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8 CHAPTER THREE EFFECTS OF PAVEMENT DEICING PRODUCTS ON AIRCRAFT COMPONENTS Alkali-metal-salt-based PDPs such as KAc and KF entered The typical C/C composite brake assembly consists of a the European market to a significant extent in the mid- to primary thrust stator and multiple stator disks in an alter- late-1990s (approximately 1995 for KAc and 1998 for KF). nating stack with multiple rotor disks (Figure 3). The float- A few years later, these modern PDPs entered the U.S. mar- ing stators are kept from rotating with the motion of the land- ket. In both cases, these salts were introduced as alternatives ing gear wheel by interlocking stator tenons and torque tube to urea and glycols used in traditional PDPs for freezing splines, while the similarly keyed rotors rotate with the wheel. point depression to mitigate the environmental concerns re- When activated, an annular piston assembly presses the thrust lated to airfield deicing and anti-icing operations. It became stator into the stack, causing very high friction force between apparent soon after their introduction that these new deicers the rotors and stators. Such friction can cause the carbon presented new challenges. For instance, introduction of the discs to reach 400°C600°C in normal operations and up to alkali-metal-salts as PDPs coincided with the increased fre- 1400°C in some extreme cases such as in the event of refused quency of failures and returns of aircraft carboncarbon (C/C) take-off (Wu 2002). brakes, whereas aircraft C/C brakes had not changed signif- icantly since their introduction in mid-1980s. Aircraft C/C A known drawback of C/C composites is their susceptibil- composite brakes mainly consist of a carbon matrix reinforced ity to thermal oxidation at brake operating temperatures. At by carbon fibers (either woven or randomly dispersed). In temperatures above 400°C, the reaction of carbon with oxy- 1985, Airbus Industries introduced the first MessierBugatti gen can easily occur and thus wears the unprotected C/C com- production carbon brakes on the A310-300 and A300-600 posites. Existing research has demonstrated that oxidation aircraft (http://www.messier-bugatti.com/article.php3?id_ (gasification) is the predominant mechanism for weight loss article=180). Other issues concerning the aircraft durability (an indicator of loss in structural integrity) of the C/C com- and operations have also contributed to the debate surround- posites under high-energy brake operation conditions (Wu ing the use of such PDPs. 2002). These conditions led to the popular practice of pro- tecting the surface of C/C composite brakes with anti-oxidant The following sections of this chapter will discuss the coatings. As such, thermal oxidation usually will not occur effects of PDPs on aircraft components, including catalytic until the anti-oxidant layer is disrupted. Dynamometer testing oxidation of C/C composite brakes, corrosion of aircraft alloys at 50% relative humidity and 100% normal aircraft landing (with a focus on cadmium plating), and interaction with air- energy led to significant oxidation of friction layer and fric- craft deicing and anti-icing products. tion debris samples obtained from commercially available C/C composite brake material, accompanied by the release of gaseous H2O, CO2, and CO (Penszynska-Bialczyk et al. CATALYTIC OXIDATION OF CARBONCARBON 2007). Therefore, some level of thermal oxidation is expected COMPOSITE BRAKES and effective oxidation protection is essential to the design of C/C composite brakes. Composites of a carbon matrix reinforced by carbon fiber (C/C composites) possess excellent mechanical and thermal As commercial and military aviation use of C/C composite properties. These C/C composites are much lighter than steel, brakes continues to grow, so does the scope of challenges asso- maintain a near-constant friction coefficient over a broad tem- ciated with the technology. Alkali-metal-salt contaminants perature range, possess a much higher heat capacity and ther- (e.g., sodium from the marine environment and potassium mal conductivity than steel, and have tensile strength gener- from cleaning and deicing chemicals) can reach the carbon ally twice that of steel at elevated temperatures. Owing to surface and act as catalysts to facilitate oxidation of C/C their advantages over metallic friction materials, C/C com- composite brakes under conditions milder than those required posites have been increasingly used on aircraft brakes (Chai for thermal oxidation. Concerns have been raised about the and Mason 1996). Since their aviation debut aboard the Con- effect of modern PDPs (mainly alkali-metal-salts) on aircraft corde and the Vickers Super VC-10 ("Company History" brakes. 2007), there has been a general transition from metallic brakes to C/C composite brakes on modern medium and large air- The rest of this section will synthesize the information on craft (Chai and Mason 1996). the validity and nature of the effect of modern PDPs (primar-
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9 Rotor disks Rotor tenons keyed to wheel splines Brake Assembly Wheel Stator disks Assembly Stators keyed to torque tube splines Primary thrust stator FIGURE 3 Typical airplane wheel and brake configuration: cross-sectional view. ily alkali metal salts) on aircraft brakes, describe the related locations, catalytic oxidation typically corresponds to local- standards and test protocols, discuss ways to prevent and mit- ized regions in cooler brake locations in contact with con- igate such effects, and identify knowledge gaps. taminant residue (e.g., aft aircraft positions) (Walker 2007). Figure 4 illustrates some C/C brake damage attributed to cat- Validity of the Effect of Modern Pavement alytic oxidation. Deicing Products on Aircraft Brakes Catalytic oxidation of C/C composite brakes owing to air- Thermal oxidation is the primary design specification gov- field PDPs has become a growing concern to be monitored in erning durability of aircraft C/C composite brakes. This is the ever-changing operation environment (McCrillis 2007). substantiated by data from Goodrich's Problem Analysis As nontraditional chemical contaminants, modern PDPs may Report System, which has monitored failures and premature be responsible for the more rapid structural failure of C/C removals of C/C composite brakes (McCrillis 2007). Whereas composite brakes in recent years. To avoid potential safety thermal oxidation can be seen uniformly at the hottest brake implications, this concern has to be mitigated through more (a) (b) FIGURE 4 Carboncarbon brake damage attributed to catalytic oxidation: (a) oxidized rotor drive lugs, (b) oxidized stator tenons (courtesy: Continental Airlines).
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10 a illustrate the catalysis of carbon oxidation by a KAc-based 1 PDP, with the C/C composite samples impregnated with the Weight / Initial weight PDP showing a higher oxidation rate and a higher weight loss 0,8 at lower temperatures (Filip 2007). New Brake 0,6 Used Brake As the brake frictional characteristics are changed by the 0,4 use of alkali-metal-salt-based PDPs, airline operators are con- cerned over the adverse effect of such PDPs on the braking 0,2 Impregnated Brake performance and safety of aircraft (Jansen 2007). In addition to reduced brake life, such effect introduces the possibility of brake failure during aborted take-off, with the concomitant 1 b risk of fire from hydraulic fluid released during such an event. Weight / Initial weight 0,8 Used Brake Since 2003, Dunlop has analyzed the contamination of New Brake material samples of heat packs returned for service using the 0,6 induction coupled plasma spectroscopy (Hutton 2007). A 0,4 plain geographic trend can be seen (as shown in Table 5), with those aircraft operated in Northern Europe experiencing Impregnated Brake 0,2 the highest rates of potassium contamination and those oper- ated in Southern Europe and the Mediterranean region expe- 0 riencing the lowest rates of contamination. According to 0 2 4 6 8 10 12 14 16 18 20 22 24 Dunlop (Hutton 2007), this trend corresponds to the reported Time (h) frequency of catalytic oxidation in the BAe 146 fleet oper- ated throughout Europe--highest in Nordic countries where FIGURE 5 Weight loss curves of samples of new, used, and KAc-impregnated aircraft brakes obtained by TGA at 500°C KAc and KF are regularly used for snow and ice control and (a) and 600°C (b) [adapted from Carabineiro et al. (1999)]. lowest in Southern European countries where PDPs are not needed. Although aircraft are moving objects exposed to var- ious PDPs, climatic conditions, and maintenance practices at different airports, the contaminants encountered other than frequent proactive maintenance and inspection activities incur- PDPs often are essentially the same type of fire extinguish- ring high direct and indirect costs. ing or cleaning fluids as defined in the manufacturers' con- sumable materials list of allowable materials. Therefore, PDPs A growing body of field evidence from airline operators play a key role in the trend seen in Table 5. suggests that the use of KAc and KF on airfield pavements leads to catalytic oxidation of C/C composite brake compo- Evidence from aircraft operators and manufacturers in nents. The thermogravimetric analysis (TGA) data in Figure 5 North America, Europe, and Asia corroborate the role of PDPs illustrate the catalysis of carbon oxidation by potassium in catalytic oxidation of C/C composite brakes. Irregular C/C species, with the new brake samples impregnated with KAc composite brake wear and damage has been reported on vir- showing high-oxidation reactivity at relatively low tempera- tually every aircraft platform equipped with them by inter- tures (Carabineiro et al. 1999). The TGA data in Table 4 also national and regional carriers (Duncan 2007). ACRP survey TABLE 4 TGA DATA AS A FUNCTION OF C/C COMPOSITE AND DEICER CONTAMINATION Onset of Oxidation Maximum Rate of Weight Loss Weight Loss Sample (°C) (% per min) (%) Virgin HWCCA 447 0.617 45 HWCCA+ PDP 421 0.902 71 Virgin HWCCD 382 0.619 53 HWCCD+ PDP 325 0.720 76 Adapted from Filip (2007). Note: HWCCA, HWCCB, and HWCCD stand for different C/C composite materials, specifically for CarbenixTM C2000 2D pitch/charred resin, CarbenixTM C2000 2D-modified pitch/charred resin, and CarbenixTM 400 3D PAN/CVD, respectively.
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11 TABLE 5 BAe 146 AIRCRAFT BRAKE CONTAMINATION ANALYSIS Base of Operations No. No. of Heat Mean K Content Operator (destinations) of Aircraft Packs Analyzed per Sample (ppm) A United Kingdom (Europe) 20 101 547 Finland B 9 44 2,183 (Scandinavia, Europe) Greece C 6 42 74 (Greek Islands, S. Europe) D Germany (Europe) 8 51 749 E Germany (Europe) 5 18 991 F Belgium (Europe) 31 93 563 G Belgium (Europe) 11 32 991 Adapted from Hutton (2007). respondents also indicated that no landing gear components oratories (Figures 6 and 7). Weight loss percentages recorded made from C/C composite were immune to catalytic oxida- by MessierBugatti were much higher than those recorded in tion. Stators and related components tend to be more prone the other two laboratories--perhaps as a result of differences to catalytic oxidation than rotors, although rotors will read- in heating temperature and air--which merit further investi- ily load catalytic contaminants when the aircraft is stationary gation. Nonetheless, there was a consistent trend in weight (Walker 2007). Direction of contaminants to brake parts can loss data confirming the catalysis of contaminants for C/C be influenced by landing gear design; redesign of landing oxidation. Figures 6 and 7 also indicate that the different C/C gear on existing aircraft, however, is not practical. composite materials (both virgin and catalyst-loaded sam- ples) experienced different levels of oxidation, which will be discussed later in this chapter. Dunlop and ABSC are currently conducting catalytic oxidation round-robin testing for the SAE A-5 committee Oxidation testing conducted jointly by the Center for Ad- (Hutton 2007), and preliminary data from both laboratories vanced Friction Studies (CAFS) at Southern Illinois Univer- indicate a significant increase in the oxidation of C/C in the sity, Dunlop, Aircraft Braking Systems Corporation (ABSC), presence of KAc and KF. In the most recent round of tests, and MessierBugatti incorporated a range of materials, cata- uniform C/C coupons (material type not specified) were pre- lyst treatment methods, and heating temperatures (Table 6). treated with 25% (by weight of solution) KAc, KF, or urea; Selected data regarding catalytic oxidation of unprotected heated at 550°C for 4 h; and then weighed. The coupons C/Cs were presented at the SAE G-12 committee meeting in treated with KF and KAc showed significantly higher weight May 2007 (Filip 2007), and the results were predictably var- loss than the control coupons, whereas those treated with urea ied owing to differences in test methods employed in the lab- showed weight loss similar to that of the control coupons TABLE 6 TESTING PARAMETERS IN VARIOUS LABORATORIES CAFS Dunlop MessierBugatti Sample Cylinder, D = 10 mm, H Cylinder, D = 49.9 mm, 40 mm × 40 mm × 40 mm, = 8 mm, mass ~ 1.2 g H = 5.94 mm, mass ~ 20 g mass ~ 50g With a KAc-based PDP-- Cryotech E36 Contamination Soaked for 60 min Soaked for 30 min and Impregnated under vacuum for dried at 150°C for 120 min 60 min and dried at 105°C for 120 min Heating 700°C for 20 min, 550°C for 4 h, 650°C for 5 h, still air still air constant O2 partial pressure CAFS = Center for Advanced Friction Studies.
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12 18% 16% 14% 12% Weight loss 10% 8% 6% 4% 2% 0% CAFS HWCCD catalyst HWCCD pure ABSC/Dunlop HWCCB catalyst HWCCB pure HWCCA catalyst HWCCA pure loaded loaded loaded FIGURE 6 Selected results from carbon oxidation testing by Center for Advanced Friction Studies (CAFS) and Aircraft Braking Systems Corporation (ABSC)/Dunlop [adapted from Filip (2007)]. HWCCA = CarbenixTM C2000 2D pitch/charred resin, HWCCB = CarbenixTM C2000 2D-modified pitch/charred resin, and HWCCD = CarbenixTM 400 3D PAN/CVD. (Figure 8). Although KAc- and KF-treated coupons experi- eral mechanisms proposed to account for the catalytic effects enced higher weight loss in the ABSC laboratory than in of metals, oxides, and salts in carbon oxidation (Walker et al. the Dunlop laboratory, the results demonstrated the roughly 1968). At approximately 800°C the diffusion of oxygen equal effectiveness of both potassium salt deicers as catalysts through the surrounding gas to the carbon surface becomes for carbon oxidation. the limiting step for thermal oxidation (Wu 2002). At typical peak operating temperatures for C/C aircraft brakes (near 500°C), the alkali-metal-salt-based PDPs (e.g., KAc) are Nature of the Effect of Modern Pavement known to decompose to alkali metal carbonates and oxides Deicing Products on Aircraft Brakes (e.g., K2CO3 and K2O). The active species (S) are believed to Existing research in the laboratory has demonstrated the cat- serve mainly as "more effective adsorption and dissociation alytic effects of potassium, sodium, and calcium on carbon agents for the gaseous reactant than carbon itself" and to oxidation (Lang and Pabst 1982; Krutzsch et al. 1996; Wu transfer the adsorbed oxygen (O) to the carbon (C). They can 2002). Oxygen transfer and electron transfer are the two gen- go through an oxidation-reduction cycle, as represented here (Wu 2002): 100% 2S + O 2 2S ( O ) (1) Pure Catalyst loaded S ( O ) + C Cf ( O ) + S (2) 80% Such an oxygen-transfer mechanism has been supported by Weight Loss 60% laboratory investigation pertinent to this subject. Environmen- tal scanning electron microscopy experiments demonstrated that K-oxide particles very effectively catalyzed the gasifica- 40% tion of isotropic carbon fibers in a C/C composite. In situ X-ray diffraction experiments suggested that a K-peroxide 20% acted as the reactive intermediate species (Carabineiro et al. 1999). Experimental observations and molecular orbital (MO) calculations supported Wu's (2002) theory as follows: the 0% presence of catalyst or inhibitor on carbon materials affects the HWCCA HWCCB HWCCD oxidation behavior by influencing the concentration and sta- FIGURE 7 Selected results from carbon oxidation testing by bility of two types of oxygen complexes on the carbon surface MessierBugatti [adapted from Filip (2007)]. during the CO2 reaction.
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13 45% 40% 35% 30% Weight loss 25% 20% 15% 10% 5% 0% KAc KF Urea Control Treatment (a) 45% 40% 35% 30% Weight loss 25% 20% 15% 10% 5% 0% KAc KF Urea Control Treatment (b) FIGURE 8 Weight loss of C/C after heating at 550°C for 4 h: (a) Dunlop data; (b) ABSC data [adapted from Hutton (2007)]. The electron-transfer mechanism, on the other hand, sug- of K salts and their decomposition products--all below 600K gests that catalysts [e.g., alkali metals (AM)] have unfilled (or 327°C)--allow them to migrate easily on the carbon sur- electron shells and accept electrons from carbon matrix, as face and form good interfacial contact with it (Wu 2002; Wu represented here (Filip 2007): and Radovic 2005), facilitating oxygen transfer (Figure 9a). In some cases heavy catalyst loading may retard the activity CO3 2 - + 2C 3CO + 2e - (3) of K species by "crowding" particles on the carbon surface (Figure 9b). Calcium oxide, with a melting point higher than 1500K (or 1227°C) (Wu 2002), relies on initial loading and 2AM + + CO 2 + 2e - AM 2 O + CO (4) impregnation to achieve the necessary surface contact. As oxidation proceeds, immobile calcium oxide may act as a AM 2 O + CO 2 AM 2 CO3 (5) barrier for additional catalysts, lowering the oxidation rate (Figure 9c). This retarded reaction recommends Ca-based This wetting ability (or lack thereof) is also relevant in the PDPs as less detrimental to C/C aircraft brakes than K-based catalysis of C/C composites. The low melting temperatures PDPs (Wu and Radovic 2005).
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14 carbon surface catalyst particles inactive catalyst active catalyst catalyst layer (a) high catalyst activity (b) intermediate catalyst activity (c) low catalyst activity FIGURE 9 Catalyst loading and activity on brake surface [adapted from Wu and Radovic (2005)]. Several other less-studied factors also deserve attention. In addition, the SAE A-5A Working Group is in the pro- Temperature has been observed to play a role in governing cess of developing an oxidation test method for anti-oxidant the intensity of catalytic oxidation. Walker et al. (1997) (AO)-treated coupons and the details are summarized as fol- observed a threshold of between 650°C and 700°C where the lows. C/C composite brake material will be cut from pro- reaction rate of catalytic oxidation of C/C friction material in duction discs and tested in the configuration of cylinders. A the presence of KAc drops off dramatically. Electrical con- generic AO treatment based on mono-aluminum phosphate, ductivity may also be an important factor, as it appears to phosphoric acid, and water (to be determined) will be used reflect catalytic potential in common pavement deicers. Con- to simulate the application of the AO protection system. ductivities of KAc and KF compared with that of urea appear Although AO treatments are typically applied to nonfriction to mirror relative catalytic abilities of these deicers (Table 7). surfaces only, it is proposed to cover all surfaces of the test coupon to demonstrate the effect of the AO treatment. Dipping coupons in the AO treatment is proposed because it is less Standards and Test Protocols operator-sensitive than other application methods (e.g., brush- ing and spraying). The AO treatment will then be cured by SAE AMS 1431C and AMS 1435B are the accepted standards heating the coupons at a ramp rate of 60°C/h to 300°C/h in for solid and liquid pavement deicers, respectively. Neither air and at 300°C for 1 h. The AO treatment will be made cen- currently contains requirements for C/C catalytic oxidation trally before the test coupons are distributed to test facilities. testing. The SAE G-12 Carbon Oxidation Working Group is To test the catalytic oxidation, runway deicing solutions will in the process of refining a carbon compatibility test protocol be used at 25% w/w concentration. The weight loss of the with assistance from the SAE A-5A Brake Manufacturers AO-treated coupons will be tested under the same tempera- Working Group for inclusion in the next revision of both ture for the testing of bare carbon; that is, 1022°F ± 10°F or standards. better (550°C ± 5°C) in still air (SAE A-5A . . . 2007). Boeing provides for testing of runway and facility deicers As discussed previously, there were large variations ob- in its comprehensive test protocol for the Evaluation of Main- served in the carbon oxidation testing results from different tenance Materials, Specification D6-17487. The current revi- laboratories. These variations highlight the need to develop sion of the protocol, released in 2003, specifies details for test- reliable, standard testing procedures to evaluate the catalysis ing solid and liquid deicers, but does not address their roles in of PDPs for carbon oxidation, which would allow better C/C composite catalytic oxidation. Boeing has no plans at this practices in preventing or mitigating such catalysis. To ensure time to add catalytic oxidation testing to D6-17487, citing a reproducible results, protocol parameters to be defined will requirement included in every Boeing brake assembly engi- include sample material, density, dimensions, coupon orienta- neering drawing: ". . . that the brake be designed to be com- tion, contamination concentration, temperature, duration, post- patible with different materials including runway de-icer flu- treatment, heating temperature, and ambient air velocity. ids" [M. Arriaga, Boeing Company, personal communication, July 2007]. Prevention and Mitigation There are potential opportunities for all stakeholder groups to TABLE 7 collaborate in addressing the catalytic oxidation issue of C/C ELECTRICAL CONDUCTIVITIES aircraft brakes with respect to aircraft and component design, OF PAVEMENT DEICERS brake testing, aircraft operations, airfield maintenance, etc. In Electrical Conductivity 1% Solution the domain of brake technologies, chemical modification of Type (µS/cm) C/C appears to offer greater potential than structural changes Urea 18 or defect elimination in mitigating catalytic oxidation (Filip KAc ~7000 2007). Chemical modification generally involves the intro- duction of groups of atoms to reduce gasification of carbon, KF ~9000 the reduction of catalyst mobility, and the formation of a
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15 barrier for transport of oxygen and reaction products (Filip combined use of P and B might offer more effective inhibi- 2007). Defect elimination generally involves the reduction of tion of catalytic carbon oxidation than used individually (Wu C/C composite porosity and the elimination of active oxida- 2002). Emphasizing the impossibility of eliminating K cata- tion sites (by means of improved crystalline order of carbon lysis of C/C composites, an inhibition system employing a and reduced defects) (Filip 2007). A firm understanding of the combination of painted or ceramic coatings on nonfriction catalysis mechanism has nurtured development of various surfaces, B-doped C/C substrate, and deposited P and/or B2O3 proprietary AO formulations tailored to material, environ- was suggested. mental, and performance needs of specific brake and wheel designs. It is also recognized that catalytic oxidation of C/C Application of the laboratory research has been met with composite friction material by some alkali-metal-salt-based mixed success. Industry experiments with B-doping have not PDPs--such as those based on KAc and KF--cannot be fully shown promise (T. Walker, Honeywell International, per- arrested in situ by current methods. sonal communication, July 2007), although elemental B has been used successfully as a barrier coating. Brush-on phos- Dozens of U.S. and European patents have been assigned phate AO coatings continue to be widely employed, with peri- to formulae and methods for blocking active oxidation sites on odically improved formulations. These AO treatments typi- and below the C/C composite brake material surface. The oxi- cally include multiple cycles of a brush-applied or sprayed dation inhibiting composition usually penetrates at least some phosphate- or phosphoric acid-based coating on the C/C of the pores of the C/C composite and, once heated, forms a material, followed by high-temperature curing. In addition, deposit within the penetrated pores and the surface of the C/C a ceramic-based oxygen barrier coating is applied to non- composite. For instance, Stover and Dietz (1995) and Stover friction surfaces (Webb 2007). C/C composites composed of (1998, 2003) created AO formulations primarily of phosphoric three-dimensional nonwoven fabric with preform chemical acid, metal phosphates, and aluminum and zinc salts in a vapor deposition matrix have generally replaced those com- polyol/alcohol base. Walker and Booker (2000) demonstrated posed of chopped polyacrylonitrile (PAN) or pitch fibers, the effectiveness of P-13 (a standard phosphoric-acid-based owing to the increased load-bearing and thermal properties of AO) and a potassium compound (KH2PO4) for inhibiting cat- the former. Combinations of these advances have resulted in alytic oxidation by KAc by blocking active oxygen transfer marked reductions in unscheduled replacements owing to cat- sites on the surface of C/C friction material. It was proposed alytic oxidation. that the addition of KH2PO4 blocked sites on the carbon sur- face that were particularly prone to "activation" by K cataly- Materials-deicer compatibility testing conducted by the sis. Phosphate-based AO paints are now a standard, effective Concurrent Technologies Corporation on behalf of the Air tool in reducing catalytic oxidation (T. Walker, Honeywell Force Research Laboratory in 2003 and 2004 included catalytic International, personal communication, July 2007). oxidation testing of four Honeywell C/C friction materials and three pavement deicers that were then new to the market Wu et al. (2001) and Wu (2002) explored inhibition by or still in development (Concurrent Technologies Corp. phosphorus (P)- and boron (B)-deposition and B-doping in 2004). For each contaminant, ten specimens--five brushed fine detail and reported several key findings. Data sug- with Honeywell P-13 phosphate-based AO compound-- gested the presence of two catalyst-deactivation mechanisms. of each friction material were soaked in deicer or deionized Surface-deposited P and B compounds were found to block water for 20 min and dried in still air at 110°F (43°C) for (with varying success) catalysts from contact with active oxi- 30 min. Specimens were then heated for two 4-h periods at dation sites on the carbon substrate. Thermally deposited 1300°F (704°C) and allowed to cool to room temperature in P compounds were demonstrated to be effective in inhibiting still air after each session. Three of the C/C materials were the carbon oxidation catalyzed by KAc and calcium acetate composed of chopped pitch or PAN fiber with a phenolic (CaAc); and the characterization of P-deposited carbon sam- char chemical vapor deposition matrix, whereas the fourth ples and ab initio molecular orbital calculations both suggested material, CarbenixTM 4000, consisted of 3D needled, non- that the inhibition effectiveness derived from the formation of woven PAN fabric with a chemical vapor deposition preform possibly C-O-PO3 groups and C-PO3 groups (Wu and Radovic matrix. As shown in Figures 10 and 11, unprotected 3D PAN 2006), which preferentially block the active carbon sites (Wu samples suffered weight and hardness losses similar to those 2002). The effect on K catalysis was much smaller owing to of the other unprotected friction materials. In contrast, AO- the high wetting ability and mobility of K species. Also sug- treated 3D PAN samples experienced the lowest weight and gested was the possibility that in sufficient concentration, hardness losses in the group. This observation suggests that these deposition compounds form stable oxide glazes over the even though the 3D material showed no inherent resistance to friction surface, acting as an oxygen barrier. B-doping pro- catalytic oxidation, one or more of its unique traits improved moted better graphitization of the C substrate, denying free the performance of the AO surface treatment. Although the electron sites to catalysts. A secondary benefit to B-doping CTC report did not offer explanations for the favorable per- was the lower curing temperature (by 400°C500°C) needed formance of the AO-treated 3D C/C composite material, for satisfactory graphitization. Wu (2002) suggested that a implementation of this improved material in the field in
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16 90 90 Average hardness loss (%) Average weight loss (%) 80 80 70 70 60 De-ionized water 60 De-ionized water Safeway KF Hot 50 Safeway KF Hot 50 AVIFORM L50 40 AVIFORM L50 40 METSS RDF-2 30 METSS RDF-2 30 20 20 10 10 0 0 Carbenix Carbenix Carbenix Carbenix Carbenix Carbenix Carbenix Carbenix 1000 2000 2110 4000 1000 2000 2110 4000 C/C friction material C/C friction material (a) (a) 90 Average hardness loss (%) 70 80 70 Average weight loss (%) 60 60 De-ionized water 50 50 Safeway KF Hot De-ionized water 40 AVIFORM L50 40 Safeway KF Hot 30 METSS RDF-2 30 AVIFORM L50 20 METSS RDF-2 10 20 0 10 Carbenix Carbenix Carbenix Carbenix 1000 2000 2110 4000 0 Carbenix Carbenix Carbenix Carbenix C/C friction material 1000 2000 2110 4000 (b) C/C friction material FIGURE 11 Average hardness loss of (a) unprotected; (b) AO- (b) treated C/C friction materials exposed to KF-based pavement FIGURE 10 Average weight loss of (a) unprotected; (b) AO- deicers and de-ionized water [adapted from Concurrent treated C/C friction materials exposed to KF-based deicers Technologies Corp. (2004)]. (Note: CarbenixTM is a Honeywell and de-ionized water [adapted from Concurrent Technologies proprietary C/C material.) Corp. (2004)]. (Note: CarbenixTM is a Honeywell proprietary C/C material.) As early as 2002, a U.S. patent was under review for an aqueous liquid aircraft runway deicer composition featuring concert with new AO systems has demonstrated similar minimal catalytic oxidation effect on C/C composites. The margins of success. New brakes with a 3D PAN preform composition contains 20%25% w/w of an alkaline earth substrate and improved AO protection fitted aboard Boeing metal carboxylate, 1%15% w/w of another alkaline earth 767s led to a 90% reduction in brake removals before end- metal carboxylate, 1%35% w/w of an aliphatic alcohol, of-service life on those aircraft, and 3D PAN substrates 0.01%1% of an alkali metal silicate, and up to about 1% have become the standard substrate on new C/C compos- w/w of a triazole (Moles et al. 2002). Through partnership ite brake designs (Walker 2007). with DuPont Tate & Lyle Bio Products LLC, this evolved into a commercial product marketed by Cryotech (BX36), Solid pavement deicer formulations were endorsed by sev- which now includes a bio-based active ingredient Susterra eral ACRP survey respondents as less aggressive catalysts, propanediol. Joint efforts by Honeywell and Cryotech led to although no specific justification was provided. Catalytic preliminary testing of BX36, which showed less conductivity oxidation of C/C brakes may also be mitigated by using more carbon-friendly PDPs on airfield pavements. Anhydrous betaine (N,N,N-trimethylglycine), a naturally occurring organic byproduct of sugar beet processing, is being developed with TABLE 8 ELECTRICAL CONDUCTIVITY AND BIOLOGICAL OXYGEN support from the Finnish Government and Finavia as a pave- DEMAND OF DEICERS ment deicer (Hänninen 2006; Simola 2006). Not yet com- mercially available, betaine has shown favorable results in Chemical Electrical Conductivity of metallic corrosion and deicing performance testing, and its Deicer Formula 1% Solution (µS/cm) BOD5 (mg O2/g) low electrical conductivity also compares favorably with Betaine C5H11NO2 99 759 other deicers (Table 8). Betaine's relatively high BOD, nitro- KAc CH3COOK ~7,000 ~300 gen content (15% by weight), and high cost may present KF HCOOK ~9,000 ~100 some challenges to using it as the sole freezing point depres- Urea H2NCONH2 18 ~2,100 sant for PDPs.