Efectos De La Tasa De Disolución En Dosis De Unidades Agroquímicas Solubles En Agua

Commercial water-soluble unit doses for agricultural active ingredients, predominantly manufactured from polyvinyl alcohol (PVOH) film via horizontal form-fill-seal thermoforming systems such as the Multivac R535 with plug-assisted deep-draw stations and sealing pressures of 4.5 bar, must dissolve rapidly in field spray-tank water to ensure uniform distribution of the active constituent throughout the application volume. The dissolution rate is not merely a convenience parameter; it directly determines whether an operator can achieve a homogeneous mixture within the standard induction-bowl recirculation window of 180 seconds to 300 seconds typical of trailed sprayers with 1,200 L to 4,000 L tank capacities. Testing protocols derived from CIPAC MT 176, adapted for a 1.0 L beaker containing 600 mL of CIPAC standard water D at 20 °C and stirred with a 50 mm magnetic bar at 600 rpm, with the unit dose loaded with 50 g of inert ballast (kaolin), define the benchmark for dissolution completeness. Film grade selection, including degree of hydrolysis, thickness, plasticiser content, and the presence of surfactants or disintegrants, allows tailoring of dissolution time from as low as 15 seconds for ultra-rapid cold-water grades to more than 8 minutes for delayed-release or low-temperature-tolerant products. The interplay between the unit dose shell and the enclosed liquid or granular agrochemical—often a highly concentrated electrolyte solution such as 41% w/w glyphosate isopropylamine salt or a 250 g/L suspension concentrate—creates a complex boundary-layer phenomenon where osmolality, pH, and ion-specific interactions modulate film hydration kinetics. Understanding these effects at the production scale demands data from gel-permeation chromatography for molecular weight distribution, differential scanning calorimetry for crystallinity index, and non-contact laser thickness gauging across the web with 0.1 µm resolution, because batch-to-batch variances exceeding ±15% in dissolution time have been traced to as little as 3 µm variation in mean film thickness during thermoforming on a 16-station rotary machine running at 28 cycles/min.

What Determines Instant Solubility in Cold-Water Environments?

Cold-water dissolution performance, particularly at temperatures between 3 °C and 8 °C encountered during early-spring herbicide applications in northern temperate zones, separates standard PVOH grades from engineered variants. A partially hydrolysed PVOH with 88 mol% hydrolysis, number-average molecular weight in the range 30,000 Da to 50,000 Da, displays a dissolution time of 65 s at 20 °C for a 40 µm thick film as determined by the CIPAC MT 176 adapted method. Lowering the water temperature to 5 °C extends the dissolution time to 310 s ± 35 s, pushing the system beyond the acceptable tank-mixing interval for high-throughput commercial sprayers. The mechanism is dominated by the reduction in water diffusion coefficient through the gel layer that forms upon initial hydration; the gel-layer thickness grows proportionally to the square root of time, and the diffusion coefficient of water in the swollen PVOH decreases by approximately 60% when the temperature drops from 20 °C to 5 °C, as calculated from gravimetric sorption data using a Fickian diffusion model with a boundary condition of constant surface concentration. Fully hydrolysed grades of 99 mol% hydrolysis, which possess higher crystallinity from syndiotactic sequencing, exhibit dramatically slower dissolution: a 40 µm cast film requires 1,020 s at 20 °C and effectively fails to dissolve completely at 5 °C within a 30-minute observation window, rendering them unsuitable for cold-water unit-dose applications unless modified with plasticisers and surfactants. The incorporation of 5 phr glycerol or 3 phr triethyl citrate reduces the low-temperature dissolution time of the 88 mol% PVOH film by an additional 25–30%, while the addition of 0.5 wt% of a non-ionic acetylenic diol surfactant further accelerates wetting and reduces the induction period by 8–12 seconds at 5 °C. Industrial formulations often combine these plasticiser-surfactant packages with a minor fraction of a low-molecular-weight PVOH (10,000–15,000 Da) to accelerate erosion. The following table summarises comparative dissolution data across film types and temperatures, measured according to the in-house method aligned with CIPAC MT 176.

When highly alkaline liquid formulations are enclosed in water-soluble unit doses, the internal chemical environment actively modifies the dissolution profile from the inside surface even before the outer film layer contacts the tank water. Concentrate solutions of potassium glyphosate or other amine-solubilised acids present aqueous-phase pH values of 4.8 to 5.2 in the isopropylamine salt form, yet higher-pH formulations based on potassium or mixed salts can reach pH 6.5–7.0. More significantly, adjuvant-loaded compositions containing alkyl amine ethoxylates or carbonate buffers can achieve bulk pH values exceeding 10.5, triggering alkaline hydrolysis of the ester linkages in modified PVOH films or of the glycosidic bonds in starch-based laminate layers. Premature film weakening has been observed within 12 weeks of ambient storage at 25 °C and 60% RH when a PVOH copolymer containing 4 mol% vinyl acetate units and 1.5 mol% itaconic acid co-monomer was exposed to an internal solution of pH 11.2. The bag burst strength, measured according to ASTM F2054 with an internal pressurisation rate of 2 psi/s, dropped from an initial 18.5 psi to 6.2 psi after 90 days, a failure mode that manifests in the field as product leakage during transport. Dissolution rate per se becomes irrelevant if the integrity of the unit dose is compromised; thus, combinations of highly basic formulations with standard PVOH films are excluded from the design space under an internal compatibility protocol requiring a minimum retained burst strength of 70% of initial value after 8 weeks at 54 °C, as per accelerated aging guidelines derived from ASTM F1980. Films formulated with high-molecular-weight PVOH and the incorporation of 2 wt% of boric acid ester crosslinkers can provide temporary reinforcement against alkaline attack, but the crosslinking density must be carefully controlled because gel fractions above 8 wt% irreversibly retard dissolution in cold water beyond 600 seconds.

Film Swelling versus Erosion: A Dichotomy in Controlled Release

The dissolution mechanism of a water-soluble agrchem unit dose proceeds through two kinetically competitive pathways—swelling and erosion—whose balance dictates not only the time to complete solubilisation but also the intermediate generation of gel slugs that can obstruct sprayer nozzle filters. Swelling-dominated dissolution, characteristic of partially hydrolysed PVOH films with a degree of hydrolysis of 85–90 mol% and containing 6–8 phr of plasticiser, involves the absorption of water into the amorphous regions of the polymer matrix, forming a swollen gel layer of thickness that can reach 3–5 times the original dry film thickness before appreciable mass loss occurs. Erosion-dominated dissolution, common in highly plasticised or surfactant-loaded films and in thermoplastic starch (TPS) blends containing 30 wt% glycerol, exhibits minimal swelling and a rapid surface-controlled dissolution front that advances at a near-constant linear velocity of 0.5–0.8 µm/s at 20 °C in stirred water. The practical consequence of a swelling-dominated mechanism in unit doses containing suspension concentrate formulations is the formation of a viscous, gel-like envelope around the particulate payload, which temporarily impedes the release of solid particles larger than 50 µm. In a field sprayer with inline strainers of 50-mesh (aperture 300 µm), partially dissolved gel fragments have been documented by end users as causing pressure build-up and triggering the bypass valve. The erosion-dominated mechanism is therefore preferred when the unit dose contains wettable powder or water-dispersible granule cores, and film formulators target a gel fraction, measured by extraction in boiling water and gravimetry, of less than 5 wt%. Manufacturers of commercial agrochemical unit doses routinely characterise the swelling/erosion balance through immersion tests in CIPAC standard water A (hardness 342 ppm as CaCO₃) and water D (hardness 20 ppm) per the procedures aligned with OECD test guideline for dissolution of water-soluble packaging, recording mass loss over time and photographing the degree of gel residue on a 500 µm sieve at 30-second intervals. Data from a 76 µm PVOH film containing 3 wt% polyethylene glycol (PEG 400) and 1 wt% sodium lauryl sulfate showed residual gel on the sieve of 42% of original mass at 30 s, decreasing to 8% at 60 s, and full clearance at 90 s; an unmodified film of the same thickness retained 65% gel at 30 s and still exhibited 12% residue at 120 s, a difference that correlates with a 3-fold reduction in filter blockage incidents during high-volume aerial spraying operations.

In twin-tank induction systems where the operator relies on a Venturi-based chemical inductor to create a pre-slurry before the main tank transfer, the dissolution rate within the first 30 seconds becomes the critical process parameter, not the time to total disappearance of the film. Equipment such as the Arag Venturi Inductor operated at a flow rate of 80 L/min generates a residence time in the induction bowl of just 18–25 seconds before the mixture is transferred. A unit dose film that merely swells without rapid disintegration will produce a jelly-like mass that clings to the strainer of the induction bowl, reducing the effective suction and causing air entrainment in the diaphragm pump. Manufacturers of unit doses intended for such equipment therefore specify a disintegration time—defined as the time for the film to rupture and release its contents—of less than 15 seconds in the induction bowl under a flow of 60 L/min of water at 10 °C. This criterion is verified not only by lab beaker tests but by a full-scale simulation using a 200 L induction hopper fitted with a 40-mesh screen, with the dissolution event recorded via a submersible camera and particle size analyser sampling the downstream pipe. The influence of water hardness on disintegration time is pronounced: at 10 °C in soft water (50 ppm CaCO₃), a PVOH hot-water-soluble grade (99 mol% hydrolysis, 50 µm thick) recorded 38 s to rupture, whereas in hard water (1,000 ppm CaCO₃) the time extended to 290 s, attributable to calcium-ion-induced cross-linking of the alcohol groups which reduces the osmotic driving force and increases the gel layer modulus. Consequently, field protocols for regions with water hardness exceeding 500 ppm CaCO₃ mandate the use of water conditioners such as ammonium sulfate at 2% w/w of the tank volume added before introducing the unit dose, restoring dissolution times to within 15% of the soft-water baseline.

Can Starch-Based Blends Satisfy OECD 301B Biodegradation While Maintaining Rapid Dissolution?

The substitution of PVOH with renewable, readily biodegradable starch-polyester blends addresses the growing regulatory pressure under the EU Fertilising Products Regulation (FPR) and the proposed microplastic restrictions under REACH, yet the dissolution rate of high-amylose thermoplastic starch composites rarely matches that of tailored PVOH. Starch-based films plasticised with 25–35 wt% glycerol, processed via twin-screw extrusion on a Leistritz ZSE 27 MAXX with an L/D ratio of 44:1 and operated at a screw speed of 250 rpm with a die temperature of 140 °C, achieve a dissolution time under CIPAC MT 176 conditions at 20 °C of 170–220 s for a 60 µm film, which is roughly three times slower than a standard 88 mol% PVOH of the same gauge. The primary limitation is the heterogeneity of the dispersed amylopectin domains, which upon hydration form a persistent gel network that resists erosion. Blending with poly(butylene adipate-co-terephthalate) (PBAT) at a 20 wt% level improves mechanical properties during thermoforming but further retards dissolution due to the hydrophobic polyester phase, increasing dissolution time by 40–60 s. Ultrafine milling of native starch to a d₅₀ of 6 µm and the addition of α-amylase at 0.2 wt% of the film weight can accelerate disintegration by enzymatically cleaving the α-1,4-glycosidic bonds; in laboratory trials, amylase-loaded starch/PBAT (80:20) films exhibited a dissolution time of 95 s at 20 °C, but the enzyme deactivates within 30 days of storage at 30 °C and 75% RH, rendering the product unstable for the typical 2-year agrochemical shelf life. The biodegradation compliance picture is summarised in the following matrix, indicating that while starch blends may meet the ultimate biodegradation criteria, simultaneous attainment of sub-120 s dissolution under field conditions remains an active development challenge.

Thermoforming unit doses on a horizontal machine such as the Hassia THM 16/28 with a forming web width of 420 mm subjects the film to localised thinning at the corners of the cavity, with draw ratios in deep pockets reaching 4:1 to 6:1. Post-thermoforming, the corner thickness may be as low as 12 µm from a starting film of 40 µm, a reduction that reduces dissolution time at that location by 40% relative to the base, thereby creating a non-uniform release profile where the payload escapes from the thinnest section first. This premature rupture, while accelerating initial release, can cause the bulk of the bag to collapse into a crumpled mass that hydrates more slowly than an intact, fully submerged unit because the water access to the interior is then limited by folds. Laser scanning of the post-thermoformed web with a Micro-Epsilon scanCONTROL 2950-100 at 50 µm lateral resolution generates a thickness map that correlates with high-speed video dissolution analysis; regions thinner than 18 µm consistently exhibited rupture at 8–11 s, whereas thicker regions of 35–40 µm persisted beyond 45 s. To mitigate this heterogeneity, film producers incorporate blends of PVOH with different melt flow indices that alter the thermoforming stretching behaviour, or they pre-stretch the film biaxially before forming to homogenise the thickness distribution. A cast PVOH film with a draw ratio of 2.2:1 in machine direction and 1.8:1 in transverse direction prior to forming showed a reduction in thickness standard deviation from 4.7 µm to 1.9 µm, reducing the overall dissolution time spread from 22–68 s to 28–52 s as measured on 30 replicates of the same cavity position. Such uniformity is essential when the unit dose contains a high-value crop protection active ingredient where dosage accuracy within ±2% of the label rate is required and incomplete bag dissolution would result in significant financial loss or sprayer calibration error.

Premature Gelation from Cross-Linking Incompatibilities

A critical rejection criterion in the formulation of water-soluble agrochemical unit doses is the chemical incompatibility between the film and co-formulants or adjuvants that may be introduced into the tank alongside the unit dose. Borate-based micronutrient solutions, commonly used as foliar fertilisers containing 10% w/w boron as sodium borate octahydrate, react with the 1,3-diol moieties of PVOH to form reversible but kinetically rapid crosslinks that gel the dissolving film. A field scenario in which a unit dose is added to a tank already containing a boron foliar feed at 1.5% v/v can result in the instantaneous formation of a gel corona around the unit dose, extending the dissolution time from a nominal 60 s to more than 600 s, and in several documented service complaints, causing the intact gel capsule to be retained on the tank outlet filter after 15 minutes of recirculation. The crosslink density achieved with as little as 100 ppm of borate ions, derived from the rapid formation of didiol-diborate complexes, raises the storage modulus of the hydrated gel by more than two orders of magnitude, transforming the dissolving film into an elastomeric membrane. Similarly, amine-based tank adjuvants containing polyalkylene amine ethoxylates with primary amine end groups can form imine-type adducts with residual carbonyl groups in PVOH that arises from oxidation during processing, insolubilising the polymer surface. An internal stability protocol therefore explicitly states: avoid combination with boron-containing inputs, amine-based additives above 500 ppm active concentration in the tank, and any product that generates formaldehyde or glyoxal as a preservative release agent, because even trace crosslinking reinstates the cold-water insolubility that the film formulation was designed to overcome. Pre-drying of PVOH film reels is mandatory when ambient relative humidity exceeds 60% during storage in uncontrolled warehouses; films conditioned to 8% moisture content exhibit a blocking tendency on the unwind stand, and the pre-drying step at 45 °C and 5% RH for 4 hours restores consistent thermoforming and dissolution performance without inducing premature crosslinking. Operation of the unit dose filling line under conditions of 23 °C and 45% RH is recommended, as excursions above 65% RH increase the film’s equilibrium moisture content beyond 5% and result in a 15–20% elongation in dissolution time due to the hygroscopically expanded free volume that paradoxically slows water ingress under dynamic mixing.