Be expected when high dosages of, for example, ozone, or other bleaching chemicals

Of high oxidation potential are applied.

The alkali resistance determined at 18% (R18) and 10% (R10) NaOH concentration

At room temperature is a measure of estimating the cellulose content (total

For R18 and long-chain cellulose for R10), and is usually used to characterize the

Degree of purity of dissolving pulps (see Tab. 11.7, Dissolving pulp characterization).

Although not very common in paper pulp analysis, the R-values provide

Insight into many aspects of pulp origin and properties, including the type of

Pulp, wood species (at least allowing a distinction to be made between hardwood

And softwood), molecular weight of carbohydrates, supramolecular structure,

Chemical composition of hemicelluloses, and functional groups (which for example

Indicate oxidative damage of carbohydrates).

In general, the R-values of paper pulps are typically at higher levels as predicted

From the cellulose content, indicating that part of the hemicelluloses are alkali-insoluble.

The low difference between R18 and R10, as observed particularly for softwood

Kraft pulps, refers to both high molecular-weight cellulose and alkali-insoluble

Hemicellulose fractions that mainly derive from xylan with a low content of

Uronic acid side chains. It is interesting to note that the pulps containing the highest

Amount of hemicelluloses, the hardwood kraft pulps, exhibit extremely high Rvalues

At the same time, completely comparable to those of dissolving pulps. In a

First approximation, this can be attributed to high molecular-weight xylan comprising

A low degree of substitution of uronic acid side chains. However, the contrary

Is true for sulfite pulps. Even though the R-values of sulfite pulps are generally

Lower as compared to kraft pulps, their cellulose content is considerably higher,

as indicated from carbohydrate analysis. Moreover, the higher values of (R18 –

R10) as compared to kraft pulps suggest a higher fraction of low molecular-weight

Cellulose and differences in the supramolecular structure.

The carbohydrate analysis of pulp provides a variety of valuable information

About the wood species, the pulping process, and even pulping and bleaching conditions.

Sulfite pulps have a significantly lower content of hemicelluloses than

Kraft pulps, which may be attributed in general to both a better preservation of

Cellulose and higher losses of hemicelluloses. It is well established that virtually

No cellulose is lost, while the hemicellulose fraction (particularly from hardwood)

Дата добавления: 2015-10-21