CW FS-II VAE Emulsion for Flexible Substrates

CW FS-II is a high-ethylene-content vinyl acetate–ethylene (VAE) copolymer emulsion, colloidally stabilised and supplied at a nominal solids level of 55 ± 1% (ISO 3251). The polymer backbone, with an ethylene incorporation of approximately 15–20 wt% (internal FTIR calibration), permanently depresses the glass transition temperature to −15 °C (DSC, midpoint, ISO 11357‑2) and reduces the minimum film formation temperature to below 0 °C (ISO 2115, white point method). These physical characteristics permit film coalescence at ambient temperature without the addition of volatile coalescing agents or external plasticisers. The emulsion is preserved with a formaldehyde-free biocide package and exhibits a pH of 4.5–5.5; Brookfield RVT viscosity (spindle 3, 20 rpm, 25 °C) is maintained within 1 500 – 3 000 mPa·s. The copolymer’s inherent film flexibility, low odour, and adhesion to a broad range of polar and non-polar surfaces position CW FS-II as a primary binder for flexible substrates that demand plasticiser-free compliance and robust mechanical integrity under dynamic flexural stresses.

How Does Ethylene Comonomer Content Modulate the Adhesion-Cohesion Balance?

In vinyl acetate–ethylene copolymers, the ethylene moieties act as an internal plasticiser, increasing free volume and chain mobility. This structural effect is directly responsible for the shift in balance between cohesive strength and surface wetting. At the 15–20 wt% ethylene level incorporated in CW FS-II, dynamic mechanical analysis (DMA, 1 Hz, tensile mode) shows a tan δ maximum at −10 °C, indicating that the material dissipates energy efficiently at ambient and sub‑ambient temperatures while retaining sufficient cohesive modulus for fibre-to‑fibre or film‑to‑film bonds. For comparison, a lower-ethylene VAE grade (CW FS‑I, ethylene content approx. 8–12 wt%) yields a Tg near +5 °C and an MFFT of +8 °C; the resulting films are stiffer and require thermal activation during lamination, typically above 90 °C.

On surface‑modified polyethylene terephthalate (PET) film, CW FS‑II delivers a 180° peel adhesion of 2.8 N/25 mm (ASTM D903, coat weight 25 g/m² dry, dried 3 min at 80 °C) and a loop tack value of 3.2 N/25 mm (PSTC‑16, identical coating conditions). The failure mode is cohesive within the adhesive layer, confirming an optimised adhesion–cohesion equilibrium. When the ethylene content is increased beyond 25 wt% (experimental benchmarks), tack values rise above 5 N/25 mm but the shear holding power (PSTC‑107, 1 kg load) drops below 1 h, indicating excessive cold flow that causes blocking in roll‑fed converting lines. Conversely, grades with less than 10 wt% ethylene require substantial coalescing solvents to achieve film integrity at 23 °C, elevating volatile organic compound (VOC) release during drying. Thus the ethylene‑segment window employed in CW FS‑II is the calibrated point where ambient‑temperature film formation, cohesive safety, and low‑VOC operation intersect.

On high‑speed nonwoven binder lines operating at web speeds of 100–300 m/min, mechanical shear stability is the primary process discriminant. CW FS‑II, when subjected to the Maron mechanical stability test (ISO 4576, 1 000 rpm for 20 min), generates less than 0.5% sieve residue on a 45 µm mesh, confirming its suitability for rotor‑stator pumps and recirculating saturator baths. Viscosity drift over an 8‑hour production shift remains below ±10% of initial values, provided the bath temperature is kept between 18 °C and 32 °C. Electrolyte tolerance, critical for spunlace lines where water hardness can fluctuate, has been quantified by the calcium‑ion stability test (DIN 53780): addition of 10 mL of 1% CaCl₂ solution to 50 g of emulsion induces no visual coagulum after 15 min of mixing. The static surface tension of the neat emulsion is 38–42 mN/m (Du Noüy ring, ISO 304), which ensures adequate spontaneous wetting onto rayon and cellulosic substrates; for highly hydrophobic fibres such as polypropylene, a non‑ionic wetting surfactant at 0.2–0.5% on wet emulsion weight is recommended, and foam generation is then controlled with a silicone‑free defoamer dosed at 0.05–0.1% to avoid surface‑energy defects in the finished nonwoven.

When CW FS-II Substitutes Plasticised PVAc in Lamination Adhesives for Paper-Based Flexible Packaging

Conventional plasticised poly(vinyl acetate) adhesives rely on dibutyl phthalate or triacetin to impart film flexibility, thereby posing long‑term migration risks under food‑contact conditions. Substituting CW FS‑II eliminates the plasticiser entirely. On a laminating line combining 40 g/m² kraft paper and a 20 µm corona‑treated polyethylene film, CW FS‑II achieves a bond strength of 4.5 N/25 mm (ISO 11339, T‑peel at 300 mm/min) after 24 h conditioning at 23 °C/50% RH. Under identical coating and drying conditions, a commercial plasticised PVAc (plasticiser content 12%) yields 3.8 N/25 mm. The oven temperature profile can be reduced from the typical 120 °C peak (required for plasticiser‑containing formulations to drive off water and minimise residual plasticiser volatiles) to a maximum of 80 °C, which lowers energy consumption by approximately 20% and reduces the risk of thermal shrinkage of the PE web.

Since CW FS‑II forms a permanently flexible film without migratory additives, the laminate complies with the extractive requirements of FDA 21 CFR §175.105 (indirect food additives) and the specific migration limits of EU Regulation 10/2011 (overall migration <10 mg/dm²). Blocking tendency, a frequent failure mode in roll‑fed packaging, is evaluated by the force required to separate two adhesive‑coated surfaces stored under load. Per ASTM D3916 (contact pressure 0.7 N/cm², 24 h at 35 °C), CW FS‑II exhibits a blocking force of <0.5 N/cm², whereas plasticised PVAc formulations typically measure above 1.2 N/cm². This low blocking characteristic is attributed to the non‑polar ethylene segments that preferentially orient at the film‑air interface during drying, reducing the surface tack without compromising interfacial adhesion.

Application via engraved roll coaters with cell volumes of 12–20 cm³/m² demands a viscosity profile that is stable under the high‑shear conditions at the doctor‑blade nip. Rheometric data (cone‑plate, ISO 3219, 0.1–1 000 s⁻¹) show that CW FS‑II exhibits shear‑thinning behaviour with a power‑law index n of 0.6–0.7. This pseudoplasticity permits a controlled pickup without dripping; at an applied nip pressure of 2–4 bar and a speed differential between applicator and transfer rolls of ±5%, a uniform dry add‑on of 2–4 g/m² is maintained on polyester films. However, if the emulsion is subjected to shear rates exceeding 10 000 s⁻¹—as can occur in poorly sized centrifugal pumps—partial desorption of the protective colloid can trigger micro‑coagulum that manifests as streaks on the coated substrate. Progressive cavity pumps are therefore specified for transfer duties. Additionally, the emulsion should be protected from repeated freeze–thaw cycles; storage below +5 °C can lead to irreversible gelation, and the ideal storage range is +10 °C to +35 °C with a shelf life of 6 months in sealed containers.

Heat Seal Activation Temperatures and Hot Tack Performance on OPP and Metallised Films

VAE copolymers display a broad thermoplastic window that can be exploited for heat‑sealable overprint varnishes and cold‑seal symmetry adhesives. CW FS‑II, coated at 2.5 g/m² dry on corona‑treated biaxially oriented polypropylene (OPP), initiates a measurable seal at 65 °C (JIS Z1707, 1.0 s dwell, 2 bar pressure). The seal strength reaches 3.1 N/25 mm at a jaw temperature of 80 °C and plateaus before declining beyond 140 °C due to thermal softening of the film substrate. Hot tack—the ability to hold a seal while still hot—measured by an Instron‑based method (jaw opening 0.2 s after seal closure, 200 mm/min pull), yields 0.8 N/25 mm at 60 °C seal temperature. This is adequate for vertical form‑fill‑seal packaging where the product drop imposes immediate stress on the still‑warm longitudinal seal. In comparative tests, conventional acrylic‑based pressure‑sensitive adhesives deliver higher ambient tack but show negligible hot tack at 60 °C, requiring additional heat‑seal lacquers.

Process engineers should note that prolonged exposure of the dry film to temperatures above 180 °C initiates deacetylation, evidenced by discolouration and a brittle residue. The practical sealing range is therefore confined to 65–160 °C, and dwell times should not exceed 2 s at the upper limit. On metallised OPP, where the aluminium vacuum‑deposition layer can act as a heat sink, the seal initiation temperature shifts upward by approximately 5 °C, and best results are achieved by pre‑heating the metallised web to 35–40 °C before the seal station. No primer is required, provided the metal surface energy is maintained above 42 mN/m through in‑line corona treatment.

In aqueous contact applications, colloidal stability must be maintained in the presence of dissolved multivalent salts. A zinc‑ion tolerance test (addition of 5 mL of 10% ZnSO₄ solution to 50 g emulsion) produces no destabilisation at 25 °C over 24 h, confirming robustness for coatings on zinc‑primed metal foils. However, direct blending with polyamine or polyimine crosslinkers leads to rapid viscosity build and eventual gelling within pot‑life; pH adjustment with ammonia or volatile buffering agents is not recommended, as the elevated pH (>7.5) accelerates ester hydrolysis and shortens wet‑state shelf life. For applications demanding enhanced water resistance, a blocked isocyanate or carbodiimide crosslinker can be post‑added at 0.5–1.0 wt% on polymer solids and must be mixed under low shear immediately before use, with a working pot‑life of 4–6 h at 23 °C.