CHAPTER ONE

1.0     INTRODUCTION

A dye is a deeply coloured organic compound, usually soluble in water. Insoluble coloured compounds are called pigments. Dyeing is the process of adding colour to textile products e.g fibers, yarns, and fabrics in a reasonable permanent fashion. Dyeing is normally done in a special solution known as the dye bath, which contains the dye and other chemical materials. After dyeing, dye molecules have an uncut or unseparated chemical bond with the fibre molecules, with temperature and time being the key factors in dyeing.

Joseph (1986) reviewed that, dye stuff is as old as the textile themselves. Predating written history, fabrics discovered in the ancient tomb in Egypt were coloured yellow with dye obtained from the sat flower plant. Until 1856 all dye stuff were produced from materials, mainly animals, vegetable matter with some few minerals for special colours.

There are two main classes of dyes, which are the natural and man-made dyes. The primary source of dye historically has generally been nature with the dyes being extracted from animals or plants. Since the mid-19thcentury, however humans have produced artificial dyes to achieve a broader range of colours and to render the dyes more stable to resist washing and other fastness properties.

Brown and Mc Gowen (1980) also review that, until the middle of the nineteenth century all dyes were derived from animals, vegetables or mineral sources, and very complicated and laborious method and formulas were worked out to get the brightest and most lasting colours from the limited sources.

Different classes of dyes are used for different types of fibres and different stages of textile production process, from loose fibres through yarns and cloth to complete garments.

Acrylic fibres are dyed with basic dyes while nylon and protein fibres such as wool and silk are dyed with acid dyes, while polyester yarn is dyed with disperse dyes. Cotton is dyed with a range of dye types, including vat dye and modern synthetic reactive and direct dyes

1.1     STATEMENT OF PROBLEM

Dyeing is an aspect of chemical processing whose rate is influenced by factors such as electrolyte concentration, liquor ratio and temperature.

In this project, temperature is being investigated to know its effect on the quality of dyed substrate.   

1.2     PURPOSE OF STUDY

This study is aimed at investigating the dyeing of a standard bleached cotton fabric at variable temperatures.

1.3     SIGNIFICANCE OF THE STUDY

The importance of temperature with regard to the rate of dying, as it  affects the quality of the dyed product, Also to give an important information on the vast use of the dye in question.

1.4     SCOPE OF THE STUDY

This project is focused on dyeing a 100% bleached cotton fabric, using variable temperature so as to investigate the most suitable temperature to use when dyeing with a soluble vat dye, and also the vatting process.

1.5     LIMITATION OF STUDY

 The limitations include insufficient funds, time and interrupted power supply. Therefore this research work is limited to the laboratory of polymer and textile technology department Kaduna Polytechnic.

1.6     DEFINITION OF TERMS

Dye: a dye is an organic chemical substance used for imparting colour to substrates, which has affinity to the substrate on which it is being applied.

Dyeing: is the process of applying colour or dye to paper, leather and substrates in a reasonable permanent fashion.

Temperature: a measure of the warmth or coldness of an object or substance with reference to some standard values

Colour: The particular visual sensation caused by the light emitted by an object is known as colour. In other words a colour is the reflection of light rays that cannot be absorbed by an object.

Absorbent: this is the ability of solid materials to retain gas or liquid on its surface.

Fibre: Fibre is a textile raw material, generally a unit of matter which is characterized by flexibility, fastness, with high ratio of length to thickness.

Fastness: This is the ability of a dyed fabric to withstand various external agencies which tend to cause fading, (simply resistance to fading)

Fade: This is any change in the colour of an object caused by light, washing, rubbing or even perspiration. It can also be regarded as the reduction in the shade of the colour of the fabric irrespective of the cause.

Substrate: Substrate is a molecule acted upon by an enzyme. Or the substance upon which an enzyme acts.

Pigment: Pigment are substance that imparts black, white or a color to other materials, especially a powdered substance that is mixed with a liquid in which it is relatively insoluble and used especially to impart colour to coating materials (such as paints) or to inks, plastic, and rubber.

Dyeing variables: Are those variables that affect the rate of dyeing e.g. temperature, effect of dye bath PH, liquor ratio and electrolyte concentration

Liquor ratio: is the proportion of water/solvent to which other dye auxiliaries are added.

Dye affinity: it is the quantitative expression of substansivity.  It is the attraction between the dye and the fibre.

Vatting: This step involves chemical reduction of vat dye to produce the water soluble, reduced, or leuco form of the dye. This is achieved by the use of sodium hydrosulphite, sodium hydroxide and water. The vatting stage also temporarily alters your original colour of the dye.

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