How does skinning arise in coatings and how can it be prevented?

Alkyd resins, based on unsaturated fatty acids, modified oils or epoxy ester resins, while in contact with air reactively crosslink to form films resistant to attack by water and other air pollutants. The slow auto-oxidation reaction can be accelerated by using driers based on salts of certain metal ions.

The formation of a partially cured surface film can unfortunately start prematurely during production of paints and during bulk coating storage due to the film's reaction with oxygen in the air. Although this skinning process inhibits air from penetrating deeper inside the paint, it also creates problems during application and promotes defects in the film's final appearance.

Anti-skinning agents can function through two mechanisms to overcome these problems. These agents can either scavenge the oxygen radicals that are formed near the surface and prevent them from prematurely crosslinking the resin, or reduce or inactivate the latent concentration of metal drier ions.

Typically these additives include nonvolatile anti-skinning compounds based on oxime or phenolic derivative chemistry. The oxime derivatives can serve to temporarily bond with the metal driers (catalysts) in the formulation to reduce the catalyzing action of the driers. Phenolic compounds are able to convert oxygen radicals into a less reactive state and are therefore widely used as anti-oxidants. Since the crosslinking process is retarded, the surface top layer is open for a longer time, providing a more uniform through-drying, and the formation of wrinkles and other surface defects is significantly reduced. As an added benefit, gloss, flow and leveling, and film hardness are improved.

Common metal driers, such as cobalt and manganese-based naphthanates or octoates bind effectively with oximes to reduce the concentration of the reactive metal state for the drying, and the storage stability of the paint is increased. These oxime complexes are sensitive to hydrolysis and, after application, they decompose and the volatile oxime evaporates quickly from the liquid film.

Evaluating Corrosion Protection of Additives for Waterbornes

What evaluation methods may be used to demonstrate the effectiveness of the corrosion-protection properties of additives for waterborne coatings on steel?

The determination of the effectiveness of additives that promote corrosion inhibition in waterborne coatings applied to metal substrates can utilize analytical instrumental evaluation methods that provide significant results on a short-term scale. As an example, many reported studies on the corrosion resistance of epoxy and urethane waterborne coatings practice using electrochemical and nonelectrochemical methods that measure the changing properties of substrate and coating components.

Electrochemical impedance spectroscopy, linear sweep voltammetry, mechanical pull-off tests and scanning acoustic microscopy combined with image analysis are examples of methods used. For these studies, two kinds of corrosion-inhibiting additives are employed: an organic inhibitor, i.e., salts based on a carboxylic acid neutralized by a polysiloxane base, and zinc-based inorganic pigment with inhibiting properties.

The results show that corrosion-inhibiting additives drastically modify the adhesion, water uptake, blistering behavior and substrate protection of waterborne coatings. Both additive types tend to show improved dry adhesion and reduced blistering under cathodic polarization conditions.

Generally, these experimental methods can be useful for additive development and for coating formulation applications because together they yield a more complete indication of the chemical processes occurring than can be obtained by single methods of how a given additive affects the coating performance in a corrosive environment.

Usefulness of Halogenated Additives

Do halogenated additives provide any particular advantages for waterborne coatings?

Halogenated additives, particularly organo-halogen compounds, have been found useful in many coating applications. These molecules can provide molecular-level, controlled polar interactions, molecular spatial characteristics and an inherent heteroatom content that supports their application as dispersants, biocides, UV stabilizers, and solvent- and heat-resistance promoters in formulated coating systems.

Despite the advantageous utility of many halogenated species in coating applications, the potential hazard offered by these chemicals has allowed the development of new coating technology advances to be employed as substitutes where practical.