1. FOREST PRODUCTS
BIOTECHNOLOGY/BIOENERGY GROUP
The influence of prehydrolysis and cold caustic extraction on dissolving cellulose
properties: Is it better to remove hemicellulose before or after pulping?
Xiaoli (Emily) Dou, Richard Chandra, and Jack Saddler
Department of Wood Science, Faculty of Forestry, University of British Columbia
E-mail: emily-dou@hotmail.com
3. Methodology and Mechanism1. Background
Abstract: The high costs and environmental impacts of cotton growth have contributed to an increased production of dissolving pulps which differ from “paper grade pulps” in requiring high
cellulose content, uniform molecular weight distribution and a high reactivity. One response to this demand has been to convert conventional Kraft pulp production to produce dissolving pulp.
However, some Kraft pulp characteristics that enhance their papermaking properties, such as high hemicellulose retention and cellulose degree of polymerization (DP) are detrimental to dissolving
pulp. Therefore, a repurposed Kraft pulping operation requires several extra steps to remove hemicellulose and tailor the cellulose to these more “speciality pulp” applications. In this work, we
compared the ability of prehydrolysis and alkaline extraction processes to remove hemicellulose and assessed their effects on the resulting dissolving grade pulp (cellulose accessibility and reactivity).
5. Conclusions
Dissolving pulp is one of the most important raw materials for the
manufacture of cellulose esters and ethers that can be subsequently
converted to products like rayon and thermoplastics such as
cellulose acetate. Due to the requirement for a homogeneous, pure
cellulose polymer, dissolving pulp is characterized by high
cellulose content, reactivity to downstream derivatization
reagents and a uniform molecular weight distribution.
To meet these criteria, Prehydrolysis (PHK) and Cold Caustic
Extraction (CCE) are used to convert Kraft pulp to dissolving pulp.
This work differentiated the effects of removing hemicellulose by
either the PHK and CCE on the pulp characteristics.
References: Randy Moores, et al. 1998. “Arrangement of fibrils, microfibrils, and cellulose in cell walls.” (http://www.bio.miami.edu/dana/226/226F07_3print.html)
Objectives:
• Understand the impact of PHK and
CCE on pulp characteristics and
enzymatic digestibility
• Figure out why CCE compromises
cellulose accessibility and
enzymatic digestibility
Substrates
Hemicellulose
%
Fibril aggregate
size nm
AFS %
Kraft pulp (KP) 19.9 16.7 13.3
5% CCE KP 12.8 21.1 10.5
7% CCE KP 8.9 25.3 8.8
9% CCE KP 4.6 28.3 7.9
Converting traditional Kraft pulp to dissolving grade/specialty pulp
Pre-
hydrolysis
Kraft
pulping
Bleaching
step
Dissolving
Pulp
Alkaline
Extraction
3. Objectives
2. Hypotheses
• PHK and CCE can both be used to remove hemicellulose but
vary in their impact on pulp properties
• CCE compromises cellulose accessibility and reactivity with
chemicals and enzymes by decreasing the accessible fibril
surface and increasing the size of cellulose microfibrils
(Reference )
CS2
NaOH
Table 1. Properties of different alkaline treated Kraft pulp
Figure 3. CP/MAS 13C NMR spectra of PHK dissolving pulp
• The acidic approach of PHK preserves cellulose reactivity while
alkaline CCE limits cellulose accessibility/reactivity
• Dissolving pulps obtained from PHK and CCE have similar
hemicellulose content, but the residual hemicellulose from a
PHK process helps to prevent the aggregation that occurs
between cellulose microfibrils.
Figure 2. Dissolving pulp manufacture and derivatization process
Figure 1. Composition of trees
4. Results: Hemicellulose content and Reactivity