An aqueous-processed vinyl alcohol homopolymer containing 1,3-diol units is blended with glycerol at 8–12 wt% and extruded into a cast film of 38 µm nominal thickness. The resulting single-dose sachet dissolves within
45 s in water at
20°C when the degree of hydrolysis is held between
86 mol% and
89 mol%, releasing a precisely metered food additive—such as a spice extract or a beverage concentrate—directly into the preparation liquid. This eliminates direct handling of fine powders, minimises cross-contamination on production lines, and delivers unit-level dose accuracy of ±
2.5% of target mass across
100,000 formed pouches per shift when vertical form-fill-seal equipment with servo-driven draw-down belts is operated at
80 cycles/min. Because the film carries FDA 21 CFR
176.170 clearance for components of paper and paperboard in contact with aqueous and fatty foods and complies with the specific migration limit for vinyl alcohol monomer of
<0.05 mg/kg under EU Regulation
10/2011 (Annex II, food simulant B, 10-day contact at 40°C), it is accepted as a food-contact material in ready-to-eat applications where the packaged substance is diluted or dispersed at the point of use.
Film-Forming Grade Parameters and Post-Consumer Fate
Commercial grades destined for single-dose food packaging are differentiated by viscosity of a
4 % aqueous solution at
20°C and by the molar fraction of residual acetate groups. A low-viscosity grade exhibiting
4.0–6.0 mPa·s (Brookfield LV, spindle 1, 60 rpm) yields machine-direction tensile strength of
38–45 MPa and elongation at break of
280–350 % when tested per ASTM
D882 on 35 µm film conditioned at
23°C and
50 % RH. A medium-viscosity grade with
20–30 mPa·s extends elongation to
350–420 % but reduces the cold-water dissolution rate; at
15°C complete disintegration of a
50 µm monofilm increases from
60 s to
240 s. In both cases the degree of hydrolysis is held at
86–89 mol% because partially hydrolysed polyvinyl alcohol retains sufficient amorphous character to permit rapid water ingress. Fully hydrolysed grades (>
98 mol%) are not used for ambient-dissolution sachets; their crystallinity raises the dissolution temperature above
70°C unless an alkalinity spike is introduced to break inter-chain hydrogen bonding.
Post-consumer biodegradation of PVA packaging is verified under controlled composting conditions according to ISO
14855-1:2012 and home-compost regimes per EN
13432. Mineralisation to CO₂ exceeds
90 % within
180 days at
58°C for films containing
5–15 % glycerol plasticiser, while aquatic aerobic biodegradation (OECD
301F) reaches
60 % in
28 days. This profile distinguishes PVA from petrochemical-derived water-soluble films such as ethylene vinyl alcohol (EVOH) copolymers with ethylene content above
29 mol%, which exhibit no meaningful mineralisation in the same timeframe. Certification against EN
13432 (OK Biodegradable Water) is specified by several European packaging waste directives, allowing unit-dose pouches to enter organic waste streams.
For film extrusion, a co-rotating twin-screw compounding step with an L/D ratio of
40:1 is employed to disperse the plasticiser and a slip/anti-block package of
0.2–0.5 % stearic acid into the PVA matrix. The pre-compounded pellets are then fed to a single-screw blown-film line with a
30:1 barrier screw, a
150 mm spiral mandrel die, and a die gap set at
0.8 mm. Because PVA’s equilibrium moisture content in a
50 % RH environment reaches
3–4 %, the resin must be dried to a residual moisture of
<0.3 % using a desiccant dryer with a dew point of
−40°C before extrusion; failure to do so generates steam bubbles and pinhole defects that compromise the hermetic seal of the sachet. The melt temperature window during blown-film processing is narrow:
180–210°C in all barrel zones, with the adapter and die held at
195–205°C. Exceeding
230°C initiates acetic acid elimination and chain scission, causing yellowing and a sharp drop in elongation at break to
<50 %. Processors therefore instrument each extrusion line with a dual-wavelength infrared pyrometer at the die exit and enforce a shutdown if measured melt temperature deviates beyond
±5°C of the setpoint. The bubble is cooled by a dual-lip air ring delivering air at
10–15°C, and the blow-up ratio is kept between
2.0:1 and
2.5:1 to maintain balanced machine-direction/transverse-direction tensile values in the finished layflat film.
What Limits the Use of PVA in High-Acid or Fat-Containing Single-Serve Food Products?
Sachet integrity under direct food contact is governed by the water activity (a
w) and acidity of the packaged content. At a
w above
0.75, the tensile strength of a
40 µm PVA film conditioned at
23°C falls by
40–50 % within
48 h, as measured by ASTM
D882 after equilibration in a saturated NaCl environment. For this reason dry-flowable ingredients such as instant coffee powder (a
w ≈
0.35) or freeze-dried fruit flakes (a
w ≈
0.12) are the principal food matrices paired with PVA pouches. Wet or semi-moist products with a
w exceeding
0.85 require an inner barrier ply—typically a metalised PET laminate—that negates the single-material dissolution advantage.
Acidic food additives below pH
3.5, such as citric acid-sweetener blends, catalyse ester hydrolysis of residual acetate groups on partially hydrolysed PVA, leading to a progressive increase in degree of hydrolysis and a concomitant rise in film crystallinity over warehouse storage periods of
12 weeks at
30°C. The result is incomplete pouch dissolution at the point of use—floating gelatinous residues are detected in brewing equipment. A formulation workaround replaces
30 % of the glycerol with a non-volatile secondary plasticiser such as sorbitol, which suppresses strain-induced crystallisation in the seal area, but the maximum achievable seal strength on a laboratory hot-bar sealer still decreases from
12 N/25 mm to
8 N/25 mm after
90 days of accelerated ageing at
40°C/75 % RH.
Fatty food simulants (vegetable oil, simulant D2 under EU
10/2011) present a different incompatibility: migration of glycerol plasticiser into the oil phase. Under worst-case testing—
10 days at
40°C with total immersion—the gravimetric fat reduction method reports a plasticiser migration of
18.5 mg/dm², which exceeds the overall migration limit (OML) of
10 mg/dm² for food-contact plastics. Therefore unit-dose packaging of oil-based concentrates or paste formulations is excluded unless the film thickness is increased beyond
80 µm and a high-molecular-weight PVA grade with a
4 %-solution viscosity above
45 mPa·s is selected to reduce plasticiser mobility. Even then, the dissolution time in cold water exceeds
300 s, making the format impractical for quick-serve beverage applications.
A Comparative Dissolution Yardstick: PVA Versus Modified Starch and Hydroxypropyl Methylcellulose
The table below consolidates relevant film-performance data for three water-soluble polymers used in unit-dose food formats, each tested on a
45 µm monofilm conditioned at
23°C/50 % RH.
| Property | PVA (86–89 % hydrolysis) | Thermoplastic starch (TPS) | Hydroxypropyl methylcellulose (HPMC) |
|---|
| Dissolution time at 20°C (s), static water | 42 ± 5 | 15 ± 3 | 120 ± 20 |
| Tensile strength MD (MPa), ASTM D882 | 38–45 | 12–18 | 28–34 |
| Elongation at break MD (%), ASTM D882 | 280–350 | 40–70 | 8–15 |
| Oxygen transmission rate (cm³·mm/m²·day·atm), 23°C/50 % RH, ASTM D3985 | 2.1–3.5 | 25–40 | 12–18 |
| Compostability certification | EN 13432, OK Biodegradable Water | EN 13432, OK Compost Industrial | EN 13432 (with specific thickness limits) |
| FDA food-contact clearance | 21 CFR 176.170 | 21 CFR 178.3520 (coating) | 21 CFR 172.874 (direct food additive) |
PVA’s strength-elongation balance permits down-gauging to
25 µm without loss of seal integrity on heat-seal jaws operating at
140°C and
0.3 MPa for
0.8 s. TPS films, while dissolving faster in cold water, lack the mechanical robustness to survive the mechanical agitation of a form-fill-seal turret at production speeds above
60 cycles/min. HPMC, despite being cleared as a direct food additive, dissolves too slowly at
20°C for instant-beverage formats unless the user stirs continuously for
90 s. PVA’s oxygen barrier appears moderate, yet it is sufficient for non-oxygen-sensitive payloads; for oxygen-sensitive enzymes (e.g., lactase, glucose oxidase), a SiOx-coated barrier ply is co-laminated, and the resulting bilayer is still accepted under a composting certification because the PVA layer constitutes more than
95 % of total film mass.
Migration kinetics into aqueous food simulants (simulant A:
10 % ethanol) were quantified by total immersion extraction at
60°C for
6 h, representing hot-fill conditions. The overall migration from a
38 µm PVA film plasticised with
10 wt% glycerol was
7.2 mg/dm², well below the OML, while the specific migration of the monomer vinyl acetate was
<0.02 mg/kg (detection limit). When the same film was laminated to a
3 µm shellac-based moisture-barrier coating, migration values were halved, but the dissolution time increased to
90 s. This trade-off is tolerated for unit-dose hot-chocolate bases intended for vending machines, where the machine’s hot-water injection at
85°C compensates for the slower breakdown. Plant-scale data from a running line producing
12,000 units per hour showed a seal-failure rate of
<0.03 % when the lamination nip pressure was maintained at
4.5 N/cm² and film edge-trim recycling was kept below
15 % regrind inclusion to avoid accumulation of heat-degraded polymer. Any attempt to push regrind beyond
20 % causes gel-particle counts—detected by an in-line camera system at
60 frames/s—to exceed the acceptable threshold of
5 particles/m² of size>
150 µm, triggering automatic rejection of the finished reel.
