Melt granulation of urea produces a dense, low-porosity core that is fundamentally different from prilled urea as a substrate for controlled-release coatings. In the granulation section, a concentrated urea melt at 98.0–99.5 wt% is sprayed through pressure nozzles into a fluidised bed granulator operating with a bed temperature of 105–115 °C and a superficial air velocity of 2.0–3.5 m/s. The process uses seed granules recycled at mass ratios of approximately 0.5:1 to 1:1 relative to product discharge. The resulting granules typically fall within a 2–4 mm screen cut, with residual moisture below 0.5 wt% and single-granule crushing strength in the range of 3.0–5.5 kg/granule. These mechanical properties are favourable for coating, but the narrow processing window between granule surface softening and coating adhesion creates significant process control requirements. Because the urea core begins to soften near 130–132 °C, melt granulated urea must be cooled and held below this temperature before entering the coating stage. The controlled-release coating is therefore applied at a granule surface temperature of 60–85 °C in most commercial configurations. The primary function of the coating is to impose a diffusion barrier that reduces the rate of water ingress, urea dissolution, and nitrogen release to soil or aqueous media. Without a coating, urea granules dissolve rapidly and nitrogen is subject to ammonia volatilisation and nitrate leaching. The low-porosity character of melt granulated urea reduces the absorption of low-viscosity coating solutions relative to more porous substrates, but surface microcracks and crystal boundary discontinuities still influence film continuity. Batch-to-batch variance in granule size distribution is a central production variable because the specific surface area per unit mass changes with granule diameter; at a constant coating weight, smaller granules receive a thicker film than larger granules. This size-dependent coating uptake directly affects the release profile and must be controlled either by narrowing the granulation screen cut or by adjusting the coating spray rate in response to the measured particle size distribution.

Why Does Granule Surface Geometry Control Coating Integrity and Lag Phase Duration?

Coating adhesion failures on melt granulated urea originate primarily from surface discontinuities produced during crystallisation and cooling. Although melt granulated urea generally has a denser and smoother surface than prilled urea, scanning electron microscopy of industrial granules has revealed microcracks, platelet boundaries, and occasional closed gas pores with effective diameters of 5–50 µm. These features create capillary wicking channels when a low-viscosity coating solution is applied. A solventborne polyurethane or polyolefin coating with viscosity below 150 mPa·s can penetrate these microcracks and leave pinholes after solvent evaporation. Pinholes shorten the lag phase because water can enter locally and dissolve urea before the intact film becomes saturated. Pull-off adhesion testing according to ASTM D4541-17 or ISO 4624:2023 is difficult to execute on curved granule surfaces, so coating integrity is more often inferred indirectly from release-profile reproducibility under ISO 21263:2017 or EN 13266:2001. Release from an intact coated granule is diffusion-controlled. The apparent lag time scales with the square of the film thickness, consistent with the plane-sheet solution for Fickian diffusion. For a 3–8 wt% polyurethane coating on a 2–4 mm urea core, water permeation and urea dissolution generate internal osmotic pressure that has been reported to exceed 2 MPa in some film systems. The film must therefore possess sufficient elongation at break to resist rupture under internal pressure. Free-film tensile properties measured by ISO 527-3:2018 demonstrate that high crosslink density raises tensile strength but reduces elongation, producing a trade-off between burst resistance and handling flexibility. Granule surface temperature during curing also governs film morphology. If the rotary drum temperature exceeds 85 °C, rapid solvent evaporation can create a porous skin layer. If the drum temperature remains below 60 °C, polyurethane cure may be incomplete within the available residence time. This processing window of approximately 60–85 °C is one of the principal threshold risks in controlled-release urea manufacturing. Published data for the specific interaction between melt granulated urea surface roughness and defect density in commercial coatings is limited, and transfer of laboratory film data to production granules requires verification on the actual granulation output.

Sulfur-Wax and Polyurethane Hybrid Systems

Sulfur-wax coating systems are produced by spraying molten sulfur at 135–145 °C onto preheated urea granules in a rotary drum. Rapid solidification of the sulfur shell produces a brittle and microporous barrier. A wax sealant at 1–3 wt% relative to sulfur is then applied to close surface cracks and reduce initial release. Sulfur-coated urea typically releases 20–40% of total nitrogen within 7 d in water at 25 °C, because the sulfur shell contains inherent defects that cannot be fully sealed by wax. Polyurethane coatings can produce lower early release, but only when a continuous film forms across the entire granule surface. Two-component aromatic polyurethane systems based on polymeric methylene diphenyl diisocyanate and a polyether or polyester polyol are common in controlled-release urea production. The pot life at 60–70 °C may be as short as 5–15 min, requiring in-line mixing close to the spray nozzle. Hybrid systems use a sulfur base coat to reduce cost and a thin polymer topcoat to seal defects in the sulfur layer. The release profile is not a simple linear function of coating mass. The same average coating loading can generate significantly different release duration depending on defect density, film thickness distribution, and sealant penetration. Trial programs therefore compare coating formulations by measuring release curves under a single specified test method, because water immersion release is more aggressive than soil incubation under unsaturated conditions. The table below summarises representative commercial ranges reported for melt granulated urea substrates.

On continuous coating lines handling 2–4 mm melt granulated urea, the rotary drum coating section commonly consists of a preheat drum, a spray chamber, and a curing or cooling drum arranged in series. Spray nozzle atomisation air pressure is typically maintained between 1.5 bar and 3.0 bar for solventless polyurethane systems. The droplet size and spray rate are adjusted to prevent droplet drying before impact and to avoid local overwetting of the granule bed. The ratio of coating mass to granule surface area controls average film thickness. Because surface area per unit mass decreases as granule diameter increases, larger granules receive a thinner film at constant coating weight. This effect can be observed as a shift in release duration between the 2.0–2.8 mm and 3.2–4.0 mm screen fractions. Narrowing the granule size distribution improves coating uniformity but reduces granulation throughput and may increase recycle load. Two-stage coating with a sealant layer can reduce release-rate sensitivity to core size, but introduces an additional curing step and, in solventborne systems, increases volatile organic compound handling requirements. The drum fill level is commonly held at 8–15% of total drum volume, and the residence time distribution determines the number of passes a given granule makes through the spray zone. A broad residence time distribution widens the coating mass distribution; internal baffles and controlled drum speed reduce axial dispersion. Industrial batch-to-batch variance is dominated by variations in granule surface temperature, ambient humidity during solvent evaporation, and the elapsed time between granulation and coating. Urea is hygroscopic above approximately 60% relative humidity, and stored uncoated granules can develop surface moisture films that inhibit coating adhesion. Pre-drying at 50–60 °C for 20–40 min is required in humid production environments. If relative humidity exceeds 60% and pre-drying is omitted, moisture on the granule surface can react with isocyanate components, forming urea linkages and causing bubble defects in the film. This incompatibility between surface moisture and isocyanate-based systems is a recognised source of batch rejection on industrial coating lines.

When Coating Thickness Exceeds 8 wt% and the Risk of Brittle Fracture Increases

Increasing coating mass to 8–15 wt% extends nutrient release duration but introduces mechanical failure modes. Sulfur-dominant coatings with limited polymer content exhibit brittle fracture during bagging, conveying, and broadcast application. Impact testing of coated granules dropped from 1.5 m onto a concrete surface reveals higher rates of shell cracking when sulfur loading exceeds 15 wt% without an elastomeric topcoat. Polyurethane films with high crosslink density and glass transition temperature above 45 °C may perform acceptably in water release tests but crack during cold-weather handling because the film is below its ductile-to-brittle transition. Free-film tensile data generated under ISO 527-3:2018 indicate that elongation at break below 10% is associated with poor impact resistance on the granule. Coating fracture creates a rapid-release fraction that is not captured by standard water release tests unless the sample is subjected to mechanical stress before testing. The conflict between thick films for extended release and brittle failure is a threshold risk for products with release durations beyond 6 months. Hybrid systems with a thick sulfur base coat and a thin high-elongation polymer topcoat reduce this failure mode, but the interfacial adhesion between sulfur and polymer becomes the limiting defect. Surface abrasion during conveyor transfer and bulk shipping can generate fines; if the coating fails at the sulfur-polymer interface, the release profile reverts toward that of unsealed sulfur-coated urea. Coating thickness distribution also affects the lag phase. A 10 wt% average coating with a coefficient of variation of 20% contains local regions where the thickness falls below the threshold for continuous film formation, causing early release. Such statistical film defects are not readily detected by optical inspection and require release testing of separated size fractions using sieves conforming to ISO 3310-1:2016. For these reasons, the acceptable upper coating limit is a function of core size distribution, coating tensile properties, and interfacial adhesion. Published data for the specific configuration of melt granulated urea with high-crosslink aromatic polyurethane is limited, so commercial qualification programs typically include mechanical stress followed by release measurement according to ISO 21263:2017.

Measuring Release Rates in Aqueous and Soil Incubation Systems

Release rate measurement for coated urea is method-dependent. Water immersion at 25 °C under ISO 21263:2017 or EN 13266:2001 accelerates release relative to unsaturated soil because the external water phase maintains a constant sink and the coating remains fully hydrated. Soil incubation at 60% water-filled pore space and 25 °C generally yields lower release rates because water availability and temperature fluctuate with the soil environment. The ratio of water release to soil release is not constant across coating types. A hydrophobic polyolefin film may show a larger difference between water and soil release than a more hydrophilic polyurethane film because the driving force for water uptake depends on water activity. Standard methods typically require separation of released urea from intact granules by filtration and subsequent measurement of nitrogen by conductivity, colorimetry, or enzymatic assay. Conductivity methods are rapid but sensitive to dissolved carbon dioxide and temperature drift; calibration against an ammonium nitrate standard is required. The duration to 80% release is commonly used as a comparative endpoint, but the early release fraction in the first 24 h is equally important for crop safety. A product with a moderate total release duration can still cause seedling damage if the 24 h release exceeds 10–15% of total nitrogen. This initial release threshold is directly relevant to turf and seedbed applications. The release curve shape is typically sigmoidal for intact polymer-coated granules, with a lag phase, a linear release phase, and a tailing phase. Deviation from this shape indicates mixed coating quality, core size effects, or mechanical damage. When using standardised release methods, the sample size should represent the full granule size distribution; testing a single screen cut can generate misleadingly uniform results. Compliance with ISO 18644:2016 requires the manufacturer to declare the release duration and the test method used. Customer specifications often require a water release curve under ISO 21263:2017 and a supporting soil incubation under EN 13266:2001 for regulatory submissions. The two methods differ in temperature control, shaking frequency, and sample preparation, and results cannot be compared directly without reporting the method designation and granule size fraction tested.

In flooded paddy rice cultivation, controlled-release urea is applied either at transplanting or as a single basal dose, and the granule remains under a saturated soil layer for most of the season. The external water phase remains near 25–30 °C under tropical lowland conditions, while oxygen tension around the granule is low. Low oxygen reduces biological nitrification of the released urea, but coating still governs the rate of urea diffusion out of the granule. In such conditions, a coating designed for water release at 25 °C releases faster at soil temperatures of 30 °C because the diffusion coefficient of water in the polymer film increases with temperature. The apparent activation energy for water permeation in polyurethane coatings is often reported in the range of 30–45 kJ/mol; a 5 °C temperature increase can therefore raise release rate substantially. This temperature sensitivity explains why release data collected at 25 °C under standard conditions must be interpreted carefully before predicting field performance in warm soils. In turf and horticultural substrates, the growth medium is frequently unsaturated and subject to repeated irrigation. Wetting-drying cycles impose additional stress on the coating because hydration and dehydration cause expansion and contraction of the urea core. Over repeated cycles, interfacial fatigue can create microcracks that are not present in continuous water immersion tests. This effect is more severe for high-glass-transition coatings with low elongation; flexible polyolefin or plasticised polyurethane topcoats exhibit less fatigue cracking but may carry higher cost and lower nitrogen content. The combination of a sulfur base coat and a flexible polymer topcoat is therefore used where repeated wetting-drying cycles are expected. Another operational boundary is storage of coated urea in bulk piles at ambient temperatures above 40 °C. Urea core softening can begin near 90–110 °C depending on moisture and applied pressure, but prolonged exposure of polymer-coated urea to high humidity can still degrade the coating-substrate interface. Warehousing at relative humidity above 70% without sealed packaging can result in surface dissolution of exposed urea at coating defects and subsequent caking. Anticaking treatments applied after coating may interfere with coating adhesion; they should be incorporated before the coating step or omitted where the coating already reduces caking under moderate load. The release profile after one season of storage may differ from the as-manufactured release profile, making logistics and warehousing conditions part of the controlled-release specification.