Evaluation of Fluorosilicone Silicone Elastomer Blend for Aeronautical Fuel System

Transcription

1 Evaluation of Fluorosilicone Silicone Elastomer Blend for Aeronautical Fuel System Evaluation of Fluorosilicone Silicone Elastomer Blend for Aeronautical Fuel System C.M. Bhuvaneswari 1 *, R. Dhanasekaran 2, S.K.R. Chakravarthy 3, Shirish S. Kale 4, and Gantayata Gouda 5 1,4 Regional Center for Military Airworthiness and Certification (F&F-FOL), CEMILAC, DRDO, Bangalore, India 2,3 Hindustan Aeronautics Ltd., (Foundry & Forge), Bangalore, India 5 Center for Military Airworthiness and Certification, DRDO, Bangalore, India Received: 28 May 2014, Accepted: 3 September 2014 Summary Elastomer blends based on fluorosilicone and silicone rubber have been investigated as a seal for aeronautical fuel system. Fluorosilicone seals are widely used as a prospective candidate in the aeronautical fuel system due to its notable resistance to aircraft turbine fuel (ATF) as well as its service in wide temperature applications. However, the FVMQ is relatively inferior in strength and high in cost compared to other ATF resistant elastomers. The drawbacks of FVMQ can be improved by blending with a High Consistent Rubber (HCR) such as vinyl methyl silicone (VMQ), which enhances the strength and reduces the cost. In the present work, an attempt has been made to study the properties and optimization of FVMQ-VMQ blend ratio by varying VMQ content. Blends were prepared in two-roll rubber mixing mill, cured and characterized for rheological, mechanical, physical, ageing and low temperature brittleness resistance. Furthermore, the effects of various aeronautical fluids such as ATF, lubricating and hydraulic oil on blends were also analysed. This paper reports the preface analysis of the properties of FVMQ-VMQ blend as a futuristic seal for the aeronautical fuel system. Keywords: Silicone, Fluorosilicone, Blend, Seal *Corresponding author. cm.bhuvaneswari@cemilac.drdo.in Smithers Information Ltd., 2015 Progress in Rubber, Plastics and Recycling Technology, Vol. 31, No. 3,

2 C.M. Bhuvaneswari, R. Dhanasekaran, S.K.R. Chakravarthy, Shirish S. Kale, and Gantayata Gouda Introduction Elastomers are class of materials that serve an enormous number of engineering needs in aeronautical field and play a key role in sealing technology. Elastomer seals are efficient in avoiding viscous, corrosive, inflammable fluids from leakage under high pressure or temperature and in static or dynamic condition with demanding load [1]. Many elastomers such as Nitrile (NBR), fluorosilicone and fluorocarbon (FKM) are widely used as seals in the aeronautical fuel system. FKM hold higher fluorine content up to 70% than FVMQ (up to 45%) 2. Hence, FKM seals have low volume swell with good sealing capability. However, the poor low temperature flexibility limits the FKM seal in many application areas. The moderate heat resistance (up to 120 C) makes NBR, a least opted seal for aircraft fuel system in high heat resistant applications. Fluorosilicones are the only fuel-resistant elastomer that will remain serviceable from -68 to 232 C. FVMQ, which is a trifluoropropyl methyl silicone, is extensively used as the seals in aeronautical system due to its wide operating temperature range [3]. FVMQ O-rings, gaskets, washers, diaphragms, and seals are used in fuel line connections, fuel control devices, electrical connectors, hydraulic line connectors, and fuel system access panels [4, 5]. However, when compared with fuel resistant FKM, FVMQ lags in strength and higher in cost. Hence, an investigation has been carried out to improve the strength of FVMQ by blending with high strength HCR silicone rubber. VMQ have excellent resistance to ozone and weathering at higher temperature. Furthermore, VMQ have same Si-O-Si backbone as FVMQ. Thus, high consistent VMQ was selected as the blend elastomer to enhance the strength and reduce cost. Blends of Silicone and Fluorosilicones have been reported by DOW Corning Corporation [6] to possess good physical properties and to be cheaper than Fluorosilicones. Ronald Romanoswski et al. [7] have reported the cost reduction of liquid FVMQ by blending with liquid silicone rubber and compared the blends with the copolymer of FVMQ/ VMQ. Michael A. DiPino [8] reported the enhanced physical properties and cost reduction of FVMQ by blending with VMQ for turbocharger hose in automotive applications. Further, it is reported that silicone rubbers even improve the low temperature flexibility of FVMQ [9]. Though the FVMQ-VMQ have been studied as an individual elastomer or blend for fluid resistance by several workers [6-9] for general applications, there has been no report considered extensively on the optimization of blend ratio for aeronautical fuel system. Hence, an attempt was made to investigate FVMQ and VMQ blend as an aeronautical fuel seal by varying VMQ from 20 to 80% by weight. The FVMQ- VMQ blend properties were examined for rheological, mechanical, physical, ageing properties, effect of aeronautical fluids and low temperature brittleness 208 Progress in Rubber, Plastics and Recycling Technology, Vol. 31, No. 3, 2015

3 Evaluation of Fluorosilicone Silicone Elastomer Blend for Aeronautical Fuel System resistance. Further, the optimized FVMQ-VMQ blend seals were evaluated for endurance in the test rig of gas turbine engine starter unit. Experimental Materials Elastosil R 401/60 grade (Wacker Silicones, Germany) of vinyl poly dimethyl siloxane (HCR-High Consistent Rubber) and FSE 7560 grade (GE Silicones, New York) of trifluoropropyl poly dimethyl siloxane were selected. ELASTOSIL PT ultramarine blue RAL 5002 (Wacker Silicones, Germany) was used as colouring agent. DI CUP 99% (dicumyl peroxide) was used as curing agent. Blend Preparation and Characterization Blends of FVMQ and VMQ, replacing FVMQ with VMQ from 20 to 80% by weight were prepared on a two roll rubber mixing mill. Six different blend ratios were selected for this study. The blends were added with curing and colouring agent. The blends were designated as given in the following Table 1. All blend compounds were tested for their curing characteristics using Oscillating Disc Rheometer (ODR) from the make of Microvision Industries, Mumbai according to ASTM D 2084 at 170ºC. The blend vulcanisates were tested for their tensile properties using Computerised Universal Testing Machine for Rubbers and Plastics Tester from the make M/s R&D Electronics, Mumbai as per ASTM D 412. Hardness was tested by IRHD (International Rubber Hardness degree tester) Microhardness Tester, Model No with make of M/s Karl Frank, Germany as per ASTM D Density was determined by hydrostatic method using Mettler Table 1. Designation of blends Sl. No Sample No Fluorosilicone (wt%) Silicone (wt%) 1. RCMA RCMA RCMA RCMA RCMA RCMA Progress in Rubber, Plastics and Recycling Technology, Vol. 31, No. 3,

4 C.M. Bhuvaneswari, R. Dhanasekaran, S.K.R. Chakravarthy, Shirish S. Kale, and Gantayata Gouda Toledo XP 105DR Densitometer according to ASTM D 297. Compression set was carried out as per the ASTM D Method B. Heat resistance was measured as per procedure outlined in ASTM D 454. The brittleness temperature of blends by impact was measured as per ASTM D 746. The blends were tested for swelling behaviour with the variety of aeronautical fuels and oils as per the Table 2. Further, optimized blend ratio seals were evaluated for endurance in the test rig of gas turbine engine starter unit as shown in Figure 1. The seals (4 Nos) were assembled as shown in Figure 2 in the test rig and exposed to aviation turbine fuel (which is also called as Jet A1), for approximately 100 start cycles. The seals were checked for cracks in HAWK made Dynascope Quadra check non contact measuring instrument with make of M/s Vision Engineers, UK. Table 2. Blends tested with fuels and oils Test condition and medium 0 C for 70 h Aviation Turbine Fuel Fuel B (70 Iso Octane:30 Toluene) Lubricating C for 70 h DERD 2487 DERD 2472 MIL PRF (JET Oil II) Hydraulic C for 70 h MIL-H-5606 Figure 1. Starter unit of an aeroengine 210 Progress in Rubber, Plastics and Recycling Technology, Vol. 31, No. 3, 2015

5 Evaluation of Fluorosilicone Silicone Elastomer Blend for Aeronautical Fuel System Figure 2. Assembly of seals Results and Discussion Study of Curing Characteristics In Table 3 the pure FVMQ, RCMA 1 showed MH as 7.7 Nm. The high strength of VMQ, RCMA 6 was evident with the MH of 8.06 Nm. An increase in MH Table 3. Curing characteristics Property RCMA 1 RCMA 2 RCMA 3 RCMA 4 RCMA 5 RCMA 6 ML (Minimum Torque, Nm) MH (Maximum Torque, Nm) (MH-ML), Nm T MIN, min T MAX, min TS 0.5, Induct Time, min TS 25, Scorch Time, min 50% curing, Tc 50, min 90% curing, Tc 90, min ET End Time, min Trend Reversion Reversion Reversion Plateau Reversion Plateau Reversion Time, min Status Pass Pass Pass Pass Pass Pass Progress in Rubber, Plastics and Recycling Technology, Vol. 31, No. 3,

6 C.M. Bhuvaneswari, R. Dhanasekaran, S.K.R. Chakravarthy, Shirish S. Kale, and Gantayata Gouda was observed with RCMA 2 and 3 with 8.46 and 9.16 Nm respectively. The RCMA 4 and 5 were also found to be in the same order of MH as RCMA 3. The higher MH values for blends can be attributed that FVMQ and VMQ were very well crosslinked with uniform curing. Uniform curing of blend could have been achieved with the FVMQ, which has similar -Si O Si- backbone structure as that of VMQ with the only difference in side chains as trifluoropropyl substituent for FVMQ and vinyl substituent for VMQ. The torque difference [10] (MH-ML) shown in the Table 3 is proportional to the extent of crosslinking in the rubber vulcanisate. The increasing trend of (MH-ML) for RCMA 2, 3, 4, 5 attributed the well compatibilisation of blends without phase separation. Induct Time (TS 0.5 ) exemplified as the incubation time before the onset of vulcanization which started above 100ºC for RCMA 1 to 6. This can be attributed that the crosslinking agent, dicumyl peroxide forms its free radicals only above 100ºC [11]. All the samples from RCMA 1 to 6 have more or less same induction time around 0.5 min. Scorch Time (TS 25 ) illustrated the prevulcanisation time or scorch safety time. This period shows the safety of the rubber compound from prevulcanization or scorching during processing. Table 3 shows that RCMA 1 to 6 have enough safety time for processing. Further, scorch time inferred that the blends can be safely moulded with compression or transfer moulding operations with adequate time for flow of rubber in the mould cavity without premature vulcanisation. The trend of scorch time and cure time with the change of blend ratio is shown in Figure 3. On further heating, the torque of RCMA 4 and 6 reached a plateau, which can be attributed to completion of curing and the formation of a stable network at 170ºC. Whereas a chain scission or crosslink breakage became dominant during prolonged heating of RCMA 1, 2, 3 and 5 above 170ºC. On blending FVMQ-VMQ, the rheological Figure 3. Scorch and cure time vs blend ratio 212 Progress in Rubber, Plastics and Recycling Technology, Vol. 31, No. 3, 2015

7 Evaluation of Fluorosilicone Silicone Elastomer Blend for Aeronautical Fuel System properties were found to compete with the rheological properties of the pure FVMQ. This can be attributed that the blends were well compatible without phase separation and can be moulded at 170ºC. Effect of FVMQ/VMQ Blends on Mechanical and Physical Properties Table 4 illustrates the mechanical and physical properties of blends. The increasing trend of tensile strength and elongation for RCMA 1 to 6 can be attributed to the high tensile strength (10.5 MPa) and elongation (438%) of VMQ. Further, it can be accredited that the entire blend range from RCMA 2 to 5, the crosslinked polymer chains were rich in dimethyl siloxy phase which was contributed by both FVMQ and VMQ. As the FVMQ content decreases the domination of dimethyl siloxy phase increases with least trifluoropropyl siloxy phase. Hence, the dimethyl siloxy phase remains continuous with isolated pockets of trifluoropropyl siloxy phase of fluorosilicone. Hence, the mechanical properties indicate co-crosslinking and consequent technological compatibility with the smooth transfer of load in the boundary of the two phases of blends. As the constituent rubbers were selected with the same hardness, there was no drastic change in the hardness range. The decreasing trend of density as shown in Table 4 from RCMA 1 to RCMA 6 can be attributed to the low density of VMQ. As VMQ content increased, the compression set was reduced. The decreasing trend of compression set from 31 to 16% can be attributed to high degree of crosslinking of VMQ and better strength compared to FVMQ. In aeronautical system, low compression set is a desirable property to maintain cushioning of elastomer seals with the good durability and rebound resilience. The higher the concentration of VMQ, the better tensile strength, elongation and low compression set were achieved. Hence, on blending FVMQ/VMQ, there was no adverse effect on mechanical and physical properties. Table 4. Mechanical and physical properties of blend Characteristics RCMA 1 RCMA 2 RCMA 3 RCMA 4 RCMA 5 RCMA 6 Tensile Strength, (MPa) Elongation (%) IRHD Compression Set at C for 70 h Density (g/cm 3 ) Progress in Rubber, Plastics and Recycling Technology, Vol. 31, No. 3,

8 C.M. Bhuvaneswari, R. Dhanasekaran, S.K.R. Chakravarthy, Shirish S. Kale, and Gantayata Gouda Effect of Aviation Turbine Fuel Table 5 shows the effect of ATF and Fuel B on RCMA 1 to 6. The fuel resistance of FVMQ was evident with the 8% volume change of RCMA 1. Fuel B also has the least effect on FVMQ. This can be attributed to the inclusion of the trifluoro propyl side group next to the methyl groups that imparts a polar nature to the silicone molecule [12], which results in a fuel resistant silicone called fluorosilicone rubber. More specifically it is the polar trifluoro group (-CF 3 ) capping the propyl branch (C 3 ) [13]. As the VMQ concentration increases, it ceases the fuel resistant property of FVMQ with the 20 to 100% volume change for RCMA 2 to 4 and complete rupture of RCMA 5 and 6. None of the blend was analogous to FVMQ in fuel resistance. However, certain aeronautical specifications allow up to 30% of volume change in fuel system. Hence, 80:20 FVMQ-VMQ blend with 27% volume change was considered for further study as a seal in aeroengine. The fuel resistance study infers that RCMA 3 to 6 with more than 20% of VMQ in FVMQ is not suitable for aeronautical fuel system. Table 5. Effect of fluids on RCMA 1 to 6 Blends C for 70 h Lubricating C for 70 h ATF Fuel II (70:30) DERD 2487 DERD 2472 MIL PRF (JET oil II) Hydraulic C for 70 h MIL H 5606 (% Volume Variation) RCMA RCMA RCMA RCMA RCMA 5 Complete rupture of RCMA 6 samples Effect of Hydraulic Oil Table 5 shows the effect of mineral base hydraulic oil with RCMA1 to 6. The hydraulic oil MIL H 5606 at 150ºC for 70 h of exposure showed a minimal swelling effect on RCMA 1. This can be attributed to the resistance of polar fluorosilicone rubber (RCMA 1) to the non-polar nature of mineral base hydrocarbon oils. As the concentration of non polar VMQ increased, more swelling effect was found with the blends RCMA 2 to 5. However, there was no drastic change or complete rupture of blends with the hydraulic oil. 214 Progress in Rubber, Plastics and Recycling Technology, Vol. 31, No. 3, 2015

9 Evaluation of Fluorosilicone Silicone Elastomer Blend for Aeronautical Fuel System Effect of Lubricating Oil Table 5 shows the percent volume change of different composition of blends with piston engine lubricating oil such as the non-polar mineral base DERD 2472, polar ester base DERD 2487 and MIL PRF at C for 70 h. A superior resistance to mineral base lubricating oil was found with fluorosilicone due to its polar nature. As the concentration of VMQ increases with RCMA 2 to 6 the polarity decreases and this was apparent with the increased percent volume change. On the contrary, VMQ showed lesser percent volume change with ester base lubricating oil. This can be attributed to the non-polar nature of VMQ. Due to the polarity, FVMQ elastomer showed less resistant to DERD 2487 and MIL PRF compared to VMQ. Results of this study showed that there was no drastic change in the percent volume variation of blends with the lubricating oil. Effect of Low Temperature on Blends FVMQ, VMQ and the blends were exposed to a temperature of -70ºC for 2 h and then tested for impact brittleness. Brittle failure was not found with any of the blend or with the constituent rubbers. This can be attributed to the flexible Si- O- Si backbone [14] of FVMQ and VMQ rubbers, which remains flexible even at low temperature. This study inferred that the blends were safe without brittle failure up to -70ºC. Effect of Heat Ageing on Blends Table 6 shows the results of ageing resistance of blends exposed to 200ºC for 70 h. The percent variation of tensile strength of FVMQ was less compared to the VMQ. This can be attributed to the presence of heat resistant trifluoropropyl groups in the side chain of the FVMQ backbone. This was even evident with the RCMA 2. However, the RCMA 3, 4 and 5 were closer to the VMQ with the increased percent variation of tensile strength. This can be accredited to the increase of dimethyl siloxy phase with less trifluoropropyl side groups, which makes the blends perform as VMQ. This study infers that compared to other blends, 20% concentration of VMQ is better for the good ageing resistance. Table 6. Ageing properties Heat Aging at 200 ºC for 70 h RCMA 1 RCMA 2 RCMA 3 RCMA 4 RCMA 5 RCMA 6 Variation in IRHD Variation in Tensile Strength (%) Variation in Elongation (%) Progress in Rubber, Plastics and Recycling Technology, Vol. 31, No. 3,

10 C.M. Bhuvaneswari, R. Dhanasekaran, S.K.R. Chakravarthy, Shirish S. Kale, and Gantayata Gouda Endurance of Seals The above studies revealed that the tensile strength enhancement of FVMQ was achieved with 20% replacement of VMQ without much compromise in fuel resistant characteristics of FVMQ. Hence, the 80:20 FVMQ-VMQ blend seals were selected for further study with the aeroengine starter unit with the exposure to ATF for 100 start cycles. The starter unit performed well without the leakage of ATF. After testing, the seals were found to be free from swelling and softening. Further, the seals were found to be free from cracks under the magnifier. CONCLUSIONS The experimental investigations carried out suggest that the strength of the Fluorosilicone rubber can be enhanced by blending with high consistent silicone rubber with negligible variation in rheological, mechanical and physical properties. However, FVMQ-VMQ blend with more than 20% concentration of VMQ is not suitable for aeronautical fuel system due to the swelling and rupture. Fluorosilicone rubber shows excellent resistance to mineral base hydraulic and lubricating oil where as silicone shows good resistant to ester base lubricating oil. Nevertheless, up to 40% of VMQ can be added to FVMQ in case of seals exposed to mineral hydraulic or mineral, ester base lubricating oil. The entire range of blend showed excellent resistance to low temperature brittleness up to 70ºC. Ageing study revealed good retention of tensile strength of blend up to 20% of VMQ. Hence, the cost effective 80:20 FVMQ-VMQ blend, with the enhanced strength and wide operating temperature can be used as the seals in aeronautical fuel system in place of inferior low temperature resistant FKM and moderate heat resistant NBR. Acknowledgements We wish to express our sincere gratitude to Dr. K. Tamilmani, Director General (Aero) and Shri P. Jayapal, Sc G, Chief Executive (Airworthiness), CEMILAC, Bangalore for permission and constant encouragement in carrying out this study. We take this opportunity to place on record our sincere thanks to Shri R. Sunder, Chief Executive and staffs, M/s Aerospace Engineers, Salem for the base raw materials, mixing and curing facility to carry out this study. We would like to express our sincere gratitude to Shri Krishna Prasad, MD, M/s Summit Tech (P) Ltd., Bangalore, for extending the ODR test facility. We wish to thank Shri.S.N Mistry, Sc F, Regional Director (Engine) and Dr. A.Sakthivel Sc F, RCMA (Aircraft) and HAL, Engine Division, Bangalore for support in 216 Progress in Rubber, Plastics and Recycling Technology, Vol. 31, No. 3, 2015

11 Evaluation of Fluorosilicone Silicone Elastomer Blend for Aeronautical Fuel System carrying out the endurance test. Special thanks to staff of CMPL, HAL (F&F), Bangalore for carrying out mechanical, physical and fluid resistance properties. References 1. Anil K. Bhowmick, Malcolm M. Hall, Henry A. Benarey, Rubber Products Manufacturing Technology, New York, USA. (1994). 2. L.D. Albin, Rubber Chem. and Technol., Vol 55, (1982), Aerospace Fluorosilicones, Technical Bulletin No. ORD 5745, Parker Hannifin Corporation, KY. 4. Scheirs J., John, Modern Fluoropolymers, Chichester, UK (1997). 5. Jiri George Drobny, Technology of Fluoropolymers, New York, USA (2009). 6. FSRs in extreme applications, Technical brochure, Dow Corning Corporation, USA, (2005). 7. Ronald Romanoswski, Paul T. Irish Charles, W. Olsen Jr. & Lauren M. Tonge, Fluororsilicone Liquid Silicone Rubber, International Silicone Conference, Akron, Ohio,USA, (2003). 8. Michael A. DiPino Advances in Fluorosilicone compounds for turbocharger Hose Liners and other transportation Applications, International Silicone Conference, Dearborn, MI-USA, (2006). 9. Zhang, H and Cloud, H Research Progress in Calenderable Fluorosilicone with Excellent Fuel Resistance, 52 nd International SMAPE Symposium & Exhibition, Maryland, USA (2007). 10. C.M.Blow and C. Hepburn (Eds), Rubber Technology and Manufacture, London, UK (1971). 11. Peter R. Dluzneski, Rubber Chem. Technol., 74(3), 2001, A. Mountney, Silicones in Transportation: Automotive and Aviation, Dow Corning Ltd., Barry, Wales. 13. Anil K. Bhowmick, Howard L. Stephens, Handbook of Elastomers: New developments and technology, New York, USA (1988). 14. Mark S.M. Alger, Polymer Science Dictionary, London, UK (1997). Progress in Rubber, Plastics and Recycling Technology, Vol. 31, No. 3,

12 C.M. Bhuvaneswari, R. Dhanasekaran, S.K.R. Chakravarthy, Shirish S. Kale, and Gantayata Gouda 218 Progress in Rubber, Plastics and Recycling Technology, Vol. 31, No. 3, 2015