Abstract
The adsorption of locust bean gum (LBG) onto Northern Bleached Softwood Kraft (NBSK) pulp improved paper tensile and burst strength and lowered refining energy by strengthening inter-fibre bonding. Adsorption kinetics and isotherms were investigated to develop a fundamental understanding of the adsorption mechanism. The adsorption rate followed pseudo-second-order kinetics and the activation energy was 99.34 kJ·mol−1, suggesting chemisorption. The adsorption rate constant increased rapidly with temperature from 25 to 45 °C (k = 1.93 to 24.03 g·mg−1·min−1), but the amount adsorbed at equilibrium decreased (qe = 1.91 to 0.48 mg·g−1 o.d. fibre). LBG adsorption to NBSK at 25 °C was consistent with the Langmuir adsorption model for LBG < 2.1 wt% of o.d. fibre, suggesting reversible, homogenous adsorption to a finite number of sites on the fibre surface. Refining to 3000 rev increased the heterogeneity of the NBSK pulp surface leading to multi-layer Freundlich adsorption with adsorption constant n = 5.00, and the equilibrium constant Kf = 2.57 mg·g−1·(mg·L−1)−1/n at 25 °C. Favorable adsorption conditions for negatively charged LBG were identified: 25 °C for 10 min, low dosage level (< 2 wt%), lightly refined (< 3000 rev) NBSK pulp at low fibre consistency (< 0.5 wt%), high agitation rate (> 150 r.p.m.), acidic or neutral conditions (pH 2–7) without salt addition.Graphic abstract
Highlights
Paper strength, essential to many applications, relies on the number of interfibre bonds, the strength of the bonds, fibre properties, and the distribution of fibres and bonds (Niskanen 1998; Lindstrom et al 2005; Leech 1953, 1954)
Softwood grown in the Northern hemisphere (e.g. British Columbia, Canada) produces high-strength bleached kraft pulp; finding ways to further enhance strength properties while reducing refining energy is a key objective for Northern Bleached Softwood Kraft (NBSK) producers
The fraction of locust bean gum (LBG) adsorbed to NBSK pulp (Eq 1) at 25 °C is plotted as a function of time with LBG dosage of 0.2 wt% relative to o.d. pulp (Fig. 1)
Summary
Essential to many applications, relies on the number of interfibre bonds, the strength of the bonds, fibre properties (e.g. fibre strength, length, and coarseness), and the distribution of fibres and bonds (sheet formation) (Niskanen 1998; Lindstrom et al 2005; Leech 1953, 1954). Paper strength can be modified through mechanical refining and the use of additives such as polysaccharides. Increasing paper strength through refining consumes large amounts of mechanical energy. Strength improvement through application of additives after refining reduces the energy required to achieve target paper strength (Bhaduri et al 1995; Silva et al 2010). The earliest paper strength additives were polysaccharides with structural affinity for cellulose such as locust bean gum (LBG) and guar gum (Leech 1953; Lindstrom et al 2005; Most 1957; Russo 1959; Swanson 1950). Paper strength enhancement by hemicellulose adsorption results from an increased number of bonds plus increased bonded area and bond strength (Leech 1954). The primary bonding between fibres and between hemicellulose and fibres is hydrogen bonding (Niskanen 1998; Leech 1954; Hannuksela et al 2002)
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