Need for Driers in Coatings Formualtions
How does drying process takes place
How does drying process takes place
After application of the paint onto the substrate the solvent is released, which is the physical part of the drying process. In order to obtain a hard paint film, further polymerization of the binder is necessary.
The chemical part of the drying process 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" and similar to the drying process in solvent-rich paints2.
This part of the drying process significantly affects film hardness.2
Autoxidation Film Formation Process of Alkyd Resins
High solids alkyd paints differ from the classical alkyd paints in having lower solvents contents as well as in resin composition. In order to reduce solution viscosity and enable higher solids contents, high solids resins are:
- Composed of lower molecular weight polymers
- demonstrate high reactivity towards autoxidation
The high chemical reactivity is required to compensate for the low molecular weight of the resin. Waterborne alkyd paints are based on alkyd resins, either emulsified in water or aqueous (typically colloidal) solutions.
Effect of Driers on Film Hardness
Effect of Driers on Film Hardness
Driers strongly speed up the autoxidation process. Without these drying catalysts the cross-linking process is very slow; the paint layer may be dry only after some months post application. With driers the formation of a hard film is accomplished typically within a few hours after paint application.
Main catalysts in use as paint drier are transitional metal based driers, such as cobalt octoate. Cobalt is the most active drying catalyst at ambient drying conditions. However, other driers play an essential role in film hardness and performance.
The general reaction catalyzed by the cobalt catalyst can be seen in the figure below:
Reaction Catalyzed by Cobalt Drier
(ROOH standing for the hydroperoxide form of the alkyd fatty acid chain)
The role of Cobalt is defined as a redox catalyst; Cobalt also contributes to the activation of oxygen to the formation of peroxides; assumed is the multivalent metal is associated to the double bond, thus increasing the oxidation susceptibility1.
Types of Driers Used in Coatings
Types of Driers Used in Coatings
Main categories of driers are composed of stabilized metal carboxylate solutions in mineral spirit.
The cation is the active part of the metal carboxylate drier. Main drier metals are:
- Cobalt
Zirconium- Calcium
- Strontium
- Manganese
- Zinc
- Barium
- Cerium
- Lithium
- Bismuth
- Vanadium
Lead driers were the most commonly used in the past, but now have been replaced in most paints by less toxic alternatives.
Essentially, the metals can be divided into two groups:
This difference should be considered arbitrary as a considerable amount of overlap exists between them.
Active & Auxiliary Driers
Active driers at ambient temperatures promote oxygen uptake, peroxide formation and peroxide decomposition. At elevated temperatures several other metals, such as bismuth, display this catalytic activity but are ineffective at ambient temperature.
Primary driers when used without through driers would simply dry the upper surface of the film,
while the coating underneath would remain wet.
Auxiliary driers do not show catalytic activity themselves at ambient temperatures, but enhance the activity of the active drier metals, especially regarding the through drying and are also referred to as "through driers" (table below).
This category of dries ensure that the drying process occurs at a uniform rate throughout the
film body and not just at the surface.3
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Secondary/ Auxilliary/ Through Driers
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Primary / Active Driers
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Barium
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Cobalt
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Zirconium
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Manganese
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Calcium
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Iron
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Bismuth
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Cerium
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Zinc
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Vanadium
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Potassium
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Lead
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Strontium
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Lithium
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Cobalt
Cobalt is the most important and most widely used drier metal of all. It is primarily an oxidation catalyst and as such acts as a surface drier.
When used alone, cobalt may have a tendency to cause surface wrinkling and poor through drying. In order to provide uniform drying cobalt is used in combination with other metals like manganese, zirconium, lead, calcium and combination driers based on these metals.
Cobalt has a strong red-violet color, which may affect the color of the liquid paint if used at higher concentrations. However, the color of the dried paint layer is hardly affected.2
Manganese
Manganese is an active drier as well, though less effective than cobalt. As an accelerator of polymerization in baking finishes manganese is more effective than cobalt. Though it is not preferred for use in white paints as it affects the color. While manganese is seldom used alone; cobalt as the primary drier with manganese is a useful modifier.
Calcium
Calcium is very effective when used in conjunction with cobalt and zirconium. It promotes drying under adverse weather conditions such as low temperature and high humidity. Loss-of-dry problems during long storage times of paints can be reduced using calcium as an auxiliary drier. Calcium drier helps to improve hardness and gloss as well as to reduce skin-formation and silking. Moreover, calcium drier is useful as
pigment wetting and dispersing agent.
Strontium
Strontium is another candidate
that substitutes lead in lead-free systems. It functions well under adverse weather conditions and promotes through drying. Compared to other candidates
that replace lead, such as zirconium, strontium has the advantage of offering
the best storage stability and lowest gelling tendency of the paints.
Zirconium
Zirconium driers are the most widely accepted replacements for lead. Zirconium improves through drying mainly by the formation of coordination bonds with hydroxyl- and carboxylic groups, available from the resin or formed during the drying process. Apart from coordination, it is assumed that zirconium also forms complexes with cobalt, which influences the catalytic effect of the primary drier.
Best properties have been obtained if used in
combination with cobalt and calcium. The results attained with this combination are very close as with cobalt / lead / calcium-combinations. In comparison with other auxiliary driers, zirconium shows
the best color, lowest yellowing tendency and best durability.
Combination of Driers
Cobalt / zirconium / calcium
combination driers are the most widely used ones. Cobalt
being the active drier, also works as "surface drier". In order to improve through-drying, hardness and stability, auxiliary driers, like zirconium and calcium, are needed. The exact dosage and ratio depends on the composition of the paint and desired properties and application conditions.
Following drier combination has found wide acceptance in low solids alkyd paints (expressed as metal % on binder solids):
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0.06% Cobalt
- 0.3% Zirconium
- 0.15% Calcium
Driers for High Solids Paints
Driers for High Solids Paints
When compared to classical alkyd resins, binders in air-drying high solids paints have a lower molecular mass and
show higher reactivity to autoxidation polymerization. As a result of the high solids content, the thickness of the applied dry film is much higher and as a consequence, the through drying properties are more critical compared to lower solids systems.
Poor through drying and hardness are main shortcomings of high solids alkyd systems.
Surface-wrinkling of the dried paint layer is another problem. Both are related to high reactivity of the binder leading to fast surface drying, which prevents further penetration of oxygen through the paint film. This is why surface drying should be controlled, allowing oxygen to saturate the complete paint layer and allowing crosslinking also in the bottom part of the film.
Main region of action of primary versus through driers
A right balance between the drier effectiveness regarding surface drying (cobalt) and through drying (auxiliary drier) is of prime importance.
Combination driers of cobalt / zirconium/calcium or also cobalt / strontium / calcium are typically used in high solids, as are drier combinations.5 As compared to lower solids alkyd paints, the level of through drier is higher, as in following example:
- 0.05% Cobalt
- 0.4% Zirconium
- 0.2% Calcium
Driers for Waterborne Paints
Driers for Waterborne Paints
Waterborne paints help meet the requirements for environment friendlier paints, with lower solvent emissions, maintaining the excellent performance of alkyd paints regarding flow and leveling, adhesion, gloss and film appeal.
Waterborne, oxidative drying alkyd paints are typically based on emulsified alkyd resins or colloidal dispersed alkyd resins. Drying takes place physically by evaporation of water, solvents and subsequently oxidative polymerization, similar to
solventborne systems.
However, considerable differences exist in initial drying speed: the water evaporation speed is related to the relative humidity: high humidity weather conditions meaning low evaporation speed and longer drying time. Driers cannot avoid this dependence.
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Note
- It is important to recognize the change in polarity during the drying. The drier should be compatible with the liquid paint, showing high polarity as well as with the drying film, being apolar.
- Furthermore, the effect of auxiliary driers is less in comparison to solventborne paints. This is explained by the high solubility of oxygen in water, which saturates the paint layer with sufficient oxygen for the autoxidation process.
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Getting the Best Performance with Driers
Best performance has been obtained using driers that are emulsified in water. Water soluble driers typically show poor compatibility with the drying paint film and solubility in this is insufficient to affect the resin molecules.
Furthermore, the effect of auxiliary driers is less in comparison to solventborne paints. This is explained by the high solubility of oxygen in water, which saturates the paint layer with sufficient oxygen for the autoxidation process.
Best drying and film appearance properties can be reached using water-emulsifiable metal-complexes in which cobalt is main drier: 0,08% Co and 0,1% Ca on binder solids is considered a good starting point for siccativation of waterborne paints.
Test Methods to Select Optimum Drier
Test Methods to Select Optimum Drier
Main test procedure for the liquid drier is the determination of solubility and compatibility with the varnish or paint. Furthermore, it is the determination of the active matter content i.e., metal percentage in the drier. However, drier manufacturers have a very strict quality control procedure, nearly excluding any out-of-spec risk towards above characteristics.
In order to select best drier combination for a paint system, a wide range of paint properties need to be considered for evaluation. Although most properties are linked to:
- Film formation criteria
- The effect of the drier on optical properties
- Storage stability, and
- Application properties
The drying recorder (ASTM D5895-96) and hardness recorder (DIN 53157) are useful instruments for this.
Driers/Siccatives for Paints & Coatings
View a wide range of driers/siccative grades available in the market today, analyze technical data of each product, get technical assistance or request samples.
References
- J H Bieleman, Ed. in "Additives for Coatings", Wiley-VCH, Weinheim, New York, 2000, chapter 7.1
- M Fabrizio, internal report ATO B.V., P.O. Box 17, 6700 AA Wageningen, Neth., January 2001
- http://www.durachem.com/docs/Metal_carboxylates_for_Coatings.pdf
- Company brochure, Patcham www.patchamltd.com
- J H Bieleman, Fatipec Brussel 1996, Congress proceedings book A, p. 112