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Cellulose (2021) 28:5775–5791
to DIN ISO 1924-2 with a ZwickZ 1 .0 with a 1 kN load cell using the software testXpert II V 3.7 1 ( ZwickRoell GmbH & Co. Kg ) in a controlled environment with 23 C and 50% r.h. Wet tensile strength measurements were conducted in analogy, after the paper samples had been submerged in H 2 O for at least five minutes. To remove any excess water, the paper samples were sandwiched between tissue paper and slightly pressed. A 20 N load cell was used for those measurements, to improve the precision of the tensile tests in the wet state. Along the measured dry and wet tensile indices, the relative wet strength is a relevant quantity, which is used in the paper formation industry and can be calculated according to this equation:
microscopy, light scattering limits axial resolution, especially with highly scattering samples, such as paper. The latter to some extent can be circumvented, by imaging thin cross-sections of paper to assess the distribution of chemical modifications across the paper width. As lateral resolution of microscopy applies, which, for confocal detection, exceeds the axial resolution by a factor of around 3, this method becomes highly suitable for distinguishing, e.g. whether the copolymer has adsorbed onto or within a fiber wall, this resolution gain is required. Addition- ally, image stacks of paper samples were made with traditional confocal detection and combined into 3D- images in order to analyze the spatial distribution of the copolymer between the fibers and inside the whole fiber network. It should be noted, that this approach reintroduces some of the challenges, which were avoided by using thin cross-sections. For confocal investigation, the prepared slices or the paper samples were mounted between two round coverslips with Type F Immersion liquid from L eica or H 2 O, respectively, as immersion medium. Using a Leica TCS SP8 (Leica Microsystems, Mannheim, Germany) CW fluorescence was excited at 405 nm and fluorescence emission was detected between 420 and 480 nm. Rhodamine B labelled copolymer, in turn, was excited at 552 nm and detected between 570 and 650 nm. Image stacks of * 50 slices with widths of 0.6 to1 l m were acquired and processed within the L eica Application Suite X to render 3D-stacks of paper samples for further analysis. It should be noted, that during image acquisition the settings for laser intensity, detector sensitivity (gain and offset) and later on during image processing, the brightness and contrast of the images were adjusted, in order to achieve the highest detail and images where the spatial distribution of the copolymers can best be compared. Thus, no quantitative information about copolymer amount in the different samples can be derived from the images provided/shown in this work. However, quantitative analysis through calibration via a concentration series can be done as was shown previously by our group (Bump et al. 2015), however it was not in focus in this work here.
wet tensile index dry tensile index
rel : wet strength ¼
ð 1 Þ
100
The definition of the dry/wet tensile index is shown in the following equation: dry = wet tensile index ¼ 100 ð 2 Þ Here Fmax is the maximum force at break in N, b is the width of the sample in mm and the grammage of the paper is given in g m - 2 . Fmax b grammage
Setup for fiber failure analysis via Zwick- videography
A commercially available full frame mirrorless cam- era from Panasonic (DC S1) with a macro lens from Canon (MP-E 65 mm f/2.8 1–5 9 Macro Photo) and an adapter from Novoflex (SL/EOS) was used. The camera was mounted on a manual x/y/z-stage on a table that was decoupled from vibrations of the tensile testing equipment. The aperture was set to 5.6, the shutter speed was 1/30 s and the ISO was set to 800. The videos were recorded with a resolution of 3840 9 2160 px at a frame rate of 29.97 frames/ second. A UV-lamp (365 nm) was used from the backside of the paper samples to excite the fluorophore (cationic dye— Pergasol Yellow F6-GZ ) of the labelled fibers (0.1 wt% of all fibers).
Results and discussion
Tensile strength analysis
As water is by far the predominant processing solvent used in the paper formation industry, we used this
Dry tensile strength of the paper samples was deter- mined as an average of at least five samples according
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