ð Fastness,
that is the resistance of dyeing and prints to external influences, is of
decisive importance for the practical use of any dyestuff.
ð Fastness
of dyestuffs is a very important property to satisfy the consumers of different
societies.
ð Technical
committee 38, Sub-committee of the International Standards Organisation (ISO)
has to date defined 45 different testing methods.
ð The
purpose for which a dyed material is to be used must always be born in mind
when dyestuffs are selected and the results of fastness tests are evaluated.
ð Fastness
properties such as resistance to light, washing, rubbing, ironing and dry
cleaning must be tested for everyday use, while resistance to boiling soda,
bleaching, potting and milling must be ascertained when dyed yarns are
submitted to process.
ð Every
test of fastness properties is made up three basic processes:-
(a) Its
implementation carried out in accordance with the pertinent standard fastness
tests;
(b) The
evaluation of its results, effected with the gray scale used to determine
changes of shade and staining;
(c) The
practical application of test results with regard to the end use of a dyeing or
print.
ð The
quantitative assessment of the colour changes is done by comparison with
standard S.D.C. grey scale, one for assessing colour changes and the second for
assessing staining.
ð Except
for fastness to light, the first scale consists of five pairs of grey scale
strips showing five extents of colour differences from nil to maximum and the
second shows five pairs of white and stained strips show no staining to maximum
staining.
Sample Preparation:-
ð A 10
X 4 cm of the dyed/printed test specimen is sandwiched between two adjacent
fabric pieces. i.e. an undyed cutting of the same material and another as
specified in standard.
ð In
the case of blended material this piece should be of the material predominant
in the blend.
ð Specifications
for these adjacent fabrics are shown in the Table.
If
First Piece is
|
Second
Piece to be
|
Cotton
|
Wool
|
Wool
|
Cotton
|
Linen
|
Cotton or
Viscose
|
Silk
|
Cotton
|
Viscose
|
Wool
|
Acetate
|
Viscose
|
Polyamide
|
Wool or
Viscose
|
Polyester
|
Wool or Cotton
|
Acrylic
|
Wool or Cotton
|
ð This
type of sample preparation is required practically for all fastness tests
except for light, rubbing and scrubbing.
Fastness to Washing
ð Machine
:- Wash wheel or Launder – o – meter
ð The
dyed sample, sandwiched between and undyed cutting of the same sample and
another as stated in the standard are placed in washing solution at appropriate
temperature in the steel jars with tight fitting lids of the machines.
ð These
jars are rotated in a constant temperature water bath for the length of time as
per the ISO standard.
ð There
are five test methods for evaluating fastness to washing. Test method no. 1 is
a mild method whereas no. 5 is a severe one as can been seen from the test
condition given in Table.
Summary
of test conditions for Fastness to Washing
ISO
Test
Method No.
|
Washing
Solution
|
Temperature
°C
|
M :
L
|
No.
of steel ball
|
Time
Minute
|
|
Saop
(g/l)
|
Soda
ash (g/l)
|
|||||
1
|
5
|
---
|
40
|
1:50
|
Nil
|
30
|
2
|
5
|
---
|
50
|
1:50
|
Nil
|
45
|
3
|
5
|
2
|
60
|
1:50
|
Nil
|
30
|
4
|
5
|
2
|
95
|
1:50
|
10
|
30
|
5
|
5
|
2
|
95
|
1:50
|
10
|
240
|
ð The
specimen are taken out, rinsed and dried below 60°C.
ð The
change in colour of the specimen and the staining on white fabrics is assessed
with reference to the two grey scales and ratings are given.
ð If
the sample is in the form of loose fibres, it is combed and pressed into sheet
and stitched between two necessary undyed fabrics.
ð If
the sample is in the form of the yarn it is either made into a sheet or knitted
into a fabric and then stitched between the undyed fabrics.
ð The
soap used for washing must be free from optical brightening agents (OBA) as their
presence may sometimes cause serious error in the evaluation of colour
difference i.e. in giving the rating.
Fastness to Rubbing
ð This
test is carried out on a Crockmeter and is particularly important for azoic
dyed and pigment printed materials.
ð A
strip of the specimen is mounted on the instrument.
ð A
piece of undyed cloth (about 5 sq cm) is clamped on the finger of the rubbing
device of the instrument.
ð The
undyed piece is rubbed to and fro 10 times against the specimen along a track
of 10 cm with a pressure of 900 g on the finger.
ð The
speed of rubbing is adjusted to one stroke (to and fro) per second.
ð The
colour transferred to white fabric (undyed piece) is then assessed by using the
grey scale for staining.
ð For
the evaluation of wet rubbing fastness, the white fabric is wetted with water
and excess water is squeezed off before clamping on the finger.
ð When
it is necessary to evaluate the rubbing fastness property of printed fabric samples
with small motifs, a rotary type crockmeter is used.
ð When
evaluating this fastness property for “Khadi” printed materials, where titanium
dioxide pigment is used in the paste, rubbing should be carried out with a
black dyed fabric, or else the white titanium dioxide pigment which will be
transferred on the white fabric will not be seen.
Fastness to Perspiration
ð Human
perspiration may be acidic or alkaline in nature depending on one’s metabolism.
ð The
preparation of the sample is similar to that in fastness to washing.
ð Fastness
to perspiration is therefore carried out at two different pH, viz. 8.0 and 5.5
using the perspiration liquor as given in Table:-
Composition
of Perspiration Liquors
Liquor
Composition
|
Solution
|
|
A
|
B
|
|
1 – Histidine
mono – hydrochloride monohydrate
|
0.5
g
|
0.5
g
|
Sodium
chloride
|
5.0
g
|
5.0
g
|
Disodium
hydrogen orthophosphate dodecahydrate
|
5.0
g
|
---
|
Sodium
dihydrogen orthophosphate dihydrate
|
---
|
2.2
g
|
Volume
(adjusted with distilled water)
|
1000
ml
|
1000
ml
|
pH (adjusted
with 0.1 N NaOH/Acetic acid)
|
8.0
|
5.5
|
ð The
composite sample is wetted out in the above perspiration solutions and
distilled water at room temperature for 30 min, material to liquor ratio being
1:50.
ð Solution
is then drained and the specimen is placed between two glass or Perspex plates
under a load of 10 lbs. (Perspirometer)
ð The
apparatus containing the composite specimen is placed in an air oven at 37 +
2°C.
ð Specimens
are then separated and dried below 60°C.
ð In
the plate method, the composite specimen after wetting out in perspiration
solutions (M : L :: 1 : 20) is placed on a flat bottomed dish and covered with
a thick glass plate weighing about 50 g.
ð The
plate is pressed lightly to drain out the liquor and the whole is placed in the
oven for 4 hours at 37 + 2°C.
ð Assessment
is done with reference to the two grey scales.
Fastness to Light
ð The
traditional method of determining this fastness property is by exposing the
specimen to daylight under a defined condition, behind glass.
ð However,
this is not only requires longer time particularly for samples of good light
fastness but it also becomes almost impossible to carry out these tests during
monsoons.
ð This
has led to the development of fading lamps, viz. carbon arc, xenon arc, etc.
ð The
use of a xenon arc lamp has been recognised by the ISO and ISI as the energy
distribution of this lamp is nearest to sun light whereas the carbon arc lamp
is extensively used in USA.
ð However,
these instruments are costly and cheaper instruments eg. Suntest (Hanau) and
Microscal are available which can be used for routine testing though these are
not yet accepted by the ISI.
ð Instrument
:- Fed – O – Meter
ð For
this purpose wool fabric samples, dyed with specific colours (known as Blue
Wool Standard (BWS)) having known light fastness property are used.
ð During
exposure, apart from the light source, the temperature and humidity are to be
controlled as they have significant bearing to the fading time.
ð A
portion of the specimen and BWS are covered and exposed to the light.
ð These
are then periodically inspected till sufficient fading occurs on the specimen.
ð The
extent to which the fading must be allowed to proceed i.e. the point at which
exposure to be terminated is determined by assessing the colour contrast
between the exposed and unexposed portions which should be grade 2 on a grey
scale.
ð In
the case of samples having exceptionally high light fastness, it may so happen
that BWS NO. 7 may fade to grey scale contrast 4 before the sample fades.
ð The
test may then be terminated at this point.
ð The
BWS which fades along with the sample is the light fastness rating for that
specimen, e.g. if the BWS NO. 5 fades along with sample the fastness rating is
5.
ð When
a large number of samples are regularly tested, it may not be possible to use
the BWS every time.
ð In
such cases the number of hr required to fade different BWS under defined
conditions of the temperature and humidity are standardised and rating are
given on the basis of exposure hr.
ð However,
in such cases, the instrument has to be periodically standardised has to be
periodically standardised using fresh BWS.
Fastness to Hot Pressing
ð It is
the resistance of the colour to ironing and processing on hot cylinders.
ð This
test may be carried out as dry pressing and wet pressing on a Scorch tester.
ð The
iron should be capable of giving a pressure of 0.43 lb/sq inch or 30 g/cm2
and temperature of :-
Cotton
and Linen 190
- 210°C
Wool,
Silk and Viscose 140
- 160°C
Cellulose
acetate, Polyamide 115 -
120°C
Polyester 180
- 190°C
ð
The cloth is pressed for 15 seconds with
a hot iron in three conditions:-
(a)
Dry pressing – Dry specimen placed on
dry cotton cloth.
(b)
Wet pressing – wet squeezed specimen on
wet squeezed cotton cloth.
(c)
Damp pressing – Dry specimen placed on
dry cotton cloth. Another wet squeezed is placed on the dry specimen.
ð
Change in colour is assessed by reference
to grey scale for assessing colour change. Samples are conditioned before
assessing.
Fastness to Sublimation
ð
This test is particularly important for
fabrics of polyester and its blends as they are dyed with disperse dyestuffs,
many of which may migrate and/or sublime during heat treatment.
ð
The presence of certain finishing
auxiliaries may enhance this problem.
ð
This test may be carried out on a scorch
tester or sublimation fastness tester as described for hot pressing.
ð
The difference between these tests is
that, in the case of hot pressing only one plate i.e. the top plate is heated
whereas in the case of sublimation, both the plates are heated.
ð
Further, in the case of the letter,
generally the test temperature is higher i.e. 150, 180 and 210°C and the time
of treatment is 30 seconds.
ð
The composite dry specimen is subjected
to the desired condition and subsequently evaluated for change in colour and
staining of the adjacent fabric.
Fastness to Bleaching
ð
Many textiles containing dyed
cellulosics, wool, acetate and silk may at some stage be treated with hydrogen
or sodium peroxide or sodium hypochlorite for bleaching main material.
ð
It is therefore necessary to know
resistance of dyes used to these bleaching agents.
Hydrogen Peroxide
ð
A composite specimen is prepared by
placing the specimen between an undyed specimen of itself and the other cotton
(if the first piece is wool, silk, linen or viscose) and viscose (if the first
piece is cotton or acetate).
ð
The 10 X 4 cm specimen is placed in a
roll form in a test tube under 30 times its own weight of the bleaching
solution.
ð
The test tube is fitted with a refluxing
arrangement to reduce evaporation of bleaching liquor. Four types of bleaching
liquors are used.
ð
The treated specimens are then taken
out, rinsed, opened on three sides and dried below 60°C.
ð
Colour changes and staining are
evaluated by reference to two grey scales.
Recipes
of Peroxide Bath for Fastness Test
Starting Bath
(per litre of distilled water)
|
(For Natural and Regenerated Cellulose)
|
(For Wool and Acetate Rayon)
Bath 3
|
(For Silk)
Bath 4
|
|
Bath 1
|
Bath 2
|
|||
30
% Hydrogen Peroxide (ml)
|
5
|
---
|
20
|
20
|
100
% Sodium Peroxide (g)
|
---
|
3
|
---
|
---
|
Sodium
Silicate (35°Be) (ml)
|
5
|
5
|
---
|
5
|
Sodium
Phosphate (g)
|
---
|
---
|
5
|
---
|
Magnesium
chloride (g)
|
0.1
|
0.1
|
---
|
0.1
|
pH
at start
|
10.5
|
11.5
|
9.3
|
10.0
|
Temperature
|
90°C
|
80°C
|
50°C
|
70°C
|
Duration
(hours)
|
1
|
1
|
2
|
2
|
Liquor
Ration
|
1
: 30
|
1 : 30
|
1 : 30
|
1
: 30
|
Sodium Hypochlorite
ð
This test is specially suggested for
natural and regenerated celluloses.
ð
Specimens are wetted out in distilled
water (or in 0.5 % soap solution if the specimen has been treated with water
repellent finish) and then treated in a bleaching solution containing 2 g/l
available chlorine brought to pH 11 with 10 g/l sodium carbonate.
ð
The container is kept covered for 1 hour
after which the samples are rinsed and treated with a solution of 2.5 ml of 30
% H2O2 per litre or 5 g sodium bisulphate per litre.
ð
Finally after rinsing, specimen is dried
below 60°C and colour change is assessed by reference to the appropriate grey
scale.
Fastness to Wet Scrubbing
ð
This test is particularly recommended
for pigment dyed/printed materials as it evaluate, the fastness property of the
binder film which ultimately holds the coloured pigments on the fabric surface
and therefore can easily differentiate the quality of binders.
ð
At present though there is no ISO for
this test, ISI will shortly come out with a standard test procedure for
scrubbing fastness.
ð
The wet scrubbing fastness is suggested
by Colur-Chem. The instrument for this purpose is similar to that of
crockmeter, but instead of a rubbing device there is a scrubbing device which
is fitted with a brush.
ð
The specimen is dipped in a bath
containing 5 g/l soap and 2 g/l soda ash at 60°C and is secured on the
instrument and subjected to scrubbing for a definite number of strokes
(generally 100 strokes).
ð
After half the number of strokes, fresh
soap soda solution at 60°C is poured on the specimen and scrubbed for the
remaining strokes.
ð
It is then washed and dried and the
change in colour is evaluated by using grey scale.
