The transformation to Na-cellulose I begins at a NaOH concentration of about

6–7% for wood pulps and beyond 10% for cotton linters, respectively, and is completed

at about 14–15% NaOH for both cellulose substrates (Fig. 11.18).

The resistance of cotton linters to mercerization is connected with its different

Supramolecular and fibrillar structure compared to dissolving pulps derived from

Pulp Properties and Applications

0 3 6 9 12 15

Cellulose II [%]

Na-Cellulose I [%]

HW-Sulfite HW-PHK HW-PHK (high P-factor) Cotton Linters

NaOH-Konzentration [%]

Fig. 11.18 Lattice transition from cellulose I to Na-cellulose I

Of HW-sulfite and HW-PHK pulps of different purity, depending

Upon steeping lye concentration at room temperature

[56]. An overlay of the lattice transition curve from cellulose I

to cellulose II of cotton linters is marked for comparison [58].

Wood or other lignocellulosic substrates. The rather uniform cellulose molecules

(low polydispersity) in cotton linters are arranged as highly organized architecture

Of fibrillar elements. The high packing density of the microfibrils paired with a

Dense and specific build-up of the primary wall in which the fibrillar elements are

Arranged in a network-like helical fashion around the secondary wall lead to a

Decreased accessibility of reagents which is (also) expressed in a reduced accessible

Pore volume as determined by the water retention value (see Tab. 11.10). Within

The wood dissolving pulps, acid sulfite pulps generally require a lower lye concentration

for the lattice conversion than PHK pulps [9]. The data in Fig. 11.19

Illustrate that the difference in lye concentration between the two types of dissolving

wood pulps necessary to transform 50% to Na-cellulose I is about 0.9% (9.4%

for sulfite versus 10.4% for PHK pulps). Compared to cotton linters, the difference

From sulfite pulps is small and can be further reduced by reinforcing the prehydrolysis

Conditions. The more complete removal of xylan by increasing prehydrolysis

intensity (see Fig. 11.18: HW-PHK high P-factor) clearly changes the supramolecular

Structure and results in a shift of the transition curve to a lower

NaOH concentration. Above 11% NaOH, the curve of the highly purified PHK

Pulp proceeds similar to that of the sulfite pulp.

Closer examination of the mercerization behavior reveals that even between different

Sulfite dissolving pulps, differences in the course of the curve have been

reported [58]. Recently, it was shown that pulping and subsequent purification

Dissolving Grade Pulp

0 2 4 6 8 10 12 14

Medium Viscosity, Low Purity Medium Viscosity, High Purity

Дата добавления: 2015-10-21