Cellulose (2021) 28:5775–5791
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important factor, if describing the tensile strength of paper in the wet state. Studies looking at the enhancement of dry strength by using carboxymethylcellulose-grafted (CMC) pulp, cationic starch (C-starch) and microfibrillated cellu- lose (MFC) found, that the enhancement was mainly due to the increase in RBA on the microscale (Lindstro¨m et al. 2016). These results are supported by experiments on the influence of the application- method of polyacrylamide (PAM) on the enhancement of dry tensile strength of paper (Mihara et al. 2008). They found, that an external application method by impregnating finished paper samples in an aqueous PAM solution yielded significantly higher dry tensile index values, compared to the mass application during paper formation. Additionally, the spatial adsorption of PAM was analyzed by controlled etching of the cellulose and concurrent ATR-FTIR measurements, which showed, that the external application method led to the deposition of PAM mainly on the fiber surface and around fiber–fiber bonds, respectively. This can explain the difference in the observed dry tensile indices of H 2 O-, IPA- and BuOH-impregnated samples, since impregnation from H 2 O yields densely homo- and hetero-cross-linked networks of copolymer inside the cellulose fibers. Therefore, the amount of copolymer contributing to cross-linking outside and in between fibers, possibly increasing the RBA and thus the dry tensile strength, can’t be as high.
The copolymer inside the fibers of H 2 O-impreg- nated samples can form homo-cross-links with its own backbone (Toomey et al. 2004; Ko¨rner et al. 2016; Prucker et al. 2018), but is also able to yield covalent hetero-cross-links with CH-groups of cellulose chains (Jocher et al. 2015). In this way, cellulose chains, penetrating into the fiber wall from adjacent fibers, can covalently link with each other and with the fibers under the influence of UV-light, thereby also strength- ening fiber–fiber bonds. Hence, the latter, may be one explanation for the increase in dry and wet tensile strength, even though most of the copolymer is located inside the fibers, ostensibly not contributing to the reinforcement of fiber–fiber bonds. Another explana- tion, how a cross-linking polymer within the fiber wall may contribute to a higher wet strength can be deduced from our video observations. Single fiber failure and in particular delamination would directly benefit from a strengthened fiber wall, i.e. reinforced by the cross- linked copolymer on and in the fiber wall. While the slipping mechanism may strongly be affected by fiber–fiber connections, it is reasonable to assume, that fiber flexibility in the dry and the wet state plays a significant role if fibers glide/slide past one another during tensile load. Combining the results of the spatial analysis and the tensile tests, it may be assumed, that the copolymer worksasa bulking wet strength agent . Luner and Zhou (1993) reported that depositing or introducing chem- icals of molecular size, small enough to penetrate the cell wall, is a method to achieve wet strengthening. By doing this, the moisture regain is reduced and there- fore the dimensional stability is increased, inhibiting swelling of the cellulose fibers. Swelling of cellulose fibers has a major effect on the morphology especially the length and width (Lindner 2018). A possible explanation for the significant impact decreased swelling and increased dimensional stability of single cellulose fibers have on the wet tensile strength of the fiber network inside paper sheets is, that the relative bonded area (RBA) isn’t decreased when the paper comes into contact with moisture. The RBA is one of the parameters of the Page equation and is a key factor influencing the dry tensile strength, next to the single fiber bond strength (Page 1969). Even though intro- ducing moisture inside a fiber network changes the situation and probably the failure mechanism under tensile load, one may still consider the RBA as an
Conclusions
The preparation of paper samples impregnated with a photo-cross-linkable fluorescent copolymer P(DMAA-co-MABP-co-RhBMA) was achieved using three different solvents H 2 O, IPA and BuOH, respec- tively. Concurrently the spatial distribution of the copolymer inside the fiber network and the fibers themselves was analyzed with confocal microscopy (CLSM) of thin cross-sections and 3D-images. This method, in contrast to commonly used imaging methods in paper science and technology, does not require sophisticated sample preparation nor any specialized very expensive equipment and would thus be of interest for many more analytical tasks in the context of paper structure analysis. Using either of the three solvents for impregnation, we were able to significantly increase the dry and wet
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