Advanced a hypothesis that, for a given degree of delignification, sulfite pulps

were more degraded on the outside of the fiber than were kraft pulps [5]. This

hypothesis was supported by Luce, who developed the method of “chemical pulping”

to study the radial distribution of different properties across the cell wall [6].

According to his results, the molecular weight of the polysaccharides of the kraft

Pulp was uniform throughout the fiber wall, while in the sulfite pulp it was low at

The outermost layers and increased towards the inner layers. The significantly

Pulp Properties and Applications

Lower molecular weight of the residual hemicellulose fraction in sulfite as compared

to kraft pulps has been also pointed out by Jayme and Koppen [7]. It can be

Concluded that (acid) sulfite cooking liquor penetrates through the pits into the

Porous middle lamella where delignification proceeds from the primary wall. The

Degradation reaction involving both lignin and carbohydrates thus proceeds from

The outer to the inner cell wall layers, and this results in a broad molecular weight

Distribution of the polysaccharide fraction. In contrast, alkaline cooking conditions

Promote rather uniform pulping reactions due to the high swelling properties.

Hamilton and Thompson have studied the main differences in the carbohydrate

Constituents of wood celluloses prepared by the sulfite and kraft pulping

processes [8]. According to their findings, xylan from hardwoods is converted to

Low molecular-weight 4-O-methylglucuronoxylan by the acid sulfite pulping procedure,

And to a rather high molecular-weight xylan with a low number of uronic

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

Дата добавления: 2015-10-21