Industrial quality raw material: Mixed PET plastic can be prepared for mechanical recycling

(01.06.2010) In Germany, 400,000 tons of PET was consumed in 2004, 385,000 tons of this (more than 96 per cent) was used for packaging. Consumption of PET packaging is growing at double-digit percentage rates. At the same time mixtures or barrier layers cause discoloration and other changes that make reuse difficult. The recorded amount of PET provided for recycling in Germany is in excess of 200,000 tons per annum. PET recycling has been state-of-the-art for many years. The collected material flows of PET are used mainly to produce polyester fibres, sheets for thermoforming, strapping tape or new bottles.

In the mechanical recycling area numerous processes for bottle-to-bottle recycling have been approved by the FDA and many of these have been used successfully for many years. In Europe the total plant capacity available for this process is about 100,000 tons per year.

For more contaminated and mixed PET waste energetic utilisation and feedstock recycling are generally recommended; however, these are not so effective in terms of resource efficiency. No adequate solutions are currently available in Central Europe for mixed PET, which would suggest that mixed PET ends up as mixed plastic fractions or in the sorting residue from packaging processing systems. Estimates for the mixed plastic flow in Germany range from about 20,000 to 30,000 tons per annum. This mixed PET is simply regarded as "too dirty" for mechanical recycling.

The Fraunhofer ICT has developed and tested a process in which mixed PET can be prepared for mechanical recycling. Within the scope of the 'LogiPET - High-performance logistics equipment from PET mixed fraction' research project, which was subsidised by the German Federal Ministry of Education and Research (BMBF), a pilot project was carried out in which a 4.5 ton batch of mixed PET was prepared and processed. The most important steps:

+ Preparing mixed PET collected as part of Germany's waste separation policy (DSD) into a compoundable material, separating unwanted materials, such as labels, adhesives, polyolefins, PVC

+ Compounding and adding the prepared and shredded material, and the associated design of the extruder and process technology

A combination of processes was chosen, with initial shredding and screening as the main step (in some cases after an Attritor to dissolve adhering contamination) followed by two-stage density-based separation in a hydrocyclone. The heavy fraction (underflow) of the cyclones was dried and any metal was then largely removed by magnetic separation and a roll-type corona-electrostatic separator, while the PVC fractions were reduced by means of an electrostatic plastic separator.

The result of this was a high-quality mixed PET compound which, with a PET content of almost 99 per cent and a chlorine concentration of at least 50 ppm, is completely suitable as an industrial quality raw material.

The prepared mixed PET was used to produce a recyclate compound. With two-stage vacuum degassing the moisture (water content 3,600 ppm) was separated so that the initial molar masses (Mw around 62,000 g/mol) did not drop despite the fact that no hydrolysis protection additives were added during compounding. An examination of the mechanical parameters of the material without additives showed a Charpy impact value of 3.915 kJ/m² (with additive, up to 4.3 kJ/m²) and an elasticity modulus of 2,380 N/mm² (with additive, up to 2,460 N/mm²).

Authors: Jörg Woidasky and Andreas Stolzenberg, Fraunhofer ICT, Pfinztal-Berghausen (Germany)

www.ict.fraunhofer.de
Foto: Koelnmesse

 

 

 

 

 

 

 

 

 

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