GOHSENOL EG-18P

GOHSENOL EG-18P is a partially hydrolysed polyvinyl alcohol powder marketed under the GOHSENOL trade name by Mitsubishi Chemical Corporation. The product is supplied as a free-flowing white to off-white powder with a 4 wt% aqueous solution viscosity at 20 °C typically reported as 20.5–24.5 mPa·s and a hydrolysis degree of 87–89 mol%. The grade also carries a pH of 5–7 in aqueous solution, a volatile matter content not exceeding 5.0 wt%, and an ash content not exceeding 0.5 wt%. Its CAS registry number is 9002-89-5. The residual acetate groups in the partially hydrolysed backbone reduce crystallinity and promote cold-water dispersibility relative to fully hydrolysed polyvinyl alcohol grades, while the intermediate molecular weight provides a balance between solution viscosity and film toughness.

The residual acetate groups in EG-18P disrupt inter-chain hydrogen bonding more effectively than acetate groups in lower-hydrolysis intermediates. This structural feature reduces the energy required for complete dissolution and lowers the tendency for hot-water gelation during size-press and coating-make-down operations. The product is therefore specified where cold-water-dispersible film formation, controlled viscosity build, and low ash residue are required simultaneously.

On production-scale make-down equipment, EG-18P is most reliably dissolved by first dispersing the powder in ambient water under high-shear agitation. A sawtooth disperser operated at a tip speed of 10–15 m/s disperses the powder without forming partially hydrated agglomerates. The dispersion is then heated to 85–95 °C and held for 30–60 min under low-shear anchor agitation to complete dissolution. Direct steam injection can be used, but local temperature excursions above 95 °C may promote hydrolysis of residual acetate groups and drift in solution viscosity. Jacketed vessels with internal coils provide tighter control in continuous coating plants. The solution should be cooled below 40 °C before adding biocide or pH adjusters. For storage longer than 24 h, a compatible biocide is required because polyvinyl alcohol solutions are susceptible to microbial degradation. Filtration through a 100–200 µm bag filter removes occasional gel specks that can cause coating defects. At relative humidity above 60%, the powder should be pre-dried or stored in sealed silos because moisture uptake can produce bridge formation in screw feeders and off-spec viscosity under gravimetric dosing.

The viscosity of a 4 wt% solution at 20 °C is controlled primarily by molecular weight and concentration. Increasing the solution concentration from 4 wt% to 8 wt% does not produce a linear viscosity increase; the shear-thinning character becomes more pronounced, and low-shear Brookfield viscosity can rise by a factor of several times. For pumping and coating design, rheological curves should be generated on site using ASTM D2196-20 rather than relying on single-point viscosity. The product is compatible with nonionic and anionic wetting agents, but cationic additives may reduce solution clarity or form complexes if the pH exceeds 7.5. Prolonged high-shear mixing after full dissolution can cause mechanical chain scission; rotor-stator mixers should be shut off or bypassed once dispersion is complete. The pH of the finished solution should remain between 5 and 7 for maximum shelf stability. Alkaline pH above 8.5 promotes gradual ester hydrolysis, shifting solubility and viscosity behaviour over time.

What Determines Surface Strength When EG-18P Is Used in Paper Sizing?

Surface sizing operations use EG-18P as a film-forming strength additive in starch-based size press formulations. At size-press solids of 4–8 wt%, addition of 0.5–2.0 parts EG-18P per 100 parts oxidised starch on a dry basis improves surface strength as measured by TAPPI T 459 or ISO 3783. The partially hydrolysed grade disperses at 20–25 °C and does not require dedicated cooking when a pre-dissolved 10 wt% stock solution is used. On high-speed film-transfer coaters running above 1,000 m/min, foaming and misting at the nip can be influenced by coating solids, roll hardness, and machine geometry; mill trials are required to define the operating window. Batches prepared with high calcium-ion mill water may show reduced compatibility with oxidised starch. Water hardness above 300 ppm CaCO3 should be evaluated because viscosity and film clarity can shift.

When EG-18P replaces a lower-viscosity partially hydrolysed grade such as GOHSENOL EG-05 at equal addition, the higher molecular weight of EG-18P generally increases size-press film toughness and surface strength but raises size-press viscosity. Published data for the exact surface strength delta in a given furnish is limited because fibre furnish composition, filler loading, and starch type mask direct comparisons. When EG-18P is compared with fully hydrolysed polyvinyl alcohol grades, the lower hydrolysis range of 87–89 mol% reduces hot-water gelation and permits easier cold-water clean-up but yields a more water-sensitive dried film. In closure operations where water resistance is required after printing, EG-18P should be blended with a fully hydrolysed grade or a crosslinker rather than used as the sole binder.

In emulsion polymerisation, EG-18P functions as a protective colloid during vinyl acetate and vinyl acetate-ethylene polymerisation. A 10 wt% aqueous stock solution is typically metered into the reactor before initiator addition at 2–6 wt% on total monomer. The residual acetate content of the grade provides a balance between sufficient grafting onto the growing particle surface and colloidal stabilisation of the latex. High-solids latices above 60 wt% may develop excessive viscosity or coagulum at the upper end of this addition range, and the optimum must be determined in the specific reactor. A glass-lined or stainless-steel reactor equipped with a pitched-blade turbine at tip speed of 1.5–2.5 m/s provides adequate monomer dispersion without generating shear-induced coagulum. The final particle size distribution should be measured by laser diffraction; no universal D50 target can be assigned to EG-18P because particle size responds strongly to initiator level, comonomer ratio, and agitation geometry. Compared with lower-molecular-weight partially hydrolysed grades, EG-18P improves dried-film strength but increases latex viscosity at equal solids.

Adhesive Formulation Variables and Film Tack Window

Water-based adhesive formulations use EG-18P to modify open time, wet tack, and set speed. In borate-tackified systems, 1–3 wt% borax on polyvinyl alcohol solids produces a viscosity increase and improves wet tack without immediate gelation at 20–25 °C. Mixing is typically performed in a high-shear disperser at 800–2,000 rpm for 20–30 min until filler agglomerates are dispersed. Temperature during mixing should not exceed 45 °C because borate-crosslinked viscosity declines with rising temperature and premature gelation can occur when the batch is cooled. Adhesion to polar substrates can be evaluated using ASTM D1002 for lap shear and ASTM D903 for peel; actual values are substrate- and formulation-dependent. Plasticisers such as glycerol or polyethylene glycol at 10–20 wt% on polyvinyl alcohol solids reduce film tensile modulus and extend open time. The product should not be combined with amine-functional additives that induce alkaline hydrolysis during storage, nor with strong oxidising agents that degrade the polymer backbone.

For ceramic green-body processing, EG-18P is employed as a temporary binder in aqueous tape-casting and spray-dried granulate formulations because its ash content is limited to ≤ 0.5 wt%. Binder addition of 2–8 wt% on ceramic solids is common, but the optimum depends on particle surface area and green density requirements. In continuous tape casting with a doctor blade gap of 200–800 µm, slurry viscosity is generally held in the 1,500–4,000 mPa·s range at 20 °C. EG-18P contributes shear-thinning behaviour that aids levelling while maintaining adequate green strength. Burnout in air is usually conducted between 250 °C and 500 °C, with residual ash controlled by the product's low ash specification. Fully hydrolysed grades can leave similarly low ash but require higher dissolution temperatures and may produce stiffer tape with higher yield stress. Hard water above 300 ppm CaCO3 should be softened because calcium salts can reduce ceramic dispersant efficiency and may interact with the polyvinyl alcohol binder during drying.

For food-contact packaging and article applications, the regulatory status of EG-18P must be confirmed for the specific end-use and region. The manufacturer can provide current declarations for FDA 21 CFR §175.105, EU Regulation (EC) No 1935/2004, and REACH. Dust control during handling is required even though the polymer is not classified as hazardous under GHS. The ash content at ≤ 0.5 wt% assists in meeting heavy-metal limits where applicable, but trace-metal certification should be requested for electronics-grade or food-contact batches.

EG-18P is stable in unopened packaging for at least 12 months when stored below 30 °C and protected from moisture. Bags should be resealed immediately after use; caking can occur after exposure to relative humidity above 60%. Silos and hoppers handling the powder should be grounded to prevent static discharge, and dust extraction systems should be used to keep respirable dust below occupational exposure limits. These operational constraints are shared with other polyvinyl alcohol powder grades and do not represent a unique limitation of EG-18P.

When Environmental Exposure Demands Moisture-Resistant Films

Films and coatings based on EG-18P retain the moisture sensitivity of partially hydrolysed polyvinyl alcohol. At 50% relative humidity and 23 °C, equilibrium moisture uptake is higher than that of fully hydrolysed grades, and wet tensile properties decline after direct water contact. Tensile properties of cast films should be measured according to ASTM D882 using specimens conditioned at 23 °C and 50% RH for not less than 40 h. When improved water resistance is required, EG-18P is blended with fully hydrolysed grades, or crosslinkers such as glyoxal are added at 5–15 wt% on polyvinyl alcohol solids and cured under heat. The product should not be processed as a thermoplastic without sufficient plasticiser; unplasticised polyvinyl alcohol decomposes near 200 °C before a stable melt is formed, so ISO 1133-1:2022 melt flow testing is not a meaningful specification for this grade. In aqueous coating applications, exposure to alkaline substrates or high-pH formulations above 8.5 can shift solubility and film performance during storage, so compatibility trials are necessary.