PETpla.net Insider 06 / 2019

MATERIALS / RECYCLING PET planet Insider Vol. 20 No. 06/19 www.petpla.net 42 Decontamination by means of vacuum during extrusion by Dr Ulrich Thiele, Polyester Technology The increasing trend towards closed recycling cycles for PET bottle waste is significantly increasing the demands placed on mechanical recycling concepts. Decontamina- tion continues to be at the focus of the recycling process in order to ensure that the recyclate can also be used in food packaging with as few restrictions as possible. In addition, the focus is increasingly on maintaining the recurring recy- clability of the polyester. Depending on the chosen recycling process, this can be considerably reduced by the addition of stabilisers and additives. When PET bottle flakes are extruded in the presence of vacuum during recycling, this is mainly done to remove the water with the aim of preventing or minimising the rapid hydrolysis of the PET melt. Depend- ing on the type of extrusion equip- ment used, success varies. The efficiency of vacuum degassing is mainly dependent on the melt surface in direct contact with the vacuum and its renewal. In the order single-screw extruder, twin-screw extruder, multi- screw extruder, the degassing power increases. In a joint project with Gneuss Kunststofftechnik GmbH, the influ- ence of process pressure during vacuum extrusion on the efficiency of decontamination was to be investi- gated. In the production and use of PET bottles, a large number of possible organic compounds are introduced either deliberately by the use of addi- tives during the production of the bottles or in the course of use in the polymer matrix. To demonstrate the suitability of a process for PET recy- cling, a challenge test is carried out in which a defined number and amount of organic compounds are added to the PET flakes whose properties simulate the possible organic impuri- ties. Following the recycling process, the amounts of these substances migrating from the polyester are quan- titatively analysed (see challenge test: https://efsa.onlinelibrary.wiley.com/doi/ epdf/10.2903/j.efsa.2011.2184). This test requires a great deal of technical, analytical and time effort and is aimed exclusively at detecting foreign substances that are harmful to health and contaminate the PET bot- tles during use. With the increase in the recy- cling rate and in particular in mate- rial cycles in which PET bottles are recycled repeatedly, in addition to the contaminants absorbed during use, repeated use of additives occurs. These are in particular substances for reducing the acetaldehyde content, as well as stabilisers and dyes. Unlike PET virgin, which has an average concentration of 0.5ppm acet- aldehyde in the granules and <3ppm in the bottle wall, higher acetaldehyde levels occur in the regranulate and the bottles made from it, depending on the recycling process used. To ensure that when using 30-50% regranulate, the low limits for acetaldehyde required in particular for still water bottles are met, additives must be added again to reduce the acetaldehyde content, so- called acetaldehyde scavenger. The project to investigate the influence of process pressure in vacuum extrusion on the efficiency of decontamination focused therefore on substances that are repeatedly added as additives in the material cycle and lead by their increasing concentration to processing and quality problems. A search was made for a possibility to investigate the effi- ciency of decontamination in vacuum extrusion as a function of process pressure with simple and inexpensive analytical methods. The main ques- tion was whether PET flakes accu- mulate a typical molecule or atom, which repeatedly enters the polyester matrix through additives and can be quantified using simple analytical methods. Ultimately, the presence of nitrogen-containing compounds was chosen as the target. Above all, the additive AA scavengers, dyes and stabilisers carry in their molecules nitrogen (N2) in bound form. But also polyamides such as MXD6, e.g. as a barrier layer or in dispersed form to improve the barrier properties are nitrogen-containing impurities. Table 1 lists some of the common N2-con- taining additives and their nitrogen content. It can be assumed that the nitro- gen content in the PET flakes is influenced primarily by the repeated addition of AA scavengers. With the Analytical Laboratories in Lindlar (http://www.analytische-laboratorien. de), a laboratory was selected that provided an analytical method using micro-Kjeldahl with which elemen- tal nitrogen in polyester could be detected reproducibly up to a detec- tion limit of 10 ppm. Trivial/Trading Name CAS-Nr. Sum Formula Molar Mass [g] Content N2 [%] Application Antharanilamide 88-68-6 C7H8N2O 136.15 20.6 AA-scavenger Polyamide MXD6 C13H18N2O2 234 12 O 2 barrier N,N'-1,6-hexanediylbis [2-amino-benzamide] 103956-07-6 C20H26N4O2 354.45 15.8 AA-scavenger C.I. Pigmentblue 15 147-14-8 C32H16CuN8 576.08 19.4 Dyestuff Estofilblue S-RBL 71872-84-9 C32H30N2O2 474.60 6 Dyestuff Table 1: Typical additives in PET bottle flakes