34 testing conditions given the lack of the formerly protective modular housing. It is not uncommon for CTP designers to require lap shear strength levels above 6 MPa after exposure to 1000 hours of ageing at 85 C and 85% RH. Historically, urethane adhesives have often had difficulty maintaining high levels of adhesion, particularly on plastic and aluminium substrates, after extended exposure at 85 C and 85% RH, and adhesives containing high levels of thermally conductive filler pose an even greater challenge. The traditional CTM gap filler exhibits an 80% drop in the laminated stack structure after only 336 hours, or two weeks, of aging and an even greater reduction after 1000 hours (six weeks). This large reduction is attributed primarily to loss of interfacial adhesion, which is reflected in the transition from mixed adhesive/cohesive failure to complete adhesive failure. Generally, the CTP adhesive exhibits high levels of Al–Al adhesion throughout an entire accelerated ageing test with values above 10 MPa. The fact that the adhesive continues to exhibit cohesive failure after 1000 hours of ageing highlights the advantage of the move to pack architecture. Cell-to-chassis All of this leads to the C2C requirements. Removing the cooling plate and replacing it with cooling channels means that there are more materials that the adhesive must bond with to provide structural strength. The biggest problem once a manufacturer has decided to pursue a C2C or differentiated design is the many unknowns regarding the rigidity or structural flexibility of the vehicle. Designing a heavy vehicle with sufficient impact resistance to keep larger battery cells in place can be a significant challenge. This process requires comprehensive vibration and shock analyses of the bonding of the materials. Tech focus | Cell-to-chassis May/June 2025 | E-Mobility Engineering Cross-section of the next generation, module-free or cell-to-pack battery pack configuration (Image courtesy of Parker Lord) Structure of a cell-to-chassis architecture (Image courtesy of Rogers)
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