Developing plasma-coated paper as a plastic alternative for the packaging industry

In recent years, the amount of plastic waste that is harmful to the environment has been increasing in Germany. Packaging generates particularly large amounts of waste. Plant-based coatings for paper packaging could be a sustainable alternative in the future.

In the BioPlas4Paper project, researchers at the Fraunhofer Institute for Surface Engineering and Thin Films IST and project partners use a coating process known as plasma polymerization to create a water-repellent, plant-based barrier coating on paper. This improves the paper’s resistance to stains. Weather effects.

Plastic packaging continues to be a major problem for the environment. Paper, on the other hand, is made from renewable raw materials. An advantage over petroleum-based plastics is that they don’t break down and remain in the ground for years.

However, uncoated paper has no barrier to moisture or oxygen. Plain paper is sensitive to temperature, reacts strongly to moisture and bacteria, and has an uneven surface.

To make the most of the material’s potential, improve recycling options, replace plastic packaging and open up new application areas, we need to improve the service life, durability and quality of paper products.

This is the challenge to which Fraunhofer IST researchers are dedicated in the BioPlas4Paper project, in close collaboration with Darmstadt University of Technology and the Thunen Wood Research Institute.

To create a homogeneous, water-repellent coating on paper, the project partners are focusing on extracts obtained from plant substances and tree barks, such as oregano and chia oil. Among other properties, these plant substances exhibit antibacterial effects.

Bio-based plasma polymer crosslinks with paper surface

“Until now, unused plant materials rich in unsaturated fatty acids have been used to make paper more hydrophobic, or water repellent. This involves exciting gases at high voltages at atmospheric pressure. Atmospheric plasma technology is employed. “In a process that produces plasma, a particle mixture of ions, free electrons, and, in most cases, neutral atoms and molecules, an electrical discharge is created between the electrodes,” says Braunschweig. says Martin Berman, researcher at Fraunhofer IST.

Addition of nitrogen converts plant matter into an aerosol, which is introduced into the plasma as a vaporized organic precursor to form a polymer network.

Experts call this process, in which precursors are activated by plasma, plasma polymerization. Micrometer-sized particles combine to form plasma polymers. Small droplets also cross-link with the paper, spreading evenly over the base paper substrate and penetrating deep into the surface pores and fibers.

“Plasma is essential for making plant molecules reactive and cross-linking them to form polymers,” Berman explains.

Innovative plasma source concept

Plasma is generated using a plasma source that ionizes a gas between two rotationally symmetrical electrodes to which a high voltage is applied. What is new is the geometry of the electrodes and the way the aerosol is introduced and the plasma is ignited.

The combination of these measures gives rise to an innovative concept developed by the researchers specifically for this project, which allows coating at higher coating speeds at atmospheric pressure while minimizing the effects of ambient air. You can maintain consistent and reproducible results.

“At high processing speeds, the roughness of the paper surface causes turbulence in the surrounding air, which changes the properties of the plasma. With our concept, these negative effects can be avoided,” says Berman. I say.

The plasma source is introduced close to the surface of the paper, resulting in complete displacement of the surrounding air. The researchers work at a plasma temperature of about 70 degrees Celsius so as not to affect the properties of the paper itself, the biological precursor molecules, or the resulting plasma polymer.

Water repellent coating using olive oil

Through numerous tests on a wide range of vegetable oils and extracts, researchers have demonstrated that plasma can be used to reproducibly and uniformly separate or deposit biobased materials.

For example, you can use olive oil or chia oil to achieve a good hydrophobic coating. Depending on the precursors and coating parameters used, researchers can influence and optimize the coating. The aim is to prepare paper for increasingly sophisticated use cases and even replace plastic materials in the future.

“One example is our mobile boxes. Our hydrophobic coating means they won’t soften even during long periods of rain. Our aim is to reduce dependence on fossil resources and support the transition to a resource-efficient economy. ,” Berman said.