Aluminum pigmenten chromate - free metal effect coatings on steel substrates

ABSTRACT: Metals such as iron and aluminum are used in industrial structures because of their mechanical properties including high strength and stiffness. These materials are very sensitive to corrosion due to aggressive environments, which cause energy and material loss. Making use of a protective film or coating is the most common route to protect metals from corrosion. A common coating used on metals is chromate coating due to its good protective and aesthetic qualities however; Cr (+6) is known and designated by EPA as one of the 17 high toxicity chemicals. It is known to be a human carcinogen and emits a toxic mist at elevated temperatures. Because of environmental and health perspective, alternative more environmentally friendly coating methods are under investigation. Sol-gel process is amongst the common methods for deposition of the coatings on metal surfaces. Such process has several advantages over those other methods such as low temperature and waste-free processing. In the current research sol-gel processing was used to deposit coatings on the steel substrates. It is known that UV curing is becoming an accepted technique and offers a variety of advantages because of its uniqueness, therefore in the current study a series of UV-curable coating with aluminum pigments was prepared in order to replace the chromium coatings, since from the health and environmental perspective, chromium can be toxic and can have significant negative effect on human beings. For this purpose, among the oligomers that can be applicable for UV-curing, aliphatic tri-functional oligomeric urethane methacrylate (ATOUA) was formulated from trimethylolpropane (TMP), isophrone diisocyanate (IPDI) and hydroxyethyl methacrylate (HEMA). Also, dibutyltin dilaurate (DBTDL) was incorporated to let the reaction to take place. As incorporated to let the reaction, the metallic shine effect was also achieved by formulating leafing and non-leafing aluminum pigments with ATOUA as binder with a proper dispersing agent, during the coating process. In this study, ATOUA was reacted with 1,6 hexanediol diacrylate (HDDA) and trimethlol propane triacrylate, serving the function of a reactive diluent, benzophenone as phtoinitiator, N-methyl diethanolamine as co-initiator, aluminum pigments, wetting agent 1OEO), to produce UV-curable aluminum pigmented coatings. SEM analysis was also performed to monitor the surface morphology of the produced coatings. SEM images of the ATOUA polymers illustrated a crack free coating involving aluminum particles. Moreover, FTIR, 1H-NMR and 13C-NMR spectroscopic measurements were employed to pinpoint the structural properties of the synthesized oligomers. Additional characteristics of the surface coatings such as chemical and mechanical properties have been investigated through the following tests: König hardness, cross cut adhesion, impact resistance, gloss, mandrel bend, cupping and salt spray. Furthermore, the DSC and TGA analysis were investigated. results clearly indicated that the cured films possess proper pendulum hardness and strong adhesion to the substrate. Moreover, coatings passed 1000 h of salt spray resistance test. Coatings have suitable abrasion resistance, gloss and flexibility and consequently can be applied in industrial sectors. Keywords: UV Curable Coating, Aluminum Pigment, Organic- Inorganic Hybrid Film, Metal Effect Coating, Gloss, and Steel. …………………………………………………………………………………………………………………………………………………………………………………………………………