Miljøprojekt, 884 – Substitution of cobalt driers and methyl ethyl ketoxime

Substitution of Cobalt Driers and Methyl Ethyl Ketoxime

Summary and conclusions

This project was undertaken to investigate the possibilities for substituting cobalt driers and methyl ethyl ketoxime in air-drying coatings. A search for alternatives to hydroquinone, used in printing inks, was performed as well. The efficiency as well as the environmental and health profile of the alternatives has been investigated and evaluated.

The number of alternatives to cobalt driers is quite limited at this stage. No non-metallic compounds, which are capable of substituting cobalt driers, have been identified so far. Of metallic compounds only vanadium (V) and manganese (Mn) possess enough catalytic effect at ambient conditions to be considered as alternatives to cobalt (Co) driers. Within the project eleven drier products have been tested as substitute to Co driers in a diversity of oxidative drying products. Eight of the driers are manganese driers and three are vanadium driers.

The number of alternatives to methyl ethyl ketoxime and hydroquinone is also quite limited. Two amino/amido based anti-skinning agents have been investigated along with two phenolic-based products, and acetone oxime. Vitamin E has also been included in the testing primarily for use in the printing inks.

The work finding a proper alternative cobalt free drier system can be laborious and as 17 products were included in the testing of the alternatives it has been impossible to work in depth with every single product optimising it with regard to drying time.

Screening

In order to determine if the alternatives will improve the overall environmental and health profile of air-drying products a screening of the alternative driers and anti-skinning agents was carried out. The alternatives were given human toxicity scores based on material safety data sheets and the list of dangerous substances. Environmental scores have only been given for very few substances.

The screening shows that if cobalt driers can be substituted with manganese and vanadium driers the health profile will improve. However, some alternative driers do contain components (organic solvents or drying accelerators) with undesirable health and/or environmental effects. In a substitution it is therefore necessary to look upon the entire drier product and not just the active metallic compound.

Only considering the active ingredients in the alternative anti-skinning agents the screening shows that the health profile of air-drying products can be improved by substituting methyl ethyl ketoxime and hydroquinone with the alternatives, although, one alternative – acetone oxime - is excluded when using the screening. However, also in this case the total health profile of the product depends on the organic solvents used in the products, but even so the profile is better than that of methyl ethyl ketoxime and hydroquinone.

Results from technical evaluation

The overall impression from the technical evaluation is that manganese driers to some extent can be used as alternatives to Co driers. The vanadium driers have not been suitable alternatives in any of the products in which they have been tested. Either did they give insufficient drying or if enough drying was induced the dry film turned out to be far too soft compared to the original products.

The efficiency of the various tested Mn driers differs quite a lot in the tested concentrations and combinations, but Mn driers can with some success be used as alternatives to Co driers depending on the specific Mn drier as well as the specific product. Do-it-yourself (DIY) coatings with tall oil alkyds seem for instance easier to substitute than coatings with linseed oil alkyds.

The industrial products were in general easier to Co substitute than the do-it-yourself products probably because these coatings either contain modified alkyds or blends of alkyds and non-oxidative drying binders, which means the amount of oxidative drying matter in the coatings is relatively low. With regard to the printing inks comparable drying time profiles and set-off effects to that of the reference inks were attainable with the alternative Mn driers.

Acetone oxime and the amino/amido based anti-skinning agents seem to prevent surface skinning on paints in closed containers to an extent, which is comparable to that of methyl ethyl ketoxime, but the results from the skinning test in open containers indicate that the amino/amido products might have a negative effect on drying time compared to the oximes. This needs to be investigated.

The amino/amido compounds do also to some extent work as antioxidants in the printing inks, so does vitamin E. It needs to be investigated if the alternatives have a negative influence on the set-off effect.

Environmental and health assessment

A more thorough assessment of the environmental and health effects of the alternatives has been carried out. For the driers, the assessment is carried out on a more general level – an assessment of the metal compounds – for two reasons. First of all, not every metal drier substance has been identified, because of confidentiality. Secondly, very few or no information on the environmental and health aspect of the specific substances was available.

The assessment of the alternative driers shows that the health profile will be less negative, if cobalt driers are substituted with manganese or vanadium driers, as only cobalt compounds are classified with regard to carcinogenic effects to humans. However, the effect is not unambiguous because both manganese and vanadium compounds have shown adverse health effects (neurotoxic effects).

Similarly, the environmental profile of air-drying products will be less negative if cobalt driers are substituted with manganese or vanadium driers. Again, the effect is not very obvious as the alternatives also are toxic to aquatic organisms. Cobalt compounds are in general considered to be very toxic to aquatic organisms whereas vanadium compounds (vanadium pentaoxide) are toxic and manganese compounds are regarded as harmful to aquatic organisms.

Overall, the environment and health profile of air-drying products will become less negative if cobalt driers are substituted with manganese or vanadium driers, especially as the driers are combined with the same secondary driers as cobalt driers and in the approximately same concentrations.

To improve the environmental and health profile as much as possible, alternative driers with the best profile of organic solvents and drying accelerators should be used if technically possible. This means that driers dissolved in solvents like petroleum distillates and 2-ethylhexanoic acid should be avoided, and that driers containing 2,2-bipyridyl as drying accelerator should be preferred to those containing 1,10-phenathroline (at present classification). As petroleum distillates today still are present in almost every drier product, both the primary and the secondary, it is impossible to avoid them completely, but products with a low content of petroleum distillates should of course be preferred. Similarly, anti-skinning agents with no or a low content of petroleum distillates should be preferred.

Assessment of the organic amino compounds shows that the health profile not necessarily will be improved if the existing anti-skinning agents are substituted with the organic amino compound. The organic amino compound is in general less toxic and less irritating than the existing anti-skinning agents, but information obtained from QSAR studies suggests that the amino compound may be genotoxic. However, with regard to the environment, the profile will improve as the amino compound has a low aquatic toxicity.

Assessment of vitamin E shows that the health profile of the air-drying product will be improved if the existing anti-skinning agents are substituted with vitamin E. Vitamin E is basically non-toxic and has shown both anti-mutagenic and anti-carcinogenic effects. No environmental information about the ecotoxicity of vitamin E was found.

Overall evaluation

The conclusion and comments made in this project can only account for the specific drier combinations and products used in the testing carried out in this project. However, some general guidelines could though be obtained and the achieved results can give some indications to the paint and ink manufacturers on whether at present it is worthwhile substituting Co driers in their air-drying product. Before substituting the manufacturers need to verify the results obtained during this project as well as perform any necessary complementary tests. Further optimising of the alternative drier systems also needs to be performed by the manufacturers.

With regard to substituting Co driers Mn driers are promising from a technical point of view as Mn driers at this point can be used as alternatives in some products, but its usability depends on the specific Mn drier as well as the product (binder type etc.). The use of V driers seems rather limited at this stage.

The health and environmental profile will be less negative, if cobalt driers are substituted with manganese driers even though manganese compounds also have shown adverse health effects and are harmful to aquatic organisms. However, with the information available at the moment the health and environmental profile of manganese compounds is better than the profile of cobalt compounds. The history of occupational manganese exposure furthermore indicates that the adverse health effects primarily are associated with intense exposure over a long period of time. In the production and use phase of paint, the exposure to the drier product will be at a minimum, for which reason a substitution of cobalt driers with manganese driers must be considered as a step in the right direction regarding the health profile of the air-drying products.

From a technical point of view the success in substituting methyl ethyl ketoxime seems rather limited. It should be noted that the alternatives have been tested in one concentration only in each air-drying paint product and a simple optimisation on the concentration might lead to more positive results. The most promising alternative is acetone oxime, which has a dubious health profile. For the amino/amido compounds, which have a reasonable anti-skinning effect in closed containers, there was a strong indication that they may influence more on the drying time than methyl ethyl ketoxime, but it has not been verified. Vitamin E needs to be investigated further in air-drying paints before it can be concluded whether it is a potential alternative to methyl ethyl ketoxime.

In the case of printing inks a certain influence on the drying is actually necessary to obtain duct stability of the ink, for which reason the most volatile alternative, e.g. acetone oxime, cannot be used. Both vitamin E and the amino/amido compounds gave promising results, but in the tested concentration they are not being as strong anti-oxidants as hydroquinone.

QSAR studies indicate that the benefit of substituting methyl ethyl ketoxime and hydroquinone with amino/amido compounds might be limited due to that they may have a genotoxic effect. The best health profile is found for vitamin E, which is practically non-toxic.

As air-drying products will remain on the coating market many years from now there is a need for continuous development of the alternatives. The use of anti-skinning agents might decrease in the future as waterborne coatings are used more and more in preference to solvent-borne. But as long as anti-skinning agents are used it would be desirable if more environmentally friendly and less harmful alternatives than methyl ethyl ketoxime and hydroquinone could be identified.

With regard to the alternative driers there is a need for making them capable of substituting Co driers in a broader spectrum of air-drying products. The impression gained during the project is that drier manufacturers actually do a lot of research and development within this area, especially with regard to Mn driers. A real break through, at least from an environmental and health point of view, would be if non-metallic alternatives could be identified. However, this might be a very distant prospect.