Cellulose (2021) 28:5775–5791
5781
tensile strength was analyzed first as a reference. It should be noted that paper without any wet strength- ening is intrinsically very weak in the presence of water and it is therefore challenging to analyze such wet paper sheets in a conventional tensile testing device, as such measurements generally lead to significant errors in the range of 10–13% (SD) for eucalyptus-sulfate paper with a grammage of 80 g m - 2 . While the influence of grammage or fiber type was not the focus of the present study, this may well be addressed in a follow-up work. Figure 3 shows examples of tensile curves in (a) the dry and (b) the (aqueous) wet-state, respectively, for paper samples that have been modified and cross- linked with the same copolymer from different solvents mentioned in the figure. The dry and wet tensile index, as derived from these measurements using Eq. (2) is shown in Fig. 4.
solvent in conjunction with copolymer treatments of paper in previous studies (Jocher et al. 2015). How- ever, while it was possible to prepare paper samples with high wet tensile indices with the copolymer dissolved in H 2 O, we also became interested in the use of other solvents in combination with copolymer immobilization. We were particularly interested in the use of solvents, that don’t lead to significant fiber swelling, in comparison with water, which is known to swell paper fibers. As model paper sheets eucalyptus- sulfate paper samples with a grammage of 80 g m - 2 were prepared and impregnated with the respective copolymer solution in the chosen solvent. In order to prevent any mechanical agitation of the cellulose fibers during treatment with the copolymer solution, an impregnation approach was used in this work. This also enables the analysis of the diffusion process of the copolymer in the different solvents throughout the fiber network and the fibers themselves, without any influence from external forces. A sieve with thin Teflon-yarn was designed for non-contact drying, to prevent further influence on the spatial distribution by capillary or gravitational forces. The copolymer modified paper sheets were illuminated by UV-light in order to cross-link the copolymer inside the sheet. Non-bound macromolecules were removed by solvent extraction. The amount of paper sheet attached, cross- linked polymer was determined by gravimetric anal- ysis and was typically on the order of 10 wt% in relation to untreated paper samples (see supporting information for details). After cross-linking and extraction, respectively, we first investigated possible failure-mechanisms by time-resolved tensile micro- scopy. Secondly, the macroscopic tensile properties in the dry and the wet state were determined via tensile strength analysis and the spatial distribution of the macromolecules inside the fiber network was finally analyzed in detail using confocal laser scanning microscopy (CLSM).
Failure mechanisms of functionalized paper sheets in the wet state
It is believed that the main failing mechanism of cellulose paper in the wet state is governed by fiber pull out. This would be intuitive, because in the wet state the fiber–fiber-bonds are weakened, and the fibers themselves become more flexible provided that the fibers do swell significantly. The failure of single fibers, on the other hand, should occur less frequently, but should be observable nevertheless (Siqueira 2012). In order to get a better understanding of the failure- mechanisms for the eucalyptus polymer-modified paper samples used in this study here, an experimental setup for observing single cellulose fibers during the tensile straining was assembled. Paper sheets, where 0.1 wt% of all fibers have been stained with fluo- rophore Pergasol Yellow F6-GZ , were analyzed in a time-resolved fashion. Figure 5 shows snapshots of tensile-test videos taken from polymer-modified paper sheets. Note, the complete video sections can be found in the supporting information. As shown in the first example in Fig. 5a, we frequently observed tearing of single fibers, while being under significant bending load. By looking at the combined length of the fiber fragments and comparing it to the initial fiber length, it becomes apparent, that the fiber-wall delaminates (Fig. 5a). Delamination of the fiber cell wall has been shown to occur in other eucalyptus paper samples with the help of electron
Tensile properties
The tensile properties of paper sheets are very sensitive to a wide variety of environmental factors, e.g. humidity, temperature, UV-exposure, swelling and drying. Since the copolymer-application includes multiple swelling and drying cycles in addition to significant UV-exposure, the influence of this treat- ment without the copolymer on the tensile and wet
123
Made with FlippingBook Online newsletter maker