Which dyes to use
Direct dyes are used on cellulosic fibres, including cotton, direct dyes are soluble in water, they are moderately fast to light but have poor wash fastness. Reactive dyes are used for cellulosic and protein fibres, i.e nylon , these are water soluble, they form strong chemical bonds with the fibres. They give bright colours which are fast to washing. Vat dyes are used for cellulosic fibres, they are not soluble in water, they give excellent light and wash fastness. To get the dye into the fibre the dye has to be made soluble, to do this the oxygen must be removed from it, when the dye is in the fibres, it is converted back into its insoluble form through oxidation which makes the molecules too big to get out of the fibres. Disperse dyes are used for acetate, polyamide, acrylic and polyester. These dyes are used to dye fibres that have hydrophobic (water hating) properties. They are almost insoluble in water, they are applied to the fibre in the form of a fine aqueous dispersion and are held inside the fibres by chemical bonds. Acid dyes are used for protein fibres and polyamide, they are soluble in water, they are applied to the fabric in an acidic dye bath. They have good fastness to light, wash fastness varies.
preparing fabrics for dyeing and printing
Before a fabric can be dyed or finished they need to have impurities removed to ensure the colour or finish can attach itself evenly to the fabric. desizeing size is a starch, gum or gelatine substance which is applied to warp yarns before they are woven into fabrics, it gives them extra strength so they can withstand the movement of the loom. Most of the substances are water soluble so they can be removed by washing.
scouring removes fatty and waxy impurities which would prevent a fabric from being 'wetted'. cotton is scoured by boiling them in caustic soda solution, not suitable for wool as the heat and alkali would damage it so they are treated by moving them through warm detergent solutions. More modern methods involve washing fabrics in solvents.
Bleaching fabrics can be bleached before colour is added so they are evenly white. cotton is bleached using hypochlorite bleach or hydrogen peroxide. Wool is often left its natural creamy white colour, it is never treated with chlorine bleaches as these will damage the scales. Can be bleached using hydrogen peroxide or by using sulphur dioxide gas, a reducing agent which works by removing oxygen from a stain. Fluroescent whitening agents used to whiten all fibre types. Colourless dyes that cause the fabric to reflect an ultraviolet light, making it appear whiter and brighter.
types of dyeing
Dope dyeing. dope or spun dying is where the dye is added to the spinning solution of man made fibres before the fibre is spun. This method of dyeing results in good light and wash fastness.
stock dyeing refers to when fibres are dyed in a loose form, this method is relatively inexpensive, and has the advantage that if any of the dye is uneven that will be removed during the blending process of fibres into yarns.
yarn dyeing yarns are dyed before they are made into fabrics.
piece dyeing, when woven or knitted fabrics are dyed in the piece
Garment dyeing where fully assembled garments are dyed. This allows manufacturers to apply colour at the last minute as and when there is a consumer demand for a certain colour.
Continuous dyeing fabric is passed through a small pad bath containing the dye then squeezed through two rollers to ensure the dye penetrates the fabric evenly. The fabric is then passed on to a fixing machine when the dye is fixed within the fabric. Batch dyeing processes in these processes a specific weight of fabric is dyed in a self contained unit using a specific weight of dye. semi-continuous dyeing dye is put onto the fabric in the same way as is done for continuous dyeing. It is then wound on to a batching roller then the dye can be fixed whilst the fabric is on the batching roller or can be fixed in a seperate process.
Fabrics can be dyed using a winch machine, this involves the fabric being sewn to make a loop of fabric, this is then passed round and round, over the winch and through the dye bath.
Jig dyeing is used for fabrics that must not be creased. The jig machine has two rollers that sit above the dye bath and fabric is wound from one to another and then the process is reversed until the desired colour is obtained.
jet dyeing is similar to winch dyeing except that the fabric is moved round the machine by a high pressure jet of the dye liquor.
screen printing :derived from the japanese technique of stencilling delicate patterns on fine fabrics, two different types FLAT screen printing- a fabric starts on a roll at the end of the printing table, it is moved along through means of a conveyer belt underneath the printing screens. When fabric stops the screens are lowered , each screen prints a different colour, squeegees move across the screen forcing the printing paste through the mesh of the screen and prints onto the fabric. From the printing table the fabric goes to a drying chamber then passes through a steaming chamber, acid baths and a scouring unit to fix the dye and remove the gum used in printing.
Rotary screen printing: most popular method because it is quick and efficient allows for greater quantities of fabric to be printed in a short time. The screen is a seamless mesh roller, all rollers are fitted on printing machine. Stationary squeegee is fitted through the centre of the roller. Dye is automatically fed through a pipe attachment. Conveyer belt carrying fabric moves in opposite way to the revolving rollers. Tranfer printing: Design is printed onto special type of paper which is wound onto a roller, fabric is placed on another roller and placed opposite to paper roller, paper and fabric are brought together in a dry heat press which transfers the design from the paper onto the the paper. The temperature of the press causes the dye to turn to a vapour which then transfers under pressure to the fabric.
Printing can involve putting colour onto a fabric, or preventing colour from going onto the fabric.
-Resist technique, the fabric is printed with chemicals which prevent dye being taken up by the treated areas.
Dyed technque, fabric is treated with a mordant and the true colour is achieved by applying the second part of the colouring compound during dyeing.
Direct technique, the design is printed directly onto the fabric.
Discharge printing, the dyed fabric is printed with a paste containing chemicals which destroy the dye in areas according to the printed design.
Other resist methods of applying colour include, Tie dye- which uses tightly tied string to resist the dye and Batik, which uses wax to resist the dye. These techniques are not often used in commercial manufacturing as they are very costly to achieve my hand.