Combining infrared and Raman spectra to assess MDI localization in novel flax-reinforced automotive composites

The present work describes the use of novel flax preforms reinforcement as substitution of glass fibres in complex composite stackings made of polyurethane foam impregnated with methylene diphenyl 4, 4′-diisocyanate (MDI), for automotive headliner parts manufacturing. Headliner structure, cohesion and mechanical performances are highly dependant of MDI distribution in both foam and fibre network. This study monitors the presence and localization of MDI in the final biobased composite by implementing a robust and innovative spectroscopic methodology. For that, Infrared and Raman spectroscopies are used to characterize the fibres and polymer chemical groups, such as the isocyanate and phenyl bonds in MDI and the urethane group. MDI in the flax preform is identified by the signal of the isocyanate group in the infrared spectrum. The complementary Raman study highlight two signatures of the interaction between polyurethane and MDI. One is the shift from 1620 to 1610 cm−1 between the urethane and the MDI aromatic vibration. The second is the aromatic ring substitution at 1530 cm−1, which increases with the MDI concentration in the composite. These investigations demonstrate that MDI is present on the surfaces of flax fibres and foam and at the foam/fibre interfaces in the composite. The results clearly demonstrate the potential of flax for a use as headliner reinforcement, without penalizing the structure of the part.