In dyeing of textile substrate it is very important, from the aspect of the uniform dyeing, to distribute dye on the surface of material in earlier stages of dyeing as evenly as possible. This is especially important with materials obtained by mixing fibres of various surface and volume characteristics, and morphological structure. According to that, dyeing the mixture of wool/ polyamide is a characteristic example. Chemical similarity of these fibres enables dyeing of the mixture by one sort of dye (usually acid), but morphological differences and the differences over molecular structure of fibres cause the differences in the speeds of dyeing, which makes nuancing "shade-to-shade" difficult.
In this article the influence of tensides of various structures on the speed of adsorption and diffusion of acid dyes in woollen and polyamide 6 fibres is examined.
Acid dyes have an important role in the dyeing of amphoteric fibres (wool, polyamide, natural silk). These dyes give colourations in a wide palette of shades (from yellow to black) characterised by exquisite brilliance, clarity and more or less good wet fastness depending on the dye structure. Chemical composition of acid dyes is sodium salts of sulfonic acids, with good solubility in water and dissociated on dyed anion and sodium cation.
According to the chromophorous system structure, they are divided into three large groups: azo dyes, anthrahinone dyes and triphenilmethane dyes." However, chemical classification of acid dyes does not significantly determine dyeing characteristics and particularly not colourfastness. According to the colouristic parameters (affinity, exhaustion velocity, migration power, colour evenness, etc), acid dyes are divided into three groups: equalising, milling and supermilling dyes. From the practical point of view, the most valuable groups are supermilling dyes, which give the highest colouration fastness, but have the lowest migration degree.
Since in wool fibres tips are damaged in comparison to roots due to atmospheric and light influences, it is often rather difficult to obtain even colouration within individual fibres. Especially in combinations of two or more dyes, it has been noticed that certain dyes can bind selectively to different spots on a fibre, thus producing so called dichroistic colourations.
An integral part of macro-molecular polyamide (PA) chains are amino groups. Therefore, PA fibre is chemically similar to wool and can be dyed with the same dyes as wool. Content of functional amino groups through which dyes are fixed to polyamide is considerably smaller in comparison to wool, and therefore maximum quantity of the absorbed dye is proportionally smaller.
Also, polyamide structure is characterised by anizotropic distribution of functional amino groups and that, in dyeing, it is manifested as a negative occurrence, so called stripe effect. 141 Problem of stripe dyeing is particularly expressed when dye mixture of different affinities is used for shading. Tendency towards uneven colouration is additionally prompted by relatively low glass temperature and lower crystalline degree of PA fibres at higher temperatures, which makes greater part of a fibre interior accessible to dye ions already at low temperatures of aqueous solution.5
The consequence of this is more intensive colouration of PA in dyeing of wool/PA mixtures in shades below the PA saturation limit. Difficulties of wool and polyamide even colouration and of "shade-to-shade" wool/PA mixture colouration in industrial dyeing have been mainly overcome through the use of leveling agents on the tenside basis recommended by a number of manufacturers.
It is a usual practice that a manufacturer offers his own auxiliary agent together with a particular assortment of dyes. Thus, for example Sandoz-Basel, for the assortment of Sandolan MF acid dyes recommend their own agent Liogen MF, Ciba-Gaigy - Basel for the assortment of Neolan A acid dyes recommend an agent Albegal NA, etc. However, in practice it is often necessary to substitute one agent with another, and therefore it is essential to know the working mechanism and optimal concentration range of individual tenside types in a given system. This article has examined the influence of various tenside types on adsorption and diffusion of acid dye into wool and polyamide by observing several kinetic parameters.
In the experiment, dyeing of wool and polyamide samples has been done under identical conditions. Australian wool 433 type and 24,2 mm fineness has been used. Scoured wool has been degreased with ethanol in a Soxhlet apparatus for 4 hours and then repeatedly rinsed with distilled water and dried at room temperature.
PA 6 fibre (made in Poland), fineness of 4,2 dtex and staple length of 80 mm has been chosen from the polyamide fibres assortment. Polyamide fibre samples have been scoured with a solution of non-ionic washing agent in Linitest apparatus at 60°C and the solution modulus of 1:40 for 30 min. After the scouring, the samples have been repeatedly rinsed with distilled water and dried at room temperature.
As auxiliary agents, two non-ionic tensides nonlipheno-lethyleneoxides (FN-10 and FN- 20, Merima - Yugoslavia), and anionic tenside - laurilpolygly-colether sulfate (Etopon LSP, Teol - Slovenia) were used. FN-10 is a viscous, light and transparent liquid, while FN-20 is in the solid state. Etopon LSP is a colourless, viscous liquid containing minimum 27% of active substance.
Dyeing was performed in a laboratory apparatus using exhaustion procedure, at a dye bath ratio of 1:80. Sample mass was 1g, and dye concentration 1 per cent with variable tenside content: 0: 0.4: 1 and 3 g/dm3. The process was isothermal (t=70°C) until equilibrium state was achieved (max 300 min). Solutions were buffered with the system CH3COOH/ CH3COONa and pH value (pH 5) was checked with a digital pH-meter HI 9025 (Hanna Instruments - Singapore).
Determination of relative exhaustion
Relative exhaustion (E) in relation to the solution prior to dyeing is determined by measuring the absorbance of dyeing solution before dyeing (A0) and after dyeing, after a certain time period (At)
Dyeing solution absorbance was measured on Spectrophotometer-meter Spekol 21 (lskra, Slovenia) in time intervals of 5, 10, 15, 20, 30, 45, 60, 120, 180 and 240 minutes from the beginning of dyeing, and at the end of dyeing at the maximum absorption wave length (? = 515 nm).
Determination of half-dyeing time
Half-dyeing time, t 1/2 is required for a fibre to absorb half the quantity of dye absorbed in equilibrium state. Half-dyeing time was determined by Bulton and Riding graphic method. According to the method, equilibrium exhaustion A has been determined first, by observing dye absorption in dependence of time, long enough to achieve the real equilibrium state. The result is graphically illustrated as the dependence of exhaustion on the time logarithm. On the basis of the equilibrium exhaustion, value A/2 has been calculated, and time t1/2 determined from the diagram.
Determination of relative diffusion coefficient
Relative diffusion coefficient Drel was obtained from the graphic presentation of the dye concentration gradient in a fibre (ctrel) in dependence on the second time root (t). According to Vickerstaf angle Tangent Square at any point of the curve in time t represents relative diffusion coefficient, and it is a measure of dye diffusion speed into the fibre and of fibre permeability. Drel for the first 20 min of dyeing is determined in the paper.
Results and discussion
Changes in adsorption in the presence of FN-10 are proportional to the additive content in the solution. In the presence of FN-20 considerable fall has been achieved at the minimal concentration already. In the presence of the anionic tenside, acid dye adsorption on wool has also slowed down, but this slowdown is considerably less than when non-ionic tensides are used, and that proves poorer efficiency of this type of agents.
When polyamide is dyed without additives, rapid adsorption of dye onto fibres takes place, and after 15 min of dyeing already exhaustion is complete. Fast adsorption of dye on polyamide is caused by viscoelastic state of the amorphous part of a PA fibre at 70°C of dye aqueous solution, and a frequent consequence of this is uneven colouration of the substrate.
The basic characteristic of the viscoelastic state is intensive mobility of macromolecular segments, which creates the so called "free volume" in a fibre, i.