Substitution of Cobalt Driers and Methyl Ethyl Ketoxime

1 Introduction

Coatings containing drying oils and fatty acid derivatives from drying oil such as alkyds, epoxy esters and urethane alkyds dry by oxidative cross-linking. Such systems are called air-drying or oxidative drying coatings and do usually contain driers and anti-skinning agents. Most air-drying systems contain cobalt driers, often in combination with other metal driers. The most commonly used anti-skinning agents in paints are volatile oximes, methyl ethyl ketoxime being the most important, whereas hydroquinone is a commonly used anti-skinning agent (antioxidant) in printing inks.

Driers are used to accelerate the drying process of air-drying systems, whereas anti-skinning agents are added to prevent the coatings from skinning during storage in closed containers. In the case of printing inks the anti-skinning agents are used to prevent the ink from drying on the ink rollers.

Due to environmental aspects as well as health concerns it would be desirable if cobalt driers and methyl ethyl ketoxime as well as hydroquinone could be avoided in coatings. Cobalt driers can cause liver and kidney damage, dermatitis and are furthermore suspected to be carcinogenic to tissues and lungs. Methyl ethyl ketoxime is a sensibiliser and an irritant. Hydroquinone is an allergen, is possibly carcinogenic and mutagenic, and is very toxic to aquatic organisms.

The consumed amounts of driers and anti-skinning agents within the coating industry are directly related to the produced amount of air-drying coating systems. Alkyd coatings are the most important. Western Europe produces about 5 million tonnes of coatings (not including printing inks) per year, which accounts for approximately 22 % of the world production. In 1996 the European coating industry consumed 1.8 million tonnes of binders for the production of paints of which 25 % were alkyds. Round 90,000 tonnes of additives were used by the European paint industries in the same period. Of these approximately 26,000 tonnes (28.5 %) were driers and 6,300 tonnes (7.1 %) anti-skinning agents, /1/.

As the paint production in Denmark in 1996 was round 2.5 % of the total amount of produced paint in Western Europe, /1/, a rough estimate of the annual consumption of cobalt metal and methyl ethyl ketoxime in the Danish paint industry can be made. Assuming that between one third and one quarter of the total amount of driers are cobalt driers with an average cobalt content of 10 weight-% and assuming that 2.5 % of these driers are used in Denmark the annual consumption of cobalt metal within the Danish paint industry is approximately 16 to 22 tonnes.

The content of cobalt metal in a specific product depends on the amount of oxidative drying matter present in the product. The level of cobalt metal does in general rarely exceed 0.075 weight-% of the total product in an air-drying coating. The common level of cobalt metal in an air-drying coating is within the range of 0.03 to 0.05 weight-% of the total amount. Methyl ethyl ketoxime is commonly used in 0.3 to 0.7 weight-% of the total product in solvent-borne air-drying systems.

The annual consumption of anti-skinning agent is approximately 160 tonnes assuming that 2.5 % of the total amount is used in the Danish production. The major part of consumed anti-skinning agent will be methyl ethyl ketoxime. The estimated level of anti-skinning agents is probably too high as Denmark produces a relatively large amount of waterborne paints compared to the European average paint production. The level of used driers and anti-skinning agents today is expected to be comparable to the levels in 1996, /1/. The used amounts for the production of printing inks are not included in the above-mentioned numbers.

1.1 Project objects

The main objects of the project were to investigate what alternative cobalt free driers and alternative anti-skinning agents that are available on the market and to investigate and evaluate the efficiency as well as the environmental and health profile of these alternatives. The main focus has been on alternative products that are already commercially available today or will become commercially available in the near future. The alternatives have been tested in a diversity of oxidative drying products, which includes common do-it-yourself (DIY) products, industrial products and printing inks. Both solvent-borne and waterborne systems were included.

Paint and printing ink manufacturers have supplied 17 oxidative drying products for testing the technical efficiency of the alternatives. The products were selected so they cover the broadest possible product range of air-drying coatings based on the products made available from the involved manufacturers also taking into account that at least two products from each manufacturer had to be present according to the project description.

1.2 Search for alternatives

The number of alternatives to cobalt (Co) driers is quite limited at this stage. A preliminary search for alternatives was performed on the Internet and followed by direct enquiries at the drier manufacturers. The outcome of the search results is that only manganese and vanadium driers are potential alternatives to cobalt driers.

The drier manufacturers were found by the Internet search or by consulting available databases on raw material suppliers for the coating industry. Less than fifteen drier manufacturers were identified on the European market during the search. The manufacturers were addressed to clarify if they were marketing alternatives for cobalt driers and/or if they were working on developing alternatives.

The minor manufacturers, which responded, did in general at that stage not work in the area of developing alternatives. The major manufacturers did all have some activity in this area. Four manufacturers supplied samples of alternatives driers to the project work. The same four drier manufacturers do more or less cover the total supply for the entire Danish coating industry. Several manganese (Mn) and vanadium (V) based driers have been tested as alternatives to Co driers. The samples supplied were a mixture of already available drier products, either as commercial or trial products, and laboratory products. Most of the investigated drier products have during the project period been upgraded to either commercial or trial products.

A search for alternatives to methyl ethyl ketoxime and hydroquinone were performed in the same way, but only a limited number of suppliers responded to the enquiries. Three different manufacturers have supplied samples to the project. Acetone oxime, amino/amido based as well as phenolic based anti-skinning agents have been investigated as alternatives to methyl ethyl ketoxime and hydroquinone. As it was the case for driers the samples were covering both commercially available products as well as laboratory products. Vitamin E (á-tocopherol) was also included as a possible alternative, mainly for use in printing inks.

1.3 Project limitations

As a high number of products (17) were included for testing the alternatives it has not been possible to optimise every single system with regard to drying time. The obtained drying time results and complementary tests should therefore only be regarded as guidelines to the manufacturers on whether a specific drier is worthwhile testing in their product or not. The same accounts for the anti-skinning agents.

How successful the outcome of an environmental and health screening and assessment is depends on the available product information, the more information given the more reliable screenings and assessments can be performed. The information about the alternative driers and anti-skinning agents has in some cases been rather limited due to confidentiality. This especially accounts for the laboratory products, but even in some of the commercial products the exact identity of the active components is confidential. In a few cases the project group was told the identity of the active components in order to verify the classification of the substances and products.

Another limiting factor in performing the environmental and health assessment is that the metallic salts in question are not very well documented with regard to environmental as well as health effects, for which reason a more general assessment had to be performed.


Version 1.0 December 2003, © Danish Environmental Protection Agency