Ningxia Dadi PVA 2688

Ningxia Dadi PVA 2688 is a partially alcoholysed polyvinyl alcohol resin in the high-viscosity segment of the manufacturer’s alcoholysis series. The grade code separates into a nominal degree of polymerization of 2600 and a nominal alcoholysis degree of 88 mol%. Supplier certificates of analysis for the grade typically list a 4% aqueous solution viscosity of 44.0–52.0 mPa·s at 20 °C under GB/T 12010.3-2010, volatile matter not exceeding 5.0 wt%, ash not exceeding 0.5 wt%, and a pH of 5.0–7.0 in a 4% aqueous dispersion. The resin is supplied as white to off-white granules. Because the alcoholysis degree is intermediate, the material is classified as partially hydrolysed; because the chain length is high, it generates more viscous aqueous solutions than lower degree-of-polymerization grades such as PVA 2488 and dissolves more readily than fully hydrolysed PVA 1799.

What Distinguishes the 2688 Grade Within the Dadi Partial-Alcoholysis Series?

The primary differentiating variable is molecular weight. In dilute aqueous solution, PVA 2688 exhibits higher solution viscosity than PVA 2488 at equal solids because the longer chain increases hydrodynamic radius and entanglement density. The residual acetate content of 12 mol% disrupts interchain crystallites and lowers the minimum dissolution temperature relative to PVA 1799, but the high degree of polymerization still requires controlled heating for complete solubilisation. Film tensile properties measured by GB/T 13022-1991 generally increase with degree of polymerization at constant alcoholysis degree, although direct quantitative comparison between 2688 and 2488 requires identical plasticizer content and conditioning at 23 °C and 50% relative humidity. Published data for the exact tensile increment of Ningxia Dadi 2688 over 2488 is limited; substitution trials should therefore include cast-film testing rather than relying solely on solution viscosity.

PVA 2688 belongs to the 26-series; the first two digits denote nominal degree of polymerization, and the final two digits denote nominal alcoholysis degree. The 88 mol% hydrolysis level is commonly selected for adhesives, sizing, and emulsion polymerization because it balances cold-water solubility with film water resistance. Fully hydrolysed 99 mol% grades require hot-water dissolution and are used where water resistance after drying is critical. Within the 88 mol% series, degree of polymerization is the main lever for solution viscosity and film strength.

Compared with PVA 1788, the 2688 grade has a longer chain and consequently higher aqueous viscosity at equivalent concentration. The practical difference is most visible in sizing and emulsion applications where the low-viscosity 1788 pumps more easily but contributes less film toughness, while 2688 increases machine load and wet pick-up. The substitution ratio is not linear; a direct equal-solids replacement can raise measured viscosity to an extent that depends on concentration, temperature, and shear rate. Published data for the exact shear-thinning index of Ningxia Dadi 2688 is limited, so pilot-scale viscosity measurement under the intended shear regime is required.

Dissolution behaviour is the main process constraint. The powder is best pre-slurried in cold water of 10–30 °C using a high-speed disperser at 500–800 rpm before steam heating to 85–95 °C. In a jacketed stainless-steel vessel with an anchor agitator running at 30–60 rpm, complete dissolution of an 8 wt% solids solution typically requires 45–60 min after the slurry reaches 90 °C. Above 10 wt% solids, vortex formation in unbaffled tanks can entrain air; a baffled vessel with low-shear impeller and vacuum degassing at −0.06 to −0.08 MPa reduces microbubble defects in subsequent film or size-press applications. The solution is shear-thinning, and the measured viscosity is dependent on spindle speed and thermal history, so the certificate value should be reproduced with the same rotational viscometer configuration before lot acceptance.

Production-scale dissolution failures usually originate from adding dry granules directly into hot water. The outer gel layer hydrates rapidly and prevents water penetration, producing semi-swollen fish-eye defects. Cold-water pre-dispersion at 10–30 °C minimizes this, and the powder should be added at a controlled rate that maintains visible surface turnover. The viscosity peak during dissolution occurs before complete solubilisation because swollen granules create transient high resistance; anchor agitator torque is typically highest around 60–70 °C before the granules fully dissolve. Monitoring torque rather than time provides a more reliable endpoint.

Viscosity, Alcoholysis Degree, and Ash Content on Typical Certificates

Typical certificate-of-analysis parameters for Ningxia Dadi PVA 2688 follow the supplier’s control ranges. The values below are representative ranges reported in technical literature for 26-series partially hydrolysed polyvinyl alcohol; they are not batch-specific guarantees.

Ash and volatile matter are relevant for continuous gravimetric feeding. Moisture uptake at storage relative humidity above 60% can raise volatile matter beyond the certificate limit and reduce the exact polymer content delivered to a loss-in-weight feeder. If storage humidity exceeds 60%, drying at 60–80 °C for 2–4 h in a forced-air tray dryer restores the targeted solids content. Batch-to-batch variance in solution viscosity of ±3 mPa·s can shift wet pick-up in size-press operations; incoming material should be checked against the internal specification at 4% solids and 20 °C before formulation adjustment.

The certificate parameters do not include molecular weight distribution. Even when degree of polymerization is fixed at 2600, the breadth of the distribution can affect high-shear viscosity and film properties. Published data for the molecular-weight distribution of this specific grade is limited; requests for gel permeation chromatography data should be directed to the supplier when the resin is used in critical high-extension film or controlled-release applications.

The grade should be dissolved with pH control during prolonged heating. At pH above 9.0 and temperatures above 80 °C, residual acetate groups can undergo further hydrolysis, shifting the effective alcoholysis degree and changing solution viscosity. Acid-catalysed chain scission may also occur if the solution is held below pH 3.0 at elevated temperature. These boundaries define the safe cooking window for processes that require extended holding before application.

In vinyl acetate and vinyl acetate-ethylene emulsion polymerization, PVA 2688 functions as a steric protective colloid when the aqueous phase is preheated and charged before initiation. Common published recipes use 0.5–5.0 phr on monomer; the high molecular weight raises latex viscosity and improves colloid stability under shear, but also increases reactor torque. In a jacketed reactor with a retreat-blade impeller below 120 rpm, adding PVA 2688 at the upper end of that range can reduce heat transfer because wall-film viscosity increases. High-shear rheology should be measured with a capillary viscometer at 10,000–100,000 s⁻¹ when the latex is formulated for blade coating.

Particle nucleation is influenced by colloid concentration. At high PVA 2688 loadings, homogeneous nucleation can be suppressed and the particle size distribution may broaden, altering the final latex rheology. Published data for the exact particle-size shift in vinyl acetate-ethylene systems using this grade is limited; pilot-scale polymerizations at 1 L and 10 L scale are recommended before production-scale commitment.

When 2688 Replaces 2488 or 1799 in Emulsion Polymerization and Paper Coating

Substitution of PVA 2688 for PVA 2488 increases dissolution time and working viscosity; therefore agitation speed and heating time are typically increased. In a dual-blade size press, higher solution viscosity at equal solids can raise blade load and reduce machine speed unless solids are reduced by 1–2 wt%. Replacement of PVA 1799 by PVA 2688 is chosen when lower dissolution temperature and easier clean-up are required, but the dried film will have lower water resistance because residual acetate groups reduce crystallinity and increase swelling.

For paper surface sizing, PVA 2688 is typically run at 6–12 wt% solids in the size press at 50–60 °C. The high molecular weight promotes film continuity and surface strength as evaluated by TAPPI T459 on conditioned handsheets, but the actual pick value is furnish-dependent and is not predicted solely from PVA viscosity. In textile warp sizing, blends with starch at 10–12 wt% total solids are used; PVA 2688 contributes film toughness and abrasion resistance, while the addition rate is limited by sizing-box viscosity and squeeze-roll pickup control.

For water-soluble film and adhesive compounding, PVA 2688 is typically used at 4–8 wt% solids after cooking; the high molecular weight increases open time and film cohesion but also reduces flow and wet-out compared with lower-viscosity grades. Formulations for paper tubes, cores, and carton adhesives may need wetting-agent adjustment because the high solution viscosity can reduce penetration into porous substrates.

Regulatory compliance is formulation-dependent. Polyvinyl alcohol is a polymer under REACH; the supplier safety data sheet is the normative source for registration status. In food-contact adhesive and paper applications, the formulated article must comply with the applicable positive-list requirements such as FDA 21 CFR 175.300, and the resin alone does not confer finished-article compliance.

Incompatibilities include borate ion and certain multivalent metal salts at elevated pH; boric acid at concentrations above a system-specific threshold causes gelation through diol complexation. The grade should not be combined with amine-based additives that shift pH above 9.0 during prolonged heating because ester groups can hydrolyse further and change the effective alcoholysis degree. Storage should avoid temperatures above 40 °C and relative humidity above 60% to prevent lumping and moisture pickup. Prolonged heating above 95 °C under alkaline conditions may shift the hydrolysis distribution and viscosity; therefore cooking should be stopped once the solution clears and no undispersed particles remain.