Cellulose (2020) 27:6961–6976
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density would be expected if the stress increase at high compression was caused mainly by the appearance of new inter-fibre contacts as proposed earlier (Ali- madadi et al. 2018). With higher initial density, the proposed densification mechanism should take place earlier during compression, and the scaled slope at 50% compression should increase with initial density. Our data does not support this idea for the compression levels studied. Even at 50% compression, densities of the studied materials are still rather low, and the mean contact stress (Subramanian and Picu 2011) remains small. Fig. 11 Comparison of the measured compression behaviour (solid curve) to the theoretical prediction (dashed curve) for structures made with unrefined Kraft (80%) and hemp (20%) with a cationic starch, b V-fines and starch, c CMF and starch, d TCNF and starch. The experimental stress–strain curves are
averages of 5 parallel measurements from the 1st (0–10%) and 2nd (10–50%) cycles. The last figure e shows the plot for the special LBG trial point TP11. The foaming agent was SDS for all samples
when the density of the final dried materials is kept on a similar level. Interestingly, the most significant impacts were from the smallest polymer components, such as the PVA and LBG. Using PVA to make wet foam leads to an uneven fibre network but to a stronger material than the one obtained using SDS foam. PVA densified the network locally and formed a film sub- structure, which affected fibre segment buckling. This was visible as a deviation from the theoretical stress– strain curve based on exponentially distributed seg- ment lengths. A similar deviation was observed when hemp bast fibres that bonded poorly with NBSK were added. On the other hand, after improving bonding and material homogeneity with fine components, the experimental stress–strain curves approached the theoretical prediction in all cases with the SDS surfactant beyond 40% strain. Among all studied samples, the average scaled slope of the stress–strain curve at 50% compression was 3.79 ± 0.33, which
Conclusions
The mechanical behaviour of foam-formed light- weight materials can be greatly affected by the multi-scale raw material components chosen, even
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