polymers
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 1 and Per Engstrand 1
1 Department of Chemical Engineering, Mid Sweden University, SE-85170 Sundsvall, Sweden; tove.joelsson@more.se (T.J.); jukka.ketoja@vtt.fi (J.A.K.); gunilla.pettersson@miun.se (G.P.); per.engstrand@miun.se (P.E.) 2 MoRe Research Örnsköldsvik AB, Box 70, SE-89122 Örnsköldsvik, Sweden 3 Department of Physics, University of Jyvaskyla, P.O. Box 35, FI-40014 Jyvaskyla, Finland; arttu.i.miettinen@jyu.fi 4 VTT Technical Research Centre of Finland Ltd., P.O. Box 1000, FI-02044 Espoo, Finland * Correspondence: amanda.mattsson@miun.se 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 − 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.
Citation: Mattsson, A.; Joelsson, T.; Miettinen, A.; Ketoja, J.A.; Pettersson, G.; Engstrand, P. Lignin Inter-Diffusion Underlying Improved Mechanical Performance of Hot-Pressed Paper Webs. Polymers 2021 , 13 , 2485. https://doi.org/ 10.3390/polym13152485
Academic Editors: Domenico Acierno and Antonella Patti
Keywords: hot-pressing; paper web; fibre; lignin; diffusion; activation energy
Received: 3 July 2021 Accepted: 23 July 2021 Published: 28 July 2021
1. Introduction Microplastic emissions are one of the world’s greatest environmental threats. The amount of these emissions has been steadily increasing for many years and is expected to continue to do so [1]. Thus, material options that are both renewable and biodegradable have been extensively searched for. A particular challenge is to develop materials that have similar or better properties in humid or wet conditions as their oil-based counterparts. This should be the case not only for strength but also for dimensional stability and barrier properties, which are important, e.g., in packaging and construction applications [2]. Recent studies have shown that hot-pressing of lignin-rich paper webs could provide at least a partial solution to the above challenge. Clear improvements are observed for both wet and dry tensile strength (later also referred to as only wet and dry strength) compared to non-treated webs [3,4], enabling applications in several packaging areas. Joelsson et al. [5] showed that the tensile strength of paper based on chemithermomechanical pulp (CTMP) could be improved even by 100% when passing the paper through a hot nip (200 ◦ C, 6 MPa) with a pressing time of 1.5 s and 70 s after hold. Moreover, by hot-pressing, the wet strength increased dramatically to a value of about 16 kNm/kg from the level of 2 kNm/kg
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Polymers 2021 , 13 , 2485. https://doi.org/10.3390/polym13152485
https://www.mdpi.com/journal/polymers
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