184 JSDC MAY 1972; BROOKS

References 1 Sheppard, Proc. Roy. Soc., A82 (1909) 256. 2 Beer, Ann. Physik. Chem., Series 2 86 (1852) 78. 3 Pfeiffer and Liebhafsky, J. Chem. Educ., 28 (1951) 123. 4 Liebhafsky and Pfeiffer, ibid., 30 (1953) 450. 5 Vierordt, in ‘Die Anwendung des Spektralapparates zur Photo-

metrie der Absorptionspectren und zur Quantitative Chemischen Analyse’ (Tubingen: H. Laupp, 1873).

6 Pugh, Giles and Duff, Trans. Faraday SOC., 67 (1971) 563. 7 Kortum, Z. Physik. Chem., B34 (1936) 255. 8 Kortum and Seiler, Angew. Chem., 52 (1939) 687. 9 Strong, Anal. Chem., 24 (1952) 338.

10 Brock, Anal. Chim. Acta, 27 (1962) 95. 11 Hardy and Young, J. Opt. SOC. Amer., 39 (1949) 265.

I2 West and Pearce, J. Physical Chem., 69 (1965) 1894. 13 Emerson, Conlin, Rosenoff, Norland, Rodriguez, Chin and Bird,

ibid., 71 (1967) 2396. 14 Mason, J.S.D.C., 84 (1968) 604. 15 McRae and Kasha, in ‘Physical Processes in Radiation Biology’,

Ed. by Augenstein, Mason and Rosenberg (New York: Academic Press, 1964), p. 23.

and Bovd. 2nd edn. 1954). Cham 3. 16 Vickerstaff, ‘The Physical Chemistry of Dyeing’ (London : Oliver

17 Coates, J.S.D.C., 85 (1969)’355. 18 Giles and Duff, ibid., 86 (1970) 405; cf. Campbell, Cathcart, Giles

19 Holmes and Standing, Trans. Faraday SOC., 41 (1945) 542, 568. 20 Alexander and Stacey, Proc. Roy. SOC., A212 (1952) 274. 27 Frank and Evans, J. Chem. Phys., 13 (1945) 507.

and Rahman, Trans. Faraday SOC., 55 (1959) 1631.

Levelness in the Dyeing of Wool Yarn and Fabric J. H. BROOKS

Division of Textile Physics, CSIRO, Ryde, N.S. W. 2112, Australia

In experiments with a thin bed of cloth, the maximum rate of uptake of dye by a wool yarn has been found to be limited by the concentration gradient of the dye across the yarn. A type of unlevelness observed in a conventional cross-wound package is attributed

to the same concentration gradient.

Introduction The economic and technical advantages of dyeing at the yarn or

cloth stages are well known. Perhaps the main disadvantage is the difficulty of achieving even distribution of dye throughout the material. Dyes of good fastness to washing, increasingly being used for machine-washable wool, in general do not level easily. Reactive dyes do not redistribute at all after reaction with the fibre. It is therefore necessary to ensure that the reactive dye is absorbed evenly before reaction occurs.

Shrink-resisted wools generally are more difficult to dye evenly by exhaustion methods because of their high rate of uptake of dye. The results presented here for fabric beds were obtained with wool shrink-resisted by the chlorine-Hercosett* process ( I ) , and the results for cross-wound yarn packages were obtained with wool that had not been shrink-resisted. However, the conditions of dyeing employed were not chosen to give a level uptake of dye but rather to illustrate (a) a mechanism that can limit the rate of uptake of dye by yarn, for which purpose the rapidly dyeing fibre was chosen, and (b) a related mechanism that can give rise to unlevelness in package dyeing and operates even with wool that has not been shrink-resisted.

Experimental Beds of fabric were made from 64s Merino wool, shrink-

resisted by the chlorine-Hercosett process ( I ) , spun into 2 x 30- tex yarn and knitted interlock with a stitch length of 0.58 cm. Each bed consisted of three fabric discs of diameter 2.85 cm. The thickness of a bed was 0 .6 cm and the weight 0.88 g. The fabric beds were dyed in a Praxitest Textile Testing Machine (Quarzlampen G.m.b.H.), modified to improve control and to enable the temperature and rate of flow to be measured accur- ately. Dye could be added automatically to keep the concentra- tion in solution at a predetermined value. The volume of liquor at the beginning was 470 ml, and increased to 490 ml during an experiment.

The yarn used for the wound packages was 2 x 30-tex from 64s Merino wool. Each package consisted of 600 g of yam, cross- wound on dye springs of diameter 50 or 62 mm. For dyeing, a package was compressed between flat impermeable end-plates of * Hercules Inc. ; a polyamide-epichlorohydrin resin

outside diameter equal to the outside diameter of the package. The mean density of a package in this situation was 0.34 g/cmY. Packages were dyed singly in a machine constructed in this laboratory; the liquor :wool ratio was 15 : 1.

The dye used in these experiments was Carbolan Blue BS ((2.1. Acid Blue 138). Recrystallisation of the dye from 90% ethanol showed the commercially available grade to have a purity of 590/,. The dye thus purified was used in the fabric-dyeing experiments, and the commercial product in package dyeing.

Dyed packages were cut in a lengthwise direction and opened out. Each package was then separated into sections so that samples of yarn could be taken for analysis from certain radial positions. A sample of yarn (0.2-0.4 g) was taken uniformly over the whole area for a radial position. The dye was extracted from the sample with a solution of 507: urea and 50,;, nonylphenol-poly(ethy1ene oxide) (30 mol. ethylene oxide) at 98°C and determined spectrophotometrically (2).

Results and Discussion FABRIC BEDS

Figure 1 shows the sorption of C.I. Acid Blue 138 by a fabric bed at rates of flow of 4, 8, 16 and 32 ml/s. Allowing for the volume of the fibre, these rates of flow correspond to mean veloci- ties in the bed of 0.8, 1.6, 3 .2 and 6.4 cm/s. The highest rate of flow used appears to be sufficient for the uptake of dye by the bed to be controlled by diffusion in the fibre, the uptake being initially proportional to the square root of time. At the lowest rate of flow used, the uptake was proportional to time for the first 3 min and was only one-third of the initial rate of uptake for the highest rate of flow. As a rate of flow of 0 . 8 cm/s is in the region that might be attained in package dyeing, it is of interest to consider what is controlling the rate of uptake of dye by the yarn.

Diffusion in the liquid can control the rate of transport of a solute to a surface. The thickness of the diffusional boundary layer around each fibre in the bed would be about 3 x lo-* cm in the experiment at 0.8 cm/s (3). The limiting rate of uptake of dye by the yarn is only about one-tenth of what would correspond to transport controlled by such a layer, so that diffusion in the liquid is clearly not controlling the rate of uptake.

LEVELNESS IN DYEING OF WOOL YARN AND FABRIC 185

T c

Time, min

Rate of flow (ml/s) 4 8 16 32 O 0 A V

Figure I-Sorption of pure C.I. Acid Blue I38 (0.053 g/ l ) by chlorine- Hercosett treaied wool fabric (0.88 g ) (at 95"C, pH 5.5, 0.6 gjl

ammonium acetate)

Alternatively, a constant rate of uptake by the bed would occur if the rate of supply of dye to the bed as a whole by flow was insufficient to maintain a significant concentration of dye in the liquor leaving the bed. Until the rate of uptake had fallen to the point where the emerging liquor had a significant concentration of dye, the observed rate of uptake of the bed would be equal to the rate at which dye was entering the bed. The question of whether this was the mechanism controlling the rate of uptake was resolved as follows. The difference in concentration across the bed was assumed to be equal to the rate of addition of dye to the system divided by the rate of flow through the bed. Calculation showed that the difference in concentration across the bed in the region of constant rate of uptake was only 0.4 times the constant concentration in the liquor being supplied to the bed. Thus, the concentration of the emerging liquor was 0 .6 times that of the incoming liquor, which eliminates the possibility of control of uptake by the rate of supply of dye to the bed as a whole.

The removal of dye must have been uneven; there must have been much greater changes than the above in the concentration of dye in the liquor somewhere in the bed, other- wise the uptake of dye would have been controlled by diffusion in the fibre and would not be linear with time. The rate of uptake would have been very close to that observed at the highest rate of flow, as the 4Oo/b drop in concentration of dye to 3 . 2 ~ g/l would be expected to give only a 15% drop in the rate of dye uptake under conditions for which rate of uptake is controlled by diffusion in the fibre (4).

It is thought that regions of the bed in which relatively high concentration gradients would be expected can be identified with the inter-fibre space in the yarn itself. On this hypothesis, the limiting initial rate of uptake observed at the lowest rate of flow arises from the fact that the rate of uptake of dye by the yam is so high relative to the rate of flow through the yam itself that the liquor emerging from the yarn anywhere in the bed is virtually free of dye. Dye-liquor passing through the periphery of a portion of the yarn will be depleted relatively little. However, an approxi- mate calculation of the velocity of flow through the yarn which would be required to give the observed result will test the above hypothesis.

For this purpose, the equation dm/dt-v,Ac was used. In this equation, dm/dt is the limiting rate of uptake observed at the lowest rate of flow and vu is the mean velocity of flow through the yarn. The projected area (A) of the yarn was taken as the product of the length of yarn (15 m) and its diameter (0.03 cm). The

concentration of dye in the liquor entering the yarn (c) was taken as the mean ofthe concentrations of dyeentering and leaving the fabric bed. The resulting value for v, was 0.04 cm/s, i.e. one-twentieth of the mean velocity in the bed. This may be compared with a value of 0 -06 cm/s obtained on purely hydrodynamic grounds by sub- stitution of the dimensions of the fibre, yam and bed and the rate of flow into equations suggested by McGregor (5). The agreement is sufficiently good to lend support to the above hypothesis.

Incidentally, these rates of flow 'through the yarn' have been calculated without consideration of the impermeability of the fibre to flow. The mean velocity of the inter-fibre rate of flow in the yarn can be obtained by dividing these rates by the intra-yarn porosity, here equal to 0.35.

A gradual change from control of the initial rate by the above mechanism to control by diffusion in the fibre would take place (a) with increase in rate of flow, as seen here, (b) with increase in the concentration of dye in the liquor being supplied to the bed or (c) with conditions (e.g. temperature or pH) giving a lower substantivity of the fibre for dye or a lower coefficient of diffusion of dye in the fibre. It appears that control by diffusion across a boundary layer in the liquid would occur only with fibre assem- blies very much thinner than commercial yarn or possibly at rates of flow and of dye uptake very much higher than are envisaged for yarn dyeing.

YARN PACKAGES

The thickness of the bed that constitutes a commercial yam package is much greater in relation to yam diameter than was the case with the fabric bed. Thus, the rate of uptake of dye by a commercial yarn package can be controlled by the difference in concentration across the package as a whole before the difference in concentration across the yarn can control the rate. However, the difference in concentration across the yam can result in unlevel distribution of dye in a package.

Figure 2 shows the radial distribution of dye in a package dyed with reversal of flow at 1-min intervals. The dye concentration at the points tested would be within + 5% of the mean value except at the outermost point of the package.

406-- -600 E L -I

200- -- --

Figure 2-Radial distribution of dye in a wool yarn package (spring diameter 50mm,O'85%on wtofyarnof Carbolan Blue BS, 70"C,pH4.0,

30 min, I0 Ilmin with flow reversal)

Figure 3 shows the dye distribution obtained for two separate packages, one dyed with liquor flowing only from inside to out- side and the other with flow from outside to inside. An anoma- lously low uptake at the outside of the package is clearly given by outside to inside flow, where the yarn exposed to the greatest concentration of dye does not have the greatest uptake.

9 040

Mass of wool (from inside af package to sampling position). g

- *

~- - . - - r 200 400 600 0 u Mass of wool (from inside of packaye to sampliiiy position). g

Flow o u t s i d e i i n Flow inside .out

Figure 3-Radial distribution of dye in a wool yarn package (spring diameter 62 mm, 1.7% on wt of yarn ofcurbofan Blue BS, 90°C, pH 5.5,

30 rnin, 5 llrnin)

C

186 JSDC MAY 1972; COATES, DAY, PROVOST AND RIGG

The low dye uptakes observed at the outer regions of the packages (Figures 2 and3) appear to be due to relatively low rates of flow through the yarn in these regions, where the yam is more widely spaced. Towards the centre of the package, the compres- sion of the package by the winding tension leads to a reduction in inter-yarn space, so that a greater proportion of the liquor passes through the yarn. Clearly, this mechanism would result in a low uptake near the outside of a package, irrespective of the direction of flow, so that flow reversal does not eliminate this source of unlevelness. * * *

I thank IMr W. B. Paluch and Mr K. Liepins for experimental assistance.

(MS. received 21 September 1971)

References I Feldtman, McPhee and Morgan, Text. Manilf., 93 (1967) 122. 2 Lewis and Seltzer, J.S.D.C., 84 (1968) 501. 3 Brooks and Nordon, ibid., 87 (Jan 1971) 12. 4 Brooks, unpublished results. 5 McGregor, J.S.D.C., 81 (1965) 429.

Publications Sponsored by the Colour Measurement Committee-IX

The Measurement and Assessment of Colour Differences for Industrial Use 111-Methods of Scaling Visual Assessments

E. COATES, S. DAY, J. R. PROVOST AND B. RIGG

School of Colour Cheniistry and Colour Technology, University of Bradford, Bradford 7

A new series of green paint samples has been prepared and used to study different methods of obtaining visual assessments of colour differences and of scaling the experimental results to give A V values directly proportional to the observed differences. The results from two standard methods (the ratio method and the paired-comparison method) were in good agreement with each other and with results from the ranking method used earlier, thus adding confidence to conclusions based on the use of the latter method. The extent of agree- ment between A E values calculated from some best-fit empirical colour-difference equations and the A V values has been calculated. Results obtained by Robinson on a series of blue-grey paint samples by a % acceptance method have been re-analysed. The scaling method used previously with visual results in the form of % acceptance values gives AY values directly proportional to calculated AE values. The visual observers can be subdivided into two groups based on whether they had experience of colour matching or not. No evidence was found for any difference between the ‘perceptibility’ and ‘acceptability’ of colour differences in the sense that different

colour-difference equations might have been required to represent the two groups of observers.

Introduction Direct comparison of visual assessments of small colour

differences with AE values calculated from instrumental results using a colour-difference equation is often not possible because the visual results are not in a convenient form. Such comparison can be made satisfactorily only if the visual results can be ex- pressed quantitatively so that the values used are directly propor- tional to the observed colour differences. Various methods of treatment of visual results have been suggested ( I , 2), each dependent on the particular technique used to obtain the results. All the methods depend on some assumptions being made about the nature of the visual judgments and these assumptions are difficult to check experimentally.

In this paper several of these scaling methods are compared with respect to their convenience in application and to their reliability in quantifying the results. The results from these ‘standard’ scaling methods are also compared with those obtained by the techniques used and reported previously (3, 4).

For the present study a new set of green paint samples (x= 0.237, y-0.351, Y-12.5) has been prepared. These samples have been used to study two standard scaling methods (the ratio method and the paired-comparison method), together with the ranking method used previously (3). Because the observers available had no industrial experience of acceptability judgments, the ‘(x acceptance’ method (4) was not attempted with this new set of samples. However, the data obtained by Robinson (5) have been treated by application of the % acceptance method and uscd to provide evidence of the validity of the method employed previously (4) .

It will be shown in a subsequent paper in this series that the results of the scaling calculations can be used to estimate the accuracy of assessments of individual observers. This information is useful particularly when considered in conjunction with the

accuracy of the AE values provided by colour-difference equa- tions. The two estimates of accuracy together enable the irtdus- trial usefulness of colour-difference equations to be appraised.

Results and Discussion GREEN PAINT SAMPLES

Eight green samples and a standard (STD) were prepared on 75-mms metal plates using the paints and techniques described previously (3). The tristimulus values of the standard and samples were measured on a Colormaster (CMV) and a Color-Eye Signature model colorimeter (CE), once each on six different days. The repeatability of the AE values calculated from the CIE (1964) colour-difference equation, given by the standard deviation of individual measurements, was 0.10 and 0.19 CIE unit for the CMV and CE colorimeters respectively. The between- instrument reproducibility was 0-20. The mean tristimulus values obtained using the CMV colorimeter are given in Table 1 , to- gether with the appropriate AE values and the standard devia- tions calculated from the individual sets of results.

Mean Tristimulus

Sample X STD 8.45 A 8-86 B 8.31 C 8.39 D 9.27 E 8.30 F 8.62 G 8.62 H 8.28

TABLE 1 Values and Colour Differences for

Samples Measured on CMV Y Z X Y

12.50 14-65 0.2374 0.3511 13.46 15.55 0.2340 0.3554 12.59 14.71 0.2334 0.3536 12.64 15.44 0.2301 0.3466 13.49 15-22 0-2441 0.3552 12.02 14.49 0.2384 0.3453 12.91 14.38 0.2400 0.3595 13.04 15.04 0.2349 0.3553 11.83 13.55 0.2460 0.3515

Green Paint

AE CI

3 . 3 0.08 1.8 0.15 2 . 4 0.10 2 .1 0-05 2 .1 0.08 2 . 0 0.05 2 . 4 0.15 3 .9 0.15

Mean 0.10