Hemicelluloses at Interfaces: Some Aspects of the Interactions - The Nanoscience and Technology of Renewable Biomaterials - page 150

6.2 Introduction

Hemicelluloses are the second most abundant plant material after cellulose (Fengel and
Wegener 1989; Alen 2000). The amount of hemicelluloses of the dry weight of wood
is usually 20-30% (Sj ostr om 1993). The most abundant hemicelluloses in trees are
xylans followed by glucomannans. In TMP process the hemicelluloses are dissolved
into the process waters during pulping and papermaking whereas the degradation of
hemicelluloses is one of the limiting factors in alkaline cooking. Hemicelluloses are
known to have positive effects on paper strength whereas it might harmfully consume
papermaking chemicals when dissolved in e.g. white waters.
There is a growing interest in utilization of by-products of forest industry in order to,
for example, develop biodegradable polymers to replace synthetic oil-based ones. Hemi-
celluloses present in wood-processing industries process waters and side streams can be
considered as large sources for this renewable raw material. Although hemicelluloses
are industrially used, for example, as food additives and thickeners there are limitations
to fully exploit these resources due to the lack of isolation, purification and modification
techniques.
The major soft wood hemicelluloses are galactoglucomannans (GGMs), arabinoglu-
curonoxylans and arabinogalactans. The dominating dissolved hemicelluloses in ther-
momechanical pulping of spruce are O -acetyl-galactoglucomannan and arabinogalactan
(Thornton et al . 1994). Arabino-4- O -methylglucuronoxylan is not notably released
during the mechanical pulping. Arabinogalactan and pectins (rhamnogalacturonans)
contribute to the anionic charge in the TMP waters. Peroxide bleaching at alkaline
conditions changes the composition of the dissolved hemicelluloses present in TMP pro-
cess waters: deacetylation of GGMs leads to their readsorption on the fiber surfaces.
Thus, the concentration of galactoglucomannans in the water phase is diminished. Con-
sequently, highly charged pectins are released which results in a higher anionic charge
of the waters (Holmbom et al . 1991; Thornton et al . 1994).
In the papermaking process water dissolved hemicelluloses often give rise to various
problems like the growth of fungi and bacteria. Due to the anionic charge of these
polyelectrolytes they tend to interact with cationic papermaking chemicals, e.g. retention
aids, increasing the consumption of these (Wagberg and Odberg 1991). On the other
hand, hemicelluloses have been found to be beneficial for paper quality and, for example,
strength properties are claimed to be improved by adsorbing galactoglucomannans and
galactomannans on fiber surfaces (Hannuksela et al . 2004).
The adsorption of dissolved hemicelluloses on extractive colloids originating from
spruce ( Picea abies ) and their stabilizing effect on colloids in TMP process waters has
been extensively studied by Johnsen et al . (2004); Sihvonen et al . (1998); Sundberg
et al . (1994a,b). The steric hindrance created around colloidal wood extractives by
dissolved hemicelluloses, especially GGMs, prevents the aggregation and accumulation
of colloids also at high ionic strengths in the presence of sodium and calcium salt
ions (Hannuksela and Holmbom 2004; Sundberg et al . 1996a). This enhanced sta-
bility of wood extractives reduces its tendency to form deposits (Otero et al . 2000).
Thus, the runnability of the papermachine is improved. GGMs have also been shown
to strongly adsorb on bleached kraft pulp fibers whose surfaces mainly consisted of