Page 115 - Basic Principles of Textile Coloration

104 ARTIFICIALLY MADE FIBRES BASED ON CELLULOSE

addition of a large amount of water, a small amount of dilute aqueous acetic acid is
added to the cellulose triacetate solution. The water reacts with the excess of
acetic anhydride and hydrolyses some of the acetate to hydroxyl groups. The final
product is called secondary cellulose acetate which is synonymous with cellulose
diacetate. Once the DS has fallen to the required value of about 2.5, the cellulose
diacetate becomes soluble in acetone. It is then precipitated by addition of water.

Because of its good solubility in acetone, filaments of cellulose diacetate can be
produced by extrusion and evaporation of this solvent. This is the dry spinning
process, which has the advantage over wet spinning that no additional treatments
such as washing are needed. Recovery of the evaporated acetone and of the acetic
acid is required for economical operation.

For dry spinning, a filtered and degassed acetone solution, containing a small
amount of water, is extruded through a spinneret. To ensure constant properties of
the filaments, the acetone solution is prepared from several blended batches of the
solid cellulose diacetate because of their slight variations in properties. If required,
pigmentation of the solution with TiO2 at this stage will decrease the lustre of the
filaments produced. The solution is spun into a rising current of hot air at 100 °C.
Some stretching occurs during this phase while the filaments are still quite plastic.
Lubricants are then sprayed or wiped on and the filaments wound up.

Cellulose diacetate is available in a range of deniers, as single or multi-
filaments, and as staple fibre. The denier depends on the solution pumping rate,
the spinneret hole size and the filament speed that causes drawing. Staple fibre is
produced by cutting a tow of mechanically crimped filaments. Slub yarns can be
made by combination of two different sets of extruded filaments.

The acetylation procedure, under acidic conditions, results in a large decrease
in the DP of the cellulose to about 300. This is controlled by pre-swelling of the
cellulose fibres, by cooling the reaction mixture and by careful hydrolysis of the
primary acetate. The large acetate side-groups prevent close packing of the
polymer chains and the development of crystallinity. Even though the filaments
are drawn during spinning, the degree of polymer chain orientation remains fairly
low. Filaments of cellulose diacetate are therefore not particularly strong. The fibre
is weaker and more extensible when wet but it does not lose as much strength as
viscose when wetted. Fabrics easily deform if over-stretched. Because this polymer
is thermoplastic, it requires careful handling when in contact with hot solutions to
avoid stretching and creasing of the material. Fabrics from this fibre have good
drape, a crisp handle, and reasonable dimensional stability and wrinkle resistance
if they are handled carefully and not over-heated.