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Colour difference analysis of lightfastness for some vat dyes
Polymer Photochemistry 7 (1986) 65-76
Colour Difference Analysis of Lightfastness for Some Vat Dyes S. S h a k r a , Textile Research Division, National Research Centre, Dokki, Cairo, Egypt
Moustafa Ibrahim Isma Dye, Dyestuff and Chemical Co., Kafr E1-Dawar, Egypt and A . B. A m e r a Photometry Laboratory, National Institute for Standards, Dokki, Cairo, Egypt (Received 16 January, 1985) ABSTRACT Fourteen vat dyes from a commercial source were used in this investigation. Their lightfastness was measured by (a) the Blue Scale, (b) the CIE system, and (c) the Hunter L,a,b scale. From the results obtained, it was found that the Hunter L,a,b scale was easier for evaluating the lightfastness than the CIE system; for individual colours AA and A E were preferred for measuring the lightfastness, and for different colours AL and AC were preferred.
INTRODUCTION In the literature, a n u m b e r of investigations are described in which instrumental assessments using various colour difference analysis methods have been compared with visual estimation for change in colour. The conclusions reached differ from one investigator to another, depending in part upon which area of the colour range has 65
66
S. Shakra, Moustafa Ibrahim, A. B. Amera
been investigated, the nature of the colour difference used and the viewing conditions. Washing tests on a wide range of dyeings on superwash wool were carried out by King and Seltzer 1 and the colour difference A E between the washed and unwashed patterns was calculated in AN40 units. The results were compared with routine visual assessments and good agreement was found between the instrumental and visual techniques. Trials were carried out to find an instrumental method for assessing the lightfastness of dyes. The authors of this paper reached a correlation between the colour difference analysis and visual measurement of the lightfastness for some direct dyes. 2 It is not yet possible, as we stated earlier, to secure absolute freedom from error when using the various colour difference formulae, as the results, i.e. the AE and AA values, do not always completely simulate human colour perception. There are numerous contributions to the literature which deal with this problem and in which colour difference formulae have been tested or compared with one another. 3-7 To elucidate the problem, we have extended the work in this field.
EXPERIMENTAL
Materials Dyes Fourteen vat dyes from a commercial source (Isma Dye, Kafr EI-Dawar, Egypt) were used in these experiments.
Cotton fabric Mill scoured and bleached plain weave cotton fabric (23 picks × 23 ends/cm) was used throughout this investigation (Misr-Helwan, Egypt).
Dyeing The cotton samples were dyed with vat dyes by a usual method, 8 to a medium shade.
Colour difference analysis of lightfastness for vat dyes
67
Lighffastness measurements
The lightfastness of the dyed samples was assessed from the mean value of three readers who compared the fading that had occurred in the dyed samples with that on the Blue Scale. 9 The fadometer used was a Xenon Tester 150 (Hanau Heraeus GmbH, FRG).
Colour measurements
A filter apparatus with seven narrow bands (Elrepho; Carl Zeiss) was used for measuring the tristimulus X, Y and Z values of the dyed samples before and after exposure to light (Table 1). A colour difference meter (D 25-2; Hunter Lab., Virginia, USA) was used for measuring L, a and b for the dyed samples before and after exposure to light (Table 2)
Calculation
Total colour difference (AE) and chromaticity difference ( AC) These were calculated from the Scofield equations:l° AE = ~/(AL) 2 + (Aa) 2 + (Ab) 2
AC= k/(Aa)2 + (Ab) 2 where L measures lightness and varies from 100 for perfect white to zero for black, approximately as the eye would evaluate it; a measures redness when positive, grey when zero, and greenness when negative; b measures yellowness when positive, grey when zero, and blueness when negative. The results are shown in Table 2.
Lightness difference (AL), chromaticity difference (AC), and total colour difference ( AA ) These were calculated from X, Y, and Z of the dyed samples as reported in the previous work. 2 The results are presented in Table 3.
S. Shakra, Moustafa Ibrahim, A. B. Amera
68
TABLE 1 X, Y and Z for the Dyeings Before and After Exposure to Light
Dye No.
Dye name
;~,~
1
Blue Scale 4
2
Blue Scale 5
3
Blue Scale 6
4
Blue Scale 7
5
Blue Scale 8
6
Isma Blue BC
719
7
Indanthrone RS
678
8
Brilliant Green F2G
--
9
Brilliant Green FB
612
10
Isma Olive Green B
695
11
Isma Yellow GCN
519
12
Isma Golden Yellow RK
481
13
Isma Golden Yellow G K
478
14
Brilliant Orange RK
556
15
Asmaden Orange R F
374
16
Isma Brilliant Pink F R
402
17
Isma Brown BR
412
18
Isma Brown RD
408
19
Isma Brown R
477
X
Y
Z
7.1897 7-6193 7.0531 7.2184 6.7081 6.6972 7.3258 7.3253 --
6.80 7.35 6.42 6.65 6.40 6.45 6.75 6.85 --
11-63285 11.39665 11.75095 11.3376 10.3928 9-8613 11.27855 11.0423 --
7.3392 7.3585 6.6007 6.6987 8.8339 9.1378 7.9143 8.1197 7-4282 7.3599 20-3525 18.5925 18.7276 18-8939 18.3183 18.1238 17.2547 17.5388 18.4213 19.7067 16.3916 16.9306 8.3161 8.4894 9.7356 10.1073 10.4765 11.0536
6.75 6.80 6.25 6.30 10.05 10.35 8.55 8.70 7.75 8.00 22.30 19.25 18.30 18.85 18.40 18.40 15.35 15.90 16.7 17.9 13.8 14.4 7.75 7-90 9-15 9.35 9.5 10.05
12.991 12.8729 10.45185 10-56995 12.57765 12.991 12.22335 12.2824 8.8575 8.9165 9.6842 9.4480 8.6213 8.6803 9-2118 9.6842 10.0975 10.3928 11-9619 13-99485 13-3453 13-99485 8.08985 8.1489 9.0937 9.2118 8.1489 8-32605
Colour difference analysis of ligh(astness for vat dyes
69
TABLE 2
L,a,b Before and After Exposure to Light, Lighffastness, AC and AE of Dyeings under Investigation
Dye No.
Lightfastness
1
4
2
5
3
6
4
7
5
8
6
7
7
6.5
8
5.5
9
6
10
6
11
6.75
12
4.75
13
5
14
6.5
15
4.25
16
5.25
17
5.25
18
4.75
19
5
L
a
b
23.2 25.5 21.9 22.9 21.9 22.2 23.1 23.5 21-6 21.5 24.8 25.1 21-5 21.4 35.5 35-2 30.3 30.7 27.3 27.9 58.1 54.7 47.9 48.8 52.4 51.4 41.6 41-5 43-0 46-1 38.6 39.6 24.0 24.6 27.8 31.2 30.3 31.1
1-0 0.8 0.8 4.9 -1.1 -1.3 3.2 2.7 0.7 0.8 5.4 4.6 5.9 5.2 -20.1 -17.5 -15.0 -14.5 -5.7 -6.2 -8.1 -3.1 12.8 13.9 5.2 3.8 26.4 25.0 20.6 21.6 33.2 30.3 6.3 7.5 9.4 12-7 11.0 12.1
-13-1 -9.5 -20.1 -15.0 -10.3 -8.8 -13.1 -12.4 -9.0 -8.8 -22.5 -21.2 -17.4 -16.0 4.2 3.4 -2.1 -1.9 2.0 2.2 32.6 29.3 27.4 27.8 27.4 25.7 17.8 17.1 15-4 13.9 2.7 3-0 4.5 5.0 6.5 11.7 10.8 11.3
AC
AE
Type of colour
3.605
4.319
Blue
6.544
6.62
Blue
1.51327
1.543
Blue
0.8602
0.949
Blue
0.224
0.245
Blue
1.5264
1.556
Blue
1.565
1-568
Blue
2.720
2.737
Green
0.5385
0.671
Green
0.5385
0.806
Green
5.9908
6.888
Yellow
1.17046
1.476
Yellow
2.20227
2.419
Yellow
1.565
1.568
Orange
1.803
3-586
Orange
2.915
3.082
Orange
1.3
1.432
Brown
5.9076
6.816
Brown
1.208
1.449
Brown
S. Shakra, Moustafa Ibrahim, A. B. Arnera
70
TABLE 3 Lightfastness, AL, AC and AA of Dyeings under Investigation
Dye No.
Lightfastness
AL
AC
AA
Type of colour
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
4 5 6 7 8 7 6.5 5.5 6 6 6.75 4.75 5 6.5 4.25 5-25 5.25 4.75 5
3.8 1-65 0.35 0.7 0 0.35 0.7 1.7 0.9 1.55 2 -0 2.2 4.6 2.65 1 1-15 3.1
7.1 4 4.35 2.8 0 0.95 1-85 1 3.6 13.2 6.1 -4.9 5.2 10-1 3.75 3-6 4.65 2-3
8 4.3 4.4 2.9 0 1 1.9 1.9 3.75 13.3 6.5 -4-9 5.6 11-1 4-5 3.75 4.8 3.9
Blue Blue Blue Blue Blue Blue Blue Green Green Green Yellow Yellow Yellow Orange Orange Orange Brown Brown Brown
Correlation coefficient r If the lightfastness figure is equal to x and AA is equal to y for a dyed sample, then JZ- E x ,
y = EY-
n
n
where n is the total number of dyeings used in this work. From the product-moment correlation
X--n
--
Colour difference analysis of light-fastness for vat dyes
71
RESULTS AND DISCUSSION In previous work, 2 a relation between lightfastness measured by the Blue scale and that by the CIE system for dyeings of direct dyes was observed. The correlation coefficient r was calculated for the different colours of dyeings (r = -0.93). This work was carried out as an attempt to find the correct relation between visual and instrumental measurement of lightfastness for each individual colour. The direct dyes were, in general, of low lightfastness. The vat dyes chosen in the present work were of high lightfastness. It is well known that colour can be classified in three groups. (i) (ii) (iii)
Primary colour (red, blue, yellow). Secondary colour (green, orange, violet). Tertiary colour (olive, brown, grey).
Dyes under investigation were of blue, yellow (primary), green, orange (secondary), olive and brown colour (tertiary).
CIE system
Primary colours From Table 4 and Fig. 1, for AL and lightfastness r for primary colours ranges between -0-885 for the Blue Scale to +0.577 for
TABLE 4 The Correlation Coefficient r (Lightfastness and A L or AC or AA or AC or AE) for the two systems CIE and Hunter L,a,b
CIE system Type of colour Blue scale Blue Green Yellow Orange Brown All colours
Hunter L, a, b system
AL
AC
&A
AC
AE
-0.885 -1.0 -1.0 +0-577 -0.196 -0-064 -0.65
-0.945 -1.0 +1.0 +1.0 -0.683 -0.446 -0.414
-0.952~ -1-0 I +1.0 +1-0 -0-727 -0-924 -0.283
-0.8693 -0.9978 -0-7468 -0.2279 -0.8575 -0.64207
-0.8198 -1.0 -1.0 +0-999 -0.976 -0.866 -0.409
S. Shakra, Moustafa Ibrahim, A. B. Amera
72
1.0 I
0-9 0.8 0.7 0.6 0.5
0-4 0"3 0.2
v
o
.-
0-1
0
o
-0.1 -0.2 -0.:3 -0-,4 -0.5 -0'6 -0.7 -0.8 -0,9
O -n
I=1
O
N
/
H
/
/
/
.~ AC
/
/
1
-I.0
Hg. 1. Relation between lightfastness and AL, AC, AA (r) for the primary (I), secondary (II), tertiary (III) and all colours by the CIE system. yellow colours, while for blue dyeings r = - 1 . 0 . The negative sign of r indicates the indirect proportionality between the lightfastness and AL, and this fits well as r approaches - 1 . 0 . For yellow dyeings r has a positive sign, i.e. there is a direct proportionality between their lightfastness and AL. For understanding this we can say, that for dyeings of high lightfastness, the lightness difference (AL) between faded and unfaded samples is very small and reaches zero for
Colour difference analysis of ligh(astnessfor vat dyes
73
lightfastness = 8 and vice versa. For yellow dyeings the opposite was found, i.e. the lightness difference is high for dyeings of high lightfastness (anomalous fading). The same phenomenon as above was observed for AC and lightfastness, but r for AC (whether negative or positive) had a higher value than for AL (cf. Table 4 and Fig. 1). Again, for AA and lightfastness, the same was observed as above, but for the blue dyeings r had increased to some extent.
Secondary colours Considering AL and lightfastness, for green dyeings r = - 1 . 0 while for orange r = -0.196, i.e. the relation fits better for green dyeings than for orange dyeings. It is worth mentioning here, that for the green c o l o u r = b l u e + y e l l o w , (noting that the orange c o l o u r = yellow + red) r fits well for blue (short wavelength region) but not for red (long wavelength region); compare Table 4 and Fig. 1. It is to be noted from the result for AC that r changed from negative to positive, i.e. the chroma difference between the faded and unfaded dyeings is high for green dyeings of high lightfastness. This phenomenon cannot be detected visually, and it may be said that the change in the yellow component of the green colour is high enough to mask the change in its blue component. For green colour, r for AA was stable as in the case of AC but for orange its value increases from -0.683 to -0.727. From the above, it can be said that AA is more fruitful than AL and AC for evaluating the change in secondary colours of dyeings due to fading. Tertiary colours For AL, r has its lowest value and equals -0.064, while it increases steadily for AC and AA, i.e. the direction of increase (negative sign) is as follows: AL---~ AC----~AA
(cf. Table 4).
Different colours From Table 4 and Fig. 1, it was found that r has its maximum value in the case of AL while it has its lowest value for AA.
S. Shakra, Moustafa lbrahim, A. B. Amera
74
From the above discussion, it is clear that: (a) (b)
AA can be taken as a measure for lightfastness of individual colours; AL can be taken as a measure for lightfastness of different colours.
Hunter L,a,b opponent-colours scale system For comparing results obtained by the CIE system with those from the Hunter L,a,b system, the L, a and b of the dyeings were measured before and after fading, then AC, AE and r were calculated as discussed above.
Primary colours From Table 4 and Fig. 2 it is clear that for AE r = -0.8198, -1.0, and +0.999 for Blue scale, blue dyeings and yellow dyeings respectively. It is worth mentioning here that the same phenomenon was observed in case of the CIE system. From Table 4, the value of r for AC has increased to some extent for the blue colour (negative) while its value changes from positive to negative for the yellow colour.
Secondary colours From Table 4, for AE r = - 1 . 0 and -0.976 for green and orange colours respectively, i.e. a very good reversible proportionality was attained between AE and the lightfastness of these colours. It was noticed that r = - 1 . 0 for both AE and AL, while it is +1.0 for AC and AA. This can be understood if we know that the green colour is a mixture of blue and yellow and the blue colour has a negative r, while the yellow has a positive one, i.e. for the blue colour the chroma and whole lightness difference is directly proportional to the lightfastness, or in other words it is reversibly proportional to the fading that occurs in these colours. For AC, r has nearly the same value for the green colour as that obtained from the CIE system, while for orange it has decreased (cf. Fig. 2).
Tertiary colours It is clear (Table 4, Fig. 2) that for AE r = -0.866 for brown colours and this indicates that for tertiary colours also the proportionality
Colour difference analysis of lightfastness for vat dyes
75
10 o.g 0.8 0-7 06 0,5 0.4 0.3 0.2 0.1
"-c 0 -0.1 -0"2
I
I
O t~
L. O
=
i t--
N
-0.3
A
-0.4
\
AE
/
-0"5
/
-0.6
/
-0.7
/
-0.8
/
-o.g -,/.(
/
Fig. 2. Relation between lightfastness and AE, AC (r) for the primary (I), secondary (II), tertiary (III) and all colours by the Hunter L,a,b system.
between AE and lightfastness is reversible, as for the primary blue colour. For AC, r has nearly the same value as for AE.
All colours For all colours r = - 0 . 4 0 9 for AE, i.e. the proportionality between AE and lightfastness is moderately reversible.
76
S. Shakra, Moustafa Ibrahirn, A. B. Amera
Its value for AC increased from r = - 0 . 4 0 9 to r = -0-64207, i.e. the reversible proportionality between the chroma difference and lightfastness increased to a greater extent in this case than in the case of AE. CONCLUSIONS From the experimental results obtained in this work, it was found that: (i) (ii)
(iii)
The colour difference meter was easier to use for evaluating results by the Scofield equation than by the CIE system. For an individual colour AA and AE can be used for evaluating the fading which occurs in dyed samples and they are preferred over the lightfastness figures. For different colours AL and AC are preferred.
REFERENCES 1. King, M. G. and Seltzer, I., J. Soc. Dyers Colourists, 90 (1974) 281. 2. Ibrahim, M. and Shakra, S., 'Colour difference analysis of lightfastness for some direct dyes', Polym. Photochem., 6 (1985) 85. 3. Simon, F. T. and Goodwin, W. J., J. Opt. Soc. Amer., 47 (1957) 1050. 4. Davidson, H. R., J. Opt. Soc. Amer., 41 (1951) 1052. 5. Brockes, A., Die Farbe, 7 (1958) 257. 6. Warburton, F. L., Die Farbe, 7 (1958) 247. 7. Williams, P. S., Die Farbe, 7 (1958) 209. 8. Trotman, E. R., Dyeing and chemical technology of textile fibres, 3rd edn., Griffin, London, 1964, pp. 441-72. 9. The standard methods for determination of colour fastness of textiles, Society of Dyers and Colourists, Bradford, Great Britain, 1962. 10. Judd, D. B. and Wysezcki, G., Colour in business, science and industry, 3rd edn., John Wiley, New York, 1975.
Colour Difference Analysis of Lightfastness for Some Vat Dyes S. S h a k r a , Textile Research Division, National Research Centre, Dokki, Cairo, Egypt
Moustafa Ibrahim Isma Dye, Dyestuff and Chemical Co., Kafr E1-Dawar, Egypt and A . B. A m e r a Photometry Laboratory, National Institute for Standards, Dokki, Cairo, Egypt (Received 16 January, 1985) ABSTRACT Fourteen vat dyes from a commercial source were used in this investigation. Their lightfastness was measured by (a) the Blue Scale, (b) the CIE system, and (c) the Hunter L,a,b scale. From the results obtained, it was found that the Hunter L,a,b scale was easier for evaluating the lightfastness than the CIE system; for individual colours AA and A E were preferred for measuring the lightfastness, and for different colours AL and AC were preferred.
INTRODUCTION In the literature, a n u m b e r of investigations are described in which instrumental assessments using various colour difference analysis methods have been compared with visual estimation for change in colour. The conclusions reached differ from one investigator to another, depending in part upon which area of the colour range has 65
66
S. Shakra, Moustafa Ibrahim, A. B. Amera
been investigated, the nature of the colour difference used and the viewing conditions. Washing tests on a wide range of dyeings on superwash wool were carried out by King and Seltzer 1 and the colour difference A E between the washed and unwashed patterns was calculated in AN40 units. The results were compared with routine visual assessments and good agreement was found between the instrumental and visual techniques. Trials were carried out to find an instrumental method for assessing the lightfastness of dyes. The authors of this paper reached a correlation between the colour difference analysis and visual measurement of the lightfastness for some direct dyes. 2 It is not yet possible, as we stated earlier, to secure absolute freedom from error when using the various colour difference formulae, as the results, i.e. the AE and AA values, do not always completely simulate human colour perception. There are numerous contributions to the literature which deal with this problem and in which colour difference formulae have been tested or compared with one another. 3-7 To elucidate the problem, we have extended the work in this field.
EXPERIMENTAL
Materials Dyes Fourteen vat dyes from a commercial source (Isma Dye, Kafr EI-Dawar, Egypt) were used in these experiments.
Cotton fabric Mill scoured and bleached plain weave cotton fabric (23 picks × 23 ends/cm) was used throughout this investigation (Misr-Helwan, Egypt).
Dyeing The cotton samples were dyed with vat dyes by a usual method, 8 to a medium shade.
Colour difference analysis of lightfastness for vat dyes
67
Lighffastness measurements
The lightfastness of the dyed samples was assessed from the mean value of three readers who compared the fading that had occurred in the dyed samples with that on the Blue Scale. 9 The fadometer used was a Xenon Tester 150 (Hanau Heraeus GmbH, FRG).
Colour measurements
A filter apparatus with seven narrow bands (Elrepho; Carl Zeiss) was used for measuring the tristimulus X, Y and Z values of the dyed samples before and after exposure to light (Table 1). A colour difference meter (D 25-2; Hunter Lab., Virginia, USA) was used for measuring L, a and b for the dyed samples before and after exposure to light (Table 2)
Calculation
Total colour difference (AE) and chromaticity difference ( AC) These were calculated from the Scofield equations:l° AE = ~/(AL) 2 + (Aa) 2 + (Ab) 2
AC= k/(Aa)2 + (Ab) 2 where L measures lightness and varies from 100 for perfect white to zero for black, approximately as the eye would evaluate it; a measures redness when positive, grey when zero, and greenness when negative; b measures yellowness when positive, grey when zero, and blueness when negative. The results are shown in Table 2.
Lightness difference (AL), chromaticity difference (AC), and total colour difference ( AA ) These were calculated from X, Y, and Z of the dyed samples as reported in the previous work. 2 The results are presented in Table 3.
S. Shakra, Moustafa Ibrahim, A. B. Amera
68
TABLE 1 X, Y and Z for the Dyeings Before and After Exposure to Light
Dye No.
Dye name
;~,~
1
Blue Scale 4
2
Blue Scale 5
3
Blue Scale 6
4
Blue Scale 7
5
Blue Scale 8
6
Isma Blue BC
719
7
Indanthrone RS
678
8
Brilliant Green F2G
--
9
Brilliant Green FB
612
10
Isma Olive Green B
695
11
Isma Yellow GCN
519
12
Isma Golden Yellow RK
481
13
Isma Golden Yellow G K
478
14
Brilliant Orange RK
556
15
Asmaden Orange R F
374
16
Isma Brilliant Pink F R
402
17
Isma Brown BR
412
18
Isma Brown RD
408
19
Isma Brown R
477
X
Y
Z
7.1897 7-6193 7.0531 7.2184 6.7081 6.6972 7.3258 7.3253 --
6.80 7.35 6.42 6.65 6.40 6.45 6.75 6.85 --
11-63285 11.39665 11.75095 11.3376 10.3928 9-8613 11.27855 11.0423 --
7.3392 7.3585 6.6007 6.6987 8.8339 9.1378 7.9143 8.1197 7-4282 7.3599 20-3525 18.5925 18.7276 18-8939 18.3183 18.1238 17.2547 17.5388 18.4213 19.7067 16.3916 16.9306 8.3161 8.4894 9.7356 10.1073 10.4765 11.0536
6.75 6.80 6.25 6.30 10.05 10.35 8.55 8.70 7.75 8.00 22.30 19.25 18.30 18.85 18.40 18.40 15.35 15.90 16.7 17.9 13.8 14.4 7.75 7-90 9-15 9.35 9.5 10.05
12.991 12.8729 10.45185 10-56995 12.57765 12.991 12.22335 12.2824 8.8575 8.9165 9.6842 9.4480 8.6213 8.6803 9-2118 9.6842 10.0975 10.3928 11-9619 13-99485 13-3453 13-99485 8.08985 8.1489 9.0937 9.2118 8.1489 8-32605
Colour difference analysis of ligh(astness for vat dyes
69
TABLE 2
L,a,b Before and After Exposure to Light, Lighffastness, AC and AE of Dyeings under Investigation
Dye No.
Lightfastness
1
4
2
5
3
6
4
7
5
8
6
7
7
6.5
8
5.5
9
6
10
6
11
6.75
12
4.75
13
5
14
6.5
15
4.25
16
5.25
17
5.25
18
4.75
19
5
L
a
b
23.2 25.5 21.9 22.9 21.9 22.2 23.1 23.5 21-6 21.5 24.8 25.1 21-5 21.4 35.5 35-2 30.3 30.7 27.3 27.9 58.1 54.7 47.9 48.8 52.4 51.4 41.6 41-5 43-0 46-1 38.6 39.6 24.0 24.6 27.8 31.2 30.3 31.1
1-0 0.8 0.8 4.9 -1.1 -1.3 3.2 2.7 0.7 0.8 5.4 4.6 5.9 5.2 -20.1 -17.5 -15.0 -14.5 -5.7 -6.2 -8.1 -3.1 12.8 13.9 5.2 3.8 26.4 25.0 20.6 21.6 33.2 30.3 6.3 7.5 9.4 12-7 11.0 12.1
-13-1 -9.5 -20.1 -15.0 -10.3 -8.8 -13.1 -12.4 -9.0 -8.8 -22.5 -21.2 -17.4 -16.0 4.2 3.4 -2.1 -1.9 2.0 2.2 32.6 29.3 27.4 27.8 27.4 25.7 17.8 17.1 15-4 13.9 2.7 3-0 4.5 5.0 6.5 11.7 10.8 11.3
AC
AE
Type of colour
3.605
4.319
Blue
6.544
6.62
Blue
1.51327
1.543
Blue
0.8602
0.949
Blue
0.224
0.245
Blue
1.5264
1.556
Blue
1.565
1-568
Blue
2.720
2.737
Green
0.5385
0.671
Green
0.5385
0.806
Green
5.9908
6.888
Yellow
1.17046
1.476
Yellow
2.20227
2.419
Yellow
1.565
1.568
Orange
1.803
3-586
Orange
2.915
3.082
Orange
1.3
1.432
Brown
5.9076
6.816
Brown
1.208
1.449
Brown
S. Shakra, Moustafa Ibrahim, A. B. Arnera
70
TABLE 3 Lightfastness, AL, AC and AA of Dyeings under Investigation
Dye No.
Lightfastness
AL
AC
AA
Type of colour
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
4 5 6 7 8 7 6.5 5.5 6 6 6.75 4.75 5 6.5 4.25 5-25 5.25 4.75 5
3.8 1-65 0.35 0.7 0 0.35 0.7 1.7 0.9 1.55 2 -0 2.2 4.6 2.65 1 1-15 3.1
7.1 4 4.35 2.8 0 0.95 1-85 1 3.6 13.2 6.1 -4.9 5.2 10-1 3.75 3-6 4.65 2-3
8 4.3 4.4 2.9 0 1 1.9 1.9 3.75 13.3 6.5 -4-9 5.6 11-1 4-5 3.75 4.8 3.9
Blue Blue Blue Blue Blue Blue Blue Green Green Green Yellow Yellow Yellow Orange Orange Orange Brown Brown Brown
Correlation coefficient r If the lightfastness figure is equal to x and AA is equal to y for a dyed sample, then JZ- E x ,
y = EY-
n
n
where n is the total number of dyeings used in this work. From the product-moment correlation
X--n
--
Colour difference analysis of light-fastness for vat dyes
71
RESULTS AND DISCUSSION In previous work, 2 a relation between lightfastness measured by the Blue scale and that by the CIE system for dyeings of direct dyes was observed. The correlation coefficient r was calculated for the different colours of dyeings (r = -0.93). This work was carried out as an attempt to find the correct relation between visual and instrumental measurement of lightfastness for each individual colour. The direct dyes were, in general, of low lightfastness. The vat dyes chosen in the present work were of high lightfastness. It is well known that colour can be classified in three groups. (i) (ii) (iii)
Primary colour (red, blue, yellow). Secondary colour (green, orange, violet). Tertiary colour (olive, brown, grey).
Dyes under investigation were of blue, yellow (primary), green, orange (secondary), olive and brown colour (tertiary).
CIE system
Primary colours From Table 4 and Fig. 1, for AL and lightfastness r for primary colours ranges between -0-885 for the Blue Scale to +0.577 for
TABLE 4 The Correlation Coefficient r (Lightfastness and A L or AC or AA or AC or AE) for the two systems CIE and Hunter L,a,b
CIE system Type of colour Blue scale Blue Green Yellow Orange Brown All colours
Hunter L, a, b system
AL
AC
&A
AC
AE
-0.885 -1.0 -1.0 +0-577 -0.196 -0-064 -0.65
-0.945 -1.0 +1.0 +1.0 -0.683 -0.446 -0.414
-0.952~ -1-0 I +1.0 +1-0 -0-727 -0-924 -0.283
-0.8693 -0.9978 -0-7468 -0.2279 -0.8575 -0.64207
-0.8198 -1.0 -1.0 +0-999 -0.976 -0.866 -0.409
S. Shakra, Moustafa Ibrahim, A. B. Amera
72
1.0 I
0-9 0.8 0.7 0.6 0.5
0-4 0"3 0.2
v
o
.-
0-1
0
o
-0.1 -0.2 -0.:3 -0-,4 -0.5 -0'6 -0.7 -0.8 -0,9
O -n
I=1
O
N
/
H
/
/
/
.~ AC
/
/
1
-I.0
Hg. 1. Relation between lightfastness and AL, AC, AA (r) for the primary (I), secondary (II), tertiary (III) and all colours by the CIE system. yellow colours, while for blue dyeings r = - 1 . 0 . The negative sign of r indicates the indirect proportionality between the lightfastness and AL, and this fits well as r approaches - 1 . 0 . For yellow dyeings r has a positive sign, i.e. there is a direct proportionality between their lightfastness and AL. For understanding this we can say, that for dyeings of high lightfastness, the lightness difference (AL) between faded and unfaded samples is very small and reaches zero for
Colour difference analysis of ligh(astnessfor vat dyes
73
lightfastness = 8 and vice versa. For yellow dyeings the opposite was found, i.e. the lightness difference is high for dyeings of high lightfastness (anomalous fading). The same phenomenon as above was observed for AC and lightfastness, but r for AC (whether negative or positive) had a higher value than for AL (cf. Table 4 and Fig. 1). Again, for AA and lightfastness, the same was observed as above, but for the blue dyeings r had increased to some extent.
Secondary colours Considering AL and lightfastness, for green dyeings r = - 1 . 0 while for orange r = -0.196, i.e. the relation fits better for green dyeings than for orange dyeings. It is worth mentioning here, that for the green c o l o u r = b l u e + y e l l o w , (noting that the orange c o l o u r = yellow + red) r fits well for blue (short wavelength region) but not for red (long wavelength region); compare Table 4 and Fig. 1. It is to be noted from the result for AC that r changed from negative to positive, i.e. the chroma difference between the faded and unfaded dyeings is high for green dyeings of high lightfastness. This phenomenon cannot be detected visually, and it may be said that the change in the yellow component of the green colour is high enough to mask the change in its blue component. For green colour, r for AA was stable as in the case of AC but for orange its value increases from -0.683 to -0.727. From the above, it can be said that AA is more fruitful than AL and AC for evaluating the change in secondary colours of dyeings due to fading. Tertiary colours For AL, r has its lowest value and equals -0.064, while it increases steadily for AC and AA, i.e. the direction of increase (negative sign) is as follows: AL---~ AC----~AA
(cf. Table 4).
Different colours From Table 4 and Fig. 1, it was found that r has its maximum value in the case of AL while it has its lowest value for AA.
S. Shakra, Moustafa lbrahim, A. B. Amera
74
From the above discussion, it is clear that: (a) (b)
AA can be taken as a measure for lightfastness of individual colours; AL can be taken as a measure for lightfastness of different colours.
Hunter L,a,b opponent-colours scale system For comparing results obtained by the CIE system with those from the Hunter L,a,b system, the L, a and b of the dyeings were measured before and after fading, then AC, AE and r were calculated as discussed above.
Primary colours From Table 4 and Fig. 2 it is clear that for AE r = -0.8198, -1.0, and +0.999 for Blue scale, blue dyeings and yellow dyeings respectively. It is worth mentioning here that the same phenomenon was observed in case of the CIE system. From Table 4, the value of r for AC has increased to some extent for the blue colour (negative) while its value changes from positive to negative for the yellow colour.
Secondary colours From Table 4, for AE r = - 1 . 0 and -0.976 for green and orange colours respectively, i.e. a very good reversible proportionality was attained between AE and the lightfastness of these colours. It was noticed that r = - 1 . 0 for both AE and AL, while it is +1.0 for AC and AA. This can be understood if we know that the green colour is a mixture of blue and yellow and the blue colour has a negative r, while the yellow has a positive one, i.e. for the blue colour the chroma and whole lightness difference is directly proportional to the lightfastness, or in other words it is reversibly proportional to the fading that occurs in these colours. For AC, r has nearly the same value for the green colour as that obtained from the CIE system, while for orange it has decreased (cf. Fig. 2).
Tertiary colours It is clear (Table 4, Fig. 2) that for AE r = -0.866 for brown colours and this indicates that for tertiary colours also the proportionality
Colour difference analysis of lightfastness for vat dyes
75
10 o.g 0.8 0-7 06 0,5 0.4 0.3 0.2 0.1
"-c 0 -0.1 -0"2
I
I
O t~
L. O
=
i t--
N
-0.3
A
-0.4
\
AE
/
-0"5
/
-0.6
/
-0.7
/
-0.8
/
-o.g -,/.(
/
Fig. 2. Relation between lightfastness and AE, AC (r) for the primary (I), secondary (II), tertiary (III) and all colours by the Hunter L,a,b system.
between AE and lightfastness is reversible, as for the primary blue colour. For AC, r has nearly the same value as for AE.
All colours For all colours r = - 0 . 4 0 9 for AE, i.e. the proportionality between AE and lightfastness is moderately reversible.
76
S. Shakra, Moustafa Ibrahirn, A. B. Amera
Its value for AC increased from r = - 0 . 4 0 9 to r = -0-64207, i.e. the reversible proportionality between the chroma difference and lightfastness increased to a greater extent in this case than in the case of AE. CONCLUSIONS From the experimental results obtained in this work, it was found that: (i) (ii)
(iii)
The colour difference meter was easier to use for evaluating results by the Scofield equation than by the CIE system. For an individual colour AA and AE can be used for evaluating the fading which occurs in dyed samples and they are preferred over the lightfastness figures. For different colours AL and AC are preferred.
REFERENCES 1. King, M. G. and Seltzer, I., J. Soc. Dyers Colourists, 90 (1974) 281. 2. Ibrahim, M. and Shakra, S., 'Colour difference analysis of lightfastness for some direct dyes', Polym. Photochem., 6 (1985) 85. 3. Simon, F. T. and Goodwin, W. J., J. Opt. Soc. Amer., 47 (1957) 1050. 4. Davidson, H. R., J. Opt. Soc. Amer., 41 (1951) 1052. 5. Brockes, A., Die Farbe, 7 (1958) 257. 6. Warburton, F. L., Die Farbe, 7 (1958) 247. 7. Williams, P. S., Die Farbe, 7 (1958) 209. 8. Trotman, E. R., Dyeing and chemical technology of textile fibres, 3rd edn., Griffin, London, 1964, pp. 441-72. 9. The standard methods for determination of colour fastness of textiles, Society of Dyers and Colourists, Bradford, Great Britain, 1962. 10. Judd, D. B. and Wysezcki, G., Colour in business, science and industry, 3rd edn., John Wiley, New York, 1975.