Insights into hydrophobic (meth)acrylate polymers as coating for slow-release fertilizers to reduce nutrient leaching†

To solve the problem of the low utilization rate of conventional fast-release water-soluble fertilizers and to minimize their negative impact on the environment, slow-release fertilizers (SRFs) have emerged as a sustainable solution to limit their losses, reduce fertilizer dosage and improve crop production. In this study, new hydrophobic (meth)acrylate polymers (poly(2,2,2-trifluoroethyle methacrylate) (PTFEMA) and poly(2-(perfluorohexyl)ethyl acrylate) (PPFEHEMA)) with different fluorinated side chains were synthesized by free radical polymerization and used as coatings for SFRs. These polymers were characterized by ¹H and ¹⁹F NMR, FTIR, WCA, TGA and DSC. Compared to PTFEMA, PPFEHEMA with a higher content of F atoms displayed improved thermal stability and an elastomer property (T_g = −10 °C) leading to satisfactory film formation. Indeed, water contact angle (WCA) measurements were carried out on films of both materials: PPFEHEMA with WCA = 109° indicated a highly hydrophobic character with an excellent water-repellent surface, resulting in a coating layer. The use of these polymers as SFR coatings was explored using dip-coating. SEM and EDX mapping were performed to study the morphology of the coated fertilizer granules and showed the formation of a cohesive film with good adhesion between the DAP fertilizer and the coating films, limiting water diffusion. The release profiles of N and P nutrients were studied, and the corresponding release times increased with coating thickness (single layer: 1L or second layer: 2L). Compared to uncoated DAP granules which are totally solubilized after less than 2 h, DAP coated with 2L of PPFEHEMA shows the slowest release of N and P nutrients, and the times to reach maximum N and P releases were 30 and 38 times higher than those of uncoated DAP. The significant delay in the release of nutrients from DAP coated with PTFEMA or PPFEHEMA is consistent with nutrient demand during crop growth and increases the efficiency of fertilizer use and therefore enhances agricultural productivity.