CW40-758 Emulsão de VAE de alta Tg para revestimentos de superfície e aplicações de papel

CW40-758 is a high-solids, carboxylated vinyl acetate-ethylene (VAE) dispersion engineered to deliver a glass transition temperature (23 °C midpoint, ISO 11357-2:2021) substantially above that of commodity VAE grades, while retaining machine-grade film formation at low minimum film-forming temperatures when properly coalesced. The product is supplied at 55 ± 1 % non-volatile content (ISO 3251:2019, 105 °C, 1 h), with a Brookfield viscosity of 800–1800 mPa·s (ISO 2555:2018, spindle 4, 20 rpm, 23 °C) and a pH of 4.5–5.5 (ISO 976:2013). Its particle size distribution, measured by laser diffraction, centres on a median diameter of 0.25–0.40 µm. CW40-758 is intended for use as a sole binder or co-binder in aqueous surface coatings and paper coating formulations where elevated service temperatures, blocking resistance, and low creep compliance are demanded without sacrificing wet-state flexibility. Unlike acrylic and styrene-acrylic latices of comparable hardness, the ethylene comonomer imparts permanent internal plasticisation that reduces the reliance on external coalescents for film integrity on porous substrates, a feature that directly influences hold-out and binder migration behaviour at high web speeds.

What Distinguishes CW40-758 from Conventional VAE Dispersions?

Conventional VAE dispersions for architectural coatings and paper converting typically exhibit a glass transition temperature below 5 °C, and in many interior decorative grades a Tg of –15 °C to 0 °C is common. Such low-Tg backbones provide inherent softness and low minimum film-forming temperature (MFFT), often 0 °C as neat dispersion, eliminating the need for coalescent in certain low-demand coating lines. The penalty is a pronounced drop in surface hardness, a low Vicat softening point, and a tendency for blocking in stacked or rolled goods when the ambient temperature exceeds 35–40 °C. CW40-758 inverts that relationship by raising the polymer Tg to 23 °C, which raises the Koenig hardness (≈ 45–55 s, ISO 1522:2007) considerably above that of a 0 °C-Tg VAE (<10 s) when films are prepared under identical coalescent-to-solids ratios. The MFFT of CW40-758 as a neat dispersion is ≈ 15–18 °C (ISO 2115:2001). Introduction of 5 wt% (on binder solids) of a standard ester-alcohol coalescent depresses MFFT to ≤ 5 °C, enabling full film consolidation on chill-roll applicators and low-energy drying hoods. This combination—high service temperature resistance from an elevated Tg combined with process-friendly film formation at modest coalescent demand—positions CW40-758 between standard VAE and harder acrylic or styrene-acrylic latices, yet without the formaldehyde-liberating crosslinking systems that are often employed to harden the latter.

For semi-transparent and opaque surface coatings where early water resistance and anti-tack properties are critical, CW40-758 is typically formulated with a co-solvent system comprising 2–5 wt% (on binder solids) of a diethylene glycol monobutyl ether-based coalescent and 0.2–0.5 wt% of a non-ionic associative thickener to impart the necessary high-shear viscosity (100–150 mPa·s at 10 000 s⁻¹, cone-and-plate geometry) for airless spray atomisation. Applied at wet film thicknesses of 75–125 µm onto primed steel or aluminium, films cured at 23 °C and 50 % RH for 7 days yield a cross-hatch adhesion rating of 4B (ASTM D3359‑17, method B) without an adhesion promoter and a direct-impact resistance exceeding 80 in‑lb (ASTM D2794‑93(2019)). The high polyethylene-like chain segments derived from the ethylene comonomer reduce the coefficient of friction to 0.25–0.35 (ASTM D1894‑14) in unplasticised films, which is a marked advantage in drawer-coating applications where stiction against elastomeric seals must be avoided.

Film Formation Mechanics on Porous Cellulosic Substrates

In blade coating of paper and board, the dynamic interaction between dispersion rheology, dewatering rate, and binder migration governs the z‑direction distribution of polymer. CW40-758, with a moderate anionic charge density from carboxylation (≈ 0.25–0.40 meq/g dry polymer, determined by conductometric titration), adsorbs partially onto cationised starch co‑binders and precipitated calcium carbonate pigments. This flocculation, when controlled, elevates the low‑shear viscosity of the coating colour to 1200–1800 mPa·s (Brookfield, 100 rpm) at 65 % total solids, reducing the drainage velocity into the base sheet and preserving a higher binder concentration at the surface. On a bent‑blade coater running at 1200 m/min with a blade angle of 30° and a bevel thickness of 0.305 mm, the immediate water‑retention behaviour of the colour can be quantified by the ÅA‑GWR water retention meter (TAPPI T 701 pm‑19). Formulations containing 12 parts CW40-758 per hundred pigment (phr) and 4 parts low‑viscosity cationic starch routinely yield retention values of 80–90 g/m² over a 90 s exposure at a pressure differential of 0.5 bar, compared with > 120 g/m² for an equivalent low‑Tg VAE binder under identical shear history. The consequence is a measurable reduction in dry‑pick, with IGT pick resistance rising from 2.5 m/s to 3.4 m/s (ISO 3783:2006, pendulum drive, 50 mm oil viscosity) when CW40-758 replaces a standard-film‑forming VAE in a double‑coated folding boxboard furnish.

In high‑speed blade coating operations, the pressure‑pulse exerted on the colour wedge beneath the blade tip traverses 0.5–5 bar in less than 0.5 ms. Under such transient extensional‑shear fields, polymeric binders with insufficient high‑shear viscosity contribution can segregate from the pigment‑co‑binder network, leading to binder‑lean regions that reduce surface strength and ink gloss uniformity. CW40-758 has been shear‑thinned through a capillary viscometer at 250 000 s⁻¹ in a coating colour pigmented with 80 parts GCC (d₅₀ = 0.7 µm) and 20 parts kaolin, revealing a high‑shear apparent viscosity of 42–48 mPa·s, which is 15–20 % above that of a comparable carboxylated low‑Tg VAE. This is attributed to the compact, less‑swollen particle morphology of particles whose hydrophobic character is enhanced by the lower ethylene content of the high‑Tg backbone. When blade load is adjusted to maintain a target 8 g/m² dry coat weight on a pre‑coated white‑top liner, the transfer efficiency of CW40-758‑based colours remains within ± 1 % across a blade‑wear cycle of 8 h, while non‑standard VAE formulations drift by 3–5 % as blade-angle compensation lags behind real‑time topography change. Film‑split patterns on the applicator roll, documented by high‑speed videography at 10 000 fps, show a shorter, more stable filament break‑up length when CW40-758 is present, correlating with reduced misting.

When Rheological Adjustments Are Dictated by Coating Speed and Blade Pressure

Although CW40-758 displays a pronounced shear‑thinning character (a power‑law index n of 0.45–0.55 derived from the shear‑rate range 0.1–1000 s⁻¹ at 60 °C), its low‑shear viscosity is sensitive to the type and dosage of alkali‑swellable associative thickeners (HASE). When formulating for rod‑metering size‑press applications on fine paper at 800 m/min, replacement of a standard HASE (hydrophobically modified alkali‑swellable emulsion) with a high‑molecular‑weight ethylene oxide‑urethane (HEUR) thickener at 0.15 wt% on total colour is necessary to maintain adequate levelling without inducing excessive water sensitivity from the thickener’s surfactant content. Under these conditions, the high‑shear viscosity of CW40-758‑based surface sizes measured on an ACAV A2 high‑shear viscometer at 1 000 000 s⁻¹ remains below 12 mPa·s, which matches the recommended working range for film‑split control on grooved metering rods.

Storage stability of CW40-758 under quiescent conditions at 5–35 °C exceeds 12 months when containers are kept sealed and protected from freezing. At temperatures below 3 °C irreversible coagulation may occur; product that has been subjected to a single freeze‑thaw cycle should be discarded. When pumping via progressive cavity pumps with a stator clearance of 0.5 mm, shear rates should be kept below 500 s⁻¹ to avoid mechanical destabilisation of the latex, and 200‑mesh bag filters are recommended downstream of the diaphragm pulsation dampener. Pre‑drying of the raw paper sheet to a moisture content below 6.5 % is recommended when the ambient relative humidity exceeds 60 % during coating application, as the high‑Tg latex surface can skin over prematurely on a slow‑moving dryer felt, causing blocking on the first calender stack.