**3. Results and discussion**

Table 1 shows the thickness of fibres forming the nonwoven fabrics.


**Table 1.** Thickness of fibres forming the nonwoven fabrics [7, 8, 23]

Figure 1 shows the structure of the nonwoven fabrics and the shape of constituent fibres.

The results show that fibres in the nonwoven fabric prepared by electrospinning a polymer solution of PEO have the lowest diameter. However, microscopic observation shows that the addition of carbon nanotubes has caused many beads, making the structure nonuniform. The carbon nonwoven fabric made of PAN has the highest fibre thickness uniformity. The meltblown nonwoven fabric shows a high spread in fibre thickness – there are both fine fibres with

**Figure 1.** SEM images of fibres forming the nonwoven fabrics: a) 98% PLA 4060D/2% MWCNT, b) 97% PEO/3% MWCNT, c) carbon (precursor 15% PAN/85% DMSO) [7, 8, 23]

a diameter of approx. 5 µm and coarse fibres with a diameter of approx. 80 µm. Table 2 shows the results of the electrical conductivity test of the nonwoven fabrics.


**Table 2.** Electrical surface resistivity of the nonwoven fabrics

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

The sensory properties of samples for toxic vapour substances were characterized by defining

*R*rel is relative changes in electrical resistance, *R*0 is initial sample resistance (Ω), *Ri*

**Type of nonwoven fabric Production technique Thickness of fibre (μm)**

Figure 1 shows the structure of the nonwoven fabrics and the shape of constituent fibres.

The results show that fibres in the nonwoven fabric prepared by electrospinning a polymer solution of PEO have the lowest diameter. However, microscopic observation shows that the addition of carbon nanotubes has caused many beads, making the structure nonuniform. The carbon nonwoven fabric made of PAN has the highest fibre thickness uniformity. The meltblown nonwoven fabric shows a high spread in fibre thickness – there are both fine fibres with

Electrospinning carbonization 0.78

98% PLA 4060D/2% MWCNT Melt blowing 17.75 97% PEO/3% MWCNT Electrospinning 0.24

Table 1 shows the thickness of fibres forming the nonwoven fabrics.

**Table 1.** Thickness of fibres forming the nonwoven fabrics [7, 8, 23]

*S R* = *rel* \* 100% (3)

0 0 – ) /( *R RR R rel i* = (4)

is final

concentration of 200 ppm.

resistance of the sample (Ω) [7, 29].

**3. Results and discussion**

Carbon (precursor 15% PAN/ 85%

DMSO)

where

268 Non-woven Fabrics

a sensory factor *S*. This was defined by formula (3) [7, 29]:

Table 2 shows that the best conductivity was observed for the PEO nonwoven fabric made by electrospinning the polymer solution, while the worst conductivity was observed for the PLA nonwoven fabric with carbon nanotubes formed using the melt-blown technique. According to EN 1149-1:2008, homogeneous materials are characterized by electrostatic properties when they show a surface resistivity of less than 2.5 × 109 Ω. In other words, all three nonwoven fabrics have electrostatic properties.

Table 3 summarizes sensory test results for the presence of polar and nonpolar solvent vapours. The sensitivity threshold of solvent vapours was based on data about the toxic effects they have on the human body. The data suggest the minimum concentration of solvent vapours having a toxic effect on the human body is 200 ppm for toluene, 300 ppm for methanol and 500 ppm for benzene and acetone [7]. Research into all three types of nonwoven fabrics was carried out at 200 ppm.

Table 3 shows average test results calculated from sensory measurements of the three non‐ woven fabrics. Figures 2–5 illustrate changes in relative electrical resistance calculated by formula (4) in terms of the influence of different solvent vapours.


**Table 3.** Sensory test results for the presence of solvent vapours in the three nonwoven fabrics [7, 23]

Sensory phenomena occurred in all three types of nonwoven fabrics. Comparison of the nonwoven fabrics presented in Table 3 shows that the nonwoven fabric manufactured by electrospinning a polymer solution containing nanotubes (97% PEO/3% MWCNT) has the higher sensitivity to vapour sensing of all the solvents (at 200 ppm concentration) then the nonwoven fabric constructed with carbon fibres carbonized from a nonwoven precursor (15% PAN/85% DMSO). This may result from the characteristics of polymer and carbon fibres. Fibre thickness may also be significant. Reducing fibre thickness increases specific surface area, which allows greater surface diffusion of solvent molecules to fibres. Furthermore, PEO/ MWCNT-penetrating molecules are known to bring about further separation of nanotubes in percolation paths that were originally formed by nanotubes separating as a result of stretching.

Results from the nonwoven fabric made of 98% PLA 4060D/2% MWCNT show that sensor response to methanol vapours is relatively low (15%), while responses to benzene, acetone and toluene vapours reach 60%, 40% and 35%, respectively. Relative changes in electrical resistance *R*rel coincide with the Flory–Huggins parameter κPLA/benzene < κPLA/acetone < κPLA/ toluene < κPLA/methanol.

Looked at from a technological viewpoint, this research demonstrates that all of the process parameters evaluated affect the properties of the manufactured products. Comparison of the response plots of produced nonwoven fabrics with chemical stimuli shows that the melt-blown nonwoven fabric made of polymer (98% PLA 4060D/2% MWCNT) respond differently than the nonwoven fabric made of polymer solution (97% PEO/3% MWCNT) and the carbon nonwoven fabric made of the precursor formed using a 15% PAN/85% DMSO solution.

Analysis has shown that differences in both the intensity and directivity of sensory properties are likely caused by the use of different technological parameters. This phenomenon may result from the structural arrangement of macromolecules and nanotubes affecting the formation of electroconductive tracks directed along the fibre axis. Too large a distance between one nanotube and another may cause a rapid increase in resistance. Finding the optimum draw ratio imposed during nonwoven fabric formation can lead to better positioning of them in the conductive network formed by well-oriented MWCNT. This implies that dispersed nanotubes can be combined or separated for such nano and submicron-composite structures.

Sensory phenomena occurred in all three types of nonwoven fabrics. Comparison of the nonwoven fabrics presented in Table 3 shows that the nonwoven fabric manufactured by electrospinning a polymer solution containing nanotubes (97% PEO/3% MWCNT) has the higher sensitivity to vapour sensing of all the solvents (at 200 ppm concentration) then the nonwoven fabric constructed with carbon fibres carbonized from a nonwoven precursor (15% PAN/85% DMSO). This may result from the characteristics of polymer and carbon fibres. Fibre thickness may also be significant. Reducing fibre thickness increases specific surface area, which allows greater surface diffusion of solvent molecules to fibres. Furthermore, PEO/ MWCNT-penetrating molecules are known to bring about further separation of nanotubes in percolation paths that were originally formed by nanotubes separating as a result of stretching. Results from the nonwoven fabric made of 98% PLA 4060D/2% MWCNT show that sensor response to methanol vapours is relatively low (15%), while responses to benzene, acetone and toluene vapours reach 60%, 40% and 35%, respectively. Relative changes in electrical resistance *R*rel coincide with the Flory–Huggins parameter κPLA/benzene < κPLA/acetone < κPLA/

Methanol 15 98 18 Acetone 40 67 19 Toluene 35 106 14 Benzene 60 102 13

**Table 3.** Sensory test results for the presence of solvent vapours in the three nonwoven fabrics [7, 23]

98% PLA 4060D/2% MWCNT

**Sensory factor (%)**

Melt blowing Electrospinning Electrospinning +

97% PEO/3% MWCNT Carbon (precursor 15%

PAN/85% DMSO)

carbonization

Looked at from a technological viewpoint, this research demonstrates that all of the process parameters evaluated affect the properties of the manufactured products. Comparison of the response plots of produced nonwoven fabrics with chemical stimuli shows that the melt-blown nonwoven fabric made of polymer (98% PLA 4060D/2% MWCNT) respond differently than the nonwoven fabric made of polymer solution (97% PEO/3% MWCNT) and the carbon nonwoven fabric made of the precursor formed using a 15% PAN/85% DMSO solution.

Analysis has shown that differences in both the intensity and directivity of sensory properties are likely caused by the use of different technological parameters. This phenomenon may result from the structural arrangement of macromolecules and nanotubes affecting the formation of electroconductive tracks directed along the fibre axis. Too large a distance between one nanotube and another may cause a rapid increase in resistance. Finding the optimum draw ratio imposed during nonwoven fabric formation can lead to better positioning of them in the conductive network formed by well-oriented MWCNT. This implies that dispersed nanotubes

can be combined or separated for such nano and submicron-composite structures.

toluene < κPLA/methanol.

**Solvent**

270 Non-woven Fabrics

**Figure 2.** Changes in relative electrical resistance of nonwoven fabrics subjected to methanol vapours at 200 ppm: a) 98% PLA 4060D/2% MWCNT, b) 97% PEO/3% MWCNT, c) carbon (precursor 15% PAN/85% DMSO)

**Figure 3.** Changes in relative electrical resistance of nonwoven fabrics subjected to acetone vapours at 200 ppm: a) 98% PLA 4060D/2% MWCNT, b) 97% PEO/3% MWCNT, c) carbon (precursor 15% PAN/85% DMSO)

**Figure 4.** Changes in relative electrical resistance of nonwoven fabrics subject to toluene vapours at 200 ppm: a) 98% PLA 4060D/2% MWCNT, b) 97% PEO/3% MWCNT, c) carbon (precursor 15% PAN/85% DMSO)

**Figure 5.** Changes in relative electrical resistance of nonwoven fabrics subject to benzene vapours at 200 ppm: a) 98% PLA 4060D/2% MWCNT, b) 97% PEO/3% MWCNT, c) carbon (precursor 15% PAN/85% DMSO)
