The push for sustainable materials has created a major challenge: how to replace durable, petroleum-based plastics with eco-friendly alternatives that don’t sacrifice performance. Wood-plastic composites (WPCs) are a popular solution, often used for outdoor decking and fencing. However, they traditionally rely on non-biodegradable plastics like polyethylene. A greener alternative involves using polylactic acid (PLA), a biodegradable plastic, combined with a widely available resource like landscaping waste (LW). The problem? This greener composite has an Achilles’ heel: it absorbs water, which causes it to swell, lose strength, and degrade. A new study in Materials & Design by Yu Xian and a team of researchers investigates a promising fix by adding another sustainable material into the mix: biochar (BC).

The team set out to find the perfect recipe, mixing poplar-branch landscaping waste and PLA with varying amounts of bamboo-derived biochar, from 0.5% to 4% by weight. In a dry state, the results were striking. A small amount of biochar dramatically improved the composite’s strength, but only up to a point. The sweet spot was 1% biochar, which maximized the material’s performance. At this concentration, the composite’s flexural (bending) strength peaked at 25.16 MPa, a 19.20% increase compared to the control group with no biochar. The impact strength also saw a modest 3.06% increase at the same 1% mark.

The reason 1% worked so well—and why adding more was less effective—lies at the microscopic level. At 1% loading, the tiny biochar particles disperse evenly, acting like rebar in concrete. They form a strong bond at the interface between the wood fibers and the PLA plastic, allowing stress to be transferred efficiently. Dynamic mechanical analysis (DMA) confirmed this superior bond: the 1% BC composite showed a 186% increase in storage modulus (a measure of stiffness) and a 66.67% reduction in its tan \delta peak (a sign that less energy was being wasted, indicating a stronger, more cohesive structure). In contrast, at 4% biochar, the particles began to clump together (“agglomeration”). Scanning electron microscope (SEM) images revealed these clumps created weak points and voids, causing the material to fail earlier under stress.

The study’s primary goal, however, was to test the material against its main weakness: water. As expected, hygrothermal aging (soaking in water) was devastating to all the samples. After saturation, the composites lost over 50% of their flexural strength. Water molecules seep into the material, causing the wood fibers to swell and breaking the bond with the PLA matrix, which leads to micro-cracks. Water also acts as a “plasticizer,” making the polymer chains more mobile and lowering the material’s glass transition temperature (Tg). This plasticizing effect did have one small, counterintuitive upside: because the material became less brittle, the impact strength was not reduced as severely as the flexural strength.

This is where the biochar truly shone as a protective additive. It acted as a physical shield, filling in microscopic pores and creating a more complex, “tortuous” path that slowed water molecules from seeping in. This was reflected in a lower Fickian diffusion coefficient, the scientific measure of how fast water penetrates a material. The practical benefits were clear: the 1% BC composite absorbed 6.14% less water overall than the control. Biochar also drastically improved dimensional stability, with composites showing progressively less thickness swelling as more biochar was added. Furthermore, it preserved the material’s appearance. The BC-free sample faded significantly , while the 4% BC sample was remarkably stable, showing a color change of only 2.61.

In conclusion, this study identifies 1% biochar as the optimal loading to create a composite that is both strong and durable. This work provides a practical, tested formula for transforming low-value landscaping waste into a high-performance, aging-resistant, and fully biodegradable material. This new composite offers a truly sustainable alternative for outdoor applications like decking and fencing, tackling both resource scarcity and plastic pollution in a single, innovative solution.

Source: Xian, Y., Li, S., Yang, X., Peng, H., Zhang, R., Li, H., Xing, Z., Wang, G., & Cheng, H. (2025). Hygrothermal aging behavior of biochar-reinforced polylactide/wood plastic composites. Materials & Design.