The overview of material flow representing manufacturing of design-for-purpose packaging products is presented in Fig. 2. The transition from open loop to closed loop systems is important in order to assure sustainable use of resources and the economic viability of modern bio-based industries. Production of packaging containers proposed here allows valorisation and utilization of waste generated during diverse industrial processes. This approach is in line with the industrial symbiosis concept, where studies regarding flow of materials between industries leads to creation of opportunities to use “waste” from one industry as a raw material for another (Cheshire 2016). In this case fit-for-purpose paper packaging containers are manufactured by combining resources generated by pulp and paper with milling industries. Increasing the recycling and reuse rates of virgin wood fibres leads to increased availability of this resource for other use. However, any recycling round requires a certain amount of virgin paper input, usually from 20% to 95% (Villanueva and Wenzel 2007). Incorporation of cereal bran into packaging products allows minimizing virgin wood fibre inputs while maintaining (or improving) the required properties. The set of expected product characteristics can be optimized by adjusting proportion of recycled fibres, virgin fibres and bran (Fig. 2). Moreover, it solves a costly disposal problem from industrial mills by providing a solution for cereal processing by-products use. Finally, control of the biodegradability rate of manufactured products allows predicting their use phase performance and duration of deterioration.
Effect of the bran additives on the properties of investigated paper products before degradation
The effect of additives on the chemical composition of manufactured paper products is summarized in Table 3. The effect was minor, but for all analysed components the differences between means were statistically significant (ANOVA, p < 0.05). The most noticeable differences are in cellulose content (88.6–91.2%). Cellulose content in control samples was highest and with increased amount of bran (both types) diminished. It was a direct effect of dilution, as the amount of cellulose is low in bran when compare to the pulp and paper. The cellulose content in bran as reported in literature ranges between 5 and 13% (Kamal-Eldin et al. 2009; Chalamacharla et al. 2018). In fact, such a high variation in composition within both wheat and rye brans is influenced by the cereal species, provenance, batch and milling techniques applied for flour production. The chemical composition of brans used in this study was researched by Modzelewska and Adamska (2006) and is summarized in Table 4.
The differences in extractives content augmented with the bran share increase.
It is an additional dilution effect of mixing components present in waste paper and bran. The bran itself contains several solvable substances that are contributing to the overall content of extractives. These, beside of cellulose include ash, dietary fibre, proteins, starch and diverse phytochemicals, among the others (Onipe et al. 2015). The relatively high amount of starch in cereal bran (estimated to be 15.8–18.9% of the dry mass) simplifies the coupling of cellulose fibres and paper forming as well as aids in filling micro-pores. However, as previously reported by the authors, the presence of starch in paper products may advance the degradation rate as it is a favourable breeding ground for bacteria and microfungi (Sandak et al. 2011).
NIR analysis of investigated papers with addition of the rye and wheat brans did not reveal any noteworthy differences between spectra due to presence of bran fillers (Sandak et al. 2011). Only slight variations were noticed for CH, CH2 and OH functional groups assigned to cellulose and holocellulose.
The influence of bran additives on selected paper properties is summarized in Table 5. The breaking length of papers with bran additives is about 25% higher than control samples. It can be explained by a positive effect of the filler increasing the number of connections between paper fibres and therefore the degree of bonding (Retulainen and Ebeling 1993). In this case the bran particles take part in the hydrogen bonds promoting consolidation of the paper structure. It was also noticed that the effect of mixing fibres with bran additions was not following the linear rule of mixture (Karlsson 2007). The breaking length increase was not significantly higher in 5% bran content compare to 3%. The breaking length was highest for paper products with rye bran.
The addition of bran seems to have rather casual influence on the paper extensibility, even if increased share of bran slightly augmented extensibility value. Conversely, tearing resistance diminished with increase of the bran content. It was expected as tearing is usually inversely correlated with the tensile strength and breaking length (Caufield and Gunderson 1988). The reduction of tearing can be explained by the fact that addition of bran particles affects fibre–fibre bonding promoting pulling up of fibres out the network (Yu 2001).
The burst resistance index, frequently used to determine the quality of paper, does not depend on the kind, but rather on the amount of filler introduced. It is lowest for the paper without additives, and slightly increases with the added bran content. The trend corresponds to that expected as the burst resistance is as well correlated to the tensile strength (Caufield and Gunderson 1988).
Air permeability and absorbance increased in papers with additives. Both properties might be desirable when increased barrier properties against water are required. In the case of waste paper, adding bran leads to early disassembly of the structure of the paper while exposed to degradation; consequently, affecting the air permeability. This peculiarity has a beneficial effect for paper pots, where high air permeability improves the natural ventilation of the root system and stimulates plant growth (Nambuthiri et al. 2015; Akelah 2013).
The results of mechanical tests for the compression strength of paper pots before degradation are presented in Table 6. The lowest crushing strength was observed for commercial pots. Pots without additives (WP) possessed the highest mechanical resistance among all laboratory-manufactured products and any bran addition lowered the crushing strength. It was also found that paper pots with added peat were brittle, as did not retain any strength after first compression cycle. Conversely, pots manufactured from waste paper with added bran were elastic, maintaining integrity after several cycles.
Changes to paper due to biodegradation in soil
Degradation processes take place in the natural environment constantly and on a large scale (Pagga 1999). Biodegradation in soil is an important end-of-life option for bio-based materials used in agricultural applications. The rate of degradation can vary significantly, depending not only on the molecular structure of the material, but also on soil characteristics and conditions (temperature, water and oxygen availability which influence microbial activity) (Briassoulis et al. 2014). Biodegradation occurs in two steps. First, the polymers are fragmented into lower molecular mass by means of abiotic reactions (oxidation, photodegradation or hydrolysis) or biotic reactions (degradations by microorganisms). Then the polymer fragments are assimilated and mineralized by microorganisms (Vroman and Tighzert 2009).
The effect of soil type on the biodegradation of investigated papers was analysed by means of NIR spectroscopy. Figure 3 presents an example of degradation progress, where the evolution of the spectra acquired after degrading sample WP5W (waste paper with addition of 5% wheat bran) in various soils is presented. All peaks mentioned in Table 2, with exception of region 5464 cm−1 (7) were affected by the degradation process. However, the spectra of paper placed in a sandy soil seems to be most similar to the control samples. It demonstrates that the sandy soil containing the lowest organic content and persistent low humidity has the lowest impact on the speed of degradation, as in Mostafa et al. (2010). In contrast, the agricultural and forest soil accelerated the degradation speed.
Analyses of the chemical composition of the papers after degradation in agricultural soil for 8 weeks are summarized in Table 3. Cellulose changed considerably after degradation, including quality and quantity alterations, as observed also by Shogren (1999). The amount of cellulose decreased in all investigated cases, where major changes (over 12%) were observed for paper with rye bran and with 5% of wheat bran. Conversely, the cellulose polymerization degree PD drop was slightly higher for papers without any bran additives. The content of extractive components was higher after biodegradation, especially in the case of extraction in 1% H2SO4. It can be explained as a result of constitutive polymers degradation due to hydrolysis and biotic factors (Witkowska et al. 1989).
Microscopic analysis allows the assessment of changes in the micro structure of cellulose fibres as results of the degradation. Selected SEM microscopic images of the WP3W paper samples (waste paper with addition of 3% wheat bran) before and after degradation 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 degradation. Table 7 presents the progression of the breaking 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 kinetics, 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 nowadays by the pulp and paper industries. The paper products differs due to composition, formula, additives, 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 residuals with other impurities introduce important discrepancy 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 overlapping signifies a high heterogeneity within all tested papers as well as varied degradation kinetics, being dependent on the paper type.
The vulnerability of the paper to degradation in soil was assessed by comparing the expected time to complete paper degradation. These degradation ratings for both sheets and pots, are summarized in Fig. 7 for all examined samples. Values were determined by visual estimation according to expert persons as the number of days until a terminal state would be reached. Disintegration of the paper sheet or decomposition > 75% of the paper pot was considered as a terminal state.
The lowest rate of degradation is expected when paper is exposed to sandy soil, where the degradation process is estimated to take between 60 and 70 days (depending on paper composition) (Fig. 7a). A higher degree susceptibility to degradation was observed for samples exposed to the forest soil, regardless of paper type; it was estimated that complete biodegradation occurred in 40–50 days for all paper types in forest soil. Agricultural soil was considered to be the most aggressive because complete paper destruction was expected after only 30 days (compared to approximately 60 days for sandy soil). It corresponds to the previous studies where Tumer et al. (2013) reported that changes related to the decomposition were most intense in organic soil, when compared to sandy soil The presence of fillers reduced life of the paper products, despite products with 5% bran filler being observed to be more resistant to biodegradation than those with 3% bran filler.
The estimated biodegradation rates of pots were similar to those of paper sheets (Fig. 7b). Commercial pots were found to decompose more rapidly than the products manufactured in the laboratory. This was due to the relatively large share of additives that increase water absorption and hasten decomposition. The laboratory-made pots with rye bran filler decomposed slightly faster than other samples. Therefore, it might be possible to adjust the degradation rate of waste paper by adding specific fillers and to deliver products most suitable for specific applications.