Development of passive anti-icing coatings by incorporating ice-binding proteins

Abstract

In cold regions, the formation and accumulation of ice can cause safety hazards and impede proper operation of equipment. By example, ice accumulation on aircraft wings can increase drag, increase weight, reduce upward force and decrease aircraft speed.

For these reasons, proper anti- or de-icing techniques need to be developed. These techniques can be divided in passive and active systems. Active systems require a supply of external energy. On the other hand, by physical or chemical surface modification, passive systems inherently possess anti-/de-icing characteristics without the requirement of external energy. For this reason, this master thesis focuses on the development of passive anti-icing coatings.

One possible new approach to develop passive anti-icing coatings could be to modify surfaces with ice-binding proteins, more specifically anti-freeze proteins (AFPs). These proteins can be found in organisms living in cold climates. They are able to inhibit freezing, thus making life in cold environments possible. Currently, limited research has been performed regarding these AFPs as anti-icing coating material. As a result, this thesis delves deeper into the effect of different environments on the behaviour of AFPs. This is fundamental knowledge that needs to be uncovered before AFPs can be used as an anti-icing material.

In a first step, AFPs are directly attached to the surface in various concentrations using a polyethylene glycol (PEG) chain with a specific chain length. From the freezing data, an unexpected phenomenon was observed. It appears that AFP-surfaces freeze faster with increasing AFP concentration. As such, they act as ice promoter instead of the expected ice inhibitor. It is hypothesized that this phenomenon could be largely attributed to the limited protein mobility on the surface. To further test this theory, AFPs with various linker chain lengths were attached to the surface. Indeed, freezing was detected at later time points with increasing linker chain length meaning that AFPs with higher mobility are able to inhibit ice growth.

In addition, the incorporationof AFPs showed another interesting phenomenon as well. Ice dendrites on the AFP surfaces appeared to grow more straight compared to their silane-treated counterpart. Because of this, dendrites on the AFP-surfaces were also more easy to blow away.

Except for attaching AFPs directly to the surface, their behaviour within a polymeric environment is also studied. For this purpose, various concentrations of AFPs were incorporated within a PEG hydrogel. DSC was used to study the different types of water within the different AFP hydrogels. Interestingly, the amount of freezable bound water increased with increasing AFP concentration. No clear trend could be found between the amount of non-freezing water and the AFP concentration. In addition, freezing tests showed that hydrogels with increasing AFP concentration inhibited ice growth. This behaviour is opposite to the behaviour that was detected for AFPs attached directly to the surface.

In a final test, the hydrogels are dehydrated and again subjected to freezing tests. Now, the freezing behaviour follows a similar trend as the AFP-surfaces, meaning that, with increasing AFP concentration, ice formation is promoted.

From these results, it is clear that the environment of the AFPs plays a crucial role to the AFP behaviour. Depending on the type of environment in which they are introduced, AFPs can either act as ice inhibitor or ice promotor.