Coatings World - June 2016

Unique Solutions to Regulatory Concerns Affecting Cobalt and MEKO
June 2016 www.coatingsworld.com Coatings World | 39

reduce long-term film yellowing.

Using Anti-Skinning
Additives in Paints

By adding driers to improve coating dry times, we also increase the tendency of the coating to cure prematurely whenever it comes in contact with air. This includes during the manufacturing, filtering and filling processes. Adding driers also means that less oxygen is needed to initiate surface curing which can affect long-term storage in the can. The coatings industry developed additives to delay the curing process to prevent skin formation.

The two main methods to prevent skin-
ning are: (1) inhibit the catalytic activity
of the metal carboxylate and (2) absorb
the free radicals during autoxidation to
prevent crosslinking.

The first method involves using a volatile material to form temporary complexes with the metal in the drier to block its catalytic activity. The inhibitor evaporates quickly during film application freeing the metal to function. The most common chemical used for this is methyl ethyl ketoxime (MEKO), or ketoximes in general. They are especially effective with cobalt, however, their high volatility makes them a health concern. The major toxicity is to erythrocytes and the hematopoietic system. Ketoximes cause similar lesions in the olfactory epithelium.

Hyperplasia of the urinary bladder transitional epithelium was observed in mice exposed to MEKO but not with a similar chemical, cyclohexanone oxime. For MEKO, the “no observed-adverse-effect level” (NOAEL) for erythrotoxicity is 312 ppm in the drinking water for rats and 2,500 ppm for mice (see National Toxicology Program, Toxicity Report Series, Number 51, July 1999, NIH Publication 99-3947).

Figure 7 shows part of the SDS for MEKO with regard to Health Hazards (2.1) and Label Elements (2.2).

The second method to prevent skinning involves using antioxidants to absorb free radicals in the coating. However, these can severely inhibit oxidation and affect curing and film appearance if excessive amounts are used. The coating can only cure after the antioxidant has been consumed (with free radicals), leaves the film through evaporation or is driven off during the bake cycle (if part of the curing system). These consist of substituted phenolics, quinones, aromatic amines, sterically-hindered aliphatic amines and hydroxyl amines.

The most preferred anti-skinning additive would be non-toxic, have no regulatory concerns, have no effect on the cured film and still prevent skin formation.

MEKO Replacement

Most antioxidants are unsuitable for architectural and industrial applications due to their side effects. However, a proprietary aminic compound has been developed which is non-toxic, volatile and consumes free radicals. This “advanced antioxidant” provides a controlled retardation of surface dry resulting in improved through dry without affecting the film hardness. Another advantage is that its activity is independent of the drier metal. This new product is more volatile than other types of antioxidants, but not as volatile as MEKO. It can be used in clear and pigmented systems and has been successfully marketed since 2002.