Thermal-oxidative aging behavior of nitrile-butadiene rubber/functional LDHs composites

Abstract

NBR-based layered double hydroxide (LDH) composites were prepared using a sodium p-styrenesulfonate hydrate (SSS) modified LDH (LDH-SSS) filler through ion exchange. The modified filler, LDH-SSS, was investigated by XRD, FTIR and TGA-DTGA, showing its layered structure was successfully obtained, as compared with the unmodified LDH-NO₃. Mechanical tests of the NBR composites showed that their thermal-oxidative aging was improved, specifically by LDH-SSS. ATR-FTIR was used to study for the aging mechanism of composites. The morphology of the composites revealed by SEM and XRD shows a stronger interaction between NBR and the LDH particles with the presence of SSS. The chemical structure and the thermal properties of the composites revealed by various techniques showed an exfoliation phenomenon of the LDH-SSS, and NBR can be branched with LDH-SSS in the aging process of the composites.

Introduction

Rubber is arguably one of the most useful class of material that finds applications in a broad range of fields, especially where toughness, elasticity and impermeability are required. Practically, rubber is blended with filler to improve the toughness and tensile strength, which provides added durability as well as reduces the cost. In the past a few years, clay nanoparticles [1], [2] have been extensively studied as fillers for rubber because of their unique structures and great potential to improve the mechanical, thermal, and gas barrier properties of the rubber. This has prompted growing research on novel fillers, such as layered double hydroxides (LDH), in elastomer composites in the last two decades [3], [4], [5], [6], [7], [8], [9], [10], [11], [12].

Layered double hydroxides (LDH) are typical crystalline materials, with a general formula of [M^II_1−xM^III_x(OH)₂]^x+(Aⁿ⁻_x/n·mH₂O)^x−, where, M^II is a divalent metal ion, such as Mg²⁺, Ca²⁺, Zn²⁺, and M^III is a trivalent metal ion, such as Al³⁺, Cr³⁺, Fe³⁺, Co³⁺, and A is an anion with valence number of (n and x/n). Due to the fact that LDH possess layered structure, simplicity of synthesis, the structural memory effect and ion exchange ability, and the manageability of particle size and aspect ratio [10], [13], various applications of LDH have been reported in many fields, such as heat/UV stabilizer [14], [15], encapsulating functional drugs or biomolecules [16], [17], heterogeneous catalysis [18], wastewater treatment [19], [20]. Many thermoplastic and thermosetting polymers including polyvinyl chloride [15], [21], polystyrene [22], polyethylene [23], poly(ethylene terephthalate) [24], have been blended with LDH, to give composites. It has been recently reported that LDH can be used as a filler in various elastomeric systems, such as carboxylated nitrile rubber (XNBR) [8], [25], ethylene propylene diene monomer rubber (EPDM) [25], [26], chloroprene rubber (CR) [27], and nitrile-butadiene rubber (NBR) [28], [29].

The properties of LDH-filled rubber depend on the dispersion of the LDH particles in rubber matrix and the interaction between them. However, the homogeneous and stable dispersion of LDH in rubber is still a challenge due to the organic-inorganic incompatibility, which could be improved by organic modification of LDH [29], [30]. For example, LDH can be modified with negatively charged organic species, like lignosulfonate [29], dodecyl sulfate [31], [32], dodecyl benzene sulphonate [32], and stearate anions [33], through anion-exchange because it includes positively charged layers [31]. Currently, many organic anions were intercalated into the LDH interlayer space through anion-exchange reaction.

In general, the research of rubber/LDH composites mainly focused on the dispersion of LDH in the rubber matrix, the mechanical properties [12], [28], [34], the light transmission properties [12], the thermal properties [29], and the rubber vulcanization process [35]. For example, a high-performance NBR/LDH-SSS composites was prepared using peroxide as a curing agent, which had two times higher tensile strength than unmodified NBR without significant loss of elongation [28]. However, the application of NBR is usually limited due to its aging failure at certain severe conditions. For example, the NBR seals cannot be used in deep oil well due to the aging failure at high temperature. To the best of our knowledge, there is nearly no research on the effect of LDH on rubber aging behavior to date.

Thus, in this study, LDH-NO₃ and Sodium p-Styrenesulfonate Hydrate (SSS) modified LDH complex (LDH-SSS) were prepared, followed by the preparation of two NBR/LDH composites using sulfur as the curing agent. The aging behavior of NBR/LDH composites was studied. After heated at 90 °C for 96 h for aging, the morphology, mechanical properties, as well as the thermal-oxidative aging properties of NBR/LDH composites were evaluated. The results showed that the LDH-SSS enhanced the properties of thermal-oxidative aging resistance. In addition, LDH-SSS were further stripped in the aging process, which improved the dispersion of LDH-SSS/NBR in the NBR matrix. On the other hand, the vinyl groups (CHCH₂) in SSS could graft onto NBR molecular chains to promote the interaction between LDH-SSS and the NBR matrix. Moreover, the grafting reaction could increase the saturation of NBR molecular chains, thus reduced chain scission in the aging process. Scheme 1 shows the diagram of two types LDH and the interaction between LDH-SSS and the NBR matrix in the aging process. To some extent, this study provides a new direction for improving the aging resistance property of NBR through the modification of inorganic fillers with organic unsaturation units.