In order to produce powder coatings pre-mix, a process is required that will be mechanical
energy from medium/high speed mixing tools. The mixing speed will be fixed, as generally
high speed is required to mix within a reasonable time scale, but low speed is preferable
for mixing in the fine raw materials and achieving the adhesion. Low speed minimises
damage to the pigment structure and flushing in the mechanical seals.
It is important to keep the pre-mix cool to avoid excessive agglomeration and lump
formation. In the extreme, some raw materials can melt and solidify on the mixer head and
agitator if overheated in premixing. This can happen if repeated additions are made to a
mix followed by repeated mixing there will be a build up of bulk temperature in the pre-mix.
Cooling of the mixer head can be achieved by introducing chilled water into the mixer head
For the manufacturing of Powder Coatings there is required a pre-mix feedstock for the
extruders that is a complete homogeneous mix of all the raw materials to be fed into the
extruder. These raw materials in thermosetting systems comprise of polymers generally of
low molecular weight, hardener, catalyst, cross-linking or curing agent, pigments and
extenders and flow control additives in a range of particle sizes >10micro-<10mm .
Solid grades of resin are used and these control properties such as melting
point, flow and film properties the resins having a softening point 68 - 1120C are mainly
used for Powder Coatings and tend to be in flake or prill form.
The choice of curing agent to be used with the particular type of polymer is
influenced by its final application and cure and the properties required in the coating. The
hardener should be unreactive at room temperature remaining latent up to1000C and
should react fully at 1800C this reaction time must be balanced within the constraints of
good flow out of resin against the commercial considerations.
Pigments used in the manufacture of liquid systems are used for
powder coatings provided they are heat resistant, chemically inert and fast to light. The
exclusive use of Titanium dioxide for the manufacture of whites and light shades. Carbon
organic and inorganic pigments are used for blacks and greys; although emphasis is now
with the organic pigments as reduction in heavy metals is made. Inorganic extenders are
now used and incorporated in formulations and are usually of high specific gravity.
Formulations usually incorporate flow control agents to manage
powder coatings finished film effects and appearance during curing without them surface
defects would appear.
With the range of the above raw materials the formulations for modern powder coatings
can include less than twelve or more than twenty different materials in any one recipe. For
good processing of the pre-mix in the extruder we have stated that this pre-mix should be
homogeneous this requires a more intensely mixed reduced and dispersed uniform premix feedstock.
To obtain this homogeneity we must pre-mix the raw materials that have been previously
weighed in their component parts to allow a dry mixing of all the materials so they will feed
into the extruder feed system and provide consistent stream of mixed raw materials with