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To get maximum performance out of the metallic pigment within a finished coatings system,it is essential to wet out metallic pigments. A dispersion process separates the flakes from each other so the metallic flakes deliver their full potential of hiding, brilliance and sheen to the coating system.
When a metallic pigment is used in a coatings system, there is the need to ensure that there are no clumps of metallic pigment that would lower the appearance quality of the metallic pigment. Inorganic / organic color pigments achieve this through milling and high speed dispersion methods. Due to the sensitivity of metallic pigments to fracture and bend with these high speed or milling operations, another method must be used to avoid shearing the metallic pigment. The pre-dispersion method, in principle a gentle mixing operation, described below properly disperses the metallic pigment without introducing the shear that would harm the particle edge and shape.
There are several factors (metallic pigment + solvent + mixing blade + shear speed + mixing time) that comprise a pre-dispersion as well as a few methods (drawdown on paper, plastic sheeting, or glass plate) to determine when pre-dispersion of the metallic paste is achieved.
One part metallic pigment paste is added to one to two parts solvent (for fine metallic pigments with average particle size d50 lower than 12 µm a one part metallic pigment and two parts solvent is preferable) and slow mixed for 10-30 minutes with a low shear speed and a low shear-mixing blade (propeller) / so-called butterfly mixers.
High shear mixing speeds (above 1500 rpm, if possible) and teeth blades (cowles) may introduce fracturing and breaking of the metallic pigment and are not recommended.
A drawdown (wire round, bird bar) on lanetta paper cards or plastic sheeting can indicate if the pigment paste is properly dispersed. The check drawdown should be free of clumps, seeds, trough marks that indicate seeds or clumps of metallic pigment.
The wetting strength of the solvent may be an issue to provide proper dispersion of the metallic pigment and a dispersing aid additive can be used at 0.2%-1% of the total weight of the pre-dispersion mix (metallic pigment + solvent) (for additive dosage follow technical data sheet). A recent study showed that PCBTF solvent had difficulty pre-dispersing metallic pigment into solvent, even after 45 minutes of dispersion time, while another solvent, n-butyl acetate dispersed the metallic pigment without the need for a dispersing agent. Pre-dispersion solvents and time should be tested in individual systems for compatibility and solvent dispersing strength.
There is been some debate on the importance of addition of a dispersing aid as part of the pre-dispersion process. Recent studies have shown that there is an advantage to including a dispersing aid. In a two component polyurethane system, the addition of a dispersing aid lowered coating viscosity as well as raised gloss levels as noted in the charts below.
While the chemistry of the dispersing aid was varied, there as unilateral improvement in gloss and viscosity including a dispersing aid as part of the pre-dispersion package across the three basic shapes of metallic pigments, cornflake, silver dollar and non-destructing flakes (larger than normal particle thickness metallic flakes).
There are many people who do not believe that pre-dispersion is needed as part of the metallic pigment addition into coatings with questions regarding time needed, equipment capacity and evaluation requirements needed to correctly pre-disperse metallic pigments. Data presented that pre-dispersion of metallic pigments is a necessary and beneficiary to successfully orient metallic particles and allow maximum benefit from this class of pigments in coatings formulations.
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