Filme PVA não solúvel em água

In packaging sectors where temporary moisture contact must not compromise film integrity, conventional water-soluble polyvinyl alcohol (PVOH) films exhibit a fatal limitation: dissolution upon contact with liquid water or condensation. The non-water-soluble PVOH film family, commercialised under the grade series NS-PVOH-20, NS-PVOH-25, and NS-PVOH-35, eliminates this failure mode through a controlled network of intermolecular acetal cross-links. This molecular architecture restricts chain solubilisation while preserving sufficient hydrophilicity to permit ultimate biodegradation in soil composting environments evaluated per ISO 14855-1:2012. The film is manufactured by blown-film extrusion on a single-screw line equipped with a grooved feed section, a barrier screw of 30:1 L/D ratio, and automatic gauge control. Production-scale data from a 45 mm extruder operating at 50 rpm with die gap 0.8 mm show that the resulting film maintains a gel content—determined by ASTM D2765-16—between 55 % and 85 %. This gel fraction correlates directly with the inability to dissolve in demineralised water at temperatures up to 40 °C. A 6-month field trial on a multi-head weigher bagging line for 25 kg ammonium nitrate prills, conducted during monsoon-season storage under unroofed sheds, recorded zero wet-bag ruptures for the non-water-soluble variety, whereas standard cold-water-soluble PVOH sacks disintegrated within 72 hours of the first heavy rain.

What governs the water insolubility of non-water-soluble PVOH film?

Water insolubility is not an intrinsic property of the vinyl alcohol backbone but is imparted by post-polymerisation acetalisation with a short-chain dialdehyde cross-linker. The process generates interchain bridges that convert the linear, water-soluble PVOH into a three-dimensional gel network. The gel fraction, quantified by extraction in boiling water for 6 hours in accordance with ASTM D2765-16 Method A, serves as the primary grade-differentiation metric. A gel fraction of 55 % in the NS-PVOH-20 grade yields a swelling ratio of 35–40 % by mass after 24-hour immersion in water at 23 °C, while NS-PVOH-35 at 85 % gel fraction swells less than 15 %. Both grades remain as integral, handleable films that can be heat-sealed without leakage, whereas a standard water-soluble PVOH film with gel fraction near zero dissolves completely within 30 seconds under the same conditions. The degree of cross-linking also modulates the water vapour transmission rate: a higher gel fraction reduces the equilibrium moisture sorption, lowering the permeation coefficient but never approaching the hydrophobic barrier of polyolefins. Consequently, the film is best characterised as a swellable, non-dissolving hydrophilic barrier rather than a moisture-proof layer.

On horizontal form-fill-seal machines running at speeds above 60 cycles per minute, the surface resistivity of non-water-soluble PVOH film can reach 10¹² Ω/sq when ambient relative humidity drops below 30 %. This charge accumulation induces static cling that interferes with photocell registration and causes machine stops. Trials on a Bosch SVE 2520 vertical bagger processing NS-PVOH-25 film of 45 µm thickness showed that the misfeed rate rose to 8 % of cycles without antistatic treatment. Application of a fatty acid amide-based antistatic masterbatch at a let-down ratio of 1.2 wt%, co-extruded as a 3 µm surface layer, reduced the incidence to <0.5 % while preserving heat-seal strength. Seal-strength testing per ASTM F88/F88M-21 at a jaw temperature of 150 °C and 0.3 MPa pressure with 1.0 second dwell time gave consistent values of 8.5 N/15 mm. No delamination of the antistatic layer was observed during sealing. The film’s surface chemistry must remain free of amine-based slip additives, as free amines catalyse transacetalisation reactions that progressively reduce gel fraction below 40 %, restoring cold-water solubility over a 3-month warehouse period.

Model Designations and Core Specifications

The three commercial grades are differentiated by cross-link density, thickness, and targeted oxygen barrier. The table below summarises key physical properties determined under standard laboratory conditions (23 °C, 50 % RH after conditioning for 48 h). All measurements represent machine-direction values unless specified otherwise.

Oxygen transmission rates measured at 23 °C and 85 % RH are typically 8–15 times higher than the 0 % RH values shown, because moisture plasticises the matrix and increases free volume. The equilibrium moisture content of the film at 85 % RH reaches 12–18 wt%, yet the film does not dissolve; it remains a pliable, non-tacky layer suitable for lamination into multi-material structures.

When the film is used as the interior liner of paper sacks for hygroscopic construction chemicals, dimensional stability under cyclic humidity must be confirmed. Laboratory conditioning between 20 % and 80 % RH in 6-hour cycles over 14 days revealed a reversible hygroexpansion of 0.9–1.3 % in the machine direction, measured by digital image correlation. This expansion reduces Elmendorf tear resistance (ASTM D1922-15) by up to 40 % from the dry-state value, placing the tear force of NS-PVOH-25 at 80-µm thickness at 420 mN after equilibration at 80 % RH, down from 700 mN at 20 % RH. Sack manufacturers compensate by increasing the basis weight of the outer kraft layer rather than altering the film grade. Published data for the long-term mechanical stability of this configuration under combined loading and moisture is limited; ongoing in-field monitoring of cement-bag breakage rates during intermodal transport at 35 °C and 85 % RH is necessary.

When the Extruder Melt Temperature Exceeds 205 °C

The processing window for non-water-soluble PVOH films is narrow and carries a high penalty for deviation. The recommended melt temperature during blown-film extrusion is 195–205 °C, measured at the adaptor with a flush-mounted thermocouple. Pre-drying of the compound is mandatory: residual moisture must be reduced to <0.08 % using a desiccant dryer operating at 80 °C for a minimum of 4 hours, with a dew point of -40 °C or lower. Failure to achieve this moisture level results in bubble instability and a hazy film with entrapped microvoids. At melt temperatures between 210 °C and 215 °C, the gel content begins to increase during the residence time inside the barrel, as residual acetal groups react further. Production logs from a 45 mm single-screw extruder (L/D 30) running NS-PVOH-25 recorded a rise in melt pressure from 210 bar to 310 bar within 20 minutes when the barrel temperature setpoint was accidentally shifted 10 °C high, accompanied by surging and a 15 % reduction in output. Above 215 °C, localized gel particles form and appear as transparent “fisheyes” in the film, and the film colour shifts to a pale amber. To mitigate this sensitivity, processors install melt filtration packs of 60/80/60 mesh and limit screw residence time by operating at moderate speeds (40–60 rpm). The addition of acidic processing aids must be avoided: acidic residues catalyse hydrolysis of the acetal cross-links at elevated temperatures, effectively reversing the insolubility and producing a film that will partially dissolve during subsequent moisture contact.

Comparative Performance Against Standard Water-Soluble PVOH and EVOH

The value proposition of non-water-soluble PVOH film becomes clear when it is positioned against chemically similar materials used for barrier packaging. The following table benchmarks a representative grade, NS-PVOH-25, against a standard cold-water-soluble PVOH film (fully hydrolysed, 98 mol%) and a commercial ethylene vinyl alcohol copolymer containing 32 mol% ethylene.

The data highlight that the non-water-soluble PVOH film retains the excellent dry-condition oxygen barrier of standard PVOH while surviving humidity regimes that would dissolve a water-soluble analogue. However, its water vapour barrier is inferior to that of EVOH, meaning the film is not suitable as a stand-alone high-moisture-barrier layer. Its primary advantage lies in providing a combination of moderate oxygen barrier, moisture-triggered fragmentation behaviour, and industrial compostability without requiring a tie-layer adhesive when laminated to paper or board.

Regulatory certification applicable to the NS-PVOH series includes compliance with EC No. 1935/2004 for food contact materials when the film is used as a functional barrier behind a layer of paper or aluminium foil. The specific migration limit for vinyl acetate monomer must not exceed 12 mg/kg food simulant under testing conditions of EN 1186-1:2002, a requirement routinely satisfied by the elevated cross-link density that restricts low-molar-mass fragment migration. Direct monolayer contact with high-acid foodstuffs having pH less than 3.5 at temperatures above 60 °C is contraindicated: acid-catalysed hydrolysis of the acetal bridges reduces gel content below the threshold of integrity, eventually resulting in partial dissolution and loss of package tightness.