Importance is in the printing and writing grades. In these grades, softwood

kraft is used for its strength characteristics, while hardwood kraft, having shorter

fibers, is used for its superior printing properties.

For dissolving pulp production, only acid sulfite cooking and the prehydrolysis

kraft process are of major practical importance. Unlike paper-grade pulping, the

acid sulfite process is the dominant system for the production of dissolving pulps,

and accounts for approximately 60% of the total production. Compared to papergrade

production, the manufacture of dissolving wood pulp represents a niche

production. However, the high demands for cellulose purity and reactivity – as

well as its manifold routes of utilization – are the reason for its advanced state of

technology within the pulp industry.

In this chapter, the main emphasis is placed on a comprehensive discussion of

the physical and chemical properties of dissolving pulps because they serve as

appropriate substrates of which many aspects of pulp characterization may also

be transferred to paper-grade pulps. The papermaking properties of pulp are

extensively described and reviewed in the relevant literature [1–3]. The following

section on paper-grade pulp is limited to a short description of certain chemical

and macromolecular properties which are rarely presented in the literature. (The

chemical and macromolecular properties of chemical paper pulps, however, were

rarely the subjects of published literature. This section is therefore concerned

with a very short description of important differences in chemical and macromolecular

properties of a selection of chemical paper pulps.)

11.2

Paper-Grade Pulp

Chemical paper-grade pulps can be categorized into hardwood and softwood sulfite

and kraft pulps. The definite differences in pulp properties between sulfite

and kraft pulps have been the subject of numerous studies [4]. Jayme and Korten

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

1010 11 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