Cure and mechanical properties and abrasive wear behavior of natural rubber, styrene–butadiene rubber and their blends reinforced with silica hybrid fillers

Highlights

• The effect of bagasse ash silica on properties of rubber composites was studied.

• The abrasive wear of rubber composites under dry and wet conditions was determined.

• The bagasse ash silica could be used as reinforcement in SBR and NR/SBR composites.

• The wear of rubbers on a fabric was greater than that on steel and concrete.

• Most rubbers abraded under wet condition showed less wear than under dry condition.

Abstract

This work investigated the mechanical properties of natural rubber (NR), styrene–butadiene rubber (SBR) and NR/SBR blends filled with silica hybrid filler, and their abrasive wear behavior against different counterface materials. The silica hybrid filler included bagasse ash silica (BASi) and precipitated silica (PSi). The BASi content used was 0, 5, 10 and 15 phr, which was incorporated into rubber compounds filled with 20 phr PSi. The wear behaviors of NR, SBR and their blends were examined using an in-house built abrader in a pin-on-plate (POP) configuration; they were tested against various counterface materials, including steel, concrete, and fabric materials under dry and wet conditions. The results suggested that BASi could be used as co-reinforcing filler with PSi for SBR and NR/SBR compounds, resulting in an improvement in the dispersive and distributive properties of PSi agglomerates in the rubber matrix, as indicated by an increase in tensile strength and elongation at break. However, this was not the case for the NR system. The BASi/rubber composites demonstrated greater abrasive wear resistance against a fabric counterface than against steel and concrete counterfaces, respectively. In the case of a fabric counterface, the addition of BASi tended to greatly increase the specific wear rate of the rubbers due to the detachment of fabric and BASi particles acting as three-body abrasive wear. In addition, all rubber composites abraded against steel, concrete and fabric under wet conditions showed less wear than composites under dry condition, except for SBR rubber composites on a concrete counterface.

Introduction

Natural and synthetic rubbers are used in numerous industrial rubber products. Natural rubber (NR) has outstanding tensile and tear strength and good abrasion resistance, qualities which are suitable for production of o-rings, seals, tires, conveyor belts, and so on. Styrene–butadiene rubber (SBR) is usually applied in wear applications, but is inferior in tensile and tear properties. SBR/NR blending and fabric reinforcement of SBR help to improve the mechanical properties [1]. Besides, the tack property of NR is necessary for SBR to adhere to the fabric.

The wear behavior of elastomeric materials can be commonly classified into different types, depending on the mechanism responsible for removal of material from the surface: (1) abrasive wear, (2) fatigue wear, and (3) roll formation [2]. The most severe wear mode, abrasive wear, is caused by the occurrence of micro-cutting and longitudinal scratches from tearing on the tips of sharp asperities. Fatigue wear happens on the blunt asperities by cyclic deformation of the rubber, leading to a small cavitation and then propagating to a definite fracture. The wear by roll formation of the compressed rubber occurs on a smooth surface, which generates wave detachment at the contact area. These waves, known as Schallamach waves, propagate across the contact zone from front to back [2]. The wear mechanism is a complex scenario that depends on many parameters, such as counterface texture, tribo-testing rig, sliding distance, applied normal load, and speed and time of running [[3], [4], [5], [6], [7], [8]]. The effect of rubber failure from wear may appear as shredding, tearing, pulling, and rolling into a curl, accompanied by local irreversible changes in elastomeric properties (e.g. tensile strength, elongation at break, hardness) [2].

Many researchers have studied the friction and wear mechanisms of rubber composites [5,8,[9], [10], [11], [12], [13], [14], [15]]. El-Tayeb et al. [5] reported that unfilled deproteinized natural rubber (DPNR) showed transverse ridges from the coalescence of micro-debris. Meanwhile, the wear due to ridge formation was reduced when carbon black (CB) was added to DPNR. Characterization of the worn surfaces of the rubber compounds containing small particle size and high CB content showed great abrasion resistance and coefficient of friction (COF), as presented by Hong et al. [11]. Felhös et al. [13] indicated that increasing the silica content of hydrogenated acrylonitrile-butadiene rubber (HNBR) enhanced the wear resistance and friction coefficient for a pin (steel)-on-plate (rubber) configuration of tribotesting. Schallamach waves appeared on the worn surface of HNBR filled with low silica content. With high silica content, large agglomerations of debris appeared instead of the Schallamach waves.

Rubber composites having good resistance to abrasive wear usually contain various reinforcing fillers [10]. Carbon black and precipitated silica (PSi) are commonly used as reinforcing fillers for black and non-black rubber products, respectively. The dispersion of precipitated silica in rubber compounds is much more difficult because the high density of silanol groups on the silica surface induces strong filler–filler interaction by hydrogen bonds [16]. Therefore, surface treatment of the silica is essential to improve filler dispersion and the mechanical properties [17,18]. Silica from natural resources, such as fly ash and bagasse ash, are by-products from power plants; these are of interest as an alternative reinforcing filler in rubber because they contain high silica contents, approximately 35% and 75%, respectively [19,20]. Sombatsompop et al. [18,21] found that the mechanical properties of NR/fly ash composites were greatly improved by the addition of bis[3-(triethoxysilyl)propyl]tetrasulfide (TESPT or Si69). The mechanical property-promoting mechanism is thought to be due to C–S bonding with rubber molecules, and siloxane linkages with the fly ash silica in the NR/fly ash composites. The optimum Si69 contents of around 2.0 and 4.0 wt% for treatment of fly ash particles were found to enhance the mechanical properties of NR and SBR, respectively. Kanking et al. [20] found that bagasse ash could potentially be used as alternative filler in NR compounds, with a recommended loading of not greater than 15 phr. Lower tensile strength of the vulcanizate with bagasse ash, as compared with PSi and CB fillers, was due to the poor dispersion level and an aggregation of the silica particles in the rubber matrix. In a hybrid filler system, bagasse ash could be used to replace both PSi and CB fillers – up to 75% of the total hybrid filler, but not exceeding 15 phr of BASi filler. The results of ultimate mechanical strength and swelling measurements clearly indicated that bagasse ash had better compatibility with PSi filler than with CB.

Based on a literature review, a number of studies have investigated the mechanical properties of silica hybrid-filled rubber composites; but the wear properties of rubber composites, which are useful in applications for seals, tires, and rubber rollers, have not yet been examined. In light of this, we investigated the mechanical and wear behavior of NR, SBR and NR/SBR blends filled with silica hybrid filler under abrasive wear against various counterface materials, such as steel, concrete and fabric, in dry and wet conditions. The silica hybrid filler included bagasse ash silica (BASi) and precipitated silica (PSi). The silicas were treated with Si69 silane coupling agent at 2% by weight of silica. The BASi content used was 0, 5, 10 and 15 phr, which was incorporated into rubber compounds filled with PSi at 20 phr.