Sustainable aluminium pigment technology

This paper describes a unique process to manufacture modern, high-performance aluminium-based special effect pigments in a safe, controllable process without the use of or emission of harmful hydrocarbon or other volatile solvents. The resultant aluminium pigment also facilitates efficient post treatments for use in different highly demanding coating applications. Steven Kerr, Metaflake Ltd

Aluminium-based effect pigments continue to be the dominant type of effect pigments used most widely in all types of coating applications globally. The unique visual properties of aluminium pigments have brought their use into many coated applications, whether for visual enhancement (major purpose) or for other protective or technical properties.

Aluminium pigments are produced almost exclusively using the Hall process, which is a wet milling process utilising large quantities of hydrocarbon solvents or other volatile solvents as the milling medium. Dry milling of aluminium is rarely done nowadays, due to safety concerns.

Metaflake has taken a radical approach to a conservative industry, developing a unique, environmentally friendly and highly innovative technology to manufacture aluminium pigments in a water milling process, 100% free of harmful hydrocarbon and other volatile solvents and solvent emissions.

There are many challenges when milling aluminium in water due to the inevitable reaction of freshly exposed aluminium surfaces with water, as illustrated by the simple chemical equation:

2 Al + 6 H₂O = 2 Al (OH)₃ + 3 H₂

Metaflake therefore undertook to design a process for production of aluminium pigments milled in water that is:

• Flexible – can be utilised to produce the required range of particle shape and size.
• Tightly controlled – to ensure highly consistent quality batch to batch.
• Safe and ecological – with the possibility to further evolve and increase efficiency.
• The future – a viable step in reducing harmful emissions and contributing to the protection of our planet.
• Simple and efficient removal of residual water – allowing various post treatments and different product delivery forms for other applications.
• Exceptionally clean products – 100% free of hydrocarbon residue and therefore suitable for the widest range of applications.

Products for liquid coatings

Via the unique water milling process, it is possible to create aluminium effect pigments with “Silver Dollar” geometry in various particle sizes which exhibit exceptionally bright metallic effects in liquid coating applications. Further, the tight process control allows the efficient manufacture of so-called Ultra-thin Silver Dollar types offering higher opacity, highly chromatic appearance and enhanced flip-flop or metallic colour travel.

The conventional silver dollar range covers a very wide spectrum of metallic particle sizes:

SD Grade	Particle size, d50	SD Grade	Particle size, d50
SD 1100	11um	SD 4000	40um
SD 1600	16um	SD 5212	50um
SD 2200	22um	SD 6500	65um
SD 2700	27um	SD 8000	80um
SD 3214	35um	SD 9900	100um

While the range of ultra-thin silver dollars available include:

TD Grade	Particle size, d50
TD 1000 type	11um, ultra-thin silver dollar
TD 1500 type	15um, ultra-thin silver dollar
TD 2100 type	21um, ultra-thin silver dollar

The features of the TD range of ultra-thin silver dollar  type include:

• Exceptionally bright metallic effects
• Narrow, tightly controlled particle size distribution
• Approximately 30-35% higher opacity than conventional silver dollar pigments
• Suitable for single coat applications
• Can formulate bright and silky-smooth effects also at low solids.
• Can achieve very high flop index with good formulation and optimised pigment orientation.
• Suitable for highly chromatic effects in combination with transparent colour pigments

Additional pigment surface treatment

These water milled aluminium pigments have been found to be highly suitable for further surface treatment modifications to allow the pigments to be used in the most highly demanding coating applications, including:

• Automotive coatings – OEM, refinish and components.
• Plastic coatings for electronic applications and appliances
• High durability coil coatings
• High durability powder coatings
• UV curable coatings and inks
• High performance water-based printing inks

An efficient application of a modified inorganic, silica-based coating is applied to the surface of the water milled pigments to meet the necessary technical requirements.

High performance water-based coatings

Aqua-Brite TCT pigments are treated with a new advanced, compact pigment encapsulation process to provide the highest levels of stability and performance in demanding applications, provided in a widely compatible, easy to disperse, paste form. Available with both Ultra-Thin Silver Dollar (TD) and standard Silver Dollar (SD) types, exceptionally high metallic brilliance with excellent opacity is retained, as is excellent adhesion, both to substrate and topcoat. Offering the highest stability in water-based paints and suitable also for high performance solvent-based coatings, as well as excellent chemical resistance and stability in the final application. The pigments are 100% free of mineral oils, mineral spirits, and hydrocarbon solvents with very good regulatory credentials – statements relating to specific regulations available upon request.

Conclusions

A unique method of milling in water has allowed the development and production of high-quality aluminium effect pigments with exceptional metallic brilliance for use in high performance coating applications. The process eliminates the use of and emission of harmful hydrocarbon or other solvents allowing a lower carbon footprint and a real progress towards a sustainable future for aluminium-based effect pigments.

Website: www.metafkale.com