Manufacturing of PP ‐Based Composite Sandwich Panels via Simultaneous Foaming and Skin‐Core Bonding

ABSTRACT Sandwich panels are skin‐core‐skin structures commonly used in aerospace applications due to their high strength‐to‐weight ratio and flexural stiffness. The most widespread manufacturing approach involves the use of fiber‐reinforced thermosetting polymer skins bonded to aramid honeycomb core. This choice generates critical issues, such as high production cost and low recyclability of main components. As an alternative, we introduced a one‐shot manufacturing technique to produce thermoplastic polypropylene (PP)‐based sandwich panels, enabling simultaneous physical foaming and its fusion with skins. Two different PPs were used in this work: a foaming‐grade polypropylene for the core and a higher melting point polypropylene used for the fiberglass composite skins. The method involved the saturation of the system, composed of two pre‐fabricated PP‐fiberglass laminates (skins) and a slab of neat PP (core), at 100 bar and 153°C, followed by controlled cooling to different temperatures and rapid depressurization to 1 bar. Under optimized conditions (final temperature 140°C), the core showed a 700% expansion ratio while the skins retained their structural integrity, allowing the formation of strong interfacial bonds without the use of adhesives. Flexural stiffness of 1822 MPa has been measured in 3‐point‐bending tests. Mode I skin/core fracture toughness tests were performed; crack propagation did not occur along the skin/core interface, as typically observed in the presence of weak interfacial adhesion. Instead, the crack deviated from this path, penetrating through the low‐density regions of foam core and into skins, indicating strong adhesion between structural layers. The method has been demonstrated to be quick and economic in producing fully thermoplastic, fiber‐reinforced sandwich panels.