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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
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Residual Lignin Structure (see Section 4.2.5) | | | General Reactions Decreasing the DP |