1. Reduction of the chips sizes to units of matches.

2. Reduction of those “matches” to fibers.

_ Stress by pressure pulsation and shearing of the fiber –

fatigue of the middle lamella

_ Thermal softening of lignin and hemicellulose

_ Rolling effect (= agglomeration of deliberated fibers)

_ Fiber cutting

3. Fibrillation of the deliberated fibers and fiber bundles.

Chemimechanical pulping involves a gentle chemical treatment stage combined

with mechanical defibration such as disc refining. The yields of these pulps are

generally in the range of 80–95%, and their properties are intermediate between

those of high-yield chemical pulps and mechanical pulps. Chemithermomechanical

pulp (CTMP) is produced with pressurized refining. Relatively low chemical

doses are applied, and the yield is typically above 90%. Chemimechanical pulp

(CMP) can be produced with refining at atmospheric pressure; the chemical treatment

stage is more severe than in the CTMP process, and the yield is typically

below 90%. CMP is also the general name for all chemimechanically produced

pulps. CTMP and CMP have been developed for the better use of hardwood and

the improvement of the bonding ability of the stiff long TMP-fibers, with the first

pulping lines beginning operation during the 1950s and 1960s for hardwood applications.

The breakthrough in chemimechanical pulping occurred during the

1970S as result of the improved tmp technology. Because the key subprocess in

chemimechanical pulping is refining, all developments of the TMP process could

also be utilized for CMP production. This caused a rapid growth in the production

of softwood CTMPs during the late 1970s and 1980s.

The difference between chemithermomechanical and chemimechanical pulping

relates mainly to the process conditions utilized (see Tab. 4.2), and is apparent

mainly in terms of the intensity of chemical treatment and pulp yield.

1101

4 Mechanical Pulping Processes

Tab. 4.2 Possible defibration conditions for the production of chemithermomechanical

pulp (CTMP) and chemimechanical pulp (CMP).

Wood type Predamping

[min]

Impregnation Preheating Cooking Yield

[%]

CTMP Softwood 10 1–5% Na2SO3 2–5 min

120–135 °C

91–96

Hardwood 10 1–3% Na2SO3

1–7% NaOH

0–5 min

60–120 °C

88–95

CMP Softwood 10 12–20% Na2SO3 10–60 min

140–175 °C

87–91

Hardwood 10 10–15% Na2SO3 10–60 min

130–160 °C

80–88

Chemimechanical pulps can be produced, in principle, by a variety of combinations

of chemical treatments and mechanical defibration. In practical operation,

sodium sulfite is the dominating chemical in softwood pulping, while sodium hydroxide

and/or sodium sulfite are the common chemicals in hardwood pulping.

Sulfonation opens the wood structure and enables the access of water to the

fiber. Atack and Heitner [20] described this procedure (see Fig. 4.25), and assumed

that the softening of lignin could be led back to an exchange of the aliphatic hydroxyl

groups or ether groups participating in hydrogen bonding between the lignin

molecule chains, by solvated groups that cannot take over any bond between

the molecule chains.

HC

CH

CH2OH

R'O

OR

OH

OMe

HC

CH

CH2OH

R'O

SO3

OH

OMe

+ HSO3

ROH

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:

1102

4.2 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

2.0% 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