Novel Coatings with low adhesion to ice
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Atmospheric icing can be a nuisance during the winter season in the Northern hemisphere, both in everyday life and for critical infrastructure. The strong adhesive forces between ice and organic coatings typically employed in the protection of steel and composites, and the subsequent ice build-up on coated structures, poses a challenge to operational, and safety aspects of machine and equipment. A promising strategy to mitigate ice build-up is the use of low ice adhesion materials. There are several promising approaches to design low ice adhesion materials ranging from surface structured surfaces, freezing point depression, lubricated surfaces, soft elastomeric materials, and amphiphilic coatings. The latter approach has received much attention recently and though the mechanism responsible for ice adhesion reduction is not fully understood, the ice shear adhesion reduction described in previous reports is likely caused by the interactions between the ice-coating interphase. The ice adhesion reduction is thus not dependent on the mechanical characteristics of the bulk coating, which makes this approach attractive for a wide range of applications ranging from hard and brittle coatings to soft-elastomers. This project focuses on the synthesis of novel polymeric structures that have amphiphilic properties and are potentially useful in ice release coatings. Amphiphilic graft-copolymers with poly(dimethylsiloxane) (PDMS) and methoxy poly(ethylene glycol) (MPEG)-grafts as the hydrophobic and hydrophilic constituents, respectively, were prepared via a grafted-through approach. The reversible deactivation radical polymerization techniques reverse iodine transfer polymerization (RITP) and atom transfer radical polymerization (ATRP) were explored as a means to improve both control and monomer conversion relative to the widely employed free radical polymerization technique. It was found that improved control and monomer conversion could be obtained with the industrially relevant RITP-technique. The ATRP-procedure gave somewhat improved control over RITP, but the monomer conversion was in most cases low and indicated that the initiators and the catalyst-ligand complexes employed in this work were not optimal for the copolymerization of methacrylate functional PDMS macromonomers and methyl methacrylate. However, the promising results for the copolymerization’s employing RITP were further explored in the preparation of poly(MMA-co-PDMSMA-co-MPEGMA) graft-type amphiphilic copolymers. This resulted in a library of amphiphilic graft-copolymers with a wide range of monomer compositions. The ice adhesion test methodology also received attention in this thesis. An adaption of a widely used ice shear adhesion test was compared to static friction measurements between ice and three commercially available coatings. The static friction measurements between ice and coatings were shown to relate to the results obtained via the ice shear adhesion and thus suggesting that for these types of coatings, on a macroscopic level, there’s a relation between ice adhesion and ice-friction. The ice shear adhesion of amphiphilic graft-copolymers incorporated into a polyurethane coating was evaluated. The composition and structure of the amphiphilic graft-copolymers were correlated to the ice shear adhesion and wetting properties of the coatings. The coatings having the lowest ice shear adhesion displayed wetting properties that are characteristic of lubricated surfaces i.e., low contact angle hysteresis. It is hypothesized that the liquid-like nature of the PDMS-grafts is responsible for the lubricating nature of these coating and the resulting low ice adhesion properties. The graft-copolymers with demonstrated ice shear adhesion reduction in a polyurethane coating were also probed as an additive that can enhance ice release properties of commercially available wind turbine blade coatings. It was found that an 85% reduction in ice shear adhesion strength could be achieved by employing 10 weight% graft-copolymer on the solids coating formulation.