Revision Flashcards
What is printing and what are the 3 stages of printing?
- Involves localised dyeing
- gives definite design/pattern
- can be applied to fabrics/garments
stages
1) Transfer
- dye transports to the fibre surface & forms a film
2) Fixation
- movement of dye from film to fibre interior
3) Wash-off
- with detergent and water (sometimes hot water) to remove excess paste
What are the ingredients in printing paste & what are their functions?
- dye or pigment
- Solvent: water - acts as a carrier for the other ingredients
- solution assistants (with higher dissolving power than water)
- thickener (ensures that appropriate vicious of the ink has thixtropy behavior)
- humectant (assists the attention of moisture)
- dye fixation chemicals (acids, alkalis, reducing/oxidising agents)
- protective agents (protects the dyes from degredation)
- miscellaneous ingredients (wetting agent, preservatives)
- binder (when pigment is used as a colourant)
Explain the relationship between printing time and viscosity of thickener?
Thixotropy
- The viscosity of the printing paste is high to begin with.
- when force is applied by the squeegee on to the screen and you start to move the squeegee across the screen, the viscosity of the thickener becomes lower.
- as soon as the force from the squeegee is removed, the viscosity of the paste goes back to being thicker again.
Name the different types of thickeners?
Natural
- plant seeds
- plant extrudes
- alginates
- starch
Synthetic
- Emulsion
- Foam
- PVA
- Polyacrylates
What are the types of dye fixation chemicals?
- Acids (eg for fixation of acid dyes to wool)
- Alkalis (e.g for fixation of reactive dyes to cotton)
- Reducing/ oxidising agents (e.g reducing agent required in Vat dyeing off cotton)
What are the preparation processes for printing?
Chlorination of wool
- oxidation process using the chemical sodium chlorate
- improves wetting properties & swelling of the fibres
- improved dye-uptake because the dye can penetrate more easily
- providing stronger, brighter colours
Bleaching of cotton
- treatment with hydrogen peroxide (an oxidation process)
- removes any natural colour on the fibre
- assists the wetting and swelling of the fibre, produces brighter printed colours
- easier dye penetration
- cleaner background
What are the 3 categories of acid dyes for printing of wool fabrics?
Acid Levelling
- brilliant colours
- gives good levelness
- relatively weakly attracted to the fibres
- gives low wash fastness
Acid Supermilling
- strongly attracted to the fibre
- excellent washfastness
- difficulties with obtaining level drying
- restricted colour range
Acid Milling
- intermediate properties
- there is a compromise between washfastness & levelling properties
- acid milling dyes are commonly used for textile printing: reasonable properties and easy to wash off unfixed dye
What should the PH condition be like when using acid dyes for printing onto wool fabrics?
- Acidic condition - Ammonium sulphate
What types of dyes can be used to colour cotton?
- Direct dye
- Vat dye
- Reactive dye
- Pigment
Explain why reactive dye produces excellent wet-fastness?
Components of a reactive dye molecule
- S - water solubilising group
- C - coloured group
- B - bridging group
- R - reactive group forms the covalent bond with the fibre
Name some examples of special effect pigments?
Thermochromic
- novelty effects
- colour change activated by temperature change
Photochromic
- novelty effects
- develop colour in UV light
Fluorescent
- light emitting
- high visual impact
Metallic
- shiny/glitter effect
What are some advantages of pigments as colourants?
- comprehensive range of colours
- excellent fastness properties (especially to light)
- relatively inexpensive
- non-toxic
- special effect types are available
- simple process
- reliable
- applicable to all fibres
- no need for substantial wash off
- fast production speeds
- little waste generated
- economical in terms of water, materials, processing and energy usage
- environmentally very acceptable
Name some examples of printing styles
- Direct printing - printing with dyes or pigments directly onto a white fabric
- Discharge printing - fabric is dyed with a solid colour & a reduction process is used to destroy locally the background colour
- Devore style - fabric used for Devore printing is composed by 2 different types of fibres, chemicals are used to destroy 1 of the fibres locally
- Resist printing - printing the fabric first with a substance called “resist” which prevents dye from being taken up in a subsequent dyeing process.
What are the differences between dyes and pigments?
Dyes
- used to colour a fabric of a single uniform colour
- penetrate into the fibres
- they are water soluble
Pigments
- create a printed design which:
- stays on the surface of the fabric
- are water insoluble
What are the important characteristics of thickeners?
a) good water solubility
b) good adhesion to fabric
c) good flexibility of dried film
d) low cost
e) non-toxic, low environmental impact
f) stability in use & storage
g) compatibility with other ingredients (dyes & pigments)
