Dyeing, printing and finishing

At what stage?

• Fabric processing

Textile processing steps

1. Yarn formation - fiber production, spinning

2. Fabric formation - weaving and knitting

3. Wet processing - dyeing, printing, finishing, fabrication

Why use pretreatments

Must be used to process the fabric to get it ready for dyeing

Pretreatments

1. Singeing

2. Desizing

3. Scouring

4. Bleaching

5. Mercerizing

Singeing

• Process using gas flame (direct) and/or hot plate (indirect) to get rid of small protruding fibers (hairs ) on the fabric

• These are normally found on spun yarns as they are made with staple fibers

• Fabric does not have hairy qualities = this is not needed

• Usually occurs with natural fibers, specifically when there is a presence of cotton

• Can occur in synthetic fibers as well

Objectives:

• Get rid of small fibers

• Smoothe fabric

• Help printing or dyestuff make clearer marks

• Improve rubbing/washing fastness of the fabric when dyed/printed

Fastness definition

Ability of a textile to resists colour change/transfer when exposed to specific conditions

Desizing process

• During weaving process, weavers will coat their warp yarn in starch or size giving them temporary strength to not damage or struggle with reeds

• Desizing process is used to get rid of this starch or size covering the warp yarns after sizing process

• This is done using enzymes, oxidizing agents or other chemicals

• If the warp yarn is not desized it will not take up the dye

• Mainly need to desize if the material includes cotton and is a woven fabric (as those are usually the ones that go through sizing process)

• Most commonly used is enzymes

Scouring process

• Common for majority of materials

• Subject to natural fibers, however factories may use for synthetic fibers as well

• A process designed to remove natural fat, wax, oil from cotton fabrics using sodium hydroxide + detergent at a boil for 20-30 minutes

• Synthetic fibers/other protein fabrics may use only sodium carbonate (weaker alkali)

• Industrial sized laundry

Bleaching process

• Process used to remove colour - make goods whiter than before

• Will also create an optic white background, allows the fabric to absorb more dyes/chemicals and allow for a brighter result after dyeing

• Cotton fabrics → hydrogen peroxide in an alkali solution at boil are most common bleaching agents

• Usually a half or full bleach will be done

• Due to chemical/water wastage usually opt for half bleach, specifically when the garment is going to be dyed a darker colour later

• White colours are only bleached and then an optical brightening agent (OPA) is used

Optical brightening agent (OPA)

• Makes already bleached fabric appear whiter and brighter

• By absorbing UV light and converting it to visible blue/violet light

• Changes the appearance of the bleached fabric

Mercerizing process

• Increases luster in cotton goods through sodium hydroxide solution in cold and tense condition

• Swells the cotton fibers increasing strength and shine

• Also increases fibers absorption of dye

Objectives:

1. Make te good more luster - raw cotton fibers → are more ribbon-like, after this process → cotton fibers will change to a more cylindrical shape. This results in more even reflected light from the goods.

2. Improve dyeability about 5-10%

Dyeing

• Fiber, yarn, fabric, garment can all be dyed, outcome differs at what stage you dye them

• Dyeing is about temperature, time and pressure

• Increase temperature = less time (and vice versa)

• Pressure is used as another source to let dyes penetrate and react

Fiber dyeing

• Very expensive and does not make a huge difference (limited colour range)

• Common in sweater/hoodie industry

• Melange - dye some fibers and mix with white fibers

• Fiber dyeing gives less even results

Yarn dyeing

• If there is a requirement of pattern design

• Relatively common

• Will give a more even result

Fabric dyeing

• Most common method

• Used for a solid coloured product

• Will give a more even result

Garment dyeing

• Usually used for aesthetic value/purposes

• Dyeing with a few garments can lead to irregularities - variation in product result

• Usually products are made in monomaterial - allow for everything to match

• Will give a less even result

Two types of colorants in dyeing

1. Dyes

2. Pigments

Dyes

• Mainly used for dyeing

• Complex organic compounds used to add colour to material by binding to them

• Dyes penetrate into the material and create a bond by chemical bonding

• Molecules dissolve in water or some other carrier allows them to penetrate the fiber

• Chemical additives are used to regulate penetration

• Great colour strength

• Small amount of dye colours large quantities

• Either used in solutions or paste

Dyes water soluble and water insoluble

1. Water soluble

• Can easily dissolve in water (do not need another carrier)

• Direct, acid, reactive, oba (mostly natural fibers)

2. Water insoluble

• Not easily dissolve in water (need another carrier)

• Vat, sulphur, disperse (mostly used for manufactured fibers)

Reactive dyeing

• Water soluble usually

• Penetrate and create bonds with the textile substrate

• Used for cellulosic, viscose and protein fibers

• Differes in temperature

• Cold, hot, high temp

Disperse dyeing

• water insoluble

• Used for acetate, polyester, triacetate, acrylic, etc.

• High temperature + high pressure

• Carrier 100C Nepthol Phenyl

• Carrier Thermosole 190-200C

• Dispersing agents are formic acis and actic acid

Process

• Make the dyes water soluble first by using agents

• Disperse dyes into body of polyester (example) at a high temperature

• Dyes reach the inside of the textile and are then made water insoluble again

• Dyes are then trapped

Pigments

• Mainly used in printing

• Insoluble colour particles are held on surface of fabric by a binding agent (which acts as glue)

• Never penetrates

• Application is quick, simple & economical

• More than 80% of printed fabrics are coloured with pigments

• Fewer sustainability issues compared to dyes

• Low colour strength - need to use more pigment

• Wash down is an issue (losing colour)

• Often used for short-run prints such as uni logos, sport logos, etc.

• Can be used for denim

• Types: vat, azoic, mineral

Exhaustion process

• Most amount of water and chemicals used

• Liquor ratio - ratio between water and goods, is high

• Emerge the goods into the dye solution for a long period of time to let the dye penetrate

• Produces more wastewater than continuous process

• Inexpensive - no need to train workers a lot

• Lots of water needed

• Slow process - 60-120 minutes

Continuous process

• Different machinery is placed into a sequence to produce dyed fabric in one continuous pass

• Fast process 10-100m/min

• Small water amount is used

• Very expensive - must train worker to look after and run it properly

Textile finishing

• Processed fabric stage

• Process of adding or enhancing an effect/property to the textile

• 3 objective: modification of surface (raising, smoothing), modification of wearing (staining, creasing, draping), modification of aftercare characteristics (ironing, shrinkage)

Types of finishing

1. Mechanical - achieved by mechanical means

• Framing (tentering)

• Raising

• Emerizing (sueding)

• Calendaring

• Embossing

2. Chemical finishing

• Burn-out technique

• Water/oil repellent

• Surface energy

Framing (tentering)

• Stabilizing fabric dimensional structure by pulling it in width and length under a heat setting (used for thermoplastic materials)

• Width may differ in fabric after construction

• Metal teeth grip fabric → stretch it → steam and shrink it → overfeed to machine

Raising

• Process of lifting a layer of fiber from the surface of fabric, stands out from fabric surface

• Gives raised, brushed appearance to the fabric

• Gives soft & lofty handle to the textile

• Improves warmth but decreases the absorbency

• Subdues the weave/knit of the fabric

• Cannot process filament yarn - need to have a natural material

Emerizing (sueding)

• Process of rubbing surface of fabric with emery paper

• Peach-skin effect

• Soft and luxurious

• Rollers mounted with emery paper, revolve at high speed

Calendaring

• Presses the fabric by passing it through heated rollers under high pressure

• Smoothens the fabric surface, increases luster, gives high gloss finish, surface pattern can be induced

• Embossing is done on the same machine

• Mainly thermoplastic and petroleum based materials are permanently changes

• Cotton cannot change its state permanently

Embossing

• Desired pattern is engraves on calendaring roller, gives emboss effect on fabric

• Fabric can be heat-set (synthetics) to make the effect permanent

Burn out technique (chemical)

• Acid paste is printed on polyester cotton blend

• Heated in the dryer it eats away at cotton leaving just the polyester behind

• Must bew ashed and treated with soda ash to neutralize after

• 70% polyester 30% cotton recommended so that the structure remains intact

Water/oil repellent finishes

• Resistant

• Repellent - takes longer for water/oil to absorb

• Proof - water/oil does not absorb at all

Surface energy

• Measure of tension in the surface of a material

• Fluids with surface energy lower than fiber will absorb into the fiber

• To repel - fiber needs to be coated with material having surface energy lower than the fluid

Finishes are either…

• Permanent

• Durable

• Temporary

• Renewable

Value addition

• Any value that wants to be added to the product afterwards

• Such as embroidery or printing

Textile printing

• Localized application of colour on a textile

• Can be done on yarn (warp) and fabrics and garments

• Bothd dyes and pigment can be used for printing

• Need to be done with a thick paste to achieve distinct and fine results

• Dyes - use if you want something to integrate with the fibers

• Pigments - use if you want something to stand out on the surface

Printing styles

1. Direct printing - leave space for subsequent colors shape

2. Over printing - print a colour shape over another colour printed shape

3. Discharge printing - dye, then print another colour shape with a chemical which will erase the dyed colour from the previous colour

4. Resist printing - print one colour shape with a chemical that will repel dyestuff when the fabric is dyes (for example wax)