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Analyzing Durability Of iNsulating materIalS

Final Report Summary - ADONIS (Analyzing Durability Of iNsulating materIalS)

Executive Summary:
The SAGE2 project was conceived with the goal of developing a novel counter-rotating open-rotor demonstrator. Implicit in its design is a system capable of deicing the rotors. The most effective method of deicing is one which uses electrical resistance to heat the rotor blades; however, in order to deploy such a deicing technique, it is imperative that the electrical systems supplying power are capable of withstanding the associated harsh environmental conditions.

The ADONIS project has as its focus the elucidation of the behavior of insulation materials with respect to high-temperature and chemical exposure aging, in order to provide information to assist in selecting the most appropriate products for use in the SAGE2 system. It evaluated the materials by exposing them to aging conditions consistent with their use under different conditions: immersion in several liquids such as oil, hydraulic fluids, and other specific products. Material analysis were then performed, following procedures previously established, in order to identify the degradation routes and analyze residues of their performances. Literature review led to the choice of characterization techniques, and all materials were analyzed using DMA, DSC, TGA, IRTF and mechanical tests.

RESCOLL is a specialist in materials testing and characterization. It holds qualifications from some of the most important companies in the aerospace industry (SAFRAN, AIRBUS, GENERAL ELECTRIC, DASSAULT AVAIATION...). RESCOLL is regularly performing expertises and developments for its customers and the French court.

The project lead to the development of a structured, rational, and multi-criteria procedure for testing and selecting insulation materials according to their specific environment. RESCOLL now an important database of materials performance with identification of their most important degradation routes. This database, ranges from standard to engineering materials such as polyimide, PEEK films, epoxy, and silicone resins. It allows RESCOLL to better respond to our customers’ needs in selecting the most appropriate candidates for their applications. Finally, all the testing procedures have been standardized and, when suitable, certified (ISO 17025) and qualified, in order to provide validation services of the robustness of final products.

The project also contributed to supporting that the Topic Manager in their material choices for designing a reliable de-icing system, permitting the use of the open rotor under harsh conditions.
Project Context and Objectives:
The ADONIS project was initiated to study the behavior of electrically insulating materials exposed to the chemical environment commonly encountered in and around aircraft engines. This aggressive environment can lead to a premature degradation of the materials leading to a modification of properties and a subsequent effect upon the operation of the rotor.

Researches in the literature performed during the first months of the project led to the choice of adequate analytical techniques to study the evolution of properties. Materials aging was described in several studies. Even if it was not possible to get information about the same resins as those selected for ADONIS project, some generalized information were obtained. The analytical techniques selected for the project were:
- TGA, to follow weight loss
- DSC, DMA and TMA to measure Tg and other properties (enthalpy, modulus, ...)
- Infrared, to observe chemical modification
- Dielectric spectroscopy, to measure permittivity and other electrical properties
- Mechanical tests, to follow specific mechanical properties
After the purchase of all materials, the first step was to improve the curing of resins in order to obtain specimens with adequate dimensions for aging and analysis. Initial state materials were characterized using all selected techniques to obtain reference values for the study.
All cured materials were subjected to a preliminary aging for 2 to 9 weeks at 170°C. This temperature is an average value typical of engine operational environments. The aim of this first aging was to observe the behavior of the materials. The aging temperature has to be high enough to induce degradation but not too high to avoid quick and total decomposition of the material.
Following the preliminary aging and baseline evaluation of the samples, a final aging of the materials was performed at the adequate temperature, depending on the results obtained during the first stage. The aim of this step was to perform a long aging and follow the evolution of material’s properties to determine the most resistant materials for this application. Concurrently with this study, aging of the interfaces between resins and metallic sheets or wires was tested. The testing of such assemblies is closer to the final use of resins and is helpful for the selection of the better materials with good adhesion properties.
Project Results:
All materials were characterized using infrared, DSC, DMA and TGA. Initial state analysis implied 1 infrared spectrum, 3 to 6 DSC thermograms, 3 DMA thermograms and 2 TGA thermograms for almost all materials. As it was not possible to analyze silicone resins by DMA (the specimens were too brittle and glass transition was not always observed by DSC), 3 TMA analyses were performed on silicone resins before aging. For films, mechanical tension tests were also performed, using 5 specimens for each material. Dielectric measurements were also performed, but their results were not relevant. As a result, it was decided not to perform further analyses.
During the preliminary aging step, mass evolution was followed by weighing 3 specimens for each material. For the first week, specimens were weighed on a daily basis. For the following weeks, only one weighing per week was performed, until stabilization was achieved. After 2 to 9 weeks, materials were characterized using infrared spectroscopy, DSC, DMA, TGA and TMA. Results are summarized in Annex 1.
According to this preliminary aging study, final aging conditions were determined:
- Silicone A : final aging at 200°C
- Silicone B : final aging at 150°C
- Epoxy A : final aging at 200°C
- Epoxy B : final aging at 170°C
- Polyamide-imide : final aging at 170°C
- Polyimide, PEEK an polyamide films : final aging at 200°C

Exposure of oil to high temperature led to a high amount of vapors. As it was not possible to use 3 ovens under aspiration, it was decided not to perform further study on Silicone B resin. As this material was already affected by oil immersion at 170°C, it has a lower resistance under engine environment than the other and consequently was not a priority.
Aging at 200°C was performed from March 2015 to March 2016. It was stopped for a few months due to technical issues with the oven. Aging at 170°C was performed from July and September 2015 to March 2016.
During the aging, several properties were measured at different times. Films were weighed 13 times in 253 days. Epoxy B, Silicone A, and Epoxy A resins were weighed 15, 8 and 5 times in 222, 162 and 134 days, respectively. Around 60 infrared spectra, 100 DSC, 80 DMA, 10 TMA, 60 ATG and 10 Shore D hardness measurements were recorded during the aging of all materials. Comparison between the initial state and post-aging properties are summarized in Annex 2.
Additionally, interfaces between resins and wires or metallic sheets were studied. It was important to select resins with good resistance to aging. According to the results, Epoxy A and B, and Silicone A resins were chosen.
Small cups with tapped lids were used to cure the resin around the wire. Curing of Silicone A resin was quite difficult, and the resin was not totally polymerized. Adhesion was then tested by pulling the wire to separate it from the resin. After aging in oil, the strength is generally lower.
The curing of resins at the surface of metallic sheets was not easy, due to the low viscosity of the resin at the temperature of curing. After several tests, it was decided to use a qualitative technique of adhesion measurement: the grid test. The adhesion of resins on copper sheets was not good after curing or after aging. Only aluminium sheets gave good results, especially with Epoxy A resin.
In conclusion, Epoxy A seem to be the most suitable resin for an application in aircraft engine. Epoxy B and Silicone A resins gave also interesting results and can be used in this kind of application.
Potential Impact:
Immersion aging tests already represent a considerable source of business for RESCOLL. RESCOLL works with many different aerospace vendors, including SAFRAN Group, AIRBUS, and AIRBUS HELICOPTERS, and has a great deal of experience in testing the chemical resistance of aerospace-grade materials to liquids such as Jet A-1, Skydrol, and oils. However RESCOLL is absent of the market related to electrical and electronic equipment of the aircrafts. As such, RESCOLL is now in an excellent position to put the knowledge and experience gained the ADONIS to immediate use to benefit the aerospace industry. The participation of RESCOLL in the ADONIS project had several benefits:
First of all, the company had benefit from European funds to finance its Research and Development work. In addition, RESCOLL:
- is now familiar and has a deep understanding on electrical insulating materials and their characterization of said materials;
- Characterisation standards and a structured procedure are now in place at company level in the 3 related laboratories (thermos-mechanical analysis, mechanical tests and spectrometry);
- Has now trained technicians and scientist able to test, to interpret results and to have a significant background to assess the performances of insulation materials working under harsh conditions.
- Develop knowledge in the field of electrical insulation characterization which a diversification market in which RESCOLL was absent. This “new” market is extremely interesting as only a limited investment is required for the company to face it.
- increased its visibility of RESCOLL at the European level and offer promotion perspectives for the project outcomes;
- Adapt and increase its current knowledge for this type of application.
RESCOLL hopes that this new market could increase by 10% the turnover in its analysis and characterisation department. This could represent the 3 to 5 positions of qualified personnel in the following years this, excluding the 3 positions that the project has already generated.
Impact on the safety of flight:
Wiring faults have been linked to several major air disasters, notable Swissair Flight 111. The development of new type of wiring insulation that are highly resistant to mechanical stresses and chemical exposure is extremely important for ensuring the safety of future flights. The ADONIS project has advanced this by bringing new methodologies for testing insulating materials which could have far-reaching implications for the way in which materials are now evaluated.
Impact over Systems for Green Operations ITD
The SAGE2 project’s counter-rotating open-rotor engine system is a key component in reaching the goals of reducing of 5% to 9% in CO2 emissions. In order to ensure a maximum of safety during operation of the engine, efficient and robust insulation materials must be found for the engine wiring. The ADONIS not only assist in the selection of such materials, but also advanced on the operational safety by raising the bar for materials testing.

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