A new water-based recycling process could offer an alternative route for converting difficult plastic waste into useful chemical building blocks. An international research team has demonstrated a catalyst-free method that uses water, oxygen, heat and stirring to break down polyethylene and other plastics into short-chain organic acids.

In testing, waste from polyethylene bags, caps and mixed streams was placed in a stirred pressure reactor with water. At around 125°C and under pressurised oxygen, the plastic melted into small droplets. This created a large interface between the polymer and water, where reactive hydroxyl radicals formed and cut long polymer chains into smaller acid molecules.

The lack of an external catalyst is a major advantage. Many chemical recycling systems rely on metal or acid catalysts that can be affected by pigments, fillers, additives and contamination. Because the new process generates reactive species directly at the water-plastic interface, it may be more tolerant of inconsistent or lower-quality waste.

The technology could be particularly relevant for flexible and multilayer packaging, which is often unsuitable for mechanical recycling. These structures may contain several polymers, coatings, inks and metallic layers. Initial trials suggest the process can handle commercial polyethylene films, mixed polyolefins and additive-containing materials, while inorganic and metal-rich residues can be removed separately.

Variations of the method were also tested on polypropylene, polystyrene, multilayer films and rubber tyres. Compared with pyrolysis, which usually requires temperatures above 400°C and produces complex hydrocarbon mixtures, the water-based reaction operates at a much lower temperature and mainly generates short-chain dicarboxylic acids for reuse in chemical manufacturing.

A five-litre reactor containing 300 grams of polyethylene achieved an 89% conversion rate after 48 hours. Although this indicates potential beyond small laboratory trials, commercial adoption will depend on solving challenges involving continuous feeding, oxygen transfer, heat control, product separation, water reuse and long-term stability.

The process must also be validated with real packaging waste. Recovered chemicals will need to meet downstream quality standards and fit into existing supply chains.

The researchers see the technology as a complement rather than a replacement for mechanical recycling. Clean, single-polymer packaging should still be recycled mechanically wherever possible. The new method could instead create a higher-value outlet for mixed, contaminated or multilayer plastics that currently have few recovery options, turning difficult packaging waste into useful industrial raw materials.