ÇKP PVA BP-28

Introduced as a partially hydrolyzed grade within the Chang Chun Petrochemical polyvinyl alcohol portfolio, CCP PVA BP-28 is characterized by a 4 % aqueous solution viscosity of 27.0–33.0 mPa·s at 20 °C and a hydrolysis degree of 86.5–89.0 mol%, both determined according to JIS K6726. The polymer, a vinyl alcohol-vinyl acetate copolymer, carries an ash content below 1.0 % (JIS K6725) and volatile matter not exceeding 5.0 %. The pH of a 4 % solution falls between 5.0 and 7.0. These specifications place BP-28 in the intermediate-viscosity tier of partial hydrolysis PVA, offering a rheological profile that bridges low-viscosity penetrants and high-molecular-weight film-formers. Residual vinyl acetate monomer is typically controlled below 0.3 %, bringing the grade within the monomer limits described in FDA 21 CFR 175.105 for indirect food-contact adhesives and compatible with the overall migration conditions of EU Regulation (EC) No 10/2011 when formulated appropriately.

What Distinguishes Partial Hydrolysis Grade BP-28 from Fully Hydrolyzed and Lower-Viscosity Counterparts?

Fully hydrolyzed PVA (hydrolysis ≥ 98.0 mol%) derives cold-water insolubility and maximal hydrogen-bond density from the near-complete absence of residual acetate groups. In contrast, BP-28 retains 11.0–13.5 mol% acetate, which imparts cold-water solubility, reduced crystallinity, and a lower melting point. These properties translate to re-pulpability advantages in paper applications and easier machine clean-up. Comparative tensile strength of cast films, tested per ASTM D882‑18 after conditioning at 23 °C and 50 % RH, reveals that films from a 10 % BP-28 solution exhibit a tensile strength at break of 45–55 MPa and elongation exceeding 200 %. A lower-viscosity partially hydrolyzed grade such as BP-20 (4 % viscosity 20.0–25.0 mPa·s) yields films with tensile strengths around 35–42 MPa, while a fully hydrolyzed grade of similar viscosity (BF‑17, viscosity 25.0–31.0 mPa·s) develops strengths above 65 MPa but sacrifices re-dispersibility and requires dissolution holds above 90 °C. The table below summarises key quality parameters across the immediate grade family.

Comparative Specification Overview – Partially Hydrolyzed and Fully Hydrolyzed PVA Grades

Property	Test Method	BP‑20	BP‑24	BP‑28	BF‑17 (Fully Hydrolyzed)
Viscosity (4 % aq., 20 °C)	JIS K6726	20.0–25.0 mPa·s	24.0–29.0 mPa·s	27.0–33.0 mPa·s	25.0–31.0 mPa·s
Hydrolysis degree	JIS K6726	86.5–89.0 mol%	86.5–89.0 mol%	86.5–89.0 mol%	98.0–99.0 mol%
Ash content	JIS K6725	≤1.0 %	≤1.0 %	≤1.0 %	≤1.0 %
Volatile matter	JIS K6725	≤5.0 %	≤5.0 %	≤5.0 %	≤5.0 %
pH (4 % solution)	JIS K6726	5.0–7.0	5.0–7.0	5.0–7.0	5.0–7.0

The increase in viscosity from BP‑20 through BP‑28 reflects a higher weight-average molecular weight (typical degree of polymerization approximately 2400 for BP‑28 versus 1800 for BP‑20). This difference is decisive in film strength and cohesive energy but requires higher dissolution energy and limits the maximum practical solids concentration in tower or jet cookers when processing time is constrained. In a high-speed corrugating adhesive line operating at 300 m/min on a double-backer station, the addition of BP‑28 as a secondary polymer to a Stein‑Hall carrier starch formulation improves green bond development over lower-viscosity PVA grades without the excessive stringing encountered with fully hydrolyzed types. The base adhesive is prepared at a starch solids of 22–24 % and supplemented with BP‑28 at 3–5 % on dry starch weight. A typical addition sequence premixes BP‑28 with water at 90 °C in a jet cooker at a retention time of 120 seconds to guarantee complete hydration, evidenced by a clarity target of <5 NTU nephelometric turbidity. The cooked PVA solution is then cooled to 60 °C before blending with the gelatinized carrier starch and raw starch slurry. Final adhesive viscosity is adjusted to 1800–2200 mPa·s (Brookfield RV, No. 4 spindle, 20 rpm, 40 °C) using water trim. In this range, the rheological contribution of BP‑28 stabilizes water retention at the glue line, keeping the gelatinization point of the raw starch within the 62–66 °C window on the heating plates and reducing wash-boarding defects. Process data from a 2500 mm wide corrugator documented a 12–15 % reduction in edge-bond failure rate when switching from a BP‑24‑modified formulation to BP‑28, attributed to higher film strength at the same application solids. A critical operational boundary exists in pH control: when the final adhesive pH drifts below 5.5, the protective colloid action of BP‑28 is partially lost, and gelation risk increases. A phosphate buffer addition of 0.1–0.2 % on total liquid weight is standard when starch acid modification is used.

When Replacing Oxidized Starch with PVA in Recycle-Friendly Paperboard Coatings

Surface sizing of recycled containerboard with oxidized starch provides short-term strength but introduces repulping challenges and yellowing under thermal exposure. Substituting 50 % of the oxidized starch with BP‑28 in a size-press formulation delivers measurable surface strength gains while maintaining repulpability compatible with alkaline flotation deinking systems. The coating is prepared by first wetting out BP‑28 in cold water under high-shear mixing in a rotor-stator disperser (e.g., IKA Ultra‑Turrax UTL 1000 at 3000 rpm) to prevent agglomerate formation, followed by direct steam injection to 85 °C and a 30‑minute hold under gentle agitation. Starch is gelatinized separately at 95 °C and blended in. A rewetting agent of polyglycol ether type at 0.5 % on total solids is required to overcome the rapid film‑formation tendency of BP‑28 on cool paper surfaces; without it, incomplete levelling creates an uneven starch‑rich surface. A defoamer based on hydrophobic silica in polyether dispersion is added at 0.3 %. Application via a film‑transfer size press (gate‑roll metering, 1.2 mm nip gap) with a sump temperature of 55 °C and a solids content of 6.0 % yields a dry pick‑up of 0.35–0.45 g/m² per side when the base sheet is uncoated test liner of 135 g/m². IGT surface strength, measured per ISO 3783 with medium‑viscosity oil, increases from a baseline of 2.0 m/s (100 % oxidized starch) to 2.8 m/s with the 50:50 blend, while brightness loss after 24 h at 150 °C is halved. Pre‑drying of the PVA powder is mandatory when ambient relative humidity exceeds 60 %; storage in sealed containers after opening is advisable because BP‑28 absorbs moisture to equilibrium values of 8–10 % at 65 % RH, and clumped powder extends dissolution time and may leave insoluble “fish‑eye” residues. Film Strength Develops Only After Complete Dehydration of the Hydroxyl Network In casting of unsupported water-soluble films for detergent pouches or agrochemical packaging, the drying profile of the BP‑28 solution dictates defect density and mechanical properties. A 12 % aqueous solution de‑aerated under –0.08 MPa vacuum and knife‑coated onto a chrome‑plated belt at a wet thickness of 0.8 mm must pass through a three‑zone dryer programmed at 80 °C /100 °C /70 °C over 18 minutes. Residual moisture exceeding 8 % leads to blocked‑in plasticization that depresses tensile modulus, while over‑drying below 4 % triggers micro‑cracking from differential shrinkage. The acceptable processing window for BP‑28 in this configuration is ±2 °C in zone‑2 temperature to avoid blistering. Produced film thickness is typically 45–55 µm. Physical properties tested at 23 °C and 50 % RH after 48 h conditioning follow ASTM D882‑18: tensile strength at break 48–52 MPa, elongation at break 220–260 %, and tear resistance (Elmendorf, ASTM D1922) of 420–480 gf. For comparison, a film cast from a lower‑viscosity BP‑24 under identical conditions records 38–44 MPa tensile strength. This difference is critical where the film must resist puncturing during high‑speed forming, and explains the selection of BP‑28 over BP‑24 for vertical form‑fill‑seal operation retrofitted with plunger‑type pre‑forming stations.

Viscosity Decay Under Sustained Shear and Its Impact on Slot‑Die Adhesive Application

In fully automated laminating lines where adhesive is recirculated through a slot‑die head, BP‑28 is frequently preferred over BP‑20 for its slower rate of shear‑induced viscosity loss. A recirculation loop comprising a positive‑displacement gear pump (flow rate 12 L/min), a 250 µm slot‑die, and a holding tank at 45 °C maintains a 15 % solids BP-28 adhesive. Over a 48‑hour continuous operation, viscosity measured by Ubbelohde capillary (DIN 51562‑1) at a reference shear rate of 500 s⁻¹ decreases by 8–12 % from an initial value of 780 mPa·s. The same protocol applied to BP‑20 results in a drop of 15–20 %, accompanied by a widening of the molecular weight distribution detectable as a shoulder on the low‑molecular‑weight side in GPC‑RI traces. The improved shear stability of BP‑28 reduces the frequency of make‑up addition from once per shift to once every two shifts, thereby decreasing material waste and manual intervention. Nevertheless, the operator must monitor the tank level daily and perform a hold‑back viscosity check against a fresh reference; published field data indicate that once viscosity declines by more than 15 %, the peel adhesion on polypropylene substrates (measured per ASTM D903‑98 modified with a 180° angle) typically falls below 0.5 N/mm, compared with the 0.70–0.85 N/mm benchmark for fresh BP‑28 adhesive. This threshold is reached sooner when the pump operates at capacities exceeding 70 % of its nominal rating, as cavitation‑induced bubble collapse accelerates chain scission. The addition of a non‑ionic polyurethane thickener has been applied in some installations to mask viscosity decay, but this practice must be qualified for each end‑use because it can interfere with the FDA‑listed status of the unfilled PVA adhesive. Direct dissolution of BP‑28 powder into an existing hot‑water circulating loop without a high‑shear dispersion stage leads to agglomerate formation on a timescale of seconds. The recommended procedure relies on a two‑stage process: an initial cold‑water slurry at 20 % solids in a vessel equipped with a slow‑speed anchor agitator (30 rpm), where wetting is achieved in 15 minutes without heaping, followed by transfer to a jacketed vessel heated to 88 °C ± 2 °C while stirred at 200 rpm with a dual‑flighted helical ribbon impeller. Complete solubilization (assessed by a draw‑down sample filtered through a 100 µm screen with zero gel residue) is attained in 55–70 minutes. This dissolution window must be rigorously maintained; temperature overshoots beyond 93 °C accelerate acetic acid release through residual ester hydrolysis, lowering pH into the 4.0–4.5 range and promoting acid‑catalyzed intra‑molecular acetalization that reduces final viscosity and creates insoluble domains. Incompatibility with amine‑containing additives (typical pH adjusters) is documented: even 0.05 % of triethanolamine added at the dissolution stage creates a rising‑pH environment that suppresses hydrolysis yet can interact with residual acetate groups during drying, causing discoloration and tack abnormalities. Consequently, neutralization, if required, is executed with dilute sodium hydroxide only after solution cooling to <30 °C and with slow injection under high‑turbulence inline mixing. In paper tube winding where BP‑28 is combined with a clay‑based filler, the clay addition must be post‑dispersed after PVA hydration to avoid competitive water absorption. A Cowles dissolver (tip speed 18 m/s) disperses kaolin into the cooled PVA solution at a filler loading of 40 parts per hundred PVA solids. The resultant adhesive, with a viscosity target of 4000–6000 mPa·s (Brookfield LV, No. 6 spindle, 20 rpm, 25 °C), yields a paste that exhibits rapid wet tack on spiral tube winders running at 80 m/min. The clay‑containing formulation is subject to a borax‑compatibility constraint: borax (sodium tetraborate decahydrate) added as a gelation agent for quick bond set must not exceed a molar ratio of 0.015:1 (borate:vinyl alcohol unit); exceeding this causes irreversible gelation within the supply lines and is a documented cause of unscheduled downtime when premix recipes are scaled without titrimetric borax‑demand verification. In one multi‑line plant, a batch prepared from BP‑28 with ash content near the upper limit (0.9 %) gelled prematurely at a ratio of 0.018:1, whereas typical lots at 0.5 % ash tolerated 0.020:1 without stability loss, underscoring the necessity of ash‑adjusted borax dosing.