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Developing a biodegradable film for controlled release of fertilizer.

With the aid of a designed experiment, a low-cost, starch-filled LDPE coating for encapsulating urea pellets was developed that releases the urea at the optimum rate for fertilizer utilization by rice plants.

Urea in the form of pellets is the major synthetic fertilizer used in rice fields. However, only half the urea applied in this manner is estimated to be absorbed by the rice plants; the other half is lost, causing water contamination. Various technologies are currently being explored to control urea release in order to maximize absorption and thereby, simultaneously, reduce contamination of the environment. One such technology is encapsulation of the urea in a permeable film that is designed to release the urea at a rate conducive to plant growth.

One important consideration in the development of this technology is the cost of the permeable film. If it is excessive, then the gain in fertilizer utilization would be outweighed by its cost. Low-density polyethylene (LDPE) is one of the least expensive polymers, and it was therefore selected as the first candidate for urea encapsulation.

Another advantage of using LDPE is its ability to photodegrade in sunlight. Degradation is known to be enhanced by the addition of starch particles. Starch degradation by exposure to soil bacteria would be expected to develop micropores in PE/starch films. The microporous film would then be capable of enhanced urea release.

Based on these considerations, a study was undertaken to determine the potential use of PE/starch films as coatings for controlled release. Initial assumptions were that smaller film thickness, higher starch content, and longer exposure of film to soil would accelerate urea release. A two-level factorial experiment was performed to examine this hypothesis. The eight treatment conditions for this design are shown as circles in Fig. 1. Variable levels are high (+) or low (-) in each treatment.


Films with 0%, 5%, and 10% starch were produced at thicknesses of 0.25 and 0.50 mils on a Haake model 40 twin-screw extruder equipped with a 4-inch slot die. The LDPE, obtained from Quantum Chemical Co., had a melt index of 24 g/10 min and a density of 0.923 g/|cm.sup.3~. Polyclean starch from Archer Daniels Midland Co. was used. The two-level factorial experimental design with three variables required that four different PE/starch films be produced and exposed to soil.

A 3.75-cm-diameter sample was cut from each film and mounted in a polycarbonate test cell illustrated in Fig. 2. The cells were designed to hold the film during soil exposure and to be used later in determining urea permeation through the film.

The mounted films were exposed on one side to soil at the International Fertilizer Development Center (IFDC), Muscle Shoals, Ala. One film group was exposed for 14 days, the other for 30 days. After soil exposure, the films were measured for urea permeability and examined with a scanning electron microscope (SEM).

Permeability tests were performed by placing 10 ml of a 10-wt% urea solution into each test cell. The cells were then positioned inside 250-ml widemouthed beakers containing 100 ml of deionized water. A refractive index (RI) detector calibrated with urea solutions of known concentration was used to measure the urea released through the PE/starch films into the 100 ml of deionized water outside the test cells. A plot of urea concentration vs. time was usually linear. The slope of this relationship gave the urea release rate recorded as grams of urea released per hour through a 1-|m.sup.2~ film area.


Film treatment conditions and urea release rates for the exposed films and four films unexposed to soil are given in Table 1. From this data was developed a two-level factorial experimental design with three independent variables: time or soil exposure (E), film thickness (T), and starch content (S). The experimental design and its results are listed in Table 2. The use of a two-level factorial experiment enables one to distinguish between significant and insignificant variables. A variable is expected to be significant if the absolute value of its estimated effect is more than two times the standard error (see G.E. Box, W.G. Hunter, and J.S. Hunter, Statistics for Experiments, John Wiley & Sons, Inc., New York, 1978).

The factorial experimental design suggested that only film thickness and possibly starch content were influential in affecting urea release. The film thickness is very significant and showed the expected negative effect, i.e., thinner film gives higher release rates.
TABLE 1. Experimental Conditions and Urea Release Rates.

 Film Starch Release rate, g/hr |center dot~ |m.sup.2~, after
 thickness, content, number of days of soil exposure
Run mils wt% 0 days 14 days 30 days

A1 0.25 5.0 - 0.22 1.15
 2 0.25 0.0 - 0.70 0.79
 3 0.25 5.0 0.04 - 0.39
 4 0.25 5.0 - - 0.81
 5 0.25 10.0 - - -
B1 0.50 5.0 - 0.16 0.19
 2 0.50 0.0 - 0.13 0.12
 3 0.50 5.0 0.09 - 0.09
 4 0.50 5.0 - 0.34 0.33
 5 0.25 5.0 - 0.89 -
C1 0.25 10.0 - 0.45 0.43
 2 0.25 0.0 0.40 - 0.55
 3 0.25 10.0 - 0.26 0.92
 4 0.25 10.0 - 0.47 0.33
 5 0.25 10.0 - 0.40 -
D1 0.50 10.0 - - 0.06
 2 0.50 0.0 0.11 0.03 0.08
 3 0.50 10.0 - - 0.09
 4 0.50 10.0 - 0.08 0.05
 5 0.25 10.0 - 0.34 -

Unexpectedly, starch content had a marginally significant negative effect on the release rate, meaning that as the starch content increased, the urea release rate decreased. This suggests that bacteria were unable to come in contact with the starch particles embedded in the film. SEM of the exposed films did not reveal any conclusive evidence of micropores in the films. This would explain why the starch content did not assume a positive role in the release of urea. The possible negative effect of starch implies that the TABULAR DATA OMITTED starch is obstructing the urea as it passes through the film. If bacteria are unable to consume the starch, then one would expect that the duration of soil exposure would not be an important factor. This is supported by the experimental design, which showed that soil exposure is insignificant. Conclusions

Thus, only film thickness was found to be significant in affecting urea release. The average release rates were 0.54 g/hr |center dot~ |m.sup.2~ and 0.13 g/hr |center dot~ |m.sup.2~ for the 0.25-mil and the 0.50-mil films, respectively. The IFDC has determined that optimum fertilizer utilization by rice plants occurs with film coatings that released 20% of the encapsulated urea after 7 days. Based on this requirement and the measured urea release rates, the theoretical urea-encapsulated sphere size and the polymer-to-urea mass ratio can be calculated. For the 0.50-mil film, the encapsulated particle size would have a spherical diameter of about 0.05 cm and the polymer-urea mass ratio would be 0.11, or 11-wt% polymer. Thus for this film, very little polymer is required, but the particle size is too small to be practical. For the 0.25-mil film, the urea particle size would be 0.21 cm in diameter, and 1.3-wt% polymer would be required. This particle size is reasonable, and a minimum amount of polymer is needed to encapsulate the urea. Urea and LDPE costs are $200 and $1000/ton, respectively. Thus, a ton of encapsulated urea would have a material cost of only $210.

Particle Diameter and Polymer Mass Calculations

Because urea concentrations above and below a film are constant, the release rate is expected to be defined by:

dM/dt = -kS

where M = urea mass encapsulated, t = time, S = film surface area, and k = release rate.

|integral of~ dM between limit |M.sub.o~ to |M.sub.e~ = -kS |integral of~ dt between limit O to |t.sub.e~

Twenty percent of the total urea must be released after a given time. Thus |M.sub.e~ = 0.8|M.sub.o~ at t = |t.sub.e~. Integration and substitution of this boundary condition gives

0.8|M.sub.o~ - |M.sub.o~ = -kS|t.sub.e~ or |M.sub.o~ = 5kS|t.sub.e~

If V is the volume of an encapsulated urea sphere of diameter d and urea density ||Rho~.sub.u~, then

|M.sub.o~ = V||Rho~.sub.u~ = 5kS|t.sub.e~ or V/S = 5k|t.sub.e~/||Rho~.sub.u~

For a sphere of diameter d, the ratio of volume to surface area is d/6. Introduction of this relationship gives

d = 30k|t.sub.e~/||Rho~.sub.u~

If T is the coating thickness and ||Rho~.sub.p~ is the polymer density, then the polymer mass used is |Pi~ ||Rho~.sub.p~|d.sup.2~T. The urea mass for this sphere is |Pi~ |d.sup.3~||Rho~.sub.u~/6. Therefore, the ratio of polymer to urea mass, R, is

R = 6||Rho~.sub.p~T/d||Rho~.sub.u~

Using the above relationship, the following table was generated for the two polymer film thicknesses at their measured release rates with |t.sub.e~ = 168 hr, ||Rho~.sub.u~ = 1.32 g/|cm.sup.3~ and ||Rho~.sub.p~ = 0.923 g/|cm.sup.3~.
 Thickness, mils
Parameter 0.50 0.25

Release rate,
g/hr |center dot~ |m.sup.2~ 0.13 0.54
Diameter, cm 0.05 0.21
Mass ratio 0.11 0.013
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Author:Posey, Ty; Hester, Roger D.
Publication:Plastics Engineering
Date:Jan 1, 1994
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