Direct functionalizing of acrylonitrile-butadiene rubber surfaces through different peroxide curing
Introduction
Oxygen-containing functional groups have the potential for surface functionalization through their reactivity as anchoring points for the formation of a wide variety of nanometer-sized structures [1]. In the fields of micro-nano electronics, solid electrolytes, computer chips, and solar cells, oxygen-containing functional groups are important for their high chemical activity for the fabrication of functional surfaces or induction of a functional molecular layer [2,3].
It is well-known that inorganic materials including metals, ceramics, and glasses contain oxide surfaces. To date, the utilization and control of the oxide surfaces of these materials have been widely actively investigated and applied [4,5]. For many surface inactive polymer materials, oxygen-containing functional groups can be obtained using a variety of methods as secondary processing surface treatments after material processing. To date, methods to form the functional groups on polymer surfaces include chemical coating, physical modification, and various combinations of these methods. Because the chemical coatings require active points to introduce the coating materials, commonly the physical surface modifications such as UV [7], corona [8] and plasma charging [[9], [10], [11]] are used after shaping to form active points of hydrogen-bonding, dipole groups, as well as oxygen-containing and other reactive functional groups. However, the energies of UV [12], corona (1–10 eV) [13,14], and plasma (>10 eV) [15], are all stronger than the chemical bond energy of CC/CH bond. Thus, during these treatments active sites are obtained by disrupting surface chemical bonds, and the surface damage observed in polymers is accompanied with additional radiation modifications [16]. Moreover, as the size of micro-nano devices continues its downward scaling and components become more complex, novel methods for the fabrication of oxygen-containing functional groups on resin and rubber surfaces must be developed to meet these challenges.
A common chemical methodology for curing of resin and rubber materials uses various organic peroxide curing agents. These chemicals are widely used in industrial rubber curing due to their beneficial effects on the reduction of gas permeability, short cross-linking time, simple compounding, heat aging resistance, permanent stretching, and small strain [[17], [18], [19]]. In the simplest and ideal scenario, oxidation reactions have been mainly investigated, showing that peroxide curing exclusively produces CC bonds during polymer chains through free-radical reaction [6]. Moreover, it has been demonstrated that the peroxide curing is intricacy due to the additional reactions, which usually occur simultaneously during curing and must be considered. Additionally, it is well-known that the polymer backbone structure, curing temperature, and peroxide concentration significantly affect the final network structure formed after peroxide curing [16]. However, to the best of our knowledge, the peroxide effects on the chemical structures of cured material bulk and surfaces remain unclear. Importantly, curing and additional reactions of organic curing agents during rubber processing may play an important role in shaping or curing complexities associated with molecular structure changes from the monomer to a 3D stereo-structure, affecting the characteristics of the bulk and the surface. In this regard, it is desirable to explore the radical reactions of peroxides during curing of material surfaces such as rubber. In the previous report [20], we used different mold materials during the rubber curing to control the surface functional groups. Surface functionalizing can be achieved without changing the formulation of the rubber, but the mold needs to be changed to satisfy different needs. Owing to the continuous interest in radical chemistry and oxide functional groups, recent progress in this field without changing the molds is reported herein.
The effects of different types of peroxides on surface functional groups during curing were investigated. The functional groups on the cured acrylonitrile butadiene rubber (NBR) surface were evaluated by X-ray photoelectron spectroscopy (XPS). Three peroxides, di-tert-hexyl peroxide (perhexyl D), tert-butyl cumyl peroxide (perbutyl C), and dicumyl peroxide (DCP) were used for curing NBR. The influences of peroxide type on the NBR curing properties and chemical composition were studied. In addition, the internal and surface reactions of NBR upon exposure to different peroxides during curing are discussed. This study provides novel insight into peroxide reactions and oxidation that occur on rubber surfaces to generate functional groups and should be of significant value for expanding rubber applications, especially in the fields of rubber integration [21].
Section snippets
Materials and reagents
The acrylonitrile-butadiene rubber (NBR) used herein was obtained from JSR Corporation Japan (AN: 35%, N230S, JSR Co.). Zinc oxide and stearic acid were obtained from Wako pure chemical industries, Ltd. Japan. The three peroxides, perhexyl D, perbutyl C, and DCP (synthesis grade) were purchased from NOF Corporation. Japan. The characteristic values of peroxides, including their active oxygen contents, residual rates, and activation energies are listed in Table 1. The thermal decomposition rate
Effect of the peroxide type and curing temperature on the elemental composition of the rubber surface
Generally, sulfur-curing packages are used if dynamically elastic and flexible properties are desired, such as in tire tread formulations [17]. Compared to sulfur curing agents, the peroxide curing system is useful when other properties such as sealing, compression set, and functionalization are necessary, especially in EPDM, SBR, and NBR curing. Herein, three peroxides were used for NBR curing to create functional groups on the cured NBR surfaces during curing without secondary processing.
Conclusions
A method was used to fabricate oxygen-containing functional groups on NBR surfaces through primary processes of curing using 3 types of peroxides at different curing temperatures. The surface and interior reactions during curing with perhexyl D, perbutyl C, and DCP with alkyl or/and benzyl union were discussed. The cumyloxy radicals preferentially underwent transfer reactions leading to a high extent of curing. Additionally, the cumyloxy radicals can react with nitrile at 180 °C on the NBR
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
This work was supported by CSTI and “Innovative Design/Manufacturing Technologies” of SIP (0930001). We gratefully acknowledge the Ministry of Economy, Trade and Industry of Japan for funding support for this research.
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