PVA for Paper: Surface Sizing & Coating Binder

Polyvinyl alcohol (PVA) is a water-soluble synthetic polymer produced by the saponification of polyvinyl acetate. In papermaking, the polymer operates as a film former in size press surface sizing and as a co-binder in pigmented coating colour. Commercial PVA grades for paper are differentiated by degree of hydrolysis (87–89 mol%, 92–94 mol%, 98–99 mol%) and by viscosity measured on a 4% aqueous solution at 20°C, typically 3–60 mPa·s. The hydroxyl groups associate with cellulose surfaces through hydrogen bonding, while the polymer backbone provides film continuity, oil holdout, and surface strength. In a size press, PVA replaces 10–30 parts of oxidized starch on a dry basis to reduce Cobb water absorption and increase IGT pick resistance. In coating colour, PVA is used at 0.5–1.5 parts per 100 parts dry pigment as a partial latex replacement that modifies high-shear rheology and water retention.

How Does Degree of Hydrolysis Shift the Surface Sizing Response?

Fully hydrolyzed PVA grades (98–99 mol%) contain residual acetyl content below 1–2 mol%. The chains crystallize during drying and produce a stiff, water-resistant film. This type is preferred for surface sizing because it reduces Cobb values and improves oil holdout without softening at calender temperatures. Dissolution requires cooking at 90–95°C for 20–30 min with moderate shear. If the solution is prepared below 80°C, microgels remain suspended and can scratch the metering rod or deposit on the size press roll. Partially hydrolyzed PVA (87–89 mol%) dissolves at 20–40°C, has lower crystallinity, and yields a more flexible film. This type is less efficient for water resistance but is preferred in coating binder applications where cold dissolution and high-solids handling are required. The surface sizing response is not linear with addition level. At 1 part PVA per 100 parts dry starch, the film is barely modified. At 15–30 parts, the film becomes continuous and hydrophobic enough to reduce Cobb water absorption. Measurement is performed according to ISO 535:2023 or TAPPI T 441 om-22 after conditioning at 50% RH and 23°C. Surface strength is monitored by IGT pick resistance under ISO 3783:2014; loose fibre removal after printing is evaluated with the same method.

Storage and solution handling impose operational boundaries. PVA powder absorbs moisture above 65% RH; caked material must be dried at 80°C for 1–2 h before use. A 4% solution of fully hydrolyzed PVA at 60°C may show viscosity below 10 mPa·s, but the same solution held without agitation or preservative for 24 h can develop microbial growth and viscosity drift. Filtration through a 100-mesh in-line screen removes undissolved granules. Borax and borate salts are incompatible because diol-borate crosslinking gels the solution and blocks supply lines. Sodium sulfate can precipitate PVA at high concentration, which is a constraint when mills close water loops and increase inorganic salt content. Defoamer selection must avoid hydrophobic silica types that destabilize PVA films; the dosage is typically 0.1–0.3% on PVA solids.

Surface Sizing Formulation Limits Appear at 1,200 m/min on Rod-Metered Film Presses

On a rod-metered film sizer running at 1,200 m/min, the surface sizing liquor is held at 55–65°C and 8–12% solids. PVA addition above 30 parts per 100 parts oxidized starch raises the film split pattern and may create an orange-peel morphology on the roll applicator. Metering rod pressure is commonly 0.8–1.5 bar, depending on rod diameter and desired film thickness. At high PVA content, rod bleeding appears when liquor viscosity exceeds the drainage capacity of the rod grooves. Supplier technical bulletins for film sizers specify shear viscosity below 150 mPa·s at 50 s⁻¹ and 60°C for stable transfer. Low-foaming grades are selected because high-speed film application entrains air and forms pinholes in the sized sheet. The sized sheet is dried to 3–6% moisture; overdrying increases curl and disrupts hydrogen bonds holding the PVA film to fibre surfaces. PVA combined with alkyl ketene dimer hydrophobic size requires separate emulsification and feed lines because AKD wax can deposit on cold PVA piping. PVA is also incompatible with some cationic starch solutions at low pH, leading to coacervate formation and filter plugging.

Surface sizing with PVA changes the surface free energy of the sheet. The polar hydroxyl groups on the dried film raise the polar component measured by contact angle goniometry; water-based inks and cold-set adhesives wet the surface more uniformly. However, PVA is hygroscopic. At storage above 70% RH, the film absorbs moisture and becomes soft, increasing blocking tendency and reducing dry pick resistance. The oxygen barrier of a PVA film is high under dry conditions but deteriorates sharply above 70% RH. Therefore, PVA surface sizing is not specified as a sole barrier layer for high-moisture packaging unless lamination or extrusion coating is applied. For moisture-vapour transmission rate measurements, ISO 2528:2017 or gravimetric cup methods can be applied, but the PVA film is not a primary moisture barrier.

When the Coating Colour Contains 0.5–1.5 Parts PVA per 100 Parts Pigment

Pigmented coating binder use is typically a partial replacement of latex at 0.5–1.5 parts PVA per 100 parts dry pigment. The PVA is first dissolved as a 10–15% aqueous solution and added to the predispersed ground calcium carbonate or kaolin slurry after latex and before thickening. Coating colour pH is held at 8.5–9.5 with sodium hydroxide or ammonia to avoid shock interaction with carboxylated latex. At 1.0 part PVA per 100 parts pigment, Brookfield viscosity at 100 rpm can increase by 200–500 mPa·s, depending on pigment shape, dispersant demand, and latex type. The high-shear viscosity measured in an Eklund capillary viscometer should remain below 60 mPa·s at 1.2×10⁵ s⁻¹ to prevent blade bleeding. To maintain blade runnability, solids content may be reduced from 62–65% to 58–62%, depending on blade angle and machine speed. Water retention improves with PVA, which reduces binder migration into the base sheet and improves final surface strength. The effect is monitored by pressure filtration under 1 bar for 90 s or by low-field NMR water mobility. Published data for each specific pigment-latex-PVA configuration are limited, and mill validation is required.

Under high drying rate, binder migration is amplified when PVA is present as a low-molecular-weight co-binder. In blade-coated paper, evaporating water carries dissolved PVA and latex toward the coating surface; the resulting surface concentration gradient can produce mottled print gloss and uneven ink acceptance. The first 20–30 m after the blade are critical. Air foil and infrared predryer settings are adjusted to keep the web surface temperature below 70°C in the initial drying zone. High PVA levels above 1.5 parts per 100 parts pigment may reduce wet coating lubricity and increase friction between the blade and coating colour, leading to blade deposits that appear as fine machine-direction streaks. Control measures include blade angle reduction, ceramic blade tip replacement, or lowering PVA to 0.8–1.2 parts. Coating colour containing PVA should not be stored more than 8 h at 35°C without agitation because viscosity drift and bacterial acidification change blade runnability.

Coating Binder Interactions with Styrene-Butadiene Latex and Acrylic Thickeners

In styrene-butadiene latex and acrylic thickener formulations, PVA interactions are dominated by competition for water and by changes in the elastic component of the coating colour. A formulation containing 8–10 parts styrene-butadiene latex and 1.0 part PVA per 100 parts pigment may show lower Brookfield viscosity but higher extensional viscosity in capillary breakup tests. Acrylic thickener demand can decrease by 0.05–0.15% on total formulation mass when PVA is present. The interaction is pH-dependent; below pH 7.0, PVA and some carboxylated lattices can form gel particles. pH is maintained at 8.5–9.5. Amine-based neutralizing agents should be avoided because amine migration can yellow the coating under hot calendering. PVA reduces the minimum film-forming temperature of latex-bound coatings, but the effect is smaller than common coalescing agents and cannot replace them. When PVA is combined with a starch-based co-binder, the starch cooking temperature must be sufficient to gelatinize starch and dissolve PVA. Addition of PVA to cooked starch near 60°C avoids thermal degradation of starch but increases solution viscosity. The mixed binder should not be held more than 4–6 h because starch retrogradation and PVA gelation create a two-phase liquid that deposits in the supply tank.

At 75° measurement geometry, print gloss and ink set-off are affected by PVA content because the dried coating surface has a narrower pore size distribution and higher film continuity. PVA can reduce dry pick and improve ink holdout, but it may lower print gloss if the coating is supercalendered at excessive moisture. Print gloss is measured by TAPPI T 480 at 75°; surface roughness is measured by ISO 8791-2:2013 or Parker PrintSurf. The acceptable PVA level for a given print gloss specification is therefore pigment-dependent. In formulations containing high platey kaolin, PVA above 1.2 parts per 100 parts pigment may reduce calender blackening but increase sheet stiffness. Published data for this specific configuration is limited.

What Limits the Repulping Efficiency of PVA-Sized Broke?

During repulping, the PVA surface film dissolves into the aqueous phase rather than remaining attached to fibres. The dissolved polymer increases the COD load of white water and can reduce the efficiency of cationic retention aids only at high addition levels. Published data for specific PVA concentrations in paper machine white water are limited because retention behaviour depends on furnish conductivity, anionic trash, and retention aid composition. In a closed water loop, PVA can build up in process water because it is not completely removed by a conventional saveall. Aerated lagoon or activated sludge treatment can degrade PVA, but the residence time required is longer than for starch. Some mills divert highly loaded broke to off-site treatment or sequence the broke addition during high wet-end stability periods. For food-contact grades, repulped PVA-sized broke remains compliant under FDA 21 CFR 176.170 provided the original PVA grade meets the same regulation.