TY - THES

T1 - Development of dismantable adhesives and reversible bonding concepts for photovoltaic applications

AU - Wanghofer, Florian

N1 - embargoed until null

PY - 2020

Y1 - 2020

N2 - The present thesis aims at the development of disbonding concepts for adhesive connections used in photovoltaic (PV) modules to enable improved recyclability and reworkability of such modules. To achieve reversible 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 thermally triggered expansion of the adhesive. 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 were further evaluated by lap shear tests. The addition of functional fillers decreased the bond strength by approximately 50 % for 50 wt.% EG and increased it by 10 % for 50 wt.% TEM. Expansion separated the bond with 30 and 50 wt.% EG and weakened it by up to 20 and 50 % for 30 and 50 wt.% TEM. Temperature cycling tests had no influence on the filled and unfilled adhesive’s strength. After damp heat tests all samples exhibited similar strengths, less than 40 % of the unaged and unfilled reference samples. In a further approach to achieve reversible and reworkable adhesive connections, vitrimeric elastomers based on functionalized poly(dimethylsiloxane) (PDMS) were realized. By exploiting reactions of epoxide functional groups with anhydrides and carboxylic acids, covalent adaptive networks (CAN) are formed in the presence of suitable catalysts. A reprocessable elastomeric CAN was successfully established by curing epoxidized PDMS with a multifunctional fatty acid and n-butylamine (BA) as a covalently bonded catalyst. In tensile tests the system showed a remaining strength of more than 85 % after reprocessing the grinded material in a hot-press for 5 h at 160°C. Lap shear tests were done to evaluate bond strength, rejoining, aging stability and the ability to weaken the adhesive bond. The elastomeric CAN achieved approximately 20 % of the reference silicone adhesive’s strength. Tests at 160°C decreased the bond strength by 60%, compared to a reduction of 40% for the reference sample. Temperature cycle tests did not influence the material strength, storage at 120°C for 930 h decreased the bond strength by approximately 20 % and in damp heat tests the material suffered from hydrolytic bond cleavage. Rejoining the lap shear samples resulted in a remaining bond strength of more than 70 %.

AB - The present thesis aims at the development of disbonding concepts for adhesive connections used in photovoltaic (PV) modules to enable improved recyclability and reworkability of such modules. To achieve reversible 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 thermally triggered expansion of the adhesive. 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 were further evaluated by lap shear tests. The addition of functional fillers decreased the bond strength by approximately 50 % for 50 wt.% EG and increased it by 10 % for 50 wt.% TEM. Expansion separated the bond with 30 and 50 wt.% EG and weakened it by up to 20 and 50 % for 30 and 50 wt.% TEM. Temperature cycling tests had no influence on the filled and unfilled adhesive’s strength. After damp heat tests all samples exhibited similar strengths, less than 40 % of the unaged and unfilled reference samples. In a further approach to achieve reversible and reworkable adhesive connections, vitrimeric elastomers based on functionalized poly(dimethylsiloxane) (PDMS) were realized. By exploiting reactions of epoxide functional groups with anhydrides and carboxylic acids, covalent adaptive networks (CAN) are formed in the presence of suitable catalysts. A reprocessable elastomeric CAN was successfully established by curing epoxidized PDMS with a multifunctional fatty acid and n-butylamine (BA) as a covalently bonded catalyst. In tensile tests the system showed a remaining strength of more than 85 % after reprocessing the grinded material in a hot-press for 5 h at 160°C. Lap shear tests were done to evaluate bond strength, rejoining, aging stability and the ability to weaken the adhesive bond. The elastomeric CAN achieved approximately 20 % of the reference silicone adhesive’s strength. Tests at 160°C decreased the bond strength by 60%, compared to a reduction of 40% for the reference sample. Temperature cycle tests did not influence the material strength, storage at 120°C for 930 h decreased the bond strength by approximately 20 % and in damp heat tests the material suffered from hydrolytic bond cleavage. Rejoining the lap shear samples resulted in a remaining bond strength of more than 70 %.

KW - polymer chemistry

KW - material science

KW - material testing

KW - photovoltaic

KW - adhesives

KW - vitrimer

KW - Chemie

KW - Polymer

KW - Kunststoff

KW - Klebstoff

M3 - Master's Thesis

ER -