Disperse Dyeing Cellulose Triacetate

These dyes have affinity for one or, usually, more types of hydrophobic fibre and they are normally applied by exhaustion from fine aqueous dispersion. Although pure disperse dyes have extremely low solubility in cold water, such dyes nevertheless do dissolve to a limited extent in aqueous surfactant solutions at typical dyeing temperatures. The fibre is believed to sorb dye from this dilute aqueous solution phase, which is continuously replenished by rapid dissolution of particles from suspension. Alternatively, hydrophobic fibres can absorb disperse dyes from the vapour phase. This mechanism is the basis of many continuous dyeing and printing methods of application of these dyes. The requirements and limitations of disperse dyes on cellulose acetate, triacetate, polyester, nylon and other synthetic fibres will be discussed more fully in Chapter 3. Similar products have been employed in the surface coloration of certain thermoplastics, including cellulose acetate, poly(methyl methacrylate) and polystyrene. 

In order to facilitate satisfactory dye uptake, the molecular size of a disperse dye must be kept small monoazo structures are therefore exceptionally important, particularly in the coloration of polyester and cellulose triacetate. In the yellow shade area, molecular size generally poses no problem and the various available coupling components can all be used without making the molecule too large. A very simple example of the type of structure employed using a phenolic coupling component is Cl Disperse Yellow 3 (4-72). This dye is known to cause skin sensitisation when on nylon [85] and can also provoke an allergic reaction [86]. 

Above Illustrating adsorption of typical non-ionic disperse dye by cellulose triacetate. Below (Schematic) Illustrating loss of affinity of polar groups by screening effect of solvated water on ionic group, in a typical sulphate-ester dye. 

Cellulose Triacetate. Cellulose acetate having 92% or more of the hydroxyl groups acetylated is referred to as triacetate. This fiber is characteristically more resistant to alkali than the usual acetate and may be scoured, generally, in open-width, with aqueous solulions of a synthetic surfactant and soda ash. Triacetate is a hydrophobic liber, as compared to secondary acetate, and consequently does not dye rapidly. It is necessary to increase the rate of diffusion of the disperse dye into the fiber by increasing the dyeing temperature to 110— 130CC or using a dye accelerant or carrier, or both. 

Dyeing Characteristics. Disperse dyes, high melting crystalline compounds w ith low solubility in the dye hath, are most frequently used for cellulose acelate and triacetate fibers. 

Disperse dyes are nonionic and have very low water solubility. They are applied as a dispersion to polyester, nylon, cellulose diacetate, triacetate, and acrylics. 

Disperse dyes are currently used to dye cellulose 2.5-acetate, cellulose triacetate, synthetic polyamides, and to a lesser degree, polyacrylonitrile and polypropylene. Their major application is clearly for dyeing polyesters. 

For printing disperse dyes on cellulose acetate, dyes are selected according to their colorfastness on 2.5-acetate and triacetate. Fixation is carried out for 2.5-acetate in saturated steam at 102°C for 20-40 min for triacetate 20-30 min at 0.25 MPa (127°C) or in superheated steam 8-6 min at 165-185 °C. 

Report of Committee Dyeing Properties of Disperse Dyes, I Cellulose Acetate, J. Soc. Dyers Colour. 80 (1964) 237-242, II Cellulose Triacetate, 81 (1965) 209-210 J.-H. Choi, A.D. Towns, Color Technol. 117 (2001), 127-133. 

The color of cellulose acetate dyed with some disperse dyes is subject to gas fading. Treatment of the dyed material with diethanolamine or melamine can overcome the problem (3). Similarly, with cellulose triacetate, gas fading of dyes can occur. It has been stated that protection can be obtained by the application of an inhibitor (3). 

A similar series of tests for the assessment of the dyeing properties of Disperse Dyes on cellulose triacetate has been published J.S.D.C., 1965, 81, 209). 

Camille and Henry Dreyfus developed the first commercial process to manufacture cellulose acetate in 1905 and commercialized the spinning of cellulose acetate fibers in 1924 in the United States. At that time, the only other human-made fiber was viscose rayon, which was still in its early stages of commercialization. The main textile fibers were natural fibers cotton, wool, silk, and flax. Cellulose triacetate textile fiber was commercialized later in the 1950s. The tremendous technical effort by the Dreyfus Brothers resulted in more than 300 patents describing such significant inventions as the dry-spinning process and disperse dyeing. 

The preferred dyes for cellulose acetate and triacetate are the disperse dyes. These consist mainly of azo, anthraquinonoid, and nitrodiphenylamine dyes that are insoluble or at best slightly soluble in hot aqueous dye-bath systems. These crystalline dyes must be ground to a very small particle size, e.g., 2 pm. They are then pasted with water and a wetting agent and added to the dye-bath through a fine mesh screen. A dispersing agent and a carrier or accelerant may be present in the dye-bath to enhance diffusion of the dye molecules into... 

Acetate and triacetate cannot be dyed by dyes used for cellulosic fibers. These fibers can be satisfactorily dyed with disperse dyes at moderate to high temperatures to give even, bright shades. The laundry colorfastness of triacetate is excellent however, dyed acetate fabrics generally have moderate to poor colorfastness. 

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