Reactions of Carbohydrates

The reactivity of carbohydrates in kraft pulping depends highly on structural features

such as morphology, crystallinity or degree of polymerization (DP). Cellulose

is more resistant towards alkaline media and suffers less degradation than hemicelluloses.

Under strongly alkaline conditions (as prevail in kraft and soda pulping), all carboxyl

groups are neutralized. Swelling promotes penetration of the cooking chemicals

into the wood matrix. The high pH at the beginning of the cook may also

ionize part of the hydroxyl groups and lead to a deacetylation (T > 70 °C) of acetyl

moieties in hemicelluloses (from softwood galactoglucomannans and hardwood

glucuronoxylans) with increasing temperature. The hemicelluloses, in particular,

undergo base-catalyzed hydrolysis of acetoxyl groups to produce acetoxyl anions,

which consume a considerable amount of base during the heating-up period of

the kraft cook, and are responsible for a decrease of about 0.35 and 0.9 mol alkali

per kg wood for hardwood and softwood, respectively [56]. In addition, acids

formed from the carbohydrates decrease the effective alkali content.

A major portion of the deacetylated hemicelluloses dissolve in the pulping

liquor. At a later stage of the cook, the hemicelluloses precipitate on the cellulose

fiber, increasing the fiber strength. During the bulk phase of the cook the carbohydrates

– including the deacetylated hemicelluloses – are reasonably stable. In the

final residual delignification phase the decrease in the concentration of alkali

tends to decline, and this leads to the aforementioned reprecipitation of hemicelluloses

on the fiber matrix. The retake of xylan (addressed in detail by Meller [57])

can be analyzed spatially on the resulting fibers by using either chemical [58]or

enzymatic peeling techniques [59,60]. Degradation of celluloses also occurs mostly

in the final phase.

In general, the following reactions proceed with carbohydrates under kraft conditions:

_ Deacetylation @T < 70 °C

_ Peeling and stopping reactions@T > 80 °C

_ Random hydrolysis (= secondary peeling) @T > 140 °C

_ Fragmentations

_ Dissolution of hemicellulose

174 4 Chemical Pulping Processes

_ Elimination of methanol from 4- O -methylglucuronic acid residues

and formation of hexenuronic acid

_ Re-adsorption of hemicelluloses on the fiber surface

_ Stabilization of carbohydrates against peeling (oxidation)

_ Formation of chromophores