Using XRF to Detect Low Levels of Magnesium to Control the Polymerization of Polyethylene and Polypropylene

The Ziegler-Natta catalyst is normally used for the polymerization of PE and PP to enable reaction enhancement. Catalyst systems of the Ziegler-Natta type are a blend of reagents used in the manufacture of polymers of the 1-alkenes type (α-olefins). Ziegler-Natta catalysts are normally based on titanium-magnesia and organometallic aluminium compounds, such as the undefined methylaluminoxane (MAO) or well-defined triethylaluminium, (C2H5)3Al.

Since the catalyst is very expensive, it is not wise to use it rapidly. The catalyst efficiency needs to be optimized to maximize the output of the polymer with a specific amount of the catalyst. In order to monitor the catalyst performance the analysis of magnesium in polymers becomes an important application. The objective is to reach superior accuracy and precision at lowest concentration levels of Mg.

ICP-OES Technique

A standard analytical technique for this application is ICP-OES. The ICP-OES is based on a time-consuming preparation using unsafe chemicals in combination with high argon gas consumption and a daily calibration is required. It is better to use efficient analytical techniques such as wavelength dispersive X-ray fluorescence spectrometry (WDXRF).

This report shows that very low levels of Mg can easily be analyzed using the wavelength dispersive X-ray fluorescence spectrometer S8 TIGER 4K. Bruker AXS’ modern sequential WDXRF spectrometer S8 TIGER utilizes an X-ray tube with up to 170 mA current at full 4 kW power. It is an optimum solution for improved accurate and precise analysis of light elements in very low concentrations.

Wavelength Dispersive X-Ray Fluorescence Spectrometer S8 TIGER

The S8 TIGER is the most versatile sequential wavelength dispersive X-ray fluorescence spectrometer. The features of the S8 TIGER include the following:

  • The instrument is fitted with an end window X-ray tube with Rh-target and a 10 position beam filter changer allowing excitation conditions to be easily adjusted to achieve optimum results.
  • Excitation power of 60 kV up to 4 kW are optimal values for the analysis of heavy elements
  • In order to further improve the analysis of traces a wide selection of up to nine different beam filters is provided to suppress, e.g. Rhodium tube lines and to improve the peak-to-background ratio.
  • The instrument has up to four collimators, eight installed analyzer crystals and two detectors by which the S8 TIGER can be adjusted for each element to match the required resolution and sensitivity

High Performance X-ray Tube

The S8 TIGER has a high intensity X-ray tube for improved sensitivity, which enables achieving very low detection limits and a very good repeatability. As compared to traditional X-ray fluorescence spectrometers, the tube of the S8 TIGER is designed in such a way as to reduce the heat dissipation to the sample. The heat emitted in the tube head during X-ray generation is effectively conducted to the cooling water circuit. This is advantageous for the analysis of hydro carbon based samples including oils and polymers. Damage to samples especially those of valuable standard is minimized hence measurement time is prolonged so as to achieve minimal detection limits or samples that can be analyzed several times. The tube is shown in Figure 1

X-ray tube of the S8 TIGER with low temperature tube head

Figure 1. X-ray tube of the S8 TIGER with low temperature tube head

Measurement and Calibration

The measurement was performed with an S8 TIGER 4K applying 23 kV and 170 mA, an XS-55 crystal which has a 2d value of 55 Å and a 0.46° collimator. These parameters ensure high resolution with the benefit that the crystal fluorescence of Si and W is neatly separated from the analyte’s (Mg) energy. Radiation of the crystal caused by the tube’s rhodium lines enables crystal fluorescence in the WDXRF spectrometer.

Four background positions were defined adjacent to the peak position so as to correct the overlay of aluminium present in the samples as shown in Figure 2.

Polyethylene sample with 10 ppm Mg beside a high Al concentration

Figure 2. Polyethylene sample with 10 ppm Mg beside a high Al concentration

The Al signal comes from the co-catalyst. Moreover, using the additional background positions, the analysis of Mg in PE and PP could be done together in one calibration especially when only polypropylene based standards are available.

The S8 TIGER is calibrated using 12 factory standards. The standard deviation of the calibration is high at 0.2 ppm. The calibration curve is displayed in Figure 3, and the calibrations details are given in Table 1.

Calibration curve of magnesium in polyethylene at a very low concentration level

Figure 3. Calibration curve of magnesium in polyethylene at a very low concentration level

Table 1. Calibration table

Standard Name Chemical Concentration (ppm) XRF Concentration (ppm) Absolute Deviation (ppm) LLD (ppm)
Std 1 1.8 1.7 -0.1 0.2
Std 2 1.6 1.6 0.0 0.2
Std 3 2.4 2.7 0.3 0.2
Std 4 3.0 3.1 0.1 0.2
Std 5 3.3 3.0 -0.3 0.2
Std 6 4.5 4.7 0.2 0.2
Std 7 4.7 4.9 0.2 0.2
Std 8 5.5 5.4 -0.1 0.2
Std 9 8.2 7.9 -0.3 0.2
Std 10 9.0 8.9 -0.1 0.2
Std 11a 10.0 9.5 -0.5 0.2
Std 12a 11.0 11.5 0.5 0.2

Results

The precision of the technique was evaluated by analyzing two factory samples. The results are listed below

  • Sample 1 contains 5.3 ppm, and sample 2 contains 1.5 ppm Mg. The reference values were obtained using ICP.
  • The XRF results were in perfect line with the ICP values.
  • The deviation is 0.3 ppm for sample 1 and 0.1 ppm for sample 2.

To test the accuracy the two samples were alternated and then analyzed, the results of the 10-fold repeatability test is shown in Table 2. An accuracy of 0.1 ppm is obtained for very low concentration ranges.

Table 2. Repeatability test

Sample 1 Mg (ppm) Sample 2 Mg (ppm)
Measurement 1 5.5 1.8
Measurement 2 5.7 1.6
Measurement 3 5.6 1.7
Measurement 4 5.4 1.5
Measurement 5 5.6 1.8
Measurement 6 5.7 1.7
Measurement 7 5.8 1.6
Measurement 8 5.7 1.5
Measurement 9 5.7 1.6
Measurement 10 5.4 1.6
Average 5.6 1.6
Standard Deviation 0.1 0.1

Conclusions

The analysis of very low levels of magnesium in polyethylene and polypropylene is precise and reliable with the S8 TIGER. The detection limits reached are very low with 0.2 ppm. The technique can be easily used to realize savings on catalysts costs giving early alerts if there are any catalyst losses. In contrast to ICP-OES, XRF is the perfect analytical tool due to its easy operation, quick sample preparation and simple integration into the plant quality and process control regime.

This information has been sourced, reviewed and adapted from materials provided by Bruker X-Ray Analysis.

For more information on this source, please visit Bruker X-Ray Analysis.

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