Dynamic light scattering (DLS)DLS analysis was utilized to determine the particle size of the prepared BC at different ball-milling times, specifically 5, 10, and 15 min. The particle size distribution curves are presented in Figs. S1–S3. The particle size is consistent with the ball-milling time, with longer times in the ball-mill resulting in smaller particle sizes. The particle sizes were 1800, 400, and 23 nm at 5, 10, and 15 min, respectively.Fourier transform infrared (FTIR) spectroscopyThe prepared BC particles were modified to enhance dispersion ability and improve the incorporation of the modified BC into the PU matrix. The chemical modification was confirmed using the FTIR technique. The FTIR spectra of untreated BC and modified BC with APTMS coupling agent are demonstrated in Fig. S4. The emergence of new bands at 1000 and 1125/cm is attributed to the Si–O–Si of APTMS48, confirming the successful incorporation of aminosilane in BC filler. The bands at 1400/cm and 876/cm are associated with C–O stretch and bending of calcite, respectively49. The band at 3422/cm is due to sorbed water. The amine group band overlaps with the adsorbed water band at 3400/cm, making it challenging to distinguish them. The aliphatic bands at 2930/cm increase in intensity after modification due to the introduction of a new propyl group to the system.The chemical structure of the prepared composites was characterized using FTIR technique. Figure 3 depicts the typical FTIR spectrum of unfilled FPU foam and selected prepared composite materials. All well-defined bands derived from FPU bonds are present. The characteristic band at 3299/cm is attributed to the N–H stretching vibration50, indicating successful urethane formation20. The bands at 2970 and 2867/cm are associated with the asymmetric and symmetric C–H stretching of the aliphatic –CH2 group10,51. The absence of the band at 2275/cm indicates that all NCO groups reacted with the polyol52. Additionally, the physical interaction between urethanes and BC functional groups can be observed utilizing the FTIR spectrum. It is widely recognized that H-bonded carbonyl groups (–C=O) exhibit lower wavenumber infrared absorbance compared to free urethane carbonyls17,52. The unfilled FPU foam showed a band at 1728/cm, indicating unbonded (–C=O) of urethanes50,53. By adding BC filler, the band shifted to 1713/cm, indicating the formation of a physical interaction, H-bonded, between BC and urethane C=O groups. The bands at 1641 and 1095/cm are ascribed to the H-bonding of –C=O urea groups and the stretching vibration of C–O–C in the polyol54,55.Figure 3FTIR spectra of (a) unfilled FPU, (b) FPU/BC0.1, (c) FPU/S_BC20, (d) FPU/S_BC400, and (e) FPU/S_BC1800.Scanning electron microscopy (SEM)The morphology of untreated BC and selected samples from modified BC at a magnification of 15,000× and working distance of 13.3 mm is illustrated in Fig. 4a–d. The images of untreated and modified BC reveal that the BC particles have an irregular surface and a granular-like structure (resembling calcite crystals)56. This is attributed to the calcination process at 550 °C that occurred before pyrolysis. Additionally, the EDX analysis confirmed the modification of BC with the APTMS coupling agent. To accurately quantify the composition of the elements, two random spots were selected, and the average values were recorded. Table S1compares the elemental composition of untreated BC and modified BC. The results show that untreated BC contains different ratios of calcium (Ca), carbon (C), and oxygen (O), attributed to calcite. In contrast, modified BC samples (S_BC20, S_BC400, and S_BC1800) have two new elements, nitrogen (N) and silicon (Si), with varying ratios, indicating the successful incorporation of APTMS moieties on the BC surface.Figure 4Typical SEM images of (a) untreated BC, (b) S_BC1800, (c) S_BC400, and (d) S_BC20.The morphology and pore architecture of the prepared foams play a crucial role in the efficient dissipation of sound waves57,58. Figure 5a–j displays the SEM micrographs of the fabricated foams. Generally, all micrographs contain oval cavities with three types of pores: closed, partially open, and completely open, depending on the thickness of the cavity walls and the PU gas drainage flow rate. The mechanism of cavity and pore formation is as follows: Closed pores develop when the polymerization reaction is completed prior to the rupture of the cavity walls; in this case, the walls of cavities are strong enough to withstand the gas pressure. Conversely, open pores arise due to the reduced strength of the walls and a high flow rate of drainage within the thin cavity walls. On the other hand, partial open pores occur when the wall strength is elevated and the drain flow rate is low; in this case, the pores may be neither fully closed nor completely open59Figure 5FESEM micrographs of (a) unfilled FPU, (b) FPU/BC0.1, (c) FPU/BC0.3, (d) FPU/BC0.5, (e) FPU/BC0.7, (f) FPU/S_BC10, (g) FPU/S_BC20, (h) FPU/S_BC30, (i) FPU/S_BC400, and (j) FPU/S_BC1800.The average values of pore and cavity diameters are summarized in Table 2. The pore size distribution for the obtained foams is illustrated in Fig. S5. It is evident that all FPU composites exhibited lower cavity diameters than unfilled FPU foam, possibly due to the filler acting as nucleation sites, supporting cavity formation. Additionally, the diameter values depend on the dispersion of filler particles within the polymer matrix. For instance, the inclusion of 0.1 wt. % of unmodified BC resulted in lower values of 0.738 and 0.183 mm for cavity and pore diameter, respectively, indicating well-dispersed BC particles within the FPU matrix. However, when the BC content exceeds 0.1 wt. %, the cavity and pore sizes of FPU/BC composites begin to increase compared to FPU/BC0.1 sample. This could be attributed to a higher concentration of BC, leading to the agglomeration of BC particles, thereby destabilizing the cellular structure of the foam and resulting in larger cavities. Moreover, the impact of different wt. % of modified BC with APTMS on the cavity and pore diameters of FPU revealed no consistent trend. Remarkably, the highest values for both cavity and pore diameters were 0.922 and 0.305 mm, respectively, observed in FPU/S_BC1800, possibly due to the large particle size disturbing the nucleation process and forming large and irregular cavities.
Table 2 Values of cavity diameter, pore diameter, gel fraction and apparent density of unfilled FPU, FPU/Untreated BC (23 nm) composites, FPU/S_BC (10, 20, and 30 wt. % APTMS) composites and FPU/S_BC with different particle sizes (400 and 1800 nm).Open porosityThe relationship between open porosity and sound absorption coefficients is significant because open porous flow paths facilitate multiple sound wave collisions16. Figure 6a–c illustrates open porosity values for both unfilled FPU and FPU composites. It is evident that open porosity decreases with up to 0.1 wt. % of unmodified BC content but increases beyond this point, as depicted in Fig. 6a. This could be attributed to well-dispersed BC filler at this limit, which resulted in thicker walls and reduced drainage flow, consequently lowering the porosity. A similar trend is observed with modified BC, showcasing minimal porosity at 20% modified BC (0.1 wt. %), as seen in Fig. 6b. Moreover, introducing different particle sizes, 400 and 1800 nm, from modified BC impacts open porosity, as demonstrated in Fig. 6c, with FPU/S_BC1800 displaying higher porosity than FPU/S_BC400. This is believed to occur due to larger particle sizes disrupting cell formation and leading to thin cavity walls unable to withstand gas pressure.Figure 6Open porosity of (a) unfilled FPU and FPU/untreated BC composites, (b) FPU/modified BC composites, and (c) FPU filled with modified BC of different particle sizes.Gel fractionThe gel fraction, a crucial parameter for understanding the cross-linking in polymer composites, was measured. The gel fraction of unfilled FPU foam was 90.54%, which increased to 92.27% with the addition of 0.1 wt. % untreated BC (23 nm), as shown in Table 2. This increase was attributed to the high surface area of untreated BC, enhancing its adhesion to the polymer matrix and reducing sorption, and the possible reaction of the surface hydroxyl groups of untreated BC with isocyanate to increase the cross-linking. However, beyond 0.1 wt. %, the gel fraction decreased, possibly due to excess BC causing agglomeration. Furthermore, incorporating modified BC with an APTMS coupling agent increased the gel fraction, with a peak at 20% modification. However, the gel fraction deceased beyond this point. Additionally, the impact of treated BC particle size on the gel fraction was investigated, showing a decrease in the gel fraction as the particle size increased. This reduction was attributed to the lower surface area of larger particles, reducing the possibility of cross-linking with the FPU matrix and, consequently, the gel fraction.Apparent densityThe density of flexible polyurethane (FPU) foam, mainly influenced by additives such as filler, is essential in measuring foam comfort and support. The density of FPU is directly proportional to its compressive properties. The densities of the obtained foams are listed in Table 2. Adding 0.1 wt. % of untreated BC filler to FPU foam increases foam density by 4.32% compared to unfilled FPU foam. However, beyond 0.1 wt. %, the density declines due to poor dispersion of the BC filler, which promotes cell rupture and more interconnected open pores, thus reducing the mass of the samples and consequently lowering the foam density. The influence of modified BC with different percentages of APTMS coupling agent on the density of FPU foam was also investigated. Adding 0.1 wt. % of S-BC10 and S-BC20 improved density compared to 0.1 wt. % of untreated BC and unfilled FPU foam. This improvement is attributed to the modification of BC with APTMS, which enhances the dispersion and compatibility of BC within the FPU matrix and produces a more crosslinking structure, thereby increasing density. However, beyond 20% modification, the density slightly decreased due to excessive modification with APTMS16,39, which facilitates particle agglomeration and poor dispersion of BC inside the FPU matrix, resulting in decreased density. The effect of BC particle size on foam density was also studied. It was observed that adding 0.1 wt. % of S-BC20 with 1800 nm particles resulted in a lower density than 0.1 wt. % of S-BC20 with 400 nm particles. This is due to the tendency of larger particles to aggregate, resulting in an inhomogeneous dispersion within the FPU matrix. Consequently, it is essential to adjust the quantity, size, and percentage of alteration to achieve the desired foam structure with optimal cellular morphology.Thermogravimetric analysis (TGA)The thermal degradation behaviour of untreated BC and the modified BC sample is demonstrated in Fig. S6. The untreated BC exhibited a major decomposition step between 600 and 800 °C, indicating the decomposition of CaCO360. On the other hand, for the S_BC20 sample, the decomposition temperature below 200 °C is attributed to moisture removal, while the decomposition between 400 and 800 °C is assigned to the decomposition of APTMS moieties61.Figure 7a–e presents TGA/DTG plots the thermal characteristics of unfilled FPU and selected composite materials. The plots emphasize the notable influence of untreated and treated BC on enhancing the thermal stability of FPU foam. Two distinct thermal decomposition phases were observed: the initial phase (250–350 °C) corresponding to the decomposition of urethane hard segments and a rapid degradation stage (350–450 °C) attributed to the decomposition of polyol soft segments11.Figure 7TGA/DTG plots of (a) unfilled FPU, (b) FPU/BC0.1, (c) FPU/S_BC20%, (d) FPU/S_BC400, and (e) FPU/S_BC1800.The detailed thermal decomposition characteristics, including the decomposition temperature at 5% and 50% mass loss (Td5% and Td50%), and residual mass percentages at 800 °C, are presented in Table 3. For instance, unfilled FPU exhibited a Td5% at 292.17 °C. The addition of 0.1 wt. % of untreated BC led to an increase in the Td5% value to 299.15 °C, indicating enhanced thermal stability. Furthermore, the addition of 0.1 wt. % of silanized BC with 20% APTMS further increased thermal stability, with the Td5% reaching 312.77 °C and 29.72% residue at 800 °C. This enhancement was attributed to the crosslinking effect of silane moieties, resulting in a more interconnected polymer network that delayed thermal decomposition. Moreover, comparing different particle sizes of silanized BC with 20% APTMS (400 nm and 1800 nm) revealed that including 400 nm particles increased the thermal stability of FPU foam compared to the larger 1800 nm particles. This improvement was primarily due to the larger surface area of the smaller particles, leading to enhanced interfacial adhesion with the FPU matrix and more effective dispersion throughout the foam, thereby delaying thermal degradation. The Td50% values exhibited the same trend.
Table 3 TGA results of unfilled FPU and FPU composites.Compression strengthFigure 8a–c presents stress–strain diagrams for unfilled FPU foam and the obtained FPU composites at 50% deformation. The diagrams display three distinct regions, each delineating its unique deformation mechanism. The initial region signifies elastic deformation, wherein the foam maintains structural integrity due to the inherent rigidity of its struts, thereby preventing collapse. The subsequent region depicts cell buckling, characterized by a length plateau in the central section. The third region denotes densification, which emerges under high-strain conditions as the polymer walls commence compressing against their adjacent counterparts10.Figure 8Typical compression stress–strain curve of (a) unfilled FPU and FPU/untreated BC composites with particle size 23 nm, (b) FPU/S_BC composites with varying APTMS ratios (10%, 20%, and 30 wt. %), and (c) FPU containing different particle sizes from S_BC (400 and 1800 nm).The results revealed that adding 0.1 wt. % of untreated BC increased compressive strength by 4.02% compared to unfilled FPU foam (Fig. 8a), which is attributed to improved dispersion and hydrogen bonding between the polymer matrix and BC. However, compressive strength decreased by 3.54%, 8.02%, and 12.65% for FPU/BC0.3, FPU/BC0.5, and FPU/BC0.7, respectively, compared with unfilled FPU foam. This decline may be attributed to BC particle agglomeration at higher concentrations, creating weak points in the foam structure that cannot withstand the applied load, resulting in reduced compressive strength. Additionally, decreased density and gel fraction contribute to this reduction. The same trend has been reported in previous studies11,55,62. Figure 8b displays the compression stress–strain curve for FPU filled with modified BC with different percentages of APTMS. The findings show that adding 0.1 wt. % of S_BC with 20% APTMS increases the compressive strength by 4.05% and 8.23% compared to FPU/BC0.1 and unfilled FPU foam, respectively. This enhancement results from BC modification with APTMS, which leads to improved dispersion and crosslinking with the FPU matrix through the NH2 groups. This, in turn, contributes to greater foam stiffness and, consequently, higher compression strength. However, when the modification exceeds 20%, the compression strength decreases. This reduction is attributed to an excessive silane percentage, which causes agglomeration due to hydrogen bonds forming between NH2 and OH groups on the surface of BC16.
Finally, we investigated how different particle sizes of S_BC20 (400 nm and 1800 nm) affect the compression strength of FPU, as shown in Fig. 8c. The results showed that incorporating S_BC20 with a particle size of 400 nm resulted in significantly higher compressive strength than the larger 1800 nm particles. This can be attributed to the smaller particles offering a larger surface area, which enhances their adhesion to the polymer matrix. Moreover, FPU containing S_BC20 particles measuring 23 nm exhibited greater compressive strength than foams with larger S_BC20 particles.Acoustic characteristicsTable 4 illustrates the normal incidence absorption coefficient (αni) per one-third octave center frequency for the three examined groups. The results of the first group, consisting of unfilled FPU and FPU/Untreated BC composites (five samples, 20 mm thick, 23 nm particle size), are depicted in Fig. 9a and reveal the following facts: In the low-frequency range, the values of αni range between 0.08 (for the unfilled FPU sample) and 0.12 (for the FPU/BC0.7 sample), with generally close values and no clear advantage for any sample. Moving to the mid-frequency range, the acoustic performance of the different samples becomes evident. The unfilled FPU exhibits the lowest performance, with an average normal absorption coefficient αni of 0.22. This value gradually increases with the percentage of added BC, reaching 0.55 for the FPU/BC0.7 composite. Excluding the FPU/BC0.7 sample, the values of αni gradually increase for the three samples (FPU/BC0.1, FPU/BC0.3, and FPU/BC0.5), indicating a progressive improvement in absorption characteristics up to the 1600 Hz band. In this mid-frequency range, it can be concluded that the best performance corresponds to the highest percentage of BC (0.5, 0.3, and then 0.1 wt. %), with the latter showing the lowest performance among the three samples, except for one exception at the 630 Hz band. The FPU/BC0.7 sample exhibits different behaviour, with notably higher absorption coefficients, but the three samples (FPU/BC0.1, FPU/BC0.3, and FPU/BC0.5) generally surpass it from the 1250 Hz band onwards. The absorption coefficient of the FPU/BC0.7 sample remains almost stable between the two bands at 1000 and 1250 Hz, followed by a sudden drop at the successive bands of 1600 and 2000 Hz, representing the threshold of the high-frequency range.
Table 4 Values of αni for the samples of the three groups, 20 mm thickness.Figure 9The normal incidence absorption coefficient (αni) for (a) unfilled FPU and FPU/untreated BC composites, (b) FPU/S_BC composites. The two samples, unfilled FPU (the black line) and the FPU/BC0.1 (the dashed red line), are illustrated for comparison. (c) FPU containing different particle sizes from S_BC. Again, the two samples, unfilled FPU (the black line) and the FPU/BC0.1 (the dashed red line), are illustrated for comparison.In the high-frequency range (above 1600 Hz), the behaviour of the unfilled FPU sample aligns with that of the other samples in this group, exhibiting a decrease in the normal absorption coefficient (αni) at the 2000 Hz band, followed by an increase. However, the unfilled FPU sample showed a noticeable decline in acoustic performance compared to the other samples. In this range, the values of αni for the four other samples under discussion strongly overlap, making it challenging to distinguish the best through detailed interpretation. Nevertheless, the mean αni (see Table 5) indicates that the FPU/BC0.3 and FPU/BC0.7 samples perform the best and are very close, with mean values of 0.83 and 0.82, respectively. Following in descending order are the FPU/BC0.1 and FPU/BC0.5 samples, with comparable mean αni values of 0.78 and 0.76, respectively. The unfilled FPU sample holds the lowest mean αni of 0.57. Generally, αni values decrease at the 2000 Hz band for all samples, with a steeper decline in the FPU/BC0.1 and FPU/BC0.5 samples, followed by a gradual increase until the 4000 Hz band. At the 5000 Hz band, all samples exhibit a decrease in αni values, followed by a subsequent rise to different values. The FPU/BC0.3 sample demonstrates exceptional performance in this high-frequency range, with a gradual decrease in performance until the 5000 Hz band, succeeded by a sudden increase at the 6300 Hz band, setting it apart from the other samples.
Table 5 The mean αni, NRC, SAA, αw, and Absorption Class of the Examined Samples.Generally, the values of the three measures SAA, NRC, and αW (see Table 5) confirm the previous findings. In descending order, the SAA proves that the FPU/BC0.7 sample is the best (0.51), followed by the FPU/BC0.3 (0.50), FPU/BC0.5 (0.48), FPU/BC0.1 (0.43), and finally the unfilled FPU sample (0.24). The values of NRC come in a close context, where the two samples, FPU/BC0.7 and FPU/BC0.3, show the best results (NRC = 0.50), then in descending order, the samples FPU/BC0.5 (0.45), FPU/BC0.1 (0.40), and finally the unfilled FPU sample (0.20). Values of αW confirm again that the FPU/BC0.7 is the best (0.35(MH), class D), followed by the three samples FPU/BC0.1, FPU/BC0.3, and FPU/BC0.5 with the same αW (0.30(MH), class D) and finally the unfilled FPU sample (0.25(H), class E). Based on the previous findings, it can be concluded that adding BC evidently improves the acoustic performance of FPU foam. This improvement increases gradually by increasing the percentage of BC in the sample up to 0.70%, see Table 4. Table 4 lists the mean αni in the three frequency ranges (low, mid, and high) in addition to the values of NRC, SAA, αW, and the absorption class of the examined samples.The second group (Fig. 9b) shows the effect of APTMS-modified BC on the acoustic performance of FPU foam. The results demonstrated that the modification enhanced the acoustic performance of FPU. The SAA and NRC values were higher for FPU/S_BC10 and FPU/S_BC20 than FPU/BC0.1, indicating improved performance. This is possibly due to the increased friction of sound waves with APTMS moieties, which facilitates the dissipation of sound waves. However, the FPU/BC30 sample exhibited the least favourable performance, with the lowest SAA and NRC values, as shown in Table 5. The two samples, FPU/S_BC10 and FPU/S_BC20, have almost identical results (SAA = 0.47, NRC = 0.55 and 0.50, respectively), with a slight increase in αw noted for both, while a remarkable decrease occurs in the sample FPU/S_BC30. (αw = 0.25(H) class E).The last group examines the effect of BC particle size on the acoustic performance of FPU foam. This group includes two samples, 400 and 1800 nm (compared to the samples in the other two groups with a particle size of only 23 nm), both of which were added with 0.1 wt. %. Results clarified that the effect of the larger size is acoustically better up to the 800 Hz band, after which the effect of the smaller size (400 nm) is significantly acoustically superior in all bands above 800 Hz, see Fig. 9c, Tables 4 and 5. Nevertheless, the two measures, NRC and SAA, are too close (0.50 and 0.48 for the FPU/S_BC400, 0.55 and 0.49 for the 1800 nm). Values of αW are (0.30(MH) class D and 0.45(H) class D for the 400 and FPU/S_BC1800, respectively).It worth recalls that the thickness of all samples under study is 20 mm. As Sabbagh and Elkhateeb45 showed, the standard absorption coefficient αS “is directly proportional to the thickness of the polyurethane foam”; a remarkable improvement is expected by increasing the thickness of the samples to 40 mm or higher.Statistical analysisA Pearson correlation study was performed to investigate the relationship between specified acoustical and mechanical parameters in this research, particularly examining how the sound absorption coefficient (SAC) correlates with mechanical properties (MECH). Regression analysis indicates a positive correlation between sound absorption coefficients (SACs) and mechanical properties (MECH.) which is significant (p < 0.05), across various composites, whether modified or unmodified, as shown in the Table 6. The table shows the values for Pearson’s correlation and p value, the correlation ranges from moderate to high in all cases, with values exceeding 0.5 and it is positive correlation in majority. The correlation between SACs is slightly stronger than between SACs and MECH. properties, or even among MECH. properties themselves, whether in modified or unmodified forms. Both the R-squared and adjusted R-squared values are very close, suggesting that the regression model is both suitable and effective. With a p-value less than 0.05, the results are statistically significant, confirming a strong correlation between sound and mechanical properties in the composites.
Table 6 Regression analysis of the SAC and MECH. properties studied in the current work.
Improving the acoustic performance of flexible polyurethane foam using biochar modified by (3-aminopropyl)trimethoxysilane coupling agent
