Preamble
This is the ninety-ninth 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
In order to understand the coloring of textile materials, we need to have some understanding of the general theory of dyeing and printing. After reading this post and if you are unsure of a definition, terminology or property of a fabric that this post assumes you should know and you don't, then study the "Preamble" above and click on any art resource post or glossary that might provide that missing information for you.
This post will continue the general theory of dyeing.
Dye Sources
Many colorants (and in fact chemicals) were once obtained from living matter. Cochineal was obtained from an insect, purple was extracted from a sea snail, and a vast number of colorants were extracted from a range of plants. Plant-derived dyes are still widely used by craft and ArtCloth practitioners.
A number of colorants were based on naturally occurring mineral compounds. These are mainly used as pigments. For example, ochres are pigments that the Australian Aboriginals used in their bark paintings.
Artist: Rover Thomas-Winbah.
Australian aboriginal ochre painting.
In the modern era, most colorants are synthesized and manufactured. Most dyes are composed of synthetic organic compounds (i.e. composed mainly of carbon, hydrogen, oxygen and nitrogen). They are manufactured from raw materials, which are ultimately derived from coal and petroleum products (e.g. anthracene, naphthalene) that were once living matter.
Getting the Dye into the Fiber
For all fibers, the basic process of dyeing is the same. The fiber is placed in a liquid that contains the dye and the dye molecules infuse into the amorphous regions of the fiber polymer system.
Dye molecules bound in the amorphous regions of the fiber polymer system [1].
The most ubiquitous carrier of a dye is water, and so it is used wherever possible. Needless to say, for non-polar fibers, non-polar dyes need to be employed. Generally, non-polar dyes are not soluble in water. There are three approaches that can be readily adopted, and they are as follows:
(i) Chemically change the insoluble dye into a water soluble form (i.e. leuco form that is usually white in color) and then once the soluble form has migrated into the fiber polymer system, chemically alter the leuco form back into the colored non-soluble form. This approach is typically used for insoluble sulfur dyes. It has the further advantage that since the dye embedded in the fiber polymer system is water repelling, such dyed fabrics have very good to excellent wash-fastness.
(ii) Using another solvent, other than water, that will dissolve the dye. As attractive as this proposition appears, it is generally not employed by artists or crafters since generally such solvents are expensive, may be toxic and so are not easily disposed.
(iii) Create an emulsion (water plus dye mixture) that can be applied to the fabric and that will carry the dye into the fiber polymer system. This approach is used in the case of non-polar disperse dyes. Note: An emulsion is a suspension of tiny droplets of the non-toxic solvent holding the dye in water. For example, oils when shaken in water form emulsions.
Solubility
With enough polar groups in the structure of a dye molecule, even large size dye molecules will be water soluble. For example, Acid Red 88 and Acid Red 13 have very polar SO3- group(s) in their structure (see below). The former dye only has one such group, whereas the latter dye has two and so Red 13 would be predicted to be more soluble in water than Red 88.
Acid Red 88 And Acid Red 13.
Note: These are "azoic" dyes due to the chromophore (i.e. the chemical group that causes the red color) is the azo link (-N=N-). They also possess an auxochrome (i.e chemical group that intensifies the red color) namely the hydroxyl (-OH) group [1].
As in the case of the dyes above, often NaSO3- is used as solubilizing groups for dyes. However, care must be taken that in solubilizing dyes that the dye-water affinity or attraction does not exceed the dye-fiber polymer attraction, otherwise the dye will not want to leave the water solution and migrate into the fiber polymer system (producing poor dye uptake) and moreover, if it does migrate with the water into the fiber polymer system, it may color a fabric that has poor wash-fastness; that is, on laundering it will leave the fabric since its affinity to water is higher than to the fabric.
The Dynamics of Sorption
Dye molecules need to leave the solvent (usually water but in sublimation dyeing it is a paper sheet) and migrate into the amorphous regions of the fiber polymer system. This process must take time since it depends on the characteristics of the dye, the fiber polymer system and the liquid medium. The rate of dye uptake by the fabric is a dynamic process: as dye molecules enter the fiber polymer system (absorption) other dye molecules will exit (desorption) and so over a period of time, ideally most of the dye will be absorbed with little being desorbed.
What characteristics assist the adsorption of dye molecules, will not assist desorption and vice versa. For example, once the dye molecule is in the fiber polymer system, forces of bonding and attraction - such as hydrogen bonding and van der Waals forces of attraction - will cohere the dye molecule to the fiber polymer system and so decrease its chances of leaving the fiber. On the other hand, if these forces of attraction are too weak and the dye molecule has a strong affinity for water, dye uptake by the fabric will be poor.
Heating the dye liquor can assist dye uptake, since it will enlarge the size of the voids or spaces within the amorphous regions of the fiber polymer system, enabling easier entry of dye molecules and moreover, not restricting entry of very large sized dye molecules. On the other hand, if the dye molecules are small in size, then heating the bath may be sufficient for dye molecules to move more easily into the fiber polymer system, but if the forces of attraction are weak, the same heat will allow them to break these forces and so to re-emerge in the dye bath and therefore out of the fiber polymer system, which would result in a poor dye uptake of the fabric.
Very strong attractive forces between the dye molecule and the fiber polymer system can create streaky or unlevel dyeing. This may be overcome, somewhat, by making sure the dye liquor is circulated exceptionally well and evenly to all parts of the fabric to be dyed. If the dye molecules can readily absorb and desorb, then there is a greater probability that by the end of the dyeing session, color will be evenly distributed throughout the fabric.
Dynamics of sorption.
Note: Unless the dye is bonded to the fiber polymer system, it will absorb and desorb from the fabric in a reversible manner. Good dyeing is an irreversible process in favor of dye uptake by the fabric [1].
If dye molecules are charged ions - as is indeed all water soluble dyes - then it is possible to force them out of the water by adding salt to the dye bath. The water molecules are more attracted to the ions of the dissolved salt than they are to large dye molecules. This also makes it harder for the dye molecules to desorb from the fiber polymer system and so the addition of salt, helps to exhaust all the dye from the dye bath onto the fabric.
Addition of salt.
Note: As salt is added the attraction between dye molecules, although weak (van Der Waals forces of attraction) are nevertheless no longer disrupted by the attraction of water (which is now more attracted to the salt ions) and so this forces the dye molecules out of solution and to form large flocs [1].
Penetrating the Fiber
The dye molecules are initially attracted to the surface of the fiber due to a range of different mechanisms that are specific to the properties of the fiber and the dye molecules. For example, different types of dye molecules can be attracted to ionic, polar or non-polar sites on the surface of the fibers etc. Once on the surface, the dye molecules can be bumped back into the dye liquor or can penetrate further into the fiber polymer system. This is because the fiber has a consistent composition throughout.
Rarely are the voids or spaces in the crystalline regions of the fiber polymer system large enough, even upon heating the dye liquor, to enable penetration of the dye molecules into these regions. However, the disordered parts of the fiber polymer system - the so called amorphous regions - have much larger voids or spaces, which are further enlarged on heating. They are enlarged when the dye liquor is heated, because the polymer units (which can be imagined as strings) vibrate more violently (just as guitar strings do when they are plucked) and so at different points in time, have large voids and other points in time, the voids get larger and smaller as the polymer units or strings part or come together. Hence, these spaces or voids can be imagined as a breathing action - becoming larger and smaller but breathing in a completely random fashion. When the voids get large, dye molecules enter and then are entangled and entrapped in these voids. They are entrapped, because the voids at any point in time can become too small to let them out. They are entangled, since when the voids get large enough for them to exit, they are attached to the fiber polymer system via bonding (e.g. covalent or ionic and/or hydrogen bonds) or by forces of attraction (polar or van der Waals forces) that may prevent them to escape. However, high temperatures of the dye liquor also allows for these forces of attraction to be broken and if the exit voids are large at the time of breakage, the dye molecule can be desorbed from the fiber polymer system.
The increased space in the swollen fiber and the increased rate of desorption are finely balanced against one another to allow a certain proportion of the dye molecules to be absorbed into the fiber. The following graph shows the effect of increased temperature on absorption of two different dyes by cotton fibers. For dye B, there is clearly an optimum temperature for the dye liquor.
The effect of temperature of dye uptake by cotton with two dyes, A and B - see reference [1]
Keeping the Dye in the Fiber
There are several types of forces that cohere the dye molecules to the fiber polymer system. For example, some fibers have charged sites, which hold the dye molecule via ionic bonding. Other dye molecules form strong covalent bonds with chemical groups on the polymer backbone. Many dyes have polar groups, which can form hydrogen bonding with similar polar groups on the fiber polymer system. Dyes are also held via weak forces of attraction, such as van der Waals forces.
Small sized dye molecules can easily move in and out of the amorphous regions of the fiber polymer system. Hence, we can increase the molecular size of these dyes by attaching a metal to them such as chromium (e.g. mordants or pre-metallized dyes) or by synthesizing much larger versions, where the dye molecule becomes a component in a much larger molecular complex (e.g. azoic dyes).
On occasions, the solubility of the dye in water is changed after absorption into the fiber polymer system (such as for vat and sulfur dyes). Once in the fiber these dyes are changed to a colored insoluble form and so become water repelling, thereby increasing their wash-fastness.
Other mechanisms to hold them into the fiber is to synthesize dye molecules, which can effectively align themselves in parallel fashion in between the fiber polymer systems, thereby increasing the number and strength of cohesive forces to the fiber. Such an effect is achieved during soaping of vat and azoic dyes.
Reference:
[1] A Fritz and J. Cant, Consumer Textiles, Oxford University Press, Melbourne (1986).
This is the ninety-ninth 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
In order to understand the coloring of textile materials, we need to have some understanding of the general theory of dyeing and printing. After reading this post and if you are unsure of a definition, terminology or property of a fabric that this post assumes you should know and you don't, then study the "Preamble" above and click on any art resource post or glossary that might provide that missing information for you.
This post will continue the general theory of dyeing.
Dye Sources
Many colorants (and in fact chemicals) were once obtained from living matter. Cochineal was obtained from an insect, purple was extracted from a sea snail, and a vast number of colorants were extracted from a range of plants. Plant-derived dyes are still widely used by craft and ArtCloth practitioners.
A number of colorants were based on naturally occurring mineral compounds. These are mainly used as pigments. For example, ochres are pigments that the Australian Aboriginals used in their bark paintings.
Australian aboriginal ochre painting.
In the modern era, most colorants are synthesized and manufactured. Most dyes are composed of synthetic organic compounds (i.e. composed mainly of carbon, hydrogen, oxygen and nitrogen). They are manufactured from raw materials, which are ultimately derived from coal and petroleum products (e.g. anthracene, naphthalene) that were once living matter.
Getting the Dye into the Fiber
For all fibers, the basic process of dyeing is the same. The fiber is placed in a liquid that contains the dye and the dye molecules infuse into the amorphous regions of the fiber polymer system.
The most ubiquitous carrier of a dye is water, and so it is used wherever possible. Needless to say, for non-polar fibers, non-polar dyes need to be employed. Generally, non-polar dyes are not soluble in water. There are three approaches that can be readily adopted, and they are as follows:
(i) Chemically change the insoluble dye into a water soluble form (i.e. leuco form that is usually white in color) and then once the soluble form has migrated into the fiber polymer system, chemically alter the leuco form back into the colored non-soluble form. This approach is typically used for insoluble sulfur dyes. It has the further advantage that since the dye embedded in the fiber polymer system is water repelling, such dyed fabrics have very good to excellent wash-fastness.
(ii) Using another solvent, other than water, that will dissolve the dye. As attractive as this proposition appears, it is generally not employed by artists or crafters since generally such solvents are expensive, may be toxic and so are not easily disposed.
(iii) Create an emulsion (water plus dye mixture) that can be applied to the fabric and that will carry the dye into the fiber polymer system. This approach is used in the case of non-polar disperse dyes. Note: An emulsion is a suspension of tiny droplets of the non-toxic solvent holding the dye in water. For example, oils when shaken in water form emulsions.
Solubility
With enough polar groups in the structure of a dye molecule, even large size dye molecules will be water soluble. For example, Acid Red 88 and Acid Red 13 have very polar SO3- group(s) in their structure (see below). The former dye only has one such group, whereas the latter dye has two and so Red 13 would be predicted to be more soluble in water than Red 88.
Note: These are "azoic" dyes due to the chromophore (i.e. the chemical group that causes the red color) is the azo link (-N=N-). They also possess an auxochrome (i.e chemical group that intensifies the red color) namely the hydroxyl (-OH) group [1].
As in the case of the dyes above, often NaSO3- is used as solubilizing groups for dyes. However, care must be taken that in solubilizing dyes that the dye-water affinity or attraction does not exceed the dye-fiber polymer attraction, otherwise the dye will not want to leave the water solution and migrate into the fiber polymer system (producing poor dye uptake) and moreover, if it does migrate with the water into the fiber polymer system, it may color a fabric that has poor wash-fastness; that is, on laundering it will leave the fabric since its affinity to water is higher than to the fabric.
The Dynamics of Sorption
Dye molecules need to leave the solvent (usually water but in sublimation dyeing it is a paper sheet) and migrate into the amorphous regions of the fiber polymer system. This process must take time since it depends on the characteristics of the dye, the fiber polymer system and the liquid medium. The rate of dye uptake by the fabric is a dynamic process: as dye molecules enter the fiber polymer system (absorption) other dye molecules will exit (desorption) and so over a period of time, ideally most of the dye will be absorbed with little being desorbed.
What characteristics assist the adsorption of dye molecules, will not assist desorption and vice versa. For example, once the dye molecule is in the fiber polymer system, forces of bonding and attraction - such as hydrogen bonding and van der Waals forces of attraction - will cohere the dye molecule to the fiber polymer system and so decrease its chances of leaving the fiber. On the other hand, if these forces of attraction are too weak and the dye molecule has a strong affinity for water, dye uptake by the fabric will be poor.
Heating the dye liquor can assist dye uptake, since it will enlarge the size of the voids or spaces within the amorphous regions of the fiber polymer system, enabling easier entry of dye molecules and moreover, not restricting entry of very large sized dye molecules. On the other hand, if the dye molecules are small in size, then heating the bath may be sufficient for dye molecules to move more easily into the fiber polymer system, but if the forces of attraction are weak, the same heat will allow them to break these forces and so to re-emerge in the dye bath and therefore out of the fiber polymer system, which would result in a poor dye uptake of the fabric.
Very strong attractive forces between the dye molecule and the fiber polymer system can create streaky or unlevel dyeing. This may be overcome, somewhat, by making sure the dye liquor is circulated exceptionally well and evenly to all parts of the fabric to be dyed. If the dye molecules can readily absorb and desorb, then there is a greater probability that by the end of the dyeing session, color will be evenly distributed throughout the fabric.
Note: Unless the dye is bonded to the fiber polymer system, it will absorb and desorb from the fabric in a reversible manner. Good dyeing is an irreversible process in favor of dye uptake by the fabric [1].
If dye molecules are charged ions - as is indeed all water soluble dyes - then it is possible to force them out of the water by adding salt to the dye bath. The water molecules are more attracted to the ions of the dissolved salt than they are to large dye molecules. This also makes it harder for the dye molecules to desorb from the fiber polymer system and so the addition of salt, helps to exhaust all the dye from the dye bath onto the fabric.
Note: As salt is added the attraction between dye molecules, although weak (van Der Waals forces of attraction) are nevertheless no longer disrupted by the attraction of water (which is now more attracted to the salt ions) and so this forces the dye molecules out of solution and to form large flocs [1].
Penetrating the Fiber
The dye molecules are initially attracted to the surface of the fiber due to a range of different mechanisms that are specific to the properties of the fiber and the dye molecules. For example, different types of dye molecules can be attracted to ionic, polar or non-polar sites on the surface of the fibers etc. Once on the surface, the dye molecules can be bumped back into the dye liquor or can penetrate further into the fiber polymer system. This is because the fiber has a consistent composition throughout.
Rarely are the voids or spaces in the crystalline regions of the fiber polymer system large enough, even upon heating the dye liquor, to enable penetration of the dye molecules into these regions. However, the disordered parts of the fiber polymer system - the so called amorphous regions - have much larger voids or spaces, which are further enlarged on heating. They are enlarged when the dye liquor is heated, because the polymer units (which can be imagined as strings) vibrate more violently (just as guitar strings do when they are plucked) and so at different points in time, have large voids and other points in time, the voids get larger and smaller as the polymer units or strings part or come together. Hence, these spaces or voids can be imagined as a breathing action - becoming larger and smaller but breathing in a completely random fashion. When the voids get large, dye molecules enter and then are entangled and entrapped in these voids. They are entrapped, because the voids at any point in time can become too small to let them out. They are entangled, since when the voids get large enough for them to exit, they are attached to the fiber polymer system via bonding (e.g. covalent or ionic and/or hydrogen bonds) or by forces of attraction (polar or van der Waals forces) that may prevent them to escape. However, high temperatures of the dye liquor also allows for these forces of attraction to be broken and if the exit voids are large at the time of breakage, the dye molecule can be desorbed from the fiber polymer system.
The increased space in the swollen fiber and the increased rate of desorption are finely balanced against one another to allow a certain proportion of the dye molecules to be absorbed into the fiber. The following graph shows the effect of increased temperature on absorption of two different dyes by cotton fibers. For dye B, there is clearly an optimum temperature for the dye liquor.
Keeping the Dye in the Fiber
There are several types of forces that cohere the dye molecules to the fiber polymer system. For example, some fibers have charged sites, which hold the dye molecule via ionic bonding. Other dye molecules form strong covalent bonds with chemical groups on the polymer backbone. Many dyes have polar groups, which can form hydrogen bonding with similar polar groups on the fiber polymer system. Dyes are also held via weak forces of attraction, such as van der Waals forces.
Small sized dye molecules can easily move in and out of the amorphous regions of the fiber polymer system. Hence, we can increase the molecular size of these dyes by attaching a metal to them such as chromium (e.g. mordants or pre-metallized dyes) or by synthesizing much larger versions, where the dye molecule becomes a component in a much larger molecular complex (e.g. azoic dyes).
On occasions, the solubility of the dye in water is changed after absorption into the fiber polymer system (such as for vat and sulfur dyes). Once in the fiber these dyes are changed to a colored insoluble form and so become water repelling, thereby increasing their wash-fastness.
Other mechanisms to hold them into the fiber is to synthesize dye molecules, which can effectively align themselves in parallel fashion in between the fiber polymer systems, thereby increasing the number and strength of cohesive forces to the fiber. Such an effect is achieved during soaping of vat and azoic dyes.
Reference:
[1] A Fritz and J. Cant, Consumer Textiles, Oxford University Press, Melbourne (1986).
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