The production of polyurethane-coated urea involves a specialized process designed to encapsulate urea particles with a polyurethane resin layer, enabling controlled nutrient release. Below is a detailed breakdown of the workflow:

1. Raw Material Preparation
   • Urea Base: Large-grain urea is used as the core substrate.
   • Polyurethane Coating: A mixture of polyurethane precursors (e.g., isocyanate, polyether glycol) is prepared, often requiring solvents like acetone for viscosity adjustment.
2. Coating Application
   • Hot-Melt Coating (Recommended Method):
     • In a heated environment (50–80°C), the polyurethane mixture is sprayed onto rolling urea particles in a fluidized bed or rotary drum.
     • The resin cures chemically on the particle surface, forming a durable film. This method ensures uniform coating thickness and true controlled-release performance.
   • Cold Stirring Coating (Limited Use):
     • A simpler process where pre-mixed resin is stirred with urea in conventional equipment. However, this method lacks effective nutrient control and is mainly for color adjustment.
3. Continuous Production System
   • Three-Stage Rotary Drum:
     • Heating/Shaping Zone: Particles are polished by friction, with optional mild water spraying for surface melting and smoothness.
     • Spray Coating Zone: Polyurethane is applied as particles tumble, building up a film to the desired thickness (3–5% of particle weight).
     • Cooling Zone: Chilled air is introduced to solidify the coating, after which particles exit the drum.
4. Key Process Parameters
   • Coating Thickness: Controls release duration (e.g., 60/90/120-day formulations).
   • Curing Conditions: Temperature (50–80°C) and reaction time (1–2 hours) optimize film integrity.
   • Additives: Nano-SiO₂ or waxes may be incorporated to enhance film durability and release control.
5. Post-Processing
   • Drying/Screening: Coated particles are dried, sized, and packaged.
   • Quality Control: Samples undergo dissolution tests to validate release rates per agricultural standards.

Advantages of Advanced Processes

• Continuous Production: Reduces equipment footprint vs. batch systems (which require 6–8 spray units).
• Precision Engineering: Membrane thickness (~14µm) and low coating ratios (3–4% of particle mass) maximize nutrient density.
• Performance Compliance: Meets EU/international standards for controlled-release fertilizers.

Technical Variants

• Temperature-Responsive Coatings: Polyurethane formulations with phase-change materials adjust nutrient release based on soil temperature (e.g., accelerated release at >33°C).

This process ensures efficient nutrient delivery, reducing leaching and enhancing crop uptake efficiency.