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X-ray vision looks inside your plastics.

Once reserved for laboratory environments, x-ray fluorescence elemental analysis can now be used by plastics formulators to check additive levels on the production floor.

Looking for a tool that can help make sure your compounds meet specifications for flame retardancy, color, or melt flow? You may want to consider adding x-ray fluorescence (XRF) analysis to QA/QC programs. A new generation of smaller, more rugged, and much more economical XRF spectrometers has emerged in the last several years that require little or no sample preparation and can be used in the lab or out on the floor by your production lines.

Up to now, XRF analysis has been primarily used by resin producers to analyze polymers for catalyst residues and unknown impurities. In more recent years, a few compounders have opted for XRF analysis as an alternative to traditional QA/QC techniques such as atomic absorption spectroscopy or specific-gravity tests.

Today, their ranks appear poised for significant growth. A new generation of compact, affordable bench-top instruments presents an opportunity for a broad spectrum of compounders to easily measure and quantify atomic elements that characterize additives such as flame retardants, pigments, stabilizers, fillers, and lubricants. The most common elements analyzed by XRF include chlorine, bromine, silicon, magnesium, barium, antimony, calcium, titanium, zinc, tin, and iron.

With XRF, sample analysis can generally be performed in less than a minute and can even be performed in a production environment. New rugged designs and greater ease of operation eliminate the need for technicians highly trained in spectroscopy. Typically, these instruments come already calibrated to detect at least two elements, and setup is included in the purchase price. Suppliers also offer troubleshooting and method refinement by modem, reducing costs for on-site technicians.

What is x-ray fluorescence?

This technique is used to identify and quantify atomic elements in a sample. Samples are irradiated by x-rays, which remove inner-shell electrons from the target atoms, forming highly unstable ions. This instability is resolved when outer-shell electrons jump from one orbit to another to fill the inner-shell vacancies. This transition is accompanied by the release of x-ray photons (fluorescence) whose energies or wavelengths are characteristic of that particular element.

The quantity of x-rays emitted by a sample is directly proportional to the concentration of the element in the sample. By collecting the emitted x-rays and measuring their energy or wavelengths against their intensities, users can identify and quantify the elements in a sample - a process automatically performed by the instrument's software.

In measuring solids such as plastic pellets and thin films, XRF has the same detection sensitivity - down to a few parts per million - as classic elemental analysis techniques such as atomic absorption spectroscopy, which require samples to be dissolved in a solvent before they can be analyzed. That sample-preparation step is both time-consuming and awkward to perform in a production area.

XRF's forte is that it permits analysis of pellets, granules, thin films, or liquids - as received. That speed and convenience is the reason why Ken Sienkowski, manager of product development at compounder M.A. Hanna Engineered Materials, is now acquiring an XRF instrument for Hanna's Technical Center in Norcross, Ga. He says XRF outperforms techniques traditionally used by compounders in ease of use, speed, and sometimes accuracy. "Typical total turnaround is 10-15 minutes with XRF," he reports. This compares with average sample-preparation and testing time of at least 8 hr with atomic absorption or other techniques.

Because atomic absorption analysis is relatively cumbersome and expensive, most compounders have traditionally relied on specific-gravity measurements for quality control of additive composition. While this may suffice when the compound has only one primary additive (e.g., Ti[O.sub.2]), it is not very accurate for measuring the quantity and ratio of two or more additives. This is particularly crucial with flame retardants, says Sienkowski. "Bromine and antimony are both heavy elements and you need to know their ratio to ensure that the compound will be adequately flame retardant."

With XRF, solid samples require a relatively flat surface for measurement. Sample surface consistency is also very important. Pellets can be pressed fiat with a "nutcracker"-type accessory or pressed into a thin film. Liquids are placed in holders fitted with x-ray transmitting windows. Loose powders may also be tested that way, although analyzing them as pressed disks yields even greater accuracy. In all cases, each measurement is performed automatically, generally in under a minute and with 90-99% accuracy.

What to look for

XRF instruments fall into two main categories according to their method of detection - wavelength dispersive (WD) and energy dispersive (ED). The WD-XRF analyzers are the oldest design and generally can detect lower concentrations of elements due to their higher resolution between elements in the sample. They also give better results in detecting low-atomic-number elements (as low as boron).

All of the floor-standing, research-grade XRF analysis systems use the WD principle. These [TABULAR DATA OMITTED] relatively bulky units are also very costly - ranging from $100,000 to $200,000 and up - which is why their interest has been limited to resin producers. XRF instrument suppliers concede that WD-XRF units generally may be "overkill" for plastics compounders.

(Suppliers of WD-XRF instruments include Applied Research Laboratories, Dearborn, Mich.; Kratos Analytical, Inc., Chestnut Ridge, N.J.; Jordan Valley AR, Inc., Austin, Texas; and Phillips Analytical X-Ray, Mahwah, N.J.)

It is the benchtop ED-XRF analyzers that have gained the most interest

recently among plastics formulators. They range from QC units that sell for as little as $20,00035,000 to higher-performance models suitable for lab or production use that cost $45,000-80,000. The accompanying table lists currently available ED-XRF instruments that are marketed for plastics uses.

All XRF instruments have an excitation source such as an x-ray tube or radioactive isotope. The key difference lies in how the x-rays get into the detector. With ED-XRF, the entire spectrum is measured simultaneously, with all the x-rays going into one detector. The detector and associated electronics then sort out which x-rays belong to which elements.

The simplest ED-XRF instruments use a gas-filled detector (proportional counter). The filler gas influences the level of response to light or heavy elements. Neon or argon is used for the lighter elements and xenon for the heavier elements.

Most ED-XRF analyzers typically measure around 80 elements from atomic numbers as low as sodium (atomic number 11) or magnesium (12) to as high as uranium (92). Some instruments can measure elements as light as beryllium (4). However, with the lower-cost benchtops, there is a limit to how many elements can be quantitatively measured at one time. The radio-isotope instruments, due to their low power, can measure no more than six elements at a time. A similar limitation applies to low-powered x-ray-tube units because their proportional-counter detectors have low resolution. The latter units are best suited to plastics-processing operations that are interested in QC of one to four elements.

Higher-end lab-grade ED-XRF systems, including some benchtop units, yield a higher resolution because of their detectors, which are typically solid-state such as lithium-drifted silicon. Many detectors of this type require refrigeration and are cooled electrically or by liquid nitrogen. This requirement increases the complexity of the analyzer. At the same time, these units can analyze the entire spectrum simultaneously, although plastics users most often select no more than about 12 elements at a time.

Hanna's Sienkowski says that although he is very impressed with the newer, lower-cost benchtop ED-XRF units, Hanna will probably opt for a laboratory scale ED-XRF analyzer. "They offer more capability when analyzing lighter elements. We can go as low as fluorine - an element in our wear-resistant compounds that contain PTFE." Hanna is evaluating instruments that can analyze compounds with very low parts per million of elements as well as compounds that are very highly loaded (e.g., with talc, silicone, Ti[O.sub.2], or magnesium). Hanna plans to use XRF to analyze flame retardants, uv stabilizers, phosphite antioxidants, lubricants such as zinc and calcium stearates, and copper-containing heat stabilizers for nylon.

Trend toward low power

Cumbersome regulations are prompting ED-XRF users to move toward low-powered x-ray tubes and away from radioactive isotopes. A radio-isotope is the most compact x-ray excitation source, as it does not require a high-voltage power supply. However, each state has its own regulations on the use of radio-isotopes. Some require operators to wear special radiation exposure-monitoring badges. Others require workers to be periodically tested.

A radioactive isotope emits x-rays at a few discrete energy values. Therefore, to cover the full range of the periodic table, several sources are needed, which can be expensive. Since most QC applications typically require a limited element range, only one excitation source is generally needed for a specific use. However, a radioactive isotope has a short life because the radioactive material is constantly decaying. Once it has lost half of its life, which can happen in under two years, it must be safely disposed of, which involves additional cost.

The x-ray tubes used on economical benchtop units are very low powered and can be turned off when not in use. They have a life span of several years. The simpler ED-XRF benchtop instruments typically use tubes of 25-35 kV, while lab-grade models typically use tubes of 40-50 kV. The cost difference is about $5000. Both use low current. (Expensive WD-XRF instruments use high-powered x-ray tubes of 60-100 kV that use high current.)

Although the trend is toward the low-powered tubes, some instrument suppliers maintain that having the option of a radioactive isotope can come in handy. "Most of the elements measured for plastics formulations have lower atomic numbers, for which the low-powered x-ray tubes do a good job," concedes Azucema Overman, product manager at Asoma Instruments. However, for heavier elements such as tin, barium (common in heat stabilizers), and antimony (flame retardants), Asoma recommends the use of a radio-isotope. Other suppliers point out that more costly 50-kV x-ray tubes can handle these heavier elements.

Overton cites one plastics compounder that uses Asoma's newest benchtop QC instrument, the Model 300 T, which has both a 35-kV x-ray tube and a radio-isotope element for higher energy to analyze medium-to-heavy elements. The compounder is using the unit to determine different levels of flame retardants, stabilizers, and lubricants in dryblend compounds.
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Title Annotation:x-ray fluorescence elemental analysis
Author:Sherman, Lilli
Publication:Plastics Technology
Date:May 1, 1998
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