GOHSENX WO-320N

GOHSENX WO-320N is a partially hydrolysed polyvinyl alcohol grade produced by Mitsubishi Chemical Corporation and supplied as a white to off-white powder. The product is positioned within the GOHSENX series as a lower-viscosity secondary suspending agent for the suspension polymerisation of vinyl chloride monomer. Published certificate-of-analysis data list a degree of hydrolysis of 71.0–75.0 mol%, a viscosity of 24.0–30.0 mPa·s for a 4% aqueous solution at 20 °C, volatile matter at not more than 5.0 mass%, and ash at not more than 0.5 mass%. These properties allow the product to modify poly(vinyl chloride) grain porosity without proportionally increasing continuous-phase viscosity during polymerisation.

What operational role does the lower-viscosity WO‑320N serve in PVC suspension polymerisation?

The suspension polymerisation of vinyl chloride is conducted in a baffled stirred autoclave in which liquid vinyl chloride is dispersed as droplets in demineralised water. A primary polyvinyl alcohol suspending agent, typically a higher-viscosity grade such as GOHSENX T‑330H, controls droplet coalescence and establishes the average droplet size. WO‑320N is then introduced as a secondary suspending agent. Because the solution viscosity of 24.0–30.0 mPa·s reflects a lower molecular weight than that of a primary grade, the secondary grade diffuses more rapidly to the vinyl chloride–water interface. The partially hydrolysed structure, with a residual acetyl group fraction of approximately 25–29 mol%, provides hydrophobic anchoring at the monomer interface while the hydroxyl groups extend into the aqueous phase. This adsorption layer modifies the interfacial tension gradient and reduces the rate of primary-particle coalescence inside the polymerising PVC grain.

During the early stages of polymerisation, vinyl chloride droplets are stabilised by a thin polyvinyl alcohol film. The agitation field imposes turbulent shear stresses that continuously deform the droplets; the adsorbed polymer layer resists coalescence through local surface elasticity. The lower-viscosity WO‑320N generates less continuous-phase thickening than a primary grade, which can reduce agitator power draw at constant tip speed. The trade-off is that if the total interfacial polyvinyl alcohol concentration drops below the required surface coverage, droplet coalescence increases and the mean grain size rises. Conversely, if the WO‑320N fraction is excessive, primary-particle packing inside the grain becomes too loose, bulk density falls below the 0.45–0.60 g/cm³ range commonly accepted for suspension PVC, and dry resin handling becomes difficult.

Excessive secondary-polyvinyl alcohol dosage can produce over-porous grains; insufficient dosage leaves grains too dense, reduces plasticizer absorption, and may increase gel-particle counts. Plant operators therefore adjust the primary-to-secondary ratio through reactor trials. Bulk density of the dry PVC resin is measured under ISO 60, plasticizer absorption under ASTM D3367, K value under ISO 1628-2, and sieve distribution under ISO 4610. In suspension resins, plasticizer absorption values typically range from 15 to 35 parts per 100 parts resin. Resins produced with excessive secondary-polyvinyl alcohol content often exceed 30 parts per 100 parts resin, while insufficient secondary content can reduce absorption below 15 parts per 100 parts resin and produce dense grains with high bulk density.

For suspension polymerisation at commercial scale, WO‑320N is normally pre-dissolved in demineralised water at a concentration of 4.0–5.0 mass%. The powder is dispersed in cold water at 20–30 °C under agitation, then heated to 85–95 °C and held for 30–60 min until a clear solution is obtained. Inline filtration through a 100 µm mesh is used to remove undissolved gel bodies before the solution is metered into the aqueous phase prior to vinyl chloride charging. Bulk transfer of the dry powder should use closed mechanical conveying with dry air or nitrogen blanketing where relative humidity exceeds 60%. Moisture uptake above the 5.0 mass% volatile limit can reduce flowability, change feeder calibration, and cause blockages in rotary valves.

Certificate-of-analysis parameters and standard test methodology

The properties that define WO‑320N as a secondary suspending agent are the degree of hydrolysis and the viscosity of a 4% aqueous solution. The degree of hydrolysis determines the hydrophobic–hydrophilic balance; the viscosity determines the molecular weight and the thickening effect in the aqueous phase. JIS K 6726 and ISO 15023-2 are the principal test methods cited for polyvinyl alcohol solution viscosity, pH, volatile matter, ash, and light transmittance. The table below summarises the typical product data.

The ash content of ≤0.5 mass% is not a low-ash specification; it is the standard ceiling for suspension-grade polyvinyl alcohol. Residual ash, largely sodium acetate, can shift the interfacial pH in the polymerising droplet and affect the hydrolysis of the polyvinyl alcohol at elevated temperature. The purity value of ≥94.0 mass% is calculated after subtraction of volatile matter and ash; it is not a chromatographic assay for polyvinyl alcohol structure. The light transmittance of ≥90% on a 4% solution is an indirect measure of undissolved or crosslinked material that could deposit on reactor baffles or condenser surfaces.

When WO‑320N is compared with GOHSENX T‑330H, the main difference is the 4% solution viscosity. T‑330H is typically specified at 44.0–52.0 mPa·s, while WO‑320N is specified at 24.0–30.0 mPa·s. Both products share a similar degree of hydrolysis of 71.0–75.0 mol%. The higher-viscosity grade is used as the primary suspending agent; the lower-viscosity grade is used as the secondary suspending agent. In a reactor formulation, a primary grade usually forms the continuous stabilising layer, and a secondary grade enters the interfacial film to create a looser packing arrangement of primary particles. This difference in function is not determined by hydrolysis alone but by molecular weight, diffusion rate, and the associated hydrodynamic volume in solution.

Compared with fully hydrolysed polyvinyl alcohol grades such as the GOHSENOL series, WO‑320N has a much higher residual acetyl group content. Fully hydrolysed grades have a degree of hydrolysis above 98 mol% and give stronger, more water-resistant films when tested under ISO 527-3. WO‑320N dissolves more readily in cold water and has surface activity that fully hydrolysed grades do not exhibit to the same extent. However, films or coatings prepared from WO‑320N have lower tensile strength and higher water sensitivity; the grade is not intended for structural film or adhesive applications. Compared with cellulose-ether suspending agents, WO‑320N is free of cellulose fibre residues and has a lower tendency to generate odour during PVC drying. However, partially hydrolysed polyvinyl alcohol solubility is temperature-dependent, so use in reactors operating above 75 °C may require a higher dilution or a shift to a more suitable grade. Published data for that specific high-temperature configuration is limited.

When the polymerisation temperature exceeds 70 °C, storage, dissolution, and compatibility limits must be reviewed

Partially hydrolysed polyvinyl alcohol grades exhibit temperature- and salt-dependent solubility. A 4% solution of WO‑320N prepared at 85–95 °C may turn turbid after cooling if the water hardness exceeds 50 mg/L as CaCO₃. Demineralised water with conductivity below 10 µS/cm is therefore used for make-down. Borate salts and borax cause immediate gelation through complexation with the 1,3-diol units of polyvinyl alcohol and must not be introduced into the suspending-agent tank. Strong oxidisers such as hydrogen peroxide or hypochlorite used for reactor cleaning should be rinsed from all feeding lines before charging the polyvinyl alcohol solution, because residual oxidant promotes chain scission and reduces solution viscosity.

Dry WO‑320N should be stored in sealed packaging at 5–35 °C. Open bags exposed to relative humidity above 60% can gain sufficient moisture to exceed the volatile matter limit within 48 h. If caking occurs, the material should not be forced through screw conveyors; compacted powder can overload the feeder drive and generate frictional heat that alters solubility. In high-temperature suspension polymerisation, the aqueous phase is often maintained at 70–80 °C. At these temperatures, the solubility window of a 72 mol% hydrolysed polyvinyl alcohol narrows, and plant operators commonly dilute the WO‑320N solution to 2.0–3.0 mass% before metering it into the reactor. The diluted solution is charged below the monomer addition point to avoid localised precipitation on cold baffles or sight glasses. Agitation is maintained at sufficient speed to disperse the polyvinyl alcohol solution before vinyl chloride charging; stagnant zones lead to wall scaling and later plate-out during PVC stripping. For the same reason, the product should not be hot-added to a reactor containing concentrated salt solutions or pH-control buffers.