Caroline Laforte

The Relationship between Water Wetting and Ice Adhesion

Ice accretion on aircraft leads to difficulties in aircraft flight control due to weight increase and change in weight distribution. Conventionally these difficulties are overcome using anti-icing or de-icing products, such as freezing depressants and heating devices. A more cost effective way to solve these problems would be to use ice repellent surfaces (ice-phobic). As a first step in this direction the relationship between water wettability and ice adhesion was investigated. Using the appropriate chemistry and tailoring the surface roughness a variety of polycarbonate-coated surfaces were created: these included ultra-hydrophilic and ultra-hydrophobic surfaces and surfaces with surface properties in between the extreme ultra-surfaces. Ice adhesion tests and contact angle measurements indicated that the higher the contact angle the lower is the adhesion of ice. The best results were obtained in the case of ultra-hydrophobic surface treatment that led to an 18 fold decrease in ice adhesion compared to the untreated aluminum surface.

Deicing Strains and Stresses of Iced Substrates

Abstract The performance of low ice adhesion or icephobic coatings might be improved by adding a mechanical energy component via an underlying low powered electro-deformable substrate. The strain could be generated with many types of smart actuators consisting of piezoelectric devices, shape memory alloys (SMAs), conductive polymers, or ionic membrane polymer composites (IMPCs). An important step in designing a new electromechanical deicing system would consist of measuring the level of strain needed within an iced substrate to break and shed ice at the interface. In this paper are presented the results of an experimental investigation, in which new set-ups were built and used to simulate the behaviour of an active iced electromechanical substrate generating three types of strains: tensile, torsion, and bending. A total of 174 icing/deicing tests were conducted with aluminum and polyamide test specimens covered with hard rime ice deposits 2, 5 and 10 mm thick and stressed at −10°C at various strain rates in the brittle ice regime. Real time strains and forces were precisely monitored using strain gages and load cells. The stress was calculated from the deicing strain ϵ deicing and force measured at time of deicing corresponding to an interfacial failure between ice and substrate and/or cohesive failure of ice. Under test conditions used, strains were found to be very similar in torsion and in tension but about ten times lower in bending. Moreover, stresses and strains at deicing were found to increase with increasing substrate roughness and decrease with increasing ice thickness.

Tensile, Torsional and Bending Strain at the Adhesive Rupture of an Iced Substrate

In order to develop an effective deicing device using mechanical deformation of substrates, the adhesive and/or cohesive strains of ice at rupture were measured for three different modes of solicitation: tensile, twisting and bending. A total of 108 icing/deicing tests were conducted with aluminum and nylon samples covered with hard rime ice deposits 2, 5, and 10 mm thick strained at various strains rates in brittle regime at −10°C. Real time deformation was precisely monitored using a strain gage fixed to the A1 interface, and force by means of load cells and a torque-meter. Deicing strain was determined at the time of ice detachment, which corresponds to a visible, instant change in the slope of stress-strain curves. The mean values of deicing strains, e %, measured in tensile, torsion and bending experiments are respectively, 0.037 ± 0.015%, 0.043 ± 0.023% and 0.004 ± 0.003% As for adhesion strength, the highest values were obtained in tension, 4 MPa ± 50%, and the lowest in bending, 0.014 MPa ± 36%. In torsion, the value was intermediary, at 1.26 MPa ± 67%. Measurements also showed that deicing stress and strain tended to increase with substrate roughness, whereas they decrease with increasing ice thicknesses. In summary, this work points out the effects of two major factors on ice adhesion strength, the solicitation mode and the ice thickness. Finally these results suggest that the first criteria for a mechanical deicing device has to satisfy to be effective is to have the capacity to generating a strain at around 0.04% ice/substrate interface.Copyright

Icephobic Coating Evaluation for Aerospace Application

This paper presents an extended study on the properties of icephobic coatings for aerospace applications, more specifically rotorcraft. Ideally, an icephobic material should be the least expensive anti-icing solution, inasmuch as the anti-icing effect is not destroyed by inclement environmental conditions. Several studies on the subject have already been published. Nevertheless, in most cases, only the results of ice adhesion on freshly coated samples are presented and discussed. In this study, the effects of aging (weathering and erosion) and of the number of ice/deicing cycles on coating durability were also considered relative to their possible use on airplanes and helicopters. In the first step, ice adhesion was measured in Centrifuge Adhesion Tests (CAT) on eight promising coatings. The ice adhesion τadh of the candidate coatings was found to vary from 0.001 MPa to 0.16 MPa. In the second step, in order to analyze ice accretion and ice shedding, four favorable coatings were evaluated in a wind tunnel on scaled-down rotor (SRB) set-ups, which were iced and rotated until shedding occurred. Regarding the environmental aspect, the durability of the utmost ice adhesion reducer coatings was evaluated under rain and sand erosion, as well as multiple icing/deicing exposures.

Issues and Testing of Non-Glycol Aircraft Ground Deicing Fluids

Deicing fluids are used to remove and prevent ice formation on aircraft before takeoff. These fluids are essentially composed of water, a freeze point depressant (FPD) usually glycol, a surfactant or wetting agent and a corrosion inhibitor. All commercial fluids are qualified to SAE (Society of Automotive Engineers) specifications, which test for aerodynamic acceptance, anti-icing endurance, corrosion inhibition, material compatibility, fluid stability and environment. However, these tests have been built around a fluid with a glycol FPD. More recently, with environmental pressure, fluids with other FPDs have been developed and qualified. The other FPDs include: acetates and formate salts, sorbitol, and other undisclosed FPDs. The acetates and formates, which came out in the early 1990s led to suspected corrosion problems. This led to the additional requirement for corrosion tests for non-glycol deicing fluids in paragraph 3.1.1 of AMS1424. This is essentially only a relevant for such a salt based non-glycol fluid. Next, came a sorbitol, or sugar, based fluid in the early 2000s. As with the formate and acetate salts, it passed all the required tests of AMS1424 including the additional corrosion test. But then in field tests, where the fluid was heated as per usual use, there were problems with foam, sticky and slippery residues. All standard specification laboratory tests are conducted on cold fluids, since this is the worst case for glycol-based fluids, where they are most viscous. However, other FPDs may have the fluid increase in viscosity with heating and evaporation. Following the failed field tests, tests were conducted in the laboratory which showed that when the fluid was heated to high levels of evaporation, the aerodynamic acceptance test was not met and with further evaporation, the fluid solidified. This does not occur with glycol-based fluids since glycol is a liquid. Furthermore, in the lab, mold developed on some exposed fluid left out in a Petri dish. The FAA has since removed this fluid from their list of qualified fluids in the official FAA Holdover Time Tables [1]. More recently, there have been newer fluids that are non-glycol-based, or have another FPD along with the glycol (low-glycol). These fluids all are qualified at least for aerodynamic performance and anti-icing endurance and two are on the current FAA list of qualified fluids. However, the specification has no tests to address stickiness, solidification or tendency for mold to form. For the foam, a test was added to the specification since this issue was arising equally with glycol-based fluids. As part of a grant from the FAA, AMIL is developing test protocols to be added to the test specifications to address the new potential issues that may be required of non-glycol fluids before their use on aircraft. Beyond the corrosion and foam issues for which tests currently exist in the specification, test for aerodynamic acceptance on evaporated heated and sheared fluids, tendency to from mold and slipperiness are proposed.

Icephobicity: Definition and Measurement Regarding Atmospheric Icing

Atmospheric ice that adheres to structures and accumulates is a critical issue in numerous northern areas. Even if different de-icing methods exist, they consume a great deal of energy or necessitate elaborate infrastructures. However, using coatings with icephobic properties could be the “miracle” solution. This chapter proposes a complete definition of icephobicity in line with the ice adhesion test methods used. The general way to assess this property is described using a holistic approach, the first step of which is a screening test campaign with many different candidate coatings evaluated in terms of their adhesion reduction factor (ARF). The relevance of this factor is also discussed. Further tests are recommended, after the better candidate coatings are identified, in an extensive test campaign performed under simulated icing and outdoor conditions prevailing in the real environment of the targeted application. Finally, a specific example of a test campaign in which the icephobic coatings are exposed to Arctic offshore conditions is presented.