Basic

Pretreatment of Fabric For Dyeing | Singeing,Desizing,Scouring

To produce colored Fabric whether dyed or printed needs pre-treatment of those fabrics of those fabrics before dyeing and printing. The pretreatment operations involved for cotton fabric are described systematically below:

Singeing

: it is the process of burning out projecting hairy fibres from surface of the grey fabric by assigns the fabric through a gas flame. Both sides of the fabric are burnt out by the flame. It helps in subsequent chemical process of the fabric. After dyeing or printing of singed fabric, brightness and luster of color and fabric becomes better.

Desizing:

it is the pr4ocess of removing starch materials present in the gray fabric. Generally enzyme is used to degrade and remove the starch present in the grey fabric. If those size materials are not removed from the grey fabrics, then the subsequent chemical treatments on the fabric will be irregular, which will cause he defective dyeing and printing.

Scouring:

It is the process of removing natural impurities present in the cotton fibre. The natural impurities are pectin’s, pectos, ash, wax, mineral compounds, etc. if those impurities present in the cotton fibres are not removed, then it will be difficult to dye or print the fabric uniformly. Normally caustic soda ash is used as main reagent fro scouring of cotton fabric.

Bleaching:

It is the process of removing natural coloring matters present in the cotton fibre. For his purpose, hydrogen peroxide, bleaching powder or other bleaching agents are used. Hydrogen peroxide is the best and most commonly used bleaching agent. If this bleaching treatment is not carried out before dyeing or printing hen the color yield and shade matching may be a great problem.

Mercerization:

It is a special chemical pre-treatment on cotton fabric t improves the properties and performance of cotton fabric. The cotton fabric is treated under tension in the 20% caustic soda solution which is called mercerization many properties of cotton fabric is improved, some of them are mentioned below:
Increases fabric strength
Increases absorbency power
Increases fabric luster
Increases fabric softness and handle property
Reduces dye consumption
Reduces chemical consumption in dyeing.

 

Different Dyes Which Are Used To Dyeing The Fabric & Yarn

Dyeing Fabric using different dyestuff is the most common things that every Textile Engineers should have to know. There are many kind of dyes are used to dyeing fabric. Maximum of them are suitable for fabric dye and yarn dyeing.

Dye staffs used to fabric dyeing are:

Direct dyes:

These dye staffs are generally sodium salt of sulphonic acid. They have direct affinity to cellulosic fibre. They are water soluble and needs salt addition in the dye bath. Should be applied at alkaline or neutral condition. Poor fastness to wash but cheaper for fabric dyeing.

Basic dyes:

These are generallyu ammonium, sulphonium or a uxonium salt. Famous for bright shade during dyeing fabric. Water soluble and application on cotton and other cellulosic fibre and leathers.

Acid dyes:

chemically acid dyes belongs to various subclasses such as nitro nitroso, monoazo, diazo, xanthance azine, quinoline, anthraquinone etc. they are water soluble and have affinity to wool, silk and nylon fibres to dyeing. They are applied to the fibres through neutral or acid dye bath.

Mordant dyes:

They are the oldest natural dyes. They have no affinity to textiles but can be applied to cellulose or protein fibres in order to dyeing fabric when they have been mordented previously with metallic salt. These dye staffs are capable of combining with metallic oxides to form insoluble colour on the fibre .

Sulpher dyes:

These are complex organic compounds containing sulphur. They are insoluble in water but sluble under reduced condition . they are usually applied cotton for cheaper shades, have high wet fastness but poor light fastness during dyeing fabric.

Azoic dyes:

These are not ready made dyes. Fibres are firstly impregnated with a coupling component like bita nepthol and then combined with a diazotized base to producer insoluble dye staffs into the fibre. Their main use on cotton but also can be used to dyeing wool silk and fur.

Vat dyes:

These are very fast color on cotton and insoluble in water. They are reduced by strong reducing agent to pr9oduce leuco vat and at this stage they are soluble in water. After impregnation they are again oxidized to their original insoluble form.

Reactive dyes:

These dye staffs directly combinds with cellulose by chemical bonding i.e. covalent bonds are produced. They have excellent wash fastness. Mainly used on cotton dyeing . can also be applied on wool, silk and nylin dyeing. Dyeing is carried out in an alkine bath.

Disperse dyes:

These dye staffs are very suitable for synthetic fibres for example polyester, nylon, acrylic, cellulosic acetate, etc. high temperature dyeing methods are suitable for these dye staffs but carrier dyeing method can also be applied.
In the above mentioned dyestuff, the Reactive Dyestuff is mostly used in today’s Textile Industry.