Those dyes applied in a bath containing mineral or organic acid like Sulphuric acid, acetic acid or formic acid and are sodium salts or organic acids in nature are called ACID DYES.

Acid Dyes

  • Affix to substrates by hydrogen bonding, Van der Waals forces and ionic bonding
  • Applied in three ways,
  • Using weak acids like acetic acid, formic acid etc. called WA dyes like Azo, hetero Azo, AQ, 2:1-symmetric metal complex acid dyes etc.
  • Using strong acids like HCl, Sulphuric acid etc. called SA dyes like 1:1-metal complex acid dyes etc.
  • Using no acid called neutral exhaust dyes called NE dyes like 2:1-asymmetric metal complex dyes etc.

Properties of Acid Dyes:

  • Soluble in water
  • Easily applied on wool, silk and nylon fibers
  • Generally applied in the presence of acids like Sulphuric, acetic or formic acid.
  • In some cases, soluble in alcohol.
  • Decolorized when treated with a reducing agent
  • Usually combined with basic dyes.
  • Good light fastness
  • Moderate washing fastness
  • Moderate leveling characteristics

Classification of Acid Dyes according to the leveling characteristics

  1. A) Acid dyes with good leveling characteristics

This type of acid dye is responsible for their good leveling characteristics. As the dye molecules have less attraction for the fiber they will migrate slowly into the polymer systems of wool or nylon fibers. However to overcome this substantivity problem, adequate exhausting agent (Sulphuric acid) is added to the dye liquor. Their lack of substantive is evidence by their poor wash fastness. However the light fastness is very good to excellent.

  1. B) Acid dyes with average leveling characteristics

The moderate substantive of this type of acid dye is responsible for average leveling characteristics. To obtain sufficient substantive and to ensure it, adequate exhausting agent, a week acid (Acetic acid or Formic acid) is added to the dye bath. The washing fastness of these dyes is fair, whilst their light fastness is good to very good.

  1. C) Acid dyes with poor leveling characteristics

These dyes are also known as fast acid dyes, and milling dyes or natural dyeing acid dyes. They have the best substantive of all the acid dyes, but have relatively poor leveling characteristics. Proper care has been taken during dyeing otherwise it may result in to too rapid uptake and consequently unleveled dyeing.

Classification of Acid Dyes according to their chemical constitution

  1. Nitro
  2. Nitroso
  3. Azo – mono, dis, tris, poly
  4. a) Acid leveling
  5. b) Acid milling
  6. Triphenylmethane
  7. Xanthane
  8. Azine
  9. Quinaline
  10. Ketorenine
  11. Anthraquinone Dyes
  12. Metal Complex Dyes
  13. a) 1:1- Metal complex
  14. b) 1:2-Asymetric metal complex
  15. c) 1:2-Symetric metal complex
  16. d) 1:2-Mix metal complex

First clarify the term, what is meant by leveling and milling

Acid leveling dyes:

They are planar dyes, tend to be small or medium sized, and show moderate inter-molecular attractions for wool fibers.  This means that the dye molecules can move fairly easily through the fibers and achieve an even color.  However, the low affinity means that these dyes are not always very resistant to washing.

Acid milling dyes:

They are larger than acid-leveling dyes, and show a much stronger affinity for wool fibers.  Because of this, the resultant color may be less even, but they are much more resistant to washing.

Now, come to important innovations part, which of course, add value to our manufacturing. Innovations depend upon market requirements.

Generally, in Industry we had been asked by our clients

CAN YOU SUGGEST SOME ACID DYES FOR SUCH APPLICATIONS

CAN YOU SUGGEST PARTICULAR SHADES FOR SUCH APPLICATIONS

then automatically this sentence is circulated/e-mailed to R&D department from marketing department and then R&D department starts functioning on that.

R&D Department must know which type of chemical constitution and which category has been used in which applications although all acid dyes applied in wool, silk, nylon, polyamide fibers without any doubt. If they know then it is very much easy to offer Acid Dyes to our client and click business immediately without any discussion.

 Acid Dyes has been divided according to their application into following categories, as per my knowledge and experience:

  1. Acid Dyes for wood staining
  2. Acid Dyes for coloration of carpet
  3. Acid Dyes for wood coating
  4. Acid Dyes for exterior finish of Aluminium
  5. Acid Dyes for double face woven cloths
  6. Acid Dyes for fur dyeing
  7. Acid Dyes for ink formulations
  8. Acid Dyes for jute dyeing
  9. Acid Dyes for thermal stable applications
  10. Acid Dyes for paper coloration
  11. Acid Dyes for polyamide printing
  12. Acid Dyes for silk dyeing and printing
  13. Acid Dyes for leather coloration
  14. Acid Dyes for wool dyeing

Now, we will discuss all value added applications one by one.

  1. Acid Dyes for wood staining:

Wood pieces are often decorated to add color and appeal. Wood products are often imparted with a wood-tone stain to enhance the natural grain or add depth or tone to the wood. Stain may alter the color and appearance of the wood or hide unattractive grain. Stains are available in a variety of wood tones, including very light, semi-transparent stains to dark, nearly opaque stains.

For these applications, generally solvent soluble acid dyes have been used.

Regardless of solvent, stains generally penetrate only the top layers of the wood. Thus, the stain can be stripped and sanded away, revealing the original color of the wood. Stain must be top coated or finished, meaning that once it is dry, some kind of surface finish is applied to protect the wood surface and stain from moisture, scratches, unwanted stains, dirt, and chemicals.

  1. Acid Dyes for coloration of carpet:

If you want your carpets to take on a new look and don’t want to spend the money to replace them you should consider carpet dye. It’s a low cost alternative that will give your carpet a total face lift.

Both nylon and wool carpets can be dyed. Polyester, polypropylene, and acrylic cannot be dyed. The current color of your carpet will determine what color you can dye your carpet. For example, if your carpet is red and yellow dye and you would get an orange carpet. Or if your carpet was blue and you added red, you would get a purple carpet. White and light beige carpet can be dyed any color but you can never get a deep black. You can never go lighter when dyeing carpet, only darker.

For this applications, generally those acid dyes which has poor washing fastness been used.

  1. Acid Dyes for wood coating:

They are selected metal complex acid dyes.

They are desalinated by a special process and are supplied in a 1-methoxy-2-propanol or 1-ethoxy-2-propanol solution form. The basic characteristics of these liquid dyestuffs are good light fastness, good over lacquering resistance, very good compatibility with a wide variety of solvents, good storage stability, low viscosity, fast drying and ease of use.

Storage stability:

-remain unchanged after 9 months storage at room temperature.

-remain unchanged after 1 day’s storage at +3˚C

-show some precipitation after 8 day’s storage at +3˚C and gel after 1 day’s storage at -10˚C

-gelling and precipitation is reversible by heating to 45˚C

Solvent compatibility:

The solvent compatibility was tested for stains containing 3% of liquid dye (delivery form) in the test solvent. The condition of the solution immediately after the formulation and after 30 days’ storage at room temperature was evaluated.

  1. Acid Dyes for coloration of Aluminium:

Those Acid Dyes having excellent exceptional light and heat fastness characteristics has been used for exterior finish of Aluminium.

  1. Acid Dyes for double face woven cloths:

Double cloth or double weave (also double cloth, double-cloth) is a kind of woven textile generally sheep skin in which two or more sets of wraps and one or more sets of weft or filling yarns are interconnected to form a two-layered cloth. The movement of threads between the layers allows complex patterns having two right sides or faces and no wrong side, and includes most trend-setting fashion (haute couture) coats, blankets, furnishing fabrics and some brocade.

For this application, generally Azo or metal complex non-bleeding dyes has been used.

  1. Acid Dyes for dyeing of fur skins:

Acid Dyes especially selected for dyeing fur skins because of the following properties:

  • good exhaustion at 60–65 °C
  • good combinability at this temperature
  • only light staining of the leather
  • similar fastness properties
  • trichromatic shades

For this application, generally nitro, Nitroso, Azo and anthraquinonoid acid dyes have been used.

  1. Acid Dyes for ink formulations:

They are generally water-based & solvent-based as well as water-solvent mix highly purified liquids, ready-to-dissolve high-purity powders, high-strength crude powders and crude wet press cake. Suitable for ink-jet printers, ink pads, roller-point pens, permanent, erasable and fluorescent markers and other specialty applications. Generally used in trichromatic forms for colored ink.

For this application, generally salt free acid dyes have been used.

  1. Acid Dyes for jute dyeing:

Jute is a ligno-cellulosic, composite natural bast fiber. Cellulose, hemi-cellulose and lignin are its major constituent components & its three dimensional structure is formed by different inter and intra-molecular forces resulted from various physical, chemical, and hydrogen bonds, between them. The commercial fiber consists of hairy strands of cylindrical networks of ultimate jute fiber. Properly retted and washed jute fibers are fairly lustrous with moderate strength but rough to touch. The color of the fiber also varies from creamy white to brown.

Golden fiber, jute, is now being utilized for a number of values added and diversified products as it has not lost its glitter.

Traditional use of jute fabric i.e., sacking and hessian will continue for its use as packaging material, carpet backing, etc.

Nontraditional use in curtains, furnishing textiles and apparel textiles will increase gradually due to its eco-friendly and biodegradable character.

So, modification of feel and look of the fabric has become essential depending on the end use requirement resulting in increasing the demand for bright, wash fast and light fast jute fabric.

For this application, those acid dyes have good light fastness have been used.

It is our privilege to know that Indian Jute crafts are much more demand all over the world. They are not only essential part of Indian culture but they earn substantial foreign exchange through exports.

  1. Acid Dyes for thermal stable applications:

To reveal the effect of acid dyes on the melt processing of recycled nylon 66, we found few acid dyes suitable to dye a molten nylon 66 yarn. The thermal stability of dyestuffs, the thermal degradation of the dyed nylon was tremendously excellent. During melt processing (melting temperature 255° C), most dyed nylon 66 changed color strength and color pattern. Few acid dyes have severe impact on the thermal stability of nylon 66 and also the existence of acid dye in nylon 66 does not influence the melting temperature of nylon 66 but some acid dyes reduce the crystallization temperature of molten nylon 66.

Generally, metal complex dyes having no solubilizing groups or less solubilizing groups are more suitable as thermostatic acid dyes.

  1. Acid Dyes for paper coloration:

Acid Dyes do not have affinity for cellulose, but are precipitated by the addition of alum & rosin size in beater. They give good level dyeing properties and generally the backwater of Acid Dyes is colored which can be minimized by the addition of fixing agents. They are not preferred for neutral or alkaline sizing.

Anionic acid dyes preferred for light & deep shades made of recycled, mechanical, bleached & unbleached pulps & in segment where fastness requirements are not high.

  1. Acid Dyes for polyamide printing:

Acid Dyes are used in printing on nylon 6 satin fabric but in this case fastness properties have no bar but dischargeability is important.

Generally, dischargeable milling Azo dyes are used.

  1. Acid Dyes for silk dyeing and printing:

Acid Dyes are used in dyeing and printing of silk fabric but in this case light fastness is more important.

Generally, all milling and metal complex acid dyes used in dyeing and printing of silk but more importance in silk printing is 1:1- metal complex.

  1. Acid Dyes for leather coloration:

The dyes were selected on the basis of their high resistance to migration.

The migration resistance depends on the type of re-tannage and fat-liquor, the quantity of dye applied and the type of dyeing process used, as well as on the selection of the correct dye.

Generally, all milling and metal complex acid dyes used but those having higher resistance to migration are preferable.

  1. Acid Dyes for wool dyeing:

Those dyes build up from a Sulphuric/formic acid bath have remarkably good migration power which makes level dyeing easily attainable, have high light fastness, moderate wet fastness properties.

Those dyes are applied from weakly acid medium have the same dyeing properties. Consequently they show excellent compatibility in combination, which together with their notably good leveling properties and all round fastness make them particularly suitable for yarn and piece dyeing.

Those dyes are applied from a neutral to weakly acid bath and find their major uses for dyeing brilliant shades and as brightening elements for 2:1 metal complex dyestuffs. Certain members of the range have extremely high wet and light fastness. They are most often dyed in self-shades and in binary combinations.

2:1-metal complex dyestuffs exhibit good light fastness down to pale depth, excellent dyeing behavior in combination, simple method of application and uniform dyeing on different wool qualities.

Generally, all acid dyes used for wool dyeing.

Dr Bhavya MODI

Multinational Color Technologist

GSM                 : 91-966-466-5448, 91-987-954-0263

Phone              : 91-79-25330696

E-mail               : [email protected]

LinkedIn           : www.linkedin.com/in/dr-modi-mba-4998a21b

Ahmedabad, Gujarat, INDIA

Roha, Raigad, MH, INDIA