7.4. Basic dyeable synthetic fibers
In the first instance, the commercially available Astrazon® powder and liquid brands can be used for dyeing acrylics which can be dyed with cationic dyes, e.g. Dralon®, Courtelle®, Orion®, Acrilan® and Exlan®, and for dyeing polyester and polyamide fibers which are dyeable with cationic dyestuffs, e. g. Dacron® 64, Dacron® 92, Nylon T 844 and Dye 1®, from PCE emulsion.
The advantage offered by the liquid brands over the powder brands is that the liquid dyestuff can be added direct to the solvent liquor which already contains water and the emulsifier. The water content of the emulsion depends on the solubility of the dyestuff if powder brands are used. In the case of the liquid brands, the water content of the emulsion should be 5-10 ml in 1 liter of liquor. Non-ionic and anionic surface active agents are suitable as emulsifiers. It has also been found useful to add amines, amides and/or amine oxides of fatty acids, and mixtures thereof. Such compounds improve the emulsion and promote level absorption and better exhaustion of the dyebath. The required amount of emulsifier depends on the water content and on the type of dyeing equipment used; it is of the order of 2-5 g/L perchloroethylene. The dyeing temperature is governed by the dependence of the rate of dyeing on the temperature for a particular kind of fiber. Basic dyeable polyamide fibers can therefore be dyed already below the azeotropic boiling point. Modified polyester fibers require a dyeing temperature of 87-95°C. In the case of acrylic fibers, the required temperature depends upon the process according to which the fiber was manufactured.
Wet spun fibers can be dyed below the azeotropic boiling point. Dry spun fibers, on the other hand, require temperatures above 92°C. The combinability of the cationic dyestuffs on polyacrylonitrile fiber materials does not correspond to the well-known rules of aqueous dyeing.
Fig 7.2 Absorption curves from perchloroethylene and water of Verofix dyestuffs on loose stock of chlorinated wool
The bath exhaustion depends upon the emulsifier and changes with the emulsifier concentration. Hence recipes established for aqueous liquors cannot be adopted directly. They must be modified to suit the special conditions which are determined by the emulsifier and the emulsifier concentration. The following disadvantages were seen when this method of working was given a thorough testing:
Basic dyestuffs which are sensitive to reduction by over boiling will tend to such reduction, i.e. in the absence of fiber material the total extinction of the liquor decreases with increasing boiling time. In the presence of fibers this decrease in dyestuff content depends on the rate of absorption, so that the shade may change if mixtures of several dye-stuffs are used. In the presence of acids the acid concentration in the limited aqueous phase of the emulsion is much higher than in a 100% aqueous dye bath. Hence dyestuff decomposition is observed particularly with those dyestuffs which are modified by the action of strong acids. This is important in the case of liquid brands which contain free acid.
Cellulosic fibers
The dyeing of cellulosic fibers has not been a focal point of interest in the field of dyeing from organic solvents. The reactive principle is the most important one among the dyeing principles which are in use today. Hence the aqueous phase must also contain alkali for the reaction, besides containing the dyestuff. In some cases it has also been found useful to improve the solubility of the dyestuff and to increase the swelling of the cellulose by adding urea or formamide.
Since the dyeing process takes place below 80°C, the dependence of the stability of the emulsion on the temperature does not cause any trouble. Anionic and non-ionic emulsifiers are chiefly suitable. The dyeing mechanism corresponds to that of a cold or warm dwell process. Hence the Levafix® E and P dyestuffs were found suitable, with yields between 80 and 95%. When polyester/cotton blends are dyed from solvents, either the polyester component is dyed first and the aqueous solution containing the reactive dye is then emulsified into the bath, or the reactive dye is dyed first from emulsion, followed by the dyeing of the polyester component in a fresh bath. The first method of working is mostly preferred because it offers easier operation. When the polyester material has a strong tendency to shrink, the cellulose component must be dyed first and the polyester component second. Since in the case of solvent dyeing equipment the bath is changed much more quickly (with cooling and heating operations) than with the conventional equipment for dyeing from an aqueous bath, the two-bath process does not have the disadvantages in solvent dyeing which it has when dyeing from water. Corresponding working methods are employed when dyeing with Sirius® and Sirius Supra dyestuffs.
Dyeing with solvent-soluble dyestuffs Polyamide
Solvent-soluble acid dye-stuffs for dyeing polyamide fibers are obtained by salt formation with primary, secondary and tertiary amines which have more than 8 carbon atoms. The salts of quaternary ammonium compounds with long-chain radicals usually have good solubility but a low absorption capacity. This means that the affinity has become so low that these dyestuffs are unsuitable for an exhaustion process.
The solubility of an acid dyestuff in perchloroethylene also depends on the constitution of the color amine. It can be modified to such an extent that with low solubility in water there is an increasing tendency to form salts which are soluble in perchloroethylene, so that even low amines will already lead to the formation of products soluble in perchloroethylene. By combining both principles it is possible to build up a range of suitable dyestuffs which will produce dyeings on polyamide with the fastness properties usually possessed by acid dyestuffs. At present we have 12 dye-stuffs developed according to this principle, which are available for testing.
Dyeing is effected at 65-98°C, depending upon the selected dyestuffs, the quality of the goods, and the type of machines employed. If necessary, the rubbing fastness of the dyeings can be improved by the presence of 0.5-2% emulsifier in relation to the weight of the goods and 0.1-8% water. The dyeing times found so far are below those required for aqueous dyeings. We could not find any reduction in bareness with the dyestuffs examined by us so far. Leaving this apart, solvent-soluble dyes have a good leveling capacity on poly-amide. The combinability depends on the choice of the amine. But the selection of suitable combinations is no problem. The drawback of this dyestuff class is the higher manufacturing cost compared with that of the water-soluble dyes.
The principle of dyeing with perchloroethylene-soluble dyestuffs is preserved even when the solvent-soluble dyestuff is produced from the free acid or from its water-soluble salts in the dye bath itself. In this case the dyestuff is added, in its well-known form, to the solution which contains a suitable amine as well as water and formic acid or acetic acid, we have no accurate knowledge of the mechanism of absorption. It appears particularly interesting to know in which form the dyestuff is present on the fiber surface and in which form it diffuses into the fiber. We are sure that this diffusion is based on an ion exchange mechanism in which the amine which is responsible for the solvent solubility is liberated and thus becomes available for dissolving additional dyestuff. Hence, in principle, dyeing can be effected even with a relatively small amount of amine, i.e. with less amine than would be required for complete dissolution of the entire quantity of dyestuff used at the beginning of the dyeing operation. In this case the amine will gradually dissolve the dyestuff from its "deposit", or "store", in the same measure as it continues to go on the fiber. The only thing we must make sure is that only such dyestuff comes into contact with the goods which have been properly dissolved: This is necessary to avoid filtration and the formation of stains. If water and emulsifier are added to dye liquor besides a water-soluble dyestuff and a suitable amine, we have a system in which the dyestuff is present both in the aqueous and in the organic phase. This system combines the lower cost of pure emulsion dyeing with the technical advantages of application of a dyestuff dissolved in the perchloroethylene phase:
As the dyeing temperature goes up and the bath exhaustion increases, there is an increase in the share of dyestuff dissolved in the organic phase in per cent of the total amount of dyestuff in the bath. This reduces the danger of separation of undissolved dyestuff when the azeotropic boiling point is exceeded. Hence level dyeings can also be obtained by this process. Polyamide dyed in perchloroethylene contains 2.8-3% solvent after simple drying. This residual solvent can be removed by drying at an elevated temperature. The dyeing method is interesting for fully fashioned articles and for beam dyeing of Perlon-poros. Softening is usually necessary.
Wool
Wool can be dyed with the perchloroethylene-soluble dyes developed for polyamide. Compared with polyamide, more water, higher dyeing temperatures and longer dyeing times are required for dyeing wool with dyestuffs soluble in perchloroethylene. Hence it is also impossible to dye wool which has not been given a non-felting finish, in drum dyeing machines by omitting water, even when perchloroethylene-soluble dyes are used. We cannot say at present how important this method will become. That is why we preferred to use water-soluble reactives in our experiments. A solvent-soluble dye can be obtained in wool dyeing by the additional use of suitable amines, similar to the procedure described for polyamide, using a dyeing system which employs water-soluble acid or reactive dyes. According to this procedure we obtained saturated, fast dyeings on knitted woolen goods which had been given a non-shrink finish, using Verofix dyes in a solvent drum dyeing machine.
The handle of non-shrink knitted woolen goods is less affected by perchloroethylene dyeing than that of synthetics. Provided suitable amines are added to the dye bath, the handle need not be corrected.
It will be necessary to test the amines for any possible action on the fastness to light. Best possible rubbing fastness values are obtained by aftertreating with emulsions to which suitable emulsifiers have been added.
Basic dyeable acrylic and polyester fibers
Perchloroethylene-soluble cat-ionic dyestuffs are salts of suitable color bases with anionic surface active agents.
The dyeing mechanism is conditioned by the presence of water in a quantity which is capable of exercising a catalytic action. The mechanism corresponds with the theoretical ideas which are now considered to apply to Astrazon dye-stuffs, i.e. with the adjustment of equilibrium for the aqueous phase. But the combinability rules for aqueous dyeings do not apply. The combinability is generally more favorable. Ten Astrazon Solvent dyestuffs which were developed according to this principle are now available for experimental purposes as samples. The dyestuffs are self-emulsifying, so that the use of an emulsifier can be restricted or omitted in the case of machines with a very strong liquor circulation. It has been found useful to add 0.1-0.5% (in relation to the weight of the goods) of a cationic auxiliary to the dyebath after the bath has been largely exhausted. This addition will serve to improve emulsification and to exhaust the residual liquor.
On account of the technical superiority of the Astrazon Solvent dyestuffs, which are soluble in perchloroethylene, to water-soluble dyestuffs, it will be hardly interesting to develop dyeing procedures in which the solvent-soluble dye is produced in its water-soluble form by a dual reaction in the dyeing system itself. But as a matter of principle it is possible to produce in the dye bath a solvent-soluble dye from a water-soluble basic dyestuff. In that case the dye bath is set with an emulsifier.
Useful dyeings can also be obtained when the equivalents of anionic compounds which produce solubility in perchloroethylene, are less than would be required for a complete transition of the water-soluble dye into a solvent-soluble dye.
The dyeing rate on polyacrylonitrile partly corresponds to that of an aqueous dyeing. For dyeing anionic modified polyester we found an increase in the dyeing rate. Both fiber types require the presence of catalytic amounts of water. Owing to this, the azeotropic boiling point of the perchloroethylene/water system is also important for dyeing with this class of dyestuffs. It is important for the quality of the dyeing that sufficient water -corresponding to 0.1-5% in relation to the weight of the goods-is available during the entire dyeing process. This amount of water can be emulsified by the dyestuff itself, depending on the depth of dyeing. Since solvent-soluble dyestuff does not require the aqueous phase as соsolvent, the water can also be blown in as steam. Dyeing can also be effected above 87°C. In that case the dyeing is no longer effected from emulsion, but from a pure solvent mixture. The Astrazon Solvent dyestuffs which are now available do not require any special preparation, nor do they have to be emulsified. There will also not be any settling out when the aqueous phase is temporarily lost. The dyed goods are rinsed in pure perchloroethylene and then dried. Depending upon the drying temperature and the kind of fiber used. The dyed goods contain perchloroethylene which must be removed to get the best possible light fastness. Of all synthetic fibers, acrylic fibers have the least solvent regain. For dry spun acrylic fibers we found a perchloroethylene content of 0.02%, while for wet spun fibers the content was 0.7%. Dacron® 64 gave a perchloroethylene absorption of 8%, Dacron® 92 one of 7.8%.
Goods dyed with perchloroethylene have a harsh, somewhat dry handle and are therefore softened with softener. High-bulk yarns can be pre-shrunk in the same dyebath. When dyeing yarns as wound packages we found a different handle inside and outside.