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.