I n d . Eng. Chem. Res. 1994,33, 1623-1630
1623
Modified Rosin-Paraffin Wax Resins as Controlled Delivery Systems for Fertilizers. Fabrication Parameters Governing Fertllizer Release in Water Elias P. Kakoulides’ and George N. Valkanas Laboratory of Organic Chemical Technology, Division IV: Synthesis and Development of Industrial Processes, Department of Chemical Engineering, 9 Heroon Polytehniou Street, Zografou Campus, 157 BO, Athens, Greece
Modified rosin-paraffin wax resins were found to be efficient as controlled delivery systems for potassium sulfate. Matrix-type formulations were fabricated and dissolution tests performed to determine the influence of individual parameters on fertilizer release rate. Differential scanning calorimetry runs were employed to investigate possible matrix-fertilizer interactions. The mechanism governing K2S04 release from the matrix was proved to be diffusion-controlled. As a consequence, the release rate was directly related t o fertilizer concentration in the matrix. Modified rosin content was of major importance to the release rate, while fertilizer particle size, as well as the geometry of the formulations played a significant role in the diffusion of K2S04 from the matrix. Moreover, the fabrication of the fertilizing forms is very simple, low in cost and energy demands.
Introduction It has been estimated that the percentage of the fertilizer dose recovered by plants when applied in conventional formsmayamountup toonly30-50% (Prasadetal., 1971). Controlled-release fertilizers provide many advantages in comparison with conventional forms: fertilizer concentration kept at effective levels in the soil solution, reduced fertilizer usage, maximal utilization of the active agent from plant systems, remarkable decrease with respect to fertilizer application rate, least possible losses of the fertilizer through volatilization or leaching, prevention of seedling damage, and full protection of the ecosystem in the case of biodegradable carriers (minimal pollution of soil, underground water, and rivers and lakes, a consequence from fertilizer leaching) (Allen and Mays, 1971). The first complete studies on the application of controlled-release technology to fertilizers are placed in 1962 (Oertli and Lunt, 1962; Lunt and Oertli, 1962). Most of the cases cited in literature have to do with reservoir-type formulations: systems where a fertilizing core is encapsulated inside an inert carrier, in other words, coated fertilizers. The release of the fertilizer is controlled by diffusion through the coating. Sulfur-coated urea (SCU) (Rindt et al., 1971;Blouin et al., 19711,polyethylene-coated urea (Salman, 1988, 19891, and coated superphosphate (Subrahmanyan and Dixit, 1988)provide typical examples for this class of formulations. Another way of regulating the release of fertilizer is accomplished by means of chemically controlled releasing products, such as ureaformaldehyde condensates (Hauk, 1974;Prasad et al., 1971; James, 1971;Stajer et al., 1984). Products falling into the aforementioned categories provide extremely prolonged fertilizer release. Matrix-type (monolithic) formulations constitute the third major category of controlled-release devices. The active agent is dispersed in the matrix and diffuses through the matrix continuum or intergranular openings, that is, through pores or channels in the carrier phase and not through the matrix itself. One of the advantages of monolithic formulations is their simple fabrication. The first comprehensive study on this type of fertilizers appeared in 1987 by Hepburn and Arizal. In comparison with the previously mentioned categories, matrix-type
fertilizer formulations have seldom been studied (Hepburn et al., l988,1989a,b; Joyce et al., 1988;Hassan et al., 1992). Various materials may be used to synthesize the matrix phase in which the fertilizer is dispersed. Natural or synthetic resins (rosin,asphalt, various waxes) and natural or synthetic polymers (starch, cellulose derivatives, polyolefins,polydiolefinsand their copolymers)are widely used in industrial practice. This study involves the use of rosin, a resin derived from pine trees. This resin and numerous derivatives have been used for the coating of fertilizers (Cambell and Belar, 1966;Jimenez et al., 1988,1989;Sellas, 1992) and the preparatian of controlled-release drugs (Nikore and Drole, 1985; Pathak and Drole, 1990). Partial differential equations with the appropriate boundary conditions according to the systems under examination, all having their origin in Fick’s laws (Crank, 1975),with exact or analytical solutions have been proposed for the analysis of controlled-release diffusion phenomena. Excellent reviews on the subject are presented by Langer (1980) and Fan and Singh (1988). However, we will be restricted to the use of three simple equations describing the release of active agents from nonswellable devices: Higuchi (1963) proposed the following relationship for one-dimensional, pseudo-steady-state release of active agents dispersed in slabs (planar case),where release occurs through the pores of the matrix:
Q = (D(t/7)(2A - tC,)C,t)”2
(1)
where Q is the amount of active agent released per unit area exposed, D is the diffusion coefficient of the active agent in the dissolution medium, t is the porosity of the matrix, T is the tortuosity of the matrix, A is the concentration of the active agent in the matrix, and C, the solubility of the active agent in the dissolution medium. It is evident that for planar cases a diffusion-controlled mechanism is declared by a square root of time dependence. Sinclair and Peppas (1984) provided a simplified, “empirical” equation, which may serve as a potential tool in revealing the mechanism that governs the diffusion of active agents from nonswellable devices (slabs, cylinders, and spheres): Qt
= kt”
0 1994 American Chemical Society 0888-5S85/94/2633-~623$04.50/0
(2)
1624 Ind. Eng. Chem. Res., Vol. 33, No. 6, 1994 Table 1. Properties of Unmodified and Modified Rosin Obtained from Pinus halepensis rosin softening point ( O C ) acid no. saponification no. iodine no.
unmodified
modified
72 181 173 203
113 144 161 129
where Qt is the fraction of active agent released at time t , k is a constant incorporating the characteristics of the carrier-active agent system, and n is the diffusional exponent, indicative of the transport mechanism. When the release of the active agent is diffusion controlled and expressed by Fick's laws, the exponent n is equal to 0.5 for slabs and cylinders with an aspect ratio (4,the ratio of the diameter to the height of a cylinder) greater than 20,0.43 for spheres and cylinders with an aspect ratio in the order of 1,and, finally, 0.45 for cylinders with q < 0.02. In the case of n = 1we have case I1 diffusion phenomena (zero-order release rate), and when 0.5< n
