Changes in characteristics of silver conductive fabrics owing to perspiration and washing

Changes in characteristics of silver conductive fabrics owing to perspiration and washing

Changes in characteristics of silver conductive fabrics owing to perspiration and washing

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Characterization of electro-conductive textile materials by its biaxial anisotropy coefficient and resistivity

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Distances d1, d2, d3 and d4 (Figs. 3, 4, 5) between electrode and the nearest circle-shaped fabric edge (see Fig. 2a) were determined and are: d1 = 35.9 mm, d2 = 21.7 mm, d3 = 7.6 mm = dc, d4 = 0.0 mm.

Distances d1, d2, d3, d4 and d5 between electrode and the nearest square-shaped fabric corner (see Fig. 2b) were determined and are: d1 = 56.6 mm, d2 = 42.4 mm, d3 = 28.3 mm = dr, d4 = 14.1 mm, d5 = 0.0 mm.

In variants 1 and 3, the voltage and current electrodes have been changed. Resistance values are comparable, because in both cases the current flows along warp direction in the case of woven fabric and along course direction in the case of knitted fabric. In variants 2 and 4, the voltage and current electrodes have also been changed. Resistance values are comparable, because in both cases the current flows along weft direction in the case of woven fabric and along wale direction in the case of knitted fabric.

The resistances R1 and R2, the modulus k determined from Eqs. (10) and (12) and the resistivity ρ calculated from Eqs. (9) and (11) for circular sample and square-shaped sample respectively, the surface resistance Rs calculated from Eqs. (14) and the coefficient R/R are presented in Table 11. The R/R is a biaxial anisotropy coefficient and is calculated as a quotient of higher and lower of resistances R1 and R2.

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The values of resistivity obtained for woven and knitted fabrics indicated that the textile materials have electrical resistivity similar to metals or semiconductors. It was found that all fabrics show electrical anisotropy with anisotropy coefficient from 1.1 to 7.2. Influence of warp and weft densities for woven fabrics and course and wale densities for knitted fabrics are presented in Fig. 9.

a Structure analysis for woven fabrics, b structure analysis for knitted fabrics

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Values of resistances R1 and R2 are dependent on principal axes of textile materials. The main axes in woven fabric were defined by warp and weft directions. The main axes in knitted fabric were defined by course and wale directions. The resistance R1 was determined when current flows between electrodes arranged on the line parallel to the warp or course direction. It was found that the resistance R1 is lower than the resistance R2 where current flows parallel to the weft or wale direction. The higher warp density means occurrence of more contact resistances between warp and weft threads and therefore an increase in woven fabrics resistances (samples WF-1 and WF-2). In case of sample WF-3 it is not so obvious. The comparable values of resistivity R1 and R2 (the biaxial anisotropy coefficient R/R was equal to 1.1) may result from a fabric weave other than a plain weave observed in WF-1 and WF-2. The higher course density means occurrence of more contact points of the same thread resulting in occurrence of contact resistances and therefore an increase in all knitted fabrics resistances.

In order to more accurately understand the impact of sample surface roughness on resistances R1 and R2, and surface resistance Rs a line profile was identified to obtain the basic structural characteristics of fabrics. Two lines of profile were received: the horizontal and the vertical ones (Tables 12, 13) based on TIF file format of images converted into grayscale. Mean file size was 1957 KB (the coefficient of variation was equal to 3%). Mean resolution of all digital images expressed by pixel count was 995684 pixels (the coefficient of variation was equal to 2%). Line profile was routed through the center of the warp/weft and parallel to the warp/weft direction for woven fabric and through the center of the course/wale and parallel to the course/wale direction for knitted fabric. Measurements were repeated three times and mean values were calculated.

Some important parameters allowing evaluation of sample surface roughness were determined based on received characteristics. Surface area S under the line profile f(l) was calculated using following formula:

$$S=\int\limits_{{lo}}^{{lm}} {f(l)dl} \quad {l_o} \le l \le l{}_{m}$$

(15)

where l—the distance; lo—the initial distance lo = 0 px; lm—the final distance.

In formula (15) lm = 387 px was assumed. Surface areas Sh and Sv were calculated corresponding to horizontal and vertical lines respectively using (15). Next spectral analysis was carried out using Statistica® to determine the occurrence of periodicity in the characteristics. The largest peak was identified in periodogram and the corresponding surface roughness frequency fh and period Th for the horizontal line and fv and period Tv for the vertical line. All calculated parameters are given in Tables 12 and 13.

Pearson’s correlation coefficient was used in statistics to measure how strong a relationship is between two parameters characterizing features of textile materials. Pairs of chosen parameters and the corresponding significant values of Pearson’s correlation coefficient (0.05 significance level was assumed) are given in Table 14.

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It was found strong downhill linear relationship between the surface area Sh or Sv and the surface resistance Rs. The surface resistance decreases when the surface area increases. The bigger surface area under the function f(l) being the line profile of the sample means smaller oscillation of the function f(l) and consequently smaller surface resistance. Thus, sample with a smooth surface will conduct electrical current better than sample with a rough one. It was noticed that the greater the disproportion between the densities (expressed by D) the greater the coefficient R/R. Thus, the textile materials are characterized by higher anisotropy of electro-conductive properties.

Strong uphill linear relationship was observed between the coefficient R/R and the surface roughness frequency fh. Therefore, if frequency fh corresponding to the largest peak of periodogram obtained for the horizontal line increases the coefficient R/R also increases. Strong uphill linear relationship was also observed between the above coefficient and the surface roughness frequency fv corresponding to the largest peak of periodogram obtained for the vertical line. Results of the conducted analysis confirmed the effect of sample surface roughness on its electro-conductive properties.

Preparation and properties of electro-conductive fabrics based on polypyrrole: covalent vs. non-covalent attachment

1757-899X/254/3/032002

Electro-conductive fabrics were prepared via in situ oxidative polymerization of pyrrole (Py) in the presence of unmodified and chemically modified cotton fabrics. Chemical modification of cotton fabric was achieved by covalent attachment of a bifunctional linker molecule to the surface of the fabric, followed by incorporation of a monomer unit onto the linker. The fabrics were characterized using Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron spectroscopy, and thermal analysis. Furthermore, the effect of Py concentration on the degree of polypyrrole (PPy) grafting, surface morphology, electrical resistivity, and laundering durability were studied for both types of cotton fabrics. Reductions of several orders of magnitude in surface and volume electrical resistivities were observed for both non-covalently and covalently linked cotton-PPy systems, whereas the effect of covalent pre-treatment of the fabric was stronger at low Py concentration. On the other hand, at higher monomer concentration, the electrical properties and laundering durability of the fabrics we comparable for both unmodified and chemically pre-treated cotton fabrics, indicating that only a small fraction of PPy chains were chemically grafted onto the fabric surface with the majority of the polymer being connected to the fabric through hydrogen bonds.