Effect of cellulose nanofiber dimensions on sheet forming through filtration

Abstract

Four different cellulose nanofibers samples were prepared from northern bleached softwood kraft fibers. Fiber diameter distributions were measured from SEM images. Fiber aspect ratios ranging from 84 to 146 were estimated from fiber suspension sedimentation measurements. Three samples had heterogeneous distributions of fiber diameters, while one sample was more homogeneous. Sheet forming experiments using filters with pores ranging from 150 to 5 lm showed that the samples with a heterogeneous distribution of fiber dimensions could be easily formed into sheets at 0.2% initial solids concentration with all filter openings. On the other hand, sheets could only be formed from the homogenous sample by using 0.5% or more initial solids content and a lower applied vacuum and smaller filter openings. The forming data and estimated aspect ratios show reasonable agreement with the predictions of the crowding number and percolation theories for the connectivity and rigidity thresholds for fiber suspensions.

Aqueous morpholine pre-treatment in cellulose nanofibril (CNF) production: comparison with carboxymethylation and TEMPO oxidisation pre-treatment methods

“…The theory, methods and equations for the determination and calculation of connectivity threshold (U c ) and rigidity threshold of CNF from sedimentation experiments have been well described by Zhang et al (2012), Varanasi et al (2013) and Raj et al (2016). The connectivity threshold is defined as ''the boundary between the dilute and semi-dilute regions, the lowest volume fraction where the particles first form a continuous network'' (Varanasi et al 2013).…”

“…The connectivity threshold is defined as ''the boundary between the dilute and semi-dilute regions, the lowest volume fraction where the particles first form a continuous network'' (Varanasi et al 2013). The connectivity threshold can be determined experimentally by fitting the plot of initial concentration of CNF vs relative sediment height with a quadratic regression, where the linear term of the regression is equivalent to the connectivity threshold (Zhang et al 2012). The rigidity threshold on the other hand is the boundary between the semi-dilute and concentrated regions, the threshold at which the fibres first begin to contribute to mechanical strength.…”

“…The plot of concentration (wt%) against the relative sediment height (h s /h 0 ) was fitted with the quadratic equation (ax 2 ? bx), where the linear fit parameter b, is equal to the connectivity threshold (Zhang et al 2012).…”

“…The aspect ratios were then estimated from the crowding number theory (CNT) and the effective medium theory (EMT) using Eqs. 5 and 6 respectively, as adopted by Varanasi et al (2013) and Zhang et al (2012) for CNF materials.…”

“…Field emission scanning electron microscopic (FE-SEM) images and transmission electron microscopic (TEM) images were used for the assessment of fibril widths and morphology. The estimation of aspect ratio by connectivity threshold analysis using sedimentation, which has been applied on uncharged CNFs (Raj et al 2016;Varanasi et al 2013;Zhang et al 2012) was extended to CNF with surface charges, by taking advantage of the screening effect of salt medium on charged CNF. Finally, the viscoelastic properties of the aqueous suspensions of the CNFs were studied.…”