Development of Composite Materials Based on Recycled Polystyrene and Wood Residues for Building Applications ()
1. Introduction
The building sector is a major contributor to global CO2 emissions, accounting for approximately 37% of energy-related and process-related emissions in 2021 [1]. This highlights the necessity to rethink construction methods and building management, emphasizing the use of environmentally friendly materials and the evaluation of locally available resources.
In Africa, a continent undergoing rapid transformation, the population is estimated to reach 2.5 billion by 2050 [2]. This demographic surge demands proactive anticipation of infrastructure and resource needs. The concept of sustainable building is crucial to address these challenges. It aims to minimize energy consumption and CO2 emissions, reduce dependence on non-renewable resources, improve occupant comfort, and optimize overall costs, principles fundamental to responsible and sustainable urban development.
In the current context, where sustainable development is imperative, especially in Africa, evaluating and valorizing local resources are essential. In Benin, wood processing waste is significantly underutilized, with over 4500 tons of sawdust produced annually [3]. This wood waste can be valorized in the manufacture of particleboard.
The development of panels from wood residues is not new. Numerous studies have utilized various matrices or adhesives such as urea-formaldehyde resins [4]-[6], phenol-formaldehyde resins [7]-[9], melamine-urea-formaldehyde, and melamine-formaldehyde [10]-[12], exploring different manufacturing techniques.
However, in Benin, these technologies are less accessible, and the materials used in adhesives are excessively expensive. Additionally, conventional adhesives have significant environmental impacts due to emissions of volatile organic compounds and formaldehyde. Therefore, finding a more accessible solution for local populations is necessary. Recycling expanded polystyrene, which is also waste needing management, becomes relevant. Benin faces major challenges in managing non-biodegradable waste, with landfills receiving over 120,000 tons of various plastics annually [13], including packaging expanded polystyrene from commercial import activities.
The fabrication of wood-polystyrene composites and exploration of various formulations demonstrate a growing interest in valorizing waste into ecological solutions. Researchers have studied the potential of composite materials using recycled polystyrene and natural or synthetic fillers to reduce environmental impact and offer sustainable construction materials.
Parikh et al. [14] demonstrated that adding wood dust-based fillers improves the mechanical and thermal properties of a PLA-wood composite, although dimensional stability issues related to water absorption persist. Adeniyi et al. [15] explored wood-polystyrene composites using Isoberlinia doka dust, optimizing mechanical properties through alkaline fiber treatment but identifying limitations in filler dispersion.
Studies by Foti et al. [16] and Rofdi et al. [17] highlighted the role of particle size and polystyrene concentration on the physical and mechanical properties of composites, achieving better water resistance and homogeneous density, though increased flammability remains a challenge. Cherkashina et al. [18] introduced agricultural waste like hazelnut shells, improving mechanical strength through chemical surface treatments. Composites incorporating flax fibers and plasticizing additives, studied by Khedr and Elnahas [19], revealed increased tensile strength and flexibility, though reliance on organic solvents is a limitation.
Work by Adeniyi et al. [20] on hybrid composites combining polystyrene with local clay and natural fibers, and by Ighal et al. [21] on wood-polystyrene composites reinforced with rice husks, confirm these materials’ effectiveness for specific applications. Mastery of manufacturing techniques, notably optimizing filler/matrix ratios and thermal treatments, is essential to balance mechanical, thermal, and moisture absorption properties.
In Benin, where advanced composite manufacturing technologies are less accessible and conventional materials are often costly, exploring local and economical alternatives becomes relevant. Recycling expanded polystyrene offers an opportunity for valorization, especially when combined with abundant sawdust waste. Integrating principles from these studies within a framework adapted to local constraints could develop materials meeting infrastructure needs while reducing environmental impacts.
This work aims to reduce the environmental impact of wood and non-biodegradable polystyrene waste by developing particleboard using adapted technology, considering local economic and environmental constraints.
2. Materials and Methods
2.1. Wood Sawdust
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.
Figure 1. Tectona grandis sawdust of various granular classes.
Table 1. Characteristics of recycled sawdust.
Code |
Granular Composition |
True Density
(g/cm3) |
Bulk Density
(g/cm3) |
GC0T |
Retained on 1.25 mm sieve |
0.258 |
0.119 |
GC1T |
Retained on 0.630 mm sieve |
0.270 |
0.126 |
GC2T |
Retained on 0.315 mm sieve |
0.278 |
0.161 |
GC3T |
Retained on 0.160 mm sieve |
0.294 |
0.171 |
GC4T |
Coarse mixture |
0.269 |
0.132 |
GC5T |
Fine mixture |
0.280 |
0.151 |
Table 2. Proportions of granular fractions in GC4T and GC5T mixtures.
Granular composition |
Coarse mixture (GC4T) |
Fine mixture (GC5T) |
GC0T (1.250 mm) |
40% |
10% |
GC1T (0.630 mm) |
30% |
20% |
GC2T (0.315 mm) |
20% |
30% |
GC3T (0.160 mm) |
10% |
40% |
2.2. Expanded Polystyrene
The expanded polystyrene was recovered from packaging materials, with densities ranging between 0.015 and 0.023 g/cm3.
2.3. Formulation of the Adhesive
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
.
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.
Density (g/cm3) |
Viscosity (Pa∙s) |
0.905 |
0.768 |
2.4. Formulation of the Composite
A dosing ratio
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.
2.5. Composite Fabrication
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].
Figure 2. Machined plates.
3. Results and Discussion
3.1. Compaction Rate
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.
Figure 3. Influence of granulometry on compaction rate.
3.2. Mass Loss
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.
3.3. Density
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.
Figure 4. Density variation by granulometry.
3.4. Mechanical Properties of the Panels
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.
Figure 5. Influence of granulometry on mechanical properties.
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.
4. Conclusions
Developing composite panels from recycled wood sawdust and dissolved polystyrene is an approach that valorizes local waste while offering alternatives to conventional materials. In developing countries, where access to advanced technologies and costly materials is limited, this approach provides an adapted response to growing needs for ecological and economical construction materials.
The results demonstrate that the panels’ mechanical properties, notably a modulus of elasticity in bending (MOE) up to 842 MPa and flexural strength (MOR) reaching 3.16 MPa, reflect the influences of granulometry, adhesive proportions, and the cold compaction process employed. Granular formulations containing coarse particles exhibit superior mechanical performance due to better stress distribution within the material.
Comparisons with previous studies reveal that, although our results show lower properties than those achieved using sophisticated processes like hot pressing or chemical reinforcements, they remain consistent within a simplified manufacturing framework. Higher performance in some studies is due to the use of plasticizers, advanced chemical treatments, or special fillers, incompatible with an approach centered on local resource valorization and accessible processes. These differences justify the performance gaps while underscoring this approach’s relevance for local applications.
The panels developed are suitable for use in lightweight partitions, furniture panels, or non-load-bearing decorative elements. These applications align with local needs for low-cost materials with reduced environmental impact. Despite the absence of advanced technologies, the obtained performance confirms the feasibility of local production of ecological composite panels. This approach addresses waste valorization challenges and the growing demand for environmentally friendly materials, particularly in developing regions.