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

Substitution of Cobalt Driers and Methyl Ethyl Ketoxime

10 Discussion

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. As it is essential that the alternatives possess some catalytic effect it narrows the substitution possibilities to the primary driers. Of these only vanadium and manganese possess enough catalytic effect at ambient conditions to be considered as alternatives to Co driers.

As drier products can vary due to differences in the used metallic salts, solvent and drying accelerator, eleven drier products have been investigated during the project, even though only two types of metals are in question as alternative. Of these are eight manganese driers and three are vanadium driers.

Between four and six alternative driers were tested in each product. In most cases they needed to be combined with one to two secondary driers. As the concentration of the driers also is important it can lead to a relatively high number of drier systems that need to be tested before a well functioning drier system is found. 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.

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.

Several of the included alternatives, this accounts for both driers and anti-skinning agents, were either trial products or even still laboratory products at the project start, for which reason there was a risk of the products being changed during the evaluation period. This turned out to be the case for a few products).

10.1 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 can though be obtained and the achieved results can give some indication to the paint and ink manufacturers on whether it at present is worthwhile substituting Co driers in their air-drying products. 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 need also to be performed by the manufacturers.

The overall impression from the results of this project is that manganese driers are the best suggestion as alternative to the Co driers. Mn driers can be used as alternatives to Co driers in some products, but its usability depends on the specific Mn drier as well as the product (binder type etc.). Vanadium driers cannot generally speaking at this stage be regarded as suitable Co alternatives. Earlier studies have also concluded that V driers are not proper alternatives to Co driers, /14/.

The health and environmental profile will be less negative, if cobalt driers are substituted with manganese driers even though the 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 for cobalt compounds.

The history of occupational manganese exposure indicates that the adverse health effects primarily are associated with intense exposure for a long period of time. In the production and use phase of paint, the exposure to the drier product will be at a minimum, and therefore 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.

In this evaluation it is necessary to remember that the content of drier metal in air-drying coatings generally lies within the range of 0.03 to 0.05 weight-% of the total amount of coating. The human exposure to the drier metals is therefore practically insignificant during the application phase of the coatings. The exposure is potentially more concentrated and more significant in the production of the driers and in the production of coatings.

To improve the environment and health profile as much as possible, alternative driers with the best profile of organic solvents and drying accelerators should be used if technical 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.

From a technical point of view the success in substituting methyl ethyl ketoxime seems rather limited. However, the alternatives have been tested in one concentration only in each air-drying paint product and a simple optimization 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, and therefore 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 as strong anti-oxidants as hydroquinone, as much higher concentrations are needed for the alternative products than for the hydroquinone product to induce the wanted delay of the drying of the printing inks.

QSAR studies indicate that 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 for the driers, the anti-skinning agents are used in small amounts in the coatings. Methyl ethyl ketoxime is for example used in 0.3 to 0.7 weight-% of the total amount in solvent-borne air-drying coatings. Hence, the exposure to anti-skinning agents during the application phase of the coatings is practically insignificant, even though the anti-skinning agent used is volatile. The exposure is potentially more concentrated and more significant in the production of the anti-skinning agents and in the production of the coatings, where the production, however, usually takes place in closed systems with a minimum of exposure.

To improve the environment and health profile as much as possible, alternative anti-skinning agents with the best profile of organic solvents should be used if technical possible. This means that anti-skinning agents with no or a low content of petroleum distillates should be preferred.

In conclusion it can be said that both alternative driers and anti-skinning agents with a better environmental and health profile than the existing alternatives are available.

10.2 Future prospects

In the future there will still be a need for using driers and anti-skinning agents within the coating industry and this especially accounts for the driers as they are used both in waterborne as well as solvent-borne systems. Alkyd binders will still be among the most used binders despite the environmental pressure, partly due to their cost-effectiveness and partly due to a continuous development of new alkyd binder types, which can be used in more environmentally friendly systems.

Therefore there will be a continuous need for developing proper alternatives to cobalt driers, which at this stage means to develop manganese and vanadium driers further. Manganese driers can at this point be used as alternatives in some products, but it would be an advantage if they could be improved even further. The impression is that drier manufacturers actually do a lot of research and development within this area, especially with regard to manganese 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.