Cellulosic paper, bulk and lightweight sheets based on the type of plant fibers, and nanopaper, thin and dense sheets made from nanocelluloses, have served as promising biomaterials for a wide range of applications, including biomedical devices (Deka et al. 2020), flexible substrates (Siegel et al. 2010), food packaging (Khwaldia et al. 2014), storage (Nyholm et al. 2011), and sensors (Dungchai et al. 2009; Liana et al. 2012). The manufacturing of paper is facile, and the raw material for paper, cellulose fiber, is the most abundant renewable biopolymer on the earth, which is sourced from wood (Boufi et al. 2016), grass (Obi Reddy et al. 2014), cotton (Shi et al. 2010), and even bacteria (Sözen et al. 2021). Although paper has numerous advantages, such as cost-effectiveness, sustainability, renewability, and high potential to replace plastic in some fields, the mechanical and physical property requirements limit its diverse applications in composites. In particular, at high relative humidity, overcoming the water absorption characteristics of paper and protecting hydrogen bonding between fibers has always been a challenge.
Traditional methods such as refining and beating can enhance the strength of paper by improving the fiber strength and fibrillation degree (Ang et al. 2019; Motamedian et al. 2019); however, they have a negative effect on the bending stiffness of paper. Thus, more chemical additives were chosen instead of beating or refining, such as dry strength agent (cationic polyacrylamide, CPAM) (Djafari Petroudy et al. 2014) and wet strength agent (polyamide amine-epichlorohydrin, PAE) (Obokata et al. 2005; Obokata and Isogai 2007; Su et al. 2012). Although the use of chemical additives has greatly improved the strength, it is not environmentally friendly, owing to the production of by-products in the manufacture and use process, and it is also not conducive to the recycling of paper (Hagiopol and Johnston 2011; Onur et al. 2019). In addition, chemicals could induce an increase in wastewater load. These serious problems have stimulated the papermaking industry and researchers to seek green, biodegradable substitutions for these chemical additives and traditional methods. So far, natural materials such as starch (Ghasemian et al. 2012; Hamzeh et al. 2013), guar gum (Xie et al. 2016), chitosan (Chen et al. 2013; Rahmaninia et al. 2018), carboxymethyl cellulose (CMC) (Uematsu et al. 2011; Siqueira et al. 2015), and nanocelluloses (nanocrystals (CNCs), and nanofibers (CNFs)) (Sun et al. 2015; Vallejos et al. 2016) have been used in papermaking to improve paper strength. Additionally, in recent years, ionic liquids, such as 1-butyl-3-methyl-imidazollium chloride ([BMIM]Cl), have been explored as a wet strength agent for paper sheets by partially dissolving the cellulose to form a thin cellulose film on the paper surface and increasing the wet strength (Ichiura et al. 2017). The use of phosphoric acid-urea solution to treat cellulose paper to increase the wet strength was also investigated (Yamamoto et al. 2019). However, there are temperature requirements (80–140 ℃) for the pretreatment of paper using these two methods.
An ideal reinforcing material for paper may (i) be sourced from natural and green processing, (ii) require no damage to pulp fibers, (iii) be easily mixed with fibers, fillers, and other additives to improve paper properties, and (iv) extend paper lifetime for recycling. Compared with other natural reinforcing materials, CNFs have attracted great attention as an alternative to traditional methods and chemical agents because they not only meet these requirements but also have excellent performance. CNFs have extremely good mechanical properties, including a high Young’s modulus of up to 150 GPa (Iwamoto et al. 2009) and excellent tensile strength up to 6 GPa (Saito et al. 2013). These unique inherent properties of CNFs can play a key role in the reinforcement of composites. Furthermore, they have diameters in the range of 5–50 nm and several micrometres in length, leading to a large specific surface area and a high aspect ratio (Moon et al. 2011; Zhu et al. 2015), which also plays an important role in improving the hydrogen bonding between fibers. The inherent tendency to form strong entangled networks also promotes interfacial contact with the fibers (Boufi et al. 2016).
CNFs as an additive for paper with and without other polymetric strength additives, including wood pulp (Kumar et al. 2016; Mocchiutti et al. 2016; Ottesen et al. 2016; Tozluoglu and Poyraz 2016), agricultural waste pulp (Balea et al. 2017), and recycled pulp (Delgado-Aguilar et al. 2015), have been studied. Generally, adding a small number of CNFs to the pulp suspensions reinforces the physical and mechanical properties of paper, resulting in reduced porosity, increased density, and the improvement of overall strength (Jonoobi et al. 2012; Boufi et al. 2016). Sehaqui et al. (2011) added 2% CNF to bleached pulp, which induced a more compact and hierarchical structure that was formed by the nanoscale CNF networks embedded into microscale pulp fiber networks. They found that all the sheets containing CNFs had a higher dry tensile strength and dry Young’s modulus compared to that of the pulp sheet without CNFs. Adding 20 wt% CNF to aramid pulp gave the composite sheet good tensile index and tear index in dry conditions, which were 2.04 times and 2.36 times that of the control sample, respectively (Lu et al. 2017). Adding 10 wt% CNF to unbeaten pulp improved the wet strength of the composite sheet by one order of magnitude, and the wet strength was further improved by approximately two orders of magnitude for the pure CNF sheet compared to non-beaten sheet (Sehaqui et al. 2013).
Although the reinforcement of CNFs on composite sheets has been studied and reported, there are few detailed reports on how a wide range (0–100%) of CNF addition affects the stiffness and strength of composite sheets. Whether the stiffness and strength of the composite sheets increase with an increase in the CNF content and whether the pure CNF sheet can have the maximum tensile strength and Young's modulus under dry and wet conditions are unknown. To better understand these questions, we designed an experiment. In particular, composite sheets made of bleached pulp fibers and CNFs were produced by vacuum filtration. The effect of 0–100% CNF on the Young’s modulus and tensile strength under dry and wet conditions was investigated by tensile testing and scanning electric microscope (SEM) observations.