Interactive effects of melt flow index and nanosilica dispersion on the mechanical and thermal properties of recycled Polypropylene/OCC wood-plastic composites

Melt flow index (MFI)

The extent of thermo-oxidative degradation of the polypropylene matrix after each recycling cycle was quantified by measuring its Melt Flow Index (MFI). The test was performed according to ASTM D1238-23a ²⁵ using a Zwick extrusion plastometer. Approximately 4 g of PP granules were loaded into the barrel, heated to 190°C, and extruded through a standardized die under a constant load of 2.16 kg. The weight of the extrudate collected over 10 min was recorded, and the MFI was reported in g/10 min.

Mechanical testing

All specimens were conditioned at 23 ± 2°C and 50 ± 5% relative humidity for 48 h prior to mechanical testing, following ASTM D618-21. ²⁶

Flammability test (Limiting Oxygen Index - LOI)

The flame retardancy of the composites was assessed by determining the Limiting Oxygen Index (LOI) according to ASTM D2863. Bar specimens (100 × 10 × 4 mm) were mounted vertically in a glass column and exposed to a mixture of oxygen and nitrogen flowing upwards. The specimen was ignited from the top, and the minimum concentration of oxygen required to sustain combustion for 3 min or burn a length of 50 mm was identified as the LOI value. A higher LOI indicates better flame retardancy.

Thermal analysis (thermogravimetric analysis - TGA)

The thermal stability and decomposition behavior of the composites were investigated using Thermogravimetric Analysis (TGA) in accordance with the general principles outlined in ASTM E1131. A PerkinElmer TGA 4000 instrument was employed. Samples weighing 5–10 mg were placed in a platinum crucible and heated from 30°C to 800°C at a constant heating rate of 20°C/min under a continuous flow of nitrogen (50 ml/min) to prevent oxidative degradation. The weight loss of the sample was recorded as a function of temperature. The data were used to determine the onset decomposition temperature, the temperature at maximum degradation rate, and the percentage of ash residue at 800°C.

Statistical analysis

A full factorial experimental design (4 levels of recycling cycles × 3 levels of nanosilica content) was implemented. All tests were performed with a minimum of three replicates per formulation, and results are reported as mean ± standard deviation (SD).

To assess the statistical significance of the main effects (recycling cycles and nanosilica content) and their interaction, a two-way Analysis of Variance (ANOVA) was performed. The F-values and p-values were estimated from the reported means and standard deviations using standard ANOVA formulas, and the significance levels are presented in Table 3. Significance was defined at p < 0.05. When significant differences were identified, Duncan’s Multiple Range Test (DMRT) was applied for post-hoc mean comparison at a 95% confidence level. For properties where the interaction effect was not significant (p ≥ 0.05), main effects were interpreted independently.

Tensile properties

As seen in Figure 2, analysis of tensile strength revealed that increasing the number of PP recycling cycles led to a continuous decline in this property, with the highest value belonging to virgin PP and the lowest to three-times recycled PP. This decline is attributed to thermo-mechanical chain scission (as discussed above), which reduces molecular weight and entanglement density, limiting stress transfer and causing premature failure.^2,3,17 Concurrently, adding nanosilica up to 3 wt% increased tensile strength, but a further increase to 5% caused it to decrease again. The improvement at 3 wt% nanosilica is attributed to two synergistic mechanisms. First, the high specific surface area of nanosilica (200 m²/g) provides strong interfacial interactions with PP chains. This restricts polymer chain mobility and increases the energy required for chain disentanglement under tensile load. Second, well-dispersed nanoparticles act as physical crosslink points, efficiently transferring stress from the soft matrix to the rigid filler.^11,12 The deterioration at 5 wt% nanosilica is inferred to result from nanoparticle agglomeration, based on the observed property trends and supporting literature.^17,18 Beyond the critical dispersion threshold (approximately 3–4 wt%), van der Waals forces cause particle clustering, reducing interfacial area, creating stress concentrators, and initiating cracks.^17,18 In the study of their combined effect, the lowest tensile strength was observed in the sample containing 5% nanosilica and three-times recycled PP, while the highest was recorded in the sample with 3% nanosilica and virgin PP. This non-monotonic response, enhancement up to an optimal loading (3 wt%) followed by deterioration at higher loading (5 wt%), is a well-documented phenomenological indicator of nanofiller agglomeration in polymer nanocomposites.^28,29 Beyond the critical dispersion threshold, nanosilica particles tend to form agglomerates that reduce the effective interfacial area, create stress concentration sites, and impair stress transfer from the matrix to the filler. ³⁰ This clearly demonstrates that both material degradation from recycling and the previously discussed dispersion-agglomeration transition^17,18 synergistically deteriorate tensile performance. The decline at 5 wt% nanosilica is consistent with the well-established behavior of polymer nanocomposites beyond the critical dispersion threshold.OPEN IN VIEWER

Similarly, as shown in Figure 3, a similar trend was observed for the tensile modulus. Increased PP recycling reduced the modulus. Adding nanosilica up to 3% improved it. Nevertheless, at 5%, it had an inverse effect. The combination of three-times recycled PP without nanosilica yielded the lowest tensile modulus, whereas virgin PP with 3% nanosilica achieved the highest value. This outcome underscores the critical roles of both polymer quality and optimal nanofiller content in maximizing the stiffness of wood-plastic composites.OPEN IN VIEWER

Flexural properties

As illustrated in Figure 4, the flexural strength followed a trend analogous to that of tensile strength. Increasing the number of PP recycling cycles resulted in a progressive decline in flexural strength, with virgin PP exhibiting the highest value and three-times recycled PP the lowest. The addition of nanosilica up to 3 wt% enhanced flexural strength, whereas a further increase to 5% led to its reduction. The synergistic interaction of these variables is evident, as the lowest flexural strength was recorded for the composite containing 5% nanosilica and three-times recycled PP, while the highest strength was achieved with 3% nanosilica in a virgin PP matrix.OPEN IN VIEWER

Similarly, Figure 5 demonstrates that the flexural modulus exhibited a similar response. Increased PP recycling reduced the modulus, while the incorporation of nanosilica up to 3% improved it, beyond which a decrease was observed. The minimum flexural modulus corresponded to the composite made from three-times recycled PP without nanosilica, and the maximum was attained using virgin PP with 3% nanosilica. These congruent trends in flexural properties arise from the same underlying mechanisms governing tensile behavior: the degradation of polymer chains with repeated recycling diminishes matrix integrity, while nanosilica at optimal dispersion (3%) reinforces the composite through enhanced stress transfer and restraint of polymer chain mobility.^11,12 Conversely, the decline at 5 wt% nanosilica follows the same trend observed in tensile properties, consistent with the dispersion-agglomeration transition behavior discussed above. Thus, the optimal flexural performance is achieved by minimizing polymer degradation and employing a well-dispersed, moderate loading of nanosilica.OPEN IN VIEWER

Impact properties

As presented in Figure 6, the notched impact strength of the composites exhibited a distinct yet explainable trend in response to the experimental variables. Contrary to the tensile and flexural properties, impact strength decreased with both increasing PP recycling cycles and with the addition of nanosilica across all tested levels (0%, 3%, and 5%). The lowest impact resistance was observed in the sample containing 3% nanosilica and three-times recycled PP. In contrast, the highest toughness was recorded for the composite made from virgin PP without any nanosilica addition.OPEN IN VIEWER

This behavior can be attributed to the fundamental mechanisms of energy absorption in polymeric composites. While nanosilica particles enhance stiffness and strength at optimal loadings, they simultaneously restrict polymer chain mobility, a critical mechanism for dissipating impact energy through plastic deformation.^11,17 These stiffening and embrittling effects are compounded by the reduced molecular weight and chain flexibility resulting from repeated PP recycling. Consequently, the combined detrimental effects of matrix degradation and nanoparticle-induced embrittlement lead to a pronounced reduction in impact toughness, even at nanosilica loadings that improve quasi-static mechanical properties.

Flammability behavior- limiting oxygen index (LOI)

As detailed in Figure 7, the Limiting Oxygen Index (LOI) of the composites showed a clear and positive response to the incorporation of nanosilica. The LOI value increased consistently with higher nanosilica content, rising from 17.5% for the sample with 0% nanosilica to 20.1% for the composite containing 5% nanosilica, when using virgin PP. Regarding the effect of PP recycling cycles on flammability, the results indicated that increased recycling negatively influenced the LOI. Although the primary factor governing flame retardancy was nanosilica content, matrix degradation due to repeated processing also played a discernible role. This reduction can be attributed to the lower molecular weight and reduced melt viscosity of recycled PP. These factors may impair the formation of a coherent and stable char layer during combustion. The degraded polymer chains are less capable of supporting the development of an effective protective barrier, thereby slightly compromising the flame-retardant efficiency of the nanosilica.OPEN IN VIEWER

This trend indicates a significant improvement in flame retardancy with nanosilica addition, while also revealing a modest detrimental effect of polymer recycling. As seen in Figure 7, the highest resistance to ignition was achieved in the sample with 5% nanosilica and virgin PP, while the lowest was observed in the composite without any nanosilica addition, regardless of recycling history. Among recycled samples without nanosilica, the LOI values remained relatively stable around 17.5%, suggesting that recycling alone, without nanofiller reinforcement, does not substantially alter the inherent flammability of the polymer matrix. The enhancement in fire performance with nanosilica addition is consistent with the protective barrier mechanism extensively documented in the literature.^14–16 As reported in previous studies using electron microscopy of combustion residues, nanosilica particles migrate to the surface during combustion and form a silicate-rich char layer that acts as a thermal insulator and mass transport barrier. Our LOI results, increasing from 17.5% to 20.1% at 5 wt% nanosilica, align with this established mechanism. However, direct validation of char morphology (e.g., SEM of residues) was not performed in this retrospective study. Consequently, a higher concentration of oxygen in the surrounding atmosphere is required to sustain combustion, which is directly reflected in the increased LOI values. However, the efficacy of this mechanism is somewhat diminished in highly recycled matrices due to polymer degradation, which may lead to a less cohesive char structure. This demonstrates the effectiveness of nanosilica as a flame-retardant filler in wood-plastic composites, improving fire safety without the use of halogenated additives. It also highlights the importance of considering matrix quality when designing flame-retardant formulations for recycled materials.

Thermal stability

As demonstrated in Figure 8, thermogravimetric analysis (TGA) revealed that increasing nanosilica content consistently enhanced the thermal stability of the composites, evidenced by higher residual ash values. For composites with virgin PP, residual ash increased from 6.29% at 0% nanosilica to 9.27% at 3% and further to 10.46% at 5% nanosilica. This improvement is consistent with the literature-reported mechanism of char formation,^14–16,19 wherein nanosilica promotes the development of a thermally stable residue that acts as a barrier against further degradation. Higher residual ash values (Figure 8) are commonly interpreted in the literature as indirect evidence of such char formation, though direct morphological analysis of the residue (e.g., SEM of TGA remnants) was beyond the scope of this study. The influence of PP recycling on thermal stability varied with nanosilica content. For composites without nanosilica, repeated recycling caused a modest decline in residual ash from 6.29% to 5.81%, attributed to reduced molecular weight and lower melt viscosity of the degraded matrix, which marginally affects char formation during thermal decomposition.OPEN IN VIEWER

For composites containing 3% nanosilica, residual ash decreased substantially from 9.27% to 8.41% after one recycling cycle, then further declined to 7.68% after three cycles. This sharper decline is due to the first recycling cycle severely compromising the polymer’s ability to interact effectively with nanosilica, hindering the formation of a stable inorganic-organic hybrid char structure.

For composites containing 5% nanosilica, residual ash dropped from 10.46% to 9.12% after one cycle, reaching 8.03% after three cycles. This sharper decline indicates that nanosilica agglomeration (as discussed in the 'Tensile Properties' section) disrupts the continuity of the char layer, reducing its protective efficiency in degraded matrices. This diminished thermal stability enhancement at 5 wt% nanosilica, despite the higher nominal filler content, is consistent with the dispersion-agglomeration transition reported in the literature.^29,30 When agglomeration occurs, the effective surface area available for char formation is significantly reduced, and the protective barrier layer becomes discontinuous, compromising thermal performance compared to a well-dispersed system at lower loading. Notably, even thrice-recycled composites with 5% nanosilica maintained higher thermal stability (8.03% ash) than virgin composites without nanosilica (6.29% ash), confirming that even in a degraded matrix, nanosilica provides substantial thermal protection, albeit diminished.