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

And density) as well as the fiber strength (dry and rewetted zero-span tensile

Strength) are unaffected by the hemicellulose content of spruce kraft pulps comprising

a cellulose:hemicellulose ratio between 3 and 9 (equal to a hemicellulose

content of 10–25%). On the other hand, the tensile index decreases by 30% when

the cellulose:hemicellulose ratio is increased from 3 to 9 when compared at the

Same density. In parallel with the decrease in tensile strength, the tear strength

increases by up to 100%. In contrast to fiber strength, the sheet strength is highly

Dependent upon the hemicellulose content. The stiffness of the fibers has been

Identified as the major influencing parameter of sheet strength properties. It has

Been shown that fibers become stiffer and more brittle with increasing proportions

Of hemicelluloses in the kraft pulp. One of several explanations for this is

that the shrinkage of the fibers during drying may increase with a decreasing cellulose:

Hemicellulose ratio. Fibers with higher shrinkage are subjected to higher

Stress during drying of sheets, which in turn increases the E-modulus of the fibers

[19]. Fibers with a high E-modulus show high tensile strength, but low tear

strength (see also Ref. [14]).

During kraft cooking, 4-O-methylglucuronic acid groups attached to xylan are

Partially converted to hexenuronic acid (HexA) groups (see Section 4.2.6.2, Modified

Kraft cooking, “Effect on carbohydrate composition”). These groups react

With electrophilic bleaching chemicals such as chlorine dioxide and ozone. Additionally,

Acid treatment at elevated temperature may also be used selectively to

Remove the HexA groups. Fully bleached kraft pulps with kappa numbers below 1

Contain only traces of residual HexA, as shown in Tab. 11.1.

In contrast to dissolving pulp specification, the standard characterization of

Paper-grade pulp does not include the determination of functional groups. However,

Many important pulp properties originate from carboxyl and carbonyl

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