Fig. 4.25 Exchange of hydroxyl groups by sulfonate groups

(according to Atack et al. [21]).

Atack and Heitner [20] also assumed that the hydrophilicity of wood is caused

by an exchange of the aliphatic hydroxyl or ether groups – acting between the

chains in hydrogen bonding – by solvated groups that cannot build bonding between

The chains, and this results in a softening of lignin. This permanent lignin

softening affects the following defibration in two important ways:

Refiner Processes

_ The addition of sulfonate groups up to about 1.2% on o.d. wood

Leads to an almost complete fiber separation and to diminution of

The disturbing influence of stiff long fibers caused by softening of

the middle lamella [20].

_ The addition of a larger quantity of sulfonate groups (>1.2% up to

On o.d. wood) makes the stiff long fibers more flexible and

Deformable, and this increases the bonding ability, caused by softening

of the fiber walls [20].

In practical CTMP and CMP operations, the chemical treatment of wood is carried

Out as pretreatment by the impregnation of wood chips. Besides refining, this

Impregnation is one of the most important process steps in chemimechanical

Pulping. Generally, impregnation is defined as penetration of the chemicals into

The microstructure of the wood. The physical processes of penetration and diffusion

Ensure the distribution of a maximum amount of chemical in a minimum of

Time evenly within the wood structure.

A practical solution to this procedure is to dampen and compress the chips on

Their way to the impregnator in the feeding screw, and then to expand them in the

Cold impregnation liquid. This expansion and condensation effect causes the

Chemical to soak into the wood structure. Depending on the chemical concentration

and impregnation time, a chemical consumption of 2% up to 20% on o.d.

wood can be achieved. This means also an increase in wood moisture of 5% to

Because of their anatomic structure, hardwoods are able to absorb more

Liquid than softwoods.

The efficacy of impregnation can be estimated by the sulfonate content in the

Pulp. This value expresses the chemical amount of sulfonate groups in lignin,

related to o.d. matter, and can be calculated as:

Sulfonate content _

Amount of SO3H _in mg_

O_d_ matter _in mg_

. 100%

A maximum sulfonate content of about 2% on o.d. matter corresponds to a degree

Of sulfonation of ca. 0.15. The degree of sulfonation is defined as the ratio between

Sulfur and methoxy groups; a value of ca. 0.15 represents complete sulfonation of

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