Review
Critical review on challenge tests to demonstrate decontamination of polyolefins intended for food contact applications

https://doi.org/10.1016/j.tifs.2015.12.003Get rights and content

Highlights

  • Diffusion in polyolefins is drastically faster compared to PET. Polyolefins can be then contaminated by high MW substances.

  • Consequently the challenge test used for PET, based on depollution of volatile substances, cannot be used for polyolefins.

  • Additionally migration tests must be based on adapted analytical methodologies to determine higher MW substances.

  • Thermal desorption, as depollution process, should not be efficient enough to remove all types of polyolefin's contaminants.

Abstract

Background

As post-consumer recycled plastics may be contaminated with chemical substances, their use for food packaging may raise food safety issues. Recycling technologies should therefore efficiently remove contaminants of concern.

Scope and approach

Usually, the abundant data available for recycled poly(ethylene terephthalate) (PET) are extrapolated to polyolefins. This paper reviews the differences of basic properties and typical contaminants of polyolefins compared to PET. The use of thermal desorption process to remove polyolefin contaminants is discussed.

Key findings and conclusions

It is suggested in this review that this extrapolation is not scientifically justified, on the basis of the comparison between the intrinsic diffusion properties of contaminants of concern in PET and in polyolefins. It is concluded that the scope of contaminants of concern considered for the safety assessment of polyolefins recycling technologies based on thermal desorption should be carefully re-examined.

Introduction

As food packaging is a rapidly growing market, the demand for post-consumer recycled plastic packaging materials also grows. However, post-consumer collected materials may be contaminated from the first usage. Contamination often occurs in an uncontrolled and unpredictable way by what is called a misuse of packaging by consumers: at the end of the previous life of packaging materials, consumers may re-use the containers to store non-food substances available in their domestic environment. Moreover, plastic waste collection and the recycling technology itself may also be contamination sources for the materials. Therefore, the safety assessment of post-consumer recycled plastics intended for food contact applications requires a careful view of each step of the process, from waste collection to the final recycled plastic output.

In order to ensure that the final material does not contain contaminants of concern, when it comes into contact with food, recycling processes must include a critical decontamination step. Each recycled polymer displays specific contamination and decontamination behaviour governed by physical processes: sorption (contamination), diffusion and migration (release into foodstuff). Overall these properties are related to structure and polarity of the polymer. Considerations underlying the design and efficiency of the decontamination are specific to each polymer to be recycled.

Up to now, recycling of post-consumer plastics into food contact materials (FCM) has focused mainly on PET. It has been shown that the post-condensation steps of PET recycling processes usually require long time high temperature and vacuum or gas flow conditions, which also allow elimination of possible contaminants of concern.

Polyolefins are the most important group of polymers used for food packaging. However only a smaller number of studies has investigated safety issues related to the use of recycled polyolefins. Most studies extrapolate to polyolefins the results obtained for PET, which may not be scientifically sound. PET is a glassy polymer at room temperature and in the vast majority of conditions of use. In contrast, polyolefins are rubbery; they display poor functional barrier properties. The diffusion coefficient of a given substance is lower in PET than in polyolefins by orders of magnitude, so that the possible migration of absorbed contaminants is much lower for PET. Furthermore, as polyolefins have a reduced thermal stability, degradation products are formed during the processes (Coulier, Orbons, & Rijk, 2007). Stabilizers, which are used to protect polyolefins from oxidation and degradation during processing also give rise to reaction products. This becomes even more important when the materials are recycled and processed several times. Neoformed substances formed from both polymers and stabilizers may migrate into the packaged food (Nerin, Alfaro, Asnar, & Domeno, 2013).

Section snippets

Current approaches for the safety evaluation of recycled polymeric materials intended to come into contact with food

The guidelines of the European Food Safety Authority (EFSA) define the major risks associated to the use of recycled polymeric materials in contact with food. These risks are linked to the possible presence in a recycled plastic of substances, following its previous lifetime, its collection, sorting and recycling processes. These substances may contaminate by migration the food that will be packed in contact with the recycled plastic. Given the variety of sources of contamination, the identity

Sorption and diffusion in PET and in polyolefins

The efficiency of cleaning and decontamination steps relates to interactions of contaminants with the plastic, the media in contact as well as to the material dimensions: sorption of substances into the plastic material during its first life, their diffusion into the mass of the material or into the media used during the washing step, their solubility in the cleaning media, their removal during decontamination steps and lastly, migration of any residual substances from the final FCM into the

Consequences for the definition of contaminants of concern for polyolefins

AFSSA (2006) correlated with a modelling approach the acceptable level in food mf,tmax and the corresponding maximum level of contaminant in the recycled plastic. The latter is in practice the Cmod later defined by EFSA (2011). Equation (3), a usual analytical solution of Fick's law was used in order to describe the migration from plastic sheets when migration follows a Fick's law and at low migration yield (Scholler, Vergnaud, Bouquant, Vergallen, & Feigenbaum, 2003).mf,tmf,=2LpDpxtπwhere Mf,t

Characterization of the actual contamination of recycled post-consumer polyolefins

In the literature, two approaches are used to evaluate the safety of recycled materials. On one hand, it can be demonstrated by a challenge test that whatever the identity of contaminants of concern, they are removed using the adequate technology, down to levels which do not raise safety concerns. On the other hand, surveys were run, in which hundreds or even thousands of bottles from a broad range of origins were analysed and the contaminants found were listed. Such surveys have been conducted

Overview of decontamination technologies for polyolefins

EFSA has examined dozens of applications (EFSA, 2015), based on criteria to be used for safety evaluation for recycled PET intended to be used for manufacture of materials and articles in contact with food (EFSA, 2011). Most technologies implemented for PET rely on thermal desorption of the contaminants present. This thermal treatment usually occurs in the solid state (in a solid state reactor) and sometimes in the melt phase (in an extruder)(Papaspyrides & Vouyiouka, 2009). A combination of

Conclusion

This paper reviews the undesired contamination of plastic packaging and the procedure to challenge the ability of recycling process to decontaminate the materials to an acceptable level. Compared to PET, collected wasted polyolefins (polyethylene and polypropylene) may be contaminated with a broad range of chemical substances, which may not be fully eliminated during the recycling process and which, finally, may contaminate the food which comes in contact with containers made from these

Declarations of interest

A. Feigenbaum has been an EFSA staff member till 2012.

B. Papaspyrides is a member of an EFSA working group on plastics recycling.

Acknowledgement

This study was supported by Fonds Unique Interministériel (FUI) and the Conseil Général de l’Ain in the frame of the REPALI 2 project (Recyclage de polyoléfines en contacts alimentaire et non alimentaire).

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