Processing and properties of a graphene-reinforced superhydrophobic siloxane

Three-dimensional superhydrophobic materials are characterized by a low surface energy and extremely low strength, and hence require reinforcement for viable applications. An experimental study is presented of the processing and mechanical properties of a graphene-reinforced superhydrophobic siloxane, synthesized by a sol–gel approach integrating alkali activation of metakaolin, hydrolysis of alkoxysilane, dispersion of graphene into the precursor, and co-condensation. To promote uniform dispersion and distribution of graphene, three processing techniques were used: while ultrasonication was adopted to disperse graphene nanoplatelets, accelerated condensation at 50 and 75 °C and varying the precursor's viscosity used to prevent graphene from floating and re-aggregation. Results of nanoindentation, porosimetry, and unconfined compression show that adding 0.9 wt% graphene increases the strength of the superhydrophobic composites from ∼ 0 to 34 MPa. Slow condensation and curing at 25 °C allow graphene to re-aggregate and float upward in the sol, leading to its heterogeneous distribution. Despite its function in improving microscale dispersion, ultrasonication detrimentally decreases the composite's strength due to acoustic cavitation. Similarly, curing at elevated temperatures accelerates co-condensation and results in a more uniform distribution of graphene, but induces thermal cavitation and bubble formation, because the threshold for acoustic and thermal cavitations is significantly reduced by superhydrophobicity.