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Microemulsions: Novel media for dye solubilisation - II

Roshan Paul

Incorporating oil-soluble dyes and/or oil miscible pigments in the oil component of the microemulsion can increase the colorant loading without substantially increasing the viscosity of the ink. Low viscosity is important for printers based on LIFT (liquid ink fault tolerant) printing principles, as the printing speed improves with low viscosity. Pigment or dyes can be simultaneously incorporated in the water phase of the microemulsion.

Microemulsion technique for inkjet inks is developed by Kabalnov, in the patent, EP 1149879 A2 20011031. Aqueous inkjet inks are formulated with oil-soluble dyes that are made more soluble in the aqueous vehicle by making a microemulsion in the ink of solvent moieties with both solubilising groups and hydrophobic groups.

Liquid-crystalline microemulsion thermal jet-printing inks are developed by Oliver et al, in the patent, CODEN: USXXAM US 5492559 A 19960220. Thermal jet-printing inks with low feathering contain aqueous phase, an oil phase, an oil-soluble dye, and a surfactant that imparts a liquid crystalline gel phase at a temperature lower than the temperature of the liquid microemulsion phase.

Textile coloration

Microemulsions and their potential applications in dyeing processes are described by Barni et al, in Journal of Dispersion Science and Technology (1991), 12 (3-4), 257-71. Among organised molecular assemblies, microemulsions, which are transparent, homogeneous, isotropic and thermodynamically stable systems, have been used in industrial activities close to dyeing. The use of microemulsions in dyeing shows encouraging results, which are obtained by thermodynamic measurements and by traditional qualitative laboratory tests.

The developments in dyeing technology based on microemulsion systems are described by Savarino et al., in Journal of Dispersion Science and Technology (1995), 16 (1), 51-68. Microemulsions, or their precursors, are proposed for improving the quality of acid and disperse dyed fabrics. Most microemulsions contain sodium alkanesulphonate systems, Brij 35, and/or octanol as an emulsifier system. The investigated substrates are polyamide (nylon 6 and 6.6), polyester, and a secondary cellulose acetate-nylon 6 blend. Besides conventional fibres, also nylon 6 and polyester microfibres are also studied. It has been possible to ascertain that suitable organised systems allowed to obtain a better uniformity of coloration without depressing dyebath exhaustion. Furthermore, in some cases, satisfactory results have been obtained at halved liquor ratios with consequent energy saving during the tinctorial cycles.

The role of cosurfactant and oil in the dyeing of cellulose acetate is studied by Savarino et al., in Journal of Dispersion Science and Technology (1993), 14 (1), 17-33. N,N-Diethylaminoazobenzene is tested, as a model dye, in quantitative studies on the dyeing of secondary cellulose acetate rayon. The effects of surfactant, cosurfactants with tuned HLB values, and long-chain alcohols on both the spectral behaviour and the partition of the dye between the bath and the substrate, are studied. Taking advantage from these results, dyeing tests are carried out on a substituted azobenzene dye possessing very good fastness properties. The tests showed the importance of sonication and of the choice of the suitable cosurfactant. The presence of an oil component promoted excellent dye uptake and uniformity of coloration, with considerable savings in chemicals.

Effects of additives on the dyeing of nylon-6 with dyes containing hydrophobic and hydrophilic moieties is discussed by Savarino et al., in Dyes and Pigments (2000), 47 (1-2), 177-188. A series of azo disperse dyes with variable hydrophobic chain length and hydrophilic (hydroxy or glucosyl group) head are synthesised. The dyeing properties on polyamide fibres are assessed with dyeing isotherms. The positive effects of additives, as ternary systems (SDS, Brij 35 and octanol) or cyclodextrins, on te dyeing uniformity are shown by tristimulus colorimetry.

Heterocyclic hydrophobic dyes and their interactions with surfactant and o/w microemulsions are studied by Savarino. et al., in Colloids and Surfaces (1990), 48 (1-3), 47-56. Starting from 2-methyl-6-(C1-15-alkylamido) quinolines, a series of styryl and symmetric polymethine dyes are synthesised. Besides the characterisation and a short discussion on physical and spectroscopic constants of dyes and their intermediates, attention is focused on the aggregation of the dyes in solution and on their interaction with amphiphilic systems. The dyes in water showed a marked tendency to form aggregates (even in the presence of fairly short alkyl chains) and the addition of anionic, cationic and nonionic surfactants (below, at, and above their critical micelle concentration) do not afford satisfactory deaggregation. Preliminary, encouraging results are attained by resorting to o/w microemulsions.

Low temperature microemulsion dyeing process for polyester fibres is developed by Yao et al., in the patent, CODEN: USXXAM US 5540740 A 19960730. The title five-step process eliminates the need for washing dyed fabric with a sodium hydrosulphite reducing agent and includes the steps of microemulsifying a swelling agent in water to form a continuous microemulsion solution, adding dyestuff and a dye solubility assistant agent selected from the group of short chain alcohols, dyeing polyester fibres at room temperature for 1-3 h, washing the polyester fibres with a nonionic washing agent, and drying the dyed polyester fibres. Because of the effect of dye-solubility assistant agent and the disperse dyestuff, the diameter of the microemulsion liquid drop is considerably smaller than that of a typical microemulsion liquid drop. Polyester fibre dyed with Dianix Yellow AC-E gave a K/S value 13.48 and wash fastness (AATCC II A) 4; vs. 11.06 and 4, respectively, for the commercial process requiring sodium hydrosulphite.

Even though, microemulsion precursors and microemulsion based systems have been employed for dyeing, it is still not very clear whether the solubilised dye in o/w microemulsion is transferred directly from the microemulsion droplets to the substrate or a destruction of the droplets is needed in order to release the dye into the dyebath so that it can interact with the substrate later. Paul et al. have first reported the dyeing of wool directly from o/w microemulsions of a water/non-ionic surfactant/polar oil system containing a solubilised natural dye C.I. Natural Orange 2, obtained from Mallotus philippinensis. The dye showed high solubility and the dyed wool samples developed good colour values. The mechanism of dye migration from the microemulsion bath to the fibre surface is also investigated.

Conclusion

Microemulsions, or their precursors, are assuming increasing importance in dyeing processes. The most important properties of microemulsions, which are of interest in the textile field are the solubilisation capacity for both water and oil soluble compounds and the extremely low values of interfacial tensions achieved in contact with aqueous or oil phases.