Polyvinyl alcohol (PVOH; CAS 9002-89-5) is a water-soluble synthetic resin produced by alcoholysis of poly(vinyl acetate). Commercial film grades are classified under ISO 15023-1:2017 by nominal degree of hydrolysis and viscosity-average degree of polymerisation. The degree of hydrolysis fixes the dissolution temperature, while molecular weight controls aqueous solution viscosity and film strength. A 4% aqueous solution of general-purpose grades at 20°C spans roughly 3 mPa·s to 70 mPa·s, with hydrolysis from 70 mol% to 99 mol%. These two parameters define the processing envelope for water-soluble packaging growth in detergent pods and agro film demand.

The resin is supplied as a white-to-cream powder or granule. Partially hydrolysed grades of 86–89 mol% are used for cold-water films; fully hydrolysed grades of 98–99 mol% are reserved for hot-water or higher-crystallinity applications. Residual acetate groups disrupt hydrogen bonding and reduce crystallinity. The crystalline fraction is the primary reason pod films require cold-water grades while some agrochemical sachets use mid-range hydrolysis to avoid premature dissolution in humid field conditions.

What Determines the Dissolution Envelope of Partially Hydrolysed Grades?

Dissolution is governed by the competition between hydrogen bonding to water and the crystalline lattice within the PVOH film. Partially hydrolysed grades of 86–89 mol% dissolve at 10–30°C, whereas fully hydrolysed grades of 98–99 mol% require 70–95°C under atmospheric pressure. The residual acetate content lowers crystallinity from approximately 30–35% in fully hydrolysed film to below 15% in partially hydrolysed film. Detergent pod film is therefore formulated around the 86–89 mol% window and plasticized to reduce the glass transition temperature below ambient storage conditions.

No single harmonised ISO dissolution method covers finished pouch break-up; supplier technical data sheets typically report disintegration time in a stirred water bath at a fixed temperature. The comparison of dissolution times therefore requires a common payload, water volume, and agitation intensity. For a 50 µm unplasticized laboratory film, cold-water disintegration may be reported between 60 s and 180 s at 20°C. Finished detergent pods may show 90 s to 300 s depending on drum geometry, load size, and water temperature.

Commercial pod film grades are further specified by 4% aqueous viscosity at 20°C from 5 mPa·s to 55 mPa·s. Viscosity-average molecular weights generally range from 20,000 g/mol to 100,000 g/mol. Higher molecular weight increases tensile strength and tear resistance but raises solution viscosity during casting, requiring lower solids or modified drying profiles. Ash content is typically below 0.5 wt%, and volatile matter after drying below 5 wt%, as set in JIS K6726.

In the conversion of unit-dose detergent pods, PVOH film is the load-bearing envelope that must survive filling, sealing, storage, and then dissolve in wash water. Film thickness typically ranges from 38 µm to 76 µm. Mechanical properties measured under ASTM D882-18 at 23°C and 50% relative humidity show machine-direction tensile strength from 25 MPa to 50 MPa and elongation at break from 200% to 400%. The film is usually cast via slot-die from aqueous solution rather than blown from melt, because melt processing requires careful plasticiser compounding to suppress thermal degradation.

The heat-seal layer is formed from the same PVOH grade. Heat-seal temperature on production lines is maintained between 120°C and 160°C, with seal-bar control held within ±5°C. A narrow processing window exists: below the lower limit, channel leakers result in detergent leakage; above the upper limit, film shrinkage and pinholes at the seal land increase reject rates. Production-scale equipment includes rotary or platen heat-seal tools with heated steel sealing faces, closed-loop temperature control, and pressure settings adjusted for film thickness and plasticizer level.

Plasticisers such as sorbitol, glycerol, and trimethylolpropane are added at 10–25 wt% of the dry resin to reduce brittleness and lower film modulus. The plasticiser type changes seal initiation and blocking tendency. Glycerol-rich films soften at lower temperatures but are more hygroscopic; sorbitol-rich films exhibit lower equilibrium moisture at 60% relative humidity but require higher seal-bar temperature. The residual moisture in finished pod film is typically maintained at 8–12 wt% to balance blocking resistance and ductility.

The unplasticized glass transition temperature of PVOH is reported near 75–85°C, but pod film is plasticized to bring the effective glass transition below 10–20°C so that storage at typical warehouse temperatures does not induce brittle fracture. Dynamic mechanical analysis of pod film shows a primary relaxation shifted by glycerol or sorbitol, with the exact shift depending on plasticizer efficiency and moisture. The film modulus at 23°C and 50% relative humidity typically falls between 500 MPa and 2,000 MPa depending on plasticiser content.

Film-Grade PVOH and the Unit-Dose Detergent Pod Conversion Landscape

Slot-die casting lines used for pod film apply a water-based PVOH solution onto a moving belt or drum, dry the film in zoned ovens, and rehumidify the web before winding. Drying conditions determine the skin-core structure and final moisture profile. If the surface dries too quickly, skinning traps water in the centre and produces haze; if the film remains underconditioned, blocking occurs in roll form. Line data from production trials show that final moisture outside the 8–12 wt% range increases defects at sealing.

The aqueous casting solution is prepared at 15–30 wt% solids, depending on molecular weight and line speed. Filtering through 10–40 µm absolute-rated filters removes undissolved gels before the slot die. Film orientation is controlled by the speed ratio between the casting drum and the winder. Machine-direction stretch up to 2.5:1 is common; transverse-direction properties are set by web width and drying shrinkage. High orientation raises strength but lowers tear resistance in the transverse direction. Production-grade pod film therefore maintains MD/TD elongation at break above 200% to accommodate stretching during liquid filling and sealing.

The agrochemical sachet film values reflect a shift toward higher hydrolysis grades that survive brief rain or high humidity exposure but still dissolve in the spray tank. These ranges are not approval limits; they are comparative values from public supplier technical bulletins and should be validated for each formulation because active ingredients and co-formulants modify film dissolution through ionic strength and pH effects.

In melt-based film production, plasticised PVOH is compounded in a corotating twin-screw extruder with an L/D ratio of 30:1 to 44:1. Barrel temperatures are commonly set from 170°C to 210°C, and the screw profile uses restrictive kneading blocks to disperse plasticiser without exceeding the thermal degradation onset of the polymer. The compounded pellets are then processed on a blown-film or cast-film line. Production-scale failure modes include melt fracture at the die, gel specks from unmixed resin, and colour development when barrel residence time exceeds 2 min at the upper temperature limit.

Finished pod integrity is checked by leak tests, often using vacuum decay or trace-gas methods. The maximum allowable leak rate is specified by the product owner, but a typical vacuum-decay threshold is below 0.05 cc/min at a pressure differential of 10 kPa. Seal-strength testing under ASTM F88/F88M-21 is performed after conditioning at 23°C and 50% relative humidity. Film tears or seal separation below 10 N/15 mm indicate insufficient seal-bar temperature or insufficient dwell time.

The conversion of laundry detergents from liquid bottles to unit-dose pods has increased demand for cold-water PVOH film, but the format also imposes chemical compatibility limits. Liquid detergent pods contain nonionic and anionic surfactants, enzymes, and solvents such as propylene glycol and glycerol. These components can plasticize the film over time, reduce seal strength, or cause swelling. Long-term package qualification therefore includes visual inspection, seal strength, and dissolution after storage at 40°C and 75% relative humidity for 8–12 weeks.

When Pesticide Sachet Films Must Resist Premature Solubilisation in High-Humidity Warehouses

Agrochemical sachets operate in a different boundary. The film must remain mechanically intact during filling of granular or liquid formulations and during storage in uninsulated warehouses, then dissolve completely in spray-tank water. A partially hydrolysed cold-water grade may soften at relative humidity above 60%, causing blocking at interlayer contact and loss of seal integrity. For this reason, sachet films are often conditioned to 5–8 wt% moisture and stored in moisture-barrier packaging with desiccant when ambient relative humidity exceeds 60%.

The interaction between PVOH and the agrochemical load is a critical quality parameter. Borate ion in micronutrient formulations crosslinks adjacent hydroxyl groups through borate-diol bonds, producing a gel skin that delays dissolution. Formulation pH below 3 or above 10 accelerates ester hydrolysis and can embrittle the film over a storage season. Published data for some active-ingredient-specific storage configurations is limited; the absence of a universal compatibility standard means that pack stability must be confirmed by measuring seal strength and dissolution time after accelerated storage at 40°C and 75% relative humidity.

Demand for PVOH agro film is concentrated in water-soluble sachets for dry flowable and water-dispersible granule pesticide formulations, seed tapes, and fertilizer sticks. The film converts a measured dose into a unit package, reduces user contact with concentrates, and allows closed transfer into the spray tank. Dissolution of the sachet in the tank is influenced by water temperature, agitation, and the ionic strength of the tank mixture. At 10°C, partially hydrolysed pod-grade film may not disintegrate fast enough under low agitation, so higher-hydrolysis sachet film is formulated to fail only after a defined wetting period rather than immediately on contact.

Spray-tank dissolution is a function of water volume, initial water temperature, and agitation. A 50 µm sachet in a 1,000 L spray tank with jet agitation at 15°C may dissolve within 2–5 min, but the same sachet in a still pail at 5°C can exceed 10 min. This is why agrochemical film is often designed with a higher degree of hydrolysis and controlled solubility delay rather than immediate cold-water solubility. Granular fill systems also impose point loading during dosing; sharp angular particles may require thicker film or an additional paper outer carton to prevent pinholes.

Agrochemical sachet film is tested for tensile properties after conditioning according to ASTM D618-21, with tensile tests under ASTM D882-18 and tear resistance under ASTM D1922-23. Dart impact values are often measured under ASTM D1709-22, with sachet film values between 100 g and 400 g for a 50 µm film depending on plasticizer and moisture content. Seal strengths of 10 N/15 mm to 25 N/15 mm are specified in many converting operations, with test methods derived from ASTM F88/F88M-21.

PVOH is biodegradable under defined aerobic conditions. Under OECD 301B, mineralisation of 60–90% within 28 days has been reported, but degradation kinetics depend on inoculum acclimation and degree of hydrolysis. Dust from powder grades may form an explosible mixture; handling systems are designed according to ATEX 2014/34/EU categories for combustible dust where applicable. The polymer is registered under REACH as a polymer, with monomer and impurity obligations addressed under the relevant registration dossiers.

In a production scenario with a high-ionic-strength liquid detergent pod, the film is pre-dried to 8–10 wt% moisture, sealed at 145°C ± 5°C, and packed in a sealed overwrap when warehouse RH exceeds 60%. In a contrasting agrochemical sachet application, the same grade family is selected at 94–96 mol% hydrolysis and 25–60 µm thickness, then subjected to seal-strength testing after a 4-day accelerated aging cycle at 40°C and 75% relative humidity. Published data for some active-ingredient-specific storage configurations is limited. The two applications therefore share PVOH chemistry but diverge in grade selection, moisture conditioning, and mechanical tolerances.