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Journal of Material Cycles and Waste Management (2025) 27:1901–1913
numerous cycles, Table 3. There is a good chance that the composition of UBKP can withstand the effects of a refin- ing activity, which manifests itself as a slight shortening of the fiber length. However, fines continued to be produced, which led to a reduction in freeness. In contrast, it was noted that SCHW had a much more significant loss in freeness versus cycle number than the UBKP. When compared to UBKP (coarseness equal to 0.42 mg/m, Table 1) SCHW is a short, thinner-walled fiber (coarseness equal to 0.27 mg/m) that is expected to be less resistant to the refining action. This is evidenced by the fact that relative to the unrefined SCHW, the SCHW after five recycles experiences a loss in fiber length of approximately 30% and an increase in fines of approximately 100%, both significantly greater than the UBKP. Wheat straw exhibited an increase in its freeness results on the first recycle loop and continued to increase even though the total amount of fines was significantly higher, Table 3. This unexpected result can be explained by an observation that fines were flowing through the screen mesh at the beginning of the freeness test before a fiber mat was formed. This may not be of practical consequence since this phenomenon was observed at very low freeness values and low wet tensile strengths of the webs which are not practical for papermaking. Table 3 shows the WRV for each type of fiber during the unrefined and recycled cycles. It was observed that the WRV of all pulps followed the same pattern, which was that it dramatically increased due to the refining process of virgin fiber at cycle 0 as expected creating swollen fiber walls, then it significantly decreased after recycling for the first recycling loops, and then was practically constant after subsequent cycles. The decrease in WRV after drying is well known as the hornification process in which the pores of the swollen fiber are irreversibly closed during drying resulting in stiff low porosity fiber walls and less conformable fib- ers. It is interesting to note that WS pulp after refining has a higher WRV than wood pulps, Table 3. This is probably due to the extensive generation of fine content for the wheat straw. Fines are known to have much larger WRV relative to their mother fibers, approximately 2.5 times higher [22]. In addition, we found a strong correlation between WRV and fines content in this study, as shown in the Supplemental Information. Figures 4 and S3 present scanning electron micrograph (SEM) images of the pulps after being subjected to refin- ing and recycling. In comparison to the fibers that have not been refined, it is observed that UBKP and SCHW maintain a satisfactory fiber shape after being subjected to a number of cycles of recycling and refining. When refining is applied to the UBKP and SCHW pulps a denser sheet structure is observed over the repeated recycle loops. The increased sheet density is reported in Table 4.
Additionally, fibrillated UBKP and SCHW fibers can be seen in every recycling cycle as small fibrils separated from the fibers. SEM images of handsheets made from WS pulp are also shown in Figs. 4 and S3. As was to be anticipated, the sheet structure of unrefined pulp displays a dense struc- ture in accordance with a high value of sheet density in Table 4. This is because the unrefined pulps are composed of short, thin-walled fibers. SEM images show very con- formable fibers and a gel-like continuous phase in some areas resulting in a smooth-appearing sheet. For the WS after refining the virgin pulp, the fibers are significantly damaged and broken as seen in the SEM images. It may be that the WS fibers are especially suited for glassine or other fiber-based low porosity, high liquid barrier types of paper films. WS density increases on the first loop and then reduces somewhat, Table 4. It is important to note that in these experiments there was an intentional allowance of fiber/ fine loss, and the surviving pulps imaged in the SEM after a recycle loop do not include the lost material. Table 4 shows some physical properties of the pulps versus number of recycles. There is a significant decrease in properties for the first cycle (from cycle 0 to 1) and then the properties stay somewhat constant. This is simi- lar to other literature results [6]. However, the handsheet properties reported are those comprising only the surviv- ing fibers. It is important to note that these constant high physical properties versus recycle number are reflective of a process that removes degraded fines and smaller fibers on each cycle, similar to what actually occurs in industrial recycling. If these degraded fines and fibers were retained, it would be expected that the paper properties versus recy- cle number would have a different relation. The relationship between the tensile index and the sheet density of unrefined and recycled handsheets made from UBKP, SCHW, and WS are plotted in Figure S4. There is a robust linear relationship between those two parameters, which represents an R-square value of 0.81 or higher for every material. It also appears that the softwood fibers fall on one line and the hardwood and WS fibers cluster about different lines. The results indicate at a common density that the softwood in all cases produces a higher tensile index. For example, at a sheet density of 600 kg/m 3 , one might anticipate that the tensile index of UBKP would be 70.2 Nm/g, greater than the tensile index of SCHW, which is 40.0 Nm/g, and the tensile index of WS, which is 35.8 Nm/g. Presumably this is because if the thicker stronger individual softwood fibers are formed into a matrix of the same density as the shorter/thinner walled fibers that the bonded area must be large and the capabil- ity of the larger fibers to withstand stress forces greater.
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