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

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.