Coatings World - June 2016

Richard Najdusak, Borchers, USA; Dr. Franjo Gol, Borchers, Germany

Introduction

Being a formulator is becoming much more difficult in light ofin- creased regulatory pressures on common raw materials. Formulators who develop coatings based on oxidatively cured (alkyd) binders face major regulatory obstacles for two key ingredients: methyl ethyl ketoxime (MEKO) anti-skinning additive and cobalt-based driers. MEKO’s toxicity makes it difficult to meet safe exposure limits due to its volatility. Cobalt carboxylates are being pressured to be classified as carcinogens. In this paper we discuss potential non-toxic alternatives to both of these materials with supporting test results.

Binders that Cure via
Oxidation

Drying oils and alkyd resins have many
advantages over other binders used in the
coatings industry. They are bio-renew-
able, inexpensive and extremely stable.
Alkyds can be used to formulate coatings
which have long open times for easier ap-
plication, yield relatively hard films for
kitchens and baths, require minimal sur-
face preparation and provide good stain
blocking properties, particularly over wa-
ter-based stains, inks and markers. Water-
based versions of these binders have been
developed to meet VOC regulations, have
low odor and water clean-up. Alkyd res-
ins are used in architectural, light duty
industrial, wood care, printing inks and
automotive aftermarket coatings.

Drying oils, alkyd resins and epoxy esters have certain things in common. They all contain some degree of unsaturation (double bonds) and hydroxyl/carboxyl functionality which are used to promote curing. In the following example of a common vegetable oil, the triglyceride contains a range of organic acids which have various degrees of unsaturation (Figure 1). Those with more potential crosslinking (unsaturation) sites, such as linolenic acid, can be expected to dry faster and yield harder finishes, while binders with high levels of oleic acid have few unsaturation sites and will yield softer, slow drying films.

The Need for Driers

Atmospheric oxygen reacts spontaneously with the unsaturated fatty acid components of oils and alkyd resins generating free radical reactions to eventually form films (Figure 2). This process is called autoxidation. However, if unaided, the reaction rates are very slow.

Surface (or oxidative) driers catalyze
reactions by: deactivating the natural an-
tioxidants found in drying oils by form-
ing hydroperoxides; accelerating oxygen
absorption and peroxide formation; re-
acting with oxygen or hydroperoxides to
form complexes that catalyze oxidation
reactions; and acting as oxygen carri-
ers. Surface driers are also susceptible to
redox reactions which can decompose hy-
droperoxides and promote crosslinking.

Metal Catalyzed Reactions Adding metal catalysts greatly improves the oxidation rate of the binder (Figure 3). When a metal catalyst is present, the activation energy for oxygen uptake is only 10% of the amount needed when none is present. Peroxide formation also proceeds more rapidly. Metal catalysts aid in the decomposition of the peroxides which accelerates free radical formation to promote crosslinking.

Another advantage is that the lower valence state of the metal is regenerated making most oxidative metal carboxylates true catalysts which, unfortunately, is not always a good thing as will be discussed.

Metals Capable of Use in Oxidative (Surface) Driers There are only five metals that promote oxidative or surface curing in some form. Cobalt is the most common oxidative drier and has been commercially available since the early 1900s. It exists in the two valance states noted in Figure 3 which helps it form coordination complexes with oxygen and organic molecules. The activity of cobalt driers is inhibited at lower temperatures and its use in high humidity drying conditions may promote film wrinkling.