Sound insulation and mechanical properties of epoxy/hollow silica nanospheres composites

Diglycidyl ether of bisphenol-A-based E-51 epoxy (EP) was purchased from Shanghai Resin Factory Co., China. Methylhexahydrophthalic anhydride (MeHHPA) hardener was obtained from Jiaxing City Ocean Chemical Co., China. Styrene (St), 2-ethyl-4-methyl-imidazole (EMI-2,4), tetra-ethoxysilane (TEOS), tetrahydrofuran (THF), polyvinylpyrrolidone (PVP, K30), ammonium hydroxide (NH₃•H₂O), tetra-ethoxysilane (TEOS), ethanol and potassium persulfate (KPS) were supplied by Beijing Chemical Reagent Co., China. The St were treated by reduced pressure distillation and cold-stored.

10.0 g St and 1.5 g PVP were added to a 250-ml three-necked flask with 90.0 g boiling H₂O. Under N₂ at 70 °C, 10.0 g of 1 wt% KPS water solution was added and stirred. After 24 h, we obtained a PS particle dispersion. 5.5 g PS particle dispersion and 0.85 g NH₃•H₂O were added to a 250 ml boiling flask with 95.0 g ethanol at 30 °C with stirring. 5 ml TEOS were dropped into the system and stirring continued for 5 h. The as-synthesized PS@silica spheres were centrifuged and dispersed in 90 ml THF to dissolve PS and obtain HSN. The dispersion was stirred at room temperature for 24 h. The HSN were collected by centrifugation and washed with THF and ethanol until the PS was cleared. The HSN were freeze-dried centrifugally for 48 h and stored [16].

20.0 g TEOS, 300 ml ethanol and 50 ml H₂O were added to a 500 ml three-necked flask and boiled with stirring at 30 °C. 12 ml NH₃•H₂O was added dropwise to the mixture. After 24 h, we obtained a dispersion of SN. Following further centrifugal separation, they were freeze dried centrifugally for 48 h and stored.

The dry silica fillers were directly mixed with EP using a Rotation-Revolution Hybrid Mixer (ARE-310, THINKY, Japan). The MeHHPA and EMI-2, 4 were added to this mixture at room temperature. After mixing for 10 min, the mixture was poured into a pre-heated steel mold coated with a release agent and cured at 50 °C/2 h, 100 °C/2 h, and 150 °C/5 h. Both EP/HSN and EP/SN composite samples were prepared under the same conditions. The composites with equal thickness and varying HSN (SN) content and equal HSN (SN) content but varying thickness were fabricated to investigate the effects of different HSN (SN) content and sample thickness on the sound insulation and mechanical properties.

Transmission electron microscopy (TEM) images were obtained on a Tecnai G220 electron microscope (FEI Co, Netherlands) using specimens cut at ambient temperature with an ultra-microtome (UCT-GA-D/E-1/00, LEICA Co., Austria) and a diamond knife. 50–100 nm thickness samples were collected in a trough filled with water and placed on the copper grids. These EP/HSN samples were dispersed in absolute ethanol, dropped onto copper grids and dried under ambient conditions. Field emission scanning electron microscopy (FE-SEM, FEI Co., Netherlands) was used to examine the morphology of the SN particles.

The STL values were measured at room temperature with impedance tubes as shown in figure 1. The test set-up (BSWA Technology Co., Ltd. China) consisted of four micro-phones, a sound calibrator, and a BSWA-SW466-type impedance tube. This STL testing method conformed to the ASTM E2611 standard. Here, E_t , E_I , E_r and E_a are transmitting, incident, reflecting and absorption sound energy, respectively, and the schematic diagram of impedance tube device and sound insulation testing process are showed in figure 1.

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Tensile tests and bending tests were performed using a universal testing machine (CMT4104, Shenzhen San Testing Machine Co., China) equipped with a 10 kN load cell at a crosshead speed of 2 mm min⁻¹. ISO 527 and ISO 14,125 testing standards were followed to conduct the tensile and bending tests, respectively. A Charpy impact test machine (ZBC-1400–1, Shenzhen San Testing Machine co., China) was used to obtain the impact energy of V-notched samples prepared according to the ISO 179 standard. Each reported value was averaged from at least eight specimens. Fracture toughness (K_IC) values were determined based on the ASTM D5045 standard using single edge notch bend (SENB) samples measuring 65 × 10 × 4 mm³ in three points bending. The K_IC values were calculated by the fracture toughness formula (1). Here, the P is critical fracture load, the letter a is initial crack length, B and W are specimen thickness and width, respectively. Every reported value was the average of at least six samples.

$$\begin{eqnarray}&&{K}_{IC}=\displaystyle \frac{{\rm{P}}}{{\rm{B}}\sqrt{{\rm{W}}}}f\left({\rm{a}}/{\rm{w}}\right)\end{eqnarray} \tag{ 1 }$$

$$\begin{eqnarray*}&&f\left(x\right)=\displaystyle \frac{6\sqrt{x}\left[1.99-x\left(1-x\right)\left(2.15-3.93x+2.7{x}^{2}\right)\right]}{\left(1+2{\rm{x}}\right)\left(1-{x}^{3/2}\right)}\left({\rm{x}}={\rm{a}}/{\rm{W}}\right)\end{eqnarray*}$$

The morphologies of both HSN and EP/HSN composites were examined via the TEM and SEM. As shown in figure 2(a), HSN with inner diameters of 160 nm and outer diameters of 200 nm were synthesized and the outer surface of the HSN was very rough and due to the deposition of tiny silica particles. The EP/HSN composites with varying particle contents from 0.4 to 2.0 vol% displayed in figures 2(b)–(e) and confirmed that the HSN particles remain hollow not filled by EP resin. These figures also indicated that the HSN has a little aggregation in nanocomposites. The figure 2(f) was the SEM image of SN particles, we could see that the morphology of these nanospheres were very uniform, and their diameter was about 200 nm, which was close to the diameter of hollow nanospheres in figure 2(a). In this work, these spheres can be used as comparative fillers to prepare the corresponding composites.

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The existence of hollow spheres in epoxy resin matrix can be directly observed from the microscopic point of view by ultra-thin slices of composites. However, the cross-section morphology of EP/HSN composites needs to be observed and characterized via SEM. Figure 3 were the SEM images of the cross-section morphology of EP/HSN composites with different loading (a) 0.4 vol%; (b) 1.6 vol%; (c) 2.0 vol%. It can be seen from this figure that with the continuous increase of the content of hollow silica spheres, the agglomeration of hollow silica spheres becomes more and more obvious. In figure 3(a), small aggregates appear in the field of vision, but the particle diameter of the aggregates was very small. When the content of hollow silica spheres increases to 1.6 vol%, more aggregates appeared in the field of vision and the largest aggregate diameter were increased to 1 micrometer. When the content of hollow silica spheres continues to increase to 2.0 vol%, the diameter of aggregates has been nearly 6 micrometers. Although we do not want the agglomeration of hollow silica nanospheres, the experimental facts tell us that the research goal of improving the dispersion of nano fillers is still very arduous. With the increasing content of hollow silica nanospheres, the phenomenon of agglomeration was intensifying. These results would affect the comprehensive properties of EP/HSN composites, especially the mechanical properties of composites, which will be discussed in section 3.3 of this work.

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It was well-known that the density of material could affect the STL value, the higher density materials possessed higher STL values. Since the densities of all the materials approached 1.19 g•cm⁻³ and these densities were showed in table 1. And the EP/SN composites had slightly higher densities than the EP/HSN composites of the same content, the effect of density change on STL values of the materials were ignored [17].

In this study, the STL values were studied via the impedance tube with four microphones as shown in figure 1. A sound wave was produced by the speaker into the sound source tube, and the sample is in the middle of the tube. The four microphones are used to measure the sound power signals which are calculated and converted to a STL value by equation (1) through the transfer function method [18, 19]. Here, E_t is transmitted sound energy, E_I incident sound energy, E_r reflected sound energy, E_a is absorbed sound energy and they are related by

$$\begin{eqnarray}&&{E}_{I}={E}_{t}+{E}_{r}+{E}_{a}\end{eqnarray} \tag{ 2 }$$

The STL values were given by this equation:

$$\begin{eqnarray}&&STL=-20\,\mathrm{lg}\,| {E}_{t}/{E}_{I}| \end{eqnarray} \tag{ 3 }$$

The STL values were obtained from 100 to 6300 Hz and the average STL values were taken as the average value over 1/3 of an octave [9, 15]. The soundproofing properties and average STL values of EP/HSN and EP/SN composites are shown in figure 4 (3–10 mm thickness, 2 vol%) and figure 4 (0.4–2.0 vol%, 10 mm thickness). We could draw conclusions from the results: (1) All STL versus frequency (Hz) curves displayed minimum STL values at the resonant frequency of 1000–1250 Hz of the composites, and the STL values were increased above these minima (figures 4 and 5). At the content of 2 vol% of the EP/SN and EP/HSN composites in figures 4(a) and (b), the STL values were improved with sample thickness from 3 to 10 mm among the range of full frequency.

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(3) The average STL values of (EP/HSN, EP/SN) composites were (44.3, 33.9 dB) and (53.9, 48.2 dB), respectively, for 3 mm and 10 mm thickness of composites samples. Therefore, the thicker samples have higher STL and are more effective in soundproofing based on the mass law. (4) For the same sample thickness in figure 4(c), EP incorporated with either HSN or SN particles has improved average STL values due to the increased composite density. Given the same incident sound energy E_I, hollow HSN yields higher average STL values than solid SN since the absorbed sound energy E_a was larger because more EP/HSN interfaces exist in the shell which will scatter, diffract and refract sound waves and there was air confined in the core which will decrease the transmitted sound waves in figure 1.

(5) At 10 mm thickness, STL of EP/HSN composites were increased with the filler content increasing from 0.4 vol% to 2.0 vol% at all frequencies, particularly at >1600 Hz, figure 5(b). Even the same trend exists, this effect was not as prominent for EP/SN composites, figure 5(a). (6) figure 5(c) showed that the average STL values of EP/HSN were larger than EP/SN for all filler loadings from 0.4 vol% to 2.0 vol% HSN. When the fillers contents were 2.0 vol%, the average STL values were 53.9 dB of the EP/HSN composites, and the average STL value was 48 dB of the EP/SN composites, respectively, and the pure EP composites average STL values only had 45.9 dB. From the table 1 results, the densities of the EP/SN composites were higher than EP/HSN at the same loading fillers according the mass law. However, the opposite result appeared in this work, which further proved that the addition of hollow filler was beneficial to the sound insulation of composites. This may be that the hollow filler could dissipate part of the sound energy in the composites and enhance the sound insulation effect [19].

(7) At the same filler loading (figure 3(c) at 2.0 vol%), the difference between of the average STL values for EP/HSN and EP/SN decreases with increasing thickness. Hence, for 3 mm thickness samples, the average STL value of EP/HSN is 11 dB higher than that of EP/SN. However, for 10 mm samples, the difference in average STL value is only 5.7 dB. So it was expected that this difference will be even smaller with thicker samples. In principle, if the sample was thick enough and no sound energy can be transmitted through the material regardless of the type of filler. Nonetheless, the materials thickness was often not un-limited under actual conditions. The hollow fillers could enhance the sound insulation performances of the composites and this view had been proved right again from this work. The mechanical properties of the EP/HSN composites were also needed to be measured as a structural sound insulation material and these results were discussed in section of 3.3.

The figure 6 showed the mechanical and fracture properties of EP/HSN composites. Fracture toughness (K_IC), impact, bending, and tensile tests were performed to evaluate the mechanical properties of the composites. The fracture toughness of EP and the EP/HSN composites with different HSN contents were showed in figure 6(a). The K_IC values of the pure EP was 0.62 MPa•m^1/2 and the EP/HSN composites with HSN content 0.8 vol% was 74 MPa•m^1/2. In figures 6(b) and (c), the bending strength and tensile strength approached the maximum values when the HSN loading was 0.8 vol%, and the bending modulus (1.81 GPa) and tensile modulus (1.87 GPa) of the EP/HSN composites were also higher than the pure EP.

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Figure 6(d) were the impact strength of the pure EP, EP/HSN, and EP/SN composites with different fillers contents. The impact strength of pure EP was 6.3 kJ•m⁻², the EP/HSN composites with 0.4 vol% HSN was 11.5 kJ•m⁻², and the EP/SN composites with 0.4 vol% SN was only 3.8 kJ•m⁻². The impact strength of the EP/HSN composites was decreased with increasing of the HSN loading, this may be due to the poor dispersion of HSN and SN in EP matrix. The HSN were better than the SN dispersed in EP matrix, because the density of the SN was much higher and the viscosity of the EP/SN system was much lower, so the SN were deposited at the bottom of the composites during EP curing.