Solvent-free coating of crosslinked and hydrophobic lignin-based biocomposite for slow-release fertilizer

Biopolymer-based slow-release fertilizers (SRFs) have attracted increasing interest because of their environmental benefits. Herein, a hydrophobic thermoplastic lignin with favorable film formation properties was synthesized through simple crosslinking followed by esterification reactions. The chemical structure, glass transition temperature, and hydrophobicity were determined using Fourier transform infrared spectroscopy, H-1 nuclear magnetic resonance spectroscopy, differential scanning calorimetry, and contact angle studies. The newly synthesized esterified crosslinked sodium lignosulfonate (ECSL) enables the encapsulation of urea via a solvent-free coating method to prepare SRFs because of its appropriate glass transition temperature (Tg) and excellent film formation properties. The ECSL-based SRF released only 86.9% of the encapsulated urea within 44 d, which is superior to most lignin-based SRFs in previous studies. The release rate can also be tuned by adjusting the ECSL ratio. The synthesis of hydrophobic lignin biocomposites with suitable Tgs for film formation resolves the bottleneck of using these types of non-thermoplastic polymers (e.g., lignin) for SRFs. This study not only maximizes the value of biowaste lignin but also offers a non-paradigm approach toward low-cost SRFs for sustainable agriculture.