Food packaging made from recycled post-consumer polypropylene is possible, but uses might be limited, according to a UK study.
The study, commissioned by WRAP — the England-based Waste & Resources Action Program — and carried out by London recycling consultant Nextek, is the latest in a series looking at the feasibility of producing recycled post-consumer PP resin approved for food contact, and developing a closed-loop PP house¬hold waste stream.
The study looked at how to recycle, process and decontaminate post-consumer PP food packaging; tested to determine if the material would meet EU food-compliance regulations and for what applications; assessed the material's ability to be made into new packaging; and determined the economics of the whole process.
The test was carried out on a large scale — it followed material from sorting to moulded container — to ensure the study's validity, said Edward Kosior, Nextek managing director, in a presentation at the Global Plastics Environmental Council, held 20-22 March in New Orleans, the US.
The study used 18,000 pounds (8,165 kilos) of post-consumer PP packaging sourced from recycler Eco Plastics of Hemswell, England. To meet EU regulations, which require all food-contact recycled plastics to be made from materials previously approved for food contact, non-food packaging had to be removed.
The material was manually sorted to remove non-food packaging and separate the remaining food packaging into clear and coloured materials, and then hand-sorted again to remove any remaining non-food packaging.
The sorting process was “intense” and resulted in 5,000 pounds (2,268 kilos) of clear, washed flake, Kosior said. To meet high purity requirements, the materials were constantly audited, he added.
After the final manual sort, the clear PP pile was 98.1% food packaging and the colour PP was 99.3% food packaging.
The material was then put through two decontamination stages.
It was first sent to equipment manufacturer Gneuss Kunststofftechnik in Bad Oeynhausen, Germany. A patented extruder system developed by Gneuss is the only one that can meet decontamination requirements and is a critical step in the process, according to Kosior.
The material was pelletized and sent to PET Processors's operation in Dumfries, Scotland, where it was put in a vacuum chamber at a high temperature for six hours. The vacuum chamber is the key to making material compliant with food requirements, he said.
The decontaminated material was then made into several food-packaging containers: injection moulded clear pots containing 25% or 50% clear recycled PP; injection moulded pots with 25% or 50% clear or coloured recycled PP; thermoformed pots containing 25% or 50% clear or coloured PET; and thermoformed trays made with varying levels of clear and coloured recycled PP.
A tray made with 50% coloured recycled PP was also coloured with a brown masterbatch to make it identical to an existing product without any special adjustments.
All of the containers could be moulded as with a typical virgin copolymer material, with very few changes. They all met physical and impact requirements as well, showing that high-speed manufacturing of recycled PP is possible, according to the study.
The containers then underwent testing to see if they would meet EU regulations.
Researchers used chocolate to see if food placed in direct contact with the containers would become tainted. They found an interesting conclusion: The chocolate placed on trays made with recycled PP actually rated better than chocolate placed on trays made of virgin material, Kosior said.
An expert panel performed blind odour tests on the containers. While experts did smell different odours for the virgin and recycled containers, none were problematic and all the containers passed sniff tests, he said.
The containers also underwent screening to see what, outside of PP, made its way into the recycled containers. Researchers were mainly focused on compounds of concern like oligomers, fatty acids, antioxidants and clarifier residues, Kosior said.
Three of the substances found in the container had restriction limits — di(2-ethylhexyl) phthalate or DEHP, Tinuvin 326: an ultraviolet absorber/antioxidant, and clarifier residue 3,4-dimethylbenzaldehyde. Researchers performed migration tests simulating severe conditions — microwaving and up to six months ambient chilled or frozen storage. They did not detect migration of DEHP or Tinuvin 326 in containers made of clear or colored recycled PP and housing aqueous or fatty foods.
The 3,4-dimethylbenzaldehyde, which is derived from a food-grade clarifier used in clear containers but is not EU-approved, did migrate into aqueous and fatty foods. Foods in clear recycled PP containers showed a higher migration rate.
The clarifier appeared at levels lower than concern, and the containers would be defined as suitable for use, but more research is needed to clarify that point, Kosior said.
The recycled PP did not pass migration tests for fatty foods at aggressive conditions.
However, containers made from recycled PP can be used as packaging for a range of aqueous and acidic foods — butter, ice cream, meat, bread, chocolate, dried foods, cheese and soup, for example — under the most extreme heating and prolonged storage conditions, according to the study.
The packaging restrictions are “characteristic of using a mixed stream,” Kosior said. Only specific food grades of virgin PP can be used to house oil, and some of the PP packages in the recycle stream, like ice cream tubs, don't fit that criteria, he added.
The study, which also looked at the market for recycled PP, is the third one on the subject completed by WRAP.
The not-for-profit group, which focuses on reducing waste and promoting sustainability, said it has partnered with the plastics industry to find ways to overcome technical barriers preventing food-approved recycled PP.
According to WRAP, the group will further investigate any potential problems that could be caused by in-mold labels or direct printing inks. It is also researching ways that PP packaging could be automatically sorted into food and non-food categories.
Currently, manual sorting is the only way to ensure that the post-consumer PP meets EU regulations. But finding an economically viable method of high-speed automatic sorting would be the “key to make the process of recycling PP commercially attractive,” the study said.