Processing effects on the kinetics morphology and properties of hybrid sol-gel superhydrophobic coatings

Abstract

Hybrid superhydrophobic sol-gel coatings were synthesized and applied on glass substrates. Low surface energy was obtained by using hydrophobic organo-functionalized silanes and roughness was formed by alkyl modified silica nanoparticles (NPs). However, these superhydrophobic coating (SHCs) are known to exhibit lack of interfacial adhesion of the NPs and limited interfacial adhesion with the underlying substrates, as demonstrated by their ease of removal in harsh environments. In this study, hybrid hydrophobic sol-gel and hydrophobic silica NPs based SHCs were prepared with the objective to obtain enhanced interfacial adhesion both between the silica NPs and the sol-gel matrix and between the sol gel matrix and glass substrates. Consequently, three curing processes were studied: Thermal, radiation, and dual curing. Dual-cured SHC presented the highest mechanical and environmental durability. It retained its superhydrophobicity (SH) for 17 tape peels and for 1000 h under accelerated weathering conditions. Failure analysis indicated that strong interfacial bonding was formed at the interface between the alkyl modified silica NPs and the hybrid hydrophobic sol gel matrix due to the interfacial free radical chemistry as well as between the sol gel coating and the glass substrate due to condensation chemistry. Moreover, the dual-cured hybrid sol-gel demonstrated transparent SHCs with over 90 % optical transmittance, water contact angle (CA) of 160˚ and a correspondingly sliding angle (SA) of ∼0˚. Morphological and roughness analyses revealed a uniform, hierarchic structure consisting of micro- and nano-sized asperities with Gaussian peak density distribution, like the Lotus leaf surface. According to ATR-IR analysis, dual curing process combines the mechanisms of chain polymerization of functional vinyl silane by radiation and thermal condensation polymerization of fluoro-silane to form a siloxane network. Hence, only chain or only condensation mechanisms lead to partial crosslinked network and low interfacial adhesion of the NPs to the sol-gel system. The dual-cured coating's failure appeared to be more ductile, compared to the brittle fracture and the delamination that were observed for thermal or radiation-cured coatings, respectively.