Self-replenishing low-adherence polymer coatings

Abstract

In light of the sustainable development, demands on polymeric materials with respect to their efficiency, ‘environmental friendliness’, etc. have increased. The service lifetime of materials would benefit from their ability to retain functioning upon damage or wear–off. Therefore, self-healing materials, having this ability, have been of the increasing research interest during the past decade. In the field of polymeric coatings, apart from sustaining the structural integrity of material, one important aspect in prolonging the service lifetime is retaining a certain amount of functional groups at different interfaces after the damage in a self-replenishing fashion. Low-adherence coatings, prepared via surface-segregation of low surfaceenergy groups during the curing process, are widely used today for their water/oil repellency, making them easily cleanable. For such coatings, low surface-energy additives are usually species containing perfluoroalkyl group at one, and hydroxyl group (or any other group that reacts with the crosslinker or resin) at the other end of a molecule. The service lifetime of these materials suffers from having a very thin fluorine-rich layer (approximately 20 nm), which can easily be removed by damage or wear off, and therefore the lowadherence character eventually may disappear. Herewith self-replenishing low-adherence coatings are those able to retain their low surface energy upon damage by recovering a sufficient amount of low surface-energy species at newly created surfaces. To summarize the findings of this research, the surface created after the damage will fully replenish in fluorinated groups if: • the bulk of the coating contains a sufficient amount of fluorinated groups • the mobility of these fluorinated groups is sufficient • the driving force for their movement towards the new interface is substantial The approach we have taken is to relatively homogeneously distribute fluorinated species that contain a long polymer spacer throughout the bulk of a film. In the case of a surface damage, that leads to the loss of the top layers of a coating, these species can reorient or even move from sublayers in order to minimize the new air/film interfacial tension. Thus the low adherence character can be sustained. The synthesis of well-defined perfluoroalkyl-end-capped precursors was performed via ‘living’ ring-opening polymerization of e-caprolactone with perfluoroalkyl-alcohol used as initiator, as described in Chapter 2. These fluorinated species were able to segregate at the surface of coatings during the curing of a film. The introduction of a polymer spacer facilitated their miscibility with the coating formulation, thus the bulk level of fluorine could be increased when compared to coatings containing fluorinated species that lack spacer as shown in Chapters 4 and 5. Once the top layer of a coating was removed by means of microtoming, the fluorinated species were reorienting spontaneously towards the new air-film interface. The F/C ratio remained similar to that of the original surface as shown by angle resolved XPS. Furthermore, the introduction of a polymer spacer appears to be the key point of the replenishing process, because it enabled the mobility of perfluoroalkyl group, as shown in Chapter 5. The driving force was the difference between the surface tension of fluorinated groups and the new interface, as briefly discussed in Chapter 2. The reorientation and movement of fluorinated species that are exposed directly at the new interface, as a result of damage, alone is not sufficient for the replenishing process. Movement of fluorinated species from sub-layers of network towards the new interface is a result of the mobility of the coating network as well. When the coating has a Tg well below room temperature, the replenishing process is autonomous. However, coatings with the Tg above room temperature do not have the ability of autonomously replenishing in a rapid fashion. Relatively fast replenishing can be triggered by the increase of temperature as described in Chapter 6. Microtoming in combination with XPS and force-displacement measurements were used to characterize the replenishing of ‘high-Tg’ coatings after annealing. The self-replenishing of low surface-energy groups placed on the dangling chains of a coating network is a process that can be further employed for sustaining different surface functionalities in a self-healing fashion.