Manufacturers that are considering manufacturing metallic effect powder coatings will
need to consider the process risks. The process will involve ‘bonding’ of fine aluminium
flake pigment onto the surface of warm thermoset powder coating particles. Fine
aluminium powder under the right conditions, has the potential to give rise to severe dust
explosion hazards. Powder coating particles themselves are also potentially flammable as
dusts. Therefore the handling and processing of these materials needs to be performed in
a way which minimises the risk of a dust explosion.
In order to produce bonded metallic powder coatings, a process is required that will heat
the powder to its softening point, usually 45-550C. The heat input will be achieved by a
combination of mechanical energy from high speed mixing tools, from a hot water jacketed
mixer, or a combination of the two. The mixing speed will be variable, as generally high
speed is required to heat up the powder within a reasonable time scale, but low speed is
preferable for mixing in the pigment and achieving the adhesion. Low speed minimises
damage to the pigment structure. When bonding is achieved, it is important to cool the mix
as quickly as possible to avoid excessive agglomeration and lump formation. In the
extreme, bonded batches can completely solidify in the bonding mixer if overheated or not
cooled fast enough. Cooling will be achieved either by introducing chilled water into the
bonding mixer jacket, or cooling in a separate cooler mixer.
The fastest cooling is likely to be achieved by using the separate mixer/cooler
combination, as the single mixer/cooler is attempting to cool an already hot mixer.
However, as the mixer batch size becomes larger, cooling using a jacket (even in a
separate cooler mixer) can be very time consuming as the cooling surface area to batch
size ratio reduces. However, the advantage of a separate mixer/ cooler is that the bonding
and cooling cycles can be de-coupled, allowing simultaneous bonding/cooling, thus
increasing capacity. A possible disadvantage is the additional cleaning time, with two
rather than one mixer to clean. However, the cooler mixer should be relatively easier to
clean, as fusion of powder to its surfaces is unlikely.
An equation for estimating cooling time is as follows :
|T||= Time (secs),|
|M||= mass of powder (kg)|
|A||= jacket heat transfer area (m2)|
|U||= Overall heat transfer coefficient (W/m2C)|
|Tw||= Chilled water temperature (C)|
|Ts||= Starting temperature (C)|
|Tf||= required final temperature (C)|
Experience has shown that this typical relationship for unsteady heat transfer in vertical
jacketed powder mixers gives reasonable approximation to cooling time using a value of U
of 100 W/m2C. Using the cooling surface area from the Plas mec TRM/RFV Mixers for
batch sizes of 6-10k and 60-90kg and using a value of powder heat capacity of 2000 J/Kg
C, gives the following cooling time predictions to cool for 55C to 20C.