Understanding EVA–Anionic PU Compatibility from a Colloidal Perspective

Blending EVA latex with anionic polyurethane (PU) dispersions is widely used in waterborne adhesives, coatings, and impregnation systems. However, formulators often encounter instability issues such as flocculation, viscosity increase, or phase separation. While pH is frequently blamed, the root cause usually lies deeper in colloidal stabilization mechanisms.

EVA latex is commonly stabilized by nonionic surfactants, PVA protective colloids, or weak anionic systems. In contrast, anionic PU dispersions rely heavily on electrostatic repulsion generated by carboxylate groups. When these two systems are combined, sudden changes in pH and ionic strength can compress the electrical double layer surrounding PU particles, reducing repulsive forces and triggering aggregation.

A common misconception is that matching pH values alone ensures compatibility. In practice, two dispersions at the same pH can still be incompatible if their stabilization mechanisms conflict. EVA systems with strong nonionic or PVA stabilization may lose stability when exposed to higher alkalinity, even if pH values appear “reasonable”.

Successful EVA–PU blending requires a holistic approach. Instead of forcing EVA to become strongly alkaline, it is often more effective to align both systems toward a neutral operating window. For EVA latexes, a pH range of 6.5–7.2 offers good tolerance, while anionic PU dispersions perform well around pH 7.5–8.0.

Equally important is blending order. Gradually adding EVA latex into PU under controlled shear minimizes ionic shock and helps maintain dispersion integrity. Introducing a small amount of nonionic compatibility buffer further reduces interparticle attraction and improves long-term storage stability.

Ultimately, EVA–PU compatibility is not defined by a single parameter. It is the result of balanced pH, controlled ionic strength, appropriate blending sequence, and respect for each dispersion’s inherent stabilization design.

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