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Chemical composition of hemicelluloses, and functional groups (which for example
Indicate oxidative damage of carbohydrates).
In general, the R-values of paper pulps are typically at higher levels as predicted
From the cellulose content, indicating that part of the hemicelluloses are alkali-insoluble.
The low difference between R18 and R10, as observed particularly for softwood
Kraft pulps, refers to both high molecular-weight cellulose and alkali-insoluble
Hemicellulose fractions that mainly derive from xylan with a low content of
Uronic acid side chains. It is interesting to note that the pulps containing the highest
Amount of hemicelluloses, the hardwood kraft pulps, exhibit extremely high Rvalues
At the same time, completely comparable to those of dissolving pulps. In a
First approximation, this can be attributed to high molecular-weight xylan comprising
A low degree of substitution of uronic acid side chains. However, the contrary
Is true for sulfite pulps. Even though the R-values of sulfite pulps are generally
Lower as compared to kraft pulps, their cellulose content is considerably higher,
as indicated from carbohydrate analysis. Moreover, the higher values of (R18 –
R10) as compared to kraft pulps suggest a higher fraction of low molecular-weight
Cellulose and differences in the supramolecular structure.
The carbohydrate analysis of pulp provides a variety of valuable information
About the wood species, the pulping process, and even pulping and bleaching conditions.
Sulfite pulps have a significantly lower content of hemicelluloses than
Kraft pulps, which may be attributed in general to both a better preservation of
Cellulose and higher losses of hemicelluloses. It is well established that virtually
No cellulose is lost, while the hemicellulose fraction (particularly from hardwood)
Is rather unstable in acid sulfite pulping, and this results in a low (hemicellulose)
Yield and low molecular weight of the residual hemicelluloses. Generally, kraft
Pulps obtain higher yields in the case of hardwoods, whereas sulfite pulps preserve
Better yields in the case of softwoods. When examining one particular cook-
Pulp Properties and Applications
ing process, for example the kraft process, the cellulose:hemicellulose ratio is
rather indicative of pulp strength [18]. Molin and Teder have shown convincingly
that the specific bond strength (relationship between Z-strength/Scott Bond energy
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Tab. 11.1 Continued. | | | And density) as well as the fiber strength (dry and rewetted zero-span tensile |