Self-replenishing ability of cross-linked low surface energy polymer films investigated by a complementary experimental-simulation approach.

Nowadays, many self-healing strategies are available for recovering mechanical damage of bulk polymeric materials. The recovery of surface-dependent functionalities on polymer films is, however, equally important and has been less investigated. In this work we study the ability of low surface energy cross-linked poly(ester urethane) networks containing perfluorinated dangling chains to self-replenish their surface, after being submitted to repeated surface damage. For this purpose we used a combined experimental-simulation approach. Experimentally, the cross-linked films were intentionally damaged by cryo-microtoming to remove top layers and create new surfaces which were characterized by water Contact Angle measurements and X-Ray Photoelectron Spectroscopy. The same systems were simultaneously represented by a Dissipative Particles Dynamics simulation method, where the damage was modeled by removing the top film layers in the simulation box and replacing it by new "air" beads. The influence of different experimental parameters, such as the concentration of the low surface energy component and the molecular mobility span of the dangling chains, on the surface recovery is discussed. The combined approach reveals important details of the self-replenishing ability of damaged polymer films such as the occurrence of multiple-healing events, the self-replenishing efficiency, and the minimum "healing agent" concentration for a maximum recovery.

Curing processes in solvent-borne alkyd coatings with different drier combinations.

The concern regarding the effect of chemicals on the environment has increased considerably in recent years. Nowadays, technological developments in the coating industry are largely influenced by environmental issues and subsequent legislation. One of these issues is the tendency to replace cobalt as a catalyst with more environmentally friendly alternatives, because studies have indicated possible carcinogenicity. Not much knowledge is available on the effects of catalysts (driers) on the in-depth drying of alkyd coatings. Therefore we have studied the effect of cobalt as a primary drier combined with Ca and Zr as secondary driers on the in-depth curing of high solid solvent-borne alkyds. The profiling of the curing of alkyd coatings is performed with a new high-spatial-resolution NMR setup. In this study, two effects observed in the solvent-borne alkyd coatings are investigated. One is that when Ca and Zr are added as secondary driers the speed of the observed cross-linking front increases. Second, in the deeper un-cross-linked region below the front, the signal of the NMR profiles was found to decrease proportional to . This could be explained by the presence of slowly reacting species that diffuse into the deeper uncured region of the coating, after which they cross-link. The model describing the effect of these reactive species also indicates that the signal decrease is inversely proportional to coating thickness L, which was confirmed by additional measurements.