Structure and reactivity of cellulose from bleached kraft pulps of different Eucalyptus species upgraded to dissolving pulp

Bleached kraft pulps from Eucalyptus benthamii, E. globulus, E. nitens and E. nitens × E. globulus hybrid were prepared in laboratory scale and upgraded to dissolving-grade via cold caustic extraction (CCE) and dilute acid hydrolysis. The CCE reduced the alkali solubility (S18) of bleached kraft pulps from 11.8–16.4% to 4.5–6.3% after extraction with 5% (w/v) NaOH, and to 0.9–1.7% after extraction with 10% (w/v) NaOH. The dilute acid hydrolysis with 3% H2SO4 reduced the intrinsic viscosity of CCE-treated kraft pulps from 800–900 to 110–450 ml/g. E. nitens and E. benthamii displayed the most contrasting values for S18 and intrinsic viscosity after CCE and acid hydrolysis treatments. The X-ray diffraction results showed that the crystallinity index of bleached kraft pulps followed the order: E. benthamii > E. globulus > En × Eg > E. nitens. Regarding cellulose reactivity, the Fock reactivity of the bleached kraft pulps was in the range of 34.7–41.3%. After CCE, the pulps reactivity was reduced to 13.7–25.4%, whereas after acid hydrolysis it was increased to 40.7–87.5%. E. benthamii and E. nitens pulps showed to be the less reactive pulps after pulp activation via acid hydrolysis, whereas E. globulus and the hybrid showed to be the most reactive pulps towards xanthation. It was observed that different Eucalyptus species can lead to kraft pulps with different requirements towards xanthation, and that the inherent features of wood and how they are processed are of outmost importance for the final use of kraft-based dissolving pulp from a given wood species.