New Technological Solutions to Prevent Ice Formation on Aircraft

TAGS:  Aerospace Coatings    

Ice formation is a serious economic and safety problem in different industries, especially the aeronautics sector. In recent decades, several fatal aircraft icing incidents have been recorded. Contrary to popular belief, these incidents do not occur only in regions with extreme weather conditions, as shown by the accident in Madrid in 2017.^[1]

Besides causing accidents, ice formation can severely affect aircraft performance^[2], especially due to ice build-up on the tail’s horizontal stabilizer, which reduces the plane’s ability to offset the nose-down pitch. Aircraft performance is also affected due to ice formation on the wing surface^[3] by increasing resistance and decreasing the ability to create lift.

A great deal of work has gone into solving this problem, based on two different strategies:

• Anti-icing or passive solutions (See image below) and 
• De-icing or active solutions

Let's explore AIMPLAS' role in the MAI-TAI project to prevent ice formation on aircrafts using new technological solutions.

The MAI-TAI Project – What is the Main Objective?

The MAI-TAI Project (Multidisciplinary Approach for the Implementation of new Technologies to prevent Accretion of Ice on aircrafts) aims to develop new ice protection systems that improve on the state of the art by considering aspects such as reducing energy consumption and facilitating application, integration, repairability and durability. This project also aims to design specific analysis to properly test the materials developed.

MAI-TAI is being developed by a consortium made up of the Universidad Pública de Navarra and three technology centers, the National Institute for Aerospace Technology (INTA), the Association of Industry of Navarre (AIN) and AIMPLAS, the Plastics Technology Center.


AIMPLAS’ role in this project is to develop:

1. Passive systems based on low surface energy polymers,
2. Active systems based on Joule effect heating that provides surface heating, as well as 
3. Hybrid systems that combine both methods.

AIMPLAS is also working on the formation of ice in climate chambers and developing a method to determine the adhesion of ice to coatings.

Anti-icing and Passive Solutions

These solutions focus on preventing surface ice formation and involve treating surfaces to prevent water build-up and increase water repellency. The hydrophobicity or water-repellency approach is based on the notion that a surface that cannot become wet has less contact with water, so ice will not be able to adhere to it.^[4] Hydrophobicity has been observed in nature, such as in the lotus plant, where the combination of non-polar substances and hierarchical roughness keeps the plant’s leaves clean even in very damp environments.

Hydrophobic surfaces have contact angles with a drop of water of between 90° and 120°. A surface is superhydrophobic when this angle is over 120° (see image below). Freezing can be prevented if the drop quickly slides off the surface. Therefore, if certain additives can be used to reduce the surface energy, the contact angle may be increased and ice adhesion reduced, thus avoiding ice nucleation.

In order to implement these surface modifications, the most common strategy is applying coatings or the additivation of compounds to minimize surface energy. Additivated materials with low surface energy include fluorinated materials, polymethylsiloxane and paraffin. Inorganic compounds can also be deposited using methods such as physical vapor deposition (PVD), chemical vapor deposition (CVD), and sol-gel synthesis.^[5]

Passive Solutions Developed by AIMPLAS

AIMPLAS’ passive solutions include the development of coatings with low surface energy polymers and nano-additives.

• Different formulations based on polyurethane paints additivated with hydrophobic nanoparticles have been tested with the aim of increasing the coating’s contact angle.
• Organosilane compounds are also being tested to improve compatibility with the polymer matrix.

In these formulations, different concentrations of these compounds are used and the most promising ones are deposited on substrates of aluminum, which is commonly used in the aeronautics sector. The image below shows a formulation in which static contact angles of 140° were obtained, which can be considered hydrophobic, as well as slipping angles of about 10°, which allow the drop of water to slide at a low angle.

De-icing or Active Solutions

Active solutions are based on eliminating ice once it has formed and preventing it from forming by applying mechanical and thermoelectric methods. Current systems have a number of drawbacks, including high energy consumption and maintenance costs. Installing these systems also increases aircraft weight, which has a negative effect on fuel consumption and, therefore, the environment. Moreover, these systems based on metal meshes have low corrosion resistance^[6].

New thermoelectric systems in which electric current is transformed into heat based on the Joule effect may have the advantage of being more lightweight and corrosion-resistant. In the Joule effect, part of the kinetic energy in the electrodes of conductive material is transformed into thermal energy, resulting in a coating in which heat is built up on the surface.

The challenge involves transforming non-conductive coatings such as paint into conductive materials through the additivation of carbon compounds, as well as properly designing the system to produce a heating device. The image below represents a heating system in which the surface of the material is heated through the application of electricity.

Active Solutions Developed by AIMPLAS

Active systems developed by AIMPLAS to prevent ice formation have focused on evaluating the formulation of heating paints based on carbonaceous nanomaterials such as:

• Graphene
• Graphite
• Carbon nanotubes, and
• Carbon blacks

When mixing additives, different proportions and dispersion methods, different mixers and calenders, and different application methods are used, but it is always important to control the thickness of the conductive layer. The image below shows a system with a heating coating, where the plastic film (PET) is used as a substrate and silver is used for the electrodes. A voltage of 40V is applied to this system to heat the surface.


In addition to the work done on formulation, progress has also been made in electrode design and layout, and in substrate pre-treatment to prevent short circuits in the final system. Parameters for measuring the heating effect obtained have been defined using an infrared camera to determine the maximum surface temperature the system can reach. The image below shows a study in which surface temperature increases when the applied voltage is increased.


Performance was assessed at room temperature and also from -20°C. The image below shows how temperatures over 0°C were reached in less than a minute when a voltage of 40V was applied.

Development of Hybrid Systems

During this project, the aim was to go one step further in the development of anti-icing systems, so the decision was made to take a synergistic approach to combine both solutions with a multilayer system. The aim of these hybrid systems is therefore to prevent ice formation and, if ice forms, to eliminate it with minimum energy consumption and weight increase. Very few devices of this kind can be found in the scientific bibliography, which means work must be done to further develop solutions of this kind.^[7,8]

Based on the passive and active systems described above, a hybrid system has been designed. In order to use this kind of system, different layers are added. The substrate is first covered with a dielectric layer to insulate it from the conductive coating, which is the second layer. Finally, a third insulating layer must be applied. In this case, it consists of the passive coating. This hybrid system maintains its heating and water-repellent properties (see image below).

Test the Effectiveness of Systems

This year, the final year of the project, the performance of these systems will be evaluated in a wind tunnel, where the real ice-formation conditions will be simulated (see image below). At the INTA facilities, parameters can be set, such as temperature, wind speed, liquid water content (LWC) and the drop median volume diameter (MVD) of water, to perform anti-icing studies to observe the formation of the ice layer and de-icing studies to assess elimination of this layer.

Conclusion

At the beginning of the MAI-TAI Project, the requirements and technical specifications were defined for ice protection systems for the aeronautics sector. Passive and active systems were then developed for combination with hybrid systems. A wind tunnel is being used to test the effectiveness of the systems developed in conditions of ice formation. The results are then analyzed and interpreted to assess the durability of the anti-icing effect achieved.

Development of the MAI-TAI Project, which falls within the framework of the national R&D Collaboration Challenges call and is funded by the Ministry of Science, Innovation and Universities, represents a major step forward towards solving the serious problem of ice formation and can be applied to other sectors, such as wind turbines and the automotive and construction industries.