The Influence of Aluminum Oxide Nanoparticles on the Characterizations and Mechanical Performance of Epoxy Matrix Composite

Epoxy resin matrix composites have been extensively used in the automobile, construction, electronics, and aerospace industries due to their excellent mechanical and thermal properties [1]. Epoxy is one of the most popular thermosetting polymers, which, after curing, forms a three-dimensional crosslinked network [2]. Because of these properties, it is used as a matrix for the production of polymer composites [1].

However, these advantages come with certain disadvantages in the mechanical properties of epoxy resin, such as low fracture toughness, low tensile strength, and brittleness, which lead to poor resistance to crack formation and propagation [3]. These limitations prohibit the use of these polymers in high-performance applications requiring mechanical strength. Expanding the range of polymer applications requires improving mechanical properties, as the materials must withstand heavy loads and challenging environmental conditions [4].

One way to address these limitations is to use various filler and reinforcing materials to enhance the properties and decrease the brittleness of cured epoxy. Therefore, recent research on epoxy polymers primarily focuses on applications employing particle reinforcement [5].

The use of particulate fillers produces consistent mechanical properties in every direction [6]. Particulate fillers include metals [7], metal oxides [8], natural rubber [9], and ceramics [10]. The dimensions of the filler particles range from microscale to nanoscale [11]. Nanoscale measures less than 100 nm in at least one dimension [12].

Significant enhancements in epoxy polymer properties have been obtained by utilizing nano-sized particles compared to micro-sized ones [5]. The incorporation of nanoparticles as a reinforcement, which presents a high surface area to volume ratio, greatly improves the interaction between the matrix and the nanoparticles [13].

Improving mechanical properties requires a high level of interfacial bonding between the polymer and nanoparticles. Due to the high viscosity of polymers and the cohesiveness that causes agglomeration, it can be difficult to disperse nanoparticles uniformly. Therefore, researchers generally need a method to ensure homogeneous distribution in a highly viscous liquid [14].

Numerous dispersion techniques have been used to combine polymers and nanoparticles, including ball milling, high shearing mixing, and ultrasonication [15]. The latter technique is very effective in achieving a homogeneous dispersion [16].

In this method, high-intensity ultrasound waves are introduced into the mixture to form cavitation bubbles. These bubbles begin to grow until they reach a specific diameter and then collapse. This collapse creates a hot zone with extremely high local temperature and pressure. These hot zones may subsequently cause the disintegration of nanoparticle aggregates in the mixture [17].

Nano-Al₂O₃ represents one of the most extensively utilized metal oxides for manufacturing polymer nanocomposites because of its exceptional mechanical properties, high hardness, and chemical and thermal stability, as well as reduced cost of Al₂O₃ nanoparticles compared to nanofillers made from carbon and nano-TiO₂ [18].

Many studies have focused intensively on this field; initially, Wu et al. [19] used epoxy resin as a matrix reinforced with 30 nm Al₂O₃ nanoparticles at various concentrations (1, 3, 5, and 7 wt.%). The mixture was mixed mechanically. Mechanical tests, including impact and tensile tests, were carried out on the samples. The findings showed a significant enhancement in the mechanical properties of the nanocomposites, with tensile strength increasing by 82.60% and impact strength by 63.58% compared to pure epoxy at a concentration of 3 wt.% of Al₂O₃.

Recent investigations by Zhang et al. [20] have used silane coupling compounds KH550, KH560, and KH570 to modify the surface of alumina nanoparticles before manufacturing epoxy nanocomposites. Mechanical properties, like hardness and tensile strength, have been evaluated. The findings demonstrated that the KH570-treated composite materials showed significant improvements in tensile strength (49.1%) and hardness (8.8%). These findings show that surface-modified Al₂O₃ nanoparticles may be used as reinforcements in epoxy composites efficiently and effectively.

There is a knowledge gap regarding the ideal balance of mechanical performance in nanocomposites for construction applications since using alumina nanoparticles to achieve significant enhancements in mechanical properties demands exact control in weight ratios and dispersion techniques.

Therefore, the primary objective of this research is to improve the mechanical properties of epoxy resin by adding Al₂O₃ nanoparticles; optimize the ratio of reinforcement that maximizes improvement and minimizes particle aggregation; evaluate hardness, flexural, impact, and tensile properties; and examine the morphological properties and particle distribution by using scanning electron microscopy (SEM).

3.1 Tensile test

The tensile strength of the sample was measured by a computerized universal testing machine, type (Yisite), model (EWN-20kN), with a crosshead speed of (5 mm/min) and a loading rate of 10 N/sec, as seen in Figure 2. The measurements of the tensile test specimen conform to ASTM D-638 [22] and are dog-bone shaped, as shown in Figure 3.

Figure 2. Computer-controlled tensile testing machine (Yisite)

(a)

(b)

Figure 3. Samples of a tensile test: (a) before the test, (b) after the test

3.2 Flexural test

The flexural test was performed according to the ASTM D-790 [22] standard by utilizing a universal testing machine type (ALFA) with a crosshead speed of (15 mm/min) and a capability of (20 kN), as shown in Figure 4. Flexural inspection of plastics and polymer composites is usually performed using a three-point test. Three samples were taken for each case, tested, and their averages were reported (see Figure 5).

Figure 4. Flexural test device (ALFA)

(a)

(b)

Figure 5. Samples of flexural test: (a) before the test, (b) after the test

3.3 Impact test

This test measures a material's strength when exposed to a sudden force. The testing was conducted using a pendulum impact tester, Izod type, model BROOKS; see Figure 6. The dimensions of the impact test sample conform to ASTM E23 [23], as shown in Figure 7.

Figure 6. Izod impact test device (Brooks)

(a)

5.1 Tensile properties of Al₂O₃-epoxy nanocomposites

The tensile tests were conducted on epoxy and nanocomposites with various nanoparticle weight percentages of alumina. Figure 13 demonstrates a brittle behavior for all the stress-strain curves, and the tensile strength gradually increases with increasing alumina nanoparticle wt.% until it reaches its maximum increase of 41.73% at 2 wt.% Al₂O₃ compared with pure epoxy, as illustrated in Table 2.

Table 2. Tensile test results of EP-Al₂O₃ nanocomposites

This improvement is attributed to the uniform distribution of nanoparticles within the epoxy and the effective interfacial adhesion between the reinforcement and the matrix, which results in facilitating stress transfer between them.

Our maximum increase in tensile strength is less than the 82.6% achieved by Wu et al. [19], because they used ultrasonic mixing for a longer period (about 4 hours), resulting in a better distribution of nanoparticles and a significant increase in tensile strength.

On the other hand, increasing the alumina content above 2 wt.% resulted in a gradual decrease in tensile strength, likely because of the agglomeration of nanoparticles within the epoxy, which increases stress concentration and subsequently leads to crack initiation and propagation. These results are consistent with the SEM images presented in the current study.

5.2 Flexural properties of Al₂O₃-epoxy nanocomposites

The flexural strength of nanocomposites was measured with different ratios of alumina nanoparticles. Figure 14 represents load-deflection curves of EP-Al₂O₃ nanocomposites.

Figure 15 shows that the flexural strength increased gradually with increasing amounts of Al₂O₃ up to 2 wt.%, then deteriorated beyond this percentage.

This increase is due to the presence of stiff and strong alumina nanoparticles within the epoxy, which limit the mobility of polymer chains under an exerted load, in addition to their resistance to deformation. However, with an increase in filler content over 2 wt.%, the aggregation of Al₂O₃ increases; this explains the deterioration of properties after this ratio. This is obvious from the SEM images presented in the current study.

The highest flexural strength was 51.6 MPa at 2 wt.% of Al₂O_3, with a 12% increase compared with pure epoxy. This value is lower than that of 135.22 MPa mentioned in the research by Abebe et al. [26], who integrated glass-wool fibers with Al₂O₃ nanoparticles (hybrid system) to strengthen the polymer, which gives superior mechanical performance.

Table 3 shows the flexural test results for different concentrations of Al₂O₃. 

The flexural stress and strain were calculated from these equations:

$\sigma_F=\frac{3 P L}{2 w B^2}$            (1)

$\epsilon_F=\frac{6 D B}{L^2}$          (2)

where,

• $\sigma_{\mathrm{F}}$= flexural stress (MPa)
• $\epsilon_{\mathrm{F}}$= flexural strain
• P = load (N)
• D = deflection (mm)
• L = length span (90 mm)
• B = thickness (5 mm)
• W = width (12.7 mm)

Table 3. Flexural test results of EP-Al₂O₃ nanocomposites

The addition of alumina nanoparticles demonstrates an enhancement in the impact properties of the nanocomposites. Figure 16 shows that the impact strength is directly proportional to nano-Al₂O₃ content.

For EP/Al₂O₃ nanocomposites, the impact strength peaked at a maximum of 42.52 kJ/m² at a concentration of 6 wt.% Al₂O₃, improved by about 38.64% compared with a pure epoxy.

This result, which is higher than the 13.34 kJ/m² reported by Farhan and Hussein [27], may be due to our use of nanoparticles instead of the microparticles used by the researcher in his study, due to their higher surface area to volume ratio, which provides excellent mechanical properties.

This increase in impact resistance, due to the inhibition of crack growth by the composite material and the resulting dislocations, leads to a change in crack direction and shape; consequently, the cracks become micro-cracks. This change in crack behavior and the loss of crack energy lead to increased toughness [27].

The impact strength is determined with the following equation:

Impact Strength $=U_c / A$                 (3)

where,

• U_c: represents fracture impact energy, which is obtained from the Izod impact test instrument in Joules.
• A: is the sample's cross-sectional area in meters.

5.4 Hardness properties of Al₂O₃- epoxy nanocomposites

The hardness test was conducted on both pure epoxy and nanocomposites at various wt.% of nanoparticles. The Shore D hardness values were measured and graphed in relation to the wt.% of Al₂O₃. Figure 17 shows the influence of the weight percentage of nanoparticles on the hardness, where the hardness increased as nanoparticle wt.% increased.

Our hardness results reached a maximum of 82 Shore D at a ratio of 6 wt.% of Al₂O₃, exceeding the value of 73.5 Shore D reported in a research by Al-Mansoori et al. [28]. This is attributed to the increased nanoparticle concentration.

The increased hardness of the nanocomposites is due to the fact that they depend on the bonds between the epoxy resin molecules and the Al₂O₃ nanoparticles. Increasing the content of Al₂O₃ nanoparticles reduces the distance between the molecules and improves the adhesion strength, thus making the nanocomposites resistant to penetration and scratching [29].

5.5 Fourier Transform Infrared analysis

FTIR spectra of epoxy and epoxy nanocomposites are shown in Figure 18. Different functional groups are noticed from the graph. A broad peak at 1106 cm^-1 refers to vibrations of C-O bonds of epoxide groups (C-O-C).

​

Figure 18. Fourier Transform Infrared (FTIR) of EP-Al₂O₃ nanocomposites

A wide peak at 2922 cm^-1 corresponds to C-H groups, while the weak peak at 1606 cm^-1 indicates vibrations of -OH groups on the surfaces of nano-Al₂O₃. A weak band at 1361 cm^-1 conformed with the vibrations of Al-O bonds. The large, wide range in the area of 555-826 cm⁻¹ is because of Al-O-Al bond vibrations.

These changes help us understand how the nanoparticles affect the way epoxy molecules bond together. Differences in the vibration structures of epoxy molecules have also been noticed. This provides us with more details on how the nanoparticles could change the way the epoxy is arranged.

5.6 Microstructural evaluation

The distribution of aluminum oxide nanoparticles (Al₂O₃) within the epoxy matrix is an essential factor influencing its mechanical behavior. SEM is necessary to evaluate the distribution of nano-Al₂O₃ in epoxy. Figure 19 illustrates the surface microstructure of epoxy/alumina nanocomposites, where we observed that the surface of pure epoxy is smooth. In contrast, the incorporation of nano-alumina increases the surface roughness.

The reinforcement shows a homogeneous distribution within the matrix; furthermore, there is strong interfacial adhesion between the reinforcement and the matrix at a ratio of 2 wt.% Al₂O₃, (see Figure 19(c)). This uniform distribution reduces stress concentration and thus improves the efficiency of load transfer from the epoxy to the alumina nanoparticles, and this phenomenon explains the enhancement in tensile and flexural properties at this concentration.

Figure 19. Scanning electron microscopy (SEM) images of the surface of epoxy nanocomposites (a) Pure epoxy; (b) 1 wt.% Nano-Al₂O₃; (c) 2 wt.% Nano-Al₂O₃; (d) 4 wt.% Nano-Al₂O₃; (e) 6 wt.% Nano-Al₂O₃

With an increase in alumina concentration above 2 wt.%, an increase in agglomerations was noticed; these agglomerations act as a stress concentration, leading to crack initiation and propagation, which leads to the deterioration of flexural and tensile properties of the nanocomposites. On the other hand, the surface hardness and impact resistance increase due to the presence of hard alumina nanoparticles, which increase the surface's resistance to scratching, penetration, and fracture. This behavior justifies increases in impact and hardness values up to 6 wt.%, as shown in Figures 19(d) and (e).