DISPERSE DYE
Disperse dye is originally developed for the dyeing of cellulose acetate. They are substantially water insoluble. The dyes are finely ground in the presence of a dispersing agent then sold as a paste or spray dried and sold as a powder. They can also be used to dye nylon, triacetate, polyester and acrylic fibres. In some cases a dyeing temperature of 130 deg C is required and a pressurised dyebath is used. The very fine particle size gives a large surface area that aids dissolution to allow uptake by the fibre. The dyeing rate can be significantly influenced by the choice of dispersing agent used during the grinding.
Disperse dyes have low solubility in water, but they can interact with the polyester chains by forming dispersed particles. Their main use is the dyeing of polyesters, and they find minor use dyeing cellulose acetates and polyamides. The general structure of disperse dyes is small, planar and non-ionic, with attached polar functional groups like -NO2 and -CN. The shape makes it easier for the dye to slide between the tightly-packed polymer chains, and the polar groups improve the water solubility, improve the dipolar bonding between dye and polymer and affect the colour of the dye. However, their small size means that disperse dyes are quite volatile, and tend to sublime out of the polymer at sufficiently high temperatures.
The dye is generally applied under pressure, at temperatures of about 130oC. At this temperature, thermal agitation causes the polymer's structure to become looser and less crystalline, opening gaps for the dye molecules to enter. The interactions between dye and polymer are thought to be Van-der-Waals and dipole forces.
The volatility of the dye can cause loss of colour density, and staining of other materials at high temperatures. This can be counteracted by using larger molecules, or making the dye more polar (or both). This has a drawback, however, in that this new larger, more polar molecule will need more extreme forcing conditions to dye the polymer2.
The most important class is the azo class. This class of azo disperse dyes may be further sub-divided into four groups, the most numerous of which is the aminoazobenzene class. This class of dye can be altered as mentioned before, to produce bathochromic shifts. A range of heterocyclic aminoazobenzene dyes are also available. These give bright dyes, and are bathochromically shifted to give blues. The third class of disperse dye is based on heterocyclic coupling components, which produce bright yellow dyes. The fourth class are disazo dyes. These tend to be quite simple in structure. Other than these, there are disperse dyes of the carbonyl class, and a few from the nitro and polymethine classes. Below is an example of a disperse dye. It is the same as the chime molecule at the top of the page. Recently, Sokolowska-Gajda and Freeman reported an effective methods for the diazotization of 2-amino-6nitrobenzothiazole in a mixture of acetic acid and dichloroacetic acid, followed by coupling with N-β-cyanoethyl-N-β-acetoxy-ethylaniline to form disperse red 177. Although the yield of this two step synthesis was good (87%), it was somewhat lower than anticipated from the work of others, in which the diazotization was conducted in the phosphoric acid (12). In addition, there is now reason to be concerned about the toxicity of an HOAc/CHCl2CO2H effluent. Similarly, the use of H3PO4 causes an environmental problem known as eutrophication.
Classes of disperse dye
Dyes may be classified in a variety of ways, some of which are unique to the particular application category. Disperse dyes are no exception. As might be anticipated, chemical classification by chromophore is generally the least useful to the dyer. However, there are some chemical differences between disperse dyes which affect their performance in dyeing. These include the ease with which they are reduced and the ease with which they are hydrolyzed.
1 Reduction clearing
Because disperse dyes have such limited solubility in water, some particulate disperse dye may still be occluded on fiber surface after the dyeing phase is complete. If this condition is suspected, the last stage of the total dyeing process may need to be one where surface dye is removed. Adverse results of excess dye on the fiber surfaces include considerably reduced wet fastness, wash fastness, sublimation and drycleaning fastness, as well as dulling of the shade.
With experience, the presence of excessive amounts of surface dye can be determined by simply agitating a sample of the dyed goods in a little cold acetone for a few seconds, when surface dye will dissolve. The acetone will not extract dye from within the fiber, which remains unswollen, but will dissolve surface color.
The usual practical procedure for removing this unwanted dye is called reduction clearing and uses a bath of about two grams per liter of both caustic soda and sodium dithionite ( hydro ); as 100% solid products, plus about one gram per liter of a stable surfactant.
A treatment for 20 minutes at approximately 70C (160F), is often sufficient to clear the fiber surfaces, but the ease of removal varies from chromophore to chromophore and dye to dye. Provided the clearing temperature is not above the dyeing transition temperature, no dye will be stripped from within the polyester fibers.
The largest majority of disperse dyes contain the azo group, -N=N-. This group is easily split into two amino groups by treatment with reducing agent:
-N=N- red -NH2+H2N
Such dyes are particularly suitable for reduction clearing since the amino residues are virtually colorless unless deliberately oxidized to form totally different products.
Some bulky azo dye molecules, notably the navies as a group, are so sensitive to reduction that under conditions of too high pH they can cleave at the azo group, even during the dyeing process, to give dyeings of a characteristic lighter, duller and greener appearance. The condition is more pronounced in polyester/cotton blends.
Another chromophore still found in some of the brighter blue, pink and red disperse dyes, despite its cost, is anthraquinone. This is more difficult to reduce and is not destroyed during normal reductive clearing. However, as in the case of vat dyes, the anthraquinone residue is at least partially and reversibly reduced to a soluble sodium leuco form which can be washed away but which on subsequent exposure to air becomes insoluble again
2 Hydrolysis of Dye Ester
Another chemical group frequently found in disperse dyes is an ester group, often an acetyl group, O-CO-CH3, and like the acetyl groups in cellulose acetate it is susceptible to hydrolysis in neutral and alkaline conditions:
Dye-O-CO-CH3 H
2O Dye-OH+HO-CO-CH
3
The products are acetic acid and a different azo disperse dye, whose color may be quite different from that of the parent dye. Usually the wavelength of maximum light absorption (minimum reflectance) is shifted to a longer wavelength. This is known as a bathochromic shift, in which colors change in the general direction: Yellow » orange » red » violet » blue » green. However two additional, widely different points are worth noting here.
The presence of hydrolysable groups in many disperse dyes and their protection is the principle reason why dyeing are generally made on the slightly acid side. The pH has no fundamental role in the dyeing mechanism as such and some disperse dyes without ester groups do not need a weakly acidic dyebath.
Dyes and dyeings in any application category have traditionally been presented- eg., in shade cards, the Color Index and until recently, in Buyers guide – in the same bathochromic series order listed above. Blacks are shown last and browns are sometimes presented after orange and sometimes between green and black. To simplify processing by computer, the Buyers guige now lists dyes alphabetically by color- i.e., black, blue, brown, green, orange, red, violet, yellow- and the color indexd is considering the possibility of doing the same. While it makes no coloristic sense, the alphabetical system simplifies the processing of the data.
A group of sisperse dyes utilizes the alkaline hydrolysis of esters in an ingenious way. These products contain ester groups in the reverse orientation relative to the dye chromophore to that shown above. During alkaline scouring, the surface dye becomes a water soluble carbxylate salt and is easily removed by washing,
Dye-CO-O-CH3+NaOH Dye-COO¯Na + CH3- OH
3 Energy Level
Most disperse dye classifications are based on some form of generalized grouping according to their rates of dyeing and resistance to sublimation. For example, a major company from the United Kingdom has chosen to classify its disperse dyes into four groups, A-D, where subgroups A contains those dyes with the highest rate of dyeing on polyester and the lowest sublimation fastness, while the dye in subgroup D are just the reverse.
The fact of the matter is that the relative dyeing, physical and most fastness properties of disperse dyes lie scattered around a line from generally small molecules, with low polarity, poor heat and sublimation resistance, rapid rates of dyeing and good leveling characteristics, to generally much larger molecules which are quite polar without being ionic, with good hesat and sublimation fastness, poor leveling and low rates of dyeing. Note that light fastness is not a property which is dependent on the molecular size or polarity.
A disperse dye is suitable for dyeing cellulose acetate, carrier dyeing polyester, high temperature dyeing of polyester and dyeing of polyester and cotton blends by thermofixation runs along the same line from A to D. But the precise position of an individual dye relative to others on the line depends on the particular physical property selected and therefore any subdivision is somewhat arbitrary.
In the U.S it is normal to classify disperse dyes into three sub-groups called low, medium and high energy. These cover the same range of properties as the A-D classification mentioned earlier. Again the groups are somewhat arbitrary. But disperse dyes within any one of the subgroups are much more likely to have similar dyeing and fastness characteristics ( other than light fastness) and are consequently more suitable for dyeing together than dyes outside the same subgroup. Because of the number of available dyes, there is still plenty of room for selecting dyes within any subgroup which can deliver the particular characteristics desirable in the final dyed product.
As an illustration, one major manufacturer carries a line of just over 30 disperse dyes principally for polyester: about 25% are low energy dyes of which six are very suitable for carrier dyeing, a different six are very suitable for high temperature dyeing and one is suitable for dyeing the polyester in polyester/cotton blends by thermofixation; about 25% are medium energy dyes of which three are recommended for carrier dyeing, nine are very suitable for high temperature dyeing and five are very suitable for thermohixation; nearly 50% are high energy dyes of whichonly one is recommended for carrier dyeing, four for high temperature dyeing and 11 for thermofixation.
4 Fastness properties on Polyester
The fastness properties of disperse dyes on polyester cover a wide enough range for an adequate dye selection to be made for most end uses. The same dyes generally show poorer fastness on nylon.
Light fastness ratings at the ISO standard depth (1/1 SD) can easily be in the 6-7 range on the Blue wool scale of 1-8, although they do drop slightly if the light source is a carbon arc, as opposed to xenon lamp. As the depth of shade decreases, light fastness drops, a phenomenon shared by dyeing of all application classes of dyes.
If extremely high light fastness is needed (automotive fabrics ), a nonionic UV inhibitor may be added to the dye bath and applied to the fiber along with the dye. These compounds, often benzotriazoles, work much like sunscreen, screening out and dissipating UV radiation to prevent sunburn.
Wet fastness tests are frequently conducted after the goods have been reduction cleared and heat set; at 180C ( 356F ) for 30 seconds, and are assessed in terms of the staining on multifiber or adjacent nylon piece goods. Ratings of 4+ out of 5 are readily achieved on regular denier fibers. What is interesting here is that the ratings are very dependent upon the extent of clearing of the fiber surfaces, the duration and temperature of the heat treatment and whether the fabric has been treated with a finish of any kind. Heating disperse dyed goods causes the dyes to tend to migrate towards the hotter fiber surfaces and some of the disperse dyes are quite soluble in hydrophobic surface films; e.g., in some softeners which may have been applied. Fastness to crocking or rubbing as well as dry-cleaning suffers if dye migrates to the fiber surface or surface layer.
For those dealing in imports and exports of dyed goods, it is vitally important to be aware that the methods of fastness testing and consequently th ratings for dyed goods, vary from country to country. The international organization for standard ( ISO ) has developed a series of fastness test which are often different from test methods used in the U.S Soil compared tests for 30 fastness properties as run in 22 countries. The U.S methods of test wee essentially the same as the ISO SN105 methods in only 7 of 30 cases. The moral is do not buy or sell to colorfastness or any other specifications you do not understand
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TextileBd
