Cellulose (2021) 28:5775–5791
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The latter may also influence the macroscopic wet- strength. As we will show in this contribution, taking a closer look at the spatial distribution inside the fiber network and the fibers themselves can improve our under- standing of the mechanisms involved in dry and wet strengthening of paper. In particular, as will be shown, fiber swelling leads to very different deposition mechanisms of a polymeric wet-strength agent and therefore affects largely the macroscopic tensile strength in wet conditions. We compare the treatment of paper samples with a light-sensitive copolymer (Toomey et al. 2004; Janko et al. 2015; Jocher et al. 2015; Bump et al. 2015) using an impregnation process with three different solvents. The solvents were chosen based on literature-data of cellulose fiber swelling (El Seoud et al. 2008; Karppinen et al. 2004) because we were particularly interested in the use of solvents, that don’t lead to significant fiber swelling, in comparison to water. The latter is known for its capability to significantly swell paper fibers thereby altering the morphology. The paper topology includes pores or grooves in which surrounding polymer molecules can be taken up during impregnation. On this basis water, 2-propanol (IPA) and 1-butanol (BuOH) were used to apply the copolymer to lab- made paper samples. By using confocal laser scanning microscopy in combination with tensile measure- ments, we study how this difference in swelling leads to different spatial deposition of the copolymer inside the paper samples and, ultimately, controls dry and wet paper sheet tensile strength.
the cellulosic fiber (homo-cross-linking additives). The other category are additives that are (also) able to form covalent bonds with the cellulose chains, so called hetero-cross-linking, like polyamidoamine epichlorohydrin (PAE). An important difference in comparison to the aforementioned formaldehyde resins is that cross-linking occurs between the poly- meric additive as well as between the additive and the fiber, respectively, thereby acting towards wet- strengthening of the sheet through a reinforcement mechanism. Finally, such reinforcement additives can be of polar nature, and may even swell in water, which is necessary for applications, where paper-water contact is crucial, such as with hygienic paper grades. Note, although there are clear evidences for these postulated mechanisms, there are still a number of open and fundamental questions that have not yet been answered to validate these models or to even pre- calculate which polymer-intrinsic and/or paper-re- lated parameters determine the wet-strength action of (pre-)polymeric additives. For example, with respect to the latter, knowledge on the exact spatial distribu- tion of the wet-strength agent in the non-woven sheet becomes crucial. Mangiante et al. (2018) prepared paper samples with alkyne-functionalized cellulose fibers, cross-linking them with difunctional PEG chains, leading to significantly improved wet tensile strength. They furthermore analyzed the spatial distribution via Raman confocal microscopy and found, that the fiber core had rich amounts of the alkyne. However, these observations weren’t further discussed with regard to the effect on the macroscopic wet strength. In previous work of our own groups, we used for the first time polymers that can cross-link within paper sheets under the influence of light (Jocher et al. 2015; Bump et al. 2015). We were able to show that this treatment increases the macroscopic tensile strength, and we analyzed the binding of the polymers to the fibers by Raman spectroscopy and imaging via confocal laser scanning microscopy, respectively. However, a systematic analysis of the spatial distri- bution of strengthening agents in the cellulosic fiber network and their impact on dry and wet strengths is still missing. Finally, it is not yet understood how fiber swelling may affect the interaction of the fiber with the polymeric additive and how the latter leads to different deposition scenarios of the polymer on/in the fiber.
Experimental section
Materials
All chemicals and solvents were purchased from Merck, Alfa Aesar, Alberdingk Boley, Fisher Scien- tific, Fluka, Covestro and TIB Chemicals, respec- tively, and were used as received, unless otherwise specified. For impregnation and extraction pure distilled water was used, which is denoted as H 2 O throughout this work.
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