The Process of bonded metallic powder coatings
The design features of the mixers have been specially optimised for metallic powder coatings. The entire bonding process can be controlled with surgical precision and offers absolute, certifiable processing reliability.
The critical phase of the bonding process is reached when the resin to which the aluminium flakes are to be bonded begins to soften. The range, within the resin changes from a powder to a doughy mass, is extremely short. If the temperature rises beyond this range, the mix can no longer be processed.
All the coolable modules of the mixer – the drive shaft and themixing tools, the jacket and the bowl bottom – can be cooled separately. No other mixing system comes as close to the glass transition temperature of the resin as the mixer. The result is a perfectly bonded metallic powder coating of fascinating brilliance.
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 define a 'basis of safety' for operation of a process, it is necessary to consider
the level of risk reduction afforded by any prevention or protection methods proposed.
This enables a judgement to be made on the adequacy of any proposed system for
reducing the risk to tolerable levels. Key information required for assessing risk in this
situation is explosion data for the materials being handled.
Both powder coatings and metal powders are potentially explosive if dispersed in air within
a certain concentration range. Ignition could result if an ignition source of sufficient igniting
power was simultaneously present.
The amount of energy required to ignite a dispersed dust cloud of aluminium is dependent
primarily upon the particle size distribution of the powder and to a lesser extent upon the
type of surface treatment. Other factors such as age, moisture content and particle shape
(i.e. sphere or flake) also play a role. The ease of ignition is defined by a parameter called
Minimum Ignition Energy (MIE) and literature values range from less than 1 mJ up to 50
mJ. Therefore, the fine leafing grades would be expected to have the lowest ignition
To put this into perspective, below is a table of possible ignition energies of electrostatic
discharges that might occur in a bonding plant.
|Spark from flange||0.5|
|Spark from a scoop/shovel||2|
|Spark from a 200 litre metal drum||40|
|Spark from a person||10-30|
|Spark from large metal items||50-100|
|Brush discharge from non-conductors (e.g. large plastic bags,|
powder surfaces and plastic ducting)
Therefore, although ignition of a dust cloud by brush discharge hasn’t been achieved in
practice, it cannot be ruled out. Ignition of aluminium by electrostatic discharge from
isolated conductors or charged human operators is clearly possible. Probability of ignition
of aluminium from mechanical sparks, friction sparks and hot surface is more difficult to
predict, and depends upon many factors. However, it is safe to assume that ignition could
occur due to sparks say from an overheated mixer bearing or an overheated motor.
If an explosion were to occur, the explosion violence of an aluminium explosion can be
very high. The explosion violence is defined by a parameter called the Kst value, which is
a function of the maximum rate of pressure rise in an unvented explosion. Reported values
for pure aluminium powder are in the range 300-1000 bar.m/s. Even at the lower end of
this range, the explosion severity will be very high and therefore significant explosion
strength can be expected, even from quantities as small as 1kg. Aluminium explosions are
quite difficult to protect against and therefore focus is generally on prevention.
Powder Coatings themselves are potentially explosive dusts. Minimum ignition energy is
dependent upon particle size and to a lesser extent, on formulation. Fine powder particles
(dv,50 3-4 microns) have been measured as 1-3 mJ (reference 2). However, as it is
unlikely that powder particles of this fineness will be used as the base for bonding, a
typical MIE range would be 10-30 mJ. Again, Powder Coating particles are susceptible
to certain types of electrostatic discharges, the exception being brush discharges that
would not be expected to ignite Powder Coating particles. In terms of explosion severity,
powder coatings typical have a range of Kst values from 100-200 bar.m/s, 200 being only
for very fine psd material. An explosion would typically be significantly less violent than an
aluminium explosion, but nevertheless, could still cause significant damage.
When aluminium powder is mixed with normal powder coating, literature data suggests
that the Kst value of the powder coating increased by about 10% when 5-6% by weight
aluminium is added. Only when about 25% by weight Aluminium is added, does the Kst
approach that of pure aluminium powder. The minimum ignition energy of the powder
coating is only really decreased when greater than 10% of the finest leafing grades are
used. Therefore in many respects, the most hazardous part of Bonding, is in the handling
of the pure Aluminium. However, it should be noted, that segregation of the aluminium
could alter the above figures in favour of higher Kst values and lower MIE values