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Whereas xylan retains its branched structure during acid sulfite pulping.

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  1. Acid side chains. The glycosidic bond between uronic acid and the xylan backbone
  2. Advanced a hypothesis that, for a given degree of delignification, sulfite pulps
  3. Alkaline Sulfite Pulping
  4. Alternative Sulfite Pulping Concepts
  5. And softwood), molecular weight of carbohydrates, supramolecular structure,
  6. Beech Sulfite Beech Kraft Eucalyptus Kraft
  7. Bleaching to the activation with Mg(OH)2 is compensated by a final hydrosulfite

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-

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.


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Читайте в этой же книге: Is rather unstable in acid sulfite pulping, and this results in a low (hemicellulose) | Groups, depending on the amount and their distribution along the polysaccharide | Beech Sulfite Beech Kraft Eucalyptus Kraft | From rather high molecular-weight cellulose and very low molecular-weight hemicellulose | Fiber length below 0.2 mm, indicating the presence of large amounts of fines derived | As resins or inorganic compounds can adversely affect the filterability of viscose, | Is an important characteristic of dissolving pulps. Finally, the qualitative and | In the past, the latest forecasts reveal a slight change in this trend (see Chapter | Lyocell | In the steeping lye inhibits cellulose degradation during ageing due to the |
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Is still under investigation.| Extractives, resins

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