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What is Metallizing and how is it used?
Metallizing (also sometimes spelled in US as metalizing or metalising) is a process of coating a non-metallic substrate with metal material. Microparticles such as microspheres, beads and spheroid powders are often metallized to impart conductivity to a non-conductive surface and create conductive spherical powder or to achieve retroreflective spherical particles. Justus von Liebig discovered the process of coating a glass surface with metallic silver, making the glass mirror one of the earliest items being metallized.
There are applications for both hollow and solid metallized glass beads. Coating the surface of hollow glass microspheres with metals or metallic compounds creates a composite material with benefits such as low density and buoyancy in aqueous systems making them suitable for a wide range of applications in areas as diverse as water purification, defence, aerospace, marine and automotive coatings, diagnostics and cosmetics.
Conductospheres™ are a range of lightweight conductive fillers consisting of hollow glass microspheres coated with a thin layer of metal.
They can be incorporated into coatings, composites and adhesives to provide these materials with electrical conductivity and shield against electromagnetic interference (EMI).
Due to their hollow glass core, Conductospheres™ have very low particle densities and so provide significant weight reductions over conventional metal fillers.
The Conductospheres™ range includes silver and nickel products with a variety of particle sizes, densities and crush strengths. Conductospheres Silver products offer the highest conductivity and stability, and are available with a range of particle sizes and densities.
Special considerations for working with metallized glass powders: Because the core of Conductospheres is hollow glass bead the physical integrity of the particles may be compromised if they are subjected to severe physical processing techniques, such as ultrasonication, high-shear mixing, grinding or milling. To maintain the free flowing nature of the product and prevent ‘caking’, avoid extended exposure of materials to high humidity and/or conditions susceptible to condensation. Once opened, storage life of the product can be maximized by resealing opened containers immediately after use.
Typical Surface Resistivities of Coatings Containing Conductospheres Silver and Conductospheres Nickel
Conductospheres Silver incorporated into a resin at 50% by volume will typically produce a composite with a density of 0.6-0.9 g/cm³ and a volume resistivity of < 0.1 O•cm. The surface resistivity of coatings containing Conductospheres Silver is typically about 1 O/sq. The graph below shows that the percolation threshold is reached at loadings of 20-30% by volume.
Another application for metalized microspheres and beads is retroreflectivity. Retroreflective microspheres are based on hemispherically aluminum-coated barium titanate glass beads. The hemispheric aluminum coating creates the mechanism for retro-reflectivity since light passing through the clear half of the glass bead “bounces” off the reflective aluminum-coated back, directing the light back to the source. Pigmented coatings allow expanded applications for hemispherically aluminum coated sphere in particular. Whereas previously their grey color limited the use of metallized spheres to only dark resin systems, pigmented coatings now allow these spheres to be added to most colored paints or plastics—even yellow—with only a slight decrease in the retro reflectivity of the colored sphere, assuring formulations that are attractive by day and brightly visible at night.
Fluorochemical and silane-based coatings can be applied to glass microspheres but their application is particularly important for metallized spheres: because of the high relative density of barium titanate glass, these spheres will typically sink in any resin system in which they are incorporated. Unless metallized spheres are at the surface of a formulation, light cannot travel through the clear hemisphere to bounce off the metallized shell and create a retro reflective effect. Metallized beads dispersed throughout a resin provide little benefit in terms of retro-reflectivity. Placement at the surface allows for a reduction in the amount of microspheres needed to achieve the desired level of retro reflectivity.
Sources:Prizmalite Industries IncMicrosphere Technology LTDCospheric is an authorized distributor for both of these companies