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Fading characteristics of some monoazo dyes on cellulose diacetate and polyamide films: Part II
Polymer Photochemistry 6 (1985) 375-383
Fading Characteristics of Some Monoazo Dyes on Cellulose Diacetate and Polyamide l 2ms: Part II
F. M . T e r a , t L . A . A b d o u , * M . N. M i c h a e l t a n d A . H e b e i s h * § t National Institute for Standards and * National Research Centre, Textile Research Division, Dokki, Cairo, Egypt (Received: 15 May 1984)
ABSTRACT The fading rate curves and the characteristic fading curves o/seven azo dyes on cellulose diacetate (CDA) and polyamide (PA) 1ilms were examined. Fading was enhanced by prolonging the time of irradiation and decreased by increasing dye concentration on the substrate, irrespective of the nature of the latter or the dye used. However, the magnitude of fading was governed by structural characteristics of both dye and substrate. The results obtained suggested that dyes of higher ability to aggregate forming large and uniform particles were more resistant to fading than those [orming smaller and/or a non-uniform size of dye aggregates. Similarly, substrates o/higher polarity and porosity acted in favour of association of dye molecules, thereby impeding fading. These suggestions were, indeed, substantiated by measurements of the extinction y/x ratio after different irradiation periods.
INTRODUCTION Light fading of azo dyes on different polymeric materials has been extensively studied, and literature relevant to the present investigation has been cited in Part I, 1 in which we showed that structural § To whom all correspondence should be addressed. 375
Polymer Photochemistry 0144-2880/85/$3.30 © Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Northern Ireland
376
F . M . Tera et al.
characteristics and concentration of the dye determined the state of dye molecules in water and methanol. Some of the dyes examined had a greater tendency to aggregate while others were present in a monodispersed state. On the other hand, all the dyes used had a strong tendency to aggregate on cellulose diacetate (CDA) and polyamide (PA) films. This was taken as indicative of alteration in the structural characteristics of the dye as a consequence of its involvement in an interaction with the substrate. In accordance with this were the results of studies dealing with evaluation of lightfastness of the same dyes on both films using natural and artificial light. This paper presents a detailed investigation on light fading of seven monoazo dyes on C D A and P A films with a view to giving more insight about the dependence of dye aggregation on the nature of both dye and substrate.
EXPERIMENTAL Cellulose diacetate (CDA) and polyamide (PA) films dyed with seven simple azo dyes at seven different concentrations were irradiated in a lightfastness tester with a M B T F lamp for 20, 40, 60, 80, 100 and 200 h. The absorption spectra were measured for both exposed and unexposed films at the different concentrations using a Shimadzo U V 300 spectrophotometer in the range 300-700 nm. The optical density at maximum absorption---expressing the concentration on both films--was determined at all exposure periods. Detailed descriptions of the materials and methods used are given elsewhere. 1
RESULTS AND DISCUSSION Fading rate curves
The fading rate curve for a dye is usually plotted when about 20% of the original dye has faded. 2"3 Figures 1-7 show the fading rate curves of the seven monoazo dyes under investigation on C D A and P A films. It is seen that the dye concentration on the substrate is decreased by increasing the duration of irradiation. It is also seen that the higher the concentration of the dye on the substrate, the lower
Fading characteristics of some monoazo dyes on C D A and P A films: Part H t2
~ ;
;
_CDA
1.4 . . . . 8
!
__
CDA
-
>,
o8
O
0
~
0
es
-
0 0
0 v
0
8
~
0
1.0
"~ 0B
- 0,4
377
-
0
-
0.6
S
e~
PA
o
1.4 1.2 1.0 08 0.6 0~, 0.2
10 08 O
06
8
--
Ok
02 0
I
I
20
6O
I
i
IO0
i
140
20O Time (hr)
l~lg. 1. Typical fading rate curve for dye 1 on cellulose diacetate and polyamide films.
Fig. 2. dye 2
-
~
I
i
20
60
I
I
100
140
|
t80 Time t hr)
Typical fading rate curve for on cellulose diacetate and polyamide films. CDA
-T 0.8
~.~__
d
CDA
~
-
"
.,.
._~- 0.4 ~ ^~ ~~ o.z
,g
X "" ~ ~:
o . 4 ~ ~
0.611.~:. -_ --=-c
-
0.6 '.'---~_~
Y.
x
~ 9
9
X
X
It
X^
~ 02 -8 0
PA
0
1.2
PA 10
1O
08~
OB
O~
0.6
0.4
O~
0.2
02
0
0
20 Fig. 3. dye 3
60
100
140
180 Time(hr.)
Typical fading rate curve for on cellulose diacetate and polyamide films,
20 Fig. 4. dye 4
60
100
140
180 Time (hr.)
Typical fading rate curve for on cellulose diacetate and polyamide films.
378
F . M . Tera et al.
CDA
1.2 O 10
C.
C,
C
'~
0
CDA
0
0
o
"~o8 =
0
C
~0.6 0.4 0.4: . . . . .
~. o O
O
C
~'
0
'~ 0'& 0.2
:"
0.2 1~ 0
~- o
O
PA
PA
0.8 05
08 0.6 0,4 i
Oz
~
0.2
o.z:
0
0 20 Fig. 5. dye 5
C
06
60
100 140
180 Time(h~)
Typical fading rate curve for on cellulose diacetate and polyamide films.
20
60
100
140
180 Time(hr)
Fig. 6. Typical fading rate curve for dye 6 on cellulose diacetate and polyamide films.
the decrement in dye concentration by fading. This indicates that, regardless of the dye and substrate used, fading is enhanced by prolonging the duration of irradiation whilst it is decreased by increasing the dye concentration. However, fading is also governed by the structural characteristics of both dye and substrate. By way of example mention is made of the following. All the dyes examined a8 0.6 '~ 0,4
o.z
{o
0.8I
PA
O~ 0.2 0 20
60
1013
140
180
Time ( ~ ) ]~o
7°
Typical fading rate curve for dye 7 on cellulose diacetate and polyamide films.
Fading characteristics of some monoazo dyes on C D A and P A films: Part H
379
faded considerably more on P A film than on C D A film. Dyes 2 and 5 were very stable to fading on C D A film whilst they faded, though very slightly, on P A film. This again brings up the point that the effect of the substrate on the two dyes is not identical. Fading rate curves of dyes 1, 3, 4 and 6 on C D A film indicate marginal fading, reflecting the effect of association of the dye molecules which predominates because of the higher polarity and porosity of C D A film.4 This is also in agreement with the results of the change in y / x ratio with dye concentration on the film, as reported earlier) Previous reports 5-7 have also emphasized that association of the dye molecules occurs during the dyeing process. The fading rate curves of dye 7 shown in Fig. 7 are characterized by an initial fast rate (first- or second-order fade) followed by a slower rate (zero-order fade). This suggests that this particular dye is perhaps present as a mixture of single molecules or very small aggregates together with large aggregates of higher predominance. Indeed, the characteristic fading grade curves have called for a similar suggestion as indicated earlier. 1 On the other hand, the fading rate curves of dye 1 on P A film (Fig. 1) showed slight fading, indicating that its molecules may be present as large aggregates. Fading rate curves of dyes 3, 4, 6 and 7 showed an initial rapid loss of dye represented by an approximately first- or second-order reaction, followed by a much slower loss approaching zero-order reaction. This would be expected to occur if the dye particles were present in the substrate as a mixture of single and aggregated molecules. The single molecules fade rapidly first, then the aggregated particles fade slowly at a constant rate. The only difference is that for dye 7 the initial fading was considerably more rapid and greater than for the other dyes during the first 20 h of exposure.
Characteristic lading curves (CF) The time required for 10% fade of the initial dye present (tt) was obtained from fading rate curves shown in Figs 1-7 and plotted against the initial dye concentration. Figure 8 shows these plots for the dyes on C D A and P A films. It should be noted that dyes 1, 2 and 5 on C D A film and dye 2 on P A film were almost fast to light and, as a consequence, CF curves of these dyes could not be obtained.
F . M . Tera et al.
380
CDA
100 80 t-
-~-~-" --
0
(4) (7)
20 o
(3,6)
10
PA 50 40 :.=
:- ~: :.,.,; zo
(1)
is) ', ' , ' . ' , ~ ( 4 ) 6
1
~
(
3
)
10 8 6
(7)
2 1
I
2 F i g ° S°
i
~ iI
I I
3 t. 5678910
i
I
20 30t,0 C.(O.O.).
Characteristic fading curves for dyes 3, 4, 6 and 7 on cellulose diacetate and dyes 1, 3, 4, 5, 6 and 7 on polyamide films.
It is clear (Fig. 8) that all dyes examined have linear CF curves with positive slopes or in s o m e cases horizontal. This is a general indication that the dyes are present in the films probably as aggregated particles and that the proportion of the particles increases as the concentration of the dye on the substrates increases or accessibility to the dye particles of the surface decreases. 2'6'8 The CF curves of dyes examined on C D A film are higher than those of the same dyes on polyamide film. This means that these dyes are more fast on C D A film than on P A film and need more time for fading on the former substrate, in accordance with our previous report. 1 Since the slope of CF curves depends u p o n the change in size distribution of the dye particles with concentration, 6'8-1~ current data (Fig. 8) would suggest that dyes 3, 4 and 6 on C D A film are present mainly as large and nearly uniform particles irrespective of concentration because their CF curves are almost horizontal. On the other hand, the slope of the CF curves of the same dyes on P A film are intermediate, indicating small differences in particle size distribution.
Fading characteristics of some monoazo dyes on C D A and P A films: Pan H
381
.9
E x >,,
b
4L.._
3
3
2
2
1
I
i
20
60
100
140
180 Time(hr)
Fig. 9.
Change of y/x extinction ratio with time of fading at different concentrations on cellulose diacetate film: (a) dye 2; (b) dye 7.
20
I
60
i
100
i
110
I
180 Time (hr.)
Fig. 10. Change of y/x extinction ratio with time of fading at different concentrations on polyamide film: (a) dye 2; (b) dye 7.
Extinction ratio (y/x) alter fading Dyes 2 and 7 were selected to represent high and low lighffastness, respectively, on C D A and P A films. The y/x extinction ratio at different dye concentrations and after each period of exposure was determined. Figures 9 and 10 show the change in this ratio with time of exposure. A close examination of these curves reveals a great similarity to the fading rate curves already discussed. As is evident, the y/x ratio of dye 7 is decreased continuously by increasing the time of exposure, though the rate of decrement does become less as the exposure is prolonged. This indicates that dye 7 may be present as a mixture of monodisperse or very small aggregates together with a large amount of aggregated particles. On the other hand, with dye 2, the change in y/x ratio with the time of exposure is nearly constant, which reflects the fact that this dye is fast to light. Figure 11 shows the change in y/x ratio with concentration--taken
F . M . Tera et al.
382
5 /., 0 •=
"0
3
/
a
)
Y I
i
02 04
I
06
i
i
02 04
,
06 O.D.( Y-bo.nd )
1~. 11. Change of y/x extinction ratio with optical density at different exposure times and at different concentrations on cellulose diacetate and polyamide films. The numbers on the curves (1-6) represent different concentrations, expressed as optical density. (a) for CDA: 1, O.D. 0.33; 2, 0.39; 3, 0.45; 4, 0.57; 5, 0.60; 6, 0.78. (b) For PA, 1, O.D. 0.25; 2, 0.35; 3, 0.44; 4, 0.55; 5, 0.70; 6, 0.77.
as optical density ( y - b a n d ) m a f t e r different periods of exposure for dye 7. Obviously, the y/x ratio on the films under investigation decreases as the optical density decreases. It is understandable that the range of d e c r e m e n t in optical density is caused by increasing the duration of irradiation. With this in mind, the data further indicate that the d e c r e m e n t in y/x ratio is greater during the initial stages of irradiation, then it b e c o m e s less affected as the duration of irradiation increases. This, indeed, w o u l d substantiate the statement already m a d e that this dye m a y b e present as a mixture of m o n o d i s p e r s e d particles together with aggregates of different particle sizes.
REFERENCES 1. Tera, F. M., Abdou, L. A., Michael, M. N. and Hebeish, A., Polymer Photochem., this issue, p. 361.
Fading characteristics of some monoazo dyes on CDA and PA j'ilms: Part H
383
2. Giles, C. H. and McKay, R. B., Text. Res. J., 33 (1963) 527. 3. Giles, C. H., Johari, D. P. and Shah, C. D., Text. Res. J., 38, (1968) 1048. 4. Giles, C. H., Text. Res. J., 49 (1979) 724. 5. Kratky, O., Mittelbach, P. and Sekora, A., Kolloid. Z. 1 (1964) 200. 6. Giles, C. H., Yabe, A. and Shah, C. D. Text. Res. J., 38 (1968) 467. 7. Ohtsu, T., Nishida, K., Nagumo, K. and Tsuda, K., Colloid and Polymer Sci., 252 (1974) 377-80. 8. Giles, C. H., Shah, C. D., Sinclair, R. S. and Watts, W. E., J. Soc. Dyers & Colourists, 88 (1972) 433. 9. Giles, C. H., J. Soc. Dyers & Colourists, 73 (1957) 127. 10. Baxter, G., Giles, C. H., McKee, M. N. and Macaulay, N., J. Soc. Dyers & Colourists, 71 (1955) 218. 11. Giles, C. H., Baxter, G., Macaulay, N. and Rahman, S. M. K., Text. Res. J., 30 (1960) 934.
Fading Characteristics of Some Monoazo Dyes on Cellulose Diacetate and Polyamide l 2ms: Part II
F. M . T e r a , t L . A . A b d o u , * M . N. M i c h a e l t a n d A . H e b e i s h * § t National Institute for Standards and * National Research Centre, Textile Research Division, Dokki, Cairo, Egypt (Received: 15 May 1984)
ABSTRACT The fading rate curves and the characteristic fading curves o/seven azo dyes on cellulose diacetate (CDA) and polyamide (PA) 1ilms were examined. Fading was enhanced by prolonging the time of irradiation and decreased by increasing dye concentration on the substrate, irrespective of the nature of the latter or the dye used. However, the magnitude of fading was governed by structural characteristics of both dye and substrate. The results obtained suggested that dyes of higher ability to aggregate forming large and uniform particles were more resistant to fading than those [orming smaller and/or a non-uniform size of dye aggregates. Similarly, substrates o/higher polarity and porosity acted in favour of association of dye molecules, thereby impeding fading. These suggestions were, indeed, substantiated by measurements of the extinction y/x ratio after different irradiation periods.
INTRODUCTION Light fading of azo dyes on different polymeric materials has been extensively studied, and literature relevant to the present investigation has been cited in Part I, 1 in which we showed that structural § To whom all correspondence should be addressed. 375
Polymer Photochemistry 0144-2880/85/$3.30 © Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Northern Ireland
376
F . M . Tera et al.
characteristics and concentration of the dye determined the state of dye molecules in water and methanol. Some of the dyes examined had a greater tendency to aggregate while others were present in a monodispersed state. On the other hand, all the dyes used had a strong tendency to aggregate on cellulose diacetate (CDA) and polyamide (PA) films. This was taken as indicative of alteration in the structural characteristics of the dye as a consequence of its involvement in an interaction with the substrate. In accordance with this were the results of studies dealing with evaluation of lightfastness of the same dyes on both films using natural and artificial light. This paper presents a detailed investigation on light fading of seven monoazo dyes on C D A and P A films with a view to giving more insight about the dependence of dye aggregation on the nature of both dye and substrate.
EXPERIMENTAL Cellulose diacetate (CDA) and polyamide (PA) films dyed with seven simple azo dyes at seven different concentrations were irradiated in a lightfastness tester with a M B T F lamp for 20, 40, 60, 80, 100 and 200 h. The absorption spectra were measured for both exposed and unexposed films at the different concentrations using a Shimadzo U V 300 spectrophotometer in the range 300-700 nm. The optical density at maximum absorption---expressing the concentration on both films--was determined at all exposure periods. Detailed descriptions of the materials and methods used are given elsewhere. 1
RESULTS AND DISCUSSION Fading rate curves
The fading rate curve for a dye is usually plotted when about 20% of the original dye has faded. 2"3 Figures 1-7 show the fading rate curves of the seven monoazo dyes under investigation on C D A and P A films. It is seen that the dye concentration on the substrate is decreased by increasing the duration of irradiation. It is also seen that the higher the concentration of the dye on the substrate, the lower
Fading characteristics of some monoazo dyes on C D A and P A films: Part H t2
~ ;
;
_CDA
1.4 . . . . 8
!
__
CDA
-
>,
o8
O
0
~
0
es
-
0 0
0 v
0
8
~
0
1.0
"~ 0B
- 0,4
377
-
0
-
0.6
S
e~
PA
o
1.4 1.2 1.0 08 0.6 0~, 0.2
10 08 O
06
8
--
Ok
02 0
I
I
20
6O
I
i
IO0
i
140
20O Time (hr)
l~lg. 1. Typical fading rate curve for dye 1 on cellulose diacetate and polyamide films.
Fig. 2. dye 2
-
~
I
i
20
60
I
I
100
140
|
t80 Time t hr)
Typical fading rate curve for on cellulose diacetate and polyamide films. CDA
-T 0.8
~.~__
d
CDA
~
-
"
.,.
._~- 0.4 ~ ^~ ~~ o.z
,g
X "" ~ ~:
o . 4 ~ ~
0.611.~:. -_ --=-c
-
0.6 '.'---~_~
Y.
x
~ 9
9
X
X
It
X^
~ 02 -8 0
PA
0
1.2
PA 10
1O
08~
OB
O~
0.6
0.4
O~
0.2
02
0
0
20 Fig. 3. dye 3
60
100
140
180 Time(hr.)
Typical fading rate curve for on cellulose diacetate and polyamide films,
20 Fig. 4. dye 4
60
100
140
180 Time (hr.)
Typical fading rate curve for on cellulose diacetate and polyamide films.
378
F . M . Tera et al.
CDA
1.2 O 10
C.
C,
C
'~
0
CDA
0
0
o
"~o8 =
0
C
~0.6 0.4 0.4: . . . . .
~. o O
O
C
~'
0
'~ 0'& 0.2
:"
0.2 1~ 0
~- o
O
PA
PA
0.8 05
08 0.6 0,4 i
Oz
~
0.2
o.z:
0
0 20 Fig. 5. dye 5
C
06
60
100 140
180 Time(h~)
Typical fading rate curve for on cellulose diacetate and polyamide films.
20
60
100
140
180 Time(hr)
Fig. 6. Typical fading rate curve for dye 6 on cellulose diacetate and polyamide films.
the decrement in dye concentration by fading. This indicates that, regardless of the dye and substrate used, fading is enhanced by prolonging the duration of irradiation whilst it is decreased by increasing the dye concentration. However, fading is also governed by the structural characteristics of both dye and substrate. By way of example mention is made of the following. All the dyes examined a8 0.6 '~ 0,4
o.z
{o
0.8I
PA
O~ 0.2 0 20
60
1013
140
180
Time ( ~ ) ]~o
7°
Typical fading rate curve for dye 7 on cellulose diacetate and polyamide films.
Fading characteristics of some monoazo dyes on C D A and P A films: Part H
379
faded considerably more on P A film than on C D A film. Dyes 2 and 5 were very stable to fading on C D A film whilst they faded, though very slightly, on P A film. This again brings up the point that the effect of the substrate on the two dyes is not identical. Fading rate curves of dyes 1, 3, 4 and 6 on C D A film indicate marginal fading, reflecting the effect of association of the dye molecules which predominates because of the higher polarity and porosity of C D A film.4 This is also in agreement with the results of the change in y / x ratio with dye concentration on the film, as reported earlier) Previous reports 5-7 have also emphasized that association of the dye molecules occurs during the dyeing process. The fading rate curves of dye 7 shown in Fig. 7 are characterized by an initial fast rate (first- or second-order fade) followed by a slower rate (zero-order fade). This suggests that this particular dye is perhaps present as a mixture of single molecules or very small aggregates together with large aggregates of higher predominance. Indeed, the characteristic fading grade curves have called for a similar suggestion as indicated earlier. 1 On the other hand, the fading rate curves of dye 1 on P A film (Fig. 1) showed slight fading, indicating that its molecules may be present as large aggregates. Fading rate curves of dyes 3, 4, 6 and 7 showed an initial rapid loss of dye represented by an approximately first- or second-order reaction, followed by a much slower loss approaching zero-order reaction. This would be expected to occur if the dye particles were present in the substrate as a mixture of single and aggregated molecules. The single molecules fade rapidly first, then the aggregated particles fade slowly at a constant rate. The only difference is that for dye 7 the initial fading was considerably more rapid and greater than for the other dyes during the first 20 h of exposure.
Characteristic lading curves (CF) The time required for 10% fade of the initial dye present (tt) was obtained from fading rate curves shown in Figs 1-7 and plotted against the initial dye concentration. Figure 8 shows these plots for the dyes on C D A and P A films. It should be noted that dyes 1, 2 and 5 on C D A film and dye 2 on P A film were almost fast to light and, as a consequence, CF curves of these dyes could not be obtained.
F . M . Tera et al.
380
CDA
100 80 t-
-~-~-" --
0
(4) (7)
20 o
(3,6)
10
PA 50 40 :.=
:- ~: :.,.,; zo
(1)
is) ', ' , ' . ' , ~ ( 4 ) 6
1
~
(
3
)
10 8 6
(7)
2 1
I
2 F i g ° S°
i
~ iI
I I
3 t. 5678910
i
I
20 30t,0 C.(O.O.).
Characteristic fading curves for dyes 3, 4, 6 and 7 on cellulose diacetate and dyes 1, 3, 4, 5, 6 and 7 on polyamide films.
It is clear (Fig. 8) that all dyes examined have linear CF curves with positive slopes or in s o m e cases horizontal. This is a general indication that the dyes are present in the films probably as aggregated particles and that the proportion of the particles increases as the concentration of the dye on the substrates increases or accessibility to the dye particles of the surface decreases. 2'6'8 The CF curves of dyes examined on C D A film are higher than those of the same dyes on polyamide film. This means that these dyes are more fast on C D A film than on P A film and need more time for fading on the former substrate, in accordance with our previous report. 1 Since the slope of CF curves depends u p o n the change in size distribution of the dye particles with concentration, 6'8-1~ current data (Fig. 8) would suggest that dyes 3, 4 and 6 on C D A film are present mainly as large and nearly uniform particles irrespective of concentration because their CF curves are almost horizontal. On the other hand, the slope of the CF curves of the same dyes on P A film are intermediate, indicating small differences in particle size distribution.
Fading characteristics of some monoazo dyes on C D A and P A films: Pan H
381
.9
E x >,,
b
4L.._
3
3
2
2
1
I
i
20
60
100
140
180 Time(hr)
Fig. 9.
Change of y/x extinction ratio with time of fading at different concentrations on cellulose diacetate film: (a) dye 2; (b) dye 7.
20
I
60
i
100
i
110
I
180 Time (hr.)
Fig. 10. Change of y/x extinction ratio with time of fading at different concentrations on polyamide film: (a) dye 2; (b) dye 7.
Extinction ratio (y/x) alter fading Dyes 2 and 7 were selected to represent high and low lighffastness, respectively, on C D A and P A films. The y/x extinction ratio at different dye concentrations and after each period of exposure was determined. Figures 9 and 10 show the change in this ratio with time of exposure. A close examination of these curves reveals a great similarity to the fading rate curves already discussed. As is evident, the y/x ratio of dye 7 is decreased continuously by increasing the time of exposure, though the rate of decrement does become less as the exposure is prolonged. This indicates that dye 7 may be present as a mixture of monodisperse or very small aggregates together with a large amount of aggregated particles. On the other hand, with dye 2, the change in y/x ratio with the time of exposure is nearly constant, which reflects the fact that this dye is fast to light. Figure 11 shows the change in y/x ratio with concentration--taken
F . M . Tera et al.
382
5 /., 0 •=
"0
3
/
a
)
Y I
i
02 04
I
06
i
i
02 04
,
06 O.D.( Y-bo.nd )
1~. 11. Change of y/x extinction ratio with optical density at different exposure times and at different concentrations on cellulose diacetate and polyamide films. The numbers on the curves (1-6) represent different concentrations, expressed as optical density. (a) for CDA: 1, O.D. 0.33; 2, 0.39; 3, 0.45; 4, 0.57; 5, 0.60; 6, 0.78. (b) For PA, 1, O.D. 0.25; 2, 0.35; 3, 0.44; 4, 0.55; 5, 0.70; 6, 0.77.
as optical density ( y - b a n d ) m a f t e r different periods of exposure for dye 7. Obviously, the y/x ratio on the films under investigation decreases as the optical density decreases. It is understandable that the range of d e c r e m e n t in optical density is caused by increasing the duration of irradiation. With this in mind, the data further indicate that the d e c r e m e n t in y/x ratio is greater during the initial stages of irradiation, then it b e c o m e s less affected as the duration of irradiation increases. This, indeed, w o u l d substantiate the statement already m a d e that this dye m a y b e present as a mixture of m o n o d i s p e r s e d particles together with aggregates of different particle sizes.
REFERENCES 1. Tera, F. M., Abdou, L. A., Michael, M. N. and Hebeish, A., Polymer Photochem., this issue, p. 361.
Fading characteristics of some monoazo dyes on CDA and PA j'ilms: Part H
383
2. Giles, C. H. and McKay, R. B., Text. Res. J., 33 (1963) 527. 3. Giles, C. H., Johari, D. P. and Shah, C. D., Text. Res. J., 38, (1968) 1048. 4. Giles, C. H., Text. Res. J., 49 (1979) 724. 5. Kratky, O., Mittelbach, P. and Sekora, A., Kolloid. Z. 1 (1964) 200. 6. Giles, C. H., Yabe, A. and Shah, C. D. Text. Res. J., 38 (1968) 467. 7. Ohtsu, T., Nishida, K., Nagumo, K. and Tsuda, K., Colloid and Polymer Sci., 252 (1974) 377-80. 8. Giles, C. H., Shah, C. D., Sinclair, R. S. and Watts, W. E., J. Soc. Dyers & Colourists, 88 (1972) 433. 9. Giles, C. H., J. Soc. Dyers & Colourists, 73 (1957) 127. 10. Baxter, G., Giles, C. H., McKee, M. N. and Macaulay, N., J. Soc. Dyers & Colourists, 71 (1955) 218. 11. Giles, C. H., Baxter, G., Macaulay, N. and Rahman, S. M. K., Text. Res. J., 30 (1960) 934.