Cellulose
images of the WP3W paper samples (waste paper with addition of 3% wheat bran) before and after degrada- tion are shown in Fig. 4. The apparent presence of both bacteria and fungi are noticed on the degraded paper surface. It was particularly evident in the outer layer, where fungal hyphae and spores are clearly visible. Swelling of the cellulose fibres caused by breaking bonds stimulated the microorganism’s growth and their further penetration toward paper bulk. It was especially noticeable in samples collected from agricultural soil, where the water reservoir was maintained at high level, and where degradation was more advanced. The mechanical properties of paper were measured on the experimental samples before and after degra- dation. Table 7 presents the progression of the break- ing length before testing, and after 2, 4, and 8 weeks of soil exposure for all investigated papers. The results obtained from samples before exposure confirm the positive impact of fillers on the mechanical resistance of paper products and corresponds to trends reported by other researchers (Nechita et al. 2010; Gonzalo et al. 2017). None of the samples were suitable for mechanical testing after 8 weeks of exposure due to excessive degradation. The highest degradation kinet- ics, as related to the breaking length loss, were observed on samples degraded in the agricultural soil. Mechanical properties had dropped significantly already after two weeks of exposure. According to Sridach et al. (2007) 50–80% of tensile strength is lost during the first week of burial process depending on the type of paper product and its composition. There are several thousands of paper types produced nowa- days by the pulp and paper industries. The paper products differs due to composition, formula, addi- tives, binders, fillers, retention agents, among the
others. However, the highest impact has a variation within cellulose mass used for paper making that is produced with variety of pulping, bleaching, sizing, strengthening, drying and/or coating processes. The raw resource used in this research was of recycled paper origin. The composition of fibers was therefore highly anisotropic as well as each resource batch may be different than another. In addition, printing resid- uals with other impurities introduce important dis- crepancy to the paper products derived. It affects also the degradation processes of papers when exposed to soil, making universal determination of the detailed degradation mechanism rather difficult. The effect of degradation duration on the NIR spectra of sample WP5W (waste paper with addition of 5% wheat bran) is shown in Fig. 5. Curves for most spectra bands differ from the control state (not exposed for degradation) after exposure for 4 weeks. Spectra are similar at 4 and 8 weeks exposure. This implies that partial decomposition of the examined paper products in forest soils had occurred by week four. The chemical decomposition of different papers can be compared with principal components analysis (PCA) derived from NIR spectra. Figure 6 presents a PCA plot for three series of spectra corresponding to samples at different degradation stages of the tested papers in forest soil. Samples before degradation are grouped together in relatively small cluster. The highest dispersion of spectra was observed at 4 weeks’ exposure, while tighter clustering was observed again after 8 weeks. The high degree of scatter observed at 4 weeks exposure, together with minor cluster over- lapping signifies a high heterogeneity within all tested papers as well as varied degradation kinetics, being dependent on the paper type.
Table 7 Breaking length (km) of investigated paper before and after soil degradation
Soil type
Before After
Forest
Sand
Agricultural
Degradation time 0
2
4
8 2
4
8 2
4
8
WP
2.85
1.37 0.38 0 1.59 0
0 1.00 0.12 0
WP3W
3.38
1.37 0.40 0 1.50 0.38 0 0.60 0.45 0
WP5W
3.40
1.39 0.72 0 1.56 0.64 0 0.78 0.42 0
WP3R
3.81
1.42 0.62 0 1.62 0.50 0 0.77 0.30 0
WP5R
3.88
1.45 0.85 0 1.69 0.80 0 0.83 0.62 0
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