Polyvinyl alcohol composite hydrogels/epoxidized natural rubber composites (CMCS/PVA/CS-ENR) with core-shell structure as biomass coating material for slow-release nitrogen fertilizer

Using natural rubber (NR)/chitosan (CS)/polyvinyl alcohol (PVA) as raw materials, biodegradable semi-IPN (semi-interpenetrating polymer network) composites were prepared by thiol-epoxy click reaction under UV light, which was used as the shell of sustained release preparation. Using polyvinyl alcohol (PVA)/carboxymethyl chitosan (CMCS) as raw material and vanillin as the cross-linking agent, hydrogen-bonded hybrid network hydrogel loaded with urea was prepared by freeze-thaw cycle, which was used as the core of sustained-release preparation. Finally, a green-coated urea fertilizer with core-shell structure was prepared to ameliorate the environmental pollution of slow-release fertilizer (for example, synthetic polymer-coated fertilizers are difficult to biodegrade), as well as to achieve the water retention and the effective slow-release. The high CS/PVA contents and the combination of the hydrogel core and the rubber shell significantly improved the mechanical properties of the coating material (tensile strength above 5 MPa, elongation at break up to 1400 %), hydrophilicity (to promote the degradation rate of the rubber shell) and soil water retention of the coating material (close to pure hydrogel), showing enhanced reaction. In terms of slow-release effect, coated nitrogen fertilizer with 3 % CS content, 5 % PVA content and 10-min coating time (SRP3/10) achieved the best slow-release effect of nitrogen (N) in water (62.7 % at 10 days), which was related to the hydrophilicity, semi-IPN skeleton and the cross-linking structure of the network, and the release conformed to the Korsmeyer-Peppas model with a non-Fickian diffusion release mechanism, indicating that the behavior of N release was mediated by a combination of drug diffusion and skeleton dissolution.