Impact strength for the composites without and with different
diisocyanates is presented in Fig. 3D. It could be found that the
diisocyanates enhanced the impact strength, and the composites
with HDI showed a significant increase of approximately 61.5%.
For the composites with IPDI and MDI, the impact strength
increased by 44.9%, and 51.4% respectively, compared to that of
the composites without diisocyanates. The increased impact
strength of the composites was attributed to the fact that the diisocyanates
improved the surface adhesion characteristics by producing
a crosslink surface morphology compared with the composites
without the diisocyanate. Fiber reinforced composites absorb
impact energy by three major mechanisms: fiber breakage, fiber
pullout, and matrix crack propagation. For high toughness, the
fracture energy must be at least equal to or larger than the surface
energy at the interface of cracking. The composites with diisocyanates
had higher fracture energy than the composite without
diisocyanates, This indicated that the reaction of isocyanate groups
and carboxyl and hydroxyl groups occurred, which resulted in
chemical bonds between the surface of ramie and PLA, leading to
high interface adhesion