1. Introduction
Today, composite material technologies focus not only on improving mechanical performance but also on criteria such as environmental sustainability, efficient use of natural resources and recyclability [1]. In this context, the modification of fossil- based polymers with natural and inorganic fillers constitutes an important research area in terms of both reducing the environmental footprint and improving functional properties [2].
Figure 1. Schematic representation of the sustainable composite material development approach
Polystyrene (PS) is a widely used thermoplastic due to its low density, good fluidity, dimensional stability and suitability for industrial production methods such as injection molding and extrusion [3]. General purpose polystyrene (GPPS) in particular is preferred in decorative and structural applications thanks to its rigid structure and surface quality. However, the relatively low impact resistance of PS and its lack of natural aesthetics limit its use in wood-like applications [4]
Figure 2. Chemical structure of polystyrene (PS) consisting of repeating units and polymer chain architecture based on styrene monomeresentation of the sustainable composite material development approach
Wood flour is widely used in polymer composites as a renewable, biodegradable and low-cost filler material . In addition to giving the polymer matrix a natural look and texture, it offers significant advantages in terms of sustainability [5]. However, the hydrophilic nature of wood dust can create negative effects such as water absorption, dimensional instability and a decrease in mechanical properties. Therefore, wood-based polymer composites require additional modifications that increase moisture resistance and improve interfacial compatibility [6].
Figure 3. Representative image of the characteristic grain pattern, surface texture, and aesthetic appearance of natural wood
Calcite (CaCO₃) is a commonly used inorganic filler in polymer composites. Although traditionally considered a cost-reducing additive, when used with appropriate particle size and surface modification, it can provide significant improvements in mechanical strength, rigidity and thermal stability [7]. In particular, calcite surface modified with hydrophobic agents such as stearic acid strengthens the interfacial bond between the polymer matrix and the filler and increases the water resistance of the composite [8].
Figure 4. Modified calcite image
The aim of this study is to develop a new generation composite material with a wood-like appearance, lightweight, high mechanical performance, and high moisture resistance, containing a GPPS matrix, natural wood powder, and modified calcite with water-repellent properties. Furthermore, the goal is to impart a realistic wood texture to the composite using a unique surface texturizing and varnishing method.
2. Materials and Methods
2.1. Materials
General-purpose polystyrene (GPPS) was used as the main matrix of the composite . Dried and ground oak wood powder with a grain size of 80–120 mesh was preferred as a natural filler. Calcite (CaCO₃) , with an average particle size of approximately 5 µm and surface modified with a stearic acid-based agent, was used as an inorganic filler and functional modifier. A transparent polyurethane- based varnish was applied for surface coating .
2.2. Formulation and Mixing
Four different formulations were prepared to investigate the effects on the mechanical and physical properties of the composites. The components were pre-mixed in a high-speed mechanical mixer to ensure a homogeneous distribution.
2.3. Extrusion and Injection Molding
The prepared mixtures were homogenized and granulated in the melt phase using a twin-screw extruder . The twin-screw extruder ensures homogeneous distribution of fillers in the polymer matrix thanks to its high shear force. The resulting granules were transformed into test specimens and panel forms conforming to ASTM standards using an injection molding machine.
2.4. Surface Texturization and Coating
The flat-surfaced panels removed from the mold were subjected to a localized surface melting process using a controlled heat source (blowtorch). This process created a surface morphology that mimicked the natural grain and pore structure of wood. After texturing, the surfaces were sanded and coated with transparent polyurethane varnish.
Figure 5. General schematic representation of the production and surface treatment process of polystyrene- based wood-look composite
Figure 6. Image of wood-look composite material
2.5. Characterization
Density measurements, tensile (ASTM D638), bending (ASTM D790), impact (ASTM D256), water absorption (ASTM D570) and surface coating adhesion tests (ASTM D3359) were performed.
3. Results and Discussion
3.1. Density and Mechanical Properties
Pure polystyrene (F1) sample exhibited a density of 1.04 g/cm³, while an increase in density values was observed with the addition of wood powder and calcite. Formulation F2 containing 30% wood powder reached a density of 1.18 g/cm³. In formulations F3 and F4 containing modified calcite, the density was measured as 1.22 and 1.26 g/cm³ respectively [9].
The results of tensile and bending tests showed that the addition of wood powder reduced the mechanical strength compared to pure PS, but the addition of modified calcite partially compensated for this loss [10]. While the bending strength in sample F2 was 42.0 MPa, this value increased to 46.5 MPa in sample F3 containing 10% modified calcite. This shows that the modified calcite improved load transfer by being better distributed in the polymer matrix [11].
3.2. Impact Resistance
The highest impact strength value was obtained in the pure PS sample (15.0 kJ /m²). With the addition of wood powder, this value decreased to 8.5 kJ /m² in the F2 formulation. However, in the F3 formulation, the impact strength increased to 9.8 kJ /m². This increase shows that fine-grained and surface -modified calcite forms a mechanism that limits crack propagation [12]. In the F4 formulation, the high calcite ratio caused the material to become more rigid, and the impact strength decreased to 8.0 kJ /m².
3.3. Water Absorption and Dimensional Stability
Water absorption tests clearly revealed the critical role of modified calcite on the moisture resistance of the composite. At the end of the 24-hour water immersion test, sample F2 showed 8.5% water absorption, while samples F3 and F4 were measured as 3.1% and 2.4%, respectively. This significant decrease shows that hydrophobic calcite particles create a barrier effect within the composite by blocking the hydrophilic nature of wood dust [13].
Table 1. Density, mechanical properties, impact strength, and water absorption values of the developed polystyrene-based composites.
3.4. Surface Coating Performance
In a shear (grid) test conducted according to the ASTM D3359 standard, an adhesion grade of 5B was obtained on textured and varnished surfaces. This result demonstrates that the developed surface texturing and coating method creates an excellent bond with the composite surface [14].
4. Conclusion
a lightweight, high-performance, and aesthetically superior polystyrene- based composite was successfully developed using natural wood powder and modified calcite with water-repellent properties, eliminating the disadvantages of traditional wood. In particular, the F3 formulation, containing 25% wood powder and 10% modified calcite, stood out as the optimum composition in terms of mechanical strength, impact resistance, low water absorption, and cost-effectiveness. The developed composite offers high potential for indoor and outdoor furniture, decorative coatings, automotive interior trim parts, and sustainable building applications.
References
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