Polyurethane-coated urea is a slow-release fertilizer that utilizes polyurethane membrane technology to control nutrient diffusion, extending the fertilizer's effectiveness period (typically 3–6 months). The core principle involves leveraging the hydrophobic and semi-permeable properties of polyurethane films to regulate urea molecule release. Below is a detailed breakdown of its production workflow and key technical considerations:

I. Raw Material Preparation

1. Primary Components

• Urea Granules:
  • Particle size: 2–4 mm or 4–6 mm (uniformity required).
  • Quality standards: Purity ≥98%, moisture content ≤0.5%.
• Polyurethane Prepolymer:
  • Synthesized from isocyanates (e.g., MDI, TDI) and polyols (e.g., polyester/polyether polyols).
  • Control isocyanate group (-NCO) content at 10–20%.
• Chain Extenders:
  • Examples: 1,4-butanediol (BDO), ethylenediamine (EDA), used to adjust polyurethane crosslinking density.
• Catalysts:
  • Organotin (e.g., dibutyltin dilaurate) or amine-based catalysts to accelerate curing.

2. Auxiliary Materials

• Solvents:
  • Ethyl acetate or acetone (some processes adopt solvent-free systems).
• Anti-caking Agents:
  • Talc or diatomaceous earth to prevent particle aggregation.
• Colorants:
  • Iron oxide red for product identification.

3. Pre-treatment

• Urea Screening:
  • Remove dust and broken granules via vibrating screens to ensure size consistency.
• Prepolymer Storage:
  • Store under dry nitrogen protection to prevent moisture-induced reactions.

II. Coating Process: Fluidized Bed Coating (Industry Standard)

This method suspends urea granules using airflow, enabling uniform polyurethane spraying and curing. Key steps include:

1. Fluidized Bed Preheating

• Load urea into a fluidized bed reactor and heat to 50–60°C using hot air (60–80°C).
• Purpose: Eliminate surface moisture to enhance polyurethane adhesion.

2. Polyurethane Solution Spraying

• Solution Preparation:
  • Mix prepolymer, chain extender, and catalyst (e.g., 9:1 ratio of prepolymer to extender).
  • Dilute with solvent (if needed) to achieve viscosity of 500–2000 mPa·s.
  • Alternative: Dual-component spraying (A: prepolymer; B: extender + catalyst) for on-site mixing.
• Spray Parameters:
  • Nozzle pressure: 0.2–0.5 MPa for fine atomization.
  • Spray rate: 10–50 kg/h (adjusted to prevent clumping).
  • Coating thickness: 50–200 μm (thicker layers slow release further).

3. Curing Reaction

• Conditions:
  • Thermal curing: 80–120°C for 5–30 minutes.
  • Moisture curing: Room temperature with catalysts (for solvent-free systems).
• Critical Controls:
  • Avoid temperatures >130°C (risk of urea decomposition).
  • Ensure complete extender reaction to prevent residual -NCO groups (which may harm roots).

4. Multi-layer Coating (Optional)

For extended release:

• Base Layer: Polyurethane (50–100 μm).
• Middle Layer: Wax or silicone oil for enhanced hydrophobicity.
• Top Layer: Inhibitor-containing coating (e.g., nitrification inhibitors) to regulate nitrogen transformation.

III. Post-treatment Processes

1. Cooling

• Cool granules to <40°C via fluidized bed cooling or conveyor systems to prevent caking.

2. Screening

• Use vibrating screens to classify particles by size (target: 2.0–4.75 mm, ≥90% compliance).

3. Anti-caking Treatment

• Apply 0.1–0.5% anti-caking agent via rotary drum mixers for uniform coverage.

4. Quality Inspection

• Nutrient Content: Nitrogen ≥42% (GB/T 2440-2017 standard).
• Controlled-Release Performance:
  • Water dissolution test: Measure nitrogen release at 24h, 7d, and 28d (e.g., ≤15% at 24h; ≥80% at 28d).
  • Soil incubation test: Simulate field conditions to assess release curves.
• Physical Properties: Particle size distribution, crush strength (≥10 N/particle), moisture (≤1%).

IV. Key Process Parameter Controls

1. Polyurethane Membrane Properties
   • Crosslinking density: Adjusted via extender dosage (higher density = slower release).
   • Porosity: Maintained at 5–15% to balance controlled release and initial nutrient availability.
2. Coating Uniformity
   • Optimize fluidized bed airflow (0.3–0.8 m/s) to avoid dead zones.
   • Position nozzles to cover the entire reactor cross-section.
3. Environmental Controls
   • Maintain workshop humidity ≤60% to prevent polyurethane moisture absorption.
   • Use negative-pressure solvent recovery systems to minimize emissions.

V. Typical Production Equipment

1. Fluidized Bed Reactor
   • Type: Horizontal multi-chamber or circulating fluidized bed.
   • Material: Stainless steel (304/316L) for corrosion resistance.
2. Spray System
   • Nozzles: Two-fluid (air + solution) for optimal atomization.
   • Pumps: Gear or diaphragm pumps for stable flow.
3. Curing Equipment
   • Hot air ovens or infrared heaters for rapid temperature control.
4. Screening & Packaging
   • Vibrating screens (linear or circular) and automatic bagging machines (25/50 kg/bag).

VI. Environmental & Safety Measures

1. Waste Gas Treatment
   • Solvent recovery: Condensation + activated carbon adsorption (recovery rate ≥90%).
   • Tail gas purification: RTO (regenerative thermal oxidizer) or catalytic combustion for VOC removal.
2. Wastewater Management
   • Treat equipment cleaning water via sedimentation/filtration for reuse.
3. Safety Protocols
   • Personal protective equipment (respirators, gloves) for handling isocyanates/solvents.
   • Emergency showers/eyewash stations and fire extinguishers (CO₂/dry powder) on-site.

VII. Process Optimization Trends

1. Solvent-Free Coating
   • Use reactive polyurethane prepolymers for direct spraying and curing, eliminating solvent pollution.
2. Smart Release Membranes
   • Develop temperature- or pH-responsive polyurethanes to synchronize nutrient release with crop demand.
3. Low-Cost Raw Material Substitution
   • Replace petroleum-based polyols with bio-based alternatives (e.g., castor oil, rapeseed oil) to reduce costs.

VIII. Applications

Polyurethane-coated urea is suitable for:

• High-value crops: Vegetables, fruits, and flowers requiring precise nutrient control.
• Field crops: Corn and wheat to minimize labor via reduced top-dressing.
• Landscaping: Lawns and ornamental plants to prevent nutrient runoff.

By enabling on-demand nutrient release, this technology improves fertilizer utilization efficiency (40–60% higher than conventional urea) while reducing ammonia volatilization and nitrate leaching, aligning with sustainable agricultural practices.

This translation maintains technical precision while ensuring clarity for an English-speaking audience in the fertilizer industry. Let me know if you need further refinements!