3474
Cellulose (2019) 26:3473–3487
Graphical abstract
Keywords
Paper properties Alginate Guar gum
strain at break significantly at the cost of tensile index and tensile stiffness (see e.g. Zeng et al. 2013; Strand et al. 2017). Stretching a shrunken fiber network is to a certain extent a reversal of shrinkage, as the applied forces tend to pull out the kinks and micro-compres- sions, both between and within the bonds. As intrafiber hydrogen bonds gradually break, the micro-compres- sions are released, allowing for a greater extension of the fiber network before it breaks due to actual failure of fiber–fiber bonds and resulting localization of stresses. For a pulp with constant refining level, a linear correlation between strain at break and negative strain, i.e. shrinkage, has previously been reported for paper, both in machine direction and cross direction (Wahlstro¨m 1999). However, recent results indicate that the relationship between shrinkage and elongation is non-linear in large scale and that the same dimen- sional contraction brought about by shrinkage can be strained out in tensile testing (Kouko and Retulainen 2018). It was shown that the strain at break of paper after unrestrained drying increases linearly with increasing refining energy and that it influences most mechanical properties of the paper considerably (Strand et al. 2018). Addition of hydrophilic polysaccharides or poly- electrolytes is known to enhance the bonding between fibers and is the most common strategy to increase paper strength (Hubbe 2006; Hubbe et al. 2009; Fornue et al. 2011). For successful wet-end additions, the additives need to be retained to the fibers and have sufficiently high molar mass to avoid migration into the fiber pores (Pelton 2004; Hubbe 2006; Fornue et al.
Chitosan Spraying Extendable fiber network Unrestrained drying Paper shrinkage
Introduction
Novel bio-based alternatives to fossil-based plastics are needed for future packaging applications. So far, the bio-based alternatives have been limited by their brittleness, low thermal stability, medium gas barrier properties, and low solvent resistance (e.g. against water) (Yu et al. 2006; Rhim 2012). Cellulose is an optimal choice of raw material for such applications, since it is biodegradable, renewable, recyclable, and currently available in large quantities. It has been reported that highly extensible cellulose-based fiber networks can be created, and can be molded into 3D shapes using available forming techniques (Svensson et al. 2013; Vishtal et al. 2014; Vishtal and Retulainen 2014b). Mechanical treatments of fibers and additions of various additives have been used in combination with unrestrained drying to improve the extensibility of fiber networks (Seth 2005; Zeng et al. 2013; Khakalo et al. 2014, 2017a; Vishtal et al. 2015; Strand et al. 2017). Paper shrinkage during unrestrained drying has been shown to cause micro-compressions in the longitudinal direction of the fibers (Page and Tydeman 1962). It is known that unrestrained drying affects the mechanical properties of paper quite severely. Unrestrained drying of paper increases its
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