Lignin and ethylcellulose as polymers in controlled release formulations of urea

Lignin and ethylcellulose (EC) have been used for the preparation of controlled release (CR) formulations of urea. The lignin matrixes were prepared by mixing the urea with kraft lignin (UL) under melting conditions. They were also crushed and sieved to obtain granules of size between 0.5-1, 1-2, 2-3 and 3-5 mm. The coated urea granules were produced in a Wurster-type fluidized-bed equipment, using an ethanolic solution of EC on two different polymer levels. Having researched the encapsulation efficiency (EE) and the homogeneity of the CR formulations, kinetic-release experiments were carried out in water. A high EE was reached, it oscillated between 95.12% for the system coated with 20% of EC and 97.18% for the 1 mm < d < 2 mm UL system. The rate of urea release from CR granules diminished in all cases in relation to nonformulated urea, being the latter completely dissolved in less than 0.5 h, but it took at least 48 h to release the 90% of urea from the EC coated formulations. Using an empirical equation, the time taken for 50% of the active ingredient to be released into water (T-50) was calculated. From the analysis of the T-50 values, we can deduce that the release rate of urea can be controlled mainly by selecting the granule size for lignin CR systems and changing the thickness of the coating film for EC coated granules. The variation order of T-50 values, UL (0.5 mm < d < 1.0 mm) < UL (1.0 mm < d < 2.0 mm) < UL (2.0 mm < d < 3.0 mm) < UL (3.0 mm < d < 5.0 mm) < UEC10 < UEC20, showed that the presence of EC in formulations retarded the release of urea in relation to those prepared with lignin. (C) 2008 Wiley Periodicals, Inc.