Lignin Inter-Diffusion Underlying Improved Mechanical Performance of Hot-Pressed Paper Webs

Broader use of bio-based fibres in packaging becomes possible when the mechanical properties of fibre materials exceed those of conventional paperboard. Hot-pressing provides an efficient method to improve both the wet and dry strength of lignin-containing paper webs. Here we study varied pressing conditions for webs formed with thermomechanical pulp (TMP). The results are compared against similar data for a wide range of other fibre types. In addition to standard strength and structural measurements, we characterise the induced structural changes with X-ray microtomography and scanning electron microscopy. The wet strength generally increases monotonously up to a very high pressing temperature of 270 °C. The stronger bonding of wet fibres can be explained by the inter-diffusion of lignin macromolecules with an activation energy around 26 kJ mol−1 after lignin softening. The associated exponential acceleration of diffusion with temperature dominates over other factors such as process dynamics or final material density in setting wet strength. The optimum pressing temperature for dry strength is generally lower, around 200 °C, beyond which hemicellulose degradation begins. By varying the solids content prior to hot-pressing for the TMP sheets, the highest wet strength is achieved for the completely dry web, while no strong correlation was observed for the dry strength.

Material-dependent ultrasonic heating behavior during the reshaping of dry paper webs

The use of ultrasonic tools during the reshaping of dry paper webs results in a temperature increase. This work aimed to determine the influence of the material and the ultrasonic process parameters of amplitude, ultrasonic duration, and static process pressure on the heating behavior of paperboard during ultrasonic-assisted reshaping. The results showed that the initial process pressure, the ultrasonic amplitude, and the compression resistance of the material noticeably influenced the heating rate. Materials with low compression resistance tended to reach higher initial heating rates during ultrasonic treatment. In addition, coating the paperboard led to an even temperature distribution in the paperboard during the ultrasonic process.

Rate-limiting mechanisms of water removal during the formation, vacuum dewatering, and wet-pressing of paper webs: A review

Because some of the critical events during the removal of water before the dryer section on a paper machine happen very rapidly within enclosed spaces – such as wet-press nips – there have been persistent challenges in understanding the governing mechanisms. In principle, a fuller understanding of the controlling mechanisms, based on evidence, should permit progress in achieving both higher rates of production of paper and more reliable control of paper attributes. In addition, energy can be saved, reducing environmental impacts. The goal of this article is to review published work dealing both with the concepts involved in water removal and evidence upon which existing and new theories can be based. The scope of this review includes all of the papermaking unit operations between the jet coming from the headbox and the final wet-press nip of an industrial-scale paper machine. Published findings support a hypothesis that dewatering rates can be decreased by densification of surface layers, plugging of drainage channels by fines, sealing effects, flocculation, and rewetting. Ways to overcome such effects are also reviewed.

Engineering fracture mechanics analysis of paper materials

The aim of the present work was to develop an analytic fracture mechanics procedure that enables accurate predictions of failure of paper materials. Analytic expressions for prediction of the critical force and critical elongation of edge-notched paper webs were developed based on isotropic deformation theory of plasticity and J-integral theory. The analytic expressions were applied to predict the critical force and elongation of paper webs with different edge-notch sizes for six different paper materials. The accuracy of the analytic failure predictions was verified by numerical predictions and experiments on edge-notched paper webs, showing that the developed engineering fracture mechanics analysis procedure predicted failure accurately.