High aspect ratio nanofillers for rubber composites

Reinforcing fillers are essential components of rubber composites, as they allow to achieve the mechanical properties required by composites’ applications. High aspect ratio fillers are of great interest, as the mechanical reinforcement increases with the aspect ratio. Over the last decades, nanometric high aspect ratio fillers have become available: clays, carbon nanotubes, graphene and graphene related materials. They can be dispersed in the polymer matrix at the level of their primary particles and, thanks to the nanosize and the high aspect ratio, they are able to establish large interfacial area with the polymer. Traditional fillers such as carbon black and silica are nanostructured: their primary particles have at least one dimension below hundred nanometers and are fused together to form aggregates with length of at least 200 nm. In principle, nanometric fillers should promote much larger mechanical reinforcement with respect to the nanometric ones. However, a key aspect to be investigated is the ability of a reinforcing filler to give rise to a stable interaction with the polymer matrix. Carbon black and silica have a “structure”, that means they have voids able to accommodate the polymer chains, restraining their mobility and promoting a firm interaction. Do nanofillers have structure? Are clays really able to accept polymer chains between their layers? And what about carbon nanotubes and graphitic nanofillers? Moreover, silica has functional groups able to react with chemicals such as silanes, coupling agents with the polymer chains. Have nanofillers suitable chemical reactivity for promoting chemical bonds with the polymers? This contribution is organized in two main parts. In the first one, the subjects reported above are discussed. Then, chemical modification of nanofillers is presented and is discussed how such functionalization can promote a better interaction and/or a coupling with the polymer chains.