Degradation kinetics can be described by means of zero order, whereby using the

model equation from Sakurada:

DPn_t _

DPn_0 _ _ kD Dn _2_

Dissolving Grade Pulp

Where kD is the reaction rate of the radiation degradation.

The exponent n takes into account the fact that the polydispersity changes during

Radiation degradation. According to Fig. 11.30, the course of chain scissions is

Comparable for both pulps investigated. Because of a larger amount of carbonyl

Groups (including reducing end groups), the degradation rate of a sulfite pulp is

More enhanced after a subsequent steeping step as compared to a PHK pulp. As

Mentioned previously, the statistical character of molecular weight degradation

Has been proposed as one important advantage of the electron beam treatment

Compared to chemical degradation processes. If random chain scission is

assumed, then nonuniformity (U = PDI – 1) would approach unity, as indicated

by Kuhn [89].

The data in Fig. 11.31 show that the MWD curves through progressive degradation

Are practically equal for both degradation processes, namely electron beam

Treatment followed by steeping and oxidative alkaline ageing. Furthermore, the

Results suggest that the statistical degradation performance applies to both degradation

Processes. The conclusion is that radiation degradation and alkaline ageing

Successively reduce the molecular weight, making the MWD progressively narrower.

In both processes, the reaction kinetics is governed by the content of carbonyl

Groups, indicating that the degradation rate of sulfite pulps is higher than

That of PHK pulps.

600 500 400 300 200

HW-Sulfite: Ageing E-beam&steeping

HW-PHK Ageing E-beam&steeping

Polydispersity Index [M

w

/M

n

]

Viscosity [ml/g]

Fig. 11.31 Polydispersity index (PDI) as a function

Of cellulose viscosity indicating the course

of degradation: comparison of electron beam

Treatment with oxidative alkaline

Treatment for both hardwood sulfite and PHK

Dissolving pulps. The PDI was determined

From GPC-MALLS measurement (according to

[68]).

Pulp Properties and Applications

Overview of Pulp Specification

As stated previously, the suitability of dissolving pulps can be adequately determined

Simply by simulating the conversion processes to the final products, at

Least on a laboratory scale. The most important property of virtually all dissolving

pulps can be expressed by the term “chemical reactivity”. Pulp reactivity, however,

Cannot be described by a single structural feature, but rather by both the physical

Дата добавления: 2015-10-21