2 Aug 2017

XRF plastics and polymers analysis | Spectroscopy Solutions

Plastics are ubiquitous in modern life with polyethylene and polypropylene being two of the most commonly used. These plastics can be found in anything from plastic carrier bags to children’s toys, to thermoplastics and consumer products.

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Polyethylene and polypropylene are polymers produced from α-olefins which are polymerized using the high-pressure Ziegler–Natta process. The catalysts used for this process are usually inorganic, containing aluminum compounds, or even metallocene complexes containing hafnium, titanium or zirconium. The nature of the catalyst used influences the chain length of the polymer and its initial structural properties.

XRF analysis of plastics, sample preparation of plastic for XRF analysis is best served using a Specac XRF press.

Following polymerization polymers such as polyethylene and polypropylene are seldom used in their ‘Raw’ state for commercial applications. They may be too waxy and brittle or too soft, be the wrong color or have a poor temperature profile. Usually to attain optimum polymer properties for product manufacture, further additives are used to modify the base polymer and provide the required properties.

f you want more information on using Specac hydraulic presses and film-makers for XRF sample preparation, or any other application, get in touch for a price and demonstration.

Modifying polymer properties with inorganic additives

As base polymers, uncross-linked polyethylene and polypropylene can flow and are regarded as ‘liquids’ with very high viscosities, so before their use they have to be compounded with various organic and inorganic additives. Inorganic additives are used to modify the physical strength and deformation resistance of polymers. Kaolin, calcium silicates and titanium dioxide are all used in this way and work by forming ‘pinning’ points along polymer chains that prevent them moving past one another too much.2,3

In addition to this, inorganic additives can be used to adjust almost every physical property of the polymers. Inorganic additives are used to control the onset temperature, extent and dimensions of the crystalline phase in the polymer.2 This is because the presence of inorganic additives determines polymer crystallite formation and nucleation during molding, and gives the polymer more uniform bulk properties.2 The inorganic constituent acts as a heterogeneous nucleation site for polymer crystallites to achieve uniformly spaced crystalline sites, which helps to balance creep resistance and flexibility of the crystalline and amorphous phases and provide the best mechanical properties.2

In addition, bromine compounds can be added to polymers to act as flame retardants.3 Polymers are often colored and in some of these cases various inorganic salts are used such as chromium, nickel, iron oxide, cadmium, molybdenum, titanium and even antimony2 to provide vivid colors (some of these can be toxic and the levels must be strictly controlled).

As polymer science has progressed the range of inorganic additives has grown to include metallic nanoparticles. Nanoparticles and nanotubes are added to the polymer to modify high temperature, electrical or magnetic performance and optical properties.

In compounding or mixing a new polymer for a specific purpose the monitoring of inorganic additives is essential to optimize the properties and safety of commercial goods. Too much of a specific additive can completely change the polymer structure, compromise its mechanical properties and lead to creep, poor weathering and ultimately degradation and structural failure.3

XRF in plastic recycling

An area where x-ray florescence has become more and more useful is in plastics recycling. Metals and metal complexes have been used for many years in plastic products. These metallic compounds, although encapsulated in polymer matrix, are not chemically bound to polymer molecules and so can gradually be released to the environment over the service life of the product. In a similar way, when plastic waste is discarded either by incineration or landfill, toxic metals released from plastics can enter the atmosphere or leach into soil.

Analyzing recycled plastic using XRF is accurate and efficient. Using XRF sample preparation techniques, such as the XRF automatic hydraulic press or XRF manual hydraulic press, is the best practice.

Environmentally friendly plastics disposal requires the monitoring of levels of potentially toxic elements. Hand held XRF systems are particularly useful for in-situ analysis with a minimum of sample preparation3,4 especially in larger electrical items with polymer components. The first rules to target heavy metals in plastics were introduced in 1994 by the European Union. The European Community ‘Packaging Directive’ - EC-Directive 94/62/EEC,6 regulated the total amount of metals such as cadmium, chromium, mercury and lead in plastic packaging materials to less than 100 mg/kg.

In the US, Proposition 65 in California went further by banning cadmium from use completely in polymers. European Council Directive 2002/96/EC on waste electrical and electronic equipment (WEEE)6,7, required the removal of all plastic containing brominated flame retardants and all mercury containing components, batteries, etc. from electrical waste. Recycling polymers is now part of mainstream waste handling and it can be seen how important it is to monitor their heavy element content either prior to re-melt and compounding or incineration.

Sample preparation for the XRF analysis of polymers

Although in polymers it has been seen that hand held XRF spectrometers can provide excellent results there are also instances where a traditional stationary XRF can give better results. However, for the best results a good level of XRF sample preparation is required.

To get the best results the sample needs to have a consistent thickness. Inadequate sample preparation can be a major source of error in XRF. This can come from the inability to take a random sample, poor mixing, grain size effects (ideal at < 50 μm) or mineralogical interference. For polymers, the analyst needs a representative sample of the whole object or the area under consideration.

The Manual Hydraulic XRF Press is ideal for preparing sample pellets ahead of X-ray Fluorescence spectroscopic analysis.

There are then several options for producing a test sample. For polymers that are thermoplastic a Mini-Film Maker may be the best option to make a random, ‘constant thickness film’ compatible with a laboratory XRF instrument (a thin-film sample).

An alternative is to use the Specac manual press on a powdered polymer sample. An automatic hydraulic press also provides exceptionally consistent results, in this case as it provides a stored, programmable load. It can also provide higher ton loads. Achieving a consistently sized pellet sample is possible if dies are used to form uniform, cylindrical pellets. If the formation of a pellet is not possible XRF sample cups can be used instead which, when pressed, gives a flat and uniform surface ready for XRF analysis.

Check out #SpectroscopySolutions for more information about the various ways spectroscopy is an ideal analytical technique, or #SpectroscopyGuides for more advice on how to perform high quality spectroscopic analysis.

References

  1. Van Grieken, R. E. and Markowicz A., Handbook of X-Ray Spectrometry, Marcel Dekker, New York,(2002)

  2. Kissin, Y. V. (2008). Alkene Polymerization Reactions with Transition Metal Catalysts. Amsterdam: Elsevier.

  3. Michael Bolgar, Jack Hubball, Joseph Groeger, Susan Meronek, Handbook for the Chemical Analysis of Plastic and Polymer Additives, CRC Press, 2008, Chapter 1 ‘Overview of polymers, additives and processing’

  4. Brian L. Riise and Michael B. Biddle, X-Ray Fluorescence Spectroscopy In Plastics Recycling, http://infohouse.p2ric.org/ref/47/46170.pdf

  5. Stanislaw Piorek, Feasibility of Analysis and Screening of Plastics for Heavy Metals with Portable X-Ray Fluorescence Analyzer with Miniature X-Ray Tube, GPEC 2004, Abstract #14, http://mie.esab.upc.es/ms/informacio/miscellanea/Heavy%20metals%20in%20plastic s.pdf

  6. European Parliament and Council Directive 94/62/EC of 20 December 1994 on packaging and packaging waste. Official Journal L 365 , 31/12/1994 P. 0010 - 0023

  7. European Parliament and Council Directive 2002/96/EC of 27 January 2003 on waste electrical and electronic equipment (WEEE). Official Journal L 37, 13/2/2003 P. 0024 – 0038

  8. C. D. Green, A.S. Vaughan, et al, Recyclable Power Cable Comprising a Blend of Slow- crystallized Polyethylenes, IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 20, No. 1; February 2013