Antioxidants for Paints

Introduction

Anti-oxidants are a group of additives, demonstrating oxidation inhibiting properties and are used in coating formulations for following two main purposes:

• To optimize stability of the liquid paint during manufacturing and storage
• To protect the paint film from decomposition

An example of first purpose is the use of anti-skinning agents in air-drying paints. Oxidation of the binder is followed by crosslinking and formation of a skin at the contact area of the liquid paint and air.

Oxidation of the paint film on exposure is followed by degradation at the cost of the protective properties and film appearance.

Composition and Mechanism

Only few chemicals are known as being efficient in providing oxidation inhibition in coatings: main products are based on phenol, as well as phosphite and thio-ethers (both known as secondary antioxidants), as well as nitrogen derivatives (MEKO).

Examples of antioxidants, based on phenolic derivatives, are hindered phenols such as:

BHT : 2,6-Di-tert-Butylphenol (also known as "butylated hydroxytoluene")

The action mechanism of the phenolic antioxidants is based on its reaction with oxygen-containing free radicals, formed during the autoxidation process; the antioxidant deactivate the formed free radicals and interrupt the autoxidation process via a kinetic mechanism (ref. 1 and 4)

where R-O-OH is a peroxide and AH a phenol antioxidant.

The antioxidant competes with the organic substances involved in the autoxidation process; however, the reaction speed of the antioxidants is much higher than the reaction speed of the autoxidation (ref. 1, 2). During this process the formed phenol radical A• is quite stable, showing low rate of reactivity with hydrogen from the polymer chain: thus preventing further participation in chain extension and crosslinking.

The stability of the phenoxy radical formed according to the equation depends on the substituents at the phenol-group and thus on the possibility of resonance stabilization (delocalization of the electron). The more stable the phenoxy radical, the less likely it is to initiate further chain reactions.

Secondary antioxidants decompose peroxides (R-O-OH) formed during the autoxidation process and extend the performance of primary antioxidants by synergistic effects.

To Protect the Liquid Paint

Oxidative drying paints typically demonstrate skinning of the liquid paint in the can during paint storage as result of oxidation and polymerization. The paint contains air drying oils or macromolecular binders, such as alkyd resin, epoxide-ester, urethane-alkyd. By influence of oxygen the liquid paint binder is polymerized and transformed into a solid layer.

In principle the skinning mechanism is identically with the film-formation mechanism.

The chemical part of the drying process of alkyd paints involves oxidative cross-linking and starts with the reaction between unsaturated parts in the binder molecule and oxygen (air). This process is known as the "autoxidation process" (figure 2).

Catalysts, known as driers or also siccatives, strongly speed up the autoxidation process.

Main catalysts in use as paint drier are transitional metal based driers, such as cobalt octoate; the active part in the carboxylate being the metal ion (ref. 3).The general reaction catalyzed by the cobalt catalyst is reported (fig.3):

Obviously the role of cobalt in this process is a redox catalyst. Besides, cobalt contributes to the activation of oxygen resulting in the formation of peroxides; assumed is the multivalent metal is associated to the double bond, thus increasing the oxidation susceptibility (ref. 1).

Skinning in the can, during storage, means quality losses in several points of view. As a result of skinning the coating material cannot be used immediately, the skin has to be removed first, with material losses. Not only a part of the coating material gets lost but it has been proved, that the strength of driers in the skin is above-average comparing to the remaining coating material, therefore the skinning could lead to a prolongation of the drying.

Parts of the skin could remain in the coating material and are visible as disturbing dots or other particles in the drying film. As a result the coating material coating has to be grinned off, connected with a considerable cost increase of the operating process.

One way to reduce the risk of skin formation in paints is including anti-oxidants in the formulation, notably phenolic compounds (see paragraph 2). Although providing effective protection against skinning during paint storage, anti-oxidants of this class have found limited use in air-drying paints as these products typically show low volatility and stay in the applied paint film, thus prolonging the drying time.

The most widely applied anti-skinning agent in air-drying paints is methyl ethyl ketoxime (MEKO). Although not being strictly considered an anti-oxidant itself, MEKO retards the oxidation process in the liquid paint by complexing the oxidation catalyst (notably cobalt) and functioning of radical cap during the autoxidation process. After film application the MEKO is removed from the drying paint by evaporation.

However, phenol-based anti-oxidants are successfully used as inhibitor for thermally induced oxidation of polymers in coatings. These antioxidants function by trapping free radicals as formed upon heating in presence of oxygen and thus prevent discoloration or alteration of mechanical properties of resins (ref. 4)

Antioxidants to Protect the Paint Film

Oxidation can impair coatings during exposure. The chemical changes it causes will eventually affect the performance and appearance of the material. Antioxidants counter the effects of oxidation of the coating layer during aging (ref. 4).

Phenolic antioxidants hinder thermally induced oxidation of polymers in coatings. They do so by trapping the free radicals that are formed on heating in the presence of oxygen, and in so doing prevent discoloration or change of the resin's mechanical properties.

Selection Criteria

Selection is made in relation to the stage of needing antioxidant, i.e.:

• A: during preparation and storage of the liquid paint
• B: during exposure of the coating (table 1)

As a next selection criteria for liquid paints the resin composition and mechanism of polymerization is considered. For air-drying paints obviously oxime (MEKO) is preferred, typically showing excellent prevention of oxidation and polymerization in the wet system, whereas the required paint drying speed, after application, is maintained. In air-drying paints, such as long or medium oil length alkyd paints, phenolic anitoxidants are considered only in case some loss of dry would be acceptable.

Thermally induced oxidation of polymers is inhibited by phenolic antioxidants. Main application areas are coil coatings and stoving enamels. Antioxidants provide protection against overbake yellowing by terminating free radicals.

For film protection, phenolic-based antioxidants have a perfect record of high efficiency. Main application area is in wood coatings. Typically compounded systems are used, containing a well-balanced mixture of various phenolic compounds and demonstrating specific solubility and compatibility aspects.

References

1. J H Bieleman, Ed. in "Additives for Coatings", Wiley-VCH, Weinheim, New York, 2000, chapter 7.1
2. M Fabrizio, internal report ATO B.V., P.O. Box 17, 6700 AA Wageningen, Neth., January 2001
3. Patcham product brochure, www.patchamltd.com
4. BASF Performance Additives Brochure: Irganox® and Irgafos®