Classification of Dyes

Classification of Dyes

Before understanding classification of dyes, we must know the difference in between dyestuff and pigment. Beside this why we do dyeing and characteristic of dyes should also be understood well. Let us discuss these points first.

Dyestuff is organic or inorganic substances which can absorb light and reflect some lights to show color.  Actually, the dyestuff is water soluble substances. In textiles, it is used to dye textile materials. They absorbed or adsorbed into the fibre

Pigment is a substance that can absorb light and reflect some lights to show color. Not water soluble and possess no specific attraction for any particular fibre type. They usually adhere to the surface of the fibre Normally it is used for printing (with the presence of binder) or mass-coloration of the synthetic fibers. They do not belong to dyes.

Why we do dyeing/printing?

•         Because Colored products are attractive and gives pleasure/ comfort

•         In textile dyeing terms: The main objectives of dyeing are:

•         Colour the fibre uniformly

•          Achieve acceptable durability of the     colour to further treatments in production and normal use    ( colour fastness properties)

Characteristics of dyes: Following are the important poits related to characteristics of dyes:

•         Intense colour

•         Solubility in water

•         Substantivity to the fibre in question

•         Durability to wet treatment

•         Durability to further treatments in  production and normal use

•         Safe, easy to handle, and reasonably priced

Intense colour

•         The dyes have intense colours.

•         Due to intense colour small amount of dyes are sufficient to obtain desired shades.

•         In practice, the increase in weight of fibrs by dyeing is between 0.1% to 5%

Solubility in water

•         Dyes are soluble or dispersible in water

•         Water is the vehicle of exhaust dyeing; it carries the dyes to the fibres.

•         Disperse dyes are sparingly soluble in water, the require dispersing agent.

Substantivity to the fibre

•         Dyes have to be substantive to the fibre in question.

•         The affinity that a dye has particular fibre is referred to by the dyers as ‘Substantivity’

•         The substantivity of a dye to a fibre must be much stronger than affinity to water

Durability to wet treatment

•         Dye must possess the ability to become durable to wet treatment

•         A high substantivity dyes some time provide sufficient durability of the colour to a limited number of mild washing

•         So after treatment step may be taken to improve upon durability

Durability to further treatment in production and normal use

•         Dyes should have fastness to further treatment in production and normal use.

•         Example: To apply durable press finish, the dyes should withstand the low pH and high temperature

•         In addition dyes should have good fastness properties

•          

Safe, easy to handle, and reasonably priced

•         The dyes should be safe, easy to handle and reasonably priced

Sunday, 18 January 2015

How to prepare polyester cotton blended fabric for dyeing?

If the fabric contains polyeset and cotton blends then we have to scour it as we do in the case of cotton scouring. For this purpose 2-3% owf sodium hydroxide  may be used depending upon the percentage of cotton in the blends at boil for 45 minutes. For example if the blend contain 50% Polyester and 50% cotton and total weight of the fabric is 100 grams. Then there will be 50 gram of cotton in the blend. If we used 3% Sodium hydroxide on weight of fabric (owf) then the total NaOH required will be 1.5 gram.


How to prepare polyester viscose blended fabric for dyeing?

As polyester and viscose both are manmade fibres, there is no need to do vigrous scouring like cotton, which has natural impurties. In that case light scouring is needed, in which detergent may be used to remove oil stains. 

Saturday, 27 December 2014

How to test scoured cotton material:

As we understand cotton is scoured using alkali at boiling temperature. During or after scouring, it is must to know whether the material scoured properly or not. To ascetain that a simple drop test can be performed. In such type of test, if the material is under scouring process, a small piece of fabric is cut from the lot of fabric which is running in the machine say jigger (after stoping it). The piece is washed properly under tap water (Soft water) and dried using iron or placing the fabric piece on hot steam pipe line. After cooling the fabric, a drop of water is allowed to fall on this piece of fabric. If the drop is absorb whithin few seconds say 3 seconds, one can be assured that scouring is perfectly done. However, if the drop is taking much time to absorb, there is a need to continue the scouring process untill the fabric acquire optimum absorbency. If the fabric is being scoured for printing process, it is required that the water drop should be absorbed within 1 second. There are standard tests are also availabe for testing absorbency of fabric in the laboratory e.g IS 2349 and AATCC 79.

Saturday, 20 December 2014

How to test desized material (Starch based size):

To test the presense of sizing material i.e starch based sizing material in the fabric iodine test is conducted. In which the fabric is spotted or treated with  Iodine solution —Iodine dissolved in an aqueous solution of potassium iodide, which reacts with the starch and produce a bluish-purple-black colour. If there is no starch the colour may be brownish-yellowish.  In the below figure, it is explained that if the iodine solution is added in the solution ( which you want to know whether there is a starch or not) if there is a starch then you may get purple/bluish colur solution. If there is no starch the colour remain same. 

TEGEWA solution is also used for the identification of starch.  For making this solution potassium iodide 10 g of KI (100%) in 100 ml of water is added with 0.6358 g of iodine (100%) and shake this well to disslove iodine crystals. Then make it up to 800 ml using distilled water and finally make it up to 1000 ml with ethanol.

Saturday, 13 December 2014

High Pressure Kier with multitubular heater:

The modern high pressure kier with multitubular heater is also known as  Walsh's kier and one of the best kier. The 2 ton capacity kier is most common which is about 9 ft high and 6.5 ft in diameter and mounted on R.C.C Coloumn. The kier works at working pressure 40 lb/sq inch at 141 degree C and is composed of mild steel plates. The kier is provided with pressure gauge, safety valve, blow off valve for air, steam trap, liquor level indicator, drain cock etc.

The goods in rope form are plaited either manually or mechanically plaiting on the perforated false bottom. The cloth can be saturated with alkaline solution containing the recipe as given above with figure.

During piling sufficient scouring liquor is   added to the kier from the below to remove entrapped air from the goods to avoid the formation of oxycellulose. The lid is then closed but air valve is kept open. Steam is turned on and the liquor is circulated by centrifugal pump coupled to an electric motor. High pressure kier operate at about 8 to 10 hours with liquor ratio 1:3 to 1:5 (MLR). After scouring the drain cock is opened at reduced pressure (5 to 6 lb/sq meter) and equivalent amount of hot water is added from the top. After circulating cold water the goods are removed and washed in a rope washing machine. The liquor is withdrawn from the bottom of the kier and forced through the heater and spread over top of the load by distributor. Fabric which are to be sold in white state are given two boils and the fabric intended for printing single boil is sufficient.

 Precaution:

 Following precaution should be taken while using kier:

• Kier should be clean
• Material should be packed evenly.
• No air packed should be formed
• The fabric should be in liquor completely.

Monday, 8 December 2014

Souring:

The fabric after bleaching contains metallic salts such as calcium carbonate, magnesium carbonate etc., and other mineral matter. These salts, if left on the fabric, will not only give a harsh feel but also affect the uniformity of the shade during dyeing. In order to make these salts soluble in water, the fabric is treated with either dilute hydrochloric acid or sulphuric acid, the former is used for fabrics meant for dyeing whereas latter, being cheap, is used for fabrics meant for white finish. Hydrochloric acid which forms soluble calcium salts is better then sulphuric acid. The concentration used is about 0.5 to 1.0% on the weight of fabric.

Sunday, 30 November 2014

Making a lot size for preparation or dyeing:

Grey stage fabric usually 60-70 m in length so that  several pieces are stitched together to make a processable length ( Lot size ).  Normally Polyester (Sewing thread ) yarn is used for stitching. Marked each piece properly for sort number, lot number etc. so that pieces can be identify at any stage of the processing. Two pieces are arranged selvedge to selvedge and the ends are stiched on a special end to end stitching machine so that pieces are joined without overlapping: Some of the important poits are given below:

•         Cotton threads: 3/40s, 4/30s, 4/36s, 6/28s and 6/36s are popular

•         Polyester threads are also used when carbonising is required

•    Stitches per inches:  6 to 7 for coarse, 7 to 8 for poplin and 8 to 10 for cambric

Stitching defects: Stitching defects are described below:

i)  Protruding Selvedge: It mostly occurs due to difference in width of the pieces. Some time careless handling also lead to a protruding selvedge.

It Causes: Fold or selvedge crease and the fold portion either less dyed or       remain undyed

ii)  Open End: Pieces are not locked in the stitch at and near the selvedge and due to careless handling.

It Causes: Such stitch will open out more and more every pull around guide roll during processing. It may induce Fold or crease formation at the selvedge, undyed/unprinted folds will result.

iii) Loose Thread: It occurs due to careless handling of stitching process.

It Causes: It may cause thread mark in dyeing and printing, lead entanglement in machine parts, etc. The loose thread may pick up soil and damage long lengths

iv) Fold: If the large folds are inserted in the stitch to accommodate width difference of two pieces, it will surely lead to damage

It Causes: It may cause warp stripes in dyeing and creases in dyeing and printing

v)Broken Stitch: It may be due to poor machine condition, poor quality of thread or poor method of operation.

It Causes: It may cause damage in dyeing and printing