Switchable hydrophobic-hydrophilic layer obtained onto porous alumina by plasma-enhanced fluorination

Abstract

Conventional lithographic printing processes using porous alumina for offset applications generally use “wet” routes. Recently “dry” processes have been developed which are based on a heat-induced hydrophilic/oleophilic conversion of one or more layers of the coating so that a stronger affinity towards ink or water fountain is created at the exposed areas with respect to the surface of the unexposed coating. Treatments involving rf plasma-enhanced fluorination (PEF) constitute exceptional tools for modifying the surface properties of materials. Many advantages of these techniques can be indeed outlined, when compared to more conventional methods: room-temperature reactions, chemical modifications limited to surface only without changing the bulk properties, possible non-equilibrium reactions. The influence of PEF treatments on porous alumina layer used in printing plates has been tested with various fluorinated gases (CF₄, C₃F₈ and C₄F₈) and characterized by XPS. The hydrophobic properties of the fluorinated layer have been deduced from contact angle measurements. Using C₄F₈ rf-PEF treatment, the outmost surface of the hydrophilic alumina substrate used for lithographic printing is hydrophobized, or in other words, the hydrophilic substrate is converted into a support with hydrophobic properties. Once being hydrophobized, the surface layer may be rendered hydrophilic using a heat pulse, thus giving rise to switchable hydrophobic-hydrophilic properties of the material.