After a two-year interruption due to coronavirus, the Winterthur Surface Day at the ZHAW Winterthur returned at the beginning of June with 95 participants. The eight presentations revolved around the topic of "Smart Coatings". 14 exhibitors were represented at the industry exhibition.
What does Smart Coatings actually mean? This new term comes from the 21st century and refers to coatings that can react to an external stimulus, such as human skin. This is why they are also called bioinspired coatings. For example, there are self-healing coatings on which scratches disappear again. The global market for smart coatings is expected to reach around 25 billion US dollars by 2025, representing massive growth.
Nicolas Spencer from ETH Zurich was the first to speak about polymer-functionalized tailor-made surfaces. He explained: "When we think of materials, the first thing that often comes to mind is their bulk properties: Strength, density, modulus of elasticity or even price. However, the surface properties, such as lubricity, biocompatibility or wettability, are often even more important for a particular application. Surface-bound polymers represent a particularly effective and versatile class of materials that can modify surfaces for specific applications. This can be achieved by attaching polymers to surfaces by a variety of methods or by growing polymers from surfaces." As a first example, he mentioned PLL-g-PEG, a bifunctional macromolecule that can bind to metal oxide surfaces on the one hand, and on the other hand makes the surface hydrophilic and biocompatible through the PEG chains. PLL-g-PEG is used in biosensors and in friction reduction; it is also self-healing. Finally, hydrogel coatings and their friction-reducing properties were presented.
Thomas Imwinkelried focused on magnesium implants, which may experience their medical technology breakthrough with a clever coating Stephen Schrettl, AMI University of Fribourg/TU Munich, spoke about polymer films with responsive functions. He began by showing polymers with non-covalent bonds, such as polymers held together by metal ions (supramolecular polymers). These have special properties, e.g. they are broken down into monomers by heat and can therefore be recycled. The properties of these polymers can also be tuned from stretchable to stiff and brittle. Their mechanical properties almost match those of commercial polymers such as PE. Scratches on supramolecular polymers heal when exposed to light or heat. Next, he presented new approaches to nanocomposites: Small platinum clusters can be created in a polymer matrix. These composites still have the catalytic effect of platinum and decompose hydrogen peroxide, for example. An external stimulus, such as irradiation with UV light, generates platinum clusters in the plastic and causes a color change from pale yellow (atomic platinum) to black (platinum clusters). Finally, he showed plastics with embedded special dyes that change color in response to an external stimulus. The dye fluoresces red when it is present in aggregates and green when it is dispersed. This can be used to produce plastics that change color in response to mechanical stimuli (stretching).
Gaffar Hossain from v-trion in Bregenz, Austria, spoke about "Smart coating of textile for advance material application". His company conducts research into self-healing, hydrophobic, hydrophilic, antimicrobial, weather-resistant and electrically conductive textiles. Smart textiles are, for example, textiles with sensors that can register and record movement, e.g. when used as a pedometer or as a machine-human interface. Self-cleaning textiles treated with fluorochemicals are a major area. Special textiles can then be used as filter material to separate mixtures of water and oil. Antiviral/antimicrobial textiles are also being produced, based on superhydrophobic surfaces.
Functional and smart coatings at IMPE was Martin Winkler's topic. The scientist from ZHAW Winterthur presented the research carried out in the laboratories of the Institute of Materials and Process Engineering (IMPE) at the host ZHAW Winterthur. The methods for surface modification can be divided into grafting from/to, adding additives to the binder matrix and modifying the binder. The first example of "grafting to" was photoreactive ski wax, which adheres to the ski twice as long as conventional ski wax thanks to the azide/nitrene bond. Medals were won with it at the 2018 Winter Olympics. As an example of additivation, he showed Easy-to-Clean coatings on conveyor belts, which become dirt-repellent thanks to a hydrophobic additive. He then talked about antimicrobial water pipes that have been biocidally treated with quaternary ammonium salts. Inorganic-organic hybrid binders can be produced using sol-gel chemistry. Wound dressings are no longer allergenic thanks to new binding agents. Finally, he spoke about hydrophilic coatings for cardiac catheters. A hydrophilic polymer is bound to the catheter by azide/nitrene chemistry, which then becomes slippery through blood contact and can therefore be easily pushed through the blood vessels.
The presentation by Sonja Neuhaus from the University of Applied Sciences Northwestern Switzerland dealt with "UV light, electron beams, plasma - powerful helpers for smart coatings on plastics". The UV LEDs available today can replace traditional mercury vapor lamps for curing coatings. However, unlike Hg lamps, LEDs emit in a narrow wavelength range. The radiation from UV-A LEDs penetrates deep into the coating, but does not harden the surface. This can be exploited and the still sticky surface can be recoated with a different material. Sonja Neuhaus calls this ReLaFun (Reactive Layer Functionalization). This can be used to create scratch-resistant, easy-to-clean or anti-fog coatings, for example.
Electron beams can then be used to immobilize enzymes on surfaces, which can be used as biosensors. Atmospheric plasma can be used to activate and coat surfaces. As an example, surfaces coated with zinc oxide particles were presented, which can neutralize bad odours with light.
Giulia Morgese, from the ZHAW Winterthur, then spoke about simple wet-chemical pre-treatment of polyolefins for coating and bonding. Despite their immense importance, polyolefins such as PE or PP are not so easy to bond. This is due to their inert surface and the resulting low surface energy. Only non-specific, drastic and non-pemanent processes such as plasma treatment make PE and PP bondable. Giulia Morgese has therefore developed a primer that can be activated by UV light, which makes it possible to bond PE and PP to each other with various adhesives (Araldite, PU adhesive, superglue). She was able to show that the adhesive strength of her primer is at least as good as that of conventional plasma processes. However, the plastic must be free of additives, otherwise the adhesive strength deteriorates. She has also shown that - in contrast to plasma treatment - her primed workpieces can be stored for months without the subsequent bonding deteriorating, so the primed workpieces are stable in storage.
Michael Steidle from Textildruckerei Mayer in Messstetten-Unterdingisheim spoke about: Sensor and actuator technology, intelligent textiles of the next generation. There is a need for textiles that conduct, support, detect (sensors), report, visualize and protect. Mayer has developed a cover material for comfortable and robust work chairs that is as soft and breathable as fabric and as robust as integral foam. This is made possible by a micro-armoring that is applied to a carrier textile. Many functional textile surfaces are required in automotive engineering. For example, for heating, sensors and actuators. These new textiles will greatly change the automobile as we know it.
Thomas Imwinkelried from the RMS Foundation Bettlach, Switzerland, posed the question: Will a clever coating help magnesium implants achieve a breakthrough? Implants today are made of magnesium because it dissolves and disappears once it has fulfilled its function. The disadvantages of magnesium are: Hydrogen gas is produced during dissolution and magnesium is susceptible to stress corrosion cracking. Magnesium screws are used in Halux operations, for example, and coronary stents are made of magnesium. The following clinical requirements can be met by a coating:
- Requirement: should disappear without undesirable side effects.
- Effect of the coating: Delay of degradation, avoidance of gas bubbles.
- Requirement: Mechanical stability, no premature failure.
- Effect of the coating: Reduction of stress corrosion cracking, extension of load-bearing capacity.
- Requirement: Desired fabric reactions.
- Effect of the coating: e.g. bone ingrowth.
Conclusion: Only a clever coating creates the technical basis to help magnesium implants achieve a breakthrough.
