KHS Plasmax looks at buffer layers in barrier coating

Beer products in Van Pur bottles

There is still potential to further enhance plasma coated barriers in PET bottles. In this feature for Plastics News Europe, David Vink reports from Aachen on FreshSafe PET development, challenges and potential solutions.

KHS Plasmax, a subsidiary of KHS Corpoplast, has been looking to further enhance its plasma process to coat stretch blow moulded PET bottle interior surfaces with a nanoscale glass barrier. The Plasmax process – introduced by KHS predecessor SIG in 2003 – adds a 0.0001mm pure silica oxide (SiOx) glass barrier layer to PET bottles, extending shelf life.

In 2013, KHS Plasmax introduced Plasmax+ for coating inner and outer surfaces. It also launched the FreshSafe PET brand name and logo for customers to use for promoting the technology’s benefits of product freshness and recyclability.

The Martens brewery group in Belgium was the first European brewery to apply KHS Plasmax technology to PET beer bottles. It was followed by Van Pur brewery in Poland which installed an InnoPET Plasmax 12D inner coating line in 2014. At that time, Jaroslaw Gajda, manager of the KHS subsidiary in Poland, said plasma-coated bottles can be recycled 100% bottle-to-bottle, with alkaline treatment to separate the glass coating.

Dr Joachim Konrad, KHS Plasmax managing director for process technology and design, talked about FreshSafe PET barrier bottle developments in February 2016 at the Plastics Technology Colloquium organised in Aachen, Germany by the IKV plastics processing institute at RWTH University.

He described how the KHS Corpoplast parent and KHS Plasmax machinery companies work together within a Bottles & Shapes programme to develop tailor-made bottle packaging solutions with FreshSafe PET barrier coating technology, also considering closures when required.

Coating involves microwave supported PICVD (plasma impulse chemical vapour deposition) in a “reliable high performance rotating machinery”, at rates from 12,000 (Plasmax 2D) to 48,000 bottles per hour (Plasmax 20QS). Mounting bottles in the machine is followed by reaction chamber and bottle evacuation, flooding with process gas, plasma ignition and coating.

Konrad said plasma-supported gaseous phase coating enables high barriers to oxygen ingress, while preventing carbon dioxide in beverages and beer from exiting. KHS Plasmax claims beer storage life of eight months. Konrad stressed high flexibility with adaptation to coating needs on a wide range of shapes and dimensions.

He said thinner, intermediate “buffer” coating with a non-barrier material increases adhesion to PET, significantly enhancing barrier effects, especially under mechanical strain. Such buffer layers “favourably” affect barrier layer properties with thickness below 20nm, but have a negative effect in thickness of 20nm and more, Konrad stated.

He said he believes a possible explanation could be that while thin buffer layers offer a sufficiently homogenous surface for application of subsequent (barrier) layers, more rugged surfaces of thicker buffer layers prevent build-up of closed following layers. Expectations that additional barrier layers, applied on a first “stack” consisting of a barrier and buffer layer, would improve overall barrier effects, have not been fulfilled. Further IKV research should establish whether replication of buffer layer surface defects in further barrier coatings are responsible for poor barriers, or if the problem comes from “a completely different mechanism”.

In a paper on improving barrier properties, IKV researcher Benjamin Twardowski said longer coating time of a thicker buffer may contribute to poorer barrier effects: higher energy transfer causing thermally induced tension and associated lower coating adhesion, as well as rougher buffer layer surfaces due to extension of surface defect “islands”.

As buffer granular structure grows, gaps and holes are created that are not completely closed by subsequent coating, Twardowski suggested, when presenting supporting SiOx barrier and inorganic silane based silica (SiOCH) non-barrier buffer layer microscopic images. One solution could be coatings applied by low pressure PECVD (plasma enhanced chemical vapour deposition). Very many thin “stacks” consisting of glass-like barrier and buffer layers should then cover defects better and reduce coating system strain.