Nanoindentation of Phase-Specific Elastic Modulus in Virgin and Recycled HDPE Wood–Plastic Composites

The structural behavior of natural fiber-reinforced polymer composites depends largely on the efficiency of stress transfer at the fiber–matrix interface. However, accurate determination of the interface thickness and its mechanical properties remains challenging in such materials. This study aimed to characterize the interface thickness and evaluate the elastic modulus of composites produced with virgin and recycled high-density polyethylene (HDPE) matrices. Nanoindentation was employed to determine the elastic modulus of each composite phase (wood fiber, interface, and matrix) individually, through sequential indentations performed across the wood fiber-interface-matrix regions. The mean elastic modulus values for the wood fiber, interface, and matrix in the 20/80 composite (20% wood fibers and 80% virgin HDPE) were 7.51, 6.17, and 2.26 GPa, respectively. For the recycled HDPE composite, the corresponding values were 8.16, 5.82, and 1.86 GPa. The mean interface thickness was 2.5 µm in the virgin-matrix composite and 0.63 µm in the recycled-matrix composite. These results demonstrate that the recycling of the polymer matrix influences both the interfacial structure and the local mechanical performance of natural fiber reinforced composites.