Universität zu Köln

Grau, Christoph (2026). Structure-Stability Relationships of Water-based Polyurethane Dispersions. PhD thesis, Universität zu Köln.

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Abstract

Water-based polyurethane dispersions (PUDs) are increasingly replacing solvent-based systems, yet the molecular factors governing their stability remain poorly understood. Despite the central role of electrostatic repulsion, growing evidence suggests that dispersion stability is shaped by more than just ionisable groups. This thesis investigates the relationship between molecular structure and dispersion stability in electrostatically stabilized, water-based polyurethane dispersions. A systematic series of well-defined PUDs is synthesized according to the acetone process to examine the effects of three key structural parameters: the polyol component (polycarbonate, polyester, and polyTHF diols), the type and concentration of hydrophilizing agents (3.0–5.5 wt% AAS and DMPA), and the hard segment content, adjusted through varying amounts of 1,4-butanediol while maintaining constant ionic group content. The dispersion stability is evaluated by critical coagulation concentration (ccc), while electrostatic interactions are characterized by potentiometric titration and electrophoretic mobility, which are analysed using both soft and hard particle models. Van der Waals interactions are quantified through Hamaker constants derived from classical DLVO theory and the van Oss–Chaudhury–Good (vOCG) method. The results are interpreted in terms of DLVO and XDLVO theory. All dispersions show monomodal particle size distributions, with particle size decreasing asymptotically as stabilizer content increased. Stability improved with higher concentrations of both AAS and DMPA, although DMPA leads to lower dispersion stability. Increasing the hard segment content leads to an improvement in dispersion stability. Among all structural parameters, the polyol component exerts the strongest impact on stability. PC based PUDs show substantially better stability than those containing PolyTHF or PE diols, indicating that the type soft segment substantially influences particle interactions. Potentiometric titration reveal that a substantial portion of ionic groups remains cannot be detected, as indicated by recovery rates of 70–85% for AAS and 50–75% for DMPA. In addition, both AAS- and DMPA functional groups show altered dissociation behaviour. Therefore, a substantial portion of ionic groups remains buried within the particle matrix, leading to heterogeneous charge distributions. Electrophoretic mobility measurements show a non-zero limiting mobility at high salt concentrations. While the soft-particle model provides qualitative insights, it fails to describe experimental data at low ionic strength and the charge distribution accurately. In contrast, hard particle model considering relaxation effects yields consistent zeta potentials. Surface potentials decrease with increasing stabilizer content, attributed to greater specific surface area, and shear plane distances align well with literature values. However, electrokinetic surface charge densities are markedly lower than those from titration, reinforcing the existence of an ion-penetrable shell. Hamaker constants calculated using DLVO theory indicate the presence of additional attractive forces. Hamaker constants derived from vOCG theory based on contact angle measurements are more consistent and typical for aqueous dispersions. Analysing these findings in terms of XDLVO theory reveals hydrophobic attractions that are dominated by the polyol type. PolyTHF based PUDs show the strongest hydrophobic interactions, followed by PE-based systems, while PC-based PUDs demonstrate the lowest hydrophobic contribution. Increasing the AAS and DMPA as well as the hard segment content results in a reduction in hydrophobic interactions in all series. The results demonstrate that the dispersion stability of electrostatically stabilized PUDs is not governed solely by electrostatic repulsion, but is significantly influenced by the structural components of the polyurethanes. This study thus provides fundamental structure–property relationships and establishes a basis for the rational design of stable, water-based polyurethane dispersions.

Item Type:	Thesis (PhD thesis)
Translated title:	Title Language Struktur-Stabilitätsbeziehungen wässriger Polyurethan Dispersionen German
Creators:	Creators Email ORCID ORCID Put Code Grau, Christoph c.grau93@web.de UNSPECIFIED UNSPECIFIED
Contributors:	Contribution Name Email Censor Schmidt, Annette M. annette.schmidt@uni-koeln.de Censor Wilkens, Jan jan.wilkens@th-koeln.de Censor Leimenstoll, Marc marc.leimenstoll@th-koeln.de
URN:	urn:nbn:de:hbz:38-802667
Date:	2026
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Chemistry > Institute of Physical Chemistry
Subjects:	Chemistry and allied sciences
Uncontrolled Keywords:	Keywords Language polyurethane dispersions English dispersion stability English structure-stability relationships English DLVO English XDLVO English Hamaker constant English Surface potential English
Date of oral exam:	22 January 2026
Referee:	Name Academic Title Schmidt, Annette M. Prof. Dr. Leimenstoll, Marc Prof. Dr. Wilkens, Jan Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/80266