Agriculture Reference
In-Depth Information
The effect of the different coating layers of DPVA on the release rate of urea from beads
is presented in Figure 25. These curves show the release of neem from the capsules, which
were controlled by the number of NR coating layers and the crosslinked density. It is clear
that with an increase in the NR coating layer from 1 to 3 layers, the rate of diffusion of the
fertilizer from the beads greatly decreased. After washing with water, the urea was exposed to
the surface of the DPVA films and could be completely removed. By measuring the urea
concentration in the water, the urea released from the films could be determined using UV-
Visible spectroscopy. The release profile from urea without the (DPVA) coating is also
shown for comparison. It is obvious that the urea release rate was reduced significantly by the
DPVA coating, which is consistent with the results of the swelling study. The DPVA film is
very strong, rigid and hard to swell, so the diffusion through this coating is the rate limiting
step for swelling and urea release. The release was prolonged by the addition of any DPVA
layers on the capsule surface. The cumulative release of urea from the capsule was derived
from the immersion times of 2, 24, 72 and 240 h in aqueous medium and found to be 31, 69,
81 and 100%, respectively. When the DPVA coating on the capsule was increased from 1 to 3
layers, the cumulative release of urea from the capsules that had been stored under the same
conditions was 8, 29, 36 and 60%, respectively. It is to be noted that with an increase in the
DPVA coating, the capsule matrix became denser and resulted in a decrease in the rate of
urea diffusion through the swollen beads, especially the beads with the third-DPVA coating.
This result was in agreement with Chen and co-worker (Chen, 2008). They found that St-g-
poly (L-lactide) (PLLA) (Chen, 2008) was an effective material for encapsulating urea for
controlling its release. The St-g-PLLA exhibited a relatively low swellability, a large
encapsulating capacity, and a slow-release rate and the water-resistance of the matrix could be
improved by increasing the PLLA graft efficiency on the St granules.
3.5. FertiliserFertilizer Encapsulated with the PVA/St/NR Blend
The PVA/St/NR blend was applied as a polymer membrane to encapsulate the fertilizer.
The problems of urea are runoff and leaching to the environment and vaporization, leading to
low utilization efficiency for the plants as previously mentioned. When the PVA/St/NR blend
was used as the matrix to encapsulate the urea particles, the encapsulated urea with this
polymer blend still remained strong and was resistant to water. Thus, the swelling ratio of the
PVA/St/NR blend was also estimated and results are shown in Figure 26. The swelling of the
polymer blend decreased as a function of the NR content. The lowest value of the swelling
ratio of the sample was found in the sample in the presence of 5%w/w NR. These phenomena
may perhaps be explained by the difference in swellability of PVA/St/NR blend matrices at
high NR because the PVA/St/NR blend with a low NR could be easily swollen by water. The
urea in the swollen PVA/St/NR blend at a low NR value can diffuse rapidly and can be
released quickly. After modifications of the PVA, both the swellability of the matrix films
decreased so that the release rate of the urea was reduced with the increasing NR content
portion in the blend. After the preparation of the polymer blend, it was used as a polymer
membrane for controlling the release of the fertilizer. The urea released from the beads was
subjected to a testing of a number of physical and chemical parameters including those
directly related to the release medium, the release conditions (temperature) and those
resulting from changes in the characteristics of the device to control the release (beads). The
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