Textile chemistry

Textile Chemistry

Contents
Textile fibres............................................................................................................................................ 2
Pretreatment of textiles.......................................................................................................................... 3
Typical pretreatment processes ............................................................................................................. 4
Desizing ............................................................................................................................................... 4
Singeing ............................................................................................................................................... 5
Scouring .............................................................................................................................................. 6
Bleaching ............................................................................................................................................. 7
Mercerisation .......................................................................................................................................... 9
Carbonising ............................................................................................................................................. 9
Pretreatment of man-made fibres ........................................................................................................ 10
Principles of dyeing ............................................................................................................................... 11
Dyeing process .................................................................................................................................. 12
Dyeing machinery and processes...................................................................................................... 12
Dye classification................................................................................................................................... 15
Dyeing progress .................................................................................................................................... 16
Printing .................................................................................................................................................. 17
Intermediate processes ........................................................................................................................ 20
Textile finishing ..................................................................................................................................... 20
Coating .................................................................................................................................................. 21
Coating process: ................................................................................................................................ 22
Lamination ............................................................................................................................................ 23
Summary ............................................................................................................................................... 24
Further reading ..................................................................................................................................... 24

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Textile fibres
Fibres are generally classified into natural fibres and synthetic/man-made fibres.

Cotton

Cotton's constituent:

• Cellulose (88-96%) • Mineral matter (0.7-1.46)

• Protein (1.1-1.9%) • Organic acids (0.8%)
• Pectic matter (0.7-1.2%) • Sugars (0.5-1.0%)
• Wax (0.4-1.0%)

Wool

Wool is protein fibre. Impurities in wool fibres are depending on fibre thickness. The finer
fibre, the higher proportion of impurities. The wool’s impurities are:

• Fat and suint (5-30%)

• Sand and dirt (5-40%)
• Vegetable matter (0.2-2.0%)

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Silk

Silk's composition:

• Fibroin (70-80 %) • Inorganic matter (1.2-1.6%)

• Sericin (20-30%) • Pigment (0.7%)
• Waxy matter (0.4-0.8%)

Sericin is the major impurity of silk fibres.

Synthetic fibres

These fibres are mostly free from natural impurities being present in most natural fibres.
But the following may be present in synthetic fibres being added during spinning, weaving
or knitting processes which may require to be removed:

• sizing materials • dirt

• anti-static agents • loom stains (lubricants).
• processing oils (especially in knits)

Pretreatment of textiles
Pretreatment/preparation means preparing the textile fibres properly for downstream
processes such as dying, printing and finishing.

Pretreatment is an important step prior to dyeing and finishing, which is a requirement and
prerequisite for good results in dyeing and finishing.

The preparation processes focuses on:

• improving absorbency (higher dye or finish uptake and more uniform treatment)
• reducing shrinkage
• removing impurities
• obtaining the desired degree of whiteness
• improving comfort.

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Typical pretreatment processes

The following shows the general preparation processes.

Desizing

The desizing process is carried out to remove the sizes and other substances applied on
warp yarns to facilitate the weaving process. For effective desizing, different desizing means
such as enzymes, oxidising agents, acid desizing or hot washing may be used, depending on
the type of sizing agents. The desizing process is essential to ensure that dyes and finishes
will bond to yarns.

Sizes: Size material forms a stiff and smooth coating on warp threads to enable them to resist the
cyclic tensions during the weaving process and diminish breakage

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Principle of desizing machine

© University of the Highlands and Islands

Singeing

The singeing process is carried out to burn-off the loose protruding fibres from the surface
of yarn/fabric which will improve the fabric luster, make the fabric surface smooth and
clean, reduce soiling and prevent pilling.

Singeing generally involves exposing either one or both sides of a fabric over a gas flame to
burn off the protruding fibres. The flame temperature is rather high and the fabric’s passing
speed over the flame is high therefore the loose protruding fibres are burnt off but the
fabric itself remains undamaged.

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Principle of gas singeing machine

© University of the Highlands and Islands

Scouring

Scouring is the process of removing all undesirable impurities such as foreign matter, soil or
processing aids (waxes, lubricants, etc.) from fabric/yarn prior to dyeing or finishing. This
pretreatment is an important stage, especially for natural fibres which have a significant
presence of natural impurities. Conventionally, scouring involves the use of wetting agents
(surfactant), Emulsifiers (Non-ionic surfactants) and alkaline solutions to remove
hydrophobic components from the primary wall. Synthetic fibres may need only a mild
detergent scour.

Scouring will make fabric sufficiently hydrophilic, remove impurities, increase absorbency
and produce clean surface for next processes.

Methods of scouring
Continuous method: J-BOX (see graphic)

• It is called J-BOX process since the scouring vessel looks like “J”.
• Feeding fabric from one end and coming out the scoured fabric from the other end.
• Impregnation box (Impregnation temperature: 70-80°C, impregnation time: 40-90
sec).

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• Pre-heating (at 90-100°C for 30 sec).

• J-BOX (1-4 hours at 100°C).
• Washing unit.

Discontinuous method: Kier Boiling (Pressure kier)

• Removal of impurities by alkaline boiling under pressure.

• Very good removal of impurities.

© University of the Highlands and Islands

Bleaching

After the desizing and scouring processes, most of the fabrics retain slight colouration. The
natural or acquired colour needs to be destroyed to bring the textiles to a white state.
Bleaching will be essential if:

• pure white colour is needed in the finished textile;

• the fabric is to be dyed to a clear pastel shade;
• the fabric is to be printed.

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Bleaching agents can be classified as:

1. Oxidative bleaching agents

o Peroxy compounds (Hydrogen peroxide (H2O2))
o Chlorine based compounds (Sodium hypochlorite (NaOCl), Sodium chlorite
(NaClO2)).
2. Reductive bleaching agents
o Sulphur dioxide, Sodium hydrosulphite, etc.

Methods of bleaching

1. Batch process:
o a defined weighed fabric is loaded into small machine;
o the process is carried out step by step.
2. Continuous process:
o the fabric will travel through a long machine made up of a series of unites;
o each unit is specified for one step of the process.

Bleached (left) and unbleached (right) fabrics

© University of Borås

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Mercerisation
Mercerisation is a process of treatment of fabric/yarn under tension and in a concentrated
solution of cold NaOH.

Mechanism of Mercerisation:

• causes the fibre cross-section to become swollen and rounder;

• increases the fibre/fabric strength;
• leading to more lustrous surface;
• higher moisture gain, reactivity and dye-uptake.

Q. Name a pretreatment process that makes the cotton stronger.

[Feedback: Mercerisation]

Carbonising
Carbonising is a chemical treatment to destroy the cellulose impurities in wool using strong
acid (sulphuric acid).

Processing stages:

Carbonising process

Carbonising process © University of the Highlands & Islands

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Q A. On which type of product carbonisation will be done?

Q B. Why do you perform this process?

[Feedback: A) Wool B) To destroy and remove cellulosic impurities.]

Pretreatment of man-made fibres

Synthetic fibres are usually free from natural impurities but they contain secondary
impurities being added during spinning, weaving and knitting processes. These include sizing
agents, spin finishes, sighting colours, lubricants, etc. The sizing agents are usually easily
removed from synthetic fibres at the desizing stage since these agents are either water-
soluble or self-emulsifiable compositions. Therefore the scouring of man-made fibres is
easier than natural fibres and most of the impurities mentioned can be removed by mild
alkaline conditions in the presence of a surfactant.

Scouring conditions for synthetic fibres:

• Polyester: Scouring can be carried out with alkali (NaOH, etc.) or anionic surfactant
depending on impurities.
• Nylon: The scouring conditions are commonly mild alkaline with presence of non-
ionic surfactants.
• Acrylic (PAN): Acrylic fibres are sensitive to alkaline conditions; therefore strong
alkalis are usually avoided for scouring.
• Viscose: These fibres can be scoured by non-ionic or ionic surfactant and they should
be rinsed in less tension as they have poor wet strength.

Bleaching conditions:

• Polyester (PET): Sodium chlorite in acidic pH is preferred for PET bleaching.

• Nylon: Sodium chlorite is the most appropriate bleaching agent for nylon. Nylon
should be bleached in acidic conditions due to its sensitivity to basic pH.
• Acrylic (PAN): Acrylic can be bleached with sodium chlorite in acidic pH.
• Viscose: Bleaching with NaOCL or H2O2 can be carried out.

Heat-setting:

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Heat-setting is a heat treatment applied for fabrics made of synthetic thermoplastic fibres.
During all of the processes from fibre to dyed fabric, the material is subjected to different
tensions having resulted in sorted stress within the fibre. If the stresses are not relieved,
they will cause an irregular fabric appearance and dimensional instability. Excellent
dimension stability and good crease-proof properties will be obtained by this heat
treatment followed by cooling at the required fibre dimension. This treatment affects
(reduces) the dye-ability of fibres thereby it is extremely important to apply heat-setting
uniformly and fabric must be treated in precisely controlled temperature and moisture
conditions. This heat-treatment is carried out by passing fabrics through a heating zone for
specified temperature (160-170°C) and time (15-50sec) depending on fibre type, fabric
density and previous heat story of the fibres.

Stenter is the machine used for heat-setting.

Flexible line for coating and printing

© University of Borås

Principles of dyeing
The aim of dyeing is to make uniform colouration of a substrate. The colour should be
uniform; it means that it should be a solid shade with no change in the shade over the
whole substrate. There are many parameters affecting the appearance of the final shade
including substrate’s texture, physical and chemical construction of substrate,
pretreatments and post treatments applied after the dyeing process.

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The substrates may be divided into two major classes: hydrophilic and hydrophobic.
Examples of hydrophilic fibres are cotton, wool, silk and etc. synthetic polyester, acrylics,
polyamides and polyolefin fibres are some examples of hydrophobic substances.

A dyed fabric sample © University of Borås

Dyeing process

Textile fabrics are dyed in aqueous solutions or dispersions of dyestuffs together with
additives such as acids, alkali, salt and other auxiliary chemicals in a dyebath.

Dyeing machinery and processes

The dyeing process can be carried out in different modes:

1. Continuous
2. Semi-continuous
3. Discontinuous (batch)

The choice of dyeing equipment depends on the type of fabric (woven, nonwoven and
knitted), fibre (natural or synthetic) and the chosen class of dye.

Continuous dyeing

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• Feeding the textile substrates continuously into a relatively small pad bath containing
the dyestuff and auxiliaries.
• Aiming high production.
• Consists of dye application, dye fixation with heat or chemicals and finally washing.
• Most suitable for woven fabrics.
• The machine's length controls the speed.

Continuous dyeing process © University of Borås

Semi-continuous dyeing

• Consists of pad batch, pad-jig and pad-roll

• Cotton, linen and viscose
• Reactive dyes

Semi-continuous dyeing process © University of Borås

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Batch dyeing process (Exhaust dyeing)

• Most common method used for dyeing of textile materials.

• All types of fibre.
• All dye classes.

Jigger © University of Borås

Jet dyeing machine © University of Borås

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Dye classification
Dyes can be categorised according to several factors such as colour, synthetic route, fibre
type, the method of application to the substrate and chemical structure, etc.

Classification of dye by method of application:

Substrate Method Fastness

Direct Mainly used for Applied directly to the substrate in Give bright shades
dyes cellulosic fibres: a neutral or alkaline bath. but exhibit poor wash
cotton, viscose, linen. and light-fastness.
Can also be applied on After-treatments with
silk and wool. dye fixing agent.
Reactive Commonly used for all Reacts chemically with the fibre Very good fastness
dyes cellulosic fibres: wool, being dyed. properties.
silk and rayon.
Vat dyes Cotton, linen, rayon, Insoluble in water. Requires High fastness to
wool, silk and reduction treatment to make chlorine, washing,
sometimes nylon. soluble in water and oxidation to light, rubbing and
resume insoluble state after boiling.
dyeing.
Sulphur Cotton, linen and Insoluble in water, similar to vat Excellent resistance to
dyes rayon. dyes. washing but poor
resistance to sunlight.
Azoic Cotton, linen and Insoluble in water. Two Good fastness
dyes viscose. components react to produce the properties.
dye (Coupling Compound
(Naphthol) and Di‐azo –
compound).
Metal Wool, nylon and silk. Pre-metallised dyes, pH<7 Good fastness
complex properties due to high
dyes molecular size and
metal complex
structure.
Acid dyes Wool, nylon and silk. Applied from a dye bath containing Fastness properties
acids. may vary among
individual dyes and
fibre types.
Disperse Nylon, polyester and Insoluble in water, pH 4-5. Diffuse Good fastness
dyes cellulose acetate. into the fibre at high temperatures. properties.
Basic Mainly used for Generally pH<7. Reaction with Good fastness
dyes Acrylic. acidic groups. properties.

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1. Which two fibres can be dyed with both reactive dyes and acid dyes?

[Feedback: Wool, silk, polyamide]

2. Which dye class is used for dyeing acrylic fibre? What kind of bond is created during
this dyeing?

[Feedback: Basic dyes / ionic bonds.]

3. Which fibre can be dyed with basic dyes?

[Feedback: Only acrylic fibre.]

4. Which dye class do you use to dye a polyester knitted sweater with 5% elastane
elastomer?

[Feedback: Disperse dyes, no specific dyes for elastane.]

5. Name 2 dye classes that can be used for dying polyamide.

[Feedback: Reactive and acid dyes. Pre-metallised dyes (similar as acid) can also be used.]

Dyeing progress

Dyeing progress © University of Borås

How to dye fabrics (YouTube, 4:05)

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Printing
Printing can be described as localised dyeing in different patterns on a substrate.

There are several basic approaches to print colour on a fabric. Click on each item below for
more information:

Direct printing

Applying the printing paste directly to the fabric surface with desired pattern.

Direct printing on fabric (YouTube, 3:40)

Discharge printing

Overprinting a plain dyed fabric with a discharge paste which changes or destroys the colour
in the designed areas

Discharge printing (YouTube, 2:07)

Resist printing

Printing with a resist paste and passing the fabric through a subsequent dyeing process (the
printed area is not coloured).

Resist printing (YouTube, 1:38)

Transfer printing

Literally moving a design from one surface to another. Printing the design on to a special
type of paper and transferring the pattern to the fabric with the help of a heated calendar.

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Heat transfer printing © University of Borås

Flock printing

Flock printing or flocking is a printing process in which glue is printed on the fabric and then
the fibre flock is applied on the adhesive-coated surface.

Flock printing (YouTube, 2:31)

Pigment printing

Pigments are colour that are insoluble and do no penetrate into the fibres. Embedding the
pigment colour with a film-forming binder.

Pigment printing © University of Borås

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Warp printing

Printing the pattern onto the warp sheet before weaving.

Warp printing © University of Borås

Roller printing

The oldest mechanised method of continuous printing. Printing the fabric with engraved
metal roller with the printing design. One roller per colour in printing design

Roller printing (YouTube, 5:07)

Hand printing

The oldest method of printing. Applying the printing paste by means of a wooden block or
by a stencil.

Block printing (YouTube, 5:29)

Screen printing

The most important printing method. Forming the design on a screen by blocking off those
parts of the screen where no printing is to happen. Coating the screen with light-sensitive
polymer and then selectively exposing through a stencil. Washing away the unexposed area
(Exposed areas are made insoluble.)

Flat screen printing

• Holding the fabric firm and flat on a conveyor blanket.

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• Supplying printing paste to the screens and forcing through the patterned areas by a
squeegee roller or blade.

Flat bed screen printing (YouTube, 2:12)

Rotary screen printing

• Continuous production.
• Pumping the printing paste from the reservoirs to the inside of the cylindrical
screens.

Rotary screen printing (YouTube, 4:56)

Intermediate processes
After printing or dyeing, the fabric needs to go through additional treatments in order to
prepare it for the final finishing stages. These treatments include:

• Fixation
• Washing
• Dewatering and drying
• Heat setting

Textile finishing
Finishing is the final process to make fabric suitable for its intended end-use.

The basic objectives of finishing are:

1. surface modification (smoothing, raising, embossing, …);

2. wearing properties modification (creasing, draping, staining, …);
3. aftercare characteristics modification (shrinking, ironing, …).

Classification of finishing

Mechanical finishing is concerned mainly with modifying the surface of fabric, while
chemical finishing aims to modify and improve the basic fibres or fabric properties.

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Coating
Coating is the application of a suitable synthetic or natural polymer on the substrate to add
or improve the function of the material by giving it specific properties.

Polymers used for coating:

• Polyvinyl chloride (PVC)

• Polyurethane (PU)
• Acrylic
• Ethylene vinyl acetate (EVA)
• Styrene butadiene rubber(SBR)
• Natural rubber
• Silicone
• Polyolefins
• Polypropylene

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A coated fabric © University of Borås

Coating process:

Coating methods:

Direct coating © University of Borås

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Indirect coating © University of Borås

Applications: Sporting, protective cloths; Leather cloth; Shower curtains; Conveyor belts;
Floor and wall covering etc.

Lamination
Lamination is a process of bonding two or more prefabricated sheet materials. The bonding
can be achieved by either applying adhesive or heating. Curing is generally not required.

Schematic of a laminated textile © University of the Highlands and Islands

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Summary
You should now be able to:

• Describe the pretreatment processes of different fibres for example:

o 100% cotton fabric: Singeing, desising, scouring, bleaching and mercerising.
o 100% silk fabric: Degumming and bleaching.
o 100% wool fabric: Carbonisation, scouring and bleaching.
• Describe the dying and printing of different fibres with different types of dye classes
and under different conditions.

Degumming: Removing sericin gum by a gentle boiling in mild soap solution.

Further reading
Sara J. Kadolph, Textiles. Pearson New International Edition, Chapter 2, 16-21

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