Reactions of Extractives

Extractives of wood can be classified according to their extraction methods. In general,

extracts are differentiated in terpenes, and resins as the nonvolatile ethersolubles

containing the fatty acids and alcohols, resin acids, and phytosterols.

Unsaponifiable substances comprise the plant hormones, but these are of minor

importance. Under acidic conditions, the various extractive classes behave differently,

though they are all highly prone to condensation reactions.

Pinosylvin (pinosylvin monomethyl ether) from the heartwood of pine species

efficiently inhibits the delignification during sulfite pulping, even at rather low

concentrations, possibly through the formation of condensates with lignin (phenol-

formaldehyde condensation) [62]. Such reactions result in larger lignin structures

with a lower degree of sulfonation, and thus a lower solubility. Interestingly,

gallic acid and its derivatives (ellagotannins) – which are a major extractive of

Eucalyptus species – are much less prone to condensation under the same conditions,

probably due to different distributions of electron densities within the aromatic

ring as a result of different substitution patterns (less-activated position in

C2 and C6) (cf. Scheme 4.57).

HO

RO

HO

OH

OH

COOH

OH

OH

OH

HO

O

O

O

O

R = H, Me

83 84 85

Scheme 4.57 Pinosylvin (pinosylvin monomethylether: R = Me), gallic acid, and ellagic acid.

4.3 Sulfite Chemical Pulping 425

Extractives such as dihydroquercetin did not show a high tendency to condense,

but they are oxidized to the corresponding quercetin (Scheme 4.58) with subsequent

reduction of hydrogen sulfite to thiosulfate, resulting in increased liquor

decomposition [65,66].

O

OH

OH

OH

OH O

HO O

OH

OH

OH

OH O

HO

86 87

Scheme 4.58 Formation of quercetin from taxifolin under acid sulfite conditions.

a-Pinene was found to be converted to cymene under sulfite conditions

(Scheme 4.59).

In acidic solution, pinene is converted to terpeniol and thereafter to terpinene,

which is finally oxidized to cymene by hydrogen sulfite [63]. Also in this case, the

hydrogen sulfite does act as oxidizing agent as it is reduced to thiosulfate. Under

acidic conditions, a number of other terpenes are unstable and undergo decomposition

and rearrangement reactions [64]. Dihydroquercetin is also oxidized under

the conditions of a technical sulfite cook to quercetin [65,66]. Taxifolin is converted

to quercetin [67].

Acidic sulfite treatment of hydroxymatairesinol yields conidendrin as the major

condensation product [5].

HSO3

-

HO

S2O3

2-

+

88 89 90 91

Scheme 4.59 Conversion of a-pinene to cymene during sulfite pulping [68].

Proanthocyanidine and catechin-based tannins can polymerize up to a molecular

weight of 7000 g mol–1, and exhibit a brown color [69]. Proanthocyanidine is

converted, under acidic conditions, to colored anthocyanidines; both are also able

to co-condensate with lignin.

Components of resins (free resin globules) have the tendency to coagulate to

larger droplets and to adhere to metal surfaces of machinery or fibrous material –

a phenomenon referred to as “pitch”. Pitch problems appear mostly during acid

pulping of coniferous wood, and this mainly limits the acid sulfite process to

426 4 Chemical Pulping Processes

hardwoods and certain softwood species. In addition to problems in the mill

operation, pitch causes specks or holes on the paper surface, and in high concentration

has also a negative effect on the viscose process.

Fatty acid esters are hydrolyzed to a great extent during the acid sulfite cook, the

saturated resin acids remain unchanged, while the unsaturated acids decrease

during cooking. Some 20–50% of the resin disappears during the cook, but no

oxidation, reduction, or polymerization was observed [1].

In contrast to alkaline pulping, where most of the extractives are either dissolved or

saponified, glycerol and sterol esters are not saponified in sulfite pulping.

4.3.5

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