Towards closed-loop recycling of plastic packages

The journey of the milk bottle

“To investigate at which point in the recycling process unintended substances enter the packaging material, we examined plastic (HDPE) milk bottles,” says researcher Ulphard Thoden van Velzen. “A plastic milk bottle encounters different environments during its life: production, life in the supermarket and of course its stay with the consumer. After that, it is discarded with the plastic waste and collected by waste management. In The Netherlands the bottle then first is transported to a cross-docking station and from there to a sorting facility. Finally, it makes its way to the recycling company to be converted into recycled plastic.”

The mystery of the contaminants

At each point in the value chain, Thoden van Velzen and his team took samples of milk bottles to investigate the presence of contaminants. Contaminants are substances that do not belong in the product, but that are nevertheless present. Thoden van Velzen: “At most points along the value chain, we found the substances we expected.” However, there were two abnormalities : upon entering the sorting facility and upon leaving.

“The bottles that arrive at the sorting facility contain very small amounts of additives that originate from the lid and label of the bottle. We weren’t expecting that.” But that wasn't even the most surprising result. When they examined the bottles taken from the end of the sorting process, they were baffled. “We were completely off track for a moment, is this really correct?”. Thoden van Velzen's voice still evokes amazement when talking about it. “Compared to the other analysis points, we suddenly saw a load of different substances here. And every time we examined another bottle, we found new types of contaminants. We were flabbergasted.”

The bale press

What turned out to be the cause of this mysterious contamination? The bale press. “At the end of the sorting process there is a machine that presses all types of sorted plastics (PE) together. This becomes a cube of compressed packaging that can easily be transported to the next step: the recycling company,” says Thoden van Velzen. Only, this compressed cube contains more than just milk bottles: “All PE plastics come together: detergent bottles, shampoo bottles, toilet cleaner bottles, you name it. And of course none of them are completely empty and clean. Due to the pressing, the residues are squeezed out and trickle down over the neighboring packages, such as milk bottles. That is where the large quantity and especially the great variety of contaminants comes from.”

On the road to safe recycling

“Now that we have identified the location in the recycling value chain that creates most contamination and prevents us from closed-loop packaging recycling, we can do something about it.” What solutions does the researcher propose? “In the route to a more circular economy, we should in any case prevent chemicals from migrating from the lid and label to the bottle. This must therefore be taken into consideration in the design process, for example by using a different type of ink to print the labels.

In addition, we should be able to sort these types of bottles separately and compress them into separate bales, so that these packages hardly come into contact with other packages. This way we can make much higher quality recycled material. Only time will tell whether the quality is really high enough to be approved for use in food packaging. The process can be made easier by, for example, packaging standardization: larger quantities of packages made of exactly the same type of plastic and with fewer disruptive components.

Now that we have located the main point of contamination in the value chain, we can create targeted solutions.”