Читайте также: |
|
Two counteracting effects. On the one hand, the removal of higher amounts of low
molecular-weight hemicelluloses, especially when using 10% NaOH, results in
The formation of a more homogeneous pulp with increased accessibility to the
Reagent. On the other hand, a denser network of hydrogen bonds ensures that
Pores are closed and accessibility becomes more difficult. In contrast, reactivity of
The kraft pulp increases continuously with increasing the NaOH concentration
Which may be accounted for by removal of the hemicelluloses. In the case of
Unpurified kraft pulp, the effect of improving reactivity due to pulp homogenization
prevails up to a NaOH concentration of about 15%. However, by exceeding
This NaOH concentration a slight decrease in reactivity is observed due to a
Decreasing accessibility.
0 5 10 15 20
0.0
Cotton Linters Bagasse-PH-Soda Bagasse-Kraft
DS-Acetyl (after 4 h)
NaOH concentration [%]
Fig. 11.20 Effect of NaOH concentration during
Cold caustic extraction on the reactivity
Towards acetylation of cotton linters, bagasse-
PH-Soda and bagasse-kraft pulps [62].
Initial pulp purities, measured in terms of
alpha-cellulose content: 99.1%, 91.9%, and
For cotton linters, bagasse-PH-Soda and
Bagasse-kraft pulps, respectively.
Dissolving Grade Pulp
Cell Wall Structure
The architecture of both native (cellulose I) and regenerated fibers (cellulose II) is
Constituted by micro- and macrofibrils. The latter are the structural elements of a
Single cellulose fiber. The cross-section of cotton linters fibers amounts to
17–27 lm, while that of spruce sulfite pulp fibers is 21–40 lm [63]. The cellulose I
Fibers are a buildup of single cell-wall layers that differ in their fibril texture, and
This contributes to the fiber properties.
The fibrillar morphology of pulps is highly dependent upon the pulping process
And pulping conditions. It is assumed that the active sulfite cooking chemicals,
Comprising hydrogen sulfite and hydrated sulfur dioxide, penetrate through the
Pits into the middle lamella where the pulping reaction starts from the primary
Wall across the cell wall. As a consequence, the primary wall is sometimes completely
Removed after acid sulfite pulping. Pulping under alkaline conditions (the
Kraft process), however, enables rather uniform pulping reactions across the cell
Wall layers due to the high swelling properties of white liquor. The morphology of
Cellulose samples is evaluated mainly using electron microscopy (scanning and
Transmission electron). A very good resolution of the fibrillar structure is obtained
Дата добавления: 2015-10-21; просмотров: 89 | Нарушение авторских прав
<== предыдущая страница | | | следующая страница ==> |
Low Viscosity, Low Purity Low Viscosity, High Purity | | | By the transmission electron microscopy (TEM) surface replica technique |