Working Report, 20/2006 – Substitution of cobalt driers in wood coatings

Substitution of cobalt driers in wood coatings

6 Conclusion

Experiences from earlier studies have shown that substitution of cobalt with alternative driers is very difficult. Although interesting results have been achieved especially with manganese driers, it is clear that cobalt is more efficient and the alternatives reveal strong limitations when it comes to substitution in a whole product range.

Thus the scope of this project was not only to look into substitution of the drier in existing products but also to get a better understanding of how different aspects of the drier systems together with different parameters of the alkyds affect the drying properties in various types of coatings.

Optimally this would lead to solution scenarios not based on trial and error but a stronger understanding of limitations and possibilities.

When alternative driers are not as efficient or “universal” as cobalt there could be possibilities through the knowledge to choose the right types of binders.

Early in the project it was clarified that looking into correlations between parameters of driers and alkyds respectively on a very detailed level was unrealistic for several reasons. Many of the parameters of the alkyds are related to each other and changing one will affect several others. The work would be enormous and the clear feeling from this project is that these correlations are very sensitive to small system changes.

In e.g. pure binders just the salt of the metal acid shows an influence on drying time with specific binders. However with the presence of other raw materials these differences seem to smoothen out and for example manganese driers, from different suppliers, behaves almost identically. Maybe there are small differences in specific measurements but overall the limitations are exactly the same.

The best candidate for a substitute to cobalt is P02(V). If it was not for the tackiness it could replace cobalt in the major part of a product range. Overall good drying properties are achieved and at a medium dosage level yellowing is not really an issue even in light opaque systems. Hardness is not developed as fast as with cobalt but ends up at the same level in 3 to 4 weeks.

In a few systems however like the red opaque base and the white opaque system with high amounts of titanium dioxide it shows some limitations compared to cobalt. Where a high dosage level of P02(V) is not a solution replacing part of the binder with a faster drying alkyd, as e.g. the urethane modified A7 tested in this project, is a possibility.

Anyway the problem with tackiness is a reality for both the vanadium driers tested in this project. The only real solution has been to partly replace the traditional binder with A7. It is a possibility for sure, but P02(V) is less attractive as a substitute when the need of special binders is necessary in all products and not only a few critical systems.

Some improvements are observed using anti-blocking agents and an approach could be to evaluate this further to get more flexibility in this scenario.

In general P07(Mn) and other manganese driers tested in this project simply lacks catalytic efficacy to be a real option as an alternative. Although the use of secondary siccatives have shown limited improvement, drying times and hardness are not at the level of Cobalt or P02(V) and especially loss of dry is a pronounced problem.

Again a fast drying alkyd like A7 can reduce these problems but the need is very consequent compared to P02(V) and the real alternative to cobalt is arguable the binder itself.

As concerns VOC2010 compliant systems it is a special situation, because these are experimental and relatively few experiences is available as regards the properties in general. Also the high solid binders are still under heavy development from the suppliers and the coating industry is facing a challenge updating the product range no matter the driers.

Overall the conclusions is the same as with other systems only with the high solid binder the hardest film is achieved with P07(Mn). As explained (5.4.4) it is probably still because of less drying efficacy and how this affects the durability is to be tested.

To summarize it has not been possible to find a drier system as efficient as cobalt. However if a modified alkyd with less need of drier is introduced a replacement seems possible although with the technologies available today some compromises will be necessary. This could e.g. be acceptance of more yellowing in some systems. The better efficacy of the drier the more the use of a special alkyd can be limited to critical systems which is a plus for the flexibility in product development in general.

It should be emphasized that at this stage durability tests, artificial or natural exposure, has not been performed and the conclusions are only based on mechanical tests of drying properties.

This project has been initiated because of the appearance of cobalt on the list of dangerous substances /1/ as being possible carcinogenic.

Very little data concerning toxicity and ecotoxicity is available for manganese and vanadium and if cobalt is forbidden as a drier in coatings there is no guarantee that these will not follow sooner or later.

Also the binder systems compliant with VOC2010 seem to be even more challenging as regards drying properties.

Thus to meet future demands development is necessary both in the binder and the drier area.