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, lowtemperature 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.
Particle morphology is a key characteristic of the waterborne polymer dispersions and plenty of effort has been dedicated to understand the mechanisms controlling the development of the morphology during polymerization. The availability of new characterization techniques that provide unprecedented quantitative details of the particle morphology have questioned the ideas about the driving forces ruling the development of the morphology. In this article, the case is considered of a seeded emulsion polymerization in which the second stage polymer (Polymer 2) is more hydrophobic than the seed polymer and a water-soluble initiator is used. Simulations of the effect of the different forces involved in the formation of the particle morphology carried by integrating the Navier-Stokes are compared with available experimental results. If is found that the interfacial tensions are responsible for the penetration of clusters of polymer 2 within the seed polymer and the spread of these clusters over the surface of the particle. On the other hand, van der Waals forces control coalescence of the clusters both at the surface and in the interior of the particle.
In the present study, a thin intermediate protective α‐alumina layer was pre‐deposited by slurry dip‐coating technique on APS–CoNiCrAlY bond coat prior depositing APS–YSZ as top coat. The alumina layer acts as barrier for oxygen diffusion to protect the metallic bond coat from excessive oxidation. The new TBC was compared with the standard TBC system which comprises of Ni‐superalloy substrate, CoNiCrAlY bond coat and YSZ top coat concerning the behavior under thermal cycling at 1150 °C. The samples were investigated before and after thermal cycling by optical microscopy, scanning electron microscopy (SEM) equipped with energy dispersive X‐ray spectroscopy (EDS), and X‐ray diffraction analysis (XRD). Image analysis was used to compare between the microstructure by measuring the crack lengths and the thermally grown oxide layer (TGO) average thickness after thermal cycling. It was observed that a more uniform and compact α‐Al2O3 TGO was formed in the new TBC system after thermal cycling comparing to the commercial one. It was, also, found that the alumina intermediate layer has the potential to reduce the length of the cracks and the average thickness of TGO by suppressing the detrimental mixed oxides, like Cr2O3, CoO, NiO, and (Ni, Co)(Cr, Al)2O4, thus, improving the TBC durability.
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