**1. Introduction**

Rubber is one of the most important polymer materials playing an essential role in dynamic applications like tires, belts, diaphragms, and seals. Rubber products are continuously subjected to repeated cyclic deformation, according to the service conditions. In most areas, due to the working conditions like rubbing, abrading, chunking, and tearing the product undergoes mechanical failure very easily. Such failure mainly occurs due to continuous friction, wear, abrasion, and crack formation. Friction and wear often occurs when two surfaces contact each other [1]. Movements between the two contacting surfaces cause cutting or damage of the rubber surface, usually known as abrasion [2]. The continuous cutting of the soft rubber material generates small cracks, which propagates faster when the rubber product is subjected repeatedly to dynamic working conditions. The mechanical performance of rubber composites depends on several material parameters like type of elastomers, type of filler, filler content, filler size and structure, state of filler dispersion, interaction between polymer and filler, type of plasticizer, and the nature of crosslinks and its density [3]. Natural rubber is one of the main rubbers used in truck tire applications due to its excellent self-mechanical reinforcement and crack growth resistance, which are attributed to its strain induced crystallization behavior [4,5]. Incorporation of fillers like carbon black and precipitated silica into the natural rubber further improves the reinforcement characteristics. In recent years, novel nanofiller systems such as layered silicates, carbon nanotubes, or graphene nanoplatelets have been emerging, which, due to their nanoscale and

their special aspect ratios, o ffer reinforcements beyond carbon black or silica [6–12]. Non-crystallizable rubbers like styrene butadiene rubber and acrylonitrile butadiene rubber have no self-reinforcement behavior. Therefore, the rubbers display poorer tear strength and crack propagation characteristics. These synthetic rubbers do not have the ability to crystallize during mechanical deformation [13]. Technically, for such non-crystallizable rubbers, the reinforcement is mainly enhanced only via a proper selection of fillers [14–16].

Carbon blacks are the major class of reinforcing materials for rubbers fulfilling the requisite properties for more than a century due to its ability to o ffer outstanding mechanical performance [17,18]. However, in the last few decades, precipitated silica is emerging as a potential candidate and has partially or fully replaced carbon black, mainly due to lower rolling resistance and improved wet grip properties in tire applications despite sensitively varying tear strength values and crack resistance properties [16,19]. However, poor nano/microscale dispersion and interaction with rubber is one of the critical issues encountered with silica and is strongly influencing the failure properties of rubber products. However, these issues have been overcome through multi-stage mixing and by incorporating suitable silane coupling agents. Scientifically, several assumptions are believed about the rubber product failure when rubbers are subjected to the mentioned severe working conditions. Some of them are: (1) molecular chains attempt to move in the stressed direction, causing chains slippage and reorientation, (2) crosslinks, entanglements, and filler particles hinder chain motion, which leads to a state of tension and local chain scission occurs, (3) scission of single polymer chain transfers the load to the neighboring chains, which propagates and the cumulative e ffect produces microvoids, and, lastly, (4) the microvoids grow to from microcracks, which is an irreversible process and these microcracks propagate and, ultimately, cause product failure [19].

For the current study, silica filled rubber composites are prepared by two di fferent methods. First, commercial precipitated silica is directly incorporated into the rubber and next silica particles are generated inside the rubber by the sol-gel method [20]. More details concerning the preparation of sol-gel silica inside the rubber could be found in our earlier work [21]. The volume fraction of silica in both nanocomposites is the same and is confirmed through thermogravimetric analysis [21]. The particle morphology (size and structure), crosslink density, and state of silica dispersion of the prepared SSBR composites are entirely di fferent. The synthesis of in-situ silica in the polymer matrix leads to trapping of polymer chains inside the silica aggregates as well as strong linkage of the polymer molecules at the silica surface. This can be considered as additional network nodes and is responsible for enhanced reinforcement of the rubber without a silane coupling agent. Usage of silane results in a further increase of the physical properties of the composites with in-situ silica due to improved silica-rubber interaction. Incorporation of a masterbatch containing in-situ silica particles leads to commercial precipitated silica to (a) enhanced silica dispersion, (b) possible higher silica loading in the elastomer matrix without impairing its processability, (c) the possibility to control and customize the particle size, and (d) energy-e ffective processing due to easy incorporation of the filler particles.

The main interest is to study how the filler size, structure, and crosslink density of the two di fferent silica filled vulcanizates contribute to friction, abrasion, and crack propagation properties.
