Green construction and release mechanism of lignin-based double-layer coated urea

Background: Currently, there are few reports about the sustained-release time of single-layer non-modified lignin-based coated urea that meets the sustained-release standard. Therefore, the unmodified lignin-based material by adding a hydrophobic polymer can improve the hydrophobicity of the coating material. Results: : A novel green double layer coating with lignin-based polyurethane (LPU) as the inner coating and epoxy resin (EP) as the outer coating was effectively constructed for coated urea. Fourier transform infrared spectra confirmed that lignin and polycaprolactone diol successfully reacted with Hexamethylene diisocyanate. The loss weight and water contact angle (WCA, 75.6-63.6°) of the LPUs decreased with the increased lignin content. The average particle hardness of the LDCU first increased from 58.1 N (lignin of 30%) to 67.0 N (lignin of 60%), but then decreased to 62.3 N (lignin of 70%) . The release longevity of the coated urea was closely related to the preparation parameters of the coating material. The optimal cumulative nutrient release rate (79.4%) of LDCU was obtained (lignin of 50%, -CNO/-OH molar ratios of 1.15, EP of 35%, and coating ratio of 5%). The aggregates of hydrone on the LDCU caused the dissolution and swelling of nutrients, and then the diffusion of nutrients through the concentration gradient. Conclusion: Although the nutrient release of the LDCUs was affected by many factors, the successful development of the LDCUs will improve the rapid development of the coated fertilizer industry.