Conservation Science

68 Chapter 4

temperature increases, adjacent hydroxyl groups on nearby chains will condense,
leading to covalent cross-linking and further rigidity. The matrix components
lignin and hemicellulose both moderate these processes – the former helps to
resist swelling, due to its hydrophobicity and structural function, whereas the
latter, with its branched chains and highly amorphous structure, will retain
water.
There is another key thermally-promoted deterioration mechanism, oxida-
tion. This is driven by free radicals and results in both cross-linking and chain
scission, as well as discolouration to yellow/brown. These reactions will
occur slowly at normal temperatures, but are accelerated by the presence of
pre-existing free radicals, such as those arising from photolysis. Lignin and,
to a lesser extent, hemicelluloses are both more susceptible to thermal
damage than is cellulose.

Acid and Alkaline Hydrolysis. Cellulose is susceptible to attack by a wide range
of chemical agents, but primarily succumbs to either acid or alkaline hydrolysis.
The damage starts in the accessible amorphous regions. The ready reaction
with acid occurs randomly throughout the chain and leads to the scission of the
glycosidic ether bond (Figure 9). The resulting fragments are termed hydrocel-
lulose and though this material tends to be highly crystalline due to extensive
hydrogen bonding after facile reorientation of the shorter chains, it is also
mechanically weak and lacks the flexibility of native cellulose. Any source of
acid is deleterious, with stronger acids, of course being the more potent, but even
organic acids and inorganic Lewis acids will be effective.
The cellulose polymer exhibits a greater resistance to alkaline attack, which,
unlike the random progress of acid hydrolysis, occurs only at the ends of

O

O

O

O

OH

OH

OH

OH

HO

HO

O

O+

H

O

O

OH

OH

OH

OH

HO

HO

O

OH

HO OH

OH

H+

H+

H 2 O

HO

O

O

OH

HO OH

O

OH

HO OH

+

Figure 9Acid-promoted hydrolysis of cellulose