Читайте также: |
|
Na-Cellulose I [%]
NaOH concentration [wt%]
Fig. 11.19 Lattice transition from cellulose I to Na-cellulose I
Of HW-sulfite dissolving pulps subjected to different pulping
And purification pretreatments in dependence on steeping lye
concentration at room temperature [56].
Procedures of hardwood sulfite dissolving pulps clearly exert an influence on the
shape of the transition curve, especially beyond an NaOH concentration of 10% at
Room temperature.
The enhanced viscosity degradation during final phase of sulfite cooking, combined
With reinforced purification during hot caustic treatment, accelerates the
transition to Na-cellulose I in the range of NaOH concentration of 10–12%. This
Type of activation towards alkali cellulose formation is also expressed in a better
Viscose filterability, thus indicating an improved reactivity.
During the course of a cold alkali extraction treatment for the manufacture of
High-purity wood dissolving pulps, a partial lattice transformation from cellulose I
To cellulose II usually occurs, depending on NaOH concentration in the aqueous
Pulp suspension (see Fig. 8.4). For the manufacture of very high-purity pulps (R18
> 98%), NaOH concentrations up to 10–11% are required, thus leading to a significant
Shift in the crystalline structure to cellulose II, as shown in Tab. 11.10 (HWPHK-
CCE). After drying, the reactivity of the mercerized dissolving pulps may
alter [59]. It is reported that dried, partly mercerized dissolving pulps cannot be
Converted to cellulose acetate under the normal processing conditions, but the
reactivity during nitration is not affected [60,61].
However, different pulps respond differently to cold alkali extraction, and hence
Behave differently towards acetylation. Thus, bond cleavage and solution of hemicelluloses
May lead to increased reactivity, while drying may lead to decreased
Pulp Properties and Applications
Reactivity. It was shown that the reactivity of a cotton linters pulp towards acetylation
Decreases steadily with increasing the concentration during cold caustic
Extraction (Fig. 11.20). The loss in reactivity is due to drying of the pulp after the
Cold caustic refining, which induces the formation of hydrogen bonding. With rising
Alkali concentration the hydrogen bonds become more dense, and this finally
Impairs the accessibility of the acetylation agent. When subjecting a low-grade dissolving
pulp with a residual pentosan content of 7.8% to cold alkali extraction, the
Дата добавления: 2015-10-21; просмотров: 103 | Нарушение авторских прав
<== предыдущая страница | | | следующая страница ==> |
The transformation to Na-cellulose I begins at a NaOH concentration of about | | | Reactivity towards acetylation changes only marginally. This may be explained by |