Sustainable bonding technology using atmospheric pressure plasma

The now traditional November workshop of the atmospheric pressure plasma user group (ak-adp) on questions of adhesion optimization with atmospheric plasmas took place in the traditional, newly renovated Volkshaus in Jena, Thuringia. The event was organized by Innovent e.V. in cooperation with Krüss GmbH, sika Deutschland GmbH, Sura Instruments GmbH and Tigres GmbH.

Adhesion optimization is a constantly recurring topic and quality criterion in many branches of industry. This is where plasmas, energetically excited gases, can help quickly, cost-effectively and environmentally friendly. Surface functionalization with atmospheric pressure plasma through ultra-fine cleaning, activation or nano-coating can replace chemical and vacuum processes or enable completely new functionalities and material combinations. Atmospheric plasmas are therefore an innovative way to achieve greater ecological sustainability, quality assurance and an increase in the economic efficiency of production processes. During the workshop, a particularly relevant focus of the plasma applications discussed was on the sustainability aspect, which is requested by many customers.

The presentations focused on topics such as efficient component cleaning, adhesion mechanisms, specific surface pre-treatments for various materials and applications, including composite materials and recyclates, as well as new methods for measuring adhesion. An evening event and the established regulars' table were also part of the workshop. Before the start of the program, there was an opportunity to visit the laboratory at Innovent e.V. Technologieentwicklung on 13 November.

Adhesion as a prerequisite for functionality

"Environmentally friendly and efficient component cleaning with atmospheric pressure plasma" was the topic of the presentation by Thomas Stehrer, Fronius International GmbH, Thalheim, Austria. He provided information on partial component cleaning with hot active plasma (HAP). The Acerios plasma torch delivers a plasma power of up to 4 kW at a working distance of 5 to 25 mm. In terms of automation, a distinction is made between component-guided systems (the plasma torch is fixed) and torch-guided systems (the plasma torch is moved, the component is fixed). The system can be integrated directly into the production line. Fluorescence analysis was used to detect organic contaminants. Surface analyses with XPS and adhesion tests (tensile/shear) demonstrated the superiority of HAP plasma cleaning over other methods. Oxide layers on aluminum are not removed by HAP.

With the technology presented, there is potential for improving sustainability in the targeted partial cleaning of components and in the replacement of chemical cleaning processes.

Functional surfaces must either have good adhesion (e.g. for bonding, painting) or poor adhesion (e.g. demolding or non-adhesion of impurities). In his article "Measuring adhesion with the force tensiometer", Daniel Frese, Krüss GmbH, Hamburg, presented various methods for measuring adhesion. Force tensiometers offer an easy-to-use, direct approach to investigating adhesion properties. They can also practically "record forces beyond the gecko's feet". A tensiometer consists of a precisely movable specimen stage and a high-resolution force sensor, with an extra camera enabling the synchronous recording of force and image data. At the heart of a force tensiometer is a high-precision spring balance for force measurement. Coupled with various test specimens (e.g. stirrup, ring, plate), the changes in weight or force (N) can be measured as a function of time (s). In addition, measured values that are otherwise difficult to measure are also accessible. Frese was able to demonstrate the diverse application potential of the method using several examples (including self-cleaning non-stick coatings for wood, sandblasting to improve adhesion, adhesion of hotmelt to paper, adhesion of polymers).

Applications for different material combinations

Martin Schollerer from the DLR Institute of Lightweight Structures in Braunschweig presented an application with very high requirements in his lecture on the "Application of AD plasma in locally surface-toughened adhesive bonds for aviation". Neither cohesion nor adhesion fractures are permitted here. Current system developments favor the use of ductile materials in the core zones of adhesion stress, including plasma pre-treatment to ensure optimum functional reliability of the bonded components.

Andreas Pfuch from Innovent provided information on the topic of "Improving the adhesion of top coats on resin-rich woods through the use of functional plasma layers". Atmospheric plasma technology can be used to adjust key properties of wood surfaces. In addition to adjusting the wettability, this also applies in particular to the adhesion of subsequently applied varnishes or glazes, e.g. to provide antimicrobial wood surfaces. Both jet plasmas and corona discharges are suitable for plasma coating. In addition to HMDSO, APTES or GLYMO can also be used as precursors. Corona discharges enable the relatively uncomplicated application of flat adhesion promoter layers on pine heartwood, Douglas fir and larch. This achieves a significant increase in adhesion of subsequently applied top coats. The increase in paint adhesion in the presence of moisture is particularly noteworthy.

"Adhesion of silicones to metals and plastics - a solvable problem" was the topic of the presentation by Norbert Eckardt, Sura Instruments GmbH, Jena. He presented the Pyrosil process, a silicatization of surfaces by flame treatment with a gas mixture containing silicon. Among other things, the layers produced provide an excellent adhesive base, increase the surface energy and offer hydrophilic and barrier properties against alkali ions and gases. The surface modification by flame/pyrosil treatment also enables good adhesion of e.g. RTV1 silicones to polypropylene. Both spot and large-area coatings are possible. Applications include improved adhesion, corrosion protection, improved moisture absorption and transmission. The Pyrosil process is an alternative process to primers and undercoats. Despite the use of fuel gas flames, the environmental balance is better than that of chemical processes for the majority of applications.

Special case of recyclates

"Plasma pre-treatment for more sustainable production" - Peter van Steenacker, Tigres GmbH, Marschacht, began his presentation by emphasizing the need for more sustainability in industry and consumption. He then went on to explain how plasma can contribute to this. The benefits include the minimization of (VOC) emissions, the use of environmentally friendly and/or more cost-effective plastics, improved recyclability by avoiding problematic substances and the use of adhesives, inks, paints, coatings and varnishes with reduced harmful substances. At Tigres, plasma nozzles (plasma jet, APPJ) or spray corona/corona jets and dielectrically impeded discharges (DBE (corona)) are available depending on the application. Both 3D bodies and films can be treated. The areas of application for plasma treatment include ultra-fine cleaning, activation, coating and deburring. Using several examples, the speaker was able to show how atmospheric pressure plasma processes can help to save material, energy and costs through sustainable processes and also comply with EU environmental legislation (reduction of the_{CO2 footprint}, REACH, etc.).

Talk on the topic of improving liability versus recyclability

A particularly exciting topic is improving adhesion in the context of sustainability and recyclability in particular. "Optimal adhesion and debonding on demand - opportunities and potential" - Michael Thomas, Fraunhofer IST, Braunschweig, and Sven Hartwig, TU Braunschweig, gave an overview of important measures for the circular economy. The functional surfaces of components and products should often have high adhesion, but at the same time be able to be separated by type during recycling. The speakers were able to show that "debonding on demand" is possible, for example via chemically reactive surfaces, but also via layers or materials such as special adhesives or low-melting alloys. The aim is to achieve a high degree of purity when recovering from complex multi-material components. Examples of debonding-on-demand systems include the low-temperature joining of credit and ID cards, adhesives in automotive engineering or functionalized thermoplastics in battery systems. It is particularly important to ensure that the triggers for debonding on demand are not too close to the application area of the component. The cost issue also plays an important role. A circular economy to conserve resources must be affordable if it is to be accepted and implemented by the industry. There is still a lot of development work to be done here and atmospheric plasmas offer great potential for new solutions.