Kontrollu salınımlı gübreler için polivinil alkol (PVA)

Polyvinyl alcohol (PVA) employed in controlled-release fertilizer systems is most commonly a partially hydrolyzed grade with a degree of hydrolysis between 86 and 89 mol%, typified by commercial designations such as PVA 17-88 (viscosity of a 4 % aqueous solution at 20 °C: 20.5–24.5 mPa·s; degree of polymerization approx. 1700–1800). Unlike fully hydrolyzed material (> 98 mol%), which requires dissolution temperatures exceeding 70 °C and yields crystalline, brittle films, the partially hydrolyzed variant dissolves readily in cold water (5–25 °C) and forms a continuous, flexible film upon drying—making it a practical candidate for water-permeable nutrient coatings. This polymer functions not as an impermeable barrier but as a semi-permeable matrix: water ingresses, dissolves the core nutrient, and an osmotic pressure gradient drives diffusive release through the swollen PVA membrane. The substitution of conventional polyolefin coatings (e.g., polyethylene, polypropylene) with PVA eliminates the need for mechanical pore-formers, as the inherent hydrophilicity of the vinyl alcohol segments provides tunable permeability without macroscopic defects. The product is supplied as a free-flowing white powder with a bulk density of 0.4–0.6 g/cm³, a maximum moisture content of 5.0 % as received, and a pH (4 % solution) of 5.0–7.0. It conforms to REACH registration and is manufactured without the use of alkylphenol ethoxylates.

What Happens When PVA Film Swells in Soil?

Upon contact with soil water, a PVA coating undergoes a two-stage swelling process that governs nutrient release kinetics. In the initial stage (0–60 min), water molecules penetrate the amorphous regions, plasticizing the vinyl alcohol segments and causing a rapid thickness expansion of 20–40 %. The equilibrium swelling ratio is strongly dependent on the degree of hydrolysis: a film cast from PVA 17-88 swells to approximately 1.8× its dry mass in deionized water at 25 °C, whereas a film from PVA 26-88 (higher molecular weight, same hydrolysis range) reaches 2.1× due to reduced crystallinity from less efficient chain packing. During the second stage, the swollen gel layer acts as a rate-limiting diffusion barrier. The effective diffusion coefficient of urea through a swollen PVA film was measured at 1.2 × 10⁻⁶ cm²/s using a side-bi-side diffusion cell (Franz cell geometry) per OECD guideline 428, a value significantly higher than that through polyethylene (<1 × 10⁻⁹ cm²/s) but lower than that through an uncoated granule. This intermediate permeability is critical: it prevents instantaneous nutrient dumping while still permitting biota-available release rates. The swelling behavior must be managed, however; a film that hydrates too rapidly loses mechanical integrity and ruptures, releasing the entire core in a single event. Addition of a secondary polymer with lower water affinity—such as poly(caprolactone) in a blend ratio of 10:90 (PCL:PVA)—depresses the equilibrium water uptake to 0.9× and extends the time to 80 % nutrient release from 7 days to 28 days in a static water column test at 25 °C following the principles of ISO 21263:2017.

In granulated urea systems coated via a top-spray fluidized bed with a 10 wt% aqueous PVA 17-88 solution containing 15 phr glycerol as plasticizer, the nutrient release profile in deionized water at 25 °C under standard extraction conditions (sample-to-water ratio 1:20) typically shows 15–25 % cumulative release after 24 h, 55–70 % after 7 days, and> 90 % by 14 days. When the same coated granules are incubated in a loamy sand soil (15 % moisture content, 25 °C), the release rate is retarded by 30–40 % relative to the water-only test due to reduced free water activity and soil colloid interactions. This discrepancy highlights the limitation of using static water extraction as a sole performance metric; a soil-column leachate test following DIN 19528 yields more agronomically relevant data. The PVA film does not disintegrate enzymatically in the initial 14 days; primary degradation is hydrolytic and begins when soil moisture and microbial colonization reach threshold levels, typically after 28–40 days in temperate agricultural soils. Published data for field-scale corn trials using PVA-coated urea at an application depth of 10 cm in a silt loam demonstrate a nitrogen use efficiency improvement of 12–18 % relative to uncoated urea, measured by the difference method (fertilizer N recovery in aboveground biomass).

Process Constraints for Aqueous PVA Solutions in Fluidized Bed Systems

Coating fertilizer granules with PVA in a Wurster-type fluidized bed apparatus imposes a narrow processing window defined by the balance between droplet drying, film coalescence, and particle agglomeration. The aqueous coating solution, typically prepared at a concentration of 8–12 wt% PVA, must be maintained at a tank temperature of 25–35 °C to avoid gelation; cooling below 10 °C can induce hydrogen-bonded physical gelation in solutions of partially hydrolyzed PVA with high syndiotacticity. The spraying nozzle (two-fluid, internal-mix) is fed with atomizing air at a pressure of 1.5–2.5 bar, yielding a median droplet diameter (Dv50) of 30–50 µm. Inlet air temperature is set between 60 and 80 °C—temperatures above 85 °C cause premature skin formation on the droplet surface, trapping water inside the film and creating vacuoles that reduce coating integrity, while temperatures below 50 °C result in insufficient evaporation and catastrophic agglomeration within 3–5 min of spraying. The bed temperature, monitored with an infrared sensor, must be held at 35–45 °C to keep the PVA film above its effective glass transition temperature (Tg of PVA 17-88 with 15 phr glycerol: approximately 28 °C as measured by DSC at 10 °C/min, second heat) for proper film coalescence, yet below the temperature that softens the urea core (melting point of urea: 132.7 °C, but softening and deformation can occur at surface temperatures above 90 °C on the hot urea particle).

A production-scale run with a batch size of 500 kg of urea prills (SGN 220) and a coating solution delivery rate of 3.0 L/min through a 1.2 mm nozzle insert typically achieves a coating thickness of 25–40 µm (as determined by cross-sectional SEM image analysis on 50 randomly selected granules) with a weight gain of 3.5–5.0 %. The coefficient of variation (CV) of coating thickness across the batch, measured with micro-CT, can reach 18–22 % if the fluidization air velocity deviates from the optimal range of 1.2–1.5 m/s. At lower velocities, uneven wetting generates doublets and triplets; at higher velocities, attrition of the already-formed coat accounts for 0.5–1.0 % mass loss per hour of processing. Pre-drying of the PVA powder before solution preparation is mandatory when ambient relative humidity exceeds 60 %, as absorbed moisture alters the gravimetric feed accuracy and can introduce a systematic error of up to 1.5 % in final coating weight. Nozzle clogging, caused by dried PVA film formation at the liquid tip during momentary spray interruptions, is mitigated by a pulsed air cap cleaner activated every 15 s for a 0.5 s burst.

If Crosslinking Agent Ratio Exceeds 5 wt%, Premature Gelation Occurs

Extending nutrient release beyond 30 days necessitates chemical crosslinking of the PVA matrix to reduce its aqueous solubility and swelling capacity. Sodium tetraborate decahydrate (borax) at concentrations of 1–3 wt% relative to PVA causes reversible di-diol complexation, reducing the cold-water soluble fraction from > 95 % to 35–50 % after 24 h immersion in water at 25 °C. However, the crosslinking reaction is instantaneous upon mixing; an aqueous solution of PVA 17-88 (10 wt%) to which borax is added at a ratio exceeding 5 wt% of PVA mass transforms into a non-sprayable thixotropic gel with a zero-shear viscosity exceeding 50,000 mPa·s within 90 s, making subsequent coating application impossible. Crosslinking must therefore be performed post-coating by spraying the borax solution onto already-formed PVA films or by incorporating the crosslinker in a subsequent topcoat. Even at 2 wt% borax, the pot life of the mixed solution is limited to 8–12 min at 25 °C, demanding in-line static mixing with a residence time of less than 2 min.

Alternative covalent crosslinkers—glyoxal (0.5–2.0 wt% of PVA) and citric acid (5–10 wt% plus sodium hypophosphite catalyst at 50 % of citric acid mass)—require a thermal curing step at 120–140 °C for 15–30 min to achieve an esterification extent sufficient to reduce water-soluble fractions below 20 %. Such a curing step is incompatible with urea prills, as urea hydrolysis begins to accelerate above 130 °C and releases ammonia, creating film blistering. This thermal incompatibility restricts covalent post-crosslinking to fertilizer cores with higher thermal stability, such as potassium chloride. Glyoxal-crosslinked PVA coatings on KCl granules (core temperature during curing held at 125 °C by short-exposure infrared radiation) produced a 75 % cumulative K release in soil after 90 days under outdoor lysimeter conditions, compared to 100 % release in 21 days for the non-crosslinked control. The toxicity profile of residual glyoxal (classified as a category 2 mutagen under CLP Regulation (EC) No 1272/2008) limits its use in European Union agricultural applications unless a post-curing washing step reduces residual free glyoxal to below the limit of quantification (0.1 mg/kg).

PVA also functions as a binder in seed coatings that incorporate micronutrient fertilizers.

The choice between PVA and starch acetate in aqueous coating systems hinges on solution stability and film retrogradation. Starch acetate with a degree of substitution (DS) of 1.5 forms a lower-viscosity solution (50–100 mPa·s at 10 % solids) than PVA 17-88 (250–400 mPa·s at 10 %), permitting higher solids loading and faster film build-up. Yet starch acetate coatings undergo progressive embrittlement due to amylose retrogradation upon storage at temperatures below 15 °C, leading to microcrack formation and loss of controlled-release function after 4–6 weeks of warehouse storage. PVA films, plasticized with glycerol, retain their elongation at break above 100 % even after 6 months of storage at 5–40 °C and 10–80 % RH, as evidenced by periodic tensile testing per ASTM D882. This storage stability is a decisive factor for distribution chains lasting multiple months. In contrast to polyurethane coatings, where the NCO index and polyol selection permit precise programming of crosslink density and thus release rate, PVA lacks such orthogonal tunability; its release profile is modulated primarily through coating thickness and plasticizer content, offering a narrower functional range. When a release duration beyond 120 days is required, PVA alone is generally unsuitable and must be layered with a hydrophobic topcoat such as a fatty acid ester of cellulose, which increases the effective diffusion path length while preserving the biodegradability of the composite coating.

Pre-treatment of fertilizer granule surfaces with a cationic surfactant (e.g., cetyltrimethylammonium bromide, 0.1 % of granule mass) improves PVA coating adhesion on potassium nitrate prills, where the inherently low surface polarity reduces wetting. Contact angle measurements (sessile drop, 5 µL) on untreated KNO₃ show a value of 65–70° for the aqueous PVA solution, dropping to 25–30° after surfactant pre-treatment, as measured with a goniometer coupled with high-speed image capture at 100 fps. This reduction translates to a coating thickness uniformity improvement of 30 % (CV reduction from 25 % to 17 %) and a corresponding decrease in burst release from 12 % to 4 % in the first 2 h of immersion.

During post-coating drying in a continuous belt dryer divided into three temperature zones—zone 1: 40 °C for 15 min, zone 2: 50 °C for 15 min, zone 3: 30 °C for 15 min with air flow at 0.8 m/s—the residual moisture in the PVA coat must reach 3–5 % before packaging. Inadequate drying leaves residual water that plasticizes the film excessively during storage, resulting in cold flow and inter-particle adhesion that can generate clumps exceeding 25 mm in diameter, jamming the distribution chutes of pneumatic fertilizer applicators. Over-drying to below 1 % moisture, conversely, raises the film's Tg above 40 °C, rendering it glassy and prone to brittle fracture under the impact loads encountered in rotary spreader vanes (impact velocity approx. 15–20 m/s).