The Evolution of Catalysis for Alkyd Coatings: Responding to Impending Cobalt Reclassification with Very Active Iron and Manganese Catalysts, Using Polydentate Nitrogen Donor Ligands

Abstract

Autoxidation processes to achieve curing of alkyd resins in paints, inks, and coatings are ubiquitous in many applications. Cobalt soaps have been employed for these applications for many decades and most of the paint and ink alkyd resin formulations have been optimized to achieve optimal benefits of the cobalt soaps. However, cobalt soaps are under increased scrutiny because of likely reclassification as carcinogenic under REACH (Registration, Evaluation, Authorisation, and Restrictions of Chemicals) legislation in Europe. This is critical, since such coatings are available for regular human contact. Alternative manganese- and iron-based siccatives have been developed to address this need for over a decade. They often show very high curing activity depending on the organic ligands bound to the metal centers. Recently, new classes of catalysts and modes of application have been published or patented to create safe paints, whilst delivering performance benefits via their unique reaction mechanisms. Besides the use of well-defined, preformed catalysts, paint formulations have also been developed with mixtures of metal soaps and ligands that form active species in-situ. The change from Co-soaps to Mn- and Fe-based siccatives meant that important coating issues related to radical-based curing, such as skinning, had to be rethought. In this paper we will review the new catalyst technologies and their performance and modes of action, as well as new compounds developed to provide anti-skinning benefits.

Recent advances on ferrocene-based photoinitiating systems

“…Thus, a severe reduction of the photoinitiating ability was determined for the mono-substituted ferrocenes compared to the disubstituted ones, with a 3-to-10-fold elongation of the reaction times. It has to be noticed that, recently, benzoylferrocenes have been revisited in the context of air-drying paints, [225] in replacement of the well-established but contested cobalt, [226][227][228][229] manganese [230][231] and vanadium [232][233][234][235][236][237]-based catalysts, classically used as primary driers for alkyd resins. However, if the ability of benzoylferrocenes to act as driers was demonstrated, no investigation of the chemical mechanism was carried out in this work.…”

Helmet Phthalocyaninato Iron Complex as a Primary Drier for Alkyd Paints

“…Our research of coordination compounds of redox-active transition metals is motivated by the recognized carcinogenic properties of soluble cobalt compounds [ 13 ], currently used as the primary driers, and a limited portfolio of cobalt-free alternatives, because none of them fully fits the demands of the paint-producing industry [ 11 ]. We note that widespread use of cobalt(II) 2-ethylhexanoate (Co-2EH, Figure 1 b) is still acceptable for common paints, but the situation might change rapidly due to the ongoing tightening of legislative restrictions [ 14 ]. Commercial cobalt-free systems usually involve manganese(II) carboxylates [ 15 ] or their mixtures with N -chelators (e.g., bipyridine) [ 16 ], but these systems suffer from discoloration and a low activity at adverse climatic conditions [ 15 ].…”

“…Commercial cobalt-free systems usually involve manganese(II) carboxylates [ 15 ] or their mixtures with N -chelators (e.g., bipyridine) [ 16 ], but these systems suffer from discoloration and a low activity at adverse climatic conditions [ 15 ]. Promising drying characteristics are further reported for manganese(III) acetylacetonate [ 17 ], manganese complexes of 1,4,7-trimethyl-1,4,7-triazacyclononane [ 14 ], oxidovanadium(IV) carboxylates [ 18 , 19 ], oxidovanadium(IV) acetylacetonates [ 20 , 21 , 22 ], and oxidovanadium(V) dithiocarbamates [ 23 ]. The field of iron-based primary driers is currently focused mainly on commercial iron bispidine complex, known under the trademark Borchi OXY-Coat, which performs at very low concentrations compared to Co-2EH [ 24 , 25 ], and its congeners with increased solubility in organic solvents [ 26 , 27 ].…”

“…The field of iron-based primary driers is currently focused mainly on commercial iron bispidine complex, known under the trademark Borchi OXY-Coat, which performs at very low concentrations compared to Co-2EH [ 24 , 25 ], and its congeners with increased solubility in organic solvents [ 26 , 27 ]. Considerable attention has further been given to iron complexes with polydentate N -chelators [ 14 ] and ferrocene derivatives bearing electron-withdrawing substituents [ 28 , 29 ]. The activity of monocyclopentadienyl compounds [(η 5 -C 5 H 5 )Fe(CO) 2 I], [{(η 5 -C 5 H 5 )Fe(CO)} 2 (μ-CO) 2 ] and [(η 5 -C 5 H 5 )Fe(η 5 -C 6 H 7 )] has been reported in patent literature [ 30 ].…”

Editorial: Cobalt and Iron Catalysis

“…This collection provides a broad overview of a multitude of processing routes. In addition to the aforementioned topics, global concerns also drive: environmentally-attractive catalysts through specific homogeneous asymmetric iron complexes [63], the development of consumer products (preservatives, cosmetics, and flavors) [64] and materials (inks, coating, and paints) [65], and effective waste water treatment [66]. Furthermore, environmental concerns necessitate not just the capture of CO 2 , but also its utilization as a means of mitigation [46,57,67].…”