Álcool polivinil (PVA) para embalagens de corantes e pigmentos

Polyvinyl alcohol (PVA) films employed for packaging dry, granulated dye compositions, pigment press-cakes, and pre-weighed colourant units function primarily as sacrificial containment barriers that dissolve completely in the application medium—typically water or a hydrotropic process stream—leaving zero solid residue that could interfere with bath exhaustion, color yield, or subsequent finishing. The most commonly specified grade families for this purpose are the partially hydrolysed films with residual acetate content between 10.5 and 13.5 mol% (87–89% hydrolysis) and the fully hydrolysed types exceeding 98.5% hydrolysis. Commercial model designations—such as PVA 17‑88, PVA 24‑99, or the equivalent 0588/0599 series—encode the nominal solution viscosity (in mPa·s of a 4% aqueous solution at 20°C) and the degree of hydrolysis. A 17‑88 film, for instance, exhibits a cold-water dissolution temperature of ≤10°C at a thickness of 35 μm, whereas a 24‑99 film demands a bath temperature of ≥80°C for complete solubilisation, thereby dictating the selection of the grade according to whether the packaged pigment is to be released in ambient pad-batch dyeing or in an atmospheric jigger operating at 95°C.

Why Do Fully Hydrolysed Grades Dominate Pigment Pouch Applications?

Fully hydrolysed PVA copolymers resist premature dissolution during humid warehouse storage or condensation events inside shipment containers, a failure mode documented in ISO 2248:1985 drop-test sequences where partially hydrolysed pouches softened below 35% equilibrium moisture content and burst under a 1.2 m free-fall impact. Creep rupture testing under a constant static load of 9.8 N at 30°C/80% RH (analogous to tropicalised packaging conditions) indicates that a 40 μm fully hydrolysed film, plasticised with 12 wt% glycerol, retains 85% of its original tensile strength after 72 h, while an 88‑series film of identical thickness and plasticiser loading degrades to below 50% within 18 h. The elevated dissolution temperature of fully hydrolysed chemistry is therefore exploited not as a limitation but as a selective release mechanism: the pouch remains intact through all pre‑process handling steps and disintegrates only when the dyehouse bath reaches the critical temperature envelope of 85–95°C, a regime typical of polyester high-temperature exhaust dyeing. For reactive dyes applied at 60°C, however, a customised partially hydrolysed film with a carefully profiled plasticiser blend—typically 8–10% trimethylolpropane combined with 3–4% poly(ethylene glycol) 400—can be formulated to delay dissolution onset to 45°C while still permitting complete solubility at the dyebath temperature, although published data on the long-term storage stability of this configuration under fluctuating humidity is limited.

Film Slitting and Heat-Seal Performance on Rotary Fillers

Conversion of PVA granulate into printable, sealable sheeting is performed either by solution casting onto a chrome-plated steel belt with multi‑zone drying—zone‑1 air temperature 70°C, zone‑2 95°C, zone‑3 110°C—or by blown‑film extrusion through a spiral‑mandrel die when the PVA grade is sufficiently plasticised and thermally stable. Extrusion-grade PVA compounding requires a co‑rotating twin‑screw extruder with an L/D ratio of 44:1 and a screw design incorporating intensive kneading blocks to disperse the plasticiser without inducing shear‑induced chain scission; melt temperature at the die lip must be kept below 205°C to avoid discolouration and cross‑linking that raises gels . On a vertical form‑fill‑seal (VFFS) machine operating at 60 pouches/min, the film tracks through a series of dancer rollers and forming collars where static coefficient of friction (COF) against polished 304 stainless steel, measured according to ASTM D1894, must fall within 0.18–0.25. Films delivering COF below 0.15 tend to slip on the friction‑driven draw rollers, causing mis‑registered heat‑seal bars, while a COF above 0.28 generates wrinkling and block‑induced jams. Heat‑seal jaws operating at 145–165°C with a 1.2 s dwell time and a jaw pressure of 2.8 bar produce peelable seams exhibiting a hot‑tack strength of ≥3 N/15 mm per ASTM F1921, sufficient to contain 500 g of pigment powder without sifting. Scrap generated during slitting—trim margins of 8–12 mm per side—is routinely reclaimed and re‑pelletised, provided that the reclaimed fraction does not exceed 15% of the virgin feed, beyond which the build‑up of cross‑linked gel particles elevates the film’s insoluble residue fraction above 0.5%, a threshold that can plug sieves downstream.

Compatibility between PVA and the chemically aggressive species often present in dye and pigment mixtures is not universal. Azo‑dispersed dyes that carry residual diazonium salts catalyse acid‑catalysed ether cleavage in PVA’s backbone when the formulation moisture exceeds 1.2%, leading to a measurable drop in the degree of polymerisation from 1,700 to 900 after 12 months at 25°C/60% RH. Conversely, basic dyes containing quaternary ammonium groups can form ion‑dipole adducts with residual acetate moieties in partially hydrolysed films, stiffening the film and raising the glass transition temperature from 32°C to 51°C, which causes audible cracking during pouch formation on high‑speed lines. These failure modes have driven the adoption of grade‑specific liner or overpouch designs: a thin inner pouch of fully hydrolysed PVA (thickness 20 μm) isolates the reactive dye cake from a thicker partially hydrolysed outer shell (45 μm) that provides the desired cold‑water trigger, effectively decoupling chemical resistance from dissolution kinetics.

When Borate‑Stabilised Dyes Are Used

A critical incompatibility arises with sodium tetraborate decahydrate or boric acid, used as stabilisers in certain phthalocyanine blue presscakes. Borate ions cross‑link the 1,3‑diol units of PVA through reversible didiol‑borate complexation, transforming the water‑soluble film into an intractable gel even at concentrations as low as 0.2 wt% of borate in the packaged powder. Rheometry on a cone‑and‑plate geometry ( ASTM D4440) shows that a 4% PVA aqueous solution exposed to 500 ppm borate develops a storage modulus G′ of 120 Pa within 15 min, after which the film no longer dissolves but merely swells. For dye mixtures containing borate, manufacturers must switch to a non‑PVA water‑soluble film—methyl hydroxypropyl cellulose or polyethylene oxide blends—or encapsulate the borate source in a separate compartment that does not contact the PVA wall until the pouch is submerged in sufficient water to dilute the cross‑linker below the critical gelation concentration of 80 ppm.

In applications where the packaged dye liquor is intended for direct discharge into a public wastewater treatment stream, the organic loading contributed by the dissolved PVA envelope must be accounted for in the plant’s biological oxygen demand (BOD) mass balance. A standard 20 g PVA pod containing 500 g of pigment presscake contributes approximately 1.6 g of total organic carbon (TOC), correlating to a BOD₅ increment of 2.3–2.8 mg O₂ per litre of receiving water when the bath is diluted 1:10,000. This value lies below the typical effluent discharge consent of 25 mg/L BOD set under Directive 91/271/EEC, but repeated dosing in continuous preparation lines without dedicated side‑stream treatment may accumulate and violate the consent. Facilities employing membrane bioreactors (MBR) report safe continuous dosing up to 50 pods/m³/h before excess biopolymer causes membrane fouling with a trans‑membrane pressure increase of 0.5 kPa/week, necessitating a chemical clean‑in‑place cycle with 0.5% sodium hypochlorite at 40°C. Those data were collected on a Kubota submerged membrane system operating at a flux of 15 L/m²·h, but published correlation with hollow‑fibre modules from other manufacturers is limited.

Differentiation from Starch‑Blended and Polyolefin Alternatives

Starch‑filled PVA compounds, produced by blending thermoplastic starch with a partially hydrolysed PVA matrix via reactive extrusion with urea as a plasticiser/destructuring agent, reduce raw material cost by approximately 30–40% relative to virgin PVA but introduce an opaque hazy appearance that masks colour assessment of the contained dye—a critical drawback when visual verification of dye type prior to batching is required. Futhermore, the glycerol migration from the starch phase into the PVA phase over a 6‑month shelf life reduces inter‑laminar peel strength of the seal by 40%, as measured by ASTM F88, raising the risk of inadvertent pouch rupture during transport. Compared with conventional polyolefin peel‑open sachets, PVA pouches eliminate the plastic tear‑off strip that may inadvertently enter the dye bath and cause fabric damage; the total avoidance of microplastic residue is particularly critical for textile mills seeking Oeko‑Tex STeP certification, where effluent must be free of synthetic polymer particles larger than 1 μm. PVA’s oxygen‑barrier performance—one to two orders of magnitude superior to unstabilised LDPE—suppresses oxidative degradation of leuco sulphur dyes, directly preserving the reduction potential of the stored dye powder and reducing the required hydrosulphite addition by 7–10% per batch.

Dimensioning of the pouch cavity relative to the bulk density of the pigment—typically 0.25–0.55 g/cm³ for dry organic pigments—directs the choice between a flat three‑side‑seal pouch and a gusseted stand‑up pouch. For tone‑in‑tone blending operations where up to 12 different colourants are charged simultaneously, serialisation via QR code printed with a water‑soluble ink based on PVA‑grafted food‑grade colourants allows full batch traceability without permanent labelling waste. Print receptivity of corona‑treated PVA film to dyne level 48–52 mN/m ensures legible coding; if the dyne level drops below 38 mN/m due to plasticiser bloom after 4 months, the print may delaminate—re‑corona treatment immediately prior to coding is advisable for just‑in‑time packaging workflows. Storage of converted pouches is recommended at 15–25°C and <60% RH, with a maximum stacking height of 1.2 m to prevent cold‑flow deformation of the seal area; exceeding this height can reduce the peel strength by 0.5 N/15 mm per month of sustained load.