Instrumental whiteness evaluation, Fluorescent Whitening Agents, Inter Instrument Agreement Tests

SENAI/CETIQT ColourInstitute, Brazil

Originally presented in AIC 2004,Interim Meeting of the International Color Association

Abstract

The instrumental evaluation of whiteobjects treated with fluorescent whitening agents, such as commonly found on substrateslike textiles, plastics and paper, is a task not as straightforward as it mightseem (ASTM 1992, Hayhurst and Smith 1995). One of the main reasons is thefluorescence of the fluorescent whitening agents (FWA) that is influenced by theamount of UV radiation in relation to the amount of radiation in the visiblerange of the spectrum available in the sample illumination. This makes itnecessary to perform a UV adjustment on the light source in reflectancespectrophotometers to establish the adequate ratio of UV and visible radiation.This study compares whiteness measurements of textile samples treated withfluorescent whitening agents made on 4 different industrial reflectance spectrophotometers,some using a traditional method of adjusting a UV filter position and othersperforming a numerical and virtual UV control. Two sets of standards were usedfor the calibration and the measurement results obtained after the differentcalibrations are compared.

1. UV Adjustment and UV Standards

For instrumental evaluation withreflectance spectrophotometers the spectral power distribution of the lightsource, usually a pulsed Xenon lamp, is rarely known to the industrial user,who generally simply adjusts the UV content of the flash during a so-called UVcalibration. There are two types of UV adjustment, the traditional filtermethod introduced by Grtner in the 1970s (Grtner and Griesser 1975) and thenumerical UV control patented by Imura (Imura 1997, USPTO 1977, 2000).

For this UV adjustment it is necessaryto use a fluorescent standard with known calibration values, and at presentthere are different standards available: paper, Teflon, plastic and textile standards.The textile standards used in this study were produced and calibrated by the TextilforschungsinstitutThringen-Vogtland (TITV), Germany. Two sets each of the cotton (CO) andpolyester (PES) were used. The calibration data available in the certificateare the CIE whiteness value, the Ganz-Griesser whiteness value (Ganz 1972) andthe spectral radiance factors for the five standards. This allows for the UVadjustment to be done according to any of the three types of calibrationvalues.

2. Samples, Instruments and Settings

The samples measured in this studyare lightweight woven textile samples with a very similar structure: one sampleis a polyester/cotton mixture treated with a FWA, one sample is a 100% cottonbleached fabric without FWA, six samples use the fabric of the previous sampleas substrate to which two different kind of FWAs and fractions of fourdifferent dyes where applied to produce different tints and five standards eachof the cotton and the polyester standard scale were also measured as samples.

Four industrial spectrophotometers,produced by Konica Minolta, were used for the measurement of the textilesamples, all of them with sphere geometry: two bench-top instruments that makeuse of the traditional filter adjustment for the UV setting, two CM- 3720dspectrophotometers, and two instruments with numerical UV control, a portableCM- 2600d and a bench-top CM-3600d.

The 18 samples were measured on the4 industrial instruments after having performed the adjustment of their UVsetting. On the two filter instruments the calibration was done according tothe CIE whiteness value (W-CIE) of the whitest standard of the scale and also accordingto the Ganz-Griesser whiteness value (W-GG) of the same sample. As mentioned previously,four calibration scales were used, a cotton and a polyester scale that were purchased8 months before being used for this study (identified as original scales) andanother cotton and polyester scale that were used immediately after theirarrival (identified as new scales). All the measurements were done during thesame week. Table 1 shows the different percentages of UV obtained as resultafter the different types of calibration.

Depending on the standard and on its calibration values used for the UV adjustment a different percentage was determined as being the adequate. In general the UV setting was lower for the new scales. This indicates that the FWA of the original scales probably had already deteriorated due to extensive use and more UV energy was required from the lamp to excite the fluorescence necessary to achieve the indicated whiteness value.

3. Measurement Results

After having adjusted the instruments as mentioned previously, the samples were then measured. The 10 samples, five standards each of the cotton and the polyester calibration scales, were measured and compared to their values as given in the calibration certificate. Table 2 shows the average differences from the calibration values of the standard samples measured on the different instruments/settings. Calibration with the original scale is slightly better than with the new scale and the cotton scale also resulted in a smaller difference from the calibration values than the PES scale.

The comparison of the three different whiteness values in Table 3 shows that the adjusted instrument-specific parameters lead to better W GG results than the standard values. These W GG adj values are better than the W CIE values, especially when taking into consideration that the W GG and W CIE scales are different. An evaluation as a percentage of the average whiteness value in the certificates, as shown in the right-hand column of Table 3 is more adequate.

A second requirement, apart from a measurement error as small as possible, where one expects the measurement results to be as close as possible to the calibration values, is the reproducibility amongst instruments. The same sample measured on different instruments should lead to results as similar as possible. The standard deviation of the average measurement results obtained on the four instruments shown in Table 4 is reasonably small. For the Ganz-Griesser whiteness values the adjusted parameters lead to closer results amongst the instruments, as is the purpose of the adjustment.

4. Conclusions

The study confirmed that it is really necessary to adjust the UV radiation of the sample illumination when fluorescent samples are to be measured a basic fact not always so obvious for the industrial user. It shows that it is necessary to adjust the instrument-specific parameters when using the Ganz-Griesser whiteness formula. If for some reason this not possible it is then more appropriate to use the CIE whiteness formula than to use the standard parameters for the Ganz-Griesser whiteness formula.

Comparing the two different types of UV setting, the filter adjustment and the numerical UV setting, it can be said that both lead to good results. From a users point of view the NUVC is much faster and easier.

We found that the portable instrument is highly compatible with the bench top instruments, even when measuring FWA-treated samples.

Our results show that the inter-instrument agreement of instrumental whiteness evaluation might not yet be as good as desirable, but compared to the visual reproducibility these results are already a significant improvement, considering that a group of observers was able to repeat their own whiteness evaluations only in approximately 50% of the cases (Gay 2004).

Acknowledgments

All the spectrophotometers used in this study are long-term loans to the Colour Institute of SENAI/CETIQT from Konica Minolta with whom SENAI/CETIQT is proud to have a cooperation agreement. The samples used in this work were prepared by Cssia Cristina Melo as part of an applied research project for which the authors acknowledge the financial help provided by FAPERJ (Fundao Carlos Chagas Filho de Amparo Pesquisa do Estado do Rio de Janeiro).

References

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