Water-Based Polyurethane Dispersions: New Insights into the Intrinsic Stabilization Using Carboxylate Groups

Area of Science:

• Polymer Chemistry
• Materials Science
• Colloid Science

Background:

• Water-based polyurethane dispersions (PUDs) require ionizable groups like dimethylolpropionic acid (DMPA) for stability.
• DMPA's pH-dependent dissociation influences electrostatic repulsion between PUD particles.
• Understanding the structure-stability relationship is crucial for optimizing PUD performance.

Purpose of the Study:

• To investigate the structure-stability relationship in PUDs with varying soft segments and DMPA content.
• To analyze the dissociation behavior of carboxyl groups and particle charge.
• To evaluate dispersion stability using critical coagulation concentration (ccc) and theoretical models.

Main Methods:

• Synthesis of PUDs with different polyols (polyether, polyester, polycarbonate) and DMPA content via the acetone process.
• Potentiometric acid-base titration to characterize carboxyl group dissociation and particle charge.
• Electrokinetic measurements and hard particle theory to determine surface potentials.
• DLVO and XDLVO theory for dispersion stability analysis.
• Contact angle measurements and van Oss-Chaudhury-Good theory for Hamaker constants.

Main Results:

• Potentiometric titration may not detect all incorporated carboxyl groups, suggesting hindered dissociation and nonuniform charge distribution.
• Titration curves fit best with at least two functional groups of different acid strengths.
• Early DMPA incorporation increases internal charge groups but minimally impacts surface potential and stability.
• Surface potentials decrease slightly with increasing DMPA content due to smaller particle size.
• Critical coagulation concentration (ccc) increases with DMPA content and is strongly influenced by the polyol component.

Conclusions:

• The dissociation of DMPA in PUDs is complex, involving multiple acid strengths and potentially hindered dissociation.
• While early DMPA incorporation affects internal charge localization, it does not significantly alter overall dispersion stability or surface potential.
• The polyol component plays a critical role in PUD dispersion stability, necessitating extensions to the DLVO theory to include hydrophobic interactions.