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Testing E-Textiles

By Diana A Wyman

AATCC research committee RA111, Electronically-Integrated Textiles, recently approved its first evaluation procedure! AATCC EP13, Evaluation Procedure for Electrical Resistance of Electronically-Integrated Textiles,  is currently available for purchase from the AATCC website as a downloadable PDF. It will also be included in the 2019 AATCC Technical Manual Mid-Year Supplement to be published this summer.

We thought it would be easy.

There was a need, willing volunteers, and methods to use as templates.

Manufacturers approached AATCC about developing standardized test methods to allow everyone—large and small companies alike—to objectively evaluate and sell e-textile products. Not surprisingly, the top priority was a laundering test. The world needs a reliable way to determine whether e-textiles can withstand repeated home laundering.

More than 100 people expressed interest in forming a committee to develop e-textile test methods. Many attended the first meeting and RA111 officially became an AATCC research committee in March 2016.

By then, a task group of volunteers was already drafting the first standard. They determined that a test method was needed to measure change in electrical resistance after home laundering. Existing AATCC documents were referenced for the laundering procedure, so the first step was to develop an evaluation procedure for electrical resistance of e-textiles. The group agreed that the first procedure should be as simple and accessible as possible—no expensive or complicated equipment; just a basic multimeter for resistance measurements.

Three test materials were selected for the first interlaboratory study of the method.

– Stainless steel thread stitched onto cotton twill fabric
– Ripstop fabric woven with metallized nylon yarns
– Silver-based ink printed on thermoplastic polyurethane (TPU) film, fused to nylon/spandex knitted fabric

We soon realized it wasn’t so easy to develop one repeatable test method to evaluate all types of e-textiles.

Units

The test method calls for reporting resistance of e-textiles in ohms (Ω). The proper measurement unit for a two-dimensional system, or “sheet resistance,” is Ω/square. Three-dimensional materials are measured for “resistivity” (Ω•m). A wire can also be evaluated as a one-dimensional system, with resistance reported in Ω/m.

As a combination of electronics and textiles, e-textiles are not simple one-, two-, or three-dimensional conductive systems.

Conductive ink does not form a solid film; there are numerous small voids in the surface. The thickness of the printed layer cannot easily be measured.

Conductive yarns are made up of multiple staple or filament fibers. The total cross-section of conductive elements is difficult to determine in this configuration.

To make things more complicated, the conductive pathways, or traces, of e-textiles may take many shapes. It is not always possible to cut a specimen with a straight trace of a particular length and width. Traces may be curved for aesthetic or functional reasons. They may cross, overlap, or diverge, as in the woven fabric with conductive yarns running in both warp and filling directions. There are multiple paths between Point A and Point B!

A similar procedure published by the European Committee for Standardization (CEN) is applicable only to those materials with individual “textile-based electrically conductive tracks” having a length to width ratio of at least 10 to 1. Measurements for this standard are reported as linear resistance in Ω/m.

Because the AATCC procedure is intended to be applicable to a wide variety of e-textiles, a simple resistance measurement in ohms is the only way to generate a value for all of them. The report section of the procedure calls for a description of each test specimen, including the distance between contact points. It may be possible to normalize some measurements along different distances for comparison, but all three dimensions must be considered.

Apparatus

There was some discussion about the required resolution and accuracy of the multimeter. Since several member companies used the same model for internal testing, this formed the basis of the first draft.

Things went smoothly with the conductive thread. Five labs reported a total of 27 original measurements. The average resistance along a 100-mm stitching line was 5.9 Ω, with a standard deviation of 0.6 Ω.

Data for the conductive fabric was less encouraging. Although a few labs reported resistance values, they were no different than those for the lead resistance in the system. A more sensitive instrument is required for materials with very low resistance (< 1 Ω).

The conductive ink is even more complicated. The multimeter reading continually drifts down. A note was added to the draft method to indicate that a reading should be recorded when it was stable for three seconds. Unfortunately, it can take several minutes for a reading to settle for three seconds. Because the reading continues to drift down, looking away can mean missing the first value that lasts three seconds, and reporting a lower value. Technicians must pay close attention to ensure accurate and repeatable resistance results.

Contact

The “three-second rule” is not an ideal solution for an unstable reading. Research showed the resistance of the ink probably does not actually decline over time. A more likely explanation is increasing contact between the conductive ink and the multimeter leads.

Several members of the task group knew from experience that surface probes produced inconsistent results if not held at exactly the same angle and pressure each time. To avoid human variability, the task group tried alligator clips in place of surface probes.

This was also a far-from-perfect solution. Alligator clips create an inconsistent connection because of their jagged design.  Smaller teeth and toothless clips improve the contact quality. Exact contact area—and the distance between contact points—is still difficult to calculate due to the nature of both the clips and the e-textiles.

Another challenge to using clips is the need to fold and pinch the e-textile material being measured. In the two-point multimeter system, only one side of each clip must contact the conductive trace. Unless the measurement points are at the very edge of the specimen, the clips will have to pinch the material from the face or back. This makes it more complicated to measure the distance between contact points, and may even damage the conductive elements.

A flat probe is the best option for most e-textile materials. Size and weight must be appropriate for the specimen.

Pressure

As observed with the surface probes, pressure—as well as contact area—is an important variable. None of the clips described above produce a consistent pressure. This is especially true when used to measure easily-deformed materials, such as a textile.  A recent study found conductance along printed traces is improved (resistance reduced) by the addition of a compressive force.

Concentric circle probes are a common means of measuring surface resistance with a known contact area and known pressure. They are less useful for e-textiles.  While one or more conductive traces may pass under the rings, entire surface area under the probe is usually not continuously conductive.

Ideal probes provide full, consistent contact with the conductive path over a known surface area, without crossing other conductive paths. Experimental probes were created using several materials. Weights were applied to foil tape contacts. Metal contacts were also embedded in weighed plastic probes. More work is needed to develop an ideal probe for use on most e-textile materials. AATCC RA111 is working with Nautilus Defense to complete some of this work.

Insulation

The three samples used for the initial interlaboratory study all had accessible conductive elements. In most consumer products, this is not the case. Conductive traces are insulated or encapsulated to protect materials from the environment and from each other (short circuit). Insulation also reduces the risk to users from burn, radiation, and other hazards of direct contact with electronic components.

Insulation is important for the function of e-textile products, but it complicates testing. Insulation must be removed, or connectors added, to provide contact points for measuring resistance. This, in turn, makes the sample less able to withstand repeated laundering.

The current evaluation procedure indicates that “Each conductive path to be measured must be accessible, at least at the two measurement points. Connectors may be used to provide accessibility.” Operators should be careful not to damage the sample if the path is not readily accessible. It may also be necessary to reseal the exposed areas for laundering. Each sample may require a slightly different approach, but operators should always consider the end use of the e-textile, if known.

Next Steps

The first AATCC evaluation procedure for resistance is approved and the full laundering procedure for e-textiles is ready for ballot. AATCC RA111 has also developed a standard for exposing e-textiles to a variety of conditions. The standard is based on existing test methods for perspiration, UV radiation, etc., with specific instructions for preparing e-textile specimens to test the change in electrical resistance.

And the committee’s work is far from done! A task group is already working on a stretch test for e-textiles. Other projects include ongoing revision of the existing standards to reflect new knowledge and new technology. Future versions will probably include more detailed descriptions of appropriate probes and use of a four-point measurement system. As the foreword to AATCC EP13 says, “At the time this evaluation procedure was developed, electronically-integrated textiles (e-textiles) were still a nascent product category…As e-textile technology evolves, more specialized methods, or options within this method, may be developed.”

Purchase AATCC EP13, Evaluation Procedure for Electrical Resistance of
Electronically-Integrated Textiles
https://members.aatcc.org/store/ep13/2515

 Attend RA111 meetings (in person or remotely)
www.aatcc.org/evnt/meetings

 RA111 updates
www.aatcc.org/test/etextiles

References

AATCC EP13 Evaluation Procedure for Electrical Resistance of Electronically-Integrated Textiles.

EN 16812:2016 Textiles and textile products. Electrically conductive textiles. Determination of the linear electrical resistance of conductive tracks.

Jur, Jesse. S., Sweet, W. J., Oldham, C. J. and Parsons, G. N. (2011), Atomic Layer Deposition of Conductive Coatings on Cotton, Paper, and Synthetic Fibers: Conductivity Analysis and Functional Chemical Sensing Using “All-Fiber” Capacitors. Adv. Funct. Mater., 21: 1993–2002. doi:10.1002/adfm.201001756