<div>Vehicle light-weighting constitutes a critical component in the automotive
sector’s drive to improve fuel economy and reduce greenhouse gas emissions.
Among the various options for lightweight materials, thermoplastic foams are
distinguished by their durability, low weight, and environmental sustainability.
This study explores the manufacturing of novel graphene-filled polypropylene
(PP) foam, employing supercritical nitrogen as an eco-friendly substitute
instead of conventional chemical foaming agents, and investigated the role of
over-molding a solid skin over a foamed core on the flexural strength of the
molded component. Our approach is broken down into four distinct
investigations—Study I investigated the effect of different graphene content by
weight percentage (wt.%), namely 0.1%, 0.5%, and 1%, on flexural properties and
foam morphology obtained for 15 wt.% reduction of the PP thermoplastic, thereby
helping identify an optimum graphene loading wt.%. Study II broadened the wt.%
reduction horizon for PP to 5 wt.%, 10 wt.%, and 15 wt.%, systematically
analyzing the impact of the optimal graphene loading and comparing their cell
morphology and flexural properties. This improvement in microstructure and
mechanical properties was confirmed in the case of graphene addition to 10 wt.%
and 15 wt.% reduction, where cell size was reduced by ~100% for 10 wt.%
reduction samples and cell density improved from 4.37 × 10<sup>5</sup>
cell/cm<sup>3</sup> to 5.42 × 10<sup>6</sup> cell/cm<sup>3</sup> for the
same when compared to baseline PP foams. Study III serves as a demonstrator for
a novel hybrid over-molding process designed to improve flexural properties.
Over-molding with solid PP was performed over a foamed PP core, generating a
composite foam with improved flexural strength and a class-A surface finish and
noticeably improved flexural strength from 23.4 MPa to 27.3 MPa, achieving an
overall 10 wt.% reduction. This is significant since it translated to a 16%
improvement in flexural strength over baseline PP foams and a flexural modulus
equivalent to solid PP. Study IV investigated the impact of this light-weighting
to assess the potential energy savings over a typical passenger vehicle’s life
cycle. The study demonstrates a viable route to achieve sustainable vehicle
light-weighting and highlights the role supercritical fluid-assisted foamed
thermoplastic nanocomposites may occupy in the vanguard of sustainable material
development.</div>
