The Dyeing Process

The main area of interest for plasticizers in PET is in the area of dyeing. Due to its lack of hydrogen bonds PET is relatively difficult to dye. Plasticizers used in this process can increase the speed and intensity of the dyeing process. The compounds used, however, tend to be of low molecular weight since high volatiHty is required to enable rapid removal of plasticizer from the product (see Dye carriers). 

Ammonium acetate has limited commercial uses. It serves as an analytical reagent, and in the production of foam mbber and vinyl plastics it is also used as a diaphoretic and diuretic in pharmaceutical appHcations. The salt has some importance as a mordant in textile dyeing. In a hot dye bath, gradual volatilization of ammonia from the ammonium acetate causes the dye solution to become progressively more acidic. This increase in acidity enhances the color and permanence of the dyeing process. 

Mordant dyes have excellent lightfastness. However, their colors are not so brilliant, and they need treatment of fibers with metal salts such as those of Cr, Al, Fe, or Ni before dyeing, which makes the dyeing process compHcated and leveling properties unsatisfactory. 

Physical and organic chemistry of dyes and the dyeing process, Dyeing of ceUulosic fibers. 

Physical and Organic Chemistry of Dyes and the Dyeing Process... 

In the dyeing process absorption from the dyebath solution to the fiber eventually stops when an equiHbrium exists between the dye in the fiber phase and the dye in the solution phase. At this point by definition (no movement of dye molecules), therefore... 

With knitted fabrics it is necessary to remove the knitting oils by either alkaH treatment or solvents. Where water-immiscible oils have been used and the fabric is to be hot dyed (80°C or above), a minimum scour to remove dirt and stains can be sufficient, the rest of the oil being removed during the dyeing process. 

Eig. 3. Amounts and forms of fiber-reactive dye on the fiber as a function of time for a low affinity dye, where X represents the reactive group. Point A represents the amount of dye exhausted in neutral conditions B is the total amount of dye exhausted at the end of the dyeing process, ie, [dye—OH] +... 

Alkali is usually added in a second stage. However, with low reactivity high affinity dyes it is possible to add the alkah at the beginning of the dyeing process and control the rate of uptake and chemical reaction by temperature control. With high affinity dyes the exhaustion takes place at low temperature rapidly before the chemical reaction becomes significant. If dyes are carefully selected or synthesized to have identical dye uptake it is possible to include all the electrolyte from the beginning and operate an "ah-in" technique. 

Dyeing Mechanism. Unmodified polyester fibers are very hydrophobic and absorb only minimal amounts of water and are therefore only dyeable with hydrophobic disperse dyes. The mechanism of dyeing is by simple partition, the so-called soHd solution mechanism. The dyeing process can be described by the general scheme... 

In continuous dyeing there are many variables and the rapidity of the dyeing process requites many adjustments during the period in which several thousand meters of textile are dyed. Instmmental science has continued to advance rapidly so that continuous ranges are available which are entirely computer-controUed except for the makeup of the dye mix. These units feature computer control and closed-cincuit television and continuous color measurement techniques. 

A third approach utilised copper salts, especially copper(II) sulphate, in conjunction with dyes containing chelatable groupings such as salicylic acid or o,o -dihydroxyazo moieties. Indeed, special ranges of copperable direct dyes, for which the treatment with copper(II) sulphate was really part of the dyeing process rather than an optional aftertreatment, were introduced. In the past the main use of this chelation treatment was to enhance light fastness, but it is little used for this purpose nowadays. 

Analysis of dyed fibres allows identification of real colouring components of natural dyestuffs taking part in the dyeing process. Wool threads dyed with madder (Rubia tinc-torum) as well as Our Lady s bedstraw (Galium verum), were studied by HPLC DAD ESI MS" (SIM mode).[8] Chromatograms of the extracts from wool dyed with madder... 

Batcheller et al. also discuss the influence of the dyeing process on the composition of the fibre s surface. They note that high alkalinity/acidity and extended processing time result in loss of surface lipid. Nevertheless, the dyeing process has limited influence on the ageing process due to light exposure. 

In the course of his studies of the dyeing process, he became deeply interested in the structure of natural fibers, and most of his efforts were directed toward this new field of research, with the help of able associates, among them R. Brill, M. Dunkel, G. von Susich, and E. Valkd. His investigation of various aspects of the problem utilized physical means (for example, x-ray diffraction, optical properties, and viscosity) and the purely chemical approach. A young scientist, H. Mark, who later became an authority in the field of high polymers, was appointed head of the physical chemistry laboratory. 

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