8

CHAPTER 2

LITERATURE REVIEW

2.1 INTRODUCTION

As the research focuses on the effect of moisture and wetness on the

coloured cotton fabrics, literature available on the various aspects related to

this area is summarized in this chapter.

2.2 COTTON FABRICS

Cotton is one of the widely used textile fibre in the world. It is the

purest form of cellulose found in nature and is the seed hair of plants of the

genus gossypium. It is a polyalcohol and chemically described as 1, 4-linked

linear polymer of -D-anhydroglucopyranose. The polymer chains of

cellulose molecules are associated with each other by forming intermolecular

hydrogen bonds and hydrophobic bonds. These coalesce to form microfibrils

that are organized into macrofibrils. The macrofibrils are organized into

fibres.

Cotton has 88-96% odwf pure cellulose with a specific gravity of

1.5, which burns freely in air with a luminous smokeless flame. Table 2.1

shows the composition of cotton fibre and the chemical structure of the fibre

is given in Figure 2.1 (Trotman 1970). Cotton is hydrophilic and swells in the

presence of water. Normally the interactions between cotton and water are

considered to occur either in intercrystalline regions or on the surfaces of the

crystallites and the gross structures. Its porous structure allows ready

9

penetration of water molecules between the fibrils and into the amorphous

regions of the polymer where they can easily form hydrogen bonds with free

cellulose hydroxyl groups. The standard moisture regain of cotton is around

8% at room temperature and it increases up to 25% at 100% relative humidity.

The wet fibres become softer and more pliable. Unusually, the strength of

cotton fibre increases when it absorbs moisture (Wakelyn et al 2007).

Table 2.1 Composition of cotton fibres

ConstituentComposition range

(% odwf)

Cellulose 88.0 – 96.0

Protein 1.1 – 1.9

Pectic substances 0.7 – 1.2

Ash 0.7 – 1.6

Wax 0.4 – 1.0

Total sugars 0.1 – 1.0

Organic acids 0.5 – 1.0

Others 0.5 – 4.3

O

CH2 OH

H

OH

H OH

H

CH2 OH

OH

OH

H

H

O

H

H H

O

O

HH

X

O

CH2 OH

H

OH

H OH

H

CH2 OH

OH

OH

H

H

O

H H

O

H

H

HO OH

H

Figure 2.1 Chemical structure of cotton

10

The cotton fibres are used to prepare a continuous strand called yarn

by twisting them together. Such yarns are used to construct textile fabrics.

The fabrics are produced by interlacing/interlooping of yarns having

substantial surface area in relation to its thickness and adequate mechanical

strength to give it a cohesive structure. Most fabrics are woven or knitted but

some are produced by non-woven processes. Since woven fabrics are widely

used for apparels and other home applications where colour plays an

important role, it was selected for this research work.

2.3 QUALITY ASSESSMENT OF DYED FABRICS

2.3.1 Colour

Colour appearance plays an important role in the production of

textile materials. Appearance is a summary of visually perceived attributes.

The appearance of an object is the response of a complex interaction of the

light incident on the object, the optical characteristics of the object and human

perception. Colour is associated with light waves specifically their

wavelength distributions. These distributions are most often referred to as the

spectrophotometric characteristics (Aspland 1993). Visible wavelengths are

those between the violet and red ends of the spectrum, near 400 and 700 nm

respectively. The selective absorption of different amounts of the wavelengths

within these limits ordinarily determines the colour of objects (Stearns 1974).

The colour of the dyed fabric can be assessed either manually or by using an

instrument called spectrophotometer.

2.3.2 Uniformity

Uniformity of dyed material is a very important parameter to be

presented to the customer. The dye uptake of the fabric during dyeing

depends on several parameters. The uniformity of the dyed fabric will be

affected if there is a deviation in any one of the dyeing parameters. Normally

11

the uniformity of the dyed fabric is affected by means of variation from

selvedge to selvedge or centre to selvedge. The colour variation of the dyed

fabric also occurs in length wise direction. The dyed fabric should be free

from such variations in colour, to take it for further processing. The

uniformity of dyed fabric is normally assessed by using the Relative

Unlevelness Index calculated from the spectrophotometer reflectance values

measured at different places (Muthukumar et al 2005).

2.3.3 Fastness

The outstandingly important property of a dyed material is the

fastness of its shade. A number of tests are necessary to cover all the

properties of any one dye. The tests may be divided into those of consumer

significance such as light, perspiration, rubbing, washing and staining.

Grey scales have been devised for a quick and simple method of

measuring the loss or variation of shade and staining of adjacent materials.

For assessing change of colour, grey scales consist of five pairs in which one

half is always the same grey and the other is graduated from white to a grey of

about equal depth. There are five grading corresponding with different

contrasts between the halves of the scales. Another scale is used for assessing

staining, in which one half of each pair is white and the other graduated from

white to grey, showing quite a strong contrast. Again there are five grading of

which 5 corresponds with virtually no staining and 1 represents poor fastness.

In the case of light there are eight graduations instead of five. The

BS 1006 test for daylight exposure specifies that the sample should be tested

together with standard dyed wool controls of light fastness 1 to 8 respectively.

The fastness is determined by ascertaining which of the eight standards has

faded to the same extent. In this test the grey scale cannot be used for grading.

The standard dyed wool samples are also exposed to light simultaneously

12

along with the sample to be tested because when daylight is the source of

illumination, the intensity and amounts of incident light will vary every time.

2.4 COLOUR MATCHING

Colour revealed by an object is the result of the interaction of a light

source, with the object and with the observer’s eye and brain. To understand

colour, it is necessary to examine the light source, the characteristics of the

object, and the human factors. When light impinges upon an object, it can be

transmitted through the material, reflected, scattered and/or absorbed

depending on the nature of the object. The human eye perceives colour as a

result of the reaction of the object with the light source. Along with the

chromatic attributes of an object, its geometric surface attributes, such as

gloss, shape, texture and pattern may affect the reaction to light and influence

the perceived colour.

The assessment of an object’s colour appearance is mostly a

subjective phenomenon, it can vary among individuals. In practice, measuring

colour in the textile industry becomes more and more essential. In the product

development process, various parts manufactured in different industries, such

as buttons, trims, and zippers, need to be assembled together to complete a

product (McDonald 1997). In order to maintain the colour of all the

components within the given tolerance, an accurate colour measurement

method must be adopted to take pass/fail decisions. In the retail environment,

textile products can be displayed under different viewing conditions. Unless a

retailer understands and controls the colour appearance under different light

sources, the consumer will perceive the colour of the product differently in

the store as compared to outside. In addition to that, there are different ways

to purchase products, such as in a retail store, from a printed catalog, or

through internet. Different presentation methods, such as of an actual product,

13

a printed picture of a product, or a presentation of a product by a display

device, significantly affect the colour appearance.

Colour of textile material is an important parameter to be presented

to the customer. Therefore, the colouration of textiles, its assessment and

matching are important phases in production of textile materials (Needles et al

1983). The orientation of molecules in the fibre also has an influence on the

reflectance value of dyed fabric (Giles et al 1961). The colour assessment and

matching can be done either visually or using instruments such as

spectrophotometer (Stearns 1974, Stanziola 1979).

2.4.1 Manual Method

The visual examination of dyed materials can be made by keeping

the production sample and standard side by side under standardized lighting

condition. The human eye and brain have incredible sensitivity to detect even

a small colour difference. But the result of visual assessment is significantly

influenced by the various parameters such as source, illuminating viewing

conditions and surroundings. Like all physiological systems, the

characteristics of the visual system vary among individuals. No two

individuals are likely to have exactly the same wavelength dependent

response characteristics (McDonald 1997). Individual observers vary

significantly in their judgment of perceived colour differences. Colour

perception abilities depend on an individual’s cone sensitivities, degree of

colour blindness, age, general health, and even attitude. Observers are also

influenced by adjoining or background colour and the relative sizes of areas

of contrasting colour. The gloss and texture of a surface also affects

perception of colour. Therefore, a standard environment is required for visual

assessment. In addition, a colour vision test for observers should be conducted

to qualify observers prior to visual perception assessment. Visual assessment

of coloured samples, for purposes of colour control and specification, requires

14

careful control of several factors (Berns 2000). Hence the conditions for the

visual colour assessment have to be standardized (Hoban 1981).

2.4.1.1 Background colour of the sample

In order to maintain the stable illuminating conditions, a light

cabinet should be used. The cabinet’s spectral power distribution and level of

illumination should exactly duplicate the lighting conditions for which the

sample is being specified. The surroundings and background of the sample

needs to be defined. In a light cabinet, the interior walls and background

should all be matte and neutral and have a middle lightness value.

2.4.1.2 Nature of light source

Once the cabinet’s background and surroundings are defined, the

spectral power distributions and the level of illumination of the light sources

selected must be standardised.

2.4.1.3 Size and distance of the samples

When observing samples of different sizes, different areas of the

retina are used and the colour appears different. Therefore, sample size should

be consistent. Observers should view the samples from a predetermined

distance such that the visual angle is not less than 2°.

2.4.1.4 Angle of illumination

Specimens should be placed on the floor of the booth so that the

illumination is centered perpendicular to the plane of the specimens. The

observation angle is 45° from normal to the specimens where normal is

considered to be perpendicular to the specimen. It is important to maintain the

same viewing conditions during visual assessment.

15

2.4.2 Instrumental Method

The colour perceived by an observer results from the interaction of a

light source with a sample and the observer. When light incident on an object,

an observer perceives colour by detecting the light reflected from that object.

The eye has sensitivity to light at three different parts of the visible spectrum.

Colour perception starts with the spectral characteristics of the light source,

which are then modified by the reflectance of the object. Hence, the perceived

colour depends on the spectral power distribution of the light source, the

reflectance of the object and the spectral response of the eye. When obtaining

spectral data, the standard light sources along with the standard reference

white, the geometry and the viewing conditions of measuring devices should

be defined. To overcome the problems associated with visual colour

assessment, instrumental colour measurement and matching is gaining

importance in all the industries dealing with the colouration of products. In

colour measuring instrument such as spectrophotometer, the colour of an

object is measured and represented by spectrophotometric curves, which are

plots of fractions of incident light as a function of wavelength throughout the

visible spectrum relative to a reference (Harold 2001). The typical reference is

a white standard that has been calibrated relative to the perfect white

reflecting diffuser (100% reflectance at all wavelengths). During colour

measurement, in principle the medium of the material is assumed to be the

same as the medium of the light and the incident light undergoes absorption,

reflection and transmission (Kubelka 1948 and Kubelka 1954). If the incident

light is diffused, there will be either a quantitative change in transmitted and

reflected radiation or a change in direction of mass action in the case of

reflection. However, the degree of diffusion remains constant (Benford 1946).

But in the case of luminescent material, the absorption and reflection will

vary from normal colouring material (Melamed 1963). The Computer Colour

16

matching technique has also lot of limitations to match the colour of the

textiles with standard (Brockes 1974).

2.5 INSTRUMENTAL COLOUR MATCHING

2.5.1 Colour Measurement

Spectrophotometers have been in use for colour measurement for

almost a half century. Spectrophotometers and colorimeters can have two

basic optical designs, monochromatic illumination (forward mode) or

polychromatic (reverse mode). For nonfluorescent samples, the measurements

are identical for both. However, if a fluorescent sample is measured by

monochromatic illumination, erroneous data will be obtained, which will

indicate a colour much lighter and duller than the sample. Several instruments

have been made which readily convert to either mode. These instruments are

quite useful in providing data on the nature of the fluorescence and

absorption, by the two-mode method (Hoban 1981).

Today, most spectrophotometers use diffraction grating. By passing

a beam of light through glass with many narrowly spaced ruled lines, the light

can be diffracted by the wavelength. Dispersed light is focused onto the

detection array and the number of detection array elements can be determined

according to the desired wavelength resolution. The electrical signal processor

amplifies, digitizes and numerically processes each signal and a spectral

reflectance or transmittance factor is yielded across the visible spectrum

(Hunter and Harold 1987, Berns 2000). Since reflectance and transmittance

are ratios, the power distribution of the spectrophotometer’s light source does

not affect reflectance and transmittance. Hence XYZ or L* a* b* coordinates

can be calculated with the particular illuminant (McDonald 1997). Depending

on the geometric attributes of the surface, such as gloss or texture, the spatial

or angular distribution of the reflected light can be changed. These geometric

17

attributes are eliminated from colour measurement. However, these surface

properties are very important in defining a material’s total appearance.

The colour appearance of textile materials is computed from their

reflectance. The relationship between the dye content and the reflectance

spectrum is of importance (Tsoutseos and Nobbs 2000). In 1931 Kubelka and

Munk expressed this relationship with the assumption of a two-flux radiation

and derived the behavior of light-scattering colourant layers. The reflected

light from the surface of a material depends on the thickness of the colourant

layer, scattering and absorption coefficients of the coloured layers and the

reflectance of the background on which the material lies. All these parameters

are summarized in equation (2.1).

g

g

RbSXctghba

)bSXctghba(R1R (2.1)

whereS

KSa

2/12 )1a(b

X – Thickness of the specimen

Rg – Reflectance of the background

K – Coefficient of absorption defined by the corresponding

thickness of layer

S – Coefficient of scattering defined by the corresponding

thickness of layer

Ctgh – hyperbolic cotangent

18

In the case of an opaque layer that is sufficiently thick so that the

reflectance of the background has no effect on the reflectance of the colourant

layer, it is given by the equation,

)(S

)(K2

)(S

)(K

)S

)(K1)(R

2

(2.2)

where R is the reflectance at infinite thickness.

Reversing this equation, the well known relationship betweenS

K

and R ,i as given in equation (2.3) is obtained.

i,

2

i,

R2

)R1(

S

K (2.3)

The theoretical model assumes that the light within the colourant layer is

completely diffused and that there can not be a change in the refractive index

at the sample’s boundaries (Berns 2000). It is assumed that the additive

function is valid for the absorption and the scattering coefficients. This means

that the absorption coefficient of a colourant layer is the sum of the weighted

absorption coefficients of the coloured materials. The mathematical

expressions are shown in equations (2.4) and (2.5).

K = C1k1 + C2k2 + …. + Cnkn (2.4)

S = C1s1 + C2s2 + …. + Cnsn (2.5)

where Ci is concentration of dye on the materials.

19

Dividing equation (2.4) by equation (2.5) yields

nn2211

nn2211

sCsCsC

kCkCkC

S

K (2.6)

The above equation (2.6) gives the two constant Kubelka Munk theory.

For materials such as textiles, where the colourants do not scatter in

comparison to the substrate, the mixing equation is simplified to equation

(2.7) (Berns 2000).

n

n

2

2

1

1s

kC

s

kC

s

kC

S

K (2.7)

This is single constant Kubelka Munk theory.

The Kubelka-Munk theory is widely used and has been applied for

colour matching in textile colouration processes including conventional

printing, dyeing and the blending of coloured fibres (Garrett and Peters 1956,

Stanziola 1979, Amirshashi and Pailthorpe 1997). In the case of textile

materials, the absorbing-scattering substrate cannot be considered as

continuous one. Moreover in the Kubelka-Munk theory the influence of

surface texture of the fabric on reflectance is not considered. Then the theory

has been mathematically expanded using empirical corrections to apply the

theory to a wider range of materials including textiles (Saunderson 1942). The

theory has also been thoroughly discussed and expanded by Nobbs (1985).

Atherton (1955) attempted to incorporate collimated beam illumination into

the theory. Preston and Tsien (1946) proposed a model where the fibre

structure is accommodated using the pile of plates approach and the Beer-

Lambert law. Some of the authors have also approached to modify the

20

Kubelka-Munk equation. Sokkar et al (1992) reviewed and expanded the

above approach.

The fibre optical sensors can also be used to measure absorbance

value of dye liquor during dyeing and it can be correlated to the K/S value of

the corresponding dyed samples (Ericson and Posner 1996). The effect of the

surface reflectance in Kubelka-Munk theory has been discussed by Stearns

(1969) and Kuehni (1975) and a correction for surface reflection was made.

2.5.2 Parameters Involved in Colour Measurement

2.5.2.1 Dyed fabric

The colour of the dyed fabric depends upon the structure of fabric,

quality of the raw material used, type of fibre and its properties (Etters 1997).

2.5.2.2 Light source

Light is a form of energy. It generates heat when it strikes an object

to such an extent. Light is part of a great spectrum of energy that varies from

x-rays to radio waves and is called electromagnetic energy. A part of this

spectrum is able to activate the retina of the eye and produce sensations of

light and colour. Light varies in many ways such as direction, intensity and

polarization. But the most important variation from the colour viewpoint is

wavelength (Stearns 1974, 1974a).

Light source should be stable, directible, should have long life and

continuous spectral power (Nickerson 1948). Main source of light are low

voltage tungsten lamp, tungsten halogen lamp, xenon arc/pulse lamp. The

new technology has provided xenon flash tube which gives flash of light of

few nano second duration, which eliminates heating and fading of colour of

samples.

21

2.5.2.3 Detector

The photo detectors are used to convert light signal into electric

signal to transmit information to signal processor. The photo detector may be

photocell, photo multiplier or solid state detector. The latest instruments

mostly employ silicon photo diode type solid state detector. In modern

instruments an array of sixteen such detectors are arranged to sense the signal

simultaneously at sixteen wavelengths covering the entire visible region.

2.5.3 Properties of Dyed Fabric

Fabrics to be dyed should have semi-oriented and amorphous

regions for easy penetration of dyes. Dyed samples should be taken from one

lot only for getting identical results. Colour recipe used for dyeing the sample

should be selected with proper care. During dyeing it is assumed that dyes

penetrate only the amorphous and semi-crystalline portions of the fibre. The

result in colour produced by a given dye on a substrate is a function of the

chemical and physical nature of the dye-substrate interaction.

2.5.3.1 Fabric geometry

It is one of the most important aspects in colour measuring

instruments. It deals with the presentation of sample to the light source and

the detector. This is the geometry by which incident light is introduced on to

the sample and by which reflected light is collected by the detector. The fabric

warp-weft density variation and fabric porosity has also influence on the

colour of the fabric (Cay et al 2007).

When an object is observed, its characteristics can be determined to

a limited extent by the way the light is reflected from the surface of that

object. The perception of surface texture via light stimuli is significant in

22

terms of assessing colour appearance (Etters 1997). Different surface features

create varying directional distributions of light allowing the surface to be

analyzed on the basis of geometrical optics. With a smooth surface the

reflection of light from the surface follows the laws of reflection so that the

angle of reflection equals the angle of incidence. Figure 2.2(a) shows a

schematic diagram of reflected light on a smooth surface. The phenomenon is

known as specular reflection. On the other hand, if the surface is

microscopically rough, the light rays will reflect and diffuse in many different

directions. Though each individual ray follows the laws of reflection, each ray

meets the rough surface with a different orientation so the normal line at the

point of incidence is different for different rays. Figure 2.2(b) describes the

diffuse reflection from a rough surface (Stearns 1969, Friedman and Miller

2004). Several reflectance models have been developed based on rough

surfaces (Beckmann 1969 and Yasuda et al 1992). These models are

especially useful in the computer graphics field to simulate objects more

realistically in the virtual world. They account for complex geometric and

radiometric phenomena such as masking, shadowing and inter reflection

between points on the surface (Nayar 1991).

The geometrical attenuation factor (masking and shadowing) was

derived by Torrance and Sparrow (1967). They modeled the light reflection

on a rough surface with a symmetrical V-groove cavity. Diffuse inter

reflection is a process whereby light reflected from an object strikes other

objects in the surrounding area and illuminating them. Therefore a rough

surface will scatter the incident light into various directions, though certain

directions may receive more energy than others, or reduce the light intensity

by blocking a portion of the incident light (Figure 2.3).

23

(a) (b)

Figure 2.2 Schematic diagram of light reflection from (a) smooth and

(b) rough surface

a) Masking effect b) Shadowing effect

c) Inter reflection effect

Figure 2.3 Schematic diagram of a) masking, b) shadowing and c) inter

reflection effect of objects

Reflected

lightIncident

lightReflected

light

Incident

light

24

Woven textiles are constructed in diverse structures from yarns

which can have different diameter and twist. The yarns are made from fibres

with various structural properties such as cross sectional shape, diameter and

longitudinal shape. Reflection from them occurs between two media namely

air and a fibre or air and a dye molecule.

Figure 2.4(a) shows light striking on a simplified fibre having

circular cross section. When a light beam strikes normal to the surface and is

passed back from the media, reflection occurs. The amount of the surface

reflected light from many textiles normally falls somewhere between 0 and

4% (Stearns 1969). Light beam hits at a glancing angle and most of it is

reflected in a forward direction to strike another fibre (Hunter 1963 and

Stearns 1969). When many fibres are grouped in a yarn, as shown in

Figure 2.4(b), some of the reflected light from the surface becomes trapped

and lost by absorption (Garret and Peters 1956, Stearns 1969). Figure 2.4(c)

presents pile fabrics, such as velvets, corduroys, and carpets, which have

more opportunity for the incident light to be trapped between the fibres or

yarns.

When textiles of different structures are dyed or printed with the

same colourants and under the same conditions, the colour appearance can

vary according to the configuration of the fabric, fibres or yarns. The micro

scale structure of the fibre, the yarn and the fabric may change the colour

appearance (Lambert et al 1986). The major elements that can affect the light

reflectance of a fibre are its length, diameter, cross sectional shape (such as

round, triangle, striated or grooved), longitudinal shape (such as crimp, spiral

or twist) and the surface texture. For instance, the length of natural fibres

varies. The grade of fibre is determined according to the average staple

length because it is related to mechanical properties such as the strength of

the yarn produced from it. However it also affects the visible colour of the

fibre, yarn and fabric.

25

a) Reflection on a fibre

b) Inter reflection among fibres

c) Inter reflection in a pile fabric

Figure 2.4 Schematic diagram of light reflection on a (a) fibre, (b) yarn

and (c) pile fabric

26

Fabric woven from a coarse yarn has more surface texture than a

fine fabric and this reinforces the dark value already fostered by the coarser

yarn’s greater diameter. Cross sectional shape varies widely and can be

influenced by finishing. For instance, mercerization of cotton fibre, which is

performed with sodium hydroxide (NaOH), changes the cross-sectional shape

to a circular form through a swelling process. Mercerization affects the light

reflection and absorption of the fibre.

Yarns composed of staple fibres are twisted to prevent fibres in the

yarn from slipping over one another. The number of fibre ends sticking out of

the yarn, the amount of twist imparted to the yarns in spinning, the direction

of the draw of the yarn, and the type of yarn construction influence the

surface texture of yarn and hence the colour appearance of yarn. For instance,

highly twisted yarns appear darker than low twist yarns of the same fibre. The

surface textures of fabrics are created by the yarn twist, yarn density and

weave structure. A commercial damask design uses the surface reflection

effect which makes one area appear darker than the other, depending on the

angles of illumination and viewing conditions (Stearns 1969). When light hits

a smooth, uniform surface, some is transmitted through the object and the rest

is reflected in an orderly way, causing the surface to appear very bright and

creating a rich colour effect.

The relation between visual texture perception and the physical

characteristics of the fabric, such as the geometric structure and the optical

properties in reflected light, was investigated by Lee and Sato (1998, 1999

and 2001). Also, they examined the reflected light characteristics of different

weave constructions by using agonio spectrophotometer. The authors

concluded that light reflection is influenced by the weave direction of the

fabric. They found that the surface colour of fabrics varied with the warp and

weft yarns on the surface and that the perceived texture could be anticipated

27

by the characteristics of the reflected light. Considering different weave

structures and yarn twists of fabric made from polyester filament yarn, Kim et

al (2004) reported that one of the important surface characteristics of textiles

is lustre.

Texture of the fabric also affects colour mainly by influencing the

effects pronounced by variations in the geometry of illumination and viewing.

Direct illumination from a single angle may result in numerous small shadows

so that at different angles of viewing the shadowed area or the illuminated

area may predominate.

2.5.3.2 Dyes used for dyeing

Depending on the characteristics of fibres a suitable class of dyes is

selected for colouration process. Dyes should have characteristics such as

a) solubility b) affinity to fibre c) intensity of colour d) compatibility with

other dyes and e) adequate fastness properties.

2.5.3.3 % shade

The intensity of dyes, used for dyeing depends on the % shade

required. The quantity of dye required to prepare dye liquor is always less for

lighter shades and more for darker shades.

2.5.3.4 Presence of other foreign materials

The colour of the object will vary, if any other foreign matter is

present in it apart from colouring matter. In the case of textiles, the fabric

which is directly taken from the dyebath may carry surface deposited unfixed

dye molecules, dyeing auxiliaries, moisture etc. These foreign materials also

can have interaction with light and inturn affect the colour of the material.

28

2.5.3.5 Moisture content

In the production process, colour of fabric is assessed either at the

end or during the process of dyeing. In both the cases the material has to be

completely dried and taken for colour measurement since the colour

appearance of textile material changes with moisture content. Even a small

amount of water can dramatically change the colour appearance of the

material (Goldfinger et al 1970, Allen et al 1972 and 1973, Smith 1979,

Dalton et al 1995, Tsoutseos and Nobbs 1998 and 2000, Manian et al 2000,

Jahagirdar et al 2002).

2.6 ON-LINE COLOUR MEASUREMENT

In continuous dyeing, colour assessment is made while processing

by reflectance measurement of the fabric at the exit of continuous dyeing

range after drying. Keesee and Aspland (1988) discussed the causes and

magnitudes of colour changes in on-line colour measurement for cotton

fabrics dyed with selected fibre reactive and sulphur dyes and finished with a

durable press finish. When using on-line colour measurement, it is important

that both the temperature and the moisture content on the goods are as

consistent as possible from side to centre to side and from piece to piece,

because the measured colour is strongly dependent on both. The colour of the

fabric also depends on the amount of water present in it after drying and the

chemicals used in dyeing process. It has been suggested that sulphur dyeing

on cotton changes shade quite markedly on ageing (Brown 1984).

Wersch (1990) discussed the various parameters involved in on-line

colour measurement with respect to machine. The dyeing machine

requirements such as automatic liquor change, constant speed, infrared dryer

and uniform drying mechanism were discussed. The material colour was

analysed at various stages of dyeing and the effect of fabric speed, low

29

residual moisture, high residual moisture, average moisture and temperature

on colour and colour difference values were also analysed. He also discussed

on-line liquor pick-up measurement with level correction and control in

dyebath. The purpose of on-line pick-up measurement through continuous

measurement and logging of the actual liquor consumption rate is to give the

production management sufficient confidence to enable it to reduce the

amount of excessive liquor formulated without any danger to the production

process. He also explained an on-line colourimetry process for measuring the

intensity of colour in the wet fabric downstream of the padder. An on-line

colourimetry system provides an indication of the distribution of the dyestuff

over the textile web.

Wills (1992) discussed the roadblocks in implementing on-line

colour monitoring instruments. Compared to laboratory colour measuring

systems, on-line systems are more complex and present a wide array of new

problems. Inter-instrument agreement, calibration, storing standards,

establishing tolerances, linking to remote system and sampling methods are

examples of some of the more common problems.

An evaluation is made immediately downstream of the padder,

indicating whether the fabric has been uniformly dyed. This is an

improvement over conventional systems where such an evaluation is only

made at the outlet of the dyeing range. This means that the colourimetry

results can be employed for production control purposes, as has already been

confirmed in practice (Wersch 1993). This system can be further expanded

for implementation of automatic padder control using computer. The data

conditioned by a computer for output to the screen and printer are further

processed in a computer and then transferred to the padder programmable

logic controller (PLC). This PLC then transmits the signals, indicating the

necessary pressure changes, to the padder. Line pressure changes in the

30

padder result in a modified colour profile on the fabric and this in turn is

detected by the colorimeter and fed back to the computer. The relevant

tolerances and increments can be preprogrammed into the system as desired

(Kazmi et al 1996).

In continuous dyeing, one of the biggest developments of the last

decade has been the automatic measurement and control of moisture or wet

pick-up. This type of control is achieved by the use of state-of-the-art non-

contact radiation-based moisture measurement device and modern squeeze

rolls. Even though this provides a dyer with an opportunity to control one of

the most important parameters in pad dyeing, it does not facilitate direct

measurement of the dye- liquor add-on. An on-line colourimetry process for

measuring the intensity of colour in the wet fabric downstream of a padder

can also be used. The result of on-line spectrophotometry is influenced by the

background of the sample to be measured, the ambient light, the distance

between measuring head and fabric, the unevenness (waviness) of the fabric

surface, fluttering of the fabric, vibration in the measuring head and

atmospheric contents and contaminants. This measurement can be useful in

the implementation of automatic control of the nip line pressure for

uniformity in colour (Kazmi et al 1996).

Pleva AF 310 (Germany) is a moisture measurement system

equipped with microwave emission units and detectors opposite to each other,

separated by a layer of the fabric to be measured. This unit can be used for

monitoring/recording only or, as some companies do, for controlling nip

pressure. Since it measures water content, it has no idea about colour. A

typical problem of continuous dyeing is tailing due to substantivity of dyes

causing change in concentration during fabric run. This instrument cannot

detect the dye distribution on the fibre, either (Murthy 2003).

31

2.6.1 Effect of Moisture Content on Colour of the Fabric

It is well known that when light falls on textile material scattering

takes place at the surface, which depends on its surface characteristics. In

addition to this, light also undergoes diffusion through the material resulting

in absorption and scattering within the material. Finally the scattered light

comes out of the material as diffuse reflection, which depends on the extent of

internal scattering that take place (Munsell et al 1933, Billmeyer and Smith

1967). This internal scattering depends on number of dye molecules present

and number of other atoms/molecules present, which may be air, water or

chemical compounds.

When dyed textiles are transferred from the dry to wet state, their

reflection behaviour changes and resulting in reduction in amount of light

reflected (Jahagirdar et al 2002) (Figure 2.5). This drop in reflectance is due

to reduced light scatter, while light absorption remains constant. Frequently

this transformation from the dry to the wet state is also accompanied by a

change in shade. The drop in reflectance due to moisture depends on the

substrate and the reflectance level but not the shade. Allen and Goldfinger

(1971) noted that a decrease in scattering efficiency would provide more

opportunity for absorption of light in the sample and thus contribute in its

brightness.

Dalton et al (1995) presented a graphical representation of

reflectance changes with varying moisture content and a method of

representing all reflectance changes for reactive dyes on wool on a single

graph. Moisture measurements and colour readings were taken at

approximately 15 min intervals for the first 30 min, then at 20 min intervals

for the next 60 min, and finally at 30 min intervals for the next 90 min during

drying process. This ensured an even spread of moisture content readings. A

final reading taken 48 h later was used as the dry colour. They concluded that

32

the reflectance change from wet to dry is independent of the colour of the dye

used, i.e. wavelength, rather it is related to the dry reflectance at discrete

wavelengths. As reflectance changes are the same for dyed and undyed

samples, the dye itself can play no part in reflectance changes at different

moisture contents.

Figure 2.5 Effect of moisture content on reflectance values of fabric

dyed with C.I. Direct Blue 77

The effect of moisture on fabric colour appearance has also been

discussed by Kazmi et al (1996) in on-line colour monitoring dyeing process.

Lee et al (2004) mentioned that the fabric with higher moisture contents

appear to be darker in colour than fabric with low moisture content.

33

A geometric model was used in the prediction of colour appearance

of dry fabrics from their measurement in a wet state (Tsoutseos and Nobbs

2000). This approach can be applied to on line colour measurement. The

model is based on the basic principles of optics so it can accommodate

changes of fibre geometry and embedding medium. The potential application

of predicting the reflectance of dyed woollen and nylon fabrics in dry state

from the reflectance measured at wet state i.e., immersed in water was tested.

The results obtained were encouraging for synthetic fibres with low levels of

delustrant but when applied to natural or dull synthetic fibres, the model tends

to underestimate the reflectance. Hence the developed geometric model may

not be useful in predicting the dry reflectance of dyed fabric from its wet state

for cotton and other natural fibres having different levels of moisture in it.

A study was carried out by Manian et al (2000) with three direct

dyes and cotton knitted fabric and the wet and dry colour of these fabrics were

analysed. A Change in colour of dyed specimens was quantified through

CIELAB measurements and reflectance spectra. The effect of print house

humidity and temperature on the colour of printed material was analysed with

three different colours on cotton, silk and nylon 6,6 fabrics by Yang et al

(2006).

The effect of moisture on colour of soil and other materials were

also discussed by various authors. A study was carried out to model

reflectance changes due to soil moisture in a real field situation using multi

resolution airborne spot data. The proposed exponential model was not valid

when all soil categories were considered together. However, when fitted to

each category, the root mean square error on moisture estimates ranged from

2.0% to 3.5% except for silty soils with crusting problems (Muller and

Decamps 2000). Barrett (2002) discussed the overall reduction of reflectance

with increasing moisture content in the well drained sandy soils. The change

34

in colour of soil with respect to the moisture content was also discussed by

O’Neal (1923). The colour of the banana pieces was also analysed by Chua et

al (2001), at various moisture levels and drying temperature. Moisture effects

on visible spectral characteristics of lateritic soils was discussed by Bedidi et

al (1992) Bhadra and Bhavanarayana (1997) discussed the estimation of the

influence of soil moisture on soil colour. Coleman and Montgomery (1987)

also discussed the effect of soil moisture, organic matter, and iron content on

the spectral characteristics of selected soil. Spectrophotometric measurement

of soil colour and its relationship to moisture and organic matter was

discussed by Shields et al (1968). A study was carried out to evaluate the

surface-soil water content by measuring the reflectance of soil by Skidmore et

al (1975). Rao et al (2009) have discussed about the influence of moisture

content of the dye powder on shade reproducibility. They concluded that the

variation in relative humidity of atmosphere influence the moisture content of

dye which inturn affects the colour reproducibility.

2.6.2 Effect of Refractive Index on Colour of the Fabric

Coloured objects will scatter part of the incident light, but in

addition they preferentially absorb certain wavelengths of the mixed radiation.

The percentage of the total incident light, which is scattered, depends on the

difference in refractive index between coloured objects and surrounding

medium.

Devore and Pfund (1947) discussed the effect of surrounding

refractive index on optical scattering of dielectric powders of uniform particle

size and they suggested an empirical relationship to determine the particle

size for white pigment paints. Garrett and Peters (1956) discussed the effect

of dye penetration on reflectance of nylon and terylene fibres dyed with

disperse dyes. In their study they assumed that the refractive index of the

35

fibres before and after dyeing remains same. Finally they concluded that the

variation in reflectance due to change in refractive index of fibre is small.

Goldfinger et al (1970) reported that the most important reason for a

fabric to look darker when wet than dry is that the ratio of the refractive

indices of the fabric to that of the continuous medium (water or air) was

significantly reduced, thus resulting in reduction of the scattering efficiency

of the fabric. According to Allen et al (1973) if the ratio of the refractive

indices of the fibre and continuous medium is 1, then the sample is black

regardless of the other optical properties of the substrate. Also, as the ratio of

refractive indices deviates more and more from unity, the sample becomes

less and less dark, since the scattering efficiency increases and light is back

scattered having had fewer opportunities to be absorbed. Allen et al (1972)

discussed the effect of refractive index of the continuous medium on colour.

In this study the light scattering-absorbing substrate used was polyester fabric

and the liquid continuous media used were water and a 63.7% sucrose

solution. The refractive index of continuous medium and textile materials was

measured with the Abbe refractometer. A theory was established by them to

predict the dry colour of a fabric from its wet colour as a function of the

refractive index of the continuous medium.

An approach has been proposed by Allen and Goldfinger (1972)

that permits independent determination of all variables such as coefficient

absorption of the dye, refractive index of the fibre, the effect of geometry of

the fabric and yarn and the distribution of the dye within the fibre. The

approach was based on ‘pile of plates’ model. However, unlike the Kubelka

method, it leaves open the possibility to consider the geometry of the array

constituting the plate. It also permits one to include the effect of the

coefficient of absorption of the fibre-dye system, the refractive indices of the

fibre, and the distribution of the dye in the fibre on the colour of the textile

36

substrate. Goldfinger et al (1973) discussed the effect of distribution of

colourant on the colour of fibres. Based on their discussion, in the wavelength

range in which no light is absorbed, the distribution of the dye can have no

effect and the reflectance ratio between ring dyed material and

homogeneously dyed material must be one. If the entire refracted light is

absorbed then only that reflected from the fibre surfaces can contribute to the

reflectance of the sample and, if there is no change in the refractive index of

the ring dyed and homogeneously dyed material, the reflectance ratio for

those materials also has to be one. Allen and Goldfinger (1973) proposed a

new approach to the prediction of the colour of absorbing-scattering

substrates such as fabrics by using the optical properties of the fibres and the

medium of observation. They also included the refractive index of the fibre

and the medium in their model.

Goldfinger et al (1974), theoretically assumed the refractive indices

of the dyed and undyed portions of the ring dyed filament to be the same. In

refractive index measurement they did not observe any effect of the dye on

the refractive index with monochromatic radiation at 436 nm. But they

observed the curved light path in the dyed portion, when radiation of 546 nm

was used. A gradual change of refractive index with penetration will give the

same effect on colour. They observed significant light absorption at 546 nm in

the dye-fibre system with the red dye. Lee and Patterson (1985) discussed the

effect of dye penetration on the resultant colour of polyester fibres using

fibre-chop method and analysed the models developed by Garrett and Peters

(1956) and Allen and Goldfinger (1972) with respect to refractive index,

diameter of the filaments and the concentration and absorption coefficient of

the dye. Manian et al (2000) and Tsoutseos and Nobbs (2000) also reported

that the change in refractive index of fabric is the reason for change in the

colour of the fabric in wet condition. Motamedian and Broadbent (2003)

suggested an optical model to predict the colour depth of an array of

37

filaments, representing textile fabric, with various distributions of dye in

either the entire filament assembly or in individual filaments based on the

light reflection and refraction in the filament.

2.7 COLOUR MEASUREMENT IN WET STATE

The wet to dry colour change slows down the process of colour

matching because, in present practice, a sample from a dyebath must be dried

before it can be assessed. The process would be much more efficient if the

colour of the sample when dry could be accurately predicted from wet sample

taken, fresh from the dyebath. Several parameters such as quantity of

moisture content, refractive index of surrounding medium and fibre directly

influence the colour of the fabric.

2.7.1 Determination of Moisture Content

To assess the dry state colour of material, quantification of moisture

in the material is necessary. Wetting of fibres is a displacement of a fibre-air

interface with a fibre-liquid interface (Kissa 1996). The temperature

influences the moisture sorption of cellulose fibres (Collins 1922) and the

chemical as well as physical property of cellulose also depends on the

moisture in the material (Fargher and Williams 1923). Moisture pick-up

measurement provides data concerning the water content per unit area of

fabric, but it is often found to be the case that uniform moisture levels do not

necessarily mean uniform dyeing. Moisture measurement is unable to provide

any direct information regarding the actual distribution of dyestuff within the

fabric. Determination of moisture content in the material can be done using

several methods.

When moisture is transferred to cotton fibre, initially water

molecules are attached with the hydroxyl groups present in the fibre. The

38

water molecules which are attached with hydroxyl groups are called ‘Bound

Water’. The bound water content in the fibre can be estimated using the

cooling curve obtained by the Differential Scanning Calorimetry (Nakamura

et al 1981 and 1983, Hatakeyama and Hatakeyama 1998, Hatakeyama et al

2000).

Gregory (1930) discussed the mechanism of water vapour diffusion

through the fabric and also suggested a test for the transfer of moisture. He

concluded that vapour diffusion through fabrics is independent of the rate of

passage of air under pressure. Tankard (1937) discussed the determination of

water in cellulose by hydration. In this study cellulose was allowed to attain

equilibrium and then was subjected to a gradually increasing pressure.

Samples were taken at intervals during the application of pressure, and their

composition determined. Using graphical method the amount of water and

other solution in the material was measured. Fourt and Harris (1947) analysed

the diffusion of water vapour through textiles. They concluded that, the

resistance towards diffusion in to a woven fabric depends on the kind of fibre,

the thickness and its tightness of weaving.

Liepins and Kearney (1971) discussed the water vapour barriers for

aromatic and aliphatic hydrocarbons, nitriles, chlorine-containing compounds

and a silane coated paper. A method was described for measuring the water

vapour resistance of textiles under variable conditions of relative humidity by

Farnworth et al (1990). This proposed method consists basically of varying

the position of the sample in an air gap between a wet and dry surface while

keeping all other conditions constant. The resistance was determined by the

rate of water loss and the temperature of the water.

The moisture content in the fabric was also determined by drying it

at various conditions. The effect of drying conditions on moisture transfer

through fibres and fabrics was studied by Fourt et al (1951) and Crow and

39

Osczevski (1998). They concluded that the fibre and fabric structure has great

influence in transporting moisture through it during drying. Several research

works have been carried out by developing mathematical models to analyse

the diffusion of heat and mass transfer through fabrics (Chen and Pei 1989,

Lee et al 2002, Etemoglu et al 2009).

2.7.2 Determination of Refractive Index

The refractive index of textile fibres can be determined using

several methods such as double variation method, Becke line method,

immersion technique (Freeman and Preston 1943, Allen et al 1973) etc. In the

double variation method, the fibre was immersed in a liquid, the refractive

index of which is near that of the fibre. If the indices of the fibre and fluid are

the same, the fibre will move to extinction (disappear) indicating a perfect

match between fibre and fluid indices (Fox 1939). His further study along

with Finch, suggested a simplified method for determining the refractive

indices of fibres which uses as its basis, a photometric match of Becke line

intensities emanating from the difference between the maximum and the

minimum refractive indices of the fibre and the index of the mounting fluid

(Fox and Finch 1940).

Preston (1947) discussed Schroder van der kolk method for

determination of refractive index of rounded cross section fibres. In this

method, the fibre acts as either a positive or a negative cylindrical lens

depending on whether its refractive index is greater or lesser than that of the

surrounding medium. The relationship between the density and refractivity of

cellulose fibres with respect to their structure is dealt by Hermans (1947).

Heyn (1952, 1953) proposed central illumination method to measure the

refractive index of fibres. In this method, the fibre acts as a lens (concave or

convex) with respect to the difference in refractive indices between those of

liquid medium and fibre. Using microscope the refractive index of the fibre

40

can be measured. Barakat and El-Hennawi (1971) used immersion technique

and multiple beam fizeau fringes to measure the refractive index of textile

fibres (Hamza and Sokkar 1981 and Hamza et al 1996). Conde et al (1996)

used refractive near-field (RNF) method to measure the refractive indices of

multicore fibres. Zhao et al (2003) suggested a non-destructive technique for

the measurement of refractive index of hollow fibres.

2.7.3 Assessment of Dry Colour of Fabric in Wet State

Several techniques were proposed to predict the dry state colour

from wet state colour of textiles. Studies on the effect of moisture content on

the colour appearance of the dyed textile materials by dyeing the fabrics,

geometric model to predict the dry reflectance value from wet materials and

the use of refractive index of the embedding medium in assessing the dry

colour, the modified Kubelka-Munk equation and statistical equation to

predict the dry reflectance value from wet materials are given below.

Prescott and Stearns (1969) described a method using software for

determining the concentration of any dye in a formula having a fixed ratio of

dyes, which would produce the maximum visual effect of an oil stain on

cotton fabrics which is opaque in nature. An SOB (soil on black) index was

also proposed to give the relative visibility of oil on any particular fabric. The

small oil spot was measured with the R-cam on a General Electric

spectrophotometer. The colour difference between the oil stained fabric and

the normal fabric was calculated using MacAdam-Friele-Chickering colour

difference formula (Chickering 1967). But the location of the soil in the

fabric, influence of illuminating and viewing conditions and the effect of

fluorescence on colour have not been investigated in the above study. The

effect of pattern has not been studied but it is believed that a pattern would

reduce the apparent colour.

41

Goldfinger et al (1970) presented an empirical equation and

experimental results relating the colour of an absorbing-scattering substrate

(cellulose triacetate dyed with disperse dye) under two different viewing

conditions that is, dry, in which case the continuous medium is air and wet,

for the sample immersed in water. These measurements have been carried

out as exploratory steps in preparation for the development of a general

treatment of the light reflectance from an absorbing-scattering sample in

which the refractive indices of the scattering particles and the continuous

medium appear explicitly. Ratio of the reflectance of the sample immersed in

water to that of the dry one plotted against the reflectance of the dry sample

and the reflectance of the sample immersed in water predicted using the

equation (2.8) plotted against the measured values are given in Figures 2.6

and 2.7 respectively.

)R20.0()R10.0(

)R1(12.0RR d

d

2

ddw (2.8)

Allen et al (1972), compared a plot of Rw/Rd or (1-Rd)/(1-Rw)

against Rd where Rw is reflectance value in wet state and Rd is reflectance

value in dry state. They do, however, cast strong doubt on the validity of the

Kubelka-Munk theory to assess the dry reflectance value of wet sample.

Allen et al (1973) tried to establish a theory to predict the dry colour

of a fabric from its wet colour as a function of the refractive index of the

continuous medium. The light scattering-absorbing substrates are darker when

the continuous medium is water instead of air. This is due to the decrease in

scattering efficiency of the scattering particles caused by the smaller value of

the ratio of the refractive indices of the scattering particles to that of water as

compared to that ratio when the continuous medium is air. Experimental

evidence for this phenomenon is presented for polyester fabric viewed

in air, water, and a concentrated sucrose solution. The wavelength range

42

Figure 2.6 Relationship between the ratio of the reflectance of the

sample immersed in water (Rw) to that of the dry one (RD)

and the reflectance of the dry sample

Figure 2.7 Relationship between the predicted and measured

reflectance of the sample immersed in water (Rw)

43

400nm to 700nm and absolute dry reflectances from 2% to 70% were

covered. The phenomenon cannot be adequately described with the Kubelka-

Munk theory of the colour of scattering-absorping substrates. Evidence is

presented that at high reflectance values, the theory describes the colour

adequately and the results are consistent with a prediction based on a

modified Mie equation for the scattering efficiency of large particles

(equation 2.9). But it was not achieved because of the inadequacy of the

Kubelka-Munk theory at low reflectance values. This is consistent with the

observation that this theory is not capable of predicting precisely high dye

concentrations on textiles but is quite adequate for low concentrations. They

also have concluded that one will be able to predict the dry colour from the

wet colour on the basis of the scattering efficiency of a substrate in the

particular medium. Noechel and Stearns (1944) also have analysed the

inadequacy of Kubelka-Munk equations.

)1m()2m(

)2m()1m(

S

S2

w

2

d

2

w

2

d

w

d (2.9)

where, S – Scattering coefficient and

m – Ratio of the refractive index of the scattering particles to that

of the continuous medium.

An empirical equation was developed, relating wet and dry

reflectance values for the common textile fibres by Smith (1979). The results

of wet and dry colour measurement obtained were furnished. The developed

equation varies with the fibre type, but independent of colour. But in general

it can be fitted to wet and dry reflectance relationship. He also suggested that

if samples were in the transparent holder, an accurate equation could be

developed provided that a constant thickness was used. But the refractive

index difference across the boundary was not included in the equation.

44

A substrate specific mathematical relationship between moist and

dry reflectance was calculated on the basis of several measurement values for

cotton and polyacrylonitrile in the form of a double-logarithmized polynomial

of fifth order, which makes it relatively simple to convert the measurement

values, obtained with moist material to those of dry material (Rieker and

Gerlinger 1984).

A relationship demonstrated between the L* as well as E* value

of dyed cotton samples, and its % moisture content as it dries after dyeing as

proposed by Manian et al (2000) with R2 value of 0.90 and 0.95 for L* and

E* respectively are given below.

L* = -11.897 + 12.202 (X) - 1.85 ln(X)

+ 9.322 10-2

(X)0.5 – 9.942 (X)0.5

(2.10)

E* = 9.459 + 9.422 (X) + 1.27 ln(X) + 1.567x10-3

(X)2

+ 8.081 (X)0.5

ln(X) (2.11)

where, X - Moisture Content

The application of a geometric model in the prediction of colour

appearance of dry fabrics from their colour in a wet state has been analysed

by Tsoutseos and Nobbs (1998 and 2000). This approach can be applied to

online colour measurement. This model assumes that the textile fabric

consists of cylindrical fibres of equal diameter and isotropic in structure and

colour. Their diameter was considered large when compared to the

wavelength of the incident light. These cylinders were considered parallel to

each other and form an array. These arrays form ‘plates’ and were immersed

in an optically transparent continuous medium. The light was incident

vertically on the first layer of the assembly as a collimated beam and was

diffused on the subsequent layers. Collimated light falls on the fibre and part

45

of it was reflected according to Fresnel laws, while the rest was diffracted

inside the fibre. In the basic form of the model there was no light scattering

inside the fibre, so light propagates linearly until it was internally refracted or

diffracted. During this propagation it was subjected to absorption according to

the Beer-Lambert law. The light that was internally reflected continues

travelling inside the fibre where it was subjected to further absorption. The

refracted light continues in the embedding medium and was considered as

transmitted if it propagates downwards, or reflected otherwise.

A numerical relationship between the reflectance of the coloured

substrate in the dry state i.e. in air, and in the wet state i.e. when immersed in

water, was established by Jahagirdar et al (2002) and is given below. To

establish the relationship mercerised cotton fabrics were dyed with five

different reactive and direct dyes. It can be used as an analytical tool to

predict the reflectance values of the dry sample from the reflectance values of

the same sample when it is in the wet state. This relationship can be fitted to

all the samples for which K/S function is linear and non-linear.

Rd = 3.03Rw3

– 4.14 Rw2+ 2.57Rw (2.12)

where, Rd – Dry reflectance value and

Rw – Wet reflectance value.

Tiwari and Jahagirdar (2007) proposed equation (2.13) to predict the

colorimetric properties of dyed polyester in dry state directly from the

corresponding wet reflectance values.

Rd = 0.9677Rw3 – 1.6256Rw

2 + 1.7306Rw + 0.0247 (2.13)

The reflectance values of dyed fabrics dyed with six different

disperse dyes at various combinations were measured over the visible region

46

at wet and dry state. Using the reflectance value the above relationship

between the wet and dry reflectance was estabilised using curve fitting

method.

2.8 SUMMARY

Survey of literature presented in this chapter covers various aspects

of structure and properties of cotton fibres. The quality assessment of dyed

fabrics is also elaborately discussed. Colour measurement and matching is

one of the important quality parameters to be taken care of in dyed fabrics.

The survey reveals that, the colour of dyed fabrics is influenced by several

factors namely structure of fibre, yarn and fabric, structure and quantity of

dye present in the fabric, method adopted for colour assessment and other

foreign matters such as moisture, chemicals etc present in the fabric. The

effect of moisture content on colour and the various methods involved in

measurement of moisture content are also elaborately discussed. The survey

also covers the various attempts made in the direction of measurement of dry

colour of dyed fabric from its wet state.

Studies carried out so far pertaining to assessment of dry colour of

dyed fabric from its wet state and models developed to meet the above

requirements, clearly brings out the fact that in these attempts the researchers

have not taken into consideration the effect of different types of water namely

bound, free and bulk water that can be present in the dyed fabric.

The present work is aimed at fulfilling the above gap as it would

give a better understanding with respect to the effect of moisture content on

the colour of dyed fabrics. Cotton fabrics with different structures dyed with

various combinations of direct dyes were chosen for the study. These dyed

fabrics were conditioned with different relative humidity levels as well as

mangled after wetting to achieve various levels of bound water and bulk water

47

content levels respectively. The wet fabrics were also subjected to drying to

arrive at various free and bound water content levels using two different

temperatures. The dry and wet colours of these fabrics were assessed in terms

of K/S value and the colour difference between them was determined in

terms of E*ab value. The various reasons for the change in colour of the

fabric when it is transformed to wet state were analysed and elaborately

discussed. Further, a suitable mathematical model developed to predict the

colour of dyed fabric at a particular moisture level from any other moisture

level is also presented.