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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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Is still under investigation. | | | Extractives, resins |