Cracking in Films Cast from Soft Core/Hard Shell Waterborne Polymer Dispersions
Hesham Abdeldaim, Bernd Reck, Konrad J. Roschmann, José M. Asúa
Abstract
Soft core-hard shell waterborne polymer dispersions are promising for achieving no-volatile organic compound, mechanically strong coatings able to form films at low temperatures. However, the resistance to deformation of the hard phase creates stresses that may lead to the cracking of the coating, which is catastrophic for substrate protection. Cracking is what hinders the broad use of soft core-hard shell latexes for demanding applications that require thick coatings. This article reports on a thorough study of the effect of particle characteristics and drying conditions on stress generation and crack formation. The morphology of particles and films is characterized in detail. The range of conditions necessary to form a crack-free, mechanically strong, low-temperature film-forming coating is determined. It is shown that the existing mathematical models for cracking are not able to explain the experimental data, and reasons for the failure are discussed. A data-derived criterion for cracking nucleation is proposed.