**2.3. Carbon nonwoven fabric produced by polyacrylonitrile electrospinning**

Another option presented in this paper is nonwoven fabrics made from submicron carbon fibres. The first step taken to produce carbon structures was to prepare a nonwoven precursor. The precursor was obtained by electrospinning fibres from a solution of polyacrylonitrile (PAN). A 15% spinning solution of PAN powder was prepared to obtain the nonwoven precursor. The solution was produced by Zoltek Zrt. (Hungary) in dimethyl sulfoxide (DMSO) manufactured by POCH (Gliwice, Poland). The intrinsic viscosity of the PAN was equal to 1.3 ± 0.02 dL/g [23, 27]. Electrospinning was performed on a large-size laboratory device largesize laboratory line for producing nonwovens from the solution electrospinning technique that had 32 capillaries at 22 ± 10°C and a relative humidity (RH) of 38% under normal atmospheric pressure [23]. Process parameters were: generator voltage 15 kV, distance between the feeding capillary and collecting drum 15 cm, capillary diameter 0.9 mm [23].

The precursor nonwoven fabric was first subjected to thermal stabilization, which consists in heating to a temperature of 200°C and thermosetting for 6 hours. It was then oxidized by heating to a temperature of 220°C and thermosetting for another 6 hours. The heating cycles were performed in a stream of air. After washing the nonwoven fabric in carbon dioxide, it was subjected to pyrolysis, which involved heating to a temperature of 600°C in an atmosphere of inert gas and annealing at the final temperature. The nonwoven fabric was then subjected to chemical activation by impregnation with a solution of potassium hydroxide using a vacuum method, and kept under reduced pressure which was slowly increased to atmospheric pressure. This was followed by soaking the nonwoven fabric in an aqueous solution of potassium hydroxide, removing the excess hydroxide solution, leaving the nonwoven fabric at room temperature, drying to a constant weight and annealing in a stream of inert gas. Finally, the nonwoven fabric was cooled and extracted using an aqueous solution of hydrochloric acid and then distilled or deionized water [28].

#### **2.4. Methods used for nonwoven fabric assessment**

to it. A ready-made nanocomposite (PLA/4% MWCNT) was then mixed with pure PLA 4060D to obtain the final composition PLA/2% MWCNT. The two-step process was designed to ensure carbon nanotubes were uniformly distributed in the structure of the product and that the formation of undesirable agglomerates was reduced. The nanotubes were 90% pure, 9.5 nm in diameter and 1.5 µm in length. The polymer selected had a molar mass of up to 87 000

**2.2. Producing a nonwoven fabric of polyethylene oxide using electrospinning and carbon**

Polyethylene oxide (PEO) with the addition of MWCNT was selected for the production of nonwovens using the electrospinning polymer solution technique [8]. The polymer had a molar mass of 400 000 Da. To produce nonwovens with this technique, a solution of PEO in distilled water at a concentration of 5 wt.% was prepared to which a homogeneous suspension of the MWCNT containing 3% by volume carbon nanotubes with respect to the polymer volume was added. Mixing the nanotube suspension was carried out using an ultrasonic homogenizer at 150 W and a frequency of 30–40 kHz. The nanotube suspension was then added to the polymer solution and mixed in an ultrasonic homogenizer at 150 W and a frequency of 3–40 kHz at a temperature of 20°C for 0.2 hours, followed by 24 hours in a magnetic stirrer. Electrospinning consisted of an electrostatic field made up of the polymer composition prepared. The electrostatic field was the result of the potential difference between the capillary feed polymer and a collecting drum. A supply voltage of 15 kV was applied to the capillary by a generator while the receiving drum was grounded. The distance between the capillary

**2.3. Carbon nonwoven fabric produced by polyacrylonitrile electrospinning**

capillary and collecting drum 15 cm, capillary diameter 0.9 mm [23].

Another option presented in this paper is nonwoven fabrics made from submicron carbon fibres. The first step taken to produce carbon structures was to prepare a nonwoven precursor. The precursor was obtained by electrospinning fibres from a solution of polyacrylonitrile (PAN). A 15% spinning solution of PAN powder was prepared to obtain the nonwoven precursor. The solution was produced by Zoltek Zrt. (Hungary) in dimethyl sulfoxide (DMSO) manufactured by POCH (Gliwice, Poland). The intrinsic viscosity of the PAN was equal to 1.3 ± 0.02 dL/g [23, 27]. Electrospinning was performed on a large-size laboratory device largesize laboratory line for producing nonwovens from the solution electrospinning technique that had 32 capillaries at 22 ± 10°C and a relative humidity (RH) of 38% under normal atmospheric pressure [23]. Process parameters were: generator voltage 15 kV, distance between the feeding

The precursor nonwoven fabric was first subjected to thermal stabilization, which consists in heating to a temperature of 200°C and thermosetting for 6 hours. It was then oxidized by heating to a temperature of 220°C and thermosetting for another 6 hours. The heating cycles were performed in a stream of air. After washing the nonwoven fabric in carbon dioxide, it was subjected to pyrolysis, which involved heating to a temperature of 600°C in an atmosphere of inert gas and annealing at the final temperature. The nonwoven fabric was then subjected

Da and a D-isomer content of 12% [7].

and the drum was 20 cm [8].

**nanotubes**

266 Non-woven Fabrics

The cross-sectional shape and thickness of fibres comprising the nonwoven fabric were examined using a JEOL JSM-5200LV scanning electron microscope (SEM). Image processing and measurements were carried out using LUCIA G image analysis software. The results are shown in Table 1 and Figure 1.

The electrical conductivity of the nonwoven fabric produced was calculated by measur‐ ing surface resistance according to EN 1149-1:2008 – Protective clothing. The electrostatic properties of the nonwoven fabric were calculated using the surface resistivity test method. An electrometric direct method using a Keithley 610C solid-state electrometer was em‐ ployed in the study. The voltage source was an RFT-4218 DC power supply with a voltage range of 0–3000 V. The electrodes and the test sample were placed in a Faraday cage. Constant conditions for conditioning and testing were: temperature 23°C, RH = 25%. The results are shown in Table 2.

Sensory tests for the presence of solvent vapours were carried out using a laboratory meas‐ urement system. Such a system allows for measurements of the humidity and temperature of the atmosphere prevailing in the system and the creation and introduction of a given concen‐ tration of liquid vapours to the measuring system. The sensory sensitivity of the nonwoven fabric produced was investigated with measuring apparatus constructed at the Department of Material, Commodity Sciences and Textile Metrology (Lodz University of Technology, Poland). This was equipped with a tank serving as a gas chamber, a pump for mixing gas fumes, and a measuring chamber containing the measuring electrodes connected to a Keithley multimeter. The gas chamber was used to evaporate the appropriate amount of solvent. The mass of solvent to be evaporated in the gas chamber to achieve a concentration of *X* ppm was calculated according to equations (1–2) [7]:

$$Y = \begin{pmatrix} X \times M \end{pmatrix} / \mathbf{22.4} \tag{1}$$

$$
\mathcal{M} = \mathcal{Y} \times V \tag{2}
$$

where *Y* is the density of solvent vapours (mg/m3 ), *X* is parts per million (1/106 , ppm), *M* is molecular weight (kg/kmol), *m* is mass of solvent to be evaporated (mg), *V* is volume of gas chamber (0.024 m3 ).

The gas chamber houses a thermometer and humidity sensor that ensure tests are conducted under identical climate conditions (temperature 23°C and RH of 25%). After evaporation of the solvent in the gas chamber, the vapour is pumped to the measuring chamber in which a test sample 2 cm × 4 cm in size is placed on the measurement electrodes. The sensory properties of the nonwoven fabric were tested for vapours of various solvents, and changes in resistance were recorded. The liquids used were typed according to EN 14605+A1:2009. The sensory properties were also investigated for vapours of both polar and nonpolar organic liquids at a concentration of 200 ppm.

The sensory properties of samples for toxic vapour substances were characterized by defining a sensory factor *S*. This was defined by formula (3) [7, 29]:

$$S = \left| R\_{rd} \right| \text{\* 100\%} \tag{3}$$

where

$$R\_{rd} = \left(R\_{\parallel} - R\_0\right) / R\_0 \tag{4}$$

*R*rel is relative changes in electrical resistance, *R*0 is initial sample resistance (Ω), *Ri* is final resistance of the sample (Ω) [7, 29].
