Dyeing of Nylon Fabric with Acid Dye

Dyeing of Nylon Fiber with Acid Dye

Rahamat Ullah Joy 
B.sc in Textile Engineering 
Daffodil International University 
Phone: +8801614445257

Acid Dyes are the dye of choice for solid color dyeing of silks, wools, alpaca, mohair and other protein fibers, as well as Nylon. Some folks even use them on leather, but test! Excellent prices, bulk pricing, handy 2oz size. 52 beautiful, vibrant, and some very hard to get colors! Can be painted on as well. A process for dyeing nylon with acid or premetallized acid dyes is provided wherein the dye bath is brought to a temperature of 160 addition of sulfuric acid to lower the pH to 3.75-2.25, thereby improving dye exhaustion.

Acid dyes are highly water soluble, and have better light fastness than basic dyes.The textile acid dyes are effective for protein fibers such as silk, wool, nylon and modified acrylics. They contain sulphonic acid groups, which are usually present as sodium sulphonate salts. These increase solubility in water, and give the dye molecules a negative charge. In an acidic solution, the -NH2 functionalities of the fibres are prorogated to give a positive charge: -NH3+. This charge interacts with the negative dye charge, allowing the formation of ionic interactions. As well as this, Van-der-Waals bonds, dipolar bonds and hydrogen bonds are formed between dye and fibre. As a group, acid dyes can be divided into two sub-groups: acid-leveling or acid-milling.

Chemical structure of acid dyes
These dyes are normally very complex in structure but have large aromatic molecules, having a sulphonyl or amino group which makes them soluble in water. Most of the acid dyes belongs to following three main structural molecules,
  1. Anthraquinon type
  2. Azo dye type
  3. Triphenylmethane type.
Different types of acid dyes
The basic dyes are classified into several groups , based on the leveling properties, economy of the dyeing and fastness properties, however generally these are classified into these three classes,

1.Neutral acid dyes :
These are supra milling or fast acid dyes, having medium to good wet fastness properties , some of the dyes have poor light fastness in pale shades . many of the dyes are used as self shades only. These are applied to the fiber in a weakly acid or neutral pH.

2.Weak acid dyes
These dyes belongs to the milling class of dyes. These dyes have good fastness properties but light fastness is moderate to poor.

3.Strong acid dyes
These dyes are applied in a strongly acidic medium and also called leveling dyes, however there wet fastness properties is a limitation. These dyes are very good to produce the combination shades.

Classification according to dyeing characteristics
Acid dyes are commonly classified according to their dyeing behavior, especially in relation to the dyeing pH, their migration ability during dyeing and their washing fastness. The molecular weight and the degree of sulphonation of the dye molecule determine these dyeing characteristics. The original classification of this type, based on their behavior in wool dyeing, is as follows:
  1. Level dyeing or equalizing acid dyes;
  2. Fast acid dyes;
  3. Milling acid dyes;
  4. Super-milling acid dyes.
Milling is the process in which a woollen material is treated, in weakly alkaline solution, with considerable mechanical action to promote felting. Dyes of good fastness to milling are essential to avoid colour bleeding during the process.

Properties of acid dyes
The main properties of acid dyes are ,
Since these are sold as a sodium salt, there fore these form a large anion in the aqueous medium. 
  1. These dyes are anionic in nature.
  2. These dyes are suitable for wool, silk, polyamide and modified acrylics.
  3. These are applied from a strongly acidic to neutral pH bath.
  4. These dyes have no affinity for cotton cellulose’s , hence not suitable for cellulosic’s.
  5. These dyes combine with the fiber by hydrogen bonds , Vander Waals forces or through ionic linkages.
Mechanism of dyeing with acid dyes
Dissolution of dyes in aqueous solvent, produces a colored anion,
The protein and polyamide fibers produce cationic sites in water under acidic conditions, as the acidity of the solution is increased more cationic sites are produced under these strongly acidic conditions. These cationic sites are thus available for the acid dye anions to combine with through hydrogen bonding, vander waals forces or ionic bonding. These linkages are strong enough to break , and thus dyeing produced are fast .
Reaction between an acid dye and wool can be represented by following equation
A dyeing cycle for nylon filament dyeing
Fastness Properties of Acid Dyes
The wet and light fastness properties of the acid dyes varies from poor to excellent , depending upon the molecular structure of the dyes.

The fastness properties as per the category are as follows
Neutral acid dyes:-since these dyes have very good leveling and migration properties ,and have a low affinity for the fiber, therefore the wet fastness properties of this class are generally poor.
Weak acid dyes or half milling dyes :- These dyes have a medium to good affinity for the fiber and are generally applied in a weakly acidic bath, shows medium to good wet fastness properties. Strong acid dyes or super milling dyes :- These dyes have poor exhaustion properties, therefore applied under very strong acidic condition , exhibit good fastness properties.

Dyeing of nylon, polyester with modifications
The production of multi-colour effects has been done by the help of differential-dyeing systems. The term “differential-dyeing” is applied to fibres which are of the same basic type, eg, nylon but with different dyeing properties. To vary the dyeing properties of the fibre (nylon), chemical modifications are employed. In one case the substantivity of the fibre for anionic dyes is modified by altering the amine end-group content. This can be done by using a stabiliser during polymerisation which has one or two carboxyl groups that can react with the amino end groups of nylon polymer. The stabiliser also has one or two sulphonate groups which make the polymer anionic. An example of such a stabiliser is disodium 3,5- disulphobenzoic acid – C6H3 (SO3Na)2 COOH, ie, Na2DSBA. The reaction of the nylon polymer with Na2 DSBA is as follows:-

HOOC – Nylon Polymer – NH2 + HOOC – C6H3 (SO3Na)2 ––>
HOOC – Nylon Polymer – NHCO – C6H3 (SO3 Na)2 + H2O (Cationic dye dyeable nylon)

Other alkali metal salts, eg, lithium salt can also be used. 5-sodium sulpho-isophthalic acid is also used to introduce sulphonate groups in nylon 6 polymer. The introduction of the sulphonate group increases the affinity for basic dyes while the affinity for acid dyes is very low. In another modification, the amine end-group content is varied which gives a differential dye pick-up of acid dyes on nylon making it regular, light-dyeable and deep-dyeable. The amine end - group content has a predominant effect on dye pick-up when dyed with acid dyes. The amino end - groups are the functional sites in nylon for the adsorption of anionic dyes in acidic solution; the polymer and acid dyes are ironically bonded in the acidic dye bath. By introducing anionic sulphonate group (SO3–), nylon polymer becomes cationic dye able. The reaction with basic dyes is as follow:-

Nylon polymer – SO3Na
Nylon polymer – SO3–+ Na+
Cationic dye – N+ Cl–
Cationic dye – N+ + Cl–
Nylon polymer – SO3 +
Cationic dye – N+
Nylon polymer – SO3—N+ -- Cationic dye (Nylon dyed with cationic dye).

In the second case, the nylon fibre is modified during manufacture by introducing anionic sulphonate groups in the nylon molecules. Such modified nylons have no substantivity for acid dyes, but can be dyed with basic dyes. They are called basic dyeable or cationic dyeable nylon.

As expected, the dyed shades obtained with basic dyes on cationic dyeable nylon are bright. The bright shades coupled with the lustre of the fabric result in a silk-like appearance of the fabric. However, when the dyed shades are further subjected to any wet treatments, there is loss of colour. The dyed shades are also unstable to high temperatures. Cationic dyeable nylon printed with basic dyes when subjected to high temperatures in the steam ager for fixation (~ 180°C), loses colour. Keeping in mind the brilliancy of the shades and the feel of the fabric, this system of basic dyes on cationic dyeable nylon can be developed to produce silk-like fabrics. Besides the appearance of the fabric, it is also economical to manufacture. The main two problems to be addressed are improving the wet fastness of the dyes and their high temperature stability. Since this problem was faced by some industrial units, a detailed study of the properties and dyeing of cationic dyeable nylon was carried out.

Cationic dyeable nylon fabric was not available in the local market so the yarn was procured. Messrs Modipon kindly supplied about 8 kg of cationic dyeable nylon yarn. The yarn which was in the form of 4 cheeses of 2 kg each was wrapped in polyethylene sheets and kept in the laboratory.

To study the properties of two different types of cationic dyeable yarns, cationic dyeable polyester fabric was also procured. Besides this, to study the physical properties of polyamides, nylon 6 and nylon 66 fabrics (10 m each) were procured. Cationic dyeable polyester, nylon 6 and nylon 66 fabrics were obtained in the pre-treated form, ie, scoured, bleached and OBA treated.

End-group analysis – Amine and carboxylic end groups of nylons

The amino (--NH2) and carboxylic (--COOH) end groups of cationic dyeable nylon, nylon 6 and nylon 66 were analysed.

Dyeing of Cationic Dyeable Nylon (CDN) and Cationic Dyeable Polyester (CDP) fabrics

Dyeing of CDN
The scoured/bleached and heatset fabrics were dyed with basic dyes. The basic dyes were selected so as to cover the entire gamut of shades. The seven basic dyes used for dyeing and their concentrations are as given below:
  1. Coracryl Yellow C7G (200%).
  2. Coracryl Golden Yellow CGL (200%) – CI Basic Yel.28.
  3. Coracryl Pink CFG (200%) Basic Red 14.
  4. Coracryl Red C2B (200%) Basic Violet 16.
  5. Coracryl Blue C2R (200%) Basic Blue 54.
  6. Basicol Violet C3R.
  7. Basicol Brilliant Blue CBR (200%).
Concentrations(%) : 0.1, 0.25, 0.5, 0.75, 1.0, 2.0, 4.0.

Dyeing was carried out in an HT/HP beaker dyeing machine at pH 7. The dyeing liquor was prepared with 2 g/l disodium hydrogen phthalate and 2 g/l sodium dihydrogen phthalate in distilled water. The material to liquor ratio was kept at 1:50. 1 ml of 1% solution of cationic retarder was added to the dyebath. The temperature cycle was:
After dyeing, the dyed samples were washed with 1 g/L non - ionic detergent at 60 - 70°C. Addition of soda ash (sodium carbonate) leads to loss of colour in some basic dyes. The dyed samples were ironed. All the samples of cationic dyeable nylon were dyed by the above method.
Dyeing of CDP
The scoured, bleached and heat-set cationic dyeable polyester (CDP) fabric was dyed with the above-mentioned basic dyes at identical concentrations. Dyeing was carried out in an HT/HP beaker dyeing machine under the following conditions:
  • M : L Ratio 1:50
  • 2 g/L Sodium acetate
  • 3 g/L Sodium sulphate
  • 0.1 g/L Retarding agent
After dyeing, the dyed samples were rinsed and washed with 2 g/L non - ionic detergent at 60°C - 70°C. The dyed samples were then subjected to mild ironing and stored in the dark.
The reflectance values of all the dyed samples of both cationic dyeable nylon and cationic dyeable polyester were measured on a ColorEye 7000A spectrophotometer (Gretag Macbeth Computer Colour Matching System).

A number of chemical treatments are available for improving the wet fastness properties of dyes on textile fibres. These treatment methods may involve increasing the molecular weight of the dye so that it is rendered less soluble and thereby improving the wet fastness. Alternatively, the after treatment agent may form a layer on the fabric which prevents diffusion of the dye out of the fabric on washing. The nature of the after treatment varies with the dyes used for dyeing and the nature of the substrate. The interaction of the after treatment agent with the dye/substrate may be a chemical reaction or by mere ion-ion interactions.

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Mazharul Islam Kiron is a textile consultant and researcher on online business promotion. He is working with one European textile machinery company as a country agent. He is also a contributor of Wikipedia.

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