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11The Use of X-RayFluorescence for CoatWeight Determinations

11.1 Introduction ......................................................................11-1

11.2 Technique...........................................................................11-1

11.3 Method...............................................................................11-2

11.4 Accuracy.............................................................................11-3

11.5 Repeatability and Reproducibility....................................11-3

11.6 Conclusion.........................................................................11-5

11.1 Introduction

The technique of elemental analysis by x-ray fluorescence (XRF) has been applied to the quality control

of coating weights at the plant level. Measurements by nonlaboratory personnel provide precise and rapid

analytical data on the amount and uniformity of the applied coating. XRF has proved to be an effective

means of determining silicone coating weights on paper and film, titanium dioxide loading in paper, and

silver on film.

11.2 Technique

XRF is a rapid, nondestructive, and comparative technique for the quantitative determination of elements

in a variety of matrices. XRF units come in a variety of packages; however, the type of unit most prevalent

in the coating industry is described in this chapter.

The XRF benchtop analyzer makes use of a low level radioisotope placed in close proximity to the

sample. The primary x-rays emitted from the excitation source strike the sample, and fluorescence of

secondary x-rays occurs. These secondary x-rays have specific energies that are characteristic of the

elements in the sample and are independent of chemical or physical state. These x-rays are detected in

a gas-filled counter that outputs a series of pulses, the amplitudes of which are proportional to the energy

of the incident radiation. The number of pulses from silicon x-rays, for example is proportional to the

silicone coat weight of the sample. Because the technique is nondestructive, the sample is reusable for

further analysis at any time.To ensure optimum excitation, alternate radioisotopes may be necessary for different applications. For

silicone coatings and titanium dioxide in paper, an iron-55 (Fe-55) source is used. Fe-55 x-rays are soft

(low energy) and do not penetrate far into a sample. For silver on film, a more energetic americanum-

241 source has been used.

Wayne E. MozerOxford Analytical, Inc.

2007 by Taylor & Francis Group, LLC

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11-2 Coatings Technology: Fundamentals, Testing, and Processing Techniques

Placing samples just a few millimeters from the excitation source enables high sensitivities to be

obtained. Irradiation of the sample is more efficient the closer the sample is brought to the source. In

the case of low energy x-rays, such as silicon, which are easily absorbed by the atmosphere, a helium

purge should be used. With optimum sample irradiation and helium purging when necessary, measure-

ments within 2 minutes of counting time are typical for most samples.

Calibration curves for different elements and materials can be stored directly in the instrument and

are available for recall. Curves are established by measuring a set of known samples or standards of the

same material. Since XRF is a comparative technique, subsequent analyses are only as good as the quality

of the calibration standards supplied.

The total accumulated intensity is actually a combination of signals from the analyte element and

from the matrix in the form of background. Background intensities may vary depending on the thickness

or the basis weight of the material. These differences may be determined by measuring uncoated or blank

samples that are automatically incorporated into the calibration.

11.3 Method

Three procedural steps must precede analysis. First, spectrum scan helps to set up various calibration

parameters. Next a calibration is established. Almost all the time the instrument is in use, it is dedicated

to analysis of production samples. Typically, spectrum scans are performed on representative standards

to identify the occurrence of elemental peaks of interest in the spectrum, as illustrated in Figure 11.1.

The scan is a qualitative tool and will show the presence of any potential interfacing element. For example,

an Fe-55 source will excite the following range of elements in the periodic scale: aluminum to vanadium

and zirconium to cerium. With the proper combination of sources, it is theoretically possible to measure

from aluminum to uranium. Once any interference has been recognized by a spectrum scan, a calibration

FIGURE 11.1 Spectra scan of silicone coatings on super calendar craft paper.

X-RayCounts/Second

100

75

50

25

100 150 200

Channel

C1Ca

Si

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The Use of X-Ray Fluorescence for Coat Weight Determinations 11-3

may be established. A sample is prepared by cutting a disk and placing it onto a nickel-coated paper

holder. The holder ensures that the sample will be held as flat as possible because an uneven surface will

introduce errors in reproducibility, caused by scattering and vacancies at the incident surface. Table 11.1

gives a typical calibration for silicon coat weights.Once a calibration has been established, two samples are selected that bracket the operational range

of the samples measured. These samples will then be used to correct for changes in instrumental

performance with time. The measurement of these two standards and of an uncoated sample for back-

ground correction purposes is a push-button procedure. Termed restandardization is performed once

per shift. Analyses may then be performed: an operator simply cuts a sample to be analyzed, inserts it

into the instrument, and initiates the measurement. Within 23 minutes, the instrument will determine

the concentration of the element of interest, both printing and displaying the results.

11.4 AccuracyTo ensure optimum accuracy, it is important to be aware of three sources of errors: suppliers of coating

material, background changes, and interferences. It has been recognized, although not explained, that

silicones of different suppliers give different sensitivities. When different silicones are analyzed on the

same basis weight paper, lines with different slopes are obtained (Figure 11.2). Thus, to avoid this source

of error, separate calibrations are established for each supplier of silicone. Second, correction for changes

in background will help reduce error.

Figure 11.3illustrates two sets of standards of the same silicone on supercalendar kraft prepared on

different dates. Quite evidently, the samples give the same slopes, they are offset. The analyzer will correct

for this background change by having the operator insert an uncoated sample from the lot of paper to

be coated. Thus, all subsequent analyses will be corrected for the apparent background change.

The final source of error is the presence of another material in high concentrations. Examples include

silicone coatings on polyvinyl chloride (PVC) or titanium dioxide filled films. As the levels in these films

change, the effect on the background for the silicon region of the spectrum will change. Thus, there will

be a raising or a lowering of the apparent silicone coat weight for that film sample. This type of interference

is easily corrected for by software that can be built into the analyzer. The interferences are recognized by

using the spectrum scan and are automatically compensated for during calibration.

11.5 Repeatability and Reproducibility

Repeatability in terms of precision of measurement is a statistical function determining the variability

of repeat measurements on the accumulated intensity and x-ray counts. Since an iron-55 source excites

titanium x-rays more efficiently than silicon x-rays, it follows that sensitivities for titania coatings are

higher, and therefore, measurements are more precise. The precision may always be improved by increas-

ing the coating time, but this does not always result in substantial improvements in accuracy. Typically,

for silicone coatings, reported standard deviations correspond to 0.01 g per square meter of silicone.

TABLE 11.1 Typical Results for a Silicone-on-Paper Calibrationa

Sample

Given Concentration

(g/m2)

X-ray Concentration

(g/m2) Error

1 0.33 0.34 0.01

2 0.41 0.41 0.003 0.54 0.52 0.02

4 0.62 0.62 0.00

5 0.70 0.72 0.02

6 1.50 1.50 0.00

a Standard error = 0.013 g/m2.

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The Use of X-Ray Fluorescence for Coat Weight Determinations 11-5

11.6 Conclusion

On-site XRF determinations are rapid enough and precise enough for effective quality control programs

for elemental determinations on a variety of substrates. The use of XRF is not limited to measuring

coatings; it is flexible enough to measure related products, such as tin, platinum, and rhodium catalysts,

and other solutions.

2007 by Taylor & Francis Group LLC