68 Recent Patents on Mechanical Engineering 2008, 1, 68-71 1874-477X/08 $100.00+.00 © 2008 Bentham Science Publishers Ltd. Active Coatings: Examples and Applications Alexandre F. Galio* and Iduvirges L. Müller Rio Grande do Sul Federal University, 90040-060, Porto Alegre/RS, Brazil Received: September 3, 2007; Accepted: November 8, 2007; Revised: November 26, 2007 Abstract: Currently, electrochemical plating, conversion coatings, anodizing, deposition of thin films (organic, inorganic or hybrid), laser surface alloying, cladding and organic coatings are employed to improve surface properties such as hardness or corrosion resistance. For example, the development of new “smart” coating systems implies not only mechanical covering of the protected surface with a dense barrier coating but also provides active properties which can allow continued physical or chemical protection, even after partial mechanical damage of the coating, by self-healing effects. In other cases, the active properties imply biological effects, by releasing of chemicals from “drug delivery” systems, or photoelectric effects due to doping the surface with adequate elements. These “smart” properties can be achieved by inclusion of specific additives into the coating system and can stimulate the superficial reactions after some damage. The sol-gel coating process is one of the most promising methods to create protecting coatings producing sensitive inorganic surfaces in general. In this environmentally friendly process, inorganic or inorganic-organic hybrid polymers are obtained by controlled hydrolysis and condensation reactions of molecular precursors. This article compares examples of the recent patents of active coatings and some of their important industrial utilizations. KEY WORDS: Surface treatments, active coatings, hybrid polymers. sol-gel processes. 1. INTRODUCTION In recent years, design of active coatings such as “artificial cells” systems have produced materials that can facilitate the healing of damaged substrates. This may be exemplified by a sol- gel system with polymer nanocontainers used as reservoirs of active nanoparticles. After a punctual damage caused by a mechanical impact, the polymeric nanocontainers crack and release a corrosion inhibitor to fill the affected zone and induce a self healing effect over it. Scientific research on “smart” coating systems is still in its initial stage. However, progress in this field is ultimately increasing the fabrication of a generation of adaptive materials that both monitor their structural integrity and mend themselves before any catastrophic failure can occur [1]. Thus, a quick search in the Patent's Worldwide database of the European Patent Office found 2074 results for the words “active coatings” in the title or abstract, however only 1612 patents are claims the properties of sol-gel chemical composition. Almost all of them were claimed in the XXI century, pointing to the current interest on coating systems with “smart” behaviour. Initially, works were concentrated on self- healing effect for corrosion protection, but actually other properties are being added into these active coatings systems, like biological activities or photoelectric effects, for example. A challenge in materials science is to design active synthetic systems that can improve this behavior, by not only feeling the presence of a defect or of localized environmental changes, but also by re-establishing the integrity and continuity of the damaged area or increasing the photoelectric effect due to light irradiation alterations. Such “smart” coatings would significantly extend the lifetime and utility of a vast array of manufactured items. The coatings should undergo repair in a relatively autonomous manner, without an external intervention. Furthermore, the coatings should ideally be able to repair damage more than once, so as new cracks or fissures appears. These self-healing effects would make the coatings ideal for surgical implants, aerospace and automotive applications and allow the fabrication of structures with enhanced sustainability. The increasing of the number of papers and patents that have appeared dealing with bulk and more sophisticated, micro- or nanostructured materials, focused on the synthetic approach or on *Address correspondence to this author at the Rio Grande do Sul Federal University, 90040-060, Porto Alegre/RS, Brazil; E-mail: afgalio@ufrgs.br the specific application, in principal on hybrid organic-inorganic coatings with “smart” behaviour. Such interest arises from the several unique features of these materials, which is often related to the increasingly important role played by the interfacial forces and by the chemistry of surface molecular layers as the size of the dispersed phase decreases [2]. In general, the inventors claim the intellectual properties as the chemical composition as the production procedures of the coatings with some specific properties. The mechanical, adhesive, cohesive, electrical, optical, photochemical, catalytic and magnetic properties of these new hybrid materials are often a synergistic combination or utterly new ones with respect to those of the constituents. Thus, organic polymers with improved toughness [3], elasticity, low surface energy [4], enhanced hardness [5], hydrophilic or hydrophobic properties [6-8], durability against ultra violet light (UV) and heat resistance, availability of reactive functional groups or catalytically active host sites, can be produced by diffusion, inclusion or dispersion of an inorganic component, where the coordinative or covalent binding provide the required stabilization of incompatible phases with large interface area. Entirely new materials and applications can be attained with respect to the traditional ones, as in the case of the combination of typical molecular properties, from electrical, photochemical, magnetic, catalytic, biological activity with the adjustable mechanical ones of organic and inorganic polymers and networks [2]. Sol-gel processes are generally used to fabricate hybrid organic- inorganic materials including self-assembled films. A sol is a liquid solution containing a colloid suspension of a material of interest dissolved in an appropriate solvent. Condensation reactions between the dissolved precursor molecules result in structures forming within the sol. The size, growth rate and morphology of these structures depend on the kinetics of the reactions within the solvent, which in turn are determined by parameters such as solution concentration, amount of water present, the temperature and pH of the solvent, agitation of the solvent and other parameters. When time enough is given, condensation reactions will lead to the aggregation of growing particles or chains until eventually, a gel is formed. The gel can be visualized as a very large number of cross- linked precursor molecules forming a continuous, macroscopic- scale, solid phase, which encloses a continuous liquid phase consisting of the remaining solution. In the final steps of the sol-gel process, the enclosed solvent is removed (generally by drying) and the precursor molecules cross link (a process called aging) resulting in the desired solid [7-8].