3.1. Morphology
The sound absorption properties of flexible polyurethane foams with a porous structure are closely linked to their morphology, which affects the interaction of sound waves with cavities and pores, leading to the dissipation of energy as thermal energy. To improve the compatibility between the PU matrix and filler, we employed a dispersant and investigated its effect on the PU reaction kinetics. Figure 1 displays the NCO conversion of PU composite foams, including 0 wt% and 3 wt% melamine particles, both with and without dispersant. The conversion rate was higher at 0 wt% melamine particles since the addition of melamine particles increases initial viscosity and hinders the drainage flow, resulting in a less smooth PU forming reaction [26]. To mitigate the decrease in NCO conversion rate due to melamine particle addition, we compared the reaction rates of PU composite foams with and without dispersant. The dispersant improves dispersity via a three-step process, in which it adheres to the surface of melamine particles to enhance wetting and compatibility with the polyol system (step 1), generates friction on the particle surface to disperse the filler particles (step 2), and prevents re-agglomeration of filler particles by generating static electricity or steric hindrance on the surface of the filler particles (step 3) [27, 28]. The NCO conversion rate of the PU composite foams containing 0 wt% and 3 wt% melamine particles with dispersant was higher than that without dispersant because the dispersant contains non-reactive amines that adsorb onto the filler particles, improving dispersion stability and decreasing viscosity to increase drainage flow, thus increasing NCO conversion rate. The formation of cavities and pores during the initial reaction rate is influenced not only by the PU matrix but also by CO2 gas movements [9, 11]. The reaction rate of the foam varies based on the addition of the dispersant and melamine particles (Fig. 1), and it further results into the different foam morphology.
SEM images of the PU composite foams with varying contents of melamine particles (0, 3, and 5 wt%) are presented in Fig. 2a, both without and with the use of a dispersant. To obtain average cavity and pore sizes, we utilized Image-pro Plus software for analysis, and the resulting measurements are displayed in Fig. 2b and 2c. The cavity size of the PU composite foams decreased as the amount of melamine particles increased, consistent with previous studies [19, 29, 30]. Fillers can serve as nucleating agents that initiate cavity formation, with an increase in the number of nucleation sites leading to more small-sized cavities. In addition, increasing filler contents also raises the viscosity of the system, making it harder for the PU matrix to flow and inhibiting cavity growth. Consequently, the average cavity size decreases with increasing filler contents. At 3 wt% filler contents, however, the decreasing tendency is reversed, likely due to filler particle agglomeration. Aggregated filler particles can reduce the available nucleation sites, thereby increasing the cavity size. High filler contents may have a negative impact on morphology development because of the agglomeration phenomena [31]. Furthermore, the cavity and pore sizes of the PU composite foams with melamine particles and dispersant were smaller compared to those without dispersant. The dispersant reduces the viscosity of the system, thereby accelerating the urethane formation reaction between isocyanate and polyol. This reaction strengthens the cavity strut, suppressing cavity growth.
Figure 3a presents the relative ratios of different pore types (open, partially open, and closed) in the PU composite foams filled with melamine particles, with and without a dispersant. The ratios of open and partially open pores exhibited significant differences depending on the filler content and the presence of a dispersant. In comparison to the non-filler case, the addition of melamine particles decreased the open pore ratio while increasing the partially open pore ratio, likely due to the increased viscosity of the PU matrix and reduced drainage flow [3, 16, 32], which delayed pore opening. However, after the 3 wt% filler content, the trend is reversed, with aggregated melamine particles poorly compatible with the PU matrix (see Fig. 4a), leading to cell collapse and increasing open pore formation during CO2 gas expansion. Furthermore, the addition of a dispersant further reduced the open pore ratio and increased the partially open pore ratio compared to the non-dispersant case. This can be attributed to the increased compatibility between the PU matrix and the surface of the filler particles. Additionally, the inclusion of the dispersant increased the NCO conversion rate (as shown in Fig. 1), causing the NCO group to react faster and resulting in stronger cell walls, leading to an increase in the partially open and closed pore ratios. However, the opposite trend is observed at 3 wt% filler contents due to the low amount of dispersant relative to the filler content. Therefore, to achieve a desirable morphology that is closely related to sound absorption properties, it is crucial to apply an appropriate amount of dispersant to a specific amount of filler when manufacturing PU composite foams. Figure 3b presents the open porosity of PU composite foams filled with melamine particles with and without dispersant. The open porosity was determined using the following Eq. (2):
Open porosity = (No + Np/2) / (No + Np + Nc) (2)
where No, Np, and Nc are the numbers of open, partially open, and closed pores. The open porosity exhibits a similar trend to the open pore ratio. The addition of melamine particles reduces the open porosity, and this effect is more pronounced in the case of melamine particles with dispersant. This is especially important for sound absorption characteristics at low frequencies because it increases the possibility of sound waves colliding with cavity struts, leading to scattering or transmission.
Figure 4 depicts representative SEM images of the PU composite foams containing 5 wt% melamine particles with and without dispersant. Figure 4a shows the aggregation of particles on the polymer matrix in the foams without dispersant. As explained in Fig. 3, excessive filler content can lead to poor compatibility between the filler surface and the PU matrix, resulting in an increase in open pore formation. Therefore, the open pore ratio of the PU composite foams containing more than 3 wt% of filler particles increased again. On the other hand, Fig. 4b illustrates that the addition of dispersant results in well-dispersed melamine particles in the PU matrix, leading to excellent interfacial compatibility and a further increase in the partially open pore ratio for filler content lower than 3 wt% (Fig. 3a). However, excessive amounts of dispersant can reduce physical properties of the foams. Therefore, it is recommended to apply optimal amounts of fillers and dispersant in the manufacture of PU composite foams to achieve the best performance.
3.2. Sound absorption properties
The sound absorption coefficient (α) is a measure of the amount of incident sound energy that is absorbed by a material. Porous materials absorb sound through three main mechanisms. First, air molecules vibrate with the pore walls, converting sound energy into heat energy. Second, the air in the pore is compressed and released, resulting in energy consumption during the energy conversion process. Third, sound energy is converted into heat energy through resonance with the pore walls [2, 10, 11]. The morphology of the material plays a crucial role in determining its sound absorption properties. To achieve optimal sound absorption performance, the material must have a large number of pores that are connected in an appropriate size for the propagation of sound waves, and there should be continuous channels between the inner pores and the outer surface [10].
Figure 5a depicts the sound absorption coefficient of PU composite foams with varying melamine particle content without dispersant. The maximum peak value of α increases from 0.88 to 0.98 as the filler particle content increases. However, it decreases again when the filler particle content exceeds 3 wt%, which can be attributed to the small cavity size (Fig. 2) and the highest partially open pore ratio (Fig. 3) at 3 wt% melamine particles contents. The small cavity size increases the air flow resistivity, increasing the residence time of sound waves in the foams and probability of collisions with air molecules, thereby improving sound absorption [17–19, 33]. In addition, the highest partially open pore ratio promotes collisions with sound waves, resulting in better dissipation of sound energy through viscous friction and heat exchange, leading to higher sound absorption characteristics. Figures 5b and 5c display the acoustic activity (AA) and noise reduction coefficient (NRC) of PU composite foams with melamine particles, both with and without dispersant. AA is calculated as the average of the sound absorption coefficient over the entire frequency range, while NRC is evaluated as the average of the sound absorption coefficient at 250, 500, 1000, and 2000 Hz [3, 19, 34, 35]. The PU composite foams including 3 wt% melamine particles with dispersant show the highest performance in terms of both AA and NRC. Consequently, the filler not only reduces the cavity size but also increases the partially open pore ratio, improving sound absorption efficiency. Both factors were higher with the dispersant than without dispersant due to the highest partially open pore ratio and the well-developed cavity and pore structure resulting from reduced particle agglomeration.
Figure 6 displays the sound absorption coefficients of the PU composite foams containing melamine particles, with and without dispersant, for the low frequency region. The mechanism of sound absorption differs based on the method of sound wave propagation, which changes depending on the frequency region. High frequencies have shorter wavelengths and stronger directionality, leading mainly to reflection. In contrast, low frequencies transmit more than they reflect due to their longer wavelengths and strong diffraction properties [36–38]. For porous structures, the highest open pore ratio is advantageous for sound absorption in the high frequency region, while the highest partially open pore ratio enhances sound absorption in the low frequency region through wall transmission. The PU composite foams with dispersant exhibit a higher partially open pore ratio than those without, indicating that transmission through the partially open pores is more effective in sound absorption than reflection through open pores in low frequency areas (as depicted in Fig. 3).