Mechanical Embossing Research Articles

Near-Surface Reconfiguration of Biopolymer Blends by Mechanical Embossment: Creation of Friction-Reduced Foils

Biopolymer blends of polylactic acid (PLA) and polybutylene adipate-co-terephthalate (PBAT) are extruded into flexible monolayer films. These blends are excellent candidates for the realization of environmentally friendly packaging applications. A necessary pre-requisite for that are appropriate tribological properties under mechanical contact. Reasonable wear resistance allows good protection of packed goods, and low friction forces reduce difficulties in stacking. In this research, mechanical embossment under high loads at room temperature was used for the modification of polymer surfaces to exhibit a significant friction reduction under dry conditions. The results particularly show a systematic decrease in the coefficient of friction for biopolymer blends containing 30 wt% and 40 wt% PBAT. FTIR was used to analyze the change in surface composition after mechanical embossing. A sophisticated FTIR calibration method revealed that the blend with 30 wt% PBAT shows a modified distribution of PBAT and PLA at the surface due to mechanical embossment. This leads to a controlled and long-lasting modification of the surface properties without a substantial change in the chemical composition of the polymer in bulk. Without the use of additional coatings, biodegradable packaging foils with improved characteristics are accessible.

Comparative Evaluation of Graphite Anode Structuring for Lithium‐Ion Batteries Using Laser Ablation and Mechanical Embossing

Lithium‐ion batteries inherently suffer from a target conflict between a high energy density and a high power density. The creation of microscopic holes in the electrodes alleviates the trade‐off by facilitating lithium‐ion diffusion. This study presents a novel concept for electrode structuring called structure calendering, combining mechanical embossing (MEC) and calendering. It is compared to the widely investigated laser ablation (LAS) process by structuring graphite anodes and examining their geometrical, mechanical, and electrochemical properties. It is shown that structure calendering results in wider and deeper holes of higher reproducibility than laser structuring. As a consequence of the different hole‐creation mechanisms, laser structuring induces a surface elevation around the holes while clogged pores are observed in the lateral walls of mechanically structured holes. In pull‐off tests, no pronounced impact of electrode structuring on the mechanical electrode properties is discerned. Structuring of electrodes using both methods yields significantly reduced electrode tortuosities and enhanced discharge rate performances. From a production engineering perspective, structure calendering has advantages over laser structuring in terms of material loss, processing rate, and investment costs, while the latter offers higher flexibility, precision, and presumably lower maintenance efforts. In conclusion, structure calendering represents an attractive process alternative to laser structuring.

Mechanical Structuring of Lithium‐Ion Battery Electrodes Using an Embossing Roller

Highly performing lithium‐ion batteries are essential for the electrification of the transport sector. However, the two performance criteria, high power density and high energy density, inversely correlate with each other via the mass loading of the electrodes. To achieve high charging and discharging rates at high energy densities, structuring of electrodes is a proven method. Currently, structures are mostly realized by laser ablation, which comes along with material loss and low process rates. Herein, a concept for electrode structuring through mechanical embossing in a high‐throughput roll‐to‐roll process is elaborated. Different integration options are described and the challenges are discussed. To provide a proof of concept, a hand‐operated embossing device is built and used to structure graphite anodes. In a rate capability test, an increase in discharge capacity at medium C‐rates by up to 14.3% is observed.

Evaluation of the Patternability of Fibre-Based Materials with Converting Experiments

Paperboard based packaging products are renewable alternatives for packages made traditionally from oil-based polymer materials and can be used for packaging of various products [1]. Embossing is used in packaging solutions to increase the functionality and appearance of the products. It can also be used to increase product safety by improving distinctiveness and identifiability of packages [2]. The aim of the study was to evaluate the patternability of various fibre-based materials. It was desired that the accuracy and details of the embossed pattern would be the same in all samples, regardless of their different material properties. The realization of this was evaluated by several analyses related to the performance of the materials in the embossing process. Eleven different sample materials were collected for the experiments so that the patternability could be studied extensively. The common denominator of the materials selected for testing was that they were all fibre-based paper and paperboard materials used in the packaging industry. Set of embossing tools were developed, and precision machined from brass, for the experiments. A laboratory scale mechanical embossing device was utilized in modification of sample surfaces to study patternability of selected fibre-based sample materials. The main variables in the forming experiments were pressing force and tool temperature. The samples were observed primarily visually - with the naked eye, with a scanning electron microscope and a 3D-profilometer which was used in the topography analysis of the achieved patterns. The results of the embossing test series confirmed that the height of the pattern increased as a function of pressing force and plate temperature and spring back occurred in all materials after the tool plates opened. It was deduced that the pattern dimensions of the embossing plate somewhat determined the achievable pattern height in the fibre-based sample materials, but the amount of springback did not change as a function of material thickness. Despite this finding, it was consistent that the amount of spring back was regularly reduced with higher tool temperatures. The optimization study of the magnitude of the forming force showed that excessive use of force is not required, which is beneficial in reducing the risk of material damage during processing and adjustment of embossing devices. All samples differing significantly from each other were found to be suitable for embossing, indicating that patterns such as those tested could be added to a variety of packaging applications.

Microstructuring of Electrodes for Li-Ion Batteries By Means of Laser Machining and Microembossing – a Promising Approach to Overcome Transport Limitations and to Improve Their Fast Charging Capability

In addition to costs, safety and durability, energy density and fast-charging capability are the key requirements for Li-ion batteries for automotive applications. To meet these requirements, new cell chemistries are being developed, such as silicon-containing anodes. Likewise, electrode designs with high mass loading, respectively high layer thickness and a high density are being researched. However, such high-energy electrodes exhibit significant transport resistances due to their design, which has a negative effect on the electrode kinetics and thus the fast-charging capability.In our presentation, we will use the example of NMC-based cathodes and graphite anodes to show how the electrode kinetics can be improved by systematically influencing the electrode microstructure and the electrode surface. Short and ultra-short pulsed near infrared laser processes as well as mechanical embossing methods using needle type tools are used. In doing so, near-surface “quasi 2D” modification and three-dimensional structuring of the electrodes active mass were investigated.We will demonstrate how a specific adjustment of the laser parameters e.g. energy per unit length allows a roughening of the electrode surface and a selective removal of the binder-conductive additive compound without damaging or significantly ablating the active material. This “quasi 2D” modification offers the advantage that the capacity is largely retained. Electrochemical analyses such as rate tests show an improvement of up to 25% in the fast-charging capability. Electrochemical impedance analyses using symmetrical cells underline these findings by revealing a reduced transport resistance. In tendency, the higher the laser energy input and surface roughness, the more pronounced the observed effects – with a loss of active material also being observed at high laser input.Comparatively, perforated (i.e. three-dimensionally microstructured) electrodes, are investigated. The results obtained are comparable to the surface-near modification. However, in the case of perforation by laser, about 10% of active material is removed, which is reflected in a corresponding loss of capacitance. Electrodes perforated by microembossing also show significantly improved electrode kinetics, but without material removal and capacity loss. This makes microembossing appear to be an interesting and alternative microstructuring process. Finally, the combination of both processes – “quasi 2D” plus 3D – is investigated to combine the beneficial effects of both.The differently processed electrodes were also subjected to various rapid aging tests such as 100 charge/discharge cycles with a current mode of C/3 as well as interspersed fast charging cycles. The modified graphite anodes exhibit a lower tendency to Li plating than unprocessed reference anodes, and the degradation is lower than for the reference electrodes. For instance, the perforated anodes exhibited a capacity retention of 96%.In addition to a significant improvement of the electrochemical performance, a clear enhancement of the electrolyte wetting is demonstrated. Droplet tests and rising height tests showed an improved wettability by a factor of 4 to 5, depending on the type of microstructuring.The process development and the electrochemical analyses were closely accompanied by microscopic and material analyses e.g. by means of white light interferometry, nano-CT, SEM and TEM as well as Raman spectroscopy to reveal the most important process-microstructure-property relationships.To sum it up, we believe that the presented microstructural modifications are very attractive – both, in terms of electrochemical performance as well as from a manufacturing point of view. Figure 1

The influence of residual stress on flux-barriers of non-oriented electrical steel

In rotating electrical machines, cutouts in the rotor laminations control the magnetic flux density distribution in the d- and q-axis of the magnetic core. Guiding the magnetic flux by cutouts leads to very narrow bridges in the electrical steel. However, at the same time, this can limit the maximum speed of a rotor e.g. due to the material’s maximum allowed mechanical stress. Therefore, the centrifugal forces confine the achievable power density. The aim of this study is the examination of the effect of flux-barriers fabricated by mechanical embossing on the magnetic properties of non-oriented electrical steel. The embossing process causes a static residual stress distribution and thereby a reduction of the permeability resulting from Villari’s effect. The non-oriented electrical steel samples are embossed with varying distances between the barriers and measured at different angles to the barrier edges on a single-sheet tester. A comparison with respect to their directional and embossing geometry-dependent effectiveness as magnetic flux barriers is carried out. A correlation of the results with measurements of the local flux density, obtained by neutron-grating interferometry, is performed in order to enable a consideration of the mechanical embossing on the local magnetic flux density distribution.

Thermoplastic, rubber-like marine antifouling coatings with micro-structures via mechanical embossing

New processing routes and materials for non-biocidal, antifouling (AF) coatings with an improved performance are currently much sought after for a range of marine applications. Here, the processing, physical properties and marine AF performance of a fluorinated coating based on a thermoplastic (non-crosslinked) fluorinated polymer are reported. It was found that the addition of lubricating oil and hydrodynamic drag reducing microstructures improved the AF properties substantially, i.e. the settlement of a marine biofilm, containing mixed microalgae including diatoms, was reduced to low levels. More importantly, the remaining fouling was removed from the coatings at low hydrodynamic shear rates and promising AF properties were obtained. Moreover, additional potential benefits were revealed originating from the thermoplastic nature of the coating material which might result in significant cost reductions.

Photonic Shape Memory Polymer with Stable Multiple Colors.

A photonic shape memory polymer film that shows large color response (∼155 nm) in a wide temperature range has been fabricated from a semi-interpenetrating network of a cholesteric polymer and poly(benzyl acrylate). The large color response is achieved by mechanical embossing of the photonic film above its broad glass transition temperature. The embossed film, as it recovers to its original shape on heating through the broad thermal transition, exhibits multiple structural colors ranging from blue to orange. The relaxation behavior of the embossed film can be fully described using a Kelvin–Voigt model, which reveals that the influence of temperature on the generation of colors is much stronger than that of time, thereby producing stable multiple colors.

Reduced friction on γ-Mo2N coatings deposited by high power impulse magnetron sputtering on microstructured surfaces

Wear-resistant molybdenum nitride (γ-Mo2N) layers are deposited on topographically modified surfaces. Mechanical embossing using diamond decorated stamps is employed to produce steel surfaces containing pit structures with typical lateral dimensions of about 10µm. Subsequent vacuum deposition of Mo2N by high power impulse magnetron sputtering (HiPIMS) allows the coating of the surfaces while preserving the surface topography. The so produced surfaces exhibit a substantial reduction of the typically high coefficient of friction (COF). This is revealed in tribological tests in comparison to coatings on non-modified surfaces. Spatially resolved investigations of the obtained abrasion pattern after tribology measurements under high loads and increased temperature allows the description of the decisive tribological processes.

Honeycomb Texturing of Silicon Via Nanoimprint Lithography for Solar Cell Applications

A novel texturing method to realize a honeycomb texture on multicrystalline silicon solar cells is presented in this paper. The demonstrated process chain is based on nanoimprint lithography (NIL), where an ultra-violet (UV)-curable polymer layer is structured by mechanical embossing in a high-throughput process. This patterned polymer layer can then be used as an etching mask for plasma or wet chemical etching processes to transfer the defined pattern into the silicon substrates. Within this study, the whole process chain of this novel texturing scheme, using interference lithography, cast moulding, NIL, and plasma etching, is described with a focus on the NIL process. The textured substrates are characterized by reflection measurements, which are compared with standard solar cell textures. Besides optical measurements, first results of honeycomb textured solar cells are presented. Short-circuit current densities above 40 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> were achieved on high-quality float zone material. To increase the feasibility of an industrial realization of this process chain, we are developing a roller-NIL tool to structure an etching mask in a continuous in-line process. First results of this novel tool are also shown in this study.