Why the use of Polypropylene (PP) in food trays is hardly ever an environmentally sound decision
Some companies of the distribution channel in certain Central European countries have considered in recent months the possibility of replacing thermoformed PET trays with trays made of PP. Due to the reasons that will be explained below, far from being a correct decision, this movement does not seem beforehand to make any sense either from the packaging or from the environmental point of view.
Except in very specific cases, in which hot food packaging is necessary or microwave heating of the package for pre-cooked food is required, PET always has better properties than PP from the point of view of packaging in thermoforming in four key issues for packaging design and functionality:
VERSATILITY
PET is very flexible to small forces, allowing great efficiency in the design and production of thermoformed containers. PP is much less versatile in this sense, which gives it a great disadvantage especially in containers requiring design with a certain detail.
TRANSPARENCY
Transparency is a very important property for manufacturers of thermoformed packaging and for packaging companies since it allows the consumer to correctly visualize the packaged food without distorting it. In this sense, while PP gives opacity to the wall of the container, PET is par excellence the best polymer for packaging in terms of transparency.
BARRIER TO GASES
This is an especially relevant property in the case of thermoformed containers since they must be capable of conferring an adequate barrier to oxygen for the correct preservation of packaged foods, extending their shelf life and avoiding food waste. In this regard, while PP has an extremely poor barrier to oxygen, PET provides an excellent barrier property to this gas [1].
IMPACT STRENGTH
Among the mechanical properties that polymers used in packaging should have, impact resistance is crucial because packaging containers are continuously suffering variable mechanical loads from handling, loading and transport to storage. The material from which the container is made should be able to support these mechanical loads, external abrasion and any other changing environmental conditions, such as temperature and pressure, to ensure a damage-free supply of the product. In this sense, PET has infinitely better mechanical properties than PP, offering much higher tensile and impact strength [2].
From an environmental point of view, it is not possible to find any property of PP that allows it to stand out from PET, but quite the opposite. Thus, there are three fundamental questions that, added to all those previously mentioned, always make this second material the preferred option for manufacturers of thermoformed containers and packaging companies to be able to respond to their commitment to the Circular Economy:
STRUCTURE AND MORPHOLOGY OF THE POLYMER
Unlike PET, PP is a polymer of much greater complexity since it has different varieties in terms of its structure and morphology, called isotactic, syndiotactic and atactic [3]. As a result, the waste generated from the products made with PP are not homogeneous and their recycling is highly complex due to the heterogeneous mixture that causes a high loss of properties of the recycled product. PET does not present this problem and, for this reason among others, it is the best recycled polymer and with a higher recycling rate in Europe.
POLYMER DENSITY
PP has a very similar density to Polyethylene (PE) [4] making its separation by flotation very complex in recycling processes when both polymers are used together in packaging solutions, either as independent elements or in multilayer structures. This causes, for example and among other issues, that containers made with PP are not recyclable by delamination when joined with layers of PE and EVOH since a mixture of polyolefins with similar densities and, therefore, not separable by flotation is obtained. This is not a problem for multilayer PET/PE and PET/EVOH/PE containers, which are perfectly recyclable by delamination (as long as the adhesive used is not polyurethane), which allows the recovery of PET and PE separately.
APPLICATIONS OF RECYCLED POLYMERS
Recycled PET is a material that has been applied in food contact applications for years, with numerous processes evaluated and authorized by the European Food Safety Agency (EFSA) for its use [5]. This allows recycled PET to be a material used in applications with high added value (“upcycling”) such as, for instance, bottles for beverages or thermoformed containers for food packaging.
In contrast with PET, in the case of PP from food packaging, the recycled material cannot be used for this same application as it is not technically feasible due to the explained limitations of the polymer itself and, therefore, there are no processes authorized by EFSA. This means that the final applications to which the recycled PP from food packaging is destined are products from other sectors with less added value (“downcycling”).
Taking in consideration all the reasons explained before, it is worth wondering if these companies of the distribution channel, which in certain Central European countries are considering the possibility of replacing thermoformed PET trays with trays made of PP, will continue with their plans. It seems clear that it is not a wise decision, neither for reasons of suitability of the properties of the polymer for food packaging nor for reasons of circularity for recycling into new food containers.
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[1] The OTR (Oxygen Transmission Rate) measures the quantity of gas that goes through a substance, per surface unit, during a specific time frame. The lower the OTR the more resistant the polymer is to gases going through it. While PP has an OTR range (cm3.m−2 per day), at 20–23°C, of 93-300, PET has a range of OTR of 1,8-7,7. Source: https://www.sciencedirect.com/topics/engineering/oxygen-transmission-rate.
[2] Tensile Strength (MPa): PET =50; PP=1-2 (atactic) // Impact Strength (J/m): PET=90; PP=(isotactic). Source: Polymers for Packaging Applications. Sajid Alavi, PhD, Sabu Thomas, PhD, K. P. Sandeep, PhD, Nandakumar Kalarikkal, PhD, Jini Varghese and Srinivasarao Yaragalla. 2015, Apple Academic Press
[3] https://www.researchgate.net/figure/The-structure-of-isotactic-syndiotactic-and-atactic-polypropene_fig1_10982133
[4] https://polymerdatabase.com/polymer%20classes/Polyolefin%20type.html
[5] https://www.efsa.europa.eu/en/topics/topic/plastics-and-plastic-recycling
