Is still under investigation.

Acid processing generally requires pulps of higher purity as compared to alkaline

Processes. Acetate pulps are high-purity pulps containing hemicelluloses in

an amount less than 1.5% and no detectable residual lignin. Experimental findings

Strongly suggest that even very small amounts of certain hemicellulose fractions

Play an important role in the formation of haze and color of cellulose triand

Diacetate solutions. Glucomannan from both acid sulfite and prehydrolysis

Kraft (PHK) pulps is a major source for diacetate haze, false viscosity, and poor

filtration [17]. False or anomalous viscosity determines production capacity, and is

Defined as the percent increase in dope viscosity compared to that of a dope prepared

From cotton linters diacetate of the same composition and intrinsic viscosity.

The effect of the pulp mannan content on false viscosity and filterability of a diacetate

Solution in acetone is illustrated in Fig. 11.6.

Glucomannan is very detrimental to diacetate filterability (obtained from the filtration

of 18% diacetate acetone dope through a cotton fabric media at 275 kPa),

Whether it comes from acid sulfite, PHK or conventional kraft pulp. Even small

0 1 2 3

False Viscosity Effect [%]

Filtration Value [g/cm2]

Mannan content [%]

Acid Sulfite PHK Kraft

Fig. 11.6 Effect of mannan content in various pulps on diacetate

filterability and false viscosity [17,19]. Pulp acetylation

according to the high-catalyst process using 14% sulfuric acid

Catalyst based on initial amount of cellulose. Diacetate dope

consists of 18% acetate, 1.35% water, and 80.65% acetone.

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,

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