On a 10 m³ stainless-steel reactor running polyvinyl acetate homopolymer synthesis at 68–72 °C, the difference between a cohesive wood adhesive and an unworkable gel observed on the scraper blade frequently originates from protective colloid selection. PVOH 745, a partially hydrolyzed polyvinyl alcohol grade with a nominal hydrolysis degree of 87–89 mol% and a 4 % aqueous solution viscosity of 5–7 mPa·s (measured per ISO 1652:2011), is pre-dissolved in demineralized water at 90–95 °C in a jacketed make-down vessel equipped with a high-torque anchor agitator. Once the 10 % stock solution cools to 40 °C, it is charged into the main reactor to give a dry PVOH loading of 3.8–4.5 wt% based on total vinyl acetate monomer. Monomer feed is introduced semi-continuously over 4–5 hours under a nitrogen blanket, with a potassium persulfate redox initiation system maintaining a steady polymerization rate. The final emulsion exhibits a Brookfield LVF viscosity of 12,000–18,000 mPa·s (ASTM D1084, spindle 4, 20 rpm), a volume median particle diameter D[4,3] below 1.4 µm, and passes 3 cycles of freeze-thaw stability testing at −10 °C without coagulum formation. This emulsion is formulated with dibutyl phthalate or a benzoate ester plasticizer at 12–18 phr and thickened with polyurethane associative thickeners to produce a D3-grade woodworking adhesive conforming to EN 204:2016 for interior joint assemblies. A critical processing boundary arises during the chase phase: if residual vinyl acetate exceeds 1.2 % and PVOH 745 grafting becomes excessive, the emulsion can exhibit a transient “viscosity hump” exceeding 45,000 mPa·s, which mechanical stirrers on 15 kW variable-frequency drives may stall unless the initiator addition rate is immediately halved. For indirect food-contact adhesive applications, the dried film complies with FDA 21 CFR 175.105 and FDA 21 CFR 176.170 (components of paper and paperboard in contact with aqueous and fatty foods) when migration testing per EU 10/2011 simulants confirms overall migration below 10 mg/dm². The finished product is supplied in 1,000 L IBCs, and batch-to-batch viscosity drift is minimized by controlling the pre-emulsion PVOH hydrolysis degree to within ±0.5 mol% of the specification midpoint.
What Happens to IGT Dry Pick When PVOH 745 Replaces 25 % of Oxidized Starch at the Size Press?
On a flooded-nip size press running recycled linerboard at 800 m/min, the surface size formulation strongly influences IGT dry pick resistance ( TAPPI T 459 with 3,900 N·s/m tack ink) and 5-min Cobb water absorptiveness ( TAPPI T 441). A typical make-down involves cooking a 9 % solids blend of oxidized corn starch and PVOH 745 at a dry weight ratio of 75:25 in a steam-injection jet cooker at 130 °C, then holding the solution at 60 °C in a supply tank with continuous low-shear recirculation to avoid retrogradation. PVOH 745 contributes a film-forming temperature just above 58–62 °C; when the starch film alone would collapse under the high-shear impulse of offset lithographic printing above 1.2 m/s, the addition of PVOH 745 raises the surface strength index from 3.2 N/m to 4.6 N/m on 135 g/m² testliner. The solution penetrates the sheet by 15–30 µm, measured by cross-sectional Rhodamine B dye staining, and the dried polymer network reduces Parker Print-Surf roughness to below 5.0 µm ( ISO 8791-4), which directly affects coat weight uniformity in downstream flexo preprint. Starch/PVOH compatibility is sensitive to the electrolyte content in the size press circuit; Ca²⁺ levels exceeding 200 ppm from broke-recovery loops will precipitate PVOH 745 as a tacky scum on press rolls, forcing a boil-out every 8 hours. When the paper machine is shut down with the size solution still in the nip, the film dries to a hard glaze that can damage the soft rubber cover (Shore A 70–75); a wash cycle of 10 % acetic acid at 40 °C is necessary to redissolve the deposits. The treated sheets are converted into heavy-duty corrugated containers for fresh produce transport, where the surface treatment must comply with BfR Recommendation XXXVI for dry and moist foodstuffs at ambient temperature.
Size recipes for 40 Ne ring-spun cotton warp yarns incorporating PVOH 745
High-speed air-jet weaving at 900 picks/min subjects warp yarns to cyclic abrasion and yarn-to-yarn friction, necessitating a size film that combines tensile toughness with low shedding. A conventional cooking kettle at 105 °C processes a slurry of PVOH 745 ( 35 kg), thin-boiling acid-modified starch ( 60 kg), and a hydrogenated tallow wax lubricant ( 3 kg) in water to a final volume of 850 L, yielding a solids content of 9.5–10.5 % and a hot viscosity of 45–55 mPa·s at 90 °C (Brookfield LV, spindle 2, 30 rpm). The PVOH 745, with a degree of polymerization around 400–500, provides a lower solution viscosity than fully hydrolyzed grades, which allows the size to penetrate the yarn core under a squeeze-roll pressure of 0.18–0.22 MPa on a multi-cylinder slasher. Size add-on is controlled to 11.0 ± 1.5 % (dry size on dry yarn). Abrasion resistance measured on a Zweigle G551 yarn abrasion tester shows a minimum of 380 cycles to failure, compared to 220 cycles for an all-starch size at the same add-on. Desizing in the finishing plant uses a continuous open-width washer with 0.5 g/L of a bacterial α-amylase at 60 °C for 15 seconds, followed by a 90 °C hot-water rinse; the PVOH 745 fraction dissolves completely, leaving a BOD5 load of approximately 0.15 kg O₂ per kg of PVA in the effluent, which requires an extended aeration treatment of 8–10 hours in the site’s activated sludge basin. The woven greige fabric is subsequently converted into denim apparel. A processing limit arises with compact-spun yarns: size film splitting at the lease rods increases if the PVOH 745/starch ratio exceeds 45:55, because the film becomes too extensible and does not break cleanly under shed opening.
Remoistenable tape lines operating at 150 m/min require an aqueous glue that dries to a non-blocking film within 12 seconds on a steam-heated drum at 120 °C, yet rewets to an aggressive tack within 2 seconds under a moistened sponge roller. PVOH 745 is dissolved at 18–22 % solids in a stirred, jacketed kettle along with glycerol or polyethylene glycol 400 as a humectant at 8–12 phr on dry PVA. The solution is filtered through a 100-mesh screen and applied via a knife-over-roll coater to 55 g/m² calcium-carbonate-filled kraft paper at 5.0–6.5 g/m² dry coat weight. Dried reels are slit and spooled without anti-blocking agents, because the PVOH 745 film at equilibrium moisture content of 4–6 % (conditioned at 23 °C, 50 % RH) maintains a blocking resistance above 45 °C as measured by a GFT heat-seal gradient tester. The rewetting time is verified by a drop penetration test on a standard paper substrate at — the contact angle must fall below 20° in less than 1.5 seconds. The final article is converted into self-adhesive envelope flaps and splicing tapes, where compliance with FDA 21 CFR 175.105 (adhesives) applies if incidental food contact cannot be excluded. Avoid combining PVOH 745 with excessive amounts of low-molecular-weight glycols at formulation temperatures above 70 °C; phase separation can occur when the Hildebrand solubility parameter of the plasticizer-PVA blend exceeds 28 MPa½, resulting in an exudate layer that impairs severability in high-speed converters.
When 20 wt% PVOH 745 is blended into a 90 mol% hydrolysis high-DP PVA for blown film, heat seal initiation temperature drops by 12 °C
Unit-dose detergent pods demand a water-soluble film that seals at a temperature well below the thermal degradation onset of the PVA backbone, so that high-speed vertical form-fill-seal machines can operate at 300–600 pouches/min without charring or pinhole formation. PVOH 745, with its lower molecular weight (M_w approximately 20,000–25,000 g/mol) and 88 mol% hydrolysis, acts as an internal processing aid when pellet-blended at 18–22 % with a high-polymerization-degree (~1,700) 90 mol% hydrolyzed PVA prior to single-screw extrusion. The screw configuration is a L/D 30:1 barrier design with a compression ratio of 3.5:1; barrel temperature zones are set at 85 °C (feed), 170 °C (compression), 185 °C (metering), and 190 °C (die head). The blend’s melt flow index at 190 °C/21.6 kg increases from approximately 4 g/10 min (neat high-DP PVA) to 14–18 g/10 min, which stabilizes the bubble at a blow-up ratio of 2.8–3.2. Film produced at 35 µm thickness exhibits a heat seal strength of 8–11 N/15 mm when sealed at 150 °C, 0.4 MPa, 1.0 s, compared to 165 °C required for identical seal strength with unmodified high-DP PVA. A tight processing window exists: if the die exit temperature drifts above 198 °C, residual acetate groups on PVOH 745 undergo elimination, generating conjugated polyene sequences that discolor the film and reduce tensile elongation at break to below 150 % (ASTM D882). Consequently, the extruder is fitted with three melt thermocouples and a closed-loop barrel cooling circuit to maintain temperature uniformity within ±3 °C. Pre-drying of all PVA pellets to below 0.3 % moisture in a desiccant hopper dryer at 80 °C for 4 hours is mandatory; otherwise, steam bubbles nucleate at adhesion promoters compounded into the film for biodegradability enhancers, creating microvoids detectable by halogen leak testing. The final film complies with the dissolution requirements of EU Regulation 648/2004 on detergents, with > 95 % disintegration in 30 °C water within 30 seconds as per the stirred immersion test method.
Spray-dried dispersible polymer powders for cementitious tile adhesives depend on the colloidal stability conferred by PVOH 745
Aqueous vinyl acetate-ethylene (VAE) copolymer dispersions stabilized solely by PVOH 745 during emulsion polymerization are converted into free-flowing redispersible polymer powders (RDP) through co-current spray drying. The feed emulsion, with a solids content of 52–55 % and a Brookfield viscosity of 2,500–4,500 mPa·s, is atomized via a rotary atomizer at 15,000 rpm into a drying chamber with an inlet air temperature of 115–125 °C and outlet temperature of 58–64 °C. PVOH 745 fulfills two roles: it prevents irreversible latex particle coalescence during the residence time of 3–5 seconds, and after the powder is mixed into a cementitious mortar at 2.5–4.0 wt% on dry cement, it rehydrates rapidly to reform a continuous polymer film that bridges microcracks. The critical parameter for powder quality is the ash content determined at 450 °C, which includes a kaolin anti-caking agent added post-drying at 8–14 wt%; total ash must stay below 15 % to ensure tensile adhesion strength exceeds 0.5 MPa after 28 days of water immersion ( EN 1348). On a Niro production-scale spray dryer, excursions in the outlet temperature above 68 °C cause partial sintering of the PVOH 745 shell around the polymer particles, which manifests as an insoluble “skin” fraction above 4 % when tested per the 150 µm wet-screening method of ASTM C 1305/C 1305M-16. Below 54 °C outlet temperature, residual moisture exceeds 1.5 % and the powder lacks sufficient flowability to pass an 8 mm orifice funnel test without vibration. The RDP is packaged in 25 kg multi-layer paper bags with a polyethylene inner liner and used in C2S1- and C2S2-classified tile adhesives, where the powder must additionally not introduce air-entrainment above 2 % air content in the fresh mortar. A persistent incompatibility arises when PVOH 745-based RDP is combined with high-alumina cement: the alkaline hydrolysis of residual acetate groups at pH > 12.8 in the pore solution releases acetic acid that retards the calcium aluminate hydrate formation, demonstrably doubling the Vicat final setting time from 180 min to over 360 min at 20 °C.
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Polyvinyl alcohol grade 745 is a partially hydrolysed, medium-viscosity resin characterised by a nominal degree of hydrolysis of 74.0 mol% (73.0–75.0 mol%) and a viscosity of 4.8–5.8 mPa·s measured as a 4 % aqueous solution at 20 °C per JIS K6726. The residual acetate content imparts controlled hydrophilicity, lowering the dissolution temperature relative to fully hydrolysed grades while suppressing gelation tendency in cold-water dispersions. Ash content is limited to 0.4 % maximum (ISO 3451‑1), volatile matter to 5.0 % maximum, and pH of the aqueous solution falls between 5.0 and 7.0. This grade serves as a functional binder, film former, and sizing agent across paper conversion, adhesive compounding, textile warp sizing, and temporary protective coatings, where the balance between cold-water dispersibility and moderate water resistance is critical.
What Distinguishes Partially Hydrolysed PVOH Grade 745 from Fully Hydrolysed Counterparts?
Partially hydrolysed grades such as 745 depart from fully hydrolysed polyvinyl alcohols (98–99 mol% hydrolysis) in three operationally decisive properties: aqueous solubility mechanism, thermal processability, and interaction with cellulosic substrates. The blocky distribution of residual acetyl groups along the chain lowers the degree of crystallinity, shifting the onset of solution precipitation to lower temperatures. Literature places the lower critical solution temperature (LCST) for a 74 mol% hydrolysed polymer at approximately 35–45 °C; fully hydrolysed types lack a practical cloud point below 100 °C. Consequently, grade 745 dissolves completely in water at 60–70 °C, whereas a fully hydrolysed grade of comparable molecular weight requires 85–95 °C for full dissolution. Films cast from 745 exhibit lower tensile strength—typically 40–55 MPa at 23 °C and 50 % RH per ASTM D882—compared with 60–80 MPa for a fully hydrolysed, similar-viscosity grade such as 505. However, the elongation at break increases from 100–150 % to 200–300 %, favouring applications demanding flexibility over tensile stiffness. The reduced hydrogen‐bonding density also depresses the melt temperature from 220–230 °C (fully hydrolysed) to 180–200 °C, substantially widening the safe processing window for extrusion and injection moulding.
Table 1 — Comparative specification profile of PVOH 745 and neighbouring grades
| Property | PVOH 745 | PVOH 505 (fully hydrolysed, low viscosity) | PVOH 117 (fully hydrolysed, medium viscosity) |
|---|
| Hydrolysis (mol%) | 73.0–75.0 | 98.0–99.0 | 98.0–99.0 |
| Viscosity, 4% aq. (mPa·s, 20 °C) | 4.8–5.8 | 5.0–6.0 | 25–31 |
| pH | 5.0–7.0 | 5.0–7.0 | 5.0–7.0 |
| Ash (%, max) | 0.4 | 0.5 | 0.5 |
| Volatile (%, max) | 5.0 | 5.0 | 5.0 |
| Dissolution temperature (°C) | 60–70 | 85–95 | 90–98 |
| Film tensile strength (MPa, ASTM D882) | 40–55 | 60–80 | 65–85 |
| Film elongation (%, ASTM D882) | 200–300 | 100–150 | 80–130 |
Processing Window and Melt Rheology Constraints
Thermoplastic conversion of grade 745 demands strict moisture management pre‑processing. The resin must be dried in a desiccant dryer at 80 °C for a minimum of 4 h to achieve a residual moisture content below 0.3 wt%. Moisture exceeding 0.5 wt% induces hydrolytic chain scission during melting, generating acetic acid and resulting in bubble‑ridden extrudate with a reduced intrinsic viscosity. Single‑screw extruders with a length‑to‑diameter ratio of 24:1 to 30:1 and a barrier screw design are preferred; the metering zone should be maintained at 190–220 °C, and the melt temperature must not exceed 225 °C. Beyond 230 °C, differential scanning calorimetry records an abrupt exothermic onset attributed to elimination of water and acetic acid and the formation of conjugated polyene sequences, visibly manifesting as amber discoloration. In injection moulding, clamp forces of 3–5 kN/cm2 projected area and a mould temperature of 30–50 °C are typical. Because the melt viscosity at the recommended processing shear rates of 100–500 s−1 lies in the range of 200–600 Pa·s, cold‑runner systems must be sized to avoid pressure drops exceeding 30 MPa. Processors blending 745 with plasticisers such as glycerol (10–20 phr) or trimethylolpropane will observe a viscosity depression of 20–40 % and a corresponding reduction of the solid‑state glass transition temperature from 58 °C to as low as 25 °C (measured by DMA per ASTM E1640). The addition level is capped at 25 phr to prevent phase exudation during long‑term storage under humid conditions.
In aqueous solution preparation, the resin must first be dispersed under vigorous agitation in water at room temperature, then heated with continuous shear to 65–70 °C to achieve full dissolution. Direct addition of dry powder to hot water above 40 °C causes premature surface swelling and agglomerate formation—commonly termed “fisheyes”—that will not dissolve without sustained high‑shear mixing at tip speeds above 15 m/s. Solutions held at 40–45 °C for more than 6 h may begin to exhibit turbidity due to the LCST; cooling to 25 °C restores clarity, but repeated thermal cycling past the cloud point gradually increases insoluble fractions. For adhesive formulations, the working viscosity is often adjusted to 1 000–3 500 mPa·s (Brookfield, 20 rpm, 25 °C) to balance penetration into porous substrates with adequate green tack.
When Viscosity at Low Shear Limits Metering in Roll Coating
Roll coating lines applying PVOH 745 as a pigment binder or surface size encounter a critical low‑shear viscosity threshold at 0.1–1 s−1. At 10 % solids, the shear‑rate‑dependent viscosity drops from approximately 2 500 mPa·s at rest to below 300 mPa·s under the dynamic gap shear of a two‑roll nip. If the solution temperature rises above the cloud point, phase‑separated micro‑domains increase apparent viscosity by 15–25 %, causing film‑weight drifts exceeding ±2 g/m2 on lightweight papers (30–45 g/m2). Closed‑loop chiller units maintaining the coating pan at 28–32 °C are therefore mandatory. Foaming is controlled through addition of silicone‑free defoamers at 0.05–0.1 % on wet coating weight; excessive defoamer causes cratering visible under ASTM D4062 levelling evaluation.
Adhesion to clay‑coated paperboard as measured by TAPPI T 569 internal bond strength reaches 180–220 J/m2 for PVOH 745 applied at 2.5 g/m2 dry coat weight, outperforming standard oxidized starch by approximately 30 %. The improvement is attributed to specific hydrogen‑bonding interactions between the residual acetyl oxygen and the clay platelet hydroxyls, which are absent in fully hydrolysed PVOH. In textile warp sizing, 8–10 % concentration at 55 °C yields yarn tenacity increases of 12–18 % (assessed by ISO 2062) and hairiness reduction of 40–50 % (Zweigle tester), with the added advantage that desizing requires water at only 60 °C, substantially lower than the 85–90 °C needed for fully hydrolysed sizes.
PVOH 745 is also utilised as a water‑soluble support material in extrusion‑based additive manufacturing, where its interlayer adhesion and dissolution kinetics are critical. Filament dried to below 0.2 % moisture and extruded at 195±5 °C through a 0.4 mm nozzle onto a bed at 45–55 °C yields a void‑free support structure that dissolves completely in agitated water at 35–40 °C within 90–120 min. Build‑plate adhesion is enhanced by pre‑applying a PVOH slurry of the same grade. Variants of this grade have been formulated into filament with a 1.27–1.29 g/cm3 density and a melt flow index (210 °C, 2.16 kg) of 8–12 g/10 min according to ISO 1133‑1.
Storage of the powder requires sealed containers at 15–30 °C and relative humidity below 60 %; exposure to 70 % RH for 48 h increases moisture content to 4–5 %, necessitating re‑drying before melt processing. The grade is incompatible with amine‑based crosslinkers that catalyse premature acetal formation, leading to gel particles that plug 25–50 µm filtration units in coating head supply lines. No adverse reactivity is reported with common polyol plasticisers, borates, or non‑ionic surfactants up to 3 % w/w admixture.
Table 2 — Regulatory compliance framework for PVOH 745 in food-contact and industrial contexts
| Standard / Regulation | Scope | Condition / Limitation | Status |
|---|
| FDA 21 CFR 175.105 | Adhesives | Indirect food contact; good manufacturing practices | Compliant |
| FDA 21 CFR 176.170 | Paper & paperboard in contact with aqueous and fatty foods | Component of coatings, size press additives | Compliant |
| FDA 21 CFR 176.180 | Components of paper & paperboard in contact with dry food | As a surface sizing or binder | Compliant |
| EU 10/2011 | Plastic materials and articles intended to come into contact with food | OML limits; specific migration limit for vinyl acetate monomer <12 mg/kg | Conforms |
| REACH (EC) No 1907/2006 | Registration, Evaluation, Authorisation of Chemicals | Monomer and polymer exempt from registration (Art. 2(9)); SVHC-free | Registered |
| RoHS 2011/65/EU | Restriction of hazardous substances | No restricted substances above threshold | Compliant |
