With this background, Guo [12] prepared slow release membrane
encapsulated, double coated urea granules with an inner shell of crosslinked
starch and an outer layer of acrylic acid and acrylamide. Soil incubation
experiments determined nitrogen release by using the Kjeldahl
method of distillation. Results indicated 10%, 15%, and 61% release rates
on days two, five, and thirty, respectively. Coated urea, thus obtained,
has cross-linked starch as inner coating layer with a copolymer of crosslinked
acrylic acid and acrylamide as an outer coating. The slow release
mechanism involves the absorption of water by the coating material
which causes it to swell and transform to a hydrogel. The core urea
then dissolves in the hydrogel's water and diffuses slowly through a
grid like system of the swollen hydrogel via mass transfer of water from
within the hydrogel to water in the soil. Another double coated ureawith an inner coating of urea-formaldehyde and an outer layer of crosslinked
poly(acrylic acid)/organo attapulgite composite was prepared by
Liang [53]. He reported a dried CRCU released of 3.9%, 7.5%, and 75%
(wt.%) at two, five, and thirty days of soil incubation, respectively. The
release mechanism was similar to Guo's study with a slight difference:
water, after diffusing through the outer coating, slowly penetrated the
urea-formaldehyde layer to dissolve the urea which then escaped slowly
by a dynamic exchange between hydrogel free water and soil moisture.
The coating's thickness and the solubility of urea-formaldehyde were
characterized as controlling factors for the slow release. Hence, higher
thickness and lower solubility produced the best slow-release outcome.