DYEING OF SYNTHETIC FIBRES As compared to natural fibres, the synthetic fibres are more compact, highly crystalline & hydrophobic in nature due to which the moisture regain values are low. There are comparatively less sites with which different classes of dyestuffs could combine. Because of these differences the rate of diffusion or penetration of dye is slow, and therefore a dye of small molecular size is preferred. Dyeing of Nylon 6 & 66: Following classes of dyestuffs are suitable for dyeing nylon: Disperse, Acid, Metal Complex, Vat, Vat Solublized Vat, Direct and Reactive dyes. Principle of Dyeing with Acid Dyes: All polyamide fibers have following groups; terminal amino group (NH2 ), terminal carboxy (-COOH) and amino groups along the polymer chain –CONH. Therefore, the simplified structure characterestic of nylon may be shown as follows: H2N-NH-COOH There characteristic groups can at different pH values react as H2N – NH - COOH A. Neutral B. Weak AcidNH3+ – NH – COO- H3N+ – NH - COOH H3N+ – N+H3 - COOH C. Acid below pH 2 meq./ gm of Nylon Dye Absorbed c b 0.02 a 0.01 0 1 2 3 4 5 6 7 8 9
This can be proved by titrating polyamides with acid & the above curve is obtained. The 3 parts of the curve are distinguished as follows: 1. Part a pH 9-6: In this area the acid is taken up i.e. the fibre accepts protons which are attached to the terminal amino groups. H2N – NH – COOH H H3N+ – NH – COOH- + - H3N – NH – COO H 2. Part b: from pH 6-2.5, further addition oe acid only lowers the pH of the dye liquor. The fibre virtually accepts no more protons. 3. Part c: below pH 2.5, in this region the fibre takes up acid again. This can only be explained by assuming that protons are attached to inside group. H3N – NH – COOH H3N+ – N+H2 – COOH ---- II The positively charged groups in structure I & II can take up anions with the formation of salt linkages. Acid ,direct, metal complex and reactive dyes contain one or more SO3Na as solubilising groups with with dye molecule D this could be represented as SO3Na SO3Na D-SO3Na, D D SO3NaMonosulphonic SO3Na SO3Na Disulphonic Trisulphonic After dissolution the dye diassociates giving charged ions. DSO3Na DSO3- + Na+
The –vely charged dye anions can react with +vely charged terminal amino group byforming salt linkage. DSO3- + +H3N – NH –COOH DSO3- +H3N – NH –COOHAs there is only one terminal amino group per polyamide chain the no. of sitesavailable for salt linkage in a polyamide filament is limited. At a pH higher than 2acid dyes can be taken up by polyamide fibres up to a saturation value. If all NH2groups are occupied, no more dye can be bound in this way.From the forgoing it is clear that one molecule of a monosulphonic dye occupies only1 amino group. DSO3- H3N+ - NH2 - COOHSulphonic Acid 2 SO3- H3N+ - NH2 - COOH D SO3- H3N+ - NH2 - COOHAnd the trisulphonic acid three SO3- H3N+ - NH2 - COOH D SO3- H3N+ - NH2 - COOH SO3- H3N+ - NH2 - COOHIn other words trisulphonic derivatives have a lower saturation value than disulphonicacids & disulphonic acids have a lower saturation values than monosulphonic acid.As a consequence of this the so called blocking effect is observed in practice. If eg.polyamide fibre is dyed with a mixture of monosulphonic & trisulphonic dye ingeneral it is the monobasic dye which will inclinely be taken up & it will have ablocking effect on the polybasic dye.
The terminal amino group content varies from fibre to fibre. The PA fibres, thestructure of which can be compare with that of wool & Silk contain much fewerterminal amino group than the two natural fibre.Fibre Amino Group (milli. eqv./gm)Wool 0.8-0.9Silk 0.12-0.2Nylon 66 0.03-0.05The saturation value of acid dye calculated from the no. of terminal amino groupshave been observed in much more cases. It is assumed that acid dyes can also bebound to the inside groups of the fibre (over dyeing effect). It would appear that thedyes with a good affinity in the neutral pH region can be linked to the fibre in thisway.Three different kinds of combinations between acid dyes & fibre are possible and theymay operate singly or side by sideI. ph 2-7 H3N+ - NH - COOH Acid Neutral DSO3-N+H3 – NH -COOH DSO3N+H3 – NH –COO-II. ph 2-7 H-bonds with imido groups H3N+ - NH – COO- DSO3Na III. ph below 2: Formation of salt linkages with imide & amino groups DSO3N+H3 – N+H2 –COOH DSO3-
The kind of binding shown in III makes possible a higher dye uptake ( i.e. higher saturation values). However, at temp. betn 90-100 0C considerable hydrolytic degradion occurs at the above pH (H2SO4) If dying is carried out for a longer periods of time (shortening of the chain length, reduction in tensile strength). Dyestuffs which combine with polyamide fibres by the formation of salt linkages or complexes produce streaky or barry dyeing effect as a rule. This is due to difference in affinity of the dye for fibres of different origin or of different spinning batches. Irregular dyeing effects can be caused by optical differences due to variation of denier, content of delusturing agent, variation of terminal amino groups (this no. varies with the type of fibre & is different for fibres of different origin & of different spinning batches). Differences in the rate of dyeing due to varying degree of crystallization or differences in fibre streaching. Acid Dye Solution Blue BNS 6% Extension 66 Dyeing Time Nylon 6 structure is more open than that of Nylon 66 & this has an effect both on the rate of dyeing & on the capacity to absorb disperse dyes. But this property is not of great importance if anionic dyes are used. With this dye classes the terminal amino end group content is of decisive importance. The no. of amino groups in Nylon 6 are somewhat higher in Nylon 66.
The principles of dyeing polyamide with direct dyes :The mechanism of the dyeing of PA fibres with direct dyes has not been muchinvestigated. It seems that direct dyes are bound to the amide groups by the H-bondsand/ or to the terminal amino groups by salt linkage. It has been shown that the rate ofdiffusion of this dyes is small & their saturation values low, the reason being that theirmolecules are large & elongated, most direct dyes are polysulphonic acids.The principles of dyeing polyamide with disperse dyes :The exact mechanism is not definitely known. It may be that the disperse dye indissolved in the PA fibre and that fixation occurs by formation of H-bonds betweenthe dye molecule & the imide groups of the fibre.The possibility of salt formation can be excluded. A saturation value of a disperse dyetherefore not dependent on the the no. of terminal amino groups, but only on theextent of non crystalline regions in the fibre. For these reasons irregularities in thechemical constitutions & in the physical state of fibres are covered up by dispersedyes. There are considerable differences between Nylon 6 & 66 regarding the rates ofdyeing & degrees of exhaustion. Nylon 6 has more open.The principles of dyeing polyamide fibres with 1:1 metal complex dyes :The mechanism of dyeing is not fully understood. Certain analogies with the bindingof these dyes to wool can be compared. Here it is assumed that on one hand a linkagebetn the sulphonic group of the dye molecule & the terminal amino groups of the fibre& on the other hand a coordinate bond betn the imide group of the fibre & the centralCr atom of the dye. The use of 1:1 metal complex dyes is limited because of slow rateof diffusion & low saturation value. The wet fastness is inadequate in many cases butas the light fastness is good even in pale shades, some selected members of this classare used.
The principles of dyeing with 1:2 metal complex dyes :The mechanism is again not clearly understood. The 1:2 complex dyes donot containsulphonic but other solubalising group. Therefore, it was first thought that saltformation with the terminal with the terminal amino group could not take place; &these consumption seemed to be in agreement with the fact that the exhaustion of thisdyes is only slightly dependent on pH. Actually, there is a preference for using themin a weakly acid, neutral or slightly alkaline bath.Recent investigations by Zollinger have shown that 2 dyeing processes occurssimultaneously. The whole dyestuff complex is –vely charged as 4 hydroxyl groupstake part in the complex formation with the trivalent metal ion. It is assumed thatthese dyes are strong acids. The commercial products are the sodium salts. It wasfound that the terminal amino groups of the polyamide molecule are neutralized bythe dye molecule & the salt formation, therefore, takes place between the terminalamino groups of the fibre & the dyestuffs. Simultaneously solution of the dye in thePA fibre takes palce. This may be a process similar to the overdye effects with aciddye.The Principles of dyeing PA with Reative Dyes:The reactive dyes contains reactive group as well as SO3Na as solubalising agent. Inthe dyeing of cotton only reactive group ip are imp. Whereas in case of Nylon bothboth SO3Na & reactive groups are important. These dyes are adsorbed on the fibrelike acid dyes & part of the dye reacts with the fibre & part of it present in the form ofsalt linkages. The saturation value of most of the prior M & H dyes is relatively low.As these dyes are polysulphonic acids, blocking effect also occurs. The matter seems to be somewhat different forvinylsulphate reactive dyes, where it is reported that more dye is combined
chemically with polyamide fibre, the dye must first be converted in to the active vinyl sulphone form in neutral or weakly alkaline medium. D - SO2 - CH = CH2 D - SO2 - CH2 – CH2 –OSO3Na The vinyl sulphone form then combines with polyamide through the terminal amino group.D - SO2 - CH = CH2 + H2N - NH - COOH D - SO2 – CH2 - CH2 - HN - NH - COOH With the vinyl sulphone types of dyes blocking effect may also occur. The proc. M, H & Remazol reactive dyes have great advantage of covering up irregularities in the PA fibres, inspite of this important advantage the disadvantages of these types of dyes (blocking effect, relatively low saturation value, moderate fastness to light) have so far prevented their wider use for the dyeing of PA fibres. It may be used occasionally to produce brighter shades on crimped PA yarn. ICI have chosen another method. They synthesized disperse dyes which carry a reactive group in the molecule. These dyes are markted under the name of Procinyl dyes. Dyeing is carried out at boil in a weakly acid bath. The disperse dyes exhaust on to PA. The reactive group does not operate at this stage. After the greater part of dye has been absorbed by the fibre the bath is made alkaline, and it is only at this stage a chemical combination of the reactive group of the dye & the terminal amino of the PA molecule occurs. That chemical combination takes palce is considered evident from the following observations.
1. The dyes can not be stripped from the fibre with chlorinated hydrocarbons, propyl alcohol or aqueous pyridine. 2. The azo compounds of the provinyl series can be split at the azo group by reducing agents & can be practically decolorized & completed with a suitable component to follow a new dyestuff which is firmly fixed in the fibre. 3. If nylon is dissolved in o-chlorophenol & the solution is poured into propyl alcohol alcohol nylon is pptd. Ordinary disperse dye dyes remain in the solution which procinyl dyes which reacted with the fibre are ppted together with nylon If all the amino grps. Are saturating the reactive groups of the procinyl dyes react much more slowly with the imino groups of the polyamide molecule.DCl2 + H2N – NH – COOH D – Cl - HN – NH – COOH + HCl DCl2 DCl – HN – N – COOH + HCl DCl As with the reactive dyes for cellulose material a certain part of the procinyl dyes is inactivated during the dyeing and no longer able to react chemically with the fibre. Contrary to the behaviour of reactive dyes on cellulose this unfixed dye can not be rinse out of PA fibres, but this small deteriration of fastness to wet treatment due to unfixed dye is in most cases tolerable. The azo types of procinyl dyes cover up yarn irregularities very well. The fastness to washing & light is very good.