deeper insights for designing the molecule structure of effective antioxidants.

Introduction

In many fields, elastomer is a type of important and irreplaceable material owing to remarkable high elasticity [1]. Among the elastomer materials, diene rubbers, such as styrene-butadiene rubber (SBR), natural rubber (NR), butadiene rubber (BR), and acrylonitrile-butadiene rubber (NBR), have been widely applied in a variety of industrial products, such as tire and latex commodities. However, because of the isolated double bond structures (Cdouble bondC groups) and the active allylic hydrogens in the molecular chain, these diene rubbers are prone to aging when exposed to heat, oxygen, light, and ozone conditions [2,3]. Particularly, the thermo-oxidative aging, one of the most common aging types, can significantly disrupt the physical and chemical properties of rubber materials, leading to premature failure of rubber products [4]. Consequently, it is essential to extend the service life of rubber materials via improving thermo-oxidative aging resistance.

In rubber formulae, antioxidants are a necessary additive to retard the aging process of rubber products. Nevertheless, most antioxidants with low molecular weight easily migrate from the rubber matrix especially at high temperatures or in liquid, thereby reducing the antioxidative efficiency of antioxidants [5], and even the escaped antioxidant can pollute the surrounding environment [6]. Therefore, some new strategies for inhibiting the migration of antioxidants have been proposed, e.g., increasing the molecular weight of antioxidant [[7], [8], [9]], slowly releasing antioxidant from hollow materials (e.g., halloysite and carbon nanotube) [10,11], and bonding antioxidant onto the filler surface [[12], [13], [14]].

With respect to increasing the molecular weight of antioxidant, Wu et al. [8] prepared two novel macromolecular hindered phenol antioxidants containing thioether and urethane groups, and their antioxidative efficiency was superior to corresponding low molecular weight antioxidant in NR composite. Wang et al. [7] synthesized a sulfur-bearing hindered phenolic antioxidant SAO with a high molecular weight, and the oxidation induction time (OIT) value of SAO/polypropylene (PP) was higher than that of PP using Chinox 1035 (a commercial stabilizer) given the slower evaporation of SAO. Yves Buchmüller et al. [15] reported that polymer-bound antioxidants can effectively mitigate oxidative aging of the polymer electrolyte in a fuel cell. Antioxidants are typically classified as primary antioxidants and secondary antioxidants according to their respective roles. Primary antioxidants, including derivatives of aromatic anime or phenol, scavenge peroxy radicals by providing hydrogen atoms, while secondary antioxidants, typically sulfur- or phosphorous-based compounds, can decompose hydroperoxide to restrain the autocatalytic oxidation process [16,17]. When primary and secondary antioxidants are simultaneously used in rubber formulae, the thermo-oxidative stabilization of rubber can be dramatically improved due to a synergistic effect. Thus, chemically bonding primary and secondary antioxidants to prepare high molecular weight antioxidants might be a promising method to restrict antioxidant migration and improve its protective efficiency in a rubber matrix.

On the other hand, with the fast development of computer technology in recent years, the structure-property relationship of materials has been extensively explored by multiscale molecular simulation [18]. On the electronic scale, quantum mechanics (QM) simulation can contribute one to uncovering the chemical reaction mechanism by computing the energy barrier of reaction for the transition state (TS) [19]. On the molecular scale, molecular dynamic (MD) simulation has been broadly employed to calculate many physical parameters [20,21], such as fraction of free volume (FFV), solubility parameter (δ), interaction energy, radial distribution function (RDF), and glass transition temperature (Tg). For example, Zhang et al. [22] comprehensively computed the elementary reaction energy of ethanol formation from syngas on the cuprum catalyst and verified the optimal reaction route using density functional theory (DFT) method. Jabeen et al. [23] calculated the reaction barrier for three different vitamins towards the short-lived oxygen centered radicals (·OH and ·OOH), thus predicting their antioxidant activity. Zheng et al. [18] roundly investigated five factors influencing the selection of antioxidants in BR composite using experiments and molecular simulation, and the relative importance of the five internal factors was clarified by gray relational analyses. In this work, molecular simulation hence serves as a useful theoretical tool to help us better understand the antioxidation behavior of bonded primary-secondary antioxidant in rubber matrix, and relevant investigations are rarely reported.

In view of the desirable features of thiol-ene addition reaction (such as simplicity, high efficiency and selectivity, and mild reaction conditions) [4], an industrialized primary antioxidant N-(4-anilino phenyl) maleic imide (MC) containing Cdouble bondC group and secondary antioxidant 2-mercaptobenzimidazole (MB) with S–H group were used to prepare a high molecular weight antioxidant, i.e., the bonded primary-secondary antioxidant (MBC). Then, the thermo-oxidative aging resistance of MBC/SBR composite was evaluated based on the temperature at oxidation exothermic peak (TO) and the change in mechanical properties during the aging process. Finally, combining experiments with molecular simulation, the antioxidation behavior of MBC in the SBR matrix was systematically discussed from the view of antioxidation mechanisms (hydrogen dissociation energy, reaction energy barrier), dispersion state (solubility parameter δ), and migration ability (MSD, interaction energy) of MBC.

Section snippets

Materials

Primary antioxidant N-(4-anilino phenyl) maleic imide (MC) was supplied by Xianyang Sanjing Trade and Industry Co., Ltd, China. Secondary antioxidant 2-mercaptobenzimidazole (MB) and catalyst triethylamine (TEA) were provided by J&K Scientific. Tetrahydrofuran (THF), methanol, and chloroform were purchased from Beijing Chemical Works. Styrene-butadiene rubber (SSBR 2466) was manufactured by TSRC Corporation. Other rubber ingredients, including zinc oxide (ZnO), stearic acid (SA), sulfur (S),

Quantum mechanics (QM) simulation

In the light of the antioxidation mechanism of primary antioxidant (AH) in Fig. 2, AH can provide a hydrogen atom (H·) to terminate the peroxy radical (ROO·), thereby weakening the reaction between ROO· and polymer chain (RH), i.e., retarding the aging of polymer materials [26,27]. Accordingly, the dissociation of active hydrogen in the antioxidant or rubber chain is very significant for understanding the antioxidation mechanism. On the other hand, a specific chemical reaction normally involves

Chemical structure of MBC

From the FT-IR spectra of MC, MB, and MBC in Fig. 4(a), the peak at 908 cm−1 that belonged to the = C–H deformation vibration of MC disappeared in the FT-IR spectrum of MBC. Compared with the Cdouble bondO group at 1757 cm−1 in MC, the absorption for the Cdouble bondO group presented a redshift phenomenon in MBC, i.e., from 1757 to 1709 cm−1. This feature was attributed to the fact that the conjugate effect between Cdouble bondC and Cdouble bondO groups vanished when the Cdouble bondC structure was subject to the thiol-ene addition reaction.

Conclusions

In this research, through the thiol-ene addition reaction between antioxidants MB and MC, a novel bonded primary-secondary antioxidant MBC was synthesized successfully. The effects and mechanisms of MBC on anti-aging performance for SBR composite were studied via experimental and computational methods. The main conclusions were summarized as follows.

  • (1)

    The antioxidant MBC could decrease the negative effect of MC on the vulcanization of rubber, thereby improving the processing efficiency and

CRediT authorship contribution statement

Kaiqiang Luo: Methodology, Data curation, Investigation, Writing - original draft. Guohua You: Software, Methodology. Sidian Zhang: Methodology, Investigation. Wei Zheng: Writing - review & editing, Supervision, Methodology, Investigation. Sizhu Wu: Conceptualization, Methodology, Software, Validation, Writing - review & editing.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 51873017).

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