An ambivalent role in dissolving pulp processing. When exceeding a certain

Threshold concentration, resins may cause severe problems along the process

Chain such as precipitation (preferably taking place through sudden changes from

Alkaline to acid pH), haze in the viscose, clogging of the spinnerets, and yellowing

Dissolving Grade Pulp

Of the yarn. On the other hand, the presence of resins considerably improves the

Accessibility of reagents to the cellulose substrate (e.g., alkali cellulose) due to lowered

Surface tension. The amount of extractives in the dissolving pulp is determined

By both the wood species used and the pulping and bleaching operations.

Acid sulfite cooking is rather sensitive to wood that is rich in resins. The major

Problems arise with phenolic extractives originating from pine, larch, Douglas fir

And other extractive-rich wood species, which tend to undergo condensation reactions

With reactive lignin structures. The formation of high molecular-weight resinous

Products may cause uncontrollable pitch problems in subsequent operation

Steps. These may be largely overcome by applying a two-stage process with an initial

Bisulfite or neutral sulfite stage. Thereby, the most reactive groups of lignin

Are protected by sulfonation. Hardwoods are generally better suited for acid sulfite

Pulping than softwoods. In the case of some resinous hardwoods, such as birch or

Some aspen species, the delignification of parenchyma cells with a high content

of resins remains incomplete. Subsequent chlorine-free bleaching sequences –

particularly those containing ozone stages – contribute significantly to reducing

The resin content. Additionally, the short parenchymal cells may be removed by

Fractionation, though this is connected with high yield loss.

Kraft cooking is less sensitive to wood raw materials that are rich in resins

Because most acidic extractives, together with part of the neutral lipophilic compounds,

Are dissolved in the cooking liquor, leaving little resin in the unbleached

Pulp. However, if the fraction of unsaponifiable compounds is high (as with many

Hardwoods), the fatty acid soaps formed do not possess sufficient micellar-forming

Properties to carry less polar compounds into solution. During PHK pulping

Of Eucalyptus globulus L., nearly all polyphenols, phenols, fatty acids are dissolved.

Two-thirds of the neutral compounds are left in the unbleached pulp, while twothirds

consists of b-sitosterol [26]. During bleaching, the unsaponifiables are modified

Дата добавления: 2015-10-21