LEhllN cYS RATTEE-“LEVELLING PROPBIRTIES OF ACID DYEY-I” 217

HUDDERSFIELD SECTION Meeting held a t Field’s Caf6, Huddersfield, on 19th October 1948, Mr. D. HANSON in the chair

Some Observations on the Levelling Properties of Acid Dyes I-Level-dyeing Acid Dyes

D. R. LEMIN and I. D. RATTEE The rates of dyeuig of n large number of level-dyeing acid dyes, measured under specific con&tions,

tend to decrease with increasing basicity of the dye. The reverse is found to be the case with regard to the relative boiling-off properties of acid dyes. With regard to the production of level dyeings, the Jatter property is of greater importance in view of the general rapidity of uptake of the level-dyeing arid dyes by wool.

The effect of pH on the exhaustion of a number of acid dyes has been examined, and the pH-sxhaustion curves have been found to follow the shape of the wid titretion curves of wool. The sensitivity of the dye exhaustion to pH has been found to increase with the basicity of the dye. A study of the effect of dye remaining in the dyebath after dyeing on the rate of levelling has been made with Naphthalene Red JS and Naphthalene Scarlet 4RS, and a hyperbolic relationship has been found to exist between the time of half-dyeing and the concentration of dye anions in the dy&ath. It is shown that good levelling a n be promoted o e at the expense of exhaustion, and that the safest way in which this can be obtained ie by nn increase in anion concentration by addition of Ulauber’s salt or a similar source of anions rather than a deerwe in the amount of acid, thereby raising the dyebath pH.

INTRODUOTION employed, together with sulphate ions from the During the past twenty years our knowledge of Glauber’s salt, and finally the dye miens. These

the dyeing of wool has been vastly increased as a diffuse towards the fibre and inside it, and approach result of the elucidation of the structure of the Closely as Possible to the positively charged fibre and an increasing understanding of the amino @’OUpS. h actual faat it is knownL that the mechanism of the dyeing of protein fibres. more mobile ions, such as sulphate, W u s e into the

The fundamental basis of the theory of wool- fibre first and are fOllowed by the dye ions O d Y a t dyeing depends upon the structure of the wool a later Period. When both ions are present in the fibre itself. It is well established that wool sub. fibre there is a competition between the dye ions stance consists of a protein made up from long and other anions for the available amino groups chains of amino acids linked together by condensa- Present in the fibre, and the final amount of dye tion of the acid and amino groups of different adsorbed by the fibre depends upon the Pro ortiom

these main chains, branches extend carrying a t relative affinities for the wool. their ends amino and carboxyl groups together The problem of obtaining a level dyeing resolves with a lesser number of other radicals such as itself into obtaining a uniform shade on all the hydroxyl groups. different types of wool fibre which may be encoun-

Adjacent peptide chains are held together by tered in any particular sample. Theoretically this several kinds of forces. In the first case, there is may be produced in two ways- the first is. to the general force of cohesion, or van der Wads apply the dye level, and the second is to rely upon force, which is operative between any long thin subsequent redistribution of the dye to cover up molecules. In addition to this there is another any initial differences. This latter process may be attractive force, known as the salt linkage, which referred to as migration, and consists in the transfer exists between the free amino groups and the of dye from heavily dyed portions to lighter areas carboxyl groups a t the ends of the side-chains. .or fibres. Finally, there is a third type of bonding between The possibility of achieving success by either of adjacent chains resulting from the presence of the these methods depends upon the fundamental amino acid cysthe. This is a double amino acid assumption that the a e i t y (using the term in its which can form B part of two separate peptide correct thermodynamic sense) of all wool fibres for chains and thus link two adjacent chains by means any particular dye is the same. There is some of a disulphide bond. This bonding is of con- justification for this assumption. If different fibres siderable importance in determining many of the have differing a&ities, then migration will lead to properties of wool as distinct from other fibres. differences between fibres with increasing time.

From the point of view of dyeing, however, the This does not appear to be the case, a t least so far most important linkage is the salt linkage. The as level-dyeing acid dyes are concerned. Thus, if widely accepted view of the dyeing process is that, h o Geranine 2GS is dyed on wool at a low tempera- when a neutral wool fibre is placed in a dyebath con- ture, a skittery result is obtained, but on treating taining acid, the hydrogen ions which are present this in a blank dyebath a t 95’0. migration occur8 in solution diffuse into the fibre and combine with and the material bedomes level. the carboxyl groups in the wool, thus neutrrtlising In the ca8e of level-dyeing acid dyes, it seems their charges. In ord‘er to maintain electriaal clear that the differences in the material which give neutrality, an equivalent number of dyebath rise to initial unlevelness must consist of differences anions must be simultaneously adsorbed. These in accessibility. Some fibres, or areas of fabric, must will usually comprise the anions of the acid be more swollen, or more easily accessible in some

molecules into the so-called peptide chains. From of the VarioUS iom Present, together wi Y h their

318 LEMIN & RATTEE--"LEVELLINQ PROPERTUS OF ACID DYES-^^ Ma# 1949

other way, than the remainder of the material, and such portions will therefore absorb dye more readily, leading to the production of a heavier dyeing in the initial stages. The dye-combining capacity of such portions, however, must be almost the same as that of the rest -of the makerial, since the level-dyeing acid dyes redistribute themselves after the initial uneven absorption until all the fibres contain almost the same proportion of dye molecules to amino goups, provided that sufficient time be given.

It appears that the migration of dye is of much greater importance than uniform application of dye in the production of level dyeings, since th9se dyes which give tho most level resulta (i.e. level- dyeing acid dyes) appear to depend for their solidity and penetration largely on migration.

Much work has been carried out in the past on the levelling properties of acid dyes, and the object of the present paper is to add further t o the available information and to correlate boiling off with other properties such as dyebath exhaustion, salt concentration, etc.

EXPERIMENTAL

As a preliminary step, the rate of dyeing of a large number of level-dyeing acid dyes was determined by the following method- 7.5 g. of wool flannel wa0 cut into 0.5-g. pieces and thoroughly wetted out in hot water. A dyebath was prepared con- taining-

IOU/, ... Ghuber's salt (calcined) based on a fibre weight of 8 g. with a liquor ratio of 50 : 1. 25 C.C. of this solution was removed and kept as a standard. The 7.6 g. of wool waa then entered into the cold dyebath, and the temperature raised by 10"~. over 5 min., when 25 C.C. of liquor and 0.6 g. of material were. removed. This process was repeated until the solution had reached the boil and was continued for a further 10 min. The exhaustion at each 10'0. interval was measured by a spectrophotometric comparison of samples of

T A B L ~ I Rate of Dyeing of LeJel-dyeing Acid Dye.

1% ... Dye 3% ... Yulphuric arid

Cdour Time of boy Dye Indez Exhaustio; Kt Baeiolty

No. (min.) Napihhnlcne Orange GS ... Liwsaniine Grcen UNS ... Mctanll Yellow YKS ... Naalithalene Paet Orange ZQS Az6 Qrrnnino 268 ...- ... Solway Bluc UNY . . . . . . Cnrmoisiue BRS . . . . . . Solway Blue SES . . . . . . Tertraeine KB . . . . . . . . . Carmoiaine WS. . . . . . . . . Lissamlne Red 8BS . . . . . . Naphthalene Black 12BS ... Solway Celestol BS . . . . . . Lisemniue Fast Yellow 2C8 ... Solway Rubinol RS . . . . . . Naphtllaleue Scarlet BS ... Solway Pur le H.8 1,lasamiiie 8ast YeUow'AES::: Nnphtlialme Red EAS ... Naphthalene Red JS ....... Ui~ulphine Green BNS ... Curniuiaiiie LS ... .,. Naptithaleue Scarlet i,i . . , Lissamine Oreen VS . . . . . . Lissamine Green BS . . . . . . Disulpliinc Blue EQS... ... Naphtllltlene Grcen QS ... Lissamine Blue BFS . . . . . . Naphthalene Bordeanx BS ... Dlaulphine Blue AN8 . . . . . . DLsulghlne Blue FBNS ... Dlaulghine Blue VNS . . . . . .

151 737 188 27 31

29 1053 087 179'

57 246 1076 039 1091 194 1073

1A2 170

1064

k

- 1 en 78 * 73 6 737 071 860 863 88

716 712

-

. , 1.4 1.6 1.9 2.0 2.3 2.7 2,7 2.9 2.0 8.0 3.8 3.4 3.6 8.8 8.9 4.0 4.8 6.0 6 3 6.6 6 6 60 6.1 0.4

-8.1 8 7 0 1 9 4 101 18.4 160 21.4

1 1 1 2 2 2 2 1 1 2 2 a 1 2 1 2 1 1 2 1 2 2 2 2 2 3 2

1 2 2

a a

90

80

70

8 60 c'

50 3 5

30

20

10

o[ , , 20 30 40 50 60 70 B O ' 90 100 100 0

Temperature, *C.

Time, min. 0 5 10 I5 20 25 30 35 40 45 50

X Naphthalene Fast Oranis ZGS A Niphthaicne Black 126s 0 Liuamine Green VS 0 Lirramlne Blue 8FS

FIG. 1

liquor taken at each temperature against the standard sample of liquor removed before the start of the dyeing operation.

In order to compare the results obtained, since h a 1 equilibrium was not attained, the time taken to reach 50% exhaustion of the bath was estimated. Selected results are given in Table I, and typical rate-of-dyeing curves in Fig. 1.

The results show that a large number of dyes ere absorbed very rapidly, the dyebath being appreciably exhausted by the time the temperature has reached 40%. (10 min. dyeing time). There is a general tendency for the rate of absorption to be slowed down with increase in basicity of the dye, as shown in Fig. 2, in which the results for sixty dyes were included. It is also of interest to note that the triphenylmethane dyes have a much lower rate of absorption than the azo or anthraquinone dyes studied.

0 I 2 3 4 5 6 7 8 9 10 Ii I2 Number ofeach Group of Five Dyer arranged In order of Rate of Dydw

- Mworul honates - - I - PolyuipRonatw

FIQ. 2.-Eff~t Of B d d t y On aSb d Dyw

a10 &fay 1049 LEMIN & RATTEE-“LEVELLING PROPERTfE8 OF’ ACID DYES-I”

EFFECT OF PH ON EXHAUSTION OF LEVEL-DYEmG ACID DYES

It is well known that the exhaustion of level- dyeing acid dyes is affected by raising the p H of the s‘olutmn, and it was decided to estimate the effect quantitatively in an attempt to correlate it with the effect of p H on levelling properties.

Conditioned wool flannel (4g.) was dyed a t a 60 : 1 liquor ratio with 06% of (lye, 10% of calcined Glauber’s salt (A.R. quality), and varying amounts of sulphuric acid at the boil under reflux until equilibrium was attained. At the end of the dyeing, the wool was removed, and the exhaust liquor filtered through a sintered glass filter funnel (pore size G4) and allowed to cool. The exhaust liquor was filtered in order to remove small wool fibres which had broken off from the main wool piece. These wool fibres, although constituting a very small weight, were able to absorb sufficient dye in the cooling bath to cause serious errors,

especially in the well exhausted baths. The exhaustion WM measured by comparing the diluted exhaust liquor against an unexhausted dyebath in the Spekker photoelectric absorptiometer, all solutions being made up to contain 26% of pyridine. The pH of the exhaust liquor was measured by means of a glass electrode and Cambridge pH-meter. The results obtained with a number of aao, anthraquinone, and triphenyl- methane dyes are given in Table I1 and Fig. 3.

These results indicate that the pH-exhaustion curves of the level-dyeing acid dyes have the same sigmoid form as the titration curves of wool with different acidsa. The pH sensitivity of the dyes is related directly to their basicity, and they may be classified in increasing order of sensitivity aa-

(1) Triarylmethane-type dyes (2) Monosulphonates (3) Disulphonates (4) Trisulphonates

Na hthalene zed 58

Dye left In

pH Bath (%)

6.68 55.5 6.91 52.7 5.51 33.2 4.26 7.4 3.88 9.82 3.04 1.2 2.83 0.73 2.67 0.50 260 0.98 2.40 0.18

Lissnmine Fast

Yellow AES Dye

left in pH Bath

(%)

13.40 50.0 6.00 32.0 4.88 213.4

6.66 62.3

4.28 18.7 3.85 17.2 3.54 11.6 3.16 7.8 244 10.9 2.78 11.7

Solway Rubinol

RS Dye

left in p H h t l l

(%) 6.80 62.3 5.33 23.0 4.00 12.0 3.36 9.1 3.00 9.2 2.70 9.3 2.36 8.3 2.06 8.3 1.96 7.8 1.71 8.75

TaBJg 11 Effect of pH on Dyebath Exhaustion

(0.5% dye at 50:l liquor ratio) Solway Solway Naphthalene Ltssamhe Celestol Blue BNS Fast Faat BY Orange 209 Yellow 2GS

left Dye In G K n %n 1%n pH Bath pH Bath pH Bath pH ,Bath

(%) (%) (%) ( %) 6.49 454 600 734 6.25 35.8 5.16 61.8 5.56 25.6 4.34 59.8 4.32 14.4 3.70 25.85 3.94 12.9 3.34 16.23 3.54 9.96 2.98 12.76 3.28 9.50 2.78 9.38 2.93 9.20 2.52 10.00 2.74 7.60 2.23 8.90 249 7.20 2.12 8.17

0 1 1 3 4 5 6 7 PH

X Dirulphlne Blue FFNS A Liuamine Fast Yellow ZGS 0 Naphthalene Red EAS U Naphthalene Scarlet 4RS A Naphthalene Red JS 0 Solway Rubinol RS

Solway Celestol 85

FIG. a

8.10 89.0 678 68.0 444 50.0 4.16 35.7 8 4 0 38.0 3.24 20.4 2.96 163 2.68 15.7 2.37 144 206 11.0

8.94 96.1 8.34 80.8 6.82 76.4 4.65 614 4.06 88.6 3.42 23.2 3.04 18.0 2.78 16.06 2.40 124 2.12 124

1

Naphthalene Red EAS

pH l%l Bath

(%) 6.78 83.6 6-86 55.6 4.71 266

8.37 4.43 2-86 273 2.71 2.12 2.30 2.01 2.14 1.60 1.98 1.38

a m 7.12

Naphthalene Scarlet 4RS

l 2 K n g H Bath

(%) 658 73.7 6.44 62.0 5.62 223 446 11.0 4.03 6.7 3.39 3.5 3.12 2 3 2.86 1.7 2.65 1.8 2.01 0.6

Dbulphlne Blue

FFNS Dye

left in pH Bath

(%) 6.74 047 6.39 92.3 6.81 85.4 4.74 78.2 4.16 72.3 3.62 67.3 3.22 61.7 272 68.1 2.41 665 206 47.9

Dbulphlne Blue AN8

I3h pH Bath

(%) 662 92.0 6.30 88.6 6.00 86.0 5.47 75.6 4.40 88.0 4.00 68.6 3.28 48.3 2434 48.6 242 48.6 1-82 44.1

It will be seen that a t the normal acid concen- tration used with level-dyeing acid dyes (final p H approx. 2.9) the exhaustion is relatively insensitive to changes in pH. With lower acid concentrations, the exhaustion becomes much more dependent on the pH and small changes can cause big variations in the final dyebath exhaustion. With all the level-dyeing acid dyes except those of the triphenyl- methane class, the exhaustion a t p H 2.90 is 90% or better, and the dyebath is-exhausted to the extent of at least 60% by the time the temperature has reached 40°u., i.e. in the h t 10 min. of dyeing. It would thus appear that the dominating factor in the production of level dyeings is the m e with which the dyes can level out on boiling. Attention was therefore turned to measuring this and to finding the relation between boiling off, p H , and exhaustion.

MEASUREbfENT OF BOILING-OFF PROPERTIES

Dyeings were prepared on log . of wool flannel of a 0.6% shade of Naphthalene Red JS in the presence of 10% of calcined Glauber’s salt in a 60 : 1 liquor containing differing amounts of sulphuric acid. Dyeing was carried out at the boil for 4 hr. under reflux, after which the material was removed, rinsed, dried, and conditioned for 24 hr. At the same time, “blank” dyeings were prepared under identical conditions. The exhaust baths were

”20 LEMIN &L RATTEE-“LEVELLING PROPERTIES OF ACID DY ES-1” Mau 1949

filtered through sintered glass and allowed to cool, and pH determinations were made in order to verify that dyebaths and sihilar blank baths were at! the Name pH. The exhaustion of the dyebaths was measured absorptiometricolly.

From the exhaust liqnors, six boiling-off baths were prepared in the’ following way- 50 C.C. of exhaust dyebath and 50 C.C. of exhaust blank bath were mixed and raised to the boil under reflux, and 1 g. each of dyed wool arid “blank-dyed” wool were entered into this batch antl boiled for a known period of time. The pieces were then removed antl rinsed, and thc. dye wi~s extracted with 25%, pyridinr. From the extracts the amount of dye tranNferretl from the origiiitllly dyed to t,hc originally undyed piece wits estimated, and from the results thc time taken for 25qo of the dye to be t ransfwretl to the originally undyed material (time of half boiliiig-off to .6) was estimated. Experiments uere carried out in order to determine to.b over a wide range of tlyebath exhaustions for Naphthalene Red ,JS. Experiments were carried out in duplicatt..

A siniilar series of experiments was carried out with Naphthnlene Scarlet 4R8. With this dye,

‘ ~ ~ U L E III Bolllng-olT of Naphthalene Red IS

I ) ) r rmir\iuiiin in Bnth T h o of Half Boiiiug~off ( Y o ) (mln.)

c ;i 10 I 8 8

260’ 2 4 0 2,20 6.30

16.7 10.6 1 0 4 8.0 6.3 4.2 2 3 0.8

TABLE I V Boillng-off of Naphthalene Scarlet 4RS

1)yc remnining in Bath Tiiiic of Half IMlinpoff (%) (tnln.) 3.7’2 8.76 3.66 3.77 4.87

28.8 21.2 10.2 17.9 11 4 .. - 9.1 6.1 7.1 8.4

11.4 12.0

0 10 20 30 40 50 60 70 80 Dye remaining In the Bath, :(,

0 Naphthalene Scarlet 4RS X Naphthalane Rod JS FIQ. 4

bp

E l - */-

0 0.1 0.2 0.3 0’4 .. (Dye remaining In Bath, ? , , ) - I

0 Naphthalene Scarlet 4RS X Naphthalene Red JS

PIG. 5

estimation of the dye on the wool was not possible by pyridine extraction, and the wool was dissolved in caustic soda and diluted to it final pyridine concentration of 25%, before a bsorptiometric es timat ion.

The results obtained are given in Tables I11 and IVY with the corresponding graphs in Fig. 4 and 5 .

The results of this work support the view previously expressed that boiling off iR a reflection of the lack of fastness of the dye to the tlyebath conditionsa. With the two dyes examined it is apparent that a hyperbolic relutionship exists between the time of half boiling-off and the con- centration R of dye in the dye liquor. In the case of Naphthalene Scarlet 4RS the position is slightly more complicated, due to the minimum value of to.s when R is approximately 10%. On both sides of this minimum the hyperbolic relation holds, but in opposite directions. When R is less than loo/, the nature of the relationship between to.6 and R is exactly the same with Nitphthitlene Scarlet 4R8 as with Naphthalene Red JS. The reversal of the effect may occur with Naphthalene Ited JS a t the higher values of R which have not been studied.

The evidence supports un cxtremrly simple view of the mechanism of the boiling-off process from the standpoint of the Gilbert concept4 of eii(.h anion occupying i t separate site on the fibre.

If we consider boiling-off itt valurn of R less than that required to give a minimum vctlue of a t tiny onc site wit,li :L dye anion iattadied to it, the probability that the anion will be displaced by i~ sulphate union is proportionitl to the number of sulphate anions in the bnth. Sirnilwly the probability of a given sulphate anion being dis- placed by a dye anion is proportional to the number of dye anions in the bath.

At a given pH a t equilibrium, the relative numbers of dye and sulphate anions on the fibre and in the bath are constant. Therefore, as a dye anion iq displaced from one part of the fibre into the liquor another dye anion must go on to another part of the fibre to maintain the equilibrium. Thus the tendency for dye anions to be transferred from u dyed part of the fibre to an undyed part of thc fibre is measured by either the sulphitte or the dye anion concentration in the liquor. This explains the already known fact that Glauber’s salt promotes levelling, and shows that the single operative factor

221 , hiau 1949 LEMIN & RATTEE-”LEVELLING PROPERTIES OF ACID DYES-11”

is the anion concentration. The tendency referreii to above is directly related to the boiling-off properties and is characterised by t .6. Thus it is to be expected that the time of halfioiling-off will be inversely proportional to the amount of the dye anion in the bath,

The nature of the process after the minimum value of to.s has been reached is more complex. A possible explanation of this effect is that there is interference between anions in the liquor above a characteristic anion concentration. Here the rab of displacement a t a given site becomes so great that the effective replacement of a sulphate by a dye anion or vice versa becomes less frequent. With a trisulphonated dye this state obtains a t a relatively low anion concentration, due to the fact that two dye anions will displace three sulphate anions, a far more complex reaction than occurs with disulphonated or monosulphonated dyes, and far more sensitive to interfering factors. The interference is directly related to anion concentra- tion, and has a retarding effect on the rate of boilingoff directly related to $. Thus the inter- ference does not affect the basic relationship between tOq6 and R, but in some cases will reduce their mutual sensitivity and in others cause a

reversal of their mutual effect, w is experienced with Naphthalene Scarlet 4RS.

CONCLUSIONS It is a p p m n t from the results that in order to

promote levelnms it is neceseary to sacrifice exhaustion to some extent. The rate of dyeing of the level-dyeing acid dyes is too rapid to allow of controlled temperature manipulation to give initial level dyeing, and levelness can be obtained only during the subsequent boiling. This can be pro- moted in two ways-(a) by raising the dyebath pH, (a) by addition of agents, such as Glauber’s salt, which decrease the exhaustion by competitiop with the dye anions. The first method would appear dangerous, since with most level-dyeing acid dyes the pH sensitivity is so great a t higher p H valuea that ditliculties may be encountered in reproducing results. The second method is safer and as reliable, and is to be recommended from all points of view. ,

Referencm Elod, -Tram. P~raday SOC., 1936,31, 305. Steinhardt andHarris, Bur. Stand. J. Rea., 1940,24, 336.

a Speakman and Clegg, this Journal, 1034, 50, 348. 4 Gilbert and Rideal, Proo. Roy. 8m., 1944, A182, 336;

Gilbert, ;bid., 1944, A183, 167.

WEST RIDING SECTION Meeting held at the Great Northern Victoria Hotel, Bradford, on 11th November 1948,

Dr. T. A. FORSTER in the chair

Some Observations on the Levelling Properties of Acid Dyes II-Prechromed Dyes D. R. LEMIN and I. D. RATTEE

The exhaustion of three aelected prechromed dyes at different dyebath pH values has been measured, anti the results show marked differences from those obtained with level-dyeing acid dyes under similar conditions. It is shown that the levelling properties of this type of dye increase with decrease in the dyebath pH. The boiling-off properties of the dyes examined have been shown to be better in the presence of sulphuric acid than hydrochloric acid at the Bame final dyebath pH.

The Ultralan dyes are chromium complexes of leke-forming azo dyes, and approach the fastness of chrome dyes while possessing the eaae of application of level-dyeing acid dyes. This type of dye is applied to wool from a bath containing 8% of sulphuric acid on the weight of wool, smaller amounts of acid leading to less level dyeings of inferior fastness. A prolonged boil is also essential for the proper development of the colour of the dyeing.

The theoretical aspects of dyeing with this type of dye have been studied by Ender and Mullerl, who pointed out that the behaviour of the moleculea will depend on whether they contain one or two sulphonic groups. The main claim of these aut,?mrs is that the free acids of the dyes are amphoteric in nature, the molecule existing &B a zwitterion. The complexes are represented as being similar to the salts of a strong acid and a weak baae. The experiments of these workers led them to the conclusion that salt formation takes place between the sulphonic acid groups in the dyes and the basic groups in the wool, and also that 10% of the basic groups in the wool are involved in a complex with

the chromium in the dye molecule. The developing action of mineral acid is attributed to a decrease in afinity between the wool and the dye, which retards the combination between the chrome complex of the dye and the basic groups in the wool by increaeing the ionisation of the latter. The development of the-true colour of the dye by 8% of sulphuric acid has been attributed by Forster2 to a change from a soluble to an insoluble form of the dye; this change does not occur if a smaller concentration of acid is present. In connection with this suggestion it should be noted that levelling of unevenly dyed pieces can be effected by boiling in a bath containing 8% of sulphuric acid on the weight of material, so that in this case an equilibrium is presumably set up between the soluble and insoluble forms of the dye.

With reference to the more practical aspects of the dyeing of wool with prechromed dyes, the literature shows a lack of published data on the exhaustion of these dyebaths a t different dyebath acidities. Similarly, no evidence is available on the levelling properties of these dyes aa compared with

A4