WET PROCESSING

• The wet processing sector of the textile industry covers all the processes on the textile that involve some form of wet or chemical treatment.

• Processes such as scouring, bleaching, dyeing, printing and finishing are carried out at different stages during textile manufacturing.

• These processes all involve some form of chemical action on the material.

• The wet processing sector can be divided into three distinct sections.

• Preparation

• Coloration

• Finishing

FABRIC PRETREATMENT

• The pretreatment process is carried out to prepare the textile material for subsequent processing, which includes dyeing, printing and finishing.

• It is the heart of textile processing.

PRE-TREATMENT

Pretreatment processes should ensure:· 1-The removal of foreign materials from the fibres in order to improve their uniformity, hydrophilic characteristics and affinity for dyestuffs and finishing treatments

2-The improvement of the ability to absorb dyes uniformly (which is the case in mercerizing)

3- The relaxation of tensions in synthetic fibres (without this relaxation of tension, unevenness and dimension instabilities can occur).

• Pretreatment processes and techniques depend:

• On the kind of fibre to be treated: for raw goods made of natural fibres such as cotton, wool, flax and silk the technical task is more difficult than for those made of synthetic and artificial fibres.

• Natural fibres in fact are accompanied by a higher amount of substances that can interfere with later processing. Man-made fibres, in turn, usually contain only preparation agents, water-soluble synthetic size and soil.

• On the form of the fibre (flock, yarn, woven or knitted fabrics)

• On the amount of material to be treated (for example, continuous methods are more efficient, but are economically viable only for large production capacities).

• Pretreatment of cotton & cellulose fibres • Principal manufacturing processes• Cotton pretreatment includes various wet

operations, namely:• Singeing• De sizing• Scouring • Mercerising (and caustification)• Bleaching.

SINGEING

SINGEING

• Technically, singeing refers to the burning-off of:

• Loose yarns not firmly bound into the fabric structure;

• Protruding fibre ends sticking out of the textile yarns and/or fabrics.

• Textiles materials are most commonly singed in woven or knitted fabric form or in yarn form.

Singeing Objectives & advantages

• Singeing of a fabric is done in order to obtain a clean fabric surface which allows the structure of the fabric to be clearly seen.

• Fabrics, which have been singed, soil less easily than un-singed fabrics.

• The risk of pilling, especially with synthetics and their blends, is reduced in case of singed fabrics.

• Singed fabrics allow printing of fine intricate patterns with high clarity and detail.

SINGEING

• It is a part of pretreatment process carried out in textile processing , is usually the first step carried out after weaving.

• Improve reflection, no frosty appearance• A smoother surface, improve visibility of fabric structure• Less pilling

Types of singeing machines

• There are three main types of singeing machines:

• Plate singeing machine

• Rotary-cylinder singeing machine

• Gas singeing machine

SIZING• During the weaving process the warp threads are

exposed to considerable mechanical strain. In order to prevent breaking, they are usually reinforced by coating (sizing) with a gelatinous substance (size).

• In the weaving of cotton blend fabrics, the size material most often contains starch in native or modified form, sometimes in combination with other polymers such as polyvinyl alcohol (PVA), poly acrylic acid (PAA) or carboxy methyl cellulose (CMC).

• Small amounts of fats or oils may be also added to the size, with the aim of lubricating the warp coat surface.

• As a consequence of the sizing, the warp threads of the fabric are not able to absorb water or finishing agents to a sufficient degree.

• This means that first the size must be removed (desizing) before finishing. Some types may be removed in a simple scouring process (water soluble sizes). But in most cases chemical breakdown of the size polymer in a separate desizing treatment is necessary in order to obtain the desired quality of the final fabric

De sizing• De sizing is used for removing sizing

compounds from woven fabric previously applied to warp and is usually the first wet finishing operation performed on woven fabric

De sizingDe sizing

• Objectives-

• Removal of added impurities- startch

• Weight loss

• Improve wettability

De sizing methods

• Hydrolytic method

• Rot steeping• Enzymatic steep• Acid steep

• Oxidative method

• Chorine• Chorite• bromite

• De sizing techniques are different depending on the kind of sizing agent to be removed.

• Currently applied techniques can be categorized as follows:

1. Techniques for the removal of starch-based sizing agents (water-insoluble sizes)

2. Techniques for the removal of water-soluble sizes

3. Techniques for the removal of water soluble and insoluble sizes.

 

Scouring

• Scouring (also known as boiling-off or kier boiling) is aimed at the extraction of impurities present on the raw fibre or picked up at a later stage such as:

• pectins• fat and waxes• proteins• inorganic substances, such as alkali metal salts, calcium

and magnesium phosphates, aluminium and iron oxides• sizes (when scouring is carried out on woven fabric

before desizing) residual sizes and sizing degradation products (when

scouring is carried out on woven fabric after desizing).

• Scouring can be carried out as a separate step of the process or in combination with other treatments (usually bleaching or desizing) on all kind of substrates: woven fabric (sized or desized), knitted fabric and yarn.

• The action of scouring is performed by the alkali (sodium hydroxide or sodium carbonate) together with auxiliaries.

• The natural and added impurities have to be removed for the following reasons:

• The sizing ingredients have to be removed for obtaining uniform reaction of chemicals during bleaching operation.

• The waxes, proteins, spinning oils need to be removed to get uniform absorbency for dyeing and printing.

• Mineral matter and natural coloring matter have to be removed to get uniform whiteness.

Scouring

• Objectives

• To remove natural fat, wax and oil material containing in the fabric without damaging the fiber.

• To accelerate wet processing.

• To improve the handle of textiles.

Bleaching agent

• OXIDATIVE REDUCTIVE• H2O2 SO2• NAOCL( Sod. Hypochlorite) CL2• NAOCL2 NA2SO4 • Sodium Perborate• KMNO4• Paracetate acid

• Calcium Hypochlorite

Bleaching agent for cellulosic

• Bleaching with sodium hypochlorite

• Bleaching with hydrogen peroxide

• Bleaching with sodium chlorate

Optical brightener• Certain organic compounds have the property of fluorescence i.e. they can absorb light of

short wavelengths and re-emit it at the longer ones. Many of such compounds absorb ultra rays and re-emit as visible light in the range of 4,000 to 7,000 A0.

• Textile fibers containing a fluorescent compound reflect more light than an untreated one and thus increase its brightness and whiteness.

• The fluorescent brightening agents called ‘optical brightening agents, increase the apparent whiteness or brightness of materials, and are used in textiles, detergent, paper, paints, etc.

 

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MERCERIZATION

• it is the action of the alkali ( caustic soda) on a fabric.• It was a revolutionary development discovered in 1853

by John Mercer , a calico printer.• It is used mainly on cotton and sometimes on linen for

many different reasons.• It increases luster and softness• It gives greater strength.• It improves the affinity for dyes and water borne finishes.• Cotton is mercerized for luster in both yarn and fabrics.

• Objectives• 1. To make the goods more luster because raw cotton fib

ers are ribbon-like. After this process, the fibers change to cylindrical shape resulting more evenness reflected light from the goods.

• Mercerising can be carried out on yarn in hanks, woven and knitted fabric through one of the following different treatments:

• mercerization with tension (stretch mercerization)• Mercerization without tension ( slack mercerization)• Ammoniating

•

• Mercerization with tension ( cold mercerization)• Caustic soda mercerizing is the most commonly applied mercerising

technique (it is not applied to flax). Cotton is treated under tension in a solution of concentrated caustic soda (270 - 300g NaOH/l, which means also 170 - 350 g NaOH/kg) for approximately 40 - 50 seconds.

• During mercerising the temperature is adjusted at low values (5 - 18 °C) when the lustre is the priority and at slightly higher levels when the improvement of the other characteristics is preferred. Because the reaction between caustic soda and cellulose is exothermic, cooling systems are applied to keep down the temperature of the bath.

• Mercerization with tension (Hot mercerization)

• The hot mercerization process can double strength of the cotton fabric and contribute resistance to shrinkage in washing.

• Beside the conventional cold treatment, a hot mercerising process is also now increasingly applied (for hanks and fabric). The material is soaked in a solution of caustic soda close to boiling point. After hot stretching, the fabric is cooled down to ambient temperature and washed under tension.

•

• Slack mercerization• The technique is applied to yarn as well as

woven and knitted fabrics.• It is also used in producing woven fabric with

stretch in the filling direction.

• Ammoniating

BIOPOLISHING

• Define

• History

• Purpose

• Properties