Given their origin and structure, natural and viscose fibers have different mechanical properties. Bast fibers form naturally, giving them distinct features, whereas viscose fibers are created purposefully, allowing their attributes to be regulated. Viscose fibers are more flexible and stretchable, while natural fibers, especially hemp, have high strength and rigidity. Artificial fibers differ from natural ones not only in structure and mechanical characteristics but also in a number of other characteristics. The concentration of alpha cellulose, resins, and fats; the amount of adsorbed moisture; and the degree of polymerization were all measured for each fiber sample in compliance with the criteria; the findings are shown in Table 1.

During pulping, low-molecular substances are removed due to their solubility in an alkaline medium, and fatty acids, esters, and waxes are saponified and hydrolyzed. In the later stages of processing, a decrease in pH can cause precipitation of these compounds on the fibers [61]. During the spinning of viscose fibers, heteropolymers are precipitated on their surface during coagulation [62]. The precipitated compounds degrade the quality of the fibers, reducing the interfiber interactions and mechanical properties due to their hydrophobicity.

Viscose fibers are spun from dissolving pulp, which has strict requirements regarding the content of organic and inorganic impurities. In natural fibers, on the other hand, various cellulose satellites may be present, and their quantity is determined by the place of growth and processing conditions. The content of inorganic impurities in the fibers used to form nonwoven materials is presented in Table 2.

The results of the elemental analysis show that the metal content depends on the type of fiber. Natural fibers are characterized by a high content of calcium and magnesium, while viscose fibers are dominated by sodium and zinc, which is due to the technological features of their production. These differences affect the properties of the fibers. For example, an iron content above 8 ppm worsens thermal stability, and sodium in cellulose should not exceed 85 ppm. Particular attention should be paid to magnesium; its content in natural fibers is 10 times higher than in viscose fibers, where it corresponds to the recommended level (up to 12 ppm). It is important to note that the chemical composition of the fibers [63,64] and crystallinity/crystal size [65,66] significantly affects the mechanical and functional properties of the resulting materials.

Since viscose and hemp (flax) fibers are utilized to make nonwovens that may be employed not only as filters, insulating materials, and so on but also as precursors to carbon nonwovens, the thermal behavior of such fibers was investigated as a first step.

Figure 1 shows the TGA and DTG thermograms for individual fibers, which are similar in nature. The curves can be divided into three main sections. In the first section, up to 30–180 °C, 4–6% of the adsorbed water mass is removed without signs of thermal destruction. In the second section, starting at 200 °C, there is a sharp loss of mass for all samples, with two competing reactions occurring: dehydration and depolymerization [67]. For viscose fibers, the destruction processes begin at lower temperatures compared to natural fibers. In the last section, in the range from 400 to 1000 °C, a gradual decrease in mass is observed, caused by the loss of oxygen and hydrogen.

To study the thermal behavior of the compositions, the required amount of flax or hemp fibers was added to the volume of viscose fibers, then the mixtures were mechanically mixed. As a result, more or less homogeneous systems of fibers of different nature were obtained with a random distribution of native cellulose I (natural fibers) in the matrix of cellulose II (viscose staple). The results of the thermal study of mixed compositions of different concentrations are presented in Table 3 and Table 4.

Table 3 shows how the pyrolysis onset temperature, carbon yield, and peak thermal decomposition rate change, depending on the composition of the samples. For example, flax and viscose have similar carbon yield values, but these values can change when the ratio of natural and manmade fibers in the mixture changes. Similar thermal properties of compositions containing hemp were also obtained and studied (Table 4).

The thermal analysis data for systems with different ratios of artificial and natural fibers are presented as concentration dependencies for visualization (Figure 2 and Figure 3).

The thermal behavior of viscose fibers starts to alter when natural fibers are added. Figure 2a illustrates how the percentage ratio of the constituents affects the water content in samples of viscose and natural (flax and hemp) fiber mixtures. Viscose fibers have the most water (about 6%), while flax and hemp fibers contain less water (3.5–4%). For up to 40–60% natural fiber concentration in mixtures, slight variations in the moisture content are seen when natural fibers are added to viscose fibers.

In mixtures with a predominance of natural fibers, the total water content decreases. Structural differences between flax and hemp fibers, as well as different chemical composition, affect the values of the total content of adsorbed moisture in the system. For mixtures with hemp fibers, higher values of adsorbed moisture are observed compared to systems with flax fibers of the same concentration.

The concentration dependence presented in Figure 2b allows us to analyze the effect of replacing viscose fibers with natural ones on the amount of carbon yield. Adding a tiny amount of natural fibers (up to 30%) to viscose results in a significant drop in carbon yield. However, increasing their concentration in the mixture leads to the creation of carbon material with a higher mass. At the same time, changes in the amount of carbon yield are almost completely independent of the type of natural fibers used, with the nature of the dependence dictated solely by the concentration of additional flax or hemp fibers.

The temperature of the onset of pyrolysis depends on the structure of the fibers and the presence of impurities in them. Bast fibers have a higher degree of ordering than viscose fibers. Therefore, with an increase in the proportion of such fibers in the system, an increase in the values of the temperature of the onset of pyrolysis is observed. Flax fibers have a lower content of calcium and magnesium compared to hemp. Hence, one can expect a greater catalytic activity of impurities, which is accompanied by a decrease in the temperature of the onset of pyrolysis [68,69,70]. When the content of bast fibers is higher than 70%, the observed difference in the temperatures of the onset of pyrolysis tends to be the maximum.

Viscose fibers contain small amounts of Ca, Mg, and Fe compared to hemp and flax [68,71,72,73]. However, the sodium content is almost an order of magnitude higher compared to bast fibers. It is possible that low-purity water was used in the process of spinning viscose fibers. Interestingly, sodium can also affect the thermal behavior of cellulose fibers and, as shown in [68], can promote accelerated decomposition, i.e., the thermal decomposition temperature of the polymer decreases. Thus, the introduction of flax and hemp into viscose fibers affects the temperature at which pyrolysis begins, shifting it to higher temperatures.

Figure 2c shows the dependence of the pyrolysis onset temperature on the flax and hemp fiber content in the blended composites. At the beginning, with a low flax and hemp percentage, the decomposition temperature remains nearly constant. As their concentration rises, a gradual temperature rise is noted, which becomes notably apparent at 80%. According to the results of the quantitative chemical analysis (Table 2), hemp has more calcium and magnesium concentration than flax, indicating better catalytic activity and a lower pyrolysis initiation temperature [68,69,70]. This is consistent with the graph, which shows that the pyrolysis initiation temperature for hemp remains lower. At the same time, viscose has significantly fewer catalytic elements (Ca, Mg, and Fe) than hemp and flax [68,71,72,73]. However, it contains an order of magnitude more sodium, which can contribute to accelerated decomposition. Sodium acts as a catalyst for cellulose pyrolysis, reducing its decomposition temperature. Thus, the introduction of these natural fibers into viscose increases the thermal stability of the material, with flax having a more pronounced effect on this indicator compared to hemp.

From the point of view of developing high-temperature heat-protective materials, natural fibers have greater heat resistance than viscose fibers. The rate of water mass loss is similar across compositions and water concentrations in fibers (Figure 3a), with a range of 79–85 °C. This indicates that adsorbed moisture is removed from the fiber surface. That is, the first stage of heating (up to 30–180 °C) occurs under the same conditions. This means that water adsorbed on the surface of the fibers is removed by similar mechanisms, regardless of the type and proportion of fibers in the system. The next section of the thermogram corresponds to the pyrolysis of cellulose. The maximum rate of mass loss in viscose is 25% lower compared to flax and hemp fibers (Figure 3b).

At a low natural fiber content, the rate of decomposition remains relatively low, with flax and hemp samples behaving similarly. With an increase in the proportion of natural fibers (flax or hemp), the maximum rate of mass loss increases, which is consistent with the previous graphs, where viscose “softens” the pyrolysis process. After exceeding the threshold of about 60% natural fibers, a sharp increase in the maximum rate is seen, with this jump being slightly more pronounced for flax than for hemp. Thus, Figure 3c illustrates the same trend: viscose at low proportions of flax or hemp reduces the rate of decomposition, but with a predominance of natural fibers, the pyrolysis process accelerates significantly, with flax demonstrating a sharper increase than hemp.

Thus, the results of thermogravimetric analysis (TGA) show the influence of the fiber type and its concentration on the thermal characteristics of the composition. It is interesting to note that, in polymer composite systems, the concentration range of 40–60% is a boundary region, where it is difficult to predict the properties of the resulting composition [74,75], and a similar situation is observed in the presented mixed systems. Systems containing flax (hemp) and viscose in the concentration range of 40–60% exhibit unstable thermal behavior (“worrying region”). A careful approach is required to obtain carbon nonwoven materials based on precursors with such a ratio of components.

In natural cellulose tows, the fiber length does not exceed 25 mm, which does not allow obtaining a nonwoven fiber with a dense mesh of interlocks. During thermal treatment of such materials, the frequency of the mesh of such interlocks can decrease, which is associated with a decrease in the length and thickness of cellulose fibers. Therefore, the formation of precursor nonwoven materials based on tows is difficult. Their reinforcement requires the presence of long fibers, which are viscose fibers. Based on this, the optimal concentration of tows in the system is 70–80%, and viscose fibers no more than 30%.

When developing new cellulose-based materials, it is important to consider the conditions of their thermal decomposition, which is especially important for biocomposites, packaging materials, heat-resistant fibers, and textiles based on them. When thermally processing such products, it is necessary to take into account not only the temperatures of the onset of pyrolysis, etc. but also the activation energy of pyrolysis. This allows choosing the optimal temperature conditions for the carbonization of nonwoven and other precursors. The Kissinger method, due to its simplicity, has become widely used to determine activation energies [76]. The Kissinger mathematical model is applied to irreversible reactions occurring during heating at a constant rate and is based on dynamic thermal analysis, such as differential scanning calorimetry (DSC) or thermogravimetric analysis.

A series of experiments was carried out with different heating rates of 5, 10, and 20 K/min for two samples (viscose and flax) and their composition (50% viscose and 50% flax). Since the thermal behavior of flax and hemp fibers is similar (Figure 1), it can be assumed that the kinetic parameters of their thermal decomposition will be the same. Figure 4 shows the TGA and DTG curves of flax, viscose, and their mixture of fibers. The temperature range of pyrolysis and the temperatures of the peaks T_p for each heating rate, determined from the first derivative of the TGA curves, were used to estimate the activation energy of thermal decomposition. According to [47], these values allow not only to estimate the energy required for the reaction but also the possible nature of the resulting product. The corresponding temperature values at which the maximum reaction rate is achieved (the peak on the DTG curve) are presented in Table 5.

As the heating rate (β) increases, the peak temperature (T_p) increases for all materials. Flax shows the highest T_p values among all the samples, while viscose shows the lowest. This difference is due to the fact that flax consists of cellulose I, which has a higher level of crystallinity, unlike viscose, which is a regenerated cellulose with a lower degree of crystallinity, making it more prone to thermal decomposition at lower temperatures [27]. The composite fiber (50% flax/50% viscose) has intermediate T_p values.

Using the obtained data, a graph of the dependence of ln(β/T_p²) on 1/T_p was then constructed, and the values of the activation energies E_a were determined from the slope of the line (Figure 5, Table 6).

The study [46] focused on the kinetics of the pyrolysis of cellulose fibers. Using a modified first-order model based on TGA data, the authors analyzed the pyrolysis of two types of fibers: natural cotton yarn and mercerized cotton. The technique allows extrapolating the data to a temperature approaching zero, which ensures the determination of the “internal” activation energy of pyrolysis. The obtained values are about 153 ± 2 kJ/mol for natural cotton yarn and 192 ± 7 kJ/mol for mercerized cotton, indicating the influence of differences in the chemical composition and structure of the fibers on their pyrolytic reactivity. Van de Velden’s studies show activation energy values of 54.1–86.4 kJ/mol for the general process of biomass pyrolysis [54]. The works [49,50,51,77,78,79] reported activation energies of cellulose pyrolysis in the range of 135–223 kJ/mol. It is noted that the obtained parameters depend on the measurement method used and the type of starting material. Thus, the values found for mixed systems based on viscose and natural fibers correlate with the values previously described in the literature for cellulose and fibers based on it.

The activation energy of the thermal decomposition of viscose can be increased by adding a material with higher ordering (crystallinity), such as flax. For a system containing 50% flax and 50% viscose fibers, the thermal behavior was studied, and the activation energy (E_a = 167.01 kJ/mol) was calculated, which is lower than the values for flax-based systems and higher than the values for viscose-based systems. Adding flax to viscose fibers changes the thermal behavior of the system with increasing activation energy values. By adjusting the concentration of flax in the system, it is possible to change E_a, i.e., the system becomes more resistant to thermal decomposition and allows obtaining materials with the required structure (network of interlocks) and mechanical properties.

Another way to modify and stabilize the pyrolysis mechanism and, accordingly, increase the activation energy is to use suitable chemicals such as impregnations or fire retardants that accelerate and catalyze dehydration reactions in order to reduce weight loss, improve yields and properties of final products, and achieve higher efficiency [80]. For example, the work [81] studied the efficiency and effect of ammonium sulfate on the oxidative thermal stabilization of activated fiber based on viscose fiber. The fibers are pre-impregnated with an ammonium sulfate solution, which promotes their stabilization before carbonization and subsequent activation. As a result of the treatment, significant physicochemical transformations are observed: the thickness and linear density of the fibers decrease, the color changes (from white to black), the crystallinity of the cellulose decreases, and the transformation of the primary hydroxyl groups into carbonyl groups is recorded, which makes them suitable for the production of carbon fibers. A more environmentally friendly way, without the use of chemical reagents, is to add reinforcing fillers [82]. The introduction of nanocellulose fibers or graphene nanoparticles, which strengthen the structure of the material, or the addition of lignin or polysaccharides, which form a more stable network inside the viscose [83,84]. These studies provide a good understanding of the energy barriers of cellulose pyrolysis and confirm that the temperature and chemical properties of the starting materials play a major role in the thermal processing of carbon fibers.

The current study did not involve the use of pyrolysis catalysts and fire retardants. Therefore, in the future, it is of interest to study the thermal behavior of nonwovens based on viscose and natural fibers with their preliminary treatment with special compounds. Such treatment will increase the observed values of carbon yield and possibly affect the activation energy of pyrolysis.