Effect of Ozonation on Strength Properties

Unfortunately, ozone delignification is accompanied by a concomitant degradation

of the polysaccharide fraction. As illustrated in Fig. 7.109, cellulose degradation

(characterized in terms of number of chain scissions) is clearly related to

ozone charge. The correlation between strength properties and carbohydrate degradation

(pulp viscosity) of ozone-bleached pulps was found to differ somewhat

from those of pulps subjected to conventional bleaching sequences [111,115].

Ozone-treated pulps are characterized by rapid beating, high-tensile strength but

low tearing resistance. In general, the tearing strength of ozone-bleached softwood

kraft pulps was found to be 10–20% lower as compared to conventionally

bleached pulps of the same provenance [116]. Lindholm has investigated the

impact of various ozone treatments on the tearing strength at a tensile strength of

70 Nm g–1 using a pine kraft pulp [116]. He reported that pulps subjected to Z,

OZ, and OZE treatments had comparable strength properties to those after (CD)E

and (CD)(EO) treatments, provided that the pulp viscosity of the ozone-treated

pulps was greater than 700 mL g–1 (Fig. 7.112). The kappa numbers were about 6

(range: 5–8) for both types of pulp.

400 600 800 1000 1200 1400

Pine kraft pulps, after treatments:

unbleached O (CD)E, (CD)(EO) Z, OZ, OZE

Tear Index [mNm2/g] at 70 Nm/g

Viscosity [ml/g]

Fig. 7.112 Tear index at 70 Nm g–1 versus viscosity for differently

treated pine kraft pulps (according to Lindholm [116]).

Figure 7.111 demonstrates a clear relationship between viscosity and tear index

at a given tensile index of the ozone-treated pulps. From this result it can be concluded

that strength properties of ozone-bleached pine kraft pulps are not deteriorated,

provided that pulp viscosity can be maintained above 700 mL g–1. Axegard

et al. reported that the tear strength at a given tensile index of an OAZQPbleached

softwood kraft pulp with a viscosity of 710 mL g–1 was only 5–10% lower

7.5 Ozone Delignification 841

as compared to an OD(EO)DD softwood kraft pulp with a viscosity of 890 mL g–1

[117]. Similar results have been reported by Dillner and Tibbling [118], indicating

that the strength–viscosity relationship presented for conventionally bleached

pulps by Rydholm [119] was not valid for TCF-bleached pulps, including ozone

treatment.

Strength properties of fully bleached hardwood kraft pulps with a sequence

including ozone were comparable to those of a conventionally bleached pulp, although

the viscosity of the ozone-bleached pulp was 20% lower [120]. The preservation

of strength properties despite cellulose degradation through ozone treatment

is also known for hardwood kraft pulps.

Quite recently, the relationships between the molecular weight distributions

(MWDs), intrinsic viscosity and zero-span tensile index of a birch kraft pulp subjected

to HC ozone bleaching were evaluated [121]. The relationship between

rewetted zero-span tensile strength and viscosity is shown graphically in

Fig. 7.113.

400 600 800 1000 1200

Zero-span tensile index [Nm/g]

Viscosity [ml/g]

Fig. 7.113 Zero-span tensile index versus viscosity for ozonetreated

birch kraft pulp (according to [121]). Unbleached

pulp: kappa number 15.5, intrinsic viscosity 1160 mL g–1.

A substantial decrease in fiber strength occurred only when pulp viscosity

decreased below 800 mL g–1. At the highest ozone dosage, the fiber strength was

still 75% of the initial value, corresponding to a viscosity of 510 mL g–1. Apparently,

ozone-treated pulps maintain their initial fiber strength at relatively high

level, despite a substantial reduction in molecular weight. Based on gel permeation

chromatography (GPC) measurements, it was shown that the degradation

pattern through ozonation of kraft pulp was different from that of cotton linters.

In contrast to unbleached birch kraft pulp, ozone-induced cellulose degradation

842 7Pulp Bleaching

did not generate a bimodality of the cotton cellulose peak. The different action of

ozone on MWD was attributed to the presence of lignin in the unbleached birch

kraft pulp, as lignin is known to promote the formation of secondary radicals during

ozone delignification [57]. Due to the enrichment of lignin at the surface of

fibers, it is suggested that cellulose degradation of an unbleached birch kraft pulp

occurs predominantly on the exterior of the fiber, thus generating two distinct cellulose

distributions [21,23].

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