Weekly

Plasma-coated paper as a plastic substitute

15 November 2024

A new process developed by the Fraunhofer Institute makes it possible to make paper water-repellent using plant-based substances. Plasma polymerisation could replace plastic packaging in the future and improve the recyclability of paper.

In order to fully utilise the potential of paper as a material, improve recycling options, replace plastic packaging and open up new fields of application, the durability, resistance and quality of paper products must be improved. Researchers at the Fraunhofer IST are tackling this task in collaboration with the Technical University of Darmstadt and the Thünen Institute of Wood Research in the BioPlas4Paper project. In order to create homogeneous, moisture-repellent layers on paper, the project partners are using plant substances such as oregano or chia oil and extracts obtained from bark material. These plant substances are characterised by their antibacterial effect, among other things.

‘We use previously unused plant substances with a high proportion of unsaturated fatty acids to make paper hydrophobic, i.e. water-repellent. To do this, we use atmospheric pressure plasma technology, in which gas is excited by means of high voltage under ambient pressure in such a way that a plasma, i.e. a mixture of particles consisting of ions, free electrons and usually also neutral atoms or molecules, ignites. A discharge occurs between the electrodes.’ - Martin Bellmann, scientist at the Fraunhofer IST in Braunschweig

The plant substances are converted into an aerosol by the addition of nitrogen and introduced into the plasma as vapour-like organic precursor compounds in order to form polymer networks. Experts refer to this process, in which the precursor compounds are activated by a plasma, as plasma polymerisation. The micrometre-sized particles bond with each other to form plasma polymers, but the tiny droplets also crosslink with the paper and lie flat on the rough paper substrate, penetrating deep into the pores and fibres of the surface. ‘Only when the plasma is used do the plant molecules become reactive and crosslink to form polymers,’ explains the researcher.

Water droplets cannot penetrate the coated paper. (Image: Fraunhofer IST)

The plasma is generated using a plasma source by ionising gas between two rotationally symmetrical electrodes to which high voltage is applied. The geometric arrangement of the electrodes and the way in which the aerosol is introduced and the plasma is ignited are new. The combination of these measures results in an innovative concept that the researchers developed specifically for the project, so that the effects of the ambient air are minimised under atmospheric pressure, even at higher coating speeds, and the results are consistent and reproducible.

‘Due to the roughness of the paper surface, the ambient air swirls at high processing speeds, which changes the plasma properties. We can avoid these harmful influences with our concept.’ - Martin Bellmann

The plasma source is brought close to the paper surface and completely displaces the ambient air. The researchers work with plasma temperatures of around 70 degrees in order not to impair the paper itself, the bio-based precursor molecules or the properties of the plasma polymers produced.

In numerous tests with a wide range of plant oils and extractives, the researchers were able to prove that biobased substances can be reproducibly and homogeneously deposited or separated using plasma. Very good hydrophobic layers could be achieved with olive and chia oil, for example. Depending on the precursor compounds used and the coating parameters, the researchers can influence and optimise the layers. The aim is to equip paper as a material for increasingly demanding utilisation scenarios and thus replace plastic materials in the future.

‘One application example is moving boxes, which can be exposed to rain for longer periods without softening thanks to our hydrophobic coatings. Our aim is to reduce dependence on fossil resources and support the transition to a resource-efficient economy.’ - Martin Bellmann

Source: Packaging-Journal