SUMIMKAFLEX S-755 Emulsão VAE

In adhesive formulations for paper and packaging, the selection of a base emulsion governs both the application rheology and the mechanical resilience of the bonded joint. SUMIKAFLEX S-755, a vinyl acetate-ethylene (VAE) copolymer dispersion stabilized by a poly(vinyl alcohol) protective colloid, is engineered for high-speed converting operations where rapid setting and broad substrate adhesion are required. The product is supplied at a nominal solids content of 55% w/w and a Brookfield LV viscosity of 3,500 mPa·s (spindle #3, 30 rpm, 25°C, ISO 2555). Its pH falls within 4.0–5.0 (ISO 976), and the minimum film-forming temperature (MFFT) measured on a gradient bar per ISO 2115 is 0°C, correlating with a glass transition temperature (Tg) of approximately 0°C determined by differential scanning calorimetry at 10 K/min (ISO 11357-2). The particle size, determined by laser diffraction (ISO 13320), lies in the submicron range, producing films with a milky appearance that clarifies upon complete water evaporation.

Unlike poly(vinyl alcohol)-stabilized VAE emulsions of lower ethylene content such as SUMIKAFLEX S-752, S-755 incorporates a higher molar fraction of ethylene in the copolymer backbone. This compositional shift depresses the Tg into the ambient range without the need for coalescing solvents, yielding a balance of cohesive strength and elongation at break that exceeds 700% (ASTM D882, 0.5 mm dry film thickness, 500 mm/min crosshead speed). The absence of external plasticizers eliminates migratory species that can compromise food-contact compliance or cause blocking in stacked substrates.

What distinguishes S-755 from other vinyl acetate-ethylene dispersions in the Sumikaflex line?

Within the Sumikaflex portfolio, S-755 occupies a mid-range position defined by its ethylene content and resulting flexibility. A structured comparison with adjacent grades clarifies the product’s design envelope.

Property	SUMIKAFLEX S-752	SUMIKAFLEX S-755	SUMIKAFLEX S-408
Solids content (% w/w)	54–56	54–56	54–56
Brookfield LV viscosity (mPa·s, #3, 30 rpm, 25°C)	2,000–3,000	3,000–5,000	1,000–2,000
Tg (DSC midpoint, °C)	approx. +5	approx. 0	approx. -15
MFFT (°C, ISO 2115)	+5	0	-15
Tensile strength (MPa, ASTM D882)	7.0–9.0	4.5–6.5	1.5–3.0
Elongation at break (%)	400–600	700–900	1,200–1,500

S-755 delivers intermediate flexibility with higher elongation than S-752 while maintaining roughly twice the tensile strength of the ultra-flexible S-408. This profile suits applications where the bondline must absorb stress without cohesive failure, such as semi-automated carton sealing on lines that stack product immediately after compression.

Coalescence physics and the absence of external plasticizers

The film formation mechanism of S-755 relies on the plastifying effect of the ethylene segments within the copolymer, reducing the minimum film-forming temperature to 0°C. At ambient temperatures above 15°C, complete interdiffusion of polymer particles occurs without coalescing aids, yielding a clear, continuous film. This characteristic is advantageous in closed-loop machine application where volatile organic compounds (VOC) from coalescent agents would accumulate in recirculated adhesive troughs. However, at processing temperatures below 5°C, the coalescence rate slows markedly, and an open time exceeding 30 seconds can result in a drastic loss of tack prior to compression. In cold-plant environments, pre-warming the emulsion to 20°C–25°C and keeping the glue station insulated is necessary to maintain consistent film formation and bond strength.

Despite its flexibility, the film of S-755 exhibits pronounced water whitening when immersed in water at 23°C for 24 h (EN 204 condition), which causes a reversible loss of clarity and a reduction in wet cohesive strength to approximately 30% of the dry value. This behavior constrains the use of the unmodified emulsion in exterior or high-humidity packaging unless a reactive crosslinker is incorporated.

When formulated as a one-component waterborne laminating adhesive for biaxially oriented polypropylene (BOPP) /paper composites, a key bottleneck arises from the insufficient anchorage on untreated film surfaces. The surface energy of corona-treated BOPP must be maintained at a minimum of 38 mN/m (ASTM D2578) immediately before coating; a drop to 34 mN/m due to aging can reduce peel strength by more than 50%. A typical process configuration on a roll-fed laminator employs a three-roll metering system applying 5–8 g/m² dry weight at line speeds of 80–150 m/min. The coated web passes through a tunnel oven set at 85°C with a residence time of 25–35 s, followed by nip lamination at 0.3 MPa linear pressure and 110°C roll temperature. Adhesion failures traced to heat-seal initiation lag were mitigated by raising the laminator nip temperature to 115°C; however, this narrows the safe window for heat-sensitive BOPP films that shrink at temperatures above 120°C.

When borax-dextrin adhesives are co-formulated

Co-formulation of S-755 with borax-modified dextrin solutions for paper tube winding introduces a distinct rheological risk. The poly(vinyl alcohol) protective colloid complexes with borate ions, causing a sharp stepwise increase in viscosity. In pilot trials on a high-speed spiral tube winder (line speed 60 m/min), addition of a 7% borax-dextrin solution to the emulsion at a ratio of 20:80 (wet weight) elevated the mixture viscosity from 4,200 mPa·s to beyond 30,000 mPa·s within 60 seconds, leading to pump cavitation and streaking on the bondline. Successful blending requires metering the dextrin solution into the emulsion under high-turbulence in-line mixing, limiting the borax concentration to below 0.3% of total formulation weight, and verifying viscosity stability over a 4-hour pot-life cycle. When these constraints are observed, the hybrid adhesive achieves an initial wet tack sufficient to hold 3-ply kraft core layers at a winding angle of 54° without telescoping.

Dynamic surface tension governs the wetting behavior of S-755 on semi-porous substrates such as clay-coated board. Measurements using a bubble pressure tensiometer (Krüss BP100, range 50–5,000 ms) show that at a surface age of 100 ms, the dynamic surface tension is approximately 45 mN/m. This value is sufficiently low to spread uniformly on substrates with contact angles below 60°. However, on heavily coated board with a surface energy of 35 mN/m, retraction of the adhesive from the surface results in a cratered application pattern unless a nonionic surfactant of low HLB (4–6) is post-added at 0.05–0.1% on wet weight. Overdosing beyond 0.2% causes excessive foam generation in the recirculating reservoir, and a silicone-free defoamer at 0.02% must then be incorporated. This delicate balance is typical of VAE emulsions that lack the inherent low-surface-tension character of all-acrylic lattices.

Heat sealing after dry lamination: threshold activation analysis

For foil-laminated packaging structures, S-755 serves as the primer coat that is subsequently heat-sealed against a polyethylene extrusion layer. The heat seal initiation temperature (HSIT) of the dry adhesive film lies between 80°C and 90°C, as measured by a gradient heat sealer at 0.5 N/mm² seal pressure and 1.5 s dwell time, according to ASTM F2029. The optimal seal bar temperature range is 110°C–130°C. Below 105°C, the peel strength on aluminum foil (EN 868-2 method) remains below the critical 2.5 N/15 mm threshold required for chevron peel pouch integrity. Above 135°C, thermal degradation of the adhesive manifests as blistering and a rapid drop in bond strength to less than 1.0 N/15 mm. Comparison with a lower-ethylene VAE (S-752) reveals that S-755’s lower Tg shifts the HSIT downward by approximately 5–8°C, widening the process window in machines with temperature gradients across the seal bar.

Adhesive specification	Substrate system	T-peel strength (N/15 mm)	Lap shear (MPa)	Conditioning /Standard
S-755 (unmodified)	kraft/kraft, 20 g/m² coat	3.8 (±0.4)	2.1 (±0.3)	ISO 4587:2003, beech wood, 0.5 mm bondline, 7 d at 23°C/50% RH
S-755 + crosslinker (glyoxal 0.5% w/w on solids)	birch/birch, EN 205	—	4.6 (±0.5) dry /2.7 (±0.3) after 4 h boil (EN 204 D2)	DIN EN 204
S-755 + rosin ester tackifier 15% on solids	corona-treated BOPP/paper	5.2 (±0.6)	—	Peel T-peel at 300 mm/min, 24 h at 23°C

Adhesive formulators requiring indirect food-contact clearance under FDA 21 CFR 175.105 can use S-755 provided the residual vinyl acetate monomer content remains below 0.1% (determined by headspace GC–MS per AOCS No. 106-99). The emulsion is manufactured without alkylphenol ethoxylates (APEOs), and the protective colloid is a fully hydrolyzed, medium-molecular-weight poly(vinyl alcohol). These attributes satisfy the guidance documents of German BfR Recommendation XIV for polymeric coatings on paper and board, though migration testing remains the responsibility of the converter when fatty foodstuffs are involved.

Enhancing water resistance with blocked isocyanates

To overcome the intrinsic water sensitivity of the PVA-stabilized VAE matrix, a post-addition of water-dispersible blocked isocyanate crosslinker can be made. At an addition level of 1.5% active isocyanate on total emulsion solids, the pot life at 25°C extends to approximately 6 hours, as tracked by a doubling of the initial viscosity. The system deblocks during thermal drying (minimum 90°C for 3 minutes), curing over 7 days at ambient humidity (ISO 291 class 2). This approach elevates the bond durability to EN 204 D2 class, withstanding a 4-hour boiling water soak and redrying cycle while retaining at least 2.5 MPa lap shear. However, the isocyanate adduct must be mixed into the emulsion using a high-shear disperser (rotor-stator tip speed 15 m/s) to avoid local gelation due to rapid reaction with hydroxyl groups on the PVA stabilizer. Aged films demonstrate a substantial reduction in water whitening: submersion for 24 h at 23°C yields only a haze increase of 5% (ASTM D1003) versus 35% for the unmodified control. This upgrade path positions S-755 as a viable binder for exterior packaging applications where occasional moisture exposure is anticipated, without sacrificing the room-temperature coalescence that distinguishes it from harder VAE grades.