To determine to what extent the structure of the nonwoven mulch, i.e., the fibers within the mulch structure, influenced the mass per unit area, thickness, air permeability, and tensile properties that should show the degradation of the mulch, an FTIR analysis was carried out.

In the 3600–3200 cm⁻¹ range, viscose fibers, primarily made of cellulose, display a distinct band indicating hydroxyl (OH) group stretching vibrations. Bands between 3000 and 2800 cm⁻¹ represent stretching vibrations of carbon–hydrogen (CH) bonds in cellulose’s aliphatic side chains, while the 1200–1000 cm⁻¹ range reflects CO bond stretching vibrations. Moreover, peaks at 1470–1420 cm⁻¹ indicate the bending vibrations of CH bonds in cellulose’s aliphatic side chains. These unique bands not only confirm the presence of cellulose, but also offer specific wavenumbers and intensity information that can indicate changes in viscose fiber degradation when exposed to environmental conditions [17,18]. A reduction in the band intensity at higher wavenumbers (3333 cm⁻¹ and 2915 cm⁻¹) in exposed viscose fibers suggests alterations or the loss of certain functional groups, leading to modifications in molecular vibrations during degradation. Slight changes are observed at lower wavenumbers (2916 cm⁻¹, 2730 cm⁻¹, and 2541 cm⁻¹), indicating the formation of new functional groups within the fiber structure. Minor changes at lower wavenumbers (below 1000 cm⁻¹) may be attributed to signals from other molecules in this range. Of particular interest is the region around 3000 cm⁻¹, where alterations in the hydroxyl band are linked to changes in the crystalline structure, indicating fiber degradation.

The analysis of viscose fibers from nonwoven mulches reveals a characteristic band at 2915 cm⁻¹, which remains unchanged after 30 days of exposure (Figure 5). However, prolonged exposure causes the band to change slightly, indicating a molecular structure change. Two additional bands at 2848 cm⁻¹ and 2730 cm⁻¹ are present in the 0 sample but disappear in the exposed fibers, indicating significant changes in molecular vibrations or functional groups. Further insight into the structural alterations comes from shifts in the band positions in the region between 1800 and 600 cm⁻¹. These shifts, attributed to variations in the hydrogen bonding scheme and changes in dihedral angles at the glycosidic linkage, reflect modifications in the bending and stretching vibrations of molecular bonds in cellulose fibers [17]. Additionally, bands around 1700–1500 cm⁻¹, corresponding to carbonyl groups and absorbed water, indicate the collapse of the cellulose structure over time, accompanied by an increase in water molecule absorption [19,20]. In a viscose FTIR analysis, the appearance of additional peaks at 1720 cm⁻¹, observed at 180 and 300 days, indicates significant structural changes. Shifts in the 1800 to 600 cm⁻¹ range suggest variations in hydrogen bonding and glycosidic linkage angles. The changes in the bending and stretching vibrations of molecular bonds in cellulose fibers, coupled with shifts around 1700–1500 cm⁻¹, signify the degradation process. Specifically, the 1720 cm⁻¹ peak highlights crucial alterations in the cellulose structure over time. Together, these FTIR spectral changes provide compelling evidence of the chemical transformations occurring in viscose fibers during the degradation process, shedding light on the dynamic evolution of their molecular and structural characteristics.

Cellulose serves as the primary structural element in bast fibers, such as hemp and jute, and it is accompanied by a smaller quantity of hemicellulose and various other non-cellulosic constituents, which include lignin, pectin, fats, waxes, water, pigments, minerals, and ashes, which are present in differing proportions. In the FTIR spectra of these bast fibers, we observe characteristic bands associated with cellulose, including OH stretching (around 3600–3300 cm⁻¹), CH stretching (around 2900 cm⁻¹), and CH₂ bending (around 1200 cm⁻¹) [21,22,23]. These FTIR profiles for hemp and jute fibers typically reflect distinctive chemical compositions and molecular structures, with band positions and intensities varying depending on factors like cultivar, agro-ecological conditions, fiber extraction, mechanical processing, and differences in the chemical composition and structure. It is crucial to note that bast fibers may also contain hemicellulose and lignin, potentially introducing additional bands in the FTIR spectra. The presence and intensity of these bands depend on the amount of pectin and lignin that is present. An interpretation of the characteristic bands of lignin and pectin requires care, as they may closely overlap. Lignin’s characteristic bands are typically found in the 1700–1600 cm⁻¹ range, signifying aromatic ring stretching vibrations, and at 900–830 cm⁻¹, indicating CH aromatic bending vibrations [21]. On the other hand, pectin’s bands may involve OH stretching in the 3600–3200 cm⁻¹ range and CO stretching in the 1200–1000 cm⁻¹ range. To differentiate between these compounds, it is essential to identify the specific functional groups associated with each band. For instance, lignin contains aromatic rings, leading to peaks related to aromatic CC bonds, whereas pectin contains carboxyl groups (COOH), and potentially yields bands in the 1750–1700 cm⁻¹ range. Lignin’s aromatic ring stretching bands are typically more intense and prominent compared to pectin’s bands [20].

Following exposure to field conditions, the FTIR spectra of jute and hemp fibers display significant changes in characteristic band positions and reduced band intensities, which are indicative of degradation processes and structural alterations, often attributed to the breakdown and loss of cellulose chains (Figure 6 and Figure 7).

In the case of hemp and jute, a decrease in intensity is visible in certain bands. For example, a decrease in characteristic bands associated with cellulose, including OH stretching, is visible, concerning the 0 sample. The intensity of the band decreases in samples that were exposed to external influences, which could suggest a move from highly crystalline to amorphous cellulose, which means there is an ongoing degradation of the material [20]. The most significant decrease in band intensity is evident in the FTIR spectrum of jute fibers after 300 days of exposure to field conditions, particularly at the 1030 cm⁻¹ and 1745 cm⁻¹ characteristic bands associated with pectin and lignin. This decrease could signify a reduction in the concentration of these chemical components [21,22,23]. Also, it is important to note that the appearance of bands around 1700 cm⁻¹ can indicate the presence of some impurities in the initial fibers, such as fats, waxes, and resins, which are particularly visible with hemp samples exposed to external influences for a longer time [20]. In addition, the materials that were tested had some impurities on them caused by exposure to soil, which the FTIR analysis picked up and presented as an increase in some bands, so any conclusions on the degradation of nonwoven material should be confirmed using other methods.

In the FTIR analysis of the nonwoven mulch made from PLA fibers before exposure, distinct bands were identified (Figure 8). These included a band around 3400 cm⁻¹, representing the stretching vibration of hydroxyl groups (OH), and a band at 1750 cm⁻¹, signifying the stretching vibration of the carbonyl group (CO) within the ester linkage of PLA. Additionally, there are bands around 2997 cm⁻¹, which are associated with the stretching vibration of CH bonds in the methylene (CH₃) groups of PLA, and bands around 1180 cm⁻¹, corresponding to the stretching vibration of CO bonds [24,25,26,27]. After 30, 180, and 300 days of fiber exposure to field conditions, minimal changes were observed in these characteristic bands, which were suggestive of minor alterations in the chemical structure but not proof of mulch degradation.

The characteristic band of cellulose-based viscose fibers, ranging from 3600 to 3200 cm⁻¹, results from the stretching vibrations of hydroxyl (OH) groups. Bands in the 3000 to 2800 cm⁻¹ range correspond to the carbon–hydrogen (CH) bond stretching vibrations in the aliphatic side chains of cellulose molecules, while the range from 1000 to 1200 cm⁻¹ is indicative of CO bond stretching vibrations. Bands in the 1470–1432 cm⁻¹ range are often associated with the bending vibrations of CH₂ and CH₃ groups in cellulose. Additionally, the band around 1735 cm⁻¹ is linked to the CO stretching vibration of acetyl groups (hemiacetals) in cellulose. Although these bands are typical for viscose fibers, those in the 2400–1200 cm⁻¹ range exhibit behavior that is more aligned with PLA fibers, suggesting a blend of the two. The band at 1750 cm⁻¹ corresponds to the stretching vibration of the carbonyl group (CO) in the ester linkage of PLA. The presence of these bands collectively suggests a combination of cellulose-based viscose fibers and PLA fibers in the nonwoven mulch (Figure 9). These results point to the heterogeneity of the samples, and the degradation of the blends itself is difficult to monitor. Taking into account the obtained results, it can be concluded that these materials contain all of the mentioned (and necessary) bands, and changes in their intensity (for example, bands between 3600 and 3200 cm⁻¹) may suggest the potential beginning of degradation. However, in most of the other characteristic bands and their intensity, there is no significant change, and it cannot be said with certainty whether degradation has occurred.

In blends of bast fibers with PLA, characteristic bands for cellulose in the region of 3600 to 3200 cm⁻¹ are visible, representing OH stretching, CH stretching (around 2900 cm⁻¹), CH₂ bending (around 1200 cm⁻¹), and CO stretching in the 1200–1000 cm⁻¹ range. The characteristic bands for PLA are also visible at around 2950 cm⁻¹, representing CH stretching, followed by 1750 cm⁻¹, corresponding to the stretching of the CO group and –CH– groups around 1400 cm⁻¹ for both samples: jute and hemp blends with PLA (Figure 10 and Figure 11). Again, these characteristic peaks indicate a homogenized sample for both the jute/PLA and hemp/PLA samples. The bands at 1748 cm⁻¹, 1651 cm⁻¹, and 1456 cm⁻¹ indicate characteristic vibrations in the jute and PLA blend with the 0 sample of jute/PLA. Some bands, such as those around 3341 cm⁻¹, 2919 cm⁻¹, and 1745 cm⁻¹, for the jute/PLA sample after 30 days, exhibit changes in absorbance compared to the 0 sample, explaining the increase in the tensile properties of the material. This trend of increased absorption due to increasing days of exposure of the mulch to field influences continues for the samples exposed for 180 and 300 days, confirming the changes caused by the exposure that led to an increase in the braking force. The hemp/PLA nonwoven mulch had more expressed bands, indicating the presence of lignin or other aromatic compounds, namely bands at 1748 cm⁻¹, 1604 cm⁻¹, and 1451 cm⁻¹, rather than the jute/PLA samples. The bands at 1358 cm⁻¹ and 1178 cm⁻¹ indicate features of cellulose and hemicellulose in hemp. The absorbance values also changed for several bands in the hemp/PLA sample after 30 days of exposure. For instance, the peak at 1749 cm⁻¹ (possibly from lignin) increased in intensity, suggesting that more lignin contributed to the higher breaking force of the material compared to the control sample. The peak at 1084 cm⁻¹ (PLA) remains intense, suggesting that PLA is relatively stable in the initial stages of exposure. As already mentioned before, the FTIR analysis confirms the presence of both fibers, and thus, a coherent blend between the two fibers. Judging by the results and their intensity in the FTIR analysis, it cannot be claimed that any degradation of the samples occurred. In addition, the presence of impurities on the material prevented an accurate reading and conclusions regarding the influence of the field conditions on the material.

The primary distinguishing feature of PE (polyethylene) is the strong peak associated with the stretching vibration of its methylene (CH₂) groups, typically in the 2916–2848 cm⁻¹ range. Another characteristic peak corresponds to the bending vibrations of the CH₂ groups, found around 1470–1465 cm⁻¹. Additionally, PE contains CH₃ groups, and their stretching vibrations result in a band at approximately 1375 cm⁻¹. The bands associated with the stretching vibrations of carbon–carbon (CC) and carbon–hydrogen (CH) bonds within the PE polymer chain are situated within the range of 1200–1100 cm⁻¹. These distinctive FTIR peaks in the PE agro foil spectrum arise from its polymer structure, which is composed primarily of long chains of carbon and hydrogen atoms [28,29]. An analysis of the FTIR spectrum of agro foil in this study indicates the absence of significant alterations in the FTIR peaks (Figure 12). This indicates that even after an exposure to environmental conditions of up to 300 days, no noticeable degradation in the PE agro foil has taken place, even though the mechanical tests suggest otherwise, namely a loss of up to 26% in the breaking force.

This behavior is consistent with the well-known chemical stability of PE, making it highly resistant to various field conditions. Furthermore, the band at around 1050 cm⁻¹ indicates the contamination of the material by the soil, which makes an accurate measurement impossible.