In the steeping lye inhibits cellulose degradation during ageing due to the

additional oxygen consumption through these degraded carbohydrates [13]. In

Addition, they react preferentially with carbon disulfide in the subsequent xanthation

Process, thus leading to inhomogeneously substituted cellulose which consequently

adversely affects viscose filterability [14]. A good quality of viscose solution,

characterized by low particle content (< 3 lm) and good viscose filterability,

Is governed by a low content of noncellulosic impurities, particularly pentosans,

certain inorganic substances, and resins [15].

Lenz et al. have shown that fiber tenacity is altered due to incorporation of low

Molecular-weight hemicellulose if they exceed a certain concentration level in the

steeping liquor (Tab. 11.6) [13].

Tab. 11.6 Influence of hemicellulose concentration in steeping

lye on fiber tenacity [13]. Pulp substrate: ECF-bleached beech

Acid sulfite dissolving pulp.

Alpha-Cellulose

[%]

Hemicellulose

Content in steeping lye

[g L–1]

Fibre-DPn Tenacity (cond)

[cN tex–1]

Elongation (cond)

[%]

90.5 17.3 254 27.5 15.3

90.5 20.5 253 26.6 15.5

90.5 37.0 261 26.0 15.0

91.3 17.6 254 28.6 16.8

91.3 35.4 255 28.0 16.2

Siclari reports that the use of dissolving pulps with increasing amounts of low

Molecular-weight carbohydrates is closely related to a decrease in the wet tenacity

of a Polynosic-type fiber [16]. An increase of the low molecular-weight fraction determined

By gel permeation chromatography (GPC) measurement and an

Increase in the xylan content in the respective viscose fibers clearly support the

Assumption that the decrease in fiber tenacity is caused by the incorporation of

The acid-insoluble hemicellulose fraction (beta-cellulose) which occurs during the

Regeneration process in the highly acidic spin bath.

Hemicelluloses also contribute to discoloration of the resulting cellulose products

During both alkaline (e.g., Viscose) and acidic conversion (e.g., acetate) steps

[13,17]. The chromophore formation of the hemicelluloses under alkaline conditions

Is closely associated with the presence of carbonyl and carboxyl groups.

Quite recently, it could be shown that hemicelluloses, particularly xylans, are

involved in the discoloration of Lyocell dopes [18]. Pentoses generally revealed a

Significantly higher rate of chromophore formation as compared to hexoses. In

Pulp Properties and Applications

Addition, the glucuronic acid side chains have been shown to be a source of

Enhanced chromophore formation, while the corresponding gluconic acid

revealed no measurable yellowing [18]. The detailed mechanism of the preferred

Chromophore formation of pentoses over hexoses in both acidic and alkaline processing

Дата добавления: 2015-10-21