Mesoporous silica as nanocarrier of antioxidant for highly anti-aging elastomer composites

Highlights

• Mesoporous silica nanoparticles (MSNs) were firstly utilized as the delivery vehicles of rubber antioxidant.

• 2-mercaptobenzimidazole (MB) was loaded into MSNs to fabricate highly anti-aging elastomer composites.

• MSNs-MB endowed SBR composites with long-term thermo-oxidative stability.

Abstract

Mesoporous silica nanoparticles (MSNs) were firstly utilized as the delivery vehicles of antioxidant 2-mercaptobenzimidazole (MB) to fabricate highly anti-aging styrene-butadiene rubber (SBR) composites. The thermo-oxidative aging behaviour of SBR composites was systematically evaluated by mechanical testing, FTIR and XPS. For the composites with MSNs-MB and free MB, though some of free MB were consumed or migrated to the elastomer surface, the released MB from MSNs-MB steadily migrated to the free rubber phase to ensure a continuous and stable antioxidant supply for a long time. Therefore, the SBR composites with binary protection from MSNs-MB and free MB exhibited superior thermo-oxidative stability than the SBR composites with only free MB. This work proposed a reliable approach to resolve the “blooming” defects of antioxidants and extend the service life of the elastomer products, which may open up a new route for the rational design and construction of highly anti-aging elastomer composites.

Introduction

Elastomers, as a kind of strategically important materials, have been widely employed in the applied polymer industry owing to the numerous remarkable properties, especially high entropic elasticity [[1], [2], [3]]. However, most of elastomers will be variable sticky or stiff when greatly suffering from harsh chemical and physical circumstances such as heat, oxygen, radiation, and stress, which severely deteriorates the chemical and mechanical properties of elastomer composites [4,5]. Some currently used commercial antioxidants can suspend elastomers aging to some extent, but unfortunately, these low molecular weight additives are subjected to volatilize and migrate to the surface of rubber products in a phenomenon called “blooming” [[6], [7], [8], [9]]. Even worse, blooming can lead to earlier failure of the elastomer composites, poor appearance performance and severe environmental pollution [10,11]. To circumvent these defects and prepare highly anti-aging elastomer composites, some scientists have devoted their research to prevent the mobility of antioxidants by chemically grafting the low molecular weight antioxidants on the surface of inorganic filler [[12], [13], [14], [15]]. However, the chemical grafted antioxidants can only perform the optimistic effect on the elastomers approaching filler, the comprehensive properties of residual free elastomer remains unsatisfactory.

Sustained-release systems have sparked tremendous interest from pharmaceutical and biomedical scientists due to their outstanding applicability in drug delivery area [[16], [17], [18], [19], [20]]. Inspired by this, scientists began to explore the applications of sustained-release systems in other fields such as pesticide, catalyst, enzyme and polymer additives [[21], [22], [23], [24], [25]]. Typically, the feasibility of antioxidants sustained-release in elastomer composites has been verified. Fu et al. firstly proposed a strategy to construct the rubber antioxidant controlled-release systems by using the antioxidant N-isopropyl-N′-phenyl-p-phenylenediamine (4010NA) loaded inside of halloysite clay nanotubes (HNTs), and prepared highly aging-resistant rubber composites [7]. After the incubation at 90 °C for 7 days, the SBR composites filled with antioxidant-loaded halloysite exhibited no surface blooming and well retained mechanical properties. Similarly, carbon nanotubes (CNTs) were also introduced to perform the same function on sustained-release of antioxidants [10,25]. Zhang et al. fabricated CNTs/phenolic antioxidants to effectively avoid the blooming of antioxidants and improve the aging resistance of polar acrylonitrile-butadiene rubber (NBR) composites [10]. These researches have efficiently laid the foundation for rubber antioxidant controlled-release and extended the service life of rubber products. Nevertheless, the poor reinforcement effect of HNTs and color contamination of CNTs in elastomers largely restrict their applications. Therefore, it is urgent to develop novel antioxidant sustained-release systems by using the light color nanocarrier with outstanding reinforcement effect.

Mesoporous silica nanoparticles (MSNs), acknowledged as promising nanocarrier without color contamination, have been a target of intensive research due to unique mesoporous structure and high surface area, especially in drug and catalyst delivery systems [[26], [27], [28], [29], [30]]. Besides, it has been confirmed in our previous work that mesoporous silica can greatly improve the mechanical properties of SBR composites [31]. Nevertheless, MSNs have to date never been utilized as sustained-release nanocarrier of rubber antioxidants to enhance the anti-aging behaviour of elastomer composites.

In this paper, to strengthen the long-term thermal-oxidative stability of styrene-butadiene rubber (SBR) composites, a novel MSNs-based antioxidant sustained-release system was firstly designed by loading the 2-mercaptobenzimidazole (MB) into MSNs nanocarrier. The thermo-oxidative aging behaviour of SBR composites was systematically investigated by mechanical testing, Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy.