Enhancing Longevity and Mechanisms of Controlled-Release Fertilizers Through High Cross-Link Density Hyperbranched Bio-Based Polyurethane Coatings

The maturation of biobased polymer synthesis process has led to the rapid adoption of biobased controlled-release fertilizers (BCRFs). Despite this progress, the loose structure, low cross-linking density, and poor water repellency of current biobased coatings result in short controlled-release periods, which do not meet the long-term nutrient demands of crops throughout their reproductive phases. Moreover, existing modifications to address these issues often involve the addition of external additives, which can increase biotoxicity and cost. In this study, castor oil-based hyperbranched polyol (COHBPs) was synthesized from castor oil (CO) using a "one-pot method." We report the development of high cross-linking density biobased hyperbranched polyurethane (COHBPPFs) as a coating for BCRFs. Gel content and SEM analysis demonstrated that COHBPPFs exhibited a high degree of cross-linking. Mechanical testing using an electronic universal testing machine, along with AFM results, highlighted the superior mechanical properties of COHBPPFs compared to traditional coatings. COHBPPFs significantly enhanced controlled release performance, with a 3% coating content providing nearly 50 days of controlled release, a marked improvement over conventional coatings. The controlled release mechanism of COHBPPFs was elucidated by modeling the cross-linking process of COHBPs with PAPI. This study offers a comprehensive examination of CRF coatings from multiple perspectives and provides valuable insights into the development of high cross-link density, environmentally friendly, and renewable biobased CRFs.