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Fig. 7 Typical cross-sectional images obtained with x-ray tomography for a refined NBSK and hemp with SDS (TP3), and b unrefined NBSK with PVA (TP4). Both samples include voids which cause deviation from the theoretical stress–strain curve
below 40% strain in Fig. 8c, d. The images are averages of 60 slices taken from a central part of the samples. Wire side is at the bottom
(TP7), and the trial points with added V-fines (TP8), CMF (TP9) and TCNF (TP10). The addition of fine components was 10% of the long fibre amount. Cationic starch was added to ensure high retention of the negatively charged fibrils. Retention of the fine Fig. 8 Comparison of the measured compression behaviour (solid curve) to the theoretical prediction (dashed curve) for a unrefined Kraft (TP1), b refined Kraft (TP2), c refined Kraft and hemp (TP3), d unrefined Kraft and PVA (TP4), e refined Kraft and PVA (TP5), and f refined Kraft, hemp, and PVA (TP6). The
experimental stress–strain curves are averages of 5 parallel measurements from the 1st (0–10%) and 2nd (10–50%) cycles. The foaming agent was SDS for the data in a – c . The parameter r 0 of Eq. (1) was fitted to the experimental curve at 50% strain
materials was not measured. However, based on the work of Turpeinen et al. (2018) and the realized grammages, we assume high, over 90% retention. This trial set was extended with a special trial point that included a partly soluble natural polymer, colloidal
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