Effects of Yellow Pigment Content on the Properties of Bamboo Flour/High-Density Polyethylene Composites

Abstract

Effects of iron oxide yellow pigment content were investigated relative to the properties of wood-plastic composites (WPCs) based on bamboo flour and high-density polyethylene (HDPE). Samples with pigment contents of 0%, 5%, 10%, and 15% were prepared, and the structure–property relationships were systematically examined. The incorporation of pigment significantly enhanced surface hydrophobicity, with the static water contact angle increasing from 79.41° to 109.15°. This was attributed to the inherent hydrophobicity of the pigment and its physical shielding of wood fibers, thereby reducing surface energy and limiting the exposure of hydrophilic groups. The composite with 5% pigment exhibited the optimum mechanical performance, whereas higher pigment loading (≥10%) resulted in reduced mechanical properties, with flexural strength decreasing by up to 14.8%. SEM analysis revealed that excessive pigment led to agglomeration, causing interfacial defects and stress concentration. The 10% pigment formulation showed a relatively uniform and well-bonded interface, while the 15% sample contained noticeable fiber exposure and voids. FTIR spectroscopy confirmed that the pigment primarily served as a physical filler without introducing new chemical functionalities. In conclusion, an appropriate amount of pigment (5%) can effectively improve the hydrophobicity and mechanical properties of bamboo flour/HDPE composites.