Suherman [61] prepared a coating solution by mixing starch, acrylic
acid, and polyethylene glycol with slow additions of water and continuous
stirring until a homogeneous mixturewas obtained. The urea coating
was carried out in a fluidized bed with a top spray of the starch based
coating solution.Water dissolution experiments revealed reduced release
rates with increased starch content of the coating. Higher temperatures
enhanced the release rate because of the pre-mature drying of the coating
droplets. Also, elevated temperatures reduced the proportion of liquid
bridges on the urea granules, thus, leaving uncoated spots that permitted
higher release rates later on.
In 2012, K2S2O8 modified starch (ST) was prepared by gelatinizing
starch with water at 80 °C followed by cooling and mixing with
K2S2O8 at 60 °C for 45min [62]. Themodified starchwas graft polymerizedwith
natural rubber (NR) latex by mixing and stirring at 60 °C for 3
h in the presence of Teric®16A16 to produce NR-g-ST. The NR-graftpolymerized
starch was then used to encase urea granules to make
CRCU. Coating was done by simple immersion of urea granules into
the graft polymer blend followed by drying. The urea release rate in
water, as determined by UV–vis spectrophotometer, was 21% in 24 h.
The diffusion mechanism of release was followed by nutrients so that
only the core's shell remained; the core being hydrophobic natural rubber
and a shell of starch. The hydrophilic nature of starch is associated
with the presence of hydroxyl functional groups [59]. Various studies
attempted to transform this hydrophilic nature to hydrophobic by the
addition of different chemicals and additives. Inmost cases, consequent
controlled release achievements did not correspondwith cropmetabolic
needs, and thus, failed to meet standards (10-12 weeks) set by the
scientific community.
Suherman [61] prepared a coating solution by mixing starch, acrylicacid, and polyethylene glycol with slow additions of water and continuousstirring until a homogeneous mixturewas obtained. The urea coatingwas carried out in a fluidized bed with a top spray of the starch basedcoating solution.Water dissolution experiments revealed reduced releaserates with increased starch content of the coating. Higher temperaturesenhanced the release rate because of the pre-mature drying of the coatingdroplets. Also, elevated temperatures reduced the proportion of liquidbridges on the urea granules, thus, leaving uncoated spots that permittedhigher release rates later on.In 2012, K2S2O8 modified starch (ST) was prepared by gelatinizingstarch with water at 80 °C followed by cooling and mixing withK2S2O8 at 60 °C for 45min [62]. Themodified starchwas graft polymerizedwithnatural rubber (NR) latex by mixing and stirring at 60 °C for 3h in the presence of Teric®16A16 to produce NR-g-ST. The NR-graftpolymerizedstarch was then used to encase urea granules to makeCRCU. Coating was done by simple immersion of urea granules intothe graft polymer blend followed by drying. The urea release rate inwater, as determined by UV–vis spectrophotometer, was 21% in 24 h.The diffusion mechanism of release was followed by nutrients so thatonly the core's shell remained; the core being hydrophobic natural rubberand a shell of starch. The hydrophilic nature of starch is associatedwith the presence of hydroxyl functional groups [59]. Various studiesattempted to transform this hydrophilic nature to hydrophobic by theaddition of different chemicals and additives. Inmost cases, consequentcontrolled release achievements did not correspondwith cropmetabolicneeds, and thus, failed to meet standards (10-12 weeks) set by thescientific community.
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