Engineers develop plasma-based coating that makes plastics less permeable

The researchers found in some instances, PET bottles have replaced glass and manufacturers are opting for plastic containers instead of jars because they are lighter and less likely to break.

Extends shelf life

However, many people are concerned about the health benefits using plastic and questions remain over the substances contained in packaging which could seep into food.

Plasma-based coatings could provide an answer and may extend the shelf life of a food product.

At the Institute for Electrical Engineering and Plasma Technology, Prof Dr Peter Awakowicz and his team has developed a process for coating surfaces with protective layers that makes the synthetic material less permeable.

“Plastics have a poor gas barrier, i.e. they are permeable to air which can pass through the material in both directions. This is why water stored in PET bottles loses carbon dioxide and why food goes bad, for example by changing its flavour​,” he said.

He added the coating his group is working on would make plastic packaging 100 times more impervious than it is today.

“Sometimes you hear people say, ‘This tastes of plastic’, and they are right,” ​said Awakowicz. “A synthetic material that does not leak ingredients of some sort does not exist. With our coating, the volume of those substances could be reduced to 1% of the volume that usually seeps out​.”

The RUB team looked at how a chosen shape made from a specific synthetic material is best coated using a plasma. For this purpose, the team used a plasma enhanced chemical vapour deposition (PECVD).

Thicker layers would tear

In one experiment, the substance hexamethyldisiloxane is vaporised, blended with oxygen and dispersed in plasma with silicon dioxide deposits on the synthetic material. The plasma provides the energy necessary for the process.

The barrier layer which the RUB team applies to the synthetic material has a thickness of 50 nanometres. Thicker layers would tear and disrupt the recycling process, because the silicon dioxide layer is wafer-thin.

Negligibly small, the quantities in the coating layers applied by the RUB researchers are not relevant for recycling. They are not applied to the synthetic material directly, however; the process begins with a protective layer.

This is because tests have shown the oxygenated plasma process for depositing silicon dioxide affects the synthetic surface.

“No matter how great a barrier layer you apply to this damaged layer, it will never hold​,” said Awakowicz.

To render this problem, the RUB team, together with colleagues from Paderborn, decided rather than applying the barrier layer to a standard synthetic surface, they used a self-assembled monolayer (SAM).

“These are molecules that assemble on an activated surface like tin soldiers and creates a regular layer​,” added Awakowicz. “In comparison, standard synthetic surfaces are nowhere near as tidy; consequently, it is impossible to quantify how much additional destruction the PECVD process causes in them.​

“In the SAM layer, even the slightest disorder is immediately apparent. The greater the mess caused by PECVD, the more strongly the surface is affected by the process. As a result, the team from Bochum and Paderborn have demonstrated the coating process destroys the topmost atom layers by oxidising surfaces​.

Optimising the process for PET bottles

“We used to think, by applying the protective layer, we are covering the synthetic material with glue, with a bonding agent. But that’s not true​.

“Experiments have shown the underlying protective layer is transformed by the growing barrier layer. Through oxidation, it gains similar properties to those of the barrier layer, thus making the synthetic material more impervious​.”

Despite these findings, different items cannot be plasma coated in the same manner. The results depend on a number of parameters, e.g. plasma density, oxygen content, and the intensity of ion bombardment.

The plasma parameters ultimately determine how impervious, hard and elastic the barrier layer becomes. Different synthetic materials, such as PET, polypropylenes or polycarbonate, require different plasma properties. The item’s shape is likewise significant.

In collaboration with partners from the industry, the team has optimised the process for PET bottles, with a grant from the Federal Ministry for Economic Affairs and Energy, Germany.

Awakowicz added: “Products in coated packaging are few and far between; PET bottles are still being made without a barrier layer. This may change in one or two years​.”

SOURCE:​ Institute for Electrical Engineering and Plasma Technology Faculty of Electrical Engineering and Information Technology Ruhr-Universität Bochum, Germany.

PUBLISHED: ​RUBIN Science Magazine.

TITLE:​ ‘Making synthetic materials more impervious’

AUTHOR(S):​ Prof Dr Peter Awakowicz