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Tear strength, freeness, scattering coefficient, and brightness, in contrast, decreased with increasing refining or dosage level. It is well-established that refining increases NBSK paper strength (Fig. 8). Tensile index and burst index increased with refining level until 6000 rev and then plateaued. Refining to 9000 rev without LBG adsorp- tion doubled the tensile index of unrefined NBSK paper. However, as highlighted by Leech (1954), paper strength does not increase continuously with bonding strength as at higher levels of bonding, handsheet strength becomes more dependent on individual fibre strength. Leech (1954) concluded that tensile and burst strength of bleached sulfite pulp plateau after refining to approximately 6000 rev, which is consistent with the results in Fig. 8a, b. Mechanical refining had a much greater effect on tensile strength than LBG adsorption. Miletzky et al. (2015) similarly reported that the effects of hemicel- lulose addition are minimal if high refining is applied. In this work, the maximum increase in tensile index due to adsorption of LBG to unrefined pulp was 20.1%. In comparison, tensile index after refining was approximately double that of unrefined pulp. How- ever, LBG adsorption can reduce the degree of refining and energy required to reach a target tensile index. For example, pulp must be refined to 2000 rev to reach a tensile index of 80 N m g - 1 without LBG adsorption but only 1000 rev were required to achieve the same index if the pulp is treated with 1wt% LBG. Swanson (1950) likewise reported that refining time was reduced by approximately 70% with addition of 0.5 wt%LBG. Burst strength demonstrated a similar trend as tensile strength (Fig. 8b). The maximum burst index also doubled after applying 1 wt% LBG to highly refined pulp (9000 rev). Enhanced tensile index and burst index could be due to either increased bonding area or an increased number of bonds and bond strength (Leech 1954). Similar trends were reported by Swanson (1950). As expected, tear index decreased with refining and LBG addition possibly due to stronger bonding between fibres (Fig. 8c). At 9000 rev, tear index decreased 62.8% after adsorption with 1 wt% LBG. A drop in tear index normally accompanies increased tensile strength (Hannuksela et al. 2004; Leech 1954; Swanson 1950). Tearing occurs due to the breakage of individual fibres or individual fibres being pulled from
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Adsorption pH
Fig. 7 The adsorbed amount of LBG on NBSK pulp fibre with varying pH of pulp suspension after 10 min at 25 C, LBG dosage 0.2 wt% of o.d. fibre
Effect of pH
LBG adsorption to NBSK pulp was investigated after 10 min at 25 C with LBG dosage 0.2 wt% of o.d. fibre (Fig. 7) while buffer pH varied from 2 to 13. The amount of LBG adsorbed ranged from 1.8–2.7 mg g - 1 of o.d. fibre with maximum standard deviation of 0.26 mg g - 1 of o.d. fibre (Fig. 7), indicating adsorp- tion was not strongly affected by pH. Adsorption was slightly higher at pH 2 and 5 due to the undissociated hydroxyl and carboxyl groups on LBG and reduced repulsive forces. At high pH, hydroxyl and carboxyl groups deprotonate more easily increasing repulsive forces between fibre and LBG thus lowering adsorp- tion. However, the differences are small due to LBG’s low negative charge density on LBG. Hannuksela et al. (2002) reported that pH (5 and 8) had no influence on guar gum adsorption to softwood kraft pulp fibre. This observation is probably due to guar gum’s weak negative charge and the limited range of pH tested.
Paper strength enhancement by LBG adsorption
Pulp and paper properties including tensile, burst, tear index, scattering coefficient, brightness, and pulp freeness were plotted as a function of PFI refining revolutions and LBG dosage in Fig. 8. Adsorption was conducted after refining. Refining and dosage posi- tively influenced tensile strength and burst strength.
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