Low-Temperature Flowable Poly(lactic acid)/Polycaprolactone Blends for the Solvent-Free Preparation of Slow-Released Urea Fertilizer in a Thermal Shear Field

This work reports a convenient and large-scale method for the preparation of slow-release urea by melt coating with modified poly(lactic acid) (PLA)/polycaprolactone blends at low temperatures. The simple process is environmentally friendly and solvent-free, which can realize the industrial production of coated fertilizers. After being modified by ferric chloride, the crystallinity and molecular weight of poly(lactic acid) (PLA) are greatly reduced and the melt fluidity of PLA is improved. The lowest melting flowable temperatures of the PLA-modified blend was decreased to 115 °C, enabling the melt coating of urea particles using a mechanical stirrer. Because of the good compatibility of PLA with urea, a complete coating layer was formed on the surface of the urea particles by simple mixing at 120 °C, and the change of coating content has a great impact on the release effect. For the slow-release urea melt-coated with 18 phr of the blend, the time of the maximum release rate was delayed significantly and the cumulative release rate was 80.63% after 28 days in soil column leaching. When the coated granular slow-release urea was hot-pressed into a large cylinder, the slow-release effect was further enhanced. Simultaneously, with the massive use of PLA, its waste disposal becomes especially important, preparation of slow-release urea by melt-coating in this environmentally friendly manner needs to adjust the molecular weight and crystallinity of PLA, and its performance requirements are low. The work in this paper may provide a new approach for the recycling and reuse of waste PLA.