CHM215
DYES
Definition of Dyes
Dyes are colored substances that can impart color to a material on a relatively permanent basis.
Dyes are soluble at certain stages of the application process.
They typically contain considerable unsaturation and often possess aromatic rings.
CLASSIFICATION OF DYES
Overview: Dyes can be classified in several ways, each having unique structures and bonding properties.
Ways to Classify Dyes
Organic/Inorganic
Natural/Synthetic
By Area and Method of Application
Chemical Classification
Based on the nature of their respective chromophores.
Electronic Excitation Classification: Lesions based on energy transfer colorants, absorption colorants, and fluorescent colorants.
Dyeing Methods:
a. Anionic (for Protein fibres)
b. Direct (for Cellulose fibres)
c. Disperse (for Polyamide fibres)
Chemical Classification of Dyes
Chromophore Group/Functional Group Types:
Nitroso: $NO$ or $N-OH$
Nitro: $NO_2$
Azo: $-N=N-$
Stilbene: $-N=N-$ and $=C=C=$
Ketonimine: $NH=C=$
Xanthene: Derivative of xanthene
Thiazole: Contains sulfur in the ring (C=N or $-S-C$)
Cyanine Dyes: Derivatives of phthalocyanine.
INDUSTRIAL CLASSIFICATION OF DYES
Classification by Industrial Uses: Based on performance in dyeing processes, particularly in the textile industry.
Azo Dyes: Represent 60% of dye stuff used worldwide in textile finishing.
Constitutes the largest chemical class among dyes, with over 66% of all colorants, characterized by one or more azo groups along with hydroxyl and amine groups as auxochromes. They are derived from aromatic amines via diazonium salts.
Other Types of Dyes
Acid Dyes:
Water-soluble anionic dyes used on silk, wool, nylon, and modified acrylic fibers using neutral to acidic dye baths.
Interact with fibers through salt formation between anionic groups and cationic groups in the fibers.
Not effective on cellulosic fibers; many synthetic food colors belong here.
Basic Dyes:
Water-soluble cationic dyes primarily applied to acrylic fibers, with limited applications to wool and silk.
Involves adding acetic acid for dye uptake, also used in paper coloration.
Vat Dyes:
Insoluble in water and require reduction in alkaline liquor to produce a soluble alkali-metal salt.
The leuco form has an affinity for textile fibers and can revert to the original insoluble dye upon oxidation. Examples include indigo.
Mordant Dyes:
Require a mordant to enhance dye fastness against water, light, and perspiration.
The choice of mordant is critical, influencing final color. Most natural dyes are mordant dyes, and synthetic mordant dyes comprise about 30% of dyes for wool, especially useful for black and navy shades. Potassium dichromate is commonly used.
Note: Some mordants can be hazardous to health.
Reactive Dyes:
Feature a chromophore that reacts directly with fiber substrates, creating covalent bonds leading to strong permanence.
"Cold" reactive dyes like Procion MX, Cibacron F, and Drimarene K can be applied at room temperature.
They are ideal for dyeing cotton and cellulose fibers.
CLASSIFICATION BASED ON APPLICATION
Dyes classified based on specific performance on materials:
a. Solvent Dyes: Used in wood staining, colored lacquers, solvent inks, and waxes.
b. Leather Dyes: Specifically for leather.
c. Fluorescent Dyes: Applied in sports goods.
d. Smoke Dyes: Utilized in military applications.
e. Leuco Dyes: Diverse applications in electronics and paper.
f. Inkjet Dyes: Used in the writing industry, particularly inkjet printers.
g. Oxidation Dyes: Primarily applied in hair products.
CLASSIFICATION OF FIBRES
Definition of Fibres:
Fibres are continuous filaments or materials arranged in elongated pieces similar to thread lengths.
Essential for holding tissues together in both plant and animal biology.
Classification of Fibres
Fibres are classified into two primary categories:
Natural Fibres:
Biodegradable, produced by plant, animal, and geological processes.
Types: a. Plant Fibres:
Vegetable Fibres: Composed mainly of cellulose, examples: cotton, linen, hemp, jute, flax, ramie, sisal.
Wood Fibres: Obtained from wood; examples include groundwood, thermomechanical pulp (TMP), and bleached/unbleached kraft or sulfite pulps.
b. Animal Fibres:Produced from animal waste and primarily consist of proteins; examples: spider silk, sinew, catgut, wool, polar bear fibers, which are hollow.
c. Mineral Fibres: Includes asbestos as the only naturally occurring long mineral fibre.
Man-Made Fibres:
Synthetic Fibres: Based on synthetic chemicals rather than natural materials, include polyamide, nylon, polyester (PET), acrylic polymers, etc.
Chemically Modified Fibres:
Mixtures of natural and synthetic fibers; examples include rayon, modal, Lyocell, cellulose acetates, fiberglass, and optical fibers.
PHYSICAL AND STRUCTURAL PROPERTIES OF FIBRE
Natural Fibres (e.g., wool, silk, leather):
Composed of proteins, which are polymers of α-amino acids categorized as $RCH(NH_2)COOH$ where R is an organic group. They contain series of amide linkages (―CO―NH―) that can serve as sites for electrostatic interactions with dyes that contain acidic or basic groups.
Polyamides (Nylons): Synthetic analogs of proteins with amide groups separated by hydrocarbon chains $(CH_2)^n$.
Polyester (PET): The main synthetic fibre, accounting for over 50% of global production. Difficult to dye due to tightly packed chains, but terminal hydroxyl groups (―OH) can act as dyeing sites.
Acrylics: Composed of hydrocarbon chains with polar nitriles made through copolymerization, improving dyeability when combined with other materials.
Cellulose: A linear polymer found in plants with multiple hydroxyl groups for hydrogen bonding; cotton is primarily pure cellulose.
Physical Properties of Fibres
Length & Shape: High length-to-width ratio is essential for spinning into yarn.
Fineness/Diameter: Affects softness, stiffness, and luster, finer fibers yield softer fabrics.
Color & Luster: Natural variability; luster defined by reflected light amount influenced by fiber shape and treatment.
Crimp: Natural waves or bends contributing to bulk, resilience, and cohesiveness.
Strength (Tenacity): Resistance against breaking under tension.
Elastic Recovery & Elongation: Ability to stretch and revert to original length.
Resiliency: Returns from wrinkles or folds.
Abrasion Resistance: Endurance against rubbing and wear.
Moisture Absorbency (Regain): Determines water absorption capacity affecting comfort and static.
Density (Specific Gravity): Impacts fabric weight and cover.
Thermal Properties: Reaction to heat influencing ironing and flammability.
Electrical Properties: Potential for static electricity buildup.
COLOUR AND CONSTITUTION
Definition and Early Theories of Colour:
Color perception linked to groups with multiple bonds (chromophores) observed by Witt (1876).
Chromophores lead to color and auxochromes deepen color effects.
Bathochromic (~red shift) and hypsochromic (~blue shift) groups influence color wavelength absorbance.
Color Deepening Spectrum:
Colors deepen based on the wavelength absorbed: Yellow → Orange → Red → Purple → Blue → Green → Black.
Chromophores:
Defined as covalently unsaturated groups responsible for UV/visible absorption, examples include: $C=C$, $C≡C$, $C=O$, $C≡N$, $N=N$, $NO_2$.
Auxochromes:
Saturated groups that modify chromophores by changing absorption wavelengths/intensity, examples include: $NH_2$, $OH$, $COOH$, and halogens. They enhance chromophore properties via resonance.
PIGMENTS
Defined as colored or colorless particulate solids that are usually insoluble and chemically unaffected by their vehicles.
Properties of Pigment Colorants:
Highly durable.
Stable to heat.
Solvent-resistant.
Good light fastness.
Migration fastness.
Difficult processing, with lower color brilliance and strength in comparison to dyes.
Characteristics of Ideal Pigments:
Practically insoluble in the application media.
Requires additional compounds to attach to substrates (via polymeric interactions).
PERCEPTION OF COLOUR
Physiology of Perception:
Color perception relies on light wavelengths within 400-700 nm, with invisible wavelengths like X-rays or radio waves outside this spectrum.
Detected by photoreceptors: cone cells for color detection and rod cells for low-light sensitivity.
Mechanism of Color Perception
Light interaction with an object leads to selective reflection and absorption of wavelengths.
Reflected wavelengths perceived as color after being focused onto cone and rod cells, processed into brain signals for color interpretation.
PHYSICAL AND CHEMICAL CAUSES OF COLOUR
Origins of color stem from light interaction with electronic structures in matter.
Processes like reflection, refraction, absorption, and re-emission (fluorescence) contribute to color perception.
Specific color appearances arise from preferential absorption and reflection across different wavelengths, notably when broad absorption bands occur in condensed matter structures.
COLOUR PSYCHOLOGY
The study of colors and their influence on human behavior, mood, and decisions. Emotions triggered by particular colors may vary due to factors like culture and personal experience.
General Effects of Colours
Red: Energy, urgency, boosts heart rate (e.g., fast food).
Blue: Calming, promotes trust (e.g., banks).
Yellow: Happiness, optimism, but can cause agitation with excess.
Green: Nature, balance, health.
Orange: Enthusiasm, creativity; enhances appetite.
Purple: Represents luxury and wisdom.
Pink: Associated with calmness, used therapeutically.
Influencing Factors
Culture: Different meanings of colors (e.g., white for mourning in some cultures).
Context: Situational effects can alter color perception.
Applications of Colour Psychology
Branding & Marketing: Use in logos and packaging to influence perceptions.
Interior Design: Choose colors to elicit desired moods.
Therapy (Chromotherapy): Utilizing colors for health and wellbeing.
UX/UI Design: Enhance user experience through strategic color application.
LIGHT AND COLOUR
Definition: Light as electromagnetic energy encompassing visible wavelengths; color as brain interpretation.
White light contains all colors; an object's appearance is due to selective absorption of light wavelengths.
Physically, light is essential for survival and existence on Earth.
SECTION A: INTRODUCTION TO DYES AND CLASSIFICATION
Definitions and Fundamentals
Define the term "Dye." What are three fundamental characteristics of dyes regarding solubility and chemical structure?
Differentiate between organic and inorganic dyes.
Chemical Classification
Identify the functional groups associated with the following dye classes:
Nitroso: {NO} or {N-OH}
Nitro: {NO_2}
Azo: -N=N-
Stilbene: -N=N- and =C=C=
Explain the significance of the chromophore in the chemical classification of dyes.
Application Methods
Describe the specific fiber types typically dyed using the following methods:
Anionic dyeing
Direct dyeing
Disperse dyeing
SECTION B: INDUSTRIAL APPLICATIONS
Major Dye Classes
Azo dyes represent approximately 60\% of the global dye market. Explain their chemical derivation and the role of auxochromes within their structure.
Compare and contrast Acid Dyes and Basic Dyes in terms of their solubility, the types of fibers they target, and the nature of their chemical interaction (e.g., salt formation).
Specialized Dyeing Processes
Vat Dyes: Explain the process of applying insoluble vat dyes to textiles. Define the "leuco" form and provide an example of a common vat dye.
Mordant Dyes: Why is a mordant required for certain dyes? Name a common metallic salt used as a mordant and discuss the potential health implications of its use.
Reactive Dyes: Describe the type of chemical bond formed between a reactive dye and a fiber substrate. List two examples of "cold" reactive dyes.
SECTION C: FIBRE SCIENCE
Classification of Fibres
Distinguish between natural, synthetic, and chemically modified fibers, providing at least two examples for each category.
Classify the following vegetable fibers: cotton, hemp, flax, and jute.
Structural Properties
Proteins are polymers of \alpha-amino acids represented by the formula {RCH(NH_2)COOH}. Explain how the amide linkages {(-CO-NH-)} in protein fibers facilitate dyeing.
Discuss why Polyester (PET) is traditionally difficult to dye and identify the specific site {(-OH)} available for dye interaction.
Physical Characteristics
Define the following physical properties of fibers and explain their importance in textile manufacturing:
Tenacity (Strength)
Crimp
Moisture Regain
Elastic Recovery
SECTION D: COLOUR THEORY, PIGMENTS, AND PERCEPTION
Constitution and Colour
Explain Witt's theory regarding chromophores and auxochromes.
Define the terms Bathochromic shift and Hypsochromic shift.
Identify three examples of covalently unsaturated groups that act as chromophores.
Dyes vs. Pigments
Provide a comparative analysis of dyes and pigments. Address solubility, durability, and the method of attachment to substrates.
What are the essential characteristics of an ideal pigment?
Physiology and Psychology of Colour
Describe the mechanism of color perception, specifically the roles of cone cells and rod cells.
Colour Psychology: Discuss the emotional and behavioral associations of the following colors in branding and therapy:
Red
Blue
Green
Purple
How do cultural factors influence the interpretation of the color white?