chemosensors

Review Health Monitoring of Aviation Hydraulic Fluids Using Opto-Chemical Sensor Technologies

Andreas Helwig 1,*, Gerhard Müller 1,2 and Sumit Paul 1,3

1 Airbus, Central Research & Technology, Materials, D-81663 Munich, Germany; [email protected] (G.M.); [email protected] (S.P.) 2 Department of Applied Sciences and Mechatronics, Munich University of Applied Sciences, D-80335 Munich, Germany 3 BMW AG, D-80788 Munich, Germany * Correspondence: [email protected]; Tel.: +49-89-607-28197



Received: 14 November 2020; Accepted: 9 December 2020; Published: 13 December 2020


Abstract: Passenger safety requires that in commercial airplanes hydraulic actuators be powered by fire-resistant hydraulic fluids. As a downside, such fluids are hygroscopic which means that these tend to accumulate humidity from the environment and that the dissolved humidity tends to produce acidity which can corrode all kinds of metallic components inside a hydraulic system. As such damage in safety-critical subsystems is hard to localize and expensive to repair, sensor technologies are required which allow the state of water contamination and fluid degradation to be routinely checked and necessary maintenance actions to be scheduled in a way that causes minimum flight interruptions. The paper reviews progress that has been made in developing such sensor systems and in commissioning these into practical flight operation. Sensor technologies that proved optimally adapted to this purpose are multi-channel non-dispersive (NDIR) systems working in the mid-infrared range. Additional options concern optical absorption sensors working in the near-infrared and visible ranges as well as fluorescence sensors.

Keywords: hydraulic fluid; fluid degradation; water contamination; total acid number; health monitoring; non-dispersive infrared absorption; fluorescence

1. Introduction In commercial aircrafts, flats, slats, tail plane fins and landing gears, i.e., all kinds of safety-critical mechanical subsystems, are powered by hydraulic actuators. The reason for using hydraulics is that it can generate large forces at comparatively low weight. As the performance of hydraulic actuators is critically dependent on those fluids that power them, important maintenance issues arise regarding such fluids. Fire-resistant phosphate-ester fluids, which are commonly used in commercial airplanes, are hygroscopic in nature [1–4] which means that these tend to accumulate humidity from the environment through air-pressurized reservoirs and leaky seals. Absorbed water vapor together with Joule heat, which is generated upon hydraulic actuation, tends to break down the base fluids into smaller molecular fragments and into several kinds of acidic phosphates which are problematic for two reasons: firstly, such fragments support autocatalytic reactions, which speed up the degradation process, and secondly, these tend to corrode all kinds of metallic components inside a hydraulic system, thus generating metallic particle debris. Once caught inside pumps, valves and actuators, such debris can cause severe abrasive damage and ultimately gross mechanical failure. As such downstream mechanical damage is hard to localize and expensive to repair, the continued performance of hydraulic fluids needs to be ascertained through repeated measurements. Conventionally this is done during C-checks—typically once in a year—by tapping fluid from the

Chemosensors 2020, 8, 131; doi:10.3390/chemosensors8040131 www.mdpi.com/journal/chemosensors Chemosensors 2020, 8, x FOR PEER REVIEW 2 of 20

Chemosensorspressurized2020 reservoir, 8, 131 and by sending the tapped fluid to specialized laboratories where a range2 of 20of relevant fluid properties can be determined. Depending on these results, either water is withdrawn pressurizedfrom the hydraulic reservoir fluid and by and/or sending the the fluid tapped is partially fluidto or specialized completely laboratories exchanged. where A key a range problem of relevantinherent fluidin this properties approach can is bethat determined. this maintenance Depending-related on information these results, typically either water becomes is withdrawn available fromwith the a delay hydraulic of one fluid week and /or the more, fluid i.e., is partially when the or completelyserviced aircraft exchanged. should A keynormally problem be inherentback in inoperation. this approach In case ismaintenance that this maintenance-related urgently needs to be information done on the typicallyfluid, the becomesplane might available get grounded with a delayin a remote of one airfield week or with more, poor i.e., infrastructure, when the serviced where aircraftsuch maintenance should normally is hard be toback perform. in operation. As such Inunscheduled case maintenance maintenance urgently is needs associated to be done with on an the interruption fluid, the plane of the might normal get grounded flight schedule in a remote and airfieldconsiderable with poorcost to infrastructure, the airline operating where such thismaintenance plane such unfavorable is hard to perform. situations As urgently such unscheduled need to be maintenanceavoided. Airliners, is associated therefore, with have an interruption asked for in- ofplane the normalfluid monitoring flight schedule systems and which considerable allow relevant cost to thefluid airline properties operating to be this measured plane such with unfavorable a daily or weekly situations frequency urgently without need to tapping be avoided. fluid Airliners,from the therefore,pressurized have hydraulic asked for system in-plane [5]. fluid Once monitoring available, systems such monitoring which allow systems relevant would fluid properties allow fluid to bedegradation measured with trends a daily to be or established weekly frequency and necessary without tapping maintenance fluid from activities the pressurized to be planned hydraulic and systemscheduled [5]. to Once occur available, at times suchand in monitoring locations systemsof the airliner would’s allowchoice fluidwithout degradation seriously trendsinterrupting to be establishedtheir flight andschedules. necessary maintenance activities to be planned and scheduled to occur at times and in locationsIn this of thepaper airliner’s we briefly choice review without the progress seriously that interrupting has been theirmade flight along schedules. this line [6–11], focusing on theIn this work paper performed we briefly at review EADS theInnovation progress Works, that has the been predecessor made along organization this line [6–11 of], focusing the current on theAirbus work Central performed Research at EADS & Technology Innovation organization. Works, the predecessorIn Section 2 we organization present a brief of the introduction current Airbus into Centralaviation Research hydraulic & fluids, Technology focusing organization. on their preparation, In Section2 chemicalwe present structure, a brief introduction and relevant into degradation aviation hydraulicprocesses. fluids, In Section focusing 3 we on theirpresent preparation, optical absorption chemical structure, data, obtained and relevant on a rangedegradation of widely processes. used Inhydraulic Section3 fluidswe present and spanning optical absorption over a wide data, range obtained of photon on a range energies. of widely In this used way hydraulic optical fluidssignatures and spanningof fluid contamination over a wide range and fluid of photon degradation energies. have In this been way identified optical signatureswhich can of be fluid used contamination to advantage andin opto fluid-chemical degradation sensor have systems. been identifiedIn Sections which 4–6, state can be of used the art to advantageoptical fluid in sensors opto-chemical and emerging sensor systems.sensor concepts In Sections are reviewed.4–6, state In of Section the art 7 opticalwe summarize fluid sensors our findings and emerging and make sensor proposals concepts for future are reviewed.research. In Section7 we summarize our findings and make proposals for future research.

2.2. Hydraulic Fluids andand FluidFluid DegradationDegradation EstersEsters inin generalgeneral areare producedproduced byby reactingreacting acidsacids (HA)(HA) andand alcoholsalcohols (R-OH),(R-OH), yieldingyielding asas reactionreaction productsproducts thethe estersesters themselvesthemselves andand reactionreaction waterwater whichwhich needsneeds toto bebe withdrawnwithdrawn toto obtainobtain thethe purepure productproduct [[1122]:]: HA + R-OH Ester (A-R) + H O. (1) HA + R-OH→ → Ester (A-R) + H22O. (1)

WhenWhen phosphoricphosphoric acid,acid, (H(H33PO4)) is is used to produce esters, three alcoholicalcoholic sideside groupsgroups cancan bebe boundbound toto aa singlesingle phosphatephosphate core.core. DependingDepending onon thethe kindskinds ofof alcoholsalcohols used,used, moleculesmolecules withwith aliphaticaliphatic andand aromaticaromatic sideside groupsgroups cancan bebe obtainedobtained asas shownshown inin FigureFigure1 .1.

Figure 1. Chemical structure of aircraft certified hydraulic fluids; (a) tributyl phosphate; (b) di-butyl Figure 1. Chemical structure of aircraft certified hydraulic fluids; (a) tributyl phosphate; (b) di-butyl phenyl phosphate; (c) triphenyl phosphate [6]. phenyl phosphate; (c) triphenyl phosphate [6]. Whereas the phosphate nucleus largely determines the fire resistance of the fluid, other fluid Whereas the phosphate nucleus largely determines the fire resistance of the fluid, other fluid properties such as mass density, viscosity, compressibility, and their respective temperature properties such as mass density, viscosity, compressibility, and their respective temperature dependencies are determined by the side groups. The key property of fire resistance of phosphate ester dependencies are determined by the side groups. The key property of fire resistance of phosphate fluids derives from the high electron affinity of the central phosphate cores and their ability to extract ester fluids derives from the high electron affinity of the central phosphate cores and their ability to electrons from excited electronic states inside the side groups towards the central core. In case of fire

Chemosensors 2020, 8, x FOR PEER REVIEW 3 of 20 Chemosensors 2020, 8, 131 3 of 20 extract electrons from excited electronic states inside the side groups towards the central core. In case of fire and high temperatures, the state of electronic excitation of the phosphate-ester molecules is andthereby high kept temperatures, to a minimum the stateand their of electronic resilience excitation against oxidative of the phosphate-ester processes is enhanced. molecules is thereby kept toCommercial a minimum aviation and their hydraulic resilience fluids against mainly oxidative consist processes of mixtures is enhanced.of tributyl phosphate, di/-butyl phenylCommercial phosphate aviation and trip hydraulichenyl phosphate fluids mainly [1–3] with consist their of mixturesmixing ratios of tributyl depending phosphate, on the di specific/-butyl phenylkinds of phosphate applications and envisaged.triphenyl phosphate Whereas [ these1–3] with constituents their mixing may ratios make depending up to 99% on of the the specific fluid, kindsaviation of applicationshydraulic fluids envisaged. also typically Whereas contain these constituents several percent may makes of additives up to 99% such of the as fluid,antioxidants, aviation hydraulicantiwear- fluidsand viscosity also typically improvers contain and several minor percents amounts of of additives metal deactivators such as antioxidants, and dyes [1 antiwear-–3]. Some and of viscositythe most improverswidely used and kinds minor of amountsaviation ofhydraulic metal deactivators fluids is marketed and dyes by [1 –Solutia3]. Some Inc. of under the most the widely brand usedname kinds Skydrol of aviation with their hydraulic main constituents fluids is marketed being listed by Solutia in Table Inc. 1. under the brand name Skydrol with their main constituents being listed in Table1. Table 1. Main ingredients in the widely used family of hydraulic fluids marketed under the brand Table 1. Main ingredients in the widely used family of hydraulic fluids marketed under the brand name Skydrol [1–3]. name Skydrol [1–3]. Substance Weight (%) Substance Weight (%) Tributyl phosphate 58% DibutylTributyl phenyl phosphate phosphate 58%20–30% Dibutyl phenyl phosphate 20–30% Butyl diphenyl phosphate 5–10% Butyl diphenyl phosphate 5–10% 2,62,6-di-tert-butyl-p-cresol-di-tert-butyl-p-cresol 1–5%1–5% CyclicCyclic Epoxy Epoxy Ester Ester 10%≤10% ≤

Once water is taken up into an aircraft hydraulic system, either during normal aircraft operation or through leaks that that might might have have occurred occurred during during operation, operation, the the dissolved dissolved water water molecules molecules tend tend to todrive drive back back the the ester ester formation formation reaction reaction (Equation (Equation (1 (1)),)), thus thus breaking breaking down down a a fraction fraction of the ester molecules into their component molecules again [[1122]. This This degradation degradation process process is is illustrated illustrated in in Figure Figure 22,, emphasizing the important fact that the freed phosphate cores togethertogether withwith thethe dissolved waterwater can form H3O++ ions which can corrode all kinds of metallic components inside the hydraulic system, thus form H3O ions which can corrode all kinds of metallic components inside the hydraulic system, thuscausing causing metallic metallic particle particle debris debris which which can damage can damage and obstruct and obstruct pumps pumps and actuators. and actuators.

Figure 2. Figure 2. ChemicalChemical reactions following the take-uptake-up of waterwater intointo aa phosphate-esterphosphate-ester basedbased hydraulichydraulic fluid, yielding split-off alcohols and partial phosphates, ultimately producing phosphoric acid and fluid, yielding split-off alcohols and partial phosphates, ultimately producing phosphoric acid and + H3O ions via reaction with the dissolved water [6]. H3O+ ions via reaction with the dissolved water [6]. In aviation hydraulic fluids the generation of corrosive hydronium ions is suppressed by epoxide In aviation hydraulic fluids the generation of corrosive hydronium ions is suppressed by additives (see Table1) which act as acid scavengers. With these acid scavengers being present, epoxide additives (see Table 1) which act as acid scavengers. With these acid scavengers being any partial phosphates that are split off from the base molecules by interaction with the dissolved present, any partial phosphates that are split off from the base molecules by interaction with the water immediately become trapped by the epoxy additives and a rapid rise in total acid number (TAN) dissolved water immediately become trapped by the epoxy additives and a rapid rise in total acid is prevented. Figure3 shows that this fluid-internal repair process continues up to an acid scavenger number (TAN) is prevented. Figure 3 shows that this fluid-internal repair process continues up to an depletion level of almost 90%. Within this long period of moderate fluid degradation, any further acid scavenger depletion level of almost 90%. Within this long period of moderate fluid degradation, degradation of the base fluid can be prevented by extracting the dissolved water, typically in an any further degradation of the base fluid can be prevented by extracting the dissolved water, typically overnight ground stop. The most important message contained in Figure3 is that once this critical

Chemosensors 2020, 8, x FOR PEER REVIEW 4 of 20 in an Chemosensorsovernight2020 ground, 8, 131 stop. The most important message contained in Figure 3 is that once4 of 20 this critical level of acid scavenger depletion has been reached, water contamination and hydrolysis lead to a rapid rise in TAN values which can only be counteracted by performing a much more expensive level of acid scavenger depletion has been reached, water contamination and hydrolysis lead to a partial or complete exchange of the fluid. As such a break point in the fluid degradation trend may rapid rise in TAN values which can only be counteracted by performing a much more expensive occur in between two C-checks, it might remain unnoticed for a long time and severe downstream partial or complete exchange of the fluid. As such a break point in the fluid degradation trend may mechanicaloccur in damage between could two C-checks, develop it while might remainthe airplane unnoticed is still for ain long operation. time and In severe summary, downstream Figure 3 vividlymechanical demonstrates damage the could importance develop while of a the more airplane or less is still continuous in operation. monitoring In summary, of Figure the degradation3 vividly process,demonstrates and in particular, the importance of arriving of a moreat early or lessand continuous reliable estimates monitoring of the of theremaining degradation acid process, scavenger reserve.and in particular, of arriving at early and reliable estimates of the remaining acid scavenger reserve.

Figure 3. Development of acidity inside aviation hydraulic fluids as a function of acid scavenger Figure 3. Development of acidity inside aviation hydraulic fluids as a function of acid scavenger depletion [11]. depletion [11]. 3. Optical Signatures of Hydraulic Fluid Degradation 3. OpticalAttempting Signatures to of find Hydraulic optical signaturesFluid Degradation of fluid contamination and fluid degradation, a large Attemptingnumber of fluid to find samples optical were signatures prepared of [10 fluid,11] and contamination contaminated and with fluid controlled degradation, amounts a oflarge water number at of fluidroom samples temperature were and prepared analyzed [10 for,11] their and water contaminated content using with conventional controlled Carl-Fischeramounts of titration water at [13 room]. Other samples had been heat-treated after water-contamination inside small, sealed glass vessels to temperature and analyzed for their water content using conventional Carl-Fischer titration [13]. Other induce hydrolysis and acidification. To speed up thermal degradation, boiling was performed at 180 C samples had been heat-treated after water-contamination inside small, sealed glass vessels to induce◦ for about 6 h, which is equivalent to several months of thermal degradation inside a heavily used hydrolysisaviation and hydraulic acidification. system [To14]. speed Again, up these thermal thermally degradation, degraded samples boiling werewas analyzedperformed for theirat 180 TAN °C for aboutvalues 6 h, usingwhich conventional is equivalent titration to several with potassium months hydroxide of thermal (KOH) degradation [12]. inside a heavily used aviation hydraulicAfter preparation, system the[14 fluids]. Again, were these inserted thermally either into degraded a Fourier-Transform samples were Infra-Red analyzed (FTIR) for or atheir TAN Near-Infrared-Visible-Ultravioletvalues using conventional titration (NIR-VIS-UV) with potassium spectrometer hydroxide and optical (KOH transmission) [12]. spectra were Aftermeasured preparation, over a spectral the fluids range were extending inserted from either about into 2.5 toa Fourier 24 µm in-Transform the mid-infrared Infra- (MIR)Red (FTIR) or 200 or a Near-toInfrared 2700 nm-Visible (UV-VIS-NIR)-Ultraviolet wavelength (NIR-VIS ranges.-UV) spectrometer After measurement, and optical all spectra transmission were normalized spectra towere measuredtheir maximumover a spectral transmission range extending values Tmax fromwhich about occurred 2.5 to in a24 wavelength μm in the rangemid-infrared which proved (MIR) to or be 200 1 unaffected by water contamination and thermal degradation λ 2670 nm (eν 3750 cm− ). Within such to 2700 nm (UV-VIS-NIR) wavelength ranges. After measurement,≈ all spectra≈ were normalized to normalized spectra the strongest absorption feature was the C-H stretching vibration of the phosphate their maximum transmission values Tmax which occurred in a wavelength range1 which proved to be ester molecules, which occurred in the vicinity of λ 3450 nm eν 2900 cm− [15]. In order to unaffected by water contamination and thermal degradation≈ 휆 ≈ 2670≈ nm (휈̃ ≈ 3750 cm−1). Within emphasize those changes in spectroscopic behavior which occurred due to the water contamination such andnormalized thermal degradation,spectra the strongest difference absorption spectra were feature generated was by the subtracting C-H stretching from each vibration measured of the phosphate ester molecules, which occurred in the vicinity of 휆 ≈ 3450 nm (휈̃ ≈ 2900 cm−1) [15]. In spectrum the reference spectrum of an undegraded fluid containing 0.2% of H2O. In this way spectra orderwere to emphasize obtained which those do changes no longer in show spectroscopic the dominating behaviour and unchanging which oc C-Hcurred vibration, due but to the which water contaminationemphasize and very thermal clearly those degradation, spectroscopic difference changes spectra that result were from generated the water by contamination subtracting andfrom the each measuredthermal spectrum degradation. the reference Some of those spectrum results of obtained an undegraded on water-contaminated fluid containing and thermally 0.2% of degradedH2O. In this way spectraSkydrol were LD4 samples obtained are which shown do in Figureno longer4. As show shown the in more dominating detail in Appendixand unchangingA, similar C data-H vibration, were but whichalso obtained emphasize on other very kinds clearly of commercial those fluids. spectroscopic Additionally, changes shown in that Figure result4 are assignments from the to water contaminationmolecular vibrationand the thermal modes which degradation. are discussed Some in moreof those detail results in Appendices obtainedB onand waterC. -contaminated and thermally degraded Skydrol LD4 samples are shown in Figure 4. As shown in more detail in Appendix A, similar data were also obtained on other kinds of commercial fluids. Additionally, shown in Figure 4 are assignments to molecular vibration modes which are discussed in more detail in Appendices B and C.

ChemosensorsFigure 4 2020, 8, 131 5 of 20

Figure 4. Difference spectra as obtained on Skydrol LD4 after controlled contamination with water (top row) and after water contamination and molecular fragmentation (bottom row). The water-contaminated samples contained up to 1.5% H2O, while the degraded ones had TAN-values between 1–3 mg KOH/g as determined by conventional titration. The spectra of the thermally degraded samples were offset by one unit of normalized absorption towards ∆Tnorm = 1 for reasons of clarity − of presentation.

4. MIR Monitoring of Aviation Hydraulic Fluids The most straight-forward way of arriving at an opto-chemical sensor system is concentrating, firstly, on those strong ground state absorptions which derive from vibrations of O-H groups which are either parts of dissolved water or split-off alcohols and, secondly, on those that derive from phosphate cores [16] which had lost their hydrocarbon side-groups. As shown in Figure5a, water contamination leads to an increase in the O-H absorption 1 strength around 3500 cm− . Upon fluid degradation, the O-H absorption feature shifts towards lower 1 wavenumbers around 3300 cm− as O-H groups, initially bound to dissolved water, become bound to alcohols which had been split off from their phosphate cores. As long as the remaining partial phosphates are trapped by acid scavenger molecules, the broad, lower-energy phosphate absorption feature, left to the dominant C-H absorption of the base molecules, remains small. A strong increase in this feature, however, occurs after all acid scavenger molecules had become consumed. In this event, ongoing fluid degradation causes the TAN value to rise and the phosphate absorption feature to increase in proportion to the low-energy O-H absorption. Figure5b shows the decomposition of a measured MIR fluid transmission spectrum into its constituent parts. In this figure circles also indicate those four spectral positions which are probed by the 4-channel NDIR sensor system prototype described below. As described in more detail in our previous publications [10,11], probing the optical transmission inside these four windows allows the water content, the state of fluid decomposition, and the acidification of the fluid to be quantitatively determined. This capability is also demonstrated in Figure6. In addition, and most interestingly, by combining the information of the phosphate and the alcohol O-H absorptions, the remaining acid scavenger reserve can be determined and estimates for the remaining fluid life can be obtained [10,11].

1

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Wavenumber [cm-1] -- 10000 5000 3333 2500 2000 1667 1429 1250 2.0

1.5 asymmetric H2O stretch

symmetric H2O stretch 1.0 H2O overtones H O bend 0.5 2 H O bend + libration 2

norm

T 0.0

D -0.5

-1.0

-1.5 multi-ring partial phosphates aromatics alcohols -2.0 0 1000 2000 3000 4000 5000 6000 7000 8000 l [nm]

Figure 4. Difference spectra as obtained on Skydrol LD4 after controlled contamination with water (top row) and after water contamination and molecular fragmentation (bottom row). The water-contaminated samples contained up to 1.5% H2O, while the degraded ones had TAN-values between 1–3 mg KOH/g as determined by conventional titration. The spectra of the thermally

degraded samples were offset by one unit of normalized absorption towards ΔTnorm = −1 for reasons of clarity of presentation.

4. MIR Monitoring of Aviation Hydraulic Fluids The most straight-forward way of arriving at an opto-chemical sensor system is concentrating, firstly, on those strong ground state absorptions which derive from vibrations of O-H groups which are either parts of dissolved water or split-off alcohols and, secondly, on those that derive from phosphate cores [16] which had lost their hydrocarbon side-groups. As shown in Figure 5a, water contamination leads to an increase in the O-H absorption strength around 3500 cm−1. Upon fluid degradation, the O-H absorption feature shifts towards lower wavenumbers around 3300 cm−1 as O-H groups, initially bound to dissolved water, become bound to alcohols which had been split off from their phosphate cores. As long as the remaining partial phosphates are trapped by acid scavenger molecules, the broad, lower-energy phosphate absorption feature, left to the dominant C-H absorption of the base molecules, remains small. A strong increase in this feature, however, occurs after all acid scavenger molecules had become consumed. In this

event,Chemosensors ongoing2020 fluid, 8, 131 degradation causes the TAN value to rise and the phosphate absorption6 of 20feature to increase in proportion to the low-energy O-H absorption.

l [nm] l [nm] 5000 4444 4000 3636 3333 3077 2857 2667 2500 5000 4444 4000 3636 3333 3077 2857 2667 2500

1.0 1.0

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0.6 0.6 W=1043 ppm, TAN=0 Figure 5b 0.4shows W=3916ppm, the decomposition of a measured0.4 MIR fluid transmission measured spectrum into its TAN=0 Skydrol LD4 C-H W=2761ppm, Skydrol LD4 Water: 694 ppm Alcohols TAN=0.27

constituent parts.Normalized transmission In this figure circles also indicate thoseNormalized transmission four spectral positions which are probed by 0.2 dopt = 100mm 0.2 TAN: 7.75 Phosphates W=694ppm, Water the 4-channel NDIR seTAN=7.75nsor system prototype described below. As described Overallin morefit detail in our 2000 2250 2500 2750 3000 3250 3500 3750 4000 2000 2250 2500 2750 3000 3250 3500 3750 4000 previous publications [10Wavenumber,11], probing [cm-1] the optical transmission insideWavenumber these [cm -1four] windows allows the water content, the state of fluid decomposition, and the acidification of the fluid to be quantitatively determined. This capability (isa) also demonstrated in Figure 6. In addition,(b) and most interestingly, by combining theFigureFigure information 5. (a 5.) (FTIRa) FTIR spectra spectraof theof ofSkydrol phosphate Skydrol LD4 LD4 samples samples and in inthe different diff alcoholerent states states Oof - waterH absorptions, contaminationcontamination (blue)(blue)the remaining and acid scavenger reservefluidand degradation can fluid be degradation determined (red, magenta) (red, magenta) and [10 ,1estimates1 [10]. ,(11b)]. Deconvolution (b) Deconvolutionfor the remaining of ofFTIR FTIR spectra spectra fluid into intolife C C-H, can-H, phosphate-phosphate be obtained- and [10,11]. waterand-related water-related spectral spectral features. features. Red circles Red circlesdenote denote sensor sensor interrogation interrogation points points used usedin the in senso the sensorr prototype describedprototype further described below further [10,11]. below [10,11].

0.30 0.5 Skydrol LD4 0.25 Skydrol 500B4 0.4 Skydrol 5

-1 0.20 Hyjet 4+ -1 Hyjet 5 0.3 0.15 0.2 Skydrol LD4

0.10 _3333cm

a a_3559cm Skydrol 500B4 0.1 Skydrol 5 0.05 (a) Hyjet 4+ (b) Hyjet 5 0.00 0.0 0 1000 2000 3000 4000 5000 6000 -2 0 2 4 6 8 10 12 14 16 18 0.40 0.40 H2O [ppm] TAN [mg KOH/g] 0.35 Skydrol LD4 0.35 Skydrol LD4 Skydrol 500B4 0.30 0.30 Skydrol 500B4

-1 Skydrol 5

-1 Skydrol 5 0.25 Hyjet 4+ 0.25 Hyjet 4+ 0.20 Hyjet 5 0.20 Hyjet 5 0.15

a_2632cm 0.15

a_2632cm 0.10 0.10 0.05 (c) (d) 0.05 0.00 0 2 4 6 8 10 12 14 16 18 0.0 0.1 0.2 0.3 0.4 0.5 TAN [mg KOH/g] a_3333cm-1

1 Figure 6. (a) Variation of normalized optical absorption in the 3559 cm− water absorption window with the water content remaining in water-contaminated fluid samples after thermal degradation; 1 1 Figure 6. (aVariation) Variation of the normalized of normalized optical absorption optical insideabsorption the 3333 cmin− thealcohol 3559 (b) andcm−1 2632 water cm− phosphateabsorption window with the waterabsorption content windows remaining (c) with the in TAN water value-contaminated in the same set of fluids;fluid ( dsamples) Correlation after of the thermal phosphate degradation; and butanol/phenol absorptions in thermally degraded aviation fluids. Above a_3333 = 0.2 a rapid −1 −1 Variation ofdepletion the normalized of the acid scavengers optical is indicated absorption [10,11 ]. inside the 3333 cm alcohol (b) and 2632 cm phosphate absorption windows (c) with the TAN value in the same set of fluids; (d) Correlation of the Figure7 shows images of the developed sensor prototype. Figure7a,b show the mechanical phosphatesetup and of thebutanol/phenol sensor system absorptions [9–11], featuring in thermally a thin fluid degraded layer sandwiched aviation fluids. between Above a thermal a_3333 = 0.2 a rapid depletionmicro-electromechanical of the acid scavengers systems (MEMS)is indicated emitter [10,1 [171].] and a 4-thermopile sensor array [18,19], covered with narrow-band filters with their transmission windows centered around those four Figurespectral 7 shows ranges images indicated of the in Figure dev5elopedb. In-depth sensor technological prototype. information Figure about 7a,b the show thermal the emitter, mechanical setup of the sensor system [9–11], featuring a thin fluid layer sandwiched between a thermal micro- electromechanical systems (MEMS) emitter [17] and a 4-thermopile sensor array [18,19], covered with narrow-band filters with their transmission windows centered around those four spectral ranges indicated in Figure 5b. In-depth technological information about the thermal emitter, the thermopile array as well as the high-pressure-resistant micro-cuvette are contained in a previous publication of our project partners [20]. Highlight of these developments was the successful fabrication of micro-structured multi-wafer stacks consisting of layers of infrared-transparent silicon and micro-structured low-temperature-cofired (LTCC) ceramic wafers to make up high-pressure resistant microcuvettes. Figure 7c shows a fully assembled MIR sensor system prototype containing these micro-components. In the final phases of the BMBF NAMIFLU project [21] these prototypes were successfully tested inside the Airbus Toulouse hydraulics test rig at internal pressures up to 400 bar. Such sensors were also successfully implemented inside the Ground Lab [9,22] shown in Figure 7d. This Ground Lab has been developed for Airbus by the hydraulics system provider HYDAC, containing the MIR sensor prototype shown in Figure 7c and an additional particle counter. Currently, such systems are being used to tap fluid from existing airplanes during ground stops and to provide rapid assessments of the chemical degradation and of the particle contamination of hydraulic fluids on ground without the need for time-consuming off-site analysis in distant laboratories. In this way maintenance activities can be speeded up and flight interruptions kept to a minimum.

Chemosensors 2020, 8, 131 7 of 20 the thermopile array as well as the high-pressure-resistant micro-cuvette are contained in a previous publication of our project partners [20]. Highlight of these developments was the successful fabrication of micro-structured multi-wafer stacks consisting of layers of infrared-transparent silicon and micro-structured low-temperature-cofired (LTCC) ceramic wafers to make up high-pressure resistant microcuvettes. Figure7c shows a fully assembled MIR sensor system prototype containing these micro-components. In the final phases of the BMBF NAMIFLU project [21] these prototypes were successfully tested inside the Airbus Toulouse hydraulics test rig at internal pressures up to 400 bar. Such sensors were also successfully implemented inside the Ground Lab [9,22] shown in Figure7d. This Ground Lab has been developed for Airbus by the hydraulics system provider HYDAC, containing the MIR sensor prototype shown in Figure7c and an additional particle counter. Currently, such systems are being used to tap fluid from existing airplanes during ground stops and to provide rapid assessments of the chemical degradation and of the particle contamination of hydraulic fluids on ground without the need for time-consuming off-site analysis in distant laboratories. In this way maintenance activities can be speededChemosensors up 2020 and, 8, x FOR flight PEER interruptions REVIEW kept to a minimum. 7 of 20

(a) (b)

(c) (d)

Figure 7.Figure(a) Micro-cuvette 7. (a) Micro-cuvette containing containing the the fluid fluid under test test and and fitted fitted in between in between a MEMS a MEMS IR emitter IR emitter and a 4-thermopile sensor array with narrow band filters transparent at the wavelengths indicated in and a 4-thermopile sensor array with narrow band filters transparent at the wavelengths indicated in Figure 5b [10,11]. (b) Mechanical set-up containing the MEMS IR emitter, the micro-cuvette (yellow) Figure5andb [ 10 the,11 thermopile]. (b) Mechanical detector array set-up inside containing a common themetal MEMS block (grey) IR emitter, that can the withstand micro-cuvette the high (yellow) and thepressures thermopile inside detector aviation hydraulic array inside systems a common [10,11]. (c) metalFully assembled block (grey) MIR sensor that can system withstand featuring the high pressuresthe inside internal aviation electronics hydraulic and fluidic systems connectors [10 [1,110,1].1]. (c ()d Fully) HYDAC assembled Ground Lab MIR for sensor on-site systemhydraulic featuring the internalfluid electronicsmonitoring on and airfields fluidic containing connectors the MIR [10, 11sensor]. (d system) HYDAC shown Ground in (a–c) Lab, andfor an on-siteadditional hydraulic fluid monitoringparticle contamination on airfields detector containing [9,22]. the MIR sensor system shown in (a–c), and an additional particleA contamination downside of the detector MIR monitoring [9,22]. system displayed in Figure 7 is that it works in a spectral range in which strong ground state absorptions dominate and where consequently sensor-internal

optical absorption paths need to be kept very short ( 푑표푝푡 < 300 μm). MIR monitoring systems therefore feature large flow resistances which preclude their direct insertion into aircraft hydraulic systems. The much-desired aircraft-internal use only becomes possible when the MIR systems are positioned inside bypass sections which can be shut-off from the aircraft hydraulic system during flight. As such bypass sections require extra space and contribute additional weight, the present MIR systems can only be incorporated into newly produced aircrafts.

Chemosensors 2020, 8, 131 8 of 20

A downside of the MIR monitoring system displayed in Figure7 is that it works in a spectral range in which strong ground state absorptions dominate and where consequently sensor-internal optical absorption paths need to be kept very short (dopt < 300 µm). MIR monitoring systems therefore feature large flow resistances which preclude their direct insertion into aircraft hydraulic systems. The much-desired aircraft-internal use only becomes possible when the MIR systems are positioned inside bypass sections which can be shut-off from the aircraft hydraulic system during flight. As such bypass sections require extra space and contribute additional weight, the present MIR systems can only be incorporated into newly produced aircrafts. Chemosensors 2020, 8, x FOR PEER REVIEW 8 of 20 5. Optical Absorption Monitoring of Aviation Hydraulic Fluids in the Visible and Near 5. OpticalInfrared Absorption Ranges Monitoring of Aviation Hydraulic Fluids in the Visible and Near Infrared Ranges ConsideringConsidering the the problem problem ofof flowflow resistance resistance in in MIR MIR systems, systems, optical optical absorption absorption monitoring monitoring in the in thenear near infrared infrared and visible and visible ranges ranges appears appears attractive. attractive. Optical absorption Optical in absorption these shorter in wavelength these shorter wavelengthranges is much ranges smaller is much and smaller larger and sensor-internal larger sensor optical-internal paths optical become paths possible. become In possible. order to assessIn order to thisassess possibility, this possibility, we show we in show more in detail more in detail Figure in 8Figure those 8 changes those changes upon water upon contamination water contamination and andthermal thermal degradation degradation in the in short-wavelengththe short-wavelength region region that occur that inoccur two widelyin two usedwidely hydraulic used hydra fluids,ulic fluids,i.e., Skydrol i.e., Skydrol LD4 and LD4 HYJET and HYJET V. V.

Wavenumber [cm-1] Wavenumber [cm-1] -- 2.0x104 1.0x104 6.7x103 5.0x103 4.0x103 -- 2.0x104 1.0x104 6.7x103 5.0x103 4.0x103 1.0 1.0 C = CH2O = H2O 0.5 0.5% - 1.5% 0.5 0.5% - 1.5%

0.0 0.0

norm norm

T -0.5 T -0.5

D D

-1.0 -1.0

TAN = -1.5 -1.5 TAN = 1-3 mgKOH/g 0-1.8mgKOH/g -2.0 -2.0 0 500 1000 1500 2000 2500 0 500 1000 1500 2000 2500 l [nm] l [nm]

(a) (b)

FigureFigure 8. 8.(a()a )Difference Difference spectra ofof water-contaminated water-contaminated and and thermally thermally degraded degraded Skydrol Skydrol LD4 in LD4 the NIRin the NIRand and VIS VIS ranges ranges (dopt (d= opt1 cm) = 1 [ 6cm)]. (b )[6 Di]. ff(erenceb) Difference spectra ofspectra water-contaminated of water-contaminated and thermally and degraded thermally

degradedHYJET V HYJET in the NIRV in andthe VISNIR ranges and VIS (dopt ranges= 1 cm) (dopt [6 ].= 1 cm) [6].

WhereasWhereas in inboth both fluids fluids water water-related-related overtone overtone absorptions absorptions of of dissolved dissolved water water of of similar similar size size and and spectral positions can be observed, significant shifts of these absorption features towards longer spectral positions can be observed, significant shifts of these absorption features towards longer wavelengths upon acidification cannot be detected. Absent also are overtone replica of the very broad wavelengths upon acidification cannot be detected. Absent also are overtone replica of the very broad phosphate absorption feature in the MIR. TAN-related absorptions, however, can be observed at optical phosphate absorption feature in the MIR. TAN-related absorptions, however, can be observed at wavelengths smaller than 500 nm. Unlike the water absorption features, these latter changes in the optical wavelengths smaller than 500 nm. Unlike the water absorption features, these latter changes VIS and UV ranges are very fluid dependent. As will become evident from the results presented in in the VIS and UV ranges are very fluid dependent. As will become evident from the results presented Section6 and in AppendixC, the fluid-dependence of these latter features derives from the fact that in diSectionfferent 6 fluids and containin Appendix different C, concentrationsthe fluid-dependence of esters of with these aromatic latter sidefeatures groups. derives Very simplefrom the and fact thateff differentective water fluids contamination contain different sensors concentrations with large internal of esters optical with paths, aromatic however, side groups. can be built Very using simple andcommercially effective water available contamination NIR LEDs andsensors NIR with detectors, large emittinginternal andoptical receiving paths, in however, the ranges can of waterbe built usingovertone comme absorptions.rcially available This latter NIR capability LEDs and is demonstratedNIR detectors, in emitting Figure9. and receiving in the ranges of water overtone absorptions. This latter capability is demonstrated in Figure 9. High-pressure resistant versions of water contamination sensors can easily be produced using the technology already developed for the MIR systems described in Section 4 and described in more detail in ref. [20]. With their wider internal optical path lengths, however, NIR sensors can be directly placed inside existing hydraulic lines and positioned close to those components, i.e., pumps and actuators, where Joule heat is generated and where hydrolysis reactions are likely to take place. Measuring the local temperature rise during actuation at these spots, the generated Joule heat can be obtained by integration and the heat input into the water-contaminated fluid can be used to estimate the volume densities of base fluid molecules that are likely to have been degraded and the amounts of acid scavenger molecules that had been consumed in counteracting the increase in free phosphates [23]. Such sensors may find applications in innovative aircraft actuation systems which replace much of the conventional hydraulics by electronics and which leave the sole responsibility of generating mechanical power to distributed hydraulic power amplifiers, which contain besides the hydraulic actuators themselves also small dedicated hydraulic reservoirs with pumps and associated health monitoring systems.

Chemosensors 2020, 8, 131 9 of 20 Chemosensors 2020, 8, x FOR PEER REVIEW 9 of 20

Photon Energy [eV] Photon Energy [eV] 0.77 0.73 0.69 0.65 0.62 0.59 0.56 1.24 0.83 0.62 0.50 0.41

1.0 1.0

0.8 0.8 0.6 0.6 0.4

Responsivity [A/W] Responsivity

LED Intensity [norm]LED 0.4 0.2

1.6 1.7 1.8 1.9 2.0 2.1 2.2 1.0 1.5 2.0 2.5 3.0 Wavelength [mm] Wavelength [mm]

(a) (b)

3

dopt = 4mm 2

d = 7mm 1 opt

Photodiode Signal [V] Signal Photodiode

dopt = 10mm

0 0.1 1 10 Water content [%]

(c) (d) Figure 9. (a) Emission spectrum of commercial NIR-LED (IR S-LED 1,94; Roithner) [6]. (b) Spectral Figure 9. (a) Emission spectrum of commercial NIR-LED (IR S-LED 1.94; Roithner) [6]. (b) Spectral sensitivity of commercial NIR photodiode (InGaAs) (FGA 20 Thorlabs) [6]. (c) Cuvette for sensor sensitivityinvestigation of commercial tests featuring NIR photodiode screw-in MEMS (InGaAs) emitter and (FGA receiver 20 Thorlabs) elements allowing [6]. (c) Cuvettefor experiments for sensor with investigationvariable tests optical featuring gap lengths screw-in [6]. (d MEMS) Variation emitter of IR and transmission receiver elementsof Skydrol allowing LD4 with for increasing experiments water with variablecontent opticalas measured gap lengthswith the [test6]. (setd)-up Variation of Figure of 9c IR and transmission the optical components of Skydrol of LD4 (a,b with) at a wavelength increasing water contentof λ = 1.94 as µm measured and at different with the absorption test set-up path of lengths Figure [69].c and the optical components of ( a,b) at a wavelength of λ = 1.94 µm and at different absorption path lengths [6]. 6. Fluorescence Monitoring of Aviation Hydraulic Fluids High-pressureFurther exploring resistant theversions possibility of water of sensing contamination fluid properties sensors in canaddition easily to be the produced water content using the technologyusing already small light developed-weight sensors for the MIRwith large systems internal described light paths, in Section we have4 and performed described fluorescence in more detail in ref. [20experiments]. With their on pure wider and internal thermally optical degraded path lengths,fluid samples. however, Key motivation NIR sensors for cansuch be experiments directly placed inside existingwas that hydraulicaviation hydraulic lines and fluids positioned contain ester close molecules to those with components, phenolic side i.e., groups pumps as andshown actuators, in where JouleFigure heat 1, which is generated as free molecules and where tend hydrolysisto fluoresce as reactions these become are likely excited to with take UV place. light Measuring[24]. the The property of fluorescence emission is illustrated in Figure 10a, which shows fluorescence local temperature rise during actuation at these spots, the generated Joule heat can be obtained by excitation and emission spectra of pure phenol. For comparison, Figure 10b shows an optical integrationabsorption and thespectrum heat of input Skydrol into LD4, the which water-contaminated also shows a strong phenolic fluid can absorption be used at toaround estimate 휆 ≈ 260 the nm volume. densitiesAdditionally, of base fluidshownmolecules in this graph thatis the arepeak likelyemission to wavelength have been of those degraded UV-LEDs and that the had amounts been used ofin acid scavengerthe fluorescence molecules thatmeasurement had been setup consumed of Figure in 10c. counteracting Using this setup, the increasethe fluorescence in free response phosphates to a [23]. Such sensorsrange of may different find hydraulic applications fluids inwas innovative tested and which aircraft were actuation subsequently systems flown which through replace its fluid much of the conventionalchamber. As expected, hydraulics the individual by electronics fluids andresponded which w leaveith different the sole levels responsibility of fluorescence of light generating as mechanicalthese powercontained to different distributed concentrations hydraulic of power esters wi amplifiers,th aromatic which side groups. contain Figure besides 10e,f theshow hydraulic that the fluorescence response significantly increases as fluids become thermally degraded and as their actuators themselves also small dedicated hydraulic reservoirs with pumps and associated health TAN values are increased. This result indicates that breaking aromatic side groups free from their monitoringparent systems. phosphate cores increases their excited state lifetimes, and thus makes them more likely to

6. Fluorescence Monitoring of Aviation Hydraulic Fluids Further exploring the possibility of sensing fluid properties in addition to the water content using small light-weight sensors with large internal light paths, we have performed fluorescence experiments on pure and thermally degraded fluid samples. Key motivation for such experiments was that aviation Chemosensors 2020, 8, 131 10 of 20 hydraulic fluids contain ester molecules with phenolic side groups as shown in Figure1, which as free molecules tend to fluoresce as these become excited with UV light [24]. The property of fluorescence emission is illustrated in Figure 10a, which shows fluorescence excitation and emission spectra of pure phenol. For comparison, Figure 10b shows an optical absorption spectrum of Skydrol LD4, which also shows a strong phenolic absorption at around λ 260 nm. ≈ Additionally, shown in this graph is the peak emission wavelength of those UV-LEDs that had been used in the fluorescence measurement setup of Figure 10c. Using this setup, the fluorescence response to a range of different hydraulic fluids was tested and which were subsequently flown through its fluid chamber. As expected, the individual fluids responded with different levels of fluorescence light as these contained different concentrations of esters with aromatic side groups. Figure 10e,f show that the fluorescence response significantly increases as fluids become thermally degraded and as their TAN values are increased. This result indicates that breaking aromatic side groups free from their parent phosphate coresChemosensors increases 2020, 8, x their FOR PEER excited REVIEW state lifetimes, and thus makes them more likely10 to of 20 undergo radiative recombination.undergo radiative As, recombination. however, this As, increase however, in this fluorescence increase in fluorescence does arise does from arise freed from aromatic freed side groups and notaromatic from side the groups barephosphate and not from cores the bare themselves, phosphate cores the observedthemselves, fluorescencethe observed fluorescence increase is more an indicator ofincrease thermal is more fluid an indicator degradation of thermal rather fluid than degradation a reliable rather measure than a reliable of the measure TAN valueof the TAN itself. value itself.

Photon energy [eV] Photon energy [eV] 6.2 5.0 4.1 3.5 3.1 6.20 5.64 5.17 4.77 4.43 4.13 100 Phenol 100 100 LED 80 Excitation 80 Emission 80 60 60 60

40 40 40

Emisison [a.u.] Emisison

Excitation [a.u.] Excitation

Absorption [a.u.] Absorption 20 20 20

0 0 0 200 250 300 350 400 200 220 240 260 280 300 Wavelength [nm] Wavelength [nm]

(a) (b)

1.2

1.0

HyJET V 0.8

0.6

Skydrol 500 B4 0.4

Skydrol LD4 0.2

Fluorescence Response [norm.] Response Fluorescence

0.0 0 5 10 15 20 Time [min]

(c) (d)

1.0 1.0

0.8 0.8 Skydrol LD4

0.6 0.6 Fresh, TAN = 0.14 TAN = 0.31 0.4 TAN = 0.67 0.4 TAN = 1.2 Skydrol LD4, TAN = 2.1 l = 295nm 0.2

Fluorescence Response [a.u.] Response Fluorescence 0.2

Fluorescene Response [a.u.] Response Fluorescene

0.0 0.0 250 275 300 325 350 375 400 425 450 0.1 1 10 Wavelength [nm] Total Acid Number [mg KOH/g]

(e) (f)

Figure 10. (a)Figure Photo 10. excitation (a) Photo and excitation emission and spectra emission of spectra free phenol of free molecules phenol molecules [24]. (b [2)4 Optical]. (b) Optical absorption absorption spectrum of Skydrol LD4 [6]. Arrows indicate the peak emission of the UV LEDs that were used in the fluorescence setup of Figure 9c. (c) Test setup for fluorescence measurements on aviation hydraulic fluids featuring four UV LEDs emitting at 휆 ≈ 275 nm and a photo-multiplier tube [6]. (d) Fluorescence response of undegraded aviation hydraulic fluids with different aromatic contents [6]. (e) Fluorescence spectra of Skydrol LD4 degraded up to increasingly larger TAN values [6]. (f)

Chemosensors 2020, 8, 131 11 of 20

spectrum of Skydrol LD4 [6]. Arrows indicate the peak emission of the UV LEDs that were used in the fluorescence setup of Figure 10c. (c) Test setup for fluorescence measurements on aviation hydraulic fluids featuring four UV LEDs emitting at λ 275 nm and a photo-multiplier tube [6]. (d) Fluorescence ≈ response of undegraded aviation hydraulic fluids with different aromatic contents [6]. (e) Fluorescence spectra of Skydrol LD4 degraded up to increasingly larger TAN values [6]. (f) Fluorescence response of Skydrol LD4 degraded up to increasingly larger TAN values and as observed with the test setup of (c)[6]. 7. Achievements and Possible Ways Forward In our work on aviation hydraulic fluids, we have been able to identify a set of optical features that allow the water content, the presence of thermal degradation products, and the TAN value of phosphate-ester hydraulic fluids to be assessed by purely optical means. Specifically, and as demonstrated in more detail in the Appendices, we could show that these features are universally present in all kinds of commercially available hydraulic fluids which is a huge advantage as in practical aircraft operation hydraulic fluids may be topped up with or exchanged by other brands of commercial fluids in case fluid maintenance needs to be done. With such universal features at hand, opto-chemical sensor technologies are in reach which allow for a continuous monitoring of relevant fluid properties without tapping fluid from pressurized hydraulic reservoirs and of sending samples to distant laboratories for off-site chemical analysis. With the help of online fluid monitoring, fluid degradation trends can be monitored during normal aircraft operation and necessary maintenance activities be scheduled to occur at routine ground stops without interrupting normal flight schedules. In this way, an important part of aircraft maintenance can be aligned with the general industry trend of replacing interval-based and corrective maintenance procedures by predictive and proactive procedures [25–28]. Among those opto-chemical sensor technologies which had been developed so far, NDIR sensors working in the MIR range have progressed to the highest level of maturity [10,11]. By probing the IR transmittance of aviation hydraulic fluids at four different wavelengths, such sensors allow all maintenance-relevant fluid properties, i.e., water content, thermal fluid degradation, TAN value and remaining acid scavenger reserve, to be determined with temporal frequencies much higher than in currently used aircraft maintenance schemes. A downside of the present MIR monitoring systems, however, is that due to the strong optical absorption in the MIR range, these feature small internal light paths (dopt < 300 µm), much smaller than the internal diameters of hydraulic lines inside commercial airplanes (d 1 cm), and that these therefore can only be operated inside bypass lines which can be int ≈ shut off during flight. In order to move closer to the requirements of predictive maintenance concepts of continuously monitoring machine data, of sending them via internet to the machine manufactures and for enabling the manufacturers to send out service recommendations, future sensor developments should aim at removing flow restrictions and to allow sensor use during normal machine operation. A first step into this direction becomes possible by making use of the NIR water monitoring sensors described in Section5. When such flow-through sensors are directly placed close to the hydraulic actuators where Joule heat is generated and where fluid degradation takes place, the generated heat can be determined from local temperature rise data and estimates of the ensuing fluid degradation and acid scavenger consumption can be obtained by calculation, making use of the known water contamination levels inside the fluid. With regard to the present state of the art, it is relevant to note that such concepts can be turned to reality by completely relying on sensor and electronics hardware which is already commercially available. Aiming at flow-through sensory capabilities beyond water monitoring, additional research is required. Looking towards the future and considering the proven potential of MIR monitoring systems, developing MIR monitoring systems based on total-internal-reflection (ATR) geometries is a very interesting option. ATR is a well-established laboratory technique, which is routinely used in the Chemosensors 2020, 8, 131 12 of 20 analysis of fluid samples and which is also increasingly used in the sensors field [29–37]. The attractivity of the ATR technology is that it uses light which is evanescently coupled from a prism or a grating into a thin surface layer of the fluid to be tested, leaving the rest of the fluid unaffected. Very interestingly, an ATR sensor system with an architecture very similar to the MIR sensor system described in Section4 has been developed by Geörg et al. [ 32,33] for the analysis of chemical reaction mechanisms. Further interesting developments are MEMS components for the realization of ATR sensor systems such as silicon microgroove ATR mirrors [34,35] and micromachined Fabry-Perot spectrometers [36,37] which potentially allow discrete multi-channel sensor systems to be turned into full-scale micro spectrometer systems. Further interesting options are fluorescence sensors. In Section6 we have demonstrated that aviation hydraulic fluids feature an intrinsic fluorescence which derives from the aromatic side groups attached to phosphate-ester molecules. As described there, a downside of this intrinsic fluorescence is that its magnitude is very fluid-dependent and that it increases as phosphate-ester molecules become broken down during fluid degradation. Further, as the fluorescence of detached aromatic side groups does not provide a direct measure of the number of partially and fully depleted phosphates, reliable measurements of the fluid TAN cannot be obtained from observations of the intrinsic fluorescence. Dealing with this problem, the Orellana group at the University of Madrid has investigated sensors which measure changes in the extrinsic fluorescence of transition-metal-based complexes upon interaction with fluid molecules. As described in detail in a review article by this group [38], a number of luminescent sensors has been developed which are of potential interest in various aeronautic fields, including hydraulic fluid monitoring. Recently, our own group has been investigating fluorescence sensors, which use GaN/InGaN nanowire arrays on silicon and sapphire substrates as luminescent materials. As described in some of our recent papers [39–42], such nanowire arrays respond with fluorescence changes when immersed into fluids with different pH values. Additionally, it has been observed that such nanowire arrays exhibit a quenching response when exposed to oxidizing gases (O2, NO2,O3) and an enhancing response when exposed to water vapor. Although not systematically tested in hydraulic fluids, these latter results indicate that such arrays will exhibit a quenching response upon interaction with partially and fully depleted phosphates and an enhancing response when interacting with dissolved water. As partial phosphates and dissolved water molecules are likely to coexist, fluorescence signals might get fully or partially compensated. Additionally, the level of extrinsic fluorescence emitted from the GaN/InGaN nanowire transduces might get superimposed by the fluid-dependent levels of intrinsic fluorescence emitted from those aromatic side groups that had been broken free from their phosphate cores. In summary, intensive research efforts are still necessary to arrive at fully functional flow-through sensor systems. Whereas in the case of ATR systems the challenges are more on the engineering side of incorporating ATR coupling elements into high-pressure resistant micro-cuvettes, the challenges with fluorescence sensors are more likely on the side of alleviating cross-sensitivity and chemical stability issues of fluorescent layers.

Author Contributions: A.H. and G.M. had been managers of research projects at EADS Innovation works (predecessor of Airbus Central Research & Technology) devoted to the development of fluid sensor technologies. S.P. performed his PhD work on hydraulic fluid monitoring. All authors have read and agreed to the published version of the manuscript. Funding: The authors gratefully acknowledge financial support under the EU project SuperSkysense (FP6-AEROSPACE 30863) and the BMBF project NAMIFLU (16SV5357). Acknowledgments: The authors benefitted from discussions and support from colleagues at EADS, from Volker Baumbach (AIRBUS Bremen), Delphine Hertens (AIRBUS Toulouse) and Thierry Lemettais and Francois Pradat, both from Airbus Group Innovations France. Angelika Hackner (spectroscopic measurements), Wolfgang Legner (sensor system integration) and Volker Hechtenberg (electronics) contributed significantly to the realization of the sensor technologies described in this work. Conflicts of Interest: The authors declare no conflict of interest. Chemosensors 2020, 8, 131 13 of 20

Appendix A. Difference Spectra of Widely Used Aviation Hydraulic Fluids To assess the general usefulness of the optical monitoring approach, spectroscopic measurements on several widely employed aviation hydraulic fluids were performed. The difference spectra derived from these data are presented in Figure A1a–c below. Table A1

(a)

(b)

(c) Table A3 Figure A1. Difference spectra as obtained on SKYDROL LD4 (a), SKYDROL 500B4 (b) and HYJET V (c) after1 controlled contamination with water (blue curves) and after water contamination and molecular

fragmentation (red curves). The water-contaminated samples contained up to 1.5% H2O, while the degraded ones had TAN-values between 1 and 3 mg KOH/g as determined by titration. The water- and TAN-spectra have been offset by 1 unit in normalized transmission from ∆Tnorm = 0 for reasons of clarity of presentation. Chemosensors 2020, 8, 131 14 of 20

Appendix B. Assignment of Observed Optical Features to Molecular Vibration Modes

Chemosensors The2020, data8, x FOR in PEER Appendix REVIEWA show that, independent of the special kind of fluid, remarkably14 of 20 similar Chemosensors 2020, 8, x FOR PEER REVIEW 14 of 20 Chemosensors 2020, 8, x FOR PEER REVIEW 14 of 20 Chemosensorsspectral 2020 features, 8, x FOR PEER exist REVIEW which signify water contamination, fluid degradation, and raised14 TAN of 20 values. TurningTurning to the tocase the of case water of contamination water contamination at room at temperature room temperature first, we first, notewe that note in the that absence in the of absence Turning to the case of water contamination at room temperature first, we note that in the absence of thermalTurningof agitationto thermal the case agitationa reactionof water a ofcontamination reaction the dissolved of the at water dissolvedroom molecules temperature water with molecules first, the wephosphate note with that the-ester in phosphate-ester the base absence fluids ofis base unlikelythermalfluids agitationto occur. is unlikely a As reaction the to occur.dissolved of the As dissolved thewater dissolved molecules water watermolecules large moleculesly withretain the largelytheir phosphate molecular retain-ester their identity, base molecular fluids water identity,is unlikely to occur. As the dissolved water molecules largely retain their molecular identity, water moleculesunlikelywater to solvated occur. molecules As in hydraulicthe solvated dissolved influid hydraulic water are expected molecules fluid are to exhibit expectedlargely the retain to same exhibit their vibrational the molecular same properties vibrational identity, as properties water as moleculeswater solvateddissolved molecules in inside hydraulic dissolved liquid fluid inside water. are liquid expected Due water. to the to Due exhibitdifferent to thethe molecule disamefferent vibrational- molecule-backgroundbackground properties interactions as water interactions in molecules dissolved inside liquid water. Due to the different molecule-background interactions in themolecules phosphatein the dissolved phosphate-ester-ester solvent, inside liquidhowever, solvent, water. however,H2O Due absorption to H the2O different absorption features moleculewill features be shifted-background will away be shifted from interactions their away liquid from in their the phosphate-ester solvent, however, H2O absorption features will be shifted away from their liquid the phosphate-ester solvent, however, H2O absorption features will be shifted away from their liquid waterthe phosphate liquidspectral water- positions.ester spectral solvent, This positions. however, expectation ThisH2O is absorption expectation confirmed features isby confirmed the datawill besummarized by shifted the data away summarizedin fromTable their A1, whichliquid in Table A1, compareswater whichspectral the compares spectral positions. positions the This spectral expectation of some positions of is the confirmed of key some vibrational of by the the key data features vibrational summarized in liquid features inwater Table in liquid [4 3A] 1to, waterwhich those [ 43] to compares the spectral positions of some of the key vibrational features in liquid water [43] to those observedcomparesthose the in observed water spectral contaminated inpositions water contaminated of some Skydrol of the LD4. Skydrol key This vibrational LD4. comparison This features comparison clearly in liquid indicates clearly water indicates that[43] to all those that key all key vibrationalobservedvibrational in features water features of contaminated liquid of liquidwater Skydrol waterare retained are LD4. retained in This water in comparison watercontaminated contaminated clearly Skydrol indicates Skydrol LD4 and that LD4 that all and keythe that the vibrational features of liquid water are retained in water contaminated Skydrol LD4 and that the associatedvibrationalassociated wavelengthfeatures wavelength of liquid shifts shifts waternever never are exceed exceedretained 3– 3–4%4 in% water of of the the contaminated wavelengths wavelengths of Skydrol ofthe the respective LD4 respective and water that water absorption the absorptionassociatedfeatures. features. wavelength The data The in shiftsdata Appendix in never AppendixA exceed show A that show3–4 these% that of arguments these the wavelengthsarguments apply equallyapply of theequally well respective also well to also a wider water to a range absorption features. The data in Appendix A show that these arguments apply equally well also to a widerabsorption ofrange phosphate-ester features. of phosphate The data fluids.-ester in fluids.Appendix A show that these arguments apply equally well also to a wider range of phosphate-ester fluids. Table TableA1. Wavelengths A1. Wavelengths of vibrational of vibrational features features in liquid in liquidwater (column water (column 3) and 3)of andwater of watermolecules molecules Table A1. Wavelengths of vibrational features in liquid water (column 3) and of water molecules dissolvedTable dissolvedA1. in Wavelengths Skydrol in Skydrol LD4 of (column LD4vibrational (column 4). Columns features 4). Columns 5in and liquid 6 5 report and water 6 reportthe (column corresponding the corresponding 3) and wavelengthof water wavelength molecules shifts. shifts. dissolved in Skydrol LD4 (column 4). Columns 5 and 6 report the corresponding wavelength shifts. dissolved in Skydrol LD4 (column 4). Columns 5 and 6 report the corresponding wavelength shifts. λH2OλH2O λSkydrolλ Δλ Δλ/λ∆H2Oλ ∆λ/λH2O Water Vibration Feature λH2O λSkydrolSkydrol_H2O Δλ Δλ/λH2O Water Vibration Feature λH2O λSkydrol Δλ Δλ/λH2O Water Vibration Feature [nm]λH2O[nm] λ[nm]Skydrol [nm][nm]Δλ Δλ[%]/λ[nm]H2O [%] Water Vibration Feature [nm] [nm] [nm] [%] [nm] [nm] [nm] [%]

BendBend νν2 60806080 6099 609919 0.3 19 0.3 Bend ν22 6080 6099 19 0.3 Bend ν2 6080 6099 19 0.3 Bend ν2 6080 6099 19 0.3

Combination CombinationCombination 4650 4791 141 3.03 CombinationBend ν2+L2 46504650 4791 4791141 3.03 141 3.03 BendBend νν22+L+L2 2 4650 4791 141 3.03 Bend ν2+L2 4650 4791 141 3.03 Bend ν2+L2

Symmetric Symmetric 3050 2952 −98 −3.21 SymmetricSSymmetrictretch ν1 3050 2952 −98 −3.21 Stretch ν1 30503050 2952 2952−98 −3.2198 3.21 StretchStretch νν11 3050 2952 −98 −3.21− − Stretch ν1 Asymmetric Asymmetric 2870 2766 −104 −3.62 AsymmetricstretchAsymmetric ν3 2870 2766 −104 −3.62 stretch ν3 2870 2766 −104 −3.62 stretch ν3 28702870 2766 2766−104 −3.62104 3.62 stretchstretch νν33 − − aν1 + ν2 + bν3. aν1 + ν2 + bν3. Combination aν1 + ν2 + bν3. 1900 1917 17 0.89 Combination aaννa1 + ++ ν bν2 += +b1ν b3ν. . 1900 1917 17 0.89 CombinationCombination a1 + b 2= 1 3 19001900 1917 191717 0.89 17 0.89 aaν a+1 ++b b =ν= 13. 1 aν1 + bν3. Combination aν1 + bν3. 1470 1471 1 0.07 Combination aaaν +ν1 +b + b=νb 23ν. . 1470 1471 1 0.07 CombinationCombination a +1 b = 2 3 14701470 1471 14711 0.07 1 0.07 a a++ bb == 2 2 Turning to the optical signatures of acid samples (TAN > 1), Figure A1 demonstrates that most of theTurning observed to theTAN optical features signatures appear of to acid be longsamples-wavelength (TAN > -1),shifted Figure replica A1 demonstrates of their parent that water most of the observedTurning TAN to thefeatures optical appear signatures to be of long acid-wavelength samples (TAN-shifted> 1), replica Figure ofA1 their demonstrates parent water that most absorptionof theof observed the ones. observed TANExceptions features TAN to features thisappear rule appear toare be the tolong extremely be-wavelength long-wavelength-shifted broad-shifted MIR absorption replica replica of features their of parent their from parent aboutwater water 38absorption00 to 5500 ones. nm Exceptions and the much to this narrower rule are VIS/UV the extremely features broad in the MIR vicinity absorption of 400 featuresnm. For fromthe specific about 3800 absorptionto 5500 nm ones. and the Exceptions much narrower to this rule VIS/UV are the features extremely in the broad vicinity MIR of absorption 400 nm. For features the specific from about example3800 to of5500 Skydrol nm andLD4, the the much corresponding narrower wavelengthsVIS/UV features and wavelengthin the vicinity shifts of 400are listed nm. For in Tablethe specific A2. example3800 of Skydrolto 5500 nmLD4,and the thecorresponding much narrower wavelengths VIS/UV and features wavelength in the vicinityshifts are of listed400 nmin Table. For A2 the. specific exampleexample of Skydrol of Skydrol LD4, the LD4, corresponding the corresponding wavelengths wavelengths and wavelength and wavelength shifts are shifts listed are in Table listed A2 in. Table A2. Table A2. Wavelengths of water vibrational features in Skydrol LD4 (column 2) as compared to those Table A2. Wavelengths of water vibrational features in Skydrol LD4 (column 2) as compared to those vibrationalTable A2. Wavelengths features that of water arise vibrational after water features contamination in Skydr ol and LD4 thermal (column degradation 2) as compared (column to those 3). vibrational features that arise after water contamination and thermal degradation (column 3). Columnsvibrational 4 and features 5 list thatthe respective arise after wavelength water contamination shifts. The features and thermal noted degradationin bold (blue) (column letters are 3). Columns 4 and 5 list the respective wavelength shifts. The features noted in bold (blue) letters are assumedColumns to4 and derive 5 list from the respectiveparent water wavelength vibrational shifts. features; The features the features noted inin italicbold (blue) letters letters (red) are assumed to derive from parent water vibrational features; the features in italic letters (red) are degradationassumed to and/or derive TAN from specific.parent water vibrational features; the features in italic letters (red) are degradation and/or TAN specific. degradation and/or TAN specific. Skydrol LD4 λpure Skydrol λacidified Skydrol Δλ Δλ/λ Skydrol LD4 λpure Skydrol λacidified Skydrol Δλ Δλ/λ Skydrol LD4 λpure Skydrol λacidified Skydrol Δλ Δλ/λ VibrationalSkydrol FeaturesLD4 λpure[nm] Skydrol λacidified[nm] Skydrol [nm]Δλ Δ[%]λ/λ Vibrational Features [nm] [nm] [nm] [%] VibrationalBend νFeatures2 [nm]6099 [nm]6416 [nm]317 [%]5.2 Bend ν2 6099 6416 317 5.2 Bend ν2 6099 6416 317 5.2 Bend ν2 6099 6416 317 5.2

Chemosensors 2020, 8, 131 15 of 20

Table A2. Wavelengths of water vibrational features in Skydrol LD4 (column 2) as compared to those vibrational features that arise after water contamination and thermal degradation (column 3). Columns 4 and 5 list the respective wavelength shifts. While most features derive from parent water vibrational features, those in italic letters are degradation and/or TAN specific.

Skydrol LD4 λSkydrol_H2O λSkydrol_acidified ∆λ ∆λ/λ Chemosensors 2020, 8, x FOR PEER REVIEW 15 of 20 Vibrational Features [nm] [nm] [nm] [%] Bend ν 6099 6416 317 5.2 Combination2 Combination 4791 5635 844 14.98 Bend ν2 + L2 4791 5635 844 14.98 Bend ν + L Partial2 2 phosphates Partial phosphates (broad) PO-H stretch - (broad) - - PO-H stretch - 3500–5500 -- 3500–5500 POH comb.POH comb. Symmetric Symmetric 2952 3773 821 27.81 Stretch ν1 2952 3773 821 27.81 Stretch ν1 Asymmetric Asymmetric 2766 2979 213 7.70 stretch ν3 2766 2979 213 7.70 stretch ν3 Combination Combination 1 2 3 aν1 + ν2 a+νbν+3 ν. + bν . 19171917 20802080 163 163 8.50 8.50 a + b = 1a + b = 1 CombinationCombination aν1 + bν3a.ν1 + bν3. 14711471 15181518 47 47 3.20 3.20 a + b = 2a + b = 2 AromaticAromatic decay products decay products -- 361361 - - - -

An important aspect of the fluidfluid degradation process is that it conserves the numbers of O-HO-H bonds during during the the degradation degradation process. process. As illustrated As illustrated in Figure in Figure A2 all O-HA2 all bonds O-H are bonds initially are associated initially withassociated dissolved with water,dissolved and water, with split-o and withff alcohols split-off and alcohols partial and phosphates partial phosphates after hydrolysis after hydrolysis has taken place.has taken As inplace. this finalAs in state this allfinal O-H state bonds all O are-H bound bonds toare larger-mass bound to larger backbone-mass molecules backbone than molecules before, thana long-wavelength-shift before, a long-waveleng of theth water-shift O-Hof the vibrational water O-H feature vibrational is expected feature [ 44is ].expected [44].

Figure A2. Exchange of water- into phenol/butanol - bonded OH groups and partial phosphates as Figure A2. Exchange of water- into phenol/butanol - bonded OH groups and partial phosphates as dissolved water is used up in molecular fragmentation events. A shift in the resonance frequency dissolved water is used up in molecular fragmentation events. A shift in the resonance frequency occurs as the force constants (bond strength) and the reduced masses of the molecular oscillators are occurs as the force constants (bond strength) and the reduced masses of the molecular oscillators are changed [44]. changed [44]. In order to test the validity of this interpretation, Skydrol LD4 samples had been contaminated withIn 1% order butanol to test or 1% the phenol, validity i.e., of typicalthis interpretation, fragmentation Skydrol products LD4 which samples are likely had been to emerge contaminated from the withdegradation 1% butanol reactions or 1% sketched phenol, i.e. in, Figure typica2l andfragmentation which had products been confirmed which are in gaslikely chromatographic to emerge from theanalyses degradation of degraded reactions aviation sketched hydraulic in Figure fluids 2 [14 and]. The which diff erencehad been spectra confirmed in Figure in gasA3 clearly chromatographic reveal that analysesbutanol andof degraded phenol addition aviation leadshydraulic to strong fluids O-H [14]. absorption The difference features spectra slightly in Figure long-wavelength-shifted A3 clearly reveal that withbutanol regard and phenol to the mainaddition stretching leads to vibrationsstrong O-H of absorption dissolved features water. Interestinglyslightly long-wavelength also, all other-shifted O-H vibrationalwith regard features, to the main which stretching can be detected vibrations in water-contaminated of dissolved water. Skydrol Interestingly LD4, are also, no longer all other observed. O-H vibrational features, which can be detected in water-contaminated Skydrol LD4, are no longer observed.

Chemosensors 2020, 8, x FOR PEER REVIEW 16 of 20

Wavenumber [cm-1]

-- 10000 5000 3333 2500 2000 1667 1429 1250 2 Chemosensors 2020, 8, 131 16 of 20

1% H3PO4 1

1.5% H2O 0

norm

Tr 1% Butanol

D -1

1% Phenol -2

dopt = 1cm dopt = 300mm -3 0 1000 2000 3000 4000 5000 6000 7000 8000 l [nm]

Figure A3. Difference spectra of phenol, butanol, and water contaminated SKYDROL LD4 as

measured relative to a reference specimen containing 0.2% H2O. The top panel displays a spectrum

of phosphoric acid that contains an amount of water that unavoidably comes with the H3PO4.

Figure A3. Difference spectra of phenol, butanol, and water contaminated SKYDROL LD4 as measured The missing O-H features, however, do turn up as small amounts of H3PO4 are added. As liquid relative to a reference specimen containing 0.2% H2O. The top panel displays a spectrum of phosphoric H3PO4 unavoidablyacid that contains contains an amount small ofquantities water that unavoidably of water comes(~15%), with we the Hattribute3PO4. the re-appearance of the missing O-H features to H3O+ ions, which form as the liquid H3PO4 undergoes electrolytic The missing O-H features, however, do turn up as small amounts of H3PO4 are added. As liquid dissociation together with the concomitantly introduced water. In this way dissolved H3O+ ions form H3PO4 unavoidably contains small quantities of water (~15%), we attribute the re-appearance of the + as sketchedmissing in Figur O-H featurese A4a. to In H case3O ions, degradation which form products as the liquid like H3PO butanol4 undergoes and electrolyticphenol are dissociation simultaneously + present, togetherthe OH withgroups the concomitantly in the split-off introduced alcohols water. can Inget this protonated, way dissolved forming H3O ions molecular form as sketched end groups as shown inin Figure Figure A4A4a.b. In As case these degradation end groups products effectively like butanol represent and phenol H3O are+ ions simultaneously tightly bound present, to heavy the OH groups in the split-off alcohols can get protonated, forming molecular end groups as shown in hydrocarbon backbones, the H-O-H end groups remain +with the same kinds of vibrational degrees Figure A4b. As these end groups effectively represent H3O ions tightly bound to heavy hydrocarbon + of freedombackbones, as the thedissolved H-O-H end H3 groupsO ions remain in Figure with the A4 samea. Due kinds to of their vibrational larger degrees effective of freedom masses, as the however, + these covalentlydissolved bonded H3O ions H in-O Figure-H end A4 a.groups Due to w theirill exhibit larger e fflowerective masses,vibrational however, frequencies these covalently as evidenced in Table bondedA2. H-O-H end groups will exhibit lower vibrational frequencies as evidenced in Table A2.

+ Figure A4.Figure (a) H A4.3O(+a )Hion3 Odissolvedion dissolved in a inphosphate a phosphate-ester-ester fluid; fluid; ( b(b) split-o) splitff-offhydrocarbon hydrocarbon side group side after group after capture of a proton from an acid partial phosphate. capture of a proton from an acid partial phosphate.

The broad absorption feature extending from about 3500 to 5000 nm, which increases as H3PO4 Theis broad added, absorption can be explained feature by stretching extending vibrations from about of OH-groups 3500 to with 5000 strong nm, hydrogenwhich increases bonds [16 as]. H3PO4 is added, can be explained by stretching vibrations of OH-groups with strong hydrogen bonds [16]. As similar bonds will also prevail in all kinds of partial2 phosphates, this broad feature appears to be the most useful one for a direct spectroscopic determination of the TAN value. A final important point, illustrated by Figure A3, is that the deliberate contamination of pure Skydrol LD4 with fluid degradation products does not provide any clue to the strong UV-VIS features that occur upon thermal degradation and in the presence of enhanced TAN values (see Figure A1a–c). The strong UV-VIS absorption therefore needs to be attributed to a molecular species that is formed from the original decay products through one or several thermally activated steps. As argued in Appendix

Chemosensors 2020, 8, 131 17 of 20

As similar bonds will also prevail in all kinds of partial phosphates, this broad feature appears to be the most useful one for a direct spectroscopic determination of the TAN value. A final important point, illustrated by Figure A3, is that the deliberate contamination of pure Chemosensors 2020, 8, x FOR PEER REVIEW 17 of 20 Skydrol LD4 with fluid degradation products does not provide any clue to the strong UV-VIS features Cthat, we occur believe upon that thermal these degradationentities consist and of in themulti presence-ring aromatic of enhanced species, TAN which values (seehad Figurebeen formed A1a–c). by selfThe-association strong UV-VIS of split absorption-off phenyl therefore groups needs and to whichbe attributed are able to a to molecular absorb in species the s thathort- iswavelength formed from the original decay products through one or several thermally activated steps. As argued in visible and long-wavelength UV ranges. AppendixC, we believe that these entities consist of multi-ring aromatic species, which had been formed by self-association of split-off phenyl groups and which are able to absorb in the short-wavelength Appendix C. UV-VIS Absorption of Single- and Multi-Ring Aromatic Species visible and long-wavelength UV ranges. Commercial aviation hydraulic fluid base stocks mainly consist of mixtures of tributyl phosphate,Appendix di/ C.- UV-VISbutyl phenyl Absorption phosphate of Single- and triphenyl and Multi-Ring phosphate, Aromatic with Speciesthe specific mix depending on the aviationCommercial hydraulic aviation fluid hydraulic product fluid [1–3 base]. stocks mainly consist of mixtures of tributyl phosphate, di/-butylAs described phenyl phosphate in the main and te triphenylxt, thermal phosphate, degradation with theof the specific base mixfluid depending splits off onthe the hydrocarbon aviation sidehydraulic groups fluid from product their parent [1–3]. phosphate cores. Aliphatic C-H side groups in turn will only absorb UV lightAs described with wavelengths in the main text,smaller thermal than degradation 200 nm [4 of5]. the A base notable fluid splits exception off the is hydrocarbon aromatics, side which containgroups conjugated from their parentC-C bonds phosphate arranged cores. in Aliphatic a ring form. C-H As side these groups conjugated in turn will systems only absorb form UVdelocalized light electronwith wavelengths systems, smaller smaller HOMO than 200-LUMO nm [45 ].splittings A notable result exception as the is size aromatics, of the delocalized which contain electron conjugated systems increasesC-C bonds [46]. arranged While phenyl in a ring side form. groups As by these themselv conjugatedes will systems hardly formabsorb delocalized light with electronwavelengths systems, longer thansmaller 300 HOMO-LUMOnm (Figure 10a), splittings larger aromaticresult as the aggregates size of the exhibit delocalized smaller electron HOMO systems-LUMO increases splittings [46 with]. opticalWhile absorption phenyl side tails groups extending by themselves out into the willVIS range hardly (Figure absorb A light5). The with comparison wavelengths in this longer latter than Figure suggests300 nm (Figurethat optical 10a), largerabsorption aromatic in the aggregates VIS range exhibit requires smaller self HOMO-LUMO-association of splittings at least withfour opticalaromatic ringsabsorption [47]. Interestingly, tails extending such out cases into of the self VIS-association range (Figure have A5 been). The observed comparison with in phenols this latter dissolved Figure in suggests that optical absorption in the VIS range requires self-association of at least four aromatic various hydrocarbon solvents [48]. As such association processes are likely to require a significant rings [47]. Interestingly, such cases of self-association have been observed with phenols dissolved in amount of thermal activation, such molecular entities can only be expected after a prolonged time at various hydrocarbon solvents [48]. As such association processes are likely to require a significant elevated temperatures or relatively short times at high temperature. Furthermore, as different hydraulic amount of thermal activation, such molecular entities can only be expected after a prolonged time fluids carry different numbers of phenyl side groups on their center phosphate groups, the probability of at elevated temperatures or relatively short times at high temperature. Furthermore, as different occurrence of multi-ring aromatic degradation products is expected to be highly fluid-dependent and also hydraulic fluids carry different numbers of phenyl side groups on their center phosphate groups, dependentthe probability on the of use occurrence profile of ofthe multi-ring particular aromaticfluids. degradation products is expected to be highly fluid-dependent and also dependent on the use profile of the particular fluids.

7

6

5

4

3

Log[ Extinction ] Log[ Extinction

2

1 200 250 300 350 400 450 500 Wavelength [nm]

Figure A5. UV absorption of multi-ring aromatic species [47]. Figure A5. UV absorption of multi-ring aromatic species [47].

References

1. Churchill, J. The Skydrol Story; Kindle Edition; 2012. Available online: https://www.amazon.com/Skydrol- Story-John-Churchill-ebook/dp/B00KGWI04K (accessed on 10 December 2020). 2. Skydrol™ aviation hydraulic fluids. Available online: https://www.skydrol.com (accessed on 10 December 2020). 3. Totten, G.E.; De Negri, V.J. Handbook of Hydraulic Fluid Technology, 2nd ed.; CRC Press Inc.: Boca Raton, Fl USA, 2011. ISBN: 978-1-4200-8526-6.

Chemosensors 2020, 8, 131 18 of 20

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