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Thermoplastic carbon nanotube composites prevent high voltage "burn-in".

The Boeing Company estimates that, on average, each airplane in service is struck by lightning twice per year. The energy from these strikes must be controlled very carefully in order to avoid system damage. This is especially important in aircraft fuel systems.

When a lightning strike occurs, it is critical that the energy not follow the fuel line path, typically consisting of aluminum tubing. The solution is to insert a section of tube called a lightning strike isolator. These isolators are currently made from a composite of carbon fiber filament wound epoxy and conductive carbon black.

The design requirements are such that in standard operation, the isolator must act to dissipate the static charges generated by the flowing fuel inside it by maintaining a volume resistivity in the range of 1E6 to 1E8 [[ohm]-cm. However, when the aircraft is struck by lightning, the isolator must still maintain this resistivity so that, relative to the electrical resistivity of the aluminum grounding straps (2.8E-6 [ohm]-cm), the isolator appears, in terms of fast voltage transients, to be an insulator.

After the energy is dissipated through the electrically conductive engineered grounding paths, the isolator must then return to its duty of dissipating static charge without suffering any "burn-in" characteristics. Burn-in is a frequently observed phenomenon in ESD materials where a static dissipative material is exposed to high voltage and a conductive path is created within the part. After several exposures, the part is no longer static dissipative, but on the contrary, rather conductive.

This resistance to burn-in is a critical element that cannot be compromised if safety of the aircraft is to be maintained. It is also important to note that these isolators are deep in the structure of the aircraft wing and are therefore non-serviceable. Should they fail, there is no cost-effective maintenance procedure for isolator replacement.

Due to the labor intensive and time consuming process currently required to produce the thermoset isolators, there was an interest in developing an injection moldable thermoplastic version that could be manufactured at a lower cost.

Background

The steps involved in raw material component selection, processing, initial testing and final component testing are presented.

Raw material selection

The base polymer selected for this development was polyetheretherketone (PEEK). This material was selected because of its good physical properties, wide operating temperature range and excellent fuel resistance. Added to the PEEK was 25% by weight glass fiber. The glass fiber was added to increase the rigidity of the PEEK without compromising the electrical resistivity of the PEEK.

This formulation of 75% PEEK polymer and 25% glass fiber was established as the mechanical baseline for electrical property modification. Initial studies focused on the addition of carbon fiber to the composite. However, it quickly became clear that there was no acceptable level of carbon fiber that would produce an ESD material capable of surviving lightning strike testing without suffering from burn-in.

A number of other additives also showed little promise of success, until a blend of milled carbon 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.

The compounded but still molten material is then forced through a 3 mm diameter hole to form a strand of plastic that is subsequently cooled with water and solidified prior to being cut into short (3 mm long) pellets by a pelletizer.

These pellets were then processed, via injection molding, to produce test specimens and isolator tubes for electrical property test measurements.

Test equipment: Molded test specimens

A Phenix Technologies Model 440-20, 0 to 40 kV DC dielectric test set, pictured in figure 2, high voltage tester was used to evaluate the burn-in characteristics of the molded specimens. The test equipment was capable of measuring the resistance of a part at a maximum test voltage of 40 kV DC.

The molded samples were 13 x 50 x 3 mm in size. Preparation consisted of sanding all of the surfaces at both ends of the bar for length of 13 mm, and then painting the exposed sanded faces with highly filled silver conductive paint. A final test specimen, in the testing fixtures, is presented in figure 3.

Once the paint had dried, the parts were connected to the dielectric test set, and voltage was applied. The typical early result was immediate burn-in above 1,000 V DC, the loss of all static dissipative characteristics and the creation of a test specimen with a volume resistivity of 1E2 [ohm]-cm or less.

Once the blend of milled carbon fiber and carbon nanotubes was evaluated, the more typical result was the ability to hold insulation characteristics at 6 kV DC for an unlimited number of test cycles, while maintaining the ESD characteristics of 1E6 -1E8 [ohm]- cm when tested at more conventional 10 V DC voltages.

Test equipment: Tube level testing

While the final application will require tubes with diameters ranging from 12.5 mm to 100 mm, the 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 laboratory conducted simulated lightning strike testing on sample tubes.

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 volume resistivity of the tube. When a tube with a failing composition is exposed to test voltage, the results are immediate and obvious, as illustrated in figure 6. In this figure, the tube is not only failing to maintain its resistivity, it is also starting to suffer from carbon arc tracking, a phenomenon where a conductive path is burned along the surface of the part.

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 four to six decades of resistance. This loss of ESD properties, illustrated in figure 7, was clearly unsuitable for the application.

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

Initial concerns centered on possible poor mixing of the materials. However, subsequent SEM analysis (figures 8 and 9) revealed that the materials were indeed well mixed.

The addition of carbon nanotubes was observed to be successful in stabilizing the post-strike volume resistivity of the molded bars and the tubes.

Figure 10 notes the resistivity of molded bar test specimens after being subjected to 10 exposures of 6 kV DC. The test time of each exposure was five seconds. While the parts do experience a decrease in resistance of approximately two decades, similar to a break-in period, the final result is a bar that is stable, and static dissipative, after any number of exposures to high voltage.

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 poststrike measurements are made using a standard electrician's Ohmmeter, with a test voltage of 10 V DC.

Further optimizations of the carbon nanotube formulations proved that it was not only possible to mold a tube that did not experience burn-in, but that it was also possible to control the resistivity of a tube directly through the careful addition of different weight percent loading of carbon nanotubes into the compounds used.

Figure 12 shows how decreasing the level of carbon nanotubes, from Mix A to Mix F, with Mix A being the richest in CNTs and Mix F having 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.

Conclusions

While the exact mechanism for how the addition of CNTs to the compound operates to reduce the burn-in effect is not clearly understood, by knowing the average diameter of the CNTs being used and the weight percentages in the formulas, it is easy to calculate that a cubic centimeter of our material will typically contain -500 to 600 m carbon nanotubes. This great length of carbon nanotubes could be acting as a network of fine electrical bridges that dissipate the surge from one milled carbon fiber to the next, thereby reducing the potential between individual milled carbon fibers to a point below which burn-in of the PEEK composite will not occur. How to test this hypothesis is beyond the scope of our current work.

What is clear is that this represents a potential new avenue for further development of compounds, not only for the aerospace industry, but for any application where burn-in is a significant issue.

References

(1.) FAA Advisory Circular Number 20-53B, "Protection of aircraft fuel systems against fuel vapor ignition caused by lightning," June 5, 2006.

(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.

by Ned Bryant, RTP Company
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Author:Bryant, Ned
Publication:Rubber World
Date:Aug 1, 2013
Words:1809
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