Cellulose (2019) 26:1995–2012
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Microfibril angle (MFA) of the S 2 cell wall layer has been proven to be one of the most important factors influencing the mechanical properties of wood and pulp fibers (Page et al. 1972; Page and El- Hosseiny 1983; Paavilainen 1993; Seth 2005; Long and Batchelor 2018). Page et al. (1972, 1977) used the mercury reflection technique in combination with a stereo-microscope for determining the fibril angle (MFA). Groom et al. (2002) used a confocal scanning laser microscope (CSLM) to determine the MFA of fiber specimens. Keunecke et al. (2008) used small- angle X-ray scattering and Wang et al. (2011) used an X-ray diffractometer to determine the MFA. Recently, Long and Batchelor (2018) measured bulk fibril angle using X-ray diffraction and the MFA of individual fibers using a confocal microscope and an analyzer crossed with polarization of the incident laser light. Fibers with a high MFA are more extensible than those with a low MFA, although variation at the same MFA can still be large (Page et al. 1972; Page and El- Hosseiny 1983; Borodulina et al. 2015; Long and Batchelor 2018). Thin-walled earlywood fibers are weaker and less stiff compared to thick-walled late- wood fibers. Thin-walled earlywood Scots pine fibers have a higher MFA compared to thick-walled late- wood Scots pine fibers, whereas in Norway spruce the opposite has been observed (Lichtenegger et al. 1999). Drying under axial tension has been shown to decrease the MFA, i.e. increase the strength and reduce elongation (Jentzen 1964). Recently, also mechano- sorptive creep rate has been measured and reported to correlate with the fibril angle (Dong et al. 2010; Olsson and Salme´n 2014). Additionally, when fibril angles approach 45 the wood fibers do not exhibit mechano-sorptive creep due to the low degree of anisotropy of the elastic and swelling properties of such high fibril angle fibers. Mechanical treatment of pulp has been shown to be an efficient way to improve the elongation of paper. Three mechanisms are responsible for this. Firstly, improving the strength of the fiber–fiber bonding also increases the extensibility and strength of the paper. Poorly bonded fiber networks fail before the full elongation potential of fibers is reached, whereas in well-bonded networks the fibers are under higher stress and are more strained before network failure. Strong arguments have been presented that the non- linearity and the viscoelastic properties of paper originate from within the fiber wall (Ebeling 1976;
Page and Seth 1980). Secondly, the creation of dislocations and microcompressions of fibers intro- duced by refining (Dumbleton 1971; Hamad et al. 2012) has also been identified as a cause for increased paper elongation. The mechanism assumed here is that these deformations are, at least partly, pulled straight during loading of the network, which increases paper elongation under load. Small fiber deformations (microcompressions and dislocations) thus lead to a higher elongation before a break of the individual fibers. Hamad et al. (2012) estimated the changes in microcompressions during single fiber tensile testing by using Raman microscopy. According to Horna- towska (2009), areas with disorders of the fiber structure such as dislocations or microcompressions behaved more elastically. Alexander and Marton (1968) stated that when it comes to dislocations, refining affects the latewood fibers more strongly. Watson and Dadswell (as cited by McIntosh and Uhrig 1968) pointed out that because of their more rigid nature, latewood fibers are damaged more during refining than the earlywood fibers. Thirdly, increased swelling and reduced axial stiffness have been named as causes of higher drying shrinkage and the conse- quent higher elongation of paper. However, influence of high consistency treatment on direct tensile prop- erties of individual fibers is poorly known. Several different fiber deformation types have been named and used in the literature, such as: curl, kinks, crimps, nodes, twists (axial and longitudinal), dislo- cations (longitudinal and transverse), microcompres- sions, misaligned zones, slip planes, and angular folds. Some of the deformation types denote similar or identical deformation types, e.g. dislocations, slip plane, and misaligned zone. Node may mean a longitudinal compression where a fiber can bend and form a kink. Mohlin et al. (1996) and Page et al. (1985) have shown that the number of kinks, folds, twists or compressions correlated with the shape factor of pulp and they have an influence on tensile strength, tensile stiffness, stretch to break, and zero-span tensile strength of paper. The objective of this study was to investigate the effect of small-scale deformations caused by mechan- ical treatment (refining) of pulps on the tensile behavior, and especially the extensibility of individual fibers and the papers made from those pulps. It is highly significant for the development of new fiber- based packaging products. This investigation was part
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