2. Materials andmethods for CRCU production
2.1. CRCU from sulfur based coating materials
Initially, sulfur was an attractive candidate for urea coating due to
its several advantages. It is generally claimed that the foremost significant
work on urea coating was accomplished by Blouin et al. for
the Tennessee Valley Authority (TVA), USA. With a goal to encase
granular urea and impart controlled release characteristics, Blouin
[6] provided a strong platform that established a cost effective, upscale
production process for the sulfur coating of urea. Initially,
urea granules were impregnated by a petroleum by-product (e.g.
petrolatum, motor oil, soft wax, etc.) to act as an impervious sealant
and sub-coating. A vacuum was then applied to cause the sealant
material to penetrate the granules more thoroughly. The sealant
was considered amobile component that prohibited urea dissolution
by filling small channels via capillary action. In turn, the urea was
tumbled in a second rolling drum and spray-coated with molten sulfur.
Finally, the sulfur coated urea (SCU) was subjected to a third
compartment wherein plasticizers (e.g. polyethylene or polyvinyl
acetate) adhered to the sulfur shell to aid the spreading and fusion
of the sulfur layer and decrease crack formation. For some products,
the addition of a plasticizer was proposed as a substitute using inexpensive,
finely divided powders (for example, talc or vermiculite) to
render a uniform sulfur layer and decrease the incidence of layer
cracking. To achieve a comparative study, urea was coated with
petrolatum-only or sulfur-only. A 24-hour dissolution test in water
was done to evaluate coating effectiveness. The authors found that
the oil-only coating was absolutely ineffective to withstand water
permeability. The sulfur-only coating was mildly effective, whereas
a combination of both gave effective controlled release results. The
coating shell with an oil to sulfur ratio of 3:21 withstood water the
most with only 1% dissolution in 24 h. Despite the controlled release
advantages, this study still had a challenge to address. The presence
of the sealant sub-coating could not negate the need for a uniform
sulfur coating. If the sulfur coating was not sufficiently uniform to
avoid fissuring, the urea substrate dissolved within minutes, even
in the presence of the sealant sub-coating.