Antioxidants for Paints

This article was originally published in 2013 and updated in 2022.

Antioxidants are a group of additives demonstrating oxidation-inhibiting properties. They are used in coating formulations for the following two main purposes:

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

An example of the first purpose is the use of anti-skinning agents in air-drying paints. At the contact area of the liquid paint and air, the binder oxidizes. This follows crosslinking and skin formation at the contact area.

For the second purpose, oxidation of the paint film on exposure follows degradation. This happens at the cost of the protective properties and film appearance.

Let’s review the role of antioxidants in the coatings industry.

Antioxidants Composition and Mechanism

Only a few chemicals are efficient in providing oxidation inhibition in coatings. The main antioxidants categories include:

• Amines,
• Lactones,
• Phosphites,
• Phenolics,
• Thioethers, and
• Blends.

Phosphites and thioethers are both known as secondary antioxidants. Other categories also include nitrogen derivatives (MEKO).

Find suitable antioxidants here »

Here, we discuss phenolic antioxidants and their mechanism of action.

Phenolic Antioxidants & Mechanism of Action

An example of phenolic antioxidants is hindered phenols. For example: BHT - 2,6-Di-tert-Butylphenol (also known as "butylated hydroxytoluene").


Figure 1: Structure of BHT
Phenolic antioxidants react with oxygen-containing free radicals during the autoxidation process. The antioxidant deactivates the formed free radicals. It also interrupts the autoxidation process via a kinetic mechanism.^1,4

R-O-O• + AH  →  R-O-OH + A•

where R-O-OH is a peroxide and AH is 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.^1,2 During this process, the formed phenol radical A• is quite stable, showing a 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,
• 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.

How Do Antioxidants Safeguard the Liquid Paint?

Autoxidation Process & Role of Catalysts

Oxidative drying paints show skinning of the in-can paint during storage. This happens as a result of oxidation and polymerization. The paint contains air-drying oils or macromolecular binders such as:

• alkyd resin,
• epoxide-ester, and
• urethane-alkyd.

By influence of the oxygen, the liquid paint binder is polymerized and transformed into a solid layer.

In principle, the skinning mechanism is identical to the film-formation mechanism.
The chemical part of the drying process of alkyd paints involves oxidative cross-linking. It starts with the reaction between unsaturated parts in the binder molecule and oxygen (air). This process is known as the "autoxidation process" (Figure 2).

Figure 2: The autoxidation film formation process of alkyd resins.
Catalysts, known as driers or also siccatives, strongly speed up the autoxidation process. The main catalysts in use as paint drier are transitional metal-based driers, such as cobalt octoate; the active part in the carboxylate is the metal ion.³ Figure 3 shows the general reaction catalyzed by the cobalt catalyst:

Figure 3: Reaction catalysis by cobalt drier.
(ROOH: hydroperoxide form of the alkyd fatty acid chain)
The role of cobalt in this process is a redox catalyst. Besides this cobalt contributes to the activation of oxygen resulting in the formation of peroxides. It is also assumed as the multivalent metal associated with the double bond, thus increasing the oxidation susceptibility.¹

How to prevent paint skinning using antioxidants?

Skinning in the can, during storage, means quality losses from several points of view. You cannot use the coating material immediately after skinning. Remove the skin first before using the paint. Learn how to improve the storage stability of your paints.

Skinning of paints causes:

• A part of the coating material gets lost.
• The strength of driers in the skin is above-average compared to the remaining coating material.

Therefore, skinning could lead to a prolongation of the drying.

Parts of the skin that remain in the coating material are visible as disturbing dots or other particles in the drying film. As a result, the material's coating has to be grinned off. This leads to a considerable cost increase in the operating process. One way to reduce the risk of skin formation in paints is to include antioxidants in the formulation, notably phenolic compounds. This class provides effective protection against skinning during paint storage. This class has found limited use in air-drying paints due to:

• low volatility and
• staying in the applied paint film, thus prolonging the drying time.

The most widely applied anti-skinning agent in air-drying paints is Methyl Emethyl ethyl ketoxime (MEKO). Although not strictly considered an antioxidant, 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, MEKO is then removed from the drying paint by evaporation.

Check out other anti-skinning agents here »

How do Antioxidants Safeguard 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.⁴

Phenolic antioxidants hinder the thermally induced oxidation of polymers in coatings.

• They trap the free radicals formed on heating in the presence of oxygen.
• Prevent discoloration or change of the resin's mechanical properties.

Selection Criteria to Consider While Choosing Antioxidants

The selection becomes easier by knowing the stage at which antioxidants are necessary.

• STAGE 1: During the preparation and storage of the liquid paint.
• STAGE 2: During exposure of the coating (Table 1).

The next criterion to consider for the following paints:

• Liquid paints: Resin composition and mechanism of polymerization.
• Air-drying paints: MEKO is the choice of antioxidant preferred. It shows excellent prevention of oxidation and polymerization in the wet system. It also maintains the required paint drying speed after application.
• For long-oil or medium-oil alkyd paints: Select phenolic antioxidants only in case some loss of drying is acceptable.

System	Aspects		Antioxidants
	Liquid	Paint Film	MEKO	Phenolics	Phosphite	Thioether	Blends
Polymer synthesis	x	n.a.	-	x	o	o	x
Alkyd, air-drying	x		x	o	-	-	o
Alkyd, air-drying		x	-	x	o	o	x
Clear wood coatings, phys. drying	n.a.	x	-	x	o	o	x
Conventional SB coatings		x	-	x	o	o
Stoving enamels	x	x	-	x	o	x	x
NC lacquers		x	-	x	o	o	x
Coil coatings		x	-	x	o	x	x
Powder coatings		x	-	x	x	-	x
x: first selection, o: optional, -: not recommended.

Table 1: Selection Aspects of Antioxidants According to Several Systems
Phenolic antioxidants inhibit the oxidation of thermally-induced polymers. The main application areas are coil coatings and stoving enamels. Antioxidants protect against overbake yellowing by terminating free radicals.

For film protection, phenolic-based antioxidants have a perfect record of high efficiency. The main application area is wood coatings. Use compounded systems containing a well-balanced mixture of various phenolic compounds. These systems also demonstrate specific solubility and compatibility aspects.

Suitable Antioxidant Grades in the Coatings Industry

View a wide range of antioxidants grades available in the market today, analyze technical data of each product, get technical assistance or request samples.