Two major characteristics. First, kraft pulps show a much higher resistance

Towards beating than do sulfite pulps. For sulfite pulps it requires less time to

Reach a given drainage resistance as compared to kraft pulps. Second, sheets from

sulfite pulps are considerably lower in tear (22%) and burst (17%) strengths, and

somewhat lower in tensile (10%) as compared to kraft pulp sheets [12]. At the

Same tensile index, kraft pulps typically exceed sulfite pulps in tearing strength by

40–80% [13]. It was reported that over the cellulose content range of 44–80%, the

Fiber strength is directly proportional to the cellulose content, but above a value of

80% cellulose content the fiber strength greatly diminishes [14]. This may be due

to the replacement of flexible cellulose–hemicellulose–cellulose bonds by the

more rigid cellulose–cellulose bonds.

Page proposed a concept based on supramolecular properties to explain differences

Between sulfite and kraft pulps. Sulfite pulp is seen as containing largely

Crystalline and paracrystalline cellulose, whereas in kraft pulp substantial transformation

Of the paracrystalline regions to the amorphous state has occurred. The

Relationship to the degree of order has been established by measuring the level-off

DP (LODP), which is a simple means of determining the mean crystalline length

[13]. Furthermore, Page suggested that paracrystalline regions in native cellulose

Fibrils are transformed into amorphous regions during pulp processing that are

Viscoelastic and capable of absorbing more energy under mechanical stress. The

Paper-Grade Pulp 1011

High LODP, the high modulus in the stress-strain curve, the excellent swelling

properties [15,16], the low tear strength, and the high beating rate of sulfite pulps

Were attributed to the greater amounts of crystalline and paracrystalline regions

Compared to kraft pulps.

Sulfite pulps contain large amounts of crystalline and paracrystalline cellulose,

While kraft pulps consist predominantly of amorphous cellulose domains. Page

Recognized an increase in tear strength of a pulp with increasing proportion of

amorphous cellulose. This is also reflected in the low modulus of the stress–strain

Curve of a single fiber, as illustrated in Fig. 11.1.

0 2 4 6 8

0,0

0,5

1,0

1,5

Bleached Kraft+16% NaOH Bleached Kraft + 7% NaOH Kraft, 60% yield

Acid Sulfite, 63% yield Holocellulose

Stress [GPa]

Strain [%]

Fig. 11.1 Stress–strain curves of single fibers prepared by

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