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CHAPTER 3

MATERIALS AND METHODS

Factors affecting light fastness of reactive dyed fabric like yarn

linear density, fabric structure, pretreatment methods, dye structure, dyeing

methods and finishing changes have been studied. Ultraviolet absorbers and

antioxidants have been applied on dyed fabric for improving light fastness.

Effect of application of ultraviolet absorbers and antioxidants on washing and

rubbing fastness has also been checked. The particulars of the materials used

and the various experimental procedures adopted in the studies are described

in this Chapter.

3.1 MATERIALS

3.1.1 Cotton Yarn Details

20 Ne, 30 Ne, 40 Ne and 60 Ne count of 100% cotton yarns were

collected from spinning mills to determine the effect of yarn linear density on

fastness. Whereas 60 Ne are combed yarns and others are carded yarns.

3.1.2 Fabric Details

Details of fabrics used to study the effect of fabric structure on light

fastness is explained below

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3.1.2.1 Woven fabrics

Woven fabrics with following specifications have been used to study the light fastness.

Table 3.1 Woven fabrics specifications

Specifications Plain Twill Warp count 60 Ne 20 Ne Weft count 60 Ne 40 Ne Ends/inch 120 60Picks/inch 103 83Areal density 80 g/m2 180 g/m2

CIE whiteness index 68 63Absorbency < 3 s < 3 s

3.1.2.2 Knitted fabrics

Knitted fabrics with following specifications have been used to study the light fastness.

Table 3.1(a) Knitted fabrics specifications

Specifications Single Jersey Pique Yarn count 30 Ne 30 Ne Courses/cm 12 12Wales/cm 9 9Areal density 150 g/m2 185 g/m2

CIE whiteness index 65 65Absorbency < 3 s < 3 s

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3.1.3 Chemicals used for Pretreatment

Sodium Hydroxide, Sodium Carbonate, Hydrogen Peroxide (50%),

Acetic acid (90%) and the enzyme pectinase from M/s Novazymes India Pvt

Ltd., Bangalore, were used for bio-scouring.

3.1.4 Chemicals used for Dyeing

Glauber’s salt (Na2SO4. 10 H2O), Sodium Carbonate, Acetic acid

(90%) and soap solution were the chemicals used, commercial grade

chemicals were used to match industrial practice.

3.1.5 Dyes

The commercial dye samples received from dyeing industry were

used. Available structures of the dye used were given in Figure 3.1.

Commercially used reactive dyes namely C.I. Reactive Yellow 84, C.I.

Reactive Yellow 205, Reactive Yellow 206, C.I. Reactive Yellow 214, C.I.

Reactive Red 22, C.I. Reactive Red 238, C.I. Reactive Red 264, C.I. Reactive

Red 271, C.I. Reactive Red 278, C.I. Reactive Red 279, C.I. Reactive Red

282, C.I. Reactive Blue 198, C.I. Reactive 230, C.I. Reactive Blue 235, C.I.

Reactive Blue 999 and C.I. Reactive Blue 261 were used without any

purification in order to match industrial practice. Some of the dye structures

were not available as they were patented by the manufacturers.

3.1.5.1 Structure of reactive dyes

The chemical structure of the reactive dyes for which the dye

structure available in the public domain are shown in Figure 3.1(a-m).

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(a) C.I. Reactive Yellow 135

(b) C.I. Reactive Yellow 176

(b) C.I. Reactive Red 22

Figure 3.1 (a-m) Chemical structures of reactive dyes

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(d) C.I. Reactive Red 23

(e) C.I. Reactive Red 49

(f) C.I. Reactive Red 194

Figure 3.1 (Continued)

77

(g) C.I. Reactive Red 195

(h) C.I. Reactive Blue 19

(i) C.I. Reactive Blue 160

Figure 3.1 (Continued)

78

(k) C.I. Reactive Blue 250

(l) C.I. Reactive Blue 198

(m) C.I. Reactive Blue 204

Figure 3.1 (Continued)

79

3.1.6 Recipe Used for Dyeing

Recipe used for producing self and combination shades are given in

Tables 3.3 (a - f).

Table 3.2 (a) Recipe used for Exhaust dyeing of combination shades

Shades

C.I. Reactive

Yellow 84(% owf)

C.I. ReactiveRed 22

(% owf)

C.I. ReactiveBlue 198 (% owf)

Sodium Sulphate

(g/l)

Sodium Carbonate

(g/l)

Light Grey 0.01 0.01 0.02 10 10Light Brown 0.01 0.03 0.01 10 10Light Olive 0.02 0.01 0.03 10 10Grey 0.10 0.10 0.20 20 10Brown 0.10 0.30 0.10 20 10Olive 0.20 0.10 0.30 20 10Dark Grey 1.0 1.0 2.0 70 20Dark Brown 1.0 3.0 1.0 70 20Dark Olive 2.0 1.0 3.0 80 20

Table 3.2 (b) Recipe used for Cold pad-batch dyeing of combination shades

Shades

C.IReactive

Yellow 84(g/l)

C.I. ReactiveRed 22

(g/l)

C.IReactiveBlue 198

(g/l)

Sodium Carbonate

(g/l)

Sodium Hydroxide

40%(ml/l)

Light Grey 0.1 0.1 0.2 10 4Light Brown 0.1 0.3 0.1 10 4Light Olive 0.2 0.1 0.3 10 4Grey 1.0 1.0 2.0 10 4Brown 1.0 3.0 1.0 10 4Olive 2.0 1.0 3.0 10 4Dark Grey 10 10 20 20 6Dark Brown 10 30 10 20 6Dark Olive 20 10 30 20 6

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Table 3.2 (c) Recipe used for Pad-humidity fix dyeing of combination shades

Shades C.I. Reactive

Yellow 84 (g/l)

C.I. ReactiveRed 22

(g/l)

C.I. ReactiveBlue 198

(g/l)

Sodium Carbonate

(g/l)

Sodium Hydroxide 40% (ml/l)

Light Grey 0.1 0.1 0.2 20 0Light Brown 0.1 0.3 0.1 20 0Light Olive 0.2 0.1 0.3 20 0Grey 1.0 1.0 2.0 20 0Brown 1.0 3.0 1.0 20 0Olive 2.0 1.0 3.0 20 0Dark Grey 10 10 20 20 2.5Dark Brown 10 30 10 20 2.5Dark Olive 20 10 30 20 2.5

Table 3.2 (d) Recipe used for self-shades in Exhaust dyeing

DyesDye

(% owf)

Sodium sulphate

(g/l)

Sodium carbonate (g/l)

Reactive Yellow 205 0.8 20 16Reactive Yellow 206 0.3 10 14Reactive Yellow 214 0.3 10 14Reactive Red 238 0.3 10 14Reactive Red 264 1.5 40 20Reactive Red 271 0.1 10 14Reactive Red 278 0.2 10 14Reactive Red 279 0.4 10 14Reactive Red 282 0.6 30 18Reactive Blue 230 2.0 40 20Reactive Blue 235 0.2 10 14Reactive Blue 999 1.6 40 20Reactive Blue 261 0.6 30 18

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Table 3.2 (e) Recipe used for self-shades in Cold pad-batch

Dyes Dye (g/l) Sodium

carbonate(g/l)

Sodium hydroxide 40%

( ml/l) Reactive Yellow 205 8 10 4Reactive Yellow 206 3 10 4Reactive Yellow 214 3 10 4Reactive Red 238 3 10 4Reactive Red 264 15 20 6Reactive Red 271 1 10 4Reactive Red 278 2 10 4Reactive Red 279 4 10 4Reactive Red 282 6 10 4Reactive Blue 230 20 20 6Reactive Blue 235 2 10 4Reactive Blue 999 16 20 4Reactive Blue 261 6 10 4

Table 3.2 (f) Recipe used for self-shades in Pad-humidity fix

Dyes Dye (g/l) Sodium carbonate (g/l)

Sodium hydroxide 40% ( ml/l)

Reactive Yellow 205 8 20 0Reactive Yellow 206 3 20 0Reactive Yellow 214 3 20 0Reactive Red 238 3 20 0Reactive Red 264 15 20 2.5Reactive Red 271 1 20 0Reactive Red 278 2 20 0Reactive Red 279 4 20 0Reactive Red 282 6 20 0Reactive Blue 230 20 20 2.5Reactive Blue 235 2 20 0Reactive Blue 999 16 20 2.5Reactive Blue 261 6 20 0

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3.1.7 Antioxidants and UV Absorbers

The antioxidants (gallic acid, vitamin C and cafeic acid) and the

UV absorbers (2-hydroxybenzophenone and phenyl salicylate) were used for

treating the cotton before exposure to light. They were obtained from

commercial sources (M/s Sigma Aldrich Ltd.). Cafeic acid (CAS

No.331.39.5), gallic acid (CAS No 149-91-7), vitamin C (ascorbic acid CAS

No. 50.81.7), phenyl salicylate (CAS No.118.55.8) and benzophenone (CAS

No. 117.99.7) were used. Chemical structure and other details of these

substances were obtained from Chemical Abstract Service index (cristae &

vilarem 2006).

3.1.8 Fixing Agent

Formaldehyde and Non-formaldehyde based fixing agents collected

from dye houses were used to study the effect of fixer on light fastness.

3.1.9 Softeners used for Finishing

Cationic, non-ionic and silicone softeners were received from the

dye house and applied on dyed fabrics. Finished samples were tested for the

change in light fastness.

3.1.10 Chemicals used for Fastness Testing

The wash fastness with European Colourfastness Establishment

detergent, without optical brightener detergent (ECE WOB) and Sodium

perborate were used for wash fastness. L-histidine monohydrochloride mono

hydrate, sodium chloride, sodium dihydrogen orthophosphate dihydrate and

disodium hydrogen orthophosphate dihydrate were used for perspiration

fastness testing.

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3.2 METHODS

3.2.1 Preparation

3.2.1.1 Grey boiling

Since water used for the wet processing has hardness compounds,

sequestering agent has to be used in scouring process. Grey boiling was

carried out with sequestering agent and non-ionic surfactant. The process was

carried out at 95°C for 40 min followed by cold wash for 10 min.

Table 3.3 Recipe used for pretreatment

Pretreatment methods Grey boiling Alkaline scouring Semi bleaching Sequestering agent 1 g/l 1 g/l 1 g/l Non-Ionic Surfactant 2 g/l 2 g/l 2 g/l Sodium Hydroxide 0 g/l 4 g/l 3 g/l Hydrogen Peroxide 0 g/l 0 g/l 2 g/l Temperature 95°C 95°C 95°CTime 40 min 40 min 40 min

3.2.1.2 Alkaline scouring

Alkaline scouring was carried out with sodium hydroxide,

sequestering agent and non-ionic surfactant. Boiling was carried out at 95°C

for 40 min, then hot wash at 80°C for 10 min and then neutralizing with acetic

acid at 55°C for 15 min.

3.2.1.2 Semi bleaching

Semi bleaching was carried with sodium hydroxide, hydrogen

peroxide, sequestering agent and non-ionic surfactant at 95°C for 40 min,

followed by hot wash at 80°C for 10 min, neutralizing with acetic acid and 1.0

g/l enzymatic peroxide killer for 15 min.

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3.2.1.4 Enzymatic scouring

Pectinase enzyme and non-ionic surfactant were used for bio

scouring. Sodium carbonate was used to set scouring-bath at a favourable pH

for enzyme to act. The pH of the scouring bath was 8.0-9.0 according to the

enzyme used in the process. 2 g/l non-ionic surfactant was used. The process

was carried out at 55°C for 40 min, then hot wash at 80°C for 10 min and then

neutralizing with acetic acid for 15 min.

Pectinase Enzyme 2% owf

Non-Ionic Surfactant 2 g/l

Soad ash 3 g/l

Temperature 95°C

Time 40 min

pH 8.0-9.0

3.2.1.5 Mercerisation and semi bleaching

Mercerisation was carried with 280 g/l caustic lye with the

parameters listed below.

Impregnation time 40 sec

Caustic Lye Temperature 18-22°C

With tension, the fabric lustre and structure were improved. After

impregnation, two hot wash, neutralization and cold wash were carried out.

Semi bleaching was carried with caustic flakes, hydrogen peroxide,

sequestering agent and non-ionic surfactant. Boiling was done at 95°C for 40

85

min, followed by hot wash at 80°C for 10 min, neutralizing with acetic acid

and 1.0 g/l enzymatic peroxide killer for 15 min.

3.2.2 Dyeing

Dyeing of the scoured fabrics was performed by both exhaust

method and continuous method (cold pad-batch and pad-humidity fix). The

details of dyeing methods followed are given in the following sections.

3.2.2.1 Exhaust dyeing

Exhaust dyeing was carried out in a Mathis Labomat machine using

as per Figure 3.2. The machine is shown in Figure 3.3. Exhaustion of dyes

was carried out using sodium sulphate (salt) for 30 min and fixation with soda

ash for 45 min at 60°C temperature. After dyeing, the samples were rinsed in

distilled water, soap solutions, rinsed again in distilled water and then dried.

Figure 3.2 Exhaust dye bath curve

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Fabric weight 10 g

Material to Liquor ratio 1:7

Salt 30 g/l

Soda 10 g/l

Figure 3.3 Lab Exhaust dyeing machine

3.2.2.2 Cold pad-batch

Cold pad-batch dyeing was carried out with Mathis padding mangle

machine. The padding machine is shown in Figure 3.4. Cold pad-batch dyeing

method is very eco-friendly dyeing method. It does not require Sodium

Sulphate to exhaust the dyes to fabric. The impregnation was done by padding

87

with dyes and alkali at 20°C with 60% pickup. After padding, the fabric is

batched for 16 hours in air tight closed condition.

Dyes As per shade

Dye: Alkaline Ratio 4:1

Padding temperature 20°C

Pick up 60%

Batching time 16 h

Figure 3.4 Padding mangle

3.2.2.3 Pad-humidity fix

Cold Pad-Humidity fix dyeing, impregnation was carried out using with padding mangle and fixation in Mathis lab humidity fix machine. The humidity fix machine is shown in Figure 3.5. Pad-humidity fix process also does not require Sodium Sulphate. The impregnation was done by padding with dyes and alkali at 20°C with 60% pickup. Fixation was done with 25%

88

humidity for 3 min. The process, which involves the application of the dye and sodium carbonate on the fabric by padding, Humidity fixing and washing-off and provides economical and more ecofriendly dyeing by continuous application.

Dyes As per shade

Dyes : Alkali Ratio 4:1

Padding temperature 20°C

Pick up 60%

Chamber Temperature 120°C

Chamber Humidity 25%

Time 3 min

Figure 3.5 Lab humidity fix machine

3.2.3 Finishing, Antioxidants and UV Absorber Application

Softener finishing was carried out by pad-dry method. Padding was

carried out by 30% effective pick up on wet on wet method and drying at

150°C for 120 seconds. The antioxidants (gallic acid, vitamin C and cafeic

acid) and the UV absorbers (2-hydroxybenzophenone and phenyl salicylate)

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were applied by Exhaust and pad-dry-cure method. The concentration of used

chemicals were 0.5%, 1.0%, 2.0% and 3.0%. It was found 1.0% (owf) giving

the saturation point for improvement in light fastness.

3.2.3.1 Exhaust method

Exhaust method of application treatment was given in the

combination of 1.0% antioxidants and 1.0 % UV absorbers. Treat the dyed

cotton fabrics in chemical solutions at 70° C for 30 min stirring. For gallic

acid, vitamin C and cafeic acid, aqueous solutions were used and for the

others water/ethanol mixture (9/1, v/v) were used. The samples were washed

and then air dried.

3.2.3.2 Continuous method of finishing

The Pad-dry method is used to study the effect of softener on

fastness. Fabric is padded with each 20 g/l of softener agent in a padding

mangle on wet-on-wet method (30% effective pick up) and dried in relax

dryer at 100°C for 50 s.

Treatment with 10 g/l Antioxidant and/or 10 g/l UV absorber

combinations with acrylic based binder 10 g/l was carried out in a pad-dry-

cure method. Wet padding with 65% pick up, dried at 100°C for 60 s and

cured at 150°C for 120 s. The samples were washed and then air dried.

3.2.4 Testing

3.2.4.1 Absorbency

Absorbency of scoured fabric was tested by AATCC test method

79. Sample was placed in an embroidery clip with all creases out of it.

90

A burette dispenses a drop of water on to the surface of the fabric from a

distance of 9.5 mm below the burette. Time is recorded until the water drop

absorbs completely. The setup used is illustrated in Figure 3.6.

Figure 3.6 Absorbency testing setup

3.2.4.2 Weight loss

It was determined using gravimetric method with weighing balance (M/s Mettler Toledo PB303, Switzerland) having an accuracy of ± 0.001 g. Weight loss of the sample was calculated from the difference between the fabric weight before and after treatment. After treatment, fabric was dried, weighed and then weight loss was determined as per the formula.

Weight loss percentage = (3.1)

3.2.4.3 Whiteness index (WI) assessment

The whiteness can be measured with computer colour matching

system. Due to their high reflectance factors throughout the visible spectrum,

white materials have high tristimulus values X, Y and Z. Different

formulations for the whiteness assessment are currently used, such as those by

Berger, Hunter and Stensby. All of them are in accordance with the illuminant

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D65/10. The calculations of whiteness indices using tristimulus values are

expressed in Equation (3.2).

WI (CIE) = Y + 800(0.3138 – X) + 1700(0.3310 – Y) (3.2)

3.2.4.4 Colour strength % measurement

This strength is also known as the chromatic colour strength. The

colour strength is calculated by using the Equation (3.3). It describes the ratio

based on the K/S value of the sample in relation to the K/S value of the

standard at a single wavelength and is expressed in percent. This calculation

is typically meaningful, if it is made at the wavelength of maximum

absorption (lowest reflectance). If standard and sample have different

wavelengths of maximum of absorption, this method would not deliver

correct results. The calculation for colour strength based on Reflectance is

(3.3)

whereas, K is absorption coefficient and S is scattering coefficient

R is the reflectance at the wavelength of maximum absorption

(20%R = 0.20R)

3.2.4.5 Colour difference (dE)

The CIE L*a*b* system describes and orders colours based on the

opponent theory of colour vision. The opponent theory is that colours cannot

be perceived as both red and green at the same time, or yellow and blue at the

92

same time. However, a colour can be perceived as a combination of red and

yellow, red and blue, green and yellow, or green and blue. In the CIE L*a*b*

colour space, the colour coordinates in this rectangular coordinate system are:

L* - the lightness coordinate.

a* - the red/green coordinate, with +a* indicating red, and -a*

indicating green.

b* - the yellow/blue coordinate, with +b* indicating yellow,

and -b* indicating blue.

CIE LAB colour difference, between any two colours in CIE 1976

colour space, is the distance between the colour locations. This distance is

typically expressed as dE*, where dE can be calculated using Equation 3.4.

(3.4)

3.3 ASSESSMENT OF FASTNESS PROPERTIES OF DYED

FABRICS

The dyed samples are tested for fastness properties according to

ISO standard methods. The ISO 105 C06 colour fastness to washing, ISO

105-X 12 colour fastness to rubbing and ISO 105 B02 colour fastness to light

standards are used (Shahidullah et al 2007 and Waheed & Asharaf 2003).

3.3.1 Tests for Wash Fastness

Wash fastness aims to determine the resistance of the colour of

textiles of all kinds and in all forms to domestic or commercial laundering

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procedures used for normal household articles. Dyed sample and multi fibre

fabric with 10 × 4 cm size are used. Then both the samples are stitched. 4.0

g/l ECE WOB detergent (European Colour fastness Establishment detergent,

without optical brightener) + 1.0 g/l sodium perborate put in water.

Temperature 60°C

Liquid volume 50 ml

Time 30 min.

Steel balls 25

pH 10.5±1

The sample was rinsed twice with cold water dried at 60°C by

hanging or by flat iron pressing at 150°C.

3.3.2 Testing of Rubbing Fastness

This test is designed to determine the degree of colour transferred

from the surface of a coloured fabric to a test white cloth during rubbing

(which could be dry and wet). In ISO-105-X12 the wet pickup of the white

rubbing cloth is 100%. Rubbing fastness tested by both dry and wet methods.

In wet rubbing, the rubbing cloth is wetted and rating given by comparing the

staining with the grey scale.

Similarly dry rubbing was tested with dry cloth and compared for

the staining with grey scale for ratings. Colour Fastness to rubbing is a main

test which is always required for every coloured fabric. If the colour fastness

to rubbing is good then it’s other properties like washing fastness and

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durability improves automatically because the rubbing is a method to check

the fixation of the colour on the fabric.

3.3.3 Testing of Light Fastness

Nowadays, the criteria of light fastness are a major concern

amongst the dyers. The stringent requirement of light fastness is getting more

and more importance in the European as well as in the American markets.

There are different test methods, rating and factors affecting light fastness.

Generally, it is difficult to achieve good grade of light fastness in light,

medium and tricky shades (khaki, olive, grey and brown). Proper dye

combination always helps to arrive at the customer requirement. Generally

two methods of testing are widely accepted by most of the customers. They

are American Test Method (AATCC 16E) and British Test Method (ISO

105/BO2). In this work AATCC 16E 20 AFU method is used to assess the

light fastness.

Figure 3.7 Light fastness tester and sample set up

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3.3.3.1 American test method (AATCC 16)

This is an accelerated test method for testing of light fastness. There

are different options in this method which are A, B, C, D, E, F, G, H, I and J.

These options differ from each other on the basis of light source, panel

temperature and humidity. Generally AATCC 16E method is widely used for

testing purpose. In this method, a test specimen is exposed under the

condition specified in various test methods for 20, 40 or 60 AATCC Fading

Units (AFU) and the factors affecting light fastness.

One AATCC Fading unit (AFU) is 1/20th of the exposure required

to produce a colour change equal to grade 4 of the grey scale for the colour

change of L4 Blue wool. AATCC L4 Blue wool and 20 AATCC Fading units

would produce a colour change equivalent to step 4 on AATCC Grey scale.

20 AFU have been determined equal to 85kj/m² at 420 nm exposure based on

inter laboratory test study.

Sample is placed in a testing mask with part of the sample exposed

and part covered as a control. The testing mask is loaded in a weather-O-

meter holder and placed in a rack in the weather-O-meter for exposure.

Sample is exposed to the required amount of AATCC fading units and colour

change of the sample is evaluated. Generally for garment sectors, the

assessment of light fastness is done after 20 AFU whereas in the case of

furnishing fabrics like car upholstery, the rating is assessed after 40-60 AFU.

Rating

Rating of light fastness in this method is given on the basis of grey

scale with rating of 1-5. Rating 1.0 being the poor and the 5.0 being the best.

Rating 4.0 is normally acceptable for most of the requirements.

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3.3.3.2 British test method (ISO 105/BO2)

The light fastness of dyed fabric is evaluated by exposing the fabric

samples to Xenon Arc lamp. Even though the light sources are same, other

conditions are different.

Rating

The fastness to light is tested in accordance with DIN 16525. The

degree of fading is assessed by comparison with the blue scale for wool (DIN

EN ISO 105-B01). The fastness to light ratings (1-8) is as per Table 3.2.

There is no direct relation between the ratings of the above two methods.

Table 3.4 Light fastness rating

Rating Property

1 very poor

2 Poor

3 Moderate

4 fairly good

5 Good

6 very good

7 Excellent

8 Outstanding

3.3.4 Repeat Laundering Stability Test

The stability of the finish is checked to ten repeat home laundering

cycles under the following condition.

97

Detergent TAED 5 g/l (Tetra acetyl ethylene diamine)

Temperature 60°C

Time 30 min

Agitation Normal

Reduction in finish caused by alkaline washing powder containing

peroxide with mechanical abrasion was tested.

3.3.5 Perspiration Fastness

Determination of staining and shade change by the action of acid

and alkaline perspiration is tested by ISO EO4 method. The sample dimension

of 10 X 4 cm stitched with multifibre adjacent fabric. The sample is soacked

in perspiration solution for 30 min, excess solution removed and kept under

pressure for 4 h with perspiration tester. Then the sample is dried at 60°C. The

staining and colour fastness are assessed by grey scale.

3.3.5.1 Acid perspiration

The acid perspiration solution is prepared with the following recipe

L-histidine monohydrochloride monohydrate -0.5 g/l

Sodium chloride -5.0 g/l

Sodium dihydrogen orthophosphate dihydrate -2.2 g/l

The pH of the solution is adjusted to 5.5 with 0.1 N acetic acid

solution

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3.3.5.2 Alkaline perspiration

The alkaline perspiration solution is prepared with the following

recipe

L-histidine monohydrochloride monohydrate -0.5 g/l

Sodium chloride -5.0 g/l

Disodium hydrogen orthophosphate di hydrate -2.5 g/l

The pH of the solution is adjusted to 5.5 with 0.1 N sodium hydroxide

solution

3.4 Statistical Analysis of Result

The statistical analysis was carried out using ANOVA. This method

used to analysis the results of two results got by changing variable is

significantly different or not significantly different.

3.4.1 Method of ANOVA Calculation

Analysis of variance (ANOVA) is one of the statistical tools developed

by Professor R.A. Fisher, plays a prominent role in experiments. In tests of

significance based on small samples, it can be shown that statistics is

adequate to test the significant difference between two sample means. In

analysis of variance, we are concerned with the testing of equality of several

population means.

99

3.4.2 Taguchi Experimental Design and Result Analysis

Taguchi approach provides a new experimental strategy in which a modified

and standardized form of design of experiment (DOE) is used. In other words,

the Taguchi approach is a form of DOE with special application principles.

This technique helps to study effect of many factors (variables) on the desired

quality characteristic most economically. By studying the effect of individual

factors on the results, the best factor combination can be determined. Taguchi

designs experiments using specially constructed tables known as “orthogonal

array” (OA). The use of these tables makes the design of experiments very

easy and consistent and it requires relatively lesser number of experimental

trials to study the entire parameter space. As a result, time, cost, and labour

saving can be achieved. The experimental results are then transformed into a

signal-to-noise (S/N) ratio. Taguchi recommends the use of the S/N ratio to

measure the quality characteristics deviating from the desired values. Usually,

there are three categories of quality characteristic in the analysis of the S/N

ratio, i.e. the-lower-the-better, the-higher-the-better, and the nominal-the-

better. The S/N ratio for each level of process parameters is computed based

on the S/N analysis. Regardless of the category of the quality characteristic, a

greater S/N ratio corresponds to better quality characteristics. Therefore, the

optimal level of the process parameters is the level with the greatest S/N ratio.

Furthermore, a statistical analysis of variance (ANOVA) is performed to see

which process parameters are statistically significant. With the S/N and

ANOVA analyses, the optimal combination of the process parameters can be

predicted.

The selection of an appropriate orthogonal array (OA) depends on the

total degrees of freedom of the parameters. Degrees of freedom are defined as

the number of comparisons between process parameters that need to be made

to determine which level is better and specifically how much better it is. In

100

this study, since each parameter has three levels except injection speed which

has two levels the total degrees of freedom (DOF) for the parameters are

equal to 11. Basically, the degrees of freedom for the OA should be greater

than or at least equal to those for the process parameters. Therefore, L 9 (3 ×

3) orthogonal array with three columns and three rows was appropriate and

used in this study.

The signal to noise ratio (S/N ratio) was used to measure the sensitivity

of the quality characteristic being investigated in a controlled manner. In

Taguchi method, the term ‘signal' represents the desirable effect (mean) for

the output characteristic and the term ‘noise' represents the undesirable effect

(signal disturbance, S.D) for the output characteristic which influence the

outcome due to external factors namely noise factors. The S/N ratio can be

defined as:

(3.5)

The aim of any experiment is always to determine the highest possible S/N

ratio for the result. A high value of S/N implies that the signal is much higher

than the random effects of the noise factors or minimum variance. As

mentioned earlier, there are three categories of quality characteristics, i.e. the-

lower-the-better, the higher-the-better, and the-nominal-the-better. To obtain

optimal light fastness, the-lower-the- dE better light fastness.