The softening temperature is understood to represent the transition of an amorphous

Polymer such as lignin from a glass-like and brittle state into a weak and

plastic one. Goring [3] identified the softening temperatures for lignin as 135 °C to

235 °C, whereas according to Styan and Bramshall [4] the softening temperatures

for softwoods are about 135 °C and for hardwoods about 100 °C. In lignin, the plasticizing

effect of water has a limit at water contents as low as 5%. For water-saturated

isolated lignin, the softening takes place at 80–90 °C, and additional water

Does not result in any considerable further softening of lignin. Under typical mechanical

Pulping conditions, the softening temperature for lignin as part of the

wood fiber matrix is higher (in the range of 100–130 °C). The lignin polymer,

Being the stiffest wood component under the conditions used in mechanical pulping,

Represents the most important component for the thermal softening of wood.

Preferably, wood for mechanical pulping should have a moisture:wood ratio of

Over 0.5.

Knowledge of the thermal processes in grinding is important step when completing

the grinding process. In evaluating investigations made by Luhde [5], the

Relationship between thermal relationships during grinding process is illustrated

Schematically in Fig. 4.4, while the five temperature-dependent regions of Fig. 4.4

Are described in more detail in Tab. 4.1.

Grinding Processes 1081

Mechanical Pulping Processes

Fig. 4.4 Thermal relationships in the grinding process.

Cool wood; 2, strongly heated wood layer; 3, actual grinding

Zone; 4, fiber suspension zone; 5, pulp stone surface.

Tab. 4.1 Temperatures at different zones during the grinding process.

Zone Temperature Process

Cold wood

Depending on

Surrounding

Temperature

(related to

Season)

The wood is cold and thermally untreated when entering

Zone 2.

Strongly heated

Wood layer

C The temperature in the wood about 0.1 mm above the

Grinding zone is high, due to by heat impound, but below

the carbonizing temperature of wood (206 °C) [6]. These high

Temperatures are the reason for the softening of lignin. The

Resulting equilibrium temperature depends on the water

Content in the wood.

Actual grinding

Zone

C (SGW)

C (PGW)

The actual grinding zone is only ca. 0.1 mm thick (according

to Steenberg and Nordstrand [7]), and has a temperature

usually close to 100 °C (higher in pressurized grinding). This

Temperature increases with increasing pit consistency and

Pit temperature. At this point, the fibers are deliberated from

The fiber compound. Here, the resulting equilibrium temperature

Also depends on the water content in the wood.

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