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FEATURE Coating & Laminating Flexibility in intermittent coating

KROENERT has launched into a new partnership to develop an intermittent coating slot die to control the flow of the coating medium for a smooth, defect-free finish

Applications in the field of electric mobility are increasing battery electrode quality requirements. For optimal

energy density setting, the metal foils coated with the electrode materials are calendered, where the electrode layer is compacted onto the metal foil. In order to guarantee optimal calendering conditions, a coating-free zone between the coated electrodes is essential. This coating-free zone serves to balance out the stresses, which occur when normally very thick coating is compacted onto metal foil. The coated and uncoated areas of the substrate behave very differently. In the peripheral areas, the metal foil begins to wrinkle and is no longer able to function as a current arrester. Due to the demands of electrode manufacturers for coating-free zones, many manufacturers are currently developing intermittent coating processes. An intermittent coating is also required, in

order to adjust the battery electrodes in the longitudinal direction to the geometric size of the battery. In spirally-wound cells, the intermittence length corresponds to the length of the spirally-wound electrode.

Slot die characteristics The following characteristics information applies to a KROENERT patented slot die technology that has been also used for the application of adhesives: - Capillary and pressure forces acting between the slot die and the substrate. - No excess coating media being required. - The system being closed and not affected by solvent evaporation or contamination. - A wide range of applications in respect of speed and operating width. - Options for chemically-reactive materials and simultaneous, multi-layer coating. Coating weight continuity in the machine

direction is a function of web velocity and the conditions under which the coating medium is fed to the die. Coating precision is defined by: the internal design of the die; the quality of the slot surfaces and the lips of the die; the positional accuracy of the die to the drum; the quality of the substrate; and the accuracy of the coating unit. Slot dies should also offer a high level of flexibility; it is possible to apply closed coating films intermittently in the transverse direction and to create combination squares

20 November 2015

with transverse intermittence. Together with TSE Troller, KROENERT has developed an intermittent slot die that is able to create geometrically accurate coatings corresponding to the required electrode dimensions and precision. The idea of joint development was not to

control stopping and starting of the coating process by means of an external upstream valve, but to interrupt the flow of the coating medium in the die. This is of advantage as the distance from the point of intermittence to the substrate transfer point is utmost short. Herewith, the stresses on visco-elastic media, such as battery slurries, are reduced. Further, there is no need to direct the coating medium through narrow channels, such as those commonly found in valves. Intermittence is achieved by a rotating bar in the middle of the die, located in front of the second distribution channel. This bar has an opening and closing motion to interrupt the flow of the coating medium to the lip of the die, which is referred to by KROENERT and TSE Troller as the ‘torsion bar die’. This motion can be variably adjusted to the speed of the web. As soon as the die has closed, the coating medium returns via a bypass circuit to the coating media storage unit. The layout must also ensure that, when in operation, the distribution channels are not clogged by particles from the media, as this can lead to production malfunctions. The optimal processing of battery coating media is dependent on many factors. Even minor alterations to the composition of the medium - changes in solid content, or even

the lengthening of storage periods prior to application - can result in major differences in processing behaviour. From this perspective, the aim when developing new coating procedures and technologies should always be to minimize the factors influencing the process. The coating technology is able to deliver the highest levels of precision required by manufacturers and ensures optimal results.

Further applications testing Intermittent slot dies are already being trialled for other applications. Tests on adhesives and PEDOT: PSS coatings have also been carried out; adhesives were found to be an optimal application. Transverse strips could be precisely applied, with none of the stringing normally associated with adhesives. The first tests on PEDOT: PSS coatings

were undertaken to establish the system limits in respect of viscosity. No tearing of the coating took place when closing the die. It was assumed that the dominance of surface tension would be problematic and would prevent the meniscus breaking off behind the die. It has to be possible to pull the liquid film back into the slot - this remains a criticism of the intermittent die. High viscosity is, therefore, the requirement for optimal deployment of the torsion bar die. KROENERT is undertaking further development work with leading German research institutes in this area.

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