In this project, conductive thread materials with electrical resistances in the high-ohm range (1-1000 kΩ/m) were implemented. The basis is formed by the developed technologies and polymer coating with conductive additives. Graphite particles and intrinsically conductive polymers were investigated as conductive additives and acrylate or polyurethane base polymers. The conductive polymer coating was applied using classic coating methods for single threads with a subsequent thermal activation process. The electrical resistances achieved in the high-ohm range as well as their low fluctuation range of < 20 % on 2 cm long thread sections enable the use of the high-ohm thread in freely cut textile heating surfaces.
Heating defined areas in functional clothing and car seats is currently a popular function. The basic prerequisite for textile-engineered and freely dimensionable heating structures are high-resistance thread materials with defined and, above all, uniform resistances over a thread length. Currently available conductive thread materials in the high-resistance range (1-1000 kΩ/m) have disadvantages for use in freely customisable heating fabrics with regard to the fluctuation range of the conductivity over a thread length of 2 cm as well as the reproducibility of the electrical characteristic values. In addition, these high-resistance thread materials have only a low thermal, chemical and mechanical resistance, which only ensures limited textile-technological processability and does not guarantee long-term stability in use. The aim of the research project is to develop a high-resistance thread with a low but homogeneous and permanent conductivity by activating a polymer layer applied to the thread with conductive additives in a thermal process.
For this purpose, on the one hand, novel thermally activatable/decomposable polymer coatings with conductive additives and, on the other hand, processes for implementing a low but constant and sustainable conductivity on textile yarn materials were developed. In addition, it was investigated whether the adhesion between conductive layer and thread can be improved by applying a SiOx coupling agent layer by means of combustion chemical vapour deposition (C-CVD). Further requirements for the high-resistance thread, in addition to achieving electrical resistances in the high-resistance range and the low fluctuation range of the resistance on small thread sections, include a low change in the textile-physical characteristic values and ensuring textile-technological processability.
Results and Applications
With the developed technology, defined and stable electrical thread resistances in the high-ohm range (1-1000 kΩ/m) can be realised on polyester mono- and multifilaments over the thread length. The technology is based on the developed polymer coatings with conductive additives, which are applied using classic yarn coating processes (patting and blocking processes) (see Figure 1).
Graphite particles and intrinsically conductive polymers were used as conductive additives. The homogeneous conductivity is achieved by adapted coating pastes and process parameters, but the thermal activation is mainly achieved by multiple coating applications. The achieved fluctuation range in the electrical characteristic values of < 20 % on 2 cm thread sections enables the use of the high-resistance thread in freely cut textile heating surfaces.
Good adhesion of the conductive polymer coating to polyester monofilaments is achieved by applying a SiOx coupling agent layer by means of C-CVD treatment (Figure 2). The conductive polymer coating subsequently applied by plough deposition is chemically bonded to the SiOx layer on the filament surface. After a coating pass through the graphite-containing polymer coating, a lower electrical resistance is achieved on the C-CVD-pretreated PES monofilament than on the PES monofilament without C-CVD treatment. This difference is relativised with increasing number of coating passes (Figure 3).
To prove the desired results, various functional samples such as conductive tape fabrics for freely cut heating applications have been developed on the basis of the developed high-resistance threads. With the implementation of the conductive tape as a heating tape, the textile-technological processability of the developed high-resistance threads has been confirmed. The heating tapes were then assembled and evaluated by means of a thermal imaging camera and electrical tests. The tape fabrics based on graphite-coated threads have a good heat distribution at a voltage of 12 V. In addition, the thermal and mechanical properties are good. In addition, there is good thermal, mechanical and chemical resistance in washing processes at 30 °C (Figure 4).
Dipl.-Ing. (FH) Antje Krahmer