Acid side chains. The glycosidic bond between uronic acid and the xylan backbone

Is particularly sensitive to alkaline cleavage. The decrease in uronic acid content

Results in a drastic decrease in the solubility of the xylan in aqueous alkaline solutions.

This leads to the well-known phenomenon of xylan reprecipitation onto the

Surface of pulp fibers during the final kraft cooking phase, as demonstrated by

Yllner-Enstrom when treating a mixture of cotton linters and xylan extracted from

birch at kraft cooking conditions [9,10]. The significantly higher enrichment of

Xylan in the surface layers of kraft pulps as compared to sulfite pulps has been

Confirmed by Dahlmann and Sjoberg, who applied an enzymatic peeling technique

To characterize carbohydrate composition across the cell wall layers of both

softwood and hardwood pulp fibers [11].

When examining the properties of paper pulps, sulfite and kraft pulps differ in

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

Дата добавления: 2015-10-21