Textiles 69
the chains and proceeds via ring opening, reorganisation and -elimination
(Figure 10); the reaction will proceed along the chain in the direction of the
exposed ‘C 1 ’ carbon. As this progressive depolymerisation (‘peeling’) occurs
one unit at a time, it will not cause a significant deterioration in the physical
properties of the fibre, unless there is pre-existing damage – the short chains
of hydrocelluloses and oxycelluloses (the products of acid hydrolysis and
photo-oxidation respectively) will degrade much more rapidly.
Photolytic Damage. Cellulose is susceptible to damage by light, especially
radiation in the ultraviolet region. On exposure, the material undergoes vari-
ous photodegradative reactions, including direct photolysis, photochemical and
radical oxidation and photosensitised degradation (Figure 11a and 11b). These
processes are accelerated by the presence of moisture and catalysts such as
transition metal-based dyes and mordants. Extensive photodegradation will
lead to depolymerisation and the formation of a variety of small, water-soluble
acidic species, which tend to be yellow or brown in colour, so leading to dis-
colouration. As a consequence of their solubility, they can readily be washed
out of degraded fabrics, but this also risks further weakening.
Biological Degradation. Microorganisms which feed on cellulose excrete
cellulase enzymes to break down the polymer. There are three types of cel-
lulase: endo--1,4 glucanase breaks the -1,4 bonds, exo--1,4 glucanase
removes cellobiose units from the chain ends, and -glucosidase cleaves the
glycosidic link in cellobiose to form glucose. By-products of the further metab-
olism, such as hydrogen peroxide and organic acids, may cause additional
OH-
OOO
OHOHOH
OHOHHOHOOOOH
OOHOH
OHOHHOHOOOHHO OHO-OH
OHOHHO OOOOH
OHOHOH
OHOHOHOFigure 10Alkaline hydrolysis of cellulose