Polysulfide

[% on wood]

H-Factor Yield

[%]

Kappa Viscosity

[mL g–1]

Cell

[% on w]

AX

[% on w]

GGM

[% on w]

COOH

[mmol kg–1 w]

0 1200 48.1 25.8 1188 37.3 3.9 3.1 50.4

2 1100 49.2 25.1 1188 39.8 4.1 3.9 48.0

4 1250 48.9 22.2 1095 39.4 4.0 4.0 41.0

The results shown in Tab. 4.37 suggest that the polysulfide addition mainly contributes

to the stabilization of the cellulose and the GGM fraction of the spruce

wood, whereas the AX yield remains almost unchanged. Because of the predominant

cellulose yield gain, it may be speculated that the polysulfide addition also

preserves the molecular weight of the cellulose fraction. The results however demonstrate

that polysulfide addition has no beneficial effect on the viscosity of the

spruce CBC pulps at a given kappa number (Fig. 4.79). In fact, the polysulfide

cooks with 4% polysulfide addition ultimately have a lower viscosity level.

292 4 Chemical Pulping Processes

10 20 30 40 50 60

CBC: no Polysulfide CBC: 2 % S

CBC: 4% S

Conventional Reference

Viscosity [ml/g]

Kappa number

Fig. 4.79 Effect of polysulfide addition on the

viscosity–kappa number relationship of CBC

kraft cooking of spruce wood (according to

[55]). Constant CBC cooking conditions: cooking

temperature 160 °C; impregnation liquor of

polysulfide cooks: [OH– ]= 0.50 mol L–1,

[HS– ]= 0.24 mol L–1; impregnation liquor for

reference CBC cooks: [OH– ]= 0.38 mol L–1,

[HS– ]= 0.20 mol L–1; cooking liquor for all CBC

cooks: [OH– ]= 0.63 mol L–1, [HS– ]= 0.34 mol L–1.

Conventional reference cooking conditions

according to [8].

Taking both the results on carbohydrate yield and viscosity measurements at a

given kappa number into account, it may be speculated that besides GGM, amorphous

cellulose with a low molecular weight is predominantly preserved during a

polysulfide CBC cook.

Strength properties

The tear and tensile indices of CBC pulps and conventional kraft pulps are shown

in Fig. 4.80. The results represent average values from four to five single measurements

for pulps in the kappa number range 25–27. The strength properties of the

CBC pulps are clearly superior as compared to those of the conventional kraft

pulps. It is interesting to note that the tensile strength develops rather comparably,

whereas the tear strength is significantly higher for the CBC pulps than for

the conventional kraft pulps (Fig. 4.80(b)). At the same kappa number level, the

CBC pulps show equal strength properties independently of the amount of additives

(polysulfide), provided that comparable cooking conditions with regard to

[OH– ]ion and [HS– ]ion and temperature are applied. The results clearly indicate

that the superior viscosity–kappa number relationship translates into superior

strength properties.

The high tear strength of the CBC pulps can be attributed to an increased fraction

of high molecular weight cellulose molecules which are organized into

4.2 Kraft Pulping Processes 293

65 74 76 78 80 82

(26)

(25)

(27)

(27) 90 Nm/g

70 Nm/g

at Tensile Index

no PS CBC: 2% S

CBC: 4% S

conventional Reference

Tear Index [mNm2/g]

Cellulose content [% od]

0 3 40 60 80 100 120

CBC: 160.C, [OH-] = 0.63 mol/l; [HS-] = 0.33 mol/l

Conventional reference

Tear Index [mNm2/g]

Tensile Index [Nm/g]

(a)

(b)

Fig. 4.80 (a) Tear indexes at tensile indices of 70 and

90 Nm g–1 related to the cellulose content of CBC, CBC-polysulfide

and conventional kraft pulps made from spruce wood

(according to [55]). Cooking conditions as for Fig. 4.79. (b)

Tear-tensile plot of CBC and conventional kraft pulps, kappa

number 27.

294 4 Chemical Pulping Processes

strands of rather undamaged cellulose microfibrils indicating rather selective

cooking conditions. Fig. 80a exemplifies that the tear strength at given tensile

index increases with increasing cellulose content which clearly confirms the view

that an even effective alkali profile provides efficient delignification while preserving

the long-chain carbohydrate fraction.

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