Page 85 - Basic Principles of Textile Coloration

74 NATURAL CELLULOSIC FIBRES

maturity. The thickness of the secondary wall of cellulose, deposited during the
second phase of growth of the fibres, can vary over a wide range. Mature fibres will
have a thick secondary wall and more convolutions per unit fibre length.
Immature fibres are those with thinner secondary walls, because of restriction of
their growth, and so-called dead fibres have essentially only the primary cell wall.
Because of their higher elasticity, immature and dead cotton fibres readily form
small bundles of entangled fibres during the mechanical processes producing yarns.
These are called neps. The thin secondary wall of nep fibres makes them more
opaque and they appear much paler than mature cotton fibres after dyeing. Dead
fibres remain undyed. Neps can therefore be readily seen on the fabric surface.
Dyeing tests are frequently used to evaluate the content of dead and immature
fibres in a cotton sample. Such fibres are always present, typically constituting
about 20% of the total weight. Immature cotton fibres will swell in 20% NaOH
solution during mercerisation (Section 5.4.6). This decreases the colour contrast
between the dyed neps and mature fibres.

5.3 CELLULOSE

5.3.1 Molecular structure of cellulose

Careful hydrolysis of cellulose gives an almost quantitative yield of glucose,
confirming that the former is a polymer of this sugar. 2,3,6-Trimethylglucose
(Figure 5.2) is the predominant product of the hydrolysis of cellulose in which all
the hydroxyl groups have been initially methylated. Therefore, the hydroxyl groups
attached to carbon atoms C-2, C-3 and C-6 in the glucose units in cellulose are
free and the oxygen atom linking the glucose units must be between carbon atoms
C-1 and C-4. The numbering of the glucose ring is shown in Figure 5.2. Cellulose
is a polymer of b-D-glucose, with the specific configuration shown in Figure 5.2.
This distinguishes it from starch (a polymer of a-D-glucose) in which the 1-
hydroxyl group has the alternative configuration. The correct name for glucose is
glucopyranose which indicates that the glucose molecule has five carbon atoms
and one oxygen atom in a ring, as in pyran. a- and b-D-glucopyranose must also
be distinguished from the respective L-glucopyranoses, their non-superposable
mirror images (enantiomers), which are not found in natural carbohydrates.

In the cellulose molecule, each successive glucose unit is rotated 180° around
the molecular axis. This gives a linear polymer chain that is almost flat. This
structure allows the close approach of neighbouring cellulose molecules necessary