Project moves to improve barrier performance of plasma coatings on PET, bioplasticsProject moves to improve barrier performance of plasma coatings on PET, bioplastics

The gas barrier performance of polyethylene terephthalate (PET) packaging can be improved in many ways including through creation of multi-layer packaging with EVOH, nylon or another material used as a barrier material, or with the addition of a plasma coating of silicon oxide to the PET. Plasma coatings often are applied to a container's interior so that they not easily scratch; scratches in the coating open a passageway for carbon dioxide egress or oxygen ingress.

The use of plasma coatings has been relatively common on blowmolded PET bottles for about ten years, with improvements made along the way, mostly in the speed with which bottles can be coated. But researchers at Germany's Institute of Plastics Processing (IKV; Aachen, Germany) are working on a new project to enhance the performance of the plasma coating. The researchers' focus first will be to improve a coating's performance on PET bottles, but then also will consider how to improve a plasma coating's gas barrier performance on bioplastics and other materials.

The Aachen-based Institute has long been involved in plasma coating technology; the Plasmax coating technology marketed by stretch blowmolding machine manufacturer Corpoplast is a commercial version of a process first tested at the IKV. In this new project, the focus of the investigations is upon the development of suitable layer systems to enhance the permeation properties of plasma-polymer coatings, as well as the conception and development of processes for the interior and exterior coating of beverage bottles or coating of films and plastic parts.

The current investigation is focused on a process to enhance the barrier performance of plasma-polymer layers by the use of a substrate-bias. Studies on films have shown that the deposition of barrier coatings using a substrate bias leads to a significant increase in the barrier properties because the negative effect of layer defects is reduced. The IKV has built a plasma reactor so that it can bias the inserted PET bottle and study the results. After a successful validation of the process a transfer ofthe technology to other materials such as polylactic acid (PLA) is planned.

In addition to the influence of the substrate bias on the coatings' performance, its influence on the coatings' flexibility also will be evaluated. A possible influence on the strain tolerance is crucial for applications such as carbonated soft drinks, since the stress caused by the internal bottle pressure may lead to a stretching and even cracking of the barrier coating. For this purpose, investigations will determine to what extent the deposition of multifunctional coatings consisting of hard and soft layers may increase the strain tolerance of the whole composite while maintaining good barrier properties at the same time.

The IKV has more than 300 employees working on improving plastics processing. Around 50% of German plastics engineers with a university degree received their education at IKV. The Institute is operated by an association of sponsors, which currently has a membership of about 250 plastics companies and institutions from all over the world.