Getting post-consumer recyclates safely into high-quality applications such as packaging requires technical finesse. Either all problematic substances must be removed from the material or it must be shielded with a layer of suitable material. In principle, all materials that completely enclose the recyclate and can withstand the application-specific loads are suitable for the second variant. A plasma coating fulfills these requirements. A team of young scientists at IKV in Aachen is working on just such a technology. In an interview with Circular Technology, Ali Cetin, Philipp Alizadeh and Jonas Franke explain what plasma actually is and why the technology is so well suited to shielding recyclates.

Circular Technology: Understandable for laypeople: What is a plasma?

The plasma leaves an extremely thin functional layer inside the vial. Photo: IKV

Ali Cetin: Plasma is the fourth aggregate state of matter alongside solid, liquid and gaseous. If the gaseous state of matter is considered, the atoms or molecules are freely movable in the space under consideration. The atoms or molecules in space are electrically neutral, which means that the number of positive protons in the atomic nuclei equals the number of negative electrons in the atomic shells.

If the gas is supplied with sufficient energy – e.g. through electromagnetic radiation – the electrons can detach from the atomic shells, resulting in a mixture of positively charged atoms (ions) and free electrons. Technical plasmas, which typically have a degree of ionization of or , are extremely reactive despite the low proportion of charged particles and enable chemical processes under simple conditions. At room temperature and low pressure, chemical reactions are made possible in remarkable applications such as Plasma Enhanced Chemical Vapor Deposition (PECVD), which can normally only take place at much higher temperatures of a few hundred or thousand Kelvin.

In contrast to the coated bottle (center), the uncoated bottle (left) shows a clear change in transparency after storage in caustic sodium hydroxide solution, which can be attributed to etching processes and degradation of the inner PET surface. In order to increase the service life of PET bottles in contact with corrosive washing lye during washing processes for reuse, a protective layer was developed which inhibits the corrosive attack of the lye on the PET inner surfaces. This is illustrated by a visual comparison of the stored coated and uncoated sample in caustic sodium hydroxide solution with the original bottle. There are no visible differences between the coated sample and the original surface, even after storage. Photo: IK

Using PECVD, a gaseous precursor is ionized into a plasma, which then reacts with a substrate surface to deposit thin films.

CT: What does the applied layer consist of?

Jonas Franke: Coatings produced by PECVD can have different chemical and physical compositions depending on the plasma process and the gases used. In such applications, plasma layers are generated which, for example are based on the element silicon. Due to the different process control during layer deposition, glassy silicon-oxide (“SiOx“) and soft silicon-organic (“SiOCH”) layers, which fulfill different functions, are applied to the plastic surfaces.

CT: Why do you use plasma coatings for recyclate in this application?

Philipp Alizadeh: Plasma-based silicon layers have very good barrier effects against gases such as oxygen or water vapor. The aim of this application is to transfer the barrier effects to a wide range of chemicals that may be contained in the recyclate and may have a negative impact on the filling material.

No influence on recyclability

CT: What are the advantages of shielding recyclates with a layer of virgin material, for example?

Philipp Alizadeh: “The coatings used, which are only a few 10 nm thick, offer an innovative and more efficient solution for recyclates, as they are applied thinly and precisely to the product surfaces by PECVD, so that material consumption is additionally reduced compared to virgin material and the recyclability of the materials used is not affected. This results in a reduction in environmental impact and a reduction in costs compared to the use of new materials. The migration barrier can be tailored to different contaminants.

Safety first

CT: How stable is the layer – is there a risk of it being damaged and substances from the recyclate being transferred to the packaged goods?

On the uncoated sample film, a distribution of water droplets can be seen on the surface, which indicates a water-attracting (hydrophilic) film. By applying a layer to the film, a water-repellent (hydrophobic) protective layer was applied, which means that the water droplets do not spread and have the least contact with the surface. Photo: IKVJonas Franke: The coatings are developed taking into account specific coating properties to enable use in the food sector, which can be achieved using the PECVD process. The protective effect, durability and elasticity are conceived and pursued from the aspect of high stability of the protective layers, with the focus on inhibiting the risk of damage. The ultimate goal is to prevent the transfer of unwanted substances, regardless of their cause.

CT: Can the thickness of the layer be varied?

Ali Cetin: The coating thickness can be controlled by varying the coating times and is very relevant in terms of coating functionalities and application. Basic functionality usually requires a minimum thickness. If the cost factor and economic efficiency are taken into account, the layer thickness is designed on the basis of the most necessary requirements and functionalities in relation to cost minimization. If barrier coatings are considered, thicknesses of only a few 10 nm are required. In comparison, scratch protection coatings have thicknesses of up to µm.

CT: How expensive is the process compared to the sandwich method with new plastics?

Jonas Franke: The PECVD process is not only functionally superior, but also more cost-effective. This is clearly demonstrated by the performance of current systems for the internal coating of PET bottles for a barrier against oxygen and carbon dioxide. If the lack of recyclability or the difficulties in recycling are included in the costs, sandwich structures also prove to be clearly inferior from a financial perspective.

Extremely powerful

CT: Can the process be integrated inline? How long does a coating process take?

Ali Cetin: Plasma reactors are remarkably efficient, with market-standard reactors being able to equip a throughput rate of over 50,000 bottles per hour with barriers. This speed corresponds to that of established industrial processes such as stretch blow molding, blow molding and filling. The high efficiency of the plasma process therefore enables seamless integration into existing production processes. Plasma coating takes place in a separate plasma reactor and is usually implemented after bottle production. The duration of the entire coating process varies depending on the process control, but is typically in the range of a few milliseconds to seconds.

Picture at the top: The project team: (from left) Jonas Franke, Philipp Alizadeh and Ali Cetin with the plasma system in IKV’s technical center. Photo: Circular Technology

By fil