Delamination and wetting behavior of natural hot-melt coating materials

In order to achieve a defined product behavior, the coating of particulate solids is an often used process in the food, chemical, and pharmaceutical industries. The wetting behavior affects the coating quality associated with hot-melt coating (HMC) considerably more than the quality associated with solvent-based coating. Thus, the uniformity and the effectiveness of the coating can be severely affected by process parameters. Furthermore, previous studies showed that delamination of the coating layer occurs frequently in HMC products [Mueller et al. 2018 (https://doi.org/10.1016/j.apt.2017.12.020); Woerthmann et al. 2021 (https://doi.org/10.1016/j.powtec.2020.09.065)]. Therefore, the successful use of natural coating materials requires a detailed understanding of the delamination and wetting behavior. The delamination observed for HMC-particles has, however, rarely been investigated, and the basis for and factors affecting this process remain unclear. However, a correlation between delamination and the wetting properties of the materials seems likely, since wetting properties provide information about the interaction between the coating and carrier material. In this work, the delamination frequency was investigated via laboratory coating experiments and via micro-computed tomographic measurements with different material combinations. In addition, the wetting behavior was investigated using a drop shape analyzer. No correlation between delamination and wetting behavior was found. All investigated coating agents exhibited excellent wetting properties with contact angles in a range of 10° to 18°, while palm fat coatings were more prone to delamination (>65%) than the rice bran (<10%) and carnauba wax coatings (<17%).