DYEING WITH DYES

-->

DYEING

The color of a textile product may be its most important marketing attribute. It is the color of the dyed (or printed) fabric that first attracts and then draws consumers to particular items for sale. It is often the color of a product that sells the product. Dyeing is impregnating fiber, yarn, fabric or garment with a dyestuff. Dyes are colorants that are applied to, or formed in, a textile substrate in a molecularly dispersed form. they derive their color from the conjugated double bonds in their molecular structure.

The definition stresses that for use in textiles, dyes must be soluble or capable of being made soluble in the medium in which they are applied, or they must themselves be molecularly dispersible into the fibers. It is this property that distinguishes them from pigments. When a dye colors a fabric directly, without the aid of a fixing agent its called Direct Dye. In some other dyeing processes, a fixing agent is required to fix dye to fiber; the fixer is called mordent

Fastness of color is its ability to remain unchanged. Color fastness may be affected by such factors such as perspiration, dry cleaning, sunlight, salt water etc. Adding color to textiles, thus making fabrics marketable as fashion component, is a sophisticated and complex area where art and creativity meet with science and technology. Although the chemistry of dyes and dyeing are extremely complex, the development of electronic and computer science applied to the dyeing process has opened the world to rapid global trading and quick response systems. Matching shades of dyeing and the approval of colors may now be executed by phone and fax (without the necessity of seeing visual samples that must be sent by mail or courier) thus saving many weeks in international and domestic trade cycling. Details covering electronic and computer usage are included in this chapter as are explanations of the traditional processes of textile dyeing.

Although color is recognized as the most important element in textile sales and merchandising, it is also the source of most problems that consumers and the textile industry encounter in the production and use of fabrics. Fading, bleeding, color staining and color streaking are typical examples. Understanding the dyes and dyeing processes discussed in this chapter can aid in reducing or eliminating many of these problems.

Colorfastness of dyeing:

A fabric that retains its color during care and use is said to be dyeing colorfast. Fastness that is affected by the factors discussed above is an important concern of consumers. Small aggregates of dye molecules distributed evenly throughout the fiber make for a more satisfactory result than do surface applications of dyes. Fabrics may be more or less colorfast to a variety of different substances or conditions.

The importance of colorfastness of dyeing depends on the use of the fabric. Colorfastness to laundering is, of course, important in those garments and household textiles that must undergo frequent laundering. Some dyes are not fast to laundering but are fast to dry cleaning, or vice versa. Perspiration may cause some color change and/or color transfer, and some colors are may be lost or diminished by hear. Dyeinf colorfastness to sunlight may be important in evaluating the - usefulness of fabrics for curtains, draperies, carpets, and outdoor clothing in case of dyeing.

Light fastness of dyeing is usually a function of the dye structure rather than its retention within the fibers. The molecular structure that provides the color can be interrupted by light, particularly ultraviolet light. Additives or finishes are available to stabilize dyes from this type of action in case of dyeing.

Some dyes tend to crock, or rub off on fabrics or other materials with which they come in contact. Others will bleed into water during laundering and may be picked up by lighter-colored fabrics. Chlorine bleaches will remove color from most dyed fabrics, but some dyes are more sensitive than others to the action of chlorine bleaches of dyeing.

You should read RELATED POST for more information.
Wish you good luck.......................................................

-->

MORDANTS; DYE FIXATION OF NATURAL DYES

BATCH DYEING OF WOOL WITH REACTIVE DYES

WOOL DYEING PROCESS WITH REACTIVE DYES
Batch dyeing of wool with reactive dyes is a very popular process. In batch dyeing process reactive dyes are usually applied to wool at pH 5–6 using ammonium salts, and acetic acid as required. At higher pH values, exhaustion is too low, and at lower values rapid dyes uptake gives unlevel dyeings in batch dyeing process. Slightly higher pH values are used for dyeing paler shades (pH 5.5–6.0) and lower values (pH 5.0–5.5) for deep shades in batch dyeing of wool. Fibre Reactive dyes often give quite good exhaustion at temperatures below the boil but the dyeing temperature will eventually be raised to 100 °C to ensure that reaction with the wool is as complete as possible. Some procedures recommend a holding stage at an intermediate temperature of 65–70 °C for 15–20 min to allow the dye to migrate before it reacts with the wool.

Batch dyeing machine for with reactive dyes.

RELATED POST 
REACTIVE DYES FOR WOOL FIBRES - Details about wool dyeing,

Because of their tendency to give unlevel, skittery dyeings, reactive dyes are usually applied to wool in the presence of proprietary levelling agents in case of batch dyeing process of wool. These are often amphoteric, having both cationic and anionic groups in the molecule. In contrast to most levelling agents, which decrease the dyeing rate, the auxiliary products for dyeing wool with reactive dyes accelerate dyeing. The anionic dye complexes with the cationic site in the auxiliary product but the remaining anionic site provide substantivity for the wool surface. The bulky dye–auxiliary complex exhausts well onto the fibre surface at relatively low temperature, better than the dye alone, but cannot penetrate into the fibres. The complex breaks down as the dyeing temperature increases so that the smaller liberated dye molecules can then absorb into the wool. The use of such products avoids unlevel, skittery dyeings and provides better compatibility of dye mixtures during the batch dyeing.

Deeply dyed wool fibre or fabric with reactive dyes in batch dyeing process must be aftertreated to remove unfixed dye so as to give the best wet fastness. This is particularly important to ensure that there is no staining of adjacent undyed material during washing. After dyeing of wool with reactive dye, the material can be washed at 80 °C for about 15 min using a dilute ammonia solution at pH 8.0–8.5, and then rinsed in water with a little acetic acid. To avoid any alkali damage to the wool after batch dyeing, washing can be done with hexamine (hexamethylenetetramine from formaldehyde and ammonia) at pH 6.5, or with sodium bicarbonate. Certain proprietary chemicals can be added to the dyebath on completion of dyeing and their hydrolysis increases the bath pH to around 7. For example, hydrolysis of sodium trichloroacetate gives chloroform, carbon dioxide, both of which are volatile, and sodium hydroxide (Scheme 16.6). The actual colour removed may consist of unreacted dye, hydrolysed dye and products of the reaction of the dye with soluble wool hydrolysis products such as ammonia and hydrogen sulphide or amino acids.

Reaction related to dyeing wool with reactive dyes:

CCl3 CO2Na + H2O = HCCl3 + CO2 +NaOH

Shrink-proof wool, which has been treated with resins in the Hercosett process, remains cationic on the surface and gives rapid uptake of reactive dyes. The usual auxiliary levelling agents may be less effective in this case. The deposited resin protects the wool from damage and the best fastness results for deep shades are obtained by dyeing at 110 °C for 30 min.

Wool dyed in deep shades with reactive dyes is better protected from damage during dyeing. A number of explanations for this have been proposed. These involve protein chain crosslinking, reaction with thiol groups that interferes with 357 the reformation of disulphide links, and reaction with non-keratinous proteins in the cell membrane complex and endocuticle. So reactive dye is best for dyeing wool fibre in batch dyeing process but proper care should be taken other wise shade will be uneven.

RELATED POST

Vinyl sulphone dyes or Remazol dyes are good quality Reactive dyes 

After Treatment And Stripping of Reactive Dye

Structural discussion of reactive dyes those suitable for cotton fibre

Informative articles on Dye reactivity, Application and Storage of Reactive dyes