Influence of different photoinitiators on the polymerization of acrylate monomers and their suitability in adhesive tapes

This work presents the results of a six months internship carried out at the Würth laboratory in Künzelsau (Germany) with the purpose of investigating the impact of different photoinitiators (PIs) on the properties of UV-cured acrylic pressure sensitive adhesives (PSA). These adhesives cure under specific UV wavelengths, making the selection of suitable PIs a crucial factor for the properties of the final material. The present study aimed to optimize existing PSAs by examining how these substances influence key characteristics like adhesion and cohesive force balancing, at the same time, factors such as cost and energy consumption. To this end, the developed experimental campaign involved the comparison of different formulations based on a standard monomer mixture but with different commercial PIs. For each formulations the samples were prepared in a coating machine using three different speeds, to provide different energy doses, and two cooling temperatures to control the exothermal polymerization. The resultant crosslinked adhesives were then characterized in terms of mechanical (tensile strength and elongation at break), adhesive (peeling test) and cohesive (static shear test) properties and to ensure complete crosslinking by spectroscopic methods. Comparison of the results revealed several ways to improve adhesive performance by adjusting PIs. High levels of an unsuitable PI caused a highly exothermic reaction up to 120°C, resulting in high bond strength but low cohesive strength. On the other hand, the combination of such PI with others gave good results: up to 50% increase in bond strength and up to 5 times longer times than the standard static shear test. These findings not only help to understand the critical role of PI in the adhesives industry, but also provide a solid basis for developing advanced adhesive formulations with optimised properties for a variety of applications.