The amount of environmentally harmful plastic waste in Germany has risen steadily in recent years. Packaging generates a particularly large amount of waste. Plant-based coatings for paper packaging could offer a sustainable alternative in the future. In the BioPlas4Paper project, researchers from the Fraunhofer Institute for Surface Engineering and Thin Films IST and project partners have used a coating process known as plasma polymerisation to create water-repellent and plant-based barrier coatings on paper, thereby improving the paper's resistance to weathering.
Plastic packaging continues to be a major environmental problem. Paper, on the other hand, is made from renewable resources. Its advantage over petroleum-based plastic is that it decomposes and does not remain in the ground for many years. The downside is that uncoated paper does not provide a barrier to moisture or oxygen. Uncoated paper is sensitive to temperature, highly reactive to moisture and bacteria, and has uneven surfaces. In order to exploit the full potential of the material, improve recycling options, replace plastic packaging and open up new areas of application, we need to improve the life, durability and quality of paper products. This is the task that researchers at Fraunhofer IST are tackling in the BioPlas4Paper project. They are working closely with the Technical University of Darmstadt and the Thünen Institute for Wood Research. To create homogeneous, moisture-repellent coatings on paper, the project partners are focusing on plant substances such as oregano and chia oil, as well as extracts from bark. Among other things, these plant substances have antibacterial properties.
Bio-based plasma polymers crosslink with the paper surface
“So far, we have been using untapped plant substances with a high proportion of unsaturated fatty acids to make the paper hydrophobic, i.e., water-repellent. For this, we employ atmospheric pressure plasma technology, where gas is excited with high voltage under ambient pressure to generate plasma, i.e., a particle mixture of ions, free electrons and, in most cases, neutral atoms or molecules. The process causes a discharge between the electrodes,” says Martin Bellmann, a researcher at the Fraunhofer IST in Braunschweig, Germany. The addition of nitrogen converts the plant materials into an aerosol and introduces them into the plasma as vaporised organic precursors to form polymer networks. Experts call this process, in which the precursors are activated by the plasma, plasma polymerisation. The micron-sized particles bind together to form plasma polymers, the tiny droplets also crosslink with the paper and spread evenly over the raw paper substrate, penetrating deep into the pores and fibres of the surface. “The plasma is essential in making the plant molecules reactive and enabling them to crosslink into polymers,” Bellmann explains.
Innovative plasma source concept
The plasma is generated in a plasma source by ionising gas between two rotationally symmetrical electrodes to which high voltage is applied. What is new is the geometric arrangement of the electrodes and the way in which the aerosol is introduced and the plasma ignited. The combination of these measures results in an innovative concept, developed by the researchers specifically for the project, which - under atmospheric pressure - minimises the effects of ambient air even at higher coating speeds, while maintaining consistent and reproducible results. “At high processing speeds, the roughness of the paper’s surface causes turbulence in the ambient air, altering the properties of the plasma. We can avoid these detrimental effects with our concept,” says Bellmann. The plasma source is introduced close to the surface of the paper, completely displacing the ambient air. The researchers work with plasma temperatures of around 70 degrees Celsius so as not to affect the paper itself, the bio-based precursor molecules or the properties of the plasma polymers produced.
Water-repellent coatings made with olive oil
Numerous tests involving a wide range of plant oils and extracts have enabled the researchers to demonstrate that bio-based materials can be separated or deposited reproducibly and homogeneously using plasma. Excellent hydrophobic coatings can be achieved with olive oil and chia oil, for example. Depending on the precursors used and the coating parameters, the researchers can influence and optimize the coatings. The aim is to prepare paper for increasingly sophisticated use cases and, in the future, to even replace plastic materials. “One example is moving boxes, which, with our hydrophobic coatings, can withstand even extended periods of rain without becoming soft. Our objective is to reduce dependence on fossil resources and support the transition to a resource-efficient economy,” says Bellmann.
