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

Substitution of Cobalt Driers and Methyl Ethyl Ketoxime

4 Anti-skinning agents

The formation of surface skin on air-drying coatings results from the same drier catalysed oxidative polymerisation processes, which normally result in drying of the coating film. Anti-skinning agents are therefore added to prevent unintended skinning on paints and printing inks. The need for preventing skin formation might range from preventing in-can skinning during storage in closed container to preventing the ink from drying in the press duct. The former may be achieved by using volatile anti-skinning agents, whereas full overnight duct stability only can be achieved by using non-volatile anti-skinning agents, more often referred to as antioxidants, /17/.

In closed containers the oxidation and thereby the skin formation takes place due to the presence of air pillows between the coating surface and the closed lid. The skinning inevitably results in a loss of coating material and a possible contamination of the bulk. Anti-skinning agents are therefore added to solvent-borne air-drying coatings to prevent in-can skinning during storage by prohibiting the drier effect until application of the coating. Methyl ethyl ketoxime is far the most used anti-skinning agent in paints, where volatile types are preferred. In the printing inks where duct stability is the issue the non-volatile hydroquinone is used to a large extent. Printing inks are though also often sprayed with volatile anti-skinning agents on the surface of the ink before storing them in cans.

Only very small amounts of anti-skinning agents are needed to prevent skin formation. Typically a fraction of less than 1% of the total formulation is necessary, /4/17/18/. Anti-skinning agents are commonly only added to solvent-borne systems as the waterborne systems are not as prone to skinning.

4.1 Function of anti-skinning agents

Anti-skinning agents react with the free radicals formed during the oxidative polymerisation processes, as they are more readily oxidised than the drying oils or drying oil derivatives present in the coating. The anti-skinning agents hereby prevent the cross-linking from taking place, for which reason the drying of the coating is stopped, /19/. The volatile anti-skinning agent might also to some extend form complexes with the primary drier metal and hereby influence the activity of the drier system, /19/. The anti-skinning agents continue to act until all the molecules of the compound are exhausted.

The use of anti-skinning agents is always a compromise between preventing skinning and retaining an adequate drying potential of the coating after application. The cross-linking should be as slow as possible during storage and then regain its full drying potential as soon as possible after application. Care must be taken to ensure that the minimum amount of anti-skinning agents is used, especially for the non-volatile types as excessive amount might have significant effect on the drying of applied coating or printing ink, /4/. The activity of anti-skinning agents should therefore preferably come to an end immediately after application of the coating, /3/4/17/18/.

4.2 Types of anti-skinning agents

The most common types of anti-skinning agents are oximes, substituted phenols or quinones. Various napthols and aromatic amines are also sometimes used.

4.2.1 Volatile anti-skinning agents

Volatile anti-skinning agents are only effective where atmospheric contact is restricted, for which reason they are mainly used for preventing skinning in closed containers. They become ineffective in open or loosely closed containers. Due to the volatile nature they evaporate shortly after application having little or no effect on the drying of the coating. Volatile oximes are primarily used in paints, methyl ethyl ketoxime being far the most important.

Cyclohexanone oximes are also used from time to time, but mainly in printing inks as they are less volatile and therefore influence negatively on the drying time, /4/19/.

4.2.2 Non-volatile anti-skinning agents (antioxidants)

Non-volatile anti-skinning agents can either be substituted phenolics or quinines, /4/. Hydroquinone is a very strong antioxidant, which severely inhibits oxidation. Non-volatile anti-skinning agents might have a marked effect on the drying of the coating after application, even if only slight excess is used. They should therefore always be used with care. Non-volatile anti-skinning agents are mainly used in printing inks. Especially hydroquinone is used to induce overnight duct stability. Obviously, an ink containing a strong antioxidant will take longer to dry than if the antioxidant was not present, but as most/many inks are printed in very thin films on absorbent substrate the retarded drying time does commonly not lead to any severe set off effects.

4.3 Alternative anti-skinning agents

Amino/amido based compounds as well as phenolic based compounds have along with acetone oxime been suggested as possible alternatives to methyl ethyl ketoxime. The amino/amido based anti-skinning agents and the phenolic based have also been investigated as alternatives for hydroquinone. Vitamin E (á-tocopherol), which is a natural antioxidant, has also been included. It was mainly tested in the printing inks, but some linseed oil paint manufacturers actually use it in their paint products, /20/.