• Formulation Specificity with Vacuum Metallised Pigments - Part 2
• A Dream of Silver Bicycles
• Silver - The Color of Speed (Interview with Honda Cheng)
• METASHEEN in XYMARA.com Materials channel - use in paints and coating applications

Formulation Specificity with Vacuum Metallised Pigments

By Alison Metzger

Giant, one of the largest bicycle manufacturers in the world, wanted to achieve the appearance of vacuum metallisation on a bicycle decal without off-line metallisation or chrome plating. The technical and application challenges that arose provided an opportunity to demonstrate how excellence in customer service and technical co-operation between supplier and consumer can result in a commercially viable product.

Vacuum-metallised pigments (VMPs) are different from “conventional” aluminium metallic pigments and formulating with them must be application specific. The nature, effects and versatility of the use of VMPs in diverse applications are described below.

Background to Vacuum-Metallised Pigments

VMPs provide coating formulators and manufacturers with an alternative means of achieving the highly reflective effects desired by end-users, consumers and specifiers.

This highly reflective effect is due to the exceptionally smooth, flat, mirror-like nature of the VMP flakes. Whereas conventional ball-milled aluminium has to be milled or ground in the production process, VMPs are produced by vacuum deposition of aluminium metal in the vapour phase on very smooth substrates. This thin layer of aluminium is then stripped from the substrate surface and broken down into particles of a size suitable for application in surface coatings.

The Technical Challenge

Technical challenges arose in two main application areas:

• The screen printing of self-adhesive decals
• Spray painting of bicycle frames

1.Screen printing

The technical challenges relating to screen-printing metallic colour on decals or badges for bicycles were to achieve:

1. an optical effect the same as, or close to, total vacuum metallisation.
2. excellent adhesion of the screen ink to the polyethylene terephthalate (PET) film.

Giant’s original decals had been produced by vacuum metallisation of the PET substrate. This gave a coating with excellent image clarity and a true mirror finish. However, difficulties arose for Giant when more complex styling for the labels was required. With a fully vacuum-metallised label, it would have proven too costly to change the design on a regular basis. So a more flexible solution to this requirement was to screen print a ‘chrome effect’ ink.

This allowed the design to be changed quickly and inexpensively and the chrome effect ink to be printed in-line with the other colours, making an extra metallization process unnecessary.

Incorporation of Ciba^® METASHEEN^® into a screen ink produces a bright, metallic coating. The best optical results are achieved when the VMP ink is reverse printed on the underside of the substrate and the ink is viewed through the transparent substrate. On printing, the VMP flakes orientate in parallel to the plane of the substrate. The end result is comparable to vacuum metallisation. The level of brilliance and image clarity attained is greatly influenced by the smoothness of the substrate and the ink formulation.

Small differences in pigment/binder ratio and solids content have a significant impact on the optical effect attainable.

The range of p:b for VMP formulations is typically between 1:1 and 1:2.5.  

The best optical effect on a reverse-printed decal is obtained at a p:b of 1:1. For surface printing, higher resin solids are necessary and a p:b of 1: 2.5 is chosen, but the visual effect is less bright and the image clarity of the coating is poor.

Ink Solids

Low-solids inks are required to give the best optical effect. The formulations above have a typical solids level of 11%, resulting in a screen ink with a viscosity of approximately 0.1 to 1 poise. Potential application problems due to the low viscosity of Ciba^® METASHEEN^® screen inks can be overcome by either:

1. Using a fine screen mesh size – typically 120 lines per cm – to achieve the definition of design required.
2. Choosing a resin that at low solids imparts high viscosity. Higher viscosity inks have a minimal effect on the VMP performance compared with the effect obtained with high total solids.

The results achieved on the screen printer with the final ink formulation met all Giant’s requirements.

>> Part 2

Source: PPCJ, December 2006