GOHSENOL NH-18

GOHSENOL NH-18 is a fully hydrolysed polyvinyl alcohol (PVOH) resin supplied as white to pale-yellow granules. The manufacturer’s technical data sheet defines the grade by a saponification degree of 98.0–99.0 mol% and a dynamic viscosity of 25.0–30.0 mPa·s for a 4% aqueous solution at 20°C, measured under JIS K6726. Volatile matter is controlled to ≤5.0%, ash as Na₂O to ≤0.4%, and pH of a 4% solution to 5.0–7.5. The “18” suffix identifies the viscosity bracket, not a concentration. NH-18 belongs to the fully hydrolysed NH series, distinguishing it from partially hydrolysed GOHSENOL GH and KH grades in cold-water solubility, dried-film water resistance, and tensile strength development. In aqueous make-down, the granulate is typically pre-slurried in cold water below 20°C before heating to 85–95°C to prevent fisheye formation.

Specification Data and Aqueous Preparation Control

Lot-to-lot control for NH-18 is anchored to JIS K6726 for saponification degree, viscosity, ash, and volatile content. The following table lists typical published ranges for selected GOHSENOL grades and should be verified against the certificate of analysis for each production batch. Variation within the 25.0–30.0 mPa·s bracket alters solution rheology during transfer; positive-displacement gear pumps with speed control are preferred over centrifugal pumps once viscosity exceeds 25 mPa·s.

Dissolution at production scale is typically performed in stainless steel jacketed vessels equipped with a slow anchor agitator at 10–20 rpm and a high-shear Cowles blade at 500–1,000 rpm. Complete hydration at 4–8% solids requires maintaining 85–95°C for 30–45 min. Viscosity rises sharply as the solution cools toward 60–70°C, and an open-tank surface can skin over when temperature drops below 75°C. The effective processing band is therefore constrained between hydration and thermal degradation; jacket control to ±2°C is used in production to avoid both undispersed gel bodies and heat-induced chain scission. Raising solution solids from 4% to 8% typically increases apparent viscosity by a factor of 2.5–3.5, which must be compensated through inline process viscometers on doctor-blade coaters or slot-die feed systems.

In paper and paperboard adhesive formulations, NH-18 is incorporated at 2–6 wt% dry basis to increase water resistance and dry tensile strength of starch or dextrin-based adhesive systems. The high degree of hydrolysis reduces swelling of the dried PVOH phase after 24 h water immersion. Bond performance is commonly evaluated according to ISO 1924-2:2008 for tensile energy absorption and TAPPI T 821 for pin adhesion in corrugated board. Production-scale mixing records show that NH-18 addition to starch adhesives in 500 L scraped-surface jacketed mixers raises clean-up demands after film drying; washdown requires hot water above 80°C and alkaline detergent. Compared with partially hydrolysed GH-17, NH-18 produces a dried film with lower equilibrium moisture pickup and lower cold-water solubility, but the granulate is not suitable for direct ambient-water dissolution without heat input.

What Distinguishes NH-18 from Partially Hydrolysed PVOH Grades?

The difference is primarily governed by residual acetate content. At 98.0–99.0 mol% hydrolysis, NH-18 contains only 1.0–2.0 mol% residual acetate groups, whereas GH-17 and KH-17 contain 11.0–13.5 mol% and 18.0–21.5 mol% residual acetate, respectively. The lower acetate content increases chain regularity, which raises crystallite density and reduces cold-water solubility. Consequently, NH-18 films exhibit higher tensile strength and lower elongation at break than partially hydrolysed grades at equal thickness when conditioned at 23°C and 50% RH and tested under ASTM D882-18. The same structural regularity raises the temperature required for full dissolution: NH-18 does not wet out adequately in water below 40°C, while KH-17 will hydrate at 20–25°C under agitation. This distinction determines equipment selection: NH-18 requires heated dissolution skids with jacket temperatures of 90°C and insulated transfer lines, whereas lower-hydrolysis grades can be handled in ambient-water make-down units.

Surface activity also differs across the hydrolysis range. Residual acetate groups increase hydrophobic character and reduce surface tension, so partially hydrolysed grades such as KH-17 behave as more effective emulsifiers and protective colloids in vinyl acetate and acrylic emulsion polymerisation. NH-18 has a higher aqueous surface tension and is selected in emulsion polymerisation primarily when the final film must exhibit lower water sensitivity and greater tensile strength after coalescence. The viscosity bracket of NH-18 provides sufficient molecular weight for film cohesion without the very high shear viscosity of NH-20, making it suitable for emulsion-polymerised adhesives where high-solids circulation and spray-drying are involved.

For textile warp sizing, NH-18 is dissolved to 6–10% solids and applied on slasher lines at 60–65°C. The higher molecular weight relative to low-viscosity PVA grades improves size film toughness but requires longer hot-water desizing. Desizing operations typically use 95°C water and 0.2–0.5 g/L non-ionic surfactant to overcome crystalline regions and remove the size film. Automatic sizing machines with ambient-water powder wetting cannot handle this grade reliably; a pre-slurry vessel or a separate cooking kettle with a high-shear disperser is required. In ceramic binder applications for alumina or zirconia green bodies, NH-18 at 0.5–2 wt% improves green strength before debinding. Thermal debinding in air above 500°C provides clean burnout, but the dried binder’s water resistance reduces moisture sensitivity of pressed parts during storage.

If Cold-Water Solubility Is Required in Automatic Dosing Systems

GOHSENOL NH-18 is not the correct grade for dosing systems that rely on ambient-water powder wetting or in-line eductor dispersion below 40°C. In those configurations, the granules remain as swollen gels, increase torque on the recirculation pump, and can block strainer baskets. When a fully hydrolysed grade is still required for end-use water resistance, the formulation should include a pre-dissolution step at 85–95°C in a jacketed tank with a rotor-stator mixer, followed by transfer through heat-traced pipework maintained above 75°C. If the process cannot accommodate heated dissolution, a lower-hydrolysis grade such as KH-17 or a cold-water-soluble PVA with a degree of hydrolysis below 82 mol% should be selected, accepting reduced film water resistance. The incompatibility of NH-18 with borate ions, glyoxal, glutaraldehyde, and strong oxidizers must also be considered; these species cause uncontrolled gelation or chain scission unless crosslinking is intentionally formulated and pH is buffered.

For food-contact paper and paperboard, GOHSENOL NH-18 is typically evaluated under 21 CFR 175.300 for resinous and polymeric coatings and 21 CFR 176.170 for components of paper and paperboard in contact with aqueous and fatty foods, subject to end-testing and residual monomer limits. Migration kinetics in polymer matrices are influenced by residual acetate content and crystallinity; extraction testing is required for the specific food type and temperature condition. The product is supplied under quality systems aligned with ISO 9001 and can be assessed under REACH Regulation (EC) No 1907/2006 for European Union inventory compliance. Heavy-metal content for electrical and electronic applications can be reviewed against RoHS Directive 2011/65/EU if the resin is used as a binder in laminated substrates, although this is not the primary application. Pre-drying at 70–80°C for 2–4 h in a desiccant dryer is recommended when ambient storage exceeds 60% RH and the granulate is intended for melt extrusion or cast film, because moisture uptake causes bubble formation and viscosity drift in the molten PVOH phase.

Rheological Consequences of an NH-20 to NH-18 Substitution in Slot-Die Coaters

Substitution of NH-20 for NH-18 within the same hydrolysis band increases the 4% aqueous viscosity from 25.0–30.0 mPa·s to 35.0–45.0 mPa·s. In high-solid paper coating and adhesive lines, this raises pump pressure, coat weight transfer, and blade or rod metering resistance. The higher viscosity grade is selected when a thicker wet film or more shear viscosity is needed, but NH-18 is preferred for formulations above 10% solids where high-solids viscosity already limits circulation. Coaters operating with slot-die heads and mass-flow-controlled supply should recalibrate flow curves when switching between NH-18 and NH-20 because the viscosity difference at shear rates below 100 s⁻¹ shifts the coating window by approximately 3–5% in wet film thickness for a fixed pump speed. The pressure drop in laminar slot-die flow scales approximately with the low-shear viscosity ratio, so a 40% increase in 4% viscosity can produce a comparable increase in die cavity pressure at constant flow. Published data for this specific substitution is limited; however, the viscosity ratio measured under JIS K6726 provides a reliable first approximation for pump sizing and die pressure drop when the shear-rate dependence of the two grades is not available.