Innovative plastics finishing thanks to plasma technology

The specific modification of plastic surfaces using plasma-supported processes has become increasingly significant in recent decades. This type of surface finishing for plastic products offers high-quality, environmentally-compatible and cost-effective solutions for the specific extension of the plastics application spectrum.

At the end of September the Institute of Plastics Processing (Institut für Kunststoffverarbeitung - IKV) at Aachen University (RWTH Aachen) invited interested parties to a specialist conference on “Surface technology - Innovative finishing for plastics”. 34 participants seized the opportunity and benefited from the latest know-how on this theme presented by the experts from industry and research.

Today the treatment of surfaces using an atmospheric discharge plasma immediately prior to coating, printing and adhesion is a widespread industrial method of improving coatings adhesion. Plasma technology today offers a diverse range of possible applications both in a vacuum as well as normal pressure applications, extending from fine cleaning through to the plasma activation of plastic parts prior to adhesion up to the corona treatment of roll goods before printing or multi-layer (barrier) coating of PET bottles.

Established technology for new materials

The themes covered in a lecture given by Dipl.-Ing. Karim Bahroun from the Institute of Plastics Processing, Aachen University, included for example the multi-layer coating of various bottle materials – also against the background of the fact that in addition to the use of PET for beverage packaging, the trend towards alternative plastics such as PLA (polylactide) or PP (polypropylene) is constantly increasing. While the plasma-supported bottle interior coating for PET bottles has been a long-established technology, its suitability for the two up-and-coming materials PLA and PP had to be first investigated. In the meantime it has been established that microwave-stimulated plasma polymerization is very suitable for barrier coating of beverage bottles. The coating enables the materials PLA and PP to achieve oxygen and carbon dioxide permeability levels approaching those of an uncoated PET bottle, this also applies to the process gas ethyne. In all the bottles investigated high performance levels and high mass flows resulted in a high CO2 barrier.

The pre-treatment of PLA can be directly integrated into the plasma polymerization process. All that is needed for successful coating is a brief oxygen pre-treatment process. In order to avoid any thermal damage to the PLA, the coating process must however be interrupted by comparatively long pauses, which could be dispensed with by using a more thermally resistant polylactide.

PP in contrast requires pre-treatment for the application of firm-bond plasma polymer layers. The use of an NF plasma in an external facility enabled a very good coating adhesion to be achieved, said Bahroun. Due to high stretching of the polypropylene the barrier coatings failed however during the bottle’s pressurization. PP bottles are however not used for carbonated beverages anyway, but more for fruit juice, milk or non-carbonated (still) water. For that reason an impermissible high expansion during the product’s life cycle can be avoided.

As far as the relatively rapid progress of the developments in the beverage packaging sector are concerned, reference was made to PET bottles: in this context, bottles of the first-generation still had to be produced with an additional base as it was not possible to initially manufacture PET with a stable base in the stretch blow-moulding process.

Special requirements for film

Dr. Klaus Vissing from the Fraunhofer Institute for Manufacturing Technology and Applied Materials Research (Institut für Fertigung und Angewandte Materialforschung - IFAM) in Bremen presented a paper on the theme of “Hard-elastic plasma polymer coatings”. Due to their processing conditions, films with a functional coating place special requirements particularly on this functional coating: it must be especially flexible, possess a low tear sensitivity, have good adhesion strength and be transparent. Its internal stress should be low, the coating process reliable and cost-effective.

Development work at the IFAM has shown that an ion-supported plasma coating process can satisfy particularly these requirements. Diverse trials have proved that ion-supported processes are suitable for producing abrasion-resistant and highly flexible coatings. The flexibility must be high enough to also enable the coating of plastic films, which can then be subsequently used in the diverse processes of plastic processing. In cooperation with the IKV initial rear injection trials were already conducted in order to establish the possible applications for this functional coating in the area of plastics processing.

Quality assurance still difficult

It was not least the introduction of normal pressure processes, which do not require (expensive) vacuum technology, which created the improved general conditions for the application of the plasma processes. These processes have in the meantime established themselves in many areas of coating and surface technology. In addition to the areas already mentioned, further examples include plasma application in the low and normal pressure process in the adhesion of thermoplasts, in the textile sector, for medical products such as contact lenses or the functionalization of pharmaceutical primary packaging. A more widespread application of plasma processes in plastics processing is currently still often hampered by a lack of online process monitoring facilities. In the IKV, although extensive investigations on process monitoring have already been conducted, plasma processes in general are influenced by a large number of different parameters. Intensive research is currently being conducted into the further development of effective quality assurance methods.

Source: interpack