which is a powder coating incorporated with metallic pigment(s), has been
gaining increased market share in recent years . Powder coatings are dry paints formulated
with resins, pigments, and additives, which are similar to the solvent borne paints with the exception of
no solvent present; while metallic pigments, such as aluminum, copper, nickel and zinc, are introduced
for providing better aesthetic appearance and protection. Metallic powder coatings are usually applied
on premium products, for instance, automotive, computer and high-end appliance parts etc.
Their final films exhibit extra shine and deepness to the color by introducing a metallic
effect. Initially, all metallic powder coatings were non-bonded, which means that a powder base coat
was prepared and then metal flakes were dry-blended into the powder to create a metallic effect
. There is no adhesion between metallic flakes and base particles. This method is simple, but
the product is unstable, due to the poor recyclability caused by non-bonding between the two.
Recyclability means that over-sprayed powders (about 10 to 40%), which failed to deposit onto a
substrate at first spray, can be re-sprayed after being recycled by a powder collecting system .
This is one of the biggest advantages of powder coating . The sharp distinctions between base
particle and metallic flake, such as shape, surface conductivity, volume and mass, result in a difference
in the concentrations of the metallic pigment in the final film and the original coating powder when
using the electrostatic spraying process, which is the main spraying method in the powder coating
industry due to its ability to realize full-automatic painting with thick films (50–300 μm) on a large
This difference of course leads to the inconsistency of the metallic pigment
concentrations between the over-sprayed powder (the part that fails to deposit onto the work piece) and
the original coating powder, making the over-sprayed non-recyclable. Additionally, the electrostatic
spraying makes the metal flakes “bunch” together due to electrostatic force, and therefore causes a nonuniform shininess over the final coating film. In addition, there are some other drawbacks with nonbonded product, such as metallic flakes accumulating on the head of the spray gun, which has a high risk of discharging and burning . On the contrary, a fully bonded product can overcome all of these problems.