Thermal-Mechanical Sensory Properties of Hot-Air Welded Textile Transmission Lines

Hot air welding technology is one of the new promising techniques for integrating conductive yarns on or into fabrics, besides weaving, knitting, printing, coating or inserting conductive yarns by the sewing and/or embroidery processes. A new issue related to the introduction of hot air welding technology for making e-textile transmission lines, i.e., the mechanical-thermal sensory properties of hot air welded e-textile transmission lines, is investigated in this study. Fabric Touch Tester (FTT) was used to evaluate thermal, compressive and bending properties of hot air welded transmission lines. The results show that the bending of welded fabric increased after hot air welding in both warp and weft directions. Furthermore, the maximum thermal ﬂ ux and thermal conductivity of welded specimens decreased in comparison to the substrate fabric.


Introduction
Th e touch feeling can be perceived through contacts between the human body skin and the fabrics surfaces. Four types of touch information including thermal, proprioceptive, cutaneous, irritating and painful arise when a fabric touches the skin [1]. A smart protective garment with integrated electrical elements brings many benefi ts and advantages to the users. During a working day, the use of a protective clothing lasts up to the eight-hour period of time. Th at is why the garment comfort requirements are very important and should be taken into account when such garment is designed. When designing smart protective garments and wearable electronics, many factors from the concept and purpose of the garment to wearable devices have to be considered. Th e embedded electronic devices and their auxiliaries have to satisfy the aesthetic and comfort requirements of end-users. Th e properties of conductive yarns integrated into fabrics can drastically aff ect the properties of these fabrics. Th ere are diff erent textile technologies that have been used to embed conductive yarn on or into textile materials: weaving and knitting technologies, sewing and embroidery techniques, printing or coating electro-conductive polymers. In the last few years, the hot-air welding technology was introduced as a new technological approach for adhering conductive yarns onto the fabric surfaces to make e-textile transmission lines [2,3]. From the construction point of view, a hot air welded e-textile transmission line can be presented as a laminated fabric, composed of a thermoplastic tape, a conductive yarn and a fabric substrate ( Figure 1).

Figure 1: Presentation of a hot air welded e-textile transmission line
Th e quality of an e-textile welded transmission line depends on the welding parameters settings. Th e bond strength between a thermoplastic tape and a substrate fabric should be evaluated, followed by the testing of electrical properties such as the conductivity and the signal transmission loss of the welded conductive yarn [2,3]. Th e hot air welded transmission line must fulfi l the following requirements: welded joint bond strength >10 N, smooth and wrinkleless surface of the welded area, no signifi cant disturbance of the welded surface due to the conductive yarn thickness, stable conductivity and signal transmission of conductive yarns. Th e aforementioned requirements can be met by proper adjustment of the hot-air welding parameters regarding the selected type of the substrate fabric, thermoplastic welding tape and conductive yarns, as well as the type of the hot-air welding machine used. Additionally, the integrated welded textile transmission lines infl uence both the aesthetic and the comfort issues of the garment. Th e results of previous experimental works [3,4] show that the hot-air welding parameters such as air temperature and pressure, rollers velocity and pressure between rollers have not any signifi cant infl uence on mechanical stresses of the conductive yarns during the process. On the contrary, these parameters have aff ected drastically the comfort quality of the welded joint as well as the welded area visual appearance. Furthermore, the bending rigidity of the embedded transmission lines depends signifi cantly on the welding parameters, the thermoplastic welding tape properties and the type of conductive yarns. Moreover, it has been shown that the constructed transmission lines have also adequate electrical properties at certain combinations of welding parameters [2,3]. In this study, the mechanical-thermal sensory properties of hot air welded e-textile transmission lines are investigated.

Experimental
In this research work the combinations of two conductive yarns (Table 1) and two welding tapes produced by Bemis company (Table 2) were used and embedded on a substrate laminated fabric. Th e latter, consisting of a 100% polyester woven fabric laminated by water-and windproof Sympatex® membrane, had a mass of 186 gm -2 . Th e purpose of the membrane and the waterproof welding tape was: (i) to protect the conductive yarn from both body moisture and environmental humidity, (ii) to provide maximum insulation and (iii) ensure uninterrupted electrical transmission line functionality during garment wearing.

Figure 3: Specimens arrangement
Th e thermal properties of the fabric specimens were evaluated by Fabric Touch Tester (FTT) [5] through the conducted measurements of the mechanical-thermal sensory properties such as fabric thickness, compression, bending, surface friction and roughness made on the same device. All tests were carried out under standard laboratory environmental conditions (20°C and 65% RH). Th e fabric sample was cut in the shape of letter "L" on two sides [6] as shown in Figure 2. Th e fabric faceside was tested fi rst, the testing of its back-side followed. FTT enabled the measurement of all fabric properties with one simple test within the duration of 2-3 minutes, and provided within one measurement 13 parameters that are presented in Table 3. Th e parameters BAR, BW, SFC, SRA and SRW are defi ned into warp and weft directions [5].

Results and discussion
Th e specimens were tested on both face and back sides in warp and weft directions. Th e results in a graphic mode present the infl uence of welding tapes and conductive yarns on bending properties (Figure 4), thickness ( Figure 5), compression work (Figure 6), thermal conductivity during compression and recovery (Figure 7) and maximum thermal fl ux (Figure 8). Due to the peculiar construction of the welded areas of the specimens, their roughness and friction properties could not be properly measured by roughness sensor and evaluated by using FTT, therefore, other testing methods should be used for this purpose. Th e bending rigidity of the welded textile transmission lines was evaluated on the face and back sides in warp and weft directions. Th e infl uence of welding tapes and conductive yarns on the bending rigidity of the welded specimens was confi rmed by previous research achievements [2][3][4]. Both the welding tapes and the conductive yarns increased the bending rigidity of welded transmission lines. Th e highest value of the bending rigidity was obtained by the combination of a three-layered welding tape (WT2, Table 2) and the thicker conductive yarn (Y2, Table 1), leading to the conclusion that the bending rigidity depends on the number of layers of welding tapes and the thickness of conductive yarn. Th e bending rigidity of the fabric in warp direction was higher than in weft direction irrespectively of the fabric side. Furthermore, the thickness of the welded specimens depends on four factors, i.e., the welding tape and conductive yarn thicknesses, the applied pressure and the temperature during the welding process. Th e heat transfer between fabrics and human skin gives the feeling of thermal comfort. Considering the eff ect of the specimen thickness, the thermal conductivity during compression (TCC) and recovery (TCR) presents a warm-cool feeling of fabrics. Th e results presented in Figure 7 show that hot-air welded specimens having thicker welding tape have higher warm-cool feeling in comparison to the substrate fabric. It is clearly evident that the fabrics aft er welding exhibit almost constant thermal conductivity during compression and recovery irrespectively of the conductive yarn thickness. Another index, named "thermal maximum fl ux", is defi ned as the maximum thermal fl ux during the measurement process. In general, the hot air welded transmission lines exhibited lower maximum thermal fl ux than the substrate fabric (Figure 8), even though the diff erences are small. Th is confi rms the fact that the hot air welding process and the used components for making textile transmission lines do not aff ect signifi cantly the heat absorption by the substrate fabric.
Infl uence of conductive yarn Y2

Conclusion
Fabric Touch Tester was used to evaluate the mechanical and thermal properties of the fabrics by examining the face and back sides in warp and weft directions at the same time. Two stainless steel conductive yarns were embedded on a waterproof polyester substrate fabric encapsulated by two waterproof welding tapes to protect the conductive yarn from air humidity, human moisture and friction during wearing. On the basis of the obtained results it can be concluded that the construction properties of the used welding tape and conductive yarns have very important infl uence on bending rigidity, thickness and thermal conductivity of the welded transmission lines. Th e properties of these components should be taken into account when they are embedded on a smart garment. Although the hot-air welding is a very fl exible technique, providing many alternatives for making etextile transmission lines, the properties of the used components and the welding parameters should be selected very carefully in order to achieve the desired three-dimensional shape and thermal comfort properties of the produced garment.