Page 116 - Basic Principles of Textile Coloration

CELLULOSE ACETATES 105

Cellulose diacetate filaments have an almost round cross-section with several
lobes, with ridges between them that run along the filament length. The
unpigmented filaments have high sheen that is valued for linings, lingerie and
dress fabrics, ribbons, and for some furnishing fabrics. Few of these have high
fastness requirements. Yarns made from multi-filaments, however, do tend to slip
easily over each other and chemical finishes may be needed to avoid this effect
and to stabilise a fabric. Cellulose diacetate is produced mainly as continuous
filament even though it is more difficult to dye level.

The temperature of materials of cellulose diacetate should not exceed 85 °C
when wet, and should be less than 140 °C in dry heat. Because bright filament is
easily delustered in hot water above 85 °C, partly by surface hydrolysis of acetyl
groups, dyeing temperatures rarely exceed this value, even if the filament has been
delustered with TiO2. The fibre surface is very easily hydrolysed by mild alkaline
scouring. This sensitivity to heat and alkaline hydrolysis should always be born in
mind. Cellulose diacetate softens on heating and fabrics can be embossed with a
pattern using heat and pressure. It is produced with almost no natural colour and
rarely requires bleaching. It is soluble, or swells considerably, on contact with a
number of common solvents.

6.3.2 Cellulose triacetate

Cellulose triacetate is obtained as described in the previous section. Before
precipitation, reaction with magnesium acetate and a little water removes any
sulphuric ester groups (Cell–OSO3H). The triacetate is then precipitated by
addition of water, filtered and washed. Alternatively, acetylation procedures are
used in which the fibrous wood pulp preserves its form and does not dissolve. The
product is soluble in chloroform but this toxic solvent is not used for dry spinning.
A solution of the triacetate in methylene chloride (dichloromethane) containing a
little ethanol is used. The safety of even this chlorinated solvent is in question and
the production of triacetate is decreasing because of this.

Fibres of the triacetate are more hydrophobic than those of secondary acetate
and they have higher chemical resistance. They are much more stable to hot water
and dilute alkali than secondary acetate. Dyeing temperatures are close to the boil
and even as high as 120 °C. Because of the more regular structure, the
crystallinity of triacetate is higher than that of the diacetate and it is more solvent
resistant.

Cellulose triacetate is thermoplastic and fabrics can be heat set or permanently