Cobalt is one of the most efficient highly potent and effective drier per unit of metal known today. It
is primarily an oxidation catalyst and as such acts as a ‘Surface’ or ‘Top Drier’. Cobalt provides fastest
surface drying properties and is required to be used in small quantities. Used alone, it may have a
tendency to cause surface wrinkling and poor through-dry. Therefore, to provide uniform drying,
Cobalt is generally used in combination with other metals, such as manganese, zirconium, lead,
calcium and combinations of these metals. Excess use of cobalt may result in premature skin
formation. This can prevent further passage of oxygen and then reduce the efficiency of drying,
ultimately resulting in formation of soft films.
| Metal Content (%) | Physical State | Colour (Gardner) | Non-volatile matter (%) | Specific Gravity (25° C) | Viscosity (cP at 25° C) |
|---|---|---|---|---|---|
| Cobalt 12% | Clear | Violet | 65 – 74 | 1.020 | 500 (max) |
| Cobalt 10% | Blue | Violet | 56 – 63 | 0.975 | 175 (max) |
| Cobalt 6% | Violet | Violet | 30 – 36 | 0.870 | 50 (max) |
| Cobalt 3% | Violet | Violet | 16 – 22 | 0.830 | 15 |
| Cobalt 1% | Liquid | Violet | 4 – 6 | 0.790 | 15 |
Lead functions as drier by promoting polymerization of drying oils to hard, insoluble film. In
contrast to cobalt and other ‘top driers’, lead causes the film to dry through its entire
thickness and is therefore known as a ‘through-drier’.
Lead also improves flexibility, toughness and durability of the film. In addition to its function
as through-drier, lead improves the water resistance and salt spray resistance of a film. It also
serves as a pigment-dispersing and wetting agent. Lead drier is generally uses in combination
with cobalt or manganese and calcium. The use of calcium with lead prevents lead
precipitation and hazing.
Lead drier should not be used in aluminum systems because of tendency to de-leaf the
aluminum and destroy the metallic luster of the pigmentation. Lead drier will cause darkening
of white paints in high sulfide environments. In adverse environmental conditions such as
low temperature and high humidity, lead performs the best with respect to any other auxiliary
drier. Toxicity of lead drier now precludes their use in many applications.
| Metal Content (%) | Physical State | Colour (Gardner) | Non-volatile matter (%) | Specific Gravity (25° C) | Viscosity (cP at 25° C) |
|---|---|---|---|---|---|
| Lead 36% | Clear liquid | 2 (max) | 70 – 78 | 1.380 | 1300 (max) |
| Lead 33% | 2 (max) | 68 – 75 | 1.280 | 300 (max) | |
| Lead 30% | 2 (max) | 58 – 66 | 1.230 | 150 (max) | |
| Lead 24% | 2 (max) | 49 – 54 | 1.110 | 50 (max) |
Calcium is one of the important auxiliary drier. It has little drying action in itself but is very
useful in combination with active driers. Calcium has strong surface active properties , hence
it is very effective as a dispersing agent. It also helps to certain extent as an anti-adsorption
agent to replace part of the lead with a larger amount of calcium to prevent precipitation of
the lead and maintain drying efficiency. The most important use of calcium is its ability to
prevent haziness or turbidity of lead phthalate observed in compositions containing dibasic
acids, like phthalic anhydride.
| Metal Content (%) | Physical State | Colour (Gardner) | Non-volatile matter (%) | Specific Gravity (25°C) | Viscosity (cP at 25°C) |
|---|---|---|---|---|---|
| Calcium 10% | Clear liquid | 2 (max) | 50 – 55 | 0.975 | 500 (max) |
| Calcium 5% | 2 (max) | 21 – 26 | 0.860 | 175 (max) | |
| Calcium 4% | 2 (max) | 19 – 23 | 0.840 | 50 (max) |
Zirconium is a most useful & efficient auxiliary drier. Zirconium like lead serves as a through
drier & is generally used in combination with Cobalt, Manganese & Calcium. Unlike Lead,
Zirconium is a poor pigment wetting & dispersing agent. Zirconium is an active cross-linking
agent & as such improves hardness of stoved films as well as their adhesions. Zirconium is
not as effective in Phenolic resin based media it is recommended as a catalyst for epscy esters.
Use of Lead driers in combination with Zirconium is to be avoided totally.
| Metal Content (%) | Physical State | Colour (Gardner) | Non-volatile matter (%) | Specific Gravity (25° C) | Viscosity (cP at 25°C) |
|---|---|---|---|---|---|
| Zirconium 6% | Clear liquid | 2 (max) | 32 – 37 | 0.87 ± 0.02 | 20 |
| Zirconium 12% | 3 (max) | 46 – 51 | 0.96 ± 0.02 | 25 | |
| Zirconium 18% | 3 (max) | 66 – 71 | 1.11 ± 0.02 | 40 |
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