Manganese and Iron Catalysts in Alkyd Paints and Coatings
Many paint, ink and coating formulations contain alkyd-based resins which cure via autoxidation mechanisms. Whilst cobalt-soaps have been used for many decades, there is a continuing and accelerating desire by paint companies to develop alternatives for the cobalt soaps, due to likely classification as carcinogens under the REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals) legislation. Alternative driers, for example manganese and iron soaps, have been applied for
... een applied for this purpose. However, relatively poor curing capabilities make it necessary to increase the level of metal salts to such a level that often coloring of the paint formulation occurs. More recent developments include the application of manganese and iron complexes with a variety of organic ligands. This review will discuss the chemistry of alkyd resin curing, the applications and reactions of cobalt-soaps as curing agents, and, subsequently, the paint drying aspects and mechanisms of (model) alkyd curing using manganese and iron catalysts. Inorganics 2016, 4, 11 2 of 18 will not discuss these additives exhaustively, additives such as anti-skinning agents, pigments and fillers will be a subject of further discussion in various sections as they may be interlinked with the performance of the siccatives or indeed the paint formulation itself. In this review, the chemistry of radical curing processes in relation to alkyd based paints and inks will be discussed first in some detail. Then, several of the mechanistic studies on cobalt soap driers will be discussed, after which publications and patent applications dealing with various classes of paint drying catalysts based on manganese and iron will be highlighted. Although several other alternative driers such as those based on copper containing laccase enzyme  or vanadium complexes  have been reported, such systems fall outside the scope of the current review. It should be noted that, in the literature, such metal catalysts (both cobalt soaps and alternative alkyd curing catalysts) are also denoted as paint drying catalysts, driers or siccatives. These terms will also be used in this review, although, strictly speaking, the catalysts merely accelerate the autoxidation reactions and do not aid in, for example, evaporation of the solvent as the names could suggest. Inorganics 2016, 4, 11 3 of 18 takes place from a double allylic -CH 2 -group, a conjugated system is often formed and this can react with O 2 (Reaction (6) ). The peroxyl radical species formed as depicted in Reaction (6) can, for example, abstract H ' radicals from the unreacted alkyd resin, yielding (conjugated) hydroperoxyl (ROOH) species (Reaction (5) ). Inorganics 2016, 4, 11 3 of 18 this can react with O2 (Reaction (6) ). The peroxyl radical species formed as depicted in Reaction (6) can, for example, abstract H • radicals from the unreacted alkyd resin, yielding (conjugated) hydroperoxyl (ROOH) species (Reaction (5) ). Scheme 1. Examples of reactions during radical curing processes in alkyd resins (L stands for organic ligand bound to metal ion) (Reactions (1)-(6)) .