80 °C [12]. Caustic treatment: 5%consistency ,

30 Min reaction time, NaOh concentrations:

20, 40, 60, 80, 100, 140, 180, 280, and 340 g L–1.

938

8.2 Reactions between Pulp Constituents and Aqueous Sodium Hydroxide Solution

dissolved hemicelluloses was analyzed by anion-exchange chromatography (AEC)

with pulsed amperometric detection (PAD) after separation into beta- and

gamma-cellulose fractions [17]. It is noted that the proportion of beta-cellulose

decreases with increasing temperature, particularly above 50 °C. While the absolute

amount of gamma-cellulose remains fairly constant at 20–50 °C throughout

the whole range of NaOH concentrations investigated, the increase in the total

amount of dissolved hemicelluloses at 80 °C is mainly attributed to an increase in

the gamma-cellulose fraction (see Fig. 8.2). The fact that up to 90% of the gammacellulose

fraction consists of degraded carbohydrates (equal to non-neutral sugars)

clearly indicates that the removal of hemicelluloses through alkaline treatment at

80 °C is mainly governed by chemical degradation reactions (e.G. Peeling reaction).

As stated previously, the extent of chemical degradation reactions decreases with

decreasing temperatures. Accordingly, the amount of degraded carbohydrates

decreases at lower temperatures. The beta-cellulose fraction originating from any

alkaline treatment consists almost exclusively of neutral sugars (except for uronic

acid side chains and oxidized end groups). As anticipated, the maximum yield of

beta-cellulose corresponds with the maximum solubility of the hemicelluloses

(sum of beta- and gamma-celluloses) at an alkaline treatment at 20 °C and 50 °C,

and with the lowest xylan content in the pulp residue at any temperature investigated.

Parallel to its highest yield, the beta-cellulose consists of the highest glucan

content (74 wt.%, 59 wt.% and 47 wt.% based on beta-cellulose for 20 °C, 50 °C,

and 80 °C, respectively). It can be assumed that the glucan fraction derives from

degraded cellulose and comprises the highest molecular weight within the betacellulose.

Surprisingly, the treatment at 80 °C also produces a beta-cellulose fraction

enriched with degraded cellulose at the same conditions where a complete

removal of alkali soluble xylan occurs. This indicates that at a lower lye concentration

the cellulose structure is opened by inter- and intramicellar swelling, even at

high temperatures. Apart from degraded cellulose, the predominant hemicellulose

fraction in beech sulfite dissolving pulps is made up of xylan, while the glucomannan

content is almost negligible. Therefore, the main objective of the alkali

purification processes comprises removal of the residual xylan content.

By comparing the amount of xylan removed from the pulp with the amount

recovered in both the beta- and gamma-cellulose fractions, it can be concluded

that most xylan is recovered in oligomeric and polymeric structures. The proportion

of degraded xylan is greater only in the lower NaOH concentration range (up

to 80 g L–1) where the easily degradable fraction is removed. Apart from the minimum

at a NaOH concentration of 100 g L–1 at 20 °C and 140 g L–1 at 50 °C and

80 °C due to the increased dissolution of degraded cellulose, the beta-cellulose

becomes increasingly enriched with xylan as both the NaOH concentration and

temperature are raised (Fig. 8.3). This means that the xylan part in the hemicelluloses

is clearly more resistant to alkaline degradation than the other carbohydrate

components. The major part of the xylan remains stable even after hot caustic

extraction (100 °C, 0.25 N NaOH, 1–4 h) as exemplified in a study conducted by

Corbett and Kidd [11].

939

8 Pulp Purification

0 100 200 300

0

20

40

60

80

100

20 єC 50 єC 80 єC

Xylan content in Beta-Cellulose [%od]

NaOH concentration [g/l]

Fig. 8.3 Xylan content in beta-cellulose as a function of

NaOH concentration and temperature [12]. Caustic treatment:

5%consistency , 30 min reaction time, NaOH concentrations:

20, 40, 60, 80, 100, 140, 180, 280, and 340 g L–1.

Model compound studies using aldobiouronic (4-O-methyl-b-d-glucuronic acid-

(1→2)-xylose) (4OMeGlcA) and aldotriouronic acid (4-O-methyl-b-d-glucuronic

acid-(1→2′)-xylobiose), confirmed that substitution at position 2 of the terminal,

reducing xylose unit strongly inhibits alkaline degradation [18]. In the absence of

a C-2 substituent, the xylose chain is rapidly shortened according to classical peeling

pathways, until the next C-2 substituted xylose unit is reached. The results

explain the observed higher stability of the xylan fraction as compared to the glucan

fraction isolated from the steeping lye. Thus, the decreased alkaline degradation

of the xylan isolated from the beta-cellulose fraction can be attributed to the

presence of side branches consisting of 4-O-methyl-glucuronic acid as detected by

FT-IR-spectra and by MALDI-MS with a 4OMeGlcA:Xylose-ratio of 5:100 at the

maximum [19].

The interaction between aqueous NaOH and cellulose also affects the supramolecular

structure of cellulose. Increasing the NaOH concentration beyond 70–

80 g L–1 at room temperature leads gradually to a change from the native cellulose

I structure into the Na-cellulose I structure. Thereby, the plane distance of the

101-lattice planes is widened from the original 0.61 nm to more than 1.2 nm due

to incorporation of the sodium hydrate ion [20]. At a NaOH concentration between

160 and 190 g L–1 the lattice transformation to Na-cellulose I is completed.

This structure gives rise to a better reactivity with chemical reactants due to the

better accessibility of the hydroxyl groups on C6 and C2 (e.g., CS2 in the case of the

viscose process). It is well known that the transition curve from cellulose I to Na-

940

8.2 Reactions between Pulp Constituents and Aqueous Sodium Hydroxide Solution

cellulose I depends also on the supramolecular structure of the dissolving pulp.

Sulfite pulps generally require a lower lye concentration to achieve full lattice conversion

than do PHK pulps [21]. The somewhat higher mercerization resistance

may be due to the less degraded primary cell wall of the latter, restricting swelling

by NaOH [20]. The changes in supramolecular structure upon alkali treatment of

two dissolving pulps, beech acid sulfite and eucalyptus PHK pulps, have been