Due to higher costs and process complexity along with issues of
environmental pollution caused by polymers, research frontiers shifted
towards developing low cost, easily fabricable and environmentally
friendly materials [37]. Although the price of starch based coating was
low, so was nutrient release longevity compared to polymer coating
formulations, and furthermore, they were occasionally incompatible
with crop metabolic needs [38]. Yang [38] employed waste polystyrene
(thermocol) as a coatingmaterial mixed with wax and polyurethane as
sealants for a more cost effective and controlled release urea fertilizer.
Initially, polyurethane was prepared by dissolving and agitating
diphenylmethane diisocyanate in ethyl acetate and castor oil, after
which polyurethane was mixed with ethyl acetate-dissolvedpolystyrene.
Urea granules were then spray coated with this solution
in a Wurster fluidized bed followed by oven drying at 40 °C for 24 h to
remove excess ethyl acetate. Nitrogen release was measured in still
water at 25 °C using the Kjeldahl method. The release rate slowed with
greater coating thickness and the addition of wax to the coating solution
did not have a significant effect. To the contrary, polyurethane effectively
enhanced controlled release characteristics.