1) Synthetic fibers like nylon are more difficult to dye than natural fibers due to their compact crystalline structure and low moisture regain. 2) Acid dyes are commonly used to dye nylon. They bond to nylon's amino groups through salt linkages below pH 2 and hydrogen bonds above pH 2. 3) Dye uptake depends on the fiber's amino group content and the dye's sulfonic acid groups, with trisulfonic dyes having the lowest saturation value.

1. 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 Acid
NH3+ – 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

2. 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 SO3Na
Monosulphonic SO3Na SO3Na
Disulphonic Trisulphonic
After dissolution the dye diassociates giving charged ions.
DSO3Na DSO3- + Na+

3. The –vely charged dye anions can react with +vely charged terminal amino group by
forming salt linkage.
DSO3- + +H3N – NH –COOH DSO3- +H3N – NH –COOH
As there is only one terminal amino group per polyamide chain the no. of sites
available for salt linkage in a polyamide filament is limited. At a pH higher than 2
acid dyes can be taken up by polyamide fibres up to a saturation value. If all NH2
groups 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 only
1 amino group.
DSO3- H3N+ - NH2 - COOH
Sulphonic Acid 2
SO3- H3N+ - NH2 - COOH
D
SO3- H3N+ - NH2 - COOH
And the trisulphonic acid three
SO3- H3N+ - NH2 - COOH
D SO3- H3N+ - NH2 - COOH
SO3- H3N+ - NH2 - COOH
In other words trisulphonic derivatives have a lower saturation value than disulphonic
acids & 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 in
general it is the monobasic dye which will inclinely be taken up & it will have a
blocking effect on the polybasic dye.

4. The terminal amino group content varies from fibre to fibre. The PA fibres, the
structure of which can be compare with that of wool & Silk contain much fewer
terminal amino group than the two natural fibre.
Fibre Amino Group (milli. eqv./gm)
Wool 0.8-0.9
Silk 0.12-0.2
Nylon 66 0.03-0.05
The saturation value of acid dye calculated from the no. of terminal amino groups
have been observed in much more cases. It is assumed that acid dyes can also be
bound to the inside groups of the fibre (over dyeing effect). It would appear that the
dyes with a good affinity in the neutral pH region can be linked to the fibre in this
way.
Three different kinds of combinations between acid dyes & fibre are possible and they
may operate singly or side by side
I. 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-

5. 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.

6. The principles of dyeing polyamide with direct dyes :
The mechanism of the dyeing of PA fibres with direct dyes has not been much
investigated. It seems that direct dyes are bound to the amide groups by the H-bonds
and/ or to the terminal amino groups by salt linkage. It has been shown that the rate of
diffusion of this dyes is small & their saturation values low, the reason being that their
molecules 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 in
dissolved in the PA fibre and that fixation occurs by formation of H-bonds between
the dye molecule & the imide groups of the fibre.
The possibility of salt formation can be excluded. A saturation value of a disperse dye
therefore not dependent on the the no. of terminal amino groups, but only on the
extent of non crystalline regions in the fibre. For these reasons irregularities in the
chemical constitutions & in the physical state of fibres are covered up by disperse
dyes. There are considerable differences between Nylon 6 & 66 regarding the rates of
dyeing & 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 binding
of these dyes to wool can be compared. Here it is assumed that on one hand a linkage
betn 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 central
Cr atom of the dye. The use of 1:1 metal complex dyes is limited because of slow rate
of diffusion & low saturation value. The wet fastness is inadequate in many cases but
as the light fastness is good even in pale shades, some selected members of this class
are used.

7. The principles of dyeing with 1:2 metal complex dyes :
The mechanism is again not clearly understood. The 1:2 complex dyes donot contain
sulphonic but other solubalising group. Therefore, it was first thought that salt
formation 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 this
dyes is only slightly dependent on pH. Actually, there is a preference for using them
in a weakly acid, neutral or slightly alkaline bath.
Recent investigations by Zollinger have shown that 2 dyeing processes occurs
simultaneously. The whole dyestuff complex is –vely charged as 4 hydroxyl groups
take part in the complex formation with the trivalent metal ion. It is assumed that
these dyes are strong acids. The commercial products are the sodium salts. It was
found that the terminal amino groups of the polyamide molecule are neutralized by
the dye molecule & the salt formation, therefore, takes place between the terminal
amino groups of the fibre & the dyestuffs. Simultaneously solution of the dye in the
PA fibre takes palce. This may be a process similar to the overdye effects with acid
dye.
The Principles of dyeing PA with Reative Dyes:
The reactive dyes contains reactive group as well as SO3Na as solubalising agent. In
the dyeing of cotton only reactive group ip are imp. Whereas in case of Nylon both
both SO3Na & reactive groups are important. These dyes are adsorbed on the fibre
like acid dyes & part of the dye reacts with the fibre & part of it present in the form of
salt 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 for
vinylsulphate reactive dyes, where it is reported that more dye is combined

8. 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.

9. 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.