PSI - Issue 37

J.M. Parente et al. / Procedia Structural Integrity 37 (2022) 820–825 J.M. Parente/ Structural Integrity Procedia 00 (2019) 000 – 000

824

5

3.6. Pre-cure temperature and graphene content. The effect of pre-cure temperature on the mechanical properties of the neat resin is shown in table 4. The results indicate that lower pre-cure temperatures improved the mechanical results. Compared with the pre-curing carried out at room temperature (25°C), when this process is carried out at 7°C, increases around 22.4%, 27.9% and 1.1% were observed for the bending stress, bending stiffness and bending strain, respectively. However, when the temperature is increased to 30 and 40ºC, all bending properties decrease except the bending strain which increases at 40ºC. The improvement observed for the lower pre-cure temperatures may be due to the slower polymerization process generate more polymerization starting points, and, consequently, the obtained polymeric network promotes a stronger material. On the other hand, temperatures higher than room temperature are detrimental to the mechanical performance of the nanocomposite. Figure 3 shows the effect of pre-cure temperature and different graphene contents on the bending properties. It is noticed that lower pre-cure temperatures improve the bending stress regardless of the grapheme content. On the other hand, the best results in terms of bending stiffness and bending strain were achieved for 30ºC and 25ºC, respectively. Finally, the decrease in mechanical properties that is observed for higher graphene contents is due to the agglomeration effect, a phenomenon that has been duly reported by other authors ( Aswathnarayan et al. (2020); Doǧan et al. (2019) ).

Table 4 Bending properties for different pre-cure temperatures. Pre-cure temperature (ºC) Bending stress (MPa) Improvement (%)

Bending stiffness (GPa)

Improvement (%)

Bending strain (%) 4.70 (0.10) 4.65 (0.15) 4.65 (0.26) 5.04 (0.14)

Improvement (%)

7

125.5 (4.2) 102.,5 (4.4) 97.8 (7.5) 91.4 (3.3)

22.4

3.5 (0.19) 2.8 (0.28) 2.7 (0.56) 2.6 (0.13)

27.9

1.1

25 30 40

-

-

-

-4.58 -10.8

-3.6 -7.2

0

8.4

() =standard deviation.

a) c) Figure 3. Effect of graphene content and pre-cure temperature on the: a) Bending stress; b) Bending strain; c) Bending stiffness. 4. Conclusion It was possible to conclude that the presence of graphene in the epoxy resin increased the viscosity and the shrinkage. Regarding the mechanical performance, longer hardener mixing times and lower pre-cure temperatures improved the mechanical properties, especially at level of bending stress. Regarding the graphene content, the highest mechanical properties were obtained with 0.5 wt.% where, depending on the pre-cure temperature increases between 8% and 31% were found for the maximum bending stress. On the other hand, although a pre-cure at 7ºC leads to b)