The sawdust used in this study comes from the Tectona grandis species ( Figure 1 ). It was collected from various sawmills and sieved to separate the material into several granular classes. Five compositions were considered: particles retained on the 0.630 mm, 0.315 mm, and 0.160 mm sieves, a coarse mixture, and a fine mixture ( Table 1 ). The different mixtures were obtained by reconstituting the particles from the sieved fractions. Percentages of the different fractions in a mixture were chosen to achieve a fineness modulus corresponding to the desired mixture type. The proportions used for the coarse (GC4T) and fine (GC5T) mixtures are presented in Table 2 .

Table 1. Characteristics of recycled sawdust.

Table 2. Proportions of granular fractions in GC4T and GC5T mixtures.

The expanded polystyrene was recovered from packaging materials, with densities ranging between 0.015 and 0.023 g/cm³.

The adhesive results from dissolving polystyrene in an organic solvent, specifically gasoline for this study. Preparation involves introducing polystyrene into gasoline and mixing until the adhesive is obtained, following the proportion $\text{k}=\frac{\text{mass}\text{of}\text{solvent}}{\text{mass}\text{of}\text{polystyrene}}$.

The ratio k was determined after several tests. A known mass m of gasoline was weighed, and polystyrene was melted until complete evaporation of the gasoline, measuring the mass of melted polystyrene. The obtained ratio k is 1.4. Ratios less than 1.4 indicate insufficient gasoline to dissolve the polystyrene, while ratios greater than 1.4 reflect excess gasoline. The adhesive’s characteristics are presented in Table 3 .

Table 3. Characteristics of the adhesive.

A dosing ratio $\text{d}=\frac{\text{mass}\text{of}\text{binder}}{\text{mass}\text{of}\text{wood}\text{sawdust}}=2$ was selected. This choice was optimized to reduce structural defects, such as crumbling, and improve the composites homogeneity. Dosages higher than 2 lead to excess binder, forming an impermeable surface layer that prevents complete solidification of the composite’s interior, weakening the material. Dosages lower than 1.5 result in a friable material.

For the composite plates, a cold compaction process was implemented. The mixture, consisting of prepared wood sawdust and the polystyrene-based adhesive, was introduced into a metal mold designed for this experiment. Compaction was performed using a hydraulic press.

After compaction, the plates were removed from the mold and left to dry at room temperature. They were weighed every 8 hours until reaching a constant mass, indicating “maturation” and complete evaporation of residual solvents. Finally, the plates were machined ( Figure 2 ) to dimensions of 11 mm thickness, 76 mm width, and 314 mm length [22] .

The compaction rate varies with the granular composition. Figure 3 illustrates the influence of granulometry on this rate. Larger particles exhibit higher compaction rates than finer particles. The compaction rates of mixtures GC1T (0.630 mm), GC2T (0.315 mm), and GC3T (0.160 mm) are 46.67%, 43.33%, and 33.33%, respectively. This is due to a higher proportion of voids in mixtures with large particles, with compaction aiming to reduce these voids.

The GC4T (coarse) mixture, although mainly composed of large particles, shows a lower compaction rate due to the presence of fine particles filling the voids, reducing compaction capacity. Conversely, the GC5T (fine) mixture presents a higher compaction rate than GC3T, mainly due to the presence of large particles promoting better granule adjustment.

All mixtures stabilized after 48 hours. Mass losses for mixtures GC1T, GC2T, GC3T, GC4T, and GC5T are 25.31%, 25.61%, 25.88%, 25.78%, and 25.55%, respectively. Minor variations indicate that granulometry does not significantly affect the composites’ mass stabilization, which lose about 26% of their weight during the process.

This mass loss is mainly due to the evaporation of residual solvents in the polymer matrix. During fabrication, polystyrene dissolved in gasoline acts as a binder, but gasoline, being a volatile organic solvent, gradually evaporates, significantly reducing the composite’s total mass. This phenomenon is uniform across different granular compositions, as the solvent’s volatile properties are not influenced by particle size.

The densities of plates GC1T, GC2T, and GC3T are 0.600 g/cm³, 0.693 g/cm³, and 0.727 g/cm³, respectively. With slight variations in moisture content, it appears that finer granulometry leads to higher composite density. Thus, density decreases with increasing particle size. This trend is also confirmed for mixtures GC4T (0.705 g/cm³) and GC5T (0.778 g/cm³), as shown in Figure 4 .

This is because mixtures with fine granulometry allow denser particle stacking, reducing voids and increasing overall composite density. In contrast, mixtures with large particles have more voids, decreasing density.

The mechanical properties of the composite panels show a significant influence of sawdust granulometry on the modulus of elasticity in bending (MOE) and flexural strength (MOR) ( Figure 5 ). Panels containing coarse sawdust (GC1T and GC4T) exhibit higher MOE and MOR values, with 842 MPa and 3.16 MPa respectively for GC1T, reflecting better structural rigidity and increased capacity to withstand mechanical loads. Conversely, panels with fine granulometry (GC3T and GC5T) show decreased mechanical properties, with MOE and MOR of 748 MPa and 2.25 MPa for GC3T, possibly due to a more compact but fragile structure limiting homogeneous stress dissipation. A balanced combination of fine and coarse particles in GC4T provides an interesting compromise with intermediate performance.

The results align with performance ranges generally observed in similar composites. For instance, Foti et al. [16] reported MOR values of 2.37 MPa for wood and recycled polystyrene composite panels, while Rofdi et al. [17] noted higher performance. Optimized density and control of hot pressing parameters improved mechanical properties in these studies. However, the composites developed here rely on a cold compaction process, adapted to local constraints and less sophisticated, justifying slightly lower mechanical results.

Studies like Cherkashina et al. [18] showed significantly higher MOR, reaching 20 MPa, by incorporating reinforcing fillers like modified hazelnut shells. These improvements result from advanced chemical treatments and better matrix-particle compatibility. Khedr et al. [19] also confirmed that adding plasticizers or coupling agents, such as titanium dioxide, can significantly increase mechanical strength and flexibility. These processes, though effective, require resources and technologies not always aligning with local economic and environmental constraints.

The properties obtained remain competitive and relevant for specific applications. The panels could be used in lightweight partitions, furniture panels, or non-load-bearing elements in construction. Akinterinwa et al. [23] , exploring composites based on rice husk and recycled polystyrene, suggested similar applications for materials with comparable mechanical and physical properties.