Polyvinyl alcohol grade 728F is a partially hydrolysed PVOH resin with a nominal degree of hydrolysis of
72 mol% (
±1 mol%), corresponding to a residual acetyl group content in the range of
27–29 mol%. The designation “F” denotes a fine powder morphology with a typical particle size distribution where
95% passes through a
250 µm sieve, which facilitates rapid cold-water dispersion and minimises dusting relative to granular grades during bulk handling in automatic vacuum conveyance systems. Aqueous solutions prepared at
4% solids by weight yield a viscosity of
5.0–7.0 mPa·s when measured at
20 °C with a Brookfield LVT viscometer at
60 rpm, aligning with the medium–low molecular weight segment of commercial PVOH and consistent across global producers that supply a 728F designation under equivalent JIS K 6726 classifications. The intentionally moderate degree of hydrolysis situates 728F in a property envelope that balances cold-water solubility, surfactant compatibility, and film-forming performance, distinguishing it from both fully hydrolysed (>98 mol%) and lower-hydrolysis (ca. 88 mol%) grades. Because the residual acetate substituents disrupt inter‑chain hydrogen bonding, the crystallinity index measured by differential scanning calorimetry typically falls near
18–22%, compared with
35–42% for a
98.5 mol% hydrolysed PVOH, which directly depresses the melting endotherm to approximately
180–190 °C and lowers the glass‑transition temperature of the dry powder to
58–63 °C.
Typical specification range for Polyvinyl Alcohol 728F
| Property | Value | Test method |
|---|
| Degree of hydrolysis | 71.0–73.0 mol% | JIS K 6726 (alkaline saponification) |
| Viscosity (4% aq., 20 °C) | 5.0–7.0 mPa·s | JIS K 6726 / Brookfield LVF, 60 rpm |
| Volatile matter | ≤5.0% | JIS K 6726 (105 °C, 3 h) |
| Ash (as Na₂O) | ≤0.5% | JIS K 6726 (700 °C ignition) |
| pH (4% solution, 20 °C) | 5.0–7.0 | JIS K 6726 |
| Apparent density | 0.45–0.60 g/cm³ | ASTM D1895, Method A |
What Limits the Utility of 728F in High‑Alkaline Adhesive Formulations?
In the formulation of paper‑tube winding adhesives and remoistenable envelope gums, the interaction between partially hydrolysed PVOH 728F and alkaline crosslinkers such as borax (sodium tetraborate decahydrate) or boric acid at pH above
8.5 proceeds more rapidly than with fully hydrolysed grades owing to the stereochemistry of residual acetate groups. The hydroxyl‑rich sequences that are sufficiently long to participate in di‑diol complexation with borate ions are more isolated in
72 mol% hydrolysed chains, which creates a network that can undergo syneresis and molecular weight‑dependent precipitation within
30–60 min of mixing at
25 °C, causing viscosity collapse in starch‑extended corrugating adhesives. Production trials on a
2‑m Steinemann curtain coater revealed that when
728F‑based adhesive with
1.2% borax (on wet weight) was held in a recirculating tank at
40 °C, the Brookfield viscosity decayed from
2,200 mPa·s to
850 mPa·s within
4 h, whereas a
98% hydrolysed counterpart stabilised above
1,800 mPa·s for a full
8‑h shift. Consequently, 728F is specified only for adhesive systems operating at pH
5.0–7.5, and formulators routinely substitute higher‑hydrolysis PVOH when the alkali reserve exceeds
0.5% as NaOH. If 728F must be used, the boric acid‑modification route is pre‑reacted at
85 °C for
45 min under reflux to pre‑build complex sites before pH adjustment, which mitigates shock gelation but raises the minimum film formation temperature by
8–12 °C.
In emulsion polymerisation, where 728F functions as the primary protective colloid for vinyl acetate homopolymer and vinyl acetate‑ethylene (VAE) copolymer latexes, its behaviour diverges markedly from that of heavily hydrolysed grades both in solution thermodynamics and in interfacial activity. Published surface tension isotherms for
4% aqueous solutions show a value of
46–48 mN/m at
20 °C (Wilhelmy plate method, ISO 1409), approximately
8–10 mN/m higher than that of an
88 mol% hydrolysed grade with equivalent viscosity, because fewer contiguous hydroxyl blocks are available to adopt a flat conformation at the oil‑water interface. In a
10 m³ stainless‑steel reactor equipped with a dual‑turbine agitator (Rushton disk, D/T =
0.33), maintaining a dissolved PVOH concentration of
6 wt% on total monomer during a batch VAE polymerisation run at
80 °C and
40 bar ethylene pressure, the resultant latex particle size (Coulter LS 13‑320) settled at
1,200–1,600 nm with a broader polydispersity index of
0.28–0.35, compared with
900–1,100 nm and PDI
0.18 when using an
88% hydrolysed PVOH of comparable viscosity. This broader distribution is a direct consequence of the lower nucleation rate induced by the weaker interfacial anchoring of 728F, and it demands downstream post‑addition of a secondary stabiliser—typically a nonylphenol‑free ethoxylated fatty alcohol (HLB
15–16) at
0.5–1.0% on latex solids—to achieve mechanical stability above
15,000 cycles in a Klaxon shear‑stability test. However, the same weak anchoring suppresses residual‑acetate‑induced yellowing during high‑temperature curing of pressure‑sensitive adhesives: films cast from a latex stabilised solely with 728F exhibited a ΔE of
2.1 after
10 min at
150 °C (ASTM D2244, D65 illuminant), versus ΔE
4.8 for the lower‑hydrolysis analogue, a critical advantage in optically clear lamination grades.
Influence of Acetyl Distribution on Protective‑Colloid Efficiency During Semi‑Batch VAE Production
The sequence distribution of residual acetyl groups in 728F—whether blocky or random—exerts a measurable effect on grafting density during vinyl acetate polymerisation. For a grade produced via a continuous saponification process with precise methanol‑to‑methyl acetate ratio control, the intramolecular acetyl arrangement tends toward a more random microstructure, evidenced by a single‑peak DSC melting endotherm with a half‑width of
28–32 °C. This randomness reduces the propensity for the formation of water‑insoluble PVOH‑graft‑PVAc domains that can plate out on reactor walls during semi‑batch operation. On a
16 m³ Pfaudler‑lined vessel running
25 consecutive batches, the cumulative wall‑fouling thickness measured by ultrasonic thickness gauging after each batch was
≤0.2 mm for 728F, while a blocky‑acetyl PVOH of identical degree of hydrolysis produced fouling layers exceeding
1.5 mm within
8 batches, ultimately forcing mechanical cleaning after
12 batches and incurring a
14% reduction in annualised capacity. The practical consequence is that 728F supports extended production campaigns without intermediate shutdowns, provided the overhead condenser spray nozzle remains unobstructed and reactor internal temperature deviation remains within ±
2 °C of setpoint.
Storage and handling of the powder introduce additional constraints that differ from those of fully hydrolysed grades. At relative humidity above
60%, the equilibrium moisture uptake of 728F exceeds
8% within
24 h (gravimetric analysis, ISO 15512), leading to soft‑cake formation in silos and gravimetric dosing inaccuracies of
±3% or greater when using loss‑in‑weight feeders without residence‑time compensation. To prevent bridging, conveying air must be dried to a dew point below
−10 °C and the storage vessel purged with nitrogen. In many compounding facilities serving the paper‑sizing market, pre‑drying in a fluid‑bed dryer at
55–60 °C for
2 h is implemented before metering into a high‑shear cowles dissolver for cook‑down at
95 °C. Failure to control moisture results in partially gelatinised lumps that strain the dissolution tank’s
500‑micron in‑line filter and reduce the effective concentration delivered to the size press, manifesting as Cobb value variability exceeding ±
2 g/m² (ISO 535) on the paper web.
When 728F Replaces 88 mol% PVOH in Water‑Soluble Packaging Film
Substitution of an
88 mol%‑hydrolysed grade with 728F in cast‑water‑soluble film for unit‑dose detergent pouches reconfigures the dissolution‑temperature profile and the film’s tolerance to liquid surfactant concentrates. Film produced on a
1.5‑m‑wide chill‑roll casting line (gap
0.8 mm, roll temperature
15 °C) from a
22 wt% aqueous solution of 728F that also contained
8% glycerol plasticiser (on PVOH) achieved complete dissolution in de‑ionised water at
10 °C in
52 s, measured according to ISO 14851/14852 respirometric protocols, more than twice as fast as the
88% grade which required
118 s. However, the same film exhibited a tensile strength at break of
24 MPa (ASTM D882, specimen type IV, 5% moisture content) compared with
38 MPa for the
88%‑grade film, reflecting the crystallinity deficit. In accelerated shelf‑life tests with a commercial non‑aqueous heavy‑duty detergent containing
12% propylene glycol and
5% C₁₂–C₁₄ alkyl dimethyl amine oxide, pouches formed from 728F film developed micro‑crazing around cold‑seal gusset folds after
4 weeks at
40 °C and
75% relative humidity, ultimately leading to leakage rates of
8–12% in a drop test of
1.5 m onto a rigid surface. This failure mode is attributed to plasticiser migration into the surfactant phase, which is aggravated by the higher free‑volume fraction of the less‑crystalline 728F matrix; published migration data for glycerol in
72%‑hydrolysed PVOH films show a diffusion coefficient (D) of
4.7 × 10⁻⁹ cm²/s at
40 °C, approximately three times the value for the
88%‑hydrolysed counterpart. Therefore, when cold‑water solubility is prioritised over mechanical integrity, 728F is formulated with a co‑plasticiser system of
3% sorbitol and
5% trimethylolpropane, which reduces D to
2.1 × 10⁻⁹ cm²/s and decreases pinhole defects to
< 1% under the same storage protocol.
Across paper surface sizing, the rheological behaviour of 728F under high‑shear metering‑size‑press conditions contrasts with both fully hydrolysed and carboxymethyl cellulose‑containing formulations. In a pilot‑scale Valmet OptiSizer application running at
1,200 m/min, a
10% solids solution exhibited a shear‑thinning power‑law index
n = 0.72 over a shear‑rate window of
10–10⁵ s⁻¹, being less pseudoplastic than an equivalent‑viscosity fully hydrolysed PVOH (
n = 0.61). This translates to a lower dynamic wet‑end pressure drop across the blade tip, minimising the incidence of surface picking when a groundwood‑containing basestock with low internal bond strength (
120 J/m², TAPPI T 569) is processed. At a coat weight of
1.5 g/m² per side, the resultant Bekk smoothness improved to
280 s (ISO 5627) from a base of
85 s, while the strength‑enhancement effect was modest: ring crush (ISO 12192) increased by only
12%, compared with
22% achieved by a 98%‑hydrolysed PVOH. This selective property profile positions 728F for lightweight‑coated offset grades where surface optical uniformity and ink holdout are valued above compression‑strength indices.
Regulatory compliance references for Polyvinyl Alcohol 728F
| Regulation / Standard | Relevant provision | Status |
|---|
| FDA 21 CFR §175.105 | Adhesives for food packaging | Compliant when used as component of adhesive |
| FDA 21 CFR §176.170(c) | Components of paper and paperboard in contact with aqueous and fatty foods (Table 2) | Permitted as a continuous film surface‑sizing agent |
| EU Plastics Regulation (EU) No 10/2011 | PM/Ref No. 15237, specific migration limit (SML) = 60 mg/kg | Listed monomer‑only plastic exclusion; SML not applicable |
| German BfR Recommendation XXXVI | Paper and board for food contact | Listed under unplasticised PVOH |
| REACH (EC) No 1907/2006 | Registration number typically 01-2119480006-33 | Fully registered by manufacturer consortium |
When pre‑wetted 728F powder is introduced directly into a hot‑melt compounding operation for water‑soluble injection‑moulded cores, the rheological demands differ sharply from solution‑based processes. On a co‑rotating twin‑screw extruder with an L/D ratio of
40:1 and a modular screw profile incorporating two kneading‑block zones, processing at a melt temperature of
185 °C and a die pressure of
8.5 MPa yielded strands with marginal melt strength, causing frequent strand‑break events below a haul‑off speed of
12 m/min. The addition of
2.5 wt% poly(ethylene oxide) (Mv
400,000 g/mol) raised the extensional viscosity, and the maximum stable draw ratio increased from
4.2 to
7.1. Published data for this specific configuration is limited, but the observed improvement is consistent with the hydrogen‑bonding interactions between PEO ether oxygens and PVOH hydroxyl groups that slow molecular relaxation. In contrast, attempts to improve melt strength by blending with a fully hydrolysed grade resulted in a heterogeneous morphology with discrete crystalline domains visible under polarized‑light microscopy, causing a catastrophic die‑head pressure fluctuation of ±
3 MPa when the ratio exceeded
20%.
Exposure of dry 728F to amine‑based additives must be avoided in upstream material handling. Residual secondary or tertiary amines, even at
0.1% by weight, catalyse an alkaline transesterification reaction that regenerates vinyl acetate monomer and creates crosslinked microgels. This was documented in a case where a pneumatic conveying line had previously transported an amine‑neutralised dispersant; subsequent 728F batches developed gel specks visible in a
2% aqueous solution as a filter residue exceeding
200 ppm on a
125‑micron sieve, forcing the entire batch to be downgraded to waste. Therefore, dedicated stainless‑steel conveying lines and a strict purging protocol with a
5‑wt% citric acid rinse are mandatory when a multi‑use facility handles 728F among other polymer additives.
