Design of debondable composite sandwich structures containing a primer based on thermally expandable microspheres: process window & thermomechanical performance evaluation

• Efficient bonding and debonding of composite laminates and sandwich structures. • Primer enables on-demand thermal separation of thermoset composite layers. • Co-curing strategy for integrating the primer into high-performance composites. • Process windows were validated via thermo-mechanical characterization. • Assessed the primer effect on sandwich structures’ thermomechanical response. Disassembly of constituent layers in composite laminates remains a major challenge in repair and end-of-life recycling of polymer matrix composites with monolithic, hybrid, or sandwich structures. Integrating a debondable primer based on thermally expandable microspheres at the constituents’ interface offers a promising solution to enable controlled separation. This work investigates critical aspects in the design of such structures, including thermal processing optimization and primer formulation for reliable interfacial strength in industrial applications, compatible with autoclave processing of aerospace grade CFRP prepreg skins bonded to a Rohacell PMI foam core under high temperature (135°C) and pressure (2.5 bars). A process window was established through thermal kinetics analysis of microsphere expansion, identifying a trade-off between maximum curing temperature and activation onset to prevent premature expansion during processing. Thermal activation at 170°C increased microsphere diameter by ∼5×, inducing controlled decohesion at the interface. Upon cooling, constituents were separated with an interfacial stress <0.5 MPa, enabling efficient end-of-life disassembly. Interfacial strength with and without the primer, both as-processed and after heat activation, was evaluated using single cantilever beam and single lap joint tests, supported by in-situ optical and electron microscopy. Additionally, four-point bending and creep tests were performed at ambient and elevated temperatures. Results confirm that the proposed debonding technology is compatible with existing composite manufacturing practices, leading to similar structural performance, and provides practical design guidelines for reliable integration into aerospace workflows