Kapton Wire: If it’s Safe to Use

The use of Kapton as electrical wire insulation dates back to 1970. Since then, DuPont has continued to supply Kapton wire to Airplane manufacturers globally. However, an event almost led to the collapse of this partnership.

On September 2, 1998, an MD-11, Swissair Flight 111, crashed into the Atlantic Ocean, killing all 229 on board.

Investigations into the matter attribute the crash to loss of control due to an in-flight fire.

Additionally, the cause of the fire stems from the faulty wiring in the plane’s in-flight entertainment system.

Furthermore, the evidence shows the wire insulation contributed to the fire that led to the catastrophic crash.

Thus, the Transport Safety Board of Canada and the US Federal Aviation Administration made nine safety changes.

This changed the testing, certification, inspection, and maintenance of plant materials.

Most notably is the impact of these changes on Kapton insulated wire for the electrical works in both civilian and military aircraft. So, is Kapton wire safe?

What are Kapton Wires?

Swissair plane in flight

Firstly, electrical connections can feature either aluminum or copper wires. Furthermore, the type of insulation will depend on their operating environment.

As such, you will find industry insiders referring to wire types by the type of insulation they employ.

Kapton wires are copper wires that feature a polyimide film that allows the conductor to operate across a vast temperature range.

Additionally, the excellent dielectric properties, incredible malleability, and low outgassing rate see Kapton wires in use across many places.

For instance, in airplanes, space crafts, and 3D printers.

In the late 1960s, American chemical company DuPont released Kapton as one of its revolutionary polymer products.

For instance, they released nylon, Teflon, and Neoprene. Also, the polymer results from the condensation of 4,4-oxydiphenylene and pyromellitic dianhydride. This happens during a process called polymerization.

The final product is an aromatic lightweight insulation material. The material can accommodate a temperature range from zero to four hundred degrees Celsius.

So, every electrician prefers Kapton wires for installations. This is especially where resistance to temperature, weight, and flexibility are mandatory.

Use of Kapton wires in several vacuum environments:

Kapton is mainly a polyimide film, which has been widely used in flexible electronics and space applications.

Invented in 1960 by the DuPont Corporation, this film remains stable in a wide range of temperatures ranging from 4-673K.

Due to its thermal properties and low-outgassing threshold, its use in high vacuum environments is quite common.

Kapton wires are very effective in a demanding temperature range and environments without humidity and restricted chafing.

Earlier, they were mainly used in aircraft but the above features make it preferable for vacuum environments for space engineering.

There have been several examples where Kapton foil has served a significant role in the successful ascent and descent of different spacecraft. 

Here are some examples.

In the descent stage and the lower part of the ascent stage of the Apollo Lunar module, aluminized Kapton film was used as a blanket for thermal insulation.

Also, in Apollo 11, Kapton wires were used in the ascent stage of the Lunar Module Eagle.

Its ability to withstand tough space environments has made it used as a plastic in solar sails by NASA’s Jet Propulsion Lab.

NASA’s other spacecraft used Kapton wire in an innovative style in its New Horizons spacecraft.

The use of Kapton in its “Thermos Bottle” design helped it retain heat and work at room temperature without using large heaters.

The main outer covering comprises multiple layers of lightweight thermal insulation which retains the what of the operating electronics.

Inside is a thermal blanket comprising 18 layers of mesh Dacron cloth. These layers are sandwiched between Kapton film and Aluminum Mylar tape.

This blanket also helped the spacecraft protect itself from tiny meteorites.

The sun shield of the James Webb Space Telescope also has 5 Kapton E-sheets. These sheets were covered with aluminum and silicon (doped) so that they could reflect heat away from the body of the spacecraft.

In the International Space Station (ISS), the crew used Kapton-made tape to repair leakage in the Soyuz spacecraft in 2018.

Again, in 2020, the Zvezda Service Module of ISS reported a leak in its transfer chamber, which was repaired by the crew using Kapton film.

Common Concerns About Kapton Wires?

Concerns about Kapton wires in aircraft electronic systems began long before the Swissair Flight 111 incident.

For example, the high frequency of in-flight fires and military aircraft crashes due to malfunctioning electrical wires in the 1980s led the US military to conduct studies on the use of Kapton insulated wire.

Subsequently, the studies reveal the following disadvantages of using Kapton insulated wire.

Charring

Charred electrical wires.

Charring refers to a situation where the wire insulation burns following an electric short.

As such, it changes to form a carbon residue that compromises the wire’s integrity. Kapton wires feature a combination of copper wire strands.

They also have an aromatic polyimide acting as the insulation material. In addition, the thickness of the Kapton insulation varies depending on the type of airplane electrical equipment.

Results from the studies reveal that a Kapton insulated wire tends to be more susceptible to charging than other wire types.

Furthermore, the resultant carbon residue is both conductive and flammable.

Therefore, any surge in the electric current can cause the entire system to ignite, as was the case in Swissair Flight 111.

Arc tracking

Man holding arcing electrical wires.

Arcing involves a situation where the current jumps between connections rather than flowing through the conductor.

Additionally, it occurs where there is damage to the wire insulation through charring or mechanical wear.

As a result, an electric charge from the exposed conductor can jump from one conducting surface to another.

Furthermore, the flashes of electricity can reach temperatures as high as 19,500 degrees Celsius, enough to cause a fire.

Keeping this in mind, Kapton insulated electrical wiring experiences a high degree of abrasion within cable harnesses.

This is a result of aircraft movement. As such, the polyamide polymer insulation breaks, exposing the copper wire within. As a result, it increases the possibility of arcing occurring.

Investigations into airplane accidents from 1980 to 1999 indicate that charring and arcing of the Kapton insulated electrical wiring led to a fire destroying the planes’ entire wire bundle.

Consequently, there was a significant loss of control, allowing the crashes to happen. 

So, What are the Best Solutions to Kapton Wire Failures?

Complete rewiring overhaul program

Rewiring of an aircraft’s control panel

The best way to mitigate the impending danger from Kapton insulated wiring is to replace the existing wiring.

Aviation authorities globally mandate both airplane manufacturers and operators to replace their existing Kapton wiring with safer alternatives.

For example, the US military now uses the TKT insulated wire in their AH1-Cobras and F-15C/D aircraft.

Similarly, all Boeing planes from 1992 also use the new TKT polyimide polymer wrapping for all their airplane wiring.

A separate emergency electrical system

MD-11 airplane on the taxiway

Alternatively, you can also install a separate emergency electrical system in your aircraft to provide safe emergency power when there is an electrical fire.

The consensus among aviation authorities is the Virgin Electrical Bus, which utilizes a separate battery and air-driven generator to power the vital controls during electric fire emergencies. 

How Do You Determine the Health of a Kapton Wire?

Like with all lab-produced products, polyimide materials degrade over time. In addition, the degradation can lead to drastic consequences, including the destruction of equipment or death, as in the case of Kapton insulated electrical wiring in planes.

Scientists checking the viscosity of a polymer

DuPont recommends the Inherent Viscosity Test as the best way to determine the health of a Kapton wire.

It involves measuring the molecular weight of different layers of polyimide. Then, comparing the results to DuPont’s minimum molecular weight measure.

Accordingly, you can schedule your monitoring, maintenance, and rewiring programs, to ensure the proper functioning of your electrical installations.

For example, 2016 saw the US military conduct IVT testing on 300 aircraft as part of their Electrical Wire Interconnection System Service Life Extension Program. 

FAQ:

Why is Kapton referred to as “POLYIMIDE” in most articles associated with electronics?

Kapton is a trademark invention of DuPont Corporation. The POLYIMIDE is a generic name that includes all producers of the material used in the wires and cables.

Should I replace the entire Kapton wires on my aircraft?

We at Cloom regularly perform wire degradation analysis on different fleets, and our analysis says it is never important to replace the entire wiring system in an aircraft.

There can be some areas in the aircraft with wire degradation, and you must replace wires on those parts only.

Should I replace the wire when the insulation starts flaking off?

No, it’s not required. First, you must understand the structure of a polyimide wire. All polyimide wires designed in MIL-DIT-81381 style have similar construction.

These wires have two polyimide wrap layers with an adhesive in between that acts as a seal. These two polyimide layers collectively form four insulation layers.

Out of these layers, the outer one is the lacquer layer, which acts as a surface printing wire identification.

The layer does not have any mechanical or electrical value. Thus, even if it flakes off, you need not go for a replacement of wire.

According to Wikipedia, an FAA-funded study reveals that kapton wire starts showing degradation in 100 hours when kept in a hot and humid environment. Is it true?

This Wikipedia content refers to the FAA-funded study conducted in 2000. The study used different methods to identify the stress factors and the intensity of degradation in the insulation of different aircraft wires.

Kapton insulated wires were one of the wires in this investigation. You can go through the report of this study on the FAA’s website.

Numerous test conditions were observed for different stresses. These test conditions changed to accelerate the aging of the sample wires.

In accelerated aging experiments, the stress factors go beyond the normal range. These limits are next to impossible in normal aircraft operations.

Thus, it is unlikely that the results of these aging experiments will apply to normal service conditions.

The conditions that showed the Kapton wire degradation in 100 hours in a hot/humid environment are:

• 95-degree Celsius temperature
• 100% relative humidity
• Wire wrapped around a mandrel of 10 times diameter

It is agreeable to have such a high temperature and physical setup, but it is never possible to have 100% relative humidity.

It is only possible when the aircraft is fully submerged in water, which is on the verge of boiling.

The study data reveals rapid degradation under such conditions but such conditions are unlikely in an aircraft’s service life.

How can I determine the health of a Kapton wire?

Northern Virginia-based engineering firm Lectromec has performed extensive research on the Kapton wire degradation process.

Under its research, the firm uses an inherent viscosity method for degradation analysis.

This is a chemical process that predicts the internal integrity of the polymer and quantifies the degradation of the material.

Aircraft’s use information, along with the result data of this study, helps to predict the service life of the aircraft.

The same technique has been used by different militaries and governments for their electrical wiring interconnect systems or EWIS testing and risk assessments.

Conclusion

So, is Kapton wire safe? Despite a shaky debut, Kapton wires continue to feature in many electrical works. For example, the KK-J-24s-SLE-100 wire features a Kapton Tape Wrap as insulation. Electricians use it as medical wire in many laboratory electric works. Finally, you can learn more about Kapton wires and IVT services by contacting Cloom’s representatives.