SELVOL Polyvinyl Alcohol MM-51

Among partially hydrolyzed polyvinyl alcohol grades, SELVOL Polyvinyl Alcohol MM-51 is supplied as a granular, water-soluble thermoplastic resin for aqueous make-down rather than direct melt processing. The grade is specified by a 4% aqueous solution viscosity of 4.5–5.5 mPa·s at 20 °C and a degree of hydrolysis of 86.0–89.0 mol%, which corresponds to a residual acetyl content of 11.0–14.0 mol%. Auxiliary published specifications normally include volatile matter not exceeding 5.0%, ash as sodium oxide not exceeding 0.5%, and a 4% solution pH of 4.5–6.5 by glass electrode. Viscosity is determined on a Brookfield rotational viscometer at 20 °C; hydrolysis is determined by saponification titration according to JIS K6726 or ISO 15023-2:2003. The combination places MM-51 between lower-viscosity partially hydrolyzed grades and fully hydrolyzed resins, with cold-water dispersibility and moderate film strength. The grade is used in suspension polymerization of vinyl chloride, paper surface sizing, remoistenable adhesives, and textile warp-size formulations.

The grade comparison below reflects representative supplier datasheet values. The key differentiation is that MM-51 provides cold-water dispersibility at a medium solution viscosity, whereas fully hydrolyzed grades require hot-water dissolution and lower-viscosity partially hydrolyzed grades provide less thickening at equal solids.

What limits cold-water make-down efficiency for MM-51?

The primary make-down fault in unheated atmospheric tanks is the formation of translucent lumps or fisheyes when dry granules are added faster than the liquid vortex can wet them. Production-scale make-down is commonly carried out in a 2,000–5,000 L jacketed stainless-steel tank equipped with a rotor-stator disperser or high-speed sawtooth impeller. Dry powder is introduced into the vortex at a rate controlled to the batch volume and mixer tip speed; water temperature is maintained between 25 °C and 35 °C during initial dispersion. Above 40 °C, partially hydrolyzed granules swell rapidly and can form a surface-gelled layer that retards further dissolution. Below 20 °C, dissolution rate slows and viscosity stabilization may require 60–90 min after the last powder addition. The solution pH is kept in the 4.5–6.5 range; alkaline conditions above pH 9.0 promote ester hydrolysis and shift residual acetyl content, while strong acid below pH 2.0 can catalyze chain scission during extended holding. Borate or borax must not be added unless a viscosity increase or gel structure is intended, because borate complexes with the diol units of partially hydrolyzed PVOH.

During high-shear dispersion, aeration can reduce coating clarity and should be minimized by controlling vortex depth and allowing deaeration before use. After the last powder addition, the solution is recirculated through a 200 µm bag filter or screened before use to remove insoluble gels and undispersed granules. In cold-water formulations, the powder is not pre-dissolved in organic solvents; the grade is insoluble in most organic solvents but soluble in water and water-alcohol mixtures at high water content. The dissolution sequence is therefore water first, then dry powder under shear, followed by pH adjustment only after complete dispersion. Surfactants and defoamers may alter cloud point and should be added after the PVOH is fully dissolved; some nonionic surfactants reduce solution clarity at elevated temperature.

Brookfield viscosity is not an intrinsic viscosity and should not be used as a direct molecular weight value. The 4% aqueous solution at 20 °C is in a low-concentration regime but can still display shear-thinning behaviour under high shear; for slot-die coating flows above 1,000 s⁻¹, the effective viscosity can be lower than the Brookfield value. Intrinsic viscosity measurements in water at 30 °C using an Ubbelohde capillary viscometer provide a better estimate of molecular weight; published data for this specific grade is limited. For thin-film tensile properties, free films are normally conditioned at 23 °C and 50% relative humidity and tested according to ASTM D882; grade-specific values for MM-51 should be generated on the intended casting line because film properties vary with drying rate and residual moisture.

Suspension Polymerization Dispersant Behaviour and Reactor Fouling Limits

For vinyl chloride suspension polymerization, MM-51 functions as a primary protective colloid that controls droplet coalescence and granule morphology. Typical industrial loading is 0.05–0.20 phr on vinyl chloride monomer, with water-to-monomer mass ratio between 1.0:1 and 1.4:1 and polymerization temperature between 45 °C and 70 °C in a glass-lined or stainless-steel autoclave. The 4% aqueous PVOH solution is charged before the initiator; the solution viscosity and hydrolysis window influence surface coverage of the monomer droplets. At 86.0–89.0 mol% hydrolysis, the residual acetate groups reduce crystallinity and provide surface activity without the hot-water handling required for fully hydrolyzed grades. Compared with a lower-viscosity partially hydrolyzed grade such as SELVOL 203, MM-51 gives higher solution viscosity at the same concentration and can improve primary particle stability at low dispersant levels. Compared with a higher-viscosity grade such as SELVOL 205, MM-51 reduces peak mixing viscosity and can lower the motor load during solution preparation and reactor charging. The operating window is nevertheless narrow: incomplete dissolution or aged solution can produce polymer scale on reactor walls, transfer piping fouling, and bimodal particle-size distribution. Batch-to-batch variation within the hydrolysis specification range shifts the interfacial behaviour of the protective colloid; published data for this specific grade in a particular reactor configuration is limited and should be validated by bench-scale autoclave trials.

For secondary dispersant modulation, MM-51 can be paired with a lower-viscosity or lower-hydrolysis grade at 0.02–0.05 phr to adjust primary particle size and porosity; the ratio of primary to secondary dispersant is more influential than the absolute loading in controlling vinyl chloride drop coalescence. Final PVC resin powder is typically assessed for bulk density, plasticizer absorption, and fish-eye count using the relevant ISO 4608 or internal suspension-PVC methods; the contribution of MM-51 is evaluated through these resin responses rather than by a single PVOH property. Reactor fouling is minimized by filtering the PVOH solution through a 200 µm screen before charging and by avoiding extended holding of the solution above 35 °C prior to use.

When fully hydrolyzed PVOH is replaced by MM-51 in paper surface sizing and remoistenable adhesive compounding, the make-down temperature and film morphology shift. A size press formulation at 6–8% PVOH solids can be made with cold water when MM-51 is used, whereas a fully hydrolyzed grade such as SELVOL 103 requires heating to 90–95 °C for complete dissolution. The residual acetate groups in MM-51 disrupt crystallinity and lower film tensile modulus, but increase water sensitivity relative to a fully hydrolyzed film. In paper surface sizing, the lower solution viscosity at equal solids permits higher size press solids without excessive rod or blade pressure, provided the metering element is designed for shear-thinning polymer solutions. Paper mill trials indicate that size press pickup at a given viscosity depends on the base sheet, metering configuration, and speed; no universal pickup figure is available. For remoistenable adhesive formulations, the grade can be dry-blended with starch or dextrin and later dissolved in cold water, but borate crosslinkers must be withheld until the desired working viscosity is reached. Although PVOH films have low oxygen transmission at low relative humidity, MM-51 without crosslinking is not suitable as a high-humidity oxygen barrier because the barrier deteriorates above 60% relative humidity. Compliance for indirect food-contact paper coatings is normally assessed under FDA 21 CFR 175.300 or 21 CFR 176.170; the supplier should confirm the specific MM-51 grade listing and extraction limits before use.

When MM-51 Replaces Lower-Viscosity PVOH in Textile Warp-Size Formulations

In cotton and polyester-cotton warp sizing, the grade is blended with starch or acrylic size at a PVOH fraction of 10–30% of total dry size. The intermediate molecular weight of MM-51 increases yarn abrasion resistance compared with low-viscosity grades at equal add-on, but the size solution must be removed by hot-water desizing at 80–90 °C. Residual size can be checked by iodine-boric acid staining of fabric; the presence of PVOH produces a characteristic colour. The typical size add-on is 8–12% on yarn weight, but this range depends on yarn count, loom speed, and shed behaviour rather than a single test standard. Because MM-51 is partially hydrolyzed, it is not recommended for alkaline peroxide size recovery systems where high pH accelerates ester hydrolysis and reduces molecular weight during recovery. Desizing effluent contributes chemical oxygen demand; activated sludge treatment is generally effective under aerobic conditions, but anaerobic degradation of PVOH is slow. For weaving operations with high relative humidity in the weave room, the film can absorb moisture; size formulations therefore require a moisture-resistant co-binder when loom shedding becomes sticky above 70% relative humidity.

Dry storage in unopened bags at ≤ 30 °C and ≤ 60% relative humidity is recommended because the granules absorb atmospheric moisture. Bags exposed to humidity above 60% can develop caking and feed irregularities in volumetric screw feeders. Pre-drying is not typically required when volatile matter remains below 5.0%; however, pneumatic conveying with unheated air can generate static and should be grounded. The powder is combustible as an organic dust; the minimum ignition energy and explosion severity are grade-dependent and should be obtained from the safety data sheet. Under REACH, polyvinyl alcohol is generally exempt from polymer registration if it meets the polymer definition, but monomer registration obligations apply; the supplier should confirm the regulatory status of the specific grade for the intended use.