Preamble
This is the one hundredth and sixth post in the "Art Resource" series, specifically aimed to construct an appropriate knowledge base in order to develop an artistic voice in ArtCloth.
Other posts in this series are:
Glossary of Cultural and Architectural Terms
Units Used in Dyeing and Printing of Fabrics
Occupational, Health & Safety
A Brief History of Color
The Nature of Color
Psychology of Color
Color Schemes
The Naming of Colors
The Munsell Color Classification System
Methuen Color Index and Classification System
The CIE System
Pantone - A Modern Color Classification System
Optical Properties of Fiber Materials
General Properties of Fiber Polymers and Fibers - Part I
General Properties of Fiber Polymers and Fibers - Part II
General Properties of Fiber Polymers and Fibers - Part III
General Properties of Fiber Polymers and Fibers - Part IV
General Properties of Fiber Polymers and Fibers - Part V
Protein Fibers - Wool
Protein Fibers - Speciality Hair Fibers
Protein Fibers - Silk
Protein Fibers - Wool versus Silk
Timelines of Fabrics, Dyes and Other Stuff
Cellulosic Fibers (Natural) - Cotton
Cellulosic Fibers (Natural) - Linen
Other Natural Cellulosic Fibers
General Overview of Man-Made Fibers
Man-Made Cellulosic Fibers - Viscose
Man-Made Cellulosic Fibers - Esters
Man-Made Synthetic Fibers - Nylon
Man-Made Synthetic Fibers - Polyester
Man-Made Synthetic Fibers - Acrylic and Modacrylic
Man-Made Synthetic Fibers - Olefins
Man-Made Synthetic Fibers - Elastomers
Man-Made Synthetic Fibers - Mineral Fibers
Man Made Fibers - Other Textile Fibers
Fiber Blends
From Fiber to Yarn: Overview - Part I
From Fiber to Yarn: Overview - Part II
Melt-Spun Fibers
Characteristics of Filament Yarn
Yarn Classification
Direct Spun Yarns
Textured Filament Yarns
Fabric Construction - Felt
Fabric Construction - Nonwoven fabrics
A Fashion Data Base
Fabric Construction - Leather
Fabric Construction - Films
Glossary of Colors, Dyes, Inks, Pigments and Resins
Fabric Construction – Foams and Poromeric Material
Knitting
Hosiery
Glossary of Fabrics, Fibers, Finishes, Garments and Yarns
Weaving and the Loom
Similarities and Differences in Woven Fabrics
The Three Basic Weaves - Plain Weave (Part I)
The Three Basic Weaves - Plain Weave (Part II)
The Three Basic Weaves - Twill Weave
The Three Basic Weaves - Satin Weave
Figured Weaves - Leno Weave
Figured Weaves – Piqué Weave
Figured Fabrics
Glossary of Art, Artists, Art Motifs and Art Movements
Crêpe Fabrics
Crêpe Effect Fabrics
Pile Fabrics - General
Woven Pile Fabrics
Chenille Yarn and Tufted Pile Fabrics
Knit-Pile Fabrics
Flocked Pile Fabrics and Other Pile Construction Processes
Glossary of Paper, Photography, Printing, Prints and Publication Terms
Napped Fabrics – Part I
Napped Fabrics – Part II
Double Cloth
Multicomponent Fabrics
Knit-Sew or Stitch Through Fabrics
Finishes - Overview
Finishes - Initial Fabric Cleaning
Mechanical Finishes - Part I
Mechanical Finishes - Part II
Additive Finishes
Chemical Finishes - Bleaching
Glossary of Scientific Terms
Chemical Finishes - Acid Finishes
Finishes: Mercerization
Finishes: Waterproof and Water-Repellent Fabrics
Finishes: Flame-Proofed Fabrics
Finishes to Prevent Attack by Insects and Micro-Organisms
Other Finishes
Shrinkage - Part I
Shrinkage - Part II
Progressive Shrinkage and Methods of Control
Durable Press and Wash-and-Wear Finishes - Part I
Durable Press and Wash-and-Wear Finishes - Part II
Durable Press and Wash-and-Wear Finishes - Part III
Durable Press and Wash-and-Wear Finishes - Part IV
Durable Press and Wash-and-Wear Finishes - Part V
The General Theory of Dyeing – Part I
The General Theory of Dyeing - Part II
Natural Dyes
Natural Dyes - Indigo
Mordant Dyes
Premetallized Dyes
Azoic Dyes
Basic Dyes
Acid Dyes
Disperse Dyes
Direct Dyes
Reactive Dyes
Sulfur Dyes
Blends – Fibers and Direct Dyeing
The General Theory of Printing
There are currently eight data bases on this blogspot, namely, the Glossary of Cultural and Architectural Terms, Timelines of Fabrics, Dyes and Other Stuff, A Fashion Data Base, the Glossary of Colors, Dyes, Inks, Pigments and Resins, the Glossary of Fabrics, Fibers, Finishes, Garments and Yarns, Glossary of Art, Artists, Art Motifs and Art Movements, Glossary of Paper, Photography, Printing, Prints and Publication Terms and the Glossary of Scientific Terms, which has been updated to Version 3.5. All data bases will be updated from time-to-time in the future.
If you find any post on this blog site useful, you can save it or copy and paste it into your own "Word" document etc. for your future reference. For example, Safari allows you to save a post (e.g. click on "File", click on "Print" and release, click on "PDF" and then click on "Save As" and release - and a PDF should appear where you have stored it). Safari also allows you to mail a post to a friend (click on "File", and then point cursor to "Mail Contents On This Page" and release). Either way, this or other posts on this site may be a useful Art Resource for you.
The Art Resource series will be the first post in each calendar month. Remember - these Art Resource posts span information that will be useful for a home hobbyist to that required by a final year University Fine-Art student and so undoubtedly, some parts of any Art Resource post may appear far too technical for your needs (skip over those mind boggling parts) and in other parts, it may be too simplistic with respect to your level of knowledge (ditto the skip). The trade-off between these two extremes will mean that Art Resource posts will hopefully be useful in parts to most, but unfortunately may not be satisfying to all!
Introduction
Dyes may be classified according to:
(i) The chemical constitution of the dye molecules or;
(ii) According to the method of application.
We shall use the second approach and classify the dyes according to their method of application. Today, we shall concentrate on acid dyes.
Acid dyes are so called because they are usually applied under acidic conditions. Acid dyes are negative charge carriers. The fibers mostly colored by acid dyes are man-made synthetic, nylon fibers and the natural protein fibers such as wool, silk, mohair etc. Acidic dyes generally have less affinity for cellulosic fibers, when compared to say, direct dyes. They have no affinity for cellulosic fibers such as cotton, linen or jute or on any fibers that are sensitive to weak acids. These dyes vary in fastness with respect to washing, laundering, dry cleaning and perspiration.
Merino Wool Dyed With Jacquard Acid Dyes.
Acid dyes are normally sodium salts of sulfonic acid - hence, their name. That is, the generalized formula for an acid dye molecule is DNa+ SO3-, where D is the color radical or component of the acid dye molecule.
The application of acid dyes to protein fibers such as wool and silk results in an ionic or salt link between the dye molecule and the fiber polymer. In the case of wool polymer, it is firstly acidified (addition of H+) in the dye bath.
Step One: The amino (NH2) group of the wool polymer is acidified.
Note: H+ is attached to the amino group of the wool polymer system.
Courtesy of reference [1].
This can be more broadly shown in terms that the wool polymer system now has positive charges on the surface of the fiber polymer system.
The acid solution places positive charges on the surface of the wool fibers.
Courtesy of reference [2].
Once acidified, the positively charged amino group in the wool polymer system is then attracted to the negative radical of the dye molecule.
Step Two: The positive charged amino group of the wool polymer forms an ionic link to the dye anion.
Note: The Na+ cation has a greater affinity for water and so goes into solution and is not involved in the dye attachment to the wool polymer system. Since it has the same charge of the fiber, it avoids it.
Courtesy of reference [1].
This can be shown more holistically as the negatively charged dye molecule being attracted to the positively charged fiber.
Addition of the anionic dye radical.
Note: this shows that the acidic dyes can only be applied to fibers, whose polymer systems have sites that will accept a positive charge from the acidic solution.
Courtesy reference [2].
The anionic (negative) dye molecule forms an ionic link with the cationic (positive) fiber sites.
Courtesy reference [2].
In addition to ionic bonds (see above diagrams) when protein and polyamide fibers are dyed with acid dyes, hydrogen bonds and van der Waals forces of attraction (not shown above) will also be formed between the acid dye molecules and chemical groups of the fiber polymer system.
Some of the acid dyes have a relatively high substantivity to protein fibers and so the fibers may be dyed unevenly. To overcome this, a retarder needs to be added to the dye liquor, such as sodium sulfate called Glauber salt (Na2SO4). The sulfate anion (SO42-) of the salt is much smaller than the dye anion and so it can move in the dye liquor much more rapidly to the positive sites on the fiber and so effectively compete with the dye anions for attachment to these sites. The dye anions have a greater affinity for the positive sites on the fiber, than the sulfate anions and so the sulphate anions retard the rate at which the dye anions can occupy the positive sites of the fiber. This produces a more uniform dyeing of the fiber. The application of heat assists the dyeing process by increasing the kinetic energy or speed of the dye anions as they move towards the positive sites on the fiber, therefore effectively overcoming the retarding effect of the sulfate anions. Moreover, the bulky, speedier dye anions can knock off the smaller attached SO42- molecules on the fiber sites and so over a longer duration in time, the addition of salt ensures that the dye gives a much more extensive and even coverage over the fabric.
Printing with Acid Dyes
Once an acid dye printing paste has been applied to the textile material, steaming the printed pattern is necessary, since the steam heated water molecules enable the dye molecules of the printing paste to transfer from the fiber surface into the fiber polymer system. It does so, by enlarging the voids in the amorphous regions of the fiber polymer system, and so enabling a greater dye penetration into the fabric. Note: On cooling, the fiber voids are shrunk in size, thereby trapping and entangling the acid dyes and so promoting not only ionic bonding, but hydrogen bonding and van der Waals forces of attraction, all of which fix the dye molecule to the fiber polymers.
The process of printing is different to dyeing, since it requires a lower liquor ratio and a thickener to ensure that the acid dye will not run when applied to the textile material.
Properties of Acid Dyes
Light Fastness
Dyed and printed acid colors have “good” light fastness, with a rating of about 4-5. The electronic stability of the chromophores of acid dyes is such that they can resist the degrading effects of UV sunlight for a considerable duration.
Wash Fastness
The wash fastness of acid dyed textile materials is rated about 2-3 for acid dyes with good leveling characteristics, 3-4 for those with average leveling characteristics, and 4-5 for those with poor leveling characteristics.
Two factors influence the wash fastness of acid dyed textile materials and these are:
(i) Acid dyed molecules, which are loosely held or which have not penetrated the fiber polymer system of wool or nylon sufficiently may be removed on laundering, due to the ionic and hydrogen bonds that cohere it to the fiber polymer system being severed via hydrolysis.
(ii) Acid dyes are acidic and so resistant to acids, but being acids will combine with alkalis (which are bases) such as the detergents used in washing apparel and so the acid-base combination will sever the cohesive bonds and assist in the dye removal from the fiber.
Leveling Characteristics
Acid dyes are divided into three groups according to their leveling characteristics namely:
(i) Acid dyes with good leveling characteristics (Acid Blue 45, 63010).
The relatively poor substantivity of this acid dye is responsible for their good leveling characteristics. As the dye molecules have less attraction for the fiber they will migrate only slowly into the wool or nylon polymer systems. However, to obtain sufficient substantivity, and to ensure adequate exhaustion, sulfuric acid is added to the dye liquor to obtain a pH between 3.5-4.5. Their lack of substantivity is evidence by the poor wash fastness. However, their light fastness is very good to excellent.
(ii) Acid dyes with average leveling characteristics (e.g. Acid Blue 83, 42660). Acetic acid is used to acidify the dye liquor to a pH between 5-6. At this pH adequate exhaustion of the dye occurs. Lowering the pH leads to uneven dyeing. The wash fastness of these acid dyes is fair, whilst their light fastness is good to very good.
(iii) Acid dyes with poor leveling characteristics (Acid Yellow 42, 22910).
These dyes are also known as fast acid dyes, acid milling dyes or neutral dyeing acid dyes. They have the best substantivity of all acid dyes, but possess poor leveling characteristics. Unless care is taken during dyeing, their relatively good substantivity for the fiber may result in too rapid a dye uptake and consequently lead to unlevel dyeing.
The excellent substantivity of these dyes is attributed to the sodium sulfonate groups (NaSO3) that are present in their molecular structure. The greater polarity of acid milling dyes is due to these extra groups that imparts to them a higher substantivity, thus requiring a pH between 6-7 in order to obtain slower exhaustion and more level dyeing. Note: pH 7 is neutral; that is, neither an acidic or basic dye liquor.
The wash fastness of these dyes is good to very good whist their light fastness is fair to good. The better wash fastness of these dyes compared to (i) and (ii) is due to the presence of a greater number of sodium sulfonate groups.
Dye Uptake
Heating of the acid dye liquor is essential to ensure satisfactory dye uptake. Below about 40oC, there is practically no transfer of dye molecules from the dye liquor to the fiber polymer system. However, the rate of dyeing increases steadily as the temperature rises. Increasing the temperature of the dye liquor provides the energy required by the dye molecules to leave the dye liquor and enter the fiber polymer system.
References:
[1] E.P.G. Gohl and L.D. Vilensky, Textile Science, Longman Cheshire, Melbourne (1989).
[2] A. Fritz and J. Cant, Consumer Textiles, Oxford University Press, Melbourne (1986).
This is the one hundredth and sixth post in the "Art Resource" series, specifically aimed to construct an appropriate knowledge base in order to develop an artistic voice in ArtCloth.
Other posts in this series are:
Glossary of Cultural and Architectural Terms
Units Used in Dyeing and Printing of Fabrics
Occupational, Health & Safety
A Brief History of Color
The Nature of Color
Psychology of Color
Color Schemes
The Naming of Colors
The Munsell Color Classification System
Methuen Color Index and Classification System
The CIE System
Pantone - A Modern Color Classification System
Optical Properties of Fiber Materials
General Properties of Fiber Polymers and Fibers - Part I
General Properties of Fiber Polymers and Fibers - Part II
General Properties of Fiber Polymers and Fibers - Part III
General Properties of Fiber Polymers and Fibers - Part IV
General Properties of Fiber Polymers and Fibers - Part V
Protein Fibers - Wool
Protein Fibers - Speciality Hair Fibers
Protein Fibers - Silk
Protein Fibers - Wool versus Silk
Timelines of Fabrics, Dyes and Other Stuff
Cellulosic Fibers (Natural) - Cotton
Cellulosic Fibers (Natural) - Linen
Other Natural Cellulosic Fibers
General Overview of Man-Made Fibers
Man-Made Cellulosic Fibers - Viscose
Man-Made Cellulosic Fibers - Esters
Man-Made Synthetic Fibers - Nylon
Man-Made Synthetic Fibers - Polyester
Man-Made Synthetic Fibers - Acrylic and Modacrylic
Man-Made Synthetic Fibers - Olefins
Man-Made Synthetic Fibers - Elastomers
Man-Made Synthetic Fibers - Mineral Fibers
Man Made Fibers - Other Textile Fibers
Fiber Blends
From Fiber to Yarn: Overview - Part I
From Fiber to Yarn: Overview - Part II
Melt-Spun Fibers
Characteristics of Filament Yarn
Yarn Classification
Direct Spun Yarns
Textured Filament Yarns
Fabric Construction - Felt
Fabric Construction - Nonwoven fabrics
A Fashion Data Base
Fabric Construction - Leather
Fabric Construction - Films
Glossary of Colors, Dyes, Inks, Pigments and Resins
Fabric Construction – Foams and Poromeric Material
Knitting
Hosiery
Glossary of Fabrics, Fibers, Finishes, Garments and Yarns
Weaving and the Loom
Similarities and Differences in Woven Fabrics
The Three Basic Weaves - Plain Weave (Part I)
The Three Basic Weaves - Plain Weave (Part II)
The Three Basic Weaves - Twill Weave
The Three Basic Weaves - Satin Weave
Figured Weaves - Leno Weave
Figured Weaves – Piqué Weave
Figured Fabrics
Glossary of Art, Artists, Art Motifs and Art Movements
Crêpe Fabrics
Crêpe Effect Fabrics
Pile Fabrics - General
Woven Pile Fabrics
Chenille Yarn and Tufted Pile Fabrics
Knit-Pile Fabrics
Flocked Pile Fabrics and Other Pile Construction Processes
Glossary of Paper, Photography, Printing, Prints and Publication Terms
Napped Fabrics – Part I
Napped Fabrics – Part II
Double Cloth
Multicomponent Fabrics
Knit-Sew or Stitch Through Fabrics
Finishes - Overview
Finishes - Initial Fabric Cleaning
Mechanical Finishes - Part I
Mechanical Finishes - Part II
Additive Finishes
Chemical Finishes - Bleaching
Glossary of Scientific Terms
Chemical Finishes - Acid Finishes
Finishes: Mercerization
Finishes: Waterproof and Water-Repellent Fabrics
Finishes: Flame-Proofed Fabrics
Finishes to Prevent Attack by Insects and Micro-Organisms
Other Finishes
Shrinkage - Part I
Shrinkage - Part II
Progressive Shrinkage and Methods of Control
Durable Press and Wash-and-Wear Finishes - Part I
Durable Press and Wash-and-Wear Finishes - Part II
Durable Press and Wash-and-Wear Finishes - Part III
Durable Press and Wash-and-Wear Finishes - Part IV
Durable Press and Wash-and-Wear Finishes - Part V
The General Theory of Dyeing – Part I
The General Theory of Dyeing - Part II
Natural Dyes
Natural Dyes - Indigo
Mordant Dyes
Premetallized Dyes
Azoic Dyes
Basic Dyes
Acid Dyes
Disperse Dyes
Direct Dyes
Reactive Dyes
Sulfur Dyes
Blends – Fibers and Direct Dyeing
The General Theory of Printing
There are currently eight data bases on this blogspot, namely, the Glossary of Cultural and Architectural Terms, Timelines of Fabrics, Dyes and Other Stuff, A Fashion Data Base, the Glossary of Colors, Dyes, Inks, Pigments and Resins, the Glossary of Fabrics, Fibers, Finishes, Garments and Yarns, Glossary of Art, Artists, Art Motifs and Art Movements, Glossary of Paper, Photography, Printing, Prints and Publication Terms and the Glossary of Scientific Terms, which has been updated to Version 3.5. All data bases will be updated from time-to-time in the future.
If you find any post on this blog site useful, you can save it or copy and paste it into your own "Word" document etc. for your future reference. For example, Safari allows you to save a post (e.g. click on "File", click on "Print" and release, click on "PDF" and then click on "Save As" and release - and a PDF should appear where you have stored it). Safari also allows you to mail a post to a friend (click on "File", and then point cursor to "Mail Contents On This Page" and release). Either way, this or other posts on this site may be a useful Art Resource for you.
The Art Resource series will be the first post in each calendar month. Remember - these Art Resource posts span information that will be useful for a home hobbyist to that required by a final year University Fine-Art student and so undoubtedly, some parts of any Art Resource post may appear far too technical for your needs (skip over those mind boggling parts) and in other parts, it may be too simplistic with respect to your level of knowledge (ditto the skip). The trade-off between these two extremes will mean that Art Resource posts will hopefully be useful in parts to most, but unfortunately may not be satisfying to all!
Introduction
Dyes may be classified according to:
(i) The chemical constitution of the dye molecules or;
(ii) According to the method of application.
We shall use the second approach and classify the dyes according to their method of application. Today, we shall concentrate on acid dyes.
Acid dyes are so called because they are usually applied under acidic conditions. Acid dyes are negative charge carriers. The fibers mostly colored by acid dyes are man-made synthetic, nylon fibers and the natural protein fibers such as wool, silk, mohair etc. Acidic dyes generally have less affinity for cellulosic fibers, when compared to say, direct dyes. They have no affinity for cellulosic fibers such as cotton, linen or jute or on any fibers that are sensitive to weak acids. These dyes vary in fastness with respect to washing, laundering, dry cleaning and perspiration.
Acid dyes are normally sodium salts of sulfonic acid - hence, their name. That is, the generalized formula for an acid dye molecule is DNa+ SO3-, where D is the color radical or component of the acid dye molecule.
The application of acid dyes to protein fibers such as wool and silk results in an ionic or salt link between the dye molecule and the fiber polymer. In the case of wool polymer, it is firstly acidified (addition of H+) in the dye bath.
Note: H+ is attached to the amino group of the wool polymer system.
Courtesy of reference [1].
This can be more broadly shown in terms that the wool polymer system now has positive charges on the surface of the fiber polymer system.
Courtesy of reference [2].
Once acidified, the positively charged amino group in the wool polymer system is then attracted to the negative radical of the dye molecule.
Note: The Na+ cation has a greater affinity for water and so goes into solution and is not involved in the dye attachment to the wool polymer system. Since it has the same charge of the fiber, it avoids it.
Courtesy of reference [1].
This can be shown more holistically as the negatively charged dye molecule being attracted to the positively charged fiber.
Note: this shows that the acidic dyes can only be applied to fibers, whose polymer systems have sites that will accept a positive charge from the acidic solution.
Courtesy reference [2].
Courtesy reference [2].
In addition to ionic bonds (see above diagrams) when protein and polyamide fibers are dyed with acid dyes, hydrogen bonds and van der Waals forces of attraction (not shown above) will also be formed between the acid dye molecules and chemical groups of the fiber polymer system.
Some of the acid dyes have a relatively high substantivity to protein fibers and so the fibers may be dyed unevenly. To overcome this, a retarder needs to be added to the dye liquor, such as sodium sulfate called Glauber salt (Na2SO4). The sulfate anion (SO42-) of the salt is much smaller than the dye anion and so it can move in the dye liquor much more rapidly to the positive sites on the fiber and so effectively compete with the dye anions for attachment to these sites. The dye anions have a greater affinity for the positive sites on the fiber, than the sulfate anions and so the sulphate anions retard the rate at which the dye anions can occupy the positive sites of the fiber. This produces a more uniform dyeing of the fiber. The application of heat assists the dyeing process by increasing the kinetic energy or speed of the dye anions as they move towards the positive sites on the fiber, therefore effectively overcoming the retarding effect of the sulfate anions. Moreover, the bulky, speedier dye anions can knock off the smaller attached SO42- molecules on the fiber sites and so over a longer duration in time, the addition of salt ensures that the dye gives a much more extensive and even coverage over the fabric.
Printing with Acid Dyes
Once an acid dye printing paste has been applied to the textile material, steaming the printed pattern is necessary, since the steam heated water molecules enable the dye molecules of the printing paste to transfer from the fiber surface into the fiber polymer system. It does so, by enlarging the voids in the amorphous regions of the fiber polymer system, and so enabling a greater dye penetration into the fabric. Note: On cooling, the fiber voids are shrunk in size, thereby trapping and entangling the acid dyes and so promoting not only ionic bonding, but hydrogen bonding and van der Waals forces of attraction, all of which fix the dye molecule to the fiber polymers.
The process of printing is different to dyeing, since it requires a lower liquor ratio and a thickener to ensure that the acid dye will not run when applied to the textile material.
Properties of Acid Dyes
Light Fastness
Dyed and printed acid colors have “good” light fastness, with a rating of about 4-5. The electronic stability of the chromophores of acid dyes is such that they can resist the degrading effects of UV sunlight for a considerable duration.
Wash Fastness
The wash fastness of acid dyed textile materials is rated about 2-3 for acid dyes with good leveling characteristics, 3-4 for those with average leveling characteristics, and 4-5 for those with poor leveling characteristics.
Two factors influence the wash fastness of acid dyed textile materials and these are:
(i) Acid dyed molecules, which are loosely held or which have not penetrated the fiber polymer system of wool or nylon sufficiently may be removed on laundering, due to the ionic and hydrogen bonds that cohere it to the fiber polymer system being severed via hydrolysis.
(ii) Acid dyes are acidic and so resistant to acids, but being acids will combine with alkalis (which are bases) such as the detergents used in washing apparel and so the acid-base combination will sever the cohesive bonds and assist in the dye removal from the fiber.
Leveling Characteristics
Acid dyes are divided into three groups according to their leveling characteristics namely:
(i) Acid dyes with good leveling characteristics (Acid Blue 45, 63010).
The relatively poor substantivity of this acid dye is responsible for their good leveling characteristics. As the dye molecules have less attraction for the fiber they will migrate only slowly into the wool or nylon polymer systems. However, to obtain sufficient substantivity, and to ensure adequate exhaustion, sulfuric acid is added to the dye liquor to obtain a pH between 3.5-4.5. Their lack of substantivity is evidence by the poor wash fastness. However, their light fastness is very good to excellent.
(ii) Acid dyes with average leveling characteristics (e.g. Acid Blue 83, 42660). Acetic acid is used to acidify the dye liquor to a pH between 5-6. At this pH adequate exhaustion of the dye occurs. Lowering the pH leads to uneven dyeing. The wash fastness of these acid dyes is fair, whilst their light fastness is good to very good.
(iii) Acid dyes with poor leveling characteristics (Acid Yellow 42, 22910).
These dyes are also known as fast acid dyes, acid milling dyes or neutral dyeing acid dyes. They have the best substantivity of all acid dyes, but possess poor leveling characteristics. Unless care is taken during dyeing, their relatively good substantivity for the fiber may result in too rapid a dye uptake and consequently lead to unlevel dyeing.
The excellent substantivity of these dyes is attributed to the sodium sulfonate groups (NaSO3) that are present in their molecular structure. The greater polarity of acid milling dyes is due to these extra groups that imparts to them a higher substantivity, thus requiring a pH between 6-7 in order to obtain slower exhaustion and more level dyeing. Note: pH 7 is neutral; that is, neither an acidic or basic dye liquor.
The wash fastness of these dyes is good to very good whist their light fastness is fair to good. The better wash fastness of these dyes compared to (i) and (ii) is due to the presence of a greater number of sodium sulfonate groups.
Dye Uptake
Heating of the acid dye liquor is essential to ensure satisfactory dye uptake. Below about 40oC, there is practically no transfer of dye molecules from the dye liquor to the fiber polymer system. However, the rate of dyeing increases steadily as the temperature rises. Increasing the temperature of the dye liquor provides the energy required by the dye molecules to leave the dye liquor and enter the fiber polymer system.
References:
[1] E.P.G. Gohl and L.D. Vilensky, Textile Science, Longman Cheshire, Melbourne (1989).
[2] A. Fritz and J. Cant, Consumer Textiles, Oxford University Press, Melbourne (1986).
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