PET hate: Checking contaminants in recycled polyethylene terephthalate

Recycled plastics: dirtiness can be an obstacle

Recycle, recycle, recycle. One of the environmental mantras that will help to "save the planet." Or, to put it more accurately, save the human race. The planet has shown over its lifetime that it can recover from disaster quite nicely, but that is another argument.

In the recycling industry, plastic is one of the more attractive options, partly because there is so much of it in circulation. About 4% of the global annual oil production is used as a feedstock for plastics production and an additional 3-4% during manufacture. So, there is a high potential for conserving fossil fuels, which are non-renewable.

A high proportion of plastic waste is used in packaging, such as food trays and bottles. A large number of plastic bottles are currently wasted, rather than recycled, which is disappointing because they are generally produced from polyethylene terephthalate (PET), a soughtafter plastic for recycling plants. It has a low content of additives and a low susceptibility for the uptake of contaminants.

Despite these attractive properties, recycled plastics are rarely used in food packaging due to concerns about food safety. Any contaminants present must be at acceptably low levels, whether they originate from the original plastics, the material in contact with the plastic article, or the recycling process itself. Subsequent migration from the recycled packaging into food must be less than 10 µg/kg, according to the European Food Safety Authority.

To this end, the EU introduced a challenge test in 2008 as a requirement for recycled plastics intended to come into contact with food. In this test, a set of contaminants are added to the plastics to see how well they are removed by the cleaning process and success relies on the ability of the screening method to measure the amounts of contaminants remaining in the plastic material.

PET challenge test: two screening methods compared

A team of scientists in Spain has compared two methods that are used to extract and measure the contaminants in recycled PET. Cristina Nerin, Juliana Felix and Pilar Alfaro from the Aragon Institute of Engineering Research at the University of Zaragoza added the surrogate compounds to virgin pellets and also analysed recycled PET flakes donated by an industrial recycling company.

Toluene, chlorobenzene, phenol, limonene and benzophenone were the chosen contaminants. In a preliminary screening procedure by headspace SPME with GC/MS analysis, all five compounds were identified but the peaks for phenol, limonene and benzophenone became saturated, even when the amount of sample was reduced, so this technique could not be used for quantification.

So, the first selected procedure involved total dissolution of PET to accomplish total release of the trapped contaminants. PET is relatively insoluble, but dichloroacetic acid (DCAA) was found to do the trick, as opposed to solvents such as toluene and dichloromethane which only achieved partial dissolution.

The solution was sampled by headspace SPME with a polydimethylsiloxane fibre, followed by GC/MS on a 5% phenyl methylsiloxane column with detection in selected ion monitoring mode.

This method proved unsatisfactory on several counts. In the first instance, limonene was degraded by DCAA into several related terpenoids, so its peak in the chromatogram was much smaller than expected. Secondly, there was no phenol peak due to conversion to a non-volatile oxonio ion by DCAA that remained in solution.

Above all this, the concentrations of the remaining compounds were lower than expected according to information from the suppliers, which was attributed to saturation of the SPME fibre.

The second challenge test procedure examined proved to be more successful. The PET samples were powdered before ultrasonic extraction with dichloromethane and GC/MS analysis on a similar column.

In this case, all of the contaminants were detected and their concentrations were all larger than those determined by the first method for contaminated PET flakes. The relative standard deviations of the measurements were low at 0.3-6.6% based on replicate analyses.

The contaminants were not detected in virgin PET flakes, as expected, and their levels in recycled PET were all below the levels of quantification, confirming that the cleaning process during recycling was highly efficient.

This research illustrates the importance of the extraction procedure. Minimising sample handling by total dissolution and SPME did not provide the best approach and would deliver misleading results. Conversely, extraction without dissolution, as in the second method, may not remove all of the contaminants but it was overcome in this work by powdering the sample first.

Adoption of the preferred procedure will provide an accurate test of recycled PET material to ensure that it has been cleaned efficiently and any contaminants have been removed before reprocessing.
Related links

Analytica Chimica Acta 2011, 687, 67-74: "Pros and cons of analytical methods to quantify surrogate contaminants from the challenge test in recycled polyethylene terephthalate"