Biodegradable IPN Hydrogels Based on Pre-vulcanized Natural Rubber and Cassava Starch as Coating Membrane for Environment-Friendly Slow-Release Urea Fertilizer

Biodegradable interpenetrating polymer network (IPN) hydrogels based on pre-vulcanized natural rubber (NR) and cassava starch (St) using sulphur (S) and glutaraldehyde (GA) as crosslinkers were developed in a solution form as a coating membrane for slow-release nitrogen fertilizer. The NR/St ratios affecting water swelling, water permeability and biodegradation were investigated. Results revealed that water swelling, water permeability and biodegradation of IPN NR/St hydrogels decreased with increasing NR content. The wax was used as outer coating shell to further improve the release characteristic of the coated urea. The urea bead (UB) coated with IPN NR/St and wax layers (W-IPN-CUB) exhibited exceptional release behavior up to 24 days in soil compared to native UB (3 days). The release mechanisms of W-IPN-CUB in both water and soil environments were a non-Fickian diffusion with n = 0.88 and 0.85, respectively, relating to the diffusion through porous membrane. Also, the W-IPN-CUB was able to slow the leaching of urea fertilizer and significantly improve the performances of corn and basil height compared to native UB (P < 0.05). Thus, the W-IPN-CUB material proved to be a good candidate material for slow release fertilizer which could be widely used in agricultural and horticultural applications.