BJ-707 Emulsão VAE para impermeabilização e aplicações gerais

The carboxylated vinyl acetate-ethylene (VAE) copolymer dispersion designated BJ-707 is engineered for cold-applied cementitious and polymer-modified waterproofing systems where cohesive film strength, low-temperature flexibility, and early wet adhesion to alkaline substrates are critical. The dispersion is supplied at a nominal solids content of 55% by weight, a Brookfield RVT viscosity (spindle 3, 20 rpm, 25°C) of 200–2000 mPa·s, and a minimum film formation temperature below 0°C when coalesced with 3–5% of a suitable ester alcohol. Unlike conventional polyvinyl acetate homopolymer or standard VAE grades with lower ethylene incorporation, BJ-707 utilises a deliberately elevated ethylene comonomer ratio (>15 wt% in the dry polymer) that depresses the glass transition onset to approximately –15°C as determined by differential scanning calorimetry per ISO 11357-2. This structural modification reduces the need for external plasticisers in thin-film waterproofing membranes, limiting plasticiser migration into adjacent substrates and maintaining stable elongation at break values in excess of 600% after 1000 h of thermal ageing at 70°C (ISO 188). The carboxylation level, quantified as an acid number of 2–4 mg KOH/g dry polymer, provides reactive sites that chelate calcium ions released during Portland cement hydration, creating an interpenetrating organic-inorganic network at the polymer-cement interface that raises pull-off adhesion to damp concrete above 2.0 MPa (ASTM D4541, Type IV adhesion tester, 50 mm dollies).

Production-scale handling of BJ-707 in continuous mixing plants demands attention to shear stability. The dispersion tolerates short-duration high-shear mixing (Cowles blade peripheral speed ≤ 18 m/s) but exhibits viscosity drift of +15–30% when subjected to extended pumping through progressing cavity pumps with stator clearances below 0.3 mm. Extended pumping loops without bypass relief valves have been observed to generate heat-induced pre-coalescence at discharge temperatures exceeding 40°C, forming micro-grit that interferes with liquid-applied membrane levelling. Batch-to-batch variation in mean particle size, as measured by laser diffraction (ISO 13320), remains within 0.8–1.2 µm, a range sufficiently narrow that co-solvent demand for film formation at low ambient temperature (5–10°C) can be fixed without per-batch rheology adjustment.

What Differentiates the BJ-707 Backbone Structure from Earlier VAE Series?

Earlier VAE grades designed for interior adhesives and interior-grade joint compounds rely on an ethylene content typically <12 wt%, yielding an MFFT near +5°C and a water absorption plateau at 15–20% after 24-h immersion (ASTM D570). BJ-707’s extended ethylene sequences create amorphous-rich domains that remain elastomeric at –10°C, enabling liquid-applied flashing membranes to bridge moving cracks in concrete tunnel linings where service temperatures periodically dip below freezing. In a comparative test protocol modelled on EN 14891:2017 for liquid-applied water impermeable products, films cast at 1.5 mm dry thickness and conditioned at 23°C/50% RH for 28 days demonstrated no water penetration when subjected to 250 kPa hydrostatic pressure for 24 h over pre-cracked concrete with a static crack width of 0.4 mm. By contrast, a lower-ethylene VAE reference with identical film thickness exhibited cohesive failure at 0.25 mm crack opening under the same pressure.

The surfactant package—a blend of anionic and non-ionic stabilisers with a cloud point above 90°C—was selected to minimise foam entrainment during high-speed mixing of self-levelling cementitious slurries without resorting to silicone-based defoamers that can interfere with subsequent tile adhesive bond strength. Residual surfactant migration to the film surface, which otherwise produces a tacky surface that picks up jobsite dust, is limited by the high polymer hydrophobicity; surface tack was rated as “none” after 7-day curing at 50% RH in a finger-tack test derived from ISO 9117-3.

Film Formation and Crosslinking Mechanisms under Ambient Humidity

In cement-free, one-component water-based waterproofing coatings, BJ-707 dries by water evaporation and particle coalescence. The absence of an oxidative curing mechanism or external crosslinker means that short-term water resistance develops immediately upon film closure, but full water-whitening resistance—indicative of complete surfactant exudation and inter-particle polymer diffusion—requires 7–14 days at 23°C. Accelerated cure at 40°C reduces the time to full clarity and T-peel strength development by a factor of 2.5. Blends containing 5–10% of a glycidyl-functional silane additive react slowly with backbone carboxyl groups, generating a post-applied crosslink density increase of 20–30% (measured by gel content per ASTM D2765) that improves immersion resistance in permanent swimming-pool applications. Published data for this specific blend configuration under continuous 60°C water immersion indicates stable adhesion to grit-blasted stainless steel beyond 5000 h without cathodic disbandment.

Operational boundaries require precise substrate moisture content control. Application to green concrete with internal relative humidity exceeding 85%, measured by sealed in-situ probe per ASTM F2170, can trap water at the interface and produce osmotic blistering within 24–72 h of membrane application, particularly when solar radiant heat elevates the film skin temperature before bulk moisture has evaporated. The blister diameter distribution, observed on low-permeability slab substrates, follows a bimodal pattern with peaks at 1–3 mm and 10–15 mm, correlating with coarse aggregate shadow voids and trowelling imperfections respectively.

The high ethylene content that confers low-temperature elasticity simultaneously reduces the cohesive energy density of the dry film, which limits the dry tensile strength to 3.5–5.0 MPa versus 7–10 MPa typical of lower-ethylene VAE polymers. This trade-off is critical in load-bearing waterproofing applications such as vehicular traffic coatings: neat BJ-707 films should not be specified where point loads exceed 1.5 MPa static compressive stress unless reinforced with fine quartz aggregate (0.1–0.3 mm) at a filler-to-binder ratio of 1:1 by volume. The aggregate-reinforced composite yields an elastic modulus increase from approximately 8 MPa to 55 MPa without compromising the crack-bridging ability below 0°C.

Compatibility with Reactive Cement Phases

BJ-707’s anionic carboxylation is intentionally buffered to pH 4.0–5.5 to permit direct addition into fresh Portland cement paste without inducing instantaneous flash setting. During the induction period of cement hydration, calcium hydroxide generated at the pore solution interface reacts with the surface-accessible carboxylate groups at a rate that is moderated by the particle-bound surfactant sheath, allowing a working period of 45–75 minutes at a polymer-to-cement ratio of 0.15 by mass (ambient temperature 23°C, w/c 0.40). Beyond 90 minutes the combination of cement hydration and polymer coalescence raises the complex viscosity (oscillatory shear rheometry, 1 Hz) by an order of magnitude, rendering the slurry unsuitable for trowel finishing. Tensile adhesion to concrete substrates after 28-day wet curing followed by 7-day drying reaches 2.3–2.8 MPa with failure predominantly cohesive within the concrete (EN 1542, direct pull-off).

Without a header, an application window observed in tunnel drainage sheet bonding warrants examination. A two-component mixture of BJ-707, 42.5R Portland cement, and graded silica sands (0–0.6 mm) applied at 1.5–2.0 kg/m² wet film weight bonds PVC-P drainage sheets within 2 h application time at 10°C and 70% RH. The film’s residual moisture, when trapped under impervious sheet material, does not cause saponification-induced adhesion loss provided the polymer-cement matrix retains a CaO-to-polymer ratio above 2.0. Below this threshold, the accumulation of free lime at the interface dissolves in condensed water films and raises the interfacial pH beyond 13.5, slowly hydrolysing the ester linkages of the VAE backbone. Field inspections after 5 years in alpine tunnel environments (consistent 7–9°C,> 90% RH) show peel adhesion retention above 80% of the initial 28-day value when the mortar mix design maintains a minimum cement content of 350 kg/m³.

In cementitious tile adhesives classified under ISO 13007-1, BJ-707 at 4–8% polymer solids on dry mix weight extends the open time from 15 minutes to over 30 minutes when measured as tensile adhesion strength of ceramic tiles after prolonged exposure (EN 1346). The anti-sag behaviour on vertical walls, quantified by the slip test of EN 1308, remains below 0.5 mm even at 6 mm notched trowel thickness, attributable to the pseudoplastic flow profile induced by the structured polymer particles. Storage stability in dry-mix formulations premixed with the dispersion is not infinite; after 6 months at 35°C in sealed silos, the redispersed powder blends show a 10–15% reduction in tensile adhesion, a drift within the permissible limit of EN 12004 for C2-class adhesives.

Incompatibilities exist with amine-based hardeners frequently used in epoxy-modified cementitious overlays. When BJ-707 latex is pre-mixed with a polyamine adduct, the amine groups strip the anionic surfactant from the particle surface, causing instantaneous coagulation into a non-redispersible crumb. Similarly, storage in unlined carbon steel tanks triggers ferric ion bridging of carboxylate groups, resulting in a viscosity spike exceeding 5000 mPa·s and irreversible grit formation within 48 h at 40°C. All contact surfaces in bulk handling equipment must be 316L stainless steel or passivated 304 stainless, with HDPE linings for static storage vessels. The flash rust potential on ferrous reinforcement steel is mitigated by the alkaline environment of the cementitious carrier; in neat polymer films applied directly to grit-blasted carbon steel, a 0.5–1.0% addition of a sodium nitrite-based corrosion inhibitor (ASTM C1582) prevents rust staining during the cure window.

Cold-weather logistics require awareness of the freeze-thaw resilience of the emulsion. BJ-707 withstands up to 3 freeze-thaw cycles at –5°C without coagulum formation when thawed under controlled, non-mechanical agitation; below –7°C ice crystal growth punctures the particle stabiliser layer, leading to a sediment that cannot be fully redispersed. Shipment in temperature-controlled tankers set at +5°C to +30°C is standard practice, and drummed material should not be stacked more than 3 pallets high to prevent container deformation that compromises the lid seal and introduces air-borne contaminants.