to the synthetic reflectance curves; the samples were considered visually satisfactory in relation to the original VDU screen colours.

CONCLUSIONS The subtractive method of producing synthetic reflectance curves has been shown to provide usable data for recipe prediction purposes. The curves are often rather undulating, but dyers rarely attempt to match the reflectance curve of a standard exactly, and in practice this has not proved a problem. A considerable number of commercial dyehouses have been faced with the task of matching synthetic reflectance data, and have coped admirably.

The research project was supported by grants from SERC ACME Division (GR/D 76545), Parkland Manufacturing plc and Pragma Ltd. Thanks are also due to my colleague Mr David Oulton for suggesting the term 'synthetic

reflectance curve' and for help in developing the Shademaster system, and to Dr J Scotney of Stevensons Dyers for permission to quote the figures in Table 6.

REFERENCES 1. C J Hawkyard and C Wilkinson, I.S.D.C., 106 (1990) 356; 107 (1991)

83. 2. W N Sproson, Colour scierice in teleoisioti arid display systems (Bristol:

Adam Hilger, 1983). 3. G W Meyer, Displays Tech. Appl., 10 (1989) 161. 4. A Mukherjee and Y V Venkatesh, Cornp. Vision, 36 (1986) 114. 5. C J Hawkyard and D P Oulton,].S.D.C., 107 (1991) 309. 6. G Wyszecki and W S Stiles, Color scierice (New York: Wiley, 1967)

184. 7. N Ohta and G Wyszecki, Color Res. Appl., 2 (1977) 183. 8. F J M Schmitt, Color 77 (Bristol: Adam Hilger, 1978) 436. 9. W Cornelius, Melliarid Textilber., 71 (1990) EX, 48.

10. E E Allen,]. Opt. Soc. Amer., 56 (1966) 1256. 11. G Tonnquist and L Heng, Displays Ech. Appl., 10 (1989) 171. 12. C J Hawkyard, PhD thesis, UMIST (1989). 13. M D H Chowdhury, MSc dissertation, UMIST (1988). 14. F W Clulow, Colour: its principles arid tkeir applicatioris (London:

Fountain Press, 1972) 171. 15. K Patterson, Undergraduate project, UMlST (1988). 16. C J Hawkyard, Rev. Prog. Coloration, 21 (1991) 43. 17. C J Hawkyard and D P Oulton, Syazio T'ssilc, (6) (1991) 65.

Indigo dyeing of cotton denim yam: correlating theory with practice

J N Etters Textile Sciences, University of Georgza, Dawson Hall, Athens, GA 30602, USA

The previously reported correlations in indigo dyeing of cotton denim between distribution of indigo in the cross-section of denim yarn, colour yield and dyebath pH are reviewed. Statistical analysis reveals that even with the experimental uncertainty of the absolute pK, values associated with the ionisation of the reduced form of indigo, distribution of indigo in the denim yarn cross-section and the resulting colour yield still are strongly correlated with dyebath pH.

INTRODUCTION During the mid 1970s indigo was in short supply. The present writer, who at that time was a research chemist in dyeing applications technology at Dan River Inc. in Danville, Virginia, was charged with the responsibility for developing new techniques for dyeing denim yarn with indigo that might result in the use of less dye to obtain a given depth of shade. He was aware of the technique of dyeing wool with indigo using soda ash to generate dyebath pH values that were lower and less protein destructive than those encountered in commercial dyeing of cotton denim yarn with caustic soda as the alkali [l]. In addition, the literature revealed work by Russian scientists, Golomb and Shalimova [2] and Mishchenko

and Artym [3], that respectively involved the investigation of dyeing protein fibres with indigo at low pH values and the effect of pH on the presence of hydrolysed and non- hydrolysed indigo. It was hoped that a series of dyeings, using various liquor ratios and concentrations of caustic soda and soda ash, might uncover novel colour yield effects.

Laboratory work eventually revealed that when about half of the sodium hydroxide in the indigo dyebath was replaced with an equal weight of sodium carbonate, the depth of shade for a given amount of fixed dye increased immensely. This purely empirical laboratory finding was applied to the company's commercial indigo dye range, resulting in the use of a significantly reduced con-

JSDC VOLUME 109 JULY/AUGUST 1993 251

centration of indigo in the dyebath. Indeed, the concentration of dye needed to produce the standard depth of shade was so very low that a visiting representative of a major supplier of indigo was quite surprised. The representative tested the character of the indigo dyebath by sticking his finger into the weakly coloured solution, and commented, upon removing his slightly discoloured digit, that he did not know what indigo dye ’extender’ was being used, but that he had never before seen such an indigo dyebath in his many years of experience in the industry. The replacement of half of the caustic soda in the dyebath with soda ash resulted in such an increased colour yield that a savings in dyestuff cost of over US $500 000 per year was reported by management! However, even though the use of the new dyeing technique resulted in a greatly reduced dyeing cost, the new system produced some strange effects.

It was noticed that dyebaths produced using the new soda ashlcaustic soda systems resulted in an indigo dye solution that was less clear or more ’muddy’ than those produced using the customary sodium hydroxide. In addition, it was observed that the polyester ‘lease lines’, which were laced into the cotton denim yarn cables to permit more easy opening of the cables after dyeing, were more heavily stained a medium blue shade than were the lease lines present in denim yarn dyed conventionally. Furthermore, periodic, routine measurement of dyebath pH revealed that the pH was significantly lower but more variable than that obtained when sodium hydroxide was the only alkali present in the bath.

Although a tremendous savings in dyestuff cost was realised by the use of the new alkali system, the technique appeared to produce a less stable dyeing environment than those based on higher concentrations of sodium hydroxide alone, and was believed to be the source of recurring yarn dyeing unlevelness, with attendant fabric seconds. The new system was never proven to be the source of the quality problems, but nevertheless was abandoned about one year after its introduction and was not pursued again either in production or in the laboratory, until recently.

DISCUSSION The initial industrial urgency to find an empirical dyeing method that would neutralise the effect of the short supply of indigo pre-empted scientific study of the physico-chemical relationships involved. It was enough that, during its brief 1970s use, the new technique worked and served a useful purpose. The reason why the new technique worked was never explored. However, after the writer joined the textile science faculty of the University of Georgia in 1987, the issue of ’why’ was at last addressed.

Soon after joining UGA, the writer was appointed research committee chairman of the Southeastern Section of the American Association of Textile Chemists and Colorists (AATCC). In his new chairman’s role he sought novel areas of research that might prove to be useful to the

section in its pursuit of an award-winning entry for the Intersectional Technical Paper competition. Based on his experience at Dan River Inc. and on the previous research [2,3], he believed that a comprehensive investigation of the interrelationship between dyebath pH, dye content of denim yarn and resulting colour yield could result in a significant contribution to the understanding of indigo dyeing. The Southeastern Section’s research project did in fact prove to be quite valuable; the work not only resulted in a first place win in the competition [4], but also continues to serve as a basis for further investigation into indigo dyeing of cotton denim yarn. For example, at the University of Georgia there is an ongoing, industry- supported project that is designed to expand the research of the Southeastern Section in the area of indigo dyeing.

In the following sections the essential relationships initially uncovered by the Southeastern Section of AATCC and now verified by others will be reviewed 151. In addition, new statistical evidence in support of the relationships will be revealed for the first time.

Colour yield Depth of shade at a given wavelength can be expressed in terms of the well known Kubelka-Munk relationship [6], subsequently modified for practical application [7] (Eqn 1):

where K I S is the ratio of light sorption ( K ) to light scattering (S), Rc is the fractional reflectance of light from a textile substrate containing a given dye concentration, R, is the light reflectance from a blank-dyed substrate, and R, is the reflectance from a substrate containing an ‘infinite’ concentration of dye, i.e. reflectance that is insensitive to further increases in dye concentration [%lo]. The quantity KIS is directly proportional to the dye content in the textile substrate and is therefore a useful measure of colour depth. The proportionality is usually expressed by Eqn 2

where C is the concentration of dye in the substrate, and a is the proportionality constant, often referred to as the reflectance absorptivity coefficient.

When WS is plotted as a linear function of dye concentration, the slope of the resulting line is equal to an absorptivity coefficient. If the intercept is statistically equal to zero, the absorptivity coefficient also is given by Eqn 3:

a = - KIS C

( 3 )

Since the absorptivity coefficient is equal to the value of KIS that is obtained for unit Concentration of dye in the substrate, the absorptivity coefficient is a measure of the ‘colour yield’ that is obtained for a given system.

252 JSDC VOLUME 109 JULY/ALJGUST 1993

Uniformity of dye distribution The initial experimentation of the Southeastern Section's research committee revealed that the apparent absorp- tivity coefficient of indigo on denim yarn is a function of dyebath pH, as illustrated in Figure 1 for a five-dip indigo dyeing process.

The absorptivity coefficient is a measure of a dye's power to absorb light and is a constant quantity for a given dye-fibre system. A true value of a dye's absorptivity coefficient can be determined only on a textile substrate in which the dye is uniformly distributed. The absorptivity coefficients represented by the slopes of the three lines in Figure 1 are merely apparent values that are the result of various degrees of ring dyeing of the denim yarn [4]. Photographic evidence that illustrates the pH-dependent ring dyeing effect is given elsewhere [5].

Although the slopes of the lines given in Figure 1 reveal that colour yield is much greater for a dyeing conducted at pH 11 than it is for a dyeing conducted at pH 13, a more detailed picture is given in Figure 2. Although there is a wide scatter of data points in Figure 2, colour yield still is revealed to be at a maximum within the pH range of about 10.5-11.5 and decreases as the dyebath pH is increased.

500

I 400

pH 11 .O / 300 -

s 200 -

0 0.5 1 1.5 2 2.5 Fixed indigo content, %

Figure 1 WS as a function of indigo dye content of denim yarn dyed at different dyebath pH values

O 0 0 0

0 0

0 o o o 0

O O O

1 I I I I

10.5 11.5 12.5 13. Dyebath pH

Figure 2 Apparent reflectance absorptivity coefficients found for a five-dip indigo dyeing process, conducted over a wide pH range; relative distribution of dye in yarn cross-section represented by schematic drawings

For example, the colour yield at pH 11.0 is about three to four times that obtained at pH 13.0.

It can be shown from a simple geometrical analysis of the distribution of dye in a denim yarn cross-section that depth of shade, expressed as KIS, is given by Eqn 4 [ll]:

r 7

(4)

where a, is the true value of the reflectance absorptivity coefficient, i.e. the value for uniform distribution of dye in the yarn cross-section, C is the concentration of dye in the yarn, and p is the fractional penetration of fixed dye in the yarn cross-section. Eqn 4 will hold approximately unless a severe concentration gradient exists within the dyed ring or the colorant layer becomes translucent.

Source of pH-dependent ring dyeing As previously noted [12], ring dyeing of cotton yarn can be caused by a high strike rate of the dye for the cotton fibre in the yarn surface, i.e. the dye exhausts rapidly onto the fibres in the outer zones of the yarn at the expense of fibres in the yarn interior. Vickerstaff has observed that [13]: '... the diffusion rate cannot indicate the actual progress of dyeing in the initial stages, as this is determined by the affinity of the dye. If two dyes are present in a binary mixture, the dye which is first adsorbed by the fibre is that having the higher affinity, irrespective of their relative diffusion rates.'

It is known that the reduced form of indigo can undergo a two-step ionisation to produce two ionic species: mono-ionic and di-ionic; the relative amount of each species is governed by the pH of the dyebath. The poorly water-soluble nonionic or 'acid leuco' form of reduced indigo can be abbreviated as H21 where H is hydrogen and I represents indigo. The first ionisation step produces the more soluble mono-ionic form of indigo, HI-, and is given by Scheme 1.

Scheme 1 HpI H+ + HI-

The associated equilibrium ionisation constant, kl, is given by Eqn 5:

(5)

The second ionisation step produces the even more soluble di-ionic form of indigo, 12-, and is given by Scheme 2.

Scheme 2 HI- H+ + 12-

The associated equilibrium ionisation constant, k2, is given by Eqn 6:

JSDC VOLUME 109 JULY/AUGUST 1993 253

Even in view of the uncertainty with regard to the thermodynamic activity of the relatively concentrated dye in solution, it is possible to estimate, at least roughly, the fraction of reduced indigo that exists as H21, HI- or Iz as a function of dyebath pH. Such estimations employ computations that make use of the negative logarithm of the equilibrium ionisation constants kl and k2. These logarithmic terms are referred to as pK, values. Although accurate experimental values of pK1 and pK2 are quite difficult to estimate for reduced indigo, the literature data for pK1 and pK2 found for the tetra-, tri-, di-, and mono- sulphonic acid forms of indigo can be used [14]. When these data are extrapolated to the zero sulphonic acid form, i.e. conventional reduced indigo, reasonable estimates for pK1 and pK2 with 95% confidence limits can be made. For pK1 the mean value is 7.97 with 95% confidence limits of 7.19 and 8.74. For pK2 the mean value is 12.68 with 95% confidence limits of 12.23 and 13.08. By use of these statistically estimated values, the fraction of reduced indigo that exists as H21, HI- or 12- can be calculated.

c ._ 5 0.8 F

0.6

Mono-ionic form of reduced indigo Previous work has shown that it is the mono-ionic form of indigo that appears to be most closely associated with the colour yield obtained in indigo dyeing [4,5]. Also the results of recent equilibrium sorption studies reveal that both equilibrium sorption and the ratio of mono-ionic to di-ionic form of indigo are higher for that pH range at which the highest colour yield is obtained in indigo dyeing [12]. When the 95% confidence limits of the first and second pK, values, found by the extrapolation technique discussed above, are substituted into Eqn 7, it is possible to estimate the 95% confidence zone in which the pH-dependent fraction of the total amount of reduced indigo exists as the mono-ionic form:

'

.

(7) 1 Mono-ionic fraction = 1 +lO'PKI - p H ) + 1 0 ( P H - P K 2 )

0

D P c 0.4 - K g 0.2 E

L L

Figure 3 shows that above about pH 11.5 the fraction of indigo that exists as a mono-anion begins to drop off rather severely, and continues to decrease with increasing pH. The reason for this behaviour is that, as the pH increases, more and more of the reduced mono-ionic indigo ionises further to produce the more soluble di-ionic form. At a dyebath pH of 12.7 about half of the indigo exists as mono-ionic form and half as the di-ionic form, within the confidence limits shown in Figure 3.

'

'

Correlation of parameters When the colour yield data of Figure 2 are converted to a fractional scale, it is possible to plot them in a common graph along with the data of Figure 3; such a plot is given in Figure 4. This shows that 16 of the 24 colour yield data points fall within the 95% confidence zone for the distribution of the mono-ionic form of indigo. The

1.0

-

- 95% confidence limits

-

-

01 I I I 10.5 11.5 12.5 13.5

Dyebath pH

Figure 3 Confidence zone for the distribution of the fraction of reduced indigo that exists as a mono-anion as a function of dyebath PH

n l I I 1

10.5 11.5 12.5 13.5 Dyebath pH

Figure 4 Distribution of fractional colour yield and mono-ionic indigo as a function of dyebath pH

graphical evidence supports a strong correlation between colour yield and the fraction of indigo that exists as the mono-ionic form. The correlatim is also revealed from the results of formal linear correlation analysis.

When fractional colour yield is expressed as a linear function of the fraction of indigo that exists as a mono- anion over a common pH range, statistical analysis reveals a correlation coefficient of 0.95 and an R2 value, adjusted for the degrees of freedom, of 0.898. This statistical fact means that about 90% of the variability of the fractional colour yield is associated with or 'explained by' the variability of the fraction of indigo that exists in the mono- ionic form in the dyebath. If the effect of pH on the ionisation of cellulose were taken into account, the correlation probably would be even higher than that revealed in this analysis. The strength of the correlation is graphically depicted in Figure 5.

CONCLUDING REMARKS It is now clear why the primitive soda ashkaustic soda process, with which the writer experimented during the mid 1970s, produces some strange effects. The 'muddy' appearance of the dyebath was caused by the presence of more of the less soluble mono-ionic and even nonionic forms of indigo than are found at higher pH values. The

254 JSDC VOLUME 109 JLJLY/AUGUST 1993

1.01 - " 1

0.8 9

- 2 0.6

.- > L

0 m - .- 0.4 F

0.2

- LL

pH range 13 .51 0.5 - Adjusted R2 = 0.898

-

-

- 0

I I I

blue stain on the polyester lease lines was probably the result of the polyester fibre being dyed by the nonionic disperse dye form of indigo, i.e. acid leuco indigo, that was formed at the lower pH values. The 'disperse dye' was adsorbed onto the fibre surface and then diffused into the fibre as it was dried on the hot drying cans. (Perhaps other hydrophobic fibres can be dyed by the use of the acid leuco form of indigo.) The fact that the dyebath pH generated by the use of soda ashlcaustic soda mixtures exhibited a high level of variation was simply due to the presence of an insufficiently buffered system.

Research at the University of Georgia and elsewhere has resulted in the formulation of buffered alkalis that permit the level of ring dyeing obtained in the commercial indigo dyeing of cotton denim yarn to be closely controlled. Dyeing systems that make proper use of the

new buffered alkalis are so resistant to pH change that the instabilities associated with the primitive 'soda ash process' of the seventies no longer exist. It is now possible for the commercial indigo dyer to obtain a level of quality control over his product that never before existed. This fact is evidence that systematic analysis of commercial dyeing processes can lead to more than just intellectual satisfaction.

* * *

Appreciation is expressed to the Virkler Company for the continuing, generous support of research at the University of Georgia.

REFERENCES 1.

2.

3. 4. 5.

6. 7.

8. 9.

10. 11. 12. 13.

14.

The applicatioti of vat dyes (Research Triangle Park, NC: AATCC, 1953) 231. L M Golomb and G V Shalimova, Technol. Text. hid . , 48 (5) (1965) 105. A V Mishchenko and M I Artym, Text. hid. USSR, 54 (4) (1971) 95. Text. Chem. Colorist, 21 (12) (1989) 25. P A Annis, J N Etters and G L Baughman, Canad. Text. I . , 108 (5) (1991) 20. P Kubelka and F Munk, Z. Technische Physik, 12 (1931) 593. R G Kuehni, Computer colorunt formulation (Lexington, Mass., 1975) 11. R H Parkand E I Stearns, 1. Opt. SOC. Amer., 34 (1944) 112. J R Aspland, Text. Chem. Colorist, 5 (10) (1973) 34. J N Etters,Amer. DyestuffRep., 80 (4) (1991) 15. J N Etters, Amev. Dyestuff Rep., 81 (3) (1992) 17. J N Etters and M Hou, Text. Research I . , 61 (1991) 773. T Vickerstaff, The physical chemistry of dyeing, 2nd Edn (London: Oliver & Boyd, 1954) 154. Indicators, Ed. E Bishop (New York Pergamon Press, 1972) 481.

BOOK REVIEW

Polyester: 50 years of achievement, Ed. J W S Hearle and D Brunnschweiler (Manchester: Textile Institute, 1993). Price: f75 hardback, ISBN 1 870812 50 6; €45 paperback, ISBN 1 870812 49 2.

The hitherto almost complete lack of response to celebrating one of our greatest inventions has certainly been rectified by this Textile Institute publication.

Many leading names from all over the world have contributed to make this a remarkable collection of articles dealing with every facet of polyester, from its birth in the frugal laboratory of Broad Oak works to its present production of nine million tonnes per annum, the manufacturing problems of converting chemicals to the new speciality fibres, and the steady proliferation of its end uses. One marvels at the number of uses of polyester described this publication.

The early history was a revelation, as we learn that terephthalate polyesters were prepared at both Du Pont and IG Farben, but both failed to produce fibres. It took the genius of J Rex Whinfield, who resolutely believed that

the combination of terephthalic acid and ethylene glycol must produce a fibre, and the practical expertise of Dixon, to succeed. Everything one needs to know about polyester is in this book; we read about the remarkable history, production problems, research, applications, economics, fashion, advertising, etc. Although polyester is a success story, it is sad that we should be reminded that ICI has stopped manufacturing, and that the centre of gravity has now passed from the UK to the Far East.

The editing of a publication of this magnitude must have been a mammoth task, and there are bound to be errors. In my copy there was an addendum of 14. I noted that there are well over 50, spelling mistakes, errors between text and tables and in one case reference to a non-existent diagram.

However, this does not detract from the value of the publication, which is a mine of information with several beautiful colour pages dealing with fashion and the very many uses of polyester. It is an excellent publication which so admirably portrays the success story of polyester.

DEREK MOORHOUSE

JSDC VOLUME 109 JULY/AUGUST 1993 255