chomic colourants in textiles

8/10/2019 Chomic Colourants in Textiles

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Application of ChromicColourants in Textiles

Presented by

Md. Ershad KhanID: 2013-2-3-001

M.Sc. in Textile Engineering

Department of Wet Processing EngineeringBangladesh University of Textiles

Welcome to the Presentationon


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What is Chromic Colourant?

Chromic colourants may be defined as dyes andpigments that exhibit a distinct colour change whenexposed to an external stimulus, especially when thechange is reversible and controllable.

A wide range of materials that exhibit colour changeeffects have been investigated in recent decadesand numerous products have been introducedcommercially.

Although these colorants have not been developedspecifically for textiles, there is growing interest intheir potential for applications in technical and smarttextile products.

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An overview of chromic materials

A wide range of chromic phenomena are now wellknown and the underlying physical and chemicalprinciples have been established.

An extensive range of materials that exhibit the

colour change effects have been investigated and,where appropriate, introduced commercially.

The industrial applications which have emerged

are generally in high technology areas, forexample in thermometry, ophthalmics,electron ics and biomedic ine, where theparticular colour change (chromic) effect producedby the stimulus is specificallyexploited.

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list of chromic phenomena & the stimuli involved

5Indeed, the list in the table is not exhaustive.

Chromic phenomenon Stimulus

Thermochromism Heat

Photochromism Light

Ionochromism Ion

Electrochromism Electric current flow

Solvatochromism Solvents

Vapochromism Solvents

Mechanochromism Mechanical action

Chronochromism Time

Radiochromism Ionising radiation

Magnetochromism Magnetic field

Biochromism Biological sources


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Chromic phenomena for textile applications

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Four chromic phenomena which have been most

extensively studied for textile applications

1. Thermochromism (colour change due to achange in temperature)

2. Photochromism (UV-light induced colourchange)

3. Ionochromism (especially halochromismwhere the colour change is related to pH

variation)4. Elect rochromism (colour change due toelectric current flow)


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Thermochromism

Thermochromic systems provide a change ofcolour as the temperature changes, commonlyreversibly, by either heating or cooling. The mainapplications of thermochromism involve a colourchange to indicate temperature variation.

Thermochromics may be categorised into two broadtypes:

1. In tr ins ic systems in which heating is the directcause of the colour change

2. Ind irect systems in which heating causeschanges in the environment in which thechromophore is located, which in turn results in acolour change.

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Types of Thermochromic Colourants

There is a wide variety of thermochromic

materials. Many of such materials commonlyoperate at high temperatures and are thus notnormally appropriate for textile applications.There have been interesting recent

developments in organic polymers that showreversible intrinsic thermochromism. Two typesof thermochromic system have been applied totextiles: (The term systemis used advisedly asneither are dyes in the conventional sense.)

1. Leuco dye

2. Liquid crystal types

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Thermochromics Leuco dyes

The most widely used industrial thermochromic system is the leuco dye

type. The term leucodescribes a dye which can acquire two forms, oneof which is colourless. This microencapsulated composite system relies

on colour formation from the interaction of three materials:

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3. A low-melting, nonvolatile hydrophobic solvent

e.g. The most common hydrophobic solvents are aliphatic alcohols of

varying chain lengths.

1. An organic colour former (the leuco dye)

e.g. Crystal violet lactone (CVL), a diarylphthalide

which is colourless in its ring-closed form (Fig.1.1(a)).

2. A developer (proton donor)

e.g. )). The most common acid developer is

Bisphenol A. Other weak acids which are

appropriate include alkylgallates,hydroxybenzoates, hydroxycoumarins and

1,2,3-triazoles.


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Figure: Schematic representation of the mechanism of colour change in a

microencapsulated organic leuco dye thermochromic system.

Thermochromics Leuco dyes


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Thermochromics Liquid crystal Dye

The second type of thermochromic system that can be

applied to textiles is based on l iqu id crysta ls.

Liquid crystals, often termed the fourth state of matter,show liquid-like behaviour but the molecules have atendency to line up in an ordered pattern, unlike normal(isotropic) liquids in which there is random orientation.

The thermochromic effect provided by certain liquidcrystals is quite different from that of leuco dye types.

They provide a continuously changing spectrum ofcolours over a range of temperatures (referred to ascolour-play) when observed against a dark (ideallyblack) background.

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As the liquid crystals

heat up, the orientationof the helices changes,

which causes the

helices to reflect a

different wavelength oflight. To our eyes, the

result is a change in

color. As the crystals

cool down, they reorientthemselves into their

initial arrangements and

the original color

returns. 12

Figure: Temperature dependence of

thereflected

wavelengths for amicroencapsulated chiral nematic

liquid crystal pigment printed on black

polyester.

Thermochromics Liquid crystal Dye


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General applications of thermochromism

Plast ic str ip thermometers

It is used for example to measure thetemperature of air or water (e.g., in an

aquarium).

Medical thermog raphyThe temperature indicating

thermochromic devices are placed on the

forehead to monitor skin temperature.

Battery testers :

It is used to monitor lifetime based on the

heating effect produced by a live battery.

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General applications of thermochromism

Food packaging

They are also commonly applied byprinting on food packaging, for example onbottles, cans or cartons of drinks toindicate correct chilling.

Archi tectural appl icat ion

There is growing interest in the use ofthermochromics in architecture, forexamples in coatings or tiles for eitherInternal or external use.

Novelty areasThere is an endless list of uses, in bathtoys, mugs, umbrellas, golf balls, jewelleryand cosmetics, to name only a few.

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Major International Manufacturers

of Thermochromic Dyes

LCR Hallcrest (UK), now incorporating

ColorChange Corporation (USA)

Matsui (Japan).

Siltech Limited (UK)

QCR Solutions Corp (USA)

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Textile applications of thermochromism

Colour changeable T-shi r ts: GlobalHypercolor brand, introduced an apparel use

of thermochromic leuco dyes is in T-shirtsinitially in 1991. These particular productswere coloured with thermochromics,commonly with an additional permanent colourto provide a colourto colourchange.

Baby c lo thes: The colour change to indicatethat the child may be too hot.

Faded indigo ef fect : Applications of leucodye systems to denim produce the blue tocolourless thermochromic effect, thus sharinga similarity with the fadedindigoeffect.

Color changing flu-masksThe color changevia Thermochromic ink is easily triggered beexhaling through the mesh of such masks.

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Photochromism

Photochromism is commonly defined as aprocess in which a compound undergoes areversible change between two species withdifferent absorption spectra, i.e., with differentcolours, on irradiation with light.

Most photochromic dyes acquire a colour whenexposed to irradiation by UV or low wavelengthvisible light and revert to their original

colourless state when the light source isremoved.

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Types of photochromic dyes

Photochromic dyes may be categorised into twobroad types.

1.T-type pho toch rom ic dyes

When the reverse reaction is thermally driven

2. P-type pho toch rom ic dyesWhen the reverse reaction is photochemically

induced, using light of a different wavelength.

While most industrial photochromic dyes are of theT-type, there is considerable ongoing researchinterest in P-type photochromics and their potentialapplications.

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Main Chemical Classes of Photochromic dyes

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1.Spiropyrans dye 2. Spirooxazines dye

3. Fulgide dye


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4. Diarylethene dye

5. Naphthopyrans dye

Main Chemical Classes of Photochromic dyes


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General applications of photochromism

Ophthalmic Sunscreening: Responsive

eyewear items include the familiar spectacleswhich become sunglasses when exposed to UV

light, and also ski-goggles and motorcycle

helmet visors.

Cosmet ic and Beauty Products: They are

used in nail varnishes which acquire colour in

the sun and have been proposed for other

cosmetics and personal care uses, such as in

hair dye formulations and sunscreen lotions.

Secur i ty Pr int ing: The photo-inducedcoloration may be used in security printing, for

example as anti-counterfeit Markers on

banknotes and documents such as passports.

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General applications of photochromism

Novelty Plast ic Items: They have also been used

for novelty effect in plastic items such as toys,combs, beads, drinkingstraws, cups, spoons and

drinks bottles.

Fish ing Lines: Another interesting functional use of

photochromism is in fishing lines which becomecoloured in sunlight and thus visible to the angler,

while below the water-line where the sun cannot

penetrate, the line is colourless, thus assisting

concealment from the fish.

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Textile applications of Photochromism

There have been comparatively few reports of the

application of photochromic compounds on textiles,although recent papers indicate growing interest,with applications envisaged in creative and intelligentdesign, and in functional or smart textile products.

There are a number of reports of investigations ofexhaust dyeing of synthetic fabrics using simplephotochromic spirooxazine dyes. Photochromic

textiles were obtained converting from colourlessor weakly coloured to blue on exposure to UVradiation.

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Photochromic dyes may be applied to textiles by

screen printing. A series of publications have beenaimed at establishing how commercialphotochromic dyes are capable of performing onfabric screen-printed using a pigment printingmethod, following optimisation of the formulations

and application conditions.

The use of photochromics in novelty printing is asignificant commercial outlet. Because of theirability to show resistance to thermal fade ratesfulgides have been the class most commonly usedfor this application. Typical uses are on childrenstoys and for logos on T-shirts.

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Textile applications of Photochromism


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Ionochromism

Ionochromism is the phenomenon in which a reversible

colour change is caused by interaction with an ionicspecies. A wide range of colour changes are availablewhich can be either from colourless to coloured or fromone colour to another.

The most common ionochromic materials, arguably thelongest-established useful group of chromic materials,are pH-sensitive dyes, used for decades as analytical pHindicators.

These dyes are sensitive to the hydrogen ion (H+

) andare referred to as halochromic. Another form ofionochromism involves a colour change due tointeraction with metal ions, referred to asmetal lochromism.

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Mechanism of ionochromic dyes

The main chemical classes of technically important pH-sensitive

dyes are phthalides (exemplified by the well-known pH indicator,phenolphthalein), triarylmethines and fluorans.

However, many other chromophores can undergo halochromism,

including a number of simple azo dyes, such as methyl orange (Fig.

(a) which provides a colour change from orange to red with the

formation of the species in Fig. (b) due to protonation as the pH is

lowered.

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Figure: The halochrom ism of m ethyl orange (a) and red (b).


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Crystal violet lactone (CVL), a diarylphthalide which is colourless in

its ring-closed form (Fig. 1.1(a) ). A ring-opened protonated species(Fig. 1.1(b) ) is formed as the pH is lowered. This species is highly

conjugated, although not coplanar (it is propeller-shaped), and is

thus coloured, reddish-blue in the case of CVL.

Figure: Protonation of crystal violet lactone (a, b)

Mechanism of ionochromic dyes


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General applications of ionochromic dyes

pH indicators

Halochromic dyes have been used extensively over the yearsin analytical chemistry as reversible pH indicators in detecting

the endpoint of acidbase titrations and in spot papers, such

as litmus.

Optical sensors

Halochromic dyes are used, for example, in absorbance-based

ion-selective optical sensors, which have applications in

chemical process control, medical diagnostics and

environmental monitoring.

Carbon less cop y paper

A familiar application of CVL and related halochromic dyes is

as colour formers in carbonless copy paper and direct thermal

printing where an irreversible change from colourless to

coloured is exploited. 29


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Textile applications of ionochromic dyes

The fabric dyeing with water-soluble disazo dye Brilliant Yellow

changes colour reversibly from yellow (pH 34) through orange (pH56) to red (pH 79).

In spite of the fact that they are the longest established class of chromic

materials, ionochromic dyes have been largely neglected for textile

applications. However, there has been recent renewed interest in their

potential for functional textile applications in which a visual indicator of pH

change is required.

Figure: Halochrom ic d yes: B r i l l iant Yel low (CI Direct Yel low 4)


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Text i le sensors

In a subsequent investigation, Brilliant Yellow and BromocresolPurple were successfully incorporated by electrospinning from

solution into non-woven nanofibrouspolyamide 6.6 structures to give

halochromic properties which were assessed as suitable for use in

textile sensors. It is envisaged that such sensors have potential for

application in medical textiles.

Heal ing indicat ing bandage

It has been reported that the pH of the skin of burn patients changes

during the healing process. It is thus conceivable that a colour

change on a wound dressing as a result of a localised change in pH

might be used to monitor the healing process without the need to riskdamaging the wound by removing the dressing.

Technical text i les

There are also potential applications for halochromic textiles in

protective clothing and geotextiles.31

Textile applications of ionochromic dyes


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Electrochromism

Electrochromism involves a reversible

colour change resulting from a flow of

electric current. The colour change is due

to electron transfer reactions, i.e.,

oxidation/reduction, occurring at anelectrodeoxidation at an anode and

reduction at a cathode.

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T f l t h i t i l


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Types of electrochromic materials

1. Solid electrochromic materials:All inorganic electrochromes exist inthe solid state in both the colourless and coloured states, e.g. PrussianBlue , cobalt oxide, nickel oxide, molybdenum trioxide, vanadium oxidetungsten trioxide and their mixtures.

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2. Solution electrochromic materials: In

these systems the soluble electrochrome

undergoes an electron transfer interaction

on the surface of the appropriate electrode,

involving either anodic oxidation orcathodic reduction, where it changes

colour and then returns back to the solution

phase, i.e. a coloured electrolyte is

produced.Figure: The electrochromism of methyl

viologen (a) and reduction at a cathode (b).

Figure: Prussian Blue produces the colourless Prussian White on reduction


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General applications of electrochromism

Anti-dazzle rear-view car m irrors : The mirrors contain an indium

tin-oxide (ITO) coated glass surface with the conductive side facinginwards as one electrode and a reflecting metal electrode at the

back. The gap between the electrodes consists of an electrolyte

solution or gel containing two soluble electrochromes, one of which

is oxidised at the anode and the other reduced at the cathode as

electric current flows. The devices can also incorporate sensors thatdetect headlight glare from following vehicles, sending a voltage to

the electrochromic system that is proportional to the level of light

detected.

Smart windows: These are generally constructed from two

transparent sheets of glass with conductive surfaces that act as theelectrodes, on which solid-state electrochromic materials are coated,

between which is sandwiched a conducting layer of a lithium ion

polymeric electrolyte. The windows darken electrochromically

according to the level of incident sunlight.

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Textile applications of electrochromism

The device, as illustrated in Figure, consists of electrodes based onfabric constructed from spandex (50% nylon/50% polyurethane)

impregnated with poly(3,4ethylenedioxythiophene)-po ly(sty renesulp honate) (PEDOT-PSS). One electrode is coated witha specifically synthesised electrochromic polymer, based on apolyth iophene, and the substrates are fused together with atransparent gel electrolyte. The device is capable of switchingelectrochromically between red and blue, based on the mechanismillustrated in Fig. 1.20.

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Figure: Schemat ic representat ion of an

electroch rom ic text i le .


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Textile applications of electrochromism

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Figure: Applying a current causes the fabric to change colour


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Future trends Materials that change colour have perennially attracted scientific

attention, not only for academic curiosity but also because of

perceived commercial potential.

There are now many varieties of chromic materials and a multimillion

dollar industry has developed for their manufacture and application.

Research into chromic materials shows no sign of diminishing and asa consequence new products with enhanced properties and offering

novel colour effects may emerge, with potential for new applications.

The future may also provide developments in chromic phenomena

which are as yet either unknown or unexploited industrially. For

example, there are probably significantopportunities for applications

of biochromism in medical textiles, where colour change might be

used to enable the monitoring of specific medical conditions or to

provide vital diagnostic information.

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C l i


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Conclusion

Research into the application of chromic materials totextiles has generally lagged behind other applications,

although recent publication activity suggests that it isgathering momentum.

However, in spite of considerable interest, commercialexploitation of chromic textiles has been rather limited. This

may be due to technical difficulties in application, a level oftechnical performance which does not yet match that oftraditional textile dyes and pigments, and their relatively highcost.

The most important current industrial applications ofchromic materials are in non-textile areas, and are generallyhighly technical in nature. These functional applications mayprovide inspiration for future uses of chromic materials ontextiles provided that the remaining technical challenges canbe met.

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References

1. M. L. Gulrajani , Advances in the dyeing and

finishingof technical text i les,Woodhead Publishing,UK (2013).

2. Peter Bamfield, Chromic Phenomena The Royal

Society of Chemistry, UK (2001).

3. www.colourchange.com [Access date: 24-04-2013]

4. www.qcrso lu t ions.com [Access date: 20-04-2013]

5. www.si l techl imited.com [Access date: 21-04-2013]

chomic colourants in textiles

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