Facile and sustainable functionalization method for preparing sp2 carbon allotropes with different solubility parameters

sp2 carbon allotropes are known as efficient reinforcing fillers for rubber materials: not only carbon black, but also nanofillers such as carbon nanotubes, graphene and graphene related materials. They can be used as such in rubber compounds and promote faster sulphur based vulcanization and substantial enhancement of mechanical properties. However, it is widely acknowledged that the introduction of functional groups on sp2 carbon allotropes allows to dramatically enlarge their properties portfolio and hence their impact on the properties of rubber materials. In the frame of a research activity aimed at generating innovative materials from renewable sources through sustainable chemistry, a facile and sustainable functionalization method, suitable for all the families of sp2 carbon allotropes, has been developed.1-3 Primary amines were combined with α,δ dicarbonyl compounds and the resulting molecules were able to functionalize sp2 carbon allotropes, with high efficiency. Indeed, the reactions, performed by simply donating either thermal or mechanical energy, were characterized by a yield up to 96% and by an atom efficiency up to 80%. The structure of the graphitic substrates remained substantially unaltered, after the reaction. Stable dispersions of functionalized carbon allotropes were prepared in solvents with different solubility parameters. From the qualitative results of such dispersion tests, estimate of Hansen solubility parameters of carbon allotropes was done by defining the Hansen sphere for the functionalized allotropes. Objective of such approach is to predict the solubility parameter of carbon allotrope, as a function of the functionalizing molecule, designing the functionalization reaction. Compatibility of the carbon allotrope with different environments, such as rubber phases, can be tuned. Rubber compounds were prepared, based on carbon black and/or silica, with functionalized carbon black. The effect on vulcanization behavior and mechanical reinforcement is discussed. The filler networking phenomenon was in particular investigated, correlating type and amount of functional groups with values of dynamic moduli and Payne effect.