A promising drying activity of environmentally friendly oxovanadium(IV) complexes in air-drying paints
Introduction
Air-drying binders have been established as a significant group of paints used in modern organic coatings. Presence of double bonds in the binder molecules enables the reaction with air oxygen to give three-dimensional polymeric structure. This radical process, called autoxidation, results in the conversion of liquid paint layer to firm coating [1], [2]. Alkyd resins modified with highly unsaturated fatty acids are able to provide polymeric film without addition of any curing agents but this process is generally slow giving the paint film with low final hardness. Nevertheless, the use of metal catalysts, so-called driers, significantly reduces binder drying time and improves physical properties of resulting polymeric film [3], [4].
Commercially used cobalt-based driers, such as cobalt(II) 2-ethylhexanoate (Co-Nuodex), belong to the most powerful drying agents. However, pronounced toxicity of cobalt(II) compounds [5], [6] stimulate a legislative pressure on the paint-producing industry to replace these compounds with less toxic alternatives. Although active driers based on manganese (e.g. Mn-bipyridine or Mn-triazacyclononane complexes) [7], [8] or iron (e.g. acylferrocenes) [9], [10] are known, the most of them still suffers from some disadvantages such as intense color, lower activity at ambient temperature or low solubility in formulation. Very recently, high drying activity was reported for Fe-bispidon complex [11]. It exhibits a considerably higher activity than common cobalt-based driers and seems to be suitable not only for solvent-borne alkyd paints but also for water-borne binders.
Our scrutiny for new drying agents leads us to vanadium compounds. Hence, oxovanadium(IV) compounds seem to be very promising candidates for this purpose because they are established as the potent oxygen-transfer catalysts in various organic reactions [12], [13], [14]. Furthermore, inorganic vanadium compounds, including diketonates, exhibit only low overall toxicity [15]. To the best of our knowledge, no original research article dealing with air-drying performance of these compounds has been published when omitting a review mentioning the ability vanadium compounds to accelerate autoxidation process [4]. So far, only the patent literature has dealt with performance of vanadium compounds, including [VO(O,O-MeCOCHCOMe)2] (VO-1) [16] and two vanadium-based trial driers of confidential composition are currently commercially available (OMG Borchers GmbH). All these vanadium driers are usually combined with secondary driers (soaps of Sr, Zn, Zr, Ca or Li) to improve through-drying of the coated film [4], [17].
We decided to start our investigations with familiar and well defined diketonate compounds of the formula [VO(O,O-R1COCHCOR2)2] (VO-1: R1 = R2 = Me; VO-2: R1 = Me, R2 = Ph; VO-3: R1 = R2 = Ph), see Fig. 1. In this paper we describe the behavior of these complexes in solvent-borne alkyd resin during the autoxidation process with the view to compare this activity with commercial cobalt-base drier. Attention is also given to elucidation of the role of oxovanadium species in the mechanism of binder drying.
Section snippets
Materials and chemicals
The complexes VO-1, VO-2 and VO-3 were prepared according to the procedure published elsewhere and repeatedly crystallized from chloroform [18]. The identity and purity of oxovanadium compounds was verified by elemental analysis and by comparing of their melting points, IR and EPR spectra with literature [18], [19]. The commercial additive-free cobalt-based drier cobalt(II) 2-ethylhexanoate (Octa-Soligen Cobalt 10 in D60, Co-Nuodex), was obtained from Borchers GmbH. The solvent-borne
Performance of oxovanadium(IV) compounds
Drying activity of the complexes VO-1, VO-2 and VO-3 has been determined in solvent-borne phtalic-type alkyd resin modified with soybean oil. Optimization of the drying process started at overall metal concentration of 0.1% in dry matter content that is typical for commercial cobalt-based drier Co-Nuodex. Lower concentrations of vanadium driers were used in attempt to reduce the metal content in the paint. Table 1 summarizes observed drying times and relative hardness for each system under the
Concluding remarks
This study clearly demonstrated that the all tested oxovanadium(IV) diketonate complexes exhibit an excellent drying activity in solvent-borne alkyd binder. At higher concentrations (0.1%), the performance is comparable with commercial Co-Nuodex but lowering the metal contents does not lead to considerable deterioration of the drying process. Hence, the performance of oxovanadium compounds at concentrations 0.03 and 0.01% seems to be satisfactory for the application in paint-producing industry
Acknowledgement
This work was supported by Ministry of Education of the Czech Republic (Project No. SG350003).
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