Extrusion International 4-2024

41 Extrusion International 4/2024 recycling. Ecologically and energeti- cally, mechanical recycling is favor - able under the circumstances of pure material streams. Including the dis- advantages of increased sorting and purification efforts. The idea of me- chanical recycling is the replacement of virgin pristine materials by recy - clates of constant quality. However, this is challenging in case of recycled post-consumer waste plastics since unknown impurities and miscibility’s of different type of polymers effect the resulting quality of the recyclates and limit their applications [SS21, FW22]. Recyclates in food packaging Since packaging plastics account for the largest share of global plas - tic production, the resulting rapid generation of plastic waste and mis- management cause environmental damage to ecosys- tems and hu- manity. Hence, rapid mechanical re- cycling for post-consumer packag- ing plastics and the use of recycled plastic in this sector is indispensable [SS21]. From a contamination per- spective, the use of post-consumer plastics in food applications is a criti - cal issue where safety assessments and analytical methods must be co - ordinated to exclude human harm, since remaining contaminations can migrate into food. The impurities can be distinguished in their ori - gins and end up in many variations in post-consumer recyclates (PCR) [UGPv23, FW22]. Under the aspects of life cycles of post-consumer plas- tic, these contaminations originate from different polymeric impuri - ties, additives, chemical contami - nants resulting from intended and unintended use, chemically formed substance during recycling process and degradations of plastic compo - sitions [RDv17, STB+23, FW22]. For the identification of these, various tech- niques are suitable to detect compounds of different origins for example GC/MS related to volatile monomers [STB+23, WF08, WF11]. Before the implementation of recycled post-consumer plastics as packaging applications, the identi- fied amounts of impurities by their migrations need to meet several re - quirements since the applications are restricted to the quantity of migrat - ing contaminants. These limitations are set by different organizations like the U. S. Food and Drug Admin- istration (FDA) or the European Food and Safety Authority (EFSA) accord- ing to the application regions [EU 11, EU 23, EU 22, Foo21]. In Europe,final products originating from recycled plastics need to meet the threshold of toxicological concerns and com - plex mixture safety assessments related to their product sections es - pecially for the Food Contact Materi - als [EU 11, EU 23, EU 22]. Therefore, intensive research for the analysis, identification and reduction of con- taminations in plastics are conduct - ed to design their life cycles [BJ23, UGPv23, Pfa22, STB+23]. Including, modelling of migration and design of different recycling paths [RDv17, Fra05, WF08]. Even though, recycling processes developed in recent years, the recycled post-consumer polypro- pylene stream for packaging materi- als don’t meet overall quality for safe use especially in the food packaging industries whilst holding the overall largest plastic share [PWNC22]. One way to enable the use of PCR pro- pylene in the Food Contact Material sector is to overcome the migration of impurities by an efficient barrier protection. Plasma barriers for food contact materials A promising path to protect ma - terials against contaminations, degradation and migration during food contact or other inorganic and organic substances is a surface coat - ing via plasma enhanced chemical vapor deposition ( see Picture 2 ) [SHT+23, WF11]. Plasma based coatings enable evading the limitations of gas per - meation of packaging materials [Pre10]. Transferring the method- ology to food containers based on recycled plastics, an alternate ap - proach to stop migrations of con - taminants can be achieved in com - parison to other e.g. multilayered plastic composite films [KBD22, WF11]. Including the advantages of low operating and material costs with homogenous properties com- pared to other barrier approaches. Gaseous monomers are excited by electromagnetic radiation to the plasma state leading to their frag - mentations in the PECVD processes. Subsequently, plasma polymeriza - tions result in recombination by a layer deposition of the plasma onto corresponding substrate surfaces [YLC+21]. Through a simple process control during PECVD, nanoscale functional coatings with a fast de- position rate, outstanding chemi - cal and physical film properties on low temperature substrates can be achieved. Process parameters such as monomer input, gas flows, en- ergy display influence the resulting chemical and physical film proper- ties e.g. chemical structure and layer thickness [AAB+23]. Signifi- cant influence on the resulting sur- face properties can be assigned to the power input and power pulsing, influencing the fragmentation and the resulting plasma polymerisa - tion during PECVD process [KBD22]. Among various coating chemistries silicon-based coatings gained high relevance due to their excellent properties and efficient process- ability in industry [CLW01]. Silicon oxide (SiOx) barrier coatings with various chemical and physical prop - erties can be quickly, nondestruc- tively and environmentally friendly deposited [VPA+22]. SiOx coatings Picture 2: Concept of plasma enhanced chemical vapor deposition