THE FADING OF BASIC DYES IN
POLYMER SUBSTRATES
A THESIS SUBMITTED FOR THE DEGREE
OF
DOCTOR OF PHILOSOPHY
AT THE UNIVERSITY OF NEW SOUTH WALES
BY SUSAN ELIZABETH MATTHEWS
·ft.
SCHOOL OF TEXTILE TECHNOLOGY
· ···JONE .. 1980 i
I hereby certify that the work described in this thesis was performed by me in the School of Textile Technology, The University of New South Wales, and has not been submitted previously for any other degree or award. ii
ACKNOWLEDGEMENTS
I wish to express my sincere thanks to my supervisors, Associate Professor C.H.
Nicholls and Dr. M.T. Pailthorpe, for their guidance and support throughout the course of this work.
I thank my colleagues and friends for their continued encouragement and interest.
I also thank my friend Anne for the typing and presentation of this thesis.
Finally, I particularly thank my fiance,
Paul, for his invaluable support during this time. iii
ABSTRACT
The reasons for the higher lightfastness of basic dyes on acrylic substrates compared to other substrates were investigated. Acrylic, nylon and poly(vinyl alcohol) substrates in film and fibre form were dyed and irradiated in order to study this phenomenon directly in the solid state. It was found that there is a difference in the initial quantum yields of dye fading of at least two orders of magnitude between the acrylic and the non-acrylic fibres.
Temperature studies indicated that the high rigidity of acrylic polymers, by virtue of their inherently compact structures and their high glass transition temperatures, contributes significantly to the high lightfastness of basic dyes present in them. This is postulated to be due to the relative inhibition of movement in the polymer, which affects both diffusional and conformational processes that are involved in the fading reactions.
Oxygen and moisture were found to have little effect on basic dye fading.
Very little difference could be found between the rate of fading of dyes bound to sulphonate and carboxylate dye sites of the same pKa. However, the presence of acid caused a significant reduction in the rate of dye fading which appears to indicate that the pK of the dye site affects a basic dye fading. This pH effect on dye fading rates suggests that electron transfer from substrate to dye could contribute iv
to the dye degradation. Thus the reduced fading in acrylics might be attributed in part to their being poor electron
donors compared with other substrates studied.
The presence of -1% cyano groups in an inert
substrate was found to significantly reduce basic dye fading, with the effect increasing with an increased cyano concen
tration of -8%. Since cyano groups comprise approximately
50% by weight of an acrylic polymer this effect would appear
to be the single most important factor contributing to the high lightfastness of basic dyes present in them. It was postulated that the formation of an excited complex between the excited dye molecule and one or more cyano groups would either inhibit the usual fading reactions or facilitate deactivation of the excited dye molecule. V
CONTENTS
DECLARATION i
ACKNOWLEDGEMENTS ii
ABSTRACT iii
CONTENTS V
CHAPTER 1 INTRODUCTION
1.1 Preamble 1
1.2 Photochemical Principles 2
1.2.1 Excited States 4
1.2.2 Photophysical Decay Pathways 5
1.2.3 Photochemical Decay Pathways 6
1.3 The Photochemistry of Dyes 9
1.3.1 Typical Photochemical Reactions of Dyes 10
1.3.1.1 Azo Dyes 11
1.3.1.2 Anthraquinone Dyes 15
1.3.1.3 Triphenylmethane Dyes 18
1. 4 Factors Influencing Dye Fading 22
1.4.1 Intensity and Spectral Distribution
of Radiation 22
1. 4 .2 The Physical State of the Dye 24
1.4.3 The Nature of the Substrate 26
1.4.4 The Dye-Fibre Bond 29
1.4.5 Oxygen 30
1.4.6 Moisture 31
1.4.7 Temperature 33
1.5 Scope of Present Work 35 vi
CHAPTER 2 QUANTUM YIELDS OF BASIC DYE FADING
2.1 Introduction 41
2.2 Fabrics 42
2.2.1 Acrilan 16 42
2.2.2 Nylon 6.6 43
2.2.3 Basic-Dyeable Nylon 43
2.2.4 Chemical Compositions 43
2.3 Dyes 44
2.4 Dyeing Methods 45
2.4.1 Acrilan 16 46
2.4.2 Nylon 6.6 47
2.4.3 Basic-Dyeable Nylon 47
2.5 Determination of Dye Concentration 48
2.6 Irradiation Technique 51
2.6.1 Calibration of Incident Light Intensity 52
2.7 Determination of Initial Quantum Yields of
Basic Dye Fading 57
2.7.1 Measurement of Fading Rates 57
2.7.2 Determination of Quanta Absorbed by Dye 59
2.7.3 Calculation of Initial Quantum Yields of
Basic Dye Fading 60
CHAPTER 3 THE EFFECT OF THE PHYSICAL STATE OF THE
SUBSTRATE ON BASIC DYE FADING
3.1 Introduction 64
3.2 Polymer Film Preparation 65
3.2.1 Nylon 6 66
3.2.2 Poly(vinyl alcohol) 66
3.2.3 Acrilan 16 67 vii
3.3 Measurement of Glass Transition Temperatures 69
3.3.l Experimental Technique 70
3.3.2 Results 72
3.4 Irradiation Technique 74
3.4.1 Method of Mounting and Heating the
Film Sample 74
3.4.2 Method of Irradiation 76
3.5 Determination of the Initial Quantum Yields
of Basic Dye Fading 77
3.5.1 Measurement of Fading Rates 77
3.5.2 Measurement of Quanta Absorbed 81
3.5.3 Calculation of the Initial Quantum
Yields of Dye Fading 83
3.6 Experimental Results 83
3.6.1 The Effect of Temperature 84
3.6.2 The Effect of Oxygen and Moisture 90
CHAPTER 4 THE INFLUENCE OF THE CHEMICAL NATURE OF
THE SUBSTRATE ON THE FADING OF BASIC DYES
4.1 Introduction 96
4.2 Irradiation Technique 97
4.3 The Identification of Reaction Intermediates
in Basic Dye Fading 98
4.3.1 The Use of Scavenging and Quenching
Agents 98
4.3.1.1 Excited State Quenching 99
4.3.1.2 Electron and Radical Scavenging 99
4.3.1.3 Experimental Technique 100
4.3.1.4 Results 102
4.3.2 Luminescence Studies 104 viii
4.4 The Influence of Dye Sites on Basic Dye Fading 104
4.4.1 Experimental Technique 105
4.4.2 Results 106
4.5 The Influence of the Cyano Group on Basic Dye Fading 109 4.5.1 Experimental Technique 109
4.5.2 Results 111
CHAPTER 5 CONCLUSION 114
REFERENCES 120 1
CHAPTER l
INTRODUCTION
1.1 Preamble
The fading of dyed materials by light, or photo fading, refers to the loss of colour or change of hue of the dye caused by its exposure to light. Being of obvious commercial interest in relation to the durability of dyed fabrics, such phenomena have long been the subject of investigation. References 1 to 8 give reviews of this research.
There are, however, still vast areas of the subject about which relatively little is known. This is due to the extremely complex nature of the photochemistry of such dye fibre systems and the number of different variables which can influence it. Further, as the concentration of dyes on fabrics is usually very low (no more than -3%) and because fading reactions have low quantum yields and occur only at the surface of the fabric only small yields of decomposition products are obtained. Consequently isolation and identification of these products is so tedious that other less direct methods have to be used to establish the mode of reaction. In addition, the reaction sequence of fading may be so complex that identification of the final products may not shed much light on the preceding reactions.
Model systems using organic solutions instead of solid substrates and simple instead of the more complex dyes have been used to simplify the systems and much information has 2
been gained in this way. However, great care must be taken
in this respect as it is not always possible to apply such
results directly to solid substrates. The fading behaviour may not necessarily be the same in the two environments due
to differing physical and/or chemical effects.
The aim of the present work was to elucidate some
aspects of the fading behaviour of one class of dyes in which large differences in lightfastness properties are
found between different fibre substrates. Solid substrates in the form of transparent polymer films and fabrics were used in order to study the problem directly and avoid the interpretation difficulties of solution models.
1.2 Photochemical Principles
The first step in a photochemical process is the
absorption of light by a molecule. This causes an electron to be promoted from its lowest energy level or ground state
to a higher energy level or "excited" state. This can be
represented by D + hv ~D.* A molecule in this excited
state is much more reactive than in its ground state having more energy, a different electron distribution and different molecular geometry. Consequently photochemical reactions of
various types, often unusual in ordinary "thermal" reactions,
can occur readily.
The lifetime of this reactive excited state is
critical in determining whether or not a photochemical
reaction, leading to dye fading, will occur. The longer
the molecule remains in its excited state the greater the 3 probability of such a reaction occurring. There are numerous alternative deactivation pathways by which the molecule can return to its ground state without any chemical change taking place and therefore the relative rates of these "photophysical" processes compared with the "photo chemical" processes will determine the rate at which the dye will fade.
The initial processes involving the electronically excited molecule are known as the primary photochemical and photophysical processes. These can then be followed by secondary or "dark" reactions of the various chemical species produced, the nature of such chemical reactions depending on the system in question.
The efficiency of the absorbed light in promoting any particular process is expressed by the quantum yield
(~) of that process and is defined as:
~=number of molecules undergoing a particular process number of quanta absorbed by the system
The sum of the quantum yields of all the different primary photochemical and photophysical processes must be equal to unity, but the quantum yield of any particular process may vary from ~ << 1 to ~ >> 1, for chain reactions. The fading reactions of dyes generally have ~ << 1 indicating that most of the absorbed energy is dissipated in other ways. 4
1.2.1 Excited States
A molecule in an electronically excited state con
tains two unpaired electrons, one of which has been excited by the absorption of light and promoted to a higher energy
level orbital leaving the second electron of the pair in the ground state orbital. When the two electrons of an excited state have the same (or parallel) spin the excited state is known as a triplet excited state, and when they have opposed
(or antiparallel) spin the excited state is called a singlet excited state (abbreviated as T and S respectively).
Excited states of organic molecules produced directly by the absorption of light are nearly always singlets. This is because virtually all their ground states are singlets
(denoted S 0 ) and conservation of spin during absorption is strongly favoured. Excited triplet states are therefore only formed via an intermediate excited singlet state. A
triplet state has a lower energy than the corresponding singlet state since according to the Pauli Exclusion
Principle its two electrons, having parallel spin, cannot occupy the same space and are therefore more separated and exert a smaller repulsive force on each other.
Depending on the energy of the quantum of light
absorbed by a molecule, an electron may be excited to the
lowest excited singlet state S 1 or a higher singlet state
S2, S 3 etc. However, if the electron is excited to a higher
state it very rapidly loses energy by radiationless decay
processes until it reaches the lowest vibrational level of
the lowest excited singlet state S 1 • From here it can be 5
converted to a triplet state. This process is known as
intersystem crossing and, although it is formally forbidden,
does occur due to a phenomenon known as spin-orbit coupling.
Again, if the state formed is a higher triplet state (T2,
T 3 etc.), rapid decay to T 1 will occur.
The extremely short lifetimes of the higher excited
states resulting from this mean that very few chemical
reactions can compete with the rapid deactivation processes.
Thus very few photochemical reactions originate from these higher excited states. In contrast, the lowest excited singlet and triplet states have relatively long lifetimes
(10- 9 to 10- 7s for S 1 , and 10- 4 to 10 2s for Ti). As a consequence it is these states that are the starting points for almost all photochemical reactions.
1.2 .2 Photophysical Decay Pathways
Photophysical deactivation processes return an electronically excited molecule to its ground state without
any chemical change occurring, and photochemical reactions have to compete with these processes. The photophysical
decay pathways which are available to an excited molecule
can be summarised as follows:
(1) Vibrational Relaxation. This involves transitions within the one excited state from higher to lower vibrational energy levels with an accompanying loss of heat.
(2) Emission of Radiation. This is known as fluorescence when the transition is from an excited singlet state to the
ground state, and phosphorescence when from an excited tri
plet state to the ground state. As triplet to singlet C a
d
So
FIGURE 1.1 Jablonski diagram illustrating energy decay
pathways for a molecule raised to its second
excited singlet state.
a absorption
b vibrational decay
C internal conversion
d fluorescence
e intersystem crossing
f phosphorescence 6
transitions are spin-forbidden requiring an inversion of spin, phosphorescence has a longer lifetime than fluorescence.
(3) Radiationless conversion. This is accompanied by a loss of heat and includes two types of processes:
(i) Internal Conversion which involves transitions
between like states i.e., singlet-singlet or
triplet-triplet.
(ii) Intersystem Crossing which involves transitions
between unlike states. The transition from singlet
to triplet occurs isoenergetically to an upper
vibrational level of the triplet state.
(4) Energy transfer to another molecule. This process is known as quenching. The second molecule may gain the energy as vibrational, rotational and translational energy or it may be raised to an excited state just as if it had absorbed a photon.
A Jablonski diagram (Figure 1.1) illustrates these various photophysical deactivation pathways. Straight lines represent emission or absorption of radiation and wavy lines are used to indicate radiationless processes.
1.2.3 Photochemical Decay Pathways
If, instead of decaying to its ground state by photophysical processes, the excited dye molecule takes part in a chemical reaction, the following types of reactions are typical of those possible: 7
A 0 + B 0 radical formation
C + D intramolecular decomposition
BA rearrangement
photoisomerisation
R 0 H-atom abstraction AB + hv -+ photodimerisation
+ e- photoionisation
B intramolecular electron transfer
intermolecular electron } transfer
The excess electronic energy of the excited state alone is often the only reason for the particular course of a photochemical reaction. At other times the change in the electron distribution and molecular geometry of the excited state also contribute.
In any particular photochemical system only a selection of these reactions is involved and minor changes in the nature of the system may mean substantial changes in
the relative rates of the various reactions. The direct involvement of outside species in some of these reactions
indicates in particular that the nature of the medium in which a molecule exists will have an influence on the type
of reactions that will occur. 8
The types of excited states of most importance in
dye fading reactions are the (n, TT)* [i.e., produced by excitation of an electron from an nor non-bonding orbital to a TT * or antibonding TT orbital, denoted an (n-TT)* transi- . * tion] , the (TT, TT ) and charge-transfer states. (The latter involves the transfer of an electron from a donor to an acceptor group.) a and a * orbitals are rarely involved because much larger energies are required to excite electrons from or to these orbitals respectively. Each of these excited states can exist in both the singlet and the triplet form.
Which of these transitions are observed in any particular molecule will depend on the basic chemical structure of the molecule. The energies of the resulting excited states will also largely depend on structure but may in addition be influenced by the nature of the medium in which it exists. For example in acid or polar media the interaction between then electron and the solvent lowers the energy of then electron and thus an (n-TT)* transition will require more energy than in non-polar and non-acid media. The polarity of solvents also affects
(TT-TT)* transitions and transitions involving charge-transfer states. In the first case the election charge distribution is more extended and more polarisable in the (TT,TT)* state
than the ground state and so polar solvents stabilise the excited state more than the ground state:
excited state ground state =c:-- non- -- I polar polar solvent solvent 9
Charge-transfer states are affected even more because they have very large dipole moments.
The basic chemical structure of a molecule will usually play a major role in determining the energy levels of the excited states, but the nature of the substituents present, especially those with electron-withdrawing or releasing characteristics, may also influence the energies, sometimes even taking over as the major determinant. Sub stituents can alter the relative positions of the excited states, possibly changing the nature of the lowest excited state. The nature of this lowest excited state will largely determine the photochemical reactivity of the molecule.
1.3 The Photochemistry of Dyes
The photochemical degradation of dyes usually occurs either by photooxidation or photoreduction reactions. Intra molecular photoreaction i.e., photodecomposition simply as a result of the instability of the excited dye molecule is
rare particularly when the dyestuff is adsorbed on rigid polymer substrates. 6
Photooxidation has usually been found to require the presence of moisture as well as oxygen. Various entities have been postulated as the oxidising agents involved,
including HO", H0 2 ", 9 H2 0 2 5 and oxygen itself which has been
shown to react with the triplet state of the dye. 10
Photoreduction requires the presence of a suitable
hydrogen or electron donating substance. The two mechanisms 10
possible can be represented as follows:
(i) hydrogen abstraction: D* + RH + DH + R
(ii) electron abstraction: o* +RH+ D 0 + RH+
Some dyes also undergo reversible fading reactions, with the dye returning to its original form when removed from the light. This phenomenon, known as phototropy, can be the result of a number of different reactions including tautomerism, cis-trans isomerisation, ring opening, free radical formation and the formation of aggregates. While not as serious a problem as permanent fading, phototropy is clearly undesirable, and dyes which are phototropic have mostly been eliminated from commercial ranges.
The fact that a dye exhibits phototropy may lead to either an increase or a decrease in its overall lightfast ness. It may decrease lightfastness if the reversible process is followed by an irreversible one:
* + D +- X + Y
or it may increase lightfastness if the transition from the excited dye state to the phototropic species is so rapid that there is little time for any other photochemical
reaction to occur from the excited state.
1.3.1 Typical Photochemical Reactions of Dyes
The types of photochemical reactions that a dye may
undergo will depend mainly on its chemical structure, as
this will determine the nature of the excited state from 11 which a reaction may proceed, the position of this state relative to other states, its reactivity and lifetime, and the probability of its formation and deactivation. With respect to their basic chemical structure the majority of dyes belong to three classes - azo, triphenylmethane and anthraquinone. A smaller number of dyes belong to a variety of other classes including methine, oxazine and xanthene.
Each of these chemical dye classes have their own characteristic photochemical reactions. Then, in addition to its basic structure, the nature of the substituents present on a particular dye will also influence its photo chemistry, sometimes even outweighing the influences of the basic chromophore. Evans and Stapleton have produced a comprehensive review 8 of these substituent effects on each chemical and dyeing class of dye where it can be seen that it is difficult to reach any general conclusions about substituent effects on dye fading.
1.3.1.1 Azo Dyes
Azo dyes {general formula ~ 1 -N = N-~2) form the largest group of all the synthetic dyestuffs, making up approximately half of the structures disclosed in the Colour
Index. A number of the different photochemical reactions which azo dyes undergo have been reviewed by Griffiths. 11
By measuring the photocurrents produced when a platinum electrode coated with an azo dye was irradiated,
Hillson and Rideal 12 found that these dyes could be either oxidised or reduced. They postulated that similar reactions 12
occurred in dyed fibres which later researchers have
verified.
Photoreduction has been found to occur either by hydrogen transfer from the substrate to an excited dye molecule or from a photochemically excited substrate to
the dye in its ground state.
The former pathway was proposed by Blaisdell 13 for
the fading of azobenzene in various solvents on the basis of the spectroscopic identification of hydrazobenzene and aniline as fading products. Hashimoto and coworkers 14 later confirmed these results by isolating and chemically identifying the same two reaction products. In addition,
under acid conditions they found that the hydrazobenzene was converted to benzidine. A more complex azo compound,
a red dye, has also been found to produce an unstable hydrazo compound on irradiation. 15 Further reaction of the hydrazobenzenes by either thermal or photochemical dispro portionation can occur and results in the irreversible
cleavage of the azo linkage:
2 q,NH-NH
The second pathway was demonstrated by Van Beek
et al., 16 who found that azo dyes in anaerobic solutions
can be reduced to amines by hydrogen donors produced by
photoexcitation of suitable simple molecules. The mechanism
proposed involves the formation of unstable hydrazyl radicals
[1] which then disproportionate to form hydrazo compounds 13
[2] and these in turn decompose to the corresponding amines
[3]. Regeneration of the dye also occurs in the last two steps: . ~1-N=N-~2. + H" + ~1-NH-N-~2 (1] 2~1-NH-N-~2 + ~1-NH-NH-~2 + ~1-N=N-~2 [2]
Oxygen was found to inhibit fading by reoxidation of the reduced dye radicals.
Photoreduction of azo dyes has also been postulated in polymeric substrates. Giles et al., 17 suggested that reductive fading of azo dyes may occur in hydrocarbon media in the absence of air, and in wool, silk and gelatin in the presence of air although this appears far from conclusive.
Kamel et al. , 18 found substituent effects of azo dyes on nylon that indicated that the fading mechanism here too is reductive.
Similar substituent investigations on other substrates have been indicative of oxidative fading e.g., the work of
Matsuoki and coworkers 19 on polyester films. Various photo oxidative mechanisms have been proposed on the basis of studies of the photolysis of various dye-solution systems.
Naumova et al., 20 studied the fading of hydroxybisazo dyes in carbon tetrachloride and proposed that cleavage of the hydroxyl hydrogen atom occurred, followed by destruction of the azo group with the formation of quinones and aromatic anhydrides. Griffiths et al., 10 proposed a mechanism in 14
which singlet oxygen attacks the hydrazone form of an azo
dye in methanol. They found that fading only occurred in
the presence of oxygen and that the introduction of a singlet oxygen quencher reduced the rate. Photoejection of a hydro gen atom from an amino group to form a radical which then disproportionates has been postulated as the mechanism of
the photooxidation of the dye Congo Red. 21
Other types of photochemical reactions have also been observed for azo dyes. Photocyclisation of azobenzene occurs in the presence of acid or a ferric chloride catalyst. 22 ' 23
It requires cis-azobenzene as the starting product and the
following mechanism has been proposed:
+ H + /H N=N/ hv
Ob 0-0H H
N=N /
0 b +H+
N=N 0--0
The final product, benzo[c]cinnoline, was identified as the
major fading product by Lewis. 22 A similar heterocyclic
decomposition product has been found following the rapid
fading of an o-aminoazo dye on cellulose 24 : 15
N=N ~NHO'--::: hv ~ 8
It is obvious from this latter fading mechanism that the ability of a dyestuff to undergo cis-trans isomerism can increase the probability of it fading rapidly. Largely to eliminate the phototropy caused by such photoisomerisation, most modern azo dyes have structures which either inhibit photoisomerisation or reduce the lifetimes of the cis-isomers.
Such structures either exist predominantly in the hydrazone form (~ 1 =N-N-~2) or have strongly electron-withdrawing groups para to the azo linkage.
1.3.1.2 Anthraquinone Dyes
Photodegradation of anthraquinone dyes (base 0 formula may also occur by either
0 reductive or oxidative mechanisms, again usually depending on the nature of the solution or substrate in which it exists.
Photoreduction may occur by an electron transfer process to produce radical ions: 16
Alternatively hydrogen abstraction can occur, in which case
semiquinone radicals (DH") are produced. 25 ' 26 These may subsequently react by a dismutation process to form the hydroquinone 27 ' 28 :
DH 0 + DH 0 + DH2 + D
In alkaline solution, depending on the substituents present on the dye, dianions (D 02 -) or the semiquinone radical anion o·- have been identified29 :
DH" + OH
D* + X + o· + x·
It is well established that the excited state of
anthraquinone dyes that is most reactive towards hydrogen or electron abstraction is the (n, TT)* triplet state. 2 ' 6
The movement of the electron away from the carbonyl oxygen on excitation makes the oxygen atom more electrophilic than
in its ground state making hydrogen or electron abstraction easier. Both the (TT,TT)* state and possible charge-transfer
states are less reactive in this respect. Consequently molecules having either of these states as their lowest
excited state will have higher lightfastness than if they
have the (n, TT)* state as the lowest energy state. As the
(n, TT)* and CT states have similar energies small differences
in substituent or solvent properties can alter their relative 17
positions and thereby greatly affect the reactivity of the excited molecule. For example, p -arninobenzophenone in iso propanol has the unreactive CT state as the lowest state while in cyclohexane the {n, TT*) state is the lowest. This
leads to hydrogen abstraction occurring in the cyclohexane but not in isopropanol. 2 Similarly the introduction of 0 X electron-donating substituents {X): X
0 change the relative positions of the energy levels so that the {TT, TT*) becomes the lowest excited state instead of the
{n, TT*) with a consequent loss of photoreactivity towards hydrogen abstraction. 6
High light stability in anthraquinones is often
achieved by 1-hydroxy substitution where intrarnolecular hydrogen bonding with the carbonyl group leads to rapid
deactivation of the excited states. 30 ' 31 ' 32 However in polar substrates, such as nylon, this substitution gives a
low lightfastness and 1,2-dihydroxy substitution is needed to stabilise the dye. 33 Allen et al., 34 found that sub stitution in the 2 and 3 positions by an amino and a hydroxy
group respectively leads to extremely rapid deactivation of
the excited states. This results in much higher light
stability than the corresponding individually 2-substituted
compounds in which deactivation is slower.
Photoreduction has also been shown to occur in poly
meric media as well as in solution. The photolysis of arnino
anthraquinones in nylon films has been shown to be 18
reductive. 35 ' 36 ' 37
Photooxidative fading of anthraquinone dyes has also
been found to occur, for example in polyester film. 38139
The photolysis of 1,4-diaminoanthraquinone inn-butyl acetate
is also oxidative involving a radical mechanism and photo
oxidation by molecular oxygen. 40 The fading of this dye and
its N-alkyl derivatives has also been investigated in poly-
(ethylene tereph thalate) films. 3 8 ' 3 9 The photoreactions
postulated included:
N-dealkylation
nuclear hydroxylation
substitution of amino groups by hydroxyl groups
introduction of oxygen at the carbon atom adjacent
to the nitrogen of N-alkylamino groups.
This led Wegerle 38 to suggest that oxidative photodegra
dation involving attack by some electrophilic agent on the
unshared electron pair of the nitrogen atom occurs with
subsequent dealkylation and the formation of a carbonyl
group at the a-carbon atom.
1.3.1.3 Triphenylmethane Dyes The two triphenylmethane dyestuffs most widely
studied are Malachite Green (C.I. Basic Green 4) and
Crystal Violet (C.I. Basic Violet 3): 19
Cl
where R = H for Malachite Green
and R = N(CH 3 ) 2 for Crystal Violet.
From the investigation of the photoproducts of these dyes
formed in solution and in various solid substrates, infor mation about the mechanisms of photodegradation and the structural factors affecting the lightfastness of triphenyl methane dyes has been obtained.
Photooxidation to the corresponding diphenylketones and leuco compounds has been observed in solution 8 ' 41 ' 42 ' 43 and in solid substrates. 42 ' 44 Porter and Spears 42 identified benzophenone, 4-methylamino- and 4-dimethylamino-benzophenone as photoproducts of Malachite Green in aqueous solutions and on modified cellulose substrates, together with p -dimethyl
aminophenol in solution only. On the basis of these photo products they proposed a photodegradation mechanism involving
absorption of light by the leuco carbinol form of the dye which then either undergoes radical fragmentation followed by reaction with oxygen and water, or reacts directly with 20 oxygen and water: QH,)2 Q-c+x--9
excited * [ J + state
(CH3) 2N -0-oH +
+ C-o-~ N/H O II - '-cH3 0
Evidence in support of the involvement of the carbinol base
(~ 3-COH) rather than the carbonium ion (~ 3-C+) included:
The rate of fading in solution was found to
decrease as the acidity of the solvent is increased. 25
When light of wavelengths absorbed by the
carbinol base (and not the coloured carbonium ion) 21
is filtered out, either by filtering the incident
light45 or by introducing a uv absorber into the
system25 the triphenylmethane dye is more stable
to light.
Bangert et al., 44 proposed similar reaction mechanisms to account for the photoproducts they identified by thin layer chromotography from the photolysis of Malachite Green and
Crystal Violet. The reactions included dealkylation via hydroxy methyl groups and oxidation to benzophenone (with no intermediate postulated).
Leaver 46 and Owen and Allen47 using spectroscopic and electron spin resonance techniques have detected triaryl methyl radicals on the photolysis of certain triphenylmethane dyes in de-aerated solution and in poly(vinyl alcohol) 46 and poly(methyl methacrylate) 47 films. Pak et al., 48 studied the leuco compounds of Crystal Violet Ar 3C-X where
Ar= o-N(CH3)2 and X = H, OH, CN,and maintained
that free radicals, cation radicals or cations can be produced from the excited triphenylmethane molecule, with the radical being formed in one of the following ways:
Ar 3C-X + e + Ar 3C 0 + X + Ar3C + e + Ar 3 C 0
and that the formation of the cation depends on the surrounding medium and conditions. 22
N-dealkylation of Malachite Green has been observed by a number of workers. 8 ' 42 ' 49 This reaction has also been observed in the photodegradation of dyes in other dye classes. 8
1.4 Factors Influencing Dye Fading
The chemical structure of a dye will basically determine the various types of photochemical reactions it is capable of undergoing. Which of these alternative reaction pathways are in fact followed in any particular system, and the rate of these reactions will depend on numerous other factors characteristic of the dye-substrate system as a whole, the surrounding atmosphere and the incident radiation.
1.4.1 Intensity and Spectral Distribution of Radiation
As the first prerequisite for a photochemical reaction to proceed is the absorption of light, obviously the dye (or the substrate, if energy transfer is possible) must absorb light of wavelengths emitted by the radiation source. That is,there must be an overlap of the absorption spectrum of the dye (and/or substrate) and the emission spectrum of the light source.
In addition, the different wavelengths of light have been found to vary in their efficiency in initiating photochemical reactions. The results of numerous investigations 43 ' 50 - 54 have shown that the quantum efficiency of fading decreases with increasing wavelength of the light - the higher energy ultraviolet and blue light 23 having the greatest effect and the lower energy red light the least. McLaren 52 using filtered sunlight on 117 Qyes
found that in general absorbed uv and violet light will
cause fading while red light will only be active if the dye is fugitive and not if it is moderately fast. Further, he
found that while fugitive dyes are faded by absorbed light of any wavelength, lightfast dyes are faded by absorbed
light only if it is below a certain critical wavelength that is different for each dye. This effect has also been observed by other researchers. Irick and Boyd54 studied disperse dyes on various hydrophobic substrates and in solution irradiated with both monochromatic and polychromatic light and found that no dye degradation occurred with light above 400nm. The quantum yield of dye degradation for monochromatic light below 400nm generally increased with decreasing wavelength while with polychromatic radiation the dye degradation rates were generally consistent with the
fraction of short wavelength emission of the light source.
Yamada et al., 43 and Maerov and Kobsa 45 observed similar behaviour in other dye-substrate systems.
Since each photon of light of the active wavelengths
can excite one molecule the rate of fading will increase with increasing intensity of this active radiation. Thus,
although Evans 55 found that uv light was the most effective
in promoting the fading of Rhodamine B, because of the very
low proportion of uv in sunlight he found that visible light
of wavelength 400-600nm although having a much lower quantum
efficiency was responsible for 84% of fading. 24
Also,it has been demonstrated that there is no threshold level of intensity below which fading will not occur56 as was originally believed.
1.4.2 The Physical State of the Dye
It is generally accepted that the more aggregated the dye the greater its lightfastness. This view was originally put forward by Giles and his coworkers 57 ' 68 on the basis that fading only occurs on the surface of a dye particle where the dye is accessible to radiation and to those agents such as oxygen and moisture which were thought to be involved in fading. More recently another mechanism for this effect has been proposed by Terenin, 59 namely that with increased aggregation there is a greater possibility of self-extinction i.e., the transfer of excitation energy between adjacent dye molecules, which results in the subsequent reduction in the quantum efficiency of dye fading.
This aggregation effect has been used to explain a variety of phenomena, including the improvement in light fastness with increasing dye concentration; 56 the improvement in lightfastness of azoic dyes on soaping; 58 ' 60 the decrease in the lightfastness of disperse dyes in the presence of certain carriers which are proposed to act as disaggregating agents; 61 and the improvement in lightfast ness of dyes in substrates with increasing pore size such as in viscose rayon compared to cotton. 56
This trend of increasing lightfastness with 25 increasing aggregation does seem to be followed in the case of dyes in hydrophilic fibres. 58 ' 61 The lightfastness of dyes was found to increase with the increasing relative moisture regain of the fibres. However, in the case of hydrophobic fibres, the opposite trend has been observed, 61 with the lightfastness of dyes decreasing with the moisture regain of the fibres. It has been postulated61 ' 62 that the significantly smaller pore sizes in hydrophobic fibres cause a second effect to predominate, the smaller pores restricting the diffusion of agents such as oxygen and moisture through the fibre to the excited dye molecule. Therefore, the smaller the pore size the greater this restriction and hence the slower the dye will fade. The higher lightfastness of reactive dyes on drawn compared to undrawn nylon has also been attributed to this "diffusion restriction effect". 63
The actual degree of aggregation of disperse dyes on hydrophobic substrates has been the subject of some controversy with Giles et al., 6 ~' 65 postulating that the dyes are aggregated, while Wegerle 38 and others 66 ' 67 maintain that they are present in monomolecular form. Both arguments are centred around the Beer-Lambert law and the ratio of the short and long wavelength bands of the dyes. Giles found that this ratio increased both with increasing concentration and on exposure to light which he attributed to an increase in the degree of aggregation of the dye. Wegerle on the other hand observed no change in the ratio with concentration, nor on fading when the absorbance of the photoproducts is taken into account. Direct physical measurements of the aggregation of these dyes gives conflicting results. 7 26
Giles et al., 56 ' 57 also maintain that the shape of a
dye's fading curve is entirely determined by the state of
aggregation of the dye which in most cases exists in a wide range of particle sizes. Kissa68 disagreed with Giles maintaining that this oversimplified the situation and that the slope of the fading curve was also dependent on the distribution and location of the dye in the fibre. Thus he
also contests the validity of the deductions Giles has made about the degree of aggregation of dyes based on the shape of their fading curves. Blair and Boyd69 contend that the increase in fastness with dye concentration is due to a
filter effect due to an increase in the amount of light
absorbed, as well as to dye aggregation.
Thus there remain some doubts as to the influence the aggregation of a dye has on its lightfastness.
1.4.3 The Nature of the Substrate
It was discovered as early as 1813 that the nature of
the substrate had a marked effect on the rate of fading of a
dye 70 and since then this has been observed by numerous
researchers. 8 ' 71 ' 72 As a consequence, it is always necessary
to specify the substrate involved when quoting lightfastness
data.
Both the chemical and physical nature of the substrate
can influence fading. Chemically, the substrate can itself
participate in the fading reactions, commonly by donating
either a hydrogen atom or an electron to the excited dye
molecule. Also, certain groups in the substrate may be able 27
to quench the excited dye molecule thereby reducing its fading.
The chemical nature of the groups which act as dye sites may also influence fading as may the nature of the dye-substrate bond. The physical characteristics of the substrate which may influence fading include its rigidity and porosity, and its permeability to various entities such as water and oxygen.
The various mechanisms involved in a number of these cases are dealt with in other sections of this review.
Chipalkatti et al., 73 divided fibre substrates into two classes on the basis of dye fading rates, namely protein and non-protein. It was suggested that the more rapid fading found on non-proteins was probably an oxidation process while on proteins fading was probably reductive involving reaction with the substrate.
As discussed in Section 1.4.2 the porosity of fibres has been postulated to influence dye fading either by limiting the degree of dye aggregation possible, or by restricting the diffusion of gases such as oxygen and moisture through the fibre.
The rigidity of the substrate has also been found to affect the photochemical reactions occurring74 ' 75 as rigidity will largely determine the possible motions, both diffusional
and conformational, of the various species present. This phenomenon is discussed further in Section 1.4.7.
It has been observed that fading behaviour can differ quite markedly in solution and in solid substrates. 27 ' 76 28
Egerton et al., 76 observed that the near uv-visible irradiation of the vat dye 1-benzamidoanthraquinone in ethanol readily produces the anthrahydroquinone whereas the same dye in cellulose film shows little or no tendency to reduction of this kind. Similarly Egerton et al., 27 found that H-abstraction occurred with 1,5-diaminoanthra quinone in organic solvents containing an abstractable H and in N-methoxy-methyl nylon films. However it did not occur in carbon tetrachloride solution, nylon and cellulose acetate films, or in the solid dye itself. They postulate that hydrogen abstraction in a polymer is dependent on the accessibility of the dye reactive group to an abstractable hydrogen with the aggregation of the dye also possibly influencing the degree of dye-fibre interaction.
Porter and Spears 42 found that altering the dye bonding site in cellulose to a sulphonate group improved the lightfastness to give a 5 to 10 times slower fading rate compared to carboxylate groups.
The polarity of the substrate has also been shown to affect fading. Using flash photolysis and luminescence spectroscopy Allen et al. , 77 ' 78 identified the excited species formed on irradiation of certain amino- and amino-
0 carboxy-anthraquinone dyes as either o· or DH • Strong transient absorption was observed in polar solvents but no transients were detected in non-polar solvents. This effect was postulated to account for the low lightfastness of these dyes on nylon 6.6 fabric, which is highly polar, compared with the high fastness on the non-polar polyester fabric. 29
The presence of other chemicals in a dyed substrate can also affect the lightfastness of the dye e.g., titanium dioxide delustrant is known to accelerate fading 7 as does the presence of certain carriers used in dyeing which are thought to act as disaggregating agents. 61
1.4.4 The Dye-Fibre Bond
The effect of covalent bonding between reactive dyes and substrate on lightfastness remains unresolved. Several workers have found improved lightfastness on fixation 79 -ai while others have obtained inconclusive results, 63 ' 82 - 8 ~ with some dye-fibre systems showing improvement in light fastness on fixing, some showing reductions and others no significant change.
The improvement of lightfastness on fixation has been attributed to the greater possibility of energy transfer away from the dye to the substrate through the covalent bond. 79 ,ao However this ignores the fact that energy transfer is unlikely because of the energy levels involved.
Giles et al., 82 attributed reductions in lightfastness with covalent bonding to the accompanying restriction to the aggregation of the dye. Krichevskii et al., 83 took a similar view but maintained that two opposing effects occurred.They postulated that closer contact between dye molecules lead to greater self-extinction in addition to the formation of aggregates reducing the surface area of the dye accessible to fading. 30
1.4.5 Oxygen
It was originally believed that oxygen always promoted the fading of dyes, with fading being retarded or even entirely prevented in its absence. 85 ' 86 Giles 56 for example found that lightfastness increased linearly with the concentration of oxygen in the atmosphere. However, other investigators have found that sometimes dyes fade more rapidly in the absence of oxygen. 3 ' 43 ' 87 Egerton et al., 3 for example found that the fading of a particular disperse dye on cellulose acetate film was negligible in dry nitrogen but significant in dry oxygen while the same dye in nylon film faded appreciably in dry nitrogen. Again it becomes obvious that observations for a particular dye-fibre system are specific to that system and are not necessarily generally applicable.
The accelerating effect of oxygen may be caused by the direct reaction of the oxygen with the excited dye molecule. Alternatively the oxygen may first quench the excited dye molecule thereby becoming excited itself and then react with the dye in either its ground or excited state. It has been shown that the excited state of oxygen involved is likely to be singlet excited oxygen. 5
Various explanations have been advanced for the retarding effect that oxygen can have. Van Beek et al., 87 postulated that oxygen inhibits the fading of azo dyes in solution with hydrogen-donor substrates by competing with the dye for hydrogen from the excited substrate molecules and by reoxidation of the reduced dye radicals back to the 31
original dye by the peroxide radicals formed. Yamada et al.,~ 3 studied triphenylmethane dyes in solution and found that the quantum yields in anaerobic solutions were ten times greater
than those in aerobic solutions. They proposed that these dyes undergo both oxidation and reduction and that the dissolved oxygen restricts the latter reactions which have higher quantum yields than the former.
1.4.6 Moisture
Lightfastness normally decreases with increasing atmospheric humidity 71 ' 88 ~ 0 with the extent of the effect being dependent on the dye-substrate system in question.
An increase in fading rate of up to sixteen times when damp compared to dry has been reported by McLaren9 0 for some dyed samples while Giles et al.,9 1 on the other hand found that dye fading on polyester film was not sensitive to relative humidity at all. There are basically two explanations that have been advanced to explain this phenomenon. The first is that the water itself or some compound formed from it takes part in the fading reactions. The second involves the physical effect that moisture has on either the substrate by swelling it and thereby increasing the rates of diffusion within it or on the dye, possibly by displacing adsorbed dye or by disaggregating it.
Giles et al.,9 2 confirmed previous observations that
the fading of dyes on cellulosic substrates was particularly
sensitive to changes in the relative humidity of the
surrounding atmosphere while dyes on proteins are relatively
insensitive to changes in humidity. Giles proposed that the 32 reason for this was that water swelled the substrates thereby increasing the diffusion of atmospheric oxygen to the excited dye molecule. In cellulosic substrates where the major degradation mechanism is oxidation the moisture content of the fibre therefore has a significant effect. However in protein substrates where reduction is the major mechanism of
fading, with hydrogen being extracted from the substrate itself, this increase in oxygen diffusion has little effect.
Disperse dyes on hydrophobic synthetic polymers have also been examined for relative humidity sensitivity in
fading.9 1 On cellulose acetate and triacetate and on nylon the dyes showed an increase in fading rate with increasing relative humidity. On polyester film on the other hand the dye fading was not sensitive to humidity, which is under standable since water does not significantly swell polyester.
The action of water in increasing fading was attributed to a
combination of the following factors:
- displacement of adsorbed dye by the water, thereby
interfering with the dye-substrate interactions,
- increased accessibility of adsorbed dye molecules
to the oxygen of the atmosphere caused by the
swelling action of water on the substrate.
Datyner et al., 84 investigated the effect of liquid
water on the quantum yields of fading of reactive dyes on
cellulose. For all the dyes examined the quantum yields
were higher wet than dry. With one dye the poor lightfast
ness in water was found to be due to the participation of 33 hydroxyl radicals produced directly from water in the fading reactions. For another dye the major fading reaction involved the photoejection of electrons which was greatly enhanced in an aqueous environment.
Thus there appears to be a number of ways in which moisture can affect the fading of dyes and it is possible that in any situation a number of these mechanisms could be operating simultaneously.
1.4.7 Temperature
At constant humidity a rise in temperature increases the rate of fading of dyes. 8 8 ' 9 1 ' 9 3 The effect is small but significant with measured activation energies ranging from approximately 4 to 76 kJmol- 1 and is more marked at high rather than low moisture regains. As photochemical reactions are activated by light rather than heat they have virtually no activation energy. Therefore, the temperature dependence of fading must be due to the temperature dependence of "dark" reactions in the fading sequence subsequent to the initial excitation of the dye molecule.
The extent of this temperature effect depends again on the dye-substrate system in question. Giles et al. ,9 1 found that the activation energies of dye fading were dependent more on the nature of the substrate and on the relative humidity than on the nature of the dye molecule. They determined activation energies ranging from approximately
8 kJmol- 1 for anionic dyes on wool to 76 kJmol- 1 for disperse dyes on nylon. From Hedges' results 88 for anionic dyes on 34 wool Giles also calculated activation energies of
approximately 5 to 13 kJmol- 1 • Shah and Srinivasan 93 on the
other hand found that activation energies were determined both by the substrate and the class of dye and gave the
following values:
Apparent Activation
Energies of Fading
(kJmol- 1 )
anionic dyes on gelatin film 13 - 19
reactive dyes on cellophane 31 - 43
disperse reactive dyes on nylon 49 - 55 disperse dyes on nylon 52 - 76
When a material is exposed to light, the temperature of the surface is increased significantly above that of its surroundings. Temperature rises of the order of 35 to 55°c were observed by Nordharnrnar 94 on exposure to sunlight.
However the moisture content of the exposed surface decreases
concurrently and so the rate of fading in practice will be
influenced by the combination of these two effects.
Temperature has also been found to influence photo
chemical reactions in another way, through its effect on
the rigidity of solid substrates. 75 ' 9 5 This effect has been extensively reviewed by Williams and Daly95 where the importance of free volume and the glass transition of polymers is emphasized. The temperature, and whether it is
above or below the polymer's glass transition temperature, will determine the amount of free volume available for the 35 movement of a reactive site. The amount of movement necessary to form the geometry of the excited or transition state and to allow some separation of the products will determine the amount of free volume required for a given photochemical reaction to take place. Typical motions which may be involved include conformational changes of a cyclohexane ring, cis trans isomerisation, rotations of a methyl or phenyl group and diffusion of a small molecule to a large molecule where reaction can take place. Depending on the amount of movement required some reactions will be unaffected by substrate rigidity while others will be considerably affected.
Thus it becomes obvious from the preceding discussion that each dye-fibre system must be considered individually as so many different factors can affect dye fading that it is impossible to generalise to any great extent.
1.5 Scope of the Present Work
The aspect of dye fading studied in this work involves a substrate effect, the aim of the research being to elucidate the anomalously high lightfastness of basic dyes observed on acrylic fibres compared to the fastness on cotton, nylon, wool and silk. The reason for particular interest in this area lies in the very desirable properties of this class of dyestuff.
By definition, basic dyes are those dyes in which the coloured chromophore is positively charged. For this reason 36 they are also known as cationic dyes and in fact this
classification has become used more frequently, especially with the more recently developed dyestuff ranges. The particular advantages of dyes of this type are their very bright colours and their very high extinction coefficients.
A large range of bright colours is available within the basic dye class and less dye is required for a given depth of shade compared with other dye classes.
Their use originally was on wool and tannin-mordanted
cotton but by the 1950's this had been virtually discontinued because of their very poor lightfastness. About this time it was discovered that newly-developed acrylic fibres could be dyed with basic dyes and that, unexpectedly, the dyes displayed very good fastness to light. This led to their reintroduction and the subsequent development of many new basic dyes with improved properties.
Acrylic fibres contain at least 85% polyacrylonitrile
(-CH 2 -CH-) . Usually the remainder consists mainly of a I n CN
copolymer with a bulky side chain such as vinyl acetate or methyl acrylate which opens up the structure to allow penetration of dyes. In addition a further copolymer
containing either acidic or basic groups to provide dye
sites is often incorporated in very low concentrations.
Of most interest for basic dyes are the cationic-dyeable
acrylics which contain acidic dye site groups, typically
as sodium styrene sulphonate. 96 37
Other synthetic fibres including polyester and nylon have also been chemically modified to produce cationic dyeable fibres. However the use of basic dyes on these substrates has been less successful because they display lower fastness to light, and, as alternative dyes are available for these substrates (unlike the acrylics} basic dyes are not often used on them.
Various explanations for the extremely high light fastness of basic dyes on acrylics compared with other substrates have been proposed but very little research has been carried out on the phenomenon. One postulate is that acrylics are less permeable to oxygen and moisture. 61 , 9 7
Schwen and Schmidt 71 found that for the range of basic dyes they studied, the presence of moisture slightly increased the rate of fading on the various substrates tested. In most cases the dyes also faded more rapidly in oxygen than in nitrogen but there were exceptions. However, even with both oxygen and moisture excluded (i.e., in a dry nitrogen environment} the lightfastness of the dyes on the non acrylic fibres, which included cotton, silk and wool, was generally not as high as on the acrylic fibre. In fact, differences in lightfastness of up to more than 6 points on the standard ISO lightfastness scale (which ranges from 1 to 8) were observed. It would appear that permeability to oxygen and moisture probably does play some role, which could be different for each dye.
It has also been postulated that the nature of the bond between the basic dye and the substrate is a reason 38 for their relatively high lightfastness on acrylics.
Wegmann98 found that the lightfastness of basic dyes increased with decreasing basicity of the dye cation which he attributed to the changing character of the dye fibre bond. He postulates that as the basicity of the dye decreases, the dye-fibre interaction becomes less electrostatic in character which in turn means increased interaction between dye and fibre. This means that the absorbed energy is more easily conducted away from the dye and therefore slows down its photodegradation. Evans and Stapleton 8 on the other hand found that while this correlation between basicity and lightfastness held for
Orlon it does not hold for wooLwhich suggests that different fading mechanisms may be operating on the two fibres.
On the basis of Wegmann's explanation Bitzeret al., 25 tried to modify cotton by the introduction of acidic groups so as to produce similar binding conditions between dye and fibre as those in acrylic fibres. Their treatments included carboxylation, cyanoethylation, carboxymethylation and treatments with butane sulfone and acrylic acid. The results obtained were variable, with each treatment giving slight improvements in the lightfastness of some dyes but not others. In all cases the improvements did not bring the lightfastness up to that on acrylic. They attributed the lower lightfastness on cotton to be due to its higher water content and to the dye being present in a different physical state. 39
Porter and Spears 42 also modified cotton, producing sulfoethyl and carboxymethyl ethers to determine the effect of sulphonate and carboxylate dye sites. They found that the triphenylmethane dye, Malachite Green, faded five to ten times slower on the cotton containing -SO 3 sites compared with that containing -COO sites or untreated cotton but that the photodecomposition products on all three were identical. They deduced that the reactions involved were the same in each case and proceeded via the carbinol form of the dye. The difference in rate was postulated to be related to the pK of the dye site a which would determine the concentration of the carbonium ion (Ar3C+) and therefore the concentration of the carbonol base present. Porter and Spears also suggest that differences in dye sites could be a major reason for the lower lightfastness of basic dyes on nylon compared with acrylics as cationic-dyeable acrylics generally have sulphonic acid groups incorporated into the polymer while the nylon utilises carboxyl groups for fixation.
Zollinger ,9 9 .. suggests an alternative reason for the difference between carboxyl or sulphonate sites. The excited dye molecule is postulated to abstract an electron from the carboxylate group to form a carboxyl radical which then decomposes to carbon dioxide and a residual radical within the fibre. Thus a recombination reaction with the dye radical is prevented and it will decompose irreversibly. Sulphonate ions on the other hand have little tendency to form radicals and so it is possible for the excited state of the dye to decay to its ground 40 state without decomposing.
Zollinger99 and his coworkers 96 disagreed with
Wegmann's basicity arguments pointing out that only dyes of similar chemical structure should be compared in such an investigation and that when discussing so-called basicity the measurable quantity pK should be used. When Johnson used Wegmann's lightfastness values and pK values determined himself he found the opposite mechanism to that proposed by
Wegmann with lightfastness increasing as the ionic character of the dye-fibre interaction increases. Subsequently
Johnson found that this relationship was also valid for a series of triphenylmethane dyes whose structure was systematically varied. 96
From the preceding discussion it can be seen that the explanation for the relatively high lightfastness of basic dyes on acrylic fibres is still far from being resolved.
It was the aim of the present work to examine the phenomenon more closely and from different angles to those already employed, to attempt to elucidate the mechanisms and factors involved. Solid substrates only, in the form of fabric and transparent polymer films were used and the factors studied included the effects of oxygen and moisture, the effect of dye sites and other groups present in the polymers and the effect of the rigidity of the substrate on fading. The latter was studied by varying the temperature of fading.
Also it was hoped to identify the excited species involved by the use of appropriate scavenging and quenching agents. 41
CHAPTER 2
QUANTUM YIELDS OF BASIC DYE FADING
2.1 Introduction
The initial task was to quantify the phenomenon to be studied. This was done by determining the quantum yields of fading of basic dyes on an acrylic substrate compared to other polymer substrates. The photochemical quantum yield of dye fading (~) gives a measure of the efficiency of the absorbed light in causing fading and is defined as:
number of dye molecules degraded ~ = number of quanta of light absorbed
Dyed fabrics were used in these quantum yield determinations as this is the dye-substrate form of most interest in practice. The difficulties involved in measuring dye fading on fabrics has meant that most recent research on solid substrates has been carried out on transparent polymer films rather than on fabrics. The technique for following the fading of dye in fabric involves the measurement of the reflectance of the fabric, a property which is not directly related to dye concentration. Recent improvements in instrumentation have greatly increased the speed and accuracy of reflectance measurements.
Preliminary fading trials were carried out on a variety of fabrics and dyes to determine those most suitable for use. Dyes chosen were ones that showed a marked difference in lightfastness on acrylic fibres compared with 42
the other fibres tested. As far as possible dyes were selected that faded "on-tone" i.e., with no accompanying change of hue and that faded within a reasonable time so
that experimental fading times would not be inconveniently long. This latter condition effectively precluded the use of the more recently developed basic dyes.
Cotton, wool, nylon and acrylic fabrics were used in these trial fadings, but as it was found that the dyed acrylic fabric had by far the superior lightfastness it was decided to restrict investigations to two fibre types i.e.,
acrylic and one other. Nylon was chosen as the second
fabric as the dyes on nylon displayed the lowest lightfast ness of the three dyed non-acrylic fabrics examined.
2.2 Fabrics
2.2.1 Acrilan 16
The acrylic polymer used was Acrilan 16, a basic dyeable acrylic manufactured by Monsanto. The exact
chemical structure of Acrilan 16 would not be disclosed, but being an acrylic polymer it would contain at least 85% polyacrylonitrile with the remainder being made up of a number of copolymers whose functions would be to open up
the structure and provide negative dye sites for the basic
dyes. It was obtained as a plain weave fabric of weight
300 ± .6 gm- 2 from Bradmill. The fabric was sufficiently
dense to be self-supporting and virtually opaque. 43
2.2.2 Nylon 6.6
Nylon 6.6 [NH(CH2) 6 NH-CO(CH ) ~CO-] was used in the 2 n form of a knitted fabric. The fabric was of locknit . F2-3/l-0 construction (Bl-O/l-2 ) produced from 44 decitex nylon 6.6 filament yarn (Fibremakers) on a "Hobley", 2 bar, 28 gauge warp knitting machine (J.Hobley & Co. Ltd., England) in these
laboratories. As the fabric construction was a loose one
(fabric weight 107 ± 0.5 gm-2) it was folded in four and
attached to a cardboard backing for irradiation in order to
reduce light penetration.
2.2.3 Basic-Dyeable Nylon
A nylon polymer modified by the introduction of
anionic dye sites to increase its substantivity to basic
dyes was obtained in yarn form from ICI Fibremakers. The yarn was a heavy one (610 ± 40 tex = g.km- 1 ) and samples
for fading were produced by winding the dyed yarn around
a cardboard backing.
2.2.4 Chemical Compositions
In an attempt to elucidate the chemical structure
of the polymers, an elemental analysis was carried out on
them by the Australian Microanalytical Service (C.S.I.R.O.
Division of Applied Organic Chemistry, Melbourne). Table
2.1 gives the results obtained together with the calculated
quantities for the polyacrylonitrile and nylon 6 homopolymers
(all expressed as percentages by weight).
In addition, the presence of carboxyl groups in
Acrilan 16 was tested by a method described by Heinkel; 00 TABLE 2.2 Structures and Lightfastness Data
of the Basic Dyes Studied.
C.I. Basic Blue 4 (oxazine)
lightfastness cotton 2-3
C.I. Basic Red 18 (azo) Cl
02N ;;--{ N=N-o~-N/C2Hs 1 \d. _ \ +/CH3 CH2CH2N'-. CH3 Cl CH3
light fastness acrylic 6-7
C.I. Basic Violet 14 (triphenylmethane)
lightfastness cotton 1, wool 1-2, acrylic 4-5 44
According to Heinkel, acrylic fibres containing carboxyl groups are very pH sensitive and by treatment for 2 minutes
0 at loo c with the dye Maxilon Black Tat pH 2 using H2 S0 4 , acrylic fibres containing carboxyl groups are stained pale blue whereas fibres which do not contain carboxyl groups vary between khaki and dark olive green. When Acrilan 16 was tested in this manner it was stained a dark olive colour indicating that carboxyl groups are not present.
The results from the staining test and the elemental analyses indicate that -so 3 and not -coo sites are present in each polymer.
2.3 Dyes
Dyes were chosen from three different chemical classes that met the required criteria i.e., azo, oxazine and triphenylmethane. Table 2.2 gives their chemical structures together with the limited lightfastness data available in the dye manufacturers' pattern cards and the
Colour Index. The preliminary fading trials with these dyes demonstrated the extremely different fading rates on the nylon and acrylic fabrics, with the difference being clearly visible by eye.
It was subsequently found that the fading rates for the triphenylmethane dye, C.I. Basic Violet 14, on nylon and acrylic fabrics could not be measured since this dye did not fade "on-tone" and the change in hue which accompanied fading was found to interfere with the determination of dye concentration. (The other dyes of the 45
triphenylmethane class that were tested in the preliminary trials also exhibited similar behaviour.) However, in subsequent work involving transparent polymer films this dye could be studied because the determination of concentration involved a different technique using a single wavelength of light rather than a wavelength band.
In order to determine the extinction coefficients of the dyes for concentration determinations dye samples were purified by recrystallisation from 50/50 (v/v) ethanol/water until the absorbance of a constant weight/ volume solution was maximised.
2.4 Dyeing Methods
All dyeings were carried out in beakers with heat supplied by hotplates. Dyestuffs were used in their commercial form (i.e., without purification) as the dyeing operation results in only the pure dye being absorbed and bound by the fibres while any impurities (which are typically salts) either remain in the dyebath or are removed from the fabric on rinsing. A range of four concentrations of each dye on each fabric was prepared. The actual concentrations used depended on the extinction coefficient of the particular dye and the amount of dye that could be adsorbed by the fabric. Very high concentrations of dye had to be avoided as these concentrations often resulted in changes in the hue of the dyeing, probably caused by aggregation of the dye. 46
2.4.1 Acrilan 16
A dyeing recipe typical of those given in dyestuff manufacturers' pattern cards for basic dyes on acrylics was followed:
Dyebath:
0 . 0 5 to 1 • 0 % dye
2.6% Astragal PAN (Bayer)
1% sodium acetate
0.9% acetic acid
10% sodium sulphate
Liquor Ratio 40:1 [=volume of dyebath (mls):
weight fabric (g)]
All percentages are on weight of goods. Astragal PAN is a cationic retarder which is used to control the rate of dyeing and thereby obtain level dyeings. Sodium acetate/ acetic acid are used to buffer the dyebath to pH 4.5-5.
Method: The fabric was entered at 50-60°c and run for a few minutes. The temperature was then raised gradually to
80-85°c over approximately 20 minutes, then to the boil over
40 minutes. Dyeing was then continued for approximately 2 hours at the boil. Finally the fabric was rinsed thoroughly in cold water.
The concentration of dye adsorbed by the fabric was determined by a mass balance technique: The absorbance and volume of the dyebath were measured before dyeing, and that of the dyebath plus rinse water after dyeing. The dye 47
concentration on the fabric thus obtained was expressed as
µmol (dye)/gram (fabric).
2.4.2 Nylon 6.6
Since unmodified nylon has a low substantivity for basic dyes, a very high concentration dyebath had to be used and the concentration range obtainable was fairly limited.
No other additives were present in the dyebath apart from the dye and again a liquor ratio of 40:1 was used. The
fabric was entered at 40-60°c, raised to the boil and then dyed at the boil for approximately 1 hour. This was followed by a thorough rinsing in cold water.
The concentration of dye on the fabric could not be determined in the same way as for the acrylic fabric as only
a very small fraction of the very large amount of dye in the dyebath was adsorbed by the fabric. Instead, the dye was
subsequently extracted from a dyed fabric sample of known weight by refluxing with 50/50 (v/v) pyridine/water. The
absorbance of the extracted dye was then measured and the
concentration of dye on the fabric calculated. Extinction coefficients of purified dye in 50/50 (v/v) pyridine/water were determined for this purpose.
2.4.3 Basic-dyeable Nylon
The acrylic dyeing recipe was found to give
satisfactory results with the modified nylon fabric. The
cationic retarder was necessary to control the rate of
dyeing and so obtain level dyeings. 48
2.5 Determination of Dye Concentration
The reflectance of the dyed fabrics was measured on a Hunterlab D25D2M Colour and Colour Difference Meter. This instrument measures reflectance in terms of the three C.I.E. Tristimulus Values X, Y and z. These X, Y and z readings give the reflectances of the sample at three different wavelength bands.
The diameter of the specimen port of the Hunterlab was reduced from 56mm to 32mm by the use of a specially constructed black annular plate, to enable the small fabric samples (37mm x 40mm exposed area) used in the fading experiments to be measured. The X, Y and Z readings obtained were corrected to account for this reduced area as the instrument is calibrated using the larger port:
a (R - b) R = reading
where R = X, y or z C. I.E. Tristimulus Value
R = x, y or z reading obtained using the reading 32mm port
a = Area correction
= 1.2 for X and Y
= 1.5 for z
b = Zero correction
= 1.4 for x, y and z
These correction factors were obtained empirically. The
X, Y and z values so obtained were then converted to percentage values as follows: Z% R = ITO 6
5
4
3
2
1 X% R = I'ITTf
1 2 3 4 5
Dye Concentration (µmol.g- 1 )
(l-R) 2 FIGURE 2.1 The Kubelka-Munk function 2R X% Y% Z% ( for R , and versus dye = 100 100' 100> concentration for C.I. Basic Red 18 on
Acrilan 16 fabric. 49
Y% = y
X X% = . 9 80 41
Z% z = 1.18103 where 100% reflectance indicates a perfect white reflector, for the light source used.
As stated previously, there is no direct relationship between the reflectance of a dyed fabric and its dye concentration. However the empirical Kubelka-Munk relation 101
[2.1] has been found to give good agreement between the two properties:
(1 - R) 2 (1 - Ro) 2 + kC [2.1] 2R = 2Ro where R = fractional reflectance of dyed fabric X% Y% Z% (in this case, R or = 100' 100 100 Ro = fractional reflectance of undyed fabric (as above)
C = concentration of dye k = the absorption constant for the given dye-substrate system.
X, Y and Z values of duplicate samples of four dye concentrations of each dye-fabric system were measured, the dye concentration determined as described in Section (1 - R) 2 2. 4 and versus dye concentration plotted. 2R A typical set of Kubelka-Munk plots, for C.I. Basic Red 18 on Acrilan 16 and on nylon 6.6 are shown in Figures 2.1 and 4.0 Z% R = 100
3.5
3.0
2.5 (-.j -p:: p:: IN r-1 2.0
1.5
1.0
X% R 0.5 = 100
1 2 3 4 5
Dye Concentration (µmol.g- 1 )
(1-R) 2 FIGURE 2.2 The Kubelka-Munk function 2R X% · Y% Z% (for R = t5o, 100 .and 100) versus dye concentration for C.I. Basic Red 18 on
Nylon 6.6 fabric. 50
2.2 respectively.
The Kubelka-Munk relation was found to be valid for
2 all the dye-fabric systems tested, (1 - R) 2R versus C giving straight line plots with correlation coefficients
{r) greater than 0.9. However, the intercept on the (1 - R) 2 {l-Ro) 2 axis {i.e., when C = 0) was not equal to 2R 2Ro - where Ro corresponds to the reflectance of the "blank dyed" fabric i.e., fabric which has been subjected to the dyeing treatment but without the dye. It was considered that the (1 - R) 2 actual intercept of the plot of 2R versus C was the more appropriate value to use in calculating dye concentrations from reflectance measurements. This was a reasonable approach providing the concentrations being determined were within the range for which the Kubelka-Munk relation had been demonstrated to be valid. Therefore equation [2.1] can be expressed in the following way:
f f. + kc = 1 f - f. and C = 1 k
(1 R)2 X% Y% Z% where f - for R = 2R = 100' 100' 100 f. value of f at C 0 axis, which is a constant 1 = = for a given dye-substrate system
C = concentration of dye k = the absorption constant for a given dye-substrate system
Hence the progress of fading was determined as follows:
drain drain
Coils Coils
to to
Cooling Cooling
II II
Tap Tap
II)..,. II)..,.
System System
Solution Solution
CuS01t CuS01t
1.0% 1.0%
Filter Filter
Sample Sample
Holder Holder
Irradiation Irradiation
Lamp Lamp
Xenon Xenon
Filter Filter
the the
Heat Heat
of of
Electrode Electrode
Negative Negative
= =
d<:)Fan d<:)Fan
Elevation Elevation
Side Side
to to
to to
I I
rP1 rP1
supply supply
power power
supply supply
power power
• •
· ·
Schematic Schematic
Cover Cover
3 3
,. ,.
2. 2.
Motor Motor
FIGURE FIGURE to to 51
Fraction of dye remaining at time t concentration of dye present at time t = concentration of dye present at zero time
f . f. time t 1 = f f. zero time 1
As with transparent media where the maximum
concentration accuracy is obtained by measurement at the wavelength of maximum absorption, so the most sensitive
reflectance region is at the wavelength of minimum reflectance. Consequently the Tristimulus Value used for each dye was the one whose wavelength band was nearest the absorption band of that dye. This gave the most accurate determination of concentration.
2.6 Irradiation Technique
A 1600W Xenon lamp (Wotan XBO 1600W ozone-free
lamp) was used as the radiation source since it has a
spectral output similar to sunlight. Its high output
ensured short fading times, but had the disadvantage of
considerable heat generation, and hence it was found necessary to filter out the heat to prevent thermal
degradation of the acrylic fabrics.
A schematic side elevation of the lamp system is
given in Figure 2.3. The Xenon lamp is located at the
centre of the system. The heat filter, consisting of two
sealed pyrex glass cylinders with the space between the
cylinders filled with 1% CuSO4 solution, surrounds the 52
lamp. This filter absorbs the heat from the lamp without
absorbing the lower wavelength radiation. The CuSO 4 solution is continuously pumped through the glass filter from a water cooled bath.
Although CuSO 4 inhibits organic growth its presence alone was not sufficient to completely eliminate all growth in the heat filter system and so a few drops of m-cresol, a strong fungicide, were also added. The filter was also regularly cleaned by pumping through 6M HNO 3 , followed by
10% NaCl, and finally rinsing it with fresh water before refilling it with freshly prepared CuSO 4 solution.
The samples for fading were clamped in metal frames and placed in vertical slots mounted on a carousel sample carriage positioned outside the heat filter. The carriage rotates slowly (1.2 r.p.m.) around the Xenon lamp to average the radiation received by each sample. The samples were positioned level with the centre of the lamp where the intensity of the incident light was a maximum. The distance of the sample from the lamp, and the area of sample illuminated were chosen so that the variation in the incident light intensity across the sample was less than 5%.
2.6.1 Calibration of Incident Light Intensity
Because of the variation in the radiant output of the Xenon lamp with time, due to changes in the city voltage supply and to aging of the lamp, it was necessary to
continuously monitor the light intensity output during
fading and a light flux integrator was constructed for this
v v
Counter Counter
I I
-
-
I0.12µF I0.12µF
I I
-
-
< <
-1-
16µF 16µF
2.2Mn 2.2Mn
0.12µF 0.12µF
RL-A RL-A
1akn 1akn
l.8Mn l.8Mn
. .
-
Integrator Integrator
50µF 50µF
L L
_ _
+ +
-
-
-
RL-B/2 RL-B/2
-
1
Flux Flux
lOkn lOkn
;J ;J
Light Light
-
21okn 21okn
100n 100n
. .
-
the the
RL-B RL-B
of of
-
-
I I
10,000µF 10,000µF
Diagram Diagram
+ +
+ +
lOOµF lOOµF
Circuit Circuit
2.4 2.4
6x4 6x4
4x0A202 4x0A202
G~ G~ FIGURE FIGURE 53 purpose. The light was detected by a photocell (RCA 935) placed in one of the sample slots on the rotating sample
carriage, and arranged so that the light dose incident on
the sample was measured. The terminals of the photocell were connected to two concentric copper tracks running
around the carriage base. Two copper contacts fixed to the
lamp cover relayed the signal from the photocell to a voltage to frequency converter and from there a cumulative
frequency count was displayed on a frequency meter (a Fluke
1900A Multi-Counter). The circuit diagram for the integrator
is given in Figure 2.4.
The cumulative light "count" obtained from the light
flux integrator was calibrated by chemical actinometry using
an acid solution of potassium ferrioxalate [K 3Fe(C2O~)3]. 102,103 The potassium ferrioxalate is reduced during
irradiation to produce Fe2+ with a known quantum efficiency.
The concentration of Fe 2+ present after exposure was
determined by adding 1,10-phenanthroline to form an intensely
red coloured complex whose concentration was determined
spectrophotometrically.
The number of Fe 2+ ions produced during irradiation
(NFe2+) was calculated as follows:
V1 . V3 2 NFe2+ = NA X X [Fe +] V2
where NA = Avogadro's Number V1 = the volume of actinometer solution irradiated (20mls) 54
V2 = the aliquot volume taken for analysis (10 mls)
V3 = the volume of the flask used in the analysis (25 mls) and [Fe 2 +] = As 1 o E: • .R, where As 1 o = the measured absorbance of the F e z+ - phenanthroline solution at the wavelength
of its maximum absorbance, 510nm.
£ = molar extinction coefficient of the
Fe 2 +-phenanthroline at 510nm •
.R, = optical path length of spectrophotometer cell used for the measurement of the
absorbance.
The value obtained for As 10 was 8.82 x 10- 3 count- 1 cm- 2 for a 1 cm cell. The value of E: = 1.06 x 10 4 .R, mol- 1 cm- 1 obtained was in good agreement with literature values
[(1.10 - 1.15) x 10 4 .R, mol- 1 cm- 1 ]. Using this data the value for NFe 2 + was calculated to be:
= 2.76 X
Now, number of quanta =
where ~a = quantum yield of the actinometer.
As ~a varies with wavelength it was necessary to determine 55
the spectral distribution of the radiation before the number of quanta could be calculated.
Light from the lamp passed through the heat filter and onto the entry slit of a Farrand Grating Monochromator
Cat. No. 117180 (Farrand Optical Co. Inc. New York) which was coupled with an RCA 1P28 Photomultiplier. The plot of intensity versus wavelength obtained was corrected for the spectral response of the photomultiplier and the percentage transmission of the monochromator grating. This information was obtained from the manufacturer's catalogues where the data was given at 20nm intervals, and interpolated to obtain values every 5nm.
The relative intensity distribution thus obtained was then calibrated in units of quanta cm- 2 count- 1 using the actinometry result obtained previously
= 2.76 X in the following manner:
Now =
where I>.. = Intensity of light at wavelength >.. (quanta cm - 2 count -i)
IR Relative intensity of light at wavelength >.. >.. = (in arbitrary units)
k conversion factor (constant) and = I ...j I 0 12 r-1 X
Q) r-1 ~ 10 rtl Cl) ...- ~ I 0 .µ .µ § 8 0 ~ u ~ ·r-1 N u I ~ I:: H u 6 ~ rtl 0 .µ •r-1 .µ ~ rtl ::, ·r-1 01 ,0 rtl 4 p::;
4-1 0
.µ:>"t ·r-1 2 !/l ~ Q) .µ ~ H
350 400 450 500 550 600 650
Wavelength (nm)
FIGURE 2.5 Intensity Distribution of Radiation from the
Xenon Lamp Incident on Sample Surface. 56
where ~aA = quantum yield of the actinometer at
wavelength A.
The upper and lower limits involved in this summation are
= 580nm, above which the quantum efficiency of the
actinometer is zero, and Al = 320nm, below which the lamp output intensity is zero.
Therefore
A=580 A=580
= ~ = ~ IR k NFe2+ l aA IA I aA . A . A=320 A=320
NFe2+ and hence k = A=580 IR I ~aA A A=320
The value of k could thus be calculated and then used to convert the relative intensity distribution to an absolute one in units of quanta cm-2 count- 1 . The summation was performed over 5nm intervals which gave adequate accuracy
(± 2%). The values for ~aA required were obtained by graphical interpolation of the values determined by
Hatchard and Parker.1021103 The resulting intensity distribution output from the lamp incident on the samples is given in Figure 2.5. It can be seen that the intensity begins to fall above approximately 590nm. This is due to the absorption of light by the copper sulphate filter. 2.0
- i::: ·r-1°' i::: •r-1 rtl s Q) p:::
Q) ::>i 1.9 Cl r .µ i::: Q) .99 u ~ .99 Q) -Ill tJ'I .99 0 r-1
• 9 7
1.8
.99 • 99
1 2 3 4
FIGURE 2.9 Semi-log Plot of the Fading Curves presented in Figure 2.8, for C.I. Basic Red 18 in:
(i) Nylon 6.6 Fabric for c 0 =: (ii) Basic-dyeable Nylon .& 1 • 18 µmo 1 • g - 1 Yarn for Co=:
1:::,. 2. 59 µmol. g - 1 • 0 • 9 2 µmo 1 • g - 1 • 4 • 0 0 µmo 1 • g - 1 D 1.91 µmol.g- 1 o 5 • 8 8 µmo 1 • g - 1 100
90
s::O'l ·r-1s:: ·r-1 "'s ~ 80 0) ~ 0 .µ s:: 0) t) 1-1 0) PI
70
60
0 1 2 3 4
Total Incident Quanta x 10-17 (cm2 )
FIGURE 2.8 Fading Curves for C.I. Basic Red 18 in: (i} Nylon 6.6 Fabric for Co=: (ii} Basic-dyeable Nylon • 1.18 µmol_.g-~ Yarn for Co=: b. 2 .59 µmol .g- 1 • 0 • 9 2 µmo 1 • g - 1 • 4 . 0 0 µmo 1 • g - 1 D 1 • 91 µmol • g - 1 o 5. 8 8 µmol. g - 1 96 s::bl ·r-1s:: ·r-1 Id El Q) ~ Q) 94 >i 0 .µ s:: Q) 0 1-1 Q) Pl 92
0 1 2 3
Total Incident Quanta x 10-17 (cm-2 )
FIGURE 2.7 Fading Curves for C.I. Basic Blue 4 in Acrilan 16 Fabric for Co=: h. 0 • 4 9 µrno 1 . g - 1 o O. 9 8 µrnol. g - 1 . -1 .A 2 .24- µrnol.g • 3.86 µmol.g-1 80 tJl i:: ·r-1 i:: ·r-1 rd I::: Q) ~ 70 Q) >i Cl .µ i:: Q) u 1-1 Q) Al 60
0.5 1.0 1.5 2.0 2.5
FIGURE 2.6 Fading Curves for C.I. Basic Blue 4 in:
{i) Nylon 6.6 Fabric, for Co=: {ii) Basic-dyeable Nylon ... 0.51 µmol.g- 1 Yarn, for Co=: D. 0. 80 µmol.g -1. • 0 • 5 9 µmol. g - 1 • 1.11 µmol.g- 1 D 0 • 9 3 µmol. g - 1 0 1.29 µmol.g- 1 57
2.7 Determination of Initial Quantum Yields of Basic
Dye Fading
2.7.1 Measurement of Fading Rates
Fabric samples were irradiated on the rotating sample carriage for an appropriate light exposure dose, then removed from their holders and their reflectance measured immediately. The samples were then returned to the lamp for a further period of irradiation, and this procedure repeated until at least five readings were obtained for each sample. Duplicates of each dyed fabric were faded and gave extremely good agreement, with a coefficient of variation of the percent dye remaining of less than 1.5% for the fading of acrylic fabrics and better than 3% for nylon fabrics.
The concentration of dye remaining relative to the initial concentration was determined after each period of irradiation using the Kubelka-Munk relation as described in Section 2.5. The results were plotted as percent dye remaining versus the exposure dose (total incident quanta) and these plots are presented in Figures 2.6 to 2.8.
It was found that plots of the log of percent dye remaining against total incident quanta gave straight lines, with correlation coefficients from linear regression, r ~ 0.96, indicating that the fadings follow apparent first order kinetics. Figure 2.9 illustrates this first order test for C.I. Basic Red 18 on nylon 6.6 and basic-dyeable nylon. 58
The slope of this line gives the apparent first
order rate constant, [ k1, cm 2 (total incident quanta)- 1]:
k 1 = 2.303 x slope
k1 was then used to calculate the initial rate of dye
degradation:
dC dio = where Co= initial dye concentration
To obtain the initial dye concentration in terms of the number of dye molecules per cm 2 of fabric surface irradiated
required an estimation of the depth of fabric involved since
the incident light does not penetrate right through the
fabric. It was decided that an appropriate fabric thickness would be to that depth of fabric by which 95% of the incident
light is absorbed. This led to the following expression:
Rate of dye degradation,
[molecules. (total incident quanta)- 1 ]
dN = dio
where N = number of dye molecules degraded
Io = total incident quanta of incident A95% = Absorbance at which 95% light is absorbed TABLE 2.3 Apparent First Order Rate Constants of Fading and
Initial Rates of Dye Degradation Determined.
(Values calculated in terms of total incident
quanta.) dN k 1 dI 0
C 0 * [cm2 (total (dye molecules Dye/Fibre (µmol.g- 1 ) incident degraded per quanta)- 1 ] total incident quanta)
C.I.Basic Blue 4 in: Acrilan 16 0.49 3.37xl0-23 2.72xl0- 7 0.98 2.40xl0-23 l.93xl0- 7 2.24 3.35xl0-23 1. 89xl0- 7 3.86 2.33xl0-23 l.88xl0- 7
Nylon 6.6 0.51 2.40xl0-21 l.93xl0- 5 0. 80 2.0lxl0-21 l.62xl0- 5 1.11 l.98xl0-21 l.59xl0- 5 1.29 l.69xl0-21 1. 36xl0- 5
Basic-dyeable 0.59 3 .62 xl0- 2 1 2.9lxl0- 5 Nylon 0. 9 3 3 .2 8 xl0- 2 1 2.65xl0- 5
C.I.Basic Red 18 in: Nylon 6.6 1.18 7.78xl0-23 2.29xl0- 6 2.59 6. 39xl0- 2 3 l.89xl0- 6 4.00 5.50xl0-23 l.62xl0- 6 4.88 5.90xl0-23 l.74xl0- 6
Basic-dyeable 0.92 l.49xl0-22 4.29xl0- 6 Nylon 1. 91 l.33xl0-22 3.92xl0- 6
* Dye concentrations were found to vary by± 1% on Acrilan 16 and basic-dyeable nylon and by± 5% on
nylon 6.6. 59
-A i.e., Fraction of light absorbed = 1 - 10 95% = 0.95 and hence = 1. 301
NA = Avogadro's Number
k1 = apparent first order rate constant
of fading [cm2 .(total incident
quanta) -i]
E = molar extinction coefficient of
dN The values obtained for k and for each of the dye/ 1 dio fibre systems are presented in Table 2.3.
2.7.2 Determination of Quanta Absorbed by Dye
The fraction of the incident quanta that is absorbed by the dye initially (F) was calculated as follows:
>-2 1 F = = Io l
where IA = total quanta absorbed by dye initially Io = total incident quanta
>. = wavelength. The summation was carried out
at 5nm intervals over the wavelength range
of the radiation incident on the sample
i.e., >. 1 = 320nm and >-2 = 650nm
= Intensity of the incident light at wavelength >. I >. RA = Initial fractional reflectance of the dyed fabric at wavelength >. TABLE 2. 4 Fraction of the Incident Quanta that is Absorbed
by the Dyes Initially
Dye/Fibre Co F (µmol. g- 1 ) (quanta absorbed per total incident quanta)
C.I.Basic Blue 4 in:
Acrilan 16 0.49 0.538
0.98 0.616
2.24 0.713
3.86 0.784
Nylon 6.6 0.51 0.510
0. 80 0.564
1.11 0.631
1.29 0.665
Basic-dyeable 0.59 0.467
Nylon 0. 9 3 0.536
C.I.Basic Red 18 in:
Nylon 6.6 1.18 0.650
2.59 0.744
4.00 0.799
5.88 0.846
Basic-dyeable 0.92 0.731
Nylon 1. 91 0.805 60
and 1 - RA = Fraction of the incident light of
wavelength A that is absorbed
Since subsequent studies of polymer films showed that the polymers being studied have a very small absorbance above
320nm, it was assumed that all the light being absorbed was absorbed by the dye.
The required reflectances were determined by measuring the reflectance spectrum of each dyed fabric on a Beckman DK2 Recording Spectrophotometer prior to fading.
The samples were measured on a white background to ensure that all the incident light was either reflected or absorbed (none transmitted). The white tile used as the reference was calibrated against freshly smoked magnesium oxide which was used as the primary reference standard.
The values obtained for the quanta absorbed by the dye initially, as a fraction of the total incident quanta, in each of the dye/fibre systems are given in Table 2.4.
2.7.3 Calculation of Initial Quantum Yields of Basic
Dye Fading
As the calculation of the initial rate of dye degradation was based on 95% of the absorbed light, so too the value of the fraction of the incident quanta that is
absorbed by the dye is 95% of the total calculated. The
use of initial rates minimises the interference from any
absorbing fading products and means that the concentration
of dye present can be accurately determined. TABLE 2.5 Initial Quantum Yields of Basic Dye
Fading
Dye/Fibre Co (µmol.g -i)
C.I.Basic Blue 4 in:
Acrilan 16 0.49 5.3 X 10- 7
0. 9 8 3.3 X 10- 7
2.24 2.8 X 10- 7
3.86 2.5 X 10- 7
Nylon 6.6 0.51 4.0 X 10- 5
0. 80 3.0 X 10- 5
1.11 2.7 X 10-5
1.29 2.2 X 10- 5
Basic-dyeable 0.59 6.6 X 10-5
Nylon 0.93 5.2 X 10- 5
C. I .Basic Red 18 in:
Nylon 6.6 1.18 3.7 X 10-6
2.59 2.7 X 10-6
4.00 2.1 X 10-6
5.88 2.2 X 10-6
Basic-dyeable 0.92 6.3 X 10-6
Nylon 1.91 5.1 X 10- 6 61
The quantum yields obtained for each dye/fibre combination are presented in Table 2.5. The quantum yields are estimated to be accurate to± 10%.
The results clearly illustrate the trend being examined. There is a hundred-fold lower quantum yield on
Acrilan fabric compared to that on the nylons for C.I. Basic
Blue 4, while for C.I. Basic Red 18 the fading rate on
Acrilan was too slow to be detected on the time scale of these experiments. No fading was detected in the latter case after an exposure dose of 3.6 x 10 20 total incident quanta.cm- 2 which corresponds to approximately seven hours exposure, while the same dye on the polyamide substrates displayed significant fading. Also, it can be seen that both dyes display significantly higher quantum yields on basic-dyeable nylon than on the conventional nylon 6.6, but within the same order of magnitude.
The values of the quantum yields given in Table 2.5 are typical of quantum yields of dye fading quoted in the literature. For example,Irick and Boyd54 found values ranging from less than 5 x 10-6 to 1.1 x 10- 3 for various disperse dyes in 90/10 (v/v) methanol/isopropyl alcohol.
Shah andSrinivasan5 3 found a quantum yield range of 10- 4 to
10- 6 for disperse dyes on various polymer films. They attributed the low values to a combination of:
dye aggregation
low rates of diffusion of gases to and from the
dye molecules 6
5
3
• 2
0 1 2 3 4 5 6
Initial Dye Concentration (µrnol.g- 1 )
FIGURE 2.11 The Initial Quantum Yields of Fading of C.I. Basic Red 18 versus Initial Dye Concentration in: • Basic-dyeable Nylon Yarn • Nylon 6. 6 Fabric ( a} 6
5
1/) 0 4 r-1
3
2
0.4 0.6 0.8 1.0 1.2
Initial Dye Concentration (µmol.g- 1 )
(b} 5
4
r-- 0 r-1 X ,e, 3
2 0 1 2 3 4
Initial Dye Concentration (µmol.g- 1 )
FIGURE 2.10 The Initial Quantum Yields of Fading of C.I. Basic Blue 4 versus Initial Dye Concentration in: ( a) • Basic-dyeable Nylon Yarn • Nylon 6 .6 Fabric (b) .a. Acrilan 16 Fabric 62
- energy transfer from the dye through the dye
substrate bonds
internal energy conversion
- the "cage" effect of the surrounding polymer
substrate
Datyner et al., 84 found quantum yields for reactive azo dyes
on cellulose of (1 to 3) x 10- 6 dry and (1.3 to 5.4) x 10-6 wet.
Figures 2.10 and 2.11 show plots of the quantum
yields against dye concentration. It can be seen that there
is a definite concentration dependence of quantum yield, with the quantum yield decreasing with increasing dye
concentration. This phenomenon has frequently been observed,
and the usual explanation is that increasing dye aggregation
accompanies increasing concentration and that this causes
the slower fading (see Section 1.4.2). However, it is
unlikely that dye aggregation explains the trends observed here as dye concentrations are quite low (in the range 0.02
to 0.25% dyeing) which means that there would probably be
sufficient dye sites available for each dye molecule to be
individually adsorbed onto a dye site thus limiting
aggregation. On the other hand, since dyeing would only take
place in the amorphous regions of the polymers it is possible
that the dye molecules may be close enough to allow self
quenching to occur by a resonance energy transfer mechanism
(see Section 1.4.2) and the probability of this occurring
would increase with increasing concentration of dye. 63
In conclusion, the initial quantum yields of basic dye fading determined demonstrate a substantial substrate effect on the fading of basic dyes. A difference of at least two orders of magnitude was observed between the quantum yields on an acrylic polymer and on the non-acrylic polymer studied, with a concentration effect also being observed. It would appear therefore that there exists some major physical or chemical difference between the two types of substrate that gives rise to such large differences in the lightfastness of basic dyes present in them. The remainder of this thesis therefore deals with the identifi cation of the physical and/or chemical factors that
influence basic dye lightfastness. 64
CHAPTER 3
THE EFFECT OF THE PHYSICAL STATE OF
THE SUBSTRATE ON BASIC DYE FADING
3.1 Introduction
The influence of the physical state of the polymer
substrate on the fading of the three basic dyes being
studied was investigated by studying the rate of fading of
dyed polymer films over a range of temperatures. Film
rather than fabric substrates were examined as the size of
a fabric sample necessary for a satisfactory measurement of
its reflectance could not be accommodated in the experimental
system required for this work. The polyamide used here was nylon 6, general formula [NH-(CH 2 ) 5 -CO-] , as nylon 6 .6 was n not available in film form. The dyes on nylon 6 film display
a relatively low lightfastness similar to that on nylon 6.6
fabric.
Varying the temperature of the polymer alters its
rigidity by changing the internal motion of the polymer
chains. A corresponding change in the "free volume" of the
polymer also occurs. In particular, the rate at which these properties change with temperature will increase at the
glass transition temperature of the polymer system i.e.,
the temperature at which the polymer changes from a "glassy"
to a viscoelastic state. Consequently it was of particular
interest to examine the fading at temperatures both above
and below the glass transition temperature of each polymer. 65
The physical state of the substrate also affects its permeability to vapours. Accordingly the fading of basic dyes in various substrates in different atmospheres containing oxygen and moisture was examined.
3.2 Polymer Film Preparation
Transparent polymer films made from Acrilan 16, nylon 6 and poly(vinyl alcohol) were used in this section of the work. Unlike dye fading on fabric, the dye fading rates of very small film samples can be determined since film size is only limited by the slit size of the spectro photometer used for the measurement of its absorbance (in this case approximately 1 x 10mm). The small sample size used also ensured that the variation in its temperature during irradiation could be kept to a minimum.
The initial concentration of the dyes present in the films (C 0 ,µmol.g- 1 ) was calculated from the initial absorbance of the dye at the wavelength of its maximum absorbance (Ao), the extinction coefficient of the dye at this wavelength (E, cm 2mol- 1 ), the thickness of the film
(£, cm) and the density of the polymer (p, gml- 1 ):
Co = X
The film absorbance was measured in a number of places using a Perkin-Elmer Coleman 55 single beam spectrophotometer which was found to be accurate to ±.002A, and the values obtained averaged. To determine the 66
absorbance due to the dye, the absorbance due to the film at the wavelength in question was subtracted from the total absorbance measured.
The extinction coefficients used were those determined previously in aqueous solution (Section 2.3).
This approach is justified as the absorbance spectra of the dyes in the polymer substrates are not significantly different from their spectra in aqueous solution. The concentrations of dye in the films were adjusted to give an initial maximum absorbance of approximately 1.
Film thicknesses were measured on a Minicom High
Speed Electric Micrometer.
3.2.1 Nylon 6
Extruded undrawn nylon 6 film, nominally 30µ thick and free of delustrant and plasticiser, was obtained from the Nihon Rayon Co. Ltd., of Japan. The film was dyed at the boil in a dyebath containing only dye, with no additives.
3.2.2 Poly(vinyl alcohol)
Poly(vinyl alcohol), (PVA), repeat unit (CH2-CH-) I n OH was selected as being a suitable solid model substrate. It is photochemically stable, transparent above 250nm and is easily prepared into films by casting from aqueous solution.
Consequently water soluble additives, such as dyes, can be incorporated into the substrate on casting, thus ensuring an homogeneous distribution of additive throughout the film. 67
A 4% (w/w) solution of PVA (Matheson, Coleman and
Bell, 99% hydrolysed) containing the appropriate amount of
dye was cast by pipette onto glass microscope slides placed
on specially levelled preparation tables and allowed to dry.
4 mls of a 4% solution cast onto a 37.5 x 50mm slide gave a
film of thickness 55 ± 10µ. The edges of the film, which were thicker than the central portion, were discarded.
3.2.3 Acrilan 16
It was not possible to obtain Acrilan 16 (or any other basic-dyeable acrylic) in film form. Consequently
films had to be prepared by dissolving the Acrilan 16 fibre
in a suitable solvent and casting the solution to form a
film. In fact, a great many difficulties were encountered
in producing transparent acrylic films. The following preparation technique was found to give the most satisfactory
results but unfortunately, not consistently:
1) Acrilan 16 fibre was dissolved in freshly distilled
dimethylformamide to produce a 4% (w/w) solution.
2) This solution was centrifuged using a Sorvall Superspeed
RC2-B Automatic Refrigerated Centrifuge to remove the
titanium dioxide delustrant, and the required amount
of dye powder added. (Attempts to dye clear film
proved unsatisfactory as the films turned opaque during
the dyeing process.)
3) 4 mls of the solution were cast onto a glass microscope
slide (50 x 75mm) placed in a levelled vacuum oven
(Virtis Unitrap Freeze-Dryer). 0 4) The oven was heated to 60 C under a vacuum of 68
approximately 1mm Hg for one hour, in order to evaporate
the solvent.
Films of approximately 65µ thickness were obtained, from which the thicker edge portion was discarded.
Difficulties persisted with producing clear uniform acrylic films until finally all the films being produced were unusable due to uneven ripples over their entire surface. The uneven thickness of the film resulted in a variable absorbance across the film as well as causing reflections of incident light at the surface. Two factors were considered to contribute to the production of these unsuitable films, viz.,
1) the adhesion of the film to the glass slide, and
2) the rate of removal of the solvent.
A skin was observed to form over the surface of the film as the dimethylformamide evaporated and this could have contributed to the problem.
Numerous variations in method were tried in an attempt to overcome these factors and included variations in the following conditions:
1) the temperature of drying, 2) the temperature program - both constant and variable
temperatures were tried,
3) the level of the vacuum applied, 69
4) the method of cleansing the slides,
5) the age of the acrylic solution used,
6) the amount of oxygen present in the acrylic solution,
7) the type of surface on which the films were cast, e.g.,
glass, stainless steel, silicone oil.
However, none of these measures were found to solve the problem and eventually the attempt to produce satisfactory films had to be abandoned. Fortunately sufficient clear films or portions of films had been produced to enable the temperature work to proceed, although insufficient films dyed with C.I. Basic Violet 14 meant that the effect of different atmospheres and concentrations could not be examined for this dye on Acrilan 16.
3. 3 Measurement of Glass Transition Temperatures
At the glass transition temperature, changes occur in the temperature dependence of various properties of the polymer that are influenced by its rigidity. These properties include its specific volume, specific heat, refractive index, elastic modulus and permeability.
Therefore, by measuring one of these properties while continuously varying the temperature an indication of the glass transition temperature can be obtained. Unfortunately there are numerous difficulties associated with such measurements:
The changes occurring at T may be too small to detect. 1) g This can be caused by a high degree of crystallinity since the physical changes are essentially associated with the 70 non-crystalline regions. Alternatively, the particular physical property being monitored may merely show only a
small change at T . g 2) The experimental conditions under which the determination
is carried out can influence the T observed including both g the temperature at which it occurs and the extent of the
associated property changes. Critical factors include the
rate at which the temperature is varied, the size of the
sample and its previous thermal history. This means, for example, that the reproducibility of a T determination g cannot necessarily be determined by repeating the
determination on the same sample, especially if it has been heated beyond its melting point during the first measurement.
3) Foreign substances present in the polymer may act as plasticisers and therefore depress T • Such substances g include various agents which may have been added to the polymer to give it specific properties, and water vapour
from the atmosphere. The latter means that the atmospheric humidity conditions during measurement can affect the Tg
found.
3.3.1 Experimental Technique
Differential thermal analysis (DTA), which is based
on the change in the specific heat of a substance with
temperature, was used here to determine the glass transition
temperatures of the polymers. This technique consists of measuring the temperature difference that occurs between
the substance under investigation and a thermally inert
substance when both are subjected to identical heating at a
The detection of T relies carefully controlled slow rate. g Exo Crystallisation { rOxidation
~ Glass b.T Transition
Decomposition Melting
Endo
Temperature (T)
FIGURE 3.1 Schematic DTA Curve of a typical polymer. 104 71
on the fact that a glass transition involves an endothermic
phase change and will therefore be seen as a change in the
baseline of a recording of 6T versus T. A schematic DTA
curve of a typical polymer, including a glass transition is
given in Figure 3.1. It should be noted that first order
changes such as melting and crystallisation give rise to
quite sharp, distinct peaks, whereas the second order glass
transition is not nearly as easy to distinguish as it
involves only a shift in the baseline which, in addition, may not be a large one.
Two different instruments were used which employed
slightly different techniques. The first was a du Pont 900
Thermal Analyzer using a conventional DTA cell, and the
second, a later and a more sophisticated and sensitive
instrument, was a du Pont 990 Thermal Analyzer which
employed a differential scanning calorimetry (DSC) cell.
The essential relevant difference between the two was in
the sample mounting and temperature measurement techniques.
In the former instrument, the samples are packed into glass
tubes, a thermocouple inserted and both mounted in a heating
block. In the latter, samples are pressed into small
aluminium discs and sealed to form a pan. These pans were
then placed on top of a constantan disc which acts as one
element of the temperature measuring thermoelectric functions
as well as the primary means of heat transfer to the sample
and reference positions.
The most desirable sample form to use was the same
as that to be used in the fading experiments i.e., polymer 72
in film form. Using the pan mounting technique this could
easily be achieved by using cut snippets of polymer film.
However, with the glass tube mounting method close contact between the sample and the thermocouple was more difficult
to achieve and other types of samples were tried, including polymer in fibre form and polymer powder obtained by
grinding film or fibre at liquid nitrogen temperatures in
a Grindex ball mill (Research and Industrial Instruments
Co. England). Both types of sample can be packed more
tightly into the sample tubes to obtain better contact and
therefore improved sensitivity. However, care does need to be taken in the interpretation of results from ground
samples as sufficient heat can be generated in the grinding process to change the sample history. 105
All samples were dried before measurement in dry nitrogen. Initially undyed samples were examined with the
intention of subsequently comparing the T's of dyed and g undyed polymers in order to determine any plasticising action
that the dyes may have on the polymers.
3.3.2 Results
Acrilan 16, nylon 6 and poly(vinyl alcohol) in
undyed film and fibre form were examined on both instruments
using a number of different instrument conditions. Changes
in the baseline indicating possible second-order phase
changes were observed in some samples e.g., for Acrilan 16
there was some evidence of a transition around 80 0 C, and
for PVA at approximately 20 0 C and 50 0 to 60 0 C. However, the
transitions could not be unequivocally distinguished from TABLE 3 .2 The Variation of the Glass Transition
Temperature of Nylon 6 with Water Content. 108
RH (%) Water Content (%W/W) T (±1°C) g
dried over silica gel 0. 35 94
12 1.17 77
33 1.99 56
44 2.70 45
55 3.47 43
66 4.45 40
86 6.61 23
97 10. 3 -6 TABLE 3.1 Analysis of Key Operational Parameters in
Differential Thermal Analysis 105
Sample Size:
Large: - Useful for detecting low level transitions
- Curve peaks are broad
- Low resolution and temperature accuracy
- Requires slow heating rate
Small: - Good resolution of curve peaks
- Peaks are sharp
- Permits fast heating rate
Heating Rate:
Fast: - Increases sensitivity
- Decreases resolution
- Decreases temperature accuracy
Maximum Maximum
Resolution Sensitivity
Sample Size Small Large
Heating Rate Slow Fast
Surface/Volume of Sample Large Small 1
~T
0 50 100
Temperature (°C)
FIGURE 3.2 Experimental DTA Curves obtained for: 1 and 2: Poly(vinyl alcohol) on the du Pont 900 Thermal Analyzer. 1 Ground film sample 2 Commercial Powder sample
3 and 4: Acrilan 16 on the du Pont 990 Thermal Analyzer - both cut film samples. 73 the variations due to instrument noise in the recordings and further, they were not sufficiently reproducible.
Figure 3.2 shows two of the DTA curves obtained for two samples of Acrilan 16 film on the du Pont 990 Thermal
Analyzer and for PVA on the du Pont 900 Thermal Analyzer, which are typical of the curves obtained. The difficulties with instrument noise and lack of reproducibility are readily apparent.
This lack of success could be due to any of the difficulties mentioned earlier. It is possible that some of the polymers were too crystalline for sufficiently large
T effects to be exhibited. Alternatively the various g experimental conditions employed may not have been optimum for the detection of T e.g., the correct combination of g sample size and heating rate in particular may not have been found. An idea of the compromises faced experimentally can be obtained from the analysis of key operational parameters given in Table 3.1.
Since this research did not yield the required glass transition temperatures, literature values were used instead to determine the approximate temperature ranges required so that fading measurements covered temperatures both above and below the T of the polymers. The following values were g representative of those reported:
Polyacrylonitrile 87° to 95°c, and 140°c 1 0 6
Poly(vinyl alcohol) asoc i o 6 , 71 oc i o 7
Nylon 6 70°c (lowered by moisture)1 0 6 -6° to 94°c depending on water content (see Table 3.2) Lamps
Unit
BE
Tube
FL
Sample
at
E •
Ice-water
G.
Reference
Film
Control
Vacuum
Heater
Films
~
Dyed
Fading
Atmospheres.
Thermocouple
r--~------4Voltmeter
for
Chamber
Irradiation
used
r------1
a------+-1--+--
Different
System
under
Heater
and
l
I
I
I
I
I
l
I
I
for
ice
room)
used
Experimental
nitrogen
or
Exchanger
the
below
..__._~..__,
temperatures
Temperatures
t~I I
(only &.------
r-
Heat
of
liquid
ethanol/dry
I
I
I
I
I I
I
I
I
_J
used
Different
Diagram
moist
lf//l'A
____
l
I
(only
,----
for
Humidifier
atmospheres)
Schematic
Flow
Meter
Water
Scinter
3
3.
Gas
FIGURE 74
The discrepancies between the literature values reported
underline again the difficulties and uncertainties associated with the measurement of T and the number of factors which g can affect the measured T of a polymer. g
Based on these values the range of temperatures
selected for use were:
Acrilan 16 o0 c to 140°c Poly(vinyl alcohol) -4o 0 c to 95°c Nylon 6 -40°c to 90°c
3.4 Irradiation Technique
3.4.1 Method of Mounting and Heating the Film Sample
The equipment used was part of the Varian Intracavity
Variable Temperature Apparatus as used in a V4531 Reflection
Cavity of an electron spin resonance spectrometer. A
schematic diagram of the experimental set-up is given in
Figure 3. 3.
The film sample to be faded was placed around the
inside of a quartz vacuum tube which was positioned in the
centre of the irradiation chamber. This tube was connected
via a heater and a heat exchanger to a gas bottle, and the
various temperatures were obtained by heating or cooling
the gas which then passed over the sample. The heater
temperature was controlled by means of a Varian EPR Heater
Control Unit. This was used in conjunction with liquid
nitrogen or an ethanol-dry ice mixture in the heat exchanger 75
to obtain temperatures below room temperature, and used on
its own to achieve temperatures above room temperature. The
temperatures obtained were measured by means of a copper
constantan thermocouple which was placed in the tube near
the top of the film sample and connected to a millivoltmeter
and an ice-water reference. The Hewlett-Packard 34703A
DVC/DCA/n meter used was highly accurate and was found to
give readings which corresponded to a temperature accuracy
of ±½0 c. The experimental set-up used was found to give a maximum variation in temperature over the sample of ±5°c.
Both dry nitrogen and dry oxygen gases were used
and their flow rate, controlled by a Fischer and Porter
Variable Area Flow Meter (Model lOA 3135M.35R2110), was set
at 2. 5 .Q, /min. To obtain temperatures below room temperature
for oxygen, an ethanol-dry ice slurry rather than liquid
nitrogen had to be used in the heat exchanger as liquid
nitrogen liquefied the oxygen. Water vapour was introduced
by bubbling the gas through liquid water. By measuring the
amount of water evaporated by a known volume of gas, as
calculated from the flow rate for a given time, it was
found that the gas became completely saturated with moisture.
The moist gases were only used at room temperature as it was
considered that sufficient information on the effect of
moisture could be obtained from this one temperature,
especially since it is the temperature of most relevance in
practice. 10
...- 8 I C) Q) Ul
<'I I 6 6 Ill .µ s:: Ill ~ 01
~... I 4 0 r-1 X :>-t .µ ·r-1 Ul s:: 2 Q) .µ s:: H
0
300 350 400 450 500
Wavelength (nm)
FIGURE 3. 4 Emission Spectrum of Radiation from sixteen
G.E. FL8E lamps incident on sample. 76
3.4.2 Method of Irradiation
An Oliphant Irradiation Chamber, Model PCR-128W which can house up to sixteen 8 watt fluorescent tubes was used as the irradiation chamber. The chamber is cylindrical with 12 inch long lamps attached vertically at equal spacings
around a reflecting wall. The vacuum tube containing the
film sample to be irradiated was mounted vertically in the
centre of the chamber, with the sample situated approximately halfway down the length of the lamps to ensure uniform
irradiation of the sample.
The lamps used were General Electric FL8E fluorescent
lamps. Their spectral distribution was determined in the
same manner as for the Xenon lamp system, using the same
actinometry method. The emission spectrum of the lamps thus
obtained is presented in Figure 3.4. It can be seen that these lamps have a much higher uv component than the Xenon
lamp system, with virtually all their emission below 450nm.
This contrasts significantly with the Xenon lamp system which has the majority of its output above 400nm, with quite
a large portion above 500nm. Obviously results obtained
from the two different lamp systems could not be directly
compared with each other unless an "action spectrum" of the particular dye's fading was measured to determine the
relative efficiency of each wavelength of light in producing
fading. However such comparisons were not necessary in this work since its purpose was to compare the effect of different
substrates on dye fading for a particular illuminant.
Despite the low power output of the fluorescent 77
lamps compared with the Xenon lamp's output (128 W compared
to 1600 W) very similar fading times were found to be
necessary. This is probably due to a much greater portion
of the emitted energy reaching the sample in the Oliphant
system and due to its much higher uv component which has been
shown in the literature to be the most effective spectral
region in producing dye fading (Section 1.4.1).
It had been found that the Oliphant lamps emit
virtually constant radiant flux during their lifetime.
Consequently it was not necessary to continuously monitor
the flux as was the case with the Xenon lamp system. The
fading intervals were therefore simply determined by the
length of time for which the lamps were switched on. There was a small error associated with this technique as all the
lamps did not always switch straight on. However the error
in timing involved was only approximately 5 seconds for
only a few of the twelve lamps at the one time, and this in
terms of irradiation times of between 2 and 60 minutes for
twelve lamps was considered to be negligible.
3.5 Determination of the Initial Quantum Yields of Basic
Dye Fading
3.5.1 Measurement of Fading Rates
Each film sample was irradiated for four consecutive
time intervals. The progress of fading was followed by
measuring the absorbance of the dyed film at the wavelength
of the dye's maximum absorbance after each period of
irradiation. In this manner a dye's fading curve was 78 obtained in the form of a plot showing the proportion of the original dye remaining versus the total irradiation time.
Prior to the first irradiation each sample was subjected to a flow of gas at the temperature and humidity at which irradiation was to take place to allow the dyed film to come to equilibrium with the desired conditions before its initial concentration was determined. This allowed any shrinkage of the film due to either the heating or drying effects of the passing gases. This phenomenon was found to occur with both of the cast films, PVA and
Acrilan, and resulted in an increase in the thickness of the film and a corresponding increase in the measured absorbance. The effect also increased with increasing temperature.
Following this pretreatment, the absorbance (at the wavelength of maximum absorbance) and the thickness of the dyed film were measured and these values used to calculate the initial dye concentration in the film as described in Section 3.2. The absorbance of the dye obtained here was also taken as the initial absorbance for the determination of the fading curve. In all cases, the absorbance of the dye was obtained by subtracting from the absorbance of the dyed film the corresponding absorbance of the undyed film, which was for all films relatively smal 1 ( A ~ 0 . 0 4) •
In order to allow the film samples to equilibrate to the conditions before each period of irradiation, the 2.0
1.95 30°c
61 °C 1.90
1.85 -t,'l s::: 97°c •r-i s::: •r-i rd s Q) 1.80 P::
Q) ~ 0 .µ s::: 1.75 Q) 0 1-1 Q) r:i.. - 1.70 t,'l 0 ,-f
1.65
1.60
138°C 1.55
0.5 1.0 1.5 2.0 2.5
FIGURE 3 .6 Semi-log plot of the Fading Curves, given in
Figure 3.5, for C.I. Basic Blue 4 in Acrilan 16
Film at various temperatures. 100
-10°C 90 30°c
61°C
80
t,'l s:: ·r-1 s:: ·r-1 m E: 70 Q) 97°c p:;
Q) >t 0 .µ s:: Q) 60 u 1-1 Q) Ill
50 ll8°C
40 138°C
0.5 1.0 1.5 2.0 2.5
FIGURE 3 .5 Fading Curves for C.I. Basic Blue 4 in
Acrilan 16 Film at various temperatures. 79 films were again subjected to the appropriately conditioned gas flow for between 10 and 30 minutes (depending on the temperature) prior to each irradiation.
In order to completely encircle the vacuum tube the film samples had to be approximately 16mm wide and for ease of measurement on the spectrophotometer at least 18mm long.
The technique developed to measure the absorbance of a film involved mounting the film at a fixed position between two glass microscope slides in order to flatten it. A number of absorbance readings were taken across the sample and averaged. The results were then expressed as follows:
Percent Dye Remaining at time t
absorbance of dye at time t = X 100% absorbance of dye at zero time
where absorbance of dye = absorbance of dyed film
absorbance of undyed film
All absorbances were measured at the wavelength of maximum absorbance of the dye.
A typical set of fading curves, for C.I. Basic Blue
4 in Acrilan 16 film, is shown in Figure 3.5. The same results are presented in Figure 3.6 plotted on a semi-log scale to demonstrate the test for the existence of first order fading kinetics. It was found that most of the fading curves followed apparent first order kinetics although a few exhibited apparent zero order kinetics. The latter were 80
assumed to be the initial apparent linear portion of a first
order reaction. This is consistent with the results obtained
for fabric fading presented in Chapter 2.
The apparent first order rate constants, k 1 , were
determined by linear regression on the log (percent dye
remaining) versus total incident quanta curve as calculated
on a Hewlett-Packard HP25 Calculator. The slope of the line
of best fit (a 1 ) determined in this way was used to calculate
the apparent first order rate constant (k 1 ): k 1 = 2.303 ai
The square of the correlation coefficients (r2 ) obtained were all better than 0.9 with most around 0.99, indicating
the extremely good fit of the results to first order plots
and better than 99% confidence in the slope of the line.
The initial rate of dye degradation,~~ (molecules
of dye degraded cm- 2 sec- 1 ), was then calculated from the
apparent first order rate constant as follows:
dN dt = = where N = dye molecules degraded per cm 2
t = time
k1 = apparent first order rate constant
Co = initial concentration of dye
Ao = initial maximum absorbance of dye
NA = Avogadro's Number
E: = extinction coefficient of dye 81
3.5.2 Measurement of Quanta Absorbed
Since the film curves around the inner wall of the
tube during irradiation the sample receives light from both
sides. This must be taken into consideration when
calculating the amount of energy absorbed by the film:
where I 0 = incident intensity
IT = transmitted
intensity
Amount of light absorbed by the
sample from outside the tube where A = absorbance of the film
Amount of light transmitted
through the sample = = I 0 I 0 (1 - 10-A) = Io X 10-A = light incident on
the back of film
Total amount of light absorbed by the film
= Io(l - 10-A) + Io X 10-A (1 - 10-A) = I o ( 1 - 10-A) (1 + 10-A) = I o (1 - 10-2A)
Thus the amount of energy absorbed in this situation is
equal to a film of double the thickness (and therefore double
the absorbance) absorbing energy from only one side. 82
The initial rate of quanta absorption by the dye dia (quanta absorbed cm- 2 sec- 1 ) was calculated as follows: dt
dI >..=520nm _..£! dt = I >..=275nm
where: Ia = quanta absorbed by dye per cm 2
t = time (sec.)
>.. = wavelength. The wavelength range used was
275nm to 520nm, which covered the total
emission of the fluorescent lamps used.
The summation was carried out at 5nm
intervals over this range.
= intensity of the incident light at
wavelength>.. (quanta cm- 2 sec- 1 ). = absorbance due to the dye only at wavelength >... = total absorbance of the dyed film at wavelength >... = + where: = absorbance due to polymer film at wavelength>... T 1 - l0-2 A>.. = fraction of incident light that is absorbed
by the dyed film.
The absorbance spectra of each dyed and undyed film was measured on a Varian Techtron 634 double beam spectro photometer which has an absorbance accuracy of better than ±.002A (at l.OA) and a wavelength accuracy of better than O. 5nm. 83
To facilitate the calculation of quanta absorbed, normalised spectra of each type of dyed and undyed film were stored on a cassette tape and a calculator program devised to use this stored data. This meant that only the maximum initial absorbance of a dyed film and its thickness
(after pretreatment with the appropriately conditioned gas but prior to irradiation) had to be fed into the calculator in order to calculate the initial rate of quanta absorption by the dye. A Hewlett-Packard 9821A Calculator was used for this purpose.
3.5.3 Calculation of the Initial Quantum Yields of
Dye Fading
The initial quantum yields of dye fading(~) were calculated as follows:
dN dt ~ =
3.6 Experimental Results Each of the three dyes being studied, c.r. Basic Blue 4, c.r. Basic Red 18 and C.I. Basic Violet 14 was examined in each of the three films, poly(vinyl alcohol), nylon 6 and Acrilan 16. Where sufficient samples of nylon
6 and Acrilan 16 films were available, dry and wet nitrogen, and dry and wet oxygen atmospheres were used in order to concurrently determine the effects of oxygen and moisture on fading. With PVA films only a dry nitrogen atmosphere was used since PVA films are relatively impermeable to oxygen and are soluble in water. Also, in some cases two 1.5
1.4
1.3
II)- 0 r-1 :< 1.2 -,e, O'I 0 r-1
1.1
1.0
0.9
FIGURE 3 .14 Arrhenius Plot of the Initial Quantum Yield of Fading of C.I. Basic Violet 14 in Acrilan 16 Film for an initial dye concentration of 2.60 µmol.g- 1 in dry nitrogen. 1.2
1.0
1/)- 0 .-I >< 0.8 -,e, O"I 0 .-I
0.6
0.4
0.2
3.0 3.5 4.0 1 T
FIGURE 3.13 Arrhenius plots of the Initial Quantum Yields of Fading of C.I. Basic Violet 14 in Nylon 6 Film for an initial dye concentration of 2.63 µmol.g- 1 • in dry nitrogen o in dry oxygen 1.6
1.4
1.2
II) 0 r-1 >< -,e, 1.0 tJ'I 0 r-1
0.8 •
0.6
0.4
3.0 4.0 1 X T
FIGURE 3.12 Arrhenius plots of the Initial Quantum Yield of Fading of C.I. Basic Violet 18 in Poly(vinyl alcohol) ·Film in dry nitrogen for initial dye concentrations of: • 2 • 4 2 µmo 1 . g - 1 • 1 • 2 8 µmo 1 • g - 1 1.6
1.4
1.2 "'- 0 .-I >< -,e, 1.0 O'I 0 .-I
0.8
0.6
0.4
2.5 3.5 1 X T
FIGURE 3 .·11 Arrhenius plots of the Initial Quantum Yield of Fading of C.I. Basic Red 18 in Acrilan 16 Film for initial dye concentrations of: 3.29 µmol.g -1 , in dry nitrogen • ' -1 0 3.29 µmol.g , in dry oxygen • 1.62 µmol.g -1 , in dry nitrogen 1.8
1.6
1.4
1.2 - "'0 r-1
~ ,e, 1.0
t,'I 0 r-1 0.8
· 0.6
0.4
0.2
3.0 3.5 4.0 1 T X
FIGURE 3.10 Arrhenius Plots of the Initial Quantum Yield of Fading of C.I. Basic Red 18 in Nylon 6 Film for initial dye concentrations of:
• 5 .18 µmol. g - 1 , in dry nitrogen o 5 .18 µmol. g- 1 , in dry oxygen • 2 • 9 4 µmol. g - 1 , in dry nitrogen and in Poly(vinyl alcohol) Film for initial dye concentrations of: .._ 4. 38 µmol. g -1 , in dry nitrogen .6. 2 .66 µmol.g- 1 , in dry nitrogen 1.6
1.4
1.2
3- 0 r-1 1.0 >< -,e, t,'I 0 r-1 0.8
0.6
0.4
FIGURE 3 .9 Arrhenius Plots of the Initial Quantum Yield of Fading of C.I. Basic Blue 4 in Acrilan 16 Film for initial dye concentrations of: • 1.21 µmol.g -1 , in dry nitrogen -1 0 1.21 µmol.g , in dry oxygen • 2.11 µmol.g -1 , in dry nitrogen 1.8
1.6
.;2-- 0 r-1 1.4 ~ -,e, tJ'I 0 r-1
1.2
1.0
0
3.0 4.0 1 X T
FIGURE 3. 8 Arrhenius Plots of the Initial Quantum Yield of Fading of C.I. Basic Blue 4 in Nylon 6 Film for Initial Dye Concentrations of: . . -1 • 1.39 µmol.g , in dry nitrogen o 1.39 µmol.g -1 , in dry oxygen . -1 • 2.60 µmol.g , in dry nitrogen 1.6
1.4
3 0 r-i >< 1.2 -,e, tJ'I 0 r-i
1.0
0.8
• 3.0 4.0 1 X T
FIGURE 3. 7 Arrhenius Plots of the Initial Quantum Yield of Fading of C.I. Basic Blue 4 in Poly(vinyl alcohol) Film in dry nitrogen for Initial Dye Concentrations of: D 1. 31 µmol. g - 1 • 2 • 9 3 µmo 1 . g - 1 84
different concentrations of dye were studied in order to
clarify the effect of dye concentration on fading.
3.6.1 The Effect of Temperature
The results obtained are graphed in the form of an
Arrhenius plot i.e., the logarithm of the initial quantum yield of dye degradation (log 10 ~) versus the reciprocal of 1 the absolute temperature (T, K- 1). In this form the apparent
activation energies of fading (E) can be calculated from the a slope of the graph, and any change in the apparent activation energy with temperatures is readily observed:
E a ~ Aexp ( Arrhenius Equation = - RT
E a log10~ = + constant 2. 30 3RT
and hence E 2. 30 3 X R x slope a = -
The graphs obtained for each dye/film combination are shown
in Figures 3.7 to 3.14. The quantum yields determined are 1 accurate to ±10% and T to ±1.5%.
It can be seen that some of the Arrhenius plots are
straight lines, implying a constant activation energy while
others are curves indicating that a change in activation energy occurs with temperature. For a curved plot it is
assumed that two activated processes are operating, and
their activation energies are estimated graphically from the
limiting slopes of the curve as indicated in the following
schematic diagram: TABLE 3.3 Inflection Temperatures and Activation Energies
of Dye Fading in Dry Atmospheres, determined
from Experimental Arrhenius Plots.
Dye/Film Co Atm. t. E (kJmol - 1 } 1. a ( µmol. g - 1 } (OC} >t. C.I.Basic Blue 4 in: Nylon 6 1.39 N2 9. 8 2.60 N2 10.5 1.39 02 6.9 Poly(vinyl alcohol} 1.31 N2 36 21 4.7 2.93 N2 31 25 7.1 Acrilan 16 1.21 lh 65 33 6.9 2.11 N2 69 25 4.5 1.21 02 7.3 C.I.Basic Red 18 in: Nylon 6 2.94 N2 7.4 5.18 N2 7.6 5.18 02 2.8 Poly(vinyl alcohol} 2.66 N2 13 15 4.8 4.38 N2 ] Acrilan 16 1.62 N2 85 54 8.5 3.29 N2 97 66 5.4 3.29 02 76 34 4.4 C.I.Basic Violet 14 in: Nylon 6 2.63 N2 9 19 5.1 2. 6 3 02 11.2 Poly(vinyl alcohol} 1.28 N2 12 23 0.7 2.42 N2 24 34 4.1 Acrilan 16 2.60 N2 8.4 85 log~ t. 1 i T The point of inflection of the curve (t., 0 c) is determined i as the intersection of the two lines as shown. The greater the difference between the two activation energies the sharper will be the inflection seen in the curve. Obviously the errors involved in the determination of the activation energies and the inflection temperatures will depend largely on the shape of the curve. Two well defined slope regions will give the more accurate values. The accuracy of the inflection temperature is also affected by the nature of the plot itself since a small error in ~(K- 1 ) can represent quite a large error when converted to T . 0 t . in c. The maximum errors found were ±7 kJ mol- 1 in the i activation energies, and ±16°c in the inflection temperature, but most errors were approximately ±5 kJ mol- 1 and ±5°c respectively. Table 3.3 lists the activation energies and inflection temperatures determined for each dye/film/ atmosphere combination. The fact that dye fadings exhibit an activation energy at all implies that a non-photochemical process is the rate-determining step, as purely photochemical reactions have essentially no activation energy because they are 86 activated by light rather than heat. This rate-controlling step may involve a diffusion or a conformation process as both would be affected by the rigidity of the substrate and therefore by temperature. The species which could be involved include the excited dye molecule, partially degraded dye fragments or reactive species within the substrate such as oxygen and water. The diffusion of partially degraded dye fragments away from each other is often necessary to prevent recombination reactions occurring, and species such as oxygen and moisture may play either a reactive or a quenching role in fading. Various conformational changes of the dye may facilitate certain reactions and the ability of the dye to alter its conformation will be dependent on the amount of free volume available and therefore also be dependent on substrate rigidity. The fact that the apparent activation energy of fading in some cases alters with temperature would seem to imply that a change in the substrate rigidity is influencing fading. It is probable that the points of inflection in the Arrhenius plots correspond to structural changes within the polymer similar to, or the same as, the glass transitions. At temperatures above this transition temperature polymer chain mobility and therefore free volume increase more rapidly with increasing temperature than they do below T g where the polymer is essentially in a glassy state. This may mean that the diffusion of some species held immobile below the transition temperature becomes possible above this temperature, or that rates of diffusion of species already mobile below the transition temperature increase more rapidly 87 with temperature above the transition temperature. Consequently the increases in activation energies above the transition temperatures that are observed may be due to either the introduction of a new mechanism with a higher activation energy, or due to an increase in the temperature sensitivity of the existing mechanism, above the transition temperature. The actual temperature at which an inflection is observed will depend on the amount of free volume necessary for the diffusion or mobility of the species involved in the rate-controlling step of the reaction. This in turn will depend on the dye fading mechanism(s) involved as this will determine either the size of the diffusing species or the amount of movement necessary for the required confor mational change. The temperature at which this free volume becomes available in the polymer will be closely related to the glass transition temperature of the polymer. In addition, the presence of dye in the polymer is likely to have a plasticising effect on the polymer which also means that each dye will probably lower the glass transition temperature to a different extent. Consequently the inflection temperature observed is characteristic of each particular dye-film system. This is seen to be the case from Table 3.3. The absence of an inflection in a number of the Arrhenius plots including C.I. Basic Blue 4 and C.I. Basic Red 18 in nylon, C.I. Basic Violet 14 in Acrilan and various fadings in an oxygen atmosphere may be due to one of the 88 following reasons: (1) only one fading mechanism that is not inhibited by the rigidity of the polymer either above or below T operating. g Such a mechanism may be an intramolecular one that is not affected by conformational or steric effects, or a mechanism involving the diffusion of relatively small species. (2) where oxygen atmospheres give rise to straight line Arrhenius plots the presence of oxygen may be quenching the second mechanism that would operate in its absence. This phenomenon is seen clearly for C.I. Basic Blue 4 in Acrilan and C.I. Basic Violet 14 in nylon where the plots in dry nitrogen do show an inflection. (3) the inflection temperature being outside the temperature range examined. The fading of two concentrations of dye, one being approximately double the other were examined for a number of the dye-film systems. It can be seen from Figures 3.7 to 3.14 that in general there is a decrease in quantum yield at the higher concentration which is the same trend as noted for the fading of the dyed fabrics. In nearly all cases the differences between the quantum yields of the two concentrations are not large, with the exception of C.I. Basic Violet 14 in PVA where the difference is quite marked. In all cases however the shapes of the two Arrhenius plots are similar and hence the activation energies and inflection temperatures are also similar for the two dye concentrations. As it is the effect of substrate on basic dye fading that is of most interest here, the results have been regrouped 1.6 1.4 1.2 II)- 0 ..-I 1.0 X -,e, O'I 0 ..-I 0.8 0.6 0.4 0.2 2.5 3.0 4.0 1 T X FIGURE 3.17 Arrhenius plots of the Initial Quantum Yield of Fading of C.I. Basic Violet 14 in dry nitrogen in: • Acrilan 16 Film (C 0 =2.60 µmol.g- 1 ) D Poly.(vinyl alcohol) Film (C 0 =2.42 µmol.g- 1) • Nylon 6 Film (C 0 =2.63 µmol.g- 1 ) 1.8 1.6 1.4 1.2 &t)- 0 r-i >< ,e, 1.0 tJ'I 0 r-i 0.8 0.6 0.4 2.5 3.0 4.0 1 X T FIGURE 3.16. Arrhenius Plots of the Initial Quantum Yield of Fading of C.I. Basic Red 18 in: • Acrilan 16 Film (C 0 =3.29 µmol.g- 1 ) o Poly(vinyl alcohol) Film (Co=2.66 µmol.g- 1 ) . . - l • Nylon 6 Film (Co= 2 .94 µmol.g ) 1.8 1.6 1.4 ~- 0 r-l 1.2 >< ,e, 01 0 ..-i 1.0 0.8 0.6 0.4 2.5 3.0 3.5 4.0 FIGURE 3 .15 Arrhenius Plots of the Initial Quantum Yield of Fading of C.I. Basic Blue 4 ~n: • Acri~an 16 Film (Co=l.21 µmol.g- 1 ) o Poly(vinyl alcohol) Film (C 0 =1.31 µmol.g- 1 ) • Nylon 6 Film (Cq=l. 39 µmol. g -i) 89 1 so that log~ versus T for similar concentrations of each dye on all three substrates are given on the one graph. (Only dry nitrogen results are presented.) These graphs are given in Figures 3.15 to 3.17. One factor that becomes very obvious is that each dye/polymer system has its own individual fading behaviour. Each dye displays quite different Arrhenius curves on each polymer with differing quantum yields and activation energies. It can be seen that the dyes also have differing effects on the apparent glass transition of the polymer. Further, the significant effects that temperature is seen to have on dye fading quantum yields demonstrates the important role that the rigidity of the substrate has in dye fading. With the exception of C.I. Basic Violet 14, the log~ versus} curve for Acrilan 16 is below the curves for PVA and nylon which is to be expected from the results in Chapter 2 and confirms that these basic dyes do have a higher lightfastness on Acrilan 16 than on nylon or PVA. (The C.I. Basic Violet 14 result is at the present time inexplicable.) In addition, at room temperature (} ~ 3.3 x 10- 3K- 1 ) the dyed Acrilan 16 films can be seen to exist below their apparent glass transition temperature whereas the other dyed films at room temperature exist above their apparent glass transition temperatures. This means that the fading of basic dyes in Acrilan 16 under normal circumstances occurs 3 3 5 5 2 2 0 82xl0-i+ 82xl0-i+ wet wet 7.7xl0-i+ 7.7xl0-i+ l.2xl0- 5.2xl0-i+ 5.2xl0-i+ 0. 0. 5.6xl0- Films Films 3 3 5 5 Dyed Dyed 02 02 Oxygen Oxygen 8xl0-i+ 8xl0-i+ for for 8xl0- dry dry 4. 4. l.3xl0- l.2xl0-i+ l.2xl0-i+ 3°c). 3°c). 6.Sxl0-1+ 6.Sxl0-1+ 5. 5. and and (q>) (q>) q> q> ± ± 3 3 5 5 5 5 2 2 (30 (30 N Fading Fading .2xl0-i+ .2xl0-i+ .2xl0- Nitrogen Nitrogen wet wet l l l.Sxl0- l.4xl0- l.lxl0-1+ l.lxl0-1+ 7.lxl0-i+ 7.lxl0-i+ 6 6 5.8xl0-i+ 5.8xl0-i+ Dye Dye Wet Wet 3 3 5 5 5 5 of of N2 N2 and and Temperature Temperature Sxl0-1+ Sxl0-1+ .Sxl0-1+ .Sxl0-1+ dry dry 7.7xl0-i+ 7.7xl0-i+ l.Sxl0- l l l.6xl0- Dry Dry 5.7xl0-i+ 5.7xl0-i+ 6.2xl0- 1. 1. Yields Yields Room Room i) i) at at - under under g g Co Co Quantum Quantum 3.29 3.29 2.63 2.63 2.94 2.94 1.21 1.21 1.39 1.39 2.60 2.60 5.18 5.18 µmol. µmol. ( ( Irradiated Irradiated Initial Initial 14 14 Atmospheres Atmospheres 4 4 4 4 18 18 18 18 .4 .4 3 3 FILMS: FILMS: Red Red Red Red Blue Blue Violet Violet Blue Blue 16 16 FILMS: FILMS: Film/Dye Film/Dye TABLE TABLE .Basic .Basic I I C. C. C.I.Basic C.I.Basic C.I.Basic C.I.Basic C.I.Basic C.I.Basic ACRILAN ACRILAN C.I.Basic C.I.Basic NYLON NYLON 90 in the region of lower activation energies and fading is therefore less affected by temperature. It also means that the greater rigidity of the acrylic polymer, because it exists below its T, is a significant factor in explaining g the greater lightfastness of basic dyes on this substrate. It is also probable that the greater inherent rigidity of acrylic polymers also contributes to the overall effect. 3.6.2 The Effect of Oxygen and Moisture The physical state of a polymer will also determine its permeability to species such as oxygen and water which may influence the fading of dyes present in the polymer. The fading of the three basic dyes being studied was therefore examined in nylon 6 and Acrilan 16 films in dry and wet oxygen and nitrogen atmospheres. The results obtained from fading the various dyed films in oxygen atmospheres at various temperatures are included in the Arrhenius plots in the preceding section (Figures 3.7 to 3.14). Table 3.4 gives the initial quantum yields of dye fading for the same dye/film systems in wet oxygen and wet nitrogen at room temperature together with the corresponding results in dry nitrogen and dry oxygen for comparison. It can be seen from the Arrhenius plots that the fading rates of basic dyes in all the cases studied is reduced by the presence of oxygen. This applies to C.I. Basic Blue 4 and C.I. Basic Red 18 in nylon 6 and Acrilan 16 films and C.I. Basic Violet 14 in nylon 6 film which represent all the dye/film systems studied under dry oxygen. This could be due to oxygen acting as a quencher of the 91 excited dye. Alternatively the presence of oxygen may restrict photoreduction reactions of the dye, with the photooxidation reactions that take over in their stead having a lower quantum efficiency. (This mechanism was proposed by Yamada43 to explain the higher quantum yields of fading of triphenylmethane dyes in anaerobic compared to aerobic solutions.) All the dry oxygen Arrhenius plots except for C.I. Basic Red 18 in Acrilan 16 give straight lines. Where the plot in dry nitrogen is also a straight line as for C.I. Basic Blue 4 and C.I. Basic Red 18 in nylon 6 the explanation for the lack of inflection would be the same as that given in the preceding section i.e., that probably only one mechanism is involved and it is not inhibited by the rigidity of the substrate. On the other hand, where the dry nitrogen plot is curved, the straight line oxygen plot would seem to indicate that the presence of oxygen prevents a second mechanism from operating. From the results given in Table 3.4 it can be seen that the presence of moisture in a nitrogen atmosphere decreases the fading rate of all three dyes in nylon films and of two of the three dyes i.e., C.I. Basic Blue 4 and C.I. Basic Red 18, in an oxygen atmosphere. The effect is most pronounced for C.I. Basic Blue 4 where a decrease of approximately 20 to 26% in quantum yield is observed in nitrogen and approximately 32% in oxygen, while the other reductions are much smaller. This decrease in quantum yield in the presence of moisture is the opposite effect to that 92 generally reported in the literature for other dye/fibre systems (see Section 1.4.6). The presence of water in a polymer will swell and plasticise it, as is evidenced by the decrease in the glass transition temperature of nylon 6 with increasing water content shown in Table 3.2. In wet nitrogen the decrease in quantum yield must in some way be due to an increase in the rate of deactivation processes that occur, possibly involving an increase in self-quenching due to an increase in the dye's mobility. In wet oxygen on the other hand the decreases in rate observed may be due to the increased rate of diffusion of oxygen which is seen to quench fading in the dry polymer. For C.I. Basic Violet 14, an increase in quantum yield is observed in wet compared to dry oxygen. In this case water itself may take part in the fading reaction either alone or in conjunction with the oxygen present. Alternatively water's effect may be a physical one either on the dye itself by displacing the dye from its site or causing disaggregation (although aggregation is unlikely at the low concentrations used here), or on the substrate with its plasticising effect increasing the rates of diffusion of various species within the substrate. In the latter case the species involved could not be oxygen as its presence alone was found to decrease fading rather ti1an increase it. The presence of moisture has little effect on the fading of C.I. Basic Blue 4 and C.I. Basic Red 18 in Acrilan 93 16. This is probably due to the lack of penetration of the water into this polymer since its moisture regain is very low - 1.4% under standard conditions (20°c, 65% RH) compared to 4.2% for nylon. Having observed some moisture effects on dyed films it was decided to extend investigations to dyed fabric. Only nylon 6.6 fabric was used, as dyed Acrilan 16 fades too slowly for effects to be easily detected and in any case its moisture absorption is very low. One concentration of each dye on nylon fabric was used and quadruplicate samples of each dyed fabric were faded under each of three atmospheres: (1) dry nitrogen (2) dry air (contains oxygen) (3) moist air (contains oxygen and moisture) All samples were conditioned in the appropriate atmosphere and then sealed in polyethylene bags for irradiation in the Xenon lamp system. The polyethylene is transparent over the whole wavelength range emitted by the lamp system and did not deteriorate with irradiation. To obtain the two dry gas atmospheres the fabric samples, attached to cardboard backings, were placed in the plastic bags open at one end and flushed with the appropriate gas for at least two hours before being sealed. The samples to be faded in moist air were simply allowed to condition TABLE 3 .5 Percent of Initial Concentration of Dye Degraded after a given Incident Light Exposure Dose in Different Atmospheres. Percent Dye Degraded Atmosphere C.I.Basic Blue 4 C.I.Basic Red 18 Dry nitrogen 18.4 (±.4) 16.9 (±1.0) Dry air 14.4 (±.7) 16 .5 (± .9) + oxygen) Moist air (+oxygen+ water) 16 .o (±1.8) 15. 3 ( ±. 5) Initial Dye Concentration (µmol.g- 1 ) 0.54 1.72 Incident Light Exposure Dose (quanta crn- 2 ) 7.7 X 10 15 1.7 X 10 17 The errors quoted are the 95% confidence limits. 94 under room conditions for at least two hours before sealing. The samples were irradiated in the Xenon lamp system for an appropriate light exposure dose, then removed from the plastic bags and their reflectance measured on a Hunterlab D25D2M Colour Difference Meter for determination of the dye degraded by the method given in Section 2.5. The experimental results obtained are given in Table 3.5. They are expressed as the percentage of the original dye present that is degraded after a given incident light exposure dose. It can be seen that oxygen decreases the fading rate of C.I. Basic Blue 4 by approximately 20% at this exposure dose but has no significant effect on C.I. Basic Red 18. The decrease in fading is consistent with the dyed film results presented earlier. The effect of oxygen and moisture together are seen to decrease the fading of C.I. Basic Red 18 by approximately 10% at this exposure dose but have no significant effect on C.I. Basic Blue 4. Again this decrease is consistent with the dyed film results. Thus these nylon fabric results confirm the results obtained on nylon film substrates. It can be seen that the effects of oxygen and moisture on basic dye fading are not large especially when compared to the differences in fading rates that occur between the different substrates. Moreover, both oxygen and moisture were usually found to decrease basic dye fading rates rather 95 than increase them which means that the low permeability of acrylic polymers to these two species can hardly account for the high lightfastness of basic dyes present in them. 96 CHAPTER 4 THE INFLUENCE OF THE CHEMICAL NATURE OF THE SUBSTRATE ON THE FADING OF BASIC DYES 4.1 Introduction The results obtained in Chapter 3 showed that the physical nature of the substrates does influence the fading of basic dyes quite significantly, and that the relatively high rigidity of acrylic compared to other substrates plays a role in reducing the rate of fading of the basic dyes present. However, it is quite possible that factors of a chemical nature also contribute to the substantially higher lightfastness of basic dyes in acrylic substrates. In this section attention was focused on various chemical factors which may influence basic dye fading. Firstly, it was hoped that the elucidation of the reaction mechanisms operating in the fading of basic dyes would give an indication of the possible types of substrate interactions. To this end the identification of various types of reaction intermediates was attempted by the use of scavenging and quenching agents in a dye-polymer system, and by a study of the luminescence of the dyes. Also, the effect on basic dye fading of various chemical groups present in the different substrates was investigated. In particular, the influence of the different dye site groups that may be present, and the influence of the cyano group which is a major constituent of acrylic 97 polymers, were studied with the aid of model systems. Any information gained from this line of research should indicate chemical factors which contribute to the particularly high lightfastness displayed by basic dyes in acrylic polymers. 4.2 Irradiation Technique The Xenon lamp system described in Section 2.6 was used for the irradiation of the fabrics, yarns and films. Films were left on the microscope slides on which they were prepared and irradiated in the same metal holders that were used for fabric mounting. The progress of fading was followed in the same manner as described in Sections 2.5 and 3.5, i.e., by measuring the reflectance of fabrics and yarn, or the absorbance of films, before exposure and after each successive irradiation. With the films, the absorbance of the dyed films at the wavelength of maximum absorbance of the dye was measured in eight separate and reproducible positions over their area. After each period of irradiation the proportion of dye remaining was calculated for each of the eight positions, and then averaged for the whole film. Variations in film thickness over its area caused a corresponding variation in the absorbance of the dye and this averaging technique gave reliable results. In all cases duplicate samples were irradiated and were found to be reproducible to within ±1.5% in percent dye remaining for films and to within ±3% for nylon fabric and yarn. 98 4.3 The Identification of Reaction Intermediates in Basic Dye Fading 4.3.1 The Use of Scavenging and Quenching Agents Some indication of the types of reactions involved in dye fading can be obtained by incorporating into the dye- substrate systems certain agents which will selectively either scavenge or quench any intermediates present. Excited states of molecules can be quenched with an accompanying transfer of energy from the excited molecule to the quencher molecule, while species such as electrons and free radicals can be scavenged (or "trapped") by appropriate agents. By interfering with the fading reactions in this manner the presence of scavengers and quenchers can produce a change in the rate of the reaction. Since the presence of agents which act as excited state quenchers in the system may also introduce additional photochemical pathways, only a decrease in rate on the addition of such an agent can be taken as evidence of the presence of a particular intermediate. An increase in rate may be due to an added reaction involving the agent itself or a by-product, but a decrease in rate cannot be attributed to this effect since if the rate of the additional reaction introduced is slower than that of the normal fading reaction, no change in fading rate will be evident. Scavenging agents may also produce an increase in rate. If such an agent is not photoreactive itself then this effect can be assumed to be due to interference with 99 the normal reactions, for example by scavenging species which prevent dark or back reactions occurring. As such, an increase in the dye's fading rate in the presence of these agents can also be taken as an indication of the presence of the particular type of intermediate being scavenged. 4.3.1.1 Excited State Quenching Various types of chemical agents have been found to be efficient excited state quenchers. They include 2-hydroxybenzophenones, hydroxybenzotriazoles, transition metal chelates especially chelates of nickel (II), metal salts and alkali metal halides, all of which are usually quoted as being excited triplet state quenchers. 109 Singlet excited oxygen quenchers include 3 -carotene, 1 1 0 sodium azide1 11 and 1, 4-diazabicyclo ( 2, 2 ,2) -octane •11 0 Nickel ( II) chelates have also been reported to be effective quenchers of singlet excited oxygen, particularly those chelates with sulphur donor ligands. 109 4. 3 .1.2 Electron and Radical Scavenging A variety of compounds have been found to function as electron and radical scavengers. Transition metal complexes have been found to be efficient electron scavengers, with Cd2 + compounds being the most efficient. 112 A number of metal chelates have been found to be efficient scavengers of "OH and "OR radicals. 113 ' 11 ~ Other radical scavengers include phenols and dialkyldithiocarbamates. The major difficulty encountered with this line of 100 research is that most of the chemicals involved can act in more than one way. For example, nickel chelates are known to be highly efficient triplet state quenchers, 10 9 but have also been observed to quench singlet excited oxygen115 and may also act as radical scavengers.10 9 Manganese salts have been postulated to be capable of destroying hydroperoxides as well as quenching excited triplet states. 116 Whereas potassium thiocyanate has been shown to scavenge hydroxyl radicals, 117 it is unlikely to be specific to this one type of radical and probably reacts with other radicals as well. Consequently a cautious approach to the interpretation of scavenging and quenching experiments is required. The possible multifunctional mode of operation of most of these agents means that no one result can be conclusive. 4.3.1.3 Experimental Technique The choice of scavenging and quenching agents was restricted by a number of factors. The use of dyed fabrics in this work meant that only substances of very low vapour pressure could be used in order to avoid evaporation losses during preparation and irradiation. Coloured compounds whose absorbance would interfere with the measurement of fabric reflectance used to calculate the dye's concentration, and substances which would compete with the dye for the incident light were not suitable. The scavengers and quenchers examined were: 1) Manganous salts (e.g., MnS04) are generally recognised as excited triplet state quenchers. 109 2) Nickel (II) diethyldithiocarbamate{[(C2Hs)2NCS2]2Ni} is 101 typical of the nickel chelates which have been found to be particularly efficient triplet excited state quenchers. 115 Unfortunately, the slight green colour of this compound was found to interfere with the measurement of the concentration of both of the dyes used here and so its use was discontinued. 3) Sodium diethyldithiocarbamate [(C 2 H5 ) 2 NCS 2 Na] has been observed to act as a radical scavenger. 4) Potassium thiocyanate (KCNS) has been demonstrated to scavenge hydroxyl radicals. 117 5) Cd2 + ions (e.g., CdS0 4 ) are highly efficient electron scavengers. 11 z 6) Sodium azide (NaN 3 ) is known to quench singlet excited oxygen. 111 However, as it was found to hydrolyse to hydrazoic acid during its application its use was discontinued. Since the very slow fading rates of basic dyes on acrylic fabrics would make it difficult to detect decreases in their fading rates it was decided to apply the various scavenging and quenching agents to nylon 6.6 fabric where basic dye fading is relatively rapid. Changes in rate, both increases and decreases, would therefore be more readily detected. The nylon 6.6 fabric was dyed with c.r. Basic Blue 4 and C.I. Basic Red 18 as described in Section 2.4.2. For all the additives, the dyed fabric was soaked in a 1% (w/v) solution of the agent for approximately 30 minutes at room temperature. (The solution could not be heated to facilitate penetration because of the very low wet fastness of these dyes on nylon.) The wet fabric was 100 95 tJ'I s:: 90 ·r-i s:: •r-i Ill E:l Q) ~ Q) 85 KCNS >t Cl .µ s:: Q) 0 Dye 1-1 Q) 80 only A.. CdSOi. and MnSOi. 75 70 0 1.0 2.0 3.0 FIGURE 4.2 Fading Curves for C.I. Basic Red 18 in Nylon 6.6 Fabric containing: • Dye only (1.72 µmol.g- 1 ); plus: o Potassium Thiocyanate (79.5 µmol.g- 1 ) • Cadmium Sulphate (33.7 µrnol.g- 1 ) o Manganous Sulphate (46 .4 µrnol.g- 1 ) • Sodium diethyldithiocarbarnate (58.5 µrnol.g- 1 ) ( a) b'I 90 i:: ·r-1 i:: ·r-1 rtl s Q) 80 p:; Q) :>t 0 .j.J 70 i:: Q) u 1--1 Q) 111 60 0 0.5 1.0 1.5 2.0 2.5 (b) 100 b'I i:: 90 ·r-1 i:: ·r-1 rtl E Q) 80 p:; Q) ::,... 0 .j.J 70 i:: Q)u J..I Q) 111 60 0 0.5 1.0 1.5 2.0 2.5 Total Incident Quanta x 10-16 (cm- 2 ) FIGURE 4.1 Fading Curves for C.I.Basic Blue 4 on Nylon 6.6 Fabric containing: (a) Dye only (b) Dye only • (0 .54 µmol.g- 1 ); plus: • (0. 80 µmol.g- 1 ); plus: D Potassium thiocyanate D Cadmium sulphate ( 79. 5 µmol. g - 1 ) ( 33. 7 µmol. g -i) ... Sodium diethyl dithio- .... Manganous sulphate carbamate (5 8 .5 µmol .g - 1 ) ( 4 6 • 4 µmo 1 • g - 1 ) 102 then passed through a padding mangle, which resulted in a 70% liquor retention, and then allowed to dry. Hence a minimum concentration of 0.7% (w/w) of each agent on the fabric was obtained. Duplicate samples of each dye/additive combination were then faded for five progressive irradiation doses. 4.3.1.4 Results The fading curves obtained for the various scavenging and quenching agents applied to nylon 6.6 fabric dyed with C.I. Basic Blue 4 and C.I. Basic Red 18 are given in Figures 4.1 and 4.2. The radical scavenger sodium diethyldithiocarbarnate increased the fading rate of both dyes quite significantly. This would seem to imply that radicals are involved in the photochemical reactions occurring. By scavenging these radicals sodium diethyldithiocarbamate may be preventing a back reaction occurring or perhaps interfering with some dark reaction to cause the observed increase in fading rate. However, in the light of subsequent results, it is possible that this could be purely a pH effect. Potassium thiocyanate, another radical scavenger also markedly affected the fading rates of both dyes. In the case of C.I. Basic Blue 4 the rate was increased while for C.I. Basic Red 18 it caused a substantial decrease in rate. In the latter case it is probable that the radicals being scavenged are reactive intermediates in the fading process so that their subsequent trapping causes a decrease 100 MnS011 90 CdS01t 80 tri i::: ·r-4 i::: ·r-4 rtj ~ Q) ~ Q) 70 !>-1 Cl .µ i::: Q)u 1-1 Q) P-4 60 50 Dye only 40 0 0.25 0.5 0. 75 1.0 0.25 FIGURE 4 .4 Fading Curves for C.I. Basic Violet 14 in Poly(vinyl alcohol) Films containing: • Dye only (2.97 µmol.g- 1 ); plus: o Cadmium Sulphate ( 38 µmol. g -i) • Manganous Sulphate (150 µmol.g- 1 ) 100 95 MnSOii 90 tJ"I s:: ·r-i s:: ·r-i rtj s CdSOii (1) p:: (1) 85 >i Cl .µ s:: (1) CJ H (1) Ill 80 Dye only 75 70 0 0.5 1.0 1.5 2.0 2.5 FIGURE 4. 3 Fading Curves for C.I. Basic Blue 4 in Poly(vinyl alcohol) Films containing: • Dye only (1.85 µrnol.g- 1 ); plus: o Cadrni~ Sulphate (38 µrnol.g- 1 ) • Manganous Sulphate (150 µrnol.g- 1 ) 103 in the rate of fading. It can be seen that both manganese sulphate and cadmium sulphate had little effect on either dye's fading. These results would seem to imply that neither triplet excited states nor electrons are involved in either fading reaction. Alternatively the agents may not be located sufficiently close to the dye molecules to have an effect on their fading reactions. In order to see the effect of an homogeneous distribution of these two agents throughout a dyed substrate, films of the photochemically inert polymer, poly(vinyl alcohol), were prepared containing both dye and quenching agent, and the rate of dye fading determined. The fading of C.I. Basic Blue 4 and C.I. Basic Violet 14 was investigated but unfortunately C.I. Basic Red 18 reacted with both of the agents in solution and so could not be used. Figures 4.3 and 4.4 show the fading curves obtained for C.I. Basic Blue 4 and C.I. Basic Violet 14 in PVA films containing 38 µmol.g- 1 CdSO 4 and 150 µmol.g- 1 MnSO 4 • (38µmol.g- 1 CdSO 4 is the limit of the solubility of CdSO 4 in PVA.) Dye concentrations were chosen to give a maximum absorbance of 1. It can be seen that the homogeneous distribution of both agents has a marked effect in reducing the fading rates of both dyes indicating the involvement of both electrons and excited triplet states in this particular system. 104 However, this does not necessarily mean that the same species are involved in the fading of these dyes in other substrates as different substrates may favour different mechanisms. It does demonstrate that electrons and excited triplet states are involved in one of the possible fading reactions of these dyes and that the lack of any observable effect of these agents in dyed nylon 6.6 fabric may be due to their uneven distribution rather than due to the absence of electrons and excited triplet states. 4.3.2 Luminescence Studies The possibilities of using luminescence studies of the dyed films to give further insight into the excited species involved in the dye fading were investigated on a spectrofluorophosphorimeter constructed in these laboratories. 118 However, in common with other dyes, any fluorescence observable was very weak and at quite long wavelengths (peaks in the range 590 - 690nm) while no phosphorescence could be detected within the wavelength range of the instrument used. 4.4 The Influence of Dye Sites on Basic Dye Fading Model compounds containing the anionic groups that are used as dye bonding sites for basic dyes were introduced into an inert solid model substrate to determine how the nature of the bonding site affects the fading of the basic dyes attached to them. Essentially only two types of dye sites appear to be used i.e., sulphonate groups (SO 3 -) and carboxylate groups (C00-). 96 It has been suggested42 (see Section 1.5) that the reason for e1e higher lightfastness of 105 basic dyes on acrylics is that these fibres contain sulphonate dye sites instead of carboxylate sites and that it is the sulphonate sites that cause the improved lightfastness of the basic dyes attached to them. 4.4.1 Experimental Technique Poly(vinyl alcohol) films were chosen as the model substrate for this work as they contain no potential sites for basic dyes other than those specifically introduced. Also, because the films are cast, additives such as the dye and the dye site models can be introduced homogeneously through the films. The model compounds chosen to simulate the dye sites, benzene sulphonic acid and trichloroacetic acid,have the same dissociation constant in water (K = 2 x 10- 1 ). a In addition, sodium benzene sulphonate and sulphuric acid were each used to determine the effect of the -SO 3 and H+ components of benzene sulphonic acid. Sodium hydroxide was used to determine the effect of base and sodium sulphate to determine the effect of the sulphate group. Each compound was added to the film-forming poly(vinyl alcohol) solution at a concentration that gave a ratio of additive molecules to dye molecules of 33:1 in the film, thereby ensuring that most dye molecules would be in close proximity to an additive molecule. Only C.I. oasic Blue 4 was used in this work since 100 95 °'s:: ·r-i 90 s:: H2S04 ·r-i ro I::: & Q) !>t Q .µ s:: Q) 85 u 1-1 Q) Al . 80 Dye only NaOH 75 0 0.5 1.0 1.5 2.0 2.5 FIGURE 4 .5 Fading Curves for C.I. Basic Blue 4 in Poly(vinyl alcohol) Films (1.85 µmol.g- 1 ) containing various additives in the molar ratio additive : dye = 33 : 1 106 C.I. Basic Red 18 was found to precipitate from PVA on the addition of acid and C.I. Basic Violet 14 changed colour in the presence of acid. An initial dye concentration of 1.85 µmol.g -1 C.I. Basic Blue 4 was used, to give an initial maximum absorbance of 1. 4.4.2 Results The resulting fading curves for 1.85 µmol.g -1 C.I. Basic Blue 4 in poly(vinyl alcohol) films containing the various additives are given in Figure 4.5. It can be seen that there is virtually no difference in the dye's fading rate between the films containing the two different dye sites i.e., there appears to be no significant difference in fading of C.I. Basic Blue 4 caused by its bonding to sulphonate or carboxylate sites. Both additives do cause a decrease in the fading rate of C.I. Basic Blue 4. However this is likely to be simply due to their acidic properties as it can be seen that a stronger acid, sulphuric acid (K = 1.20 x 10-2 ) a has an even greater effect in decreasing the rate, whereas the sodium salt of benzene sulphonic acid has no significant effect. Sodium hydroxide and sodium sulphate are seen to have very little effect on the dye's fading. The sodium sulphate result would seem to imply that the effect of sulphuric acid in decreasing the fading rate is due to the + 2- effect of the H rather than the SO 4 ions. Hence these results for C.I. Basic Blue 4 in the model substrate PVA provide no evidence that the type of dye site affects the lightfastness of the dye bonded to it providing that their pKa's are the same. 100 95 s::tJ'I ·r-1s:: ·r-1 ro El Q) p:; Q) 90 >t 0 .µ s:: Q) u 1-1 Q) Ill 85 80 0 0.5 1.0 1.5 2.0 2.5 Total Incident Quanta x 10-16 (cm- 2 ) FIGURE 4.6 Fading Curves for C.I. Basic Blue 4 in Poly(vinyl alcohol) Films containing various amounts of Sulphuric Acid. -1 • Dye only (0.93 µmol.g ) ; • Dye only (1.85 µmol.g- 1 ); plus: plus: -1 D 61.8 µmol.g- 1 H2SO~ • 5.6 µmol.g H2S0 4 6 15 µmol.g- 1 H2S0~ o 30.6 µmol.g- 1 H2S04 T 61.8 µmol.g- 1 H2SO~ V 91.8 µmol.g- 1 H2S04 107 In order to determine the effect of the pH of the system on the fading of C.I. Basic Blue 4 in PVA, various concentrations of sulphuric acid were added to dyed films and the films irradiated. The sulphuric acid concentration in the film was varied from 5.6 to91.8µmol/g PVA while the concentration of dye was kept constant at 1.85 µmol/g PVA. This represents a range in the molar ratio of acid to dye of approximately 3:1 up to 50:1. The concentration of dye was also halved for one acid concentration (0.93 µmol.C.I. Basic Blue 4/g PVA +61.8µmol.H 2 SO~/g PVA) which represents a molar ratio of acid to dye of approximately 70:1. The resulting fading curves are given in Figure 4.6. It can be seen that the rate of fading of C.I. Basic Blue 4 in PVA does decrease with increasing sulphuric acid concentration and that the effect is quite a significant one. This decrease in rate is also seen for the lower concentration of dye examined. Having noted the effect of acid on the fading of C.I. Basic Blue 4 in the model substrate PVA, it was decided to examine the effects of acid and base on the fading of dyed fibre substrates. Nylon 6.6 fabric and basic-dyeable nylon yarn dyed with one concentration of C.I. Basic Blue 4 and C.I. Basic Red 18 were examined. Unfortunately only the basic-dyeable nylon yarn could be used as attempts to apply acid to the dyed nylon 6.6 fabrics resulted in excessive leaching of the dye out of the fabric. There was a slight leaching of dye out of the basic-dyeable nylon but it was possible by restricting treatment time to keep this leaching { a} t,"Is:: ·r-1s:: •r-1 Ill 80 s & Q) ~ +l s:: 60 Q) u l--1 Q) Al 40.,__ ___--+-----+------f-----4------1- 0 0.5 1.0 1.5 2.0 2.5 {b} 10 tJ'I s:: ·r-1s:: ·r-1 Ill s & Q) >t Q +l s:: Q)u l--1 Q) Al 0 0.5 1.0 1.5 2.0 2.5 Total Incident Quanta x 10- 17 (cm- 2 ) FIGURE 4. 7 Fading Curves for {a) C.I. Basic Blue 4 {0.84 µmol.g- 1 ) {b) C.I. Basic Red 18 (1.9 µmol.g- 1 ) in Basic-dyeable nylon yarn containing: • Dye only; plus: 1 D H 2 SO 4 { 410 µmo • g 1 - ) 1:::,,. NaOH ( 2 7. 3 µmol. g -i) 108 to an acceptable level while still obtaining sufficient acid take-up. The dyed yarn was soaked for 15 minutes in lM NaOH or 1 minute in 0.5M H2 SO 4 at room temperature. The sample was then briefly immersed in ice-cold water to remove the non-absorbed acid or base from the fibre's surface. The yarn was allowed to dry and then wound onto a cardboard backing for irradiation. The amount of acid or base absorbed by the fibres was determined by the back titration of similarly treated samples using a Sargent-Welch Model PBL pH meter to detect the end point. The fading curves obtained for 0.84 µmol.C.I. Basic Blue 4 per gram and 1.9 µmol.C.I. Basic Red 18 per gram of basic-dyeable nylon containing 410 µmol(H+)/g as sulphuric acid and27.3µmol(OH )/gas sodium hydroxide are given in Figure 4.7. It can be seen that the presence of acid decreases the fading rate of C.I. Basic Blue 4 in basic-dyeable nylon which corresponds to the effect seen for this dye in poly(vinyl alcohol) films. The presence of base is seen to markedly increase its rate of fading here whereas in poly(vinyl alcohol) films no change in rate was observed. For C.I. Basic Red 18, acid has no significant effect on its fading but like C.I. Basic Blue 4 it displays a marked increase in fading rate in the presence of base. 109 These results demonstrate that pH can have a marked effect on the fading of basic dyes both in model polymer films and in fibre substrates. It appears that in general the presence of acid tends to reduce the dye's fading rate and that of base to increase it. The extent of the effect appears to depend on the dye/substrate system in question. This pH effect observed means that the pK of the dye site a is likely to influence the fading of basic dyes attached to them. As it is quite probable that the pK 's of the same a site in different polymers or of different sites in the one polymer will differ, it is therefore possible that the nature of the dye site may by virtue of its pK influence the fading a of basic dyes. 4.5 The Influence of the Cyano Group on Basic Dye Fading The influence of the main side group present in acrylic fibres, the cyano group (-C=N), on dye fading was investigated by incorporating a model compound containing this group into dyed poly(vinyl alcohol) films and examining the effect on the dyes' fading. 4.5.1 Experimental Technique The model compound chosen was succinonitrile (NCCH2CH2CN) which is similar to the repeat unit of the polyacrylonitrile polymer (CH 2-CH-) and contains only the I n CH CN group as the possible active component. Since acrylic polymers consist of at least 85% polyacrylonitrile, at least 49% by weight of an acrylic polymer consists of -CN. A maximum of 1% CN by weight could be added to poly(vinyl 110 alcohol) films, as this represented the limit of solubility of succinonitrile in poly(vinyl alcohol). Above this concentration the succinonitrile precipitated out producing an opaque film. Malononitrile (NCCH 2 CN) was found to be more soluble in the film, but due to its volatility evaporated from the film during the drying. Consequently, the use of other polymer substrates was investigated and it was found that films of methyl cellulose could be prepared containing up to 8% CN by weight as succinonitrile without precipitation occurring. The method of preparation of these films involves first swelling the polymer by the addition of a little boiling water, followed by dissolution with the added dye and succinonitrile in cold water. A 2% (w/w) solution of methyl cellulose (Fluka Methocell MC 400cP) was prepared and 6 mls of this solution cast on a 50 x 37.5mm slide. This produced a film of thickness 30 ± 5 µ. It was found subsequently that the succinonitrile gradually evaporated from the polymer films. The dyed films were therefore irradiated as soon as possible after they had dried but unfortunately this evaporation meant that the concentration of succinonitrile present in the dyed film at the time of fading could not be determined accurately. Consequently the values given are only approximate and, being the concentration of succinonitrile introduced into the film on casting, they represent the maximum possible concentration that could be present. This evaporation effect was much greater in the relatively porous methyl cellulose (a) 100 t,"I 95 s:: ·r-1s:: ·r-1 =IO 90 ~ Q) >t Q 85 .µ s:: Q) CJ 1-1 Q) 80 ~ 0 0 .• 5 1.0 1.5 2.0 2.5 Total Incident Quanta x 10- 16 (cm-2 ) (b) 0 0.5 1.0 1.5 2.0 2.5 FIGURE 4 .9 Fading Curves for C.I. Basic Blue 4 (1.85 µmol.g- 1 ) in: (a) Poly(vinyl alcohol) Film (b) Methyl Cellulose Film, containing: • Dye only; plus: a Up to 1% CN (as succinonitrile) • Up to 8% CN (as succinonitrile) ( a) 100 s::t,"I 98 ·r-is:: ·r-i rt! s 96 & (I) ~ 0 94 .j.J s:: (I)u 1-1 (I) Ill 92 90 0 0.5 1.0 1.5 . 2.0 2.5 3.0 (b) 100 t,"I 90 s:: ·r-is:: ·r-i rt! s 80 & (I) £: 70 .j.J s:: (I)u 1-1 (I) Ill 60 50 0 0.25 0.5 0.75 1.0 1.25 FIGURE 4.8 Fading Curves for (a} C.I. Basic Red 18 (1.69 µmol.g- 1 ) (b) C.I. Basic Violet 14 (1.85 µmol.g- 1 ) in Poly(vinyl alcohol) Films containing: • Dye only; plus: t::. up to 1% CN (as succinonitrile} 111 films than in the poly(vinyl alcohol). Consequently, the effects of succinonitrile on the lightfastness of all three dyes were determined in poly(vinyl alcohol), but in methyl cellulose only the dye C.I. Basic Blue 4 could be studied as it required a much shorter irradiation time than the other two dyes. (Irradiation does result in a slight temperature rise of the sample which over the longer exposure times caused marked evaporation of the succinonitrile.) The use of less volatile models for the cyano group such as phthalo- N ni trile ( O( ) was investigated but due to their low N solubility in water they could not be incorporated into the films by the common solvent technique of forming films. In any case, the use of succinonitrile does demonstrate the influence of the cyano group on fading even though the effect cannot be strictly quantified. 4.5.2 Results The resulting fading curves obtained for similar concentrations of C.I. Basic Blue 4, C.I. Basic Red 18 and C.I. Basic Violet 14 in PVA film and for C.I. Basic Blue 4 in methyl cellulose film for the dye alone and in the presence of succinonitrile are presented in Figures 4.8 and 4. 9. It can be seen that in all cases the presence of cyano groups decreased the rate of fading of the dye concerned. For C.I. Basic Blue 4 and C.I. Basic Violet 14 in PVA the effect is small but for Basic Red 18 it is quite 100% 100% X X 4 4 .6 .6 11.9 11.9 21.6 21.6 10. 10. 58 58 54.6 54.6 l l kD+S kD+S k? k? l l - D D k alone, alone, alcohol), alcohol), 18 18 18 18 Dye Dye ) ) 8 8 2 9 9 1 1 1 18 18 lQ- 10- the the fading fading cm 1 X X X X 10- 10- 10- presence presence for for X X dye dye X X X X poly(vinyl poly(vinyl k?+S k?+S 8 8 4.48 4.48 2.07 2.07 0 0 in in the the of of (quanta- 4.92 4.92 8. 8. 1.44 1.44 groups. groups. by by (i) (i) (ii) (ii) Fading Fading ) ) Dye Dye maximum maximum 7 7 2 8 8 constant constant Cyano Cyano l l 19 19 18 18 1 1 caused caused cm of of CN CN 1 of of succinonitrile succinonitrile 10- 10- 10- 10- rate rate 1% 1% rate rate X X X X X X X X k? k? of of 3 3 to to 0 0 00 00 in in (quanta- 3.17 3.17 5.58 5.58 1. 1. 5. 5. order order presence presence Constants Constants presence presence the the ) ) first first Rate Rate 1 in in alone alone cellulose. cellulose. decrease decrease - in in .g .g corresponding corresponding dye dye Co Co dye dye and and Order Order 1.85 1.85 1.85 1.85 1.84 1.84 1.69 1.69 succinonitrile succinonitrile methyl methyl (µmol (µmol for for for for of of percent percent Apparent Apparent in in First First = = = = = = = = S S CN CN + + 8% 8% 100% 100% D D D D k1 k1 x x 14 14 Film: Film: concentrations concentrations 4 4 4 4 Apparent Apparent (ii) (ii) 18 18 Film: Film: 1 1 kD+S kD+S l l and and _ _ Red Red kD kD Blue Blue Violet Violet where where Blue Blue 1 1 4.1 4.1 alcohol) alcohol) kD kD 1% 1% Basic Basic Basic Basic Cellulose Cellulose Basic Basic Basic Basic . . . . . TABLE TABLE Dye/Film Dye/Film (yj_nyl (yj_nyl (i) (i) succinonitrile succinonitrile & & C.I. C.I. C.I. C.I. C.I. C.I. C.I. C.I. for for and and Poly Poly Methyl Methyl 112 large. Further, it can be seen that by increasing the concentration of -CN by a factor of approximately 8, in methyl cellulose, the rate of fading of Basic Blue 4 is further reduced. Considering that the concentration of succinonitrile in the methyl cellulose films still represents only 8% CN at most, it would be expected that in an acrylic polymer where the concentration of CN is approximately 49% that a much larger effect still would occur. The apparent first order rate constants for these curves were calculated from the slope of log (percent dye remaining) versus total incident quanta curves by linear regression. These results, together with the percentage change in the rate constant in the presence of succinonitrile are given in Table 4.1. Expressed in this manner the significance of these results is even more apparent. Even the small effects represent an appreciable decrease in the rate constant (and therefore also the rate itself) of 12 to 22%, for C.I. Basic Violet 14 and C.I. Basic Blue 4 respectively in the presence of a maximum of 1% CN, while for C.I. Basic Red 18 the decrease is up to 55%. Then, on increasing the concentration of CN by a factor of approximately 8, the decrease for C.I. Basic Blue 4 changes from 10% to 59%. This effect could be explained by the CN groups acting as excited state quenchers and deactivating the excited dye molecules. However, the very small energies of the lowest excited states of the dyes 113 compared to the relatively high energy levels of any of the groups in the polymer, including the cyano group, make energy transfer highly unlikely. Alternatively, the cyano groups could reduce dye fading by combining with the excited dye to form an excited complex, or exiplex. Such a complex, which could involve one or more cyano groups, could have more numerous or more rapid photophysical deactivation processes than the excited dye molecule alone. This would lead to more rapid deactivation of the dye from the excited state and thereby reduce the probability of fading. Alternatively the structure of the exiplex may in some way inhibit the normal fading reactions of the dye. 114 CHAPTER 5 CONCLUSION Published data reveals the high lightfastness of basic dyes in acrylics compared with other substrates. The three dyes studied here are typical, with quoted lightfastness of between 4 and 7 on acrylics and approximately 1 to 3 on other fibres. This information has been quantified by the determination of the initial quantum yields of the dyes' fading in Acrilan 16, nylon 6.6 and basic-dyeable nylon fibres, and differences of at least two orders of magnitude between the quantum yields of the dyes in acrylic and the non-acrylic fibres were detected. A definite concentration dependence of quantum yield was also observed, with the quantum yields decreasing with increasing dye concentration. This is the trend that has generally been observed but the usual explanation of the phenomenon as being due to increasing dye aggregation with increasing concentration is unlikely to apply here because of the very low dye concentrations used. The possibility that the large difference in fading rates between acrylic and non-acrylic substrates could be due to differences in the physical structure of the polymer substrates was examined by determining the effect of temperature on fading rates. A definite temperature dependence of quantum yield was observed and many dye/film systems displayed an inflection in their Arrhenius plots 115 which was postulated to correspond to a physical transition in the polymer equivalent to its glass transition. The presence of these inflection points indicates that the physical structure of the polymer has a significant effect on dye fading, the more rigid the structure, the lower the rate of fading. Thus the greater inherent rigidity of acrylics compared with polyamides would contribute to the improved lightfastness. Furthermore, the fact that in most cases the inflection points found for the acrylic substrate were above ambient temperature while those for the other substrates were at or below ambient temperature suggests that fading occurs with the acrylic in its most rigid form, while the other substrates are in a more flexible state. This effect is probably even more pronounced in practice as the glass transition temperatures of polymers such as nylon will be lowered by the presence of moisture much more than that of acrylic polymers. This is demonstrated by the fact that the T of Acrilan is lowered from 90°c dry to 57°c wet 119 while g the T of nylon 6 is 94°c dry but -6°c at 97% R.H. 108 g Since a purely photochemical reaction has essentially zero activation energy, then activation energies associated with photochemical reactions must be due to some diffusional or conformational process involved in the degradation reaction. Hence the high rigidity of acrylics is likely to affect dye fading by restricting these motions. The activation energies determined ranged from 116 approximately 1 to 10 kJmol- 1 below the inflection temperatures and 15 to 66 kJmol- 1 above them. These values are of the same order of magnitude as values reported for diffusion processes in polymers. For example, small dye molecules such as vat and direct dyes, and azoic coupling components have been reported to have activation energies of diffusion, relating to the dyeing process, in the range 42 to 59 kJmol- 1 in the relatively porous polymer viscose rayon. 120 On the other hand the larger basic dyes in the relatively non-porous Orlon 42 have much higher activation energies, reported as 250 kJmol- 1 • 121 It would appear from this that any rate-determining diffusional processes involved in basic dye fading are unlikely to involve whole dye molecules. Pace 122 related data for the diameters (d) of simple penetrant molecules to their activation energies of diffusion (Ea), and found that, in the case of polyethylene, the very small helium atom (d = 0.22nm) had an Ed of approximately 20 kJmol- 1 • The relatively high activation energies for such a small particle would seem to indicate that activation energies below 20 kJmol- 1 are probably unlikely to be due to diffusional processes, except in the case of even smaller particles such as electrons, and are more likely to be due to conformational processes. This effect of substrate rigidity and the explanation for it have frequently been quoted in studies into photo degradation of polymers, 7 5 ' 95 but has not previously been linked with dye fading studies. 117 The low permeability of acrylics to oxygen and moisture has been postulated by some 61 ' 97 to contribute to the high lightfastness of basic dyes present, since some fading reactions of dyes have been found to involve these species. However, as reported in Chapter 3, moisture and oxygen were found to have little effect on basic dye fading and furthermore, where an effect was observed, their presence generally reduced rather than increased dye fading. Some workers 42 ' 99.have observed that the nature of the dye site in the polymer affects basic dye fading. In particular, sulphonate sites appear to give rise to slower fading for dyes adsorbed onto them than the other typical dye site, viz., carboxylate. However, in these studies no difference was detected between the two different sites providing their pK 's were the same. On the other hand, a the pH of the polymer was found to have a marked effect, with an acidic environment reducing fading and a basic environment generally increasing fading. It is quite probable that the different dye site/polymer combinations give rise to quite different pH's and that the nature of the dye site will therefore by virtue of its pK affect a basic dye fading. This pH effect on dye fading rates suggests that electron transfer from substrate to dye could contribute to the dye degradation. The reduction in fading of dyes in poly(vinyl alcohol) in the presence of an electron scavenger (cadmium ions) also supports this suggestion. Thus the reduced fading in acrylics might be attributed in part to them being poor electron donors compared with the other substrates studied. 118 A major factor influencing basic dye fading was found to be the quenching action of the cyano group which constitutes approximately 50% by weight of an acrylic polymer. Although experimental difficulties limited the amount of information which could be obtained, the results did show that approximately 1% concentrations of cyano groups in an inert substrate produced a significant reduction in fading rate with the effect becoming greater with a cyano concen tration of approximately 8%. This implies that the high cyano group concentration in acrylics would result in quite a significant effect on dye fading. The mechanism by which the cyano group acts to reduce fading is unlikely to involve energy transfer because of the energy levels involved. Enhanced electron transfer from substrate to dye or vice versa is also unlikely since this action would result in an increase in fading rate. It has therefore been postulated that an excited complex is formed between the excited dye molecule and one or more cyano groups which may facilitate rapid deactivation of the dye or alternatively the structure of the complex may inhibit the normal fading reactions of the dye. In summary, it can be seen from the above that several independent properties contribute to the high lightfastness of basic dyes on acrylic fibres. The physical nature of the acrylic polymers, by virtue of their high rigidity plays an important role. In addition, the low pK a of their dye sites, and their poor electron donating properties may also contribute. However, the major influence 119 would appear to be the high cyano group content of the polymer. 120 REFERENCES 1. Giles,C.H., and McKay,R.B., Text.Res.J., 31, 527 (1963). 2. 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