Measured local paper structure—i.e. local basis weight, local thickness, local density and local fiber orientation—has been linked to local strain and local material failure (local temperature increase due to energy dissipation upon fiber–fiber bond failure) measured during tensile testing. The data has been spatially linked through data map registration delivering several thousand $$1\times 1\,\hbox{mm}^2$$ 1 × 1 mm 2 paper regions, each containing all measured properties. The relation between local paper structure and resulting local deformation and failure is studied with regression models. Multiple linear regression modeling was used to identify the paper structure related drivers for local concentrations of strain under load and local concentrations of material failure, which are both starting to occur considerably before rupture of the paper. Analyzing the development of local strain in paper we found that regions with higher basis weight and higher fiber orientation in load direction tend to exhibit considerably lower strain during tensile testing. Furthermore, the relation between local strain and local grammage can be predicted with the statistical theory of elasticity. Also regions with higher density have lower local strain, but not as pronounced. The findings for local fiber–fiber bond failure of paper are similar but not equivalent. The strongest correlation exists with local grammage. Local density and local fiber orientation show in turn weaker correlation with local bond failure. Local variations in paper thickness were not relevant in any case. These findings are highlighting the relevance of local fiber orientation and local density variations as structural mechanisms governing paper failure. In the past the focus has been mostly on paper formation. Together with local grammage (formation) they are responsible for the weak spots in paper, and thus cause local concentrations of paper strain and the initiation of failure under tensile load.
The shear modulus of pulp fibers is difficult to measure and only very little literature is available on this topic. In this work we are introducing a method to measure this fiber property utilizing a custom built instrument. From the geometry of the fiber cross section, the fiber twisting angle and the applied torque, the shear modulus is derived by de Saint Venant’s theory of torsion. The deformation of the fiber is applied by a moving coil mechanism. The support of the rotating part consists of taut bands, making it nearly frictionless, which allows easy control of the torque to twist the fiber. A permanent magnet moving coil meter was fitted with a sample holder for fibers and torque references. Measurements on fine metal bands were performed to validate the instrument. The irregular shape of the fibers was reconstructed from several microtome cuts and an apparent torsion constant was computed by applying de Saint Venant’s torsion theory. Fibers from two types of industrial pulp were measured: thermomechanical pulp (TMP) and Kraft pulp. The average shear modulus was determined as (2.13 $${\pm }$$ ± 0.36) GPa for TMP and (2.51 $${\pm }$$ ± 0.50) GPa for Kraft fibers, respectively. The TMP fibers showed a smaller shear modulus but, due to their less collapsed state, a higher torsional rigidity than the kraft fibers.
Print quality demands are steadily growing. Several print quality problems are known to be closely related to coating layer uniformity (e.g., print or gloss mottle). This study evaluates the potential for using ultraviolet (UV) scanner imaging as a fast method for characterization of the spatial coating layer formation. The method measures the visual light response of the excited fluorescent whitening agent (FWA) in the coating color. UV scanner imaging was compared to two reference methods for determining the coating layer uniformity, the burnout test, and β-radiography. Within the study, a commercially produced lightweight coated paper, a laboratory coated commercial woodfree base paper, and laboratory coated handsheets were examined. Image analysis showed that UV scanner imaging indeed reveals the coating layer formation, especially for lower coat weights. Furthermore, the influence of varying amounts of FWA in the coating color and in the base paper on the measurement results was investigated. The results indicate that UV scanner imaging of coating layer uniformity works best for coat weights below 20 g/m² and moderate FWA concentrations in the coating color.
The shear modulus of pulp fibers is difficult to measure and only very little literature is available on the topic. In this work we are introducing a method to measure this highly relevant fiber property utilizing a custom built instrument. From the geometry of the fiber, the fiber twisting angle and the applied torque, the shear modulus is derived by de Saint Venant's theory of torsion. The deformation of the fiber is applied by a moving coil mechanism. The support of the rotating part consists of taut bands, making it nearly frictionless, which allows easy control of the torque to twist the fiber. A permanent magnet moving coil meter was fitted with a sample holder for fibers and torque references. Measurements on fine metal bands were performed to validate the instrument. The irregular shape of the fibers was reconstructed from several microtome cuts and an apparent torsion constant was computed by applying de Saint Venant's torsion theory. Fibers from two types of industrial pulp were measured: thermomechanical pulp (TMP) and Kraft pulp. The average shear modulus was determined as 2.13 GPa for TMP and 2.51 GPa for kraft fibers, respectively. The TMP fibers showed a smaller shear modulus but, due to their less collapsed state, a higher torsional rigidity than the kraft fibers.
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