In an article published on January 11, 2016 in the peer-reviewed journal Trends in Food Science & Technology, Stella Palkopoulou and colleagues from the Laboratoire BioDyMIA (Bioingénierie et Dynamique Microbienne aux Interfaces Alimentaires), University of Lyon, France, and other institutions in France and Greece, provide a critical review of decontamination processes for polyolefins.
Decontamination is an important step in the recycling of plastics, particularly those intended for use in food contact applications after recycling. The substances that can contaminate plastics are quite diverse and include those arising from use and misuse by consumers (i.e. petrol or domestic-use pesticides), those introduced during collection and recycling processes (e.g. detergents from the washing step), and those newly formed during the recycling process (e.g. degradation products of plastic components). The efficiency of the decontamination step during recycling is commonly demonstrated by performing so-called challenge tests, where plastics are “spiked” (i.e. chemicals are added) with a set of representative surrogate contaminants at known concentrations. Then, the spiked plastics are subjected to the recycling process and analyzed afterwards to demonstrate the removal of surrogate contaminants to level considered of no health concern. For more information on plastics recycling, see FPF dossier.
The polyolefins polyethylene (PE) and polypropylene (PP) are widely used for food packaging. So far, the procedures proposed for recycling of polyolefins are largely based on those developed for polyethylene terephthalate (PET) plastics. However, PET and polyolefin polymers have different physical properties. This has major consequences for both proper decontamination and compliance assessment procedures for polyolefins recycling, which should likely differ from those used for PET, the authors argue.
In particular, compared to PET, polyolefins display poorer functional barrier properties and a faster migration of absorbed contaminants within the polymeric material. The consequence of this is that polyolefins are potentially highly contaminated due to fast sorption of contaminants, which, moreover, can be of higher molecular weight than those typical for PET. As a result, polyolefins also need to be decontaminated to a high degree, since the migration of residual contaminants into food is likely to be fast and hence of higher concern. Another property of polyolefins that needs to be considered is their reduced thermal stability. This results in higher amounts of degradation products formed during the decontamination processes, especially the one based on thermal desorption, which is used most widely.
Based on the above, the authors suggest that the surrogate compounds used for challenge tests in the recycling of polyolefins need to cover a wider molecular weight range. The set of volatile substances typically used for PET is not sufficient for polyolefins, and has to be supplemented with higher molecular weight compounds (up to 1000 g/mol). This also means that the analytical techniques based on gas chromatography, routinely used for analysis of contaminants released from PET, may not be sufficient for detection of all substances which may potentially migrate from polyolefins. Consequently, additional analytical strategies need to be developed, specifically those able to analyze the substances of high molecular weight.
Following a thorough review of existing literature on the recycling of polyolefins, the authors conclude that thermal desorption is likely not efficient enough for removing all types of contaminants that can be present in polyolefins. However, to establish this with certainty, more data are needed. In particular, additional challenge tests should be performed using higher molecular weight compounds as surrogate contaminants. All studies available so far used the set of surrogates established for analysis of PET decontamination, which is not suitable for polyolefins. Depending on the results of the adequate analysis of current recycling technologies, additional processes may need to be developed to ensure proper decontamination of polyolefin plastics during recycling.
Palkopoulou, S., et al. (2016). “Critical review on challenge tests to demonstrate decontamination of polyolefins intended for food contact applications.” Trends in Food Science & Technology 49:110-120.