The impermeable film on urea granules formulated by Eudragit
L30-D55® was not effective for soils with a pH N5.5. Also, low temperatures
and high flow rates caused a rough coating surface. Therefore,
the need arose to optimize processing conditions using
different coating compositions as mentioned in Table 1 [33]. In this
case, nutrient release was measured by a static capture system in
which filter paper soaked in H2SO4 was used to capture evolved nitrogen
and an empirical processwas used to measure its loss. The optimal
fluidizing air temperature was 74 °C; the optimal suspension
flow rate was 11 ml/min; and the optimal atomizing air pressure
was 68.95 kPa; which produced a controlled release of evolved nitrogen
at 3–57%. The authors extended this research by using vinasse as
the solvent instead of water for the aqueous polymeric suspension
[34]. Vinasse is an effluent of the ethyl alcohol industry that prevents
pollution when used as an ingredient of the coating solution. Additionally,
it also contains the plant nutrients nitrogen, potassium, calcium
and magnesium. Therefore, Rosa [34] used vinasse instead of
water to prepare a coating suspension from Eudragit with the same
composition (Table 1) as previously reported [33]. The equipment,
as well as the coating method and nitrogen volatilization measurements
were also the same. The coating process was successfully carried
out with the use of vinasse as a solvent and achieved a decrease
in nitrogen volatilization up to 57%.
The permeability of water and urea in the coating film is a factor
that governs release rate, release time, and release pattern. Lan [35]
studied the effects of various process parameters on the film's structure
and permeability by coating urea granules in a Wurster type,
fluidized bed apparatus. Polyacrylic acid latex with 40% solids was
used as the coating solution. At elevated fluidizing air temperature
and atomizing gas pressure, the coating film had a porous structure
attributed to the poor spreading and pre-mature drying of droplets.
Similarly, higher spraying rates resulted in reduced dewatering capacity
by forming large pores on the coating's surface leading to
poorly controlled release.
Wu[36] reported that thicker coating layers may damage soil quality
if they are not degraded in parallel with nutrient release. With this in
mind, urea coating with polyurethane is costly but its thinner coating
layer was said to reduce coating cost by coating greater quantities of
urea granules with less material. Coating was done in a rotating drum
so that isocyanate, polyols and wax were added to urea granules for a
certain period. The reaction between isocyanate and polyols formed a
10–15 μm thick polyurethane layer on the granules while paraffin
acted as lubricant to facilitate the process. Water dissolution and soil
incubation experiments revealed a 10% dissolution over the first ten
days with 70–80% dissolution in thirty days followed by total release
by forty–fifty days. The release mechanism was the same asmentioned
by Shaviv [5].