Wurster Coating (Fluid Bed Coating)
The Wurster process has been used for years to coat
particles, spheres, granules, and tablets. Systems have been developed for use
with a variety of coating formulations, including aqueous/organic solvents, hot
saturated solutions and hot melts. The basic concept in Wurster coating is to
separate the particles in the fluid bed from one another in an air (gas)
stream. While the particles are suspended, a coating formulation is sprayed
from the bottom of the bed up onto the particles (bottom-up spray).
It is characterized by the setting of a spray nozzle at the
bottom of the fluidized bed. The particles are moved with a fluidizing air
stream that is intended to induce a cyclic upward flow of particles, past the
spray nozzle. The nozzle sprays atomized droplets of coating solution or
suspension concurrently with the particle flow, depositing droplets on the
surfaces of the particle as they pass upward into an expansion chamber. This
expansion chamber reduces air velocity to allow particles to circulate back to
the coating chamber. It also allows particles to temporarily separate from one
another, minimizing the potential for particle agglomeration and accretion.
Organic or aqueous coating solutions evaporate as the particles move into and
through the expansion chamber, leaving non-volatile coating formulation ingredients
on the particle surface as part of the developing film coat. Process parameters
are designed for optimal solution evaporation and film coat characteristics.
This batch process is continued until each particle is uniformly coated to the
desired coat percentage or film thickness.
In a similar fashion, the Wurster fluid bed process can be
used to apply a hot melt coating, such as wax. Wax is heated to a molten state
and sprayed in the same manner as a solution suspension. Process parameters are
adjusted to congeal molten wax droplets on the surfaces of the circulating
particles.
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