The interfacial adhesion between fiber/interphase and interphase/matrix were the important factors for
the effectiveness of flexible interphase on mechanical property improvement. It can be seen that at the
surface of treated bamboo fiber and coconut fiber in Fig. 5a-5b, small amount of flexible epoxy resin can
be observed on the surfaces of the fibers. Good adhesion between reinforcing fiber and flexible epoxy
interphase resulted in tensile strength improvement of bamboo fiber and coconut fiber reinforced PLA
composites. However, in case of treated vetiver grass fiber in Fig. 5c, no flexible epoxy resin appeared on
the fiber surfaces. It can be assumed that the interfacial bonding between flexible resin and PLA was
constant with different natural fibers. These two different interfacial adhesions resulted in the different
fracture mechanism when comparing treated vetiver grass fiber reinforced PLA composite with other
reinforcements. It is possible to note that the interfacial adhesion between vetiver grass fiber and flexible
interphase was weaker than the interfacial bonding between flexible interphase and PLA matrix. The
weak bonding of vetiver grass fiber and flexible epoxy resin resulted in slightly different tensile strength
improvement versus other natural fibers.
The tensile modulus and tensile strength improvement performance of untreated composites were
compared with flexible epoxy resin treated composites in Fig. 7. The comparison was obtained from the
differential ratio in percentage of the tensile properties at 40 % by weight natural fiber content and in
reference to the properties of PLA matrix. The tensile modulus of bamboo fiber and vetiver grass fiber
were strongly affected by flexible epoxy surface treatment. On the other hand, the modulus of coconut
fiber was unaltered by the coating of flexible epoxy interphase as shown in Fig. 7(a). Among all
reinforcement bamboo fiber was the most effective reinforcement to enhance the tensile strength of PLA