LESSON 12 TESTS TO DETERMINE COLOUR FASTNESS STRUCTURE 12.0
OBJECTIVES
12.1
INTRODUCTION
12.2
AN APPROACH TO TESTING COLOUR FASTNESS
12.3
TESTS FOR WASH FASTNESS 12.3.1 12.3.2 12.3.3
TEST NOS. 1,2 & 3 TEST NO. 4 TEST NO. 5
12.4
LIGHT FASTNESS
12.5
RUBBING FASTNESS 12.5.1 12.5.2
12.6
DRY RUBBING TEST WET RUBBING TEST
ASSIGNMENTS 12.6.1 12.6.2
CLASS ASSIGNMENTS HOME ASSIGNMENTS
12.7
SUMMING UP
12.8
POSSIBLE ANSWERS TO SELF-CHECK QUESTIONS
12.9
TERMINAL QUESTIONS
12.10
REFERENCES & SUGGESTED FURTHER READING
12.11
GLOSSARY
1
12. TESTS TO DETERMINE COLOUR FASTNESS This is the third and the final lesson of the unit on ‘Textile Chemical Processing’. You were introduced to the preparatory processes followed by a description of the dyes used and the dyeing process in lesson 10. Lesson 11 dealt with printing and finishing processes. In this final lesson of the unit, the tests made to determine colour fastness when the fabric is washed and/or subjected to abrasion by rubbing, are described.
12.0 Objectives After going through this lesson, you will be able to understand: •
The test methodology involved in colour fastness of textile materials.
•
The tests and instruments involved in determining wash fastness, light fastness and rubbing fastness of textile materials.
12.1 Introduction In Lesson 10, while discussing dyes and the dyeing process, it was stated that once the dye has entered the fibre, it is important that it should stay there and not come out of the fibre. The factors which help the retention of the dye in the fibre were discussed and it was seen that the bonding between the fibre and the dye, the size of the dye molecule and its solubility characteristics in water were important factors. For example, the chemical bonding of reactive dyes and the aggregation of vat dyes forming large dye aggregates in cotton and also its insolubility in water led to excellent colour fastness. Colour fastness of a textile product is of great practical interest, particularly against events or agencies that it daily experiences, e.g. washing, sunlight and rubbing. Besides these, colours must also not fade when the fabric is subjected to ironing, steaming, dry cleaning or when it comes in contact with perspiration, chlorine, etc.
12.2 An Approach to Testing Colour Fastness In view of the universal importance of assessing colour fastness of textile materials, and the significant international trading in textiles, the importance of uniformity of test methods for assessing colour fastness is self evident. The various agencies in different countries which control quality and lay down test methods therefore collaborate with the world body called International Standards Organization (ISO) to formulate standards. In India this role is played by the Bureau of Indian Standards or BIS, which was previously known as Indian Standards Institute or ISI. The test methodology involves taking a piece of the coloured fabric and covering half of it on both sides, say with a specified white fabric and leaving the other half free. Then it is subjected to a prescribed test method which may be washing, for example. Two 2
types of measurable events occur — first, the coloured cloth which was left on its own may show a change in shade and second the white cloth holding the coloured cloth may get stained. The assessment of fastness involves the visual determination of change in shade and the staining of the adjacent white cloth. For this a grey scale for assessing change in shade (Fig. 12.1a) and another grey scale for assessing staining (Fig. 12.1b) are available (these are standard scales and can be purchased).
Fig. 12.1a Grey scale to assess change in shade
Fig. 12.2b Grey scale to assess staining
The extent of change in shade compared to the original is assessed with the help of the grey scale shown in Fig. 12.1a. The fastness property is then expressed in terms of grade 1 to 8 in case of light and a scale of 1 to 5 in case of agencies other than light. Similarly the extent of staining on adjacent white material is assessed with the help of the grey scale shown in Fig. 12.1b and expressed in the same scale as for shade variation. We will now consider the test methods prescribed for assessing fastness against washing, light and rubbing, respectively.
Self-check Questions 1.
Why does a standard wash fastness test of a coloured fabric include a white fabric also?
2.
What are the two grey scales used for?
12.3 Tests for Wash Fastness For testing wash fastness, a wash fastness tester which is known as Launderometer is used (Fig. 12.2). It is a stainless steel double-walled container equipped with a rotor on which eight jars are provided. The container can store water and is 3
equipped with a heating coil for heating the water to the specified/required temperature. A thermostat controls the temperature.
Fig. 12.2 A Launderometer
There are five (5) wash tests which are application-specific. These will now be briefly considered.
12.3.1 Test numbers 1, 2 & 3 A specimen measuring 10 cm × 4 cm is cut out from the fabric to be tested for wash fastness. It is placed between two pieces of un-dyed (white) fabric measuring 10 cm × 4 cm and three pieces are held together by stitching round the edges. In the case of yarn, a white yarn is entwined to make a composite sample, whereas in case of fibre, it is compressed in the form of a sheet of 10 cm × 4 cm and held in place by sewing in between pieces of cloth measuring 10 cm × 4 cm. One of the white materials enclosing the specimen is the same as the dyed sample and the other is as indicated in Table 12.1. After the specimens have been prepared, a solution is made containing 5 gram per litre of good quality washing soap/detergent. Then the tests are performed as per the test procedures of Bureau of Indian Standards.
4
Table 12.1 Composition of the second sandwiching fabric
First piece
Second piece
Cotton
Wool
Wool
Cotton
Silk
Wool
Linen
Wool
Viscose
Wool
Polyester or Polyester / Cotton
Wool or Cotton
Acrylic
Wool or Cotton
The tests are all made in a launderometer under conditions listed in Table 12.2. Table 12.2 Conditions for making tests in the Launderometer
Test Number
Temperature (°C)
Time (Min.)
M:L Ratio
1 (IS 687)
40 ± 2
30
1:50
2 (IS 3361)
50 ± 2
45
1:50
3 (IS 764)*
60 ± 2
30
1:50
* In this case, 2 gram per litre of anhydrous sodium carbonate is added to the soap solution.
After the treatment is over, the composite samples aree rinsed twice in cold water and then for 10 minutes in cold running tap water. After squeezing, the stitching is removed in such a manner that the dyed and un-dyed materials are sewn together only along one short side. After opening out, the pieces are dried in air at room temperature. The change in colour of the uncovered portion of dyed/printed specimen was assessed with grey scale for change in shade and staining of the un-dyed material with grey scale for staining. Now the remaining two tests will be described. 12.3.2 Test number 4 (IS 765) The specimen is prepared in the same way as in the previous tests. But in this test, the composite sample is placed in the launderometer together with 10 stainless steel balls of specified size and sufficient solution containing 5 grams soap and 2 grams anhydrous sodium carbonate per litre to give liquor to material ratio of 50:1. The test is carried out at 95 ± 2°C for 30 minutes. The rest of the procedure is the same as for the first three samples.
12.3.3 Test number 5 (IS 3417) 5
The composite sample is prepared as in test no. 4 and liquor to material ratio of 50:1 is used with a solution containing 5 grams soap and 2 grams anhydrous sodium carbonate per litre. The test is carried out at 95 ¹ 2°C for 4 hours. The rest of the procedure is similar to that for the previous tests.
Activity 1. Take 2 dyed samples; one dyed with direct dyes and the other with vat dyes. Perform the wash fastness tests as per the prescribed procedures and later compare the results obtained in the two cases.
Self-check Questions 3. What is a launderometer? 4. Why do the standards prescribe a number of tests? 5. Fill in the blanks: i) ISO stands for _________________ and ISI stands for ________________. ii) The fastness property of a textile material is judged with the help of a _________. iii) Wash fastness tester is known as a ____________. iv) Grey scale determines the fastness property in terms of grade __________ to ____________. v) A __________ controls the temperature of the water in the launderometer.
12.4 Light Fastness The test is carried out in light fastness tester (Fig. 12.3). In this method, the test specimen is tested together with standard dyed blue wool samples of light fastness grade 1 to 8. The test specimen and the standards are mounted on a cardboard as shown in Fig. 12.4. Half of it is covered with an opaque sheet (black paper), as shown in the figure. This covered assembly of samples is exposed to the light of light fastness tester (which simulates sunlight). After 24 hours, the assembly is removed and the covered and uncovered samples are compared. The cycle is repeated for another 24 hours and continued comparison is made
6
Fig. 12.3 A Light fastness tester
till the difference in shade between the exposed and un-exposed portions is equivalent to grade 3 of grey scale. The light fastness grade is given by comparing with the fading of the standard wool sample.
12.5 Rubbing Fastness The instrument used for assessing the dry and wet rubbing fastness of fabrics is known as the crockmeter (Fig. 12.5). To make this test, two pieces each not less than 14 cm Ă— 5 cm are cut from the fabric sample for dry rubbing and two pieces for wet rubbing. One specimen of each pair is cut in the warp direction and second in the weft direction. In case of yarn and fibre, combing is done to get a sheet of 14 cm Ă— 5 cm from them. The test specimen is fixed onto the crockmeter by means of clamps. In case of printed fabric, the sample should be so fixed that all colours of the design are rubbed during the test. If the area of colours is large, more test specimens are taken and individual colour assessed separately. A crock test is used to determine the amount of colour that may be transferred from a sample fabric to another fabric by rubbing. 12.5.1 Dry rubbing test
Fig. 12.4 Mounting of the specimen for grading of light fastness
A piece of white cotton cloth (usually provided by instrument manufacture) is attached at the end of the finger of the crockmeter. The test sample is rubbed 10 times to and fro in a straight line along a track 10 cm. long on the dry specimen. The amount of colour transferred due to dry rubbing is assessed by comparing it with the grey scale for evaluating staining. 12.5.2 Wet Rubbing Test The test done above is repeated for a fabric that is rubbing against a white fabric that has been wetted with water and squeezed so as to retain its own mass of water. The sample is dried after rubbing and the staining of the white fabric assessed as in the previous case.
Fig. 12.5 A Crockmeter
7
Activity 2 1.
For the samples of Activity 1, perform the dry and wet rubbing tests and briefly comment on the results.
Self-check Questions 6. Fill in the blanks: i) In testing the light fastness of a textile material, the specimens are tested with standard dyed __________ ___________ samples. ii) The instrument used to assess the rubbing fastness of a textile fabric is known as a __________. iii) Two different types of rubbing fastness tests are _______ rubbing test and ________ rubbing test. iv) The samples mounted for rubbing fastness tests are of the size _____ cm. x ____ cm.
12.6 Assignments 12.6.1 Class assignments i)
Print a fabric and fix the fabric under different conditions. Evaluate the wash and rubbing fastness of the fabrics.
12.6.2 Home assignments i)
Draw a schematic sketch of a launderometer and label its parts.
12.7 Summing Up Tests, which are made to determine colour fastness against washing, light and rubbing, are described. It is shown that using a launderometer, a light fastness tester and crockmeter these three tests can be made and grey scales for assessing change in shade and change in staining of adjacent specimen allow quantification of colour fastness.
12.8 Possible Answers to Self-check Questions
8
1.
The degree of staining of the white fabric during the test is also a measure of colour fastness.
2.
The first grey scale is for determining shade variation while the second is used for measuring staining.
3.
A launderometer is a stainless steel double-walled container equipped with a rotor on which a number of jars (for example, eight jars) are provided. The container can store water and is equipped with a heating coil to heat the water to the specified / required temperature. A thermostat controls the temperature.
4.
The standards prescribe a number of tests so that they are application-specific. In some applications, the washing, for example, may be at room temperature; in others it may be at 60°C. The tests are therefore performed at different temperatures.
5.
Fill in the blanks: i) ii) iii) iv) v)
6.
International Standards Organization and Indian Standards Institute Grey scale Launderometer 1, 8 Thermostat
Fill in the blanks: i) ii) iii) iv)
Blue wool Crockmeter Dry, wet 14, 5
12.9 Terminal Questions 1. How does a Launderometer work? 2. Why do we use the rubbing test to assess colour fastness? Name the other types of colour fastness tests conducted in a laboratory.
12.10
References and Suggested Further Reading
1. Anonymous. Standards issued by Bureau of Indian Standards IS 687, IS 764, IS 765, IS 3361, IS 3417. Bureau of Indian Standards, New Delhi. 2. Manufacturers’ Instruction Manuals on Launderometre, Light Fastness Tester and Crockmeter.
12.11 Glossary 1. Fastness
The quality or condition of being fast
2. Assessing
Estimating, setting a value
3. Fade
To lose colour intensity 9
10
4. Specified
Specifically indicated
5. Shade
Degree of darkness of a colour
6. Stain
Discolouring or spotting
7. Composite
Formed of distinct parts