Linkages between Pigments-Dyes, Chemistry tutorial

Introduction

Both dyes and pigments appear to be coloured since they absorb several wavelengths of light preferentially. In contrast through a dye, a pigment usually is insoluble, and has no affinity for the substrate. Though, there has been an interchange of usage between the 2. Pigment dyeing is a reasonably recent addition.

Definition of Dyes

A dye can generally be explained as a coloured material that has an affinity for the substrate to which it is being applied.  The dye is usually applied in an aqueous solution, and might need a mordant to develop the fastness of the dye on the fiber.

Features of Dyes

At the extremely basic level, utilize of colour in identifying individual components of tissue sections can be completed primarily through dyes. Even though there are other means,  Dyes are though,  the largest group that  can  simply  be  manipulated  to  our  liking.  Dyes are applied to several substrates for instance to textiles, leather, plastic and paper in liquid form. One trait of dye is that the dyes must get totally or at least partially soluble in what it is being put to. The rule that we pertain to other chemicals is similarly applicable to dyes also. For example, certain dyes can be toxic, carcinogenic or mutagenic and can be hazardous to health.

How Can the Colour of the Dyes be Altered

The answer lies in the modifiers. Colour modifiers as methyl or ethyl groups can in fact alter the colour of dyes. They do so via altering the energy in the delocalised electrons. It has been found that through addition of a particular modifier there is a progressive alteration of colour. An instance can be known for methyl violet series.

The subsequent step describes what occurs to the colour of the dyes when modifiers are added

Step A: When no methyl group is added, the original dye Pararosanil as it is termed is red in colour.

Step B: As 4 methyl groups are added ruddy purple dye Methyl Violet is got.

Step C: Through the addition of more methyl groups a purple blue dye Crystal Violet is obtained. It has in it 6 such groups.

Step D:  Further addition of a 7th methyl group the dye that is got is termed Methyl green.

What Gives Dyes Solubility and Cohesiveness?

The answer to this riddle lies in substance termed Auxochrome. Auxochromes have the ability to intensify colours. It is a group of atoms that attaches to non-ionizing compounds yet has to ionize. Auxochromes are of 2 kinds, positively charged or negatively charged.

Dyeing and Fabric

This is an application of dyes by respect to the fabrics.

Table: Kinds of Dyes and Fabric

Group

Application

Direct

Cotton, cellulosic and blended fibres

Vat dyes

Cotton, cellulosic and blended fibres

Sulphur

Cotton, cellulosic fibre

Organic pigments

Cotton, cellulosic, blended fabric, paper

Reactive

Cellulosic fibre and fabric

Disperse dyes

Synthetic fibres

Acid Dyes

Wool, silk, paper, synthetic fibres, leather

Azoic

Printing Inks and Pigments

Basic

Silk, wool, cotton

Conventional pigment dyeing system

There is a challenge in pigment dyeing. In the procedure of pigment dyeing no real chemical reaction occurs between the dye and the fabric. In its place, what happens is that the pigments get seated on the fabric through the assist of binders.

Pigments aren't soluble in water and show no affinity for fibre. So, conventional   dyestuff-based dyeing conditions aren't feasible for pigment dyeing. To come to terms through these limitations, a new type of pigments have been formulated for utilize in fibres. Such are continued in a stable dispersion in the medium of water via anionic surfactants. This kind of pigment is recognized as pigment resin color (PRC), primarily utilized in printing. Several of the popular pigments utilized in fabrics are specified here.

a. Yellow colour: Acetoacetic acid anilide pigments

b. Red: Azoic pigments

c. Blue or green: Phthalocyanine pigments

A typical procedure of pigment dyeing for cellulosic textile substances consists of padding the textile materials through a dye bath. The dye bath encloses anionic or neutral colour dispersions of pigments. Along by the pigments it as well contains anionic binders, acid-liberating catalyst, anti-migrating agents, and other kinds of additives. Then, the textile materials are dried at high temperatures, this cures the film-forming binders and pigment colours resolutely on the textiles.

Differences between Dyes and Pigments

The main dissimilarities between the Dyes and Pigments are highlighted below

Table: Differences between Dyes and Pigments

Points of  Difference

Dyes

Pigments

Solubility

They are soluble

Pigments are colourants  that

are  insoluble  in  water  and

most solvents

Number

Available in large number

Comparatively lesser  in

number

Product resistance

Lower as compared  to

pigments

Very high

Light fastness

Lower Dyes are very much

vulnerable.   Lights   destroy

coloured    objects   by

breaking open electronic

bonding   within the

molecule

Traditionally pigments have been found to be more lightfast than dyes

Size

Dye molecules  are

comparatively smaller   it's

like  comparing  a  football

(pigment) to say a head of a

pin (dye)

Pigment particles are about

1-2 microns in size.  (1

micron  =1/1000  meter).   It

means that the particles  can

be seen  under  a magnifying

glass

Bonding

aking    the    example   of

dyeing a wood surface,  the

dye and   the    substrate

(wood) that  is   dyed are

chemicals that  have certain

features    called functional

groups.  At    the   level  of

molecules  these   groups

serve  as  open  pockets   of

electrostatic charges (+ or -

). The  functional  group   in

dyes,  serve  as  a  point  for

attaching the dye to the wood

Taking the  example  of   a

wood surface  pigment

requires the help of a binder

for  gluing. As it is an  inert

substance  which  is  merely

suspended in  a

carrier/binder

Structure during  the

application process

During application  process

there is a  temporary

alteration in the structure  of

the dyes

During application,

pigments have the capacity

to   retain   its  particulate  or

crystalline structure

Imparting of Colours

Dyes can only impart colour

by selective absorption

Pigments impart colours by

either  scattering of light  or

by selective absorption

Combustible  properties

Taking the  example  of   a

candle   making  process,   if

the  candles  are  dyed  it   is

easily combustible  and  can

be  applied  throughout   the

candle

In the example of a candle making as pigments are coloured particles, they tend to clog a wick when burned. this makes them undesirable for a candle if it is coloured throughout and used for burning

Chemical Composition

Usually the dyes are organic

(i.e. carbon-based)

compounds

While pigments are

normally  inorganic

compounds, often involving

heavy toxic metals

Longevity factor

The dye based printing  inks

do  not  last  as  long  as  the

pigment inks

In case of link based printing prints made with pigments last longer

Printing  on substrates

Compatible with almost  all

the  substrates that needs  to

be dyed

Owing to the physical

makeup of the pigment  inks

the range   for  suitable

substrates are limited

Colour gamut

Taking the case of  printing

inks, dye based inks offers a

wide variety

As compared to dye-based inks, pigment inksets somewhat lags behind, on the same paper stock

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