• Formulation Specificity with Vacuum Metallised Pigments - Part 1
• METASHEEN in XYMARA.com Materials channel - use in paints and coating applications

Formulation Specificity with Vacuum Metallised Pigments - Part 2

2. Electrostatic Spray Paint Coating

Having achieved the desired bright metallic effect on their decal, Giant wanted to obtain the same result in their electrostatic spray process using a VMP-based paint on the frame of the bicycle.

The desired effect was achieved, following several technical refinements of the complete paint and coating process.

Primer Coat

Giant was already using a solvent-based primer with a high gloss finish that gave a very smooth surface, ideal for the subsequent application of the VMP paint.

Surface defects and scratches on the primer or substrate are exaggerated when covered by the basecoat on account of the very low solids systems and the inherent brilliance of VMPs.

Basecoat

The orientation of the flakes is crucial to the brilliance of the coating. A coating with a monolayer of flakes in perfect alignment would be extremely bright. Perfect recreation of the mono-layer, however, can never be achieved. In order to compensate for this, a degree of over-pigmentation is required. Inevitably, flakes will overlap each other and extra flakes will be needed to achieve full opacity and optimise coverage. However, the flake edges have a negligible effect on the brilliance of the coating.

For a VMP, solids should be 0.3–0.7% metal in the final basecoat. Even at this apparently low pigment content, full opacity is achievable. This is due to the surface area of the VMPs, which is approximately 10 times larger than that of a conventional ball-milled aluminium of the same particle diameter.

The resin is another important component of the basecoat system. Its primary function is to bind the pigment to the substrate, but high resin solids can prevent smooth settlement and orientation of the VMP flakes at the substrate surface. A typical p:b for VMP-based spray paints should be between 1:6 and 1:10, which equates to 3-5% resin solids.

The choice of solvent depends on the solubility of the resin. The evaporation rate of the solvent combination affects VMP performance. Fast solvents evaporate too quickly, preventing the flakes from orienting correctly, resulting in a dull, hazy finish. Suitable solvents are amyl acetate, Dowanol PM or PMA.

Topcoat (clear coat)

Ideally the solvents used in the topcoat should be incompatible with the resins in the basecoat. Any solubilisation of the basecoat causes the flakes to become mobile, which disturbs the orientation of the flakes and the resulting effect is dull and hazy. If solvent choice is limited, re-solubility can be minimised by using:

1. A 2k basecoat that can be cured, locking the flakes in place.
2. A powder clear coat, so no solvent is introduced to the basecoat.

Typical Basecoat Spray paint Formulation:

Conclusion

VMPs are a versatile, highly reflective alternative to conventional metallic pigments and can be used in a variety of coatings applications. The liquid metal effect, which replaces the grainy appearance produced with the coarser conventional aluminium pigments, is extremely popular with consumers as it matches current shade trends and give a high-quality, high-tech feel to the finished product.

Giant is now using the VMP pigment in their production models.

>> Back to part 1

Source: PPCJ, December 2006