Pressure–Volume–Temperature and Two-Domain Tait Model as a Characterization Tool of Hybrid Polypropylene/Glass-Fiber/Graphene Composites for Injection Molding

Dimensional control in injection-molded polypropylene (PP) parts remains challenging when conventional characterization overlooks pressure-dependent behavior. In this study, neat polypropylene (PP) and three composite formulations─PP + 10 wt % glass fiber (GF), PP + GF + 2 wt % maleic-anhydride-grafted PP (MAPP), and PP + GF + MAPP + 0.125 wt % graphene nanoplatelets (GNP)─were compounded, injection-molded, and systematically analyzed. Isothermal pressure–volume–temperature (PVT) measurements were fitted with the two-domain Tait equation, while flexural, creep, and DSC and SEM tests provided complementary property maps. GF decreases low-pressure volumetric shrinkage by ∼2.5 percentage points and doubles Young’s modulus. MAPP further reduced melt compressibility and increased flexural strength, while GNP increased the crystallinity without extra shrinkage. Overall, the Tait parameters revealed complementary structure–property trends, demonstrating an alternative use for PVT-analysis data and offering a potential tool for optimizing hybrid composites.