Analysis of the Interdependence of Surface-Induced Pilling and Electrical Resistance of Cotton Knitwear

Abstract

The occurrence of pilling affects the appearance and aesthetic properties of knitwear and leads to a shortened lifespan of underwear, which is usually worn directly on the skin and under the outer layers of clothing and is exposed to direct contact with various textile materials in a dynamic microclimate. The interdependence of surface-induced pilling and electrical resistance (i.e., conductivity), which also affects wearing comfort, has not been sufficiently investigated. This paper therefore analyzes how surface-induced pilling of different intensities affects the surface resistivity and vertical resistance, physical properties and moisture content of double jersey cotton knitwear under different relative humidity conditions (25%, 40%, 65% and 80%) using Pearson’s correlation coefficient and coefficient of determination. Pilling was induced using the modified Martindale method and two types of abrasives, with higher intensity and larger pills obtained with a rougher wool reference abrasive. It was found that the surface resistivity and vertical resistance of cotton knitwear increased after prolonged wear due to surface-induced pilling and that mass, thickness and moisture content were not directly related to changes in electrical resistivity. The results of the Pearson correlation analysis showed a strong and quantifiable correlation between the intensity of surface pilling and surface resistivity at relative humidity up to 65%, despite their high moisture absorption. This statistically confirms that the occurrence of pilling reduces the electrical conductivity of cotton knitwear, resulting in a lower wearing comfort of cotton-based underwear. This finding can be useful in the development of underwear with high durability and comfort.

1. Introduction

During use, the surface of textile materials is exposed to various external influences (e.g., mechanical, chemical, thermal [1,2,3,4]), which can lead to wear of the material and change the appearance of the surface. The most common form of wear is mechanical wear, i.e., the abrasion that occurs when the surfaces of two textile materials are in direct physical contact [5,6]. However, abrasion also causes the fibers to detach from the fabric structure and migrate to the surface, where surface changes such as pilling occur [7,8].

According to EN ISO 12945-4:2020 [9], pilling in textiles is defined as the tangling of fibers into balls (pills) that protrude from the fabric surface and have such a density that the light cannot penetrate and casts a shadow. The occurrence of pilling in textile materials is influenced by numerous factors, including the fiber properties, the yarn and fabric structure and the type of finish. Fabrics made from fine staple fibers such as cotton or merino wool are particularly susceptible to pilling because their surface has more protruding fiber ends [10]. In addition, fibers with high flexibility and low tensile strength, such as viscose, tend to detach more easily from the fabric structure when abraded [11]. In fabrics made from blends, weaker fibers that detach during abrasion become entangled with stronger synthetic fibers such as polyester or polyamide 6.6, which act as anchors and hold the pills in place over a longer period of time [12]. Yarns with low twist and high hairiness are more prone to pilling. Ring-spun yarns have a more compact structure compared to open-end or air-jet spun yarns, but are hairier and often more prone to pilling [13,14]. Fabrics with a looser structure, such as single or double jersey knits, allow greater fiber mobility, while denser, woven structures restrict the movement of the fibers and reduce pilling [15]. Finishing processes such as singeing can reduce pilling by stabilizing the fibers and reducing surface hairiness, especially in synthetic fabrics [15,16].

Textile materials that are more susceptible to surface pilling are knitted materials [10,12,17], which consist of one or more yarn systems formed into loops and are intermeshed into stitches. This makes it easier for loose or damaged fibers to reach the fabric surface, as the structure of knitted fabrics is more open and flexible than that of woven fabrics [10,12,18]. Weft-knitted materials in single or double jersey patterns are often used for the production of next-to-skin garments such as underwear or T-shirts. Single jersey refers to weft-knitted fabrics where the technical face shows weft stitches (courses), while the back shows reverse stitches (wales), whereas double jersey refers to fabrics consisting of two independent layers of single jersey knitted face-to-face, showing face stitches on the outside and the reverse stitches on the inside [19]. This type of knitwear is usually worn directly on the skin and under the outer layers of clothing, so that its surface is exposed to contact with different materials and a dynamic microclimate [20]. Knitwear is often made from cotton fibers, which still accounts for about 20% of total global fiber production [21] and an estimated 68.6% of global men’s underwear production [22] and 47.2% of women’s underwear production [23], mainly because of its wearing comfort. Cotton is a natural cellulose fiber consisting mainly of cellulose with a degree of polymerization of 2000 to 3000. Cotton fibers are characterized by their good moisture properties, with a regain of 7–9% and a water retention capacity of 38–55%. They have moderate extensibility, with an elongation of 7–10% when dry and 12–14% when wet. Cotton fibers also have good strength, with a dry tenacity of 24–36 cN/tex and a slightly higher wet tenacity, ranging from 26–40 cN/tex. This makes cotton a versatile fiber suitable for the production of absorbent and comfortable knitwear [24,25].

The occurrence of pilling affects the appearance of the surface of knitwear, resulting in a shortened product life. The tendency of knitted fabrics to pilling is usually tested using the modified Martindale method (EN ISO 12945-2:2020 [12,17,26,27,28]) and is usually analyzed as part of the materials usage quality [13,29,30,31]. The influence of pilling on the physical properties of textile materials, such as mass and thickness, has been extensively studied. Research shows that pilling can lead to a reduction in fabric mass and compressibility, which can also affect other properties such as moisture absorption [30,32,33].

Pilling is mainly investigated in recent studies as a surface property of textiles that is analyzed with imaging techniques using digital cameras or scanners [28] that only capture its two-dimensional parameters. However, measurement systems that use tactile sensors to directly detect the three-dimensional topographic geometry (length, width and height) of the surface of textiles are not commonly used, although such devices can detect subtle surface irregularities such as fingerprints on various materials [34].

The interdependence of pilling and electrical resistance (i.e., conductivity), which also affects wearing comfort, has not been sufficiently investigated. This is probably due to the fact that pilling is mainly analyzed as an aesthetic problem, while the fact that it is mostly caused by material abrasion is often overlooked. Abrasion affects the electrical resistance of textiles mainly by damaging the surface of fabrics and causing breakage, fibrillation and displacement of fibers [24,35,36]. Therefore, the influence of abrasion on the electrical resistance of textile materials has been researched, but mainly for textiles containing electrically conductive components or coatings [37,38,39], as abrasion leads to greater degradation of the surface [38,39,40,41]. Textiles investigated in this area are usually made of polyester, polyamide and polypropylene fibers, as they absorb little moisture and have a high electrical resistance [38,39,40,41]. The occurrence of pilling was investigated in a study by Tunáková et al. [42], in which the authors tested the influence of washing and drying cycles on the electrical properties of knitted and woven fabrics with conductive yarns. They reported that the knitted material was more susceptible to pilling, resulting in a higher increase in surface and volume resistivity as well as a loss of electromagnetic shielding properties. A similar description was found in some other studies, which also reported the occurrence of pilling while determining the usage durability of fabrics with added electrically conductive components against abrasion [35,39,43].

As smart, conductive fabrics are an active field of research nowadays due to their increasing application in flexible electronics, wearable devices and electronic sensors, cotton composites are often investigated due to their natural origin and pleasing properties [44]. When cotton is functionalized with conductive polymers such as poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) or carbon-based nanomaterials, it shows a significant improvement in conductivity and lower electrical resistance [45]. By integrating recyclable and environmentally friendly nanomaterials, the latest approaches not only improve the tunable electrical performance of cotton-based fabrics under various external stimuli, but also meet the sustainability requirements of the next generation of smart textiles [44,46].

The electrical resistance of textile materials is usually determined by two standard test methods, EN 1149-1:2006 [47] for surface resistivity and EN 1149-2:1997 [48] for vertical resistance. These standards are intended for testing electrostatically dissipative protective clothing or fabrics [49,50,51,52] and are carried out under defined atmospheric conditions, i.e., at a higher temperature of 23 ± 1 °C and a lower relative humidity of 25 ± 5%. Textiles that have poor electrical conductivity and high electrical resistance are usually made of synthetic fibers [49,53,54,55,56]. However, this does not mean that the electrical resistance of these or other textiles cannot change during use [25,56,57,58], which is particularly observed in atmospheric conditions with lower relative humidity [49,50,51,52,53,54]. Atmospheric conditions have a strong influence on the electrical properties of textiles, but there are also many other factors that can affect them, such as the type of yarn [35,38,50], the structure of the fabric [56,59], surface roughness [37,60], thickness, moisture absorption [36,55,61,62], porosity [41,60] or the addition of a pretreatment [41,43,60].

This study therefore investigates how surface-induced pilling of varying intensity affects the electrical resistance of cotton knitwear. Since knitwear is usually in direct contact with the user’s skin or between the outer layers of garments, it is exposed to different microclimates during use. To simulate these conditions, surface resistivity and vertical resistance were measured on specimens conditioned under atmospheric conditions with different relative humidity (25%, 40%, 65% and 80%). Since the occurrence of pilling can lead to additional changes in the properties of the knitted fabric, which in turn can influence the electrical resistance, the physical and moisture-absorbing properties of the knitted fabric were also determined under the different conditions of relative humidity. The dependence of surface-induced pilling and the physical and moisture-absorbing properties on the surface resistivity and vertical resistance of the knitted fabric was evaluated using the Pearson correlation coefficient and coefficient of determination [63,64,65]. The material selected for this study was an untreated, weft-knitted cotton double jersey fabric made from single ring-spun yarn, as cotton is still one of the most commonly used fibers in the manufacture of next-to-skin garments. The occurrence of pilling on the surface of the selected fabric was induced using the Martindale pilling test device according to EN ISO 12945-2:2020 [26]. To facilitate the traceability of the results, two different abrasives were used—the first was the same cotton knitted fabric that was tested and the second was a reference wool woven fabric to achieve different pilling intensities.

2. Materials and Methods

For this study, an untreated 100% combed cotton double jersey fabric made from a single ring-spun cotton yarn with a linear density of 20.1 ± 0.26 tex, a Z-twist of 800 ± 17.75 m⁻¹, a tensile strength of 302.0 ± 5.0 cN and an elongation at break of 3.7 ± 0.1% was used [13]. The untreated fabric contains no residues of brighteners, finishing additives or chemically damaged fibers, limiting the number of factors that could potentially influence the measured electrical properties. The basic fabric properties are listed in

Table 1. Basic properties of double jersey weft-knitted fabric (calculated on the basis on n ≥ 3 replicates) with the corresponding standard deviation where applicable [13].

Property	Cotton Knitted Fabric
Thickness, mm	0.70 ± 0.02
Bulk Density, g/m3	0.26
Overall Porosity, %	83.04
Areal Density, g/m2	167.2 ± 1.1
Wale/Course Density, cm−1	21.5 ± 0.5/12.5 ± 0.5
Stitch Density, cm−2	268.8 ± 10.2
Lengthwise/Widthwise Tensile Strength, N	415.7 ± 42.3/121.2 ± 11.6
Lengthwise/Widthwise Elongation at Break, %	50.6 ± 2.2/142.4 ± 3.1

. The abrasion resistance is up to 60,000 rubs until the sample breaks down [13], while the propensity to pilling is high and the degree of pilling decreases continuously during the test. Since weft-knitted fabrics are more susceptible to various dimensional and spiral changes [10,12,18], a double jersey knitted fabric with a more stable structure was chosen to ensure the stability of the sample during testing and conditioning under various relative humidity conditions. Figure 1 shows optical microscopy images of the single spun cotton yarn (Figure 1a) and a double jersey weft-knitted fabric (Figure 1b) used for this analysis. The images were taken with a Dino-Lite microscope, catalog number AM413ZT, AnMo Electronic Corporation, Taiwan, at ×60 magnification.

The double jersey knitted fabric was produced under laboratory conditions at the University of Zagreb Faculty of Textile Technology on a double bed circular knitting machine with gauge E17, eight knitting systems and a needle bed diameter of 200 mm (including 432 × 2 needles) [13]. Dry relaxation of the row-untreated fabric was performed by laying it flat in standard atmospheric conditions according to EN ISO 139:2005/A1:2011 [66] (20 ± 2 °C temperature and 65 ± 4% relative humidity) for approximately 96 h.

2.1. Induction and Assessment of Pilling of Knitwear Specimens

In order to analyze the interdependence of surface pilling and the electrical properties of cotton double jersey knitted fabric, the occurrence of pilling on the face of the fabric had to be induced. For this purpose, the Martindale method according to EN ISO 12945-2:2020 [26] was applied, which was performed using the Martindale abrasion and pilling tester, catalog number 2561E, Mesdan S.p.A, Raffa, Italy (Figure 2a) under standard atmospheric conditions according to EN ISO 139:2005/A1:2011 [66]. The Martindale method and the device used allowed controlled induction of pilling by moving the tested knit specimens according to the Lissajous curve (Figure 2b) and rubbing them against an abrasive surface. The Lissajous curve corresponds closely to the pattern of rubbing that normally occurs in the specific areas of the next-to-skin garment where pilling first occurs.

To achieve different pilling intensities and thus facilitate traceability of results, two different abrasives were used—the first was the same cotton knitted fabric that was tested (Cot) and the second was a reference wool woven fabric (Wo) purchased from SDC Enterprises Limited, Holmfirth, UK, which is mainly used to determine the abrasion resistance of the fabric according to EN ISO 12947-1 [67]. For more intensive pilling induction, the test specimens were loaded with an additional load disk weighing 260 g during the rubbing in both cases, with the weight of the entire specimen holder and load disk being 415 g.

The number of pilling rubs after which the specimens were removed from the Martindale tester, visually assessed and subjected to further testing was 125, 500, 1000, 2000, 5000 and 7000. For each defined number of pilling rubs, three specimens with a diameter of 140 ± 5 mm were prepared, resulting in a total of 18 specimens rubbed with the tested cotton double jersey fabric and a total of 18 specimens rubbed with a reference wool abradant fabric.

The visual assessment was carried out according to the EN ISO 12945-4:2020 standard [9] by placing the specimens in a custom made viewing unit (Figure 3a), which was located in a darkened room and in which the surface of the specimen was illuminated with a light source with artificial daylight D65. The illuminated surface of the specimens was assessed by three experts who compared the surface of the rubbed fabric (Figure 3a, 1) with that of the non-rubbed fabric (Figure 3a, 2) and gave a rating (grade) from 5 to 1 using reference photographs of pilling (Roaches SM54 knitted standards, catalog number 5000-0006, Holmfirth, UK, Figure 3a, 3). The numerical ratings indicate the intensity of pilling formation as described: 5—no change (Figure 3b), 4—partially formed pills, 3—moderate pilling (pills of varying size and density partially covering the surface of the specimen), 2—distinct pilling (pills of varying size and density covering a large part of the surface of the specimen) and 1—severe pilling (pills of varying size and density covering the entire surface of the specimen (Figure 3c)). The assessments of the three experts should not differ by more than ±1, otherwise the assessment process must be repeated for all experts. The standard test method [9] allows the ratings for the specimen to be expressed as decimal numbers (e.g., “x.0”), so that a half rating (e.g., “x.5”) can be given if the ratings of the three experts lie between two adjacent values (±1). After the pilling induction and assessment process, the cotton knit specimens were subjected to a conditioning process.

2.2. Conditioning of Knitwear Specimens Under Different Relative Humidity Conditions

Since cotton underwear is highly absorbent and is exposed to different microclimatic conditions on a daily basis, the knitwear specimens were conditioned under atmospheric conditions with four different relative humidities before further testing. The temperature of the selected conditions was kept constant at 20 ± 2 °C [66] to further limit the number of factors affecting the electrical properties of the tested fabric. The first relative humidity selected was that of standard methods EN 1149-1 and EN 1149-2 [47,48] at 25 ± 5% (HR25), which is used for measuring the electrical resistance of textiles, and the second was that of EN ISO 139:2005/A1:2011 [66], which is used for conditioning and testing textiles, with a relative humidity of 65 ± 4% (HR65). The other two selected conditions were then interpolated around these values: 40 ± 5% (HR40) and 80 ± 5% (HR80). These conditions were chosen to reduce the difference between two neighboring conditions and to obtain more accurate information about the measurement at lower and higher relative humidities.

Conditioning of all unrubbed and rubbed cotton specimens was carried out in three steps. In the first step, the specimens were dried for 24 h at a temperature of 105 °C until they reached an absolutely dry state, which ensured the moisture absorption of the specimens at different relative humidities. The second step was cooling in a desiccator (20 L) with silica gel for 1 h so that the high temperature would not affect the microclimate in the desiccators with the selected relative humidities. The last step was the conditioning process, which was carried out for 24 h in the desiccator (20 L) with sulfuric acid at different concentrations: 57% (for HR25), 47% (for HR40), 36% (for HR65) and 27% (for HR80), selected according to Wilson, R. E. Humidity Control by Means of Sulfuric Acid Solutions, with Critical Compilation of Vapor Pressure Data from 1921 [68].

2.3. Electrical Resistance Measurement of Knitwear Specimens

The electrical properties of cotton double jersey knitted fabric specimens were determined by measuring the surface resistivity and vertical resistance according to the standard methods EN 1149-1 and -2 [47,48]. The instrument used for both measurements was a conductivity meter, Static Lab, catalog number 291B, Mesdan S.p.A, Raffa, Italy (Figure 4a), equipped with an electronic control panel (Figure 4a, 1), a circular upper electrode (Figure 4a, 2), a lower electrode (specimen holder, Figure 4a, 3) and an insulating plate (Figure 4a, 4). If the specimen is placed on the lower electrode of the conductivity meter with the insulating plate, the surface resistivity can be measured, and if the specimen is placed directly on the lower electrode, the vertical resistance can be measured. The usefulness of the conductivity tester was confirmed by comparing the diameter of the inner circular electrode (50 mm) and the outer circular electrode (90 mm) of the conductivity tester with the diameter of the circular area of the cotton double jersey fabric specimen affected by pilling (90 mm) (Figure 4b). This confirms that the measurements can be carried out mainly in the area of the specimen affected by pilling.

The time for measuring surface resistivity and vertical resistance was set at 30 s, as the measurement procedure was carried out in the test laboratory under standard atmospheric conditions (relative humidity was 65 ± 4%) [66]. In the first 15 s after the specimens were placed on the meter, a stabilization of the device measurements was performed, after which five consecutive measurements were performed in the next 15 s. Before each electrical resistance measurement, the conditioning process of the specimens was repeated and the conductivity meter was calibrated. Calibration was performed either by placing the circular electrode on the insulating plate to confirm that the measured surface resistivity was above the maximum of 10 TΩ, or by placing the circular electrode on the specimen holder to confirm that the measured vertical resistance value was below the minimum of 10 kΩ.

According to the measurement procedure, the values from five consecutive measurements of surface resistivity are expressed as a geometric mean, while the values of vertical resistance are expressed as an arithmetic mean, according to standard methods [49,50]. The value of the standard deviation was also calculated for each geometric and arithmetic mean value.

2.4. Physical and Moisture Absorbing Properties Measurement of Knitwear Specimens

The occurrence of pilling due to rubbing on the surface can lead to additional changes in the physical properties of the knitted fabric [37,40,42,43], which in turn can influence the electrical properties. Therefore, the selected physical properties and moisture absorption of the specimens of knitted cotton double jersey were determined under the four defined conditions of relative humidity before and after each defined number of pilling rubs: mass of specimen (g), thickness (mm, EN ISO 5084:2003 [69]), number of wales and courses (cm⁻¹, EN 14971:2008 [70]), stitch density (cm⁻², EN 14971:2008 [70]) and moisture content (%, ASTM D 2654-22 [71]).

The mass of each specimen was measured on an analytical measuring balance KERN catalog number ALS 250-4A, KERN & SOHN GmbH, Balingen, Germany, with an accuracy of 0.0001 g. The thickness of the specimen was determined with a thickness tester, catalog number DM 2000, Wolf Mess-technik GmbH, Freiberg, Germany, with an accuracy of 0.01 mm at ten different points on the surface of the specimen with induced pilling. The number of wales and courses on the double jersey knitted specimens was determined using a magnifying glass with ×8 magnification over a length of 1 cm at five different points on the specimens. The stitch density is calculated as the product of the number of wales and the number of courses per square centimeter of the specimen area, as it is commonly associated with many other properties of knitted fabrics, e.g., thickness, breathability, and electrical conductivity [54]. The moisture content of the specimens is calculated according to Equation (1) and expressed as a percentage.

Mc_HRx (%) = (m_HRx − m_ad)/m_ad × 100

(1)

where

• Mc_HRx—moisture content of the specimen determined after a different number of pilling rubs (from 125 to 7000) and under different conditions of relative humidity (HR₂₅, HR₄₀, HR₆₅ and HR₈₀);
• m_HRx—mass of the specimen determined after different number of pilling rubs (from 125 to 7000) and under different conditions of relative humidity (HR₂₅, HR₄₀, HR₆₅ and HR₈₀);
• m_ad—mass of the absolutely dry specimen after a different number of pilling rubs (from 125 to 7000).

The mass of the absolutely dry specimen (m_ad) was determined by drying the specimens for 24 h at 105 °C and then weighing them with an accuracy of 0.0001 g.

The arithmetic mean and the standard deviation value were calculated for all measured property results.

2.5. Coefficients of Correlation and Determination

The Pearson correlation coefficient (Pc) and coefficient of determination (Pc²) with a 95% confidence interval were used to investigate and better understand how pilling intensity, physical properties and moisture absorption affect the electrical properties of cotton double jersey fabrics after a certain number of pilling rubs under different relative humidity conditions and whether there is a linear relationship between any of these variables. All the analyses were based on a total of 7 data points (n = 7), and correlations with p-values below 0.05 were considered statistically significant.

The Pearson correlation coefficient (Pc) indicates the strength and direction of the relationship between independent and dependent measures. Its values range from 0 to ±1, with 0 indicating no correlation and ±1 indicating a very strong correlation. A positive sign (+) means that both variables increase and decrease together, while a negative sign (−) means that one variable increases while the other decreases.

The coefficient of determination (Pc²) indicates the proportion of variance in the dependent variable that can be explained by the independent variable. In this study, it was used to assess how much of the variation in the dependent electrical properties (surface resistivity and vertical resistance) was due to the independent surface-induced pilling or the change in physical properties and moisture absorption.

The values of Pc are generally categorized as follows: negligible correlation (0.00–0.10), weak correlation (0.10–0.39), moderate correlation (0.40–0.69), strong correlation (0.70–0.89) and very strong correlation (0.90–1.00). The Pc² values are categorized in a similar way: low (<0.25), moderate (0.26–0.50), high (0.51–0.75) and very high (0.76–1.00) [63,64,65].

In cases where high values were obtained for the correlation coefficient and the coefficient of determination, the results were presented both numerically and graphically in the form of correlation diagrams, where the linear regression line indicates the relationship between two properties together with the corresponding regression equation (Equation (2)) and the coefficient of determination (Pc²). The regression equation is expressed as:

y = ax + b

(2)

where a and b are the two coefficients of change in the independent variables, where a is the slope and b is the intercept. The equation is usually accompanied by the standard error of regression (SE), which quantifies the accuracy of the graphical regression model. The SE is calculated according to the following Equation (3).

$$SE=\sqrt{{\displaystyle \frac{\Sigma (y_i-{\hat{y}}_i)}{n-2}}}$$

(3)

where y_i are the observed values, ${\hat{y}}_i$ are the predicted values and n is the number of observations.

3. Results

The results include assessments of pilling intensity and measurements of the electrical, physical and moisture-absorbing properties of unrubbed and rubbed specimens with the same tested cotton double jersey fabric and a reference wool woven fabric after a specified number of pilling rubs (from 125 to 7000) under specified relative humidity conditions (HR₂₅, HR₄₀, HR₆₅ and HR₈₀). The dependence of the surface-induced pilling intensity and the wear-related changes in the measured physical and moisture-absorbing properties of the tested cotton double jersey fabric on the investigated electrical properties is discussed on the basis of the determined values of the Pearson correlation coefficient and the coefficient of determination.

3.1. Pilling Intensity Ratings of Cotton Knitwear Specimens

The pilling intensity ratings given by three experts for specimens of a cotton double jersey fabric rubbed with the same test fabric (Cot) and with a reference wool abrasive fabric (Wo) after a certain number of pilling rubs at a relative humidity of HR₆₅ are shown in

Table 2. Pilling intensity ratings (grades) of the unrubbed (0) and rubbed knitwear specimens (based on n = 3 replicates) with the tested double jersey fabric (Cot) and the reference fabric (Wo) at a relative humidity of HR₆₅ after a certain number of pilling rubs (from 125 to 7000).

Abradant Fabrics	Number of Pilling Rubs
	0	125	500	1000	2000	5000	7000
	Surface-Induced Pilling Intensity Ratings
Cot	5.0	4.0	4.0	3.5	2.5	2.0	1.0
Wo	5.0	3.5	3.0	2.5	2.0	1.5	1.0

.

The results show that the pilling on the surface of the cotton knit specimens was more intense after prolonged wear simulation with increasing number of pilling rubs using both abradant fabrics. Different pilling intensities can be observed between the specimens rubbed with the fabric under the test and reference wool fabric. Previous studies have determined that the surface of abrasive materials has a great influence on the occurrence of pilling [1,6,8,30]. In this case, a reference wool fabric of rougher surface caused a more intense occurrence of pilling than that of tested knitted fabric. This becomes particularly clear at the beginning of the test, where a strong tendency towards surface pilling was observed in the specimens rubbed with the wool fabric, which was assessed with moderate pilling grades of 3.5 and 3 after just 125 and 500 pilling rubs. The specimens that were rubbed with the tested Cot fabric showed partially formed pills at the beginning of the rubbing process (rating 4.0), which is probably due to the smoother surface of the tested cotton knitted fabrics, which causes a slower wear process and thus a lower number of formed pills. After 1000 rubs, the development of pilling on the fabric surface was linear for both specimens rubbed with the abrasives Cot and Wo, which was confirmed by the continuous decrease in the degree of pilling.

In addition to the different intensity of the induced pilling, the formation of pills of different sizes on the surface of the cotton knitted fabric specimens was also observed. Furthermore, it should be noted that rubbing with a reference wool fabric caused larger pills on the surface of the knitted cotton fabric specimens than rubbing with the tested Cot fabric. In addition,

Table 3. Surface appearance of the entire and enlarged area (×20, 1.2 × 1.0 cm²) of the knitwear specimen rubbed with the tested cotton double jersey fabric (Cot) and a reference wool fabric (Wo) after 125, 1000 and 7000 pilling rubs.

Number of Pilling Rubs	Abradant Fabric
	Cot		Wo
125
Pilling Intensity Rating	4.0		3.5
1000
Pilling Intensity Rating	3.5		2.5
7000
Pilling Intensity Rating	1.0		1.0

shows the appearance of the entire surface of the specimen after 125, 1000 and 7000 pilling rubs as well as a smaller area of the specimen (1.2 × 1.0 cm²) taken with the Dino-Lite AM413ZT digital microscope under ×20 magnification, clearly showing the different intensity of pilling formation and the shapes of the intertwined pills.

With a higher number of pilling rubs (5000 and 7000), a stronger tendency to surface pilling was observed in the tested knit specimens, where both abrasives were used. At the end of the test, where 7000 pilling rubs were achieved, the cotton knit specimens rubbed with Cot and Wo abradant fabric were completely covered with pills and received the lowest final ratings for pilling tendency (Table 2). When analyzing the surface of the rubbed cotton knit specimens (Table 3), the interlaced pills negatively affect the smoothness and thus the aesthetic properties [13], but also confirm the appropriate selection of cotton knitwear for this analysis.

3.2. Interdependence of Surface-Induced Pilling and Surface Resistivity of Cotton Knitwear Specimens

Table 4. Geometric mean with corresponding standard deviation of surface resistivity values (expressed in GΩ, based on n = 5 replicates) for unrubbed (0) and knitwear specimens rubbed with the tested cotton double jersey (Cot) and reference wool abradant (Wo) fabric, determined under different relative humidity conditions (HR₂₅, HR₄₀, HR₆₅ and HR₈₀) after a defined number of pilling rubs (from 125 to 7000).

Surface Resistivity, GΩ		0	125	500	1000	2000	5000	7000
Cot	HR25	9700.00 ± 582.00	6633.33 ± 506.13	6100.00 ± 457.60	12,700.00 ± 952.10	16,000.00 ± 576.00	20,000.00 ± 1200.00	27,000.00 ± 1890.00
	HR40	260.00 ± 16.64	560.00 ± 26.88	516.67 ± 36.17	760.00 ± 53.20	530.00 ± 20.14	850.00 ± 68.00	1550.00 ± 124.00
	HR65	11.95 ± 0.72	16.10 ± 0.97	17.85 ± 1.07	18.20 ± 1.28	18.00 ± 0.99	19.90 ± 1.39	22.00 ± 1.43
	HR80	0.96 ± 0.07	0.86 ± 0.06	0.76 ± 0.06	0.75 ± 0.05	0.77 ± 0.05	0.85 ± 0.06	0.80 ± 0.05
Wo	HR25	9700.00 ± 582.00	12,100.00 ± 847.00	13,200.00 ± 594.00	18,300.00 ± 1281.00	19,100.00 ± 954.30	31,000.00 ± 2170.00	34,333.33 ± 2396.67
	HR40	260.00 ± 16.64	599.00 ± 38.94	686.67 ± 44.47	930.00 ± 65.10	740.00 ± 51.80	1040.00 ± 72.80	2000.00 ± 140.00
	HR65	11.95 ± 0.72	17.90 ± 1.25	19.10 ± 1.06	19.50 ± 1.27	19.10 ± 1.15	20.70 ± 1.45	23.00 ± 1.61
	HR80	0.96 ± 0.07	0.80 ± 0.06	0.68 ± 0.05	0.69 ± 0.04	0.75 ± 0.05	0.79 ± 0.06	0.78 ± 0.06

shows the geometric mean values of five consecutive measurements of surface resistivity calculated for each cotton knit specimen after a certain number of pilling rubs and after conditioning at a certain relative humidity.

The results clearly indicate a strong influence of relative humidity (HR25, HR40, HR65 and HR80) on the surface resistivity values of cotton double jersey fabric. This effect was expected [55,59] and can be observed for both unrubbed (0) and rubbed specimens (from 125 to 7000 pillings), as the resistivity values decrease rapidly at higher relative humidity (e.g., for unrubbed specimens: by 9440.00 GΩ for HR40, 9688.10 GΩ for HR65 and 9699.00 GΩ for HR80). The investigated interdependence of surface-induced pilling was found in the value of surface resistivity, as this value changed after each number of pilling rubs performed, both for specimens rubbed with cotton double jersey (Cot) and with wool abrasive fabric (Wo). The knit specimens rubbed with Cot fabric showed a decrease in surface resistivity after 125 and 500 pilling rubs (due to a lower tendency to pilling on the surface, Table 2) and then an increase from 1000 to 7000 pilling rubs over the three of four relative humidity conditions (HR25, HR40 and HR65) with a decrease in pilling grades. The specimens rubbed with Wo fabric under the same three relative humidity conditions (HR25, HR40 and HR65) showed a consistent increase in surface resistivity from 125 to 7000 pilling rubs (as well as a continuous increase in pilling grades, Table 2). In addition, the specimens rubbed with Wo fabric, which had a higher intensity of induced pilling and larger pills (see Section 3.1), showed a more pronounced increase in surface resistivity under the HR₂₅, HR₄₀ and HR₆₅ relative humidity conditions than the specimens rubbed with Cot fabric when the same number of pilling rubs were performed.

Comparing the values between successive rubbings, the difference in surface resistivity becomes smaller at higher relative humidity (from HR₆₅ to HR₈₀) for both specimens rubbed with Cot and Wo abradant fabrics. At HR₈₀, the surface resistivity values of all specimens decrease significantly. The previously observed trend of an increase in surface resistivity with a higher number of pilling rubs, which was observed for HR₂₅, HR₄₀ and HR₆₅, was no longer present. This indicates that the influence of surface irregularities at high relative humidity is superimposed by the high moisture content and higher electrical conductivity of cotton materials [55].

To further investigate the relationship between the surface resistivity and pilling intensity values obtained for both the Cot and Wo rubbed specimens, the values were plotted (Figure 5a–d). As a potential linear correlation was observed, the Pearson correlation coefficient and coefficient of determination were calculated and are shown in

Table 5. The values of Pearson correlation coefficient (Pc), coefficient of determination (Pc²) and regression parameters (SE, a, b and p) calculated between the geometric mean surface resistivity values (based on n = 5 replicates) and the pilling intensity ratings (based on n = 3 replicates) of the knitwear specimens.

Abradant Fabric		Pc	Pc2	SE *	a **	b **	p ***
Cot	HR25	−0.92	0.84	3297.79	−5066.0	29,941.0	0.004
	HR40	−0.87	0.75	224.69	−260.3	1536.2	0.011
	HR65	−0.91	0.83	1.41	−2.08	24.263	0.004
	HR80	0.42	0.18	0.074	0.023	0.750	0.349
Wo	HR25	−0.89	0.80	4657.84	−6334.6	36,418.0	0.007
	HR40	−0.84	0.70	326.46	−341.8	1797.1	0.018
	HR65	−0.95	0.91	1.14	−2.41	25.12	0.001
	HR80	0.57	0.32	0.084	0.039	0.675	0.183

* SE—standard error of regression; ** a and b—coefficient of change due to independent variables; *** p—significance (p < 0.05).

.

The high Pearson correlation coefficient (Pc) and coefficient of determination (Pc²) values obtained for the HR₂₅, HR₄₀ and HR₆₅ conditions indicate a strong (from 0.70 to 0.89) to very strong (from 0.90 to 1.00) and consistent relationship between surface resistivity and the intensity of surface pilling in cotton knitwear (Figure 6a–c). However, this was not the case for condition HR₈₀ (Figure 6d), indicating a much stronger influence of relative humidity, under which the surface resistivity of cotton knitwear is very low. The relationship is indicated both by the values of the standard error of regression (SE), which reflect the accuracy of the model in predicting surface resistivity under defined conditions, and by the p-values, which confirm the statistical significance of the observed correlations (p < 0.05).

It follows that cotton knitwear worn in direct contact with the skin and exposed to prolonged wear increases its surface resistivity, with the intensity of surface-induced pilling and the size of the pills increasing at a relative humidity of up to 65% in the microclimate, which significantly reduces the wearing comfort and electrical conductivity of cellulose-based next-to-skin garments despite their high moisture absorption.

3.3. Interdependence of Surface-Induced Pilling and Vertical Resistance of Cotton Knitwear Specimens

Table 6. Average vertical resistance with corresponding standard deviation (expressed in MΩ, based on n = 5 replicates) for unrubbed (0) and knitwear specimens rubbed with the tested cotton double jersey (Cot) and reference wool abradant (Wo) fabric, determined under different relative humidity conditions (HR₂₅, HR₄₀, HR₆₅ and HR₈₀) and after a defined number of pilling rubs (from 125 to 7000).

Vertical Resistance, MΩ		0	125	500	1000	2000	5000	7000
Cot	HR25	5430.00 ± 352.95	3780.00 ± 204.12	7583.33 ± 531.25	8313.33 ± 598.56	6706.67 ± 402.40	4960.00 ± 347.20	1930.00 ± 123.52
	HR40	1035.00 ± 72.45	800.00 ± 56.00	1650.00 ± 124.58	649.00 ± 42.79	1230.00 ± 73.80	843.50 ± 60.73	607.50 ± 42.53
	HR65	9.25 ± 0.56	16.45 ± 1.03	19.17 ± 1.34	18.95 ± 1.23	16.10 ± 0.97	21.00 ± 1.33	22.20 ± 1.33
	HR80	1.13 ± 0.07	1.21 ± 0.06	1.33 ± 0.09	1.63 ± 0.10	1.37 ± 0.09	1.83 ± 0.12	1.21 ± 0.08
Wo	HR25	5430.00 ± 352.95	4877.78 ± 342.78	10,411.11 ± 583.02	11,133.33 ± 758.27	9600.00 ± 643.20	7600.00 ± 486.40	7533.33 ± 467.00
	HR40	1035.00 ± 72.45	833.00 ± 61.04	1696.67 ± 128.57	803.33 ± 57.04	1073.33 ± 70.84	1320.00 ± 92.40	1866.67 ± 130.67
	HR65	9.25 ± 0.56	15.70 ± 1.02	16.40 ± 1.15	20.20 ± 1.31	17.55 ± 1.10	19.90 ± 1.39	27.35 ± 1.91
	HR80	1.13 ± 0.07	1.23 ± 0.08	1.60 ± 0.10	1.51 ± 0.10	1.27 ± 0.08	1.55 ± 0.10	1.31 ± 0.09

shows the average mean values of five consecutive measurements of the vertical resistance values calculated for each cotton knit specimen after a certain number of pilling rubs and after conditioning at a certain relative humidity.

The values for the vertical resistance (MΩ) are significantly lower than the values for the surface resistivity (GΩ, Table 4), which indicates that the electrical volume conductivity in the knitted fabric is significantly higher than on the surface [25]. This may be related to the electrical properties of cotton, but also to its moisture content, which strongly influences the electrical conductivity of the fibers under different conditions of relative humidity [50]. Additional changes in vertical resistance values after each number of pilling rubs were observed in both the specimens rubbed with cotton double jersey (Cot) and those rubbed with the reference wool abradant fabric (Wo). When the changes in the measured values under different relative humidity conditions were further examined, it was found that the values for the samples rubbed with the Cot fabric were not the same at lower (HR₂₅ and HR₄₀) and higher (HR₆₅ and HR₈₀) relative humidity. At lower relative humidity, the vertical resistance after the initial number of pilling rubs (125) decreased (by −1650.0 MΩ for HR₂₅ and by −235.0 MΩ for HR₄₀), while at higher relative humidity the vertical resistance increased (by +7.2 MΩ for HR₆₅ and by +0.1 MΩ for HR₈₀). The values of the resistances between successive pilling rubs (from 500 to 7000) were not uniform, and their progression could not be determined possibly due to moisture dynamics, which strongly influences the values. After 7000 pilling rubs, the values at low relative humidity had decreased by −3500.0 MΩ for HR₂₅ and by −427.5 MΩ for HR₄₀, while the values at higher relative humidity had increased by +13.0 MΩ for HR₆₅ and minimally +0.1 MΩ for HR₈₀.

The changes in the vertical resistance values of the specimens rubbed with Wo fabric after the initial number of pilling rubs (125) were similar to those of the specimens rubbed with Cot, i.e., they decreased at lower relative humidity values (by −552.2 MΩ for HR₂₅ and by −202.0 MΩ for HR₄₀) and increased at higher relative humidity values (by +6.5 MΩ for HR₆₅ and by +0.1 MΩ for HR₈₀). A different behavior was observed after the last number of pilling rubs (7000), as the values obtained under all selected relative humidity conditions increased (by +2103.3 MΩ for HR₂₅, by +831.7 MΩ for HR₄₀, by +18.1 MΩ for HR₆₅ and by +0.2 MΩ for HR₈₀), suggesting that the surface changes obtained by abrasion with a rougher surface cause a higher increase in the vertical resistance of the material. The increase between successive pilling rubs (from 125 to 7000) was lower at a relative humidity of HR₆₅ and HR₈₀, regardless of the higher intensity of surface-induced pilling achieved at the end of the test (from 5000 to 7000 pilling rubs).

This indicates that the vertical resistance measurements of the cotton knit specimens are very sensitive to the conditions of relative humidity (non-monotonic variation with increasing rubs) under which they are tested. At low relative humidity and a low number of pilling rubs, the influence of surface-induced pilling is superimposed by the change in moisture content of the cotton material. At higher relative humidity and a higher number of pilling rubs, the moisture content is already high and the fluctuations in the vertical resistance caused by the moisture content of the material are lower, so that the influence of the surface-induced changes is noticeable as a slight increase in the vertical resistance of the tested knitted fabric.

The correlation between the vertical resistance and the pilling intensity of cotton double jersey is generally weak and inconsistent at different relative humidity conditions, regardless of the rubbing material used (

Table 7. The values of Pearson correlation coefficient (Pc) and the coefficient of determination (Pc²) between the values of average vertical resistance (based on n = 5 replicates) and pilling intensity ratings (based on n = 3 replicates) of the knitwear specimens.

Abradant Fabric		Pc	Pc2
Cot	HR25	0.47	0.22
	HR40	0.39	0.16
	HR65	−0.78	0.60
	HR80	−0.34	0.11
Wo	HR25	−0.42	0.17
	HR40	−0.48	0.23
	HR65	−0.92	0.85
	HR80	−0.46	0.21

). Pearson correlation coefficients (Pc) range from moderately positive to strongly negative (from 0.47 to −0.92), with most coefficients of determination (Pc²) not exceeding 0.23. These results suggest that surface-induced pilling has no discernible effect on the vertical resistance of cotton double jersey fabric and the interdependence between the two is low.

However, it was found that the vertical resistance of cotton knitwear increases even after prolonged wear in contact with various coarser garment materials due to surface-induced pilling at different relative humidity in the microclimate. This ultimately leads to a reduction in the electrical conductivity and overall comfort properties of cotton knitted underwear.

3.4. Interdependence of Physical, Moisture Absorbing and Electrical Properties of Cotton Knitwear Specimens

The average values of physical properties (mass, thickness, number of wales and courses and stitch density) and moisture content of cotton double jersey knit specimens determined under different relative humidity conditions (from HR₂₅ to HR₈₀) and after a certain number of pilling rubs (from 125 to 7000) are shown in

Table 8. Average mass and standard deviation (expressed in grams, based on n = 3 replicates) for unrubbed (0) and knitwear specimens rubbed with the tested cotton double jersey (Cot) and reference wool abradant (Wo) fabric, determined under different relative humidity conditions (HR₂₅, HR₄₀, HR₆₅ and HR₈₀) and after a defined number of pilling rubs (from 125 to 7000).

Mass of Specimen, g		Number of Pilling Rubs
		0	125	500	1000	2000	5000	7000
Cot	HR25	2.5624 ± 0.06	2.5808 ± 0.07	2.5816 ± 0.03	2.5783 ± 0.03	2.5821 ± 0.03	2.5812 ± 0.05	2.5815 ± 0.06
	HR40	2.7002 ± 0.08	2.6638 ± 0.04	2.6683 ± 0.03	2.6673 ± 0.08	2.6705 ± 0.04	2.6692 ± 0.07	2.6684 ± 0.06
	HR65	2.7360 ± 0.05	2.7314 ± 0.08	2.7334 ± 0.03	2.7343 ± 0.05	2.7346 ± 0.08	2.7291 ± 0.06	2.7268 ± 0.07
	HR80	2.7712 ± 0.08	2.7725 ± 0.04	2.7734 ± 0.07	2.7738 ± 0.03	2.7772 ± 0.05	2.7746 ± 0.05	2.7755 ± 0.03
Wo	HR25	2.5624 ± 0.06	2.5674 ± 0.03	2.5638 ± 0.03	2.5738 ± 0.06	2.5772 ± 0.06	2.5764 ± 0.06	2.5745 ± 0.04
	HR40	2.7002 ± 0.08	2.6440 ± 0.07	2.6408 ± 0.06	2.6466 ± 0.03	2.6451 ± 0.03	2.6455 ± 0.07	2.6436 ± 0.04
	HR65	2.7360 ± 0.05	2.7309 ± 0.08	2.7327 ± 0.05	2.7327 ± 0.07	2.7315 ± 0.04	2.7290 ± 0.07	2.7226 ± 0.08
	HR80	2.7712 ± 0.08	2.7697 ± 0.06	2.7686 ± 0.08	2.7695 ± 0.05	2.7728 ± 0.03	2.7755 ± 0.05	2.7705 ± 0.06

,

Table 9. Average number of wales and courses per knits centimeter, stich density with corresponding standard deviation values (expressed in cm⁻², based on n = 5 replicates) for unrubbed (0) and knitwear specimens rubbed with the tested cotton double jersey (Cot) and reference wool abradant (Wo) fabric, determined under different relative humidity (HR₂₅, HR₄₀, HR₆₅ and HR₈₀) conditions and after a defined number of pilling rubs (from 125 to 7000).

Abradant Fabric		Number of Pilling Rubs
		0–7000	0–7000
		Number Wales/Courses, cm−1	Stich Density, cm−2
Cot and Wo	HR25	22.5 ± 0.5/13.5 ± 0.5	303.75 ± 7.6
	HR40	21.5 ± 0.0/12.5 ± 0.0	268.75 ± 0.0
	HR65	21.5 ± 0.0/12.5 ± 0.0	268.75 ± 0.0
	HR80	22.5 ± 0.5/13.5 ± 0.5	303.75 ± 7.6

and

Table 10. Average moisture content and standard deviation (expressed in %, based on n = 3 replicates) for unrubbed (0) and knitwear specimens rubbed with the tested cotton double jersey (Cot) and reference wool abradant (Wo) fabric, determined under different relative humidity conditions (HR₂₅, HR₄₀, HR₆₅ and HR₈₀) and after a defined number of pilling rubs (from 125 to 7000).

Moisture Content, %		Number of Pilling Rubs
		0	125	500	1000	2000	5000	7000
Cot	HR25	1.2 ± 0.03	1.3 ± 0.01	1.3 ± 0.02	1.3 ± 0.02	1.3 ± 0.03	1.3 ± 0.03	1.3 ± 0.04
	HR40	4.2 ± 0.05	4.6 ± 0.08	4.8 ± 0.05	4.9 ± 0.07	4.8 ± 0.10	4.8 ± 0.05	4.8 ± 0.07
	HR65	6.8 ± 0.16	7.2 ± 0.15	7.3 ± 0.11	7.5 ± 0.16	7.3 ± 0.19	7.1 ± 0.07	7.0 ± 0.18
	HR80	8.1 ± 0.19	8.8 ± 0.15	8.9 ± 0.12	9.0 ± 0.26	9.0 ± 0.15	8.9 ± 0.11	9.0 ± 0.11
Wo	HR25	1.2 ± 0.03	1.3 ± 0.03	1.4 ± 0.04	1.3 ± 0.03	1.3 ± 0.03	1.3 ± 0.04	1.3 ± 0.02
	HR40	4.2 ± 0.05	4.5 ± 0.12	4.5 ± 0.10	4.3 ± 0.12	4.1 ± 0.09	4.2 ± 0.10	4.1 ± 0.04
	HR65	6.8 ± 0.16	7.8 ± 0.12	7.9 ± 0.09	7.6 ± 0.11	7.4 ± 0.09	7.4 ± 0.12	7.2 ± 0.16
	HR80	8.1 ± 0.19	9.4 ± 0.16	9.5 ± 0.13	9.0 ± 0.14	9.0 ± 0.26	9.2 ± 0.21	9.1 ± 0.20

and Figure 6.

With increasing relative humidity, all tested specimens (unrubbed and rubbed) show a minimal increase in mass (Table 8), which is primarily due to an increase in the content of absorbed moisture (Table 10). The mass increase in unrubbed specimens caused by different relative humidity conditions was slightly higher (by +0.1378 g for HR₄₀, by +0.1736 g for HR₆₅, and by +0.2088 g for HR₈₀) than the average mass increase in the Cot rubbed specimens (by +0.0870 g for HR₄₀, by +0.1507 g for HR₆₅ and by +0.1936 g for HR₈₀) and Wo rubbed specimens (by +0.0720 g for HR₄₀, by +0.1577 g for HR₆₅ and by +0.1989 g for HR₈₀), determined after defined number of pilling rubs (from 125 to 7000).

When pilling was induced (from 125 to 7000 pilling rubs), the observed changes in the mass of the tested specimens were also minimal, with the average difference in mass between the specimens rubbed with Cot and Wo fabric after a certain number of pilling rubs being almost zero (Cot = +0.0033 g, Wo = −0.0001 g).

The average thickness values of the knitted fabric specimens after each specified number of pilling rubs and for each atmospheric condition are shown in Figure 7. It can be seen that no significant changes were observed under the different conditions of relative humidity or according to the number of pilling rubs performed compared to the average value of the unrubbed fabric (0.70 ± 0.02 mm). The values for the number of wales and courses as well as the stitch density also do not change with increasing number of pilling rubs. Only a slight change was observed under the conditions of HR₂₅ and HR₈₀ (Table 9), which indicates that the different conditions of relative humidity have an influence on the values obtained.

Compared to the values determined for HR₂₅, the number of wales and courses has decreased for HR₄₀ and HR₆₅, while it has remained the same for HR₈₀ as for HR₂₅. The calculated values for the stitch density of cotton fabrics therefore only vary between two values 303.75 cm⁻² and 268.75 cm⁻² (Table 9).

The values for the moisture content of the knitted cotton fabric specimens (Table 10) were influenced by both the different relative humidity conditions and the pilling induction process (from 125 to 7000 pilling rubs).

With increasing relative humidity, all tested specimens (unrubbed and rubbed) show an increase in the absorbed moisture content (Table 10). The influence of the different relative humidity is most evident when the moisture content values determined are compared with those determined under HR₂₅. The analysis showed that as the relative humidity increased (from HR₂₅ to HR₈₀), the moisture content of the knitted fabric also increased, from 1.2% at HR₂₅ to 4.2% at HR₄₀, 6.8% at HR₆₅ and 8.1 at HR₈₀.

The influence of induced pilling on the surface of the cotton double jersey specimens was also observed as an increase in the moisture content of the specimens. After 125 pilling rubs, a sharp increase in moisture content is observed, which is particularly evident at a higher relative humidity of HR₆₅ (Cot = 7.2%, Wo = 7.8%) and HR₈₀ (Cot = 8.8%, Wo = +9.4%). This indicates easier accessibility and greater openness of the structure of the knitted fabric due to the surface wear of the cotton fabric at the beginning of the test. In addition, the average increase in the moisture content of the knitted fabric after the pilling rubs performed (from 125 to 7000) was +0.11% for HR₂₅, +0.62% for HR₄₀, +0.48% for HR₆₅ and +0.79% for HR₈₀ at Cot and +0.15% for HR₂₅, +0.10% for HR₄₀, +0.77% for HR₆₅ and +1.0.4% for HR₈₀ at Wo abradant.

Since the number of wales and courses and stitch density were not changed after the surface pilling induction process (Table 9), their influence could not be evaluated with the Pearson correlation. Therefore, the values of the Pearson correlation coefficient and the coefficient of determination for the variables mass, thickness and moisture content were compared separately with the surface resistivity (

Table 11. The values of the Pearson correlation coefficient (Pc) and the coefficient of determination (Pc²) between the average values of mass, thickness, moisture content (all based on n ≥ 3 replicates) and geometric mean of surface resistivity (based on n = 5 replicates) of the knitwear specimens.

Abradant Fabric		Measured Property of the Specimens
		Mass		Thickness		Moisture Content
		Pc	Pc2	Pc	Pc2	Pc	Pc2
Cot	HR25	0.28	0.08	0.57	0.32	0.36	0.13
	HR40	−0.46	0.21	0.60	0.36	0.51	0.26
	HR65	−0.78	0.61	0.46	0.21	0.42	0.18
	HR80	−0.65	0.43	−0.69	0.47	−0.86	0.74
Wo	HR25	0.75	0.57	0.47	0.22	0.23	0.06
	HR40	−0.50	0.25	0.73	0.53	−0.32	0.10
	HR65	−0.84	0.70	0.56	0.32	0.40	0.16
	HR80	0.29	0.08	−0.81	0.65	−0.82	0.67

) and with the vertical resistance values (

Table 12. The values of the Pearson correlation coefficient (Pc) and the coefficient of determination (Pc²) between the average values of mass, thickness, moisture content (all based on n ≥ 3 replicates) and average vertical resistance (based on n = 3 replicates) of the knitwear specimens.

Abradant Fabric		Measured Property of the Specimens
		Mass		Thickness		Moisture Content
		Pc	Pc2	Pc	Pc2	Pc	Pc2
Cot	HR25	−0.05	0.00	0.08	0.01	0.08	0.01
	HR40	0.13	0.02	−0.08	0.01	0.05	0.00
	HR65	−0.77	0.59	0.50	0.25	0.36	0.13
	HR80	0.31	0.09	0.51	0.26	0.64	0.41
Wo	HR25	0.35	0.12	0.37	0.13	0.36	0.13
	HR40	−0.26	0.07	−0.04	0.00	0.26	0.07
	HR65	−0.91	0.83	0.15	0.02	0.54	0.29
	HR80	0.01	0.00	0.64	0.41	0.52	0.27

).

From the results presented in Table 11, the Pearson correlation coefficients between mass, thickness, moisture content and surface resistivity are scattered and inconsistent at the different relative humidity conditions (from HR₂₅ to HR₈₀) for specimens rubbed with both Cot and Wo abrasive fabric. The values for mass range from weak (Cot Pc = +0.28, Wo Pc = +0.29) to strongly positive (Wo Pc = +0.75) to strongly negative (Cot Pc = −0.78, Wo Pc = −0.84). The thickness values range from moderate (Cot Pc = +0.46, Wo Pc = +0.47) to moderate (Cot Pc = −0.69, Wo Pc = +0.73) to strong (Wo Pc = −0.81), while the values for moisture content ranged from weak (Cot Pc = +0.36, Wo Pc = +0.23) to moderate (Cot Pc = +0.51, Wo Pc = +0.43) to strong (Cot Pc = −0.86, Wo Pc = −0.82).

These results are reflected in the scattered and inconsistent values of the coefficient of determination, which range from low (<0.25) to moderate (around 0.50) to high (around 0.75). It can therefore be assumed that the values for mass, thickness and moisture content are not directly related to the changes in the surface resistivity of cotton knitwear that occur after a different number of pilling rubs.

Table 12 shows the values of Pc and Pc² used to evaluate the relationship between the values of mass, thickness, moisture content and vertical resistance. It can be seen that the Pearson correlation coefficients (Pc) between mass, thickness, moisture content and vertical resistance are scattered and inconsistent at the different relative humidity conditions (from HR₂₅ to HR₈₀) for specimens rubbed with both Cot and Wo abrasive fabric. The values for mass range from negligible (Cot Pc = −0.05, Wo Pc = +0.01) to weak (Cot Pc = +0.31, Wo Pc = +0.35) to strongly negative (Cot Pc = −0.77) and very strongly negative (Wo Pc = −0.91). The thickness values range from negligible (Cot Pc = −0.08, Wo Pc = −0.04) to weak (Wo Pc = +0.37, Cot Pc = +0.15) to moderate (Cot Pc = +0.51, Wo Pc = +0.64), while the moisture content values ranged from negligible (Cot Pc = +0.05, Wo Pc = +0.08) to weak (Cot Pc = +0.36, Wo Pc = +0.26) to moderate (Cot Pc = +0.64, Wo Pc = +0.54).

These results are also inconsistent when it comes to the values of the coefficient of determination (Pc²), which ranges from low (<0.25) to moderate (around 0.50) and high (around 0.75) to very strong (close to ±1.00). Therefore, due to the inconsistent and widely scattered Pearson correlation coefficients and coefficients of determination, the values for mass, thickness and moisture content of the tested cotton knitwear cannot be used to explain the changes in vertical resistance that occurred after a different number of pilling rubs under different relative humidity conditions.

4. Conclusions

The interdependence of surface-induced pilling and the electrical properties of cotton double jersey weft-knitted fabric was analyzed under four different relative humidity conditions. The following conclusions can be drawn from the results presented.

The cotton knitwear tested showed a high tendency to surface-pilling during the pilling induction process using the modified Martindale method, both when rubbed with the tested cotton double jersey fabric and with the reference wool abradant fabric. The intensity of surface-induced pilling was more pronounced when the specimens were rubbed with the wool fabric, which has a more abrasive surface.

The strong influence of different relative humidity conditions (from HR₂₅ to HR₆₅) was observed for both surface resistivity and vertical resistance, as both resistances decreased rapidly with increasing relative humidity. The pilling induction process (from 125 to 7000 pilling rubs) led to an increase in the electrical resistances of the cotton double jersey specimens, resulting in a decrease in the electrical conductivity and overall comfort properties of the knitwear. The higher values of the electrical resistances were found for the specimens rubbed with wool fabric. A strong and consistent correlation coefficient and coefficient of determination was found between the values of surface resistivity and surface-induced pilling for three of the four relative humidity conditions used (HR₂₅, HR₄₀, HR₆₅). It was found that as the intensity of surface-induced pilling and the size of the pills increases, the wearing comfort and electrical conductivity of next-to-skin cotton knitwear is significantly reduced at a relative humidity of up to 65% in the microclimate.

When analyzing other tested properties of cotton knitwear, a strong dependence between the different conditions of relative humidity (from HR₂₅ to HR₆₅), mass and moisture content values was found, as well as the interdependence of induced pilling intensity and moisture content values [32,33,34]. However, based on the dispersed and inconsistent Pearson correlation and coefficients of determination, which ranged from low to strong, it was concluded that mass, thickness and moisture content could not be directly related to the changes in surface resistivity or vertical resistance of the tested cotton double jersey fabric.

The overall results of this analysis indicate that surface-induced pilling can affect the electrical properties of cotton knitwear and alter its electrical resistance values, particularly the surface resistivity, despite the high moisture content of the material at relative humidity levels of up to 65%. This means that the occurrence of pilling in knitwear is not only an aesthetic problem that leads to wear of the material over time, but also a factor that significantly affects the electrical properties of the material. The results of this analysis provide valuable information for future, more comprehensive studies that should be conducted using different methods to induce pilling, as real-life wear is much more diverse and includes other textile materials with different structures and made from different types of fibers and yarns. These findings can be useful and applied in the development of underwear with high usage durability and wearing comfort.