Testing colour fading of cotton fabrics by activated oxygen bleach-containing detergent: an interlaboratory trial

Duncan Phillips, Michael Duncan*, Andrew Graydon*, Geoff Bevan+, John Lloyd$, Chris Harbod and Jurgen Hoffmeister 5

Dept of Textiles, UMIST, PO Box 88, Manchester M60 1 QD, UK *Procterand Gamble Ltd, Whitely Road, Longbenton, Newcastle upon Tyne NE12 9TS, UK +Lever Brothers Ltd, PO Box 69, Port Sunlight, Merseyside L62 4 2 0 , UK

SUnilever Research Laboratory, Quarry Road East, Bebington, Merseyside L63 3JM! UK SHenkel KGaA, WEW-Waschtechnik, Henkelstrasse 367,040192 Dusseldo~ Germany

A single diagnostic wash test to identify coloured cotton fabrics susceptible to activated oxygen bleach was subjected to an interlaboratory trial (ring test). The results from the trial are described and a procedure developed for use as a British Standard is outlined.

INTRODUCTION In a previous article [l], work was described leading to the development of a new diagnostic single wash test protocol (referred to as COX) to identify coloured cotton fabrics susceptible to oxygen bleach fading through repeated washing with domestic detergents. Prior to submitting this new test protocol to BSI for official recognition, it was important that other interested parties (retailers, dye manufacturers, dyers and finishers, garment suppliers and test houses) had the opportunity to examine its applicability and reproducibility. An inter-laboratory trial, called a 'ring test' (RT), was therefore organised in which the fading of a number of dyed fabrics was tested using the new protocol by 19 different laboratories. In addition, eleven of the laboratories also carried out machine washes to confirm the applicability of the test protocol as a predictor of the fading that can arise after repeated laundering. This paper, sponsored by the Fastness Tests Committee of the Society of Dyers and Colourists under the guidance of the Wash Fastness Working Group, describes the results obtained from both aspects of the study and their implications for the proposed test protocol.

TEST METHOD

Design approach The primary objective of the ring test was to obtain a measure of the repeatability of the proposed protocol

within a single laboratory and its reproducibility between different laboratories.

The study was carried out in two phases. In phase 1, participating laboratories were asked to follow a protocol where the test solutions were made up at 40 'T and the washing carried out while heating up to 60 'T over 10 min and during a further 30 min at this temperature. Use of steel balls for agitation was voluntary but laboratories were asked to record whether or not they had been used. In addition, and where possible, participating laboratories were also asked to compare the results from the test protocol with 20 machine washes using 60 'T wash cycles.

In phase 2 the protocol was revised to incorporate detergent dissolution at ambient temperature. The fabric specimen was added and the liquor heated up from an ambient temperature to 60 "C with agitation in the laundering device. The wash was then continued for a further 30 min at the final temperature of 60 "C. This protocol was employed to achieve optimum bleaching performance from the peracetic acid generated from the reaction between tetra-acetylethylenediamine (TAED) and the hydrogen peroxide resulting from hydrolysis of the sodium perborate. Steel balls were omitted from this phase.

Twelve dyed cotton fabrics were selected from those examined in the development of the test protocol to include 'high', 'medium' and 'low' bleach response [l]. All dyeings of each fabric were carried out at the same time by a single laboratory and, unless specified otherwise, were dyed at a level of 4% 0.w.f. The dyes selected are listed in Gble 1.

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Table 1 Test fabrics

Code Dye

1 2 3 4 5 6 7 8 9 10 11 12

CI Sulphur Black 1 CI Sulphur Red 10 CI Vat Blue 4 CI Reactive Red 120 CI Reactive Red 195 CI Reactive Red 228 CI Reactive Orange 16 CI Reactive Blue 225 CI Reactive Blue 224 CI Reactive Blue 171 CI Reactive Black 5 (1 YO) CI Reactive Black 5 (4%)

Table 2 Formulation of ECE non-phosphate reference detergent

Component Mean alkane Amount chain length (“/o)

(a) Base detergent Linear sodium alkyl benzene sulphone Ethoxylated fatty alcohol Sodium soap

SIK (foam inhibitor concentrate

Sodium aluminium silicate zeolite 4A Sodium carbonate Sokalan CP5 (BASF) sodium salt of acrylidmaleic acid copolymer

Sodium silicate (SiOp:Na 0 = 3.3:l) Carboxymethylcellulose bMC) Dequest 2066 (Monsanto) Diethylene triaminepenta

Sodium sulphate Water

8% silicon on inorganic carrier)

(methylene phosphonic acid)

6.5 32.5 11.8

5.2 3.4 1.3

0.8 7.8

12.2

(b) Sodium perborate tetrahydrate As separate addition

(c) TAED As separate addition

Participants in the ring test were provided with a sufficient quantity of each dyed fabric and the ECE reference detergent to carry out three replicated washes with the new test protocol and 20 repeated machine washes. The ECE non-phosphate reference detergent (Table 2) was supplied in three parts: (a) base detergent, (b) sodium perborate tetrahydrate and (c) TAED. Full instructions on the amount of each to use was also stated.

Colour fading resulting from application of the test protocol or repeat machine washes were determined instrumentally using the CMC(2:l) equation as the metric of the colour change, AE.

EXPERIMENTAL

Phase 1 A 50 x 100 mm piece of fabric was weighed and the

appropriate volume of washing liquor added to maintain a liquor ratio of 100:l. A suitable mechanical laundering device consisted of a water bath containing a rotatable shaft which supported, radially, stainless steel containers (75 k 5) x (125 f 10) mm high of capacity 550 f 50 ml, the bottom of the containers being 45 k 10 mm from the centre of the shaft. The shaWcontainer assembly was rotated at a frequency of 40 k 2 per min. The temperature of the water bath was thermostatically controlled to maintain the test solution at the prescribed temperature to within 2 degC.

The wash liquor was prepared by dissolving 10 g of the base detergent, 1.8 g TAED (100% activity) and 12 g sodium perborate tetrahydrate per litre of grade 3 water (see BS EN IS0 105:AOl). The base detergent and TAED were first stirred vigorously using a mixer 1100 k 100 rpm in grade 3 water at room temperature and heated rapidly to 40 k 2 “C. Sodium perborate tetrahydrate was then added and the solution stirred for a further minute.

One fabric specimen was placed in each container and a sufficient volume of wash liquor was added to give a 1OO:l 1iquor:specimen ratio. After checking that the temperature was at 40 k 2 “c, 50 steel balls were added to increase agitation. The container was closed, placed in the laundering device and rotation commenced.

The temperature was raised at a maximum rate of 2 “c per min to the required 60 k 2 “c and the test continued for a further 30 f 1 min at this temperature.

The specimen was removed at the end of the wash cycle, rinsed for 1 min in warm running water, then in cold running water for 10 min. The specimens were dried by pressing between two white absorbent cotton cloths to remove excess water and then hung in air at a temperature not exceeding 60 “c. The change in colour of the specimen relative to the original fabric, AE, was assessed instrumentally.

Phase 2 The wash liquor was prepared as in phase 1 but with stirring for 10 min at an ambient temperature of 20 f 2 “c using an ultrafast mixer rotating at 5000 f 100 rpm. The water bath of the laundering device was also at ambient temperature. The fabric specimen was added to the container and the temperature raised at a maximum rate of 2 degC per min to the required temperature of 60 f 2 “c. As in phase 1, the test was continued for a further 30 f 1 min at this temperature. Steel balls were omitted in all tests.

RESULTS AND DISCUSSION

Correlation of COX test fading with machine wash results In total, eleven laboratories carried out the 20 cycle, 60 “c machine washes as well as the COX protocol. The machines used included both domestic and Wascator types. The relationship between the fading observed after repeated machine washing and the COX test averaged over all of the laboratories is shown in Figure 1. The

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Figure 1 Comparison of fading in machine and COX tests

expected three groupings of low, medium and high colour changes derived from single washes using the test protocol are clearly reflected in the actual aggregate response from repeated washing.

Individual laboratories also achieved very good correlation, the COX protocol predicting well the effect of multiple machine washing. However, different linear relationships were obtained depending on the washing machine characteristics employed. In particular, it was found that the eight laboratories using domestic washing machines obtained mean colour changes in the order of 11.3 units while the three laboratories using Wascator equipment obtained a mean colour change of only 5.7 units. The mean colour changes for the COX tests from these two sets of laboratories were very close, that is, 4.5 and 4.0 units, respectively. This should be noted by laboratories using Wascator equipment for the performance testing of textiles.

Repeatability and reproducibility The two measures of Variability of signhcance to the ring test can be defined as follows:

Repeatability The variability observed between replicate runs of the same fabric within the same laboratory. This gives an indication of the standard errors that could be expected for a test performed in a single laboratory, in other words, the 'intralab variability'.

Reproducibility The variability observed in the fading of a given test fabric between the results obtained from different laboratories. This indicates the agreement likely to be obtained between different laboratories for a protocol involving a given number of replications and is the 'interlab variability'.

The statistical quantities associated with these definitions are the repeatability standard deviation or and the reproducibility standard deviation o, respectively. Details of their calculation and statistical analysis for the

presence of any outliers are described in the Appendix. For a direct comparison between any two laboratories the total variance will be a function of the sum of the squares of these two quantities.

Each laboratory carried out three replicate washes on the twelve different fabrics. Consequently, it is possible to derive estimates for the repeatability and reproducibility for each individual fabric aggregated over the partici- pating laboratories and for the repeatability and reproducibility for each individual laboratory aggregated over the different fabrics. Both provide insights into the reliability and consistency of the test method.

Fading results from phase 1 of COX test Data using the first dissolution procedure was returned from 17 laboratories. There was an essentially equal split between laboratories electing to include, and those electing to omit, steel balls from the method and one laboratory ran tests with both. In total, some eight different makes of laundering device were employed with 60% of the equipment having programmable heat controllers. Mean results for the colour change on the individual fabrics are summarised in Bble 3.

Table 3 Mean colour change using 40 "C dis- solution procedure

Code A€ (CMC(2:l))

1 16.89 2 5.18 3 1 .81 4 4.31 5 1.83 6 4.44 7 3.74 8 0.93 9 3.76

10 1.05 11 4.79 12 3.42

Results for the repeatability standard deviation, averaged over the participating laboratories, are summarised in Figure 2. Fabric 1 stands out in respect of having a very much higher variability than the others and was the fabric giving the highest level of fading. All laboratories agreed with this finding, the minimum colour change being 10.1 units.

Results for the reproducibility standard deviation for each laboratory are summarised in Figure 3. One laboratory clearly stands out as obtaining quite different results from the general consensus. Inspection of this laboratory's repeatability and mean fading did not yield any systematic reason for this and consequently the data from this company were excluded from further analysis.

A critical area, from the viewpoint of the methodology, is the ability of the test to distinguish successfully between

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Figure 2 Individual fabric repeatabilities

as the Variability within a laboratory. In addition, the variability is also higher in the group of fabrics giving medium sensitivity in the COX test indicating, as might be expected, an approximate relationship between the variability and the magnitude of the colour change obtained.

Analysis of the data was also undertaken to determine if factors associated with the test equipment were giving systematic contributions to the results. No evidence could be found for systematic contributions associated with the make of laundering device nor with the incorporation of steel balls. In the latter case, for example, mean colour changes of 3.2 were obtained with steel balls and 3.3 without the steel balls. However, laboratories employing equipment with controlled heating programmes produced, on average, 0.8 unit smaller colour differences than those without. This effect was consistent across the majority of fabrics.

Fading results from phase 2 of COX test In total, 11 laboratories returned data for this second phase and all tests were carried out in the absence of steel balls. Mean results for each fabric are summarised in Table 5.

Table 6 compares the results from the two dissolution procedures for the low (fabrics coded 3, 5, 8 and lo), medium (fabrics 2,4,6,7,9,11 and 12) and high (fabric 1) bleach sensitivity fabrics, respectively. Fading results are within 0.3 colour difference units of each other for the low sensitivity fabrics indicating that the testing methodology is very robust for fabrics in this group. The difference

Figure 3 Individual laboratory reproducibility Table 5 Mean colour change using ambient dissolution procedure

Table 4 Summary of fading results and variability from phase 1 of COX test for fabrics showing low and medium bleach sensitivity

Group A€(CMC 2:l) or 0 s

1: CMC(2:1)<2.0 1.40 0.275 0.475 2: CMC(2:l) > 2.0 4.24 0.464 1.131

Code AE(CMC(2:l))

fabrics of low risk to fading in the presence of a TAED containing oxygen bleach and those where the stability is more borderline. Further analysis, therefore, focused on the variability of the test when applied to the 11 fabrics giving colour changes in the small to medium range. Eble 4 summarises the average repeatability and reproduci- bility standard deviations obtained for these fabrics grouped (Figure 1) according to whether the colour

Table 6 Effect of dissolution procedure on mean fading (A€, CMC(2:l))

1 18.17 2 5.18 3 1.48 4 5.48 5 1.07 6 4.95 7 5.15 8 0.97 9 2.78

10 0.86 11 5.52 12 4.66

Dissolution Low A€ Medium A€ High A€ change obtained was greater (seven fabrics) or smaller (four fabrics) than two units. For both groups of fabrics,

40 “C 1.40 4.24 16.89 the reproducibility standard deviation is larger than the repeatability standard deviation, indicating that the vari- Ambient 1.09 4.82 18.17 ability between laboratories is at least as sigruficant a factor

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increases as the sensitivity to bleach increases and, for fabrics of medium sensitivity, all but one showed a higher fading when ambient temperature dissolution, consistent with more efficient bleach generation, was used

The effect of the revised dissolution procedure on the repeatability and reproducibility standard deviations, grouped as before into small and medium colour changes, are summarised in Table 7. No consistent change is observed for the fabrics insensitive to bleach - the repeatability standard deviation increasing slightly and the reproducibility standard deviation decreasing slightly. However, by comparison with Table 4, both measures of variability have decreased for the fabrics giving medium levels of fading, with the magrutudes of the mean colour changes being larger than in phase 1 of the trial.

Table 7 Summary of fading results and variability from phase 2 of the COX test for fabrics showing low and medium bleach sensitivity

Group AE(CMC(2:l)) u, 0,

1 : CMC(2:l) c 2.0 1.09 0.311 0.435 2: CMC(2:l) z 2.0 4.82 0.383 0.958

Effect of alternative d o u r equations For this work it was agreed by the participating laboratories to use the CMC(21) colour difference equation since this is now a common unit for pasdfail decisions. Other colour difference equations can be employed and, indeed, in the original study the CIELAB system was used. The effect of using different colour equations is summarised in 'kble 8 for the fabrics giving low and medium colour changes, respectively. This includes conversion of the results to an instrumental grey scale rating, following the procedure given in IS0 105:A05.

Table 8 Repeatability and reproducibility of bleach fading results as a function of colour difference equation used for ambient temperature dissolution

Equation Mean A € 4 0s

Fabrics giving small AEa CIELAB 10" D6, 1.75 CMC(1:l) 1.54 CMC(2:l) 1.09 A05 grey scaleb 4.28

Fabrics giving medium AEC CIELAB 10" D65 9.15 CMC( 1 :1) 6.91 CMC(2:l) 4.82 A05 grey scaleb 1.96

0.455 0.572 0.31 1 0.21 9

0.750 0.596 0.383 0.136

0.621 0.903 0.435 0.358

2.043 0.996 0.958 0.242

a AECMC(2:I) less than 2 units b Instrumentally assessed c AECMC(21) greater than 2 units

The choice of colour equation has a marked effect on the mean colour change obtained for fabrics designated as having medium colour change: in general, both measures of variability increasing as the magnitude of the colour difference increases. The opposite effect can be seen with the calculated grey scale, the variability estimates being higher for the fabrics giving small colour changes. Again, this is consistent, as it is in the grey scale range of 4 to 5 that the equation provides the highest sensitivity to changes in colour.

Ideally one would compare these results to similar ring test data derived from established test methods, such as from the IS0 105:C06 series, to assess if the variability is within an acceptable range. However, no equivalent published data are available that could be used for this purpose. As a rough guide, it would be reasonable to expect a robust test to allow an individual laboratory to be able to produce mean colour changes where the uncertainty in the mean, as calculated for a 95% level of confidence, is no greater than 0.5 grey scale units. Assuming a three-replicate procedure, as applied here, the calculated grey scale confidence limits for repeatability are 0.35 and 0.22 for small and medium colour changes, respectively. The proposed tes;, therefore, meets this criterion. It is more difficult to make judgements on the reproducibility but, for a comparison between two laboratories, confidence limits are in the order of 0.71 for small changes and 0.47 for medium changes, which does not seem excessive. Most importantly, disagreements between laboratories are likely to be infrequent regarding fabrics showing large colour differences when instrumental grey scale ratings form the basis of the measured change in shade.

CONCLUSIONS A new diagnostic single wash test (COX) to identify coloured cotton fabrics susceptible to fading when washed repeatedly with domestic laundry detergents containing activated oxygen bleaches has now been tested across a number of laboratories. The repeatability and reproducibility of the test method has been quantified using two different procedures for the detergent components dissolution step. The main conclusions to be drawn are as follows:

Fading obtained as a result of oxidative bleach action in the COX test shows a close linear relationship to that obtained after multiple washes in a domestic washing machine The repeatability and reproducibility of the COX is acceptable for purposes of interlaboratory comparisons The test is insensitive to the presence of steel balls in the wash vessel The dissolution procedure used for the detergent components is not a critical factor influencing fading, but variability will be reduced and the fading of sensitive fabrics increased if ambient dissolution is employed

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- Wascator test equipment is likely to underestimate the extent of fading arising in the domestic wash although it shows a similarly good correlation with the COX test The choice of colour difference equation employed has

Committee of the Society it is proposed to submit the procedure to IS0 at the July 1998 meeting.

- * * * a major effect on the magnitude of calculated fading and should always be specified when reporting results. The authors gratefully acknowledge the assistance of

those companies who participated in the study.

The procedure is being developed as part of BS 1006, ’UK- TO colour fastness to domestic laundering - oxidative bleach response’. With the guidance of the Fastness Tests

REFERENCE 1. D A S Phillips et al., J.S.D.C., 112 (1996) 287

APPENDIX Statistical analysis Repeatability standard deviation The repeatability standard deviation sp,r for a given laboratory p and one of the test fabrics is calculated from Eqn 1:

where n is the number of replicates, xi the result of replicate i and x the arithmetic mean value of the n replicates.

The repeatability standard deviation or for each test fabric is then calculated from Eqn 2

where p represents the number of participating laboratories.

Similar equations can be derived for the repeatability standard deviation by laboratory using summations over the Werent fabrics.

Reproducibility standard deviation The reproduability standard deviation sB, for a given fabric is calculated from Eqn 3

where xm, the mean value over the participating laboratories,is given by Eqn 4

and all other symbols are as before.

(4)

Outlier analysis The presence of a small number of outlying values can result in a large increase in the underlying estimates of variability in a procedure. Therefore, it is important to examine the data for outlying values to see if the variation is due to general differences between results or can be attributed to just a few observations. In the present work, outlier analysis was restricted to the estimates of the repeatability standard deviation.

The repeatability for a given laboratory suspected to be an outlier can be compared with a combined estimate of repeatability calculated from all other laboratories. Assuming the repeatability for all of the laboratories are equal and that there are no outliers, then the ratio of the square of these estimates, expressed as Eqn 5:

will have an F distribution with (n - 1) and (n - 1) times (p - 1) degrees of freedom. Outliers can be identified as those companies with an F statistic greater than a critical value.

A company could be flagged as being an outlier for repeatability either because of poor repeatability or because of the result of a single outlying replicate. Because of the small number of replicates in this study it is not be possible to suggest which of these two explanations of the increased variation is correct.

The critical value employed in the present work was that the repeatability was significantly different at the 99% level of confidence. Data meeting this criteria were excluded from the analysis.

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