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

**Amanda Mattsson 1,\* , Tove Joelsson 1,2, Arttu Miettinen 3,4 , Jukka A. Ketoja 1,4 , Gunilla Pettersson <sup>1</sup> and Per Engstrand <sup>1</sup>**


**Abstract:** 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−<sup>1</sup> 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.

**Keywords:** hot-pressing; paper web; fibre; lignin; diffusion; activation energy
