Manufacturing of physically unclonable fluorescent markings by in-mould functionalization (IMF): particle coating, adhesion properties and interphase formation

Unique and unclonable fluorescent particle-based markings enable the individualization of parts for track and trace applications and benefit from a more robust optical detection when compared with markerless techniques. The marking of surfaces by coating methods however requires additional fabrication steps and limits scalability. We therefore investigate a modified in-mould coating process to alleviate these drawbacks for injection moulded parts. Fluorescent particles are thereby incorporated into the surfaces of the parts by spraying them first into the mould and transferring them then permanently onto the parts surface by overmoulding so that no separate processing step for the application of a marking is needed.
However, while the ability of the particles to transfer and adhere to the parts surface is considered a key factor for the usability and robustness of a marking, the highly dynamic wall temperatures and the cooling of the melt make adhesion a non-trivial task and expose a significant knowledge gap. The adhesion of the particles was therefore analysed to reveal the principal adhesion mechanisms and to investigate tailored organic particle coatings to improve adhesion.
We identified mechanical interlocking and interfacial chemical reactions as key adhesion mechanisms. Peel tests showed that particle adhesion improves significantly when modifying the fluorescent particles with a polymeric multilayer shell using covalent layer-by-layer (LbL) assembly of polyethyleneimine and polyethylene-alt-maleic acid (up to four layers). Amide formation within the polymeric shell contributes to a high shell stability and establishes covalent links between the particles and the plastic part. The analysis of the interfacial reactions disclosed the formation of urethane and urea groups in case of polycarbonate. It was demonstrated that a second fluorescent particle type with identical excitation wavelength but different emission wavelength increases the information depth of product markings.