11.3 Dissolving Grade Pulp

amounts of glucomannan seem to deteriorate filtration of the dope. It is assumed

that glucomannan is competing against cellulose for acetylation, leaving swollen

fiber fragments in the solution. Xylan causes only a moderate drop in filtration in

the range below 2%, but this is the major contributor to acetate color and thermal

Instability. Diacetate color is measured by determining the yellowness coefficient

[Cy = (T640 – T440)/T640] of a 12% diacetate dispersion in a mixed solvent of dichloromethane

and methanol. The xylan originating fromPHK pulps causes more intense

yellowing than that present in acid sulfite pulps (Fig. 11.7). This behavior has been

explained by the lower content of uronic acid side chains of the kraft xylan compared

to sulfite xylan which, on acetylation, produces hazy solutions [20]. Kraft cooking conditions

favor the removal of 4-O-methylglucuronic acid substituents from xylan,

whereas xylan retains its branched structure during acid sulfite pulping.

0 1 2 3

0.0

0.1

0.2

0.3

Acid Sulfite PHK

Yellowness Coefficient

of Diacetate

Xylan content [%]

Fig. 11.7 Effect of the xylan content originating from acid

sulfite and PHK pulps on yellowness coefficient of diacetate

solution [17,19]. For pulp acetylation, see Fig. 11.6.

Moreover, the higher molecular weight of the kraft xylan as compared to the

sulfite xylan gives rise to a higher yellowness coefficient. It has been assumed that

the diacetate color is caused by undissolved particles in the size range of 1 lm,

since centrifugation eliminated both color and fiber fragments [21].

The yellowness of the cellulose triacetate solution is also closely related to the xylan

content of the initial pulps, and again, at equal xylan content, the PHK pulps reveal a

higher yellowness coefficient than the corresponding sulfite pulps (Fig. 11.8).

These findings agree with the results reported by Conca and associates, who

showed that xylans of low uronic acid content gave poor acetate solution properties

[23]. Later, Wells and coworkers [24], as well as Gardner and Chang [25], con-

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11 Pulp Properties and Applications

0 1 2 3 4 5

0.0

0.2

0.4

0.6

0.8

1.0

HW-, SW-PHK SW-Acid Sulfite Eucalyptus Acid Sulfite

Yellowness Coefficient

of Triacetate

Xylan content [%]

Fig. 11.8 Effect of xylan content originating from acid sulfite

and PHK pulps on yellowness coefficient of triacetate solution

[22]. Pulp acetylation according to the low-catalyst process

using 0.3% sulfuric acid catalyst based on initial amount of

cellulose.

firmed the poor acetylation performance of xylans derived in particular from alkaline

processes and which are known to possess a lower degree of branching and

have a higher molecular weight as compared to acid sulfite xylan.

Quite recently, residual chromophores in cellulose triacetate were isolated by

treating the cellulosic material with a boron trifluoride:acetate acid complex,

BF3.2HOAc, in the presence of small amounts of sodium sulfite. The identified

structures of the compounds causing the yellowish appearance indicate that the

formation pathways of chromophores involves thermal condensation of degraded

cellulose or hemicellulose compounds to form aromatic condensation products

(primary chromophores such as 2,5-dihydroxybenzoquinone, o-dihydroxybenzene,

and others), self-condensation of acetic acid derivatives and the reaction of primary

chromophores with acetic acid [11].

The detailed mechanismof the preferred chromophore formation of pentoses over

hexoses in both acidic and alkaline processing is, however, still under investigation.

Extractives, resins

Pulp resins, determined as acetone or dichloromethane (DCM) extractives, play

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

1030