Esterification modified starch by phosphates and urea via alcohol solvothermal route for its potential utilization for urea slow-releasing

The natural starch (NS) is modified by an esterification process which is accomplished by reacting the NS and phosphate together with urea via a facile alcohol solvothermal method. After modification, a series of obvious variations can be easily confirmed for the resulted starch phosphate carbamides (denoted as SPC) compared with that of NS, such as the introduction of new groups of CO, PO, P-O-C and P-O-H together with new elements of N and P in starch molecular structure unit confirmed in FT-IR and XPS analyses and the decreased crystallinity along with formed surface defect demonstrated in XRD and SEM measurements. Furthermore, the formed SPC has a higher viscosity of 480 mPa.s-1 and lower gelatinization temperature of under 10 °C than that of the NS. More importantly, when the SPC is utilized as outer coating material together with ethylcellulose (EC) as inner coating material for preparing double-layer slow-release urea (denoted as EC/SPC based SRU), the EC/SPC based SRU has a desirable slow-release behavior with release percentages of 40.9% for 12 h in water and merely 59.6% for 20 day together with even exceeding 30 days in soil. Conclusively, this work provides a facile preparation approach for the SPC and its creative application for the preparation of SRU.