PO and PS Recycling: Hot Caustic Process as a Game Changer

Driven by the goal of further increasing the proportion of recyclates in plastic packaging, the demand for transparent and white recyclates is experiencing strong growth. However, it is not yet clear how a secure supply of these uncoloured recyclates can be guaranteed in the long term. As a result, some design-for-recycling guidelines have updated their recommendations for white polyolefin (PO) and polystyrene (PS) packaging and declared directly printed white cups as non-recyclable. “This decision surprised many companies, mainly because the claim was not supported by data,” says Dr Andrey Charkovskiy, Senior Business Partner Recycling and Polymers at Siegwerk, one of the world's leading suppliers of printing inks and coatings for packaging applications and labels.

With this in mind, the company has teamed up with packaging manufacturer Greiner Packaging to gather data on the issue and tackle the challenge head-on. “Krones, recognised in the industry as one of the market leaders for the provision of hot washing systems for both PET and PO recyclers, had already worked with us in the past on several deinking tests in their state-of-the-art industrial pilot washing systems. It was therefore a clear decision to involve them in this project,” explains Dr Andrey Charkovskiy.

Adaptation of Existing Recycling Technologies

The main aim of the initiative was to highlight the discrepancy between the design-for-recycling guidelines and the actual state-of-the-art recycling processes, particularly in relation to printing inks. This is because, according to Charkovskiy, no directive has yet recognised the crucial role of deinking in the circular economy of packaging, even though the recycling industry is already driving this process forward. “This misalignment is a significant problem, as it could lead to the value chain investing significant resources in ineffective solutions, which ultimately undermines sustainability efforts,” emphasises the Siegwerk expert.

The three partners quickly realised that the hot caustic washing process with an adaptation to polyolefins could offer an ideal solution to this problem. “This process is traditionally used in PET bottle recycling and is increasingly being adopted by PO and PS recyclers,” says Jörg Sabo, Global Director Marketing & Innovation at Greiner. Different machine manufacturers offer different designs of these lines, with customisations aimed at achieving the most effective cleaning.

Partners Call for Guidelines to be Adapted

The initiative was able to show that recycling directly printed PP and PS cups is not a real challenge if state-of-the-art equipment is used. “To our surprise, we didn't have to change the washing conditions or the structure of the printing inks,” says Dr Andrey Charkovskiy. The material was first tested in the laboratory in accordance with DIN SPEC 91496. This showed that the printing inks were removed in the laboratory without any bleeding.

“These encouraging results led to the next phase of validating these findings under industrial conditions at Krones,” says Jörg Sabo, Global Director Marketing & Innovation at Greiner, adding: “The continuous hot washing process also proved to be extremely effective in removing the standard offset printing inks. These results show that deinkability is possible without having to deviate from standard washing conditions.” This recycling technology can also be extended to other plastics. According to Sabo, however, further trials are still needed.

Most packaging segments require recyclates that are not only transparent or white, but also thoroughly decontaminated – free from printing inks, adhesives, coatings and residues of the contents. Under these conditions, the project participants believe that the hot caustic process can establish itself as an important pillar of the circular economy for PP and PS cups.
However, Charkovskiy and Sabo emphasised that the relevant laws and guidelines must be adapted accordingly. This is because the current design-for-recycling guidelines assume that the post-consumer waste material is only washed with water at room temperature. “However, with the increasing capacity for hot washes, the possibility of deinking needs to be considered. It is only a matter of time before this becomes obvious and we are committed to ensuring that this realisation occurs as soon as possible,” says Charkovskiy.