álcool polivinil PVA

Polyvinyl alcohol (PVA, or PVOH in the context of film-forming grades) exists as a collection of synthetic, water-soluble macromolecules whose functionality is governed not by a single compositional value but by two independent parameters: the degree of hydrolysis (DH) of the parent polyvinyl acetate and the weight-average degree of polymerization (P_w). Commercial PVA resins are produced through a controlled alcoholysis of polyvinyl acetate, usually in a methanolic medium with an alkali or acid catalyst. The resulting polymer chains retain a quantifiable fraction of acetate groups, and the distribution of these residual groups—random or blocky, depending on the saponification conditions—defines the cold-water solubility, thermal behaviour, and tensile properties of the final material. Standard industrial nomenclature classifies grades by a four-digit code: the first one or two digits indicate the viscosity of a 4% aqueous solution at 20 °C (a proxy for molecular weight), and the remaining two digits denote the degree of hydrolysis. For instance, a grade designated “17-88” exhibits a viscosity near 17 mPa·s and a DH of 88%. The table below summarizes typical specification ranges for three broad categories of PVA encountered in industrial supply chains.

Grade Category	Degree of Hydrolysis	Viscosity (4% aq., 20 °C)	Ash Residue (max.)	pH (4% solution)	Reference Standard
Partially hydrolysed (low MW)	86.0 – 89.0%	3.0 – 5.0 mPa·s	0.5%	5.0 – 7.0	JIS K6726
Partially hydrolysed (high MW)	86.0 – 89.0%	20.0 – 30.0 mPa·s	0.5%	5.0 – 7.0	JIS K6726
Fully hydrolysed	98.0 – 99.5%	5.0 – 65.0 mPa·s	0.8%	5.0 – 7.0	JIS K6726

How Does the Degree of Hydrolysis Dictate Cold-Water Solubility and Film Mechanical Response?

The transition from a water-soluble thermoplastic to a hot-water-dispersible material occurs across a narrow hydrolysis window. Partially hydrolysed grades, with a DH between 85% and 89%, possess sufficient residual acetate groups to disrupt inter-chain hydrogen bonding, allowing complete dissolution in cold water (≤ 25 °C) within 15–30 minutes under mild agitation. A shift to 95% DH renders the polymer only swellable at 20 °C; at 98% DH and above, the crystalline fraction exceeds 60%, and the dissolution temperature jumps to > 70 °C. This solubility cliff is the primary control lever for applications: the rapid cold-water disintegration of partially hydrolysed PVA makes it the backbone of single-use detergent pods and agrochemical sachets, whereas fully hydrolysed grades, requiring elevated temperature for dissolution, are selected for solvent-resistant adhesive films and barrier coatings. Film tensile properties, measured per ASTM D882, reflect this structural shift. A fully hydrolysed film conditioned at 50% RH can exhibit a tensile strength of 55–70 MPa with elongation at break below 150%; the partially hydrolysed counterpart loses approximately 30% of its tensile strength but gains elongation values exceeding 250%, a direct consequence of the plasticizing effect of acetate side groups. These data originate from continuous solution-cast films dried on polished steel belts at line speeds of 8–12 m/min and subjected to orientation drawing at ratios between 1:1.5 and 1:3.0.

When Melt Extrusion Replaces Solution Casting: Thermal Stability and Plasticizer Migration

Conventional PVA film production relies on aqueous solution casting because the theoretical melting point of fully hydrolysed PVA (~228 °C, per ISO 11357-3) lies perilously close to its degradation onset temperature (approximately 200–210 °C in air). Melt extrusion becomes feasible only through intensive plasticisation, which suppresses the melt temperature below the degradation threshold. Glycerol, sorbitol, or trimethylolpropane are compounded into PVA powder at loading levels between 10 phr and 30 phr using a co-rotating twin-screw extruder with an L/D ratio of 40:1 and a barrel temperature profile spanning 90 °C (feed) to 195 °C (die). Screw speed is maintained at 200–400 rpm to minimise residence time below 100 seconds. The process window is unforgiving: a deviation of ±3 °C at the metering zone can initiate yellowing and crosslinking, evidenced by a rapid torque increase. Furthermore, plasticizer blooming remains a documented failure mode. At relative humidity above 60%, glycerol can migrate to the film surface within 72 hours, forming a tacky, dust-attracting layer that reduces printability and peel adhesion in laminated structures. Measurements of surface energy, per ASTM D5946, drop from an initial 58 mN/m to below 42 mN/m after blooming, compromising corona treatment retention.

A distinctive feature that separates polyvinyl alcohol from most other synthetic water-soluble polymers is its capacity for crosslinking into an insoluble yet hydrophilic network without losing its inherent transparency. This is exploited in the manufacture of polarizer films for liquid-crystal displays, where a stretched PVA film doped with iodine or dichroic dyes is subjected to boric acid treatment. The boric acid forms reversible boronate-ester crosslinks between adjacent 1,3-diol units on the polymer backbone, locking in the anisotropic molecular orientation generated during uniaxial stretching at ratios of 1:5 to 1:7. The process is conducted in a heated bath at 50–60 °C immediately after the dyeing step, and the tension must be held constant within ±2 N/cm across a web width of 3 meters to prevent localised shrinkage that would manifest as mottle in the final display panel. PVA’s structured behaviour in this role is not replicated by substitutes like ethylene-vinyl alcohol copolymer (EVOH) because EVOH lacks the pendant hydroxyl density required to form an optically clear crosslinked matrix with iodine species. Published data for this specific configuration remains confined largely to patent literature and equipment manufacturers’ technical bulletins; independent peer-reviewed kinetic studies are sparse.

Barrier Performance and Gas Transmission in PVOH Films

The oxygen transmission rate (OTR) of a pure, unplasticized PVA film aligns more closely with that of EVOH than with polyolefins, but only under desiccated conditions. At 0% RH and 23 °C, OTR measured on a 25 μm film per ASTM D3985 can reach 0.4–0.8 cm³/(m²·day·atm), a value comparable to EVOH with 32–38 mol% ethylene content. As the environmental humidity rises past 50% RH, the OTR increases exponentially; at 75% RH, it commonly exceeds 15 cm³/(m²·day·atm), rendering monolithic PVA films inadequate for high-moisture food packaging without lamination to a hydrophobic substrate such as polypropylene or polyethylene terephthalate. This humidity sensitivity contrasts with the more stable barrier performance of chlorinated polymers like PVDC, whose OTR at 75% RH remains below 2 cm³/(m²·day·atm). In retortable pouch structures, a sandwiched PVA layer between two polypropylene layers will retain acceptable oxygen barrier for approximately 90 days at ambient shelf storage, but only if the tie-layer adhesive (typically maleic anhydride-grafted polypropylene) is applied at a coat weight exceeding 3 g/m² to prevent delamination induced by the differential swelling of PVA.

Controlling Ash Content and Methanol Residue for Pharmaceutical Excipient Compliance

Grades intended for pharmaceutical film coating or as a binder in tablet granulation must conform to the Pharmacopoeial monograph for Polyvinyl Alcohol (e.g., USP-NF, Ph. Eur.). The sulfated ash limit is set at 0.2% for the low-ash type, driven by the need to avoid catalytic degradation of active pharmaceutical ingredients sensitive to metal ions. Methanol impurity, a process residual from the alcoholysis, is restricted to 100 ppm or less by headspace gas chromatography per general chapter USP 〈467〉. Loss on drying at 105 °C is controlled to a maximum of 5.0% because the powder readily sorbs atmospheric moisture; pre-drying in a fluid-bed dryer at 80 °C for 4 hours is standard before blending with moisture-sensitive drugs. The dissolution behaviour in simulated gastric fluid (pH 1.2) at 37 °C shows that fully hydrolysed PVA remains intact for over 2 hours, whereas partially hydrolysed grades begin to solubilise within 30 minutes, a factor influencing controlled-release matrix integrity.

A separate operational domain where PVA diverges sharply from other hydrocolloids is in paper surface sizing and coating. Compared with oxidised starch, which at equal 5% solids deposition yields a surface strength (IGT pick resistance) of typically 60–80 cm/s per TAPPI T499, a partially hydrolysed, low-MW PVA applied at the same solids improves the pick resistance to 180–240 cm/s due to its superior film-forming capacity and specific adhesion to cellulose fibres. During application on a metered size press, PVA solution at 4–8% solids is held at 50–60 °C to prevent skin formation on the rolls; the viscosity remains stable within a window of ±0.5 mPa·s over an 8-hour production shift, whereas starch-based formulations often require constant enzyme desizing or temperature adjustment to counteract retrogradation. This rheological predictability reduces web breaks on high-speed machines operating above 1200 m/min, but it comes with a trade-off: PVA is not easily removed during repulping and contributes to stickies if broke enters recycled furnish without adequate screening at slot widths below 0.10 mm.

Property	Polyvinyl Alcohol (PVOH)	Sodium Carboxymethyl Cellulose (CMC)	Polyacrylamide (PAM, Non-ionic)
Solubility in cold water	DH-dependent; rapid at 87-89% DH	Full dissolution; may require 40 °C for high DS types	Readily soluble; slow wetting requires dispersion aids
Film tensile strength (ASTM D882)	40-70 MPa	30-45 MPa	15-25 MPa
Shear viscosity build (5% solution at 100 s⁻¹)	10-60 mPa·s (Newtonian plateau wide)	50-500 mPa·s (pseudoplastic, marked shear-thinning)	80-300 mPa·s (pseudoplastic, high shear sensitivity)
Surface sizing pick resistance (IGT, TAPPI T499)	180-240 cm/s	90-130 cm/s	Not typically used
Readily biodegradable per OECD 301B	Yes (> 60% in 28 days for partially hydrolysed)	Variable; depends on DS	Generally recalcitrant; <20%

What Distinguishes Polyvinyl Alcohol from Ethylene-Vinyl Alcohol Copolymer in Flexible Packaging Laminates?

Despite shared hydroxyl functionality, PVA and EVOH occupy divergent positions on a packaging converter’s shelf. EVOH is a melt-processable random copolymer containing 27–44 mol% ethylene, yielding a semicrystalline barrier layer that can be coextruded directly between polyolefin skins without a separate casting step. Its OTR at 0% RH, typically 0.1–0.5 cm³·mm/(m²·day·atm), slightly outperforms PVA, and its moisture-induced barrier loss is less catastrophic because the crystalline regions resist swelling; EVOH retains useful barrier up to 85% RH in retort applications. PVA, conversely, requires separate aqueous casting or plasticized extrusion, imposing a conversion complexity that limits its use to niche ultra-high-barrier structures where the film is embedded deep within a laminate and shielded from moisture ingress. The key advantage of PVA over EVOH in such designs is its lower cost per oxygen barrier unit at ≤ 0.2 cm³·mm/(m²·day·atm) when dry, combined with the complete absence of organic co-monomers that could raise migration concerns under food contact regulation EU 10/2011 for unplasticized grades. However, the moisture sensitivity cap makes PVA unsuitable as a direct food contact layer for moist contents with a water activity above 0.75; any design brief proposing such a configuration must account for an interlayer of aluminium-metallized PET or a PVDC coating.

The application of PVA as a temporary bonding agent in advanced ceramic tape casting warrants a distinct processing note. In producing alumina substrates for electronic modules, a slurry of ceramic powder, solvent, dispersant, and PVA binder is doctor-bladed onto a polyethylene carrier film. The PVA grade selected is typically a partially hydrolysed, medium-MW type (viscosity 12–18 mPa·s) that provides sufficient green strength for handling and punching, yet burns out cleanly during the subsequent debinding step at 500–600 °C under an oxidising atmosphere. Thermogravimetric analysis per ISO 11358 must show a single, narrow decomposition event between 250 °C and 350 °C, with ash residue below 0.1% to prevent contamination of the alumina grain boundaries. Any residual sodium acetate from the alcoholysis catalyst, if present above 0.3%, will lower the sintering onset by 20–40 °C and cause warpage in multilayer structures. In this context, PVA is preferred to acrylic binders because acrylics often require a slower, multi-stage burnout profile to avoid carbon residues, adding 6–10 hours to the thermal cycle.