Tomography-based Micromechanical Analysis of Novel Composite Material

Abstract: This study explores the performance of porous Paptic paper materials composed of a mixture of softwood and lyocell fibers. The investigation involves laboratory experiments and numerical simulations to analyse the impact of various parameters on the paper's characteristics. Tensile and hygroexpansion tests were conducted on sheets with different binder quantities and drying methods. VTT aided in the analysis of mechanical properties using tomography images. The objective was to determine the optimal binder content, to understand the behaviour of the paper under different drying conditions and to optimise the pulp mixture through numerical simulations. Experimental tests involved producing paper sheets with varying binder amounts and different drying methods. Tensile tests were conducted to assess the elastic stiffness, strength, and strain at break. Constrained dried and freely dried papers were compared to evaluate the influence of drying conditions. Hygroexpansion tests were performed to examine the water storage behaviour of papers with added binders. Tomography measurements provided the density profile, which was replicated in the numerical sheets. A micro-mechanical model was employed for numerical simulations, representing each fiber as a beam. The model was calibrated using stress-strain data from VTT's tensile testing of the paper with the highest binder content. The influence of altering the amount and length of lyocell fibers was examined to optimise the pulp. From the tensile tests, an optimal binder content was identified that yielded the highest elastic stiffness while considering the density increase caused by binders. Further additions of binders did not enhance elastic stiffness. However, no optimal value was found for strength and strain at break, as both parameters continued to increase with additional binders. Tensile tests comparing constrained dried and freely dried papers showed similar behaviour, suggesting inadequate constraint in the former. Hygroexpansion tests confirmed the similarities between the two drying methods and revealed that papers with added binders stored less water at a given humidity. Additionally, the drying-moistening cycling exhibited an unusual behaviour not observed in conventional paper, with irreversible expansion occurring during the first drying cycle. Numerical simulations using a micro-mechanical model demonstrated that higher amounts of lyocell fibers improved performance, increasing strength and strain at break. However, varying fiber length did not yield significant improvements in these parameters, although stiffness showed a slight increase. While the literature suggests that the addition of long lyocell fibers decreases paper strength, this study found that when maintaining constant bulk, strength increased under the assumption that the bonding strength was unaffected by lyocell fibers. Furthermore, numerical simulations indicated that an even density profile throughout the paper thickness resulted in higher strength and strain at break. The tomography data revealed that the density profile is affected by the binder quantity. With the addition of binders, the thickness decreased even though the grammage increased. The density is high on the top and bottom surface of the papers which contain more binders while the density is lower in the middle. This difference in density is more pronounced with higher amounts of binders.