Designing for effective controlled release in agricultural products: new insights into the complex nature of the polymer–active agent relationship and implications for use

Abstract

BACKGROUND Various active chemical agents, such as soil microbial inhibitors, are commonly applied to agricultural landscapes to optimise plant yields or minimise unwanted chemical transformations. Dicyandiamide (DCD) is a common nitrification inhibitor. However, it rapidly decomposes under warm and wet conditions, losing effectiveness in the process. Blending DCD with an encapsulating polymer matrix could help overcome this challenge and slow its release. Here, we encapsulated DCD in a biodegradable matrix of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and investigated the effects of DCD crystal size and loading rates on release rates. RESULTS Three DCD crystal size fractions (0–106, 106–250 and 250-44 μm) were blended with PHBV at 200, 400, 600 and 800 g.kg−1 loadings through extrusion processing and release kinetics were studied in water over eight weeks. For loadings ≥600 g.kg−1, more than 95% release was reached within the first seven days. By contrast, at 200 g.kg−1 loading only 10%, 36% and 57% of the DCD was mobilized after eight weeks in water for 0–106 μm, 106–250 μm and 250–420 μm crystal size fractions, respectively. CONCLUSION The lower percolation threshold for this combination of materials lies between 200 and 400 g.kg−1 DCD loading. The grind size fraction of DCD significantly affects the quantity of burst release from the surface of the pellet, particularly below the lower percolation threshold. The results presented here are likely translatable to the encapsulation and release of other crystalline materials from hydrophobic polymer matrixes used in controlled release formulations, such as fertilisers, herbicides and pesticides. This article is protected by copyright. All rights reserved.