Page 43 - Basic Principles of Textile Coloration

32 FIBRES AND TEXTILES: PROPERTIES AND PROCESSING

absorbed energy into a visible blue-violet fluorescence in the 400–500 nm region.
This emission of light offsets the absorption of the fibres in this wavelength region,
even producing an effect where the reflected light plus the fluorescence causes
more visible light to leave the fabric surface than is apparently incident upon it.
This gives the impression of a brilliant white. Such compounds are called
fluorescent whitening agents or optical brighteners (Section 5.4.5). They are
widely used for textiles, paper and in domestic detergents.

Cellulosic and protein fibres are particularly vulnerable to growth by micro-
organisms under warm humid conditions. Even though the synthetic fibres are
fairly resistant to bacterial degradation, a variety of residual chemicals on the fibre
surfaces can serve as nutrients for air-born micro-organisms. Rapid growth of
colonies of bacteria produces coloured spots and often a distinct odour. The odour
of mildew is from the growth of fungi. Once formed, the coloured pigments from
bacterial metabolism are often resistant to vigorous bleaching. This problem can
be minimised by ensuring that stored fabric is completely dry and relatively clean.
Use of an anti-bacterial agent such as a chlorophenol or a quaternary ammonium
compound will prevent this problem.

Effective preparation before dyeing is essential, no matter what the physical
form of the textile material, but particularly for fabrics. Uneven preparation
invariably results in unlevel dyeing. For loose dyed fibre, and to a lesser extent for
dyed yarn, uneven colour is blended by the mixing that occurs in subsequent
manufacturing processes. This is impossible for dyed fabric and unlevel colour
usually leads to rejection of the goods. In later chapters, we will discuss the
preparation of various different types of fibres and see more of the impact of
preparation on dyeing processes.

2.5 DYEING AND FINISHING

Textile finishing consists of a variety of mechanical, thermal and chemical
processes that improve the appearance, texture or performance of a textile
material. Dyeing is usually the first finishing process. Textile materials come in a
wide variety of forms and coloration occurs at almost any stage from loose fibres
right through to fully formed fabrics, or even made-up articles. Coloration involves
dyeing or printing in either batch or continuous processes, depending upon the
quantity of material to be processed. The objective of dyeing is to colour the entire
material so that the dye has completely penetrated into the fibres and the visible
surface has a completely uniform colour. Good penetration of the dyes ensures