Thermoplastic Carbon Nanotube Composites Prevent High Voltage “Burn In”

RTP Company

RTP Company Corporate Headquarters • 580 East Front Street • Winona, Minnesota 55987 USA website: www.rtpcompany.com • email: [email protected] • Wiman Corporation • +1 320-259-2554 TELEPHONE: U.S.A. SOUTH AMERICA MEXICO EUROPE SINGAPORE CHINA +1 507-454-6900 +55 11 4193-8772 +52 81 8134-0403 +33 380-253-000 +65 6863-6580 +86 512-6283-8383 Thermoplastic Carbon Nanotube Composites Prevent High Voltage “Burn In”

Ned Bryant, Sr. Product Development Engineer RTP Company, 580 E. Front St., Winona, Minnesota, USA Telephone: +1 (507) 454-6900, Internet: www.rtpcompany.com

Abstract – During a lightning strike event, lightning strike isolators are critical parts of aircraft fuel- line safety. Current technology is based on carbon fiber and carbon black filled epoxies. Recent development efforts have achieved burn-in resistant, injection moldable thermoplastics using carbon nanotube additives, capable of maintaining ESD characteristics after multiple ~10 kV DC strikes.

I.) INTRODUCTION several exposures the part is no longer static dissipative, but on the contrary, rather conductive. The Boeing Company estimates that on average, This resistance to burn-in is a critical element that each airplane in service is struck by lightning twice cannot be compromised if safety of the aircraft is to per year. The energy from these strikes must be be maintained. It is also important to note that these controlled very carefully in order to avoid system isolators are deep in the structure of the aircraft wing damage. This is especially important in aircraft fuel and are therefore non-serviceable. Should they fail systems. there is no cost effective maintenance procedure for isolator replacement. When a lightning strike occurs, it is critical that the energy not follow the fuel line path, typically Due to the labor intensive and time consuming consisting of aluminum tubing. The solution is to process currently required to produce the thermoset insert a section of tube called a lightning strike isolators, there was an interest in developing an isolator. These isolators are currently made from a injection moldable thermoplastic version that could composite of carbon fiber filament wound epoxy and be manufactured at a lower cost. conductive carbon black. II.) BACKGROUND The design requirements are such, that in standard operation, the isolator must act to dissipate the static The steps involved in raw material component charges generated by the flowing fuel inside it, by selection, processing, initial testing, and final maintaining a volume resistivity in the range of 1E6 component testing. to 1E8 Ω-cm. However, when the aircraft is struck by lightning, the isolator must still maintain this Raw Material Selection resistivity so that, relative to the electrical resistivity of the aluminum grounding straps (2.8E-6 Ω-cm), The base polymer selected for this development was the isolator appears, in terms of fast voltage polyetheretherketone (PEEK). This material was transients, to be an insulator. selected because of its good physical properties, wide operating temperature range and excellent fuel After the energy is dissipated through the electrically resistance. Added to the PEEK was 25% by weight conductive engineered grounding paths, the isolator glass fiber. The glass fiber was added to increase the must then return to its duty of dissipating static rigidity of the PEEK without compromising the charge, without suffering any “burn-in” electrical resistivity of the PEEK. characteristics. Burn-in is a frequently observed This formulation of 75% PEEK polymer and 25% phenomenon in ESD materials, where a static glass fiber was established as the mechanical dissipative material is exposed to high voltage and a baseline for electrical property modification. Initial conductive path is created within the part. After studies focused on the addition of carbon fiber to the

© Copyright, RTP Company Page 1 composite. However it quickly became clear that Test Equipment: Molded Test Specimens there was no acceptable level of carbon fiber that would produce an ESD material capable of surviving A Phenix Technologies, Model 440-20, 0 to 40 kV lightning strike testing without suffering from burn DC Dielectric Test Set, pictured in Figure 2, high in. voltage tester was used to evaluate the burn in characteristics of the molded specimens. The test A number of other additives also showed little equipment was capable of measuring the resistance promise of success, until a blend of milled carbon of a part at a maximum test voltage of 40 kV DC. fiber and carbon nanotubes was evaluated.

Compounding Equipment

The plastic materials used in this study were compounded on a 40 mm twin-screw extruder. This type of extruder is standard equipment in the plastics compounding industry where it is routinely used to melt and blend thermoplastic resins with a wide variety of additives. The extruder melts and mixes the individual components of the compound into a single homogenous material. Figure 1 is a general diagram of the plastic compounding process. Figure 2: Portable Hi-Pot Tester.

Figure 3: Typical Test Specimen.

1 Polymer pellets 7 Gravimetric feeder The molded samples were 13x50x3 mm in size. 2 Pellet mill 8 Twin-screw extruder Preparation consisted of sanding all of the surfaces at 3 GF additive 9 Vacuum pump both ends of the bar for length of 13 mm, and then 4 CNT additive 10 Water bath painting the exposed sanded faces with highly filled 5 Polymer powder 11 To pelletizer silver conductive paint. A final test specimen, in the 6 Premix blender testing fixtures, is presented in Figure 3. Figure 1: General diagram of the compounding process. Courtesy of Coperion GmbH. Once the paint had dried the parts were connected to

the Dielectric Test Set, and voltage was applied. The compounded but still molten material is then The typical early result was immediate burn-in forced through a 3 mm diameter hole to form a above 1000 V DC, the loss of all static dissipative strand of plastic that is subsequently cooled with characteristics, and the creation of a test specimen water and solidified prior to being cut into short (3 with a volume resistivity of 1E2 Ω-cm or less. mm long) pellets by a pelletizer.

These pellets were then processed, via injection Once the blend of milled carbon fiber and carbon molding, to produce test specimens and isolator nanotubes was evaluated the more typical result was tubes for electrical property test measurements. the ability to hold insulation characteristics at 6 kV

DC for an unlimited number of test cycles, while

© Copyright, RTP Company Page 2 maintaining the ESD characteristics of 1E6 -1E8 Ω- volume resistivity of the tube. When a tube with a cm when tested at more conventional 10 V DC failing composition is exposed to test voltage, the voltages. results are immediate and obvious, as illustrated in Figure 6. In this figure the tube is not only failing to Test Equipment: Tube Level Testing maintain its resistivity, it is also starting to suffer from carbon arc tracking, a phenomenon where a While the final application will require tubes with conductive path is burned along the surface of the diameters ranging from 12.5 mm to 100 mm, the part. focus of this work was on the 50 mm diameter tube. After molding, the tubes were prepared for testing by building the actual isolator to be commercialized. This involved turning the tubes in a lathe, in order to create the exact inside and outside diameters required for both the inside and outside faces, and then machining threads into both ends. A highly conductive silver filled epoxy adhesive was then applied to the threads and metal threaded ferrules were screwed on to each end. Once the epoxy had completely cured, the parts were ready for electrical testing. Completed isolators were then tested by both the Dielectric Test Set, with a test voltage of ~10 kV DC, and by simulated lightning strike methods, in order to evaluate the burn in characteristics of the molded tubes. The electrical test waveform is depicted in Figure 4. An outside Figure 5: Example of a successful tube level test. lab conducted simulated lightning strike testing on sample tubes.

Figure 6: Example of a failing tube level test.

Figure 4: Waveform B from SAE ARP5412 Rev. A.

During a successful test there is very little obvious action. The tube is placed in the test fixture and then subjected to a series of identical voltage pulses as shown in Figure 5. Subsequent low voltage resistivity testing will result in no change in the

III. RESULTS

Because of the two level nature of the testing involved, the results are best summarized in two parts.

Results of Molded Test Specimens

While initial trials of PEEK with glass fiber and carbon fiber composites were successful in producing static dissipative molded test specimens, increasing the test voltage resulted in the historical burn-in phenomenon. The burn-in result meant that the test specimens no longer possessed volume resistivities in the electrically dissipative range, but were in fact conductive, having lost 4 to 6 decades of resistance. This loss of ESD properties, illustrated in Figure 8: SEM showing the well-blended nature of the carbon Figure 7, was clearly unsuitable for the application. and glass fibers in the PEEK matrix.

Figure 7: Typical high voltage molded bar test specimen burn-in failure.

Retesting of the above specimen will immediately result in current flow at less than 100 V DC.

Figure 9: SEM showing the glass fibers (light gray) and the Initial concerns centered on possible poor mixing of carbon fibers (dark gray) in the PEEK matrix. the materials. However, subsequent SEM analysis (Figures 8 & 9) revealed that the materials were The addition of carbon nanotubes was observed to be indeed well mixed. successful in stabilizing the post-strike volume resistivity of the molded bars and the tubes.

Figure 10: Resistivity of Tested Molded Bars (4 identical bars numbered 5561 to 5564).

Figure 10 notes the resistivity of molded bar test Figure 11: Pre & Post Strike Tubes. specimens after being subjected to 10 exposures of 6 kV DC. The test time of each exposure was 5 Further optimizations of the carbon nanotube seconds. While the parts do experience a decrease in formulations proved that it was not only possible to resistance of approximately 2 decades, similar to a mold a tube that did not experience burn-in, but that break in period, the final result is a bar that is stable, it was also possible to control the resistivity of a tube and static dissipative, after any number of exposures directly through the careful addition of different to high voltage. weight percent loading of carbon nanotubes into the compounds used. Results of Tube Level Testing

While the above testing has focused on small molded test specimens, some of the failing compounds were also molded into tubes to better understand the magnitude of the problem.

Figure 11 illustrates the dramatic changes in resistance after an ~10 kV DC lightning strike test for a failing test formula. The pre-strike and post- strike measurements are made using a standard electrician’s Ohmmeter, with a test voltage of 10 V DC. Figure 12: Resistivity Control.

Figure 12 shows how decreasing the level of carbon nanotubes, from Mix A to Mix F, with Mix A having the richest in CNTs and Mix F being the poorest carbon nanotube loadings, affects the volume resistivity of the molded isolators. Demonstrating the ability to control the final resistivity of the isolator tubes after test exposure at ~10 kV DC.

IV. CONCLUSIONS VII.) ABOUT THE AUTHOR

While the exact mechanism for how the addition of Ned Bryant graduated from the Georgia Institute of CNTs to the compound operates to reduce the burn- Technology, Atlanta, GA with a Bachelors of Textile in affect in not clearly understood. Knowing the Engineering. He has experience working in spinning average diameter of the CNTs being used and the high performance fibers, manufacturing magnet wire weight percentages in the formulas, it is easy to for oil extraction pumping systems, product calculate that a cubic centimeter of our material will development of plastics based on carbon nanotubes typically contain ~500 to 600 m carbon nanotubes and electronic device materials selection and This great length of carbon nanotubes could be manufacturing in Asia. He holds four patents and is acting as a network of fine electrical bridges that currently a Senior Product Development Engineer dissipate the surge from one milled carbon fiber to with custom compounder RTP Company in Winona, the next, thereby reducing the potential between MN. individual milled carbon fibers to a point below which burn-in of the PEEK composite will not VIII.) ABOUT RTP COMPANY occur. How to test this hypothesis is beyond the scope of our current work. RTP Company is a global compounder of custom engineered thermoplastics. Private ownership allows What is clear is that this represents a potential new the company to be independent and unbiased in its avenue for further development of compounds, not selection of resins and additives while its materials only for the aerospace industry, but for any experts formulate thousands of compounds each year application where burn-in is a significant issue. that meet specific end-use application requirements.

V.) ACKNOWLEDGEMENTS Thermoplastic compounds can be tailored to provide multiple property enhancements in a single material. RTP Company would like to acknowledge the help RTP Company’s customization capabilities include and support of Eaton Aerospace in conducting these color, conductivity, elastomeric, flame retardant, experiments. high temperature, structural, and wear resistant performance. VI.) REFERENCES Headquartered in Winona, Minnesota, RTP 1. FAA Advisory Circular Number 20-53B, Company operates 12 full-service manufacturing “Protection of Aircraft Fuel Systems against facilities located throughout North America, Europe, Fuel Vapor Ignition Caused by Lightning”, June and Asia. Each facility has complete make-to-order 5, 2006. manufacturing, color lab, product development, and technical service on-site. 2. U.S. Patent 8,003,014 “Dielectric Isolators”, August 23, 2011.

3. The ESD Association, “ESD Phenomena and the Reliability for Microelectronics”, October, 2002.

4. DOT/FAA/CT-94/74 – “Aircraft Fuel System Lightning Protection Design and Qualification Test Procedures Development”, September 1994.

5. SAE ARP5412 Rev. A – “Aircraft Lightning Environment and Related Test Waveforms”, February, 2005.