Research

INNO-KOM 49MF200090

Abstract
When it comes to connecting conductor tracks and electrical components in smart textiles, adhesion and contact resistance play an important role. Currently, loose contacts and poor contact resistance are a recurring problem because adhesion is not yet sufficient while maintaining flexibility. By applying a thermally activatable and simultaneously adhesive coating to conductive thread materials, it is possible to ensure secure and reliable contact. This surface functionalization with and without additional conductive particles is carried out using a conventional thread coating process. After textile processing, the coating is activated by pressure and temperature, thus creating a strong adhesive contact at the intersections of the conductive thread materials. The improvement in adhesion achieved depends on the type of coating paste and the amount applied. However, a compromise must be found between the amount applied and the resulting abrasion in order to achieve the best possible result.

Problem
The aim is to develop surface functionalization of electrically conductive yarns with thermally activatable adhesive coatings for secure and reliable contacting. The application of the thermally activatable adhesive coating primarily serves to create a firm and reliable, yet flexible contact between two conductor tracks within a stick- or web-produced textile or between two textile surfaces/units and electrical components. This prevents “loose contacts” and thus poor contact resistance between the conductive components, and contact resistance is reduced in particular by adding electrically conductive particles to the polymer mass.
Secondarily, the coating serves as physical and chemical protection for the thread surface against environmental influences such as friction, scratches, corrosion, and UV radiation. An extended service life of the conductive thread materials and surfaces can also be expected, e.g., longer stability of the contacts under the influence of movement and washing cycles.

Solution
The developed surface functionalization with and without additional conductive particles was applied to conductive thread materials using a conventional thread coating process via galette. After knitting and weaving, the applied coating is activated by pressure and temperature, thus creating a strong adhesive contact at the intersections of the conductive thread materials. The polyurethane-based coating without conductive particles alone can already reduce the contact resistance at these intersections. The addition of conductive particles leads to a further reduction. The project succeeded in using ultrasound to produce a uniform coating paste with conductive particles. However, sedimentation occurs during storage and use. Further optimization is therefore required to produce a homogeneous and storage-stable aqueous coating paste. It is also possible to apply the conductive particles in powder or aqueous form after application to the still moist adhesive coating. With some conductive filament materials, this results in a demonstrable reduction in electrical resistance. However, these layers are very uneven, and the layers based on conductive powders in particular have poor adhesion. In addition, technical adjustments to the equipment are necessary for the subsequent application of the conductive particles, among other things to reduce tunneling in the powder supply container.
The conductive threads developed with a thermally activatable, adhesive coating based on polyurethane can be processed using stick and weaving techniques. The applied PU coating results in a rough thread surface, which is reflected in higher friction forces and thread tensions. This problem can be controlled by applying an additional friction-minimizing layer. The conductive thread material and the coating layer of the activatable coating play a decisive role in textile processing and the improvement of the contact points. A higher coating thickness improves the adhesion of the contact points, but leads to difficulties in textile processing and increases the transition resistance due to insulation phenomena. Furthermore, an improvement is not always detectable. Ultimately, based on the results, it is not possible to make a clear recommendation; further detailed test series are necessary for this. The desired improvement in chemical resistance is evident in the reduction of corrosion current by approximately 87%.

Results and Applications
The main objective of developing surface functionalization of electrically conductive yarns with thermally activatable adhesive coatings for secure and reliable contacting in the field of smart textiles has been achieved. However, further tests are necessary to justify the additional costs for the extra coating step. In particular, activation and assembly processes as well as textile structures and modifications to the coating and plant technology need to be reconsidered.

Figure 1: Scanning electron microscope images of ELITEX® with surface functionalization,
left and center: longitudinal view, right: cross-section

We would like to thank the Federal Ministry for Economic Affairs and Climate Protection for its financial support of research project 49MF200090 “eELITEX,” which was funded by the federal budget.