Investigation of expandable fillers for reversible adhesive bonding in photovoltaic modules

Photovoltaic (PV) is a sustainable energy source and an efficient tool towards CO2 neutrality. However, recycling of PV modules remains challenging due to the strong connection of the multi-material assembly. The present work aims at the development of disbonding concepts for adhesive connections used in PV modules to enable improved recyclability and repairability of such modules. To achieve stimuli-responsive adhesive connections, thermally expandable fillers were implemented in a condensation-curing alkoxy-based silicone adhesive. 10–50 wt% of expandable graphite (EG) as well as 10–50 wt% of thermally expandable microspheres (TEM) were incorporated to obtain a controlled expansion of the adhesive at elevated temperature. The expansion ratio in dependence on time and temperature was examined. Furthermore, nanoscale magnetite (Fe3O4) passivated with a layer of SiO2 was added to the formulation to trigger the expansion of TEMs, and therefore separation of adhesive bonds by inductive heating. In this approach an efficient heating and expansion of the adhesive layers was feasible by applying an external alternating magnetic field with a ring coil. Aging stability, influence of filler content and expansion of the adhesive layers were further evaluated in lap shear tests. The addition of functional fillers decreased the bond strength by approximately 50% for adhesive layers containing 50 wt% EG. In contrast, adhesives with 50 wt% TEM exhibited a higher bond strength (by 10%) compared to the unfilled silicone system. Thermally triggered expansion significantly lowered the bond strength (20–30%) for TEM-filled adhesives and led to separation for EG-filled adhesives. Temperature cycling tests had no influence on the filled and unfilled adhesive's strength, whereas after damp heat tests all samples exhibited a lower (less than 40% of the unaged, pristine reference samples) but similar bond strength.