Swapping old for new - when it comes to recycling PET beverage bottles from the deposit system, this can be taken literally. A look at the new Interzero plant in Liebenau, Lower Saxony, shows what is possible today with state-of-the-art processes. What you don't see here, however, are empty bottles. The material had already gone through several process steps when it was delivered here. The collected PET bottles have previously been shredded, sorted by material and colour and then cleaned by scalding.

1. delivery and quality control

The so-called PET flakes arrive at the recycling plant in big bags weighing 1,000 kilograms. The Interzero team takes a sample from every second bag, tests the material for contaminants and analyses chemical residues, moisture content and viscosity. The actual processing only begins when quality control gives the green light.

2. homogenisation

The flakes are first homogenised in mixing silos to absorb fluctuations in the composition of the material and thus stabilise subsequent processes. The screw conveyor then moves on to a further, even more precise sorting stage. Here, Interzero relies on near-infrared (NIR) technology.

3. Near-infrared separator

The NIR separator is fully automated and separates any impurities and undesirable colours such as green, yellow or dark blue. Only light blue flakes are allowed to remain in the process, as they positively affect the clear colour shades required for competitive production-grade plastics.

4 Extrusion

Now the actual extrusion stage begins, which turns the pre-treated PET flakes into high-quality pellets for production. To do this, the plastic is melted at around 290 °C and fed into another screw conveyor. This viscous mass passes through several filters before finally being squeezed through a perforated panel to be crushed by a granulator. To make the granulate suitable for use as a food contact material, one final processing step is required, which takes place in a solid state polycondensation (SSP) reactor. The material remains in the reactor for about 10 hours at a temperature of about 180°C, in a vacuum/nitrogen atmosphere that allows oxygen molecules to escape and avoids undesirable reactions with the material.

5. return to circulation

The finished pellets are then filled back into large bags and sent for further processing. Interzero's customers use food-grade rPET to produce so-called 'preforms'. These 'bottle blanks' are produced using injection moulding and look a bit like test tubes: they can be blown into full-size bottles and - once filled - end up back on the shelves in shops, completing the bottle-to-bottle cycle. The cycle starts again when consumers return their bottles to collect their deposit.