C. Becker-Willinger, B. Ali, J. Brunke, G. Wenz
INM - Leibniz Institute for New Materials,
Keywords: coating, self-healing, polyrotaxane, nanocomposite
Summary:Rigid surface coatings with glossy appearance showing self-healing behavior of superficial micro-scratches, that usually are visible with the naked eye, are desired for many applications such as e.g. car body paints or smartphone covers because of aesthetic reasons. A physical approach towards self-healing based on a stress relaxation mechanism is provided by so called “slide-ring gels” (SRGs), which are composed out of cross-linked polyrotaxanes (PRs). PRs are supramolecular assemblies which comprise a ring-shaped molecule and a polymer. The ring-shaped molecules (e.g. cyclodextrins (CDs)) are threaded onto the polymer main chain like beads on a necklace. The PRs are accessible by one-pot emulsion polymerization where monomers are complexed by CDs and afterwards co-polymerized with a bulky co-monomer. In the present work nanocomposite coating materials were synthesized from mixtures of cyclodextrin based PRs from emulsion polymerization, functional CeO2-nanoparticles having a heteropolysiloxane surface modification and hexamethylen diisocyanate (HMDI) oligomers as cross-linkers. Scratching and self-healing experiments were performed on nanocomposite coatings applied as top-coat after spray coating on pre-painted surfaces and thermal curing up to 120 °C. Different mixtures out of PRs and increasing amount of surface modified CeO2-nanoparticles were investigated. The coatings show a Martens hardness between 150 MPa and 250 MPa depending on the composition. Partial hydrophobization of the threaded CDs with monofunctional isocyanates prior to coating and cross-linking not only resulted in improved hydrolytic stability, while maintaining the mechanical surface properties, but also led to faster self-healing compared to non-hydrophobized systems. Complete self-healing of superficial micro-scratches was achieved by thermal treatment at 90 °C for only 60 s. The incorporated CeO2-nanoparticles significantly improved the UV-stability as well as the weather-ability. Dynamic differential calorimetric analysis (DSC) and dynamic-mechanical-thermal analysis (DMTA) revealed different phase transition temperatures that can be correlated with the observed self-healing behavior.