An Introduction to Textile Coloration E-Book

Roger H. Wardman

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Serie: SDC - Society of Dyers and Colourists
• Sprache: Englisch

Seitenzahl: 625

Veröffentlichungsjahr: 2017

Beschreibung

An Introduction to Textile Coloration: Principles and Practice

The Publications Committee of the Society of Dyers and Colourists (SDC) has been aware for some time of the need to produce a book at an introductory level aimed at personnel working in textile dyeing or printing companies as well as those interested in entering into the field. The SDC runs a course for dyehouse technicians leading to the award of its Textile Coloration Certificate and this book is intended to be helpful for candidates following the course.  Additionally, it will be helpful for professionals in textile companies who do not have a strong scientific background, so that they may attain a better understanding of the chemical principles of colour application.

Starting with the basic science underlying dyeing and printing processes, this comprehensive book explains the fundamentals of dye and pigment chemistry and the various application techniques and processes. It offers chapter coverage of the general chemistry related to textiles, textile fibres, chemistry of dyes and pigments, industrial coloration methods, textile printing, theoretical aspects of dyeing, the measurement of colour and fastness testing. Reference is made to developments that have taken place in the coloration industry in recent years, not least of which have been the challenges imposed by the drive towards environmentally-friendly processes and restrictions on the use of certain chemicals.

An Introduction to Textile Coloration: Principles and Practice

• Covers atomic structure, chemical reactions, and acids, bases, and salts
• Explains the nature of fibre-forming polymers and the conversion of synthetic polymers into fibre filaments
• Educates on the classification of colorants and the commercial naming of dyes and pigments
• Introduces readers to the dye application processes and dyeing machinery
• Instructs on dye aggregation, factors affecting colour appearance, the principles of colour fastness testing, and more

“…this is the sort of book any dyer, technician, student, academic will want to always have as an ready reference to everything pertaining to textile coloration.”

Richard S. Blackburn, School of Design, University of Leeds, Leeds, LS2 9JT, UK

Leseprobe

Table of Contents

Cover

Title Page

Society of Dyers and Colourists

Preface

1 General Chemistry Related to Textiles

1.1 Introduction

1.2 Atomic Structure

1.3 Periodic Table of the Elements

1.4 Valency and Bonding

1.5 Chemical Reactions

1.6 Acids, Bases and Salts

1.7 Redox Reactions

1.8 Organic Chemistry

1.9 The Use of Chemicals by Industry

2 Textile Fibres

2.1 Introduction

2.2 Nature of Fibre‐Forming Polymers

2.3 Properties of Textile Fibres

2.4 Mechanical Properties of Textile Fibres

2.5 Chemistry of the Main Fibre Types

2.6 Protein Fibres

2.7 Regenerated Fibres

2.8 Synthetic Fibres

2.9 Conversion of Synthetic Polymers into Fibre Filaments

2.10 Fibre Cross‐Sectional Shapes

2.11 Microfibres

2.12 Absorbent Fibres

2.13 Drawing of Synthetic Fibre Filaments

2.14 Conversion of Man‐Made Fibre Filaments to Staple

2.15 Imparting Texture to Synthetic Fibres

2.16 Fibre Blends

2.17 Textile Manufacturing

Suggested Further Reading

3 Chemistry of Dyes and Pigments

3.1 Introduction

3.2 Classification of Colorants

3.3 Colour in Organic Molecules

3.4 Classification of Dyes According to Chemical Structure

3.5 Classification of Dyes According to Application Class

3.6 Commercial Naming of Dyes and Pigments

3.7 Strength and Physical Form of Colorants

References

4 Industrial Coloration Methods

4.1 Introduction

4.2 Dye Application Processes

4.3 Dyeing Machinery

4.4 Supercritical Fluid Dyeing

References

Suggested Further Reading

5 Textile Printing

5.1 Introduction

5.2 Print Paste Formulation

5.3 Thickeners

5.4 Binders

5.5 Pigments and Dyes

5.6 Printing Screens

5.7 Stages of Printing

5.8 Printing Styles

5.9 Printing Methods

Suggested Further Reading

6 Theoretical Aspects of Dyeing

6.1 Introduction

6.2 Kinetic Aspects of Dyeing

6.3 Dye Aggregation

6.4 Diffusion

6.5 Rate of Dyeing

6.6 Adsorption

6.7 Thermodynamic Information Derived from Equilibrium Studies of Dyeing Systems

References

Suggested Further Reading

7 The Measurement of Colour

7.1 Introduction

7.2 Describing Colour

7.3 Additive and Subtractive Colour Mixing

7.4 The Colour Solid

7.5 Factors Affecting Colour Appearance

7.6 The CIE System of Colour Specification

7.7 Applications of the CIE System

7.8 Solution Colour Measurement

Suggested Further Reading

8 Fastness Testing

8.1 Introduction

8.2 Standards Related to Coloration

8.3 Resistance of Coloured Fabric to Harmful Agencies

8.4 Principles of Colour Fastness Testing

8.5 Fastness Tests

8.6 Test Organisations for Sustainable Textile Manufacture

References

Appendix: Some Textile Terms and Definitions

Index

End User License Agreement

List of Tables

Chapter 01

Table 1.1 Filling of the shells by electrons in the first 30 elements.

Table 1.2 Common functional groups in organic compounds.

Chapter 02

Table 2.1 Codes for some man‐made fibres.

Table 2.2 Worldwide production of some textile fibre types, thousand metric tonnes, 2014.

Table 2.3 Important fibre properties.

Table 2.4 Moisture regain values of some textile fibres (measured at 20 °C and 65% relative humidity).

Table 2.5 Examples of different types of amino acids occurring in protein fibres.

Chapter 03

Table 3.1 Colour Index application classes.

Table 3.2 Example of the information given in a CIGN entry in the Colour Index.

Table 3.3 Characteristics of acid dyes.

Table 3.4 Characteristics of fibre‐reactive groups.

Chapter 04

Table 4.1 Summary of dye types used on wool.

Chapter 05

Table 5.1 Guidelines for the temperature and pressure of dry saturated steam.

Chapter 06

Table 6.1 Aggregation behaviour of dye application classes.

Table 6.2 Size/charge ratios of dyes showing different aggregation behaviour.

Table 6.3 Diffusion coefficients of two disperse dyes on different fibres [2].

Table 6.4 Activation energies of diffusion of different dye/fibre systems [4].

Table 6.5 Time of half‐dyeing of direct dyes on viscose dyed at 90 °C [6].

Table 6.6 Structures of some typical anionic dyes.

Table 6.7 Standard affinities of some direct dyes applied to cotton and viscose at various temperatures [9].

Table 6.8 Standard affinities of C.I. Direct Blue 1 applied to viscose and Tencel at 100 °C, in the presence of 0.02 mol/l NaCl (viscose) and 0.08 mol/l NaCl (Tencel) [10, 11].

Table 6.9 Standard affinities of some polycyclic vat dyes applied to cotton at 40 °C [9, 12].

Table 6.10 Thermodynamic values for some anthraquinone dyes applied to wool at 50 °C and pH 4.6 [13].

Table 6.11 Standard affinities of a series of dyes of increasing hydrophobic character applied to nylon 6.6 at 95 °C at pH 6.1 [14].

Table 6.12 Effect of increasing basicity on standard affinity of acid dyes on nylon 6.6.

Chapter 07

Table 7.1 Colour temperatures of typical light sources.

Table 7.2 Weighted ordinates for Illuminant D65, 10° observer (values are shown at 20 nm intervals).

Table 7.3 Calculation of tristimulus values (Illuminant D65, 10° observer) for a green‐coloured fabric.

Table 7.4 Tristimulus values of the perfect reflecting diffuser viewed under different illuminants (10° observer).

Table 7.5 Values of absorbance for different values of %

T

.

Chapter 08

Table 8.1 Colour fastness standards.

Table 8.2 ISO 105‐A series of standards.

Table 8.3 Colour‐difference values of the grey scale for change in colour and staining.

Table 8.4 Terms used for the qualitative description of colour changes.

Table 8.5 ISO 105‐B series of standard tests for light fastness.

Table 8.6 ISO 105‐C series of standard tests for wash fastness.

Table 8.7 Standard adjacent fabrics for use in fastness testing.

Table 8.8 Multifibre adjacent fabrics.

Table 8.9 Conditions specified in BS EN ISO 105‐C10 standard test for wash fastness.

List of Illustrations

Chapter 01

Figure 1.1 The three p orbitals.

Figure 1.2 Periodic table of elements.

Figure 1.3 Electronic arrangements of the sodium and chlorine atoms (the ↑↓ arrows indicate electrons with opposite spins).

Scheme 1.1 Reaction between sodium and chlorine.

Scheme 1.2 Reaction between calcium and chlorine atoms.

Scheme 1.3 Dissociation of the sulphonate group.

Scheme 1.4 Formation of the chlorine molecule.

Scheme 1.5 Formation of methane (outer electrons only are shown); x electron from carbon, • electron from hydrogen.

Figure 1.4 Tetrahedral molecular structure of the methane (CH

4

) molecule.

Figure 1.5 Electronic structure of the carbon atom.

Figure 1.6 Electronic structure of the sp

3

‐hybridised carbon atom.

Scheme 1.6 Covalent bonding by nitrogen and oxygen atoms with hydrogen atoms.

Figure 1.7 Dipole–dipole attraction between molecules.

Figure 1.8 Hydrogen bonding in water (─ covalent bond, ‐‐‐‐ hydrogen bond).

Figure 1.9 π–H bonding.

Scheme 1.7 General formula representing a synthesis reaction.

Scheme 1.8 Formation of ammonia from nitrogen and hydrogen.

Scheme 1.9 General formula representing a decomposition reaction.

Scheme 1.10 Decomposition of calcium carbonate.

Scheme 1.11 Single and double displacement reactions.

Scheme 1.12 General formula representing a general reaction in which a reactant changes to a product.

Scheme 1.13 General formula representing a reversible reaction.

Figure 1.10 Effect of temperature on the energy of molecules: blue line low temperature; red line high temperature.

E

is the activation energy.

Figure 1.11 Energy pathway of a chemical reaction. (a) An exothermic reaction and (b) an endothermic reaction.

Scheme 1.14 Formation of carbon dioxide from carbon and oxygen.

Scheme 1.15 Burning of methane in oxygen.

Scheme 1.16 Reduction of vat dyes.

Scheme 1.17 Redox reactions of sulphur dyes.

Figure 1.12 Zigzag form of hydrocarbon chains.

Figure 1.13 sp

2

hybridisation.

Figure 1.14 σ and π bonds in ethane.

Scheme 1.18 Dissociation of carboxylic acids.

Scheme 1.19 Formation of esters.

Scheme 1.20 Formation of amides.

Chapter 02

Figure 2.1 The textile and clothing industries and their supporting functions.

Figure 2.2 Classes of textile fibres.

Figure 2.3 Normal and skewed distributions of molecular masses in polymers.

Figure 2.4 Crystalline and amorphous structure of polymer molecules in fibres.

Figure 2.5 Typical stress–strain diagrams of (a) natural and (b) synthetic fibres.

Scheme 2.1 Formation of cellobiose from two glucose units.

Figure 2.6 Cotton bolls.

Figure 2.7 Cotton fibre, (a) structure represented diagrammatically, (b) as a scanning electron photograph.

Scheme 2.2 Degradation products due to over‐oxidation of the glucose rings of cellulose.

Scheme 2.3 Formation of a dipeptide from two amino acid molecules.

Scheme 2.4 Growth of a polypeptide chain.

Figure 2.8 (a) Diagrammatic representation of the structure of a wool fibre. (b) Electron microscope photograph of a merino wool fibre.

Figure 2.9 Distribution of the orthocortex and paracortex in a wool fibre.

Scheme 2.5 Degradation of the polypeptide chain in wool by alkali (or strong acid).

Figure 2.10 Rupture and re‐formation of cross‐links.

Figure 2.11 Stages of manufacture of viscose and lyocell fibres (the pink arrows show the use and recovery of the NMMO solvent).

Scheme 2.6 Synthetic route for the formation of polylactic acid.

Figure 2.12 Routes to the formation of synthetic fibres.

Figure 2.13 The stepwise construction of a ‘hook and eye’ chain.

Scheme 2.7 Reaction between a dicarboxylic acid and a diamine.

Scheme 2.8 Formation of a nylon polymer.

Scheme 2.9 Formation of nylon 6.6.

Scheme 2.10 Formation of nylon 6.

Scheme 2.11 Synthesis of (a) a

m

‐aramid, for example, Nomex, and (b) a

p

‐aramid, for example, Kevlar.

Scheme 2.12 Synthesis of polyester.

Scheme 2.13 Polymerisation of ethene to give polyethylene.

Scheme 2.14 Addition polymerisation of cyanoethene.

Figure 2.14 Different methods of fibre spinning: (a) melt spinning, (b) dry spinning, (c) wet spinning, (d) dry‐jet wet spinning.

Figure 2.15 Orifice shape and the cross sections of the resulting filaments.

Figure 2.16 Drawing fibre filaments.

Figure 2.17 Changes in molecular orientation in filaments during drawing: (a) at the filament emerges from the spinneret, orientation is mainly random (amorphous); (b) on cooling and stretching some crystalline order appears, with a degree of general orientation along the fibre axis; (c) further stretching aligns amorphous and crystalline regions along the fibre axis.

Figure 2.18 Schematic diagrams of fibre crimping methods: (a) stuffer box, (b) knife edge, (c) Gear crimp, (d) air jet, (e) false twist.

Figure 2.19 S‐twist and Z‐twist.

Figure 2.20 Winding from bobbins on to a beam.

Figure 2.21 Rear view of a loom showing the beam and warped threads.

Figure 2.22 Plain weave types (a) plain weave – one over one then one under one, (b) hopsack two over two then two under two.

Figure 2.23 (a) 2 × 1 twill weave, (b) 2 × 2 twill weave.

Figure 2.24 Satin weaves (a) warp‐faced satin, (b) weft‐faced satin.

Figure 2.25 Structure of a weft knitted fabric.

Figure 2.26 A warp knitting machine.

Figure 2.27 Structure of a warp knitted fabric.

Chapter 03

Scheme 3.1 Diazotisation of primary amines to form a diazonium salt.

Scheme 3.2 Diazo coupling to form an azo dye.

Scheme 3.3 Coupling reaction under acid and alkaline conditions.

Scheme 3.4 Synthesis of anthraquinone by oxidation of anthracene.

Figure 3.1 Electronic configurations of states of chromium.

Figure 3.2 Relationship between migrating power and wet fastness properties of acid dyes.

Scheme 3.5 Nucleophilic substitution and nucleophilic addition reactions of α‐bromoacrylamido reactive dyes with protein fibres.

Scheme 3.6 Nucleophilic substitution reaction of chlorodifluoropyrimidine reactive dyes with protein fibres.

Scheme 3.7 Nucleophilic addition reaction of vinyl sulphone reactive dyes with protein fibres.

Scheme 3.8 Intramolecular hydrogen bonding in a vat dye molecule.

Scheme 3.9 Formation of a sulphuric ester of a leuco vat dye.

Figure 3.3 General structure of a fibre‐reactive dye.

Scheme 3.10 Nucleophilic substitution reaction of chlorotriazinyl reactive dyes with cellulosic fibres.

Scheme 3.11 Nucleophilic addition reaction of vinyl sulphone reactive dyes with cellulosic fibres.

Scheme 3.12 Synthesis of C.I. Sulphur Black 1.

Chapter 04

Scheme 4.1 Adsorption of hydrogen ions by protein fibres.

Scheme 4.2 Adsorption of dye anions by protein fibres under acid conditions.

Figure 4.1 Dyeing profile for the application of acid dyes to wool. 1: Add Na

2

SO

4

.10H

2

O, 5–10% owf. 2: Add dyes. 3: Boil. 4: Rinse.

Scheme 4.3 Adsorption of dichromate ions by wool under acid conditions.

Scheme 4.4 Reduction of adsorbed dichromate ions by wool under acid conditions.

Figure 4.2 Change in shade of C.I. Mordant Blue 9 during after‐chroming.

Figure 4.3 Dyeing cycle for application of reactive dyes to wool.

Figure 4.4 Typical dyeing profiles for the three classes of direct dyes.

Figure 4.5 On‐tone build‐up of a trichromatic mixture of direct dyes belonging to the same class.

Scheme 4.5 Chemistry of vat dyeing – vatting to generate the soluble leuco form and subsequent oxidation back to the parent insoluble dye.

Scheme 4.6 Vatting of indigo.

Scheme 4.7 Oxidation of sodium dithionite during vat dyeing process.

Figure 4.6 Typical dyeing profiles for reactive dyes on cellulosic fibres.

Scheme 4.8 Reduction reaction of sulphur dyes.

Scheme 4.9 Mechanism of adsorption of disperse dyes into hydrophobic fibres.

Figure 4.7 Relationship between sublimation temperature and dyeing properties.

Figure 4.8 Schematic representation of the pad‐Thermosol method.

Figure 4.9 Dyeing profiles of (a) groups 1 and 2 dyes and (b) group 3 dyes.

Scheme 4.10 Protonation of amino end groups in nylon fibres under acid conditions.

Scheme 4.11 Reaction of chlorotriazinyl reactive dyes with nylon fibres by nucleophilic substitution.

Scheme 4.12 Reaction of vinyl sulphone reactive dyes with polyamide fibres by nucleophilic addition.

Scheme 4.13 Ion‐exchange mechanism of dyeing acrylic fibres with basic (cationic) dyes.

Figure 4.10 Schematic diagram of a conical pan dyeing machine.

Figure 4.11 Nine cages, each containing five bump tops, ready to be hoist into the dyeing machine.

Figure 4.12 Bump tops after dyeing.

Figure 4.13 Cheese packages on rigid plastic formers, after dyeing. In the background are dyed wool tops.

Figure 4.14 Packages of wool yarn on the spindles of a package dyeing machine of 900 kg capacity.

Figure 4.15 Loading a package dyeing machine.

Figure 4.16 Horizontal loading package dyeing machine.

Figure 4.17 A radio‐frequency drying machine.

Figure 4.18 Schematic diagram of a Hussong hank dyeing machine.

Figure 4.19 Small‐scale hank dyeing machine.

Figure 4.20 Winch dyeing machine for multiple lengths of fabric.

Figure 4.21 Schematic diagram of a jig dyeing machine.

Figure 4.22 High‐pressure jig dyeing machine.

Figure 4.23 Sectional diagram of a high‐temperature beam dyeing machine.

Figure 4.24 Jet dyeing machines.

Figure 4.25 Principle of the soft‐flow jet dyeing machine.

Figure 4.26 Schematic diagram of a side‐paddle garment dyeing machine. (a) Top view. (b) Side view.

Figure 4.27 Rotary drum garment dyeing machine.

Figure 4.28 Principle of pad mangle operation: (a) two‐bowl mangle, (b) three‐bowl mangle.

Figure 4.29 Phase diagram for carbon dioxide.

Figure 4.30 DyeCoo scCO

2

beam dyeing machines installed at Yeh Group factory in Thailand.

Chapter 05

Scheme 5.1 Conversion of starch to British gum showing increased branching of the polymer chains following conversion.

Figure 5.1 Print screen registration.

Figure 5.2 Galvano rotary screen construction.

Figure 5.3 Etched grooves on a copper roller.

Figure 5.4 Engraved cells and depth of shade on a copper roller.

Figure 5.5 The discharging of the background colour during discharge printing.

Figure 5.6 Example of an illuminated discharge print.

Figure 5.7 Schematic of a flat screen printing machine. Rollers 1 and 2 maintain the motion of the blanket. Roller 3 is a pressure roller. Roller 4 applies adhesive to the blanket. Roller 5 is the blanket washer.

Figure 5.8 Commercial flat screen printing.

Figure 5.9 Cross‐sectional diagram of a rotary screen.

Figure 5.10 Commercial rotary screen printing.

Figure 5.11 Diagram of a two‐colour roller printer. A, pressure bowl (impression cylinder); B, endless printing blanket; C, back grey; D, fabric being printed; E, engraved printing cylinder; F, furnishing roller; G, colour box; H, colour doctor; K, lint doctor.

Figure 5.12 Continuous transfer printing.

Figure 5.13 Continuous ink jet printing system.

Figure 5.14 Diagram of a thermal ink jet printing head.

Figure 5.15 Diagram of a piezo ink jet printing head.

Figure 5.16 Digital ink jet printing.

Chapter 06

Figure 6.1 Transfer of dye molecules from dye solution into a fibre. A: Transfer of molecules to the fibre surface. B: Adsorption of dye molecules at the fibre surface. C: Diffusion of dye molecules into the fibre matrix.

Figure 6.2 Transport of dye from bulk solution to interior of a fibre.

Figure 6.3 Diffusion of dye into a fibre from an infinite dyebath.

Figure 6.4 Diffusion of dye molecules into fibrous substrates. (a) and (b): pore model, (c) and (d): segmental‐mobility model.

Figure 6.5 Disruption of molecular stacking due to steric hindrance by bulky sulphonate groups.

Figure 6.6 Overlapping structures of dye molecules in aggregates.

Figure 6.7 Effect of dye aggregation on Beer’s law plots.

Figure 6.8 Absorbance spectrum of a dye forming a dimer at high concentrations.

Figure 6.9 Effect of salt concentration on dye aggregation.

Figure 6.10 Effect of temperature on the aggregation of C.I. Direct Blue 1.

Figure 6.11 The concentration gradient.

Figure 6.12 Rate of dyeing curve and time of half‐dyeing.

Figure 6.13 Illustration of a situation in which two different dyes of different build‐up properties have the same

t

1/2

values.

Figure 6.14 Effect of temperature on dyebath exhaustion.

Figure 6.15 Ionic attractions between dyes and fibres. (a) Wool or nylon fibre. (b) Acrylic fibre.

Figure 6.16 Polar forces of attraction. (a) Dipole–dipole attraction, (b) dipole‐induced dipole attraction.

Figure 6.17 The Nernst isotherm.

Figure 6.18 The Freundlich isotherm.

Figure 6.19 The Langmuir isotherm.

Figure 6.20 Distribution of co‐ions and counter‐ions at a cellulose fibre surface. (a) Helmholtz model, (b) Gouy–Chapman model, (c) Stern model.

Figure 6.21 (a) Concentration of ions at a cellulosic fibre surface, (b) decrease in the zeta potential ξ on moving away from the fibre surface.

Figure 6.22 Attractive and repulsive forces between a cellulosic fibre and an anionic dye molecule.

Figure 6.23 Distribution of ions near a cellulosic fibre surface, in the case of unimolecular adsorption.

Figure 6.24 Distribution of sodium, chloride and dye ions on either side of a semipermeable membrane at equilibrium.

Figure 6.25 Diffuse adsorption of anionic dye ions on cellulose fibres.

Figure 6.26 Distribution of ions in an anionic dye/fibre system giving diffuse adsorption.

Figure 6.27 Distribution of ions in unimolecular adsorption, 3‐phase model.

Figure 6.28 Dye adsorption at specific sites in a fibre.

Chapter 07

Figure 7.1 Additive colour mixing – mixing red, green and blue lights.

Figure 7.2 Subtractive colour mixing.

Figure 7.3 The electromagnetic spectrum.

Figure 7.4 Sensitivity of the eye to visible light.

Figure 7.5 (a) The colour circle; (b) the colour solid.

Figure 7.6 Structure of the Munsell Book of Color.

Figure 7.7 The Munsell hue circle, showing the five principle and five secondary hues of hue pages 5 and 10 (the 2.5 and 7.5 hue pages are omitted).

Figure 7.8 Diagram of a sample page from the Munsell Book of Color (in the Munsell book the neutral samples, where Chroma = 0, are not shown to avoid unnecessary duplication; instead they are mounted on a separate card).

Figure 7.9 The viewing geometry.

Figure 7.10 Spectral power distributions of tungsten and average daylight.

Figure 7.11 Spectral power distribution of a normal fluorescent light.

Figure 7.12 Spectral power distribution of a white LED source.

Figure 7.13 CIE recommended illuminating/viewing geometries for the measurement of reflectance by spectrophotometers.

Figure 7.14 Measurement of reflectance with the specular component of reflection either included (left) or excluded (right).

Figure 7.15 Spectral reflectance curves of typical white, grey and black colours.

Figure 7.16 Spectral reflectance curves of typical chromatic colours.

Figure 7.17 Measured spectral reflectance curves of fluorescent colours used in high visibility clothing.

Figure 7.18 Measured spectral reflectance curves of white cotton fabrics.

Figure 7.19 Diagram of the horizontal cross section of the human eye.

Figure 7.20 Principle of additive colour mixing to match spectral light.

Figure 7.21 Results of colour‐matching wavelengths of visible light using real [R], [G], [B] primaries and location of the [X], [Y], [Z] imaginary primaries.

Figure 7.22 10° standard observer colour‐matching values.

Figure 7.23 Chromaticity diagram, 10° standard observer.

Figure 7.24 Reflectance curves of C.I. Acid Blue 40 applied to wool at different depths of shade.

Figure 7.25 (a) % Reflectance plotted against concentration (% o.w.f.) of C.I. Acid Blue 40 on wool, at wavelengths of minimum (480 nm) and maximum (620 nm) absorption of light; (b) corresponding graphs for the Kubelka–Munk function of reflectance (

K

/

S

) plotted against concentration of dye, o.w.f.

Figure 7.26 Dye colour (CIE

a

*,

b

*) map for selected vat dyes on cotton. A: C.I. Vat Yellow 2; B: C.I. Vat Orange 7; C: C.I. Vat Orange 17; D: C.I. Vat Brown 30; E: C.I. Vat Red 14; F: C.I. Vat Red 1; G: C.I. Vat Green 30; H: C.I. Vat Green 1; I: C.I. Vat Blue 6; J: C.I. Vat Violet 21.

Figure 7.27 Calculation of colour difference, Δ

E

between a standard and a batch. Δ

E

2

 = 

c

2

 + 

d

2

, and since

d

2

 = 

a

2

 + 

b

2

, Δ

E

2

 = 

c

2

 + 

a

2

 + 

b

2

.

Figure 7.28 Application of Pythagoras theorem to three‐dimensional

X

,

Y

,

Z

colour space.

Figure 7.29 The CIELAB colour space.

Figure 7.30 Tolerance limits of acceptability in CIELAB colour space.

Figure 7.31 Dimensions of the tolerance ellipsoid around a standard colour.

Figure 7.32 Reflectance curves of a metameric pair of grey colours.

Figure 7.33 SDC Metameric samples.

Figure 7.34 Using a colour‐difference formula as a metameric index.

Figure 7.35 The ‘555’ system of shade sorting.

Figure 7.36 A glass cell for solution colour measurement. For most measurements a cell path length of 1 cm is used.

Figure 7.37 Schematic diagram of a double‐beam recording spectrophotometer.

Figure 7.38 Absorbance curves for solutions of C.I. Acid Red 249, at the concentrations indicated.

Figure 7.39 Beer’s law plot for solutions of C.I. Acid Red 249 at 525 nm.

Chapter 08

Figure 8.1 Staining of adjacent fibres during washing.

Figure 8.2 Grey scales: (top) the scale used to assess staining and (bottom) the scale used for the assessment of colour change in the sample.

Figure 8.3 The Hilfstypen series of standard depths.

Figure 8.4 Society of Dyers and Colourists standard depths.

Figure 8.5 Example of light fastness testing of an orange coloured fabric (top) and blue wool standards for assessment of fading (bottom).

Guide

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Titles in the Society of Dyers and Colourists – John Wiley Series

An Introduction to Textile Coloration: Principles and PracticeRoger H. Wardman

Physico‐chemical Aspects of Textile ColorationStephen M. Burkinshaw

Standard Colorimetry: Definitions, Algorithms and SoftwareClaudio Oleari

The Coloration of Wool and Other Keratin FibresDavid M. Lewis and John A. Rippon (Eds)

An Introduction to Textile Coloration

Principles and Practice

Roger H. Wardman

Formerly Head of School of Textiles and DesignHeriot‐Watt UniversityEdinburgh, UK

This edition first published 2018© 2018 John Wiley & Sons Ltd

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Society of Dyers and Colourists

Society of Dyers and Colourists (SDC) is the world’s leading independent educational charity dedicated to advancing the science and technology of colour. Our mission is to educate the changing world in the science of colour.

SDC was established in 1884 and became a registered educational charity in 1962. SDC was granted a Royal Charter in 1963 and is the only organisation in the world that can award the Chartered Colourist status, which remains the pinnacle of achievement for coloration professionals.

We are a global organisation. With our head office and trading company based in Bradford, United Kingdom, we have members worldwide and regions in the United Kingdom, China, Hong Kong, India and Pakistan.

Membership: To become a member of the leading educational charity dedicated to colour, please email [email protected] for details.

Coloration Qualifications: SDC’s accredited qualifications are recognised worldwide. Please email [email protected] for further information.

Colour Index: The unique and definitive classification system for dyes and pigments used globally by manufacturers, researchers and users of dyes and pigments (www.colour‐index.com).

Publications: SDC is a global provider of content, helping people to become more effective in the workplace and in their careers by educating them about colour. This includes textbooks covering a range of dyeing and finishing topics with an ongoing programme of new titles. In addition, we publish Coloration Technology, the world’s leading peer‐reviewed journal dealing with the application of colour, providing access to the latest coloration research globally.

For further information please email: [email protected] or visit www.sdc.org.uk.

Preface

In 1993 the Society of Dyers and Colourists published the book Colour for Textiles: A User’s Handbook by Dr Wilfred Ingamells, which covered the basic science and technology of textile coloration. Dr Ingamells wrote the book at a level at which those without a detailed scientific background could understand the fundamental principles underlying dyeing and printing processes and it proved to be a very successful publication.

During the 25 years since the publication of the book, there have been a considerable number of developments in the dyeing and printing industry, not least of which have been the challenges imposed by the drive towards environmentally friendly processes and restrictions on the use of certain chemicals. In response, the Publications Committee of the Society of Dyers and Colourists (SDC) considered it necessary to produce an updated version of the book, and I, together with Dr Matthew Clark, agreed to take on the task. Unfortunately Dr Clark had to withdraw from the project at an early stage, but did complete Chapter 5 on textile printing, and I gratefully acknowledge his contribution.

One of the aims of rewriting Dr Ingamells’ book was to create a supporting textbook for the course ‘Textile Coloration Certificate’, which had been developed by Dr Clark and Mr Filarowski at the SDC and introduced in 2011. This course is aimed at personnel working in textile dyeing or printing companies, but who do not have a strong scientific background, so that they may attain a good understanding of the chemical principles involved in the processes with which they are involved. Accordingly, given the slightly different aim of this book, there is slightly more technical detail than in Dr Ingamells’ book.

It is the intention that candidates who successfully complete this course will be very well prepared to continue their studies to enter the Society’s examinations for Associateship, an honours degree level qualification. It is hoped therefore that this book additionally will provide a sound basis for students preparing for the ASDC qualification, though of course it is expected that they will also consult books available on the specialised topics related to textile coloration. At the introductory level of this book, it has not been possible to cover the dyeing of all the variants of textile fibre types. For example, there are many different types of polyester fibres and numerous types of fibre blends, and it is unrealistic to detail the processes involved in all these cases.

It was necessary for me to make some decisions about nomenclature in writing the book. In the teaching of organic chemistry in schools, the names of chemicals established by the rules of the International Union of Pure and Applied Chemistry (IUPAC) are used. In industry the original (trivial) names are still widely used, so to avoid confusion I have used these names also. Thus, for example, I have used ‘ethanoic (acetic) acid’, instead of just ‘ethanoic acid’. Another issue to address was that of commercial names for products. I have tried to avoid giving commercial names as much as possible, so, for example, all dye structures are labelled with their Colour Index numbers.

Finally, in preparing the book, I am grateful to Dr Ingamells for allowing me to use, where I considered it appropriate, parts of his text and diagrams. I gratefully acknowledge the help of the library staff at the Scottish Borders campus of Heriot‐Watt University, especially Mr Peter Sandison, Mr Jamie MacIntyre and Mrs Alison Morrison. I am particularly indebted to Mr Andrew Filarowski at the Society of Dyers and Colourists for so carefully going through the manuscripts of Chapters 1–6 and suggesting countless, yet very pertinent, modifications. His advice, support and links with the dyeing companies have been invaluable in the preparation of the book. Dr Jim Nobbs, formerly of Leeds University, very carefully scrutinised Chapter 7 and similarly provided many useful comments. Finally, I would like to thank Mr Alan Ross of High Street Textile Testing Services Ltd for carrying out very thoroughly a similar task on Chapter 8 and for making many useful corrections.

Roger H. Wardman

1General Chemistry Related to Textiles

1.1 Introduction

This chapter provides a background to the chemical principles involved in coloration processes, which will be beneficial to those with little working knowledge of dyeing chemistry. Chemistry has been classically divided into three branches: inorganic chemistry, organic chemistry and physical chemistry. Inorganic chemistry is the study of elements and their compounds. However carbon is so unique in the breadth of the compounds it forms (chiefly with hydrogen, oxygen, nitrogen and, to a lesser extent, sulphur) that it has its own branch – organic chemistry. Physical chemistry is concerned with the influence of process conditions such as temperature, pressure, concentration and electrical potential on aspects of chemical reactions, such as how fast they proceed and the extent to which they occur.

There are no clear distinctions between the three branches. For example, organometallic compounds are important substances that combine organic and inorganic chemistry, and the principles of physical chemistry apply to these two branches as well. Fundamental to all these branches of chemistry is an understanding of the structure of matter, so the chapter begins with this important aspect.

1.2 Atomic Structure

Modern chemistry is based on the belief that all matter is built from a combination of exceedingly minute particles (atoms) of the various chemical elements. Many different elements are found in nature, each possessing characteristic properties; the atoms of any one element are all chemically identical. An element is a substance made up of only one type of atom, for example, carbon is only made up of carbon atoms, and sodium is only made up of sodium atoms. Atoms combine together to form molecules of chemical compounds. A molecule is the smallest particle of a chemical element or compound that has the chemical properties of that element or compound.

A single atom consists of a very dense central core or nucleus, which contains numbers of positively charged particles called protons and uncharged particles, called neutrons. Protons and neutrons have equal mass and together they account for the atom’s mass. A number of very small negatively charged particles, called electrons, circulate around the nucleus in fixed orbits or ‘shells’, each orbit corresponding to a certain level of energy: the bigger the shell (the further away from the nucleus it is), the greater the energy. These shells are labelled n = 1, 2, 3, etc., counting outwards from the nucleus, and each can hold a certain maximum number of electrons, given by 2n2. The movement of an electron from one energy level to another causes the absorption or emission of a definite amount of energy. Atoms are electrically neutral, so the number of electrons in an atom is exactly the same as the number of protons in its nucleus. The total number of electrons within an atom of a particular element is called the atomic number of the element. This is the same as the number of protons in its nucleus. It is the arrangement of the electrons around the nucleus of an atom that determines the chemical properties of an element, especially the electrons in the outermost shells.

It is possible that some of the atoms of an element have a different number of neutrons in their nucleus, but their numbers of protons and electrons are still the same. These atoms are called isotopes, and although they have the same chemical properties as the other atoms, their atomic masses are different. Also recent research into atomic structure has shown that the three subatomic particles are themselves made up of other smaller particles such as quarks, but for this book it is sufficient to only consider atoms in terms of protons, neutrons and electrons.

The simplest atom is that of hydrogen, which has a nucleus consisting of just one proton with one electron orbiting around it and has an atomic number of 1. In deuterium, an isotope of hydrogen, there is one neutron and one proton in its nucleus. So its atomic mass is 2, but its atomic number is still only 1. There are roughly 6400 atoms of ‘normal’ hydrogen for every atom of deuterium. Another example is chlorine, which has two stable isotopes – one with 18 neutrons and the other with 20 neutrons in the nucleus. Because each has 17 protons, their atomic weights (the combined weights of protons and neutrons) are 35 and 37, respectively. These two forms are labelled 35Cl and 37Cl. Approximately 75.8% of naturally occurring chlorine is 35Cl and 24.2% is 37Cl, and this is the reason why the periodic table of the elements shows the atomic weight of chlorine to be 35.45.

Within a shell there are orbitals, each of which can hold a maximum of two electrons. Within an orbital, the two electrons are distinguished by the fact that they are spinning around their own axis, but in opposite directions. In illustrating this diagrammatically the electrons in an orbital are often shown as upward and downward arrows ↑↓, for example, as in Figure 1.3. The orbital nearest the nucleus is called an s orbital, followed by p, d and f orbitals, which are occupied in the larger atoms. These orbital types have different shapes. The s orbitals are spherical, whilst the p orbitals have two lobes and are dumbbell shaped. The three p orbitals are all perpendicular to each other, in x, y, z directions around the nucleus, so are often labelled px, py and pz (Figure 1.1). There are five d and seven f orbitals and these have more complex shapes.

Figure 1.1 The three p orbitals.

The first shell (n = 1) can accommodate only two electrons (according to the 2n2 rule) and there is just the s orbital. The next element, that of atomic number 2 (helium), has two electrons, both occupying the s orbital. In lithium (atomic number 3), its first shell contains two s electrons, but because that is now full, the third electron goes into the s orbital of the next shell. This second shell (n = 2) now fills up, and after the s orbital is full, further electrons go into the p orbitals, as shown in Table 1.1. The p orbitals can hold a maximum of six electrons and after they are full the third shell (n = 3) begins to fill.

Table 1.1 Filling of the shells by electrons in the first 30 elements.

Atomic number

Element

Orbit,

n

1

2

3

4

1

Hydrogen

1s

2

Helium

2s

3

Lithium

2s

1s

4

Beryllium

2s

2s

5

Boron

2s

2s 1p

6

Carbon

2s

2s 2p

7

Nitrogen

2s

2s 3p

8

Oxygen

2s

2s 4p

9

Fluorine

2s

2s 5p

10

Neon

2s

2s 6p

11

Sodium

2s

2s 6p

1s

12

Magnesium

2s

2s 6p

2s

13

Aluminium

2s

2s 6p

2s 1p

14

Silicon

2s

2s 6p

2s 2p

15

Phosphorus

2s

2s 6p

2s 3p

16

Sulphur

2s

2s 6p

2s 4p

17

Chlorine

2s

2s 6p

2s 5p

18

Argon

2s

2s 6p

2s 6p

19

Potassium

2s

2s 6p

2s 6p

1s

20

Calcium

2s

2s 6p

2s 6p

2s

21

Scandium

2s

2s 6p

2s 6p 1d

2s

⋮

30

Zinc

2s

2s 6p

2s 6p 10d

2s

Table 1.1 shows that once the three p orbitals of the third shell (n = 3) are full in argon, the electron of the next element, potassium, goes into the fourth shell, instead of continuing to fill the third shell, which can hold a maximum of 18 electrons. However, after calcium, further electrons go into the third shell, into its d orbital, of which there are five, thus holding a total of 10 electrons. After the d orbitals are all filled, at zinc, further electrons then fill up the 4p orbitals from gallium to krypton. Thereafter electrons go on to occupy the 5th orbit in a similar order, starting with the 5s orbital (rubidium and strontium).

1.3 Periodic Table of the Elements

During the nineteenth century, as new elements were discovered, chemists attempted to classify them according to their properties, such as metals and non‐metals, or on the basis of their atomic weights. One of the most important methods of classification was Newlands’ law of octaves, which he developed in 1865. Newlands considered that elements with similar chemical characteristics differed by either seven or a multiple of seven and created a table comprising rows of the known elements, in sevens. Going down the columns of his table gave, for example, hydrogen (H), fluorine (F) and chlorine (Cl) with similar chemical properties in the first column, then lithium (Li), sodium (Na) and potassium (K) in the second and so on. However Newlands’ table was not entirely correct, but his ideas were taken further by Mendeleev who developed what has become known as the periodic table of elements. Mendeleev, in focussing on arranging the elements into families with the same valencies (see Section 1.4), produced a more accurate table and left spaces in it for elements he considered had yet to be discovered. In the years since many new elements have indeed been discovered and a complete version of the periodic table is shown in Figure 1.3.

Figure 1.2 Periodic table of elements.

The periodic table lists the elements in columns called groups and rows called periods. Moving across the table from left to right, in any given period the atomic number increases incrementally, meaning the size of the atom increases. Moving down the table from top to bottom, in any given group the elements have the same number of electrons in their outer shells. In group 1 all the elements – hydrogen (H), lithium (Li), sodium (Na) and so on – have one electron in their outer shell and are all very chemically reactive and readily form ionic bonds (see Section 1.4.1). In group 2, the elements beryllium (Be), magnesium (Mg), calcium (Ca) and so on all have two electrons in their outer shell and, whilst chemically reactive, are not quite as reactive as the group 1 elements, but still form ionic bonds. The elements in group 17 (called the halogens) all have one electron short of a complete outer shell and readily form ionic bonds with elements of groups 1 and 2. The elements in group 16 are two electrons short of a complete outer shell and again readily form ionic bonds with elements of groups 1 and 2. The elements in groups 13, 14 and 15 do not gain (or lose) electrons easily, so they tend to form covalent bonds with other elements (see Section 1.4.2). The elements in group 18 have completely full outer shells and are unreactive. These elements – helium (He), neon (Ne), argon (Ar) and so on – are called inert gases.

The elements in groups 3–12 are the transition elements. Those in period 4 – scandium (Sc) to zinc (Zn) – involve the filling of the inner d orbitals with electrons, as described in Section 1.2. Those elements in period 5 – yttrium (Y) to cadmium (Cd) – involve the filling of f orbitals. In the higher periods (periods 6 and 7), many of the elements are unstable and gradually break down through radioactive decay.

The periodic table in Figure 1.3 shows a solid black line, labelled the Zintl border. This line represents the boundary between metals and non‐metals: the elements to the left and below the line are metals, and those above it and to the right are non‐metals.

1.4 Valency and Bonding

The number of electrons in the outermost shell considerably influences the chemical reactivity of the elements. For example, those elements with just one electron in their outermost shell (hydrogen, sodium, lithium, potassium) are very reactive, whilst those with eight electrons are very unreactive (the ‘inert’ gases helium, neon, argon, etc.), having what is termed stable octets. Although something of an oversimplification, it is convenient to assume that when bonds form between atoms of different elements, the atoms achieve an electronic configuration of a stable octet in their outermost shell. The achievement of a stable octet can be brought about either by atoms giving or receiving electrons or by the sharing of electrons.

1.4.1 Giving or Receiving of Electrons: Formation of Ionic Bonds

A good example of ionic bond formation is that of sodium chloride, formed by the reaction between sodium and chlorine atoms. As can be seen in Table 1.1, the sodium atom has an electronic configuration of 2.8.1, which means there are two (s) electrons in the first shell, eight (two s and six p) electrons in the second shell and a single (s) electron in its outermost shell. The chlorine atom has a configuration of 2.8.7 with seven (two s and five p) electrons in its outermost shell. Their electronic configurations are represented in Figure 1.3.

Figure 1.3 Electronic arrangements of the sodium and chlorine atoms (the ↑↓ arrows indicate electrons with opposite spins).

In the reaction between the two, one electron (the outermost) is transferred from the sodium atom to the chlorine atom. Sodium is then left with the configuration 2.8 and chlorine with 2.8.8, thus both having stable octets (Scheme 1.1).

Scheme 1.1 Reaction between sodium and chlorine.

(The electron on the sodium atom represented by x is identical with those on the chlorine atom represented by •: they are only given different notations to show where the electrons come from.)

Since atoms started out electrically neutral, the loss of one electron in sodium leaves the atom positively charged (it is now a positive ion, or cation) and the gain of one electron leaves the chlorine atom negatively charged (a negative ion or anion). Crystals of sodium chloride are therefore made up of equal numbers of sodium and chloride ions. Since opposite charges attract each other, there is strong electrostatic attraction between the two kinds of ions, which makes sodium chloride a very stable compound. This type of bonding is called ionic bonding and is typical of compounds called electrolytes. When simple electrolytes of this type dissolve in water, they split up (dissociate) into their separate ions. It is for this reason that they allow an electric current to pass through water.

Sodium and chlorine react in equal numbers because they each have a valency of one, the sodium atom needing to lose one electron and the chlorine atom needing to gain one electron. In the case of atoms of an element with two electrons in their outermost shell, it is necessary for them to lose these two electrons to achieve a stable octet, so they have a valency of two. For example, a calcium atom needs to react with two atoms of chlorine, with the result that the ionic compound formed, calcium chloride, has the formula CaCl2(Scheme 1.2):

Scheme 1.2 Reaction between calcium and chlorine atoms.

It is reasonable to expect that atoms of elements with three or four electrons in their outermost shells will need to react with three or four atoms of chlorine, respectively. However, in these cases, the removal of so many electrons is less easy and such elements tend to form covalent bonds (Section 1.4.2) where electrons are shared instead.

There are many simple electrolytes and the two most commonly used in dyeing and printing are sodium chloride (NaCl) and sodium sulphate (Na2SO4), the latter being known as Glauber’s salt. The reason why there are two ions of sodium to one sulphate ion in Glauber’s salt is that the sulphate ion is a complex ion, but requires two electrons to achieve stability: it has a valency of 2. A sodium atom has one electron in its outer shell (a valency of 1) and so two sodium atoms are required to satisfy this valency requirement. However, because a calcium atom has two electrons in its outer shell (it also has a valency of 2), it can react with the sulphate on a 1 : 1 basis, so calcium sulphate has the formula CaSO4.

Water‐soluble dyes are also electrolytes, but in this case the coloured part of the molecule is very large and usually an anion, whilst the cation, usually a sodium ion, is very small by comparison. In fact water‐soluble dye molecules are all synthesised to contain at least one group of atoms known to confer water solubility on the dye molecule through the formation of ions. Very often this is the sulphonic acid (or sulphonate) group, ─SO3H, or the carboxylic acid group, ─COOH, both of which form sodium salts that dissociate in water. In each case, their valency is one, so they form salts with sodium ions in a 1 : 1 ratio. The dissociation of a dye molecule with a sulphonate group is shown in Scheme 1.3, in which D represents the coloured part of the dye molecule.

Scheme 1.3 Dissociation of the sulphonate group.

1.4.2 Sharing of Electrons: Formation of Covalent Bonds

A covalent bond differs from the bonding in an ionic compound in that there is no transfer of electrons from one atom to another. Instead two atoms share two electrons, each atom providing one electron of the pair. Scheme 1.4 represents a covalent bond in the simple inorganic molecule of chlorine (Cl2) as an example. As in Schemes 1.1 and 1.2, although electrons are given different symbols on the two atoms, this is just to show where they come from; in practice there is no difference between them.

Scheme 1.4 Formation of the chlorine molecule.

By sharing a pair of electrons each of the chlorine atoms achieves a stable octet of electrons in their outer shells.

Elements whose atoms have four electrons in their outermost shells need to either gain or lose four electrons if they are to achieve a stable octet by forming ionic bonds. To do this requires too much energy and so instead they react with other atoms by forming covalent bonds instead, through the sharing of electron pairs. Such elements have a valency of 4. Typical of such an element is carbon, and the structures of all organic molecules, including dye molecules, are based on carbon atoms linked by covalent bonds.

The simplest organic compound is methane, CH4(Scheme 1.5). In methane there are four covalent bonds from the carbon atom, one to each hydrogen atom, arranged in the form of a symmetrical tetrahedron with the carbon atom in the middle (Figure 1.4).

Scheme 1.5 Formation of methane (outer electrons only are shown); x electron from carbon, • electron from hydrogen.

Figure 1.4 Tetrahedral molecular structure of the methane (CH4) molecule.

The carbon atom has the electronic structure shown in Figure 1.5. There are only two unpaired electrons (in the px and py orbitals), so before the four covalent bonds can be formed, one of the 2s electrons must be promoted to the vacant pz orbital, giving the electronic arrangement shown in Figure 1.6.

Figure 1.5 Electronic structure of the carbon atom.

Figure 1.6 Electronic structure of the sp3‐hybridised carbon atom.

There are now four unpaired electrons, giving carbon a valency of four. The formation of the four orbitals, which now each contain one electron, is called hybridisation. The four sp3 hybrid orbitals are equivalent and oriented tetrahedrally from the nucleus of the carbon atom. In methane the four bonds with hydrogen are formed by the overlap of the four sp3 hybrid orbitals with the s orbitals of the hydrogen atoms.

On bonding with four hydrogen atoms, the carbon atom has eight electrons in its outermost shell and each hydrogen atom has two. The electronic requirements of both carbon and hydrogen atoms are satisfied and the compound is very stable. Such bonds do not dissociate in water; indeed most covalent compounds are insoluble in water and do not conduct electricity. Covalent bonds are the most stable of all chemical bonds and they cannot be broken easily. As indicated in Figure 1.4, the bonding is directional and covalent compounds therefore exist as molecules with a definite shape.

Organic chemistry is essentially the chemistry of carbon and the compounds it forms with atoms of other elements, most notably hydrogen, but also oxygen and nitrogen. Nitrogen with five electrons in its outermost shell and oxygen with six readily form covalent bonds, for example, with hydrogen to form ammonia and water, respectively (Scheme 1.6).

Scheme 1.6 Covalent bonding by nitrogen and oxygen atoms with hydrogen atoms.

Nitrogen and oxygen atoms (and also sulphur atoms) can also form covalent bonds readily with carbon atoms, so there is a vast range of organic compounds involving these elements. Dye and organic pigment molecules are comprised mainly of atoms of these elements covalently bound to each other. For all of these compounds, carbon has a valency of 4, so all C atoms can form four single bonds. The atoms of nitrogen, with a valency of 3, have three single bonds; of oxygen (valency 2) two single bonds, and of hydrogen (valency 1) one single bond. The classes of organic compounds are dealt with later in this chapter.

1.4.3 Secondary Forces of Attraction

Ionic and covalent bonds are referred to as primary forces. Most application classes of dyes are attracted to fibres by ionic bonds (e.g. acid dyes on wool and nylon, basic dyes on acrylics) and by covalent bonds (reactive dyes). In addition to these types of attractive forces between molecules, there exist other types called secondary forces because they are usually weaker than the primary forces. Although weaker, they have an important influence on the physical properties of organic compounds and indeed serve to enhance the attraction between dye molecules and fibres. The main types of secondary forces of attraction are dipolar forces, hydrogen bonding, π–H forces (pronounced ‘pi–H’) and dispersion forces. Collectively, these forces are often referred to as van der Waals forces.

Dipolar Forces

These forces occur between molecules that are polar in character. A molecule is polar if there exists some charge separation across it where some parts of the molecule are partially positive in character (δ+) and other parts are partially negative (δ−). Polarity in a molecule occurs when there is unequal sharing of the electrons of the covalent bond between two different atoms, because one has greater electronegativity than the other, the latter tending to pull the electron pair towards it from the other less electronegative atom. The polar molecules then attract each other, the slightly positive part of one molecule being attracted to the slightly negative end of the other (Figure 1.7).

Figure 1.7 Dipole–dipole attraction between molecules.

Hydrogen Bonding

Hydrogen bonding can be regarded as a special case of dipole–dipole attraction. It is so named because it involves hydrogen and because hydrogen is such a small simple atom, when it is bound to another atom that is electronegative, such as oxygen, the shared electron pair forming the covalent bond between them is pulled so much more towards the oxygen that a strong charge separation occurs, creating polarity. The hydrogen atom then serves as a bridge, linking two other electronegative atoms. The most common example of a molecule that readily forms hydrogen bonds is water (Figure 1.8). Aside from forming between neighbouring molecules (called intermolecular hydrogen bonds), these bonds can also occur between different functional groups of more complex organic molecules such as dyes and pigment molecules. This is called intramolecular hydrogen bonding and is often responsible for the good technical performance, especially the lightfastness, of many dyes and pigments.

Figure 1.8 Hydrogen bonding in water (─ covalent bond, ‐‐‐‐ hydrogen bond).

π–H Bonding

This is a variation on hydrogen bonding in that it involves the interaction between the π‐electron (pronounced ‘pi’ electron) system of an aromatic ring (see Section 1.8.1.2