Nanomaterials 2023 , 13 , 1931
2of 19
hot-pressing, it is possible to densify the wood by almost 300% up to 1300 kg/m 3 and to improve the tensile strength by over 1000% to 587 MPa [9]. This was obtained when 12% of the lignin was left in the wood prior to hot-pressing, which was optimal to maximize both density and strength values. On the other hand, Cristescu et al. (2015) concluded that temperature (up to 250 ◦ C), compared to pressure and pressing time, is the most influential parameter in hot-pressing laminated beech, as it provides the highest density, strength, and lowest water sorption compared to the samples pressed at lower temperatures [10]. During hot-pressing, heat and mass-transfer processes interact with each other, in combination with deformation and chemical reactions, making the mechanism complex and challenging to investigate [11]. Therefore, simulations of hot-pressing have also been carried out to further understand the mechanisms at work [12,13]. Similar optimal amounts of lignin have been reported to maximize the reinforced effect of hot-pressed paper [14]. The benefit of using this hot-pressing method in combination with other methods to increase the strength of the sheet, such as the addition of chemicals, is that it is environmentally friendly and also that the sheet keeps its properties better over time [15]. In recent years, several studies have demonstrated that the hot-pressing of papers obtained with HYPs enhances their properties. Clear improvements are observed in both wet and dry tensile strength compared to non-pressed papers [16,17]. Joelsson et al. (2020) showed that the dry tensile strength of paper produced with various pulps, chemi-thermomechanical pulp (CTMP), thermomechanical pulp (TMP) and others, could be improved even by 100% when passing the paper through hot nips (200 ◦ C, 6 MPa) [18]. Moreover, the wet strength increased dramatically from 2 kN · m/kg to about 16 kN · m/kg after hot-pressing. A high compression strength was also achieved, probably due to the high bending stiffness of the lignin-rich CTMP fibers compared to lignin-free chemical pulp fibers. Furthermore, promising results have also been found regarding other impor- tant properties of the paper for the final applications, such as water resistance. Contact angle measurements showed increased values for the hot-pressed paper samples, which suggests a more hydrophobic surface due to the increased density and smoothness of the paper [18–20]. The quality profile of these laboratory paper sheets is in line with or superior to which is demanded today for commercial advanced sustainable packaging paper materials (even without any addition of chemicals, such as wet-strength agents), where very high strength is highly prioritized, in products such as liners, and paper bags. Moreover, it was lately shown that it is possible to achieve wet-strength levels of over 50% of the dry-strength level by combining the hot-pressing of CTMP or lignin-rich kraft pulp with sizing agents such as ASA[15]. Since the different levels of improvement were related to the lignin content [21], the prerequisite to achieve these improved properties, both for wood and paper materials, is that optimal conditions during the press-drying of sheets are achieved, specifically that the temperature exceeds the softening temperature of lignin and hemicellulose. In 2021, Joelsson indicated that even better strength improvements might be possible at further increased press-drying temperatures [22]. On the other hand, in recent decades, it has also been proved that nanocellulose (NC) significantly increases mechanical and barrier paper properties, allowing the use of paper in applications covered in our days by other materials. Numerous studies related to the production, characterization, and use of NC in different applications of interest are described in the literature [23–26]. This is due to the excellent properties of this family of products, including the ability to form stable three-dimensional networks, its great mechan- ical resistance, colloidal properties [27], its high specific surface, adsorption capacity [28] or the capacity of functionalization, among others [29,30]. All these properties together with the great availability, and biodegradable, biocompatible and environmentally friendly prop- erties of these materials make them of great interest in an endless number of applications like biomedicine [26,31], nanocomposites [32], reinforced inorganic matrix [33], rheology
Made with FlippingBook flipbook maker