Extrusion International 6-2022

63 Extrusion International 6/2022 nm, the mono-material container re- mains fully recyclable. Their coatings offer gas, solvent, and water vapour barrier, while also providing protec- tion against aggressive corrosive sub- stances. They are food safe and can be applied on any plastic material. Plasma process and technology – What is a Plasma? Plasma is one of the four funda- mental states of matter. It is often de- scribed as ionised gas, because it con- tains a significant portion of charged and freely as well as arbitrarily moving particles. The properties of a plasma are essentially defined by the interac - tions of charged particles. Continuous elastic and inelastic collisions between these freely moving particles can lead to further ionisation processes. If certain so called monomer gases are excited to a plasma, these ionisa- tion processes lead to the formation of reactive particles, which can be adsorbed on surfaces to form a coat- ing. This process of plasma assisted coating application is called plasma polymerisation or plasma-enhanced chemical vapour deposition (PECVD). Plasma-enhanced chemical vapour deposition (PECVD) Plastics are produced by conven- tional polymerisation, a chemical pro- cess in which monomers are bonded together forming long molecule chains (polymers). Plasma polymeri- sation, on the other hand, is a non- specific polymerisation of fragments formed in a plasma, resulting in a polymer structure that consists of par- tially old and new functional groups from a gaseous monomer. These mol- ecule fragments formed in the plasma can recombine in the plasma bulk and are then adsorbed on a substrate sur- face to form a coating. The properties of the coatings are highly dependent on the plasma process parameters, namely process gas mixture and mass flow, plasma energy density, process pressure and coating time. For temperature-sensitive substrates such as plastics, only low-temperature plasmas can be used. One way to tech- nically generate a low-temperature plasma is to ignite the plasma under low-pressure conditions. Typical tech- nical low-pressure plasmas are operat- ed in the pressure range of a few pas- cals, so that the process temperature is only slightly above ambient tem- perature due to the increased mean free path length of the gas particles, regardless of the process gases used. IonKraft uses microwave excita- tion to form the plasma, as particu- larly high deposition rates can be achieved. Performance of the IonKraft coating technology In the following, exemplary results on gas and solvent barrier as well as a demonstration of the corrosive resis- tance of the IonKraft coatings against alkaline solutions are presented. In these examples, the coatings were applied on 1 liter HD-PE bottles. Oxygen Transmission Rate (OTR) The oxygen transmission rate (OTR) is the steady-state rate at which oxy- gen gas permeates through a sub- strate at specified conditions of tem - perature and relative humidity. The OTR is a key performance indicator for the gas barrier of the coatings. The graph (image 1) shows bench- mark measurements comparing the OTR values of 1-litre bottles from HD- PE with IonKraft coating against un- coated containers and multi-material systems with Polyamide (PA) and Eth- ylene Vinyl Alcohol (EVOH) as barrier plastics. It can be seen, that the IonKraft- coatings have the potential to even outperform the oxygen barrier of multi-layer systems with EVOH. Chemical stability as proven by re- tention of oxygen barrier One key aspect for the wide appli- cation of the IonKraft barrier tech- nology is the chemical stability of the coating against corrosive media. To demonstrate chemical resis- tivity, a test was performed using caustic soda with pH = 13 as corro - sive testing liquid. 1L HD-PE bottles were coated and the barrier against oxygen was measured. The same bottles were then filled with aggres - sive caustic soda and stored at 40 C° for 4 weeks. After that, the OTR was measured again and the barrier val- ues increased only slightly, which clearly indicates that the coatings re- mained stable and intact (Image 2). Solvent Barrier The key performance indicator for barrier against solvents is the weight reduction due to migration loss. IonKraft examined their coat- ing system in collaboration with Image 1