By Joshua Halstead, Ph.D., Borchers Americas, Inc., A Subsidiary of Milliken & Company, United States
Alkyd-based coatings cure via both physical and chemical drying processes. The natural drying time of an alkyd can be weeks to months, which is not desirable from a practical point of view. In practice, the chemical crosslinking process is accelerated using catalysts, commonly referred to as driers. Generally, these are transition metal complexes with organic ligands. The most widely known and commonly used driers are based on cobalt carboxylates. While cobalt driers lead to highly crosslinked hard films, cobalt-based siccatives have recently faced reclassification as class 1b carcinogens by the Cobalt REACH consortium,1 a nonprofit group tasked with preparing the registration dossiers for cobalt and cobalt compounds. As a carcinogen, cobalt that is used to cure coatings and inks can be a risk to human health as humans can be in frequent contact with these substances (especially when applying paints or scraping off old paint layers). Many regions recognize that the use of cobalt in this industry must be reduced as part of a movement toward a sustainable future.
The two leading technologies to replace cobalt driers are based on either manganese or iron. Manganese carboxylates (Mn3+) have long been known to exhibit good drying activity, although generally to a somewhat lesser degree than that observed with cobalt. Furthermore, the formation of Mn3+ species often leads to a brown coloration of the formulation, which precludes its use at high levels or in light-colored coatings.
Most iron driers show poor activity, especially in relation to cobalt or manganese analogs at ambient temperatures. Iron carboxylates work well at high temperatures and are often used in stoving enamels where curing occurs at 80–250 °C. Iron has a distinct yellow-brown color that can lead to significant discoloration in light-colored formulations as well.
An exception to this is the recent invention of an iron-bispidon complex (Figure 1), which shows vastly improved catalytic activity at very low iron weight percentages (wt %).2 For example, in a medium-oil alkyd resin, the level of iron needed to obtain a good level of curing activity can be reduced from 0.08 wt % on resin solids, a typical drier loading for many formulations, to 0.0007 iron wt % on resin solids.3 As a result of this low dosage, color intensity in a paint formulation is greatly reduced, often to a level below that of cobalt-based coatings.
Fe-bispidon complexes have emerged as a preeminent cobalt alternative for the drying of waterborne and solventborne alkyd paints. Due to very high efficiency at low metal wt %, these complexes are often referred to as High-Performance Catalysts (HPC). When tested in model systems, HPCs often provide superior performance in drying speed as well as many physical coating properties when compared with cobalt-based driers.4-6 While model systems provide valuable insight into the underlying chemistry, the goal in this study was to understand the effect of HPCs on the chemistry and coating properties of real-world alkyd paint formulations made with industry-leading resins. A series of experiments was conducted to determine the optimum dosage of HPCs for four commercially available alkyd emulsions using representative formulations of the types used in architectural trim paints. These formulations were then evaluated in an array of standard industry tests versus equivalent cobalt-catalyzed formulations. Differences in the types and levels of chemical crosslinks were studied through Fourier Transform Infrared (FTIR) spectroscopy experiments on cured films. Possible variations in the β-scission reaction pathway were evaluated through gas chromatography/mass spectroscopy (GC/MS).
Finally, to demonstrate the relevance of these formulations for architectural trim applications, a section of popular commercial alkyd and acrylic paints was benchmarked and compared.
Continue reading in the May-June 2023 digital issue of CoatingsTech.