ELVANOL 50-42

ELVANOL 50-42 is a partially hydrolyzed polyvinyl alcohol resin supplied as a white to cream granular powder. The grade designation belongs to the Elvanol portfolio and identifies a low-viscosity polymer with a nominal 4% aqueous solution viscosity of 5.0–6.0 mPa·s at 20 °C and a degree of hydrolysis of 87.0–89.0 mol%. The residual acetate functionality is not a contaminant; it is a compositional variable that controls cold-water solubility, film flexibility, and interaction with cellulosic and inorganic substrates. Because the grade is partially hydrolyzed, the dry-state crystallinity is lower than that of fully hydrolyzed polyvinyl alcohol, which reduces the energy required for dissolution and changes the film’s water sensitivity after drying.

What the 50-42 designation encodes in the Elvanol nomenclature

In the Elvanol grade system, the first two digits are commonly associated with the nominal solution viscosity band, while the final digits denote the degree of hydrolysis. For 50-42, the 5 prefix places the product in the low-viscosity range, and the 42 suffix denotes a partially hydrolyzed species. Published quality-control data list the 4% aqueous solution viscosity at 5.0–6.0 mPa·s using Brookfield LVF spindle geometries per JIS K6726; the hydrolysis range is 87.0–89.0 mol%, determined by saponification value or equivalent method. The pH of a 4% aqueous solution is typically 4.5–6.5; volatile matter is controlled to a maximum of 5.0%, and sulfated ash to a maximum of 0.5%. Because the polymer is partially hydrolyzed, it retains hydrophobic acetate groups that reduce hydrogen-bonding density relative to fully hydrolyzed grades. This compositional difference is the primary driver of its cold-water solubility and lower aqueous solution viscosity at equivalent molecular weight.

When the grade is selected over fully hydrolyzed and higher-viscosity counterparts

Partially hydrolyzed PVOH of this viscosity band is selected when the processing sequence requires dissolution at ambient or near-ambient temperatures. Fully hydrolyzed grades with hydrolysis above 98 mol% typically demand heating to 80–85 °C for complete dissolution, whereas 50-42 disperses in water at 20–25 °C and forms clear solutions without a high-temperature hold, provided the powder is added to a well-formed vortex. The trade-off is film water resistance: fully hydrolyzed grades produce dried films with lower cold-water sensitivity and higher tensile strength per ASTM D882, while 50-42 yields films that remain redispersible until insolubilized with glyoxal, zirconium salts, or formaldehyde-free crosslinkers. Compared with higher-viscosity partially hydrolyzed grades of equivalent hydrolysis, the 5.0–6.0 mPa·s solution viscosity of 50-42 permits higher solids loading in size-press and coating formulations before the viscosity exceeds the transfer limits of metering equipment.

When intermediate water resistance is required, 50-42 is blended with fully hydrolyzed PVOH. Blends in the range of 70:30 to 30:70 by weight shift film redispersibility and block resistance. The exact ratio is determined by the required water absorption value and the specific substrate. Such blends are prepared as separate solutions and combined after dissolution to avoid competition for water during hydration.

On production lines equipped with high-shear dispersers or turbine agitators, the resin should be added slowly to the vortex of cold water. If the powder is dumped into stagnant water, partially hydrated skins form that shield the core from further hydration, producing fisheyes that persist through a 200–400 µm filter screen. A typical make-down sequence uses a 1.5–2.5 wt% initial slurry in cold water, followed by in-line heating to 40–50 °C for 20–30 min under agitation. The solution remains stable for several weeks if biocide is present; unpreserved solutions at 20–25 °C can develop microbial growth within 48–72 h. Recirculation through a 100 µm in-line filter removes residual gels before coating or adhesive use.

Viscosity-temperature response remains predictable only below 40 °C

Rheological evaluation of a 4% aqueous solution with a Brookfield LVF spindle at 60 rpm shows a nearly Newtonian plateau at shear rates below 100 s⁻¹. The viscosity decreases as temperature increases, following typical Arrhenius behavior for PVOH solutions; no anomalous gelation occurs below 40 °C in the absence of borate ions or polyvalent salts. Holding the solution above 60 °C for extended periods can accelerate ester hydrolysis and pH drift, particularly if alkaline additives are present. The addition of borax at 0.1–0.5 wt% on resin solids increases viscosity through reversible diol complexation; measurable gel formation occurs at higher concentrations and pH above 9.0.

Adhesive compounding with 50-42 frequently proceeds through a high-shear Cowles blade or rotor-stator mixer. Formulations for remoistenable envelope adhesives and tube winding typically combine 10–20 wt% PVOH with plasticizers, defoamers, and preservatives. The low solution viscosity reduces air entrainment during mixing at 1000–1500 rpm, but vacuum deaeration is still required for thick films. Borate addition follows the same rheological limits described for aqueous blends; excess borax above roughly 1.0 wt% can cause irreversible gelation or pH drift above 9.0. The residual acetate groups moderate the crosslink density of borate networks, making 50-42 less prone to abrupt gelation than fully hydrolyzed grades.

How does the product function as a protective colloid in emulsion polymerization?

In vinyl acetate and vinyl acetate-ethylene emulsion polymerization, partially hydrolyzed PVOH functions as a protective colloid and particle stabilizer. The 87.0–89.0 mol% hydrolysis range provides sufficient grafting through residual acetate groups while retaining aqueous solubility. Reactor charging of 2–6 wt% PVOH on total monomer is typical; the low solution viscosity of 50-42 permits high-shear circulation without overloading the reactor agitator. Emulsion viscosity, particle size distribution, and coagulum content are monitored via ISO 2555 or Brookfield methods and laser diffraction per ISO 13320. Compared with higher-viscosity protective colloids, 50-42 produces emulsions with lower final viscosity and smaller mean particle diameters under identical agitation, but published data for this specific configuration is limited. Mechanical stability can be reduced when the PVOH concentration is below 1.5 wt%.

Batch temperature is maintained at 60–80 °C with a reflux condenser, and initiator feeds are adjusted to maintain a starved monomer condition. The grafted PVOH layer contributes to steric stabilization; particle size is typically measured by dynamic light scattering per ISO 22412. If coagulum exceeds 0.1% on total latex, the PVOH addition point is moved from the initial reactor charge to a delayed feed.

Paper surface sizing and functional coating binder behaviour

In paper surface sizing, the product is applied at 2–8 wt% solids on a metering size press or conventional size press. The low-viscosity profile allows rod-metered transfer at speeds above 800 m/min without excessive misting. The partially hydrolyzed polymer penetrates the sheet to 10–40 µm depending on internal sizing and base-sheet porosity, increasing IGT surface strength and reducing dusting. Sizing efficiency can be measured by TAPPI T 530 size test or Cobb water absorption per TAPPI T 441; the expected Cobb values are formulation-dependent and are not specified by the resin manufacturer. Insolubilization with 0.5–1.5 wt% glyoxal on resin solids or zirconium ammonium carbonate shifts the dried film from water-sensitive to water-resistant.

Pigmented paper coatings employing 50-42 as co-binder are formulated at 0.5–3.0 parts PVOH per 100 parts pigment. The addition contributes to high-shear viscosity and water retention, measured by TAPPI T 701 water retention meter. Because the grade is low-viscosity, the impact on low-shear Brookfield viscosity is smaller than that of high-molecular-weight grades, allowing higher solids coatings to be run on blade coaters.

In textile warp sizing, the grade is combined with starch or modified starch at 60–80 °C in jet cookers or open kiers. The PVOH fraction improves abrasion resistance of sized yarns as measured by loom stop count and yarn hairiness tests, but published data for this specific configuration is limited. Desizing is achieved by washing in hot water at 70–90 °C, because the partially hydrolyzed film does not require oxidative desizing chemistry. The residual acetate groups reduce film brittleness at low loom-humidity conditions, which lowers end-break rates in weaving.

Does residual acetyl content create processing constraints?

The residual acetyl content reduces crystallinity and dry-state glass transition temperature, but it also lowers thermal stability relative to fully hydrolyzed PVOH. Thermogravimetric analysis in air shows measurable weight loss above 150 °C and rapid degradation above 200 °C, with acetic acid release. Melt processing without plasticizer is not recommended because the thermal degradation window is narrow; plasticized extrusion with polyols at 10–25 phr can be performed at barrel temperatures below 190 °C, but published data for this specific configuration is limited. The powder should be stored in sealed containers at relative humidity below 60%; prolonged exposure leads to lumping and reduced flow in screw feeders. Incompatibility with strong oxidizing agents, peroxides, and certain transition metal salts should be assumed unless compatibility testing indicates otherwise. Dust explosion hazards should be evaluated according to process safety data; the manufacturer’s dust cloud ignition parameters should be obtained before pneumatic conveying or vacuum transfer.

For compliance verification, the following matrix summarizes the primary assessment routes.

In multi-ply tissue laminating and paper tube winding, the grade is applied at 6–12 wt% solids and dried on steam-heated drums at surface temperatures of 110–130 °C. The partially hydrolyzed film develops sufficient green tack when rewetted, but remains fully repulpable under mill conditions defined by TAPPI T 275 screening. Use of 0.2–0.5% of a suitable defoamer on formulation weight is common to prevent foam entrainment in high-speed laminating trays.