2.2. CRCU from polymer based coating materials
Following the affair with sulfur, polymeric materials were widely
used to coat urea since sulfur coatings were easily disrupted by microorganisms
whereas polymer coatings were not. The nutrient release
from polymer coating is affected by diffusion as a function of
coating thickness and soil temperature. However, polymer spray coating
involves organic solvents that not only inflict additional costs of the lean
solvent and solvent recovery, but also cause hazardous environmental
emissions. Hence, the use of aqueous polymeric solutions was initiated
to counter these issues.Donida [32] studied urea coating using a commercially
available aqueous polymeric material called Eudragit L30-D55®
(methacrylic acid copolymers) in a two dimensional spouted bed with
top spray orientation. The coating's composition is given in Table 1.
Eudragit L30-D55® is mixed with water in addition to: talc, esthearates
of magnesium, triethyl citrate, polyethylene glycol, and titanium dioxide
to produce the CRCU. Higher atomizing air pressure and fluidizing air
temperature produced a uniform coating film due to the production of
smaller droplets and improved spreading of the suspension, respectively,
resulting in a homogeneous layer. The coating thickness also imparted
controlled release characteristics as it increased in thickness at a higher
coating suspension rate and atomizing air pressure, but decreased with
increased fluidizing air flow rate and temperature. However, elutriation
was also caused at elevated air temperatures due to the pre-mature drying
of droplets before contacting the granules' surface.