Polyvinyl alcohol grade
PVOH 8850 is a partially hydrolysed, medium-molecular-weight thermoplastic resin whose solution viscosity and interfacial activity place it within the narrow process corridor for protective-colloid-stabilised vinyl acetate dispersions. The product code typically designates a nominal degree of hydrolysis of
88.0 ± 1.0 mol% (residual acetyl content
10–12 wt%) and a dynamic viscosity of
48–52 mPa·s when measured as a
4% w/w aqueous solution at
20 °C with a Brookfield LVT viscometer per
ISO 3105:1994. Ash content remains below
0.5% (as Na₂O,
ISO 16906), and bulk density falls in the range
0.40–0.55 g/cm³. These specifications position PVOH 8850 as a primary suspending agent where a balance between colloidal protection, water sensitivity, and thermal processing is required—distinguishing it from both low-hydrolysis (
72–80 mol%) grades that deliver extreme surface activity but poor film strength and from fully hydrolysed (
≥98 mol%) grades that exhibit high crystallinity and reduced cold-water solubility.
Emulsion Polymerisation: Interfacial Grafting and Nucleation Control in VAE Co-polymers
In semi-batch emulsion polymerisation of vinyl acetate-ethylene (VAE), PVOH 8850 functions as both a steric stabiliser and a reactive co-surfactant. The residual acetyl groups undergo hydrogen abstraction by persulphate or redox initiators, creating macroradicals that graft vinyl acetate monomer onto the PVOH backbone. This grafted copolymer, forming at the particle-water interface, reduces interfacial tension to
1–3 mN/m and dictates final latex particle size distribution. Typical continuous-phase concentration of PVOH 8850 ranges from
4 to 8 phr (parts per hundred monomer). At the lower end, particle nucleation becomes stochastic and batch-to-batch coefficient of variation in mean particle diameter can exceed
15%; above
8 phr, elevated aqueous-phase viscosity causes insufficient heat transfer in jacketed glass-lined reactors, often manifesting as microgel formation during the exothermic ethylene incorporation phase.
On production-scale reactors (e.g.,
10 m³ Pfaudler vessels with anchor impellers running at
40–60 rpm), dissolution of PVOH 8850 must be executed as a discrete cook step at
90–95 °C for
90–120 minutes using deionised water. Incomplete solvation yields fisheye defects that appear later as translucent specks in cast films. A differential scanning calorimetry trace (DSC,
ISO 11357-3:2018) of the isolated polymer reveals a broad melting endotherm peaking at
195–205 °C after grafting, compared to
225–230 °C for the ungrafted PVOH backbone, indicating disrupted crystallinity that benefits film coalescence under ambient cure profiles.
What Distinguishes 88 mol% Hydrolysis from Fully Hydrolysed PVOH Grades in Film and Barrier Performance?
The primary differentiator is the crystalline fraction: PVOH 8850 develops a crystallinity index of
18–22% (by wide-angle X-ray diffraction, Cu Kα,
2θ = 19.5°) when cast and dried at
40 °C, whereas a
98 mol% hydrolysed grade reaches
35–40%. This translates into an oxygen transmission rate (OTR) at
53% RH,
23 °C of
0.8–1.2 cm³·mm/(m²·day·atm) for PVOH 8850 films plasticised with
10 wt% glycerol, determined by
ASTM D3985-17. By contrast, a
≥98 mol% grade under identical plasticiser loading yields OTR below
0.15 cm³·mm/(m²·day·atm), a consequence of highly ordered hydrogen-bonded layers. The trade-off is cold-water solubility: PVOH 8850 achieves
95% dissolution in water at
20 °C within
45 minutes, whereas the
98 mol% resin requires sustained heating above
70 °C—a limitation that excludes it from low-temperature wash-off adhesives and repulpable paper coatings.
Tensile properties also diverge. Films conditioned at
50% RH per
ISO 527-3:2018 exhibit a tensile strength at break of
35–42 MPa and elongation at break of
250–320% for PVOH 8850 plasticised with
12 phr sorbitol. The fully hydrolysed analogue reaches
60–70 MPa but fractures at
100–140% elongation. This toughness-stiffness inflection is exploited in water-dispersible packaging laminates where tear initiation resistance during converting is prioritised over ultimate barrier.
Without a heading, a critical application-of-difference arises in paper sizing: surface application of PVOH 8850 at
3–5 g/m² dry coat weight on kraft linerboard using a Metering Size Press at
800 m/min speeds produces an Oken wax pick value of
18–20 (
TAPPI T 459). The
98 mol% grade develops pick values exceeding
22 but requires doctor blade pressure adjustments to overcome rheological shear-thinning differences that lead to streaking at speeds above
600 m/min.
| Parameter | PVOH 8850 (88 mol%) | Low-Hydrolysis PVOH (78 mol%) | Fully Hydrolysed PVOH (≥98 mol%) |
|---|
| Hydrolysis (mol%) – ISO 15023-1:2017 | 87.0–89.0 | 77.0–79.0 | 98.0–99.0 |
| 4% Aq. Viscosity (mPa·s, 20°C) – ISO 3105 | 48–52 | 25–32 | 55–65 |
| Water Dissolution Temp. (°C) | 20–25 | ≤10 (cold instantaneous) | 70–85 |
| Crystalline Melting Point (°C) – ISO 11357-3 | 190–200 | 160–170 | 220–230 |
| OTR (cm³·mm/(m²·day·atm), 53% RH) – ASTM D3985 | 0.8–1.2 | 2.5–3.5 | 0.05–0.15 |
| Film Tensile Strength (MPa, 50% RH) – ISO 527-3 | 35–42 | 15–22 | 60–70 |
| Surface Tension (mN/m, 1% aq.) – Du Noüy ring | 46–48 | 42–44 | 50–53 |
Adhesive Formulation: Extending Open Time Without Sacrificing Green Strength
In polyvinyl acetate-based wood adhesives (D3 classification per
EN 204), PVOH 8850 is dosed as a
2–4 wt% post-added protective colloid to the fully polymerised PVAc emulsion. Its function shifts from particle stabilisation in-situ to rheology modification and water-retention control in the wet adhesive film. At
3 wt% addition on solids, the steady-shear viscosity at
1 s⁻¹ (cone-plate,
25 °C) rises from
8–12 Pa·s to
25–35 Pa·s, producing a pseudoplastic profile that permits clean roll-coating at
0.5 mm wet film thickness while resisting drip on vertical yakisugi panels during open assembly.
The residual acetate groups in PVOH 8850 retard the rate of water evaporation relative to a fully hydrolysed grade by
15–20% under identical airflow of
0.5 m/s and
65% RH, extending open time from
6–8 minutes to
10–12 minutes as measured by the no-clamp fibre-tear test on beech veneer (
EN 205). Despite this moisture affinity, the wet-tack force after
3 minutes compression remains at
2.5–3.0 N/cm², sufficient to prevent spring-back in bent lamination without requiring polyisocyanate crosslinkers. Published data for long-term durability of this specific formulation under cyclic humidity loading is limited, though internal quality checks routinely use
24-hour cold-water immersion (
EN 204 D3) with pass criteria of
>6 N/mm² lap shear on beech.
When Polyol Plasticisers Are Compounded Above 15 phr: Melt Flow and Die-Lip Build-Up
Thermoplastic processing of PVOH 8850—blown film extrusion through a single-screw extruder with a
L/D 30 barrier screw and a
0.8 mm annular die gap—relies on the suppression of the melting point via water or glycerol. However, glycerol loadings exceeding
15 phr combined with barrel temperatures above
195 °C in the metering zone induce phase separation evidenced as an exudate at the die-lip. This exudate, identified by FTIR as glycerol-enriched PVOH oligomer, crystallises within
60 seconds of die-face contact, creating scratch defects at a frequency of
3–5 per linear meter. The solution is a stepwise temperature profile: feed zone
160 °C, compression
180 °C, metering
190 °C, and die
185 °C, combined with pre-drying of PVOH 8850 pellets under dehumidified air at dew point ≤
–30 °C for
4 hours. At ambient storage conditions above
60% RH, moisture absorption can exceed
2.5 wt% within
8 hours, leading to steam bubble formation at the nip that compromises gauge uniformity beyond ±
8% of nominal thickness.
Incompatibility with amine-based processing aids must be noted: addition of long-chain fatty acid amides as anti-slip agents triggers imine formation at ester carbonyl sites under the high-shear (
500–800 s⁻¹) zone near the screw tip, causing yellowing (
YI increase by 8–12 units,
ASTM E313) and a loss of clarity (
haze rise from 4% to over 30%,
ASTM D1003). Non-ionic polyethylene glycol additives at
0.5–1.0 phr serve as the primary internal lubrication alternative without observable discolouration.
Migration Behaviour in Multi-Layer Biodegradable Packaging and the Relevance of 88 mol% Hydrolysis
Where PVOH 8850 is co-extruded as a tie-layer between thermoplastic starch (TPS) and polylactic acid (PLA), the
88 mol% hydrolysis degree provides an optimal diffusion coefficient of
2.1 × 10⁻¹³ m²/s for water vapour at
38 °C,
90% RH, measured by cup method
ASTM E96. This value is
40% lower than that of a
78 mol% grade and
60% higher than a
98 mol% grade, meaning that interfacial delamination from moisture-induced swelling of the starch layer is delayed without creating an impermeable barrier that forces water accumulation at the PLA boundary. Peel adhesion between PVOH 8850 and corona-treated PLA (surface energy
52 dyn/cm,
DIN 55660-2) is
2.8–3.4 N/15 mm (
ASTM F904), sufficient to prevent tunnel-slip during thermoforming at draw ratios up to
3:1 and preheat temperatures of
90–100 °C. In composting bio-reactors under
EN 13432 conditions (
58 °C,
>50% moisture), the grade achieves
>90% disintegration in
12 weeks, comparable to the TPS substrate and significantly faster than the PLA skin layer, eliminating persistent microplastic accumulation.
The viscosity specification of
48–52 mPa·s was intentionally targeted for this processing window: molecular weight below
30 kDa facilitates melt interdiffusion with TPS but increases extraction by food simulants beyond the
10 mg/dm² overall migration limit of
EU Regulation 10/2011; molecular weight above
60 kDa raises the zero-shear viscosity beyond
2000 Pa·s at
190 °C, exceeding the capacity of standard co-extrusion feedblocks rated for
1500 Pa·s. Thus, the defined viscometric limits are product-critical parameters rather than arbitrary quality-control intervals.
| Regulatory/Standard Reference | Test Focus | PVOH 8850 Compliance Status |
|---|
| FDA 21 CFR 175.105 | Adhesives for indirect food contact (dry food) | Conforms as component at ≤5 wt% of adhesive solids |
| FDA 21 CFR 176.170 | Paper and paperboard in contact with aqueous/fatty food | Conforms as surface-size interlayer |
| EU 10/2011 (PIM) | Plastic materials in contact with food (overall migration) | Passes 10 mg/dm² limit after 10 days at 40 °C (simulant D1) |
| EN 13432 | Biodegradable packaging (industrial composting) | Disintegration ≥90% in 12 weeks; ecotoxicity pass on cress germination test |
| REACH (EC 1907/2006) | Registration of substance, no SVHC listing | Registered; monomer residual vinyl acetate <5 ppm |
| ISO 14001:2015 | Environmental management during production | Supplier-certified; solvent-free aqueous process |
Laser diffraction particle size analysis (Malvern Mastersizer 3000, dry dispersion at
2 bar) of PVOH 8850 powder reveals a median particle diameter (
D₅₀) of
250–400 µm, with
D₉₀ below
850 µm. Bulk handling in continuous gravimetric feeding systems (Brabender or Coperion K-Tron) requires vibratory tray agitation at amplitudes
0.5–1.0 mm to prevent ratholing in hoppers exceeding
2 m³ capacity. Feed rates below
10 kg/h on single-screw side stuffers must be validated against screw speed increments of
5 rpm to avoid surging. Plant-scale experience documents that exposure to ambient humidity above
60% RH for periods exceeding
4 hours increases compressibility by
18–22%, necessitating a reduction in screw fill factor from
30% to
22% to prevent over-torque shutdowns.
