Effects of nano-SiO₂ on mechanical and hygric behaviors of glass fiber reinforced epoxy composites

2.1 Materials

An epoxy resin system, CYD-128/GA-327, is used as the matrix material, which is produced by Sinopec Baling petrochemical Co., Ltd. (Yueyang, China). CYD-128 is a bisphenol-A type epoxy resin and GA-327 is a polyamine hardener. The mix mass ratio of CYD-128/GA-327 is 100:45±2. Purity and molecular formula of the abovementioned materials are tabulated in Table 1. Nano-SiO₂ powder (RNS-A) is produced by WangWu Nano-Technology Co. Ltd. (Jiyuan, China). The nano-SiO₂ particles are sphere shape with diameter ranges from 25 to 50 nanometers. Type-E glass fibers (1200 Tex) produced by ZhongCai Science and Technology Co. Ltd. (Beijing, China) is used as reinforcement material.

Table 1:

Purity and molecular formula of CYD-128/GA-327.

Name	Purity	Molecular formula
CYD-128	Industrial grade
GA-327	Industrial grade

2.2 Preparation of GFRP composites

Two resin systems, the epoxy system and the SiO₂ modified epoxy system, were prepared to fabricate glass fiber prepregs through a wet-winding process. Then, the prepared prepregs were used to fabricate unmodified and nano-SiO₂ modified GFRP composites by the hot-compression molding process in this paper.

The epoxy system is the epoxy (CYD-128)/hardener (GA-327) system, the mass ratio of which is 100:45±2. The epoxy (CYD-128) was uniformly mixed with the hardener (GA-327) by 5 min stirring (1000 r/s). Then the epoxy system was used to prepare its glass prepreg by wet-winding process.

The nano-SiO₂ powder is necessary to disperse uniformly into the epoxy resin system before the prepreg preparation. First, the epoxy (CYD-128) and the nano-SiO₂ was uniformly mixed by 60 min mechanical-stirring (15000 r/s) under the 50°C constant-temperature water condition. The mass ratio of CYD-128/nano-SiO₂ is 100:1. Second, the epoxy with nano-SiO₂ was uniformly mixed with the hardener (GA-327) by 5 min stirring (1000 r/s). Then the SiO₂ modified epoxy system was used to prepare its glass prepreg by the wet-winding process.

Then, the prepared prepregs were used to fabricate unmodified and nano-SiO₂ modified GFRP composites. The GFRP is unidirectional composite and its thickness is 3 mm. The predetermined fiber volume fraction of all the fabricated composites is 60%. To verify proper laminates fabrication and quality, visual inspection and thickness measurements are performed on each laminate produced. Test samples are clear and free of voids, dry spots, or any other contaminants. The coefficient of thickness variation shall be within 3% of that predetermined thickness. The curing schedule is shown in Figure 1.

Figure 1:

The curing schedule of the GFRP composites.

2.3 Water absorption

The water immersion tests of the GFRP composites were carried out based on Chinese Standard GB1462-88. The dimension of the water-absorption sample is 50×50×3 mm. All the specimens were dried in an oven at 60°C for 24 h and then cooled to room temperature. This process was repeated until the mass of the specimens changed by <0.1 mg. Then each sample was weighed by the electronic balance and the weight was recorded as M₀. Afterward, the dry specimens were immersed in a water bath at 50°C. After that, the immersed sample was periodically taken from the water bath, dried with the filter paper and weighed for 10 min, and recorded its weight as M_t.

The moisture absorption (W_r) of the GFRP composites can be calculated by the following equation:

Where M₀ is the weight of the dry sample, and M_t is the weight of the hygric-aging sample.

2.4 Swelling determination

The volume swelling of the immersed composites was determined by the drainage method. The dimension of the immersed sample is also 50×50×3 mm. The immersed sample was tied with nylon silk and put on the bottom of the beaker, which has been filled with water and laid on the electronic balance. After the balance was reset, the sample was lifted until it was completely suspended in the water and the readings of the balance were recorder at the moment. The recorded weight was the weight of the sample in water. According to the drainage method, the volume swelling (V_s) of the sample can be calculated as the following equation:

Where M₀ and M_w0 are the weight of the un-aged sample in air and in water, respectively; M_t and M_wt are the weight of the immersed sample in air and in water, respectively.

2.5 Mechanical tests

The immersed composite was periodically taken from the water bath and cut into pieces within 60 min for mechanical testing.

The tensile and flexural properties of the composites were tested according to the Chinese Standard GB/1447-2005 and GB/1449-2005 using a CMT 5105 (Electromechanical Universal Testing Machine), which was produced by MTS Systems (China) Co., Ltd. (China). In these tests, the cross-head speed was 2 mm/min. The length, span, width and thickness of the tensile specimen were 250, 100, 25, and 3 mm, respectively. While the length, width and thickness of the flexural specimen were 80, 15, and 3 mm, respectively. The ratio of span to thickness of the flexural specimen was 16:1. Each reported data is the average of more than six successful samples.

A short beam shear test was conducted to determine the interlaminar shear strength (ILSS) values of the composites based on the standard ASTM D2344/D2344M (standard test method for short-beam strength of polymer matrix composite materials and their laminates) using CMT 5105. The thickness of the ILSS specimens is 3 mm, and the width is 6 mm. The ratio of span to thickness is 4:1. The crosshead speed is 0.2 mm/min. Each reported data is the average of more than six successful samples.

2.6 Scanning electron microscopy analysis (SEM)

For SEM a FEI Quanta 200, was used to observe the cross section of the composites. Cross sections of the composite samples were coated with gold and then analyzed at 20 kV.

3.1 Effects of nano-SiO₂ on the mechanical properties of the GFRP composites

Nano-particle has high surfactivity due to its high specific surface area and a large number of unsaturated bonds. Nano-particles can be considered as the cross-linking points of the molecular chain when it uniformly dispersed in the composite matrix. When the propagating crack encountered nano-particles with high strength and stiffness, crack termination, turning and offset may occur to prevent the crack propagation, thereby, the tensile and flexural properties of the GFRP composites can be effectively improved [14], [15]. It is supposed that the nano-particles with high surface energy can help to strengthen the matrix, reduce stress concentration and prevent crack propagation, which contribute to the increasing of tensile and flexural strength for the GFRP composites.

Figure 2 shows the tensile, flexural and ILSS of the GFRP and nano-SiO₂ modified GFRP. Where the GFRP and nano-SiO₂ modified GFRP represents the unmodified GFRP and 1 wt% nano-SiO₂ modified GFRP, respectively. It can be clearly seen that the tensile and flexural strength of the nano-SiO₂ modified GFRP is 9.1% and 7.9% higher than that of the GFRP, respectively. While the ILSS of the nano-SiO₂ modified GFRP is 10.6% lower than that of the GFRP.

Figure 2:

Effects of nano-SiO₂ on the mechanical properties of the GFRP composites.

The ILSS of the composites not only depends on the performance of the matrix, but also depends on the fiber/resin adhesion interface and strength. The presence of the nano-particles would reduce the adhesion area of the fiber and resin, which could result in the ILSS decrease of the composites.

The typical load-displacement curves of the GFRP and nano-SiO₂ modified GFRP under the tensile, flexural and short-beam load condition are shown in Figures 3–5, respectively.

Figure 3:

Typical load-displacement curves of the GFRP and nano-SiO₂ modified GFRP under the tensile load condition.

Figure 4:

Typical load-displacement curves of the GFRP and nano-SiO₂ modified GFRP under the flexural load condition.

Figure 5:

Typical load-displacement curves of the GFRP and nano-SiO₂ modified GFRP under the short-beam load condition.

3.2 Effects of nano-SiO₂ on the hygric behavior of the GFRP composites

Figure 6 shows the water absorption as a function of the immersion time for the GFRP composites. The curves show that the water-absorption rate of the GFRP is faster than that of the nano-SiO₂ modified GFRP during the initial-aging period. This could be explained by the diffusion model for the nano-composites in [14]. When the water molecules encounter the impermeable nanoparticles, it would take a tortuous pathway to bypass the nanoparticle and continue its diffusing process. That would increase the distance of the diffusing pathway and delay the diffusing development.

Figure 6:

Water absorption as a function of the immersion time for the GFRP composites.

After 25 days, the water absorption reached maximum, which was 3.08% and 3.26% for the GFRP and nano-SiO₂ modified GFRP. Then the water absorption of the GFRP decreased gradually with the aging time, while the water absorption of the nano-SiO₂ modified GFRP was almost unchanged. The reason could be that the resin hydrolysis results in the dissolution of the low-molecular substance from the GFRP, while the nanoparticles can delay this dissolution in the nano-SiO₂ modified GFRP.

After the water-absorption testing, the specimen aged 45 days were dried in an oven at 105°C for 72 h and then cooled to room temperature. This process was repeated until the mass of the specimens change <0.1 mg. Then the residual weight of the aged specimen can be calculated by the weighing measurement. And it is 98.9% and 99.2% of the un-aged specimen for the GFRP and nano-SiO₂ modified GFRP.

It could be concluded that the nano-SiO₂ modification can improve the moisture resistance and inhibit the matrix hydrolysis for the GFRP composites.

The effect of the nano-SiO₂ on the hydrolysis resistance of the GFRP composites could be explained by the results of the swelling experiments. Figure 7 shows the volume swelling as a function of the immersion time. It can be clearly seen that the swelling rate is very fast during the initial aging stage, then gradually slows down and finally approaches equilibrium for both the GFRP and the nano-SiO₂ modified GFRP. The difference is that the maximum swelling of the GFRP is 2.6 times as that of the nano-SiO₂ modified GFRP. It indicates that the volume swelling of the GFRP composites can be effectively prevented by the nano-SiO₂ particles.

Figure 7:

Volume swelling as a function of the immersion time for the GFRP composites.

3.3 Effects of hygric aging on mechanical properties of the GFRP composites

The normalized ILSS as a function of the immersion time for the GFRP composites is shown in Figure 8, where the normalized cardinal is the ILSS of the un-aged specimen. It can be observed that the hygric aging results in a sharp decrease of the normalized ILSS in the initial 20 days, then the decrease rate gradually slows down and finally approaches equilibrium for both the GFRP composites. However, the maximum decrease is distinctly different for the GFRP and the nano-SiO₂ modified GFRP. The normalized-ILSS decrease of the nano-SiO₂ modified GFRP is only 12% after 138 days aging, while that of the GFRP reaches 31%.

Figure 8:

Normalized ILSS as a function of the immersion time for the GFRP composites.

The normalized flexural strength as a function of the immersion time for the GFRP composites is shown in Figure 9, where the normalized cardinal is the flexural strength of the un-aged specimen. It can be clearly seen that the normalized flexural strength of the GFRP decreases with the increasing immersion time, while that of the nano-SiO₂ modified GFRP increases in the initial 50 days and then gradually decreases as the immersion time. After 95-days hygric-aging, the decrease of the normalized flexural strength is 15.3% for the GFRP, while the normalized flexural strength of the nano-SiO₂ modified GFRP still maintains an increase of 5.0%.

Figure 9:

Normalized flexural strength as a function of the immersion time for the GFRP composites.

3.4 SEM analysis

Based on the above discussion, it can be concluded that the nano-SiO₂ particle has a positive effect on the mechanical and hygric properties of the GFRP composites. Figure 10 shows the cross-section SEM-micrographs of the GFRP and nano-SiO₂ modified GFRP after 95-days aging. Continuous and larger cracks can be clearly observed in the cross-section of the GFRP as shown in Figure 10A, while only some sporadic and scattered cracks are distributed in the cross-section of the nano-SiO₂ modified GFRP as shown in Figure 10B. It could be concluded that the introduction of the nano-SiO₂ particles with high surface energy and activity can significantly decrease the volume swelling of the GFRP composites by reducing stress concentration and preventing crack propagation.

Figure 10:

SEM images of the cross-section of the GFRP composites after 95-days aging. (A) GFRP. (B) Nano-SiO₂ modified GFRP.